Directionality of organic reactions in solution

Reactions of Charged Substrates. 8. The Nucleophilic Substitution Reactions of (4-Methoxybenzyl)dimethylsulfonium Chloride

Displacement reactions on the title compound (1) occur only for nucleophiles with intermediate hardness. Nucleophiles that react display a range of mechanisms. 1 reacts with the neutral nucleophile pyridine-d(5) through a mixed S(N)1/S(N)2 mechanism; salt added to control ionic strength affects the rate for the unimolecular process, but has no effect on the bimolecular rate constant. The mechanism of displacement by N(3)(-) and SO(3)(2)(-) depends on the presence or absence of exogenous salt. At constant ionic strength, the mechanism is mixed S(N)1/S(N)2 over most of the range of [Nu]. With nucleophile only present, plots of k(obsd) vs [Nu] exhibit severe breaks that are not the result of salt effects. Analysis of rate constants and product ratios suggests that at low [Nu] reaction occurs simultaneously through concerted Hughes-Ingold S(N)2 and preassociation-concerted mechanisms. At high [Nu], displacement occurs only through the preassociation-concerted mechanism. Comparison of these results with data for gas-phase dissociation of benzyl dimethylsulfoniums and with solution results for benzyl pyridiniums suggests that the intrinsic stability of the intermediate does not necessarily determine the mechanism.

Theory and modeling of stereoselective organic reactions

Theoretical investigations of the transition structures of additions and cycloadditions reveal details about the geometries of bond-forming processes that are not directly accessible by experiment. The conformational analysis of transition states has been developed from theoretical generalizations about the preferred angle of attack by reagents on multiple bonds and predictions of conformations with respect to partially formed bonds. Qualitative rules for the prediction of the stereochemistries of organic reactions have been devised, and semi-empirical computational models have also been developed to predict the stereoselectivities of reactions of large organic molecules, such as nucleophilic additions to carbonyls, electrophilic hydroborations and cycloadditions, and intramolecular radical additions and cycloadditions.