Preparation of camphene

Many early literature reports describe the use of clays for this transformation. An interesting example is a 1957 paper by Wystrach et al. [3] who described the isomerization of 1 over 1 % calcined Attapulgus clay to yield 2 and 3 as the major products, and also postulated a mechanism for this transformation [4]. It is also worth noting that they were the first group to realize the importance of the stereochemistry of adsorption in studies relating to isomerization over clays. 

Ivanova, Borovskaya, and Rudakov investigated the liquid phase isomerization of yS-pinene in the presence of the solid-acid catalysts titanium dioxide, and alumi- 

Because acidified titanium oxide is the catalyst usually employed commercially for the transformation of 1 into 2 [8] there has been much investigation of this catalytic system [9]. A 1995 paper by Stefanis et al. [10] reported an investigation of the reaction of 1 in several alumina-pillared clays (PILCs montmorillonite- and beidellite-based, and their and Ca -exchanged congeners) under Lewis acid conditions (solid is activated by heat to remove all water). The results were compared with those obtained by use of medium-pore zeolites USY, NH4+-ZSM-5, and H-mordenite. Conversion to 2 50% was always observed. The aim of the work was to clarify differences between site availability and acidity for the two types of solid. 

A recent study [11] into the isomerization of 1 into 2 and 3 using both aeid treated polycation-exchanged bentonite clays and their non-polyeation-exchanged equivalents found that use of polycation-exehanged elays significantly increased the yield of 2. For further references related to the use of polycation-exchanged clays the reader is referred to the diseussion section in Ref. 11. 

Another recent publication concerning the isomerization of 1 into 2 describes a selective liquid-phase reaetion over dealuminated mordenites, Y-zeolites, and a 13 % alumina amorphous aluminosilicate [12]. It found that over microporous zeolite structures the main products were 2 and 3, but that the wider the pore diameter of the catalyst the greater the amount of unwanted and unidentifiable products obtained. This was worst for the amorphous aluminosilicate, which has a meso-porosity of 90 %. Interesting selectivity was noticed when the dealuminated mordenites were used the combined yield of 2 4- 3 was 68 % and the ratio 2/(2 4- 3) was 0.54. 

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