Photosensitized Electron Transfer Promoted Reductive Activation of Carbon−Selenium Bonds To Generate Carbon-Centered Radicals:  Application for Unimolecular Group Transfer Radical Reactions

Abstract

The investigation presented in this paper explores the mechanistic aspects and synthetic potentials of photosensitized electron transfer (PET) promoted reductive activation of organoselenium substrates. PET activation of substrates 1-5 is achieved through a photosystem comprised of light-absorbing 1,5-dimethoxynaphthalene (DMN) as electron donor and ascorbic acid as co-oxidant. The fluorescence quenching of (1)DMN by organoselenium compounds 1-5, correlation of fluorescence quenching rate constant with the reduction potentials of 1-5, and the dependence of photodissociation quantum yields of 1-5 on their concentration suggests the occurrence of electron-transfer (ET) processes between (1)DMN and 1-5. Steady state photolysis of organoselenium substrates (R(2)CHSePh) in the presence of (1)DMN and ascorbic acid leads to the cleavage of the -C-Se- bond to produce a carbon-centered radical and PhSe(-) species via the intermediacy of R(2)CH-SePh&uprhbr;(-)(*). The mechanistic interpretation for the reductive activation of -C-Se- bonds and the synthetic utility of observed cleavage pattern is extended for the unimolecular group transfer radical sequences.

Organic Photoredox Catalysis

“…564 Some of the first use of this class of photoreductants as catalysts was for the generation of carbon radicals from alkyl selenides (186.1) (Scheme 186). 565 Pandey and coworkers employed DMN as the photoredox catalyst in conjunction with ascorbic acid as the stoichiometric reductant to effect atom transfer radical cyclizations. Five-membered rings cyclized in high efficiency to afford the expected atom transfer cyclization adducts (186.2), whereas 6-exo-type radical cyclizations at best gave 10% yields.…”

Photocatalytic Cleavage of Aryl Ether in Modified Lignin to Non-phenolic Aromatics

“…The desired products were produced in low yields in the absence of ascorbic acid (10 and 23%, 2a and 3a , respectively; entry 3). Replacing ascorbic acid with sodium thiosulfate retained the products moderately (entry 4), indicating that ascorbic acid could act as the reductant to participate in the oxidative cycling step of the PTH catalyst . The addition of formic acid was proposed to promote the hydrolysis of the imine intermediate to release arylamine and ketone products.…”

“…Replacing ascorbic acid with sodium thiosulfate retained the products moderately (entry 4), indicating that ascorbic acid could act as the reductant to participate in the oxidative cycling step of the PTH catalyst. 64 The addition of formic acid was proposed to promote the hydrolysis of the imine intermediate to release arylamine and ketone products. Without formic acid, the yields of desired products slightly decreased (entry 5), and it could be replaced by sodium dihydrogen phosphate (entry 6).…”

Generation of Alkyl Radicals: From the Tyranny of Tin to the Photon Democracy

“…The alkyl radical generation is granted by the very versatile photochemical tool. This feature includes particular cases such as C–Se (in alkyl selenides), , C–Te [in (aryltelluro)­formates, , for a previous thermal generation of alkyl radical from diorganyl tellurides, see ref ], and C–Si (in tetra alkyl silanes and bis-chatecolates) − to be added to C–Sn (in alkyl stannanes). , Interestingly, even the more resilient C–H , or C–C ,,,− bonds may be cleaved for alkyl radical generation, opening up new exciting possibilities for the synthetic (photo)­chemist (Figure ).…”