Allylbarium chlorides

Allylbarium reagents homoallylic alcohols. The reaction of anhydrous Bal2 with lithium biphenylide (2 cquiv.) in THF provides a reactive barium species that reacts with allylic chlorides at —78° to form allylbarium chlorides. These reagents react with high a-selcctivity with carbonyl compounds to form homoallylic alcohols with retention of configuration. 

An unusual temperature dependence on the regioselective behavior of an allylbarium species has been demonstrated in asymmetric allylation with the optically active imine SAMP-hydrazone [SAMP = (A)-(—)-l-amino-2-meth-oxymethylpyrrolidine] (Scheme 10).322 Its reaction with prenylbarium chloride at 0°C produced an a-allylated hydrazine in 60% diastereotopic excess, but at — 78 °C, the y-adduct was generated with 98% diastereotopic excess. The temperature dependence of the ct/y ratio may reflect competition between a kinetically favored y-adduct at low temperature and a thermodynamically preferred ct-form at higher temperatures. 

Allylation. Active barium is generated by reduction of Bah with 2 equivalents of Li biphenylide in THF at room temperature. Allylic chlorides are converted to the allylbarium reagents, which can be used to effect allylation with excellent regio- and stereoselectivities. r/ireo-Homoallylic alcohols from a y-selective reaction with aldehydes are obtained. ... 

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