0013^1686 84 S3.00 + 0.00 1984- Pereamon Press Ltd.

Electrochimica Acta. Vol. 29. No. 8, pp. Printed in Great Britain.

ELECTROOXIDATION OF TERPENES—I. SYNTHESIS OF DIHYDROCARVONE AND 1-HYDROXYNEODIHYDROCARVEOL BY ANODIC OXIDATION OF LIMONENE V. M O N T I E L , M. L Ó P E Z - S E G U R A and A. A L D A Z *

Departamento de Química Física, Facultad de Ciencias, Universidad de Alicante, Apartado 99, Spain M.

GRANDE

Departamento de Química Orgánica, Facultad de Ciencias, Universidad de Alicante, Spain and F. BARBA

Departamento de Química Orgánica, Facultad de Ciencias, Universidad de Murcia, Spain (Received 3 January 1984) Abstract—The oxidation of (+) and ( - ) limonene, 1, in T H F - H 2 0 (25:1), NaC10 4 on a graphite electrode gives, with a relative stereoselectivity, ( + ) and ( —) dihydrocarvone, 5, and ( + ) and ( —) 1-hydroxyneodihydrocarveol, 7, respectively as the major products. A logical route for this oxidation via a protonated epoxide as intermediate is proposed.

predominant products being dihydrocarvone, 5, and 1hydroxyneodihydrocarveol, 7, in relation 2 : 1 .

INTRODUCTION There are few studies on the electrochemical oxidation of terpenes, most of them from a preparative point of view in which a mixture of reaction products were found. So, in the electrochemical oxidation of a- and /?piñenes the products were a mixture of ring-opened c o m p o u n d s [ l ] and in the anodic oxidation of limonene in methanol[2] the final products are a mixture of different compounds formed by endocyclic double bond oxidation. In the present paper the electrolytic oxidation of ( + ) and ( —) limonene in T H F - w a t e r is studied. Remarkable stereoselectivity in the process is found, the

RESULTS A N D D I S C U S S I O N In the anodic oxidation of limonene on graphite paste electrode in T H F - H 2 0 (25 :1) with N a C 1 0 4 as supporting electrolyte, the reaction products were a mixture of dihydrocarvone, 5, and isodihydrocarvone, 6, in relation 5 :1 (45 %) and 1-hydroxyneodihydrocarveol, 7, (25 %). Shono el al.[2] proposed, for the anodic oxidation of nonconjugated dienes, the reaction pathways shown in Scheme 1.

To whom correspondence should be addressed.

^

-C-" NuH

• C -C+

•c— Scheme 1. 1123

C—Nu I +

C—Nu ,Cs—Nu

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V. MONTIEL, M. LÓPEZ-SEGURA, A. ALDAZ, M. GRANDE AND F. BARBA

With Scheme 1 in mind, the electrooxidation of limonene can be explained by the process shown in Scheme 2.

• ^

L -^5.

(-)1

J

^4%.

2 CH 3

6

recovering 90 % of the initial starting material. With the same charge using a graphite paste electrode this value is 12%.

'^%

3

/ 4

C H

. 3

/

(-)7

9

' ^ \-H

+

(-)5

\CH 3

(-)S

Scheme 2.

However the formation of only trans diaxial glycol, 7, shows that route B is not, in our case, a single step. This is due to the fact that the oxidation of 3 followed by a nucleofilic attack of H 2 0 should give: (i) the ris-isomer, C-l hydroxyl equatorial and C-2 hydroxyl axial (1-hydroxyisodihydrocarveol), if the nucleofilic attack is on the adsorbed species 3 and (ii) a mixture of the cis and trans diols, 1-hydroxyisodihydrocarveol and 1-hydroxyneodihydrocarveol, if after oxidation of 3 the cationic intermediary is desorbed and the nucleofilic attack takes place in the solution. Because only 7 is found it must be accepted that B is not only a single step but that there is formation of an epoxide, 4, as an intermediate species. This epoxide is originated by donation of a pair of electrons of oxygen atom of the OH group to tertiary carbocation electrogenerated.

The use of (—) limonene as starting material leads to ( —) dihydrocarvone, 5, and ( —) 1-hydroxyneodihydrocarveol, 7; only traces of the corresponding diastereomer, 6, was detected (GLC and lH-nmr spectroscopy were used). In the same way the use of ( + ) limonene leads to the formation of the enantiomers ( + ) 5 and ( + ) 7. This stereoselectivity can be explained by assuming the existence of a heterogenous process caused by absorption of limonene; this adsorption takes place on one of the sides of the double bond. The formation of an intermediate adsorbed on the electrode, 2, forces the nucleofile (H 2 0) to attack on the side not protected by the electrode. Therefore, for example, to explain oxidation products of (—) limonene the adsorption must take place preferentially by the side Ire, 2si as is shown in Scheme 3.

Scheme 3. From protonated species 4 it can rationalize the formation of dihydrocarvone, 5, through an 1,2 hydrogen rearrangement and deprotonation. The trans diaxial opening of 4, by attack in ami of the nucleofile, led to formation of 7[3-5]. We have identified the protonated epoxide through electrolyzing (—) limonene, 1, in basic medium ( T H F - H 2 0 + NaOH). In these conditions the cisepoxide 8 is isolated (IR: 942cm" 1 ; nmr 5 = 3.00 ppm, t, J = 2 Hz, lit[5,6], together with trans-epoxide, 4, (IR: 835cm" 1 , 752cm" 1 ; nmr ¿ = 2.95ppm, d, J = 4.5Hz[5,6]) in relation 5:1. This is in agreement with the proposed reaction pathway. The nature of the electrode material affects on the quantity of limonene transformed as final products. So, when a platinum anode is used almost all current is employed to decomposed the SSE, for which reason the quantity of limonene transformed is very low,

EXPERIMENTAL ( - ) and ( + ) limonene (a)¿5 = - 9 0 . 3 (c = 1.1, CHC13) and (
Electrooxidation of terpenes—I H 2 0 and 2g of NaC10 4 . This solution was electrolyzed until a charge of 2.39 F m o P 1 was passed (/ = 143 Am" 2 ) using graphite paste electrodes. The solvent was eliminated under reduced pressure. A dark oil (8.47 g) was obtained after three extractions with diethyl ether, dried over anhydrous sulphate and eliminated under reduced pressure of the ether. The gas chromatogram of the oil (Carbowax 20 M, internal standard) gives: 10 ° 0 of limonene non transformed, 45 % of a mixture of (—) dihydrocarvone, 5 (3 = 1.01, d, J — 6 Hz, Me-7), and isodihydrocarvone, 6 (<5 = 1.07, d, J = 7 Hz, Me-7) in relation 5:1, 25 % of ( —) 1-hydroxyneodihydrocarveol, 7. When the dark oil is extracted three times with hot water (90°C) and cooled, a white solid m.p. 55~C was obtained. The non-soluble portion was distilled under reduced pressure using a short Vigreux column. Fraction 21-23°C (2mmHg) 1.2 g was identified as ( —) limonene. Fraction b.p. 56CC (2mmHg) 2.5 g was ( —) dihydrocarvone with minute quantities of other non identified products.

( —) 1-Hydroxyneodihydrocarveol, 7 The white solid m.p. 55=C is the glycol hydrate of 7. It was dissolved in diethyl ether and dried over anhydrous sodium sulphate. The solvent was eliminated under reduced pressure and 0.265 g of white needles were obtained. A posterior recrystallization from petroleum ether gives white prisms, m.p. 73°C[7], 70-72=C. Anal. Caled, for C 1 0 Hi 8 O 2 : C, 70.59; H, 10.59. Found: C, 70.49; H, 10.60. GLC: Carbowax 20 M, 80°C, 4°Cmin- 1 , nitrogen 20 ml min" 1 , Rt = 29 min. MS. m/z (%): 170(0.8, M + ), 152(28), 137(19), 123(11), 108(41), 93(27), 82(27), 71(100), 58(22). 43(70), 27(15), 18(4). IR (Nujol): 3380, 2960,1640,1460,1380,1190,1080, 1050, 890cm"'[ó]. ^-nmríCDClj), 5 = 4.74 ppm (pseudo d, W¡,2 = 6 Hz, =CH 2 ), 3.63 (t, J = 3 Hz. equator. H-2), 1.72 (t, J = 1 Hz, Me-10), 1.25 (s, Me-7).

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Semicarbazone of {—) dihydrocarvone. Ethanol, m. p. 178:C (dec), flit.[12] 189~;C. lit.[14] 189-190=C). Anal. Cale, for C n H ^ N j O : C, 63.15; H, 9.09; N, 20.09. Found: C. 63.03; H. 9.12; N, 20.11. MS. m/z (°0): 209(31. M + ), 194(6), 181(4), 166(17), 150(70), 134(50), 123(27), 115(32), 107(45), 93(59), 79(47). 67(61-, 55(54), 41(100), 27(38), 18(16).

( —) Isodihydrocarvone, 6 The presence of 6 in the fraction with b.p. 56=C (2 mm Hg) was detected by its characteristic signals in l U-nmr (d. 8 = 1.07ppm, J = 7Hz[9,10]. Electrooxidation of ( + ) limonene, 1 The same conditions that for ( —) limonene were used. The products found were: ( + ) dihydrocarvone and ( + ) 1-hydroxyneodihydrocarveol which spectroscopic data are identical to those of ( —) 5 and ( —) 7 respectively. Semicarbazone of ( + ) dihydrocarvone. Ethanol, m.p. 178=C (dec), (ht.[13] 190^19rC and 145-150°C). Anal. Cale, for C n H ^ N j O ; C, 63.15; H, 9.09; N, 20.09. Found: C, 63.10; H, 9.15; N, 20.15. MS. fragmentation and intensities identical to that of the isomer ( —). (+) 5. was chemically synthetized [ 11 ] for reduction of ( - ) carvone, (a)2/ = -46.4 (c = 0.9, CHC13) with Zn/KOH-EtOH. The properties (spectroscopic and chromatographic data) of this ( + ) dihydrocarvone were identical to the dihydrocarvone obtained by electrolysis. ( —) Limonene cis-epoxide, 8

When the electrolysis of limonene is carried out in basic medium 8 is formed. 12.22g (8.98 x 10" 2 moles) of limonene were electrolyzed with anode and cathode of graphite paste in a SSE composed by 125 ml of THF, 5 ml of NaOH 30 % and 0.05 g of NaC10 4 in 10 ml of H 2 0 . After a charge of 2.42Fmol" 1 (i = 1.66 x 10" 2 A m " 2 ) was circulated the solvent was eliminated under reduced pressure, after this the mixture was extracted three times ( —) Dihydrocarvone, 5 with ethyl-ether and dried over anhydrous sodium It was noted that the fraction b.p. 56=C sulphate. Two new compounds 8 and 9 were detected (2mmHg)[8] was slightly impure with ( —) lim- by means of GLC (Rt = 8 and 9 min respectively, onene and ( —) isodihydrocarvone (GLC, TLC, relation of area 5:1). 1 H-nmr). Posterior purification by column chroBy distillation under reduced pressure (44-46°C, matography over silica gel (eluent hexane and 2 mm Hg;[4] b.p. 92-94, 20 mm Hg) and later silica gel hexane-ether 5:2) gives 5, ( a ) " = — 9.7 (c = 1.1, chromatography (eluant hexane-ether 5:1), 8 slightly CHC13). impurified with trans-epoxide 9 was obtained. IR (film): 3082, 1635, 942 (cis), 880, 835, Anal. Cale, for C 1 0 H 1 6 O: C, 78.95; H, 10.53. Found: 752cm" J ,[6] and[5]. C, 78.95; H, 10.51. 'H-nmr (CDC13): <5 = 4.60 ppm (m, 2H-9), 3.00 (t, GLC: Carbowax 20 M, 80=C, 4 c Cmin" 1 , nitrogen J = 2 Hz, H-2 cis), 2.95 (d, J = 4.5 Hz, H-2 trans), 1.68 20mlmin" 1 , Rt = 13 min. MS. m/z (°,0): 152(11, M + ), 141(1), 133(11), 123(13), (s. br, Me-10), 1.25 (s. Me-7). 109(29), 95(50), 81(38), 67(68), 55(39), 43(100), 27(35), 15(9). IR (film): 3080, 1705, 1640, 1370, S Q O c m " 1 ^ 10], Acknowledgements—We are indebted to ACEDESA for the 'H-nmr (CDC13), d = 4.76 (q, br, W 12 = 7Hz. kindly supplied samples of limonene. This study was sup=CH 2 ), 1.74 (t, J = 1 Hz, Me-10), 1.01 (d. J = 6 Hz. ported in part by the Comisión Asesora de Investigación Científica v Técnica. Me-7).

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8. G. Farges and A. Kergomard, Bull. Soc. chim. Fr. 51 (1963). 9. N. Yoshiaki, N. Seiichi, U. Hiroo and T. Chuji. Agr. biol. Chem. 38(4), 735 (1974). 10. R. Mestres, M. C. Polo and M. J. Valero, An. Quim. 75, 970 (1979). 11. Wallach, Schrader. Ann. 279, 378 (1984). 12. H. Schmidt, Chem. Ber. 83, 193 (1950). 13. Nagasawa, Rep. Osaka ind. Res. Inst. 19 (1938); 219 (1940). 14. R. G. Johnston and J. Read, J. chem. Soc. 233 (1934).