9. i0. ii. 12. 13. 14. 15. 16.

G. I. Nikishin, M. N. Elinson, and I. V. Makhova, Izv. Akad. Nauk SSSR, Ser. Khim., 1919 (1984). H. Lund, Acta, Chem. Scand., Ii, 1323 (1957). W. C. Neikam, G. R. Dimeler, and M. M. Desmond, J. Electrochem. Soc., iii, 1190 (1964). M. Fleischman and D. Pletcher, Tetrahedron Lett., 6255 (1968). M. Rakoutz, D. Michelet, B. Brossard, and J. Varagnat, Tetrahedron Lett., 3723 (1978)o C. Walling and W. Thaler, J. Am. Chem. Soc., 83, 3877 (1961). H.-F. Gruzmacher and J. Winkler, Org. Mass Spectrom., 295 (1968). A. Nilsson, U. Palmquist, T. Pettersen, and A. Ronlan, J. Chem. Soc., Perkin Trans. I,

708 (1978).

ELECTROCHEMICAL HYDRODIMERIZATION OF CARBONYL COMPOUNDS IN APROTIC MEDIA V. P. Gul'tyai, A. S. Mendkovich, and T. Ya. Rubinskaya

UDC 541.138.3:547.572.1

In the investigation of the process of the electroreduction (ER) of aromatic carbonyl compounds in aprotic media, it was established that the primary products at the first stage of the electrode reaction are anion-radicals (ARs); the products of the second stage are dianions [i, 2]. The assumption is thereby made that the main reaction of the ARs in the aprotic media is dimerization, which leads to pinacone (PC). Under these conditions, the dianion, which possesses significantly higher basicity, can be protonated with the formation of the corresponding alcohol. We previously proposed [3, 4] a method of calculating the rate constants for the reactions of the ARs and dianions under conditions where the specific interactions of these entities with the components of the solution (e.g., in the solutions of tetraalkylammonium salts in dry DMF) can be ignored. In the framework of such an approach to the ER of aromatic carbonyl compounds, neither PC nor alcohol can be the main products of the electrolysis due to the competing rections of the "head-tail" type under the indicated conditions. This conclusion is consistent with the results of the preparative ER of carbonyl derivatives of the thiophene series [5, 6]. However, the practically quantitative yield of PC is ascertained as the result of the electrolysis of acetophenone (I) in dry MeCN and DMF in the presence of quaternary ammonium salts in [7, 8]. Moreover, the solutions of the tetraethylammonium salts in the dry aprotic solvents are recommended in [8] as the optimal media for the preparative isolation of the aliphatic-aromatic PCs. This conclusion is in contradiction with the data of [9], where it was shown that the cyanidation products are formed together with the PCs by the electrolysis of aromatic ketones in MeCN. The given work is devoted to a more detailed investigation of the process of the ER of (I) in "dry" DMF at a controlled potential. EXPERIMENTAL Reagents. The DMF was first dried over molecular sieves; it was then distilled in vacuo. The water content according to Fischer was <0.04%. In a series of cases, the DMF was passed through a layer of calcined neutral AI203 instead of the distillation. The salts of the electrolyte present were dried in vacuo at 60~ before utilization. The commercial acetophenone (I) was purified by fractional distillation in vacuo. Electrolysis. The electrolysis was performed at a controlled potential with the aid of a P-5848 potentiostat (Gomel') in cells with a glass diaphragm. The cathode was mercury; the anode was platinum gauze. The potentials were measured relatively to the adjustable N. D. Zelinskii Institute of Organic Chemistry, Academy of Sciences of the USSR, Moscow. Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 7, pp. 1576-1579, July, 1987. Original article submitted December 29, 1985.

0568-5230/87/3607-1455512.50 9 1988 Plenum Publishing Corporation

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TABLE i. Yield of 2,3-Diphenylbutane-2,3-diol (II) in the Electroreduction of Acetophenone in "Dry" DMF

r

E, V (rel. to

Background electrolyte adjustable electrode) 0,2 M Bu~NI 0,2 M Bu~NI, t0~ EtOH 0,2 M Bu4NI, 0,2 M LiC104 0,2 M Bu~NI

i,95 1,95 1,90 2,35

F

Yield, percent n exptl, 0,? 1,3

theor.

30 45 70

26

25

25,

Note. The cathode potential during the electrolysis is given by E; the number of the electrons is given by n. electrode connected with the investigated solution by a bridge with the glass diaphragm, and filled with the background electrolyte. Treatment of the Reaction Mixture. After the electrolysis, the solution was evaporated in vacuo. The residue was diluted with water, acidified by dilute HCI to the pH 6.0, and extracted by ether. The chromatographic determination of the yield of 2,3-diphenylbutane2,3-dioi (II) was performed on a column of i = i m with 3% OV-17 on Chromaton N-AW-HMDS using a flame ionization detector at 190~ the standard was o-nitrodiphenyl. DISCUSSION The preliminary experiments on the ER of (I) in "dry" DMF in the presence of Bu~NI at the potential of the limiting current of the first stage showed that the yield of the PC of (II) is low under these conditions. According to the TLC data, a significant amount of resinforming substances and minor amounts of unstable compounds are formed. This contradicted the conclusions of [8] on the quantitative formation of PCs under the analogous conditions. The analysis of the works [7, 8] showed that the error of the authors in the determination of the yield of PC cannot be excluded, since their method [i0] was based on the utilization of the proportion of the integral values of the signals in the PMR spectrum of the product obtained in t h ~ treatment of the reaction mixture. / The quaStltative data for the dependence of the yield of (II), based on the amount of (I) introduced into the reaction, on the composition of the electrolyte present was obtained by us using the method of GLC (Table i). As can be seen from Table i, there is adequately good conformity between the yields of (II), calculated on the basis of the theoretical values of the rate constants of the subsequent reactions of the ARs and the dianions (I) [4], and the experimental values. It was previously noted [6] in a study of the hydrodimerization of thiophene-2-aldehyde that the change in the yield of the corresponding PC on changing the composition of the solution was determined by the redistribution of the electron density in the AR. The formation of associates (ion pairs of the AR with alkali metal cations, H-bonds between the AR and water molecules) assists the increase of the electron density on the CO group, which leads to an increase in the rate constant of the pinaconization reaction. The results of the ER of (I) in the presence of Li cations and alcohol (el. Table i) indicate that (I) does not differ from the carbonyl derivatives of thiophene in this regard. /

.

It follows from theoretical presentations that the low yield of the PC in "dry" DMF is determined by the presence of competing reactions leading to the products of the "head-tail" and " t a i l ~ a i l " type. The products of such a type were rapeatedly recorded previously in the ER of aromatic and heteroaromatic carbonyl derivatives, e.g., o-aminoacetophenone [ii], lacetylnaphthalene [12], 9-formylanthracene [3], thiophene-2-aldehyde [13], and esters of anthracene acids [14]. The attempts which we made to stabilize the possible dimeric derivatives of (I), which differ from (II), by the treatment of the postelectrolysis solution with bromine water (according to the method of [15]) were unsuccessful, since (II) is oxidized by bromine water with the formation of products of an unestablished structure. It should also be noted that the dimeric products of the "head~ail" type in the case of (I) should readily enter the reaction leading to resin-forming products. It indicates the presence of the reactions of resin formation when the electrolysis is performed with 0.2 M Bu4NI (cf. Table i) and that n < i in this case.

1456

The mechanism of the ER of aromatic carbonyl compounds in aprotic media at the potentials of the second stage of the process, corresponding with the formation of the dianion, merits special consideration. It was previously noted [3, 16] that the value of the limiting current of the second wave on the polarograms of a series of aromatic ketones depends on the concentration of the depolarizer, and that the total limiting current of the two waves does not reach the two-electron level in some cases. In [16], this phenomenon was explained by the fact that the dianion which is formed possesses a high basicity and can therefore break off the protons from the depolarizer molecule. However, it is readily shown by utilizing the approach described in [3, 4] that the difference in the basicity of the AR and the dianion cannot be large in the case of the orbitally controlled protonization reaction - according to our evaluations, the rates of the protonization of the AR and the dianion when acted on by (I) should not differ by more than twofold to threefold. Therefore, it seems more probable that the dimeric dianions of three possible structures [3], also including the dianion of the PC, are the products of the reactions of the dianion with the depolarizer molecule in the case of the aromatic ketones~ The carrying out of the electrolysis of (I) at the potentials of the second stage of reduction (cf. Table i) is accompanied by the significantly more rapid decrease in the current than is observed in the case of the process which is uncomplicated by the subsequent reactions. This phenomenon is analogous to the anomalously low slopes of the logarithmic plots of the current-time dependences on the dropping mercury electrode, which we previously observed [3, 17] for the processes in which the product of the electrode reaction reacts with the depolarizer with the formation of electrochemically inactive products. The data of polarography and GLC indicate that the complete depletion of the depolarizer occurs during the electrolysis. The detection of (I) is also unsuccessful after the acidification of the electrolysis solution; this may possibly be expected in the presence of the selfprotonization proposed in [16]o After the treatment of the reaction mixture, we only observed (II) (cf. Table i) in the absence of methyl phenyl carbinol; the amounts were determined by the method of GLC. These experimental results are in conformity with theoretical representations [3]. Therefore, there is conformity between the data obtained for the preparative ER of (I) in DMF in the presence of the tetralkylammonium salts and the results of the theoretical calculation of the rate constants of its AIR and the dianion. The main reactions of the primary products of the ER at the potentials of the limiting current of the first and second polarographic waves of (I) are the dimerization of the two ARs and the reaction of the dianion with (I), respectively. The PC is formed in both cases, but it is not the only product due to the presence of competing reactions leading to resin-forming products. The optimal conditions for the isolation of the aliphatic-aromatic PCs in the ER of aromatic carbonyl derivatives in aprotic solvents are characterized by the presence, in the electrolyte solution, of components (water, alcohols, metal cations) which are able to form associates with the products of the electron transfer reactions. CONCLUSIONS i. On the basis of the results of the theoretical calculation and the data on the electrolysis of acetophenone in DMF in the presence of a tetrabutylammonium salt, it was shown that the main stable product at the potentials of both the first and the second stages of the electroreduction is 2,3-diphenylbutane-2,3-diol (25-30% yield). 2. Under the conditions of the formation of associates between the anion radicals of acetophenone and the lithium cations or the ethanol molecules, the yield of the pinacone increases by the factor of 1.5-2. LITERATURE CITED I. 2. 3. 4. 5.

C . K . Mann and K. K. Barnes, Electrochemical Reactions in Nonaqueous Systems, Marcel Dekker, New York (1970). D . H . Evans, Encyclopedia of Electrochemistry of the Elements, Vol. 12, Marcel Dekker, New York (1978), p. 1 7 0 . A . S . Mendkovich, A. P. Churilina, L. V. Michalchenko, and V. P. Gul~tyai (Gultyai)~ Sandbjerg Meeting 1982 on Organic Electrochemistry, Abstracts of Paper, Aarhus (1982), p. 23. V.P. Gul'tyai (Gultyai) andA. S. Mendkovich, J. Electroanal. Chem., 45, 201 (1983). V . P . Gul'tyai (Gultyai), L. M. Korotaeva, A. S. Mendkovich, and I. V. Proskurovskaya, Izv. Akad. Nauk SSSR, Ser. Khim., 834 (1981).

1457

6. 7. 8. 9. i0. ii. 12. 13. 14. 15. 16. 17.

L. N. Nekrasov, I. N. Vykhodtseva, L. M. Korotatayeva, and V. P. Gul'tyai (Gultyai), J. Electroanal. Chem., 138, 177 (1982). J. H. Stocker and R. M. Jenevein, Collect. Czech. Chem. Commun., 36, 925 (1971). W. J. V. Tilborg and C. J. Smit, Tetrahedron Lett., 3651 (1977). E. M. Abbot, A. J. Bellamy, J. B. Kerr, and L. S. Mackirdy, J. Chem. Soc. Perkin Trans. 2, 425 (1982). J. H. Stocker, D. H. Kern, and R. M. Jenevein, J. 0rg. Chem., 33, 412 (1968). H. Lund and A. Thomsen, Acta Chem. Scand., 23, 3567 (1969). J. Grimschow and E. J. F. Rea, J. Chem. Soc. C, 2628 (1967). V. P. Gul'tyai, L. M. Korotaeva, A. P. Rodionov, and A. M. Moiseenkov, Izv. Akad. Nauk SSSR, Ser. Khim., 1150 (1981). C. Z. Smith and J. H. P. Utley, J. Chem. Res., 18 (1982). G. R. Strelets and D. V. Ioffe, Zh. Org. Khim., 9, 2432 (1973). M. A. Michel, G. Mousset, and J. Simonet, J. Electroanal. Chem., 98, 319 (1979). A. S. Mendkovich, L. V. Martynova, V. N. Leibzon, and V. P. Gul'tyai, Elektrokhimiya, 19, 264 (1983).

INTRAMOLECULAR NUCLEOPHILIC

SUBSTITUTION OF FLUORINE

IN ~-PENTAFLUOROPHENYL-N-PHENYLNITRONE UDC 542.91:547.587.1'161:547.815.1'161

N. I. Petrenko and T. N. Gerasimova

Intramolecular nucleophilic substitution reactions in polyfluoroaromatics most often involve N- or O- containing groups [i, 2] as the nucleophile, in particular the N-oxide function [3]. In this study we have used ~-pentafluorophenyl-N-phenylnitrone (I) [4] as a representative polyfluoroaromatic nitrone. The usual conditions for this reaction include heating in DMF, sometimes in the presence of base (KF, K2C03, NaH) [i, 2]. We have shown that heating I in DMF leads to separation of a fluoride ion to give a mixture of octafluoroxanthone (II) [5] and tetrafluorosalicylanilide (III)

F

CGF~CH=NPh

DMF

F

0

F

I

II

I

//

F

F

F

F

F

I

C

I

OH

O

)

i00--ii0 ~

(I)

F

0

F

F

F (If)

(IIl)

Formation of carboxanilides from aromatic nitrones in acid or basic media has been frequently reported [6, 7]. In the presence case it can be suggested that the intermediate 2,3-dihydro-l,2-benzisoxazole (A) is formed as a result of intramolecular attack of the 0 atom at a C atom of the pentafluorophenyl ring with decomposition under the reaction conditions by intramolecular elimination. FH F

F

/OH C

I~20

(III)

(A) Novosibirsk Institute of Organic Chemistry, Siberian Branch of the Academy of Sciences of the USSR. Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 7, pp. 1579-1582, July, 1987. Original article submitted November i0, 1985.

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0568-5230/87/3607-1458512-50

9 1988 Plenum Publishing Corporation