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

Photocatalytic 1,3-oxyheteroarylation of aryl cyclopropanes with azine N-oxides

Cyclopropanes, valuable C3 building blocks in organic synthesis, possess high strain energy and inherent stability. We present an efficient, environmentally benign 1,3-oxyheteroarylation of aryl cyclopropanes using azine N-oxides as bifunctional reagents under visible light irradiation. This metal-free method yields β-pyridyl ketones under mild conditions. Mechanistic studies reveal a photo-induced radical pathway involving single-electron oxidation of both aryl cyclopropanes and azine N-oxides, followed by stepwise ring opening. The dual oxidation mechanism accommodates diverse cyclopropane and azine N-oxide combinations based on their oxidation potentials. This green chemistry method enhances the synthetic utility of aryl cyclopropanes while introducing an efficient strategy for their difunctionalization. The methodology aligns with sustainable organic synthesis principles, offering an environmentally conscious route to valuable synthetic intermediates.

Allylsilane as a versatile handle in photoredox catalysis

Organosilanes have secured a special place in the synthetic world for several decades. However, among them, allylsilanes are a choice reagent for organic chemists to develop novel organic transformations. In recent years researchers have proved that visible-light photoredox catalysis has emerged as one of the most mild, sustainable, straightforward, and efficient strategies to construct simple to complex molecules with or without enantioselectivity. This review provides an in-depth analysis of recent advances and strategies employed in visible-light photoredox catalysis for allylsilane and its analogues for the development of various organic transformations. The review is divided into sections, each focused on a specific reactivity of allylsilane under light irradiation with C(sp2) center arene or alkene, C(sp2) center carbonyl, and C(sp3) center carbon functionality. In this review, we present optimization data, reaction scope, and mechanistic aspects to bring forward specific reactivity and selectivity trends of allylsilane in photoredox conditions.

Photocatalytic and Electrochemical Borylation and Silylation Reactions

Due to their high versatility borylated and silylated compounds are inevitable synthons for organic chemists. To escape the classical hydroboration/hydrosilylation paradigm, chemists turned their attention to more modern and green methods such as photoredox chemistry and electrosynthesis. This account focuses on novel methods for the generation of boryl and silyl radicals to forge C-B and C-Si bonds from our group.

2-Silylated Dihydroquinazolinone as a Photocatalytic Energy Transfer Enabled Radical Hydrosilylation Reagent

The hydrosilylation of alkenes is one of the most important methods for the synthesis of organosilicon compounds. In addition to the platinum-catalyzed hydrosilylation, silyl radical addition reactions are notable as economic reactions. An efficient and widely applicable silyl radical addition reaction was developed by using 2-silylated dihydroquinazolinone derivatives under photocatalytic conditions. Electron-deficient alkenes and styrene derivatives underwent hydrosilylation to give addition products in good to high yields. Mechanistic studies indicated that the photocatalyst functioned not as a photoredox catalyst but as an energy transfer catalyst. DFT calculations clarified that the triplet excited state of 2-silylated dihydroquinazolinone derivatives released a silyl radical through the homolytic cleavage of a carbon-silicon bond, and this was followed by the hydrogen atom transfer pathway, not the redox pathway.

Visible-Light-Driven Silyl or Germyl Radical Generation via Si-C or Ge-C Bond Homolysis

We report a simple, rapid, and selective protocol for visible-light-driven generation of silyl radicals through photoredox-induced Si-C bond homolysis. Irradiating 3-silyl-1,4-cyclohexadienes with blue light in the presence of a commercially available photocatalyst smoothly generated silyl radicals bearing various substituents within 1 h, and these radicals were trapped by a broad range of alkenes to afford products in good yields. This process is also available for efficient generation of germyl radicals.

Photocatalytic Generations of Secondary and Tertiary Silyl Radicals from Silylboranes Using an Alkoxide Cocatalyst

Silyl radicals are valuable species to prepare diverse organosilicon compounds. However, unlike stable tertiary silyl radicals, the use of secondary silyl radicals has been problematic in silylation reactions due to their instability. Here, we present photocatalytic in situ generations of both secondary and tertiary silyl radicals by one-electron oxidation of ate complexes, formed from silylboranes and an alkoxide cocatalyst, achieving highly efficient hydrosilylation and deuterosilylation of electron-rich alkenes and dienes as well as electron-deficient alkenes. The theoretical studies show that anionic borate complexes activated with an alkoxide have lower oxidation potentials than neutral borate complexes, allowing the formation of secondary silyl radicals. The calculated reaction pathways reveal that anionic conditions using the conjugate acid-base pair of NaOEt (cocatalyst) and EtOH (solvent) are the key to expanding the scope of silyl radicals and alkenes.

Light-Mediated Defluorosilylation of α-Trifluoromethyl Arylalkenes through Hydrogen Atom Transfer

Herein, we report a direct, light-mediated defluorosilylation protocol for converting α-trifluoromethyl arylalkenes and alkyl silanes into γ,γ-difluoroallylic compounds via a combination of photoredox catalysis and hydrogen atom transfer. The clean, convenient protocol can be scaled to the gram level, and its mild conditions make it very suitable for late-stage functionalization of complex natural products and drugs.

Visible-light-mediated metal-free C–Si bond formation reactions

Conserving the environment is one of the most imperative goals in recent days among the chemists throughout the world. Swiftly increasing the environmental awareness also increases the demand to build new approaches for synthesizing the same active molecules with zero-waste and pollution. In this background, visible-light-mediated synthesis and functionalization of diverse organic compounds has been established as a tremendously successful topic and has achieved a remarkable stage of superiority and efficiency in the last 20 years. Alternatively, organosilicon derivatives are gradually aspiring leaves among chemists because of their significant application on synthetic, medicinal, and material chemistry. In this scenario, the addition of Si–H group to carbon−carbon multiple bonds (alkenes, hetero-arenes, alkynes, allenes, carboxylic acids, enynes, and dienes) provides an extremely step- and atom-efficient method to obtain silicon-containing compounds. Several attempts for the development of mild, robust, and efficient green protocol were taken in the last two decades. In spite of substantial advancement/research on C–Si bond formation using transition metal catalysis, a green and metal-free approach is highly essential considering its application in the field of medicine and with respect to environmental aspects as well. In this article, we will summarize the reports considering suitable visible-light-mediated metal-free silylation of C–C multiple bonds that includes alkenes, hetero-arenes, alkynes, allenes, enynes, and dienes.

Photochemical Organocatalytic Regio- and Enantioselective Conjugate Addition of Allyl Groups to Enals

We report the first catalytic enantioselective conjugate addition of allyl groups to α,β‐unsaturated aldehydes. The chemistry exploits the visible‐light‐excitation of chiral iminium ions to activate allyl silanes towards the formation of allylic radicals, which are then intercepted stereoselectively. The underlying radical mechanism of this process overcomes the poor regio‐ and chemoselectivity that traditionally affects the conjugate allylation of enals proceeding via polar pathways. We also demonstrate how this organocatalytic strategy could selectively install a valuable prenyl fragment at the β‐carbon of enals.

Visible-light-induced silylation: an update

This review summarizes the recent findings and developments in the emerging area of photocatalytic silylation with literature coverage mainly extending from 2014 to February 2021.

Application of the stabilization effect of a silyl group in radical-polar crossover reactions enabled by photoredox-neutral catalysis

Taking advantage of the stabilization effect of a silyl group, with a bulky but positive nature, a range of less investigated organosilanes could be generally accessed via photoredox-catalysed radical-polar crossover reactions.

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

The hydroalkylation and hydroacylation of electron-deficient alkenes proceeded smoothly by using benzothiazoline derivatives as radical-transfer reagents under thermal conditions without light irradiation or any additive. Both benzyl and benzoyl moieties were transferred efficiently.

Radical coupling of β-ketoesters and amides promoted by Brønsted/Lewis acids

Recent advances in photocatalysis have enabled radical methods with complementary chemoselectivity to established two electron bond forming approaches. While this radical strategy has previously been limited to substrates with favorable redox potentials, Brønsted/Lewis acid activation has emerged as a means of facilitating otherwise difficult reductions. We report herein our investigations into the Lewis acid-promoted redox activation of β-ketocarbonyls in a model photocatalytic radical alkylation reaction. Rapid evaluation of substrates and reactions conditions was achieved by high throughput experimentation using 96-well plate photoreactors.