Oxidative Heck-type Reaction Research Articles

Mechanochemical Oxidative Heck-Type Reaction Assisted by Cyclodextrin

Mechanochemical Oxidative Heck-Type Reaction Assisted by Cyclodextrin

Acetoxy Group-Directed Regioselective C2 Alkenylation of Indoles via Pd-Ag Bimetallic Catalysis.

Regioselective C-H functionalizations of indoles reported to date with directing groups at C3 mainly rely on functional groups that are linked to the indole via C-C bonds. However, groups that are linked to the indole core by C-X linkages are also attractive due to the possibility of further modifications of the C-X bond. Herein, we report a 3-acetoxy directing group for the regioselective C2 alkenylation of indoles via a C-H activation-based, cross-dehydrogenative, oxidative Heck-type reaction. The reaction is catalyzed by Pd(II) and Ag(I) with stoichiometric Cu(II) as the oxidant and provides the 2-alkenylated indoles in yields of 52-84%. The reaction conditions are compatible with several functional groups at different positions as well as different N-protecting groups or free NH groups on the indole core. With respect to the alkene coupling partners, the reactions are successful with acrylates, vinyl sulfates, and phosphates. Specifically designed experiments, as well as density functional theory (DFT) computational studies, reveal that a heterodinuclear [Pd(μ-OAc)3Ag] bimetallic species is the actual catalyst responsible for the C-H alkenylation. A mechanistic path involving this catalytic species was also found to be favorable over other possible pathways for explaining the observed regioselectivity through DFT studies.

Cross-Dehydrogenative Coupling Polymerization via C-H Activation for the Synthesis of Conjugated Polymers.

Owing to their versatile (opto)electronic properties, conjugated polymers have found application in several organic electronic devices. Cross-coupling reactions such as Stille, Suzuki, Kumada couplings, and direct arylation reactions have proved to be effective for their synthesis. More atom-efficient oxidative direct arylation polymerization has also been reported for making homopolymers. However, growing interest toward donor-acceptor polymers has led to the recent emergence of cross-dehydrogenative coupling (CDC) polymerization to synthesize alternating copolymers without any prefunctionalization of monomers. Metal-catalyzed cross-coupling of two simple arenes via double C-H activation, or of an arene with an alkene via oxidative Heck-type reaction have been used so far for CDC polymerization. In this article, we discuss the development of CDC polymerization protocols along with the relevant small molecule CDC reactions for an improved understanding of these reactions.

Recent Developments in Heck-Type Reaction of Unactivated Alkenes and Alkyl Electrophiles

AbstractThe Mizoroki–Heck reaction is considered as one of the most ingenious and widely used methods for constructing C–C bonds. This reaction mainly focuses on activated olefins (styrenes, acrylates, or vinyl ethers) and aryl/vinyl (pseudo) halides. In comparison, the studies on unactivated alkenes and alkyl electrophiles are far less due to the low reactivity, poor selectivity, as well as competitive β-H elimination. In the past years, a growing interest has thus been devoted and significant breakthroughs have been achieved in the employment of unactivated alkenes and alkyl electrophiles as the reaction components, and this type of coupling is called as Heck-type or Heck-like reaction, which distinguishes from the traditional Heck reaction. Herein, we give a brief summary on Heck-type reaction between unactivated alkenes and alkyl electrophlies, covering its initial work, recent advancements, and mechanistic discussions.1 Introduction2 Intramolecular Heck-Type Reaction of Unactivated Alkenes and Alkyl Electrophiles2.1 Cobalt-Catalyzed Intramolecular Heck-Type Reaction2.2 Palladium-Catalyzed Intramolecular Heck-Type Reaction2.3 Nickel-Catalyzed Intramolecular Heck-Type Reaction2.4 Photocatalysis and Multimetallic Protocol for Intramolecular Heck-Type Reaction3 Intermolecular Heck-Type Reaction of Unactivated Alkenes and Alkyl Electrophiles3.1 Electrophilic Trifluoromethylating Reagent as Reaction Partners3.2 Alkyl Electrophiles as Reaction Partners4 Oxidative Heck-Type Reaction of Unactivated Alkenes and Alkyl Radicals5 Conclusions and Outlook

Rhodium(iii)-catalyzed C-H activation at the C4-position of indole: switchable hydroarylation and oxidative Heck-type reactions of maleimides.

A Rh(iii)-catalyzed C-H activation of indole at the C4-position leading to novel and switchable functionalization has been reported by employing a weakly co-ordinating COCF3 group as a directing group. An additive plays an important role in switching the selectivity between 1,4-addition products and Heck-type products. An acid additive led to the formation of 1,4-addition products whereas a base additive promotes the formation of Heck-type products. Deuteration studies and control experiments were helpful to propose the mechanism.

Recent Developments in Palladium-Catalyzed Asymmetric Intermolecular Heck Reaction

Some important progress has been made in palladium-catalyzed asymmetric intermolecular Heck reaction in re- cent years. The breakthrough was achieved in the use of acyclic olefins, the aryl halide and benzylic electrophiles in asym- metric intermolecular Heck reaction with some newly developed chiral ligands by Jung, Sigman and Zhou groups. Asymmet- ric intermolecular oxidative Heck-type reaction of acyclic α,β-unsaturated carbonyls and boronic acids was realized by Jung group in high enantioselectivity in the presence of chiral NHC-amidate-alkoxide Pd(II) complexes as catalyst. The Heck re- action of acyclic hydroxyl alkenes and allyl alcohol aryldiazonium salts was achieved by Sigman group with a catalyst de- rived from Pd2dba3 and chiral pyridine oxazoline by using a redox-relay strategy, affording β-, γ-, and δ-aryl carbonyl prod- ucts in excellent enantioselectivity. Sigman group also reports a catalytic and enantioselective intermolecular Heck-type reac- tion of trisubstituted-alkenyl alcohols with aryl boronic acids, providing direct access to diverse molecular building blocks containing an enantiomerically enriched quaternary carbon center. The first examples of asymmetric Mizoroki-Heck reaction using benzyl electrophiles was reported by Zhou group with their newly developed phosphoramidite as the optimal chiral ligand. With this strategy, a key intermediate in asymmetric synthesis of (+)-anisomycin was quickly afforded when p-methoxybenzyl trifluoroacetate was adopted as starting material. Zhou group also realized the first Heck reaction of aryl bromides and chlorides with various cyclic olefins in high enantioselectivities with (R)-Xyl-SDP(O) as the ligand. The use of alcoholic solvents and alkylammonium salts were essential to creating cationic aryl-Pd species for enantioselective olefin insertion. Zhou group has also realized desymmetrization of substituted cyclic olefins successfully via asymmetric Heck reac- tion and found that the use of bisphosphine mono oxide as ligand was important. The Heck reaction of substituted cyclopen- tenes gives almost exclusively trans isomers and establishes two stereocenters in high ee. Under the effect of some newly developed chiral ligands, fused carbo- and heterocycles are synthesized in high ee via asymmetric domino cyclizations. The method is applied to a short synthesis of chiral diamine en route to (-)-martinellic acid. Keywords palladium-catalysis; asymmetric catalysis; Heck reaction; intermolecular reaction

Copper-Catalyzed Oxidative Heck Reactions between Alkyltrifluoroborates and Vinyl Arenes

We report herein that potassium alkyltrifluoroborates can be utilized in oxidative Heck-type reactions with vinyl arenes. The reaction is catalyzed by a Cu(OTf)2/1,10-phenanthroline with MnO2 as the stoichiometric oxidant. In addition to the alkyl Heck, amination, esterification, and dimerization reactions of alkyltrifluoroborates are demonstrated under analogous reaction conditions. Evidence for an alkyl radical intermediate is presented.

ChemInform Abstract: Pd(II)‐Catalyzed Oxidative Heck‐Type Reaction of Triarylphosphines with Alkenes via Carbon—Phosphorus Bond Cleavage.

Abstract A Pd(II)-catalyzed oxidative Heck-type reaction of triarylphosphines with alkenes via carbon–phosphorus bond cleavage was reported. Under the optimal reaction conditions, most reactions proceeded smoothly to give the expected coupling products in acceptable to high yields.

An efficient palladium(II) catalyst for oxidative Heck-type reaction under base-free conditions

An efficient catalytic system for the palladium-catalyzed oxidative Heck reaction between arylboronic acids and various alkenes has been developed. Using the bis(aminoalkoxy)palladium complex of N,N-dimethylethanolamine as a catalyst, a series of substituted alkenes were obtained in moderate to excellent yields under mild reaction conditions. This protocol could be tolerated to arylboronic acids containing electron-donating or withdrawing groups and a wide range of olefins, such as acrylate, allyl esters, allyl ethers and alkenylphosphonate. A plausible reaction mechanism of the oxidative Heck reaction was proposed.

Pd(II)-catalyzed oxidative Heck-type reaction of triarylphosphines with alkenes via carbon–phosphorus bond cleavage

A Pd(II)-catalyzed oxidative Heck-type reaction of triarylphosphines with alkenes via carbon–phosphorus bond cleavage was reported. Under the optimal reaction conditions, most reactions proceeded smoothly to give the expected coupling products in acceptable to high yields.

ChemInform Abstract: Pd‐Catalyzed Olefination of Perfluoroarenes with Allyl Esters.

AbstractThe oxidative Heck‐type reaction proceeds via a β‐H elimination rather than a β‐O‐Ac elimination.

Silanol as a Removable Directing Group for the PdII‐Catalyzed Direct Olefination of Arenes

Need some direction? Silanol was developed as a directing group for the Pd(II)-catalyzed oxidative Heck-type reaction of arenes. A one-pot C-H activation/desilylation process of benzyldiisopropylsilanol was also demonstrated, providing a powerful approach for the synthesis of ortho-alkenyl-substituted alkylarenes. Considering the easily attachable and readily removable properties of the silanol group, this reaction will find broad synthetic applications.

Generally applicable and efficient oxidative Heck reaction of arylboronic acids with olefins catalyzed by cyclopalladated ferrocenylimine under base- and ligand-free conditions

A mild and efficient method for oxidative Heck-type reaction of arylboronic acids with olefins catalyzed by cyclopalladated ferrocenylimine was developed. The results represent the first examples involving the palladacycle as the catalyst for such couplings. Moreover, the catalytic system could tolerate various functional groups, such as F, Cl, Br, NO 2 and CH 3O, and the substrates could be extended to a wide range of olefins from acrylic esters, α,β-unsaturated ketones to alkenes. Furthermore, the olefination could proceed well under base- and ligand-free conditions and employ oxygen as the environmentally benign oxidant.

Oxidative Heck‐Type Reaction Involving Cleavage of a Carbon—Phosphorus Bond of Arylphosphonic Acids.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Oxidative Heck-Type Reaction Involving Cleavage of a Carbon−Phosphorus Bond of Arylphosphonic Acids

Cleavage of a carbon-phosphorus bond is achieved under palladium catalysis in an oxidative Heck-type reaction which exploits arylphosphonic acids. The reaction of arylphosphonic acids with alkenes provides arylation products in good yields in the presence of TBAF with trimethylamine oxide as an oxidant.