Studies of the electron-promoted cope cyclization of 2,5-phenyl-substituted 1,5-hexadiene radical anions

Article abstract of DOI:10.1021/jo0618775

This work describes studies of the electron-promoted Cope cyclization of 2,5-phenyl-1,5-hexadiene radical anions in a flowing afterglow triple quadrupole mass spectrometer. The electronic properties of the hexadienes have been systematically modified by u

Full text of DOI:10.1021/jo0618775

Studies of the Electron-Promoted Cope Cyclization of
2,5-Phenyl-Substituted 1,5-Hexadiene Radical Anions
Silvi A. Chacko and Paul G. Wenthold*
The Department of Chemistry, Purdue UniVersity, West Lafayette, Indiana 47906-2084
pgw@purdue.edu
ReceiVed September 11, 2006
This work describes studies of the electron-promoted Cope cyclization of 2,5-phenyl-1,5-hexadiene radical
anions in a flowing afterglow triple quadrupole mass spectrometer. The electronic properties of the
hexadienes have been systematically modified by using aromatic substituents at the 2- and 5-positions of
the hexedienes, including those with nitro, trifluoromethyl, fluoro, chloro, and acetyl groups. Ions were
formed by the thermal attachment of electrons in the gas phase. Structures of the molecular radical anions
were probed to determine whether they undergo cyclization to a cyclohexane-1,4-diyl anion structure by
examining chemical reactivity with neutral reagents including carbon dioxide, carbon disulfide, and nitric
oxide. First-order rate constants for the reactions of ions were measured, and the reaction efficiencies
were determined. Based on the reactivity results, a thermochemical model has been developed, which
predicts the reaction thermochemistry by using thermochemical properties of model systems. The observed
reactivity from ion-molecule reactions and the study of reaction rates show that the ion of
2,5-dicyanohexadiene and 2,5-di(4,4′-trifluoromethyl phenyl)-1,5-hexadiene undergo Cope cyclization,
whereas the radical anions having substituents such as the fluoro, nitro, chloro, and acetyl groups do not.
Introduction
co-workers and olefin cyclodimerization9 of 1,3-cyclohexa-
dienes. The discovery of the hole-catalyzed Diels-Alder
reaction paved the way for the exploration of other hole-
catalyzed cation-radical pericyclic reactions.7
Over six decades ago, Cope reported the thermal rearrange-
ment of hexa-1,5-dienes,10-15 and since its discovery, the Cope
rearrangement has diversified into many variants, including the
oxy-Cope16 and the anionic oxy-Cope17 versions. The Cope
rearrangement is part of a family of related [3,3] sigmatropic
Theoretical and experimental studies in the field of cationic
reactions1-7 have led to increased understanding of the phen-
omenon of “hole catalysis”.8 In hole-catalyzed systems, ioniza-
tion lowers the activation energy for a reaction and ultimately
leads to reaction acceleration. Two notable examples are the
hole-catalyzed Diels-Alder reaction5,7 developed by Bauld and
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10.1021/jo0618775 CCC: $37.00 © 2007 American Chemical Society
Published on Web 12/22/2006
494
J. Org. Chem. 2007, 72, 494-501

ReactiVity Studies of Substituted Hexadiene Radical Anions
rearrangements that include the Claisen,18 Caroll,19,20 and
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The Cope rearrangement has been extensively studied over
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timates50 support the assessment that the hydrocarbon undergoes
rearrangement through an aromatic transition state,51 although
FIGURE 1. Schematic potential energy surface for neutral and cationic
1,5-hexadiene Cope rearrangement. The ionization energy of the
hexadiene is obtained from photoelectron spectroscopy experiments,66
whereas the ionization energy of the transition state is estimated to be
similar to that of cyclohexyl radical.68 The exothermic rearrangement
of the cyclohexene-1,4-diyl radical cation is also indicated. Values are
in kcal/mol.
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J. Org. Chem, Vol. 72, No. 2, 2007 495

Chacko and Wenthold
hexadiene radical cation has been provided by Williams and
co-workers65,69 in their matrix isolation studies and by Wenthold
and Hu70 in their gas-phase experiments. Matrix EPR studies
of the1,5-hexadiene radical cation carried out by Williams and
co-workers65,69 have shown that it spontaneously undergoes
cyclization to the cyclohexane-1,4-diyl radical cation, which
rearranges to cyclohexene radical cation. Subsequent studies of
1,5-hexadiene radical cation in a flowing afterglow mass
spectrometer are also consistent with the existence of the
inverted potential energy surface, and a similar reaction sequence
has been proposed to occur.70 Photoinduced electron-transfer
(PET) studies of 2,5-diaryl-1,5-hexadienes derivatives by Mi-
yashi and co-workers71 have shown that hole catalysis can
indeed be used to effect slow Cope reactions.
In contrast to the phenomenon of hole catalysis is electron
catalysis, where reduction is used to accelerate the reaction.
Indeed, examples of electron-promoted electrocyclic72-83 and
cycloaddition84-87 reactions have been previously reported.
Sigmatropic rearrangement of radical anions88,89 have also been
proposed but likely occur through dianionic states rather than
from radical anions. Perhaps, the most common example of
electron catalysis is electron-induced olefin polymerization.90
Electronic structure calculations predict that, in select cases,
single-electron reduction can effect an inverted potential energy
surface for the Cope rearrangement.91 This happens because the
electron binding energy of the cyclic intermediate is greater than
the electron affinity of the closed-shell hexadiene, leading to
an inverted potential energy surface as shown in Figure 2.91
For example, cyclization of the radical anion of 2,5-dicyano-
1,5-hexadiene, 1, to form the corresponding cyclohexane-1,4-
diyl radical anion, 1-, is calculated at the B3LYP/6-31G* level
of theory to be exothermic by 16.2 kcal/mol (eq 1).91
FIGURE 2. Potential energy surface for neutral and anionic 2,5-
dicyano-1,5-hexadiene Cope rearrangement, calculated at the B3LYP/
6-31 + G* level of theory, adapted from ref 91.
SCHEME 1
of 1 in the gas phase,92 providing the first example of an
electron-promoted Cope cyclization.92 In this work, we examine
the scope of this reaction and the effect that substituents have
on it, with the goal of elucidating the role that electronic
structure has on the potential energy surfaces for reaction of
1,5-hexadiene radical anions.
For this study, the electronic properties of the hexadienes have
been systematically modified by using aromatic substituents at
the 2- and 5-positions, 2, as shown in Scheme 1. The aromatic
substituents include those with nitro, trifluormethyl, fluoro,
chloro, and acetyl groups in the meta and para positions. Here
we describe the results of reactivity studies of substituted
hexadiene radical anions, with particular focus on determining
whether they have acyclic or cyclic cyclohexane-diyl anion
structures. From these reactivity studies, we propose a simple
thermochemical model to predict whether an anion will have
an inverted potential energy surface and hence spontaneously
cyclize.
We have recently reported experimental studies that confirm
1- as the structure of the ion formed upon one-electron reduction
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496 J. Org. Chem., Vol. 72, No. 2, 2007

ReactiVity Studies of Substituted Hexadiene Radical Anions
TABLE 1. Measured Rate Constants and Efficiencies for the Reactions between Diphenylhexadiene Radical Anions and CO2, CS2, and NOa
rate coefficient (cm3/s) (efficiency, eff)
diene
X
CO2
CS2
NO
2b
2c
2d
2e
2f
2g
2h
2i
p-F
m-F
p-NO2
p-Cl
p-COCH3
m-Cl
m-CF3
p-CF3
CNc
b
b
b
b
b
b
1.4 ( 0.4 × 10-12 (0.002)
2.2 ( 0.4 × 10-12 (0.002)
2.2 ( 0.5 × 10-12 (0.002)
2.0 ( 0.2 × 10-11 (0.02)
3.4 ( 1 × 10-12 (0.004)
3.2 ( 1 × 10-11 (0.04)
4.8 ( 0.5 × 10-12 (0.01)
1.7 ( 0.4 × 10-11 (0.02)
1.4 ( 0.3 × 10-10 (0.14)
1.4 ( 0.3 × 10-11 (0.02)
2.6 ( 1 × 10-12 (0.002)
3.1 ( 0.8 × 10-12 (0.005)
2.4 ( 0.8 × 10-11 (0.04)
2.3 ( 0.8 × 10-12 (0.003)
4.9 ( 0.5 × 10-12 (0.01)
2.8 ( 0.8 × 10-12 (0.01)
2.1 ( 1 × 10-10 (0.34)
1.8 ( 0.4 × 10-10 (0.28)
2.7 ( 1.5 × 10-12 (0.004)
2.5 ( 0.4 × 10-11 (0.04)
3.4 ( 0.3 × 10-10 (0.49)
1
a Uncertainities correspond to one standard deviation. b Rate of the reaction is too slow to be measured (<10-12 cm3/s). c Substituted at the diene.
but they were prepared by the ZnCl2-based condensation108 reaction
of the substituted acetophenone and the corresponding bromoac-
etophenone followed by olefination by using Nysted reagent.109 The
bromoacetophenones that were not commercially available were
synthesized by using known procedures.110-112 All synthesis
procedures and characterization data are included in Supporting
Information. Helium was purified via a liquid nitrogen trap
containing molecular sieves. Gas purities are as follows: He
(99.995%), CO2 (99%), CS2 (99%), NO (99%), and NO2 (99%).
Experimental Section
All experiments were carried out at room temperature (298 ( 2
K) in a flowing afterglow triple quadrupole mass spectrometer that
has been described previously.107 The radical anions of the 1,5-
hexadienes were generated in the high-pressure flow reactor (P[He]
) 0.400 Torr, flow[He] ) 200 std/cm3) by electron ionization (EI)
of neutral 2,5-diphenyl-1,5-hexadienes, added 15 cm downstream
of the electron source. Ion-molecule reactions were investigated
by adding neutral reagent vapors through either of two fixed-
position ring inlets situated downstream in the flow tube. Reactions
were characterized by the loss of the reactant ion and the formation
of the product. The ions in the flow tube, collisionally cooled to
ambient temperature by the helium buffer gas, were extracted
through a 1 mm orifice and focused into a low-pressure region
containing a triple quadrupole mass analyzer. Reaction rates were
measured by monitoring the yield of the reactant ion as a function
of neutral flow.107 The efficiency of the reaction, eff, was obtained
by comparing the measured rate constant for the reaction with the
collision rate constant, calculated by using the average dipole
orientation (ADO) approach,93 with eff ) k/kcoll. None of the
hexadienes investigated in this work are commercially available,
Results
Most of the substituted diphenylhexadienes listed in Scheme
1 undergo electron attachment when added to the flow tube in
the presence of thermal electrons. Of those listed in Scheme 1,
only 2a, 2,5-diphenyl-1,4-hexadiene, does not give a molecular
radical anion, 2a-, to any extent. Yields of diphenylhexadiene
anions for the other systems vary from very high (2h-, 2i-) to
very low (2e-, 2c-). Fragmentation is also observed upon
ionization, with fragmentation patterns dependent on the sub-
stituents. Full lists of EI products and relative yields are provided
in Supporting Information.
Structures of the molecular radical anions, M•-, were probed
by examining chemical reactivity with neutral substrates,
including CO2, CS2, and NO. First-order rate constants for the
reactions of ions 2b--2i- and 1- are listed in Table 1. Also
included in Table 1 is the reaction efficiency, the ratio of the
measured rate to the ADO collision rate.93 Most of the reactions
examined in this work are relatively slow, with efficiencies
generally below 0.05. Notable exceptions include the reactions
of 1- and the reaction of 2i- with NO.
The products in the observed reactions are listed in Table 2.
In most cases, the observed products correspond to simple
adducts. The exceptions include the reaction of 1- with NO,
which occurs by addition and loss of HCN to form the oximate
anion as described previously,92 and the reaction of 2d- with
CS2, which occurs by sulfur atom abstraction.
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J. Org. Chem, Vol. 72, No. 2, 2007 497

Chacko and Wenthold
TABLE 2. Ion-Molecule Reactivity with Neutral Reagent Gases for Diphenylhexadiene Radical Anions
CO2 + NOb
CO2 + NO2
b
diene
X
CO2
CS2
NO
2b
2c
2d
2e
2f
2g
2h
2i
p-F
m-F
p-NO2
p-Cl
p-COCH3
m-Cl
m-CF3
p-CF3
CNc
a
a
a
a
a
a
[M + CS2]
[M + CS2]
[M + S]
[M + CS2]
[M + CS2]
[M + CS2]
[M + CS2]
[M + CS2]
[M + CS2]
[M + NO]
[M + NO]
[M + NO]
[M + NO]
[M + NO]
[M + NO]
[M + NO]
[M + NO]
[M + NO - HCN]
[M + NO]
a
a
a
a
a
a
[M + NO]
[M + NO]
[M + NO]
[M + NO]
[M + NO]
[M + CO2]
[M + CO2]
[M + CO2]
[M + CO2 + NO]
[M + CO2 + NO]
[M + CO2 + NO]
[M + CO2 + NO2]
[M + CO2 + NO2]
[M + CO2 + NO2]
1
a No reaction is observed (see Table 1). b Indicates the product of sequential reaction of CO2 and NO or NO2. c Substituted at the diene.
Ion 1- is used as a model for the cyclized product. The cyclic
SCHEME 2
structure of 1- has been established previously92 by using the
reaction with nitric oxide to convert it to an oximate (eq 3) that
has CID behavior indistinguishable from the authentically
prepared ion.
Characteristic reactions for 1- include efficient reactions with
CO2, CS2, and NO. Moreover, the CO2 adducts of 1- are found
to undergo subsequent reactions with radical substrates, includ-
ing NO and NO2 (Table 2). The reactivity can be readily
understood in the light of the electronic structure of the cyclic
be electronically very similar to the radical anion generated from
anion. Reaction with CO2 results in the formation of the
the corresponding methyl styrene.
carboxylate radical, 1a-, that can undergo radical coupling with
Therefore, reactions were also carried out with conventional
other open-shell reagents (eq 4). This reaction sequence is
radical anions, including 3 and 4. These ions were found to be
common for distonic radical anions,94 and has been observed
essentially unreactive with CO2, CS2, and NO. Similarly,
previously for ions including o-, m-, and p-benzyne anions95,96
because these are all phenyl-substituted systems, the cyclized
and the m-xylylene radical anion.92,95,96
ion would be expected to undergo benzyl-anion-like reactivity.
The reactivity of 1- with CO2 can be contrasted with that
First-order rate constants for reaction of p-NO2, p-CF3, and
for conventional radical anions, such as R-methyl-p-nitrostyrene
p-COCH3 benzyl anions with neutral reagents CO2, CS2, and
radical anion, 3, which serves as a model for ring-opened
NO have been measured and are listed in Table 3.
structures of the hexadiene anions, and fumaronitrile anion, 4,
unreactive with CO2. Unlike most benzylic ions, conventional
radical anions with olefinic and styrenyl moieties do not appear
to react with CO2 to any appreciable extent, and we interpret
the formation of adduct in reaction with CO2 for those systems
that form a stable radical anion as evidence for an ion with
closed-shell character, which would be the case for the cyclized
product.97 Thus, we can use the reaction with CO2 as a probe
for cyclization.
Discussion
Reactivity as a Probe of Ion Structure. The reactivity
described in the previous section provides insight into the
structure of hexadiene radical anions. The most important
question addressed by these studies is whether the ion has
undergone Cope cyclization to the cyclohexane-diyl structure
(eq 2).
The reaction with NO also provides insight into the structure
of the ion.94 Although reaction of NO is found to occur with
all the ions in Table 1, it appears to be significantly more
efficient with the cyclized dicyano system. This is also true for
CS2, although the reaction of 1- with CS2 is not as efficient as
those with CO2 or NO. Of the diphenylhexadiene anions listed
in Table 1, only 2i- (X ) p-CF3) is clearly evidenced to have
a cyclic structure. It undergoes a marginally efficient reaction
498 J. Org. Chem., Vol. 72, No. 2, 2007

ReactiVity Studies of Substituted Hexadiene Radical Anions
TABLE 3. Reactivity of Benzyl Anions with Neutral Reagentsa
rate coefficient, (cm3/s) (efficiency, eff)
benzyl anion
CO2
CS2
NO
p-NO2
p-CF3
p-COCH3
1.3 ( 0.3 × 10-12 (0.002)
2.2 ( 0.9 × 10-10 (0.32)
1.8 ( 1.3 × 10-11 (0.10)
1.3 ( 0.2 × 10-10 (0.13)
3.3 ( 1.3× 10-10 (0.34)
8.4 ( 1 × 10-11 (0.08)
0.9 ( 0.7 × 10-11 (0.01)
3.4 ( 1.5 × 10-11 (0.05)
3.7 ( 1.2 × 10-11 (0.05)
a Uncertainities correspond to one standard deviation
TABLE 4. Predicted Electron Affinity (EA) Values for the
Reactivity Modela
EA
(benzyl)
EA
(styrene)
∆EA
(eV)
diene
X
cyclizes?
2a
2b
2c
2d
2e
2f
2g
2h
2i
H
p-F
m-F
p-NO2
p-Cl
p-COCH3
m-Cl
m-CF3
p-CF3
CNc
1.0
1.0
1.0
2.1
1.2
1.7
1.2
1.4
1.5
1.54d
0
0
0
1.0
0
0.5
0
0
0
0
1.0
1.0
1.0
1.1
1.2
1.2
1.2
1.4
1.5
1.54
n/a
no
no
no
no
no
no
maybeb
yes
yes
1
a Styrene EAs correspond to EAs of the corresponding substituted
benzene. Benzyl EAs are estimated using the gas-phase acidities99 and the
procedure described by Fattahi and Kass,98 unless otherwise noted.
b Inconclusive as to whether the ion is cyclized or not. c Substituted at the
diene. d Reference 100.
favorable, the electron binding energy of the cyclic anion, EBE-
(cyclic), must be greater than the combined electron affinity of
the hexadiene, EA(diene), and the activation energy for the Cope
rearrangement of the hexadiene, ∆Eq, as reflected in eq 5a.
Alternatively, the cyclization is energetically favorable if the
difference between EBE(cyclic) and EA(diene) is greater than
the activation energy for the Cope rearrangement, as shown in
eq 5b.
FIGURE 3. Parameters used to predict thermochemistry of the Cope
cyclization of radical anions. The key parameters are the electron affinity
of the hexadiene, EA(diene), the vertical electron binding energy of
the cyclic anion, EBE(cyclic), and the activation energy for the neutral
Cope rearrangement, ∆Eq.
with CO2, although it is about 10 times less efficient than the
reaction of CO2 with 1-. Similarly, the efficiency for the reaction
of 2i with CS2 is 10 times lower than that for 1-. However, 2i-
also undergoes efficient addition of NO, occurring in about one-
third of all collisions. Moreover, as indicated in Table 2, the
CO2 adduct of 2i- undergoes addition with both NO and NO2.
The results are consistent with those expected for cyclic ion
but not for ring-opened radical anion.
EBE(cyclic) > EA(diene) + ∆Eq
EBE(cyclic) - EA(diene) > ∆Eq
(5a)
(5b)
Unfortunately, none of these hexadienes have known gas-
phase EAs, and the EBE values for the cyclic ions are similarly
unknown. However, we can estimate these values by using the
thermochemical properties of comparable systems. For example,
the electron affinity of the hexadiene should be similar to that
of the corresponding styrene (CH2CHY), indicated in Scheme
2, and consequently the corresponding substituted benzene. For
the EBE of the cyclic ion, we use the EA value estimated for
the triplet biradical. Because these are phenyl-substituted
systems, the electron affinity of the biradical is approximately
that of the corresponding substituted benzyl radical, which is
more likely to be known or readily estimated from the acidity.98
The predicted electron affinity values are listed in Table 4.
With these approximations, in light of eq 5b, we predict the
cyclization to be energetically favorable when the relation in
eq 6 is satisfied. The model is reasonably consistent with known
The results for 2h- (X ) m-CF3) are less clear. Although it
does undergo slow reaction with CO2 and even undergoes
sequential addition of CO2 and NO/NO2, neither the reaction
with CS2 nor that with NO is indicative of a cyclic structure.
Hence, the results are inconclusive, potentially indicating an
exceptionally reactive ring-opened structure, an unreactive cyclic
structure, or even a mixture of two forms. In contrast to the
above, 2b--2g- radical anions are completely unreactive to
CO2 and exhibit very little reactivity with CS2 and NO. It is
most likely that they have conventional π-radical anion struc-
tures.
Thermochemical Model of Reactivity. From the reactivity
studies, we conclude that ions 1- and 2i- likely have cyclic
structures and 2b--2g- are acyclic hexadiene ions, whereas
the structure of 2h- is unclear. The observed cyclization
reactivity of the hexadiene radical anions can be explained by
using a simple model based on potential energy surfaces as
shown in Figure 3.
EA(CH2Y) - EA(CH2CHY) ) ∆EA > ∆Eq
(6)
experimental results and/or reported electronic structure calcula-
tions. For example, for the dicyano-substituted diene, 1, the
electron affinity of the diene is estimated to be that of the
acrylonitrile, CH2CHCN. Acrylonitrile does not form a valence
From Figure 3 we identify three important thermochemical
properties for predicting the thermochemistry of the cyclization
reaction. In order for the cyclization to be energetically
J. Org. Chem, Vol. 72, No. 2, 2007 499

Chacko and Wenthold
bound state but only forms a dipole bound anion with a small
binding energy of 7 meV.101 The electron affinity of cyano-
methyl radical (EA(CH2CN)) has been measured by using
negative ion photoelectron spectroscopy to be 1.543 ( 0.014
eV.100 Therefore, according to eq 4, the cyclization of the ion
derived from hexadiene 1 (Y ) CN) should be favorable because
∆EA, ca. 1.54 eV, is greater than the barrier for Cope
rearrangement of 1, 23.2 kcal/mol (1.0 eV).52,102 Similarly, for
the 1,5-hexadiene hydrocarbon (Y ) H), the values are EA-
(CH3) ) 1.8 kcal/mol103 and EA(HC2dCH2) ) 0 kcal/mol,
giving ∆EA ) 1.8 kcal/mol, well below the 33.3 kcal/mol Cope
rearrangement barrier.31 Indeed, electronic structure calculations
predict that the cyclohexan-1,4-diyl anion is unbound with
respect to electron detachment and ring opening.91 We can also
use this thermochemical model with substituted phenyl hexa-
dienes to predict the reactivity of their anions toward Cope
cyclization. For example, for diphenylhexadiene 2a (X ) H),
the value of EA(YCH2) is that for benzyl, 0.912 ( 0.006 eV,104
and the value of EA(CH2CHY) is that for styrene, which does
not form a stable anion and has EA ) 0. This gives ∆EA )
0.9 eV (21 kcal/mol), slightly less than the 25 kcal/mol barrier
for the Cope rearrangement of 2,5-dipenyl-1,5-hexadiene.41 To
estimate the ∆EA for 2,5-di(4,4′-nitro phenyl)-1,5 hexadiene,
2d, we need the electron affinity of p-nitrobenzyl radical.
Although this has not been measured, it can be estimated by
using the approach described by Fattahi and Kass98 to give a
value of 49 kcal/mol (2.1 eV). Similarly the electron affinity
of p-nitrostyrene is not experimentally known but is likely
similar to that of nitrobenzene, 1 eV.105 The difference in EA
is thus found to be 1.1 eV ) 25 kcal/mol, similar to the barrier
for Cope rearrangement for aromatic substituted hexadiene.59
However, empirically it is found that 2d- does not cyclize. EA
values for the substituted hexadiene systems, ∆EA values,99 and
cyclization results for 1- and 2b--2i- are shown in Table 4.
A clear pattern is observed, as the ∆EA values for the
phenylhexadienes that do not cyclize are all in the range of 1.0-
1.2 eV. For those that cyclize, which include 2i- (p-CF3) and
1- (-CN) the ∆EA values are ca. 1.5 eV. The m-CF3-substituted
system, 2h, has a ∆EA value of 1.4 eV and appears to be near
the threshold at which cyclization occurs. Therefore, on the basis
of our experimental results, it appears that the cyclization occurs
only if ∆EA > 1.4 eV (32 kcal/mol).
FIGURE 4. Revised thermochemical model for predicting the ther-
mochemistry of the Cope cyclization of radical anions, in which the
quantity EA(diene) is estimated to be 0.3 eV higher than the electron
affinity of the corresponding styrene, the electron binding energy of
the cyclic anion is estimated to be that of the model benzyl anion.
Reaction Dynamics. The thermochemical model described
in the previous section predicts the reactivity of hexadiene
radical anions solely on the basis of reaction thermochemistry,
in that a reaction is predicted to occur if it is exothermic but
not if it is endothermic. It does not take into account any energy
barriers that separate the ring-opened anion from the cyclic
structure. Calculations of the acyclic dicyanohexadiene radical
anion predict that the ion is a stable minimum on the potential
energy surface,91 which means there exists a barrier for the
cyclization. Potential energy surface calculations carried out in
our lab106 suggest that the barrier for cyclization is very small
(<1 kcal/mol) and thus is not large enough to have a significant
effect on the rate of the cyclization reaction at room temperature.
Moreover, the energy of electron attachment to the hexadiene
provides energy that can be used to overcome larger barriers
for the reaction, such that reaction thermochemistry is the
overriding consideration.
According to eq 5, the cyclization reaction should be
exothermic if ∆EA is greater than ∆Eq, which is ca. 25 kcal/
mol for the aromatic substituted system.59 The difference
between ∆Eq and the empirical threshold of 32 kcal/mol likely
results from systematic errors in the estimation that is used and
particularly in the approximation that the electron affinity of
the diene is similar to that of the styrene. The increased size
and the potential group interactions of the diene are expected
to lead to a larger EA for the hexadiene than for the styrene.
For example, the electron affinity of of 2,5-dicyanohexadiene
radical anion to form the ring-opened structure is calculated to
be 13 kcal/mol higher than that of acrylonitrile.91 The experi-
mental results in this work suggest that the EAs of diphenyl-
hexadiene are ca. 0.3-0.4 eV higher than those of the
corresponding styrenes. This leads us to a revised thermochemi-
cal model, where EA(diene) is estimated as EA(CH2CHY) +
0.3 eV as shown in Figure 4. With this revision, the reaction is
predicted to occur if eq 7 is satisfied.
Conclusions
Reactivity studies of ionized 2,5-diphenyl-1,5-hexadienes
indicate that electron-promoted cyclization occurs for those
radical anions with trifluoromethyl substituents on the aromatic
ring. Systems with electron-withdrawing (nitro, acetyl) or
electron-donating (halogens) substituents form stable radical
anions that apparently do not cyclize. The occurrence of
cyclization can be predicted by using a simple thermochemical
model based on electron affinities of model substrates and the
activation energies of the neutral Cope rearrangement. Accord-
ing to this model, CF3 groups are optimal because they stabilize
the benzyl-like anion without extensively stabilizing the open-
shell, π-radical anion.
Although the other radical anions examined in this work do
not form stable cyclic structures, this does not preclude the
possibility that the ionization does not accelerate the Cope
rearrangement, as compared to the neutral substrates. Establish-
ing this to be the case will require the determination of the
kinetics for rearrangement.
EA(CH2Y) - EA(CH2CHY) > ∆Eq + 0.3 eV
500 J. Org. Chem., Vol. 72, No. 2, 2007
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ReactiVity Studies of Substituted Hexadiene Radical Anions
Supporting Information Available: Fragmentation patterns and
syntheses for the 2,5-biphenyl hexadienes 2b-2i. This material is
available free of charge via the Internet at http://pubs.acs.org.
Acknowledgment. Funding for this work was provided by
the National Science Foundation (CHE04-54874). We also thank
the donors of the Petroleum Research Fund, administered by
the American Chemical Society, for partial support.
JO0618775
J. Org. Chem, Vol. 72, No. 2, 2007 501