Liberation of N-heterocyclic carbenes (NHCs) from thermally labile progenitors: protected NHCs as versatile tools in organo- and polymerization catalysis

Abstract

This mini-review describes the thermal in situ generation of N-heterocyclic carbenes (NHCs) from various heat sensitive precursors for use in organocatalysis and in polymer chemistry. Thermally labile progenitors such as azolium hydrogen carbonates, NHC-CO₂ adducts, alcohol-protected NHCs and NHC-metal complexes are discussed and compared in their scope and limitations. Advantageous aspects like enhanced stability, controlled release by external stimuli, latency and co-catalysis by the protecting groups are illuminated in numerous examples, stressing the versatility and adaptability that can be achieved by protected NHCs. Emerging strategies to harness the reactivity of NHCs are also briefly discussed to provide an outlook on where future developments are to be expected.