Spectroscopic and calorimetric studies on the mechanism of methylenecyclopropane rearrangements triggered by photoinduced electron transfer

Article abstract of DOI:10.1021/ja0277982

2-(Dideuteriomethylene)-1,1-bis(4-methoxyphenyl)cyclopropane (d₂-1) undergoes degenerate rearrangement in both singlet- and triplet-sensitized electron-transfer photoreactions. Nanosecond time-resolved absorption spectroscopy on laser flash pho

Full text of DOI:10.1021/ja0277982

Published on Web 07/08/2003
Spectroscopic and Calorimetric Studies on the Mechanism of
Methylenecyclopropane Rearrangements Triggered by
Photoinduced Electron Transfer
Hiroshi Ikeda,*,† Kimio Akiyama,§ Yasutake Takahashi,†,| Tatsuo Nakamura,†
Satoshi Ishizaki,† Yukio Shiratori,† Hitoshi Ohaku,† Joshua L. Goodman,‡
Abdelaziz Houmam, Danial D. M. Wayner, Shozo Tero-Kubota,§ and
Tsutomu Miyashi*,†
Contribution from the Department of Chemistry, Graduate School of Science, Tohoku UniVersity,
Sendai 980-8578, Japan, Institute of Multidisciplinary Research for AdVanced Materials,
Tohoku UniVersity, Sendai 980-8577, Japan, Department of Chemistry, UniVersity of Rochester,
Rochester, New York 14627, and National Institute for Nanotechnology, National Research
Council of Canada, Ottawa, Ontario, Canada K1A 0R6
Received July 22, 2002; E-mail: ikeda@org.chem.tohoku.ac.jp
Abstract: 2-(Dideuteriomethylene)-1,1-bis(4-methoxyphenyl)cyclopropane (d2-1) undergoes degenerate
rearrangement in both singlet- and triplet-sensitized electron-transfer photoreactions. Nanosecond time-
resolved absorption spectroscopy on laser flash photolysis of the unlabeled 1 with 9,10-dicyanoanthracene,
1,2,4,5-tetracyanobenzene, or N-methylquinolinium tetrafluoroborate as an electron-accepting photosen-
sitizer gives rise to two transients with λmax at 500 and 350 nm assigned to the dianisyl-substituted largely
twisted trimethylenemethane cation radical (6•+) and the corresponding diradical (6••), respectively. These
intermediates are also detected, respectively, by steady state and nanosecond time-resolved EPR with
chloranil or anthraquinone as a sensitizer. The degenerate rearrangement of d2-1 thus proceeds via these
two different types of intermediates in a cation radical cleavage-diradical cyclization mechanism. Energetics
based on nanosecond time-resolved photoacoustic calorimetry support this mechanism. A comparison of
the reactivities and the spectroscopic results of 1, 1,1-bis(4-methoxyphenyl)-2-methylenespiro[2.2]pentane
(2), and 1-cyclopropylidene-2,2-bis(4-methoxyphenyl)cyclopropane (3) suggest that the reversible meth-
ylenecyclopropane rearrangement between 2 and 3 proceeds via a similar mechanism.
Chart 1.a
Introduction
We have reported that 2,5-diaryl-3,3,4,4-tetradeuterio-1,5-
hexadiene undergoes photoinduced electron-transfer (PET)
degenerate Cope rearrangement, involving a 1,4-diarylcyclo-
hexane-1,4-diyl cation radical and a 1,4-diarylcyclohexane-1,4-
diyl.1 An important reaction in this mechanism is the highly
exergonic back electron transfer from a sensitizer anion radical
to the 1,4-diarylcyclohexane-1,4-diyl cation radical to form the
1,4-diarylcyclohexane-1,4-diyl. In view of the expectation that
the degenerate rearrangement of 2-(dideuteriomethylene)-1,1-
bis(4-methoxyphenyl)cyclopropane (d2-1 in Chart 1)2,3 and the
† Graduate School of Science, Tohoku University.
§ Institute of Multidisciplinary Research for Advanced Materials, Tohoku
University.
a Abbreviation: Ar, 4-MeOC6H4; TMM, trimethylenemethane.
‡ University of Rochester.
reversible methylenecyclopropane rearrangement between 1,1-
bis(4-methoxyphenyl)-2-methylenespiro[2.2]pentane (2) and
1-cyclopropylidene-2,2-bis(4-methoxyphenyl)cylopropane (3)3
occur by a similar mechanism, the PET reactions of 1,1-bis(4-
methoxyphenyl)-2-methylenecyclopropane (1), 2, and 3 were
investigated by means of nanosecond time-resolved (TR)
absorption spectroscopy, EPR, and photoacoustic calorimetry
on laser flash photolysis. TR-EPR studies of 4 and (Z)- and
(E)-1-ethylidene-2-(4-methoxyphenyl)cyclopropane (Z-5 and
National Research Council of Canada.
| Present address: Department of Chemistry for Materials, Faculty of
Engineering, Mie University.
(1) (a) Ikeda, H.; Minegishi, T.; Abe, H.; Konno, A.; Goodman, J. L.; Miyashi,
T. J. Am. Chem. Soc. 1998, 120, 87-95. (b) Miyashi, T.; Ikeda, H.;
Takahashi, Y. Acc. Chem. Res. 1999, 32, 815-824.
(2) Takahashi, Y.; Miyashi, T.; Mukai, T. J. Am. Chem. Soc. 1983, 105, 6511-
6513.
(3) A part of this work was presented at the 8th IUPAC Conference on Physical
Organic Chemistry (Japan, 1986)4 and at the 10th International Conference
on Physical Organic Chemistry, Organic Chemistry Division, IUPAC
(Israel, 1990)5 and reported as a preliminary communication.6
9
10.1021/ja0277982 CCC: $25.00 © 2003 American Chemical Society
J. AM. CHEM. SOC. 2003, 125, 9147-9157
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A R T I C L E S
Ikeda et al.
Scheme 1. CRCL-DRCY Mechanism for PET
Scheme 2. Degenerate Methylenecyclopropane Rearrangement
of d2-1 and Oxygenation of 1 Triggered by PETa
Methylenecyclopropane Rearrangementa
a Sens: DCA, TCNB, or NMQ+BF4
.
-
Table 1. Halfwave Oxidation Potentials (Eox1/2) of 1, Free Energy
Changes (∆Get) Associated with Electron-Transfer Reactions of 1
with the Excited State of Sensitizers, and Rate Constants (kq) for
the DCA-Fluorescence Quenching with 1 in Various Solvents
∆G b/eV
et
NMQ BF -
kq/1010 M-1 s-1
ox
+
solvent
E
1/2(1)a/V
DCA
TCNB
4
CH3CN
CH2Cl2
C6H6
+1.35
+1.53
c
-0.61
-0.68
-1.72
-1.93
-1.32
-1.67
1.6
1.2
0.9
a In CH3CN containing 0.1 M Et4N+ClO4- or in CH2Cl2 containing 0.1
-
M n-Bu4N+ClO4
.
b ∆Get ) Eox1/2(1) - Ered1/2(Sens) - E0-0(Sens) - e2/
ꢀr. Ered1/2(DCA) is -0.95 and -0.89 V vs SCE and E0-0(DCA) is 2.91 and
2.87 eV in CH3CN and CH2Cl2, respectively. Ered1/2(TCNB) is -0.73 and
-0.57 V vs SCE and E0-0(TCNB) is 3.80 and 3.80 eV in CH3CN and
CH2Cl2, respectively. Ered1/2(NMQ+BF4-) is -0.80 and -0.51 V vs SCE
and E0-0(NMQ+BF4-) is 3.47 and 3.48 eV in CH3CN and CH2Cl2,
respectively. The coulomb term (e2/ꢀr) is disregarded in CH3CN and is
0.23 eV in CH2Cl2. c No attempt.
a Abbreviations: CRCL, cation radical cleavage; DRCY, diradical
cyclization; PET, photoinduced electron transfer; Sens, sensitizer; Ar,
4-MeOC6H4.
The oxidation potential (Eox1/2) of 1 (+1.35 V vs SCE in
acetonitrile) is low enough to quench the excited singlet DCA,
TCNB, and NMQ+BF4 in acetonitrile, dichloromethane, or
-
E-5) established the stereochemical consequences for the initial
ring cleavage to form a trimethylenemethane (TMM) cation
radical intermediate. Herein we report spectroscopic and
calorimetric evidence together with results of product analysis,
which support the participation of two different types of
intermediates in the PET methylenecyclopropane rearrangements
of 1, 2, and 3 shown in Scheme 1.
benzene by electron transfer as suggested by the calculated free
energy changes (∆Get) and the observed quenching rate
constants (kq) shown in Table 1. In fact, as shown in Table 2,
the DCA-sensitized photoreactions of d2-1 in acetonitrile,
dichloromethane, and benzene under nitrogen occur in excellent
yields via the degenerate rearrangement, giving rise to a near
57:439 photostationary mixture of d2-1 and d2-1′. Similar
photoreaction of 1 under oxygen gives 3,3-bis(4-methoxyphe-
nyl)-4-methylene-1,2-dioxolane (8)2,10 quantitatively as a major
product in acetonitrile or dichloromethane, but in a poor yield
in the less polar solvent benzene. The TCNB-sensitized pho-
toreactions in acetonitrile give similar results, though oxygen-
ation in dichloromethane is less efficient. In contrast, the
degenerate rearrangement of d2-1 with NMQ+BF4- and toluene
(TOL, as a cosensitizer) in acetonitrile is extremely slow, while
oxygenation proceeds rapidly to form 8 quantitatively. From
those experimental results, it is conceivable that an immediate
precursor for the degenerate rearrangement may be different
from one captured by oxygen to form 8. Under those sensitized
conditions, TR absorption spectroscopy on laser flash photolysis
of 1 was performed as described later.
Results and Discussion
PET Degenerate Rearrangement of d2-1 and Reversible
Methylenecyclopropane Rearrangement between 2 and 3.
We have reported the degenerate rearrangement of d2-1 and
oxygenation of 1 triggered by PET using a triplet sensitizer such
as chloranil (CA) or anthraquinone (AQ).2,7 For spectroscopic
convenience8 in laser flash photolyses, electron-transfer pho-
toreactions of d2-1 and oxygenations of 1 were investigated
using a singlet sensitizer such as 9,10-dicycanoanthracene
(DCA), 1,2,4,5-tetracyanobenzene (TCNB), or N-methylquino-
linium tetrafluoroborate (NMQ+BF4-) (Scheme 2).
(4) Miyashi, T.; Takahashi, Y.; Kamata, M.; Yokogawa, K.; Ohaku, H.; Mukai,
T. Phys. Org. Chem. 1986, Stud. Org. Chem. 1986, 31, 363-368.
(5) Miyashi, T.; Takahashi, Y.; Ohaku, H.; Ikeda, H.; Morishima, S.-i. Pure
Appl. Chem. 1991, 63, 223-230.
(9) The ratio seems to be affected by the kinetic isotope effects on the CRCL
and DRCY step. Similar kinetic isotope effects have been found to be
responsible for the 52:48 ratio observed in the PET degenerate Cope
rearrangement between 3,3,4,4-tetradeuterio-2,5-diphenyl-1,5-hexadiene and
the corresponding 1,1,2,2-tetradeuterio derivative which proceeds in a cation
radical cyclization-diradical cleaVage mechanism.1
(10) (a) Miyashi, T.; Kamata, M.; Mukai, T. J. Am. Chem. Soc. 1986, 108, 2755-
2757. (b) Miyashi, T.; Kamata, M.; Mukai, T. J. Am. Chem. Soc. 1987,
109, 2780-2788.
(6) Ikeda, H.; Nakamura, T.; Miyashi, T.; Goodman, J. L.; Akiyama, K.; Tero-
Kubota, S.; Houmam, A.; Wayner, D. D. M. J. Am. Chem. Soc. 1998, 120,
5832-5833.
(7) Miyashi, T.; Takahashi, Y.; Mukai, T.; Roth, H. D.; Schilling, M. L. M. J.
Am. Chem. Soc. 1985, 107, 1079-1080.
(8) The use of TCNB and NMQ+BF4- makes it easy to observe TR absorption
spectra at the region below 450 nm, while the lifetime of cation radicals is
extended by coexistence of BP with DCA.
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PET Methylenecyclopropane Rearrangements
A R T I C L E S
Table 2. Photostationary Ratios (d2-1:d2-1′) in the Degenerate Methylenecyclopropane Rearrangement of d2-1, Yields of 8 in the
Photooxygenation of 1, and Transient Absorption Maxima (λ500
of 1 under Various PET Conditions
and λ350max) around 500 and 350 nm Observed in Laser Flash Photolyses
max
c
rearrangementa
photooxygenationb
transient absorption
350 d
conditions
DCA/CH3CN
DCA/CH2Cl2
DCA/C6H6
DCA-BP/CH3CN
TCNB/CH3CN
TCNB/CH2Cl2
TCNB-BP/CH2Cl2
NMQ+BF4-/CH3CN
NMQ+BF4--TOL/CH3CN
time/h
d2-1:d2-1′
time/min
yield of 8/%
λ500max/nm
λ350max/nm
∆OD500/∆OD
4.5e
2
58:42
56:44
57:43
15
15
120
15
20
30
15
15
5
100
19
3
100
100
4
96
100
100
f
f
f
g
g
g
3
slowe
4.5e
3
494
500
f
508
500
498
g
54:46
56:44
56:44
351
354
354
352
350
1.3
∼0
15
2
slowe
slowe
3.6
>10
a Under N2. [d2-1] ) 0.1 M in deuterated solvents. b Under O2. [1] ) 0.01 M. c Under N2 or Ar. [1] ) 0.001 M. d Ratio of optical density (∆OD) of
transient absorption at λ500 to that at λ350 at 200 ns after excitation. e In degassed CH3CN. f No transient absorption was observed. g Not observable
max
max
under the conditions employed.
Scheme 3. Methylenecyclopropane Rearrangement and
Oxygenation of 2 and 3 Triggered by PETa
Scheme 4. Assignment of the Transients Observed by TR
Absorption Spectroscopy on Laser Flash Photolyses of 1 under
Various PET Conditions
-
a Sens: DCA, TCNB, or NMQ+BF4
.
The reversible methylenecyclopropane rearrangement of 2 and
3 is also triggered by PET. The DCA-sensitized photoreaction
of 2 in acetonitrile gives a 40:60 photostationary mixture of 2
and 3 under nitrogen and gives 4-cyclopropylidene-3,3-bis(4-
methoxyphenyl)-1,2-dioxolane (9)4,5 under oxygen (Scheme 3).
The similarity in the reactivities of 1, 2, and 3 under PET
conditions suggests that 1, 2, and 3 undergo rearrangement in
a similar reaction sequence.
is used as a sensitizer in acetonitrile, the 500 nm transient is
predominant with the large ∆OD500/∆OD350 (Figure 1c).
The observed experimental and spectroscopic results shown
in Table 2 provide a mechanistic clue to the assignment of the
500 and 350 nm transients. In the TCNB-sensitized reactions
in dichloromethane where the 350 nm transient is predominant,
rearrangement occurs efficiently but oxygenation does not. In
contrast, in the NMQ+BF4--TOL-cosensitized reactions where
the 500 nm transient is predominant, oxygenation occurs rapidly,
while rearrangement is extremely slow. These facts reasonably
suggest that the 500 and 350 nm transients are the immediate
precursors, respectively, for oxygenation and rearrangement. In
fact, both oxygenation and rearrangement occur efficiently with
TCNB in acetonitrile, where both the 500 and 350 nm transients
are observed as shown in Figure 1a.
TR Absorption Spectroscopy on Laser Flash Photolyses
of 1, 2, and 3. In an effort to characterize the proposed TMM
intermediates in the rearrangements and oxygenations of 1, 2,
and 3, nanosecond TR absorption spectroscopy on laser flash
photolysis was performed with DCA, TCNB, or NMQ+BF4
-
as a sensitizer. The spectrum obtained on laser flash photolysis
of 1 with DCA in acetonitrile exhibits no clear transient
absorption band, but that of 1 with DCA and biphenyl (BP, as
a cosensitizer) exhibits a transient with λmax at 494 nm in
acetonitrile. Since the region below 400 nm is not transparent
in the DCA-sensitized reactions, relevant results were obtained
The identities of the 500 and 350 nm transients may be
inferred from λmax of 1,1-bis(4-methoxyphenyl)ethyl cation 10+
(484 nm in acetonitrile and 499 nm in dichloromethane11), the
corresponding radical 10• (349 nm in acetonitrile and 352 nm
in dichloromethane), and 1,1,4,4-tetrakis(4-methoxyphenyl)-
butane-1,4-diyl cation radical 11•+ (500 and 350 nm in 1,2-
with TCNB or NMQ+BF4 (Scheme 4). As shown in Figure
-
1a, laser excitation (308 nm) of TCNB with 1 in acetonitrile
gives two transient absorption bands with λmax at 500 and 351
nm, showing the ratio of optical densities ∆OD500/∆OD350
)
1.3, whereas only the intense 350 nm transient is observed in
dichloromethane (Figure 1b). The 500 nm transient corresponds
-
to the 494 nm transient. On the other hand, when NMQ+BF4
(11) Heublein, G.; Helbig, M. Tetrahedron 1974, 30, 2533-2536.
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A R T I C L E S
Ikeda et al.
Scheme 5. PET Reactions of 1 in the Presence of DCDMF or O2
that 6•• with λmax at 350 nm is formed from 6•+ with λmax at
500 nm by back electron transfer. Although a kinetic analysis
of the rate of back electron transfer is not successful, clean first-
order kinetics is obtained for the decay of the 350 nm transient
observed with TCNB as a sensitizer in dichloromethane. The
temperature dependence of the first-order rate constants mea-
sured at six points between 266.7 and 299.5 K provides the
Figure 1. TR absorption spectra observed at a delay time of 60 ns (i), 200
ns (ii), 1 µs (iii), and 5 µs (iv) after the laser flash photolyses (308 nm
excitation for TCNB and NMQ+BF4-, 355 nm excitation for DCA) of 1
under various PET conditions.
Arrhenius parameters Ea ) 2.9 kcal/mol and A ) 107.2 s-1
.
These values can be ascribed to the activation parameters for
dichloroethane, 490 and 340 nm in acetonitrile12). Therefore,
we assign λmax at 500 and 350 nm to the 1,1-bis(4-methoxy-
phenyl)ethyl cation moiety of the largely twisted TMM cation
radical 6•+ and the 1,1-bis(4-methoxyphenyl)ethyl radical moiety
of the largely twisted TMM diradical 6••, respectively. The most
reasonable process to form 6•• is back electron transfer from a
sensitizer anion radical to 6•+ via a contact or solvent-separated
ion radical pair [6•+/Sens•-]. Back electron transfer from
TCNB•- to 6•+ proceeds in dichloromethane much more
efficiently than in polar acetonitrile, while that from NMQ• to
6•+ must be inefficient because of the ready cage-escape of
NMQ• and 6•+. The cage-escaped 6•+ is then efficiently captured
by oxygen.
cyclization of 6•• to 1.
Transient spectra observed in the TCNB-sensitized laser flash
photolyses of 2 and 3 in acetonitrile are shown in Figure 2a,b.
Transient spectra from 2 and 3 bear close resemblance,
indicating the generation of a similar transient species from 2
and 3. By comparison with λmax of 6•+ and 6••, the transient
absorption bands at 500 and 350 nm are assigned to TMM cation
radical 7•+ and diradical 7••, respectively (Scheme 6). The effects
of solvent and sensitizer on the generation of the transient
species from 2 and 3 are similar to those observed from 1. Thus,
the TCNB-sensitized laser flash photolyses of 2 and 3 in
dichloromethane give only the 350 nm transient as shown in
Figure 2c,d, whereas the intense 500 nm transient is predominant
-
with NMQ+BF4 and TOL in acetonitrile as shown in Figure
The decay kinetics of the 500 and 350 nm transients support
their assignment to 6•+ and 6••. The 500 nm transient observed
with DCA and BP persists for several microseconds under
nitrogen in acetonitrile, but disappears completely within 200
ns under oxygen, whereby a new transient absorption band with
2e. Therefore, it is evident that two different types of TMM
intermediates participate in the singlet-sensitized electron-
transfer photoreactions of 1, 2, and 3 as shown in Scheme 1.
EPR Spectroscopy for PET Reactions of 1-5. (1) TR-
EPR for 1-3: Evidence for the Formation of Twisted TMM
Cation Radicals. Methylenecyclopropanes 1, 2, and 3 were
subjected to EPR spectroscopic studies to help determine the
structures of TMM cation radical intermediates.14 Figure 3a
shows the CIDEP spectrum observed at delay time of 1 µs after
pulsed 355 nm irradiation of a DMSO solution of 1 and CA at
ambient temperatures. The E*/E spectrum is easily reproduced
by the superposition of triplet (E) and radial pair (E/A)16 signals
as shown in Figure 3b, where E*, E, and A denote excess
emission, emission, and enhanced absorption of the microwave
radiation, respectively. The intense signal peak at 343.135 mT
(g ) 2.0058) is assigned to CA•-. The well-separated five lines
are analyzed with two hyperfine splitting (hfs) constants (RH )
1.438 and 1.380 mT) corresponding to 6•+ (g ) 2.0031)
λ
max at 518 nm arises, as shown in Figure 1d. This transient is
presumably due to the peroxide cation radical 14•+, leading to
8. The 350 nm transient observed with TCNB in dichlo-
romethane decays with the first-order rate constant k ) 1.1 ×
105 s-1 at 20 °C in the absence of a trapping reagent, whereas
it decays with the second-order rate constant k ) 5.2 × 106
M-1 s-1 in the presence of dicyanodimethylfumarate (DCDMF),
indicating chemical capture of 6•• by DCDMF. In fact, under
those sensitized conditions the [3 + 2] cycloadducts,13 trans-
and cis-1,2-dicarbomethoxy-1,2-dicyano-3,3-bis(4-methoxyphe-
nyl)-4-methylenecyclopentane (trans-12, cis-12) and trans-1,2-
dicarbomethoxy-1,2-dicyano-4-(bis(4-methoxyphenyl)methylene)-
cyclopentane (13), are formed in 23, 23, and 46% yields,
respectively, in dichloromethane (Scheme 5). Fast and slow
decay profiles of the 500 and 350 nm transient, respectively,
shown in Figure 1a, are not inconsistent with the conclusion
(14) For an EPR spectrum of the parent TMM cation radical, see ref 15.
(15) Komaguchi, K.; Shiotani, M.; Lund, A. Chem. Phys. Lett. 1997, 265, 217-
223.
(12) Fujita, M.; Shindo, A.; Ishida, A.; Majima, T.; Takamuku, S.; Fukuzumi,
S. Bull. Chem. Soc. Jpn. 1996, 69, 743-749.
(13) For the mechanism of the [3 + 2] cycloadditions of 1, see ref 10.
(16) McLauchlan, K. A. In Modern Pulsed and Continuous-WaVe Electron Spin
Resonance; Keva, L., Bowman, M. K., Ed.; Wiley: New York, 1990; pp
285-363.
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PET Methylenecyclopropane Rearrangements
A R T I C L E S
Figure 3. CIDEP spectrum of 6•+ observed at a delay time of 1 µs after
the laser excitation (355 nm) of CA (10 mM) with 1 (50 mM) in DMSO at
ambient temperature (a) and its simulation with the parameters given in
Scheme 7 (b). Asterisks (*) represent emissions due to CA•-
.
Scheme 7. Assignment of the Transients Observed by TR-EPR
on Laser Flash Photolyses of 1 under Various PET Conditions
b
a In mT. See ref 17.
with a previous proposal based on the CIDNP experiments by
Miyashi, Roth, and co-workers.7
Figure 2. TR absorption spectra observed at a delay time of 200 ns (i), 1
µs (ii), and 5 µs (iii) after the laser flash photolyses (308 nm excitation) of
2 and 3 under various PET conditions.
The CIDEP spectrum of 7•+ is similarly obtained on laser
excitation (355 nm) of a propylene carbonate (PC) solution of
CA and 2 or 3 at ambient temperatures (Figure 4, Scheme 8).
As expected from TR absorption spectroscopy on laser flash
photolysis of 2 and 3, the CIDEP spectrum from 2 nearly
coincides with that from 3, showing slight differences in the
contribution ratio of triplet (E) and radical pair (E/A) mecha-
nisms as shown in Figure 4a-c.
Scheme 6. Assignment of the Transients Observed by TR
Absorption Spectroscopy on Laser Flash Photolyses of 2 and 3
under Various PET Conditions
(2) TR-EPR for 4, Z-5, and E-5: Stereochemistry for the
Formation of Twisted TMM Cation Radicals at the CRCL
Step. Recent calculations of Cramer and Smith21 have shown
that in the parent TMM the planar structure is more stable than
the bisected one by ca. 14 kcal/mol. Nevertheless, the dianisyl-
substituted TMM diradicals 6•• and 7•• are twisted largely.
Because their cation radical precursors 6•+ and 7•+ are also the
largely twisted species, the stereochemistry of cleavage at the
CRCL step is most responsible for the formation of the twisted
structures of 6•• and 7••. As a previous CIDNP study7 proposed,
the less bulky C-3 methylene group of 1•+ preferentially rotates
to form 6•+ in the least motion pathway, keeping the σ-orbital
of the C-1 dianisylmethylene group orthogonal to the methylene
π-system as shown in Scheme 1. Probably, the double rotation
at C-1 and C-3 would cause significant steric destabilization of
the resulting planar TMM cation radical. To gain further insight
into the stereochemistry of cleavage at the CRCL step, we
investigated the CA-sensitized photoreactions of the monoanisyl
derivatives 2-(4-methoxyphenyl)-1-methylenecyclopropane (4),
Z-5, and E-5 by TR-EPR.
(Scheme 7). The contribution from the 1,1-bis(4-methoxyphe-
nyl)ethyl moiety to the spectrum is in the range of the line width,
indicating a negligibly small population of the unpaired electron
in this moiety. Since the hfs constants and the g-value are close
to those of the parent allyl radical,17,19 the unpaired electron
must be located mainly on the allyl moiety of 6•+, indicating
its largely twisted structure. This conclusion is thus consistent
(17) Krusic, P. J.; Meakin, P.; Smart, B. E. J. Am. Chem. Soc. 1974, 96, 6211-
6213. See also ref 18.
(18) Fessenden, R. W.; Schuler, R. H. J. Chem. Phys. 1963, 39, 2147-2195.
(19) See also the data of 2-methylallyl radical20 shown in Scheme 9.
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A R T I C L E S
Ikeda et al.
Figure 4. CIDEP spectra of 7•+ observed at a delay time of 500 ns after
the laser excitation (355 nm) of CA (5 mM) with 2 (10 mM) (a) and of CA
(20 mM) with 3 (20 mM) (b) in PC under Ar at ambient temperature and
its simulation with the parameters given in Scheme 8 (c). Asterisks (*) and
pound signs (#) represent emissions due to CA•- and a radical species
derived from PC, respectively.
Scheme 8. Assignment of the Transients Observed by TR-EPR
on Laser Flash Photolyses of 2 and 3 under Various PET
Conditions
Figure 5. CIDEP spectra of 15•+ (a), Z-16•+ (c), and E-16•+ (e) observed
at a delay time of 500 ns after the laser excitation (355 nm) of CA (20
mM) with 4 (10 mM), Z-5, and E-5, respectively, in DMSO under Ar at
ambient temperature and simulated spectra of 15•+ (b), Z-16•+ (d), and
E-16•+ (f) with the parameters given in Scheme 9. Asterisks (*) and pound
signs (#) represent emissions due to CA•- and a radical species (probably
b
a In mT. See ref 17.
Figure 5a shows a CIDEP spectrum obtained on laser
excitation of CA with 4 in DMSO at ambient temperatures. The
structure of 15•+ is confirmed by three hfs constants shown in
Scheme 9 obtained from the hyperfine structure and spectral
simulation shown in Figure 5b. Similar CIDEP spectra obtained
by stereoselective cleavage of Z-5•+ and E-5•+ are shown in
Figure 5 together with simulated spectra. The structures of
Z-16•+ and E-16•+ are similarly determined by comparison of
their hfs constants with those of (Z)- and (E)-1-methylallyl
radicals20 shown in Scheme 9. Apparently, the unpaired electrons
in 15•+, Z-16•+, and E-16•+ are localized predominantly on the
allyl or methylallyl radical moiety. However, very small hfs
•
CH3SOCH2 ) derived from DMSO, respectively.
constants are observed for the hydrogen (HC-2 in Scheme 10)
on the carbon bearing the anisyl group in 15•+, Z-16•+, and
E-16•+.
On the assumption that the hfs of HC-2 of 15•+ originates
mainly from spin density at C-1 of the allyl radical part as does
that of 2-methylallyl, the dihedral angle (θ in Scheme 10) be-
tween two planes of the allyl radical moiety and the Ar-C+-H
moiety in 15•+ is estimated using the following equation:23
aH(HC-2) ) [B0 + B2 cos2(90 - θ)]F(C-1), where aH(HC-2), B0
(21) Cramer, C. J.; Smith, B. A. J. Phys. Chem. 1996, 100, 9664-9670. See
(20) Kochi, J. K.; Krusic, P. J. J. Am. Chem. Soc. 1968, 90, 7157-7159.
also ref 22.
9
9152 J. AM. CHEM. SOC. VOL. 125, NO. 30, 2003

PET Methylenecyclopropane Rearrangements
A R T I C L E S
Table 3. ZFS Values of 6••, 7••, and the Parent TMM Diradical
Scheme 9. Assignment of the Transients Observed by TR-EPR
on Laser Flash Photolyses of 4, Z-5, and E-5 under PET
Conditions
|D/hc|/cm-1
0.0116
0.012
0.024
|E/hc|/cm-1
0.0038
0.0039
0
6••
7••
parent TMMa
a See ref 27.
orientation of the anisyl and methyl groups. Probably, a potential
rotation of the anisylmethylene group of 4•+, Z-5•+, and E-5•+
and of the dianisylmethylene group of 1•+, 2•+, and 3•+ would
induce steric compression and thereby inhibit the formation of
planar TMM cation radical intermediates.
(3) Steady State (SS)-EPR for 1-3: Observation of the
Twisted TMM Generated in the PET Reactions. For the
spectroscopic identification of diradicals 6•• and 7•• from 1 and
2, the SS-EPR measurements were performed at cryogenic
temperature. Upon the pulsed 355 nm irradiation of AQ with 1
in a dichloromethane matrix at 22 K, a characteristic EPR
spectrum of a randomly oriented triplet species ascribed to 6••
is observed together with a signal due to 6•+. In addition to the
|∆Ms| ) 1 transition signals, a weak |∆Ms| ) 2 transition is
observed at 167.3 mT. The triplet signal of 6•• persists at
cryogenic temperature, and the Curie plot26 of the |∆Ms| ) 2
transition signal intensities gives a straight line between 4.2 and
40 K, indicating the triplet ground state of 6•• as the usual TMM
diradical derivatives. The AQ-sensitized photoreaction of 2 gives
a similar EPR spectrum with a |∆Ms| ) 2 transition at 167.5
mT, which is ascribed to 7••.
b
c
a Conditions; hν/CA, DMSO, 20 °C. In mT. See ref 20.
Scheme 10. Numbering and Definition of θ in 15•+, Z-16•+, and
E-16•+
As shown in Table 3, the zero-field splitting (ZFS) parameters
of 6•• and 7•• are characterized by relatively small |D/hc| and
large |E/hc| values as compared with those of the parent planar
TMM diradical.27 Small |D/hc| values may reflect the nonplanar
structures of 6•• and 7••. The deduction of the |D/hc| values
caused by the deviation from planar conformation has been
reported for a series of biphenyl derivatives,28 conjugated
enones,29 and conjugated TMM diradical derivatives.30
The most likely process to form 6•• and 7•• is back electron
transfer from a sensitizer anion radical to 6•+ and 7•+ without
structural change, respectively. Taking account of the fact that
TR absorption spectra of 6•• and 7•• are coincident with that of
10•, it is reasonable to conclude that both 6•• and 7•• are also
twisted. Consequently, small |D/hc| values of 6•• and 7•• are
consistent with their large |E/hc| values which are related to
the molecular symmetry. EPR spectroscopic results clearly
indicate that two different types of TMM intermediates are
formed also in the triplet-sensitized electron-transfer photore-
actions of 1, 2, and 3.
and B2, and F(C-1) are the hfs constant of HC-2, constant terms,
and spin density of C-1, respectively. When the spin polarization
effects are very small for HC-2, the value B0 can be disregarded.
The value B2F(C-1) ) 0.638 mT is obtained from the hfs
constant for the methyl proton of the 2-methylallyl radical [aH-
(CH3) ) 0.319 mT],20 in which cos2(90 - θ) = cos2(90 - θ)
) 0.5, assuming free rotation of the methyl group of the
2-methylallyl radical.24 Using the estimated value B2F(C-1) )
0.638 mT and experimental value aH(HC-2) ) 0.239 mT, θ in
15•+ is calculated to be 38°. Since the Ar-C+-H group in 15•+
is less bulky than the Ar-C+-Ar group in 6•+, θ of 15•+ must
be reduced as compared with that of 6•+. In fact, a preliminary
calculation using AM1 UHF parameters25 suggests that θ of
6•+ and 15•+ are ca. 44° and 24°, respectively. Nevertheless,
the unpaired electrons in Z-16•+ and E-16•+ are localized
predominantly on the methylallyl radical moiety. Note that they
are formed respectively from Z-5•+ and E-5•+ by rotating the
less bulky C-3 methylene groups and keeping intact the relative
Mechanism of the Degenerate Methylenecyclopropane
Rearrangement Based on Energetics. (1) Determination of
the Energy of [6•+/DCA•-] by Photoacoustic Calorimetry.
A reasonable mechanism to explain the spectroscopic detection
of 6•• and 6•+ is a cation radical cleavage (CRCL)-diradical
cyclization (DRCY) mechanism shown in Scheme 1a. In this
(26) A figure of the Curie plot for 6•• is available as Supporting Information in
ref 6.
(22) (a) Davidson, E. R.; Borden, W. T. J. Am. Chem. Soc. 1977, 99, 2053-
2060. (b) Davis, J. H.; Goddard, W. A., III. J. Am. Chem. Soc. 1977, 99,
4242-4247.
(27) (a) Dowd, P. J. Am. Chem. Soc. 1966, 88, 2587-2589. (b) Dowd, P. Acc.
Chem. Res. 1972, 5, 242-248. (c) Dixon, D. A.; Dunning, T. H., Jr.; Eades,
R. A.; Kleier, D. A. J. Am. Chem. Soc. 1981, 103, 2878-2880.
(28) Tanigaki, K.; Taguchi, N.; Yagi, M.; Higuchi, J. Bull. Chem. Soc. Jpn.
1989, 62, 668-673.
(23) Heller, C.; McConnell, H. M. J. Chem. Phys. 1960, 32, 1535-1539.
(24) A PM3 UHF calculation suggests that there is no significant activation
energy for free rotation of the methyl group in the 2-methylallyl radical.
See also ref 25.
(29) Yamauchi, S.; Hirota, N.; Higuchi, J. J. Phys. Chem. 1988, 92, 2129-
(25) A preliminary semiempirical calculation was carried out using MOPAC
ver. 6 on a CAChe WorkSystem.
2133.
(30) Bushby, R. J.; Jarecki, C. Tetrahedron Lett. 1986, 27, 2053-2056.
9
J. AM. CHEM. SOC. VOL. 125, NO. 30, 2003 9153

A R T I C L E S
Ikeda et al.
∆Hirp([6•+/DCA•-]) is determined to be 37.0 ( 0.8 kcal/mol.38
Since the free energy change for the formation of [1•+/DCA•-]
is 53 kcal/mol,39 the ion radical pair [6•+/DCA•-] lies 16 kcal/
mol lower in energy than [1•+/DCA•-]. This is supported by an
MNDO UHF calculation25 which predicts that 6•+ lies 18 kcal/
mol lower than 1•+. This suggests that cyclization of d2-6•+ to
d2-1•+ (or d2-1′•+) is significantly endergonic and unlikely to
operate in the degenerate rearrangement of d2-1.
(2) Energetics from [6•+/DCA•-]. In contrast, the back
electron transfer from DCA•- to 6•+ to form 6•• is estimated to
be exergonic by 21 kcal/mol, using the oxidation potential of
10• (Eox ) -0.06 V vs SCE in acetonitrile) determined by
1/2
photomodulation voltammetry, on the assumption that Eox1/2 of
the largely twisted 6•• is comparable with that of 10•. Accord-
Figure 6. Schematic diagram of photoacoustic calorimetry for the 1/DCA-
BP system. Relative energy is represented in kcal/mol. The value, 37.0 kcal/
mol, is obtained using eq 1. For other values, see text and footnote 39.
ingly, this electron-transfer process should take place rapidly
40,43
with an estimated rate constant (kbet
)
of 1.0 × 1010 s-1 in
mechanism, cation radical d2-6•+ is formed from d2-1•+ and d2-
1′•+ in the CRCL step and cyclization of diradical d2-6••
regenerates d2-1 and d2-1′ in the DRCY step. The key process
in this mechanism is thus the formation of d2-6•• from d2-6•+
by back electron transfer, which will formally compete with
cyclization of d2-6•+ to regenerate d2-1•+ and d2-1′•+. However,
this cyclization should be energetically unfavorable on the basis
of the calculations of Borden31 and Kikuchi32 that suggest
exergonic ring cleavage of methylenecyclopropane cation
radicals. To investigate the relative energetics of these processes,
photoacoustic calorimetry was employed.33
acetonitrile at 20 °C. Although the ground state of 6•• is probably
triplet (36••) as shown above, 6•• right after back electron transfer
under those conditions must be singlet (16••) because a fast
sequence of the CRCL and back electron transfer in [1•+/
1
DCA•-] conserves the singlet multiplicity originated in 1DCA.45
Diradical 6•• is thus estimated to lie 16 kcal/mol higher in
1
energy than 1, as shown in Figure 7. According to the calculation
of Cramer and Smith,21 the energy gap between the parent
bisected-singlet and -triplet TMM diradicals is less than 2 kcal/
mol. If the energy gap between the largely twisted 16•• and 36••
3
is also small, 6•• will lie 15 ( 1 kcal/mol47 higher in energy
Photoacoustic calorimetry is a useful technique to determine
the energetics of various photoreactions, especially the enthalpy
of formation of the ion radical pair (∆Hirp) of PET reactions,
as has been reported previously.1,33-36 For reasons of experi-
mental limitation, photoacoustic calorimetry was performed for
the 1/DCA-BP system in acetonitrile according to the reported
procedure.1,33-36 Figure 6 shows a schematic diagram for the
1/DCA-BP system. Actually, the value determined by photo-
acoustic calorimetry under these conditions is the enthalpy of
than 1 and will cyclize to 1.
(35) (a) Rothberg, L. J.; Simon, J. D.; Bernstein, M.; Peters, K. S. J. Am. Chem.
Soc. 1983, 105, 3464-3468. (b) Goodman, J. L.; Peters, K. S. J. Am. Chem.
Soc. 1986, 108, 1700-1701. (c) Ci, X.; da Silva, R. S.; Goodman, J. L.;
Nicodem, D. E.; Whitten, D. G. J. Am. Chem. Soc. 1988, 110, 8548-
8550. (d) Zona, T. A.; Goodman, J. L. Tetrahedron Lett. 1992, 33, 6093-
6096. (e) LaVilla, J. A.; Goodman, J. L. J. Am. Chem. Soc. 1989, 111,
712-714.
(36) Ikeda, H.; Takasaki, T.; Takahashi, Y.; Konno, A.; Matsumoto, M.; Hoshi,
Y.; Aoki, T.; Suzuki, T.; Goodman, J. L.; Miyashi, T. J. Org. Chem. 1999,
64, 1640-1649.
(37) At the same time, a parameter τ2 ) 237 ( 110 ns was obtained.
(38) Although BP coexisted with 1 and DCA, the value of ∆Hirp([6•+/DCA•-])
can be obtained independently of BP as the free enthalpy changes between
1 + DCA + BP and 1•+ + DCA•- + BP states.
formation of free ion radicals (6•+ + DCA•-), ∆Hfir(6•+
+
DCA•-). It can be regarded, however, as ∆Hirp of the ion radical
pair [6•+/DCA•-], ∆Hirp([6•+/DCA•-]), because energy gaps
between an ion radical pair and the corresponding free ion
radicals are generally quite small in acetonitrile. The ∆Hirp([6•+/
DCA•-]) value can be expressed by eqs 1 and 2,35 where hν,
Φ, and E([BP•+/DCA•-]) are photon energy (420 nm, 68.1 kcal/
mol), the quantum yield to form [BP•+/DCA•-], and the ion
radical pair energy (66.2 kcal/mol) of [BP•+/DCA•-], determined
(39) The value, 53 kcal/mol, can be calculated using the redox potentials of 1
and DCA.
(40) The rate constant (kbet) for the back electron transfer in [6•+/DCA•-] at 20
°C in acetonitrile was calculated using the following equations (3,41 4,41
and 5) and parameters reported by Kikuchi and co-workers.42
(λs + ∆Gbet + whν)2
1/2
∞
4π3
e-ssw
2
kbet
)
|V|
exp
-
(3)
∑
(
)
(
)
{
}
h2λskbT
w!
4λskbT
w ) 0
S ) λv/hν
(4)
(5)
from the redox potentials of BP (Eox ) +1.92 V vs SCE in
1/2
∆Gbet(eV) ) -[Eox1/2(10•) - Ered1/2(DCA)]
acetonitrile) and DCA, respectively.
in which parameters V, λs, λv, ν, and ∆Gbet are, respectively, an electronic
coupling matrix element (39 cm-1), solvent reorganization energy (1.45
eV), vibration reorganization energy (0.3 eV), single average frequency
(1500 cm-1), and free energy change for electron-transfer process (-0.89
eV). In addition, h, kb, and T are Planck’s constant, Boltzmann’s constant,
and temperature (293 K), respectively.
∆Hirp([6•+/DCA•-]) ) hν(1 - R1 - R2)/Φ
Φ ) hν(1 - R1)/E([BP•+/DCA•-])
(1)
(2)
(41) (a) Miller, J. R.; Beitz, J. V.; Huddleston, R. K. J. Am. Chem. Soc. 1984,
106, 5057-5068. (b) Siders, P.; Marcus, R. A. J. Am. Chem. Soc. 1981,
103, 741-747. (c) Siders, P.; Marcus, R. A. J. Am. Chem. Soc. 1981, 103,
748-752. (d) Van Duyne, R. P.; Fischer, S. F. Chem. Phys. 1974, 5, 183-
197. (e) Ulstrup, J.; Jortner, J. J. Chem. Phys. 1975, 63, 4358-4368.
(42) Niwa, T.; Kikuchi, K.; Matsushita, N.; Hayashi, M.; Katagiri, T.; Takahashi,
Y.; Miyashi, T. J. Phys. Chem. 1993, 97, 11960-11964.
From the deconvolution fitting parameters (R1 ) 0.27 ( 0.03
and R2 ) 0.28 ( 0.04)37 obtained from several experiments,
(31) Du, P.; Borden, W. T. J. Am. Chem. Soc. 1987, 109, 5330-5336.
(32) Takahashi, O.; Morihashi, K.; Kikuchi, O. Tetrahedron Lett. 1990, 31,
5175-5178.
(43) If parameters by Farid and co-workers44 are used, kbet is calculated to be
(33) (a) Rudzki, J. E.; Goodman, J. L.; Peters, K. S. J. Am. Chem. Soc. 1985,
107, 7849-7854. (b) Herman, M. S.; Goodman, J. L. J. Am. Chem. Soc.
1989, 111, 1849-1854. (c) Griller, D.; Wayner, D. D. M. Pure Appl. Chem.
1989, 61, 717-724.
3.1 × 108 s-1
.
(44) Gould, I. R.; Ege, D.; Moser, J. E.; Farid, S. J. Am. Chem. Soc. 1990, 112,
4290-4301.
(45) No efficient intersystem crossing from 1DCA* to 3DCA* has been observed
in acetonitrile (Φisc ) 0.003).46
(34) Peters, K. S. In Kinetics and Spectroscopy of Carbenes and Biradicals;
Platz, M. S., Ed.; Plenum: New York, 1990; pp 37-49.
(46) Hanaoka, K. Master Thesis, Tohoku University, 1991.
9
9154 J. AM. CHEM. SOC. VOL. 125, NO. 30, 2003

PET Methylenecyclopropane Rearrangements
A R T I C L E S
3
by TR-EPR must be the cage-escaped 6•+ from [6•+/CA•-].
3
3
The energy of 6•• formed by back electron transfer in [6•+/
CA•-]48 can be calculated to be ca. 16 kcal/mol because the
back electron transfer is expected to be endergonic by ca. +1
kcal/mol using Eox1/2(10•) and the reduction potential of CA.47
Thus, the energy level of 6•• formed in this process will be
nearly the same as that of 6•• formed in the DCA-sensitized
3
3
photoreaction. This agreement strongly suggests that evaluation
of the above energy gap between 16•• and 36•• is reasonable and
that the intersystem crossing between them is facile in both the
DCA- and CA-sensitized photoreactions. The triplet species
detected by the SS-EPR must be the resulting 36••, which rotates
the methylene group while cyclizing to regenerate the singlet
1. The Arrhenius A factor obtained for the decay of 6•• formed
in the TCNB reaction supports this possibility. The observed A
factor (107.2 s-1) is quite small and actually even smaller than
the value, A ) 109.1 s-1, observed for the decay of the parent
planar-triplet TMM diradical.49 Unusually small A factors are
often observed with reactions involving intersystem crossing.51
A plausible process to reproduce 1 at the DRCY step is, thus,
the spin-forbidden cyclization of the triplet diradical 6•• as shown
in Figure 7.
Conclusion
Figure 7. Energy diagram for the DCA- (left) and CA-sensitized (right)
PET degenerate methylenecyclopropane rearrangement of 1 in acetonitrile.
Relative energy is represented in kcal/mol. The values, 15 ( 1 and ca. 16
kcal/mol, for 36•• were obtained by analyzing the diagram of the DCA- and
CA-sensitized reaction, respectively.47
We are the first to observe both reactive cation radical and
diradical intermediates simultaneously in the same PET isomer-
ization system. By the combination of product analysis,
spectroscopy, and calorimetry, it can be concluded that the PET
degenerate rearrangement of d2-1 proceeds in a CRCL-DRCY
mechanism shown in Scheme 1a, involving the largely twisted
TMM cation radical and diradical intermediates d2-6•+ and d2-
6••, respectively. Cation radical d2-6•+ is formed in the least
motion pathway at the CRCL step in which only the C-3
methylene group rotates but the bulkier C-1 dianisylmethylene
group does not, evading stereochemical disadvantages caused
by the anisyl groups in a hypothetical planar TMM cation
radical. The back electron transfer from a sensitizer anion radical
to d2-6•+ is the most important process in this rearrangement.
Cyclization of the resulting d2-6•• completes the degenerate
rearrangement, regenerating d2-1 and d2-1′ at the DRCY step.
Close resemblances in reactivities and spectroscopic results
among 1, 2, and 3 support the operation of a CRCL-DRCY
mechanism in the reversible methylenecyclopropane rearrange-
ment between 2 and 3 as shown in Scheme 1b.
In consequence, energetics based on photoacoustic calorim-
etry suggests that back electron transfer to form diradical d2-6••
operates in preference to cyclization of d2-6•+, supporting a
CRCL-DRCY mechanism shown in Scheme 1a. Similarities
in reactivities and spectroscopic results among 1, 2, and 3
reasonably suggest that the reversible rearrangement between
2 and 3 proceeds with similar energetics.
(3) Common Mechanistic Channel between the DCA- and
CA-Sensitized Photoreactions of 1 and Role of the Triplet
TMM Diradical at the DRCY Step. Since the degenerate
methylenecyclopropane rearrangement of d2-1 occurs in both
singlet- and triplet-sensitized electron-transfer photoreactions,
there must be a common mechanistic channel between these
different photoreactions. A conceivable common process will
be the DRCY process, in which the triplet diradical d2-36••
cyclizes to regenerate d2-1 and d2-1′. Taking account of the
results of photoacoustic calorimetry of the DCA-sensitized
photoreaction, plausible mechanistic relationships between the
DCA- and CA-sensitized photoreactions are depicted in Figure
Experimental Section
General Method. See the Supporting Information.
Syntheses. See the Supporting Information for details. Physical data
of key compounds are as follows.
(i) 1,1-Bis(4-methoxyphenyl)-2-methylenecyclopropane (1): mp
31-32 °C (colorless prisms from n-hexane); 1H NMR (200 MHz,
CDCl3) δ 1.81 (dd, J ) 2.6, 2.0 Hz, 2 H), 3.77 (s, 6 H), 5.66 (t, J )
2.0 Hz, 1 H), 5.77 (d, J ) 2.6 Hz, 1 H), 6.81 (AA′BB′, J ) 8.0 Hz, 4
H), 7.20 (AA′BB′, J ) 8.0 Hz, 4 H); MS (60 °C, 13.5 eV) m/z (relative
intensity) 266 (100, M+), 251 (41), 251 (50).
3
7. The triplet diradical 6•• can be formed also in the CA-
sensitized photoreaction. The initial electron transfer from 1 to
3CA* forms 3[1•+/CA•-], which is calculated to lie 31 kcal/mol
higher in energy than 1 by redox potentials of CA (Ered
)
1/2
0.00 V vs SCE in acetonitrile) and 1. Then, the 16 kcal/mol
exergonic ring cleavage of 1•+ forms the triplet ion radical pair
3[6•+/CA•-], which is thus estimated to lie 15 kcal/mol higher
in energy than 1 (Figure 7). The cation radical species captured
(48) An alternative pathway for the formation of 36•• is a sequence of (i)
intersystem crossing of 3[6•+/CA•-] to 1[6•+/CA•-], (ii) spin-allowed back
electron transfer within 1[6•+/CA•-], and (iii) intersystem crossing of the
resulting 16••. This may be operative if the singlet-triplet energy gap
between two ion radical pairs is small.
(49) Calculated values using kinetic parameters reported in ref 50.
(50) Dowd, P.; Chow, M. J. Am. Chem. Soc. 1977, 99, 6438-6440.
(51) Salem, L.; Rowland, C. Angew. Chem., Int. Ed. Engl. 1972, 11, 92-111.
(47) To verify the relatively low energy value (15 ( 1 or ca. 16 kcal/mol) for
36••, one of the reviewers suggested comparing the energy value with the
estimated value obtained using calculation results for the parent TMM.
Although this estimate involves many assumptions and considerable
uncertainty, it suggests that the energy of 36•• is 10-24 kcal/mol, which is
not inconsistent with the value obtained for 36•• in this work. Details are
given in the Supporting Information.
9
J. AM. CHEM. SOC. VOL. 125, NO. 30, 2003 9155

A R T I C L E S
Ikeda et al.
(ii) 1,1-Bis(4-methoxyphenyl)-2-methylenespiro[2.2]pentane (2):
mp 62 °C (colorless needles form EtOH); 1H NMR (90 MHz, CCl4) δ
1.13 (m, 2 H), 1.23 (m, 2 H), 3.70 (s, 6 H), 5.16 (s, 1 H), 5.48 (s, 1 H),
6.65 (m, 4 H), 7.00 (m, 4 H); MS (80 °C, 25 eV) m/z (relative intensity)
293 (10), 292 (40, M+), 277 (23), 265 (23), 264 (100), 249 (28), 233
(19), 229 (12), 221 (13), 178 (16), 121 (15); UV (CH3CN, λmax/nm,
ꢀ/M-1cm-1) 235 (19800), 277 (2860).
and sensitizer (0.01 mmol, 0.002 M52 for DCA, 0.0005 mmol, 0.0001
M for TCNB, 0.005 mmol, 0.001 M for NMQ+BF4-) with or without
cosensitizer (0.1 mmol, 0.02 M for BP and TOL) in a Pyrex test tube
(diameter 1 cm) was saturated with oxygen by bubbling for 5 min.
These solutions were irradiated through a cutoff filter (λ > 360 nm for
DCA or λ > 300 nm for TCNB and NMQ+BF4-) with a 2 kW Xe
lamp at 20 ( 1 °C under oxygen. The yields of products were
determined by 1H NMR analyses after removal of the solvent and
cosensitizer by evaporation in vacuo and column chromatography,
respectively. The products 82 and 9 were separated from the reaction
mixture by further chromatography. Physical data of 9 are as follows.
Compound 9: colorless oil; 1H NMR (90 MHz, CCl4) δ 0.93 (m, 2
H), 1.02 (m, 2 H), 3.75 (s, 6 H), 4.78 (m, 2 H), 6.70 (m, 4 H), 7.13 (m,
4 H); 13C NMR (50 MHz, CDCl3) δ 1.82 (t), 2.98 (t), 55.23 (q), 74.61-
(t), 91.51 (s), 113.25 (d), 114.46 (s), 129.43 (d), 132.96 (s), 137.94
(s), 159.61 (s); MS (90 °C, 25 eV) m/z (relative intensity) 325 (10),
324 (33, M+), 308 (21), 292 (8), 243 (74), 205 (52), 135 (100).
TCNB-Sensitized Photoreaction of 1 with in the Presence of
DCDMF. A dichloromethane solution (3 mL) containing 1 (13.3 mg,
0.05 mmol), TCNB (2.7 mg, 0.015 mmol), and DCDMF (19.4 mg, 0.1
mmol) in a Pyrex test tube (diameter 1 cm) was saturated with argon
by bubbling for 5 min. The sample solution was irradiated for 2 h at
20 ( 1 °C under argon with a 2 kW Xe lamp through a cutoff filter (λ
> 330 nm). The yields of products were determined by 1H NMR
analyses after evaporation in vacuo. Details of the procedure to separate
trans-12, cis-12, and 13 are described below.
Separation of trans- and cis-1,2-Dicarbomethoxy-1,2-dicyano-3,3-
bis(4-methoxyphenyl)-4-methylenecyclopentane (trans-12 and cis-
12) and trans-1,2-Dicarbomethoxy-1,2-dicyano-4-(bis(4-methoxy-
phenyl)methylene)cyclopentane (13). An acetonitrile solution (20 mL)
containing 1 (226 mg, 0.85 mmol), DCA (9.7 mg, 0.042 mmol), and
DCDMF (180 mg, 0.93 mmol) in a Pyrex test tube (diameter 5 cm)
was saturated with argon by bubbling for 5 min. The sample solution
was irradiated for 15 h at 20 ( 1 °C under argon with a 2 kW Xe
lamp through a cutoff filter (λ > 360 nm). Removal of the solvent in
vacuo gave 420 mg of a brown oil. Repetitive HPLC followed by
recrystallization gave pure trans-12 (35 mg, 0.076 mmol, 9% yield),
cis-12 (43 mg, 0.093 mmol, 11% yield), and 13 (12 mg, 0.026 mmol,
3% yield).
(iii) 1-Cyclopropylidene-2,2-bis(4-methoxyphenyl)cylopropane
1
(3): mp 41 °C (colorless solid from n-hexane); H NMR (90 MHz,
CCl4) δ 1.27 (m, 4 H), 1.81 (m, 2 H), 3.70 (s, 6 H), 6.67 (m, 4 H),
7.07 (m, 4 H); MS (80 °C, 25 eV) m/z (relative intensity) 292 (6, M+),
264 (30), 220 (18), 205 (100), 176 (52). This material was labile at
room temperature and thus stored in a freezer under nitrogen.
(iv) 2-(4-Methoxyphenyl)-1-methylenecyclopropane (4): bp 45-
47 °C (1 Torr); 1H NMR (400 MHz, CDCl3) δ 1.13 (m, 1 H), 1.68 (m,
1 H), 2.57 (m, 1 H), 3.78 (s, 3 H), 5.56-5.57 (m, 2 H), 6.81 (AA′BB′,
J ) 8 Hz, 2 H), 7.08 (AA′BB′, J ) 8 Hz, 2 H); 13C NMR (100 MHz,
CDCl3) δ 14.14, 19.40, 55.25, 104.35, 113.78 (2 C), 127.42 (2 C),
133.78, 135.58, 157.89; MS (70 eV) m/z (relative intensity) 160 (94,
M+), 159 (100).
(v) (Z)-1-Ethylidene-2-(4-methoxyphenyl)cyclopropane (Z-5): col-
1
orless oil; H NMR (200 MHz, CDCl3) δ 1.06 (m, 1 H), 1.65 (m, 1
H), 1.79 (m, 3 H), 2.54 (m, 1 H), 3.78 (s, 3 H), 5.94 (m, 1 H), 6.81
(AA′BB′, J ) 8.8 Hz, 2 H), 7.04 (AA′BB′, J ) 8.8 Hz, 2 H); 13C
NMR (50 MHz, CDCl3) δ 14.69, 16.98, 18.65, 55.24, 113.84 (2 C),
114.69, 126.42, 127.33 (2 C), 134.31, 157.81; UV (CH2Cl2, λmax) 296
nm.
(vi) (E)-1-Ethylidene-2-(4-methoxyphenyl)cyclopropane (E-5):
1
colorless oil; H NMR (200 MHz, CDCl3) δ 1.03 (m, 1 H), 1.61 (m,
1 H), 1.88 (m, 3 H), 2.53 (m, 1 H), 3.77 (s, 3 H), 5.96 (m, 1 H), 6.80
(AA′BB′, J ) 8.6 Hz, 2 H), 7.06 (AA′BB′, J ) 8.6 Hz, 2 H); 13C
NMR (50 MHz, CDCl3) δ 13.03, 16.78, 19.31, 55.27, 113.78 (2 C),
114.44, 126.88, 127.48 (2 C), 134.66, 157.88; MS (70 eV) m/z (relative
intensity) 174 (54, M+), 159 (100), 143 (25), 128 (24); UV (CH2Cl2,
λmax/nm) 298.
Analyses of Time-Dependent Change in the Product Ratios for
the PET Degenerate Methylenecyclopropane Rearrangement of d2-
1. A 0.5 mL CD3CN solution containing d2-1 (0.05 mmol, 0.1 M) and
sensitizer (0.01 mmol, 0.02 M52 for DCA, 0.0005 mmol, 0.001 M for
TCNB, 0.005 mmol, 0.01 M for NMQ+BF4-) with or without
cosensitizer (0.1 mmol, 0.02 M for BP-d10 and TOL-d8) in a Pyrex
NMR tube was degasseed by five repeated freeze (-196 °C)-pump
(10-2 Torr)-thaw (0 °C) cycles and then sealed at 10-2 Torr. Similar
sample solution in CD2Cl2 or C6D6 was just saturated with nitrogen by
moderate bubbling for 5 min. These solutions were irradiated through
a cutoff filter (λ > 360 nm for DCA or λ > 300 nm for TCNB and
NMQ+BF4-) with a 2 kW Xe lamp at 20 ( 1 °C. The ratios, d2-1:d2-
trans-12: mp 143-146 °C (colorless cubes from CHCl3-n-hexane);
1H NMR (400 MHz, CDCl3) δ 3.37 (d, J ) 16.0 Hz, 1 H), 3.50 (dd,
J ) 16.0, 2.1 Hz, 1 H), 3.62 (s, 3 H), 3.78 (s, 3 H), 3.83 (s, 3 H), 3.85
(s, 3 H), 4.85 (d, J ) 1.2 Hz, 1 H), 5.66 (dd, J ) 2.1, 1.2 Hz, 1 H),
6.79 (AA′BB′, J ) 6.8, 2.0 Hz, 2 H), 6.91 (AA′BB′, J ) 6.8, 2.0 Hz,
2 H), 7.19 (AA′BB′, J ) 6.8, 2.4 Hz, 2 H), 7.57 (AA′BB′, J ) 6.8, 2.4
Hz, 2 H); 13C NMR (100 MHz, CDCl3) δ 42.25, 53.37, 54.06, 54.79,
55.21, 55.23, 66.19, 69.34, 113.11 (2 C), 113.23 (2 C), 115.70, 116.52,
117.80, 131.17 (2 C), 131.46 (2 C), 131.67, 131.76, 148.36, 159.13,
163.88, 164.63; MS (70 eV) m/z (relative intensity) 460 (M+, 100); IR
1
1′, during the photoreaction were determined by 200 MHz H NMR
analyses. Results are shown in Table 2.
(KBr) 2232, 1760 cm-1
.
Analyses of Time-Dependent Change in the Product Ratios for
the PET Methylenecyclopropane Rearrangement of 2. A 0.5 mL
CD3CN solution containing 2 (0.05 mmol, 0.1 M) and sensitizer (0.01
mmol, 0.02 M52 for DCA, 0.0005 mmol, 0.001 M for TCNB, 0.005
mmol, 0.01 M for NMQ+BF4-) with or without cosensitizer (0.1 mmol,
0.02 M for BP-d10 and TOL-d8) in a Pyrex NMR tube was saturated
with nitrogen by moderate bubbling for 5 min. These solutions were
irradiated through a cutoff filter (λ > 360 nm for DCA or λ > 300 nm
for TCNB and NMQ+BF4-) with a 2 kW Xe lamp at 20 ( 1 °C. The
ratios, 2:3, during the photoreaction were determined by 200 MHz 1H
NMR analyses.
cis-12: mp 161-162 °C (colorless cubes from CHCl3-n-hexane);
1H NMR (400 MHz, CDCl3) δ 3.36 (ddd, J ) 16.8, 2.0, 2.0 Hz, 1 H),
3.42 (s, 3 H), 3.80 (s, 3 H), 3.82 (s, 3 H), 3.83 (s, 3 H), 3.91 (ddd, J
) 16.8, 2.0, 2.0 Hz, 1 H), 4.89 (dd, J ) 2.0, 2.0 Hz, 1 H), 5.54 (dd,
J ) 2.0, 2.0 Hz, 1 H), 6.82 (AA′BB′, J ) 9.2, 2.0 Hz, 2 H), 6.89
(AA′BB′, J ) 9.2, 2.2 Hz, 2 H), 7.20 (AA′BB′, J ) 9.2, 2.4 Hz, 2 H),
7.67 (AA′BB′, J ) 9.2, 2.4 Hz, 2 H); 13C NMR (100 MHz, CDCl3) δ
41.64, 53.81, 53.93, 54.50, 55.23, 55.26, 65.90, 69.87, 113.10 (2 C),
113.26 (2 C), 116.03, 116.19, 116.68, 130.75, 131.09 (2 C), 131.53,
131.83 (2 C), 148.86, 159.13, 159.31, 165.56, 166.63; MS (70 eV)
General Procedure of the PET Reactions of 1, 2, and 3 under
Oxygen. A 5 mL solution containing substrate (0.05 mmol, 0.01 M)
m/z (relative intensity) 460 (M+, 100); IR (KBr) 2236, 1748, 1733 cm-1
.
Compound 13: mp 49-50 °C (colorless cubes from ether); 1H NMR
(400 MHz, CDCl3) δ 3.32 (d, J ) 18.2 Hz, 2 H), 3.42 (d, J ) 18.2 Hz,
2 H), 3.81 (s, 6 H), 3.92 (s, 6 H), 6.86 (AA′BB′, J ) 8.8, 2.0 Hz, 4 H),
7.08 (AA′BB′, J ) 8.8, 2.0 Hz, 4 H); 13C NMR (100 MHz, CDCl3) δ
41.54 (2 C), 53.81 (2 C), 54.56 (2 C), 55.26 (2 C), 113.85 (4 C), 115.41
(52) The sample solution was slightly suspended with DCA. The actual
concentrations of DCA are the saturated ones, which are ca. 4 × 10-5, 2
× 10-3, and 3 × 10-3 M at 20 °C, in acetonitrile, dichloromethane, and
benzene, respectively.53
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9156 J. AM. CHEM. SOC. VOL. 125, NO. 30, 2003

PET Methylenecyclopropane Rearrangements
A R T I C L E S
(2 C), 126.66 (2 C), 129.97 (4 C), 133.40 (2 C), 139.98 (2 C), 156.91
Energy Determination by TR Photoacoustic Calorimetry. Details
of the experimental procedure for photoacoustic calorimetry have been
given previously.1a,33a,b,35b Values were obtained by at least two separate
runs, and errors are within 1σ. A chart of the photoacoustic calorimetry,
waveforms, and deconvolution fitting parameters for the 1/DCA-BP
system is available as Supporting Information in ref 6.
(2 C), 164.77 (2 C); MS (70 eV) m/z (relative intensity) 460 (M+, 100);
IR (KBr) 2216, 1752 cm-1
.
Nanosecond TR Absorption Spectroscopy of PET Reactions on
Laser Flash Photolysis. A sample solution (2-4 mL) containing a
substrate (1 mM), a sensitizer, whose absorbance was adjusted to be
ca. 1.5 at the excitation wavelength, and a cosensitizer (0.2 M) in a
glass cell was saturated with argon, nitrogen, or oxygen by bubbling
for 5 min just before use. The sample solution was irradiated with 10
ns pulse beams obtained from an Excimer or YAG laser system. The
probe assembly consisted of a monitoring lamp, a polychromator
equipped with an image intensifier coupled with an image sensor, and
a personal computer. The monitoring beam was adjusted to be
perpendicular to the excitation pulse. The combination of a monochro-
mator, a photomultiplier tube, and a digitizing oscilloscope was used
for the measurements of decay kinetics. An analogous laser flash
photolysis system has been described elsewhere.54
Acknowledgment. We gratefully acknowledge financial
support by a Grant-in-Aid for Scientific Research on Priority
Areas (417) and others (Nos. 08740560, 09740536, 10640507,
122440173, and 14050008) from the Ministry of Education,
Culture, Sports, Science, and Technology (MEXT) of the
Japanese Government. We also acknowledge Professors J. A.
Berson (Yale University) and J. P. Dinnocenzo (University of
Rochester) for valuable discussions, Professor M. Kamata
(Niigata University) for informing us of a new procedure for
the preparation of 1, and Dr. Y. Hoshi (Tohoku University) for
technical assistance with laser flash photolysis experiments.
Observation of 10• on Laser Flash Photolysis. Radical 10• was
generated by laser flash photolysis (excimer, XeCl, λex ) 308 nm) of
1,1-bis(4-methoxyphenyl)ethane (0.022 M) with di-tert-butyl peroxide
(0.5 M) in acetonitrile and dichloromethane. The system for nanosecond
TR absorption spectroscopy is similar to that for PET reactions.
Supporting Information Available: Details of the experiment
including general methods, syntheses, and physical data for
substrates, and verifying the energy of 36•• (PDF). This material
is available free of charge via the Internet at http://pubs.acs.org.
(53) Ikeda, H.; Aburakawa, N.; Tanaka, F.; Fukushima, T.; Miyashi, T. Eur. J.
Org. Chem. 2001, 3445-3452.
(54) Ushida, K.; Nakayama, T.; Nakazawa, T.; Hamanoue, K.; Nagamura, T.;
Mugishima, A.; Sakimukai, S. ReV. Sci. Instrum. 1989, 60, 617-623.
JA0277982
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