Convenient Partial Reduction of CO₂ to a Useful C₁ Building Block: Efficient Access to ¹³C‐Labelled N‐Heterocyclic Carbenes

Abstract The selective, transition metal‐free hydrosilylation of CO 2 to CH 2 (OSiEt 3 ) 2 has been achieved under mild conditions and in high isolated yields (up to 90%) by using Et 3 SiH and the simple, easily prepared borohydride catalyst Li + [HB(C 6 F 5 ) 3 ] − . The resulting CO 2 ‐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 13 CO 2 this protocol can be exploited to prepare NHC isotopologues that are selectively 13 C labelled at the key, ligating C2 position. This provides an electronically responsive 13 C NMR spectroscopic handle with dramatically enhanced sensitivity, which can directly benefit reactivity studies in both organo‐ and organometallic catalysis, where NHC use is ubiquitous.