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MacMillan reported the Diels‐Alder reaction using a chiral imidazolidinone, which reacts with an α,β‐unsaturated aldehyde to afford an iminium ion. The iminium ion is a highly electron‐withdrawing group; its alkene has a lower LUMO level than the parent α,β‐unsaturated aldehyde. Thus, the Diels‐Alder reaction proceeds via the iminium ion intermediate not from the α,β‐unsaturated aldehyde. By the appropriate design of the chiral amine moiety, an enantioselective reaction can be realized. In fact, the chiral imidazolidinone catalyst afforded an excellent enantioselectivity (see above; Eq. 1.3) [4]. Many reactions have been developed involving the iminium ion as a key intermediate.