Chiral bicyclo[3.3.0]octa-2,5-dienes as steering Ligands in substrate-dependent rhodium-catalyzed 1,4-addition of arylboronic acids to enones

The synthesis of disubstituted chiral diene ligands (3aR,6aR)- and (3aS,6aS)-10 with a pentalene backbone from the corresponding bicyclo[3.3.0]octa-1,4-diones 7 is described. The latter were accessible by enzymatic resolution of the racemic diol rac-5 and subsequent Swern oxidation. The efficiency of the ligands 10 in the rhodium-catalyzed 1,4-addition of arylboronic acids 12 to cyclic and acyclic enones 11 and 15 could be demonstrated. In the case of cyclic enones 11 the enantiomeric diphenyldienes (3aR,6aR)- and (3aS,6aS)-10a were more selective than the corresponding dibenzyldiene ligands 10b. When acyclic enones 15 were employed this result, however, reversed. Ligands 10a were nearly inactive whereas dibenzyldienes IN afforded the addition products 16 with enantioselectivities up to 91 %.