Enantioselective hydrogenation steric factor

As with enantioselective hydrogenation, we see that several factors are involved in the high efficacy of the Ti(OiPr)4-tartrate epoxidation catalysts. The metal ion has two essential functions. One is the assembly of the reactants, the allylic alcohol, and the hydroperoxide. The second is its Lewis acid character, which assists in the rupture of the 0—0 bond in the coordinated peroxide. In addition to providing the reactive oxidant, the r-butyl hydroperoxide contributes to enantioselectivity through its steric bulk. Finally, the tartrate ligands establish a chiral environment that leads to a preference for one of the diastereomeric TSs and results in enantioselectivity. 

Ferrocenyl-based ligands comprise a versatile class of auxiliaries because they can be easily modified at the benzylic position with retention of configuration and can incorporate both central and planar chiralities. The appropriate balance of steric and electronic factors has provided ferrocenyl derivatives featuring chelating P,N properties that proved beneficial in numerous enantioselective transformations [50]. Among more recent applications, they could be utilized very efficiently in Pd-catalyzed hydrosilylation (14 >99% ee) [51] and hydroboration (>94% ee) [52] of olefins, allylic amination (99 % ee) [53], Suzuki cross coupling reactions (Section 2.11) [54], and enamide hydrogenation (>99% ee) [55]. 

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