Chromium, allylic enantioselective addition reactions

Allylic t ganranetallics modified at the metal center by chiral adjuvants add to aldehydes and ketones to provide optically active hranoallylic alcohols. This process has been described for reagents containing boron, tin and chromium metd centers. Gore and coworkers have shown that a chrranium-medi-ated addition reaction of allylic brranides to simple aldehydes that uses a complex of lithium N-methyl-nt ephedrine and chromium(Il) chloride occurs with modest (6-16% ee) enantioselectivity (equation 61, Table 8). 

Umani-Ronchi adapted the Furstner protocol to achieve the first catalytic, enantioselective variant of this reaction. The chiral chromium salen complex was prepared from the in situ reduction of the anhydrous CrCb to CrCl2 with an excess of manganese metal, followed by complexation with the salen ligand 8 in the presence of catalytic triethylamine." Then the addition of allylic chloride (9) to aldehydes 10 to give the allylic alcohols 11 in moderate yields and in up to 95% ee. The same groups employed the same conditions for the addition of 2-butenyl bromides to aldehydes to achieve up to 83 17 syn/anti of allylic alcohol products and for the addition of 1,3-dichloropropene to aromatic aldehydes to obtain the syn chlorohydrin adduct in modest yield which were further converted to optically active vinyl epoxides. The [Cr(salen)]-catalyzed addition of propargyl halides to aromatic aldehydes allowed the synthesis of enantiomerically enriched homopropargyl alcohols in moderate yields with up to 56% ee. ... 

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