Brønsted Acid/Silane Catalytic System for Intramolecular Hydroalkoxylation and Hydroamination of Unactivated Alkynes

Total Synthesis of Pyrrolidine and Piperidine Natural Products via TMSOTf-Mediated "5/6-endo-dig" Reductive Hydroamination of Enynyl Amines

Stereoselective syntheses of pyrrolidines and piperidines bearing hydrophobic chains have been achieved through a metal free, Lewis acid-mediated 5/6-endo-dig reductive hydroamination cascade of enynyl amines. The brevity of the developed strategy allowed for the collective stereoselective total synthesis of various alkaloids, including (±)-pyrrolidine cis-225H, (±)-epi-197B, (±)-epi-225C, the family of (+)-solenopsins and (+)-isosolenopsins, and the formal synthesis of (±)-bgugaine and (+)-azimic acid.

Triflic Acid/Silane Promoted Deoxygenative Transformation of Ketones via Carbocations

Deoxygenative transformation of ketones into value-added products often suffers from precious-metal-based catalysts or complicated operational conditions. Triflic acid and silane serve as a cooperative reductant to transform ketones via carbocations that undergo β-H elimination much faster than hydride transfer from silane to produce alkenes with high selectivity. Alternatively, the presence of indoles would incept the protonated ketones to generate carbocations, allowing access to Csp2-Csp3 bond formation with high reactivity and selectivity in "one pot".

Mechanistic Insight into Palladium/Brønsted Acid Catalyzed Methoxycarbonylation and Hydromethoxylation of Internal Alkene: A Computational Study

Density functional theory (DFT) calculations were performed to study the palladium/Brønsted acid-catalyzed methoxycarbonylation and hydromethoxylation reactions of internal alkene. The calculated results show that the pyridyl group (N atom) in bidentate phosphine ligand with built-in base (L1) plays a crucial role in controlling the selectivity. With the help of the pyridyl group, the methanolysis steps in the methoxycarbonylation reaction and the hydromethoxylation reaction become easy, and both the linear ester methyl 3,4-dimethylpentanoate (P1) and the hydromethoxylation product 2-methoxy-2,3-dimethylbutane (P2) could be obtained. In contrast, the possibility of leading to branched ester P1' was ruled out according to our calculations. The steric effect could account for the observed selectivity. In the presence of the DPEphos ligand (L2) that does not bear the pyridyl group, the methanolysis step in the methoxycarbonylation reaction becomes the rate-determining step with a high overall energy barrier. Neither linear nor branched methoxycarbonylation product could be generated. The palladium/Brønsted acid co-catalyzed hydromethoxylation also become difficult without the assistance of the pyridyl group in the presence of the L2 ligand. Instead, TsOH-catalyzed hydromethoxylation reaction could take place to generate the ether product P2.

Electrochemically enhanced deoxygenative cross-coupling of aryl ketones with heteroarenes through in situ generated benzyl carbocations

Triflic acids/silanes as cooperative reductants enable the convenient transformation of CO bonds through a multistep reaction pathway in one pot. Electrolysis of the acidic reaction mixture significantly improved carbonyl reduction and thus facilitated the generation of benzyl carbocations, which show high reactivity towards electron-rich heteroarenes for C-C bond formation.

Vinyl cation-mediated intramolecular hydroarylation of alkynes using pyridinium reagents

Once considered to be exotic species of limited synthetic utility, vinyl cations have recently been shown to be highly versatile intermediates in a variety of processes. Here, we report a method for the synthesis of aryl-substituted benzocycloheptenes and -hexenes using the hydrotriflate salt of an electron-poor pyridine as a uniquely efficient proton source for a vinyl cation mediated Friedel-Crafts cyclization. The mild conditions made possible by this reagent allowed a range of simple and functionalized alkynes bearing pendant aryl groups to serve as suitable substrates for this scalable and convenient protocol.

Differential Dihydrofunctionalization: A Dual Catalytic Three-Component Coupling of Alkynes, Alkenyl Bromides, and Pinacolborane

A new method for differential dihydrofunctionalization of terminal alkynes enables the synthesis of allylic boronate esters through reductive three-component coupling of terminal alkynes, alkenyl bromides, and pinacolborane. The transformation is promoted by cooperative action of a copper/palladium catalyst system and results in hydrofunctionalization of both π-bonds of an alkyne. The synthesis of allylic boronate esters can be accomplished in the presence of a wide range of functional groups, including, esters, nitriles, alkyl halides, sulfonyl esters, acetals, protected terminal alkynes, aryl halides, and silyl ethers. Mechanistic experiments reveal the importance of subtle ligand effects on the performance of the palladium co-catalyst.

Enantio- and Diastereoselective, Complete Hydrogenation of Benzofurans by Cascade Catalysis

We report an enantio- and diastereoselective, complete hydrogenation of multiply substituted benzofurans in a one-pot cascade catalysis. The developed protocol facilitates the controlled installation of up to six new defined stereocenters and produces architecturally complex octahydrobenzofurans, prevalent in many bioactive molecules. A unique match of a chiral homogeneous ruthenium-N-heterocyclic carbene complex and an in situ activated rhodium catalyst from a complex precursor act in sequence to enable the presented process.

Rh(iii)-Catalyzed Csp2–Csp3 bond alkoxylation of α-indolyl alcohols via C–C σ bond cleavage

A Rh(iii)-catalyzed direct Csp²–Csp³ bond alkoxylation of α-(2-indolyl)alcohols with alcohols has been achieved via C–C sigma bond/C–O single bond switch.

Brønsted Acid-Catalyzed Enantioselective Cycloisomerization of Arylalkynes

The first example of an enantioselective carbocyclization of an alkyne-containing substrate catalyzed by chiral Brønsted acids was achieved. The use of the 2-hydroxynaphthyl substituent on the alkyne as a directing group constituted the key parameter enabling both efficient regioselective protonation of the carbon-carbon triple bond and chiral induction. The key cationic intermediate could be depicted either as a cationic vinylidene ortho-quinone methide or a stabilized vinyl cation. Atropoisomeric phenanthrenes derivatives were produced in high yields and good enantioselectivities under mild, metal-free reaction conditions in the presence of chiral N-triflylphosphoramide catalysts. The carbenic nature of the cationic intermediate was also exploited to describe an example of alkyne/alkane cycloisomerization.

Silver(i)-catalyzed selective hydroalkoxylation of C2-alkynyl quinazolinones to synthesize quinazolinone-fused eight-membered N,O-heterocycles

A silver-catalyzed selective 8-endo-dig cyclization of C2-alkynyl quinazolinones was developed to prepare a series of novel quinazolinone-fused eight-membered N,O-heterocycles in good-to-excellent yields under mild reaction conditions.

Differential Dihydrofunctionalization of Terminal Alkynes: Synthesis of Benzylic Alkyl Boronates through Reductive Three-Component Coupling

The differential dihydrofunctionalization of terminal alkynes is accomplished through the reductive three-component coupling of terminal alkynes, aryl halides, and pinacolborane. The transformation results in hydrofunctionalization of both π-bonds of an alkyne in a single reaction promoted by cooperative action of a copper/palladium catalyst system. The differential dihydrofunctionalization reaction has excellent substrate scope and can be accomplished in the presence of esters, nitriles, alkyl halides, epoxides, acetals, alkenes, aryl halides, and silyl ethers. Mechanistic experiments indicate that the reaction proceeds through copper-catalyzed hydroboration followed by a second hydrocupration. The resulting heterobimetallic complex is the key intermediate that participates in the subsequent palladium-catalyzed cross-coupling, which furnishes benzylic alkyl boronate products.

Strategies for the synthesis of spiropiperidines - a review of the last 10 years

Spiropiperidines have gained in popularity in drug discovery programmes as medicinal chemists explore new areas of three-dimensional chemical space. This review focuses on the methodology used for the construction of 2-, 3- and 4-spiropiperidines, covering the literature from the last 10 years. It classifies the synthesis of each of the types of spiropiperidine by synthetic strategy: the formation of the spiro-ring on a preformed piperidine ring, and the formation of the piperidine ring on a preformed carbo- or heterocyclic ring. While 3- and 4-spiropiperidines are predominantly synthesised for drug discovery projects, 2-spiropiperidines are synthesised en route to natural products. The lack of 2-spiropiperidines in drug discovery is presumably due to limited general procedures for their synthesis.

Bismuth(iii)-catalyzed cycloisomerization and (hetero)arylation of alkynols: simple access to 2-(hetero)aryl tetrahydrofurans and tetrahydropyrans

2-(Hetero)aryl tetrahydrofurans and tetrahydropyrans were successfully synthesized using Bi(OTf)3-catalyzed hydroalkoxylation (cycloisomerization) of alkynols (via 5 or 6 exo-dig cyclization) and intermolecular (hetero)arylation. This reaction involves a highly efficient cascade process, where initially the alkynol undergoes a cycloisomerization step via activation of the triple bond and generates the oxocarbenium ion, which subsequently participates in the (hetero)hydroarylation step with electron-rich arenes. Simple to complex suitably functionalized alkynols (4-pentyn-1-ols and 5-hexyn-1-ols) and electron-rich aromatic compounds were found to be reliable substrates in this cascade transformation and furnished a wide range of oxygen heterocycles. This practical tandem process provides a means to build libraries related to pharmacologically active molecules and natural product like scaffolds.

Computational and experimental investigation on the BCl3 promoted intramolecular amination of alkenes and alkynes

The BCl₃ promoted aminoboration of alkenes and alkynes was investigated both computationally and experimentally, leading to the discovery of a metal-free hydroamination of alkynes.

Diastereoselective synthesis of 1,3-disubstituted isoindolines and sultams via bronsted acid catalysis

Isoindolines and sultams formed under mild conditions.

Transfer hydrogenation of nitrogen heterocycles using a recyclable rhodium catalyst immobilized on bipyridine-periodic mesoporous organosilica

Transfer hydrogenation of unsaturated nitrogen heterocycles using a rhodium catalyst immobilized on bipyridine-periodic mesoporous organosilica (BPy-PMO) is described.