Kinetic and thermodynamic aspects of the carbon-sulfur bond activation reactions of asymmetrically substituted thiophenes with rhodium and platinum metal complexes

The kinetic and thermodynamic aspects of the C-S bond activation reactions of asymmetrically substituted thiophenes with [(C5Me5)Rh(PMe 3)] and [Pt(dippe)] fragments were studied both experimentally and theoretically.;The reactions of 2-cyanothiophene and 2-methoxythiophene with the [(C 5Me5)Rh(PMe3)] fragment gave only one product resulting from the insertion of the rhodium atom into the more hindered substituted C-S bond, whereas 2-methylthiophene gave the product resulting from activation of the unsubstituted C-S bond. The ground state energies calculated in the gas phase were found to be consistent with these observed selectivities. The ∼1:1 ratio of products with 3-cyano and 3-methyl substituted thiophenes are also consistent with the ground state energies calculated for these insertion products. For all the substituents the kinetic selectivities predicted based on the calculated C-S bond activation barriers were found to be different from the experimentally observed ones.;Exchange reactions of 2-/3-cyanothiophene and 2-methylthiophene with thiophene in the thiaplatinacycle, Pt(dippe)(SCH=CHCH=CH), were performed to probe the kinetic and thermodynamic selectivity of the above C-S bond activation reactions. The reaction with 2-cyanothiophene gave a kinetic selectivity of 3:1 and a thermodynamic selectivity of 1:49 for the products derived from cleavage of the unsubstituted and substituted C-S bonds, respectively. In the reaction with 3-cyanothiophene, although there is a greater kinetic selectivity of 10:1, the thermodynamic selectivity is only 2:1 favoring the cleavage of the substituted C-S bond. The reaction with 2-methylthiophene did not go to completion, and only one product resulting from the activation of the unsubstituted C-S bond was observed. The ground states of these products as well as the transition states leading to their formation were calculated by including solvation effects. The relative energies of both the ground states and the transition states were found to be consistent with the observed selectivity of these C-S bond activations.;These exchange reactions were extremely useful to understand the kinetic and thermodynamic aspects of the C-S bond cleavage process and showed that the selectivities seen in the C-S bond activation reactions of substituted thiophenes with these platinum and rhodium complexes are under thermodynamic control.