Facile and Efficient Pinacol Rearrangement Using Tungstophosphoric Acid (H3PW12O40) under Solvent-free Conditions

Abstract

The pinacol-pinacolone rearrangement is a typical 1,2rearrangement reaction of vicinal diols under acidic conditions which has been known since 1960. This rearrangement involving a dehydration and skeletal rearrangement is one of the earliest documented Wagner-Meerwein rearrangements in carbocation chemistry and has been used as a key step in organic synthesis. The intermediate cations are usually generated under Lewis or Bronsted acid conditions which often result in regioisomeric mixtures of ketones and aldehydes when unsymmetrical diols are used. Regioselectivity is determined by stability of the primary formed carbocation as well as migratory aptitude of the diol substituents which in turn are predicted by stereoelectronic factors. Because of the influence of these factors, it has not been possible to determine an unambiguous migatory order for substituents in the pinacol rearrangement. However, some general trends are discernible. Various studies on the relative migratory aptitudes in the pinacol rarrangement have been conducted. In general, the relative migratory tendency of aryl > alkenyl > hydride > alkyl is observed. Organic reactions are found to occur efficiently under solvent-free conditions and many examples have been reported so far. Heteropoly acids are the object of research for a long time. Much work has been done and there are several commercial processes utilizing heteropoly compounds. In this work we report the usage of heteropoly acids such as tungstophosphoric acid (H3PW12O40) as solid acid for pinacol rearrangement. It is a commercially available and cheap crystalline compound. Also, it is easy to handle and in comparison to the results obtained by carrying out the reaction using other acids, the reaction using tungstophosphoric acid (H3PW12O40) proceeds more efficiently under much milder conditions during a shorter reaction time. When a mixture of 1 : 3 molar ratio of diol and H3PW12O40 are ground at room temperature the products of rearrangement are obtained in high yield. Similar results are obtained by tungstosilicic acid but the conversion percents are lower and leads to formation of some byproducts. Heteropoly acids as solid acids in Pinacol rearrangement affects the migratory aptitude of groups as well as conversions and facility of the reaction process. As the results of Table 1 show the aryl anions migrate more easily than hydride does. Diol 1 led to aldehyde 1a, exclusively, which results from para-methoxyphenyl migration and the product of hydride migration was not obtained under the reaction condition. Diol 4 led to rearrangement products, aldehyde 4a and ketone 4b, which are obtained in ratio of 1:2.1 by paramethoxyphenyl and phenyl migrations, respectively. As it can be seen again no hydride migration is observed under the reaction condition. In the case of 4 because of the greater resonance stabilization of the carbonium ion (4c, Scheme 1), caused by methoxy group, gives mainly the thermodynamically controlled product 4b. Therefore, the reaction of 4 is accelerated because of the greater stability of the carbcatonic intermediate. Pinacol 5 is rearranged to aldehyde 5a and ketone 5b in the ratio of 1:0.5 which are obtained by paramethylphenyl and phenyl migrations, respectively. In the case of 5 the carbocation adjacent to the two phenyl groups is more stable and the reaction led to thermodynamically controlled product 5a. Diol 3 led to aldehyde 3a and ketone 3b in the ratio of 1:0.37. Aldehyde 3a is obtained by phenyl migration and ketone 3b may be obtained by phenyl and/or hydride migration. However, we have no evidence to show that if phenyl or hydride migration has occurred to produce ketone 3b but if we compare this reaction with entries 4 and 5, we may anticipate that the ketone is obtained by phenyl migration and the reaction is thermodynamically controlled. The same trend was observed for rearrangement of 6 which led to 6a in 95% yield. Anthrapinacol 8 gave bianthryl as a dehydration product under the reaction condition. In summary we have shown that pinacol rearrangement is easily carried out by tungstophosphoric acid (H3PW12O40) under solvent-free conditions. Aryl groups migrate more easily than hydride does and the reaction is thermodynamically controlled.