Formation of Enamines via Catalytic Dehydrogenation by Pincer-Iridium Complexes

Di-isopropylphosphino-substituted
pincer-ligated iridium catalysts
are found to be significantly more effective for the dehydrogenation
of simple tertiary amines to give enamines than the previously reported
di-t-butylphosphino-substituted species. It is also
found that the di-isopropylphosphino-substituted complexes catalyze
dehydrogenation of several β-functionalized tertiary amines
to give the corresponding 1,2-difunctionalized olefins. The di-t-butylphosphino-substituted species are ineffective for
such substrates; presumably, the marked difference is attributable
to the lesser crowding of the di-isopropylphosphino-substituted catalysts.
Experimentally determined kinetic isotope effects in conjunction with
DFT-based analysis support a dehydrogenation mechanism involving initial
pre-equilibrium oxidative addition of the amine α-C–H
bond followed by rate-determining elimination of the β-C–H
bond.