Kinetics and Mechanisms of the Gold-Catalyzed Hydroamination of Axially Chiral 1-Aryl-1,2-butadienes with Aniline

Abstract

The mechanisms of the hydroamination of enantiomerically enriched, axially chiral (R)-1-aryl-1,2-butadienes (R)-(4-C 6 H 4 R)C(H)=C=C(H)-Me [R = Ph (1a), OMe (1b)] with aniline catalyzed by (IPr)AuOTf have been investigated employing a combination of kinetic and stereochemical analyses and in situ spectroscopy of catalytically active mixtures. These experiments were consistent with mechanisms involving essentially irreversible displacement of aniline from [(IPr)Au(NH 2 Ph)] + by allene (R)-1 to form the enantiomerically enriched gold−π-allene intermediate {(R)-(IPr)Au[η 2 -(R)-ArC(H)=C=C(H)-Me]} + [(R)-I], followed by unimolecular stereomutation to form the racemic gold−π-allene intermediate rac-I. Intermediate rac-I, in turn, undergoes either allene displacement to form racemic allene rac-1 or C−N bond formation/protodeauration to form racemic (E)-N-(4-aryl-3-buten-2-yl)aniline (rac-2), favoring the former pathway by a factor of ∼2.4. As such, approximately 30% of the racemic product rac-2 is formed directly from (R)-1 without the intermediacy of rac-1.