Stereochemical and mechanistic aspects of the aprotic conjugate addition reactions of the carbanions of octenyl sulfides and octenyl thiocarbamates with 4-T-butoxycyclopent-2-enone in the presence of hexamethylphosphoric triamide

The carbanions of (E)- and (Z)-l-(phenylthio)oct-2-ene, (E)-l-(methylthio)- and l-(t-butyl- thio)oct-2-ene, N, N-dimethyl 5’-[(E)-oct-2-enyl]thiocarbamate and (E)-oct-2-enyl benzothiazole undergo conjugate addition to 4-t-butoxycyclopent-2-enone in the presence of hexamethylphosphoric triamide (hmpa) in tetrahydrofuran at -70° to give as predominant products diastereomeric mixtures of syn and anti allylic sulfides arising from reaction through C 1 of the octenyl carbanions. Also formed in some cases are small amounts of diastereomeric mixtures of syn and anti vinylic sulfides arising from reaction through C 3 of the octenyl carbanions. In the absence of the hmpa, carbonyl addition is the major reaction pathway of the carbanions of each of the (E)- and (Z)-octenyl thiocarbamates. The constitutions and stereostructures of all the products have been established through chemical shift and coupling constant correlations of their ¹H n.m.r. data obtained at 400 MHz. The relative proportions of the regioisomeric allylic and vinylic sulfides are dependent both upon the nature of the non-allylic substituent attached to the sulfur atom, and the geometry of the double bond in the starting octenyl sulfide. While there is no apparent connection between the electronic properties or sizes of the non-allylic substituent and the diastereomer ratios of the allylic and vinylic sulfides, it is noted that the (Z)-(phenylthio)octene delivers an allylic sulfide mixture enriched in the syn isomer. It is concluded that the carbanions are configurationally stable at -70° and that the configurations of the vinylic sulfides reflect the configurations of the carbanions from which they are derived. It is also concluded that the reactions proceed under orbital control, as this best accounts for the formation of the vinylic sulfides and is in accord with the results of sto-3g calculations carried out on a series of sulfur-bearing allylic carbanions. The likely angle of attack of the carbanions on the enone system is briefly discussed in terms of a published model relating to the attack of a simple nucleophile on an electrophilic double bond.