Silyl Radicals

Chatgilialoglu, Chryssostomos; Schiesser, Carl H.

doi:10.1002/0470857269.ch4

Cited by 21 publications

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“…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. , The silicon and phosphorus atom radical formation via a hydrogen atom abstraction from hydro-silanes and phosphine oxides are well-known. − To the best of our knowledge, use of a visible-light excited catalyst for direct silicon and phosphorus atom radical generation is rare.…”

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confidence: 99%

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

Hou

Yang

et al. 2020

Org. Lett.

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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

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confidence: 99%

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

Hou

Yang

et al. 2020

Org. Lett.

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“…This halogenation reaction is well known as a general reaction of silyl radicals. 2,5 In summary, we have succeeded in the synthesis, isolation, and structural characterization of the first stable Si-centered triradical 2. The EPR spectrum of 2 showed the preference for the high-spin (quartet) ground state.…”

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“…9 In the solid state, the geometry around the Si radical center was nearly trigonal-planar, with the sum of the bond angles at Si1 triradical 2 and biradical II was observed for the dihedral angles between the ( t Bu 2 MeSi) 2 Si unit and the phenyl ring. In biradical II, the 3p(Si) orbital is nearly perpendicular (87°) to the π-system of the aromatic ring; in contrast, in 2, these dihedral angles are 6270°to minimize the steric repulsion between the bulky substituents.…”

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1,3,5-[(tBu2MeSi)2Si]3C6H3: Isolable Si-centered Triradical with a High-spin Quartet Ground State

2014

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Reductive deiodination of 1,3,5-tris[2,2-di-tert-butyl-1-(ditert-butylmethylsilyl)-1-iodo-2-methyldisilanyl]benzene (1) with KC 8 results in the formation of isolable Si-centered triradical 1,3,5-[( t Bu 2 MeSi) 2 Si] 3 C 6 H 3 (2), which was characterized by X-ray crystallography and electron paramagnetic resonance (EPR) spectroscopy. The quartet ground state for triradical 2 was established using EPR spectroscopy by Curie plots (560 K) for the characteristic ¦m = 2 and ¦m = 3 transitions.

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“…There is, in fact, a third pathway to aryl C–H silylation. That is free-radical-promoted homolytic aromatic silylation. , It was considered unpractical because of very low yields and poor selectivities. Recently, Grubbs and Stoltz et al realized a series of highly efficient radical heteroaryl C–H silylations initiated by alkali metal oxides, which made a significant breakthrough in this area.…”

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Free-Radical-Promoted Site-Selective C–H Silylation of Arenes by Using Hydrosilanes

Chai

Liu

2017

Org. Lett.

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A free-radical-promoted aryl/heteroaryl C-H silylation using hydrosilane was developed. This cross-dehydrogenative silylation enables both electron-rich and electron-poor aromatics to afford the desired arylsilanes in unique selectivity. A "para-selectivity" was observed by examination of over 54 examples. This exceptional orientation is quite different from that in Friedel-Crafts C-H silylation or transition-metal-catalyzed dehydrogenative silylation.

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Silyl Radicals

Cited by 21 publications

References 0 publications

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

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

1,3,5-[(tBu2MeSi)2Si]3C6H3: Isolable Si-centered Triradical with a High-spin Quartet Ground State

Free-Radical-Promoted Site-Selective C–H Silylation of Arenes by Using Hydrosilanes

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