Chiral Olefins as Steering Ligands: Syntheses of C1-Symmetric Dibenzo[a,e]cyclooctenes (Rdbcot)

Abstract

A simple three-step synthesis of chiral dibenzo[a,e]cyclooctenes (dbcot) starting from commercially available dibenzosuberenone was developed. These compounds give highly stable and robust rhodium(I) and iridium(I) diene complexes of the type [M(Rdbcot)(L1)(L2)] (M = Rh, Ir; L1, L2 = MeCN, Cl, diamine, chloride). The complex [Rh((R)-Phdbcot)((R)-(+)-1,1‘-binaphthyl-2,2‘-diamine)]+OTf- could be obtained in enantiomerically pure form and catalyzes the enantioselective 1,2-addition of PhB(OH)2 to α,β-unsaturated ketones with good activity and acceptable enantiomeric excess (62%). The iridium complex [Ir(Phdbcot)(MeCN)2]+OTf- catalyzes the hydrogenation of dimethylitaconate with good activity, while the rhodium complexes are almost inactive. Likewise, the complex [Ir(Phdbcot)(H2NCH2CH2NH2)]+OTf- serves as a rather efficient catalyst precursor with an activity 4 orders of magnitude higher than for the analogous rhodium complex. These experiments further establish the use of dienes as steering ligands in catalysis.