A general nickel-catalyzed hydroamination of 1,3-dienes by alkylamines: Catalyst selection, scope, and mechanism
A simple colorimetric assay of various transition-metal catalysts showed that the combination of DPPF, Ni(COD)2, and acid is a highly active catalyst system for the hydroamination of dienes by alkylamines to form allylic amines. The scope of the reaction is broad; various primary and secondary alkylamines react with 1,3-dienes in the presence of these catalysts. Detailed mechanistic studies revealed the individual steps involved in the catalytic process. These studies uncovered unexpected thermodynamics for the addition of amines to π-allyl nickel complexes: instead of the thermodynamics favoring the reaction of a nickel allyl with an amine to form an allylic amine, the thermodynamics favored reaction of a nickel(0) complex with allylic amine in the presence of acid to form a Ni(II) allyl. The realization of these thermodynamics led us to the discovery that nickel and some palladium complexes in the presence or absence of acid catalyze the exchange of the amino groups of allylic amines with free amines. This exchange process was used to reveal the relative thermodynamic stabilities of various allylic amines. In addition, this exchange reaction leads to racemization of allylic amines. Therefore, the relative rate for C−N bond formation and cleavage influences the enantioselectivity of diene hydroaminations.
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