The phosphorus-oxygen bond of diastereomerically pure menthyloxyphosphine-boranes was reductively cleaved at -78 "C by lithium naphthalenide or Li-NH3 in excellent yields with almost complete retention of configuration at phosphorus. Chiml tetracoordinate organophosphorus compounds containing a leaving group are well known to undergo nucleophilic substitution reactions, and numerous examples of these reactions have been described in literature. 1 However, comparatively little work has been done on the reductive removal of the leaving group,*?3 and, to our knowledge, the stereospecific cleavage of the bond between phosphorus and the alkoxy group has not yet been reported. We report herein the first successful example of this type of stereospecific reduction.
In association with our ongoing interest in the synthesis and reactions of optically active phosphineboranes,4 we have envisioned that diastereomerically pure menthyloxyphosphine-boranes might be converted into optically active secondary phosphine-boranes by reductive removal of the menthyloxy group. Our initial trial was undertaken with treatment of diastereomerically pure (Rp)-menthyloxy(methyl)phenylphosphine-be
(1)4aand (Sp)-menthyloxy( omethoxyphenyl)phenyIphosphine-borane (2)4a with various reducing agents, and it was found that the desired reductions could be accomplished by the use of one-electron reductants such as lithium naphthalenide, lithium 4,4'-di-tefibutylbiphenylide (LDBB), and Li-NH3. 5,6 Thus, the reductions with these reducing agents proceeded rapidly at -78 "C and were complete within 5 min to generate anionic tricoordinate phosphorus species, which were subsequently allowed to react with methanol or alkyl halides at the same temperature to afford optically active secondary or tertiary phosphine-bomnes (Scheme 1).7 The results obtained under various reaction conditions are summarized in Table 1.8
Scheme 1 yH3 .