Zinc—carbon bonds formation

Experimental and estimated thermodynamic data of homoleptic dialkylzinc compounds are listed in Table 3. Like many organometallic compounds, the lower dialkylzincs have a positive enthalpy of formation, and only the incorporation of silicon atoms in the /3-position imparts significant thermodynamic stability. The mean Zn-C bond rupture enthalpies, all of which are quite low, follow a similar trend as the bond lengths in these compounds. Thus, the presence of methyl substituents in the a-position weakens the zinc-carbon bonds, while silyl substituents strengthen them. 

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). 

These results indicate that zinc ions formed by oxidation of the anode do not play a part or only have side effects in the direct electroreductive carbon—carbon bond formation carried out with a zinc anode and a nickel catalyst. In these reactions, a nickel organometallic is involved. 

The Pechmann synthesis is unsuitable for acid-sensitive phenols, as for example the furo[2,3 -6]benzofuran derivative (373). An alternative approach uses the enhanced electrophilic character of a vinyl bromide in the presence of zinc carbonate to construct a suitable side-chain adjacent to the phenolic group (71JA746). In the examples cited, ring closure occurred under the mild conditions to form the pyranone ring of the aflatoxins (374). Since neither sodium nor potassium carbonate proved effective, it was considered that chelation of the zinc facilitated the carbon-carbon bond formation (Scheme 115). 

The zinc-mediated Reformatsky reaction is one of the classical methods for carbon-carbon bond formation. To date, various main group metals and transition metals have been used for this reaction. Rieke s activated indium powder mediates readily the coupling of ethyl a-bromoacetate and a variety of carbonyl compounds yielding /3-hydroxy esters in good yields (Scheme 87).3 Later, commercially available indium powder has been found to be equally effective for the indium-based Reformatsky reaction in THF.28 This indium Reformatsky reaction is accelerated by ultrasound irradiation (Scheme 88).322,323 Indium(i) iodide also mediates the Reformatsky reaction of aldehydes and ketones to give /3-hydroxy esters, presumably via organoindium(m) diiodide (Scheme 89).27... 

Organozinc reagents have played an important role in the development of efficient catalysts for enantioselective carbon-carbon bond-formation by lj4-addition to a, -unsaturated compounds. Important advantages of the use of organozinc reagents are the option of starting with alkenes (throu hydroboration-zinc transfer procedures) and the tolerance towards functional groups. 

Zinc-mediated Alkylations of C=N Bonds Yet Another Tool in the Carbon-Carbon Bond Formation Toolbox... 

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). 

In a further refinement to the synthesis, Sygula and Rabideau discovered an appealing alternative method (Method C, Scheme 4) to prepare 1,2,5,6-tetrabromo-corannulene 18 from 17 by the use of sodium hydroxide to deprotonate the remaining benzylic hydrogen and initiate carbon-carbon bond formation [36-38], The desired molecule 1 was formed (90%) by treatment of 18 with zinc and potassium iodide [37, 38],... 

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