Reactions of Tris(oxazolinyl)phenylborato Rhodium(I) with C−X (X = Cl, Br, OTf) Bonds: Stereoselective Intermolecular Oxidative Addition

The achiral and enantiopure chiral compounds ToMRh(CO)2 (3) and ToPRh(CO)2 (4) (ToM = tris(4,4-dimethyl-2-oxazolinyl)phenylborate; ToP = tris(4S-isopropyl-2-oxazolinyl)phenylborate) were prepared to investigate stereoselective oxidative addition reactions and develop new catalytic enantioselective bond functionalization and cross-coupling chemistry. Reactivity at the rhodium center is first shown by the substitution of the carbonyl ligands in 3 and 4 in the presence of the appropriate ligand; thus treatment of ToMRh(CO)2 with P(OMe)3 provides ToMRh(CO)[P(OMe)3] (5). However, reaction of ToMRh(CO)2 and MeOTf (Tf = SO2CF3) affords the complex [{N-Me-κ2-ToM}Rh(CO)2]OTf (6), resulting from N-oxazoline methylation rather than oxidative addition to rhodium(I). In contrast, ToMRh(CO)2 reacts with allyl bromide and chloroform, forming the rhodium(III) species (κ3-ToM)Rh(η1-C3H5)Br(CO) (7) and (κ3-ToM)Rh(CHCl2)Cl(CO) (8), respectively. Interestingly, the chiral ToPRh(CO)2 and CHCl3 react to give one diastereomer of (κ3-ToP)Rh(CHCl2)Cl(CO) (9; 100:3 dr) almost exclusively. To evaluate the reactivity of these rhodium(I) compounds, the carbonyl stretching frequencies have been examined. The data for the mono- and trivalent rhodium oxazolinylborate compounds indicate that the electron-donating ability of [ToM]− is slightly greater than that of [ToP]−, and both ligands provide electronic environments that can be compared to the tris(pyrazolyl)borate ligand family.