Science of Synthesis

Davies, A. T.; Smith, A. D., Science of Synthesis: N-Heterocyclic Carbenes in Catalytic Organic Synthesis, (2016) 2, 435.

In most organocatalytic transformations using NHCs, the corresponding azolium salts tend to be used as a stable precursor, which can be deprotonated in situ to generate the free carbene. Bearing this in mind, the pKa of the azolium salts used may have an influence on the reaction itself and on the base required for the deprotonation. Full examination of this was overlooked until the publication of Smith, OʼDonoghue, and co-workers in 2012. This work described pKa studies on 19 commonly used triazolium salts, with an interesting variation observed depending on the electronic characteristics of the N-aryl group (‌Scheme 45‌).[‌222‌] Triazolium salts containing electron-poor N-aryl substituents have noticeably lower pKas than those with electron-rich N-aryl substituents, whilst minimal difference is observed when varying the counterion from tetrafluoroborate to chloride.

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References