(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

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"'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.

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(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)

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:;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(/)

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(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.

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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°.

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(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.

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