Blueprints for the Geometric Control of NHC-CDI Isomers

Rational control of the 3-dimensional presentation of atoms—stereochemistry—lies at the heart of syn-thetic organic and materials chemistries. Here, we report detailed computational studies on conforma-tional isomerism in N-heterocyclic carbene–carbodiimide (NHC–CDI) zwitterionic adducts. By varying the steric and electronic parameters of the NHC and CDI components, criteria for controlling isomerization (thermo)dynamics and predicting energetically favorable conformations were identi-fied. These criteria were validated experimentally using a novel synthetic approach to NHC-CDIs, which exploits the thermodynamic equilibrium be-tween sterically unencumbered NHC dimers to ac-cess NHC-CDI adducts with low barriers to confor-mational isomerization, including the first example of an (E/E)-NHC-CDI.