The dimerisation of 2-methoxycarbonylbuta-1,3-diene: The importance of paralocalisation energy in assessing diene reactivity

Article abstract of DOI:10.1039/a708199e

The dimerisation and competitive cycloaddition of 2-methoxycarbonylbuta-1,3-diene with electron-rich dienes has been investigated. Experimental results provide evidence that the enthalpy of the π-system significantly influences the energy of the transition state of cycloadditions of this type. This has been corroborated by ab initio calculations. We propose an early reorganisation of the π-electrons in such cycloadditions to explain the influence stated above.

Full text of DOI:10.1039/a708199e

View Article Online / Journal Homepage / Table of Contents for this issue
The dimerisation of 2-methoxycarbonylbuta-1,3-diene: the
importance of paralocalisation energy in assessing diene reactivity
,
a
a
b
b
Claude Spino,* Jason Crawford, Yunping Cui and Melinda Gugelchuk
a
Université de Sherbrooke, Département de Chimie, Sherbrooke, Qc, Canada J1K 2R1
University of Michigan, Department of Chemistry, Ann Arbor, MI 48109-1055, USA
b
The dimerisation and competitive cycloaddition of 2-methoxycarbonylbuta-1,3-diene with electron-rich
dienes has been investigated. Experimental results provide evidence that the enthalpy of the ð-system
significantly influences the energy of the transition state of cycloadditions of this type. This has been
corroborated by ab initio calculations. We propose an early reorganisation of the ð-electrons in such
cycloadditions to explain the influence stated above.
Introduction
dienes 5a and 5b. Note that the reactivity of diene 1 is under-
estimated here since diene 1 is generated slowly (3–4 h) and
must be at a lower concentration relative to the competing
diene. In the same vein, only dimer 4 was obtained when 1 was
generated by method B in the presence of butadiene, isoprene
or 1-methoxycarbonylbuta-1,3-diene (1-MCBD) as indicated
in the last three entries. We also know that the ethyl ester ana-
logue of 1 dimerised in preference to reacting with 2,3-
dimethylbuta-1,3-diene but it reacted as a dienophile with
The facile dimerisation of 2-methoxycarbonylbuta-1,3-diene
(
2-MCBD) 1 to give dimethyl mikanecate 4 has been known for
1–6
some time (Scheme 1). Although the unusual ease with which
CO2Me
Method A
toluene
110 °C
S
CO2Me
O2
10
excess cyclopentadiene at 25 ЊC in quantitative yield. Overall,
2
CO2Me
1
(as a diene) seems to have a reactivity in between that of
3
2
cyclopentadiene and diene 5a.
4
1
These results were puzzling in view of the fact that 2-MCBD
is an electron-poor diene and we wanted to be sure that they did
not arise from some artefact. Firstly, there was no evidence of
hydrolysis of dienes 5 during the reaction. Secondly, we ruled
out the possibility of a local concentration effect (whereby
molecules of diene 1 would dimerise faster because they were
generated in proximity of one another) by stirring 1 equiv. each
of sulfolene 2 and diene 5b with excess methyl acrylate (10
equiv.) in refluxing toluene (Scheme 2, bottom). After 4 h, both
MeO2C
Method B
1
CO2Me Et N, CH Cl
3
2
2
4
25 °C
Br
3
Scheme 1
this [4 ϩ 2]-cycloaddition takes place has been noted, a ration-
alisation of this phenomenon has yet to be offered. A com-
munication by Jung and Zimmerman provided the first kinetic
CO2Me
OSiMe3
7
data on this dimerisation. They proposed that the high dieno-
O
O
O
philic character of the C1᎐C2 bond in 1 and an increased
population of its s-cis conformation are major factors in
accelerating this Diels–Alder reaction. We later provided evi-
dence that 2-MCBD is also an activated diene from results
obtained in its cross Diels–Alder reactions with electron-rich
dienes. We then proposed an early reorganisation of the
1 + 5b
+
+ 4 + 6b
(
1 equiv.)
O
MeO
O
Toluene, ∆
O
O
O
O
7a
8a
8
π-network to explain the activation of 1. We report herein a
full account of this work which includes additional experi-
CO2Me
OSiMe3
mental results and ab initio calculations on the dimerisation of
1
supporting our hypothesis. We also extend our discussion
of the rationale to include the concept of paralocalisation of
electrons.
CO2Me
+
+ 4 + 6b
1
+ 5a
(
10 equivs.)
Toluene, ∆
MeO
CO2Me
CO2Me
7
b
8b
Results and discussion
Scheme 2
A 96% yield of dimer 4 is obtained by heating sulfolene 2 at
1
1
3
10 ЊC (Scheme 1, method A) or by treating the allylic bromide
dienes had completely reacted to give adducts 7 and 8 respect-
ively (GC analysis). Also detected were traces of 6b and 4.
Much more dimer would have been detected if a proximity
effect were operative. We believe that diene 1 reacted as it was
generated since it or dimer 4 were not detected in significant
2
with base at 25 ЊC (Method B). However, in the presence of
9
the electron-rich diene 5a (Y = Me) and the very reactive diene
5
b (Y = OMe), either method led to a mixture of dimer 4 and
cross cycloadduct 6 (Table 1). In most cases, the dimerisation
product prevailed unless a large excess of the electron-rich
diene was used. A lower temperature seems to favour dimeris-
ation over cross cycloaddition (entries 3, 4, 5 and 8). These
results indicate that the dienophile 1 reacts more or as rapidly
with itself, an electron-deficient diene, than with electron-rich
amounts during the reaction (the extrusion of SO in 2 takes ca.
2
4 h). Thirdly, an ionic mechanism is not possible for this dimer-
isation as suggested by the regiochemistry of dimer 4 (the other
regioisomer could not be detected). To verify if a diradical
mechanism were involved we repeated the dimerisation of 1
J. Chem. Soc., Perkin Trans. 2, 1998
1499

View Article Online
Table 1 Cross Diels–Alder reactions of diene 1
X
MeO2C
X
+
Y
MeO2C
5
CO2Me
CO2Me
Y
1
4
6
Combined yield
of adducts (%)
a
b
c
d
Entry
Diene (equiv.)
Y
X
Method
Ratio 4:6
e
1
2
3
4
5
6
7
8
9
5a (1)
5a (5)
5a (1)
5a (5)
5a (>5)
5b (1)
5b (5)
5b (1)
5c (5)
5d (5)
5e (5)
Me
Me
Me
Me
Me
OMe
OMe
OMe
H
OSiMe3
OSiMe3
OSiMe3
OSiMe3
OSiMe3
OSiMe3
OSiMe3
OSiMe3
H
A
A
B
B
2:1e
95
95
94
93
98
89
80
80
95
96
93
1:4
f
9:1f
3:1f
2:1f
1:2f
1:9f
3:1
g
B
A
A
B
B
B
B
1:0
1:0
1:0
1
1
0
1
H
CO Me
Me
H
2
a
b
c
Molar equivalents with respect to 2 or 3. Method A: Toluene, reflux. Method B: Et N, CH Cl , room temp. Adducts 6a and 6b were isolated as
3
2
2
d
e
the hydrolysed product after chromatography on silica gel. Isolated yields based on 2 or 3 after chromatography. Ratios of isolated adducts.
f
1
g
Ratios determined by H NMR integrations of crude mixture. Slow addition of 3 over 12 h.
from 2 or 3 and its cycloaddition with methyl acrylate in the
absence of light and the presence of 2,6-di-tert-butylphenol or
hydroquinone with no effect on the rate of reaction. Yields of
dimerisation are high regardless of diene concentration (includ-
ing in the neat form) with no trace of [2 ϩ 2]- or [4 ϩ 4]-
cycloaddition or polymerisation products. More convincingly,
4 (73%), the expected cycloadduct 7b (8%) and cross cyclo-
adduct 6b (7%), but no trace of reaction between 5b and methyl
acrylate to give 8b (yields based on 3). Diene 5b, in a separate
experiment, did not react at all with methyl acrylate under those
conditions. Nonetheless, at that temperature 1 dimerised faster
than it could add to methyl acrylate because it is a better
dienophile.
(
Z)-2-methoxycarbonylhexa-1,3-diene 13 (see Scheme 3) does
Different experiments were needed to provide evidence of the
activation of 2-MCBD by the ester at the 2-position. Com-
parison between the reactivity of 2-MCBD and some related
molecules was helpful. For example, 1-methoxycarbonylbuta-
1,3-diene (1-MCBD) is reluctant to dimerise and could not
compete with dienes 5a and 5b in cycloadditions with electron-
poor dienophiles. The interesting 2,3-bis(methoxycarbonyl)-
buta-1,3-diene also undergoes dimerisation and reacts with
CO2Me
CO2Me
MeO)2CHBF4
CH2Cl2
CO2Me
BF4–
(
Et3N
CH2Cl2
S
S+
SMe
Me
9
10
11
CH2Cl2 or CDCl3 Neat,
12
electron-poor as well as electron-rich dienophiles readily. We
prepared dienes 11 and 15 to compare their reactivities with 1
5–7 days
90 min
CO2Me
(Schemes 3 and 4). Dihydrothiophene 9 was prepared from
CO2Me
Me
13–16
methyl acrylate following the method of Belleau.
After
17
methylation with Borch’s salt, the resulting sulfonium salt 10
SMe
MeS
MeO2C
CO2Me
CO2Me
n
13
12
2 x Bu Li
Toluene
2
Scheme 3
ClCO2Me
∆
CO2Me
CO2Me
S
O
O
1
1
not dimerise and is quite stable. This compound would behave
just like diene 1 if a diradical mechanism were operational but it
is not expected to dimerise easily via a concerted cycloaddition
because of the Z-geometry of the double bond. Therefore, the
above reactions appear to proceed via a concerted [4 ϩ 2]-
cycloaddition mechanism.
14
15
Toluene
∆
O
O
O
MeO2C
O
H
O
N
MeO2C
We then attempted to make dienes 1 and 5b compete for a
separate dienophile but the results were somewhat thwarted for
two reasons: on the one hand, 1 is also a powerful dieno-
phile and will compete; on the other hand, its in situ generation
takes time. So, when 1 equiv. of maleic anhydride was heated in
toluene with equimolar amounts of 2 and 5b, it yielded mostly
dimer 4 and cycloadduct 8a along with small amounts of
adducts 7a and 6b. But diene 5b alone reacted in less than 1 h
with maleic anhydride at that temperature. Given the fact that
Toluene
O
∆
H
O
1
6
O
MeO2C
MeO2C
MeO2C
N
H
MeO2C
+
the extrusion of SO from 2 takes up to 4 h, it appears that the
2
dienophile was consumed faster than 1 was generated. In add-
ition, the reaction of 3 and 5b with excess methyl acrylate
17
18
(
10 equiv.) at 25 ЊC following method B (3 h) gave mostly dimer
Scheme 4
1
500
J. Chem. Soc., Perkin Trans. 2, 1998

View Article Online
Table
2
Calculated energies of 2-methoxycarbonylbuta-1,3-diene
conformations
a
a
Structure
Conformation
Etotal
Erel
4
4
4
4
1
2
3
4
s-cis
s-cis
s-trans
s-trans
Ϫ379.435 308 1
Ϫ379.436 318 8
Ϫ379.434 337 4
Ϫ379.437 058 8
0.76
0.13
2.31
0.00
a
Etotal = total energy (in atomic units); E = energy relative to structure
rel
Ϫ1
4
4 (in kcal mol ); Erel includes zero point vibrational energy (ZPE)
correction.
Fig. 1 RHF/3-21G optimised conformations of 2-methoxycarbonyl-
buta-1,3-diene
underwent an elimination reaction with triethylamine at 25 ЊC
to yield the (Z)-enethiol ether 11. Upon standing at 25 ЊC in a
chlorinated solvent, 11 dimerised to give a 9:1 mixture of
stereoisomeric adducts 12 in 5–7 days. When concentrated, 11
1
dimerised in 90 min in 89% yield. The H NMR spectra of both
isomers of 12 were consistent with (Z)-enethiol ether struc-
tures, indicative of a concerted cycloaddition mechanism. A
radical or ionic mechanism would have led to isomerisation to
the more stable (E)-enethiol ethers. The dimerisation of 11 is
noteworthy because of severe steric interactions when in the
cisoid conformation. Clearly the 2-methoxycarbonyl group acti-
vates 11 toward dimerisation since the known (Z)-1-methyl-
thiobuta-1,3-diene does not undergo Diels–Alder reactions at
1
8
all. However, the sulfur also plays an activating role since the
Fig. 2 RHF/3-21G optimised transition structures of the dimeris-
ation of 1 having cisoid dienophile conformations
carbon analogue of 11, (Z)-2-methoxycarbonylhexa-1,3-diene
11
1
3, does not dimerise (Scheme 3).
En route to diene 15, the dianion of sulfolene 2 was acylated
with methyl chloroformate to give a 78% yield of 14 (Scheme
shows the four conformations of 2-methoxycarbonylbuta-1,3-
diene that were investigated. We chose to leave the O᎐C᎐O᎐Me
torsional angle to 0Њ in all structures since it is well known that
19
4
). Upon heating in toluene for 12 h, 14 underwent a chele-
22,23
tropic elimination of SO to afford the stable, but volatile, diene
this conformation is prevalent in all esters.
Therefore only
2
1
5 as a 1:1 mixture of E- and Z-isomers. Heating 14 for 12 h in
the rotational preference of the carbonyl group with respect to
the buta-1,3-diene system was examined. Table 2 lists the cal-
culated energetic parameters. For the conformations of 2-
methoxycarbonylbuta-1,3-diene, the lowest energy structure
was the s-trans form 44 in which the C᎐O bond eclipses the
toluene containing maleic anhydride gave 87% of two isomeric
cycloadducts 16 in a ca. 1:1.5 ratio, along with a small amount
of unreacted (E)-15. Chromatography on silica gel of these
adducts led to decomposition but the crude mixture was of
sufficient purity for identification. Again, the reaction of the
C2᎐C3 bond of the butadiene moiety. The s-cis conformation
1
Ϫ1
E-isomer is noteworthy. Contrary to diene 1, dienes 15 could
42 (C᎐O eclipsed with C2᎐C3) was only 0.13 kcal mol
higher.
also cycloadd with an electron-rich dienophile. Heating 14
in toluene in the presence of 1-morpholinocyclopentene gave
Due to conformational and stereochemical issues there is a
large number of possible concerted transition structures. This
was one consideration for not using a higher level of theory for
the calculations. In any case, geometries and bond lengths of
Diels–Alder transition structures calculated at the 3-21G level
7
5% of the cycloaddition/elimination product 17 along with
approximately 5% of isomeric cycloadducts 18. The regio-
chemistry observed in adduct 17, which was unambiguously
established by NMR experiments, does not preclude an ionic
mechanism in this case.
24
are very close to those at the 6-31G level.
Figs. 2–4 show the
ten transition structures we have examined with the activated
C1᎐C2 double bond reacting as the dienophile. To keep the
In the light of all of the above results, we concluded that the
ester in 2-MCBD activates it toward cycloaddition and 1 is a
diene truly more reactive than it should be as predicted by
᎐
number of structures down, we have excluded other regio-
isomers as well as cycloadducts with the C3᎐C4 double bond
᎐
20,21
Theoretical cal-
playing the role of dienophile, all of which were not observed
experimentally. One marked feature of all the transition states
calculated is that they are quite asynchronous. The energetic
parameters of these ten transition structures are given in Table
Frontier Molecular Orbital (FMO) theory.
culations on this dimerisation have not been reported before so
we decided to undertake this task at the ab initio level.† Fig. 1
3
. Six of the ten structures (45–50) were very similar in energy,
†
All calculations have been carried out with the G92/DFT program (1)
Ϫ1
being within 0.55 kcal mol of each other. The other four
structures (51–54) were also similar in energy, being within 2
kcal mol of each other and within 3 kcal mol of the former
group. We calculated the dimerisation of butadiene to assess
using the 3-21G basis set at the RHF level. Transition state and
ground state structures were fully optimised with analytical gradient
methods without symmetry constraints and characterised by analytical
frequency calculations.
Ϫ1
Ϫ1
J. Chem. Soc., Perkin Trans. 2, 1998
1501

View Article Online
Table 3 Calculated energetic parameters for transition states 19–28
a
a
a
Structure
Etotal
Erel
Ea
4
4
4
4
4
5
5
5
5
5
5
6
7
8
9
0
1
2
3
4
Ϫ758.832 750
Ϫ758.831 738
Ϫ758.831 683
Ϫ758.832 290
Ϫ758.832 647
Ϫ758.831 681
Ϫ758.831 104
Ϫ758.829 096
Ϫ758.829 510
Ϫ758.828 012
0.00
0.35
0.42
0.33
0.00
0.53
1.15
2.18
1.87
2.83
27.10
27.98
27.52
27.93
28.11
28.17
28.75
29.77
32.43
28.77
a
Etotal = total energy (in atomic units); E = energy relative to structure
rel
Ϫ1
4
5 (in kcal mol ); E = activation energy calculated relative to con-
a
Ϫ1
formation 44 (in kcal mol ); E and E include ZPE correction.
a
rel
a
Table 4 Frontier molecular orbital energies of monomers
Structure
HOMO
LUMO
s-cis butadiene
s-trans butadiene
Ϫ8.85
3.59
3.36
2.73
2.70
2.51
2.50
Ϫ8.85
Ϫ9.03
Ϫ9.04
Ϫ9.12
Ϫ9.14
4
4
4
4
3
4
1
2
a
Fig. 3 RHF/3-21G optimised transition structures of the dimeris-
ation of 1 having cisoid dienophile conformations (continued)
Energies are given in eV.
energy gap exists between the LUMO of 1 and the HOMO of
buta-1,3-diene the cross cycloadducts should have predomin-
ated. In fact, all of the results from our cross Diels–Alder
experiments cannot be reconciled with FMO arguments. The
highly polarised C1᎐C2 bond of 1 may make it an excellent
᎐
dienophile and contribute to the low activation barrier to
dimerisation, but it does not explain why its cross cycloaddition
1
with electron-rich dienes or dienophiles is not favoured over
dimerisation. Orbital coefficient matching should also favour
2
the cross-cycloadditions (e.g. Σc = 0.143, 0.222 and 0.253
respectively for the dimerisation of 1, its cross cycloaddition
with 5b, and its reaction with methyl vinyl ether).
Could an increased population of the cisoid conformation
be responsible for the higher reactivity of 2-MCBD? Studies
on the dimerisation of 2-cyanobuta-1,3-diene and its cyclo-
addition reactions with electron-deficient dienophiles indicate
27
that it has a comparable reactivity to 1. Yet, 2-cyanobuta-1,3-
diene does not experience a strong destabilisation of its s-trans
conformation as 2-MCBD does.‡ In addition, the famous
28
dimerisation of isomers 55 to Thiele’s ester 56 demonstrates
that of the three dienes in fast equilibrium, isomer 55c is a more
reactive diene than the other two (Scheme 5).§ All of these dienes
have a fixed cisoid conformation. Moreover, we have observed
that the sulfolene 2 extrudes SO in 3–4 h in refluxing toluene.
2
In comparison, the unsubstituted dihydrothiophene 1,1-
Fig. 4 RHF/3-21G optimised transition structures of the dimeris-
ation of 1 having transoid dienophile conformations
dioxide extrudes SO four times slower at 150 ЊC to give buta-
2
diene. Thus, the principle of microreversibility allows us to
deduce that s-cis-1 reacts faster than s-cis-butadiene in a con-
certed reaction with SO2. This cheletropic addition is very
much akin to the Diels–Alder reaction. All of this strongly sug-
gests that dienes like 1 owe their enophilicity to a factor other
than an enhanced population of the s-cis conformation.
the validity of our transition structures for 2-MCBD. Our
calculations gave transition geometries very close to those
calculated by Houk and the lowest calculated energy of acti-
20
Ϫ1
vation was over 10 kcal mol higher than the lowest calculated
energy of activation for the dimerisation of 2-MCBD, in line
The paralocalisation energy (P ) of dienes as described by
Ϫ1 7,25
1,4
with the experimental value of ca. 11 kcal mol .
Brown and the P1,2 energy of dienophiles represent more or less
The highest occupied molecular orbital (HOMO) of dienes
like 1 has been shown to be of lower energy than that of buta-
TM
1
,3-diene by approximately 0.2 eV, and the lowest unoccupied
‡ Molecular mechanics calculations using CSCChem 3D Pro soft-
Ϫ1
26
ware gives a 1.2, 1.3 and 2.3 kcal mol preference to the s-trans con-
formations of 2-cyanobuta-1,3-diene, isoprene and buta-1,3-diene,
respectively.
Isomers 55a and 55c are at low concentration, but according to the
Curtin–Hammett principle 55c must lead to the lowest-energy transi-
molecular orbital (LUMO) is lower by approximately 1 eV.
Our own calculations confirm these values (Table 4). When
compared to buta-1,3-diene, a stronger HOMO–LUMO inter-
action is expected for 2-MCBD’s dimerisation and this can be a
factor lowering the activation barrier. However, since a lower
§
tion structure.
1
502
J. Chem. Soc., Perkin Trans. 2, 1998

View Article Online
2
4,32,33
•
•
lated Diels–Alder transition structures.
Hehre and Salem
P1, 4
P1, 2
calculated a very short C2᎐C3 bond in their transition state for
the reaction between buta-1,3-diene with ethylene. They even
2
R
6
32
1
3
suggested that ‘the C1C2C3 fragment resembles its geometry in
product cyclohexene far more than that in reactant butadiene’.
Indeed, we propose that these changes in length are caused
by an early reorganisation of π-electrons. By contrast, sigma
bonds are still long at 2.0–2.4 Å for 1 and ca. 2.2 Å for buta-
diene. Bonds between C1᎐C2 and C3᎐C4 range between 1.35–
5
•
4
TS
•
Fig. 5 Paralocalisation energy and our proposed transition state for
the Diels–Alder cycloaddition
1
.38 Å for 1 (highly asynchronous) and ca. 1.36 Å for buta-
CO2Me
CO2Me
CO2Me
diene. Again, one will find close agreement between our values
and those calculated by others for several Diels–Alder sys-
24,32,33
tems.
On the basis of the above discussion, we propose
that the structure TS in Fig. 5 represents a more accurate
description of the transition structure for the concerted
cycloaddition than the usual one with π-electrons delocalised
over the whole structure. In our TS model, there is a large
double bond character between C2 and C3 which may provide
further stabilisation when conjugating groups (for example) are
attached to those carbons. This view of the transition structure
also finds support in the theoretical treatment of ethylene–
55a
55b
55c
Exclusive product
MeO2C
CO2Me
56
33
butadiene offered by Bach et al. In their treatment, they
Scheme 5
deduced, based on geometrical and electronic standpoints,
a transition state that is ‘σ-early’ and ‘π-late’ similar to that
described in this paper.
the energy needed to reorganise the π-bonds in the reaction
29,30
(
Fig. 5).
Energetically, it represents the portion of the over-
In addition to the cross-cycloaddition experiments described
above, other experimental observations lend support to our
views. For example, the unreactive 1-MCBD has a higher
paralocalisation energy and also a lower lying HOMO and
LUMO, both of which will depress the reaction rate. 2,3-Bis-
(methoxycarbonyl)buta-1,3-diene would be predicted to have a
lower P1,4 energy than 2-MCBD but also a lower lying HOMO
and LUMO. Each will affect the reaction rate in opposite
senses. Indeed, this diene dimerises at a slower rate than 1 and
reacts both with electron-rich and electron-deficient dieno-
all enthalpy of cycloaddition brought about by the cleavage and
formation of the π-bonds only. While it is certain that some of
this energy is spent on the way to the transition state for any
pair of diene/dienophile, historically paralocalisation energy
alone failed to predict the reactivity of most common dienes.
The FMO theory proved far superior in such predictions. Per-
haps this is why the paralocalisation energy factor was largely
ignored outside of Brown’s systems, although Kiselev and
Konovalov reported in 1989 that FMO and paralocalisation
energies taken together did explain the order of reactivity of
certain substituted anthracenes that did not follow the order
20
12
philes. The same could be said of diene 14. Diene 11 must
have a higher lying HOMO and a low P1,4 energy and indeed it
turns out to be highly reactive despite severe steric interactions
in its cisoid conformation. Compelling experimental evidence
31
predicted by the frontier orbital theory alone. During cyclo-
addition, diene 1 goes from a cross-conjugated 6π-electron to a
fully conjugated 4π-electron system and we calculated its para-
also comes from the known intense reactivity of o-quino-
Ϫ1
34,35
localisation energy to be 35.3 kcal mol . Butadiene, on the
dimethanes [eqn. (1)].
These dienes would develop aro-
other hand, goes from a conjugated 4π-electron to a non-
Ϫ1
conjugated 2π-electron array and must spend 40.5 kcal mol as
paralocalisation energy according to our calculations. The P1,4
(1)
energy and other factors mentioned above combine to give the
dimerisation of 1 the distinctly low barrier of activation of
Ϫ1 7
1
1.12 kcal mol . One could perhaps reach the same conclu-
maticity during the reaction if a transition state such as we
propose is operative. Opposite to that are aryls with their high
paralocalisation energy which are very unreactive dienes.
sion by calculating the heat of reaction for the dimerisation
of 1. However, the latter combines π- and σ-bond energies
together and is difficult to estimate qualitatively. Cyclopenta-
diene, in comparison, dimerises with an energy of activation
In conclusion, we submit that a lower paralocalisation energy
explains the difference between the observed and expected
reactivity of 1 and of its analogues. We propose that the transi-
tion structures of many Diels–Alder reactions must look like
that in Fig. 5 with an early reorganisation of π-electrons. We
have found support for this idea both in experiments and in
theory. Admittedly, it is difficult to quantify the actual contri-
bution of the P1,4 energy to the transition state energy at the
moment so the current analysis is largely qualitative. We are
currently studying the competitive cycloaddition of other simi-
larly activated dienes with restricted conformations in order
to better isolate and quantify this phenomenon. Nevertheless,
paralocalisation energies can be roughly assessed qualitatively
and taking it into consideration will help predict the reactivities
of starting dienes. A wider acceptance of this concept will lead,
we believe, to a better prediction of diene reactivity in general.
Ϫ1
of about 16 kcal mol while the dimerisation of butadiene
Ϫ1
25
requires ca. 22 kcal mol of activation energy.
Of course, the question of exactly how much localisation
energy is spent at the transition state level is not easily answered
at this stage, but the sooner the π-electrons reorganise the larger
the influence of P1,4 on the TS energy. In other words, if the π-
bonds have reorganised to a significant extent in the transition
state and greatly precede the formation of σ-bonds, then conju-
gating substituents (or others) may affect the energy of the TS
in ways opposite to predictions on the basis of FMO or other
theories. We were therefore looking for evidence of such an
early reorganisation. Our calculations revealed C5᎐C6 bond
lengths of 1.38–1.39 Å for the dienophiles in the transition state
for cycloadditions involving either butadiene or 1 (see Fig. 5 for
numbering). That is a ca. 32% increase from their starting
C5᎐C6 bond length of 1.32 Å. Similarly, C2᎐C3 bond lengths
are calculated at ca. 1.39 Å for both dimerisations or a decrease
of nearly 50% from their original length. Gratifyingly, similar
bond lengths are found in a large number (if not all) of calcu-
Experimental
Unless otherwise stated all reactions were run under an atmos-
phere of argon using sodium–benzophenone dried solvents,
J. Chem. Soc., Perkin Trans. 2, 1998
1503

View Article Online
with the exception of dichloromethane which was dried using
calcium hydride. Gas chromatography was conducted using a
Reaction of (Z)-1-bromo-2-methoxycarbonylbut-2-ene 3 with
2-trimethylsilyloxypenta-1,3-diene 5a
1
5 m, 25µ DB-1 capillary column connected to a FID detector
Bromide 3 (124 mg, 0.70 mmol) and diene 5a were treated as
described in method B. After a careful filtration on silica gel
eluting with ethyl acetate the dimer 4 and adduct 6a were
obtained in the indicated yield and ratio as determined by inte-
gration of appropriate signals in their proton NMR spectra
(Table 1). The adduct 6a was found to be composed of a 3:1
ratio of stereoisomers.
with electronic integration. Flash chromatography was per-
formed by using Kieselgel 60 (230–400 mesh, Merck) silica gel.
NMR spectra were recorded on a Bruker 250 or 360 MHz
instrument, while infrared and mass spectra were done on a
Perkin-Elmer paragon 1000 FT-IR and a Kratos Concept-H
double focusing mass spectrometer, respectively. J Values are
given in Hz.
Reaction of 3-methoxycarbonyl-3-sulfolene 2 with 1-methoxy-3-
trimethylsilyloxybuta-1,3-diene 5b
General procedure for the cross Diels–Alder cycloadditions of
diene 1 and electron-rich dienes
Sulfolene 2 (176 mg, 1.0 mmol) and diene 5b were treated as
described in method A. After column chromatography on silica
gel eluting with hexanes–ethyl acetate (3:1), the dimer 4 and
4-methoxycarbonyl-4-vinylcyclohex-2-enone 10 (the hydrolysis-
elimination product of 6b) were obtained in the indicated yield
and ratio (Table 1). Adduct 6b could be isolated if only a
short filtration on silica gel was performed and it was found to
be mostly one stereoisomer as determined by proton NMR
analysis. For 6b: δ (250 MHz, CDCl ) 5.66 (dd, 1H, J 10.2,
Method A. 3-Methoxycarbonyl-3-sulfolene 2 and the appro-
priate diene were heated to reflux in dry toluene for the
indicated length of time. In some experiments 2,6-di-tert-
butylphenol or hydroquinone (10 mol%) were added as radical
scavengers. Then the solvent was removed in vacuo and the
products were purified by column chromatography on silica gel
eluting with the indicated solvents.
Method B. (Z)-1-Bromo-2-methoxycarbonylbut-2-ene 3 and
H
3
the appropriate diene were dissolved in dichloromethane at
18.0), 5.16 (d, 1H, J 10.2), 5.14 (d, 1H, J 18.0), 5.11 (d, 1H,
J 5.9), 3.92 (d, 1H, J 5.9), 3.67 (s, 3H), 3.27 (s, 3H), 2.3–1.9 (m,
4H), 0.16 (s, 9H). For 10: δ (250 MHz, CDCl ) 6.95 (d, 1H,
2
5 ЊC and triethylamine (3 equiv. wrt 3) was slowly added. It
was stirred at that temperature for the indicated length of
time. Then the mixture was filtered through a thin pad of silica
gel eluting with ethyl acetate. The solvents were removed in
vacuo and the products were purified by chromatography as
indicated.
H
3
J 10.2), 6.05 (d, 1H, J 10.2), 5.94 (dd, 1H, J 10.7, 18.0), 5.27 (d,
1H, J 10.7), 5.13 (d, 1H, J 18.0), 3.72 (s, 3H), 2.5–2.0 (m, 4H);
Ϫ1
νmax(CHCl )/cm 1730 (s), 1680 (s); m/z (CI) 209 (10%,
3
M ϩ 29), 181 (100, M ϩ 1), 149 (13), 121 (10) (HRMS calc. for
C H O : 181.0865. Found: 181.0878).
10
12
3
Reaction of 3-methoxycarbonyl-3-sulfolene 2 with 2-trimethyl-
silyloxypenta-1,3-diene 5a
Sulfolene 2 (124 mg, 0.70 mmol) and diene 5a were treated as
described in method A for 4 h. Separation by column chrom-
atography on silica gel eluting with hexanes–ethyl acetate (5:1)
gave the dimer 4 and 4-methoxycarbonyl-3-methyl-1-trimethyl-
silyloxy-4-vinylcyclohexene 6a in the indicated yield and ratio
Reaction of (Z)-1-bromo-2-methoxycarbonylbut-2-ene 3 with
1-methoxy-3-trimethylsilyloxybuta-1,3-diene 5b
Bromide 3 (100 mg, 0.52 mmol) and diene 5b were treated as
described in method B. After a short filtration on silica gel
eluting with ethyl acetate, the dimer 4 and adduct 6b were
obtained in the indicated yield and ratio as determined by
proton NMR integration (Table 1). Product 6b was found to be
mostly one stereoisomer.
(
Table 1). Product 6a was found to be composed of a 2:1 ratio
of stereoisomers as determined by integration of proton NMR
signals. Adduct 6a was characterised only by proton NMR
because it was difficult to avoid partial hydrolysis of the silyl
enol ether. For characterisation purposes, it was completely
hydrolysed by treating it with 1 drop of conc. hydrochloric acid
in a suspension of silica gel in 4 ml of ethyl acetate for 1 h
followed by filtration on a pad of silica gel eluting with ethyl
acetate. After evaporation of the solvent the product was puri-
fied by column chromatography on silica gel, eluting with
hexanes–ethyl acetate (5:1) to yield 4-methoxycarbonyl-3-
methyl-4-vinylcyclohexanone 9 as a colourless oil (65%
yield). This product was also found to be a 2:1 mixture of
stereoisomers by integration of proton NMR signals. For the
major isomer of 6a: δ (250 MHz, CDCl ) 5.75 (dd, 1H, J 10.2,
Competition between 3-methoxycarbonyl-3-sulfolene 2 and
1-methoxy-3-trimethylsilyloxybuta-1,3-diene 5b for methyl
acrylate
3-Methoxycarbonyl-3-sulfolene 2 (100 mg, 0.57 mmol), diene
5b (98 mg, 0.57 mmol) and methyl acrylate (488 mg, 5.7 mmol)
were heated to reflux in 3 ml of toluene. The reflux was stopped
after 30 min, 1 h, 2 h, 3 h, 4 h and 5 h and a GC trace of the
reaction mixture was taken each time. After 4 h no more prod-
uct 7 could be observed forming. After 5 h product 8 had
formed in 97% yield. The solvents were removed in vacuo and
the product directly chromatographed on silica gel using
hexanes–ethyl acetate (3:1) as eluent to yield 91 mg (81%) of
1,4-bis(methoxycarbonyl)cyclohexene 7 and 73 mg (83%) of a
2.5:1 inseparable mixture of the known 4-methoxycarbonyl-
cyclohex-2-enone and 4-methoxycarbonylcyclohex-3-enone
(hydrolysis/elimination products of 8). Adduct 7 was found to
H
3
1
7.8), 5.13 (dd, 1H, J 0.5, 10.2), 5.12 (dd, 1H, J 0.5, 17.8), 4.81
br d, 1H, J 5.6), 3.61 (s, 3H), 2.5–1.6 (m, 5H), 0.88 (d, 3H,
J 6.5). For the minor isomer of 6a: δ (250 MHz, CDCl ) 5.78
(
H
3
(
dd, 1H, J 10.2, 17.0), 5.10 (dd, 1H, J 0.5, 10.2), 5.06 (dd, 1H,
J 0.5, 17.0), 4.85 (dt, 1H, J 0.7, 5.6), 3.68 (s, 3H), 2.5–1.6 (m,
H), 0.86 (d, 3H, J 6.5). For the major isomer of 9: δ (250
13
be a mixture of inseparable regioisomers by C NMR analysis.
1
5
Neither gas chromatography nor H NMR spectroscopy could
H
MHz, CDCl ) 5.94 (dd, 1H, J 10.8, 17.6), 5.32 (d, 1H, J 10.8),
be used to identify these two regioisomers. A trace (<5%) of the
dimer 4 accompanied adduct 7 and could be seen in both pro-
ton and carbon NMR spectra. No effort was made to separate
this mixture.
3
5
.28 (d, 1H, J 17.6), 3.64 (s, 3H), 2.5–2.0 (m, 7H), 0.92 (d, 3H,
J 7.5); δ (62.9 MHz, CDCl ) 210.5 (s), 173.8 (s), 139.0 (d),
C
3
1
16.9 (t), 52.0 (s), 51.2 (q), 44.6 (t), 38.5 (d), 37.3 (t), 27.5
Ϫ1
(
t), 16.6 (q); νmax(CHCl )/cm 1720 (s), 1630 (m); m/z (CI)
The reaction was repeated with sulfolene 2 and methyl
acrylate and then, separately, with diene 5b and methyl acrylate.
Each reaction was monitored by GC as above. The lengths of
the reactions were the same as for the above experiment. For 7:
δ (250 MHz, CDCl ) 6.92 (m, 1H), 3.69 (s, 3H), 3.66 (s, 3H),
3
2
1
1
5
37 (5%, M ϩ 41), 225 (15, M ϩ 29), 197 (100, M ϩ 1),
65 (5), 137 (10) (HRMS calc. for C H O : 197.1178. Found:
11
17
3
97.1145). For the minor isomer of 9: δ (250 MHz, CDCl )
H
3
.82 (dd, 1H, J 10.8, 17.6), 5.24 (dd, 1H, J 0.5, 10.8), 5.13
H
3
Ϫ1
(
dd, 1H, J 0.5, 17.6), 3.70 (s, 3H), 2.5–2.0 (m, 7H), 0.84 (d,
2.6–1.6 (m, 7H); νmax(CHCl )/cm 1730–1705 (br s), 1645 (s);
3
3
1
1
H, J 7.5); δ (62.9 MHz, CDCl ) 210.4 (s), 174.5 (s), 137.9 (d),
16.8 (t), 52.0 (s), 51.2 (q), 45.7 (t), 38.0 (t), 37.0 (d), 26.9 (t),
5.4 (9).
m/z (CI) 239 (5%, M ϩ 41), 227 (8, M ϩ 29), 199 (5, M ϩ 1),
C
3
167 (100), 139 (90) (HRMS calc. for C H O : 167.0708. Found:
9
11
3
167.0702). For the major isomer of 7: δ (75.5 MHz, CDCl )
C
3
1
504
J. Chem. Soc., Perkin Trans. 2, 1998

View Article Online
Ϫ1
1
75.4 (s), 167.3 (s), 137.4 (d), 129.7 (s), 51.8 (q), 51.5 (q), 38.18
νmax(CHCl )/cm 1730 (br s), 1650 (w), 1110 (s); m/z (EI) 316
3
ϩ
(
d), 27.8 (t), 24.8 (t), 23.4 (t). For the minor isomer of 7: δ (75.5
(8%, M ), 269 (43, ϪSMe), 158 (68), 140 (100) (HRMS calc. for
C
MHz, CDCl ) 175.3 (s), 167.2 (s), 138.8 (d), 128.6 (s), 51.7 (q),
C H O S : 316.0803. Found: 316.0764). For the minor isomer:
3
14 20
4 2
5
1.5 (q), 38.7 (d), 26.4 (t), 24.9 (t), 23.9 (t). For the major
δ (360 MHz, CDCl ) 6.87 (dm, 1H, J 5.4), 5.94 (d, 1H, J 10.5),
H 3
hydrolysis product of 8: δ (250 MHz, CDCl ) 6.95 (m, 1H),
5.15 (d, 1H, J 10.5), 3.99 (br d, 1H, J 5.2), 3.67 (s, 3H), 3.64 (s,
H
3
3
2
.72 (s, 3H), 3.00 (d, 2H, J 1.0), 2.75 (t, 2H, J 6.5), 2.5–2.4 (m,
3H), 2.4–1.7 (m, 4H), 2.17 (s, 3H), 2.05 (s, 3H); δ (75.5 MHz,
C
Ϫ1
H); νmax(CHCl )/cm 1725 (s), 1670 (br s); m/z (EI) 155 (100,
CDCl ) 172.8 (s), 166.8 (s), 135.2 (d), 130.9 (d), 130.1 (s), 126.4
3
3
M ϩ 1). For the minor hydrolysis product of 8: δ (250 MHz,
(d), 51.8 (q), 51.5 (q), 51.3 (s), 49.2 (d), 22.2 (t), 21.4 (t), 17.2 (q),
16.3 (q).
H
CDCl ) 7.05 (dd, 1H, J 1.0, 10.5), 6.04 (dd, 1H, J 1.0, 10.5), 3.72
3
(
s, 3H), 3.40 (m, 1H), 2.6–2.1 (m, 4H).
2
,3-Bis(methoxycarbonyl)-3-sulfolene 14
Competition between (Z)-1-bromo-2-methoxycarbonylbut-2-ene
and 1-methoxy-3-trimethylsilyloxybuta-1,3-diene 5b for methyl
acrylate
To a stirred solution of 3-methoxycarbonyl-3-sulfolene 2 (1.6 g,
9.08 mmol) in 200 ml of dry THF at Ϫ78 ЊC was added 10.1 ml
of Bu Li (1.8  in hexanes, 18.16 mmol). After stirring for 5
3
n
Bromide 3 (100 mg, 0.52 mmol), diene 5b (89 mg, 0.52 mmol)
and methyl acrylate (446 mg, 5.2 mmol) were dissolved in 3 ml
of dichloromethane. Triethylamine was slowly added and the
reaction was left to stir at 25 ЊC for 3 h. After filtration on a pad
of silica gel eluting with ethyl acetate, the solvents were
removed in vacuo and the product directly chromatographed on
silica gel using hexanes–ethyl acetate (3:1) as eluent to yield 42
mg (73%) of dimer 4, 8 mg (8%) of 7 and 7 mg (7%) of 8.
min at that temperature, 0.35 ml (4.54 mmol) of methyl chloro-
formate were slowly added and stirring was continued for 5
more min. The yellow solution turned bright orange after the
addition of the chloroformate. Quenched with 1  HCl at
Ϫ78 ЊC, the solution was then allowed to warm to room tem-
perature. The aqueous phase was separated and extracted with
diethyl ether, and the combined organic portions were washed
with brine, dried over MgSO , filtered and evaporated under
4
reduced pressure. Chromatography on silica gel using hexanes–
ethyl acetate (3:1 and then 1:1) furnished 684 mg of pure
product 14 and 445 mg of a 1:1 mixture of product and start-
ing material for a total yield of 85% based on the quantity of
methyl chloroformate; δ (250 MHz, CDCl ) 7.17 (dt, 1H, J 1.1,
3
-Methoxycarbonyl-2,5-dihydro-1-methylthiophenium tetra-
fluoroborate 9
Dimethoxycarbonium tetrafluoroborate (699 mg, 4.32 mmol)
was added to a solution of dihydrothiophene 2 (500 mg, 3.45
mmol) in 3 ml of dry dichloromethane. After stirring overnight
at 25 ЊC, 5 ml of ethyl acetate were added and the mixture was
stirred vigorously. When the stirring was stopped, the upper
layer of solvent was carefully removed from a yellowish oil
using a Pasteur pipette. Then fresh ethyl acetate (5 ml) was
added and the mixture stirred vigorously for 5 min. The upper
layer of solvent was removed and this process was repeated
three times. Then the oily residue was put under high vacuum
for 24 h leaving the slightly hygroscopic sulfonium salt 10 as a
white crystalline compound. The product could be used with-
out further purification in the next step. For characterisation
H
3
2.6), 4.80 (br s, 1H), 3.96–4.15 (m, 2H), 3.83 (s, 3H), 3.79 (s,
ϩ
3H); m/z (EI) 234 (23%, M ) (HRMS calc. for C H O S:
8
10
6
234.0198. Found: 234.0189).
1,2-Bis(methoxycarbonyl)buta-1,3-diene 15
2,3-Bis(methoxycarbonyl)-3-sulfolene 14 (50 mg, 0.21 mmol)
was stirred in 3 ml of dry toluene and heated to reflux for 12 h.
The solvent was then removed under reduced pressure and the
product directly chromatographed on silica gel using hexanes–
ethyl acetate (8:1 and then 3:1) to give 36 mg of isomer A and
17 mg of isomer B. Both were slightly volatile. The proton
NMR spectrum of the crude mixture, however, indicated a 1:1
mixture of the two volatile compounds. The less polar A was
identified as the (E)-isomer because some of it was recovered
after the mixture was reacted with maleic anhydride in a Diels–
Alder reaction. In addition, its signal at δ 6.35 must be that of a
vinyl proton cis to a carbonyl. The same signal for the (Z)-
purposes it could be recrystallised from hot ethyl acetate; mp
2
8
(
1
2–84 ЊC; δ (250 MHz, [ H ]DMSO) 6.93 (br s, 1H), 4.55–4.10
H
6
Ϫ1
m, 4H), 3.73 (s, 3H), 2.78 (s, 3H); νmax(KBr)/cm 1710 (br s),
650 (br s), 1050 (br s); m/z 159 (7%, M ), 158 (67), 143 (100)
ϩ
(
HRMS calc. for C H O S: 159.0480. Found: 159.0440).
7
11
2
3
-Methoxycarbonyl-1-methylthiobuta-1,3-diene 11
isomer is found at δ 5.86. Less polar (E)-isomer A: δ (250 MHz,
H
Triethylamine (617 mg, 6.09 mmol) was added to a suspension
of sulfonium salt 10 at 25 ЊC. It was stirred until the mixture
became homogeneous (15–30 min). The mixture was then fil-
tered through a pad of silica gel eluting with dichloromethane
taking care to keep the product in solution at all times. The
diene 11 thus obtained can be characterised by NMR analysis
with little dimerisation; δ (250 MHz, CDCl ) 6.50 (s, 1H), 6.33
CDCl ) 7.44 (ddd, 1H, J 1.4, 10.5, 16.9), 6.35 (br s, 1H), 5.89
3
(dm, 1H, J 16.9), 5.60 (dt, 1H, J 1.4, 10.5), 3.82 (s, 3H), 3.75 (s,
ϩ
3H); m/z (EI) 170 (35%, M ) (HRMS calc. for C H O :
8
10
4
170.0579. Found: 170.0584). More polar (Z)-isomer B: δ (250
H
MHz, CDCl ) 6.37 (dd, 1H, J 10.6, 17.5), 5.86 (br s, 1H), 5.54
3
(d, 1H, J 10.6), 5.49 (d, 1H, J 17.5), 3.88 (s, 3H), 3.71 (s, 3H).
H
3
(
s, 2H), 5.86 (s, 1H), 3.75 (s, 3H), 2.35 (s, 3H); δ (75.5 MHz,
Cycloadduct 16
C
CDCl ) 166.8 (s), 134.9 (s), 132.7 (d), 126.4 (d), 119.1 (t), 60.0
An equimolar mixture of (E)- and (Z)-2,3-bis(methoxycarb-
onyl)-3-sulfolene 14 (50 mg, 0.21 mmol) and maleic anhydride
3
(
q), 18.7 (q).
(
71 mg, 0.73 mmol) in 3 ml of dry toluene was stirred and
1
,4-Bis(methoxycarbonyl)-3-methylthio-4-{1-methylthio-
ethenyl}cyclohexene 12
-Methoxycarbonyl-1-methylthiobuta-1,3-diene 33 was stirred
heated to reflux for 12 h. The solvent was then removed under
reduced pressure but the product proved labile on silica chrom-
atography. The isomeric mixture was characterised as the crude
mixture. Both isomers: δ (250 MHz, CDCl ) 7.28 (dd, 1H,
J 2.7, 7.8), 7.2–7.3 (m, 1H), 4.64 (d, 1H, J 6.1), 4.52 (br s, 1H),
4.05 (dd, 9.9 Hz), 3.80 (s, 3H), 3.77 (s, 3H), 3.68 (s, 3H), 3.63 (s,
3H), 3.31–3.25 (m, 2H), 3.0–2.8 (m, 1H), 2.46 (ddd, 1H, J 0.5,
7.4, 17.2), 2.6–2.5 (m, 1H), 2.46 (ddd, 1H, J 3.0, 5.6, 17.2). One
signal from one isomer is missing, believed buried under the
ester peaks.
3
in a dichloromethane or deuterated chloroform solution for 3
days to yield 12 as a 4:1 mixture of α- and β-stereoisomers.
Alternatively, diene 11 was left in the neat form for 60–90 min,
after which time dimerisation was complete. The product was
then purified by column chromatography on silica gel eluting
H
3
with hexanes–ethyl acetate (3:1). For the major isomer: δ (360
MHz, CDCl ) 6.97 (dt, 1H, J 1.7, 5.2), 6.05 (d, 1H, J 10.5), 5.44
H
3
(
d, 1H, J 10.5), 3.99 (br d, 1H, J 5.2), 3.62 (s, 6H), 2.4–1.7 (m,
4
1
H), 2.18 (s, 3H), 2.12 (s, 3H); δ (75.5 MHz, CDCl ) 173.1 (s),
66.9 (s), 137.6 (d), 130.5 (d), 127.7 (s), 127.5 (d), 52.2 (q), 51.4
Cycloadduct 17
C
3
An equimolar mixture of (E)- and (Z)-2,3-bis(methoxy-
carbonyl)-3-sulfolene 14 (50 mg, 0.21 mmol) and 1-morpho-
(
q), 50.4 (s), 46.6 (d), 27.9 (t), 21.7 (t), 17.8 (q), 17.2 (q);
J. Chem. Soc., Perkin Trans. 2, 1998
1505

View Article Online
linocyclopentene (65 mg, 0.43 mmol) in 3 ml of dry toluene was
stirred and heated to reflux for 12 h. The solvent was then
removed under reduced pressure and the product directly
chromatographed on silica gel using hexanes–ethyl acetate (8:1
and then 3:1) as eluent to yield 17 mg of pure cycloadduct 17
and 31 mg of mostly compound 17 contaminated with some
morpholine adduct 18. Only 17 was fully characterised since
5 O. Goldberg and A. S. Dreiding, Helv. Chim. Acta, 1976, 59, 1904.
6
7
8
A. I. D. Alanine, C. W. G. Fishwick, A. D. Jones and M. B. Mitchell,
Tetrahedron Lett., 1989, 30, 5653.
M. E. Jung and C. N. Zimmerman, J. Am. Chem. Soc., 1991, 113,
7
813.
C. Spino and J. Crawford, Can. J. Chem., 1993, 71, 1094.
9 H.-J. Liu, T. K. Ngooi and E. N. C. Browne, Can. J. Chem., 1988, 66,
3143.
1
1
1
0 D. Martina and F. Brion, Tetrahedron Lett., 1982, 23, 865.
1 H. Düttmann and P. Weyerstahl, Chem. Ber., 1979, 112, 3480.
2 B. Tarnchompoo, C. Thebtaranonth and Y. Thebtaranonth,
Tetrahedron Lett., 1987, 28, 6671.
3 J. F. Honek, M. L. Mancini and B. Belleau, Synth. Commun., 1984,
14, 483.
14 M. Franck-Neumann, D. Martina and M.-P. Heitz, J. Organomet.
Chem., 1986, 301, 61.
15 T.-S. Chou and S.-C. Hung, J. Org. Chem., 1988, 53, 3020.
1
8 eliminates the morpholine moiety upon further chromato-
graphy. δ (360 MHz, CDCl ) 7.59 (br s, 1H), 3.87 (s, 3H), 3.85
H
3
(
s, 3H), 3.06 (m, 1H), 2.93 (dt, 4H, J 7.3, 15.0), 2.08 (quin, 2H,
J 7.3), 2.1–1.5 (m, 2H) [the last 2 protons are among a small
amount of inseparable impurity and it is difficult to assign
them]; δ (75.7 MHz, CDCl ) 169.6 (s), 167.4 (s), 148.1 (s), 143.9
1
C
3
(
2
s), 142.9 (s), 125.0 (d), 52.3 (q), 52.3 (q), 31.73 (t), 31.70 (t),
Ϫ1
Ϫ1
5.5 (t), 24.6 (t); νmax(Cϩ HCl )/cm 1725 (br s), 1655 (s) cm ;
3
1
6 M. Franck-Neumann, D. Martina and F. Brion, Angew. Chem., Int.
Ed. Engl., 1981, 20, 864.
m/z (EI) 236 (60%, M ) (HRMS calc. for C H O : 236.1049.
13
16
4
Found: 236.1046).
1
1
7 R. F. Borch, J. Org. Chem., 1969, 34, 627.
8 R. L. Crumbie and D. D. Ridley, Aust. J. Chem., 1981, 34, 1017.
Adduct from 1-methoxycarbonylbuta-1,3-diene and maleic
anhydride
19 T.-S. Chou, C.-Y. Tsai and L.-J. Huang, J. Org. Chem., 1990, 55,
5
410.
2
2
0 I. Fleming, Frontier Orbitals and Organic Chemical Reactions, Wiley
Interscience, Chichester, 1976.
1 T. A. Nguyên, Orbitales frontières, Manuel pratique, Interéditions,
CNRS éditions, Paris, 1995.
1
-Methoxycarbonylbuta-1,3-diene (100 mg, 0.89 mmol) was
mixed with maleic anhydride (262 mg, 2.67 mmol) and stirred at
room temperature for several days. The reaction was followed
by GC and after 15 h less than 50% product was formed (this
is approximate since no standard was used). After several
days, some starting diene still remained. The reaction could be
pushed to completion by refluxing in toluene for 15 h and
evaporating the solvent under reduced pressure. The resulting
product was not amenable to chromatography and was only
characterised by proton NMR analysis of the crude mixture;
δ (250 MHz, CDCl ) 6.2–6.1 (m, 2H), 3.65 (s, 3H), 3.65–3.50
22 E. L. Eliel and S. H. Wilen, Stereochemistry of Organic Compounds,
Wiley, New York, 1994.
3 E. Juaristi, Stereochemistry & Conformational Analysis, Wiley Inter-
science, New York, 1991.
4 K. N. Houk, Y. Li and J. D. Evanseck, Angew. Chem., Int. Ed. Engl.,
2
2
2
1
992, 31, 682.
5 Y. Li and K. N. Houk, J. Am. Chem. Soc., 1993, 115, 7478.
26 K. N. Houk, J. Am. Chem. Soc., 1973, 95, 4092.
27 P. G. Baraldi, A. Barco, S. Benetti, S. Manfredini, G. P. Pollini,
D. Simoni and V. Zanirato, Tetrahedron, 1988, 44, 6451.
H
3
(
m, 2H), 3.5–3.35 (m, 1H), 2.5–2.4 (m, 2H).
2
2
3
8 G. L. Dunn and J. K. Donohue, Tetrahedron Lett., 1968, 3485.
9 R. D. Brown, J. Chem. Soc., 1950, 2, 691.
0 Streitwieser, Molecular Orbital Theory for Organic Chemists, Wiley,
New York, 1961.
Acknowledgements
We thank the Natural Sciences and Engineering Council of
Canada, the University of Sherbrooke and the University of
Waterloo for the financial support of this work, as well as the
University of Victoria for a scholarship to J. C.
31 V. D. Kiselev and A. I. Konovaloc, Russ. Chem. Rev., 1988, 58, 230.
32 R. E. Townsend, G. Ramunni, L. E. Overmann, W. J. Hehre and
L. Salem, J. Am. Chem. Soc., 1976, 98, 2190.
3
3
3
3 R. D. Bach, J. J. W. McDouall and H. B. Schlegel, J. Org. Chem.,
1
989, 54, 2931.
4 W. Carruthers, Cycloaddition Reactions in Organic Synthesis, ed.
J. E. Baldwin and P. D. Magnus, Pergamon Press, Oxford, 1991.
5 F. Fringuelli and A. Taticchi, Dienes in the Diels–Alder Reaction,
Wiley, New York, 1990.
References
1
2
J. M. McIntosh and R. A. Sieler, J. Org. Chem., 1978, 43, 4431.
H. M. R. Hoffmann and J. Rabe, Angew. Chem., Int. Ed. Engl., 1983,
2
2, 795.
3
4
W. Poly, D. Schomburg and H. M. R. Hoffmann, J. Org. Chem.,
Paper 7/08199E
Received 12th November 1997
Accepted 2nd April 1998
1
988, 53, 3701.
L. K. Sydnes, L. Skattebøl, C. B. Chapleo, D. G. Leppard, K. L.
Svanholt and A. S. Dreiding, Helv. Chim. Acta, 1975, 58, 2061.
1
506
J. Chem. Soc., Perkin Trans. 2, 1998