Imine Hydrogenation Technical Process

Metolachlor is the active ingredient of Dual , one of the most important grass herbicides for use in maize and a number of other crops. In 1997, after years of intensive research. Dual Magnum, with a content of approximately 90% (I S)-diastereomers and with the same biological effect at about 65% of the use rate, was introduced into the market. This chiral switch was made possible by the new technical process that is briefly described below. The key step of this new synthesis is the enantioselective hydrogenation of the isolated MEA imine, as depicted in Figure 1.3. 

Tab. 3 gives an overview of the timetable and the milestones for the development of a technical process for the production of enriched (S)-metolachlor. It took so many years to reach the ambitious goal because the efforts to find a suitable catalyst system for the enantioselective imine hydrogenation had to start almost from zero. The final result of our efforts can be seen in Fig. 14 which depicts a partial view of the production unit. 

Keywords Imine hydrogenation, Ir diphosphine complexes, Ir ferrocenyldiphosphine complexes, Rh diphosphine complexes. Technical process. Industrial application, MEA imine, Ir ferrocenyldiphosphine complexes, (S)-Metolachlor, Chiral switch... 

Two alternatives to the homogeneous hydrogenation of the isolated MEA imine have been investigated (see Figure 1.4). The first method involved the direct ami-nation of MEA with methoxyacetone in order to avoid isolation of the MEA imine [27] the second method involved the use of immobihzed josiphos Hgands in order to avoid product distillation [28], While both variants led to the desired product with similar ee-values as the homogeneous process, both the TON and TOF were insufficient for a technical application. 

As an aside, decades of work have gone into the study of enantioselective homogeneous hydrogenation processes in both organic and aqueous systems. There is increasing commercial interest in this field spurred by the spectacular, time-encrusted development of a complex catalyst for the enantioselective hydrogenation of an imine to a chiral amine needed for manufacture of the important herbicide, (,S )-Metolaclor. The technical success of this program (Scheme l)25 owed much to the perserverance... 

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