Catalytic and stoichiometrically directed synthesis of less accessible bromothiophenes and bromobithiophenes. Trapping and characterization of catalytic intermediates of trans-PdBr(C4H4-nBrn-1S-C)(PPh3)(2) (n = 1-4), trans-PdBr(C8H4BrS2-C)(PPh3)(2), and trans,trans-Pd2Br2(mu-C8H6-nBrn-2S2-C,C ')(PPh3)(4) (n = 2, 4)

Abstract

A facile one-pot procedure for synthesizing isomerically pure bromothiophenes and bromobithiophenes was achieved by palladium-catalyzed hydrodebromination. The compounds synthesized include 2,3,4-tribromothiophene (2), 3,4-dibromothiophene (3), 2,3-dibromothiophene (7), 3-bromothiophene (4), 3,3′,5-tribromo-2,2′-bithiophene (10), 3,3′-dibromo-2,2′-bithiophene (11), and 3-bromo-2,2′-bithiophene (12). The regioselectivity is catalytically dictated by the metalation site, whereas the extent of debromination is conveniently controlled by the stoichiometry and substrate quantity. Under favorable conditions, complete debromination takes place, giving rise to thiophene and bithiophene. The catalytic intermediates trans-PdBr(3,4-C4HBr2S-C)(PPh3)2 (16), trans,trans-Pd2Br2(μ-C8H2Br 2S2-C,C′)(PPh3)4 (17), trans,trans-Pd2Br2(μ-C8H 4S2-C,C′)(PPh3)4 (18), trans-PdBr(C8H4BrS2-C)(PPh3) 2 (19), trans-PdBr(C4H3S-C)(PPh3)2 (20), and trans-PdBr(3,4,5-C4Br3S-C)(PPh3)2 (21) were isolated. Palladium insertion invariably occurs at the C(α)-Br bonds of the thiophene or bithiophene rings. Single-crystal X-ray crystallography analyses of 20, 16, and 21 gave three mononuclear square-planar Pd(II) thienyl structures with phosphines in trans orientations.