Synthesis of Dialkyl Ethers from Organotrifluoroborates and Acetals

SOMOphilic alkyne vs radical-polar crossover approaches: The full story of the azido-alkynylation of alkenes

We report the detailed background for the discovery and development of the synthesis of homopropargylic azides by the azido-alkynylation of alkenes. Initially, a strategy involving SOMOphilic alkynes was adopted, but only resulted in a 29% yield of the desired product. By switching to a radical-polar crossover approach and after optimization, a high yield (72%) of the homopropargylic azide was reached. Full insights are given about the factors that were essential for the success of the optimization process.

Azido-alkynylation of alkenes through radical-polar crossover

We report an azido-alkynylation of alkenes allowing a straightforward access to homopropargylic azides by combining hypervalent iodine reagents and alkynyl-trifluoroborate salts. The design of a photocatalytic redox-neutral radical polar crossover process was key to develop this transformation. A variety of homopropargylic azides possessing electron-rich and -poor aryls, heterocycles or ether substituents could be accessed in 34-84% yield. The products are synthetically useful building blocks that could be easily transformed into pyrroles or bioactive amines.

Total Synthesis of Strempeliopidine and Non-Natural Stereoisomers through a Convergent Petasis Borono-Mannich Reaction

Strempeliopidine is a member of the monoterpenoid bisindole alkaloid family, a class of natural products that have been shown to elicit an array of biological responses including modulating protein-protein interactions in human cancer cells. Our synthesis of strempeliopidine leverages palladium-catalyzed decarboxylative asymmetric allylic alkylations to install the requisite all-carbon quaternary centers found in each of the two monomeric natural products, aspidospermidine and eburnamine. Initial studies employing Suzuki-Miyaura cross-coupling followed by diastereoselective hydrogenation provided evidence for a structural reassignment of the natural product. Our final synthetic sequence employs a diastereoselective Petasis borono-Mannich reaction to couple eburnamine to a trifluoroborate aspidospermidine derivative. These convergent approaches enabled the synthesis of eight diastereomers of this heterodimer and offer support for the reassignment of the absolute configuration of strempeliopidine.

Reaction of Alkynyl- and Alkenyltrifluoroborates with Propargyldicobalt Cations: Alkynylation, Alkenylation, and Cyclopropanation Product Pathways

The Lewis acid-mediated Nicholas reactions of propargyl acetate-Co₂(CO)₆ complexes with a series of potassium alkynyltrifluoroborates and potassium alkenyltrifluoroborates are described. Alkynyltrifluoroborates directly alkynylate the intermediate propargyldicobalt cations. In contrast, alkenyltrifluoroborates proceed through one of the three modes of dominant reactivity: C-2-substituted alkenyltrifluorobrates directly alkenylate, predominantly with the retention of stereochemistry. C-1-substituted alkenyltrifluoroborates alkenylate at C-2. Potassium vinyltrifluoroborate incorporates a cyclopropane at the site propargyl to alkynedicobalt. Computational analysis of these systems explains the differential modes of reactivity of alkenyltrifluoroborates and outlines the probable mechanisms for the formation of each product.

Regioselective Cross-Coupling of Isatogens with Boronic Acids to Construct 2,2-Disubstituted Indolin-3-one Derivatives

Herein we present a transition-metal-free cross-coupling reaction of isatogens with boronic acids through a 1,4-metalate shift of a boron "ate" complex. This coupling reaction provides a feasible method to deliver valuable 2,2-disubstituted indolin-3-one derivatives with excellent regioselectivity, which exhibit operational simplicity, good functional group tolerance, and a broad substrate scope.

Metal-Free C-H Borylation of N-Heteroarenes by Boron Trifluoride

Organoboron compounds are essential reagents in modern C-C coupling reactions. Their synthesis via catalytic C-H borylation by main group elements is emerging as a powerful tool alternative to transition metal based catalysis. Herein, a straightforward metal-free synthesis of aryldifluoroboranes from BF3 and heteroarenes is reported. The reaction is assisted by sterically hindered amines and catalytic amounts of thioureas. According to computational studies the reaction proceeds via frustrated Lewis pair (FLP) mechanism. The obtained aryldifluoroboranes are further stabilized against destructive protodeborylation by converting them to the corresponding air stable tetramethylammonium organotrifluoroborates.

Organocatalytic Enantioselective Conjugate Alkynylation of β-Aminoenones: Access to Chiral β-Alkynyl-β-Amino Carbonyl Derivatives

Readily available potassium alkynyltrifluoroborates were used for organocatalytic asymmetric conjugate alkynylation of β-enaminones. The interception of a modified binaphthol catalyst and in situ generated organodifluoroboranes proved important to access functionalized β-alkynyl-β-amino carbonyls and derivatives with improved chemo-reactivity and enantio-induction. Mechanistic studies revealed the impact of molecular sieves on efficiency and stereocontrol. The products undergo additional functionalization to yield a diverse set of valuable β-alkynyl-β-amino carbonyl scaffolds.

Enantioselective synthesis of α-amino esters through Petasis borono-Mannich multicomponent reaction of potassium trifluoroborate salts

Enantioselective synthesis of α-amino esters have been achieved through the Petasis borono-Mannich multicomponent reaction using ( R)-BINOL-derived catalysts with stable heteroaryl and alkenyl trifluoroborate salts under mild conditions. The reaction provides direct access to optically active α-amino esters with moderate to good yields and enantioselectivities.

Copper-catalyzed highly diastereoselective cross-dehydrogenative coupling between 8-hydroxyisochromanes and 1,3-dicarbonyl compounds

A novel copper-catalyzed highly diastereoselective cross-dehydrogenative coupling reaction for the access of tricyclic chromanes from 8-hydroxyisochromanes and 1,3-dicarbonyl compounds.

Selective Functionalization of Achmatowicz Rearrangement Products by Reactions with Potassium Organotrifluoroborates under Transition-Metal-Free Conditions

The repertoire of synthetic transformations of the products of the Achmatowicz rearrangement has been expanded by exploring their reactivity with potassium organotrifluoroborates in the absence of transition metals. Depending on the reaction conditions and the substitution pattern of the starting material, the reaction may lead to the stereoselective synthesis of dihydropyranones (2,6- trans), tetrahydropyranones (2,3- cis-2,6- cis) or functionalized 1,4-dicarbonyl compounds. The method has also been adapted for the one-pot synthesis of functionalized pyrroles.

Highly Discriminative and Chemoselective Deprotection/Transformations of Acetals with the Combination of Trialkylsilyl Triflate/2,4,6-Collidine

Acetals are the most useful protecting groups for carbonyl functional groups. In addition to the role of protection, they can also be used as synthons of carbonyl functions. Previously, we developed a chemoselective deprotection and nucleophilic substitution of acetals from aldehydes in the presence of ketals. This article describes the highly discriminative and chemoselective transformations of acetals bearing different substitution patterns, different types of acetals, as well as mixed acetals. These reactions can achieve the transformations that cannot be attained by conventional methods, and their results strongly suggest the combination of R₃SiOTf/2,4,6-collidine to promote such unprecedented phenomena.

Evolution of a Strategy for the Enantioselective Total Synthesis of (+)-Psiguadial B

(+)-Psiguadial B is a diformyl phloroglucinol meroterpenoid that exhibits antiproliferative activity against the HepG2 human hepatoma cancer cell line. This full account details the evolution of a strategy that culminated in the first enantioselective total synthesis of (+)-psiguadial B. A key feature of the synthesis is the construction of the trans-cyclobutane motif by a Wolff rearrangement with in situ catalytic, asymmetric trapping of the ketene. An investigation of the substrate scope of this method to prepare enantioenriched 8-aminoquinolinamides is disclosed. Three routes toward (+)-psiguadial B were evaluated that featured the following key steps: (1) an ortho-quinone methide hetero-Diels-Alder cycloaddition to prepare the chroman framework, (2) a Prins cyclization to form the bridging bicyclo[4.3.1]decane system, and (3) a modified Norrish-Yang cyclization to generate the chroman. Ultimately, the successful strategy employed a ring-closing metathesis to form the seven-membered ring and an intramolecular O-arylation reaction to complete the polycyclic framework of the natural product.

Conjunctive functionalization of vinyl boronate complexes with electrophiles: a diastereoselective three-component coupling

A method for the conjunctive functionalization of vinyl boronate complexes with electrophiles is described. The overall process represents a three-component coupling between a vinyl boronic ester, carbon nucleophile and an electrophile, thus affording complex multifunctionalized products from simple starting materials. The diastereoselectivity (syn or anti) of this process is strongly dependent upon the nature of the electrophile.

Copper-catalyzed electrophilic amination using N-methoxyamines

Copper-catalyzed electrophilic amination of a triarylboroxin using an N-methoxyamine to give quick access to a variety of anilines was reported. The reaction was especially useful for syntheses of functionalized anilines when combined with our previously reported nucleophilic addition to N-methoxyamides.

Transition-Metal-Free Stereospecific Cross-Coupling with Alkenylboronic Acids as Nucleophiles

We herein report a transition-metal-free cross-coupling between secondary alkyl halides/mesylates and aryl/alkenylboronic acid, providing expedited access to a series of nonchiral/chiral coupling products in moderate to good yields. Stereospecific SN2-type coupling is developed for the first time with alkenylboronic acids as pure nucleophiles, offering an attractive alternative to the stereospecific transition-metal-catalyzed C(sp(2))-C(sp(3)) cross-coupling.

Enantioselective Copper(I)-Catalyzed Alkynylation of Oxocarbenium Ions to Set Diaryl Tetrasubstituted Stereocenters

An enantioselective, copper(I)-catalyzed addition of terminal alkynes to isochroman ketals to set diaryl, tetrasubstituted stereocenters has been developed. The success of this reaction relies on identification of a Cu/PyBox catalyst capable of distinguishing the faces of the diaryl-substituted oxocarbenium ion. This challenging transformation enables efficient conversion of readily available, racemic ketals into high-value enantioenriched isochroman products with fully substituted stereogenic centers. High yields and enantiomeric excesses are observed for various isochroman ketals and an array of alkynes.

Dialkyl Ether Formation by Nickel-Catalyzed Cross-Coupling of Acetals and Aryl Iodides

A new substrate class for nickel-catalyzed C(sp(3)) cross-coupling reactions is reported. α-Oxy radicals generated from benzylic acetals, TMSCl, and a mild reductant can participate in chemoselective cross-coupling with aryl iodides using a 2,6-bis(N-pyrazolyl)pyridine (bpp)/Ni catalyst. The mild, base-free conditions are tolerant of a variety of functional groups on both partners, thus representing an attractive C-C bond-forming approach to dialkyl ether synthesis. Characterization of a [(bpp)NiCl] complex relevant to the proposed catalytic cycle is also described.

Enantioselective copper-catalyzed alkynylation of benzopyranyl oxocarbenium ions

We have developed highly enantioselective, copper-catalyzed alkynylations of benzopyranyl acetals. By using a copper(I) catalyst equipped with a chiral bis(oxazoline) ligand, high yields and enantioselectivities are achieved in the alkynylation of widely available, racemic isochroman and chromene acetals to deliver α-chiral oxygen heterocycles. This method demonstrates that chiral organometallic nucleophiles can be successfully used in enantioselective additions to oxocarbenium ions.

Metal-free synthesis of ynones from acyl chlorides and potassium alkynyltrifluoroborate salts

Ynones are a valuable functional group and building block in organic synthesis. Ynones serve as a precursor to many important organic functional groups and scaffolds. Traditional methods for the preparation of ynones are associated with drawbacks including harsh conditions, multiple purification steps, and the presence of unwanted byproducts. An alternative method for the straightforward preparation of ynones from acyl chlorides and potassium alkynyltrifluoroborate salts is described herein. The adoption of organotrifluoroborate salts as an alternative to organometallic reagents for the formation of new carbon-carbon bonds has a number of advantages. Potassium organotrifluoroborate salts are shelf stable, have good functional group tolerance, low toxicity, and a wide variety are straightforward to prepare. The title reaction proceeds rapidly at ambient temperature in the presence of a Lewis acid without the exclusion of air and moisture. Fair to excellent yields may be obtained via reaction of various aryl and alkyl acid chlorides with alkynyltrifluoroborate salts in the presence of boron trichloride.

Transition-metal-free C–C bond forming reactions of aryl, alkenyl and alkynylboronic acids and their derivatives

Transition-metal-free C–C bond forming reactions of boronic acids are new emerging tools in organic synthesis which complement metal-based procedures.

Bimolecular Coupling Reactions through Oxidatively Generated Aromatic Cations: Scope and Stereocontrol

Chromenes, isochromenes, and benzoxathioles react with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone to form stable aromatic cations that react with a range of nucleophiles. These oxidative fragment coupling reactions provide rapid access to structurally diverse heterocycles. Conducting the reactions in the presence of a chiral Brønsted acid results in the formation of an asymmetric ion pair that can provide enantiomerically enriched products in a rare example of a stereoselective process resulting from the generation of a chiral electrophile through oxidative carbon-hydrogen bond cleavage.

Aromatic cations from oxidative carbon-hydrogen bond cleavage in bimolecular carbon-carbon bond forming reactions

Chromenes and isochromenes react quickly with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) to form persistent aromatic oxocarbenium ions through oxidative carbon-hydrogen cleavage. This process is tolerant of electron-donating and electron-withdrawing groups on the benzene ring and additional substitution on the pyran ring. A variety of nucleophiles can be added to these cations to generate a diverse set of structures.

Nickel-catalyzed cross-coupling of chromene acetals and boronic acids

A modular and highly efficient protocol for the synthesis of 2-aryl- and heteroaryl-2H-chromenes is described. Under base-free conditions, readily accessible 2-ethoxy-2H-chromenes undergo C(sp(3))-O activation and C(sp(3))-C bond formation in the presence of an inexpensive nickel catalyst and boronic acids. This new strategy enables broad access to 2-substituted-2H-chromenes and has been applied to the late-stage incorporation of complex molecules, including the pharmaceuticals loratidine and indomethacin methyl ester.