Palladium(II)-catalyzed oxidative transformation of allylic alcohols and vinyl ethers into 2-alkoxytetrahydrofurans: catechol as an activator of catalyst

[chemical reaction: see text]. A highly effective synthesis of 2-alkoxytetrahydrofurans from allylic alcohols and vinyl ethers was achieved by using catalytic amounts of Pd(OAc)2, Cu(OAc)2, and catechol (1:1:2) under O2. The use of catechol as an activator of Pd(II)-Cu(II) catalyst has been unprecedented. The 2-alkoxytetrahydrofurans are formed via oxypalladation of allylic alcohols toward vinyl ethers followed by 5-exo cyclization of the resulting oxypalladation intermediate and subsequent beta-Pd-H elimination. No 6-endo cyclization of the oxypalladation intermediate occurs.     

An Ir-Pt catalyst for the multistep preparation of functionalized indoles from the reaction of amino alcohols and alkynyl alcohols

The 1,2,4-trimethyltriazolylidene (ditz) ligand allows the preparation of homo- and heterodimetallic complexes of Pt(2) and Ir-Pt. These two complexes have been characterized by means of spectroscopic and diffractommetric techniques. The catalytic activity of these complexes, together with that of other Pt-based compounds, has been explored in the cyclization-addition of alkynyl alcohols and indoles. The Ir-Pt complex [{PtI(2)(py)}(μ-ditz){IrI(2)(Cp*)}] (py=pyridine; Cp*=pentamethylcyclopentadienyl) allows the combination of an iridium-mediated oxidative cyclization of 2-(ortho-aminophenyl)ethanol to form indoles, with a further step employing a Pt-based multistep reaction that functionalizes indoles. Our results show that the Ir-Pt complex is a very active catalyst in this new multistep preparation of functionalized indoles from the reaction of an amino alcohol with alkynyl alcohols.     

Scope and mechanism of the Pd(II)-catalyzed arylation/carboalkoxylation of unactivated olefins with indoles

Treatment of 1-methyl-2-(4-pentenyl)indole (5) with a catalytic amount of [PdCl2(MeCN)2] (2; 5 mol %) and a stoichiometric amount of CuCl2 (3 equiv) in methanol under CO (1 atm) at room temperature for 30 min gives methyl (9-methyl-2,3,4,9-tetrahydro-4-carbazolyl)acetate (6), which was isolated in 83% yield. A number of 2- and 3-alkenyl indoles undergo a similar palladium-catalyzed cyclization/carboalkoxylation to give the corresponding polycyclic indole derivatives in moderate to excellent yields with excellent regio- and diastereoselectivity. Under similar conditions, vinyl arenes undergo intermolecular arylation/carboalkoxylation with indoles to give 3-(1-aryl-2-carbomethoxyethyl) indoles in moderate yield with high regioselectivity. Stereochemical analyses of the palladium-catalyzed cyclization/carboalkoxylation of both 2- and 3-alkenyl indoles are in agreement with mechanisms involving outer-sphere attack of the indole on a palladium-olefin complex followed by alpha-migratory insertion of CO and methanolysis of the resulting acyl palladium intermediate. CuCl2 functions as the terminal oxidant in this palladium-catalyzed cyclization/carboalkoxylation of alkenyl indoles and also significantly increases the rate of reaction of 2 with the alkenyl indole to form the corresponding acyl palladium complex. Spectroscopic studies are in agreement with the intermediacy of a heterobimetallic Pd/Cu complex as the active catalyst in this reaction.     

Pd(II) acts simultaneously as a Lewis acid and as a transition-metal catalyst: synthesis of cyclic alkenyl ethers from acetylenic aldehydes

The reaction of carbon-tethered acetylenic aldehydes with alcohols in the presence of a catalytic amount of Pd(OAc)2 in 1,4-dioxane at room temperature gave the 5- or 6-membered acetal products in high yields. The 13C NMR studies suggested that a Pd(II) catalyst exhibited dual roles in the present reaction; the attack of ROH to aldehyde is catalyzed by Lewis acidic Pd(OAc)2, and the nucleophilic oxygen of the resulting hemiacetal reacts with alkyne complexed by Pd(II), giving the alkenyl ethers.     

Palladium-catalyzed cyclization/carboalkoxylation of alkenyl indoles

Reaction of 1-methyl-2-(4-pentenyl)indole with a catalytic amount of PdCl2(CH3CN)2 (5 mol %) and a stoichiometric amount of CuCl2 (3 equiv) in methanol under CO (1 atm) at room temperature for 30 min led to cyclization/carboalkoxylation to form the corresponding tetrahydrocarbazole in 83% isolated yield as a single regioisomer. Palladium-catalyzed cyclization/carboalkoxylation of 2-(4-pentenyl)indoles tolerated substitution along the alkenyl chain and at the internal and cis-terminal olefinic positions. Palladium-catalyzed cyclization/carboalkoxylation tolerated a range of alcohols and was effective for the cyclization of 2-(3-butenyl)indoles, 3-(3-butenyl)indoles, 3-(4-pentenyl)indoles, and 2-(5-hexenyl)indoles.     

Heterogenized polymetallic catalysts: Part III. Catalytic air oxidation of alcohols by Pd(II) complexed to a polyphenylene polymer containing β-di- and tri-ketone surface ligands

This paper describes further studies on mono- and bi-metallic catalysts attached to a polymer support by β-di- and tri-ketone surface ligands. The previous two papers described the oxidation of catechol by the heterogeneous catalysts using Cu(II), Fe(III) and Pd(II) as the metal species. The present study expands these studies to a series of mono- and polyfunctional alcohols using Pd(II) as the metal species. The final catalytic surfaces were prepared by treatment of the modified polymer with a very reactive form of Pd(II), [Pd(CH₃CN)₄]²⁺. The simple alcohols gave increases in rates of up to 5-fold for the bimetallic systems. As might be expected glycols and - -glucose gave even higher increases in rate in going from the mono- to the bi-metallic catalyst. For ethylene glycol the factor was 30. Unsaturated alcohols gave the most dramatic results. With the monometallic catalyst, the products from allyl alcohol consisted of 25% acrolein resulting from direct alcohol oxidation and 75% 3-hydroxypropanal resulting from Wacker-type oxidation of the double bond. With the bimetallic catalyst the overall rate increased by a factor of 10 and the products consisted of 80% acrolein and 20% 3-hydroxypropanal. The actual rate increase for the direct alcohol oxidation is calculated to be a factor of 32. 4-Penten-2-ol and 4-penten-1-ol gave rates that were lower than the monofunctional alcohols. This is attributed to inhibition by olefin π-complex formation with the Pd(II).     

Highly active Au(I) catalyst for the intramolecular exo-hydrofunctionalization of allenes with carbon, nitrogen, and oxygen nucleophiles

Reaction of benzyl (2,2-diphenyl-4,5-hexadienyl)carbamate (4) with a catalytic 1:1 mixture of Au[P(t-Bu)2(o-biphenyl)]Cl (2) and AgOTf (5 mol %) in dioxane at 25 degrees C for 45 min led to isolation of benzyl 4,4-diphenyl-2-vinylpyrrolidine-1-carboxylate (5) in 95% yield. The Au(I)-catalyzed intramolecular hydroamination of N-allenyl carbamates tolerated substitution at the alkyl and allenyl carbon atoms and was effective for the formation of piperidine derivatives. gamma-Hydroxy and delta-hydroxy allenes also underwent Au-catalyzed intramolecular hydroalkoxylation within minutes at room temperature to form the corresponding oxygen heterocycles in good yield with high exo-selectivity. 2-Allenyl indoles underwent Au-catalyzed intramolecular hydroarylation within minutes at room temperature to form 4-vinyl tetrahydrocarbazoles in good yield. Au-catalyzed cyclization of N-allenyl carbamates, allenyl alcohols, and 2-allenyl indoles that possessed an axially chiral allenyl moiety occurred with transfer of chirality from the allenyl moiety to the newly formed stereogenic tetrahedral carbon atom.     

One-pot synthesis of indole-fused scaffolds via gold-catalyzed tandem annulation reactions of 1,2-bis(alkynyl)-2-en-1-ones with indoles

The gold-catalyzed tandem cyclization of 1,2-bis(alkynyl)-2-en-1-ones with indoles offers an efficient and straightforward route to indole-fused polycyclic systems. The process is realized through a cascade carbonyl-yne cyclization/Friedel-Crafts/indole-yne cyclization sequence catalyzed by a single-pot catalyst of gold.     

Palladium-catalyzed functionalization of indoles with 2-acetoxymethyl-substituted electron-deficient alkenes

New functionalizations of indoles via palladium-catalyzed reaction of indoles and 2-acetoxymethyl-substituted electron-deficient alkenes are reported. It was found that for N-protected indoles the reaction proceeded smoothly in the presence of 5 mol % of Pd(acac)2 and 10 mol % of PPh3 at 80 degrees C in HOAc, while for N-unprotected indoles, the reaction was carried out by using 5 mol % of Pd(dba)2 or 2.5 mol % of Pd2(dba)3.CHCl3 with 10 mol % of 2,2'-bipyridine as the catalyst in toluene. This strategy allows the selective installation of electron-deficient olefin functionality at the 3-position of indoles, which might be difficult to obtain by other methods and can be further elaborated.     

Stereoselective synthesis of 3-alkylideneoxindoles by palladium-catalyzed cyclization reaction of 2-(alkynyl)aryl isocyanates with organoboron reagents

A palladium(0)/monophosphine catalyst promotes a cyclization reaction of 2-(alkynyl)aryl isocyanates with organoboron reagents to produce stereodefined 3-alkylideneoxindoles. The alkynyl and isocyanato groups undergo oxidative cyclization with Pd(0) to form an oxapalladacycle intermediate. Subsequent transmetalation and reductive elimination afford the product.     

A homogeneous catalyst made of poly(4-vinylpyridine-co-N-vinylpyrrolidone)-Pd(0) complex for hydrogenation of aromatic nitro compounds

Poly(4-vinylpyridine-co-N-vinylpyrrolidone)(VPy-co-NVP) and its palladium complex (VPy–NVP–Pd) were prepared. The palladium complex was used as catalyst for the hydrogenation of some nitroaromatics. The molar content of VPy units in VPy-co-NVP was determined as 31.25% by ¹H NMR. VPy–NVP–Pd can be easily resolved in ethanol forming a homogeneous catalytic hydrogenation system together with substrates. The optimum catalytic activity for hydrogenation of nitrobenzene appeared when VPy/Pd molar ratio was 2. The catalytic behavior of the catalyst was found to be greatly affected by the type and concentration of added alkalies. The highest hydrogenation rate for nitrobenzene was found in a 0.1 mol/l ethanol solution of potassium hydroxide. The catalytic stability was examined by using nitrobenzene and 4-nitroanisole as substrates.     

Highly enantioselective copper(I)-fesulphos-catalyzed 1,3-dipolar cycloaddition of azomethine ylides

The catalyst system formed by Cu(CH3CN)4ClO4 and the planar chiral P,S-ligand Fesulphos behaves as a very efficient chiral Lewis acid in the catalytic asymmetric 1,3-dipolar cycloaddition of azomethine ylides. This catalyst shows a remarkable reactivity at low catalyst loading (0.5-3 mol %), affording in good yields the endo adducts with exceptional levels of enantioselectivity (up to >99% ee). This catalytic asymmetric procedure has a broad structural scope with regard to both azomethine and dipolarophile substitution. The first examples of catalytic asymmetric 1,3-dipolar cycloaddition with ketimine-derived azomethines are reported.     

Friedel-Crafts-type allylation of nitrogen-containing aromatic compounds with allylic alcohols catalyzed by a [Mo3S4Pd(η3-allyl)] cluster

With the direct use of allylic alcohols as allylating agents, the Friedel-Crafts-type allylic alkylation of nitrogen-containing aromatic compounds catalyzed by a [Mo(3)S(4)Pd(η(3)-allyl)] cluster is achieved. With a 3 mol % catalyst loading in acetonitrile at reflux or 60 °C, a variety of N,N-dialkylanilines and indoles reacted smoothly with allylic alcohols to afford the Friedel-Crafts-type allylation products in good to excellent yields with high levels of regioselectivity.     

Palladium-catalyzed C-H activation/intramolecular amination reaction: a new route to 3-aryl/alkylindazoles

A method for the catalytic C-H activation of hydrazone compounds followed by intramolecular amination is described. It requires the use of a catalytic amount of Pd(OAc)2 in the presence of Cu(OAc)2 and AgOCOCF3, which efficiently effects the cyclization to afford variously substituted indazoles. The reactions proceed under relatively mild conditions and thus tolerate a variety of functional groups, including alkoxycarbonyl and cyano groups and halogen atoms.     

Palladium-catalyzed synthesis of 2-substituted benzothiazoles via a C-H functionalization/intramolecular C-S bond formation process

Catalytic synthesis of 2-substituted benzothiazoles from thiobenzanilides was achieved in the presence of a palladium catalyst through C-H functionalization/C-S bond formation. This method features the use of a novel catalytic system consisting of 10 mol % of Pd(II), 50 mol % of Cu(I), and 2 equiv of Bu4NBr that produced variously substituted benzothiazoles in high yields with good functional group tolerance.     

Mechanism of Pd(OAc)2/DMSO-catalyzed aerobic alcohol oxidation: mass-transfer-limitation effects and catalyst decomposition pathways

Pd(OAc)(2) in DMSO is an effective catalyst for the aerobic oxidation of alcohols and numerous other organic substrates. Kinetic studies of the catalytic oxidation of primary and secondary benzylic alcohol substrates provide fundamental insights into the catalytic mechanism. In contrast to the conclusion reached in our earlier study (J. Am. Chem. Soc. 2002, 124, 766-767), we find that Pd(II)-mediated alcohol oxidation is the turnover-limiting step of the catalytic reaction. At elevated catalyst loading, however, the rate of catalytic turnover is limited by the dissolution of oxygen gas into solution. This mass-transfer rate is measured directly by using gas-uptake methods, and it correlates with the maximum rate observed during catalysis. Initial-rate studies were complemented by kinetic analysis of the full-reaction timecourses at different catalyst concentrations. Kinetic fits of these traces reveal the presence of unimolecular and bimolecular catalyst decomposition pathways that compete with productive catalytic turnover.     

Cu(I)-catalyzed oxidative cyclization of alkynyl oxiranes and oxetanes

In the presence of a Cu(I) catalyst and a pyridine oxide, alkynyl oxiranes and oxetanes can be converted into functionalized five- or six-membered α,β-unsaturated lactones or dihydrofuranaldehydes. This new oxidative cyclization is proposed to proceed via an unusual allenyloxypyridinium intermediate.     

负载型Cu-Pd双金属纳米氧化物催化剂催化合成碳酸二乙酯：Cu（I）作为活性中心

催化反应活性与催化剂活性组分的存在价态密切相关,所以探讨催化剂在反应过程中的活性中心及其价态变化,对于催化反应机理和催化剂的研究都显得十分重要.目前对于氧化羰基合成碳酸二甲酯催化剂的机理的探讨很多,主要存在的争议是Cu+还是Cu2+作为活性中心,以及铜物种的配位状态.大多体系都是以分子筛为载体的铜基催化剂,其活性中心的研究存在铜离子在分子筛中的定位问题,而且催化活性也会受到分子筛结构的影响.采用这种方法研究活性中心的影响因素较多,存在一定的局限性.因此,直接制备纳米级的铜基氧化物用于本催化体系,有利于更直观简单地探索其活性中心.纳米级金属氧化物材料是一种新型的功能性材料,而纳米铜基氧化物(CuO和Cu2O)因其独特的物化性质和结构而引起广泛关注.我们采用水热法制备纳米CuO及其它氧化物,研究了NaOH浓度对催化剂的催化性能的影响;葡萄糖是一种还原性较强的还原剂,其用量必定会对所制备的氧化物的物种有所影响.为了探究Cu0和Cu+在本体系中的作用,采用不同葡萄糖用量制备了具有不同Cu2O含量的PdCl2/Cu-Cu2O催化剂.在上述研究基础上,我们采用X射线衍射、场发射扫描电子显微镜、热重分析、等离子体原子发射光谱等表征手段研究了负载型纳米铜基氧化物催化剂在合成碳酸二乙酯反应中催化性能差异的原因,旨在直接考察活性中心主要是Cu+还是Cu2+,避免分子筛等体系中载体结构的影响,研究结果更具参考性.结果发现, NaOH浓度为5 mol/L时制备的PdCl2/CuO和PdCl2/Cu-Cu2O催化剂的性能优于其他浓度下制备的催化剂,这可能是由于不同浓度的碱溶液会对铜离子的沉淀过程产生不同的影响;相同NaOH浓度下制备的催化剂中, PdCl2/Cu-Cu2O催化剂的催化性能明显优于PdCl2/CuO催化剂,这可能是由于PdCl2/Cu-Cu2O催化剂更有利于反应过程中电子的传递,从而表现出更好的催化性能,我们推测Cu0和Cu+可能更有利催化乙醇氧化羰基合成DEC;表征分析发现PdCl2/CuO和PdCl2/Cu-Cu2O均具有很好的热稳定性,两种催化剂中PdCl2负载量几乎相同,因此,主要影响催化性能的因素是载体CuO和Cu-Cu2O中铜的价态.采用不同葡萄糖用量制备了含有不同Cu2O含量的PdCl2/Cu-Cu2O催化剂,其中, PdCl2/Cu-Cu2O-2催化剂中含有更多的Cu2O,在反应中乙醇转化率达到了7.2%, DEC的选择性为97.9%, DEC的时空收率可达到151.9 mg·g–1·h–1.由此可见在乙醇气相氧化羰基合成DEC体系中, Cu+是主要的活性中心.     

Platinum-catalyzed intramolecular alkylation of indoles with unactivated olefins

Reaction of 1-methy-2-(4-pentenyl)indole (1) with a catalytic amount of PtCl2 (2 mol %) in dioxane that contained a trace of HCl (5 mol %) at 60 degrees C for 24 h led to the isolation of 4,9-dimethyl-2,3,4,9-tetrahydro-1H-carbazole (2) in 92% yield. Platinum-catalyzed cyclization of 2-(4-pentenyl)indoles tolerated substitution at each position of the 4-pentenyl chain. Furthermore, the protocol was applicable to the synthesis of tetrahydro-beta-carbolinones and was effective for cyclization of unprotected indoles. 2-(3-Butenyl)indoles underwent platinum-catalyzed cyclization with exclusive 6-endo-trig regioselectivity. Mechanistic studies established a mechanism for the platinum-catalyzed cyclization of 2-alkenyl indoles involving nucleophilic attack of the indole on a platinum-complexed olefin.     

Synthesis of 4-(1-alkenyl)isoquinolines by palladium(II)-catalyzed cyclization/olefination

A variety of 4-(1-alkenyl)-3-arylisoquinolines have been prepared in moderate to excellent yields by the Pd(II)-catalyzed cyclization of 2-(1-alkynyl)arylaldimines in the presence of various alkenes. The introduction of an o-methoxy group on the arylaldimine promotes the Pd-catalyzed cyclization and stabilizes the resulting Pd(II) intermediate, improving the yields of the isoquinoline products. Ketone-containing isoquinolines 36 and 49-51 have also been prepared by this process when unsaturated alcohols are employed as the alkenes.