CROATICA CHEMICA ACTA CCACAA 46 (1) 71-78 (1974)

CCA-821 YU ISSN 0011-1643

541.14:547.725 Original Scientific Paper

Photochemical Isomerizations of Furylacrylic Acidsa>b>

G. Karminski-Zamola and K. Jakopcic Institute of Organic Chemistry and Biochemistry, University of Zagreb, and Laboratory of Organic Chemistry, Faculty of Technology, Uni-versity of Zagreb,

41000 Zagreb, Cr oatia, Yugoslavi a

Received October 11, 1973

It was shown that cis-3-(2' -furyl)acrylic acid (lb), cis-2-methyl-3-(2' -furyl)acrylic acid (Ilb), cis-3-(5' -methyl-2' -furyl) acrylic acid (IIIb) and cis-2-methyl-3-(5'-methyl-2' -furyl)acrylic acid (!Vb) can be conveniently prepared by photochemical isome­rization of the corresponding trans acids (Ia-IVa). The rate of isomerization and the composition of the mixture at photoequi-1ibrium in water-filtered ultraviolet is not influenced by methyl substitution. The isomeric acids were separated through their cyclohexylammonium salts and characterized by elemental ana­lyses, uv. and ir. spectra.

The photochemistry of conjugated arylalkenes has been widely investi­gated. Stilbene and its derivatives have been a centre 0£ interest for decades and many aspects of their photochemistry are now understood1•2• Simpler conjugated monophenylalkenes have been studied to a limited extent3•4•

More attention has been paid to the photochemistry of cinnamic acids. The irradiation of crystal1ine trans-cinnamic acid with sunlight led mostly to dimeric truxillic and truxinic acids5, but to a small extent isomerization to cis-cinnamic acid was also observed6• On the other hand filtered or unfiltered ultraviolet irradiation of substituted or unsubstituted trans-cinnamic acids in solution caused trans-cis isomerizations7 , accompanied in some instances by lactonisation8•

As a part of our studies of furans 9 we turned our attention to photo­chemical transformations of furylacr ylic acids. On irradiation with sunlight or light of a high pressure mercury lamp crystalline 3-(2' -furyl)acrylic acid dimerized to 2-furyl analogue of truxinic acid10• Similar results were obtained by Lahav and Schmidt11•

T.o the best of our knowledge there are no reported data about photo­chemical transformations of unsubstituted or substituted furylacrylic acids in solutions; in the present work we wish to report on photochemically induced trans-cis isomerizations of such acids.

Much has been done to increase the understanding of the mechanisms of direct or sensitized cis-trans photoiisomerizationrs of olefiniic compounds, and to identify the states involved2• By direct excita·tion an electron is

• l Studies in the Furan Series. XIV. bl Taken in part from the Ph.D. Thesis of G. Karm~nski-Zamola, University of

Zagreb, 1972.

I

72 G. KARMINSKI-ZAMOLA AND K. JAKOPCIC

promoted from the highest filled bonding n orbital to the lowest antibonding n* orbital. It can be shown that the most stable geometric configuration of both the singlet and triplet (n, n*) states of ethylene and its derivati'ves are highly twisted from the planar ground state about the double bond axis . In this form, the distinction between cis and trans is lost and the excited molecule can easily return to either cis or trans olefin ground state, though no't with equal probability12• Since the isomer of lower ground state stability is frequently formed in higher yield, the process is useful synthetically.

The various syntheses of unsubstituted or alkyl substituted fury1acrylic acids usually give the more stable trans isomer. A successful photochemical isomerization of generally easy obtainable trans-furylacrylic acids to their cis isomers followed by appropriate separation seems to be a very useful method for preparing cis acids of this type, since the only reported example of cis-acid was cis-3-(2' -furyl)acrylic acid prepared besides trans isomer by Liebermann 13,14•

Another reason for undertaking this research was to study the influence of methyl substitutron on photochemical isomerizatton of trans-furylacrylic acids in solution:

Sandris15 found that methyl substitution either in position 2 or 3 of trans-cinnamic acid had no influence on the rate of photochemical isomeri­zation induced by uv.-irradiation of 4 X 10-4 M solutions, but the percentage of cis isomer in 'the photostationary state was influenced by such substitution.

In our investigations on furylacrylic acids, neither the rate of isomeri­zation nor the percentage of cis and trans isomer was influenced by methyl substitution. The photostationary state under our e:x:perimental conditions was established at about 200/o cis isomer after a 5 h of water-filtered ultra­violet irradiation of 100/o solutions in gl,acial acetic add.

The rate of photochemically induced isomerization and the composition of the photoequilibrium mixture was determined by GLC-analyses of esterified aliquots taken during irradiation of the particular furylacrylic acid (Fig. 2.).

cis-Furylacrylic acids were separated from the pho.tochemical mixture on the basis of different solubilities of the isomeric acids in benzene or petroleum ether. Final purification was performed by fractional crystallizations of corresponding piperidine or cyclohexylamine salts followed by regeneratiun of acids.

The acids, their salts and methyl esters were characterizect by elemental analyses, uv. and ir. spectra.

(

%cis

40

30

20

3

ISOMERIZATIONS OF FURYLACRYLIC ACIDS 73

······C·· · · ··· II

---..----111

---<>--IV

0 , ,, ......•..• .. • ··D ~-- ..... ·- ....:_,,.. ____ _

10 15 20 25 hrs

Fig . 1. Relative contribution of cis acids (as Me-esters) in photochemical mixture. 10°/o solutions in glacial acetic acid, water cooled ORIGINAL HANAU TQ-150 high pressure mercury lamp.

It should be noted that uv. maxima of cis and correspondim.g trans acids are very near each other, but cis acids posses lower absorption intensities (Table I) .

TABLE I

trans cis

Acid Amax logs Amax logs .run nm

I 296 4.37 299 4.14 II 292 4.41 294 4.15

III 308 4.48 310 4.21 IV 305 4.43 302 4.15

EXPERIMENTAL

Melting points and boiling points are uncorrected. Ir. spectra were taken on a Perkin-Elmer 137 Infracord spectrophomometer using mull technique. Uv. spectra were recorded on Perkin-Elmer 124 spectrophotometer. Ultraviolet irradiations were performed using a water-cooled immersion ORIGINAL HANAU TQ-150 quartz high pressure mercury arc lamp. GLC were performed on an Aerograph 660 chromato-­graph fitted with a flame ionization detector and a 1/s" X 5' steel column packed with 30/o SE-30 on Aeropak 30 or 30/o Carbowax 20 M on Chromosorb T.

trans Acids used in this work, except !Va, were reported and were prepared by known procedures1s-1s.

trans-2-Methyl-3-(5' -methyl-2' -furyl)acrylic acid (IVa)

A mixture of 50.0 g (0.46 mol) freshly distilled 5-methylfurfural, 65.0 g (0.46 mol) propionic anhydr,ide and 65.0 g anhydrous potassium propionate was heated 20 h at 160 °c. The reaction mixture was poured into 100 ml water and neutralized with saturated solution of sodium carbonate. Crys~alline trans acid melting at 115-121 °c was obtained on acidification of the decolourized solution with hydrochloric acid, and was recrystallized f.rom aqueous methanol. Yield 22.0 g, m . p. 13()-131 °C.

Anal. C9H100 3 (166.2) calc'd.: C 65.03; H 6.06-0/o found: C 65 .04; H 6.25%

Ir. spectrum : vmax 1670 cm-1 (C=O) and 1630 cm-1 (C=C).

74 G. KARMINSKI-ZAMOLA AND K. JAKOPCIC

Irradiation experiments

A 100/o solution of trans acid (Ia-IVa) in glacial acetic acid was irradiated at 20--25 °c until there was no change in melting points of evaporated small aliquots (about 20 h). The dark residue after removal of the solvent, containing trans and cis acid was treated according to stated procedure.

Procedure A. - A photochemical mixture of cis and trans acid was triturated Wiith benzene. The less soluble trans acid was filtered off. The additional quantity of trans acid was separated after the addition of petroleum ether, and the solution evaporated to dryness. The crude cis acid was purified by trituration with hot petroleum ether and recrystallization of the evaporation product.

Procedure B. - A photochemical mixture of cis and trans acid was triturated wd.th hot petroleum ether. The solution was evaporated. After fractional crystal­lization from petroleum ether crude cis acid was obtained. To the hot ethylacetate solution of the acid an equimolar quantity of cyclohexylamine was added. The cyclohexylammonium salt of trans acid which crystallized on cooling was filtered off and the filtrate evaporated. To the residue a 50/o aqueous potassium hydroxyde was added, and the solution extracted with ether. Pure, cryistalldne cis acid was obtained on acidification with 5% sulfuric acid.

cis-3-(2' -Furyl)acrylic acid (lb) .

From 7.0 g trans-3-(2'-furyl)acrylic acid16 (m. p. 139-140 °C). The mixture· of cis and trans acid was elaborated according to Liebermann's procedure13,14• The crystalline Ib (1.0 g) melting at 106-108 °c was obtained*.

cis-2-Methyl-3-(2' -furyl)acrylic acid (Ilb)

From 20.0 g trans-2-methyl-3-(2'-furyl)acrylic acid17 (m. p. 118-119 °C). Proce­dure B. Y·ield 2.0 g, m. p. 72-73 °c.

Anal. C8Hs03 (152.1) calc'd.: C 63.15; H 5.32% found: C 63.26; H 5.590/o

Ir. spectrum: vmax 1670 cm-1 (C=O) and 1600 cm-1 (C=C).

cis-3-(5' -Methyl-2' -furyl)acrylic acid (IIIb)

By irradiation of 4.5 g trans-3-(5'-methyl-2'-furyl)acrylic acid18 (m. p. 156--157 oc). Procedure A. Yield 0.6 g, m . p. 110--112 °c.

Anal. C8H 80 3 (152.1) calc'd.: C 63.15; H 5.32% found: C 63.38; H 5.020/o

Ir. spectrum: vmax 1680 om-1 (C=O) and 1625 cm-1 (C=C).

cis-2-Methyl-3-(5' -methyl-2' -furyl)acrylic acid (IVb)

From 20.0 g trans-acid IVa (m. p. 130-131 °C). Procedure B. Yield 2.9 g, m. p. 80--82 °c.

Anal. C9H100 3 (166.2) calc'd.: C 65.03; H 6.060/o found: C 64.90; H 6.210/o

Ir. spectrum: vmax 1680 cm-1 (C=O) and 1600 cm-1 (C=C).

*Liebermann13 stated m. p. 104--105 oc for the sample of cis acid Ib prepared besides trans isomer Ia by decarboxylation of furfurylidenemalonic acid and separation of both isomers.

TA

BL

E I

I

Ph

en

ylh

yd

razo

niu

m a

nd

Cyclo

he

xyla

mm

on

ium

S

alt

s

R. -

().-

cH

=C

-CO

O-H

x+

1 0

I Rz

R1

R2

iso

mer

x

M.p

. F

orm

ula

M

ol.

oc

wei

gh

t

I H

H

tr

an

s

CuH

sN2'

I

- 88

-90

C

1aH

!14N

20a

246.

2

H

H

cis

CaH

sN2

83

-84

C

13H

i.4N

20a

246.

2

H

CH

s tr

an

s

CuH

sN2

79

-80

C

uH

1aN

20

s 26

0.3

H

CH

s tr

an

s

CuH

1sN

b 1

71

-17

2

C14

H21

NO

s 25

1.3

H

CH

s ci

s C

aH13

N

15

0-1

51

C

uH21

NO

s 25

1.3

CH

s H

tr

an

s

CuH

sN2

67

-68

C

14H

16N

20s

260.

3

CH

s H

ci

s C

uHsN

2 7

6-7

8

i C

14H

1uN

20s

260.

3 '

'. ---

--~,

. ,-

-.. --

' ..

I.

''

.. •,,

-.·.:

CH

s C

Hs

tra

ns

CaH

sN2

70_:

_72

C1s

H1s

N20

s 27

4.3

CH

s C

Hs

tra

ns

CuH

1sN

1

53

-15

4

C1s

H2s

NO

s 26

5.3

• P

hen

ylh

yd

razi

ne

b C

yclo

hex

yla

m!n

e

An

al.

calc

'd

fou

nd

O/o

c I

O/o

H

I 63

.40

5.73

63

.18

5.53

63.4

0 5.

73

63.1

9 5.

85

64.6

0 6.

20

64.8

6 6.

49

66.9

0 8.

42

67.2

1 8.

10

66.9

0 8.

42

66.9

5 8.

21

64.6

0 6.

20

64.5

7 5.

92

64.6

0 6.

20

64.2

3 5.

92

65.6

7 6.

61

65.4

3 6.

82

67.8

9 8.

74

68.0

5 8.

50

O/o

N

11.3

8 11

.28

5.57

5.

42

5.28

5.

48

H

[fl 0 :;: ~ N >

~ 0 z [fl 0 ':J ~ ~ t" > ~ t"

H n ?> n H

t)

[fl

-.:i

C

l

TA

BL

E II

I

Met

hyl

est

ers

of

cis-

an

d t

rans

-fur

ylac

ryli

c ac

ids

RD

CH

=C

-CO

OC

H

1 0

I 3

R2

An

al.

calc

'd

R1

R2

iso

mer

y

ield

B

.p.

Fo

rmu

la

Mol

. fo

un

d

O/o

°C/m

mH

g

wei

gh

t.

O/n

c I

0 /o

H

H

H

tran

s 57

9

2-9

5/6

•·b'

CsH

aOs

152.

1

H

H

cis

33

75--

76/5

C

sHsO

s 15

2.1

63.1

5 5.

32

62.9

6 5.

50

H

CH

s tr

ans

36

99

-10

1/5

') C

9H10

0s

166.

2

H

CH

s ci

s 58

7

0-7

1/1

0

C9H

100s

16

6.2

65. 0

3 6.

06

65.0

0 6.

30

CH

s H

tr

ans

55

89

-90

/5d,

e) C

oH10

0s

166.

2

CH

s H

ci

s 55

7

2-7

3/5

C

9H10

0s

166.

2 65

.03

6.06

64

.68

6.36

CH

s C

Hs

tran

s 35

6

8-6

9/5

C

10H

120s

18

0.2

66.6

5 6.

71

66.3

5 6.

70

CH

s C

Hs

cis

92

81

-85

/5

C10

H12

0s

180.

2 66

.65

6.71

66

. 39

6.59

• m

. p.

29 o

c; b

L

it.H

b

. p

. 9

5-9

7 •

C/ 6

mm

H•

; •

Lit

.21

b.

p.

89

-90

•C

/ 5 m

mH

s;

d M

. p

. 3

9-4

0 •

c;

• L

it."

b

. p

. 8

8-9

0 O

C/5

mm

Hg

; '

M. p

. 27

•c

.

-:i

O':I

S1 ~ >

:d ii:: ~ [/l ~ N

>

ii:: 0 t-<

)> > z t:J

~ ... >

~

0 '1l

()

H

(')

ISOMERIZATIONS OF FURYLACRYLIC ACIDS 77

The preparation of salts (Table II)

Phenylhydrazonium salts .. - To the saturated ether solution of a particular acid an equimolar quantity of phenylhydrazine was added. The salt soon crystallized, and was usually of analytical purity. It should be noted that the salts of cis acid with methyl substituent in position 2 (IIb and IVb) could not be obtained.

Cyclohexylammonium salts. - To the solution of the acid in a minimum amount of ethylacetate an equimolar quantity of cyclohexylamine was added. A quantitative yield of crystalline salt was separated and recrystallized from benzene. Crystalline cyclohexylammonium salt of IVb could not be obtained.

The preparation of methyl esters (Table III)

To the etheral solution of the acid an excess of diazomethane* was added. After evaporation in vacuo methyl ester was distilled twice at reduced pressure.

Infrared absorption of esters at 1725-1690 cm-1 and 1650-1620 cm-1 (C=O and C=C).

Acknowledgement. This work has been financially suppo,rted by Research Fund of Croatia. We are indebted to late Prof. V. Hahn for suggesting the problem and for stimulating discussion in the early stage of the work. The authors wish to thank Mrs. I. Gustak-Masek for the microanalyses.

REFERENCES

1. See e.g. a) G. S. Hammond, J. Sa 1 tie l, A. A. Lam o 1 a, N. J. Turro, J . S.

Br ads haw, D. 0. Cowan, R. C. Couse 11, V. Vogt, and C. D a 1-t o In,, J. Amer. Chem. Soc. 86 (1964) 3197.

b) J. Sa 1 tie l, ibid. 90 (1968) 6394. 2. J. Sa 1tie1 and E . D. Meg a r it y, ibicl. 94 (1972) 2742. 3. C. S. Nag aw a and P. J. Sig a 1, J. Chem. Phys. 58 (1973) 3529. 4. M. G. Ro c k 1 e y and M. S a 1 is bury, J. Chem. Soc. Perkin Trans. II (1973)

1582, and references cited therein. 5. M. D. Cohen, G. M. J. Schmidt, and F . I. Sontag, J . Chem. Soc. (1964)

2000. 6. A. W. K. de Jong, Chem. Weekbl. 26 (1929) 270. 7. a) M. B. Hocking, Canad. J. Chem. 47 (1969) 4567, a nd references cited therein.

b) R. Stoermer, Chem. Ber. 44 (1911) 637. c) R. Stoermer, F. Grimm and E. La age, ibid. 50 (1917) 959. d) H. Burton and C. W. Shoppe e, J . Chem. Soc. 1935 1156. e) Y. Ur us hi bar a and M. Hirota, Nippon Kagaku Zasshi 82 (1961) 351,

354, 358; cit. from Chem. Abstr. 56 (1962) 10025g. 8. 0 . L. Ch a pm an n and W. R. Ad ams, J. Amer. Chem. Soc. 90 (1968) 2333. 9. D. Bi 1 o vi c and V. Hahn, Croat. Chem. Acta 39 (1967) 189 and preceeding

papers of the series. 10. G. Karminski-Zamola, M.Sc. Thesis, Univ. Zagreb 1969. 11. M. Lah av and G. M. J. Schmidt, J. Chem. Soc. (B) 1967 239. 12. N. J. Tu r r o, Photochemistry and Photobiology 9 (1969) 555. 13. C. Lieb er m an n, Chem. Ber. 27 (1894) 286. 14. C. Liebermann, ibid. 28 (1895) 131. 15. C. S and r i s, Tetrahedron 24 (1968) 3569. 16. J . R. Johnson, Org. Synth. Coll. Vol. 3 (1955) 426. 17. R. Lukes and J. Sr o g 1, Collection Czech. Chem. Commun. 26 (1961) 2238. 18. R. Kuhn, F. Koh 1 er, and L. Koh 1 er, Z. Physiol. Chem. 247 (1937) 192. 19. T. J. De Boer and H. J. Backer, Org. Synth. Coll. Vol. 4 (1963) 250. 20. E. I. du P on t d e Ne mo u rs and C o., Brit. Patent 529 (1945); cit. from

Chem. Abstr. 42 (1948) 928g. 21. I. F. Be 1 ski, N. I. Sch u i kin, W. M. Sch o st a k ow ski, and S. N.

Ch ark o v, Zhur. Prikl. Khim. 32 (1962) 1030.

* The diazomethane solution was prepared accordin to DeBoer and Backer19 •

78 G . KARMINSKI-ZAMOLA AND K. JAKOPCIC

SAZETAK

Fotokemijska izomerizacija furilakrilnih kiselina

G. Karminski-Zarnola i K. Jakopcic

Ultraljubicastim osvjetljavanjem lako pristupacnih trans-izomera (Ia-IVa) u ledenoj octenoj kiselini s pomocu vodom hladena ORIGINAL HANAU TQ-150 zivina Iuka visokog tlaka i naknadnom separacijom izomera dobivene su cis-3-(2'-furil) akril.Jna (lb), cis-2-metil-3-(2' -furil)akrilna (Ilb) , cis-3-(5' -metil-2'-furil)akrilna (IIIb) i cis-2-metil-3-(5'-metil-2'-furil)akrilna kiselina (!Vb). Pokazalo se da brzina fotoke­mi}ske izomerizacije kao i sastav smjese u fotoravnotezi pod primijenjenim uvje­tima ne ovise o metil-supstituciji. Izomerne kiseline karakterizirane su elementar­nom analizom, ir. i uv. spektrima, te kao metilini esteri, cikloheksilamonijeve ili fenilhidrazonijeve soli.

INSTITUT ZA ORGANSKU KEMIJU I BIOKEMIJU I Primljeno 11. listopada 1973.

ZAVOD ZA ORGANSKU KEMIJU, TEHNOLOSKI FAKULTET SVEUCILISTA U ZAGREBU