Chirality in the Absence of Rigid Stereogenic Elements: The Design of Configurationally Stable C3‐Symmetric Propellers |
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Authors: | Tiziana Benincori Prof Andrea Marchesi Dr Tullio Pilati Dr Alessandro Ponti Dr Simona Rizzo Dr Francesco Sannicolò Prof |
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Institution: | 1. Dipartimento di Scienze Chimiche ed Ambientali, Università dell'Insubria via Valleggio 11, 22100 Como (Italy);2. Laboratori Alchemia s.r.l. via S. Faustino 68, 20134 Milano (Italy);3. Istituto di Scienze e Tecnologie Molecolari, Consiglio Nazionale delle Ricerche via Golgi 19, 20133 Milano (Italy), Fax: (+39)?0250314300;4. Dipartimento di Chimica Organica e Industriale, C.I.MA.I.NA., Università di Milano via Venezian 21, 20133 Milano (Italy), Fax: (+39)?0250314139 |
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Abstract: | Residual stereoisomerism is a form of stereoisomerism scarcely considered so far for applicative purposes, though extremely interesting, since the production of stereoisomers does not involve classical rigid stereogenic elements. In three‐bladed propeller‐shaped molecules, a preferred stereomerization mechanism, related to the correlated rotation of the rings, allows the free interconversion of stereoisomers inside separated sets (the residual stereoisomers) that can interconvert through higher energy pathways. In light of possible future applications as chiral ligands for transition metals in stereoselective processes, some C3‐symmetric phosphorus‐centered propellers, which could exist as residual enantiomers, are synthesized and the possibility of resolving their racemates into residual antipodes is explored. While the tris(aryl)methanes are configurationally stable at room temperature, only selected tris(aryl)phosphane oxides display a configurational stability high enough to allow resolution by HPLC on a chiral stationary phase (CSP HPLC) at a semipreparative level at room temperature. Stability was evaluated through different techniques (circular dichroism (CD) signal decay, dynamic CSP HPLC (CSP DHPLC), dynamic NMR analysis (DNMR)) and the results compared and discussed. Phosphanes were found much less stable than the corresponding phosphane oxides, for which preliminary calculations suggest that the three‐ring‐flip enantiomerization mechanism (M0) would be easier than phosphorus pyramidal inversion. The parameters affecting the configurational stability of the residual enantiomers of C3‐symmetric propellers are discussed. |
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Keywords: | chirality circular dichroism NMR spectroscopy phosphane oxides phosphanes residual enantiomers |
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