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

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.

Recent progress in the catalytic enantioselective reactions of 1,1-diborylalkanes

Organoboron compounds are environmentally benign, have low toxicity and are versatile reagents that are extensively employed in organic synthesis, especially in the realm of asymmetric synthesis. The last several decades have witnessed a tremendous outburst of asymmetric reactions based on various organoboron compounds. Among them, 1,1-diborylalkanes, which contain two boryl groups at the same sp3-carbon atom, are regarded as some of the most versatile and powerful reagents for their unique structure and unusual reaction mode in organic synthesis. Moreover, owing to the stabilizing effect of the empty p-orbital of the neighboring boron atoms and the inherent good steric-hindrance, 1,1-diborylalkanes often exhibit extraordinary reactivity and stereoselectivity compared to other kinds of organoboron compounds in asymmetric synthesis. Herein, the present highlight summarizes and discusses the recent progress achieved in the catalytic enantioselective reactions of 1,1-diborylalkanes during the past decade.

Asymmetric Synthesis of Chiral 1,2-Bis(Boronic) Esters Featuring Acyclic, Non-Adjacent 1,3-Stereocenters

The construction of acyclic, non-adjacent 1,3-stereogenic centers, prevalent motifs in drugs and bioactive molecules, has been a long-standing synthetic challenge due to acyclic nucleophiles being distant from the chiral environment. In this study, we successfully synthesized highly valuable 1,2-bis(boronic) esters featuring acyclic and nonadjacent 1,3-stereocenters. Notably, this reaction selectively produces migratory coupling products rather than alternative deborylative allylation or direct allylation byproducts. This approach introduces a new activation mode for selective transformations of gem-diborylmethane in asymmetric catalysis. Additionally, we found that other gem-diborylalkanes, previously challenging due to steric hindrance, also successfully participated in this reaction. The incorporation of 1,2-bis(boryl)alkenes facilitated the diversification of the alkenyl and two boron moieties in our target compounds, thereby enabling access to a broad array of versatile molecules. DFT calculations were performed to elucidate the reaction mechanism and shed light on the factors responsible for the observed excellent enantioselectivity and diastereoselectivity. These were determined to arise from ligand-substrate steric repulsions in the syn-addition transition state.

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.

A Catalytic Method for the Enantioselective Synthesis of α-Quaternary Ketones, α-Ketoesters and Aldehydes

A practical method for the efficient and enantioselective preparation of versatile ketones and aldehydes that contain an α-quaternary stereocenter is described. Reactions utilize simple carboxylic acid or ester starting materials, a monodentate chiral phosphine, and afford a variety of aryl, alkenyl, alkynyl, and alkyl-substituted ketone and aldehyde products in 25-94 % yield and 90 : 10 to >99 : 1 enantiomeric ratio. Reactions proceed by acyl substitution with in situ formed chiral allylic nucleophiles, and display selectivity and conversion dependence on a protic additive. The utility of the approach is demonstrated through several product transformations.

Dual Ruthenium-Catalyzed Alkene Isomerization-Hydrogen Auto-Transfer Unlocks Skipped Dienes as Pronucleophiles for Enantioselective Alcohol C-H Allylation

The first use of 1,4-pentadiene and 1,5-hexadiene as allylmetal pronucleophiles in regio-, anti-diastereo-, and enantioselective carbonyl addition from alcohol proelectrophiles is described. As corroborated by deuterium labeling experiments, primary alcohol dehydrogenation delivers a ruthenium hydride that affects alkene isomerization to furnish a conjugated diene, followed by transfer hydrogenative carbonyl addition. Hydrometalation appears to be assisted by the formation of a fluxional olefin-chelated homoallylic alkylruthenium complex II, which exists in equilibrium with its pentacoordinate η1 form to enable β-hydride elimination. This effect confers remarkable chemoselectivity: while 1,4-pentadiene and 1,5-hexadiene are competent pronucleophiles, higher 1,n-dienes are not, and the olefinic functional groups of the products remain intact under conditions in which the 1,4- and 1,5-dienes isomerize. A survey of halide counterions reveals iodide-bound ruthenium-JOSIPHOS catalysts are uniquely effective in these processes. This method was used to prepare a previously reported C1-C7 substructure of (-)-pironetin in 4 vs 12 steps.

Unusual Enantiodivergence in Chiral Brønsted Acid‐Catalyzed Asymmetric Allylation with β‐Alkenyl Allylic Boronates

We report herein a rare example of enantiodivergent aldehyde addition with β-alkenyl allylic boronates via chiral Brønsted acid catalysis. 2,6-Di-9-anthracenyl-substituted chiral phosphoric acid-catalyzed asymmetric allylation using β-vinyl substituted allylic boronate gave alcohols with R absolute configuration. The sense of asymmetric induction of the catalyst in these reactions is opposite to those in prior reports. Moreover, in the presence of the same acid catalyst, the reactions with β-2-propenyl substituted allylic boronate generated homoallylic alcohol products with S absolute configuration. Unusual substrate-catalyst C-H⋅⋅⋅π interactions in the favoured reaction transition state were identified as the origins of observed enantiodivergence through DFT computational studies.

Chiral Phosphoric Acid-Catalyzed Enantio- and Diastereoselective Allylboration of Aldehydes with β,γ-Substituted Allylboronates

The catalytic asymmetric addition of β,γ-substituted allylboronates to aldehydes has been described. Promoted by 5 mol % chiral phosphoric acid, the reactions were broadly applicable, scalable, and efficient, allowing for the formation of 3,4-anti/syn-homoallylic alcohols bearing adjacent tertiary or quaternary stereogenic centers in a highly enantio- and diastereoselective manner (≤99% ee and dr >20:1). The rigid chairlike transition state involving the chiral phosphoric acid contributed to the highly controlled reaction.

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.

Stereodivergent Access to Trisubstituted Alkenylboronate Esters through Alkene Isomerization

We report an efficient method for the preparation of synthetically valuable trisubstituted alkenylboronate esters through alkene isomerization of their readily available 1,1-disubstituted regioisomeric counterparts. Either stereoisomer of the target alkenylboronate motif can be obtained at will from the same starting material by employing different isomerization catalysts.

Stereochemical Control via Chirality Pairing: Stereodivergent Syntheses of Enantioenriched Homoallylic Alcohols

We report herein the development of stereodivergent syntheses of enantioenriched homoallylic alcohols using chiral nonracemic α-CH₂ Bpin-substituted crotylboronate. Chiral phosphoric acid (S)-A-catalyzed asymmetric allyl addition with the reagent gave Z-anti-homoallylic alcohols with excellent enantioselectivities and Z-selectivities. When the enantiomeric acid catalyst (R)-A was utilized, the stereoselectivity was completely reversed and E-anti-homoallylic alcohols were obtained with high E-selectivities and excellent enantioselectivities. By pairing the chirality of the boron reagent with the catalyst, two complementary stereoisomers of chiral homoallylic alcohols can be obtained selectively from the same boron reagent. DFT computational studies were conducted to probe the origins of the observed stereoselectivity. These reactions generate highly enantioenriched homoallylic alcohol products that are valuable for rapid construction of polyketide structural frameworks.

Catalytic asymmetric transformations of racemic α-borylmethyl-(E)-crotylboronate via kinetic resolution or enantioconvergent reaction pathways

We report herein catalytic asymmetric transformations of racemic α-borylmethyl-(E)-crotylboronate. The Brønsted acid-catalyzed kinetic resolution-allylboration reaction sequence of the racemic reagent gave (Z)-δ-hydroxymethyl-anti-homoallylic alcohols with high Z-selectivities and enantioselectivities upon oxidative workup. In parallel, enantioconvergent pathways were utilized to synthesize chiral nonracemic 1,5-diols and α,β-unsaturated aldehydes with excellent optical purity.

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.

α-Boryl Organometallic Reagents in Catalytic Asymmetric Synthesis

Recent years have witnessed an increase in the popularity of α-boryl organometallic reagents as versatile nucleophiles in asymmetric synthesis. These compounds have been adopted in chemo- and stereoselective coupling reactions with a number of different electrophiles. The resulting enantioenriched boronic esters can be applied in stereospecific carbon-carbon or carbon-heteroatom bond construction reactions, enabling a two-step strategy for the construction of complex structures with high efficiency and functional group compatibility. Due to these reasons, tremendous effort has been devoted to the preparation of enantiomerically enriched α-boryl organometallic reagents and to the development of stereoselective reactions of related racemic or prochiral materials. In this review, we describe the enantio- or diastereoselective reactions that involve α-boryl organometallic reagents as starting materials or products and we showcase their synthetic utility.

Stereo- and Enantioselective Synthesis of Propionate-Derived Trisubstituted Alkene Motifs

We report a new method for constructing propionate-derived trisubstituted alkene motifs in a stereoselective manner. 1-Substituted 1,1-di(pinacolatoboryl)-(E)-alk-2-enes are generated in situ from 1-substituted 1,1-di(pinacolatoboryl)alk-3-enes through ruthenium(II)-catalyzed double-bond transposition. These species undergo a chiral phosphoric acid catalyzed allylation reaction of aldehydes to produce the E isomers of anti-homoallylic alcohols. On the other hand, the corresponding Z isomers of anti-homoallylic alcohols are obtained when a dimeric palladium(I) complex is employed as the catalyst for this double-bond transposition. Thus, both E and Z isomers can be synthesized from the same starting materials. A B-C(sp² ) bond remaining with the allylation product undergoes the Suzuki-Miyaura cross-coupling reaction to furnish a propionate-derived trisubstituted alkene motif in a stereo-defined form. The present method to construct the motifs with (E)- and (Z)-alkenes are successfully applied to the syntheses of (+)-isotrichostatic acid, (-)-isotrichostatin RK, and (+)-trichostatic acid, respectively.

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.

Diastereo-, Enantio-, and anti-Selective Formation of Secondary Alcohol and Quaternary Carbon Stereocenters by Cu-Catalyzed Additions of B-Substituted Allyl Nucleophiles to Carbonyls

A general method for the synthesis of secondary homoallylic alcohols containing α-quaternary carbon stereogenic centers in high diastereo- and enantioselectivity (up to >20:1 dr and >99:1 er) is disclosed. Transformations employ readily accessible aldehydes, allylic diboronates, and a chiral copper catalyst and proceed by γ-addition of in situ generated enantioenriched boron-stabilized allylic copper nucleophiles. The catalytic protocol is general for a wide variety of aldehydes as well as a variety of 1,1-allylic diboronic esters. Hammett studies disclose that diastereoselectivity of the reaction is correlated to the electronic nature of the aldehyde, with dr increasing as aldehydes become more electron poor.

Enantioselective anti- and syn-(Borylmethyl)allylation of Aldehydes via Brønsted Acid Catalysis

The enantioselective anti- and syn-(borylmethyl)allylation of aldehydes via phosphoric acid catalysis is reported. Both (E)- and (Z)-γ-borylmethyl allylboronate reagents were prepared via the Cu-catalyzed highly stereoselective protoboration of 1,3-dienylboronate. Chiral phosphoric acid-catalyzed aldehyde allylation with either the (E)- or (Z)-allylboron reagent provided 1,2-anti- or 1,2-syn-adducts in good yields with high enantioselectivities. The application to the synthesis of morinol D was accomplished.

Highly Diastereo- and Enantioselective Synthesis of 3,6'-Bisboryl-anti-1,2-oxaborinan-3-enes: An Entry to Enantioenriched Homoallylic Alcohols with A Stereodefined Trisubstituted Alkene

A Cu-catalyzed regio-, diastereo-, and enantioselective carboboration of 1,1-bisboryl-1,3-butadiene is developed to generate enantioenriched 3,6'-bisboryl-anti-1,2-oxaborinan-3-enes. DFT calculations indicate that the initial diene 1,2-borocupration forms a ³ η-allylic copper as the most stable intermediate. Subsequent aldehyde addition, however, operates under Curtin-Hammett control via a more reactive α,α-bisboryl tertiary allylcopper species to furnish products with high enantioselectivities. The three boryl groups in the products are properly differentiated and can undergo a variety of chemoselective transformations to produce enantioenriched homoallylic alcohols with a stereodefined trisubstituted alkene.

Bi(cyclopentyl)diol-Derived Boronates in Highly Enantioselective Chiral Phosphoric Acid-Catalyzed Allylation, Propargylation, and Crotylation of Aldehydes

In this study, we disclose the catalytic addition of bi(cyclopentyl)diol-derived boronates to aldehydes promoted by chiral phosphoric acids, allowing for the formation of enantioenriched homoallylic, propargylic, and crotylic alcohols (up to >99% enantiomeric excess (ee), diastereomeric ratio (dr) >20:1). These boronate substrates provided superior enantioselectivities, allowing for the reactions to proceed with low catalyst loading (0.5-5 mol %) and reduced reaction time (15 min at room temperature for aldehyde allylboration). A wide substrate scope was exhibited, and the novel boronates provided high enantiocontrol. Reactions with substituted allylboronates and aldehydes yielded vicinal stereogenic alcohols bearing β-tertiary or quaternary carbon centers. High enantio- and diastereoselectivities were found due to the closed six-membered chair-like transition state, with backbone modifications of the boronate and its interactions with the chiral phosphoric acid being the most likely contributing factor.

Development of α,α-Disubstituted Crotylboronate Reagents and Stereoselective Crotylation via Brønsted or Lewis Acid Catalysis

The development of α,α-disubstituted crotylboronate reagents is reported. Chiral Brønsted acid-catalyzed asymmetric aldehyde addition with the developed E-crotylboron reagent gave (E)-anti-1,2-oxaborinan-3-enes with excellent enantioselectivities and E-selectivities. With BF₃·OEt₂ catalysis, the stereoselectivity is reversed, and (Z)-δ-boryl-anti-homoallylic alcohols are obtained with excellent Z-selectivities from the same E-crotylboron reagent. The Z-crotylboron reagent also participates in BF₃·OEt₂-catalyzed crotylation to furnish (Z)-δ-boryl-syn-homoallylic alcohols with good Z-selectivities. DFT computations establish the origins of observed enantio- and stereoselectivities of chiral Brønsted acid-catalyzed asymmetric allylation. Stereochemical models for BF₃·OEt₂-catalyzed reactions are proposed to rationalize the Z-selective allyl additions. These reactions generate highly valuable homoallylic alcohol products with a stereodefined trisubstituted alkene unit. The synthetic utility is further demonstrated by the total syntheses of salinipyrones A and B.

Enantioselective Syntheses of (Z)-6'-Boryl-anti-1,2-oxaborinan-3-enes via a Dienylboronate Protoboration and Asymmetric Allylation Reaction Sequence

The enantioselective synthesis of 6'-boryl-anti-1,2-oxaborinan-3-enes is reported. A Cu-catalyzed highly stereoselective 1,4-protoboration of 1,1-bisboryl-1,3-butadiene is developed to generate (E)-α,δ-bisboryl-crotylboronate. The chiral phosphoric-acid-catalyzed asymmetric allylboration of aldehydes with the boron reagent produces 6'-boryl-anti-1,2-oxaborinan-3-enes with excellent Z-selectivities and enantioselectivities. The product contains a vinyl and alkyl boronate unit that can directly participate in a variety of subsequent transformations.

Synthesis of Quaternary Carbon Stereogenic Centers by Diastereoselective Conjugate Addition of Boron-Stabilized Allylic Nucleophiles to Enones

A method for the site-selective and diastereoselective conjugate addition of boron-stabilized allylic nucleophiles to α,β-unsaturated ketones is disclosed. Transformations involve easily prepared γ,γ-disubstituted allyldiboron reagents and proceed in the presence of a fluoride activator at 80 °C. Reactions proceed with a wide variety of enones and allyldiboron reagents efficiently to deliver ketone products that contain otherwise difficult-to-access vicinal β-tertiary and γ-quaternary carbon stereogenic centers and an alkenylboron moiety. The utility of the method is highlighted by several transformations, including cross-coupling and carbocyclizations.

Ru-catalyzed isomerization of ω-alkenylboronates towards stereoselective synthesis of vinylboronates with subsequent in situ functionalization

The stereoselective preparation of synthetically versatile vinylboronates from ω-alkenylboronates is achieved through a ruthenium-catalyzed isomerization reaction. A variety of di- and trisubstituted vinylboronates were conveniently produced and could be used as a new starting point for subsequent in situ remote functionalization through either a sequential Ru/Pd or Ru/Cu double catalytic system.

A regio- and stereoselective ruthenium-catalyzed isomerization of ω-alkenyl boronates into stereodefined di- and trisubstituted alkenylboronate derivatives is reported.

1,n-Bisborylalkanes via Radical Boron Migration

A systematic study of radical boron migration in diboronate complexes to form synthetically valuable 1,n-bisborylalkanes is reported. The boronate complexes are readily generated by reaction of commercial bis(pinacolato)diboron with alkyl Grignard compounds. C-radical generation at a defined position with respect to the diboron moiety is achieved either via intermolecular H-abstraction with a CF3-radical or via alkene perfluoroalkyl radical addition. It is shown that radical 1,2- and 1,4-boron migrations to provide geminal and 1,3-bisborylalkanes are efficient transformations. The 1,5-boron migration in the homologous series leading to 1,4-bisborylalkanes is also occurring, albeit with lower efficiency. Experimental results are supported by DFT calculations which also reveal the corresponding 1,3-boron migration in such diboronate complexes to be feasible.

Photoredox-Mediated Reaction of gem-Diborylalkenes: Reactivity Toward Diverse 1,1-Bisborylalkanes

The use of gem-diborylalkenes as radical-reactive groups is explored for the first time. These reactions provide an efficient and general method for the photochemical conversion of gem-diborylalkenes to rapidly access 1,1-bisborylalkanes. This method exploits a novel photoredox decarboxylative radical addition to gem-diborylalkenes to afford α-gem-diboryl carbon-centered radicals, which benefit from additional stability by virtue of an interaction with the empty p-orbitals on borons. The reaction offers a highly modular and regioselective approach to γ-amino gem-diborylalkanes. Furthermore, EPR spectroscopy and DFT calculations have provided insight into the radical mechanism underlying the photochemistry reaction and the stability of the bis-metalated radicals, respectively.

A Catalytic Dual Isomerization/Allylboration Sequence for the Stereoselective Construction of Congested Secondary Homoallylic Alcohols

A catalytic sequence for the diastereo- and enantioselective preparation of homoallylic alcohols with an adjacent quaternary (stereo)center is reported. The one-pot process relies on the use of a single (achiral or chiral) iridium complex to catalyze the concomitant isomerization of primary allylic alcohols and homoallylboronates into (chiral) aldehydes and allylboronates, respectively. In the same flask, a chiral Brønsted acid is added next to engage the isomerization products into a stereocontrolled allylboration reaction. Structural variations have been performed on both the allylic alcohols and the homoallylboronates. This mild process affords an array of stereochemically congested and complex chiral secondary homoallylic alcohols in high yield, excellent diastereoselectivity, and usually high enantioselectivity.

Cu-Catalyzed Diastereo- and Enantioselective Reactions of γ,γ-Disubstituted Allyldiboron Compounds with Ketones

A catalytic diastereo- and enantioselective method for the preparation of complex tertiary homoallylic alcohols containing a vicinal quaternary carbon stereogenic center and a versatile alkenylboronic ester is disclosed. Transformations are promoted by 5 mol % of a readily available copper catalyst bearing a bulky monodentate phosphoramidite ligand, which is essential for attaining both high dr and er. Reactions proceed with a wide variety of ketones and allylic 1,1-diboronate reagents, which enables the efficient preparation of diverse array of molecular scaffolds.

Diastereo- and Enantioselective Synthesis of Homoallylic Amines Bearing Quaternary Carbon Centers

A Cu-catalyzed method for the efficient enantio- and diastereoselective synthesis of chiral homoallylic amines bearing a quaternary carbon and an alkenylboron is disclosed. Transformations are promoted by a readily prepared (phosphoramidite)-Cu complex and involve bench-stable γ,γ-disubstituted allyldiborons and benzyl imines; products are obtained in up to 82% yield, >20:1 dr, and >99:1 er. Reactions proceed via stereodefined boron-stabilized allylic Cu species formed by an enantioselective transmetalation. Utility of the 1-amino-3-alkenylboronate products is highlighted by a variety of synthetic transformations.

Diastereo- and Enantioselective Synthesis of (E)-δ-Boryl-Substituted anti-Homoallylic Alcohols in Two Steps from Terminal Alkynes

We report the highly diastereo- and enantioselective preparation of (E)-δ-boryl-substituted anti-homoallylic alcohols in two steps from terminal alkynes. This method consists of a cobalt(II)-catalyzed 1,1-diboration reaction of terminal alkynes with B₂ pin₂ and a palladium(I)-mediated asymmetric allylation reaction of the resulting 1,1-di(boryl)alk-1-enes with aldehydes in the presence of a chiral phosphoric acid. Propyne, which is produced as the byproduct during petroleum refining, could be used as the starting material to construct homoallylic alcohols that are otherwise difficult to synthesize with high stereocontrol.

Catalytic Enantioselective Synthesis of 1,4-Keto-Alkenylboronate Esters and 1,4-Dicarbonyls

A catalytic enantioselective method for the synthesis of 1,4-keto-alkenylboronate esters by a rhodium-catalyzed conjugate addition pathway is disclosed. A variety of novel, bench-stable alkenyl gem-diboronate esters are synthesized. These easily accessible reagents react smoothly with a collection of cyclic α,β-unsaturated ketones, generating a new C-C bond and stereocenter. Products are isolated in up to 99 % yield with greater than 20:1 E/Z and greater than 99:1 e.r. Mechanistic studies show the site-selectivity of transmetalation and reactivity is ligand dependent. The utility of the approach is highlighted by gram-scale synthesis of enantioenriched cyclic 1,4-diketones, and stereoselective transformations of the products by hydrogenation, allylation, and isomerization.