Effects of ligands on the migratory insertion of alkenes into rhodium-oxygen bonds

Abstract

Migratory insertions of olefins into metal-oxygen bonds are elementary steps of important catalytic processes, but well characterised complexes that undergo this reaction are rare, and little information on the effects of ancillary ligands on such reactions has been gained. We report a series of alkoxo alkene complexes of rhodium(i) that contain a range of bidentate ligands and that undergo insertion of the alkene. Our results show that complexes containing less electron-donating ancillary ligands react faster than their counterparts containing more electron-donating ancillary ligands, and that complexes possessing ligands with larger bite angles react faster than those with smaller bite angles. External added ligands had several effects on the reactions, including an inhibition of olefin isomerisation in the product and acceleration of the displacement of the product from complexes of ancillary ligands with small bite angles. Complementary computational studies help elucidate the details of these insertion processes.

Scheme 1. Insertion of alkenes into metal–alkoxo bonds.

Fig. 1. Synthesis and reaction of rhodium alkoxo alkene complexes 3a–m. Rate constants refer to the reaction of alkoxo–alkene complexes 3a–m at 35 °C in tol-d₈ with 20 equiv. of added PPh₃. Bite angles are those from published work. Yields were determined by ¹H NMR spectroscopy using 1,3,5-trimethoxybenzene (TMB) as internal standard. ^aTHF-d₈ used as solvent; ^b0 °C was used as the reaction temperature.

Scheme 2. Products of the reaction of alkoxo alkene complexes 3a–m.

Scheme 3. Proposed mechanism for the reaction of 3a–m to form 4a.

Fig. 2. Calculated reaction pathway for the reaction of complex 3f. Energies were calculated at the PBE0 level of theory. Additional energy values from single point calculations at the M06 level of theory are given in parentheses.

Scheme 4. Formation of tetrahydrofuran 6 by quenching the reaction of alkoxo alkene complex 3f with an excess of hydrochloric acid. Yields were determined by ¹H NMR spectroscopy using TMB as internal standard. The proposed structure of the observed complexes is also shown.