Triflate, Zinc

Lewis acids such as zinc triflate[16] and BF3[17] have been used to effect the reaction of indole with jV-proiected aziridine-2-carboxylate esters. These alkylations by aziridines constitute a potential method for the enantioselective introduction of tryptophan side-chains in a single step. (See Chapter 13 for other methods of synthesis of tryptophans.)... 

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)... 

Scheme 5.1-27 The zinc triflate-catalyzed coupling of alkynes with aldehydes to give propargyl alcohols in an ionic liquid.

Figure 6 compares voltammograms for P(EO)9(ZnCl2)08 CsCl which contains a mixture of ZnClj and ZnCl4 and for a zinc triflate derivative [95], The metal desposition process can be written as Eq. (6),... 

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]. 

A highly fluorinated bis(pyrazolyl)borate ligand, dihydridobis(3,5-bis(trifluoromethyl)pyrazo-lyl)borate has been synthesized. The zinc triflate complex was prepared from the potassium salt of the ligand and exhibits distorted tetrahedral coordination which allows comparison of the amount of distortion, in relation to the electronic properties of substituents, with the methylated analog.165... 

A few further general examples of zinc catalytic activity or reactivity include the following. Other zinc-containing systems include a zinc phenoxide/nickel(0) catalytic system that can be used to carry out the chemo- and regioselective cyclotrimerization of monoynes.934 Zinc homoenolates have been used as novel nucleophiles in acylation and addition reactions and shown to have general utility.935,936 Iron/zinc species have been used in the oxidation of hydrocarbons, and the selectivity and conditions examined.362 There are implications for the mechanism of metal-catalyzed iodosylbenzene reactions with olefins from the observation that zinc triflate and a dizinc complex catalyze these reactions.937... 

The reaction of nitrones with terminal alkynes proceeds in excellent yields and high purity, in the presence of stoichiometric quantities of diethylzinc and zinc triflate (219, 661-663). To optimize the process of diastereoselective addition of terminal alkynes to chiral nitrones, ZnCl2 and NEt3 in toluene were used. This reaction protocol is facile to perform, cost-effective and environmental friendly (664). 

The templates used in these reactions have a significant impact on the reaction outcome and in determining product enantioselectivity. Sibi et al. also showed that changing the oxazolidinone template as in 119 to a 3,5-dimethylpyrazole in 131 resulted in a reversal of stereochemistry using the same chiral Lewis acid (Scheme 34) [91]. Additions in the presence of stoichiometric amounts of zinc triflate and ligand 19 gave good yields and moderate selectivities of 132. These acylated pyrazoles 131 form 5-membered... 

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. 

Tetrahydropyrimidines 949 are produced by the formal [4- -2] cycloaddition of A -tosylazetidines 948 with nitriles under the influence of boron trifluoride or zinc triflate catalysis <2004OL4829, 2005JA16366, 2006TL5393>. 

Bandini and co-workers studied the zinc triflate-bis(oxazohne)-catalyzed reduction of a-alkoxy-ketones with catecholborane. The example in Figure 9.76 shows the reduction of a-methoxyacetophenone 252. l-Phenyl-2-methoxyethanol 254 was isolated in 78% yield and 82% ee. 

A related one-pot, three-component synthesis of fi-amino carbonyl compounds has been achieved using a cascade reaction of anilines with aromatic aldehydes and car- (g) bonyl compounds, catalysed by zinc triflate.19... 

A cheap and efficient enantioselective aza-Henry reaction of nitromethane with a variety of A-protected arylaldimines has been reported.73 Using zinc triflate and (-)-A-methylephedrine at -20 °C, yields and ees of up to 99% have been achieved with wide tolerance of aryl substituent in terms of both electronic nature and position. The auxiliary is also easily recycled. 

An enantioselective aza-Henry reaction has been performed in the presence of zinc triflate and A-methylephedrine 47 This method features tolerance to imines that bear aryl groups of diverse electronic nature and substitution patterns. 

An enantioselective reaction also occurs in a system in which the alkene itself does not contain any chiral auxiliary, but a chiral ligand is used in combination with a Lewis acid. In the following example, a chiral bisoxazoline (BOX) ligand is successfully employed together with zinc triflate as a Lewis acid to achieve an enantiomeric excess as high as 90% (Scheme 6.21) [37]. 

Lewis acids, such as boron trifluoride etherate, zinc triflate <2000H(53)1421>. N-Unsubstituted tetrazoles 24 can be alkylated with primary aliphatic alcohols using the Mitsunobu protocol (Equation 26) <1996SC2687>. 

Initial attempts to prepare a self-assembled spheroidal cage using zinc triflate yielded a colorless solid which was insoluble in organic solvents. 

Figure 3 The rearrangement of a-pinene oxide catalysed by supported zinc triflate...

The aza-Diels-Alder reaction of substituted lf/-indole-2-carbaldehydes 1323 with Danishevsky s diene 1324 proceeds with a high degree of diastereoselectivity providing highly functionalized l-R -2-(Tmethyl-lf/-indol-2-yl)-2,3-dihydro-4(l//)-pyridinones 1325 which were further elaborated into novel polycyclic heterocycles (Equation 289) <2002TL29>. Attempts to form the imine of 12/-indole-2-carbaldehyde 1326 in situ in the presence of diene 1324 and zinc chloride or zinc triflate at 0 °C or at room temperature did not afford the expected cycloadduct 1327. Instead, 2-(177-indol-2-yl)-2,3-dihydro-477-pyran-4-one 1328 and the 5-hydroxy-l-methoxy-5-(l/7-indol-2-yl)-Tpenten-3-one 1329 were isolated in 45% and 25% yields, respectively (Equation 290). 

A metal-catalyzed tandem 1,4-addition/cyclization of propargyl alcohols with Michael acceptors such as alkylidene-malonates has been developed. In the presence of catalytic amounts of zinc triflate and triethylamine, various 2-alkylidene-l,3-dicarbonyl compounds react with propargyl alcohols to give 3- or 4-methylenetetrahydrofurans in excellent yields (Equation 96) <20040L2015>. 

Zinc triflate-catalyzed condensation of propargyl alcohols and various phenols furnishes 2-methylbenzofurans with an alkyl or aryl group attached at the 3-position in excellent yields (Scheme 88) <2006JOC4951>. 

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. 

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