Nucleophilicity of cyclic conjugated silylenes using DFT method

Abstract

AbstractAssuming 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, 1sσ2 (N?), 2sπ2 (ω?), 3sσ2 (N?), and 4sπ2 (ω?), respectively. Our calculations show silacyclopropenylidene (1sσ2) and silacyclopenta‐2,4‐dienylidene (2sπ2) 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 (3sσ2) meets our prediction of being N, despite deviating from planarity. Contrary to our expectation, silacyclonona‐2,4,6,8‐tetraenylidene (4sπ2) 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, 2sπ2 appears planar while its carbenic analog is nonplanar. Nucleophilicity for singlets increases as a function of the ring size 4s > 3s > 2s > 1s, and it decreases as the aromaticity increases. Our triplet silylenes show higher N than their corresponding singlet structures.