Cationic intermediates in the Pd-catalyzed negishi coupling. kinetic and density functional theory study of alternative transmetalation pathways in the Me-Me coupling of ZnMe2 and trans-[PdMeCl(PMePh2)2

The complexity of the transmetalation step in a Pd-catalyzed Negishi reaction has been investigated by combining experiment and theoretical calculations. The reaction between trans-[PdMeCl(PMePh(2))(2)] and ZnMe(2) in THF as solvent was analyzed. The results reveal some unexpected and relevant mechanistic aspects not observed for ZnMeCl as nucleophile. The operative reaction mechanism is not the same when the reaction is carried out in the presence or in the absence of an excess of phosphine in the medium. In the absence of added phosphine an ionic intermediate with THF as ligand ([PdMe(PMePh(2))(2)(THF)](+)) opens ionic transmetalation pathways. In contrast, an excess of phosphine retards the reaction because of the formation of a very stable cationic complex with three phosphines ([PdMe(PMePh(2))(3)](+)) that sequesters the catalyst. These ionic intermediates had never been observed or proposed in palladium Negishi systems and warn on the possible detrimental effect of an excess of good ligand (as PMePh(2)) for the process. In contrast, the ionic pathways via cationic complexes with one solvent (or a weak ligand) can be noticeably faster and provide a more rapid reaction than the concerted pathways via neutral intermediates. Theoretical calculations on the real molecules reproduce well the experimental rate trends observed for the different mechanistic pathways.     

Total synthesis of pyranicin

[Reaction: see text] A stereocontrolled convergent synthesis of the annonaceous acetogenin pyranicin (1) is presented. Asymmetric Horner-Wadsworth-Emmons (HWE) reactions were used to access key intermediates. The tetrahydropyran derivative 2 was obtained via an asymmetric desymmetrization of the meso-dialdehyde 6, and the butenolide fragment was constructed using a stereoconvergent reaction sequence involving a parallel kinetic HWE resolution followed by a Pd-catalyzed allylic substitution. The C10/C15 1,6-diol motif was installed using Carreira's asymmetric acetylide addition methodology.     

Stereoselective intermolecular allylic C-H trifluoroacetoxylation of functionalized alkenes

Pd-catalyzed allylic C-H trifluoroacetoxylation of substituted alkenes was performed using PhI(OCOCF(3))(2) as the oxidant and acyloxy source. Trifluoroacetoxylation of monosubstituted cyclopentenes and cyclohexenes proceeds with excellent regio- and diastereoselectivity. Studies with one of the possible (η(3)-allyl)Pd(II) intermediates suggest that the reaction proceeds via stereoselective formation of Pd(IV) intermediates and subsequent stereo- and regioselective reductive elimination of the product.     

钯催化氧化/水解2-炔基芳基叠氮一锅法合成2-苯甲酰胺基苯甲酸

描述了通过钯催化2-炔基芳基叠氮一锅法氧化/水解制备2-苯甲酰胺基苯甲酸的合成方法.在温和的反应条件下,反应以良好至极好的产率完成.     

Synthesis of 2-vinylic indoles and derivatives via a Pd-catalyzed tandem coupling reaction

A novel one-step synthesis of valuable 2-vinylic indoles and their tricycle derivatives is described. This reaction, which utilizes a gem-dibromovinyl unit as a readily available starting material, occurs via an efficient Pd-catalyzed tandem Buchwald-Hartwig/Heck reaction.     

10-hydroxy-10,9-boroxarophenanthrenes: versatile synthetic intermediates to 3,4-benzocoumarins and triaryls

10-Hydroxy-10,9-boroxarophenanthrenes were obtained as unexpected major products upon BBr(3)-mediated O-demethylation of 2-methoxybiaryls. The formation likely proceeds via intramolecular electrophilic aromatic cyclization of a reactive dibromoaryloxyborane intermediate. Essentially quantitative yields of 10-hydroxy-10,9-boroxarophenanthrenes were also obtained from 2-hydroxybiaryl and BCl(3)/AlCl(3) with use of a modified literature procedure. As synthetic intermediates, 10-hydroxy-10,9-boroxarophenanthrenes were efficiently converted to 3,4-benzocoumarins and triaryls through Pd-catalyzed CO insertion and Suzuki reaction.     

Palladium-catalyzed intermolecular directed C-H amidation of aromatic ketones

Pd-catalyzed directed ortho C-H amidation of aromatic ketones with both sulfonamides and amides has been accomplished. The use of an electron-deficient Pd complex, Pd(OTf)(2), is crucial for the success of this transformation. Some key intermediates of the reaction, that is, the cyclopalladation complexes of ketones, have been characterized by X-ray crystallography. Experimental analysis of these palladacycles and also the experimental results with N-methyl sulfonamides indicate that the new reaction does not seem to proceed through a nitrene intermediate. The utility of the newly developed reaction was demonstrated for the synthesis of useful organic intermediates such as 2- and 3-alkyl indoles and 2-aminophenyl ketones.     

Synthesis of self-doped polyaniline bearing phosphonic acid moiety via Pd-catalyzed phosphonation of poly(2-bromoaniline)

Self-doped conducting polyaniline bearing phosphonic acid moiety was synthesized via the Pd-catalyzed phosphonation of poly(2-bromoaniline) as a key reaction.     

In situ generation of vinyl allenes and its applications to one-pot assembly of cyclohexene, cyclooctadiene, 3,7-nonadienone, and bicyclo[6.4.0]dodecene derivatives with palladium-catalyzed multicomponent reactions

A novel tandem Pd-catalyzed cross-coupling and [4 + 4] cycloaddition sequence allows the rapid synthesis of eight-membered carbocycles starting from alpha-bromovinyl arenes and propargyl bromides in one reaction vessel. It is noteworthy that four components are assembled into one molecule via this procedure. In contrast to alpha-bromovinyl arenes, alpha-bromovinyl alkanes afforded tandem cross-coupling and homo [4 + 2] cycloaddition products. Subjecting an equimolar mixture of alpha-bromostyrene and 2-bromo-1-octene to propargyl bromides furnished the tandem Pd-catalyzed cross-coupling and hetero [4 + 2] cycloaddition product. Exposure of equimolar mixtures of alpha-bromovinyl arenes to allenylindium resulted in tandem a Pd-catalyzed cross-coupling and hetero [4 + 4] cycloaddition products. Synthesis of vinylallene from the reaction of vinyl triflate with allenylindium followed by Pd-catalyzed carbon monoxide insertion reaction gave the corresponding 3,7-nonadienone product via tandem Pd-catalyzed cross-coupling and [4 + 4 + 1] annulation. Tandem Pd-catalyzed cross-coupling, [4 + 4] cycloaddition, and [4 + 2] cycloaddition provided the rapid synthesis of bicyclo[6.4.0]dodecene derivatives starting from alpha-bromovinyl arenes, propargyl bromides, and dienophiles in one operation, in which five components were integrated into one molecule.     

Palladium-catalyzed asymmetric synthesis of axially chiral allenylsilanes and their application to SE2' chirality transfer reactions

Stepwise application of the Pd-catalyzed S(N)2' reaction and the desilylative S(E)2' reaction to the ambivalent 2-bromo-1-silyl-1,3-dienes provides a novel route to the highly enantioselective construction of tertiary and quaternary propargylic stereogenic centers via axially chiral allenylsilanes.     

Silica gel-promoted convenient synthesis of 2-bromo-3-hydroxybenzoate derivatives

A convenient silica gel-promoted synthesis of 2-bromo-3-hydroxybenzoate derivatives has been developed via the Diels–Alder reaction of furans with methyl 3-bromopropiolate, followed by a ring-opening aromatization. In addition, 2-bromo-3-methoxybenzoate, derived from 2-bromo-3-hydroxybenzoate with iodomethane, was found to be a good substrate for Pd-catalyzed cross-coupling reactions, despite its sterically hindered structure.     

Pd-catalyzed carbonylation of diazo compounds at atmospheric pressure: a catalytic approach to ketenes

The carbonylation of carbenes through catalytic cycles is highly desirable due to the importance of ketene-mediated reactions in organic synthesis. In this investigation, a highly efficient and mild catalytic approach toward ketene intermediates has been developed based on Pd-catalyzed carbonylation of diazo compounds with CO. When α-diazocarbonyl compounds or N-tosylhydrazone salts are heated in the presence of a palladium catalyst under atmospheric pressure of CO, ketene intermediates are formed in situ, where they undergo further reactions with various nucleophiles such as alcohols, amines, or imines. The Pd-catalyzed tandem carbonylation-Staudinger cycloaddition gives β-lactam derivatives in good yields with excellent trans diastereoselectivity. The results from DFT calculation on the reaction mechanism suggest that Pd is involved in the [2 + 2] cycloaddition process and affects the diastereoselectivity of the β-lactam products by assisting isomerization of the addition intermediate. On the other hand, the acylketenes generated from the Pd-catalyzed carbonylation of α-diazoketones react with imines in a formal [4 + 2] cycloaddition manner to afford 1,3-dioxin-4-one derivatives. This straightforward carbonylation provides a new approach toward highly efficient catalytic generation of ketene species under mild conditions.     

Synthesis and characterization of monodisperse oligofluorenes

An efficient synthesis of 9,9-bis(2-ethylhexyl)fluorene oligomers up to the heptamer is reported, with repetitive Suzuki and Yamamoto coupling reactions employed in the synthesis. The key steps for preparation of the essential intermediates include Pd-catalyzed transformation of aryl bromides to aryl boronic esters (Miyaura reaction) and the application of the much higher reactivity of aryl boronic esters over aryl bromides in the Pd-catalyzed cross-coupling reaction with aryl diazonium salts. Variation of the UV/Vis absorption and photoluminescence characteristics with chain length is reported. Moreover, glass transition and liquid-crystal characteristics of the oligomers are described and compared with those of the polymer.     

Palladium catalyzed bicyclization of 1,8-diiodonaphthalene and tertiary propargylic alcohols to phenalenones and their applications as fluorescent chemosensor for fluoride ions

Phenalenone derivatives were efficiently constructed from 1,8-diiodonaphthalene and tertiary propynols via a one-pot domino reaction which eventually included Pd-catalyzed Sonogoshira coupling, Pd-catalyzed allylic oxidation and Pd-catalyzed C(sp(2))-H activation. Moreover, the synthesized phenalenone derivative presented a practical application as a fluorescent chemosensor for fluoride anion with high sensitivity and selectivity.     

Vinyl ketones to allenes: preparation of 1,3-dien-2-yl triflates and their application in Pd-catalyzed reactions with soft nucleophiles

[chemical reaction: see text]. A general route of converting alkenyl ketones to functionalized allenes was developed. Substituted 1,3-dien-2-yl triflates, which were prepared from the alkenyl ketones via silyl dienol ethers, were excellent substrates for the palladium-catalyzed reaction with soft nucleophiles giving the multisubstituted allenes in high yields. Comparison between the dienyl triflates and analogous bromodienes in the Pd-catalyzed reaction was studied as well.     

Synthesis of Spiroindenyl-2-Oxindoles through Palladium-Catalyzed Spirocyclization of 2-Bromoarylamides and Vinyl Bromides

An expeditious approach to the construction of spiroindenyl-2-oxindoles was developed via a palladium-catalyzed spirocyclization reaction of 2-bromoarylamides with vinyl bromides. The reaction formed spiropalladacycles as the intermediates via carbopalladation and the C–H functionalization of 2-bromoarylamides. The spiropalladacycles reacted with vinyl bromides to form spiroindenyl-2-oxindoles. A Heck process rather than vinylic C–H functionalization was involved in the reaction.     

Regio- and stereoselective introduction of 15β-hydroxy group and side chains to steroids by the palladium-catalyzed reaction of 1,3-diene monoepoxide

The Pd-catalyzed reaction of 1,3-diene monoepoxides with carbonucleophiles is applied to the regio- and stereoselective introduction of 15β-hydroxy group and side chains to steroid nuclei. 3β-Hydroxyandrost-5-en-17-one (15) was converted to 15,16β-epoxy-Δ¹⁷⁽²⁰⁾ isoheptylidene steroid 20 and ethylidene steroid 21. The former was subjected to the Pd-catalyzed reaction with dimethyl malonate and then converted to 15β-hydroxycholesterol (29). Similarly, 15β-hydroxyisocholesterol (32) was obtained from the ethylidene steroid 21 using the Pd-catalyzed reaction of methyl 3-oxo-5-methylhexanoate (24) as a key reaction.     

Mechanism of the MoO2Cl2-catalyzed hydrosilylation: a DFT study

A detailed mechanistic study of the hydrosilylation of benzaldehyde catalyzed by MoO2Cl2 is reported. On the basis of DFT calculations (B3LYP/6-311++G**), a reaction pathway via initial silane activation is proposed. SiH and CH activation energies by other metal-oxo compounds like RuO4, OsO4, and MnO4- are compared to MoO2Cl2. MO4-type metal-oxo compounds follow a different mechanism, forming highly exergonic intermediates, which prevent a catalytic reaction, whereas MoO2Cl2 prefers a reaction pathway via thermoneutral intermediates.     

A general approach to cyathin diterpenes. Total synthesis of allocyathin B(3)

[reaction: see text] The synthesis of allocyathin B(3) from an advanced intermediate possessing the ring system and relative stereochemistry but lacking the isopropyl and hydroxymethyl groups is reported. The isopropyl group was introduced by radical cyclization of a methyl propargyl acetal of an alpha-bromo ketone, and the hydroxymethyl group was generated by Pd-catalyzed carbonylation of a vinyl triflate. The route provides functionalized intermediates that could allow access to more complex members of the cyathin family of diterpenes.     

Pd-catalyzed tandem C-N/C-C coupling of gem-dihalovinyl systems: a modular synthesis of 2-substituted indoles

2-substituted indoles were synthesized via a Pd-catalyzed tandem C-N/Suzuki-Miyaura coupling from readily prepared ortho-gem-dihalovinylanilines. Optimal conditions used a Pd(OAc)(2)/S-Phos catalyst in the presence of K(3)PO(4).H(2)O and an organoboron reagent, which included boronic acids, esters, alkyl 9-BBN derivatives, and trialkylboranes. Yields of the desired indoles were good to excellent using low catalyst loadings (typically 1 mol %). [reaction: see text]