Visible‐Light‐Mediated Regioselective Allylation, Benzylation, and Silylation of Methylene‐Malononitriles via Photoredox‐Induced Radical Cation Fragmentation

Abstract

Visible‐light‐mediated regioselective allylation, benzylation, and silylation of methylene‐malononitriles using allylic/benzylic silane and disilane reagents have been developed, delivering the corresponding allylated, benzylated, and silylated products in moderate to excellent yields. These reactions proceed smoothly with exclusive regioselectivity by employing only an organo‐photoredox catalyst under very mild conditions. A photoredox‐induced radical cation fragmentation is proposed for the generation of the key allylic, benzylic, and silyl radicals.

Radical Hydroalkylation and Hydroacylation of Alkenes by the Use of Benzothiazoline under Thermal Conditions

“…The generated cation radical species 1 •+ releases a radical species (R • ), and the radical species adds to substrate 2 . This is followed by a reduction via single-electron transfer (SET) with benzothiazoline 1 to generate cation radical 1 •+ . Thus, the reaction proceeds as a radical chain reaction.…”

Asymmetric Addition of Allylsilanes to Aldehydes: A Cr/Photoredox Dual Catalytic Approach Complementing the Hosomi–Sakurai Reaction

“…Bearing the literature background on organochromium species 38,40−45 and the SET oxidation of allylsilanes 46 in mind, a dual catalytic approach seemed feasible. Indeed, when irradiating a mixture of the organic dye 9-mesityl-10methylacridinium tetrafluoroborate (PC1) together with allyltrimethylsilane, CrCl 2 , and aldehyde 1 with blue LEDs in acetonitrile, product formation was observed, but the reaction quickly decolorized before full conversion was achieved.…”

“…This indicates a chain mechanism that constantly has to be re-initiated to work efficiently and becomes more relevant at higher conversions. As the oxidative power of Cr III (E 1/2 ox = −0.51 V vs SCE in DMF) 55 does not match the potential of allyltrimethylsilane (E 1/2 ox = +1.54 V in MeCN), 46 a chain closure by a radical chain mechanism is thermodynamically disfavored. Instead, considering the high oxophilicity of silicon (BDE [Si−O] ≈ 190 kcal/mol vs BDE [Cr−O] ≈ 110 kcal/mol), 56 we propose a formal σ-bond metathesis taking place.…”

Asymmetric Addition of Allylsilanes to Aldehydes – A Cr/Photoredox Dual Catalytic Approach Complementing the Hosomi–Sakurai Reaction

“…Bearing the literature background on organochromium-species 38,[40][41][42][43] and the SET-oxidation of allylsilanes 44 in mind, a dual catalytic approach seemed feasible. Indeed, when irradiating a mixture of the organic dye 9-mesityl-10-methylacridinium tetrafluoroborate together with allyltrimethylsilane, CrCl2 and aldehyde 1 with blue LEDs in acetonitrile, product formation was observed, but the reaction quickly decolorized before full conversion was achieved.…”