Intra/intermolecular direct allylic alkylation via Pd(II)-catalyzed allylic C-H activation

The first catalytic direct alkylation of allylic C-H bonds via Pd(II)-catalysis is described in the absence of base. Polysubstituted cyclic compounds can also be constructed by the intramolecular direct allylic alkylation.     

Pd(II)-catalyzed para-selective C-H arylation of monosubstituted arenes

Pd-catalyzed highly para-selective C-H arylation of monosubstituted arenes (including toluene) is developed for the first time using an F(+) reagent as a bystanding oxidant. This finding provides a new retrosynthetic disconnection for para-substituted biaryl synthesis via C-H/C-H cross-coupling.     

Rhodium(I)-catalyzed direct carboxylation of arenes with CO2 via chelation-assisted C-H bond activation

Rh-catalyzed direct carboxylation of unactivated aryl C-H bond under atmospheric pressure of carbon dioxide was realized via chelation-assisted C-H activation for the first time. Variously substituted and functionalized 2-arylpyridines and 1-arylpyrazoles underwent the carboxylation in the presence of the rhodium catalyst and a stoichiometric methylating reagent, AlMe(2)(OMe), to give carboxylated products in good yields. The catalysis is proposed to consist of methylrhodium(I) species as the key intermediate, which undergoes C-H activation to afford rhodium(III), followed by reductive elimination of methane to give nucleophilic arylrhodium(I). This approach demonstrates promising application of C-H bond activation strategy in the field of carbon dioxide fixation.     

Iron-facilitated direct oxidative C-H transformation of allyl arenes to alkenyl aldehydes

A direct oxidative approach to alkenyl aldehydes from allyl arenes via allyl sp³ C-H functionalization was disclosed. An inexpensive iron catalyst was employed to facilitate this transformation. The mechanistic studies indicate that the cleavage of the allyl sp³ C-H bond is involved in the rate-determining step.     

New reactivity of oxaziridine: Pd(II)-catalyzed aromatic C-H ethoxycarbonylation via C-C bond cleavage

A novel Pd(II)-catalyzed aromatic C-H ethoxycarbonylation with oxaziridine involving C-C bond cleavage is described. Various aromatic 2-phenylpyridines and related compounds as well as aryl ureas can be effectively ethoxycarbonylated. A catalytic cycle involving Pd(II) and Pd(IV) is proposed.     

2,2-Difunctionalization of alkenes via Pd(II)-catalyzed aza-Wacker reactions

N-Ts and N-Boc derivatives of 1,2-diamines and 1,2-amino alcohols are shown to undergo efficient Pd(II)-catalyzed aza-Wacker reactions with a large range of electron-deficient alkenes. The resulting enamine intermediate generally undergoes cyclization with the second heteroatom to form 1,3-heterocycles. The sequence facilitates the rapid synthesis of saturated oxazolidines, imidazolidines, and their derivatives. Use of N-L-valinol derivatives results in highly diastereoselective reactions, where the net stereochemical outcome diverges between N-Ts and N-Boc.     

Pd(II)-catalyzed direct C5-arylation of azole-4-carboxylates through double C-H bond cleavage

The first palladium-catalyzed direct C5-arylation of azole-4-carboxylates with simple unactivated arenes through double C-H bond cleavage is realized. This protocol provided a straightforward access to diverse 5-arylsubstituted azole-4-carboxylic derivatives with good functional group tolerance.     

Palladium(II)-catalyzed direct alkenylation and arylation of arenes: removable 2-pyridylsulfinyl group assisted C-H bond activation

Palladium-catalyzed C-H activation reactions directed by a removable 2-pyridylsulfinyl group were developed. Aromatic olefination products were formed in good yields on treatment of 2-(phenylsulfinyl)pyridines with alkenes in the presence of a Pd catalyst. The reaction tolerates a wide range of substituted alkenes, including various acrylates and styrenes. The controlled experiments indicated that the 2-pyridyl moiety, rather than the sulfinyl, played the role of ligand. The final reductive desulfonylation affords the stilbenes, sulfides, and disulfides with different reductive conditions, respectively. More importantly, this transformation could also be applied in arylation through dual C-H activation.     

Oxidative Pd(II)-catalyzed C-H bond amination to carbazole at ambient temperature

We report a new Pd(II)-catalyzed C-H bond amination reaction to form carbazoles, an important motif that is prevalent in a range of systems. The catalytic amination process operates under extremely mild conditions and produces carbazole products in good to excellent yields. Carbazoles possessing complex molecular architecture can also be formed using this reaction, highlighting its potential in natural product synthesis applications. Preliminary mechanistic investigations reveal the reaction proceeds through a Pd(II)/Pd(IV) manifold and that reductive elimination from a high oxidation state Pd(IV) complex facilitates the mild conditions of this transformation.     

Pd(II)-catalyzed enantioselective C-H activation of cyclopropanes

Systematic ligand development has led to the identification of novel mono-N-protected amino acid ligands for Pd(II)-catalyzed enantioselective C-H activation of cyclopropanes. A diverse range of organoboron reagents can be used as coupling partners, and the reaction proceeds under mild conditions. These results provide a new retrosynthetic disconnection for the construction of enantioenriched cis-substituted cyclopropanecarboxylic acids.     

Cu(I)/Cu(II)-catalyzed allylic amination of alkenes

Allylic hydrocarbons are selectively converted to the corresponding allyl amines in good to excellent yield by reaction with aryl hydroxylamines catalyzed by a 1:1 mixture of CuCl and CuCl₂ (10 mol %). Under these conditions unsymmetrical olefins react highly regioselectively with N-functionalization at the less substituted vinylic carbon. Trapping experiments indicate that a free nitrosoarene is not an intermediate in these reactions.     

Pd(OAc)(2)-catalyzed oxidative C-H/C-H cross-coupling of electron-deficient polyfluoroarenes with simple arenes

Pd(OAc)(2)-catalyzed intermolecular C-H/C-H cross-coupling reactions between electron-deficient polyfluoroarenes and simple arenes for the synthesis of fluorinated biaryls have been developed. Deuterium-labeling experiments suggested that C-H bond cleavage of the simple arenes rather than the polyfluoroarenes is involved in the rate-limiting step.     

Pd(0) or Pd(II)-catalyzed ring-opening reactions of benzylidene- and alkylidenecyclopropyl ketones and aldehydes

Pd(0) catalyzed reactions of methylenecyclopropyl carbonyl compounds afforded a convenient method for the synthesis of conjugate (E,E)-1,3-diene derivatives 2 in good to excellent yields. Moreover, we also found that Pd(II)-catalyzed reactions of methylenecyclopropyl carbonyl compounds with water gave 1,5-diketones in good to high yields via a carbene-palladium intermediate. The plausible reaction mechanisms have also been provided on the basis of control and ¹⁸O-labeling experiments.     

Cobalt(II)-catalyzed chemoselective synthesis of acetals from aldehydes

The acetalization of aldehydes has been studied with methanol, ethanol, and 2-propanol using CoCl₂ in high yields under reflux conditions. The reaction is simple, efficient, chemoselective and does not involve any other additive.     

Rh(III)-catalyzed oxidative coupling of unactivated alkenes via C-H activation

Oxime directed aromatic C-H bond activation and oxidative coupling to alkenes is reported using a cationic Rh(III) catalyst. Significantly, the method can be used to oxidatively couple unactivated, aliphatic alkenes.     

Silanol as a removable directing group for the Pd(II)-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.     

Intramolecular Pd(II)-catalyzed aerobic oxidative amination of alkenes: synthesis of six-membered N-heterocycles

Use of a base-free Pd(DMSO)(2)(TFA)(2) catalyst system enables the synthesis of six-membered nitrogen heterocycles via a Wacker-type aerobic oxidative cyclization of alkenes bearing tethered sulfonamides. Various heterocycles, including morpholines, piperidines, piperazines, and piperazinones, are accessible by this method.