J . Org. Chem., Vol. 36, No. 1, 1971 193
ALLYLATION OF HINDERED PHENOXIDES
The Effect of Pressure on the Allylation of Hindered Phenoxides' W. J. LE NOBLE,*T. HAYAKASVA,~~ A. K. SEN,^^
AND
Y. TATSUKAMI~~
Department of Chemistry, State University of New York at Stony Brook, Stony Brook, New York,
11790
Received April 99, 1970 The allylations of all 4, 2,6, and 3,5 methyl-, ethyl-, isopropyl-, and tert-butyl-substituted phenols have been carried out in alkaline aqueous medium. The 4-alkylphenols give rise to 2-allylphenols and the allyl phenyl ethers as well as products resulting from further allylation. The 2,6-dialkylphenoxide ions yield allyl ethers, 4.-allylphenols, and 0-dienones as well as products containing more than one allyl group, and similar results were obtained with the 3,5-dialkylphenols. Some of these products are sensitive to thermal rearrangements. The product distributions have been measured over a pressure range of 1 atm to several kilobars, and the difference in partial molal volume of the isomeric transition states has been calculated in each reaction. In all cases, the transition states leading to ether formation are larger than those on the way to the ortho-substituted products, which in turn are larger than the transition states leading to the p-allylphenols; thus, massive support is provided for the contention that solvation of the 0 atom in hydroxylic media is of crucial importance in determining the allylation ratios. No systematic correlation with steric hindrance was found, however; the postulate, so well documented in the case of the Menshutkin reaction, that crowded transition states become favored a t high pressure' is not borne out in this reaction, so that its general validity must be questioned.
If the effect of pressure on a rate constant is known, the activation volume of the reaction can be calculated by means of the expression
where k is expressed in concentration units at 1 atm. AI,'*,, can be predicted for most mechanisms with fair accuracy on the basis of (a) comparisons with pressure data for reactions of well-known mechanisms, (b) volume changes in equilibria, and (c) densities and parachor data of stable substances. This ability allows one to use the pressure coefficient of a rate constant as a mechanistic criterion in many cases. Several features make important contributions to AV*,. Paramount among these are bond formation and cleavage, and charge separa.tion and neutralization. Displacements in which there is no net change in the number of charges have small negative volumes of activation ( w - 5 to - 10 cm3/mo1), suggesting that bond formation is more advanced in the transition state than the concurrent bond f i ~ s i o n . ~ A factor of great potential interest is that of steric hindrance. Crowded compounds usually have somewhat greater densities than their unhindered isomers, and it mould seem reasonable to suppose that hindered transition sta1,es would similarly have smaller volume requirements than the unhindered substrates from which they are formed. If this is so, hindered reactions should be accelerated to a greater degree than their unhindered analogs, clearly a possibility of much interest. Evidence for it has been reported by several groups. The first such claim was made by Perrin and Williams4 in 1937 and quite recently Gonikbergs concluded that "the more sterically hindered a chemical reaction, the
* To whom correspondence should be addressed. (1) (a) Presented in part a t the Second International High Pressure Conference a t Schloss Elmau, Germany, M a y 1968; (b) paper X X I I in the series, "Chemical Reactions Under High Pressure." (2) (a) On leave trom Mitsui Toatsu Chemicals, Inc., Yokohama, Japan, 1967-1969; (b) on leave from the Indian Institute of Technology, Kharagpur, India, 1967-1968; (c) on leave from the Sumitomo Chemical Company, Osaka, Japan, 1967--1969. (3) W.J. le Noble, P r ~ g r P . h y s . Org. Chem., 5, 207 (1967); Cf.also the several excellent reviews a n d books referred t o in the opening paragraph of t h a t paper. (4) M . W. Perrin a n d E. G. Williams, Proc. R o y . Soc., Ser. A , 169, 162 (1937). (5) M. G . Gonikherg, R u s s . J . P h y s . Chern., 97, 248 (1963).
greater the degree to which it should be accelerated with increasing pressure." I n spite of the evidence,'j it is probably still too early for such a generalization. Almost all of the examples that have been found are Menshutkin reactions, and even in that reaction the evidence is sometimes more apparent than real since one is often forced to compare data gathered in different solvents, at different temperatures and over different pressure ranges. Also, while it is true that crowded compounds are more dense than their unhindered isomers, the differences in molar volume seem rarely to be more than a cm3 or two; but the AAV* values reported are often much larger than that. The potential of this phenomenon, a selective increase of the rate of sterically hindered reactions, appeared to us great enough t o warrent a systematic investigation. We report here our results for the alkylation of substituted phenoxide ions in aqueous medium.
Discussion Allyl chloride was chosen as the alkylating agent and water as the solvent because it is known' that, at least with phenoxide ion itself, initially three products form in reasonable amounts under such conditions: allyl phenyl ether, and o- and p-allylphenol. Since those mixtures can be readily analyzed, it appeared that the effect of pressure on the competition of those three reactions, already known in the case of phenoxide itself, might provide us with a well-documented example of the relation between that effect and steric hindrance in a displacement reaction. I n its execution, the problem was complicated somewhat by the possibilities of further alkylation of the phenolic products, and of rearrangements. Secondary alkylation (see Scheme I) was not found to present serious analytic difficulties in any case; thus, any allyl o-allylphenyl ether formed is simply considered o-allylphenol, since it must have arisen from that phenol. Any o- or p-diallylphenol was considered to be formed from both allylphenols; the ratio of the contributions was crudely calculated on the assumption that the rates of allylation at these positions are not affected by the presence of the m-allyl group already there. I n nearly (6) Summarized b y W.J. le Noble and Y. Ogo, Tetrahedron, 26, 4119 (1970). (7) N. Kornblum, P. J. Berrigan, a n d W. J. le Iloble, J . Amer. Chern. SOC., sa, 1257 (1960).
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J . Org. Chem., TToZ. 36, No. 1, 1971
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