Heck-type 5-endo-trig cyclizations promoted by vinylic fluorines: Ring-fluorinated indene and 3H-pyrrole syntheses from 1,1-difluoro-1-alkenes

Stereoselective Palladium-Catalyzed C-F Bond Alkenylation of Tetrasubstituted gem-Difluoroalkenes via Mizoroki-Heck Reaction

A highly diastereoselective Pd(0)-catalyzed Mizoroki-Heck reaction of gem-difluoroalkenes is described. Unlike previously reported C-F bond functionalization with organometallic reagents, this reaction takes place between two different alkenes to achieve a formal C-F and C-H bond cross-coupling via a distinct pathway. Monofluorinated 1,3-diene products can be synthesized with control of the geometry of each alkene and good functional group tolerability.

Alkyne Aminopalladation/Heck and Suzuki Cascades: An Approach to Tetrasubstituted Enamines

Alkyne aminopalladation reactions starting from tosylamides are reported. The emerging vinylic Pd species are converted either in an intramolecular Heck reaction with olefinic units or in an intermolecular Suzuki reaction by using boronic acids exhibiting broad functional group tolerance. Tetra(hetero)substituted tosylated enamines are obtained in a simple one-pot process.

Pd/Cu-Catalyzed Enantioselective Sequential Heck/Sonogashira Coupling: Asymmetric Synthesis of Oxindoles Containing Trifluoromethylated Quaternary Stereogenic Centers

An asymmetric palladium and copper co-catalyzed Heck/Sonogashira reaction between o-iodoacrylanilides and terminal alkynes to synthesize chiral oxindoles was developed. In particular, a wide range of CF₃ -substituted o-iodoacrylanilides reacted with terminal alkynes, affording the corresponding chiral oxindoles containing trifluoromethylated quaternary stereogenic centers in high yields with excellent enantioselectivities (94-98 % ee). This asymmetric Heck/Sonogashira reaction provides a general approach to access oxindole derivatives containing quaternary stereogenic centers including CF₃ -substituted ones.

Synthesis of Unsaturated Silyl Heterocycles via an Intramolecular Silyl-Heck Reaction

We report the synthesis of unsaturated silacycles via an intramolecular silyl-Heck reaction. Using palladium catalysis, silicon electrophiles tethered to alkenes cyclize to form 5- and 6-membered silicon heterocycles. The effects of alkene substitution and tether length on the efficiency and regioselectivity of the cyclizations are described. Finally, through the use of an intramolecular tether, the first examples of disubstituted alkenes in silyl-Heck reactions are reported.

Direct synthesis of indenes via a rhodium-catalyzed multicomponent Csp2-H annulation reaction

A highly efficient, direct and multicomponent route for the synthesis of indenes is reported herein. This process is catalyzed by a rhodium(iii) complex and conducted under very mild conditions, making the overall process atom and step-economical. DFT mechanistic studies were performed to explore the mechanism of this reaction system.

DFT Studies on Ni-Mediated C-F Cleavage for the Synthesis of Cyclopentadiene Derivatives

Density functional theory calculations have been performed to study the detailed mechanism of Ni-mediated [3+2] cycloaddition of 2-trifluoromethyl-1-alkenes with alkynes via cleavage of two C-F bonds. It was found that the reaction pathway involves oxidative cyclization, the first β-fluorine elimination, and then intramolecular 5-endo insertion of difluoroalkene, followed by the second cleavage of C-F bond, and finally the dissociation of difluorides yields the fluorine-containing product cyclopentadienes in sequence. The overall rate-determining step is the combined processes of the β-fluorine elimination and the 5-endo insertion. Furthermore, we investigated the effect of different ligands and the regioselectivity of asymmetric alkynes. The detailed energy profiles and structures are presented in this study.

Synthesis of β,β-Disubstituted Indanones via the Pd-Catalyzed Tandem Conjugate Addition/Cyclization Reaction of Arylboronic Acids with α,β-Unsaturated Esters

Under Pd catalysis with a newly synthesized electron-deficient heterocycle, 2-(4,5-dihydroimidazol-2-yl)pyrimidine (as the ligand), the reaction of α,β-unsaturated esters with arylboronic acids afforded a wide range of 3,3-disubstituted indan-1-ones bearing a quaternary carbon in high yields. Mechanistic studies revealed that the reaction involves a tandem conjugate addition/1,4-Pd shift followed by a cyclization.

Recent developments in the use of aza-Heck cyclizations for the synthesis of chiral N-heterocycles

Aza-Heck cyclizations are an emerging method for the construction of chiral N-heterocyclic systems. In these processes, an activated N-O bond replaces the C-X bond (X = halide, OTf) used in conventional Heck reactions, with the associated aza-Pd(ii)-intermediate engaging pendant alkenes in a Heck-like manner. This perspective article commences with an historical overview of the area, which stems from Narasaka's seminal studies using oxime esters as the initiating motif. The scope and mechanism of associated chiral N-heterocyclic methodologies are then outlined, including cascade processes that enable diverse alkene 1,2-carboaminations. The recent emergence of new N-O donors and the realization of highly enantioselective aza-Heck cyclizations are then discussed. Collectively, these studies suggest that the aza-Heck approach can underpin a broad family of redox-neutral and enantioselective C-N bond forming processes.

Olefin Insertion into a Pd-F Bond: Catalyst Reactivation Following β-F Elimination in Ethylene/Vinyl Fluoride Copolymerization

The discrete (phosphinoarenesulfonate)Pd fluoride complex (PO^Bp,OMe )PdF(lutidine), where PO^Bp,OMe =(2-MeOC₆ H₄ )(2-{2,6-(MeO)₂ C₆ H₃ }C₆ H₄ )(2-SO₃ -5-MeC₆ H₃ )P, inserts vinyl fluoride (VF) to form (PO^Bp,OMe )PdCH₂ CHF₂ (lutidine) and inserts multiple ethylene (E) units to generate polyethylene that contains -CH₂ F chain ends. These results provide strong evidence that the -CHF₂ and -CH₂ F chain ends in E/VF copolymer generated by (phosphinoarenesulfonate)PdR catalysts form by β-F elimination of Pd(β-F-alkyl) species, VF or E insertion of the resulting (PO)PdF species, and subsequent chain growth. These results also imply that β-F elimination is not an important catalyst deactivation reaction in this system.

Palladium-Catalyzed Defluorinative Coupling of 1-Aryl-2,2-Difluoroalkenes and Boronic Acids: Stereoselective Synthesis of Monofluorostilbenes

The palladium-catalyzed defluorinative coupling of 1-aryl-2,2-difluoroalkenes with boronic acids is described. Broad functional-group tolerance arises from a redox-neutral process by a palladium(II) active species which is proposed to undergo a β-fluoride elimination to afford the products. The monofluorostilbene products were formed with excellent diastereoselectivity (≥50:1) in all cases, and it is critical, as traditional chromatographic techniques often fail to separate monofluoroalkene isomers. As a demonstration of this method's unique combination of reactivity and functional-group tolerance, a Gleevec® analogue, using a monofluorostilbene as an amide isostere, was synthesized.

Synthesis of Functionalized gem-Difluoroalkenes via a Photocatalytic Decarboxylative/Defluorinative Reaction

A photocatalytic decarboxylative/defluorinative reaction of α-trifluoromethyl alkenes with α-keto acids and α-amino acids has been developed. The reaction occurs at room temperature under visible light irradiation, affording various γ,γ-difluoroallylic ketones and 1,1-difluorohomoallyl amines in good yields. The synthetic applications of the resulting functionalized gem-difluoroalkenes were also described.

One-pot syntheses of N-(α-fluorovinyl)azole derivatives from N-(diphenylmethylene)-2,2,2-trifluoroethanamine

N-(Diphenylmethylene)-2,2,2-trifluoroethanamine acts as a versatile fluorine-containing building block, and from which N-(α-fluorovinyl)azole derivatives were prepared in a one-pot process.

Competing reaction pathways of 3,3,3-trifluoropropene at rhodium hydrido, silyl and germyl complexes: C–F bond activation versus hydrogermylation

The reactivity of the Rh complexes [Rh(L)(PEt₃)₃] (L = H, Si(OEt)₃, GePh₃) towards CH₂CHCF₃ was investigated which involve C–F bond activation and germylation reactions.

Catalytic defluorinative [3 + 2] cycloaddition of trifluoromethylalkenes with alkynes via reduction of nickel(ii) fluoride species

A catalytic synthesis of 2-fluoro-1,3-cyclopentadienes was achieved via [3 + 2] cycloaddition of 2-trifluoromethyl-1-alkenes and alkynes using a nickel catalyst and a diboron-based ternary additive system.

Synthesis of N-(α-fluorovinyl)azoles by the reaction of difluoroalkenes with azoles

A mild and versatile method for the construction of C-N bonds by the reaction of (2,2-difluorovinyl)arenes with various N-H-containing heterocycles in the presence of K3PO4 has been developed. The reaction proceeded efficiently at room temperature (25 °C) affording the (E)-N-α-fluorovinyl derivatives of azoles 3 in good to excellent yields with relatively high stereoselectivity.

Gold(I) carbenes by retro-Buchner reaction: generation and fate

The fate of the aryl gold(I) carbenes generated by retro-Buchner reaction of ortho-substituted 7-aryl-1,3,5-cycloheptatrienes is dependent on the constitution of the ortho substituent. Indenes and fluorenes are obtained by intramolecular reaction of highly electrophilic gold(I) carbenes with alkenes and arenes. According to density functional theory calculations, the gold-catalyzed retro-Buchner process occurs stepwise, although the two carbon-carbon cleavages occur on a rather flat potential energy surface.

Mizoroki-Heck reaction of (1-fluorovinyl)methyldiphenylsilane with aryl iodides

The Mizoroki-Heck reaction of (1-fluorovinyl)methyldiphenylsilane with a variety of aryl iodides was accomplished under the conditions composing Pd(OAc)(2), Ag(2)CO(3) and MS 4 Å in 1,4-dioxane to give the corresponding (E)-β-aryl-(α-fluorovinyl)methyldiphenylsilanes with excellent stereoselectivity.