THE PREPARATION AND SYNTHETIC APPLICATIONS OF THEXYLCHLOROBORANE

Under selected conditions, the reaction of 2,3-dimethyl-2-butene with one equivalent of BH(,2)Cl(.)SMe(,2) proceeds cleanly and quantitatively, either in solution or under neat conditions, to give exclusively the monohydroboration product, thexylchloroborane-methyl sulfide (ThxBHCl(.)SMe(,2)). Solutions of this reagent displayed an unusual thermal stability at ambient temperatures or below. A detailed study of the hydroboration properties of this reagent revealed that ThxBHCl(.)SMe(,2) is a much more selective hydroborating agent than thexylborane. The reaction between reactive alkenes (terminal or simple disubstituted olefins) and ThxBHCl(.)SMe(,2) proceeds cleanly and quantitatively to produce isomerically pure thexylalkylchloroborane intermediates. With less reactive olefins, redistribution of the product occurs at a significant level, due to pre-equilibration of the reagent. The relative reactivities of different types of alkenes toward hydroboration by ThxBHCl(.)SMe(,2) were determined under competitive conditions. Subsequently, the reaction between ThxBHCl(.)SMe(,2) and a number of nonconjugated dienes demonstrated conclusively that this reagent could be used to selectively hydroborate terminal alkenes in the presence of other olefin structures. A quantitative stepwise hydroboration process was then developed utilizing ThxBHCl(.)SMe(,2). Reduction of thexylalkylchloroboranes in the presence of a second olefin proceeded cleanly and quantitatively with pure potassium triisopropoxyborohydride to form the corresponding unsymmetrical thexyldialkylboranes. Subsequent carbonylation or phase-transfer catalyzed cyanidation of these intermediates led to an essentially one-pot, high yield synthesis of unsymmetrical aliphatic and alicyclic ketones. The method successfully incorporates a wide variety of alkene structures and is compatible with a number of functional groups. Methanolysis of thexylalkylchloroboranes proceeds cleanly and quantitatively to produce methyl thexylalkylborinates. Iodination of the corresponding lithium alkynyl ate complexes of these intermediates produces the desired unsymmetrical alkynes in good yield. The combined use of the thexyl and methoxy moieties as blocking groups in this transfer process results in the efficient utilization of the starting olefin in the reaction