Oxidation of thioketones by singlet oxygen

Abstract

The thesis entitled “Oxidations of Thioketones and Thioketenes by Singlet Oxygen” consists of 9 chapters. The first two chapters are a general introduction to the work presented in the thesis, and Chapter 6 is a brief introduction to the oxidation of cumulenes and heterocumulenes by singlet oxygen. Chapters 3–5 present the work carried out on two thioketones, namely di-t-butylthioketone (1) and 2,2,4,4-tetramethylcyclobutanethioketone (2). Chapters 7–9 describe the studies on the oxidation of two thioketenes, namely di-t-butylthioketene (3) and 2,2,6,6-tetramethylcyclohexylidenemethanethioketone (4) by singlet oxygen.

Chapter 1 gives a brief introduction to the chemistry of singlet oxygen, which is used as a reagent in our studies. This chapter is divided into five sections: (i) history, (ii) electronic structure, (iii) methods of generation, (iv) quenching of singlet oxygen, and (v) reactions of singlet oxygen.

Chapter 2 deals with the photoreactions of thioketones. The work thus far reported on the oxidation of thioketones is presented in detail in order to provide a background for the work described in the later parts of the thesis. The need for a systematic investigation to understand the mechanism of oxidation of thioketones by singlet and triplet oxygen is stressed.

Results, discussion, and experimental aspects on the oxidation of di-t-butylthioketone (1) and 2,2,4,4-tetramethylcyclobutanethioketone (2) are presented in Chapters 3, 4, and 5, respectively. Studies were carried out in detail on the photooxidation and oxidation by singlet oxygen of the highly hindered di-t-butylthioketone and the similar but less hindered 2,2,4,4-tetramethylcyclobutanethioketone (2). Surprisingly, these two thioketones showed marked differences in their behavior toward singlet and triplet oxygen.

Oxidation of 1 and 2 by singlet oxygen, generated through dye sensitization and decomposition of triphenyl phosphite ozonide, yielded the corresponding sulfine and ketone. In the case of 1, sulfine is the major product, whereas in the case of 2, ketone is the major product. Along with sulfine and ketone, sulfur dioxide and sulfur have been isolated. Estimation of the former gave a clue to the 1,2,3-dioxathiolane being the possible intermediate in the formation of ketone. Zwitterionic/diradical peroxides are believed to be common primary intermediates for both sulfine and ketone. Steric influence is felt both during primary interaction between singlet oxygen and thioketone and during partitioning of the peroxide intermediate. (The measured rates of quenching of singlet oxygen by 1 and 2 are 1.0 × 10 M¹ s¹ and 3.5 × 10 M¹ s¹, respectively.) Steric interaction is suggested as the reason for variation in product distribution between 1 and 2. A proposed general mechanism for the oxidation of thioketones by singlet oxygen is shown in Scheme 1.

Thioketones 1 and 2 are also oxidized in an aerated atmosphere upon direct excitation. By sensitization and quenching studies, the reactive state of thioketone is identified to be triplet n*. The oxidizing species is inferred to be singlet oxygen through quenching experiments. In addition to the pathway involving singlet oxygen, a direct reaction between triplet thioketone and triplet oxygen, leading to ketone, is also recognized. Singlet oxygen is proposed to result from the energy transfer from triplet thioketone to triplet oxygen. The proposed mechanism for the oxidation of thioketone during direct excitation is shown in Schemes 2a and 2b. Moreover, more experimental work is needed to clearly define various aspects of oxidation of thioketones by triplet oxygen.

Chapters 7 to 9 describe the results, discussion, and experimental part on the oxidation of the two thioketenes, namely di-t-butylthioketene (3) and 2,2,6,6-tetramethylcyclohexylidenemethanethioketone (4), by singlet oxygen. Oxidation of 3 in CHCl/CCl yielded the corresponding thioketene-S-oxide (5), thioketone-S-oxide (6), and 2,2,7,7-tetramethyl-3,6-dione-trans-4-octene (7), and that in methanol gave 6, l-t-butyl-l-pivaloylmethylmethanesulfinate (8), and l-methoxy-3,3-dimethylbutano-2-one (9). Similar oxidation of 4 in CHCl/CHCl gave the corresponding thioketene-S-oxide (10), thioketone-S-oxide (11), and ketone (12), and in methanol ketone (12) and 3,3,7,7-tetramethyl-2-methylsulfinylcycloheptane (13) were formed.

Low-temperature and trapping studies were carried out to understand the mechanism of oxidation. No conclusive evidence in favor of a singlet mechanism was obtained, and an attempt has been made to rationalize the formation of various products during the oxidation of 3 and 4. A general mechanism, shown in Scheme 3, is utilized to understand the formation of various products.

In addition to the above products, isolated gaseous products such as CO, COS, CS, isobutylene, and sulfur have been qualitatively detected and quantitatively estimated. Such estimated yields and their matching yields with the above isolated products provide significant support for the proposed mechanism. Formation of the above products has been rationalized to arise through the involvement of zwitterionic intermediates (Scheme 3), resulting from the attack of singlet oxygen on the sulfur lone pair of the thioketene chromophore. Thionodioxetane is proposed to be the precursor for the ketone.

The difference in behavior between 3 and 4 in the formation of the corresponding ketone is suggested to be due to the difference in the nature of cleavage of the suspected thionodioxetane. The results presented in this part of the thesis have clearly established that the behavior of thioketones is indeed different from that of ketones and ketenimines. A need for further work to obtain a clear picture on the complicated photooxidation mechanism is emphasized.