Ene and 1,3-Dipolar Reactions

In recent years there has been increasing interest, particularly by research workers at the Malaysian Rubber Producers Research Association, (MRPRA), in certain reactions that involve the direct reaction between molecules and do not involve active species such as ions and free radicals as intermediates. Among the advantages of such reactions are the fact that catalysts are not involved and that they proceed smoothly, predictably and in a reasonably efficient manner. The reactions of particular interest are those which involve highly substituted ethylenes. These are of particular relevance to polyisoprenes such as natural rubber, which may be considered to be a poly-trisubstituted ethylene, as a means of introducing interesting groups into the polymer structure. 

One reaction of this type is the so-called ene reaction which proceeds schematically as follows  

This has a very strong formal resemblance to the better known Diels-Alder reaction which is of general form  

Because of the similarities of the two reactions it is not surprising to find that dienophiles effective in Diels-Alder type reactions are also good acceptors (enophiles) in ene syntheses. In the case of the ene syntheses it is also to be noted that the atom Y should be capable of forming good bonds with hydrogen. 

Amongst materials which have been reacted with trialkyl ethylenes in this way are maleic anhydride, acrylonitrile, acrylic acid and certain isocyanates. In principle it should be possible to effect many of these reactions with natural rubber carrying out the reaction at elevated temperatures in bulk rather than in solution in, for example, an internal mixer. It seems probable that the high temperature 

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