2-Diazo-1-(4-hydroxyphenyl)ethanone: A Versatile Photochemical andSynthetic Reagent

摘要

α-Diazo arylketones are well-known substrates for Wolff rearrangement to phenylacetic acids through a ketene intermediate by either thermal or photochemical activation. Likewise, α-substituted p-hydroxyphenacyl (pHP) esters are substrates for photo-Favorskii rerrangements to phenylacetic acids by a different pathway that purportedly involves a cyclopropanone intermediate. In this paper, we show that the photolysis of a series of α-diazo-p-hydroxyacetophenones and p-hydroxyphenacyl (pHP) α-esters both generate the identical rearranged phenylacetates as major products. Since α-diazo-p-hydroxyacetophenone (>1a, pHP N2) contains all the necessary functionalities for either Wolff or Favorskii rearrangement, we were prompted to probe this intriguing mechanistic dichotomy under conditions favorable to the photo-Favorskii reangement, i.e., photolysis in hydroxylic media. An investigation of the mechanism for conversion of >1a to p-hydroxyphenyl acetic acid (>4a) using time-resolved infrared (TRIR) spectroscopyclearly demonstrates the formation of a ketene intermediate that is subsequentlytrapped by solvent or nucleophiles. The photoreaction of >1a isquenched by oxygen and sensitized by triplet sensitizers and the quantum yieldsfor >1a–>c range from 0.19 to a robust 0.25. Thelifetime of the triplet, determined by Stern-Volmer quenching, is 15 ns with arate for appearance of >4a of k = 7,1× 10⁶ s⁻¹ in aq. acetonitrile (1:1 v:v).These studies establish that the primary rearrangement pathway for>1a involves ketene formation in accordance with the photo-Wolffrearrangement. Furthermore we have also demonstrated the synthetic utility of>1a as an esterification and etherification reagent with avariety of substituted α-diazo-p-hydroxyacetophenones,using them as synthons for efficiently coupling it to acids and phenols toproduce pHP protect substrates.