Formation of (SiN2SiC) five-membered rings by intramolecular insertion into the Si-N bond: A quantum chemical study

The formation of the diazadisilacyclopentane 2 in a ring expansion reaction is the first example of an SiR2-CH2 insertion into the nitrogen-silicon si ngle-bond accompanied by migration of a hydrogen atom from a carbon to a ni trogen atom [J. Organonzet. Chem. 1994, 482, 285]. Density functional calcu lations support the experimental findings and suggest a reaction mechanism which includes two four-center transition states and an intermediate silahy drazone (3b). The reaction enthalpy Delta H-R degrees (298 K) of the isomer ization reaction converting the three-membered ring (Me3SiN)(2)SiH2 (1b) in to the diazadisilacyclopentane Me2Si(NHNSiMe3CH2)SiH2 (2b) is calculated to be -23 kcal mol(-1). Another intermediate of the system shows an unexpecte d electronic structure with a lone electron pair at the SiH2 silicon atom. The transition states and the unimolecular reaction mechanism are discussed in detail, The calculated geometry for the product 2b is in very good agre ement with X-ray structure data.