An efficient entry to planar chiral organometallic complexes via Pd-catalyzed asymmetric hydrogenolysis.

Planar chiral chromium- and ruthenium-based arene complexes were prepared with high levels of enantioselectivity via a Pd-catalyzed asymmetric hydrogenolysis reaction using a bulky chiral phosphoramidite ligand. Key elements for the efficiency of the process are the use of DABCO as borane-trapping reagent as well as substantial kinetic resolution, which was found to enhance the stereochemical outcome of the reaction.

Asymmetric catalytic hydrogenolysis of aryl-halide bonds in fused arene chromium and ruthenium complexes.

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).

Synthesis of highly enantiomerically enriched planar chiral ruthenium complexes via Pd-catalysed asymmetric hydrogenolysis.

Key elements in this communication are a very efficient microwave synthesis of [RuCp(naphthalene)][PF(6)], the precursor of [RuCp(CH(3)CN)(3)][PF(6)], and a Pd-catalysed asymmetric hydrogenolysis to afford planar chiral ruthenium complexes with high levels of enantioselectivity using a bulky chiral phosphoramidite ligand.

Chiral palladium template promoted asymmetric hydrophosphination reaction between diphenylphosphine and vinylphosphines.

An organopalladium complex containing ortho-metalated (S)-(1-(dimethylamino)ethyl)naphthalene as the chiral auxiliary has been used to promote the asymmetric hydrophosphination reactions between diphenylphosphine and (E)- or (Z)-diphenyl-1-propenylphosphine in high regio- and stereoselectivities under mild conditions. Hydrophosphination of (Z)-diphenyl-1-propenylphosphine with diphenylphosphine gave (S)-(-)-prophos as the major product. Using the same chiral metal template, the corresponding hydrophosphination reaction with (E)-diphenyl-1-propenylphosphine gave (R)-(+)-prophos predominantly. The hydrophosphination reactions generated the asymmetric diphosphines as bidentate chelates on the chiral naphthylamine palladium templates. The template products obtained undergo cis-trans isomerization in solution to form an equilibrium mixture of regioisomers. X-ray analysis of the major template products obtained from the hydrophosphination of (Z)-diphenyl-1-propenylphosphine reveals that the two regioisomers are cocrystallized in a 1:1 ratio. The naphthylamine auxiliary could be removed chemoselectively from the template products by treatment with concentrated hydrochloric acid to form the corresponding optically pure neutral complexes [(R)- or (S)-(prophos)PdCl(2)]. Subsequently, the (R)- and (S)-dichloro complexes undergo ligand displacement with aqueous cyanide to generate the corresponding optically pure diphosphine ligands in high yields. Mechanistic pathways explaining the stereoselectivity of the chiral organopalladium template promoted hydrophosphination reactions are also proposed.