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The reaction of the chiral tripod ligands X 2 POCH(CH 2 P(Ph) 2 )(CH 2 PR 2 ) ((±)- and R - 1a,b : X 2 = 2,2′-biphenoxy-; PR 2 = P(4-Me-C 6 H 4 ) 2 ( 1a ), PR 2 = dibenzophospholyl(DBP)- ( 1b )) and MeC(CH 2 P(Ph) 2 )(CH 2 PR′ 2 )(CH 2 PR″ 2 ) ((±)- and R - 2 : PR′ 2 = P(3,5-Me 2 -C 6 H 3 ) 2 ; PR″ 2 = DBP) with [Rh I (COD)Cl] 2 is investigated. It yields tripod rhodium COD complexes [( 1a,b )Rh I (COD)]PF 6 ((±)- and R - 3a,b ) and [( 2 )Rh I (COD)]PF 6 ((±)- and R - 4 ). The tripod ligand is coordinated by all three donor groups in each case, irrespective of the different donor capabilities of the coordinating groups. The complexes 3a,b and 4 are catalysts for the hydrogenation of the prochiral olefins acetamidocinnamic acid and dimethylitaconate. Both the poor activity and the low enantioselectivity compared to rhodium complexes containing chiral bisphosphino ligands can be explained by the presence of a third coordinating donor group in the ligand. Thus the trihapto coordination of the tripodal ligands is shown to be more of an impediment for this type of catalytic transformation. |
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