1.6- dienes, cyclo-isomerisation

There are rather few reactions that can be described as fully atom economical , i.e. when there are no co-products and all the atoms in the starting material(s) appear in the product(s). However, all isomerisation reactions necessarily fall into this category. The use of a transition metal to catalyse such a process with an appropriate substrate brings the possibility of effecting asymmetric isomerisation, a very efficient method to generate enantiomerically enriched products. Indeed, the asymmetric Rh-catalysed isomerisation of an allylamine to an enamine, which proceeds in over 96% ee, was scaled up a number of years ago for industrial production. The enamine product forms a multi-tonne feedstock for menthol and perfumery synthesis. In contrast, the cyclo-isomerisation of dienes, an equally atom-economical process that generates synthetically useful cyclic products, has seen relatively little development despite the reaction having been known for some 30 years. 

Scheme 12.6 Pro-catalyst-dependent regioselectivity in the Pd-catalysed cyclo-isomerisation of 1,6-dienes DCE = 1,2-dichloroethane (solvent) see the text for catalysts.

We shall consider reactions catalysed by two different types of pro-catalyst the first (type A) employs Pd-allyl cations ([Pd(a]lyl)(PCy3)]+/Et3SiH or [Pd(allyl)(MeCN)2] + ), and the second (type B) employs Pd-alkyl or chloro complexes ([(phen)Pd(Me)(MeCN)]+, where phen = phenanthroline, and [(RCN)2PdCl2]). These two types of catalysts give very different products in the cyclo-isomerisation of typical 1,6-dienes such as the diallyl-malonates (10), Scheme 12.6. Since there is known to be a clear order of thermodynamic stability 11 < 12 <13, with a difference of ca. 3-4 kcal mol 1 between successive pairs, any isomerisation of products under the reaction conditions will tend towards production of 12 and 13 from 11 and 13 from 12. Clearly, when 11 is the major product (as with pro-catalysts of type A), it must be the kinetic product (see Chapter 2 for a discussion of kinetic and thermodynamic control of product distributions). However, when 12 is generated selectively, as it is with pro-catalysts of type B, there is the possibility that this is either generated by rapid (and selective) isomerisation of 11 or generated directly from 10. 

Scheme 12.7 Conclusions from the studies of the kinetics of the Pd-catalysed cyclo-isomerisation of 1,6-diene 10 (E = CO2R) to give 11, 12 and 13.

On an unspecified supported palladium catalyst at 323 K, 1,5-cyclo-octadiene isomerised to the 1,3-form at about the same rate as it was hydrogenated 5tot was about 90%, and some mathematical modelling was attempted. Other cyclic dienes have also been examined. ... 

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