Structure organozincates

By reaction of an a-halo ester 1 with zinc metal in an inert solvent such as diethyl ether, tetrahydrofuran or dioxane, an organozinc compound 2 is formed (a Grignard reagent-like species). Some of these organozinc compounds are quite stable even a structure elucidation by x-ray analysis is possible in certain cases ... 

By reaction of zinc-copper couple with diiodomethane 2 an organozinc species 4 is formed, similar to a Grignard reagent. Its structure cannot be fully described by a single structural formula. The actual structure depends on the reaction conditions—e.g. the solvent used this corresponds to the Schlenk equilibrium as it is observed with the Grignard reaction ... 

Because of the separation of this chapter into fundamental synthetic and structural aspects of organozinc compounds and the applications of these compounds in organic synthesis, many topics are treated twice, but with decidedly different emphases. By way of example, the important organozinc alkoxides are covered first in the inorganometallic section, where the emphasis is on their syntheses, structures, and applications other than in organic synthesis. Later, in Section 2.06.16.2, the uses of such compounds as chiral catalysts in asymmetric addition reactions are discussed. 

A large number of all citations to organozinc compounds deal with the use of these compounds as precursors for solid-state materials, particularly for MOCVD applications. The latter area has become so interdisciplinary and grown to such an extent that the editors of this edition considered it prudent to devote an entire volume (Volume 12) to the use of organometallic compounds in materials chemistry. While the syntheses and structures of potential organozinc precursors for such applications will be treated in this chapter, the detailed aspects of the conversion of organozinc compounds to solid-state materials are the topic of Volume 12. 

Despite the extensive history of organozinc chemistry, the first solid-state structure of a Lewis base-free diorganozinc compound with cr-bound ligands, namely diphenylzinc, was not reported until 1990.37 Diphenylzinc was synthesized by the transmetallation of diphenylmercury with metallic zinc (Scheme 10) and, after multiple sublimations, obtained halide free in 67% yield. 

Organozinc cations are compounds of the type RZn+, but the true structure of these charged species is more complex than indicated by this simple formula. Thus, the zinc atom is never one coordinate, but it is always associated with additional ligands or solvent molecules. These cations are therefore more accurately represented as [RZn(L)]+. Truly... 

Heterobimetallic clusters (Figure 58,125 and 126) with solvent-dependent structures were also obtained upon mixing alkali metal tert-butoxides and -trimethylsiloxides in THF, TMEDA, and toluene.184 The common occurrence of heterocubes shows that there is a strong driving force for the formation of heterocubic structures in organozinc alkoxides. Solvent effects are important, however, as demonstrated by the formation of seeo-diheterocubic compounds in TMEDA. 

The mono- and dinuclear ethylzinc complexes 137 and 138, respectively, were obtained when a 1,3-dimethylether /j-BT-calixMarene was treated with one and two equivalents of diethylzinc (Scheme 87).198 Excess [ZnEt2(tmeda)], in turn, reacted with this calixarene to furnish a pentanuclear dicalixarene. The syntheses and structures of related diorganozinc calixarenes, featuring both identical and non-identical organozinc moieties, were reported recently.199 The solid-state structure of the bis(ethylzinc)-l,3-dibenzylether-/)-But-calix[4]arene 139, which bears a close resemblance to 138, is shown in Figure 64. 

Downs et al. reported the synthesis of methylzinc tetrahydridoborate 164 by the two routes shown in Scheme 99.230 The compound is an extremely moisture and oxygen sensitive, colorless, polycrystalline solid, whose solid-state structure was determined by X-ray analysis. Figure 76 shows that 164 consists of helical polymers of alternating methylzinc and tetrahydridoborate ions. The zinc atom is formally pentacoordinate, making this one of the few organozinc compounds with five-coordinate zinc atom. 

The report of the first zinc compound with a Zn-Zn core elicited a number of critical comments on the structure and bonding of decamethyldizincocene, and the interpretation of the results.236,237 None of the authors of these commentaries questioned the data or their interpretation. Parkin, however, has pointed out that the formal oxidation state of +1 for zinc in this compound is merely due to the convention that metals are assigned an oxidation state of 0 when they form bonds with like atoms.237 If the conventional definition of valence, namely the capacity of atoms to form bonds to other atoms is used, then the zinc atoms in decamethyldizincocene are not monovalent, but divalent. The synthesis of a paramagnetic organozinc compound in which zinc uses only one of its two 4s electrons will remain an interesting challenge to many synthetic organometallic chemists. 

In the early days the organozincates and organozinc compounds were regarded as academic curiosities whose structural characterization was particularly hampered by their extreme sensitivity towards oxygen and moisture. This phenomenon had already been recognized by Frankland and has plagued chemists ever since. ... 

---