IT has been observedlJa that pyrolysis of 1,ldisubstituted-1-silacyclobutanes gives the corresponding substituted lJ3-disilacyclobutanes and ethylene.

Silicon and carbon belong to group IVB of the periodic table, and both have the same constitution of valence shell electrons:C ls22s22p2, Si:ls22s22p63s23p2. Most compounds of silicon have a formal valence number of four due to the transition of one 3s electron to a 3p orbital. Both silicon and carbon readily form single bonds resulting from sp3 orbital overlap. However, despite the great abundance of stable multiple-bonded compounds of pr-pT type for carbon, there has been a complete absence of them for silicon, although silicon does have d orbitals available for pT-d"-bonding.1 <2 Until recently silicon analogs of unsaturated compounds were believed to be nonexistent.1 In this Account we describe the evidence for generation of this new class of organosilicon compounds and discuss their synthetic utility. Our main emphasis is on compounds of silicon which are analogous to olefins and ketones.3 This means that formally, at least, their structure can be described in terms of -bonded silicon. At present there are no data unequivocally indicating this type of chemical bond with silicon: alternative structures may also be correct.R2Si-CH2 -*-* R2Si=CH2R2Si-Ch2We do not consider such 7r-bonded intermediates as :SiO and :SiS, which are discussed in the fairly recent review4 on divalent radicals of group IVB.The first structures with silicon double bonds were assigned to compounds obtained in reactions of ammonia with silicon tetrachloride, trichlorosilane,5 etc. (HN=Si=NH; N=SiNH2). In 1912 Schlenk and Renning6 described the synthesis of a stable compound with a silicon-carbon double bond by dehydration of diphenylmethylsilanol. However, their work was reexamined by Kipping,7 who proved that they had been dealing with a mixture of diphenyldimethylsilane, diphenylmethylsilanol and diphenyl. By the end of the second decade of this century, it had become clear that earlier works in which structures with multiple bonds to silicon were assigned to stable compounds were wrong and reactions which lead to formation of double bonds with carbon are Leonid E. Gusel'nikov was born in Novosibirsk in 1939, graduated from Gubkin Institute for Petrochemical and Gas Industry, and worked for his Ph.D. degree under the supervision of Professor N. S. Nametkin. In 1967 he was on a year's postdoctoral fellowship at Southampton University. His research interests center around reactions of reactive intermediates of group IVB elements.

No abstract

No abstract

ERRATA. Section B1961, page 4923. The values of k and krel in the Table refer to a temperature of 50.0°, rtot 25" as stated. 1966, page 4667, line 27. For [ailD -19.1 read [a], +l9.1.

π-Bond energies in doubly bonded silicon species, H 2 SidX (X ) E 14 H 2 , E 15 H, E 16 ), were determined using G2 and G3 procedures. In addition, the distribution of electron density in the internuclear area between silicon and the element in H 2 SidX and H 3 Si-XH was estimated using quantum-topological approaches AIM (MP2/6-311++G(2d,p)) and ELF (HF/6-311++G(2d,p)). Both parameters, an increase of electron density at the critical point of the silicon-element bond and π-bond energy, appeared to correlate linearly, increasing on going from group 14 to group 16 elements. The contribution of the ion-component to the SidE π-bond energy increases as the negative inductive effect of the element grows, attaining the maximum for silanone.

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.

No abstract