The main disadvantage of using a chiral auxiliary is that additional steps are required in the synthesis for appendage and removal of the chiral group. One method of overcoming this is to use a chiral catalyst to promote the conversion of an achiral starting material into a chiral product. By definition, the advantage is that only a small molar ratio of catalyst is required, which can be recovered and reused. One of the most widely exploited areas of catalytic asymmetric synthesis is hydrogenation using a chiral transition metal complex. Two important ligands in this area are BINAP and DIPAMP (Figure 5.14), which are often complexed to rhodium (II) or ruthenium (II) acetate. DIPAMP is used in the industrial synthesis of (S)-3,4-dihydroxyphenylalanine (l-DOPA), an anti-Parkinson's agent.
FIGURE 5.14 Two commonly utilized chiral catalysts for performing asymmetric hydrogenations. BINAP (right) is shown as its octahedral rhodium acetate complex.
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