Stable Cobalt Organoazide Complexes and Their Reactivity in Stoichiometric and Catalytic C–H Amination

Catalytic C–H amination using
organic azides is
attractive
for C–N bond formation. Here, Co(HMDS)2 has been
investigated as a simple yet appealing catalyst for this reaction.
Upon exposure to organoazides, Co(HMDS)2 forms an organoazide
adduct that was characterized spectroscopically and crystallographically.
With adamantyl-N3 as an organoazide that cannot undergo
intramolecular C–H bond amination, gradual interligand C–H
bond activation occurred without any spectroscopically detectable
intermediate. Photocrystallography uncovered the imido intermediate
unambiguously in the solid state. Using (4-azido-4-methylpentyl)-benzene
as a substrate induced intramolecular C–H amination and formation
of a pyrrolidine adduct. Under catalytic conditions, turnover frequencies
(TOFs) up to 300 h–1 were reached, three times higher
than that of the fastest iron catalyst known to date. However, turnover
numbers (TONs) stalled at a mediocre 35, which was attributed to catalyst
decomposition to a tetrazido complex according to spiking experiments.
Pyridine as a solvent mitigated catalyst decomposition by competitive
bonding to the active site and gave ∼90 TON. DFT calculations
suggest that the high TOF originates from a lower barrier of N2 loss compared to iron (29.1 kcal/mol vs 31.7 kcal/mol). Moreover,
the catalytically competent species was predicted to be the bis(pyrrolidyl)
cobalt complex, generated after two turnovers by H(HMDS) release.