The dissociation constants of theE- andZ-alpha;beta;-diarylacrylic acids

摘要

1980 161 The Dissociation Constants of the E-and Z-a@-Diarylacrylic Acids By Emanuele Maccarone," Antonino Mamo, Domenico Sciotto, and Michele Torre, lstituto dipartimentale di Chimica e Chimica industriale Universita di Catania, Viale A. Doria 6, 95125 Catania, Italy The pK values of 36 ap-diarylacrylic acids, in both Z-and Z-configurations, have been measured in 80 aqueous 2-methoxyethanol at 25 "C. The Z-isomers are stronger acids than the euro;-isomers because of the deconjugation of the carboxy-group which is out of the plane of the aromatic rings. In agreement with these conformational factors, the transmission of electrical effects (Hammett p-values) is more effective in E-than in Z-isomers. The heterocycle- containing acrylic acids (2-fury1, 2-thienyl, selenophen-2-yl) are weaker than the corresponding benzene deriva- tives. This has been ascribed to a combination of polar and steric effects, as shown by correlation with a two-para-meter equation.The experimental pK values, corrected for the contribution due to the different size of P-aryl ring systems, follow the order of polar effects. THEdissociation constants of carboxylic acids (RC0,H) are a fundamental property which may provide an insight into the structural effects of the R gr0ups.l In particular, five-membered heterocycles are considered electron-withdrawing groups since they show an acid-strengthening effect compared with the benzene ring. Positive Hammett constants relative to benzoic acid (cr~~)are associated with the heteroatoms in furan, thiophen, and selenophen.2 Electron-withdrawing effects are also indicated by Taft's polarity constants (o*) derived from rates of ester hydr~lysis.~~~ Although five-membered heterocycles show a strong mesomeric effect in electrophilic substitution^,^ the results just mentioned indicate that these rings do not facilitate x-electron delocalization towards the carboxy- group in the ground state.In addition, spectral red-shifts with respect to cin- namic acid are observed in p-2-f~ryl-,~ P-2-thienyl-,' and p-selenophen-2-yl-acrylicacid ;8 however, this through- resonance is not reflected by their dissociation constants, which are strictly similar.9 Moreover, in the a-phenyl-p- para-X-substituted-phenylacrylic acids the transmission of the electrical effects (px) is different for E (1) and 2 (2) Y (1 1 (2) X = OMe,, Me,, H, C1, NO,; Y= H X = H; Y = OMe, Me, C1, NO, isomers owing to steric effects.1deg; A combination of polar and steric effects could affect the pK values of the E-and Z-a@-diarylacrylic acids (1)-(4), as a result of differences in stereochemistry.ll9 l2 We are interested in the acid-catalysed isomer-izations of E-ap-diarylacrylonitriles ;l3?l4 any infor-mation on electrical and conformational effects in the parent diaryl-acrylic acids obtained from pK measure- inents could be useful for a wider understanding of the isomerization process.We now report the pK values of 36 acids of the types (1)-(4) determined for solutions in aqueous 2-methoxyethanol at 25 "C.These dissociation constants are not thermodynamic quantities, but they can be analysed analogously by linear free-energy relation~1iips.l~ Y f Y I (3) (4) 2 = 0, S,Se; Y = Me, H, el, NO, RESULTS AND DISCUSSION The pK values of the acrylic acids in Table 1 were measured with the usual potentiometric apparatus, using a procedure requiring correction for the hydrogen-ion concentration.16 TABLE1 pK values of E-and Z-ap-diarylacrylic acids in S070 aqueous 2-methoxyethanol at 25 "C a X Me Me H C1 HNO, Y H H H H H Me E (1) 7.31 (6.99) 7.28 (7.23) 7.03 (7.00) 6.05 (6.66) 6.32 (6.33) 7.31 (2) 5.75 (5.73) 5.73 (5.71) 5.60 (5.59) 5.39 (5.36) 5.05 (5.08) 5.82 H H Mec1 7.40 6.83 5.86 5.50 H NO* 6.20 5.00 z 0 Y Me E (3)7.40 (4)6.25 0 H 7.36 6.15 0 c1 7.17 5.95 0 S NO, Me 6.44 7.50 5.42 6.36 S H 7.35 6.24 S C1 7.17 6.00 S Se NO,H 6.51 7.30 5.29 6.40 Literature pK values are in parentheses.'* TABLE2 Application of the Hammett equation to the pK values of E-and Z-ap-para-substituted-diphenyland a-para-substituted-phenyl-P-arylacrylicacids E (1) (2)Y=H PX -0.942 -0.692 PFO 7.09 5.58 r 0.986 0.992 Sb 0.094 0.051 X=H PY -1.13 -0.816 PKO 7.08 5.65 ra 0.986 0.988 Sb 0.11 0.075 E (3) (4)z=o PY -1.06 -0.892 PFO 7.32 6.13 r 0.980 0.997 Sb 0.15 0.047 z=s Py -1.05 -1.15 PKO 7.35 6.21 ra 0.996 0.995 Sb 0.07 0.08 a Linear correlation coefficient.b Standard deviation of estimated p value.The Z-isomers are stronger acids than the E-isomers, the differences being 1.6-1.2 pK units for substituted diphenylacrylic acids and 1.2-0.9 yK units for the heterocyclic derivatives. This is due to specific con- formational factors of the p-aromatic ring. In the Z-acids both aromatic rings are coplanar with the ethylenic fragment, while the carboxy-group is deconjugated, being non-coplanar.l1Yl2 In the E-acids the p-aromatic ring is almost coplanar with the carboxy-group, and conjugative effects weakening acidity become possible. The i.r. stretching frequencies of the carbonyl group are a measure of conjugative interactions between the aromatic rings and the carboxy-group, a lower frequency being associated with such interaction^.^' This be-haviour has been observed in a-phenyl-p-arylacrylic acids,12*18 where the vco ranges are 1660-1 680 and 1 685-1 710 cm-l for E-and Z-isomers, respectively.The pK values vary linearly with the Hammett 0-values for the substituents, electron-withdrawing groups facilitating the acid dissociation. Hammett p-values for the series of acrylic acids examined are in Table 2. The transmission of electrical effects of the X-substituents is more effective in E-than in Z-isomers for the stereo- chemical reasons already indicated. In the Y-substituted acids py-values do not show significant variations, in agreement with their structural analogies. Moreover, in the Z-isomers py-values are higher than px (absolute values) because of the proximity of the a-phenyl ring to the carboxy-group.The heterocycle-substituted acrylic acids, for both the E-and the Z-configurations, are weaker than the cor- responding benzene derivatives. The obvious explan- ation of a greater electron-donating effect of the hetero- cyclic nucleus to the carboxy-group is really only applicable to the E-isomers, where functional conju- gation is possible; this explanation is unreliable for the 7 met(E,Z): 2-fury1 (-0.29, -0.79) ; 2-thienyl (-0.28,-0.92); selenophen-2-yl (-0.22, -1.16). J.C.S. Perkin I1 Z-isomers, where the carboxy-group is twisted out of the plane. Electron-withdrawing effects in the ground state cannot themselves explain the observed trends in the pK values, since a strengthening of the acidity would be expected compared with the phenyl derivatives.It is reasonable that a combination of polar and steric effects may contribute to the observed pK values. There are two alternative methods for examining this problem : the Hammett treatment and the separation of polar and steric effects. Application of the Hammett equation to equilibria or reactivity data for five-membered heterocyclic com-pounds could provide information on the structural effects of heteroatom~.~,~~ In fact, if we consider the heterocyclic nucleus as a substituted benzene, constants for the replacement of a CH=CH group of the benzene ring by the heteroatom can be calculated by using the appropriate px and pKo values in the Hammett equation (1).The calculated 0het values indicate a chet = (PK -pamp;)/PX (1) greater electron-donating effect of the heteroatoms in the Z-isomers.? This is misleading since it is in contrast with the stereochemical data.11T121s The different size of p-aromatic ring systems does not allow a unified treatment for derivation of reliable dhet constants; in fact, differences in steric effects, within the series examined, usually cause deviations from linear free-energy relationships. 2o Then Thije and Janssen have pointed out that failure of the Hammett equation might be expected for 2-substituted five-membered heterocyclic compounds, and suggested correlation of the data by use of Taftrsquo;s G* constants. The pK values of arylcarboxylic acids are correlated well by these constant^.^,^ However, in a-phenyl-P-arylacrylic acids the occurrence of steric as well as polar effects complicates the application of cor-relation analysis.The Taft-Pavelich equation (2),21 which takes into account both polar (o*) and steric log klk, = p*~*+ BE, (2) effects (amp;), appears to be a better model for relating pK values to substrate structure. Nevertheless, the E, values for aromatic and unsaturated groups cannot be considered to be totally reliable measures of steric effects including conjugative contributions.21 The practical utility of this equation for five-membered heterocyclic compounds is then greatly reduced. To elucidate quantitatively the role of steric as well as polar effects, we have tested the application of equation (3),which has already been used in a kinetic study of the acidic isomerization of E-crp-diarylacry10nitriles.l~ pK == PO + $0 + constant (3) The electronic effects of the heteroatoms in the ground state are represented well by aBA-values,f while the dif- $+ asa-values : 2-furyl, 1.04; 2-thienyl, 0.67 ; selenophen-2-y1, 0.60.2 ference between the internal angles in the heterocycles LZ-C(2)-C(3) * and the phenyl ring (120') was chosen as a measure of steric effects (0).The p and t,b para-meters are the susceptibilities of the equilibrium to polar and steric effects. In this way the value of the constant term affords the calculated pK, for the unsubstituted phenyl derivative, whose G and 8 values are zero.We have used a computer to analyse the relationship between the pK values for a-phenyl-p-pma-X-substituted phenylacrylic acid (X = OMe, Me, H, C1, NO,) and p-2- t -5.0 1 I I I I 1 l.--L- -0.2 0.0 0.2 0.4 0.6 0.8 1.0 0 Correlation of (pK -$0) with 5 values for the ionization of E-and Z-a-phenyl-p-arylacrylic acids furyl, p-2-thieny1, and p-selenophen-2-yl compounds and the appropriate CI and 0 values. Regression analysis provided the empirical equations (4) and (5). E isomers: PK = -0.843~ -0.10~+ 7.07 (4) (Y L-0.950; S, = 0.095; s,,, = 0.010; s,,K, 0.05)=z 2 isomers: pk' = -0.6990 -0.1398 + 5.58 (5) (Y == 0.996; S, = 0.036; S,J = 0.021; s~,bsol;,= 0.02). The negative signs of both p and # constants are coii- sistent with the expected effects, since the increase in G and 6 values tends to decrease the pK values.The more effective transmission of polar effects in E isomers lp~ pzl is confirmed by this extended treatment including heterocyclic compounds. Also, the greater t,b value for the 2 isomers $El is in agreement with the foregoing stereochemical arguments. The chemical meaning of these equations is that the observed pK values, corrected for contributions due to steric effects (pK -$O), follow the order of the 0-values (Figure). It is noteworthy that the oxygen, sulphur, and selenium atoms in 2-furyl, 2-thienyl, and selenophen- 2-yl rings, which seem to behave as electron-donating * Internal angles 22 and 0-values are 110.68"and -9.32 for the furan, 111.47" and -8.53 for the thiophen, and 111.56" and -8.44 for the selenophen.substituents using the Hamniett treatment, appear to be electron-withdrawing as suggested by their QBA values. EXPERIMENTAL Materials.-The E-and Z-ap-diarylacrylic acids (1)-(4) were synthesized by Perkin condensation of the appropriate arenecarboaldehyde and phenylacetic acid in acetic anhy- dride with triethylanline or pyridine as catalyst, following a procedure already 23 The previously reported acids had n1.p.s in agreement with the E-a-phenyl-P-selcnophen-2-ylacryEic acid had m.p. 189 'C (from benzene-light petroleum) ; vco 1 660 cni-l (KBr disc) (Found: C, 56.2; H, 3.7. C13H,o0,Se requires C, 56.3; H, 3.67;) ; Z-a-pJieny~-P-selenop~~en-2-ylacrylicacid had m.p.130 "C (from benzene-light petroleum); vco 1 685 cn1-l (KRr disc) (Found: C, 56.45; H, 3.5); Z-a-p-toZy1-P-fihenylacrylic acid had m.p. 128 "C (from benzene-light petroleum) (Found: C, 80.5; H, 6.0. Cl6H14O2requires C, 80.65; H, 5.9); Z-a-p-chlorophenyl-p-PhenyZacrylicacid had m.p. 126 "C (from benzene-light petroleum) (Found: C, 69.5; H, 4.4. C1,HllC1O2 requires C, 69.6; H, 4.3). The stereochemical purity of the crystallized acids was checked by 'H n.m.r. spectroscopy in each case.1deg;.12 2-Methoxyethanol (commercial sample) and doubly dis- tilled water (' carbon dioxide free ') were used to prepare 80.4 (w/w) aqueous 2-methoxyethanol. Acidity Constants.-pK Measurements were made in duplicate or triplicate runs by potentiometric titration of the acid by a digital pH-meter (Amel 333) equipped with an automatic burette using a combined glass-calomel electrode.The acrylic acid concentrations were about 2 x 10-3~. The solutions (100 ml) were titrated in a thermostatted cell (25 f 0.1 "C) under nitrogen with 0.1~sodium hydroside. The electrode was standardized with buffer solutions before and after each titration. The pK values were calculated by equation (6) l6 where the hydrogen ion concentration was pK = pH + log(CH* -CH+)-lOg(CA-+ CH+) (6) talten to be antilog pH, and can be neglected above pH 6. The maximum error was amp;0.03pK units for all compounds. We thank the Consiglio Nazionale delle Ricerche (Rome) for financial support.9/373 Received, 7th March, 19791 REFERENCES 1 C. K. Ingold, ' Structure and Mechanism in Organic Clicmistry,' 2nd edn., Bell, London, 1969, ch. 14. 2 P. Tomasik and C. D. Johnson, Adv. Heterocyclic Chem., 1976, 20, 38. 3 1'. A, Then Thije and M. J. Janssen, Rec. Trav.chim., 1965, 84, 1169. 4 A. Arcoria, E. Maccarone, and A. Mamo, J.C.S. Perkin 11, 1979, 1347. G. Marino, Adv. Heterocyclic Chem., 1971, IS,235. 6 A. Andrisano and A. Tundo, Atti Accad. naz. Lincei, Rend. Classe Sci. 3s. mat. nut., 1952, 13, 158. G. Pappallardo, GazzPtta, 1959, 89, 540.* L. Chierici and G. Pappallardo, Gazzetta, 1960, 90, 69. 9 C. C. Price and E. A. Dudley, J. Amer. Chem. Soc., 1956, 78, 68. lo K.Bowden and D.C. Parkin, Canad. J. Chem., 1968, 46, 3909. l1 H.E.Zimmermann and L. Ahramjian, J. Amer. Chem. Soc., 1959, 81, 2086. l2 S. Fisichella, G. Scarlata, and D. Sciotto, Ann. Chim. (Italy),1973, 63,55; S. Fisichella, G. Mineri, G. Scarlata, and D. Sciotto, ibid., p. 779; Tetrahedron, 1975, 31, 2445. l3 E. Maccarone, A. Manio, G. Scarlata, and M. Torre, Tetra-hedron, 1978, 34, 3531. l4 E. Maccarone, A. Manio, G. Scarlata, and M. Torre, J. Org.Chcnz., 1979, 44, 2896. l5 W. Simon, Angew. Chem. Internat. Edn., 1964, 13, 661. l6 A. Albert and E. P. Serjeant, ' Ionization Constants of Acid and Bases,' Methuen, London, 1962, ch. 2. l7 M. St. C. Flett, Trans. Faraday SOC., 1948, 44, 767. C. W. Bird and E. M. Briggs, Spectrochim. Acla, 1969, 25A, 899. l9 H. H. Jaffe and H. L. Jones, Adv. Heterocyclic Chem., 1964, 3, 221. J.C.S. Perkin I1 2u L. P. Hanirnett, ' Physical Organic Chemistry,' 2nd edn., McGraw-Hill, New York, 1970, ch. 11. 21 R. W. Taft, jun., ' Steric Effects in Organic Chemistry,' ed. M. S. Newman, Wiley, New York, 1956, ch. 13. 22 17. Fringuelli, G. Marino, and A, Taticchi, Adu. HeterocycZic Chem., 1977, 21, 121. 23 R. Ketcham and D. Jambotkar, J. Org. Chem., 1963, 28, 1034, and references cited therein. 25 C. W. Bird and E. M. Briggs, J. Chem. SOC. (C), 1967, 1265, and refcrences cited thercin.