Revisiting the stability of endo/exo diels-alder adducts between cyclopentadiene and 1,4-benzoquinone

Article abstract of DOI:10.1590/S0103-50532010000100017

In this work it is presented a detailed theoretical analysis of the relative stability of endo/exo Diels-Alder adducts formed by the reaction between cyclopentadiene (1) and 1,4-benzoquinone (2). The intrinsic reaction coordinate (IRC) showed the existence of only one transition state for the reaction studied, for both endo 3 and exo 4 adducts. The energies of both adducts were obtained at high level of theory (CBS-Q) confirming that the endo adduct is more stable than exo, which is in the opposite way to the observed in reactions that usually follow Alder's rule. An electronic structure analysis was performed through NBO methodology, indicating that the attractive delocalization interaction predominates over the steric repulsive interaction in the endo adducts. In summary, for the studied cycloaddition reaction the endo adduct is the thermodynamic and kinetic product, which can be also confirmed by experimental data mentioned in this work.

Full text of DOI:10.1590/S0103-50532010000100017

J. Braz. Chem. Soc., Vol. 21, No. 1, 112-118, 2010.
Printed in Brazil - ©2010 Sociedade Brasileira de Química
0
103 - 5053 $6.00+0.00
Revisiting the Stability of endo/exo Diels-Alder Adducts between
Cyclopentadiene and 1,4-benzoquinone
a
b
,c
Claudio F. Tormena, Valdemar Lacerda Jr. and Kleber T. de Oliveira*
a
Departamento de Química Orgânica, Instituto de Química, Universidade de Campinas -
UNICAMP - CP 6154, 13083-970 Campinas-SP, Brazil
b
Departamento de Química, Centro de Ciências Exatas da Universidade Federal do Espírito Santo -
UFES, Av. Fernando Ferrari 514, 29060-900 Vitória-ES, Brazil
c
Centro de Ciências Naturais e Humanas, Universidade Federal do ABC -
UFABC, Rua Santa Adélia 166, Bangu, 09210-170 Santo André-SP, Brazil
Neste trabalho é apresentada uma análise teórica detalhada da estabilidade relativa dos
adutos endo/exo formados entre o ciclopentadieno (1) e a 1,4-benzoquinona (2). As coordenadas
intrínsecas de reação (IRC) indicaram a presença de apenas um estado de transição para a reação,
mostrando que se trata de um mecanismo concertado para ambos os adutos, endo 3 e exo 4. As
energias dos adutos foram calculadas com um alto nível de teoria (CBS-Q) confirmando que o
aduto endo é mais estável que o exo, o que está em desacordo com o que é observado para reações
que usualmente seguem a regra deAlder. Uma análise estrutural eletrônica foi realizada através da
metodologia NBO, a qual indicou que interações atrativas predominam sobre as interações estéricas
repulsivas no aduto endo. Em resumo, para a reação de cicloadição estudada o aduto endo é o
produto termodinâmico e cinético, o que pode ser confirmado também pelos dados experimentais
mencionados neste trabalho.
In this work it is presented a detailed theoretical analysis of the relative stability of endo/exo
Diels-Alder adducts formed by the reaction between cyclopentadiene (1) and 1,4-benzoquinone
(2). The intrinsic reaction coordinate (IRC) showed the existence of only one transition state for the
reaction studied, for both endo 3 and exo 4 adducts. The energies of both adducts were obtained at
high level of theory (CBS-Q) confirming that the endo adduct is more stable than exo, which is in
the opposite way to the observed in reactions that usually followAlder’s rule.An electronic structure
analysis was performed through NBO methodology, indicating that the attractive delocalization
interaction predominates over the steric repulsive interaction in the endo adducts. In summary,
for the studied cycloaddition reaction the endo adduct is the thermodynamic and kinetic product,
which can be also confirmed by experimental data mentioned in this work.
Keywords: Diels-Alder reaction, endo/exo stability, theoretical calculation, NBO analysis
Introduction
structure of the obtained adduct were inconsistent. After
several studies, in 1928 Otto Diels and Kurt Alder,
1
0
The Diels-Alder reaction is one of the most interesting
established a correct structure for the mono- and bis-adducts
formed by the reaction between these compounds through a
[4+2] cycloaddition, in opposition to the reactional course
suggested byAlbrecht: an 1,4-addition of cyclopentadiene
(1) into 1,4-benzoquinone (2). Since then, the Diels-Alder
reaction has appeared in more than 32,000 papers involving
th
and useful reactions found in organic chemistry in 20
1
-8
century. This cycloaddition is widely used to construct,
in a regio- and stereo-controlled way, a six-membered
1
-8
ring with up to four stereogenic centers. Historically,
9
in 1906 Albrecht published the reaction between the
1
1
cyclopentadiene (1) and 1,4-benzoquinone (2) as a 1:1
synthetic and theoretical approaches.
adduct. However, Albrecht’s considerations about the
In general, Diels-Alder reactions are excellent
reactional models for the transition states calculations
due to the nature of their mechanism (usually concerted).
*
e-mail: kleber.oliveira@ufabc.edu.br

Vol. 21, No. 1, 2010
Tormena et al.
113
1
Besides this, the estimation of several chemical properties
and reactivities indexes of dienes and dienophiles can
be performed with good experimental agreement.
the endo form. Actually, we observed from the H NMR
spectrum of the reactional mixture, 98% of compound 3
and 2% of compound 4 (Fig. 1).
1
2-15
An evidence of the importance of this reaction to the
development and application of some computational
calculations is the large number of published papers
involving considerations about the concerted and non-
concerted, synchronous and asynchronous mechanism
Thus, in principle, the results for cycloaddition between
compounds 1 and 2 follow the Alder’s rule, however some
surprising results were observed when an intrinsic reaction
coordinate (IRC) and the NBO analysis of the endo/exo
transition states and products were performed at high level
of theory. These results are discussed in the present study.
1
5-18
nature of some Diels-Alder reactions.
Most of
these studies attempt to explain some experimental
results non-consistent with empirical rules or expected
results concerning the selectivity. While experimental
measurements can provide accurate rate constants for the
different reaction pathways, high-level quantum chemical
calculations are often necessary to explain the observed
phenomena at an electronic level, to predict the substituent
effects and also to evaluate steric interactions between
the participating species. In this context, advances in
the quantum-chemical calculations methodology (e.g.,
the development of new DFT functional methods) and
the improvements of computational power provided the
accuracy needed to quantitatively explain experimental
observations. Moreover, convenient methods for the
analysis of correlated wave functions, as natural bond
Materials and Methods
Chemicals
All solvents and reagents were purchased from Merck,
Acros or Aldrich. The cyclopentadiene (1) was freshly
distilled and then used. p-Benzoquinone (2) was purified
by sublimation in an appropriated apparatus (Aldrich).
2
,7
Synthesis of endo-tricyclo[6.2.1.0 ]undeca-4,9-diene-
3,6-dione (3)
To a solution containing 541 mg (5.0 mmol) in 18 mL
of dry methanol at -78 ˚C under nitrogen atmosphere, was
added cyclopentadiene freshly distilled (344 mg, 5.2 mmol,
in 4 mL of dry methanol) also cooled to -78˚C. Then, the
reaction mixture was allowed to reach 0 ˚C (approx. 1h).
After that, the solvent was removed under reduced pressure
and the product was crystallized by using hexane, yielding
the yellow crystals (854 mg, 4.9 mmol, 98%). mp. 63-65 ˚C.
1
9,20
orbital analysis (NBO),
numerous stereoelectronic interactions, including
secondary orbital interactions (SOI). These interactions
can be used to evaluate the
2
1,22
23,24
2
5
(
SOI) were proposed by Woodward and Hoffmann
to rationalize the empirical endo principle of addition
2
6-29
(
Alder’s rule) formulated by Alder and Stein,
but they
3
0
1
had been object of some criticism. According to Alder’s
rule the major stereoisomer in Diels-Alder reactions is the
one that is formed by maximum accumulation of double
H NMR (CDCl , 400 MHz), d (ppm) 1.44 (dt, 1H, J8.4 Hz
3
and J 1.7 Hz; 1.55 (dt, 1H, J 8.4 Hz, J 1.7 Hz), 3.20–3.26
(m, 2H), 3.53 – 3.58 (m, 2H), 6.07 (t, 2H, J 1.7 Hz), 6.58
2
6-29,31-34
13
bonds in the transition state (through-space).
(sl, 2H). C NMR (100 MHz, CDCl ), d (ppm) 48.3, 48.7
3
On the other hand, the endo product is usually less stable
than exo.
(2 x CH and 1 x CH ), 135.3, 142.0, 199.4.
2
While the stereoelectronic interactions, involved in
the transition state stability, are widely studied in the
Computational Details
1
4,15,35
literature,
the same attention has not been dedicated
All calculations were performed with Gaussian 03
5
3
to understand the stability of the products. One of the
most used examples of the Diels-Alder reaction, which is
program. Full geometry optimization for adducts were
5
4,55
56,57
performed applying HF, B3LYP,
specifies that all electrons are included in a correlation
calculation) and CBS-Q methods and Dunning’s
correlation consistent basis set
method was used to solve the major source of error in
most ab initio calculations of molecular energies, which
is due to truncation of the one-electron basis set, and the
mean absolute deviation in electronic energy, using CBS-Q
MP2 full
(full
36-38
listed in several Organic Chemistry textbooks,
is that
5
8
between cyclopentadiene (1) and maleic anhydride which,
at room temperature, gives only the endo adduct that is then
5
9,60
were used. The CBS-Q
o
converted at 200 C to the thermodynamically more stable
4
exo adduct through a retro Diels-Alder reaction. According
39-48,49-52
to several authors,
the Diels-Alder reaction between
cyclopentadiene (1) and 1,4-benzoquinone (2) (Scheme 1),
gives, in a similar way, only the kinetic endo adduct 3,
although the exo adduct 4 has higher stability than the endo
adduct 3 due to the steric repulsive interaction present in
-
1 58
method, is less than 1 kcal mol .
The transition states calculations (TS) and NBO
6
1
analysis were performed at the B3LYP/cc-pVTZ level

1
14
Revisiting the Stability of endo/exo Diels-Alder Adducts between Cyclopentadiene and 1,4-benzoquinone J. Braz. Chem. Soc.
1
Figure 1. H NMR of the obtained products 3 (98%) and 4 (2%).
of theory, while Intrinsic Reaction Coordinate (IRC)
calculation was performed at the B3LYP/6-31g (d,p) level.
All stationary points were characterized as minima or
transition structures by calculating the harmonic vibrational
frequencies (ZPE), expected for MP2 full calculations.
both adducts, which suggests that for this reaction the
mechanism is concerted. It can also be observed, that the
transition state energy to form the endo adduct is smaller
than for exo adduct.
Results and Discussion
The [4+2] cycloaddition between cyclopentadiene
(
1) and 1,4-benzoquinone (2) can lead to endo and exo
products, as shown in Scheme 1.
7
7
O
O
9
3
H
H
4
3
4
2
2
10
8
+
5
9
+
5
1
6
11
1
6
O
8
O
H
11
H
O
O
1
2
3
4
Endo
Exo
9
8
2
Figure 2. IRC profile for the endo and exo adducts of the [4+2]
Scheme 1. Diels-Alder reaction between 1 and 2.
cyclopentadiene-1,4-benzoquinone cycloaddition.
The IRC calculations (Fig. 2) for both reaction pathways
were performed at the B3LYP/6-31g (d,p) level and it
was observed that there is only one transition state for
The geometries and energies for reagents, products and
transition state structures were optimized at the B3LYP/
cc-pVTZ level and the energy profiles are shown in Fig. 3.

Vol. 21, No. 1, 2010
Tormena et al.
115
It can be observed (Fig. 3) from the theoretical
products it was observed that the endo adduct (Fig. 4b) is
calculation that the energy for exo transition state structure
is 1.8 kcal mol higher than for endo (Fig. 4a), which
more stable (thermodynamic product) than exo, which is
-
1
39-48
in disagreement with literature data.
It is reported in
3
9-48
is in agreement with the obtained experimental results
literature
that exo is the thermodynamic and endo is
6
2
(
Scheme 1). However, analyzing the energies of the
the kinetic product.
Theoretical calculations at high level of theory (Table 1)
were performed for adducts endo and exo to get more
accurate energies values to check if the endo adduct is in
fact the thermodynamic product. It can be seen from Table
1
that in all levels of theory applied in the present study,
the endo adduct is the more stable than exo.
-1
Table 1. Calculated energies (kcal mol ) for adducts endo and exo at
different level of theory
Method
HF
Basis set
Eendo
0.0
0.0
0.0
0.0
0.0
0.0
Eexo
0.9
0.9
0.2
0.3
1.1
1.1
DH
0.9
0.9
0.2
0.3
-
cc-pVTZ
HF
aug-cc-pVTZ
cc-pVTZ
B3LYP
B3LYP
MP2 full
CBS-Q
aug-cc-pVTZ
cc-pVDZ
Figure 3. Energies for endo and exo reaction pathways of the [4+2]
cycloaddition reaction of cyclopentadiene and 1,4-benzoquinone
calculated at the B3LYP/cc-pVTZ level.
1.1
Figure 4. Optimized structures for: a) transition states; b) products.

1
16
Revisiting the Stability of endo/exo Diels-Alder Adducts between Cyclopentadiene and 1,4-benzoquinone J. Braz. Chem. Soc.
Table 2. Energies obtained from NBO deletion calculation for endo and exo adducts at the HF and B3LYP method using cc-pVTZ basis set
Parameters
HF/cc-pVTZ
B3LYP/cc-pVTZ
endo
-572.244626
-571.131031
1.113595
4.7
exo
endo
exo
Total SCF energy (a.u)
Deletion energy (a.u)
-572.243196
-571.137107
1.106089
0.0
-575.778572
-574.731736
1.046836
3.3
-575.778335
-574.736773
1.041562
0.0
a
Energy change (a.u)
-
1 b
DE (kcal mol )
a
b
Energy change = Total SCF energy – Deletion energy. DE = energy change for endo – energy change for exo.
-1
Structure stability is usually explained by stereoeletronic
interactions [attractive (delocalization interaction) or
Table 3. Delocalization orbital interaction energies (kcal mol ) from NBO
analysis for endo and exo adduct, calculated at the HF/cc-pVTZ level
63,64
repulsive (steric interaction)]. The NBO analyses can be
invokedtoexplaintheunexpectedstabilityobservedforendo
adduct and quantify the delocalization interaction. There
is a simple way to determine which interaction (attractive
or repulsive) is more pronounced in any structure. First
the NBO analysis of a full wave function was performed.
Then, only the attractive delocalization interactions are
deleted (NBOdel) and the energy is recalculated without
these delocalization interactions, which means that only
steric interactions will be present. The energy change
values (Table 2) can be used to determine the amount
of attractive or repulsive interactions in endo and exo
adducts. This energy value is the difference between a full
molecular electronic energy and the molecular electronic
energy calculated deleting the delocalization interactions,
and the larger is this energy change the large attractive
delocalization interaction the structure has.
Orbital interaction
endo
4.6
1.3
3.3
1.5
3.3
8.2
4.8
2.9
29.9
exo
4.3
0
pC1=C2 →s*C5-C6
pC1=C2 →s*C5-H9
sC4-C5 →s*C8=O
3.3
0
sC5-C8 →s*C6-C7
sC5-H9 →s*C6-C1
sC5-H9 →p*C8=O
sC6-C7 →s*C5-C8
sC5-C6 →p*C8=O
Sum
2.0
7.8
3.1
3.3
23.8
other parts of molecule system should also increase the
stabilization of endo.
The results discussed in the present paper can be used for
a correct explanation of the experimental results obtained
4
8
When these energy changes for endo and exo are
computed (Table 2) at the HF/cc-pVTZ and B3LYP/
cc-pVTZ level, deleting the delocalization interactions
by Yates and Switlak for thermal stereoisomerization of
48
adduct endo 3. In their study, the authors reflux a solution
of adduct endo in toluene under argon for 36 h and the
products of this thermal isomerization consisted of 1:9
mixture of adduct exo and endo (50%), one hydroquinone
derivative (1,4-dihydro-1,4-methanonaphthalene-5,8-diol)
and 1,4-benzoquinone. The authors suggest that the small
amount of exo adduct is due to the position equilibrium
was not established under that conditions. However, the
reason for small amount of exo adduct obtained in the
thermo stereoisomeration reaction performed byYates and
(
NBOdel), the exo adduct becomes more stable than endo
-
1
by 4.7 and 3.3 kcal mol , respectively. The data from
Table 2 corroborate that in the endo adduct the steric
repulsive interactions are larger than in exo, however, the
attractive delocalization interaction in endo is higher than
in exo and compensates the steric repulsion. This is the
reason why the endo 3 is also the thermodynamic product
and not the exo 4, as it is reported in the current literature.
The most important delocalization orbital interactions
from NBO analysis involving bonding and antibonding
orbital, responsible for the stabilization of endo adduct,
are listed in Table 3; similar interactions for exo adduct
were also collected. The interactions listed at Table 3, are
those only for the molecular fragment that differentiate
endo from the exo adduct.As can be seen from Table 3, the
attractive interactions (delocalization orbital interactions)
are higher for endo than exo adduct, and probably are
responsible for extra stabilization acquired for endo in
comparison to exo. Probably, other interactions involving
4
8
Switlak is simply due to the fact that the endo adduct is
the thermodynamic and also the kinetic product.
Conclusions
The study described in the present work was performed
at high level of theory using sophisticated theoretical
methods and shows some evidences that the oldest
Diels-Alder reaction follow Alder rule. However, the
adduct endo 3 is not only the kinetic product, but also
the thermodynamic product, which is in agreement with

Vol. 21, No. 1, 2010
Tormena et al.
117
experimental data.Also, this study demonstrates that some
stabilizing stereoeletronic interactions are present in the
endo product similar to observed in the endo transition
state.
The most important point that we want to emphasize is
that the stability acquired by any structure will depend of an
energy balance between attractive and repulsive interactions
present in that structure.
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Acknowledgments
2
The authors thank the Fundação deAmparo à Pesquisa
do Estado de São Paulo (FAPESP) for financial support
for this research (05/59649-0 and 06/03980-2), for post-
doctoral scholarship (07/03589-4) and for financial support
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Received: January 8, 2009
Web Release Date: October 23, 2009
FAPESP helped in meeting the publication costs of this article.