Synthesis of (P)- and (M)-6,7-Bis[(diphenylphosphanyl)methyl]-8,12-diphenylbenzo[a]heptalenes - Potential Ligands for Homogeneous Asymmetric Catalysis
作者:Peter Mohler、Andreas Johannes Rippert、Hans-Jürgen Hansen
DOI:10.1002/(sici)1522-2675(20000119)83:1<258::aid-hlca258>3.0.co;2-a
日期:2000.1.19
The title bis(phosphane) ligands have been prepared starting from optically pure diisopropyl (P)- and (M)-8,22-diphenylbenzo[a]heptalene-6,7-dicarboxylaes ((P)-1b and (M)-1b) that had been obtained by HPLC separation of rac-1b on a semi-preparative Chiralcel OD column. Reduction of (P)-1b and (M)-1b with diisobutylaluminum hydride (DIBAH) gave optically pure (P)- and (M)-dimethanols 3 (Scheme 6 and Fig. 5). Unfortunately, the almost quantitative chlorination of rac-3 with PCl5 in CHCl3 at - 60 degrees led with (M)-3 to nearly complete loss of optical integrity However, mesylate formation of (P)-3. followed by phosphanylation with LiP(BH3)Ph-2 gave (P)-6 with only a small loss of optical activity. Optically pure (P)-6 was obtained by crystallization from Et2O/hexane, which removed the nearly insoluble rac-6. The pure bis(phosphane) ligands (P)-2 and (M)-2 can be liberated quantitatively from 6 by warming 6 in toluene in the presence of 1,4-diazabicyclo[2.2.2]octane (DABCO). First Rh'-catalyzed asymmetric hydrogenation reactions of (Z)-alpha-(acetamido)cinnamic acid ((Z)-14) in the presence of (P)-2 led to (R)-N-acetylphenylalanin ((R)-15) in optical purities up to 77% (see Table 1).