Photochemical Functionalization of Heterocycles with EBX Reagents: C−H Alkynylation versus Deconstructive Ring Cleavage**

Abstract

The development of novel methodologies for the functionalization of saturated heterocycles is highly desirable. Herein, we reportacheap and efficient photochemical methodf or the CÀHf unctionalization of saturated O-heterocycles, as well as the deconstructive ring-cleavage of S-heterocycles, employing hypervalent iodine alkynylation reagents (ethynylbenziodoxolones, EBX). This photochemical alkynylation is performed utilizing phenylglyoxylic acid as the photoinitiator,l eadingt ot he correspondingp roducts in good to high yields, under household fluorescent light bulb irradiation. When O-heterocycles were employed, the expected a-CÀHa lkynylation tookp lace. In contrast, oxidative ring-opening to form at hioalkyne and an aldehyde was observedw ith S-heterocycles. Preliminary mechanistic experimentsa re presented to give first insights into this puzzling divergentreactivity.

Photochemical Reaction of N,N‐Dimethylanilines with N‐Substituted Maleimides Utilizing Benzaldehyde as the Photoinitiator

“…We initiated our study utilizing 4‐bromo‐ N , N ‐dimethylaniline ( 1 a ) and N ‐phenylmaleimide ( 2 a ), in order to find the optimum catalyst (Table 1). Thioxanthen‐9‐one ( 4 a ) [10k,l] and phenylglyoxylic acid ( 4 c ), [10a–i] afforded mediocre yields (Table 1, entries 1–3). Aromatic aldehydes, as aromatic ketones, have shown catalytic activity upon irradiation in organic synthetic reactions or in polymerization reactions [3f,10j] .…”

Iodine-based reagents in photoredox-organocatalysis

“…65 For the first time, in 2016, Cheng and coworkers reported a method for decarboxylative alkynylation of carboxylic acids 12 using organo-photocatalyst (Figure 6). 52 They have used 5 mol % of DCA as organo-photocatalyst, 1.5 equiv of hypervalent iodine reagent ethenyl benziodoxolones (EBX) [66][67] as alkyne source, and 2.0 equiv of K2CO3 under 24 h blue light or natural sunlight irradiation to get the decarboxylative alkynylation products 13 via vinylic radical intermediate 14. Here, EBX reagent acted as a group transfer reagent.…”

Photochemical C–H acetalization of O-heterocycles utilizing phenylglyoxylic acid as the photoinitiator

“…Usually transition metal-based catalysts are employed, but in the last decade, a significant number of researchers have focused their attempts on the use of small organic molecules as photocatalysts, a field called Photoorganocatalysis [26][27][28][29][30][31][32][33]. In recent years, our group has focused its interest on utilizing effectively a variety of photocatalysts-photoinitiators in many different chemical tranformations [34][35][36][37]. We have developed a cheap, easy to operate photochemical protocol, where phenylglyoxylic acts as the photoiniator under irradiation by common household lamps.…”

“…We have developed a cheap, easy to operate photochemical protocol, where phenylglyoxylic acts as the photoiniator under irradiation by common household lamps. During the last 7 years, this venture led to a plethora of succesful organic tranformations [34][35][36][37]. A photochemical synthesis of lactones via C-H activation of alcohols using phenylglyoxylic acid as the photoinitiator has been reported [34].…”

“…A photochemical synthesis of lactones via C-H activation of alcohols using phenylglyoxylic acid as the photoinitiator has been reported [34]. Recently, via this PhCOCOOH-mediated photochemical protocol, C-H functionalization at the α-position of heterocycles became possible, paving the way for the design of other synthetic pathways [36,37]. In these cases, the THF radical was employed as the nucleophile, reacting with Michael acceptors or other electrophilic reagents.…”