Organozinc reagents formation reactions

The organozinc reagent formation and concentration is best determined by performing an iodolysis and a hydrolysis of a reaction aliquot. These experiments... 

Formation of heterocycles in reactions using organozinc reagents 96MI8. 

Feringa and co-workers described the tandem addition-aldol cyclization protocol leading to the formation of 6,6-, 6,7-, and 6,8-annulated bicyclic systems (Scheme 68).39 Using Cu(n)-29 as catalyst and functionalized organozinc reagents as nucleophiles, the conjugate addition reaction followed by aldol cyclization can offer highly enantioselec-tive annulation products (up to 98% ee). This method can be used in the synthesis of carbocyclic compounds, such as steroids, terpenes, and other natural products. 

This reaction was successfully applied to various hydrazones derived from acyclic or cyclic ketones and extended to a variety of unsubstituted vinylsilanes as acceptors (trialkyl, triphenyl and alkoxy vinylsilanes). In all cases, the regioselectivity of the addition reaction was consistent with the formation of a silicon-stabilized organozinc reagent. 

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. 

The success of the reaction is crucially dependent on the cleanliness of the zinc surface, prior to the formation of the organozinc reagent sonication has been suggested as a valuable aid to this reaction (cf. Expt 7.14). Under standard conditions, the hydroxy ester is the major product when the reaction is carried out in ether or benzene the contaminants which frequently arise during distillation are the a,0- and 0,y-unsaturated esters. Dehydration can be deliberately effected with the aid of reagents such as fused potassium hydrogen sulphate or acetic anhydride. Catalytic hydrogenation of the mixture of unsaturated esters fol-... 

P. Knochel, J. J. A. Perea, P. Jones, Organozinc Mediated Reactions, Tetrahedron, 1998, 54, 8275-8319. P. Knochel, Carbon-Carbon Bond Formation Reactions Mediated by Organozinc Reagents, in Metal-catalyzed Cross-coupling Reactions (F. Diederich, P. J. Stang, Eds.), Wiley-VCH, Weinheim, 1998, 387-416. 

Aryl-aryl coupling by reaction of halides with organozinc reagents is a useful and mild route for the formation of some mixed biaryls. Such a process has been used in the reaction of pyridylzinc reagents with various iodoaromatic compounds giving good yields of coupled products (equation 65)484 87. 

The stereochemistry of addition of an allylic organozinc reagent to a carbonyl group has received considerable attention. Both diallyl- and dicrotylzinc in their reactions with alkyl-substituted cyclohexanones display a strong preference for equatorial attack (formation of the rfl -alcohol) on the carbonyl group (5, 6), e.g.,... 

Carbon-Carbon Bond Formation Reactions Mediated by Organozinc Reagents... 

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