Nucleophilicity of cyclic conjugated silylenes using DFT method

Assuming aromaticity (cyclic continuous conjugation, planarity, and obeying the Hückel 4n + 2 rule), nucleophilic (N), and electrophilic (ω) characters are anticipated to alternate for σ² and π² singlet 3-, 5-, 7, and 9-membered silylenes, 1_sσ² (N?), 2_sπ² (ω?), 3_sσ² (N?), and 4_sπ² (ω?), respectively. Our calculations show silacyclopropenylidene (1_sσ²) and silacyclopenta-2,4-dienylidene (2_sπ²) as N and ω, respectively, for exhibiting all the above criteria, at B3LYP/6-311++G**, B3LYP/6-311++G(2df,2p), and M06/6-311++G^** levels of theory. Silacyclohepta-2,4,6-trienylidene (3_sσ²) meets our prediction of being N, despite deviating from planarity. Contrary to our expectation, silacyclonona-2,4,6,8-tetraenylidene (4_sπ²) not only does not turn out to be ω but also because of its intrinsic angle strain and boat-like structure, turns out as the most N among singlet silylenes. In all cases, singlet silylenes appear as ground states, exhibiting more stability than their corresponding triplet states. In contrast to our previous report on the 3-, 5-, 7-, and 9-membered carbenes (Kassaee, et al, Tetrahedron, 1985), all the above singlet and triplet silylenes appear as minima on their energy surfaces. Besides, no allenic isomer is found as a minimum in the silylene series, and finally, 2_sπ² appears planar while its carbenic analog is nonplanar. Nucleophilicity for singlets increases as a function of the ring size 4_s > 3_s > 2_s > 1_s, and it decreases as the aromaticity increases. Our triplet silylenes show higher N than their corresponding singlet structures.