Werner Complexes with ω‐Dimethylaminoalkyl Substituted Ethylenediamine Ligands: Bifunctional Hydrogen‐Bond‐Donor Catalysts for Highly Enantioselective Michael Additions |
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| Authors: | M?Sc Subrata K Ghosh Dr Carola Ganzmann Dr Nattamai Bhuvanesh Prof?Dr John A Gladysz |
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| Institution: | 1. Department of Chemistry, Texas A&M University, College Station, TX, USA;2. Institut für Organische Chemie and Interdisciplinary Center for Molecular Materials, Friedrich-Alexander-Universit?t Erlangen-Nürnberg, Erlangen, Germany |
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| Abstract: | The racemic carbonate complex Co(en)2O2CO]+ Cl? (en=1,2‐ethylenediamine) and (S)‐H3NCH((CH2)nNHMe2)CH2NH3]3+ 3 Cl? (n=1–4) react (water, charcoal, 100 °C) to give Co(en)2((S)‐H2NCH((CH2)nNHMe2)CH2NH2)]4+ 4 Cl? ( 3 a – d H4+ 4 Cl?) as a mixture of Λ/Δ diastereomers that separate on chiral‐phase Sephadex columns. These are treated with NaOH/Na+ BArf? (BArf=B(3,5‐C6H3(CF3)2)4) to give lipophilic Λ‐ and Δ‐ 3 a–d 3+ 3 BArf?, which are screened as catalysts (10 mol %) for additions of dialkyl malonates to nitroalkenes. Optimal results are obtained with Λ‐ 3 c 3+ 3 BArf? (CH2Cl2, ?35 °C; 98–82 % yields and 99–93 % ee for six β‐arylnitroethenes). The monofunctional catalysts Λ‐ and Δ‐Co(en)3]3+ 3 BArf? give enantioselectivities of <10 % ee with equal loadings of Et3N. The crystal structure of Δ‐ 3 a H4+ 4 Cl? provides a starting point for speculation regarding transition‐state assemblies. |
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| Keywords: | cobalt complexes enantioselective catalysis hydrogen bonding Michael additions nitroalkenes |
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