Photochemical generation of electron poor reactive intermediates: Photochemistry of S-alkoxy dibenzothiphenium tetrafluoroborates, N-alkoxy pyridinium perchlorates, and Sulfonium ylides
Photolysis of thiophene ylides such as dibenzothiophene ylide (DBTY) and 2,5-dichlorothiophene ylide (DCTY) provides a route to certain carbenes with adjustable photophysical parameters, in that the absorption spectrum of the precursor and the initial spin population of the carbene can be varied. However, the accumulated thiophene from extensive photolysis can act as a triplet sensitizer leading to the generation of relatively high proportion of triplet carbene. It is demonstrated that dibenzothiophene has this property and 2,5-dichlorothiophene does not.
S-alkoxy dibenzothiophenium and N-alkoxy pyridinium salts are prepared and photolyzed to explore the potential homolytic and heterolytic paths of the photochemical S-O cleavage of the thiophenium salts. The heterolytic path would generate a highly unstable electron-deficient alkyl oxenium ion and the corresponding neutral leaving group. Homolysis would generate an alkoxy radical and the corresponding radical cation of the leaving group. The evidence obtained suggests the homolytic formation of alkoxy radical and radical cation of the leaving group though some fraction of the oxenium and the neutral leaving group cannot be ruled out completely.