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Anodic oxidation dialkyl diselenides in aprotic media
J. Elecrroanal Chem., 199 (1986) 211-218 Elsewer Sequoia S.A.. Lausanne Prmted
211 I” The Netherlands
Short communication ANODIC OXIDATION
CHANTAL
DEGRAND
OF DIALKYL DISELENIDES
and MOHAMMED
Laboraim-e de Synlhtk ef d’Electrosynthbe Scrences, 6 bd Gabriel, -71100 Dqon (France) (Received
11 th October,
IN APROTIC MEDIA
NOUR Organom&zlhques
asso&- nu CNRS
(U.A
33). Furulrk des
1985)
INTRODUCTION
Basic organoselenium compounds such as diselenides have been widely used fat the synthesis of more complex organoselenium derivatives [l-7]. Thus, novel C-Se bonds have been built from electrophilic Se reagents. In the field of electrochemistry, the anodic oxidation of diphenyl diselenide which is commercially available, has been shown to generate a cation PllSe’ which can be trapped by aikenes or aikynes [8-141. In acetonitrile, acetamidoselenation of double and triple bonds was achieved in high yields [8,9,14]. We describe below the anodic behaviour of the dialkyl diselenides la-d in aprotic media. These compounds can be prepared easily by the ultrasound-induced electrochemical reduction of grey Se powder to Se;‘- [15], followed by addition of an alkyl halide [16]. It is shown that the anodic oxidation of la and lb leads to the corresponding RSe+ cation which can be trapped in acetronitrile in the presence of cyclohexene or KCN, and isolated as acetamidoselenides and alkyl selenocyanates, respectively. In the case of lc and Id, a cleavage of the C-Se bonds occurs preferentially.
i
a : R = PhCH,-
/b:R=NC-@-(.H,Ic:R=
Ph ,CH-
RSeSeRl 1
I
d:R=FlCH-=
YCy H
0022-0728/86/$03.50
IQ 1986 Elsevier Sequcna S.A
212 EXPERIMENTAL
Diselenides la [17], lb [16], lc [18] and Id [16] were prepared by the ultrasoundinduced electrochemical reduction of grey Se powder to Se;[15], followed by addition of benzyl chloride, 4-cyanobenzyl bromide, benzhydryl chloride and 9bromofluorene, respectively. Halogenoalkanes and diphenyl diselenide were purchased from Fluka. Solvents (DMF and CH,CN) of analytical grade were dried carefully on neutral alumina. Elemental analyses were performed by Service Central d’Analyses, C.N.R.S., Lyon. Spectra were recorded by means of the following instruments: infrared, Perkin-Elmer 580 B; ‘H NMR, Jeol FX 100; mass spectra. Finnigan 3002. A glassy carbon or Pt disc electrode was used as the working electrode in voltammetry. Voltammograms at a rotating disc electrode (RDE) and cyclic voltammograms at a stationary disc electrode (SDE) were obtained with a Tacussel UAP 4 unit and a GSTP function generator and recorded on an Ifelec 2025-CXY recorder. An Amel 552 potentiostat and a Tacussel IG 5N integrator were used in large-scale electrolysis. All the potentials referred to the aqueous saturated calomel electrode (SCE). Large-scale electrolyses were carried out in an H-type cell filled with MeCN containing 0.1 M Bu,NBF, or Bu,NPF,. The anode was a glassy carbon cloth or a Pt grid and the cathode a Pt grid. The anolyte was stirred by sonication or mechanically and was deaerated with argon. Its volume was 60 ml. Unless otherwise stated, 1 mmol of diselenide and 5 equivalents of cyclohexene or potassium cyanide were initially present in the anolyte. In most of the experiments, the electrolysis was stopped after passage of 2 coulomb equivalents of charge per mole of diselenide. The anolyte was diluted with water and the electrolysis products were extracted with diethyl ether. After the solution was dried, the ether was removed and a ‘H NMR spectrum of the crude product was recorded. For the diselenides la and lb, the experimental conditions of electrolysis are summarized in Table 2. The crystalline compounds 2a, b, 3a m.p. 72°C (diethyl ether + hexane; ref. 17: m.p. 71.5”C) and 3b were isolated by column chromatography on silica gel with an acetone + hexane mixture as eluant and/or by recrystallization. 2-Acetamido-I-benzylselenocyclohexane (2a) White powder; m.p. 128°C (acetone + hexane); ‘H NMR 6 0.80-2.40 (nz, 8H, 4CH,), 1.90 (s, 3H, CH,), 2.58 (m, lH, CHN), 3.80 (m. lH, CHSe), 3.81 (s. 2H, CH,Ph), 5.22 (m, lH, NH), 7.29 (s, 5H, phenyl H); IR (KBr) 3306 (NH) and 1644 (C=O) cm-‘; MS m/e 311 (7.5, M+), 309 (3.8, M+), 252 (23.1, M+-CH,CONH,), 250 (11.1, M+-CH&ONH,), 91 (100, PhCH:). Anal. Calcd.: C, 58.06; H, 6.77; N, 4.51; Se. 25.48. Found: C, 58.16; H, 6.15; N, 4.56; Se, 25.47. 2-Acetamido-I-(p-cyanobenzyl)selenoqclohexane (26) White powder; m.p. 166°C (CH,Cl, + hexane); ‘H NMR S 1.00-2.36 (m, 8H. 4CH,), 1.97 (s, 3H, CH,). 2.68 (m. lH, CHN), 3.84 (m, lH, CHSe), 3.84 (s, 2H.
313
CH,Ph). 5.39 (m, lH, NH), 7.41 (d, J = 8.0 Hz, 2H, ArH), 7.57 (d, J = 8.0 Hz, 2H, ArH); IR (KBr) 3286 (NH), 2231 (C=N) and 1631 (C=O) cm-‘: MS m/e 336 (10.8. M+), 334 (5.1, M+), 277 (51.6, M+-CH,CONH,), 275 (25.6. M+-CHJONH,), 220 (82.1, M+-CH,PhCN), 218 (42.1, M+-CH?PhCN), 116 (100, +CH2PhCN). Anal. Calcd.: C, 57.31; H, 5.97; N. 8.36: Se, 23.58. Found: C, 57.41; H, 6.00; N. 8.23; Se, 23.25.
p-Cyanobenz_vlselenocyanate (3b) White powder; m.p. 137°C (CH,Cl, + hexane): ‘H NMR 6 4.25 (s, 2H, CH?), 7.45 (d, J = 8 Hz, 2H. ArH), 7.68 (d, J = 8 Hz, 2H, ArH); IR (KBr) 2224 (C-EN) and 2140 (SeeeN) cm:‘; MS m/e 222 (3.4, M+), 220 (1.7, M+), 116 (100, +CH2PhCN). Anal. Calcd.: C, 48.64; H, 2.70; N, 12.61; Se, 36.03. Found: C. 48.86; H, 2.57; N, 12.00; Se, 36.50. Large-scale electrolyses of dibenzhydryl diselenide (lc) at a Pt anode. in the presence of cyclohexene or KCN, could not be achieved. The faradaic current fell rapidly as the Pt grid became red, indicating a poisoning and a passivation of the electrode surface by a film containing some red selenium. The electrolysis of Id in the presence of cyclohexene was performed at a carbon anode. The applied potential was moved from 0.9 to 1.2 V. A mixture of at least nine compounds was obtained, as observed by thin layer chromatography. Starting material (Id), fluorenone and fluorenol were identified as minor compounds, together with the amide 4, m.p. 250°C (ref. 19: m.p. 260-261°C). which was isolated by column chromatography. Its mass spectrum and elemental analysis revealed that the amide was soiled by a trace of difluorenyl selenide ( < 1%).
RESULTS
Linear sweep uoltammet~ Figure 1 shows voltammograms obtained for la in acetonitrile at a glassy carbon RDE. An anodic wave with E,,? = 1.20 is observed, the badly defined limiting current of which is slightly less than the corresponding limiting current of the reduction wave. At the cathode, the overall process has been shown to correspond to a two-electron process, with formation of selenolate [16]. Addition of cyclohexene in excess doubles the limiting anodic current (Fig. 1). In linear sweep voltammetry at a SDE, an irreversible peak corresponds to the anodic wave (Fig. 2a). The values E,,, of the anodic peak potentials in DMF and MeCN of the substrates la-d are listed in Table 1. Depending on the nature of the electrode, a cathodic peak B may appear in cyclic voltammetry (Fig. 2b) which, however, is not related to a simple redox process since the peak separation always exceeds 0.8 V. Peak B is not observed at a glassy carbon anode whereas at a Pt anode, it tends to increase during repetitive cycling (compare Figs. 2a and 2b).
240-
/
I Fig. the the 105
and in 1. Voltammograms m MeCN at a glassy carbon RDE, of la (1 mM) in the absence ( -) presence (. - - ) of cyclohexene in excess (10 mM and 20 mM). The dashed line corresponds to voltammogram acquired m the presence of the supportmg electrolyte alone. The angular velocity IS s-‘.
E/V
1
0
1
I ’ PA
(a)
(b)
Fig. 2 Repetitive voltammograms at a SDE of la (1 mM): (a) in MeCN at a glassy carbon electrode. (b) m DMF at a Pt electrode. Scans no. 1 (+), 3 (-+#-). 10 and more (+). Scan rate 200 mV s-’ The dashed hne corresponds to the cychc voltammogram of the supporting electrolyte alone.
215 TABLE
1
Values of the anodlc
peak potential
Substrate
la-d, measured
at a glassy carbon
anode
WV
la lb lc Id ’ Prepeak
E,,, of the diselenides
DMF
MeCN
1.35 1.40 1.45 1.35
1.37 1.50 1.50 d 1.44
at 1.36 V.
Large-scale
electroijses
Preparative electrolyses of la and lb in MeCN, in the presence of cyclohexene or KCN in excess (5 equivalents), are reported in Table 2. In most of the cases, the electrolyses correspond to the passage of 2 coulomb equivalents of charge per mole of diselenide. The acetamides 2a, b and the selenocyanates 3a [17] and 3b are isolated in the presence of cyclohexene and potassium cyanide, respectively. Large-scale electrolysis of dibenzhydryl diselenide (lc) in the presence of cyclohexene or KCN is accompanied by a passivation of the Pt anode surface by a film containing red selenium. The faradaic current drops very rapidly. Preparative electrolysis of difluorenyl diselenide (ld) in the presence of cyclohexene leads to a cleavage of the C-Se bonds since fluorenone, fluorenol and N-fluorenyl acetamide (4) [19] are identified among several minor electrolysis products.
ArCH,SeCN
qp
\ H
NHCCH, J
The compounds 2a,b can be oxidized anodically. In cyclic voltammetry at a glassy carbon SDE, the amide 2a exhibits an irreversible peak at 1.42 V which is badly defined, a shoulder being observed at 1.24 V. DISCUSSION
The anodic behaviour of the diselenides la and lb is quite similar to that of diphenyl diselenide [8,9,14]. The results indicate that the process occurring at the
C,H,o
la lb lb PhSeSePh’ la lb g
C,H,, C,H,, C,H,, KCN KCN
added compound
RSeSeR
From ‘H NMR. On the basts of 2 e- consumed. Based on RSeSeR consumed. Stirring by sonication. 2.9 mmol. ’ 77% m ref. 8. a 1.27 mmol.
a ’ ’ ’ ’
Run no.
Pt Pt C” Pt Pt d Pt d
1.0-l 6 1 .o-2.2 0.9-1.0 1.3-1.4 1.0-I .4 0.9-1.3
E/V
1a.b and diphenyl
electrode
Electrochemtcal oxtdation of the drselenides 1 mmol of dtselemde was initrally present
TABLE 2
2.4 2.0 2.0 5.8 2.0 2.0 traces 0.25 0.06 0.98 0.43 0.37
0.04 0.08
RSeR /mm01
2b 2b 2c 3a 3b
(0.96) (1.20) (2.90) (0.55) (0.96)
43 48 60 50 27.5 48
2a (1.03)
current
effictenty
a’
stated.
52 64 65 77 r 48 54.5
/%
yield L
or KCN. Unless otherwise
/%
product
of cyclohexene
/mmol
additton
Isolated seienide derwattves RSeSeR /mmol
QF- ’ /mm01
of 5 equivalents
in the presence
dtselenide
217
anode is an oxidative cleavage with formation of RSef (R = benzyl and 4-cyanobenzyl) in an overall two-electron process (ECE mechanism [8,9,14]). In MeCN, addition of cyclohexene or KCN allows the formation of 2a,b or 3a,b. The yields of 2a,b based on RSeSeR consumed are reasonable, although slightly less than in the case of 2c (in Table 2, compare runs l-3 with run 4 and ref. 8). They suggest, as already shown for 2c [lo], a further oxidation of the amides 2a,b to selenoxide derivatives, in an overall two-electron process. This would also explain the moderate values of the current efficiency. Due to the known fragility of the u C-Se bonds [l-7], a competitive oxidative cleavage of the C-Se bonds contained in la and lb cannot be excluded. Such a process occurs preferentially in the case of lc and Id. Indeed, red selenium is deposited on the anode in the case of lc, whereas the fluorenyl cation is trapped by a molecule of solvent, leading to 4 in the case of Id. The effect of water content on the anodic oxidation process of PhSeSePh was studied in MeCN [14]. The formation of phenylseleninic acid PhSeO,H in an overall six-electron process was postulated. Similarly. anodic oxidation of la,b in the presence of residual water would lead to the corresponding seleninic acids and would account for the moderate values of the current efficiency in Table 2. 2 RSe+ RSeSeR - 2 e-r+
2Rf+2Se
Whether or not the reduction of RSe+ occurs at the potentials of peak B in cyclic voltammetry (Fig. 2b) has not yet been established. The fact that peak B appears only at a Pt electrode is puzzling and merits further studies. In the case of PhSeSePh, a similar peak was observed at a Pt electrode in MeCN and attributed to the reduction of PhSe+ [14]. By modulated specular reflectance spectroscopy, the rate constant of the decay of PhSe+ could be measured [9]. Its value was 4.9 s-i in MeCN. ACKNOWLEDGEMENT
We are grateful to Mrs. Fouquet for her technical assistance. The financial supports of the Centre National de la Recherche Scientifique and of the Agence Francaise pour la Maltrise de 1’Energie (PIRSEM grant no. 06931) are greatly appreciated. REFERENCES
1 D.L. Klayman and W.H.H. Giinther. Organic Selenium Compounds: Their Chermstry and Biology, Wiley, New York, 1973. 2 D.L.J. Clive, Tetrahedron, 34 (1982) 1049. 3 H.J. Reich in W.S. Trahanovsky (Ed.), Oxidation in Organic Chemistry,.Academic Press, New York, 1978. part C, Ch. 1. 4 H.J. Reich, Act. Chem. Res., 12 (1979) 22. 5 D. Liotta, Act. Chem. Res., 17 (1984) 28.
218 6 A. Krief and L. Hevesi. Janssen Chim. Acta. 2 (1984) 3. 7 P.D. Magnus in D.H.R. Barton and W.D. OIlis (Eds.), Comprehenstve Organic Chemistry. Vol. 3, Pergamon Press, New York, 1980, Ch. 12. 8 A. Bewick. D.E. Coe, G.B. Fuller and J.M. Mellor. Tetrahedron Lett., 21 (1980) 3827. 9 A Bewick. D.E. Coe, M. Libert and J.M. Mellor. J. Electroanal. Chem.. 144 (1983) 235 10 S. Torii. K. Uneyama and M. Ono. Tetrahedron Lett.. 21 (1980) 2653. 2741. 11 S. Torii. K. Uneyama. M. Ono and T. Bannon. J. Am. Chem. Sot.. 103 (1981) 4606. 12 K. Uneyama. K. Takano and S. Toru, Tetrahedron Lett.. 23 (1982) 1161. 13 K Uneyama, M. Ono and S. Torii, Phosphorus Sulfur, 16 (1983) 35; Chem. Abstr.. 100 (1984) 22 285g. 14 A. Kunai. J. Harada, J. Izumi, H. Tachihara and K. Sasaki, Electrocmm. Acta. 28 (1983) 1361. 15 B Gautheron, G. Tainturier and C. Degrand. J. Am. Chem. Sot., 107 (1985) 5579. 16 C. Degrand and M. Nom. J. Electroanal. Chem., 190 (1985) 213. 17 C.L. Jackson. Justus Liebigs Ann. Chem.. 179 (1975) 1. 18 D.S. Margohs and R.W Pittman, J Chem. Sot.. (1957) 799. 19 J. Schmrdt and H. Sthtzel. Ber. Dtsch. Chem. Ges.. 41 (1908) 1243.
211 I” The Netherlands
Short communication ANODIC OXIDATION
CHANTAL
DEGRAND
OF DIALKYL DISELENIDES
and MOHAMMED
Laboraim-e de Synlhtk ef d’Electrosynthbe Scrences, 6 bd Gabriel, -71100 Dqon (France) (Received
11 th October,
IN APROTIC MEDIA
NOUR Organom&zlhques
asso&- nu CNRS
(U.A
33). Furulrk des
1985)
INTRODUCTION
Basic organoselenium compounds such as diselenides have been widely used fat the synthesis of more complex organoselenium derivatives [l-7]. Thus, novel C-Se bonds have been built from electrophilic Se reagents. In the field of electrochemistry, the anodic oxidation of diphenyl diselenide which is commercially available, has been shown to generate a cation PllSe’ which can be trapped by aikenes or aikynes [8-141. In acetonitrile, acetamidoselenation of double and triple bonds was achieved in high yields [8,9,14]. We describe below the anodic behaviour of the dialkyl diselenides la-d in aprotic media. These compounds can be prepared easily by the ultrasound-induced electrochemical reduction of grey Se powder to Se;‘- [15], followed by addition of an alkyl halide [16]. It is shown that the anodic oxidation of la and lb leads to the corresponding RSe+ cation which can be trapped in acetronitrile in the presence of cyclohexene or KCN, and isolated as acetamidoselenides and alkyl selenocyanates, respectively. In the case of lc and Id, a cleavage of the C-Se bonds occurs preferentially.
i
a : R = PhCH,-
/b:R=NC-@-(.H,Ic:R=
Ph ,CH-
RSeSeRl 1
I
d:R=FlCH-=
YCy H
0022-0728/86/$03.50
IQ 1986 Elsevier Sequcna S.A
212 EXPERIMENTAL
Diselenides la [17], lb [16], lc [18] and Id [16] were prepared by the ultrasoundinduced electrochemical reduction of grey Se powder to Se;[15], followed by addition of benzyl chloride, 4-cyanobenzyl bromide, benzhydryl chloride and 9bromofluorene, respectively. Halogenoalkanes and diphenyl diselenide were purchased from Fluka. Solvents (DMF and CH,CN) of analytical grade were dried carefully on neutral alumina. Elemental analyses were performed by Service Central d’Analyses, C.N.R.S., Lyon. Spectra were recorded by means of the following instruments: infrared, Perkin-Elmer 580 B; ‘H NMR, Jeol FX 100; mass spectra. Finnigan 3002. A glassy carbon or Pt disc electrode was used as the working electrode in voltammetry. Voltammograms at a rotating disc electrode (RDE) and cyclic voltammograms at a stationary disc electrode (SDE) were obtained with a Tacussel UAP 4 unit and a GSTP function generator and recorded on an Ifelec 2025-CXY recorder. An Amel 552 potentiostat and a Tacussel IG 5N integrator were used in large-scale electrolysis. All the potentials referred to the aqueous saturated calomel electrode (SCE). Large-scale electrolyses were carried out in an H-type cell filled with MeCN containing 0.1 M Bu,NBF, or Bu,NPF,. The anode was a glassy carbon cloth or a Pt grid and the cathode a Pt grid. The anolyte was stirred by sonication or mechanically and was deaerated with argon. Its volume was 60 ml. Unless otherwise stated, 1 mmol of diselenide and 5 equivalents of cyclohexene or potassium cyanide were initially present in the anolyte. In most of the experiments, the electrolysis was stopped after passage of 2 coulomb equivalents of charge per mole of diselenide. The anolyte was diluted with water and the electrolysis products were extracted with diethyl ether. After the solution was dried, the ether was removed and a ‘H NMR spectrum of the crude product was recorded. For the diselenides la and lb, the experimental conditions of electrolysis are summarized in Table 2. The crystalline compounds 2a, b, 3a m.p. 72°C (diethyl ether + hexane; ref. 17: m.p. 71.5”C) and 3b were isolated by column chromatography on silica gel with an acetone + hexane mixture as eluant and/or by recrystallization. 2-Acetamido-I-benzylselenocyclohexane (2a) White powder; m.p. 128°C (acetone + hexane); ‘H NMR 6 0.80-2.40 (nz, 8H, 4CH,), 1.90 (s, 3H, CH,), 2.58 (m, lH, CHN), 3.80 (m. lH, CHSe), 3.81 (s. 2H, CH,Ph), 5.22 (m, lH, NH), 7.29 (s, 5H, phenyl H); IR (KBr) 3306 (NH) and 1644 (C=O) cm-‘; MS m/e 311 (7.5, M+), 309 (3.8, M+), 252 (23.1, M+-CH,CONH,), 250 (11.1, M+-CH&ONH,), 91 (100, PhCH:). Anal. Calcd.: C, 58.06; H, 6.77; N, 4.51; Se. 25.48. Found: C, 58.16; H, 6.15; N, 4.56; Se, 25.47. 2-Acetamido-I-(p-cyanobenzyl)selenoqclohexane (26) White powder; m.p. 166°C (CH,Cl, + hexane); ‘H NMR S 1.00-2.36 (m, 8H. 4CH,), 1.97 (s, 3H, CH,). 2.68 (m. lH, CHN), 3.84 (m, lH, CHSe), 3.84 (s, 2H.
313
CH,Ph). 5.39 (m, lH, NH), 7.41 (d, J = 8.0 Hz, 2H, ArH), 7.57 (d, J = 8.0 Hz, 2H, ArH); IR (KBr) 3286 (NH), 2231 (C=N) and 1631 (C=O) cm-‘: MS m/e 336 (10.8. M+), 334 (5.1, M+), 277 (51.6, M+-CH,CONH,), 275 (25.6. M+-CHJONH,), 220 (82.1, M+-CH,PhCN), 218 (42.1, M+-CH?PhCN), 116 (100, +CH2PhCN). Anal. Calcd.: C, 57.31; H, 5.97; N. 8.36: Se, 23.58. Found: C, 57.41; H, 6.00; N. 8.23; Se, 23.25.
p-Cyanobenz_vlselenocyanate (3b) White powder; m.p. 137°C (CH,Cl, + hexane): ‘H NMR 6 4.25 (s, 2H, CH?), 7.45 (d, J = 8 Hz, 2H. ArH), 7.68 (d, J = 8 Hz, 2H, ArH); IR (KBr) 2224 (C-EN) and 2140 (SeeeN) cm:‘; MS m/e 222 (3.4, M+), 220 (1.7, M+), 116 (100, +CH2PhCN). Anal. Calcd.: C, 48.64; H, 2.70; N, 12.61; Se, 36.03. Found: C. 48.86; H, 2.57; N, 12.00; Se, 36.50. Large-scale electrolyses of dibenzhydryl diselenide (lc) at a Pt anode. in the presence of cyclohexene or KCN, could not be achieved. The faradaic current fell rapidly as the Pt grid became red, indicating a poisoning and a passivation of the electrode surface by a film containing some red selenium. The electrolysis of Id in the presence of cyclohexene was performed at a carbon anode. The applied potential was moved from 0.9 to 1.2 V. A mixture of at least nine compounds was obtained, as observed by thin layer chromatography. Starting material (Id), fluorenone and fluorenol were identified as minor compounds, together with the amide 4, m.p. 250°C (ref. 19: m.p. 260-261°C). which was isolated by column chromatography. Its mass spectrum and elemental analysis revealed that the amide was soiled by a trace of difluorenyl selenide ( < 1%).
RESULTS
Linear sweep uoltammet~ Figure 1 shows voltammograms obtained for la in acetonitrile at a glassy carbon RDE. An anodic wave with E,,? = 1.20 is observed, the badly defined limiting current of which is slightly less than the corresponding limiting current of the reduction wave. At the cathode, the overall process has been shown to correspond to a two-electron process, with formation of selenolate [16]. Addition of cyclohexene in excess doubles the limiting anodic current (Fig. 1). In linear sweep voltammetry at a SDE, an irreversible peak corresponds to the anodic wave (Fig. 2a). The values E,,, of the anodic peak potentials in DMF and MeCN of the substrates la-d are listed in Table 1. Depending on the nature of the electrode, a cathodic peak B may appear in cyclic voltammetry (Fig. 2b) which, however, is not related to a simple redox process since the peak separation always exceeds 0.8 V. Peak B is not observed at a glassy carbon anode whereas at a Pt anode, it tends to increase during repetitive cycling (compare Figs. 2a and 2b).
240-
/
I Fig. the the 105
and in 1. Voltammograms m MeCN at a glassy carbon RDE, of la (1 mM) in the absence ( -) presence (. - - ) of cyclohexene in excess (10 mM and 20 mM). The dashed line corresponds to voltammogram acquired m the presence of the supportmg electrolyte alone. The angular velocity IS s-‘.
E/V
1
0
1
I ’ PA
(a)
(b)
Fig. 2 Repetitive voltammograms at a SDE of la (1 mM): (a) in MeCN at a glassy carbon electrode. (b) m DMF at a Pt electrode. Scans no. 1 (+), 3 (-+#-). 10 and more (+). Scan rate 200 mV s-’ The dashed hne corresponds to the cychc voltammogram of the supporting electrolyte alone.
215 TABLE
1
Values of the anodlc
peak potential
Substrate
la-d, measured
at a glassy carbon
anode
WV
la lb lc Id ’ Prepeak
E,,, of the diselenides
DMF
MeCN
1.35 1.40 1.45 1.35
1.37 1.50 1.50 d 1.44
at 1.36 V.
Large-scale
electroijses
Preparative electrolyses of la and lb in MeCN, in the presence of cyclohexene or KCN in excess (5 equivalents), are reported in Table 2. In most of the cases, the electrolyses correspond to the passage of 2 coulomb equivalents of charge per mole of diselenide. The acetamides 2a, b and the selenocyanates 3a [17] and 3b are isolated in the presence of cyclohexene and potassium cyanide, respectively. Large-scale electrolysis of dibenzhydryl diselenide (lc) in the presence of cyclohexene or KCN is accompanied by a passivation of the Pt anode surface by a film containing red selenium. The faradaic current drops very rapidly. Preparative electrolysis of difluorenyl diselenide (ld) in the presence of cyclohexene leads to a cleavage of the C-Se bonds since fluorenone, fluorenol and N-fluorenyl acetamide (4) [19] are identified among several minor electrolysis products.
ArCH,SeCN
qp
\ H
NHCCH, J
The compounds 2a,b can be oxidized anodically. In cyclic voltammetry at a glassy carbon SDE, the amide 2a exhibits an irreversible peak at 1.42 V which is badly defined, a shoulder being observed at 1.24 V. DISCUSSION
The anodic behaviour of the diselenides la and lb is quite similar to that of diphenyl diselenide [8,9,14]. The results indicate that the process occurring at the
C,H,o
la lb lb PhSeSePh’ la lb g
C,H,, C,H,, C,H,, KCN KCN
added compound
RSeSeR
From ‘H NMR. On the basts of 2 e- consumed. Based on RSeSeR consumed. Stirring by sonication. 2.9 mmol. ’ 77% m ref. 8. a 1.27 mmol.
a ’ ’ ’ ’
Run no.
Pt Pt C” Pt Pt d Pt d
1.0-l 6 1 .o-2.2 0.9-1.0 1.3-1.4 1.0-I .4 0.9-1.3
E/V
1a.b and diphenyl
electrode
Electrochemtcal oxtdation of the drselenides 1 mmol of dtselemde was initrally present
TABLE 2
2.4 2.0 2.0 5.8 2.0 2.0 traces 0.25 0.06 0.98 0.43 0.37
0.04 0.08
RSeR /mm01
2b 2b 2c 3a 3b
(0.96) (1.20) (2.90) (0.55) (0.96)
43 48 60 50 27.5 48
2a (1.03)
current
effictenty
a’
stated.
52 64 65 77 r 48 54.5
/%
yield L
or KCN. Unless otherwise
/%
product
of cyclohexene
/mmol
additton
Isolated seienide derwattves RSeSeR /mmol
QF- ’ /mm01
of 5 equivalents
in the presence
dtselenide
217
anode is an oxidative cleavage with formation of RSef (R = benzyl and 4-cyanobenzyl) in an overall two-electron process (ECE mechanism [8,9,14]). In MeCN, addition of cyclohexene or KCN allows the formation of 2a,b or 3a,b. The yields of 2a,b based on RSeSeR consumed are reasonable, although slightly less than in the case of 2c (in Table 2, compare runs l-3 with run 4 and ref. 8). They suggest, as already shown for 2c [lo], a further oxidation of the amides 2a,b to selenoxide derivatives, in an overall two-electron process. This would also explain the moderate values of the current efficiency. Due to the known fragility of the u C-Se bonds [l-7], a competitive oxidative cleavage of the C-Se bonds contained in la and lb cannot be excluded. Such a process occurs preferentially in the case of lc and Id. Indeed, red selenium is deposited on the anode in the case of lc, whereas the fluorenyl cation is trapped by a molecule of solvent, leading to 4 in the case of Id. The effect of water content on the anodic oxidation process of PhSeSePh was studied in MeCN [14]. The formation of phenylseleninic acid PhSeO,H in an overall six-electron process was postulated. Similarly. anodic oxidation of la,b in the presence of residual water would lead to the corresponding seleninic acids and would account for the moderate values of the current efficiency in Table 2. 2 RSe+ RSeSeR - 2 e-r+
2Rf+2Se
Whether or not the reduction of RSe+ occurs at the potentials of peak B in cyclic voltammetry (Fig. 2b) has not yet been established. The fact that peak B appears only at a Pt electrode is puzzling and merits further studies. In the case of PhSeSePh, a similar peak was observed at a Pt electrode in MeCN and attributed to the reduction of PhSe+ [14]. By modulated specular reflectance spectroscopy, the rate constant of the decay of PhSe+ could be measured [9]. Its value was 4.9 s-i in MeCN. ACKNOWLEDGEMENT
We are grateful to Mrs. Fouquet for her technical assistance. The financial supports of the Centre National de la Recherche Scientifique and of the Agence Francaise pour la Maltrise de 1’Energie (PIRSEM grant no. 06931) are greatly appreciated. REFERENCES
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