Excited state behaviour of substituted cyclopropenethiones and thioketenes and photochemical investigations in cyclodextrins

Abstract

The thesis entitled “Excited State Behaviour of Substituted Cyclopropenethiones and Oxyketenes and Photochemical Investigations in Cyclodextrins” is divided into three parts. Part A deals with the strain?assisted ??cleavage reactions of substituted cyclopropenethiones. Part B with the higher excited?state behaviour of thioketenes and Part C with the effect of cyclodextrin encapsulation on the Norrish Type?II and Type?I reactions of arylalkyl ketones in aqueous media. Strain?assisted ??cleavage reactions of thiocarbonyls have been of considerable interest in our laboratory. In order to generalise this newly discovered ??cleavage reaction originally observed in the case of cyclobutanethiones, another strained system namely substituted cyclopropenethiones was selected. Part A deals with the study of ??cleavage reactions of substituted cyclopropenethiones (1a–e). Chapter?1, which forms a general introduction to Part A and Part B, is a concise review on the photochemistry of thiocarbonyl compounds. Chapter?2 presents a summary of photochemical investigations on substituted cyclopropenethiones (1a–e). Occurrence of ??cleavage in these systems has been established; the reactive state has been identified to be (??*). Diphenylcyclopropenethione (1a) on photoexcitation yields 2–4, while alkylphenylcyclopropenethiones (1b–e) afford 5–6. These products are suggested to originate via the ??cleavage process and their formation is believed to involve diradicals and carbene as reactive intermediates. Alkylphenylcyclopropenethiones (1b–e) undergo regioselective ??cleavage to form the less stable diradical/carbene. This anomalous regioselective ??cleavage has been rationalised on the basis of funnel theory. Singlet oxygen oxidises substituted cyclopropenethiones to form cyclopropenones which on photolysis afford acetylenes. Photoextrusion of carbon monosulfide is not observed. Characterisation of the products and the mechanism of their formation is discussed. Chapter?3 is a brief exposition dealing with the light?induced reactions of allenes, ketenes and heterocumulenes. Different approaches to the generation of 4??antiaromatic thiirenes are briefly presented and the photochemistry of the parent thioketene at low temperatures in argon and nitrogen matrices is highlighted. Chapter?4 summarises the higher excited?state reactions of sterically encumbered thioketenes (7a–c) in solution. The reactive state is suggested to be (??*). Substituted thiiranes (8), thioesters (9), thioacetophenones (10) and diones (11) are formed on photoexcitation of thioketenes (7) in hydroxyllic and non?hydroxyllic solvents. Thioketene (7) is unreactive upon excitation to produce thiirenylylidene carbene and zwitterionic intermediates upon excitation to S?. Characterisation of products and the mechanism of their formation are discussed. A comparative account of the excited?state behaviour of ketenes, allenes and thioketenes is presented. Chapter?5 is a comprehensive account dealing with the structure and properties of inclusion complexes and their influence on organic reactions. The emphasis is on reactions conducted in cyclodextrins. Environmental effects on thermal and photochemical reactions occurring in various organised media (e.g., chlorination, photo?Fries rearrangement, etc.) are presented. The differences between the reactions studied in homogeneous solutions and in inclusion complexes are highlighted. Chapter?6 deals with the effect of cyclodextrin encapsulation on the photoreactivity of arylalkyl ketones (12a–e). The structure of the complex has been inferred from ¹H?NMR. The dissociation constants were also measured. The effect of the size of the guest molecule is manifested in the variation in the elimination?to?cyclisation ratio of the Norrish Type?II products in p?cyclodextrin complexes of arylalkyl ketones (12a–e). Norrish Type?I reaction of ?,??diethyl?substituted ketones is significantly retarded inside the cyclodextrin cavity. A reasonable explanation to the above observations has been forwarded.