4-hydroxy-2-quinolones 170. Synthesis and bromination of N-allylisatin

Article abstract of DOI:10.1007/s10593-010-0413-5

Independently of the reaction conditions N-allylisatin forms only the 2, 3-dibromo derivative upon treatment with molecular bromine in contrast to N-allyl-substituted quinolin- or pyrid-2-ones which readily undergo halocyclization to the corresponding 2-bromomethyl oxazoles.

Full text of DOI:10.1007/s10593-010-0413-5

Chemistry of Heterocyclic Compounds, Vol. 45, No. 10, 2009
4-HYDROXY-2-QUINOLONES
170*. SYNTHESIS AND BROMINATION
OF N-ALLYLISATIN
I. V. Ukrainets1**, N. L. Bereznyakova1, O. V. Gorokhova1, and S. V. Shishkina2
Independently of the reaction conditions N-allylisatin forms only the 2,3-dibromo derivative upon
treatment with molecular bromine in contrast to N-allyl-substituted quinolin- or pyrid-2-ones which
readily undergo halocyclization to the corresponding 2-bromomethyl oxazoles.
Keywords: N-allylisatin, alkylation, bromination, halocyclization, X-ray structural analysis.
Decolorization of bromine is one of the widest known analytical reactions for alkenes but does not
always prove to be due to the conventional addition of halogen to an unsaturated bond. With the existence of
certain structural features such reactions can be accompanied by the formation of novel 5-, 6- and even
7-membered rings thanks to which they are often referred to as bromo- or halocyclizations [2-7].
This type of chemical reaction occurs in the bromination of N-allyl-substituted 4-hydroxy-2-quinolones
which is, in principle, a simple method for preparing 4R-2-bromomethyl-1,2-dihydrooxazolo[3,2-a]quinolines
[8-10]. For evaluating the synthetic potential of this interesting reaction and to work out a fuller and more
proven idea of the actual mechanism occurring in these chemical processes there is much value in the
accumulation and subsequent analysis of experimental material including the maximum use of a broad range of
model compounds.
This report is a part of such an investigation, the aim of which was a study of the behavior of
N-allylisatin 1 (with a structural similarity to 4-hydroxy-2-quinolones) under conditions of bromination using
molecular bromine. According to a known outline [11], the alkylation of isatin 2 principally needs its conversion
to a sodium salt and used sodium ethylate in absolute ethanol or, indeed, NaH in anhydrous DMF. Preference is
usually given to the latter variant since, in this case, the initial separation of the salt is not essential. However,
from our data, the use of this method is far from always justified. At least with allyl bromide this reaction is
much more conveniently carried out using the system DMSO/K2CO3. The N-alkylation occurs very rapidly at
room temperature and in virtually quantitative yield.
_______
* For Communication 169 see [1].
** To whom correspondence should be addressed, e-mail: uiv@kharkov.ua.
1National University of Pharmacy, Kharkiv 61002, Ukraine.
2Institute for Single Crystal, National Academy of Sciences of Ukraine, Kharkiv 61001, Ukraine, e-mail:
sveta@xray.isc.kharkov.com.
__________________________________________________________________________________________
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1546-1553, October, 2009.
Original article submitted December 18, 2008.
0009-3122/09/4510-1241©2009 Springer Science+Business Media, Inc.
1241

As had been expected, the N-allylisatin 1 was brominated very readily by molecular bromine in glacial
acetic acid, the brown color of the bromine disappearing immediately after mixing the reagents. In principle this
reaction can occur by two alternative routes via halocyclization to 2-bromomethyl-9-oxo-3,9-dihydro-
2H-oxazolo[3,2-a]indol-4-ylium bromide (3) or by the simple addition of the bromine to the allyl double bond
giving N-(2,3-dibromopropyl)isatin (4). A parallel formation of both of these compounds cannot be ruled out.
O
Br
+
N
O
O
O
Br
All-Br
Br2
3
O
O
DMSO,
K2CO3
N
N
O
H
O
2
1
N
Br
Br
4
A positive Beilstein test and TLC monitoring showed that the bromination of the N-allylisatin 1 occurs
selectively since a single product was obtained. Both possible bromo-substituted products 3 and 4 contain the
same number of protons with quite similar chemical environments. For this reason it was not possible to
establish the true structure of the compound formed on the basis of just a simple 1H NMR spectrum.
An unambiguous solution to this structural problem was achieved through X-ray structural analysis (see
Figure 1 and Tables 1 and 2). It was found that the product obtained is N-(2,3-dibromopropyl)isatin (4). The
independent part of the unit cell for this compound consist of two molecules (A and B) differing in several
geometric parameters. For both molecules the bicyclic fragment and atoms O(1), O(2), and C(9) lie in a single
plane to within an accuracy of 0.02 Å. The C(7)–C(8) (1.558(6) in molecule A and 1.538(6) Å in B is somewhat
lengthened when compared with the mean value [12] for a Csp2–Csp2 bond of 1.455 Å and this has also been
observed previously in studied isatins [13-16].
The repulsion between the substituent on the N(1) atom and the benzene ring with a shortened
intramolecular contact H(10)···C(2) of 2.85 Å in A and 2.82 Å in B (sum of van der Waal radio [17] = 2.87 Å)
leads to the 2,3-dibromopropyl fragment being positioned virtually perpendicularly to the bicyclic ring plane
(torsional angle C(1)–N(1)–C(9)–C(10) 72.9(5)º in A and 71.6(5)º in B) and occurs in an ap-conformation
(torsional angle N(1)–C(9)–C(10)–C(11) = -171.6(4)º in A and -168.5(3)º in B). The bromine atoms occupy a
mutual +sc-conformation to one another (torsional angle Br(1)–C(10)–C(11)–Br(2) 66.2(3)º in A and 68.7(3)º
in B). A shortened intramolecular contact for H(9a)···Br(2) of 2.95 in molecule A and 2.94 Å in B.
A series of shortened intermolecular contacts is seen for the A and B molecules in the crystal of
compound 4: H(3a)···Br(2a)' (0.5-x, -0.5+y, 0.5-z) 3.12 (3.23); H(4a)···Br(1b)' (-0.5+x, 0.5-y, 0.5+z) 3.03 (3.23);
H(11b)···Br(2b)' (0.5-x, -0.5+y, -0.5-z) 3.11 (3.23); H(3b)···Br(1)' (0.5-x, -0.5+y, -0.5-z) 3.09 (3.23); H(3b)···Br(2b)'
(0.5-x, -0.5+y, -0.5-z) 3.10 (3.23); H(4b)···Br(1a)' (0.5+x, 0.5-y, -0.5+z) 3.17 (3.23); H(11c)···Br(1a)' 3.05 (3.23);
H(11a)···Br(2a)' (-0.5+x, 0.5-y, -0.5+z) 2.99 (3.23); Br(1a)···Br(2b)' 3.73 (3.94 ); Br(1a)···Br(2b)' (-x, 1-y, -z) 3.57
(3.94) and Br(2a)···Br(1b)' (1-x, 1-y, -z) 3.74 Å (3.94 Å).
Hence the bromination of the N-allylisatin 1 at room temperature in glacial acetic acid occurs exclusively
via a classical addition of halogen to the double bond. Bearing in mind that the course of a similar reaction is
sometimes significantly affected by the solvent and temperature used [18, 19] we additionally carried out this
synthesis in carbon tetrachloride at -20ºC. However, in this case only compound 4 was formed.
1242

One obstacle to bromocyclization of N-allylisatin 1 to the oxazoloindole 3 can firstly be identified as
relating to the size of the ring (already having several stresses in its pyrrole fragment) when attaching a further
five-membered ring to it and so making it energetically less favored than the process of forming the dibromo
derivative 4. However, a more detailed analysis shows that this quite logical explanation, at first glance, does not
agree with numerous experimental results. In particular, it is well known that successful halocyclizations can
accompany the formation of heterocyclic systems of this type, i.e. those containing five-membered rings
mutually annelated [2, 18-24]. On the other hand, it turns out that not each six-membered 1-N-allyl-substituted
Fig. 1. Structure of the N-2,3-dibromopropylisatin (4) structure with atomic numbering.
TABLE 1. Bond Lengths (l) in the N-(2,3-Dibromopropyl)isatin (4) Structure
Bond
l, Å
Bond
l, Å
1.952(4)
1.366(5)
1.443(5)
1.210(5)
1.406(6)
1.373(7)
1.385(6)
1.558(6)
1.497(6)
1.945(4)
1.427(5)
1.196(5)
1.372(5)
1.376(6)
1.376(6)
1.468(5)
1.522(5)
1.937(5)
1.411(5)
1.201(5)
1.370(6)
1.390(7)
1.382(6)
1.450(6)
1.522(5)
1.961(4)
1.369(5)
1.450(5)
1.202(5)
1.382(6)
1.382(6)
1.381(6)
1.538(6)
1.474(6)
Br(1A)−C(10A)
N(1A)−C(8A)
N(1A)−C(9A)
O(2A)−C(8A)
C(1A)−C(6A)
C(3A)−C(4A)
C(5A)−C(6A)
C(7A)−C(8A)
C(10A)−C(11A)
Br(2B)−C(11B)
N(1B)−C(1B)
O(1B)−C(7B)
C(1B)−C(2B)
C(2B)−C(3B)
C(4B)−C(5B)
C(6B)−C(7B)
C(9B)−C(10B)
Br(2A)−C(11A)
N(1A)−C(1A)
O(1A)−C(7A)
C(1A)−C(2A)
C(2A)−C(3A)
C(4A)−C(5A)
C(6A)−C(7A)
C(9A)−C(10A)
Br(1B)−C(10B)
N(1B)−C(8B)
N(1B)−C(9B)
O(2B)−C(8B)
C(1B)−C(6B)
C(3B)−C(4B)
C(5B)−C(6B)
C(7B)−C(8B)
C(10B)−C(11B)
1243

TABLE 2. Valence Angles (ω) in the Compound 4 Structure
Angle
ω, deg
Angle
ω, deg
110.8(4)
124.7(4)
128.4(5)
116.5(5)
120.1(5)
121.0(4)
107.7(4)
124.2(4)
127.5(4)
106.1(4)
114.3(4)
109.4(3)
110.7(4)
124.9(3)
127.5(4)
116.9(4)
119.2(5)
119.5(4)
107.7(4)
124.5(4)
127.5(4)
105.9(4)
114.4(3)
109.4(3)
124.4(4)
121.2(4)
110.4(4)
123.3(5)
117.9(5)
131.3(5)
130.8(5)
105.0(4)
126.4(4)
113.5(3)
111.0(3)
114.7(3)
124.4(4)
122.2(4)
110.3(4)
122.6(4)
119.6(4)
132.8(4)
130.1(5)
105.4(4)
126.6(4)
113.0(3)
111.8(3)
114.5(3)
C(8A)−N(1A)−C(1A)
C(1A)−N(1A)−C(9A)
C(2A)−C(1A)−N(1A)
C(1A)−C(2A)−C(3A)
C(3A)−C(4A)−C(5A)
C(5A)−C(6A)−C(1A)
C(1A)−C(6A)−C(7A)
O(1A)−C(7A)−C(8A)
O(2A)−C(8A)−N(1A)
N(1A)−C(8A)−C(7A)
C(11A)−C(10A)−C(9A)
C(9A)−C(10A)−Br(1A)
C(8B)−N(1B)−C(1B)
C(1B)−N(1B)−C(9B)
C(2B)−C(1B)−N(1B)
C(1B)−C(2B)−C(3B)
C(5B)−C(4B)−C(3B)
C(5B)−C(6B)−C(1B)
C(1B)−C(6B)−C(7B)
O(1B)−C(7B)−C(8B)
O(2B)−C(8B)−N(1B)
N(1B)−C(8B)−C(7B)
C(11B)−C(10B)−C(9B)
C(9B)−C(10B)−Br(1B)
C(8A)−N(1A)−C(9A)
C(2A)−C(1A)−C(6A)
C(6A)−C(1A)−N(1A)
C(4A)−C(3A)−C(2A)
C(4A)−C(5A)−C(6A)
C(5A)−C(6A)−C(7A)
O(1A)−C(7A)−C(6A)
C(6A)−C(7A)−C(8A)
O(2A)−C(8A)−C(7A)
N(1A)−C(9A)−C(10A)
C(11A)−C(10A)−Br(1A)
C(10A)−C(11A)−Br(2A)
C(8B)−N(1B)−C(9B)
C(2B)−C(1B)−C(6B)
C(6B)−C(1B)−N(1B)
C(2B)−C(3B)−C(4B)
C(4B)−C(5B)−C(6B)
C(5B)−C(6B)−C(7B)
O(1B)−C(7B)−C(6B)
C(6B)−C(7B)−C(8B)
O(2B)−C(8B)−C(7B)
N(1B)−C(9B)−C(10B)
C(11B)−C(10B)−Br(1B)
C(10B)−C(11B)−Br(2B)
2-oxoheterocycle can undergo halocyclization during bromination*. Hence the failure we have experienced
when trying to transform the N-allyl derivative 1 to oxazoloindole 3 called for totally different reasoning.
Attention was turned to the fact that bicyclics of the type discussed are only readily formed in those
conditions where a sulfur or nitrogen atom takes part in the halocyclization (see structures 5-8). Closing of the
second ring at an oxygen atom is encountered very rarely and then as the result of the iodination of carboxylic
Alk
S
S
S
R
N
N
N
N
H
8 [23]
Me
Ts
5 [18]
6 [19, 20]
7 [21, 22]
Me
N
OH
COOH
O
COOH
O
O
N+
9 [24]
10 [25]
11
_______
*We have shown that the reaction of 1-allyl-3-(2-hydroxyethyl)-1H-quinazoline-2,4-dione with molecular
bromine in glacial acetic acid also occurs with addition of the halogen to the allyl double bond, i.e. the only
product is the corresponding 1-N-(2,3-dibromompropyl) derivative.
1244

acids containing an olefine bond (structure 9). A feature of such reactions is the participation of the oxygen atom
of a hydroxyl group in the cyclization to form lactones in the final step, hence being referred to as
iodolactonization. But here examples of the formation of similar bicycles involving the carbonyl oxygen atom
are generally unsuccessful.
In the halocyclization reactions the carbonyl oxygen atom is thought to become a nucleophilic center via
the formation of stable α-deprotonation products, i.e. the hydroxy form [2]. For N-allylisatin 1 a similar reaction
is impossible in principle and this might be taken as a quite convincing reason for the failure of
bromocyclization. However, in the case of 1-allyl-4-methyl-2-oxo-1,2-dihydroquinoline-3 carboxylic acid (10),
which is also unable to form a 2-hydroxy derivative, its brominative cyclization to 2-bromomethyl-4-carboxy-
5-methyl-1,2-dihydrooxazolo[3,2-a]quinolinium bromide is in no way hindered. This was explained by X-ray
analysis which showed a marked contribution of the bipolar aromatic form 11 in the resonance hybrid [25].
These facts allow us to propose that the route of the reaction studied by us depends to a marked extent
not so much on the sizes of the rings created or the possibility of enol forms as on a dependence of the structure
of the molecule as a whole on the polarizability of potential nucleophilic centers, i.e. the sulfur, nitrogen, or
oxygen atoms. For isatins the contribution of a polar form of type 11 is evidently not large, hence bromination
of its N-allyl-substituted derivative 1 occurs only via the usual addition of bromine to an allyl double bond. It is
likely that, for this reason, the potassium or sodium salts of isatin (even though formally ambident nucleophiles)
are none the less alkylated exclusively at the nitrogen atom. For comparison, the pyrid- and quinolin-2-ones (the
1-N-allyl-substituted derivatives of which are brominated very readily) form a mixture of the N- and O-alkyl-
substituted isomers under the same conditions [26, 27]. In other words, the polarizability of the 2-carbonyl
oxygen atoms in the pyrid- and quinolin-2-ones is markedly greater than in isatins and this particular feature is
reflected in their chemical properties.
EXPERIMENTAL
1H NMR spectra for the compounds synthesized were recorded on a Varian Mercury-VX-200 (200
MHz) instrument using DMSO-d6 solvent and TMS as internal standard.
N-Allylisatin 1. Finely divided K2CO3 (2 g) was added to a solution of isatin 2 (1.47 g, 0.01 mol) in
DMSO (15 ml), stirred, and treated with allyl bromide (0.93 ml, 0.011 mol). The reaction product changed color
from violet to red virtually instantaneously, after which it was diluted with cold water. The bright red precipitate
of N-allylisatin 1 was filtered off, washed with cold water, and dried. Yield 1.83 g (98%); mp 86-88ºC (ethanol).
1H NMR spectrum, δ, ppm (J, Hz): 7.62 (1H, t, J = 7.8, H-6); 7.54 (1H, d, J = 7.3, H-4); 7.11 (1H, t, J = 7.6,
H-5); 7.04 (1H, d, J = 8.0, H-7); 5.84 (1H, m, CH=CH2); 5.31 (1H, d, J = 17.3, NCH2CH=CH trans); 5.16 (1H,
d, J = 10.4, NCH2CH=CH cis); 4.28 (2H, d, J = 4.8, NCH2).
N-(2,3-Dibromopropyl)isatin (4). A. Bromine (0.52 ml, 0.01 mol) was added with stirring to a solution
of N-allylisatin 1 (1.87 g, 0.01 mol) in acetic acid (20 ml) and the color disappeared immediately. The reaction
mixture was diluted with water. The red precipitate was filtered off, washed with cold water, and dried. Rf 0.48
1
(Silufol UV-254, hexane–CH2Cl2–acetone, 5:5:1). Yield 3.26 g (90%); mp 129-131ºC (ethanol). H NMR
spectrum, δ, ppm (J, Hz): 7.67 (1H, tt, J = 7.8 and 1.4, H-6); 7.56 (1H, dt, J = 7.3 and 1.4, H-4); 7.25 (1H, dd,
J = 7.8 and 2.0, H-7); 7.13 (1H, tt, J = 7.5 and 1.3, H-5); 4.67 (1H, q, J = 6.4, NCH2CHBr); 4.15 (2H, d, J = 7.1,
NCH2); 4.02 (2H, dd, J = 5.9 and 2.1, CH2Br). Found, %: C 37.93; H 2.50; N 4.13. C11H9Br2NO2. Calculated,
%: C 38.07; H 2.61; N 4.04.
B. A solution of bromine (0.52 ml, 0.01 mol) in CCl4 (20 ml) was added with vigorous stirring and
cooling to -20ºC to a solution of N-allylisatin 1 (1.87 g, 0.01 mol) in CCl4 (150 ml) also cooled to -20ºC. After
addition of all of the bromine the cooling was removed. Solvent was removed under reduced pressure to a
volume of ~ 30 ml then cooled. The red precipitate was filtered and dried. Yield 3.40 g (94%). A mixed sample
1245

1
with compound 4 made by method A did not give a depression of melting point. The H NMR spectra and Rf
values of these compounds were identical.
X-ray Crystallographic Investigation. Crystals of N-(2,3-dibromopropyl)isatin (4) were monoclinic
(acetone). At 20ºC: a = 12.988(2), b = 13.013(3), c = 15.092(3) Å, β = 113.41(2)º, V = 2340.7(7) Å3,
Mr = 347.01, Z = 8, space group P21/n, dcalc = 1.969 g/cm3, µ(MoKα) = 6.910 mm-1, F(000) = 1344. The unit
cell parameters and intensities of 12819 reflections (4071 independent with Rint = 0.033) were measured on an
Xcalibur-3 diffractometer (MoKα radiation, CCD detector, graphite monochromator, ω-scanning to
2θmax = 50º). The absorption was calculated by a semiempirical method from the results of multiscanning
(Tmin = 0.208, Tmax = 0.501).
The structure was solved by a direct method using the SHELXTL program package [28]. The positions
of the hydrogen atoms were revealed from electron density difference synthesis and refined using the "riding"
model with Uiso = 1.2 Ueq for a non-hydrogen atom bound to the given hydrogen. The structure was refined via
F2 full-matrix least-squares analysis in the anisotropic approximation for non-hydrogen atoms to wR2 = 0.071
for 4054 reflections (R1 = 0.034 for 2430 reflections with F>4σ(F) and S = 0.838). The full crystallographic
information was placed in the Cambridge Structural Data Base (reference CCDC 717535). Interatomic distances
and valence angles are given in Tables 1 and 2.
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