Nucleophilicity of cyclic conjugated silylenes using DFT method

摘要

Assuming aromaticity (cyclic continuous conjugation, planarity, and obeying the Huckel 4n + 2 rule), nucleophilic (N), and electrophilic (omega) characters are anticipated to alternate for sigma(2) and pi(2) singlet 3 -, 5 -, 7, and 9 -membered silylenes, 1(s)sigma(2) (N?), 2(s)pi(2) (omega?), 3(s)sigma(2) (N?), and 4(s)pi(2) (omega?), respectively. Our calculations show silacyclopropenylidene (1(s)sigma(2)) and silacyclopenta -2,4-dienylidene (2(s)pi(2)) as N and omega, 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(s)sigma(2)) meets our prediction of being N, despite deviating from planarity. Contrary to our expectation, silacyclonona-2,4,6,8-tetraenylidene (4(s)pi(2)) not only does not turn out to be omega 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(s)pi(2) 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.