title: Regio- and stereoselectivity in the decatungstate photocatalyzed alkylation of alkenes by alkylcyclohexanes.
abstract: Tetrabutylammonium decatungstate (WO) photocatalyzes radical alkylation reactions starting directly from alkanes. When using tert-butylcyclohexane and methylcyclohexane as the radical precursors, the addition to electrophilic alkenes (beta,beta-dialkylmethylenemalononitriles, acrylonitrile, methyl acrylate, methyl vinyl ketone) is regioselective and exclusively gives 3- and 4-substituted cyclohexyl adducts, with no significant functionalization of the other positions. Furthermore, when a beta,beta-disubstituted alkene is used, the reaction is stereoselective (cis stereochemistry for the 1,3-cyclohexane derivatives and trans for the 1,4 isomers). Some of the reactions have also been carried out by using benzophenone as the photocatalyst, giving the same product distribution. However, the decatungstate anion is a superior catalyst from a preparative point of view, because it is efficient at low concentrations (0.002 m, 2 mol %) and allows for a simple work up. From a mechanistic point of view, the role of both the alkyl radicals and the radical adducts has been assessed by trapping experiments in the presence of suitable additives (alpha-phenyl-N-tert-butylnitrone, PBN, and 2-methyl-2-nitrosopropane, MNP) and by EPR spectroscopic detection of the resulting nitroxides in solution. Furthermore, trapping by the nitroxide TEMPO (TEMPO = 2,2,6,6-tetramethylpiperidine N-oxide) gives O-(tert-butylcyclohexyl)hydroxylamines, again as only the 3- and the 4-substituted isomers. We conclude that the observed regioselective activation originates from the initial hydrogen abstraction step (the statistically corrected ratio for positions 3 and 4 ranges from 1.1 to 1.35 for all of the trapping products). The selectivity is mainly due to steric hinderence.
authors.affiliation: Department of General Chemistry v. Taramelli 12, 27100 Pavia (Italy).
authors.forename: Daniele
authors.initials: D
authors.lastname: Dondi
authors.affiliation: Department of Organic Chemistry v. Taramelli 10, 27100 Pavia (Italy), Fax: (+39) 0382-987323.
authors.forename: Davide
authors.initials: D
authors.lastname: Ravelli
authors.affiliation: Department of Organic Chemistry v. Taramelli 10, 27100 Pavia (Italy), Fax: (+39) 0382-987323.
authors.forename: Maurizio
authors.initials: M
authors.lastname: Fagnoni
authors.affiliation: Department of Organic Chemistry v. Taramelli 10, 27100 Pavia (Italy), Fax: (+39) 0382-987323.
authors.forename: Mariella
authors.initials: M
authors.lastname: Mella
authors.affiliation: Department of Chemistry, v. Borsari 46, 44100 Ferrara (Italy).
authors.forename: Alessandra
authors.initials: A
authors.lastname: Molinari
authors.affiliation: Department of Chemistry, v. Borsari 46, 44100 Ferrara (Italy).
authors.forename: Andrea
authors.initials: A
authors.lastname: Maldotti
authors.affiliation: Department of Organic Chemistry v. Taramelli 10, 27100 Pavia (Italy), Fax: (+39) 0382-987323.
authors.forename: Angelo
authors.initials: A
authors.lastname: Albini