Vinyl ether hydrolysis. XI. The effect of a-phenyl substitution1

Y. CHIANG, W. K. CHWANG, A. J. KRESGE,' L. H. ROBINSON, D. S . SAGATYS, and C. I . YOUNG

Deptrrt~uort c!f'Clre~tri.stry, Utriver.siry t [ f 'To~.o~lro, St.orbororrg1~ College, Wesr H i l l , Otrr., Ctrtlcrtltr M I C /A4 crtrtl Drpcrrtr,retrt of Clrc,t,list~y, Illitrois 1tr.stitrrte of Tc,cl~rrology. Cllictrgo, 1llitloi.s 60616 U.S.A.

Received August 29, 1977

Y. CHIANG, W. K. CHWANG, A. J. KRESGE, L. H. ROBINSON, D. S. SAGATYS, and C. I. YOUNG. Can. J. Chem. 56,456 (1978).

The unexpected inability of an a-phenyl group to accelerate the rate of vinyl ether hydrolysis more strongly than an a-methyl substituent, k(CH2=CPhOEt)/k(CH2=CMeOEt) = 0.2 for catalysis by H 3 0 f at 2S°C, is examined and is found to be the result of two effects: (i) pre- ferential initial state stabilization by phenyl and (ii) weak resonance interaction with the developing alkoxy carbonium ion in the transition state induced by the reactant-like character of the latter. A curved Brernsted relation for the hydrolysis of 3-methoxyindene catalyzed by carboxylic acids and monohydrogen phosphonate anions gives the Marcus theory parameters AGO* = 3 + 1 kcal/mol and wr = 12 + 1 kcal/mol.

Y. CHIANG, W. K. CHWANG, A. J. KRESGE, L. H. ROBINSON, D. S. SAGATYS et C. I. YOUNG. Can. J. Chem. 56,456 (1978).

Les vitesses d'hydrolyse, catalysee par H,O+ et a 2S°C, d'tthers vinyliques substitues en a par des groupes phenyle ou methyle ne sont pas tres differentes; k(CH,=CPhOEt)/k(CH,= CMeOEt) = 0.2; on a trouve que ce rCsultat inattendu est dQ a deux effets: (i) une stabilisa- tion preferentielle de 1'Ctat initial par le phenyle et (ii) une interaction faible de resonance avec le carbocation alcoxy qui se developpe dans 1'Ctat de transition et qui est induit par le caractkre ressemblant au reactif de 1'Ctat de transition. La courbe representant la relation de Brernsted pour I'hydrolyse du mkthoxy-3 indkne catalysee par les acides carboxyliques et les anions monophosphate conduit a des paramktres pour la theorie de Marcus de AGO* = 3 + 1 kcal/mol et ,or = 12 + 1 kcal/mol.

[Traduit par le journal]

Carbonium ions are generally stabilized to a greater extent by cr-phenyl substituents than by cr-methyl groups. The triphenylmethyl cation, 1, for example, is formed from triphenylmethanol in

50% aqueous H,SO, but 'superacids' such as FS0,H-SbF, or HF-SbF, are needed to generate the tert-butyl cation, 2, from tert-butyl alcohol (1) and cumyl chloride, 3, undergoes solvolysis in ethanol solution 5000 times more rapidly than tert-butyl chloride, 4 (2).

We were somewhat surprised, therefore, when we discovered (3) that an cr-phenyl group was actually less effective than an cr-methyl group in promoting vinyl ether hydrolysis. This is a reaction which takes place via rate-determining formation of an alkoxy carbonium ion (eq. 1j (4) and yet the ratio of specific rates for R = C6H, to R = CH, in 5 is 0.2. In this

'For part X, see ref. 13d. 'Author to whom correspondence should be addressed at

the University of Toronto.

H' [ I ] CH2=COC,H, I ; slow

C H ~ C ~ C ~ H I %+ fast CH,CR + H O C ~ H ,

R

paper we report our further study of this unexpected phenomenon.

Results and Discussion Sigma-Rho Relationships

Phenyl groups stabilize adjacent carbonium ion centers by a conjugative or resonance mechanism; when resonance is inhibited, a destabilizing inductive effect takes over., The fact that an cr-phenyl group does facilitate reaction in the present case (although not as much as cr-methyl): the rate ratio for R = C6Hs to R = H in 1 is 70 (3), indicates that some conjugative stabilization is available. To assess the magnitude of this resonance effect, we examined the influence of substituents in the para position of the phenyl group on the rate of methyl cr-phenylvinyl ether hydrolysis catalyzed by the hydronium ion.

3For a recent example of such inhibition of resonance, see ref. 5.

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CHIANG E 1T AL. I 457

TABLE 1. Summary of rate constants for the hydrolysis of some vinyl ethers in aqueous solution at 25'C under catalysis by Hf

Substrate k H + (M-I S-l)

Ethyl a-p-nitrophenylvinyl ether 2.01" Ethyl a-p-bromophenylvinyl ether 36.1b Ethyl a-phenylvinyl ether 118" Ethyl a-p-methylphenylvinyl ether 277* Ethyl a-p-methoxyphenylvinyl ether 64Sb 3-Methoxyindene 57. gb Methyl a-phenylvinyl ether 53.3"

OReference 3. 'This work.

The results are detailed in Table S14 and are summarized in Table 1. They show a response to substitution, especially to introduction of methyl and methoxyl groups, which is rather less than expected for a strong resonance interaction, and the specific rates in fact correlate better with o than with o f . Treatment of the data by the Yukawa- Tsuno method (6) gives 0.14 + 0.07 for the reson- ance parameter r , which rates the conjugative effect on a scale from zero for no resonance to unity for a resonance interaction equivalent to the rather strong conjugative effect in the cumyl cation 6. The value

CH3 + / C6H5-C

'CH3

6

of p is also low: p = -2.23 + 0.14. Thus, these experiments reveal a rather weak resonance inter- action.

3-Methoxyindene Resonance effects of phenyl groups have a strong

conformational requirement, and reduced con- jugation may often be traced back to steric effects which prevent the system from adopting its optimum configuration. In the present case there would seem to be no obvious interference of this kind: the a-phenylvinyl group is not a crowded one, and the benzene ring should easily be able to assume the necessary arrangement coplanar with the carbon- carbon double bond.

It is, however, a simple matter to test this idea experimentally; for example, by examining a sub- stance in which coplanarity is forced upon the system through the presence of a small ring, as in 3-methoxyindene, 7. We therefore measured the rate of hydrolysis of this substance. The data, detailed in Table S24 and summarized in Table 1, show the rate

4Tables S1, S2, and S3 of rate data are available, at a nominal charge, from the Depository of Unpublished Data, CISTI, National Research Council of Canada, Ottawa, Ont., Canada K I A 0S2.

of this reaction to be virtually identical with that of its acyclic analog: for 3-methoxyindene k,+ = 58 M - l s - l while for methyl a-phenylvinyl ether, 8, k,+ = 53 M-Is-'. Steric inhibition of resonance may therefore be ruled out as the cause of the weak resonance effect.

Initial State Stabilization An additional factor which must be taken into

account in the present comparison of phenyl and methyl group effects is the interaction between these groups and the carbon-carbon double bonds in the vinyl ether substrates of the initial states of these reactions. It is well known that phenyl groups stabilize double bonds; a recent study (7) puts this effect, in free energy terms, at 6,G = 4.9 kcal/mol. Methyl groups also lower the energy of carbon- carbon double bonds but by a smaller amount: 6,G = 3.2 kcal/mol. The difference between these two values, 1.7 kcal/mol, is the amount by which the initial state for methyl a-phenylvinyl ether hydrolysis is stabilized relative to that for ethyl a-methylvinyl ether hydrolysis. This energy difference is equivalent to a rate factor of 20 at 25"C, which is more than enough to offset the rate ratio of 0.2 actually found for these two compounds.

This analysis suggests, therefore, that an a-phenyl group is in fact better at stabilizing positive charge in an alkoxy carbonium ion than is a methyl group in the same position but this superiority is not reflected in rates of vinyl ether hydrolysis because of overwhelming initial state effects. This conclusion is supported by the fact that benzaldehyde diethyl acetal, 9, undergoes acid-catalyzed hydrolysis 30

times faster than does acetaldehyde acetal, 10 (8). These reactions occur by rate-determining formation of alkoxy carbonium ions, eq. 2,5 similar to those generated in vinyl ether hydrolysis but the substrates

5For a recent review of the evidence, see ref. 9.

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458 C A N . J. CHEM. VOL. 56. 1978

/ H - [21 RCH -1 R C H ~ C ~ H , + C,H,OH

\ OCZHS

here do not contain carbon-carbon double bonds and the initial state effects found in vinyl ether hydrolysis are therefore absent. Phenyl substitution in the ketals 11 and 12, on the other hand, is less effective at promoting hydrolysis than is methyl sub- stitution: the rate ratio 11:12 is 0.3 (10). But this system is much inore crowded than the acetal series 9 and 10, and complicating steric effects may well be present (10, 11).

It is interesting also that 'inverted' phenyl to methyl rate ratios have been found in a number of other reactions involving rate-determining proton addition to carbon-carbon double bonds. In fact, of the six systems for which data are available (Table 2), only one gives a phenyl to methyl rate ratio greater than unity.

Btpfuted Relation Weak phenyl group resonance effects such as those

observed here seem to be characteristic of fairly stable carbonium ions. They have been observed before in reactions which give alkoxy carbonium ions and have been attributed to a substituent saturation effect or diminished demand for additional stabilization in already quite stable species (12). Some insight into how this saturation effect operates is provided by a Bronsted relation for one of the present vinyl ether hydrolysis reactions.

This Brarnsted relation was constructed using 3-methoxyindene as the substrate. Catalytic co- efficients were determined for carboxylic acids and for nlonohydrogen phosphonate anions by measur- ing rates of hydrolysis in buffer solutions of these acids. The data, summarized in Table S3,4 when treated by methods we have described before (13), give the rate constants listed in Table 3.

Here, as before in the hydrolysis of other vinyl ethers (13c and d), the monohydrogen phosphonate anions prove to be better catalysts than neutral carboxylic acids of the same acid strength; this difference may be understood as the result of electro-

TABLE 2. Comparison of phenyl and methyl substituent effects in double bond protonation reactions

System k(C6H5)/k(CH3) (Ref. No.)

FIG. 1. Separate linear Br~lnsted relations for the hydrolysis of 3-methoxyindene catalyzed by carboxylic acids and by monohydrogen phosphonate anions.

static interactions in the transition states of these reactions (13b and c). The two kinds of catalysts give separate Bronsted relations (Fig. 1) of different slope, a = 0.64 f 0.05 for RC0,H and a = 0.78 f 0.02 for RP0,H-. These may be regarded as linear segments of more extensive curved correlations, and, as before (13c and d) a single curved Brarnsted relation (Fig. 2) may be obtained by a simple vertical displacement of the phosphonate anion points.

This curved Bronsted relation shows that a diminishes with increasing catalyst acid strength. For an acid as strong as H,Of, the catalyst which was used in the rate comparisons and o-p correlations discussed above, the value a = 0.39 may be extra- polated. Insofar as Bronsted exponents may be used as a measure of the degree of proton transfer (14), this implies that the proton is only some 40% trans- ferred, and the carbonium ion only 40% developed, in the transition states of these reactions. Resonance stabilization of this incipient carbonium ion can therefore be no more than 40% of that in a fully developed carbonium ion, and it could be less if resonance effects in carbonium ion-forming reactions lag behind proton transfer just as they seem to do in carbanion formation (14, 15). The weak resonance effects observed in these vinyl ether hydrolyses thus appear to be natural consequences of the early transition states these reactions possess.

Curved Bronsted relations such as that of Fig. 2, when analyzed using Marcus rate theory, give certain parameters by which proton transfer re- actions may be characterized (15b, 16). This treat- ment in the present case provides the intrinsic barrier AGO* = 3.4 f 0.9 kcal/mol and the work term wr = 12.2 -f 1.2 kcal/mol. Thus, in the hydrol- ysis of 3-methoxyindene, just as in most other vinyl ether hydrolysis reactions (3, 13c), rather more energy is required to bring the catalyst and the substrate together and form them into a reaction

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CHIANG ET AL. I 459

TABLE 3. Summary of catalytic coefficients for the hydrolysis of 3-methoxyindene in aqueous solution at 25°C

Catalyst pK, (Ref. No.) k,,, (M-I s-l)

FIG. 2. Curved Brunsted relation for the hydrolysis of 3-methoxyindene by carboxylic acids and monohydrogen phosphonate anions.

complex (w') than is needed to surmount the kinetic barrier to proton transfer within this complex (AGO*).

Experimental Materials

Vinyl ethers were prepared from the corresponding ketones via the dimethyl or diethyl ketals. The latter were made by treating the ketones with trimethyl or triethyl orthoformate in alcohol (methanol or ethanol) solution in the presence of a catalytic amount of p-toluenesi~lfonic acid; the ketals were isolated and characterized, and alcohol was then eliminated, by heating under reflux withp-toluenesulfonic acid catalyst, to produce the vinyl ethers. The identity of all ketals and vinyl ethers was confirmed by their nmr and ir spectra, and satis- factory carbon and hydrogen analyses were obtained for all new substances.

Phosphoric acids were samples prepared previo~lsly for pK, determinations (17); all other substances were best available commercial grades. Solutions were prepared using deionized water p~lrified filrther by distillation from alkaline perman- ganate.

Kitzetics Rates were measured spectroscopically by monitoring the

increase in product ketone absorption in the region 240-270

nm. Runs with half-lives greater than 2-3 min were performed using a Cary 118 spectrometer, and faster reactions were measured with a Durrum-Gibson stopped-flow spectrometer; both instruments were thermostatted at 25.0 k O.I0C. Rate constants were evaluated graphically, either as plots of In (A -A,) vs. time (using infinite-time readings made after 10-12 half-lives) or by the Guggenheirn (18) or Swinbourne (19) methods.

Acknowledgements We are grateful to the National Research Council

of Canada, the National Science Foundation of the United States, and the donors of the Petroleun~ Research Fund administered by the American Chemical Society for their generous support of this research.

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