A Stable N‑Heterocyclic Carbene Organocatalyst for Hydrogen/Deuterium Exchange Reactions between Pseudoacids and Deuterated Chloroform

It was observed that the stable and commercially available N-heterocyclic carbene (NHC) 1,3-bis(2,6-di­iso­propyl­phenyl)­imidazol-2-ylidene, the so-called IDipp, catalyzes hydrogen/deuterium exchange reactions between pseudoacids and chloroform-d 1, while the analogous saturated NHC 1,3-bis(2,4,6-tri­meth­yl­phenyl)­imidazolin-2-ylidene, the so-called SIMes, is inefficient for the same transformation. Experimental and computational DFT studies allowed these differences of reactivity to be attributed to the relative stability of the corresponding azolium–trichloromethyl anion ion pairs: in the former case, the complex evolves toward dissociation of the ions to produce an aromatic azolium cation and a basic trichloromethyl anion, while in the latter case, it evolves by ion recombination to give the product of formal carbene C–H insertion into the C–H bond of chloroform. These results provide a rationale for some early intuitions and observations of Wanzlick, Arduengo, and others on the reactivity of NHCs with chloroform as well as a simple organocatalytic method for the deuteration of pseudoacids (pK a,DMSO = 14–19) with chloroform-d 1.