Department of Chemistry, University of Kansas, Lawrence, Kansas 66045, USA.
Photochem Photobiol Sci. 2014 Feb;13(2):324-41. doi: 10.1039/c3pp50305d.
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.
α-重氮芳基酮是众所周知的底物，通过热或光化学激活，通过烯酮中间体转化为苯乙酸。同样，α-取代的对羟基苯乙酰基（pHP）酯是通过不同途径进行光-Favorskii 重排为苯乙酸的底物，据称该途径涉及环丙酮中间体。在本文中，我们表明，一系列α-重氮对羟基苯乙酮和对羟基苯乙酰基（pHP）α-酯的光解均以主要产物的形式生成相同的重排苯乙酸酯。由于α-重氮对羟基苯乙酮（1a，pHP N2）包含进行 Wolff 或 Favorskii 重排所需的所有官能团，因此我们在有利于光-Favorskii 重排的条件下，即羟基介质中的光解，探究了这种有趣的机制二分法。使用时间分辨红外（TRIR）光谱法研究 1a 转化为对羟基苯乙酸（4a）的反应机制，清楚地表明形成了烯酮中间体，随后被溶剂或亲核试剂捕获。1a 的光反应被氧气猝灭，并被三重态敏化剂敏化，1a-c 的量子产率范围从 0.19 到稳健的 0.25。通过 Stern-Volmer 猝灭确定三重态的寿命为 31 ns，在 aq.乙腈（1：1 v：v）中 4a 的出现速率为 k = 7.1×10(6) s(-1)。这些研究表明，1a 的主要重排途径符合光-Wolff 重排，涉及烯酮的形成。此外，我们还证明了 1a 作为酯化和醚化试剂的合成实用性，与各种取代的α-重氮对羟基苯乙酮一起使用，将其作为有效的缩合剂与酸和酚结合，以产生 pHP 保护基底物。
