Photons in Action: Asymmetric Synthesis and Polymer Degradation

Abstract

Light is an environmentally benign and renewable source of energy that finds wide application in the field of science. This dissertation explores two areas of chemistry that utilizes light extensively viz., asymmetric phototransformations and photodegradation of polymers. Phototransformation is an elegant method to construct structurally complex and diverse organic scaffolds. However, controlling the excited state in phototransformation to manipulate its stereochemical outcome is a challenge. This dissertation discloses a unique method employing atropisomeric chromophore to tackle asymmetric induction from the excited state. Photodegradation is a safe method to breakdown polymers that pose huge environmental and ecological concerns. Apart from designing polymers with a phototrigger that initiates the breakdown in a programmed fashion, this thesis also demonstrates recovery and reuse of monomers making the strategy sustainable. Chapter 1 describes the importance of light and basic principles involved in organic phototransformations. In this section, principle differences between asymmetric thermal and photochemical transformations are introduced, methodologies developed in asymmetric phototransformations and the role of light in medicinal/biological system and material science is presented. Chapter 2 evaluates metal free, thiourea/urea organocatalyst in enantioselective 6π- photocyclization of acrylanilides. Preliminary investigations revealed that the asymmetric induction imparted by thiourea was low. Detailed photophysical analysis provided valuable information on the excited state interaction of the substrate with the catalyst, opening avenues for future development of this strategy. Chapter 3 demonstrates “axial-point chiral” strategy towards atropselective Paternò-Büchi reactions of oxoamides and chain length dependent [2+2] vs. [5+2]-photocycloaddition of atropisomeric maleimides. Axial chirality dictated high enantioselectivity (>97 %) in the photoproduct, while the solvent and substitution in the reactant controlled the diastereomeric ratio in photoproducts respectively. Chapters 4-5 report the photodegradation of bio-derived polymers using phototriggers. This method not only enabled us to deconstruct the polymers to its functional monomer(s) but also enabled a pathway to recover and recycle the monomer highlighting the sustainability of the strategy. In summary this thesis details the role of light in asymmetric phototransformations using organocatalyst and atropisomeric chromophores leading to chiral photoproducts. Further, it describes the photodegradation of biomass-derived polymers and its recoverability and reusability of the monomer as a sustainable approach.