Direct observation of two different types of TMM intermediates in the photoinduced electron-transfer degenerate methylenecyclopropane rearrangement [14]

Full text of DOI:10.1021/ja973760r

5832
J. Am. Chem. Soc. 1998, 120, 5832-5833
Chart 1
Direct Observation of Two Different Types of TMM
Intermediates in the Photoinduced Electron-Transfer
Degenerate Methylenecyclopropane Rearrangement
Hiroshi Ikeda,† Tatsuo Nakamura,† Tsutomu Miyashi,*,†
Joshua L. Goodman,‡ Kimio Akiyama,§ Shozo Tero-Kubota,§
Abdelaziz Houmam, and Danial D. M. Wayner
Department of Chemistry, Graduate School of
Science, Tohoku UniVersity, Sendai 980-8578, Japan
Department of Chemistry, UniVersity of Rochester
Rochester, New York 14627
Institute for Chemical Reaction Science
Tohoku UniVersity, Sendai 980-8577, Japan
Steacie Institute for Molecular Sciences
National Research Council of Canada
Scheme 1
Ottawa, Ontario, Canada K1A 0R6
ReceiVed October 31, 1997
We previously reported that 4,4-dideuterio-2,2-bis(4-methoxy-
phenyl)-1-methylenecyclopropane (d2-1)1 undergoes the degener-
ate methylenecyclopropane (MCP) rearrangement, involving the
bisected trimethylenemethane (TMM) cation radical intermediate
(d2-2•+)4 under the triplet-sensitized photoinduced electron-transfer
(PET) conditions (Chart 1). We now report further mechanistic
studies based on nanosecond laser flash photolysis (LFP), EPR
spectroscopy, and time-resolved photoacoustic calorimetry (PAC)
that support a new, energetically favorable mechanism that
requires both TMM cation radical d2-2•+ and TMM d2-2 as key
intermediates in the rearrangement sequence.
species is a precursor for rearrangement product 1. The identities
of the 350 and 500 nm transients may be inferred from the
absorption spectra of cation 4+ (484 nm in CH3CN and 499 nm
7
in CH2Cl2 ) and radical 4• (349 nm in CH3CN and 352 nm in
8
CH2Cl2 ). Thus, the λmax around at 500 and 350 nm can be
unambiguously assigned to the 1,1-bis(4-methoxyphenyl)ethyl
cation moiety of the bisected TMM cation radical 2•+ and the
1,1-bis(4-methoxyphenyl)ethyl radical moiety of the bisected
TMM 2.9 The most reasonable process to form 2 is back electron
transfer (BET)11 from the sensitizer anion radical to 2•+ within a
contact or solvent separated ion radical pair [2•+/sens.•-].
Table 1 shows photostationary ratios (d2-1:d2-1′) of the
degenerate MCP rearrangement of d2-1, yields of dioxolane (3)
in oxygenation of 1 and transient absorption maxima (λmax
)
The most reasonable mechanism, based on the spectroscopic
evidence and the product analysis, that accounts for the participa-
tion of two different types of TMM intermediates is a cation
radical cleavage-diradical cyclization (CRCL-DRCY) mechanism
shown in Scheme 1. Cation radical d2-2•+ formed by the CRCL
of d2-1•+ or d2-1′•+ does not directly recyclize to d2-1•+ and d2-
1′•+ but undergoes BET to form d2-2. The degenerate MCP
rearrangement is then completed by the DRCY of d2-2 to d2-1
and d2-1′. This mechanism is supported by time-resolved PAC.12
Using PAC for the 1-DCA-biphenyl (BP) system, ∆Hirp([2•+/
DCA•-]) was determined to be 37.0 ( 0.8 kcal/mol. This result
indicates that recyclization of d2-2•+ to d2-1•+ and d2-1′•+ is at
least 16 kcal/mol endothermic because ∆Hirp([1•+/DCA•-]) is
calculated to be 53.0 kcal/mol.13,15 A MNDO UHF calculation16
also supports the suggestion that the recyclization of d2-2•+ at
the cation radical stage is significantly endothermic: at this level
observed in LFP of 1 under the 9,10-dicyanoanthracene (DCA)-,
1,2,4,5-tetracyanobenzene (TCNB)-, or N-methylquinolinium tet-
rafluoroborate (NMQ+BF4-)-sensitized conditions. A mechanistic
connection among rearrangement, oxygenation, and transient
absorption provides evidence for the participation of TMM cation
radical 2•+ and TMM 2 in the degenerate rearrangement sequence.
Relevant results were obtained under the TCNB- and NMQ+-
BF4--sensitized conditions (entries 3-6). Oxygenation of 1 to
give 31,5 and the degenerate rearrangement of d2-1 occur efficiently
under the sensitized conditions in which two transient absorptions
at λmax ) 351 and 500 nm were observed by LFP (entry 3 or 5).
Interestingly, under the sensitized conditions in which only the
transient absorption at λmax ) 354 nm was observed the degenerate
rearrangement occurs efficiently but not oxygenation (entry 4).
Conversely, oxygenation proceeds rapidly but not rearrangement
under the sensitized conditions in which the transient, λmax ) 498
nm was predominant (entry 6). These results suggest that the
air-sensitive,6 longer wavelength transient is a precursor for
oxygenation product 3, whereas the shorter wavelength transient
(6) Under the DCA-BP-cosensitized conditions in CH3CN in the presence
of oxygen, LFP of 1 exhibited a new transient absorption with λmax at 518
nm. The observed transient species may be assigned to a peroxy cation radical,
a precursor of dioxolane 3.
(7) Heublein, G.; Helbig, M. Tetrahedron 1974, 30, 2533-2536.
(8) Radical 4• was generated by photolysis of bis(4-methoxyphenyl)methane
with di-tert-butyl peroxide (0.5 M) in CH3CN and CH2Cl2.
† Graduate School of Science, Tohoku University.
‡ University of Rochester.
§ Institute for Chemical Reaction Science, Tohoku University.
National Research Council of Canada.
(9) Intersystem crossing (ISC) of diradicals with small spin-orbit coupling
interaction10 proceeds generally with small Arrhenius A factor. From the
Arrhenius plot of the decay rate constants of TMM 2 in the TCNB-sensitized
LFP between 266.7 and 299.5 K in CH2Cl2, an activation energy (Ea) was
determined to be 2.9 kcal/mol and small A factor (107.2 s-1) was obtained. If
the observed small A factor may be explained by ISC and large structural
change from triplet TMM 2 to singlet 1, TMM 2 observed in LFP is triplet
and gives the singlet 1 via a surface crossing mechanism.
(1) Takahashi, Y.; Miyashi, T.; Mukai, T. J. Am. Chem. Soc. 1983, 105,
6511-6513. For the thermal degenerate MCP rearrangement of the diphenyl
derivative and reviews for the MCP rearrangement, see refs 2 and 3,
respectively.
(2) Gilbert, J. C.; Butler, J. R. J. Am. Chem. Soc. 1970, 92, 2168-2169.
(3) Berson, J. A. In Rearrangements in Ground and Excited States; Mayo,
P. de, Ed.; Academic: New York, 1980; Vol. 1; pp 311-390. Berson, J. A.
In Diradicals; Borden, W. T., Ed.; Wiley: New York, 1982; pp 151-194.
Gajewski, J. J. In Organic Chemistry, A Series of Monographs 44, Hydro-
carbon Thermal Isomerizations; Wasserman, H. H., Ed.; Academic: New
York, 1981; Vol. 5, pp 43-70.
(4) Miyashi, T.; Takahashi, Y.; Mukai, T.; Roth, H. D.; Schilling, M. L.
M. J. Am. Chem. Soc. 1985, 107, 1079-1080.
(5) Miyashi, T.; Kamata, M.; Mukai, T. J. Am. Chem. Soc. 1986, 108,
2755-2757, and 1987, 109, 2780-2788.
(10) Salem, L.; Rowland, C. Angew. Chem., Int. Ed. Engl. 1972, 11, 92-
111.
(11) Du, P.; Borden, W. T. J. Am. Chem. Soc. 1987, 109, 5330-5336.
(12) Rudzki, J. E.; Goodman, J. L.; Peters, K. S. J. Am. Chem. Soc. 1985,
107, 7849-7854. Herman, M. S.; Goodman, J. L. J. Am. Chem. Soc. 1989,
111, 1849-1854. Peters, K. S. In Kinetics and Spectroscopy of Carbenes and
Biradicals; Platz, M. S., Ed.; Plenum: New York, 1990; pp 37-49. Griller,
D.; Wayner, D. D. M. Pure Appl. Chem. 1989, 61, 717-724.
S0002-7863(97)03760-8 CCC: $15.00 © 1998 American Chemical Society
Published on Web 05/28/1998

Communications to the Editor
J. Am. Chem. Soc., Vol. 120, No. 23, 1998 5833
Table 1. Results of the Degenerate MCP Rearrangement of d2-1,a Oxygenation of 1,b and LFP of 1c under Various PET Conditions
entry
conditions
DCA/CH3CN
DCA-BP/CH3CN
TCNB/CH3CN
d2-1:d2-1′ (time/h)
yield of 3/% (time/min)
λ
max(2)/nm
λ
max(2•+)/nm
∆OD(2•+)/∆OD(2)d
1
2
3
4
5
6
58:42 (4.5)
slow
54:46 (4.5)
56:44 (3)
56:44 (2)
slow
100 (15)
100 (15)
100 (20)
4 (30)
96 (15)
100 (5)
e
e
f
494g
500
f
351
354
354
350h
1.3
∼0
TCNB/CH2Cl2
TCNB-BP/CH2Cl2
508
2
NMQ+BF4--toluene/CH3CN
498h
>10
a Under N2. [d2-1] ) 100 mM. Deuterated solvent and cosensitizer were used. b Under O2. [1] ) 10 mM. c Under N2. [1] ) 1 mM. d Ratio of
∆OD of 2•+ to that of 2 at 200 ns after excitation. e Not observable. f No transient absorption was observed. g See footnote 6. h Under air.
On the other hand, irradiation of anthraquinone with 1 in a
CH2Cl2 matrix at 20 K provided a characteristic EPR spectrum
of randomly oriented triplet species ascribed to 2 along with 2•+.
In addition to the |∆Ms| ) 1 transition signals, a weak |∆Ms| )
2 transition was observed at 0.1673 T. The zero-field splitting
Figure 1. CIDEP spectrum (left) and its simulation (right) of 2•+. An
parameters were estimated to be |D/hc| ) 0.0116 and |E/hc| )
0.0038 cm-1 from the spectrum. The |D/hc| value is small25
asterisk, *, represents an emission due to chloranil anion radical.
2•+ is located 18 kcal/mol lower in energy than 1•+. In contrast,
compared with those of other phenyl-substituted TMM deriva-
BET from DCA•- to d2-2•+ is estimated to be about 20.5 kcal/
tives.27 The triplet EPR signal of 2 persisted at cryogenic
temperature, and the Curie plot of the |∆Ms| ) 2 transition line
intensity gave a straight line between 4.2 and 50 K, indicating
that the ground state of 2 is triplet as usual TMMs. It is
noteworthy that while the ground state of the parent TMM28 is
triplet with a planar structure in accord with calculation,28b the
structure of TMM 2 is bisected regardless of its triplet ground
state. Since 2 is formed by BET without significant conforma-
tional change, the bisected structure29 of 2 is most likely due to
that of 2•+ formed by the least motion ring cleavage4 of 1•+ which
requires only the rotation of the methylene group but not of the
bulkier diarylmethylene group of 1•+.
The proposed rearrangement sequence including a diradical-
forming BET process30 was also suggested to similar PET MCP
rearrangements of 2-aryl-1-methylenecyclopropane, 2,2-diaryl-
1-methylenespiropentane,32 and 1-cyclopropylidene-2,2-diarylcy-
clopropane.32 The results herein provide the first observation of
the interconversion of the relevant intermediates.
mol exothermic using the oxidation potential of 4• (Eox ) -
1/2
0.06 V vs SCE in CH3CN) as determined by photomodulation
voltammetry.18 Thus the highly exothermic BET presumably
occurs rapidly19 to form d2-2, which is 16.5 kcal/mol higher in
energy than either d2-1 or d2-1′.
The participation of two types of TMM intermediates in the
degenerate MCP rearrangement of d2-1 was further directly
confirmed by EPR spectroscopy using chloranil or anthraquinone
as sensitizers.1 Figure 1 (left) shows the time-resolved EPR
spectrum of 2•+ observed at a delay time of 1 µs after the laser
excitation of chloranil (10 mM) with 1 (50 mM) in DMSO22 at
ambient temperature. The hyperfine structure (hfs) was analyzed
with two splitting constants corresponding to 2•+ [aH (2H) ) 1.38
mT, aH (2H) ) 1.44 mT, and g ) 2.0026]. The observed
spectrum was well reproduced by simulation, in which both the
triplet (E) and radical pair mechanisms (E/A)23 are taken into
account [Figure 1 (right)]. Since the hfs constants and g-value
of 2•+ are close to those of the neutral allyl radical,24 it follows
that the unpaired electron is mainly distributed over the allyl part
and the positive charge is localized on the bis(4-methoxyphenyl)-
methyl moiety. The structure of bisected TMM cation radical
2•+ elucidated by time-resolved EPR well agrees with that from
LFP and CIDNP.4
Acknowledgment. We gratefully acknowledge financial support from
the Ministry of Education, Science, Sports and Culture (Grant-in-Aid for
Scientific Research Nos. 08740560 and 09740536). We also thank
Professor Y. Takahashi for valuable discussions.
Supporting Information Available: Transient absorption spectrum
(2•+ and 2), deconvolution fitting parameters for the PAC waveforms
(1-DCA-BP), and the Curie plot for 2 (4 pages, print/PDF). See any
current masthead page for ordering and Web access instructions.
(13) ∆Hirp([1•+/sens.•-]) ) 23.06 [Eox1/2(1) - Ered1/2(sens.) ] - C (in kcal/
mol), where Eox1/2(1) ) +1.35 V vs SCE, Ered1/2(DCA) ) -0.95 V, and
Ered1/2(NMQ+PF6-) ) -0.90 V in CH3CN and the Coulomb term (C) was
ignored after Farid’s example.14
(14) Gould, I. R.; Ege, D.; Moser, J. E.; Farid, S. J. Am. Chem. Soc. 1990,
112, 4290-4301.
JA973760R
(15) For the 1-NMQ+PF6--toluene system in CH3CN, ∆Hirp([2•+/NMQ•-
PF6-]) was determined to be 35.0 ( 0.7 kcal/mol by PAC, and thus
endothermicity for the cation radical cyclization of d2-2•+ is suggested to be
about 17 kcal/mol endothermic based on ∆Hirp([1•+/NMQ•PF6-]), 51.9 kcal/
mol.13
(25) A small |D/hc| value of 2 can be ascribed to its bisected form. If
diphenylmethylenecyclopentane-1,3-diyl is planar,26a a decrease in |D/hc| value
of 2 is probably caused by molecular distortion of 2 as exemplified by a series
of biphenyl derivatives,26b conjugated enones,26c and conjugated TMMs.26d
(26) (a) Turro, N. J.; Mirbach, M. J.; Harrit, N.; Berson, J. A.; Platz, M. S.
J. Am. Chem. Soc. 1978, 100, 7653-7658. (b) Tanigaki, K.; Taguchi, N.;
Yagi, M.; Higuchi, J. Bull. Chem. Soc. Jpn. 1989, 62, 668-673. (c) Yamauchi,
S.; Hirota, N.; Higuchi, J. J. Phys. Chem. 1988, 92, 2129-2133. (d) Bushby,
R. J.; Jarecki, C. Tetrahedron Lett. 1986, 27, 2053-2056.
(27) Platz, M. S.; McBride, J. M.; Little, R. D.; Harrison, J. J.; Shaw, A.;
Potter, S. E.; Berson, J. A. J. Am. Chem. Soc. 1976, 98, 5725-5726. Hirano,
T.; Kumagai, T.; Miyashi, T.; Akiyama, K.; Ikegami, Y. J. Org. Chem. 1992,
57, 876-882. Dougherty, D. A. In Kinetics and Spectroscopy of Carbenes
and Biradicals; Platz, M. S., Ed.; Plenum: New York, 1990; pp 117-142,
and references therein.
(28) (a) Dowd, P. J. Am. Chem. Soc. 1966, 88, 2587-2589. (b) Dixon, D.
A.; Dunning, Jr. T. H.; Eades, R. A.; Kleier, D. A. J. Am. Chem. Soc. 1981,
103, 2878-2880.
(29) A study on the multiplicity-structure relationship of phenyl-substituted
TMM is in progress.
(30) Similar electron-transfer mechanism including a diradical-forming BET
process is operative in the PET degenerate Cope rearrangement of 2,5-diaryl-
1,5-hexadiene derivatives.31
(16) Similar energy difference for the 2,2-diphenyl derivative was previously
calculated to be 24.0 kcal/mol17 by MNDO UHF.
(17) Takahashi, O.; Morihashi, K.; Kikuchi, O. Tetrahedron Lett. 1990,
31, 5175-5178.
(18) Wayner, D. D. M.; McPhee, D. J.; Griller, D. J. Am. Chem. Soc. 1988,
110, 132-137.
(19) According to theoretical equations20 and reported parameters by Farid14
and Kikuchi,21 a rate constant for the BET in [2•+/DCA•-] at 20 °C was
estimated to be 3.0 × 108 and 1.0 × 1010 s-1, respectively, in CH3CN.
(20) Miller, J. R.; Beitz, J. V.; Huddleston, R. K. J. Am. Chem. Soc. 1984,
106, 5057-5068. Siders, P.; Marcus, R. A. J. Am. Chem. Soc. 1981, 103,
741-747 and 748-752. Van Duyne, R. P.; Fischer, S. F. Chem. Phys. 1974,
5, 183-197. Ulstrup, J.; Jortner, J. J. Chem. Phys. 1975, 63, 4358-4368.
(21) Niwa, T.; Kikuchi, K.; Matsushita, N.; Hayashi, M.; Katagiri, T.;
Takahashi, Y.; Miyashi, T. J. Phys. Chem. 1993, 97, 11960-11964.
(22) The degenerate MCP rearrangement of d2-1 similarly occurs in DMSO-
d6 under the DCA-sensitized conditions.
(23) McLauchlan, K. A. In Modern Pulsed and Continuous-WaVe Electron
Spin Resonance; Keva, L., Bowman, M. K., Eds.; Wiley: New York, 1990;
pp 285-363.
(24) Fessenden, R. W.; Schuler, R. H. J. Chem. Phys. 1963, 39, 2147-
2195. Krusic, P. J.; Meakin, P.; Smart, B. E., J. Am. Chem. Soc. 1974, 96,
6211-6213.
(31) Ikeda, H.; Minegishi, T.; Abe, H.; Konno, A.; Goodman, J. L.; Miyashi,
T. J. Am. Chem. Soc. 1998, 120, 87-95.
(32) Miyashi, T.; Takahashi, Y.; Ohaku, H.; Ikeda, H.; Morishima, S. Pure
Appl. Chem. 1991, 63, 223-230.