213
J . A m . Chem. SOC. 1989, 1 11, 2 13-223
Solvent and a- and ,&Substituent Effects on the Association with Hydrogen Bond Accepting Solvents and the OH Conformation of Stable Simple Enols’j2 Ella B. Nadler and Zvi RapPoport* Contribution from the Department of Organic Chemistry, The Hebrew Unicersity of Jerusalem, Jerusalem 91 904, Israel. Receiued April 21, 1988. Recised Manuscript Received August 15, 1988
Abstract: The association of stable simple enols with hydrogen bond accepting solvents was investigated by ’H NMR. &(OH) values for P,P-dimesityl-cu-arylethenols 2 (Ar = Mes, p-An, p-Tol, Ph, p-CF3C6H4,3,5-Br&H,), a,@-dimesityl-P-R-ethenols 3 ( R = Ph, Me, Et, i-Pr), and P,P-dimesityl-a-(trimethylsi1yl)ethenol (4c) in CC14-DMSO-d, mixtures were measured. The O H signal is sharp and enol-enol association is absent. The large changes in 6(OH) are ascribed to the formation of a 1 : 1 enol.DMSO hydrogen-bonded association complex, and association constants K,,,, were calculated from the &(OH)values. For compounds 2, K,,,, = 1.6-5.2 and it increases with the polar electron withdrawal of the a-Ar, and p = 0.35. In DMSO-d6, 298% of the enol is associated. The calculated &(OH)values for the associated form are linear with Hammett’s u values. Steric effects of the a-mesityl group on K,,,, or &(OH)are minor. The &(OH)for compounds 3 are linear with the &(OH) of the P,@-dimesityl-a-Ranalogues 4, but K,,,, values of 3 (1.7-2.8) are 1.3-2.3-fold higher. K,,,, values for 4c (0.52) and for 4, R = t-Bu, are close. The changes in &(OH)and 3J(HCOH) for P,P-dimesitylethenol (1) were determined in binary and K,,,,(J) values were determined. CC1,-solvent mixtures in eight solvents and found to be parallel in most cases. Kassn(&) For each solvent they are reasonably similar in most cases and are solvent dependent. K,,,,(J) values range from 0.05 (C6D6) to 9.7 (DMSO-d,). The log K,,,, values are approximately linear with the hydrogen bond accepting ability parameter ,6 of the solvent. The dihedral C=C-0-H angle increases with 0, e.g., the conformation is anti-periplanar in C6D6or dioxane-& but anti-clinal in DMSO-d, and DMF-d,. In CC1, the conformation is syn-planar with an intramolecular r(Ar)-.HO hydrogen bonding. &(OH)and 3J(HCOH) values were also measured in several solvent-DMSO-d, mixtures. The Karrnvalues with acetone for P,P-dimesityl-cu-methyl (0.25) (and a-tert-butyl, 0.09) ethenols in CCI4-acetone mixtures are lower than those for association with DMSO.
Recently we investigated the association of P,@-dimesitylethenol (1) with various solvents and the solvent-dependent conformation
of its O H group by a combination of ‘ H NMR and IR technique^.^ From the correlation between the H-C-0-H dihedral angle and the 3J(H C O H ) coupling constant^,^ the C=C-OH conformation around the C - 0 bond is syn-planar ( l a ) in non-hyMes
\ /c=c
/H
Mes\
\
9
Mes H’
/c=c
Mes
ib
la Mes = mesityl
2,4,6-Me3CeH2
drogen bond accepting solvents, (e.g., CCI,) and anti-clinal ( l b ) in strong hydrogen bond accepting solvents, (e.g., D M S O ) . In moderate hydrogen bond accepting solvents (e.g., acetone), the two conformers are present in appreciable population and are in a rapid equilibrium. The solvent-dependent conformation is due mainly to hydrogen bonding: intermolecular with the solvent S (C-O-H--S) in hydrogen bond accepting solvents and intramolecular [r(Ar).-HO] between the OH and the cis-mesityl group. The association constant K,,,, for the 1:l solvate (of lb) with DMSO was obtained from the changes of 6(OH) and 3J(HCOH) for 1 with the composition of the CCl,-DMSO-d, mixtures. The reported K,,,, values based on 3 J ( H C O H ) and &(OH)were 9.35 The analogous 6(OH) and K,,,, values for and 7.9, re~pectively.~ a-alkyl-/3,P-dimesitylethenols were recently determined.] The
decrease of the Karsnvalues with the increased bulk of the a-alkyl group was ascribed to a combination of predominant electronic effects and steric effects.’ These studies raised several questions: These include (a) the contribution of polar effects to K,,, in the absence of steric effects, (b) the dependence of K,,,, on the structure of isomeric dimesitylethenols, (c) the correlation of K,,,, values with solvent parameters, (d) the dependence of the O H conformation on the solvent, and (e) the comparison of the effect of an a-Si substituent and related a-alkyl substituents on K,,,,. Some of the answers are given in the present paper which describes the effect of a-aryl and a-trimethylsilyl and of &phenyl and /?-alkyl substituents in the enols on K,,,, in D M S O and the solvent effect on the K,,,, values of 1 and derivatives.
Results The change in the hydroxyl chemical shift & ( O H ) for @,P-dimesityl-a-aryl (2a-f) and a-trimethylsilyl and of (Z)-a,P-dimesityl-@-alkyland p-phenyl (3a-d) ethenols in binary CC1,Mes M e s ,C=C
‘ 0 H
2a. Ar b , Ar c , Ar d , Ar e , Ar
0002-7863/89/1511-0213$01.50/0
R
/OH \Mes
3s. R = Ph
= Mes
=p-MeOC6H, ( p - A n ) =p-MeCeH, [ p - T o l ) = Ph =P-CFSC~H, f , Ar =3.5-BrzC6H3
b, R = M e c. R =Et d . R = 1-Pr
4 - MeC6H4
\
/c=c
MeszC=C
( I ) Part 21: Stable Simple Enols. Part 20: Rappoport, Z . ; Nugiel, D. A,; Biali, S. E. J . Org. Chem. 1988, 53, 4814. (2) Presented in part in (a) Rappoport, Z. Kyushu “Zen” Symposium on Physical Organic Chemistry, Kyushu University, Fukuoka, Japan, September 1986; Abstract pp 53-62. (b) Nadler, E. B.; Rappoport, Z., The 53rd Meeting of the Israel Chemical Society, Beer-Sheva, Israel, February 9-10, 1988. (3) Biali, S. E.; Rappoport, Z. J . A m . Chem. SOC.1984, 106, 5641. (4) (a) Fraser, R. R.; Kaufman, M.; Morand, P.; Govil, G. Can. J . Chem. 1969, 4 7 , 403. (b) Watanabe, S. J . Mol. S f r u c f .1980, 64, 285.
\
/c=c
/Ar
\OH
4 a , R = Me b, R = f-Bu c , R = SiMe3
/H
\O
Mes
H
5
DMSO-d, mixtures and for a-methyl- and a-tert-butyl-P,@-dimesitylethenols (4a and 4b) in CC14-acetone mixtures was studied. Both 6(OH) and the ) J ( H C O H ) coupling constants for &@-di-
C 1989 American Chemical Society
214
J. Am. Chem. SOC.,Vol. I l l , No. I , 1989
Nudler and Ruppoport
mesitylethenol (1) were determined in several binary solvent mixtures and for (Z)-/3-tolyl-~-mesitylethenol( 5 ) in CCl, and in DMSO-d,. First we will describe the method of calculation of the association constant from the N M R parameters; then the results for each series will be presented separately. Absence of Intermolecular Hydrogen Exchange. The absence of an intermolecular enol-enol association is deduced from the absence of exchange of the O H groups of different enol molecules on the N M R time scale, as demonstrated by the following observations: (a) sharp OH signals; (b) a 10-fold increase in the concentration of 1 in CC1, resulted in a negligible change in 6(OH) and 3J(HCOH) (cf. Table IX); (c) a mixture of 1 and 2d ( 5 mg each) in CC14 (0.5 mL) a t 292 K displays an OH doublet a t 6 4.45, 3 J ( H C O H ) = 14.5 Hz, and an O H singlet a t 6 5.01 which are a t the positions of the individual O H signals of 1 and 2d, respectively. When 0.1 m L of DMSO-d, was added to this mixture, the separate signals shifted to 6 8.58 ( 3 J = 6.3 Hz) and 6 7.88, which are the approximate 6(OH) values of 1 and 2d in 80:20 CC1,-DMSO-d6 (v/v) mixtures. Calculation of the Association Constant K,,,,. When a nonsolvated (I) and a 1:l solvated (11) enolic ( R O H ) species exist in equilibrium in a solvent S (eq I ) , the equilibrium constant K,,,
value is then inserted into eq 8, and the derived linear plot gives a slope of 1 / ( ~ 3 ~ ,6,) and an intercept of l/Kassn(611 - 6,). Since for all our experimental points [11] 0.99, still has several drawbacks. First, the simultaneous calculation of K,,,, and 6,, from the slope and intercept of the same plot leads to mutually dependent errors in both terms. The errors due to this and to the small intercept will not be discussed here since they were treated extensively both for hydrogen bond associationsbi and in relation to the BenesiHildebrand equation.’ Second, when small changes in the solvent compositions result in large changes in tjOkall the first experimental points are grouped together near the intercept in these plots. This treatment accords equal weight to every point. Since there are more points for lower concentrations of base, these points are ultimately more influential in determining K,,,,. Third, it does not include iterative search for the best 6I value. Consequently, following a related treatment by Taft and co-workers,I0 we used eq 2 and 6 to derive eq 9 which gives dabs as a function of 61, 6,,, gobs
I + S S I I (1) is defined by eq 2, where [SIf is the nonassociated solvent and
=
[(6ll - 61)/2[ROHlOI [[ROHIO+ [SI, + I/Kassn - 2[SIo[ROHIo + 2[Sl0/Kassn + 2[ROHl0/Kassn + 1/Kassn2)”*1 + 61 (9)
and K,,,. By using 6, = 6ccl, and K,, and 6,, values derived from eq 8, a calculated value for 6,bs is obtained. A least-squares (2) procedure, which minimizes C(A6,,),2 is then applied where Sob, Fs are the corresponding mole fractions. Equations 3 and 4 is the difference between the measured and the calculated 6,, (with represent the mass balance where the subscript 0 designates the the parameters derived from eq 8) where i runs over the data points initial concentration of the species before complexation. for the values. The RNLIN subroutine from the IMSL package, which fits a nonlinear model using least squares, was employed. [ROH], = [I] [11] (3) RNLIN is based on HINPACK routines LMDIF and L M D E R ” ~and uses [SI0 = [SI, + [I11 (4) a modified Levenberg-Marquardt”” method to generate a sequence of approximations to a minimum point. The following The following analysis applies for a case when I and 11 are in statistical quantities are derived directly: DFE, degree of freedom a rapid equilibrium on the N M R time scale and the solvent effect for errors; SSE, sums of squares for errors; Rank of matrix R, on the N M R parameters 6(OH) or 3J(HCOH), which is not where R is the upper triangular matrix containing the Q R derelated to solvation, is small. Although the equations are written composition of the Jacobian; RB,the multiple correlation coeffor the parameter 6, the same analysis applies for 3J(HCOH). ficient; and SD, the standard deviation of Examples are given Each observed value (60bs) corresponding to a different in the Experimental Section. The K,, and 6 values derived from CCI,-solvent mixture is a weighted average of the 6 values of I eq 8 and 9 are given in the tables. and I1 (61, 611) (eq 5). Inserting eq 3 into 5 gives eq 6 and 7. Kass$PP values obtained on assuming a complete association in = - Fll) + 6llFlI (5) any pure solvent (Le., 6,, = 6,) are sometimes given for comparison. Systems. The 6(OH) values of the tridabs = 6l([ROHlO - [III)/[RoHIO + ~ l l [ ~ ~ l / ([ 6 ~ ) ~ ~ l a-Aryl-Substituted O arylethenols 212 in CC14 are 4.96-5.10 ppm. Addition of a small (7) [I11 = [ROHlO(6abs - 61)/(6Il - 61) amount of DMSO-d, to a solution of 2 in CC14 results in a relatively large downfield shift of the sharp O H signal. E.g., addition A priori, neither 6I nor 6,, are known. IR studies indicate that of 0.2% (v/v) DMSO-d, brings about downfield shift of ca. 0.2 in CCI, the enols are 298% intramolecularly a s ~ o c i a t e dand ~*~ ppm in systems with electron-donating a-aryl groups (2a-c), and hence the error introduced by assuming that 6, = bCClr is small. 0.4-0.6 ppm in 2e and 2f with the a-electron-withdrawing aryls. This is consistent with the absence of intermolecular association In 99% CC14-1% DMSO-d, (v/v), 6(OH) values are already with CCI4, a solvent with a hydrogen bond accepting parameter 5.72-5.86 for 2a-2d and 6.36-6.74 for 2e and 2f. Since 6(OH) of 0 in the Kamlet-Taft solvatochromic equation,, and with the extensive use of CCI, as a solvent for association studies of alcohols and phenols.7a Both K,,,, and are unknown and cannot be (8) (a) Nakano, M.; Nakano, N. I.; Higuchi, J. J . Phys. Chem. 1967, 71, 3954. (b) Berkeley, P. J., Jr.; Hanna, M. W. J . Phys. Chem. 1963, 67, 846. assumed. By combination of eq 2, 4, and 7, eq 8 is obtained. A Kassn = [I11/ [I1 [SI, = F I I / F I [ S I ~
+
[s10/(60bs - 61) = ( [ R O H I O+ [SI,
- [111)/(611 - 61) +
l/Kassn(ht - 61) (8)
plot of the experimentally known [s]O/(6,b, - 6,) values vs the [ R O H I o [SI, - [11] term should give both KaSsnand 1 3 ~ The ~. unknown [11] term is estimated by an iteration procedure: [s]O/(6,b, - 6,) is plotted first against ([ROH], [SI,) values, and the obtained slope is used to calculate [11] from eq 7. This
+
+
~
~~
~~~
~
~
~
~
~
(5) Buswell, A. M.; Rodebush, W. H.; Whitney, R. McL. J . A m . Chem. SOC.1947, 69, 770. (6) (a) Kamlet, M. J.; Abboud, J.-L. M.; Taft, R. W. Prug. Phys. Org. Chem. 1981, 13, 485. (b) For a list of recent p values, see: Kamlet, M. J.; Doherty. R. M.; Abraham, M. M.; Carr, P. W.; Doherty, R. F.; Taft, R. W. J . Phys. Chem. 1987, 91, 1996. Kamlet, M. J.; Taft, R. W. Acta Chem. Scand. 1986, 8 4 0 , 619. (7) (a) Joesten, M. D.; Schaad, L. J. Hydrogen Bonding; Marcel Dekker: New York, 1974; pp 293-363; (b) pp 173-175.
_
(c) Hanna, M. W.; Ashbaugh, A. L. J . Phys. Chem. 1964, 68,811. (d) Mathur, R.; Becker, E. D.; Bradley, R. B.; Li, N. C. J . Phys. Chem. 1963, 67,2190. (e) Foster, R.; Fyfe, C. A. Trans. Faraday SOC.1965, 61, 1626. (f) Goldstein, M.; Mullins, C. B.; Willis, H. A. J . Chem. SOC.B 1970, 321. ( 9 ) Siung, J. H.; Goldberg, E.; Miller, S. 1. Org. Magn. Reson. 1972, 4, 683. (h) Slejko, F. L.; Drago, R.S. Inorg. Chem. 1973, 1 2 , 176. (i) Rosseinsky, D. R.; Kellawi, H . J . Chem. SOC.A 1969, 1207. (9) (a) Benesi, H. A,; Hildebrand, J. H. J. Am. Chem. SOC.1949, 71, 2703. (b) Mulliken, R. S.; Person, W. B. Molecular Complexes; Wiley: New York, 1969; Chapter 7, pp 81-90. (c) Person, W. B. J . A m . Chem. Soc. 1965,87, 167. (d) Hammond, P. R. J . Chem. SOC.1964, 479. _ _ ( I O ) _(a) Taft, R._ W.; ~Klingensmith, G. B.; Ehrenson, S . J . A m . Chem. SOC. 1965, 87, 3620. (b) Gurka, D.; Taft, R. W. Ibid. 1969, 91, 4794. (1 1) (a) Dennis, J. E., Jr.; Schnabel, R . B. Numerical Merhods f o r Unconstrained Optimization and Nonlinear Equarions; Prentice-Hall: Englewood Cliffs, NJ, 1983. Levenberg, K. Q.Appl. Marh. 1944, 2, 164. Marquardt, D. W. S I A M J . Appl. Marh. 1963, 1 1 , 431. (b) More, J . J.; Garbow, B. S.; Hillstrom, K . E. User Guide for MINPACK-1; Argonne National Labs Report AKL-80-74, Argonne, IL, 1980. (c) Daniel, C.; Wood, F. S. Fitting Equations to Data; Wiley: New York, 1971. (12) Nadler, E. B.; Rappoport, Z . J . A m . Chem. Sor. 1987, 109, 2 1 12.
J . Am. Chem. Soc., Vol. 111, No. I , 1989 215
Enol-H Bond Accepting S o h e n t Associations
Table I. 6(OH) Values for cu-Aryl-/3,/3-dimesitylethenols 2 in Binary CC1,-DMSO-d6 Mixtures % DMSO-d, in % DMSO-d, in
CCId-DMSO-d, (v/v)
[DMSO-d,], M 2aa 2bb 2ca 2d'
0 0.2 0.4 0.6 0.8
0 0.028 0.057 0.085 0.11 0.14 0.28 0.57 0.71
1 2 4 5
5.10 5.31 5.44 5.55 5.13 5.84 6.16
4.96 5.15 5.29 5.44 5.54 5.72 6.07
4.97 5.16 5.31 5.51 5.57 5.76 6.23
5.00 5.23 5.42 5.57 5.69 5.86 6.22
CCI,-DMSO-& Zed 5.08 5.52 5.77 5.96 6.16 6.36 6.74
2P
(v/v)
5.03 5.67 6.06 6.22
7 IO 20 40 50 60 80 100
6.75 7.27 7.76
[DMSO-d,], M 2aa 2bb 2ca 2dC Zed 2P 0.99 1.41 2.83 5.66 7.07 8.48 11.31 14.14
7.44 7.31 7.41 7.54 7.76 7.72 7.76 7.91 8.02 8.02 8.05 8.14 8.08 8.17 8.10 8.19 8.22 8.25 8.27 8.29 8.33 8.43 8.36 8.38 8.43 8.53
7.92 7.91 8.13 8.22 8.38 8.43 8.58 8.60 8.71 8.65 8.79 8.78 8.91
7.00 6.84 6.93 7.08 7.49
[2] = 0.027 M; measured at 289 K . [2] = 0.026 M; measured at 292 K . [2] = 0.028 M; measured at 290 K. 293 K. c[2] = 0.019 M; measured at 289 K.
[2] = 0.024 M; measured at
Table 11. Parameters of the Linear Correlations 6(OH)[Mes2C=C(OH)Ar] = A6(OH)[1] + B in Binary CC14-DMSO-d6 Mixtures",b c, , ~ i i d ~ u d Ar A B R U A'C B!C RC Rd b 0.36 3.47 0.9933 0.04 0.9991 0.01 Mes 0.77 1.17 0.9677 0.32 1.25 -2.91 0.34 0.38 3.20 0.9920 0.05 1.48 -4.92 0.9957 0.03 p-An 0.81 0.80 0.9664 0.43 2.99 0.9858 0.07 1.41 -4.24 0.9987 0.02 0.9721 0.31 P-ToI 0.82 0.81 3.11 0.9966 0.03 -3.50 0.9984 0.02 0.32 0.42 1.34 Ph 0.83 0.82 0.9718 0.9962 0.03 1.09 0.9874 0.21 0.56 2.61 0.9991 0.02 1.16 -1.69 p-CF?C6H4 0.83 0.9982 0.02 m,rn-Br,CsH, 0.83 1.29 0.9976 0.09 0.76 1.69 0.9976 0.06 1.05 -0.57 "Data for enols 2 from Table I and for 1 from ref 3. bFor 12 6(OH) values, except for m,m-Br&H, where 13 points were used. 'Parameters of the correlation for the first six points only. dParameters of the correlation for the last six points only.
8V
b
a
f
I
7-
c
5g IL 4i
/
l i 1 -
4
IO
6(3H;
'
1 40
1 60
Yo DMSO in CC14-DMS0
11
.
I20 ;v/v)
Plot of 6(OH) values of Mes2C=C(OH)Ar vs percent DMSO-d, (v/v) in CC14-DMSO-d6. A ( 0 )Ar = 3,5-Br,C6H3 (2c); B (0)A r = p-MeOC,H, (2f). Figure 1.
in pure DMSO-d, is 8.36-8.91, a large percentage of the shift is already achieved on addition of the first 1% of DMSO-d,; 185% of the change takes place at up to 60% cc14-40% DMSO-d,. This is demonstrated for 2b and 2f in Figure 1. The 6(OH) values for 2a-f in 14-15 binary CC14-DMSO-d6 mixtures [5-6 at &I% ([0.14 MI V/V)DMSO-d,, 5 a t 1-10% DMSO-d,, and the rest a t higher [DMSO-d,]] are given in Table I. Plots of 6(OH) for 2 vs 6(OH) of 1 in the binary mixtures are linear according to their reasonable correlation coefficients (eq 10 and Table 11). However, visual inspection (cf. Figure 2) shows 6(OH)(2) = A6(OH)(1) B (10) that the plots are actually concave (e.g., Figure 2A for the a-p-tolyl derivative 2c), and near linearity ( A = 0.83, R = 0.9976) was observed only for the most electron-withdrawing m,mBr,C,H,-substituted system 2f (Figure 2B). A division into two linear plots, one up to 6(OH)[1] values of ca. 7.5 and one a t the higher 6(OH) values, gives better correlation coefficients for each part of the plot as shown in Table 11, where the A', B'and the A", B"values relate to the first and the second part of the plot, respectively. Plots according to eq 8 are linear (Figure 3), and the slope and intercept remain unchanged after one or two iterations. The derived K,,,, and 6(OH) values for conformer I1 together with the 6cc4and dDMsovalues and the fraction of species I1 in D M S O are given in Table 111. Also given are KansnaPP values calculated
54 f3r
6
0
13
1
Plots of 6(OH) for Mes,C=C(OH)Ar vs d(OH) for Mes2C=CHOH in CCI,-DMSO-d, mixtures A, Ar = p-MeC,H, (Zc), B, Ar = 3,5-Br2C6H3(2f)
Figure 2.
47 /
3
+
5
IO
[ R O H I o + [DMSO];
I5
20
[I11
Figure 3. Plot of [DMS0]o/(6,b - 6cch)vs [ROH], + [DMSO], - [II] for Mes2C=C(OH)C6H3Br2-3,5in CC14-DMSO-d6mixtures.
for the region of 0-10% DMSO-d, in K,, values calculated according to eq 9 varied by 0-5% for 2a-2e compared to K,,,, values according to eq 8. However, K,,,, for 2f was much larger, Le., 5.16. The 611.DMso value was 8.73, 0.18 ppm lower than the observed value in the pure solvent, whereas 61.c~14was 5.19, 0.16 ppm higher than the observed value. The calculated 61.cc4 values were also higher by 0.05-0.17 ppm than the observed rJCcq values. Since a justification of the 611.DMso values according to eq 9 is not obvious and the only significant difference in K,,,, is for 2f, and
216 J. Am. Chem. SOC.,Vol. 111, No. I , 1989 Table 111. 6(OH) (in ppm) and K, in CC14-DMSO-d6 Mixtures
Ar na 6CCl4
~DMSO KaJs$PP
6 1 1 . ~ (eq ~~0 8) Kasm
(eq 8)
O/c IImDMSO in
Nadler and Rappoport
Values for MeszC=C(OH)Ar
Mes p-An p-To1
Pm,mCF3C6H4 BrzC6H3
Ph
14 15 15 14 5.10 4.96 4.97 5.00 8.38 8.43 8.53 8.36 1.58 1.73 1.84 1.84 8.42 8.45 8.49 8.58 1.82 1.69 1.79 1.94 98.2 98.0 98.3 98.6
14 5.08 8.78 2.33 8.79 3.00 99.7
14 5.03 8.91 2.84 8.89 3.87 100
DMSO-d, (eq 8) 5.03 5.03 5.08 5.25 5.19 6 1 . ~ (eq ~ ~ 9) 4 5.15 8.69 8.73 6 1 1 . ~ ~ (eq ~ 09) 8.36 8.41 8.43 8.51 1.84 1.93 2.99 5.16 K a s m (eq 9) 1.82 1.61 "Number of experimental points. bCalculated by using the first nine points in the range of 0-10% DMSO.
0.5
02
t-
r-;,
-0.4 -0.2
/
, 0.0
0.2 U
, 0.4
3.6
i
,
0.8
Figure 4. Log Ka, with DMSO for 2a-f (from eq 8) vs u plot ( 0 ) .The open circle is for Mes with u = -0.42.
since previous reported values were derived from eq 8, we used values from eq 8 for the correlations below. Correlations Involving log K,,,, and 8(OH). Log K,,,, values were plotted against Hammett's u values. The u values recommended by ExnerI3 were used except for p-MeO, where a recommended value is not given, and hence we used the value based on the pK, of p-MeOC6H4COOH in water (-0.28).13 The linear plot (eq 11 and Figure 4) shows a low response to the substituents log K,,,, = 0.350 0.31 ( R = 0.9926; S D = 0.017) (11)
+
( p = 0.35).14, Addition of the point for mesityl by arbitrarily using u = -0.42 (30pMe) results in deterioration of the plot ( p = 0.31; R = 0.9636; S D = 0.036). A uMeSvalue of -0.1 will put the point on the line. The B(OH) values for compounds 2 in D M S O (BII.DMso) increase with the increased electron withdrawal by the a-aryl substituent (Table 111). A plot of 6II.DMSO vs u valuesI3 shows excellent linearity (eq 12),'4b and the point for the mesityl deviates 6 1 1 . ~ ~= ~0 0. 4 3 ~ 8.51 ( R = 0.9983; S D = 0.01) (12)
+
only slightly (slope = 0.42; R = 0.9966; S D = 0.015). Consequently, a plot of log K,, vs 6ll.DMSO is also linear, and the point for mesityl deviates (eq 13, Figure 5). log K,,,, = 0.82611.DM~~ - 6.68 ( R = 0.9916; S D = 0.018) (13)
(Z)-@-Alkyl- and @-Phenyl-a,@-dimesitylethenols. All the signals of these enols are sharp singlets. The B(OH) values for (Z)-@-phenyl-(3a), @-methyl- (3b), @-ethyl-(3c), and P-isopropyl-, (3d) a,@-dimesitylethenols were determined in 13-1 7 binary
8.4
6.5
8.6
8.7
8.8
8.9
~1:.0uso
Figure 5. Plot of log K, for 2a-f in DMSO vs The open circle is for Ar = Mes.
from eq 8 ( 0 ) .
6 1 1 . ~ ~ ~ 0
CCi4-DMSO-d6 mixtures (Table IV), which enabled comparison with B ( 0 H ) values of the a-alkyl-' and a-phenyl-@,@-dimesity13 isomers. The derived K,, 61.ccL, and 6II.DMSO values, which were calculated by eq 9, are given in Table V. The K,,,, values for the fl derivatives decrease in the order M e > Et > i-Pr as was found for the isomeric a-substituted series, but they are generally higher. Plots of the 6(OH) values for the @-alkylor phenyl derivatives 3 vs the 6(OH) values for the isomeric a-alkyl or a-phenyl derivative 4a, 4 (R = Et, i-Pr),' and 2d are linear with slopes of unity (Table VI). The values for the @-alkyl-substituted systems are shifted by 0.7 ppm to higher field. These enols are much less stable than the a derivatives and they decompose slowly on standing.15 Consequently, the measurements were conducted immediately after dissolution, in order to avoid a concentration change that affects Bobs. Association of 1 in Binary Solvent Mixtures. The previous study of the association of 1 with DMSO-d, in binary CC1,-DMSO-d6 mixtures enabled comparison of the &(OH)and the 3J(HCOH)3 and gave information on the conformation of the C=C-OH moiety. The present work examines 6(OH) and 3J(HCOH) values for 1 in seven new CCI,-solvent mixtures and in mixtures of (CD3)2C0, CD3CN, and C6D6 with DMSO-d, (Tables VI1 and VIII). If a 1:l association complex of 1 with the solvent exists under all conditions, this should be reflected in unaltered values of B(OH) and 3 J ( H C O H ) on changing the concentration of 1. A 10-fold increase in [ l ] from 0.036 M to 0.36 M in 0.4% DMSO-d, [0.057 MI-99.6% CCl, shifted B(OH) downfield by 0.55 ppm and decreased 3J(HCOH) by l Hz, as expected from eq 6 (Table IX). With the increase in concentration, the O H signal became broader and the error in 3J became significant. Finally, a broad singlet was obtained. The broadening appears a t lower [ 11 values in 99: 1 CC14-DMSO-d6. We attribute this behavior to increased viscosity a t the higher concentration. Therefore, the K,,,, values obtained from each experiment by using eq 2-5 are not concentration-independent. However, the substantial broadening of the signals, the nonconformational solvent effects on the signals which are demonstrated by the change of B(CH) in CC14 with the concentration, and the small [DMSO-d61fterm at high [I] lead to large errors in the calculated K,,,, values. Nevertheless, a t the low concentrations of 1 they are reasonably constant. Two sets of K,,, values were calculated from the data of Table VII. K,,,,(B) values were calculated by using eq 9 and K,,,,,(.J) values from an equation analogous to eq 9. .III.,was obtained by an iterative procedure from eq 14. The plots were all linear with - Jobs) =
(13) Exier, 0. In Correlation Analysis in Chemistry; Chapman, N. B., Shorter, J., Eds.; Plenum Press: New York, 1978; Chapter 10, p 438. (14) (a) By using the log K,,, calculated by eq 9 the correlation deteriorates: p = 0.45, R = 0.9666, SD = 0.048. (b) Use of the 611.DMS0values calculated by eq 9 gives p = 0.33 ( R = 0.9911, SD = 0.018).
I
Il0/
o,2LA---
([ROHIO +
[SI0 -
[111)/(J1
- JIIJ+ l/Kassn(JI
- J11.s) (14)
(15) (a) Fuson, R. C.; Byers, D. .I.Rabjohn, ; N. J . Am. Chem. Sac. 1941, 63, 2369. (b) Nadler, E. B. Unpublished results.
J . Am. Chem. Sot.. Vol. 111, No. I, 1989 217
Enol-H Bond Accepting Solvent Associations
Table IV. 6(OH) Values for (Z)-MesC(R)=C(OH)Mes (3a-d) and for Mes,C=C(OH)SiMel (4c) in CCL-DMSO-dh Mixtures
6.7 0.947 6.55 6.46 6.17 8 1.13 6.61 6.56 6.24 10 1.41 7.47 6.76 5.95 12.6 1.78 6.87 6.79' 6.58 20 2.83 7.81 7.04 7.01 6.73 6.36 5.03 4.99 4.70 40 5.66 8.03 7.27 7.26 7.08 6.84 5.57 5.56 5.15 60 8.48 8.18 7.39 7.40 7.23 7.02 6.15 6.04 80 11.31 8.26 7.46 7.51 7.32 7.22 7.02 6.28 6.23 5.87 5.55 100 14.1 8.28 7.52 7.54 7.37 7.32 "At 292 K. [3a] or [4c] = 0.028 M. [3] = 0.034 M. d A t 289 K. 'The OH signal is not observed and it is apparently hidden below an aromatic singlet as deduced by integration (aryl region, CH3CH, = 3.1:5). /Relatively broad signal with various batches of DMSO. 0 0.2 0.4 0.6 0.8 1 2 4 5
4.58 4.85 5.04 5.26 5.45 5.65 6.23
0 0.028 0.057 0.085 0.113 0.141 0.283 0.566 0.707
4.12 4.32 4.54 4.69
4.13 4.33 4.52 4.66
4.06 4.21 4.35 4.46
4.66 4.74.' 4.79 4.84 4.91 4.93 5.15
Table V. K,,,, and 6(OH) Values for (Z)-MesC(R)=C(OH)Mes 3 in CC14-DMSO-d6 Mixtures at 289-292 K R =
d
i-Pr (3d) na 14 17 16 14 4.58 4.12 4.13 4.06 6CCb 4.60 4.12 4.16 4.07 6I.CCl: 8.28 7.52 7.54 7.37 ~DMSO 8.31 7.53 7.56 7.45 611.DMSOb Kass: 2.75 2.58 2.27 1.65 % IIaDMSO in DMSO-d,b 99.2 99.7 99.4 97.6 "Number of points used for the iterative calculations. bFrom eq 9. parameter
Ph (3a)
Me (3b)
Et (3c)
J/"
1
Table VI. Parameters of the Linear Correlation of 6(OH)(3) vs 6(OH)(4) for Different R Groups
R
n"
Ph Me Et i-Pr
14 6 6 7
slope 1.07 1.00 1.07 1.10
intercept -0.64 -0.5 1 -0.72 -0.66
Rb 0.9972 0.9937 0.9854 0.9630
Figure 6 . Plot of A'J(HC0H) vs A6(OH) for binary CC14-solvent mixtures. A (0) CCl,-CD,CN; B ( 0 ) CCI4-CD3COCD3; C (A) CCI,-THF-d,.
SDb 0.10 0.12 0.18 0.26
Number of points in the correlation. b R , correlation coefficient; SD. standard deviation.
R = 0.99, except for benzene and nitrobenzene. The values together with 611.s,61ccq, File, and KassCPPvalues are given in Table X. Kassn(J)and KaSsn(6)show reasonable agreement in most of the solvents, except in dioxane where KaSsn(6)= 2.4Ka,,(J). The relatively small changes in 6(OH) and 3Jfor nitrobenzene and benzene resulted in an appreciable scatter in the plots according to eq 8 and corresponding errors and differences between the two K,,,,values. 6(OH) in benzene changed in the opposite direction so that only Kassn(J)was calculated. Both 6(OH) and 3 J ( H C O H ) are temperature dependent. On lowering the temperature, 6(OH) shifts downfield and 3J(HCOH) decreases (Table XI). The two changes are approximately parallel, and they become more pronounced with the increase in the hydrogen bond accepting ability of the solvent. 3J(HCOH) vs 6(OH) Correlation. The changes in the 3J( H C O H ) and the 6(OH) values in the binary CC14-solvent mixtures are parallel. Plots of A J = Jcc,,- J o b s vs A6 = Bobs 6ccq are linear ( R > 0.995; eq 15) in all the binary mixtures (Table 3 J ( H C O H ) = C6(OH)
+D
(15)
XI1 and Figure 6), as found previously for CC14-DMS0 mixture^,^ except for the CCI4-dioxane plot, which is curved. Enol 1 in S0i~ent-DMSO-d~ Mixtures. The changes in 6(OH) and 3 J ( H C O H ) values for 1 in C6D6-DMSO-d6, CD3CNDMSO-d,, and CD3COCD3-DMSO-d6 mixtures were also studied (Table VIII). As expected, the addition of small percentage of DMSO-d, to the solvent resulted in a larger change for C,D,DMSO-d, and a small change for CD3COCD3-DMSO-d6. A3J(HCOH) vs A6(OH) plots for CC14-CD3CN, CD3CNDMSO-d,, and CC14-DMSO-d6 mixtures were h e a r . In these binary mixtures the equilibria involve three species, I (with 61), the solvent-associated species 11-S (with 611.s),and the
DMSO-d,-associated species 11-DMSO (with 6 1 1 . ~ ~ Equations ~ ) . 16 and 17 give the observed 6(OH) value and the mass balance. bobs
=
61FI
+ ~II.sFII.s + 611.DMSOFlI.DMS0
FI + F I I .+~ FII.DMSO =1
(16) (17)
The 61, 611.s, and 61I.DMSO values are known from Tables VI1 and X . Defining Kassn3by eq 18 and 19, approximating [SI, by [SIo 11-S
+ DMSO
Km3
11-DMSO
+S
Kassn3= [II-DMSO][SI,/ [II.S][DMSO]f
(18) (19)
and [DMSOIf by [DMSOIo and rearranging the equation, gives eq 20 where Kassn'and Kassn2are the association constants for 1 Kassn3 = Kassn2/Kassn'
(20)
with the solvent and D M S O respectively. A large error in Kassn3 calculated from Kassnland Kassn2values from Table X is due to the combined errors in these values and to the small changes in 6obs and Jobsvalues for (CD3),CO-DMSO-d6. Hence, the conclusion is only qualitative: even a small percentage of added DMSO-d, associates effectively with the free 1 in the mixture. 6(OH) and K,,,, for the cu-Trimethylsilyl Enol 4c. The 6(OH) values for the a-silylenol 4c in 12 binary CC14-DMSO-d6 mixtures are given in Table IV. The values in pure CCI, (4.66) and pure D M S O (7.32) differ slightly from those for 4b (4.77 and 7.30, respectively). A plot of 6(OH) vs & ( O H ) for 1 is nonlinear, resembling Figure 2A. The linear plot according to eq 8 gives a K,,,, = 0.71 and 6 1 1 . ~ ~=~7.52, 0 and F,I.DMSOis 0.93. However, although R > 0.99 for the plot, the first two experimental points are below the regression line. Application of eq 9 gives K,,,, = 0.52, 6 1 1 . ~ ~ = 7.60, and 61.Cc14= 4.72. 4a and 4b in Binary CC14-CD3COCD3Mixtures. The 6(OH) values for the a-methyl- (4a) (at 292 K) and a-tert-butyl- (4b) P,P-dimesitylethenols (at 290 K) in binary CCI4-acetone-d6
~ 0
Nadler and Rappoport
218 J . Am. Chem. SOC.,Vol. I l l , No. I , 1989
? Y 9
922 n m -
$26; -m
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d
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m
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J. Am. Chem. Soc., Vol. 111, No. 1, 1989 2 1 9
Enol-H B o n d Accepting Solvent Associations Table IX. Effect of Concentration of 1 on 6, J , K , and K,,,
Values T , 6(OH), 6(CH), 3J(HCOH), K,, K,,,, solvent K ppm ppm Hz CCI,' 292 4.45 6.30 13.2 CCl) 292 4.48 6.29 13.5 292 4.50 6.28 11.6b CCI," 292 4.51 6.28 CCI," 290 4.56 6.13 13.4' CCI,' 99.6:0.4 CC14- 289 6.03 6.32 10.8 11.1 15.0 DMSO-d6' 6.30 10.9 12.8 20 99.6:0.4 CC14- 289 5.94 DMSO-d6" 6.27 11.6 20.9 30 99.6:0.4 CC14- 289 5.78 DMSO-d6"
T, NOH), 6(CH), 'J(HCOH), solvent K PPm PPm Hz 0.360 99.6:0.4 CC14289 5.48 6.18 9.V DMSO-d6" 289 5.41 6.04 0.720 99.6:0.4 CC1,DMSO-d64 292 7.01 6.35 8.8 0.036 99:l CCI4DMSO-d6 ( v / v ) ~ 8.8 292 6.85 6.33 0.072 99:l cc1,DMSO-d, ( v / v ) ~ 0.072 0.108 99:l CCI,292 d,e 6.34 d,e DMSO-d6 ( v / v ) ~ 0.144 0.144 99:l CCI,292 d,e 6.33 d,e DMSO-d6 ( v I v ) ~ [DMSO-d6] = 0.057 M. [DMSO-d6] = 0.143M. 'Broad singlet at reduced resolution with a considerable error. d6(OH)and 'J(HC0H) for the calculations are from Table X. 'Poor resolution and the OH signal coincides with the aromatic signal.
[I], M 0.036 0.072 0.108 0.144 0.360 0.036
(ad
PI, M
(I
Table
K,,,,
K,,,,
(ad
10.0 15.0
10.8 18.5
values used
X. K,,,,, 6(OH), and ,J(HCOH) Values in Various Solvents from Data in CCI,-Solvent Mixtures a t 20 OC" solvent = parameter K,,,, (6)
C6D6
C6D5N02 0.08 7.68 0.12 9.0 4.47 13.86 0.46 0.53 0.07 0.07 5.94 11.3
6ll.S J1l.S
Kasm
0.053 11.58
6I.CCl4 Jl.CC14
14.04
(J)
Fll.s from 6 F,l.sfrom J KassnaPP(6) KarsnaPP(J) 6,d" JsOiv
Data from eq 9 .
0.46
CDICN 0.47 6.41 0.58 7.7 4.45 14.06 0.91 0.89 0.14 0.36 6.23 8.4
CD,COCD, 0.61 8.15 0.67 7.4 4.46 14.00 0.92 0.90 0.44 0.47 7.86 8.1
0.01 1 4.28 13.1 From ref 1. Calculated by eq 8.
dioxane-d8 1.1 6.99 0.46 9.9 4.49 13.87 0.95 0.85 0.39 0.20 6.86 10.5
THF-d, 1.1 7.68 0.87 8.7 4.52 13.89 0.96 0.95 0.55 0.41 7.56 9.0
DMF-d, 4.6 9.1 1 5.0 6.7 4.57 13.85 1 .oo 0.99 3.30 5.36 9.22 6.8
DMSO-d, 7.4 (5.2)b 8.86 9.7 5.9 4.51 13.96 1 .oo 1 .oo 3.79 9.21 9.01 5.9
Table XI. Temperature Dependence of N O H ) and l J ( H C 0 H ) for 1" in CC1,-Solvent Mixtures
% S in CCL-S (VIV)
solvent (S)
10
(CD,)2CO CD3CN dioxane-d,
10
10 30 50
C6D5N02
CDA
T. K
6(OH), DDm
'J(HC0H). Hz
274 276 29 3 276 276
6.44 5.53 5.88 5.21 4.34
10.7 11.4 12.4 12.5 13.5
6(OH),
T, K 307 308 306 308 308
ppm
)J(HCOH), Hz
5.93 5.24 5.76 5.02 4.33
11.2 11.0 12.6 12.7 13.5
" [ l ]= 0.036 M Table XII. Correlation of 3J(HCOH) vs 6(OH)
for 1 (eq 15) in
CCI,-Solvent Mixtures solvent
no
Table XIII. 6(OH) Values for Mes2C=C(OH)R 4" in
CCI4-CD3COCD3 Mixtures
c6
Db
Rb
SDb
DMSO-d6' DMF-d, CDjCOCDS THF-dg dioxane-d, CD3CN
0.47 14 1.79 0.9962 0.24 15 1.55 0.01 0.9978 0.18 11 1.80 -0.002 0.9988 0.11 13 1.61 -0.07 0.9977 0.12 -0.21 11 1.43 0.9894 0.19 0.09 13 3.31 0.9984 0.13 0.09 0.9949 0.10 11 1.93 C6D5N02 QNumber of experimental points. b C , slope; D,intercept; R,correlation coefficient; SD, standard deviation of the linear regression (eq 15). cData from ref 3. mixtures ( T a b l e X I I I ) c o m p l e m e n t similar d a t a for 1 in Table V I I . H a l f of t h e c h a n g e in 6 ( O H ) occurs a t 20% a n d 40% CD3COCD3for 4a a n d 4b,respectively whereas t h e corresponding change in 6 ( O H ) for 1 is a t 10% C D 3 C O C D 3 . T h e K,,,, a n d 611.acetone values were calculated by both e q 8 a n d 9 a n d a r e given together with t h e FIIvalues in T a b l e X I V . Enol 5. Enol 5 w a s investigated only in t w o solvents a t 2 9 3 K. In C C l l d ( O H ) = 4.03 a n d 3 J ( H C O H ) = 13.85 a n d in DMSO-d, 6 ( O H ) = 8.81 a n d 3 J ( H C O H ) = 5.92.
Discussion T h e present d a t a extend o u r knowledge on t h e association of s t a b l e enols with hydrogen bond accepting solvents in several directions. T h e polar contribution of a substituents is shown t o b e relatively small from t h e effect of a - a r y l substituents o n K,,,,.
% CDjCOCD,
[CD,COCD,],
(v/v)
M
b(OH) for R = t-Bu (4b)C
Me (4a)b
0 0.5
0.0 4.74 0.07 4.80 1 0.14 4.8 1 2 0.27 4.88 5 0.68 5.16 10 1.36 5.40 20 2.72 5.86 40 5.44 6.22 60 8.16 6.42 80 10.9 6.58 100 13.6 6.87 "[4] = 0.034 M. b A t 292 K . c A t 290 K .
4.17 4.80 4.83 4.86 5.00 5.18 5.45 5.62 6.00 6.1 1 6.44
Table XIV. 6(OH) (in ppm) and K,,,, Values for 4a and 4b in
CCl,-acetone-d,
ne
49'
4bb
11 4.74 6.87 7.39 0.24
11 4.77 6.44 6.93 0.17
49" Fll.acetone
4bb
0.80 0.77
in acetone (eq 8) (eq 9) 4.74 4.81 6rI.acetane (eq 8) 611.acetonc(eq 9) 7.34 7.60 K,,,, (eq 9) 0.25 0.09 K,,,, (eq 8) F11.acetonc 0.82 0.58 in acetone (eq 9) Q A t 292 K. b A t 290 K . eNumber of points in the correlations. &C14
~DMSO
220
J . A m . Chem. SOC.,Vol. 1 I I , No. 1 , I989
A moderate sensitivity to the arrangement of substituents is deduced by comparing K,,,, values of isomeric enols. The data for 1 point to the importance of hydrogen bond accepting ability of the solvent in determining KaSnand give a qualitative relationship moiety between K,,,, and the conformation of the C=C-0-H in different environments. The similarity of the association behavior of a-alkyl and a-aryl-P,P-substituted enols is extended to CD3COCD3. Finally, the effects of an a-Me3Si and of a-alkyl substituents are compared. a-Aryl-Substituted Systems. The steep change in 6(OH) for compounds 2 on addition of only a small percentage of DMSO-d, (Figure 1) indicates clearly that the phenomenon probed is chemical association rather than a polar effect of the solvent on the &OH) value. This is supported by the regularity of the K,,,, values. Several important conclusions emerge from the data of Tables I and 111. First, the sharp OH signals even a t low temperature indicate that equilibration of the I and I1 conformers is rapid on the time scale of the measurement. Second, the effect of metaand para-substituted a-aryl substituents on the association is rather small: K,,,, (from eq 8) increases only 2.3-fold from the a-anisyl to a-3,5-dibromophenyl. The linear Hammett plot suggests that polar effects are responsible for the change in K,,,,. The positive p is consistent with expectations since electron withdrawal increases the acidity and the hydrogen bond donating ability of a n O H group. The low p value of 0.35 reflects the fact that the substituents are remote from the association center. The OH group is attached to a vinylic carbon which itself enhances the hydrogen bond donating ability, but we did not find a Hammett p value for the association of X C 6 H 4 C H 2 0 Hwith solvents which would enable an estimation of these two factors. W e note that a similar plot for a-alkyl-/3,/3-dimesitylethenols (4) vs u* gives a slope of 1.30.' Third, the steric effect on the association is negligible. The K,,,, values for 2a (a-Mes) and 2c (a-p-Tol) are almost identical, suggesting that the steric effects of the two o-methyl groups increase the extent of association with DMSO by the same amount that their electronic effects decrease it. From Table I11 this is 1.17-fold, i.e., only 0.1 kcal mol-'. Finally, the iraction of species I1 in DMSO (20.98) is very close to unity. This is reminiscent of the behavior of 1 in DMSO, but differs from that of a-alkyl-[ and a-(trimethylsily1)-P,P-dimesitylethenols where F l , . D M s o in D M S O in 10.95.The nonlinearity of most of the plots of 6(OH)(2) vs 6(OH)(1) reflects the fact that most of the changes in 6(OH) for 1 occur a t a smaller percentage of DMSO-d, than required for corresponding changes for 2, resulting in the shallow first part of the curves in Figure 2A. When 1 is almost completely associated, b(OH)(1) reaches a plateau, but 6(OH) for 2 continues to increase due to increased percentage of the associated species, and Figure 2A displays the steep part of the curve. The Nature of Species I1 of Enols 2. There is little doubt that the large chemical shift of the O H is due to hydrogen bonding association of the hydrogen bond donor enol with the hydrogen bond acceptor DMSO. This is corroborated by the linear correlation of 6(OH) for 1, 2a, 3a, or a-anthryl-P,P-dimesitylethenol with Kamlet-Taft hydrogen bond accepting parameter of the solvent @ in a large number of solvent^.^ The chemical shifts cannot give information on the conformation of the C=C-OH moiety of either species 11, or of I in CC14. However, the stretching absorption of the OH in the I R spectra of 2b-f a t u = 3518-3524 cm-l l 2 is ascribed to an intramolecular hydrogen bonding to the 7~ system of the mesityl group cis to the O H , which is possible only for a syn arrangement (6a) of the Mes c,\ =c Mes
Mes
>=C,
/Ar
' 0
H
/
Mes
/Ar
,,H.*.0SMe2
0 6b
60
C=C-0-H moiety. The 3J coupling constant for 1 in solution' (see below) and data on trimesitylethenol in the solid statei6 and
Nadler and Rappoport
-~c -
"'
/
34-
i
4 1 . EM53
Figure 7. Plot of log K,,,, vs 611 (both calculated by eq 9) for Me&= C(0H)R. R: 1, H; 2, Me; 3, Et; 4, i-Pr; 5, t-Bu; 6 , Me3Si; 7, Mes; 8, p-An; 9, p-Tol; 10, Ph; 11, p-CF3C6H,; 12, 3,5-Br2C,H3. Point 6 was not used for drawing the slope.
on isotopomeric ( E ) - and (Z)-2,4,6-(CD3),C6H2c(Mes)=C(OH)Mes17 strongly indicate that the conformation in CCI, is 6a for Ar = Mes. By analogy, and in line with the IR data, we suggest that this is true also for compounds 2. Calculations on very simple enols suggest that the two most stable conformers of the C=C-0-H are syn and anti.I8 Hence, it is reasonable that the second conformer will be similar to 6b, since a linear hydrogen bond to the solvent in the plane of the 0-H bond in 6a is sterically impossible. However, the energy gain by hydrogen bond formation exceeds the calculated energy difference between unsolvated syn and anti conformers in simple systems, and other conformations, even one with a C=C-0-H dihedral angle of 90' cannot be excluded. Analogy with 1 suggests that I1 is a periplanar or clinal anti-type conformer, but direct evidence is lacking. Log Ka,,,-6(OH) Correlations. Table 111 and Figure 5 show a linear correlation of log KaSsn values in D M S O with the 611,DMsO values for the same substrates for 2a-f. The slope (0.82) and intercept (-6.68) of eq 13, resemble those (0.77 and -6.3, respectively) of a similar linear plot for compounds 4 ( R = Me, Et, i-Pr, t-Bu) and 1 in CCl,-DMSO.' Indeed, Figure 7 shows that a plot (data from eq 9) for the six aromatic (2a-f), the five aliphatic (4a-c. 4, R = Et, i-Pr), and the 1 H (1) derivatives of Mes2C=C(OH)R is reasonably linear (eq 21), although inspection log K,,,, = 0 . 8 3 6 1 1 . ~ ~-~6.6 0 ( R = 0.9782; S D = 0.07) (21) shows separation to a-aryl and a-alkyl derivatives and deviation of 4c from the line. Log Kassnshows large sensitivity to 611.DMSO values: 6(OH) changes by ca. 1.2 pprn for a change of 1 order of magnitude in Kassn.The linearity may indicate that the same (presumably polar) effect operates for all these substituents, and it will be interesting to delineate the reason for the deviations from the line by a study of additional substituents. (Z)-MesC(R)=C(OH)Mes Systems. The exchange of positions of an a-alkyl or a n a-aryl substituent in 4 or 2 with the @-mesitylgroup cis to it leads to two main changes. First, 6(OH) in CCI4 decreases by 0.42 ppm for a-phenyl and by 0.614.62 ppm for the a-alkyl substituents, and 6,, in D M S O decreases by 0.21 ppm for a-Ph and by 0.40-0.63 pprn for a-alkyl groups. Second, the Kansnvalues, which follow the order p-Ph > @-Me > P-Et > P-i-Pr for R, are 1.3-, 1.7-, 2.0-, and 2.3-fold higher than for the a - R derivatives. (16) Kaftory, M.; Biali, S. E.; Rappoport, Z. J . Am. Chern. SOC.1985, 107, 1701. (17) Biali, S. E.; Rappoport, Z.; Hull, W. J. J . Am. Chem. SOC.1985, 107, 5450. (18) (a) Samdal. S.; Seip, H. M . J . Mol. Strucr. 1975, 28, 193. (b) Bouma, W . J.; Radom, L. Ibid. 1978, 43, 267. (c) Nobes, R. H.; Radom, L.; Allinger, N . L. Ibid. 1981, 85, 185. (d) Schleyer, P. v. R. Pure Appl. Chem. 1987, 59, 1647. (e) Arad, D.; Apeloig, Y.; Rappoport, Z. Unpublished results.
Enol-H Bond Accepting Solcent Associations The increase of Kwn values of 3 relative to those for 4 is ascribed to electron withdrawal by the aryl compared with alkyl groups. This effect should roughly bring K,,,, for all the P-R derivatives to that (1.82) of trimesitylethenol (2a) in the absence of other effects. However, despite the larger distance from the association center, K,,,, for P-R’s is not constant and follows the order P-i-Pr < P-Et < P-Me. This can be explained by the geometry of 4. If the 6-Mes-C=C dihedral angle increases with the size of the P-alkyl group, the intramolecular a(P-Mes)-.HO association is strengthened, leading to an accompanying decrease in K,,,,. The decrease of 8(OH) in CCI, and DMSO-d6 can be related to the geometry of the system. W e assume a ( Z ) structure for all compounds 3 although complete X-ray structure is available only for 3a,16and a qualitative X-ray information indicates a ( Z ) structure for 3c as In CCI4, the higher the P-Mes-C=C torsional angle the stronger is the intramolecular a-P-Mes-.HO interaction which is responsible for the trend in 6(OH). For 3a, this dihedral angle is larger than for all the @,P-dimesityl-a-R derivative^.'^^'^ If this behavior is general, then a larger P-Mes-C=C angle in 3 is responsible for a decrease in 6(OH) relative to 4 in CC14. In DMSO, the decrease of 6(OH) in the order Me > Et > i-Pr probably reflects a major contribution of the ring current by the a-mesityl ring on the 6(OH). This is dependent on the aMes-C=C angle, which probably increases with the bulk of the @-alkylgroup. This effect is manifested by the A.6(OH) of 0.42 ppm between 1 and 5 , although the change in the P-aryl ring is a t a remote position from the O H . Trimethylsilyl Enol 4c. The study of 4c complements the previous study on a-alkyl derivatives.’ A linear correlation between log K,,,, for compounds 4 and 1 (calculated by eq 8) and cr* was previously found.’ Silicon is more electropositive than carbon, and whereas cr*(Me,Si) is not available, comparison of cr* values” of Me3SiCH2 (-0.26), t-BuCH, (-0,165), and t-Bu (-0.30) suggests that a*(Me,Si) = -0.47. Since the K,,,, of 0.71 (by eq 8) for 4c is similar to that for 4, R = i-Pr and u*(i-Pr) = -0.19, the point for 4c shows a high positive deviation from the plot. A correlation of log K,,,, with the uI values13 is linear with a slope of 14.6,’ and crl(Me3Si) = -0.06 is required to put 4c on the regression line. Six different u,(Me,Si) values ranging from 0.02 to -0.13 were determined*I and their average is -0.08, but this has little mechanistic significance since the spread in uI values is much higher than the difference cr,(Me,Si) - u,(i-Pr). The effect of Me@ on Kassnmay be alternatively evaluated by comparing the acidities of R,SiCH,OH and R 3 C C H 2 0 H . Unfortunately, quantitative data for R,SiCH,OH are not available.2k Other data are contradictory. Ph,SiOH is 0.22 pK, unit more acidic than Ph3COH22bin DMSO, and Ph,SiCOOH is 0.4722‘ or 0.6822dpK, unit more acidic than Ph,CCOOH in aqueous EtOH, but pK,(Me,SiCOOH) - pK,(Me,CCOOH) = 0.19 in 45% EtOH and 0.22 in 76% EtOH.22d Obviously, a clear conclusion from these values is difficult, except that the somewhat higher K,,,, value for 4c than that for the carbon analogue 4b is not inconsistent with expectation based on the electron-donating ability of carbon and silicon substituents. The longer C,-Si bond has little influence, in agreement with the small contribution of steric effects on K,,,, discussed previously’ and above. Association and Conformation of 1 in Several Solvents. Enol 1 was investigated previously very extensively due to the availability (19) Kaftory, M.; Biali, S. E.: Nugiel, D. A,; Rappoport, 2. Unpublished results. (20) Taft, R. W. In Steric Effects in Organic Chemistry; Newman, M. S., Ed.: Wiley: New York, 1956; Chapter 13. (21) For a tabulation and discussion of various u values of silicon substituents. see: Bassindale, A. R.; Taylor, P. G. In The Chemistry of Organic Silicon Compounds; Patai, S., Rappoport, Z., Eds.; in press. (22) (a) For a discussion of acidity of organic silicon compounds, see: Bassindale, A . R.; Taylor, P.G . In The Chemistrji of Organic Silicon Compounds; Patai, S., Rappoport, Z., Eds.; in press. (b) Steward, 0. W.; Fussaro, D. R. J . Organomet. Chem. 1977. 129, C28. (c) Steward, 0. W.; Erwin, H. W.; Gartska, R. A.: Frohlinger, J . J . Chem. Soc. A 1968, 3119. (d) Steward, 0. W.: Dziedzic, J.: Johnson, J. S . ; Frohlinger, J. 0.J . Org. Chem. 1971, 36, 3480.
J . Am. Chem. Soc., Vol. 1 1 1 , No. 1 , 1989
-3
221
O t
i.; *
1
. -.--_L_l---.L
‘3
c.2
24
2.6
0.8
0
Figure 8. Plot of log K,,(J) vs Kamlet-Taft’s @. Numbering of solvents: 2, C6D6:3, C6D,N02; 4, CD,CN: 5, dioxane-d8; 6, (CD,),CO; 7, T H F d,; 8, DMF-d,; 9, DMSO-d6.
of both the 6(OH) and ,J(HCOH) probes., Correlation of both parameters with the Kamlet-Taft solvatochromic equation6 shows that the hydrogen bond accepting ability of the solvent is mainly responsible for the chemical shift. From the ,J probe it was deduced that the conformation is l a in CCI,, l b in DMSO, and their mixture in most other solvents and that the association of 1 with the pure solvents (except for D M S O and D M F ) is not ~omplete.~ I n binary CC1,-solvent mixtures three conformers should be considered: one with “free” O H which constitutes 5 2 % for 1 in CC14,3,5an intramolecularly hydrogen bonded conformer which is the main one present in CCl,, and a conformer that is hydrogen bonded to the other solvent component of the mixture. The rapid equilibration of the two latter conformers under our conditions enables us to obtain K,,,, and , J ( H C O H ) for the latter, solvent-bound conformer, and hence the conformations of 1 in the various solvents. We have seen that the change of 6(OH) and ,J(HCOH) values in CCI,-solvent mixtures qualitatively parallels the reported behavior in CCI,-DMSO-d6 mixture^.^ However, whereas in CC1,-DMSO-d6 and CCI,-DMF-d, mixtures addition of < I % of the cosolvent raises 6(OH) and lowers ,J(HCOH) appreciably, the changes are more moderate in solvents that are less efficient hydrogen bond acceptors. Several important features emerge from Table X : (a) K,,,, values change appreciably with the solvent. (b) Although AjJ(HCOH) and A6(OH) values are linear (Figure 6), Kas,,(6) and K,,,,(J) are not identical. Kass,(J) is 1.1-1.45-fold higher than Kass,(6), except in T H F and dioxane (which is the only solvent where the A3J(HCOH) vs A6(OH) plot is not linear) and where Kass,(6) = 2.4Ka,,,(J). (c) In most cases, the “improvement” in calculating K,,,, by eq 9 rather than by eq 8 changes K,,,, by 51095, which is the magnitude of the error due to the intercept of eq 8. The improvement is more significant for C 6 D 5 N 0 2and for C6D6, where the overall changes in 6 and J are relatively small, and for Kass,(J) in DMSO-d,, where the corresponding changes on addition of a small amount of the solvent are large. (d) The fraction of the anti-type conformer obtained by both probes is very similar. It is 20.9, except in benzene and nitrobenzene (and the value calculated from in dioxane). (e) 6,, and JIIfor the anti conformers differ from 6,,So and JoMso,but not to the same extent. (f) values for the anti conformer are solvent dependent and are roughly proportional t o log K,,,,. A plot of log Ka,,,(J) vs Kamlet-Taft’s /3 is monotonous and approximately linear (eq 22 and Figure 8). A plot of log Kass,(6) vs /3 shows a larger scatter (slope 3.1; H = 0.868; S D = 0.30), log Kassn(J) = 3.266 - 1.62
( n = 8; R = 0.9520;SD = 0.21) (22)
but if the points for dioxane and acetonitrile are excluded a linear correlation (eq 23) is obtained for the other five solvents. The slope of eq 24 resembles the slope of a similar correlation for the
Nadler a n d Rappoport
222 J . A m . Chem. SOC.,Vol. 1 I I , No. I , 1989 log Kassn(6)= 4.3p - 2.34
(n = 5; R = 0.9979; S D = 0.59) (23)
log Kassn(6) = 3.438 - 0 . 8 3 ~ *- 1.05
(n = 7; R = 0.9203; S D = 0.31) (24)
association of 4-fluorophenol with hydrogen bond accepting solvents but not for CF3CH20H.23 A dual-parameter equation involving also the solvent polarity ( T * ) is~ poorer than eq 22 ( R = 0.523), whereas a slight improvement over eq 23 (cf. eq 24) was achieved. Consequently, the main parameter affecting the association with the solvent is its hydrogen bond accepting ability. The linear correlation between A3J(HCOH) and A6(OH) values (Figure 6) for almost all the solvents suggests that the hydrogen bond formation is accompanied by a conformational change that also involves a change in the orientation of the OH in relation to the P-mesityl ring. Since the observed J and 6 values are weighted averages of the values for the two conformers, we conclude that there is no unusual change either in the conformation or in the hydrogen bond association with a gradual change in the solvent compositions. W e have no explanation for the curved plot for dioxane, although it may be connected with the presence of two association sites. Due to the linearity of the two parameters it is reasonable to = K,,,,,(J), but this is not always the case expect that Kassn(6) because A3J(HCOH)/(Jccl, - JI1.,)and A6(OH)/(611., - 6ccJ are not parallel. In spite of the improvement that eq 9 gives over eq 8 the error in K,,, which is determined simultaneously with 611.s, can be appreciable, and on the basis of previous treatments8 we suggest that an error of *lo%, Le., a difference of up to 20% between Ka,,,(6) and K,,,(J), could be attributed to this reason. With this arbitrary criterion the agreement between the K,,,,(J) and K,,,(6) values is reasonable for four out of the seven solvents for which data are available. The large discrepancy between the values for dioxane may result from the nonlinearity of the A3J( H C O H ) vs A6(OH) plot. The small deviation of Kassn(J)from the plot of Figure 8 as compared with the large deviation of the Ka,,,(6) vs (3 plot suggests that for dioxane Kassn(J)is a better representation of K,,,,. For C6D5NO2,K,,,(J) = 1.5Ka,,(6), and it seems that the use of a relatively large number of experimental points that do not differ much from one another leads to high error in both values. The situation is exactly opposite for DMSO, for which K,,,,(J) = l,3Kas~,,(6) (by eq 9) since 6 and J change appreciably on addition of a very low percentage of DMSO-d6. However, eq 9 gives the same weight to values a t both high and low [DMSO] regions. The resulting appreciable error is reflected in the increase of Kass,(6) by 40% by using eq 9 a t the expense of a change of only 0.15 ppm in 611.DMSO. That 6II.DMSO calculated by eq 9 is slightly lower than the observed value (Table X) is inconsistent with the assumption that the solvent effect on K,,,, is negligible. In this highly associating solvent, where the 1:l association is almost complete a t low solvent concentrations, higher associates, e.g., 1:2 enol to solvent, with different 6's may be formed. Such association, which is not incorporated in the equations used for calculating K,,,,, can lead to the discrepancy in the two values. KasSn(J)is much less sensitive than Kassn(6) to the calculation method, and for D M S O it is taken as a more reliable measure of Kassn.24 The fraction of the associated conformer in all the solvents, except for CClp and the aromatic solvents, is high (>0.9). The lower values in the aromatic solvents reflect competition with intramolecular association to the neighboring P-mesityl. The Fll., values are higher than those previously calculated for 1 in the same solvents by using the incorrect assumption that 61~.s= ~ D M S Oand JII.,= JDMso in all solvents. W e see now that and JIl.svalues change appreciably with the solvent, from 6 6.41 (MeCN) to 9.1 1 (23) For these and other correlations with a, see: Taft, R. W.; Gramstad, T.; Kamlet, M . J. J . Org. Chem. 1982, 47, 4557. Abboud, J. L. M.; Sraidi, K.; Guiheneuf, G.; Negro, A.; Kamlet, M. J.; Taft, R. W. Ibid. 1985, 50, 2870. (24) In correlations involving K,,,,or d values for both I and other substrates the KaS8"(6) value for 1 is used for consistency with values for other substrates.
14
CI
J
I2
07
+.
7
IO
8
6
4 0
0.2
0.4
0.6
P Figure 9. Plot of JII.$ vs Kamlet-Taft's 8. Numbering
0.8
as in Figure 8 and
solvent 1 is CCI4.
( D M F ) and from J = 11.6 (C&) to 5.9 in D M S O (Table X). Whereas there is no correlation between Kassn(6) and SIl., ( R = 0.636), there is a rough correlation between Kassn(J)and JII., values (eq 25). The most associated solvents (DMSO, D M F ) (n = 8; R = log Kassn(J)= -0.37J 0.29 0.9005; S D = 0.31) (25)
+
have the lowest 3J(HCOH) values, and the least associated solvents ( P h N 0 2 , C&) have the highest Ss. The relation between 3J( H C O H ) values and the HCOH dihedral angle is through the Fraser equation4 (eq 26), and its application suggests that the 3 J ( H C O H ) = 10.4 cos2 0 - 1.5 cos 6'
+ 0.2
(in Hz)
(26)
conformation of 1 in CC1, is syn-planar, since 3J is even higher than the expected value according to eq 26. The plot of eq 26 is close to a parabola, which means that in solvents with higher Kassn the H C O H dihedral angle is higher than in solvents with lower K,,,,. If we accept the reasonable assumption3 that I1 is an anti-type conformation, then (by eq 26) in C6H6, C6H5NO2, dioxane, MeCN, and THF the conformation is anti-periplanar, whereas in DMF, DMSO, and probably in acetone it is anti-clinal. Since for good conjugation there is a preference of the anti-planarover perpendicular- or anti-clinal-type conformations in the absence of solvent, as shown by calculations,'& we conclude that association with the solvent reduces the conjugation of the O H with the double bond. However, the calculated potential energy change with the HCOH dihedral angle for vinyl alcohol is a relatively shallow function. The energy difference between the anti-planar and an anti-clinal conformation with H C O H angle of 40-50' (the calculated dihedral angle of l in D M S O and D M F ) in the parent enol is ca. 1 kcal mol-'.18c The association with a perpendicular O H (dihedral H C O H angle 90') is sterically the least hindered process. Although we deduced earlier that steric effects in the enol are not the dominant factor on the association,' some contribution from them is still expected. Apparently, the energy loss associated with the deconjugation of the planar arrangement is more than compensated by the stronger hydrogen bond formed with the solvent in an anti-clinal conformation. CorrelationsInvolving and JIIs.In view of the approximate correlation between the experimental J or 6 and /3 reported earlier for 1,3 it was of interest to discern whether the use of Jll., or all., would improve the approximate correlations between Job and aok and P. Figures 9 and 10 and eq 27 and 28 show that the corre(n = 9; R = 0.9009;SD = 1.05) JIl., = -9.lp 12.6 (27) (n = 8; R = 0.9340; S D = 0.50) 611., = 5.7p + 4.9 (28)
+
J . A m . Chem. SOC.,Vol. 11 I , No. 1 , 1989 223
Enol-H Bond Accepting Sohent Associations '01
I
The combination of electron-donating a-alkyl substituent on the enol and a moderate hydrogen bond acceptor solvent results in an appreciable fraction of the nonassociated conformer in pure acetone: FI = 0.18 for 4a and 0.42 for 4b in acetone, whereas it is 1 0 . 1 2 in DMSO. The above mentioned differences have implications to solvent dependence of the values of the keto + enol equilibrium constants ( K e q ) . The carbonyl derivative is expected to be less associated than the enol with hydrogen bond accepting solvents. If the hydrogen bond donating ability of the solvent is less important, Keq should be higher in a solvent with higher K,,,, value and the differences in K,, in various solvents should decrease with the increased bulk of the cy substituent. The results of the single work are that gives the solvent dependence on Kq for a simple in agreement with the above prediction, but more Kes values are required for evaluating the generality of this behavior.
I
9-
6-
,,/'
-
*I
I
4'
o
I
oz
04
0.6
Experimental Section
0.8
P Figure 10. Plot of 611., vs Kamlet-Taft's and solvent 1 is CCI,.
P. Numbering as in Figure 8
lations are still approximate ( R = 0.90-0.93). Both JII., and &I., are influenced by other effects in addition to the hydrogen bond accepting ability of the solvent. With dual-parameter equations including the solvent polarity parameter a*,the correlation of JII.s improves and that of 611.sdeteriorates (eq 27a and 28a). Better Jll.s
= -6.5p - 4.2a*
( n = 9; R = 0.9488; SD = 0.93) (27a)
611.s
= 13.5p
+ 0 . 2 1 ~ -* 1.3
( n = 8;R = 0.8187; SD = 2.4) (28a)
correlations of NMR parameters with /?,or p and a* for t-BuOH and P h O H than those showr, in eq 27, 27a, 28, and 28a were previously reported.25 K,,,, Values for 4a and 4b in CCI4-Me2CO. The study of 4a and 4b in CC1,-acetone-d6 mixtures was undertaken in order to see if the differences of K,, values for 1 with DMSO and acetone are parallel for a-alkyl derivatives of 1. The previously determined K,,, values with DMSO (eq 8) were 1.38 for 4a and 0.47 for 4b, which are higher than the respective K,,, values of 0.24 and 0.17 (eq 8) with acetone (Table XIV) by 5.8- and 2.8-fold or by 6.19and 4.9-fold from the values determined for both systems by eq 9. Since this ratio is 12.1 for 1 (Table X), the ratio between the K,,, values in DMSO and in acetone decreases with the increased bulk of the a substituent. The less associated enol is the less discriminating between solvents with which it associates. ~~~
(25) Taft, R. W.; Kamlet, M . J. Org. Magn. Reson. 1980, 14, 485.
Materials and Solvents. The a-aryl-/3,/3-dimesitylethenols, 12a-f,I2 the a-methyl- (4a),27aa-rert-butyl- (4b),27ba-(trimethylsily1)- (4c),27cand a-H- (1) /3,/3-dimesitylethenols,' and (Z)-P-phenyl-a$-dimesitylethenol (13a)) were prepared by us previously. P-Methyl- and P-ethyl-a,@-dimesitylethenols (3b, 3c) and /3-mesityl-/3-tolylethenols (5) were prepared according to Fuson2, and c o - w o r k e r ~ . Preliminary '~~ X-ray data suggest that 5 is the ( Z ) isomer. Preparation of (Z)-P-isopropyl-a,/3-dimesitylethenol (3d) will be reported elsewhere. The deuteriated solvents, which were the best quality commercial grade (Aldrich), were dried before use over 4A molecular sieves. Statistical Parameters Derived from Eq 9. Statistical parameters according to eq 9 are as follows. The Rank value (number of parameters to be optimized) is 3. The D F E s are the differences between the number of points and the Rank. The SSE values are 0.057 (Zd), 0.1292 (2e), 0.2242 (Zf), and 0.0272 (4c) in CCI,-DMSO-d,, 0.0303 (4a) and 0.0434 (4b) in CC1,-CD,COCD,, and 0.018 (C6Ds, J ) , 0.040 and 0.0464 (CD,CN, 6 and J ) , 0.0239 (CD3COCD3,6). 0.0246 and 0.0695 (dioxane-d,, 6 and J ) , 0.3474 (THF-d8,6) and 0.1454 (CD,SOCD,) for 1 in CC14-solvent mixtures. Spectra. N M R spectra were recorded with a Bruker W.P200 SY pulsed FT spectrometer operating at 200.133 MHz. The OH signals were either sharp singlets (enols 2-4) or well-separated doublets (enols 1 and 5).
Acknowledgment. W e are indebted to Marian Dodu ( M S c . in mathematics) from the Hebrew University Computation Center for help with the statistical treatment and to D r . Silvio Biali for discussions. This work was supported by a grant from the United States-Israel Binational Science Foundation (BSF), Jerusalem, Israel, to which we are grateful. (26) Miller, A. R. J . Org. Chem. 1976, 41, 3599. (27) (a) Nugiel, D. A.; Rappoport, Z. J . Am. Chem. SOC.1985,107, 3669. (b) Nadler, E. B.; Rappoport, Z.; Arad, D.; Apeloig, Y. Ibid. 1987, 109, 7873. (28) Fuson, R. C.; Maynert, E. W.; Tan, T. L.; Trumbull, E. R. J . Am. Chem. SOC.1957, 79, 1938.
