Norrish type 1 Cleavage reactions of cyclobutanethiones and media

Abstract

The thesis entitled "Norrish Type I ?-Cleavage Reactions of Cyclobutanethiones and Photochemical Studies in Micellar Media" has been divided into two parts. Part I deals with the strain-assisted ?-cleavage reactions of three cyclobutanethione derivatives and Part II, the photodimerization and fluorescence studies of coumarins in organic, aqueous, and micellar media. The strained ring system discussed in the first half of this thesis is the cyclobutane ring. The ?-bonds of the cyclobutane ring are weaker due to Baeyer’s strain involved in the system and hence the properties of the cyclobutane ring system are different from those of other higher cycloalkane rings. In the photochemical studies of cyclic ketones, this difference has been noticed to lead to the difference in the mode of reaction, the nature of reactive states, and the efficiency of the ?-cleavage process. Chapter 1 of this thesis is a brief review dealing with the photochemistry of electronically excited thiocarbonyls and ?-cleavage reactions of ketones. Various reactions of thiocarbonyls reported to date are summarized and compared with those of carbonyls. The Norrish Type I ?-cleavage reaction is the least studied transformation of thiocarbonyls. The various factors responsible for the ?-cleavage process in carbonyl systems are outlined. Chapter 2 describes the photochemistry of 2,2,4,4-tetramethylcyclobutanethione (3a). The reactive state is identified as O(n, ?*) triplet. Formation of product 2 suggests the occurrence of ?-cleavage in 4. Diradical (3a) is suggested to be the primary intermediate. Failure of this diradical intermediate towards ring expansion to thiacarbene 4, one of the common reactions of 1,4-diradical (3a, S=O) derived from cyclobutanone (1, S=O), has been analyzed using Salem correlation diagrams for ?-cleavage of cyclic ketones. The diradical 3 derived from the excited state (T?) of 3a is suggested to be a different diradical. Chapter 3 presents Norrish Type I ?-cleavage reactions of dimethylthioketene dimers 5a and 6a. Occurrence of ?-cleavage process in these systems has been established. Excited n,?* triplet and singlet states respectively are the reactive states. Diradicals (7a & 8a) and thiacarbenes (9a & 10a) are the reactive intermediates. Unlike the diradicals (7d & 8d) generated from 1,3-cyclobutanedione (5d) and ?-lactone (6d), these (7a & 8a) undergo ring expansion to thiacarbenes (9a & 10a) — the precursors of the observed cyclic thioacetals. Concerted formation of thiacarbene 9a from the excited state of 6a has been suggested, at least in part, for the formation of one of the thioacetals. Characterization of the products and the mechanism of their formation are discussed. Chapter 4 discusses the photofragmentation reactions of tetramethyl-3-thio-1,3-cyclobutanedione (5b) and 3-mercapto-2,2,4-trimethyl-3-pentenoic acid ?-thiolactone (6b). Thiodione 5b shows wavelength dependency and undergoes ?-cleavage with respect to both thiocarbonyl and carbonyl chromophores. Diradicals (7b, 7c and 8c) and carbenes (9b, 9c and 10c) are demonstrated to be the reactive intermediates. Comparative behaviors of various diradicals generated photochemically from 5a–5d and 6a–6d are described. Chapter 5 is a comprehensive account dealing with the structure and properties of micelles and organic photochemical reactions carried out in this medium. Environmental effects on photochemical reactions occurring in various organized media with emphasis on the achievement of modification and control of reactivity through the use of organized media are presented. The differences between photoreactions studied in homogeneous solutions and in constrained media are highlighted. Photodimerization of coumarin (11a) in aqueous and micellar media is the subject described in Chapter 6. Enhanced dimerization and selectivity in the formation of dimer (syn head-head dimer) are attributed to the polar environment in which coumarin undergoes dimerization. Fluorescence emission spectra of coumarin in various media are provided. The extent of water penetration into the micellar aggregates is discussed. The need to explore the unique influence exerted by water on the reactivity of organic molecules is emphasized. Chapter 7 describes the photodimerization of various substituted coumarins (11b–11i) in homogeneous organic solvents and in ionic micellar media (SDS & CTAB). Many of these coumarins upon both direct and sensitized irradiations dimerize to head-tail dimers. Characterization of the product dimers and the origin of their formation are outlined. The nature of the dimers formed in homogeneous solutions and in micellar media are compared. The results presented in this chapter clearly suggest the absence of preorientational effects of micellar aggregates on the solubilized substrates. Since the recent reports on photoannelation reactions in micelles have demonstrated the utilization of micellar alignment effect, an investigation attempting regioselective dimerization of coumarins was carried out. The results of photodimerization of long-chain alkoxycoumarins (11k–11n) presented in Chapter 8, clearly demonstrate the limitations of micellar alignment effect and indicate that the micellar orientational effect is important only in those systems where the inherent molecular forces controlling regiochemistry are weaker than hydrophobic association energies introduced by the alkyl chains. In Chapter 9, solvent polarity-dependent fluorescence emissions of alkoxycoumarins (11i–11n) in organic solvents, solvent-water mixtures, and various micellar media are described. Evidence to the solubilization sites of these coumarin molecules in the micelles is provided. Polarity of the micellar interfacial region is discussed.