Anion radicals from imidazobenzoquinones

JOURNAL

OF MAGNETIC

12,331-336

RESONANCE

(1973)

Anion Radicalsfrom Imidazobenzoquinones G. F. Istiruro

di Chimica

PEDULLI

A.

AND

Organica dell’Universit& 40136 Bologna, Italy

SPISNI Viale Risorgimento,

4,

P. VIVARELLI lstituto

di Chimica

Organica

P.

dell’lJniversirri,

DEMBECH

AND

Via Campi,

G.

183, Modena,

Italy

SECONI

Laboratorio dei composti de1 carbonio conrenenri eteroatomi, e loro applicazioni de1 C.N. R., Ozzano Emilia, Italy

ReceivedMay 22,1973 The electron resonancespectraof the radicalsgeneratedby reducingwith potassium t-butoxide in DMSO imidazo-orrho- and para-benzoquinones and the corresponding l-methyl and 2-methyl derivativesare reported. From the relative values of the hypertine splitting constants, evidencehas been obtained that the radicalsfrom N-unsubstituted quinonespossessa dianionic structure which results from the sum of two processes:reduction of the parent moleculeto the monoanionic form and ionization of the iminic proton.

An electron resonance study of the radical produced by reducing electrolytically the imidazo-p-benzoquinone has been recently reported (I). The room temperature ESR spectrum showed coupling of the unpaired electron with two equivalent nitrogens, two equivalent protons and another single proton having hyperfine splitting constants of 0.94, 3.50, and 0.37 gauss, respectively, in DMF solution. The smaller splitting which could be due either to the proton in position 2 or to the proton attached to the nitrogen, was assigned to the latter as HMO calculations showed that the orbital containing the unpaired electron has a node on the imidazolic carbon. This however would imply the nonequivalence of the splittings from the two nitrogens and also from the 5 and 6 protons. The observed equivalence was explained in terms of the fast exchange of the proton in position 1 between the two nitrogens as shown in Scheme 1.

SCHEME

1

The rate constant of this process should be greater than lo9 set-’ because of the absence of any line width alternation effect. Copyright 0 1973 by Academic Press, Inc. All rights of reproduction in any form reserved. Printed in Great Britain

331

332

PEDULLI

ET

AL.

This interpretation seems rather unlikely since NMR measurements carried out on substituted benzimidazoles (2) showed that the rate of the proton exchange was of the order of IO2 see-’ at room temperature, that is very much smaller than in the imidazo-pbenzoquinone radical. A factor of 10’ or more between these two rate constants can hardly be justified even considering the differences of the two systems and of the solvents employed. By repeating the experiment on the same compound Ia and on the related derivatives listed below, we obtained evidence that the above interpretation is in fact not correct

I II

(a) R=R1=H

(a) R-R1=H

(b) R=H,R,=CHJ (c) R=CHI,Ri=H

(b) R=H,R*=CH~

and that the radicals from Ia, b and IIa, b have deprotonated

dianionic structures.

EXPERIMENTAL

Compounds Imidazo-p-benzoquinone Ia and its 2-methyl derivative Ib were obtained by oxidation of the corresponding 4,7-dihydroxybenzimidazolium bromide or chloride with ferric chloride or cromic anhydride, respectively, according to known methods (3). 1-Methylimidazo-p-benzoquinone Ic was prepared by the following procedure. A solution of l-methyl 4,7-dimethoxybenzimidazole (4) (0.02 mol) in. aqueous hydrobromic acid (48 %, 35 ml) was refluxed for 1 h. On cooling, l-methyl 4,7-dihydroxybenzimidazolium chloride separates almost quantitatively; mp 265-267” (decomp.) (from dry ethanol-petroleum ether) (lit., 247-252” decomp). This compound was oxidized to the corresponding quinone by a literature procedure (4). The crude material (mp 169-172”) was crystallized by adding ligroine to a chloroform solution; mp 176177” (decomp.) (lit., 165170”). (Found: C, 59.4; H, 3.6; N, 17.3. Calc. for C,H,N,02: C, 59.3; H, 3.7; N, 17.3%). The bromohydrate of 5,6-dihydroxybenzimidazole IIa was prepared by literature methods (3). The chlorohydrate of 2-methyl5,6-dihydroxybenzimidazole IIb was prepared by the reaction of 2-methyl 5,6-dimethoxybenzimidazole (3) (0.01 mol) with aqueous hydrochloric acid (35 %, 15 ml) in a sealed glass tube heated at 100” for 20 h. The crude material (mp 320” decomp.), separated on cooling, was crystallized from dry ethanoldiethyl ether; yield 67x, mp 336” (decomp.). (Found: C, 48.1; H, 4.5; Cl, 17.9; N, 14.1. C,H,ClN,O, requires: C, 47.9; H, 4.5; Cl, 17.7; N, 13.96%).

ANION

RADICALS

FROM

333

IMIDAZOBENZOQUINONES

Anion Radicals

The radicals were generated by treating the quinones or the hydroquinones with potassium t-butoxide in dimethyl sulphoxide (DMSO) solution. This method was preferred to the electrolytic reduction since it has the advantage of giving radicals stable for several days and much better resolved ESR spectra. The radicals of the ortho derivatives II were produced from the hydroquinones since they can be prepared more easily than the corresponding quinones. However, it is well known that quinones and hydroquinones, in basic solution, give origin to the same semiquinone radicals (5). In some cases the radical were also produced electrolytically in deoxygenated acetonitrile containing (n-butyl),NClO, as supporting electrolyte. The main features of the ESR spectra were always the same observed in DMSO. RESULTS

AND

DISCUSSION

The hyperfine splitting constants of imidazo-p-benzoquinone benzohydroquinone IIa are listed in the table.

Ia and imidazo-o-

TABLE HYPERFINE

SPLITTING CONSTANTS (IN GA@ DERIVED FROM Ia, c AND Ila,

OF THE RADICALS

b

Ia

0.50

0.80

3.82

lb

3.77 3.67 2.96

IIa IIb

5.04 -

0.81 0.94 0.35 1.06 1.06

-

IC

0.35

-

0.53 0.33

-

0.69 1.09

5.02

In both cases the spectra arise from the interaction of the unpaired electron with two equivalent nitrogens, two equivalent protons and a single proton as previously observed for compound Ia by Nair, Santhanam, and Venkataraman (I). The latter proton has a hyperfine splitting constant of 0.50 and 5.04 gauss in the radicals from Ia and IIa, respectively. Since proton and nitrogen of the -NH- group usually experience very similar coupling constants (6), being the nitrogen splitting 1.06 gauss in Ha, it seems rather unlikely to admit that the observed splitting of 5.04 gauss comes from the attached proton. Moreover, simple HMO calculations carried out on the imidazo-obenzosemiquinone IIa using the same parameters of Ref. (I) for compound Ia, give a fairly large unpaired electron density on the carbon 2. Therefore the 5.04 gauss splitting should be due to the proton in the 2 position. To verify this point we examined the 2-methyl substituted derivatives Ib and IIb. Since methyl substitution does not alter the spin distribution too much in respect to the parent compound (7, 8) and because of the similarity of the QcH, and Qn values, the doublet splitting of the substituted proton will be replaced in the methyl derivative by a

334

PEDULLI

ET AL.

1: 3 : 3 : 1 quartet having almost the same separation, while the coupling constants of the other nuclei should remain practically unchanged. Of course, if the proton 2 is not coupled with the unpaired electron, no methyl splitting is expected and the spectra of the radicals from IIa and IIb should be almost identical. Actually this is not the case as the ESR spectrum of IIb (Fig. 1) shows a

4 FIG.

gauss

1. ESR spectrum of the radical from 2-methyl-5,6-dihydroxybenzimidazole

Ilb.

quartet separated by 5.02 gauss; similarly in the spectrum of Ib the 0.50 gauss doublet is replaced by a 0.69 gauss quartet. Consequently the interpretation given in the paper of Ref. (I) must be incorrect, as the smaller splitting in the radical from the imidazo p-benzoquinone should be assigned to the proton in position 2. The absence of coupling between the iminic proton and the unpaired electron in the radicals from Ia, b and IIa, b, suggests that they cannot retain the structure of the parent compounds. Our results can be better explained if the deprotonated dianionic structures III and IV are assigned to the observed radicals. Ionization of the hydrogen in position 1

0. :p,

R=H,CH3

0III

Iv

is not really unexpected, owing to the acidic character of the iminic group. Although pK, data for the present compounds are lacking, a value of 12.3 is reported in the literature for benzimidazole (9). In our systems, the presence of the electron-withdrawing quinonic function should decrease the above value by perhaps a couple of pK, units. A similar decrease has been actually observed on going from 2-chlorobenzimidazole (10) or I-methyl-2-methoxybenzimidazolium salts (II) to the corresponding 5-nitro-substituted derivatives. The dianionic character of our radicals is confirmed by the ESR spectrum of the radical produced by reduction in DMSO solution from 1-methylimidazo-p-benzoquinone Ic. In this case the substitution of the iminic hydrogen by a methyl-group will prevent any ionization and we should observe an asymmetric

ANION

RADICALS

FROM

335

fMIDAZOBENZOQUINONES

monoanion radical having the aromatic protons and the two nitrogens nonequivalent. The relative spectrum, reported in Fig. 2 together with the simulated one, shows in fact that the two nitrogens have different hyperhne splitting constants, one being 0.35 and the other 0.94 gauss, and that also the aromatic proton splittings differ by 0.71 gauss. Moreover the reasonably large coupling constant of the methyl protons (1.09 gauss)

FIG. 2. Experimental quinone Ic.

and simulated ESR spectrum of the radical from I-methylimidazo-p-benzo-

indicates that we should be able to observe a definite coupling from the proton in position 1, in the unsubstituted compounds, if deprotonation should not occur. We also tried to produce the radical from compound Ia by electrolytic methods (see Experimental), expecting that, by increasing the voltage very slowly, ionization of the hydrogen in position 1 could be avoided. Unfortunately at all reduction potentials employed we observed the dianion radical III. Therefore, it is impossible to state if ionization of the iminic proton precedes or succeeds the reduction to the radical form.

REFERENCES 1. M. K. V. NAIR, K. S. V. SANTHANAM, AND B. VENKATARAMAN, J. Magn. Mol. Phys. 19,585 (1970). 2. R. BENASSI, P. LAZARE~I, L. SCHENETTI, F. TADDEI, AND P. VIVARELLI,

(1971). 3. L. WEINBERGER AND A. R. DAY, J. Org. Chenz. 24,1451 4. E. R. ZAKHS AND L. S. EFKOS, Zh. Org. Khim. 2,1095

(1959). (1966).

Resonance Tetrahedron

9,229 Lett.

(1973); 3299

336 5. 6. 7. 8.

PEDULLI B. VENKATARAMAN,

B. G. SEGAL, AND G. K. FRAENKEL, J. Chem. Phys. 30,1006 (1959). M. T. MELCHIOR AND A. H. MAKI, J. Chem. Phys. 34,471 (1961). R. E. Moss, N. A. ASHFORD, R. G. LAWLER, AND G. K. FRAENKEL, J. Chem. Phys. 51,1765 P. CAVALIERI ~‘0~0, R. DANIELI, G. MACCAGNANI, P. PALMIERI, AND G. F. PEDULLI. Mol.

20,365 (1971). 9. H. F. W. TAYLOR, J. Chem. IO. A. Rrca AND P. VIVARELLI, II.

ET AL.

P. DEMBECH,

A. Rrccr,

Sot. 765 (1948). BON. Sci. Fat. Chim. Ind. Bologna 24,249 (1966). G. SECONI, AND P. VIVARELLI, J. Chem. Sot. (B), 2299 (1971).

(1969). Phys.