Photochemical studies on thiocarbonyl compounds

Abstract

The thesis entitled "Photochemical Studies on Thiocarbonyl Compounds" consists of five chapters. The photochemical processes studied are Norrish Type I (?-cleavage) and photooxidation reactions. The thiocarbonyls chosen for the investigation of ?-cleavage reaction are ?,??-dispiro-substituted cyclobutane-1,3-dithiones and thiocarboxylic acid derivatives which include ?-dithiolactones and thioesters. The photooxidation reaction of cyclobutanethioketones revealed the effect of steric and electronic influences on the product distribution. In Chapter I, the photochemical reactions of thiocarbonyl compounds and ?-cleavage reaction of ketones are briefly discussed. Of the various photoprocesses discussed and compared with those of carbonyls, the ?-cleavage reaction is the least studied transformation of thiocarbonyls. Except for a few, most of the thiocarbonyls investigated so far have resisted ?-cleavage reaction from both lower and higher excited states. Based on the studies of the photochemical behaviour of di-t-butylthioketone, the resistance of thiocarbonyls to ?-cleavage is attributed to two factors, namely the thermodynamic and electronic factors. Possible methods of bringing about ?-cleavage in thioketones are also outlined. In Chapter II, the photochemistry of ??-dispiro-substituted cyclobutane-1,3-dithiones is presented. The interest in the cyclobutane system is due to the characteristic photobehaviour exhibited by the cyclobutanones. The differences in the photobehaviour of cyclobutanones with respect to higher cycloalkanones have been attributed to the ring strain. Further, the photochemical reactions of ??-dispiro-substituted cyclobutane-1,3-diones have established the effect of spiro ring sizes on the nature of the photoproducts and on the site preferred for the ?-cleavage. The possibility of ?-cleavage has already been established for cyclobutanethioketones 5 and 6; hence the present investigation on dispiro-substituted cyclobutanethioketones 1–3 is carried out to obtain information regarding the effect of spiro ring sizes on their photobehaviour. Attempts to synthesise the diketone analogue of 5 were unsuccessful; therefore, the photobehaviour of 3 could not be examined. The diketone 4 was chosen to obtain information regarding the selectivity on the site of cleavage. The cyclobutane dithiones 1, 2, and 4 produced respective sulfur-incorporated adducts and ?-dithiolactones 8 when irradiated in both hydroxylic and non-hydroxylic solvents. The formation of thioacetals 9 and 25 occurred when the irradiations were carried out in hydroxylic solvents. The formation of these is interpreted in terms of a cleavage of the ?-bond to the thiocarbonyl chromophore producing a 1,4-diradical as the primary intermediate VI. The formation of thioacetal is interpreted to arise from thiacarbene intermediates 13 and 14 and ?-dithiolactones from the diradical 12, which is believed to be in equilibrium with the initially formed 1,4-diradical VI. The photobehaviour of dithiones 1, 2, and 4 is found to be identical to that of 2,2,4,4-tetramethylcyclobutanethiones 5 and 6. The present study has established that the ?-cleavage is a common process among cyclobutanethiones. Chapter III discusses the ?-cleavage reaction of ?-dithiolactones preceded by an introduction to photochemical reactions of carboxylic acid derivatives like lactones and anhydrides. Spiro-substituted ?-dithiolactones 15–17 were chosen for the investigation. The results obtained revealed that these thiolactones undergo cleavage selectively at the C–S bond. The nature of the photoproducts is found to be identical to those that have been isolated from their respective cyclobutanedithiones (1, 2, and 3) and from the ?-dithiolactone 18. The formation of sulfur-incorporated adducts 7 and respective cyclobutanedithiones is explained by a stepwise mechanism involving 1,4-diradicals 12 and 14. The formation of thioacetal of the type 10 is attributed to a concerted process in which the excited-state ?-dithiolactone molecules produce thiacarbene 14. In Chapter IV, after a brief introduction to photochemical reactions of carboxylic esters and thionoesters, the investigations carried out with a few aryl alkyl and dialkyl thioesters are presented. Earlier reports on thiocarbonates and thioxanthates, which contain C–X groups, suggested the occurrence of photoelimination reaction by the ?-cleavage. Further, our investigations on ?-dithiolactones for which the ?-cleavage reaction has been established prompted us to explore the photochemical reactions of thioesters. Photolyses of ethyl dithiobenzoate (19) in alcoholic solvents produced respective thionoesters (23). Irradiation in cyclohexane gave rise to an aryl alkyl ketone (24). The transesters formed upon photosolvolysis reaction are attributed to a process in which the nucleophilic solvent molecule attacks the polarised thiocarbonyl carbon atom under the excitation condition. However, the formation of aryl alkyl ketone 24 can occur only if the dithioester undergoes cleavage of a C–S bond to produce a thiobenzoyl and alkylthiyl radicals. The thiobenzoyl radical coupled with the solvent radical would produce an aryl alkyl thioketone which probably undergoes photooxidation under the experimental conditions. This is supported by the isolation of other oxidation products such as ethylthiolobenzoate (25) and methyl benzoate (26). Methyl dithioacetate (20) when photolysed in alcoholic solvents produced respective thionoesters only in methanol and ethanol. Similarly, ethyl dithiopivalate (21) exhibited photosolvolysis only in methanol. The observed photobehaviour of 20 and 21 can be attributed to the steric hindrance provided by the groups ? to the thiocarbonyl carbon atom to the incoming nucleophilic solvent molecule. Photolyses of 20 and 21 in solvents such as acetonitrile and cyclohexane resulted in total Polymerisation. Methyl thionobenzoate (22), similar to its dithioester, produced phenyl cyclohexyl ketone when photolysed in cyclohexane solution. The formation of transester is observed only in methanol irradiation. This is rationalised in terms of the effect of the electronegative oxygen atom on the polarised thiocarbonyl chromophore. Selective excitation and sensitisation experiments have shown that the thioesters exhibit solvolysis and ?-cleavage reactions from ?,?* level depending upon the nature of the solvent as well as the substrate. In Chapter V, the photooxidation process of cyclobutanethiones is discussed. Thiones 1, 2, 6a and 7 were chosen for the study. Based on the knowledge of the photooxidation of thioketones that have already been studied, the products of photooxidation of the above thioketones are rationalised. The oxidation was brought about by dye sensitisation, triphenylphosphite ozonide decomposition and direct excitation. The results obtained established the influence of steric interactions provided by the various sizes of the substituents placed at the ? and ?? centres as well as the electronic effect due to the presence of an additional chromophore (thiocarbonyl and carbonyl) at the ?-position of the cyclobutanethiones.