An Organozinc Reagent for Cyclopropane Synthesis

Zinc reacts with alkyl halides in a manner similar to that of magnesium. 

Victor Grignard was led to study organomagnesium compounds because of earlier work he performed with organic derivatives of zinc. 

Zinc is less electropositive than lithium and magnesium, and the carbon-zinc bond is less polar. Organozinc reagents are not nearly as reactive toward aldehydes and ketones as Grignard reagents and organolithium compounds. 

An organozinc compound that occupies a special niche in organic synthesis is iodomethylzinc 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. 

What makes iodomethylzinc iodide such a useful reagent is that it reacts with alkenes to give cyclopropanes. 

There is often little advantage in choosing one route over another when preparing a particular target alcohol. For example, all three of the following combinations have been used to prepare the tertiary alcohol 2-phenyl-2-butanol, and each is effective. 

Use retrosynthetic analysis to develop a synthesis of 2-phenyl-2-butanol from benzene and 2-butyne as the source ot all of the carbons and write a series of equations for the synthesis showing all necessary reagents. 

PROBLEM 14.9 Suggest a combination of organic haiide and cuprate reagent appropriate for the preparation of each of the foiiowing compounds  

SAMPLE SOLUTION (a) First inspect the target moiecule to see which bonds are capabie of being formed by reaction of an aikyi haiide and a cuprate, bearing in mind that neither the aikyi haiide nor the aikyi group of the lithium dialkylcuprate shouid be secondary or tertiary. 

A bond between a — methyl group and a methylene group can be formed. 

None of the bonds to the methine group can be formed efficiently. 

There are two combinations, both acceptable, that give the CH3—CH2 bond (CH3)2CuLi + BrCH2CH(CH3)2 

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Organozinc compounds, XZnCRR (RR = H, alkyl, X halogen), are widely used for the synthesis of cyclopropanes as Simmon-Smith reagents [12,36,37,79- 93]. Simmon-Smith reagents are prepared by two processes as shown in eqs. (5.32) and (5.53). The first one is the reaction of diazomethane with zinc halides. The second is the reaction of methylene iodide with an activated zinc. Zinc containing 10% of... 

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