Complexations of 1,4-dienes to rhodium(I) pentane-2,4-dionates; thermal rearrangements of the complexes

Abstract

As models for natural products containing a ‘skipped’ diene fragment (–CHCHCH₂CHCH–), a series of hepta-2,5-dienes, hepta-2,5-dien-4-ols, and hepta-2,5-dien-4-ol acetates have been prepared. Some of these dienes react with either bis(ethylene)(pentane-2,4-dionato)rhodium(I) or bis(ethylene)(1,1,1,5,5,5-hexafluoropentane-2,4-dionato)rhodium(I) to afford 1 : 1 complexes, e.g.[(E,E)-hepta-2,5-diene](1,1,1,5,5,5-hexafluoropentane-2,4-dionato)rhodium(I). Other dienes, e.g.(Z,Z)-hepta-2,5-diene, form 2 : 1 complexes in which for each molecule of diene, one double bond is co-ordinated to rhodium, whereas the other is not. The (Z,Z)-dienes do not form 1 : 1 complexes because there would be a severe steric interaction between the terminal substituents of each double bond in such complexes. For the complexes of the dienols and certain allylic alcohols (e.g. prop-2-en-1-ol), evidence was obtained for the presence of a stabilising intramolecular hydrogen bond between each OH and its nearest CO of the pentane-2,4-dionate. When heated in benzene with 5 mol % of bis(ethylene)(pentane-2,4-dionato)rhodium(I) each dienol rearranged to give an enone as, the main product [e.g.(E,E)-hepta-2,5-dien-4-ol →(E)-hept-2-en-4-one (85%)]. Their acetates rearranged to isomeric conjugated dienes [e.g.(Z,Z)-4-acetoxyhepta-2,5-diene →(3E,5Z)-2-acetoxyhepta-3,5-diene]. The hepta-2,5-dienes were recovered unchanged. The mechanisms of the rearrangements observed are explained in terms of intermediate (π-allyl)rhodium complexes.