Trimethylsilyl Substituent Effects on the Triazolium-catalysed Benzoin Condensation

Abstract

In the benzoin condensation catalysed by N-heterocyclic carbenes (NHCs) there exists a unique 2-substituent effect. 2-Substituted aldehydes with 2-Lewis basic substituents will preferentially take part in the first addition step of NHC to aldehyde, though the relative contributions of steric and electronic factors are unknown. The aim of this project was to further probe the origin of this 2-substituent effect, and to investigate the influences of sterics and Lewis acidity on the N-aryl substituent of pyrrolidine derived triazolium NHCs. By substituting trimethylsilyl groups into the 2-position of benzaldehyde we sought to identify the nature of the 2-substituent effect. The substituent effect was investigated by examining 2-silyl substituent effects on homo and cross benzoin type condensations. It was discovered that a 2-substituent reduces the rate of the benzoin condensation in the case of a 2-trimethylsilyl substituent. Thus, it appears that the 2-substituent effect varies depending on electronic nature, as benzaldehyde with sterically crowded and Lewis acidic 2-positions have reduced activity in the benzoin condensation, as opposed to what has been observed for the 2-substituent effect with Lewis basic substituents. Two NHC precursors, 6,7-Dihydro-2-(3-chlorophenyl)-5H-pyrrolo[2,1-c]-1,2,4-triazolium tetrafluoroborate 142 and 6,7-Dihydro-2-(4-bromophenyl)-5H-pyrrolo[2,1-c]-1,2,4-triazolium tetrafluoroborate 144, were isolated with the intent to substitute trimethylsilyl groups onto the N-aryl group. The pKas of the novel NHCs were identified, and efforts to substitute trimethylsilyl groups onto these are ongoing. These results indicate that that silyl groups present challenges for examining the benzoin condensation, and that the 2-silyl substituent effect differs from other 2-heteroatoms examined to date. Further work is necessary to explore the origin of this difference in behaviour.