Activation of C−H Bonds in Thiophenes by Rh(PMe3)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 products: two conformers of the six-coordinate complex trans -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 conformer of trans -Rh(PMe 3 ) 2 (CO)(Cl)(2,5-dimethyl-3-thienyl)H decays back to 1 at a rate of (2.9 ± 0.2) × 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 thiophene affords products arising from addition at both the 2- and 3-positions 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 thiophene or 2,5-dimethylthiophene under nitrogen with UV light at low temperatures also gives rise to products arising from thiophene carbonylation.