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Zinc halides: acetalization catalysts

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)... [Pg.114]

The yield of the arylcyclopropanes is very sensitive to the catalyst employed. Very low yields (< 10%) were observed when lithium salts were employed. Magnesium, cobalt and copper salts gave better results, but the best results were achieved with zinc halides and rho-dium(II) acetate. This was apparent when 7-phenylbicyclo[4.1. OJheptane (4) was prepared from cyclohexene and phenyldiazomethane. ... [Pg.349]

One of the most general approaches to the synthesis of this type of compounds involves the Lewis acid promoted rearrangement of acetals of a-substituted propiophenones [4,5]. Besides silver salts, a large variety of soft and borderline Lewis acids have been found to be convenient catalysts for the 1,2-aryl shift, noticeably zinc halides in substoichiometric amounts [6]. As final products need to be free of metal traces for human consume, the use of Lewis acids supported on microporous solids can be advantageous since a better recovering of the catalyst can be anticipated. [Pg.654]

Ion 21 can either lose a proton or combine with chloride ion. If it loses a proton, the product is an unsaturated ketone the mechanism is similar to the tetrahedral mechanism of Chapter 10, but with the charges reversed. If it combines with chloride, the product is a 3-halo ketone, which can be isolated, so that the result is addition to the double bond (see 15-45). On the other hand, the p-halo ketone may, under the conditions of the reaction, lose HCl to give the unsaturated ketone, this time by an addition-elimination mechanism. In the case of unsymmetrical alkenes, the attacking ion prefers the position at which there are more hydrogens, following Markovnikov s rule (p. 984). Anhydrides and carboxylic acids (the latter with a proton acid such as anhydrous HF, H2SO4, or polyphosphoric acid as a catalyst) are sometimes used instead of acyl halides. With some substrates and catalysts double-bond migrations are occasionally encountered so that, for example, when 1 -methylcyclohexene was acylated with acetic anhydride and zinc chloride, the major product was 6-acetyl-1-methylcyclohexene. ... [Pg.784]

Alkenes react with acyl halides or acid anhydrides in the presence of Lewis acid catalysts. The reaction works better with cyclic alkenes than for acyclic ones. A mechanistically significant feature of this reaction is the kinetic preference for formation of unsaturated ketones. Mechanistic studies using acetic anhydride and zinc chloride as the reagent system are consistent with a two-step mechanism in which the location of the double bond in the product is determined by ease of deprotonation of the initial adduct. A related reaction occurs when alkenes react... [Pg.464]


See other pages where Zinc halides: acetalization catalysts is mentioned: [Pg.806]    [Pg.541]    [Pg.454]    [Pg.210]    [Pg.722]    [Pg.242]    [Pg.69]    [Pg.881]    [Pg.664]    [Pg.132]    [Pg.157]    [Pg.452]    [Pg.655]    [Pg.40]    [Pg.155]    [Pg.291]    [Pg.14]    [Pg.207]    [Pg.611]    [Pg.29]    [Pg.249]    [Pg.452]    [Pg.260]    [Pg.514]    [Pg.351]    [Pg.25]    [Pg.44]    [Pg.974]    [Pg.139]    [Pg.220]    [Pg.262]    [Pg.241]    [Pg.207]   
See also in sourсe #XX -- [ Pg.268 ]

See also in sourсe #XX -- [ Pg.268 ]




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Acetals catalyst

Halide catalysts

Zinc acetate catalyst

Zinc catalysts

Zinc halides

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