title: Metalloradical approach to 2H-chromenes.
abstract: Cobalt(III)-carbene radicals, generated through metalloradical activation of salicyl N-tosylhydrazones by cobalt(II) complexes of porphyrins, readily undergo radical addition to terminal alkynes to produce salicyl-vinyl radical intermediates. Subsequent hydrogen atom transfer (HAT) from the hydroxy group of the salicyl moiety to the vinyl radical leads to the formation of 2H-chromenes. The Co(II)-catalyzed process can tolerate various substitution patterns and produces the corresponding 2H-chromene products in good isolated yields. EPR spectroscopy and radical-trapping experiments with TEMPO are in agreement with the proposed radical mechanism. DFT calculations reveal the formation of the salicyl-vinyl radical intermediate by a metalloradical-mediated process. Unexpectedly, subsequent HAT from the hydroxy moiety to the vinyl radical leads to formation of an o-quinone methide intermediate, which dissociates spontaneously from the cobalt center and easily undergoes an endocyclic, sigmatropic ring-closing reaction to form the final 2H-chromene product.
authors.affiliation: Homogeneous Catalysis Group, van 't Hoff Institute for Molecular Sciences, University of Amsterdam , Science Park 904, 1098 XH Amsterdam, The Netherlands.
authors.forename: Nanda D
authors.initials: ND
authors.lastname: Paul
authors.forename: Sutanuva
authors.initials: S
authors.lastname: Mandal
authors.forename: Matthias
authors.initials: M
authors.lastname: Otte
authors.forename: Xin
authors.initials: X
authors.lastname: Cui
authors.forename: X Peter
authors.initials: XP
authors.lastname: Zhang
authors.forename: Bas
authors.initials: B
authors.lastname: de Bruin
mesh_terms: D001578:Benzopyrans / Q000737:chemistry*; D002384:Catalysis; D003035:Cobalt / Q000737:chemistry*; D005609:Free Radicals / Q000737:chemistry; D008697:Methane / Q000031:analogs & derivatives* / Q000737:chemistry; D008958:Models, Molecular; D008968:Molecular Conformation; D009942:Organometallic Compounds / Q000737:chemistry*; D013237:Stereoisomerism; D013379:Substrate Specificity
chemical_list: D001578:Benzopyrans; D005609:Free Radicals; D009942:Organometallic Compounds; C030011:carbene; D003035:Cobalt; D008697:Methane