σ-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

Abstract

Oxidative addition reactions of Pd(PPh3)4 with bromothiophenes C4H4-nBrnS (n = 1-4) occur readily at RT to give σ-thienyl complexes trans-PdBr(C4H4-nBrn-1S-C)(PPh3)2 in yields ≥ 85%. The products are intermediates in the regioselective reduction of bromothiophenes catalyzed by Pd(0). 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 = 10.160(1), c = 16.451(1)Å, Z = 4, 3106 reflections, R = 0.0614; for 6: space group P21/n, a = 11.361(2), b = 13.049(3), c = 26.670(5)Å, β = 102.00(3)°, Z = 4, 3577 reflections, R = 0.0375. Reduction of PdBr(3,4-dibromo-2-thienyl)(PPh3)2, 5, by NaBH4 gives 2,3-dibromothiophene.