Furan derivatives. XXXIX. The isomerization of substituted 5-phenyl-2-furfurylthiocyanates

to the corresponding isothiocyanates

A. KRUTOŠÍKOVÁ, J . KOVÁČ, A. PIKLEROVÄ, and R. FRIMM

Department of Organic Chemistry, Slovak Technical University, 880 37 Bratislava

Received 15 July 1971

The isomerization of substituted 5-phenyl-2-furfurylthiocyanates to the corresponding isothiocyanates was investigated from the kinetic point of view. The measured rate constants were correlated with Hammett a con­stants.

The isomerization of thiocyanates to isothiocyanates was already known in the last century; the rate and the mechanism of this reaction was, however, studied only recently. Thus Rao and Balasubrahmanyan [1] reported the first kinetic measurements of iso­merization of methylthiocyanate. The isomerization of allyl and benzyl substituted thiocyanates was investigated more intensively than that of saturated thiocyanates [2 — 9]. Some authors rationalized this fact by a greater ability of unsaturated thiocyanate systems to isomerize and furnish proper isothiocyanates [10, 11]. The isomerization mechanism of some types of thiocyanates was already suggested by Iliceto et al. [2—7] .and also by Smith and Emerson [11, 12]. The above-mentioned authors [2 — 7] explained the isomerization of allyl substituted thiocyanates by an intramolecular mechanism involving a transitive formation of a six-membered ring. When investigating the iso­merization of benzhydrylic thiocyanates by means of kinetic measurements using 3 5 S, these authors found this reaction to proceed by a monomolecular mechanism. Bimolecular mechanism was considered to be improbable due to the low nucleophility of the nitrogen atom in the organic thiocyanate.

The structure of the appropriate carbenes formed during the isomerization was evi­denced by Spurlock and co-workers [13—15] on the basis of results obtained when exa­mining single types of thiocyanates. Also the rearrangement of furfuryl thiocyanate to furfurylisothiocyanate was reported by the same author [14]. The rearrangement of 5-substituted furfurylthiocyanates was studied by further authors [16, 17] as well.

Experimental

The preparation of 5-phenyl-2-furfurylthiocyanates and some reference isothiocyanates was published earlier [18]. The purity of thiocyanates was checked on a silica gel thin--layer plate before each kinetic measurement [19].

The determination of rate constants

The isomerization rate constants of substituted 5-phenyl-2-furfurylthiocyanates to the proper isothiocyanates were determined by means of infrared spectroscopy with an SP-100 Unicam Spectrophotometer in the main NCS-band region at approximately 2060 cm - 1 . The apparatus was calibrated with a polystyrene foil 25 [xm thick.

Chem.zccsti 27 (1)119-121 (1973) 1 1 9

A. KRUTOŠÍKOVÁ, J . KOVÁČ, A. PIKLEROVÁ, R. FEIMM

The kinetic examination procedure

T h e proper t h i o c y a n a t e was dissolved in a spectra l g r a d e chloroform t o give a 5 x X 10~3 mol l " 1 solut ion a n d p o u r e d in to a flask p r o v i d e d w i t h a reflux condenser. T h e

flask was p laced in a b a t h t h e t e m p e r a t u r e of which was k e p t a t 60 i 1°C. S a m p l e s were w i t h d r a w n in 10 —60 m i n . per iods according t o pre l iminar i ly e s t i m a t e d r a t e s of r e a r r a n g e m e n t . T h e r e a r r a n g e m e n t was m e a s u r e d in cells 5.276 m m in w i d t h t o a con­s t a n t absorbance.

R e s u l t s a n d d i s c u s s i o n

W e found t h a t s u b s t i t u t e d 5-phenyl-2-furfuryl t h i o c y a n a t e s easily u n d e r g o isomeriza­t ion d u r i n g p r e p a r a t i o n . T h e course of i somerizat ion of t h e synthes ized t h i o c y a n a t e s was invest igated in t h e m a i n NCS-band region a t a p p r o x i m a t e l y 2060 c m - 1 [18]. I n s t e a d

A

D.8

0.6

0A

0.2

n

1 I

n

- /

/ Г , ,

1 1 !

1 1 i

1

-

-

1

0 10Ü 200 300 400 500 600 [mm]

А 0.8

0.6

ол

0.2

0

-

-j

0 I

20

I

i

40

I

i

60

1

i

80

1

, Ю0

1

r

120

I

-

-

-

-,

140 [miň

Fig. 1. T h e isomerizat ion course of 5-(4--bromophenyl) - 2 -furfury l th iocyana t e t o

i s o t h i o c y a n a t e . T h e t ime-depending absorbance a t

v = 2071 c m - 1 .

Fig. 2. T h e i somerizat ion course of 5-(4--tolyl)-2-furfurylthiocyanate t o isothio­

c y a n a t e . T h e t ime-depending absorbance a t

v = 2069 c m " 1 .

Table 1

K a t e c o n s t a n t s of r e a r r a n g e m e n t of s u b s t i t u t e d 5-(X-phenyl)-2-furfurylthiocyanates t o t h e corresponding i sothiocyanates

Measured in CHCI3 a t 60°C. T h e initial concentra t ion of t h i o c y a n a t e 5 x 1 0 - 3 mol l - 1

No. k x 10'1 m i n - 1

/ //

IV v

VI VII

4-NO, 3-NO, 4-C1 3-C1 4-Br H 4-CH3

4.37 5.06

41.6 34.0 44.8

184.0 423.2

0.778 0.71 0.227 0.37 0.232 0

- 0 . 1 7

1 2 0 Cham, zresti27 (1> 11S> 121 (1973)

FURAX DERIVATIVES. X X X I X

of concentration, we put the absorbance into the kinetic equation since from the calibra­tion curve it is evident that there is a linear dependence between concentration and absorbance in the concentration region under study. The relationship between con­centration and absorbance of 5-(4-nitrophenyl)-2-furfurylisothiocyanate was the basis for construction of the calibration curve. The course of some kinetic measurements could be seen in Figs. 1 and 2. The measured data were evaluated using the first order kinetic equation, since it was found that the rate constant is not concentration dependent. The measured values of rate constants are listed in Table 1.

The statistical evaluation of the measured values of rate constants with Hammett Op and Gm constants gave the following results: q = —2.1, correlation coefficient r = = —0.99 and standard deviation 0.083. These results indicate the effect of various substituents attached to benzene ring to be transmitted through the conjugated benzene and furan system on the rhodanomethyl group and that substituents markedly influence its reactivity.

From the measured rate constants (Table 1) it could be assumed that a monomolecular reaction course is involved in the case of isomerization of substituted 5-phenyl-2-furfu-rylthiocya nates. In support of this assumption electron-donor substituents favour the isomerization, whereas electron-acceptor substituents retard the rearrangement what is evident from the values of rate constants.

References

1. Rao C. X. R., Balasubrahmanyan S. N., Chem. Ind. (London) I960, 625. 2. Iliceto A., Fava A., Mazzucato U., Tetrahedron Lett. 11, 27 (1960). 3. Iliceto A., Fava A., Mazzucato U., Radici P., Gazz. Chim. Ital. 90, 919 (1960). 4. Iliceto A., Fava A., Mazzucato U., Rossetto O., J. Amer. Chem. Soc. 83, 2729 (1961). 5. Iliceto A., Fava A., Mazzucato U., J. Org. Chem. 25, 14457 (1960). 6. Iliceto A., Gaggia G., Gazz. Chim. Ital. 90, 262 (1960). 7. Fava A., Iliceto A., Ceccon A., Koch P., J. Amer. Chem. Soc. 87, 1045 (1965). 8. Fava A., Iliceto A., Bresndola S., J. Amer. Chem. Soc. 87, 4791 (1965). 9. Fava A., Tonellato U., Congin L., Tetrahedron Lett. 1657 (1965).

10. Tonellato U., Rossetto O., Fava A., J. Org. Chem. 34, 4032 (1969). 11. Smith P. A. S., Emerson D. W., J. Amer. Chem. Soc. 82, 3076 (1960). 12. Emerson D. W., Booth J . K., J. Org. Chem. 30, 2480 (1965). 13. Spurlock L. A., Cox W. G., J. Amer. Chem. Soc. 91, 2961 (1969). 14. Spurlock L. A., Fayter R. G., J. Org. Chem. 34, 4035 (1969). 15. Spurlock L. A., Parks T. E., J. Amer. Chem. Soc. 92, 1279 (1970). 16. Jurášek A., Thesis. Slovak Technical University, Bratislava, 1964. 17. Jurášek A., Kováč J., Chem. Zvesti 19, 840 (1965). 18. Krutošíková A., Kováč J., Frimm R., Chem. Zvesti 27, 107 (1973). 19. Komanová E., Krutošíková A., Kováč J., Chem. Zvesti 27, 112 (1973).

Translated by Z. Votický

Chem. zvesti 27(1) 119-121 (1973) 121.