Computational Studies of the Relative Rates for Migratory Insertions of Alkenes into Square-Planar, Methyl, −Amido, and −Hydroxo Complexes of Rhodium

The relative rates of the migratory insertions of alkenes into the M-X bonds of (PMe(3))(2)Rh(eta(2)-CH(2) horizontal lineCHR)(X) (R = H, Me; X = CH(3), NH(2), OH) have been analyzed by DFT calculations. These insertions are computed to form metallacycles containing a metal-carbon bond and either an agostic interaction, a dative metal-nitrogen bond, or a dative metal-oxygen bond. The computed barriers for migratory insertion into the metal-hydroxo and metal-amido bonds are lower than those for insertion into the metal-methyl bond. Application of Bader's atoms-in-molecules analysis and natural localized molecular orbital analysis implies that the barriers for alkene insertion into M-X bonds are controlled by the degree of M-X bonding in the transition state, which correlates with the degree of M-X bonding in the initial product. The Rh-X bond orders in the transition states for migratory insertion of ethylene into the Rh-NH(2) and Rh-OH bonds in (PH(3))(2)Rh(eta(2)-C(2)H(4))(NH(2)) and (PH(3))(2)Rh(eta(2)-C(2)H(4))(OH) are much larger than that in the transition state for insertion into the Rh-C bond of (PH(3))(2)Rh(eta(2)-C(2)H(4))(CH(3)). The free energy barriers for 1,2- and 2,1-insertions of propene into the rhodium complexes were also calculated, and the barrier for 1,2-insertion was found to be lower than that for 2,1-insertion. Most striking, the DeltaDeltaG(double dagger) values for 1,2- versus 2,1-insertion of propene into these rhodium complexes were calculated to increase in the order X = CH(3) < NH(2) < OH. The increasing stability of the 1,2-insertion product with increasing polarity of the C-X bonds parallels the relative stabilities of linear versus branched alkanes, amines, and alcohols.