Structure organozinc compounds

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 ... 

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. 

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. ... 

H and NMR spectroscopic studies of 64 showed that the structure present in the solid state is retained in solution. That the reactivity of 64 is different compared to that of normal organozinc compounds became evident from a hydrolysis reaction of 64 with a carboxylic acid affording the corresponding zinc salt of the carboxylic acid and metallic zinc in a 1 1 ratio and pentamethylcyclopentadiene. This product formation suggests the... 

Although the simple alkylzinc halides RZnX (R = Me or Et, X = Cl, Br, I) were among the first known organozinc compounds, for a long time nothing was known about the actual structures of such compounds in solution or in the solid state. The constitution of these compounds in solution seemed to depend on the particular solvent employed " . It should be noted that in ethereal solutions, the possibility of the presence of a Schlenk equilibrium should always be considered. In these solvents the organozinc halides most... 

The second structural motif, B (Figure 72), involves also a dimeric structure in which two alkoxy oxygen atoms bridge between two zinc atoms, but now an additional intramolecular coordinating substituent is present, resulting in the formation of four-coordinate zinc atoms. Most of these strucmral investigations are aimed at the elucidation of the mechanism of the enantioselective addition of organozinc compounds to aldehydes, a topic that will be discussed in Chapter 13. 

A few organozinc compounds containing the f-BuPOs bis-anion have been re-ported . They form complex aggregated structures containing up to ten zinc atoms, but are of less interest for organozinc chemists. 

Only for a few organozinc compounds, in which in addition to the zinc-carbon bond a zinc-heteroatom bond other than oxygen or nitrogen is present, have structures in the solid state been determined. These structures include those of organozinc compounds in which, in addition to the zinc-carbon bond, a zinc-sulfur, zinc-selenium, zinc-phosphorus or zinc-arsenic bond is present. 

M. Melnik, J. Skorsepa, K. Gyoryova, C. E. Holloway, Structural Analyses of Organozinc Compounds, ... 

Table 9 Average bond lengths for some structurally characterized organozinc compounds containing a zinc-oxygen bond...

Structurally related bis(dichloromethyl)zinc (prepared from dichloromethyllithium and zinc chloride) gave, with cyclohexene (80°C, benzene), 7-chlorobicyclo[4.1.0]heptane (15%). With the exception of the reactions above, the usefulness of organozinc compounds for the synthesis of chlorocyclopropanes has not been explored. 

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