Ab initio computational study of 1-methyl-4-silatranone and attempts at its conventional synthesis

Abstract

1-Methyl-4-silatranone could exhibit the structural aspects of a typical silatrane including a short N–Si bond distance reflecting a dative bond. But given the significant amide resonance in a [3.3.3] bridgehead bicyclic lactam, the lone pair could be shared with the carbonyl group leading to a very long N–Si bond, essentially a “non-silatrane.” Ab initio calculations (MP2/6-311 + G*) predict that ground state conformations of this molecule are best regarded as lactams rather than silatranes, the most stable having a calculated N–Si bond length of 2.902 A and an N–CO bond length of 1.387 A. The calculated transition state for inversion of the amide ring retains very little amide resonance (N–CO, 1.440 A). Some of this loss is compensated through tightening of the N–Si bond (2.422 A), leading to a net energy of activation of ca 8 kcal/mol. Attempts to synthesize 1-methyl-4-silatranone using conventional pathways successful for 1-methylsilatrane [condensations employing N,N-bis(2-hydroxyethyl)glycolamide in place of tris(2-hydroxyethyl)amine] were unsuccessful. This is due to the net loss in resonance energy of the amide reactant relative to that in the [3.3.3] system, the essential absence of the N–Si dative bond, and the rigidity introduced by the planar amide linkage in the starting material. A more likely pathway to successful synthesis should be formation of the amide linkage in the final step.