Computational studies of nucleophilic substitution reactions, particularly involving cinchona alkaloid catalysis.

摘要

This dissertation describes computational studies of several organic reactions, with an emphasis on nucleophilic substitution, amines, and enantioselectivity induction by cinchona alkaloids.; Chapter 1 on provides an introduction to application of computational methods in chemistry.; Chapter 2 studies energetics of complex formation between Cram's carceplex I and acetonitrile at high pressures. The conclusion is that the complex formed at 12 kbar and 100°C contains three acetonitrile molecules and the conformation in which at least one of the "French doors" is open.; Chapter 3 suggests that enantioselective alpha-ketoesters hydrogenation on cinchona alkaloid modified platinum proceeds through nucleophilic addition of an alkaloid to an ester. This is in contrast with the widely accepted mechanism involving hydrogen bonded complex formation. The conclusion is drawn based on B3LYP density functional calculations and literature review.; Chapter 4 is a DFT comparison of nucleophilic versus base catalyzed additions of alcohols to ketenes. The trimethylamine, pyridine and N-methylimidazole are used to model cinchonidine and "planar-chiral" bases developed by Fu. The results indicate that in non-polar solvents, trialkylanmines catalyze the reaction acting as bases, while for stronger nucleophiles, the two possible mechanisms compete. The competition results in strong dependence of enantioselectivity on temperature.; Chapter 5 estimates the origins of steric effects and the role of solvation in SN2 reactions. The models studies are ethyl and neopentyl chlorides. The CBS-QB3 calculations and explicit solvation computed by Monte Carlo simulations show that solvation effects have minor, if any, influence on steric retardation in SN2 reactions.; Chapter 6 estimates the influence of alkoxy- and thio- substituents as well as ring strain on the rates of oxy-Cope rearrangements, by means of both experiment and theory.