Isomerisation and addition reactions of some monoterpene–tricarbonyliron complexes

Abstract

Reactions of myrcene,cis-ocimene, cis- and trans-allo-ocimene, and α-phellandrene with iron carbonyls gave dienetricarbonyliron adducts, the structures of which were deduced from n.m.r. spectra and from identification of the olefins recovered after decomposition of the complexes.

The cis-ocimene and cis-allo-ocimene adducts were isomerised on heating, on treatment with alumina, or on reaction with triphenylmethyl tetrafluoroborate followed by sodium borohydride to give tricarbonyliron complexes in which the ligand had undergone both skeletal and double-bond rearrangement. The mechanism of the thermal isomerisation was delineated by tracer studies using carbon-14 and hydrogen-3.

Hydroboronation of the adducts formed by myrcene and cis- or trans-allo-ocimene gave derivatives that could be decomposed under exceptionally mild conditions to give excellent yields of novel dihydromonoterpenes in which the isopropylidene double bond had been reduced. The iron-containing portion of the myrcene complex similarly acted as a protecting group during reduction with di-imide, and during hydroacetoxylation and hydration. The products of the last two additions were pyrolysed to give good yields of α-myrcene.