Zinc—carbon bonds reactions with

Properties of zinc salts of inorganic and organic salts are Hsted in Table 1 with other commercially important zinc chemicals. In the dithiocarbamates, 2-mercaptobenzothiazole, and formaldehyde sulfoxylate, zinc is covalendy bound to sulfur. In compounds such as the oxide, borate, and sihcate, the covalent bonds with oxygen are very stable. Zinc—carbon bonds occur in diorganozinc compounds, eg, diethjizinc [557-20-0]. Such compounds were much used in organic synthesis prior to the development of the more convenient Grignard route (see Grignard reactions). 

Predictably, alkyltrisilanols react with dialkylzincs to yield larger aggregates, such as 133, which are much more inorganic in nature than those formed by mono- and disilanols (Scheme 84). Often these zinc siloxanes are without direct zinc-carbon bonds and resemble the silicates for which they serve as model compounds.192 The specific structures of these products depend heavily on the substituents of the silanetriols and diorganozincs, as well as the reaction stoichiometries.193... 

This review intends to summarize the data available on the mass spectrometry of organozinc compounds. Analytical applications of MS to the characterization of compounds having a Zn—C bond will be the major focus of the present work. Physico-chemical parameters of organozinc derivatives will also be reported, along with gas phase reactions resulting in the dissociation or formation of zinc-carbon bond(s). 

A further asset of this class of reagents is the covalent character of the zinc-carbon bond, this allows these organozinc reagents to undergo trans-metallation reactions smoothly and cleanly with a wide variety of transition metal salts. Thus, the zincated phosphine 5 can be reacted with (acetonitrile)... 

Fluorine migration from the CF3 group to the metal occurs with Cr", Fe ", Co ", Pt" and Zn" but not with Ru ", Rh" , Ni" and Pd". The reaction pathway for the zinc complexes is shown in Scheme 3. The molecular ion (48) loses a CFj radical from one monothio-)8-diketone moiety to give the ion (49), which then loses carbon monoxide to yield the fragment (50). Sulfur is ejected from (50) with the concomitant formation of a zinc-carbon bond in the species (51), which can subsequently lose the alkyne RC=CH to give the mono-ligand ion [ZnL] (52). The second ligand moiety loses Cp2 with the formation of a zinc-fluorine bond in the ion (53). 

Due to the preferred kinetic reactivity of the zinc-nihogen bonds in comparison with the zinc-carbon bonds, amidozincates are more reactive bases than aUcylzincates. When used in THF at room temperature, (t-Bu)2Zn(TMP)Li is able to deprotonate functionahzed substrates such as alkyl benzoates and benzonitrile the generated arylzincates can be quenched by iodine, benzalde-hyde (Table 27.12), and bromine [29,112-116]. Crystals of the base were isolated and its structure identified by X-ray diffraction as an ion-contacted zincate [117]. In addition, different reaction outcomes being obtained by changing the nature of the alkali metal (sodium vs. lithium), a reactivity as solvent-separated ion pairs was found unlikely [118]. 

One of the other reactions with organozinc compounds are insertion reactions to sulfur dioxide or oxygen. For example, dialkylzinc reacts with sulfur dioxide to give almost quantitatively an insertion product to the zinc-carbon bond as shown in eq. (5.37) [94],... 

The structure of fois(iodozincio)methane (4), which possesses double nucleophilic sites on one carbon, has a possibility to react with two different electrophiles sequentially. It will act as a molecular hinge that connects two molecules. It is found that reactivity of one C-Zn bond of 4 is much higher than that of methyl-zinc in the reaction with water or iodine (Scheme 8.25) [34,35]. These results indicate that it is possible to use two C-Zn bonds individually. 

Coupling reactions and related fluoroalkylations with polytTuoioalkyl halides are induced by vanous reagents, among them metals such as copper and zinc, or by an electrochemical cell. More recently, examples of carbon-carbon bond forma tion by coupling of unsaturated fluorides have been reported Both acyclic and cyclic fluoroolefins of the type (Rp)2C=CFRp undergo reducUve dimerization on treatment with phosphines [42] (equation 33) The reaction shown in equation 33 IS also accompbshed electrocheimcally but less cleanly [43]... 

The classical Reformatsky reaction consists of the treatment of an a-halo ester 1 with zinc metal and subsequent reaction with an aldehyde or ketone 3. Nowadays the name is used generally for reactions that involve insertion of a metal into a carbon-halogen bond and subsequent reaction with an electrophile. Formally the Reformatsky reaction is similar to the Grignard reaction. 

Asymmetric conjugate addition of dialkyl or diaryl zincs for the formation of all carbon quaternary chiral centres was demonstrated by the combination of the chiral 123 and Cu(OTf)2-C H (2.5 mol% each component). Yields of 94-98% and ee of up to 93% were observed in some cases. Interestingly, the reactions with dialkyl zincs proceed in the opposite enantioselective sense to the ones with diaryl zincs, which has been rationalised by coordination of the opposite enantiofaces of the prochiral enone in the alkyl- and aryl-cuprate intermediates, which precedes the C-C bond formation, and determines the configuration of the product. The copper enolate intermediates can also be trapped by TMS triflate or triflic anhydride giving directly the versatile chiral enolsilanes or enoltriflates that can be used in further transformations (Scheme 2.30) [110],... 

Chiral methylzinc aminoalkoxides 146a-c were obtained from the reaction of ZnMe2 with aminoalcohols, having chiral centers in their carbon backbones (Scheme 92).211 The methylzinc aminoalkoxides crystallize dimeric and trimeric with the formation of intermolecular zinc-oxygen bonds and creation of additional chiral centers. 

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 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. 

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