Synthesis and reactions of new phosphine - boranes
Synthesis and reactions of new phosphine-boranes have been studied. Diastereomerically pure menthyloxyphenylphosphine-boranes were prepared from dichlorophenylphosphine. These secondary phosphine-boranes underwent stereospecific alkylation and arylation reactions. It is noted that the stereochemistry of palladium-catalyzed cross-coupling reactions was largely affected by the solvent and the base used. Thus, the reaction in acetonitrile proceeded with net retention of configuration. In sharp contrast, the reaction in THF in the presence of K2CO3 occurred with inversion of configuration with high degree of stereospecificity. The phosphorus-oxygen bond of the resulting phosphine-boranes was reductively cleaved at -78 "C by lithium naphthalenide or lithium in liquid ammonia in almost quantitative yields with an excellent degree of stereospecificity. On the other hand, the reactions at elevated temperatures provided the products with almost complete racemization. This racemization was proved to occur via pyramidal inversion of the anionic tricoordinate phosphorus species. Based on these results, we developed new methods for the synthesis of bidentate phosphine ligands with homochiral phosphine centers. Optically pure bisphosphinoethanes and bisphosphinobutanes possessing phenyl, t-butyl, cyclohexyl, emethomhenyl, methyl, or 2methoxyethyl group were prepared via phosphine-boranes. Functionalizations of the boranato group of phosphine-boranes have been investigated. Trimethylphosphine-borane reacted with methanesulfonic acid with evolution of hydrogen. The resulting trimethylphosphine-methylsulfonyloxyborane was subjected to nucleophilic substitution reaction on the boron atom with arenthiols or secondary phosphine-boranes. A new phosphine-borane having a P-B-P-B-P-B-P-B bond linkage was synthesized. Novel boron anion species were generated by the reaction of phosphine-monoiodoboranes with lithium 4,4'-dit -butylbiphenylide.