Electrochemical reduction of some pyridinium and sulfonium ylides

E/ec?roc/timica Acfa,

Vol.36, No. 1, pp. 101-107, 1991

0013-4686/91 $3.00 + 0.00

Printed in Great Britain.

0 1990.Pergamon Press plc.

ELECTROCHEMICAL REDUCTION PYRIDINIUM AND SULFONIUM

OF SOME YLIDES

GEORGEKOKKINIDIS,*EVANGELUHATZIGmGoRrOu,t DIMITRA SAZOU*

and ANASTASIOS VfivoGLIst *Laboratory of Physical Chemistry, TLsboratory of Organic Chemistry, Aristotle University of Thesssloniki, GR 540 06 Thessalonlki, Greece (Received 30 January 1990; in revisedform 13 March 1990) Ah&-The electrochemical reduction of some pyridinium and sulfonium dicyanomethylides ss well as pyridinium ylides coming from an amidic precursor on mercury cathodes has been investigated by employing polarography, cyclic voltsrnmetry and controlled potential electrolysis. In neutral solutions, pyridinium ylides are reduced in one single step. The primary radical anion formed after the first electron uptake either dimerizes or undergoes further reduction through protonation and cleavage of the C-N bond after the second electron uptake. In acid solutions, protonation in the bulk solution is followed by electroreduction exclusively through cleavage of the C-N bond. Electroreduction of sulfonium dicyanomethylides leads also to the cleavage of the sulfur-carbon bond by means of a two-electron reaction. Key worrls:polarography,

cyclic voltammetry, radical anions, dimerixation, bond cleavage.

coming from an amidic precursor have also been included in the present study, in order to compare their behaviour with that of 2-pyridinio-1,3-indandionides.

INTRODUCTION In a recent investigation of the electrochemical prop erties of certain phenyliodonium ylides[l] we found that ylides generally have hardly attracted any attention from the electrochemical viewpoint. The only pertinent study we were able to spot concerns the behaviour of some 2-pyridinio-1,34ndandionides[2]. Since a large variety of ylides of several elements such as S, Se, Te, N, P, As, Sb, Bi and I are presently known and their chemistry is actively explored, we decided to initiate a thorough study on the electrochemistry of such compounds. Whatever the cationic moiety of ylides, the anionic moiety is mostly carbanionic bearing in a-position one or two electron withdrawing groups which confer stability to the xwitterion by charge delocalixation. Such a carbanionic part is the dicyanomethylene group, which when is linked to cationic N, S, P and As (but not I) leads to the formation of stable ylides. Pyridinium dicyanomethylides, 1, have been the subject of extensive chemical investigations[fd] and their electrochemical reduction is presently reported, along with that of some sulfonium dicyanomethylides, 2. In addition, two pyridinium ylides, 3, NC,_ -C-N+

NC'

//

’

x /._.A

EXPERIMENTAL Conventional dc polarograms were obtained by using a Radiometer polarograph “Polariter PO4”. The capillary constant (mz’3t’/6), was 3.1 x 10e2. The polarograms were taken in reference to aqueous calomel electrode saturated with NaCl. Special care was taken to ensure that both water and Clions from the reference electrode did not reach the compartment with the Hg electrode. Cyclic voltammograms were obtained using a hmde from Metrohm. The area of the mercury drop was 3.5 x lo-*cm*. The experimental set-up consisted of a Wenking potentioscan, type POS72, and a Houston 2000x-y recorder. All measurements were carried out at 22 f O.l”C. The solutions were thoroughly deoxygenated by purging the system with ultrapure nitrogen. The supporting electrolyte was prepared from absolute methanol (Merck p.a.) and recrystallized

NC,..

-c-q

lR1

NC'

R2

1

z

1 (a)

X=H

(a)

R,=R2

=CH2

(a)

X=H

(b)

X = p-CH2

(b)

R,=R2

=C6H5CH2

(b)

X = p-NH2

(c)

X=m-CN

(=I

Rl,R2

VI-I214

101

GEORGEKOKKINIDISet al.

102

and has intermediate values for the other pyridinium ylides. The one-electron reduction waves of compounds lc and 3a show very good reversibility, as indicated from the slopes dE/d log(i/i, - i) N 60 mV dec-‘. The nature of electroreduction intermediates and products was studied by cyclic voltammetry (see Figs 1 and 2 and Table 2). All pyridinium ylides give one reduction peak (I,) at potentials corresponding to the E,,, of their conventional polarograms. The peak current scales linearly with u”~ only for compounds lc, 3a and 3b, while for compounds la and lb some deviation is observed indicating semikinetic character for the overall cathodic process under potentiodynamic conditions. With the exception of 3b, an anodic peak (I,,) appears in the cyclic voltammograms. Figure 3 shows the dependence of the ratio (i,)./(i,), on scan rate. The higher is the number of electrons transferred during the reduction of each ylide, the lower is the ratio of the anodic and cathodic peak currents. The values of (E,), are significantly more positive than the electroreduction peak potentials (about 0.8-l .OV). This indicates that the anodic peak should

LiC104 (Fluka, purum p.a.). HC104 used was from Merck (puriss p.a.). All ylides were known compounds, prepared according to standard procedures[6, 71. RESULTS AND DISCUSSION Voltammetric behaviour methanol-LiCIO,

of pyridinium

ylides

in

Typical dc polarograms for pyridinium dicyanomethylide la and for 4-pyridinio-l,Zdiphenyl4Hpyrazolidine-3,5-dionide 3a are given in Figs 1 and 2, together with the corresponding cyclic voltammograms. A single reduction wave appears in the polarograms. One wave is also observable on polarographic reduction of ylides lb, lc and 3b (Table 1). The wave is diffusion controlled for all compounds, as indicated by the linear dependence of the limiting current both on the ylide bulk concentration and the square root of the height of the mercury head. The number of electrons calculated from coulometric data (see below) is n = 1 for ylides lc and 3a, n = 2 for ylide 3b I

9

I

aL ._

8

I

I

I

fa)

0

.

I

- 1.6

- 0.8

t

81

I 0

I

I - 0.8

1

I -1.6

1

1

E/Vseo Fig. 1. (a) Polarogram for ylide la (lo-” M) in 0.1 M LiCIO, in methanol. Curve (---) was obtained in the presence of 10e5M C,H5CH2P+(C,H,)3CI-. Inset: plot of id OSconcentration of la. (b) Cyclic voltammogram for ylide la on a hmde in 0. I M LiC104 in methanol. Scan rate u = 200 mV s-‘. Inset: plot of i, us “‘I2for ylides la, lb and lc.

Electrochemical reduction of some ylides

6-

10 d .s .9

aa \ ._

:’ I

0 ET7 0

3-

0

241

61

103

ICY

‘I

1 2 c/ mM

I 0

I -1.6

- 0.9

I

1

I

I

0

-

I

I

I

I

I

I

I

I

I

-1.6

0.6

Fig. 2. (a) Polarogram for ylide 3~ (10-r M) in 0.1 M LiClO, in methanol. Curve (---) was obtained in the presence of 10e5M C,H5CH,P+(C6H,),Cl-. Inset: plot of ia vs concentration of 3r. (b) Cyclic voltammogram for ylide 3a on a hmde in 0.1 M LiClO, in methanol. Scan rate IJ = 200 mV s-l. Inset: plot of ip vs u’/*. correspond

to the oxidation

of a secondary

inter-

mediate, presumably a dimer, which may be formed by bimolecular dimerization of the primary radical, ie the one-electron reduction intermediate. This reaction should be very rapid, since no oxidation peak corresponding to the primary radical is observed up to scan rate, o = 10 Vs-‘. A biomolecular reaction

involving dimerization of primary radicals has been previously observed in the course of electroreduction of pyridinium salts[8,9] and l-methyl-nicotinamide[lO], as well as of some pyridinium-1,34ndandionides [2]. The anodic wave observed in the cyclic voltammograms of pyridinium dicyanomethylide, la and its m-cyano derivative, lc, consists of two close

Table 1. Polarographic data of pyridinium and sulfonium ylides (10-l M) in methanol with 0.1 M LiClO, and lo-’ M C,H,CH2P+(C,H5)3CldE

b=

.

dh+ Compound

PA

19 lb

7.6 10.0 6.0 5.1 10.1 12.8 10.5 12.6

IC

3 3b

;kr 2c

Electrons d

id El12 Iv,

-1.41 -1.52 -0.95 -1.40 -2.00 -1.72 - 1.34 -1.62

per

mV dec-’

(ar)

molecule*

50*5 65 f 5 60*5 60*5 65 f 5 120 f 10 240* 10 .110* 10

1.18 0.91 0.91 0.49 0.25 0.54

1.3 1.7 1 1 2

Capillary constant (m2/3 r’16)= 3.1 x lo-*. *From coulometric data.

: 2

GEOROE KOKKINIDIS

104

et al.

Table 2. Cyclic voltammetric data of pyridium and sulfonium ylides (10e3 M) in methanol with 0.1 M LiCIOd at scan rate u = 200 mV s-’

Compound la lb lc 3a 3b 2s 2b 2c

Cathodic peak potential (E,),IV,,

Anodic peak potential (Ep ), IV,.

-1.45 -1.56 -0.98 -1.43 -2.04 -1.76 -1.40 - 1.67

-0.46 -0.47 -0.11 -0.57 -

0

b = d(Wc 2 dlogu (i,)./(i,),

(E,), - (&),ImV

0.18 0.07 0.55 0.46 -

35 40 30 30 55 85 -

200

/mV dec-’ 25 * 30 f 0 0 35 f 65 f -

5 5

5 10

(ar) 1.18 0.98 0.84 0.45 -

400 4

fllVi’

Fig. 3. Dependence of (it,),&,,), us scan rate, u. peaks (see Fig. lb) corresponding

probably to two dimeric products. Indeed, pyridinium derivatives are known to undergo dimerization at the a and yposition with respect to nitrogen atom[l 11. The peaks at around 0 V and 0.2 V which appear in the cyclic voltammograms of ylides la (Fig. lb) and lb correspond to the anions - CH(CN), and CH,O-, respectively, formed during the reduction of these ylides, as proved by using authentic samples. These ions may depolarize the mercury electrode by forming insoluble mercurous salts[l2]. Obviously, the anion of malonitrile is formed by cleavage of the C-N bond after the second electron uptake. This charge transfer reaction occurs in parallel to the dimerization of the primary radical via its protonated form (see

below). Indeed, in acetonitrile where protonation is excluded, all ylides undergo one-electron reduction and the ratio (i,),/($), takes the same values independently of the substttuents in the pyridine ring. Controlled potential elctrolysis of pyridinium ylides in

methanol-LiClO, Electrolysis of pyridinium ylides ( 10m3M) in 0.1 M LiC104 was carried out on the plateau of their waves using as a cathode a stirred mercury pool of N 10 cm* area. The identification of products was based on thin layer chromatography against authentic samples and the polarographic behaviour of the electrolysed solution. The results are summarized in Table 3. The dimeric products of ylides are stable enough in

Table 3. Results of controlled potential electrolysis of pyridinium ylides on mercury pool in methanol-LiCIO, Compound la lb lc 3a

Electrolysis potential/V,, -1.63 -1.75 - I .20 -1.65

Electrons per molecule 1.3 1.7 1 1

Products (yield based on consumed ylide, %) Pyridine* (25) Malononitrilet (25) Dimerj (70) p-Methyl-pyridine* (70), Malononitrilet (70), Dimer$ (25) Dimerj (95) Dimeri (100)

Identification was made: *from comparison with authentic sample; tboth from the polarogram and product isolation; Sfrom the polarographic behaviour of the electrolysed solution.

105

Electrochemical reduction of some ylides

( z

12-

.?

._

/

530

.--

: i I

10 0

6

0

:' I

1

2

e/mM

--i/i

Y-

0

I

0

t 0

-0.8

1

-1.6

I -0.8

I

I -1.6

I

E / Vsca Fig. 4. (a) Polarogram for ylide 2a (lo-’ M) in 0.1 M LiClO, in methanol. Curve (---) was obtained in the presence of lO-‘M C6H,CH2P+(C,H,)ICl-. Inset: plot of id us concentration of 21. (b) Cyclic voltammogram for ylide 2r on a hmde in 0.1 M LiClO, in methanol. Scan rate u = 200 mV s-l. Inset: plot

of iPus u’/*. deaerated solutions to be identified polarographitally. Malononitrile was isolated after neutralization of the electrolysed solution. Voltammetric behaviour of suvonium ylides in merhanol/LiClO, All sulfonium ylides are reduced in one two-electron step in methanol containing LiClO, as supporting electrolyte (Fig. 4 and Tables 1 and 2). The waves show poor reversibility in the charge-transfer controlled potential region but their limiting currents and peak currents exhibit diffusion-controlled characteristics. The reduction yields the corresponding sulfides and malononitrile, by cleavage of the C-S bond. Voltammetric behaviour of pyridinium ylides in methanol-LiClO, + HCIO., The effect of addition of HCIO, on the polarographic behaviour of the pyridinium ylides la and lb is illustrated in Fig. 5. One wave appears before the potential of hydrogen evolution the height of which is increased, although not proportionally, with increasing acid concentration, The wave exhibit

semi-kinetic character, as indicated by the non-linear dependence of its limiting current on the square root of the height of the mercury head. However, at acid concentrations greater than 2.5 x lo-* M for compound lb and IO-’ M for compound la, the limiting current reaches a two-electron level and becomes purely diffusion-controlled. Compound lc also gives a reduction wave but at much higher acid concentrations. The other pyridinium ylides as well as the sulfonium ylides studied do not give any reduction wave before the potential of hydrogen evolution. The cyclic voltammetric behaviour of compound la in excess of HCIO, concentration is shown in Fig. 6. Only one cathodic peak is obtained. The peak current exhibits semikinetic character and becomes diffusion-controlled at acid concentrations higher than those required under polarographic conditions (see the iPus II’/*plots in the inset diagram of Fig. 6). In the presence of HCIO,, the pyridinium dicyanomethylides are in equilibrium with their conjugated acids which undergo an irreversible two-electron reduction. It is obvious that an electron donor, such as the methyl group, favours protonation, whereas an

GEORGE KOKKINIDIS et al.

106

ca) 12 i \ ._

8-

2.2

2.:2

E/ bx

E / Vsce

Fig. 5. Polarograms for ylides (a) la (lo-’ M) and(b) 2b (10e3 M) in 0.1 M LiClO, in methanol in presence of HClO,: (1) 0; (2) 2.5 x lo-‘; (3) 5 x lo-‘; (4) lo-*; (5) 5 x 10-2; (6) lo-’ M.

electron

acceptor,

such as the cyan0 group, has the

opposite effect. Mechanisms for electroreduction sulfonium ylides on Hg

of

pyridinium and

On the basis of the above mentioned results, a mechanistic picture emerges, as shown in Scheme 1. All pyridinium ylides are initially reduced to a radical anion, the fate of which depends both on the structure of the carbanionic moiety and the substituents in the pyridine ring. In the case of ylides lc

and 3a, the radical anion rapidly dimerizes, probably from the a-position, by analogy with 2-pyridinio-1,3indandionides[2]; the dimeric product can be oxidized back

to the initial compound

at less negative

potentials. In the case of ylides la and lb, further reduction of the primary radical occurs, parallel to dimerization, via protonation and cleavage of the C-N bond after the second electron uptake, although first C-N cleavage of the radical followed by the electron uptake from the radical species CH(CN), is equally possible. This reaction pathway is favoured

32-

24-

IL \ *-

4 a \

.,P

16-

8-

o-

,, , 0.4

0

-04

-0.8

I

I

-1.2

E / he

Fig. 6. Cyclic voltammograms for ylide Ia (IO-) M) on a hmde in 0.1 M LiClO, and 0.1 M HClO, in methanol. Scan rate, u: (1) 25; (2) 50; (3) 100; (4) 200; (5) 300 mV s-l. Inset: plots of in OSu”* for different concentrations of HCIO,: (1) 0.1; (2) 0.3; (3) 1.0 M.

-1e-

\

Y

l/2

=-CN, -CON (Ph) -

Scheme 1

,2e-+

H+

NC, NC’

X

Scheme 2 by electron donors in the pyridine ring and the reduction of ylide 3b occurs exclusively by this route. In acid solutions, pyridinium dicyanomethylides are protonated in the bulk solution and their electroreduction proceeds solely through C-N bond cleavage (Scheme 2). At acid concentrations lower than those required for complete protonation, the reduction is partially governed by the kinetics of the preceding chemical reaction. All three sulfonium dicyanomethylides tested are cleaved by means of a two-electron process, identical to that of the pyridinium analogues. No dimerization is possible here, since there is no resonance stabilization. Despite the limited data available so far, it is clear that, in contrast to I-ylides[l], the anionic part of N-ylides does not influence E,,* values, since compounds la, 3~ and also 2:pyridinio-1,3-indandionide (2) have, respectively, E,,* values equal to - 1.41, -1.40 and - 1.45 V. The cationic part of S-ylides and especially of N-ylides has a pronounced effect on ,?& values, ranging between -0.95 and -2.OOV; if the most easily reducible I-ylides are also taken into account (eg the phenyliodonio analogue of 3 has El,* value of -0.29 V), the range of electroreduction of ylides generally is further expanded. Therefore, by choosing suitable &ionic-anionic partners, it will be possible to obtain ylides with an expected electron affinity. These may be useful in effecting specific

chemical transformations, such as controlled radicalinduced thermal or photo-polymerizations[ 131.

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5482 (1973).

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