Home > Contenuti utili > ENVIRONMENT INNOVATION AT EXPOSHANGHAI 2010
ENVIRONMENT INNOVATION AT EXPOSHANGHAI 2010
<p><strong>Design and synthesis of new water soluble organo-inorganic systems as potential catalyst. </strong><em>Chirality amplification of water soluble Rhodium complexes</em><strong> </strong></p>
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<p><em>Silvia Bordoni, Dept of Physical and Inorganic Chemistry, Faculty of Industrial Chemistry, University of Bologna, Italy</em></p>
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<p>The recently reported1, one pot, carbon-economic and diasteroselective synthesis of the amphiphilic racemic complex (nbd)RhCpooo, nbd=norbornadiene, bearing the propeller-like ligands, Cpooo = (1,2,4)-C5H2[CH(CH2)3CHOH]3, shows central, planar (<em>Sp) </em>and helical chirality. The X-ray molecular structure evidences a 3D macrocycles (18 and 24 atoms) supramolecular network, adopting a folded conformation by interaction of the hydroxy vicinal H-bonding, further monitored in solution by nOe and VT (±60 C) NMR experiments.</p>
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<p>Figure 1: <em>X-ray diffraction one configuration of the </em><em>racemic </em><em>pS-1R,2S,4R)</em>C5H2<em>[CH(CH2)3CHOH]3 Rh(nbd)</em> <em>showing different stereogeometries </em></p>
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<p>In the absence of a chiral tag, the ORD (Optical Rotatory Dispersion) and ECD (Electronic Circular Dichroism) spectra show unexpected solvent-driven chiral response, depending on the polarity of the solvent which promotes H-binding network able to alter the stable configurations. The chiral behaviour is confirmed by asymmetric induction (e.e. 63%) in the catalytic hydrogenation of methyl phenyl ketone in mild conditions (r.t., 5 atm H2).</p>
<p>The use of strongly polar and basic solvents (dimethylsulfoxide, acetonitrile) drives the helical stereomutation2 intramolecularly, by the OH-bonding interactions, or intermolecularly by mediation of the solvent in the coordinating sphere.</p>
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<p>The solvent effect results in a significant amplification of the chirality observed, in spite of its racemic nature. The efficiency of the procedure has been proved by using differently substituted Rh and Ir fragments as Rh(CO)2, Rh(et)2, Rh(COD), Rh(CO)PPh3 and Ir(COD), IrI2(pyH) [COD = cyclottadiene, et = CH2CH2, pyH =pyrazole] as well as new perspective towards more eco-compatible metals as Ti, Mn or Fe to obtain half sandwich or sandwich complexes.</p>
<p>The reason of this “chiral economy” is currently investigated by analysing different stereochemical conformations through:</p>
<ul>
<li>VCD (Vibrational Circolar Dichroism).</li>
<li>DFT (density functional theory) calculations, by introducing the solvent effect as PCM (polarized continuum model) and by approaching discrete solvent molecules to identify the more stable conformations (COSMO-ADF approach)2. </li>
<li>Enantiomeric resolution by coordinating an enantiomeric pure chiral co-ligand at Rh centre, as the (S,S) diphenyl bicycloctadiene.</li>
<li>The feasibility of graphting the hydroxyl functionalities on Ti or Si compounds, as molecular model for titania or silica surface.</li>
</ul>
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<p>1) <em>ORGANOMETALLICS. (2008) vol.27 pp.945 – 954; 2) ORGANOMETALLICS (2009) vol </em>28, <em>pp</em>. <em>5382–5394</em></p>
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