C S I R O P U B L I S H I N G

Australian Journal of Chemistry

Volume 53, 2000© CSIRO 2000

A journal for the publication of original research in all branches of chemistry and chemical technology

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the Australian Academy of Science

Aust. J. Chem., 2000, 53, 109–121

10.1071/CH00027 0004-9425/00/020109

Geometrically Specific Hydrogen Transferin the Reaction of Terminally Alkyl-Substituted1,3-Dienes with 1,4-Quinones

Donald W. Cameron A,B and Ross M. Heisey A

A School of Chemistry, The University of Melbourne, Parkville, Vic. 3010.B Author to whom correspondence should be addressed.

Reaction of certain geometrically defined 1,1-dioxy-4-alkyl- and -4,4-dialkyl-substituted buta-1,3-dienes withhalogenated quinones does not involve Diels–Alder or Michael addition chemistry. Instead, rapid competitive oxi-dation of the dienes to give 2,4-dienoate esters was observed. This new reaction involves strong spatial associationbetween diene and quinone, hydrogen being transferred specifically from the (4E)-alkyl group. Its scope is com-pared with addition of the same terminally substituted dienes towards the reactive non-quinonoid dienophile tetra-cyanoethylene.

Keywords. Dienes; quinones; cycloaddition; hydrogen transfer.

IntroductionTerminally dialkyl-substituted 1,3-dienes have poor inter-

molecular reactivity towards dienophiles in Diels–Alderchemistry. This comes from steric constraint, affectingdevelopment of the characteristic endo-cis transition stateand deriving particularly from the terminal (Z)-substituent.1Additional oxy substituents on the diene assist such addi-tions but, even so, reactions with conventional dienophilesremain slow. Thus [4+2] cycloaddition of the alkoxy diene(1) to acrylonitrile and to maleic anhydride required severaldays heating at 110°, conditions under which reaction of theanalogous silyloxy diene (2) led only to decomposition.2

Reaction of diene (1) with the quinonoid dienophile (3), aprocess that might usefully have led to the bicyclic system(4) incorporating the AB-ring skeleton of higher terpenoids,did not afford conventional addition.2 Prolonged heatinggave the analogous adduct (5) as the only isolable product.Its formation was suggested as involving redox interactionbetween the two reactants (1) and (3).

Earlier work here3 showed that quinone (3) underwentconventional cycloaddition to the less substituted (E)-methyldiene (6), on being heated at 110° for 7 days, giving theadduct (7) (73%). Even the (Z)-methyl isomer (8) was foundto react analogously, to give the isomeric adduct (9) (45%)under more forcing conditions (150°).

These observations showed that protection of dienes (6)and (8) and their respective cycloadducts (7) and (9) as t-butyldimethylsilyl ethers gave them considerable thermalstability. The present investigation began by assessing howan analogous system, the new dimethyl-substituted diene(10), would behave on being heated with dienophile (3).

Reaction of Terminally Substituted Dienes withQuinones

The diene (10) was obtained from aldehyde (11), preparedby acid-catalysed reaction of isobutyraldehyde with ethylvinyl ether. Whilst the isolated intermediate was originallyformulated as a 1,3-dioxan (12),4 spectroscopic analysis sug-gested it was a complex mixture of oligomers. Nevertheless,on vacuum distillation from p-toluenesulfonic acid it gave(11) (28%) as the only product. Its olefinic resonances (�6.07, 6.81) in the 1H n.m.r. spectrum showed trans coupling(J 15.8 Hz), while the aldehydic proton resonated as adoublet (� 9.50, J 8.1 Hz). On being treated with triethyl-amine and t-butyldimethylsilyl triflate, it gave (10) (66%).The gem-dimethyl protons resonated as two broad singlets (�1.74, 1.68) and (E)-geometry was assigned to the 1,2-doublebond by comparison of the associated coupling constant (J1,211.5 Hz) with literature for other silyloxy dienes.5

Heating (10) with the quinone (3) led to an unexpectedoutcome. Raising the temperature above only 70° led toextensive decomposition of both components. This was rapidby 100°. The combination of the dimethyl diene (10) with (3)was thus surprisingly more reactive than those involving (3)and the simpler dienes (6) or (8), in that it occurred at lowertemperature. However, it did not lead to cycloadduct (13) orto any other isolable product. Since (10) was largely stable at150° over many days, its destructive interaction with (3) wasinferred to have involved something other than Diels–Alderchemistry.

The possibility that combination of (10) with (3) mighthave entailed hydrogen transfer between the electron-richdiene and electron-deficient quinone was then investigated.

Manuscript received 21 February 2000 © CSIRO 2000

D. W. Cameron and R. M. Heisey110

111

Both components were modified in the hope of accentuatingany such process, enabling it to be observed under milderconditions. The new 1,1-dioxy diene (14), a more electron-rich system than (10), was obtained from the �,�-unsaturatedester (15)6 by enolization with lithium diisopropylamide(Pri

2NLi) in the presence of hexamethylphosphoric triamidefollowed by silylation. Its 1H n.m.r. spectrum showed broad-ened C-methyl resonances at � 1.76 and 1.67. 1,2-(Z)-Geometry was assigned by n.O.e. difference spectroscopy.Thus, irradiation of the methoxy resonance (� 3.56) causedenhancement of the signal for H 2 (� 4.47) by 7%.

The new diene (14) was treated with p-chloranil (16), aquinone of high redox potential but limited dienophilicity. Inequimolar proportions in (D)chloroform at room tempera-ture, the mixture instantly gave an intense blue colour, thatfaded within 10 s to amber. The blue intermediate parallelsthe charge-transfer complexes observed on treatment ofcertain oxy dienes and quinonoid dienophiles, as a visualpreliminary to Diels–Alder cycloaddition but from (14) theoutcome was different. The 1H n.m.r. spectrum of themixture, acquired after 4 min, showed virtually completeconversion into the dehydrogenated ester (17). The associ-ated colour changes implied the process to have been largelycomplete within a few seconds. The same outcome wasobserved in (D6)benzene and it was unaffected by exclusionof light. The olefinic �-proton H 3 (� 7.37) of the productshowed trans-coupling (J2,3 15.9 Hz) to H 2 (� 5.86). An estermethoxy resonance was observed at 3.77 and the C-methylgroup as a broadened signal at � 1.88, while the terminalmethylene protons appeared as two broad singlets (� 5.36,5.35). The product (17) was identical with material indepen-dently synthesized by Wittig chemistry.7 As expected, it wasitself a poor diene in the Diels–Alder sense, failing to reactseparately with either (3) or the more dienophilic 2,6-dichloro quinone (18). Heating such mixtures only led todecomposition. As an extended acrylate system, (17) was rel-atively unstable.

Stoichiometry suggests that the other product of redoxinteraction between (14) and (16) should be a silyl ether ofthe quinol (19). However, the only product that could be iso-lated from a mixture of the two reactants in pentane was thequinol itself 8 (56%), presumably reflecting adventitioushydrolysis during workup.

With hydrogen transfer chemistry thus demonstrated forthe dimethyl diene (14) under very mild conditions, attentionwas redirected to its monooxy analogue (10). Its reactionwith p-chloranil (16) was monitored in (D)chloroform by 1Hn.m.r. spectroscopy. At room temperature the solution dark-ened and a very slow reaction was observed. After 10 daysthere was some decomposition but only two componentswere spectroscopically identifiable: unreacted diene (10)(64%) and the dehydrogenated pentadienal (20)9 (15%). Thealdehydic proton (� 9.59) resonated as a doublet coupled toH 2 (� 6.16, J1,2 8.0 Hz). (E)-Geometry was shown by thecoupling between H 2 and H 3 (� 7.20, J2,3 15.4 Hz). The ter-minal methylene protons were observed as two broad sin-glets (� 5.49, 5.47), while the methyl protons appeared as abroadened signal at � 1.93.

While this conversion into (20) was not preparativelyuseful, it was consistent with the 1-oxy diene (10), like its1,1-dioxy analogue (14), undergoing redox interaction withquinonoid dienophiles in preference to cycloaddition; butdoing so more slowly. As an extended acrolein, the product(20) apparently underwent concomitant decompositionduring the prolonged conditions of its formation, a processthat would have been facilitated by heating in the originalreaction between (10) and quinone (3).

The dioxy diene (14) was then treated at room tempera-ture with 2,6-dichloro-1,4-benzoquinone (18), a compoundof lower redox potential than p-chloranil (16). Reactionoccurred more slowly. An intense blue colour formed imme-diately but this persisted for 40 s before fading.Spectroscopic monitoring showed the formation and decayof an unstable intermediate that was unobservable in thefaster reaction involving (16). After 13 min, signals for thisintermediate had disappeared, to be replaced by resonancesfor the ester (17) (95%). The latter was isolated (74%) withsome loss from polymerization, together with the dichloroquinol (21) (50%).

The short-lived intermediate was too labile to be isolated.At room temperature in (D)chloroform or (D6)benzene, spec-troscopic monitoring indicated a maximum accumulation of30 or 60% respectively. However, its spectrum clearlyshowed a system in which hydrogen transfer from the start-ing diene (14) had already occurred. It contained C-methyland vinylic methylene resonances (� 1.79, 5.09 respec-tively). There was also trans-coupling (J 15.9 Hz) betweenH 2 and H 3 (� 5.53, 6.62 respectively), compared with thesmaller value (J 10.7 Hz) for the corresponding protons of(14). The lower degree of deshielding of the �-proton H 3,relative to that of the ester (17) (� 7.37), may indicate theabsence of a carbonyl group at position 1, as may shieldingof the methoxy protons (� 3.37) relative to those of (17) (�3.77). A tentative structure accommodating these compo-nents is the orthoester (22). Not surprisingly, addition of tri-fluoroacetic acid to a solution containing this intermediateresulted in instant conversion into (17).

Other high-potential 1,4-benzoquinones had paralleleffects. The 2,3-dichloro-5,6-dicyano, tetrafluoro, andtrichloro quinones all oxidized (14) within seconds to give(17) virtually quantitatively. The action of 2,5-dichloro-1,4-benzoquinone resembled that of its 2,6-dichloro isomer (18)in that oxidation proceeded more slowly than for the morehighly substituted quinones; and a short-lived intermediatesimilar to (22) was detected by 1H n.m.r. spectroscopy.

Despite this parallel, the hydrogen transfer process did notaltogether correlate with redox potential of the quinonoidcomponent. Treatment of (14) with o-chloranil (23), a betteroxidant than (16), did not promote it at all. Instead, rapidreaction occurred without the visible involvement of acharge-transfer complex but this led to the cycloadduct (24),a process involving inverse electron demand, such as hasbeen observed for other o-quinones.10 The methoxy protonsof the adduct resonated at � 3.59 and the t-butyl protons at �0.91, while the two silyl methyl groups were differentiated (�0.20, 0.18). The olefinic proton was strongly shielded (�

Geometrically Specific Hydrogen Transfer

D. W. Cameron and R. M. Heisey112

3.56), confirming that the 1,2-double bond of the originaldiene remained intact. This proton was vicinally coupled (J9.3 Hz) to the adjacent methine proton (� 4.74).

Mild treatment of (24) with trifluoroacetic acid effectedhydrolysis to the isolable methyl ester (25) (71% overall).The methine proton (� 4.46) resonated as a doublet of dou-blets, the two adjacent methylene protons being spectro-scopically differentiated.

Other behaviour also suggested that the hydrogen transferprocess might depend on factors additional to the oxidizingcapability of the coreactant. Thus treatment of (14) with thehydride abstractor trityl fluoroborate also gave no diene ester(17). Instead, rapid reaction at room temperature in (D)chlo-roform resulted only in adventitious conversion into the �,�-unsaturated ester (26). Its methylene protons resonated as abroad doublet (� 3.00, J 7.2 Hz), while the olefinic H 3appeared as a broad triplet (� 5.27). It was identical with asynthetic sample prepared as for the corresponding ethylester.11

Hydrogen transfer was also not observed when (14) wasreplaced by its lower vinylogue (27), derived by enol silyla-tion of methyl isobutyrate.12 Despite being activated, like(14), by the conjugative influence of two oxy groups, (27)did not form a change-transfer coloration nor react with p-chloranil (16) under conditions where reaction of (14) wasrapid. The failure of (27) thereby to be oxidized to methylmethacrylate suggested that, relative to the hydrogen accep-tor, there was a specific spatial requirement between the acti-vating 1,1-dioxy group of the electron-rich system and thealkyl group from which hydrogen transfer occurred.

To help identify any such spatial requirement, attentionwas directed to two factors: varying geometry at the dioxyterminus of the diene; and ascertaining whether hydrogentransfer from the gem-dialkyl group was geometrically spe-cific. The (E)-diene (28), geometrically isomeric with (14),was prepared by adapting Ireland’s procedure for controlledenolization of simple alkanoate esters,13 the Pri

2NLi-derivedenolate of the �,�-unsaturated ester (26), generated in theabsence of hexamethylphosphoric triamide, being quenchedby subsequent addition of t-butyldimethylsilyl chloride.While this gave a poor yield (16%), the major component wasassigned as (28) on the basis of its spectroscopic differencefrom the (Z)-diene (14). The absence of hexamethylphospho-ric triamide, dictated for purposes of geometrical control, hasthe disadvantage of slowing silylation by the sterically hin-dered reagent. Adding hexamethylphosphoric triamide at thefinal silylation stage was found to increase the yield of diene(30%) but also, unacceptably, to lead to a mixture of the twogeometrical isomers (28) and (14) (1 : 4), in which the latterpredominated. Enol silylation of (26), involving Pri

2NLi inthe presence of hexamethylphosphoric triamide from theoutset, proceeded as expected13 to give only the (Z)-diene(14) (56%), as noted earlier.

The (E)-diene (28), when treated with p-chloranil (16) in(D)chloroform at room temperature, reacted analogously to(14) and almost as rapidly. Within 10 s the initial bluecharge-transfer colour faded, with virtually quantitative con-version into ester (17).

Ascertaining whether hydrogen transfer from diene (14)involved differential reactivity of the two C-methyl groupswas assessed by investigating the component behaviour ofits geometrically defined, monomethyl homologues, the (E)-and (Z)-methyl dienes (29) and (30) respectively. While rec-ognizing that their individual C-methyl groups could notnecessarily be considered simply as additive components of(14), it seemed likely that if one of them underwent hydro-gen transfer to a quinonoid acceptor under conditions wherethe other did not, or if both did so at greatly different rates, itwould be a significant observation.

The two dienes (29) and (30) were obtained by enoliza-tion (Pri

2NLi, hexamethylphosphoric triamide) and silyla-tion of the respective (Z)-14 and (E)-esters (31) and (32)according to established selectivities.15,16 The (1Z,3E)-product (29) was obtained geometrically pure within thelimits of spectroscopic detection. Its 1H n.m.r. spectrumshowed an n.O.e. of 7% for H 2 (� 4.39) when the methoxyresonance (� 3.54) was irradiated, while H 3 (� 6.18) and H 4(� 5.31) showed trans-coupling (J 15.5 Hz). The (1Z,3Z)-product (30) was contaminated with a trace (5%) of itsisomer (29). The 1H n.m.r. spectrum of the major componentshowed an n.O.e. of 4% for H 2 (� 4.54) on irradiation of themethoxy resonance (� 3.60), while resonances for H 3 (�6.18) and H 4 (� 5.10) showed the expected cis-coupling (J10.8 Hz).

The (E)- and (Z)-methyl dienes (29) and (30) were thenseparately treated with the two quinones (16) and (18) andthe mixtures were spectroscopically monitored by 1H n.m.r.spectroscopy. Addition of (29) to a suspension of p-chloranilin (D)chloroform at room temperature immediately gave anintense blue-green colour that faded to an amber solutionafter 10 s. The spectrum of the mixture, taken after 4 min,showed clean conversion into the hydrogen transfer product(33). On a preparative scale this was isolated in 76% yield.Its �-enoate proton H 2 (� 5.91) resonated as a doubletshowing trans-coupling (J 15.4 Hz) to H 3 (� 7.27), whichwas further split by H 4 (� 6.45, J3,4 10.6 Hz). These andother physical data were in satisfactory agreement with liter-ature values.17

The same (E)-methyl diene (29) was then treated with2,6-dichloro-1,4-benzoquinone (18) under the same condi-tions. After 10 min all of the diene was consumed but thereaction took an altogether different course from that with p-chloranil (16), reflecting the stronger dienophilicity of (18)and its poorer electron-accepting capability. The only recog-nizable product was the unstable cycloadduct (34) in modestyield (25%), together with unidentified by-products.However, none of the oxidized ester (33) was evident, withinthe limits of spectroscopic detection. The proton resonancesdiagnostic for structure (34) were the two olefinic signals ofthe dioxy-substituted ring (� 5.85, 5.60), which showed cis-coupling (J 10.7 Hz), together with appropriate long-rangesplitting, and a doublet C-methyl resonance (� 1.42, J 7 Hz).These characteristics were analogous to those for the isolatedadduct (7).3

Reaction of the (Z)-methyl diene (30) with p-chloranil(16) proceeded much more slowly than for its (E)-isomer

113

(29). There was little development of colour. Spectroscopicmonitoring suggested the formation of a single product (35)(60%), which required several hours for completion andwhich involved Michael addition/elimination chemistry,such as has been observed for other 2,3-dichloro quinonesreacting with polarized dienes.18 No hydrogen transfer chem-istry was detected.

For the 1H n.m.r. spectrum of the product (35) the olefinicprotons of the side chain (� 5.90, 7.10) were trans-coupled (J15.6 Hz). The adjoining methine proton (� 4.21) resonated asan apparent quintet of doublets, reflecting equivalentcoupling (J 6.8 Hz) to the four vicinal protons and allyliccoupling (J 1.5 Hz). The C-methyl group (� 1.47) corre-spondingly was observed as a doublet and the ester methylgroup (� 3.73) as a singlet.

The (Z)-methyl diene (30), when treated with the dichloroquinone (18), spectroscopically showed extensive decompo-sition after 5 min at room temperature. No cycloaddition orhydrogen transfer products could be identified from thismixture. Michael addition, analogous to the first stage of thereaction leading to (35), would have involved the unsubsti-tuted 3(5)-position of (18) leading to a substituted quinol andpossible complications from subsequent redox chemistry. Nosimple Michael product could be detected.

While some of the reactions thus studied between dienes(29) and (30) with quinones (16) and (18) were inconclusive,the fact that the (E)-diene (29) with p-chloranil (16) effi-ciently underwent oxidation to (33) under conditions whereits (Z)-isomer (30) did not, encouraged the supposition thathydrogen transfer chemistry was occurring exclusively fromthe (E)-methyl group. This has been supported by investigat-ing higher homologues of (29) and (30).

The new (E)- and (Z)-4-ethyl dienes (36) and (37) weresynthesized as for their respective methyl homologues (29)and (30). Enolization/silylation of the (Z)-ester (38)14 gave(36), while the (E)-ester (39) similarly gave (37). Irradiationof the methoxy resonance (� 3.54) of the 1H n.m.r. spectrumof (36) showed a 7% n.O.e. of the signal for H 2 (� 4.41),while the signal for H 4 (� 5.38) showed trans-coupling (J15.4 Hz) to H 3 (� 6.17). For (37) there was a parallel 7%n.O.e., while J3,4 10.7 Hz indicated cis-coupling. The twodienes (36) and (37) were therefore assigned the expected(1Z,3E)- and (1Z,3Z)-geometries respectively.

Reaction of the (E)-diene (36) with p-chloranil (16) paral-leled the behaviour of its methyl analogue (29) but towardsthe dichloro quinone (18) it showed an interesting difference.Its reaction with (16) proceed through an initially blue solu-tion, at a comparably rapid rate to (29), to give a clean con-version into methyl sorbate (40). On a preparative scale (40)was thereby isolated in 72% yield, its formation involvingtransfer of a methylene hydrogen for the first time. Theproduct was identical with authentic material. Comparisonof its 1H n.m.r. spectrum with published spectra19 for all fourgeometrical isomers of (40) confirmed its all-trans structureand the absence of any other isomer within detectable limits.

However, whereas its methyl analogue (29) reacted withthe dichloro quinone (18) chiefly by cycloaddition, the newdiene (36) gave no cycloadduct but instead underwent rapid

hydrogen transfer, again forming methyl sorbate (40) (30%)as the only spectroscopically detectable product, togetherwith decomposition. This difference may reflect lower reac-tivity of (36) towards cycloaddition, because of the greatersteric demand of an ethyl group over a methyl; and also itshigher reactivity towards redox chemistry, reflecting transferof a secondary hydrogen versus a primary one. Accuratecomparison of hydrogen transfer reactivity of the two dienes(36) and (29) towards the common acceptor p-chloranil (16)is difficult because, in (D)chloroform, both reacted substan-tially to completion in shorter times than were required forconventional spectrometric scanning of the solutions.However, if it can be assumed that formation of the respec-tive ester products (40) and (33) parallels visual disappear-ance of charge-transfer colour, then the ethyl diene is indeedthe more reactive. In toluene solution at room temperature, inwhich the lifetime of the blue intermediate was prolonged,the approximate times for comparable discharge of colourfor reactions of (36) and (29) with (16) were 30 s and 4 minrespectively.

The differing outcomes—hydrogen transfer versuscycloaddition, described earlier for the respective quinones(16) and (18) reacting with the same diene (29), and now forthe respective dienes (36) and (29) reacting with the samequinone (18)—indicate the two processes to be competitive.

Towards the standard quinones (16) and (18) the (Z)-ethyldiene (37) behaved as for its methyl counterpart (30).Hydrogen transfer chemistry could not spectroscopically bedetected. Reaction of (37) with (18) showed extensivedecomposition after 5 min. Reaction with (16) requiredseveral hours for completion, with the addition/eliminationproduct (41) (40%), detected by 1H n.m.r. spectroscopy, themajor outcome. Its olefinic protons (� 5.88, 7.10) showedtrans-coupling (J 15.6 Hz). The adjoining methine proton (�3.97) resonated as an apparent quartet (J 7.5 Hz) of doublets(J 1.5 Hz). The remainder of the spectrum was consistentwith the assigned structure.

The foregoing observations of the (E)-ethyl diene (36) notundergoing cycloaddition to either of the chloro benzo-quinones (16) and (18) but reacting instead by hydrogentransfer, contrast with the reported behaviour of a morehighly oxygenated ethyl diene (42) cycloadding to the chloronaphthoquinone (43), in the course of synthesizing theanthraquinone (44).20 The next three paragraphs show thatthis difference in outcome probably derives from the changeto a naphthoquinonoid dienophile. This was assessed byinvestigating reaction between the simpler (E)-ethyl diene(45) and the simpler chloro naphthoquinone (46).

A diene assigned structure (45) has previously beenreported by silylation following hexamethylphosphoric tri-amide mediated lithiation of the (E)-ester (39).21 However,from the published data (J3,4 10 Hz), the diene so derivedwould appear to be the expected (3Z)-isomer (47). Synthesisof (45) itself (79%; J3,4 15.4 Hz) was effected by Pri

2NLi-enolization of the (Z)-ester (38) followed by treatment withchlorotrimethylsilane. The trimethylsilyl analogue of (36)was chosen for this aspect of the work for convenience ofsynthesis. The 1H n.m.r. and 13C n.m.r. spectra of (45) resem-

Geometrically Specific Hydrogen Transfer

D. W. Cameron and R. M. Heisey114

bled those of (36) apart from appropriate resonances associ-ated with the silyl group. In addition, like (36), the diene (45)was oxidized nearly quantitatively to the ester (40) oncontact with (16), while with (18) it gave predominantly thesame ester (40) (40%), together with decomposition prod-ucts.

Towards the chloro naphthoquinone (46), however, thediene (45) reacted at room temperature chiefly by cycloaddi-tion. Aromatization of the crude adduct gave the ethylanthraquinone (48) (36%) as the major product, withoutdetectably competing hydrogen transfer chemistry. Thelimited yield of (48) was consistent with steric inhibition atthe addition stage. However, the observation that cycloaddi-tion was the chief process occurring in the formation of (48),as for the reported formation of (44),20 identifies the lowerredox potential of chloro naphthoquinones (46) and (43) rel-ative to the chloro benzoquinones (16) and (18)22 as the mostlikely responsible factor.

The 1H n.m.r. spectrum of the anthraquinone (48) showedtwo chelated hydroxy resonances (� 12.62, 12.01) andappropriate quartet methylene and triplet methyl signals (�3.12, 1.25 respectively). The product underwent oxidation,as for the anthraquinone (44),20 by bubbling oxygen throughan ethanol solution irradiated with visible light. This gavethe acetyl anthraquinone (49) (74%). The ethyl resonances of(48) were replaced, in the 1H n.m.r. spectrum of the product,by an acetyl signal (� 2.51), while its 13C n.m.r. spectrumshowed a third carbonyl resonance (� 204.0).

With allowance having been made for competitivecycloaddition, the foregoing data show (4E)-monoalkylgroups undergoing hydrogen transfer to appropriatequinonoid acceptors under conditions where their (4Z)-counterparts did not. Extrapolating to 4,4-dialkyl dienes like(14) would suggest that only the (E)-alkyl substituent wasinvolved in analogous chemistry there. This has been sup-ported by investigating mixtures of the geometrically iso-meric 4-ethyl-4-methyl dienes (50) and (51), which have notbeen able to be obtained individually pure. The likelihoodthat, for mixtures of the two isomers, competing hydrogentransfer of methylene versus methyl hydrogens wouldproceed at different rates was not expected to be a complica-tion, given the use of at least equivalent concentrations ofquinonoid acceptor to carry them both to completion.

Conventional methods, involving conjugate enolizationof �,�-unsaturated esters with Pri

2NLi/hexamethylphospho-ric triamide followed by silylation, largely failed whenapplied to the synthesis of dienes (50) and (51). This waspresumably for steric reasons. Thus the (E)-ester (52),7,23

prepared by Wittig chemistry, gave the two dienes (50) and(51) (3 : 1) but in a yield of only 4%. The sterically crowded(Z)-ester (53),14 prepared by Favorskii chemistry, could notbe made to give a diene at all. Its methine proton H 4 (� 3.39)was considerably deshielded by the ester carbonyl, relativeto the corresponding proton of its isomer (52) (� 2.21).

The most efficient synthesis of dienes (50) and (51) pro-ceeded from the (E)- and (Z)-isomeric �,�-unsaturated esters(54) and (55),24,25 consistent with enolate formation beingmore effective by deprotonation at a methylene group. The

starting esters (54) and (55) (2 : 1) were obtained as an insep-arable mixture by adapting a Knoevenagel procedurereported for the corresponding ethyl esters.11 The 2 : 1 ratiofavouring the (E)-ester (54) over its (Z)-isomer (55) wasestablished by the 1H n.m.r. spectrum of the mixture and isthermodynamically consistent with the greater stericdemand of an ethyl group relative to methyl. The (Z)-methylgroup of the major isomer (54) (� 1.63) resonated as a broadsinglet, while the (E)-methyl group of (55) (� 1.73) appearedas a distinct doublet (J 1.2 Hz). The larger allylic couplingand relative deshielding for the methyl protons syn to theolefinic proton (H 3) are consistent with literaturemodels.26,27

Enolization of the mixed esters (54) and (55) withPri

2NLi/hexamethylphosphoric triamide followed by silyla-tion satisfactorily gave the dienes (50) and (51) (2 : 1) in 61%yield. The ratio of isomers was independently determined by1H and 13C n.m.r. spectroscopy, analysis being assisted byreference to spectra of the several homogeneous dienesalready described. For the major isomer (50), irradiation ofthe proton resonance for the branching 4-methyl substituent(� 1.67) resulted in an n.O.e. of 4% for the H 2 signal (� 4.45)but none for the adjoining H 3 (� 5.94). For the minor isomer(51), parallel irradiation of the 4-methyl protons (� 1.74)resulted in a significant n.O.e. (2%) only for H 3 (� 5.89).The effect on H 2 for isomer (50) reflects the s-trans confor-mation thermodynamically favoured by substituted butadi-enes.28,29 The expected (Z)-geometry about the 1,2-doublebond was confirmed for at least the major isomer (50) by anobserved n.O.e. (4%) of the coincident signals for H 2 (�4.45) by co-irradiating the methoxy resonances at � 3.58 and3.57.

The 2 : 1 mixture of dienes (50) and (51) was then treatedwith p-chloranil (16) at room temperature in (D)chloroform.Rapid hydrogen transfer occurred through a blue charge-transfer colour. Analysis of the product by 1H n.m.r. spec-troscopy showed a 2 : 1 mixture of the diene esters (56)25 and(57)30 in a combined yield of 90%. The 1H n.m.r. spectrum ofthe major component (56) appropriately showed two C-methyl resonances, one a broad singlet (� 1.76) and the othera doublet (� 1.81) coupled to H 5 (� 5.98, J 7.1 Hz). For theminor component (57) there was a broad methylene quartet(� 2.23) coupling with a methyl triplet (� 1.11, J 7.5 Hz),consistent with the ethyl substituent. The 3 : 1 mixture ofdienes (50) and (51) from enolization of (52), when treatedwith p-chloranil (16), similarly underwent conversion into a3 : 1 mixture of (56) and (57). Formation of (56) entailedtransfer of a methylene proton from (50), while that of (57)involved a methyl proton from (51). These outcomes leavelittle doubt that the hydrogen transfer process was geometri-cally specific in favour of the (4E)-substituent.

Reaction of Terminally Substituted Dienes withTetracyanoethylene

Most of the 1,1-dioxy 4-substituted dienes of the previoussection have also been added to the non-quinonoiddienophile tetracyanoethylene (TCNE). The rapid reactionsthat ensued in (D)chloroform at room temperature were

115

monitored by 1H n.m.r. spectroscopy. While accompanied bythe development and dissipation of blue-green coloration,they did not lead to hydrogen transfer chemistry. Stericfactors notwithstanding, nucleophilic attack by the dieneoccurred so as invariably to involve the branched carbon-4.

Previously reported chemistry of TCNE additions wouldanticipate reaction of dienes (14), (29), (30), (36) and (37) toproceed through zwitterionic intermediates (58) and lead tothe two isomeric [4+2] and/or [2+2] cycloadducts shown inScheme 1.28,29 With concertedness diminished because of theinvolvement of zwitterions (58), the stereochemical selectivityof conventional Diels–Alder chemistry is expected to be lost.

The most fully defined outcome in the present investiga-tion was observed for the 4-ethyl diene (36). After 5 mincontact with TCNE, its spectrum indicated efficient formationof the two [4+2] cycloadducts (59) and (60) (4 : 3).Individual diastereoisomeric structures were unable to beassigned. The olefinic resonances overlapped as a multiplet(� 5.92), as did signals for the adjoining methine proton (�2.99). Only the two methoxy resonances (� 3.48, 3.69) (4 : 3)were differentiated and interpretation was assisted by com-parison with sharper data for the homologous system (61)and (62), discussed later.

No [2+2] cycloadduct was identified from the reaction of(36) or of any of the other 1,1-dioxy dienes in this investiga-tion. However, after the foregoing mixture of (59) and (60)continued standing at room temperature for 24 h, its spec-trum was almost completely changed to that of a newproduct, assigned as the isomeric C-silyl ester (63). The twoolefinic protons (� 6.16, 6.68) were trans-coupled (J 15.6Hz). The adjoining methine proton H 4 (� 2.72) showedvicinal coupling to H 3 (J 9.7 Hz) and to the differentiatedprotons of the methylene group (J 9.2, 3.1 Hz). Formation of

this product presumably implies equilibrium between thekinetic adducts (59) and (60) and the zwitterion (58), throughwhich inter- or intra-molecular silyl migration occurredleading to (63). Its structure was supported by treatment withtrifluoroacetic acid, protiodesilylation thereby affording thestable end-product (64). This was the only compound of theseries that could be fully characterized. Its 1H n.m.r. spec-trum paralleled that of (63) except for a new methine singlet(� 4.45) replacing resonances associated with the silyl group.Like the proton resonance of malononitrile, this methinesignal was D2O-exchangeable.

Other 1,1-dioxy dienes reacted with TCNE compatibly with(36) but differed from it in that not all of the intermediatescorresponding to (59), (60) and (63) accumulated to spectro-scopically identifiable concentrations at different times.Thus the (Z)-diene (37) reacted with TCNE, initially to give aspectroscopically complex mixture of unidentified products.But this clarified after 24 h to show resonances for (63)(80%), which on treatment with trifluoroacetic acid gave(64). The 4,4-dimethyl diene (14), on the other hand, wentdirectly to the silyl-rearranged ester (65). A 1H n.m.r. spec-trum, scanned as soon as practicable after mixing the tworeactants, indicated formation of this product almost exclu-sively. Apart from signals appropriate to the ester side chain,the gem-dimethyl system resonated as a sharp six-protonsinglet (� 1.46). Addition of one drop of trifluoroacetic acidgave the desilylated product (66), whose spectrum incorpo-rated a new, D2O-exchangeable singlet (� 4.46) for themethine proton.

Another diene that gave a silyl-rearranged ester directlywas (45), its difference from (36) presumably reflectingeasier rearrangement for trimethylsilyl than for t-butyldimethylsilyl ethers. Thus (45) with TCNE rapidly gave(67). On addition of trifluoroacetic acid this again underwentdesilylation to (64).

Similar treatment of the 4-methyl diene (29) with TCNErapidly gave a spectroscopically identifiable mixture of threecompounds: the silyl-rearranged ester (68) and the twodiastereoisomeric [4+2] cycloadducts (61) and (62) (6 : 4 : 3).For the major diastereoisomer (61) the olefinic protons (�5.87, 5.89) showed cis-coupling (J 10.7 Hz) and each wascoupled to the aliphatic methine proton (� 3.33, J 2.4, 1.9 Hzrespectively). The C-methyl group resonated as a doublet (�1.58, J 7.1 Hz). These signals nearly overlapped those for theminor isomer (62), the largest difference between thediastereoisomers involving the methoxy resonances: � 3.47and 3.68 for (61) and (62) respectively. The spectrum of (68)showed an ester methoxy resonance (� 3.77) and two trans-coupled olefinic protons (� 6.16, 6.84, J 15.4 Hz). Theadjoining methine proton (� 3.03) showed vicinal couplingto both H 3 (J 8.5 Hz) and the C-methyl group (� 1.48, J 6.8Hz). The spectrum of this mixture deteriorated over 2 daysand attempted isolation led to decomposition. However,addition of trifluoroacetic acid at that point caused it toclarify, showing substantially one product, the desilylatedester (69). Its spectrum resembled that of (68), together witha new D2O-exchangeable methine singlet (� 4.51), replacingthe silyl group resonances.

Geometrically Specific Hydrogen Transfer

D. W. Cameron and R. M. Heisey116

The isomeric (Z)-diene (30) also reacted rapidly withTCNE. But this led to a spectroscopically complex mixture ofunidentified components until the addition of trifluoroaceticacid, whereupon it was smoothly converted into the samedesilylated product (69) (estimated 80%) as obtained above.

ConclusionsTowards the non-quinonoid dienophile TCNE, the numer-

ous 4-alkyl- and 4,4-dialkyl-substituted dienes assessed inthis study thus underwent essentially orthodox additionchemistry. In all cases, addition involved bonding at carbon4 despite steric impedance at that centre, particularly for the4,4-disubstituted series.

Towards the 1,2-quinone (23), reaction of diene (14) alsoproceeded in orthodox fashion, formation of the derivedcycloadduct (24) occurring with involvement of the 3,4-double bond.

However, towards 1,4-quinones there was competitionbetween conventional Diels–Alder or Michael chemistryand the hydrogen-transfer process now identified.

So far as the 1,4-quinonoid component of such interactionis concerned, its tendency to promote hydrogen transfer, rel-ative to addition chemistry, required relatively high redoxpotential coupled with limited dienophilicity. The fully sub-stituted p-chloranil (16) was thus a strong hydrogen acceptorrelative to 2,6-dichloro-1,4-benzoquinone (18) and evenmore so relative to the chloro naphthoquinone (46).

So far as the diene component is concerned, the structuralrequirements for hydrogen transfer have turned out to bevery specific. It is to have been expected that 1,1-dioxydienes would be more reactive than their 1-oxy analogues.But no literature parallel can be found for the strong spatialelement now identified, in which there is particular involve-ment of (4E)-alkyl geometry, likely activated throughcharge-transfer interaction with the 1,4-quinone.

Defining such an interaction needs to accommodate thefollowing factors: (i) no diene without a (4E)-alkyl sub-stituent has been observed to undergo hydrogen transfer; (ii)the (4Z)-monoalkyl dienes, despite their poor Diels–Alderreactivity for steric reasons, have shown no compensatinghydrogen transfer reactivity—this applies to the point whereMichael addition or complex decomposition may occur pref-erentially; (iii) dienes that contain a (4E)-monoalkyl sub-stituent and that accordingly lend themselves to hydrogentransfer, often do not show it because (E)-geometry alsofacilitates competitive cycloaddition.

These factors indicate that 4,4-dialkyl-substituted dienespossess particularly appropriate structural features for pro-moting hydrogen transfer; the (4Z)-alkyl component sup-presses competing cycloaddition, while its (4E)-alkylpartner effects the transfer. A further structural factor that hasnot been considered so far is that the product esters (17), (33)and (40) formed by hydrogen transfer have invariably pos-sessed 2,3-(E)-geometry. Given the mildness of conditionsleading to their formation and the fact that related isomericester systems possessing 2,3-(Z)-geometry are stable enoughfor independent characterization,17,19 this (E)-configurationseems likely to have been formed directly. This makes it dif-

ficult to see the hydrogen-transfer process occurring throughthe conventional charge-transfer complex associated withDiels–Alder chemistry, which necessarily involves an s-cisconformation about the 2,3-bond of the diene.

To accommodate these factors in outline, Fig. 1 comparestwo competing interactions between a 1,1-dioxy-4,4-dialkyl-substituted diene and chloranil. The first interaction (70)would conventionally afford the Diels–Alder adduct (71) butit is suppressed by severe steric impedance. The alternativeinteraction (72), suggested for hydrogen transfer, parallelsthe first, except for the diene component being in the steri-cally unrestricted s-trans conformation about its 2,3-bond.Models suggest that such an interaction can readily involvea quinonoid oxygen and the ketal carbon of the dieneapproaching bonding distance to one another, while at thesame time, the other quinonoid oxygen and specifically oneof the (4E)-akyl hydrogens do likewise. Hydrogen transfer tothe lower quinonoid oxygen can then be combined withinvolvement of the upper one in formation of an orthoester(73) analogous to (22). Concertedness is not implied andthere is no identified basis for expressing a preferencebetween one- and two-electron chemistry in the overalltransformation. However, the process appears to represent anew highly specific example of redox chemistry involving1,4-quinones.

ExperimentalGeneral

Melting points were determined on a Kofler hot-stage and areuncorrected. Microanalyses were carried out by National AnalyticalLaboratories, Melbourne. Electronic spectra were recorded in ethanolby using a Varian Super Scan 3 spectrophotometer. Infrared spectrawere recorded with a Perkin–Elmer 983G infrared spectrophotometer.Solids were recorded as potassium bromide disks and liquids as thinfilms between sodium chloride plates. Proton (1H) and carbon (13C)n.m.r. spectra were recorded on a JEOL JNM-GX-400 spectrometer,operating at 400 MHz for proton and 100 MHz for carbon. Unlessotherwise stated, the solvent was (D)chloroform. High- and low-reso-lution mass spectra were recorded either on a VG Micromass 7070Fmass spectrometer or on a JEOL AX-505H instrument at 70 eV, unless

Fig. 1. Competing interactions between a 1,1-dioxy-4,4-dialkyl-sub-stituted diene and chloranil.

117

otherwise stated. In general only peaks greater than 20% are quoted,apart from the molecular ion (M). Preparative thin-layer chromatogra-phy (t.l.c.) was conducted using 20 by 100 cm glass plates coated withMerck Kieselgel 60 GF254 and the plates were eluted with the solventsystem indicated. Flash chromatography was carried out using Mercksilica No. 9385. Tetrahydrofuran was purified by distillation frompotassium benzophenone ketyl. Petrol and light petrol refer to thehydrocarbon fractions boiling in the range 60–80 and 40–60° respec-tively. Reactions involving the use of air-sensitive reagents were per-formed in flame-dried glassware under an atmosphere of dry nitrogen.Organic extracts were generally dried over magnesium sulfate beforeevaporation under reduced pressure.

Diene Synthesis from Esters: GeneralTo a stirred solution of diisopropylamine (2.4 ml, 1.1 equiv.) in

tetrahydrofuran (10 ml) at 0° was added butyllithium (2.2 M in hexanes,7.8 ml, 1.1 equiv.). The mixture was cooled to –78° and subjected tosuccessive dropwise addition, at 15-min intervals, of hexamethylphos-phoric triamide (4.07 ml, 1.5 equiv.), the methyl alkenoate (15.6 mmol,1.0 equiv.) and a solution of t-butyldimethylsilyl chloride (2.34 g, 1.0equiv.) in tetrahydrofuran (4 ml). It was then allowed to warm to roomtemperature over 40 min and the solvent was removed under vacuum.The residue was dissolved in pentane (30 ml), washed with water(2×30 ml), brine (30 ml) and dried and concentrated to give the diene asa yellow oil.

1H N.M.R. Spectroscopic Monitoring: General1H n.m.r. spectroscopic experiments were performed in (D)chloro-

form or (D6)benzene. To a solution/suspension of the dienophile (0.10mmol) in the solvent (0.7 ml) was added a solution of the diene (1.0 Min the same solvent, 100 �l, 0.10 mmol). The mixture was monitored atambient temperature by 1H n.m.r. spectroscopy at 400 MHz until com-plete reaction or decomposition had occurred. Cited percentage yieldsare based on peak integration of dienes and diene-derived products.

(E)-1-(t-Butyldimethylsilyloxy)-4-methylpenta-1,3-diene (10)The pentenal (11) was synthesized by adapting a literature proce-

dure.4 A solution of isobutyraldehyde (67 g), ethyl vinyl ether (27 g) anddry ether (60 ml) was added dropwise over 1.5 h to a stirred solution ofboron trifluoride etherate (0.25 ml) in dry ether (100 ml) at 0°. The solu-tion was then allowed to warm to room temperature and was stirred for12 h, after which time sodium carbonate (2 g) was added and the solu-tion was filtered and evaporated under reduced pressure to yield a crudeoil (79 g). To the oil was added p-toluenesulfonic acid (200 mg) and themixture was distilled at 100°/30 mm over 4 h to afford the pentenal (11)(10.1 g, 28%) as a colourless liquid, b.p. 50–51°/20 mm (lit.4 41°/11mm). �max 1695, 1470, 1365, 1135 cm–1. �H 9.50, d, J 8.2 Hz, H 1; 6.81,dd, J 15.8, 6.2 Hz, H 3; 6.07, ddd, J 15.8, 8.1, 1.5 Hz, H 2; 2.59, septdd,J 6.5, 6.2, 1.5 Hz, H 4; 1.11, d, J 6.5 Hz, CH(CH3)2. �C 194.3 (C 1),164.8 (C 3), 130.1 (C 2), 31.3 (C 4), 20.9 (CH(CH3)2). m/z 98 (M, 11%),97 (42), 71 (37), 69 (48), 59 (32), 55 (24), 43 (75), 41 (100).

To a stirred solution of triethylamine (13.9 ml) in dry ether (15 ml)at 0° was slowly added t-butyldimethylsilyl trifluoromethanesulfonate(11.5 ml) followed by the 4-methylpentenal (11) (4.9 g), and the two-phase system was heated at 40° for 12 h. The lower phase was discardedand then the top phase was diluted with light petrol (10 ml). It waswashed with aqueous sodium bicarbonate (5 ml), and brine (5 ml), thendried and concentrated to a pale yellow oil (7.0 g, 66%). Distillationgave the title diene (10) (3.6 g, 34%) as a colourless oil, b.p. 66–67°/2mm (Found: C, 68.0; H, 11.5. C12H24OSi requires C, 67.9; H, 11.4%).�max 1660, 1620, 1470, 1255, 1175, 840 cm–1. �H 6.47, d, J 11.5 Hz, H 1;5.88, t, J 11.5 Hz, H 2; 5.67, br d, J 11.5 Hz, H 3; 1.74, 1.68, br s, br s,=C(CH3)2; 0.92, s, OSiC(CH3)3; 0.15, s, OSi(CH3)2. �C 142.5 (C 1),129.8 (C 4), 120.2 (C 3), 110.4 (C 2), 26.0 ((E)-CH3), 25.6(OSiC(CH3)3), 18.3 ((Z)-CH3), 18.1 (OSiC(CH3)3), –5.3 (OSi(CH3)2).m/z 212 (M, 31%), 155 (43), 75 (100), 73 (100), 73 (54).

(Z)-1-(t-Butyldimethylsilyloxy)-1-methoxy-4-methylpenta-1,3-diene (14)

The 4,4-dimethyl diene (14) was prepared from methyl (E)-4-methylpent-2-enoate (15)6 (2.0 g, 15.6 mmol) as described in theGeneral Procedure for Diene Synthesis. Distillation of the crude yellowoil (4.07 g) furnished the title diene (14) as a colourless oil (2.22 g,59%), b.p. 78–79°/0.9 mm (Found: C, 64.5; H, 10.6. C13H26O2Sirequires C, 64.4; H, 10.8%). �max 1665, 1625, 1370, 1225, 970 cm–1. �H5.92, br d, J 10.7 Hz, H 3; 4.47, d, J 10.7 Hz, H 2; 3.56, s, OCH3; 1.76,1.67, br s, br s, =C(CH3)2; 0.96, s, OSiC(CH3)3; 0.17, s, OSi(CH3)2.N.O.e. OCH3→H 2, 7%. �H [(D6)benzene] 6.39, br d, J 10.5 Hz, H 3;4.55, d, J 10.5 Hz, H 2; 3.11, s, OCH3; 1.85, 1.73, br s, br s, =C(CH3)2;1.00, s, OSiC(CH3)3; 0.21, s, OSi(CH3)2. �C 157.2 (C 1), 125.0 (C 4),119.1 (C 3), 76.2 (C 2), 54.6 (OCH3), 26.0 ((E)-CH3), 25.6(OSiC(CH3)3), 18.1 ((Z)-CH3), 18.0 (OSiC(CH3)3), –4.3 (OSi(CH3)2).m/z 242 (M, 12%), 96 (100), 89 (30), 73 (79).

Methyl (E)-4-Methylpenta-2,4-dienoate (17)(A) To a stirred solution of 2,6-dichloro-1,4-benzoquinone (18)

(212 mg) in dichloromethane (3 ml) was added the 1,1-dioxy-4,4-dimethyl diene (14) (290 mg). The resultant intense blue colour fadedto amber after 40 s and the solution was then diluted withdichloromethane (10 ml). It was washed with 10% aqueous sodiumhydroxide (2×10 ml), water (10 ml) and brine (10 ml), then it was driedand concentrated. Bulb-to-bulb distillation (100°/4 mm) gave themethyl pentadienoate (17) (111 mg, 74%) as a colourless oil, b.p.57–58°/11 mm. �max 1710, 1630, 1605, 1434, 1270, 1170 cm–1. �H 7.36,d, J 15.9 Hz, H 3; 5.87, d, J 15.9 Hz, H 2; 5.35, 5.34, br s, br s, =CH2;3.76, s, OCH3; 1.88, br s, 4-CH3. �H [(D6)benzene] 7.48, d, J 15.8 Hz,H 3; 5.86, d, J 15.8 Hz, H 2; 4.95, 4.91, br s, br s, =CH2; 3.44, s, OCH3;1.47, br s, 4-CH3. The product was chromatographically and spectro-scopically indistinguishable from authentic material prepared asdescribed in the literature.7

(B) Reaction of (14) with (18) in (D)chloroform as in the GeneralProcedure for Spectroscopic Monitoring showed after 5 min a mixtureof the ester (17) (60%) and the intermediate tentatively formulated as(E)-4-(1�-(t-butyldimethylsilyloxy)-1�-methoxy-4�-methylpenta-2�,4�-dienyloxy)-2(3),6(5)-dichlorophenol (22) (30%). �H 7.03, s, 2×ArH;6.62, d, J 15.9 Hz, H 3�; 5.53, d, J 15.9 Hz, H 2�; 5.09, br s, =CH2; 3.37,s, OCH3; 1.79, br s, 4�-CH3; 0.87, s, OSiC(CH3)3; 0.17, s, OSi(CH3)2.After 13 min, the spectrum showed the disappearance of (22) in favourof (17) (95%), which was indistinguishable from the 1H n.m.r. spectrumof material from (A).

(C) Reaction of (14) with (18) in (D6)benzene as in the GeneralProcedure for Spectroscopic Monitoring showed after 15 min a mixtureof the ester (17) (16%) and the intermediate (22) (64%). �H 6.81, d, J15.9 Hz, H 3�; 6.20, s, 2×ArH; 5.63, d, J 15.9 Hz, H 2�; 4.96, 4.88, br s,br s, =CH2; 3.13, s, OCH3; 1.57, br s, 4�-CH3; 0.93, s, OSiC(CH3)3;0.18, s, OSi(CH3)2. After 20 min, trifluoroacetic acid (10% in(D6)benzene, 1 drop) was added and the spectrum showed the disap-pearance of (22) in favour of (17) (80%), indistinguishable from the 1Hn.m.r. spectrum of authentic material.

(D) Reaction of (14) in (D)chloroform with p-chloranil (16),2,3-dichloro-5,6-dicyano-1,4-benzoquinone, tetrafluoro-1,4-benzo-quinone, or trichloro-1,4-benzoquinone, as in the General Procedure forSpectroscopic Monitoring, all showed after 4 min formation of the ester(17) (100%).

(E) Reaction of (14) with 2,5-dichloro-1,4-benzoquinone in(D)chloroform as in the General Procedure for SpectroscopicMonitoring showed after 3 min a mixture of the ester (17) (30%)and the intermediate tentatively formulated as (E)-4-(1�-(t-butyldimethylsilyloxy)-1�-methoxy-4�-methylpenta-2�,4�-dienyloxy)-2,5-dichlorophenol (60%). �H 7.00, br s, 2×ArH; 6.68, d, J 15.8 Hz,H 3�; 5.58, d, J 15.8 Hz, H 2�; 5.07, br s, =CH2; 3.40, s, OCH3; 1.80, brs, 4�-CH3; 0.87, s, OSiC(CH3)3; 0.17, s, OSi(CH3)2. After 10 min, thespectrum showed the disappearance of this intermediate in favour of(17) (90%), which was indistinguishable from the 1H n.m.r. spectrumof material from (A).

Geometrically Specific Hydrogen Transfer

D. W. Cameron and R. M. Heisey118

2,6-Dichlorobenzene-1,4-diol (21)To a stirred suspension of 2,6-dichloro-1,4-benzoquinone (18) (177

mg) in pentane (3 ml) was added the 1,1-dioxy-4,4-dimethyl diene (14)(242 mg). After 10 min, the intense blue colour had faded to amber andthe solution was filtered. The filtrate was left at –18° for 48 h and thenthe resultant precipitate was filtered off, to give the quinol (21) (89 mg,50%) as beige needles, m.p. 157–159° (lit.8 158°). �max 3370, 1580,1435, 1200, 790 cm–1. �H 6.81, s, H 3, H 5; 5.43, s, 1-OH; 4.55, s, 4-OH.m/z 182 (M[37Cl2], 11%), 180 (M[37Cl35Cl], 66), 178 (M[35Cl2], 100),114 (53), 86 (43), 79 (52), 51 (60). It was chromatographically andspectroscopically indistinguishable from authentic material.8

2,3,5,6-Tetrachlorobenzene-1,4-diol (19)To a stirred suspension of p-chloranil (16) (25 mg) in chloroform (1

ml) was added the 1,1-dioxy-4,4-dimethyl diene (14) (24 mg). After 5min, the intense blue colour had faded to amber and the solution was fil-tered. Petrol (5 ml) was added to the filtrate and the resultant precipi-tate was filtered off to give the quinol (19) (14 mg, 56%) as pale tanneedles, m.p. 227–229° (lit.8 232°). �max 3400, 1410, 1310, 1205, 885cm–1. �H 5.66, s, 2×OH. m/z 252 (M[37Cl3

35Cl], 11%), 250(M[37Cl2

35Cl2], 52), 248 (M[37Cl35Cl3], 100), 246 (M[35Cl4], 85), 147(41), 111 (23), 89 (28), 87 (83). It was chromatographically and spec-troscopically indistinguishable from authentic material.8

Methyl 2-(5�,6�,7�,8�-Tetrachloro-3�,3�-dimethyl-2�,3�-dihydro-1�,4�-benzodioxin-2�-yl)acetate (25)

(A) To a solution of o-chloranil (23) (400 mg) in dichloromethane(10 ml) was added the 4,4-dimethyl diene (14) (500 mg). After stirringfor 3 min, trifluoroacetic acid (4 drops) was added and a brick redcolour developed. The solvent was evaporated and the residue was dis-solved in ethyl acetate (50 ml). The resultant solution was washedrapidly with 10% aqueous sodium hydroxide (2×20 ml), water (20 ml),10% aqueous citric acid (20 ml), and brine (20 ml). It was then driedand the solvent was evaporated to yield amber-coloured oily crystals(609 mg). Flash chromatography (15% ethyl acetate in petrol) of theresidue gave the title compound (25) (425 mg, 71%) as colourlessprisms from ethyl acetate/petrol, m.p. 119–120° (Found: C, 41.8; H,3.1; Cl, 37.8. C13H12Cl4O4 requires C, 41.7; H, 3.2; Cl, 37.9%). �max1730, 1555, 1425, 1265, 1045, 985 cm–1. �H 4.46, dd, J 8.8, 4.6 Hz, H 2�;3.76, s, OCH3; 2.65, m, CH2; 1.44, 1.30, s, s, 2×3�-CH3. N.O.e.H 2�→CH2, 6%; H 2�→(2×3�-CH3), 4%. �C 170.0 (C 1), 138.8, 138.2,124.9, 124.2, 120.5, 120.3, 77.1, 76.3, 52.3 (OCH3), 35.5 (C 2), 24.1,20.6. m/z 378 (M[37Cl3

35Cl], 2%), 376 (M[37Cl235Cl2], 8), 374

(M[37Cl35Cl3], 16), 372 (M[35Cl4], 13), 127 (82), 95 (38), 85 (100), 69(32), 58 (22).

(B) Reaction of (14) with (23) in (D)chloroform as described in theGeneral Procedure for Spectroscopic Monitoring showed after 4 min(Z)-2-(2�-t-butyldimethylsilyloxy)-2�-methoxyethenyl)-5,6,7,8-tetra-chloro-3,3-dimethyl-2,3-dihydro-1,4-benzodioxin (24) (90%). �H 4.74,d, J 9.3 Hz, H 2; 3.59, s, OCH3; 3.56, d, J 9.3 Hz, H 1�; 1.40, 1.28, s, s,2×3-CH3; 0.91, s, OSiC(CH3)3; 0.20, 0.18, s, s, OSi(CH3)2. After 20min, trifluoroacetic acid (10% in (D)chloroform, 1 drop) was added andthe spectrum showed the disappearance of (24) in favour of the ester(25) (90%), which was indistinguishable from the spectrum of materialfrom (A).

(E)-1-(t-Butyldimethylsilyloxy)-1-methoxy-4-methylpenta-1,3-diene (28)

Methyl 4-methylpent-3-enoate (26) was synthesized by a literaturemethod for the analogous ethyl ester.11 It distilled as a colourless oil,b.p. 66–68o/45 mm (lit.31 145–148o/640 mm). �max 1740, 1435, 1165cm–1. �H 5.27, br t, J 7.2 Hz, H 3; 3.66, s, OCH3; 3.00, br d, J 7.2 Hz,CH2; 1.73, 1.62, s, s, =C(CH3)2.

To a stirred solution of diisopropylamine (1.2 ml) in tetrahydrofuran(5 ml) at 0o was added butyllithium (2.2 M in hexanes, 3.9 ml). Themixture was cooled to –78o and after 15 min the methyl alkenoate (26)(1.0 g) was added, followed 15 min later by a solution of t-butyldimethylsilyl chloride (1.17 g) in tetrahydrofuran (2 ml). The reac-

tion mixture was stirred for an additional 15 min at –78o, allowed towarm to room temperature over 40 min and subsequently treated asdescribed in the General Procedure for Diene Synthesis. Bulb-to-bulbdistillation (100o/0.2 mm) of the yellow oil (1.4 g) gave the title diene(28) (310 mg, 16%) as a colourless oil. �H 5.91, br d, J 10.5 Hz, H 3;4.54, d, J 10.5 Hz, H 2; 3.59, s, OCH3; 1.74, 1.63, br s, br s, =C(CH3)2;0.97, s, OSiC(CH3)3; 0.23, s, OSi(CH3)2. This spectrum was differen-tiable from that of the (Z)-isomer (14), in mixtures of the two com-pounds. Both reacted indistinguishably towards p-chloranil (16), givingthe same ester (17).

Reaction of the 1-Oxy-4,4-dimethyl Diene (10) with p-Chloranil (16)Reaction of (10) with (16) in (D)chloroform as in the General

Procedure for Spectroscopic Monitoring showed, after 10 days, pre-dominantly a mixture of the starting diene (10) (64%) and (E)-4-methylpenta-2,4-dienal (20)9 (16%). �H 9.59, d, J 8.0 Hz, CHO; 7.20,d, J 15.4 Hz, H 3; 6.16, dd, J 15.4, 8.0 Hz, H 2; 5.49, 5.47, br s, br s,=CH2; 1.93, m, 4-CH3.

(1 Z,3 E)-1-(t-Butyldimethylsilyloxy)-1-methoxypenta-1,3-diene (29)Methyl (Z)-pent-2-enoate (31) was synthesized by Fischer esterifi-

cation of (Z)-pent-2-enoic acid.16 The (1Z,3E)-4-methyl diene (29) wasprepared from (31) (2.0 g) as described in the General Procedure forDiene Synthesis. Distillation of the crude yellow oil (4.0 g) afforded thetitle diene (29)14 (1.9 g, 48%) as a colourless oil, b.p. 71–72°/1.0 mm.�H 6.18, ddq, J 15.5, 10.5, 1.5 Hz, H 3; 5.31, dq, J 15.5, 6.5 Hz, H 4;4.39, d, J 10.5 Hz, H 2; 3.54, s, OCH3; 1.72, dd, J 6.5, 1.5 Hz, =CHCH3;0.96, s, OSiC(CH3)3; 0.17, s, OSi(CH3)2. N.O.e. OCH3→H 2, 7%. �C157.1 (C 1), 126.1 (C 3), 118.4 (C 4), 79.2 (C 2), 54.7 (OCH3), 25.7(OSiC(CH3)3), 18.4 (C 5), 18.1 (OSiC(CH3)3), –4.3 (OSi(CH3)2).

(1 Z,3 Z)-1-(t-Butyldimethylsilyloxy)-1-methoxypenta-1,3-diene (30)The (1Z,3Z)-4-methyl diene (30) was prepared from methyl (E)-

pent-2-enoate (32) (1.7 g) as described in the General Procedure forDiene Synthesis. Distillation of the crude yellow oil (2.5 g) afforded thetitle diene (30)15 (1.9 g, 56%) as a colourless oil, b.p. 72–73o/1.5 mm.�H 6.18, tq, J 10.8, 1.6 Hz, H 3; 5.10, dq, J 10.8, 7.4 Hz, H 4; 4.54, d, J10.8 Hz, H 2; 3.60, s, OCH3; 1.68, dd, J 7.4, 1.6 Hz, =CHCH3; 0.95, s,OSiC(CH3)3; 0.18, s, OSi(CH3)2. N.O.e. OCH3→H 2, 4%. �C 158.4(C 1), 124.2 (C 3), 116.4 (C 4), 75.2 (C 2), 54.7 (OCH3), 25.6(OSiC(CH3)3), 18.1 (OSiC(CH3)3), 13.2 (C 5), –4.3 (OSi(CH3)2).

Methyl (E)-Penta-2,4-dienoate (33)(A) To a suspension of p-chloranil (16) (245 mg) in dichloromethane

(10 ml) was added the (1Z,3E)-4-methyl diene (29) (228 mg). Theresultant intense blue-green colour faded after 10 s to amber and thesolution was then diluted with dichloromethane (10 ml). It was washedwith 10% aqueous sodium hydroxide (2×10 ml), water (10 ml), andbrine (10 ml), then dried and concentrated. Bulb-to-bulb distillation(100o/4 mm) gave the methyl pentadienoate (33) (85 mg, 76%) as acolourless oil. �max 1720, 1645, 1600, 1270, 870 cm–1. �H 7.26, dd, J15.4, 10.6 Hz, H 3; 6.44, ddd, J 15.6, 10.6, 10.0 Hz, H 4; 5.90, d, J 15.4Hz, H 2; 5.60, d, J 15.6 Hz, H 5Z; 5.48, d, J 10.0 Hz, H 5E; 3.74, s,OCH3. �C 167.3 (C 1), 144.9, 134.7, 125.5, 121.7, 51.6 (OCH3). Its 1Hn.m.r. spectrum was in satisfactory agreement with reported data.17

(B) Reaction of (29) with (16) in (D)chloroform as described in theGeneral Procedure for Spectroscopic Monitoring showed, after 4 min, theester (33) (90%). Its spectrum was indistinguishable from that from (A).

Reaction of the (1 Z,3 E)-1,1-Dioxy-4-methyl Diene (29) with2,6-Dichloro-1,4-benzoquinone (18)

Reaction of (29) with (18) in (D)chloroform as described in theGeneral Procedure for Spectroscopic Monitoring showed after10 min, among decomposition products, (4a�,5�,8�,8a�)-8-(t-butyldimethylsilyloxy)-2,8a-dichloro-8-methoxy-5-methyl-4a,5,8,8a-tetrahydronaphthalene-1,4-dione (34) (25%). �H (partial) 7.05, br s,H 3; 5.85, br dd, J 10.7, 2.2 Hz, H 7; 5.60, dd, J 10.7, 2.9 Hz, H 6; 3.72,s, OCH3; 1.42, d, J 7.6 Hz, 5-CH3.

119

Reaction of the (1 Z,3 Z)-1,1-Dioxy-4-methyl Diene (30) withp-Chloranil (16)

Reaction of (30) with (16) in (D)chloroform as described in theGeneral Procedure for Spectroscopic Monitoring showed, after 15 min,a mixture of unreacted diene (30) (50%) and methyl (E)-4-(3�,5�,6�-trichloro-1�,4�-benzoquinon-2�-yl)pent-2-enoate (35) (30%). �H 7.10,dd, J 15.6, 6.6 Hz, H 3; 5.90, dd, J 15.6, 1.5 Hz, H 2; 4.21, quin d, J 6.8,1.5 Hz, H 4; 3.73, s, OCH3; 1.47, d, J 7.1 Hz, CHCH3.

After 2 days, the spectrum showed disappearance of the remainingdiene (30) in favour of the ester (35) (60%).

(1 Z,3 E)-1-(t-Butyldimethylsilyloxy)-1-methoxyhexa-1,3-diene (36)Methyl (Z)-hex-2-enoate (38) was synthesized by Fischer esterifica-

tion of (Z)-hex-2-enoic acid.14 The (1Z,3E)-4-ethyl diene (36) was pre-pared from the ester (38) (1.9 g) as described in the General Procedurefor Diene Synthesis. Distillation of the crude yellow oil (3.4 g) affordedthe title diene (36) (1.95 g, 54%) as a colourless oil, b.p. 76–77°/0.6 mm(Found: C, 64.7; H, 11.1%; M+••, 242.1704. C13H26O2Si requires C,64.4; H, 10.8%; M+••, 242.1702). �max 1665, 1625, 1440, 1370, 1210cm–1. �H 6.17, ddt, J 15.4, 10.3, 1.5 Hz, H 3; 5.38, dt, J 15.4, 6.6 Hz, H 4;4.41, d, J 10.3 Hz, H 2; 3.54, s, OCH3; 2.07, qdd, J 7.4, 6.6, 1.5 Hz,CH2CH3; 0.99, t, J 7.4 Hz, CH2CH3; 0.96, s, OSiC(CH3)3; 0.18, s,OSi(CH3)2. N.O.e. OCH3→H 2, 8%. �C 157.3 (C 1), 126.2 (C 3), 123.9(C 4), 79.3 (C 2), 54.6 (OCH3), 26.0 (C 5), 25.7 (OSiC(CH3)3), 18.1(OSiC(CH3)3), 14.2 (C 6), –4.3 (OSi(CH3)2). m/z 242 (M, 21%), 96(83), 89 (50), 81 (45), 73 (100).

(1 Z,3 Z)-1-(t-Butyldimethylsilyloxy)-1-methoxyhexa-1,3-diene (37)The (1Z,3Z)-4-ethyl diene (37) was prepared from methyl (E)-hex-

2-enoate (39) (1.9 g) as described in the General Procedure for DieneSynthesis. Distillation of the crude yellow oil (3.2 g) afforded the titlediene (37) (2.2 g, 61%) as a colourless oil, b.p. 78–80o/0.7 mm (Found:C, 64.4; H, 10.8%; M+••, 242.1699. C13H26O2Si requires C, 64.4; H,10.8%; M+••, 242.1702). �max 2930, 1650, 1610, 1340, 1210, 975 cm–1.�H 6.12, tt, J 10.7, 1.6 Hz, H 3; 5.02, dt, J 10.7, 7.6 Hz, H 4; 4.52, d, J10.7 Hz, H 2; 3.59, s, OCH3; 2.10, quind, J 7.6, 1.6 Hz, CH2CH3; 1.00,t, J 7.6 Hz, CH2CH3; 0.95, s, OSiC(CH3)3; 0.17, s, OSi(CH3)2. N.O.e.OCH3→H 2, 7%. �C 158.5 (C 1), 124.6 (C 3), 122.6 (C 4), 75.3 (C 2),54.7 (OCH3), 25.7 (OSiC(CH3)3), 21.0 (C 5), 18.1 (OSiC(CH3)3), 14.4(C 6), –4.2 (OSi(CH3)2). m/z 242 (M, 19%), 96 (73), 89 (44), 81 (38), 73(100).

Methyl (2 E,4 E)-Hexa-2,4-dienoate (Methyl Sorbate) (40)(A) To a suspension of p-chloranil (16) (245 mg) in dichloromethane

(10 ml) was added the (1Z,3E)-4-ethyl diene (36) (242 mg). The resul-tant intense blue-green colour faded after 10 s to amber and the solutionwas then diluted with dichloromethane (10 ml). It was washed with10% aqueous sodium hydroxide (2×10 ml), water (10 ml) and brine (10ml), then dried and concentrated. Bulb-to-bulb distillation (100o/3 mm)gave methyl sorbate (40) (91 mg, 72%) as a colourless oil. �H 7.25, dd,J 15.4, 10.0 Hz, H 3; 6.16, m, H 4, H 5; 5.77, d, J 15.4 Hz, H 2; 3.73, s,OCH3; 1.85, d, J 6.8 Hz, =CHCH3. The ester (40) was chromatograph-ically and spectroscopically indistinguishable from authentic mate-rial,19 prepared by Fischer esterification of sorbic acid.

(B) Reaction of (36) or (45) with (16) in (D)chloroform as describedin the General Procedure for Spectroscopic Monitoring both showedafter 4 min the ester (40) (95%). Its spectrum was indistinguishablefrom that from (A).

(C) Reaction of (36) or (45) with (18) in (D)chloroform as describedin the General Procedure for Spectroscopic Monitoring both showedafter 5 min, among many products, the ester (40) (30, 40% respec-tively). Its spectrum was indistinguishable from that from (A).

Reaction of the (1 Z,3 Z)-1,1-Dioxy-4-ethyl Diene (37) withp-Chloranil (16)

Reaction of (37) with (16) in (D)chloroform as described in theGeneral Procedure for Spectroscopic Monitoring showed after 20 mina mixture of unreacted diene (37) (50%) and methyl (E)-4-(3�,5�,6�-

trichloro-1�,4�-benzoquinon-2�-yl)hex-2-enoate (41) (20%). �H 7.10,dd, J 15.6, 7.5 Hz, H 3; 5.88, dd, J 15.6, 1.2 Hz, H 2; 3.97, qd, J 7.5, 1.2Hz, H 4; 3.72, s, OCH3; 1.95, m, CH2CH3; 0.95, m, CH2CH3.

After 24 h, the spectrum showed disappearance of the remainingdiene (37) in favour of the ester (41) (40%).

(1 Z,3 E)-1-Methoxy-1-trimethylsilyloxyhexa-1,3-diene (45)To a stirred solution of diisopropylamine (1.79 ml) in tetrahydrofu-

ran (7 ml) at 0o was added butyllithium (2.4 M, 5.32 ml). The solutionwas cooled to –78o and chlorotrimethylsilane (1.62 ml) was added, fol-lowed by methyl (Z)-hex-2-enoate (38) (1.5 g). The mixture wasallowed to warm to room temperature over 40 min and then the solventwas removed under vacuum. The residue was dissolved in pentane (25ml) and left at –18o overnight before filtration through Celite.Concentration of the filtrate to a pale yellow oil (3.03 g) followed bydistillation yielded the title diene (45) (1.85 g, 79%) as a colourless oil,b.p. 43–44o/0.3 mm (Found: C, 59.8; H, 10.2. C10H20O2Si requires C,59.9; H, 10.1%). �max 1665, 1625, 1370, 850 cm–1. �H 6.12, ddt, J 15.4,10.3, 1.5 Hz, H 3; 5.37, dt, J 15.4, 6.6 Hz, H 4; 4.41, d, J 10.3 Hz, H 2;3.55, s, OCH3; 2.06, qdd, J 7.3, 6.6, 1.5 Hz, CH2CH3; 0.98, t, J 7.3 Hz,CH2CH3; 0.22, s, OSi(CH3)3. �C 157.1 (C 1), 126.5 (C 3), 124.0 (C 4),79.7 (C 2), 54.8 (OCH3), 26.0 (C 5), 14.3 (C 6), 0.4 (OSi(CH3)3). m/z200 (M, 10%), 96 (42), 89 (53), 81 (67), 75 (33), 73 (100), 58 (40), 57(35).

1-Ethyl-4,5-dihydroxy-9,10-anthraquinone (48)To a stirred solution of 2-chloro-8-hydroxy-1,4-naphthoquinone

(46) (100 mg) in benzene (4 ml) under nitrogen was added the 4-ethyldiene (45) (400 mg). After 30 min the solvent was removed and thegreen-brown residue was taken into tetrahydrofuran (5 ml).Concentrated hydrochloric acid (2 drops) was added to the solution fol-lowed by sodium acetate (100 mg) and ethanol (2 ml), and then it waswarmed to 50o for 10 min. Reacidification (10% hydrochloric acid) wasfollowed by dichloromethane extraction (3×15 ml). The combinedextracts were dried and evaporation gave a brown oily solid.Preparative t.l.c. (15% ethyl acetate in petrol) of the residue gave amajor yellow band (RF 0.8). Recovery then gave the title quinone (48)(46 mg, 36%) as orange needles from ethyl acetate/petrol, m.p. 98–100o

(Found: C, 71.7; H, 4.7%; M+••, 268.0738. C16H12O4 requires C, 71.6; H,4.5%; M+••, 268.0736). �max (log ) 457sh, 437, 282, 250 nm (3.84, 3.91,3.87, 4.21). �max 1625, 1455, 1280 cm–1. �H 12.62, 12.01, s, s, 2×OH;7.73, dd, J 7.6, 1.2 Hz, H 8; 7.65, dd, J 8.3, 7.6 Hz, H 7; 7.50, d, J 8.8Hz, H 2; 7.23, dd, J 8.3, 1.2 Hz, H 6; 7.19, d, J 8.8 Hz, H 3; 3.12, q, J 7.3Hz, CH2CH3; 1.25, t, J 7.3 Hz, CH2CH3. �C (partial) 181.1 (C=O),161.9 (C=O), 141.0, 137.1, 124.7, 123.5, 119.7, 28.4 (CH2CH3), 14.8(CH2CH3). m/z 268 (M, 100%), 253 (75).

1-Acetyl-4,5-dihydroxy-9,10-anthraquinone (49)Oxygen was bubbled through a boiling solution of the ethyl

anthraquinone (48) (18 mg) in ethanol (20 ml) for 2 h while the mixturewas irradiated with a 200 W tungsten lamp. The solvent was evaporatedand the residue was subjected to preparative t.l.c. (15% ethyl acetate inpetrol) which furnished a major yellow band (RF 0.21). This affordedthe title quinone (49) (14 mg, 74%) as orange needles from ethylacetate/petrol, m.p. 195–197o (Found: M+••, 282.0524. C16H10O5requires M+••, 282.0528). �max (log ) 453sh, 433, 283, 252 nm (3.68,3.77, 3.75, 4.12). �max 1695, 1625, 1455, 1255 cm–1. �H 12.28, 11.95, s,s, 2×OH; 7.79, dd, J 7.8, 1.2 Hz, H 8; 7.71, dd, J 8.3, 7.8 Hz, H 7; 7.45,d, J 8.8 Hz, H 2 or H 3; 7.34, dd, J 8.3, 1.2 Hz, H 6; 7.34, d, J 8.8 Hz,H 2 or H 3; 2.51, s, CH3CO. �C (partial) 204.0 (C=O), 192.8 (C=O),162.6 (C=O), 137.6, 134.4, 125.1, 124.9, 120.4, 30.6 (CH3CO). m/z 282(M, 19%), 267 (100).

(1 Z,3 E)- and (1Z,3Z)-1-(t-Butyldimethylsilyloxy)-1-methoxy-4-methylhexa-1,3-dienes (50) and (51)

(A) The 4-ethyl-4-methyl dienes (50) and (51) were prepared frommethyl (E)-4-methylhex-2-enoate (52)7,23 (2.2 g) as described in theGeneral Procedure for Diene Synthesis. Bulb-to-bulb distillation(100o/0.2 mm) of the crude viscous yellow oil gave a mixture of the two

Geometrically Specific Hydrogen Transfer

D. W. Cameron and R. M. Heisey120

title dienes (50) and (51) (3 : 1, 140 mg, 4%) as a colourless oil. Thecompounds were inseparable by fractional distillation, b.p. 85–86o/0.6mm (Found: M+••, 256.1856. C14H28O2Si requires M+••, 256.1858). �max1665, 1625, 1440, 1225 cm–1. �H (50) 5.94, br d, J 10.5 Hz, H 3; 4.45,d, J 10.5 Hz, H 2; 3.58, s, OCH3; 2.05, br q, J 7.3 Hz, CH2CH3; 1.67, d,J 1.2 Hz, 4-CH3; 1.01, t, J 7.3 Hz, CH2CH3; 0.96, s, OSiC(CH3)3; 0.17,s, OSi(CH3)2. N.O.e. OCH3→H 2, 4%; 4-CH3→H2, 4%. �H (51) 5.89,br d, J 10.5 Hz, H 3; 4.45, d, J 10.5 Hz, H 2; 3.57, s, OCH3; 2.08, br q,J 7.5 Hz, CH2CH3; 1.74, d, J 1.2 Hz, 4-CH3; 0.99, t, J 7.5 Hz, CH2CH3;0.95, s, OSiC(CH3)3; 0.16, s, OSi(CH3)2. N.O.e. 4-CH3→H 3, 2%. �C(50) 157.4 (C 1), 130.6 (C 3), 117.4 (C 4), 76.2 (C 2), 54.6 (OCH3), 32.5(C 5), 25.6 (OSiC(CH3)3), 18.0 (OSiC(CH3)3), 16.4 (4-CH3), 12.7 (C 6),–4.3 (OSi(CH3)2). �C (51) 157.3 (C 1), 130.9 (C 3), 118.5 (C 4), 75.8(C 2), 54.6 (OCH3), 25.7 (OSiC(CH3)3), 25.5 (C 5), 23.2 (4-CH3), 18.0(OSiC(CH3)3), 12.4 (C 6), –4.3 (OSi(CH3)2). m/z 256 (M, 2%), 75(100), 56 (22), 54 (23).

(B) The methyl (E)- and methyl (Z)-4-methylhex-3-enoates (54) and(55)24,25 were prepared as a mixture (2 : 1) from 2-methylbutyraldehydein an adaptation of the method of Mikolajczak and Smith11 for an anal-ogous ethyl ester system. The mixed esters (54) and (55) distilled as acolourless oil, b.p. 71–72o/15 mm (lit.24 75–78o/22 mm). �H (54) 5.30,br t, J 7.1 Hz, H 3; 3.68, s, OCH3; 3.05, br d, J 7.1 Hz, CH2CO2CH3;2.03, br q, J 7.6 Hz, CH2CH3; 1.63, br s, 4-CH3; 1.00, t, J 7.6 Hz,CH2CH3. �H (55) 5.27, br t, J 7.3 Hz, H 3; 3.67, s, OCH3; 3.04, br d, J7.3 Hz, CH2CO2CH3; 2.04, br q, J 7.5 Hz, CH2CH3; 1.73, d, J 1.2 Hz,4-CH3; 0.97, t, J 7.5 Hz, CH2CH3. The mixed esters (54) and (55) (2 : 1,2.2 g) were enolized and silylated as described in the General Procedurefor Diene Synthesis. Bulb-to-bulb distillation (120o/0.1 mm) of thecrude yellow oil (4.0 g) afforded a mixture of the two title dienes (50)and (51) (2 : 1, 2.4 g, 61%) as a colourless oil. They were indistinguish-able by 1H n.m.r. spectroscopy from the mixed dienes (50) and (51) pre-pared in part (A).

Reaction of the Mixed Dienes (50) and (51) with p-Chloranil (16)Reaction of the mixed dienes (50) and (51) (2 : 1) with (16) in

(D)chloroform as described in the General Procedure for SpectroscopicMonitoring showed, after 4 min, two products. (i) Methyl (2E,4E)-4-methylhexa-2,4-dienoate (56)25 (60%), �H 7.32, d, J 15.6 Hz, H 3; 5.98,br q, J 7.1 Hz, H 5; 5.78, d, J 15.6 Hz, H 2; 3.75, s, OCH3; 1.81, d, J 7.1Hz, =CHCH3; 1.76, br s, 4-CH3. (ii) Methyl (E)-4-methylenehex-2-enoate (57)30 (30%), �H 7.34, d, J 15.9 Hz, H 3; 5.92, d, J 15.9 Hz, H 2;5.35, m, =CH2; 3.76, s, OCH3; 2.23, br q, J 7.5 Hz, CH2CH3; 1.11, t, J7.5 Hz, CH2CH3.

Reaction of Dienes with Tetracyanoethylene (TCNE)(A) The 4-ethyl diene (36) (25 mg) was added to a suspension of

TCNE (13 mg) in chloroform (2 ml). To the resultant clear yellow solu-tion was added trifluoroacetic acid (2 drops) and the solvent was evap-orated. The residue was triturated with ethyl acetate/petrol and filtered,and the residue was recrystallized from ethyl acetate/petrol to yieldmethyl (E)-5,5,6,6-tetracyano-4-ethylhex-2-enoate (64) (17 mg, 66%)as fawn needles, m.p. 116–117o (Found: C, 61.2; H, 4.8; N, 22.1.C13H12N4O2 requires C, 60.9; H, 4.7; N, 21.9%). �max 2220, 1720, 1435,1245 cm–1. �H 6.59, dd, J 15.5, 10.7 Hz, H 3; 6.27, d, J 15.5 Hz, H 2;4.32, br s, H 6; 3.82, s, OCH3; 2.91, ddd, J 11.2, 10.7, 3.2 Hz, H 4; 2.14,dqd, J 13.4, 7.3, 3.2 Hz, CHAHBCH3; 1.85, ddq, J 13.4, 11.2, 7.3 Hz,CHAHBCH3; 1.05, t, J 7.3 Hz, CH2CH3. �C 164.3 (C 1), 137.3 (C 3),130.9 (C 2), 109.8 (CN), 108.9 (CN), 107.1 (CN), 106.6 (CN), 52.4(OCH3), 49.6 (C 6), 43.1 (C 5), 30.9 (C 4), 24.5 (CH2), 11.4 (CH2CH3).m/z 225 (M – 31, 22%), 170 (22), 169 (44), 159 (100), 58 (26).

(B) Reaction of (36) with TCNE in (D)chloroform as in the GeneralProcedure for Spectroscopic Monitoring showed after 5 min a mixtureof the two diastereoisomeric 3-(t-butyldimethylsilyloxy)-6-ethyl-3-methoxycyclohex-4-ene-1,1,2,2-tetracarbonitriles (59) and (60) (48and 36% respectively). �H (59) (partial) 5.92, m, H 4, H 5; 3.48, s,OCH3; 2.99, m, H 6; 1.95, m, CH2; 1.25, d, J 7.0 Hz, CH2CH3. �H (60)(partial) 5.92, m, H 4, H 5; 3.69, s, OCH3; 2.99, m, H 6; 1.95, m, CH2;1.25, d, J 7.0 Hz, CH2CH3. After 24 h, the spectrum showed these prod-ucts being replaced by methyl (E)-6-(t-butyldimethylsilyl)-5,5,6,6-

tetracyano-4-ethylhex-2-enoate (63) (80%). �H 6.68, dd, J 15.6, 9.7 Hz,H 3; 6.16, d, J 15.6 Hz, H 2; 3.78, s, OCH3; 2.72, ddd, J 9.7, 9.2, 3.1 Hz,H 4; 2.15–1.90, m, CH2; 1.00, t, J 7.0 Hz, CH2CH3; 1.00, s, SiC(CH3)3;0.41, s, Si(CH3)2. Upon addition of trifluoroacetic acid (10% in(D)chloroform, 1 drop) the spectrum showed the disappearance of (63)in favour of the desilylated ester (64) (80%), characterized in (A).

(C) Reaction of the 4,4-dimethyl diene (14) with TCNE as in (B)showed, after 5 min, methyl (E)-6-(t-butyldimethylsilyl)-5,5,6,6-tetra-cyano-4,4-dimethylhex-2-enoate (65) (90%). �H 6.98, d, J 15.9 Hz, H 3;6.06, d, J 15.9 Hz, H 2; 3.77, s, OCH3; 1.46, br s, 2×4-CH3; 1.00, s,SiC(CH3)3; 0.40, s, Si(CH3)2. After 20 min trifluoroacetic acid wasadded as in (B) and the spectrum showed the disappearance of (65) infavour of methyl (E)-5,5,6,6-tetracyano-4,4-dimethylhex-2-enoate (66)(90%). �H 6.93, d, J 15.9 Hz, H 3; 6.18, d, J 15.9 Hz, H 2; 4.46, br s, H 6;3.80, s, OCH3; 1.47, br s, 2×4-CH3.

(D) Reaction of the trimethylsilyloxy diene (45) with TCNE as in(B) showed, after 5 min, methyl (E)-5,5,6,6-tetracyano-4-ethyl-6-trimethylsilylhex-2-enoate (67) (90%). �H 6.68, dd, J 15.6, 9.7 Hz, H 3;6.16, d, J 15.6 Hz, H 2; 3.78, s, OCH3; 2.72, ddd, J 9.7, 9.2, 3.1 Hz, H 4;2.15–1.90, m, CH2; 1.00, t, J 7.0 Hz, CH2CH3; 0.48, s, Si(CH3)3. Uponaddition of trifluoroacetic acid as in (B), the spectrum showed the dis-appearance of (67) in favour of the desilylated ester (64) (80%), char-acterized as in (A).

(E) Reaction of the 4-methyl diene (29) with TCNE as in (B) showedafter 5 min a mixture of three products. Methyl (E)-6-(t-butyldimethylsilyl)-5,5,6,6-tetracyano-4-methylhex-2-enoate (68)(42%), �H 6.84, dd, J 15.4, 8.5 Hz, H 3; 6.16, d, J 15.4 Hz, H 2; 3.77, s,OCH3; 3.03, m, H 4; 1.48, d, J 6.8 Hz, 4-CH3; 1.00, s, SiC(CH3)3; 0.41,s, Si(CH3)2. 3-(t-Butyldimethylsilyloxy)-3-methoxy-6-methylcyclo-hex-4-ene-1,1,2,2-tetracarbonitrile (61) (28%), �H (partial) 5.87, dd, J10.7, 2.4 Hz, H 4(5); 5.80, dd, J 10.7, 1.9 Hz, H 5(4); 3.47, s, OCH3;3.33, qdd, J 7.1, 2.4, 1.9 Hz, H 6; 1.58, d, J 7.1 Hz, 6-CH3. Thediastereoisomeric cyclohexene (62) (21%), �H (partial) 5.87, dd, J 10.7,2.4 Hz, H 4(5); 5.80, dd, J 10.7, 1.9 Hz, H 5(4); 3.68, s, OCH3; 3.29,qdd, J 7.1, 2.4, 1.9 Hz, H 6; 1.59, d, J 7.1 Hz, 6-CH3. After 2 days tri-fluoroacetic acid was added as in (B). The spectrum then showed thedisappearance of (68), (61) and (62) in favour of methyl (E)-5,5,6,6-tetracyano-4-methylhex-2-enoate (69) (90%). �H 6.72, dd, J 1.54, 9.8Hz, H 3; 6.26, d, J 15.4 Hz, H 2; 4.51, br s, H 6; 3.80, s, OCH3; 3.26, dq,J 9.8, 6.8 Hz, H 4; 1.62, d, J 6.8 Hz, 4-CH3.

AcknowledgmentsWe acknowledge the financial support of the Australian

Research Council and an Australian Postgraduate ResearchAward (to R.M.H.). We are grateful to Dr P. G. Griffiths fordiscussion. We thank the Research School of Chemistry,Australian National University, for a Visiting Fellowship (toD.W.C.), during which part of this paper was written.

References1 Roush, W. R., and Barda, D. A., J. Am. Chem. Soc., 1997, 119, 7402.2 Kienzle, F., Mergelsberg, I., Stadlwieser, J., and Arnold, W., Helv.

Chim. Acta, 1985, 68, 1133.3 Cameron, D. W., Feutrill, G. I., Heisey, R. M., and Teo, A. A. L.,

unpublished data.4 Chretien-Bessiere, Y., and Leotte, H., C. R. Seances Acad. Sci.,

1962, 255, 723.5 Cazeau, P., Duboudin, F., Moulines, F., Babot, O., and Dunogues, J.,

Tetrahedron, 1987, 43, 2089.6 Piers, E., and Phillips-Johnson, W. M., Can. J. Chem., 1975, 53,

1281.7 Bohlmann, F., and Zdero, C., Chem. Ber., 1973, 106, 3779.8 Akita, T., Yakugaku Zasshi, 1962, 82, 91 (Chem. Abstr., 1962, 57,

9711c).9 Spangler, C. W., McCoy, R. K., and Karavakis, A. A., J. Chem. Soc.,

Perkin Trans. 1, 1986, 1203.

121

10 Ansell, M. F., and Bignold, A. J., J. Chem. Soc., Chem. Commun.,1969, 1096.

11 Mikolajczak, K. L., and Smith, C. R., J. Org. Chem., 1978, 43, 4762.12 Ireland, R. E., Mueller, R. H., and Willard, A. K., J. Am. Chem. Soc.,

1976, 98, 2868.13 Ireland, R. E., Wipf, P., and Armstrong, J. D., J. Org. Chem., 1991,

56, 650.14 Rappe, C., and Adestrom, R., Acta Chem. Scand., 1965, 19, 383.15 Cameron, D. W., Looney, M. G., and Pattermann, J. A., Tetrahedron

Lett., 1995, 36, 7555.16 Kende, A. S., and Toder, B. H., J. Org. Chem., 1982, 47, 163.17 Cardellach, J., Estopa, C., Font, J., Moreno-Manas, M., Ortuno, R.

M., Sanchez-Ferrando, F., Valle, S., and Vilamajo, L., Tetrahedron,1982, 38, 2394.

18 Cameron, D. W., Crosby, I. T., and Feutrill, G. I., Tetrahedron Lett.,1992, 33, 2855.

19 Fujimoto, Y., Kihara, T., Isono, K., Tsunoda, H., Tatsuno, T.,Matsumoto, K., and Hirokawa, H., Chem. Pharm. Bull., 1984, 32,1583; Lewis, F. D., Howard, D. K., Barancyk, S. V., and Oxman, J.D., J. Am. Chem. Soc., 1986, 108, 3016.

20 Banville, J., and Brassard, P., J. Chem. Soc., Perkin Trans. 1, 1976,1852.

21 Hoffman, R. V., and Kim, H.-O., J. Org. Chem., 1991, 56, 1014.22 Aumuller, A., and Hunig, S., Liebigs Ann. Chem., 1986, 165.23 Guerriero, A., D’Ambrosio, M., Cuomo, V., Vanzanella, F., and

Pietra, F., Helv. Chim. Acta, 1989, 72, 438.24 Lozanova, A. V., Moiseenkov, A. M., and Semenovskii, A. V., Izv.

Akad. Nauk SSSR, Ser. Khim., 1980, 1932; Struchkova, M. I.,Lozanova, A. V., Moiseenkov, A. M., and Semenovskii, A. V., Izv.Akad. Nauk SSSR, Ser. Khim., 1980, 2655.

25 Camps, F., Coll, J., and Guitart, J., Tetrahedron, 1987, 43, 2329.26 Jackman, L. M., and Wiley, R. H., J. Chem. Soc., 1960, 2881.27 Chuman, T., and Noguchi, M., Tetrahedron Lett., 1977, 18, 3045.28 Josey, A. D., Angew. Chem., Int. Ed. Engl., 1981, 20, 686.29 Drexler, J., Lindermayer, R., Hassan, M. A., and Sauer, J.,

Tetrahedron Lett., 1985, 26, 2555; Drexler, J., Lindermayer, R.,Hassan, M. A., and Sauer, J., Tetrahedron Lett., 1985, 26, 2559.

30 Arbuzova, I. A., and Nikolyan, V. N., Zh. Org. Khim., 1969, 5,2125.

31 Rogan, J. B., J. Org. Chem., 1962, 27, 3910.

Geometrically Specific Hydrogen Transfer