A THEORETICAL-STUDY OF ALKYL SUBSTITUENT EFFECTS IN INTRAMOLECULAR SN2 REACTIONS .2. SUBSTITUTIONS IN NONIDENTICAL EXCHANGE-REACTIONS

This paper examines the effects of alkyl substituents on S(N)2 transit ion structures, using NH3, NH2Me, NHMe2 and NMe3 as nucleophiles and M eCl, EtCl, i-PrCl, i-BuCl and t-BuCl as substrates. The reactions of M e- and Et- with MeCl and EtCl are taken as model systems that give ear ly transition states. In late transition states, according to ab initi o (3-21 G or 6-31 G) calculations, substituting the nucleophile lengt hens the (nucleophile-reaction center) bond and shortens the (reaction center-leaving group) bond by approximately 0.115 angstrom. Monosubst itution of the reaction center lengthens the Nu-C and C-Cl bonds by ap proximately 0.10 angstrom and approximately 0.05 angstrom, respectivel y. The leaving group is also deviated by approximately 15-degrees. Dis ubstitution of the reaction center leads to an angular deviation of ap proximately 10-degrees for both the nucleophile and the leaving group. The bond lengthening is approximately 0.20 angstrom for Nu-C and appr oximately 0.10 angstrom for C-Cl. In early transition states, substitu ting the nucleophile leaves the TS practically unchanged. The reason f or this is that steric hindrance is markedly reduced by the ''outwards '' pyramidalization of the reagent and by the large intermolecular dis tance. Monosubstitution of the reaction center leads to a deviation of approximately (0.10-degrees for the nucleophile and no Nu-C bond leng thening. The leaving group is deviated by approximately 5-degrees and the C-Cl bond lengthens by approximately 0.05 angstrom. The estimated maximal values are approximately 20-degrees for the angular deviation and approximately 0.20 angstrom for the bond lengthening. These are sl ightly larger than the values observed for identity exchanges and are due to the dissymmetrical nature of the transition structures. Substit uting both the nucleophile and the reaction center gives approximately additive effects. Even with neutral nucleophiles likes amines, AM1 ca lculations still overestimate the basicity and underestimate the nucle ophilicity. Compared with the 6-31G results, the AM1 central barriers , which were slightly on the lower side for charged nucleophiles, beco me systematically too high when the reagent is neutral. This may be tr aced back to the fact that AM1 transition states are not only tighter but also much later than ab initio (6-31 G or 3-21 G) transition stat es, as indicated by the more advanced Walden inversion and LG departur e. The combination of tightness and lateness enhances steric hindrance and raises the central barrier. The 3-21G barriers are again too low, although not so badly for neutral as for charged nucleophiles. In fac t, with the standard 3-21G basis set, carbanions react with RCl withou t a barrier and, in order to obtain transition states, the electronega tivity of the charged carbon atom must be raised (by setting its nucle ar charge Z equal to 6.5 or 6.8).