Aliphatic thiols, photolysis

Irradiation of aliphatic thiols, sulfides, and disulfides with a mercury lamp produces gaseous products identified by the mass spectrograph. Thiols are the least stable to light, with the formation of hydrogen95-129 as the main product. Sulfides and disulfides yield, as the predominant products, saturated hydrocarbons of structures corresponding to the smallest alkyl radical attached to sulfur. Haines et al.95-97 have offered a mechanism to explain the predominant production of hydrogen during photolysis of thiols. 

Analogous [4-1-1] annulations were also obtained starting from isonitriles and f-cyano-substituted sulfanyl radicals, generated either by hydrogen abstraction from aliphatic thiols or (more profitably) through photolysis of aromatic disulfides [18]. The reactions afforded thieno- (24) and benzothienoquinoxalines (25), respectively (Scheme 10). 

The photolysis of 0-acyI benzophenone oximes in isopropyl alcohol in the presence of r-butyl thiol leads to overall reductive decarboxylation. Yields for simple aliphatic and amino acids are reported to be good. It is especially noteworthy that reductive decarboxylation of 2- and 4-quinolinecarboxylic acids (although not of the 3-isomer, nor 1- or 2-naphthalenecarboxyIic acids) was achieved in moderate yield by this method (equation 9). 

Hydrogen atom donors such as non-nucleophilic tertiary thiols or tri-n-butyltin hydride are extremely efficient traps for the capture of the alkyl radical R derived from O-acyl thiohydroxamates, thus providing a very efficient method for reductive decarboxylation (Scheme 3). In practical terms, the use of the mercaptan is preferred since the tertiary alkyl pyridyl disulfide can be easily removed during work up by a simple acid extraction. The reaction has been successfully applied to a very wide range of complex substrates [8] possessing primary, secondary, or tertiary aliphatic carboxylic acids, and reactions at room temperature or below require only photolysis from a simple tungsten lamp and often involve in situ O-acyl thiohy-droxamate derivatization. 

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