Excited state behaviour of a,b – unsaturated thiones

Abstract

The thesis entitled “Excited State Behaviour of ?,?-Unsaturated Thiones” consists of six chapters. Chapter I provides a brief introduction to the work presented in this thesis. Chapters II–V describe the photophysical and photochemical studies carried out on ?,?-unsaturated thiones, while the final chapter (Chapter VI) presents a perturbation molecular orbital (PMO) analysis of thermal and photochemical cycloaddition reactions of thioketones. Thioenones, which are formal analogues of enones, have received relatively little attention compared to enones. Considering the extensive chemistry exhibited by enones, a systematic study of thioenones is of considerable interest. After careful examination of a large number of thioenones, a systematic investigation was initiated on the excited-state behaviour of thioenones 1–5 (Fig. S). The studies carried out on these systems are presented in this thesis. Chapter I includes a brief survey of the literature on the photochemistry of thiocarbonyls. The main features of their characteristic reactions and the differences in reactivity from the lower and upper excited states of thiocarbonyls are highlighted. Results from earlier scattered reports on thioenones are discussed, and the need for a systematic investigation to understand their excited-state behaviour is emphasized. Chapter II summarizes the photophysical studies carried out on thioenones 1–5. In particular, the absorption and emission characteristics of thioenones 1–5 and the results of time-resolved studies on compounds 2–4 are presented. The important features recognized in the photophysical behaviour of thioenones include: a) enhanced oscillator strength for absorption, b) fluorescence from the second excited singlet state (S? ? S?), c) phosphorescence from the lowest triplet state (T? ? S?), d) efficient quenching of triplet states by oxygen and ground-state thioenone (self-quenching), and e) high efficiency of intersystem crossing (???c ? 1) and singlet oxygen generation. Chapter III presents a detailed study on the photooxidation of ?,?-unsaturated thiones. Direct excitation of compounds 1–5 in an aerated atmosphere into the n??* band results in the formation of the corresponding ketones. These thiones do not form the corresponding sulfines under these conditions, although such products are expected based on earlier studies on the oxidation of diaryl, arylalkyl, and dialkyl thiones. The absence of sulfines in these cases is neither due to their instability nor due to their ready oxidizability, as confirmed by independent experiments. The involvement of singlet oxygen as the oxidizing species is suggested by singlet oxygen quenching and sensitization experiments. Formation of singlet oxygen is proposed to occur through energy transfer from the lowest n?* excited triplet state of the thioenone. The mechanism of triplet thioenone-to-triplet oxygen energy transfer is discussed based on the high values of the rate constant for quenching of thione triplets by O? and the efficiency of singlet oxygen generation, as obtained from laser flash photolysis studies. The measured rates of singlet oxygen quenching by compounds 1–5 are similar to those reported for arylalkyl and dialkyl thiones. Formation of ketones appears to be preceded by a 1,2,3-dioxathietane intermediate, as sulfur and sulfur dioxide—products of its decomposition—have been isolated from all the thiones investigated. Possible reactive intermediates resulting from the interaction between the n-orbital of the thione and singlet oxygen are discussed. This study demonstrates that the thiocarbonyl chromophore is the primary site of attack by singlet oxygen, while the adjacent C=C double bond remains inert under these conditions. Chapters IV and V describe the photochemical behaviour of ?,?-unsaturated thiones toward olefins. Among the thioenones 1–5, 1,1,3-trimethyl-2-thioxo-1,2-dihydronaphthalene (compound 4) was found to be most suitable for these studies. Chapter IV presents studies with electron-deficient olefins. Upon excitation, electron-deficient olefins undergo cycloaddition to the thiocarbonyl chromophore of thioenone 4, preferentially from the less hindered side, yielding thietanes (compounds 6–19, Fig. S). Thietane formation is both stereospecific and regioselective. This addition is suggested to originate from the second excited singlet state. Possible routes for the interaction of the excited singlet thioenone and olefin, and for thietane formation, are discussed. Exciplex intermediacy is inferred from the dependence of fluorescence quenching rate constants on the electron-acceptor properties of the olefin. The observed site specificity and regioselectivity are rationalized using PMO theory. Interestingly, the photochemical behaviour of thioenones toward electron-poor olefins is unique and differs significantly from that of enones. In enones, electron-poor olefins typically add at the olefinic centre to yield cyclobutanes, and oxetane formation rarely competes. Chapter V summarizes the photochemical behaviour of thioenone 4 toward electron-rich olefins. Addition occurs at the thiocarbonyl chromophore, yielding thietanes (compounds 20–23) and 1,4-dithianes (compounds 24–26). The ratio of thietane to dithiane depends on irradiation conditions and the nature of the olefin. Product characterization and mechanistic details are discussed. This cycloaddition is suggested to originate from the lowest triplet state (T?, n?*). A 1,4-prethietane biradical is proposed as an intermediate, which can be trapped by a second molecule of thioenone to yield 1,4-dithiane. Results from photocycloaddition of compound 4 to electron-rich olefins contrast sharply with those for electron-poor olefins. Chapter VI presents a comprehensive qualitative molecular orbital treatment of thermal and photochemical cycloaddition reactions of saturated and conjugated thioketones. Orbital coefficients and energies of several saturated, ?,?-unsaturated, and aromatic thiones, olefins, and dienes are calculated using the MINDO/3 formalism. These data on isolated reactants are used to explain reactivity and regioselectivity. Seven categories of thermal (4+2) cycloaddition reactions involving thiocarbonyl compounds—classified based on substituents on the diene and dienophile—are considered. The PMO method, combined with MINDO/3 molecular orbital energies and coefficients, successfully rationalizes the regiochemistry of thermal cycloadditions of thiocarbonyls. In particular, the predicted regiochemistry for (1) (4+2) cycloaddition reactions of saturated thiones with 2-substituted dienes and (2) dimerization of ?,?-unsaturated thiones agrees with experimental observations. This analysis also provides unequivocal predictions for thermal cycloadditions not yet studied experimentally. Key predictions include: 1. Thermal cycloaddition reactions of saturated thiones with 1-substituted dienes should be highly regioselective. 2. The dienophilic nature of transoid ?,?-unsaturated thiones and aromatic thiones should resemble that of saturated thiones; hence, their thermal cycloadditions with substituted dienes should exhibit similar regioselectivity. 3. Thermal cycloaddition of cisoid ?,?-unsaturated thiones to 1- or 2-substituted dienes should preferentially occur at the more substituted double bond of the diene, although regioselectivity is predicted to be low. Predictions of reactivity (n?* vs ??* excited states) and regioselectivity of saturated thiones in photocycloadditions with electron-rich and electron-poor olefins are also consistent with experimental results. Furthermore, the unique excited-state behaviour of ?,?-unsaturated thiones toward electron-poor olefins is examined, with evidence presented for the important role of secondary orbital interactions. In conclusion, the PMO model convincingly rationalizes three levels of selectivity observed in these systems: • Preferred reaction site (C=C vs C=S), • Regiochemistry of products (2,3-disubstituted thietane vs 2,2?-disubstituted thietane), and • Preferred orientation of substituents (cis vs trans). Based on the work reported here, the related publications are listed below.