Abstract
Cyclonona-3,5,7-trienylidene (1) changes from being a transition state (TS) to minimum states when substituted by α-methyl groups and ά-X, where X = CMe2, NMe, PMe, O, S, cyclopropyl, and SiMe2 (2, 3, 4, 5, 6, 7, 8, respectively) at density functional theory. Specifically, the parent carbene 1 exhibits a negative vibrational force constant and proves to be an unreachable electrophilic TS while shows Cs symmetry with an NBO atomic charge of +0.70 on its carbenic center. It has a triplet ground state with a rather small singlet-triplet energy gap (ΔEs–t = −4.1 kcal/mol). In contrast, all of its seven scrutinized derivatives enjoy reachable global minima, with C1 symmetry, desired nucleophilicity, and singlet closed shell (Scs) ground states (for all but 8 which remains triplet). Stability is indicated by relative ΔEs–t values: 2 > 3 > 4 > 5 > 6 > 7 > 1 > 8. The highest ΔEs–t as well as NBO carbenic atomic negative charge (−0.74) are displayed by 2. Our carbenes (2, 3, 4, 5, 6, 7) appear more nucleophilic than the synthesized N-heterocyclic carbenes (imidazol-2-ylidenes). Copyright © 2013 John Wiley & Sons, Ltd.
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