Substituent effects and chiral discrimination in the complexation of benzoic, 4-methylbenzoic and (RS)-2-phenylpropanoic acids and their conjugate bases by β-cyclodextrin and 6A-amino-6A-deoxy-β-cyclodextrin in aqueous solution: potentiometric titration and1H nuclear magnetic resonance spectroscopic study
- 1 January 1993
- journal article
- research article
- Published by Royal Society of Chemistry (RSC) in Journal of the Chemical Society, Faraday Transactions
- Vol. 89 (7) , 1035-1040
- https://doi.org/10.1039/ft9938901035
Abstract
A potentiometric titration study in aqueous solution (l= 0.10 mol dm–3, KCl) of the complexation of benzoic, 4-methylbenzoic and (RS)–2-phenylpropanoic acids (HA) and their conjugate bases (A–) with β-cyclodextrin, βCD, and its substituted analogue, 6A-amino-6A-deoxy-β-cyclodextrin, βCDNH2, in which a primary hydroxy group is replaced by an amino group which may be protonated to produce a singly charged species, βCDNH+ 3, is reported. At 298.2 K the stability constants for the complexes have the values (in dm3 mol–1) shown in parentheses: benzoic acid ·βCD (K1HA= 590 ± 60); benzoate ·βCD (K1A= 60 ± 10); benzoic acid ·βCDNH+ 3(K2HA= 340 ± 30); benzoate ·βCDNH+ 3(K2A= 120 ± 20); benzoate ·βCDNH2(K3A= 50 ± 20); 4-methylbenzoic acid ·βCD (K1HA= 1680 ± 90); 4-methylbenzoate ·βCD (K1A= 110 ± 1); 4-methylbenzoic acid ·βCDNH+ 3(K2HA= 910 ± 20); 4-methylbenzoate ·βCDNH+ 3(K2A= 330 ± 20); and 4-methylbenzoate ·βCDNH2(K3A= 100 ± 20). These data indicate that for a given cyclodextrin the guest carboxylic acids form complexes of higher stability than do their conjugate base analogues, and that βCDNH+ 3 forms more stable complexes with the conjugate bases than do βCD and βCDNH2. These trends are also observed for the complexation of (RS)-2-phenylpropanoic acid and (RS)-2-phenylpropanoate where the complexes indicated are characterised by the stability constants (in dm3 mol–1) shown in parentheses: (RS)-2-phenylpropanoic acid ·βCD (K1RHA= 1090 ± 30, K1SHA= 1010 ± 40); (RS)-2-phenylpropanoate ·βCD (K1RA= 63 ± 8, K1SA= 52 ± 5); (RS)-2-phenylpropanoic acid ·βCDNH+ 3(K2RHA= 580 ± 20, K2SHA= 480 ± 10); (RS)-2-phenylpropanoate ·βCDNH+ 3(K2RA= 150 ± 8, K2SA= 110 ± 10); and (RS)-2-phenylpropanoate ·βCDNH2(K3RA= 36 ± 6, K3SA= 13 ± 7). These data also show that while K1RHA and K1SHA, and K1RA and K1SA are indistinguishable for (RS)-2-phenylpropanoic acid ·βCD and (RS)-2-phenylpropanoate ·βCD, chiral discrimination is indicated by K2RHA > K2SHA for (RS)-2-phenylpropanoic acid ·βCDNH+ 3, K2RA > K2SA for (RS)-2-phenylpropanoate ·βCDNH+ 3, and K3RA > K3SA for (RS)-2-phenylpropanoate ·βCDNH2. The 1H NMR spectra of the methyl groups of the enantiomers of (RS)-2-phenylpropanoic acid appear as two separate doublets, indicating chiral discrimination when complexed by βCD or βCDNH+ 3, but such chiral discrimination is not observed for (RS)-2-phenylpropanoate when complexed by βCDNH+ 3. The implications of these observations are discussed.Keywords
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