Ligands steric interactions

Catalytic asymmetric hydrogenation was one of the first enantioselective synthetic methods used industrially (82). 2,2 -Bis(diarylphosphino)-l,l -binaphthyl (BINAP) is a chiral ligand which possesses a Cg plane of symmetry (Fig. 9). Steric interactions prevent interconversion of the (R)- and (3)-BINAP. Coordination of BINAP with a transition metal such as mthenium or rhodium produces a chiral hydrogenation catalyst capable of inducing a high degree of enantiofacial selectivity (83). Naproxen (41) is produced in 97% ee by... 

A concerted [2 + 2] cycloaddition pathway in which an oxametallocycle intermediate is generated upon reaction of the substrate olefin with the Mn(V)oxo salen complex 8 has also been proposed (Scheme 1.4.5). Indeed, early computational calculations coupled with initial results from radical clock experiments supported the notion.More recently, however, experimental and computational evidence dismissing the oxametallocycle as a viable intermediate have emerged. In addition, epoxidation of highly substituted olefins in the presence of an axial ligand would require a seven-coordinate Mn(salen) intermediate, which, in turn, would incur severe steric interactions. " The presence of an oxametallocycle intermediate would also require an extra bond breaking and bond making step to rationalize the observation of trans-epoxides from dy-olefms (Scheme 1.4.5). 

Structures 5 and 6 display the solid state structures of two representative distibine and dibismuthine adducts. The ligands bound to the central Sb and Bi atoms adopt a staggered conformation in relation to one another, with the bulky M(t-Bu)3 groups arranged in a trans-position. This is likely due to repulsive steric interactions. The central Sb—Sb [281.1(1) 32 283.9(1) pm 35] and Bi—Bi bond distances [298.3(1) 36 and 298.4(1) pm 37] are almost unchanged compared to the uncomplexed distibines and dibismuthines, as can be seen... 

Related platinum compounds such as 69 have thus been synthesized from a-keto or cyano-stabilized ylides. A selective C-coordination trans to the P phosphorus atom of the ligand is observed, probably resulting from steric interactions. 

Davies [30] studied the PyBOx-induced conformational effects by testing several ligands sterically hindered on the oxazoUne moieties (Scheme 11, structures 18 and 19). However, these new ligands gave poorer results in terms of yields and enantioselectivities than ligand 16 for the Ru-catalyzed cyclopropanation reaction, indicating unfavorable steric interactions between styrene and the carbene complex. 

A model consistent with the stereochemical outcome of the reaction and with the role of the halide has been proposed (Fig. 10). This model suggests side-on olefin binding and reorganization of the halide ligands. In such geometry, a steric interaction between the unbound olefin and apical halide may justify the dramatic increase in enantioselectivity observed upon changing the halide from cr to r. 

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