Asymmetric Isomerization and Mechanism

Like the synthesis of L-DOPA by asymmetric hydrogenation, the manufacture of L-menthol hy Takasago Company is also one of the early examples of an industrial process where asymmetric isomerization is a key step. The desired isomerization reaction is one of the steps of the overall synthetic scheme. The synthesis of L-menthol from diethyl geranylamine is shown by 9.2. The formal electron pair pushing mechanism for the isomerization of the allylic amine to the enamine proceeds according to reaction 9.3. 

The enamine in 9.31 can coordinate to the metal center in three different ways. It can act as a monodentate ligand by coordinating either through the N atom lone pair or the double bond. It can also act as a chelating ligand. As we will see, the proposed catalytic intermediates in the catalytic cycle have all three different types of coordination. 

A simplified proposed catalytic cycle is shown in Fig. 9.6. The precatalyst, an analogue of 9.14 with S-BINAP, undergoes reaction 9.3 to generate 9.32, where the enamine acts as a chelating ligand. Note that in 9.32 asymmetric isomerization has already taken place. How this may come about will be dis- 

Note that in 9.33 hydride abstraction generates quaternary nitrogen containing conjugated allyl amine ligand. This prochiral molecule coordinates to the 

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