Alkyl zinc complexes, synthesis

Scheme 13. Synthesis of alkyl zinc complexes with his(pyrazol-l-yl) acetato ligands (47).

The synthesis of the zinc complex 115 according to Scheme 3 presents an efficient alternative to the previous method based on reduction of carbon diselenide by sodium metal <1990JCD115>. Compounds 22, 98-101, and 116 containing either (2-methoxycarbonyl)ethyl or 2-cyanoethyl as thiolate protecting groups are versatile compounds for the synthesis of a large selection of functionalized tetraselenafulvalenes via the (1) base-promoted deprotection and (2) thiolate alkylation protocol. 

The reaction of the carbonato complex with C02 has allowed the demonstration of a facile insertion-deinsertion equilibrium. The study of the deinsertion reaction has allowed estimation of the activation parameters as being = 130 4.0 kJ mor and AS = 121.6 11.9 J moP K . From the above values the authors have calculated an approximate value of the equiUbrium cmistant for the carboxylation reaction equal to 3 x 10 M at 195 K (or a AG value for the same reaction of < 50 kJ moP ), showing that the insertion of CO2 into the M-O-alkyl bond is both kineticaUy and thermodynamically very favored. This trend has also been confirmed for the insertion of CO2 into the Nb-OR bond in [Nb(OR)5]2 (R = methyl, ethyl, aUyl) (see Sect. 6.2.2.1), a catalyst for the synthesis of dialkyl carbonates [67]. Very recently, the facile insertion of CO2 into metal-phenoxide bonds has been reported [68] for cobalt and zinc complexes (Fig. 4.2). It should be noted that such metal systems are used as catalysts in the copolymerization of CO2 with epoxides. 

Synthesis. These macrocycles are prepared from seven-membered ring dinitrile complexes, 84a-84c (Scheme 17), which contain either methylene, sulfur or oxygen in the five position (129). These cyclic dinitriles are synthesized by alkylating maleonitrile dithiolate or derivatives thereof with the corresponding dihalide. The dinitriles 84a-84c can be cyclized in magnesium propoxide to form porphyrazines 85a (33%), 85b (19%), and 85c (27%) (Scheme 17), which can be demetalated with trifluoroacetic to form 86a-86c. Additionally, 86a has been remetalated with nickel (87a, 92%), copper (88a, 95%), and zinc (89a, 94%). The sulfur and oxygen derivatives 85b, 85c, 86b, and 86c are of low solubility and are not suitable for further manipulation. 

The preparation of a novel pentadentate ligand, 2-hydroxy-5-methylisophthalalde-hyde bis(/7-methoxy thiobenzoylhydra-zone), was described together with the chloro-bridged Zn(II) complexes [127]. Synthesis and electrochemical properties of some long-chain 1,4,8,11,15,18,22,25-octa-alkylated metal-free and zinc phtha-locyanines were presented by Swarts et al. [128]. The zinc derivatives show higher Kquid crystalline behavior than the free-metal compounds. 

Among isolable metal homoenolates only zinc homoenolates cyclize to cyclo-propanes under suitable conditions. Whereas acylation of zinc alkyls makes a straightforward ketone synthesis [32], that of a zinc homoenolate is more complex. Treatment of a purified zinc homoenolate in CDC13 with acid chloride at room temperature gives O-acylation product, instead of the expected 4-keto ester, as the single product (Eq. (22) [33]). The reaction probably proceeds by initial electrophilic attack of acyl cation on the carbonyl oxygen. A C-acylation leading to a 4-keto ester can, however, be accomplished in a polar solvent Eq. (44)-... 

Zero-valent nickel complexes are known to reduce 1,2-dihalides to olefins and to mediate C,C-coupling reactions of vinyl halides. Based on these facts, lyoda and coworkers developed a two-step, one-pot synthesis of alkyl-substituted [4]radialenes which starts from 2,3-dihalo-l,3-butadienes and 1,4-dichloro-2-butyne derivatives and circumvents the isolation of the butadiene intermediates. Furthermore, the synthesis can be made catalytic in nickel when the Ni(0) complex is generated from NiBr2(PPh3)2 with a more than stoichiometric quantity (based on the dihalide) of zinc. Again, the formation of radialene 94 must compete with that of 95 and 96. With preformed Ni(PPh3)4 and Ni(PBu3)4, the [4]radialene is normally favored in benzene solution, but formation of 95 and/or 96 becomes important in the more polar solvents THF and DMF. With a catalyst... 

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