Effect of geminal substitution at silicon on 1-sila- and 1,3-disilacyclobutanes' strain energies, their 2+2 cycloreversion enthalpies, and Si=C pi-bond energies in silenes.

Abstract

To evaluate the effect of geminal substitution at silicon on 1-sila- and 1,3-disilacyclobutanes' strain energies, their 2+2 cycloreversion enthalpies, and Si=C pi-bond energies in silenes, an ab initio MO study of silenes, R2Si=CH2 (1), 1-silacyclobutanes, cyclo-R2Si(CH2)3 (2), and 1,3-disilacyclobutanes, cyclo-(R2SiCH2)2 (3), was performed using the level of theory denoted MP4/TZ(d)//MP2/6-31G(d) (TZ means the 6-311G(d) basis set for elements of the second period and hydrogen, and the McLean-Chandler (12s,9p)/[6s,5p](d) basis set for the third period elements). In the series R = H, CH3, SiH3, CH3O, NH2, Cl, F, the growth of the reaction enthalpies and strain energies is proportional to the substituents' electronegativities. 2+2 cycloreversion of 2 is endothermic by 40.6-63.1 kcal/mol, whereas that of 3 is endothermic by 72.7-114.2 kcal/mol. On going from a silicon to a fluorine substituent at the sp2-hybridized silicon atom, the pi-bond energy in 1 weakens by 11.3 kcal/mol, and the Si=C bond length shortens by 0.053 A. The effect of substituents' electronegativities at the double-bonded silicon atom in silenes is formulated as follows: the higher electronegativity, the shorter and the weaker the Si=C pi-bond. The latter is rationalized in terms of more strained geometry resulting from the energetic cost for planarizing the R2SiC moiety. The enthalpies of the ring-opening reaction are 68.0-80.1 kcal/mol (a cleavage of the Si-C bond in 3), 65.0-76.4 kcal/mol (a cleavage of the Si-C bond in 2), and 58.0-64.9 kcal/mol (a cleavage of the C-C bond in 2). The pronounced difference in the enthalpies of 2+2 cycloreversion of 1-sila- and 1,3-disilacyclobutanes is mainly due to the difference in the enthalpies of diradicals' decomposition. The decomposition of diradicals resulting from a cleavage of C-C and Si-C bonds in 2 is exothermic by 24.3-3.3 kcal/mol (apart from the difluoro derivative which is endothermic by 5.1 kcal/mol) and 27.0-13.3 kcal/mol, respectively. The decomposition of a 1,4-diradical resulting from ring opening of 3, apart from the disilyl derivative, is the endothermic process for which the enthalpy varies from 10.6 to 40.4 kcal/mol.