Crystal Reformatsky reagent

Fig. 7.5. Crystal structure of Reformatsky reagent of (-butyl bromoacetate crystallized from THF. Reproduced from J. Chem. Soc., Chem. Commun., 553 (1983), by permission of the Royal Society of Chemistry.

The Reformatsky reaction is related to both organometallic and aldol addition reactions and probably involves a cyclic TS. The Reformatsky reagent from /-butyl bromoacetate crystallizes as a dimer having both O—Zn (enolate-like) and C—Zn (organometallic-like) bonds (see Figure 7.5).165... 

Due to the disclosure of the first crystal structure of a Reformatsky reagent by Boersma and coworkers [81], it was noticed as a special feature of the relatively electropositive metal zinc to form an enolate with a carbon metal bond. Indeed, in the dimeric structure zinc is bound to bromine and the a-carbonyl carbon atom and additionally coordinated to THF and the carbonyl oxygen of a second ester molecule to give a C,0-bridged dimeric structure, as shown in Figure 3.14. 

Zinc enolates can be prepared in solvents of greatly different polarity, including THF, DME, Et20, 1,4-dioxane, benzene, toluene, dimethoxy-methane, DMF, B(OMe)3, DMSO, and mixtures thereof, just to mention the most frequently used ones.1-3 Reformatsky reactions in the absence of a solvent have also been described.7 The reagent is dimeric in the crystal state and in solution except for the most polar media.5,8... 

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