Preparation organozinc reagents

Organozinc reagents prepared from ultrasonic irradiation of organic halides with Li in the presence of ZnBr2 have recently been used for conjugate addition to a-enones [Eqs. (47) and (48)] (234,235). In the... 

Organozinc reagent prepared from the 2-(trimethylsilyl)allyl bromide 156 and zinc react with alkynes or carbonyl compounds as well as nitriles to give the corresponding coupling products in moderate to good yields (equation 130)225. 

Optically active a-hydroxy carboxylic acids. The organozinc reagents prepared in situ from RMgX and ZnCl2 or ZnBr2 add selectively to the keto group of - l-menthyl phenylglyoxalate (1). The adducts are hydrolyzed to optically active a-hydroxy carboxylic acids (3).1... 

Prepare a solution of 3-methyl-2-cyclopenten-1-one (0.384 g, 4 mmol) and Ni(acac)2 (21 mg, 0.08 mmol) in 5 1, toluene tetrahydrofuran (2 mL), and slowly add this solution to the organozinc reagent prepared above at 0°C. 

Reactions of organozinc reagents, prepared from organomagnesium compounds in situ (see Section 16), with a-sulfonyl ethers and carbamates [40] can give excellent results, e.g. [41]. 

Organozinc reagents can also be prepared from trialkylboranes by exchange with dimethylzinc.136... 

This route can be used to prepare enantiomerically enriched organozinc reagents by asymmetric hydroboration (see Section 4.5.3), followed by exchange with diisopropyl-zinc. Trisubstituted cycloalkenes such as 2-methyl or 2-phenylcyclohexene give an enantiomeric purity greater than 95%. The exchange reaction takes place with retention of configuration.137... 

Organozinc reagents have been used in conjunction with a-bromovinylboranes in a tandem route to Z-trisubstituted allylic alcohols. After preparation of the vinylborane, reaction with diethylzinc effects migration of a boron substituent with inversion of configuration and exchange of zinc for boron.176 Addition of an aldehyde then gives the allylic alcohol. The reaction is applicable to formaldehyde alkyl and aryl aldehydes and to methyl, primary, and secondary boranes. 

Lack of cooling during preparation of the Simmons-Smith organozinc reagent caused the reaction to erupt. The possibly pyrophoric nature of organozinc compounds and the presence of ether presents a severe fire hazard [1]. An alternative, safer method of activating the zinc for the reaction involves use of ultrasonic irradiation rather than the copper-zinc couple [2]. 

Because of the enormous volume of information regarding the use of organozinc derivatives in organic chemistry, this survey is focused only on recent major trends in this area. Valuable information about preparation methods of major types of organozinc compounds and their reactions can be found in two recent books246 247 and reviews. There have been a number of reviews devoted to specific applications of organozinc reagents... 

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