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)­(η6-FC6H5)]­[BArF4] [L = Ph2P­(CH2)3PPh2, ArF = 3,5-(CF3)2C6H3] with 2 equiv of H3B·PR2H affords
[Rh­(L)­(H)­(σ,η-PR2BH3)­(η1-H3B·PR2H)]­[BArF4]. These complexes undergo dehydrocoupling to give the diboraphosphine
complexes [Rh­(L)­(H)­(σ,η2-PR2·BH2PR2·BH3)]­[BArF4]. With electron-withdrawing groups on the phosphine–borane there is the parallel formation
of the products of B–P cleavage, [Rh­(L)­(PR2H)2]­[BArF4], 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}]­[BArF4], that undergoes B–P
bond cleavage to give [Rh­(L)­{η1-H3B·P­(Ad)2H}­{P­(Ad)2H}]­[BArF4]. 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.