Effect of Geminal Substitution at Silicon on 1-Sila- and 1,3-Disilacyclobutanes' Strain Energies, Their 2+2 Cycloreversion Enthalpies, and SiC π-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.

Hetero‐π‐systems from 2 + 2 cycloreversion, part 2.1Ab initio thermochemical study of heterocyclobutanes 2 + 2 cycloreversion to form heteroethenes H2C=X (X=NH, O, SiH2, PH, S)

“…The activation energies of oxetane's cycloreversion (62.6 and 63.1 kcal/mol via C C and C O bond cleavage, respectively [76]), are also close to the predicted ring-opening enthalpies, 60.7/60.2 and 64.7/60.2 kcal/mol. Taking into consideration the correction on the stabilization effects of silicon atom (see footnote to Table 12), our calculations predicted for siletane the initial cleavage of C C bond, a conclusion widely accepted in the literature [2,3,[77][78][79][80][81][82][83]. The enthalpy of the most economical ring opening via an initial cleavage of C C bond kcal/mol, is only a little preferred to the C N homolysis, 58.3/56.8 kcal/mol.…”

Decomposition of 1,1‐dimethyl‐1‐silacyclobutane on a tungsten filament—evidence of both ring CC and ring SiC bond cleavages

“…For comparison, we have also determined the activation energy ( E a = 69.8 ± 2.8 kJ/mol) for the formation of methyl radicals from the decomposition of TMDSCB on the W filament. The similarity in the activation energies is due to the fact that both DMSCB and TMDSCB decompose to form methyl radical via the cleavage of the SiCH 3 bond and it suggests that the difference in the ring strain energy of DMSCB (108.7 kJ/mol32) and TMDSCB (71.9 kJ/mol33) does not affect much the activation energy of the methyl radical formation.…”

Zwitterionic Silenes: Interesting Goals for Synthesis?

“…It is noteworthy that the same trend in E dim is followed by the zwitterionic silenes 21 as the naturally polarized silenes previously studied by Gusel'nikov and co-workers. [47] Dimerization is only one of the reaction channels leading away from the silene; the others are various rearrangements. A detailed exploration of all possible rearrangement pathways is outside the scope of the present study.…”

“…Inductively electrondonating substituents at Si lower the dimerization energy, whereas the opposite is true for inductively withdrawing substituents, in a similar manner as previously concluded by Gusel'nikov. [47] The charge at the Si atom of zwitterionic silenes resembles that of silyl anions, and the influence of Si substituents on the properties of silyl anions and zwitterionic silenes is similar. Inductively electron-withdrawing substituents at Si raise their E inv values, and silenes with inversion barriers as high as 35 kcal mol À1 are found computationally.…”

“…Gusel'nikov et al recently found that substituents R influence the dimerization energy (E dim ) of R 2 SiCH 2 silenes into 1,3-disilacyclobutanes. [47] Silyl groups lower E dim by % 4 kcal mol À1 when compared to the parent silene (E dim À 78.3 kcal mol À1 at the MP4/6-311G(d)//MP2/6-31G(d) level). On the other hand, when R F, E dim is even more exothermic by % 37 kcal mol…”