7690
Skeletal Rearrangement of 3C Labeled Methylpentenes and Methylhexenes on Supported p-Toluenesulfonic Acid Franqois Fajula and Franqois G. Gault* Contributionfrom the Laboratoire de Catalyse, Universit; Louis Pasteur de Strasbourg, Strasbourg, France. Received March 29, I976
Abstract: The skeletal rearrangements of 2-methylpentenes, 3-methylpentenes, 2-methylhexenes (exclusively alkyl shifts), and the double bond migrations which occur simultaneously have been studied at 74 OC over p-toluenesulfonic acid supported on silica. The rates are of the same order of magnitude for both types of reactions, except for the interconversion between 2-methylpent-2-ene and 2-methylpent- I-ene which is extremely fast. T h e use of I3C labeled hydrocarbons demonstrates the occurence, besides the 1,2-alkyl shift, of direct 1,3-methyl shift and of even more intricate skeletal rearrangements. For example, 2-methylpentenes-3-I3C and 2-methylpentenes-5-I3C are formed in equal amounts from 2-methylpentenes-I - I 3C. Side-protonated, corner-protonated, and 1,3-C-methonium ion a r e required as intermediates to account for these products. Moreover 2-methylhexenes-2- 13C yields, besides the 3-methylhexenes-2- I3C (60%) expected by 1,2-alkyl shift, 3-methylhexenes-5- I3C (20%) and 3-methylhexenes-3- I3C(20%). The formation of these “abnormal” products necessitates, besides the classical carbenium ions, the occurrence of 1,4-C-methonium ion and of side- or corner-protonated cyclopentanes. The location of the carbon-I 3 in each individual isomeric olefins has been determined in the isomerization of 2-methylpent-I-ene-2- I3C and 2-methylpent-2-ene-2- I3C.Appreciable amounts of 2-methylpent-2-ene-4- I3Cand 4-methylpent-2-ene-2- I3C were found in the approximate ratio 20 to 1, while 2-methylpent-1-ene-4- I3Ccould not be detected. These results, together with the initial product distributions obtained in the isomerization of 4-methylpent-2-ene and 2-methylpentenes-2- I3C show the following sequence for the rates of the various elementary steps: desorption of a tertiary carbenium >> desorption of a secondary carbenium ion to form a trisubstituted olefin > alkyl shifts > tertiary-secondary hydride shift > secondary-secondary hydride shift >> desorption of a secondary carbenium ion to form a disubstituted olefin. It is suggested that this latter process, or the reverse (protonation of a disubstituted olefin), or a secondary-tertiary hydride shift is the rate-determining step in the isomerization of methylpentenes on supported p-toluenesulfonic acid. Some of the striking differences observed between the reactions on supported PTS acid and the ones occurring in acidic solution may be explained by the particularities of the heterogeneous system: A strong bond between the organic cation and the rigid counteranion could account for the very slow desorption of the disubstituted olefins, while the 1,3-alkyl shift and other “abnormal” skeletal rearrangements could be explained by 1,3- and 1.4-hydride shifts involving a concerted participation of several adsorbed p-toluenesulfonic acid molecules.
Introduction The nature of the active sites, of Bronsted or Lewis types, on a solid acid catalyst such as silica-alumina, zeolites and so on, is still a matter of controversy. It is therefore interesting to use p-toluenesulfonic (PTS) acid supported on silica to demonstrate the typical behavior of a pure Bronsted acid catalyst. Recently a careful study of the isomerization of butenes on supported PTS acid showed that the intramolecular hydride shift in a carbonium ion on the solid catalyst was much slower than that in the liquid phase.’ This very important result, if it can be extended to other olefins, makes possible and attractive a study of the mechanisms of the skeletal rearrangements which occur when olefins are contacted with heterogeneous Bronsted acid catalysts. I n the present work, the isomerization of methylpentenes and methylhexenes labeled with carbon-13 in various positions has been investigated. The advantage of using a I3C tracer technique is apparent from previous works in the field of metal cataIysis.’.3 Experimental Section Materials. Alcohols. The synthesis of selectively l3C labeled methylpentanols has been described.2”The same procedure, i.e., the reaction of ketone, aldehyde, or ester with alkylmagnesium halide, has been used to prepare the various I3C labeled 2-methyl- and 3methylhexanols with a reasonable yield (65-70%) (Table I).4 The isotopic purity of the starting materials, methyl (I3C)iodide, ethylI - I3C and -2- I3C iodide, acetone-2- I3C. and ethyl acetate-I-I3C, obtained from Merck Sharp & Dohme (Canada), was in some cases around 60% and in some cases around 90%. Hydrocarbons. All the possible I3C labeled 3-methylhexanes and the 2-methylhexane-2- 13Cused for the catalytic experiments and the mass spectrometrical calibrations were prepared as already described2”.)from the appropriate labeled alcohols by catalytic dehydration and hydrogenation on highly selective catalysts in a pulse flow system.
Journal of the American ChemicalSociety
The labeled olefins were obtained similarly by catalytic dehydration of the labeled alcohols and gas chromatographic separation of the positional isomers. The unlabeled hydrocarbons were Fluka puriss grade and used without further purification. Catalyst. p-Toluenesulfonic acid supported on silica was prepared according to Sakurai et al:’ 3 g of silica were impregnated with a solution of 3 g of monohydrated paratoluene sulfonic acid (Fluka puriss grade) in 80 ml of anhydrous ether. The ether was then evaporated and the catalyst outgassed overnight under Torr at 40 O C . The silica used was pure Davisson grade (surface area: 700 m2/g; porous volume: 40 ml/g; particle size: 60-200 mesh). Apparatus and Procedure. The catalytic reactions were carried out in an all-glass, grease-free flow system. A small amount of reactant (ca. 4 mg) was injected a t a constant pressure (6 Torr for hexenes, 1 Torr for heptenes), in a flow of purified hydrogen (1 atm, 10 ml/min) in the reactor where the catalytic bed (500 mg). isothermal and isobaric, operated under differential conditions. The details of the apparatus as well as the device to measure the hydrocarbon flow rate and the procedure to collect samples for GLC and mass spectrometrical analysis a r e described in ref 3. CLC Analysis. A 5 m X in, column of DC-ZOO/firebrick operating at 45 “C was used for the analysis of the various olefins and a 5 m X ‘14 in. column of SE-30/firebrick for the preparative separation at 24 “ C . Mass Spectrometrical Analysis. a. Procedures. Two procedures were used to obtain the mass spectra of the various reaction products. In the first one, the various olefins were separated by gas chromatography, each molecule hydrogenated on 10% Pt-AI203 at 120 O C , and the mass spectrum of the resulting paraffin was recorded after introduction into the ionization chamber through the direct inlet system of a CH7 mass spectrometer. When using this procedure, the accuracy of the location of the labeling carbon-13 was f0.5%.In some control experiments, hydrogenations were effected on platinum black at 50 OC in order to check that no I3C scrambling due to some rearrangement on the support was occurring during the hydrogenation on Pt-AI203. In most experiments, the mixture of olefins was hydrogenated, and the resulting paraffins were analyzed by using a mass spectrometer-
/ 98:24 / November 24, 1976
769 1 Table I. Synthesis of Labeled C, Alcohols Carbon skeleton
RC(=O)R'
R"MgX
RR'R"C0H
/Lv
Acetone-2%
Br Mg-n-B u
2-Methyl-2-hexanol-Z-' 'C
7
2-Methylpentanal
IMg"CH,
3-Methyl-2-hexanol-l r 3 C
2-Methyl-2-pentanone
IMgl3CH2CH,
3-Methyl-3 -hexanol-2-' 3C
Ethyl acetate-l-''C
3-Methyl-3-hexano1-3-' 'Ca
7 1""
^("
3-Hexanone
BrMgEt BrMgF'r IMgl'CH,
/\rv
2-Methylbutanal
IMg"CH,CH,
4-Methyl-3-hexan01-2-~'C
7-
3-Methylbutanal
IMgCH 21 'CH,
4-Methyl-3-hexanol-l -l ' C
3-Methyl(' 'C)-3-hexanol
a Together with 3-methyl-3-pentan01-3-'~C and 4-methyl-4-heptan01-4-~~C. The separation of the resulting labeled saturated hydrocarbons obtained by dehydration and hydrogenation of the mixture of alcohols is easily effected by gas chromatography.
Table 11. Values of a,, for the Various Labeled 3-Methylhexanes
0.096 0.9595 0.9005
'6
a5 a4
0.997 0.1105 0.921
0.946 0.1105 0.947
1.000 0.95 0.100
1.000 0.933 0.929
0.96 0.95 0.100
1.000 0.99 1 0.105
Table 111. Isomerization of 2-Methylpentenes Product Distributions
A, Run
I
A.-.
Reaction hydrocarbon
An
L /LA
i
r-i
1
I
I=
A//\ P'
%
%
0.18 0.16
10 4.3
6.4 1.8
1.56 2.38
0.17
4.1
1.2
3.4,
0.17 0.18 0.20 0.17
3.6 3.8 4.3 11.8
1.6 1.1 2 9
2.25 3.45 2.15 1.31
0.20
75.7
4.2
gas chromatograph coupling device. The experimental conditions of the chromatography ( 5 m X '/g in. 5% DC-200/firebrick; helium flow rate: 8 ml/min; entrance pressure: 2 atm, - 10 "C) and the scanning speed (1.2 s/decade) used to record the mass spectra were chosen so that (a) the accuracy of the measurement on the mass spectra was only limited by the determination of the height of the peaks; (b) the change of the total ion current was smaller than 0.5% for a group of ten masses, even on the steepest part of the chromatograph elution peak.3b In spite of these precautions, the absolute uncertainty in determining the location of the labeling atom was higher than with the preceding procedure ( 1 to 2%), presumably on account of a change in the source pressure and temperature when the hydrocarbons were eluted from the chromatography column. b. Calculations. From the recorded mass spectra, after the usual corrections for background, naturally occuring isotopes, and fragmentation, the percentage of each isotopic species in a mixture of hydrocarbons labeled in the various positions was obtained: (a) by calculating the mole fractions a, of the ions C,Hzn+ I which have retained the carbon- 13 during their formation from an isotopically pure labeled hydrocarbon and (b) comparing these values (a,) with the ones obtained for the various isotopic species (a,,,): +
Zc,a,, = a,
zc, = 1 c, and a,, a r e the concentrations and the mole fractions of the heavy I + ions in the ith isotopic species,respectively. The location of the carbon-I3 in the labeled 2-methyl- and 3methylpentanes has already been d i s c u ~ s e d .In ~ .a~ mixture of labeled
C,H2,,+
Fajula, Gault
3-methylhexanes, the location of the carbon-1 3 is determined by using the ChH13+, CsH, I+. and C4Hg+ ions ( 0 6 , a5. a4. respectively). The a,, values of the reference hydrocarbons are reported in Table When a value of 1 is found for ab. species I-X, 1-2,and I-W are absent from the reaction mixture, and as. a4 allow one to calculate accurately the percentage of I-Y, I - T , and of the sum I-U I-V.
+
Results Isomerization of 2-Methylpentenes. The isomerization (double bond migration and skeletal rearrangement) of 2methylpent-1-ene (2MP-1), 2-methylpent-2-ene (2MP-2), and 4-methylpent-2-ene (4MP-2) in the presence of supported p-toluenesulfonic acid has been investigated a t 74 OC.The distributions of the products, which include 2-methyl- and 3-methylpentenes exclusively, are given in Table 111. The equilibrium between 2-methylpent- 1-ene and 2-methylpent-2-ene and between 3-methylpent-2-ene (3MP-2) and 2-ethylbut- I-ene (2EB- 1) are achieved in any experiment whatever the reacting olefin might be. Values of 0.18 f 0.02 and 25 f 2 were found for the equilibrium ratios 2MP-1/ 2MP-2 and 3MP-2/2EB-1, respectively. On the other hand, the amounts of 4-methylpent-2-ene and 3-methylpentenes obtained from 2MP-1 and 2MP-2 are of the same order of magnitude. The ratio p = 4MP-2/3MP-2 does not depend upon the nature of the reacting olefin but increases when the conversion is decreased. The extrapolation of p a t zero conversion gives a value of 5 f 1 (Figure 1).
1 Rearrangement
of 3CLabeled Methylpentenes and Methylhexenes
7692 Table IV. Isomerization of Labeled 2-Methvloentenes Distribution of the Various Species ~
2-Methylpentenes
L L L Run
T T”r - 7 [Ll= 7 1 I J= To or
Total %
Reacting hydrocarbon 7.5
0
2.5
89
11
0
3
4
0
3
88
12
0
1.3
5.5
0
2
88
12
0
2.7
3
3.5
0
1.2
85
15
0
3
5
2
0
1.1
89
11
0
1.9
93.3
0
11
88
12
0
0.6
92.5
6
46.5
12
41.5
1.2
92.5 10
+ (25%)
11
12
&
6.7
13
3.5 i 0.8
4
i
0.8
~~~~
P’
or
or
~
3-Methylpentenes
(75%)
UMost probably in run 13.
In order to check the 1,3-methyl shift, the isomerization of 2-methylpent-2-ene-4- I3C and of a mixture 3/1 of 2-methylpent- 1-ene and 2-methylpent-2-ene-l - I3C were effected under the same conditions. 2MP-4- I3C yields 2MP-2- I3C besides 3-methylpentenes-2- I3Cand -3- I3C. Unexpectedly, the 2-methylpentenes labeled a t carbon 1 yields in approximately equal amounts 2-methylpentene-5- I3C and 2-methylpentene-3- I3C (or -2- I3C since both hydrocarbons are not distinguishable by mass spectrometry). Among the 3-methylpentenes, besides 3-methylpentene-1- I3C, methyl(I3C)-3pentenes are also obtained in appreciable amount. In some experiments performed with 2-methylpent- 1-ene or pent-2-ene-2- I3C,the various olefins were separated by gas chromatography, and the location of the carbon-13 was determined in each molecule by mass spectrometry. Three such experiments are reported in Table V. In the two experiments at low conversion, the 4-methylpent-2-ene and the 2-methylpent-2-ene both contained appreciable amounts of species labeled on carbon 2 and 4, respectively, i.e., resulting from a 1,3-methyl shift while the 2-methylpentene-J - I3Cretains all the label on carbon 2. In run 14, where the conversion to 3methylpentenes is much higher, an appreciable amount of 2-methylpent-1-ene-4- I3C is also formed, which results presumably from the consecutive isomerization of 2-methylpent-2-ene-4- I 3C:
j 10
E
