A new pathway for hydroamination. Mechanism of palladium-catalyzed addition of anilines to vinylarenes

The mechanism of the hydroamination of vinylarenes with anilines catalyzed by phosphine-ligated palladium triflates was uncovered. eta3-Arylethyl diphosphine palladium triflate complexes, which result from migratory insertion of vinylarene into a palladium hydride triflate, were shown to be the resting state of the catalytic cycle. A series of these complexes has been isolated and fully characterized. The [(R)-Tol-BINAP][1-(2-naphthyl)ethyl]palladium triflate derivative 1a was crystallographically characterized. This complex reacted with aniline to form the N-phenethylaniline product in 83% yield. Reaction of the benzylic derivative [(R)-Tol-BINAP](eta3-benzyl)palladium triflate with aniline also formed the N-benzylaniline product in a high 87% yield. Predominant inversion of configuration from the reaction between 1a, which is enantiopure, and aniline showed that external attack of the amine on the eta3-arylethyl ligand occurred to form the product. This product from reaction of aniline with 1a is the opposite enantiomer to that obtained from the catalytic process. Thus, a minor diastereomer gives the major enantiomer in the catalytic cycle, and the major diastereomer gives the minor enantiomer. Consistent with this assertion, kinetic studies showed that the major diastereomer formed product with a rate constant roughly 3.5 times slower than the rate constant for the catalytic process that contains all diastereomers.     

Transcyclometalation,a versatile methodology for multiple metal-carbon bond formation with multisite ligands

Transcyclometalation has been successfully applied for the multi-platination and -ruthenation of 'chartwheel'-type ligand systems containing six potential metal binding sites, thus providing a method for multiple metal-carbon bond formation via C-H bond activation which is superior to established cyclometalation protocols.     

A palladium-catalyzed carbon-carbon bond formation of conjugated dienones : A macrocyclic dienone lactone model for the carbomycins

A Pd catalyzed coupling of terminal vinylic iodides with methyl vinyl ketone and related enones to produce dienones is described. The application of this method to macrocyclization is demonstrated in a model system for the agylcone of carbomycin B.     

Efficient preparation of monohydrosilanes using palladium-catalyzed Si-C bond formation

The arylation of dihydrosilanes with aryl iodides or heteroaryl iodides in the presence of a palladium catalyst provides the corresponding monohydrosilanes in good to high yield. Moderate to good yields are obtained even in the presence of a variety of reactive functional groups, such as -NH2, -OH, or -CN, without their protection.     

A new route to phenylenedimalononitrile and the analogues using palladium-catalyzed carbon-carbon bond formation

Phenylenedimalononitrile and the analogues, which are key intermediates in the synthesis of TCNQ and the analogues, are prepared directly by the Pd-catalyzed reaction of-diiodoarenes with malononitrile anion.     

A novel method for carbon-carbon bond formation palladium-catalyzed cross-coupling reaction of phenyl fluoroalkanesulfonates with organozinc reagents

Cross-coupling reaction of phenyl fluoroalkanesulfonates with three kinds of organozinc reagents in the presence of palladium gives the corresponding alkylbenzenes in good yields. A possible mechanism is suggested.     

A thiophosphinidene complex as a vehicle in phosphinidene transmetalation: easy formation and cleavage of a P-S bond

A method for the generation of transition metal-phosphorus multiple bonds has been developed using the reactions of a novel thiophosphinidene-bridged dimolybdenum complex with different metal carbonyls. The overall process could be considered as a transmetalation of the phosphinidene ligand involving the activation of P-S and P-Mo bonds.     

Carbon-carbon bond formation in regio- and stereoselective palladium-catalyzed cyclization of allene-substituted conjugated dienes.

Regio- and stereoselective palladium-catalyzed reactions of allene-substituted 1,3-dienes 1 in acetic acid at room temperature lead to cyclization with formation of a carbon-carbon bond between the middle carbon of the allene and the terminal carbon of the 1,3-diene. Two different types of reactions, both that constitute 1,4-carboacetoxylations of the 1,3-diene, have been developed. In one of the reactions, Pd(II) catalyzes the oxidation of 1 to bicyclic compounds 2, and in the other, Pd(0) catalyzes the transformation of 1 to bicyclic compounds 3. The products 2 are useful for further synthetic transformations and undergo Diels-Alder reactions with dienophiles to give polycyclic ring systems.     

Selective palladium-catalyzed C-F activation/carbon-carbon bond formation of polyfluoroaryl oxazolines

A selective palladium-catalyzed Suzuki-Miyaura coupling reaction of polyfluorophenyl oxazolines through ortho C-F activation is described. It was found that reactions with DPPF as the ligand occurred much faster than those with other ligands. A variety of arylboronic acids including challenging functionalized arylboronic acids such as enolizable ketones, aldehyde, cyano, ester, and trifluoromethyl groups were tolerated with the reaction conditions.     

A domino palladium-catalyzed C-C and C-O bonds formation via dual O-H bond activation: synthesis of 6,6-dialkyl-6H-benzo[c]chromenes

An efficient Pd-catalyzed domino reaction of α,α-dialkyl-(2-bromoaryl)methanols to 6,6-dialkyl-6H-benzo[c]chromenes is presented. Their formation can be explained via a five membered Pd(II)-cycle that efficiently involves a domino homocoupling with the second molecule, β-carbon cleavage, and finally intramolecular Buchwald-Hartwig cyclization. This domino process effectively involves breaking of five σ-bonds (2C-Br, 2O-H, and a C-C) and formation of two new σ-bonds (C-C and C-O). This mechanistic pathway is unprecedented and further illustrates the power of transition metal catalysis.     

Carbon-carbon bond formation: palladium-catalyzed oxidative cross-coupling of N-tosylhydrazones with allylic alcohols

In the zone: Pd-catalyzed oxidative cross-coupling of N-tosylhydrazones with allylic alcohols leads to C?C bond formation. A palladium-carbene migratory insertion is proposed to play the key role in this transformation. The reaction proceeds with readily available starting materials to afford substituted alkenes in a highly stereoselective manner.     

Observation of a hidden intermediate in the Stille reaction. Study of the reversal of the transmetalation step

A study of the reaction of cis-[PdRf2(AsPh3)2] (Rf = 3,5-C6Cl2F3) with ISnBu3 (that is the reversal of the natural Stille reaction of [PdRfI(AsPh3)2] with RfSnBu3) allows for the observation of cis-[PdRf2(AsPh3)(ISnBu3)], the expected intermediate from a cyclic transmetalation in the direct Stille reaction, thus providing experimental support to the operation of cyclic transmetalation pathways.     

Computational characterization of a complete palladium-catalyzed cross-coupling process: the associative transmetalation in the Stille reaction

[reaction: see text] The species presumably involved in the associative ligand substitution mechanism for the Stille cross coupling of vinyl bromide and trimethylvinyl stannane with Pd(PMe(3))(2)/PMe(3) as catalysts in DMF (as ligand and solvent) have been structurally and energetically characterized. The cyclic four-coordinate transition state for the rate-determining transmetalation step explains the retention of configuration in the Stille coupling of chiral nonracemic alkyl stannanes.     

Copper- and palladium-catalyzed intramolecular C-S bond formation: a convenient synthesis of 2-aminobenzothiazoles

Copper- and palladium-catalyzed intramolecular C-S bond formation by cross-coupling between an aryl halide and thiourea functionality is demonstrated for the synthesis of 2-aminobenzothiazoles, wherein the Cu-catalyzed protocol is generally superior and more cost effective than the Pd-catalyzed protocol; the Cu-catalyzed reaction also further expands recent studies exploring the utility of Cu salts as replacements for Pd in carbon-heteroatom bond-forming reactions. A one-pot variant combining the synthesis of the thiourea and the cyclization was also demonstrated.     

Aromatic hydroxylation: oxygen insertion into the metal-carbon bond of thioazobenzene palladacycles

Summary High-yield regiospecific oxidation, termed aromatic metaloxylation, (Ar-Pd Ar-OPd) of cyclopalladated thioazobenzenes (2) by peracids (YCO₃H) in CHCl₃ solution is described. The oxidized pink-violet product (3) has been characterized by spectroscopy. The free hydroxyazo ligands (4) are liberated from (3) by reductive (hydrazine) elimination of the metal. The kinetics of metaloxylation have been examined using different peracids and yield the rate law: d[(3)]/dt = k[(2)]²[YCO₃H]. The net H (20–50 kJ) is small and S (-115 to -130 J deg^–1 mol^–1) is large. This suggests an associative mechanism. Comparatively less negative S (-30 to -47 J deg^–1 mol^–1) for (2d) and (2e) may be due to the steric retardation of molecular association in the rate determining step provided by the benzyl group. It is proposed that in the transition state YCO₃H binds to two metal centres bridging via peroxo oxygen, followed by heterolytic cleavage of the O-O bond and oxygen insertion into the Pd-C bond.     

Synthesis of (1-alkynyl)dicarbonylcyclopentadienyliron complexes by palladium-catalyzed Sonogashira-type carbon-iron bond formation

Treatment of [CpFe(CO)₂I] with terminal alkynes in the presence of catalytic amounts of dichlorobis(triphenylphosphine)palladium and copper iodide in aliphatic amine/THF results in Sonogashira-type carbon-iron bond formation to yield [CpFe(CO)₂(CCR)] in good yields.