81722-03-4Relevant articles and documents
A quantitative examination of the photoisomerization of some protonated phenols
Childs, Ronald F.,George, Baha E.
, p. 1343 - 1349 (2007/10/02)
The photoisomerization of a series of protonated, methyl substituted phenols to protonated bicyclohexenones has been examined.These reactions, which were carried out in CF3SO3H as a strong acid solvent at ambient temperatures, provide a convenient route to a variety of bicyclohexenones.The quantum yields for these photoisomerizations vary from 0.018 for protonated 3,5-dimethylphenol to 0.65 for protonated 2,6-dimethylphenol.This variation in efficiency can be understood in terms of a competition between ring opening, to regenerate the starting phenol, or cyclopropyl migration, to give product, of aninitially formed intermediate.
The use of homogeneous and heterogeneous Lewis acids to catalyse the photoisomerizations of phenols
Chadda, Satish K.,Childs, Ronald F.
, p. 3449 - 3455 (2007/10/02)
The reaction of methyl substituted phenols with CH2Cl2 solutions of Al2Br6 leads to the formation of two types of complexes.In both of these the aluminum is bound to the oxygen atom of the phenol but they differ in that the OH proton can either remain on oxygen, oxonium complex, or migrate to one, usually the para, ring carbon, keto complex.The equilibrium position between the two types of complex depends on the position of the methyl groups.Irradiation of a complex of the keto type leads initially to the formation of a complexed bicyclohexenone and subsequently to the formation of an isomeric complexed phenol.Only in the case of the tetramethylphenols is the yield of the bicyclic ketone high enough to warrant the use of this photoisomerization as a preparative procedure, although the technique can be used to isomerize a range of phenols to their 4-substituted isomers.The photoisomerizations are not restricted to the use of a homogeneus Lewis acid such as Al2Br6 but heterogeneus aluminosilicates can also be used.The adsorption of 2,3,5,6-tetramethylphenol, 1h, on an aluminosilicate was monitored quantitatively and it was shown that a monolayer was formed in which the phenol was at least partially present in the keto form.Irradiation of 1h in the presence of stirred slurries of the aluminosilicate led to the formation of 2,3,4,6-tetramethylphenol.No bicyclic ketones were present in the contacting solution but traces were detected on the catalyst when the irradiations were carried out on adsorbed material in the absence of a solvent.In the case of heterogeneous acids, the selectivity in the adsorption of the starting phenol and the photoproducts is important in determining the composition of the final mixture.
Photochemical Transformationos of Protonated Phenols. A One-Step Synthesis of Umbellulone from Thymol
Baeckstroem, Peter,Jacobsson, Ulla,Koutek, Bohumir,Norin, Torbjoern
, p. 3728 - 3732 (2007/10/02)
UV irradiation of thymol (7) at 254 or 300 nm in trifluoromethanesulfonic acid affords ten products, eight of which have been isolated and characterized.Four competitive processes are suggested to be operating in the formation of the photoproducts: (i) regioselective type A rearrangement leading to umbellulone (8, about 10percent, (ii) formal C2->C3 migration by type A rearrangement and ring opening which affords the principal products, 3-isopropyl-5-methylphenol (12, 17percent), (iii) intermolecular transalkylation leading to diisopropylphenols 13-15 (17percent), and (iv) formation ofpiperitenone (10, 5percent) initiated by hydrogen abstraction.A mechanism for the formation of 10 is proposed.Both para- and ortho-protonated 7 are suggested to be involved in product formation.
