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whereas a Pd(II)/NO
- n catalytic system promotes its stereoselective oxidative
coupling and further oxidation of the resulting diene in a novel ?β,?γ-unsaturated
ketone. We have recently studied by in situ NMR the interaction of camphene
with Pd(II) complexes and discovered a Pd catalyzed reaction of its highly regio
and stereoselective deuteriation. Palladium hydride and palladium deuteride
intermediates have been detected by 1 H and 2 H NMR, respectively. Similar
mechanisms for the deuteriation and oxidative coupling of camphene have been
proposed, which involve the formation of ?σ-vinyl Pd hydride intermediates.
Cobalt catalyzed oxidation
A CoCl 2 catalyzed homogeneous autoxidation of limonene, ?α-pinene and
β?-pinene have been studied. In acetic acid, the reactions give a complex mixture
of products, while in acetonitrile, good chemoselectivities for epoxidation and/or
allylic oxidation products have been obtained. Limonene and α?-pinene give both
allylic oxidation and epoxidation products (ca. 1/1). On the other hand, oxidation
of β?-pinene leads essentially to allylic products, i.e., highly valuable pinocarveol,
pinocarvone, myrtenol and myrtenal, with selectivity of up to 85 %.
Later, we have developed heterogeneous versions of this system, which offer
not only important technological and environmental advantages, being very easily
separated and recovered compared to their homogeneous counterparts, but also
present better catalytic performance showing higher reaction selectivities (up
to 95 %). Sol-gel Co/SiO 2 and magnetites Fe 3-x M x O 4 doped with Co or Mn have
been used as active and recyclable heterogeneous catalysts for the oxidation of
some monoterpenes under solvent free conditions. The catalysts undergo no metal
leaching and can be recovered either by filtration or magnetically (in the case of
magnetites) and reused.
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