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Publication 148  


  1. “Asymmetric Catalytic Hydrogenolysis of Aryl—Halide Bonds in Fused Arene Chromium and Ruthenium Complexes”
    A. Mercier, X. Urbaneja, W.C. Yeo, P.D. Chaudhuri, G.R. Cumming, D. House, G. Bernardinelli, E.P. Kündig,
    Chem. Eur. J. 2010, 16, 6285-6299.

Access to highly enantioenriched planar chiral [Cr(5-bromonaphthalene)(CO)3] (6), [Ru(η5-C5R5)(5-bromonaphthalene)][PF6] (42) and [Ru(η5-C5R5)(4-bromoindene)] (44) was sought using asymmetric hydrogenolysis of [Cr(5,8-dibromonaphthalene)(CO)3] (5), [Ru(η5-C5R5)(5,8-dibromonaphthalene)] (39) and [Ru(η5-C5R5)(4,7-dibromoindene)] (40), respectively. Initial efforts focused on the chromium complex 5. Pd0 catalysts with dimethoxyethane as the solvent and LiBH4 or NaBH3CN 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 °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)][PF6] (39 a; 75 %, 90 % ee), [RuCp*(5-bromonaphthalene)] [PF6] (39 b; 88 %, 99 % ee), [RuCp(4-bromoindenyl)] (44 a; 72 %, 96 % ee), and [RuCp*(4-bromoindenyl)] (44 b; 62 %, 68 % ee).

DOI : 10.1002/chem.201000011 

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