The activation of a section, or of all the carbons of thiophene by bromine in lithium-halogen exchange reactions, was implemented in a step-wise manner to facilitate the preparation of novel multiple Fischer carbene-bearing thiophene compounds.Mononuclear complexes with two Fischer carbene ligands are far less studied than the corresponding alkoxy or amino monocarbene complexes.1 Carbon-carbon coupling reactions from reactive biscarbene intermediates facilitated by catalysts 2 or from non-catalytic processes have been reported.3 The synthesis of mononuclear biscarbene chelate complexes are scarce 4 and their potential in template reactions in organic chemistry has yet to be recognized.5 The classical Fischer method of carbene synthesis require organolithium reagents, metal carbonyls and a strong electrophile or alkylating agent.6 Hence, for arene substrates to form the backbone of a biscarbene chelate complex the creation of two adjacent carbanions is a necessity which can be synthetically challenging. Fischer reported the first example of such a biscarbene chelate by reacting 1,2-dilithiobenzene with chromium hexacarbonyl.4b The 1,2-dilithiation of benzene was troublesome and incorporated the formation of mercury polymers of benzene and a subsequent reaction with lithium metal.7 Reactions of 1,2-halobenzenes with butyllithium yielded polymeric material due to the reactivity of mixed lithium halogen intermediates. Heteroarene substrates such as thiophene (A, Fig. 1) and furan have two activated carbon atoms but these reside on positions 2 and 5. 8A common feature of metallations of furan and thiophene is the preference for the lithiations to occur at positions 2 and 5. Pretoria, Pretoria, 0002, Fax: +27 12 The relative kinetic acidities between the protons in the 2 and 3 position was determined by H/D exchange experiments and shows the 2 proton to be 500 times more acidic than the 3 proton. Department of Chemistry, University of 9The degree of activation of adjacent carbons 2 and 3 towards direct deprotonation with butyllithium differ too much to allow for sequential lithiation and an alternative strategy is required to synthesize biscarbene chelates. Monolithiation in the 2 position of thiophene is readily achieved at -30• C in THF by n-butyllithium. 10 Harsher reaction conditions are required to effectively dilithiate thiophene in both the 2 and 5 positions with BuLi. Typically this reaction was performed in hexane in the presence of TMEDA at 60• C and used in our laboratories to synthesize 2,5-binuclear biscarbene rods (B). 11Using 2,5-dibromothiophene as a precursor allows for dilithiation in much higher yields and at much lower temperatures in THF. 12A strategy to synthesize an unsymmetric biscarbene chelate would be to activate the 3 position to such an extent that lithiation occurs at this site before position 2 or 5. Blocking of position 5 by a methyl substituent and then attempting to dilithiate at positions 2 and 3 followed by the Fischer protocol did not afford biscarbene chelates and only mixtures...

Access to multicarbene complexes of a fused thienothiophene substrate was obtained by the use of the tetrabrominated thieno [2,3-b]thiophene precursor in a lithium-bromide exchange reaction, followed by nucleophilic attack on metal hexacarbonyls (M = Cr, W). Subsequent alkylation afforded unique triscar- were also obtained in high yields, and the molecular structures of the tungsten complexes, with the exception of 9 and 11, were confirmed by single crystal X-ray diffraction studies.

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.

Novel chromium and tungsten mono-and multiethoxycarbene complexes were synthesized from tris(4-bromophenyl)amine and tri(2-furyl)phosphine substrates. A comparative study between the amine and phosphine multicarbene complexes revealed small differences between the respective compounds. In solution, an equilibrium situation was observed between the mono-and bis-carbene complexes of tri(2-furyl)phosphine. The monocarbene amine ligand differs from the analogous phosphine ligand in the way the heteroatom lone pair electrons are able to interact with the carbene functionality. Unlike for the phosphine, the greater electron delocalization of the amine affected the coordination ability of the ligand when reacted with [Pt(COD)Cl 2 ]. These properties of the monocarbene-carrying ligands were confirmed and supported by solid state structural studies, spectroscopic data, and density functional theory (DFT) calculations.

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