Organozinc-oxygen compounds

Unsolvated organozinc compounds of the type RZnOR are associated in solution and the formation of trimeric aggregates has been established by cryoscopic measurements for EtZnOCHPh2 and [RZnOC6H4NMe2]3 in benzene solution [4, 5], The structures of these trimers are unknown (although presumably cyclic) and X-ray crystal structure determinations of a significant number of solid state com-poxmds reveal a much more complex reality, with dimers, trimers, and tetramers being identified. 

Derivatives of aminoalcohols, in which chelate rings are formed by nitrogen donation, are also dimers formed by O — Zn donation, and several examples, e.g. 4, are known [10-12]. 

The ethylzinc derivative of 2-(l-methyliminoethyl)phenol is also a dimer with non-equivalent Z-O bonds (Zn-O 1.965 and 2.064 A) [13]. 

The structure of bis[phenyl(acetylacetonato)zinc][bis(acetylacetonato)zinc], 2PhZn(acac)-Zn(acac)2, 5, also contains a Zn202 ring [14, 15] as does the ethylzinc derivative of pivaloylacetone [16]. 

Six-membered zinc-oxygen rings are rare - only one example has been authenticated. Pyrazolylboratozinc hydroxide, [ // -H2B(3-Bu Pz)2 Zn(y -OH) ]3, is a cyclic trimer, 6, with four-coordinate zinc atoms and alternating zinc-oxygen bond lengths (1.90 and 1.97 A) [17]. 

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Organozinc Compounds Bearing Simple Alkoxides and Related Monodentate Oxygen Donors... 

Reports on the organozinc chemistry with heavier group 16 donor ligands for the reviewed period were limited to sulfur donors. As with the organozinc chemistry of anionic oxygen donors, three themes dominate this area of research, namely the use of organozinc compounds ... 

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. 

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. ... 

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. 

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. 

Organometallic compounds asymmetric catalysis, 11, 255 chiral auxiliaries, 266 enantioselectivity, 255 see also specific compounds Organozinc chemistry, 260 amino alcohols, 261, 355 chirality amplification, 273 efficiency origins, 273 ligand acceleration, 260 molecular structures, 276 reaction mechanism, 269 transition state models, 264 turnover-limiting step, 271 Orthohydroxylation, naphthol, 230 Osmium, olefin dihydroxylation, 150 Oxametallacycle intermediates, 150, 152 Oxazaborolidines, 134 Oxazoline, 356 Oxidation amines, 155 olefins, 137, 150 reduction, 5 sulfides, 155 Oxidative addition, 5 amine isomerization, 111 hydrogen molecule, 16 Oxidative dimerization, chiral phenols, 287 Oximes, borane reduction, 135 Oxindole alkylation, 338 Oxiranes, enantioselective synthesis, 137, 289, 326, 333, 349, 361 Oxonium polymerization, 332 Oxo process, 162 Oxovanadium complexes, 220 Oxygenation, C—H bonds, 149... 

An unusual syn addition to epoxides occurs when 1,3-diene monoepoxides are treated with organozinc reagents. Thus, the cyclic vinyl epoxide 72 was converted to the cis-ethyl-cyclohexenol 75 with diethyl zinc in methylene chloride and trifluoroacetic acid. The syn addition is believed to derive from an initial coordination of the oxiranyl oxygen to the organozinc compound, which then delivers the alkyl group to the same face. This transfer is facilitated by a relaxation of the sp3 hybridization brought about by the Lewis acidic zinc center and the allylic character of the incipient carbocation <020L905>. 

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

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