Gas-phase intramolecular anion rearrangements of some trimethylsilyl-containing systems revisited. A theoretical approach

Ab initio calculations at the CCSD(T)/6-311++G(2d,p)//B3LYP/6-311++G(d,p) level of theory have been carried out for three prototypical rearrangement processes of organosilicon anion systems. The first two are reactions of enolate ions which involve oxygen-silicon bond formation via three- and four-membered states, respectively. The overall reactions are: CH(2) = C(O(-))Si(CH(3))(3) --> (CH(3))(3)SiO(-) + CH(2)C, and (CH(3))(3)SiCH = CHO(-) --> (CH(3))(3)SiO(-) + C(2)H(2). The DeltaG (reaction) values for the two processes are +175 and +51 kJ mol(-1), with maximum barriers (to the highest transition state) of +55 and +159 kJ mol(-1), respectively. The third studied process is the following: (CH(3)O)C(=CH(2))Si(CH(3))(2)CH(2)(-) --> (CH(3))(2)(C(2)H(5))Si(-) + CH(2)CO, a process involving an S(N)i reaction between -CH(2)(-) and CH(3)O- followed by silicon-carbon bond cleavage. The reaction is favourable [DeltaG(reaction) = -39 kJ mol(-1)] with the barrier for the S(N)i process being 175 kJ mol(-1). The previous experimental and the current theoretical data are complementary and in agreement.