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; <i>p</i>-(CF₃)­C₆H₄; <i>p</i>-(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)­(η⁶-FC₆H₅)]­[BAr^F₄] [L = Ph₂P­(CH₂)₃PPh₂, Ar^F = 3,5-(CF₃)₂C₆H₃] with 2 equiv of H₃B·PR₂H affords [Rh­(L)­(H)­(σ,η-PR₂BH₃)­(η¹-H₃B·PR₂H)]­[BAr^F₄]. These complexes undergo dehydrocoupling to give the diboraphosphine complexes [Rh­(L)­(H)­(σ,η²-PR₂·BH₂PR₂·BH₃)]­[BAr^F₄]. 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^F₄], 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)­{η²-H₃B·P­(Ad)₂H}]­[BAr^F₄], that undergoes B–P bond cleavage to give [Rh­(L)­{η¹-H₃B·P­(Ad)₂H}­{P­(Ad)₂H}]­[BAr^F₄]. 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