Aryl cations from aromatic halides. Photogeneration and reactivity of 4-hydroxy(methoxy)phenyl cation.
The photochemistry of 4-chlorophenol (1) and 4-chloroanisole (2) has been examined in a range of solvents and found to lead mainly to reductive dehalogenation, through a homolytic path in cyclohexane and a heterolytic path in alcohols. Heterolysis of 1 and 2 in methanol and 2,2,2-trifluoroethanol offers a convenient access to triplet 4-hydroxy- and 4-methoxyphenyl cations. These add to pi nucleophiles, viz., 2,3-dimethyl-2-butene, cyclohexene, and benzene, giving the arylated products in medium to good yields. Wagner-Meerwein hydride and alkyl migration are evidence for the cationic mechanism of the addition to alkenes. Arylation (with no rearrangement) was obtained to some extent also in nonprotic polar solvents such as MeCN and ethyl acetate, reasonably via an exciplex and with efficiency proportional to the nucleophilicity of the trap (2,3-dimethyl-2-butene > cyclohexene >> benzene).
Protti
Stefano
S
Dipartimento di Chimica Organica, Università, V. Taramelli 10, 27100 Pavia, Italy.
Fagnoni
Maurizio
M
Mella
Mariella
M
Albini
Angelo
A