Discovering the role of N-heterocyclic carbene as hydrogen borrowing organocatalyst: metal-free, direct N -alkylation of amines with benzyl alcohols

A highly sustainable, bench-stable, N-heterocyclic carbene based organocatalyst has been demonstrated to impersonate the role of a transition-metal catalyst which had already been proven to be a milestone in the hydrogen borrowing reaction of amines with alcohols. In the present study, an inexpensiv...

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Veröffentlicht in:Organic Chemistry Frontiers 2023-01, Vol.10 (3), p.730-744
Hauptverfasser: Sharma, Richa, Rahaman T A, Abdul, Sen, Janmejaya, Mashevskaya, Irina V., Chaudhary, Sandeep
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Sprache:eng
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Zusammenfassung:A highly sustainable, bench-stable, N-heterocyclic carbene based organocatalyst has been demonstrated to impersonate the role of a transition-metal catalyst which had already been proven to be a milestone in the hydrogen borrowing reaction of amines with alcohols. In the present study, an inexpensive, non-toxic and commercially available N-heterocyclic carbene based organocatalyst i.e. , 1,3-bis(2,4,6-trimethylphenyl)imidazolinium chloride, has been demonstrated to show the efficient storage of hydrogen (in the form of 2H + + 2e − ) generated from alcohol oxidation (dehydrogenation step) temporarily through a SET process which then redelivers the same to the in situ generated imine intermediate (hydrogenation step), subsequently leading to the N -alkylation of amines. The established practical catalytic methodology works efficiently with a wide variety of aromatic and hetero-aromatic amines with high functional group tolerance in good to excellent yields. The protocol is operationally simple and is feasible under metal-free mild reaction conditions. The gram-scale synthesis and the intermolecular cyclization to 2-phenyl quinoline shed further light on the versatility of the developed protocol. Isolation of a radical intermediate trapped with a TEMPO free-radical scavenger together suggest a radical pathway. The SET (single electron transfer) to benzyl alcohol from NHC occurs through the formation of a cation radical intermediate. The hydrogenation–dehydrogenation of in situ generated N-heterocyclic carbene N -alkylation of several substituted amines with various substituted benzyl alcohols.
ISSN:2052-4129
2052-4110
2052-4129
2052-4110
DOI:10.1039/D2QO01522F