Cyclohexane-1,4-dienes rearrangement

In acyclic systems, Claisen rearrangements show a well-established prefoence for chair-like transition states. With crotyl propenyl ether, the chair selectivity amounts to 97-98% at 142 C, which corresponds to an approx. 3 kcal mol difference between the fiee energy of activation (AAG ) of chair and boat TS (equation 26). The preference for a chair-like geometry in the TS is even more pronounced in the Cope reaiT ement 99.7% of the 3,4-dimethylhexa-1,5-diene rearranges at 225 C via a chair-like TS, corresponding to a AAG chair-boat of -5.7 kcal mol" . - The latter result closely parallels the difference in energy of the chair and boat conformations of cyclohexane (5-6 kcal mol" ). ... 

Benzylenol ethers rearrange in an apparently similar fashion via photolytic fission of the benzyl-oxygen bond and subsequent recombination steps. Irradiation in quartz of a cyclohexane solution of 3-benzyloxycholesta-3,5-diene (250) leads to 23% (251), 13% (252) [presumably formed from (251) during workup] and 10% (253). ... 

The fluorescence of DCA is also quenched efficiently by 2,5-diphenyl-l,5-hexadiene with a nearly diffusion-limited rate constant in MeCN (1.1 x 10lodm3 mol-1 s ), since the photoinduced electron transfer from the diene ( ° = 1.70 V vs. SCE) to DCA (E ed = 1.91 V vs. SCE) [170] is exergonic [184], The photoinduced electron transfer induces Cope rearrangement of the diene via the cyclohexane-1,4-radical cation intermediate. In... 

Homologation of 1,3-dienes.1 The reagent is particularly useful for cyclohomologation of 1,3-dienes. Thus the adduct a of a Diels-Alder reaction of 1 with 1,2-bis(methylene)cyclohexane undergoes Ramberg-Backlund rearrangement to the tri-ene 2. Repetition of the sequence provides the tetraene 3. The sequence is applicable to cyclopentadiene, furan, and 2,3-dimethyl-l,3-butadiene. 

One significant aspect of the cyclohexane-1,4-diyl radical cation has been a point of contention the question whether it undergoes cleavage to the hexa-1,5-diene radical cation, i.e. whether it completes the Cope rearrangement. Results obtained in the laboratory of Miyashi, particularly the exchange of a deuterium label between the terminal olefinic and allylic positions, seem to suggest such a... 

As an example, the rate of rearrangement of 4,4-dicyano-5-ethylhepta-l,5-diene changes only by a factor of 3.8, even when the solvent is changed drastically by going from cyclohexane to dimethyl sulfoxide (Eq. (5-49)) [151]. 

The 1,4-diyl species, 73 +, is obtained also upon electron transfer from 1,5-cyclohexadiene, 77, both in cryogenic matrices [143] and in solution [201]. Solution experiments provided chemical evidence for the cyclohexane-1,4-diyl structure type in an elegant study of the electron-transfer-initiated photochemistry of 2,5-diphenylhexa-1,5-diene, 78, and derivatives in the presence of molecular oxygen [201-203]. The intermediate 1,4-cyclohexanediyls, 79 , were intercepted by O2 the stereochemistry of the en /o-peroxide products, 80, showed that the initial cycloaddition occurred in the same stereospecific manner established for the thermal rearrangement of the neutral parent [204]. 

A combination of the alternative pathways illustrated in Schemes 13 and 15 explains why the derivative of dimethyl fra 5-cyclohexane-l,2-dicarboxylate (22) fails to give any of the silylated coupled enediol even at 25 °C, using sodium-potassium alloy in benzene, thermal rearrangement to an octa-1,3-diene occurs, whereas use of sodium in liquid ammonia, at -78 °C, cleaves the bond joining the two functionalized carbon atoms, leading to dimethyl 2,7-dimethyloctane-l,8-dioate. ... 

Two common six-membered rearrangements of 1,5-dienes have Y as either oxygen (Claisen rearrangement) or carbon (Cope rearrangement). Both are reversible and favor the more stable product. The preceding example problem demonstrated that they were thermally allowed An+2 suprafacial retention processes. As in the Cope example problem above, the transition state for the reaction commonly resembles the chair conformation of cyclohexane. Figure 12.29 shows a biochemical example of the Claisen rearrangement. 

A study of the photochemical Cope reaction of the hexadienes 40 has been carried out under photoinduced electron-transfer conditions. Evidence was gathered for the formation of a chair cyclohexane-1,4-radical cation 41 °. In snch systems, where the radical cation is formed using DCA as the sensitizer, a degenerate Cope process is operative. Thus when the tetradeuterio derivative 42 is used, rearrangement affords a (52 48) mixture of the two dienes 42 and 43. Related to this general problem, DCA-sensitized reactions of the isomeric dienes 44 and , -45 and the cyclization prodnct, the bicyclohexane 46, have been studied in considerable detail. At low conversions, the irradiation of 46 affords a mixture of the dienes 44 and , -45 in ratios that are independent of temperature. The influence of the position of the aiyl groups on the diene skeleton has also been studied. This does not appear to affect the conversion to a cyclic radical cation. Thus the SET-induced reaction of the diene 47 has shown that the open chain radical cation of the diene 48 cyclizes preferentially to the radical cation 49. ... 

Thus, the case for a non-concerted 3,3-shift via a cyclohexane-1,4-diyl is weak. Nonetheless, substituent effects on the rate of the 3,3-shift were intially interpreted in terms of the diyl species. In particular, Dewar found the 2-phenyl and 2,5,-diphenyl-l,5-hexadiene rearrange 40 and 1600 times, respectively, more rapidly than that of the parent diene. Further, semi-empirical MINDO/3 calculations supported the proposition that even the parent species proceeded via the chair-like cyclohexane-1,4-diyl. These observations and calculations provided stimulus for a substantial effort in the subsequent years to address the question of transition state structure in and the energy surface for the 3,3-sigmatropic shift of 1,5-hexadiene. 

In the clay-catalysed reaction even oleate will furnish a cyclic dimer along with monomer which is a mixture of saturated and unsaturated (mainly tram), straight-chain and branched-chain Cis compounds formed by hydrogen transfer and rearrangement. Dimers are formed by diene synthesis (Diels Alder reaction) between a monoene and a conjugated diene produced from monoene by hydrogen transfer. The cyclohexene derivatives are converted by hydrogen transfer to cyclohexane and benzene derivatives. These monocyclic dimers are accompanied by acyclic and bicyclic dimers such as those formulated below. Linoleate reacts in a similar manner. 

Photochemically generated diatomic sulphur added to l,2-bis(methylene)-cyclohexane to afford a 3,6-dihydro-l,2-dithiin. The dihydrodithiin (20) was formed by thermal rearrangement of a 1,3-dithiin. Addition of disulphur monoxide to 2,3-disubstituted buta-1,3-dienes yielded the rather unstable dihydrodithiin 1-oxides (21). 

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