sigma-Thienyl complexes of palladium(II), trans-PdBr(C4H4-nBrn-1S-C)(PPh3)(2) (n=1-4) from oxidative addition of bromothiophenes to Pd(PPh3)(4). X-ray structural identification of key intermediates of catalytic debromination of 2-bromothiophene and tetrabromothiophene

Oxidative addition reactions of Pd(PPh3)(4) with bromothiophenes C4H(4-n)Br(n)S (n=1-4) occur readily at RT to give sigma-thienyl complexes trans-PdBr(C4H4-nBrn-1S-C)(PPh3)(2) in yields greater than or equal to 85%. The products are intermediates in the regioselective reduction of bromothiophenes catalyzed by Pd(O). Palladation invariably occurs at the carbon adjacent to the thienyl sulfur. Single crystal X-ray crystallographic determination was carried out on PdBr(2-thienyl)(PPh3)(2), 1, and PdBr(3,4,5-tribromo-2-thienyl)(PPh3)(2), 6. Crystal data for 1: space group Pbcn, a = 19.354(1), b = 10.760(1), c = 16.451(1)Angstrom, Z = 4, 3106 reflections, R = 0.0614; for 6: space group P2(1)/n, a = 11.361(2), b = 13.049(3), c = 26.670(5)Angstrom, beta = 102.00(3)degrees, Z = 4, 3577 reflections, R = 0.0375. Reduction of PdBr(3,4-dibromo-2-thienyl)(PPh3)(2),5, by NaBH4 gives 2,3-dibromothiophene.