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 |
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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. |
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ISSN: | 2052-4129 2052-4110 2052-4129 2052-4110 |
DOI: | 10.1039/D2QO01522F |