USE OF PROCHIRAL PHOSPHAALKENE COMPLEXES IN THE SYNTHESIS OF OPTICALLY-ACTIVE PHOSPHINES

Prochiral L-Menthyl phosphaalkene complexes were prepared from ''phospha-Wittig'' reagents, [MenP(H)-P-(O) (OR)2]M(CO)5 (M = Mo, W), and several aldehydes. Their catalytic hydrogenation using RhL2+ catalysts and their [2 + 4] cycloaddition with cyclopentadiene proceeded with full diastereoselectivity. A molecular model of such a phosphaalkene complex showed that a preferred conformation exists that minimizes the combined interactions of the isopropyl substituent of the L-menthyl group with the complexing group and the phosphavinylic C-H bond. In this conformation, only the si face of the phosphaalkene is free for the incoming reagents. Both hydrogenation and [2 + 4] cycloaddition with cyclopentadiene selectively take place on this face as demonstrated by the X-ray crystal structure analysis of two of the resulting complexes. A two-step procedure was devised for the conversion of [MenPH2]Mo(CO)5 into optically pure phosphines. In the first step, the primary phosphine complex was phosphorylated, the resulting phospha-Wittig reagent was allowed to react with an aldehyde, and the phosphaalkene complex thus formed was trapped by cyclopentadiene. The decomplexation of the resulting molybdenum complex was carried out by heating with diphos. An optically pure 2-L-menthyl-2-phospha-5-norbornene was thus prepared.