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-hemi-aminal (SH) and silanoic amide (SA) tautomers (Xiong Y, Yao S, Muller R, Kaupp M, Driess M, J Am Chem Soc 132: 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 center dot center dot center dot 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.