Thermal [2 + 2] cycloreversion of a cyclobutane moiety via a biradical reaction

Article abstract of DOI:10.1021/jp111577j

The nature of the Woodward-Hoffmann-forbidden, thermal activated cycloreversion mechanism of cyclobutane has long been the subject of speculation and intense research. We were now able to prove the theoretically postulated biradicalic mechanism directly from radical scavenging reactions and electron paramagnetic resonance (EPR) experiments on [2 + 2] heterodimers of 5-fluoro-1-heptanoyluracil and 7-methoxy-1,1-dimethylnaphthalenon. The dimers show both the "allowed" photochemically as well as the "forbidden" thermally triggered [2 + 2] cycloreversion of the cyclobutane ring. The quantum efficiency of the photochemical cleavage is about 1%. The thermal cycloreversion reaction is independent from solvent and occurs at low activation energies of about 13 kcal/mol, even in the solid state. The radical scavenger and EPR results are further supported by the finding, that the reaction products are solely the educts for the anti-head-to-tail heterodimer. But for the syn-head-to-head heterodimer two additional products are observed, which require a sufficiently stable biradical intermediate to facilitate the required intramolecular rearrangements. Because of the surprisingly high lifetime of the radical species of these heterodimers it was possible to prove the long-discussed biradical mechanism experimentally.

Full text of DOI:10.1021/jp111577j

ARTICLE
pubs.acs.org/JPCA
Thermal [2 þ 2] Cycloreversion of a Cyclobutane Moiety via
a Biradical Reaction
J. Liese† and N. Hampp*,†,‡
†Department of Chemistry, Philipps-University Marburg, Department of Chemistry, Hans-Meerwein-Str. Bldg.
H, D-35032 Marburg, Germany
‡Material Sciences Center, Philipps-University Marburg, D-35032 Marburg, Germany
S Supporting Information
b
ABSTRACT: The nature of the WoodwardꢀHoffmann-forbidden,
thermal activated cycloreversion mechanism of cyclobutane has long
been the subject of speculation and intense research. We were now able
to prove the theoretically postulated biradicalic mechanism directly from
radical scavenging reactions and electron paramagnetic resonance (EPR)
experiments on [2 þ 2] heterodimers of 5-fluoro-1-heptanoyluracil and
7-methoxy-1,1-dimethylnaphthalenon. The dimers show both the
“allowed” photochemically as well as the “forbidden” thermally triggered
[2 þ 2] cycloreversion of the cyclobutane ring. The quantum efficiency
of the photochemical cleavage is about 1%. The thermal cycloreversion
reaction is independent from solvent and occurs at low activation
energies of about 13 kcal/mol, even in the solid state. The radical
scavenger and EPR results are further supported by the finding, that the
reaction products are solely the educts for the anti-head-to-tail heterodimer. But for the syn-head-to-head heterodimer two
additional products are observed, which require a sufficiently stable biradical intermediate to facilitate the required intramolecular
rearrangements. Because of the surprisingly high lifetime of the radical species of these heterodimers it was possible to prove the
long-discussed biradical mechanism experimentally.
’ INTRODUCTION
cycloreversion is not to be expected. An indirect proof of the
validity of the “diradical hypothesis” was reported by Zewail et al.
monitoring the Norrish-type R decarbonylation reaction of
cyclopentanone via femtosecond mass spectroscopy which yields
tetramethylene-biradicals forming cyclobutane by ring closure or
two ethylene molecules by fragmentation.14 However, biradical
intermediates of direct thermal activated cyclobutane cyclorever-
sions have not been observed yet, whenever attempted lifetimes
of these species were found to be too short to be detected by
radical scavenging reactions or electron paramagnetic resonance
(EPR) experiments.12,23
Here we report direct proof for the existence of a biradical
intermediate species in the thermally induced cycloreversion of
two stereoisomers of the [2 þ 2] cyclodimers of 7-methoxy-1,1-
dimethylnaphthalenon (MDN, 1) and 5-fluoro-1-heptanoyl-
uracil (H5FU, 2) via radical scavengers and EPR experiments.
Thermal cycloreversion is observed at rather low temperatures
starting from 80 °C. The products yielded depend on the isomer
species. The results obtained prove the long proposed biradical
reaction pathway experimentally.
[2 þ 2] cycloreversion reactions of cyclobutane derivatives are
of fundamental as well as practical importance regarding DNA
photoenzymatic repair,1,2 photochemical energy storage,3 and
organic synthesis.4 According to WoodwardꢀHoffmann the
concerted mechanism is photochemically allowed but thermo-
chemically forbidden5 (Figure 1).
Soon after Woodward and Hoffmann published their
findings5,6 examples of “forbidden” thermal [2 þ 2] cyclorever-
sions were described.7,8 For example, the thermal dissociation of
the simplest system c-C4H8 of two ethylene molecules has been
extensively studied theoretically and experimentally.9ꢀ17 The
reaction mechanism was postulated to occur via a stepwise
biradical mechanism.14,18 A rather high activation barrier of
62.5 kcal/mol19 was found. Further [2 þ 2] cyclodimers of
several arenes have been synthesized, and their abilities to
undergo thermally induced [2 þ 2] cycloreversions were
examined.20,21 Thermal cycloreversions of compounds which
lead to two aromatically stabilized products were observed, and
rather low activation energies around 20ꢀ30 kcal/mol16,19 were
measured. When the components are oriented so that the orbital
lobes are able to react from opposite sides, so-called antarafacial,
a thermal cycloreversion is allowed. However, in the cases re-
ported here this is sterically not possible, so a thermal [2 þ 2]
Received: December 6, 2010
Revised:
February 23, 2011
Published: March 23, 2011
r
2011 American Chemical Society
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Scheme 1. Synthesis of Heterodimers of MDN and H5FUa
Figure 1. Allowed and forbidden cleavage pathways of
cyclobutane rings.
’ EXPERIMENTAL METHODS
All chemicals were purchased from commercial suppliers and
used without further purification. Organic solvents were received
in technical quality and were purified by filtration and distillation.
Standard NMR spectra were recorded on a Bruker Avance 300 B
spectrometer (300 MHz), 2D-NMR spectra on a Bruker DRX-
400 NMR (400 MHz). The thermal cycloreversion reaction was
monitored via high-performance liquid chromatography
(HPLC) on an UltiMate 3000 System (Dionex) equipped with
a Nucleosil column (RP18, 5.0 μm, 250 mm ꢁ 4 mm, Bischoff
Chromatography), to determine reaction rates and activation
energies. LC/MS experiments were measured on an ESI/MS
LCQ Duo mass spectrometer (Thermoelectron) connected to a
1050-HPLC (Agilent Technologies) with a Nucleosil 100-3-C18
column (3 μm, 250 mm ꢁ 4 mm, Bischoff Chromatography).
The ORCA 2.6ꢀ3524 software package was used to perform
single-point density functional theory (DFT) calculations at the
B3LYP level on the structures of the dimers using the SVP basis
set. For thermochemistry issues frequency calculations were
carried out on the B3LYP level using the TZVP basis set. EPR
experiments were carried out in quartz glass tubes from Wilmad
on a Bruker ESP 300 E with a Varian X-Band resonator including
a heating coil using forming gas.
a Two different isomers of the MDN-H5FU heterodimer were obtained
as main products from the photochemical [2 þ 2] cycloaddition. Their
stereochemistry was assigned by 2D-NMR.
separable from the main products due to their very similar
chemical properties. The absolute amounts are very small; they
undergo [2 þ 2] cycloreversion as well and do not have any effect
on the reaction mechanism. 0.5 mL (c = 5 ꢁ 10ꢀ3 mol Lꢀ1 in
3
methanol) was incubated at 373, 383, 393, 403, and 413 K,
respectively, in HPLC vials in a heating bath in the dark. The
samples were removed from the bath and quickly cooled down
for measurements. These reactions are independent of the
solvents used and occur even without any solvent, therefore
being of first order.
’ SYNTHESIS OF H5FU-MDN HETERODIMERS
(SCHEME 1)
Monomers MDN and H5FU and dimers of s-hhHD and of
a-htHD were synthesized according to procedures described
earlier.25,26 Configurations of dimers were determined via 2-D
NMR experiments as syn-head-to-head (s-hhHD) and anti-head-
to-tail (a-htHD) regarding the position of the carbonyl functions
next to the cyclobutane ring (see Supporting Information for
details). The products s-hhHD and a-htHD were obtained as a
yellow solid (602 mg, 1.35 mmol, 23%, mp = 149,5 °C) and as a
yellow oil (634 mg, 1.42 mmol, 24%), respectively. The struc-
tures of both dimers were determined via 2D-NMR experiments.
s-hhHD. 1H NMR (300 MHz, CDCl3): 8.03 (s, 1H), 7.34 (dd,
J = 3.66, 8.38 Hz, 1 H), 6.93ꢀ6.87 (m, 2H), 5.38 (ddd, J = 15.24,
8.70, 1.20 Hz, 1H,), 4.25 (dd, J = 8.61, 0.63 Hz,1H), 3.85 (s, 3 H),
3.08ꢀ2.79 (m, 3H), 1.67 (s, 6H), 1.32ꢀ1.25 (m, 8 H),
0.89ꢀ0.85 (m, 3 H).
Initial reaction rates were calculated and the activation energy
EA was obtained via the Arrhenius plot ln(k) vs 1/T for the
given temperatures resulting in EA = 54.1 ( 3.4 kJ/mol =
12.9 ( 0.8 kcal/mol. The entropy of activation employing
the activated-complex theory of Eyring was calculated to
ΔSq = ꢀ32.1 ( 4.5 e.u.
Different solvents for the thermal cycloreversion at 403 K were
examined according to those in methanol. For measuring sam-
ples without any solvent, the dry dimer was incubated, dissolved
in methanol for HPLC measurement, and then dried in vacuo for
the next incubation step.
Products of the cycloreversion were identified via HPLC
(75:25 MeOH/water), LC/MS (75:25 acetonitrile/water), and
NMR. The monomers H5FU and MDN were experimentally
verified to be stable under the applied conditions not undergoing
any decomposition or rearrangement reactions, thus the HMDN
and MPA are direct products of the cycloreversion reaction
observed.
a-htHD. 1H NMR (300 MHz, CDCl3): 8.01 (s, 1H), 7.34 (dd,
J = 3.66, 8,34 Hz, 1H), 6.92ꢀ6.87 (m, 2H), 5.38 (ddd, J = 15.22,
8.72, 1.15 Hz, 1H), 4.25 (d, J = 8.56 Hz, 1H), 3.85 (s, 3H),
3.08ꢀ2.84 (m, 3H), 1.67 (s, 6H) 1.32 (s, 8H), 0.91ꢀ0.84
(m, 3H).
’ THERMAL CYCLOREVERSIONS OF s-hhHD and a-
htHD
’ ANALYSIS OF 5FU, MPA, MDN, AND HMDN
MPA, MDN, and HMDN were isolated from a 200 mg
thermal cycloreversion reaction of s-hhHD and purified via
preparative HPLC (45:55 acetonitrile/water). Products were
identified via NMR and LC/MS (75:25 acetonitrile/water). The
very light sensitive product HMDN was identified via HPLC and
The thermal cycloreversion reactions were monitored via
analytical RP-HPLC. Smaller signals next to the dimers show
one or three other stereoisomers of the dimer, which are
produced in traces during photodimerization and are not
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LC/MS in direct comparison with a sample of the pure substance
from the synthesis procedure.
MDN. 1H NMR (300 MHz, CDCl3): 7.39 (d, J = 9.8 Hz, 1H);
7.26 (d, J = 8.4 Hz, 1H); 7.09 (d, J = 8.2 Hz, 1H); 6.98 (d, J = 2.4
Hz, 1H); 6.04 (d, J = 9.8 Hz, 1H,); 3.86 (s, 3H); 3,04 (t, J = 6.4 Hz,
7.3 Hz, 2H); 2,66 (t, J = 6.5 Hz, 7.3 Hz, 2H,); 1,46 (s, 6H). LCMS
(m/z) = [Mþ] calcd. for C13H14O2, 202.10; found, 203.11.
H5FU. 1H NMR (300 MHz, DMSO): 3,44 (s, 1H); 3,48 (s,
1H); 7,74 (d, 1H). MS (m/z) = [Mþ] calcd. for C11H15FN2O3
(H5FU), 242.25; found, 243.02. LCMS (m/z) = [Mþ] calcd. for
C4H3FN2O2 (5FU), 130.08; found, 129.04.
HMDN. LCMS (m/z) = [Mþ] calcd. for C13H16O2, 204.12;
found, 203.51.
MPA. 1H NMR (300 MHz, DMSO): 1.45 (s, 3H);3.82 (s, 3H);
6.82 (s, 1H); 6.87 (d, J = 2.2 Hz, 1H); 6.91 (dd, J = 2.2 Hz, 8.4 Hz,
1H); 7.10 (d, J = 2.3 Hz, 1H); 7.16 (d, J = 8.5 Hz, 1H); 7.34 (d, J =
8.5 Hz, 1H); 8.40 (d, J= 2.4 Hz, 1H); 11,03 (s, 1H). LCMS (m/z) =
[Mþ] calcd. for C13H14O2, 202.10; found, 203.50
Figure 2. Thermal cycloreversion of a-htHD and s-hhHD and identi-
fication of the products obtained. HPLC chromatograms of a-htHD and
s-hhHD before (0 min) and after heating. Upward arrows indicate rising
peaks and downward arrows decreasing peaks. The first two chromato-
grams show the cleavage of a-htHD which disappears completely when
heated to 380 K for 200 min. The products obtained are solely the
monomers H5FU and MDN. The decrease of s-hhHD when incubated
at 410 K for various times (15ꢀ50 min) is accompanied by the increase
of four product peaks MDN (1), H5FU (2), HMDN (5), and MPA (6).
Smaller signals next to the main product in the 0 min measurements
belong to minor amounts of other stereoisomers of the dimers.
’ RADICAL SCAVENGING REACTIONS
To a sample of 0.5 mL of a 5 ꢁ 10ꢀ3 mol Lꢀ1 methanol
3
solution of respectively s-hhHD or a-htHD in a HPLC vial was
added hydroquinone. The samples were incubated for 5 h at
403 K. Both were analyzed via HPLC (75:25 MeOH/water), and
two new additional peaks at 2.85 and 5.79 min were detected in
both samples, whereas the formation of MDN (and MPA for s-
hhHD) was quite weak. The peaks were further analyzed via LC/
MS (75:25 acetonitrile/water), and hydroquinone adducts of
dimer and MDN were identified.
The thermal cycloreversion of a-htHD yields two products
only, MDN and H5FU, independent of the solvents used, exactly
as in the photochemical cleavage. The same cleavage products are
obtained when solid a-htHD is heated. However, s-hhHD is more
complex. In methanol and CꢀH acidic acetonitrile four products
are obtained. In addition to H5FU and MDN two derivates of
MDN are observed, 7-methoxy-1,1-dimethyl-3,4-dihydro-
naphthalen-2(1H)-one (HMDN) and 3-(4-methoxy-2-(prop-
1-en-2-yl)phenyl)acrylaldehyde (MPA) (Figure 2). With the
sample being dissolved in chloroform or being in solid form,
HMDN was not formed at all. This means that HMDN forma-
tion requires a protic solvent. As the monomers H5FU and MDN
are stable under the applied conditions, not undergoing any
decomposition or rearrangement reactions, HMDN and MPA
are direct products of the cycloreversion reaction.
To confirm the thermal cleavage pathway the activation
energy EA of the cycloreversion was determined. Solutions of
s-hhHD in methanol were heated to various temperatures in the
range from 373 to 413 K for defined time intervals, and the
decrease in dimer concentration was traced via HPLC (Figure 3).
Samples were kept in the dark throughout the whole process
to exclude any photoinduced cycloreversion. From the Arrhenius
plot the activation energy was determined to be EA = 12.9 ( 0.8
kcal/mol, which is the lowest value for a thermally induced
cycloreversion so far. The cleavage of the symmetric dimer
dicoumarin reported by Yonezawa et al.27 required temperatures
from 473 to 573 K. Even the activation energies for the thermally
induced cycloreversion of intrinsically unstable molecules are
considerably higher, 26.8 kcal/mol21 for dibenzene and 62.5
kcal/mol15,28 for cyclobutane.
MDN þ hydroquinone: LCMS (m/z) = [Mþ] calcd. for
C19H20O4, 312.14; found, 313.60.
Dimer þ hydroquinone: LCMS (m/z) = [Mþ] calcd. for
C30H35FN2O7, 554.24; found, 552.50
MDN and H5FU were both tested with hydroquinone
separately to ensure that no other reactions of these components
occur, and they were heated under the same reaction conditions
like the scavenging experiments. No chances in the composition
of the solutions were detected. This supports the conclusion that
the adducts of MDN and hydroquionone occur solely from the
[2 þ 2] cycloreversion.
’ EPR EXPERIMENTS
For s-hhHD a quartz glass tube was filled with about 130 mg
solid. Experiments were carried out at 296ꢀ400 K. a-htHD was
measured as a solution of 120 mg in 0.2 mL acetonitrile (c = 1.35
mol Lꢀ1) in a tube ablated at about 14 cm height, because the
3
highly viscous oily dimer could not be transferred into the EPR
tube without any solvent. Experiments were carried out at
380ꢀ410 K.
’ RESULTS AND DISCUSSION
The photochemical properties of both dimers were found to
be identical within the experimental limits. With quantum
efficiencies of about 1% the photochemical [2 þ 2] cyclorever-
sion reverts back to the educts H5FU and MDN solely. (For
details, see Supporting Information.)
Heating s-hhHD and a-htHD in various solvents, i.e., metha-
nol, acetonitrile, or chloroform, as well as without any solvent to
temperatures above 353 K causes thermally induced cleavage of
the dimers.
The entropy of activation employing the activated-complex
theory of Eyring was calculated to ΔSq = ꢀ32.1 ( 4.5 e.u. This is
quite low compared to the values for dibenzenes,20 ꢀ2.2 to 0.9e.u.,
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Figure 3. Determination of the activation energy EA. (a) Decreasing dimer concentrations in dependence on the incubation time and the temperature.
(b) The Arrhenius plot ln(k) vs 1/T, slope = ꢀ6505.67(1/K), EA = ꢀ1 ꢁ slope ꢁ R = 54.09 ( 3.44 kJ/mol = 12.9 ( 0.8 kcal/mol.
Scheme 2. Proposed Reaction Mechanism for the Thermal Cycloreversion of a-htHD and s-hhHDa
a Three possible reaction pathways yield the products MDN, MPA, and HMDN, whereas H5FU is always formed independent of the reaction pathway.
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Figure 4. EPR measurements of s-hhHD and a-htHD. (a) Temperature-dependent EPR measurements of solid s-hhHD. The first spectrum (0 h) was
measured at 380 K. After 2 h the temperature was raised from 380 to 400 K and the signal increased. Then the sample was cooled down to room
temperature (296 K) by convection within 2 h (4 h). The radical signal was still observed. (b) EPR measurements of a-htHD at 420 K in acetonitrile
solution. The signal was significantly weaker than that of s-hhHD although the absolute amount of dimer in the experiments was identical for both
dimers.
and for cyclobutanes,15,21 ꢀ2.7 to 1.6 e.u. Strong negative
activation entropies indicate a more ordered structure and
stronger solvation effects in the transition state than in the
ground state of the dimer. Simple decompositions of ring
structures to noncyclic products are expected to have a slightly
positive entropy of activation, but this depends on the grade of
cyclization in the transition state. If the cyclization in the
transition state is still strong, the activation entropy may be
negative.
The mechanistic interpretation of the s-hhHD and a-htHD
reactions requires a rather stable biradical intermediate which
allows intramolecular rearrangements. For a-htHD the thermally
induced [2 þ 2] cycloreversion occurs via a biradical intermedi-
ate back to the monomer educts H5FU and MDN. The biradical
is most likely formed at the side of the fluorine in the H5FU,
because fluorine stabilizes radicals in R-position.28 This radical
stabilizing effect is associated with the resonance delocalization
of the unpaired electron which is able to form a two-center three-
electron bond.
The first step in the radical bond cleavage of s-hhHD is
likewise at the fluorine side, but the resulting biradical inter-
mediate has—different than the a-htHD—several possible reac-
tion pathways (scheme 2). In all cases H5FU is formed as one
component. First the radical intermediate cleaves via a second
homolytic scission into the monomers H5FU and MDN just like
a-htHD does. Only in protic solvents the second pathway is
observed. The dimethylnaphtalenoneꢀradical abstracts hydro-
gen from the solvent, and HMDN is formed. The third possibility
is that the biradical intermediate undergoes internal rearrange-
ment to stabilize the radical at the tertiary carbon atom in the
MDN. The second bond scission then leads to a 1,7-H-shift and
yields 3-(4-methoxy-2-(prop-1-en-2-yl)phenyl)acrylaldehyde
(MPA).
From these results a rather stable biradicalic intermediate
needs to be postulated which should be detectable by EPR. As the
s-hhHD stabilizes through molecular rearrangement, the radical
intermediate is expected to have a longer lifetime than that of the
a-htHD intermediate. The reaction with the radical scavenger
hydroquinone led to the formation of two new products which
were identified as the hydroquinone adducts of MDN and
s-htHD and a-hhHD, respectively. It should be mentioned that
no hydroquinone adducts with H5FU were observed. The radical
at the MDN molecule is apparently significantly more reactive
than that at the H5FU, which is able to stabilize via the R-fluorine
group28 and resonance structures in the uracil.29 EPR experi-
ments were accomplished at temperatures from 380 to 420 K.
Temperatures were adjusted through a heating coil around the
EPR tube in order to allow for monitoring the reaction from the
very first beginning. Higher temperatures increase the number of
radicals produced per unit time but decreased their lifetimes.
Therefore a saturation-limited dimer concentration needed to be
used, which led to line broadening and the deprivation of any fine
structures in the signal (Figure 4). The g-values of about 2 are
characteristic for organic radicals. The a-htHD was analyzed as a
concentrated solution in acetonitrile at 420 K and the s-hhHD as
a solid in the range from 380 to 400 K. Both dimers showed a
temperature-dependent EPR signal from around 380 K onward.
Solid s-hhHD was heated from 380 to 400 K, and the signal
increased with increasing temperature. Finally the sample was
cooled to room temperature, and the EPR signal remained
detectable, albeit weaker. Within the first hour the signal lost
about 90% of its intensity and faded to zero within another hour.
Following the EPR measurements, the s-hhHD and a-htHD
samples were analyzed via HPLC as described above, and the
predicted product distribution patterns were found, i.e., for the
s-hhHD sample H5FU, MDN, and MPA, and for the a-htHD
sample H5FU and MDN only. Tracing the EPR signal from zero
to maximum signal and back to zero is a direct experimental proof
for the radical nature of the thermal [2 þ 2] cycloreversion
reaction.
’ DISCUSSION
EPR measurements as well as radical scavenger reactions
brought direct experimental proof of the existence of a biradical
intermediate in the thermal cyclobutane cycloreversion reaction
pathway. Two different isomers of MDN and H5FU, s-hhHD
and a-htHD, were examined photochemically as well as
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(13) Segal, G. A. J. Am. Chem. Soc. 1974, 96, 7892–7897.
thermally. The photolytic [2 þ 2] cycloreversion of their
cyclobutane moieties is identical as expected. The educts
MDN and H5FU were regained with the same quantum
efficiency for both isomers. The thermal [2 þ 2] cycloreversion,
however, showed differences. A low activation energy of about 13
kcal/mol was found, but the product patterns differ. Whereas
from a-htHD only the educts were formed, from s-hhHD two
additional products were obtained, HMDN and MPA. The
difference in the product patterns results from the different
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lifetime of the s-hhHD biradical an intramolecular rearrangement
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’ ASSOCIATED CONTENT
S
Supporting Information. Supporting Information pro-
b
vides additional data regarding the photochemical cyclorever-
sion, thermochemistry, and structure calculations, 2D-NMR
data, and detailed information regarding the radical scavenging
reactions. This material is available free of charge via the Internet
at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: hampp@staff.uni-marburg.de.
’ ACKNOWLEDGMENT
This work was financially supported through Grant FKZ
13N8978 from the German Federal Ministry of Education and
Research. We thank Dr. Olaf Burghaus (EPR/ENDOR service
department, University of Marburg) for his help with the EPR
experiments.
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