Solvent effect in pericyclic reactions, XI. - The electrocyclic reactions

Article abstract of DOI:10.1002/(sici)1099-0690(199908)1999:8<1921::aid-ejoc1921>3.0.co;2-n

The rate constants of the electrocyclic ring closure of (1Z,3Z,5E)- 1,2,6-triphenylhexa-1,3,5-triene (1) and of the ring opening of dimethyl 3,4- dimethyl-1,2-diphenylcyclobutene-cis-3,4-dicarboxylate (3) were determined in 15 and 16 solvents, respectively. The ring closure of 1 shows, in spite of theoretical predictions, neither solvent nor salt effects. For the ring opening of 3, small solvent and salt effects were found, suggesting the possibility of observing small acceleration due to specific solute-solvent or salt interactions.

Full text of DOI:10.1002/(sici)1099-0690(199908)1999:8<1921::aid-ejoc1921>3.0.co;2-n

FULL PAPER
Solvent Effect in Pericyclic Reactions, XI[°]
The Electrocyclic Reactions
Giovanni Desimoni,[a] Giuseppe Faita,*[a] Sergio Guidetti,[a] and Pier Paolo Righetti[a]
Keywords: Cyclizations / Electrocyclic reactions / Solvent effects / Salt effect
The rate constants of the electrocyclic ring closure of
(1Z,3Z,5E)-1,2,6-triphenylhexa-1,3,5-triene (1) and of the
ring opening of dimethyl 3,4-dimethyl-1,2-diphenylcyclo-
butene-cis-3,4-dicarboxylate (3) were determined in 15 and
of theoretical predictions, neither solvent nor salt effects. For
the ring opening of 3, small solvent and salt effects were
found, suggesting the possibility of observing small
acceleration due to specific solute–solvent or salt inter-
16 solvents, respectively. The ring closure of 1 shows, in spite actions.
Introduction
Results
Two different reactions were chosen as models of the
Reactions involving activation complexes isopolar with
the reagents, such as pericyclic processes, are usually ex-
pected, on the basis of the HughesϪIngold model, to exhi-
bit negligible solvent effects.[1] Nevertheless, it has been
found that significant solvent dependencies can be observed
when specific soluteϪsolvent interactions are involved. In
particular, DielsϪAlder (DA) cycloadditions[2][3] and ene
reactions[4] are usually characterized by specific solvent ef-
fects and the reaction rates increase (or decrease) with the
increase in the solvent acidity (or basicity). 1,3-Dipolar
cycloadditions,[5] Claisen,[6][7] retro Claisen,[8] diaza Claisen
rearrangements,[9] as well as cheletropic reactions[10] belong
to a second group of pericyclic processes characterised by
generally small solvent effects which are functions of sol-
vent polarity.
above processes. The first was the 1,6-electrocyclization of
(1Z,3Z,5E)-1,2,6-triphenylhexa-1,3,5-triene (1) to cis-1,5,6-
triphenylcyclohexa-1,3-diene (2)[16] (Equation 1). The sec-
ond reaction was the ring opening of dimethyl 3,4-dimethyl-
1,2-diphenylcyclobutene-cis-3,4-dicarboxylate (3) to di-
methyl (2E,4Z)-2,5-dimethyl-3,4-diphenylmuconate (4)
(Equation 2).[17] The former electrocyclic process involves a
totally aliphatic system, the latter a substrate characterised
by the presence of carbonyl substituents able to show polar
and/or specific interactions in determing the solvent effect.
Even though electrocyclic reactions are important both
from a theoretical[11] and a synthetic point of view,[12] there
have been few detailed investigations of the influence of the
solvent on reactivity; only two studies on the effect of a
limited number of solvents on two different pericyclic pro-
cesses have been reported in the literature. The conrotatory
cyclization of all-cis-2,4,6,8-tetraene to trans-7,8-dimeth-
ylcycloocta-1,3,5-triene has been investigated in five sol-
vents of different polarity and a negligible increase of the
rate was observed (krel ഠ 1.2).[13] The effect of the solvent
on the ring opening of 2-methyl-4,4-diphenylcyclobutenone
has also been determined in seven different solvents, and
again it was found to be small (krel ഠ 3.0).[14]
Since the experimental investigation of small solvent ef-
fects in pericyclic reactions would provide an interesting
comparison with results derived from ab initio calcu-
lations,[10][15] the rate of a six-electronϪsix-atom ring clos-
ure and of a four-electronϪfour-atom ring opening was de-
termined for a large set of solvents.
The kinetic determinations, run at 80°C in 15 solvents,
were performed by UV/Vis-spectroscopic analysis of the
1
disappearing chromophore or by H-NMR analysis of the
reaction mixture. The reactions were monitored to 90Ϫ95%
completion (see Experimental Section for details). The first-
order rate constants, expressed as the average of at least
three independent kinetic runs, are reported in Table 1.
Both electrocyclic processes are influenced very little by
the solvent, but a significant difference between the two re-
actions was observed. The correlation of the kinetic data of
the ring closure of 1 with different solvent parameters gave
very poor results,[18] and, in our opinion, when a reaction
run in 15 solvents has a k1 average of 1.27 sϪ1 with a stand-
ard deviation of ±0.24, any attempt to find relationships
[°] Part X: Ref.[10]
[a]
`
Dipartimento di Chimica Organica, Universita di Pavia,
V.le Taramelli 10, I-27100 Pavia, Italy
E-mail: faita@chifis.unipv.it
Eur. J. Org. Chem. 1999, 1921Ϫ1924
WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1999
1434Ϫ193X/99/0808Ϫ1921 $ 17.50ϩ.50/0
1921

G. Desimoni, G. Faita, S. Guidetti, P. P. Righetti
FULL PAPER
Table 1. Rate constants of the electrocyclic ring closure of 1 at 80°C within the solvent, while the ring opening of 3 shows the
and of the electrocyclic ring opening of 3 at 70°C in different sol-
lowest activation enthalpy in methanol, the solvent with the
vents
highest HBD ability. In both cases, the values of ∆H϶ and
∆S϶ are in substantial agreement with the values reported
in the literature for hexatriene cyclizations and cyclobutene
ring opening.[20]
Solvent
104 ϫ k1(1) [sϪ1
]
106 ϫ k1(3) [sϪ1
]
Cyclohexane
1.44 ± 0.14
3.57 ± 0.04
3.50 ± 0.20
3.78 ± 0.05
3.80 ± 0.05
Ϫ
4.97 ± 0.04
3.76 ± 0.08
3.20 ± 0.05
3.42 ± 0.04
3.20 ± 0.08
3.30 ± 0.05
Ϫ
3.51 ± 0.08
3.60 ± 0.15
6.20 ± 0.05
7.70 ± 0.08
5.90 ± 0.10
6.30 ± 0.10
Carbon tetrachloride 1.09 ± 0.05
Benzene
1.27 ± 0.07
1.58 ± 0.05
1.07 ± 0.07
0.88 ± 0.02
Ϫ
1,4-Dioxane
Chlorobenzene
Chlorofom
Pyridine
Discussion and Conclusion
The very small solvent effect for the electrocyclic ring
closure of 1 and the absence of any significant correlation
between kinetic data and solvent polarity, suggested no
change of the polarity during the activation process for the
reaction of 1. This was confirmed by the computation
(AM1 method) of the dipole moments of 1 and 2. The cal-
culated values of µ1 and µ2 are 0.18 and 0.12 D, respectively,
and therefore the resulting ∆µ(1Ǟ2), Ϫ0.06 D, is very close
to zero.
1,2-Dichloroethane
Nitrobenzene
Acetone
DMF
tert-Butyl alcohol
DMSO
0.95 ± 0.13
Ϫ
1.20 ± 0.20
1.17 ± 0.07
1.42 ± 0.12
1.08 ± 0.08
1.58 ± 0.08
1.40 ± 0.10
Ϫ
Acetonitrile
2-Propanol
Acetic acid
Ethanol
1.26 ± 0.09
1.69 ± 0.06
Methanol
In a recent paper[21] large accelerating effects by cationic
lithium, sodium, and magnesium catalysis were predicted
by computation for the cyclization of 1,3,5-hexatriene, with
the cation strongly interacting with the π-electron cloud in
the transition state. The electrocyclic conversion of 1 to 2 is
therefore the appropriate model for an experimental test of
calculations, even if the absence of correlation between rate
constants and acidic character of the solvent do not sup-
port the expectation of a large cationic catalysis.[22Ϫ25]
On the other hand, the small solvent effect observed in
the ring opening of 3 which is dependent on the HBD sol-
vent character, suggested testing the rate-increasing proper-
ties of inorganic cations on this reaction, too.
with solvent parameters is meaningless and the reaction can
only be classified as one with no solvent effect.
The reaction of 3 was also very little influenced by the
solvent, but some statistically significant correlations with
solvent parameters were observed. Although a specific sol-
uteϪsolvent interaction shows only a small solvent effect,
the Hydrogen Bond Donor (HBD) ability of a solvent may
be observed to have a clear influence, since faster reactions
occur in alcohols and acetic acid. When the kinetic data of
the electrocyclic ring opening of 3 are plotted versus a typi-
cal HBD parameter as α,[19] the linear regression in Figure
1 is obtained.
The rate constants of the reactions of 1 and 3 were deter-
mined at 70°C in 0.5  acetonic solutions of lithium (LP)
and magnesium perchlorate (MP); the results are collected
in Table 3. Taking the rates in acetone as the references, no
increase of the rate was observed when the reaction was
performed in the presence of inorganic perchlorates. The
lithium and magnesium cations do not, therefore, have a
catalytic effect.
The use of 1 equiv. of MP in noncoordinating solvents
such as dichloromethane has recently been found to give
higher accelerations than those observed in MP/acetone or
LP/acetone solutions,[26] even if, given the intrinsic insolu-
bility of the inorganic salt in dichloromethane, the reacting
substrates must behave as a ligand to obtain homogeneous
solutions. Since substrate 1 does not dissolve MP, the effect
of one equiv. of MP was tested in dichloromethane in the
presence of 15 equiv. of dimethyl carbonate (DMC) as
auxiliary ligand. Whereas the data reported in Table 3 do
not support the theoretical predictions since the ring clo-
sure of 1 is not catalyzed by MP, the rate of the ring open-
ing of 3 was increased by a factor of 4, a value that is about
Figure 1. Rate constants of the electocyclic ring opening of 3 at
70°C plotted against the α parameter of the solvent
Activation Parameters
The activation parameters of the electrocyclic reactions two times higher than the solvent effect.
of 1 and 3 were determined in acetone, benzene, DMSO,
In conclusion the electrocyclic reactions investigated in
and methanol by running the reaction at four different tem- this paper can be regarded as pericyclic processes with no
peratures; the results are collected in Table 2. The activation solvent and no cationic catalysis effect in the case of totally
parameters of the electrocyclization of 1 are nearly constant aliphatic substrates. However, in the presence of polar
1922
Eur. J. Org. Chem. 1999, 1921Ϫ1924

Solvent Effect in Pericyclic Reactions, XI
FULL PAPER
Table 2. Activation parameters of the electrocyclic reactions of 1 and 3 in different solvents
1 Ǟ 2
3 Ǟ 4
Solvent
∆H϶ [kcal molϪ1
]
∆S϶ [cal KϪ1 molϪ1
]
∆H϶ [kcal molϪ1
]
∆S϶ [cal KϪ1 molϪ1
]
Benzene
Acetone
DMSO
22.7 ± 0.5
22.0 ± 1.0
22.0 ± 1.0
22.0 ± 1.0
Ϫ12.1 ± 1.4
Ϫ15.0 ± 1.5
Ϫ15.0 ± 1.5
Ϫ14.0 ± 2.0
27.4 ± 0.2
28.1 ± 0.5
27.5 ± 0.3
26.8 ± 0.3
Ϫ3.5 ± 0.5
Ϫ1.8 ± 1
Ϫ3.5 ± 0.8
Ϫ4.4 ± 0.8
Methanol
flask was then filled with the solvent. Seven samples were prepared
for each run. At t ϭ 0, the samples were placed in a thermostat at
the required temperature, and the initial absorbance of the solution
was determined on a further sample. At appropriate time intervals
the reaction was quenched and the residual absorbance of the chro-
mophore at 360Ϫ380 nm (for 1) or 310Ϫ330 nm (for 3) was deter-
mined. The activation parameters reported in Table 2 were ob-
tained from the rate constants reported in Tables 4 and 5.
Table 3. Rate constants of the reaction of 1 [k(1)] and 3 [k(3)] at
70°C in acetone, in acetonic solution of lithium perchlorate (LPAC)
and magnesium perchlorate (MPAC) (0.5 ), and in dichloro-
methane/diemethyl carbonate (DMC) solutions
Solvent
105 ϫ k (1) [sϪ1
]
krel
106 ϫ k (3) [sϪ1
]
krel
Acetone
4.8
5.2
5.0
1
3.2
3.8
3.5
1
LPAC 0.5 
MPAC 0.5 
1.08
1.04
1.19
1.09
3.96[b]
MP (1 equiv.)/ 2.8
CH2Cl2
0.53[a] 19.0
Table 4. Rates and activation parameters of the electrocyclic reac-
tion of 1 in different solvents
[a]
T [°C]
105 ϫ k [sϪ1
]
Calculated by taking as reference the rate in pure dichloro-
methane at 70°C (5.3 ϫ 10Ϫ5
reference the rate in pure dichloromethane at 70°C (4.8 ϫ 10Ϫ6
Ϫ1).
s
Ϫ1). Ϫ
Calculated by taking as
[b]
Benzene
Acetone
DMSO
Methanol
s
60
70
80
90
2.03 ± 0.06
5.04 ± 0.12
12.7 ± 0.7
39.7 ± 0.3
1.8 ± 0.1
4.8 ± 0.1
12.0 ± 0.2
30.8 ± 0.2
2.3 ± 0.1
6.1 ± 0.1
10.8 ± 0.8
40.1 ± 0.6
2.9 ± 0.3
6.6 ± 0.4
16.9 ± 0.6
49.4 ± 0.2
methoxycarbonyl substituents with donor properties small,
but nevertheless significant, contributions both from the
acidic solvent character and the inorganic cations acting as
Lewis acid are observed.
Table 5. Rates and activation parameters of the electrocyclic reac-
tion of 3 in different solvents
T [°C]
106 ϫ k [sϪ1
]
Benzene
Acetone
DMSO
Methanol
Experimental Section
Materials: (1Z,3Z,5E)-1,2,6-Triphenylhexa-1,3,5-triene (1) was pre-
pared as described in the literature; m.p. 81Ϫ82°C (ref.[16]
78Ϫ80°C). Dimethyl 3,4-dimethyl-1,2-diphenylcyclobutene-cis-3,4-
dicarboxylate (3) was prepared as described in the literature; m.p.
136Ϫ137°C (ref.[17] 136Ϫ138°C). The solvents for the kinetic runs
were deuterated grade (% D у 99%) reagents (for the 1H-NMR
kinetic determinations) or distilled anhydrous UV/Vis-spectro-
scopic grade reagents. The metal perchlorates were A. C. S. grade
reagents; lithium perchlorate (LP) was dried under vacuum at
140°C for 8 h. Caution: All perchlorates are potential explosives
and must be handled with care.[27]
60
70
80
90
1.15 ± 0.01 0.99 ± 0.02 1.06 ± 0.01 2.02 ± 0.06
3.78 ± 0.05 3.20 ± 0.08 3.51 ± 0.08 6.3 ± 0.1
12.8 ± 0.1 11.5 ± 0.1 11.5 ± 0.2 20.8 ± 0.2
37.9 ± 0.2 35.6 ± 0.7 35.7 ± 0.3 61.4 ± 0.3
Acknowledgments
Thanks are due to Prof. Rolf Huisgen for helpful discussions and
suggestions. Financial support for this work was provided by the
1H-NMR Kinetic Determinations: The overall reaction rates were
measured by monitoring the change of the H-NMR spectrum of
`
Ministero dellЈUniversita e della Ricerca Scientifica e Tecnologica
1
(MURST), by the Consiglio Nazionale delle Ricerche (CNR) “Pro-
getto Strategico Tecnologie Chimiche Innovative”, and by the Uni-
versity of Pavia.
the reaction mixture recorded with a Bruker AC 300 spectrometer.
About 10 mg of 3, dissolved in the required deuterated solvent
(about 0.5 mL), was heated in a sealed NMR tube in an oil ultra-
thermostat at 70.0 ± 0.1°C. At appropriate time intervals (from
400 to 800 min) the tube was cooled at room temperature and the
1H-NMR spectrum was recorded. The relative concentrations of 3
and 4 were determined from the integral ratios of their respective
methyl and methoxy signals.
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[O99048]
1924
Eur. J. Org. Chem. 1999, 1921Ϫ1924