Remarkable rate acceleration of Pd(0)-catalyzed hydrogermylation of alkynes and dienes in water

[reaction: see text] Without the use of hydrophilic cosolvents or phase transfer catalysts, Pd(0)-catalyzed hydrogermylation of alkynes in water provides dienylgermanes efficiently with high stereo- and regioselectivity. The reaction in water proceeds much faster than the reaction under neat conditions. By using an aqueous system, hydrogermylation can be conducted at a low catalyst loading.     

Mechanism of Pd(NHC)-catalyzed transfer hydrogenation of alkynes

The transfer semihydrogenation of alkynes to (Z)-alkenes shows excellent chemo- and stereoselectivity when using a zerovalent palladium(NHC)(maleic anhydride)-complex as precatalyst and triethylammonium formate as hydrogen donor. Studies on the kinetics under reaction conditions showed a broken positive order in substrate and first order in catalyst and hydrogen donor. Deuterium-labeling studies on the hydrogen donor showed that both hydrogens of formic acid display a primary kinetic isotope effect, indicating that proton and hydride transfers are separate rate-determining steps. By monitoring the reaction with NMR, we observed the presence of a coordinated formate anion and found that part of the maleic anhydride remains coordinated during the reaction. From these observations, we propose a mechanism in which hydrogen transfer from coordinated formate anion to zerovalent palladium(NHC)(MA)(alkyne)-complex is followed by migratory insertion of hydride, after which the product alkene is liberated by proton transfer from the triethylammonium cation. The explanation for the high selectivity observed lies in the competition between strongly coordinating solvent and alkyne for a Pd(alkene)-intermediate.     

Pd(0)-catalyzed reaction of alkynes with trifurylgermane and CO providing acylgermanes: the example of hydrometalcarbonylation of alkynes

A direct synthesis of acylmetals from alkynes with group 14 metal hydride and CO is reported. The Pd(0)-catalyzed reaction of alkynes with tri(2-furyl)germane under ambient pressure of CO provides alpha,beta-unsaturated acylgermanes in good yields. Conversion of acylgermanes to the corresponding amides is achieved.     

Gold(I)-catalyzed asymmetric cyclopropenation of internal alkynes

Highly enantioselective cyclopropenation of internal alkynes with aryldiazoacetates was achieved using the binuclear gold catalyst (S)-xylylBINAP(AuCl)(2), activated by silver hexafluoroantimonate.     

Regiocontrolled Cu(I)-catalyzed borylation of propargylic-functionalized internal alkynes

Good to excellent reactivity and regiocontrol have been achieved in the Cu(I)-catalyzed borylation of dialkyl internal alkynes with bis(pinacolato)diboron. The presence of a propargylic polar group (OH, OR, SAr, SO(2)Ar, or NHTs), in combination with PCy(3) as ligand, allowed maximizing the reactivity and site-selectivity (β to the propargylic function). DFT calculations suggest a subtle orbitalic influence from the propargylic group, matched with ligand and substrate size effects, as key factors involved in the high β-selectivity. The vinylboronates allowed the stereoselective synthesis of trisubstituted olefins, while allylic substitution of the SO(2)Py group without affecting the boronate group provided access to formal hydroboration products of unbiased dialkylalkynes.     

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.     

Chemoselective Pd(0)-catalyzed peptide coupling in water

[reaction: see text] Highly charged peptides ranging in length from 17 to 33 residues have been efficiently tricoupled to a trialkyne nucleus by using a Sonogashira Pd(0) coupling strategy under both acidic (pH 5.0) and basic (pH 7.5) conditions. These results demonstrate the utility of Pd(0) to construct protein-sized structures (12,000 mol wt) in aqueous milieu.     

Expeditious enyne construction from alkynes via oxidative Pd(II)-catalyzed Heck-type coupling

The enyne, ubiquitous in natural products, can be a challenge to generate since these moieties require many synthetic transformations to assemble them. We developed a simpler protocol to construct enynes while we found that this oxidative reaction was tolerant in substrate scope. In addition, the utility of this reaction was demonstrated through the attempt in synthesizing antifungal agent Lamisil^™.     

Evaluation of disaccharide-based ligands for Pd(0)-catalyzed asymmetric allylations

A series of phosphines based on disaccharides containing a d-glucosamine framework were prepared and tested for their abilities as ligands for the palladium(0)-catalyzed asymmetric allylic allylation of racemic 1,3-diphenyl-2-propenyl acetate with various nucleophiles. In contrast to previous results exploiting monosaccharides, the iminophosphines generally afforded higher enantiomeric excesses, up to 99%.     

Pd(0)-catalyzed amphiphilic activation of bis-allyl alcohol and ether

Pd(0).Et3B catalyzes amphiphilic activation of symmetric allylic diol 1 to promote electrophilic allylation at the alpha-position of aldehydes and nucleophilic allylation at the aldehyde CO, furnishing 3-methylenecyclopentalols 2 and thus generation of a zwitterionic trimethylenemethane species from the commercially available diol 1.     

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.     

Microwave-assisted, Pd(0)-catalyzed cross-coupling of diazirines with aryl halides

Pd(0)-catalyzed cross-coupling reactions of diazirines with aryl halides under microwave heating conditions afford a series of substituted olefins. A reaction mechanism involving the migratory insertion of the Pd carbene intermediate is proposed.     

CuCl-K(2)CO(3)-catalyzed highly selective borylcupration of internal alkynes - ligand effect

An efficient and practical copper-catalyzed highly regio- and stereoselective borylcupration of internal alkynes with bis(pinacolato)diboron using a catalytic amount of K(2)CO(3) as base producing Z-alkenylboron compounds has been demonstrated by applying the ligand effect: commercially available electron-rich tris(p-methoxyphenyl) phosphine ensures a smooth and efficient reaction. Functionalized alkynes, such as propargylic alcohols and derivatives as well as N-propargyl tosylamide, may also be used with excellent selectivity.     

Gold(I)-catalyzed arylmethylation of terminal alkynes

AuCl/AgOTf catalyzes the reaction of terminal alkynes with aryl trichloroacetimidate to afford arylmethylation products in moderate to good yields.     

Rhodium(III)-catalyzed synthesis of isoquinolines from aryl ketone O-acyloxime derivatives and internal alkynes

A synthetic method of isoquinolines from aryl ketone O-acyloxime derivatives and internal alkynes has been developed using [Cp*RhCl(2)](2)-NaOAc as the potential catalyst system. The present transformation is carried out by a redox-neutral sequence of C-H vinylation via ortho-rhodation and C-N bond formation of the putative vinyl rhodium intermediate on the oxime nitrogen, where the N-O bond of oxime derivatives could work as an internal oxidant to maintain the catalytic cycle.     

Pd(0)-catalyzed diarylation of sp3 C-H bond in (2-azaaryl)methanes

A highly efficient and selective palladium-catalyzed diarylation of (2-azaaryl)methanes at the methyl group is described. Aryl chlorides proved reactive enough. A palladium η(3)-azaallyl intermediate has been identified on the basis of DFT studies.     

A new access to 3,5-disubstituted piperazinones via Pd(0)-catalyzed amination

An original synthetic route toward 3,5-disubstituted piperazinones has been developed. The method relies upon a 6-exo intramolecular process between a sulfonylated nitrogen atom of amino acid derivation and an η³-allyl-palladium moiety. This cyclization process generates the two possible (cis and trans) diastereoisomers whose ratio depends on the amino acid employed. The bulkier the amino acid residue, the higher the observed cis:trans ratio. Convincing evidence for reversible intramolecular addition of the nitrogen nucleophile to the η³-allyl-palladium complex is put forward.