Simple access to elusive α-boryl carbanions and their alkylation: an umpolung construction for organic synthesis

Diverse Synthesis of (Thio)ethers and (Thio)esters Using Halodiborylmethane as a Transformable C1 Building Block

The development of effective strategies to forge C-O and C-S bonds in diverse chemical spaces is of considerable interest in synthetic organic chemistry. Herein we report a versatile approach for the modular synthesis of structurally diverse (thio)ethers and (thio)esters via homologative coupling of α-halodiborylmethane followed by transformation of the introduced diborylmethyl group. This method accommodates a wide array of oxygen- and sulfur-containing molecules, including biologically active compounds. The initial coupling exhibits a broad substrate scope, while subsequent diversification of the diborylmethyl moiety enables access to various structural motifs through deborylative alkylation, Zweifel olefination, and boron-Wittig reaction. This protocol efficiently generates diversely functionalized (thio)ethers and (thio)esters, expanding the toolkit for accessing biologically relevant scaffolds.

Photoredox/Cu-Catalyzed Decarboxylative C(sp3)-C(sp3) Coupling to Access C(sp3)-Rich gem-Diborylalkanes

gem-Diborylalkanes are highly valuable building blocks in organic synthesis and pharmaceutical chemistry due to their ability to participate in multi-step cross-coupling transformations, allowing for the rapid generation of molecular complexity. While progress has been made in their synthetic metholodology, the construction of β-tertiary and C(sp3)-rich gem-diborylalkanes remains a synthetic challenge due to substrate limitations and steric hindrance issues. An approach is presented that utilizes synergistic photoredox and copper catalysis to achieve efficient C(sp3)-C(sp3) cross-coupling of alkyl N-hydroxyphthalimide esters, which can easily be obtained from alkyl carboxylic acids, with diborylmethyl species, providing a series of C(sp3)-rich gem-diborylalkanes with 1°, 2°, and even 3° β positions. Furthermore, this approach can also be applied to complex medicinal compounds and natural products, offering rapid access to molecular complexity and late-stage functionalization of C(sp3)-rich drug candidates. Mechanistic experiments revealed that diborylmethyl Cu(I) species participated in both the photoredox process and the key C(sp3)-C(sp3) bond-forming step.

Electrochemistry-Enabled C-Heteroatom Bond Formation of Alkyl Germanes

Because of their robustness and orthogonal reactivity features, alkyl germanes bear significant potential as functional handles for the construction of C(sp3)-rich scaffolds, especially in the context of modular synthetic approaches. However, to date, only radical-based reactivity has been accessible from these functional handles, which limits the types of possible decorations. Here, we describe the first general C(sp3)-heteroatom bond formation of alkyl germanes (-GeEt3) by leveraging electrochemistry to unlock polar reactivity. This approach allowed us to couple C(sp3)-GeEt3 with a variety of nucleophiles to construct ethers, esters, amines, amides, sulfonamides, sulfides, as well as C-P, C-F, and C-C bonds. The compatibility of the electrochemical approach with a modular synthetic strategy of a C1 motif was also showcased, involving the sequential functionalization of Cl, Bpin, and ultimately GeEt3 via electrochemistry.

Copper-catalysed asymmetric hydroboration of alkenes with 1,2-benzazaborines to access chiral naphthalene isosteres

Bioisosteric replacement has emerged as a clear strategy for drug-structure optimization. Naphthalene is the core element of many chiral pharmaceuticals and drug candidates. However, as a promising isostere of naphthalene, the chiral version of 1,2-benzazaborine has rarely been explored due to the lack of efficient synthetic methods. Here we describe a copper-catalysed enantioselective hydroboration of alkenes with 1,2-benzazaborines. The method provides a general platform for the atom-economic and efficient construction of diverse chiral 1,2-benzazaborine compounds (more than 60 examples) that bear a 2-carbon-stereogenic centre or allene skeleton in high yields and excellent enantioselectivities. Three 1,2-benzazaborine analogues of bioactive chiral naphthalene-containing molecules have been prepared, and a series of transformations around chiral 1,2-benzazaborines have also been developed. Notably, the hydroboration process of this study reveals that the identity of 1,2-benzazaborine plays an essential role in the rate-determining step and catalyst resting state.

PolyBorylated Alkenes as Energy-Transfer Reactive Groups: Access to Multi-Borylated Cyclobutanes Combined with Hydrogen Atom Transfer Event

While polyborylated alkenes are being recognized for their elevated status as highly valuable reagents in modern organic synthesis, allowing efficient access to a diverse array of transformations, including the formation of C-C and C-heteroatom bonds, their potential as energy-transfer reactive groups has remained unexplored. Yet, this potential holds the key to generating elusive polyborylated biradical species, which can be captured by olefins, thereby leading to the construction of new highly-borylated scaffolds. Herein, we report a designed energy-transfer strategy for photosensitized [2+2]-cycloadditions of poly-borylated alkenes with various olefins enabling the regioselective synthesis of diverse poly-borylated cyclobutane motifs, including the 1,1-di-, 1,1,2-tri-, and 1,1,2,2-tetra-borylated cyclobutanes. In fact, these compounds belong to a family that presently lacks efficient synthetic pathways. Interestingly, when α-methylstyrene was used, the reaction involves an interesting 1,5-hydrogen atom transfer (HAT). Mechanistic deuterium-labeling studies have provided insight into the outcome of 1,5-hydrogen atom transfer process. In addition, the polyborylated cyclobutanes are then demonstrated to be useful in selective oxidation processes resulting in the formation of cyclobutanones and γ-lactones.

Photogeneration of α-Bimetalloid Radicals via Selective Activation of Multifunctional C1 Units

Light-driven strategies that enable the chemoselective activation of a specific bond in multifunctional systems are comparatively underexplored in comparison to transition-metal-based technologies, yet desirable when considering the controlled exploration of chemical space. With the current drive to discover next-generation therapeutics, reaction design that enables the strategic incorporation of an sp3 carbon center, containing multiple synthetic handles for the subsequent exploration of chemical space would be highly enabling. Here, we describe the photoactivation of ambiphilic C1 units to generate α-bimetalloid radicals using only a Lewis base and light source to directly activate the C-I bond. Interception of these transient radicals with various SOMOphiles enables the rapid synthesis of organic scaffolds containing synthetic handles (B, Si, and Ge) for subsequent orthogonal activation. In-depth theoretical and mechanistic studies reveal the prominent role of 2,6-lutidine in forming a photoactive charge transfer complex and in stabilizing in situ generated iodine radicals, as well as the influential role of the boron p-orbital in the activation/weakening of the C-I bond. This simple and efficient methodology enabled expedient access to functionalized 3D frameworks that can be further derivatized using available technologies for C-B and C-Si bond activation.

Simultaneous Stereoinvertive and Stereoselective C(sp3)-C(sp3) Cross-Coupling of Boronic Esters and Allylic Carbonates

With increasing interest in constructing more three-dimensional entities, there has been growing interest in cross-coupling reactions that forge C(sp3)-C(sp3) bonds, which leads to additional challenges as it is not just a more difficult bond to construct but issues of stereocontrol also arise. Herein, we report the stereocontrolled cross-coupling of enantioenriched boronic esters with racemic allylic carbonates enabled by iridium catalysis, leading to the formation of C(sp3)-C(sp3) bonds with single or vicinal stereogenic centers. The method shows broad substrate scope, enabling primary, secondary, and even tertiary boronic esters to be employed, and can be used to prepare any of the four possible stereoisomers of a coupled product with vicinal chiral centers. The new method, which combines the simultaneous enantiospecific reaction of a chiral nucleophile with the enantioselective reaction of a chiral electrophile in a single process, offers a solution for stereodivergent cross-coupling of two C(sp3) fragments.

Synthesis of Borylated (Aminomethyl)cyclopropanes Using C1-Bisnucleophiles

Lithiated 1,1-diborylalkanes have been used as nucleophilic coupling partners with a range of oxygen-based electrophiles, including esters, carbonyls, and epoxides. However, their reactivity with nitrogen-based electrophiles, such as aziridines, has remained relatively understudied. Herein, we show that lithiated 1,1-diborylalkanes react with α-halo and α-tosyl aziridines to yield borylated (aminomethyl)cyclopropanes-a privileged scaffold within medicinal chemistry. The reaction displays high levels of diastereoselectivity, enabling careful control of up to three stereocenters within a single transformation. DFT studies provide insight into the reaction mechanism, which diverges from that observed with analogous epihalohydrin starting materials. Derivatization studies were also performed on the products to demonstrate the utility of the boron and amine handles.

Palladium-Catalyzed Cascade Heck Coupling and Allylboration of Iododiboron Compounds via Diboryl Radicals

Geminal bis(boronates) are versatile synthetic building blocks in organic chemistry. The fact that they predominantly serve as nucleophiles in the previous reports, however, has restrained their synthetic potential. Herein we disclose the ambiphilic reactivity of α-halogenated geminal bis(boronates), of which the first catalytic utilization was accomplished by merging a formal Heck cross-coupling with a highly diastereoselective allylboration of aldehydes or imines, providing a new avenue for rapid assembly of polyfunctionalized boron-containing compounds. We demonstrated that this cascade reaction is highly efficient and compatible with various functional groups, and a wide range of heterocycles. In contrast to a classical Pd(0/II) scenario, mechanistic experiments and DFT calculations have provided strong evidence for a catalytic cycle involving Pd(I)/diboryl carbon radical intermediates.

Sodium-Mediated Reductive C-C Bond Cleavage Assisted by Boryl Groups

In contrast to the well-established oxidative C=C double bond cleavage to give the corresponding carbonyl compounds, little is known about reductive C=C double bond cleavage. Here we report that C-C single bond cleavage in 1,2-diaryl-1,2-diborylethanes proceeds by reduction with sodium metal to yield α-boryl benzylsodium species. In combination with our previous reductive diboration of stilbenes, the overall transformation represents reductive cleavage of the C=C double bonds of stilbene to yield α-boryl-α-sodiated toluenes. This reductive two-step C=C double bond cleavage is applicable to ring-opening or ring-expansion reactions of polycyclic aromatic hydrocarbons.

Triply Selective & Sequential Diversification at Csp 3: Expansion of Alkyl Germane Reactivity for C-C & C-Heteroatom Bond Formation

We report the triply selective and sequential diversification of a single Csp 3 carbon carrying Cl, Bpin and GeEt3 for the modular and programmable construction of sp3-rich molecules. Various functionalizations of Csp 3-Cl and Csp 3-BPin (e.g. alkylation, arylation, homologation, amination, hydroxylation) were tolerated by the Csp 3-GeEt3 group. Moreover, the methodological repertoire of alkyl germane functionalization was significantly expanded beyond the hitherto known Giese addition and arylation to alkynylation, alkenylation, cyanation, halogenation, azidation, C-S bond formation as well as the first demonstration of stereo-selective functionalization of a Csp 3-[Ge] bond.

α-Boryl Carbanions: The Influence of Geminal Heteroatoms in C-C Bond Formation

The wide applications of alpha-boryl carbanions in selective coupling with organohalides, imines/carbonyls and conjugated unsaturated substrates has become an interesting tool for organic synthesis. Strategically, the inclusion of heteroatoms, such as Si, S, N, F, Cl, Br and I in the alpha position opens a new venue towards multifunctionalities in molecular design. Here, a conceptual and practical view on powerful carbanions, containing α-silicoboron, α-thioboron, α-haloboron and α-aminoboron is given, as well as a prespective on their efficient application for selective electrophilic trapping.

Regioselective 1,2-Alkylboration of Benzylidenecyclopropanes: Access to Csp3-Enriched Cyclopropyl Boronic Esters

We describe a novel, regioselective alkylboration of versatile (hetero)benzylidenecyclopropanes with β-H-containing alkyl iodides and bis(pinacolato)diboron enabled by copper catalysis. This three-component method allows for consecutive B-Csp3 and Csp3-Csp3 bond formation to access Csp3-enriched diverse tertiary cyclopropyl boronic esters with broad functionality tolerance, and the so-formed C-B bond is amenable to further structural diversification. Radical clock experiment, Hammett analysis, and DFT calculation suggest a mechanism of polar, rather than radical manifold, and SN2-type C-C bond formation was found to be the rate-limiting step instead of migratory alkene insertion.

A General Method to Access Sterically Encumbered Geminal Bis(boronates) via Formal Umpolung Transformation of Terminal Diboron Compounds

General methods for the preparation of geminal bis(boronates) are of great interest due to their widespread applications in organic synthesis. While the terminal gem-diboron compounds are readily accessible, the construction of the sterically encumbered, internal analogues has remained a prominent challenge. Herein, we report a formal umpolung strategy to access these valuable building blocks. The readily available 1,1-diborylalkanes were first converted into the corresponding α-halogenated derivatives, which then serve as electrophilic components, undergoing a formal substitution with a diverse array of nucleophiles to form a series of C-C, C-O, C-S, and C-N bonds. This protocol features good tolerance to steric hindrance and a wide variety of functional groups and heterocycles. Notably, this strategy can also be extended to the synthesis of diaryl and terminal gem-diboron compounds, therefore providing a general approach to various types of geminal bis(boronates).

Diborodichloromethane as Versatile Reagent for Chemodivergent Synthesis of gem-Diborylalkanes

The development of boron reagents is crucial for synthetic chemistry. Herein, we present a scalable and practical synthesis of diborodichloromethane (DBDCM) through the reaction of trichloromethyllithium with bis(pinacolato)diboron (B2 pin2 ). The resulting DBDCM reagent serves as a basic synthetic unit for the construction of various structurally diverse gem-diborylalkanes through controllable C-Cl functionalizations. Moreover, we have developed consecutive tetra-functionalizations of DBDCM for the construction of diverse tertiary and quaternary carbon containing molecules. The use of isotopically enriched 13 C-chloroform and 10 B2 pin2 enables the synthesis of isotopically enriched 13 C-DBDCM and 10 B-DBDCM reagents, which are beneficial for the convenient synthesis of carbon-13 and boron-10 molecules.

Generation and Application of Homoallylic α,α-Diboryl Radicals via Diboron-Promoted Ring-Opening of Vinyl Cyclopropanes: cis-Diastereoselective Borylative Cycloaddition

Carbon-centered radicals stabilized by adjacent boron atoms are underexplored reaction intermediates in organic synthesis. This study reports the development of vinyl cyclopropyl diborons (VCPDBs) as a versatile source of previously unknown homoallylic α,α-diboryl radicals via thiyl radical catalyzed diboron-directed ring opening. These diboryl stabilized radicals underwent smooth [3+2] cycloaddition with a variety of olefins to provide diboryl cyclopentanes in good to excellent diastereoselectivity. In contrast to the trans-diastereoselectivity observed with most of the dicarbonyl activated VCPs, the cycloaddition of VCPDBs showed a remarkable preference for formation of cis-cyclopentane diastereomer which was confirmed by quantitative NOE and 2D NOESY studies. The cis-stereochemistry of cyclopentane products enabled a concise intramolecular Heck reaction approach to rare tricyclic cyclopentanoid framework containing the diboron group. The mild reaction conditions also allowed a one-pot VCP ring-opening, cycloaddition-oxidation sequence to afford disubstituted cyclopentanones. Control experiments and DFT analysis of reaction mechanism support a radical mediated pathway and provide a rationale for the observed diastereoselectivity. To the authors' knowledge, these are the first examples of the use of geminal diboryl group as an activator of VCP ring opening and cycloaddition reaction of α-boryl radicals.

Rapid and Practical Synthesis of gem-Dibromoalkanes from Aldehydes by Tribromide Reagent

gem-Dibromoalkanes are important synthetic building block in organic chemistry, but their preparation is still troublesome. Herein, we have developed a simple and practical protocol for the synthesis of gem-dibromoalkanes from aldehydes using tetrabutylammonium tribromide and triphenyl phosphite. A variety of alkyl and aromatic aldehydes can be transformed into the corresponding products within 10 minutes. This protocol is also applicable to alcohols, and the configuration of chiral alcohol is inverted during the process with excellent enantiopurity.

Palladium-Catalyzed Cross-Coupling of gem-Difluorocyclopropanes with gem-Diborylalkanes for the Synthesis of Boryl-Substituted Fluorinated Alkenes

This study presents a novel method for the regioselective coupling of gem-difluorinated cyclopropanes with gem-diborylmethane, utilizing a Pd-catalyst system. This innovative approach enables the synthesis of 2-fluoroalkenyl monoboronate scaffolds with high Z-selectivity. The resulting products undergo further transformations, including oxidation, Suzuki cross-coupling, and trifluoroborylation, all of which are achieved with good yields. This work introduces a valuable synthetic pathway to access important fluorinated compounds for various applications in organic chemistry.

Base-mediated C-B bond activation of benzylic boronate for the rapid construction of β-silyl/boryl functionalized 1,1-diarylalkanes from aromatic alkenes

The effect of tBuOK on the existing state of benzylic boronates in the solution phase has been investigated in detail by NMR analysis and DFT calculations. It was determined that simply using an excess of tBuOK (2.0 equivalents) can result in the full deborylation of benzylic boronates to afford free benzyl potassium species. These mechanistic insights were leveraged for the facile construction of β-silyl/boryl functionalized 1,1-diarylalkanes from aromatic alkenes via the combination of base-mediated silylboration or diborylation of aromatic alkenes and nucleophilic-type reactions with various electrophiles. Based on further machine-learning-assisted screening, the scope of electrophiles for this transformation can be generalized to the challenging aromatic heterocycles. Late-stage functionalization performed on several drug-relevant molecules generates the highly valuable 1,1-diaryl framework.

Copper-Catalyzed Asymmetric Acylboration of 1,3-Butadienylboronate with Acyl Fluorides

We report herein a Cu-catalyzed regio-, diastereo- and enantioselective acylboration of 1,3-butadienylboronate with acyl fluorides. Under the developed conditions, the reactions provide (Z)-β,γ-unsaturated ketones bearing an α-tertiary stereocenter with high Z-selectivity and excellent enantioselectivities. While direct access to highly enantioenriched E-isomers was not successful, we showed that such molecules can be synthesized with excellent E-selectivity and optical purities via Pd-catalyzed alkene isomerization from the corresponding Z-isomers. The orthogonal chemical reactivities of the functional groups embedded in the ketone products allow for diverse chemoselective transformations, which provides a valuable platform for further derivatization.

Photochemical [2+2] Cycloaddition of Alkynyl Boronates

A photochemical [2+2] cycloaddition of alkynyl boronates and maleimides is reported. The developed protocol provided 35-70 % yield of maleimide-derived cyclobutenyl boronates and demonstrated wide compatibility with various functional groups. The synthetic utility of the prepared building blocks was demonstrated for a range of transformations, including Suzuki cross-coupling, catalytic or metal-hydride reduction, oxidation, and cycloaddition reactions. With aryl-substituted alkynyl boronates, the products of double [2+2] cycloaddition were obtained predominantly. Using the developed protocol, a cyclobutene-derived analogue of Thalidomide was prepared in one step. Mechanistic studies supported the participation of the triplet-excited state maleimides and ground state alkynyl boronates in the key step of the process.

Multicomponent Reaction: Pd/Cu-Catalyzed Borocarbonylation of Aryldiazonium Salts with Aliphatic Terminal Alkynes to gem-Bis(boryl) Ketones

We report the development of Pd/Cu-catalyzed selective 2,1-borocarbonylation reactions of aliphatic terminal alkynes with aryldiazonium salts and B2 Pin2 to prepare gem-bis(boryl) ketones in one-pot. A series of corresponding products are obtained with good to excellent yields under a carbon monoxide atmosphere (10 bar). In addition, wide functional-group tolerance can be observed. Preliminary mechanistic studies reveal that ethyl acetate serves as a proton source in the reaction.

Photocatalyzed Borylcyclopropanation of Alkenes with a (Diborylmethyl)iodide Reagent

Cyclopropane skeletons play a prominent role in the development of organic synthesis and pharmaceutical chemistry. Herein, we report the design and synthesis of a stable, multifunctional (diborylmethyl)iodide reagent (CHI(Bpin)2 ) for the photoinduced cyclopropanation of alkenes, providing an array of 1,2-substituted cyclopropylboronates in good yields. This α-haloboronic ester can be readily synthesized on a multigram scale from commercially available starting materials. Furthermore, the protocol displays high chemo- and diastereoselectivity, excellent functional-group tolerance, and allows for late-stage borylcyclopropanation of complex molecules. Mechanistic studies reveal that the borylcyclopropanation proceeds through a radical addition/polar cyclization pathway mediated by the photocatalyst fac-Ir(ppy)3 and visible light.

Synthesis of Quaternary and Tertiary Carbon-Substituted Arenes by Lewis Base Promoted Site-Selective Coupling with Allylic Nucleophiles

A practical method for the preparation of quaternary and tertiary allyl-substituted heteroarenes by site-selective couplings of heteroaryl nitriles and allylic nucleophiles is disclosed. Transformations utilize readily accessible stable reagents, proceed in the presence of a Lewis base activator, and undergo aryl-C(sp3 ) quaternary and tertiary carbon formation with high γ-selectivity (up to >98 : 2 γ : α).

Facile preparation of organosilanes from benzylboronates and gem-diborylalkanes mediated by KOtBu

Methods to efficiently synthesize organosilanes are valuable in the fields of synthetic chemistry and materials science. During the past decades, boron conversion has become a generic and powerful approach for constructing carbon-carbon and other carbon-heteroatom bonds, but its potential application in forming carbon-silicon remains unexplored. Herein, we describe an alkoxide base-promoted deborylative silylation of benzylic organoboronates, geminal bis(boronates) or alkyltriboronates, allowing for straightforward access to synthetically valuable organosilanes. This selective deborylative methodology exhibits operational simplicity, broad substrate scope, excellent functional group compatibility and convenient scalability, providing an effective and complementary platform for the generation of diversified benzyl silanes and silylboronates. Detailed experimental results and calculated studies revealed an unusual mechanistic feature of this C-Si bond formation.

Highly Stereoselective Syntheses of α,α-Disubstituted (E)- and (Z)-Crotylboronates

We report herein stereoselective syntheses of α,α-disubstituted (E)- and (Z)-crotylboronates. Starting from α-boryl (E)- or (Z)-crotylboronate, base-mediated alkylation occurred exclusively at the position α to the boryl groups to give targeted boronates while retaining the geometries of the alkenes in the starting crotylboronates. Under proper conditions, the resulting α,α-disubstituted crotylboronates underwent aldehyde addition to give allylated products with high stereoselectivities.

Nickel-Catalyzed Negishi Cross-Coupling of Alkyl Halides, Including Unactivated Tertiary Halides, with a Boron-Stabilized Organozinc Reagent

Nickel-catalyzed cross-coupling of unactivated tertiary alkyl electrophiles with alkylmetal reagents is still a challenge. We report herein a nickel-catalyzed Negishi cross-coupling of alkyl halides, including unactivated tertiary halides, with boron-stabilized organozinc reagent BpinCH₂ZnI, yielding versatile organoboron products with high functional-group tolerance. Importantly, the Bpin group was found to be indispensable for accessing the quaternary carbon center. The synthetic practicability of the prepared quaternary organoboronates was demonstrated by their conversion to other useful compounds.

Synthesis of gem-Diboromethyl-Substituted Bicyclo[1.1.1]pentanes and Their Application in Palladium-Catalyzed Cross Couplings

We describe the first general transition-metal-free synthesis of gem-diboromethyl-substituted bicyclo[1.1.1]pentane (BCP) and other related C(sp³)-rich carbocyclic benzene bioisosteres from their corresponding p-tosylhydrazones. These novel functionalized benzene bioisosteres demonstrated unique reactivities toward palladium-catalyzed C(sp²)-C(sp³) cross couplings. The overall transformation can be applied to relatively complex substrates with potential utility in drug discovery.

Synthesis of 3-borylated cyclobutanols from epihalohydrins or epoxy alcohol derivatives

There is an increasing interest in cyclobutanes within the medicinal chemistry community. Therefore, methods to prepare cyclobutanes that contain synthetic handles for further elaboration are of interest. Herein, we report a new approach for the synthesis of 3-borylated cyclobutanols via a formal [3 + 1]-cycloaddition using readily accessible 1,1-diborylalkanes and epihalohydrins or epoxy alcohol derivatives. 1-Substituted epibromohydrin starting materials provide access to borylated cyclobutanols containing substituents at three of the four positions on the cyclobutane core, and enantioenriched epibromohydrins lead to enantioenriched cyclobutanols with high levels of enantiospecificity (>98%). Finally, derivatization studies demonstrate the synthetic utility of both the OH and Bpin handles.

A Stereodivergent Approach to the Synthesis of gem-Diborylcyclopropanes

We report a designed stereodivergent strategy for the synthesis of gem-diborylcyclopropanes. The reaction provides a highly modular approach to prepare cyclopropane ring variants bearing gem-(Bpin,Bpin), gem-(Bpin,Bdan), and gem-(Bpin,BF₃ K), with outstanding levels of stereocontrol. This was achieved by diastereoselective Pd-catalyzed cyclopropanation reactions of gem-diborylalkenes with α-diazoarylacetates and α-diazoaryl-trifluoromethyl. The key to the success of this general protocol was the diastereoselective trifluorination reaction of gem-diborylcyclopropanes, followed by the stereospecific interconversion of the trifluoroborate salts into the Bdan group.

Direct deoxygenative borylation

Direct deoxygenative borylation is a highly enabling chemical transformation considering the attractive synthetic features of oxygenous feedstocks and organoboron compounds. Despite ranking among the synthetic ideality in different settings, such chemical space remained largely uncharted and underutilized until recent decades. This short review will summarize some key advances in the field of direct deoxy-borylation of alcohols, ethers, aldehydes, ketones, and carboxylic acids and organize these contributions based on substrate classes. In each representative, the general features, including reaction conditions, product scopes and mechanistic insights, will be highlighted and discussed.

Diastereodivergent Synthesis of Cyclopentyl Boronic Esters Bearing Contiguous Fully Substituted Stereocenters

The synthesis of molecules bearing two or more contiguous, quaternary stereocenters is challenging, owing to the difficulty in controlling stereochemistry whilst simultaneously constructing a sterically congested motif. Herein, we report the electrophile-induced ring contractive 1,2-metallate rearrangement of 6-membered cyclic alkenyl boronate complexes for the synthesis of cyclopentyl boronic esters bearing two contiguous, fully substituted stereocenters with high levels of stereocontrol. Remarkably, simple variation of the reaction solvent enabled their diastereodivergent construction with facile access to complementary diastereomeric pairs. The utility of our methodology is demonstrated in the asymmetric total synthesis of (+)-herbertene-1,14-diol.

Boron-Promoted Deprotonative Conjugate Addition: Geminal Diborons as Soft Pronucleophiles and Acyl Anion Equivalents

Conjugate addition of α-boron-stabilized carbanions is an underexplored reaction modality. Existing methods require deborylation of geminal di-/triboryl alkanes and/or the presence of additional activating groups. We report the 1,4-addition of α,α-diboryl carbanions generated via deprotonation of the corresponding geminal diborons. The methodology provided a general route to highly substituted and synthetically useful γ,γ-diboryl ketones. The development of geminal diborons as soft pronucleophiles also enabled their use as acyl anion equivalents via a one-pot tandem conjugate addition-oxidation sequence.

Stereoselective Cyclopropanation of 1,1-Diborylalkenes via Palladium-Catalyzed (Trimethylsilyl)diazomethane Insertion

Palladium catalyzes the cyclopropanation of 2-substituted 1,1-diborylalkenes with (trimethylsilyl)diazomethane. The relative stereoselectivity is controlled via a carbene insertion sequence generating an exclusive anti conformation between the R and SiMe₃ substituents. Mixed 1,1-diborylalkenes also contributed to the formation of stereoselective B, B, Si-cyclopropanes. Orthogonal activation with NaO^tBu gives protodeborylation preferentially on the boron moiety syn to the aryl group. Further oxidation gives access to polyfunctional cyclopropyl alcohols with controlled enantioselectivity when chiral boryl motifs are involved.

Palladium-Catalyzed Cross-Coupling Reaction of Diarylmethanol Derivatives with Diborylmethane

A palladium-catalyzed cross-coupling reaction of diarylmethanol derivatives with diborylmethane has been developed. The reaction proceeds chemoselectively to deliver the corresponding homobenzylic boronates in good yields.

Simmons-Smith Cyclopropanation of Alkenyl 1,2-Bis(boronates): Stereoselective Access to Functionalized Cyclopropyl Derivatives

A Simmons-Smith stereodefined procedure for the synthesis of cyclopropyl-1,2-bis(boronates) has been developed starting from the corresponding alkenes. The resulting compounds were then subjected to regioselective Suzuki-Miyaura couplings to produce diversely tri- or tetra-substituted arylcyclopropanes in good yields. Further functionalization with 2-lithiothiophene provided 1,2-bis(aryl)cyclopropanes.

Capturing Reactive Carbanions by Microdroplets

Carbanions appear in many organic or biological reactions as fleeting intermediates, prohibiting direct observation or spectroscopic measurement. An aqueous environment is known to rapidly annihilate a carbanion species, reducing its lifetime to as short as picoseconds. We report that aqueous microdroplets can capture and stabilize reactive carbanion intermediates isolated from four classic organic reactions, aldol and Knoevenagel condensations, alkyne alkylation, and the Reimer-Tiemann reaction, enabling the detection of their carbanion intermediates by desorption electrospray ionization mass spectrometry. This is accomplished in real time of the reaction, allowing new insights into reaction mechanisms to be obtained. The efficacy of microdroplets in capturing such elusive species was examined by varying the solvent and the microdroplet negative charge density. We observed that microdroplets composed of water-methanol outperform other solvents, such as pure water, in capturing carbanions, which is in contrast to the earlier report that presented the highest performance of pure water microdroplets in capturing carbocations. We offer some mechanistic insights to explain the discriminatory behavior of these two oppositely charged species in microdroplets.

Synergistic Hydrocobaltation and Borylcobaltation Enable Regioselective Migratory Triborylation of Unactivated Alkenes

The structural diversity of sp³ -triorganometallic reagents enhances their potentiality in the modular construction of molecular complexity in chemical synthesis. Despite significant achievements on the preparation of sp³ 1,1,1- and 1,1,2-triorganometallic B,B,B-reagents, catalytic approaches that enable the installation of multiple boryl groups at skipped carbons of unactivated alkenes still remain elusive. Herein, we report a cobalt-catalyzed selective triborylation reaction of unactivated alkenes to access synthetically versatile 1,1,3-triborylalkanes. This triborylation protocol provides a general platform for regioselective trifunctionalization of unactivated alkenes, and its utility is highlighted by the synthesis of various value-added chemicals from readily accessible unactivated alkenes. Mechanistic studies, including deuterium-labelling experiments and evaluation of potential reactive intermediates, provide insight into the experimentally observed chemo- and regioselectivity.