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Comprehensive mechanistic study of ion pair S N 2 reactions of lithium isocyanate and methyl halides
Author(s) -
Sun YingXin,
Ren Yi,
Wong NingBew,
Chu SanYan,
Xue Ying
Publication year - 2006
Publication title -
international journal of quantum chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.484
H-Index - 105
eISSN - 1097-461X
pISSN - 0020-7608
DOI - 10.1002/qua.20914
Subject(s) - chemistry , isocyanate , isomerization , halide , cyanate , nucleophile , heterolysis , transition state , lithium (medication) , dissociation (chemistry) , ion , medicinal chemistry , photochemistry , inorganic chemistry , catalysis , organic chemistry , medicine , polyurethane , endocrinology
The anionic S N 2 reactions NCO − + CH 3 X and ion pair S N 2 reactions LiNCO + CH 3 X (X = F, Cl, Br, and I) at saturated carbon with inversion and retention mechanisms were investigated at the level of MP2/6‐311+G( d,p ). There are two possible reaction pathways in the anionic S N 2 reactions, but eight in the ion pair S N 2 reactions. Calculated results suggest that the previously reported T‐shaped isomer of lithium isocyanate does not exist. All the retention pathways are not favorable based on the analysis of transition structures. Two possible competitive reaction pathways proceed via two six‐member ring inversion transition structures. It is found that there are two steps in the most favorable pathway, in which less stable lithium cyanate should be formed through the isomerization of lithium isocyanate and nucleophilic site (N) subsequently attacks methyl halides from the backside. The thermodynamically and kinetically favorable methyl isocyanate is predicted as major product both in the gas phase anionic and the ion pair S N 2 reactions. In addition, good correlations between the overall barriers relative to separated reactants, ΔH ≠ ovr, with geometrical looseness parameter %L ≠ and the heterolytic cleavage energies of the CX and LiN (or LiO) bonds are observed for the anionic and ion pair S N 2 reactions. The trend of variation of the overall barriers predicts the leaving ability of X increase in the order: F < Cl < Br < I. The polarized continuum model (PCM) has been used to evaluate the solvent effects on the two inversion pathways with six‐member transition structures for the reactions of LiNCO + CH 3 X. The calculations in solution indicate that solvent effects will retard the rate of reactions and the predicted product, methyl isocyanate, is same as the one in the gas phase. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006