Wenhong Lin

Date of Award

9-13-2023

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

Advisor(s)

John Chisholm

Keywords

imidates;organic chemistry

Subject Categories

Chemistry | Physical Sciences and Mathematics

Abstract

Imidate alkylating reagents are known to be effective for a wide range of nucleophiles under acidic or thermal conditions. These alkylation reactions may even proceed spontaneously in some cases, as has been observed with the formation of esters from carboxylic acids. This work is focused on further exploring the synthetic utility of imidate alkylating reagents. A general overview of imidate is given in Chapter 1. Some of the important transformations using imidates are briefly discussed. This background provides a basis for understanding the reactivity of this functional group, as both a nucleophile and a leaving group. Building on this work, some esterifications using N-aryl trifluoroacetimidates under promoter free condition was explored. This involved the synthsis of a number of N-aryl trifluoroacetimidates and the evaluation of their reactivity with some carboxylic acids. In Chapter 2, a general method for preparing 2-trimethylsilylethyl (TMSE) esters using 2-(trimethylsilyl)ethyl 2,2,2-trichloroacetimidate under thermal conditions is described. The 2-(trimethylsilyl)ethyl 2,2,2-trichloroacetimidate was easily formed under mild conditions with catalytic DBU from 2-(trimethylsilyl)ethanol. The use of TMSE protecting group was advantageous during some total syntheses, as this protecting group may be easily cleaved using fluoride ions under mild conditions that do not affect other alkyl and benzyl esters. Previously a number of studies on the use of 2,2,2-trichloroacetimidates as electrophiles in N-alkylation reactions have been disclosed. More recently, alkylation of ambident nucleophiles using imidates has been shown to lead to unusual selectivity under some reaction conditions. Building on this work, a new alkylation method for 1,2,3-triazoles using 2,2,2-trichloroacetimidate electrophiles is described in Chapter 3. This alkylation is catalyzed by a Brønsted acid and is regioselective with a strong preference for the N2 alkylation product. This unusual regioselectivity as the N2 alkylation product is typically the minor product from alkylations under basic conditions employing alkyl halides. The isomeric ratio is sensitive to both the reaction solvent and reaction concentration. Optimal results were obtained with a non-polar solvent at low concentrations.

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