3991
Organic and Biological Chemistry The Role of Neighboring Groups in Replacement Reactions. XXVII.’ 5-Methoxyl Participation in Some Solvolysis Reactions’ Evan L. Allred and S. Winstein Contribution No. 2053 from the Department of Chemistry, University of California, Los Angeles, California 90024, and the Department of Chemistry, University of Utah, Salt Lake City, Utah. Received February 25,1967 Abstract: Me0-5 participation in the solvolysis of 6-methoxyalkylp-bromobenzenesulfonateshas been scrutinized to provide information regarding the factors affectingring closure and the reactivity of the resulting intermediates. Rates of solvolysis of 4-methoxy-l-pentyl (IP) and 5-methoxy-2-pentyl(IS) p-bromobenzenesulfonates show that an a-or &methyl group is rate enhancing by a factor of ca. 6 in acetic acid and ethanol. These results are discussed in terms of k,, k,, and ka. The effect of both a-and &methyl groups is approximately additive for threo-5-methoxy-2hexyl p-bromobenzenesulfonate. However, the erythro isomer is slower than the threo by a factor of 2.4. The differencecan be ascribed most plausibly to steric retardation. This suggeststhat the O-methyl and &methyl groups prefer to be in a trans orientation. Both IP and IS give identical ratios of CH~OCH(CH~)(CHZ)ZCH~OS and CH30CH2(CHz)2CH(CH3)OS as solvolysis products (60:40 for EtOH and 40:60 for AcOH). A trace of 2-methyltetrahydrofuran was detected in ethanolysis. These results collectively have been taken as evidence for the formation of O-methyl-2-methyltetrahydrofuraniumion which undergoes nucleophilic attack at the methyl and primary and secondary C-0 carbon atoms. The change in product ratio on going from ethanol to acetic acid may be looked upon as the consequence of a shift to a less nucleophilic solvent.
E
arlier, in a preliminary communication, we summarized some of our observations regarding methoxy1 participation in solvolytic substitution reactions.lb These showed that participation is substantial for the Me0-5 and Me0-6 cases. Since the first report we have completed a more extensive investigation of the Me0-5 participation mechanism. The investigation of this system was of special interest to us because of the importance of the results in several directions. Such a study has a bearing on the phenomenon of neighboring group participation and furnishes information regarding the factors which control ring closure and the reactivity of any resulting intermediate. In another aspect, it represents a structural extreme among possible RX substrates in solvolysis and provides an important calibration point for our understanding of ion pairs and dissociated ions in solvolysis.3 This paper reports the portions of our study concerned with a broad survey of the Me0-5 mechanism in solvolysis, and particularly with the structural factors which influence participation and subsequent product formation. In subsequent papers we will describe ion pair phenomena connected with Me0-5 participation in acetolysis and the observation that (1) (a) Part XXVI: C. B. Anderson, E. C. Friedrich, and S. Winstein, Tetrahedron Letters, No. 29, 2037 (1963); (b) part XXV: S. Winstein, E. Allred, R. Heck, and R. Glick, Tetrahedron, 3, 1, (1958); some of the material of the present manuscript has been presented previously at the symposium on “Dynamic Stereochemistry,” Manchester, England, March 31, 1954: see Chem. Ind. (London), 562, 569 (1954); and S. Winstein, Experientia Suppl., 2, 137 (1955); (c) part XXIV: R. M. Roberts, J. Corse, R. Boschan, D. Seymour, and S. Winstein, J . Am. Chem. Soc., 80, 1247 (1958). (2) This research supported by the National Science Foundation. (3) S. Winstein, et al.: (a) Chem. Ind. (London), 664 (1954); (b) J . Am. Chem. Soc., 78, 328 (1956); (c) ibid., 80, 169 (1958); (d) ibid., 86, 2072 (1964); (e) ibid., 86, 2720 (1964); (f) Special Publication No. 19, The Chemical Society, London, 1965, p 109.
participation accompanies lithium aluminum hydride reduction in ether. Results Substrate Systems and Rates of Solvolysis. The required methyl-substituted methoxybutanols and the corresponding p-bromobenzenesulfonates were readily available through well-known synthetic routes. In the case of the 5-methoxy-2-hexyl system, the study was made with the pure erythro isomer and with a mixture of threo and erythro isomers. Pure dl-erythrodmethoxy-2-hexanol was made via the action of methyl iodide on the monosodium salt of meso-2,5-hexanediol. The diastereomeric mixture of 5-methoxy-2-hexanols was prepared from acetaldehyde and the Grignard reagent of 1-chloro-2-methoxybutane. Kinetic results from the acetolysis of the mixed p-bromobenzenesulfonates indicated an erythro :threo ratio of ca. 0.8. Pertinent kinetic data are summarized in Table I for the various methyl-substituted methoxybutyl p-bromobenzenesulfonates. This table also lists the values of the thermodynamic quantities of activation, AH* and AS*. The observed solvolysis kinetics were first order within an experimental error of less than =t2x except in acetolysis of the diastereomeric mixture of 5methoxy-2-hexyl p-bromobenzenesulfonates. In this case, the integrated titrimetric rate constants calculated from eq 1 drifted downward during a run in a manner typical of two materials reacting at slightly different rates. Comparison of these results with the lower, but extremely steady first-order rate constant of erythrod-methoxy-2-hexyl p-bromobenzenesulfonate made it apparent that the downward drift was caused by a difference in the reaction rates of the two dia-
kt = In [a/(a
-
x)]
(1)
Allred, Winstein 1 MeO-5 Participation in Sokvo[ysis Reactions
3992 Table I. Summary of Solvolysis Rates
Compound CH~OCH(CH~)(CHZ)ZCHZOBS (IP) CHDOCHZ(CHZ)ZCH(OBS)CH~ (IS)
Solvent
Temp, "C
ROBS concn, 102M
AcOH AcOH EtOH AcOH AcOH EtOH
25.20 50.00 25.20 25.20 50.03 25.20
3.19 3.25 3.36 3.37 3.25 3.30
k , sec-1 (1.71=tO0.O2)X10-6 1 2 . 8 7 i 0 . 0 6,).x. 10-4 (3.87 =t 0.02) X 10-5 (1.72 i0.02) X 10-6 (3.02 f 0.02) x 10-4 ( 3 . 6 7 i 0.04) x 10-6
AcOH AcOH AcOH
25.20 50.00 25.20
3.23 3.09 3.15
(3.93 i 0.02) x 10-6 (6.77i-0.07) X 10-4 ( 9 . 4 7 i 0 . 0 5 ) X 10-6
CH~OCH(CH~)(CH~)~CH(OBS)CH~ dl-erythro dl-threo" 0
~
AH*, kcal/ mole
AS*, eu
20.8 -~
-10.9
21.1
-9.4
21 .o
-8.2
~
Rate constant calculated by eq 2.
stereomers. Using eq 2, which is derived in the Experimental Section, and the known rate constant of the erythro isomer, it is possible to obtain the rate constant of the threo constituent. In this equation, x and y
k,t = -In { ( z / x o )- [(zo/xo)- l]e-k~'] (2) refer to the amounts of the threo and erythro isomers, respectively, while z is the total amount ( x y), all at time t. The initial quantities are denoted as xo, y o , and zo, respectively. The rate constant for threo-5-methoxy-2-hexyl p-bromobenzenesulfonate was calculated using a best value of x o obtained by carrying out successive approximations until k , remained constant throughout a kinetic run. Average deviation for nine points covering the range 2 0 4 5 % reaction was *OS%. The results indicate that the threo isomer solvolyzes faster by a factor of 2.4. It was somewhat surprising to find that 4-methoxy-lpentyl p-bromobenzenesulfonate (IP) and 5-methoxy-
+
IP
J
\
IS
II
1SOCH3
H+o+H kH3
H
2-pentyl p-bromobenzenesulfonate (IS) have identical rate constants for acetolysis at 25 O and differ by only 5 for ethanolysis. This raised the question whether the bromobenzenesulfonate samples used were indeed difJournal of the American Chemical Society
89.16
ferent compounds. However, examination of the usual properties revealed that the samples designated IP and IS were different. Unequivocal evidence that the bromobenzenesulfonate samples were different, pure, and of the indicated structure came from the observation that during acetolysis the two materials rearrange into each other and actual rates of isomerization can be measured. A detailed account of these results is presented in a subsequent paper. Solvolysis Products. Products of complete solvolysis of 4-methoxy-I-pentyl (IP) and 5-methoxy-2pentyl (IS) p-bromobenzenesulfonates were analyzed and the results are summarized in Table IT. For acetolysis, the product acetates from both IP and IS were isolated after 11 reaction half-lives in 85 and 86 % yields, respectively. Analysis of the two product samples by gas chromatography showed each to be a mixture of the same two components. These were identified as 5-methoxy-2-pentyl and 4-methoxy-lpentyl acetates by comparison of infrared spectra and gas chromatographic retention times with pure authentic samples of each acetate, Both IP and IS yielded essentially identical mixtures of secondary and primary acetates in a 60:40 ratio. Analysis of a known mixture checked the weighed composition to within 0.1 %. A control experiment in which 4-methoxy-1-pentyl acetate was kept in an acetic acid solution containing 0.11 M p-bromobenzenesulfonic acid for a time corresponding to eight reaction half-lives demonstrated that this observed ratio was inherent in the acetolysis reaction and was not due to isomerization of the acetate product in a strongly acid medium. Gas chromatographic analysis of acetic acid distillates from the total acetolysis of both bromobenzenesulfonates failed to reveal the presence of any 2-methyltetrahydrofuran or methyl acetate. However, a check on the analysis with controls of known concentration indicated that yields of 2-methyltetrahydrofuran below ca. 5 % could not be detected. The kinetic infinity titers for acetolysis of IP and IS were 97.8 f 0.3% and 98.0 i 0.2%, respectively. Since methyl pbromobenzenesulfonate reacts at least I O 4 times slower than these c o m p o ~ n d s , the ~ upper limit of methyl ester formation is 2%. This means that no more than a 2 % yield of 2-methyltetrahydrofuran could arise from this route. In the case of ethanolysis the high boiling ether products from IP and IS were recovered in about 86 yield
August 2, 1967
(4)
S. Winstein and H.Marshall, J . Am. Chem. SOC.,74, 1120 (1952).
3993 Table II. Summary of Products from the Total Solvolysis of 4-Methoxy-1-pentyl (IP) and 5-Methoxy-2-pentyl (IS) pBromobenzenesulfonates
-
Compd IP IS 4-Me0-1Dentvl OAc
Compd IP IS
Added solute
...
... 0.11 M HOBS
--
Acetic acid, [ROBs] 0 . 1 M , 25.2" Compostn of acetate mixta 4-Me0-15-Me0-2Reaction Yield of pentylOAc, pentylOAc, wt % half-lives ROAc, % wt % 11 11 8
85 86 93
39.54 39.80 100.00
60.46 60.20 0
Ethanol, [ROBs] 0 . 1 M , 25.2' --Compostn of ether mixta-2-Me0-5-EtO1-Me0-4-Et0pentane, pentane, Reaction Yield of half-lives wt ROEt, % wt
z
z
14.5 13.5
86 85
60.23 58.14
Estimate based on a 97.8 i~ 0.3% kinetic infinity titer. a Determined by gas chromatography. detected by gas chromatography. However, for acetolysis this technique was only sensitive to ca. 5 %. kinetic infinity titer.
39.17 41.86 d
2-Methyltetrahydrofuran, % of theory
