PhSeSiR₃-Catalyzed Group Transfer Radical Reactions

Abstract

A novel design for initiating radical-based chemistry in a catalytic fashion is described. The design of the concept is based on the phenylselenyl group transfer reaction from alkyl phenyl selenides by utilizing PhSeSiR(3) (1) as a catalytic reagent. The reaction is initiated by the homolytic cleavage of -C-Se- bond of an alkyl phenyl selenide by the in situ generated alkylsilyl radical (R(3)Si(*)), obtained by the mesolysis of PhSeSiR(3)](*)(-)( )()(1(*)(-)). The oxidative dimerization of counteranion PhSe(-) to PhSeSePh functions as radical terminator. The generation of 1(*)(-) is achieved by the photoinduced electron transfer (PET) promoted reductive activation of 1 through a photosystem comprising of a visible-light (410 nm)-absorbing electron rich DMA as an electron donor and ascorbic acid as a co-oxidant (Figure 1). The optimum mole ratio between the catalyst 1 and alkyl phenyl selenides for successful reaction is established to be 1:10. The generality of the concept is demonstrated by carrying out variety of radical reactions such as cyclization (10, 15-18), intermolecular addition (25), and tandem annulations (32).

Recent Developments in the Photo‐Mediated Generation of Silyl Radicals and Their Application in Organic Synthesis

“…Two early reports in the late 1990s by the Pandey group showcased the production of silyl radicals via mesolysis of PhSeSiR 3 .− produced by photo‐induced electron transfer (PET) under visible‐light (410 nm). Starting from a bromide or a selenium compound, the same furanic product could be obtained (Scheme ) …”

Formation of a Five‐Membered Ring

“…The radical anion 211 adds on to the tethered olefin to produce 1, 2-trans-dialkylated cyclopentane derivatives 212 in good yield. A further improvement in the photosystem has been considered (Figure 8.3) [97] by utilizing 1,4-dimethoxynapthalene (DMN) as the primary electron donor (towards 1 DCA * ) and ascorbic acid as the sacrificial electron donor.…”

“…An intramolecular H-bonding of ascorbic acid with intermediate 211 is suggested to be the reason for the reversal in stereochemistry. Pandeys group has also developed an electron-transfer reductive activation of the CÀSePh group and phenylselenyl group transfer radical cyclization reaction to produce five-membered ring compounds, employing the photosystem shown in Figure 8.4 [97]. Cyclopentanols 214 and 217 are synthesized by intramolecular ketyl radical addition to tethered olefin by a photochemically induced electron-transfer activation of d,w-unsaturated ketones 213 and 216, respectively, using either hexamethylphosphoric triamide (HMPA) or triethylamine in CH 3 CN [98] as solvent (see Scheme 8.59).…”

Radical Cyclization of Haloacetals: The Ueno-Stork Reaction

“…Pandey has introduced the use of PhSeSi(t-Bu)Ph 2 as a reagent to perform transfer of a phenylseleno group under photoelectron transfer conditions. [76][77][78] Cyclization of bromoacetals leads to cyclic alkylselenide (Scheme 19) and alkenyl selenide. The mechanism of this tranformation involves the formation of a silyl radical and PhSefrom Ph-SeSi(t-Bu)Ph 2 under photoelectron transfer (PET) conditions.…”