Asymmetric Catalytic Hydrogenolysis of ArylHalide Bonds in Fused Arene Chromium and Ruthenium Complexes

Abstract

Access to highly enantioenriched planar chiral [Cr(5-bromonaphthalene)(CO)(3)] (6), [Ru(eta(5)-C(5)R(5))(5-bromonaphthalene)][PF(6)] (42) and [Ru(eta(5)-C(5)R(5))(4-bromoindene)] (44) was sought using asymmetric hydrogenolysis of [Cr(5,8-dibromonaphthalene)(CO)(3)] (5), [Ru(eta(5)-C(5)R(5))(5,8-dibromonaphthalene)] (39) and [Ru(eta(5)-C(5)R(5))(4,7-dibromoindene)] (40), respectively. Initial efforts focused on the chromium complex 5. Pd(0) catalysts with dimethoxyethane as the solvent and LiBH(4) or NaBH(3)CN as a hydride source worked best. Nineteen chiral bidentate phosphorus ligands were screened in this reaction. Asymmetric induction was low to modest with product ee's in the range of 4 to 52% and yields of 6 of up to 70%. Chiral phosphoramidite ligands proved superior and a bulky ligand derived from a Whitesell amine and 3,3'-diphenyl-binaphtol afforded 6 with an ee of 97%. The high enantioselectivity is largely due to the initial desymmetrization reaction though kinetic resolution also plays an important role as shown by the determination of a selectivity factor s=8.5 at -10 degrees C. Initially high ligand loadings (4 equiv/Pd) were necessary to achieve good asymmetric induction. This could be traced to the trapping of the chiral ligand by borane formed in the reaction. Addition of 1,4-diazabicyclo[2.2.2]octane (DABCO) suppressed this, and its addition led to the use of Pd and chiral ligand in a 1:1.2 ratio. Asymmetric hydrogenolysis of cationic dibromonaphthalene and neutral dibromoindenyl complexes of Ru cyclopentadienyl complexes was investigated and afforded the following results: [RuCp(5-bromonaphthalene)][PF(6)] (39a; 75%, 90% ee), [RuCp*(5-bromonaphthalene)] [PF(6)] (39b; 88%, 99% ee), [RuCp(4-bromoindenyl)] (44a; 72%, 96% ee), and [RuCp*(4-bromoindenyl)] (44b; 62%, 68% ee).