Ate Complexes of Secondary Boronic Esters as Chiral Organometallic-Type Nucleophiles for Asymmetric Synthesis

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.

Stereospecific Phosphination and Thioetherification of Organoboronic Esters

Alkyllithium-activated organoboronic esters are found to undergo stereospecific phosphination with copper chloride and chlorophosphines. They also react with thiolsulfonate electrophiles under copper catalysis. These reactions enable stereospecific phosphination and thiolation of organoboronic esters, which are further applied in preparation of chiral ligands and biologically active molecules.

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.

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).

Dual Function of N-Iodosuccinimide for C(sp3)-B Bond Activation

A practical method for C(sp3)-B bond activation was developed. Using a combination of alkyl trifluoroborates and N-iodosuccinimide (NIS), various C(sp3)-heteroatom bonds were readily generated in an efficient manner. Mechanistic studies revealed the bifunctional ability of NIS: mediating the formation of reactive halogenated intermediates and activating them via halogen bonding. This electrophilic activation of the reaction center enables the utilization of general heteroatom nucleophiles, which are used in a limited capacity in traditional 1,2-metalate rearrangements.

Chemoenzymatic Synthesis of Cylindrocyclophanes A and F and Merocyclophanes A and D

Incorporating enzymatic reactions into natural product synthesis can significantly improve synthetic efficiency and selectivity. In contrast to the increasing applications of biocatalytic functional-group interconversions, the use of enzymatic C-C bond formation reactions in natural product synthesis is underexplored. Herein, we report a concise and efficient approach for the synthesis of [7.7]paracyclophane natural products, a family of polyketides with diverse biological activities. By using enzymatic Friedel-Crafts alkylation, cylindrocyclophanes A and F and merocyclophanes A and D were synthesized in six to eight steps in the longest linear sequence. This study demonstrates the power of combining enzymatic reactions with contemporary synthetic methodologies and provides opportunities for the structure-activity relationship studies of [7.7]paracyclophane natural products.

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.

Boron-Mediated Regioselective Aromatic C-H Functionalization via an Aryl BF2 Complex

An efficient regioselective functionalization of 2-aryl-heteroarenes and aryl aldehydes via an azaaryl BF₂ complex has been developed. Mechanistically the reaction comprises fluoride to bromide ligand exchange on an aryl boron species and consecutive C-B bond cleavage to deliver a broad range of functionalized products. The reaction is high yielding, has a broad substrate scope where several different heteroarenes can be functionalized with chloro, bromo, iodo, hydroxyl, amine and BF₂ in a highly regioselective fashion. The method can be applied for late-stage functionalization or for rapid skeleton remodeling with for instance cross-couplings.

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.

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.

Enantioselective Copper-Catalyzed sp2/sp3 Diborylation of 1-Chloro-1-Trifluoromethylalkenes

Fluorine-containing organoboron compounds have emerged as novel building blocks in chemical synthesis; among them, fluorinated sp²/sp³ diborylated compounds are particularly appealing, since they might undergo chemoselective and diversified transformations of different C-B bonds with fluorinated functionality, thus bringing versatility and complexity to the eventual products. However, expedient, synthetic strategies for the construction of such fluorinated diborylative compounds are very sparse. Herein, we disclose enantioselective Cu-catalyzed sp²/sp³ diborylations of 1-chloro-1-trifluoromethylalkenes, leading to diborylated compounds bearing a gem-difluoroalkenyl moiety; most intriguingly, the new formed C-B bonds include one stereoselective and optically pure Csp³-B bond. Further transformations on the eventual products demonstrated the values of our presented strategy.

Dual Nickel/Photoredox-Catalyzed Site-Selective Cross-Coupling of 1,2-Bis-Boronic Esters Enabled by 1,2-Boron Shifts

Site‐selective transition‐metal‐catalyzed mono‐deboronative cross‐couplings of 1,2‐bis‐boronic esters are valuable methods for the synthesis of functionalized organoboron compounds. However, such cross‐couplings are limited to reaction of the sterically less hindered primary boronic ester. Herein, we report a nickel/photoredox‐catalyzed mono‐deboronative arylation of 1,2‐bis‐boronic esters that is selective for coupling of the more sterically hindered secondary/tertiary position. This is achieved by taking advantage of a 1,2‐boron shift of primary β‐boryl radicals to the thermodynamically favored secondary/tertiary radicals, which are subsequently intercepted by the nickel catalyst to enable arylation. The mild conditions are amenable to a broad range of aryl halides to give β‐aryl boronic ester products in good yields and with high regioselectivity. This method also allows stereodivergent coupling of cyclic cis‐1,2‐bis‐boronic esters to give trans‐substituted products.

Enantioselective Cobalt-Catalyzed Hydroboration of Ketone-Derived Silyl Enol Ethers

Catalytic asymmetric hydroboration of alkenes is a powerful tool for the synthesis of natural products, agrochemicals, and pharmaceuticals via the versatile transformations of chiral alkyl boronic esters. However, the scope of available alkenes is limited to styrenes, activated alkenes, and compounds with directing groups. The catalytic enantioselective hydroboration of heteroatom-substituted alkenes is rarely explored and those catalyzed by earth-abundant metals are yet to be reported. Herein, we report a cobalt-catalyzed asymmetric hydroboration of ketone-derived silyl enol ethers and provide a convenient approach to access valuable enantiopure β-hydroxy boronic esters. This protocol features mild reaction conditions, a broad substrate scope, and excellent enantioselectivities (up to 99 % ee). This approach was applied in the successful synthesis of salmeterol and albuterol, demonstrating its potential to streamline complex molecule synthesis.

Remote Site-Selective Asymmetric Protoboration of Unactivated Alkenes Enabled by Bimetallic Relay Catalysis

A remote C(sp³ )-H bond asymmetric borylation of unactivated alkenes was achieved by bimetallic relay catalysis. The reaction proceeded through reversible and consecutive β-H elimination/olefin insertion promoted by CoH species generated in situ, followed by copper-catalyzed asymmetric protoboration. The use of this synergistic Co/Cu catalysis protocol allowed the enantioselective protoboration of various unactivated terminal alkenes and internal alkenes, as well as an unrefined mixture of olefin isomers, at the distal less-reactive β-position to a functional group, leading to chiral organoboronates.

Photo-induced trifunctionalization of bromostyrenes via remote radical migration reactions of tetracoordinate boron species

Tetracoordinate boron species have emerged as radical precursors via deboronation by photo-induced single electron transfer (SET) pathway. These reactions usually produce an alkyl radical and boron-bound species, and the valuable boron species are always discarded as a by-product. Given the importance of boron species, it will be very attractive if the two parts could be incorporated into the eventual products. Herein we report a photo-catalyzed strategy in which in situ generated tetracoordinated boron species decomposed into both alkyl radicals and boron species under visible light irradiation, due to the pre-installation of a vinyl group on the aromatic ring, the newly generated alkyl radical attacks the vinyl group while leaving the boron species on ipso-position, then both radical part and boron moiety are safely incorporated into the final product. Tertiary borons, secondary borons, gem-diborons as well as 1,2-diborons, and versatile electrophiles are all well tolerated under this transformation, of note, ortho-, meta- and para-bromostyrenes all demonstrated good capabilities. The reaction portraits high atom economy, broad substrate scope, and diversified valuable products with tertiary or quaternary carbon center generated, with diborons as substrates, Csp²-B and Csp³-B are established simultaneously, which are precious synthetic building blocks in chemical synthesis.

Tetracoordinate boron species are common radical precursors in organic synthesis, but the boron species are discarded as by-products. Herein the authors report a strategy to incorporate both the alkyl moiety and boron species into the eventual products, yielding organoboron compounds.

Recent Advances in the Construction of Fluorinated Organoboron Compounds

Fluorinated organoboron compounds are important synthetic building blocks that combine the unique characteristics of a fluorinated motif with the versatile synthetic applications of organoboron moiety. This review article guides the research on fluorinated organoboron compounds mainly from four aspects in recent years: selective monodefluoroborylation of polyfluoroarenes and polyfluoroalkenes, selective borylation of fluorinated substrates, selective fluorination of organoboron compounds, and borofluorination of alkynes/olefins. In addition, this review will provide a necessary guidance and inspiration for the research on the valuable synthetic building block fluorinated organoboron compounds.

Contra-thermodynamic Olefin Isomerization by Chain-Walking Hydroboration and Dehydroboration

We report a dehydroboration process that can be coupled with chain-walking hydroboration to create a one-pot, contra-thermodynamic, short- or long-range isomerization of internal olefins to terminal olefins. This dehydroboration occurs by a sequence comprising activation with a nucleophile, iodination, and base-promoted elimination. The isomerization proceeds at room temperature without the need for a fluoride base, and the substrate scope of this isomerization is expanded over those of previous isomerizations we have reported with silanes.

Ligand-controlled cobalt-catalyzed remote hydroboration and alkene isomerization of allylic siloxanes

The Co-catalyzed remote hydroboration and alkene isomerization of allylic siloxanes were realized by a ligand-controlled strategy. The remote hydroboration with dcype provided borylethers, while xantphos favored the formation of silyl enol ethers.

Three-component 1,2-carboamination of vinyl boronic esters via amidyl radical induced 1,2-migration

Three-component 1,2-carboamination of vinyl boronic esters with alkyl/aryl lithium reagents and N-chloro-carbamates/carboxamides is presented. Vinylboron ate complexes generated in situ from the boronic ester and an organo lithium reagent are shown to react with readily available N-chloro-carbamates/carboxamides to give valuable 1,2-aminoboronic esters. These cascades proceed in the absence of any catalyst upon simple visible light irradiation. Amidyl radicals add to the vinylboron ate complexes followed by oxidation and 1,2-alkyl/aryl migration from boron to carbon to give the corresponding carboamination products. These practical cascades show high functional group tolerance and accordingly exhibit broad substrate scope. Gram-scale reaction and diverse follow-up transformations convincingly demonstrate the synthetic potential of this method.

Saturated Boronic Acids, Boronates, and Trifluoroborates: An Update on Their Synthetic and Medicinal Chemistry

This review discusses recent advances in the chemistry of saturated boronic acids, boronates, and trifluoroborates. Applications of the title compounds in the design of boron-containing drugs are surveyed, with special emphasis on α-amino boronic derivatives. A general overview of saturated boronic compounds as modern tools to construct C(sp³ )-C and C(sp³ )-heteroatom bonds is given, including recent developments in the Suzuki-Miyaura and Chan-Lam cross-couplings, single-electron-transfer processes including metallo- and organocatalytic photoredox reactions, and transformations of boron "ate" complexes. Finally, an attempt to summarize the current state of the art in the synthesis of saturated boronic acids, boronates, and trifluoroborates is made, with a brief mention of the "classical" methods (transmetallation of organolithium/magnesium reagents with boron species, anti-Markovnikov hydroboration of alkenes, and the modification of alkenyl boron compounds) and a special focus on recent methodologies (boronation of alkyl (pseudo)halides, derivatives of carboxylic acids, alcohols, and primary amines, boronative C-H activation, novel approaches to alkene hydroboration, and 1,2-metallate-type rearrangements).

Cu-Catalyzed Cross-Coupling of Benzylboronic Esters and Epoxides

A reaction between epoxides and benzylboronic acid pinacol esters is described. CuI was found to be an effective catalyst of this transformation upon activation of the benzylboronic ester with an alkyllithium reagent. The reaction was very efficient and a variety of substituted epoxides were found to be good substrates with good regioselectivity for substitution at the less substituted side of the epoxide. A reaction using an enantioenriched secondary benzylboronic ester was found to not be stereospecific.

Transition Metal Free sp3-sp3 carbon-carbon coupling between Benzylboronic Esters and Alkyl Bromides

A transition metal free coupling reaction of benzylboronic esters and alkyl halides has been developed. Both alkyl bromides and alkyl iodides were found to be competent substrates with the nucleophilic boronate intermediate generated from the combination of benzylboronic ester and an alkyllithium. Good chemoselectivity was observed in substrates with a second electrophile. Both secondary and tertiary benzylboronic esters were effective nucleophiles in the reaction with primary alkyl halides. Mechanistic observations are consistent with a radical mechanism.

Construction of Complex Cyclobutane Building Blocks by Photosensitized [2 + 2] Cycloaddition of Vinyl Boronate Esters

Cyclobutyl moieties in drug molecules are rare, and in general, they are minimally substituted and stereochemically simple. Methods to assemble structurally complex cyclobutane building blocks suitable for rapid diversification are thus highly desirable. We report herein a photosensitized [2 + 2] cycloaddition with vinyl boronate esters affording straightforward access to complex, densely functionalized cyclobutane scaffolds. Mechanistic studies suggest an activation mode involving energy transfer to the styrenyl alkene rather than the vinyl boronate ester.

Metal-Free ortho-Selective C-H Borylation of 2-Phenylthiopyridines Using BBr3

A novel route for ortho-selective C-H borylation of 2-phenylthiopyridines using BBr₃ as the boron source under metal-free conditions has been reported. The reaction exhibited site exclusivity, and the synthesized aryl boronates were freely converted to various useful intermediates. Thus, this facile method would be beneficial to synthesize structurally diversified phenylthioethers derivatives and other materials containing boron-nitrogen coordination.

Strain-release 2-azaallyl anion addition/borylation of [1.1.1]propellane: synthesis and functionalization of benzylamine bicyclo[1.1.1]pentyl boronates

We report a 3-component reaction between N-benzyl ketimines, [1.1.1]propellane, and pinacol boronates to generate benzylamine bicyclo[1.1.1]pentane (BCP) pinacol boronates. These structures are analogous to highly sought diarylmethanamine cores, which are common motifs in bioactive molecules. We demonstrate the versatility of the boronate ester handle via downstream functionalization through a variety of reactions, including a challenging Pd-catalyzed (hetero)arylation that exhibits a broad substrate scope. Together, these methods enable the synthesis of high-value BCP benzylamines which are inaccessible by existing methods. Furthermore, we demonstrate the successful application of these newly developed (hetero)arylation conditions to a variety of challenging tertiary pinacol boronates, including nitrogen-containing heterocycles, 1,1-disubstituted cyclopropanes, and other BCP cores.

C-H Borylation of Diphenylamines through Adamantane-1-carbonyl Auxiliary by BBr3

A method for ortho-C-H borylation of diphenylamines using BBr₃ as the boron source has been reported. The noncatalytic adamantane-1-carbonyl directed reaction exhibited site exclusivity and good functional group tolerance. Generally, the borylation occurred at the more electron-rich aromatic ring and the borylated products could be converted to various useful intermediates. Besides, the derived arylation and removal of auxiliary of the product could be achieved in a one-pot fashion.

Diverse functionalization of strong alkyl C-H bonds by undirected borylation

The selective functionalization of strong, typically inert carbon-hydrogen (C-H) bonds in organic molecules is changing synthetic chemistry. However, the undirected functionalization of primary C-H bonds without competing functionalization of secondary C-H bonds is rare. The borylation of alkyl C-H bonds has occurred previously with this selectivity, but slow rates required the substrate to be the solvent or in large excess. We report an iridium catalyst ligated by 2-methylphenanthroline with activity that enables, with the substrate as limiting reagent, undirected borylation of primary C-H bonds and, when primary C-H bonds are absent or blocked, borylation of strong secondary C-H bonds. Reactions at the resulting carbon-boron bond show how these borylations can lead to the installation of a wide range of carbon-carbon and carbon-heteroatom bonds at previously inaccessible positions of organic molecules.

Iridium-Catalyzed γ-Selective Hydroboration of γ-Substituted Allylic Amides

Reported here for the first time is the Ir-catalyzed γ-selective hydroboration of γ-substituted allylic amides under mild reaction conditions. A variety of functional groups could be compatible with reaction conditions, affording γ-branched amides in good yields with ≤97% γ-selectivity. We have also demonstrated that the obtained borylated products could be used in a series of C-O, C-F, C-Br, and C-C bond-forming reactions.

Nucleophilic Addition of Benzylboronates to Activated Ketones

A method has been developed for the addition of benzylboronic acid pinacol ester to activated ketones including trifluoromethyl ketones in good yields. The use of DABCO as an additive was found to enhance the rate and efficiency of this reaction. In reactions of ketones with a second carbonyl group present such as an ester or amide, good chemoselectivity for the ketone was observed. Competition experiments suggest an electrophile relative reactivity order of CF2H ketone > CF3 ketone > aldehyde under these reaction conditions.

How to make C–N bonds using boronic acids and their derivatives without transition metals

No need for transition-metal catalysis in amination, amidation, nitration or nitrosation reactions with boron derivatives as reagents.

Three-Component Olefin Dicarbofunctionalization Enabled by Nickel/Photoredox Dual Catalysis

An intermolecular, photocatalytic dicarbofunctionalization (DCF) of olefins enabled by the merger of Giese-type addition with Ni/photoredox dual catalysis has been realized. Capitalizing on the rapid addition of 3° radicals to alkenes and their reluctance toward single electron metalation to Ni complexes, regioselective alkylation and arylation of olefins is possible. This dual catalytic method not only permits elaborate species to be assembled from commodity materials, but also allows quaternary and tertiary centers to be installed in a singular, chemoselective olefin difunctionalization. This multicomponent process occurs under exceptionally mild conditions, compatible with a diverse range of functional groups and synthetic handles such as pinacolboronate esters. This technology was directly applied to the synthesis of an intermediate to a preclinical candidate (TK-666) and its derivatives.

Silaboration of [1.1.1]Propellane: A Storable Feedstock for Bicyclo[1.1.1]pentane Derivatives

The silaboration of [1.1.1]propellane enables direct introduction of B and Si functional groups onto the bicyclo[1.1.1]pentane (BCP) scaffold in high yield under mild, additive-free conditions. The silaborated BCP can be obtained on a gram-scale in a single step without the need for column-chromatographic purification, and is storable and easy to handle, providing a versatile synthetic intermediate for BCP derivatives. We also describe various conversions of the C-B/C-Si bonds on the BCP scaffold, including development of a modified Suzuki-Miyaura cross-coupling reaction at the highly sterically hindered bridgehead sp³ carbon center of the BCP skeleton using a combination of highly activated BCP boronic esters, copper(I) oxide, and a PdCl₂ (dppf) catalyst system.