The interaction of carbonyl-containing compounds with organometallic reagents

2 THE INTERACTION OF CARBONYL-CONTAINING COMPOUNDS WITH ORGANOMETALLIC REAGENTS 

Other ethereal solvents (e.g. tetrahydrofuran, dibutyl ether) may also be used indeed tetrahydrofuran is the required solvent for the formation of vinylmag-nesium halides. 

The true structural nature of the reactive species in solution is uncertain and 

Two principal approaches to the synthesis of an optically pure chiral secondary or tertiary alcohol from the reaction of an organometallic reagent with an aldehyde or ketone respectively are of current interest. In the first approach an alkyllithium or dialkylmagnesium is initially complexed with a chiral reagent which then reacts with the carbonyl compound. In this way two diastereo-isomeric transition states are generated, the more stable of which leads to an enantiometic excess of the optically active alcohol. This approach is similar in principle to the asymmetric reductions discussed in Section 5.4.1 (see also p. 15). Two chiral catalysts may be noted as successful examples, (10) derived 

In the second approach the carbonyl function is incorporated in a chiral adjuvant (or auxiliary) which then stereoselectively directs a preferred attack of the organometallic reagent on the si- or re-faces of the carbonyl group, as determined by steric and electronic interactions. This results in two diastereoisomeric products in a ratio dependent on the relative free energies of activation. One such auxiliary is (12), derived from the readily available and optically pure ( + )-pulegone. 

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The interaction of carbonyl-containing compounds with organometallic reagents (Expts 5.39 to 5.43). 

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