The use of homogeneous and heterogeneous Lewis acids to catalyse the photoisomerizations of phenols

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 bicyclo[3.1.0]hexenone 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 homogeneous Lewis acid such as Al2Br6 but heterogeneous 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