106539-32-6Relevant articles and documents
Aryl Cations from Aromatic Halides. Photogeneration and Reactivity of 4-Hydroxy(methoxy)phenyl Cation
Protti, Stefano,Fagnoni, Maurizio,Mella, Mariella,Albini, Angelo
, p. 3465 - 3473 (2007/10/03)
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 π 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).
Electron-transfer oxidation of chlorophenols by uranyl ion excited state in aqueous solution. Steady-state and Nanosecond flash photolysis studies
Sarakha, Mohamed,Bolte, Michele,Burrows, Hugh D.
, p. 3142 - 3149 (2007/10/03)
The oxidation of chlorophenols by photoexcited uranyl ion was studied in aqueous solution at concentrations where the ground-state interactions were negligible. Nanosecond flash photolysis showed that a clean electron-transfer process from the chlorophenols to the excited uranyl ion is involved. This is suggested to lead to the formation of a U(V)/chlorophenoxyl radical pair complex. The efficiency of this charge-transfer process is unity for the three chlorophenols. However, low product yields suggest that in the absence of oxygen, back electron transfer, both within the radical pair and from separated uranium(V) to phenoxyl radicals, appears to be the major reaction pathway. In the presence of oxygen the quantum yields of disappearance of chlorophenol and of photoproduct formation increased. This leads to the conclusion that oxygen favors reaction with uranium(V) and/or the uranium(V) - phenoxyl radical pair, leading to the formation of the superoxide anion and its conjugate acid, HO2*, which then regenerate UO22+. Based on this, a catalytic cycle for chlorophenol photooxidation involving uranyl ion and molecular oxygen is proposed.
Formation and reactivity of 4-oxocyclohexa-2,5-dienylidene in the photolysis of 4-chlorophenol in aqueous solution at ambient temperature
Grabner, Gottfried,Richard, Claire,K?hler, Gottfried
, p. 11470 - 11480 (2007/10/02)
Nanosecond laser flash photolysis of an aqueous solution of 4-chlorophenol (λexc = 266 nm) produces, at pulse end, a transient with absorption maxima at 384, 370, and ca. 250 nm; upon addition of an H-donor such as 2-propanol, this spectrum is converted into that of the phenoxyl radical (λmax = 400 and 385 nm), and in presence of O2, it is converted into a transient with a broad absorption band peaking at 460 nm. This reaction behavior can be understood by assuming formation of the carbene, 4-oxocyclohexa-2,5-dienylidene, by elimination of HCl from excited 4-chlorophenol; the pulse end transient spectrum is assigned to this species, while the 460 nm band is assigned to benzoquinone O-oxide formed by addition of O2 to the carbene. Both phenoxyl radical and benzoquinone O-oxide are produced upon photolysis of 4-chlorophenol in neat alkanols as well. On the other hand, photolysis in n-hexane yields the triplet-triplet absorption, which is absent in polar solvents, and no indication of carbene formation. It can be concluded that the primary step of 4-chlorophenol photolysis in aqueous or alcoholic solution is heterolytic C-Cl bond scission; a quantum yield of 0.75 is determined for it in neutral or acid aqueous medium upon excitation at 266 nm. Photolysis of chlorophenolate produces the same transients, but with a markedly lower yield, and, in addition, eaq- and 4-chlorophenoxyl radicals. The proposed reaction mechanism provides a straightforward explanation of the results of photoproduct analysis, published by previous authors as well as contributed in the present work. In particular, formation of p-benzoquinone in the presence of O2 can be accounted for by intermediate formation of benzoquinone O-oxide. Production of 4-oxocyclohexa-2,5-dienylidene with high yield allows, for the first time, extensive investigation of the kinetics and mechanism of the reactions of a carbene in an aqueous environment. In the present work, we have studied (a) the addition reaction with O2 on the one hand and with halides on the other; (b) H abstraction reactions with alkanols; (c) reaction with 4-chlorophenol itself; and (d) reaction with H2O. The rate constants for reaction with O2 (3.5 × 109 M-1 s-1) and with I- (4.6 × 109 M-1 s-1) are close to the diffusion-controlled limit, whereas reactions with Br- (6.8 × 107 M-1 s-1) and Cl- (5 M-1 s-1) are slower. Rate constants for reaction with alkanols follow the pattern known for their reactions with radicals, with values ranging from 5 × 105 M-1 s-1 for tert-butyl alcohol to 1.9 × 107 M-1 s-1 for 2-butanol. All these observations are consistent with the triplet character of the carbene. A rate constant of 1.5 × 103 M-1 s-1 has been determined for reaction with H2O. This reaction is not accompanied by formation of OH radicals; it is concluded that it proceeds by insertion into the O-H bond rather than by O-H cleavage. The exceptional stability of the carbene in aqueous solution is thus mainly attributed to the high barrier for O-H rupture in the water molecule. Additionally, a specific carbene-H2O interaction is revealed by semiempirical calculations, which could contribute to energetic and orientational hindrance of the reaction. Further theoretical results support the interpretation of both spectroscopic and kinetic properties of the carbene.
