Crystal zinc enolates

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... 

Goel, S. C., Chang, M. Y., and Buhro, W. E., Preparation of soluble and volatile zinc dialkoxides. X-ray crystal structures of an (amido)zinc alkoxide and a ho-moleptic zinc enolate Inorg. Chem., 29, 4646 (1990). 

Later studies revealed however that the C-bound tautomer is not a general feature of zinc enolates. Thus, O-bound tautomer were a-aminoester zinc enolates 48 were found to form tetramers 49 as the most stable aggregates with a Zn-O-bridged eight-membered core unit, as detected by a crystal structure... 

But also simple ketone enolates without a chelating a-substituent were found to exist under the form of the O-bound tautomers. This is clearly evidenced by crystal structures of bis(amidoamine)-complexed zinc enolates 49 and 50 derived of diisopropyl ketone and acetophenone, respectively. They feature carbon-carbon double bonds, and their core unit is formed by a square or parallelogramshaped Zu202 skeleton. NMR studies revealed related structures for zinc enolates of isobutyrophenone, cyclohexanone, and 2,2-dimethylcyclopentanone (Scheme 3.16) [83]. The mononuclear enolate 51 of methyl mesityl ketone wherein zinc is chelated by TMEDA has the O-bound structure [84a] as well as mixed alkali zinc enolates derived of the same ketone [84]. 

Methylene difluorocyclopropanes are relatively rare and their rearrangement chemistry has been reviewed recently [14]. In addition, electron deficient alkenes such as sesquiterpenoid methylene lactones may be competent substrates. Two crystal structures of compounds prepared in this way were reported recently [15,16]. Other relatively recent methods use dibromodifluoromethane, a relatively inexpensive and liquid precursor. Dolbier and co-workers described a simple zinc-mediated protocol [17], while Balcerzak and Jonczyk described a useful reproducible phase transfer catalysed procedure (Eq. 6) using bromo-form and dibromodifluoromethane [18]. The only problem here appears to be in separating cyclopropane products from alkene starting material (the authors recommend titration with bromine which is not particularly amenable for small scale use). Schlosser and co-workers have also described a mild ylide-based approach using dibromodifluoromethane [19] which reacts particularly well with highly nucleophilic alkenes such as enol ethers [20], and remarkably, with alkynes [21] to afford labile difluorocyclopropenes (Eq. 7). 

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. 

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