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)-I (R1 = Ph, PhCH2) with a pentalene backbone from the corresponding bicyclo[3.3.0]octa-1,4-diones II is described. The compds. II were in turn accessible by enzymic resoln. of the corresponding racemic diol and subsequent Swern oxidn. The efficiency of the ligands I in the rhodium-catalyzed 1,4-addn. of arylboronic acids ArB(OH)2 (Ar = Ph, 4-MeC6H4) to cyclic and acyclic enones, namely, 2-cyclopentenone, 2-cyclohexenone, 2-cycloheptenone, and R2CH:CHCOMe (R2 = Me, 2-furyl), was demonstrated. In the case of cyclic enones, the enantiomeric di-Ph dienes (3aR,6aR)- and (3aS,6aS)-I (R1 = Ph) were more selective than the corresponding dibenzyl analogs. When acyclic enones were employed, the result was, however, reversed: the ligands I (R1 = Ph) were nearly inactive whereas I (R1 = PhCH2) afforded the corresponding conjugate addn. products with enantioselectivities up to 91 %.