Iminium activation conjugate Friedel-Crafts

As indicated from computational studies, the catalyst-activated iminium ion MM3-2 was expected to form with only the (E)-conformation to avoid nonbonding interactions between the substrate double bond and the gem-dimethyl substituents on the catalyst framework. In addition, the benzyl group of the imidazolidinone moiety should effectively shield the iminium-ion Si-face, leaving the Re-face exposed for enantioselective bond formation. The efficiency of chiral amine 1 in iminium catalysis was demonstrated by its successful application in several transformations such as enantioselective Diels-Alder reactions [6], nitrone additions [12], and Friedel-Crafts alkylations of pyrrole nucleophiles [13]. However, diminished reactivity was observed when indole and furan heteroaromatics where used for similar conjugate additions, causing the MacMillan group to embark upon studies to identify a more reactive and versatile amine catalyst. This led ultimately to the discovery of the second-generation imidazolidinone catalyst 3 (Fig. 3.1, bottom) [14],... 

Friedel-Crafts-type vinylogous conjugate addition of 2-vinyl pyrroles to enals was achieved site-selectively with the use of diphenylprolinol trimethylsilyl ether 14 as an iminium-enaniine activation catalyst (Scheme 41) [71]. Stepwise, formal [2-1-2] cycloaddition would be a plausible outcome of the reaction for constructing stereochemically enriched cyclobutanes. The polarity of the solvent had a critical impact on the catalytic efficiency. Trace amounts or none of the desired product was formed when less polar toluene or dichloromethane was used. Increasing the polarity of the solvent led to enhancement of the turnover frequency of 14 the polar protic solvent, ethanol, was optimal. 

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