Substituted six-membered ring carbenes: the effects of amino and cyclopropyl groups through DFT calculations

DFT calculations are employed to compare and contrast six-membered ring carbenes including 1,3-dimethyltetrahydropyrimidin-2-ylidene (1a), 1-methyl-3-cyclopropyltetrahydropyridine-2-ylidene (2a), and 1,3-dicyclopropylcyclohexane-2-ylidene (3a) as well as their unsaturated analogues 1b, 2b, 3b, and 2c. The amino groups exert singlet–triplet energy separation (∆Es−t) of 60.9 kcal/mol to 1a while cyclopropyls induce a ∆Es−t of 14.8 kcal/mol to 3a. The simultaneous presence of amino and cyclopropyl in 2a leads to a ∆Es−t of 43.3 kcal/mol. Unsaturation slightly increases the ∆Es−t of 1a and 3a but not that of 2a. Our thermodynamic, kinetic, and reactivity results are compared with those of synthetic five-membered ring N-heterocyclic carbenes.