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 To(M)Rh(CO)(2) (3) and To(P)Rh(CO)(2) (4) (To(M) = tris(4,4-dimethyl-2-oxazolinyl)phenylborate; To(P) = 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 To(M)Rh(CO)(2) with P(OMe)(3) provides To(M)Rh(CO)[P(OMe)(3)] (5). However, reaction of To(M)Rh(CO)(2) and MeOTf (Tf = SO2CF3) affords the complex [{N-Me-kappa(2)-To(M)[Rh(CO)(2)]OTf (6), resulting from N-oxazoline methylation rather than oxidative addition to rhodium(I). In contrast, To(M)Rh(CO)(2) reacts with allyl bromide and chloroform, forming the rhodium(III) species (kappa(3)-To(M))Rh(eta(1)-C3H5)Br(CO) (7) and (kappa(3)-To(M))Rh(CHCI2)CI(CO) (8), respectively. Interestingly, the chiral To(P)Rh(CO)(2), and CHCI3 react to give one diastereomer of (kappa(3)-To(P))-Rh(CHCI2)CI(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 [To(M)](-) is slightly greater than that of [To(P)](-), and both ligands provide electronic environments that can be compared to the tris(pyrazolyl)borate ligand family.