Copper(I)-Catalyzed Enantioselective Synthesis of α-Chiral Linear or Carbocyclic (E)-(γ-Alkoxyallyl)boronates

P-Stereogenic Phosphorus Ligands in Asymmetric Catalysis

Chiral phosphorus ligands play a crucial role in asymmetric catalysis for the efficient synthesis of useful optically active compounds. They are largely categorized into two classes: backbone chirality ligands and P-stereogenic phosphorus ligands. Most of the reported ligands belong to the former class. Privileged ones such as BINAP and DuPhos are frequently employed in a wide range of catalytic asymmetric transformations. In contrast, the latter class of P-stereogenic phosphorus ligands has remained a small family for many years mainly because of their synthetic difficulty. The late 1990s saw the emergence of novel P-stereogenic phosphorus ligands with their superior enantioinduction ability in Rh-catalyzed asymmetric hydrogenation reactions. Since then, numerous P-stereogenic phosphorus ligands have been synthesized and used in catalytic asymmetric reactions. This Review summarizes P-stereogenic phosphorus ligands reported thus far, including their stereochemical and electronic properties that afford high to excellent enantioselectivities. Examples of reactions that use this class of ligands are described together with their applications in the construction of key intermediates for the synthesis of optically active natural products and therapeutic agents. The literature covered dates back to 1968 up until December 2023, centering on studies published in the late 1990s and later years.

Stereocontrolled Pericyclic and Radical Cycloaddition Reactions of Readily Accessible Chiral Alkenyl Diazaborolidines

In this paper is described an easily synthesized chiral diazaborolidine that is inexpensive, stable, and provides excellent stereoselection across a number of reaction classes. These versatile compounds possess utility in four different classes of cycloaddition reactions, offering good yield and stereoselectivity. X-ray structure analysis provides insight about the origin of stereocontrol.

Enantioselective Cu(I)-catalyzed borylative cyclization of enone-tethered cyclohexadienones and mechanistic insights

The catalytic asymmetric borylation of conjugated carbonyls followed by stereoselective intramolecular cascade cyclizations with in situ generated chiral enolates are extremely rare. Herein, we report the enantioselective Cu(I)-catalyzed β-borylation/Michael addition on prochiral enone-tethered 2,5-cyclohexadienones. This asymmetric desymmetrization strategy has a broad range of substrate scope to generate densely functionalized bicyclic enones bearing four contiguous stereocenters with excellent yield, enantioselectivity, and diastereoselectivity. One-pot borylation/cyclization/oxidation via the sequential addition of sodium perborate reagent affords the corresponding alcohols without affecting yield and enantioselectivity. The synthetic potential of this reaction is explored through gram-scale reactions and further chemoselective transformations on products. DFT calculations explain the requirement of the base in an equimolar ratio in the reaction, as it leads to the formation of a lithium-enolate complex to undergo C-C bond formation via a chair-like transition state, with a barrier that is 22.5 kcal/mol more favourable than that of the copper-enolate complex.

Rapidly building molecular structures with both elements of complexity and flexibility is a key goal of organic synthesis. Here the authors show a tandem copper-catalyzed β-borylation/Michael addition on prochiral enone-tethered 2,5-cyclohexadienones, to generate bicyclic borylated products in high yield and enantioselectivity.

Development of α-Borylmethyl-(Z)-crotylboronate Reagent and Enantioselective Syntheses of (E)-δ-Hydroxymethyl-syn-homoallylic Alcohols via Highly Stereoselective Allylboration

We report herein the development of α-borylmethyl-(Z)-crotylboronate reagent and the application in highly stereo- and enantioselective syntheses of (E)-δ-hydroxymethyl-syn-homoallylic alcohols. Starting from 1,4-pentadiene, α-borylmethyl-(Z)-crotylboronate was synthesized in two steps with high Z-selectivity and enantioselectivity. Subsequent aldehyde allylboration with the developed boron reagent gave highly enantioenriched (E)-δ-hydroxymethyl-syn-homoallylic alcohols upon oxidative workup.

Recent advances in asymmetric borylation by transition metal catalysis

Chiral organoboronates have played a critical role in organic chemistry and in the development of materials science and pharmaceuticals. Much effort has been devoted to exploring synthetic methodologies for the preparation of these compounds during the past few decades. Among the known methods, asymmetric catalysis has emerged as a practical and highly efficient strategy for their straightforward preparation, and recent years have witnessed remarkable advances in this respect. Approaches such as asymmetric borylative addition, asymmetric allylic borylation and stereospecific cross-coupling borylation, have been extensively explored and well established employing transition-metal catalysis with a chiral ligand. This review provides a comprehensive overview of transition metal-catalysed asymmetric borylation processes to construct carbon-boron, carbon-carbon, and other carbon-heteroatom bonds. It summarises a range of recent achievements in this area of research, with considerable attention devoted to the reaction modes and the mechanisms involved.

Asymmetric Syntheses of (E)-δ-Hydroxymethyl-anti-homoallylic Alcohols via Highly Enantio- and Stereoselective Aldehyde Allylation with α-Borylmethyl-(E)-crotylboronate

Highly stereo- and enantioselective synthesis of (E)-δ-hydroxymethyl-anti-homoallylic alcohols is reported. Under the developed conditions, reactions between aldehydes and chiral nonracemic α-borylmethyl-(E)-crotylboronate upon oxidative workup gave δ-hydroxymethyl-anti-homoallylic alcohols with high E-selectivities and enantioselectivities.

Highly stereoselective syntheses of (E)-δ-boryl-anti-homoallylic alcohols via allylation with α-boryl-(E)-crotylboronate

A highly stereoselective synthesis of (E)-δ-boryl-anti-homoallylic alcohols is developed. In the presence of a Lewis acid, aldehyde allylation with α-boryl-(E)-crotylboronate gave δ-boryl-anti-homoallylic alcohols in good yields with excellent E-selectivity. The E-vinylboronate group in the products provides a useful handle for cross-coupling reactions as illustrated in the fragment synthesis of chaxamycins.

Backbone-Modified C2-Symmetrical Chiral Bisphosphine TMS-QuinoxP*: Asymmetric Borylation of Racemic Allyl Electrophiles

A new C₂-symmetrical P-chirogenic bisphosphine ligand with silyl substituents on the ligand backbone, (R,R)-5,8-TMS-QuinoxP*, has been developed. This ligand showed higher reactivity and enantioselectivity for the direct enantioconvergent borylation of cyclic allyl electrophiles than its parent ligand, (R,R)-QuinoxP* (e.g., for a piperidine-type substrate: 95% ee vs 76% ee). The borylative kinetic resolution of linear allyl electrophiles was also achieved using (R,R)-5,8-TMS-QuinoxP* (up to 90% ee, s = 46.4). An investigation into the role of the silyl groups on the ligand backbone using X-ray crystallography and computational studies displayed interlocking structures between the phosphine and silyl moieties of (R,R)-5,8-TMS-QuinoxP*. The results of DFT calculations revealed that the entropy effect thermodynamically destabilizes the dormant dimer species in the catalytic cycle to improve the reactivity. Furthermore, in the direct enantioconvergent case, detailed calculations indicated a pronounced enantioselective recognition of carbon-carbon double bonds, which is virtually unaffected by the chirality at the allylic position, as a key for the borylation from both enantiomers of racemic allyl electrophiles.

Nickel-Catalyzed Highly Selective Hydroalkenylation of Alkenyl Boronic Esters to Access Allyl Boron

Allyl boron derivatives are valuable building blocks in the synthesis of natural products and bioactive molecules. Herein, a practical strategy of nickel-catalyzed highly selective hydroalkenylation of alkenyl boronic esters was developed. Under the mild reaction conditions, a variety of allyl boronic esters were accessed with excellent chemo- and regioselectivity. The mechanism of this transformation was illustrated by control experiments and kinetic studies.

Sulfonate N-Heterocyclic Carbene-Copper Complexes: Uniquely Effective Catalysts for Enantioselective Synthesis of C-C, C-B, C-H, and C-Si Bonds

A copper-based complex that contains a sulfonate N-heterocyclic carbene ligand was first reported 15 years ago. Since then, these organometallic entities have proven to be uniquely effective in catalyzing an assortment of enantioselective transformations, including allylic substitutions, conjugate additions, proto-boryl additions to alkenes, boryl and silyl substitutions, hydride-allyl additions to alkenyl boronates, and additions of boron-containing allyl moieties to N-H ketimines. In this review article, we detail the shortcomings in the state-of-the-art that fueled the development of this air stable ligand class, members of which can be prepared on multigram scale. For each reaction type, when relevant, the prior art at the time of the advance involving sulfonate NHC-Cu catalysts and/or subsequent key developments are briefly analyzed, and the relevance of the advance to efficient and enantioselective total or formal synthesis of biologically active molecules is underscored. Mechanistic analysis of the structural attributes of sulfonate NHC-Cu catalysts that are responsible for their ability to facilitate transformations with high efficiency as well as regio- and enantioselectivity are detailed. This review contains several formerly undisclosed methodological advances and mechanistic analyses, the latter of which constitute a revision of previously reported proposals.

Beyond Carbon: Enantioselective and Enantiospecific Reactions with Catalytically Generated Boryl- and Silylcopper Intermediates

Catalytic asymmetric C-C bond formation with alkylcopper intermediates as carbon nucleophiles is now textbook chemistry. Related chemistry with boron and silicon nucleophiles where the boryl- and accordingly silylcopper intermediates are catalytically regenerated from bench-stable pronucleophiles had been underdeveloped for years or did not even exist until recently. Over the past decade, asymmetric copper catalysis employing those main-group elements as nucleophiles rapidly transformed into a huge field in its own right with an impressive breadth of enantioselective C-B and C-Si bond-forming reactions, respectively. Its current state of the art does not have to shy away from comparison with that of boron's and silicon's common neighbor in the periodic table, carbon. This Outlook is not meant to be a detailed summary of those manifold advances. It rather aims at providing a brief conceptual summary of what forms the basis of the latest exciting progress, especially in the area of three-component reactions and cross-coupling reactions.

Methodologies and Strategies for Selective Borylation of C-Het and C-C Bonds

Organoborons have emerged as versatile building blocks in organic synthesis to achieve molecular diversity and as carboxylic acid bioisosteres with broad applicability in drug discovery. Traditionally, these compounds are prepared by the substitution of Grignard/lithium reagents with electrophilic boron species and Brown hydroboration. Recent developments have provided new routes for the efficient preparation of organoborons by applying reactions using chemical feedstocks with leaving groups. As compared to the previous methods that used organic halides (I, Br, and Cl), the direct borylation of less reactive C-Het and C-C bonds has become highly important to get efficiency and functional-group compatibility. This Review aims to provide a comprehensive overview of this topic, including (1) C-F bond borylation, (2) C-O bond borylation, (3) C-S bond borylation, (4) C-N bond borylation, and (5) C-C bond borylation. Considerable attention is given to the strategies and mechanisms involved. We expect that this Review will inspire chemists to discover more efficient transformations to expand this field.

Rh-Catalyzed Asymmetric Hydrogenation of α,β- and β,β-Disubstituted Unsaturated Boronate Esters

A highly enantioselective hydrogenation of α,β-unsaturated boronate esters catalyzed by Rh-(S)-DTBM-Segphos complex has been developed. Both (Z)-α,β- and β,β-disubstituted substrates can be successfully hydrogenated to afford chiral boronates with excellent enantioselectivities, up to 98 % ee. Furthermore, the obtained chiral boronate esters, as important versatile synthetic intermediates are successfully transformed to the corresponding chiral alcohols, amines and other important derivatives with maintained enantioselectivities.

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

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

Copper-catalyzed borylamidation of vinyl arenes with isocyanates

A copper-catalyzed borylative amidation of vinyl arenes with isocyanates and bis(pinacolato)diboron has been developed.

Cu-Catalyzed Asymmetric Hydroboration of Naphthylallylic Compounds for Enantioselective Synthesis of Chiral Boronates

A Cu-catalyzed regio- and enantioselective hydroboration of various naphthylallylic compounds affording chiral boronates with high yields and excellent enantioselectivities (up to 96% ee) was presented. The utility of the boronated products is further illustrated by other stereospecific C-B bond transformations to produce amino alcohols and other useful compounds.

Iron-catalyzed boration of cinnamyl carbonates: a highly stereoselective approach to cyclopropylboronates

An efficient approach to cyclopropylboronates with excellent stereoselectivities via iron-catalyzed boration/cyclopropylation of cinnamyl carbonates is developed.

Which future for stereogenic phosphorus? Lessons from P* pincer complexes of iron(ii)

Locking the chelate conformation and supplying steric bulk for enantiodiscrimination: a tough task for stereogenic phosphorus in multidentate ligands.

Copper-catalyzed diastereoselective synthesis of β-boryl-α-quaternary carbon carboxylic esters

Cu(i)-Catalyzed diastereoselective carboboration of α-alkyl-substituted α,β-unsaturated carboxylic esters to produce β-boryl-α-quaternary carbon esters was developed. The carbon skeletons of dialkyl sulfates, primary allyl halides, and benzyl bromides were transferred to the α-position of the substrates to provide products in moderate to good yields with a diastereoselectivity of >95% in most cases. Substrates bearing a β-(hetero)aryl substituent gave higher diastereoselectivities than those bearing a linear β-alkyl substituent. The crystal structure of the potassium trifluoroborate derivative shows that the reactions probably go through a copper(i) enolate intermediate and the diastereoselectivity arises from the electrophilic attack of electrophiles to the less hindered side of the enolates.

Kinetic Resolution of Secondary Allyl Boronates and Their Application in the Synthesis of Homoallylic Amines

Highly enantioenriched, chromatographically-stable secondary allyl boronates featuring a 1,1,2,2-tetraethyl-1,2-ethanediol fragment (Epin) were obtained by kinetic resolution of their racemic mixtures. The Epin group at boron considerably improved stability of allyl boronates allowing them to be readily isolated by chromatography on silica. The resolved reagents were applied in stereoselective synthesis of homoallylic amines with an internal double bond employing unprotected imines formed in situ from aldehydes and ammonia. The reactions proceeded with an excellent transfer of chirality.

Copper-boryl mediated organic synthesis

Organoboron compounds are valuable synthetic intermediates that find application in a diverse variety of processes including both C-X and C-C bond-forming transformations. This has been achieved by using a variety of boron derivatives. Of these, boronate esters are probably the most versatile and, reflecting this, methods for the generation of boronate esters are of considerable current interest. Given the mild reaction conditions, good functional group tolerance, and low cost of the metal catalyst, the use of copper-boryl reagents is particularly attractive. In this review, methodologies in copper-boryl chemistry are discussed and the many different transformations possible are surveyed.

Radical Borylative Cyclization of 1,6-Dienes: Synthesis of Boron-Substituted Six-Membered Heterocycles and Carbocycles

A radical borylative cyclization reaction of 1,6-dienes was developed to assemble boron-handled six-membered heterocycles and carbocycles. This reaction was initiated by the chemo- and regio-controlled addition of an N-heterocyclic carbene-boryl radical to one of the alkene tethers, followed by an intramolecular 6- exo cyclization to afford a six-membered ring framework. The utility of this method was demonstrated in the synthesis of diverse paroxetine analogues through late-stage derivatization of the boryl functional unit.

Copper-catalyzed asymmetric dearomative borylation: new pathway to optically active heterocyclic compounds

Chiral N-heterocyclic organoboronates represent promising intermediates for the preparation of various bioactive and pharmaceutical compounds. We recently reported the first asymmetric dearomative borylation of indoles by copper-catalyzed borylation. Then we further developed dearomatization/enantioselective borylation sequence. Chiral 3-boryl-tetrahydropyridines and chiral boryl-tetrahydroquinolines via the copper(I)-catalyzed regio-, diastereo- and enantioselective borylation of 1,2-dihydropyridines and 1,2-dihydroquinilines, which were prepared by the partial reduction of the corresponding pyridine or quinoline derivatives. This dearomatization/enantioselective borylation procedures provide a direct access to chiral piperidines and tetrahydroquinolines from readily available pyridines or quinolines in combination with the stereospecific transformation of the stereogenic C–B bond.

Enantioselective Synthesis of anti-1,2-Oxaborinan-3-enes from Aldehydes and 1,1-Di(boryl)alk-3-enes Using Ruthenium and Chiral Phosphoric Acid Catalysts

A cationic ruthenium(II) complex catalyzes double-bond transposition of 1,1-di(boryl)alk-3-enes to generate in situ 1,1-di(boryl)alk-2-enes, which then undergo chiral phosphoric acid catalyzed allylation of aldehydes producing homoallylic alcohols with a (Z)-vinylboronate moiety. 1,2-Anti stereochemistry is installed in an enantioselective manner. The (Z)-geometry forged in the products allows their isolation in a form of 1,2-oxaborinan-3-enes, upon which further synthetic transformations are operated.

Iron-catalyzed boration of allylic esters: an efficient approach to allylic boronates

The first general iron-catalyzed boration of allylic esters has been developed.