Page 230 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

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CHAPTER 2
Stereochemistry,
Conformation,
and Stereoselectivity

Fig. 2.26. Space-filling model of crystallographic structure of S-
enantiomer 2,2,2-trifluoro-1-(9-anthryl)ethanol showing hydrogen bonding
to the A-ring of anthracene. Reproduced from J. Chem. Soc., Chem.
Commun., 765 (1991), by permission of the Royal Society of Chemistry.

Chemical shifts sufficient for analysis can sometimes be achieved without the
need for covalent bonding. If solvation is strong enough, the chiral additive induces
sufficiently different chemical shifts in the two enantiomeric complexes to permit
analysis. One such compound is called the Pirkle alcohol (Entry 5 in Scheme 2.12). 188
The structurally important features of this compound are the trifluoroethanol group,
which provides a strong hydrogen bond acceptor, and the anthracene ring, which
generates anisotropic shielding. In some cases, there may also be - stacking inter-
actions. The structure of the compound has been determined in both the crystalline
state and solution. Figure 2.26 shows an intermolecular hydrogen bond between the
hydroxyl group and the anthracene ring for the S-enantiomer. 189
Various amines and amides that can serve as hydrogen bond donors are used
as chiral solvating agents for carboxylic acids and alcohols. One example is 1,2-
diphenylethane-1,2-diamine (Entry 4 in Scheme 2.12). 190 The alkaloid quinine also
shows enantioselective solvation with certain alcohols. 191 It is also possible to design
molecules to act as chiral receptors, such as structure 8, which incorporates a binding
environment for the carboxylic acid group and gives good NMR resolution of chiral
and prochiral carboxylic acids. 192

188 W. H. Pirkle and S. D. Beare, J. Am. Chem. Soc., 91, 5150 (1969); W. H. Pirkle and D. J. Hoover, Top.
Stereochem., 13, 263 (1982).
189 H. S. Rzepa, M. L. Webb, A. M. Z. Slawin, and D. J. Williams, J. Chem. Soc., Chem. Commun., 765
(1991); M. L. Webb and H. S. Rzepa, Chirality, 6, 245 (1994).
190
R. Fulwood and D. Parker, Tetrahedron: Asymmetry, 3, 25 (1992); R. Fulwood and D. Parker, J. Chem.
Soc., Perkin Trans., 2, 57 (1994); S. E. Yoo and S. I. Kim, Bull. Kor. Chem. Soc., 17, 673 (1996).
191 C. Rosini, G. Uccello-Barretta, D. Pini, C. Abete, and P. Salvadori, J. Org. Chem., 53, 4579 (1988).
192
T. Stork and G. Helmchen, Rec. Trav. Chim. Pays-Bas, 114, 253 (1995).