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
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1921