THEORETICAL INVESTIGATIONS ON THE MECHANISM OF ACTIVATION OF AMMONIA BY A p-DIMETHYLAMINOPYRIDINE COORDINATED SI=O DOUBLE BOND

Recently, Xiong et al. reported an activation of ammonia by p-dimethylaminopyridine (DMAP) coordinated silanone (DMAP→Si(L)=O, DS) affording a unique pair of sila-hemiaminal (SH) and silanoic amide (SA) tautomers (Xiong Y, Yao S, Müller R, Kaupp M, Driess M, J Am Chem Soc132:6912, 2010). In this paper, the mechanisms of the activation of ammonia affording SH and SA, the successive generation of hydrogen bonded complex pair SH–SA and the tautomerization between SH and SA have been intensively investigated computationally for the first time at MP2/6-311+G(2d,p)//B3LYP/6-31+G(d,p) level in toluene. The concerted Paths C and D with ammonia assistance are determined by our calculations to be the dominant pathways corresponding to forming SH and SA from DS, respectively. The free energy barrier of Path C affording SH from DS is 14.45 kcal/mol, and that of Path D affording SA is 21.46 kcal/mol. So it is determined theoretically that Path C is dynamically dominant over Path D. And the pair SH–SA is formed then spontaneously by intermolecular hydrogen bond (O–H ⋯ O′) without any barrier. While the tautomerization between SH and SA is nonsignficant resulting from the corresponding relative high barriers (23.79 kcal/mol for process from SH to SA and 26.52 kcal/mol for process from SA to SH). Our results are in good agreement with and good interpretation of the experimental results by Xiong et al.