title: 2-Diazo-1-(4-hydroxyphenyl)ethanone: a versatile photochemical and synthetic reagent.
abstract: 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 rearrangements 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 rearrangement, 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) spectroscopy clearly demonstrates the formation of a ketene intermediate that is subsequently trapped by solvent or nucleophiles. The photoreaction of 1a is quenched by oxygen and sensitized by triplet sensitizers and the quantum yields for 1a-c range from 0.19 to a robust 0.25. The lifetime of the triplet, determined by Stern-Volmer quenching, is 31 ns with a rate for appearance of 4a of k = 7.1 × 10(6) 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-Wolff rearrangement. Furthermore we have also demonstrated the synthetic utility of 1a as an esterification and etherification reagent with a variety of substituted α-diazo-p-hydroxyacetophenones, using them as synthons for efficiently coupling it to acids and phenols to produce pHP protect substrates.
authors.affiliation: Department of Chemistry, University of Kansas, Lawrence, Kansas 66045, USA. givensr@ku.edu.
authors.forename: Sanjeewa N
authors.initials: SN
authors.lastname: Senadheera
authors.forename: Anthony S
authors.initials: AS
authors.lastname: Evans
authors.forename: John P
authors.initials: JP
authors.lastname: Toscano
authors.forename: Richard S
authors.initials: RS
authors.lastname: Givens
mesh_terms: D000098:Acetophenones / Q000737:chemistry*; D001391:Azo Compounds / Q000737:chemistry*; D060326:Chemistry Techniques, Synthetic; D006863:Hydrogen-Ion Concentration; D007202:Indicators and Reagents / Q000737:chemistry; D055668:Photochemical Processes; D013055:Spectrophotometry, Infrared
chemical_list: D000098:Acetophenones; D001391:Azo Compounds; D007202:Indicators and Reagents; C031335:4-hydroxyacetophenone