Zinc bonding

Organozinc Compounds with a Direct Zinc-Zinc Bond The First Molecular... 

An X-ray crystal strnctnre determination of the product obtained from the reaction of methyl A-phenylcarbamate with Et2Zn (equation 24) shows the formation of a complex aggregate containing two zinc-bonded ethyl groups, four zinc atoms and six deprotonated carbamate molecules, instead of the anticipated ethylzinc carbamate . ... 

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. 

II. DYNAMICS OF CARBON-ZINC BOND EXCHANGE C-Zn EXCHANGE... 

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. 

In this chapter the term zinc enolate refers both to molecules like A, containing a carbon—zinc bond at the a position of an electron-withdrawing group, and to species which contain an oxygen—zinc bond (Figure 1). 

Carbozincations are reactions that involve the addition of the carbon—zinc bond of an organozinc reagent 1 across a carbon—carbon multiple bond 2, leading to a new organozinc species 3 (equation 1). 

Mechanistic studies suggested that a full equivalent of Cp2ZrI2 was not required but the reaction proceeded rather sluggishly in the presence of a lesser amount. Moreover, addition of Et2Zn to 1-octyne could be catalyzed by Cp2ZrMeI and led to an exclusive ethylzincation process, suggesting that the addition of a carbon—zinc bond rather than a carbon—zirconium bond was involved. 

When diphenylzinc was reacted with gold(III) chloride, a [Ph3ZnAu] species was obtained (117, 118). Molecular weight measurements showed this product to be dimeric, and the H-NMR spectrum indicated the presence of two types of aryl groups in a 1 2 ratio, whereas the l3C-NMR data showed these to be due to zinc-bonded and bridging aryls, respectively. Structure 3 was postulated. The same complex was obtained when Ph2Zn was reacted with [AuCl(CO)] thep-tolyl analog was also prepared. 

The reactivity of the 1,1 -diorganometallics (61) has been extensively investigated by Knochel and co-workers.9293 Since the reactivity of a carbon-lithium or a carbon-magnesium bond is considerably different from that of a carbon-zinc bond, a selective reaction of (61) with two different electrophiles is often possible. The general reaction pathways are summarized in Scheme 33. The addition of allylic zinc... 

The hydroboration of allylic silanes proceeds with high diastereoselectivity as demonstrated by Fleming and Lawrence.87 It is difficult to use the newly formed carbon-boron bond for making new carbon-carbon bonds due to its moderate reactivity. However, the B/Zn exchange converts the unreactive carbon-boron bond to a reactive carbon-zinc bond, as in compound 24. A further transmetallation with the THF soluble salt CuCN-2LiCl provides copper reagents, which can be allylated, alkynylated, or acylated (Scheme 6). 

The high covalent degree of the carbon-zinc bond and the small polarity of this bond lead to a moderate... 

In the different examples presented above, the coordination at zinc suggests that both a- see a-Bond) and jr-type interactions see n-Bond) contribute to the cyclopentadiene-zinc bonding. 

A series of compounds with general formula [RMAs(t-Bu)2] (R = Me, r-Bu M = Zn, Cd, Hg) has been studied and it was found that their stability decreases in the order Zn > Cd] Hg. Table 12 summarizes some representative average bond distances and bond angles found in organozinc compounds containing a phosphorus-or arsenic zinc bond. 

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