Zinc derivatives

It is probable that the reaction proceeds through an organic zinc derivative, analogous to a Grignard reagent, formed by interaction of the a-halogen ester... 

By analogy with the Reformatsky reaction, the zinc derivative of a p-bromoester would do ... 

The transmetallation of lithio derivatives with either magnesium bromide or zinc chloride has been employed to increase further their range of synthetic application. While the reaction of l-methyl-2-pyrrolyllithium with iodobenzene in the presence of a palladium catalyst gives only a poor yield (29%) of coupled product, the yield can be dramatically improved (to 96%) by first converting the lithium compound into a magnesium or zinc derivative (Scheme 83) (81TL5319). 

In the case of the osmium(VIII) oxide oxidation of 5,10,15,20-tetraphenylchlorin,3b the metal-free macrocycle gives the bacteriochlorin 6, whereas the corresponding zinc derivative leads to an isobacteriochlorin. 

Figure 6. Voltammetry at microeleclrode of different zinc derivates in PEO the CsCI adduct corresponds to P(EO)9(ZnCI2 CsCl. For the triflate derivative, the current is magnified by a factor of 10. Reprinted from D. Baril, C. Michot. M. Armand, Solid State Ionics 1997, 94, 35, Copyright 1997, with kind permission of Elsevier Science—NL, Sara Burgerhartstraat 25, 1055 KV Amsterdam, The Netherlands.

The molecular structures of several [TpBut]ZnX derivatives have been determined by x-ray diffraction. For example, x-ray diffraction studies confirm that the acetate ligand in [TpBut]Zn(r)1-02CMe) is bound to zinc in a unidentate mode, similar to that proposed for [TpBut]Mg(7j1-02CMe), but in contrast to the bidentate coordination proposed for the copper analogue [TpBut]Cu(T)2-02CMe) (86,87). Such a change in coordination mode for copper and zinc derivatives is to be anticipated on the basis of structural studies on the nitrate derivatives [TpBut]M(N03) (M = Co, Ni, Cu, Zn), as described in Section V,B,2,e. The thioacetate [TpPh]-Zn V-SC(0)Me (81), and cyanoacetate [Tp lZnlr -C CCH N) (88) derivatives also exhibit unidentate coordination. 

The reactivity of the zinc alkyl derivatives [TpBut]ZnR is typically lower than that of the corresponding magnesium derivatives (Section III,A,2,c.). For example, whereas [TpBut]MgMe undergoes insertion of C02 into the Mg-C bond at room temperature, no reaction is observed between [TpBut]ZnMe and C02 at 140°C. Similarly, whereas the magnesium alkyl derivatives [TpBut]MgR (R = Me, Et, Pr1, Bul) react immediately with 02 at room temperature to give alkylperoxo derivatives [TpBut]MgOOR, solutions of the zinc derivative [TpBut]ZnEt are stable in the presence of 02 at 100°C. 

The ability of cobalt(II), nickel(II), and copper(II) to exhibit a greater tendency than Zn(II) towards bidentate coordination is further illustrated by structural comparisons within a series of bridging carbonate complexes (188). For example, of the complexes [TpPr 2]M 2(/x-C03) (M = Mn, Fe, Co, Ni, Cu, Zn), only the zinc derivative does not exhibit bidentate coordination at both metal centers (151,153). Furthermore, the carbonate ligand in the complexes [TpPr 2]M 2(/x-C03) (M = Mn, Fe, Co, Ni, Cu) also exhibits varying degrees of asymmetry that closely parallel the series of nitrate complexes described earlier (Fig. 47 and Table IX). 

The zinc derivative of dithizone explodes at 215°C. See Other A-METAL derivatives, metal hydrazides... 

The selectivity of the aldol addition can be rationalized in terms of a Zimmer -man-Traxler transition-state model with TS-2-50 having the lowest energy and leading to dr-values of >95 5 for 2-51 and 2-52 [18]. The chiral copper complex, responsible for the enantioselective 1,4-addition of the dialkyl zinc derivative in the first anionic transformation, seems to have no influence on the aldol addition. To facilitate the ee-determination of the domino Michael/aldol products and to show that 2-51 and 2-52 are l -epimers, the mixture of the two compounds was oxidized to the corresponding diketones 2-53. 

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... 

The other is to convert organylzirconocene derivatives to the corresponding zinc derivatives in situ and to carry out their addition to aldehydes [123—125] (Scheme 1.32). 

Since the early disclosure by Negishi that zinc halide salts accelerate Pd(0) -catalyzed crosscouplings between vinyl zirconocenes and various halides [78], several methods have been developed that extend the utility of this metathesis process from a zirconium chloride to a zinc chloride (79 Scheme 4.47). Alternatively, routes to more reactive diorganozinc intermediates, e. g., using Me2Zn, convert readily available zinc derivatives to mixed species 80, which selectively couple with various electrophiles [14]. 

Consecutive CO2 insertion has not been observed (and is assumed to be thermodynamically unfavored). However, consecutive epoxide ring opening is common, particularly for Lewis acidic catalysts like zinc derivatives (Scheme 4). 

Fluoral hydrate and hemiacetals are industrial products. They are stable liquids that are easy to handle, and they react as fluoral itself in many reactions. Thus, in the presence of Lewis acids, they react in Friedel-Crafts reactions. They also react very well with organometallics (indium and zinc derivatives) and with silyl enol ethers.Proline-catalyzed direct asymmetric aldol reaction of fluoral ethyl hemiac-etal with ketones produced jS-hydroxy-jS-trifluoromethylated ketones with good to excellent diastereo- (up to 96% de) and enantioselectivities. With imine reagents, the reaction proceeds without Lewis acid activation. The use of chiral imines affords the corresponding 8-hydroxy ketones with a 60-80% de (Figure 2.49). ° ... 

The preparation of a novel pentadentate ligand, 2-hydroxy-5-methylisophthalalde-hyde bis(/7-methoxy thiobenzoylhydra-zone), was described together with the chloro-bridged Zn(II) complexes [127]. Synthesis and electrochemical properties of some long-chain 1,4,8,11,15,18,22,25-octa-alkylated metal-free and zinc phtha-locyanines were presented by Swarts et al. [128]. The zinc derivatives show higher Kquid crystalline behavior than the free-metal compounds. 

First, when 1,1-dihaloalkanes are reduced with low-valent titanium derived from TiCU and zinc, it is necessary to add a catalytic amount of PbCU (or Pb) for reproducibility (Equations (5) and (6)). Two kinds of zinc powder are available for laboratory use electrolytic zinc derived by hydrometallurgy and distilled zinc derived by pyrometallurgy. 

SCHEME 11. Preparation and uses of alkyd zinc derivatives starting from alkyl sulfonates... 

Several methods have been described for preparing allylic zinc derivatives. In contrast to alkylzincs, allylic zinc reagents are much more reactive due to the more ionic nature of the carbon-zinc bond in these organometallics. The chemistry displayed by these reagents is not representative of the usually moderate reactivity of organozinc derivatives. Tamaru and coworkers have converted various allylic benzoates to the corresponding organozinc intermediates in the presence of palladium(O) as catalyst. 

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