Controlling First-Row Catalysts: Amination of Aryl and Heteroaryl Chlorides and Bromides with Primary Aliphatic Amines Catalyzed by a BINAP-Ligated Single-Component Ni(0) Complex

First-row metal complexes
often undergo undesirable one-electron
redox processes during two-electron steps of catalytic cycles. We
report the amination of aryl chlorides and bromides with primary aliphatic
amines catalyzed by a well-defined, single-component nickel precursor
(BINAP)­Ni­(η2-NC-Ph) (BINAP = 2,2′-bis­(biphenylphosphino)-1,1′-binaphthalene)
that minimizes the formation of Ni­(I) species and (BINAP)2Ni. The scope of the reaction encompasses electronically varied aryl
chlorides and nitrogen-containing heteroaryl chlorides, including
pyridine, quinoline, and isoquinoline derivatives. Mechanistic studies
support the catalytic cycle involving a Ni(0)/Ni­(II) couple for this
nickel-catalyzed amination and are inconsistent with a Ni­(I) halide
intermediate. Monitoring the reaction mixture by 31P NMR
spectroscopy identified (BINAP)­Ni­(η2-NC-Ph) as the
resting state of the catalyst in the amination of both aryl chlorides
and bromides. Kinetic studies showed that the amination of aryl chlorides
and bromides is first order in both catalyst and aryl halide and zero
order in base and amine. The reaction of a representative aryl chloride
is inverse first order in PhCN, but the reaction of a representative
aryl bromide is zero order in PhCN. This difference in the order of
the reaction in PhCN indicates that the aryl chloride reacts with
(BINAP)­Ni(0), formed by dissociation PhCN from (BINAP)­Ni­(η2-NC-Ph), but the aryl bromide directly reacts with (BINAP)­Ni­(η2-NC-Ph). The overall kinetic behavior is consistent with turnover-limiting
oxidative addition of the aryl halide to Ni(0). Several pathways for
catalyst decomposition were identified, such as the formation of the
catalytically inactive bis­(amine)-ligated arylnickel­(II) chloride,
(BINAP)2Ni­(0), and the Ni­(I) species [(BINAP)­Ni­(μ-Cl)]2. By using a well-defined nickel complex as catalyst, the
formation of (BINAP)2Ni­(0) is avoided and the formation
of the Ni­(I) species [(BINAP)­Ni­(μ-Cl)]2 is minimized.