1.3- Dizincs

Some other sp3 organogembismetallic compounds were reported as the synthesis of 1,3 dizinc derivatives using the boron-zinc exchange24 (Protocol 11), or the synthesis of l,n-heterobimetallic reagents.25 In the latter case, the reactivity of these derivatives is mainly the same as that of the monometallic compound. 

Dizincs. The preparation of one such species is illustrated below. 

The preparation of 1,3-dizinc species is illustrated in Schane 7.37. It shows the reaction of the corresponding diethylborane 100 species with dialkylzinc, which in turn was prepared from the reaction of diethylhydroborane with 99 (Eick and Knochel 1996). 

Eick, H. and Knochel, P. 1996. The preparation of 1,3-dizinc organometallics via a boron-zinc exchange. Angew. Chem. 108 229-231. 

The pyrazole phosphazine ligand, hexakis(3,5-dimethylpyrazolyl)cyclotriphosphazene (16) forms a dizinc complex, that has been structurally characterized, and the analogous copper zinc complex.158 Two ZnCl2 units are coordinated, one distorted tetrahedral and one distorted trigonal-bipyramidal, separated by 7.51 A. Cu1 and ZnCl2 can also be coordinated giving a heteronuclear compound with a Zn Cu separation of 6.798 A. 

Kimura and co-workers have made a major contribution to this area with dizinc azamacrocycle complexes characterized including phosphate ester X-ray structures.445,446 They have also studied ligands with pendent alcohol groups.44... 

Two dinuclear complexes, with five-coordinate zinc centers, derived from tris((2-pyridyl)-methyl) amine were synthesized and bridging phosphate or phosphate ester groups. The X-ray structure of the phosphate monoester complex shows a syn-anti bridging mode in contrast to alkaline phosphatase in which it is syn-syn 448 Fenton and co-workers have also studied other related dizinc species of compartmental ligands 449... 

As already mentioned, Lippard and co-workers have also studied the -hydroxy dizinc unit found in metallohydrolases in a model system [Zn2L(/x-OI I)(/x-02PPh2)]2+, L = 2,7-bis [2-(2-pyridylethyl)-aminomethyl]-l,8-naphthyridine) 454 Hydrolysis of phosphodiesters and beta-lactams was studied and related to the PI nuclease and beta-lactamase enzymes. The dinuclear complex... 

Dinuclear complexes with hydroxide bridged dizinc centers were synthesized with a ligand based on the phthalazine framework.455 The dizinc complex of 1,4-bis(2,2 -dipyridylmethyl)phtha-lazine (49) is bridged by the phthalazine moiety, a water molecule, and a hydroxide ion. Both centers are pseudooctahedral with two pyridine donors and a water molecule occupying the remaining coordination sites. 

A dithiolate ligand forms a structurally characterized dizinc bisligand complex with both of the ligands bridging, bis //-[(dimercaptomethylene)propanedinitrilato-1S, S ,] tetrakis(4-methylpyridine) dizinc.578... 

A few further general examples of zinc catalytic activity or reactivity include the following. Other zinc-containing systems include a zinc phenoxide/nickel(0) catalytic system that can be used to carry out the chemo- and regioselective cyclotrimerization of monoynes.934 Zinc homoenolates have been used as novel nucleophiles in acylation and addition reactions and shown to have general utility.935,936 Iron/zinc species have been used in the oxidation of hydrocarbons, and the selectivity and conditions examined.362 There are implications for the mechanism of metal-catalyzed iodosylbenzene reactions with olefins from the observation that zinc triflate and a dizinc complex catalyze these reactions.937... 

Fournier and Charette proposed a new gem-dizinc carbenoid, IZnCHIZnI 279, for alkene cyclopropanation.389 They reported that EtZnI reacted with CHC13 to form unstable 279, which was capable of reacting with the Unprotected allylic alcohols 280a-c. The final step of the reaction sequence was quenching the Zn-containing intermediate 281a-c with an electrophile (Scheme 147). 

Scheme 4. Exchange of carboxylate ligands in dizinc complexes 49a-f. The bowl-shaped representation of the host molecules should not be confused with the one used for the cyclodextrins.

Dizinc hexaborate heptahydrate, 4 242t DLVO (Derjaguin, Landau, Vervey,... 

Several zinc enzymes that catalyse the hydrolysis of phosphoesters have catalytic sites, which contain three metal ions in close proximity (3-7 A from each other). These include (Figure 12.11) alkaline phosphatase, phospholipase C and nuclease PI. In phospholipase C and nuclease PI, which hydrolyse phosphatidylcholine and single-stranded RNA (or DNA), respectively, all three metal ions are Zn2+. However, the third Zn2+ ion is not directly associated with the dizinc unit. In phospholipase C, the Zn-Zn distance in the dizinc centre is 3.3 A, whereas the third Zn is 4.7 and 6.0 A from the other two Zn2+ ions. All three Zn2+ ions are penta-coordinate. Alkaline phosphatase, which is a non-specific phos-phomonoesterase, shows structural similarity to phospholipase C and PI nuclease however,... 

Figure 16.17 Amine-terminated polypropylene imine) dendrimers act as tridentate ligands for the complexation of transition metals [217] (a), and can function as templates for the assembly of Troger s base dizinc(ll) bis-porphyrin molecules, (b) [218]...

Dizinc-monoethoxide complex 58 (Fig. 11) reported by Hillmyer and coworkers [81] displayed rapid polymerization of rac-lactide in CH2CI2 at room temperature with a [LA]o/[l]o ratio of 300 and [LA]q = 1 M greater than 90% conversion to PLA occurred within 30 min. PLA derived from L-LA was isotactic, lactide, signifying an absence of epimerization of stereogenic centers during the polymerization. In a similar study, zinc alkyl complexes (59a, b) and the alkoxo bridged... 

Divalent metal ions, reversible binding, 38 153 Dixenon cation, 46 68 Dizinc enzymes, 40 351-354 DMA, see Dicarbomethoxy acetylene DMAD complexes, see Dicarboxymethoxy dithiolene complexes DMAE, see Dimethylarsinoylethanol DMF, reduction potentials, 33 57 DMSO, see Dimethylsulfoxide DNA... 

The use of iodoform as the reagent precursor under Furukawa s conditions gives rise to a more complex scenario, since the additional C—I bonds can further react with an ethylzinc species (equation 8)" . The reaction of the iodo-substituted zinc carbenoid with an alkene will generate an iodo-substituted cyclopropane, whereas that involving the gem-dizinc carbenoid will lead to a cyclopropylzinc product. The evidence for the formation of a. gem-dizinc carbenoid was obtained not only by the analysis of the cyclopropanation products but also by the formation of rfi-iodomethane upon quenching the reagent with D2O/DCI. 

Improved preparation and reactivity of the gem-dizinc reagent was accomplished by using a mixture of Znl2, EtZnI 20Et2 and CHIs. The presence of Znl2 allows for shorter reaction times and cleaner reactions, particularly with less reactive substrates. 

The cyclopropanation reaction using gem-dizinc carbenoids has also been theoretically studied by Phillips and coworkers. The gem-dizinc carbenoids react with ethylene via a synchronous attack with an activation energy of about 15 kcalmoR (Figure 3, D). It was also predicted that the reaction could be accelerated by the addition of zinc iodide. 

The synthesis of halo-substituted cyclopropanes using zinc carbenoids can be accomplished using three different approaches by the cyclopropanation of a halo-substituted alkene, by the cyclopropanation using a halo-substituted zinc carbenoid, or by the cyclopropanation using, gem-dizinc carbenoids followed by trapping the cyclopropylzinc with an electrophilic halide source. 

An alternative stereocontrolled approach involves the use of em-dizinc reagents to generate the corresponding cyclopropylzinc species, which was then trapped with iodine... 

A modification to the original gem-dizinc protocol to improve the scope of the reaction was also reported. Ethylzinc iodide was used as the carbenoid precursor, and the reaction was run in the presence of zinc iodide. Under these conditions, simple aUylic ethers could be converted into iodo-cyclopropanes in high yields and diastereocontrol (equation 35). 

Charette and coworkers have shown that the diastereoselective cyclopropanation of chiral allylic alcohols with gem-dizinc carbenoids proceeds with high diastereocontrol for protected 2-butene-1,4-diol derivatives (equation 63). The minimization of the A-1,3 strain and the formation of a zinc chelate in the product is believed to be responsible for... 

A selective reaction of 1,4-brmetallic alkanes with CuCN 2LiCl allows the preparation of a range of new polyfunctional zinc-copper reagents. Thus, the reaction of 1,4-dizincated butane (94) with CuCN 2LiCl, followed by cyclohexenone in the presence of TMSCl (2 equiv), provides the new zinc-copper reagent 95 which reacts with 3-iodo-2-cyclohexenone, furnishing the diketone 96 in 64% yield (Scheme 33). ... 

SCHEME 33. Preparation and uses of alkylzinc-copper reagents via the selective reaction of 1,4-dizincated butane... 

Not only a zinc-copper couple, but also a zinc-lead couple forms a gem-dimetal species from diiodomethane, according to the 1975 Nysted patent5. He also insisted that treatment of dibromethane with a zinc-lead couple in THF at 80 °C forms a characteristic gem-dizinc species 1 (equation 3). However, there was no further evidence concerning the structure except H NMR data, which was not enough for the complete structural determination. The obtained compound was definitely a gem-dizinc species, but the written structure 1 was not fully characterized. The white solid 1 is obtained as a dispersion in THF, and does not dissolve in DMF and DMI (l,3-dimethyl-2-imidazolidinone). This THF dispersion is commercially available from Aldrich Co. as Nysted reagent. Nysted also showed that this dizinc compound is effective for the methylenation of a-hydroxy ketone moiety in steroid derivatives5. 

Eisch and Piotrowski reported the preparation of gem-dizinc compounds from diiodomethane and zinc powder in the titanocene chloride mediated methylenation of ketones in 1983 (equation 5)7. In this case, it was not mentioned that they had used pyrometallurgy zinc. The Tebbe-type reagent 2 was shown as an intermediary species. Before addition of titanocene chloride, the amount of methane was measured after hydrolysis of the reaction mixture to determine the formation of gem-dizinc species7. 

In 1998, Matsubara, Utimoto and coworkers reported a general procedure for the preparation of bis(iodozincio)methane, which was obtained as a THF solution (equation 6)9. The detailed structural study of the solution by SANS (small angle neutron scattering), AXS (anomalous X-ray scattering) and EXAFS (extended X-ray absorption fine structure) implied that the g< wt-dizinc species prepared from diiodomethane and zinc is the monomeric bis(iodozincio)methane (3). Formation of polymethylene zinc 4 via Schlenk... 

These side reactions, however, can be suppressed by addition of TMEDA. As shown in equations 9-11, several types of gem-dizinc compounds are prepared in the presence of... 

---