Effect of the Phosphine Steric and Electronic Profile on the Rh-Promoted Dehydrocoupling of Phosphine-Boranes

Abstract

[Image: see text] The electronic and steric effects in the stoichiometric dehydrocoupling of secondary and primary phosphine–boranes H(3)B·PR(2)H [R = 3,5-(CF(3))(2)C(6)H(3); p-(CF(3))C(6)H(4); p-(OMe)C(6)H(4); adamantyl, Ad] and H(3)B·PCyH(2) to form the metal-bound linear diboraphosphines H(3)B·PR(2)BH(2)·PR(2)H and H(3)B·PRHBH(2)·PRH(2), respectively, are reported. Reaction of [Rh(L)(η(6)-FC(6)H(5))][BAr(F)(4)] [L = Ph(2)P(CH(2))(3)PPh(2), Ar(F) = 3,5-(CF(3))(2)C(6)H(3)] with 2 equiv of H(3)B·PR(2)H affords [Rh(L)(H)(σ,η-PR(2)BH(3))(η(1)-H(3)B·PR(2)H)][BAr(F)(4)]. These complexes undergo dehydrocoupling to give the diboraphosphine complexes [Rh(L)(H)(σ,η(2)-PR(2)·BH(2)PR(2)·BH(3))][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)(PR(2)H)(2)][BAr(F)(4)], while with electron-donating groups no parallel product is formed. For the bulky, electron rich, H(3)B·P(Ad)(2)H no dehydrocoupling is observed, but an intermediate Rh(I) σ phosphine–borane complex is formed, [Rh(L){η(2)-H(3)B·P(Ad)(2)H}][BAr(F)(4)], that undergoes B–P bond cleavage to give [Rh(L){η(1)-H(3)B·P(Ad)(2)H}{P(Ad)(2)H}][BAr(F)(4)]. The relative rates of dehydrocoupling of H(3)B·PR(2)H (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. H(3)B·PCyH(2) 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