Asymmetric Hydrogenation of Tetrasubstituted Olefins

Schrems et al. identified a number of P,N ligands, which have given encouraging results for this class of substrate [70]. Surprisingly, the structurally simple readily accessible phosphinooxazoline 12a, originally reported by Sprinz and Helmchen [71], and subsequent analogs 12b-e proved to be the most efficient ligands for several substrates (Table 8). 

For a few select cases, the cyclic aUcene 62a-c with simple primary alkyls as substituents were readily hydrogenated with SimplePHOX ligand 7a and 7c. Importantly, no epimerization at the benzylic position was observed, and hydrogenation gave entirely cis product with most substrates. Aromatization of the dihydronapthe-lene substrates 63a-b was a frequent side reaction, even at high pressures. 

For some substrates, a dependence of enantioselectivity on pressure was observed with low to moderate pressures being optimal for many combinations. Ligand 12c in combination with substrate 62a gives an enantioselectivity of 92% at 50 bar of pressure and 97% at 1 bar, similar to the trend found in the early studies of the l,l -disubstituted substrates. 

Mastery of tetrasubstituted olefins greatly expands the tools available for the synthesis of chiral carbon skeletons and tetrasubstituted olefins have the added value of possibly creating two stereogenic centers simultaneously. 

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The ferrocenylbisphosphines 8f—h bearing the amino pendant side chain are unique ligands that effect the rhodium-catalyzed asymmetric hydrogenation of tetrasubstituted olefins 57 (Scheme 2-43) [61], Thus, the hydrogenation of a-aryl-acrylic acid 57a in the presence of a cationic rhodium catalyst coordinated with 8g gives a quantitative yield of carboxylic acid 58a with 98.4% ee. Other tetra-... 

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