Table 3 Comparison of the substituent effects (in kJ molϪ1) of the first
(X) and second (Y) ortho-substituents on the enthalpy of isomerization
of allyl aryl ethers to (Z)-prop-1-enyl aryl ethers in DMSO solution
the parent compounds 1 by 1.4 kJ molϪ1. Placing two addi-
tional Me groups at the meta-positions of the 2-Me derivatives,
37 leads to a change of ca. Ϫ0.3 kJ molϪ1 in ∆G –– – (see
᭺
m
50a→50b); this agrees with the corresponding changes in
First ortho-
substituent
Second ortho-
substituent
∆G –– – on replacement of the meta-hydrogens of the parent
᭺
m
Difference
compounds 1a and 1b by Me or But groups (cf. reactions
X
SE(X)
Y
SE(Y)
SE(Y) Ϫ SE(X)
14a→14b and 15a→15b with 1a→1b).
4-Me- and 4-But-substituents have only negligible effects on
the thermodynamics of isomerization of the parent com-
pounds 1. However, for the o-But-substituted derivatives the
4-Me- and 4-But-substituents lead to a slight increase of ca.
Ϫ0.5 kJ molϪ1 in the negative character of both the Gibbs
energy and enthalpy of isomerization, without any detectable
effect on the entropy of isomerization. A 2,3-fusion of a
6-membered saturated carbocyclic ring to the parent com-
Me
Me
Pri
1.8
1.8
Ϫ0.7
Ϫ1.1
2.1
4.5
3.8
Me
But
Pri
Me
MeO
F
Ϫ5.2
Ϫ7.4
Ϫ2.3
Ϫ4.5
Ϫ3.7
Ϫ0.6
Ϫ5.3
Ϫ7
Ϫ9
Ϫ2
Ϫ3
Ϫ6
Ϫ5
Ϫ9
But
MeO
F
Cl
Cl
pounds, leading to 51, gives rise to a value of ∆G –– – of
᭺
m
isomerization comparable to that of the 2-Me derivatives 37.
compounds (63–70) are characterized by markedly negative
–– –
–– –
᭺
–– –
᭺
᭺
On the other hand, the values of ∆H
and ∆S
for
reaction enthalpies. The most exothermic reaction, ∆H
=
m
m
m
51a→51b are more similar to those of the 2-Et derivatives 38
suggesting, not unexpectedly, that the steric effects of the
2-Et group in 38 are comparable to those of the fused carbo-
cyclic ring in the tetralin moieties of 51a and 51b.
Ϫ22.7 kJ molϪ1, was found for the 2-But-6-Me derivatives 67.
Similarly, the other 2,6-dialkyl-substituted compounds, as well
as the 2,6-di-MeO (68) and 2,6-di-Cl (70) derivatives, also have
high negative reaction enthalpies of Ϫ19 to Ϫ21 kJ molϪ1. Thus
the low reaction enthalpy, Ϫ13.2 0.3 kJ molϪ1, for the 2,6-di-
F derivatives is clearly exceptional in this group of reactions,
being ca. 6 kJ molϪ1 less negative than that for the 2,6-di-Cl
derivatives.
The relatively high exothermic character of these isomer-
ization reactions shows, not unexpectedly, that ortho-
substituent effects are not additive. Introduction of two
ortho-substituents to 1a and 1b has a marked destabilizing
effect on both isomers, but the destabilization is seen to be
larger for the allyl aryl ethers. This is reasonable, because the
loss of stabilization arising from a marked reduction of the
strength of p–π conjugation in the ArO moiety of these non-
planar compounds is greater in the allylic ethers. In the
absence of two ortho-substituents the allylic ethers may
Introduction of a second MeO substituent to the 3-position
of the 2-MeO derivatives 43 decreases the reaction entropy
from ca. 6.2 to 0.1 J KϪ1 molϪ1 (reaction 54a→54b). Simul-
taneously, a 1.7 kJ molϪ1 enhancement in the exothermic char-
acter of the reaction is observed so that the Gibbs energy of
isomerization remains essentially unaffected. In both isomers
of 54 the 2-MeO group is likely to have an almost orthogonal
orientation with respect to the plane of the aromatic system (cf.
the orientation of the 2-MeO group in 1,2,3-trimethoxy-
benzene6). It is surprising to find that the effect of the 3-MeO
group in 54 is to decrease the reaction entropy by ca. 6 J KϪ1
molϪ1, whereas the effect of the 4-MeO group in the 3,4,5-tri-
MeO derivatives 20 is to increase the reaction entropy by
approximately the same amount. (The structural feature com-
mon to 20 and 54 is the presence of a perpendicular MeO group
between two other ethereal moieties.)
Expectedly, introduction of an additional F atom to the
4-position of the 2-F derivatives 46 has only a marginal effect
on the thermodynamics of isomerization, see reaction
56a→56b. However, the 2,3-di-F substitution in 55, relative to
the 2-F substitution in 46, decreases the exothermic character
of the reaction by ca. 1.6 kJ molϪ1, whereas the reaction
entropy remains unchanged. The reaction enthalpy of Ϫ11.0 kJ
molϪ1 for the 2,3-di-F derivatives is the least negative one
among the reactions studied in this work. For these com-
pounds, the reaction entropies are generally more positive
than those for the other reactions studied. The relatively mild
exothermic character of these reactions, together with the
markedly positive reaction entropies, suggest that the allylic
isomers of these fluorine-containing compounds are more
strongly stabilized by solvation with the DMSO molecules
than the other allyl aryl ethers.
assume planar (or nearly planar) structures of the C ᎐C –O–
᎐
ar
ar
C moiety, with no steric hindrance for efficient p–π conju-
gation. On the other hand, the loss of conjugative stabilization
in the ArO moiety of the b isomers may be compensated for
by an increased conjugative interaction of the O atom with
the olefinic C᎐C bond.
᎐
The non-additivity of ortho-substituent effects on the
thermodynamic data of isomerization is illustrated quanti-
tatively in Table 3. The substituent effect on reaction enthalpy
on introduction of the first ortho-substituent X to the parent
compounds 1 is given by the term SE(X) in Table 3; this term
is generally positive, excluding the cases X = Pri and X = But
for which it is slightly negative. On introduction of the second
ortho-substituent Y, invariably negative substituent effects
SE(Y) are found. The highest of these, Ϫ7.4 kJ molϪ1, applies
to the case (Y = But, X = Me), the lowest (Ϫ0.6 kJ molϪ1) to
Y = X = F. The values of SE(Y) for Y = Me are remarkably
similar, ca. Ϫ5 kJ molϪ1, for both X = Me and X = But. On
the other hand, although there is no significant difference in
the values of the term SE(X) between X = F and X = Cl, the
value of SE(Y) is ca. 5 kJ molϪ1 more negative for Y = X = Cl
than for Y = X = F. This is likely to arise from the bulkiness
of the Cl atom which forces the allyloxy group of the 2,6-di-
Cl derivative 70a to assume a more nonplanar conformation
about the Car–O bond than that in the respective 2,6-di-F
derivative 69a.
If an additional halogen atom is introduced to the 2-Cl
derivatives, the reaction entropy remains essentially unchanged
whereas the reaction becomes 2–3 kJ molϪ1 less exothermic. In
the 2,4-dichloro derivatives (60), the effect of ϩ1.6 kJ molϪ1 of
the 4-Cl substituent on the Gibbs energy of isomerization is
comparable to that, ϩ1.4 kJ molϪ1, of the Cl atom in the 4-Cl-
substituted derivatives of the parent compounds 1a and 1b.
However, the corresponding effect of ϩ1.0 kJ molϪ1 of the 4-F
atom in the 2-Cl-4-F derivatives (62) is unexpectedly large in
comparison with the substituent effect of only ϩ0.2 kJ molϪ1 of
the 4-F atom in 10.
The difference, SE(Y) Ϫ SE(X), between the substituent
effects of the second and first ortho-substituents is shown in the
last column of Table 3. Among the symmetrically 2,6-
disubstituted compounds, the difference is largest (Ϫ9 kJ
molϪ1) for X = Y = Cl and, surprisingly, smallest (Ϫ2 kJ molϪ1
)
Compounds with two ortho-substituents
for X = Y = Pri. Intermediate values of Ϫ5 to Ϫ7 kJ molϪ1 of
the difference SE(Y) Ϫ SE(X) are found for the F-, MeO- and
Me-substituents.
Excluding the a→b reaction of the 2,6-difluoro derivatives 69,
the other reactions in this group of ortho,orthoЈ-disubstituted
J. Chem. Soc., Perkin Trans. 2, 2001, 1824–1834
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