Allylamines equilibration

The equilibration of allylamines like 176, which have an isolated C=C double bond, to conjugated enamines under basic conditions has been studied extensively139,292 297. With only one exception298, that of l-(indol-3-ylethyl)-3-pyrroline (181), which is only partially transformed into its enamine 182 (equation 15), the equilibrium generally lies far on the side of the enamine l-JV,JV-dimethylamino-2-propene (183) is isomerized292 with potassium terf-butoxide in DMSO, first to the Z-enamine (184) and then to the thermodynamically more stable is-enamine293,294 (185, see equation 16) as was shown by XH-NMR spectroscopy. 

Several studies204 257-265 have demonstrated that allylamines are thermodynamically less stable than the corresponding enamines, and that equilibration can be achieved and favours the enamine over the allylamine. A stabilizing overlap between the nitrogen lone... 

Price and Snyder257 were the first to show that the higher relative stability of enamine versus allylamine could be used preparatively for the equilibration of the latter into the former. 

These experiments clearly show that piperidin-2-enes are thermodynamically more stable than the piperidin-3-ene (allylamine) isomers and that they can be produced from them by equilibration. 

As will be further detailed in Section 4.1.2.2, it is possible to effect equilibration of allylamines and the corresponding enamines (for example, equation 11). Such a procedure obviously lacks generality, as in the majority of cases the allylamine would be more difficult to prepare than the enamine. Furthermore, and more importantly, such a sequence for forming enamines removes the ketone functionality from its central role in the synthetic sequence. 

Other determinations of the energy difference between an allylamine and an enamine include the equilibration of the species in equations (11), (13) and (14). ... 

The equilibration of allylamines like 176, which have an isolated C=C double bond, to conjugated enamines under basic conditions has been studied extensively ... 

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