title: Activating tert-butyl hydroperoxide by chelated vanadates for stereoselectively preparing sidechain-functionalized tetrahydrofurans.
abstract: tert-Butyl hydroperoxide (TBHP) stereoselectively oxidizes substituted 4-pentenols, when activated by (ethyl)[cis-(piperidine-2,6-diyl)dimethyl] vanadates. The reaction affords (tetrahydrofuran-2-yl)methanols in up to 89% yield, and in stereoselectivity ranging between moderate (cis:trans=32:68) to excellent (>99:1). Correlating structures of 4-pentenols, differing by substitution at tetragonal and trigonal stereocenters, to configuration of products obtained from oxidative cyclization provides a reaction model explaining the origin of stereoselectivity by (i) intramolecular oxygen atom transfer to (ii) a chair-like folded alkenol, being (iii) hydrogen-bonded to one of the two aminodiolate oxygens of the chelated vanadate, having (iv) substituents in the chair-like transition structure preferentially aligned equatorially. Substituents at trigonal stereocenters improve 2,5-cis- and 2,4-trans-selectivity for oxidative 4-pentenol cyclization in case of (Z)-configuration. An (E)-substituent does not alter selectivity exerted by a terminal (Z)-substituent of similar steric size. Larger (E)-groups increase the fraction of 2,5-trans-cyclized products. The reaction model additionally implements results from vanadium-51 NMR spectroscopy and density functional theory. According to theory, the (dialkoxy)(oxo)vanadium substituent exerts in the preferred end-on conformation almost no effect on structure and bonding of the peroxide group in tert-butylperoxy vanadates. Changing conformation to a higher in energy side-on arrangement puts the vanadate-bound tert-butylperoxy group into a position to serve in a concerted reaction as combined electron acceptor and oxygen atom donor.
authors.affiliation: Fachbereich Chemie, Organische Chemie, Technische Universität Kaiserslautern, Erwin-Schrödinger-Straße, D-67663 Kaiserslautern, Germany.
authors.forename: Maike
authors.initials: M
authors.lastname: Dönges
authors.affiliation: Fachbereich Chemie, Organische Chemie, Technische Universität Kaiserslautern, Erwin-Schrödinger-Straße, D-67663 Kaiserslautern, Germany.
authors.forename: Matthias
authors.initials: M
authors.lastname: Amberg
authors.affiliation: Fachbereich Chemie, Organische Chemie, Technische Universität Kaiserslautern, Erwin-Schrödinger-Straße, D-67663 Kaiserslautern, Germany.
authors.forename: Mark
authors.initials: M
authors.lastname: Niebergall
authors.affiliation: Fachbereich Chemie, Organische Chemie, Technische Universität Kaiserslautern, Erwin-Schrödinger-Straße, D-67663 Kaiserslautern, Germany. Electronic address: hartung@chemie.uni-kl.de.
authors.forename: Jens
authors.initials: J
authors.lastname: Hartung
mesh_terms: D002614:Chelating Agents / Q000737:chemistry; D056831:Coordination Complexes / Q000737:chemistry; D005663:Furans / Q000737:chemistry*; D014638:Vanadates / Q000737:chemistry*; D020122:tert-Butylhydroperoxide / Q000737:chemistry*
chemical_list: D002614:Chelating Agents; D056831:Coordination Complexes; D005663:Furans; D014638:Vanadates; D020122:tert-Butylhydroperoxide
keywords: Alkyl hydroperoxide; Aminodiolate ligand; Density functional theory; Oxidation catalysis; Stereoselective cyclization; Vanadium