Asymmetric Hydrogenation of Acyclic N-Aryl Imines

The structure of metoiachior and the importance of stereochemistry in the activity of this herbioide.  

This reaction is the largest scale asymmetric process and is conducted on a scale of 10,000 tons/year. Tlie reaction occurs under optimized conditions with 1,000,000 turnovers and with an initial turnover frequency of 1,800,000 h T The product generated from the opposite enantiomer of the amine is inactive and nontoxic. Thus, the enantioselectiv-ity of 79% was sufficiently high for production, and the use of enantioenriched material reduces the amount of product that must be applied to the field. Other complexes catalyze this reaction with high enantioselectivities, but without the same activity. 

The mechanism of this hydrogenation has not been revealed in detail, but several aspects are noteworthy in the context of the mechanistic discussion earlier in this chapter. First, this reaction almost certainly occurs by the insertion of an imine into a metal hydride (see Chapter 9), rather than by the transfer of a hydride and a proton by a metal-ligand bifunctional system. This iridium catalyst does not contain a protic ligand. Second, acid and iodide are needed as promoters to obtain the fast rates. The iodide is thought to help stabilize an iridium(III) species and the acid is thought to help in the release of the amine product from the metal. 

---