Nickel organozinc reagents

Nickel acetylacetonate, Ni(acac)2, in the presence of a styrene derivative promotes coupling of primary alkyl iodides with organozinc reagents. The added styrene serves to stabilize the active catalytic species, and of the derivatives examined, m-trifluoromethylstyrene was the best.274... 

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. 

III. ELECTROCHEMICAL PREPARATION OF ORGANOZINC REAGENTS USING NICKEL OR COBALT CATALYSIS... 

Organozinc reagents, including the Reformatsky reagents, are extensively used in transition metal catalysed coupling reactions with aryl halides or triflates, vinyl halides, and allylic halides or acetates, as reviewed by Erdik156. Nickel and palladium complexes are... 

The most often-reported metallic systems used in the formation of carboxylic acids are most certainly those systems that involve a zero-valent nickel species as the active intermediate. Ochiai et al. reported on a bimetallic catalytic system which allowed the synthesis of various saturated carboxylic acid in good yields, under very mild conditions [53] (0.1 MPa C02, 4—8 h reaction time, temperatures ranging from room temperature to 323 K). The catalytic system was based on the use of organozinc reagents as carbon nucleophiles, which could be selectively carboxy-lated in the presence of Ni(acac)2 as the main catalyst. 

In their catalytic cycle, the authors proposed the formation in situ of a zero-valent nickel species by the reduction of Ni(acac)2 with the organozinc reagent. Since the pioneering studies of Aresta et al., which detailed the isolation of the first nickel-C02 complex, such zero-valent species have been known easily to activate C02 [54]. Transfer of the so-formed Ni(II)-carboxylate occurred following a transmetallation... 

Fig. 1 Nickel-catalyzed Negishi coupling reactions of alkyl halides 1 with organozinc reagents 2...

The electrosynthesis of /Miydroxy esters (220), 2,3-epoxy esters (221) and /Miydroxy nitriles (222) was achieved under nickel-catalyzed conditions, obviating Reformatsky reaction 70(equation 112-114). The reaction proceeds in excellent yield when a sacrificial zinc rod is used as the anode. A mechanism has been proposed which involves reduction of a Ni(II) complex to a Ni(0) complex, oxidative addition of the a-chloroester to the Ni(0) complex, and a zinc (II)/Ni(II) exchange, leading to an organozinc reagent, in analogy to the Reformatsky reactions. 

The copper-catalyzed conjugate addition reaction of organozinc reagents has reached a prominent level of success by the development of sophisticated chiral phosphorus ligands. Nickel catalysts have also been surveyed by several groups. 

Other coupling partners to organozinc reagents include heterocyles such as 2-methylthiobenzothiazole, alkenyl aryl iodonium triflates (alkenyl group transfer for synthesis of trisubstituted alkenes), and aryl heteroaryl ethers. " Improved nickel-catalyzed cross-coupling conditions between oz-rto-substituted aryl iodides-nonaflates and alkyl-zinc iodides in. solution and in the solid phase have been defined. ... 

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