Bonds carbon-zinc

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. 

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 all these reactions, the cis stereoselectivity of the major isomer observed is attributed to steric interactions which favor a geometry in which the R substituent preferentially occupies an outside position in the chair-like transition state. The pronounced cis stereoselectivity of the reaction is the opposite of the traits stereoselectivity usually observed [56]. It may be that organolithium or organomagnesium cyclizations, in contrast to the organozinc one, proceed through a rather product-like transition state. The highly covalent nature of the carbon-zinc bond, as well as the favorable intramolecular association of the zinc atom with the double bond [50], may explain the cis stereoselectivity observed here [52] (Scheme 49). 

Mixed bimetallic reagents possess two carbon-metal bonds of different reactivity, and a selective and sequential reaction with two different electrophiles should be possible. Thus, the treatment of the l,l-bimetailic compound 15 with iodine, at — 78"C, affords an intermediate zinc carbenoid 16 that, after hydrolysis, furnishes an unsaturated alkyl iodide in 61% yield [Eq. (15) 8]. The reverse addition sequence [AcOH (1 equiv), —80 to — 40 C iodine (1 equiv)] leads to the desired product, with equally high yield [8]. A range of electrophile couples can be added, and the stannylation of 15 is an especially efficient process [Eq. (16) 8]. A smooth oxidation of the bimetallic functionality by using methyl disulfide, followed by an acidic hydrolysis, produces the aldehyde 17 (53%), whereas the treatment with methyl disulfide, followed by the addition of allyl bromide, furnishes a dienic thioether in 76% yield [Eq. (17) 8]. The addition of allylzinc bromide to 1-octenyllithium produces the lithium-zinc bimetallic reagent 18, which can be treated with an excess of methyl iodide, leading to only the monomethylated product 19. The carbon zinc bond is unreactive toward methyl iodide and, after hydrolysis, the alkene 19... 

Zinc is less electropositive than lithium and magnesium, and the carbon-zinc bond is less polar. Organozinc reagents are not nearly as reactive toward aldehydes and ketones as Grignard reagents and organolithium compounds. 

Scheme 3.3 Oleflnic jc-bond insertion into the carbon—zinc bond.

Other possibilities for the transition state, like carbanions, [Ar ]" " or concerted insertion of Zn in the C—Br bond, seem unlikely. The moderate p value does not indicate the development of a carbanion or a carbon—zinc bond in the transition state. In such a case, the carbon wotdd bear a large negative charge, and a larger value for p would be expected [235]. 

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