J . Am. Chem. SOC.1988, 110, 6514-6521
6514
slower this step will be, and thus the more important that it is catalyzed effectively. In a single-base enzyme, this can be done by the general acid generated by the removal of the C-H proton, as long as the leaving group oxygen is syn to it. For an unstable intermediate, spontaneous (water-catalyzed) C - 0 cleavage would be faster and anti stereochemistry more necessary, as described above. Eventually, as the general level of catalytic efficiency rose,
evolutionary pressure would increase for catalysis even of relatively fast C - 0 cleavage, leading to enzymes with a second active site catalytic group, but naturally retaining anti stereochemistry. Acknowledgment. W e are grateful to the Science and Engineering Research Council for a studentship to T.L.A. and to Dr. J. P. Richard for helpful discussion.
Rate and Equilibrium Constants of Ionization of (a-Cyanodiphenylmethane)bis( tricarbonylchromium( 0)) in Me2SO-Water Mixtures. Unusual Solvent Effect on the Intrinsic Rate Constant Claude F. Bernasconi,*,*Richard D. Bunnel1,f and Franqois Terrier? Contribution from the Thimann Laboratories of the University of California, Santa Cruz, California 95064, and the Laboratoire de Physicochimie des Solutions, UA CNRS 403, ENSCP. 1 1 Rue Curie, 75231 Paris Cedex 05, France. Received March 17, 1988
Abstract: The kinetics of the ionization of (a-cyanodiphenylmethane)bis(tricarbonylchromium(O)) by carboxylate ions, primary amines, and the piperidine/morpholine pair have been measured in 50%, 70%, and 90% aqueous Me2S0. The Bransted plots for the carboxylate ion reactions in 70% and 90% Me2S0 show some downward curvature which is reminiscent of recent reports of such curvature in the ionization of acetylacetone and 1,3-indandione, and is attributed to a solvation effect. Intrinsic rate constants, defined as ko = k l B / q= kIBH/p at ApK + log @ / q ) = 0, were determined by suitable interpolation or extrapolation of the Brernsted plots. For the piperidine/morpholine pair in 50% Me2S0 log ko = 3.30. Even if one allows for a rate lowering steric effect of up to one log unit, giving a corrected log ko = 4.3, this value is still close to that for 4-nitrophenylacetonitrile (log ko = 3.95). This suggests that the resonance effect of the Cr(C0)3-phenyl groups is substantial and comparable to that of the combination of a 4-nitrophenyl and a cyano group, in agreement with some literature reports but in disagreement with others. The effect of increasing the Me2S0 content of the solvent is to significantly decrease ko for both the carboxylate ion and amine reactions. This contrasts with either sharp increases in ko or solvent independent ko values with six other carbon acids for which similar data are available. The decrease in ko can be attributed to carbanion solvation being substantially stronger in MezSO than in water, as confirmed by measurements of the solvent activity coefficient for the transfer from 50% to 70%and from 50% to 90% MezSO. Since it appears that carbanion solvation generally lags behind proton transfer at the transition state, this lag produces a decrease in ko in the better solvating medium. Surprisingly, the solvent effect on ko is essentially independent of the ionizing agent, again in contrast to findings in other systems where the addition of M e 8 0 enhanced ko for the carboxylate ion reactions more strongly than for the amine reactions. A possible explanation of our results is that the main factor usually responsible for the larger solvent effects in the carboxylate ion reactions, Le., the lag of the proton transfer behind the desolvation of the carboxylate ion, becomes insignificant because proton transfer is nearly complete at the transition state, as implied by very high Brernsted &values.
The tricarbonylchromium(0) group (Cr(CO),) is known to be strongly electron withdrawing as reflected in a pK, value of Cr(CO),-phenylacetic acid that is virtually identical with that of 4-nitrophenylacetic acid.l Despite its strong electronic effect it has not been widely used as an activating group in the study of proton transfers from carbon acids. In a first quantitative study Terrier et a].* recently compared kinetic and thermodynamic acidities of the Cr(CO),-substituted di- and triphenylmethanes 1-3 with the corresponding 4-nitro-substituted analogues 4-6 in Me2SO-methanol mixtures. Their major findings can be summarized as follows. (1) The nitro group is more effective in stabilizing the diphenyl and triphenylmethane anions as reflected in much higher acidity constants of 4 vs 1, 5 vs 2, and 6 vs 3. (2) One Cr(C0)3-phenyl group is kinetically more acidifying than one 4-nitrophenyl group, as reflected in an approximately 3-fold higher rate constant of deprotonation of 1 vs 4 by methoxide ion. (3) In the comparisons 2 vs 5 and 3 vs 6 the respective nitro compound is deprotonated slightly more rapidly than the Cr(CO)3 compound, but if the results are adjusted to compensate for the of California. UA CNRS 403, ENSCP.
Chart I
2
1
3
5
0 6
different thermodynamic acidities, 2 is faster than 5 while 3 and 6 show comparable kinetic reactivities.
1 University
(1) Nicholls, B.; Whiting, M. C. J . Chem. Soc. 1959, 551.
0002-7863188115 IO-6514$01.50/0
0 1988 American Chemical Society
J. Am. Chem.Soc., Vol. 110, No. 19, 1988 6515
(a-Cyanodiphenylmethane)bis(tricarbonyIchromium(OJ)
These results show that the intrinsic rate constants ( k for K = 1) for the Cr(C0)3-phenyl activated carbon acids are higher than for the 4-nitrophenyl ones. The reduced kinetic advantage of the Cr(C0)3 compounds in the series 1 2 3 was attributed to a steric effect that progressively lowers the rate of proton transfer. Since low intrinsic rate constants of proton transfers are generally associated with the formation of resonance stabilized carbanion^,^,^ it was suggested that, in contrast to the nitro group, the mode of action of the C r ( C 0 ) 3 group is more by an inductive effect than by a resonance effect? This conclusion is in agreement with findings based on rate effects in E2-elimination reaction^,^ but it is difficult to reconcile with the effect of the Cr(C0)3group on the pKa of phenol: or the rate of SNAr reactions6 and other phenomena.' In an attempt to shed more light on the mode of action of the Cr(C0)3 group in proton transfers we have studied the ionization of 7 in Me2SO-water mixtures. A major virtue of 7 is that it is
Table I. Rate Constants and pK, Values for Equation 1 in 50% Me2SO-50% Water (v/v) at 20 OC, p = 0.5 M klB,bM-l k_,BH,b ApKC B pKBBHa s-' M-I s-l log (P/4)
--
+
piperidine 11.05 1.20 X lo4 8.70 2.66 X lo2 morpholine n-BuNH2 10.68 4.25 X lo' CH30CH2CH2NH2 9.63 5.89 X lo2 H2NCOCH2NH2 8.28 7.75 X 10' 7.83 3.71 X 10' EtOOCCH2NH2 5.39 4.40 X lo-' NCCH2NH2 "From ref I l b . bEstimated error limits pKnCHwith pKnCH= 10.26.
7
M KOH solution and then reacting it with the appropriate buffer. M e 2 S 0 solutions (50%) of 7- were quite unstable and completely decomposed within 2 to 3 min. This problem was circumvented by generating 7- in a multimixing stopped-flow apparatus that allowed reaction of 7-with the buffer to be carried out a few seconds after 7- was generated. In 70% and 90% Me2S0 the decomposition of 7- was significantly slower and did not require the use of the multimixing stopped-flow apparatus. In 50% Me2S0 rates could only be measured with amine buffers but not with carboxylic acids because these latter were outside the time resolution of the stopped-flow method. In the other solvents these rates were much slower, mainly because of lower p e H and higher pK:H values, and thus these rates were measurable. A typical set of experiments with a given buffer consisted of measuring kobd for five different buffer concentrations at a constant buffer ratio. Plots of koM vs buffer concentration were all linear; in most cases, particularly in the Me2SO-rich solvents, intercepts were quite small or negligible, indicating that ky [ B] + k!p[BH] >> k y o kyHa + kH,a + + kH0, and no attempts were made to measure $ ,'; kyH,k?,, or k$O. The buffer ratio was usually 1:l and hence a plot of koW vs [ B ] (or [ B H + ] ) would provide a slope given by eq 3
~
DyG
Cr(C0)3
R:".On+
R-:an
+ R-:20
k,8tBI + k-I
Bti
CBHl
CN 7
7-
+
Under pseudo-first-order conditions that were used throughout, the first-order rate constant for equilibrium approach is given by
kobd = kyz0
+ kYHaoH-+ k?,UH+ + k ? p + k y [ B ] + k_Bp[BH]
(2) When the pK, of the buffer (pK:") was higher than the pKa of the carbon acid ( p p H ) the e uilibrium was approached from left to right. When pK$ < p e the equilibrium was approached from right to left by first generating 7-in a 2 X M or ~~~
_
_
_
_
+ k!,"
(3)
ky/k!," = gH/K:"
(4)
slope = ky
a
(2) Terrier, F.; Farrell, P. G.; LeliBvre, J.; Top, S.; Jaouen, G. Organometallics 1985, 4, 1291. (3) (a) Eigen, M. Angew. Chem., I n t . Ed. Engl. 1964, 3, 1. (b) Bell, R. P. The Proton in Chemistry, 2nd ed.;Cornell University: Ithaca, NY, 1973; Chapter 10. (c) Crooks, J. E. In Proton Transfer Reactions; Caldin, E., Gold, V., Eds.; Wiley: New York, 1975; p 153. (d) Hibbert, F. Compr. Chem. Kinet. 1977,8,97. (e) Bernasconi, C F. Pure Appl. Chem. 1982,54, 2335. (4) (a) Bernasconi, C. F. Acc. Chem. Res. 1987,20,301. (b) Bernasconi, C. F. Tetrahedron 1985,41, 3219. ( 5 ) Ceccon, A.; Catelani, G. J. Organomef. Chem. 1974, 72, 179. (6) Brown. D.A. J. Chem. SOC.A 1966. 40. (7j Top, S:; Jaouen, G.; Say&,B. G.; McGlinchey, M. J. J. Am. Chem. SOC.1983, 105, 6426. (8) Bernasconi, C. F., Terrier, F. J . Am. Chem. SOC.1987, 109, 7115.
1.09 -1.26 0.90 -0.15 -1.50 -1.95 X IO4 -4.39 17%. 'ApK = pKnBH-
(I
Results Kinetic Measurements. Rates were measured in the stoppedflow apparatus. The reaction can be described by eq 1. #t:20+
X lo3 X 10' X lo3 X lo3 X lo' X lo3
Table 11. Rate Constants and pK, Values for Equation 1 in 70% Me,SO-30% Water (vlv) at 20 OC. u = 0.25 M klB,6 M-' k-,BH,b ApKc + B D K . ~ ~ S-I~ M-1 s-l loe ( D I U ) piperidine 10.66 1.77 X lo4 7.72 X IO2 1.72 8.66 7.04 X lo2 3.96 X lo' morpholine -0.28 n-BuNH2 10.49 6.94 X lo' 3.99 X lo2 1.73 7.04 X lo2 CH30CH2CH2NH2 9.57 1.50 X lo' 0.81 H2NCOCH2NH2 8.34 2.62 X lo2 2.13 X lo3 -0.42 EtOOCCH2NH2 7.75 1.03 X lo2 5.05 X 10' -1.01 NCCH2NH2 5.42 1.70 X 10' 1.51 X lo4 -3.34 -2.64 6.90 1.76 X lo2 2.66 X lo4 AcOCH3OCH2COO5.67 2.09 X 10' 5.50 X lo4 -3.87 CICH2C004.65 2.37 X loo 7.85 X lo4 -4.89 NCCH2COO4.13 7.43 X lo-' 7.29 X lo4 -5.41 From ref 11b. Estimated error limits 110%. ApK = pKaBHpKnCHwith pKaCH= 9.24.
more acidic than 1-3, which allows kinetic measurements with the same amine and carboxylate buffers used to measure intrinsic rate constants of ionization of several other carbon acids studied recently. Our work includes an investigation of the effect of changing the solvent from 50% Me2SO-50% water to 70% Me2SO-30% water and 90% Me2SO-10% water on rate and equilibrium constants, and thus on the intrinsic rate constant, of the ionization of 7. Coupled with a determination of solvent activity coefficients for the transfer of 7 and its anion from 50% MezSO-50% water to the other solvents, this solvent effect study is aimed at testing a recently proposed theoretical framework4,* for understanding solvent effects on intrinsic rate constants of proton transfers.
Cr(C013
1.94 8.61 1.60 2.40 7.07 8.50 3.00
In conjunction with eq 4
~
~
ky and k!? could be determined from the known pK:H and p e H , the latter being obtained as follows. Since the instability of 7 precluded a precise spectrophotometric determination of p p " , this quantity was measured kinetically in buffers whose pK$ was close to p e " . For these experiments koM was determined as a function of buffer concentration a t several different buffer ratios. The slope of a plot of koW vs [ B ] is then given by slope = k?
+ k!paH+/K:"
(5)
From the pH dependence of this slope ky and k!? could be obtained and thus p C H could be determined by solving eq 4 for
e".
6516
Bernasconi et al.
J . Am. Chem. Soc.. Vol. 110, No. 19, 1988 6
Table 111. Rate Constants and pK, Values for Equation 1 in 90% Me2SO-10% Water (v/v) at 20 "C, p = 0.06 M klB,b M-' k-lBH,b ApKc + B pKaBHa s-' M-1 s-I 1% (Pld piperidine 10.74 8.91 morpholine n-BuNH2 10.96 CH30CH2CH2NH2 10.16 H2NCOCH2NH2 9.19 EtOOCCHZNH2 8.22 NCCH2NH2 5.94 AcO9.53 CH'OCH2COO8.17 6.84 CICH2C006.25 NCCH2COO4.68 ClzCHC00"From ref l l b . bEstimated pKaCHwith pKaCH= 9.12.
-1
70% M e 2 S 0 - 30% Water
-e
1.92 10' 10' 0.09 lo2 2.32 1.52 lo2 0.55 lo2 -0.42 10' -2.70 lo' 103 0.11 lo4 -1.25 -2.58 lo4 -3.17 lo' -4.74 lo4 error limits 510%. cApK = pKaBH1.87 X 1.15 X 1.11 X 3.36 X 5.82 X 1.68 X 2.89 X 1.77 x 2.24 X 1.21 X 2.71 X 7.87 X
lo4 10' lo4 10' lo2 lo2 loo 104 IO' lo2 10' lo-'
5.28 X 2.06 X 2.21 X 3.52 X 4.52 X 1.40 X 3.72 X 5.00 x 1.29 X 1.71 X 1.56 X 1.85 X
mx v
0I
-6
-4
ApK
A/
A Pip/Mor
ARNH2
Ai' -2
-4
2-
0
0 -
ApK
log6
I I 0
2
+ Iogg
=
7OpH/SOpH
50 90
=
90pH/SOpH
-2
-4
ApK
(6)
+ log s07c+ - log "7& + log 50y9H9,- log "4"
2
0
+
log;
Figure 3. Bransted plots in 90% Me2SO-10% water. Table IV. Intrinsic Rate Constants (log k,) and pK, Values for the Ionization of Cyano Carbon Acids by Amines in Water or 50% Me,SO-50% Water acid solvent temp, OC pK. log kn HCNC H20 25 9.0 =7.5" 25 11.1 =7.0b H20 CH2(CN)2d O2N--@-CHpCN
(8)'
50% Me2S0
20
12.62
3.95'
50% Me2S0
20
8.06
2.906
(7)
with 'OPH,a n d gopH respectively being t h e partition coefficients between n-heptane and 50%, 70%, and 90% Me$O. 5oPH = 1.02 X lo-], 70PH= 2.00 X and 9oPH < 3 X lo4 were determined as described in the Experimental Section, from which we obtain 507&= 1.96 X lo-, (log = -1.71) and so-& < (log 'O7;g < -2.53). 2.94 X From the relationship between t h e solvent effect on p e " a n d t h e solvent activity coefficients of the carbon acid, the carbanion, and the hydronium ion,l0 eq 8 and 9, we obtained log 50y5p_= -1.83 < -2.56 for 7-. a n d log
$"@
+
I I I I
SO,&
soA90pK$H = log
I
2
90% Me2SO-10% Water
Our results are summarized in Tables I (50% Me,SO), I1 (70% M e 2 S O ) , and 111 (90% M e 2 S O ) . Solvent Activity Coefficients. The solvent activity coefficients for t h e transfer of 7 from 50% Me2SO-50% water t o 70% Me,SO-30% water (S07&)and from 50% Me2SO-50% water to 90% Me2SO-10% water were determined from partition experiments between n-heptane and the above Me2SO-water mixtures. T h e solvent activity coefficients9 are given by
s o A 7 0 p c H= log "7;q
1
0
6
Figure 1. Bronsted plots in 50% Me2SO-50% water.
YCH
4
-2
Figure 2. Brmsted plots in 70% Me2SO-30% water.
50% M e 2 S 0 - 50% Watei
/A
4-
(8) (9)
Discussion Tables 1-111 summarize the rate constants ky and k!p defined in eq 1, obtained in the three solvents 50%, 70%, and 90% M e 8 0 , while Figures 1-3 show t h e B r ~ n s t e dplots for k?. A s has been (9) Parker, A. J. Chem. Rev. 1969, 69, 1. (10) Wells, C. F. In Thermodynamic Behavior in Electrolyres in Mixed Soloents-II; Furter, W. F., Ed.; American Chemical Society: Washington, DC, 1979; Advances in Chemistry 177; p 53.
L
-
o z N ~ c H z c N NO2
a Primary and tertiary amines. Piperidine/morpholine. 17. dReference 3d. 'Reference 16. /This work.
Reference
observed in numerous cases,11-13t h e three families of bases form separate B r m s t e d plots, with t h e statistically corrected r a t e constants for a given ApK log @/q) being in the order RCOO> secondary amines > primary amines. Another similarity with recently reported results on the ionization of acetylacetonella a n d 1,3-indandione'Ib is the downward curvature of the B r ~ n s t e dplots for the carboxylate ions in 70% and 90% M e 2 S 0 . This downward
+
( 1 1 ) (a) Bernasconi, C. F.; Bunnell, R. D. Isr. J . Chem. 1985,26, 420. (b) Bernasconi, C. F.; Paschalis, P. J . Am. Chem. Soc. 1986, 108, 2969. (12) Bernasconi, C. F.; Kliner, D. A. V.; Mullin, A. S.; Ni, J. J . Org. Chem. 1988, 53, 3342. (13) Terrier, F.; LeliBvre, J.; Chatrousse, A.-P.; Farrell, P. G.J . Chem. SOC.,Perkin Trans. 2 1985, 1479.
J. Am. Chem.SOC.,Vol. 110, No. 19, 1988 6517
(a-Cyanodiphenylmethane)bis(tricarbonylchromium(O))
Table V. Intrinsic Rate Constants, log kn/ and Solvent Effects (6 log WkoD)d % Me2SO (XMc~SO) RCOO-C RNH2 7 50 (0.203) 2.94 f 0.04 2.65 f 0.03 (-0.29 f 0.07) 70 (0.372) 4.59 f 0.08 2.42 f 0.03 (-0.52 f 0.07) 4.35 f 0.10 (-0.24 f 0.18) 90 (0.696) 9-Cyanofluorened (IO) 3.62 f 0.01 3.76 f 0.09 (0.14 f 0.16) 3.57 f 0.09 (-0.05 f 0.16)
10 (0.028) 50 (0.203) 90 (0.696)
pip/mor 3.30 f 0.06 3.04 f 0.06 (-0.26 f 0.12) 2.98 f 0.10 (-0.32 f 0.16) Z4.44 4.58 f 0.05 ( ~ 0 . 1 4 ) z 4.39 ( ~ - 0 . 0 5 )
9-Carbomethoxyfl~orene~ (1 1) 2.84 f 0.06 3.09 f 0.03 (0.25 f 0.09)
50 (0.203) 90 (0.696) 10 (0.028) 50 (0.203) 70 (0.372) 90 (0.696)
2.64 f 0.26 3.18 f 0.10 (0.54 f 0.36) 3.77 f 0.10 (1.13 f 0.36) 4.53 f 0.05 (1.89 f 0.31)
0 (0) 50 (0.203) 90 (0.696)
~3.14 23.80 ( ~ 0 . 6 6 ) ~ 5 . 2 7(22.13)
0 (0) 50 (0.203) 90 (0.696)
-2.10 f 0.05 -0.57 (e2.67) 1.88 f 0.05 (3.98 f 0.10)
1,3-Indandionee (12) 2.27 f 0.13 2.44 f 0.17 (0.17 f 0.30) 2.59 i 0.18 (0.32 f 0.31) 2.97 f 0.05 (0.70 f 0.18)
2.97 3.13 3.33 3.85
Acetylacetond (13) 1.90 f 0.03 2.06 f 0.02 (0.16 f 0.05) 2.91 f 0.05 (1.01 f 0.08)
2.60 f 0.06 2.75 f 0.06 (0.15 f 0.12) 3.64 f 0.06 (1.04 f 0.12)
f 0.10 f 0.10 (0.16 f 0.20) f 0.10 (0.36 f 0.20) f 0.10 (0.88 f 0.20)
Phenylnitromethan8 (14) 0.97 f 0.07
-1.22 f 0.10 -0.25 f 0.10 (0.97 f 0.20) 1.75 f 0.10 (2.97 f 0.20)
Nitromethan8 (15) 0 (0) 50 (0.203) 70 (0.372) 90 (0.696)
2.77 f 0.05
-0.59 f 0.10 0.73 f 0.10 (1.32 f 0.20) 1.76 f 0.10 (2.35 f 0.20) 3.06 f 0.10 (3.65 f 0.20)
OError limits are standard deviations except for pip/mor where the limits are estimated. b 6 log WkoD= log ko (high Me2S0 content) - log ko (lowest Me2S0 content), values in parentheses. 'Based on the linear portion of the Brensted plots. dReference 8. eReference 1lb. /Reference 1la. SReference-12.
curvature was shown" not to constitute a Hammond effectI4 or "Marcus c u r ~ a t u r e " ' ~ but - ' ~ to be a solvation effect. The same interpretation of the curvature seems to be called for in the present case. Resonance or Inductive Effect by the Cr(CO), Group? From the B r ~ n s t e dplot with piperidine and morpholine one finds log ko = 3.30 for the statistically corrected intrinsic rate constant (log ko = log (ky/q) a t ApK log ( p / q ) = 0) for the ionization of 7 with secondary cyclic amines. It is instructive to compare this parameter with log ko for the ionization of other cyano carbon acids under similar reaction conditions. Table IV summarizes such data along with p e " values. W e note that log ko for 7 is of the same order of magnitude as for 4-nitrophenylacetonitrile (8) and 2,4-dinitrophenylacetonitrile(9),16but much lower than log ko for malononitrileM or HCN." Low intrinsic rate constants are generally attributed to substantial resonance stabilization of the carbanion while high intrinsic rate constants are found in systems in which resonance plays no role (CN-) or only a modest one (CH(CN)2-).3$4v'7According to this criterion the resonance effect of the Cr(CO),-phenyl groups in 7- is quite substantial and lies approximately halfway between that of the 4-nitrophenyl and that of the 2,4-dinitrophenyl group. The above analysis fails to consider steric effects, though. Comparisons between 1 and 2, and particularly between 2 and 3, have shown that steric hindrance reduces the intrinsic rate constant for deprotonation of bis(Cr(CO)3-phenyl) activated CH
+
(14) Jencks, W. P. Chem. Rev. 1985, 85, 511. (15) Marcus, R. A. J. Phys. Chem. 1968, 72, 891. (16) Bernasconi, C. F.; Hibdon, S. A. J. Am. Chem. Soc. 1983,105,4343, (17) Bednar, R. A.; Jencks, W. P. J . Am. Chem. SOC.1985, 107, 7117. (18) The fact that resonance lags behind proton transfer has been recognized by numerous authors, see, e.g., ref 19-23. (19) Bordwell, F. G.; Boyle, W. J., Jr. J. Am. Chem. SOC.1972, 94, 3907. (20) Kresge, A. J. Can. J. Chem. 1975, 52, 1897. (21) Jencks, D. A.; Jencks, W. P. J . Am. Chem. SOC.1977, 99, 7948. (22) Bell, R. P.; Grainger, S. J. Chem. Soc., Perkin Tram. 2 1976, 1367. (23) Pross, A.; Shaik, S. S.J . Am. Chem. SOC.1982, 104, 1129.
acids substantially, but not for the deprotonation of bis(4-nitrophenyl) activated CH acids.* The reduction in ko for 3 relative to 2 can be estimated to be 10- to 20-fold. It is more difficult to estimate the steric effect on ko in the deprotonation of 7 compared to 8 or 9. Since the cyano group is much less bulky than a phenyl group the steric crowding in 7 is expected to be significantly less than in 3, with a correspondingly less dramatic reduction in ko. However, having two Cr(CO),phenyl groups in 7 compared to one 4-nitrophenyl or 2,4-dinitrophenyl group in 8 or 9, respectively, may put 7 a t a further kinetic disadvantage compared to 8 or 9. W e shall assume that the combined steric factors reduce ko for 7 by not more than 10-fold compared to ko for 8 and 9. The assumption of a much larger steric effect would be difficult to reconcile with the fact that log ko for the piperidine/morpholine reactions is higher than log ko for the reactions with the less bulky primary amines, as is commonly observed for proton transfers (see Table V, below). The difference log ko (pip/mor) - log ko(RNH2) of about 0.4 for 7 is only slightly smaller than that observed for the other carbon acids in Table V ( ~ 0 . to 7 0.8),suggesting that the steric effect, even though real, is not dramatic. A log ko value which after correction for the steric effect would be about 4.3 is still fairly close to that for 8 (log ko = 3.95) and still indicates a substantial resonance effect in the anion of 7. This contrasts with an estimated 1.O to 1.5 log units difference in ko between 1 and 4 or 2 and 5.2 A possible reason why the difference in the resonance effect between the Cr(CO),-phenyl and 4nitrophenyl groups appears so much smaller in the comparison of 7 with 8 than in the comparisons of 1 with 4, and of 2 with 5 is the presence of the a-cyano group in 7 and 8. By virtue of its strong electron-withdrawing effect the cyano group diminishes the charge stabilizing role played by the Cr(CO),-phenyl and Cnitrophenyl groups. The reduced demand placed on these groups would decrease the resonance effect of both of them, thereby attenuating the superiority of the 4-nitrophenyl over the Cr(CO)3-phenyi group.
Bernasconi et al.
6518 J . Am. Chem. SOC.,Vol. 110, No. 19, 1988 6
Table VI. Bransted 4 Values" % MezSO RCOO-b
I U
0
50 70 90
4-
CH3COCHzCOCH3
10 50 90
3
Y
01
-
0
0.2
04
0.8
0.6
I 1.0
XMne,SO
Figure 4. Solvent effects on log ko for the piperidine/morpholine reactions.
c I
I
/
A
pip/mor
0.94 f 0.02 1.00 f 0.03
0.75 f 0.02 0.71 f 0.02 0.71 f 0.02
0.71 f 0.03 0.70 f 0.03 0.66 f 0.03
9-Cyanofluorene' (10) 0.53 f 0.12 0.47 f 0.08 0.58 f 0.04
0.43 f 0.03
50 90
9-Carbomethoxyfl~orene~ (11) 0.49 f 0.04 0.60 f 0.02
10 50 70 90
1,3-Indandioned (12) 0.50 f 0.05 0.43 f 0.03 0.42 f 0.03 0.53 f 0.03 0.49 f 0.04 0.63 f 0.02 0.74 f 0.01 0.53 f 0.02
0.41 f 0.03 0.40 f 0.03 0.45 f 0.03 0.47 f 0.03
0 50 90
AcetylacetoneC (13) 0.61 f 0.03 0.40 f 0.02 0.45 f 0.01 0.69 f 0.04 0.83 f 0.01 0.53 f 0.02
0.37 f 0.03 0.42 f 0.03 0.51 f 0.03
0 50 90
Phenylnitromethand (14) 0.54 f 0.03 ~0.60 0.75 f 0.02 0.65 f 0.02
0.48 f 0.03 0.52 f 0.03 0.69 f 0.03
D
I
RNH,
7
A CH3COCHzCOCH3
Nitromethand (15)
0 50 70 90
0.69 f 0.02
0.59 0.62 0.63 0.69
f 0.03 f 0.03 f 0.03 f 0.03
a Error
limits are standard deviations except for pip/mor where the limits are estimated. bBased on the linear portion of the Bransted plots (low pKaBHvalues). 'Reference 8. dReference Ilb. 'Reference l l a . fReference 12.
I
0
02
04
06
I 08
10
x Me;,SO Figure 5. Solvent effects on log ko for the carboxylate ion reactions.
Another difference between the two studies is that the data for 1, 2, 4, and 5 were obtained in 98% Me2SO-2% MeOH; in the absence of a solvent effect study on ko for 8 it is difficult to assess how much of the difference in behavior between the two systems is a consequence of using different solvents. Solvent Effects on Intrinsic Rate Constants. The log ko and Bransted values for the ionization of 7 by carboxylate ions, primary amines, and piperidine/morpholine in the various solvents are summarized in Tables V and VI, respectively, along with the corresponding parameters for 9-cyanofluorene (lo), 9-carbomethoxyfluorene (1l), 1,3-indandione (12), acetylacetone (13), phenylnitromethane (14), and nitromethane (15). The dependence of log ko on the solvent for the piperidine/morpholine reactions is shown graphically in Figure 4 and for the carboxylate ion reactions in Figure 5. The most striking observation is that among all the systems studied so far, 7 is the only one for which log ko clearly decreases with increasing M e 2 S 0 content of the solvent. Another feature which sets 7 apart from the other systems is the very high B r ~ n s t e d values, in particular for the carboxylate ion reactions, but also for the amine reactions in water-rich solvents. The question we wish to address now is whether the reactivity pattern of 7, even though quite different in several respects from that of the other carbon acids, may still be understood within the conceptual framework developed for these other systems. The main points of this framework4v8can be summarized as follows. The reason why carbon acids that form resonance stabilized carbanions have lower intrinsic rate constants for proton transfer
than carbon acids that form purely inductively stabilized carbanions is that development of resonance lags behind proton transfer a t the transition state. In contrast, no such lag seems to occur in the development of the inductive effect. The reduction of the intrinsic rate constant by the lag in the resonance development can be understood as a manifestation of the Principle of Nonperfect Synchronization4 (PNS). As shown elsewhere: this reduction in ko may be expressed by eq 10 where 6 log k;=(C-) represents the change in log ko compared to a system in which the carbanion is only inductively stabilized, 6 log K,(C-) is the enhancement of the equilibrium constant that can be attributed to the resonance stabilization of the carbanion, p is the Bransted p value which is taken as an approximate measure of the progress of proton transfer at the transition state, : , not to be confused with the Bransted a,measures the while a progress in the resonance development at the transition state. Since 6 log K,,(C-) > 0, and the lag in resonance development means :a < 6, we see that 6 log k;=(C-) < 0, Le., ko is lower for the resonance stabilized system. The log ko values listed in Table V range from -2.10 (PhCH2N02with RCOO- in water) to 4.58 (9-cyanofluorene with pip/mor in 50% Me2SO-50% water), Le., they span more than six powers of ten, suggesting large differences in the degree of resonance stabilization of the various carbanions. The solvation of the carbanion is another factor that affects ko strongly. The most easily identified effect is hydrogen bonding solvation of carbanions that delocalize their charge onto oxygen atoms, as is the case with enolate ions and even more so with nitronate ions. One way of understanding this factor is to consider it part of what makes resonance forms such as RCH=N02- so unusually stable in protic solvents, and thus to include it into the resonance effect. This point of view is implied in the above discussion which equated the differences between the log ko values
J . Am. Chem.SOC.,Vol. 110, No. 19, 1988 6519
(a-Cyanodiphenylmethanejbis(tricarbonylchromium(0))
listed in Table V and log ko = 7 . 5 (amines) to 8.0 (oxyanions) for HCN1' with different degrees of resonance stabilization of the carbanions. If this point of view is adopted it should be reflected in eq 10 by including the subscript and superscript "sol", e.g.,
6 log ~$'so~(c-)= (a~i,sol-P)S log Kres,sol(C-)
( 11
The increase in ko for the nitroalkanes and diketones upon addition of M e 2 S 0 to the solvent can thus be understood to be essentially the result of a reduction in 6 log Kra,ml(C-). The absence of a similar solvent effect on ko for the other carbon acids may then be traced to a 6 log Krs,ml(C-) that depends little on the solvent, which is plausible since hydrogen bonding solvation cannot play an important role in these cases. The above point of view, even though qualitatively reasonable, assumes that the lag in solvation of the carbanion is equal to the lag in the resonance development, Le., if we introduce an a s as measure of solvation a t the transition state we would have a s -- $cres = ares,ml. Since solvation is known to lag behind bond formation even in cases where resonance or charge delocalization does not occur24-26(more on this below), it is likely that the lag in the solvation of the carbanion is actually greater than the lag in the charge delocalization. Hence, when dealing with solvent effects on ko, a more satisfactory approach is to separate the two factors. Thus, we shall understand eq 10 as referring to a nonsolvating medium while the effect of solvation on ko, relative to the nonsolvating medium, is represented by a separate equation, eq 12.
6 log kr'(C-) = (a$; - @)6 log Km,(C-)
(12)
The separation of the two effects allows for the expectation that solvation lags more than charge delocalization, Le., a s C a::. Of equal importance, eq 12 provides a convenient formalism to deal with solvent effects on ko, including solvation effects that are unrelated to hydrogen bonding. Assume eq 13 refers to a Me2SO-rich solvent (D) and eq 14 to a water rich solvent (W).
6 log krl(C-)D = (a&;- @ID6 log Ksd(c-)D 6 log kio'(C-)w = (CY;; - @)w6log KwI(C-)w The effect of ko of adding MezSO is then given by 6c- = 6 log kryc-)D - 6 log krI(C-)w =
(13)
(14)
L
-
JZ
9-CN-FI- (lo-)' 9-COOMe-FI- (ll-)'
CH3COCHCOCH3- (13-) PhCH=N02- (14-)'
If we approximate ( a s - P ) = ~ (a:: - plW= a s - P,~'eq 15 simplifies to 6c- = (a;; - @)(-log wrg-) (16) with W$- being the solvent activity coeffcient9 for the transfer of the carbanion from W to D. log Wrg-for the carbanions derived from 7 and 10-1528are summarized in Table VII. For a carbanion that is less solvated in D than in W (e.g., nitronate ions) log Wr:-> 0, hence with a:; C P one obtains 6 , > 0, Le., the retarded solvation of the carbanion contributes to an increase in ko, as observed. Even though 6c- is a major contributor to the observed solvent effect on ko there are other factors as well. For reactions that involve oxyanions (B-) as the ionizing agent the desolvation of the oxyanion appears to be ahead of proton transfer a t the transition ~ t a t e . ~ ~This , ~ effect ~ * ~can ~ be ~ *described ~ ~ by eq 17.
(17)
(24) (a) Hupe, D. J.; Jencks, W. P. J . Am. Chem. SOC.1977,99,451. (b) Hupe, D. J.; Wu, D. Ibid. 1977, 99, 7653. (c) Pohl, E. R.; Wu, D.; Hupe, D. J. Ibid. 1980, 102, 2759. (25) (a) Jencks, W. P.; Brant, S.R.; Gandler, J. R.; Fendrich, G.; Nakamura, C. J. Am. Chem. SOC.1982,104,7045. (b) Jencks, W. P.; Haber, M. T.; Herschlag, D.; Nazaretian, K. L. Ibid. 1986, 108, 479. (26) Huskey, W. P.; Schowen, R. L. Gazz. Chim. Ital. 1987, 117, 409. (27) These approximate equalities do not require that at:, and 6 individually are solvent independent, only that their difference is insensitive to the solvent change. (28) Bernasconi, C. F.; Bunnell, R. D. J . Am. Chem. Soc. 1988,110,2900.
50 90 50 90 50 70 90 50 90 50 70 90 50 70 90
-1.37d -4.02d
-2.65 -2.12
0.38d 0.76d 1.79d 2.07 4.50 I .99 2.59 4.09 2.87 4.51 6.70
"Reference 10. bThis work. 'Reference 28. 90% water.
0.38 1.41 2.43 0.60 2.10 1.64 3.83 W = 10% Me2SO-
Equation 17 is analogous to eq 16 and is derived in a similar way, with being a measure of the progress of the desolvation of B- a t the transition state. Since log W r i -is typically > O (see B= AcO- in Table VII) and advanced desolvation means a& > @, we have 6g- > 0. When the ionizing agent is an amine, late development of the solvation of the incipient ammonium ion3b929(NH') contributes to the solvent effect on ko by a term given by eq 18 in which aEy (CP) measures the progress in the solvation of the ammonium 6NH+
(a$; - P)D6 logKml(c-)D - (a&;- P)W6 log KSOI(~-)W (l5)
6B- = ( a b - 6 ) log w-yg
Table VII. Solvent Activity Coefficients for the Transfer of Ions from Water to Me2SO-Water Mixtures (log WyD), from 50% Me2S0 to 70% Me2S0 (log and from 50% Me2S0 to 90% at 20 'C Me2S0 (log ion % Me2S0 log wyD log 50y70log 50 3.08 AcO-" 70 ~4.50 ~1.42 90 r6.50 ~3.42 -1.81 50 70 =-2.4 =-0.6 =-1 .o 90 ;.-2.8 b 50 -1.93 70 90 0 (eq 19) but 6NH+ < 0 (eq 20). (3) In the reactions of 9-cyanofluorene the combination of a strongly negative 6c and a somewhat negative 6NH+ should add up to a substantial decrease in ko in 90% Me2S0. The fact that log ko is essentially solvent independent suggests that a positive 6SR term contributes significantly to 6 log wkF(CH/N), eq 20. A similar conclusion may be drawn for the reaction of 9-carbomethoxyfluorene with amines where a slightly negative 6 log wk[(CH/N) would have been predicted on the basis of tic- and 6NH+ alone, but a slightly positive value is observed. Is the Behavior of 7 Anomalous? Turning now to 7 we note that log ko for both the amine and the carboxylate ion reactions is lower in the Me2SO-richer solvents. This is consistent with the negative log WrE-values (Table VII) which lead to < 0. The rather large decrease in log ko requires that log Wy&is strongly negative, more so than for 9-cyanofluorene. This is borne out by our measurements, e.g., log soyp < -2.56, which is probably significantly more negative than log 50y$E = -2.65 for 9-cyanofluorene. What is puzzling, though, is that 6 log w k t ( C H / N ) and 6 log wkt(CH/B-) are, within experimental error, the same. To some extent this result may be an artifact caused by the rather large experimental uncertainty. This uncertainty allows for the possibility that log 7 0 k r ( C H / N ) could be as much as 0.23 log units more negative than log 70kr(CH/B-), in qualitative agreement with expectation. Experimental error cannot be the entire explanation, though. Added insight can be gained from a semiquantitative analysis of the relative contributions of 6c-, hB-, bNH+, and ljSR. Such an analysis has recently been carried out for 10-15: with the results summarized in Tables VI11 and IX. According to eq 19 and 20 the difference in the solvent effects between the carboxylate ion and amine reactions is given by
+
66 = 6 log Wkg(CH/B-) - 6 log w k t ( C H / N ) = 6g- - 6NH+ 6SR(CH/B-) - 6SR(CH/N) (21) For the change from 50% to 90% MQSO the parameters in Tables
VI11 and IX (6sR(CH/B-) - GsR(CH/N) = 0.22)8 yield 66 = 0.31
+ 0.09 + 0.22 = 0.62. Since 6g-, 16NH+I, and 6SR are expected to be smaller for the change from 70%to 90% M e 2 S 0 (only comparison available for carboxylate ions), 66 is expected to be 0 (eq 19). This behavior, which is unique among all systems studied so far, is attributed to the very large Bransted @ values, particularly for the carboxylate ion reaction, which should make bg very small and possibly bNH+ less negative than in other systems. Experimental Section Materials. (a-Cyanodiphenylmethane)bis(tricarbonylchromium(O)) was prepared according to a previously reported method,', mp 166 OC (lit. mp 166 OC).',
The purification of the amines and carboxylic acids, where needed, has been described previous1y.I" Me,SO was stored over a 4-A molecular sieve prior to use. Kinetic Measurements. The methods used were basically the same as described before.'Ia The reaction was monitored at 414 nm which cor(37)
Top, S.; Jaouen, G. J . Organomet. Chem. 1979, 182, 381
6521
responds to one of three maxima in the spectrum of the carbanion. In some of the experiments where the equilibrium was approached from the carbanion side the carbanion was generated in a multimixing DurrumGibson stopped-flow apparatus by reaction of the substrate with 0.002 or 0.01 M KOH. The acidic buffer solution was then added in the same apparatus a few seconds later and the decay of the carbanion monitored in the usual way. This technique was used in those cases where the carbanion solution was unstable which was a most serious problem in 50% Me2S0. Solvent Activity Coefficients. The solvent activity coefficients for the transfer of 7 from 50% to 70% Me2S0 (50-y&) and from 50% to 90% Me2S0 were determined by measuring partition coefficients for 7 between n-heptane and the various Me2SO-water mixtures. The partition coefficients were measured as described previously;** they are defined as "pH= [CH]H/[CH],,, = [CH],/[CH],,, and 9oPH= [CHIH/[CHI,, respectively, with [CHI, being the equilibrium concentration of the carbon acid in n-heptane, and [CHI,, [CHI,,, and [CHI, being the equilibrium concentrations of the carbon acid in the respective Me2SO-water mixtures. Acknowledgment. This research was supported by Grant CHE-8517370 from the National Science Foundation and by the donors of the Petroleum Research Fund administered by the American Chemical Society. We also thank Dr. S. Top for a sample of (a-cyanodiphenyl)bis(tricarbonylchromium(O)).
Capacity of Quadricyclane Frameworks Related to syrz-Sesquinorbornatriene for Excited-State Rearrangement Leo A. Paquette,* Hermann Kunzer,' and Mark A. Kesselmayer Contribution from the Evans Chemical Laboratories, The Ohio State University, Columbus, Ohio 43210. Received February 3, 1988
Abstract: By means of a sequence of reactions involving sequential below-plane Diels-Alder cycloaddition to tricyclo-
[5.2.1 .02q6]deca-2,5,8-triene, sensitized irradiation to transform the norbornadiene part structure into a quadricyclane, and introduction of a double bond into the noncyclopropanated bridge, it is possible to arrive efficiently at hexacyclic systems typified by 6, 12, and 18. When the unsaturated center is substituted with an electron-withdrawing group, direct and sensitized irradiation results in quantitative translocation of the two cyclopropane rings. The pair of quadricyclane-containingvalence isomers can be thermolyzed to the same syn-sesquinorbornatriene,irradiation of which in turn delivers only the photostable quadricyclane derivative. Monodeuteriated compound 18 undergoes excited-state isomerization only when directly irradiated. In contrast, syn-sesquinorbomatrene-l-d (20) engages in ring closure when sensitized (efficient ring closure) or directly irradiated (competitive polymer formation). Mechanistic analyses of this chemical behavior are provided, along with quantitative measurements of selected processes.
The structural uniqueness of the quadricyclane ring system has prompted many researchers to elucidate methods for its construction and to understanding its chemical reactivity. The parent hydrocarbon is available by the direct2 or preferably sensitized photoisomerization' of norbornadiene. Substitution of one of the norbornadiene double bonds bv one or two electron-withdrawing groups facilitates the valence isomerization! While it is possible to reverse the light-induced ring closure by heating a t 140-190
OC,sthe cycloreversion occurs rapidly a t ambient temperature in the presence of transition m e t a k 6 As a consequence of this reversible electrocyclization, the norbornadiene-quadricyclane couple has been intensively investigated as a promising tool for solar light energy c o n ~ e r v a t i o n . ~
Y
(1)
Postdoctoral fellowship awardee of the Deutsche Forschungsgemeins-
chaft, 1985-1986.
(2) Dauben, W. G.; Cargill, R. L. Tetrahedron 1961, 15, 197. (3) (a) Hammond, G. S.; Turro, N. J.; Fischer, A. J . Am. Chem. SOC. 1961,83,4674. (b) Hammond, G.S.; Wyatt, P.; DeBoer, C. D.; Turro, N. J. Ibid. 1964, 86, 2532. (c) Smith, C. D. Org. Synfh. 1971, 51, 133. (4) (a) Cristol, S. J.; Snell, R. L. J . Am. Chem. SOC.1954, 76, 5000. (b) Edman, J. R. J . Org. Chem. 1967, 32, 2920. (c) Gorman, A. A.; Sheridan, J. B. Tetrahedron Left. 1969, 2569. (d) Jones, G.,11; Xuan, P. T.; Schwarz, W. Ibid. 1982, 5505. (e) Ikezawa, H.; Kutal, C.; Yasufuku, K.; Yamazaki, H. J . Am. Chem. Soc. 1986, 108, 1589 and references cited in these papers. (0 Cristol, S. J. Tetrahedron 1986, 42, 1617. (9) Maruyama, K.; Tamiaki, H.; Kawabata, S. J . Chem. SOC.,Perkin Trans. Ii 1986, 543.
0002-7863/88/1510-6521$01.50/0
(5) Frey, H. M. J . Chem. SOC.1964, 365. ( 6 ) Review: Bishop, K. C., 111 Chem. Reo. 1976, 76, 461. (7) (a) Hautala, R. R.; Little, J. L.; Sweet, E. M. Solar Energy 1977, 19, 503. (b) Jones, G.,11; Rinehard, T. E.; Bergmark, W. R. Ibid. 1978,20, 241. (c) Kutal, C.; Schwendiman, D. P.; Grutsch, P. A. Ibid. 1977, 19, 651. (d) Kutal, C. Solar Energy Chemical Conversion and Storage; Hautala, R. R., King, R. B., Kutal, C., Eds.; Humana Press: Clifton, NJ, 1979. (e) Maruyama, K.; Terada, K.; Yamamoto, Y. J . Org. Chem. 1981, 46, 5294. (0 Sasaki, S.; Ohtsubo, K.; Fujiwara, F.; Ohyoshi, A. J . Mol. Catal. 1982, 16, 181. (g) Maruyama, K.; Tamiaki, H.; Kawabata, S . J . Org. Chem. 1985, 50, 4742. (h) Maruyama, K.; Tamiaki, H.; Kawabata, S. J . Chem. SOC.,Perkin Trans. II 1986, 543. (i) Nishino, H.; Toki, S.;Takamuku, S. J . Am. Chem. Soc. 1986,108,5030. 6 ) Draper, A. M.; de Mayo, P. Tetrahedron Lett. 1986, 6157. (k) Gassman, P. G.;Hershberger, J. W. J . Org. Chem. 1987.52, 1337. (I) Arai, T.; Oguchi, T.; Wakabayashi, T.; Tsuchiya, M.; Nishimura, Y.; Oishi, S.; Sakuragi, H.; Tokumaru, K. Bull. Chem. SOC.Jpn. 1987, 60, 2937.
0 1988 American Chemical Society
