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(diehenylphosphino)methanes 5a-c. Compounds 4 and 5 were fully characterized by multinuclear NMR spectroscopy (H-1, P-31, C-13, B-11), mass spectrometry, and elemental analysis, and the X-ray crystal structures of 4a,c and Sa,b have been established. Ligands 5a,b were used to prepare the cationic rhodium complexes [Rh(eta(4)-COD)(5a)](+) (8a'), [Rh(eta(4)-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(eta(4)-COD)(4)](+) chelate complexes 10a-f, resulting from coordination of the phospholyl part and the BH3 group via a 172 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.