Stereochemical models hydrogenation

This stereochemical model explains the stereochemistry of the hydrogenation of MAA over TA-Ni system. It also predicts that the TA-Ni catalyst can be effective for the enantioselective hydrogenation of some prochiral ketones with excellent e.e. values (70%). 

The absolute configuration of the amine 7 may be explained by a stereochemical model based on the X-ray crystal structure of the chiral BINOL-phosphate (Fig. 4). In the transition state the ketimine is activated by the Brpnsted acid in such a way, that the nucleophile has to approach from the less hindered si face as the re face is effectively shielded by the large aryl substituent of the catalyst (Fig. 4, left). Furthermore, a bifunctional activation seems to be plausible, where next to the ketimine protonation, the dihydropyridine is activated through a hydrogen bond from the Lewis basic oxygen of the phosphoryl group. 

Acyclic p-aminokelones (297) are alkylated from the less hindered side (A) of the carbonyl plane, similarly to hydrogenation, but with higher stereoselectivity. The pro-po,sed stereochemical model involves a cyclic transition state in which the magnesium... 

In the enantio-differentiating hydrogenation of substrates that meet with the "Stereochemical Model", certain types of substrates give almost prefect e.d.a. s. but all others give around a 75 to 90% e.d.a. This section briefly deals with a supplementary procedure, which allows for the imperfect nature of MNi and provides optically pure compounds from the reaction products. 

The multiplet theory permits the building of stereochemical models of active complexes of hydrogenation of the compounds (VII) and (VIII). Since the molecules of triptycene possess a rigid structure, except for flattening or inversion according to the Sn2 mechanism, the molecule cannot accommodate an index group on an even surface. Therefore, one should assume the existence of elevations on the surface of the catalyst. As the Cn=Ci6 bond is internal, the molecule must superimpose on the elevation that carries the (111) facet (see above). 

Figure 4.18. Stereochemical models of intermediate complexes in the hydrogenation of ethyl acetoacetate on crystalline Ni modified with (2i ,3i )-tartaric acid (a) and with amino acid (b) (according to Klabunovskii,Vedenyapin ).

An examination of the above stereochemical formula or its model would easily show that the two hydrogens attached to the a carbon are stereochemically different. So its NMR spectrum will have 4 signals and this is actually so. 

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