Triarylsulphonium salts

A 50 ml flask equipped with a reflux condenser and a thermometer was charged with thiophenol (10 mmol), the iodonium salt (20 mmol), tributylamine (2.22 g, 10 mmol) and cupric benzoate (0.2 g). The mixture was stirred and heated at 120-125°C for 3 h under nitrogen. On cooling, the reaction mixture was washed with anhydrous ether (3 x 50 ml) to remove the iodoarenes. The residue was recrystallized from ethanol to give triarylsulphonium salts (48-87%). Similar conditions with slight variations were used when sulphides were the starting material. 

Photo-initiators for cationic polymerization may be classified in four groups the diazonium salts, the diaryliodonium salts, the triarylsulphonium salts, and the mixed ligand arene cyclopentadienyl Fe11 salts. 

Two mechanistic studies relevant to photocuring processes have appeared. One deals with the efficient photoinduced generation of radical cations in solvents of medium and low polarity ". These cations act as sensitizers of the polymerization of N-methylacridinium hexafluorophosphate. The other is a study of the photochemistry of triarylsulphonium salts . 

The photolysis of triarylsulphonium salts yields diarylsulphides and products of lateral nuclear shift reactions which are ortho, meta, and para aryl substituted diaryldisulphides such as (342). A by-product in these reactions is a proton because of this these reactions have been applied to photoinitiation of cationic polymerisations. A full paper describing a detailed study of the reaction mechanism has been published.In addition, the product distribution obtained by photolysis of triphenylsulphonium salts in films of the polymer of 4-(tert -butoxycarbonyloxy)styrene has been compared with that obtained in solution.The synthesis of some new triarylsulphonium salts and their application for photoinitiation of cationic polymerisation has also been reported.The formation of the products arising from lateral nuclear shifts in sulphonium salts occurs under direct photolysis but not under triplet sensitisation. 

Benzobenzvalene (100) has been shown to add sulphur dioxide to give the sulphone (101) and the 7-sultine (102) both adducts extrude sulphur dioxide on direct photolysis regenerating benzobenzvalene accompanied by naphthalene. Competing homolytic and heterolytic bond cleavages have been observed in singlet excited triarylsulphonium salts, and intramolecular electron transfer is involved in the photodissociation of p-nitrobenzyl 9,10-dimethoxy-anthracene-2-sulphonate.A photochemical method for the detosyl-ation of sulphonamides has been described,and the thermolysis and photolysis of some thiourea derivatives have been compared. 

Factors influencing the reactivity of triarylsulphonium salts include counterion type and the nature of ring substituents. For a series of triphenyl sulphonium salts, reactivity was found to vary with the counterion in the order SbF > AsF > PFj > BFj". A reaction scheme proposed to account for the activity of these salts as initiators of both free-radical and cationic polymerizations is outlined in reactions (D—(3),... 

Photo-initiation for A -vinylcarbazole with azoisobutyronitrile or benzoyl peroxide leads to simultaneous radical and cationic polymerizations, as when the peroxide is used as a thermal initiator with this monomer. Triarylsulphonium salts can initiate both types of polymerization in some systems. The use of sources of radicals for indirect initiation of cationic polymerization has been reviewed. ... 

A reaction scheme similar to that outlined above has been proposed to account for the photo-initiation of cationic polymerizations by triarylsulphonium salts, and a systematic comparison of the [x>Iymerization behaviour of various epoxides in the presence of sulphonium and iodonium salts has been conducted. More recently the concept of sulphonium salt photo-initiators has been extended to include thiopyrylium salts. ... 

Some comments should be added for a few cases. The reaction of diphenyliodo-nium tetrafluoroborate with sodium thiocyanate, and sodium phenylsulphinate, was also carried out in chloroform-water, at reflux, with very high yields similar efficient phenylation occurred with other anions [84]. Although these reactions were not performed on a preparative scale, it is likely that similar conditions may be applicable to other iodonium salts as well, for example in the thiocyanation of di-(3-thienyl)iodonium ion which gave 3-thiocyanatothiophene (43%), the best yield compared with other methods [18]. Concerning the preparation of triarylsulphonium... 

Mechanistic studies with sulphonium salts concern decomposition kinetics in hydroxylic solvents H- H exchange of the a-protons of methyl and allyl-sulphonium salts, the relative rates of which are strongly dependent upon both solvent and micellar effects radical chain reactions of triarylsulphonium halides and sodium alkoxides, which lead to arenes, anisoles, diaryl sulphides, and aldehydes or ketones and the catalysis of hydrocarbon autoxidation by... 

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