Aromatic vs aliphatic C-H bond activation by rhodium(I) as a function of agostic interactions: Catalytic H/D exchange between olefins and methanol or water

The aryl-PC type ligand 3, benzyl(di-tert-butyl)phosphane, reacts with [Rh(coe)₂(solv)_n]BF₄ (coe = cyclooctene, solv = solvent), producing the C-H activated complexes 4a-c (solv = (a) acetone, (b) THF, (c) methanol). Complexes 4a-c undergo reversible arene C-H activation (observed by NMR spin saturation transfer experiments, SST) and H/D exchange into the hydride and aryl ortho-H with ROD (R = D, Me). They also promote catalytic H/D exchange into the vinylic C-H bond of olefins, with deuterated methanol or water utilized as D-donors. Unexpectedly, complex 2, based on the benzyl-PC type ligand 1 (analogous to 3), di-tert-butyl(2,4,6-trimethylbenzyl)phosphane, shows a very different reversible C-H activation pattern as observed by SST. It is not active in H/D exchange with ROD and in catalytic H/D exchange with olefins. To clarify our observations regarding C-H activation/reductive elimination in both PC-Rh systems, density functional theory (DFT) calculations were performed. Both nucleophilic (oxidative addition) and electrophilic (H/D exchange) C-H activation proceed through η²-C,H agostic intermediates. In the aryl-PC system the agostic interaction causes C-H bond acidity sufficient for the H/D exchange with water or methanol, which is not the case in the benzyl PC-Rh system. In the latter system the C-H coordination pattern of the methyl controls the reversible C-H oxidative addition leading to energetically different C-H activation processes, in accordance with the experimental observations.