Efficient Generation and Synthetic Applications of Alkyl‐Substituted Siloxycarbenes: Suppression of Norrish‐Type Fragmentations of Alkanoylsilanes by Triplet Energy Transfer

Organocatalyzed Hydroacylation of Enones by Photosensitization of Acyl Silanes

A mild protocol for hydroacylation of enones through photosensitization of acyl silanes with thioxanthone under blue light (455 nm) irradiation is reported. A Brønsted acid is used as a cocatalyst in the reaction. The versatility of the method is demonstrated through inter- and intramolecular hydroacylation reaction. The hydroacylation product is applied for synthesizing an anti-HCV agent. Mechanistic insights are also provided through control experiments.

Synthesis of chroman-annulated cyclopropanols via photoinduced intramolecular [2 + 1]-cycloaddition of 2-allyloxybenzaldehydes

2-Allyloxybenzaldehydes undergo [2 + 1] cycloadditions under 365 nm LED irradiation to form the corresponding chroman-fused cyclopropanols. The reaction proceeds easily without any catalysts or additives in dimethyl sulfoxide.

Benzoyldiisopropylchlorosilane: a visible light photocleavable alcohol protecting group

Silyl chlorides are highly valuable and popular reagents for the protection of alcohols. In this edge article we introduce a photocleavable alcohol protecting group on the basis of acyl silanes. To achieve this, acylchlorosilanes that represent a new class of acylsilanes were developed. They can be easily synthesized in a concise sequence of three steps in high overall yield. Alcohol silyl protection takes place under established mild conditions, akin to those associated with classical silicon-based protecting groups. The removal of the Si-group is achieved at room temperature through exposure to visible light (456 nm) in methanol. We demonstrate a broad spectrum of substrates with remarkable tolerance toward diverse functional groups, highlighting a substantial level of orthogonality with respect to other protecting groups. Furthermore, we showcase the robustness of this approach against various transformations.

Acylsilanes in Transition-Metal-Catalyzed and Photochemical Reactions: Clarifying Product Formation

Acylsilanes are able to react as nucleophilic carbene precursors, electrophiles, and directing groups in C-H functionalization. To date, some of the products reportedly formed during transition-metal-catalyzed and photochemical reactions involving acylsilanes have been incorrectly assigned. To provide clarity, we herein address these structural misassignments and detail the revised structures. New insights into the reactivity of acylsilanes were also afforded via the discovery that light-induced siloxy carbenes participate in intramolecular 1,2-carbonyl addition to proximal esters.

Cu/SaBox-Catalyzed Photoinduced Coupling of Acylsilanes with Alkynes

The transition metal-catalyzed cross-coupling reaction with Fischer metal carbene intermediates bearing an electron-rich alkoxyl or siloxyl group remains a big challenge due to the lack of readily available corresponding carbene precursors. Herein, we report the coupling of alkynes with the Fischer-type copper carbene species bearing a α-siloxyl group, which could be in situ generated from acylsilanes catalytically under photoirradiation and redox-neutral conditions. The side-arm modified bisoxazoline (SaBox) ligands prove to be crucial for this coupling reaction, which provides the corresponding alkynyl alcohol in high yields with remarkable heterocycle tolerance and broad substrate scope.

Cascade cyclization of alkene-tethered acylsilanes and allylic sulfones enabled by unproductive energy transfer photocatalysis

Developing photo-induced cascade cyclization of alkene-tethered acylsilanes is challenging, because acylsilanes are unstable under light irradiation. Herein, we report that the energy transfer from excited acylsilanes to a photocatalyst that possesses lower triplet energy can inhibit the undesired decomposition of acylsilanes. With neutral Eosin Y as the photocatalyst, an efficient synthesis of cyclopentanol derivatives is achieved with alkene-tethered acylsilanes and allylic sulfones. The reaction shows broad substrate scope and the synthetic potential of this transformation is highlighted by the construction of cyclopentanol derivatives which contain fused-ring or bridged-ring.

Acylsilanes decompose under light irradiation, and this limits their use in light-induced organic transformations. Here the authors report a strategy to inhibit the light-induced decomposition of acylsilanes, enabling the photochemical synthesis of cyclopentanol derivatives from alkene-tethered acylsilanes and allylic sulfones.

Electron Shuttle in N-Heteroaromatic Ni Catalysts for Alkene Isomerization

Simple N-heteroaromatic Ni(II) precatalysts, (L)NiMe2 (L = bipy, bipym), were used for alkene isomerization. With an original reduction method using a simple borane (HB(Cat)), a low-valent Ni center was formed readily and showed good conversion when a reducing divalent lanthanide fragment, Cp*2Yb, was coordinated to the (bipym)NiMe2 complex, a performance not achieved by the monometallic (bipy)NiMe2 analogue. Experimental mechanistic investigations and computational studies revealed that the redox non-innocence of the L ligand triggered an electron shuttle process, allowing the elusive formation of Ni(I) species that were central to the isomerization process. Additionally, the reaction occurred with a preference for mono-isomerization rather than chain-walking isomerization. The presence of the low-valent ytterbium fragment, which contributed to the formation of the electron shuttle, strongly stabilized the catalysts, allowing catalytic loading as low as 0.5%. A series of alkenes with various architectures have been tested. The possibility to easily tune the various components of the heterobimetallic catalyst reported here, the ligand L and the divalent lanthanide fragment, opens perspectives for further applications in catalysis induced by Ni(I) species.

Photoinduced Intramolecular Cyclization of Acylsilanes Bearing a Boronate Moiety: Construction of a Highly Strained trans-Fused Bicyclo[3.3.0]octane Skeleton

A reliable strategy for the construction of trans-fused bicyclo[n.3.0] skeletons was explored by utilizing photoinduced cyclization of acylsilanes bearing a boronate. The substrates having an acylsilane and a boronate in a 1,2-trans relationship were prepared via hydroboration of cycloalkene derivatives. The desired cyclization was efficiently promoted by photoirradiation to give the trans-fused bicyclo[n.3.0] derivatives as a single diastereomer. It is noteworthy that this methodology enables the efficient construction of a highly strained trans-5-5 ring system.

Accessing Diverse Cross-Benzoin and α-Siloxy Ketone Products via Acyl Substitution Chemistry

An approach to diverse cross-benzoin and α-siloxy ketone products which leverages a simple yet underutilized C-C bond disconnection strategy is reported. Acyl substitution of readily accessible α-siloxy Weinreb amides with organolithium compounds enables access to a broad scope of aryl, heteroaryl, alkyl, alkenyl, and alkynyl derivatives. Enantiopure benzoins can be accessed via a chiral pool approach, and the utility of accessible cross-benzoins and α-siloxy ketones is highlighted in a suite of downstream synthetic applications.

Visible-Light-Induced Organocatalyzed [2+1] Cyclization of Alkynes and Trifluoroacetylsilanes

The synthesis of common cyclopropenes has been widely studied, but the synthesis of cyclopropenols is a significant challenge. Herein, we highlight our recent work on the synthesis of trifluoromethylated cyclopropenols through [2+1] cycloaddition reaction between alkynes and trifluoroacetylsilanes under visible-light-induced organocatalysis. The novel amphiphilic donor-acceptor carbenes derived from trifluoroacetylsilanes can react effectively with both activated and unactivated alkynes. Broad substrate scope and good functional group tolerance have been achieved. Besides, the synthetic potential of this reaction was highlighted by a gram-scale reaction and the one-pot diastereoselective synthesis of trifluoromethylated cyclopropanols.

Synthesis of Cyclopropenols Enabled by Visible-Light-Induced Organocatalyzed [2+1] Cyclization

Although the synthesis of common cyclopropenes has been well studied, the access to cyclopropenols is rather limited. Herein, we report the first synthesis of α-trifluoromethylated cyclopropenols via 2+1 cycloaddition reactions between alkynes and trifluoroacylsilanes, enabled by visible-light-induced organocatalysis. The novel ambiphilic donor-acceptor carbenes derived from trifluoroacetylsilanes reacted efficiently with both activated and non-activated alkynes. The reaction features simple operation, mild conditions, broad substrate scope and good functional group tolerance. The synthetic potential of the reaction is highlighted by the gram-scale reactions and first synthesis of α-trifluoromethylated cyclopropanols through the combination of the 2+1 cyclization and high diastereoselective hydrogenation reaction in one pot.

Photoinduced Copper-Catalyzed Asymmetric Acylation of Allylic Phosphates with Acylsilanes

We report a visible-light-induced copper-catalyzed highly enantioselective umpolung allylic acylation reaction with acylsilanes as acyl anion equivalents. Triplet-quenching experiments and DFT calculations supported our reaction design, which is based on copper-to-acyl metal-to-ligand charge transfer (MLCT) photoexcitation that generates a charge-separated triplet state as a highly reactive intermediate. According to the calculations, the allylic phosphate substrate in the excited state undergoes novel molecular activation into an allylic radical weakly bound to the copper complex. The allyl radical fragment undergoes copper-mediated regio- and stereocontrolled coupling with the acyl group under the influence of the chiral N-heterocyclic carbene ligand.

Generation of (amino)(boryloxy)carbenes from carbamoylboranes and their coupling reaction with aldehydes

Carbamoylboranes were found to react with various aldehydes under heating conditions to give α-hydroxycarboxamides in good yields. A DFT study supports the mechanism, which involves thermally generated (amino)(boryloxy)carbene intermediates. To our knowledge, this is the first report on the generation of (amino)(boryloxy)carbene intermediates from carbamoylboranes and its application to carbon-carbon bond-forming reactions.

Intramolecular Photochemical [2+1]-Cycloadditions of Nucleophilic Siloxy Carbenes

Visible light induced singlet nucleophilic carbenes undergo rapid [2 + 1]-cycloaddition with tethered olefins to afford unique bicyclo[3.1.0]hexane and bicyclo[4.1.0]heptane scaffolds. This cyclopropanation process requires only visible light irradiation to proceed, circumventing the use of exogenous (photo)catalysts, sensitisers or additives and showcases a vastly underexplored mode of reactivity for nucleophilic carbenes in chemical synthesis. The discovery of additional transformations including a cyclopropanation/retro-Michael/Michael cascade process to afford chromanones and a photochemical C–H insertion reaction are also described.

Visible light induced singlet nucleophilic carbenes undergo rapid [2 + 1]-cycloaddition with tethered olefins to afford unique bicyclo[3.1.0]hexane and bicyclo[4.1.0]heptane scaffolds.

Visible-Light-Induced In Situ Generation of Fischer-Type Copper Carbene Complexes from Acylsilanes and Its Application to Catalytic [4 + 1] Cycloaddition with Siloxydienes

A novel methodology for in situ generation of Fischer-type metal-carbene complexes was developed. Photoirradiation to a mixture of an acylsilane and a cationic copper complex cleanly gave a Fischer-type copper-siloxycarbene complex, which was detected by spectroscopic methods. This carbene complex reacted with siloxydienes in a [4 + 1] cycloaddition manner to give cyclopentene derivatives. It is noteworthy that this reaction proceeds with a catalytic amount of copper through in situ generation of a Fischer-type copper-siloxycarbene complex intermediate.

Acyl silane directed Cp*Rh(III)-catalysed alkylation/annulation reactions

Studies into the Cp*Rh(iii)-catalysed hydroarylation of alkenes with aryl acyl silanes led to the discovery of a new synthetic strategy to access unique silicon derived indene frameworks. Rather than protodemetalation of the metal enolate formed following insertion of an alkene into the aryl C-H bond, intramolecular aldol condensation of the acyl silane occurred to generate a series of 2-formyl- and 2-acetyl-3-silyl indenes. This represents only the second example of rhodium-catalysed C-H functionalisation employing acyl silanes as weakly coordinating directing groups and the intramolecular aldol condensation strategy was extended to access analogous silicon derived benzofurans.

Visible-Light-Induced Cycloaddition of α-Ketoacylsilanes with Imines: Facile Access to β-Lactams

We report the synthesis of β-lactams from α-ketoacylsilanes and imines, which proceeds via a formal [2+2] photochemical cycloaddition with in situ generation of siloxyketene. This mild and operationally simple reaction proceeds in an atom-economic fashion with broad substrate scope, including aldimines, ketimines, hydrazones, and fused nitrogen heterocycles, affording a variety of important β-lactams with satisfactory diastereoselectivities in most cases. This reaction also features good functional-group tolerance, facile scalability and product diversification. Experimental and computational studies suggest that α-ketoacylsilanes can serve as photochemical precursors by engaging in a 1,3 silicon shift to the distal carbonyl group.

Fluorinated Ketones as Trapping Reagents for Visible-Light-Induced Singlet Nucleophilic Carbenes

Singlet nucleophilic carbenes (SNCs) containing only one heteroatom donor remain underutilized in chemical synthesis. We recently discovered that visible-light-induced SNC intermediates can be trapped by fluorinated ketones via 1,2-carbonyl addition to afford benzoin-type products. This discovery represents a rare example of nucleophilic carbenes reacting with ketones and delivers an efficient, user-friendly, and scalable process for accessing fluorinated tertiary alcohol derivatives driven only by light circumventing the use of exogenous catalysts or additives.