The reactions of metal complexes of monoterpenes continue to be actively explored and many specific examples will be found in later sections. Of general interest are the dimerization of π-allyl-Pd complexes of α- and β-pinenes and of carvone that are effected by irradiation at 366 nm and the thermal decompositions of (π-allyl)nickel halide complexes of, ., isoprene (33), to form myrcene. Hydrosilylation of 1,3-dienes ( . isoprene, myrcene, ocimene) was found to be a regiospecific 1,4-addition for Pd complexes but followed the alternative route for Rh compounds; a good discussion is appended. A series of dimers of isoprene bonded at 1–2, 1–3, 1–4, 2–4, 3–4, and 4–4 positions were prepared by suitable regiocontrolled catalysis by transition metals of the coupling of 2-methylbut-2-ene-1,4-diylmagnesium or 3-methylbut-2-enylmagnesium chloride with C5-alkenyl halides. Various terpene amines have been obtained in excellent yields by Pd-catalysed telomerization of isoprene with NH3.
Isoprene itself does not undergo the building process, but rather activated forms, isopentenyl pyrophosphate (IPP or also isopentenyl diphosphate) and dimethylallyl pyrophosphate (DMAPP or also dimethylallyl diphosphate), are the components in the biosynthetic pathway. IPP is formed from acetyl-CoA via the intermediacy of mevalonic acid in the HMG-CoA reductase pathway . An alternative, totally unrelated biosynthesis pathway of IPP is known in some bacterial groups and the plastids of plants, the so-called MEP(2-Methyl-D-erythritol-4-phosphate)-pathway, which is initiated from C5-sugars. In both pathways, IPP is isomerized to DMAPP by the enzyme isopentenyl pyrophosphate isomerase.