Zinc elimination mechanism

SCHEME 14.3 Postulated fluoroalkylation and zinc elimination mechanism. 

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. ... 

Zn(Cu)] and the Wittig-Schwarzenbach reagent [CH2N2 4- Znl2] give rise to the formation of one and the same carbenoid. Zinc chloride catalyzes the reaction (5) and an explanation in terms of ZnClg-assisted elimination of chloride ion via a transition state (X) was suggested to be most probable (546). The possibility of a two-step (addition and elimination) mechanism [Eq. (6)], 200) which was proposed for the reaction of... 

The extension of this cross-coupling to iodoalkenes is also possible. If the iodo-, bromo-, or chloroalkene is further conjugated with an electron-withdrawing group, a facile substitution via an addition-elimination mechanism is observed. Typically, 3-iodo-2-cyclohexen-l-one 24 [55] reacts with a zinc-copper reagent such as 25 furnishing the expected cross-coupling product 26 (see Section 9.6.5 Scheme 25) [13]. 

An important general method of preparing indoles, known as the Fischer Indole synthesis, consists in heating the phenylhydrazone of an aldehyde, ketone or keto-acld in the presence of a catalyst such as zinc chloride, hydrochloric acid or glacial acetic acid. Thus acrtophenone phenylhydrazone (I) gives 2-phenyllndole (I V). The synthesis involves an intramolecular condensation with the elimination of ammonia. The following is a plausible mechanism of the reaction ... 

Dezincification Dezincification is corrosion of a brass alloy containing zinc in which the principal product of corrosion is metallic copper. This may occur as plugs rilling pits (plug type) or as continuous layers surrounding an unattacked core of brass (general type). The mechanism may involve overall corrosion of the alloy followed by redeposition of the copper from the corrosion products or selective corrosion of zinc or a high-zinc phase to leave copper residue. This form of corrosion is commonly encountered in brasses that contain more than 15 percent zinc and can be either eliminated or reduced by the addition ox small amounts of arsenic, antimony, or ph osphorus to the alloy. 

Magnesium alkoxides (formed by ROH- -Me2Mg —>ROMgMe) have been decomposed thermally, by heating at 195-340°C to give the alkene, CEU, and MgO. Syn elimination is found and an Ei mechanism is likely. Similar decomposition of aluminum and zinc alkoxides has also been accomplished. ... 

The mechanism of carbometallation has been explored computationally.77 The reaction consists of an oxidative addition to the triple bond forming a cyclic Cu(m) intermediate. The rate-determining step is reductive elimination to form a vinyl magnesium (or zinc) reagent, which then undergoes transmetallation to the alkenyl-copper product. 

The proposed reaction mechanism is as follows (Scheme 16.83). Zinc metal reduces Ni(II) species to Ni(0). A nickelacyclopentadiene may be produced via coordination of two molecules of propiolates and regioselective head-to-head oxidative cyclometallation. Coordination and subsequent insertion of an allene into the Ni(II)-carbon bond give rise to a nickelacycloheptadiene intermediate. Finally, a benzene derivative is produced via reductive elimination followed by isomerization. 

The same mechanism is operative for the preparation of squaric acid derivatives of type 112. Treatment of 3,4-dicMorocyclobutene-l,2-dione with two different zinc-copper reagents provides the double addition-elimination product 112 in 67% yield (Scheme 2.41) [87]. 

The prominent role of alkyl halides in formation of carbon-carbon bonds by nucleophilic substitution was evident in Chapter 1. The most common precursors for alkyl halides are the corresponding alcohols, and a variety of procedures have been developed for this transformation. The choice of an appropriate reagent is usually dictated by the sensitivity of the alcohol and any other functional groups present in the molecule. Unsubstituted primary alcohols can be converted to bromides with hot concentrated hydrobromic acid.4 Alkyl chlorides can be prepared by reaction of primary alcohols with hydrochloric acid-zinc chloride.5 These reactions proceed by an SN2 mechanism, and elimination and rearrangements are not a problem for primary alcohols. Reactions with tertiary alcohols proceed by an SN1 mechanism so these reactions are preparatively useful only when the carbocation intermediate is unlikely to give rise to rearranged product.6 Because of the harsh conditions, these procedures are only applicable to very acid-stable molecules. 

A fully concerted mechanism for reaction 299 has been eliminated as inconsistent with 14C and 15N KIEs and also with the observed inverse solvent D2O effect. The reaction path for the deamination of AMP has been formulated613 as a stepwise conversion involving the formation of tetrahedral intermediate 515 characterized by full-bonded hydroxyl and amino groups (equation 300). The TS for slow formation of 515, resulting from the attack of the hydroxyl from enzyme zinc-activated water at the C(6), is characterized by the C(6) OH bond order of 0.8 0.1 (late TS) and fully bonded NH2, that is by the nearly complete conversion to sp3 at C(6), and by nearly complete protonation of Nq), 516, The protonation of NH2 (in 515) and departure of NH3 (with TS 517) take place in the subsequent rapid steps as shown in equation 300, Zinc hydroxide is formed prior to attack514 at C(6). Enzymatic degradation of [6-14C]AMP has been carried out to prove the position of the radiolabel in 513 (equation 301). No radioactivity in the allantoin... 

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