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

Yeo, Wee Chuan
Chaudhuri, Piyali Datta
Cumming, Graham R.
Published in Chemistry - A European Journal. 2010, vol. 16, no. 21, p. 6285-6299
Abstract 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).
Keywords Asymmetric catalysisChromiumEnantioselectivityHydrogenolysisIndenylNaphthaleneRuthenium
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MERCIER, Audrey Catherine Marie et al. Asymmetric catalytic hydrogenolysis of aryl-halide bonds in fused arene chromium and ruthenium complexes. In: Chemistry - A European Journal, 2010, vol. 16, n° 21, p. 6285-6299. https://archive-ouverte.unige.ch/unige:6696

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Deposited on : 2010-05-25

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