Redox auxiliary mediated catalysis of organic thermal reactions. Stilbene isomerization and Aza-claisen rearrangement

Loading...
Thumbnail Image
Date
2018
Journal Title
Journal ISSN
Volume Title
Publisher
University of Alabama Libraries
Abstract

This work focuses on the development of new general methods of reducing the activation barriers of organic thermal reactions using a diarylamino redox auxiliary (RA) group. Electron removal from the RA weakens bonds and/or changes the conformation of the appended structure to activate its chemical transformation. The electron transfer activation is catalytic, hence the name redox auxiliary catalysis (RAC). This work involves the development of RAC for two thermal organic reactions which normally require high temperatures: stilbene isomerization and the aza-Claisen rearrangement. The outcome is to enable these reactions at room temperature. Stilbenes may exist as cis and trans isomers, and photochemical switching between these forms allows for stilbenes use in molecular machines, light-controlled storage devices, and molecular switches including light responsive biological systems. The thermal cis to trans isomerization of stilbenes requires a large activation barrier of ∼46 kcal/mol, which corresponds to a half-life of ∼1012 years at 303 K. As a result, the thermal isomerization of cis-stilbene is negligible at room temperature. This barrier to isomerization can be dramatically reduced using RAC, thus yielding a photo electrochromic stilbene which is switched in one direction with photons and in the other direction with electron loss. The oxidized RA unit (RA.+) preferentially stabilize twist transition state, thereby reducing the isomerization barrier. Electron transfer between RA.+-trans-stilbene and RA-cis-stilbene completes a catalytic cycle. To date, we have synthesized eight different RA-stilbenes containing electron donating and withdrawing substituents. The RAC cis to trans stilbene isomerization rate enhancement is found to be ≥ 105. RA catalysis has also been investigated as a way to promote [3,3]-sigmatropic rearrangement of 1,5-hexadienes (Cope rearrangement) at room temperature. The parent Cope substrate has an activation barrier of ∼33.5 kcal/mol and requires temperatures above 200 ºC. Our RA-appended diene undergoes a facile [3,3]-sigmatropic rearrangement at room temperature upon a one electron oxidation of the RA to give predominantly an aza-Claisen product instead of the anticipated Cope product.

Description
Electronic Thesis or Dissertation
Keywords
Organic chemistry
Citation