The use of light to induce dehalogenation of chlorophenols received much attention being both a clean procedure for the destruction of widespread pollutants1 and a tool for the production of intermediates useful for synthetic applications.2 There is general agreement that the mechanism involves heterolytic dissociation of the C-Cl bond with production of an aryl cation.3,4 In water fast deprotonation of the cation leads to the formation of a carbene, which is the key intermediate, as the final products result mainly from the interaction of the carbene with oxygen, solvent and reactant itself. In the presence of H-donors the carbene is believed to reduce to phenoxyl radical and produce the corresponding phenol.3 However product studies indicate that the aryl cation also may be trapped by suitable nucleophiles and the importance of such path is influenced by the medium acidity and the nature of the additive. Aryl cations derived from chloroanilines were shown to react with ? nucleophiles.5 In the frame of a comprehensive study of these intermediates, we present a laser flash photolysis investigation of the transient species produced in the photodechlorination of 4-chloro-2,6-dimethyl-phenol in methanol (MeOH) and in trifluoroethanol (TFE), in absence and presence of allyl-trimethyl-silane (ATMS) as nucleophilic additive. In accord with steady state results, time resolved data support the involvement of either the 2,6-dimethyl-4-phenyl cation or 2,6-dimethyl-4-oxoyclohexadienylene in the formation of the products, depending on the conditions. This conclusion is further confirmed by laser flash photolysis and preparative studies on 4-chloro-2,6-dimethyl-O-(SiMe2-tBu)-phenol, in which the aryl cation formed by photodechlorination cannot convert to carbene.
A laser-flash photolysis study of the photodechlorination of 4-chloro-2,6-dimethyl-phenol in alcoholic media
I Manet;S Monti;
2003
Abstract
The use of light to induce dehalogenation of chlorophenols received much attention being both a clean procedure for the destruction of widespread pollutants1 and a tool for the production of intermediates useful for synthetic applications.2 There is general agreement that the mechanism involves heterolytic dissociation of the C-Cl bond with production of an aryl cation.3,4 In water fast deprotonation of the cation leads to the formation of a carbene, which is the key intermediate, as the final products result mainly from the interaction of the carbene with oxygen, solvent and reactant itself. In the presence of H-donors the carbene is believed to reduce to phenoxyl radical and produce the corresponding phenol.3 However product studies indicate that the aryl cation also may be trapped by suitable nucleophiles and the importance of such path is influenced by the medium acidity and the nature of the additive. Aryl cations derived from chloroanilines were shown to react with ? nucleophiles.5 In the frame of a comprehensive study of these intermediates, we present a laser flash photolysis investigation of the transient species produced in the photodechlorination of 4-chloro-2,6-dimethyl-phenol in methanol (MeOH) and in trifluoroethanol (TFE), in absence and presence of allyl-trimethyl-silane (ATMS) as nucleophilic additive. In accord with steady state results, time resolved data support the involvement of either the 2,6-dimethyl-4-phenyl cation or 2,6-dimethyl-4-oxoyclohexadienylene in the formation of the products, depending on the conditions. This conclusion is further confirmed by laser flash photolysis and preparative studies on 4-chloro-2,6-dimethyl-O-(SiMe2-tBu)-phenol, in which the aryl cation formed by photodechlorination cannot convert to carbene.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
