Copper-Catalyzed 1,2-Addition of Nucleophilic Silicon to Aldehydes: Mechanistic Insight and Catalytic Systems

Electroreductive Cross-Coupling between Aromatic Aldehydes and Chlorosilanes Enabling the Synthesis of α-Silyl Alcohols

α-Silyl alcohols are powerful structural motifs for pharmaceutical chemistry, materials chemistry, and organic synthesis. The limitations of current synthetic techniques encompass a requirement for difficult-to-obtain silyl precursors, noble-metal catalysts, and narrow substrate scopes. Here, we developed a general synthetic method for α-silyl alcohols through electroreductive cross-coupling of aldehydes and chlorosilane. This method features easily available reagents, mild conditions, and a wide substrate scope. The establishment of this protocol will provide an alternative for access to α-silyl alcohols.

Copper-catalyzed silylation of aryl and alkenyl triflates with silylboronic esters avoiding base-mediated borylation

Silylation of aryl and alkenyl triflates is found to occur readily with silylboronic esters as a silicon source under copper catalysis. The silyl moieties are exclusively installed into the organic frameworks through the preferential generation of a silylcopper species, wherein base-mediated direct borylation is totally suppressed. The combined use of tri-n-butylphosphine and 4,4'-diphenyl-2,2'-bipyridine as a ligand combination turned out to be indispensable for achieving the high catalytic activity.

The structures and reactivity of NHC-supported copper(i) triphenylgermyls

Deprotonation of triphenyl germane with NHC-supported copper alkoxides afforded four novel (NHC)CuGePh3 complexes. Of these, (IPr)CuGePh3 (IPr = :C{N(2,6-iPr2C6H3)CH}2) was selected for further investigation. Analysis by EDA-NOCV indicates it to be a germyl nucleophile and its σ-bond metathesis reaction with a range of p-block halides confirmed it to be a convenient source of [Ph3Ge]-. The Cu-Ge bond of (IPr)CuGePh3 underwent π-bond insertions with tBuNCS, CS2, and PhNCO to furnish a series of germyl substituted carboxylate derivatives, (IPr)CuXC(Y)GePh3 (X = S, NPh; Y = S, NtBu, O), which were structurally characterised. (IPr)CuGePh3 inserted phenyl acetylene, providing both the Markovnikov and anti-Markovnikov products. The (NHC)CuGePh3 compounds were validated as catalytic intermediates; addition of 10 mol% of NHC-copper(i) alkoxide to a mixture of triphenyl germane and a tin(iv) alkoxide resulted in a tin/germanium cross coupling with concomitant formation of alcohol. Moreover, a catalytic hydrogermylation of Michael acceptors was developed with Ph3GeH adding to 7 activated alkenes in good conversions and yields in the presence of 10 mol% of NHC-copper(i) alkoxide. In all cases, this reaction provided the β-germylated substrate implicating nucleophilicity at germanium.

Copper(I)-Catalyzed Regio- and Stereoselective Silaboration of Terminal Allenes

Organic compounds bearing both silyl and boryl groups are important building blocks in organic synthesis because of the adequate reactivity of the silyl and boryl groups and high stereospecificity in their derivatization reactions. The difference in reactivity between the silyl and boryl groups enables stepwise derivatization of these groups to afford complex molecules. Here, we report the copper(I)-catalyzed silaboration of terminal allenes to produce multisubstituted allylic boronates embedded with an alkenyl silane structure. The reaction can proceed with a variety of allenes and silylboranes. Furthermore, the silyl and boryl groups were successfully converted into other functional groups, while retaining the stereochemistry of the alkene moiety.

Sodium silylsilanolate as a precursor of silylcopper species

Silylcoppers function as convenient and effective sources of silicon functional groups. Commonly used precursors for those species have been limited to certain symmetric disilanes and silylboranes. This fact renders the development of silylcopper precursors desirable so that more diverse silyl groups could be efficiently delivered. Here we extend the utility of sodium silylsilanolates as competent precursors of silylcoppers. A silanolate unit operates as an auxiliary to transfer a variety of silyl groups to the copper centre, which was demonstrated in the copper-catalysed hydrosilylation of internal alkynes, α,β-unsaturated ketones, and allenes. Our mechanistic studies through DFT calculation suggested that a copper silylsilanolate undergoes intramolecular oxidative addition of the Si–Si bond to the copper centre to generate a silylcopper, in contrast to the typical formal σ-bond metathesis mechanism for conventional disilanes or silylboranes and copper alkoxides. Accordingly, sodium silylsilanolate has been established as an expeditious precursor of a variety of silylcopper species.

Sodium silylsilanolates are demonstrated as useful silylating reagents for copper-catalysed hydrosilylation of unsaturated bonds via the formation of reactive silylcopper species that can deliver a series of silyl groups.

Challenges and recent advancements in the transformation of CO2 into carboxylic acids: straightforward assembly with homogeneous 3d metals

Transformation of carbon dioxide (CO2) into valuable organic carboxylic acids is essential for maintaining sustainability. In this review, such CO2 thermo-, photo- and electrochemical transformations under 3d-transition metal catalysis are described from 2017 until 2022.

Sodium silylsilanolate enables nickel-catalysed silylation of aryl chlorides

Structurally diverse aryl chlorides were silylated with sodium silylsilanolate reagents in the presence of a Ni(cod)2 catalyst complexed with a phosphine ligand; PMe2Ph for electron-rich substrates, and PCy2Ph for electron-deficient ones. The mild reaction conditions allowed the silylation of various aryl chlorides including functionalised drug molecules.

Copper-Catalyzed Chemoselective Silylative Cyclization of 2,2'-Diethynylbiaryl Derivatives

In this protocol, copper-catalyzed diverse silylative carbocyclization reactions of 2,2'-diethynylbiaryl derivatives with silaboronate were reported. Three new and novel types of domino reactions for the copper-catalyzed transformation of silaboronate were discovered. The corresponding cyclobuta[l]phenanthrene, bis((silyl)methyl)phenanthrene, and silyl-substituted exocyclic diene products were chemoselectively formed with high efficiency.

Catalytic Reductive Cross-Coupling between Aromatic Aldehydes and Arylnitriles

A reductive cross-coupling reaction between aromatic aldehydes and arylnitriles using a copper catalyst and a silylboronate as a reductant is reported. This protocol represents an unprecedented approach to the chemoselective synthesis of α-hydroxy ketones by electrophile-electrophile cross-coupling.

Activation of the Si–B interelement bond related to catalysis

Covering the past seven years, this review comprehensively summarises the latest progress in the preparation and application of Si–B reagents, including the discussion of relevant reaction mechanisms.

Reductive umpolung for asymmetric synthesis of chiral α-allenic alcohols

In this communication, we report a chiral copper/NHC-catalyzed reaction between aromatic aldehydes and propargylic phosphates using a silylboronate, providing enantioenriched chiral α-allenic alcohols with complete regioselectivity and moderate to high enantioselectivity. The reaction pathway involves the catalytic formation of a nucleophilic α-alkoxylalkylcopper(i) species from aldehydes followed by its S_N2' type substitution reaction with propargylic phosphates.

Mononuclear Cu Complexes Based on Nitrogen Heterocyclic Carbene: A Comprehensive Review

During the last decade, organometallic, coordination, and catalytic chemistry of the three-dimensional metals such as copper (Cu) has been greatly affected by the emergence of nitrogen heterocyclic carbene (NHC) complexes. The NHCs, and in particular the mononuclear Cu^I-based ones, have been proven vastly useful in several applications such as in biosynthesis, catalysis, photochemistry, etc. This review tries to thoroughly describe a series of mononuclear Cu^I NHC complexes and their subcategories such as heteroleptics, and bidentate and tridentate heteroatom complexes, and give some detailed insights on their development, emergence, and applications. A brief outlook is also disclosed to enable other researchers to further develop a platform for future advances and studies in the field of Cu^I-based NHCs.

Recent advances in Cu-catalyzed C(sp3)-Si and C(sp3)-B bond formation

Numerous reactions generating C-Si and C-B bonds are in focus owing to the importance of incorporating silicon or boron into new or existing drugs, in addition to their use as building blocks in cross-coupling reactions en route to various targets of both natural and unnatural origins. In this review, recent protocols relying on copper-catalyzed sp3 carbon-silicon and carbon-boron bond-forming reactions are discussed.

Reductive Coupling of Carbon Dioxide and an Aldehyde Mediated by a Copper(I) Complex toward the Synthesis of α-Hydroxycarboxylic Acids

Copper-mediated reductive coupling between CO₂ and an aldehyde to form α-hydroxycarboxylic acid was achieved using silylborane as a reductant. CO₂ cleanly inserted into a copper-carbon bond that was formed by the reaction between a silylcopper-NHC complex and an aldehyde. A series of reactions that regenerate the silylcopper complex were developed for the synthesis of an α-hydroxycarboxylic acid. After repeating each step iteratively for two cycles, 0.62 equiv of α-hydroxycarboxylic acid based on the copper complex was obtained.

Allylic cross-coupling using aromatic aldehydes as α-alkoxyalkyl anions

The allylic cross-coupling using aromatic aldehydes as α-alkoxyalkyl anions is described. The synergistic palladium/copper-catalyzed reaction of aromatic aldehydes, allylic carbonates, and a silylboronate produces the corresponding homoallylic alcohol derivatives. This process involves the catalytic formation of a nucleophilic α-silyloxybenzylcopper(I) species and the subsequent palladium-catalyzed allylic substitution.

Copper-Catalyzed Enantioselective Reductive Cross-Coupling of Aldehydes and Imines

A copper-catalyzed enantioselective reductive cross-coupling using aromatic aldehydes and imines, producing chiral β-amino alcohols, is described. The catalytic formation of enantioenriched chiral α-alkoxyalkylcopper(I) species from aromatic aldehydes and the subsequent reaction with imine electrophiles are attractive features of this protocol.

Ligand-Dependent-Controlled Copper-Catalyzed Regio- and Stereoselective Silaboration of Alkynes

A copper-catalyzed highly regio- and stereoselective silaboration of alkynes was developed. In this work, direct cis-difunctionalization of alkynes was realized with silaboronate reagent and copper catalyst in aprotic solvents. The regiodivergent silaborations were controlled by tuning the copper catalysts and phosphine ligands used in reactions. This protocol provides an efficient and practical method to synthesize the multisubstituted functionalized alkenes with specific stereoselectivity.

N-Heterocyclic Carbene Complexes of Copper, Nickel, and Cobalt

The emergence of N-heterocyclic carbenes as ligands across the Periodic Table had an impact on various aspects of the coordination, organometallic, and catalytic chemistry of the 3d metals, including Cu, Ni, and Co, both from the fundamental viewpoint but also in applications, including catalysis, photophysics, bioorganometallic chemistry, materials, etc. In this review, the emergence, development, and state of the art in these three areas are described in detail.

Nickel/copper-cocatalyzed decarbonylative silylation of acyl fluorides

A transformation of acyl fluorides with silylboron via nickel/copper-cocatalysed carbon–fluorine bond cleavage and a sequential decarbonylation, which provides an efficient protocol to functionalize arylsilanes, has been disclosed.

Highly regio- and diastereoselective Cu-catalyzed hydroborylation and hydrosilylation of difluorocyclopropenes with B2pin2 and PhMe2Si–Bpin

An efficient method for the synthesis of trans-difluorocyclopropyl boronates and silanes through a Cu-catalyzed hydroborylation and hydrosilylation of difluorocyclopropenes was reported.

Asymmetric Catalysis Using Aromatic Aldehydes as Chiral α-Alkoxyalkyl Anions

We have developed a new umpolung strategy for catalytically forming a chiral α-alkoxyalkyl anion from an aromatic aldehyde for use in asymmetric synthesis. The reaction between aromatic aldehydes and aryl or allyl electrophiles with a silylboronate utilizing a chiral copper-N-heterocyclic carbene catalyst and a palladium-bisphosphine catalyst in a synergistic manner occurred with high enantioselectivities to deliver the three-component coupling products, chiral silyl-protected secondary alcohol derivatives. Our method features the catalytic generation of enantioenriched chiral α-alkoxyalkylcopper(I) intermediates from aldehydes and their subsequent palladium-catalyzed stereospecific cross-coupling.

Synergistic palladium/copper-catalyzed Csp3-Csp2 cross-couplings using aldehydes as latent α-alkoxyalkyl anion equivalents

The first Csp3-Csp2 cross-coupling using aldehydes as latent α-alkoxyalkyl anion equivalents has been achieved. The synergistic palladium/copper-catalyzed reaction of aromatic aldehydes and aryl bromides with a silylboronate afforded the three-component coupling products, silyl-protected benzhydrol derivatives. The reaction pathway involves the catalytic formation of a nucleophilic α-silyloxybenzylcopper(i) species followed by its palladium-catalyzed cross-coupling with aryl bromides.

[(18-C-6)K][(N≡C)CuI-SiMe2Ph], a Potassium Silylcyanocuprate as a Catalyst Model for Silylation Reactions with Silylboranes: Syntheses, Structures, and Catalytic Properties

Cu^I-catalyzed silylation reactions involving silylboranes (in particular, pinB-SiMe₂Ph (1)) as silyl sources have recently gained considerable attention. One of the most efficient and versatile and yet simplest catalyst systems consists of CuCN/NaOMe; however, nothing is known about the catalytically relevant species. Using an NHC-based model catalyst, copper silyl complexes of the type [(NHC)Cu-SiMe₂Ph] have been established to be crucial species in these catalytic processes. The well-defined and spectroscopically and structurally characterized complex [(18-C-6)K][NC-Cu-OtBu] (2), as a model for the catalytic system, CuCN/NaOMe, shows comparable catalytic activity toward established, exemplary substrates (aldehydes, imines, α,β-unsaturated carbonyls) and in extension allows the efficient silylation of ketones. In addition, a number of peculiarities of the catalytic reaction are readily rationalized on the basis of the mechanistic insight already established using [(NHC)Cu-SiMe₂Ph] as a model catalyst. Analogously to the NHC model system, the reaction of 2 with the silylborane 1 furnishes the silylcyanocuprate [(18-C-6)K][NC-Cu-SiMe₂Ph] (3) as a potential crucial intermediate in these silylation reactions also suggesting mechanistic similarities between (NHC)Cu- and CuCN/NaOMe-based catalyst systems. Moreover, 3 and [(NHC)Cu-SiMe₂Ph] complexes also share structurally distinctive features. In the solid state 3 either exists as a linear, two-coordinated copper complex or, depending on the conditions of crystallization, forms binuclear μ-silyl bridged dimers exhibiting very short Cu···Cu distances. Both structural motifs are also known for [(NHC)Cu-SiR₃] complexes. These findings give an initial insight into the versatile structural chemistry of certain silylcyanocuprates; in particular, the finding of dinuclear silylcuprates gives rise to the question whether these dimeric species are of mechanistic relevance for the catalytic processes. However, all peculiarities of the investigated catalytic reaction can readily be rationalized on the basis of the mechanistic details established using (NHC)Cu model complexes.

Decarbonylative Silylation of Esters by Combined Nickel and Copper Catalysis for the Synthesis of Arylsilanes and Heteroarylsilanes

An efficient nickel/copper-catalyzed decarbonylative silylation reaction of carboxylic acid esters with silylboranes is described. This reaction provides access to structurally diverse silanes with high efficiency and excellent functional-group tolerance starting from readily available esters.

[(NHC)Cu(I)-ER3] Complexes (ER3 = SiMe2Ph, SiPh3, SnMe3): From Linear, Mononuclear Complexes to Polynuclear Complexes with Ultrashort Cu(I)···Cu(I) Distances

A series of complexes of the type [(NHC)Cu-ER3] (NHC = IDipp, IMes, ItBu, Me2IMe, and ER3 = SiMe2Ph, SiPh3, SnMe3) and [(NHC)Cu-R'] (NHC = IDipp, Me2IMe and R' = Ph, C≡CPh) was synthesized in good yields by the reaction of the corresponding [(NHC)Cu-OtBu] complex with the respective silylborane pinB-ER3 (pin = OCMe2CMe2O; ER3 = SiMe2Ph, SiPh3), the stannylborane ((C2H4)(iPrN)2)B-SnMe3, or a boronic acid ester pinB-R' (R' = Ph, C≡CPh). Solid structures of all complexes were systematically studied by X-ray diffraction analysis. The solid state structures of the complexes [(NHC)Cu-ER3] show a dependence of the structural motif from the steric properties of the NHC ligand. The sterically demanding NHC ligands (IDipp, IMes, ItBu) lead to monomeric, linear complexes [(NHC)Cu-ER3], while with the less demanding Me2IMe ligand, polynuclear, μ-ER3-bridged complexes with ultrashort Cu···Cu distances are observed. For the related complexes [(NHC)Cu-R'] no analogous complexes with bridging anionic ligands are realized. Instead, irrespective of the NHC ligand, linear coordinated copper complexes of different types are formed. (29)Si heteronuclear solution NMR spectroscopic data on [(NHC)Cu(I)-SiR3] exhibit distinctly different chemical shifts for the (in the solid state) monomeric and dimeric complexes suggesting different structure types also in solution. This agrees well with the observation of a trinuclear complex [(Me2IMe)Cu-SnMe3]3 both in the solid state and in solution. Initial catalytic studies suggest that [(NHC)Cu-OtBu] complexes (NHC = ItBu, Me2IMe) are, in addition to the established [(IDipp)Cu-OtBu] complex, efficient precatalysts for the silylation of aldehydes and α,β-unsaturated ketones with pinB-SiMe2Ph.

Intermolecular Three-Component Arylsilylation of Alkynes under Palladium/Copper Cooperative Catalysis

An intermolecular three-component arylsilylation of alkynes has been developed under mild palladium/copper cooperative catalysis. The reaction proceeds through syn-addition of an aryl group and a silyl group across the carbon-carbon triple bond of an alkyne. This represents the first transition-metal-catalyzed fully intermolecular arylsilylation of alkynes, and transformations of the resulting products have also been demonstrated.

Methane activation by metal-free Lewis acid centers only – a computational design and mechanism study

The design strategy and mechanism of methane activation by metal-free Lewis acidic silylboranes is investigated.

Copper-catalyzed silylation of p-quinone methides: new entry to dibenzylic silanes

An efficient and general copper(I)-catalyzed silylation of p-quinone-methides is described. Non-symmetric dibenzylic silanes are obtained in high yields under mild reaction conditions. These compounds can be used as bench-stable benzylic carbanion precursors.

Synthesis, structural characterization and thermal properties of copper and silver silyl complexes

A series of copper and silver-silyl complexes containing N-heterocyclic carbene or N-donor ligands were synthesized and characterized in the solid state. A number of different structural forms were observed and many compounds were shown to be volatile.