Organozincates preparation

The organozinc prepared from propargyl bromide did not react with terminal alkynes because, under the reaction conditions, abstraction of the acidic terminal acetylenic hydrogen from the propargylic metabotropic form presumably occurred (equation 154)185. 

Palladium catalysed acylation of organozincs preparation of S-nonyn-2-one45... 

Notably, by this means, alkenes become suitable starting materials for organozinc preparation by a sequence entailing hydroboration (for instance, with EtjBH [64]) and subsequent treatment with Et2Zn [64] or iPr2Zn [66]. The preparation of oligofunctional dialkylzincs such as 38 [64b-c], 39 [64b-c, 66], 40 [67], and 41 [68] showcases the potential of this approach (Scheme 4.10). 

An organozinc compound that occupies a special niche in organic synthesis is iodo-methylzinc iodide (ICH2ZnI). It is prepared by the reaction of zinc-copper couple [Zn(Cu), zinc that has had its surface activated with a little copper] with diiodomethane in diethyl ether. 

Recently, the required heteroaromatic organozinc halides for the Negishi reaction have also been prepared using microwave irradiation [23]. Suna reported that a Zn - Cu couple (activated Zn), prepared using a slightly modified LeGoff procedure from Zn dust and cupric acetate monohydrate, allowed the smooth preparation of (3-pyridinyl)zinc iodide and (2-thienyl)zinc iodide... 

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... 

Aspects of the scale-up of aminoalcohol-catalyzed organozinc reactions with aldehydes have been investigated using A lV-diethylnorephedrine as a catalyst.153 In addition to examples with aromatic aldehydes, 3-hexanol was prepared in 80% e.e. 

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]. 

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... 

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