Theoretical study of the deprotonation of nitriles, RCH(2)CN: Ab initio and PM3 calculations of intermediate aggregates and transition states

The intermediates and transition state for the deprotonation of acetonitrile, 7, with dimeric lithiumamide [LiNH2](2), 8, which leads to the dianion complex [CH2CN/NH2](2-) 2 Li+, 14, have been calculated by ab initio (MP2(full)6-31+G*//MP2(full)6-31+G*), DFT (Becke3LYP/6-311+G*, DGAUSS DZP/A1), and semiempirical methods. This reaction is initiated by the formation of a 4-ring-dimer/nitrile complex, 9, which rearranges to give an ''open dimer'' complex 10. This step is (energetically) the most expensive, requiring 11.4 kcal/mol. The complex 10 is a suitable precursor for easy C to N proton transfer via transition state 11, which lies just 1.2 kcal/mol above 10. The reaction proceeds via 12, a second ''open dimer'' and finally, after extrusion of NH3, yields 14. The PM3 and ab initio results for the relative energies of these structures are in acceptable agreement; the only exception is the overestimation of the stability of the complex 9 by PM3 (Figure 1). Therefore, PM3 allows comparison of the inter- and intramolecular versions of the deprotonation step with inclusion of solvation, Schemes 3 and 4 and Figure 2, indicating that the intramolecular pathway is preferred by 9 kcal/mol. To assess the reliability of the PM3 method in predicting the geometries of larger complexes in this series, PM3 and MNDO were applied to simulate the X-ray structures of model intermediates such as LiC6H5CHCN . LDA . 2 TMEDA (Figure 3), [t-BuCN . LiN(SiMe(3))(2)](2) (Figure 4), and [(LiC6H6CHCN . TMEDA)(2) . C6H6] (Figure 5). In all cases the LiX subunits are described significantly better by PM3 than by MNDO.