HARLAN L. GOERING ,4ND
1992
2-m. stainless steel di-n-decyl phthalate column (Type A) was employed with hydrogen a t 15-20 lb. p.s.i. gauge pressure as the carrier gas. Quantitative estimation of areas was made by tracing the curves on filing cards, cutting out the pertinent section and weighing. In most instances, it correction factor (c.f.) needed to multiply the observed composition t o obtain the true composition was determined by calibration with known mixtures. Photolysis of Diethyl Diazomalonate (I) in 2 +Dimethylbutane.--.& solution of 13.0 g. (0.07 mole) of diazomalonic ester in 550 ml. ( 5 . 4 mols) of 2,3-dimethylbutane (Phillips Petroleum Co.; pure grade, > 99yo) was irradiated according to the previously described procedure for diazoacetic ester.4 The crude malonic esters remaining after distillation of the solvent were refluxed 3 hr. with 125 g. of 10% ethanolic potassium hydroxide. Removal of the ethanol left a solid residue which was dissolved in 50 ml. of water and acidified. Ether extraction afforded the crude malonic acids which were decarboxylated by heating a t 160-180". The resulting monocarboxylic acids were dissolved in ether and extracted with 6 1V sodium hydroxide. After two ether extractions, the aqueous solution of sodium salts was acidified t o liberate the free acids, which were then converted by excess ethereal diazomethane t o the methyl esters; 4.09 g. (37.2yOoftheory), b.p. 56-63" (15mm.). G.1.p.c. separation of the liquid esters a t 133' gave two fractions, of which the first (67.6%) was isolated and shown to have the same infrared spectrum as methyl 3,3,4-trimethylpentanoate (c.f. 1.03)4 and the second (32.4YG)had the same infrared spectrum as authentic methyl 4,5-dimethylhexanoate (c.f. 0.97) . 4 Calculation of Reactivity Ratio.-The reactivity ratio of different groups (or types of hydrogen atoms), a i l a , is defined as the ratio of the rate of reaction (of the carbene) with the group i t o t h a t with group a taken as standard. The reactivity ratio is therefore equal t o fixa/%ija where f is the fraction of total product and n the number of each type of group. Photolyses in Isobutane. (a) Authentic Materials.Methyl 2,2-dimethylbutanoate, b.p. 133" ( i 6 0 mm.), was prepared according t o Homeyer, et ~ 1 . ' Diethyl ~ tbutylmalonate was prepared from t-butylacetic acid by the procedure of BushIe; b.p. 102-103" a t 11 mm. Methyl 4methylpentanoate, b.p. 146" (760 mm.), and diethyl isobutylmalonate were prepared by a standard malonic ester synthesis. A mixture of 61.970 of methyl t-butylacetate (15) A. H. Homeyer, F. C. Whitmore and W. H. Wallingford, J . A m . Chem. Soc., 66, 4209 (1933). (16) M. T. Bush, ibid., 61, 637 (1939).
[CONTRIBUTION FROM
THE
DEPARTMENT O F CHEMISTRY O F
LIARTIN
F. SLOAN
Vol. 83
and 38.1'% of methyl 4-methylpentanoate was analyzed by g.1.p.c. a t 100'; three determinations gave relative compositions of 61.9 and 38.1Y0, 61.6 and 38.4y0, and 61.3 and 38.770. Correction factors (c.f.) of 1.005 and 0.992, rerespectively, were derived and applied in the analytical procedure. (b) Methyl Diazoacetate.-Irradiation of 10.0 g. of methyl diazoacetate in 1000 ml. of isobutane gave 6.48 g. of product, b.p. 114-117' a t 760 mm., which was shown by g.1.p.c. (100') to consist of 25.4% of methyl t-butylacetate and '74.6% of methyl 4-methylpentanoate. The pure compounds were separated by g.1.p.c. and were identified by their infrared spectra. (c) Ethyl Diazoma1onate.-The product from irradiating 8.0 g. of ethyl diazomalonate in 1000 ml. of isobutane was saponified with 120 g. of refluxing ethanolic potassium hydroxide. The crude acids were decarboxylated and methylated with ethereal diazomethane giving 2.74 g. of mixed esters, b.p. 150-180'. Separation by g.l.p.c., identification by infrared spectrum and analysis by g.1.p.c. showed the mixture t o consist of i O . 0 7 0 of methyl t-butylacetate (c.f. 1.01) and 30.0% of methyl 4-methylhexanoate (c.f. 0.99). (d) Validation of Analytical Procedure.-A synthetic mixture of 2.120 g. ( 7 0 . 0 ~ oof ) diethyl t-butylmalonate and 0.910 g. (30.0cG)of diethyl isobutylmalonate was treated in the same manner as above, then saponified, decarboxylated, methylated and analyzed by g.1.p.c. a t 100". The mixture, after applying the correction factor, appeared t o consist of €19.37~of methyl t-butylacetate and 3O.7yG of methyl isobutylacetate. Photolyses in %-Butane. ( a ) Authentic Materials.Methyl n-hexanoate (IX) was obtained from commercial hexanoic acid and excess ethereal diazomethane: b .p. 152" (760 mm.), n Z 61.4038. ~ Methyl 3-methylpentanoate (VIII) was prepared from sec-butyl chloride by a standard malonic ester synthesis: b.p. 142' (760 mm.), n2% 1.4050. Triplicate analyses by g.1.p.c. a t 100' of a synthetic mixture of 69.07, of TI11 and 31.0% of IX indicated 69.8, 69.8 and 69.5y0 of 17111, corresponding to c.f. of 0.990 for VI11 and 1.022 for IX. ( b ) Methyl Diazoacetate.-The reaction of 9.5 g. of methyl diazoacetate afforded 6.02 g. (4970) of mixed esters consisting of 60.9% of VI11 and 39.1% of IX. The pure products were separated by g.1.p.c. and identified by their infrared spectra. (c) Ethyl Diazoma1onate.-The product from 15.0 g. of I was worked in the usual manner t o give 2.06 g. of mixed esters, b.p. 75-154', consisting of 84.870 of VIII and 15.270 of IX (determined by g.1.p.c. with hydrogen a t 15 lb. p.s.i. (gauge) and 75").
THE ~ N I V E R S I T VO F LYISCONSIN,
MADISON6, \TIS.]
Ionic Reactions in Bicyclic Systems. 111. Solvolysis of Bicyclooctanyl and Bicyclooctenyl p-Toluenesulfonatesl BY HARLAN L. GOERING AND MARTIN F. SLOAN~ RECEIVED OCTOBER 17, 1960 First-order rate constants for acetolysis and ethanolysis of (a) endo-bicyclo[2.2.2]oct-5-en-2-yl p-toluenesulfonate ( I ) ,( b ) bicyclo[2.2.2]octan-2-ylp-toluenesulfonate (VII) and (c) exo(axial)-bic~clo[3.2,l~octan-2-J.I p-toluenesulfonate (VIII) have been determined. Rate constants for the solvolysis of 1'11 in 80% aqueous acetone and acetolysis of endo(eguaforial)bicyclo[3.2.l]octan-2-yl p-toluenesulfonate (XVJ) have also been determined. Ionization of endo-bicyclo[2.2.2]oct-~-en-2-~-l p-toluenesulfonate (I), which results in complete rearrangement t o the bicyclo[3.2.1] oct-3-en-2-yl system, is anchimerically assisted. Solvolysis of bicyclo[2.2.2]octan-2-yl p-toluenesulfonate (VII) and axial-bicyclo [3.2.1]octan-2-yl p-toluenesulfonate (1'111) is accompanied by internal return which results in the interconversion (equilibration) of the two isomers during solvolysis. The reactivities of these isomers are consistent with the idea that ionization results in the formation of a common bridged ("non-classical") carbonium ion; i.e.,anchimeric acceleration is indicated.
Introduction In the first paper in this series3 it was shown that acetolysis of endo-bicyclo [2.2.2]oct-5-en-2-yl p (1) This work was supported in p a r t by the Office of Ordnance Research.
toluenesulfonate (I) gives axial-bicyclo [3.2.1Ioct3-en-2-yl acetate (Ix). Deamination Of endo-5arninobicyclo [2.2.2]oct-2-ene also results in complete rearrangement to the bicyclo [3.2.1Ioct-3en-2-yl system. 3 , 4 On the other hand, solvolysis
(2) Wisconsin Alumni Research Foundation Fellow 1956-1958; National Science Foundation Fellow 1'358-1960.
(3) H. L. Goering, R . W. Greiner and M. F. Sloan, S o r . , 83, 1391 (1961).
J. Am. Chem.
April 20, 1961
1993
SOLVOLYSIS OF BICYCLO~CTENYL p-TOLUENESULFONATES
of the saturated analog, bicyclo [2.2.2]octan-2yl e-toluenesulfonate (VII), or deamination of the corresponding amine results in only partial rearrangement to the bicyclo [3.2.l]octan-2-y1 system.j The Wagner-Meerwein conversion of the endobicyclo [2.2.2]oct-5-en-2-~1system to the bicyclo[3.2.1]oct-3-en-2-yl system is not unexpected. By analogy with related systems, ionization of I would be expected to give the bicyclo[3.2.1]allylic cation 11. Evidence has been presented that syn-7-norbornenyl p-toluenesulfonate (111)6,'
It was discovered that VI1 undergoes a reversible stereospecific isomeric rearrangement to axialbicyclo [3.2.1loctan-2-yl p - toluenesulf onate (VII I) during solvolysis and for this reason solvolysis of the latter compound, and its epimer XVI, were also investigated.
Results
endo-Bicyclo [2.2.2]oct-5-en-2-01, prepared and purified by a method described earlier,'O gave a p-toluenesulfonate derivative which contained a reactive contaminant (probably the exo isomer) .3,11 Pure endo-bicyclo[2.2.2]oct-5-en-2-yl p-toluenesulfonate (I) was obtained by recrystallization of this material from ethanol. When Compound I was derived from pure endo-bicyclo [2.2.2]oct-5en-2-01~the ethanol recrystallization was not necesI1 sary. Homogeneous samples of I, prepared by and trans-5,6-dibromobicyclo [2.2.2]oct-2-ene (IV)* either of these ways, showed excellent first-order ionize in polar media to give the allylic carbonium solvolytic behavior. The p-toluenesulfonate deions. In each of these cases participation by the rivative I was shown to have the bicyclo [2.2.2]methylene group is manifested by a high reactivity. oct-5-en-2-yl structure by conversion (hydrogenaThe conversion of syn-7-camphenyl p-toluene- tion) to bicyclo [2.2.2Ioctan-2-yl p-toluenesulfonate (VII) .5 TsO? Specific first-order rate constants for acetolysis \nh (30' and 42') and ethanolysis (30' and 49') are given in Table I. In each experiment the reaction was followed to a t least 757, completion and nine or more values of the first-order constant were I11 determined from appropriately spaced titrations. No trends in these values were observed and the rate constants were reproducible. A kinetic experiment is included in the Experimental section. Br With this compound, and all of the others investigated in the present work, the infinity titers corresponded to theory. sulfonate (V) to l,l-dimethyl-2-hydroxymethyl- With one exception, the constants in Table I bicyclo [3.2.0]hept-2-ene (VI) by hydrolysiss indi- are average values (and deviations from the cates that here also ionization may involve direct TABLE I formation of the allylic carbonium ion. However, S O L V O L Y S I S O F endO-BICYCLO[2.2.2]OCT-5-EN-2-YL p-TOLUin this case there is no apparent rate enhancementENESULFONATE (1)" the acetolysis rate of V a t 140' is slightly smaller 105 k, Temp., than that of its saturated ana10g.~ nb sec. -1
n
n
O C .
LOTS
Acetolysis 2
30.07
30.07 42.06
AH*
=
1" 2
23.1kcal./mole; A S *
2.97=t0.03 3.10=k .02 .05
13.2 =
-3.0 e.u. at 30"
Ethanolysis
In the present work the solvolytic reactivity of 3 0.3234= 0.002 30.07 endo-bicyclo [2.2.2]oct-5-en-2-y1 p-toluenesulfonate 49.03 2 3.93 .05 (I) in ethanol and acetic acid was determined to AH* = 25.0 kcal./mole; AS* = - 1.2 e.u. at 30" obtain information concerning the nature of Initial concentration of I was 0.025 &fin all experiments. carbonium ion reactions in this system. To assess Number of independent kinetic experiments. c Conthe rate enhancement resulting from methylene tained 0.055 M sodium acetate. participation the rates of acetolysis and ethanolysis of the saturated analog, bicyclo [2.2.2]octan-2- average) of two or three independent kinetic exy1 p-toluenesulfonate (VII),j were also determined. periments. The other is the average of nine determinations made during the single experiment. ( 4 ) W. C. Wildman and D. R . Saunders, J . Am. Chem. SOC., 76, 946 The activation parameters included in the table
*
(1954). (5) H.L.Goering and M. F. Sloan, ibid., 83,1397 (1961). (6) S. Winstein and E. T. Stafford, ibid.. 79, 505 (1957). (7) S. Winstein and C. Ordronneau, i b i d . , 82, 2084 (1960). (8) A. Gagneux and C. A. Grob, Xels. Chim. Acta, 48, 1763 (1959). (9) E.E.van Tamelen and C. I. Judd, J . A m . Chem. Soc., 80,6305 (1958).
(10) W. C. Wildman and D. R. Saunders, J . Ovg. Chem., 19, 381 (1953). (11) This material, even after several recrystallizations from e t h e r pentane, gave apparent integrated first-order rate constants (ethanolysis and acetolysis) which decreased more than 60% during the first 50% reaction.
1.994
HARLAN L. GOERINGAND MARTINF. SLOAX
were computed in the usual manner.IZ The second entry in Table I shows that 0.055 Msodium acetate causes a slight positive salt effect. It was under these conditions that the product was shown to be exo(axial)-bicyclo [3.2.l]oct-3-en-2-y1 acetate (IX).3 To determine the effect of the double bond in I on the reactivity, the rates of acetolysis and ethanolysis of the saturated analog, bicyclo [2.2.2]octan-2-yl p-toluenesulfonate (VII), were investigated. The p-toluenesulfonate VI1 was prepared from pure bicyclo [2.2.2]octan-2-01.~ In this case the integrated first-order rate coefficient, k (eq. l), for both acetolysis and ethanolysis decreased during the reaction. Both the initial rate constants (obtained by extrapolating plots of percentage reaction 71s. integrated first-order rate coefficient to 0% reaction) and the rate and magnitude of the downward drift were reproducible. The integrated constants drifted down throughout the entire reaction. However, plots of log [ROTS]us. time became linear after about 3oy0 reaction for acetolysis and about 40% reaction for ethanolysis. I n other words, the instantaneous first-order rate constant, klnst (eq. 2) drifted down during the earIy stages of the reaction and then became constant. In all cases, integrated constants calculated from data for the last 50% of reaction were steady.
Vol. 83
TABLE I1 SOLVOLYSIS
O F BICYCLO[2.2.2]OCTAN-2-YL ~ T O L U E N E S U L FONATE
(sTII)"
Temp., 'C.
Itb
30.07 49.03 49.03 49.03 49.03
Acetolysis 2 0.68 i 0.01 0.448f 0,001 2 7.4 i .1 5.40 f .04 2 2.99 NaOAc 7.6 f .1 5 . 4 2 f .05 2 5.50 NaOAc 7.8 .1 5.36 f .01 1" 5.50 KaOAc 7 . 7 f . 2 5.33 f .07
49.03
2
Added salt, 104 Ai
106 ki[y,ml, c sec. - 1
10' k! id scc.
Ethanolysis
1.32 i 0.01 0.978 f 0.004
80% aqueous acetone' 49.03 1 9 . 1 f 0.2 a Initial concentration of VI1 was 0.025 M. Nurnber of independent kinetic experiments. Initial rate constant. Rate constant after 50Yo reaction ( i . e . , zeropoint titer taken a t about 50% reaction). OSample of VI1 used in this experiment was prepared by recrystallizing analytically pure material (material used in other experiments) four times. f Composition based on volumes of pure components a t 25" before mixing.
the acetolysis rate behavior and physical properties of an analytically pure sample of VI1 were unchanged by four additional recrystallizations (fifth entry in Table 11). k = ( l / t ) In ( [ R O T S J O / [ R O T S ~ ~ ) (1) The less reactive isomer formed during the kinat = -(d[ROTs]/dt)/[ROTs]t (2) solvolysis of bicyclo [2.2.2]octan-2-yl p-tolueneThe pertinent kinetic experiments are sum- sulfonate (VII) was identified as uxial-bicyclomarized in Table 11. Two values of the rate con- [3.2.l]octan-2-yl p-toluenesulfonate (VIII) in the following way. Unreacted p-toluenesulfonate was stants are given in the table. The first, k i ~ ~ . 2 . 2is ], the initial rate constant (obtained by extrapolat- isolated from an acetolysis reaction after 50% ing the integrated coefficients to 0% reaction) and reaction. After recrystallization, the yield of the second, kf,is the average of five or more values recovered p-toluenesulfonate was 63%. The melt(and average deviation) of the integrated first- ing point (11' lower than that of pure VII) and order constant for the final 50% of the reaction infrared spectrum of this material were indistin(;.e., the zero-time titer was taken a t about 50y0 guishable from those of a pure synthetic binary reaction). The latter values correspond to the mixture of 30% V I 1 and 70% VIII. Bands instantaneous first-order rate constant after it characteristic of equatorial-bicyclo 13.2.1loctan-2becomes steady--if integrated first-order constants yl p-toluenesulfonate (XVI) mere absent. are steady, k(eq. 1) = kinet (eq. 2). The data in The fact that the instantaneous first-order Table I1 show that the acetolysis behavior of VI1 is constants become steady after the initial downward not affected by the presence of 0.05 M sodium drift indicates that a steady-state ratio of conacetate. One rate constant for the hydrolysis of centrations of VI1 and VI11 is obtained; ;.e., VI1 in SOYG acetone is included in Table 11. In the rearrangement is reversible and reaches equithis case the rate constant also drifted downward librium. This was confirmed by investigating (from 9.1 X set.-', 0% reaction, to 6.5 X the solvolytic behavior of axial-bicyclo [3.2.1] sec.-l, 75y0reaction). These are the condi- octan-2-yl p-toluenesulfonate (VIII). tions under which the product was shown to be a Pure VI11 was prepared from pure axial-bicyclobinary mixture of bicyclo 12.2.2loctan-2-01 and [3.2.l]octan-2-01.~ In this case the integrated axial-bicyclo [3.2.1]octan-2-01.~ first-order coefficients drifted upward for both The decrease in the rate constants for the sol- ethanolysis and acetolysis. As for the solvolysis volysis of bicyclo [2.2.2]octan-Z-y1 P-toluenesulfo- of VII, plots of log[ROTs] VS. time become linear nate (VII) is due to rearrangement during sol- after about 30% acetolysis and 45y0 ethanolysis. volysis to the slightly less reactive axial-bicyclo- Thus in this case the instantaneous rate constants [3.2.1loctan-2-yl p-toluenesulfonate (VIII). T h a t show initial upward drifts and then become steady. VI11 is produced during solvolysis, rather than Data for the acetolysis and ethanolysis of VI11 being present in the p-toluenesulfonate (VII) are given in Table 111. from the outset, mas established as follows. The That the upward drift in the rate was indeed infrared spectrum of V I 1 used in the kinetic due to isomerization to the more reactive bicycloexperiments did not contain bands characteristic [2.2.2] isomer VI1 was demonstrated by isolation of the bicyclo[3.2.1] isomer VIII. Moreover, of the unsolvolyzed p-toluenesulfonate from an ethanolysis experiment after 50% solvolysis. This (12) A. A. Frost and R. G. Pearson, "Kinetics and Mechanism," material, isolated in 78y0 yield, had an infrared John Wiley and Sons, Inc., New York, N. I 1953.
SOLVOLYSIS OF BICYCLO~CTENYL p-TOLUENESULFONATES
April 20, 1961
1995
( F ) in the equilibrated unsolvolyzed p-toluenesulfonate is 0.32 f 0.04 for acetolysis a t 49' and 0.45 SOLVOLYSIS O F UX~ULBXCYCLO [3.2.1] OCTAN-2-YL p-TOLUENEf 0.04 for ethanolysis a t 49'. These values SULFONATE (VIII) AT 49.03'" correspond to equilibrium constants ( K e g ) of 105 ~ I I ~ . P . b~ I , 105 k f , b Solvent sec. sec. - 1 2.1 f 0.4 for acetic acid and 1.2 f 0.3 for ethanol HOAc 4.3 5.23 f 0.07 for the interconversion of VI1 and VIII. This HOAc 4.3 5.28 2~ .03 steady-state composition differs slightly from the EtOH 0.67 0.928 f ,020 thermodynamic equilibrium composition because EtOH 0.64 0.986 f .034 the isomers are not consumed (solvolyzed) a t equal rates. The equilibrium mixture would conInitial concentration of VIII was 0.025 M. * See footnotes c and d , Table 11. tain slightly more of the more reactive isomer VII; ;.e., F would be higher and K,, would be lower. spectrum which corresponded in detail to a binary From the present data it is clear that the difference mixture of 75 5% VI11 and 25 f 5% VII. in thermodynamic stability between VI1 and VI11 The data presented in Tables I1 and I11 clearly under these conditions is
