A THEORETICAL-STUDY OF ALKYL SUBSTITUENT EFFECTS IN INTRAMOLECULAR SN2 REACTIONS .1. TRANSITION STRUCTURES OF (CL-+RCL) REACTIONS

In order to assess the feasibility of intramolecular S(N)2 reactions, it is necessary to determine: (a) the transition structures for substi tuted systems in the absence of any constraints; and (b) the ''permiss ible'' deviations from the ideal transition structures. In an attempt to partially answer point (a), this paper examines the influence of al kyl substituted reaction centers on the geometry of the transition sta tes. The problem of substituted nucleophiles and point (b) are address ed in the two accompanying papers. 3-21G and/or AM1 SCF calculations w ere carried out for the reactions of Cl- with MeCl, EtCl, n-PrCl, i-Pr Cl, s-BuCl, i-BuCl, t-BuCl, 2-Me-BuCl and t-Bu-CH2Cl. AM1 calculations , which apparently underestimate the nucleophilicity and overestimate the basicity of the chloride ion, give tighter transition structures a nd higher ''central barriers'' (i.e. energy differences between transi tion states and nucleophile-substrate clusters) than 3-21G calculation s. Comparison with higher level calculations suggests that 3-21G energ ies and AM1 structures are not very reliable. Solvating the system or improving the basis set substantially raise the central barrier but wi ll produce little effect on the transition structures of identity exha nges. If, however, only the nucleophile (leaving group) is solvated, t he TS will tighten (or loosen) by approximately 0.05 angstrom. The str uctural and energetic trends are nevertheless almost the same for AM1 and 3-21G calculations. Putting one methyl group on the reaction cente r induces an angular deviation of approximately 80-degrees, and a leng thening of the C-Cl bond of approximately 0.06 angstrom. With two meth yl groups, the angular deviation and the bond lengthening increase to approximately 10-degrees and approximately 0.13 angstrom, respectively . Excluding S(N)2 reactions with tertiary halides, the maximal C-Cl le ngthening is extimated to be ca. 0.15 angstrom and the maximal angular deviation ca. 20-degrees. Each methyl substituent raises the central barrier by approximately 4.5 kcal/mol. Replacement of a methyl by an e thyl group leaves the transition geometry practically unaltered but in creases the central barrier (1-3 kcal/mol). The reliability of these t rends is discussed.