Dizinc complexes

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. 

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 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... 

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.

Reaction of isoquinoline with allylzinc bromide in THP gave the dizinc complex 99 which on heating at 65 C for 30 minutes furnished the dizinc complex of the isoquinuclidine ring system (101). Work up with MeOH and base yielded a 7 3 mixture of the ISQ isomers, endo-X l and exo-102 (Scheme 13) [98]. Transformation of 99 to the tricyclic compound 101 appears to proceed by the addition of the benzylzinc fragment to the double bond of the 1-allyl group. The predominant formation of endo-102 is most likely caused by the electronic effect of the benzene ring on the orientation of the double bond and the 4-ZnBr group in the transition state [98]. 

Scheme 5.82 Trost s direct aldol reaction catalyzed by the dizinc complex of ligand 290.

Scheme 5.83 Catalytic cycle proposed for the aldol reaction mediated by dizinc complexes of ligand 290.

It is almost always the case that the overall increase in reaction rate is greater than that for either catalyst alone when researchers use more than one catalyst to speed up a chemical reaction [5]. As a consequence, much effort has been made in creating designer catalyst systems in the past decade, such as Yamamoto s designer acids [6], Shibasaki s bimetallic systems [7], Trosfs dizinc complexes... 

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... 

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... 

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. 

Recently, we discovered that phosphate P-O bond cleavage is promoted by an alkox-ide bridged dizinc(II) complex with an hydroxyloctaazacryptand. In this case weakly coordinating secondary nitrogen attacks the substrate phosphomonoester, and the bridging alkoxide () anion between two zinc(II) ions has no nucleophilic activity, which is somewhat similar to our dinuclear complex 13 T. Koike, M. Inoue, E. Kimura, and M. Shiro, J. Am. Chem. Soc., 118 3091-3099 (1996). 

Coates proposed that the mechanism involved a bimetallic pathway (Fig. 19), with one metal coordinating the epoxide, whilst the second metal bound the carbonate group (growing polymer chain) which attacked and ring-opened the epoxide. This mechanistic proposal has since led to the preparation of many dizinc (and other dinuclear) complexes (see Sect. 3.4). 

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