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Solvent and Temperature Effects on Diastereofacial Selectivity: Amines as Co‐Solvents in n‐ Butyllithium Addition to α‐Chiral Aldehydes
Author(s) -
Cainelli Gianfranco,
Giacomini Daria,
Galletti Paola,
Quintavalla Arianna
Publication year - 2003
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/ejoc.200200579
Subject(s) - chemistry , hexane , selectivity , amine gas treating , solvation , solvent , solvent effects , organic chemistry , aldehyde , tertiary amine , butyllithium , medicinal chemistry , catalysis
Temperature‐dependent selectivity in nucleophilic additions to α‐chiral aldehydes is affected by solvent mixtures. In this context, we investigated the effect on diastereoselectivity of tertiary amines as co‐solvents. Addition of 5 mol% of an amine to hexane strongly influences the anti / syn ratio in the additions of n BuLi to 2‐phenylpropanal and to (2 S )‐2‐[( tert ‐butyl)dimethylsilyloxy]‐2‐propanal. In these hexane/amine mixtures as reaction solvents, we observed diastereoselectivities higher than those obtained in hexane. In the corresponding Eyring plots of ln( anti / syn ) against the reciprocal temperature, we observed inversion temperatures ( T inv ), these generally occurring at higher temperatures in the hexane/amine mixtures than in pure hexane. The presence of the inversion temperatures confirms that these tertiary amines take part in the solvation of the starting aldehyde, thus contributing to a solvent effect on facial selectivity. In some cases, the differential entropy of activation is the predominant factor in differentiating the diastereoselectivity among these hexane/amine mixtures. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)