Oxide catalysts, cyclization

Silica-supported cyclized polyacrylonitrile (PAN) metal complexes, claimed to be better oxidation catalysts than the polyphthalocyanine-metal complexes, have been obtained shown below as [11] ... 

As is true for other classes of aromatic nucleophilic substitution, the halogen displacement can frequently be catalyzed by copper or copper(I) salts. Using sodium hydride as the base and copper(I) iodide as catalyst, a series of o-bromophenylethylamine derivatives, including both amides and carbamates, have been cyclized. Oxidation to the indole can be effected with manganese dioxide (81JCS(P1)290). 

The direct formation of the S—C bond to form thiophenes is better illustrated by the conversion of 2-alkylthiophenols (1) to benzothiophenes (2) (48JA1561). Passing (1 n =2) over a chromium-aluminum oxide catalyst at 475 °C gave (2 R = R = H) in about 20-25% yield, while similar treatment of (1 n - 3) gave (2 R = H, R = Me) in only 10-12% yield. 2-Mercaptostyrene also cyclized to (2) over this same catalyst, and a number of substituted derivatives of (1), where n =2 and R = 5-NH2, 3-NH2, 5-CN or 5-C02Me, were also converted to benzo[6]thiophene derivatives in low yield (56JOC265). 

In his 1940 review Plate subjected the experimental material on dehydrocyclization of paraffins published to that time to a critical analysis (304) and concluded that aromatization of paraffins at the temperatures employed will depend upon the selection of proper catalysts in order to suppress the competing reactions, that the multiplet theory satisfactorily explains the mechanism of cyclization, and that intermediate formation of olefins is conceivable on oxide catalysts but can hardly occur on platinum. 

Cyanoethyl)pyridine was reductively cyclized to indolizidine (octahydropyrrocoline) by carrying out the reaction in aqueous alcohol containing two molar equivalents of hydrochloric acid in the presence of platinum oxide catalyst (133). The authors suggest that preferential ring reduction is essential for cyclization and propose the following reaction scheme ... 

Cyclization. The catalyst induces endo-trig cyclization of 2-hydroxy-3-butenylamine derivatives at room temperature to afford 3-pyrrolines. An oxidative cyclization of tertiary o-allylbenzylamines occurs when Ph,P is added after complexation. 

In the presence of oxide catalysts such as chromium oxide at 500°C., 7i-heptane will cyclize to toluene. There are also many other possible reactions, and Taylor and Turkevich (107) have examined the thermodynamics of the reaction and its alternatives. They concluded that cyclization was the favored reaction above 300° C., and pointed out the need to choose a catalyst which activated the C—H rather than the C—C bond. 

O. de Paohs, J. Baffoe, S. M. Landge, B. Torok, Synthesis 2008, 3423-3428. Multicomponent domino cyclization-oxidative aromatization on a bifunctional Pd/C/K-10 catalyst an environmentally benign approach toward the synthesis of pyridines. 

Das B, Venkateswarlu K et al (2007) An efficient and convenient protocol for the synthesis of quinoxalines and dihydropyrazines via cyclization-oxidation processes using HClOySiO as a heterogeneous recyclable catalyst. Tetrahedron Lett 48 5371-5374... 

Other related cyclic olefins, 1,3-cyclohexadiene and vinylcyclohexene, are reported to be formed as main products from butadiene over tungsten oxide catalysts [24]. Under the reaction conditions, hexatriene, resulting from the intermolecular metathesis of butadiene, underwent subsequent cyclization to 1,3-cyclohexa-diene ... 

Silica-supported perchloric acid (HC104-Si02) has also been used as an effective, heferogeneous, recyclable catalyst for the S5mthesis of quinoxalines and dihydropyrazines using a-bromokefones and 1,2-diamines [48] (Scheme 28). The reaction takes place at room temperature presumably via cyclization-oxidation. 

In 2010, Gouverneur and coworkers reported a novel cascade cyclization-intermolecular alkynylation reactions toward the synthesis of five-membered 0-heterocycles. /i-Alkynyl-y-butenolides 2 could be efficiently prepared from allenoates 1 and terminal alkynes in the presence of a gold catalyst with the use of Selectfluor as an external oxidant (Scheme 12.1) [5]. This process involves a direct C(sp)-H functionalization of terminal alkynes. This novel gold-catalyzed cascade cyclization oxidative cross-coupling process provides a basis for the development of novel cascade reactions combining traditional gold catalysis and intermolecular oxidative alkynylation. 

Pd-cataly2ed reactions of butadiene are different from those catalyzed by other transition metal complexes. Unlike Ni(0) catalysts, neither the well known cyclodimerization nor cyclotrimerization to form COD or CDT[1,2] takes place with Pd(0) catalysts. Pd(0) complexes catalyze two important reactions of conjugated dienes[3,4]. The first type is linear dimerization. The most characteristic and useful reaction of butadiene catalyzed by Pd(0) is dimerization with incorporation of nucleophiles. The bis-rr-allylpalladium complex 3 is believed to be an intermediate of 1,3,7-octatriene (7j and telomers 5 and 6[5,6]. The complex 3 is the resonance form of 2,5-divinylpalladacyclopentane (1) and pallada-3,7-cyclononadiene (2) formed by the oxidative cyclization of butadiene. The second reaction characteristic of Pd is the co-cyclization of butadiene with C = 0 bonds of aldehydes[7-9] and CO jlO] and C = N bonds of Schiff bases[ll] and isocyanate[12] to form the six-membered heterocyclic compounds 9 with two vinyl groups. The cyclization is explained by the insertion of these unsaturated bonds into the complex 1 to generate 8 and its reductive elimination to give 9. 

Tandem cyclization/3-substitution can be achieved starting with o-(trifluoro-acetamido)phenylacetylenes. Cyclization and coupling with cycloalkenyl trif-lates can be done with Pd(PPh3)4 as the catalyst[9]. The Pd presumably cycles between the (0) and (II) oxidation levels by oxidative addition with the triflate and the reductive elimination which completes the 3-alkenylation. The N-protecting group is removed by solvolysis under the reaction conditions, 3-Aryl groups can also be introduced using aryl iodides[9]. 

Catalysts. Silver and silver compounds are widely used in research and industry as catalysts for oxidation, reduction, and polymerization reactions. Silver nitrate has been reported as a catalyst for the preparation of propylene oxide (qv) from propylene (qv) (58), and silver acetate has been reported as being a suitable catalyst for the production of ethylene oxide (qv) from ethylene (qv) (59). The solubiUty of silver perchlorate in organic solvents makes it a possible catalyst for polymerization reactions, such as the production of butyl acrylate polymers in dimethylformamide (60) or the polymerization of methacrylamide (61). Similarly, the solubiUty of silver tetrafiuoroborate in organic solvents has enhanced its use in the synthesis of 3-pyrrolines by the cyclization of aHenic amines (62). 

In an extension of this work, the Shibasaki group developed the novel transformation 48—>51 shown in Scheme 10.25c To rationalize this interesting structural change, it was proposed that oxidative addition of the vinyl triflate moiety in 48 to an asymmetric palladium ) catalyst generated under the indicated conditions affords the 16-electron Pd+ complex 49. Since the weakly bound triflate ligand can easily dissociate from the metal center, a silver salt is not needed. Insertion of the coordinated alkene into the vinyl C-Pd bond then affords a transitory 7t-allylpalladium complex 50 which is captured in a regio- and stereocontrolled fashion by acetate ion to give the optically active bicyclic diene 51 in 80% ee (89% yield). This catalytic asymmetric synthesis by a Heck cyclization/ anion capture process is the first of its kind. 

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