The oxidation of olefins promoted by transition metal complexes
The oxidation of olefins by transition metal complexes leading to epoxide or ketonic products has been reviewed, with an emphasis on non-free radical processes. The formation of cycloadducts by reaction of bis (triphenylphosphine )- platinum(O)dioxygen with the α,β-unsaturated carbonyl compounds trans but-2-enal, propenal, cyclohex-2-en-1-one, exo 2-methvlenecyclohexan-1-one and 3-phenylprop- 2-enal was investigated by H- and 31P- nuclear magnetic resonance (n.m.r.). The cycloadducts derived from trans but-2-enal and propenal were thermally unstable in solution and further reacted to form stable, isolable complexes. The mechanism by which the cycloadduct cis [4-trans(prop-2-enyl )-1,1-bis( triphenyl - phosphine)platina-2,3» 5-trioxacyclopentane] , I , was transformed into cis [trans- (5-formyl-4-methyl)-1,1-bis(triphenylphosphine )platina-2 t 3-dioxacyclopentane] , II , was investigated by H- and H-n.m.r. utilising trans 4-[2H3]but-2-enal. A mechanism which involved both exchange of free trans but-2-enal with I and reaction to give II was proposed; a computer model, based upon numerical integration of the alternative rate equations was in good agreement with this proposition. The chemical reactivity of II with a variety of reagents was explored. In particular, the reaction of II with trifluoroacetic acid gave trans 2-formyl- 3-methyloxirane in high yield and stereoselectivity. The dynamic behaviour of cis[bis(triphenylphosphine)(23,3'-m)(trans - but-2-enal )platinum(0)] was observed by variable temperature P-n.m.r. and explained by the interconversion of carbonyl rotamers. The preparation of iridium(I) olefin and peroxo complexes has been described Chlorobis triphenylphosphine) (1,2-m) (ethene )dioxygen iridium( I) , III , and related complexes were found to oxidise terminal olefins to their methyl ketones stoichiometrically, by a non-free radical process. The mechanism has been discussed.