Preparation and use of sterically hindered organobis(2,4,6-triisopropylphenyl)hydroborates and their polystyrene derivatives for the diastereoselective reduction of ketones

Preparations of benzyl and phenylbis(2,4,6-triisopropylphenyl)hydroborates [organoditripylhydroborates] are outlined. The lithium and potassium benzyl ditripylhydroborates reduce substituted cyclohexanones with diastereoselect ivities comparable to those obtained with the most selective reagents known (e.g. 99% cis-4-methylcyclohexanol from 4-methylcyclohexanone). Lithium ph enylditripylhydroborate also reduces ketones with significant selectivity. For example, 4-methylcyclohexanone is reduced to cis-4-methylcyclohexanol i n 88% isomeric purity. Unlike with most other highly selective reagents the reactions take place at room temperature and have the additional advantage that the boron reagent can be recovered quantitatively. Coupling of Merrif ield's resin with ditripylfluoroborane in the presence of lithium naphthale nide affords (ditripylborylmethyl)polystyrene. Similarly, coupling of bromo polystyrene with ditripylfluoroborane in the presence of n-BuLi affords (di tripylboryl)polystyrene. Reactions of these polymeric organoboranes with t- BuLi give the corresponding polymer-supported lithium hydroborates. Lithium ditripylhydroboratylmethylpolystyrene reduces cyclic ketones in identical fashion to its non-polymeric counterpart, giving the corresponding thermody namically less stable alcohols in 99% or better isomeric purity. Similarly, lithium ditripylhydroboratylpolystyrene behaves like its non-polymeric cou nterpart and reduces 4-methylcyclohexanone to cis-4-methylcyclohexanol in 8 9% isomeric purity. Recovery and reuse of the organoboranes are even easier for the polymeric reagents.