Reaction of the electrogenerated cyanomethyl anion with carbonyl compounds: A clean and safe synthesis of β-hydroxynitriles

Article abstract of DOI:10.1002/ejoc.200900527

The electrogenerated cyanomethyl anion reacts with carbonyl compounds to yield the corresponding β-hydroxymtriles in moderate to high yields. The reported methodology is very clean and safe, avoiding the use of any classical base or catalyst.

Full text of DOI:10.1002/ejoc.200900527

SHORT COMMUNICATION
DOI: 10.1002/ejoc.200900527
Reaction of the Electrogenerated Cyanomethyl Anion with Carbonyl
Compounds: A Clean and Safe Synthesis of β-Hydroxynitriles
Gabriele Bianchi,[a] Marta Feroci,[b] and Leucio Rossi*[a]
Keywords: Green chemistry / Electrochemistry / Nucleophilic addition / Carbanions / Cyanides
The electrogenerated cyanomethyl anion reacts with car-
bonyl compounds to yield the corresponding β-hydroxyni-
triles in moderate to high yields. The reported methodology
is very clean and safe, avoiding the use of any classical base
or catalyst.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2009)
In order to avoid these limitations we have developed a
new electrochemical methodology for the synthesis of β-hy-
droxynitriles under mild conditions, preventing the use of
toxic and hazardous reagents. Among the different green
methodologies recently reported, organic electrochemis-
try[15] could be rightly considered a powerful tool for clean
and safe organic syntheses, providing a valuable alternative
to the use of conventional reagents for fine chemical synthe-
sis. In particular, the use of carbanions stabilized by elec-
tron-withdrawing groups has received, in the last years,
great attention, especially because of their use in the forma-
tion of carbon–carbon bonds.
Introduction
β-Hydroxynitriles are useful and versatile synthetic inter-
mediates in organic synthesis.[1] The nitrile group is a valu-
able precursor of both amino and carbonyl compounds[2]
and β-hydroxynitriles can be easily converted into useful
building blocks, such as β-hydroxycarboxylic acid deriva-
tives and γ-amino alcohols.[3] β-Hydroxynitriles are usually
prepared by ring-opening reactions of epoxides with cya-
nides (HCN,[4] NaCN,[1,5] KCN,[6] LiCN,[7] cyanide ex-
change resin,[8] TMSCN[9]) or with acetone cyanohydrin
under basic conditions.[10] Cyanomethylation of carbonyl
compounds is another useful method for the synthesis of β-
hydroxynitriles. Simple alkylnitriles are among the least
acidic carbon pronucleophiles, and the preparation of α-
cyano carbanions from alkylnitriles usually requires more
than stoichiometric amounts of a strong base in a separate
process.[11] The reaction is also performed by using a pro-
azaphosphatrane as a Brønsted base catalyst in the presence
of magnesium sulfate.[12] Alternatively, the reaction was car-
ried out by CuF-catalyzed cyanomethylation by
TMSCH2CN.[13] The CuI-catalyzed direct enantioselective
cross-aldol-type reaction of acetonitrile has been recently
reported.[14] All the reported methods suffer from several
drawbacks including severe reaction conditions, high reac-
tion temperatures, expensive and hazardous reagents, hy-
groscopic and nonrecyclable catalysts, and the use of vola-
tile and toxic HCN. In addition, the above-mentioned ring-
opening reactions of epoxides usually require protic sol-
vents and additives and result in the formation of mixtures
of regioisomers.[5a,6a,9a]
Results and Discussion
The cyanomethyl anion, usually obtained by deproton-
ation of acetonitrile (pKa = 31.3) by suitable bases (alkali
metal hydrides, amides, and alkoxides), has found wide ap-
plication as a nucleophile in several reactions such as the
addition reaction to carbonyl compounds and imines and
Michael-type addition reactions. Recently, our research
group reviewed the generation of the cyanomethyl anion by
nonconventional electrochemical procedures.[16] In particu-
lar, we found that the cyanomethyl anion generated by di-
rect electrolysis, under galvanostatic control, of CH3CN/
Et4NBF4 (TEATFB = tetraethylammonium tetrafluoro-
borate) solutions can be used as a base in a large number
of organic reactions. In combination with carbon dioxide,
the electrogenerated cyanomethyl anion proved to be a
powerful carboxylating reagent for the synthesis of carb-
amates,[17] oxazolidine-2-ones,[18] and oxazolidine-2,4-di-
ones.[19] In addition, it has proved to be effective, as a base,
for the selective activation of amidic and aminic N–H[20]
and C–H[21] bonds in cyclization reactions, leading to the
formation of β-lactams,[22] butenolides, pyrrolin-2-ones,[23]
indoles,[24] and so on.
[a] Dipartimento di Chimica, Ingegneria Chimica e Materiali, Uni-
versità degli Studi dell’Aquila
Monteluco di Roio, 67040 L’Aquila, Italy
Fax: +39-0862434203
E-mail: leucio.rossi@univaq.it
[b] Dipartimento di Ingegneria Chimica Materiali Ambiente, Uni-
versità “La Sapienza”
via del Castro Laurenziano 7, 00161 Roma, Italy
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© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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G. Bianchi, M. Feroci, L. Rossi
SHORT COMMUNICATION
In order to exploit the reactivity of the electrogenerated Table 1. Reaction of benzaldehyde with the electrogenerated cya-
nomethyl anion under different reaction conditions.[a]
cyanomethyl anion as a nucleophile, we considered the pos-
sible reaction of CH2CN– with carbonyl compounds (alde-
hydes and ketones) for the synthesis of β-hydroxynitriles.
The electrolysis of carbonyl compounds (acetophenone,
benzaldehyde, and benzophenone) in dry CH3CN (1.0  in
TEATFB) solution was first reported by Bellamy.[25] The
reaction gave significant yield of different cyanomethylation
products not including β-hydroxynitriles. The author ex-
cluded the generation of the cyanomethyl anion by direct
or indirect reduction of the solvent. Bellamy also studied
the indirect electrogeneration of the cyanomethyl anion by
reduction of a probase (azobenzene) in CH3CN or by re-
duction of cyanomethyl phosphonium or arsonium salts in
super-dry DMF[26] and its reaction with carbonyl com-
pounds. Alternatively, Troupel[27] indirectly obtained the cy-
anomethyl anion by using the electrogenerated base (EGB)
phenyl anion by using a sacrificial magnesium anode. Mod-
erate to high yields of β-hydroxynitriles were obtained. The
electrochemical synthesis of 3-phenylcinnamonitrile by re-
duction of benzophenone in acetonitrile was also reported.
Entry Q [Fmol–1][b] Method[c] T [°C]
t [h]
Yield [%][d]
1
2
3
4
5
6
7
1.2
2.0
3.0
2.0
2.0
2.0
2.0
A
A
A
A
B
B
B
20
20
20
80
20
0
24
24
24
20
0.5
2
64 (25)
71 (18)
67 (23)
77
87
87
–20
4
81
[a] The electrolyses were carried out, under galvanostatic control,
in a divided cell equipped with Pt electrodes at 0 °C. [b] The value
Q refers to the amount of benzaldehyde (0.5 mmol) used in the
experiment. [c] Method A: benzaldehyde in anhydrous MeCN
(3.0 mL) was added in “one-portion” to the cathodic solution of
0.1  MeCN in TEATFB (2.0 mL); Method B: benzaldehyde in
anhydrous MeCN (2.0 mL) was added dropwise over 30 min to the
cathodic solution of 0.1  MeCN in TEATFB (3.0 mL). [d] Yields
refer to isolated products. Yields in parentheses refer to the reco-
vered benzaldehyde.
The cyanomethyl anion was obtained by benzophenone tion” to the catholyte containing electrogenerated –CH2CN,
radical anion that acted as the EGB.[28]
and method B that consists of the dropwise addition of the
Here we report a clear and safe method for the synthesis aldehyde solution to the catholyte over 30 min. Reactions
of several β-hydroxynitriles by nucleophilic addition of the carried out with method B provided higher reaction yields
electrogenerated cyanomethyl anion to carbonyl com- than those obtained with method A. In addition, tempera-
pounds in moderate to very high yields. The anion was sim- ture also played an important role: in method A raising the
ply obtained, according to the usual way, by electrochemi- temperature from room temperature to 80 °C gave an in-
cal reduction, under galvanostatic control, of CH3CN/TE- crease in the reaction yields. Conversely, method B did not
ATFB solutions (Scheme 1). 3-Aminocrotonitrile was also seem to be affected by temperature variation; reaction
produced in small amounts during the electrolysis. Con- yields were very good even when low temperatures were
cerning the reaction mechanism, a mechanistic hypothesis used. Only an increase in reaction time was observed.
has been proposed.[18b]
The procedure was then tested over a large number of
aliphatic and aromatic aldehydes and ketones. Results are
reported in Table 2. Beside sterically hindered 2-naphth-
aldehyde (57% yield), both aromatic and aliphatic alde-
hydes provided very good yields of hydroxynitriles (67–92%
isolated yield). Method B is preferred over method A, as it
provides better reaction yields and the formation of byprod-
ucts (double substitution and aldolic condensation prod-
ucts; Table 2, Entries 2 and 5, respectively) that appear
when the reaction is carried out at high temperature is
avoided. Cyclic ketones usually react very well even when
Scheme 1. Reaction of the electrogenerated cyanomethyl anion
with carbonyl compounds: synthesis of β-hydroxynitriles.
In order to optimize the reaction conditions, benzalde- method A is adopted (Table 2, Entries 13–16); only with
hyde was used as a model compound in a series of experi- substituted cyclohexanones (Table 2, Entries 17–20) was
ments that showed that better reaction yields were obtained method B preferred to method A, although the reactions
at room temperature by using method B and an amount of needed to be carried out at room temperature. Aromatic
electricity of 2.0 Fmol–1 relative to the starting benzalde- and linear ketones (Table 2, Entries 20–22) provided only
hyde (R1 = Ph, R2 = H; Table 1, Entry 5).
moderate yields of the corresponding β-hydroxynitriles.
Electrolysis of 0.1  CH3CN solutions in TEATFB were
A further investigation was carried out by using different
carried out, under galvanostatic control, in a divided cell nitriles than acetonitrile. Results are reported in Scheme 2.
equipped with platinum electrodes at 0 °C. Reaction yields Benzaldehyde was used as the starting compound. Both lin-
are related to the amount of electricity supplied to the cath- ear propionitrile and branched 2-methylpropionitrile re-
ode. Best yields were obtained when Q = 2.0 Fmol–1 of al- acted with benzaldehyde to yield the corresponding β-hy-
dehyde were used. Another interesting factor that affects droxynitriles in quite good yields. This result opens the way
the reaction yield was the rate of aldehyde addition. Two to an effective method for the synthesis of α-branched β-
methods are reported: method A, in which benzaldehyde hydroxynitriles. Further improvements in this methodology
dissolved in anhydrous CH3CN was added in “one-por- are now under study.
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Eur. J. Org. Chem. 2009, 3863–3866

Clean and Safe Synthesis of β-Hydroxynitriles
Table 2. Reaction of different carbonyl compounds with the elec-
Experimental Section
trogenerated cyanomethyl anion: synthesis of β-hydroxynitriles.[a]
General Procedure for Electrochemical Addition of Cyanomethyl
Anion to Carbonyl Compounds
Entry R1
R2
Method[b] T [°C] t [h] Yield [%][c]
1
2
3
4
5
6
7
8
2-furyl
2-furyl
2-furyl
n-heptyl
n-heptyl
n-heptyl
4-CH3OC6H4
4-CH3OC6H4
2-naphthyl
2-naphthyl
4-CH3C6H4
4-ClC6H4
(CH2)5
(CH2)4
(CH2)4
(CH2)6
H
H
H
H
H
H
H
H
H
H
H
H
A
A
B
A
A
B[f]
A
B
20
80
–20
20
80
–20
20
–20
20
–20
–20
–20
20
20
–20
20
20
1
1
20
1
2
16
2
4
4
2
77
62[d]
82
Method A: A solution of anhydrous CH3CN (0.1  in TEATFB,
10.0 mL) was electrolyzed in a divided cell under galvanostatic con-
trol [Pt electrodes; J = 50 mAcm–2; Q = 2.0 Fmol–1 (t = 33 min)
relative to carbonyl compound] at 0 °C under an atmosphere of
argon. At the end of electrolysis the carbonyl compound
(0.5 mmol) in anhydrous CH3CN (1.0 mL) was added to the cath-
odic compartment, and the reaction was prolonged at the tempera-
ture and for the time reported in Table 2. The solvent was then
evaporated under reduced pressure, and the crude reaction mixture
was purified by flash chromatography on silica gel (ethyl acetate/
hexane).
61
53[e]
73
67
92
11
57
81
67
90
70
76
81
51
73
49
78
35 (49)
76 (12)
9
A
B
B
10
11
12
13
14
15
16
17
18
19
20
21
22
B
4
2
A
A
B
A
A
B
48
24
48
24
24
24
24
8
Method B: A solution of CH3CN (0.1  in TEATFB) was electro-
lyzed according to the conditions reported in method A. At the
end of electrolysis, the carbonyl compound (0.5 mmol) in CH3CN
(5.0 mL) was added to the cathodic solution over 30 min. The reac-
tion mixture was allowed to stand at the temperature and for the
time reported in Table 2, after which the mixture was subjected to
the work up procedure reported in method A.
(CH2)2CH(C6H5)(CH2)2
(CH2)2CH(C6H5)(CH2)2
(CH2)2CH(tBu)(CH2)2
(CH2)2CH(tBu)(CH2)2
C6H5
C6H5
20
20
20
20
20
–20
A
B
CH3
CH3
A
B[f]
15
2-(1-Hydroxy-4-phenylcyclohexyl)acetonitrile: Yield: 79 mg, 73%.
1
[a] The electrolyses were carried out, under galvanostatic control,
in a divided cell equipped with Pt electrodes at 0 °C. An amount
of electricity of 2.0 Fmol–1 was supplied to the electrodes;
0.5 mmol of carbonyl compound was used in each experiment.
[b] Method A: substrate in anhydrous MeCN (3.0 mL) was added
in “one-portion” to the cathodic solution of 0.1  MeCN in TE-
ATFB (2.0 mL); Method B: substrate in anhydrous MeCN
(2.0 mL) was added dropwise over 30 min to the cathodic solution
of 0.1  MeCN in TEATFB (3.0 mL). [c] Yields refer to isolated
products. Yields in parentheses refer to recovered starting material.
[d] 3-(2-Furyl)pentanedinitrile was isolated in 28% yield as a by-
product. Substrate was added dropwise over 30 min. [e] 2-Hex-
ylnon-2-enal was isolated as a byproduct in 18% yield. [f] Substrate
was added dropwise over 60 min.
M.p. 105–106 °C. H NMR (200 MHz, CDCl3): δ = 1.41–2.08 (m,
8 H, CH2CCH2), 2.15–2.30 (br. s, 1 H, OH), 2.55 (s, 2 H, CH2CN),
7.22–7.35 (m, 5 H, arom.) ppm. 13C NMR (50.3 MHz, CDCl3): δ
= 27.8, 32.2, 35.8, 42.3, 68.2, 116.3, 125.2, 125.7, 127.4, 145.2 ppm.
MS (EI): m/z (%) = 215 (28) [M+], 91 (98), 73 (100). C14H17NO
(215.29): calcd. C 78.10, H 7.96, N 6.51; found C 77.88, H 7.77, N
6.44.
2-(1-Hydroxycycloheptyl)acetonitrile: Yield: 62 mg, 81%. M.p. 41–
42 °C. 1H NMR (200 MHz, CDCl3): δ = 1.11–1.33 (m, 2 H,
CHCCH), 1.38–1.73 [m, 10 H, CH(CH2)4CH], 2.47 (s, 2 H,
CH2CN), 2.63–2.68 (br. s, 1 H, OH) ppm. 13C NMR (50.3 MHz,
CDCl3): δ = 21.7, 25.0, 31.7, 36.8, 69.9, 117.6 ppm. MS (EI): m/z
(%) = 98 (85), 81 (100), 57 (63). C7H11NO (125.17): calcd. C 70.55,
H 9.87, N 9.14; found C 69.23, H 9.71, N 9.19.
Acknowledgments
This work was supported by the Ministero dell’Università e della
Ricerca (MIUR) PRIN 2006 and CNR (Rome).
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–
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3865

G. Bianchi, M. Feroci, L. Rossi
SHORT COMMUNICATION
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Received: May 11, 2009
Published Online: June 30, 2009
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