Catalysis of gas and liquid phase ionic and radical rearrangements of alpha- and beta-pinene by metal(IV) phosphate polymers

Gas-phase rearrangements of alpha- and beta-pinene catalysed by metal( IV) phosphate polymers have been studied. The phosphates catalyse carbocation rearrangements in both substrates as expected, but catalysis of a modified radical rearrangement of alpha-pinene to yield alpha-pyronene has also been observed. Reduction in the acidity of the catalysts by exchange of protons with copper or ammonium ions, or by heating them to convert phosphates into pyrophosphates suppresses the carbocation reaction whilst having little effect on the rate of the radical reaction. Reasons for the alteration in the mode of the radical reaction, which normally yields ocimene and alloocimene, are discussed.