Alkenylboronic Acids Research Articles

Copper‐Mediated Difunctionalization of Alkenylboronic Acids: Synthesis of ɑ‐Imino Ketones

AbstractVarious ɑ‐imino ketones were prepared in good yields through a copper‐mediated difunctionalization of alkenylboronic acids with benzotriazolamine in air. Mechanistic studies showed that ɑ‐imino ketones formation occurred through an initial copper‐mediated coupling reaction to form an enamine, followed by homolysis of the C−Cu bond to produce an ɑ‐radical imine, and finally radical oxidation by air. The ɑ‐imino ketones were easily converted to various useful scaffolds through further transformations.magnified image

Sulfonamides as Amine Component in the Petasis-Borono Mannich Reaction: A Concise Synthesis of α-Aryl- and α-Alkenylglycine Derivatives­

A catalyst-free three-component synthesis of α-aryl- and α-alkenylglycine derivatives starting from glyoxylic acid, sulfonamides, and aryl- or alkenylboronic acids is described. This operationally simple method tolerates a broad range of functional groups and enables the generation of a wide array of α-amino acids. Sulfonamides were utilized as amine component in the classic Petasis reaction for the first time.

Efficient Homocoupling of Aryl- and Alkenylboronic Acids in the Presence of Low Loadings of [{Pd(μ–OH)Cl(IPr)}2

NHC–palladium(II) complex [{Pd(μ–OH)Cl(IPr)}2] (IPr = bis(2,6-di­isopropylphenyl)imidazolin-2-ylidene) catalyzes the oxidative coupling of a broad spectrum of aryl- and alkenylboronic acids at loadings down to 5 ppm. At the concentration of 0.05 mol% the catalyst permits an efficient reaction under base-free conditions.

Direct Copper-Catalyzed Three-Component Synthesis of Sulfonamides

First introduced into medicines in the 1930s, the sulfonamide functional group continues to be present in a wide range of contemporary pharmaceuticals and agrochemicals. Despite their popularity in the design of modern bioactive molecules, the underpinning methods for sulfonamide synthesis are essentially unchanged since their introduction, and rely on the use of starting materials with preinstalled sulfur-functionality. Herein we report a direct single-step synthesis of sulfonamides that combines two of the largest monomer sets available in discovery chemistry, (hetero)aryl boronic acids and amines, along with sulfur dioxide, using a Cu(II) catalyst, to deliver a broad range of sulfonamides. Sulfur dioxide is provided by the surrogate reagent DABSO. The reaction tolerates broad variation in both coupling partners, including aryl, heteroaryl and alkenyl boronic acids, as well as cyclic and acyclic alkyl secondary amines, and primary anilines. We validate the method by showing that a variety of drugs, and drug-fragments, can be incorporated into the process.

Ni(II)-catalyzed asymmetric alkenylations of ketimines

Chiral allylic amines are not only present in many bioactive compounds, but can also be readily transformed to other chiral amines. Therefore, the asymmetric synthesis of chiral allylic amines is highly desired. Herein, we report two types of Ni(II)-catalyzed asymmetric alkenylation of cyclic ketimines for the preparation of chiral allylic amines. When ketimines bear alkyl or alkoxycarbonyl groups, the alkenylation gives five- and six-membered cyclic α-tertiary allylic amine products with excellent yields and enantioselectivities under mild reaction conditions. A variety of ketimines can be used and the method tolerates some variation in alkenylboronic acid scope. Furthermore, with alkenyl five-membered ketimine substrates, an alkenylation/rearrangement reaction occurs, providing seven-membered chiral sulfamide products bearing a conjugated diene skeleton with excellent yields and enantioselectivities. Mechanistic studies reveal that the ring expansion step is a stereospecific site-selective process, which can be catalyzed by acid (Lewis acid or Brønsted acid).

Ni(II)/mono-RuPHOX-catalyzed asymmetric addition of alkenylboronic acids to cyclic aldimines

A Ni(II)-catalyzed asymmetric addition of alkenylboronic acids to aldimines has been developed employing a mono-RuPHOX ligand. A series of cyclic aldimines and alkenylboronic acids are tolerated under our catalytic system, giving various chiral allylamines in good yields and ees.

A Tunable Route to Prepare α,β‐Unsaturated Esters and α,β‐Unsaturated‐γ‐Keto Esters through Copper‐Catalyzed Coupling of Alkenyl Boronic Acids with Phosphorus Ylides

AbstractA tunable strategy to prepare α,β‐unsaturated esters and α,β‐unsaturated‐γ‐keto esters in good to excellent yields was developed through copper‐catalyzed oxidative coupling of phosphorus ylides with alkenyl boronic acids under mild conditions. The reaction without water afforded α,β‐unsaturated esters, ketones, and amides while α,β‐unsaturated‐γ‐keto esters, 1,4‐α,β‐unsaturated diketones and α,β‐unsaturated‐γ‐keto amides were obtained when using 5.0 equiv. of water. H2O18 labeling experiments showed that water played an important role in the formation of α,β‐unsaturated‐γ‐keto esters. A plausible formation mechanism for α,β‐unsaturated esters and α,β‐unsaturated‐γ‐keto esters was proposed based on mechanistic studies. Phosphonium salts could also be used directly as coupling partners instead of phosphorus ylides. The reaction exhibited a broad substrate scope, good functional group tolerance, good regioselectivity, and diverse coupling products.magnified image

Transition‐Metal‐Catalyst‐Free Cross‐Coupling Reaction of Secondary Propargylic Acetates with Alkenyl‐ and Arylboronic Acids

A cross‐coupling reaction between secondary propargylic acetates and alkenylboronic acids proceeded to give 1,4‐enynes in good yields without addition of transition metal catalyst and base. This simple protocol was also applicable to arylboronic acids, which gave 3‐arylated alkynes in good yields. The observed induction period suggested that the reaction of propargylic acetates and organoboronic acids was affected by the in‐situ generated AcOH as a catalyst, which was confirmed by a separate experiment.

Rh-Catalyzed Conjugate Addition of Aryl and Alkenyl Boronic Acids to α-Methylene-β-lactones: Stereoselective Synthesis of trans-3,4-Disubstituted β-Lactones.

A one-step preparation of 3,4-disubstituted β-lactones through Rh-catalyzed conjugate addition of aryl or alkenyl boronic acids to α-methylene-β-lactones is described. The operationally simple, stereoselective transformation provides a broad range of β-lactones from individual α-methylene-β-lactone templates. This methodology allowed for a direct, final-step C-3 diversification of nocardiolactone, an antimicrobial natural product.

N-heterocyclic Carbene–Cu-Catalyzed Enantioselective Conjugate Additions with Alkenylboronic Esters as Nucleophiles

Catalytic enantioselective conjugate additions with easily accessible alkenylboronic acid pinacol esters as nucleophiles promoted by chiral copper complexes of N-heterocyclic carbenes are presented. These processes constitute an unprecedented instance of conjugate additions of a variety of functionalized alkenyl groups and afford desired products that are otherwise difficult to access in up to 98% yield and 99.5:0.5 enantiomeric ratio. The origins of ligand-controlled enantioselectivity are elucidated with density functional theory (DFT) calculations.

Cu-Catalyzed Stereoselective Borylation of gem-Difluoroalkenes with B2pin2.

A novel and efficient method for the synthesis of (Z)-fluorinated alkenylboronic acid pinacol esters via Cu-catalyzed stereoselective borylation of gem-difluoroalkenes using bis(pinacolato)diboron (B2pin2) as the boron source with the assistance of NaOtBu and Xantphos at room temperature was developed.

Copper-Catalyzed Selective N-Vinylation of 3-(Hydroxyimino)indolin-2-ones with Alkenyl Boronic Acids: Synthesis of N-Vinyl Nitrones and Spirooxindoles.

A copper-catalyzed selective cross-coupling reaction of 3-(hydroxyimino)indolin-2-ones with alkenyl boronic acids to access (E)-N-vinyl oxindole nitrones has been achieved under mild conditions. The studies showed that catalytic copper salt selectively gave mono N-vinylation products, while 2.0 equiv of copper salt provided double N-vinylation products. The control experiments revealed that the carbonyl group in 3-(hydroxyimino)indolin-2-one played important roles on N-vinylation. Furthermore, the prepared N-vinyl oxindole nitrones could be converted to spirooxindoles in good yields under thermal conditions.

Palladium-Catalyzed Cross-Coupling of N-Sulfonylaziridines and Alkenylboronic Acids: Stereospecific Synthesis of Homoallylic Amines with Di- and Trisubstituted Alkenes

A palladium-catalyzed cross-coupling of 2-alkylaziridines with alkenylboronic acids to give homoallylamines is presented. The reaction is highly regioselective and stereospecific and provides convenient access to enantioenriched amines with 1,1-disubstituted, 1,2-disubstituted, and trisubstituted alkenes. The modular synthesis of a 2,5-disubstituted pyrrolidine natural product was completed in three steps and 67% overall yield.

Single‐Step Modular Synthesis of Unsaturated Morpholine N‐Oxides and Their Cycloaddition Reactions

A single-flask procedure for the generation of α-keto-N-alkenylnitrones through a Chan-Lam coupling and subsequent spontaneous 6π electrocyclization of these intermediates for the synthesis of 2H-1,4-oxazine N-oxides has been developed for a variety of α-ketooximes and alkenylboronic acids. This transformation provides a new approach to C-substituted unsaturated morpholine derivatives that are poised to undergo further functionalization for the preparation of a diverse array of novel heterocyclic structures. The scope of the new method for the synthesis of 2H-1,4-oxazine N-oxides is discussed, in addition to initial studies describing the cycloaddition reactivity of these new heterocyclic intermediates.

Single‐Step Modular Synthesis of Unsaturated Morpholine N‐Oxides and Their Cycloaddition Reactions

AbstractA single‐flask procedure for the generation of α‐keto‐N‐alkenylnitrones through a Chan–Lam coupling and subsequent spontaneous 6π electrocyclization of these intermediates for the synthesis of 2H‐1,4‐oxazine N‐oxides has been developed for a variety of α‐ketooximes and alkenylboronic acids. This transformation provides a new approach to C‐substituted unsaturated morpholine derivatives that are poised to undergo further functionalization for the preparation of a diverse array of novel heterocyclic structures. The scope of the new method for the synthesis of 2H‐1,4‐oxazine N‐oxides is discussed, in addition to initial studies describing the cycloaddition reactivity of these new heterocyclic intermediates.

Synthesis of α-oxygenated ketones and substituted catechols via the rearrangement of N-enoxy- and N-aryloxyphthalimides

A common approach to the synthesis of α-oxygenated carbonyl compounds and catechols is the treatment of a carbonyl compound or a phenol with an electrophilic oxygen source. As an alternative approach to these important structures, formal [3,3]-rearrangements of N-enoxyphthalimides, N-enoxyisoindolinones, and N-aryloxyphthalimides have been explored. When used in combination with an initial Chan-Lam coupling, these transformations facilitate the dioxygenation of alkenylboronic acids for the synthesis of α-oxygenated ketones and the dioxygenation of arylboronic acids for the synthesis of catechols. The rearrangements of N-enoxyisoindolinones have also been shown to be diastereoselective.

A deciduous directing group approach for the addition of aryl and vinyl nucleophiles to maleimides.

A Rh(iii)-catalyzed C-H activation followed by conjugate addition to maleimides, using carboxylic acid as a traceless/deciduous directing group, to formally furnish a Csp2-Csp3 bond is presented. For the first time, derivatives of acrylic acid have been shown as surrogates for the unstable and expensive alkenyl boronic acids.

ChemInform Abstract: Direct Cross‐Couplings of Propargylic Diols.

[Pd(PPh3 )4 ] catalyzes a Suzuki-Miyaura-like twofold cross-coupling sequence between underivatized propargylic diols and either aryl or alkenyl boronic acids to furnish highly substituted 1,3-dienes. Thus, 2,3-diaryl-1,3-butadienes and their dialkenic congeners ([4]dendralenes) are delivered in a (pseudo)halogen-free, single-step synthesis which supersedes existing methods. Allenols are also readily formed. Treatment of these single- and twofold cross-coupled products with acid leads to remarkably short syntheses of highly-substituted benzofulvenes and aryl indenes, respectively.

Rh(I)-Catalyzed 1,4-Conjugate Addition of Alkenylboronic Acids to a Cyclopentenone Useful for the Synthesis of Prostaglandins

An efficient and trans-diastereoselective Rh(I)-catalyzed 1,4-conjugate addition reaction of alkenylboronic acids and a homochiral (R)-4-silyloxycyclopentenone useful for the synthesis of derivatives of prostaglandins E and F is described for the first time. The reaction functions under mild conditions and is particularly rapid (≤6 h) under low power (50 W) microwave irradiation at 30 °C in MeOH in the presence of a catalytic amount of KOH. Under these conditions, 3 mol % of [RhCl(COD)]2 is typically required to produce high yields. The method also functions without microwave irradiation at 3 °C in the presence of a stoichiometric amount of KOH. Under these conditions, only 1.5 mol % of [RhCl(COD)]2 is needed, but the reaction is considerably slower. The method accepts a range of aryl- and alkyl-substituted alkenylboronic acids, and its utility has been demonstrated by the synthesis of PGF2α (dinoprost) and tafluprost.

Efficient N-heterocyclic carbene nickel pincer complexes catalyzed cross coupling of benzylic ammonium salts with boronic acids

Pyridine-bridged bis-benzimidazolylidene nickel complexes exhibited very high catalytic activity toward cross coupling of inactive (hetero)aryl benzylic ammonium salts with (hetero)aryl and alkenyl boronic acids under mild reaction conditions. Even at 2mol% catalyst loading, a wide range of substrates for both coupling partners with different steric and electronic properties were well tolerated.