Carbon zinc organometallics

Thermochemical attention in this chapter is directed towards compounds with carbon—zinc bonds, i.e. species that are usually labeled organometallic. The thermodynamic properties that we discuss are restricted to the enthalpy of formation (often called the heat of formation ), enthalpy of vaporization and carbon—zinc bond energies. We forego discussion of other thermochemical properties such as entropy, heat capacity or excess enthalpy. The energy units are kJmoU where 4.184 kJ is defined to equal 1 kcal. 

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. 

The high covalent degree of the carbon-zinc bond and the small polarity of this bond leads to a moderate reactivity of these organometallics toward many electrophiles. Only powerful electrophiles react in the absence of a catalyst. Thus, bromolysis or iodolysis... 

The synthesis of functionalized zinc organometallics can be accomplished with a variety of methods that have been developed in recent years. The intrinsic moderate reactivity of organozinc reagents can be dramatically increased by the use of the appropriate transition metal catalyst or Lewis acid. Furthermore, the low ionic character of the carbon-zinc bond allows the preparation of a variety of chiral zinc organometallics with synthetically useful configurational stability. These properties make organozinc compounds ideal inteimediates for the synthesis of complex and polyfunctionalized organic molecules. 

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. 

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... 

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