196602111_ftp

Diels- Alder-Reac tions Part I: New Preparative Aspects[*]

BY PR1V.-DOZ. DR. J. SAUER

INSTITUT FUR ORGANISCHE CHEMIE DER UNIVERSITAT MUNCHEN (GERMANY)

Cycloadditions of conjugated dienes, named after their discoverers, have claimed preparative and mechanistic interest for nearly 40 years. The almost inexhaustible variability of the components of these one-stage reactions ofers entry to important classes of compounds. The systematization of the preparative uses of these reactions which is offered in this paper relates predominantly to recent results. The mechanistic aspects of the Diels-Alder reactions will be discussed later in the secondpart of this contribution.

A. Definition and History

B. The Diene Components

1. Acyclic Dienes 2. Acyclic Dienes with Heteroatoms 3. Polyenes 4. 1,2-Dimethylenecycloalkanes 5. Alicyclic Dienes 6. Cyclopentadienones 7. o-Quinones and o-Quinonoid Systems 8. Aromatic Compounds

A. Definition and History

The combination of conjugated dienes with olefins is only a special case of the more general reaction of cycloaddition 111 ; in the formation of the six-membered ring ( I ) , two new o-bonds are formed at the expense of two x-bonds.

Other examples of cycloadditions are epoxidations and carbene additions, dimerizations of olefins to cyclobutane derivatives, and 1,3-dipolar addition reactions [1,21.

Individual examples of diene additions according to the Diels-Alder scheme are found in the literature as early as the turn of the century. Although Zincke[31 correctly explained the dimerization to perchloroindenone (2) of the tetrachlorocyclopentadienone occurring as an intermediate in the pyrolysis of l-hydroxyperchloro- cyclopent-3-enecarboxylic acid, an elucidation of the structure of the 1 : 1 adduct (3) of p-benzoquinone and cyclopentadiene was first given by Diels and Alder[41;

[*I Part I1 will appear shortly in this Journal. [ I ] R. Huisgen, R. Grashey, and J. Sauer in S. Patai: The Chem- istry of Alkenes. Interscience, London 1964, p. 739. [2] R. Huisgen, Angew. Chem. 75,604,742(1963); Angew. Cheni. internat. Edit. 2, 565, 633 (1963). [3] T. Zincke and H. Giinther, Liebigs Ann. Chern. 272, 243 (1893); T. Zincke, ibid. 296, 135 (1897); T. Zincke and K. H. Meyer, ibid. 367, 1 (1909).

9. Heterocyclic Compounds

C. The Dienophilic Components 10. Olefins and Non-Conjugated Dienes

1 . Acyclic Alkenes and Alkynes 2. Allenes 3. Cyclic Dienophiles 4. Cyclic Azo Compounds 5 . Other Dienophiles with Heteroatoms

D. Retro-Diels-Alder Reactions E. Future Prospects

structures proposed earlier by Stuudinger (with a cyclobutane ring) and Albrecht proved to require revision.

r 1

(3 )

H O

The structure of the 1 : 1 addition compound (4) from cyclopentadiene and diethyl azodicarboxylate 151 first indicated that these addition reactions of dienes were cases

(41

[4a] 0. Dieis and K. Alder, Liebigs Ann. Chem. 460, 98 (1928).

Angew. Cliem. iilternot. Edit. ,! Vol. 5 (1966) J No. 2 21 1

of a more general reaction principle. The classical paper of Diels and Alder [4al provided the mental breakthrough and introduced the fruitful preparative and mechanistic investigations of these authors. Complete treatment of the extensive experimental ma- terial is possible only within the scope of a monograph. In this survey, the reviews that appeared before 1955 [6-13bl and a recent article 111 will be supplement- ed by newer examples from the literature.

B. The Diene Components

1. Acyclic Dienes

Owing to the possibility of rotation about the single bond between the two conjugated double bonds, open- chain dienes can occur in two conformations. With butadiene and its simple alkyl derivatives, the trunsoid form (5 ) frequently predominates in the conformer equilibrium [141. However, only the cisoid conformer (6) is capable of taking part in the Diels-Alder reaction.

t r a n s o i d (5 ) c i s o i d (6)

Substituents in butadiene influence the rate of cycloaddition to form six-membered rings, both through their electronic nature and by displacing the conformational equilibrium. By combining butadiene with dimethyl maleate or fumarate in toluene at 155 "C the cis- (7) and the trans- form (8), respectively, of the cyclohexene-4,5-dicarboxylic ester are readily accessible via a stereospecific cis- addition [151.

[4b] 0. Diels, K. Alder, G . Stein, P . Pries, and H . Winckler, Ber. dtsch. chem. Ges. 62, 2337 (1929). [5] 0. Diels, J . H . Blom, and W. Koll, Liebigs Ann. Chem. 443, 242 (1925). [6] K. Alder in W. Foerst: Neuere Methoden der Praparativen Organischen Chemie. Verlag Chemie, Berlin 1943, Part I, p. 251. [7] K. Alder in K. Ziegler: Praparative Organische Chemie. Ver- lag Chemie, Weinheim 1953, Part 11, p. 125. [8] K. Alder and M . Schumacher in L. Zechmeister: Fortschritte der Chemie Organischer Naturstoffe. Springer-Verlag, Vienna 1953, Vol. X, p. 1. [91 K. Alder in Experientia Supplementum 11, 86 (1955). [lo] J . A . Norton, Chem. Reviews 31, 319 (1942). [ I l l M . C . Kloetzel, Org. Reactions 4, 1 (1948). [I21 H . L. Holmes, Org. Reactions 4, 60 (1948). [I31 L. W . Butz and A. W . Rytina, Org. Reactions 5, 136 (1949). [13a] A . S . Onishchenko: Dime Synthesis. Translation from the Russian by the Israel Program for Scientific Translations, Jeru- salem, 1964; obtainable through Oldbourne Press, London. [ 13bl A. Wassermann: Diels-Alder Reactions. Elsevier, New York 1965. [I41 J. Gresser, A . Rajbenbach, and M . Szwarc, J. Amer. chem. SOC. 82, 5820 (1960); D. Craig, J . J . Shipman, and R. B. Fowler, ibid. 83, 2885 (1961); W . B. Smith and J. L . Massingill, ibid. 83, 4301 (1961). [I51 A. A . Pefrov and N. P . Sopov, Sb. Statei Obshch. Khim. 2, 853 (1953); Chem. Abstr. 49, 5329 (1955).

1 -Substituted butadienes can be used almost without exception provided the substituent R is in the trans- arrangement (9), while the cis-form (10) generally undergoes a Diels-Alder reaction only with poor yield.

H

(10)

The extremely acid-sensitive 1-diethylaminobutadiene (9), R = N(C2H5)2, adds to ethyl acrylate in benzene at 20 "C to form the 1 : 1 cis-adduct ( I I ) , which changes readily into 2,3-dihydrobenzoic acid 1171. In the analogous reactions of 1,4-bis(dialkylamino)butadiene the two-fold elimination of amine from the Diels-Alder adduct yields the aromatic system [181.

(11) 94%

1,4-Dihydroarornatic compounds, e.g. (12), are obtained in excellent yields when butadienes react with dienophiles containing triple bonds 1191 (at about 150 "C in toluene in the example illustrated).

112) 81%

l-Hydroxybuta-1,3-dienes can be regarded a s tautomers of the a,$-unsaturated aldehydes and ketones. As an example, we select a reaction of 2-methylpent-2-enal (2-ethyl-1-methyl- acrolein) [71; here the primary adduct (13) can be isolated [201.

[I61 Review of the reactions of I-substituted butadienes: I . I . Gurseinov and G. S. Vnsil'ev, Russ. chem. Rev. 32, 20 (1963). (English transl. of Usp. Khim.). [I71 S. Hiinig and H. Kahanek, Chem. Ber. 90, 238 (1957). [ 181 M . F. Fegley, N. M . Bortnick, and C. H. McKeever, J. Amer. chem. SOC. 79, 4736 (1957). [I91 See, e.g. , N . P. Sopov and V . S . Miklashevskaya, Zh. obshch. Khim. 26, 1914 (1956); Chem. Abstr. 51, 4968 (1957). [20] H . Meerwein, Ber. dtsch. chem. Ges. 77, 227 (1944).

212 Angew. Chem. internat. Edit. / Vol. 5 (1966) / No. 2

The mechanistic relationship of this sequence of reactions to the Diels-Alder scheme has not, however, been shown. Re- cently, anthrone has been made t o react with ethylene in the presence of bases according t o the same scheme t o give (14) in 60 76 yieldr211. (18) OAc OAc

The adducts (15) of 2-alkoxybutadienes with maleic anhydride and its derivatives, which a re obtained in high yields, can, as enol ethers, easily be hydrolysed to cyclo- hexanone derivatives [221. I n the case of 2-methylene-l,4-

COzCH3 ~ll,O,C-C=C-CO,CR, i - . Q OAc COzCH3 4 9%

Tetraalkylbutadienes are eminently suitable for diene additions provided they do not exhibit the structural ele- ment (10) of cis-1-substituted butadienes; 1,l'-bicyclo- pentenyl and 1 ,l'-bicyclohexenyl (19) ~ 8 1 lead to polycyclic compounds 1291, as illustrated here for nitroethyl- ene as the dienophile.

pentadiene (2-vinylbutadiene), the reaction with maleic anhydride does not s top at the stage of the 1 : 1 adduct (16) ; the new diene system adds on a second molecule of the dieno-

Hexacyanobutadiene (20) 1301 acts not as a diene but as a dierophile with respect to butadiene, i. e. it behaves as

phile t o give the 1 : 2 adduct [231.

Disubstituted dienes have been used very frequently; Section '). 2,3-diphenylbutadiene, for example, gives o-terphenyl H C & H ~ NC,@N

butadiene (18) with methyl acrylate forms the first step

a derivative of the very reactive tetracyanoethylene (see

derivatives 1241. The reaction of fruns,tvuns-l,4-diacetoxy-

of a total synthesis of shikimic acid (17) [*51; a second NC ' C N N

HC ' Y H 2 + I iCOkjjOcN - qN (20)

OAc

?Ac

route to this compound begins with the Diels-Alder reaction between butadiene and propiolic acid (yield 85 "/o) 1261. Diacetoxybutadiene (f8) has very recently been recognized as a suitable diene component for the synthesis of benzene derivatives without isolation of intermediate products [271.

[21] J. S . Meek, W. Brice, V . Evans Godefroi, W. R. Benson, M . F. Wilcox, W. G . Clark, and T. Tiedeman, J. org. Chemistry 26, 4281 (1960). [22] M . S. Newmnri and H. A. Lloyd, J . org. Chemistry 17. 577 (1952). [23] A. T. Blornquist and J. A. Verdol, J . Amer. chcrn. Soc. 77, 8 I (1955). [24] K . Alder and J. Haydn, Liebigs Ann. Chem. 570, 201 (1950). [25] E. E. Stnissman, J. T. Sah, M . Oxman, and R . Daniels, J . Arner. chern. Soc. 84, 1040 (1962). [26] R . Grewe and I . Hinrich, Chem. Ber. 97, 443 (1964). [27] R . K. Hill and R . M . Carlson, Tetrahedron Letters 1964, 1157; J . org. Chemistry 30, 2414 (1965).

2. Acyclic Dienes with Heteroatoms

Whereas one or both carbon atoms in the dienophile component can be replaced by heteroatoms (see Sec- tions C. 4 and C. 5), there are fewer analogous examples in the case of the dienes1311. Replacement of carbon atom 1 in 1,3-dienes by oxygen yields cr$-unsaturated aldehydes and ketones which combine preferentially with electron-rich double bonds, enol ethers, olefins 1321, enamines [331, N-vinylcarbamic esters, and N-vinyl- ureas 1341 to form 1 :1-adducts; these addition compounds arise formally from a Diels-Alder reaction. Hydrolysis of the enamine adducts (21) and the enol ether adducts yield 1,5-dicarbonyl compounds, which are valuable starting compounds for syntheses of al- kaloids 1351. With acrolein (22) as the diene component,

[28] For example: E. D. Bergmann, H . Davies, and R. Pappo, J. org. Chemistry 17, 1331 (1952). [29] N. L . Drake and C. M . Kraebel, J. org. Chemistry 26, 41 (1961). [30] 0. W. Websrer, J . Amer. chern. SOC. 86, 2898 (1964). [3 I ] A review of heterodienes and heterodienophiles is given by S. B. Needleinan and M. C . Chang Kuo, Chern. Reviews 62, 405 (1962). [32] C . W . Smith, D. C . Norton, and S. A . Ballard, J. Amer. chem. SOC. 73, 5267. 5273 (1951). [33] G. Opitz and I . Luschnzanri, Angew. Chem. 72, 523 (1960). [34] R . C . Schnlz and H . Hartmann, Chem. Ber. 95, 2735 (1962). [35] K. Alder, H . Betzing, R. Kuth, and H. A . Dorrmann, Liebigs Ann. Chem. 620, 73 (1959).

Angew. Chem. internat. Edit. / Vul. 5 (1966) No. 2 213

(22) (21) 7970

the use for less reactive dienophiles is limited by the competing dimerization of the unsaturated aldehyde, which formally is also a Diels-Alder reaction [361.

The reaction of compound (23), prepared in situ from methanesulfonyl chloride and triethylamine, with vinylogous amides can also be regarded as a Diels- Alder reaction [371.

(23) R = CH,, R ’ = H, 80%

3. Polyenes

The mode of reaction of conjugated polyenes in diene addition can be understood by regarding them as substituted butadienes. The case of 2-vinylbutadiene has already been mentioned in Section B. l. trans-Hexa-1,3,5- triene reacts as 1-vinylbutadiene; the hydrogenation product (24) of the Diels-Alder adduct with maleic anhydride can also be obtained from trans-hexa-1,3- diene [381.

cis-Hexa-l,3,5-triene, as a cis-I-substituted butadiene ( lo ) , R = CH=CHz, does not react with dienophiles, so that it can be separated from the trans isomer by the Diels-Alder reaction 1391. Of the trans-w,w,w’w’-tetra- phenylpolyenes (25), only the representatives with

1361 K. Alder and E. Ruden, Ber. dtsch. chem. Ges. 74,920 (1941); K. Alder, H. Offermanns, and E. Riiden, ibid. 74, 905 (1941). [37] G. Opitz and E. Tempel, Angew. Chem. 76, 921 (1964); An- gew. Chem. internat. Edit. 3, 754 (1964). [38] K. Alder and H. von Brachel, Liebigs Ann. Chem. 608, 195 (1957). [39] J. C. H . Hwa, P . L. De Benneville, and H . J . Sims, J. Amer. chem. SOC. 82, 2537 (1960).

n > 1 are capable of adding dienes; with large values of n, Alder and Schamacher observed a relationship between the chain length and the position of addition of the dienophile 1401.

C= CH- ( CH= CH)n- CH= c 6 H t

6Hg

4. 1,2-Dimethylenecycloalkanes

1,2-Dimethylenecycloalkanes (26), in which the cisoid conformation (6) necessary for reaction is fixed by the linkage of the 2,3-positions of the diene through a ring, are characterized by high reactivity 1411. This, and the ready accessibility of this class of compounds by the pyrolysis of esters or the Hofman elimination of suitable bifunctional quaternary ammonium salts 1421, make the 1,2-dimethylenecycloalkanes (26) the favorite dienes for the synthesis of polycyclic compounds, e.g. (27) ; the reaction of 1,2-dimethylenecyclohexane [(26), n = 41 may serve as an example [431.

(27) 8570

With (28), the Diels-Alder addition goes as far as the 1 : 2- adduct 1441; with (29) only two of the three diene systems a r e

(28J

used for the addition of the dienophiIe[451. The dioxolane derivatives (30) react only with the “record” dienophile tetracyanoethylene [461.

The four-membered cyclic diene (26), n = 2, adds to one equivalent of maleic anhydride at 20-80 OC; for the formation of the 1 : 2-adduct an opening of the four-mem-

[40] K. Alder and M. Schumacher, Liebigs Ann. Chem. 570, 178 (1950). For further examples of polyene addition, see 181. 1411 Kinetic measurements: J. Sauer, D. Lang, and A. Mielerf, Angew. Chem. 74, 352 (1962); Angew. Chem. internat. Edit. 1, 268 (1962); D. Lang, Dissertation. Universitat Miinchen 1963. 1421 Review: Y. A. Titov, Russ. chem. Rev. (Engl. transl. of Usp. Khim.) 30, 327 (1961). [43] W. J. Bailey and H. R. Golden, J. Amer. chem. SOC. 79, 6516 (1957). [44] W. J. Bailey, E. J . Feffer, and J. Economy, J. org. Chemistry 27, 3479 (1962). [45] H. Hopffand G. Kormany, Helv. chim. Acta 46, 2533 (1963). 1461 J. B. Miller, J. org. Chemistry 25, 1279 (1960).

214 Angew. Chem. internot. Edit. 1 Vol. 5 (1966) No. 2

bered ring of the 1 : 1-addition compound (31) has been discussed 1471. In the reactions of benzocyclobutene derivatives (32) with dienophiles (see Table l), a primary

ring opening to the 1 ,Zdimethylene derivative (33), which here has an o-quinonoid structure, is also as- sumed [4*,491. The diiodo compound (32) ( X = I) reacts similarly with N-phenylmaleimide [491 to give 35 % of the naphthalenedicarboxylic derivative.

- moo (35)

0

Table 1. anhydride.

Reaction of benzocyclobutene derivatives (32) with maleic

(32), X I TrT 1 Yield of (34) [ % I I Yield of (3.51 [%I

Huisgen and SeidI[5ol have recently shown that a valence-tautomeric equilibrium (32) + (33), in which the four-membered ring opens stereospecifically, pre- cedes the Diels-Alder reactions of cis- and truns-2,3-di- phenylbenzocyclobutenes [(32), x = C ~ H S ] .

“0-Quinonoid” I ,2-dimethylenecycloalkenes have been postulated as intermediates in other reaction sequences. Thus the highly reactive 5,6-dimethylenecyclohexa-l,3-diene (o-quino- dimethane) (36), which cannot be isolated, can be trapped in

1471 A. T. Blornquist and J. A . Verdol, J. Amer. chem. SOC. 77, 1806 (1955); 78, 109 (1956); K. Alder and 0. Ackermann, Chem. Ber. 87, 1567 (1954). [48] F. R . Jensen, W. E. Coleman, and A. J. Berlin, Tetrahedron Letters 1962, 15. [49] M . P. Cava and M . J. Mitchell, J. Amer. chem. SOC. 81,5409 (1959); M . P. Cava, A . A . Deana, and K. Muth, ibid. 81, 6458 (1959); M . P. Cava, R . L. Shirlty, and B. W. Erickson, J. org. Chemistry 27, 755 (1962). [SO] R. Huisgen and H. Seidf, Tetrahedron Letters 1964, 3381; cf. also G. Quinkert, K . Opitz, W . W. Wiersdorff, and M . Finke, Tetrahedron Letters 1965,3009; G. Quinkert, Pure appl. Chem. 9, 607 (1964).

solution with maleicanhydride, N-phenylmaleimide, oracrylo- nitrile [51,521. The “in siru” occurrence of 2,3-dihydronaphthalene and 2,2-dimethyl-2H-indene has been shown similarly

by means of trapping reactions [53,511. Both compounds, like (36), were obtained by dehalogenation of dibromo compounds. An intermediate (39) with the dihydronaphthalene unit occurs also in the two-stage valence isomerization of the benzocyclobutadiene dimer (37) to dibenzocyclooctatetra- ene (38). Compound (39) can be detected by a Diels-Alder reaction with N-phenylmaleimide 1541.

The expectation that the Diels-Alder reaction of (40) with tetracyanoethylene would lead to a derivative of cyclobutadiene proved incorrect; instead an adduct with spiro structure 1551 is formed.

€ H5ct- .,-

+

5. Alicyclic Dienes

The Diels-Alder reactions of alicyclic dienes claim both historic and preparative interest. Diels and Alder recognized the general validity of the reaction scheme with cyclopentadiene [41. For example, with maleic anhydride, the 1 : 1-adduct (41) arises stereospecifically in quantitative yield.

d

The preferred endo-orientation in the addition is observed with many reactions of cyclic dienes, and is a characteristic of this type of reaction [561. Thus endo-di-

[51] K . Alder and M . Fremery, Tetrahedron 14, 190 (1961). [52] M . P. Cava, A . A . Deana, and K. Muth, J. Amer. chem. SOC. 81, 6458 (1959). [53] I . G. Dinulescu, M . Avram, and C . D . Nenitzescu, Chem. Ber. 93, 1795 (1960); M . Avram, I . G. Dinulescu, D . Dinu, and C . D . Nenitzescu, Chem. and Ind. 1962, 555. [54] M . Avram, I . G. Dinuslescu, D . Dinu, G. Mateescu, and C. D . Nenitzescu, Tetrahedron 19, 309 (1963). [ 5 5 ] A . T. BIomquist and Y. C. Meinwald, J. Amer. chem. SOC. 81, 667 (1959); the reaction of the corresponding dimethyl compound is described by R. Criegee, Angew. Chem. 74, 703 (1962); Angew. Chem. internat. Edit. I , 519 (1962). [56] Review of the stereochemistry of Diels-Alder reactions: J. G. Martin and R. K . Hill, Chem. Reviews 61, 537 (1961).

Angew. Chem. internat. Edit. / Vol. 5 (1966) No. 2 215

cyclopentadiene (42) is formed in high purity from the monomer; in this reaction, cyclopentadiene acts both as a diene and as a dienophile. This phenomenon, already

observed with acrolein, will be encountered again with cyclopentadienones (see Section B.6), o-quinones (see Section B.7), and thiophene 1,l-dioxide (see Section B.9). Compound (42) serves as a source of cyclopentadiene, which can be obiained from it at 180 “C in a retro-Diels-Alder reaction. The combination of cyclopentadiene with alkenes and alkynes leads in single-stage syntheses and in high yields to derivatives of the angularly strained bicyclo[2.2.l]hept-2-ene and bicyclo[2.2.l]hepta-2,5-diene. Substituted cyclopentadienes yield camphor derivatives in a few reaction steps [El, Bicyclo-

C L C l

(431 (430)

heptadiene itself is made industrially f rom cyclopentadiene and acetylene in 60 to 65 % yieldr571; with hexachlorocyclopentadiene [581 it gives quantitatively compound (43), which has become known a s a n insecticide under the name Aldrin; many other insecticides and heat-resistant polymers are derived from the Diels-Alder adducts of hexachlorocyclopentadiene.

Like cyclopentadiene itself, hexafluorocyclopentadiene (43a), which has been synthesized only very recently, tends to undergo dimerization [58al, while hexachlorocyclopentadiene is stable as a monomer. The reactions of (430) with dienophiles such as ethylene, maleic anhydride, or bicyclo[2.2.l]hepta-2,5-diene take place smoothly; on the other hand, no reaction is obtained with tetrafluoroethylene or tetracyanoethylene. With respect to dienes (e.g. cyclopentadiene or anthracene), (43a) behaves sometimes as a diene and sometimes as a dienophile. The reactivity decreases on passing to higher monocyclic conjugated dienes ; cyclooctadiene has almost no ability for additionl91; dienes with 14- and 15-membered rings again react with dienophiles, although under relatively severe conditions. The Diels-Alder adducts of cycloheptatriene and cyclo- octatriene are derived from the valence-bond tautomers (44) and (45), respectively [59-621. Whereas the tauto-

[57] US.-Pat. 2875256 (Feb. 24th, 1959), J. Hyman, E. Freireich, and R. E. Lidov; Chem. Abstr. 53, 13082 (1959). [58] Reviews of the reactions of hexachlorocyclopentadiene: H . E. Ungnade and E. T. McBee, Chem. Reviews 58, 249 (1958); C. W . Roberts, Chem. and Ind. 1958, 110. [%a] R. E. Banks, A . C . Harrison, R. N . Hasreldine, and K. G. Orrell, Chem. Commun. (London) 1965, 41. [59] K . Alder and G. Jacobs, Chem. Ber. 86, 1528 (1953). [60] R. Huisgen and W . D. Wirth, unpublished work. [61] W . Reppe, 0. Schlichting, K . Klager, and T. Toepel, Liebigs Ann. Chem. 560, 1 (1948). [62] A. C . Cope, A . C . Haven, F. L . Ramp, and E. R. Trumbull, J. Amer. chem. SOC. 74, 4867 (1952).

meric equilibrium between the monocyclic and the bicyclic compound can be detected in the eight-membered cyclic triene [6*1(15 % of the bicyclic tautomer is present in the equilibrium mixture at lOO”C), the existence of such an equilibrium for the seven-membered cyclic triene is still uncertain; it seems likely, however, from the recent investigations of Vogel et al. 1631 on oxepin [(48), R = HI and its dimethyl derivative [(48), R = CH31.

(46) 77% (44)

(45)

f49) (50) R = CH3: 100%

The Diels-Alder adducts are again derived from the bicyclic compound (49) ; spectroscopic investigations have shown a valence-tautomeric equilibrium (48) $ (49) (in each case R = H), which is established very rapidly. However, spectroscopic detection fails with bicyclo- [4.2.0]octa-2,5,7-triene, a valence-bond isomer of cyclooctatetraene. Nevertheless, Huisgen and Mietzsch demonstrated a small equilibrium concentration (ca. 0.01 %) by kinetic studies [641. At 165-170 OC, a product analogous to (47) but poorer by 2 H is formed in 87 yield with maleic anhydride. Heating of cyclooctatetraene gives several dimers [651.

G. Schriider recently revised the structure proposed for one of them, and assigned to it the constitution (51) M I ; he used this compound for his elegant synthesis of bull- valene (52) 1671. The typical Diels-Alder additions of the

___ [63] E. Vogel, R . Schubart, and W . A . Boll, Angew. Chem. 76, 535 (1964); Angew. Chem. internat. Edit. 3, 510 (1964); E. Vogel, W . A . Boll, and H. Giinther, Tetrahedron Letters 1965, 609. [64] R. Huisgen and F. Mietzsch, Angew. Chem. 76, 36 (1964); Angew. Chem. Internat. Edit. 3, 83 (1964). [65] W . 0. Jones, Chem. and Ind. 1955, 16. [66] G. Schroder, Chem. Ber. 97, 3131 (1964). [67] G. Schroder, Chem. Ber. 97, 3140 (1964).

216 Angew. Chem. internot. Edit. 1 Vol. 5 ( I966) No. 2

cyclohexadiene system in (51) with dienophiles have facilitated the elucidation of the structure of the dimer

It may be recalled that cycloheptatriene reacts with ni- trosobenzene [681 or azodicarboxylic ester 1691 to give l : 1- adducts (53) and (54)respectively, which are not derived from the valence tautomer (44) . Compound (54) opens up a productive route to tropylium salts L691.

(51).

monomers (58) or dimers (59) [74J. Like cyclopentadiene, cyclopentadienones may act as diene and dienophile components.

(58) R'

I

Table 2. Reaction 2 (58) + (59) as a function of the substituents.

Fulvenes can be regarded as derivatives of the alicyclic cyclopentadiene; dienophiles add onto the cisoid diene system of the ring[70J*J. The splitting into the starting materials, which takes place with the Diels- Alder adducts of the fulvenes even under mild conditions, makes it difficult in many cases to elucidate the configuration of the products; a critical study by Wood- ward and Baer has clarified the situation with the penta- methylenefulvene (55) maleic anhydride system 1721. The reaction leading to the endo-adduct (56) at low temperatures is reversible; this enables the formation of the thermodynamically more stable exo-adduct (57) at 80°C.

Q

The position of the adduct + components equilibrium and the rate of the Diels-Alder addition have been measured in the diarylfulvene-tetracyanoethylene and diarylfulvene-acetylenedicarboxylic ester systems 1731.

6. Cyclopentadienones

The diene activity of cyclopentadiene is retained in cyclopentadienone and its derivatives (58). Depending on the substituents (see Table 2), cyclopentadienones exist as

[68] G. Kresze and G . Schulz, Tetrahedron J2, 7 (1961). [69] J . M . Cinnamon and K. Weiss, J . org. Chemistry 26, 2644 (1961). [70] Review on fulvenes: J. H. Day, Chem. Reviews 53, 167 (1953). [71] Examples: K. Alder and R . Riihmann, Liebigs Ann. Chem. 566, 1 (1950); D . Craig, J . J . Shipman, J . Kiehl, F. Widmer, R . Fowler, and A. Hawthorne, J. Amer. chem. SOC. 76, 4573 (1954); C. H . DePuy and P. R . Story, ibid. 82, 627 (1960). 1721 R. B. Woodwardand H. Baer, J. Amer. chem. Soc. 66, 645 (1944). [73] G. Kresze, S . Rau, G . Sabelus, and H . Goetr, Liebigs Ann. Chem. 648, 57 (1961).

Forms present (58), R' I R2 I R3 I R4 I

2 (58) \+ (59)

The tendency towards dimerization is particularly pronounced with the unsubstituted compound (58a). All attempts to prepare this compound - e.g. by a retro- Diels-Alder reaction - gave only the dimer [751. However, the brief occurrence of cyclopentadienone (58a) - i. e. a finite lifetime for this compound - is suggested by a trapping experiment [76,77J; ( M a ) again exhibits diene and dienophile properties, in this case in the reaction with cyclopentadiene. The N,N-dimethylhydrazone of

(58a), on the other hand, exists in the monomeric forrn[781. The tendency of (58a) to dimerize can be de- duced from its electron distribution 1791. Tetrachloro- cyclopentadienone could be trapped as a monomer with cyclopentadiene and norbornadiene L79al.

All cyclopentadienone derivatives existing in the monomeric form and the dimers which dissociate can take part

... ~

[74] C . F. H . Allen and J. A . van Allan, J. Amer. chern. SOC. 72, 5 165 (1950); review of reactions of cyclopentadienones: C. F. H . Allen, Chem. Reviews 37, 209 (1945); 62, 653 (1962). [75] K. Alder and F. H. Flock, Chem. Ber. 87, 1916 (1954); C. H . DePny and C. E. Lyons, J. Amer. chem. SOC. 82, 631 (1960). [76] K . Hafner and K. Goliasch, Chem. Ber. 94, 2909 (1961). [77] C. H. DePuv, M . Isaks, and K. L. Eilers, Chem. and Ind. J961, 429; C. H. DePuy, M . Isaks, K. L. Eilers, and G . F. Morris, J. org. Chemistry 29, 3503 (1964). [78] K. Hafner and K. Wugner, Angew. Chem. 75, 1104 (1963); Angew. Chem. internat. Edit. 2, 740 (1963). [79] R . D . Brown, J . chem. SOC. (London) 1951,2670; E. D . Berg- mann, G. Berthier, D . Ginsburg, Y. Hirshberg, D . Lavie, S . Pin- rhas, P . Pullman, and A. Pullman, Bull. Soc. chim. France (5) 18, 661 (1951). [79a] W . H . Dietsche, Tetrahedron Letters 1966, 201.

217 Aiigcw. Cheni. internut. Edit. 1 Vol. 5 (1966) ! No. 2

in Diefs-Alder reactions as diene components. With double-bond dienophiles as co-reactants, the 1 : 1-adducts occurring as primary products can as a rule be isolated; when the temperature is raised, the CO bridges break [741.

The primary adducts with alkynes can be isolated only in exceptional cases; the tendency towards aromatization possibly brings about the spontaneous elimination of C 0 [ 7 4 , 8 0 1 . Even the highly strained 1,2,4,5- tetra-t-butylbenzene can be obtained in good yield in this way from (60) [811.

An interesting complication occurs in the reaction of tetraphenylcyclopentadienone with bicyclo[2.2.l]heptadiene (in CHC13 under reflux) [821. The angulary strained 1 : 1-adduct (61) undergoes a retro-Diels-Alder reaction under astonishingly mild conditions; here the tendency towards aromatization is an additional driving force.

0 9 2 % 3 2%

+ co

It is not only cyclopentadienone (58a) that is characterized by an extraordinary tendency to undergo dimerization, but also the ketals (62) derived from it. Earlier attempts to obtain the monomers (62d) gave only the dimers [8%l.

(62) RO OR

a R = CHI c R-R = -(CH2)3- b R = CzH5 a R-R = - (cH~)~-

Eaton and Hudson [82bl recently showed that the ketals (62a) -(62d) have finite lifetimes. Trapping experiments with some dienophiles gave the expected Diels-Alder

[80] J. J . Dudkowski and E. I . Becker, J. org. Chemistry 17, 201 (1 952). [81] C. Hoogzand and W. Hubel, Tetrahedron Letters 1961, 637. [82] K. Mackenzie, J. chem. SOC. (London) 1960, 473. [82a] E. Vogel and E. G . Wyes, Angew. Chem. 74, 489 (1962); Angew. Chem. internat. Edit. I, 404 (1962); C. H. DePuy, B. W. Ponder, and J. D . Fitzpatrick, J. org. Chemistry 29, 3508 (1964). [82b] P. E. Eaton and R. A . Hudson, J. Amer. chem. SOC. 87, 2769 (1965).

adducts in good yields. The rate of dimerization of corn pounds (62) depends on the group R; (62d) dimerizes about 1100 times faster than (62a), and (626) almost 500 times faster than cyclopentadiene.

7. o-Quinones and o-Quinonoid Systems

The dimerization observed with many o-quinones shows that these compounds are in principle capable of reacting as dienes and dienophiles, e.g. to give (63) [83,841; however, as a rule, their capacity for acting as dienes predominates 1851. Thus the structure (64) of the 1 : 1-adduct

(63) 30-5070

from cyclopentadiene and tetramethyl-o-benzoquinone [861 proved to require revision : cyclopentadiene takes part in the reaction as the dienophile and (65) is formed [871.

o-Quinones contain two diene systems - the carbocyclic system of the six-membered ring and that of the 1,2- diketo grouping with two heteroatoms. Horner and Merz [881 observed an interesting competition of these two

64% 2 3%

c1

[83] J. Harley-Mason and A. H. Laird, J. chem. SOC. (London) 1958, 1718. [84] L. Horner and K. Sturm, Liebigs Ann. Chem. 597, 1 (1955); L . Horner and W. Diirckheimer, Chem. Ber. 95, 1219 (1962). [85] L. W. Butzand A . W. Rytina, Org. Reactions 5, 136 (1949); J . A . Barltrop and J. A . D. Jeffreys, S. chem. SOC. (London) 1954, 154. [86] L. I. Smith and L. R. Hac, J. Amer. chem. SOC. 58, 229 (1936). [87] L. Horner and W. Spietschka, Liebigs Ann. Chem. 579, 159 (1953). [88] L. Horner and H. Merz, Liebigs Ann. Chem. 570, 89 (1950).

218 Angew. Chem. internat. Edit. I Vol. 5 (1966) J No. 2

diene systems in tetrachloro-o-benzoquinone (66). Ad- dition to the cyclohexadiene system or to the hetero- diene system takes place depending on the nature of the dienophile (styrene, 1 , I -diphenylethylene); with cyclopentadiene as the dienophile (all reactions at 80°C in benzene) both reactions occur to comparable extents. Tetrachloro-o-benzoquinone is, however, inert to typical dienophiles such as acrylonitrile and maleic anhydride.

On the basis of the addition reactions with cyclopentadiene and maleic anhydride as dienophiles, it has been found that the chlorination products of phenol and some methylphenols possess the o-quinonoid structure (67) and not the hypohalite structure (68) (R1-R4 = C1 or CH3) 1891.

The tendency of the o-quinones to dimerize is retained even in 6,6-disubstituted cyclohexadienones [90,911, as can be seen from the example of methyl-o-quinol acetate (69) 1921.

6870

It is also reasonable to assume an o-quinonoid intermediate (70) for reactions of certain Mannich bases

1 8 ooc

70) 5 0 - 6 5 %

[89] 5’. Kumamoto, J. chern. SOC. Japan, ind. Chem. Sect. (Kogyo Kagaku Zasshi) 64, 188 (1961); Chem. Abstr. 58, 2391f (1963). 1901 D. Y. Curtin and R. R . Fraser, J. Amer. chem. SOC. 81, 662 (1959). [91] K . Alder, F. H . Flock, and H . Lessenich, Chem. Ber. 90, 1709 (1957). [92] W . Metlesics and F. Wessely, Mh. Chem. 88, 108 (1957); W. Metlesics, F. Wessely, and H. Budzikiewicz, ibid. 89, 102 (1958); F. Wessely and H . Budzikiewicz, ibid. 90, 62 (1959).

with dienophiles 1931. The reaction again takes place preferentially with electron-rich dienophiles such as butadiene, styrene (55 %), isobutylene (45 %), and 1,l-diphenylethylene (87 %). The intermediate (70) is also accessible in situ by intramolecular elimination of water from (71) . Analogous reactions are possible in the benzene series with o-dialkyl(aryl)aminomethylphenols or o-hydroxy- methylphenol as sources of dienes, although the yields are only moderate 1931.

8. Aromatic Compounds

In the adducts from aromatic compounds and dienophiles, the low-energy state of cyclic conjugation is completely destroyed. Since the loss of aromatic resonance may already be quite considerable in the transition state of cycloaddition, high activation energies, i. e. low reactivities, are to be expected for the Diels-Alder additions of this class of compounds. In agreement with theoretical considerations [941, the diene properties increase in the sequence benzene < naphthalene < anthracene. As expected, benzene itself takes part in Diels-Alder reactions only very sluggishly, and no 1 : 1- adducts have yet been isolated; the formation of 1,2- bis(trifluoromethy1)benzene in the reaction of benzene with hexafluorobut-2-yne can, however, be rationalized as a normal diene addition with subsequent elimination of acetylene 1951. With durene (72) as the diene component, the 1 : 1-adduct (73) is obtained; this is a derivative of the interesting bicyclic compound barrelene [95,961.

R

The reactions of naphthalene with dienophiles, e.g. maleic anhydride, still require drastic reaction conditions (100 “C) [97,981; with maleic anhydride a total yield of 5 % of the adducts (74) and (75), approximately in a 1 : 1 ratio, is obtained. 1,2,3,4-TetramethylnaphthaIene

[93] J. Brugidou and H. Cristol, C. R. hebd. Seances Acad. Sci. 256, 3149, 3326 (1963). [94] R. D. Brown, J. chem. SOC. (London) 1950,691,2730; 1951, 1612. A. Streitwieser: Molecular Orbital Theory for Organic Chemists. John Wiley, New York 1961, p. 432. [95] C. G. Krespan, B. C. McKusick, and T. L. Cairns, J. Arner. chern. SOC. 83, 3428 (1961). [96] C. D. Weis, J. org. Chemistry 28, 74 (1963). [97] M . C. Kloetzel and H. L. Herzog, J. Amer. chem. SOC. 72, 1991 (1950). [98] K . Takeda, K. Kitahonoki, M . Sugiura, and Y. Takano, Chem. Ber. 95, 2344 (1962).

Angew. Chem. internat. Edit. 1 Vol. 5 (1966) / No. 2 219

reacts substantially faster than naphthalene. Surprisingly an excess of the dienophile is said to slow down the reaction with naphthalene [981. In contrast to other examples[99al, the reaction is not accelerated by the action of light [99bl.

Similarly to naphthalene, the 1,6-methanocyclodeca- pentaene (76) recently synthesized by VogeI and Roth [loo] has a cyclic conjugated 10 x-electron system with aromatic character. Diene addition with maleic anhydride sets in only at about 150 “C and stops at the 1 : 1- adduct; here the reaction involves the valence tautomer (77) (two cyclohexadiene systems). The I : 1-addition compound (77a) from (76) and dimethyl acetylenedi- carboxylate readily undergo thermolysis to give dimethyl phthalate and benzocyclopropene (see Section D).

R P H H

On the basis of the structure of the resulting 1 : 1-adducts, Alder, Pascher, and Vagt [lo11 postulated for the addition reactions of indene (78) a preliminary transition into 2H-indene (79), which adds dienophiles in the positions

I 78) (79)

indicated. The fact that such an isomerization with a hydrogen shift is possible in the temperature range of diene additions (200 “C) has recently been shown by Roth in an elegant NMR study on deuterated indene[102]. Berson and Aspelin[lo*al have come to the same con- clusion concerning the reaction mechanism. The formation of (82) in small amount in the reaction of benzyne (80) with 1,l-dimethylcyclopropene can also be explained readily by a 2H-indene intermediate (81) [1031.

[99a] G. 0. Schenck, Angew. Chem. 74,81 (1962); G. 0. Schenck, S. P . Mannsfeld, G. Schomburg, and C. H. Krauch, Z . Natur- forsch. 196, 18 (1964); D. Valentine, N. J. Turro, and G. S. Ham- mond, J . Amer. chem. SOC. 86, 5202 (1964). [99b] G. 0. Schenck, J . Kuhls, S. P. Mannsfeld, and C. H. Krauch, Chem. Ber. 96, 813 (1963). [I001 E. Vogel and H. D. Roth, Angew. Chem. 76, 145 (1964); Angew. Chem. internat. Edit. 3, 228 (1964); E. Vogel, personal communication. [loll K . Alder, F. Pascher, and H. Vagt, Ber. dtsch. chem. Ges. 75, 1501 (1942). [I021 W. R. Roth, Tetrahedron Letters 1964, 1009; G. Eergson, Acta. chem. scand. 18, 2003 (1964). [102a] J. A. Berson and G. E . Aspelin, Tetrahedron 20, 2697 (1964). [I031 J. A . Berson and M. Pomerantr, J. Amer. chem. SOC. 86, 3896 (1964).

As with other additions (e.g. halogenations to 9,lO-di- halogeno-9,10-dihydroanthracenes), with anthracene and its derivatives the dienophile usually adds in positions 9 and 10. With 1,l-dicyanoethylene, 9,lO-dimethylanthracene yields 95 % of (83) even at room temperature, while reaction with the angularly strained norbor- nene to yield 88 % of (84) requires heating to 140 “C [1041.

The addition in dioxane of the compound c 6 0 6 (8.5), a bis-anhydride of ethylenetetracarboxylic acid, to 9,lO- dialkoxyanthracenes in the 9,lO-positions is reversible at 20°C; at 100°C the dienophile adds irreversibly in the 1,4-positions of the anthracene system[losl. This reaction resembles that of 9,lO-diphenylanthracene with maleic anhydride; here a Diels-Alder addition takes place only in the melt, again in the 1,4-position [106!.

0 0

In this case of the naphthacene derivative (86), as with 9,10-diphenylanthracene, steric factors are probably re- sponsible for the fact that the dienophiles add to the “external” diene systems in the positions indicated [1071.

[I041 J. Sauer, H . Wiest, and A. Mitlert, Chem. Ber. 97, 3183 (1 964). [I051 J. Sauer, B. Schroder, and A. Mielerr, unpublished work; J. Sauer, Angew. Chem. 75, 1123 (1963); Angew. Chem. internat. Edit. 3, 150 (1964). [I061 J. W. Cook and L. Hunter, J. chem. SOC. (London) 1953, 4109. In some disubstituted anthracenes, competing 9,lO- and 1,4-addition occurs with benzyne; E. H. Klanderman, J. Amer. chem. SOC. 87, 4649 (1965). [I071 J. Rigaudy and N. K . Cuong, C. R. hebd. Seances Acad. Sci. 254, 4184 (1962). Other anthracene derivatives preferring 1,4- addition are described by J. Rigaudy and N. K. Cuong, ibid. 260, 1705 (1965); J . Rigaudy and K. V. Thong, ibid. 260, 2527 (1965).

220 Angew. Client. iiirci,nat. Edit. 1 Vol. 5 (1966) f No. 2

The red-orange naphthothiadiazole (87) is less reactive than anthracene, but, like the latter, adds e.g. maleic anhydride to the central ring [1081.

The reactions of styrene and its derivatives, in which only one double bond of the diene system belongs to the aromatic ring, sometimes give yields too low for preparative pur- poses because of the tendency of styrene to undergo polymeri- zation or copolymerization. As a rule, the 1 : I-adduct first produced is not isolated; this can either add a second molecule of the dienophile [e.g. to give (@)] or can undergo aromatization with a hydrogen shift [e.g. t o (89)1[109.1101.

H C H 3 - H2c33-& 0

o o (89) 40-5070

In contrast, 2-vinylnaphthalene and tetracyanoethylene give 95 % of the 1: 1-adduct (90) at room temperature 1 11

Phenols react with dienophiles only under drastic conditions [11*-1141; the adducts are derived from the tauto-

[I081 M . P. C a w and R. H . Schlessinger, Tetrahedron Letters 1964, 3815. [I091 K. Alder and R. Schmitz-Josten, Liebigs Ann. Chem. 595, 1 (1955); K. Alder and H . Niklas, ibid. 585, 97 (1954). [I101 B. J. F. Hudson and R. Robinson, J. chem. SOC. (London) 1941, 715. [ I 1 I ] W . J. Middleton, R . E. Heckert, E. L. Little, and C. G. Kres- pan, J. Amer. chem. SOC. 80, 2783 (1958). [ 1121 K. Takeda and K . Kitahonoki, Liebigs Ann. Chem. 606, 153 (1957). [ I 131 R . C. Cookson and N . S . Wariyar, J. chem. SOC. (London) 1957, 327. [ I 141 K. Takeda and K. Kitahonoki, J. pharmac. SOC. (Yakuga- kuzasshi) 73, 280 (1953); Chem. Abstr. 48, 2019 (1954); K. Take- da, S . Nagakura, and K. Kitahonoki, Pharmac. Bull. (Tokyo) I , 135 (1953); Chem. Abstr. 49, 3103 (1955).

meric keto forms of the phenols, as in the case of 2,5-dirnethylhydroquinone and rnaleic anhydride: at 200°C, 21 % of (91) is obtained and not the primary adduct [1121.

Of the pseudoaromatic compounds azulene and tropolone, only some derivatives of the latter are suitable for the Diels-Alder reaction [1151.2-Chlorotropolone fll6land y-tropolone methyl ether [I171 react with maleicanhydride to give respectively (92) and (93).

9. Heterocyclic Compounds

Since the discovery of the Diels-Alder reaction, the heterocycles furan, pyrrole, and thiophen have been thoroughly investigated with respect to their suitability as diene components ; their reactivity decreases in the above sequence.

Furan adds rnaleic acid derivatives even at room temperature [118, 1191 ; oxabicycloheptene derivatives are readily accessible in this way [e.g. (94) by reaction with maleimide in ether at 20 "C]. The tendency of the adducts to redissociate is as pronounced as with the fulvene ad-

ducts (cf. Section B.5). Like the alkyl derivatives, the derivatives of cyanofuran [1201 and chlorofuran [1211, are capable of undergoing cycloaddition.

Some intramolecular diene additions of furan derivatives take place particularly readily ; thus the non- activated olefinic part of (95) adds to the furan nucleus even at room temperature; dissociation takes place on

.CH2

H ~ C = HC '

[I 151 T. Nozoe in D. Ginsburg: Non-benzenoid Aromatic Com- pounds. Interscience Publishers, New York 1959, p. 396; K. Haf- ner and K . L. Morrtz, Liebigs Ann. Chem. 650, 92 (1961); W. Treibs, Naturwissenschaften 47, 156 (1960). [116] T. Nozoe and Y. Toyooka, Bull. Chem. SOC. Japan 34, 623 (1961); Chem. Abstr. 56, 12767b (1962). [I171 0. L. Cltupinun and D. J. Pasto, J. Amer. chem. Sor. 81, 3696 (1959). [118] With maleic anhydride: R. B. Woodward and H . Baer, J. Amer. chem. SOC. 70, 1161 (1948). With maleimide: H. Kwart and I . Burchuk, J. Amer. chem. SOC. 74, 3094 (1952). [I191 With maleic acid: J. A . Berson and R . Swidler, J. Amer. chem. SOC. 75, 1721 (1953). [I201 C. D . Weis, J. org. Chemistry 27, 3514, 3520 (1962). [I211 H. Krzikalla and H. Linge, Chem. Ber. 96, 1751 (1963).

Angew. Cliem. iiiternnt. Edit. .' Vol. 5 (1966) / No. 2 22 1

vacuum distillation 11221. Furan itself combines with acetylenedicarboxylic ester to give a 2: 1-adduct (96) [1231.

R = COzCH, 0

Diphenylisobenzofuran (97), which itself is easy to obtain by a sequence including a Diels-Alder reaction [1241, is a valuable diene component ; the benzocyclobutadiene (98) appearing as an intermediate has recently been

b 5

(99)

trapped in the form of its diene adduct (99) with the aid of this substance 1125-1271. The addition of (98) to furan itself is possible, but gives a yield of only 5 % [1281.

Pyrrole and its derivatives have for many years had the repu- tation of being unsuitable for Diels-Alder additions; the elec- trophilic substitution readily occurring in the a-position of the base substance gave rise to side reactions: with maleic anhydride cr-pyrrylsuccinic acid was obtained after hydrolysis. Some examples appearing in the last few years show that pyrrole derivatives also possess, in principle, the capacity for taking part in Diels-Alder reactions : N-benzylpyrrole [L29,1301, N-methoxycarbonylpyrrole [1311, N-triphenylmethylpyrrole, and N-l-naphthylpyrrole[1321 can be converted into Diels-Alder adducts, particularly with acetylenedicarboxylic

l l0l ) f l 0 2 ) R = H: 43% (103) R = CH,: 51%

R’ = COzCH,

[I221 D. Bilovic, 2. Stojanac, and V. Hahn, Tetrahedron Letters 1964, 2071. [123] 0. Diels and S. Olsen, J. prakt. Chem. (2) 156, 285 (1940). The second stereoisomer (96a) possible has recently also been isolated; D. Gagnaire, personal communication. [I241 R. Adams and R. B. Wearn, J. Amer. chem. SOC. 62, 1233 (1940). [125] M . P. Cava and R. Pohlke, J. org. Chemistry 27, 1564 (1962). [126] M . Avram, I. G. Dinulescu, D. Dinu, and C. D . Nenitzescu, Chem. and Ind. 1962, 555. [I271 M . P. Cava and M . J. Mitchell, J. Amer. chem. SOC. 81, 5409 (1959); M . Avram, G. D. Mateescu, D. Dinu, I. G. Dinulescu, and C. D. Nenitrescu, Acad. Rep. Populare Romine, Studii Cercetzri stiint. 9, 435 (1961); Chem. Abstr. 57, 4605a (1962). [I281 M . P . Cava and M . J. Mitchell, J. Amer. chem. SOC. 81, 5409 (1959). [129] L. Mandell and W. A. Blanchard, J. Amer. chem. SOC. 79, 6198 (1957). [I301 R. M. Acheson and J . M . Vernon, J. chem. SOC. (London) 1962, 1148. [131] R. M . Achrson and J. M . Vernon, J. chem. SOC. (London) 1961, 457; cf. also N. W . Gabel, J. org. Chemistry 27, 301 (1962). [I321 L. Mandell, J. U. Piper, and C. E. Pesterfield, J. org. Chem- istry 28, 574 (1963).

ester. In the reaction of N-methoxycarbonylpyrrole [(loo), R = HI, the primary adduct (101) cannot be isolated because it eliminates acetylene at the high temperature.

It has so far been impossible to cause thiophene to react with dienophiles; in fact, with hexachlorocyclopentadiene the heterocycle acts as a dienophile with the formation of a bis- adduct (1331. 2- and 3-vinylthiophenes, in which only one double bond belongs to the diene system of the heterocycle, react in a manner similar to that of styrene “341. Of a number of benzo[c]thiophenes [1351, the diphenyl derivative (104) may be selected as an example [1361.

f104j 1105) 79%

Thiophene 1,l-dioxide (108) can be prepared “in situ” in several stages from the butadiene-sulfur dioxide adduct (107); all attempts to isolate or trap (108) have so far failed 11371, since dimerization outweighs all compet-

1107) f108)

ing reactions. In the case of the stable 3,4-dichloro derivative (109), on the other hand, additions are success- ful, (109) taking part in the reaction sometimes as a diene and sometimes as a dienophile. For example, the adducts (l09a) and (1096) are formed in exothermic reactions in 61 and 16 % yields, respectively, based on (109). Dimerization occurs only at a relatively high temperature 11381.

[I331 H. Hamadait and M . Neentan, Israel. Pat. 9749; Chem. Abstr. 52, 1263 (1958). [I341 W. Davies and Q . N. Porter, J. chem. SOC. (London) 1957, 4958, 4961; J. F. Scully and E. V . Brown, J. Amer. chem. SOC. 75, 6329 (1953); J. Szmuszkovicz and E. J . Modest, J. Amer. chem. SOC. 72, 571 (1950). [I351 0. Dann, M. Kokorudz, and R. Gropper, Chem. Ber. 87, 140 (1954). [I361 G. Wittig, E. Knauss, and K. Niethammer, Liebigs Ann. Chem. 630, 10 (1960). [137] W. J. Bailey and E. W. Cummins, J. Amer. chem. SOC. 76, 1936 (1954). An account of the relative rates of addition of SO2 and maleic anhydride to dienes is given by 0. Grummitt and A. L. Endrey, J. Amer. chern. SOC. 82, 3614 (1960). [I381 H. Bluestone, R. M . Bimber, R . Berkey, and Z . Mnndel, J. org. Chemistry 26, 346 (1961); US.-Pat. 3 I10739 (Nov. 12th, 1963), Diamond Alkali Comp., inventor: R . M. Bimber; Chem. Abstr. 60, 2870b (1964).

222 Angew. Chem. internat. Edit. 1 Vol. 5 (1966) 1 No. 2

1109) (l09a)

The reaction of arylphospholes with acetylenedicarboxylic ester can be formulated by means of a diene synthesis as the initial reaction step [139J; for example, the pentaphenyl derivative ( I 10) forms tetraphenylphthalic ester in 88 % yield; the fate of the P-containing fragment has not been elucidated [1401.

1110) R‘ = COZCH,

CsH5

H5C6 6%

- H5c,,

1-Ethoxyphosphole 1-oxide, like thiophene 1,l-dioxide (108) and cyclopentadienone (Ma) , has a pronounced tendency to dimerize and can be trapped in the form of its 1 : 1-adduct (111) only’with a 50- to 100-fold excess of cyclopentadiene [1411.

3

P

( I l l ) 0 1 2 )

Of the five-membered heterocycles with several heteroatoms, only oxazoles [142-1451 can be used successfully in Diels-Alder additions, while pyrazoles, thiazoles, imidazoles, and isoxa- zoles fail [146J. Pyridine derivatives can be obtained in good yields by hydrolytic treatment of the adducts of the oxazoles with dienophiles. Of the six-membered heterocycles, the a-pyrones (112) deserve to be mentioned; they react with olefins to give 1 : l-adducts, while the Diels-Alder adducts from (112) and acetylenes spontaneously lose C 0 2 and undergo aromatization [1471.

[I391 I. G. M . Campbell, R. C . Cookson, and M . B. Hocking, Chem. and Ind. 1962, 359. [I401 E. H. Braye, W. Hubel, and I. Caplier, J. Amer. chem. Soc. 83, 4406 (1961). [I411 D. A . Usher and F. H . Westheimer, J. Amer. chem. SOC. 86, 4732 (1964). [I421 G. Y. Kondrat’eva, Izv. Akad. Nauk S.S.S.R., Ser. khim. Nauk 1959, 484; Chem. Abstr. 53, 21 940 (1959). [1431 C . D . Nenitzescu, E. Cioranescu, and L. Birladeanu, Com- mun. Acad. Rep. Populare Romine 8, 775 (1958); Chem. Abstr. 53, 18003 (1959). [I441 An elegant pyridoxine synthesis via the diene addition of an oxazole derivative is described by E. E. Harris, R. A . Firestone, K . Pfster, R . R. Boettcher, F. J . Cross, R. B. Currie, M . Monaco, E. R. Peterson, and W . Reuter, J. org. Chemistry 27, 2705 (1962). [I451 G. Y. Kondrat’eva and C . H . Huang, Dokl. Akad. Nauk S . S . S . R . 142, 593 (1962); Chem. Abstr. 57, 2204g (1962). [I461 I. I . Grandberg and A . N. Kosr, Zh. obshch. Khim. 29, 1099 (1959); Chem. Abstr. 54, 1500g (1960). [I471 J . D . Bu’Lock and H . G. Smith, J. chem. S O C . (London) 1960, 502.

(109b)

The diene activity is retained in perchloro-a-pyrone “481. Like anthracene, acridinium bromide adds dienophiles to the central ring[1491. The fact that the addition compounds from 2,3- dimethylquinoxaline and alkenes are not true Diels-Alder adducts has been shown only recently[I*ol.

The reactions of symmetrical tetrazines with simple olefins, discovered by Carboni and Lindsey [1513, deserve interest; Avram, Dinulescu, Marica, and Nenitzescu “521

have studied some conversions of the very reactive 1,2,4,5-tetrazine-3,6-dicarboxylate. Nitrogen is evolved in an exothermic reaction under surprisingly mild conditions, sometimes at room temperature. 1,6Dihydro- pyridazines, whose structures have been established by NMR spectroscopy [153,1541, are produced in high yields. The reaction sequence formulated with a primary dienophile addition in the 3,6-positions of the tetrazines to give

1 R

(113) was elucidated on the basis of mechanistic investigations [1551. When enamines, enol ethers, or esters and ketene acetal are used, substituted pyridazines such as (116) and (117) are obtained directly by single-stage syntheses in excellent yields [1541.

r o OCZHS J, H,C=C, .NJ CbH, y 5 c 6 H 5 OCzH, N’ N OC2H, H ~ C s < - I I1 -

N \ N N \

R $ N, R Y R

1116) 1117)

R = C6H5, 100% R = C02CH3, 100%

[I481 G. Murkl, Chem. Ber. 96, 1441 (1963). “491 C. K . Bradsherand T. W. G. Solomons, J . Amer. chem. SOC. 80, 933 (1958). [150] C. W . Bird and G. W . H . Cheeseman, J . chem. SOC. (Lon- don) 1962, 3037; E. C . Taylor and E. Smakula-Hand, J . org. Chemistry 27, 3734 (1962); Tetrahedron Letters 1962, 1225. [151] R. A . Carboni and R. V. Lindsey, J. Amer. chem. SOC. 81, 4342 (1959). [I521 M . Avram, 1. G. Dinulescu, E. Marica, and C. D . Nenitzescu, Chem. Ber. 95, 2248 (1962). [I531 M . Avrain G . R. Bedford, and A . R. Katritzky, RecueilTrav. chim. Pays-Bas 82, 1053 (1963).

Angew. Chem. internat. Edit. VoI. 5 (1966) 1 No. 2 223

10. Olefins and Non-Conjugated Dienes

Some particularly reactive dienophiles also take part in reactions with simple olefins and non-conjugated dienes, giving 1 : 1-adducts. The mechanistic relation to Diels- Alder diene additions [I] justifies a brief discussion. The reaction of 1-hexene with dimethyl acetylenedi- carboxylate “561, as an example, includes all the charac- teristics of this type of reaction which Alder, in analogy with “diene synthesis” has called the “ene synthesis”, or “indirect substitution addition” [*I : the product of ally1 substitution always arises with double displacement in a stereospecific cis-addition. The experimental results, to- gether with an elegant study of the asymmetric induction

80%

in “ene syntheses” [1571, suggest a multi-center mechanism. “Indirect substitution addition” also takes place with azodicarboxylic ester, as Hiiisgen and Pohl[15*1 have shown with 1,3-diarylpropenes and 1,2- and 1,4-di- hydronaphthalenes, as the olefinic components; radical and ionic modes of reaction were excluded. The reaction of the azo ester with simple olefins (1-hexene, 1-pentene, cyclopentene, and cyclohexene) also is an “ene synthesis” 11591. The C=O and C-S functions in chloral [1601, carbonyl cyanide [1611, pyruvic acid “621, perfluorocyclobutanone ( I IS) [1631, and hexafluorothioacetone (119) [I641 can

11541 D. Lang, Dissertation, Universitat Miinchen 1963 ; J. Sauer, A. Mielert, D . Lang, and D. Peter, Chem. Ber. 98, 1435 (1965). [I551 J. Suuer and D. Long, Angew. Chem. 76, 603 (1964). [I561 K. Alder and H. von Brachel, Liebigs Ann. Chem. 651, 141 (1962); cf. also J. C . Suuer and G . N. Snusen, J. org. Chemistry 27, 2730 (1962). [157] R. K . Hill and M . Rnbinovitz, J. Amer. chem. SOC. 86, 965 (1964); J . A . Berson, R . G . Wall, and H . D . Perlmtrtter, ibid. 88,187 (1966). [158] R. Hiiisgen and H . Poltl, Chem. Ber. 93, 527 (1960). [I591 0. Achmatowicz and 0. Achmutowicz, Roczniki Chem. 36, 1791 (1962); -?7,317(1963); Chem. Abstr.59,8610b, 12655e (1963). [160] M . Vilkas, G. Dupont, and R. Dulou, Bull. Soc. chim. France 1955, 799. [161] G. I. Birnbaum, Chem. and Ind. 1961, 1 1 16. [I621 R. T. Arnold and P . Veeravngu, J. Amer. chem. SOC. 82, 5411 (1960). [I631 D. C. Euglnud, J. Amer. chem. SOC. 83, 2205 (1961). [I641 W. J. Middleton, E. G . Howard, and W . H . Sharkey, J . Amer. chem. SOC. 8.7, 2589 (1961); J. org. Chemistry 30, 1375 ( 1965) ; W . J . Middletoit and W. H . Sharkey, ibid. 30, I384 ( I 965); W. J . Middleton, ibid. 30, 1390, I395 ( I 965).

also take part in ene syntheses; however, ketones and thioketones sometimes undergo a change in orientation.

The definition of Diels-Alder additions given in Section A (two x-bonds changing into two o-bonds) does not apply to the reactions of dienophiles with homodienes, in which the two double bonds are separated by a tetrahedral center. In the homo-Diels-Alder additions of bicyclohep?adiene, three new a-bonds are formed at the expense of th:ee ;c-bonds in a 1,5- addition. The addition of maleic anhydride, the first example of this type of reaction, takes place in low yield “651. A sub-

,& C,H,, 80%) & 1 (NC)z (CN)z

(NC)&=C(CN)z (120) 100%

stantially higher yield is given by reaction with the “record dienophiles” tetracyanoethylene[J661, 4-phenyl-l,2,4-triazoline-3,5-dione [1671, and the bis-anhydride (85) [1’351, the products being respectively (120), (121), and (122). Acryloni- trilerl681, azodicarboxylic ester [1691, dicyanoacetylene [961, and some other highly reactive cyanoethylenes “69al react similarly. The combination of barrelene (bicyclo[2.2.2]octa-2,5,7-triene), which has one more double bond, with alkynes also leads to the formation of homo-Diels-Alder adducts [17’31.

The 1,7-addition of tetracyanoethylene t o 1,3,5,7-tetra- methylenecyclooctane (124) in tetrahydrofuran takes place in accordance with the same structural principle: 3 n-bonds -f 3 a-bonds 11711.

1124)

[I651 E. F. Ulltnan, Chem. and Ind. 1958, 1173 [166] A. T . Blomyuist and Y. C . Meinwald, J. Amer. chem. SGC. 81, 667 (1959). [167] R. C . Cookson, S. S . H. Gilani, and I. D. R . Stevens, Tetra- hedron Letters 1962, 615. [I681 H . K . Hull, J. org. Chemistry 25, 42 (1960). In the presence of the bisacrylonitrile-Ni(0) complex the yield is quantitative: G. N . Schrnuzer and S. Eichler, Chem. Ber. 95, 2764 (1962). [I691 S. J. Cristol, E. L. Allred, and D. L. Wetzel, J. org. Chem- istry 27, 4058 (1962); R . M . Moriarty, ibid. 28, 2385 (1963). [169a] W. J . Middleton, J. org. Chemistry 30, 1402 (1965). [I701 H . E. Zimmermnn and G. L. Grunewald, J. Amer. chem. SOC. 86, 1434 (1964). [I711 J . K . Willionis and R. E. Benson, J. Amer. chem. SOC. 84, 1257 (1962).

224 Angew. Chcm. ititerntit. Edit. Vol. 5 (1966) / N o . 2

C . The Dienophilic Components

The enormous preparative importance of Diels-Alder additions is based on the great variability of both the reactants; the discussion above has given a small insight into the variability of the dienes. A short systematization of the dienophiles will now follow. Without going into mechanistic details at this point, it may be mentioned that the reactivity of the dienophiles is a function of the diene component: with electron-rich dienes (e.g. cyclopentadiene or 9,10-dimethylanthracene), electron-at- tracting groups X on the double bond accelerate the reaction '1041 ; with electron-deficient dienes (e.g. hexachlo- rocyclopentadieneand 1,2,4,5-tetrazines),electron-donat- ing substituents in the dienophile have a promoting effeCt[154,155,1721.

1. Acyclic Alkenes and Alkynes

The low reactivity of ethylene and of allylic compounds with respect to electron-rich dienes limits the use of these dienophiles ; the necessity of working in autoclaves at high temperatures is also troublesome in the laboratory. Of the monosubstituted ethylenes, acrolein, methyl acrylate and acrylonitrile, styrene derivatives, and ni- troolefins [I731 enjoy greater favor. 1,l-Disubstituted olefins (e.g. dimethyl methylenemalonate and 1, l-dicyanoethylene) are more reactive than their 1,2-disubstituted isomers (dimethyl fumarate and fumaronitrile) L1041. Tri- and tetrasubstituted olefins with several activating substituents increase the preparative possibilities to a considerable extent. The interesting reactions of tri- and tetracyanoethylene, in particular, may be recalled at this

Acetylene itself reacts with electron-rich dienes only under severe conditions; working with relatively large amounts of acetylene under pressure in the laboratory is not without danger. The reaction of dienes with vinyl bromide and the subsequent elimination of HBr from the Diels-Alder adduct, however, leads to the same product as the direct addition of acetylene. Among the dienophiles derived from mono- and disubstituted alkynes propiolic acid, phenylpropiolic acid, and acetylenedicarboxylic ester may be mentioned in particular. Di- cyanoacetylene and hexafluorobut-2-yne have been discussed above in Sections B. 10 and B. 8 [95,961.

point 1174,1751.

2. Allenes

Allene itself exhibits sufficient activity only with respect to electron-deficient dienes - e. g. hexachlorocyclopentadiene 11761. Reference may be made to the addition

[I721 J. Sauer and H. Wiest, Angew. Chern. 74, 353 (1962); An- gew. Chern. internat. Edit. /, 269 (1962); H. Wiest, Dissertation, Universitat Miichen 1963. [I731 Review of diene syntheses with nitro compounds: S. S. Novikov, G. A. Shvekhgeimer, and A. A. Dudinskaya, Russ. chern. Reviews (Engl. transl.) 29, 79 (1960). [ 1741 C. L. Diekinsort, D. W. Wiley, and B. C. McKusick, J. Amer. chcrn. Soc. 82, 6132 (1960). [I751 T.L. Cairnsand B.C. McKusick, Angew. Chem. 73,520(1961). [I761 H. Pledger, J. org. Chemistry 25, 278 (1960).

of ally1 bromide with subsequent elimination of HBr, which was used in the example below for establishing the structure of the allene adduct (125) [1771.

c 1

clQClz

c1 c 1

(125)

Cyclopentadiene combines with allenecarboxylic acid (126) to give a 1 : 1-adduct in 84 % yield; in this process the dienophile reacts as a derivative of acrylic acid at the double bond indicated [1781.

The absolute configuration of allenedicarboxylic acid (127) was determined by Agosta[1791; a diene addition to cyclopentadiene formed the introductory step of the sequence of reactions. Extensive investigations by Staudinger [1801 have shown that for ketenes the four-center cycloaddition to form cyclobutane derivatives is preferred to the formation of a six-membered ring. Buvtlett et d. pointed out an artifice for the preparation of the Diels-Alder adducts of ketene itself [1811. The diene is treated with 1- cyanovinyl acetate (128) to give an adduct which on alkaline hydrolysis yields the desired ketone (129).

128) 62% 82% 1/29)

3. Cyclic Dienophiles

Among the cross-conjugated dienophiles, the greatest importance is attached to the derivatives of p-benzoquinone 1131. Quinones substituted with electron-attract- ing groups prove to be particularly reactive with respect

[177] R. Riemschneidtr, F. Herzel, and H. J . Koetstlt, Mh. Chern. 92, 1070 (1961). [I781 E. R. H. Jones, G . H . Mansfield, and M . C . Whiting, J. chern. SOC. (London) 1956, 4073. [I791 W. C. Agosta, J. Amer. chern. SOC. 84, I10 (1962); 86. 2638 (1964). [I801 H. Staudinger: Die Ketene. F. Enke, Stuttgart 1912; see also H. L. Drvclen, J. Amer. chern. SOC. 76, 2841 (1954); A. T. Blomqtrist and J . Kwiatrk, ibid. 73, 2098 (1951); IV. Rrllenstnanu and K . Ha/tier, Chern. Ber. 95, 2579 (1962); see also [ I ] . [I811 P. D . Bnrtlett and B. E. Tote, J . Amer. chern. Soc. 78, 2473 (1956); C. H . DePuj and P . R. Story, ibid. 82, 627 (1960).

Aiigew. Cltetn. interiittt. Edit. / Vol. 5 (1966) 1 No. 2 225

to electron-rich dienes. The dicyanoquinone (129) reacts more slowly than tetracyanoethylene, whereas quinonedicarboxylic anhydride (130) does so conside- rably faster [1821.

0 0

(1311 f 132)

In (129), (130), and 1,4,5,8-naphthodiquinone (131) [18*1, as expected, cyclopentadiene, 2,3-dimethylbutadiene[l8zJ, and 1,2-dimethylenecyclobutadiene 11831 add t o the more highly activated double bond [in the 2,3-position in (129) and (130) and the 9,lO-position in (131)] to form the monoadduct. A n interesting difference is found with butadiene: (129) forms two monoadducts by the reaction of the 5,6- and the 2,3- positions (yields 16 and 62 %, respectively) [1841, whereas (131) gives a 2: I-adduct (addition in the 2,3- and 6,7-positions) and a 1:l-adduct (reaction of the 9,lO-double bond) with comparable yields[1821. On the other hand, (130) adds butadiene in the 2,3-position with a yield of about 75 0/0[18*J.

There is still n o satisfactory explanation for this phenomenon. The dienophilic activity of p-benzoquinones is even retained in N,N'-disulfonylquinonedi-imines [e. g. (132)]"851.

Through its diene additions, cyclopent-l-ene-3,5-dione (133), which exists completely in the diketo form, makes enolized 1,3-dioxocyclopentanes readily accessible [1861.

f133) 100%

A fairly large number of cyclic olefins and acetylenes with pronounced angular strain have found application as dienophiles in the last few years. Already in the transition state, adduct formation is associated with a re- duction in angular deformation; this acts as a driving force in the Diels-Alder addition. Cyclopropene [(134), R = HI combines with cyclopentadiene even at 0 "C rapidly and stereospecifically to give the endo adduct [(135), R = HI [1871. Methylcyclopropene reacts similarly to give (135) [R = CH3][1881, but di-

[I821 J. Sausr and B. Schroder, unpublished work; B. Schroder, Dissertation, Universitat Miinchen, 1965. 11831 H . D. Hartzler and R. E. Benson, J . org. Chemistry 26, 3507 (1961). [184] M . F. Ansell, B. W . Nash, and D. A . Wilson, J. chem. SOC. (London) 1963, 3006, 3012. [I851 Review: R. Adams and W . Reijschneider, Bull. SOC. chim. France 1958, 23. [I861 C. H . DePuy and E. F. Zaweski, J. Amer. chern. Soc. 81, 4920 (1959); C. H. DePuy and C. E. Lyons, ibid. 82, 631 (1960), 11871 K . E . Wiberg and W. J. Eartley, J. Amer. chem. SOC. 82, 6375 (1960). [I881 G. L. Closs, L. E. Closs, and W . A. Bull, J. Amer. chem. SOC. 85, 3796 (1963); see also M . A . Buttiste, Tetrahedron Letters 1964, 3795.

methylcyclopropene, probably for steric reasons, does notre act [1881. Triphenylcyclopropene and tetraphenylcyclopentadienone give heptaphenylcycloheptatriene by the elimination of CO from the primary 1 : I-adduct [1891.

Diphenylcyclopropenone and 1-diethylaminobutadiene in boiling benzene give 2,7-diphenylcyclohepta-2,4,6-tri- enone (136) in a single-stage reaction with a yield of almost 70 % [189al; the adduct (136a) probably occurring as an intermediate undergoes elimination and valence-isomerization to (136).

In cyclobutene derivatives, as well, the angular strain has a reaction-promoting effect; 3,3,4,4-tetrafluoro- cyclobutene reacts with dienes under remarkably mild conditions [1901, as do unsaturated four-membered cyclic sulfones [190al. Trapping reactions with the benzocyclobutadiene probably appearing "in situ" (cf. Section B. 9) take place at room temperature. The isolable cyclobutadiene derivative (137) also proves to be a very reactive dienophile [1911. The corresponding monobromonaph- thocyclobutadiene and the dibromo compound are too short-lived, and could only be detected by trapping 1191-4.

The sp-hybridization determines the linear structure of acetylene and permits the strain-free incorporation of a triple bond only in rings with nine or more members. The increase in the angular deformation with decreasing ring size in the sequence cyclooctyne --f cyclopentyne is shown in an increasing tendency to undergo additions; cyclooctyne exhi bits pronounced dienophilic properties. Lower cycloalkynes have not been isolated, but their existence has been proved by trapping reactions [19*1; for

[I891 M . A. Buttiste, Chem. and Ind. 1961, 550. [189a] J. Cicibattoni and G. A . Eerchtold, J. Amer. chem. SOC. 87, 1404 (1965). [I901 R. J. Shozda and R. E. Putnam, J . org. Chemistry 27, 1557 (1962). [190a] For example, L. A . Paquette, J. org. Chemistry 30, 629 (1965); D. C. Dittmer and N. Takashina, Tetrahedron Letters 1964, 3809. [I911 M . P. Cavn, B. Hwang, and J. P. van Meter, J. Amer. chem. SOC. 85, 4032 (1963). [191a] M . P. Cnvn and M . Hwang,Tetrahedron Letters 1965,2297. [I921 G. Wittig and A. Krebs, Chem. Ber. 94, 3260 (1961); G. Wirtig and R. Pohlke, ibid. 94, 3276 (1961); further studies on cyclic alkynes: G. Wirrig and J. Weinlich, ibid. 98, 471 (1965).

226 Angew. Chem. internat. Edit. 1 VoI. 5 (1966) 1 No. 2

example, cyclopentyne gives the bis-adduct (138) with diphenylisobenzofuran.

The cyclic compounds (142) [2001 and (143) [1671, on the other hand, can be isolated. Compound (142) adds to cyclopentadienc reversibly ; slightly above room temper-

1138) * / Arynes, e.g. dehydrobenzene (80) (see also Section B. 8), represent cyclic acetylenes in one resonance form. In brilliant investigations, Wittig and his school [I931 have demonstrated the Diels-Alder additions of the “de- hydroaromatic” intermediates. Cyclopentadiene ‘1931,

cyclohexadiene [1941, and even benzene and naphthalene [I951 add to the highly reactive species C 6 H 4 (80). The dienophilic activity is also retained in the “dehydro- aromatics” of the pyridine series and in quinoline derivatives [1961.

4. Cyclic Azo Compounds

Some cyclic azo compounds, which have been synthesized only recently, possess high dienophilic activity; however, only some of them are stable. Thus indazol-3-one (139) [1971, 2,3-diaza-p-benzoquinone (140) 11981, and 2,3-diaza-1,4-naphthoquinone have been detected by their reactions with dienes at low temperatures; in all cases the azo compounds were prepared by oxidation of the corresponding cyclic hydrazo compounds. The dienes add to the -N=N- bond system of (139) and (140). In the dehalogenation of the

m 0 6 N 1140)

n

pyrazolinone (141), it is reasonable to postulate an intermediate with a cyclic azo grouping [1991.

1/41) ‘70-90qo

[I931 G. Wittig, Angew. Chem. 69, 245 (1957); 74, 479 (1962); Angew. Chem. internat. Edit. I, 415 (1962); G. Wittig, W. Uhlen- brock, and P. Weinhold, Chem. Ber. 95, 1692 (1962). [I941 H. E. Simmons, J. Amer. chem. SOC. 83, 1657 (1961). (1951 R. G. Miller and M . Stiles, J. Amer. chem. SOC. 85, 1798 (1963). [196] T. Kauffmann and F. P. Boettcher, Chem. Ber. 95, 949, 1528 (1962); R. J. Martens and H. J. den Hertog, Tetrahedron Letters 1962, 643. [I971 E. F. UIIman and E. A . Bartkus, Chem. and Ind. 1962, 93. 11981 T. J. Kealy, J. Amer. chem. SOC. 84, 966 (1962); R. A. Clement, J. org. Chemistry 25, 1724 (1960); 27, 1115 (1962). I1991 L. A. Carpino, P. H. Terry, and S. D . Thatte, Tetrahedron Letters 1964, 3329.

ature (142) offers a high-yield route to dehydrobenzene (80).

I - N,, - S O , (80)

4-Phenyl-l,2,4-triazoline-3,5-dione (143) was found in kinetic experiments to be a dienophile with extremely good addition properties, many times better than tetracyanoethylene [1821; it combines with cycloheptatriene even at -50°C to give (144) [1671. The Diels-Alder adducts with 9,lO-dialkoxyanthracenes undergo an acid- catalysed rearrangement at room temperature to give (146) [1821.

1 143) 1144)

RO 0

H ,o@ - 1 I 45) 1146) ’O

5 . Other Dienophiles with Heteroatoms

The replacement of carbon in the diene chain by heteroatoms is possible only in individual cases, as shown by the examples of the u,P-unsaturated aldehydes and ketones and the 1,2,4,5-tetrazines (cf. Sections B. 2 and B. 9). In contrast, in the dienophiles one or both C atoms of the dienophilic multiple bond can frequently be replaced by heteroatoms without loss of activity. This has already been illustrated by the cyclic azo compounds described in Section C. 4. Carbonyl functions in aldehydes and ketones add to dienes particularly well if the C=O double bond is de- prived of electrons by means of suitable substituents. Formaldehyde reacts slowly [2011; the reactivity in- creases on passing to chloral [2021 and mesoxalic ester or

[200] G. Wittig and R. W. Hoffmann, Chem. Ber. 95,2718 (1962). [201] D. G. Kubler, J. org. Chemistry 27, 1435 (1962); T. L. Gres- ham and T. R. Steadman, J. Amer. chem. SOC. 71, 737 (1949). [202] W. J. Da/e and A . J. Sisti, J. Amer. chem. SOC. 76,81 (1954).

Angew. Chem. internat. Edit. / VoI. 5 (1966) 1 No. 2 227

mesoxalonitrile [2031, and reaches a maximum with per- fluorinated cyclobutanone (118) [204,1041; other fluoro- ketones (147) also react readily [2051. Glyoxylic ester and

tely formed 12093. cc-Ketonitriles and cyanopyridines react at temperatures as low as about 250 "C [*lo]. 1118) 100% (148)

R RFNR

its semiacetals also react with dienes to give partially hydrogenated pyrans [*061.

(153) 5 3 1 , ~ I -

While the C=S bond system has proved to be a highly dipolarophilic grouping [21, only a few examples of a Some of the longest-known addition reactions are those dienophilic activity are known ; the dienophilic systems Of azodicarboxylic ester 151. Tetrahydropyridazines such (119), (149), and (150) again bear electron-attract- as (154) call be isolated, usually in excellent ing groups

B F"Y2 F3

More recent investigations [164,206a] have shown that simple thioketones - for example, thiofluorenone - react with dienes to give Diels-Alder adducts even at room temperature. Obviously the dienophilic nature of compounds with the C=S double bond has been overlooked, not least because of their unpleasant odor. Here is pre- sumably a wide field of preparative application.

Among Schiff bases [2071, immonium salts [2081, and ni- triles [2091 (the latter react only at high temperatures) C=N or CE N heterodienophiles are occasionally found. (151) generally combines with dienes to give Diels-Alder adducts in high yields (> 90 %)[2071. The immonium salts that may be formed as intermediates from cc-halogenoamines such as (152) make spiro- cyclic ammonium compounds accessible 12081. Simple ni- triles, such as benzonitrile, acetonitrile, and dicyanogen, add to dienes only at about 400°C; the products of a subsequent dehydrogenation, e.g. (153), are immedia-

[203] 0. Achnzatowicz and A . Zamojski, Bull. Acad. polon. Sci., CI. 111, 5, 927 (1957); Chem. Abstr. 52, 6333 (1958). 12041 D. C. England, J. Amer. chem. SOC. 83, 2205 (1961); US.- Pat. 3036091 (May 22nd, 1962); Chem. Abstr. 57, 16567e (1962).

[205] W . J. Linn, J. org. Chemistry 29, 3111 (1964). [206] Y. A . Arbuzov, E. M . Klirnor, and E. I. Kliinova, Dokl. Akad. Nauk S . S . S . R . 142, 341 (1962); Chem. Abstr. 57, 765, (1962).

HF-c02c2H5 ---t NH "C02CzH5

H3C H H3C H

yields[211,2121, and they can be converted into cyclic hydrazine derivatives. The Diels-Alder adducts, e.g. (4) , lead in many cases to new azo compounds, such as (155) which, in turn, are capable of undergoing interesting reactions and make new compounds accessible [213,2141.

(4) ' 155) 94%

[209] G. J, Janz and A . G . Keenan, Canad. J. Chem. 25 B, 283 (1947); G. J. Janz and W. J. H . McCulloch, J. Amer. chem. SOC. 77, 3143 (1955); G. J. Janz and A . R. Monahan, J. org. Chemistry 29, 569 (1964). [210] Examples: W . Polaczkowa and J . Wolinski, Roczniki Chem. 26, 407 (1952); Chem. Abstr. 48, 11359 (1954); W . Polaczkown, T. Jaworski, and J. Wolinski, ibid. 27, 468 (1953); Chem. Abstr. 49, 3181 (1955); T. Jaworski and W . Polaczkowa, ibid. 34, 887 (1960); Chem. Abstr. 55, 8407d (1961); T. Jaworski, ibid. 35, 1309 (1961); Chem. Abstr. 57, 58881 (1962). [21 I ] P. Baranger and J. Levisalles, Bull. SOC. chim. France 1957, 704; P . Baranger, J. Levisalles, and M . Vuidart, C. R. hebd. Seances Acad. Sci. 236, 1365 (1953). [212] K . Alder and H. Niklas, Liebigs Ann. Chem. 585, 81, 97 (1954); J. C. J. McKenzie, A. Rodginan, and G. F. Wright, J. org. Chemistry 17, 1666 (1952); A. Rodgman and G. F. Wright, ibid. 18, 465 (1953); Y. S . Shabarov, N. I. Vasil'ev, and R. Y. Levina,

[206a] A. Sclionberg and B. Konig, Tetrahedron Letters 1965, Dokl. Akad. Nauk S . S . S . R . 229, 600 (1959); Chem. Abstr. 54, 3361. 14259 (1960). [207] G. Kresze and R. Albrecht, Chem. Ber. 97, 490 (1964); see [213] R. Criegee and A. Rirnmelin, Chem. Ber. 90, 414 (1957). also R. Albrecht and G. Kresze, ibid. 98, 1431 (1965). [214] Experiments with hexachlorocyclopentadiene and azo [208] H. Bohme, K. Hartke, and A. Miller, Chem. Ber. 96, 601 compounds: J. G. Kuderna, J . W . Sims, J. F. Wikstroin, and (1963). S. B. Soloway, J. Amer. chem. SOC. 81, 382 (1959).

Br'

92%

228 Aiigew. Chem. internot. Edit. , Vol. 5 (1966) No. 2

The possibility, mentioned in Section B. 10, of a radical ally1 substitution[2151 o r a multi-center reaction with a shift of the double bond in the reaction of azodicarboxylic ester with olefins also exists in principle with dienes, and sometimes makes it difficult to establish the constitution of the Diels-Alder 1 : I- adducts. Apparently slight changes in the reaction conditions lead t o the formation of different products [2161. Thus azodicarboxylic ester and cyclohexa-1,3-diene a t 2OoC in cyclo- hexane give the compounds (156) and (157) in a n exothermic reaction (total yield 89 %). Without a solvent, between 20 and YO O C , only (158) is formed.

I A

Possibly the influence of light, recently discovered by As- kani [216al, offers a n explanation of the apparent discrepancy: on irradiation with a high-pressure mercury lamp, cyclohexa- d i e m and azodicarboxylic ester form only the adduct (1.56) in 87 "/o yield.

Finally, there is also a possibility of replacing the two C atoms of the double bond in the dienophile by two different heteroatoms : nitroso compounds (159) and N-sulfinyl compounds (160), on addition to dienes, form six-membered heterocycles 131,2171. The adducts of the nitrosoaromatic compounds can easily be converted by hydrogenolysis into cis-l,4-aminoalco- hols [2181, and the diene adducts of N-sulfinyl compounds such as (161) give rise to a wealth of new reactions 12171, two of which are illustrated here.

R-N=O (159) R-N=S=O (160)

The 1 : 1-addition compounds from dienes and molecular oxygen, which frequently are formed on ir-

[215] R . Huisgen, F. Jacob, W. Siegel, and A. Cadus, Liebigs Ann. Chem. 590, 1 (1954); R. Huisgen and F. Jacob, ibid. 590, 3 1 (1954). [216] S. G. Cohen and R . Zand, J. Amer. chem. SOC. 84, 586 (1962); B. T. Gillis and 5 . E. Beck, J. org. Chemistry 27, 1947 (1962); 5 . Franzus and J. H. Surridge, ibid. 27, 1951 (1962); see also M . Cais in S. Patai: The Chemistry of Alkenes. Interscience Publishers, New York 1964, p. 739. [216a] R. Askani, Chem. Ber. 98, 2551 (1965). cis-Azodicarbot- ylate: G. 0. Schenck et al . , Z . Naturforsch. 206, 637 (1965). [217] G. Kresze, A. Maschke, R . Albrecht, K . Bederke, H. P . Patzschke, H. Stnalla, and A . Trede, Angew. Chem. 74, 135 (196%); Angew. Chem. internat. Edit. I, 89 (1962); G. Kresze and J. Firl, Angew. Chem. 76, 439 (1964); Angew. Chem. internat. Edit. 3, 382 (1964). E. S. Levchenko, J . G. Baron, and A . V. Kirsanov, J. gen. Chem. U.S.S.R. (Engl. trans]. of Zh. obshch. Khim.) 33, 1546 (1963). [218] G. Kresze and G. Schrilz, Tetrahedron 12, 7 (1961); Chem. Ber. 96, 2165 (1963).

radiation [2191, can be regarded formally in some cases as Diels-Alder adducts. These reactions will not be discusssed in this review.

D. Retro-Diels-Alder Reactions

The diene additions are reversible, often under surprisingly mild conditions; for the fulvenes and furans (Sections B. 5 and B. 9) we have already demonstrated this capacity for undergoing the retro-Diels-Alder reaction. Many adducts can be modified chemically and yet still undergo cleavage in a retro-Diels-Alder reaction. In this way a modified diene and/or dienophile are obtained. The fact that the retro-Diels-Alder reactions frequently claim preparative interest may be illustrated by the following example. The alkali-lability of the simple p-benzoquinones inter- feres with their epoxidation with alkaline H202. On the other hand, the benzoquinone adducts (162) with cyclopentadiene can be easily epoxidized ; pyrolysis at 420 "C/10 mm smoothly yields the desired quinone epoxides [2201.

R = H, CHs

n

4 0 + '@O 80-95% H R

0

As further examples may be mentioned: the preparation of 6,6-disubstituted cyclohexadienones from dimeric fulvene epoxides 1911, the preparations of o-quinodimeth- ane 148,491, and of acetylenedicarbonyl dichloride from the anthracene adduct of the free acid 12211, and the con- version of 1-vinylcyclohexene into vinylcyclohexane via the anthracene adduct 12221. The determination of the position of groups in the cycloheptatriene ring has been carried out through the addition of acetylenedicarboxylic ester and pyrolysis of the adduct to substituted phthalic esters 12231. The transient existence of isobenzo-

[219] G. 0. Schenck, Naturwissenschaften 35, 28 (1948); Angew. Chem. 64, 12 (1952); A . Schonberg: Priparative Organische Photochemie. Springer-Verlag, Berlin 1958; see also [8]; K. Gull- nick and C. 0. Schenck in: Organic Photochemistry. Internat. IUPAC Symposium, Strasbourg 1964; Butterworths, London 1965. [220] K. Alder, F. H . Flock, and H . Benmling, Chem. Ber. 93, 1896 (1960). [221] 0. Diels and W. E. Thiele, Ber. dtsch. chem. Ges. 71, 1173 (1 938). [222] L. H. Slaugh and E. F. Magoon, J. org. Chemistry 27, 1037 (1962). [2%3] K. Alder, R . Muders, W. Krane, and P . Wirfz , Liebigs Ann. Chem. 627, 59 (1959); K. Alder, H . Jungen, and K. Rust, ibid. 602, 94 (1957); W. von E. Doering, G. Laber, R . von der Wahl, N . F. Chamberlain, and R . 5. Williams, J. Amer. chem. SOC. 78, 5448 (1956).

Angew. Chem. internat. Edit. f Vol. 5 (1966) No. 2 229

furan [(163), R = HI and its dimethyl derivative was also shown for the first time by the retro-Diels-Alder reaction with trapping experiments 12241.

R R

R

R = H, CH3

A 95%

L J

1163) R = H: > 92%

It is not always the bonds formed in the addition that are split on pyrolysis, as can be seen from an example due to Criegee[2251. The adduct (164) decomposes at about 2OO0C/18 mm into dimethyl phthalate and diacetoxybutadiene. Tetramethoxyethylene (166) was first obtained by Hofmann and Hauser [2261 by a retro-diene

urably fast 12271; the preparation of the compound C6O6 (85) in solution (Section B. 8) in a similar manner may be recalled. In warm ethanol, the hydrolysis product (167) of the adduct from azodicarboxylic ester and anthracene gives the diimide; which is capable of stereospecific cis-addition [2281.

(167)

Compounds (168) and (169), which were isolated as the only products, probably arise from many addition and splitting steps: the reversibility of the addition step possibly ensures the establishment of thermodynamic equilibrium 12291.

(1681 30% (169) 35%

A particularly fine example of a retro-Diels-Alder reaction has recently been contributed by Vogel et al. [2301.

The adduct (77a) (see Section B. 8) decomposes at 100 "C/1 mm to give a 45 % yield of benzocyclopropene, which is very interesting in relation to bond theory, and phthalic ester.

COzCH3 F0ZCH3 H

F COzCH3 A c ; b + F - Ac$$-jJ

Ac 0 Ac 0 C02CH3

f l 6 4 j

synthesis from the easily accessible adduct (165) of 5,5- dimethoxytetrachlorocyclopentadiene and phenylacet- ylene.

(165) cH3q C= CCH3 c H,O' oc H3

1166)

The adducts of tri- and tetracyanoethylene, dicyano-p- benzoquinone, and dicyanomaleimide with 9,lO-dialkoxyanthracenes decompose into the components in

E. Future Prospects

Any short account of the Diels-Alder reaction must necessarily be incomplete. However, the discussion of the modes of reaction of diene and dienophile, possible here only in a sketchy fashion, shows that the preparative possibilities of diene additions are still not exhausted. It has not been possible to discuss Diels-Alder reactions initiated by irradiation. In addition to preparative investigations, since the discovery of this cycloaddition to form six-membered rings attempts have been made to clarify the reaction mechanism. An account of systematic mechanistic studies with the inclusion of stereochemical and kinetic results and of orientation and catalysis phenomena will be given in the second part of this paper.

Received: March 2nd, 1965 [A 486/279 IE] German version: Angew. Chem. 78, 233 (1966)

Translated by Express Translation Service, London . . solution, even at room temperature, sometimes immeas-

12241 L. F. Fieser and M. J. Huddadin, J. Amer. chem. SOC. 86, 2081 (1964). 12251 R. Criegee, W. HorauL and W. D. Schellenberg, Chem. Ber. 86, 126 (1953).

[227] J. Sauer, R. Wiemer, and A . Mielert, unpublished work. [228] E. J. Corey and W . L. Mock, J. Amer. chem. SOC. 84, 685 (1962); see also J. K. Stille and T. Anyos, J. org. Chemistry 27, 3352 12291 C. D . Weis, J. org. Chemistry 27, 3693 (1962).

12261 R. W. Hoffmann and H. Hauser, Tetrahedron Letters 1964, 197. 1965, 3625.

[230] E. Vogel, W. Grimme, and S. Korte, Tetrahedron Letters

230 Angew. Chem. internut. Edit. 1 Vol. 5 (1966) No. 2

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