Synthesis of profluorescent isoindoline nitroxides via palladium-catalysed Heck alkenylation

Article abstract of DOI:10.1039/b504354a

The synthesis of a new structural class of isoindoline nitroxides (aminoxyls), accessible via the palladium-catalysed Heck reaction, is presented. Reaction of the aryl bromoamine, 5-bromo-1,1,3,3-tetramethylisoindoline (4) or dibromoamine, 5,6-dibromo-1,1

Full text of DOI:10.1039/b504354a

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A R T I C L E
Synthesis of profluorescent isoindoline nitroxides via
palladium-catalysed Heck alkenylation
Daniel J. Keddie, Therese E. Johnson, Dennis P. Arnold and Steven E. Bottle*
School of Physical and Chemical Sciences, Queensland University of Technology,
GPO Box 2434, Q4001, Australia. E-mail: s.bottle@qut.edu.au
Received 29th March 2005, Accepted 10th May 2005
First published as an Advance Article on the web 13th June 2005
The synthesis of a new structural class of isoindoline nitroxides (aminoxyls), accessible via the palladium-catalysed
Heck reaction, is presented. Reaction of the aryl bromoamine, 5-bromo-1,1,3,3-tetramethylisoindoline (4) or
dibromoamine, 5,6-dibromo-1,1,3,3-tetramethylisoindoline (5) or the analogous bromonitroxides 6 and 7 with
methyl acrylate gives the acrylate substituted tetramethylisoindoline amines 8 and 10 and nitroxides 12 and 14.
Similarly, the reaction of the aryl bromides and dibromides 4–7 with methyl 4-vinylbenzoate gives the carboxystyryl
substituted tetramethylisoindoline amines 9 and 11 and the analogous nitroxides 13 and 15. The carboxystyryl
tetramethylisoindoline nitroxides demonstrate strongly suppressed fluorescence, which is revealed upon removal of
the free radical by reduction or radical coupling.
acrylate (2) or methyl 4-vinylbenzoate (3) as the olefin starting
materials allows for the production of isoindoline amines and
nitroxides containing substituted styryl and substituted stilbene
Introduction
The isoindoline class of nitroxides (aminoxyls), such as 1,1,3,3-
tetramethylisoindolin-2-yloxyl1 (1) (TMIO), possesses some
functionality respectively. These molecules combine the advan-
advantages2–5 over commercially available nitroxides. These
tages of the isoindoline class of nitroxides with a robust carbon–
advantages arise largely from the fused aromatic moiety, which
carbon bonded fluorophore–nitroxide linkage. The free radical
imparts rigidity to the ring system. Isoindoline nitroxides
compounds display weak fluorescence which can be switched
can exhibit superior electron paramagnetic resonance (EPR)
on by reduction to generate highly fluorescent hydroxylamines.
linewidths,6–9 resulting in increased accuracy in EPR oximetry,
Compounds which contain the stilbene functionality are often
as relative line broadening is more easily measured on narrower
usedas fluorescentprobe molecules inthe study ofphotophysical
lines.10 Isoindoline systems may also allow the synthesis of
properties of various media, due to the characteristically high
more complex structures, through substitution onto the ring.11
quantum yields and short excited state lifetimes of stilbene
This is particularly significant in the generation of structures
derivatives.40 The incorporation of a stilbene moiety imparts
where extended conjugation can give rise to useful spectroscopic
properties.
these key fluorescent properties to the novel nitroxide probes,
albeit suppressed by the nitroxide function. Upon reaction at
the nitroxide moiety, the fluorescence potency of the stilbene is
revealed.
Results and discussion
Due to the relative stability of bis(tert-alkyl) nitroxides, many
reactions can be performed in the presence of the free radical
moiety. Reactions performed on the isoindoline systems have
included nitration,6 Friedel–Crafts alkylation,41 as well as many
functional group interconversions. Little work to date has
dealt with palladium-catalysed coupling reactions which are a
powerful tool in current synthetic methodology. Whilst Kalai
et al.42 have successfully applied such coupling reactions to
pyrroline nitroxides, here we report the first examples where
palladium-catalysis has been performed on the isoindoline class
of nitroxides.
In this context, nitroxide radicals have long been recognised as
effective quenchers of excited states of fluorescent moieties.12–18
Following the seminal work of Blough et al.19–23 several nitroxide
molecules containing a covalently linked fluorophore have
been synthesised. The covalent linkage creates a permanent
“collision complex” between the nitroxide free radical moiety
and the fluorophore resulting in almost complete quenching
of the expected fluorescence output.19 The conversion of
such “profluorescent” nitroxides into diamagnetic species by
radical scavenging or redox activity eliminates this process
and consequently allows for their detection by fluorescence
spectroscopy. These nitroxide–fluorophore species can therefore
be used as sensitive probes for the investigation of processes
involving reactive free radical/redox species. Few of the proflu-
orescent nitroxides synthesised to date involve robust carbon
frameworks linking the fluorophore and the nitroxide,24–26 and
usually contain more labile linkages such as esters,19–21,27–31
amides32–34 and sulfonamides35–38 or utilise comparatively labile
fluorescamine/amine adducts.22,39 In this context we have re-
cently demonstrated the use of a novel profluorescent nitroxide
based on a stable fused phenanthrene structure to probe the
thermo-oxidative degradation process in polypropylene.26
Here we describe the first application of palladium-catalysed
alkenylation of isoindoline nitroxides to generate a range of
novel profluorescent nitroxides and amines. The use of methyl
Palladium-catalysed Heck alkenylation of 5-bromo-1,1,3,3-
tetramethylisoindoline (4) and 5-bromo-1,1,3,3-
tetramethylisoindolin-2-yloxyl (6) with methyl acrylate (2)
and methyl 4-vinylbenzoate (3)
The reaction of bromoisoindoline 4 and the nitroxide analogue 6
with methyl acrylate (2) in the presence of 2 mol% palladium cat-
alyst in anhydrous N,N-dimethylformamide (DMF) at 100 ◦C,
gave the acrylate substituted isoindoline (8) and isoindoline
nitroxide (12) in modest yields of 29% and 13% respectively
◦
(Scheme 1, Table 1). Due to its low boiling point (80.7 C),43
a large excess of the olefin was used to maximise the available
concentration in the reaction mixture.
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 2 5 9 3 – 2 5 9 8
2 5 9 3
©

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Table 1
Starting
material
Reagents/
conditions
Reaction
time/h
Purified
yield (%)
R1
Br
Br
Br
Br
R2
H
R3
H
Alkene
Product R3
R4
R5
=
4
5
6
7
a
b
c
d
e
f
g
h
2
3
2
3
2
3
2
3
8
9
H
H
H
CH CHCO2CH3
CH CH(C6H4)CO2CH3
CH CHCO2CH3
CH CH(C6H4)CO2CH3
CH CHCO2CH3
CH CH(C6H4)CO2CH3
CH CHCO2CH3
CH CH(C6H4)CO2CH3
H
160
96
66
142
72
72
29
33
15
11
50
85
62
53
=
H
R4
R4
H
=
Br
H
H
10
11
12
13
14
15
=
H
O•
O•
O•
O•
O•
O•
=
=
H
R4
R4
72
72
=
Br
=
Scheme 1 Reagents and conditions: (a) K2CO3 (1.8 eqv.), Pd(OAc)2 (0.025 eqv.), PPh3 (0.04 eqv.), 2 (9.6 eqv.), DMF, 100 ◦C, 160 h (b) K2CO3 (1.8 eqv.),
Pd(OAc)2 (0.025 eqv.), PPh3 (0.04 eqv.), 3 (1.25 eqv.), DMF, 100 ◦C, 96 h (c) K2CO3 (3.0 eqv.), Pd(OAc)2 (0.06 eqv.), PPh3 (0.10 eqv.), 2 (19.1 eqv.),
DMF, 100 ◦C, 66 h (d) K2CO3 (3.0 eqv.), Pd(OAc)2 (0.06 eqv.), PPh3 (0.1 eqv.), 3 (2.5 eqv.), DMF, 100 ◦C, 142 h (e) K2CO3 (1.95 eqv.), Pd(OAc)2
(0.05 eqv.), PPh3 (0.10 eqv.), 2 (9.6 eqv.), DMF, 120 ◦C, 72 h (f) K2CO3 (1.9 eqv.), Pd(OAc)2 (0.05 eqv.), PPh3 (0.10 eqv.), 3 (1.3 eqv.), DMF, 120 ◦C,
72 h (g) K2CO3 (3.26 eqv.), Pd(OAc)2 (0.10 eqv.), PPh3 (0.20 eqv.), 2 (19.1 eqv.), DMF, 120 ◦C, 72 h (h) K2CO3 (3.27 eqv.), Pd(OAc)2 (0.10 eqv.), PPh3
(0.20 eqv.), 3 (2.6 eqv.), DMF, 120 ◦C, 72 h.
In a similar reaction, this bromoisoindoline (4) and the
nitroxide analogue (6) successfully reacted with methyl 4-
vinylbenzoate (3) to give the carboxystyryl-substituted isoin-
doline (9) and isoindoline nitroxide (13) in yields of 33% and
34% respectively (Scheme 1). These initial reactions were not
optimised, with the methyl acrylate being used to demonstrate
the viability of Heck methodology to achieve this coupling.
Palladium catalysed coupling with 1◦ and 2◦ amines in this
context can be problematic, leading to amine cross coupling
via Buchwald–Hartwig amination of the aryl halide. This may
not be a major factor under the conditions used here due to the
high steric demand of the amine in these tetamethylisoindoline
systems.
This reaction was quite sensitive to the conditions used, as
subtle alterations were shown to lead to substantial changes
in the isolated yields. For instance, with the nitroxides, optimal
reaction conditions (5 mol% palladium catalyst and temperature
120 ◦C) improved the syntheses of the acrylate substituted
nitroxide (12) and carboxystyryl substituted nitroxide (13) from
the 13% and 34% described above to 50% and 85% respectively.
tive addition of the halide to the palladium complex becomes
more difficult in the presence of electron-donating substituents.
This is also evident from the yields of the disubstituted Heck
products (10, 11, 14 and 15) which, under the same conditions,
are obtained in comparable amounts to the monosubstituted
products (8, 9, 12 and 13). Vinylation would be expected to lower
electron density in the ring and therefore promote reactivity for
the second substitution.
Fluorescence
A measure of the fluorescence suppression of nitroxides 13 and
15 can be determined by in situ reduction with ascorbic acid
measured directly in a spectrofluorimeter.
Complete reduction of the mono and disubstituted nitroxides
13 and 15 to give the hydroxylamines 16 and 17 is readily
achieved, with maximum fluorescence being observed ∼3 hours
after addition of ascorbic acid (see Scheme 2). As expected,
the reduced mono-carboxystyryl substituted nitroxide (16) has
a maximum emission at a shorter wavelength (378 nm) than the
corresponding reduced disubstituted nitroxide (17) (436 nm),
due to the more extended conjugation present in the latter.
Interestingly, the monosubstituted nitroxide displays a much
stronger relative suppression (225 fold) compared to the disub-
stituted species (cf. 25 fold). Notably the disubstituted system
generates the stronger emission intensity and displays greater
fluorescence than the monosubstituted compound. Presumably
incomplete excited state quenching by the nitroxide radical,
when linked to a strongly fluorescent aromatic structure such
as the one present in 15, does not allow complete suppression of
fluorescence emission.
Palladium-catalysed Heck alkenylation of
5,6-dibromo-1,1,3,3-tetramethylisoindoline (5) and
5,6-dibromo-1,1,3,3-tetramethylisoindolin-2-yloxyl (7) with
methyl acrylate (2) and with methyl 4-vinylbenzoate (3)
The reaction of the aryl ortho-dibromides 5 and 7 with methyl
acrylate (2) at 100 ◦C, in the presence of 5.5 mol% palladium
catalyst, successfully gave the bis-acrylate substituted isoindo-
line (10) and isoindoline nitroxide (14) in yields of 15% and 32%
respectively (Scheme 1).
Similarly, reaction of aryl ortho-dibromides 5 and 7 with
methyl 4-vinylbenzoate (3) at 100 ◦C gave the bis-carboxystyryl-
substituted isoindoline (11) and isoindoline nitroxide (15) in
yields of 11% and 36% respectively (Scheme 1).
Again this reaction was quite sensitive to the conditions used,
and optimal reaction conditions improved the syntheses of the
extended aryl nitroxide products (14) and (15) from the 32% and
36% described above to 62% and 53% respectively.
The difference in the fluorescence intensity between the
profluorescent nitroxides (13 and 15) and the diamagnetic
hydroxylamines (16 and 17) indicates that these compounds
may be useful fluorescent probes for the detection of reactive
radical–redox species (Figs. 1 and 2).
Radical coupling
Electron-rich aryl-halides, such as 4, 5, 6 and 7, are somewhat
As further proof of the structure of the nitroxides generated
using this Heck methodology, the carboxystyryl substituted
deactivated with respect to the Heck reaction,44 because oxida-
2 5 9 4
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 2 5 9 3 – 2 5 9 8

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Scheme 2 Reduction of nitroxides to hydroxylamines with ascorbic acid.
Fig. 2 Fluorescence spectra of 15 (—) and 17 (---) normalised to 1 lM.
Fig. 1 Fluorescence spectra of 13 (—) and 16 (---) normalised to 1 lM.
Experimental
nitroxide (13) was reacted with methyl radicals generated
from dimethylsulfoxide and hydrogen peroxide to give the
methoxyamine 18 (Scheme 3). The fluorescence intensity of 18
mirrored the hydroxylamine 16 with an excitation at 342 nm
producing a strong fluorescence at 392 nm.
1H NMR spectra were recorded on a Bruker Avance 400
spectrometer in CDCl3 and 13C NMR spectra on a Varian Unity
300 spectrometer in CDCl3. Coupling constants are given in
Hz. IR spectra were recorded on a Nicolet 870 Nexus Fourier
Scheme 3 Coupling of carboxystyryl substituted nitroxide (13) to give the methyl radical adduct.
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 2 5 9 3 – 2 5 9 8
2 5 9 5

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=
Transform infrared spectrometer equipped with a DTGS TEC
detector and an ATR objective. High resolution mass spectra
were recorded on a Kratos Concept ISQ mass spectrometer,
utilising a direct insertion probe and operating at 70 eV, 5.3 kV
accelerating voltage and a source temperature of 200 ◦C, at
a resolution of 6000 ppm, with perfluorokerosene used as an
internal mass reference. Spectrofluorimetry was performed on a
Varian Cary Eclipse fluorescence spectrophotometer equipped
with a standard multicell Peltier thermostatted sample holder. 5-
Bromo-1,1,3,3-tetramethylisoindoline11 (4), 5,6-dibromo-1,1,3,3-
tetramethylisoindoline11 (5) 5-bromo-1,1,3,3-tetramethylisoin-
dolin-2-yloxyl11 (6), 5,6-dibromo-1,1,3,3-tetramethylisoindolin-
2-yloxyl11 (7) and methyl 4-vinylbenzoate45 (3) were synthesised
by methods previously described in the literature.
CH); dC (75.430 MHz; CDCl3) 31.9 (CH3), 52.1 (OCH3),
=
63.0 (C-1 and C-3), 120.8 ( CH), 121.2 (C-5 and C-6), 134.1
=
(C-4 and C-7), 141.9 ( CH), 151.7 (C-3a and C-7a), 167.1
(C O); mmax (ATR) 3365 (NH), 2961 (alkyl CH), 1716 (C O),
=
=
−1
=
1637 (C C), 1463 and 1436 (aryl C–C), 1167 (OCH3) cm ;
EI MS found (M–H)+ 342.17020 (0.96 ppm from calc. mass of
C20H24NO4 (M–H)); m/z 342 ((M–H)+, <1%), 328 (100), 268 (9).
5-(2-Methylcarboxyethenyl)-1,1,3,3-tetramethylisoindolin-2-
yloxyl (12)
5-Bromo-1,1,3,3-tetramethylisoindolin-2-yloxyl (6) (100 mg,
0.38 mmol), K2CO3 (100 mg, 0.72 mmol), Pd(OAc)2 (4 mg,
0.008 mmol), PPh3 (8 mg, 0.031 mmol) and methyl acrylate
(2) (0.32 cm3, 3.55 mmol) were treated as above and heated
in anhydrous DMF (5 cm3) for 72 hours at 120 ◦C. Column
chromatography (30% EtOAc, 70% n-hexane) and recrystalli-
sation from MeCN gave yellow irregular platelets of the
monosubstituted acrylate nitroxide 12 (52 mg, 0.189 mmol, 50%)
Heck reactions with methyl acrylate (2)
The procedure for the synthesis of acrylate substituted isoin-
doline amines 8 (monosubst.) and 10 (disubst.) and nitroxides
12 (monosubst.) and 14 (disubst.) from methyl acrylate (2) and
the brominated amines 4 and 5 or nitroxides 6 and 7 is shown
below, using the synthesis of the monosubstituted acrylate
isoindoline amine 8 as an example. Alterations of the procedure
for the synthesis of disubstituted amine 10 and the mono and
disubstituted nitroxides, 12 and 14 respectively, are also shown
below. Notably, standard oxidations of the substituted amines
to generate the nitroxides were not successful, possibly due to
oxidation of the alkene moiety.
◦
mp 162–164 C; mmax (ATR) 3023 (aryl CH), 2973 (alkyl CH),
=
=
1704 (C O), 1641 (C C), 1492 and 1432 (aryl C–C), 1359 and
1330 (NO), 1164 (OCH3) cm−1; EI MS found M+ 274.14427
(0.18 ppm from calc. mass of C16H20NO3); m/z 274 (M+, 66%),
259 (100), 244 (83), 229 (60).
5,6-Bis-(2-methylcarboxyethenyl)-1,1,3,3-tetramethylisoindolin-
2-yloxyl (14)
5,6-Dibromo-1,1,3,3-tetramethylisoindolin-2-yloxyl (7) (100 mg,
0.28 mmol), K2CO3 (154 mg, 1.11 mmol), Pd(OAc)2 (8 mg,
0.016 mmol), PPh3 (16 mg, 0.062 mmol) and methyl acrylate
(2) (0.5 cm3, 5.55 mmol) were treated as previously indicated
above and heated in anhydrous DMF (5 cm3) for 72 hours at
120 ◦C. Column chromatography (30% EtOAc, 70% n-hexane)
and recrystallisation from MeCN gave yellow needles of the
disubstituted acrylate nitroxide 14 (62 mg, 0.173 mmol, 62%)
5-(2-Methylcarboxyethenyl)-1,1,3,3-tetramethylisoindoline (8)
To 5-bromo-1,1,3,3-tetramethylisoindoline (4) (300 mg,
1.18 mmol), K2CO3 (300 mg, 2.17 mmol), Pd(OAc)2 (6 mg,
0.027 mmol) and PPh3 (12 mg, 0.046 mmol) in a Schlenk
vessel, under argon were added methyl acrylate (2) (0.96 cm3,
10.66 mmol) and anhydrous DMF (15 cm3) and the resulting
solution was degassed via several freeze–evacuate–thaw cycles
then sealed under argon and stirred at 100 ◦C for 160 hours.
Water was then added to the resultant brown solution and
the reaction mixture was extracted with DCM. The com-
bined organic phases were washed with brine, dried (Na2SO4)
and the solvent was removed under reduced pressure. 5-(2-
Methylcarboxyethenyl)-1,1,3,3-tetramethylisoindoline (8) was
purified by column chromatography (30% EtOAc, 70% n-
hexane) to give an orange oil (86 mg, 0.34 mmol, 29%) which
did not crystallise; dH (400.162 MHz; CDCl3) 1.45 (6H, s, CH3),
1.46 (6H, s, CH3), 1.81 (1H, br, NH), 3.80 (3H, s, OCH3), 6.46
mp 216–218 ◦C (decomp.); mmax (ATR) 3066 ( CH), 3033 (aryl
=
=
=
CH), 2967 (alkyl CH), 1702 (C O), 1627 (C C), 1488 and 1436
(aryl C–C), 1367 and 1328 (NO), 1166 (OCH3) cm−1; EI MS
found M+ 358.16511 (0.95 ppm from calc. mass of C20H24NO5);
m/z 358 (M+, 90%), 343 (14), 328 (100), 283 (53), 268 (75),
253 (44).
Heck reactions with methyl 4-vinylbenzoate (3)
The procedure for the synthesis of the carboxystyryl isoindoline
amines 9 and 11 and nitroxides 13 and 15 from methyl 4-
vinylbenzoate (3) and the brominated amines 4 and 5 or
nitroxides 6 and 7 is shown below using the synthesis of
the monosubstituted carboxystyryl isoindoline amine 9 as an
example. Alterations of the procedure for the synthesis of
the disubstituted carboxystyryl isoindoline amine 11 or the
mono and disubstituted carboxystyryl nitroxides, 13 and 15
respectively, are also shown below.
=
(1H, d, J 15.9 Hz, CH), 7.14 (1H, d, J 7.8 Hz, 7-H), 7.27 (1H,
d, J 1.5 Hz, 4-H), 7.42 (1H, dd, J 7.8 Hz and 1.5 Hz, 6-H), 7.75
=
(1H, d, J 15.9 Hz, CH); dC (75.430 MHz; CDCl3) 29.9 (CH3),
32.0 (CH3), 51.9 (OCH3), 63.1 (C-1), 117.4 ( CH), 121.3 (C-5),
=
=
122.3 (C-6), 127.9 (C-7), 133.9 (C-4), 141.9 ( CH), 149.8 (C-7a),
151.6 (C-3a), 167.1 (C O); mmax (ATR) 3369 (NH), 2960 (alkyl
CH), 1716 (C O), 1636 (C C), 1463 and 1436 (aryl C–C), 1164
(OCH3) cm−1; EI MS found (M–H)+ 258.14939 (0.04 ppm from
calc. mass of C16H20NO2 (M–H)); m/z 258 ((M–H)+, ∼1%), 244
(100), 229 (33).
=
=
=
5-[2-(4-Methylcarboxyphenyl)ethenyl]-1,1,3,3-
tetramethylisoindoline (9)
To 5-bromo-1,1,3,3-tetramethylisoindoline (4) (300 mg,
1.18 mmol), K2CO3 (300 mg, 2.17 mmol), Pd(OAc)2 (6 mg,
0.027 mmol) and PPh3 (12 mg, 0.046 mmol) in a Schlenk vessel
under argon were added methyl 4-vinylbenzoate (3) (240 mg,
1.47 mmol) and anhydrous DMF (15 cm3) and the resulting
solution was degassed via several freeze–evacuate–thaw cycles,
sealed under argon and stirred at 100 ◦C for 96 hours. Water
was added to the resultant dark brown solution and this
reaction mixture was extracted with DCM. The combined
organic phases were washed with brine, dried (Na2SO4) and
the solvent removed under reduced pressure. The product, 5-[2-
(4-methylcarboxyphenyl)ethenyl]-1,1,3,3-tetramethylisoindoline
(9), was purified by column chromatography (98% CHCl3,
2% MeOH) and recrystallised from EtOH, forming colourless
crystals (132 mg, 0.394 mmol, 33%) mp 144–146 ◦C; dH
5,6-Bis-(2-methylcarboxyethenyl)-1,1,3,3-tetramethylisoindoline
(10)
,6-Dibromo-1,1,3,3-tetramethylisoindoline (5) (300 mg,
0.90 mmol), K2CO3 (378 mg, 2.73 mmol), Pd(OAc)2 (12 mg,
0.054 mmol), PPh3 (24 mg, 0.092 mmol), methyl acrylate (2)
(1.44 cm3, 16 mmol), were treated as above and heated in
anhydrous DMF (15 cm3) for 66 hours at 100 ◦C. Column
chromatography (95% CHCl3, 5% MeOH) gave the disubstituted
acrylate isoindoline amine 10 as a yellow oil (45 mg, 0.132 mmol,
15%), which did not crystallise; dH (400.162 MHz; CDCl3) 1.46
(12H, s, CH3), 1.92 (1H, br, NH), 3.82 (6H, s, OCH3), 6.35 (2H,
=
d, J 15.8 Hz, CH), 7.29 (2H, s, Ar–H), 8.04 (2H, d, J 15.8 Hz,
2 5 9 6
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 2 5 9 3 – 2 5 9 8

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=
=
(400.162 MHz; CDCl3) 1.47 (6H, s, CH3), 1.50 (6H, s, CH3),
1.61 (1H, br, NH), 3.92 (3H, s, OCH3), 7.11 (1H, d, J 16.1 Hz,
(ATR) 2973 (alkyl CH), 1708 (C O), 1602 (C C), 1484 and
1403 (aryl C–C), 1373 and 1359 (NO) cm−1; EI MS found M+
510.22812 (0.14 ppm from calc. mass of C32H32NO5); m/z 510
(M+, 15%), 495 (9), 480 (100), 465 (10), 330 (40).
=
CH), 7.12 (1H, d, J 7.8 Hz, 7-H), 7.25 (1H, d, J 16.1 Hz,
CH), 7.28 (1H, d, J 1.2 Hz, 4-H), 7.42 (1H, dd, J 1.2 Hz
=
and 7.8 Hz, 6-H), 7.57 (2H, d, J 8.3 Hz, ArH), 8.02 (2H, d, J
8.3 Hz, ArH); dC (75.430 MHz; CDCl3) 31.8 (CH3), 31.9 (CH3),
5-[2-(4-Methylcarboxyphenyl)ethenyl]-2-methoxy-1,1,3,3-
tetramethylisoindoline (18)
=
=
52.0 (OCH3), 62.7 (C-1 and C-3), 119.6 ( CH), 121.8 ( CH),
126.2 (C-6), 126.3 (C-4), 127.0 (C-7), 128.8 (ArC), 130.0 (ArC),
131.3 (ArC), 136.1 (C-5), 142.0 (ArC), 149.3 (C-7a), 149.5
To a solution of the carboxystyryl substituted nitroxide 13
(40 mg, 0.114 mmol) and FeSO4·7H2O (64 mg, 0.230 mmol)
in DMSO (4 cm3) was added H2O2 (30%, 26 lL) dropwise and
the mixture was stirred at room temperature for 30 minutes.
The resultant solution was poured onto NaOH (1 M, 30 cm3)
and subsequently extracted with Et2O (3 × 50 cm3) and dried
(Na2SO4). Removal of solvent under reduced pressure gave the
carboxystyryl substituted N-methoxyisoindoline 18 (36.8 mg,
0.101 mmol, 89%). Recrystallisation from EtOH gave colourless
crystals of 18 (32.2 mg, 0.088 mmol, 77%); dH (400.162 MHz;
CDCl3) 1.45 (12H, br s, CH3), 1.61 (1H, br, NH), 3.80 (3H, s,
NOCH3), 3.92 (3H, s, ester OCH3), 7.10 (1H, d, J 16.2 Hz,
=
(C-3a), 166.9 (C O); mmax (ATR) 3351 (NH), 2962 (alkyl CH3),
=
=
1719 (C O), 1604 (C C), 1477 and 1435 (aryl C–C), 1178
(OCH3) cm−1; EI MS found (M–H)+ 334.18164 (2.81 ppm from
calc. mass of C22H24NO2 (M–H)); m/z 334 ((M–H)+, 14%), 320
(100), 305 (23), 304 (18), 144 (10).
5,6-Bis-[2-(4-methylcarboxyphenyl)ethenyl]-1,1,3,3-
tetramethylisoindoline (11)
5,6-Dibromo-1,1,3,3-tetramethylisoindoline (5) (300 mg,
0.90 mmol), K2CO3 (378 mg, 2.73 mmol), Pd(OAc)2 (12 mg,
0.054 mmol), PPh3 (24 mg, 0.092 mmol) and methyl 4-
vinylbenzoate (3) (360 mg, 2.21 mmol) were treated as described
above and heated in anhydrous DMF (15 cm3) for 142 hours at
100 ◦C. Column chromatography (70% EtOAc, 30% n-hexane;
then 80% EtOAc, 20% EtOH) and recrystallisation by mixed
solvent layer recrystallisation from DCM–n-pentane gave
off-white crystals of the disubstituted carboxystyryl isoindoline
amine 11 (50 mg, 0.101 mmol, 11%) mp 176–178 ◦C; dH
(400.162 MHz; CDCl3) 1.53 (12H, s, CH3), 1.74 (1H, br, NH),
=
CH), 7.11 (1H, d, J 8.1 Hz, 7-H), 7.23 (1H, d, J 16.2 Hz,
CH), 7.27 (1H, d, J 1.3 Hz, 4-H), 7.40 (1H, dd, J 1.3 Hz
=
and 8.1 Hz, 6-H), 7.56 (2H, d, J 8.3 Hz, ArH), 8.02 (2H, d,
J 8.3 Hz, ArH); dC (75.430 MHz; CDCl3) 25.2 and 29.1 (CH3
groups, broadened through ring inversion), 52.5 (ester OCH3),
65.9 (NOCH3), 67.8 (C-1 and C-3, broadened through ring
inversion), 120.0 ( CH), 122.3 ( CH), 126.7 (C-6), 126.8 (C-4),
127.5 (C-7), 129.2 (ArC), 130.4 (ArC), 131.7 (ArC), 136.6 (C-5),
=
=
=
142.3 (ArC), 146.2 (C-7a and C-3a), 167.3 (C O); mmax (ATR)
=
3.93 (6H, s, OCH3), 7.04 (2H, d, J 15.8 Hz, C–H), 7.34 (2H, s,
4-H and 7-H), 7.57 (2H, d, J 15.8 Hz, C–H), 7.59 (4H, d, J
=
=
2978 (alkyl CH3), 1714 (C O), 1603 (C C), 1493 and 1435 (aryl
C–C), 1178 (OCH3) cm−1; EI MS found M+ 365.19948 (1.05 ppm
from calc. mass of C23H27NO3); m/z 365 (M+, 5%), 350 (100),
319 (26), 304 (23).
=
8.2 Hz, ArH), 8.04 (4H, d, J 8.2 Hz, ArH); dC (75.430 MHz;
CDCl3) 31.7 (CH3), 52.1 (OCH3), 62.9 (C-1 and C-3), 119.8
=
=
( CH), 119.9 ( CH), 126.4 (C-4 and C-7), 129.1 (ArC), 130.1
(ArC), 130.3 (ArC), 135.4 (C-5 and C-6), 141.8 (ArC), 149.3
Fluorescence measurements
=
(C-3a and C-7a), 166.8 (C O); mmax (ATR) 3345 (NH), 3018
5-[2-(4-Methylcarboxyphenyl)ethenyl]-1,1,3,3-tetramethyliso-
indolin-2-yloxyl (13). The carboxystyryl nitroxide 13 (0.4 mg)
was dissolved in HPLC grade MeOH (100 cm3) in a volumetric
flask. The resulting solution (4.0 mg dm−3, 1.14 × 10−5 M)
was deoxygenated by bubbling with argon for 15 min. Any
subsequent solvent loss was replaced to maintain a known
concentration. Reduction of the carboxystyryl nitroxide 13 to
the hydroxylamine 16 was achieved by addition of a saturated
solution of ascorbic acid in HPLC grade MeOH (50 lL) to a
solution of 13 in MeOH (3 cm3) with stirring.
=
=
(aryl CH), 2955 (alkyl CH), 1708 (C O), 1603 (C C), 1479
and 1433 (aryl C–C), 1178 (OCH3) cm−1; EI MS found (M–H)+
494.23332 (0.38 ppm from calc. mass of C32H32NO4 (M–H)):
m/z 494 ((M–H)+, ∼1%), 480 (100), 465 (8), 320 (24).
5-[2-(4-Methylcarboxyphenyl)ethenyl]-1,1,3,3-
tetramethylisoindolin-2-yloxyl (13)
5-Bromo-1,1,3,3-tetramethylisoindolin-2-yloxyl (6) (100 mg,
0.38 mmol), K2CO3 (100 mg, 0.72 mmol), Pd(OAc)2 (4 mg,
0.018 mmol), PPh3 (8 mg, 0.031 mmol) and methyl 4-vinyl
benzoate (3) (80 mg, 0.491 mmol) were treated as above
and heated in anhydrous DMF (5 cm3) for 72 hours at
120 ◦C. Column chromatography (70% EtOAc, 30% n-hexane)
and recrystallisation from EtOH gave light orange needles
of the monosubstituted carboxystyryl isoindoline nitroxide 13
(113 mg, 0.323 mmol, 85%) mp 164–165 ◦C (found: C, 75.2; H,
6.9; N, 3.9. C22H24NO3 requires C, 75.4; H, 6.9; N, 4.0%); mmax
5,6-Bis-[2-(4-methylcarboxyphenyl)ethenyl]-1,1,3,3-tetrameth-
ylisoindolin-2-yloxyl (15). In a similar procedure to the one
described above, disubstituted carboxystyryl nitroxide 15
(0.4 mg) was dissolved in HPLC grade MeOH (100 cm3) in
a volumetric flask. This solution was further diluted by a
factor of 5. The resulting solution of 15 in MeOH (0.8 mg
dm−3, 1.57 × 10−6 M) was deoxygenated by bubbling with
argon for 15 min. Any subsequent solvent loss was replaced to
maintain the concentration. Similar to above, the reduction of
disubstituted carboxystyryl nitroxide 15 to the hydroxylamine
17 was achieved by addition of a saturated solution of ascorbic
acid in HPLC grade MeOH (50 lL) to the solution of 15 in
MeOH (3 cm3) with stirring.
=
=
(ATR) 2976 (alkyl CH), 1715 (C O), 1604 (C C), 1492 and
1434 (aryl C–C), 1373 and 1358 (NO), 1177 (OCH3) cm−1; EI MS
found M+ 350.17546 (0.46 ppm from calc. mass of C22H24NO3);
m/z 350 (M+, 30%), 335 (25), 320(100), 305 (42).
5,6-Bis-[2-(4-methylcarboxyphenyl)ethenyl]-1,1,3,3-
tetramethylisoindolin-2-yloxyl (15)
Acknowledgements
5,6-Dibromo-1,1,3,3-tetramethylisoindolin-2-yloxyl
(7)
(100 mg, 0.28 mmol), K2CO3 (154 mg, 1.11 mmol), Pd(OAc)2
(8 mg, 0.036 mmol), PPh3 (16 mg, 0.062 mmol) and methyl
4-vinylbenzoate (3) (120 mg, 0.737 mmol) were◦treated as above
and heated in anhydrous DMF (5 cm3) at 120 C for 72 hours.
Column chromatography (30% EtOAc, 70% n-hexane) and
subsequent recrystallisation from MeCN gave fine light yellow
needles of the disubstituted carboxystyryl isoindoline nitroxide
15 (78 mg, 0.152 mmol, 54%) mp 212–214 ◦C (decomp.); mmax
The authors acknowledge the financial support of the Australian
Research Council (DP0211669) and the National Cancer Insti-
tute (PO1 CA91597).
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