Regioselective reduction of 2-cyanoprop-2-enethioamides with sodium borohydride

Article abstract of DOI:10.1007/s11172-012-0321-1

Treatment of 2-cyanoprop-2-enethioamides with NaBH₄ in EtOH gave C-alkylated cyanothioacetamides in 62-69% yields.

Full text of DOI:10.1007/s11172-012-0321-1

Russian Chemical Bulletin, International Edition, Vol. 61, No. 12, pp. 2261—2264, December, 2012
2261
Regioselective reduction of 2ꢀcyanopropꢀ2ꢀenethioamides
with sodium borohydride
V. V. Dotsenkoa,b and S. G. Krivokolyskoa
aChemEx Laboratory, V. Dal´ EastꢀUkranian National University,
20A Molodyozhny kv., 91034 Lugansk, Ukraine.
Eꢀmail: Victor_Dotsenko@bigmir.net
bState Enterprise "Luganskstandartmetrologiya",
50 ul. Timiryazeva, 91021 Lugansk, Ukraine
Treatment of 2ꢀcyanopropꢀ2ꢀenethioamides with NaBH4 in EtOH gave Cꢀalkylated cyanoꢀ
thioacetamides in 62—69% yields.
Key words: cyanothioacetamide, 2ꢀcyanopropꢀ2ꢀenethioamides, reduction, the Hantzsch
thiazole synthesis, 2ꢀ(1ꢀcyanoalkyl)thiazoles.
Reductive alkylation is a very popular method of C—C
and C—N bond formation in various organic comꢀ
pounds.1—3 In the case of active methylene compounds,
Cꢀalkylation is usually effected in two steps: (1) the Knoeꢀ
venagel condensation with a carbonyl compound and (2)
reduction of the resulting electronꢀdeficient alkene using
a wide range of reducing agents. This approach has been
used to obtain monoalkylated derivatives of malononiꢀ
trile,4,5 ethyl cyanoacetate,5,6 cyanoacetamide,7 barbituꢀ
ric acid,8 Meldrum´s acid,9 ethyl acetoacetate,10 ethyl
malonate,6,11 indaneꢀ1,3ꢀdione,12 and other 1,3ꢀdiketonꢀ
es.10a,13 It allows selective and highꢀyielding synthesis of
Cꢀalkylation products from active methylene compounds
and is especially useful when direct alkylation is not selecꢀ
tive or gives C,Cꢀdialkylation products. As a rule, direct
Cꢀalkylation of active methylene thioamides is also hinꢀ
dered by regiospecific alkylation at the S atom of the thioꢀ
amide group. Known exceptions include successful synꢀ
theses of ,ꢀunsaturated thioamides by tandem alkylaꢀ
tion of thioamide dianions with allylic electrophiles folꢀ
lowed by the thioꢀClaisen rearrangement of the resulting
Sꢀalkylated derivatives14 and formal Cꢀalkylation in the
formation of stable acyclic adducts under the conditions
of the Michael reaction.15,16 Despite an obviously conveꢀ
nient introduction of an alkyl fragment into active methylꢀ
ene thioamides by a tandem process involving the Knoeꢀ
venagel condensation/reduction of the C=C bond, we
found only one example of such a transformation in the
literature.17
ether proceeds nonselectively to give a complex mixture of
products, regardless of the reaction conditions and the
ratio of the reagents. However, unsaturated thioamides 1
and 2 smoothly and selectively react with excess NaBH4
in EtOH to form the expected thioamides 3 and 4 in good
yields (Scheme 1). The compounds obtained can be used
in the Hantzsch condensation with ꢀhalo ketones. This
was illustrated with the synthesis of 2ꢀ(1ꢀcyanoethyl)ꢀ
thiazole 5 from thioamide 3a and 4ꢀmethylphenacyl
bromide.
It should be noted that reduction of compounds 1e,f
with excess NaBH4 in EtOH did not produce thioamides
3e,f: acidification of the reaction mixture results in its
resinification. Apparently, when compounds contain reꢀ
ducible substituents, the reduction is not selective and proꢀ
ceeds any further. Compounds 3 and 4 are pale yellow fine
crystalline solids that are well soluble in DMSO, ethyl
acetate, acetone, and hot EtOH but are insoluble or poorly
soluble in benzene and water. The structures of the comꢀ
pounds obtained were confirmed by IR and 1H NMR specꢀ
troscopy as well as by HPLC/MS and elemental analysis.
The 1H NMR spectra of compounds 3 show a characꢀ
teristic ABX pattern: a multiplet for the diastereotopic
methylene C(3)H2 protons at  3.09—3.43 and a douꢀ
blet of doublets for the C(2)H proton at  4.20—4.33
(3J = 7.2—8.6 Hz). The signals for the protons of the thioꢀ
amide group appear as two broadened peaks at  9.46—9.57
and 9.80—9.92. The IR spectra of compounds 3 and 4
contain no absorption bands due to conjugated cyano
groups at 2200 30 cm–1; the lowꢀintensity bands at
2244—2256 cm–1 correspond to the stretching vibrations
of the nonconjugated CN group.
In the present work, we tried to study the possibility of
obtaining Cꢀalkylated derivatives of cyanothioacetamide
via reduction of products of its condensation with carbonꢀ
yl compounds. It turned out that reduction of 2ꢀcyanoꢀ
propꢀ2ꢀenethioamides 1 with LiAlH4 in boiling diethyl
To sum up, reactions of easily accessible derivatives of
2ꢀcyanopropꢀ2ꢀenethioamide with excess NaBH4 in EtOH
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2240—2243, December, 2012.
1066ꢀ5285/12/6112ꢀ2261 © 2012 Springer Science+Business Media, Inc.

2262
Russ.Chem.Bull., Int.Ed., Vol. 61, No. 12, December, 2012
Dotsenko and Krivokolysko
Scheme 1
propꢀ2ꢀenethioamides 1a—d (see Ref. 18) and (cyano)(cycloꢀ
alkylidene)thioacetamides 2a,b (see Ref. 19) were prepared by
condensation of cyanothioacetamide20 with carbonyl compounds
according to known procedures.
(E)ꢀ2ꢀCyanoꢀ3ꢀ(5ꢀnitroꢀ2ꢀfuryl)propꢀ2ꢀenethioamide (1e).
A mixture of cyanothioacetamide (1.00 g, 10 mmol) and 5ꢀnitroꢀ
furfural (1.41 g, 10 mmol) in EtOH (15 mL) was stirred at ~30 C
to complete homogenization. Then one drop of Nꢀmethylmorꢀ
pholine was added to the vigorously stirred resulting solution.
A precipitate of the product formed within 1—2 min. The mixꢀ
ture was stirred for an additional 1 h and kept at 20 C for 4 h.
The precipitate was filtered off and washed with EtOH. Yield
1.97 g (88%), brown powder, m.p. 195—200 C (decomp.)
(cf. Ref. 21: m.p. 218 C). Found (%): C, 43.19; H, 2.38; N, 18.60.
C8H5N3O3S. Calculated (%): C, 43.05; H, 2.26; N, 18.83. IR
(Nujol), /cm–1: 3366, 3263, 3150 (N—H); 2214 (CN); 1458,
1
1377 (NO2). H NMR (300 MHz, DMSOꢀd6), : 7.59 (d, 2 H,
C(3)Hfur, 3J = 3.9 Hz); 7.84 (d, 2 H, C(4)Hfur, 3J = 3.9 Hz); 8.00
(s, 1 H, —CH=); 9.70, 10.27 (both br.s, 1 H each, C(S)NH2).
MS, m/z (ESꢀAPI): 224.3 [M + H]+.
(E)ꢀ3ꢀ(1ꢀBenzylꢀ1Hꢀbenzimidazolꢀ2ꢀyl)ꢀ2ꢀcyanopropꢀ2ꢀeneꢀ
thioamide (1f) was obtained from 1ꢀbenzylꢀ1Hꢀbenzimidazoleꢀ
2ꢀcarbaldehyde (1.11 g, 4.71 mmol) and cyanothioacetamide
(0.47 g, 4.70 mmol) as described for thioamide 1e. Yield 1.16 g
(78%), brickꢀred powder, m.p. 192—194 C, Rf 0.61 (acetone—hexꢀ
ane, 1 : 1). Found (%): C, 67.79; H, 4.53; N, 17.74. C18H14N4S.
Ar = 4ꢀMeOC6H4 (1a, 3a), 3,4ꢀ(MeO)2C6H3 (1b, 3b),
2ꢀfuryl (1c, 3c), 2ꢀClC6H4 (1d, 3d), 5ꢀnitroꢀ2ꢀfuryl (1e),
1ꢀbenzylbenzimidazolꢀ2ꢀyl (1f)
n = 1 (2a, 4a), 2 (2b, 4b)
Calculated (%): C, 67.90; H, 4.43; N, 17.60. IR (Nujol), /cm–1
:
3399, 3259, 3148 (N—H); 2208 (CN). 1H NMR (500 MHz,
DMSOꢀd6), : 5.77 (s, 2 H, CH2Ph); 7.17 (d, 2 H, Ar, 3J = 7.3 Hz);
7.27—7.40 (m, 5 H, Ar); 7.66 (d, 1 H, Ar, 3J = 7.8 Hz); 7.83
(d, 2 H, Ar, 3J = 7.8 Hz); 8.19 (s, 1 H, —CH=); 9.80, 10.29
(both br.s, 1 H each, C(S)NH2). 13C NMR (125 MHz, DMSOꢀd6),
: 46.90 (CH2Ph); 112.05 (=C—CN); 115.58 (CAr); 116.63
(CN); 121.01 (CAr); 124.27 (CAr); 125.63 (CAr); 127.18 (CAr);
128.30 (CAr); 129.35 (CAr); 131.09 (CAr); 136.03 (CAr); 137.22
(CAr); 143.55 (CAr); 145.53 (CAr); 192.12 (—CH=C—CN);
194.59 (C=S).
Reduction of unsaturated thioamides 1 and 2 (general proceꢀ
dure). Sodium borohydride (302 mg, 8 mmol) was added to
a stirred suspension of an unsaturated thioamide 1a—f or 2a,b
(4 mmol) in 96% EtOH (10—15 mL). A brief induction period
was followed by an exothermic reaction (cooling on an ice water
bath is required!) accompanied by evolution of H2 and H2S,
dissolution of the starting thioamide, and decoloration of the
solution. The reaction mixture was stirred for 1.5 h and diluted
with water (10 mL). Then dilute (1 : 1) HCl was added dropwise
to pH ~3—4. The reaction mixture was left for 2—4 days for
completion of crystallization. The product was filtered and
washed with water, 50% aqueous EtOH, and benzene (for 4a,b,
with benzene—light petroleum, 1 : 2) to complete decoloration
of the filtrate. Compounds 3a—d and 4a,b were obtained in the
analytically pure state. Reduction of thioamides 1e,f followed by
acidification with HCl resulted in strong resinification precludꢀ
ing isolation of the target products 3e,f.
allow reduction of the C=C bond to Cꢀmonoalkylated
cyanothioacetamides, which hold promise as lowꢀmolecꢀ
ularꢀweight building blocks for the synthesis of heteroꢀ
cyclic compounds.
Experimental
1
H NMR spectra were recorded on Bruker Avance DPXꢀ300
(300.16 MHz) (for 1e), Bruker DRXꢀ500 (500.13 MHz) (1f),
and Bruker DPXꢀ400 instruments (400.40 MHz) (the other comꢀ
pounds) in DMSOꢀd6 with Me4Si as the internal standard. The
13C NMR spectrum of compound 1f was recorded on a Bruker
DRXꢀ500 instrument (125.76 MHz) in DMSOꢀd6, with Me4Si
as the internal standard. IR spectra were recorded on an Inꢀ
fralum FTꢀ801 FTIR spectrometer (KBr pellets) (for 3a), a Maꢀ
gnaꢀIR 750 spectrophotometer (KBr) (4a), and an IKSꢀ29 specꢀ
trophotometer (Nujol) (the other compounds). HPLC/MS analꢀ
ysis of compound 3b was carried out using an Agilent 1200
instrument (Rapid Resolution HT Cartridge column, 4.6×30 mm,
1.8 m, Zorbax SBꢀC18, ESꢀAPI ionization mode) equipped
with DAD and MS detectors. The other compounds were anaꢀ
lyzed with a Shimadzu LCꢀ10AD liquid chromatograph equipped
with Shimadzu SP Dꢀ10A UVꢀVis (254 nm) and Sedex 75 ELSD
detectors; the chromatograph was combined with a Perꢀ
kin—Elmer SCIEX API 150EX mass spectrometer. Elemental
analysis was carried out on a Carlo Erba Strumentazione 1106
analyzer. The individuality of the compounds was checked by
TLC on Silufol UVꢀ254 plates with acetone—hexane (1 : 1) as an
eluent; spots were visualized with the iodine vapor and under
UV light. Melting points were measured on a Kofler hot stage
and are given uncorrected. The starting reagents 3ꢀarylꢀ2ꢀcyanoꢀ
2ꢀCyanoꢀ3ꢀ(4ꢀmethoxyphenyl)propanethioamide (3a). Yield
69%, light yellow powder, m.p. 156—158 C (subl.), Rf 0.60
(acetone—hexane, 1 : 1). Found (%): C, 60.18; H, 5.33; N, 12.94.
C11H12N2OS. Calculated (%): C, 59.97; H, 5.49; N, 12.72. MS,
m/z (ESꢀAPI): 221.3 [M + H]+, 238.1 [M + NH4]+, 441.4
[2 M + H]+, 458.3 [2 M + NH4]+. IR (KBr), /cm–1: 3366.6,
3281.9, 3165.6 (N—H); 2914.3, 2836.8 (C—H); 2247.1 (CN).

Reduction of 2ꢀcyanopropꢀ2ꢀenethioamides
Russ.Chem.Bull., Int.Ed., Vol. 61, No. 12, December, 2012
2263
1H NMR (400 MHz, DMSOꢀd6), : 3.09—3.15 (m, 2 H,
C(3)H2); 3.76 (s, 3 H, MeO); 4.20 (dd, 1 H, C(2)H, 3J = 8.6 Hz,
3J = 8.6 Hz); 6.86, 7.22 (both d, 2 H each, Ar, 3J = 8.9 Hz); 9.46,
9.80 (both br.s, 1 H each, C(S)NH2).
filtered off and washed with EtOH and light petroleum. Yield
54%, m.p. 102.5—104.5 C. Found (%): C, 71.68; H, 5.47; N, 8.50.
C20H18N2OS. Calculated (%): C, 71.83; H, 5.42; N, 8.38.
IR (Nujol), /cm–1: 2249 (CN). 1H NMR (400 MHz, DMSOꢀd6),
: 2.34 (s, 3 H, Me); 3.28—3.39 (m, 2 H, C(3)H2); 3.72 (s, 3 H,
2ꢀCyanoꢀ3ꢀ(3,4ꢀdimethoxyphenyl)propanethioamide (3b).
Yield 66%, pale yellow powder, m.p. 171.5—173.5 C (acetone—
toluene), Rf 0.60 (acetone—hexane, 1 : 1). Found (%): C, 57.39;
H, 5.78; N, 11.30. C12H14N2O2S. Calculated (%): C, 57.58; H, 5.64;
N, 11.19. MS, m/z (ESꢀAPI): 151.1 [3,4ꢀ(MeO)2C6H3CH2]+,
251.1 [M + H]+. IR (Nujol), /cm–1: 3391, 3312, 3240 (N—H);
2256 (CN). 1H NMR (400 MHz, DMSOꢀd6), : 3.09—3.19
(m, 2 H, C(3)H2); 3.73, 3.74 (both s, 3 H each, MeO); 4.29 (dd,
1 H, C(2)H, 3J = 7.5 Hz, 3J = 7.8 Hz); 6.81 (d, 1 H, Ar,
3J = 8.0 Hz); 6.89 (d, 2 H, Ar, 3J = 8.0 Hz); 6.94 (s, 1 H, Ar);
9.53, 9.92 (both br.s, 1 H each, C(S)NH2).
3
MeO); 3.83 (dd, 1 H, C(2)H, J = 6.7 Hz, 3J = 8.3 Hz); 6.88,
7.24 (both d, 2 H each, Ar, 3J = 8.6 Hz); 7.27, 7.86 (both d,
2 H each, Ar, 3J = 7.9 Hz); 8.04 (s, 1 H, C(5)Hthiazolyl).
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Received August 22, 2012;
in revised form November 19, 2012