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Aromatization of cyclohexadienes by TEMPO electro-mediated oxidation: Kinetic and structural aspects
Electrochemistry Communications 7 (2005) 1445–1448 www.elsevier.com/locate/elecom
Aromatization of cyclohexadienes by TEMPO electro-mediated oxidation: Kinetic and structural aspects Tony Breton a
a,*
, Denis Liaigre a, El Mustapha Belgsir
b
Laboratoire de Catalyse en Chimie Organique, UMR 6503 CNRS, 40 Avenue du Recteur Pineau, 86022 Poitiers Cedex, France b BioCydex, 40 Avenue du Recteur Pineau, 86022 Poitiers Cedex, France Received 23 August 2005; received in revised form 29 September 2005; accepted 29 September 2005 Available online 9 November 2005
Abstract Cyclohexadienes are easily converted into the corresponding aromatics in excellent yield (>90%) in the presence of 2,2,6,6-tetramethyl-1-oxopiperidinium ion (TEMPO+). The TEMPO radical was used in catalytic amount and was electrochemically regenerated in the presence of 2,6-lutidine as a base in hydro-organic medium (AcCN/H2O 95/5). This work has been focused on the kinetic aspects. We have demonstrated that the reactivity of different cyclohexadienes is strongly dependent on the configuration of the double bonds and on the nature of the substituents. Competition between allylic functionalization and aromatization has been observed during the oxidation of 1,2-dihydro-4-phenylnaphthalene. 2005 Elsevier B.V. All rights reserved. Keywords: Electrosynthesis; Aromatization; Oxidation; Cyclohexadienes; TEMPO
1. Introduction We have previously reported the ability of oxoammonium ions 1 to oxidize activated alkenes into corresponding alkenones [1]. This paper concerns the oxidation of various cyclohexadienes using the TEMPO 2 electro-mediated system [2]. Oxoammonium ions, obtained from nitroxyl radicals are known to oxidize alcohols into the corresponding carbonyls [3,4]. The early work of Olah and Friedmann [5] has demonstrated the ability of oxoammonium ions to oxidize activated structures as isopropylbenzene in benzilic position. Recently, Koop et al. [6] have reported the aromatization of dihydropyridine derivatives with the same nitrosonium ions but also with TEMPO+, BF4 in excellent yield. Such oxidation of bi-unsaturated heterocyles could be transposed to various diethylenic rings and become a relevant issue for the aromatization of cyclohexadienes. We have investigated the effect of the configuration of diethylenic structures on their reactivity with 1. *
Corresponding author. Tel.: +33 549 453 731; fax: +33 549 453 580. E-mail address: [email protected] (T. Breton).
1388-2481/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2005.09.029
Electrolyses of various cyclohexadienes were carried out in the presence of 2,6-lutidine in hydro-organic medium. The effect of the conjugation of the double bonds, the substitution of the dienic ring and the conjugation of aryl groups has been studied in order to outline the activation parameters of the allylic position. 2. Experimental All the experiments were carried out in acetonitrile/water media using ultrapure water (Milipore system). All the cyclohexadienes and the bidistilled 2,6-lutidine (99,5%) were purchased from Sigma–Aldrich. Electrochemical experiments were carried out in a divided three electrode Pyrex cell (2 · 50 cm3). The working electrode consisted of two vitreous carbon plates having a geometric surface area of 32 cm2. A 16 cm2 platinum sheet and a silver nitrate (0.1 M)/silver electrode served as counter and reference electrodes, respectively. The electrolysis equipment was composed of a potensiostat (Wenking PGS 77) monitored by a micro computer. The quantity of electricity was directly measured by a coulometer (Wenking EVI 80).
T. Breton et al. / Electrochemistry Communications 7 (2005) 1445–1448
3. Results and discussion The reactivity of the cyclohexadienes was evaluated by cyclic voltammetry. The increase of the anodic current corresponding to the oxidation of the nitroxyl radicals into oxoammonium ions is related to the rate of the chemical step i.e., oxidation of the olefine by 1 in the bulk. It was also found that the regeneration of 1 from its reductive form (hydroxylamine) is limited by the diffusion step. The oxidation of the 1,3-cyclohexadiene, 3, and 1,4cyclohexadiene, 4, via the electrochemical system is confirmed by cyclic voltammetry. An increase of the current densities was observed during the positive sweep in the presence of the cyclohexadienes compared with the curves recorded without any substrate (Fig. 1). The activation process (0.25–0.4 V (Ag/AgNO3) is followed by a diffusion plateau. The current density reached in the presence of the conjugated isomeric 3 (7 mA cm 2) is higher than in the presence of the non conjugated isomeric 4 (6 mA cm 2). This difference of reactivity is due to the diethylenic configuration (conjugated or not conjugated system). The voltammetric studies of the TEMPO-mediated system in the presence of a-terpinene 5 and c-terpinene 6, respectively, conjugated and not conjugated, have also shown the same difference of reactivity (Table 1). In all the cases, the aromatization of the ring is complete and we have obtained benzene 7 and p-cymene 8 in excellent yield (Table 2). The consumption of 2 F is consistent with the mechanism involving an allylic hydride abstraction followed by proton elimination. Depending on the
18
(e)
16 14 12 10 -2
The vitreous carbon anodes were pre-treated in sulphuric acid solution (30%) and polished. Cyclohexadienes were electrolysed in 50 cm3 acetonitrile/water (95/5) at room temperature at 0.55 V (Ag/AgNO3). Chromatographic analyses were achieved on a Hewlett– Packard capillary gas chromatograph (5890 series) equipped with a DB-5, 95% dimethyl, 5% diphenyl polysiloxane bounded capillary column (30 m, 0.25 lm film thickness) with H2 as carrier gas. The products were also identified by GC–MS (EI: 70 eV, 1200L Varian) by comparing the mass spectra and the retention times with references. In the case of the electrolysis of 1,2-dihydro-4-phenylnaphthalene, the functionalized product was analysed by HPLC– MS (Spray voltage: 4 KV, Termo Electron) using a C18 column (Bio-Rad). NMR analyses were carried out on a Brucker 300 MHz with CDCl3 as solvent and TMS as internal standard. After electrolysis, acetonitrile was removed under vacuum, then 15 mL of HCl solution (5%) was added, the products were extracted with 3 · 20 mL of diethyl ether and dried over anhydrous magnesium sulphate. After evaporation of the solvent, the residue was purified by flashchromatography on silica-gel 60; 15–40 lm (Merck). The oxidation product of the 1,2-dihydro-4-phenylnaphthalene was eluted with light petroleum/diethyl ether (3/1) RF = 0.65.
j / mA cm
1446
(d)
8
(c) (b) (a)
6 4 2 0 -0,1
0,0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
E / V (Ag/AgNO3) Fig. 1. Cyclic voltammetry of TEMPO (0.3 mmol) recorded at 50 mV s 1 in AcCN/H2O (95/5), 0.2 M NaClO4, under stirred condition on a vitreous carbon anode containing 2 mmol of 2,6-lutidine (a), in presence of 2 mmol of 1,4-cyclohexadiene (b), 1,3-cyclohexadiene (c), c-terpinene (d) and a-terpinene (e).
Table 1 Current densities recorded in cyclic voltammetry at 50 mV s 1 in AcCN/ H2O (95/5), 0.2 M NaClO4, under stirred condition on a vitreous carbon anode containing 2 mmol of 2,6-lutidine and 2 mmol of each compound Compound
Current density (mA cm 2) at 0.7 V (Ag/AgNO3)
3 4 5 6
7 6 17 8
vinylic system, the activated hydride is located in a-position of the doubles bonds (conjugated system) or between the two double bonds (not conjugated system). We have previously shown that the allylic activation is more effective when the diethylenic system is already delocalized [7]. This result is not related to any steric effect because of the quasiplanar geometry of the molecules. Presuming the partial charges of the allylic hydrogen atoms equal, the difference of reactivity is more probably due to the mechanism involved. In the case of 1,3-systems, the stabilization of the transition state should be better than in the case of 1,4-systems because of the linear conjugation. Furthermore, the comparison of the reactivity of cyclohexadienes and terpinenes outlines the role of the substituents. Compounds 5 and 6 are, respectively, more reactive than 3 and 4 (Fig. 1). The greater reactivity of 6 when compared to 3 is due to the stabilization of the cyclohexadienyl cation formed from 6 by the alkyl substituents and this effect is more important than the effect of the conjugation of the double bonds observed between 3 and 4. The difference or reactivity could also be explained by the abstraction of a hydride on the isopropyl. Deprotonation of the conjugated carbocation formed gives a conjugated triene, which isomerizes to the more stable p-cymene through base-catalysed proton transfer.
T. Breton et al. / Electrochemistry Communications 7 (2005) 1445–1448
1447
Table 2 Aromatization of cyclohexadienes by TEMPO electromediated system at 0.55 V (Ag/AgNO3) Substrate
Yielda/%
Product
3
Faradic yield / %
Q/F (mol 1)
b
97
95
2.0
95b
98
2.0
63.96b
94
2.1
60.94b
95
2.1
85.10
92
2.3
92.5
98
2.2
7
4
7
5
8
6
8
O
9 10
φ
φ
11
φ O
12 a
13
14
Isolated. By GC.
b
In the case of 5, conjugated and substituted, the shape of the oxidation wave and the absence of diffusion step over 0.5 V (Ag/AgNO3) indicates a rapid overall reaction rate. Cyclic voltammetry on 3, 4, 5 and 6 only reveal a weak difference related to reaction rates (Fig. 2). During the electrolyses, the concentration of 1 was kept null. The maximum reaction rate reached 0.63 mmol L 1 min 1
5
30
3.75
20
2.50
10
1.25
-2
0
j / mA cm
C / mmol L
-1
40
0
0
20
40
60
80
100
120
t / min
Fig. 2. Material balance curves (—–) and current densities (- - - -) plotted during the TEMPO electro-mediated oxidation (0.3 mmol) of 2 mmol of 1,4-cyclohexadiene (–s–) and 2 mmol of 1,3-cyclohexadiene (–d–) into benzene (–h–/–j–) at 0.55 V (Ag/AgNO3) in AcCN/H2O (95:5), NaClO4 0.2 M with 2 mmol of 2,6-lutidine.
and 2 F was involved in all cases in 2 h. In fact, the overall transformation is a diffusion-controlled process. We have observed that the current intensity of all electrolyses was identical few minutes after t = 0. It is expectable that the reaction rates could be drastically increased in a filter press reactor under which, the diffusion step is limited. In order to confirm the results obtained in cyclic voltammetry, a simultaneous electrolysis of each pair of cyclohexadienes was carried out independently (Fig. 3). For 3 and 4, no difference of reaction rate was observed at the initial time (0.52 mmol L 1 min 1). Conversely, the C = f(t) curves plotted during the simultaneous oxidation of the two terpinenes exhibited very different reaction rates: 0.386 mmol L 1 min 1 for 6 and 0.711 mmol L 1 min 1 for 5. The voltammetric studies of a disubstituted cyclohexadiene as bicyclo(3,4,0)nona-3,6(1)-diene, 9, in the presence of 2 have shown a stronger reactivity than for 6 (Fig. 4). This activation can be attributed to the ortho position of the alkyl substituents but it could be also due to the fact that the ring of 9 is more planar than that of 6 and, as a result, the overlap between the sigma orbital of the pseudo axial C–H bond and the orbitals of the double bonds would be better in 9 than in 6. The electrolysis carried out during 2 h leads to the expected indane 10 but indanone 11 has also been isolated in small amount. The ratio of 10/11 was found to be dependant on the water proportion. Under dry conditions, 10 was isolated in 97% yield and 11 was not detec-ted. Conversely, when the electrolysis was carried out in acetonitrile/water (50/ 50), 11 was isolated in 5% yield.
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T. Breton et al. / Electrochemistry Communications 7 (2005) 1445–1448
30
[terpinens] / mmol L
-1
25
20
15
10
5
0 0
20
40
60
80
100
120
140
160
t / min Fig. 3. Concentrations of a-terpinene (–h–) and c-terpinene (–s–) plotted during the simultaneous electro-mediated oxidation of an equimolar mixture of a- and c-terpinene (1.5 mmol) at 0.55 V (Ag/AgNO3) in AcCN/ H2O (95:5), NaClO4 0.2 M, in the presence of 0.3 mmol of TEMPO and 8 mmol of 2,6-lutidine.
11 10 9 8
j / mA cm
-2
7 6 5 4 3 2 1 0 -1 -0,2
0,0
0,2
0,4
0,6
0,8
superposed (Fig. 4). After 6 h of electrolysis, 4-phenylnaphthalene, 13, was isolated in 80% yield and 1,2-dihydro-3oxo-4-phenylnaphthalene, 14, was obtained in 5% yield. The competition between aromatization and functionalization seems to be dependent on the electrolysis conditions and particularly on the presence of water. In dry acetonitrile, no functionalization was observed and 13 was isolated in 94% yield. In this case, the proton elimination is probably rate determining and could be explained by the particular stability of the conjugated intermediate carbocation. The secondary product comes basically from the addition of water at position 2 of the cyclohexadienyl cation formed by abstraction of the allylic hydride at position 4, followed by isomerization to an enol conjugated with the aromatic ring, and finally, by tautomerization to the more stable keto form. 4. Conclusion This study has outlined the synthetic potential of 1 in the aromatization of cyclohexadienes. Electro-mediated oxidation system TEMPO/lutidine allowed the aromatization of several cyclohexadienes in excellent yield and short reaction times. The voltammetric studies have outlined the highest reactivity of substituted cyclohexadienes compared to the non substituted ones. On the other hand, the 1,3-dienic systems were found to be more reactive than the 1,4 ones. With 5, where the dienic system is conjugated and substituted with alkyl groups, we have observed a strong synergic effect with a current density reaching more than 17 mA cm 2 at 0.7 V (Ag/AgNO3) in cyclic voltammetry. In the particular case of 12, the aromatization reaction involving proton elimination is in competition with the nucleophilic attack of hydroxyl ions. The reaction leads to a functionalized product in very poor yield. The succesive aromatization and benzylic oxidation was also observed during the electrolysis of 9.
E / V (Ag/AgNO 3) Fig. 4. Cyclic voltammetry of 0.3 mmol of TEMPO recorded at 50 mV s 1 in AcCN/H2O (95/5), 0.2 M NaClO4, under stirred condition on a vitreous carbon anode with 2 mmol of 2,6-lutidine ( ), in the presence of 2 mmol of bicyclo(3,4,0)nona-3,6(1)-diene (- - - -) and in the presence of 2 mmol of 1,2-dihydro-4-phenylnaphthalene (—–).
In the same conditions, electrolysis of commercially available 10 has been carried out in order to confirm the origin of 11. 7% of 10 were converted into 11: this result seems to prove that 10 is an intermediate product in the oxidation of 9 and that benzilic oxidation occurs. The oxidation waves recorded in the presence and in the absence of 1,2-dihydro-4-phenylnaphthalene, 12, are
References [1] T. Breton, D. Liaigre, E.M. Belgsir, Tetrahedron Lett. 46 (2005) 2487. [2] D. Liaigre, T. Breton, E.M. Belgsir, Electrochem. Commun. 7 (2005) 312. [3] A.E.J. de Nooy, A.C. Bessemer, H. van Bekkum, Synthesis (1996) 1153, and the literature cited therein. [4] K. Schnatbaum, H.J. Scha¨fer, Synthesis 5 (1999) 864. [5] G.A. Olah, N. Friedmann, J. Am. Chem. Soc. 88 (1966) 5330. [6] B. Koop, A. Straub, H.J. Scha¨fer, Tetrahedron: Asymmetr. 12 (2001) 341. [7] T. Breton, D. Liaigre, B. Kokoh, C. Lamy, E.M. Belgsir, Functionalization of activated C–H mediated by oxoammonium, in: Joint International Meeting, San Francisco (USA), September 2001.
Aromatization of cyclohexadienes by TEMPO electro-mediated oxidation: Kinetic and structural aspects Tony Breton a
a,*
, Denis Liaigre a, El Mustapha Belgsir
b
Laboratoire de Catalyse en Chimie Organique, UMR 6503 CNRS, 40 Avenue du Recteur Pineau, 86022 Poitiers Cedex, France b BioCydex, 40 Avenue du Recteur Pineau, 86022 Poitiers Cedex, France Received 23 August 2005; received in revised form 29 September 2005; accepted 29 September 2005 Available online 9 November 2005
Abstract Cyclohexadienes are easily converted into the corresponding aromatics in excellent yield (>90%) in the presence of 2,2,6,6-tetramethyl-1-oxopiperidinium ion (TEMPO+). The TEMPO radical was used in catalytic amount and was electrochemically regenerated in the presence of 2,6-lutidine as a base in hydro-organic medium (AcCN/H2O 95/5). This work has been focused on the kinetic aspects. We have demonstrated that the reactivity of different cyclohexadienes is strongly dependent on the configuration of the double bonds and on the nature of the substituents. Competition between allylic functionalization and aromatization has been observed during the oxidation of 1,2-dihydro-4-phenylnaphthalene. 2005 Elsevier B.V. All rights reserved. Keywords: Electrosynthesis; Aromatization; Oxidation; Cyclohexadienes; TEMPO
1. Introduction We have previously reported the ability of oxoammonium ions 1 to oxidize activated alkenes into corresponding alkenones [1]. This paper concerns the oxidation of various cyclohexadienes using the TEMPO 2 electro-mediated system [2]. Oxoammonium ions, obtained from nitroxyl radicals are known to oxidize alcohols into the corresponding carbonyls [3,4]. The early work of Olah and Friedmann [5] has demonstrated the ability of oxoammonium ions to oxidize activated structures as isopropylbenzene in benzilic position. Recently, Koop et al. [6] have reported the aromatization of dihydropyridine derivatives with the same nitrosonium ions but also with TEMPO+, BF4 in excellent yield. Such oxidation of bi-unsaturated heterocyles could be transposed to various diethylenic rings and become a relevant issue for the aromatization of cyclohexadienes. We have investigated the effect of the configuration of diethylenic structures on their reactivity with 1. *
Corresponding author. Tel.: +33 549 453 731; fax: +33 549 453 580. E-mail address: [email protected] (T. Breton).
1388-2481/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2005.09.029
Electrolyses of various cyclohexadienes were carried out in the presence of 2,6-lutidine in hydro-organic medium. The effect of the conjugation of the double bonds, the substitution of the dienic ring and the conjugation of aryl groups has been studied in order to outline the activation parameters of the allylic position. 2. Experimental All the experiments were carried out in acetonitrile/water media using ultrapure water (Milipore system). All the cyclohexadienes and the bidistilled 2,6-lutidine (99,5%) were purchased from Sigma–Aldrich. Electrochemical experiments were carried out in a divided three electrode Pyrex cell (2 · 50 cm3). The working electrode consisted of two vitreous carbon plates having a geometric surface area of 32 cm2. A 16 cm2 platinum sheet and a silver nitrate (0.1 M)/silver electrode served as counter and reference electrodes, respectively. The electrolysis equipment was composed of a potensiostat (Wenking PGS 77) monitored by a micro computer. The quantity of electricity was directly measured by a coulometer (Wenking EVI 80).
T. Breton et al. / Electrochemistry Communications 7 (2005) 1445–1448
3. Results and discussion The reactivity of the cyclohexadienes was evaluated by cyclic voltammetry. The increase of the anodic current corresponding to the oxidation of the nitroxyl radicals into oxoammonium ions is related to the rate of the chemical step i.e., oxidation of the olefine by 1 in the bulk. It was also found that the regeneration of 1 from its reductive form (hydroxylamine) is limited by the diffusion step. The oxidation of the 1,3-cyclohexadiene, 3, and 1,4cyclohexadiene, 4, via the electrochemical system is confirmed by cyclic voltammetry. An increase of the current densities was observed during the positive sweep in the presence of the cyclohexadienes compared with the curves recorded without any substrate (Fig. 1). The activation process (0.25–0.4 V (Ag/AgNO3) is followed by a diffusion plateau. The current density reached in the presence of the conjugated isomeric 3 (7 mA cm 2) is higher than in the presence of the non conjugated isomeric 4 (6 mA cm 2). This difference of reactivity is due to the diethylenic configuration (conjugated or not conjugated system). The voltammetric studies of the TEMPO-mediated system in the presence of a-terpinene 5 and c-terpinene 6, respectively, conjugated and not conjugated, have also shown the same difference of reactivity (Table 1). In all the cases, the aromatization of the ring is complete and we have obtained benzene 7 and p-cymene 8 in excellent yield (Table 2). The consumption of 2 F is consistent with the mechanism involving an allylic hydride abstraction followed by proton elimination. Depending on the
18
(e)
16 14 12 10 -2
The vitreous carbon anodes were pre-treated in sulphuric acid solution (30%) and polished. Cyclohexadienes were electrolysed in 50 cm3 acetonitrile/water (95/5) at room temperature at 0.55 V (Ag/AgNO3). Chromatographic analyses were achieved on a Hewlett– Packard capillary gas chromatograph (5890 series) equipped with a DB-5, 95% dimethyl, 5% diphenyl polysiloxane bounded capillary column (30 m, 0.25 lm film thickness) with H2 as carrier gas. The products were also identified by GC–MS (EI: 70 eV, 1200L Varian) by comparing the mass spectra and the retention times with references. In the case of the electrolysis of 1,2-dihydro-4-phenylnaphthalene, the functionalized product was analysed by HPLC– MS (Spray voltage: 4 KV, Termo Electron) using a C18 column (Bio-Rad). NMR analyses were carried out on a Brucker 300 MHz with CDCl3 as solvent and TMS as internal standard. After electrolysis, acetonitrile was removed under vacuum, then 15 mL of HCl solution (5%) was added, the products were extracted with 3 · 20 mL of diethyl ether and dried over anhydrous magnesium sulphate. After evaporation of the solvent, the residue was purified by flashchromatography on silica-gel 60; 15–40 lm (Merck). The oxidation product of the 1,2-dihydro-4-phenylnaphthalene was eluted with light petroleum/diethyl ether (3/1) RF = 0.65.
j / mA cm
1446
(d)
8
(c) (b) (a)
6 4 2 0 -0,1
0,0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
E / V (Ag/AgNO3) Fig. 1. Cyclic voltammetry of TEMPO (0.3 mmol) recorded at 50 mV s 1 in AcCN/H2O (95/5), 0.2 M NaClO4, under stirred condition on a vitreous carbon anode containing 2 mmol of 2,6-lutidine (a), in presence of 2 mmol of 1,4-cyclohexadiene (b), 1,3-cyclohexadiene (c), c-terpinene (d) and a-terpinene (e).
Table 1 Current densities recorded in cyclic voltammetry at 50 mV s 1 in AcCN/ H2O (95/5), 0.2 M NaClO4, under stirred condition on a vitreous carbon anode containing 2 mmol of 2,6-lutidine and 2 mmol of each compound Compound
Current density (mA cm 2) at 0.7 V (Ag/AgNO3)
3 4 5 6
7 6 17 8
vinylic system, the activated hydride is located in a-position of the doubles bonds (conjugated system) or between the two double bonds (not conjugated system). We have previously shown that the allylic activation is more effective when the diethylenic system is already delocalized [7]. This result is not related to any steric effect because of the quasiplanar geometry of the molecules. Presuming the partial charges of the allylic hydrogen atoms equal, the difference of reactivity is more probably due to the mechanism involved. In the case of 1,3-systems, the stabilization of the transition state should be better than in the case of 1,4-systems because of the linear conjugation. Furthermore, the comparison of the reactivity of cyclohexadienes and terpinenes outlines the role of the substituents. Compounds 5 and 6 are, respectively, more reactive than 3 and 4 (Fig. 1). The greater reactivity of 6 when compared to 3 is due to the stabilization of the cyclohexadienyl cation formed from 6 by the alkyl substituents and this effect is more important than the effect of the conjugation of the double bonds observed between 3 and 4. The difference or reactivity could also be explained by the abstraction of a hydride on the isopropyl. Deprotonation of the conjugated carbocation formed gives a conjugated triene, which isomerizes to the more stable p-cymene through base-catalysed proton transfer.
T. Breton et al. / Electrochemistry Communications 7 (2005) 1445–1448
1447
Table 2 Aromatization of cyclohexadienes by TEMPO electromediated system at 0.55 V (Ag/AgNO3) Substrate
Yielda/%
Product
3
Faradic yield / %
Q/F (mol 1)
b
97
95
2.0
95b
98
2.0
63.96b
94
2.1
60.94b
95
2.1
85.10
92
2.3
92.5
98
2.2
7
4
7
5
8
6
8
O
9 10
φ
φ
11
φ O
12 a
13
14
Isolated. By GC.
b
In the case of 5, conjugated and substituted, the shape of the oxidation wave and the absence of diffusion step over 0.5 V (Ag/AgNO3) indicates a rapid overall reaction rate. Cyclic voltammetry on 3, 4, 5 and 6 only reveal a weak difference related to reaction rates (Fig. 2). During the electrolyses, the concentration of 1 was kept null. The maximum reaction rate reached 0.63 mmol L 1 min 1
5
30
3.75
20
2.50
10
1.25
-2
0
j / mA cm
C / mmol L
-1
40
0
0
20
40
60
80
100
120
t / min
Fig. 2. Material balance curves (—–) and current densities (- - - -) plotted during the TEMPO electro-mediated oxidation (0.3 mmol) of 2 mmol of 1,4-cyclohexadiene (–s–) and 2 mmol of 1,3-cyclohexadiene (–d–) into benzene (–h–/–j–) at 0.55 V (Ag/AgNO3) in AcCN/H2O (95:5), NaClO4 0.2 M with 2 mmol of 2,6-lutidine.
and 2 F was involved in all cases in 2 h. In fact, the overall transformation is a diffusion-controlled process. We have observed that the current intensity of all electrolyses was identical few minutes after t = 0. It is expectable that the reaction rates could be drastically increased in a filter press reactor under which, the diffusion step is limited. In order to confirm the results obtained in cyclic voltammetry, a simultaneous electrolysis of each pair of cyclohexadienes was carried out independently (Fig. 3). For 3 and 4, no difference of reaction rate was observed at the initial time (0.52 mmol L 1 min 1). Conversely, the C = f(t) curves plotted during the simultaneous oxidation of the two terpinenes exhibited very different reaction rates: 0.386 mmol L 1 min 1 for 6 and 0.711 mmol L 1 min 1 for 5. The voltammetric studies of a disubstituted cyclohexadiene as bicyclo(3,4,0)nona-3,6(1)-diene, 9, in the presence of 2 have shown a stronger reactivity than for 6 (Fig. 4). This activation can be attributed to the ortho position of the alkyl substituents but it could be also due to the fact that the ring of 9 is more planar than that of 6 and, as a result, the overlap between the sigma orbital of the pseudo axial C–H bond and the orbitals of the double bonds would be better in 9 than in 6. The electrolysis carried out during 2 h leads to the expected indane 10 but indanone 11 has also been isolated in small amount. The ratio of 10/11 was found to be dependant on the water proportion. Under dry conditions, 10 was isolated in 97% yield and 11 was not detec-ted. Conversely, when the electrolysis was carried out in acetonitrile/water (50/ 50), 11 was isolated in 5% yield.
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T. Breton et al. / Electrochemistry Communications 7 (2005) 1445–1448
30
[terpinens] / mmol L
-1
25
20
15
10
5
0 0
20
40
60
80
100
120
140
160
t / min Fig. 3. Concentrations of a-terpinene (–h–) and c-terpinene (–s–) plotted during the simultaneous electro-mediated oxidation of an equimolar mixture of a- and c-terpinene (1.5 mmol) at 0.55 V (Ag/AgNO3) in AcCN/ H2O (95:5), NaClO4 0.2 M, in the presence of 0.3 mmol of TEMPO and 8 mmol of 2,6-lutidine.
11 10 9 8
j / mA cm
-2
7 6 5 4 3 2 1 0 -1 -0,2
0,0
0,2
0,4
0,6
0,8
superposed (Fig. 4). After 6 h of electrolysis, 4-phenylnaphthalene, 13, was isolated in 80% yield and 1,2-dihydro-3oxo-4-phenylnaphthalene, 14, was obtained in 5% yield. The competition between aromatization and functionalization seems to be dependent on the electrolysis conditions and particularly on the presence of water. In dry acetonitrile, no functionalization was observed and 13 was isolated in 94% yield. In this case, the proton elimination is probably rate determining and could be explained by the particular stability of the conjugated intermediate carbocation. The secondary product comes basically from the addition of water at position 2 of the cyclohexadienyl cation formed by abstraction of the allylic hydride at position 4, followed by isomerization to an enol conjugated with the aromatic ring, and finally, by tautomerization to the more stable keto form. 4. Conclusion This study has outlined the synthetic potential of 1 in the aromatization of cyclohexadienes. Electro-mediated oxidation system TEMPO/lutidine allowed the aromatization of several cyclohexadienes in excellent yield and short reaction times. The voltammetric studies have outlined the highest reactivity of substituted cyclohexadienes compared to the non substituted ones. On the other hand, the 1,3-dienic systems were found to be more reactive than the 1,4 ones. With 5, where the dienic system is conjugated and substituted with alkyl groups, we have observed a strong synergic effect with a current density reaching more than 17 mA cm 2 at 0.7 V (Ag/AgNO3) in cyclic voltammetry. In the particular case of 12, the aromatization reaction involving proton elimination is in competition with the nucleophilic attack of hydroxyl ions. The reaction leads to a functionalized product in very poor yield. The succesive aromatization and benzylic oxidation was also observed during the electrolysis of 9.
E / V (Ag/AgNO 3) Fig. 4. Cyclic voltammetry of 0.3 mmol of TEMPO recorded at 50 mV s 1 in AcCN/H2O (95/5), 0.2 M NaClO4, under stirred condition on a vitreous carbon anode with 2 mmol of 2,6-lutidine ( ), in the presence of 2 mmol of bicyclo(3,4,0)nona-3,6(1)-diene (- - - -) and in the presence of 2 mmol of 1,2-dihydro-4-phenylnaphthalene (—–).
In the same conditions, electrolysis of commercially available 10 has been carried out in order to confirm the origin of 11. 7% of 10 were converted into 11: this result seems to prove that 10 is an intermediate product in the oxidation of 9 and that benzilic oxidation occurs. The oxidation waves recorded in the presence and in the absence of 1,2-dihydro-4-phenylnaphthalene, 12, are
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