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Cyclic voltammetry and spectral study of the electrochemical reduction of some dibenz[b,e]-thiepinone derivatives
201
J. Electroanal. Chem., 291 (1990) 201-215 Elsevier Sequoia S.A., Lausanne
Cyclic voltammetry and spectral study of the electrochemical reduction of some dibenz[b,e]-thiepinone derivatives Mariana Ciureanu, Mihaela Hillebrand and Elena Volanschi Department of Physical Chemistry, Polytechnic Institute Bucharest, B-dul Republicii 13, Bucharest (Rumania) (Received 16 March 1990; in revised form 16 April 1990)
ABSTRACT Cyclic voltammetry, ESR and optical spectroscopy experiments performed on some derivatives in the dibenz [b,e]-thiepinone series are presented and discussed. On the basis of these results, a mechanism for the electrochemical reduction of these compounds is suggested which accounts for the cleavage of the seven-membered ring and the appearance of anthrone and/or anthraquinone as transient or final products.
INTRODUCTION
Previous ESR investigation of several seven-membered heterocycles in the [b,e]thiepinone series [l] has indicated that chemical or electrochemical reduction determines the cleavage and rearrangement of the central ring leading to the anthraquinone radical anion. The presence of this radical anion was proved by comparison of the ESR spectrum with that provided by an authentic sample of anthraquinone; thin-layer chromatography of the reaction mixture revealed, after prolonged electrolysis, the presence of anthraquinone. An intermediate radical species with an unusually large g-factor value (2.0096) and hyperfine splittings from four quasi-equivalent protons with uH = 1.39 G was also noted for dibenz [b,e]-tbiepinone and assigned tentatively to the radical anion of the parent compound; however, owing to the high symmetry of the spectrum and a g-value’ considerably higher than that found for other sulphur-containing ketyl radical anions [2] (i.e. thioxanthone) a different assignment should be considered. The aim of the present study was to obtain more complete information on the mechanism of the electrochemical reduction of these compounds. The series investigated, including carbonyl and thiocarbonyl derivatives (expected to have greater g values than the corresponding oxygen derivatives) is presented in Scheme I. Besides ESR experiments, cyclic voltammetric and optical spectroscopic studies were also carried out.
lX=O;Y=O 2X=S;Y=O 3X=S;Y=S Scheme I.
In order to support the assignment of the hyperfine splittings, MO calculations in the frame of the Hiickel and McLachlan methods were also performed.
EXPERIMENTAL
The electrochemical measurements were performed with an M 173 Princeton potentiostat equipped with a Radelkis VLF function generator and an ENDIM 620.02 X-Y recorder. The working and counter-electrodes were made of platinum and the reference electrode was Ag/AgBr; the solvent was spectrograde dimethyl sulphoxide (DMSO), used without further purification; the residual water content amounted to several mm01 1-l. The supporting electrolyte was a mixture of 0.1 M tetra-n-butylammonium perchlorate (TBAP) and 0.05 M tetra-n-butylammonium bromide (TBAB) in DMSO. The electrochemical reduction was investigated by ESR and absorption spectroscopy using in situ techniques previously described [1,3]. The ESR spectra were recorded on a JES-3B spectrometer in the x-band frequency, using peroxylaminedisulphonate as standard (a = 1.3 mT). Optical spectra were recorded on a SPECORD UV-VIS spectrophotometer in DMSO and dimethyl formamide (DMF).
RESULTS
ESR spectra The ESR spectrum obtained when the electrochemical reduction of dibenz [b,e]-thiepin-11-thione (3) in DMSO is started is presented in Fig. 1 and was rationalized in terms of the hyperfine (hf) splitting constants listed in Table 1. As the g-factor value of 2.0073 suggests a thioketyl-type radical, the assignment of the three pairs of equivalent splittings to the ring protons was made by analogy with the hf pattern of thiobenzophenone thioketyl [4,5], also included in Table 1, and is supported by the hf splittings calculated according to McLachlan. The remaining splittings were ascribed to the seven-ring methylene protons. Their inequivalence and the lack of line-width effects suggest that, at room temperature,
203
a)
b)
g =2.0073 Fig. 1. The low-field half of the ESR spectrum obtained for the chemical reduction of 3 in DMSO (species A). (a) Experimental (the cross indicates the centre of the spectrum); (b) simulated assuming a Lorentzian line shape.
a). B
g-2.0096
I,
g,=2.0096
g-2.0073
g-2.OOL3
g2.2.00L3
Fig. 2. ESR spectrum obtained from 3 with prolonged electrolysis. (a) After about 30 min, mixture of radical species B (g = 2.0096), A (g = 2.0073) and C (g = 2.0043). (b) After ca. 1 h.
204
205
the seven-membered ring has a non-planar conformation which is frozen on the ESR time scale. Comparison of the hf pattern of species A obtained from 3 with that of thiobenzophenone reveals smaller ring proton splittings, indicating that the geometry required by the methylene and sulphur bridge determines a high degree of localization of the odd electron on the central ring. This is also reflected by the relatively high value of the methylene proton splittings. The calculated hf pattern agrees reasonably well with the experimental one and reflects the trends discussed above. With prolonged electrolysis the time evolution of the ESR spectrum presented in Fig. 2 is observed. Spectrum a in Fig. 2 was rationalized in terms of a mixture of three radical species characterized by g values of 2.0096, 2.0073 (species A) and 2.0043. In time, spectrum b was obtained, indicating the disappearance of the initial radical (species A) centred at g = 2.0073. The two other radical species (g = 2.0096 and g = 2.0043) were found to be identical to those obtained previously starting from dibenz [b,e]-thiepinone (2) and will be therefore be denoted as species B and C. Species C was identified unambiguously as the radical anion of anthraquinone; the ESR experiment alone could not provide an unambiguous assignment for radical species B. The time evolution of the ESR spectrum obtained from 3 towards the mixture of radicals B and C obtained when starting from 2 reveals that replacement of the thiocarbonyl by a carbonyl group occurs before the cleavage and desulphurization reaction of the central ring leading to anthraquinone as a final product. Cyclic voltammetry Representative cyclic voltammograms of the compounds under investigation are presented in Figs. 3-5. For dibenz[b,e]-oxepinone (1) the general pattern of the cyclic voltammogram is rather simple (Fig. 3). In the first scan an intense cathodic wave, denoted by A, was found at about - 1.2 V; this wave showed no anodic counterpart in the reverse scan in the entire scan rate range investigated. A second wave (B) was observed only at high rates (> 0.3 V s-l), just prior to the background. In the second and subsequent scans two small reversible couples DD’ and EE’ are apparent, and are intensified after controlled potential electrolysis at - 1.2 V. Since previous ESR experiments [l] revealed the presence of the anthraquinone radical anion (AQ-) as the only paramagnetic product after prolonged electrolysis of 1, the couples DD’ and EE’ were assigned to the consecutive monoelectronic charge transfers of anthraquinone (AQ). This assignment was supported by the match of waves D and E with those obtained in a cyclic voltammogram of an authentic sample of AQ recorded under the same experimental conditions. For dibenz[b,e]-thiepin-11-one (2) the voltammogram in the first negative scan is rather similar to that presented by 1, with the difference that wave B appears at
206
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0
-
0.5
-1.0
I
- 1.5
I
-2
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Fig. 3. Cyclic voltammograms of 1 in DMSO. () First scan; (- - -) second scan. (a) v = 0.022 V s-l; (b) u = 0.36 V s-‘; (c) u = 0.072 V s-l after controlled potential electrolysis at -1.2 V.
more positive potentials (Fig. 4a); this wave is rather small at low scan rates (0.004-0.04 V SC’) but with increasing rate undergoes considerable amplification at the expense of wave A (Fig. 4b). In the second scan, the occurrence of two new anodic waves G’ and F’ (the former having a less intense cathodic counterpart demonstrates the appearance of two new redox systems as a result of the chemical reactions following the electrochemical process (Fig. 4a). It was shown that these waves appeared only if the negative scan was reversed after peak A; however, the intensity of the waves was rather small unless the potential sweep was reversed after peak B (Fig. 4~). After prolonged electrolysis at the peak potential of A, the voltammogram of 2 also showed a DD’ couple (Fig. 4d), at the same potential as the first reversible reduction step of anthraquinone. Therefore we assigned this redox couple to anthraquinone, in agreement with the ESR data discussed above.
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First scan; (- - -) second scan, v = 0.036 Fig. 4. Cyclic voltammograms of 2 in DMSO. (a) ( -) V s-l. (h) 0 = 0.36 V s-l; (c) u = 0.036 V s-l, negative sweep stopped before wave B; (d) u = 0.036 V s-l,
after prolonged electrolysis at the peak potential of A.
For dibenz[b,e]-thiepin-ll-thione (3) the general pattern of the voltammogram is more complicated (Fig. 5). In the first negative scan four waves were observed, denoted by A, B, C and H (Fig. 5a). Wave A has practically the same location as that for compounds 1 and 2. Waves B and C have a rather small intensity at low sweep rates and undergo a considerably increase at higher scan rates. The most important difference between the voltammograms of 3 and 2 is provided by the intensive couple HH’, which appears at a potential more positive than A. This wave showed a perfectly reversible behaviour with no decrease in the second scan if the potential sweep was stopped before wave A (Fig. 5b). As no such reversible wave was demonstrated by the voltammograms of 1 and 2 in the first scan, wave HH’ was assigned to reversible formation of the relatively stable radical anion of 3 (species A), as already identified by the ESR spectrum.
T-r-Fig. 5. Cyclic voltammograms of 3 in DMSO. (a) () First scan; (- - -) second scan; u = 0.036 V s-l. (b) Negative scan stopped at -0.7 V; (c) with TBOH, same conditions as those for (a).
When the negative sweep was reversed only after reaching the peak potential B, wave H showed a different behaviour: in the second and subsequent cycles, the peak exhibited a considerable decrease, accompanied by a negative shift of about 50 mV (Fig. 5a). This shift can be rationalized in terms of superposition of the decreasing wave H with a new wave G, due to a product generated at wave B; since the peak potential of G matches that of the corresponding wave observed for 2, it is reasonable to assume that these waves are due to the formation of the same radical species B, which was observed in the ESR spectra of both compounds. As can be observed from a comparison of Figs. 4 and 5, in the second and subsequent scans, the general pattern of the voltammogram of 3 shows a close resemblance to that of 2. In the presence of a base (TBOH), the behaviour of 3 is
209 TABLE 2 Voltammetricdata for wave A Compound -(E&.4
-
“/v
1
2
3
1.260
1.215
1.220
&B’d=-’
0.030 0.060
(4 - EC,, Y/v
0.060
0.060
a Measuredat a sweeprate of 0.036 V s-l.
similar to that of 2, i.e. waves GG’ and F’ disappear whereas DD’ remains unchanged (Fig. 5~). The fundamental process characterizing the systems investigated is associated with the main wave A, which appears at practically the same potential for the three compounds. Table 2 presents the results of the usual electrochemical tests used to characterize an electrode process: both from the slope of the linear E,-f(log u) dependence and from the breadth of the wave at half width, it may be inferred that the reaction responsible for wave A is a reversible electron transfer. Since wave A shows no anodic counterparts, even at high scan rates, it may be inferred that the electron transfer is followed by chemical reactions which are faster than the reverse electron transfer. Further information may be obtained from the observation that the peak intensity of this wave undergoes a decrease of about 50% for a ten-fold increase of the scan rate. The voltammetric data for wave H obtained for compound 3 are presented in Table 3. The usual electrochemical tests indicate a reversible monoelectronic charge transfer. The rate constant for this charge transfer was calculated by the $ technique [6] from the difference of the anodic and cathodic peak potentials. DISCUSSION The reaction sequence presented in Scheme II is proposed for compound 2. The reversible one-electron reduction of 2 leads to the radical anion 2a, which is able to
TABLE 3 Voltammetricdata for wave H
-(E,x)H
a
-(Ec-Ec,da
/V
/v
0.710
0.070
(d&),4
-~ dlogV /V
dec-’
0.014
C-4,- Ed
a
lo3 k,
/V
/cm s-l
0.085
6.5
’ Measuredat a sweeprate of 0.036 V s-* in the first cycle.
2t =
0”
Scheme II.
evolve further via two competitive pathways: the first pathway (I) involves protonation of 2a, followed by disproportionation: a homogeneous electron transfer between the resulting neutral ketyl radical 2b and the radical anion 2a. The negative charge of anion 2e favours a Wittig rearrangement of the latter to give 2f. An important point to be stressed is that the protons required for the protonation step are likely to be supplied by the substrate itself. Owing to the special structural features which enable keto-enol tautomerization, 2 is expected to have sufficient acidic character to protonate its own radical anion; it is well known that such self-protonation reactions are characteristic for ketones having a CH, group with acidic properties [7,8]. Pathway II involves tautomerization of the radical anion 2a followed by a Wittig rearrangement; the resulting radical anion may undergo a heterogeneous charge transfer, performed at the peak potential B, followed by protonation of the substrate to give 2f. Several arguments support the proposed mechanism: (a) The increase of the intensity of peak B, accompanied by a decrease of peak A with increasing sweep rate, is consistent with competition between pathways I and
211
bH
g Scheme III.
II: at small sweep rates, the system has enough time to evolve via pathway I, wave B is rather small; at higher scan rates, there is no time for 2b to disproportionate, so the intensity of wave A decreases and the system evolves preferentially via pathway II. (b) According to the present mechanism, the final product of both reaction pathways is anion 2f. The latter is responsible for both the reversible couple GG’ and the anodic wave F’ which were observed in the second cycle(see Scheme III). It is now easy to understand why waves GG’ and F’ appear as the potential scan is reversed after wave A, or after wave B, but with a higher intensity in the latter case: from Scheme II it is obvious that 2f is the reaction product for both pathways I and II. (c) In the presence of small quantities of a base tetra-n-butylammonium hydroxide (TBOH) - waves GG’ and F’ were shown to disappear completely: this observation is consistent with the mechanism presented in Scheme II, since in basic media the reaction product 2f is prevented from appearing, owing to the slowing down of the protonation steps in both pathways I and II. (d) The present mechanism is supported by the ESR data: no signal could be observed for the radical anions 2a and 2b, owing to the rapidity of the chemical follow-up reactions. The signal observed for species B could be assigned to the neutral radical 2j, in agreement with the unusually large g value (close to that observed for saturated sulphide radicals) and with the symmetry of the observed hfs pattern. A possible explanation for the appearance of anthraquinone after prolonged electrolysis could be the following: the anion of the substrate, 2g, undergoes a Wittig rearrangement followed by hydrolysis in the presence of residual water in the solvent. The resulting dihydroxyanthracene 2i is able to disproportionate to form anthrone (4) and anthraquinone (5). Therefore anthraquinone is not a reduction product, but the result of a reaction sequence of the anion in the presence of electrogenerated bases. This idea is supported by the fact that AQ- was obtained also by the action of a strong base on 2 (i.e. KOH in DMSO), even without electrochemical reduction [l]. For 1 the reaction mechanism should be similar to that described for 2, with the difference that the reaction product is expected to be dihydroxydihydroanthracene (If), instead of an anion similar to 2f, owing to the different basicities of the SH and
I
s3a
I 0-
2s
Scheme IV.
OH groups. This compound is electrochemically inactive in the potential range investigated, so that no wave similar to GG’ and F’ is likely to appear. The electrochemical behaviour of 3 is more complicated. Compound 3 undergoes a reversible monoelectronic reduction (wave HH’) to form the thioketyl-type radical anion 3a, identified in the ESR spectra as the radical species A. This radical anion has a lower basicity than 2a, so that it is not protonated but is able to hydrolyse in the presence of small amounts of water in the solvent to give the radical anion 2a; the latter, after protonation and disproportionation, is able to form the neutral compound 2 (Scheme IV, which evolves, in the second cycle, according to Scheme II. The perfect resemblance between the cyclic voltammograms of 2 and 3 after the first scan supports this reaction sequence. Small quantities of the radical anion 3a are able to undergo a Wittig rearrangement, followed by an electron transfer which may be assigned to wave C. The proposed reaction schemes are further supported by the optical spectra. Optical spectra The optical spectra were recorded during the in situ electrochemical reduction of compounds l-3 in aprotic solvents. Anthrone (4) and anthraquinone (S), which are suspected products in the mechanism discussed above, were also investigated under the same working conditions. For compound 1, inspection of the spectrum presented in Fig. 6 reveals that by controlled potential electrolysis the peak at 355 nm, characteristic for the neutral molecule, decreases in favour of two new absorption bands located in the 380-400 nm and 480-550 nm ranges.
213
28
Fig. 6. Visible spectrum recorded during the electrochemical on electrolysis. Fig. 7. Absorption spectrum of 2 during electrochemical electrolysis (10W3 M); (7-9) on electrolysis (low2 M).
2L
20
16
12
reduction of 1 in DMSO. (1) Initial; (2, 3)
reduction
in DMSO.
(1) Initial;
(2-6)
on
The spectrum recorded during the electrochemical reduction of 2 also presents absorption bands in the same ranges, with a shape similar to that obtained starting from 1, especially when concentrated (10e2 M) samples are used. For both com-
I
I
30
26
, 6’ 22
18
10-3S/cm4
Fig. 8. (a) Electrochemical reduction of anthraquinone: (la) initial; (2-5) with time, on electrolysis. (b) Decomposition of the electrochemically reduced solution of anthrone: (lb) initial; (2) after electrolysis; (3-7) with time, in air.
214
Fig. 9. Absorption spectra recorded during the reduction of 3. (a) (1) initial; (2) with Na, DMF; (3’-5’) with time, in air. (b) Electrochemical reduction in DMSO: (lb) initial; (2b-5b) with time, on electrolysis.
pounds 1 and 2, when the current is switched off and the solution is exposed to air, all these absorption bands disappear, but the neutral compound is not recovered; this is further experimental evidence that the electrogenerated radical anion is subject to a sequence of following reactions. The spectra of the electrolysed samples of 1 and 2 show a striking similarity to those obtained under similar conditions for anthrone (4) and anthraquinone (5) (Fig. 8). This observation demonstrates the presence of these compounds (4 and 5) in the reaction mixture obtained by electrochemical reduction of 1 and 2, as discussed in the previous section. Anthrone is clearly not a final product in the reaction scheme and is further transformed into anthraquinone *; this seems obvious by examining the decomposition of the coloured species in the electrolysed samples (Fig. 8): for 5 the parent
* The formation of the anthraquinone radical anion in the electrochemical reduction of anthrone was previously observed using ESR spectroscopy; a similar behaviour has been noted for other aromatic ketones comaining CH, groups (9,101. The mechanism of this process is rather complicated and has not yet been elucidated.
215
compound is recovered, demonstrating that the reduction is reversible, whereas for 4 the final product has the same spectrum as that of anthraquinone. In both cases the spectra are rather complex, suggesting the presence of many intermediate species; the single peak which could be assigned unambiguously, by comparison with literature data [ll], is the 545 nm band, due to the anthraquinone radical anion. For compound 3, the spectrum obtained in the chemical or electrochemical reduction is rather complex and the relative intensities of the bands are dependent on the reduction methods (Fig. 9). The bands of the parent compound (429 and 600 nm) disappear and new bands at 641, 500-550 and 400 nm (shoulder) are apparent. The peak at 641 nm, more intense when alkali metal is used as the reducible agent, was assigned to the thioketyl radical anion (Fig. 9a, 2). In time, this peak decreases in intensity in favour of the band at 500-550 nm, a process enhanced in the presence of KOH (Fig. 9a, 2’-5’). If electrochemical reduction is used, especially for dilute starting solutions, spectra similar to those in Figs. 6 and 7 are obtained, but with stronger absorption at.640 nm. This is further experimental support for a hydrolysis reaction occurring at the C-S group, after the formation of the thioketyl-type radical anion and leading to the same final products as compound 2 (Scheme IV). In conclusion, it may be stated that the optical spectra recorded during the electrochemical reduction of compounds l-5 represent additional experimental support for the reaction mechanism suggested above, on the basis of the ESR and cyclic voltammetric data. REFERENCES 1 2 3 4 5 6 7 8 9 10 11
E. Volanschi, C. Volanschi and C. Vl~descu, Rev. Roum. Chim., 19 (1974) 755. E.T. Kaiser and D.H. Eargle, J. Am. Chem. Sot., 89 (1967) 5179. M. Ciureanu, M. Hillebrand, A. Meghea and E. Volanschi, J. Electroanal. Chem., 239 (1988) 227. L.J. Aarons and F.C. Adams, Can. J. Chem., 50 (1972) 1390. L. Lunazzi, G. Maccagnani, G. Mazati and G. Plaucci, J. Chem. See. B, (1971) 162. R.S. Nicholson, Anal. Chem., 37 (1965) 1351. K. Ahndal, H. Eggert and 0. Hammerich, Acta Chem. Stand., Ser. B, 40 (1986) 230. R.C. Buchta and D.H. Evans, J. Electrochem. Sot., 117 (1970) 1494. R. DehJ and G.K. Fraenkel, J. Chem. Phys., 39 (1963) 1793. B. Tabner and RD. Zdysiewics, J. Chem. Sot. B, (1971) 1659. L. Ciurea and V. Em. Sahini, Rev. Roum. Chim., 14 (1969) 1093.
J. Electroanal. Chem., 291 (1990) 201-215 Elsevier Sequoia S.A., Lausanne
Cyclic voltammetry and spectral study of the electrochemical reduction of some dibenz[b,e]-thiepinone derivatives Mariana Ciureanu, Mihaela Hillebrand and Elena Volanschi Department of Physical Chemistry, Polytechnic Institute Bucharest, B-dul Republicii 13, Bucharest (Rumania) (Received 16 March 1990; in revised form 16 April 1990)
ABSTRACT Cyclic voltammetry, ESR and optical spectroscopy experiments performed on some derivatives in the dibenz [b,e]-thiepinone series are presented and discussed. On the basis of these results, a mechanism for the electrochemical reduction of these compounds is suggested which accounts for the cleavage of the seven-membered ring and the appearance of anthrone and/or anthraquinone as transient or final products.
INTRODUCTION
Previous ESR investigation of several seven-membered heterocycles in the [b,e]thiepinone series [l] has indicated that chemical or electrochemical reduction determines the cleavage and rearrangement of the central ring leading to the anthraquinone radical anion. The presence of this radical anion was proved by comparison of the ESR spectrum with that provided by an authentic sample of anthraquinone; thin-layer chromatography of the reaction mixture revealed, after prolonged electrolysis, the presence of anthraquinone. An intermediate radical species with an unusually large g-factor value (2.0096) and hyperfine splittings from four quasi-equivalent protons with uH = 1.39 G was also noted for dibenz [b,e]-tbiepinone and assigned tentatively to the radical anion of the parent compound; however, owing to the high symmetry of the spectrum and a g-value’ considerably higher than that found for other sulphur-containing ketyl radical anions [2] (i.e. thioxanthone) a different assignment should be considered. The aim of the present study was to obtain more complete information on the mechanism of the electrochemical reduction of these compounds. The series investigated, including carbonyl and thiocarbonyl derivatives (expected to have greater g values than the corresponding oxygen derivatives) is presented in Scheme I. Besides ESR experiments, cyclic voltammetric and optical spectroscopic studies were also carried out.
lX=O;Y=O 2X=S;Y=O 3X=S;Y=S Scheme I.
In order to support the assignment of the hyperfine splittings, MO calculations in the frame of the Hiickel and McLachlan methods were also performed.
EXPERIMENTAL
The electrochemical measurements were performed with an M 173 Princeton potentiostat equipped with a Radelkis VLF function generator and an ENDIM 620.02 X-Y recorder. The working and counter-electrodes were made of platinum and the reference electrode was Ag/AgBr; the solvent was spectrograde dimethyl sulphoxide (DMSO), used without further purification; the residual water content amounted to several mm01 1-l. The supporting electrolyte was a mixture of 0.1 M tetra-n-butylammonium perchlorate (TBAP) and 0.05 M tetra-n-butylammonium bromide (TBAB) in DMSO. The electrochemical reduction was investigated by ESR and absorption spectroscopy using in situ techniques previously described [1,3]. The ESR spectra were recorded on a JES-3B spectrometer in the x-band frequency, using peroxylaminedisulphonate as standard (a = 1.3 mT). Optical spectra were recorded on a SPECORD UV-VIS spectrophotometer in DMSO and dimethyl formamide (DMF).
RESULTS
ESR spectra The ESR spectrum obtained when the electrochemical reduction of dibenz [b,e]-thiepin-11-thione (3) in DMSO is started is presented in Fig. 1 and was rationalized in terms of the hyperfine (hf) splitting constants listed in Table 1. As the g-factor value of 2.0073 suggests a thioketyl-type radical, the assignment of the three pairs of equivalent splittings to the ring protons was made by analogy with the hf pattern of thiobenzophenone thioketyl [4,5], also included in Table 1, and is supported by the hf splittings calculated according to McLachlan. The remaining splittings were ascribed to the seven-ring methylene protons. Their inequivalence and the lack of line-width effects suggest that, at room temperature,
203
a)
b)
g =2.0073 Fig. 1. The low-field half of the ESR spectrum obtained for the chemical reduction of 3 in DMSO (species A). (a) Experimental (the cross indicates the centre of the spectrum); (b) simulated assuming a Lorentzian line shape.
a). B
g-2.0096
I,
g,=2.0096
g-2.0073
g-2.OOL3
g2.2.00L3
Fig. 2. ESR spectrum obtained from 3 with prolonged electrolysis. (a) After about 30 min, mixture of radical species B (g = 2.0096), A (g = 2.0073) and C (g = 2.0043). (b) After ca. 1 h.
204
205
the seven-membered ring has a non-planar conformation which is frozen on the ESR time scale. Comparison of the hf pattern of species A obtained from 3 with that of thiobenzophenone reveals smaller ring proton splittings, indicating that the geometry required by the methylene and sulphur bridge determines a high degree of localization of the odd electron on the central ring. This is also reflected by the relatively high value of the methylene proton splittings. The calculated hf pattern agrees reasonably well with the experimental one and reflects the trends discussed above. With prolonged electrolysis the time evolution of the ESR spectrum presented in Fig. 2 is observed. Spectrum a in Fig. 2 was rationalized in terms of a mixture of three radical species characterized by g values of 2.0096, 2.0073 (species A) and 2.0043. In time, spectrum b was obtained, indicating the disappearance of the initial radical (species A) centred at g = 2.0073. The two other radical species (g = 2.0096 and g = 2.0043) were found to be identical to those obtained previously starting from dibenz [b,e]-thiepinone (2) and will be therefore be denoted as species B and C. Species C was identified unambiguously as the radical anion of anthraquinone; the ESR experiment alone could not provide an unambiguous assignment for radical species B. The time evolution of the ESR spectrum obtained from 3 towards the mixture of radicals B and C obtained when starting from 2 reveals that replacement of the thiocarbonyl by a carbonyl group occurs before the cleavage and desulphurization reaction of the central ring leading to anthraquinone as a final product. Cyclic voltammetry Representative cyclic voltammograms of the compounds under investigation are presented in Figs. 3-5. For dibenz[b,e]-oxepinone (1) the general pattern of the cyclic voltammogram is rather simple (Fig. 3). In the first scan an intense cathodic wave, denoted by A, was found at about - 1.2 V; this wave showed no anodic counterpart in the reverse scan in the entire scan rate range investigated. A second wave (B) was observed only at high rates (> 0.3 V s-l), just prior to the background. In the second and subsequent scans two small reversible couples DD’ and EE’ are apparent, and are intensified after controlled potential electrolysis at - 1.2 V. Since previous ESR experiments [l] revealed the presence of the anthraquinone radical anion (AQ-) as the only paramagnetic product after prolonged electrolysis of 1, the couples DD’ and EE’ were assigned to the consecutive monoelectronic charge transfers of anthraquinone (AQ). This assignment was supported by the match of waves D and E with those obtained in a cyclic voltammogram of an authentic sample of AQ recorded under the same experimental conditions. For dibenz[b,e]-thiepin-11-one (2) the voltammogram in the first negative scan is rather similar to that presented by 1, with the difference that wave B appears at
206
I
I
0
-
0.5
-1.0
I
- 1.5
I
-2
EV
Fig. 3. Cyclic voltammograms of 1 in DMSO. () First scan; (- - -) second scan. (a) v = 0.022 V s-l; (b) u = 0.36 V s-‘; (c) u = 0.072 V s-l after controlled potential electrolysis at -1.2 V.
more positive potentials (Fig. 4a); this wave is rather small at low scan rates (0.004-0.04 V SC’) but with increasing rate undergoes considerable amplification at the expense of wave A (Fig. 4b). In the second scan, the occurrence of two new anodic waves G’ and F’ (the former having a less intense cathodic counterpart demonstrates the appearance of two new redox systems as a result of the chemical reactions following the electrochemical process (Fig. 4a). It was shown that these waves appeared only if the negative scan was reversed after peak A; however, the intensity of the waves was rather small unless the potential sweep was reversed after peak B (Fig. 4~). After prolonged electrolysis at the peak potential of A, the voltammogram of 2 also showed a DD’ couple (Fig. 4d), at the same potential as the first reversible reduction step of anthraquinone. Therefore we assigned this redox couple to anthraquinone, in agreement with the ESR data discussed above.
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I I
,,%__/’
,I/
e-’
/
/’ b).
,C’----/
,’ ‘-,H---------% \d
0
I-
I
- 0.5
-1.0
I
-1.5
EV
First scan; (- - -) second scan, v = 0.036 Fig. 4. Cyclic voltammograms of 2 in DMSO. (a) ( -) V s-l. (h) 0 = 0.36 V s-l; (c) u = 0.036 V s-l, negative sweep stopped before wave B; (d) u = 0.036 V s-l,
after prolonged electrolysis at the peak potential of A.
For dibenz[b,e]-thiepin-ll-thione (3) the general pattern of the voltammogram is more complicated (Fig. 5). In the first negative scan four waves were observed, denoted by A, B, C and H (Fig. 5a). Wave A has practically the same location as that for compounds 1 and 2. Waves B and C have a rather small intensity at low sweep rates and undergo a considerably increase at higher scan rates. The most important difference between the voltammograms of 3 and 2 is provided by the intensive couple HH’, which appears at a potential more positive than A. This wave showed a perfectly reversible behaviour with no decrease in the second scan if the potential sweep was stopped before wave A (Fig. 5b). As no such reversible wave was demonstrated by the voltammograms of 1 and 2 in the first scan, wave HH’ was assigned to reversible formation of the relatively stable radical anion of 3 (species A), as already identified by the ESR spectrum.
T-r-Fig. 5. Cyclic voltammograms of 3 in DMSO. (a) () First scan; (- - -) second scan; u = 0.036 V s-l. (b) Negative scan stopped at -0.7 V; (c) with TBOH, same conditions as those for (a).
When the negative sweep was reversed only after reaching the peak potential B, wave H showed a different behaviour: in the second and subsequent cycles, the peak exhibited a considerable decrease, accompanied by a negative shift of about 50 mV (Fig. 5a). This shift can be rationalized in terms of superposition of the decreasing wave H with a new wave G, due to a product generated at wave B; since the peak potential of G matches that of the corresponding wave observed for 2, it is reasonable to assume that these waves are due to the formation of the same radical species B, which was observed in the ESR spectra of both compounds. As can be observed from a comparison of Figs. 4 and 5, in the second and subsequent scans, the general pattern of the voltammogram of 3 shows a close resemblance to that of 2. In the presence of a base (TBOH), the behaviour of 3 is
209 TABLE 2 Voltammetricdata for wave A Compound -(E&.4
-
“/v
1
2
3
1.260
1.215
1.220
&B’d=-’
0.030 0.060
(4 - EC,, Y/v
0.060
0.060
a Measuredat a sweeprate of 0.036 V s-l.
similar to that of 2, i.e. waves GG’ and F’ disappear whereas DD’ remains unchanged (Fig. 5~). The fundamental process characterizing the systems investigated is associated with the main wave A, which appears at practically the same potential for the three compounds. Table 2 presents the results of the usual electrochemical tests used to characterize an electrode process: both from the slope of the linear E,-f(log u) dependence and from the breadth of the wave at half width, it may be inferred that the reaction responsible for wave A is a reversible electron transfer. Since wave A shows no anodic counterparts, even at high scan rates, it may be inferred that the electron transfer is followed by chemical reactions which are faster than the reverse electron transfer. Further information may be obtained from the observation that the peak intensity of this wave undergoes a decrease of about 50% for a ten-fold increase of the scan rate. The voltammetric data for wave H obtained for compound 3 are presented in Table 3. The usual electrochemical tests indicate a reversible monoelectronic charge transfer. The rate constant for this charge transfer was calculated by the $ technique [6] from the difference of the anodic and cathodic peak potentials. DISCUSSION The reaction sequence presented in Scheme II is proposed for compound 2. The reversible one-electron reduction of 2 leads to the radical anion 2a, which is able to
TABLE 3 Voltammetricdata for wave H
-(E,x)H
a
-(Ec-Ec,da
/V
/v
0.710
0.070
(d&),4
-~ dlogV /V
dec-’
0.014
C-4,- Ed
a
lo3 k,
/V
/cm s-l
0.085
6.5
’ Measuredat a sweeprate of 0.036 V s-* in the first cycle.
2t =
0”
Scheme II.
evolve further via two competitive pathways: the first pathway (I) involves protonation of 2a, followed by disproportionation: a homogeneous electron transfer between the resulting neutral ketyl radical 2b and the radical anion 2a. The negative charge of anion 2e favours a Wittig rearrangement of the latter to give 2f. An important point to be stressed is that the protons required for the protonation step are likely to be supplied by the substrate itself. Owing to the special structural features which enable keto-enol tautomerization, 2 is expected to have sufficient acidic character to protonate its own radical anion; it is well known that such self-protonation reactions are characteristic for ketones having a CH, group with acidic properties [7,8]. Pathway II involves tautomerization of the radical anion 2a followed by a Wittig rearrangement; the resulting radical anion may undergo a heterogeneous charge transfer, performed at the peak potential B, followed by protonation of the substrate to give 2f. Several arguments support the proposed mechanism: (a) The increase of the intensity of peak B, accompanied by a decrease of peak A with increasing sweep rate, is consistent with competition between pathways I and
211
bH
g Scheme III.
II: at small sweep rates, the system has enough time to evolve via pathway I, wave B is rather small; at higher scan rates, there is no time for 2b to disproportionate, so the intensity of wave A decreases and the system evolves preferentially via pathway II. (b) According to the present mechanism, the final product of both reaction pathways is anion 2f. The latter is responsible for both the reversible couple GG’ and the anodic wave F’ which were observed in the second cycle(see Scheme III). It is now easy to understand why waves GG’ and F’ appear as the potential scan is reversed after wave A, or after wave B, but with a higher intensity in the latter case: from Scheme II it is obvious that 2f is the reaction product for both pathways I and II. (c) In the presence of small quantities of a base tetra-n-butylammonium hydroxide (TBOH) - waves GG’ and F’ were shown to disappear completely: this observation is consistent with the mechanism presented in Scheme II, since in basic media the reaction product 2f is prevented from appearing, owing to the slowing down of the protonation steps in both pathways I and II. (d) The present mechanism is supported by the ESR data: no signal could be observed for the radical anions 2a and 2b, owing to the rapidity of the chemical follow-up reactions. The signal observed for species B could be assigned to the neutral radical 2j, in agreement with the unusually large g value (close to that observed for saturated sulphide radicals) and with the symmetry of the observed hfs pattern. A possible explanation for the appearance of anthraquinone after prolonged electrolysis could be the following: the anion of the substrate, 2g, undergoes a Wittig rearrangement followed by hydrolysis in the presence of residual water in the solvent. The resulting dihydroxyanthracene 2i is able to disproportionate to form anthrone (4) and anthraquinone (5). Therefore anthraquinone is not a reduction product, but the result of a reaction sequence of the anion in the presence of electrogenerated bases. This idea is supported by the fact that AQ- was obtained also by the action of a strong base on 2 (i.e. KOH in DMSO), even without electrochemical reduction [l]. For 1 the reaction mechanism should be similar to that described for 2, with the difference that the reaction product is expected to be dihydroxydihydroanthracene (If), instead of an anion similar to 2f, owing to the different basicities of the SH and
I
s3a
I 0-
2s
Scheme IV.
OH groups. This compound is electrochemically inactive in the potential range investigated, so that no wave similar to GG’ and F’ is likely to appear. The electrochemical behaviour of 3 is more complicated. Compound 3 undergoes a reversible monoelectronic reduction (wave HH’) to form the thioketyl-type radical anion 3a, identified in the ESR spectra as the radical species A. This radical anion has a lower basicity than 2a, so that it is not protonated but is able to hydrolyse in the presence of small amounts of water in the solvent to give the radical anion 2a; the latter, after protonation and disproportionation, is able to form the neutral compound 2 (Scheme IV, which evolves, in the second cycle, according to Scheme II. The perfect resemblance between the cyclic voltammograms of 2 and 3 after the first scan supports this reaction sequence. Small quantities of the radical anion 3a are able to undergo a Wittig rearrangement, followed by an electron transfer which may be assigned to wave C. The proposed reaction schemes are further supported by the optical spectra. Optical spectra The optical spectra were recorded during the in situ electrochemical reduction of compounds l-3 in aprotic solvents. Anthrone (4) and anthraquinone (S), which are suspected products in the mechanism discussed above, were also investigated under the same working conditions. For compound 1, inspection of the spectrum presented in Fig. 6 reveals that by controlled potential electrolysis the peak at 355 nm, characteristic for the neutral molecule, decreases in favour of two new absorption bands located in the 380-400 nm and 480-550 nm ranges.
213
28
Fig. 6. Visible spectrum recorded during the electrochemical on electrolysis. Fig. 7. Absorption spectrum of 2 during electrochemical electrolysis (10W3 M); (7-9) on electrolysis (low2 M).
2L
20
16
12
reduction of 1 in DMSO. (1) Initial; (2, 3)
reduction
in DMSO.
(1) Initial;
(2-6)
on
The spectrum recorded during the electrochemical reduction of 2 also presents absorption bands in the same ranges, with a shape similar to that obtained starting from 1, especially when concentrated (10e2 M) samples are used. For both com-
I
I
30
26
, 6’ 22
18
10-3S/cm4
Fig. 8. (a) Electrochemical reduction of anthraquinone: (la) initial; (2-5) with time, on electrolysis. (b) Decomposition of the electrochemically reduced solution of anthrone: (lb) initial; (2) after electrolysis; (3-7) with time, in air.
214
Fig. 9. Absorption spectra recorded during the reduction of 3. (a) (1) initial; (2) with Na, DMF; (3’-5’) with time, in air. (b) Electrochemical reduction in DMSO: (lb) initial; (2b-5b) with time, on electrolysis.
pounds 1 and 2, when the current is switched off and the solution is exposed to air, all these absorption bands disappear, but the neutral compound is not recovered; this is further experimental evidence that the electrogenerated radical anion is subject to a sequence of following reactions. The spectra of the electrolysed samples of 1 and 2 show a striking similarity to those obtained under similar conditions for anthrone (4) and anthraquinone (5) (Fig. 8). This observation demonstrates the presence of these compounds (4 and 5) in the reaction mixture obtained by electrochemical reduction of 1 and 2, as discussed in the previous section. Anthrone is clearly not a final product in the reaction scheme and is further transformed into anthraquinone *; this seems obvious by examining the decomposition of the coloured species in the electrolysed samples (Fig. 8): for 5 the parent
* The formation of the anthraquinone radical anion in the electrochemical reduction of anthrone was previously observed using ESR spectroscopy; a similar behaviour has been noted for other aromatic ketones comaining CH, groups (9,101. The mechanism of this process is rather complicated and has not yet been elucidated.
215
compound is recovered, demonstrating that the reduction is reversible, whereas for 4 the final product has the same spectrum as that of anthraquinone. In both cases the spectra are rather complex, suggesting the presence of many intermediate species; the single peak which could be assigned unambiguously, by comparison with literature data [ll], is the 545 nm band, due to the anthraquinone radical anion. For compound 3, the spectrum obtained in the chemical or electrochemical reduction is rather complex and the relative intensities of the bands are dependent on the reduction methods (Fig. 9). The bands of the parent compound (429 and 600 nm) disappear and new bands at 641, 500-550 and 400 nm (shoulder) are apparent. The peak at 641 nm, more intense when alkali metal is used as the reducible agent, was assigned to the thioketyl radical anion (Fig. 9a, 2). In time, this peak decreases in intensity in favour of the band at 500-550 nm, a process enhanced in the presence of KOH (Fig. 9a, 2’-5’). If electrochemical reduction is used, especially for dilute starting solutions, spectra similar to those in Figs. 6 and 7 are obtained, but with stronger absorption at.640 nm. This is further experimental support for a hydrolysis reaction occurring at the C-S group, after the formation of the thioketyl-type radical anion and leading to the same final products as compound 2 (Scheme IV). In conclusion, it may be stated that the optical spectra recorded during the electrochemical reduction of compounds l-5 represent additional experimental support for the reaction mechanism suggested above, on the basis of the ESR and cyclic voltammetric data. REFERENCES 1 2 3 4 5 6 7 8 9 10 11
E. Volanschi, C. Volanschi and C. Vl~descu, Rev. Roum. Chim., 19 (1974) 755. E.T. Kaiser and D.H. Eargle, J. Am. Chem. Sot., 89 (1967) 5179. M. Ciureanu, M. Hillebrand, A. Meghea and E. Volanschi, J. Electroanal. Chem., 239 (1988) 227. L.J. Aarons and F.C. Adams, Can. J. Chem., 50 (1972) 1390. L. Lunazzi, G. Maccagnani, G. Mazati and G. Plaucci, J. Chem. See. B, (1971) 162. R.S. Nicholson, Anal. Chem., 37 (1965) 1351. K. Ahndal, H. Eggert and 0. Hammerich, Acta Chem. Stand., Ser. B, 40 (1986) 230. R.C. Buchta and D.H. Evans, J. Electrochem. Sot., 117 (1970) 1494. R. DehJ and G.K. Fraenkel, J. Chem. Phys., 39 (1963) 1793. B. Tabner and RD. Zdysiewics, J. Chem. Sot. B, (1971) 1659. L. Ciurea and V. Em. Sahini, Rev. Roum. Chim., 14 (1969) 1093.