Synthesis and application of carbene complexes with heteroaromatic substituents

Abstract

This study involves the synthesis and characterisation of Group VI Fischer carbene complexes with heteroaromatic Iigands to explore aspects of structure and reactivity. Chromium and tungsten carbene complexes with furan, N-methylthieno[3,2-b]pyrrole, bithiophene, 2-(2'-thienyl)furan and N-methyl-2-(2'-thienyl)pyrrole as substituents were prepared. The thienopyrrole condensed ring structure was prepared by employing the condensation of an aromatic aldehyde with an azido-acetate as the key step. The conjugated ring systems with different heteroaromatic rings were synthesized via a metal catalysed cross-coupling methodology. Initially the biscarbene metal complexes were prepared via- the classical Fischer method in which the precursor heteroaromatic derivative is doubly lithiated, reacted with a metal hexacarbonyl and quenched with an alkylating reagent such as triethyl oxonium tetrafluoroborate. In most cases mono- and biscarbene metal complexes was isolated as the major products. An adapted method entailing sequentiallithiation and derivitisation increased the yields of the biscarbene metal complexes and provided access to heteronuclear bimetallic complexes. The yield of the biscarbene metal complexes was further improved by treating a dilithiated precursor with sequential equivalents of metal hexacarbonyl and washing the ionic product with hexane in-between metal additions. In the conjugated ring systems it was found that the proton a to sulfur was most acidic in all cases. In 2-(2'-thienyl)furan the second lithiation took place α to oxygen whilst in the analogous N-methyl-2-(2'-thienyl)pyrrole a second proton could not be removed from the anionic intermediate under the conditions employed. In the condensed ring system, N-methylthieno[3,2-b]pyrrole, for the second lithiation the proton β to sulfur was comparable in acidity to the proton ex to nitrogen and monometallic biscarbene complexes were formed as by-products. These complexes were thoroughly characterised by spectroscopic techniques and crystal structure determinations. This data also indicated structural features conducive to extended electron delocalisation throughout the molecules. This was confirmed by the observation of weak π-donor-acceptor interaction in the electronic spectral studies of these compounds. Although the solvatochromism of the MLCT bands are fairly small, these complexes are good candidates for NLO chromophores due to their strong ligand charge transfer absorption bands. Exploratory reactivity studies showed clear trends with regards to reactivity and selectivity whilst the use of bimetallic complexes created the opportunity to employ one carbene moiety as a reactive centre and yield unique modified monocarbene complexes. The Dötz benzannulation of heteronuclear bimetallic complexes showed that the chromium metal fragment was more reactive than the tungsten metal fragment. Also, the thienyl moiety was more reactive than the furan moiety in mixed heterobiaryl metal complexes. The palladium catalysed dimerisation of biscarbene metal complexes yielded di-ester derivatives in which both metal fragments reacted with oxygen as the major product. The reaction with monocarbene metal complexes yielded carbene dimers with high selectivity towards the trans product.