Photochemical Sciences Ph.D. Dissertations

Light as a Reagent for Chemical Reactions-Excited State Manipulation to Discover New Reactivity

Date of Award

2019

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Photochemical Sciences

First Advisor

Jayaraman Sivaguru (Advisor)

Second Advisor

Mihai Staic (Other)

Third Advisor

Malcolm Forbes (Committee Member)

Fourth Advisor

R. Marshall Wilson (Committee Member)

Abstract

Photochemical reactions provide a complementary strategy for the synthesis of complex organic molecules as it often involves multiple bond formation in a single step. However, use of organic photoreaction as a common tool in synthesis is limited due to challenges associated with controlling their excited state properties. Altering the molecular framework has the potential to alter their excited state features opening new reactive pathways. This requires in depth understanding of reactivity and excited state dynamics. This dissertation details an effort to engineer new excited state reactions of organic molecules.

Fundamentals of organic photochemistry and various strategies that were developed in this field are discussed in the first chapter. It details concepts related to organic photoreactions and asymmetric photochemical transformations, which are known to be quite challenging due to short life time and high energetics of the reactive excited states. The chapter details various methodologies that are developed and successfully implemented for controlling photochemical reactions.

Second chapter describes photocycloaddition of an excited alkene to a carbonyl group. Due to the difference in the nature of orbital that initiates the cycloaddition, the reaction was labeled as Transposed Paternò-Büchi reaction. The strategy developed in this chapter is different from normal Paternò-Büchi, where an excited state carbonyl functionality adds to a ground state alkene. In-depth investigations were carried out to decipher the excited state in the reaction pathway with a likely mechanistic rationale. Chapter three details the photocycloaddition of excited alkenes with statistical and nonstabilized imines. This newly uncovered reaction was labeled as Aza Paternò-Büchi reaction. This new reaction was deciphered using photophysical studies. The reactive modes were analyzed by detailed mechanistic investigations.

Fourth chapter details entropic effects as a tool to control chemoselectvity involving intramolecular [2+2] vs [3+2]-photocycloaddition. The physical properties for entropic control were deciphered due to [2+2] and [3+2] cycloaddition.

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