Carbanions. 24. Rearrangements of (E)- and (Z)-2,2-Diphenyl-3-pentenyl Alkali Metal Compounds

Article abstract of DOI:10.1021/jo00268a033

<(E)- and (Z)-2,2-diphenyl-3-pentenyl>lithium (15 and 35) were prepared by reaction of (E)- and (Z)-5-chloro-4,4-diphenyl-2-pentene, respectively, with lithium at -75 deg C in diethyl ether or tetrahydrofuran (THF).These organolithium compounds when warmed in diethyl ether solution to 35 deg C undergo <1.2>-sigmatropic rearrangement of the propenyl group with >95percent retention of geometrical configuration of the trans-propenyl group and about 90percent retention of configuration of the cis-propenyl group.The failure to incorporate an n-butyl group when 15 is warmed from -75 deg C to 35 deg C in the presence of an excess of n-butyllithium implies that these rearrangements are not likely to occur by elimination and then readdition of cis- or trans-propenyllithium.Instead an intramolecular syn-addition of the lithiomethyl group to the double bond of 15 or 35 followed by an anti-elimination in the intermediate cyclopropylcarbinyllithium (33 or 36) is proposed to account for the retention of configuration during propenyl migration (see Scheme V).A similar mechanism (Scheme VI) is suggested to account for retention of configuration during <1,2>-propenyl migration in the previously known Wittig rearrangements of benzyl propenyl ethers and methallyl propenyl ethers.In the solvent THF, rearrangement of 15 and 35 occurs on warming to 0 or 10 deg C; however, here the E isomer 15 undergoes about a 50/50 ratio of <1,2>-phenyl to -propenyl migration.The Z isomer similarly undergoes both phenyl and propenyl migration in THF but also undergoes evidently an intramolecular <1,5>-proton migration to the allylic anion 42 (see Scheme VII).If 15 is treated with potassium tert-butoxide in THF at -75 deg C, exclusive <1,2>-phenyl migration is observed.Possible interpretations of these cation and solvent effects are given.