Convenient partial reduction of CO2 to a useful C1 building block: efficient access to 13C‐labelled N‐heterocyclic carbenes

The selective, transition metal‐free hydrosilylation of CO2 to CH2(OSiEt3)2 has been achieved under mild conditions and in high isolated yields (up to 90%) using Et3SiH and the simple, easily prepared borohydride catalyst Li+[HB(C6F5)3]−. The resulting CO2‐derived bis(silyl)acetal product – whose mechanism of formation has been interrogated through detailed computational and experimental studies – can be rapidly valorized through the facile synthesis of N‐heterocyclic carbenes, via their corresponding imidazolium salts. By using relatively inexpensive, isotopically enriched 13CO2 this protocol can be exploited to prepare NHC isotopologues that are selectively 13C labelled at the key, ligating C2 position. This provides an electronically responsive 13C NMR spectroscopic handle with dramatically enhanced sensitivity which can directly benefit reactivity studies in both organo‐ and organometallic catalysis, where NHC use is ubiquitous.