Some aspects of the chemistry of diazoalkanes

Abstract

The discussion of the research reported in this thesis is prefaced by a review on diazoalkane chemistry. The initial object of this work was to continue and expand upon the work previously reported on thiophenium ylids. 3-Substituted thiophenium ylids had not previously been synthesized. Attempts to synthesize these compounds using reported conditions met with limited success, only low yields of the desired ylids were obtained. The use of rhodium-(II)-hexanoate as a catalyst in the ylid forming reactions was found to be more effective in certain circumstances. The thermal rearrangement of thiophenium ylids to thiophen-2-malonates was found to proceed via a 2(H)-thiopyran intermediate. Nmr studies of the substituted ylids showed non equivalent ester signals with a barrier to their exchange of 47-60 kJ mole-1. Two exchange processes were found to be taking place, inversion at sulphur and rotation about the ylid bond, with inversion at sulphur having the lower barrier. The rotation about the ylidcarbon-carbonylcarbon bond was also hindered, this having a barrier of 38 kJ mole-1. Reaction of dimethyl diazomalonate with aliphatic amines resulted in the formation of 1,2,3-triazoles in good yield. However, the reaction of other diazoesters with the amines did not give the corresponding triazoles An attempt was made to synthesize the phytoalexin Wyerone using the rearrangement of an a-diazomethylthiophen to generate the cis-ene-yne side chain. The intermediate diazoalkane could not be prepared in sufficient yield to allow further transformation. The rhodium-(II)-acetate catalysed reaction of a closely related model compound resulted in the formation of the corresponding ketone.