C-S BOND SCISSION OF SUBSTITUTED THIOPHENES AT RHODIUM - FACTORS INFLUENCING THE REGIOSELECTIVITY OF THE INSERTION AND THE STABILITY OF THE RESULTING METALLATHIACYCLES

The fragment [(triphos)RhH], generated in situ by thermolysis of the trihydride (triphos)RhH3 in refluxing tetrahydrofuran, reacts with a variety of substituted thiophenes to give C-S insertion products of the formula (triphos)Rh(eta(3)-SCR=CR'CH-CH2) (R' = H, R = Me, Et, COMe, CO(2)Et; R = H, R' = Me, COMe, OMe; triphos = MeC(CH(2)PPh(2))(3)). Irrespective of the position and electronic character of the substituent in the thiophene, insertion is seen exclusively into the C-S bond away from the substituent, consistent with a determinant steric control. The electronic properties of the substituent in the thiophene play an important role in determining the stability of the C-S insertion products in chlorinated solvents. Among the thiophenes investigated, only 2,5-Me(2)T, for steric reasons, and 2-OMeT, for electronic reasons, do not form butadienethiolate complexes by reaction with [(triphos)RhH]. The competitive reactivity of various substituted thiophenes toward the fragment [(triphos)RhH] has been studied by NMR spectroscopy. The reactivity decreases in the order: 2-CO(2)EtT > 2-COMeT > 3-COMeT much greater than 3-OMeT > T congruent to 2-MeT > 3-MeT, consistent with a predominant electronic effect.