Effect of the phosphine steric and electronic profile on the Rh-promoted dehydrocoupling of phosphine-boranes.

The electronic and steric effects in the stoichiometric dehydrocoupling of secondary and primary phosphine-boranes H3B·PR2H [R = 3,5-(CF3)2C6H3; p-(CF3)C6H4; p-(OMe)C6H4; adamantyl, Ad] and H3B·PCyH2 to form the metal-bound linear diboraphosphines H3B·PR2BH2·PR2H and H3B·PRHBH2·PRH2, respectively, are reported. Reaction of [Rh(L)(η(6)-FC6H5)][BAr(F)4] [L = Ph2P(CH2)3PPh2, Ar(F) = 3,5-(CF3)2C6H3] with 2 equiv of H3B·PR2H affords [Rh(L)(H)(σ,η-PR2BH3)(η(1)-H3B·PR2H)][BAr(F)4]. These complexes undergo dehydrocoupling to give the diboraphosphine complexes [Rh(L)(H)(σ,η(2)-PR2·BH2PR2·BH3)][BAr(F)4]. With electron-withdrawing groups on the phosphine-borane there is the parallel formation of the products of B-P cleavage, [Rh(L)(PR2H)2][BAr(F)4], while with electron-donating groups no parallel product is formed. For the bulky, electron rich, H3B·P(Ad)2H no dehydrocoupling is observed, but an intermediate Rh(I) σ phosphine-borane complex is formed, [Rh(L){η(2)-H3B·P(Ad)2H}][BAr(F)4], that undergoes B-P bond cleavage to give [Rh(L){η(1)-H3B·P(Ad)2H}{P(Ad)2H}][BAr(F)4]. The relative rates of dehydrocoupling of H3B·PR2H (R = aryl) show that increasingly electron-withdrawing substituents result in faster dehydrocoupling, but also suffer from the formation of the parallel product resulting from P-B bond cleavage. H3B·PCyH2 undergoes a similar dehydrocoupling process, and gives a mixture of stereoisomers of the resulting metal-bound diboraphosphine that arise from activation of the prochiral P-H bonds, with one stereoisomer favored. This diastereomeric mixture may also be biased by use of a chiral phosphine ligand. The selectivity and efficiencies of resulting catalytic dehydrocoupling processes are also briefly discussed.