The mechanism of the Ir(III) and Rh(III)-mediated C-N coupling reaction, which is the key step of catalytic C-H amidation, was investigated in an integrated experimental and computational study. Novel amidating agents containing a 1,4,2-dioxazole moiety allowed for designing a stoichiometric version of the catalytic C-N coupling reaction and giving access to reaction intermediates that reveal details about each step of the reaction. Both DFT and kinetic studies strongly point to a mechanism where the M(III) complex engages the amidating agent via oxidative coupling to form a M(V)-imido intermediate, which then undergoes migratory insertion to afford the final C-N coupled product. For the first time, the stoichiometric versions of the Ir and Rh-mediated amidation reaction were compared systematically to each other. Iridium reacts much faster than rhodium (~ 1100 times at 6.7 °C) with the oxidative coupling being so fast that the activation of the initial Ir(III)-complex becomes rate-limiting. In the case of Rh, the Rh-imido formation step is rate-limiting. These qualitative difference stems from a unique bonding feature of the dioxazole moiety and the relativistic contraction of the Ir(V), which affords much more favorable energetics for the reaction. For the first time, a full molecular orbital analysis is presented to rationalize and explain the electronic features that govern this behavior.
In place of functional groups that impose different inductive effects, we immobilize molecules carrying thiol groups on a gold electrode. By applying different voltages, the properties of the immobilized molecules can be tuned. The base-catalyzed saponification of benzoic esters is fully inhibited by applying a mildly negative voltage of –0.25 volt versus open circuit potential. Furthermore, the rate of a Suzuki-Miyaura cross-coupling reaction can be changed by applying a voltage when the arylhalide substrate is immobilized on a gold electrode. Finally, a two-step carboxylic acid amidation is shown to benefit from a switch in applied voltage between addition of a carbodiimide coupling reagent and introduction of the amine.
While numerous organo(metallic)catalyst systems were documented for dearomative hydroboration of N‐aromatics, alkoxide base catalysts have not been disclosed thus far. Described herein is the first example of alkoxide‐catalyzed hydroboration of N‐heteroaromatics including pyridines, providing a broad range of reduced N‐heterocycles with high efficiency and selectivity. Mechanistic studies revealed an unprecedented counterintuitive dearomatization pathway, in which 1) pyridine‐BH3 adducts undergo a hydride attack by alkoxyborohydrides, 2) in situ generated BH3 serves as a catalytic promoter, and 3) 1,4‐dihydropyridyl borohydride is in a predominant resting state.
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