The Elusive Antiaromaticity of Maleimides and Maleic Anhydride: Enthalpies of Formation of N-Methylmaleimide, N-Methylsuccinimide, N-Methylphthalimide, and N-Benzoyl-N-methylbenzamide

Article abstract of DOI:10.1021/jo9621985

In order to understand the antiaromaticity of maleimides, the enthalpies of formation and sublimation of N-methylmaleimide, N-methylsuccinimide, N-methylphthalimide, and N-benzoyl-N-methylbenzamide were measured. The numerical values of enthalpies of formation for these compounds in the solid state are -329,3 ± 1.4, -469.8 ± 1.6, -325.0 ± 2.1, and -239.6 ± 3.8 kJ mol^-1, respectively, while the corresponding values in the gaseous state are -256.0 ± 1.5, -389.7 ± 1.6, -233.9 ± 2.2, and -119.5 ± 3.8 kJ mol^-1, respectively. The values of enthalpies of sublimation for the same compounds are 73.3 ± 0.5, 80.1 ± 0.3, 91.1 ± 0.5, and 120.1 ± 0.4 kJ mol^-1, respectively. We find that the antiaromaticity of maleimides is only modest.

Full text of DOI:10.1021/jo9621985

2732
J . Org. Chem. 1997, 62, 2732-2737
Th e Elu sive An tia r om a ticity of Ma leim id es a n d Ma leic An h yd r id e:
En th a lp ies of F or m a tion of N-Meth ylm a leim id e,
N-Meth ylsu ccin im id e, N-Meth ylp h th a lim id e, a n d
N-Ben zoyl-N-m eth ylben za m id e
Mar´ıa Victoria Roux,*,† Pilar J ime´nez,† Maria AÄ ngeles Mart´ın-Luengo,† J uan Z. Da´valos,†
Zhiyuan Sun,‡,§ Ramachandra S. Hosmane,*,‡ and J oel F. Liebman*,‡
Institute of Physical Chemistry “Rocasolano”, CSIC, Serrano 119, 28006 Madrid, Spain, and
Department of Chemistry and Biochemistry, University of Maryland Baltimore County,
1000 Hilltop Circle, Baltimore, Maryland 21250
Received November 25, 1996X
In order to understand the antiaromaticity of maleimides, the enthalpies of formation and
sublimation of N-methylmaleimide, N-methylsuccinimide, N-methylphthalimide, and N-benzoyl-
N-methylbenzamide were measured. The numerical values of enthalpies of formation for these
compounds in the solid state are -329.3 ( 1.4, -469.8 ( 1.6, -325.0 ( 2.1, and -239.6 ( 3.8 kJ
mol-1, respectively, while the corresponding values in the gaseous state are -256.0 ( 1.5, -389.7
( 1.6, -233.9 ( 2.2, and -119.5 ( 3.8 kJ mol-1, respectively. The values of enthalpies of sublimation
for the same compounds are 73.3 ( 0.5, 80.1 ( 0.3, 91.1 ( 0.5, and 120.1 ( 0.4 kJ mol-1, respectively.
We find that the antiaromaticity of maleimides is only modest.
In tr od u ction
erocycles and see what they indicate about the degree of
antiaromaticity in maleimides and maleic anhydride.
The variously N-substituted maleimides and maleic
anhydride are well established to be highly reactive
dienophiles in the Diels-Alder reaction.1 Their ring
opening2 and addition across the carbon-carbon double
bond3 are well-known to be highly facile. These phe-
nomena have been interpreted as manifestations of
antiaromaticity4,5 arising from the formal presence of 4π
ring electrons.6 The current study discusses this anti-
aromaticity in terms of measured (condensed and gas
phase) enthalpies of formation of N-methylmaleimide,
maleic anhydride, and some related species.7
Resu lts
The results of the combustion experiments carried out
with N-methylmaleimide, N-methylsuccinimide, N-meth-
ylphthalimide, and N-benzoyl-N-methylbenzamide are
given in Table 1. The symbols in this table have the same
Since we wish to consider the intrinsic destabilization
of the maleimides and maleic anhydride, implicitly all
thermochemical quantities below will refer to species in
their gas phase under the standard conditions of 25 °C
and 1 atm (or more “properly” 298.15 K and 101.325 kPa,
respectively). We will discuss various precedented ap-
proaches to aromaticity in various carbocycles and het-
† Institute of Physical Chemistry “Rocasolano”.
‡ Department of Chemistry and Biochemistry.
meaning, and the experimental values have been derived
as in ref 8a. The specific energy of combustion of the
four compounds are referred to the final temperature of
the experiments, 298.15 K. Table 2 gives the standard
molar energy and enthalpy of combustion and the stan-
dard molar enthalpy of formation of N-methylmaleimide,
N-methylsuccinimide, N-methylphthalimide, and N-ben-
zoyl-N-methylbenzamide in the crystalline state at T )
298.15 K.
The uncertainties of the standard molar energy and
enthalpy of combustion are twice the final overall stan-
dard deviation of the mean and were estimated as
outlined by Olofsson.8b The values for the standard molar
enthalpies of formation of H2O(l) and CO2(g) at T )
§ Present address: Institute of Elemento-Organic Chemistry, Nan-
kai University, Tianjin, P. R. China.
X Abstract published in Advance ACS Abstracts, April 1, 1997.
(1) (a) Konopelski, J . P. Tetrahedron Lett. 1993, 34, 4587. (b) Hu,
Z.; Lashmikantham, M. V.; Cava, M. P.; Becher, J .; Hansen, T.;
J orgensen, T. J . Org. Chem. 1992, 57, 3988.
(2) (a) Mumper, M. W.; Aurenge, C.; Hosmane, R. S. Bioorg. Med.
Chem. Lett. 1993, 3, 2847. (b) Mumper, M. W., Ph.D. Thesis, University
of Maryland Baltimore County, 1996.
(3) (a) Mumper, M. W.; Aurenge, C.; Hosmane, R. S. J . Biomol.
Struct. Dyn. 1994, 11, 1107. (b) Nahm, S.; Cheng, H. N. J . Org. Chem.
1986, 51, 5093.
(4) Garratt, P. J . Aromaticity; J ohn Wiley & Sons: New York, 1986.
(5) Tobia, D.; Harrison, R.; White, T. L.; DiMare, M.; Rickborn, B.
J . Org. Chem. 1993, 58, 6701.
(6) This assignment arises from counting the π electrons as two from
the carbon-carbon double bond and two from the nitrogen or oxygen
as is done to arrive at an aromatic count to six electrons in pyrrole or
furan, respectively. The two carbonyl groups are considered not to
contribute electrons at all as done to arrive at six electrons for either
2- or 4-pyridone to be considered aromatic.
(7) We accept the value of the enthalpy of formation of maleic
anhydride from the literature, as well as that of many ancillary
substances. Unless otherwise said, these and any other unreferenced
data will be taken from the following archive: Pedley, J . B. Thermo-
chemical Data of Organic Compounds, 3rd ed.; Thermochemical
Research Center: College Station, TX, 1994.
(8) (a) Westrum, E. F., J r. Presentation of combustion calorimetric
data in the primary literature. In Combustion Calorimetry; Sunner,
S., Ma¨nsson, M., Eds.; Pergamon Press: Oxford, 1979; Chapter 7. (b)
Olofsson, G. Assignment of uncertainties. In Combustion Calorimetry;
Sunner, S., Ma¨nsson, M., Eds.; Pergamon Press: Oxford, 1979; Chapter
6. (c) CODATA. Recommended key values for thermodynamics, 1975.
J . Chem. Thermodyn. 1976, 8, 603.
S0022-3263(96)02198-6 CCC: $14.00 © 1997 American Chemical Society

Antiaromaticity of Maleimides and Maleic Anhydride
J . Org. Chem., Vol. 62, No. 9, 1997 2733
Ta ble 1. Resu lts of Com bu stion Exp er im en ts (p0 ) 101.325 k P a )a
N-Methylmaleimide
0.63949
m′ (compound)b/g
0.63722
0.04982
1.1111
0.64340
0.05412
1.1353
0.63686
0.05590
1.1313
0.63477
0.05430
1.1230
m′′ (polyethylene)b/g
∆tc/K ) (tf - ti + ∆tcorr)/K
ꢀ(calor)c (-∆tc)/kJ
0.05365
1.1282
-16.1201
-0.0184
0.0472
-15.8758
-0.0181
0.0414
-16.2218
-0.0185
0.0484
-16.1645
-0.0185
0.0439
-16.0463
-0.0183
0.0474
ꢀ(cont)d (-∆tc)/kJ
∆Uigne/kJ
∆Udec (HNO3)f/kJ
0.0414
0.0110
2.3102
0.0414
0.0111
2.4879
0.0414
0.0112
2.5098
0.0443
0.0111
2.5924
0.0420
0.0110
2.5178
∆U(corr to std states)g/kJ
-m′′∆cu°(polyethylene)/kJ
∆cu°(compound)/(kJ g-1
)
-21.1699
-21.1822 ( 0.0033
-21.1902
-21.1836
-21.1841
-21.1831
<∆cu°(298.15 K)>/(kJ g-1
)
N-Methylsuccinimide
m′ (compound)b/g
0.62097
0.05491
1.1418
0.62307
0.05894
1.1587
0.62382
0.04289
1.1086
0.59409
0.04819
1.0785
0.60463
0.04971
1.1011
0.60310
0.04626
1.0871
0.59845
0.05075
1.0936
m′′ (polyethylene)b/g
∆tc/K ) (tf - ti + ∆tcorr)/K
ꢀ(calor)c (-∆tc)/kJ
-16.3276
-0.0186
0.0469
-16.5699
-0.0189
0.0493
-15.8551
-0.0180
0.0465
-15.4237
-0.0175
0.0477
-15.7476
-0.0179
0.0471
-15.5472
-0.0176
0.0471
-15.6400
-0.0177
0.0480
ꢀ(cont)d (-∆tc)/kJ
∆Uigne/kJ
∆Udec(HNO3)f/kJ
0.0406
0.0097
2.5462
0.0424
0.0429
0.0406
0.0428
0.0414
0.0414
∆U(corr to std states)g/kJ
-m′′∆cu°(polyethylene)/kJ
0.0098
0.0096
0.0092
0.0093
0.0093
0.0093
2.7329
1.9890
2.2346
2.3051
2.1449
2.3534
∆cu°(compound)/(kJ g-1
)
-22.0668
-22.0799 ( 0.0056
-22.0752
-22.0979
-22.0658
-22.0981
-22.0894
-22.0663
<∆cu°(298.15 K)>/(kJ g-1
)
N-Methylphthalimide
0.61039
m′ (compound)b/g
0.61278
0.61031
1.1227
-16.0411
-0.0180
0.0450
0.0337
0.0115
-26.1652
0.61017
1.1222
-16.0347
-0.0180
0.0467
0.0325
0.0115
∆tc/K ) (tf - ti + ∆tcorr)/K
ꢀ(calor)c (-∆tc)/kJ
ꢀ(cont)d (-∆tc)/kJ
∆Uigne/kJ
1.1270
-16.1029
-0.0181
1.1230
-16.0453
-0.0180
0.0468
0.0473
0.0325
0.0116
∆Udec(HNO3)f/kJ
0.0331
0.0115
∆U(corr to std states)g/kJ
∆cu°(compound)/(kJ g-1
)
-26.1588
-26.1667
-26.1599
<∆cu°(298.15 K)>/(kJ g-1
)
-26.1627 ( 0.0019
N-Benzoyl-N-methylbenzamide
m′ (compound)b/g
0.53870
0.06103
1.3848
0.50709
0.04825
1.2742
0.49262
0.04665
1.2369
0.49271
0.05449
1.2629
0.49215
0.04715
1.2381
m′′ (polyethylene)b/g
∆tc/K ) (tf - ti + ∆tcorr)/K
ꢀ(calor)c (-∆tc)/kJ
-19.8111
-0.0226
0.0474
-18.2299
-0.0206
0.0455
-17.6963
-0.0199
0.0480
-18.0677
-0.0204
0.0442
-17.7125
-0.0200
0.0469
ꢀ(cont)d (-∆tc)/kJ
∆Uigne/kJ
∆Udec(HNO3)f/kJ
0.0284
0.0111
2.8302
0.0248
0.0103
2.2375
0.0236
0.0254
0.0254
∆U(corr to std states)g/kJ
-m′′∆cu°(polyethylene)/kJ
0.0100
0.0101
0.0100
2.1634
2.5268
2.1866
∆cu°(compound)/(kJ g-1
)
-31.4026
-31.4193
-31.4060
-31.4213
-31.4207
<∆cu°(298.15 K)>/(kJ g-1
)
-31.4140 ( 0.0040
a
b
For the definition of the symbols, see refs 4, 7. Tth ) 298.15 K; Vbomb ) 0.380 l. Corrected masses obtained from weight by calculating
the buoyancy. c ꢀ(calor) is the energy equivalent of the whole system minus the content of the bomb. ꢀ(cont) energy equivalent of the
d
contents of the bomb ꢀ(cont)(-∆Tc) ) ꢀicon(Ti - 298.15 K) + ꢀfcon(298.15 K - Tf + ∆Tcorr). e Experimental energy of ignition. f Experimental
g
energy of formation of HNO3. ∆U(corr to std states) is the sum of items 81-85, 87-90, 95, and 94 in ref 25.
Ta ble 2. Sta n d a r d Mola r En er gies of Com bu stion a n d
En th a lp ies of Com bu stion a n d F or m a tion a t
Tem p er a tu r e T ) 298.15 K
loss during the time t, Wa is the Clausing coefficient of
the Knudsen cell orifice, a is the area of the effusion
orifice, R is the gas constant, T is the temperature, and
M is the molar mass of the studied compound.
An equation of the type
∆cU°m/
∆cH°m/
∆fH°m/
compound
(kJ mol-1
)
(kJ mol-1
)
(kJ mol-1
)
N-methylmaleimide
-2353.4 ( 1.3 -2352.8 ( 1.3 -329.3 ( 1.4
lg(p/Pa) ) -B(T/K)-1 + A
(2)
N-methylsuccinimide -2497.5 ( 1.5 -2498.1 ( 1.5 -469.8 ( 1.6
N-methylphthalimide -4216.4 ( 1.8 -4217.0 ( 1.8 -325.0 ( 2.1
N-benzoyl-N-methyl- -7516.5 ( 3.2 -7520.9 ( 3.2 -239.6 ( 3.8
benzamide
was fitted to the results of Table 4 by the least-squares
method. The quantities δp/p are the fractional deviations
of the experimental vapor pressures from those computed
using eq 2. The highest percentage error for the vapor
pressure in Table 3 is 0.5, arising from all quantities in
eq 1. The parameters A and B of the equations corre-
sponding to N-methylmaleimide, N-methylsuccinimide,
N-methylphthalimide, and N-benzoyl-N-methylbenza-
mide are given in Table 4. The enthalpies of sublimation,
corresponding to the mean temperature θ of their ex-
perimental ranges, have been calculated from the corre-
sponding B values and are also collected in Table 4. The
uncertainties assigned to the values of ∆subH°m are based
on the standard deviations of B values.
298.15 K: -(285.830 ( 0.042) and -(393.51 ( 0.13) kJ
mol-1, respectively, were taken from CODATA.8c
The results of our Knudsen effusion experiments are
summarized in Table 3; temperature is reported in K,
time in seconds, and mass of sublimed substance in
milligrams.
The vapor pressures were calculated by means of the
equation
p ) (∆m/Waat)(2πRT/M)1/2
where p represents the vapor pressure, ∆m is the mass
(1)

2734 J . Org. Chem., Vol. 62, No. 9, 1997
Roux et al.
Ta ble 3. Va p or P r essu r es
102δpe/p Ta/K
N-Methylmaleimide
Ta/K
tb/s
∆mc/mg
pd/Pa
tb/s
∆mc/mg
pd/Pa
102δpe/p
276.15
278.24
280.30
15780
15360
10320
4.71
5.89
4.98
1.41
1.81
2.29
-0.3
-0.01
-0.4
282.87
286.01
289.07
11760
11880
12840
7.58
10.71
16.32
3.07
4.32
6.13
-0.5
-1
-0.03
N-Methylsuccinimide
280.17
282.98
285.96
288.97
15780
13380
12600
10800
3.95
4.65
6.23
7.51
0.265
0.37
0.53
0.75
0.9
-0.09
-0.2
-0.8
291.89
294.85
297.95
14160
14400
15780
13.86
19.40
30.05
1.06
1.46
2.08
0.7
-0.7
0.5
N-Methylphthalimide
298.30
301.38
303.22
307.26
16140
21780
20940
22620
0.91
1.73
2.09
3.57
0.0513
0.0730
0.0919
0.147
1
310.42
313.30
316.26
23820
27300
16320
5.37
8.50
7.13
0.21
0.29
0.41
-0.7
-0.2
1
-0.7
0.3
-0.2
N-Benzoyl-N-methylbenzamide
345.57
348.74
351.84
352.67
354.70
12960
19440
21300
20820
23640
4.26
9.15
14.16
15.32
21.44
0.266
0.382
0.542
0.600
0.742
0.7
-0.07
-0.6
0.2
357.80
359.82
363.67
366.88
35780
10140
11400
14640
15.87
16.18
27.56
49.78
1.06
1.32
2.00
2.83
0.1
-0.09
0.1
1
-1
a
d
Temperature in K. b Time length of the experiment in seconds. c Mass of sublimed substance in mg. Vapor pressure in Pa. e Fractional
deviations of the experimental vapor pressures from the values computed using eq 2.
Ta ble 4. Mola r En th a lp ies of Su blim a tion
leaving aside questions of endo/exo adduct ratiossthe
differences are ca. 13 and 17 kJ mol-1, respectively. It
may be argued that these reactions ultimately relate to
saturation of the formal CdC bond and with this, all
conjugation to the imide and thus antiaromatic destabi-
lization ceases.
∆
subH°m(θ)/
compound
θ/K
A
B
(kJ mol-1
)
N-methylmaleimide
282.61 14.4 ( 0.1 3935 ( 26
75.3 ( 0.5
80.6 ( 0.3
91.1 ( 0.5
N-methylsuccinimide 289.06 14.4 ( 0.1 4208 ( 17
N-methylphthalimide 307.28 14.6 ( 0.1 4750 ( 26
N-benzoyl-N-methyl- 356.23 17.1 ( 0.1 6070 ( 21 116.8 ( 0.4
As such, somewhat more direct, and thus simpler, is
the comparison of the enthalpy of hydrogenation to the
corresponding succinimides and succinic anhydride rela-
tive to that of cyclopentene and other species with five-
membered ringsswe recognize this as paralleling the
understanding of the aromaticity of benzene in terms of
the enthalpy of hydrogenation of benzene relative to those
of cyclohexene and other species with six-membered
rings. Because the enthalpy of formation of hydrogen is
0.0 by definition, the enthalpy of hydrogenation of an
arbitrary compound is the difference of its enthalpy of
formation and that of its hydrogenated product. In
particular, the enthalpy of hydrogenation of N-methyl-
maleimide is precisely the difference of its enthalpy of
formation and that of N-methylsuccinimide. From num-
bers presented in the current paper, the difference is
found to be 136.7 ( 2.2 kJ mol-1. Using enthalpies of
formation from the archival and current literature,11 the
enthalpy of hydrogenation of maleic anhydride is likewise
benzamide
The enthalpy of sublimation at T ) 298.15 K has been
computed using the same equation as in ref 27b. The
C0p,m(cr) values have been determined by DSC, and
C0p,m(g) values were calculated using the group-contribu-
tion scheme of Rihani and Doraiswamy.9
The experimental molar heat capacities obtained for
the four solid compounds were corrected by a program
that includes the calibration of temperature and power
scales of our DSC-2C. These values were fitted by a
least-squares method. Due to the temperature interval
being restricted, the molar heat capacities may be
expressed as a function of temperature by the equation
Cp,m/J K-1 mol-1 ) a + b(T/K) + c(T/K)2
(3)
In Table 5 are listed the interval of temperature, ∆T,
on which the measurements were done, the parameters
a, b, and c, and the coefficients of determination R2 for
the four compounds.
The standard molar enthalpies of formation for both
crystalline and gaseous phases, and the standard molar
enthalpies of sublimation for each of the four compounds
at T ) 298.15 K, are reported in Table 6.
No vapor pressures or combustion enthalpies of N-
methylmaleimide, N-methylsuccinimide, N-methylphthal-
imide, and N-benzoyl-N-methylbenzamide have been
found in the literature for comparison with our results.
129.6 ( 5.4 kJ mol-1
. By contrast, the enthalpies of
hydrogenation of the obviously “unconjugated” cyclopen-
tene and “merely conjugated” cyclopentadiene (to cyclo-
pentene) are 110.3 ( 1.6 and 100.4 ( 2.1 kJ mol-1. If it
is argued that the 110.3-100.4 ≈ 10 kJ mol-1 difference
between the hydrogenation enthalpy values for cyclopen-
tene and cyclopentadiene reflects the conjugation energy
of the latter species, then the 110.3-136.7 ≈ -26 and
110.3-129.6 ≈ -19 kJ mol-1 reflect the antiaromatic
destabilization of N-methylmaleimide and maleic anhy-
dride.
Discu ssion
(10) These two results are from ref 5. Encouragingly, much the same
difference is derivable from reactions of 1,3-diphenylbenzoisofuran with
maleic anhydride and a whole series of variously substituted N-
phenylmaleimides, cf.: Kiselev, V. D.; Ustyugov, A. N.; Breus, I. P.;
Konovalov, A. I. Dokl. Chem. 1977, 234, 320.
We recall the recent discussion of the enthalpy of
Diels-Alder reactions involving maleic anhydride and
N-methylmaleimide as competing dienophiles.5,10 With
furan and 1,3-diphenylisobenzofuran as the dienessand
(11) The enthalpy of formation of maleic anhydride is taken from
Pedley, op. cit., while that of succinic anhydride (-527.9 ( 1.7 kJ mol-1
)
is from the following: Yang, M.-Y.; Pilcher, G. J . Chem. Thermodyn.
1990, 22, 893.
(9) Rihani, D. N.; Doraiswamy, L. K. Ind. Eng. Chem. 1965, 8, 17.

Antiaromaticity of Maleimides and Maleic Anhydride
J . Org. Chem., Vol. 62, No. 9, 1997 2735
Ta ble 5. P a r a m eter s of Eq 3 for th e Mola r Hea t Ca p a cities Cp ,m
compound
∆T/K
a
b
c
R2
N-methylmaleimide
N-methylsuccinimide
N-methylphthalimide
N-benzoyl-N-methylbenzamide
270-339
270-339
270-339
270-363
-3.2957 × 102
-2.5978 × 102
-2.5532 × 102
2.9212 × 102
2.8373
2.2906
2.2665
-4.1621 × 10-3
-3.0740 × 10-3
-2.6241 × 10-3
3.2761 × 10-3
99.32
99.50
99.66
99.64
-0.9812
Ta ble 6. Sta n d a r d Mola r En th a lp ies of F or m a tion
∆fH°m (cr ) a n d of Su blim a tion ∆su bH°m of th e Solid a n d
∆fH°m (g) of th e Ga s a t T ) 298.15 K a n d p0 ) 101.325 k P a
the difference of the enthalpies of formation of cyclopen-
tene and 1,3-diphenylpropane.15 This suggests maleic
anhydride is destabilized by 8 kJ mol-1. Relatedly, the
difference of the enthalpies of formation of N-methyl-
maleimide and N-benzoyl-N-methylbenzamide provides
for the destabilization for the former. From our numbers
for both imides, the difference is found to be 136.5 ( 4.1
kJ mol-1, which, taken at face value, suggests N-
methylmaleimide is stabilized by ca. 50 kJ mol-1. We
find stabilization, much less of this magnitude, to be quite
inconceivable: What is the origin of the discrepancy? Is
it our measured enthalpy of formation of N-benzoyl-N-
methylbenzamide that is problematic? To first ap-
proximation we should think that the following “formal”
phenyl/methyl exchange reaction should be approxi-
mately thermoneutral.
∆fH°m(cr)/
(kJ mol-1
∆
subH°m/
∆fH°m(g)/
(kJ mol-1
)
compound
)
(kJ mol-1
)
N-methylmaleimide
-329.3 ( 1.4 73.3 ( 0.5 -256.0 ( 1.5
N-methylsuccinimide -469.8 ( 1.6 80.1 ( 0.3 -389.7 ( 1.6
N-methylphthalimide -325.0 ( 2.1 91.1 ( 0.5 -233.9 ( 2.2
N-benzoyl-N-methyl- -239.6 ( 3.8 120.1 ( 0.4 -119.5 ( 3.8
benzamide
Taking all of the numbers literally (i.e., by ignoring
error bars), we would conclude that the maleimide is
more destabilized, and thus more antiaromatic, than is
maleic anhydride by ca. 7 kJ mol-1. This result is in
satisfactory agreement with the ca. 11 kJ mol-1 difference
for maleic anhydride and the parent maleimide given by
quantum chemical calculations5 at both the HF/6-31G*
and MP2/6-31G*//HF/6-31G* levels. It also follows from
the general conclusion that if pyrrole is more aromatic
than furan, the corresponding diones are sensibly anti-
aromatic in the reverse order.12
C6H5N(COCH3)2 + (C6H5CO)2O + CH3NH2 f
CH3N(COC6H5)2 + (CH3CO)2O + C6H5NH2 (4)
From the archival and our newly measured gas phase
enthalpies of formation (see Table 6), the preceding
reaction is found to be ca. 10 kJ mol-1 endothermic in
rough accord with our expectations. The enthalpies of
formation of the other imides were earlier shown to be
consistent as well. As we are not yet ready to jettison
the model presented in ref 14 because of its successes
with aromaticity, we leave our measured values and
derived analyses intact. In doing so, we leave ambiguous
the degree of antiaromaticity, and merely even of the
degree of destabilization, of maleic anhydride and the
various maleimides.
It has long been recognized13 that benzoannelation of
a relatively unstrained, unconjugated olefin is accompa-
nied by a ca. 28 kJ mol-1 increase in the enthalpy of
formation. For example, the formal transformation of
(Z)-2-butene and cyclopentene to o-xylene and indane
show changes of 26.2 ( 1.5 and 26.8 ( 2.2 kJ mol-1. Even
the “merely” conjugated cyclopentadiene and the result-
ant indene show the comparable difference of 29.1 ( 2.6
kJ mol-1. By contrast, benzoannelation of the aromatic
benzene to form naphthalene shows a much larger, more
positive difference, 67.7 ( 1.7 kJ mol-1. Intuitively, we
might think that benzoannelation of destabilized or even
antiaromatic species should show a smaller, less positive,
difference than the unstrained, unconjugated olefins. We
find for the benzoannelation of p-benzoquinone and 1,4-
naphthoquinone results in differences of 25.4 ( 4.0 and
21.8 ( 3.5 kJ mol-1, respectively, showing but a small
effect of benzoannelation for destabilized species. Indeed,
the transformation of maleic anhydride into phthalic
anhydride gives an enthalpy of formation change of 26.9
( 5.5 kJ mol-1 while the difference for the N-methyl
derivatives of maleimide and phthalimide is 22.1 ( 2.7
kJ mol-1. This analysis suggests maleic anhydride and
maleimides are not aromatic, but distressingly little else
has been learned.
Con clu sion
Variously N-substituted maleimides and maleic anhy-
dride are well-established to be highly reactive species
in Diels-Alder, ring-opening, and nucleophilic addition
reactions. This has been earlier ascribed to their anti-
aromaticity. Our thermochemical measurements show
this antiaromaticity to be quite modest.
Exp er im en ta l Section
Ma ter ia ls. N-Meth ylm a leim id e was prepared according
to the literature procedure16 by the reaction of maleic anhy-
dride (0.1 mol) with methylamine (0.1 mol) at 0 °C for 1 h,
followed by cyclization of the resultant N-methylmaleamic acid
by heating with excess acetic anhydride in the presence of
sodium acetate (0.05 mol) in a steam bath for 1 h. The product,
obtained in 60% yield, was recrystallized three times from
Using a recent Dewar/Breslow related model of aro-
maticity and antiaromaticity,14 the destabilization of
maleic anhydride may be ascertained using the difference
of its enthalpy of formation and that of benzoic anhy-
dride. The former difference is 79.3 ( 6.8 kJ mol-1, some
8 kJ mol-1 less than the ca. 87 kJ mol-1 derived from
(15) The needed enthalpy of formation of gaseous 1,3-diphenylpro-
pane was derived in two ways: summing the enthalpy of formation
for the liquid (Serijan, K. T.; Wise, P. H. J . Am. Chem. Soc. 1951, 73,
4766) with an estimated enthalpy of vaporization using the simple
equation of Chickos, J . S.; Hyman, A. S.; Ladon, L. H.; Liebman, J . F.
J . Org. Chem. 1981, 46, 4294, and by summing the archival enthalpy
of formation of 1,2-diphenylethane and the “universal methylene
increment” (or ca. -20.6 kJ mol-1) discussed in the following: Cox, J .
D.; Pilcher, G. Thermochemistry of Organic and Organometallic
Compounds; Academic Press: New York, 1970.
(12) This parallels customary assumptions as to the relatively
greater aromatic stabilization of benzene over naphthalene and the
relatively greater destabilization of p-benzoquinone over 1,4-naphtho-
quinone.
(13) See, for example, the use of this regularity in the following:
Liebman, J . F. The Cyclophanes; Keehn, P. M., Rosenfeld, S. M., Eds.;
Academic Press: New York, 1983.
(14) Hosmane, R. S.; Liebman, J . F. Tetrahedron Lett. 1992, 33,
2303.
(16) Mehta, N. B.; Phillips, A. P.; Fu, F.; Brooks, R. E. J . Org. Chem.
1960, 25, 1012-1015.

2736 J . Org. Chem., Vol. 62, No. 9, 1997
Roux et al.
diethyl ether to obtain colorless crystals: mp 96 °C (lit.16 mp
96 °C); 1H NMR (CDCl3) δ 3.0 (s, 3 H, CH3), 6.67 (s, 2 H, CH);
MS (70 eV) m/e 111 (M+, 100), 83 (M+ - CO, 23), 82 (24), 54
(87).
Ta ble 7. P h ysica l P r op er ties a t 298.15 K (Va lu es in
P a r en th eses Wer e Estim a ted )
M/
F/
(δu/δp)T/
cp/
compound
polyethylene
(g mol-1
)
(g cm-3
)
(J kPa-1 g-1
)
(J K-1 g-1
)
N-Meth ylsu ccin im ide was purchased from Aldrich Chemi-
cal Co. and was recrystallized three times from a mixture of
diethyl ether-petroleum ether (bp 40-60 °C) into colorless
13.558
111.100
0.918
1.30
1.34
1.37
1.26
(-0.00023)
(-0.00010)
(-0.00008)
(-0.00008)
(-0.00009)
2.0
N-methylmaleimide
1.32
1.33
1.15
1.20
N-methylsuccinimide 113.116
N-methylphthalimide 161.160
N-benzoyl-N-methyl- 239.274
benzamide
1
needles: mp 71 °C (lit.17 mp 71 °C); H NMR (CDCl3) δ 2.73
(s, 4 H, CH2), 2.99 (s, 3 H, N-Me).
N-Meth ylp h th a lim id e was prepared according to the
literature procedure18 by condensation of phthalic anhydride
(0.1 mol) with 33% aqueous methylamine (0.15 mol), followed
by removal of water by azeotroping with toluene, using a
Dean-Stark apparatus. The product, obtained in 91% yield,
was recrystallized from ethyl alcohol into colorless needles:
The energy of combustion of the compounds was determined
by burning the solid sample in pellet form in oxygen in a bomb
with 1 cm3 of water added. Due to the relatively high vapor
pressure of the N-methylsuccinimide, N-methylmaleimide, and
N-benzoyl-N-methylbenzamide, the pelleted compounds were
burnt enclosed in polyethylene bags. The combustion bomb
was flushed and filled with oxygen, previously freed from
combustible impurities, up to a pressure of 3.04 MPa, at T )
298.15 K. The initial temperature was 296.95 K. The energy
of reaction was always referred to the final temperature 298.15
K.
The energy equivalent of the calorimeter was determined
from the combustion of benzoic acid, NIST standard reference
sample 39i, having a specific energy of combustion under the
conditions specified on certificate of -(26434 ( 3) J g-1. Three
values of the energy equivalent of the calorimeter were used
in computing the combustion experiments: ꢀ(calor) ) (14299.9
( 1.5) J K-1 for N-methylsuccinimide, ꢀ(calor) ) (14288.5 (
2.3) J K-1 for N-methylmaleimide and N-methylphthalimide,
and ꢀ(calor) ) (14306.5 ( 1.5) J K-1 for N-benzoyl-N-methyl-
benzamide from 15, 12, and 15 experiments respectively,
where the uncertainties quoted are the standard deviations
1
mp 134 °C (lit.18 mp 134 °C); H NMR (CDCl3) δ 3.19 (s, 3 H,
Me), 7.7-7.84 (m, 4 H, Ar-H).
N-Ben zoyl-N-m eth ylben za m id e19,20 was prepared by the
reaction of benzoyl chloride with N-(phenylmethylene)methan-
amine N-oxide with benzoyl chloride as follows:
N-(P h en ylm eth ylen e)m eth a n a m in e N-Oxid e.19 Ben-
zaldehyde (9.7 g, 0.091 mol) was placed in a 250-mL three-
necked flask, and 9.6 g (0.115 mol) of N-methylhydroxylamine
hydrochloride in 145 mL of methylene chloride was added.
Sodium bicarbonate (24.1 g, 0.287 mol) was added with
stirring. The reaction mixture was refluxed for 15 h. The
reaction mixture was cooled in an ice bath and filtered to
remove salt. The filtrate was evaporated under reduced
pressure, 20 mL of hexanes was added to the residue, and the
mixture was cooled in an ice bath. The solid precipitated was
filtered and washed with hexanes to give a pale yellow solid
(6.5 g, 53%): mp 79-81 °C (lit.19,20 mp 82-84 and 84-86 °C);
1H NMR (CDCl3) δ 3.86 (s, 3 H, Me), 7.39 (m, 4 H, Ar-H), 8.22
(m + s, 2 H, Ar-H + NdCH).
of the means. The specific energy of combustion and empirical
formula of polyethylene are -(46371 ( 4) J g-1 and C0.961H2.000
.
24
N-Ben zoyl-N-m et h ylb en za m id e.21 In a 250-mL three-
necked flask were placed 2.7 g (0.02 mol) of N-(phenylmeth-
ylene)methanamine N-oxide and 2.2 g (0.022 mol) of triethyl-
amine in 160 mL of dry dioxane. Benzoyl chloride (3.1 g, 0.022
mol) was added dropwise at room temperature during a period
of 5 min. Triethylamine hydrochloride began to precipitate
within minutes. The reaction mixture was stirred overnight
at room temperature. The amine hydrochloride salt was
removed by filtration, and the filtrate was evaporated under
reduced pressure. The solid residue was recrystallized from
ethanol three times to obtain colorless crystals: mp 90-92 °C
(lit.21 mp 90-92 °C); 1H NMR (CDCl3) δ 3.52 (s, 3 H, Me), 7.23
(m, 6 H, Ar-H), 7.46 (m, 4 H, Ar-H).
P r oced u r e for Th er m och em ica l Mea su r em en ts. Con-
trol of purity, as assessed by differential scanning calorimetry
(DSC) and the fractional fusion technique,22a indicated that
the mole fraction impurities in each compound was <0.001.
The samples were studied by DSC22b over the whole working
temperature range, and no phase transitions in the solid state
were observed.
The corrections for nitric acid formation were based on -59.7
kJ mol-1 for the molar energy of formation of 0.1 mol dm-3
HNO3(aq) from N2(g), O2(g), and H2O(l). All samples were
weighed with a Mettler AT-21 microbalance. For the correc-
tion of apparent mass to mass, conversion of the energy of the
actual bomb process to that of the isothermal process, and
correction to standard states, we have used the values of
density F, specific heat capacity cp, and (δu/δp)T, respectively,
as given in Table 7.
The densities were measured in our laboratory. Heat
capacities were determined by DSC. Corrections to standard
states were made according to suggestions by Hubbard et al.25
The atomic weights of the elements were those recommended
by IUPAC in 1991.
A Perkin-Elmer DSC-2C, connected to a Model 3600 Data
Station and provided with an Intracooler-2 unit, was used for
the DSC measurements. Its temperature scale was calibrated
by measuring the melting point of the recommended high-
purity standards: n-octadecane, n-octadecanoic acid, benzoic
acid, indium, and tin.22b The power scale was calibrated using
high-purity indium (>99.999 mol % of In). Heat capacities
were determined as in ref 26. Synthetic sapphire and benzoic
acid were used as standard materials.22c The complete tem-
perature range for determination of the heat capacities was
divided into two intervals for N-methylmaleimide, N-methyl-
succinimide, N-methylphthalimide, and three intervals for
N-benzoyl-N-methylbenzamide of approximately 40 K, over-
lapping by 15 K from one interval to another. The heat
The combustion experiments of the four compounds were
performed with a static bomb calorimeter. Apparatus and
procedure have been described in ref 23.
(17) CRC Handbook of Chemistry and Physics; Weast, R. C., Astle,
M. J ., Eds.; CRC Press, Inc.: Boca Raton, FL, 1981; C-525.
(18) Vasilevskaya, T. N.; Yakovleva, O. D.; Kobrin, V. S. Synth.
Commun. 1995, 25, 2463.
(19) Dicken, C. M.; Deshong, P. J . Org. Chem. 1982, 47, 2047.
(20) Hwu, J . R.; Robl, J . A.; Wang, N.; Anderson, D. A.; Ku, J .; Chen,
E. J . Chem. Soc., Perkin Trans. 1 1989, 1823.
(23) (a) Colomina, M.; J ime´nez, P.; Roux, M. V.; Turrio´n, C. An.
Quim. 1986, 82, 126. (b) Colomina, M.; Cambeiro, M. An. Real Soc.
Espan˜. Fis. Quim. 1959, 55B, 501. (c) Boned, M. L.; Colomina, M.;
Pe´rez-Ossorio, R.; Turrio´n, C. An. Real Soc. Espan˜. Fis. Quim. 1964,
60B, 459-468. (d) Colomina, M.; Roux, M. V.; Turrion, C. J . Chem.
Thermodyn. 1974, 6, 140. (e) Roux, M. V.; J ime´nez, P.; Da´valos, J . Z.;
Abboud, J .-L. M.; Molina, M. T. J . Chem. Thermodyn. 1996, 28, 1029.
(24) J ime´nez, P.; Roux, M. V.; Turrio´n, C. J . Chem. Thermodyn.
1987, 19, 985-992.
(25) Hubbard, W. N.; Scott, D. W.; Waddington, G. In Experimental
Thermochemistry; Rossini, F. D., Ed.; Interscience: New York, 1956;
Chapter 5.
(26) J ime´nez, P.; Mene´ndez, V.; Roux, M. V.; Turrio´n, C. J . Chem.
Thermodyn. 1995, 27, 679.
(21) Heine, H. W.; Zibuck, R.; Williams, J . A.; Heavel, V. J . Am.
Chem. Soc. 1982, 104, 3691.
(22) (a) Marti, E. E. Thermochim. Acta 1973, 5, 173. (b) The DSC
calibration data was obtained from (i) n-octadecane: NPL Certificate
of Measurement CRM No M 14-11, Set of Ten Melting point Standards,
National Physical Laboratory, Teddington, 1980. (ii) n-octadecanoic
acid: Lopez de la Fuente F. L. Ph.D. Thesis, Facultad de Ciencias
Qu´ımicas, Universidad Complutense, Madrid, 1989. (iii) Benzoic acid:
Serge, S.; Cammerga, H. K. Thermochim. Acta 1985, 94, 17-31. (iv)
Indium and tin: standard material and melting point supplied by
Perkin-Elmer. (c) Head, A. J .; Sabbah, R. In Recommended Reference
Materials for the Realization of Physicochemical Properties; Marsh, K.
N., Ed.; Blackwell Scientific Publications: Oxford, 1987; Chapter 9.

Antiaromaticity of Maleimides and Maleic Anhydride
J . Org. Chem., Vol. 62, No. 9, 1997 2737
capacity values were the average of five experiments done for
every interval of temperature. Fresh samples of mass ∼10
mg were scanned using a heating rate of 0.17 K s-1 and a
sensitivity of 0.008 W full scale. The estimated error of the
Wa ) (0.958 ( 0.009) for N-methylmaleimide, respectively. The
enthalpies of sublimation were computed from standard rela-
tions between pressure and temperature.
molar heat capacities was between 0.01Cp,m and 0.02Cp,m
.
Ack n ow led gm en t. This research was supported by
the following grants: The National Institutes of Health
(RO1 CA 71079 and GM 49249) to R.S.H., The National
Institute of Standards and Technology (70NANB4H1539)
to J .F.L., and the Spanish DGICYT (Project PB93-0289-
CO2-02) to M.V.R. The mass spectrum was obtained
from the Michigan State University Mass Spectrom-
etry Facility, supported in part by a grant (no.
P41RR004800054) from the National Institutes of Health.
The vapor pressures for the four compounds were measured
by the Knudsen effusion method as previously described.27ab
The effusion orifice area, a, and the Clausing coefficient,27c Wa,
were a ) (3.47 ( 0.02) × 10-3 cm2 and Wa ) (0.94 ( 0.01) for
N-methylsuccinimide, N-methylphthalimide, and N-benzoyl-
N-methylbenzamide, and a ) (0.799 ( 0.03) × 10-3 cm2 and
(27) (a) Colomina, M.; J ime´nez, P.; Turrio´n, C.; Ferna´ndez, J . A.;
Monzo´n, C. An. Quim. 1980, 76, 245. (b) Colomina, M.; J ime´nez, P.;
Roux, M. V.; Turrio´n, C. J . Chem.Thermodyn. 1984, 16, 379. (c)
Freeman, R. D.; Searcy, A. W. J . Chem. Phys. 1954, 772.
J O9621985