Pasto, Gadberry
/
Additions of Acetic Acid to ( E ) - and (2)-2-Butene
1469
Stereoselectivity and Mechanisms of Acid-Catalyzed Additions of Acetic Acid to ( E ) -and (2)-2-Butene in Acetic Acid' Daniel J. Pasto* and James F. Gadberry Contribution from the Department of Chemistry, University of Notre Dame, Notre Dame, Indiana 46556. Receiced June 9, I977
Abstract: The stereoselectivity and mechanisms of acid-catalyzed additions of acetic acid to ( E ) - and (Z)-2-butene in acetic
acid have been studied. With the weak acid catalysts deuterium chloride, deuterium bromide, and methanesulfonic acid-0-d in acetic acid-0-d, the additions proceeded 84 f 2% anti with no alkene diastereomerization or positional isomerization, or hydrogen-deuterium exchange with solvent. These reactions are proposed to proceed via concerted AdE3 processes. I n contrast. with the very strong acid trifluoromethanesulfonic acid-0-d in acetic acid-0-d. lower stereoselectivity, alkene diastereomerization and positional isomerization, and hydrogen-deuterium exchange are observed. This reaction is proposed to proceed via an AdE2 pathway involving reversible formation of a tight-ion pair, or free carbenium ion.
Introduction Previous investigations in our laboratories have been devoted to the study of the mechanism of the addition of hydrogen bromide, and the hydrogen bromide catalyzed addition of acetic acid, to the ( E ) -and (2)-2-butenes and -3-hexenes, and cyclopentene.* Kinetic studies showed that alkyl bromide and alkyl acetate were formed according to the rate expressions
-d[RBrl - k[HBr]*[alkene] dt
d'RoAcl = k [ HBr] [alkene] dt respectively. The lack of an acidity function correlation with rate of reaction3 and less than unity H-D ( k ~ / k oisotope ) effects dictated that proton, or deuteron, is delivered by undissociated HBr, or DBr. The lack of H-D exchange in unreacted alkene, alkene isomerization (both positional and diastereomerization), and hydride shift rearrangement products i n the 3-hexene system militated against formation of a tight-ion pair or free carbenium ion. The stereochemistry of the addition of both DBr and DOAc to both ( E ) - and ( 2 ) 2-butene was found to be 84 f 2% anti, the stereoselectivity remaining constant over a 100-fold concentration range of DBr. AdE3 mechanisms were considered to be the most consistent with all of the experimental data, the products being formed via the syn and anti transition states 1 and 2.
to ( E ) - and (2)-2-butene, the results of which are reported herein.
Results The deuterium chloride, deuterium bromide, methanesulfonic acid-0-d, and trifluoromethanesulfonic acid-0-d catalyzed additions of acetic acid-0-d to ( E ) - and (2)-2-butene were studied. Owing to the essentially identical results obtained with the first three acids, the results of those studies will be discussed together, with the trifluoromethanesulfonic acid catalyzed reaction being discussed separately. DCI, DBr, and CHJSO~DCatalyzed Additions. Solutions of the acid in acetic acid-0-d were prepared (see Experimental Sections) and the 2-butene added. The reactions were allowed to proceed to -50% completion at 25 "C and the unreacted butene was pumped from the reaction mixture and analyzed by GLC. N o diastereomerization or positional isomerization was detected. Mass spectral analysis of the butenes showed that no deuterium incorporation had occurred. The acetate fractions were isolated by extraction techniques and were purified by G L C or distillation. The acetates were converted to the corresponding alcohols by reduction with lithium aluminum hydride, and the diastereomeric composition of the alcohol fractions was determined by N M R techniques using a chemical shift reagent. The results of the stereochemical analyses are given i n Table I . The acetate fractions were also analyzed by mass spectrometry and compared with authentic 3-deuterio-2-butanol prepared by the deuterioboration of 2-butene followed by oxidation and acetylation. The relative intensities of the M+-, (M I)+., and ( M 2)+. peaks in the acetates formed in the acid-catalyzed addition reactions were within experimental error with authentic 3-deuterio-2-butyl acetate indicating the incorporation of only a single deuterium atom. The diastereomeric stability of the 3-deuterio-2-butyl acetate in the presence of methanesulfonic acid in acetic acid was determined by monitoring the rotation of I-2-octyl acetate in acetic acid in the presence of methanesulfonic acid; no change in rotation was observed over a time period equivalent to that required for the addition reaction. (The optical stability of 2-octyl acetate and bromide in the presence of hydrogen bromide was previously established.)' 2-Butyl methanesulfonate was shown not to be formed during the addition reaction by monitoring reacting solutions by N M R . 2-Butyl methanesulfonate was prepared and subjected to acetolysis under the conditions of the addition reactions. The major products formed were the butenes (by
+
2
1
Xu = Br or OXc
The identical stereoselectivities of addition of DBr and DOAc to the two 2-butenes was surprising. Limited stereochemical data indicated that the addition of DBr and DOAc to the 3-hexenes was essentially identical with that of the 2butenes (84-85% anti); yet the additions to cyclopentene were found to be 96 f 4% anti. Although there is some effect of alkene structure on stereoselectivity of addition, it was felt that initial complexing of the alkene with one molecule of hydrogen bromide resulted in polarization of the H system which dictated the stereoselectivity of the addition in the rate-determining transition state. Accordingly, we initiated a study of the stereoselectivity of various acid-catalyzed additions of acetic acid 0002-7863/78/l500-1469$01.00/0
+
0 1978 American Chemical Society
Journal of the American Chemical Society
1470
r
0“5
0
0
Table 111. Rate Constants for Trifluoromethanesulfonic Acid Catalyzed Addition and Isomerization Starting alkene
100
200 300 time ( m i n u t e r )
400
500
Figure 1. Triflic acid catalyzed addition of acetic acid to (E)-2-butene.
Table 1. Stereoselectivities of Acid-Catalyzed Additions of Acetic Acid-0-d to ( E ) - and (Z)-2-Butene Acid DCI DCI DBr CH3S03D
1 100:5 1 March I , 1978
catalyst (concn)
2Butene
(0.87 M ) (0.87 M) (0.55 ‘M) (0.39 M) (0.63 M) (0.39 M ) (0.39 M)
(E) (2) (E) (E)
(E) (Z) (Z)
% anti
(concn)
addn
(1.5 M) (1.5 M) (-1.0 M ) ” (1.67 M ) (1.67 M) (1.67 M) (1.67 M)
84 83 84h
84 84 83 81
a Approximate concentration of a triply freeze-degassed solution of (€)-2-butene in acetic acid-0-d. Previous result 84.0 f 2.0% (see ref 1 ).
N M R ) . The rate of acetolysis is slower than the rate of acidcatalyzed addition of acetic acid and appeared to be autocatalytic, the initial rate of acetolysis being greater in the presence of methanesulfonic acid, but yet slower than acidcatalyzed addition. Trifluoromethanesulfonic Acid Catalyzed Addition. Recovery of unreacted butene from addition reactions catalyzed by trifluoromethanesulfonic acid-0-d in acetic acid-0-d followed by GLC analysis indicated that both diastereomerization and positional isomerization had occurred, the extent of which increased with reaction time (see Table 11). Separation of the recovered butene fractions by GLC followed by mass spectrometric analysis showed that H-D exchange had occurred (runs 2 and 4 in Table 11). The stereoselectivity of the addition reaction decreased with increasing time of reaction (see Table 11). Pseudo-first-order rate constants were measured for the acid-catalyzed addition reactions (see Figure I and Table 111). The H-D isotope effect for the catalyzed addition to ( 2 ) - 2 butene was found to be 1 .8.4 “Apparent rate constant^"^ for isomerization of (Z)-2-butene to the E isomer were also determined. The observed isotope effect of 2.0 is within experimental error with that for the addition. Attempts to prepare 2-butyl trifluoromethanesulfonate at l o w temperature resulted in the apparent formation of the
k H , M-’ S - ’ x 105
ko, M-ls-‘
x
105
(E) (Z)
Addition 2.98 2.15
1.53
(2)
lsomerizationh 3.87
1.93
U
a Not measured. The “rate constant” for isomerization of ( E ) to (Z)-2-butene could not be measured with reasonable accuracy owing to the small amount of isomerized alkene formed and the experimental uncertainty in the analysis.
desired ester. However, warming to -20 to 0 “C resulted in the decomposition of the sulfonate and formation of a mixture of butenes. The butene mixture was isolated and analyzed by GLC showing the presence of 69% ( E ) - and 29% (Z)-2-butene and 2% I-butene. In a control experiment, the optical rotation of a solution of l-2-octyl acetate in acetic acid in the presence of trifluoromethanesulfonic acid remained constant over a period of time longer than that required for the addition reaction to take place.
Discussion The lack of alkene diastereomerization, positional isomerization, and hydrogen-deuterium exchange in unreacted alkene in the addition reactions catalyzed by HCI, HBr, and CH3S03H is fully consistent with the operation of concerted AdE3-type processes as proposed earlier,’ and is inconsistent with the reversible formation of intermediate tight-ion pairs or free carbenium ions. Contrary to expectation, the stereoselectivity of the catalyzed addition reactions is not a function of relative acidity. These acids are relatively weak acids in acetic acid,6 decreasing in the order CH3S03H > H B r > HCI spanning an acidity range of 1 02- 1 03. The only effect of the relative acidities of the acids on the addition reactions is on the rate of reaction, the rates decreasing with decreasing acidity. The stereoselectivity of these addition reactions must therefore result from effects of alkene structure and/or solvent properties. Future efforts will be devoted to studies i n these areas. In contrast to the results derived with the weak acids described above, catalysis by the very strong acid trifluoromethanesulfonic a ~ i dresults ~ . ~ in alkene diastereomerization, positional isomerization, and hydrogen-deuterium exchange, along with a much lower degree of stereoselectivity of addition. These results are consistent with the reversible formation of a tight-ion pair or free carbenium ion (see Scheme 1). The slightly greater extent of anti addition can be attributed to steric shielding of the syn side of the carbenium ion in the tight ion pair. The changeover in mechanism must be due to the greater acidity of trifluoromethanesulfonic acid relative to the weaker acids discussed above.8
Table 11. Trifluoromethanesulfonic Acid Catalyzed Addition of Acetic Acid-0-d to ( E ) - and (Z)-2-Butene % recovered
butene
2-
1-
Run
Butene
Butenea
(€)-2-Butene
(Z)-2-Butene
O/O anti addn
I 2 3 4
(Z) (Z)
0.5 0.5 0.5 0.5
12.4 21.5 (25%dl) 95.9 90. I
87.1 78.0 3.6 9.4 (20% d l )
60 57 12 71
(E)
(E)
Deuterium content not analyzed for. Corrected for do and d2 content.
Pasto, Gadberry
/ Additions of Acetic Acid to ( E ) - and (Z)-2-Butene
1471
Table IV. Mass Spectral Relative Intensities of 3-Deuterio-2-butyl Acetates and 2-Butyl Acetate Alkene precursor
100
(E) (Z) Standard
101
Re1 intensities at m/e 102
103
I04
100.0 100.0 6. I
6.1 6.0 I .o
1 .o 1 .o
7.0 7.2 100.0
1 .o
Table V. Mass Spectral Relative Intensities of 3-Deuterio-2-butyl Acetates Derived from Methanesulfonic Acid Catalyzed Additions Re1 intensities at m/e 2-Butene
[CHW3HI, M
(Z) (Z) (E) (E)
0.39 0.39 0.39 0.63 2-Butyl acetate
100
101
102
103
104
100.0 100.0 100.0 100.0 7 .O
7.0 7.3 6.8 7.6
0.4 0.8
1.1
5.3 2.8 4.9 2.7 100.0
Table VIII. Optical Rotation of I-2-0ctyI Acetate in Acetic Acid in the Presence of Trifluoromethanesulfonic Acid
Table VI. Optical Rotation of I-2-0ctyI Acetate in Acetic Acid in the Presence of Methanesulfonic Acid Time
Rotation, deg
Time
Rotation, deg
0 8 h 59 min 22 h 23 rnin
-0.537 -0.534 -0.542
93 h 44 rnin 31 1 h 56 min 800 h
-0.532 -0.536 -0.540
\
c=c
H/
/cH
CH CHDCH=CH.
P
\H
CH
\
c=c
H/
4 1)
4-
\
D/
0 47 h 46 rnin 101 h
-0.670 -0.664 -0.672
/CH
\CH
0
Jr
_+
H'
0 0.006 0.024 0.044
66 I10
CH CHDCHCH
I OCOCH
[ 2-Butyl acetate], M
11
CH
C=C
Rotation. deg
Time, min
/H
(I
CH
Time
Table IX. Kinetic Data for the Trifluoromethanesulfonic Acid Catalyzed Addition of Acetic Acid to (Z)-2-ButeneU
Scheme I CH
1.3
1.1
\
c=c /"
D/ A
erythro and threo
= CF
\CH
SO
[2-Butyl acetate], M
244 586
0.093 0.205 0.333 0.374
1018
1249
[CF$O3H] = 0.276 M , [butene10 = 0.8514 M.
Table X. Kinetic Data for the Trifluoromethanesulfonic Acid-U-d Catalyzed Addition of Acetic Acid-0-d to (Z)-2-Butene" Time, rnin
[2-B~tyl acetate], M
Time, min
[2-ButyI acetate], M
0 3 29 122
0 0.0165 0.0169 0.0423
294 565 72 1
0.068 1 0.1 102 0.1 352
Experimental Section Reagents. (E)-2-Butene ( C P grade 99.0% min, found 99.0% ( E ) ,
