Novel annulation reactions using cobalt stabilised carbonium ions

This programme of work was aimed at the synthesis of a range of linear hydrocarbon molecules bearing functionality at each end. It was envisaged that by the application of an intramolecular Nicholas reaction the linear chain would cyclise to afford an [alpha],[beta] disubstituted cycloalkanone. The Nicholas reaction is based on the propargyl carbonium ions, stabilised with binuclear cobalt species, which are subject to nucleophilic attack at the propargyl position. This reaction has previously been exploited in syntheses in an intermolecular fashion, however, few approaches involving an intramolecular reaction have appeared in the literature. Thus, in the present study, investigation of the latter was undertaken. In this novel approach a series of 5,6 or 7 membered carbocyclic compounds were synthesised from precursor molecules containing 8,9,10 or 11 carbon chains. During this exercise one or two asymmetric centres were also created in a series of reactions that are the equivalent of a 1,4-addition of ethyne to an enone. All the synthetic steps were thoroughly investigated and optimised. It was also demonstrated that for unsymmetrical linear ketones, the kinetic enolates seem to predominate irrespective of the reaction conditions employed. This was in contrast to enolate formation in unsymmetrically substituted cyclic ketones, where selectivity between the kinetic and thermodynamic enolates has been observed by appropriate choice of base. Infra-red, [sup]1H N.M.R., [sup]13C N.M.R. and Mass spectroscopy analyses were performed on all the compounds synthesised. However cobalt complexed compounds showed some paramagnetism, possibly due to the presence of some paramagnetic impurities or by-products. This led to difficulties in interpretation of spectra as a result of line broadening. Future work might usefully investigate the use of these methodologies in the synthesis of fused carbocycles, heterocycles and highly functionalised fused ring systems, generated from a tandem intramolecular reaction involving a Nicholas reaction followed bya Pauson-Khand reaction