Indirect electrochemical oxidation of aryl alkyl ketones mediated by NaI-NaOH system: Facile and effective way to α-hydroxyketals

Article abstract of DOI:10.1016/S0040-4020(00)00951-0

Indirect electrochemical oxidation of aryl alkyl ketones in methanol in an undivided cell in the presence of sodium iodide-sodium hydroxide leads to the corresponding α-hydroxyketals in 75-85% substance yield (70-75% current yield). (C) 2000 Elsevier Scie

Full text of DOI:10.1016/S0040-4020(00)00951-0

TETRAHEDRON
Pergamon
Tetrahedron 56 (2000) 9999±10003
Indirect Electrochemical Oxidation of Aryl
Alkyl Ketones Mediated by NaI±NaOH System:
Facile and Effective Way to a-Hydroxyketals
*
Michail N. Elinson, Sergey K. Feducovich, Alexander S. Dorofeev, Anatolii N. Vereshchagin
and Gennady I. Nikishin
N. D. Zelinsky Institute of Organic Chemistry, Leninsky prospect 47, 117913 Moscow B-334, Russia
Received 6 July 2000; revised 20 September 2000; accepted 5 October 2000
AbstractÐIndirect electrochemical oxidation of aryl alkyl ketones in methanol in an undivided cell in the presence of sodium iodide±
sodium hydroxide leads to the corresponding a-hydroxyketals in 75±85% substance yield (70±75% current yield). q 2000 Elsevier Science
Ltd. All rights reserved.
The oxidation of ketones is a method for preparing
carboxylic acids and their derivatives, bifunctional
compounds such as a-hydroxyketones, diketones and
other useful intermediates in organic synthesis.1 The
formation of adipic acid from cyclohexanone is an impor-
tant industrial process. a-Hydroxyketones are signi®cant
`building blocks' in the construction of natural products
and ®ne chemicals.2,3 In the case of aryl alkyl ketones, the
corresponding a-hydroxyketones and a-hydroxyketals are
convenient compounds for synthesis of the pharmaco-
logically active 2-arylalkanoic acids.4,5
out in acetonitrile or tri¯uoroacetic acid as a result of the
subsequent transformation of the initially produced cation
radical R1R2CvO1z.11,12
In some oxidative transformations of ketones, such as in the
haloform reaction, the a-halogenation of ketones is an
important step.13 So for certain cases, the selective indirect
electrooxidation of ketones with the electrochemically
generated halides is also possible. Thus, the electrocatalytic
variant of the haloform reactionÐthe procedure to prepare
carboxylic acid esters by the electrooxidation of methyl
alkyl and methyl aryl ketones in methanol in the presence
of alkali metal bromides is well known.14 Aryl alkyl ketones
electrolysed in methyl orthoformate in the presence of
iodides give rise to methyl 2-arylalkanoates.15
The advance of electrooxidation in recent years has
provided organic chemists with a new versatile synthetic
device of great promise.6 Despite the long history of
the electroorganic chemistry, most of the electroorganic
reactions which could provide product-selectivity have
been developed only within last twenty years, and research
on various applications has spread gradually to cover many
areas of fundamental and industrial organic chemistry.
The mediatory system NaI±NaOH for the effective indirect
electrochemical oxidation of carbonyl compounds is also
known. It was employed for the indirect electrochemical
oxidation of aldehydes16 and cyclic ketones.16,17
However, in the case of the electrochemical oxidation of
ketones, only rare examples of the procedures which
could ensure product-selectivity are known.
Continuing our studies on the electrooxidation of
ketones,18±20 we have recently accomplished the indirect
electrochemical oxidation of aliphatic ketones 1a±e into
unsaturated conjugated esters 2a±f in methanol in an
undivided cell in the presence of the NaI±NaOH system21
(Scheme 1).
The direct electrochemical oxidation of ketones led to the
formation of a mixture of acids, saturated and unsaturated
hydrocarbons, carbon monoxide and dioxide.7±10 Remote
non selective oxidative functionalization of aliphatic
ketones was observed when electrooxidation was carried
Now we wish to report our results on the indirect electro-
chemical oxidation of aryl alkyl ketones 3a±f into
a-hydroxyketals 4a±f in methanol in an undivided cell in
the presence of the mediatory system NaI±NaOH (Table 1)
(Scheme 2).
Keywords: electrochemical reactions; aryl alkyl ketones; a-hydroxyketals.
* Corresponding author. Tel.: 17-095-938-35-42; fax: 17-095-135-58-
23; e-mail: elinson@cacr.ioc.ac.ru
0040±4020/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020(00)00951-0

10000
M. N. Elinson et al. / Tetrahedron 56 (2000) 9999±10003
Scheme 1.
Table 1. Electrooxidation of aryl alkyl ketones 3a±f (8 mmol of ketone, 10 mmol of NaI, 1 mmol of NaOH in 20 ml MeOH, Fe-cathode, C-anode, undivided
cell, 308C; conversion of 3a±f 98±100%)
N
Ketone
Current density mA/cm2
Electricity passed, F/mol
Product, yield [%]a
Current yield [%]
1
2
3
4
5
6
7
8
3ab
3a
100
100
200
200
200
300
400
200
200
200
200
200
200
200
2.3
2.3
2.3
2.3
2.3
2.3
2.3
3.5
4.5
2.3
2.3
2.3
2.3
2.3
4a, 52
4a, 64
4a, 82 (75)
4a, 68
4a, 80 (72)
4a, 78
4a, 65
4a, 79
4a, 83
4b, 86 (77)
4c, 75 (70)
4d, 84 (76)
4e, 78 (72)
4f, 85 (74)
45
56
71
59
70
68
57
45
37
75
65
73
68
74
3a
3ab
3ac
3a
3a
3ad
3ae
3b
3c
9
10
11
12
13
14
3d
3e
3f
a Determined by gas chromatography and NMR spectra, isolated yields are given in parentheses.
b 16 mmol of 3a.
c 608C.
d Without addition of NaOH.
Scheme 2.
Results and Discussion
out stirring of the reaction mixture, as well as evolution of
hydrogen at the cathode.
The reactions on electrodes, which take place during the
process are usual for the mediatory system NaI±NaOH in
methanol and lead to the formation of iodine at anode and
methoxide anions at cathode (Scheme 3).
The formation of iodine at the anode is observed by the
corresponding colour when electrolysis is conducted with-
Scheme 3.

M. N. Elinson et al. / Tetrahedron 56 (2000) 9999±10003
10001
Scheme 4.
Then a-monoiodination of the enol form of ketone takes
place in the solution in the same manner as it was previously
established for the cases of cyclic17 and aliphatic ketones.21
a-Iodoketone thus formed undergoes CH3O2 anion attack
on carbonyl group with further cyclization (Scheme 4).
thus facilitates the reaction of iodination of ketone. So the
addition of NaOH increases the general current ef®ciency of
the indirect electrooxidation of ketone in the undivided cell.
The system NaBr±NaOH is less effective for the indirect
electrochemical oxidation of aryl alkyl ketones compared to
the system NaI±NaOH. In the absence of NaOH and in the
presence of only NaBr in the analogous process, 4a was
obtained in 78% substance and 20% current yield. When
the mediatory system NaBr±NaOH was used, 4a was
formed in 83% substance and 37% current yield (exp. 9,
Table 1). Nevertheless, it should be also mentioned that
electrooxidation of 3a under the conditions of exp. 1
(Table 1) using the NaBr±NaOH system after passing
4.5 F/mol of electricity gave rise to 4a in 73% yield (32%
current yield). This result might indicate that with the
system NaBr±NaOH (even at the current density of
100 mA/cm2) the potential of the anode is suf®cient for
the direct oxidation of 5a into 3a in accordance with the
well-known reactions of the electrochemical cleavage of
1,2-diols.25,26 To con®rm this proposal the electrochemical
oxidation of 5a in the presence of NaBr±NaOH system
was performed and 4a was obtained in 62% yield (5 F/mol).
On the other hand, 5a is stable at the current density of
200 mA/cm2 in the presence of NaI±NaOH system.
The subsequent interaction of cyclic intermediate with the
second CH3O2 anion results in the formation of the end
product of the processÐthe corresponding a-hydroxyketal
(Scheme 5).
The analogous mechanism of the indirect electrooxidation
was previously demonstrated for the electrolytic trans-
formation of cyclohexanone into 2,2-dimethoxycyclo-
hexanol.17 The same mechanism is also known for the
reaction of a-halogenoketones with alkoxide anions in
usual organic chemistry.22
The electrooxidation of 3a when its concentration is 0.8 M
(16 mmol of ketone in 20 ml of MeOH) and at the current
density 100 mA/cm2 gave rise to 4a (52%) along with 3,4-
diphenyl-3,4-hexanediol 5a in 45% yield (exp. 1, Table 1).
The formation of 5a is a result of concurrent cathodic di-
merization of 3a23 under the conditions studied (Scheme 6).
The two-fold decrease in 3a concentration to 0.4 M (8 mmol
of ketone in 20 ml of MeOH) and an increase in the current
density up to 200 mA/cm2 depress cathodic dimerization of
3a to a suf®cient degree and the main cathodic process
becomes the formation of CH3O2 anions. Under the last
conditions, 4a was formed in 82% yield and 5a only in
13% yield (exp. 3, Table 1). Further increasing the current
density gives rise to a decrease in the selectivity of 4a
formation because of the pathways of direct electrochemical
oxidation7±10 and electrochemically induced oligomeri-
sation24 of ketones, which take place under these conditions.
The temperature of the process has no suf®cient in¯uence on
the electrooxidation 3a into 4a in the temperature interval of
30±608C.
Benzyl ethyl ketone 6, under the conditions studied, was
transformed into methyl 2-methyl-3-phenylpropionoate 7
in 88% yield. The more acidic benzylic proton in 6 changes
the reaction route and the Favorskii rearrangement becomes
the main route for the process. In this case, the CH3O2 anion
attacks not the carbonyl group, but eliminates the benzylic
proton in the intermediate a-iodoketone followed by cycli-
zation into unstable cyclopropanone 8 (Scheme 7).
Under the optimal conditions of 3a oxidation into 4a, other
aryl alkyl ketones 3b±e were converted into 4b±e in
75±86% substance yield (65±75% current yield).
The analogous mechanism has already been proposed for
the electrochemical oxidation of benzyl alkyl ketones in the
presence of NaBr.27
Using NaI without addition of NaOH decreases the current
yield of a-hydroxyketals from 70% to 45%. The addition of
NaOH has two effects. First, it increases the concentration
of CH3O2 anions in solution and thus accelerates all
processes with participation of CH3O2 anions. Second, it
increases the quantity of enol form in the starting ketone and
One more interesting fact is that dialkyl ketones 1 were
transformed, under the same conditions, into unsaturated
Scheme 5.
Scheme 6.

10002
M. N. Elinson et al. / Tetrahedron 56 (2000) 9999±10003
Scheme 7.
conjugated esters 2.21 In this case, the Favorskii rearrange-
ment also takes place, but the Favorskii rearrangement of
the double iodinated ketone (Scheme 8).
Experimental
GC analysis was carried out on Hewlett±Packard Model
5890 chromatograph with a ¯ame-ionisation detector.
Columns: (1) fused-silica capillary column HP-1 (5 m£
0.53 mm£2.65 mm), 2) glass column 3 m£3 mm with
10% FFAP on Chromaton N-Super (0.13±0.16 mm).
1H NMR spectra were run for solutions in CDCl3 and
recorded with Varian Unity 200 (200 MHz) spectrometer
[1H NMR spectrum of 7 on Varian Unity 500-PLUS
(500 MHz) spectrometer]. Chemical shifts are presented in
d scale with tetramethylsilane (TMS) used as an internal
standard. Mass-spectra (70 eV) were recorded with chroma-
tographic injection using Hewlett±Packard Model 5988A
spectrometer.
Thus, in the case of dialkyl ketones, the attack of CH3O2
anion on the carbonyl group have suf®cient sterical dif®-
culties as compared with aryl alkyl ketones having one
¯at substituent.
Another explanation of the different reaction ability of
dialkyl and aryl alkyl ketones, under the conditions studied,
could be a higher reactivity of carbonyl group in aryl alkyl
ketones owing to the conjugation with aryl-group.
In conclusion it should be mentioned that NaI±NaOH
electrocatalytic system can produce under mild conditions
direct `one-pot' effective transformation of aryl alkyl
ketones 3a±e into corresponding a-hydroxyketals 4a±e in
high substance and current yields. This process is also a
facile and economical method for the transformation of
aryl alkyl ketones into a-hydroxyketalsÐconvenient inter-
mediates for the synthesis of the pharmacologically active
2-arylpropanoic acids.4,5 The procedure utilises inexpensive
reagents, simple equipment and undivided cell, it is easily
carried out, and the work up is not complicated.
3,4-Diphenyl-3,4-hexanediol 5a was obtained by the known
method.32
General electrolysis procedure
A solution of ketone (8 mmol), NaI (10 mmol) and NaOH
(1 mmol) in methanol (20 ml) was electrolysed in an undi-
vided cell equipped with C-anode and Fe-cathode at 308C
under constant current density indicated in Table 1 until the
quantity of the electricity indicated in Table 1 was passed.
The reaction mixture was neutralised by dilute HCl, the
solvent was then removed, and the reaction mixture was
extracted with ether, washed with a solution of Na2S2O3 in
water, then with water, and dried with Na2SO4. After distil-
lation, compounds 4a±f were isolated.
Three step or two step methods of a-hydroxyketals 4a,e,f
conversion into 2-arylpropanoic acids include rearrange-
ment with 1,2-migration of the aryl group.28±31 In this
manner, for example, 4f can be transformed into (2-meth-
oxynaphth-2-yl)propanoic acid (Naproxen) in 80±85%
yield.28,29,31
1,1-Dimethoxy-1-phenyl-2-propanol (4a).33,34 Bp 75±
788C (0.08 Torr), [lit.34 bp 94±958 (0.5 Torr)]; H NMR
1
(CDCl3): d 0.93 (d, 3H, CH3, Jˆ6.5 Hz), 2.59 (d, 1H,
OH, Jˆ4.3 Hz), 3.18 (s, 3H, OCH3) 3.33 (s, 3H, OCH3),
4.10 (m, 1H, CH), 7.15±7.58 (m, 5H, C6H5).
1,1-Dimethoxy-1-phenyl-2-pentanol (4b).35 Bp 93±958C
(0.08 Torr), [lit.35 bp 80±828 (0.05 Torr)]; 1H NMR
(CDCl3): d 0.83 (t, 3H, CH3, Jˆ7.4 Hz), 1.20±1.58 (m,
4H, CH2), 2.53 (d, 1H, OH, Jˆ4.4 Hz), 3.21 (s, 3H,
OCH3) 3.33 (s, 3H, OCH3), 3.94 (m, 1H, CH), 7.16±7.60
(m, 5H, C6H5).
1,1-Dimethoxy-3-methyl-1-phenyl-2-butanol (4c).35 Bp
80±828C (0.06 Torr), [lit.35 bp 98±1008 (0.3 Torr)]; 1H
NMR (CDCl3): d 0.69 (d, 3H, CH3, Jˆ6.9 Hz), 0.88 (d,
3H, CH3, Jˆ6.9 Hz), 1.45 (m, 1H, CH), 2.63 (d, 1H, OH,
Jˆ4.2 Hz), 3.22 (s, 3H, OCH3) 3.28 (s, 3H, OCH3), 3.72 (m,
1H, CH), 7.16±7.60 (m, 5H, C6H5).
Scheme 8.

M. N. Elinson et al. / Tetrahedron 56 (2000) 9999±10003
10003
5. Rieu, J. P.; Boucherle, A.; Cousse, H.; Mouzin, G. Tetrahedron
1986, 42, 413.
1,1-Dimethoxy-1-phenyl-2-hexanol (4d). Bp 90±938C
(0.05 torr), 1H NMR (CDCl3): d 0.81 (t, 3H, CH3,
Jˆ6.6 Hz), 1.12±1.50 (m, 6H, CH2), 2.28 (d, 1H, OH,
Jˆ4.3 Hz), 3.21 (s, 3H, OCH3) 3.33 (s, 3H, OCH3), 3.89
(m, 1H, CH), 7.15±7.58 (m, 5H, C6H5); 13C NMR (CDCl3):
d 13.98 (q), 22.60 (t), 28.50 (t), 30.49 (t), 49.26 (q), 49.86
(q), 74.52 (d), 103.39 (s), 127.68 (d), 127.76 (d), 127.88 (d),
137.56 (s); MS (70 eV): m/z (relative intensity): 207
(M2OCH13 , 2), 152 (8), 150 (100), 121 (14), 105 (33), 91
(12), 77 (21), 59 (4). Anal. Calcd for C14H22O3: C, 70.56; H,
9.31. Found: C, 70.82; H, 9.39.
6. Shono, T. Electroorganic Chemistry as a New Tool in Organic
Synthesis. In Reactivity and Structure: Concepts in Organic
Chemistry; Springer: Berlin, 1984; Vol. 20.
7. Bradt, W. E.; Opp, C. J. Trans. Electrochem. Soc. 1931, 59, 237.
8. Yokoyama, M. Bull. Chem. Soc. Jpn 1933, 8, 71.
9. Pirrone, F. Gazz. Chim. Ital. 1936, 66, 244.
10. Shipley, J. W.; Rogers, M. T. Can. J. Res. 1939, 17B, 147.
11. Becker, J. Y.; Byrd, L. R.; Miller, L. L.; So, Y.-H. J. Am.
Chem. Soc. 1975, 97, 853.
12. Campbell, C. B.; Pletcher, D. Electrochim. Acta 1978, 23, 953.
13 Rappe, C. The Chemistry of the Carbon±Halogen Bond, pt. 2;
Patai, S. Ed.; Wiley: New York, 1973; p 1071.
14. Nikishin, G. I.; Elinson, M. N.; Makhova, I. V. Angew. Chem.
1988, 100, 1716.
1,1-Dimethoxy-1-(4-methylphenyl)-2-propanol (4e). Bp
72±758C (0.08 Torr), H NMR (CDCl3): d 0.96 (d, 3H,
1
CH3, Jˆ6.5 Hz), 2.34 (s, 3H, CH3), 2.38 (d, 1H, OH,
Jˆ4.1 Hz), 3.21 (s, 3H, OCH3) 3.34 (s, 3H, OCH3), 4.07
(m, 1H, CH), 7.07 (d, 2H, Ar, Jˆ7.5 Hz), 7.25 (d, 2H, Ar,
Jˆ7.5 Hz); 13C NMR (CDCl3): d 16.46 (q), 21.05 (q), 49.26
(q), 49.93 (q), 70.73 (d), 103.59 (s), 127.81 (d), 128.40 (d),
134.26 (s), 137.65 (s); MS (70 eV): m/z (relative intensity):
179 (M2OCH13 , 8), 165 (100), 150 (24), 149 (78), 119 (79),
105 (15), 91 (75), 77 (14), 57 (84). Anal. Calcd for
C12H18O3: C, 68.55; H, 8.63. Found: C, 68.73; H, 8.87.
15. Shono, T.; Matsumura, Y.; Katoh, S.; Fujita, T.; Kamada, T.
Tetrahedron Lett. 1989, 30, 371.
16. Shono, T.; Matsumura, Y.; Inoe, K.; Iwasaki, F. J. Chem. Soc.,
Perkin Trans. 1 1986, 73.
17. Barba, F.; Elinson, M. N.; Escudero, J.; Feducovich, S. K.
Tetrahedron 1997, 53, 4427.
18. Elinson, M. N.; Makhova, I. V.; Nikishin, G. I. Izv. Akad. Nauk
SSSR, Ser. Khim. 1991, 122.
1,1-Dimethoxy-1-(6-methoxynaphtyl-2)-2-propanol (4f).
Bp 102±1058C (0.05 Torr), viscous oil, H NMR (CDCl3):
19. Elinson, M. N.; Lizunova, T. L.; Nikishin, G. I. Izv. Akad.
Nauk SSSR, Ser. Khim. 1992, 154.
1
d 1.11 (d, 3H, CH3, Jˆ6.5 Hz), 2.64 (d, 1H, OH, Jˆ4.3 Hz),
3.37 (s, 3H, OCH3) 3.53 (s, 3H, OCH3), 4.01 (s, 3H, OCH3),
4.30 (m, 1H, CH), 7.20±8.00 (m, 6H, Ar); 13C NMR
(CDCl3): d 16.53 (q), 49.35 (q), 50.01 (q), 55.20 (q),
70.81 (d), 103.60 (s), 105.28 (d), 118.71 (d), 125.97 (d),
126.03 (d), 127.40 (d), 128.56 (s), 129.85 (d), 132.62 (s),
134.12 (s), 157.87 (s); MS (70 eV): m/z (relative intensity):
276 (M1, 4), 245 (M2OCH13 , 3), 231 (13), 216 (4), 185 (100),
157 (38), 142 (25), 114 (31), 89 (41), 45 (28). Anal. Calcd
for C16H20O4: C, 69.55; H, 7.30. Found: C, 69.73; H, 7.41.
20. Elinson, M. N.; Barba, F.; Galakhov, M. V.; Feducovich, S. K.;
Bushuev, S. G. Mendeleev Commun. 1995, 186.
21. Barba, F.; Elinson, M. N.; Escudero, J.; Guirado, M.;
Feducovich, S. K. Electrochim. Acta 1998, 43, 973.
22. Kende, A. S. In Organic Reactions, Wiley: New York, 1960;
Vol. 11, p 261.
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1969, 34, 2810.
24. Elinson, M. N.; Lizunova, T. L.; Nikishin, G. I. Izv. Akad.
Nauk SSSR, Ser. Khim. 1991, 2536.
Methyl 2-methyl-3-phenylpropionate (7).36,37 Bp 64±668
25. Shono, T.; Matsumura, Y.; Hashimoto, T.; Hibino, K.;
Hamaguchi, H.; Aoki, T. J. Am. Chem. Soc. 1975, 97, 2546.
26. Torii, S.; Uneyama, K.; Ueda, K. J. Org. Chem. 1984, 49, 1830.
27. Elinson, M. N.; Makhova, I. V.; Nikishin, G. I. Izv. Akad. Nauk
SSSR, Ser. Khim. 1989, 1208.
1
(0.08 Torr), [lit.37 bp 90±938 (1 Torr)]; H NMR (CDCl3):
1.15 (d, 3H, CH3, Jˆ7.0 Hz), 2.66 (d,d 1H, CH, J1ˆ8.0 Hz,
J2ˆ13.5 Hz), 2.88 (m, 1H, CH) 3.25 (d,d 1H, CH,
J1ˆ7.0 Hz, J2ˆ13.5 Hz), 3.63 (s, 3H, OCH3), 7.15±7.28
(m, 5H, Ar).
28. Tsuchihashi, G.; Kitajima, K.; Mitamura, S. Tetrahedron Lett.
1981, 22, 4305.
29. Schdemer, G.C. (Syntex Pharm. Int.), EP 64,394 (10 Nov.
1982), Chem. Abstr. 98, 143130b.
Acknowledgements
30. Sonawane, H. R.; Najundiah, B. C.; Kulkarni, D. G.; Ahuja,
R. J. Tetrahedron 1988, 44, 7319.
The authors gratefully acknowledge the ®nancial support of
the Russian Foundation for Basic Research (Project No.
00-03-32913a).
31. Yamauchi, T.; Hattori, K.; Ikeda, S.; Tamaki, K. J. Chem.
Soc., Perkin Trans. 1 1990, 1683.
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