Preparation of Organozinc Halides using Transmetallation Reactions

1222 Preparation of Organozinc Halides using Transmetallation Reactions 

The mixed 1,2-bimetallic Zn/Si-reagent 47 is a versatile species that reacts with aldehydes in high diastereoselectivity [59]. It is prepared by a bromine-lithium-exchange reaction starting from 48 followed by a transmetallation with ZnClj. The reaction with acetaldehyde is leading initially to the alkenylzinc species 49, which reacts with MejSiCl, providing the alkenylsilane 50 in 41% yield and a diastereoselectivity 9 1. 

Similarly, zincated hydrazone derivatives of type 56 undergo an intermolecular carbozincation of strained cyclopropene rings such as 57 leading to the adduct 58 with 92% yield [63]. This type of addition can be extended to ethylene ]63c]. It proceeds with an excellent stereoselectivity allowing the enantioselective synthesis of a-substituted ketones. Allylic zinc species also add to cyclopropenone acetals 

Transmetallations starting from alkenylzirconium species that are obtained by hydrozirconation using H(Cl)ZrCp2 are readily accomplished [68,69]. Ichikawa and Minami have elegantly shown that diflurovinylzinc iodide 64 is obtained by the addition of ZrCpj to the alkenyl tosylate 65 [68]. In situ transmetallation reactions have also been reported. 

The reaction of 73 with electrophiles affords products of type 74 with variable enantioselectivities. Quenching of the copper-zinc reagent 73 with reactive electrophiles proceeds with retention of configuration with up to 95% ee. 

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Preparation of organozinc halides using transmetalation reactions. . . 296... 

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