ACTIVATION OF C-H BONDS IN THIOPHENES BY RH(PME(3))(2)(CO)CL - A LOW-TEMPERATURE NMR-STUDY

Irradiation of trans-Rh(PMe(3))(2)(CO)Cl (1) at 230 K in the presence of 2,5-dimethylthiophene or thiophene affords products arising from C- H insertion. Irradiation of 1 in 2,5-dimethylthiophene affords three p roducts: two conformers of the six-coordinate complex ns-Rh(PMe(3))(2) (CO)(Cl)(2,5-dimethyl-3-thienyl)H, related by rotation about the Rh-C bond, and trans-Rh(PMe(3))(2)(CO)(2,5-dimethyl-3-thienyl). The major c onformer of ans-Rh(PMe(3))(2)(Co)(Cl)(2,5-dimethy1-3-thienyl)H decays back to 1 at a rate of (2.9 +/- 0.2) x 10(-4) s(-1) at 273 K, whereas the minor conformer and trans-Rh(PMe(3))(2)(CO)(2,5-dimethyl-3-thienyl ) are relatively stable. Irradiation of (1) in the presence of thiophe ne affords products arising from addition at both the 2- and 3-positio ns in the thiophene ring: trans-Rh(PMe(3))(2)(CO)(Cl)(2-thienyl)H, Rh( PMe(3))(2)(CO)(Cl)(3-thienyl)H and trans-Rh(PMe(3))(2)(CO)(2-thienyl). The six-coordinate Rh(III) species each exist as two rotamers at 230 K related by rotation about the Rh-C-aryl bond. Irradiation of 1 in th iophene or 2,5-dimethylthiophene under nitrogen with UV light at low t emperatures also gives rise to products arising from thiophene carbony lation.