Zinc triflate catalyst

More recently spinning disc reactors have been used by Wilson et alP0) to carry out catalytic reactions using supported zinc triflate catalyst for the rearrangement of a-pinene oxide to yield campholenic aldehyde. The results of this study, presented in Table 20.1, suggest that by using a supported catalyst on a spinning disc reactor it is possible to... 

Wilson, K., Renson, A. and Clark, J. H. Catalysis Letters 61 (1-2) (1999) 51-55. Novel heterogeneous zinc triflate catalysts for the rearrangement of alpha-pinene oxide. 

Vicevic, M., Boodhoo, K.V.K. and Scott, K. (2007). Catalytic isomerisation of Oi-pinene oxide to campholenic aldehyde using silica-supported zinc triflate catalysts. II. 

Magnesium triflate and zinc triflate are outstanding catalysts for the intro duction of the thioketal group for the protection of the ketone function [126] The reaction of a variety of ketones with ethane 1,2-dithiol in the presence of these triflates proceeds under mild conditions to form the corresponding thioketals in high yield (equation 62)... 

The catalyst used was zinc triflate supported on silica, which was glued onto the surface of the SDR. Although total conversion could be achieved in a single pass, selectivity was highly dependent on residence time, increasing with shorter times. In a direct comparison with a batch process using the same catalyst at the same conversion and residence time a 200 times increase in throughput could be obtained. 

A combination of the Lewis acid zinc triflate and the bases NEt, or pyridine acted as an achiral catalyst for this reaction. Instead, using a chiral base which incorporates a bipy ligand to bind zinc gave 26% ee of the product (Scheme 5-42a). Alternatively, diethylzinc was an active precatalyst, but attempts to use chiral amino alcohols as ligands in this system gave low ees (Scheme 5-42b) [31]. 

Takacs, J.M., Lawson, E.C., Reno, M.J., Youngman, M.A. and Quincy, D.A. (1997) Enantioselective Diels—Alder reactions Room temperature bis(oxazoline)-zinc, magnesium, and copper triflate catalysts. Tetrahedron Asymmetry, 8, 3073—3078. 

The ethylene thioketal of the unstable, acid- and base-sensitive ketone 1 can be obtained in 85% yield with zinc triflate as catalyst in CHjClj at 23°. ... 

Thioketalization.1 Zinc (or magnesium) triflate is an efficient catalyst for thioke-talization with 1,2-ethanedithiol, particularly in the cases of acid-sensitive ketones and highly hindered ketones (e.g., camphor ethane dithioketal, 99% yield). A4-Cholestenone is converted into the corresponding dithioketal cleanly with zinc triflate, but into a 4 1 mixture of the A4- and A5-cholestenone dithioketals with magnesium triflate. In general, zinc triflate is more effective than magnesium triflate. 

Indoles react with epoxides and aziridines in the presence of Lewis acids (see 20.4.1 for reaction of indolyl anions with such reactants) with opening of the three-membered ring and consequent 3-(2-hydroxyethylation) and 3-(2-aminoethylation) of the heterocycle. Both ytterbium triflate and phenylboronic acid are good catalysts for reaction with epoxides under high pressure silica gel is also an effective catalyst, but reactions are slow at normal pressure and temperature. Reaction with aziridines can be catalysed by zinc triflate or boron trifluoride. °... 

Alkylation by aziridines is of interest because it can introduce the 2-aminoethyl group which is found in many biologically active indoles. Zinc triflate was found to be the best catalyst for effecting enantiospecific alkylation of indoles by l-(Cbz)aziridine-2-carboxylate esters <89TL4073>. Aziridine (31) was used to stereospecifically introduce an a-methyltryptophan side-chain using BF3 as the catalyst (Equation (80)) <90CPB564>. 

Asymmetric addition of alkynylzinc reagents to aldehydes has been developed with A-methylephedrine (or other chiral amino alcohol) as the chiral ligand. The alkynylzinc reagent is prepared in situ from the terminal alkyne and this allows use of the metal salt (zinc triflate) as a catalyst in substoichiometric amount (1.145). 

In addition to metal-based catalysts, organocatalysts are also selective promoters of asymmetric Diels-Alder reactions. Several groups reported the use of cinchona alkaloid catalysts in standard Diels-Alder reactions. Deng combined 2-pyrones with a,P unsaturated ketones, while Bernard and Ricci focused on the reactions of vinylindoles with quinones and maleimides. Lectka reported enantioselective inverse electron demand hetero Diels-Alder reactions of ketene enolates and o-benzoquininone diimides catalyzed by a combination of benzoylquinidine and zinc triflate. For example, subjecting diimide 51 to the standard reaction conditions yields cycloadduct 52 as a single stereoisomer, which can be easily converted to... 

The reaction of 1-adamantane carboxylic acid proceeded smoothly at 80 °C, providing Boc-protected amine 92 in high yield, but the slow step of the process was addition of the ert-butoxy nucleophile (butoxide or butanol) to the isocyanate intermediate. Of numerous catalysts and conditions screened to accelerate that addition, zinc triflate in tetrahydrofuran solvent proved optimal, allowing the process to occur smoothly at 40 °C for a range of alkyl carboxylic acids. In some cases, such as the stereoretentive conversion of or-methyl-y -siloxy propionic acid 93 to Boc-protected chiral amine 94, inclusion of additional tert-butanol was required. Catalysis was proposed to occur via an isocyanate-derived, zinc-complexed carbamoyl bromide. The Lebel conditions were also applied to malonate monoesters as a route to cr-disubstituted-cr-amino acids. 

A combination of ytterbium (or zinc) triflate and chiral C2-symmetric prolinamide ligand leads to high enantioselectivities in direct aldol reactions under aqueous conditions (Scheme 27). The presence of 5 mol% of the catalyst affords an asymmetric intermolecular aldol reaction between umnodified ketones and aldehydes to give antiproducts with excellent enantioselectivities up to 99% ee. 

Carvone-derived 2,3-epoxy alcohol derivatives (317) have been found to rearrange with stereoselective formation of ring-contracted ketones (318) in a process where the stereochemical result seems to be independent of epoxide configuration. On the basis of this study, the authors concluded that the rearrangement of tetrasubstituted cyclic epoxy alcohol derivatives remains a transformation for which both regiose-lectivity and stereochemical outcome are difficult to predict. New solid acid catalysts based on silica-supported zinc triflate have been prepared for use in the rearrangement of a-pinene oxide to campholenic aldehyde. ... 

Generally, alcohols, readily generating carbenium cations in the presence of acidic catalysts, were found to react with A H-unsubstituted 5-trifluoromethyltetrazole 96 yielding N -alkylated products 112 [28, 103], The reaction can be carried out in neutral organic solvents (chloroform, dichloromethane, acetonitrile, nitromethane) in the presence of catalytic amounts of sulfuric or p-toluenesulfonic acids as well as Lewis acids like boron trifluoride etherate or zinc triflate. 

This cydization protocol could be substantially improved by using potassium osmate as the catalyst and by replacing the Broensted acid by the oxophiUc zinc triflate. 

Vinylic copper reagents react with CICN to give vinyl cyanides, though BrCN and ICN give the vinylic halide instead." Vinylic cyanides have also been prepared by the reaction between vinylic lithium compounds and phenyl cyanate PhOCN." Alkyl cyanides (RCN) have been prepared, in varying yields, by treatment of sodium trialkylcyanoborates with NaCN and lead tetraacetate." Vinyl bromides reacted with KCN, in the presence of a nickel complex and zinc metal to give the vinyl nitrile. Vinyl triflates react with LiCN, in the presence of a palladium catalyst, to give the vinyl nitrile." ... 

Although more hydrolytically sensitive than the phosphine boranes, diorganochlorophosphines can be more accessible than diorganophosphines and are not pyrophoric. Thus, the reaction of a chlorophosphine with an aryl halide or aryl triflate in the presence of zinc as a reducing agent and (DPPE)NiCl2 as catalyst provides a convenient procedure for P—C coupling (Equation (49)).150 A related nickel-catalyzed process driven by electrochemical reduction has also been reported 151... 

Because of the high nucleophilicity and reactivity of diazoalkanes, catalytic decomposition occurs readily, not only with a wide range of transition metal complexes but also with Brpnsted or Lewis acids. Well-established catalysts for diazodecomposition include zinc halides [638,639], palladium(II) acetate [640-642], rhodium(II) carboxylates [626,643] and copper(I) triflate [636]. Copper(II)... 

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