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%) by 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.