3464
HARLAN L. GOERIKGAKD RICHARDW.GREINER [CONTRIBUTIOU FROM
THE
Irol. 70
DEPARTMEUT OF CHEMISTRY OF THE UNIVERSITY OF \TISCOSSI~]
Stereochemistry of Allylic Rearrangements. IX. The Solvolysis of trans-cu,~-DimethylallylAcid Phthalate in Aqueous Acetone BY
HARLAN
L. GOERINGAND RICHARDIv.GREINER~ RECEIVED JASUARY
14, 1957
The polarimetric (k,) and titrimetric (kt) rate constants for the solvolysis of trans-cu,r-dimethylallyl acid phthalate ( I ) in aqueous acetone have been determined. At concentrations up to 0.1 both k, and k, are steady during the reaction and k , > kt under all conditions at which the two were compared. The greater rate of loss of optical activity than of formation of solvolysis products is due t o partial racemization of the reactant, by an isomeric intramolecular (Ssi’) allylic rearrangement, prior to solvolysis. The data are consistent with a picture involving the reversible formation of a symmetrical ionpair intermediate which is irreversibly converted to solvolysis products.
In earlier papers in this series3m4it was pointed out that a n intermediate (an “internal ion-pair’’ intermediate according to R‘instein’s classifications) is involved in the ionization-dissociation step of carbonium-ion reactions in allylic systems. This intermediate can return to the original allylic compound or its allylic isomer by internal returnpresumably this represents the mechanism of the SNI rearrangement of allylic compounds3-or dissociate t o the allylic cation and the accompanying anion. I n non-reactive solvents, e.g., non-hydroxylic solvents, the dissociated carbonium ion and anion recombine; however. in hydroxylic solvents this recombination is completely diverted by sol~ o l y s i s .Thus, ~ ~ ~ in hydroxylic solvents the SNi‘ rearrangement is cleanly isolated from an intermolecular isomeric rearrangement. Optically active symmetrical allylic systems, i.e., systems with similar a - and yalkyl substituents, are especially well suited for studying the Sxi’ rearrangement (internal return) which accompanies solvolysis. I n such systems the SNi‘ rearrangement results only in the interconversion of enantiomers and thus does not disturb the firstorder rate of product formation but results in a larger rate of loss of optical activity (polarimetric rate) than of product formation (titrimetric rate). The first-order rate constant for the conversion of one enantiomer to the other by the Ssi’ rearrangement is (1 ’2j(k, - kt) and thus can be determined readily from the polarimetric (a,) and titrimetric ( k t ) rate c o ~ i s t a n t s . ~ ~ ~ As described in earlier papers in this series, internal return is involved in the solvolysis of the isomeric 5-methyl-2-cyclohexenyl chlorides in ethanol and acetic acid4 and the corresponding acid phthalate+ and p-nitrobenzoates3b in aqueous acetone. I n the present work we have examined the solvolysis of a ,y-dimethylallyl acid phthalate in aqueous acetone to determine the importance of internal return in this acyclic symmetrical allylic system relative to that in the S-methyl-2-cyclohexenyl system. (1) This work x a s supported by t h e Office of Ordnance Research and b y t h e Research Committee of t h e Graduate School with funds given b y the Wisconsin Alumni Research Foundation. ( 2 ) Socony-1Iobil Co. Fellow, 1856-19.57. 1 3 ) (a) H . I.. Goering and E. F Silversmith, THIS J O U R N A L , 77, 1149 (1836); (b) 77, (3249 (1933). (4) H . L. Goering, T D . S e v i t t and E. F . Silversmith, ;bid., 77, 5026 (1953). (i) S.\\“instein, E. Clippinger, A . Fainberg, €2. Heck and G. Robinson, ;bid,, 78, 328 (19Sf). ( 0 ) C . R. Ingold, “Structure and Mechanism in Organic Chemistry,” C,rnell ITnivernity Press, Ithac:,, S . Y . , 1933, p 59fi
Results The a ,y-dimethylallyl acid phthalate used in the present work was apparently the pure tvnns isomer. T h a t the material was homogeneous was indicated by a rather sharp reproducible melting point (89-90.5’)which was unchanged by recrystallization. The infrared spectrum of a,y-dimethylallyl alcohol obtained by reduction of the pure acid phthalate with lithium aluminum hydride had a strong absorption band a t 10.4 p and no bands above 14 p, which is characteristic of k ~ n s - l , 2 disubstituted ethylenes.7 The neutral equivalents of the samples of racemic and optically active acid phthalate used in the kinetic experiments were within 0.37, of the theoretical value. The first-order polarimetric (k,) and titrimetric (kt) rate constants for the solvolysis of tmns-a,ydimethylallyl acid phthalate (I) in ‘TO, SO and !IO:)$ aqueous acetone are given in Table I. The (lata are arranged so that the polarimetric rate constants are for the same conditions as the preceding titrimetric rat e con stants. The polarimetric constants were determiiied from the rate of loss of optical activity of solutions of (+)-I. In all cases solvolysis resulted in coinplete loss of optical activity, and the polarimetric constants were steady over the range that the reaction was followed (88 to 94rG completion). The observed first-order rate constants were reproducible and most of the values for kcr given in Table I are average values of two independent kinetic experiments. -1typical kinetic experiincnt is i n cluded in the Experimental section. The titrimetric constants ( k t ) were determined from the rate of increase in acidity of solutions of racemic I. In all cases the observed initial titer, ;.e., the titer of freshly prepared solutions prior to heating, agreed well with the calculated value. The observed infinity titers, however, were found to be approximately 2YGless than the calculated values (twice the initial titer). Evidently this discrepancy is due to conversion of about l‘,:C of the phthalic acid to phthalic anhydride under the COIIditions of the solvolysis experiments. I n a control experiment the titer of a 0.03.X solution of phthalic acid in 907, acetone a t 100’ was determined periodically. The titer of this solution decreased slo.cvly and after a period of time, corresponding t o infinity time for the solvolysis, the percentage difference between the initial and final titers was the same as between the observed and calculated infinity titcrs. ( 7 ) R . S Rasmusien, R . IZ nrnttnin ;,nil P. S Z ~ i r r i i ., l , < ‘ I > , , I I TT. ’ h v ~ . , 1 5 , 13.i ( 1 9 i 7 ) .
July 5, 1957
SOLVOLYSIS O F i?U?ZS-a,
7-DIMETHYLALLYL ACID PHTHALATES
TABLE I POLARIMETRIC AND TITRIMETRIC RATECONSTANTS FOR THE SOLVOLYSIS O F trans-ol,r-DIMETHYLALLYLACID PHTHALATE I N A%QUEOUS-4CETONE
Acid phthalate, 102 .M
7.50 10.0 10.0 5.00 5.00 2.50 2.50 5.00' 5.00' 5.00" 5 . 0Oh 5.00 5.00
k,cld
Methodb
ka/kt'
102 hr. -1
90% (vol.") acetone, 99.81" T 110 f 6 j T 117 d= 4 P 295 f 8 j
T P T P T P T P
108 f 4 291 f 4 102 f 2 289 =t111 11.9 f 0 . 6 29.8 i 0 . 1 3.97=!=0.16 9.66 f 0 . 1 1
807, (vol.") acetone, 79.58' T 52.9 i 0 . 9 P 100 f 1
70y0 ( ~ 0 1 . acetone, ~) 79.58" T 142 f 2 P 238 f 2
2.52
* 0.16
1 2 . 6 9 f 0.14
I
2.83 f 0 . 1 7 2.50 f 0.14 2.43 i.0.13
'I
i1.89
0.05
).
5.00 1 . 6 8 =IC0 . 0 3 5.00 a Based on volumes of pure components a t 25" before T = titrimetric and P = polarimetric. These mixing. are values for k , (polarimetric determinations) and kt (tiThese values are the average trimetric determinations). (and mean deviation) of two or more independent kinetic experiments, except as noted. E Indicated uncertainty determined from limiting values for k , and k t . f These values are the average (and mean deviation) of from seven to thirteen determinations within a kinetic experiment. g Temperature, 79.58". Temperature, 70.12'.
Because the conversion of phthalic acid to phthalic anhydride is slow, the calculated rather than the observed infinity titers were used to calculate the first-order titrimetric constants. The constants obtained in this way did not show any significant trends in reactions followed to 75-85% completion. With one exception the values for kt in Table I are average values of two or more independent kinetic experiments. A typical kinetic experiment is included in the Experimental section. I n order to establish that the solvolysis of I in aqueous acetone involves alkyl-oxygen cleavage, the rate of solvolysis of the saturated analog, 2pentyl acid phthalate, in 90% acetone at 100" was determined. The rate constant for this compound under these conditions is 7.4 f 0.4 X hr.-I or 1/45 of that for I. Since there is no apparent reason why an allylic ester should undergo acyloxygen cleavage more readily than the saturated analog, the observed factor of 145 appears to be compelling evidence that the solvolysis of I involves exclusive alkyl-oxygen cleavage. ils shown in the final column of Table I the polarimetric rate exceeds the titrimetric rate under all conditions where the two were compared. The polarimetric rate constants increase slightly as the initial concentration of acid phthalate is increased, and the titrimetric constants show a somewhat larger increase with increase in concentration. This increase in rate presumably is due to a slight increase in the ionizing power of the medium as the
3465
Ih' ACETONE
acid phthalate concentration is increased. Both k a and kt show marked increases as the ionizing power of the solvent is increased; cf. rates in 7070 and 90y0 aqueous acetone. The titrimetric rate is slightly more sensitive than the polarimetric rate to increase in ionizing power of the solvent. Thus kcr/kt decreases as the ionizing power increases. This behavior is similar to that observed prcviously for the solvolysis of cis-5-methyl-2-cyclohexenyl acid phthalate in aqueous acetone. 3a Excellent linear plots are obtained when log k a or log kt is plotted against the Y valuess of the solvents. The slopes of these plots, m,8 are 0.542 for kt and 0.454 for k,. These values are similar to those calculated* for the solvolysis of cis-5methyl-2-cyclohexeny1 acid phthalate in aqueous acetone3" (m = 0.472 for kt and 0.429 for k a ) . Thus both k , and kt show about the same sensitivity to changes in the ionizing power of the solvent in the two systems. The activation parameters for loss of optical activity ( k a ) , solvolysis ( k t ) and racemization or SNi' rearrangement (k, - k t ) in 90% acetone are given in Table 11. These data were determined from least-square plots of log k lis. 1/ T . TABLE I1 ACTIVATIONPARAMETERS FOR THE SOLVOLYSIS OF 0.05 ,If DIMETHYLALLY ALLYL AcId PHTHALATE IN "90%" AQUEOUS ACETONE'
ka kt
(krr
-
kt)
Ea b kcal./mole
log
A
kcal./mole
e.u.
29.1i0.2 2 8 . 3 2~ 0 . 6 29.8 i 0 . 8
12.974 12.077 13.156
28.4iO.2 27.6 zt 0 . 6 29.0 i 0 . 8
-1.6110.51 - 5 , 7 1 11 . 7 9 -0.77 k 2.0
4H*,b,C
AS*,b,C
These values w r e calculated from the rate constants a t 70, 80 and 100' given in Table I. b T h e indicated uncertainties were calculated from the limiting values of the rate constants a t 70 and 100". These values were calculated for 100'.
The present system is a little more reactive than the 5-methyl-2-cyclohexenyl system. In 90yoaqueous acetone a t looo, k a , kt and k a - k t are larger for I than for the isomeric 5-methyl-2-cyclohexenyl acid phthalates by factors of about 5 , 3 and 7, respectively.
Discussion The solvolysis of a,ydimethylallyl acid phthalate (I) is similar to that of the isomeric 5-methyl-2cyclohexenyl acid phthalates3" in that k , > kt in each case. It appears that the present data are consistent with the mechanism suggested prev i ~ u s l y for ~ , ~the solvolysis and rearrangement of allylic compounds. According to this picture, which is illustrated below for the present systemthe indicated rotations for the enantiomers of I have been chosen arbitrarily-an "internal ionpair'I5 intermediate (11) is common to solvolysis and intramolecular (SNi') rearrangement. The first-order rate constant for loss of optical activity is k,. That for formation of phthalic acid, i.e., the titrimetric rate constant (kt) is k , k2/(2kl k2), and that for the conversion of one enantiomer to the other is 112 (k, - k t ) .
+
(8) A H Fainberg and S Winstein, T n n
J O T T R & + I , 78,
2770 ( I S X )
HARLAN L. GOERING AND RICHARD W. GREINER
34GG H
I
CH~ c /CH3 )CH \C-OCOAr H \H (+)-I FI
ka
J_ ki
11
br
TABLE I11 TITRIMETRIC RATE OF SOLVOLYSIS OF 0.05 hr t r ~ n s - a , y - D ~ METHYLALLYL ACIDPHTHALATE IX 90% AQL-EOUS ACETONE AT 99.81' 1,
hr.
I
solvolysis products
Vol. 79
init. 0.00 0 . 75 1.50 3.00 4.50 6.00 7.50 9.00 10.50 12.10 13.50 15.00 011s. 02 Theor.
NaOH, ml.
10%k + . a
4.745 4.910 5.210 5.580 6.205 6.690 7.150 7.530 7.800 8.060 8.310 8.470 8.600 9,300 9.490
.. ..
hr. -
1
109 105 111 109 112 113 111 111 112 111 109
In the present system the intramolecular rearrangement results in the interconversion of enantiomers providing t h a t the planar allylic moiety in I1 has the indicated configuration. The steady polarimetric and titrimetric rates suggest that I reacts via the conformation corresponding to the .. indicated configuration for 11, i.e., the configuration .. with the a- and y-methyl groups cis. If I1 had the Av. 109 f 3 other configuration ( i e . , with the methyl groups Calculated from theoretical infinity titer. trans) the SNi' rearrangement would result in the TABLE IV conversion of optically active trans-acid phthalate to optically active cis-acid phthalate. This would POLARIMETRIC RATEOF SOLVOLYSIS OF 0.05 lu(+)-transIN 90% AQUEOUS likely disturb the polarimetric and possibly also the DIMETHYLALLYL ALLYL ACID PHTHALATE titrimetric rate. Scale models also indicate that the configuration indicated for the a,y-dimethylallylcarbonium ion in I1 is the more stable 0.000 1,955 .. I t has been shown recently that in the intramo0,500 + 1.72:3 303 lecular rearrangement of a-phenyl-~-methylallyllOa +1.3G3 1 ,500 2%; and a-phenylallyl p-nitrobenzoatelob the carbonyl + o , 955 302 3.083 oxygen atom in the reactant becomes the alkyl + i5l 29 1 4.500 oxygen atom in the product. This suggests that 6.000 ,603 285 there is some covalent bonding between the allylic + ,497 287 7.500 carbon atoms and the oxygen atoms in the symf ,308 .. metrical intermediate 11. However, from the way Solvent -+ ,305 in which k , responds to changes in the ionizing Xv. 2 9 4 k 6 power of the solvent, it appears t h a t there must be considerable charge separation in the intermediate. found 233.9, calcd. 234.2. The acid phthalate was re-
+ +
OD
solved according to the method of Kenyon, et uZ.12 The ( +)-trans-a,y-dimethylallyl acid phthalate obtained in this way had a melting point of 78-83.6" and [ a ] " D 35.4' ( I = Materials.-trans-a,y-Dimethylallyl alcohol, b.p. 1164 , c 1.1, ethanol) (1it.l2 m.p. 82-84", [ a ]38.3" ~ for optically 121°, x Z o1.4280 ~ (lit." b.p. 116-121°, n Z o 1.4280) ~ was pure material). prepared according to the previously described method.1iz12 Reduction of a,*pDimethylallyl Acid Phthalate with The alcohol was converted12 to dZ-frans-a,y-dimethylallyl Lithium Aluminum Hydride.-The ester was reduced acacid phthalate, m.p. 89-90.5" (lit.I290-90.5'); neut. equiv. cording to the method of Doering and 2 e i ~ s . I ~ a,y-zimethylallyl alcohol, b.p. 116-121°, was isolated in 66/0 yield. (3) T h e stereochemical relationship between t h e reactant and The infrared spectrum of this material was found to have product for a n S N i ' process in t h e Ivans-cr,~-dimethylallylsystem has a strong band a t 10.4 p. been discussed by W. G. Young, et a l . , THISJOURNAL, 7 7 , 4 1 8 2 ( 1 9 5 5 ) . Kinetic Experiments.-The kinetic runs were carried out ( 1 0 ) (a) TV. E. Doering, private communication: (b) E. A. Braude as described p r e v i o ~ s l y . ~Typical ~ kinetic experimeiits are and D. TV. Turner, Chemistry &lndzrsluy, 1 2 2 3 (1355). summarized in Tables 111 and JV. ( 1 1 ) E. R. Alexander and R , W. Kluiber, THISJOURNAL, 73, 4304 (1951). MADISON,WISCONSIN
Experimental
(12) H. W. J. Hill, J. K e n y o n and H . Phillips, J . Chem. Soc., 576 (1%3li).
(13) W.E. Doering and H. H. Zeiss, THISJ O U R N A L , 72, 117 (1950)
