Photoisomerization of 4-Hydroxypyrylium Cations in Concentrated Sulfuric Acid

Article abstract of DOI:10.1021/jo00197a040

Irradiation of di-, tri-, and tetraalkyl-4-hydroxypyrylium cations in concentrated sulfuric acid leads to the formation of 2-hydroxypyrylium cations as phototransposition products and, in certain cases, to furyl cations by a photo-ring-contraction reaction.Product analysis by spectroscopic techniques, deuterium labeling, and unambiguous synthesis reveals that 2-hydroxypyrylium cations are formed by two distinct transposition patterns and accordingly, by two distinct mechanistic pathways.The bond-forming and -breaking requirements of the major transposition pattern, which constitutes approximately 95percent of the reaction, are consistent with a mechanism initiated by 2,6-bridging in the first excited state of the 4-hydroxypyrylium cation.Similarly, the bond-formation and -breaking requirements of the minor pattern are consistent with a mechanism involving 2,5-bridging in the first excited state of the starting cation.