Thiophene site activation: Synthesis and structures of Fischer carbene complexes with 3,3′- and 2,3′-bithienyl substituents

Abstract

Deprotonation of the reactive α-thiophene ring positions of 3,3′-bithiophene and 2,3′-bithiophene afforded, after metalation with group 6 transition metal carbonyls and alkylation, a diverse range of mono- and biscarbene complexes. The bithienyl isomers differ in the number of active α-positions but representative examples of Fischer monocarbene complexes of all possible isomers were obtained. For 3,3′-bithienyl, the carbene fragment is in the electronically favoured C2-positions on the “inside” of the bithienyl and is structurally and electronically significantly different from the carbene substituent in the “outside” C5-positions. In the unsymmetrically substituted 2,3′-bithienyl scaffold, in addition to [M(CO)5{C(OEt)-2′-(2,3′-BT)}] and [M(CO)5{C(OEt)-5′-(2,3′-BT)}], a carbene moiety in the C5-position of the T2-thienyl ring, [M(CO)5{C(OEt)-5-(2,3′-BT)}], was also formed. In the same reaction mixture of the mono(ethoxycarbene) complexes, biscarbene complexes were obtained. In contrast, the major biscarbene complexes display the carbene fragments in the sterically favoured outside C5,C5′-positions. The unexpected formation of the 2,2′-bithienyl biscarbene complex, [{M(CO)5C(OEt)}2-5,5′-(2,2′-BT)] as well as the formation of the biscarbene complexes [{M(CO)5C(OEt)}2-2,5-(3,3′-BT)] and [{M(CO)5C(OEt)}2-2′,5′-(2,3′-BT)] with both carbene moieties on the same thiophene ring, were observed. By a modified method of synthesis, a mixed metal biscarbene complex of chromium and tungsten in the C2,C5-positions on the same thiophene ring could be isolated for both 3,3′-bithienyl and thiophene [{Ma/b(CO)5C(OEt)}2-2,5-(T/3,3′-BT)]. The formation of a monocarbene complex of 2,3′-bithienyl, [W(CO)5C(OEt)-5-(2,3′-BT){C(O)(OEt)}2-2′,5′] betrays the presence of an unstable triscarbene intermediate [{W(CO)5C(OEt)}3-5,2′,5′-(2,3′-BT)]. The molecular structures of most of the major isomers were confirmed by single crystal X-ray studies.