Four-Selective Pyridine Alkylation via Wittig Olefination of Dearomatized Pyridylphosphonium Ylides

Methods to synthesize alkylated pyridines are valuable because these structures are prevalent in pharmaceuticals and agrochemicals. We have developed a distinct approach to construct 4-alkylpyridines using dearomatized pyridylphosphonium ylide intermediates in a Wittig olefination-rearomatization sequence. Pyridine N-activation is key to this strategy, and N-triazinylpyridinium salts enable coupling between a wide variety of substituted pyridines and aldehydes. The alkylation protocol is viable for late-stage functionalization, including methylation of pyridine-containing drugs. This approach represents an alternative to metal-catalyzed sp² -sp³ cross-coupling reactions and Minisci-type processes.

Silver-Catalysed Hydroarylation of Highly Substituted Styrenes

Hydroarylation is an effective strategy to rapidly increase the complexity of organic structures by transforming flat alkene moieties into three-dimensional frameworks. Many strategies have already been developed to achieve the hydroarylation of styrenes, however most of these reports examine the hydroarylation of unpolar, β-mono- or β-unsubstituted styrenes, while exploring mainly electron-rich benzene nucleophiles. Herein, we report a mild and general catalytic system for the selective hydroheteroarylation of multiply substituted styrenes and heteroaromatic styrenes. Mechanistic analysis of the reaction led to the discovery of commercially available 2,2':5',2''-terthiophene as a key reagent.

Olefin-Stabilized Cobalt Nanoparticles for C=C, C=O, and C=N Hydrogenations

The development of cobalt catalysts that combine easy accessibility and high selectivity constitutes a promising approach to the replacement of noble-metal catalysts in hydrogenation reactions. This report introduces a user-friendly protocol that avoids complex ligands, hazardous reductants, special reaction conditions, and the formation of highly unstable pre-catalysts. Reduction of CoBr₂ with LiEt₃ BH in the presence of alkenes led to the formation of hydrogenation catalysts that effected clean conversions of alkenes, carbonyls, imines, and heteroarenes at mild conditions (3 mol % cat., 2-10 bar H₂ , 20-80 °C). Poisoning studies and nanoparticle characterization by TEM, EDX, and DLS supported the notion of a heterotopic catalysis mechanism.

Electronic Control on Linear versus Branched Alkylation of 2-/3-Aroylbenzofurans with Acrylates: Combined DFT and Synthetic Studies

Investigations on the factors that govern unusual branched alkylation of 2-aroylbenzofurans with acrylates by Ru-catalyzed carbonyl-directed C-H activation has been carried out by calculating the kinetics associated with the two key steps-the coordination of the acrylate with the intermediate ruthenacycle and the subsequent migratory insertion reaction-studied with the help of DFT calculations. Eight possible orientations for each mode of alkylation have been considered for the calculations. From these calculations, it has been understood that there is a synergistic operation of the steric and electronic effects favoring the branched alkylation. Further DFT investigations on the alkylation of the isomeric 3-aroylbenzofurans indicated a preference for the linear alkylation and this has been verified experimentally. Overall, the observed/calculated complementary selectivity in the alkylation of 2-/3-aroylbenzofurans with acrylates reveals that the substrate-dependent charge distribution of the Ru-C bond in the intermediate ruthenacycle is an important determining factor and thus the current work opens up a new domain of substrate design for controlling regioselectivity.

Theoretical Study of Nickel-Catalyzed Selective Alkenylation of Pyridine: Reaction Mechanism and Crucial Roles of Lewis Acid and Ligands in Determining the Selectivity

Selective alkenylation of pyridine is challenging in synthetic organic chemistry due to the poor reactivity and regioselectivity of the aromatic ring. We theoretically investigated Ni-catalyzed selective alkenylation of pyridine with DFT. The first step is coordination of the pyridine-AlMe₃ adduct with the active species Ni⁽⁰⁾(NHC)(C₂H₂) 1 in an η²-fashion to form an intermediate Int1. After the isomerization of Int1, the oxidative addition of the C-H bond of pyridine across the nickel-acetylene moiety occurs via a transition state TS2 to form a Ni^(II)(NHC) pyridyl vinyl intermediate Int3. This oxidative addition is rate-determining. The next step is C-C bond formation between pyridyl and vinyl groups leading to the formation of vinyl-pyridine (P1). One of the points at issue in this type of functionalization is how to control the regioselectivity. With the use of Ni(NHC)/AlMe₃ catalyst, the C⁴- and C³-alkenylated products (ΔG°^⧧ = 17.4 and 21.5 kcal mol^-1, respectively) are formed preferably to the C² one (ΔG°^⧧ = 22.0 kcal mol^-1). The higher selectivity of the C⁴-alkenylation over the C³ and the C² ones is attributed to the small steric repulsion between NHC and AlMe₃ in the C⁴-alkenylation. Interestingly, with Ni(P(i-Pr)₃)/AlMe₃ catalyst, the C²-alkenylation occurs more easily than the C³ and C⁴ ones. This regioselectivity arises from the smaller steric repulsion induced by P(i-Pr)₃ than by bulky NHC. It is notable that AlMe₃ accelerates the alkenylation by inducing the strong CT from Ni to pyridine-AlMe₃. In the absence of AlMe₃, pyridine strongly coordinates with the Ni atom through the N atom, which increases Gibbs activation energy (ΔG°^⧧ = ∼27 kcal mol^-1) of the C-H bond activation. In other words, AlMe₃ plays two important roles, acceleration of the reaction and enhancement of the regioselectivity for the C⁴-alkenylation.

Branch-Selective Alkene Hydroarylation by Cooperative Destabilization: Iridium-Catalyzed ortho-Alkylation of Acetanilides

An iridium(I) catalyst system, modified with the wide-bite-angle and electron-deficient bisphosphine d^Fppb (1,4-bis(di(pentafluorophenyl)phosphino)butane) promotes highly branch-selective hydroarylation reactions between diverse acetanilides and aryl- or alkyl-substituted alkenes. This provides direct and ortho-selective access to synthetically challenging anilines, and addresses long-standing issues associated with related Friedel–Crafts alkylations.

Nickel-Catalyzed Insertion of Alkynes and Electron-Deficient Olefins into Unactivated sp(3) C-H Bonds

Insertion of unsaturated systems such as alkynes and olefins into unactivated sp(3) C-H bonds remains an unexplored problem. We herein address this issue by successfully incorporating a wide variety of functionalized alkynes and electron-deficient olefins into the unactivated sp(3) C-H bond of pivalic acid derivatives with excellent syn- and linear- selectivity. A strongly chelating 8-aminoquinoline directing group proved beneficial for these insertion reactions, while an air-stable and inexpensive Ni(II) salt has been employed as the active catalyst.

C-H alkenylations with alkenyl acetates, phosphates, carbonates, and carbamates by cobalt catalysis at 23 °C

Inexpensive cobalt catalysts with N-heterocyclic carbene ligands enable direct arene alkenylations with easily accessible alkenyl acetates through regioselective C-H/C-O functionalizations in a stereoconvergent fashion. The versatile cobalt catalyst was broadly applicable and thus also allowed for the efficient conversion of alkenyl phosphates, carbonates, and carbamates at ambient temperature.