Visible-Light Mediated Hydrosilylative and Hydrophosphorylative Cyclizations of Enynes and Dienes

Abstract

Described herein is a visible-light mediated intermolecular radical cyclization approach to access heterocycles. Heteroatom radicals, such as silicon and phosphorus atom radicals, were generated via direct hydrogen atom abstraction by the photoexcited catalyst species with hydro-silanes and phosphine oxides. The radical addition/cyclization/ HAT (hydrogen atom transfer) reaction sequences of 1,6-enynes and 1,6-dienes were highly efficient delivering the desired heterocycles in good yields.H eterocyclic scaffolds are ubiquitous in natural products and exhibit a broad range of pharmacological properties, which attracted great attention for the drug discovery and as building blocks for the synthesis of pharmaceutically relevant compounds and other highly functionalized molecules. 1 The intermolecular radical addition/cyclization cascade presents a powerful and efficient synthetic strategy to access complex organic heterocycles. 2,3 Among the various element radicals, heteroatom radicals, such as silicon and phosphorus atom radicals, recently have drawn great attention in the organic transformation. 4,5 Specifically, visible-light photocatalysis has demonstrated to be a powerful modus for radical generation. 6 Although there are a number of reports on visiblelight mediated radical addition across the π-system, few radical cyclization reactions have been reported. 7,8 However, most of the radicals were generated by a single-electron transfer process, or assisted by a hydrogen atom transfer cocatalyst. 9 Recently, the group of Wu demonstrated that the photoexcitation of Eosin Y could act as a hydrogen atom transfer (HAT) catalyst, where the HAT with oxygen adjacent to a C− H bond or aldehydes generated the carbon radicals. 10,11 The silicon and phosphorus atom radical formation via a hydrogen atom abstraction from hydro-silanes and phosphine oxides are well-known. 12−15 To the best of our knowledge, use of a visible-light excited catalyst for direct silicon and phosphorus atom radical generation is rare.Given the similar bond dissociation energies (BDEs) of the Si−H of 1,1,1,3,3,3-hexamethyl-2-(trimethylsilyl)trisilane (BDE = 79 kcal/mol) and P−H bond of phosphine oxide (BDE ≈ 85 kcal/mol) with C−H (BDE ≈ 90 kcal/mol), 16 both hydrosilylative and hydrophosphorylative cyclizations of enynes under visible-light photo-HAT catalysis were evaluated.The visible light irradiation of the organic dyes, such as Eosin Y, could generate the photoexcited catalyst species Eosin Y* (Scheme 1). 10 The hydrogen atom abstraction of Eosin Y* with hydro-silanes or phosphine oxides (FG-H) would give the silicon or phosphorus atom radicals (FG• radical), leading the formation of Eosin Y−H, the radical addition of FG• across the CC π-bond of the 1,6-enyne which would give the vinyl radical intermediate I-1; the 5-exo radical cyclization of the

Copper‐Photocatalyzed Hydrosilylation of Alkynes and Alkenes under Continuous Flow

“…(5)] [12] . Regarding the photocatalytic hydrosilylation of alkyne, Wang, Yao, [13] and Zhu [14] independently reported the synergistic use of photocatalyst and HAT catalyst to achieve the process [Eq. (6)].…”

Advances in radical phosphorylation from 2016 to 2021

“…Although significant progress has been achieved, the elaboration of the phosphorylation products of alkynes remains highly desirable. In 2017, Zhu et al [88] reported a silver-mediated radical 5-exo-dig cyclization of 2alkynylbenzonitrile with disubstituted phosphine oxides and H 2 O for the synthesis of phosphinylated 1indenones [Figure 55]. Multiple bonds, including C-P, C-C and C-O bonds, were successively constructed under atmospheric conditions.…”

Electrophotocatalytic Si–H Activation Governed by Polarity-Matching Effects

“…However, the substrates were invariably limited to phenylsilanes or (TMS) 3 SiH with labile Si-H bonds (Scheme 1b). 11,[29][30][31][32][33][34][35] As pioneered by Fagnoni et al, 36 decatungstate was found to be an effective hydrogen atom transfer (HAT) photocatalyst for aromatic tertiary silanes activation (Scheme 1c). Nevertheless, only poor selectivities were observed for trialkylsilanes owing to the comparably high bond dissociation energies (BDEs) of Si-H and α-Si-C-H bonds (>92 kcal mol −1 ).…”