Alkylzinc halides

Alkylzinc halides have also been prepared under microwave irradiation. The Reformatsky reagents (2-t-butoxy-2-oxoethyl)zinc bromide and [(2-dibenzylamino)-2-oxoethyl]zinc bromide were synthesized from the corresponding bromides via reaction with zinc in THF (Scheme 5) [24], The oxidative addition was executed at 100 °C in 5 min. The obtained reagents were subsequently used in Negishi reactions on 2-bromopyridine, 3-bromopyridine, 2-bromo-5-nitropyridine, and 2-bromo-5-trifluoromethyl-pyridine using Pd(PPh3)4 as a catalyst (Scheme 5). 

The possible mechanism for the reaction is that an alkylzinc halide forms first and then reacts with the acid to produce the alkane ... 

Catalytic reactions of alkylzinc halides with a-chloroketones 410... 

Another Pd-catalyzed reaction of aryl- and alkylzinc halides used carboxylic anhydrides as starting organic compounds (Scheme 127).328 One of the advantages of this method is that anhydrides can be synthesized in situ from the corresponding sodium salts of carboxylic acids and ethyl chloroformate. The scope of the method includes aliphatic and aromatic anhydrides, phenyl-, ethyl-, isopropyl-, and n-butylzinc iodides. 

Development of new methodologies for formation of carbon-carbon bonds has been one of the major tasks in organic chemistry. Obviously, organometallic compounds, particularly zinc derivatives, have found great use in such reactions. During the past several years, there have been several significant reports of nickel- and palladium-catalyzed reactions of dialkylzincs and alkylzinc halides with alkyl halides of diverse structure. A detailed account of most of these studies can be found in a recent review by Knochel et al,246... 

Miscellaneous Reactions of Organozinc Reagents 2.06.16.9.1 Catalytic reactions of alkylzinc halides with a-chloroketones... 

Although the simple alkylzinc halides RZnX (R = Me or Et, X = Cl, Br, I) were among the first known organozinc compounds, for a long time nothing was known about the actual structures of such compounds in solution or in the solid state. The constitution of these compounds in solution seemed to depend on the particular solvent employed " . It should be noted that in ethereal solutions, the possibility of the presence of a Schlenk equilibrium should always be considered. In these solvents the organozinc halides most... 

Interestingly, silylated propargylic zinc reagents, such as 342, may be better viewed as the allenic zinc reagent 343 which reacts with an aldehyde via a cyclic transition state, affording only the a f/-homopropargylic alcohol 344 with 90% yield (Scheme 91) 22. Alkylzinc halides react only sluggishly with aldehydes or ketones. This reactivity can be... 

Uneyama has described some interesting reactions of the N-aryl imines of ethyl trifluoropyruvate. Tandem alkylation/defluorination occurred (Eq. 97) upon exposure to diethylzinc, via attack at nitrogen and SN2 displacement of fluoride anion [279]. Interestingly, an alkylzinc halide reagent attacked regioselec-tively at carbon, perhaps the more expected outcome. 

RX + Zn diethylether> RZnX Alkyl halide Zinc Alkylzinc halide... 

The first microwave-assisted Negishi couplings were recently published both aryl and alkylzinc halides were used as substrates42 (Scheme 2.14). 

Reformatsky first introduced electron-withdrawing substituents on the a-carbon of an organozinc halide, leading to the more reactive and thermally stable a-(alkoxycarbonyl)alkylzinc halides (123).107 Typically these reagents react with aldehydes or ketones to afford 3-hydroxy esters while nitriles afford 3-keto esters (Blaise reaction),l07b>c but 1,4-conjugate additions to select a,3 unsaturated ketones are precedented (Section 1.2.2.2.2). 

Alkyl units, unactivated, functionalization, 10, 102 Alkylzinc bromides, preparation, 9, 84 Alkylzinc halides... 

Exchange of zinc-carbon bonds with zinc-halogen bonds has been observed to yield alkylzinc halides, RZnX (274). 

Diorganozinc reagents display an enhanced chemical reactivity compared to alkylzinc halides and furthermore, they are better suited for applications in asymmetric synthesis.38,39... 

Lithium triorganozincates are best prepared by the reaction of an alkyllithium (3 equiv.) with zinc chloride or by the addition of an alkyllithium to a dialkylzinc in an ethereal solvent.304,3042-3043 Lithium and magnesium trialkylzinc-ates are more reactive compared to dialkylzincs or alkylzinc halides due to the excess of negative charge at the metallic zinc center, which confers a higher nucleophilicity to the organic substituents. 

This method was originally used by Frankland to prepare diethylzinc. It is described as an oxidative insertion see Insertion) of zinc metal inside the carbon-halogen bond of an organic halide. The initial product of the reaction is an alkylzinc halide derivative (RZnX), from which the diaUcylzinc compound (R2Zn) is formed as the Schlenk-type equilibrium establishes (equations 1 and 2). 

As a result of the Schlenk-type equilibrium (equation 2), alkylzinc halide solutions are believed to contain some amounts of the corresponding dialkylzinc compound and zinc halide salt. The equilibrium usually lies on the left side in such a way that the alkylzinc halide compound is favored. Nevertheless, as already mentioned in Section 2, the equilibrium can be shifted to the right by distillation of the more volatile dialkylzinc derivative or by addition of complexing agents to remove the zinc dihahdes from the solution. 

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