Revised Structures of N-Substituted Dibrominated Pyrrole Derivatives and Their Polymeric Products. Termaleimide Models with Low Optical Band Gaps

Article abstract of DOI:10.1021/jo9722055

This paper describes an unexpected rearrangement/oxidation of N-substituted 2,5-dibromopyrroles upon treatment with HNO₃. The bromides migrate from the 2,5-positions to the 3,4-positions with subsequent oxidation at the 2,5-positions to afford N-substituted 3,4-dibromomaleimides; the structure was confirmed by single-crystal X-ray analysis. The maleimides were then polymerized to the poly(N-substituted-3,4-maleimide)s with copper bronze. This constitutes a revision of structure for the monomers and polymers. The propensity for the dibromide migration was further confirmed by treatment of N-benzyl-2,5-dibromopyrrole under nonoxidative acidic conditions (p-TsOH) to afford N-benzyl-3,4-dibromopyrrole; both the starting material and product structures were confirmed by single-crystal X-ray analysis. Several termaleimides were prepared from pyrrole, maleic anhydride, and citraconic anhydride. These trimeric compounds underwent enormous shifts in their optical absorbance maxima (ca. 200 nm) when bases or nucleophilic solvents were added. Therefore, the termaleimides served as excellent models for the polymeric systems that had undergone shifts of 350-400 nm upon treatment with the same additives. Ab initio Hartree-Fock and density functional theory were utilized to assess the minimum conformation of the trimeric system. Both terminal maleimides appear canted 37° relative to the central maleimide unit. As the two end maleimide units were computationally forced into closer proximity, there was a dipolar stabilization that ensued between the two terminal maleimides with the formation of a 1,3-dioxetane. However, it is unlikely that there could be the formation of an isolable 1,3-dioxetane due to the large energy difference between the canted structure and the dioxetane. A significant decrease in the HOMO-LUMO energy of 13 kcal/mol was calculated upon formation of the 1,3-dioxetane, suggesting that nucleophiles likely move the canted structure more toward a planar form via addition to the α,β-unsaturated carbonyl units.

Full text of DOI:10.1021/jo9722055

2646
J . Org. Chem. 1998, 63, 2646-2655
Revised Str u ctu r es of N-Su bstitu ted Dibr om in a ted P yr r ole
Der iva tives a n d Th eir P olym er ic P r od u cts. Ter m a leim id e Mod els
w ith Low Op tica l Ba n d Ga p s
Dong-Sook Choi, Shenlin Huang, Mingsheng Huang, Thomas S. Barnard,
Richard D. Adams,*,† J orge M. Seminario,* and J ames M. Tour*
Department of Chemistry and Biochemistry, University of South Carolina,
Columbia, South Carolina 29208
Received December 4, 1997
This paper describes an unexpected rearrangement/oxidation of N-substituted 2,5-dibromopyrroles
upon treatment with HNO3. The bromides migrate from the 2,5-positions to the 3,4-positions with
subsequent oxidation at the 2,5-positions to afford N-substituted 3,4-dibromomaleimides; the
structure was confirmed by single-crystal X-ray analysis. The maleimides were then polymerized
to the poly(N-substituted-3,4-maleimide)s with copper bronze. This constitutes a revision of
structure for the monomers and polymers. The propensity for the dibromide migration was further
confirmed by treatment of N-benzyl-2,5-dibromopyrrole under nonoxidative acidic conditions (p-
TsOH) to afford N-benzyl-3,4-dibromopyrrole; both the starting material and product structures
were confirmed by single-crystal X-ray analysis. Several termaleimides were prepared from pyrrole,
maleic anhydride, and citraconic anhydride. These trimeric compounds underwent enormous shifts
in their optical absorbance maxima (ca. 200 nm) when bases or nucleophilic solvents were added.
Therefore, the termaleimides served as excellent models for the polymeric systems that had
undergone shifts of 350-400 nm upon treatment with the same additives. Ab initio Hartree-
Fock and density functional theory were utilized to assess the minimum conformation of the trimeric
system. Both terminal maleimides appear canted 37° relative to the central maleimide unit. As
the two end maleimide units were computationally forced into closer proximity, there was a dipolar
stabilization that ensued between the two terminal maleimides with the formation of a 1,3-dioxetane.
However, it is unlikely that there could be the formation of an isolable 1,3-dioxetane due to the
large energy difference between the canted structure and the dioxetane. A significant decrease in
the HOMO-LUMO energy of 13 kcal/mol was calculated upon formation of the 1,3-dioxetane,
suggesting that nucleophiles likely move the canted structure more toward a planar form via
addition to the R,â-unsaturated carbonyl units.
Due to their stable semiconducting characteristics,
unique optical properties of the polymers. The model
trimers proved to be interesting in their own right,
exhibiting enormous optical band gap decreases of 200
nm upon the addition of nucleophiles, an exceptionally
large value for simple trimeric systems.
there has been considerable interest in the preparation
and use of pyrrole-derived polymers, especially the parent
poly(2,5-pyrrole), which is made by electropolymerization
or simple FeCl3-induced oxidative polymerization of
pyrrole.1 Recently, there have been several syntheses of
pyrrole-derived polymers from the well-documented N-
substituted 2,5-dibromopyrroles.2 Starting from these
dibromopyrroles, we previously reported the preparation
of polymers with unique sensor-like properties possessing
pH, solvent, or ion-induced absorption shifts of λmax from
the UV or visible to the near-IR (750-900 nm) spectral
regions.3 We report here a reassignment for the mono-
mer and polymer structures. We also prepared several
termaleimides to serve as models for understanding the
In our initial studies on pyrrole-derived polymers, we
had suggested that NBS-induced 2,5-dibromination of
N-substituted pyrroles followed by HNO3 oxidation yielded
the zwitterionic dibromopyrroles 3a -c.3 However, recent
work has shown that HNO3 oxidation of N-substituted
2,5-dibromopyrroles (2a -c) does not generate the zwit-
terionic structures 3a -c but instead generates the
constitutional isomers 4a -c, respectively (Scheme 1).4
This results from an unexpected yet high-yielding trans-
position of both bromides to the 3,4-position with subse-
quent oxidation at the 2,5-positions to afford the N-sub-
stituted 3,4-dibromomaleimides.5 We confirmed the
structure of 4a by single-crystal X-ray diffraction analy-
sis.6 The mechanism for the rearrangement could be as
† To whom correspondence regarding crystal structure details should
be addressed.
(1) Street, G. B. In Handbook of Conducting Polymers; Skotheim,
T. J ., Ed.; Dekker: New York, 1986. (b) Bureau, J . M.; Gazard, M.;
Champagne, M.; Dubois, J . C.; Tourillon, G.; Garnier, F. Mol. Cryst.
Liq. Cryst. 1985, 118, 235. (c) Street, G. B.; Lindsey, S. E.; Nazzal, A.
I.; Wynne, K. J . Mol. Cryst. Liq. Cryst. 1985, 118, 137.
(2) Groenendaal, L.; Peerlings, H. W. I.; van Dongen, J . L. J .;
Havinga, E. E.; Vekemans, J . A. J . M.; Meijer, E. W. Macromolecules
1995, 28, 116. (b) Pomerantz, M.; Yang, H.; Cheng, Y. Macromolecules
1995, 28, 5706.
(4) Beddoes, R. L.; Zhao, Y.; J oule, J . A. Acta Crystallogr. 1996, C52,
2313. This reference contains the corrected structure for the Diels-
Alder adduct, confirmed by X-ray analysis, resulting from the [4 + 2]
cycloaddition of 4a and cyclopentadiene; however, the structures
suggested by Beddoes et al. for the NBS-bromination products of
N-alkylpyrroles are incorrect.
(3) Brockmann, T. W.; Tour, J . M. J . Am. Chem. Soc. 1995, 117,
4437. (b) Brockmann, T. W.; Tour, J . M. J . Am. Chem. Soc. 1994, 116,
7435.
(5) Gilow, H. M.; Burton, D. E. J . Org. Chem. 1981, 46, 2221. (b)
Artico, M. In Heterocyclic Compounds, Pyrroles; J ones, R. A., Ed.;
Wiley: New York, 1990; Vol. 48, Part I.
S0022-3263(97)02205-6 CCC: $15.00 © 1998 American Chemical Society
Published on Web 03/24/1998

Termaleimide Models with Low Optical Band Gaps
J . Org. Chem., Vol. 63, No. 8, 1998 2647
Sch em e 1
Sch em e 2
small differences between the 1H NMR spectra of the
monomers and their corresponding polymers.
In an effort to understand the unique sensor-like
optical properties in the polymers, we sought to make
well-defined oligomeric species, specifically trimers, that
could be studied optically. Oligomers can often be formed
by shortening reaction times and temperatures of the
corresponding polymerization reactions.7 Unfortunately,
as is commonly noticed with this method of oligomer
synthesis, we were unable to isolate any of the desired
homogeneous oligomeric species after fractionation and
chromatographic separations. We then proceeded to
prepare N-butyl-3-bromomaleimide for use as a chain-
terminating agent in the Ullmann polymerization; how-
ever, again no desired trimeric species could be isolated.
Seeking a more controlled cross-coupling approach rather
than a homocoupling approach, Stille coupling methods
were used to prepare termaleimides; however, it became
evident that the R,ω-unsubstituted termaleimides were
too unstable to isolate. Therefore, the N-substituted 3,4-
dibromomaleimides 6a -f were prepared from pyrrole by
bromination with NBS followed by rearrangement and
oxidation with HNO3 or, more simply, from maleic
anhydride by reaction with a primary amine and a
bromonium ion source (Br2/NaOAc/AcOH or Br2/AlBr3).8
Similarly, the N-substituted 3-methyl-4-(trimethylstan-
nyl)maleimides 7a -f were prepared from citraconic
anhydride by reaction with a primary amine, the same
bromonium ion sources listed above, and then conversion
of the bromide to the trimethylstannane with (Me3Sn)2
under Pd-catalyzed conditions.9 These two components
were then cross-coupled with Pd catalysts to afford the
methyl-terminated termaleimides 8a -i (Scheme 3). Un-
fortunately, we have been unable to obtain any of these
compounds in a crystalline form suitable for structural
analysis by X-ray diffraction.
shown in Scheme 2.5 Oxidation at the 5-position might
precede the second bromide migration; nevertheless, the
bis(bromide) rearrangement and oxidation was facile and
high yielding even at 0 °C.
To ensure that the initial NBS-bromination of the
N-alkylpyrroles occurred at the 2,5-positions, the struc-
ture of 2d was determined by single-crystal X-ray analy-
sis.6 Furthermore, that acid-induced rearrangement
could occur was further confirmed by treatment of 2d
under nonoxidative acidic conditions with p-TsOH to
afford N-benzyl-3,4-dibromopyrrole whose structure was
also determined crystallographically (eq 1).6 The 1H
NMR chemical shift change for the pyrrole resonances
(δ 6.26 for 2d and δ 6.66 for N-benzyl-3,4-dibromopyrrole)
provide a useful handle for following the migration.
However, the aforementioned crystallographic determi-
nations were necessary to ensure the initial assignments
of the bromide locations.
Ullmann polymerization of the dibromides 4a -c af-
forded, therefore, the poly(N-substituted-3,4-maleimide)s
(eq 2). The spectroscopic and size-exclusion chromatog-
raphy data for these polymers have been described
previously.3 In brief, after one fractional precipitation,
Mn ) 5000-10 000 and PDI ) 1.2-1.5. There were only
While the polymaleimides had λmax values ca. 520 nm
in THF, the trimers, with their shorter degrees of
extended conjugation, understandably had smaller opti-
cal absorbance maxima (ca. 390 nm). The polymaleim-
(6) Crystal data for 2d : space group P21/n, a ) 12.324(5) Å, b )
6.284(3) Å, c ) 14.580(6) Å, â ) 93.69(3)°, Z ) 4, 717 reflections I >
3σ(I), R ) 0.053, and Rw ) 0.052. (b) Crystal data for N-benzyl-3,4-
dibromopyrrole: space group P21/n, a ) 12.165(7) Å, b ) 6.443(4) Å,
c ) 14.609(6) Å, â ) 95.27(4)°, Z ) 4, 845 reflections I > 3σ(I), R )
0.053, Rw ) 0.056. (c) Crystal data for 4a : space group P21/c, a )
7.427(2) Å, b ) 18.564(3) Å, c ) 8.154(2) Å, â ) 111.18(2)°, Z ) 4,
1097 reflections I > 3σ(I), R ) 0.039, and Rw ) 0.043.
(7) Tour, J . M. Chem. Rev. 1996, 96, 537.
(8) Earl, R. A.; Clough, F. W.; Townsend, L. B. J . Heterocycl. Chem.
1978, 15, 1479. (b) Brown, J . P. Chem. Abstr. 1966, 65, 2131a.
(9) Sandosham, J . Undheim, K. Acta Chem. Scand. 1989, 43, 684.

2648 J . Org. Chem., Vol. 63, No. 8, 1998
Choi et al.
Sch em e 3
F igu r e 1. UV-vis spectrum of 8i in THF, DMSO, NMP, and
aqueous NaOH/THF.
F igu r e 2. UV-vis spectrum of 8c (2.5 mL, 0.30 mM in THF)
with an added solution of aqueous NaHCO3 (0.03 mL, 0.1 M)
and monitoring as a function of time.
ides exhibited bathochromic shifts in their absorption
maxima of 350-400 nm when bases or strong donor
solvents were added. The trimers similarly showed
bathochromic shifts of ca. 200 nm when exposed to the
same nucleophilic environments (Figure 1). All of the
termaleimides exhibited nearly identical behavior in
these various media. None of these solvent systems are
likely to act as one-electron donor or acceptor reagents
promoting a doped-like state in the trimers; therefore,
the bathochromic shifts are probably conformationally
induced rather than oxidatively or reductively induced.
Although the trimers did not show the low HOMO-
LUMO gaps as in the band gaps of the polymers, their
bathochromic shifts were unusually pronounced for short
conjugated systems; optical band gaps of ca. 1.8 eV were
obtained. Therefore, a reduction of ca. 1.0 eV in the
HOMO-LUMO gaps was afforded upon treatment of
these trimers with nucleophilic solvents.
As typically observed when comparing polymers and
small molecules, the trimers were less resilient than the
polymers; hence, decomposition of the trimers was ob-
served over a period of 24 h when exposed to aqueous
NaOH/THF. However, the trimers were stable in mild
alkaline environments such as aqueous NaHCO3/THF,
although the response times to these milder bases were
slow (Figure 2).
F igu r e 3. UV-vis spectral trends of 8c (2.5 mL, 0.30 mM in
THF) upon the alternating addition of aqueous NaHCO3 (0.1
M) and aqueous HCl (0.1 M) at 5-10 min cycles.
the expected intense bathochromic shifts to 445 and 580
nm, while addition of aqueous HCl did not restore the
original spectral characteristics. Instead, upon acid
addition, the absorption intensities plummeted through-
out the UV-vis spectral region. Addition of base afforded
some recovery of the 445 and 580 nm absorptions, which
could again be lost with the addition of acid. By the
hypochromicity of the third cycle, it was clear that the
material was decomposing (Figure 3). These spectral
trends are difficult to rationalize, but they are interesting
nonetheless.
While the polymers showed reversible sensor-like
activity with alternating basic and acidic additions,
reversibility was not noticed with these less stable
oligomeric versions. Addition of sodium bicarbonate gave

Termaleimide Models with Low Optical Band Gaps
J . Org. Chem., Vol. 63, No. 8, 1998 2649
Ta ble 1. Tota l, HOMO, a n d LUMO En er gies for th e
Tw o Con for m a tion s of 8a (Op en , 37°-Ca n ted Ter m in a l
Ma leim id es) a n d 14 (Closed , 1,3-Dioxeta n e)
Usin g HF /3-21G
property
open (hartrees)
closed (hartrees)
∆ (kcal/mol)
energy
HOMO
LUMO
∆
-1258.030 78
-0.356 85
0.000 15
-1257.986 55
-0.341 68
-0.005 79
0.335 89
27.8
9.5
-3.7
13.2
0.357 00
represent 7-exo-trig ring-closure processes, while eqs 4
and 6 have 6-exo-trig closures; therefore, all four routes
are stereoelectronically favorable by Baldwin’s rules.12
Finally, conjugate addition followed by bond migration
would lead to severe buttressing of the carbonyl moieties
(Scheme 4, eq 7); thus, this manifold is unlikely to occur
to any significant extent. Hence, several nucleophilic
routes to highly conjugated structures are possible that
would lower the optical band gap of the material, though
configurations 9 and 11 appear most likely to afford the
highly conjugated systems with low HOMO-LUMO
gaps.
F igu r e 4. Reaction progress of 8i (3.0 mL, 0.23 mM in THF)
upon the addition of NaSMe (0.1 mL, 0.01 M in methanol) as
a function of time.
Since no crystal data could be obtained, we proceeded
to study 8a using ab initio Hartree-Fock (HF) and
density functional theory (DFT).13 Compound 8a , in its
minimum energy conformation, had both terminal male-
imides canted 37° relative to the central maleimide unit.
As the two end maleimide units were computationally
forced into closer proximity, there was a dipolar stabili-
zation that ensued between the two terminal maleimides
with the formation of a 1,3-dioxetane that had the three
original maleimide rings held in a coplanar conformation
as in 14. However, it is unlikely that there could be the
F igu r e 5. Reaction progress of 8c (3.0 mL, 0.50 mM in THF)
upon the addition of n-C4H9Cu (0.05 mL, 0.01 M in diethyl
ether) as a function of time.
Upon the addition of sodium thiomethoxide or n-
butylcopper, both of which are typically 1,4-addition
reagents, intense bathochromic shifts were observed that
generated stable structures (monitored for 5-24 h) with
absorption maxima ca. 580 nm (Figures 4 and 5). This
may suggest a 1,4-addition mechanism, though a one-
electron addition is possible with the copper reagent.
To rationalize these large changes in the HOMO-
LUMO gaps of the trimers, we considered the possible
effects of nucleophiles on these compounds. It has been
reported that even weak nucleophiles can react with
maleimides by a 1,4- or 1,2-addition to one of the R,â-
unsaturated carbonyl groups.11 Therefore, considering
1,2-addition to a maleimide unit on the trimer, the newly
generated oxy anion is in position to attack the carbonyl
moiety of a maleimide two units away, thus fixing the
trimer into a near-planar configuration and maximizing
the extended conjugation (Scheme 4, eq 3). 1,2-Addition
followed by an intramolecular 1,4-addition would form a
spiro-6,5 ring system 10 (Scheme 4, eq 4). Though attack
at a â,â-disubstituted system could be slow, 1,4-addition
would again lead to an intramolecular reaction to give
the near planar trimer 11 or another spiro-6,5 ring
system 12 (Scheme 4, eqs 5 and 6). Equations 3 and 5
formation of an isolable 1,3-dioxetane from the near-
planar intermediates resulting from 9 and 11 since the
relative stability between the minimized structure for 8a
(37° canted terminal maleimides) and 14 was 28 kcal/
mol using HF/3-21G (Table 1) and 14 could readily open
to form a less strained intermediate. Using structures
similar to 8a and 14, except with substitution of the
methyl groups with hydrogen atoms to limit the size of
the system, and using the more precise B3PW91/6-31G**,
which increases the size of the basis, adds polarization
functions, and includes electron correlation, the relative
(12) Baldwin, J . E. J . Chem. Soc., Chem. Commun. 1976, 734.
(13) Frisch, M. J .; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.;
J ohnson, B. G.; Robb, M. A.; Cheeseman, J . R.; Keith, T. Petersson, G.
A.; Montgomery, J . A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski,
V. G.; Ortiz, J . V.; Foresman, J . B.; Ciolowski, J .; Stefanov, B. B.;
Nenayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen,
W.; Wong, M. W.; Andres, J . L.; Replogle, E. S.; Gomperts, R.; Martin,
R. L.; Fox, D. J .; Binkley, J . S.; Defrees, D. J .; Baker, J .; Stewart, J .
P.; Head-Gordon, M.; Gonzalez, C.; Pople, J . A. Gaussian 94, Revision
E.1, Gaussian, Inc., Pittsburgh, PA, 1996. (b) Hehre, J .; Radom, L.;
Schleyer, P. v. R.; Pople, J . A. Ab Initio Molecular Orbital Theory;
Wiley: New York, 1986. (c) Modern Density Functional Theory: A Tool
for Chemistry; Seminario J . M., Politzer, P., Eds.; Elsevier: Amster-
dam, 1995. (d) Recent Developments and Applications of Modern
Density Functional Theory; Seminario, J . M., Ed., Elsevier: Amster-
dam, 1996.
(10) Stille, J . K. Angew. Chem., Int. Ed. Engl. 1986, 25, 508. (b)
Echavarren, A. M.; Stille, J . K. J . Am. Chem. Soc. 1987, 109, 5478.
(11) Khan, M. N. J . Chem. Soc., Perkin Trans. 2 1987, 819.

2650 J . Org. Chem., Vol. 63, No. 8, 1998
Choi et al.
Sch em e 4
Ta ble 2. Tota l En er gy, Usin g B3P W91/6-31G**, for th e
Tw o Con for m a tion s of a Ter m a leim id e An a logou s to 8a
Op en (37°-Ca n ted Ter m in a l Ma leim id es) a n d 14 (Closed ,
1,3-Dioxeta n e) bu t w ith Hyd r ogen Atom s Rep la cin g All
th e Meth yl Gr ou p s
Hartree-Fock and density functional theories were
utilized to assess the minimum energy conformations of
the trimeric system, which suggest that there is a dipolar
stabilization between the two terminal maleimides with
a concomitant decrease in the HOMO-LUMO energy.
Nucleophiles probably move the canted structure toward
a planar form via addition to the R,â-unsaturated car-
bonyl units. Therefore, the study of these trimeric
systems has afforded a rationale for the extraordinary
optical trends exhibited by their polymeric analogues.
open (hartrees)
closed (hartrees)
∆ (kcal/mol)
energy
-1075.520 10
-1075.488 48
19.8
energy difference between the 37° canted structure and
the 1,3-dioxetane system was 20 kcal/mol as shown in
Table 2. A HOMO-LUMO energy difference between
the minimized 37°-canted structure and the 1,3-dioxetane
14 was calculated to be 13 kcal/mol (Table 1), suggesting
that minimizing the twisting would cause a significant
decrease in the HOMO-LUMO gaps; a trend readily
observed experimentally.
In summary, an unexpected yet fascinating rearrange-
ment/oxidation of N-substituted 2,5-dibromopyrroles re-
sults in the formation of N-substituted 3,4-dibromomale-
imides, which were polymerized to the poly(N-substituted
3,4-dibromomaleimide)s under Ullmann conditions. Sev-
eral model termaleimides were prepared that exhibited
a 200 nm bathochromic shift in their optical absorbance
maxima upon treatment with nucleophiles; a trend
similar to that exhibited by the polymers. Ab initio
Exp er im en ta l P r oced u r es
Gen er a l Meth od s. All nonaqueous operations were carried
under a dry, oxygen-free, nitrogen atmosphere. Copper bronze,
N-methylpyrrole, citraconic anhydride, aluminum tribromide,
benzyl bromide, iodine monochloride, p-methylbenzylamine,
and p-methoxybenzylamine were purchased from Aldrich
Chemical Co. N-Phenylmaleimide was purchased from Fisher
Scientific. Hexamethyldistannane was purchased from Ald-
rich Chemical Co. or Fluka Chemical Co. Reagent-grade
diethyl ether and tetrahydrofuran (THF) were distilled under
nitrogen from sodium benzophenone ketyl. Reagent-grade
acetonitrile, benzene, dichloromethane, and toluene were
distilled under nitrogen from calcium hydride. Bulk-grade
hexane was distilled prior to use. Gravity column chroma-

Termaleimide Models with Low Optical Band Gaps
J . Org. Chem., Vol. 63, No. 8, 1998 2651
tography and flash chromatography were carried out on silica
gel (230-400 mesh from EM Science). The syntheses of 1a -
c, 2a -c, 4a -c, and 5a -c have been described previously.3
Although some small impurity peaks were noted in the 1H
NMR or 13C NMR spectra of the new compounds, the purity
levels were >94% in all cases. Specific minor impurity peak
locations can be noted by referring to the actual spectra in
the Supporting Information.
was collected and purified by recrystallization from benzene.
The details are described below for each material.
3,4-Dibr om om a leic An h yd r id e.8b See procedure A. Ma-
leic anhydride (9.81 g, 100 mmol), aluminum tribromide (0.302
g, 1.13 mmol), and bromine (10.3 mL, 200 mmol) were used to
give 18.2 g (71%) of the title compound as a pale yellow solid
after recrystallization: mp 105-111 °C (lit.8b mp 105-115 °C);
FTIR (film) 1857, 1823, 1778, 1592, 1279, 1237, 1180, 1158,
971, 924, 827, 758, 724, 684 cm-1 13C NMR (75 MHz, CDCl3)
;
δ 158.6, 131.4.
N-Ben zylp yr r ole (1d ).14 The procedure by Wang et al.14
was modified as follows. A nitrogen-purged flask equipped
with a reflux condenser was charged with dichloromethane
(50 mL), tetrabutylammonium bromide (9.69 g, 30.1 mmol),
pyrrole (2.10 mL, 30.2 mmol), and benzyl bromide (4.0 mL,
33.6 mmol), and the mixture was cooled to 0 °C using an ice
bath. To this solution was added dropwise 50% aqueous
sodium hydroxide (30 mL). After the addition, the reaction
mixture was heated to reflux and allowed to stir for 24 h. The
mixture was cooled and diluted with water (20 mL) and the
aqueous phase was extracted with methylene chloride (3×).
The combined organic extracts were washed with 3 N hydro-
chloric acid, water, and brine and dried over magnesium
sulfate. The solvent was removed in vacuo, and the crude
product was purified by distillation to afford 3.95 g (84%) of
the title compound as a colorless liquid: FTIR (neat) 3098,
3061, 3027, 2918, 1497, 1453, 1394, 1355, 1284, 1088, 1067,
3-Br om o-4-m eth ylm a leic An h yd r id e. See procedure A.
Citraconic anhydride (4.50 mL, 50.1 mmol), aluminum tribro-
mide (0.151 g, 0.57 mmol), and bromine (2.60 mL, 50.1 mmol)
were used to afford 6.59 g (69%) of the title compound as a
white solid after recrystallization: mp 74-76 °C; FTIR (film)
2139, 1914, 1857, 1829, 1768, 1485, 1430, 1380, 1275, 1237,
1163, 1048, 1031, 905, 855, 763, 723, 676 cm-1; 1H NMR (400
MHz, CDCl3) δ 2.13 (s, 3 H); 13C NMR (75 MHz, CDCl3) δ
163.5, 160.1, 145.6, 126.8, 11.4; HRMS calcd for C5H379BrO3
189.9266, found 189.9262.
N-P h en yl-3-m eth ylm a leim id e.16 To a stirred solution of
citraconic anhydride (2.70 mL, 30.0 mmol) in an ice-cooled bath
in acetic acid (60 mL) was added aniline (2.70 mL, 30.7 mmol)
dropwise. The reaction mixture was heated to 90 °C for 2 h.
After the mixture was cooled to 0 °C, the solvent was removed
to give the crude product, which was purified by recrystalli-
zation from ethanol to afford 4.78 g (85%) of the title compound
as a pale yellow solid: mp 96-98 °C (lit.16 mp 98 °C); FTIR
(film) 3085, 2989, 2924, 1744, 1704, 1642, 1596, 1506, 1455,
1406, 1290, 1186, 1115, 1072, 1037, 1000, 888, 873, 769, 753,
1
1028, 968, 813, 724, 626 cm-1; H NMR (300 MHz, CDCl3) δ
7.40-7.11 (m, 5 H), 6.71 (t, J ) 2.1 Hz, 2 H), 6.21 (t, J ) 2.2
Hz, 2 H), 5.08 (s, 2 H).14
N-Ben zyl-2,5-d ibr om op yr r ole (2d ).15 The procedure used
was analogous to Khoury et al.’s15 method and was modified
as follows. To a nitrogen-purged flask charged with THF (100
mL) was added N-benzylpyrrole (1.57 g, 10.0 mmol), and the
mixture was then cooled to -78 °C. N-Bromosuccinimide (3.48
g, 19.6 mmol) was added, and the reaction mixture was allowed
to warm to room temperature. After the mixture was stirred
at room temperature for 12 h, the solvent was removed in
vacuo, and the crude product was dissolved in hexane and
filtered. The solvent was removed from the filtrate to give a
residue that was purified by flash chromatography on silica
gel (hexanes) to afford 2.66 g (84%) of the title compound as a
pale yellow solid: FTIR (film) 3114, 3031, 2922, 2852, 1545,
1496, 1442, 1419, 1374, 1357, 1286, 1221, 1150, 1102, 1074,
1
708, 691, 652, 617 cm-1; H NMR (400 MHz, CDCl3) δ 7.46-
7.31 (m, 5 H), 6.46 (q, J ) 1.8 Hz, 1 H), 2.15 (d, J ) 1.8 Hz, 3
H); 13C NMR (75 MHz, CDCl3) δ 170.7, 169.6, 145.8, 131.7,
129.1, 127.7, 127.5, 126.0, 11.2; LRMS calcd for C11H9NO2 187,
found 187.
p-(Tr im eth ylsilyl)ben zyla m in e.17 The procedure used
was analogous to Sakata et al.’s17 method and was modified
as follows. To a stirred suspension of lithium aluminum
hydride (1.64 g, 43.2 mmol) in diethyl ether (100 mL) in an
ice bath was added dropwise p-(trimethylsilyl)benzonitrile
(4.81 g, 27.4 mmol) in diethyl ether (20 mL). The reaction
mixture was heated to reflux for 3 h. After cooling, mixture
was sequentially quenched with ethyl acetate, ethanol, ice
chips, and water. The resulting salt was removed by filtration,
and the filtrate was extracted with diethyl ether (3×). The
combined organic layers were washed with brine and dried
over magnesium sulfate. After evaporation, the residue was
purified by column chromatography on silica gel (hexane/ethyl
acetate 9:1) and then distillation (98 °C, 4 mmHg)17 to afford
2.11 g (43%) of the title compound as a colorless oil: FTIR
(neat) 3351, 3067, 3018, 2954, 2896, 2630, 1627, 1602, 1548,
1477, 1396, 1378, 1320, 1296, 1248, 1220, 1107, 1037, 992, 954,
838, 800, 755, 725, 692, 650 cm-1; 1H NMR (300 MHz, CDCl3)
δ 7.50 (d, J ) 8.02 Hz, 2 H), 7.30 (d, J ) 8.06 Hz, 2 H), 3.85
(s, 2 H), 0.26 (s, 9 H); 13C NMR (75 MHz, CDCl3) δ 144.0, 138.8,
133.7, 126.5, 46.5, -1.1; LRMS calcd for C13H12BrNO2 293;
found 293.
996, 972, 902, 842, 762, 728, 690 cm-1 1H NMR (300 MHz,
;
CDCl3) δ 7.35-7.26 (m, 3 H), 7.05 (d, J ) 7.2 Hz, 2 H), 6.26 (s,
2 H), 5.23 (s, 2 H).6a,15
N-Ben zyl-3,4-dibr om opyr r ole. To a nitrogen-purged flask
charged with acetonitrile (70 mL) and benzene (70 mL) was
added N-benzyl-2,5-dibromopyrrole (4.41 g, 14.0 mmol), and
the mixture was then cooled to 0 °C. To this solution was
added p-toluenesulfonic acid (5.33 g, 28.0 mmol), and the
reaction mixture was allowed to warm to room temperature.
After the mixture was stirred at room temperature for 18 h,
the mixture was poured into cold saturated sodium bicarbonate
and extracted with ether (3×). The combined organic layers
were washed with brine and dried over magnesium sulfate.
Removal of solvent gave the crude product, which was purified
by flash chromatography on silica gel (hexanes) to afford 2.30
g (52%) of the title compound as a pale yellow solid: mp 59-
62 °C; FTIR (film) 3120, 3032, 3008, 2921, 1604, 1494, 1453,
1438, 1394, 1354, 1304, 1194, 1124, 1109, 1077, 1031, 974, 960,
904, 850, 782, 768, 756, 729, 694, 629 cm-1; 1H NMR (300 MHz,
CDCl3) δ 7.37-7.11 (m, 5 H), 6.66 (s, 2 H), 4.95 (s, 2 H); 13C
NMR (75 MHz, CDCl3) δ 136.2, 129.0, 128.3, 127.3, 121.1, 98.6,
54.5.6b
Gen er a l P r oced u r e for th e P r ep a r a tion of Mon obr o-
m om a leim id es fr om Cyclic An h yd r id es (P r oced u r e B).16
To a stirred solution of citraconic anhydride in acetic acid was
added alkylamine (1 equiv) dropwise in an ice-cooled bath. The
mixture was then heated to reflux for 2 h. After cooling,
sodium acetate and bromine were added. The mixture was
then heated to reflux for another 2 h. After cooling, the
mixture was poured into water and extracted with ethyl
acetate (3×). The combined organic phase was dried over
magnesium sulfate, and then solvent was removed to give the
crude product, which was purified by recrystallization. The
details are described below for each material.
Gen er a l P r oced u r e for th e Br om in a tion of Cyclic
An h yd r id es (P r oced u r e A).8b To a nitrogen-purged screw
cap tube was sequentially added maleic anhydride or citraconic
anhydride, aluminum tribromide (1.1 mol %), and bromine
(1-2 equiv). The reaction mixture was then heated to 120 °C
overnight. After cooling to 0 °C, the resulting solid product
N-Meth yl-3-br om o-4-m eth ylm a leim id e. See procedure
B. Citraconic anhydride (4.5 mL, 50 mmol), acetic acid (50
(14) Wang, N.; Teo, K.; Anderson, H. J . Can. J . Chem. 1977, 55,
4112.
(15) Khoury, Y.; Kovacic, P.; Gilow, H. M. J . Polym. Sci., Polym.
Lett. 1981, 19, 395.
(16) Mehta, N. B.; Phillips, A. P.; Lui, F. F.; Brooks, R. E. J . Org.
Chem. 1960, 25, 1012.
(17) Sakata, Y. Yakugaku Zasshi 1962, 82, 929.

2652 J . Org. Chem., Vol. 63, No. 8, 1998
Choi et al.
mL), methylamine (25 mL, 50 mmol, 2 M in THF), sodium
acetate (4.1 g, 50 mmol), and bromine (4.1 mL, 79.7 mmol)
were used to afford 6.2 g (61%) of the title compound as a
yellow solid after recrystallization from ethyl acetate: mp 44-
46 °C; FTIR (film) 1773, 1716, 1646, 1440, 1386, 1244, 1003,
934, 816, 727, 668 cm-1; 1H NMR (300 MHz, CDCl3) δ 3.04 (s,
3 H), 2.03 (s, 3 H); 13C NMR (75 MHz, CDCl3) δ 169.5, 165.6,
142.3, 124.8, 24.6, 10.6; HRMS calcd for C6H679BrNO2 202.9582,
found 202.9594.
130.2, 128.2, 125.0, 114.0, 55.3, 41.8, 10.7; HRMS calcd for
C13H1279BrNO3 309.0001, found 308.9996.
N-[p -(Tr im et h ylsilyl)b en zyl]-3-b r om o-4-m et h ylm a le-
im id e. See procedure C. Used were 3-bromo-4-methylmaleic
anhydride (0.788 g, 4.40 mmol), acetic acid (6 mL), and
p-(trimethylsilyl)benzylamine (0.996 g, 5.21 mmol) to give 1.04
g (67%) of the title compound as a white solid after flash
chromatography on silica gel (hexane/ether 6:1): FTIR (film)
3070, 3019, 2954, 2896, 1778, 1716, 1645, 1603, 1558, 1502,
1433, 1393, 1348, 1303, 1249, 1211, 1106, 1035, 965, 905, 839,
797, 756, 737, 694, 656, 632 cm-1; 1H NMR (300 MHz, CDCl3)
δ 7.46 (d, J ) 8.1 Hz, 2 H), 7.33 (d, J ) 8.1 Hz, 2 H), 4.67 (s,
2 H), 2.02 (s, 3 H), 0.23 (s, 9 H); 13C NMR (75 MHz, CDCl3) δ
169.1, 165.2, 142.3, 140.4, 136.3, 133.8, 127.9, 125.1, 42.3,
10.71, -1.2; HRMS calcd for C15H2079BrNO2Si 351.0290, found
351.0282.
N-Ben zyl-3-br om o-4-m eth ylm a leim id e. See procedure
B. Citraconic anhydride (15.0 mL, 167 mmol), acetic acid (100
mL), benzylamine (18.0 mL, 167 mmol), sodium acetate (13.7
g, 167 mmol), and bromine (12.9 mL, 251 mmol) were used to
afford 40.2 g (86%) of the title compound as a yellow solid after
recrystallization from ethyl acetate: mp 60-62 °C; FTIR (film)
3470, 3059, 2938, 2848, 1955, 1907, 1776, 1715, 1651, 1600,
1494, 1436, 1402, 1338, 1288, 1255, 1207, 1183, 1156, 1107,
N-Meth yl-3,4-d ibr om om a leim id e18 (6a ). To a nitrogen-
purged flask charged with THF (100 mL) was added N-
methylpyrrole (4.0 mL, 45 mmol), and the mixture was then
cooled to -78 °C. To this solution was added N-bromosuccin-
imide (16.0 g, 90.0 mmol), and the mixture was allowed to
warm to room temperature. After the mixture was stirred at
room temperature for 16 h, the solvent was removed in vacuo
from the mixture, and the crude product was dissolved in
hexane and filtered. The solvent was removed from the filtrate
to give N-methyl-3,4-dibromopyrrole, which was cooled to 0
°C immediately, and 5 mL nitric acid (70%) was added
dropwise.3 After addition of nitric acid, the mixture was
poured into ice. The aqueous phase was extracted with ethyl
acetate (3×), and the combined organic extracts were washed
with brine. The organic phase was dried over magnesium
sulfate, and the solvent was removed in vacuo. The residue
was purified by flash chromatography on silica gel (hexane/
ethyl acetate 9:1) to afford 4.5 g (43%) of the title compound
as a yellow solid: mp 114-116 °C (lit.18 mp 120 °C); FTIR
(film) 3484, 1783, 1717, 1667, 1596, 1488, 1471, 1449, 1267,
1070, 1030, 963, 925, 897, 860, 824, 754, 731, 701, 636 cm-1
;
1H NMR (400 MHz, CDCl3) δ 7.35-7.26 (m, 5 H), 4.67 (s, 2
H), 2.02 (s, 3 H); 13C NMR (75 MHz, CDCl3) δ 169.0, 165.1,
142.4, 136.0, 128.8, 128.6, 128.0, 124.9, 42.3, 10.7; HRMS calcd
for C12H1079BrNO2 278.9895, found 278.9886.
N-(p-Meth ylben zyl)-3-br om o-4-m eth ylm a leim id e. See
procedure B. Citraconic anhydride (0.1 mL, 10 mmol), acetic
acid (35 mL), p-methylbenzylamine (1.3 mL, 10 mmol), sodium
acetate (0.94 g, 10 mmol), and bromine (0.7 mL, 15 mmol) were
used to afford 0.91 g (31%) of the title compound as a pale
yellow solid after recrystallization from ethanol: mp 81-82
°C; FTIR (film) 3470, 3032, 2939, 1777, 1714, 1651, 1614, 1538,
1512, 1431, 1396, 1344, 1304, 1253, 1206, 1181, 1094, 1037,
990, 962, 900, 850, 829, 802, 756, 725, 709, 693, 634 cm-1; 1H
NMR (400 MHz, CDCl3) δ 7.23 (d, J ) 7.9 Hz, 2 H), 7.10 (d, J
) 7.9 Hz, 2 H), 4.63 (s, 2 H), 2.30 (s, 3 H), 2.01 (s, 3 H); 13C
NMR (75 MHz, CDCl3) δ 169.1, 165.2, 142.3, 137.8, 133.0,
129.4, 128.7, 125.0, 42.1, 21.16 10.7; HRMS calcd for C13H12
-
79BrNO2 293.0051, found 293.0062.
1
1165, 998, 822, 728, 714 cm-1; H NMR (300 MHz, CDCl3) δ
N-P h en yl-3-br om o-4-m eth ylm a leim id e. To a stirred
solution of N-phenyl-3-methylmaleimide (4.78 g, 25.5 mmol)
and anhydrous sodium acetate (2.46 g, 30.0 mmol) in glacial
acetic acid (30 mL) was added dropwise a solution of bromine
(2.30 mL, 45.0 mmol) in acetic acid (45 mL) at 0 °C. The
reaction mixture was then heated to reflux for 3 h. The
precipitate was removed by filtration, and the filtrate was
evaporated to dryness. The residue was extracted with ethyl
acetate (3×), and the combined organic layers were washed
with brine and dried over magnesium sulfate. Removal of the
solvent gave the crude product, which was purified by recrys-
tallization from ethanol to give 4.92 g (72%) of the title
compound as a yellow solid: mp 88-89 °C; FTIR (film) 1714,
1646, 1591, 1503, 1454, 1401, 1382, 1284, 1259, 1179, 1128,
1069, 1050, 1030, 964, 913, 892, 824, 783, 755, 730, 703, 687,
3.12 (s, 3 H); 13C NMR (75 MHz, CDCl3) δ 164.0, 129.4, 25.5;
HRMS calcd for C5H379Br2NO2 266.8531, found 266.8523.
N-P h en yl-3-br om om a leim id e.19a To a stirred solution of
N-phenylmaleimide (5.08 g, 29.3 mmol) in glacial acetic acid
(6 mL) at 90 °C was added dropwise a solution of bromine (2.20
mL, 42.7 mmol) in acetic acid (6 mL). After being stirred for
15 min, the mixture was cooled to 0 °C, and the solvent was
removed to give the crude brominated N-alkylsuccinimide,
which was treated with triethylamine (11.2 mL, 80.0 mmol)
in ether (150 mL) at room temperature for 30 min. The
resulting salt was removed by filtration, and the filtrate was
evaporated to dryness. The residue was purified by recrys-
tallization from ethanol to give 3.57 g (53%) of the title
compound as a pale yellow solid: mp 150-153 °C (lit.19a mp
155 °C); FTIR (film) 3098, 3055, 1782, 1710, 1591, 1505, 1454,
1397, 1290, 1243, 1192, 1172, 1149, 1048, 974, 917, 865, 848,
1
636 cm-1; H NMR (400 MHz, CDCl3) δ 7.46-7.30 (m, 5 H),
2.13 (s, 3 H); 13C NMR (75 MHz, CDCl3) δ 168.3, 164.4, 142.4,
1
799, 751, 718, 699 cm-1; H NMR (400 MHz, CDCl3) δ 7.48-
131.4, 129.2, 128.1, 125.9, 125.5, 10.9; HRMS calcd for
7.30 (m, 5 H), 7.01 (s, 1 H); 13C NMR (100 MHz, CDCl3) δ 167.4,
164.2, 131.9, 131.8, 131.0, 129.3, 128.4, 126.1; HRMS calcd
for C10H679BrNO2 250.9582, found 250.9577.
C
11H879BrNO2 264.9738, found 264.9738.
Gen er a l P r oced u r e for th e P r ep a r a tion of Mon obr o-
m om a leim id es fr om Cyclic An h yd r id es (P r oced u r e C).
To a stirred solution of 3-bromo-4-methylmaleic anhydride (1.0
equiv) in acetic acid was added the alkylamine (1-2 equiv) in
an ice-cooled bath. The mixture was then heated to reflux 2
h. After cooling, the solvent was removed to give the crude
product, which was purified by column chromatography. The
details are described below for each material.
N-(p-Meth oxyben zyl)-3-br om o-4-m eth ylm aleim ide. See
procedure C. 3-Bromo-4-methylmaleic anhydride (2.9 g, 15.3
mmol), acetic acid (50 mL), and p-methoxybenzylamine (2.0
mL, 15.3 mmol) were used to afford 4.1 g (82%) of the title
compound as a white solid after flash chromatography on silica
gel (hexane/ethyl acetate 9:1): FTIR (film) 2941, 2838, 1776,
1716, 1649, 1612, 1584, 1511, 1459, 1429, 1397, 1339, 1295,
1248, 1210, 1175, 1091, 1032, 961, 899, 850, 805, 760, 726,
634 cm-1; 1H NMR (300 MHz, CDCl3) δ 7.30 (d, J ) 8.7 Hz, 2
H), 6.83 (d, J ) 8.7 Hz, 2 H), 4.61 (s, 2 H), 3.76 (s, 3 H), 2.00
(s, 3 H); 13C NMR (75 MHz, CDCl3) δ 169.1, 165.2, 159.4, 142.3,
N-P h en yl-3,4-d ibr om om a leim id e19b (6b). To a stirred
solution of the N-phenyl-3-bromomaleimide (0.50 g 1.98 mmol)
and anhydrous sodium acetate (0.166 g, 2.03 mmol) in glacial
acetic acid (2 mL) was added dropwise a solution of bromine
(0.17 mL, 3.30 mmol) in acetic acid (5 mL) at 0 °C. The
reaction mixture was then heated to reflux for 3 h. The
precipitate was removed by filtration, and the filtrate was
evaporated to dryness. The residue was extracted with ethyl
acetate (3×), and the combined organic layer was washed with
brine and dried over magnesium sulfate. Removal of the
solvent gave the crude product, which was purified by flash
chromatography on silica gel (hexane/ethyl acetate 3:1) to
afford 0.66 g (98%) of the title compound as a yellow solid: mp
(18) Scharf, H.-D.; Korte, F. Chem. Ber. 1965, 24, 764.
(19) Balasubramaniyan, V.; Laddha, G. K.; Argade, N. P. Org. Prep.
Proc. Intl. 1991, 23, 388. (b) Martin, E. L.; Dickinson, C. L.; Roland, J .
R. J . Org. Chem. 1961, 26, 2032.

Termaleimide Models with Low Optical Band Gaps
J . Org. Chem., Vol. 63, No. 8, 1998 2653
160-166 °C (lit.19b mp 166-168 °C); FTIR (film) 1722, 1682,
135.6, 133.9, 129.5, 128.1, 43.2, -1.2; HRMS calcd for
C14H15Br2NO2Si 414.9219, found 414.9227.
1599, 1503, 1488, 1392, 1288, 1215, 1134, 1115, 1069, 913, 828,
1
765, 731, 688 cm-1; H NMR (400 MHz, CDCl3) δ 7.49-7.31
Gen er a l P r oced u r e for th e P r ep a r a tion of th e (Tr i-
m eth ylsta n n yl)m a leim id es (P r oced u r e E).9,10 A mixture
of the N-substituted 3-bromo-4-methylmaleimide (1.0 equiv),
hexamethyldistannane (1-2 equiv), and bis(triphenylphos-
phine)palladium(II) chloride (5 mol %) in toluene was heated
to 60-70 °C overnight in a nitrogen-purged screw-cap tube.
After cooling, the mixture was filtered and washed with ethyl
acetate (2×). The filtrate was evaporated to remove the
solvent, and the residue was purified by column chromatog-
raphy on silica gel. The details are described below for each
material.
N-Meth yl-3-Meth yl-4-(tr im eth ylstan n yl)m aleim ide (7a).
See procedure E. N-Methyl-3-bromo-4-methylmaleimide (1.6
g, 7.8 mmol), hexamethyldistannane (1.7 mL, 7.8 mmol),
bis(triphenylphosphine)palladium(II) chloride (0.55 g, 0.78
mmol), and toluene (4 mL) were used to give 1.5 g (61%) of
the title compound as a colorless liquid after flash chroma-
tography on silica gel (hexane/ethyl acetate 9:1): FTIR (film)
3437, 2984, 2918, 1756, 1698, 1611, 1438, 1385, 1274, 1244,
1191, 1119, 1041, 990, 924, 778, 734, 673, 628 cm-1; 1H NMR
(300 MHz, CDCl3) δ 2.96 (s, 3 H), 2.07 (s, 3 H), 0.35 (s, 9 H);
13C NMR (75 MHz, CDCl3) δ 175.6, 172.1, 154.6, 144.3, 24.0,
12.3, -8.7; HRMS calcd for C8H12NO2116Sn 269.9885, found
269.9899.
(m, 5 H); 13C NMR (75 MHz, CDCl3) δ 162.9, 130.9, 129.9,
129.4, 128.7, 126.1; LRMS calcd for C10H5Br2NO2 331, found
331.
N-Ben zyl-3,4-d ibr om om a leim id e20 (6c). To a nitrogen-
purged flask were added maleic anhydride (15.0 g, 153 mmol),
acetic acid (100 mL) and benzylamine (16.7 mL, 153 mmol).
The mixture was heated to reflux for 2 h. After cooling using
an ice bath, sodium acetate (12.6 g, 153 mmol) and bromine
(23.6 mL, 459 mmol) were added to the mixture and heated
to reflux for another 2 h. The mixture was poured into water
and extracted with ethyl acetate (3×). The combined organic
phase was dried over magnesium sulfate. The solvent was
removed in vacuo. The residue was crystallized from ethyl
acetate to afford 43.8 g (83%) of the title compound as a yellow
solid: mp 101-103 °C; FTIR (film) 2941, 1783, 1716, 1599,
1430, 1391, 1335, 1163, 1101, 1065, 909, 813, 754, 729, 700,
630 cm-1; 1H NMR (400 MHz, CDCl3) δ 7.34 (m, 5 H), 4.73 (s,
2 H); 13C NMR (100 MHz, CDCl3) δ 164.0, 135.6, 129.9, 129.3,
129.2, 128.7, 43.6; HRMS calcd for C11H779Br2NO2 342.8844,
found 342.8829.
Gen er a l P r oced u r e for th e P r ep a r a tion of Dibr om o-
m a leim id es fr om Dibr om ocyclic An h yd r id es (P r oced u r e
D).21 To a stirred solution of 3,4-dibromomaleic anhydride (1.0
equiv) in acetic acid was added the alkylamine (1-2 equiv) in
an ice-cooled bath. The mixture was then heated to reflux
overnight. After cooling, the solvent was removed to give the
crude product, which was purified by column chromatography
or recrystallization. The details are described below for each
material.
N-(p -Met h ylben zyl)-3,4-d ibr om om a leim id e (6d ). See
procedure D. 3,4-Dibromomaleic anhydride (0.986 g, 3.85
mmol), acetic acid (8 mL), and p-methylbenzylamine (0.501 g,
3.89 mmol) were used to give 0.90 g (65%) of the title compound
as a white solid after recrystallization from ethanol: mp 117-
120 °C; FTIR (film) 3034, 2942, 1782, 1726, 1600, 1508, 1429,
1387, 1336, 1304, 1206, 1158, 1086, 1041, 912, 860, 839, 810,
755, 723, 706, 628 cm-1; 1H NMR (400 MHz, CDCl3) δ 7.26 (d,
J ) 7.7 Hz, 2 H), 7.11 (d, J ) 7.7 Hz, 2 H), 4.69 (s, 2 H), 2.30
(s, 3 H); 13C NMR (75 MHz, CDCl3) δ 163.6, 138.2, 132.3,
129.52, 129.49, 128.8, 43.0, 21.2; HRMS calcd for C12H979Br2NO2
356.9000, found 356.8982.
N-P h en yl-3-m eth yl-4-(tr im eth ylstan n yl)m aleim ide (7b).
See procedure E. N-Phenyl-3-bromo-4-methylmaleimide (1.07
g, 4.02 mmol), hexamethyldistannane (2.75 g, 8.39 mmol),
bis(triphenylphosphine)palladium(II) chloride (0.103 g, 0.147
mmol), and toluene (9 mL) were used to afford 1.06 g (76%) of
the title compound as a pale yellow solid after flash chroma-
tography on silica gel (hexane/ethyl acetate 8:1): mp 65-69
°C; FTIR (film) 3060, 2989, 2919, 1777, 1756, 1698, 1614, 1596,
1503, 1454, 1424, 1387, 1289, 1196, 1114, 1073, 1012, 910, 886,
1
775, 747, 694, 636 cm-1; H NMR (400 MHz, CDCl3) δ 7.55-
7.24 (m, 5 H), 2.17 (s, 3 H), 0.41 (s, 9 H); 13C NMR (75 MHz,
CDCl3) δ 174.3, 170.7, 154.7, 144.9, 132.4, 129.0, 127.3, 125.7,
12.6, -8.5; HRMS calcd for C14H17NO2Sn 347.0277, found
347.0264.
N-Ben zyl-3-m eth yl-4-(tr im eth ylstan n yl)m aleim ide (7c).
See procedure E. N-Benzyl-3-bromo-4-methylmaleimide (5.53
g, 19.7 mmol), hexamethyldistannane (4.20 mL, 19.7 mmol),
bis(triphenylphosphine)palladium(II) chloride (0.69 g, 0.99
mmol), and toluene (10 mL) were used to give 5.0 g (70%) of
the title compound as a colorless liquid after flash chroma-
tography on silica gel (hexane/ethyl acetate 9:1): FTIR (neat)
3440, 3033, 2985, 2919, 1763, 1697, 1610, 1496, 1456, 1434,
1398, 1348, 1263, 1192, 1124, 1069, 1030, 962, 921, 886, 778,
735, 698, 670, 637 cm-1; 1H NMR (CDCl3) δ 7.30 (m, 5 H), 4.61
(s, 2 H), 2.07 (s, 3 H), 0.35 (s, 9 H); 13C NMR (75 MHz, CDCl3)
δ 175.4, 172.0, 155.0, 144.9, 137.3, 129.0, 128.9, 128.0, 42.2,
N-(p-Meth oxyben zyl)-3,4-d ibr om om a leim id e (6e). See
procedure D. 3,4-Dibromomaleic anhydride (3.90 g, 15.3
mmol), acetic acid (50 mL), and p-methoxybenzylamine (2.00
mL, 15.3 mmol) were used to afford 4.9 g (85%) of the title
compound as a white solid after flash chromatography on silica
gel (hexane/ethyl acetate 9:1): mp 116-118 °C; FTIR (film)
3480, 3008, 2943, 2837, 1781, 1721, 1610, 1598, 1513, 1459,
1430, 1388, 1335, 1302, 1250, 1210, 1176, 1161, 1085, 1031,
915, 856, 814, 760, 725, 628 cm-1; 1H NMR (300 MHz, CDCl3)
δ 7.31 (d, J ) 8.8 Hz, 2 H), 6.84 (d, J ) 8.8 Hz, 2 H), 4.67 (s,
2 H), 3.76 (s, 3 H); 13C NMR (75 MHz, CDCl3) δ 163.7, 159.6,
130.4, 129.5, 127.5, 114.2, 55.3, 42.7; HRMS calcd for
12.8, -8.3; HRMS calcd for
346.0198, found 346.0190.
C
14H16NO2116Sn (M - CH3)
N -(p -Me t h ylb e n zyl)-3-m e t h yl-4-(t r im e t h ylst a n n yl)-
m a leim id e (7d ). See procedure E. N-(p-Methylbenzyl)-3-
bromo-4-methylmaleimide (0.303 g, 1.03 mmol), hexameth-
yldistannane (0.568 g, 1.73 mmol), bis(triphenylphosphine)-
palladium(II) chloride (0.0406 g, 0.0578 mmol), and toluene
(2.4 mL) were used to afford 0.19 g (50%) of the title compound
as a pale yellow solid after flash chromatography on silica gel
(hexane/ethyl acetate 8:1): mp 92-97 °C; FTIR (film) 3439,
2984, 2920, 1761, 1698, 1610, 1515, 1433, 1395, 1347, 1305,
1263, 1192, 1126, 1090, 1023, 961, 892, 844, 778, 756, 731,
670, 634 cm-1; 1H NMR (400 MHz, CDCl3) δ 7.23 (d, J ) 7.97
Hz, 2 H), 7.09 (d, J ) 7.81 Hz, 2 H), 4.56 (s, 2 H), 2.29 (s, 3 H),
1.56 (s, 3 H), 0.34 (s, 9 H); 13C NMR (75 MHz, CDCl3) δ 175.1,
171.7, 154.6, 144.4, 137.3, 133.9, 129.3, 128.6, 41.6, 21.2, 12.4,
-8.6; HRMS calcd for C14H17NO2Sn 360.0355, found 360.0330.
N-(p -Met h oxyb en zyl)-3-m et h yl-4-(t r im et h ylst a n n yl)-
m a leim id e (7e). See procedure E. N-(p-Methoxybenzyl)-3-
bromo-4-methylmaleimide (1.6 g, 4.9 mmol), hexamethyldi-
stannane (1.1 mL, 5.2 mmol), bis(triphenylphosphine)pal-
ladium(II) chloride (0.34 g, 0.48 mmol), and toluene (4 mL)
were used to afford 1.1 g (58%) of the title compound as a
C
12H979Br2NO3 372.8949, found 372.8936.
N-[p-(Tr im eth ylsilyl)ben zyl]-3,4-dibr om om aleim ide (6f).
See procedure D. 3,4-Dibromomaleic anhydride (1.43 g, 7.98
mmol), acetic acid (11 mL), and p-(trimethylsilyl)benzylamine
(2.55 g, 9.97 mmol) were used to give 2.91 g (87%) of the title
compound as a pale yellow solid after column chromatography
on silica gel (hexane/ethyl acetate 6:1): mp 87-89 °C; FTIR
(film) 3495, 3020, 2954, 1786, 1728, 1594, 1433, 1386, 1343,
1303, 1248, 1211, 1185, 1108, 1086, 1022, 954, 921, 837, 798,
757, 737, 693, 855, 628 cm-1 1H NMR (300 MHz, CDCl3) δ
;
7.47 (d, J ) 7.9 Hz, 2 H), 7.34 (d, J ) 7.8 Hz, 2 H), 4.73 (s, 2
H), 0.23 (s, 9 H); 13C NMR (75 MHz, CDCl3) δ 163.6, 140.8,
(20) Xie, G.; Lown, J . W. Tetrahedron Lett. 1994, 35, 5555.
(21) Edge, S.; Charlton, A.; Hansen, T. K.; Varma, K. S. Underhill,
A. E.; Kathirgamanathan, P.; Becher, J . Chem. Ind. 1991, 4, 130.
(22) Felix, G.; Dunogues, J .; Pisciotti, F.; Calas, R. Angew. Chem.,
Int. Ed. Engl. 1977, 16, 488.

2654 J . Org. Chem., Vol. 63, No. 8, 1998
Choi et al.
colorless liquid after flash chromatography on silica gel
(hexane/ethyl acetate 9:1): FTIR (neat) 3436, 2918, 2835, 1759,
1693, 1612, 1586, 1513, 1433, 1395, 1347, 1292, 1248, 1176,
1125, 1088, 1035, 960, 892, 780, 731, 670, 632 cm-1; 1H NMR
(300 MHz, CDCl3) δ 7.30 (d, J ) 8.7 Hz, 2 H), 6.82 (d, J ) 8.7
Hz, 2 H), 4.54 (s, 2 H), 3.76 (s, 3 H), 2.05 (s, 3 H), 0.34 (s, 9 H);
13C NMR (75 MHz, CDCl3) δ 175.1, 171.7, 159.6, 154.6, 144.4,
130.0, 129.2, 113.9, 55.2, 41.3, 12.4, -8.7; HRMS calcd for
135.8, 135.4, 131.4, 128.9, 128.7, 128.6, 128.4, 128.3, 128.1,
127.9, 42.1, 42.0, 12.0; HRMS calcd for C35H27N3O6 585.1900,
found 585.1919.
N,N′,N′′-Tr is(p -m et h ylb en zyl)-4,4′′-d im et h ylt er m a le-
im id e (8d ). See procedure F. N-(p-Methylbenzyl)-3,4-dibro-
momaleimide (0.116 g, 0.323 mmol), N-(p-methylbenzyl)-3-
methyl-4-(trimethylstannyl)maleimide (0.148 g, 0.392 mmol),
bis(triphenylphosphine)palladium(II) chloride (0.0296 g, 0.0422
mmol), and THF (5 mL) were reacted for 2 d to give 0.036 g
(21%) yield of the title compound as a yellow solid after gravity
column chromatography on silica gel (hexane/ethyl acetate,
9:1) and then preparative TLC (hexane/methylene chloride
2:1): FTIR (neat) 2924, 2855, 1772, 1708, 1644, 1516, 1434,
1398, 1347, 1307, 1184, 1073, 1022, 987, 900, 845, 806, 756,
C
16H21NO3116Sn 391.0539, found 391.0522.
N-[p-(Tr im eth ylsilyl)ben zyl]-3-m eth yl-4-(tr im eth ylstan -
n yl)m a leim id e (7f). See procedure E. N-[p-(Trimethylsilyl)-
benzyl]-3-bromo-4-methylmaleimide (0.610 g, 1.73 mmol),
hexamethyldistannane (0.858 g, 2.62 mmol), bis(triphenylphos-
phine)palladium(II) chloride (0.122 g, 0.173 mmol), and toluene
(3 mL) were used to afford 0.52 g (69%) of the title compound
as a colorless oil after flash chromatography on silica gel
(hexane/ether 5:1): FTIR (neat) 2955, 2921, 1762, 1698, 1609,
1431, 1394, 1302, 1248, 1192, 1108, 1087, 963, 892, 839, 779,
739, 693, 656, 633 cm-1; 1H NMR (300 MHz, CDCl3) δ 7.47 (d,
J ) 8.11 Hz, 2 H), 7.34 (d, J ) 8.08 Hz, 2 H), 4.61 (s, 2 H),
2.08 (s, 3 H), 0.37 (s, 9 H), 0.24 (s, 9 H); 13C NMR (75 MHz,
CDCl3) δ 175.1, 171.7, 154.6, 144.5, 139.8, 137.3, 133.7, 127.9,
41.8, 12.4, -1.1, -8.6; HRMS calcd for C17H24NO2SiSn 418.0594,
found 418.0590.
Gen er a l P r oced u r e for th e P r ep a r a tion of th e Ter -
m a leim id es (P r oced u r e F ).10 To a nitrogen-purged screw-
cap tube was added the N-alkyl-3,4-dibromomaleimide (1
equiv), N-alkyl-3-methyl-4-(trimethylstannyl)maleimide (1
equiv), and bis(triphenylphosphine)palladium(II) chloride (10
mol %) in THF. The mixture was then stirred at room
temperature 1-2 days. The mixture was filtered and washed
with ethyl acetate (2×), and then the filtrate was concentrated
to dryness. The crude product was purified with column
chromatography on silica gel and/or preparative TLC. The
details are described below for each material.
N,N′,N′′-Tr im eth yl-4,4′′-dim eth ylter m aleim ide (8a). See
procedure F. N-Methyl-3,4-dibromomaleimide (1.4 g, 5.2
mmol), N-methyl-3-methyl-4-(trimethylstannyl)maleimide (1.5
g, 5.2 mmol), bis(triphenylphosphine)palladium(II) chloride
(0.37 g, 0.52 mmol), and THF (5 mL) were reacted for 2 d to
afford 0.55 g (59%) of the title compound as a yellow solid after
gravity column chromatography on silica gel (hexane/ethyl
acetate 9:1): FTIR (film) 3470, 2951, 1774, 1704, 1439, 1386,
1276, 1196, 1163, 1090, 1009, 940, 805, 722, 670 cm-1; 1H NMR
(300 MHz, CDCl3) δ 3.17 (s, 3 H), 2.97 (s, 6 H), 2.25 (s, 6 H);
13C NMR (75 MHz, CDCl3) δ 170.7, 169.2, 167.8, 144.7, 131.6,
128.3, 24.8, 24.4, 11.7; HRMS calcd for C17H15N3O6 357.0961,
found 357.0956.
1
720 cm-1; H NMR (400 MHz, CDCl3) δ 7.25-7.05 (m, 12 H),
4.70 (s, 2 H), 4.44 (s, 4 H), 2.30 (s, 3 H), 2.26 (s, 6 H), 2.23 (s,
6 H); 13C NMR (100 MHz, CDCl3) δ 170.2, 168.7, 167.3, 144.3,
138.0, 137.5, 132.7, 132.4, 131.2, 129.4, 129.2, 128.47, 128.2,
128.0, 42.4, 41.8, 21.3, 21.3, 12.2; HRMS calcd for C38H33N3O6
627.2369, found 627.2363.
N,N′,N′′-Tr i(p-m et h oxyb en zyl)-4,4′′-d im et h ylt er m a le-
im id e (8e). See procedure F. N-(p-Methoxybenzyl)-3,4-
dibromomaleimide (0.53 g, 1.4 mmol), N-(p-methoxybenzyl)-
3-methyl-4-(trimethylstannyl)maleimide (1.1 g, 2.8 mmol),
bis(triphenylphosphine)palladium(II) chloride (0.20 g, 0.28
mmol), and THF (4 mL) were reacted for 2 d to afford 0.55 g
(59%) of the title compound as a yellow solid after gravity
column chromatography on silica gel (hexane/ethyl acetate
9:1): FTIR (film) 2936, 2837, 1772, 1712, 1613, 1586, 1514,
1435, 1398, 1347, 1294, 1249, 1178, 1103, 1033, 899, 820, 765,
1
720 cm-1; H NMR (CDCl3) δ 7.28 (d, J ) 8.5 Hz, 2 H), 7.16
(d, J ) 8.6 Hz, 4 H), 6.83 (d, J ) 8.5 Hz, 2 H), 6.79 (d, J ) 8.6
Hz, 4 H), 4.67 (s, 2 H), 4.43 (s, 4 H), 3.76 (s, 3 H), 3.72 (s, 6 H),
2.23 (s, 6 H); 13C NMR (75 MHz, CDCl3) δ 170.4, 169.0, 167.5,
159.5, 159.3, 144.4, 131.4, 130.1, 129.6, 128.4, 128.0, 127.7,
114.21, 114.0, 55.3, 55.3, 42.1, 41.5, 12.0; HRMS calcd for
C
38H33N3O9 675.2217, found 675.2231.
Gen er a l P r oced u r e for th e P r ep a r a tion of th e Iod o-
ter m a leim id es (P r oced u r e G).10,22 To a nitrogen-purged
screw-cap tube was added the N-alkyl-3,4-dibromomaleimide
(1 equiv), the N-substituted 3-methyl-4-(trimethylstannyl)-
maleimide (1-2 equiv), and bis(triphenylphosphine)palla-
dium(II) chloride (10 mol %) in THF. The mixture was then
stirred at 23-60 °C for 1-2 days. After cooling, the mixture
was filtered and washed with methylene chloride. The filtrate
was concentrated to dryness, and the residue was purified by
flash chromatography on silica gel to remove the unreacted
compounds along with the catalyst. The collected product
portions (dimer and trimer mixture) were then treated with
iodine monochloride (1.5-4.5 equiv) in carbon tetrachloride
at room temperature. The mixture was heated to reflux
overnight. After cooling, it was concentrated to dryness. The
crude product was purified by column chromatography and
then preparative TLC. The details are described below for
each material.
N,N′,N′′-Tr iph en yl-4,4′′-dim eth ylter m aleim ide (8b). See
procedure F. N-Phenyl-3,4-dibromomaleimide (0.18 g, 0.53
mmol), N-phenyl-3-methyl-4-(trimethylstannyl)maleimide (0.19
g, 0.53 mmol), bis(triphenylphosphine)palladium(II) chloride
(0.11 g, 0.157 mmol), and THF (7 mL) were reacted for 1 d to
give 0.080 g (28%) of the title compound as a yellow solid after
gravity column chromatography on silica gel (hexane/ethyl
acetate 9:1) and then preparative TLC (hexane/methylene
chloride 2:1): FTIR (film) 2923, 2851, 1774, 1709, 1654, 1597,
1502, 1457, 1388, 1289, 1198, 1130, 1080, 1068, 1028, 898, 851,
N ,N ′′-Dip h e n yl-N ′-(p -iod ob e n zyl)-4,4′′-d im e t h ylt e r -
m a leim id e (8g). See procedures F and G. N-[(Trimethylsi-
lyl)benzyl]-3,4-dibromomaleimide (0.224 g, 0.537 mmol), N-phen-
yl-3-methyl-4-(trimethylstannyl)maleimide (0.223 g, 0.637
mmol), bis(triphenylphosphine)palladium(II) chloride (0.0453
g, 0.0645 mmol), and THF (3 mL) were used to afford the crude
N,N′′-diphenyl-N′-[p-(trimethylsilyl)benzyl]-4,4′′-dimethylter-
maleimide (8f) that was enhanced in purity using silica gel
(hexane/ethyl acetate 3:1) and then treated directly with iodine
monochloride (0.260 g, 1.60 mmol) and carbon tetrachloride
(5 mL) to give 0.0734 g (20%) of the title compound as a yellow
sticky oil after gravity column chromatography on silica gel
(hexane/ethyl acetate 5:1) and then preparative TLC (hexane/
methylene chloride 1:1): FTIR (neat) 2923, 2851, 1774, 1709,
1654, 1597, 1502, 1457, 1388, 1289, 1198, 1130, 1080, 1068,
827, 760, 714, 690, 676 cm-1 1H NMR (400 MHz, CDCl3) δ
;
7.44-7.19 (m, 15 H), 2.39 (s, 6 H); 13C NMR (125 MHz, CDCl3)
δ 170.1, 169.3, 167.4, 145.7, 131.9, 130.1, 129.9, 129.9, 129.4,
129.1, 129.0, 128.8, 126.8, 126.6, 12.9; HRMS calcd for
C
32H21N3O6 543.1430, found 543.1429.
N,N′,N′′-Tr iben zyl-4,4′′-dim eth ylter m aleim ide (8c). See
procedure F. N-Benzyl-3,4-dibromomaleimide (3.96 g, 11.5
mmol), N-benzyl-3-methyl-4-(trimethylstannyl)maleimide (4.18
g, 11.5 mmol), bis(triphenylphosphine)palladium(II) chloride
(0.81 g, 1.15 mmol), and THF (10 mL) were reacted for 2 days
to afford 1.73 g (51%) of the title compound as a yellow solid
after gravity column chromatography on silica gel (hexane/
ethyl acetate 9:1): FTIR (film) 3468, 3033, 2929, 1771, 1714,
1645, 1497, 1456, 1434, 1398, 1350, 1143, 1125, 1067, 1030,
984, 921, 888, 822, 723, 698, 669, 638 cm-1; 1H NMR (400 MHz,
CDCl3) δ 7.25 (m, 15 H), 4.76 (s, 2 H), 4.50 (s, 4 H), 2.27 (s, 6
H); 13C NMR (100 MHz, CDCl3) δ 170.4, 169.0, 167.5, 144.5,
1028, 898, 851, 827, 760, 714, 690, 676 cm-1 1H NMR (300
;
MHz, CDCl3) δ 7.69 (d, J ) 8.4 Hz, 2 H), 7.48-7.15 (m, 12 H),
4.75 (s, 2 H), 2.34 (s, 6 H); 13C NMR (75 MHz, CDCl3) δ 169.4,
168.5, 167.4, 144.7, 138.1, 134.9, 131.6, 131.2, 130.7, 129.2,

Termaleimide Models with Low Optical Band Gaps
J . Org. Chem., Vol. 63, No. 8, 1998 2655
128.33 128.30, 126.1, 94.0, 42.2, 12.2; HRMS calcd for
4.48 (s, 4 H), 2.24 (s, 6 H); 13C NMR (100 MHz, CDCl3) δ 170.3,
168.8, 167.4, 144.6, 138.1, 135.7, 135.0, 131.4, 130.6, 128.7,
128.3, 128.1, 127.9, 94.1, 42.1, 42.0, 12.0; HRMS calcd for
C
33H22IN3O6 683.0553, found 683.0546.
N ,N ′′-Dib e n zyl-N ′-(p -iod ob e n zyl)-4,4′′-d im e t h ylt e r -
m a leim id e (8i). See procedures F and G. N-[(Trimethylsi-
lyl)benzyl]-3,4-dibromomaleimide (0.420 g, 1.01 mmol), N-ben-
zyl-3-methyl-4-(trimethylstannyl)maleimide (0.731 g, 2.01
mmol), bis(triphenylphosphine)palladium(II) chloride (0.144 g,
0.205 mmol), and THF (9 mL) were used to afford crude N,N′′-
dibenzyl-N′-[p-(trimethylsilyl)benzyl]-4,4′′-dimethyltermaleim-
ide (8h ) that was enhanced in purity using silica gel (hexane/
ethyl acetate 3:1) and then treated with iodine monochloride
(0.200 g, 1.23 mmol) and carbon tetrachloride (15 mL) to give
0.158 g (22%) of the title compound as a yellow sticky oil after
gravity column chromatography on silica gel (hexane/ethyl
acetate 5:1) and then preparative TLC (hexane/methylene
chloride 1:1): FTIR (neat) 3467, 3063, 3032, 2926, 2845, 1772,
1707, 1643, 1588, 1486, 1434, 1398, 1348, 1267, 1143, 1125,
1068, 1008, 984, 918, 888, 844, 797, 735, 723, 698, 668, 637
C
35H26IN3O6 711.0866, found 711.0873.
Ack n ow led gm en t . We thank the Office of Naval
Research for support of this work. We also thank the
NAS at NASA for use of their supercomputing facilities.
We thank Professor A. Padwa for helpful discussions.
Su p p or tin g In for m a tion Ava ila ble: Detailed crystal-
lographic data for 2d , 4a , and N-benzyl-3,4-dibromopyrrole in
addition to 1H NMR or 13C NMR spectra for all new compounds
(49 pages). This material is contained in libraries on micro-
fiche, immediately follows this article in the microfilm version
of the journal, and can be ordered from the ACS; see any
current masthead page for ordering information.
1
cm-1; H NMR (300 MHz, CDCl3) δ 7.65 (d, J ) 8.3 Hz, 2 H),
7.61-7.23 (m, 10 H), 7.10 (d, J ) 8.3 Hz, 2 H), 4.67 (s, 2 H),
J O9722055