Metallo-Claisen reaction

Diastereoselective metallo-Claisen reactions can be used to prepare aldol-type products if the 1,1-dimetallic reagents 91 obtained are oxidized to aldehydes or ketones (equation 43)37f. 

Two main mechanistic hypotheses were considered for this reaction type38, a zinca ene 39 reaction and a metallo-Claisen rearrangement (equations 31 and 32). 70 and 71 are drawn as monomers for the sake of simplicity. The former probably exists in oligomeric form (see Section . . ), whereas the transition state of the metallo-Claisen rearrangement may involve two molecules of 7138. Since the simplified structures are perfectly suitable to rationalize the selectivity and reactivity of these reactions, they are used throughout this chapter. 

The oldest reaction involving a rearrangement of an organozinc compound is probably the Reformatsky reaction54 55. It took one hundred years until its key step could be classified as a [3,3)-sigmatropic shift ( metallo-Claisen rearrangement) on the basis of computational studies (equation 51)56. 

A zinca ene reaction is by definition (M. B. Smith and J. March, Advanced Organic Chemistry—Reactions, Mechanisms, and Structure, 5th edition, Wiley, New York, 2001, p. 1377) not a rearrangement, because it involves two different molecules. Nevertheless, reactions of the type described in equation 30 are included in this review regardless whether the authors rationalized them by a metallo-ene or metallo-Claisen pathway. This is justified since the original mechanistic assumptions were modified later (see Section II.A.3). 

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