Modular Phosphole-Methano-Bridged-Phosphine(Borane) Ligands. Application to Rhodium-Catalyzed Reactions

The synthesis of the phospholyl­(phosphinoborane)­methane air- and moisture-stable hybrid ligands 4a–f, starting from 1-phenylphospholes 1a–d, was performed via P–C bond coupling on the methano bridge. Two strategies were investigated, according to the phospholyl moiety used as a nucleophilic or an electrophilic reagent. In the first pathway, the phospholyl anions react with the easily available (chloromethyl)­diphenylphosphine–borane 3 to afford ligands 4a–d in 29–67% isolated yields. In the second pathway, the phospholyl­(dicyclohexylphosphinoborane)­methane ligands 4e,f were synthesized in 18–23% yields, through a nucleophilic substitution on the cyanophosphole. Removal of the BH3 moiety on 4a–c assisted by DABCO leads to the hybrid phospholyl­(diphenylphosphino)­methanes 5a–c. Compounds 4 and 5 were fully characterized by multinuclear NMR spectroscopy (1H, 31P, 13C, 11B), mass spectrometry, and elemental analysis, and the X-ray crystal structures of 4a,c and 5a,b have been established. Ligands 5a,b were used to prepare the cationic rhodium complexes [Rh­(η4-COD)­(5a)]+ (8a′), [Rh­(η4-COD)­(5b)]+ (8b), [Rh­(5a)2]+ (9a′), and [Rh­(5b)2]+ (9b), containing four-membered chelate rings with BF4– or CF3SO3– as counterions. Ligands 4a–f were also used to synthesize the [Rh­(η4-COD)­(4)]+ chelate complexes 10a–f, resulting from coordination of the phospholyl part and the BH3 group via a η2 mode with two bridging B–H–Rh 3c–2e bonds, as shown by the X-ray crystal structures of the complexes 10b,c. Rhodium complexes 8 and 10 isolated or formed in situ with ligands 4 and 5 were studied for catalytic hydrogenation of methyl 2-(acetamidomethyl)­acrylate (11) and hydroboration of styrene (13) with catecholborane. In both reactions, the catalytic systems prepared either from the phospholyl­(phosphinoborane)­methane ligands 4 or the corresponding free ligands 5, gave good to excellent conversions. In addition, the regioselectivity of the catalyzed hydroboration is slightly influenced using these ligands. Finally, the use of hybrid phospholyl­(phosphinoborane)­methanes as ligands offers a new, efficient way to improve catalytic processes, for designing both the structure and the electronic properties of the catalyst, or still to implement it without removing the borane protecting group.