title: Cobalt-Catalyzed Annulation of Salicylaldehydes and Alkynes to Form Chromones and 4-Chromanones.
abstract: A unique cobalt(I)-diphosphine catalytic system has been identified for the coupling of salicylaldehyde (SA) and an internal alkyne affording a dehydrogenative annulation product (chromone) or a reductive annulation product (4-chromanone) depending on the alkyne substituents. Distinct from related rhodium(I)- and rhodium(III)-catalyzed reactions of SA and alkynes, these annulation reactions feature aldehyde C-H oxidative addition of SA and subsequent hydrometalation of the C=O bond of another SA molecule as common key steps. The reductive annulation to 4-chromanones also involves the action of Zn as a stoichiometric reductant. In addition to these mechanistic features, the Co(I) catalysis described herein is complementary to the Rh(I) - and Rh(III) -catalyzed reactions of SA and internal alkynes, particularly in the context of chromone synthesis.
authors.affiliation: Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore, 637371, Singapore.
authors.forename: Junfeng
authors.initials: J
authors.lastname: Yang
authors.affiliation: Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore, 637371, Singapore. nyoshikai@ntu.edu.sg.
authors.forename: Naohiko
authors.identifier: http://orcid.org/0000-0002-8997-3268
authors.initials: N
authors.lastname: Yoshikai
keywords: C-H functionalization; aldehydes; alkynes; cobalt; heterocycles