Anhydrous zirconium(IV) sulfate and tin(IV) sulfate: solid Lewis acid catalysts in liquid-phase hydro-acyloxy-addition reactions?

Abstract

Anhydrous zirconium(IV) and tin(IV) sulfate [Zr(SO₄)₂ and Sn(SO₄)₂] were prepared from their corresponding metal tetrachlorides and fuming H₂SO₄/10% SO₃. Properties of interest for their potential use as heterogeneous Lewis acid catalysts, such as their susceptibility to undergo hydrolysis, are studied. In the presence of water the metal sulfates are converted into their metal oxides (ZrO₂ ·2H₂O and SnO₂·2H₂O) and the Brønsted acid H₂SO₄, which can also be catalytically active. This is illustrated by the solvent-free liquid-phase hydro-acyloxy-addition of acetic acid (2) to camphene (1) (molar ratio 1: 2 = 1:1) giving the fragrance isobornyl acetate (3). Both anhydrous Zr(SO₄)₂ and Sn(SO₄)₂ are inactive as heterogeneous solid Lewis acid catalysts; no conversion of 1 into 3 is found. Upon addition of water, however, 1 is efficiently converted into 3. Moreover, the conversion of 1 into 3 continues even after removal of the solid Lewis acid catalyst by filtration. These results show that after addition of water the reaction becomes homogeneously catalysed by dissolved H₂SO₄.