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(E- Z)-ISOMERIZATION OF UNSATURATED ESTERS X--CH= CH-COOCH 3
Vaclav VSETECKA, Jaroslav PECKA and Milos PROCHAZKA Department of Organic Chemistry, Charles University, 12840 Prague 2
277
Received November 6th, 1980
Total energy differences for a series of (E) and (Z) isomers of unsaturated esters X- CH=CH- COOCH3 have been calculated by the CNDO /2 method. The applicability of this method to the prediction of relative stabilities of (E) and (Z) isomers and their optimal conformations was checked by comparison with the experimental data .
The (E - Z) isomerization equilibria for many methyl propenoate derivatives (X-CH= CH-COOCH3) were investigated by us l (X = CI, Br, I, CN, OCH3 ,
SCH 3 , NO z), as well as other authors (X = CH 3 , ref. Z-
4, X = CzHs, ref.4 ; X =
= C6 Hs, ref. s; X = OCH3 , ref. 4; X = COOCH3, ref. 6
•7). We have now attempted
to compare the experimentally determined enthalpy differences with those calculated by the CNDO/2 method and to consider the possible use of this method in predicting the relative stabilities of the (E) and (Z) isomers and their optimal conformations. We performed quantum-chemical calculations using the MO-LCAO-CNDO/2 methodS in the Santry-Segal parametrization9 for compounds X-CH= CH-COOCH3 where X was CI, CN, OCH 3 , SCH3 , CH 3 and COOCH3. Geometric parameters, used in these calculations, were taken from the experimentally determined values for analogous compounds. According to preliminary calculations, in the optimum conformation of (E) as well as (Z)-isomers of methyl 3-chloropropenoate the C= C- C= O system is planar and the O = C-O- C grouping adopts an sp arrangement (Scheme 1, Table I) . In both the (E) and (Z) isomers the sp conformation of the C= C- C= O grouping is slightly preferred (about 3 kJ mol - I), any conformation of the (E) isomer being more stable than that of the (Z)-isomer by about 8 kJ mol - 1. Calculation, using the spd-basis, does not agree with the experimental datal because it predicts greater stability of the (Z)-isomer.
Planar arrangement of the C= C- C= O system was also assumed in the calculation of total energy of methyl 3-cyanopropenoate. We studied also the dependence of f1E on rotation of the CH30 group (Table II and III). According to the calculation, the sp conformation of the CH30 group relative to the C= O bond is preferred. The energy difference between the optimal conformations, f1E(Z - E), was calculated to be 11·2 kJ mol-I. The experimental values are f1G473 = -8,3 kJ mol - l
Collection Czechoslovak Chem. Commun. [Vol. 47] [1982]
278 Vsetecka, Pecka, Prochazka:
and 11H = -4·0 kJ mol-l, the (Z)-isomer predominating1• As calculated for the
sp and ap conformations of the C=C-C= O system in the (Z)- and (E)-isomers, in both compounds the sp forms predominate (11E = 4·0 kJ mol- 1 and 5·2 kJ mol- 1
,
TABLE I
Total energy calculation (in kJ mol- 1) for methyl 3-chloropropenoate
Parameter
spa apa
tJ.E(sp-ap)
spa apa
tJ.E(sp-ap)
a Conformation C=C-C=O.
TABLE II
(Z)-Isomer 1a
CNDO/2
- 12848·9 - 12846·0
-2·9
(E)-Isomer 1b
-12856·9 -12853·8
-3·1
CNDO/2 with d-orbitals
-13196·5 -13 168·2 -13191·6 -13 165·0
-4·9 -3·2
tJ.E(E- Z)
- 8·0 - 7·8
28·3 26·6
Total energy calculation (in kJ mol- 1) for methyl 3-cyanopropenoate
Rotation of -OCH3
(Z)-Isomer (E)-Isomer
(Z)-Isomer (E)-Isomer
sp conformation of C=C- C=O
-16248·8 -16226·4 -16237-6 -16215·3
ap conformation of C=C-C= O
- 16244·8 -16238·9 -16113·0 -16232·4 -16226·8 -16191·8
Collection Czechoslovak Chern. Commun. [Vol. 47] [1982]
(E- Z)-Isomerization
2.9 kJ mol- 1
3.1 kJ mol- I
18.0 kJ mol-I
10.7 kJ mol - 1
5.2 U mol - I
19 .6 kJ mol- 1
6.4 kJ mol- I
Collection Czechoslovak Chern. Cornrnun. [Vol. 47] [19821
"'c "",,' / ' o "-H C=O
"C=C/
"- / " C H 1 \
279
(fa)
(1b)
(2a)
(2b)
(3a)
(3b)
(4)
280 Vsetecka, Pecka, Prochazka:
respectively). Although the barrier of rotation in this system is not known, the difference between standard heats of hydrogenation of (E)-2-butene and (E)-2-butenal shows that the conjugation energy is at most 10 kJ mol-I. We can therefore assume a practically free rotation in the system C=C-C= O at 293 K, with a slight excess of the sp conformer. In both the conformations, the (Z)-isomer is calculated to be more stable mainly due to the attractive interaction C=O···C=N.
TABLE III
One-center and two-center contributions to the total energy in methyl 3-cyanopropenoate (in kJ mol-I)
TABLE IV
Energy Monocentric Bicentric Total
sp conformation of C= C- C= O
(Z)-Isomer (E)-Isomer llE(Z-=- E)
55\0·0 5 509· 5
0·5
-21758 ,8 -21 747·1
- 11,7
-16248,8 -16237,6
-1 1,2
ap conformation of C=C-C=O
(Z)-Isomer (E)-Isomer llE(Z- E)
5508·1 5510'5 -2,4
-21 752'9 - 16244,8 - 21742·9 - 16232'4
-10·0 - 12-4
One-center and two-center contributions to the total energy of methyl 2-butenoate (in kJ mol-I)
Energy Monocentric Bicentric Total
(Z)-Isomera 5131 ·9 - 21569·3 - 16437,4 (E)-Isomera 5143·2 - 21575 '4 - 16432'2 llE(Z-E) -11,3 6· 1 -5,2
(Z)-Isomerb 5125·6 - 21542-4 - 16416,8 (E)-Isomerb 5141·1 - 21577·5 - 16436-4 llE(Z- E) -15' 5 35·1 1%
a Conformation 3a; b conformation 3b.
Collection Czechoslovak Chern . Cornrnun. [Vol. 47] [1982]
(E- Z)-Isomerization 281
Optimal conformations obtained for the isomeric methyl 2-butenoates are given in Scheme 2 (Table IV). A 30° deviation of the CH30-groUP from the O- C=O plane increases the energy of the system mainly by suppressing the C(9f' ·0(7) bonding interactions and reducing the C(6)- 0(8) bond strength. The CH3COO- group adopts therefore the planar sp arrangement (Scheme 3). In analogous arrangements differing in rotation of the CH3COO- group by 180°, the calculation prefers for the sp conformation of the HCH2- C= grouping an sp arrangement of the C= C--C - C= O system (7·2 kJ mol -1 for the (Z)-isomer and 5·6 kJ mol - 1 for the (E)-isomer). For the ap conformation of HCH2-C= , the sp arrangement of C=C- C= O is again preferred (11 kJ mol- 1 in the (Z)-isomer and 5·5 kJ mol - 1 in the (E)-isomer). The ap conformation of the segment HCH2-C= is preferred by 24-4 kJ mol- 1
or 20·6 kJ mol - 1 for the (Z)-isomers, however, for the (E)-isomers the sp conformation of the HCH2- C= segment is by 4·2 or 4·3 kJ mol - 1 more stable, depending on the C=C-C= O conformation. If we compare conformations of lowest energy
~-Ca ~ -Z.r-t a '-1\ 1\
-1 7330
r E -16944
- 17360
z
E
-17410 '-----,0..,..-L--=6'=-0 ."..· -'-----''---'---'1'''8"'''0.:-'
FIG. 1
Dependence of total energy of methyl 3-methoxypropenoate on rotation of the CH3 0 group (synperiplanar conformation of C=C -C=O)
Collection Czechoslovak Chern . Cornrnun . [Vo l. 47] [1982J
:;zan )(0-(_1-(-/\ /\
- 16039
t -"14772
E
-1 6047
-1 6055 '--Ob.,..-.L..--L--;;9:':::0;'-. -'------''--1~8c;;0c;;-. --'
FIG. 2
Dependence of total energy of methyl 3-methylpropenoate on rotation of the CH 3S group (synperiplanar conformation of C=C -C=O)
282 Vsetecka, Pecka, Prochazka:
(Scheme 4), we see that the (Z)-isomer should be more stable by 6·4 kJ mol- l which does not agree with the experimental data. According to equilibration measurements3, !lG < 9 kJ mol-I; this vaiue corresponds rather to the !lE value according to Scheme (2a) or (3b), i.e. + 18-19 kJ mol-I.
In the case of methyl 3-methoxypropenoate we studied dependence of the energy content on rotation of the -OCH3 group. As seen from Fig. 1, conformations with 180° and 120° angles between the planes are operating in both isomers (Scheme 5). Calculation shows that an sp conformation of the CH30- group is not advantageous in either isomer. Equilibration measurements I found that !lG403 = 15·7 kJ mol- l
and !lH = 2·5 kJ mol - l with a significant contribution of the entropy term. Also for methyl 3-methylthiopropenoate the dependence of the total energy
on rotation of the - SCH3 group was calculated (in the sp conformation of the C=C-C=O system; Fig. 2). For the (E)-isomer two stable conformations with torsion angles 0° and 180° are possible (Scheme 6) whereas iI! the (Z)-isomer only conformation with a 0° angle is stable and rotation of the SCH3 group results in a steep energy increase. If we compare conformers with sp and ap arrangement of the C=C-C=O system and with the same conformation of the -SCH3 group, we see that the compounds should exist in both these conformations because !lE(sp-ap) values for the (Z) and (E) isomers are 5·3 kJ mol- l and 1·8 kJ mol - I, respectively (Scheme 7). The experimental value of !lG423 (9·9 kJ mol-I) and !lH (2'6 kJ mol-I, with a high entropy term) fairly agree with the value of !lE (0-4 kJ . . mol-I).
(5a) 2 kJ rnol- I
I kJ rnol- 1 (5b )
(6(/)
Collection Czechoslovak Chern. Commun . [Vol. 47J [1982]
(E-Z)-Isomerization 283
0.4 kJ mo]-l
(6h)
3.9 kJ mo(1
(7)
33.6 kJ mol - I
(8a)
TABLE V
Total energy calculation for dimethyl butenedioate (in kJ mol- 1)
Conformation (8a) (8b) (8e)
(Z)-Isomer - 20524' 3 - 20494-4 - 20522,1 (E)-Isomer - 20557,9 -20548,2 -20553'3 />"E(Z-E) 33·6 53·8 31·2
TABLE VI
Comparison of the experimental enthalpy differences with the calculated total energy differences for selected su bstituents (kJ mol- 1)
X />"H />"E (/>"H-/>"E)
CI 10·8 8·0 2·0 CN --4,0 -11,2 7·2 CH3 18 - 19 <9 CH3 0 2' 50 - 2·0 4'5 CH3S 2'60 0-4 2·2 COOCH3 32·2 33-6 -1,4
o The value suffers from a considerable experimental error.
Collection Czechoslovak Chern . Cornrnun. [Vol. 47] [19821
284 Vsetecka , Pecka , Prochazka :
For (Z) and (E) dimethyl butanedioate, total energy was calculated for conformations depicted in Scheme 8 (Table V). Calculation shows that deviation of both the -COOCH3 groups from planarity by ± 20° results in an energy increase of about 3·4 kJ mol- 1 for the (Z)-isomer but 18·2 kJ mol- 1 for the (E)-isomer. We cannot therefore exclude that, contrary to the (E)-isomer, the (Z)-isomer is not planar. The value f,.E = 33·6 kJ mol - 1 agrees very well with the experimental value of f,.H
(32'2 kJ mol-I; Table VI).
We can conclude that the experimentally found values of f,.H agree well with the calculated f,.E values, except the data calculated for the methoxy group which wrongly predict a predominance of the (Z)-isomer.
53.8 kJ mol- 1
31.2 kJ mol-I
EXPERIMENTAL
CHJ
0 /
H "C= O " / C=C "
/ " O= C H
" ° / HJC
(8b)
(8e )
(9)
The geometric parameters of the molecules were based on the experimental data found for analogous compounds lO . Basic data for methyl 2-propenoate (Scheme 9): C(I )-C(2) 1' 34; C(2)- C(6) 1-46; C(6)-0(7) 1'22; C(6)-0(8) 1'36; 0(8)- C(9) 1'43; C(l)- H(3) 1'08; C(1)- H(4) 1'08; C(2)- H(5) 1'08; C(9)- H(lO ) 1'09; C(9)-H(1l) 1'09; C(9)- H(12) 1'09; ~ C(6)- 0(8)- C(9) 109'5°. Data for substituents: 3-CH3: C(l)- C 1'51; C-H 1·09; 3-Cl: C(l)- Cl 1'72; 3-CN: C(I)- C 1'45, C-N 1·16; 3-CH30: C(l)- O 1' 36, C- O 1-43, C- H 1,08, ~ C(I)-O-C 109'5°; 3-CH3S: C(l)- S 1'748, S- C 1-81, C-H 1'09, ~ C(l)- S-C 104'5°.
Collection Czechoslovak Chern. Cornrnun. [Vol. 47] [1982]
(E- Z)-Isornerization 285
REFERENCES
1. Topek K., Vsetecka V., Prochazka M. : This Journal 43, 2395 (1978). 2. Uchytilova V.: Thesis. Charles University, Prague 1969. 3. Butler J. N ., Small G . J. : Can. J . Chern. 41, 2492 (1963). 4. Rhoads S. J., Jitendra K., Chattopadhyay, WaaJi E. E.: J. Org. Chern. 35, 3352 (1970). 5. Hocking M. B.: Can. J. Chern. 47, 4567 (1969). 6. Davies M., Eva ns F. P., Trans . Faraday Soc. 61 , 1506 (1955). 7. Nelles M.: Z . Phys. Chern. (Leipzig) 1931, 369. 8. Pople J . A., Beveridge D. L.: Approximate M olecular Orbital Theory . McGraw-Hili, New
York 1970. 9. Santry D . P., Segal G . A. : J . Chern . Phys. 47, 158 (1967) .
10. Int eratomic Distances Supplement (L. E. Sutton, Ed .), Spec. Pub!. 18. The Chemical Society, London 1965.
Translated b y M . Tichy.
Collection Czechos lovak Chem. Commun. [Vol. 47J [1982J