Selectivity in the Oxidative Addition of C-S Bonds of Substituted Thiophenes to the (C5Me5)Rh(PMe3) Fragment: A Comparison of Theory with Experiment

Theoretical studies were performed on the C-S bond activation reactions of 2-/3-cyanothiophene, 2-/3-methoxythiophene, and 2-/3-methylthiophene with the [Rh(PMe3)(C5Me5)] fragment to compare with the selectivity of these reactions observed in the experimental study, with the goal of determining whether the latter represent kinetic or thermodynamic products. Density functional theory (DFT) calculations have been used to optimize the ground-state structures of the two possible insertion products and the transition state structures leading to the formation of the products arising from the above cleavage reactions to address this question. With the 2-cyano and 2-methoxy substituents, the observed formation of one product resulting from the exclusive insertion of the rhodium into the more hindered substituted C-S bond was found to be consistent with the calculated energy differences between the ground states of the two possible products (7.6 and 2.6 kcal mol(-1)). With 2-methylthiophene, the product resulting from the activation of the unsubstituted C-S bond is calculated to be favored by 5.8 kcal mol(-1), in agreement with observed results. The similar to 1:1 ratio of products with 3-cyano and 3-methyl substituted thiophenes are also found to be consistent with the small calculated energy differences (0.4 and 0.8 kcal mol(-1)) between the ground states of the two insertion products. Although the observed high selectivity in the formation of a single C-S bond activation product with 3-methoxythiophene appears to be underestimated in the calculations, the observed products for all substituted thiophenes correlate with the calculated thermodynamic products. In addition, the kinetic selectivities predicted based on the calculated C-S bond activation barriers are different from those observed experimentally. Consequently, these investigations demonstrate that DFT calculations can be used reliably to differentiate if an experimentally observed C-S bond activation reaction proceeds under thermodynamic or kinetic control.