Group 13 chiral Lewis acids MCl2OR (M = B, Al, Ga) complexed with substituted enal dieneophiles have been used in asymmetric Diels-Alder reactions; however, the factors that influence stereochemical control are not yet well understood. A “formyl hydrogen bond” between a boron Lewis acid and the formyl hydrogen of an enal has been proposed to explain selectivity. New competing interactions between the enal vinylic alpha- and beta-hydrogens and the Group 13 chiral Lewis acids have been investigated. Second-order Moller-Plesset theory and M06-2X density functional with Dunning basis sets have been used to study these three bidentate complexes. It is found that the previously unconsidered, alpha-hydrogen effectively competes for complexation in the chiral Lewis acid complexes. Evidence is found for competition between the formyl and alpha-hydrogen interactions in aluminum and gallium chiral Lewis acid complexes, but not with boron. These two organizing elements explain experimental selectivity reported by Koga.

Group 13 chiral Lewis acid stereoselective control of enal Diels-Alder reactions

249th National Meeting (2015)

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