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.

本研究报道了一种可靠的钯催化芳基氯（aryl chlorides）与氨的氨基羰基化（amino-carbonylation）反应的机理信息。该反应以乙烯配合物1（ethylene complex 1）为催化剂，通过分离催化中间体与动力学测量获取了反应机理数据，其中包括首次报道的一氧化碳（carbon monoxide，缩写CO）存在下，芳基氯氧化加成（oxidative addition）至钯(0)配合物（palladium(0) complex）的机理数据。本研究合成了芳基钯（arylpalladium）与苯甲酰基钯卤化物（phenacyl-palladium halide）中间体，并通过测量这些中间体的生成与反应动力学，确定了CO存在下芳基氯与芳基溴氧化加成至二羰基钯(0)物种的反应机理，以及氨与Pd(II)苯甲酰基配合物（Pd(II) phenacyl complex）反应生成苯甲酰胺（benzamide）的反应机理。实验确定，芳基氯与芳基溴的氧化加成步骤的决速步为卤代芳烃与携带双齿膦配体与一个CO配体的三配位钯(0)物种的反应。一级13C动力学同位素效应（13C kinetic isotope effect）表明，该步骤涉及碳卤键（carbon–halogen bond）的断裂。本研究数据显示，苯甲酰基钯卤化物配合物与氨反应生成苯甲酰胺的路径为：氨可逆置换苯甲酰基钯氯化物中的氯离子，结合态氨发生去质子化生成苯甲酰基钯氨基配合物，随后经还原消除（reductive elimination）形成C-N键。与该机理相符的是，芳基钯氨基配合物与CO反应可生成相应的一级苯甲酰胺。本研究同时阐明了一条涉及Pd(I)二聚体（Pd(I) dimer）形成的催化剂失活路径（catalyst deactivation pathway）。