Regio- and stereoselectivity in the decatungstate photocatalyzed alkylation of alkenes by alkylcyclohexanes.
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.
Dondi
Daniele
D
Department of General Chemistry v. Taramelli 12, 27100 Pavia (Italy).
Ravelli
Davide
D
Department of Organic Chemistry v. Taramelli 10, 27100 Pavia (Italy), Fax: (+39) 0382-987323.
Fagnoni
Maurizio
M
Department of Organic Chemistry v. Taramelli 10, 27100 Pavia (Italy), Fax: (+39) 0382-987323.
Mella
Mariella
M
Department of Organic Chemistry v. Taramelli 10, 27100 Pavia (Italy), Fax: (+39) 0382-987323.
Molinari
Alessandra
A
Department of Chemistry, v. Borsari 46, 44100 Ferrara (Italy).
Maldotti
Andrea
A
Department of Chemistry, v. Borsari 46, 44100 Ferrara (Italy).
Albini
Angelo
A
Department of Organic Chemistry v. Taramelli 10, 27100 Pavia (Italy), Fax: (+39) 0382-987323.