Organozinc compounds nickel catalysts

B. Electrochemical Preparation of Organozinc Compounds using Nickel as Catalyst... 

The electrochemical preparation of organozinc compounds obtained from the corresponding aromatic halides and with the use of a nickel complex as catalyst is only efficient in dimethylformamide as solvent. Moreover, in most cases and as described previously, the reaction requires the presence of excess 2,2 -bipyridine (five molar equivalents with respect to nickel) to achieve the transmetallation reaction leading to the organozinc compound and to avoid the formation of biaryl, Ar-Ar (equation 53). 

D. Cross-coupling Reactions of Organozinc Compounds in the Presence of Nickel or Palladium Catalysts... 

The past decade has seen extensive development of cross-coupling reactions of organozinc compounds and organic halides catalyzed by nickel or palladium catalysts. Although nickel-based catalysts are more reactive with respect to the organic halide partner, the number of failures with these catalysts and the greater selectivity realized with palladium-based catalysts have resulted in the almost exclusive use of the latter group of catalysts for these reactions. 

The cross-coupling of organozinc compounds has been accomplished using a number of metal enolate complexes, namely Rh(acac)(H2C=CH2)2 (6) , Co(acac)3 (30) °, Ni(acac)2 (47) , Li(acac) (107) and Cr(acac)3 (108) . For instance, complex 47 proved to be an extremely effective catalyst system for the Negishi cross-coupling of arylzinc halides (109) with aryl (110), heteroaryl and alkenyl halides, triflates and nonaflates to give the corresponding biaryl compounds (111) (equation 30). The solvent played an important role in these reactions and optimal conditions were found with 8 1 mixtures of THF and A-ethylpyrrolidinone (nep). Only 0.05 mol% of the nickel complex... 

Organozinc compounds, as described above, become highly reactive by using catalysts such as palladium, nickel and copper compounds. Therefore, even if the reaction could not proceed since the reactivity of the organozinc compound is too low, it is able to be carried out. Recently, these reactions using catalysts are noted because the selectivity of the reaction is high [15,35—47]. 

With organozinc compounds in the presence of palladium, nickel or copper compounds as the catalysts, highly selective additions to carbonyl groups or carbon-carbon double bonds proceed. For example, an addition of chalcone (Ph-COCH=CH-Ph) proceeds with dialkylzinc in the presence of a nickel catalyst. The asymmetric addition proceeds by using a nickel catalyst having the chiral ligand as shown in eq. (5.17) [38,43]. 

Instead of quenching with deuterium chloride, the intermediary organozinc compound 16 can be used as a nucleophile. Not only allylic halide but also alkenyl or aryl halide can be used as an electrophile in the reaction with gem-dizinc. In Scheme 8.27, the sequential coupling reactions of fois(iodozincio)methane are summarized. In the case of the coupling with a bromoalkene, a nickel catalyst was more effective than a palladium catalyst. 

Inspired by the well-established nickel-catalyzed co-oligomerization of 1,3-dienes with CO2, which proceeds via bis-TT-allyl intermediate, Mori has developed a powerful intramolecular version of this process (Scheme 103). After insertion of C02 into the bis-vr-allyl complex, a transmetallation with an organozinc reagent takes place to generate the Ni(0) catalyst. Highly functionalized carbo- and heterocyclic compounds with complete stereocontrol can372 be synthesized by this method. 

Since transmetalation of organozinc halides in the presence of palladium or nickel compounds easily proceeds, the coupling reaction is carried out by the catalytic reaction via formation reaction of these organometallic compounds. For example, an aromatic /i-ester is prepared in a high yield by a reaction with a palladium catalyst as shown in eq. (5.31) [78]. 

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