aromaticity in macrocyclic polypyrrolic ring...

AROMATICITY IN MACROCYCLIC POLYPYRROLICRING SYSTEMS

A. W. JOHNSON

School of Molecular Sciences, University of Sussex, Falmer,Brighton, BN1 9QJ, UK

ABSTRACTThe aromatic character of 18(annulene) is maintained in porphin, which isstabilized by the presence of the four bridging nitrogen atoms. Numerousvariations on the porphin ring structure are described and the aromaticproperties of the new ring systems are assessed by their spectral characteristics

and by their chemical behaviour.

Convincing experimental verification of the application of Hückel's rulesto conjugated ring systems larger than benzene has been provided bySondheimer1 in his elegant studies on annulenes. Many of these compoundsare conformationally mobile as well as chemically unstable which tends tocomplicate the interpretations of physical data and severely limits theexploitation of chemical reactivity. In this lecture, I shall discuss the stabiliza-tion of annulenes by the use of bridging atoms and the consequent effects onthe physical and chemical properties, particularly aromaticity, of the resultingring systems.

In the medium rings, especially the [10] and [12]annulenes, the existenceof bridging atoms often alleviates, and even eliminates, the crowding causedby the inner hydrogen atoms, thus favouring planarity and aromatic charac-ter. This concept has been well illustrated by Vogel and his collaborators2by the preparation and properties of 1,6-methano-[10]annulene (I; X =CH2) as well as the corresponding 1,6-oxido- (I; X = 0)23 and 1,6-imino-(I; X = NH)4 bridged compounds. The stability of these compounds, whichexhibit the expected delocalization of the lOit electron system on the basis ofphysical properties such as u.v., e.s.r., magnetic c.d.5 and especially n.m.r.6spectra, and x-ray crystallography7, is in marked contrast to that of theunbridged [10]annulene which is chemically unstable8. Another bridged[10]annulene is cycl[3,2,2]azine(II)9, which, from its n.m.r. spectrum, wasshown to exhibit an appreciable diamagnetic ring current'°.

(I) (II)

195

(III)

A. W. JOHNSON

The properties of cycl[3,2,2]azine(II) contrast sharply with those ofcycl[3,3,3]azine(III)'1 which is a stabilized form of the unknown non-aromatic [12]annulene. In the aromatic [14]annulene series, two arrange-ments, (IV) and (V), of bridging atoms are known, and both structuresexhibit typically aromatic properties. The planarity and bond lengths of thetrans- 15,16-dialkyldihydropyrenes'2(IV) follow from x-ray studies'3 and inthe second series (V; X = CH2,O)2'3'

14 an additional feature is the flippingof the relative positions (i.e. syn and anti isomers) of the two bridging groupsat moderate temperatures. An x-ray examination'5 of the dioxido compound(V; X = 0) showed that the carbon—carbon bond lengths were all 1.39 ±o.iA.

IRII X X

(V)

(IV)(VI)

In the [18]annulene series, the simplest bridged compounds are those ofstructure (VI; X = 0, NH, or S). However, in these compounds, particularlywhen two or all three of the bridging atoms are sulphur' 6, the presence of thebulky bridges may cause greater sterie hindrance within the ring than thesix inner hydrogens of [18]annulene itself and the compound (VI; X =S) isfound to be non-planar and therefore non-aromatic. On the other hand whenat least two of the bridging atoms are oxygen, the molecule can assume aplanar configuration and exhibit aromatic character, which is borne out byn.m.r. measurements.

Of the bridged (18)annulenes, the most important are porphin(VlI) andrelated ring systems such as chlorin(VI1I), and phthalocyanin(IX), largelybecause of their stability and the biochemical importance of their metalcomplexes in several vital natural products including haemoglobin, myo-globin, the cytochromes, and chlorophyll. Tetracoordinate square planar,pentacoordinate square pyramidal, and hexacoordinate octahedral covalentmetal complexes are all known in the porphin series, the formation of anyparticular type depending on the nature of the metal and the experimentalconditions. Metals of group I may replace the porphin imino hydrogens toyield ionic complexes of the M2P type.

The evidence of x-ray crystallography shows that porphin is essentially aplanar molecule (deviations from planarity are, however, observed in certainsubstitution products and metal derivatives) with a fourfold axis of symmetrywith respect to bond angles and distances, and that the f3-carbons of thepyrrolic rings are equivalent and separated only by 1.34 A, approximatingto an olefinic carbon—carbon bond length. For this reason as well as theoreti-cal considerations'8 a better pictorial representation of porphin is probably

196

AROMATICITY IN MACROCYCLIC POLYPYRROLIC RING SYSTEMS

(X)

(X) where the ring current involves mainly a 16-membered ring with twoextra electrons from the nitrogen atoms utilized to make up the aromatic1 8it system. The symmetry of the inner hydrogens, i.e. a statistical equivalentof a half-hydrogen associated with each of the four nitrogen atoms alsoaccords with x-ray diffraction data19. From the point of view of the organicchemist, the characteristic visible spectra (especially the intense Soret bandnear 400nm) of porphins and the modified spectra of their metal derivatives20and of the chiorins are of prime importance, both as aids to detection and tostructure analysis. Studies on photosynthesis have generated much intereston the luminescence and fluorescence21 spectra of several metal porphinderivatives, as well as their photochemically induced redox reactions. Then.m.r. spectra of porphins are also characteristic22 and show a wide spreadof the positions of the proton signals ranging from those associated with themeso protons at Ca. r —0.1 to those of the imino protons at Ca. r 14. Thesesignals are not changed appreciably in the chiorins or dihydrochlorins23indicating the retention of the aromatic l8ic system. Variations in the n.m.r.spectra with change of concentration of the porphins has been ascribed tothe formation of aggregates24, and indeed these molecules are known to beprone to the formation of charge transfer complexes, some of which havebiological significance. Numerous other spectral and other physical proper-ties of porphins have been examined but special mention should be made ofmass spectral fragmentation patterns because of their application in analysisand structural determination, particularly of traces of porphins in petroleumresidues and geological sampi es2 .

197

(VII)(VIII)

(IX)

A. W. JOHNSON

All the physical evidence suggests that the porphins and their metalcomplexes possess a high degree of aromatic character, in spite of smalldeviations from planarity in certain derivatives, and that the conformationalmobility of the annulenes is virtually eliminated in the porphins. In order toassess the contribution of the bridging nitrogen atoms to the aromaticity ofthe porphin structure we have prepared a number of related macrocyclescontaining mixed bridges using variations of established synthetic routes toporphins. These include the cyclization of 1-bromo-19-methyl-1,19-dide-oxybiladiene-ac dihydrobromides(XI) by heat2ó as well as the condensationof a 5,5'-diformyldipyrromethane with a dipyrromethane-5,5'-dicarboxylicacid in the presence of hydrogen bromide27:

Brft

+ '- R1 R2 R3 R4 R5 R6 R7 R8

R5__R6 R,R ,2BrNiLL).N

LNJLN MeH H (XI)

Br

R1 R2 R3 R4

OHCN1)1.NJJCHO \,R5 R6 R7 R8 R8 / R5

HO2CNJLINJJCO2H R6

Drs King, Gourley and Broadhurst at Nottingham have shown28 that thecondensation of 5,5'-diformyldifurylmethane(XII) with a dipyrromethane-5,5'-dicarboxylic acid yielded a 21,22-dioxaporphin which was obtainedeither as the mono- or di-hydrobromide (e.g. XIII), and the structure of thelatter has been confirmed by x-ray analysis.

198


Me

0 HN) 28rJ.>-_9 HT\>

Me

The development of a 1 + 3 condensation, rather than the foregoing 2 + 2method, has enabled us to obtain 21-oxaporphins (e.g. XIV; X = 0) and21-thiaporphins (e.g. XIV; X = S), and a further modification of the methodhas yielded the 21,23-dioxaporphins, 21-oxa-23-thiaporphins, and 21,23-dithiaporphins(XV ; X = Y = 0;X = 0,Y = S ; X = Y = S respectively)29.

Me Et Et Me Et Me

HO2Cft)jLNJJ1NJCOH+

OHCIX)CHO

C1CHVJCH2C1

OHCiXJCHO +

Me Ft

H

Me

(XIV)

Me

Although the general appearance of the spectra of the new macrocyclescompared with those of aetioporphyrin I left no doubt about their aromaticnature, some interesting variations were reveiled. Thus, in trifluoroacetic

199

OHC1LJL.JL0JICHO

(XII)+

Me Et Ft Me

HO2CI..NJL,JLN JCO2HH H (XIII)

Ft Me

+ 2Ft Me

N CO2RH

H Ft

(XV)

A. W. JOHNSON

acid solution, where all the products were present as dications, the Soretband was shifted to shorter wavelengths in the oxa compounds, but to longerwavelengths in the thia derivatives and the intensities of absorptions weremuch lower than those observed for porphins (Table 1).

Table 1. Soret bands (B-bands) in TFA (nm)

Compound )L maxU

21,23-Dioxaporphin (XV;X = Y 0) 369.5 (sh.), 376.5(196000) , (248000)

21,22-Dioxaporphin (XIII) 375.5 ' 385.5(212000) , (229000)

21-Oxaporphin (XIV; X = O)385.5 (sh.), 389.5

(220000) (23000)

Aetioporphyrin I 395.5(467 500)

21-Oxa-23-thiaporphin (XV; X = 0,Y = S) 398

(205000)

21-Thiaporphin (XIV; X = S) 410(175000)

21,23-Dithiaporphin (XV; X = Y = S) 412(156200)

Another interesting effect was noted in the position of the quartet of so-called Q bands (480—680 nm: Table 2) of the visible spectra of the compoundsin pyridine solution. Separation of the (I and II) and the (III and IV)bandsremained constant while the (II) and (III) band separation increased markedlywith the number of nitrogen atoms replaced (Table 3). Also the intensityof band (IV) relative to those of the other bands increased similarly (Table 4).No marked variations of the proton chemical shifts were observed relativeto those of aetioporphyrin I, but the unusually high basicity of the dioxa-porphins was unexpected and the free bases were not obtained in the purestate. The mass spectra revealed that the most intense ion in the molecularion region (M + 2 for the oxa compounds with significant peaks for M + 4)did not necessarily represent the molecular weight, although the relativeintensities varied considerably with experimental conditions. Finally it was

Table 2. Q Bands in pyridine (nm)

Compound Band IV Band III Band II Band I

21-Oxaporphin

21-Oxa-

491e16020

488.5

5239320

518.5

590.5, 5994160 3550

616,5, 627 649.5

6511880680.5

23-thiaporphin c15 900 6750 1 830 , 1 720 445 440Aetioporphyrin I 500

e11890533

8340571

5460627

436021-Thiaporphin 500

17820528.5

11340596 , 603

4320 ,3440

625818

6561605

21,23-Diathia- 494 521 613 , 621 645 674.5

porphin e11030 4140 1140 ,1006 252 640-

200


Table 3. Approximate* band separations (nm) in pyridine

Compound (I — II) (III — IV) (II — III)

21-Oxaporphin21-Oxa-23-thiaporphinAetioporphyrin I21-Thiaporphin21,23-Dithiaporphin

5658565656.5

.32303328.527

711033871.595

Measured from centre of doubtet of band TI.

Table 4. Relative intensities of Q bands in pyridine

Compound Band IV Band III Band II Band I

21-Oxaporphin 8.55 4.95 2.2 1

21-Oxa-23-thiaporphin 35.2 15.3 4.1 1Aetioporphyrin I 2.7 1.9 1.25 1

21-Thiaporphin 11.0 7.1 2,4 1

21,23-Dithiaporphin 17.25 6.45 1.7 1

apparent that, of the new macrocycles, only the 21-monoxa derivative formedstable metal complexes; although a zinc derivative was obtained from the 21-monothia compound, it was stable only in the presence of excess zinc ions.

The chemistry of the porphins likewise leaves little doubt about thearomatic nature of the ring system. The extreme stability of the ring and itsease of recognition, coupled with the essential biological role of its derivatives,leads to the existence of porphins being used as evidence for the existence oflife at various periods in the history of the earth. In this connection the resultsof porphin analyses of samples of the moon's surface3° are of topical interestas are the so-called prebiotic syntheses of porphins from methane—ammonia—water mixtures31. There are several reactive sites in the porphin molecule, andchemical reactions may involve substitutions at the meso positions, thepyrrolic p-positions, the imino groups or additions to the -double bonds.The imino groups of the metal-free porphins are susceptible to deuterationand methylation32 but most other electrophilic substitutions, e.g. halogena-tion33, nitration34, formylation35 involve the meso positions. If thesereactions, e.g. deuteration36, cause formation of the porphin dication, therate of substitution may be slow. Although most porphins produced bylaboratory syntheses have all eight n-positions substituted by alkyl groups,some, e.g. deuteroporphyrin IX (XVI), contain free l-positions, and thenthe reactivity of the 3-positions towards electrophilic substitution is roughlycomparable with that of the meso positions, and either or both may beinvolved in these reactions37, i.e. a certain amount of aromatic character isassociated with the individual pyrrole rings embodied within the aromaticmacrocycle36.

The chemical reactivity of a porphin may be modified appreciably byconverting it to its metal complex and the assessment of this property has

201

CO2MeMeO2C,MeO2C( \\ Me, '\)

\\ /NJMeO2C'—,( I II

MNH NS—N HN

Me Me

PMe CHfCH2CO,Me Et

OHNN Et

(XX)

Et Et

EtCH.CO2Etr Et

NNEtI 'EtEl Et

EtEt(XXI)

Reactions which involve the I33.-double bonds of the porphin moleculeare largely oxidations and reductions, but others have been described whichinvolve the formation of new carbon—carbon bonds, e.g. the Diels—Alderreaction of protoporphyrin IX dimethyl ester with dimethyl acetylene-

202

A. W. JOHNSON

been investigated in some detail, as well as reactions which involve the metalitself. Such reactions include the formation of compounds containingmetal—carbon cr-bonds and examples have been quoted of derivatives ofcobalt, iron38 and rhodium39. These metal—alkyl groups are embeddedwithin the t-cloud of the aromatic system and the alkyl protons are subjectto strong shielding in the n.m.r. spectrum, e.g. the cobalt(iii)—methyl deriva-tive (XV1I) of aetioporphyrin I shows the methyl signal at r 14.5. Althoughsome metals are removed from their complexes by acids, others, e.g. palladium,are resistant to acid treatment and this may sometimes be advantageous,e.g. palladium octaethylporphin may be methylated at the meso-position bymethyl fluorosuiphonate, a reagent which causes reaction at nitrogen in themetal-free porphin40.

Me Et Me

Me Me Et>—N eN(/ \I' \s Co\ ,N

Me Me Et . Me\ .—_/Me Et

(XVII)

= CH2.CH2• CO2H

(XVI)

Me Me(XVIII)

(XIX)


dicarboxylate to yield (XVIII)4' and the reaction of copper octaethylporphinwith ethyl diazoacetate to give (XIX) (as well as causing meso-substitution)42Oxidations at the (3-positions include hydroxylations which can involve atleast three of the four pyrrolic rings in porphins. The x-glycols (partial struc-ture, XX) formed in the reaction readily undergo the pinacol rearrangementto give the ketones (partial structure, XXI)43'

Under other experimental conditions, oxidations and reductions involvethe porphin meso positions. Thus, whereas catalytic hydrogenation normallyyields porphyrinogens (XXII), controlled reduction of porphins usingisobutyl aluminium hydride yields the 5,15-dihydro' derivatives (XXIII),isomeric with the chlorins (Vu!)45. Under other conditions a third group ofdihydroporphins can be formed, the so-called phiorins (XXIV; X = CH2)46where only one meso position is reduced, and it is of interest that electro-chemical reduction of chiorins yields the chlorin-phlorins44. Meso-oxidationof porphins has been studied in connection with the biologically importantoxidation of porphins to bile pigments, e.g. biliverdin (XXV), and earlierstudies on the oxidation of the iron porphins had clarified the main pathway.More recently this work has been extended and, in particular, a new type ofporphin oxidation product, the keto-phiorins (XXIV; X = CO) have beenisolated and characterized, and their reactions, including conversion toderivatives of meso-hydroxyporphin, have been documented47. Spectro-scopic evidence48 suggests that the hydrochloride of 5-amino-octaethyl-porphin has a chromophore of the type (XXIV; X = C=NH2).

Et Et

(XXII)(XXIII)

(xxiv)X is absent (corrole) or +X = CO. CH2. S, or C=NH2

(XXV)

203

Cu

A. W. JOHNSON

In the phiorins, the macrocyclic conjugated system is broken by themethylene group but in the keto-phiorins, a formal aromatic structure canbe derived either by polarization of the carbonyl group or by tautomerismto the enolic meso-hydroxyporphin. The structure (XXIV; X =CO) is anexample of a bridged annulenone of a size (4n + 3 macrocycle) which shouldbe aromatic49 (cf. cyclopropenone and tropone) although the n.m.r. signalsat t 2.2—3.1 associated with the meso protons suggest that this property isnot pronounced.

In the course of our studies we have synthesized two other ring systemsof type (XXIV), the first with a sulphur bridge (XXIV; X = S) and the second,corrole, which has the linking group omitted, i.e. a direct linkage between twoof the pyrrolic rings. For the formation of the macrocycle (XXIV; X =S)containing the sulphur bridge, the required intermediate (XXVI) was obtained,albeit in low yield, by reaction of 2-formyl-3,4-dimethylpyrrole with sulphurmonochioride. The diformyl compound (XXVI) was condensed with certain3,3',4,4'-tetra-alkyldipyrrometbane-5,5'-dicarboxylic acids at — 100 whenmoderate yields of purple-red products were obtained together with unstablegreen compounds50. The latter, regarded as thiaporphins, formed relativelystable charged zinc complexes formulated as (XXVII; R = Me, Et) on thebasis of analyses, spectra and mass spectra (parent ion at m/e M—X, whereX = Cl). The meso proton signals at t 0.11 (1H) and 0.37 (2H) indicated anaromatic structure with it-electron delocalization through sulphur. Asimilar structure for the corresponding hydrobromide (XXVIII) has beendescribed recently by Harris5 .

Me RMe Me Me Me \ .

OHCtL..Nj... ))ciio

MeEt(XXVI) Me I / Et

Me RR = Me, Et

(XXVII)

Me R

Me/ I "EtH N HN

Br s—N HN

Me "EtMe R

(XXVIII) (XXIX)

204

R Me

R Me

Meç HNMe Et

R Me

(XXXII)

Me

Me

R Me

(XXXI)

Et

Et

EtO2CMe Me

r—- CO2Et

OHCl[J SIN)CHOH H

(XXXIII)

In another series the bis(5-formylpyrrolyl) sulphide (XXXIII) was condensedwith 3,3'-diethyl-4,4'-dimethyldipyrromethane-5,5'-dicarboxylic acid andgave a deep blue product, which proved to be the meso-thiaphiorin (XXX;R = CO2Et). The n.m.r. positions of the meso proton signals at r 2.43(2H)and 3.7(1H) and the imino protons at -r 5.44 demonstrated the absence of aring current in the molecule. The free base formed greenish brown mono-cationic salts, the visible spectra of which resembled those of the phiorinsalts46 and in strong acid, diprotonation occurred to give a species formulatedas (XXXIV) on the basis of its n.m.r. spectrum. Oxidation of the thiaphiorinwith 2,3-dichloro-5,6-dicyano-1,4-benzoquinone gave the correspondingthiaporphin, which was again isolated as the zinc salt. When the thiaphiorin

205


The structures of the purple-red compounds have been tentatively formula-ted as (XXIX), isomers of the mesothiaphiorins. The n.m.r. spectra indicatethe non-aromatic nature of the compound, a low-field signal at -r 0.68 beingassociated with the angular proton at C1. Treatment of solutions of (XXIX)with zinc acetate gave up to 30 per cent of the thiaporphin zinc complex(XXVII). When the new macrocycle (XXIX) was heated in refluxing bromo-benzene, a low yield (one to two per cent) of corrole was obtained; this wasraised to 15 per cent by the addition of triphenyiphosphine, but whether thelatter reacts with the sulphur before or after its removal from the macrocycleis unknown. If the macrocycle (XXIX) were to tautomerize to the thiaphiorin(XXX; R Me) on heating, it has been calculated that the highest filledmolecular orbital of structure (XXX) has the correct symmetry for a disrota-tory ring closure to the valence tautomer (XXXI) which can then lose sulphurby a cheletropic process to generate the 18 n-electron aromatic corrole(XXXII).

Me

Me

(Xxx)

R Me

A. W. JOHNSON

was heated in refluxing o-dichlorobenzene for two hours, the correspondingcorrole (cci. 40 per cent) was obtained. This yield was unaffected by thepresence of free radical scavengers (e.g. tert-butylcatechol) but raised to60 per cent in presence of triphenylphosphine. An N-methyl derivative of thethiaphiorin also lost sulphur on heating and gave the corresponding 21-N-methylcorrole.

The corrole ring system, e.g. (XXXII), can be prepared by brief irradiationof the corresponding 1,19-dideoxybiladiene or alternatively several metalderivatives can be obtained by aeration of the biladiene in basic solution inthe presence of metal ions52. Corrole contains an aromatic 18 ,t-electronchromophore and its spectroscopic properties support this, e.g. the meso-proton signals at r 0.6—1.2 and imino proton signals near r 13.5, the strongSoret band near 400 nm and the molecular ion (M) being the base peak inthe mass spectrum. An x-ray examination of the corrole (XXXII) has shownthat ring A is twisted by ca.8° out of the general plane53. Corroles readilyform stable aromatic anions, and undergo monoprotonation to the aromaticsalts (XXXV). The susceptibility of corrole to both proton abstraction andaddition is a consequence of the delocalization of the change on to all fournitrogen atoms thus creating additional resonance structures over those ofneutral corrole. In strong acids (H2S04, FSO3H etc.) a diprotonated speciesis obtained which is formulated as the non-aromatic blocked structure(XXXVI) largely on the basis of n.m.r. spectra.

Reaction of the corrole anions with alkyl(methyl, ethyl, allyl, and 3,3-dimethylallyl) halides causes formation of a separable mixture of the N(21}.(XXXVII) and N(22)-alkylcorroles (XXX VIII), of which the latter appeared topossess a higher degree of aromatic character (possibly a measure of deviation.from the planar structure) as judged by n.m.r. and visible spectra. Withacetyl chloride only the N(21)-acetylcorrole was characterized. Furthermethylation of the N(21)-methylcorrole gave the N,N(21,22)-dimethylcorrolesalt which was thermally unstable and reverted to the N(2 1)-monomethylderivative, the more stable isomer, on heating. This conclusion was reachedalso through a study of N(22)-allylcorrole which, in refluxing toluene, formed24 per cent of the N(21)-isomer and 29 per cent of the parent corrole. Thisrearrangement was shown to be intermolecular as a similar reaction of theN(22)-3,3-dimethylallyl derivative proceeded without inversion, and theyield of the rearranged product was reduced appreciably in the presence ofcumene, a radical trapper32.

The formulation of the divalent transition metal corrole complexes posesthe problem of the location of the original third imino hydrogen, now locatedon carbon, but in the case of cobalt, it is the cobalt(Iii) derivative which isobtained where all three imino hydrogens are replaced by the metal. Un-fortunately both the nickel(Ii) and copper(II) corroles are paramagnetic andn.m.r. studies with a view to locating the 'extra' hydrogen have been of noavail, although from the visible spectra, the nickel and palladium complexesmay differ in this respect from the copper complex54. The 'extra' hydrogencan be removed readily by the action of base with the formation of aromaticmetal corrole anions and palladium corrole anions have been isolated ascrystalline dihydrates of the pyridinium salts.

Alkylation of the nickel corrole anions also gave aromatic alkyl derivatives

206


Me Me

Me<\ I I )Et

2X \rNH HN1

MefrjTIhhiJEtMe Me

(XXXIV) (XXXV)

MeH H Me

MeNHHN

2)C

Me / NH HN— Et

Me Me

(XXXVI)

Me Me

Me((f"\EtNH RN-_c

(XXXVII) (XXX VIII)

where the new alkyl group was appreciably shielded (the methylationproduct showed a singlet at r 12.6 for the new methyl group). Althoughthese products were formulated originally as nickel—alkyl derivatives55 thiswas modified to nickel N(21)-methylcorroles after an x-ray examination ofthe analogous copper derivative56, and a later x-ray study of the nickel corroleethylation product (XXXIX) confirmed this54. Palladium and copper, butnot nickel, could be introduced directly into N(21)-methylcorrole and copperN(22)-methylcorrole could also be obtained. As the visible spectra of coppercorrole and of copper N(21)-methylcorrole were similar it is possible thatthe 'extra' hydrogen of copper corrole (but not nickel corrole) is located atN-2154.

207

P.AC.—28/3—E

A. W. JOHNSON

Me Et Et

Me Et Me Et

Me Me Me Me

(XXXIX)(XL)

When the nickel N(21)-methylcorroles were heated in boiling chioro-benzene they rearranged to isomeric products (e.g. XL) containing gem-dialkyl groups at C-355. This was proved by the use of ethyl marker groupsand by n.m.r. studies. The corresponding N(21)-ethyl and -propyl derivativesrearranged even in refluxing benzene and a reaction of the nickel corroleanion with ally! bromide gave the nickel 3-alkyl-3-allylcorrole directlytogether with several nickel meso- allylcorroles. The size of the alkyl groupis an important feature in these alkylation—rearrangement reactions for asimilar reaction of the nickel corrole anion with isobutyl iodide in refluxingacetone gave the 3,3-disubstituted product in 58percent yield with none of theN(21)-isobutyl derivative. The n.m.r. meso-proton signals rise from r 0.6in the N(21)-alkyl derivatives to 2.43—3.32 in the rearranged products, andthe driving force to the rearrangement clearly does not involve a gain inaromatic character and must involve factors such as the formation of astrong carbon—carbon a-bond from a weak carbon—nitrogen cT-bond, reliefof steric strain, and the gain in ligand field stabilization energy when nickelreturns to the more ideal square planar geometry assumed in the product.The rearrangement is regarded as two consecutive (1,5)-sigmatropic shifts ofthe alkyl group across the face of the molecule and the absence of 'crossed'products in the thermolysis of mixtures is confirmation of the intramolecularnature of the reaction as also was the insensitivity of the reaction to thepresence of radical trapping agents54.

Methylation of the palladium corrole anion caused the formation of theN(21)-methyl derivative (6 per cent) together with the palladium 3,3-dialkyl-corrole (24 per cent) and both products were shpwn to be stable under thereaction conditions. In refluxing o-dichlorobenzene the palladium N(21)-methyl derivatives underwent rearrangement but on the other hand, thermo-lysis of the copper N(21)-methyl corroles caused loss of the methyl groupsand regeneration of the parent copper corrole54.

Just as in the porphin series we have investigated the substitution of oneand two of the corrole nitrogen atoms by oxygen. Because only low yieldsof 21,24-dioxacorroles(XLI) were obtained from the condensation of5,5'-diformyl-2,2'-bifuran(XLII) with dipyrromethane-5,5'-dicarboxylic acids,it was decided to use the sulphur extrusion method. 5,5'-Diformyldifurylsuiphide was synthesized and condensed with certain 3,3',4,4'-tetra-alkyl-dipyrromethane-5,5'-dicarboxylic acids when the dioxacorroles were ob-tained in 20 to 30 per cent yield. The products had typical corrole-like visible

208


spectra, the n.m.r. spectra showed the existence of a strong ring current,and analysis and mass spectra left no doubt that sulphur had been extrudedwith the formation of the direct linkage between rings A and D. As byproducts,traces of porphin were observed, but also one to two per cent of the 21-monoxacorroles (XLIII), derived from cleavage and recombination reactionsof the diformyldifuryl sulphide. Structure (XLIII) was deduced from inter-pretations of the n.m.r. spectrum, and it was found that unlike the dioxa-corroles, the monoxa derivatives formed metal complexes57.

Me

t: HN OHCIOL-tOJCHO

(XLII)R=Me,Et Me

(XLI)

Me Me

Et Et

Me", ,/Et ", ,, /EtEt Me Me

(XLIII) (XLIV)

The visible spectra of the dioxacorroles in N,N-dimethylformamidesolution was unchanged on addition of aqueous sodium hydroxide, whichcontrasts markedly with the properties of corrole. N-metbylation of dioxa-corrole yielded a mixture of the mono-N-methyl and di-N-methyl (XLIV)derivatives and as expected the N-alkyl groups, located in the centre of amacrocyclic aromatic system, were highly shielded, the n.m.r. signals forthe N-alkyl protons being at Ca. 'r 14.7_15.532. Molecular models indicatedthat, for the two methyl groups to be accommodated in the centre of themacrocycle, the hybridization of the nitrogen atoms has to approachsp3 in order to maintain planarity of the conjugated periphery. Moreoverthe disposition of the methyl groups must be trans and this was confirmedby a resolution of (XLIV) as the D-camphor- 1O-sulphonate. Certain chemicalreactions of (XLI) were investigated. Treatment of the base with acetylchloride and aluminium chloride at 80° for one hour gave the 5-monoacetylderivative together with a diacetyl compound with the acetyl substituents

209

A. W. JOHNSON

located either at 2,18- or 3,17-. No deuterium exchange in deuteriotri-fluoroacetic acid occurred with the dioxacorrole at 35° over several hours;at 100°, the meso protons exchanged in one hour but no exchange of thep3-protons was observed even after 100 hours at 1000.

1O-Oxa(XLV; X =0), 10-aza(XLV; X = NH or tautomer), 10-methyl-aza(XLV; X = NMe), and 10-thia-corroles(XLV ; X = S)havebeen obtainedtogether with their metal complexes by cyclization of metal derivatives ofbis(5-bromo-2,2'-dipyrrometheny1s (XLVI) in the presence of hydrochloricacid, ammonia, methylamine and sodium suiphide respectively. Thesemacrocycles showed typical Soret bands in their visible spectra58.

Et Et Et Et

Me<\ I I \)Me Me

'i—NH N-<x

Me Me Me

(XLV) (XLVI)

(X = NH, NMe, 0, S)

The formation of 21,24-dioxacorrole (XLI) in low yield (one to five percent) from the condensation of 5,5'-diformyl-2,2'-bifuran (XLII) withdipyrromethane-5,5'-dicarboxylic acids has been mentioned above, but inthe reaction a second green product (about seven per cent) was also formed.This compound showed an intense Soret-like band at 435.5 nm in the visiblespectrum, and a considerable induced diamagnetic ring current was evidentfrom the n.m.r. spectrum (meso and 3-protons signals at t — 2.2 to —1.2;imino protons at 'r 15.1 and 16.5) and on the basis of this the product wasformulated as the salt (XLVII) of a 26,30-dioxasapphyrin, an aromatic22-it electron macrocycle, its formation being due to a cleavage—recombina-tion reaction of the dipyrromethane. Sapphyrin (XLVIII) the parent systemcontaining five nitrogen atoms, also was first observed as a result of cleavagereactions. Structure (XLVII) has been confirmed by a rational synthesisfrom (XLII) and the tripyrrane acid (XLIX) when the dioxasapphyrin wasobtained in 40 per cent yield. Like the dioxaporphyrin (XIV), the dioxa-sapphyrin was strongly basic and pure samples of the free base were notobtained: nor were any metal complexes. The disalts also gave strongM + 2 ion peaks in the mass spectrum as did the dioxaporphyrins. A similarcondensation of 5,5'-diformyl-2,2'-dipyrrole with the acid (XLIX) gave thesapphyrin in 46 per cent yield29' 57•

We have also investigated the preparation of 22-ic electron macrocyclescontaining two direct linkages between the smaller rings and for these wehave used the [2,5']-pyrrolyldipyrromethanes (L), obtained from 2,2'-bipyrroles by condensation with 2-formylpyrroles59 and subsequent reduc-

210

Et Et


MeMe2X

R Me —Me Ft

R = Me. Et

(XLVII)(XLVIII)

Me Et Et Me Ft Me Me Et Et Me Me Et

HO2CiJLJLJLJ!JCO2H U

(XLIX) (L)

tion. These were condensed with 5,5'-diformyl-2,2'-bifuran(XLII) in thepresence of hydrogen bromide when the dioxanorsapphyrin free bases (ca.20 per cent) (LI) were obtained. In acid solution, the Soret bands formed asharp doublet centred at Ca. 450 nm, and the n.m.r. spectrum of the free baseshowed the meso and 13-protons at r — 0.5 to 0.4 and the imino protons at'r 14.85 thus confirming the aromatic nature of the product. These 22-itelectron macrocyclic systems are bridged derivatives of the unknown(22)-annulene, which according to the calculations of Dewar and Gleicher60,is the highest aromatic (4n + 2)-annulene.

/ Me Me

Me{iN'EtEt/N\ TMC Me / / / Et

TIJ" Et—

Me

Et Me R—Me,Et

(LI)(LII)

Because of the structural connection with the chromophore of vitaminB12 we have synthesized and examined the reactions of the metal complexesof corrole-type macrocycles containing angular substituents at positionsC-i and C-19, which thus become sp3 carbon atoms. The nickel 1-methyl-tetradehydrocorrins (e.g. LII) are obtained by the cyclization of 1-bromo-19-methyl-1,19-dideoxybiladienes-ac (XI) in the presence of base and nickel

211

A. W. JOHNSON

ions, and in the absence of air61. It will be recalled that the salts of theseintermediates (XI) yield porphins when they are heated26. The angular methylgroup of (LII) introduces a strong element of asymmetry into the moleculeas well as constituting a block in the conjugated system. The n.m.r. signalscorresponding to the meso protons range from t2.8 to 4.0 which gives littleevidence of aromatic character and this is supported by the chemicalbehaviour. Thus, reaction with dimethyl acetylenedicarboxylate occurs inDiels—Alder fashion to yield (LIII; isolated as the corresponding salts)62by a [16 + 2] type thermal addition. When the nickel 1-alkyltetradehydro-corrins were heated, the products were the same nickel 3,3-dialkylcorroles(e.g. XL), as were formed from the thermolytic rearrangements of the nickelN(21)-alkylcorroles (e.g. XXXIX)63. By the use of ethyl groups as markers,and taking into account the greater migratory aptitude of ethyl comparedwith methyl, it was shown that the C-i substituent migrated first to C-2and then to C-3 by a concerted mechanism. In extensions of these observa-tions, it was shown that C-i allyl groups underwent rearrangement to C-3particularly easily, and that 3,3-dimethylallyl groups were not inverted inthe course of the rearrangement. Moreover, ethoxycarbonyl angular sub-stituents could also be rearranged from C-i to C-3, and these novel migra-tions also proceeded under experimental conditions milder than thoserequired for the rearrangement of alkyl groups. The nickel 1-alkyltetra-dehydrocorrins (LII) were found to be susceptible to protonation or alkylationat C-1964, and in the latter case, the salts of the nickel 1,i9-dialkyltetra-dehydrocorrins (LIV) were obtained. These salts may also be obtained readilyby aeration of alkaline solutions of i,19-dialkyl-1,19-dideoxybiladienes-acin the presence of nickel (or cobalt) ions6 5•

Me Me Me R MeMe Me Et

MCOZCII)NcII�Me Et Me Et

Et Me Me R' Me

(LIII) (LIV)

Me

MeMe

:)rN /NMe

(Lv)

212


The trans disposition of the C-i and C-19 angular methyl substituents in(LIV: R = = H) has been proved by the resolution of the D-camphor-sulphonates62 and this stereochemical assignment is incorporated in theassigned structure of (LIII). Experimental conditions for the stepwisehydrogenation of these salts have been established and lead to the isolationof the ACD-tridehydro- and AD-bisdehydrocorrin (LV) salts66 and morerecently hydrogenation of the 13-free nickel tetradehydrocorrin salt has beenshown to yield the nickel 1,19-dimetbylcorrin isolated as its crystallineperchlorate (LVI)67. This total synthesis of the corrin ring system involvesonly three stages (cf. ref. 68).

H+

H __ MejLL+4jiL+JMe2 sE

Me[JCHO

—4MetN\v)

(LVI)

In contrast to the thermal rearrangement of the nickel 1-alkyltetradehydro-corrins (above), the nickel 1,19-dialkyltetradehydrocorrin salts (LTV;R = = H) give porphins when they are heated. Of the two angularalkyl groups, one, usually methyl, is the source of the porphin fourth mesocarbon and, depending on the nature of the original anion, the other alkylgroup is either retained as a meso substituent (e.g. LIX) (anion = per-chlorate) or expelled (anion other than perchlorate). The stepwise course ofthis interesting rearrangement has been elucidated (e.g. LIV —' LVII -.LVIII —* LIX)69.

The cationic ring system of the nickel 1,19-dialkyltetradehydrocorrinsalts is readily susceptible to nucleophilic attack and, for example, in thepresence of bases it is converted to the neutral keto derivative (LX: R =H).Although spectroscopic evidence for protonation at oxygen, with the forma-tion of a meso-hydroxy derivative of the original salt, was obtained when theketo derivative was treated with trifluoroacetic acid, the derivative couldnot be isolated in the free state. The reaction of the nickel tetradehydrocorrinsalt with methanolic potassium cyanide was also very rapid, and gave threemain products. One of these was the neutral 15-cyano-5-keto derivative

213

A. W. JOHNSON

(LX; R = CN) but the two major products have been shown to be freeradicals which yield the 5-cyano (LIV; R = CN, R' = H) and the 5,15-dicyano (LIV; R = R' = CN) derivatives of the original salt respectivelywhen they are treated with acid70.

The nickel tetradehydrocorrin salts are also susceptible to electrophilicsubstitution and, for example, the 5,15-dibromo derivative (LIV; R = R' =Br), isolated as the perbromide, could be readily obtained. The brominesubstituents were very reactive and could be removed even by heatingsolutions in chloroform. Reaction of the dibromo compound with lithiumcopper methyl gave the 5,1 5-dimethyl derivative (LIV; R = R' = Me)(ci ref. 65) in good yield. Nitration of the original salt gave the 5-mononitroderivative (LIV; R = NO2, R' = H) and although this was rather unstableit was easily converted by oxidation in the presence of base to the 5-keto-15-nitro compound (LX; R = NO2)70.

Et Me

Me Et

Me-t\+ /KNiX PH— N

Et\ I yMe\ _,__/Me Me

Et Me

Mec I I EtN N

The chemical and spectral properties of the nickel tetradehydrocorrinsalts (LIV) could be taken as an indication of a certain amount of aromaticcharacter within the molecule, and the break in the conjugated systemcaused by the angular methyl groups might be bypassed by invoking themetal (as in LXI) or regarding the peripheral system as homoaromatic(LXII). However, it is probably more accurate to regard the molecules ascharged metal derivatives of 1,19-dideoxybilatrienes and such aromaticcharacter as exists is to be associated with the individual five-membered

214

Et Me

— HX

x

Me Me

(LVII)(Liv)

Et Me

Me Et

Me( N/N/R'Et&' I I 'Me

Me Me

(LVIII)

Me Me

(LIX)


rings36. The integrity and resistance to further chemical attack of the sixconjugated double bonds of the corrins (LVI) is noteworthy and the reasonsfor this must await further study71.

Me 0 Me

Me R Me

(Lx)

(LXII)

Me Me

Me Et

Me%N./\

Me,l)Ft

Me Me

(LXI)

The results described in this paper represent the efforts of a large number ofcolleagues in Nottingham and Sussex, and to all of them I accord my sincerethanks.

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aromaticity in macrocyclic polypyrrolic ring...

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