Stereodivergent Asymmetric Synthesis of α,β-Disubstituted β-Aminoalkylboronic Acid Derivatives via Group-Selective Protodeboronation Enabling Access to the Elusive Anti Isomer

Suppressing Protodeboronation in Cu-Mediated 19F/18F-Fluorination of Arylboronic Acids: A Mechanistically Guided Approach Towards Optimized PET Probe Development

Fluorinated arenes play a crucial role in drug discovery, specialty materials, and medical imaging. Although several variants for Cu-mediated nucleophilic fluorination of arylboronic acids and derivatives have been developed, these protocols rarely address the occurrence and control of protodeboronation, which greatly complicates product separation and can compromise the effectiveness of a radiotracer for in vivo imaging. Consequently, simpler and more efficient procedures are needed to allow rapid 18F/19F-fluorination of both arylboronic acids and esters while minimizing protodeboronation. Mechanistic controls revealed that in addition to a high temperature, strong donor ligands such as acetonitrile and pyridine accentuate a Cu-mediated protodeboronation. This observation guided the optimization of a ligandless procedure, with t-BuOH as solvent, to activate fluoride under milder conditions at lower temperatures minimizing protodeboronation. Additionally, a new copper salt, Cu(ONf)2 was employed to further improve the fluorination efficiency. A large range of functional groups are tolerated under the new procedure, which is complete within 30 minutes at a temperature of 60 °C, and affords fluorinated arenes and heteroarenes in 39 % to 84 % yield. With minimal modifications, the protocol can also be applied in 18F-radiofluorination, affording radiochemical conversions (RCCs) between 17 and 54 % with minimal protodeboronation compared to previously established protocols.

Highly Selective Boron-Wittig Reaction: A Practical Method to Synthesize Trans-Aryl Alkenes

Olefins play an essential role in synthetic chemistry, serving not only as important synthons but also as key functional groups in numerous bio-active molecules. Consequently, there has been considerable interest in the development of more powerful methods for olefins. While the Wittig reaction stands as a prominent choice for olefin synthesis due to its simplicity and the ready availability of raw materials, its limitation lies in the challenge of controlling cis-trans selectivity, hampering its broader application. In this study, a novel Boron-Wittig reaction has been developed utilizing gem-bis(boryl)alkanes and aldehydes as starting materials. This method enables creating favourable intermediates, which possess less steric hindrance, and leading to trans-olefins via intramolecular O-B bonds elimination. Notably, synthesis studies have validated its good efficacy in modifying bioactive molecules and synthesizing drug molecules with great trans-selectivity. Furthermore, the reaction mechanism was elucidated based on intermediate trapping experiments, isotope labelling studies, and kinetic analyses.

Enantioselective Synthesis of β-Aminoboronic Acids via Borylalkylation of Enamides

Aminoboronic acids represent a class of significant compounds that have attracted significant attention in the fields of drug discovery and organic synthesis. Despite notable progress in their synthesis, the efficient construction of chiral β-aminoboronic acids with alkyl side chains remains a challenging endeavor. Here, we introduce an unprecedented nickel-catalyzed asymmetric borylalkylation of enamides, employing a simple chiral diamine ligand, readily available B2pin2, and alkyl halides as coupling partners. This reaction serves as an efficient platform for assembling a diverse range of β-aminoboronic acid derivatives with flexible alkyl side chains, displaying exceptional regio-, stereo-, and enantioselectivities. Moreover, this transformation exhibits a broad substrate scope and remarkable tolerance toward various functional groups. Theoretical calculations demonstrate that the benzyl group on the ligand is the key to the high enantiocontrol in this transformation. Additionally, we exemplify the practical application of this strategy through the concise synthesis of complex bioactive molecules.

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.

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.

Asymmetric Synthesis of β-Aminoboronates via Copper-Catalyzed Reductive Coupling of Vinyl Boronates with Imines

We report a copper-catalyzed asymmetric reductive coupling of vinyl boronates with imines, which directly access enantiomerically enriched β-aminoalkylboronates. Stereoselective addition of the in situ generated chiral α-borylalkyl copper to N-phosphinoyl imines provided target products in good yields with high diastereo- and enantioselectivity. Vinyl boronate with methylated acenaphthoquinone as a boron ligand was essential to efficiently spawn asymmetric products, and organic transformations of the boron moiety, along with the easily removable N-protecting group, proved their synthetic utility.

Structural Features of Diacyldodecaheterocycles with Pseudo-C2-Symmetry: Promising Stereoinductors for Divergent Synthesis of Chiral Alcohols

Pseudo-C₂-symmetric dodecaheterocyclic structures, which possess two acyl/aroyl groups disposed on either a cis- or trans-relative configuration, were prepared from the naturally occurring (-)-(1R)-myrtenal. Addition of Grignard reagents (RMgX) to the diastereoisomeric mixture of these compounds unexpectedly showed that nucleophilic additions to the two prochiral carbonyl centers gave the same stereochemical result in both cis/trans diastereoisomers, making unnecessary the separation of this mixture. Noticeably, both carbonyl groups showed different reactivity because one of them is attached to an acetalic carbon and the other to a thioacetalic carbon. Furthermore, addition of RMgX to the carbonyl attached to the former carbon takes place through the re face, while addition to the second one proceeds through the si face, thus affording the corresponding carbinols in a highly diastereoselective process. This structural feature allowed the sequential hydrolysis of both carbinols, yielding separately (R)- and (S)-1,2-diols after reduction with NaBH₄. The mechanism of the asymmetric Grignard addition was explained by density functional theory calculations. This approach contributes to the development of the divergent synthesis of structurally and/or configurationally different chiral molecules.

Palladium-Catalyzed trans-Hydroalkoxylation: Counterintuitive Use of an Aryl Iodide Additive to Promote C–H Bond Formation

We report an enantioselective palladium-catalyzed trans-hydroalkoxylation of propargylic amines with a trifluoroacetaldehyde-derived tether to build chiral oxazolidines. Diastereoselective hydrogenation using a heterogeneous palladium catalyst then gave access to protected benzylic amino alcohols in 45–87% yields and 84–94% ee values. Hydroalkoxylation of the alkynes required a catalytic amount of aryl iodide, highlighting the counterintuitive key role played by a putative Pd(II)/ArI oxidative addition complex to promote oxypalladation/protodemetalation.

Primary trifluoroborate-iminiums enable facile access to chiral α-aminoboronic acids via Ru-catalyzed asymmetric hydrogenation and simple hydrolysis of the trifluoroborate moiety

This work describes the first preparation and application of primary trifluoroborate-iminiums (pTIMs) as a new, easily accessible and valuable class of organoboron derivatives. An array of structurally diverse pTIMs was prepared from potassium acyltrifluoroborates in excellent yields. Highly efficient and enantioselective [(R,R)-TethTsDpen-RuCl] complex-catalyzed hydrogenation of pTIMs provided direct access to chiral primary trifluoroborate-ammoniums (pTAMs). Moreover, facile synthesis of a series of structurally diverse chiral α-aminoboronic acids from chiral pTAMs was accomplished through novel, operationally simple and efficient conversion using hexamethyldisiloxane/aqueous HCl. Using no chromatography at any point, this work allowed easy access to chiral α-aminoboronic acids, as exemplified by the synthesis of optically pure anti-cancer drugs bortezomib and ixazomib.

A tandem process for the synthesis of β-aminoboronic acids from aziridines with haloamine intermediates

An unprecedented synthetic strategy is devised to generate β-aminoboronic acids from aziridines via a sequential process involving 1,2-iodoamine formation and radical borylation under light irradiation. A variety of aziridines including multiply substituted aziridines have been successfully employed as synthetic precursors, expanding their synthetic utility compared to previous methods. Mechanistic studies suggest that the boron source plays a unique role in the borylation step, and in the formation of haloamine intermediates.

Diastereo- and Enantioselective Synthesis of Borylated 3-Hydroxyoxindoles by Addition of gem-Diborylalkanes to Isatins

The catalytic asymmetric synthesis of borylated 3-hydroxyoxindoles by addition of gem-diborylalkanes to isatins is disclosed. Chiral 3-hydroxyoxindoles bearing two contiguous stereogenic centers were produced in up to >20:1 dr and 99% ee. The synthetic utility of the corresponding products is presented through several transformations of the boryl moiety. This report provides an efficient strategy to incorporate a boryl functional group toward the synthesis of 3-hydroxyoxindoles.

Recent developments in the asymmetric synthesis and functionalization of symmetrical and unsymmetrical gem-diborylalkanes

gem-Diborylalkanes are an important class of organoboron compounds as they function as a key building block in organic synthesis. This review summarizes recent developments of the enantioselective synthesis of gem-diborylalkanes and application in asymmetric synthesis.

A DFT examination of the role of proximal boron functionalities in the S-alkylation of sulfenic acid anions

Sulfenic acid anions represent an emerging nucleophile for the preparation of sulfoxides. Their S-functionalization chemistry can often be influenced by a nearby group that interacts with the component atoms of the sulfenate through non-bonding interactions. This study uses DFT methods to assess the importance of proximal boron-containing functional groups to direct S-alkylation chemistry of selected sulfenate anions. Several structural variations were modelled at the B3LYP/6-311++G(d,p) level, with the boron species positioned 3 to 5 carbons away from the alkylation site. Transition state free energies of S-alkylation transition states were located with and without sulfenate oxygen precomplexing to the nearby boron atom. The outcomes suggest that an ortho-substituted boronate ester on benzyl bromide can direct and accelerate an alkylation reaction principally due to a reduction of the entropic barrier. It was also determined that an intermolecular precomplex imparts too much stabilization to the sulfenate, thereby reducing its reactivity. The modelling suggests a possible aryl migration of the boronate/sulfenate complex is not competitive with S-alkylation.

Catalytic Chemo- and Enantioselective Transformations of gem-Diborylalkanes and (Diborylmethyl)metallic Species

Chemo- and stereoselective transformations of polyborylalkanes are powerful and efficient methods to access optically active molecules with greater complexity and diversity through programmed synthetic design. Among the various polyborylalkanes, gem-diborylalkanes have attracted much attention in organic chemistry as versatile synthetic handles. The notable advantage of gem-diborylalkanes lies in their ability to generate two key intermediates, α-borylalkyl anions and (gem-diborylalkyl) anions. These two different intermediates can be applied to various enantioselective reactions to rapidly access a diverse set of enantioenriched organoboron compounds, which can be further manipulated to generate various chiral molecule libraries via stereospecific C(sp³)-B bond transformations.In this Account, we summarize our recent contributions to the development of catalytic chemo- and stereoselective reactions using gem-diborylalkanes as versatile nucleophiles, which can be categorized as follows: (1) copper-catalyzed enantioselective coupling of gem-diborylalkanes with electrophiles and (2) the design and synthesis of (diborylmethyl)metallic species and their applications to enantioselective reactions. Since Shibata and Endo reported the Pd-catalyzed chemoselective Suzuki-Miyaura cross-coupling of gem-diborylalkanes with organohalides in 2014, Morken and Hall subsequently developed the first enantioselective analogous reactions using TADDOL-derived chiral phosphoramidite as the supporting ligand of a palladium catalyst. This discovery sparked interest in the catalytic enantioselective coupling of gem-diborylalkanes with electrophiles. Our initial studies focused on generating chiral (α-borylmethyl)copper species by enantiotopic-group-selective transmetalation of gem-diborylalkanes with chiral copper complexes and their reactions with various aldimines and ketimines to afford syn-β-aminoboronate esters with excellent enantio- and diastereoselectivity. Moreover, we developed the enantioselective allylation of gem-diborylalkanes that proceeded by reaction of in situ-generated chiral (α-borylalkyl)copper and allyl bromides. Mechanistic investigations revealed that the enantiotopic-group-selective transmetalation between gem-diborylalkanes and the chiral copper complex occurred through the open transition state rather than the closed transition state, thereby effectively generating chiral (α-borylmethyl)copper species. We also utilized (diborylmethyl)metallic species such as (diborylmethyl)silanes and (diborylmethyl)zinc halides in catalytic enantioselective reactions. We succeeded in developing the enantiotopic-group-selective cross-coupling of (diborylmethyl)silanes with aryl iodides to afford enantioenriched benzylic 1,1-silylboronate esters, which could be used for further consecutive stereospecific transformations to afford various enantioenriched molecules. In addition, we synthesized (diborylmethyl)zinc halides for the first time by the transmetalation of isolated (diborylmethyl)lithium and zinc(II) halides and their utilization to the synthesis of enantioenriched gem-diborylalkanes bearing a chiral center at the β-position via an iridium-catalyzed enantioselective allylic substitution process. In addition to our research efforts, we also include key contributions by other research groups. We hope that this Account will draw the attention of the synthetic community to gem-diboryl compounds and provide guiding principles for the future development of catalytic enantioselective reactions using gem-diboryl compounds.

Highly diastereoselective entry to chiral oxindole-based β-amino boronic acids and spiro derivatives

We here describe the first Cu-catalysed, diastereoselective 1,2-addition of 1,1-diborylmethane to chiral ketimines for the synthesis of quaternary stereocenters and spiro compounds. The method provides easy access to a range of chiral, highly functionalized compounds, namely oxindole-based β,β'-disubstituted β-amino boronates, boron-containing peptidomimetics and six-, seven-membered spirocyclic hemiboronic esters. Such unprecedented compounds are mostly obtained in high yields and easily isolated as single diastereoisomers, paving the way to a more intense exploitation of boron-containing compounds in diversity-oriented chemistry and drug-discovery programs. Concerning stereochemistry, the application of Ellman's auxiliary strategy allows in principle to access both steric series of target compounds.

Selective hydroboration of equilibrating allylic azides

The iridium(I)-catalyzed hydroboration of equilibrating allylic azides is reported to provide only the anti-Markovnikov product of alk-1-ene isomers in good yields and with good functional group tolerance.

Stereoselective Diels-Alder Reactions of gem-Diborylalkenes: Toward the Synthesis of gem-Diboron-Based Polymers

Although gem-diborylalkenes are known to be among the most valuable reagents in modern organic synthesis, providing a rapid access to a wide array of transformations, including the construction of C-C and C-heteroatom bonds, their use as dienophile-reactive groups has been rare. Herein we report the Diels-Alder (DA) reaction of (unsymmetrical) gem-diborylalkenes. These reactions provide a general and efficient method for the stereoselective conversion of gem-diborylalkenes to rapidly access 1,1-bisborylcyclohexenes. Using the same DA reaction manifold with borylated-dienes and gem-diborylalkenes, we also developed a concise, highly regioselective synthesis of 1,1,2-tris- and 1,1,3,4-tetrakis(boronates)cyclohexenes, a family of compounds that currently lack efficient synthetic access. Furthermore, DFT calculations provided insight into the underlying factors that control the chemo-, regio-, and stereoselectivity of these DA reactions. This method also provides stereodivergent syntheses of gem-diborylnorbornenes. The utility of the gem-diborylnorbornene building blocks was demonstrated by ring-opening metathesis polymerization (ROMP), providing a highly modular approach to the first synthesis of the gem-diboron-based polymers. Additionally, these polymers have been successfully submitted to postpolymerization modification reactions. Given its simplicity and versatility, we believe that this novel DA and ROMP approach holds great promise for organoboron synthesis as well as organoboron-based polymers and that it will result in more novel transformations in both academic and industrial research.

Copper-catalyzed 1,6-conjugate addition of para-quinone methides with diborylmethane

Presented herein is the first 1,6-conjugate addition of diborylmethane. This reaction features high yields, mild reaction conditions, and broad functional group compatibilities.

Enantioselective Carboetherification/Hydrogenation for the Synthesis of Amino Alcohols via a Catalytically Formed Chiral Auxiliary

Chiral auxiliaries and asymmetric catalysis are the workhorses of enantioselective transformations, but they still remain limited in terms of either efficiency or generality. Herein, we present an alternative strategy for controlling the stereoselectivity of chemical reactions. Asymmetric catalysis is used to install a transient chiral auxiliary starting from achiral precursors, which then directs diastereoselective reactions. We apply this strategy to a palladium-catalyzed carboetherification/hydrogenation sequence on propargylic amines, providing fast access to enantioenriched chiral amino alcohols, important building blocks for medicinal chemistry and drug discovery. All stereoisomers of the product could be accessed by the choice of ligand and substituent on the propargylic amine, leading to a stereodivergent process.