Fischer Dinuclear and Mononuclear Bis-Carbene Complexes of Thiophene and Thiophene Derivatives

Abstract

The reaction of dilithiated thiophene and thiophene derivatives with group 6 transition metal carbonyl precursors and subsequent alkylation afforded linearly arranged Fischer 2,5-bis-carbene and the rare unsymmetrical 2,3-bis-carbene chelated complexes. The latter requires a second lithiation to occur at an adjacent, less reactive site on the thiophene ring. The control of reactivity sites was investigated and achieved by either blocking more reactive positions with substituents or activating less reactive positions by lithium-halogen exchange reactions. A series of Fischer bis-carbene chelates were synthesized by manipulating the above variables. Structural features of Fischer mono-carbene, mononuclear bis-carbene, and bis-carbene chelated complexes were studied by IR, NMR, and singlecrystal X-ray diffraction.

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

“…Notably no monocarbene complexes, with the carbene carbon in a β-position, were isolated while using n-BuLi. This is in line with our previous studies showing that lithium-halogen exchange is a requirement for β-carbon lithiation under the reaction conditions employed [12,19].…”

“…The complexes with the carbene moiety in the C5-position represent a more linear (open) arrangement for the dithienyl and ethoxy substituents of the carbene ligand. In 2a, 2b, 4a and 4b the ethoxy substituents point towards the sulphur atom of the adjacent thienyl ring, which is the electronically favoured conformation [6,7,19]. However for the aminolysed complex (7a), the nitrogen atom is on the opposite side of the thienylene sulphur atom; emphasizing the electronic and steric differences between an amino-and an ethoxycarbene substituent in Fischer carbene complexes.…”

“…The method of synthesis used was the classical Fischer method of deprotonation by an organolithium base, reaction with a metal carbonyl and subsequent alkylation of the metal acylate to afford the neutral carbene complexes [19]. In this study, reactions were performed at low temperatures in THF to try and discriminate between the different α-active sites and study novel products that form.…”

Fischer mono- and biscarbene complexes of tungsten with mono- and dimeric heteroaromatic substituents

“…The detailed synthesis and characterization of most of the complexes of this study have been reported previously [13][14][15][16][17][18][19][20]. Complex (5) was synthesized according to the analogous methoxy complex reported in literature [13] and characterization data are similar.…”

Fischer carbene complexes remain favourite targets, and vehicles for new discoveries

“…43 Using another approach, chelated biscarbene complexes of chromium and tungsten could be formed upon consecutive CO functionalizations. 44 3-Bromo-5-methyl-thiophene was deprotonated by LDA and reacted with M(CO) 6 to form a 2-carbeniate. Subsequent Li/Br exchange with n-BuLi effected a second nucleophilic attack on a cis-positioned CO ligand, and alkylation yielded the product (Scheme 3).…”