Heck-type Mechanism Research Articles

Hydride Relay Exchange Mechanism for the Heterocyclic C-H Arylation of Benzofuran and Benzothiophene Catalyzed by Pd Complexes.

The mechanism and regioselectivity of the heterocyclic C-H arylation of benzofuran and benzothiophene catalyzed by Pd(OAc)2 complexes were investigated using the density functional theory (DFT) method. The Pd(0)L2(PhI) complex (L = HOAc) is proposed to be the catalytic species. Compared to the traditional Heck-type mechanism, concerted metalation-deprotonation (CMD) mechanism, and electrophilic aromatic substitution (SEAr) mechanism for the C-H arylation, a new hydride relay exchange mechanism was proposed for the benzoheterocyclic C-H arylation catalyzed by Pd complexes, which consists of two redox processes between Pd(II) and Pd(0) species to complete the regioselective C-H activation. The calculated results indicate that the reaction along the hydride relay exchange mechanism is more favorable than those along other mechanisms, including the traditional Heck-type mechanism and the base-assisted anti-H elimination mechanism. This agrees well with the experimental results. Meanwhile, the origin for the regioselective C-H arylation was unveiled in which the α-C-H arylation products are major for the heterocyclic C-H arylation of benzofuran, but the β-C-H arylation products are major for that of benzothiophene. This study might provide a deep mechanistic understanding on the regioselective C-H activation and arylation of benzoheterocycle compounds catalyzed by transition-metal complexes.

Ru-Catalyzed C-H Arylation of Acrylic Acids with Aryl Bromides.

In the presence of a [Ru(p-cymene)Cl2]2/triethylphosphine/lithium carbonate catalyst system, aryl bromides undergo (Z)-selective couplings with unprotected 2-arylacrylic acids to form (Z)-diarylacrylic acids. This vinylic C-H functionalization proceeds in high yields of up to 94% and (Z/E)-ratios of up to 99:1, tolerating a wide range of functional groups. Mechanistic studies indicate that the vinylic C-H activation proceeds via base-assisted cyclometalation rather than via a Heck-type mechanism, which explains its orthogonal stereoselectivity.

Palladium Catalyzed Stereoselective Arylation of Biocatalytically Derived Cyclic 1,3-Dienes: Chirality Transfer via a Heck-Type Mechanism

Microbial arene oxidation of benzoic acid with Ralstonia eutropha B9 provides a chiral highly functionalized cyclohexadiene, suitable for further structural diversification. Subjecting this scaffold to a Pd-catalyzed Heck reaction effects a regio- and stereoselective arylation of the cyclohexadiene ring, with 1,3-chirality transfer of stereogenic information installed in the microbial arene oxidation. Quantum chemical calculations explain the selectivity both by a kinetic preference for the observed arylation position and by reversible carbopalladation in competing positions. Further product transformation allowed the formation of a tricyclic ketone possessing four stereogenic centers. This demonstrates the capability of the method to introduce stereochemical complexity from planar nonchiral benzoic acid in just a few steps.

Bench-Stable Nickel Precatalysts with Heck-type Activation.

Herein, we report the synthesis and characterization of a new class of air- and moisture-stable phosphine-containing nickel(II) precatalysts, which activate through a Heck-type mechanism. The activities of the precatalysts are demonstrated with a carbonyl–ene coupling reaction.

A Nickel-Catalyzed Carbonyl-Heck Reaction.

The use of transition-metal catalysis to enable the coupling of readily available organic molecules has greatly enhanced the ability of chemists to access complex chemical structures. In this work, an intermolecular coupling reaction that unites organotriflates and aldehydes is presented. A unique catalyst system is identified to enable this reaction, featuring a Ni0 precatalyst, a tridentate Triphos ligand, and a bulky amine base. This transformation provides access to a variety of ketone-containing products without the selectivity- and reactivity-related challenges associated with more traditional Friedel-Crafts reactions. A Heck-type mechanism is postulated, wherein the π bond of the aldehyde takes the role of the olefin in the insertion/elimination steps.

Stereocontrolled Formation of Styrenes by Pd(0)-catalyzed Cross-coupling of Photoactivated (E)-Alkenylgermanes with Aryl Bromides

Abstract The stereocontrolled synthesis of (E)-configured styrenes via Pd(0)-catalyzed cross-couping of (E)-alkenylgermanes with aryl bromides is described. The germanes employed have bis(naphthalen-2-ylmethyl) substitution to allow photooxidative activation toward coupling and a C8F17-fluorous tag to facilitate purification by fluorous solid-phase extraction (F-SPE). The selectivities obtained suggest that a germyl-Stille rather than Heck-type mechanism predominates in the coupling step.

Iridium-Catalyzed (Z)-Trialkylsilylation of Terminal Olefins

A complex of commercial [Ir(OMe)(cod)](2) and 4,4-di-tert-butyl-2,2-bipyridine (dtbpy) catalyzes the Z-selective, dehydrative silylation of terminal alkenes, but not 1,2-disubstituted alkenes, with triethylsilane or benzyldimethylsilane in THF at 40 degrees C. Yields and Z-stereoselectivity were significantly improved by 2-norbornene, in contrast with other sacrificial alkenes. The reaction is compatible with many functional groups including epoxides, ketones, amides, alcohols, esters, halides, ketals, and silanes. alpha,beta-Unsaturated esters were unreactive. The reaction probably proceeds through a Heck-type mechanism.

Direct Access to Ketones from Aldehydes via Rhodium‐Catalyzed Cross‐Coupling Reaction with Potassium Trifluoro(organo)borates.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Direct Access to Ketones from Aldehydes via Rhodium-Catalyzed Cross-Coupling Reaction with Potassium Trifluoro(organo)borates

A direct cross-coupling reaction of aromatic aldehydes with potassium trifluoro(organo)borates afforded ketones in high yields and under mild conditions in the presence of a rhodium catalyst and acetone. This new reaction, involving a formal aldehyde C-H bond activation, is believed to proceed via a Heck-type mechanism followed by hydride transfer to acetone.