Facile and Fast Pinacol Rearrangement by AlCl3 in the Solid State

A facile and efficient synthetic procedure for effecting the pinacol rearrangement catalyzed by AlCl3 in the absence of solvent is developed. The rearrangement product is obtained at room temperature in a few minutes and in almost quantitative yield. Benzylic pinacols rearrange under these conditions, while aliphatic pinacols do not react.


Introduction
Organic reactions are found to occur efficiently in the solid state and many examples have been reported so far [1][2][3][4][5]. The pinacol rearrangement is usually carried out under drastic conditions, like heating in H 2 SO 4 . Toda found that the reaction proceeds faster and more selectively in the solid state by using organic acids like CCl 3 CO 2 H and p-TsOH [6]. It is possible to use a strong Lewis acid for the pinacol rearrangement and to the best of our knowledge, there has been no report on pinacol rearrangement by using a Lewis acid like AlCl 3 . This reagent has previously been used for Friedel-Crafts alkylation and acylation in the solid state [7] and also in Beckman rearrangements [8] under similar conditions. The novel utility of aluminium chloride as an efficient reagent for pinacol rearrangement is reported in this paper.

Results and Discussion
Benzylic pinacols react with AlCl 3 to give the corresponding rearrangement product in almost quantitative yield after only a few minutes. The reaction is easily carried out in an agate mortar and the reaction conditions and the subsequent workup are simple and quick. The pinacol compound is mixed with AlCl 3 in a ratio of 1:3 respectively and is then thoroughly ground at room temperature. A fast reaction happens, HCl evolves and a colored mass is immediately obtained. Workup of the reaction mixture by adding dilute H 2 SO 4 and extracting with chloroform affords the rearrangement product in good yield. By adopting the above method, benzylic pinacol compounds have been rearranged to the corresponding pinacolones and it is found that the method is much better than all the methods known so far for its simplicity and high conversion. The list of substrates that have been rearranged by this method is given in Table 1. For anthrapinacol, entry 7, the only product detected was bianthryl which shows that this procedure can be used in systems that aromatize by dehydration. For tetralone pinacol, entry 5, the product of rearrangement was obtained in fairly good yield compared to the procedures described previously [9]. In series of compounds 2-6 it was found that the selectivity is affected by AlCl 3 and for entry 6 the rearranged product, 8, undergoes keto-enol tautomerization to give 10phenyl-9-phenanthrol, 6 (Scheme 1).

Scheme 1
Entry 8 shows that for rearrangement under these conditions, the existence of an aryl group adjacent to carbon which bears the OH group is necessary. The reaction is expected to proceed via the formation of an aluminium adduct 9 which is formed by reaction of aluminium chloride with the OH group followed by liberation of HCl gas. In the second step the rearrangement takes place by the migration of alkyl, aryl or hydride groups (Scheme 2).

Acknowledgments
Parviz Rashidi Ranjbar is grateful to the Research Council of Tehran University for financial support. Professor Tony Durst is highly acknowledged for providing us access to NMR and HRMS facilities.

General
Solvents and reagents for the preparation of starting materials were used as received from Aldrich. 1 H-and 13 C-NMR spectra were recorded on Varian Unity Plus instruments (500 and 200 MHz). FT-IR spectra was obtained using a Shimadzu 4300 spectrophotometer.

General synthetic procedure
A mixture of the appropriate pinacol compound and anhydrous aluminium chloride (Aldrich, 99.99 %) was thoroughly ground in an agate mortar for a few minutes under an efficient hood. Hydrogen chloride is evolved violently. The reaction mixture was worked up by adding H 2 SO 4 (2N) and extracting with chloroform. The organic layer was dried over Na 2 SO 4 and the solvent was removed in vacuo to give the products. Pinacol starting materials were prepared by the methods of Toda [10] and Khurana [11].