Mechanistic Studies of Palladium-Catalyzed Aminocarbonylation of Aryl Chlorides with Carbon Monoxide and Ammonia

Mechanistic information on a reliable,
palladium-catalyzed amino­carbonylation
of aryl chlorides with ammonia is reported. The reaction occurs with
ethylene complex 1 as catalyst, and mechanistic information
was gained by isolation of catalytic intermediates and kinetic measurements,
including the first mechanistic data on the oxidative addition of
aryl chloride to a palladium(0) complex in the presence of CO. Arylpalladium
and phenacyl­palladium halide intermediates were synthesized,
and kinetic measurements of the formation and reactions of these intermediates
were undertaken to determine the mechanism of the oxidative addition
of aryl bromides and chlorides to a Pd(0) dicarbonyl compound in the
presence of CO and the mechanism of the reaction of ammonia with a
Pd­(II) phenacyl complex to form benz­amide. The oxidative addition
of aryl chlorides and aryl bromides was determined to occur with rate-limiting
reaction of the haloarene with a three-coordinate Pd(0) species bearing
a bidentate phosphine and one CO ligand. A primary 13C
kinetic isotope effect suggested that this step involves cleavage
of the carbon–halogen bond. Our data show that the formation
of benz­amide from the reaction of phenacyl­palladium halide
complexes with ammonia occurs by a pathway involving reversible displacement
of chloride from a phenacyl­palladium chloride complex by ammonia,
deprotonation of the bound ammonia to form a phenacyl­palladium
amido complex, and reductive elimination to form the C–N bond.
Consistent with this mechanism, the reaction of an aryl palladium
amido complex with CO formed the corresponding primary benz­amide.
A catalyst deactivation pathway involving the formation of a Pd­(I)
dimer also was elucidated.