Theoretical Study of the Deprotonation of Nitriles, RCH₂CN:  Ab Initio and PM3 Calculations of Intermediate Aggregates and Transition States

Abstract

The intermediates and transition state for the deprotonation of acetonitrile, 7, with dimeric lithiumamide [LiNH 2 ] 2, 8, which leads to the dianion complex [CH 2 CN/NH 2 ] 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 NH 3, 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 LiC 6 H 5 CHCN·LDA·2 TMEDA (Figure 3), [ t -BuCN·LiN(SiMe 3 ) 2 ] 2 (Figure 4), and [(LiC 6 H 5 CHCN·TMEDA) 2 ·C 6 H 6 ] (Figure 5). In all cases the LiX subunits are described significantly better by PM3 than by MNDO.