Bromozinc enolate

The methods previously used to obtain single aldol products (or their dehydrated derivatives) from reactants where several aldol products are possible8 include the reaction of bromozinc enolates, from a-bromo-ketones, with aldehydes 9 the reaction of bromomagnesium enolates, from either a-bromoketones or from ketones and bromomagnesium... 

The 1,4-conjugate addition of ester enolates to a, 3-enones was first reported by Kohler in 1910,138a c as an anomalous Reformatsky reaction, but chemoselectivity was dependent on the structure of the a,(3-enone and restricted to bromozinc enolates obtained from either a-bromoisobutyrate or bromomalonate esters (Scheme 66).138d,e Further evaluation, with lithio ester enolates and lithio amide enolate additions, has resulted in identification of four factors that affect the chemoselectivity and diastereoselectivity of additions to a, 3-enones.139 These factors are (a) enolate geometry, (b) acceptor geometry, (c) steric bulk of the -substituent on the acceptor enone and (d) reaction conditions. In general, under kinetic reaction conditions (-78 °C), ( )-ester enolates afford preferential 1,2-addition products while (Z)-ester enolates afford substantial amounts of 1,4-addition products however, 1,2 to 1,4 equilibration occurs at 25 C in the presence of HMPA. The stereostructure of the 1,4-adducts is dependent on the initial enolate structure for example, with ( )-enones, (Z)-ester enolates afford anti adducts, while (E)-ester enolates afford syn adducts (Scheme 54). In contrast, amide enolates show a modest preference for anti diastereomer formation. 

The organozinc reagent from ethyl a-bromoisobutyrate has been found to comprise two entities one of these, the bromozinc enolate (66), reacted as expected a dimeric structure (67) has been assigned to the other, inactive component. 

No crystallographic structure has been reported for zinc ketone enolates. Although carbon-bonded structures analogous to (5) have been proposed, spectral data obtained for the bromozinc and ethylzinc... 

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