FACTORS AFFECTING CONFORMATIONS OF (R,R)-TARTARIC ACID AMIDES

Marcin Hoffmann, Urszula Rychlewska, Jacek Rychlewski

Faculty of Chemistry, A. Mickiewicz University, ul. Grunwaldzka 6, 60-780 Poznan, Poland hoffmann@man.poznan.pl, urszula@krystal.amu.edu.pl, rychlew@man.poznan.pl

Keywords: quantum chemistry, molecular modelling, ab-initio, solvatation

(R,R)-tartaric acid and its salts and derivatives hold a crucial role in the history of stereochemistry. Results of studies on them allowed Pasteur to discover enantiomers and Bijvoet et al. to assign an absolute configuration to the first ever organic molecule. Nowadays (R,R)-tartaric acid and its derivatives are widely utilised as chiral auxiliaries in several important syntheses and in resolution of racemic mixtures. Conformations of primary, secondary and tertiary amides of (R,R)-tartaric acid both symmetrically and asymmetrically substituted have been studied crystalographically. Moreover ab initio studies up to MP2/6-31G*//RHF/6-31G* level for both the diamide of (R,R)-tartaric acid and N,N,N',N'-tetramethyldiamide of (R,R)-tartaric acid have already been carried out.

X-ray results have shown that primary and secondary amides of (R,R)-tartaric acid tend to adopt a conformation with the extended carbon chain - the Taa structure. In this Taa conformation both the -hydroxyamide moieties are planar and the structures gain stabilisation from hydrogen bonding between a donor the NH and an acceptor the proximal OH group. Moreover the Taa structure is favourably stabilised by the attraction of antiparallel local dipoles formed along distal C_sp3-H and C_sp2=O bonds. In the case of tertiary amides due to the bulky substituents the planar arrangement of -hydroxyamide moieties is not favoured. What is more the repulsion between the N-alkyl substituents and the hydrogen atom attached to the distal C_sp3 atom destabilise the T structures of tertiary amides of (R,R)-tartaric acid. As the result the N,N,N',N'-tetramethyldiamide of (R,R)-tartaric acid is found in the G^-p⁺p⁺ conformation, in which the main carbon chain is bent and the -hydroxyamide moiety is not planar but the O-C_sp3-C_sp2=O torsion angle is about 90^o.

On the contrary ab initio calculations of the isolated molecules of primary amide, (R,R)-tartaric acid diamide, and tertiary amide, the N,N,N',N'-tetramethyldiamide of (R,R)-tartaric acid, indicated that the G⁺aa structures are energetically favoured. The driving forces being (1) the attraction of antiparallel local dipoles formed along C_sp3-H and C_sp2=O bonds, (2) hydrogen bonds between distal OH and C=O groups and (3) in the case of the primary amide hydrogen bonds with N-H group as a donor and proximal OH as an acceptor.

The differences between the crystallographical results and ab inito calculations appears to be in line with the NMR studies. The NMR analysis indicated that in a polar solvent the T conformer predominates, whereas in a nonpolar chloroform solution there is a significant amount of the G⁺ conformer.

These differences stimulated our interest and prompted us to model the conformers of (R,R)-tartaric acid amides utilising ab initio calculations which are performed in the presence of solvent using Self Consistent Isodensity Polarisable Continuum Model (SCIPCM). In addition we have attempted to recrystallise the N,N,N',N'-tetramethyldiamide of (R,R)-tartaric acid from a non polar solvent in order to obtain a polymorphic structure. In this communication we will report the results obtained and compare with the existing data.