Advances in the stereoselective synthesis of antifungal agents and ...

Chapter 2 44Chiral phosphine complexes of late transition metals in a low oxidationstate catalyze enantioselective reductions in a homogeneous phase. Noyorideveloped an interesting approach for the enantioselective reduction ofprochiral α-substituted ketones employing an enantiopure Ru (II) complexwith 2,2'-Bis-(diaryl phosphino) 1,1'Binaphtyl (BINAP) 21e/d.These organometallic catalysts are endowed with functionality andaxial chirality and allow a differentiation between diasteroisomeric transitionstates differing in energy of only 10 KJ/mol. Such molecular catalysts arenot only able to accelerate the reaction rate, but also to control thestereochemical outcome of the reaction in an absolute sense.BINAP 21e, a fully arylated, C2 symmetrical chiral diphosphine, is oneof the most effective chiral ligands that have been designed 13 (Fig. 2.4).BINAP ligands can accommodate a wide variety of transition metals toform conformationally unambiguos seven membered chelate ringscontaining only sp 2 carbons.PAr 2PAr 2PAr 2PAr 2(R)-BINAP(S)-BINAPFigure 2.4: R- and S-BINAP 21e.The single crystal X-Ray analysis of certain square planar oroctahedral BINAP complexes indicates that the seven membered rings havea higly skewed configuration that provides a distinct chiral microenviromentin which the phosphophenyl groups are oriented into axial and equatorialdirections 14 ( Fig. 2.5 ).