Nucleophilic substitution of chloro-substituted enaminones that are derivatives of imidazolidine nitroxides - The catalytic effect of the cyanide ion

Article abstract of DOI:10.1002/ejoc.200200722

The cyanide ion is a catalyst for reactions of nucleophiles with chloro-substituted enaminones derived from imidazolidine nitroxides, which result in the formation of formal nucleophilic substitution products. ( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Wei

Full text of DOI:10.1002/ejoc.200200722

FULL PAPER
Nucleophilic Substitution of Chloro-Substituted Enaminones
that are Derivatives of Imidazolidine Nitroxides ؊ The Catalytic Effect of the
Cyanide Ion
Galina I. Roshchupkina,[a] Tatyana V. Rybalova,[b] Yury V. Gatilov,[b] and
Vladimir A. Reznikov*[b]
Keywords: Enaminones / Nitrogen heterocycles / Nitroxides / Nucleophilic substitution
The cyanide ion is a catalyst for reactions of nucleophiles
with chloro-substituted enaminones derived from imidazolid-
ine nitroxides, which result in the formation of formal nucleo-
philic substitution products.
( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2003)
Introduction
philic addition to the carbonyl carbon atom (I). Secondly,
epoxide cycle formation results from an intramolecular nu-
cleophilic substitution of chloride ion by the anionic center
(II). It seems improbable that this step would occur without
double-bond migration within the heterocycle as a result
of a conjugation failure. In the case where a double-bond
migration is impossible (e.g., when there is a disubstituted
enamine nitrogen atom), the substitution does not proceed.
Thirdly, the opening of the epoxide cycle follows by the ac-
tion of one more equivalent of the nucleophile (III).
The nucleophilic substitution reaction is one of the
classic reaction types for the functionalization of organic
molecules. These transformations are constantly the sub-
jects of intense studies to elucidate mechanisms and meth-
ods for designing complex organic molecules having con-
trolled geometries and properties. The reactivity of α-halo-
substituted carbonyl compounds towards nucleophiles is
well known and it is believed that the mechanism includes
initial attack at the carbonyl carbon atom, subsequent
epoxide ring closure, and then its ring opening under the
action of another equivalent of the nucleophile.[1]
Nitriles 2 are of interest as paramagnetic ligands for the
synthesis of coordination compounds with unusual mag-
netic properties.[4] Additionally, it has been shown recently
that they are promising spin probes because they have pH-
sensitive EPR spectra, which makes them suitable for pH
measurements in molecular biophysics because their pK val-
ues are in the range 7Ϫ10.[5] These two applications are
mostly results of introducing the nitrile group, which, on
the one hand, increases the acidity of these compounds and,
on the other, provides additional possibilities for partici-
pation in coordination to metal ions. We believe that enami-
nones of imidazolidine nitroxides having other substituents
in the same position would be also of interest as paramag-
netic ligands and, at first sight, there seem to be no ob-
stacles for involving enaminones 1 in reactions with other
nucleophiles.
Results and Discussion
Previously, we have found that chloro-substituted enami-
nones 1, which are derivatives of imidazolidine nitroxides,
react unexpectedly well with cyanide ion with the formation
of the formal nucleophilic substitution products, the nitriles
2.[2] Reaction of enaminones 1 with azide ion proceeds in a
similar manner, but the resulting azides 3 are unstable and
are transformed spontaneously into monoimines of α-dike-
tones (4).[3] A study of these reactions resulted in the con-
clusion that halo-substituted enaminones 1 react with these
nucleophiles to form the intermediate epoxides. There are
three key steps in this transformation. The first is a nucleo-
We have found, however, that, under the same conditions,
enaminone 1a is inert towards different nucleophiles, such
as amines, NO2Ϫ, NCOϪ, NCSϪ, AcOϪ, and [(methylsulfi-
nyl)methyl]sodium in DMSO solutions. In the reaction of
1a with methylmagnesium iodide in diethyl ether, only a
partial reduction of the nitroxyl group and its conversion
into a methoxyl group (5) take place with the enaminone
group remaining intact. It is interesting to note that the
[a]
Novosibirsk State University,
630090 Novosibirsk, Pirogova str. 2, Russian Federation
[b]
N. N. Vorozhtsov Institute of Organic Chemistry, Siberian
Division of the Russian Academy of Sciences,
630090 Novosibirsk, Acad. Lavrent’ev ave. 9, Russian
Federation
E-mail: mslf@nioch.nsc.ru
Eur. J. Org. Chem. 2003, 3599Ϫ3602
DOI: 10.1002/ejoc.200200722
 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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G. I. Roshchupkina, T. V. Rybalova, Y. V. Gatilov, V. A. Reznikov
FULL PAPER
˚
Figure 1. Crystal structure of radical 7a; selected bond lengths [A]:
S1ϪC2 1.765(4), S1ϪC6 1.761(4), C2ϪN2a 1.398(5), N2aϪC3
1.477(5), N2aϪC5a 1.357(5), C3ϪN4 1.484(6), N4ϪC5 1.478(6),
N4ϪO8 1.263(4), C5ϪC5a 1.512(5), C5aϪC6 1.361(6)
Scheme 1
To date, there have been no 6,7-dihydro-5H-imidazo[1,5-
c][1,3]thiazole derivatives reported in the Cambridge Struc-
tural Data Base.[6] The geometry of the thiazole ring is al-
most the same as that found in 3-(methoxycarbonylamino)-
5-(methoxycarbonyl)-4-methyl-2-(phenylimino)-2,3-
reaction of the dichloro derivative 6 with excess MeMgI
yields monochloro derivative 1a along with the products
described above of the further transformation of 1a.
The reason for the inactivity of enaminones 1 towards
nucleophiles that are strong bases, such as MeMgI, [(meth-
ylsulfinyl)methyl]sodium, and, probably, amines, is the
readiness for deprotonation of their comparatively acidic
enaminone groups, which would reduce their affinity
towards nucleophiles. The inactivity of 1 in reactions with
other nucleophiles can be rationalized by considering the
reaction mechanism. Step I — the addition to the carbonyl
group — should proceed more readily with hard nucleo-
philes and, conversely, step III — opening of the epoxide
cycle — should be realized more easily with soft nucleo-
philes. Different steps of the reaction could be limiting de-
pending on the nature of the nucleophile, but all the trans-
formations require the nucleophile to be relatively effective
in both steps I and III. Of all the nucleophiles we tested for
this reaction, only the azide and cyanide ions seem to be
suitable for realizing both steps.
dihydrothiazole.[7] The bicyclic fragment is nearly planar
˚
within Ϯ0.038(13) A and forms an angle of 88.2(8)° with
the plane of the phenyl ring. Infinite chains of molecules
are formed in the crystal along the a axis by means of inter-
molecular N7ϪH···O8 hydrogen bonds [H···O8 distance,
˚
2.19(5) A; N7ϪH···O8 angle, 159(8)°].
Scheme 2
We found out that the epoxide isolated as a reaction
product of enaminone 1 (R ϭ tert-butyl) with sodium cyan-
Reactions of enaminones 1b,c with KNCS in the presence
ide undergoes spontaneous transformation to its respective of NaCN proceed similarly with the subsequent formation
nitrile 2 — even in the solid state. The transformation pro- of the 6,7-dihydro-5H-imidazo[1,5-c][1,3]thiazole deriva-
ceeds quantitatively and seems to be result of an autocata- tives 7b,c. We note that the rate of reaction with KNCS in
lytic process related to an initial liberation of a trace the presence of NaCN is noticeably lower (a few weeks at
amount of cyanide ion that causes the reaction. Thus, we room temp.) than that with cyanide alone (2Ϫ3 h), but,
believed that the cyanide ion could be a catalyst for such nevertheless, the catalytic effect is quite appreciable, because
transformations, particularly in the cases of soft nucleo- in the absence of cyanide no product formation was ob-
philes. Actually, the reaction of enaminone 1a with KNCS served during that period of time. The use of this approach
in the presence of a catalytic amount of sodium cyanide permits the reaction of chloro-substituted enaminones 1
leads to compound 7a. The elemental analysis data of this with other nucleophiles — NO2Ϫ, OCNϪ, and AcOϪ — to
product corresponds to that expected for the substitution yield the subsequent substitution products, enaminones
product 8a, but, according to X-ray analysis, compound 7a 8Ϫ11.
is formed as a product of a further transformation of the
In contrast to the cyanide ion, the azide ion does not
initially formed thiocyanate 8a, namely a nucleophilic ad- catalyze the reaction of enaminones 1 with nucleophiles.
dition of the endocyclic nitrogen atom at the thiocyanate The reasons for these phenomena may be because of the
group, to yield a derivative of 6,7-dihydro-5H-imidazo[1,5- different limiting steps of the reactions with these two nu-
c][1,3]thiazole (Figure 1).
cleophiles; in other words, the very short lifetime of the
3600
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Eur. J. Org. Chem. 2003, 3599Ϫ3602

Nucleophilic Substitution of Chloro-Substituted Enaminones
FULL PAPER
Radical 7a: A mixture of enaminone 1a (0.6 g, 2 mmol), potassium
isothiocyanate (0.78 g, 9 mmol), and NaCN (20 mg, 0.3 mmol) in
anhydrous DMSO (20 mL) was stirred at 20 °C for 96 h and then
cooled to 0 °C and poured into cold brine (30 mL). The precipitate
was filtered off, washed with water, and dried. The crude product
was purified chromatographically on a silica gel (CHCl3/MeOH,
50:1) to give 7a (0.5 g, 75%). M.p. 159Ϫ161 °C (from ethyl acetate/
hexane). IR: ν˜max ϭ 3322, 3291, 3206 (NH), 1639, 1610, 1564 (Oϭ
CϪCϭCϪN, CϭC) cmϪ1. UV/Vis (EtOH): λmax (lg ε) ϭ 250
(3.96), 357 (3.71) nm. C16H18N3O2S (316.4): calcd. C 60.74, H 5.73,
N 13.28; found C 61.29, H 5.55, N 13.23.
epoxide bearing the azido group substituent leads to its
irreversible transformation into the imine 4.
Conclusion
The cyanide anion catalyzes the reaction of nucleophiles
with chloro-substituted enaminones of imidazolidine nitro-
xides. On one hand, these reactions confirm the postulated
mechanism for the transformation and, on the other, pro-
vide a method for introducing different functional substitu-
ents into paramagnetic enaminones.
Crystallographic Data for Compound 7a: C16H18N3O2S (316.39),
crystal class orthorhombic, space group Pna21, a ϭ 8.201(2), b ϭ
3
˚
˚
20.042(4), c ϭ 9.778(2) A, V ϭ 1607.3(5) A , Z ϭ 4, dc ϭ 1.308 Mg
mϪ3 µ ϭ 0.212 mmϪ1, λ ϭ 0.71073 A, crystal size 0.08 ϫ 0.15 ϫ
˚
2.0 mm. A correction for absorption was made by the integration
method (transmission 0.9813Ϫ0.9888). The structure was solved by
direct methods and refined by a full-matrix least-squares aniso-
tropic/isotropic procedure (for hydrogen atoms) using the
SHELXL-97 program. The hydrogen atom positions were located
geometrically. The final indexes are wR2 ϭ 0.1481, S ϭ 1.040 for all
1496 F2 and R1 ϭ 0.0493 for 1019 Fo Ͼ 4σ. The absolute structure
parameter (Flack parameter) is equal to 0.6(4). Atomic coordi-
nates, thermal parameters, bond lengths and bond angles have been
deposited at the Cambridge Crystallographic Data Center.
CCDC-194015 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge at
www.ccdc.cam.ac.uk/conts/retrieving.html [or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; Fax: (internat.) ϩ 44-1223/336-033; E-mail:
deposit@ccdc.cam.ac.uk].
Experimental Section
General: IR spectra were recorded with a Bruker IFS 66 spec-
trometer as KBr pellets (concentration, 0.25%; thickness of pellet,
1 mm). UV spectra were measured in EtOH with a Specord M-40
spectrophotometer. 1H and 13C NMR spectra were run with a
Bruker WP 200 SY spectrometer with 5Ϫ10% solutions in CDCl3
and [D6]DMSO using HMDS as the internal standard. High-reso-
lution mass spectra were recorded with a Finnigan MAT 8200 mass
spectrometer with direct sample injection. A Bruker P4 single-crys-
tal diffractometer with graphite-monochromated Mo-Kα radiation
was used to measure the unit cell dimensions and to collect data
(θ-2θ scans, θ Ͻ 50°). Melting points were measured with a Boe¨tius
plate and are uncorrected. Thin layer chromatography was carried
out with the use of Silufol UV-254 plates with chloroform and
chloroform/methanol (30:1 or 20:1) as eluents. DMSO was dried
with NaOH and distilled in vacuo from BaO. In all cases concen-
tration was carried out under reduced pressure. The synthesis of
the starting enaminones 1 was performed as published in ref.[2] and
that of the dichloro derivative 6 as in ref.[8]
7a,b: Reactions of enaminones 1b,c with potassium isothiocyanate
were carried out as described above. 7b: Yield ϭ 80%. M.p.
136Ϫ138 °C (from hexane). IR: ν˜max ϭ 3429 (OH), 3296 (NH),
1683, 1644, 1613, 1585, 1556 (OϭCϪCϭCϪN, CϭN, CϭC) cmϪ1
UV/Vis (EtOH): λmax (lg ε) ϭ 220 (3.71), 337 (3.29) nm.
C11H13F3N3O2S (308.3): calcd. C 42.85, H 4.25, N 13.63; found C
42.76, H 4.21, N 13.29. 7c: Yield ϭ 30%. M.p. 127Ϫ131 °C (from
hexane). IR: ν˜max ϭ 3426 (OH), 3282 (NH), 1659, 1624, 1609, 1562
(OϭCϪCϭCϪN, CϭN, CϭC) cmϪ1. UV/Vis (EtOH): λmax (lg
ε) ϭ 335 (4.04) nm. C13H20N3O2S (282.4): calcd. C 55.29, H 7.14,
N 14.88; found C 54.94, H 7.05, N 14.63.
2-Chloro-2-(1-methoxy-2,2,5,5-tetramethylimidazolidin-4-ylidene)-1-
phenylethanone (5): Enaminone 1a (0.5 g, 1.7 mmol) was added
portionwise to a solution of methylmagnesium iodide, prepared
from Mg (0.2 g, 8.3 mmol) and CH3I (0.53 mL, 8.5 mmol) in anhy-
drous diethyl ether (20 mL). The reaction mixture was stirred for
1 h under reflux, 7 h at room temp., and then was kept overnight
before being treated with saturated aqueous NH4Cl (15 mL). The
organic layer was separated and aqueous phase was extracted with
CHCl3 (2 ϫ 20 mL). The combined extracts were dried with
MgSO4 and filtered and then MnO2 (2 g) was added to the re-
sulting solution and the mixture stirred for 30 min at room temp.
The excess oxidant was filtered off, the filtrate was concentrated,
and a residue was separated on a silica gel column with CHCl3 as
eluent to provide sequentially the methoxy derivative 5 (0.2 g, 40%)
and starting enaminone 1a (0.3 g). M.p. of 5: 127Ϫ129 °C (from
Radical 9a: A mixture of enaminone 1a (0.3 g, 1 mmol), NaNO2
(0.14 g, 2 mmol), and NaCN (10 mg, 0.15 mmol) in anhydrous
DMSO (10 mL) was stirred at 20 °C for 96 h and then cooled to 0
°C and poured into cold brine (30 mL). The precipitate was filtered
off, washed with water, and dried. The mixture of unchanged 1a
(25 mg, 8%) and nitro derivative 9a (0.12 g, 40%) was separated
chromatographically on silica gel with CHCl3/MeOH (50:1) as elu-
ent. M.p. 180Ϫ184 °C (ethyl acetate/hexane). IR: νmax ϭ 3425
(OH), 3221 (NH), 1675, 1602, 1588 (OϭC, CϭC) cmϪ1. UV/Vis
(EtOH): λmax (lg ε) ϭ 254 (4.31), 336 (4.18) nm. C15H18N3O4
(304.3): calcd. C 59.20, H 5.96, N 13.81; found C 59.55, H 6.03,
N 13.58.
˜
1
hexane). H NMR (CDCl3, 200.13 MHz): δ ϭ 1.48 (s, 6 H), 1.62
[br. s, 6 H, 2,5-(CH3)2], 3.70 (s, 3 H, OCH3), 7.37 (m, 3 H), 7.57
(m, 2 H, C6H5) ppm. IR: ν˜max ϭ 3210 (NH), 1594, 1538 (Oϭ
CϪCϭCϪN, CϭC, CϭN) cmϪ1. UV/Vis (EtOH): λmax (lg ε) ϭ
243 (3.58), 343 (4.69) nm. C16H21ClN2O2 (308.8): calcd. C 62.24,
H 6.81, N 9.08; found C 62.53, H 6.85, N 9.07.
Radical 10a. Method A: A mixture of enaminone 1a (0.6 g,
2 mmol), potassium cyanate (0.83 g, 10 mmol), and NaCN (20 mg,
0.3 mmol) in anhydrous DMSO (15 mL) was stirred at 20 °C for 2
weeks and the poured into ice-cold brine (40 mL). The resulting
mixture was extracted with chloroform (3 ϫ 20 mL) and the com-
bined extracts were washed with brine (3 ϫ 10 mL) and water (3
ϫ 10 mL) and then dried (MgSO4). The residue obtained after re-
Under the same conditions, the reaction of dichloro derivative 6
with methylmagnesium iodide provided the monochloro derivative
1a, which was isolated in 30% yield, along with the starting mate-
rial 6 (50%). The formation of methoxy derivative 5 was observed
chromatographically.
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G. I. Roshchupkina, T. V. Rybalova, Y. V. Gatilov, V. A. Reznikov
FULL PAPER
moval of the solvent was purified chromatographically on silica gel
(CHCl3/MeOH, 50:1) to give 10a (0.31 g, 50%). Method B: A mix-
ture of enaminone 1a (0.2 g, 0.68 mmol), potassium cyanate
(0.28 g, 3.4 mmol), and NaCN (10 mg, 0.15 mmol) in anhydrous
DMSO (10 mL) was heated to 110 °C with stirring for 30 min and
then kept at this temperature for 30 min. After cooling, the reaction
mixture was treated as described above to yield 10a (0.12 g, 60%).
M.p. 192Ϫ195 °C (ethyl acetate/hexane). IR: ν˜max ϭ 3203 (NH),
1707, 1644, 1597, 1573 (OϭC, CϭC) cmϪ1. UV/Vis (EtOH): λmax
(lg ε) ϭ 251 (3.83), 323 (3.97) nm. MS: calcd. for C16H18N3O3 m/
z ϭ 300.13524; found m/z ϭ 300.13481.
(C-5), 80.4 (C-2), 117.3 (ϭCNO2), 128.3, 129.2, 132.8, 137.7 (Ph),
163.3 (C-4), 188.0 (CϭO) ppm.
10a (NOH): 1H NMR (CDCl3, 200.13 MHz): δ ϭ 1.36 [s, 6 H, 5,5-
(CH3)2], 1.58 [s, 6 H, 2,2-(CH3)2], 6.9 (broad s, 1 H, OH), 7.40 (m,
2 H), 7.48 (m, 1 H), 7, 62 (m, 2 H, Ph), 9.31 (broad s, 1 H, NH)
ppm. 13C NMR (CDCl3, 50.32 MHz): δ ϭ 23.0 [5,5-(CH3)2], 23.7
[2,2-(CH3)2], 64.1 (C-5), 80.1 (C-2), 113.2 (ϭC-O), 127.6, 128.2,
131.8, 138.0 (Ph), 140.0 (OCN), 148.3 (C-4), 183.2 (CϭO) ppm.
10c (NOH): 1H NMR ([D6]DMSO, 200.13 MHz): δ ϭ 0.87 (t, J ϭ
7 Hz, CH3CH2), 1.33 (s, 6 H, 5,5-(CH3)2], 1.41 (s, 6 H, 2,2-(CH3)2],
1.53 (m, CH3CH2CH2), CH2CO signal masked by solvent signal,
8.12 (br. s, 1 H, OH), 10.54 (br. s, 1 H, NH) ppm. 13C NMR
([D6]DMSO, 50.32 MHz): δ ϭ 13.1, 16.8 (C3H7, CH2CO signal
masked by solvent signal), 23.0 [5,5-(CH3)2], 23.6 [2,2-(CH3)2], 62.6
(C-5), 78.6 (C-2), 113.1 (ϭCϪO), 138.3 (OCN), 147.7 (C-4), 187.9
(CϭO) ppm.
10c: Reaction of enaminone 1c with potassium cyanate was carried
out in the same manner as described for enaminone 1a to yield 10c
(15%). IR: ν˜max ϭ 3366 (OH), 3164 (NH), 1702, 1674, 1624 (Oϭ
C, CϭC) cmϪ1 UV/Vis (EtOH): λmax (lg ε) ϭ 214 (3.98), 298 (4.74)
nm. MS: calcd. for C13H20N3O3 m/z ϭ 266.15046; found m/z ϭ
266.15044.
11a (NOH): 1H NMR (CDCl3, 200.13 MHz): δ ϭ 1.45 [s, 12 H,
2,5-(CH3)2], 1.91 (s, 3H, CH3CO), 5.5 (br. s, 1 H, OH), 7.35 (m, 3
H), 7.61 (m, 2 H, Ph), 10.30 (br. s, 1 H, NH) ppm. 13C NMR
(CDCl3, 50.32 MHz): δ ϭ 20.2 (CH3CO), 23.1 [5,5-(CH3)2], 26.4
[2,2-(CH3)2], 67.8 (C-5), 79.6 (C-2), 118.3 (ϭCϪO), 126.8, 127.3,
129.5, 138.7 (Ph), 159.8 (C-4), 170.0 (COO), 188.2 (PhCO) ppm.
Radical 11a: A mixture of enaminone 1a (1.0 g, 3.4 mmol), anhy-
drous sodium acetate (0.84 g, 10 mmol), and NaCN (20 mg,
0.3 mmol) in anhydrous DMSO (15 mL) was stirred at 20 °C for
16 d. The reaction mixture was poured into cold brine (30 mL) and
the precipitate was filtered off, washed with brine and water, and
then dried on air. The crude acetate was dissolved in chloroform
(20 mL) and filtered through silica gel (5 cm) with chloroform as
eluent to give 11a (0.9 g, 80%). M.p. 155Ϫ157 °C (ethyl acetate/
hexane). IR: ν˜max ϭ 3262 (NH), 1759 (OϭCϪO), 1627, 1580, 1547
(OϭC, CϭC) cmϪ1. UV/Vis (EtOH): λmax (lg ε) ϭ 242 (3.85), 332
(4.45) nm. C17H21N2O4 (317.4): calcd. C 64.34, H 6.67, N 8.83;
found C 64.45, H 6.76, N 8.82.
Acknowledgments
We are indebted to the Russian Foundation for Basic Research for
covering the license fee of the Cambridge Structural Database
(Project 99-07-90133).
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7a (NOH): H NMR ([D6]DMSO, 200.13 MHz): δ ϭ 1.42 [s, 6 H,
5,5-(CH3)2], 1.51 [s, 6 H, 2,2-(CH3)2], 7.50 (m, 2 H), 7.57 (m, 1 H),
7.66 (m, 2 H) ppm. 13C NMR ([D6]DMSO, 50.32 MHz): δ ϭ 22.2
[5,5-(CH3)2], 22.9 [2,2-(CH3)2], 64.4 (C-5), 81.4 (C-2), 101.7 (Cϭ
CS), 127.3, 128.3, 131.8, 139.6 (Ph), 154.6, 155.6 (C-4, SCϭN),
184.4 (CϭO) ppm.
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9 (NOH): H NMR (CDCl3, 200.13 MHz): δ ϭ 1.19 (s, 6 H), 1.52
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V. A. Reznikov, T. I. Reznikova, L. B. Volodarskii, Izv. Sib.
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(broad s, 2 H, Ph), 10.21 (broad s, 1 H, NH) ppm. 13C NMR
(CDCl3, 50.32 MHz): δ ϭ 23.3 [5,5-(CH3)2], 25.7 [2,2-(CH3)2], 69.2
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Received December 29, 2002
3602
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Eur. J. Org. Chem. 2003, 3599Ϫ3602