Synthesis of 1,3-Bis-(boryl)alkanes through Boronic Ester Induced Consecutive Double 1,2-Migration

Photocatalytic Multisite Functionalization of Unactivated Terminal Alkenes by Merging Polar Cycloaddition and Radical Ring-Opening Process

Although highly appealing for rapid access of molecular complexity, multi-functionalization of alkenes that allows incorporation of more than two functional groups remains a prominent challenge. Herein, we report a novel strategy that merges dipolar cycloaddition with photoredox promoted radical ring-opening remote C(sp3)-H functionalization, thus enabling a smooth 1,2,5-trifunctionalization of unactivated alkenes. A highly regioselective [3+2] cycloaddition anchors a reaction trigger onto alkene substrates. The subsequent halogen atom transfer (XAT) selectively initiates ring-opening process, which is followed by a series of 1,5-hydrogen atom transfer (1,5-HAT) and intermolecular fluorine atom transfer (FAT) events. With this method, site-selective introduction of three different functional groups is accomplished and a broad spectrum of valuable β-hydroxyl-ϵ-fluoro-nitrile products are synthesized from readily available terminal alkenes.

Site-Selective Activation and Stereospecific Functionalization of Bis(boronic Esters) Derived from 2-Alkenes: Construction of Propionates and Other 1,2-Difunctional Motifs

Non-directed regioselective activation of bis(boronic esters), followed by functionalization, is reported. A bulky activator is shown to selectively activate the less hindered boronic ester enabling it to undergo stereospecific cross-coupling to a variety of electrophiles. This steric-based regioselectivity provides a simple and efficient method to prepare highly functionalized, enantiomerically enriched products starting from simple alkenes.

N-Heterocyclic carbene catalytic 1,2-boron migrative acylation accelerated by photocatalysis

The transformation of organoboron compounds plays an important role in synthetic chemistry, and recent advancements in boron-migration reactions have garnered considerable attention. Here, we report an unprecedented 1,2-boron migrative acylation upon photocatalysis-facilitated N-heterocyclic carbene catalysis. The design of a redox-active boronic ester substrate, serving as an excellent β-boron radical precursor, is the linchpin to the success of this chemistry. With the established protocol, a wide spectrum of β-boryl ketones has been rapidly synthesized, which could further undergo various C─B bond transformations to give multifunctionalized products. The robustness of this catalytic strategy is underscored by its successful application in late-stage modification of drug-derived molecules and natural products. Preliminary mechanistic investigations, including several control experiments, photochemistry measurements, and computational studies, shed light on the catalytic radical reaction mechanism.

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).

Overcoming Radical Stability Order via DABCO-Triggered Desulfurization: Visible-Light-Promoted 1,2,4-Trifunctionalization of Butenyl Benzothiazole Sulfone with Thiosulfonate

A radical 1,2,4-trifunctional reaction of thiosulfonate to unactivated olefin is achieved by a migration strategy under mild conditions. In this reaction, the more unstable primary free radicals are in situ generated after the migration of heteroaryl groups in the presence of DABCO. This trifunctionalization of unactivated olefins involves two C-S bond formations and one C-C bond formation.

Ni-Catalyzed 1,2-Alkyl Borylation and Silylation of Allenes En Route to [1,3]-Bis-Organometallic Reagents

A nickel-catalyzed multicomponent reaction that rapidly and reliably accesses [1,3]-bis-organometallic reagents from allenes is reported. The protocol exhibits a predictable regioselectivity pattern that enables the incorporation of B,B(Si) fragments across the allene backbone under mild conditions, thus offering a complementary platform for accessing polyorganometallic reagents possessing both sp2 and sp3 hybridization from readily available precursors.

Nickel-Catalyzed 8-Aminoquinoline Directed Reductive Dialkylcyclization/Homodialkylation of Unactivated Alkenes

Chemo and regioselective dialkylation of alkene is an efficient protocol for constructing useful chemicals, but challenges remain in the unrestricted application of alkylating reagents. Alkyl bromide belongs to the easy-to-access and operable alkyl electrophiles that can be used in reductive coupling with alkenes. Here, we reported convenient strategies for dialkylcyclization and homodialkylation of unactivated β,γ- and γ,δ-unsaturated alkenyl amides with 1,3-dibromoalkanes or primary alkyl bromides under nickel-catalyzed reductive conditions that exhibited high regioselectivity and functional-group tolerance.

Construction of α-Halogenated Boronic Esters via Visible Light-Induced C-H Bromination

α-Halogenated boronic esters are versatile building blocks that can be diversified into a wide variety of polyfunctionalized molecules. However, their synthetic potential has been hampered by limited preparation methods. Herein, we report a visible light-induced C-H bromination reaction of readily available benzyl boronic esters. This method features high yields, mild conditions, simple operation, and good functional group tolerance. The analogous chlorides and iodides can be accessed via Finkelstein reaction. Synthesis of halogenated geminal diborons has also been demonstrated.

Decarboxylation of β-boryl NHPI esters enables radical 1,2-boron shift for the assembly of versatile organoborons

In recent years, numerous 1,2-R shift (R = aliphatic or aryl) based on tetracoordinate boron species have been well investigated. In the contrary, the corresponding radical migrations, especially 1,2-boryl radical shift for the construction of organoborons is still in its infancy. Given the paucity and significance of such strategies in boron chemistry, it is urgent to develop other efficient and alternative synthetic protocols to enrich these underdeveloped radical 1,2-boron migrations, before their fundamental potential applications could be fully explored at will. Herein, we have demonstrated a visible-light-induced photoredox neutral decarboxylative radical cross-coupling reaction, which undergoes a radical 1,2-boron shift to give a translocated C-radical for further capture of versatile radical acceptors. The mild reaction conditions, good functional-group tolerance, and broad β-boryl NHPI esters scope as well as versatile radical acceptors make this protocol applicable in modification of bioactive molecules. It can be expected that this methodology will be a very useful tool and an alternative strategy for the construction of primary organoborons via a novel radical 1,2-boron shift mode.

Enantioselective Construction of Carbocyclic and Heterocyclic Tertiary Boronic Esters by Conjunctive Cross-Coupling Reaction

Synthesis of stereodefined carbocyclic and heterocyclic tertiary boronic esters is accomplished by performing a conjunctive cross-coupling reaction on preformed cyclic boron ate complexes. Boronates bearing spirocyclic and aryl bicyclic skeletons can be synthesized enantioselectively using a chiral PHOX-ligated Pd catalyst with achiral starting material, while substrates bearing continuous stereogenic centers can be generated diastereoselectively. A variety of aryl and alkenyl electrophiles are incorporated.

Uncatalyzed Carbometallation Involving Group 13 Elements: Carboboration and Carboalumination of Alkenes and Alkynes

Carbometallation of alkenes and alkynes are powerful carbon-carbon bond-forming reactions. The use of compounds containing bonds between carbon and group 13 elements, particularly boron and aluminum, are particularly attractive because of the versatility of subsequent transformations. Uncatalyzed carboboration and carboalumination represent less common classes of reactions. This Short Review discusses uncatalyzed carboboration and carboalumination reactions of alkenes and alkynes, including the reaction design and mechanism.

Silaborative Assembly of Allenamides and Alkynes: Highly Regio- and Stereocontrolled Access to Bi- or Trimetallic Skipped Dienes

A highly stereo- and regiocontrolled multicomponent approach to skipped 1,4-dienes decorated with one boryl and two silyl functionalities is described. This Pd-catalyzed atom-economical union of allenamides, alkynes, and Me₂ PhSiBpin (or Et₃ SiBpin) proceeds without the use of phosphine ligands, instead relying on chelation through the internal amide group of the allenamide sulfonyl. A variety of alkynes, including those derived from complex bioactive molecules, can be efficiently coupled with allenamides and Me₂ PhSiBpin in good yields and with excellent selectivity. The synthetic potential was demonstrated through multiple valuable chemoselective transformations, establishing new disconnections for functionalized dienes. Density functional theory calculations revealed that the reaction first proceeded through borylation of the allenamide, followed by silylation of the alkyne and then reductive elimination, which convergently assemble the skipped 1,4-diene.

Stereoselective gem-C,B-Glycosylation via 1,2-Boronate Migration

A novel protocol is established for the long-standing challenge of stereoselective geminal bisglycosylations of saccharides. The merger of PPh₃ as a traceless glycosidic leaving group and 1,2-boronate migration enables the simultaneous introduction of C-C and C-B bonds at the anomeric stereogenic center of furanoses and pyranoses. The power of this method is showcased by a set of site-selective modifications of glycosylation products for the construction of bioactive conjugates and skeletons. A scarce metal-free 1,1-difunctionalization process of alkenes is also concomitantly demonstrated.

Annulative Coupling of Vinylboronic Esters: Aryne-Triggered 1,2-Metallate Rearrangement

A stereoselective annulative coupling of a vinylboronic ester ate-complex with arynes producing cyclic borinic esters has been developed. An annulation reaction that proceeded through the formation of two C–C bonds and a C–B bond was realized by exploiting a 1,2-metallate rearrangement of boronate triggered by the addition of a vinyl group to the strained triple bond of an aryne. The generated aryl anion would then cyclize to a boron atom to complete the annulation cascade. The annulated borinic ester could be converted to boronic acids and their derivatives by oxidation, halogenation, and cross-coupling. Particularly, halogenation and Suzuki–Miyaura coupling proceeded in a site-selective fashion and produced highly substituted alkylboronic acid derivatives.

A stereoselective annulative coupling of a vinylboronic ester ate-complex with arynes producing cyclic borinic esters has been developed.

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.

Visible-Light-Triggered Sulfonylation/Aryl Migration/Desulfonylation and C-S/Se Bond Formation Reaction: 1,2,4-Trifunctionalization of Butenyl Benzothiazole Sulfone with Thiosulfonate/Selenosulfonates

A visible-light-triggered radical cascade sulfonylation/aryl migration/desulfonylation and C-S/Se bond formation reaction of butenyl benzothiazole sulfone with thiosulfonates or selenosulfonates is developed. This study affords the 1,2,4-trifunctionalization of butenyl benzothiazole sulfone derivatives under mild conditions.

Regio- and Stereoselective 1,2-Carboboration of Ynamides with Aryldichloroboranes

Catalyst‐free 1,2‐carboboration of ynamides is presented. Readily available aryldichloroboranes react with alkyl‐ or aryl‐substituted ynamides in high yields with complete regio‐ and stereoselectivity to valuable β‐boryl‐β‐alkyl/aryl α‐aryl substituted enamides which belong to the class of trisubstituted alkenylboronates. The 1,2‐carboboration reaction is experimentally easy to conduct, shows high functional group tolerance and broad substrate scope. Gram‐scale reactions and diverse synthetic transformations convincingly demonstrate the synthetic potential of this method. The reaction can also be used to access 1‐boraphenalenes, a class of boron‐doped polycyclic aromatic hydrocarbons.

Copper-Catalyzed Highly Selective Protoboration of CF3 -Containing 1,3-Dienes

The copper-catalyzed highly selective protoboration of CF₃ -containing conjugated diene with proton source and B₂ Pin₂ has been developed. This chemistry could suppress the well-known defluorination and provide borated reagents with an intact CF₃ -group. Further studies indicated that the functional group tolerance of this chemistry is very well, and the products could be used as versatile precursors for different types of transformations. Importantly, using chiral diphosphine ligand, we have developed the first example for using such starting material to synthesis allylic boron-reagents which bearing a CF₃ -containing chiral center. Notably, the reaction mechanism was intensively studied by DFT calculations, which could reveal the reason that defluorination was inhibited.

Cyclic (Alkyl)(amino)carbene Ligands Enable Cu-Catalyzed Markovnikov Protoboration and Protosilylation of Terminal Alkynes: A Versatile Portal to Functionalized Alkenes*

Regioselective hydrofunctionalization of alkynes represents a straightforward route to access alkenyl boronate and silane building blocks. In previously reported catalytic systems, high selectivity is achieved with a limited scope of substrates and/or reagents, with general solutions lacking. Herein, we describe a selective copper-catalyzed Markovnikov hydrofunctionalization of terminal alkynes that is facilitated by strongly donating cyclic (alkyl)(amino)carbene (CAAC) ligands. Using this method, both alkyl- and aryl-substituted alkynes are coupled with a variety of boryl and silyl reagents with high α-selectivity. The reaction is scalable, and the products are versatile intermediates that can participate in various downstream transformations. Preliminary mechanistic experiments shed light on the role of CAAC ligands in this process.

1,2-Aryl Migration Induced by Amide C-N Bond-Formation: Reaction of Alkyl Aryl Ketones with Primary Amines Towards α,α-Diaryl β,γ-Unsaturated γ-Lactams

Rearrangement reactions incorporated into cascade reactions play an important role in rapidly increasing molecular complexity from readily available starting materials. Reported here is a Cu-catalyzed cascade reaction of α-(hetero)aryl-substituted alkyl (hetero)aryl ketones with primary amines that incorporates an unusual 1,2-aryl migration induced by amide C-N bond formation to produce a class of structurally novel α,α-diaryl β,γ-unsaturated γ-lactams in generally good-to-excellent yields. This cascade reaction has a broad substrate scope with respect to primary amines, allows a wide spectrum of (hetero)aryl groups to smoothly undergo 1,2-migration, and tolerates electronically diverse α-substituents on the (hetero)aryl ring of the ketones. Mechanistically, this 1,2-aryl migration may stem from the intramolecular amide C-N bond formation which induces nucleophilic migration of the aryl group from the acyl carbon center to the electrophilic carbon center that is conjugated with the resulting iminium moiety.

Rhodium-catalysed selective C-C bond activation and borylation of cyclopropanes

Transition metal (TM)-catalysed directed hydroboration of aliphatic internal olefins which facilitates the construction of complex alkylboronates is an essential synthetic methodology. Here, an efficient method for the borylation of cyclopropanes involving TM-catalysed directed C-C activation has been developed. Upon exposure to neutral Rh(i)-catalyst systems, N-Piv-substituted cyclopropylamines (CPAs) undergo proximal-selective hydroboration with HBpin to provide valuable γ-amino boronates in one step which are otherwise difficult to synthesize by known methods. The enantioenriched substrates can deliver chiral products without erosion of the enantioselectivities. Versatile synthetic utility of the obtained γ-amino boronates is also demonstrated. Experimental and computational mechanistic studies showed the preferred pathway and the origin of this selectivity. This study will enable the further use of CPAs as valuable building blocks for the tunable generation of C-heteroatom or C-C bonds through selective C-C bond activation.