Journal of Organic Chemistry vol:59 issue:26 pages:8015-8022
The model reaction of hydrogen isocyanide and silene (HN=C + CH2=SiH2) has been studied by theoretical methods. Geometries of the stationary points have been optimized at the MP2/6-31G** level while relative energies have been estimated using QCISD(T) wave functions in conjunction with the 6-31G** basis and corrected for zero-point energies. The primary cycloadduct, siliranimine, is calculated to be slightly less stable than silaziridine, the isomeric alternative having an exocyclic C=C double bond, in agreement with experiment on substituted systems. The third alternative species with an exocyclic C=Si double bond lies higher in energy. The [2 + 1] cycloaddition of HN=C to H2C=SiH2 is a concerted but asynchronous and stereoselective process in which the carbon lone pair of HN=C attacks first the Si atom. The favored unimolecular rearrangement siliranimine-silaziridine occurs in two distinct steps involving a four-membered cyclic carbene as the intermediate. The carbene has a relative stability similar to siliranimine, and its formation constitutes the rate-determining step of the entire transformation. Although the carbene is a formal product of a [2 + 2] cycloaddition of HNC + H2C=SiH2, the relevant transition structure could not be located at the levels employed. Some properties of the rings considered including the ring strains, the barriers to nitrogen inversion, and the proton affinities at nitrogen have also been examined.