Revealing the P–B coupling event in the rhodium catalysed dehydrocoupling of phosphine–boranes H3B·PR2H (R = tBu, Ph)

Abstract

We demonstrate that [Rh(h 6 -FC 6 H 5 )(Ph 2 PCH 2 CH 2 CH 2 PPh 2 )][BAr F 4 ] is a competent catalyst for the dehydrocoupling of H 3 B$PR 2 H to give H 3 B$PR 2 BH 2 $PR 2 H (R ¼ Ph, t Bu). The isolation of intermediates (R ¼ t Bu, Ph) and kinetic/isotopic labelling experiments (for R ¼ Ph) point to a mechanism, for R ¼ Ph, in which the rate-limiting process for dehydrocoupling involves B-H activation, while the turnover limiting process for catalysis involves the substitution of the oligomeric product at the metal, which forms a strong chelate complex with a Rh(III) centre, by H 3 B$PPh 2 H. When R ¼ t Bu no chelate product complex forms, and it is speculated that the dehydrocoupling process now becomes turnover limiting. These observations provide, for the first time we believe, data on the P-B bond forming events that are central to the dehydrocoupling and dehydropolymerisation of phosphine-boranes.

The Synthesis, Characterization and Dehydrogenation of Sigma‐Complexes of BN‐Cyclohexanes

“…[57, 60] Related phosphido–borane complexes have been isolated and shown to undergo rapid and reversible P–H/B–H bond activation as probed by H/D scrambling experiments. [61] As significant D–incorporation into the PCy 3 ligand is also observed we cannot discount more complicated mechanisms for H/D exchange that involve cyclometallated phosphine ligands.…”

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

“…31 The primary phosphine–borane 5 has also been used. 37 Compounds 2 (33) and 3 (38) are known adducts and offer electron-withdrawing and donating aryl groups, respectively.…”

“…31 The 31 P{ 1 H} NMR spectrum shows four different phosphorus environments. Two of these signals are well-resolved and show coupling to 103 Rh, δ 46.6 [ J (RhP) 111 Hz] and δ 12.8 [ J (RhP) 91 Hz], and are attributed to the chelating phosphine ligand.…”

The Catalytic Dehydrocoupling of Amine–Boranes and Phosphine–Boranes

“…In a further development of this system, Huertos and Weller were able to form a more stable catalytic fragment by replacement of the monodentate phosphine ligands with a chelating phosphine ligand 1,3-bis(diphenylphosphino)propane (Ph 2 PCH 2 CH 2 CH 2 PPh 2 , dppp) [174]. Under the harsh melt conditions required for catalysis to occur, the chelating ligand was not displaced by any free phosphine formed from P-B cleavage of the substrate, allowing further investigation into the [Rh(dppp)] + fragment as the active catalytic species.…”

“…The difference in oxidation state of the rhodium centre in these cases is likely due to the acidity of the P-H bond. When electron-withdrawing substituents are present on phosphorus, the P-H bond more readily undergoes P-H oxidative addition, making Rh III species favoured [174]. In a follow-up report, secondary phosphine-boranes bearing fluorinated substituents H 3 B · P( p-F 3 C-C 6 H 4 ) 2 H and H 3 B · P(m-(F 3 C) 2 C 6 H 3 ) 2 H were found to dehydrocouple at a faster rate than H 3 B · PPh 2 H using the same [Rh(dppp)] + system [175].…”