Toward stable N-heterocyclic silylenes at theoretical levels

To appreciate the chemistry of N-heterocyclic silylenes, eight silylenic isomers derived from 2-, 3-, and 4-silapyridines (1, 2, and 3, respectively) are compared and contrasted at B3LYP/AUG-cc-pVTZ//B3LYP/6-31+G∗ and MP2/6-311++G∗∗//B3LYP/6-31+G∗ levels of theory. Specifically three of eight silylenic isomers are derived from 1: 1H-, 3H-, and 5H-2-silapyridine-2-ylidene (1a, 1b, and 1c, respectively), three from 2: 2H-, 4H-, and 6H-3-silapyridine-3-ylidene (2a, 2b, and 2c, respectively), and finally, two silylenic isomers from 3: 1H-, and 5H-4-silapyridine-4-ylidene (3a, and 3b, respectively). Various thermodynamic parameters are calculated for these eight silylenic minima, along with a kinetic focus on their intramolecular rearrangements to the corresponding silapyridines (1-3). From a thermodynamic point of view, 1a and 3a are the most stable with singlet-triplet energy gaps near to that of Denk's synthesized silylene, i.e. 59.0 and 46.6 vs. 53.0 kcal/mol, respectively. The calculated energy barrier for the [1,2]-H shift of 1a to 1 is 63.4 kcal/mol compared to 80.3 kcal/mol for the [1,4]-H shift of 3a to 3. This study signifies the thermodynamic and kinetic stabilities of 3a which appears compatible with the reported 1a. Considering the σ-donor characteristic of the stable silylenes, the highest nucleophilicity and the lowest electrophilicity is calculated for 3a.