Chemical capture of an unprecedented oxatetramethyleneethane radical cation with a through-space electronic coupling

Article abstract of DOI:10.1016/j.tetlet.2005.02.109

A photoinduced electron-transfer reaction of 2,2-dianisyl-4-isopropylidene- 3,3-dimethylcyclobutanone (5) in acetonitrile containing molecular oxygen or water gave 4,4′-dimethoxybenzophenone (7) and 2,2-dianisyl-4- isopropylidene-5,5-dimethylhydrofuran-3-one (8), demonstrating the chemical capture of an unprecedented oxatetramethyleneethane-type radical cation intermediate (6^?+). A density functional theory calculation suggests through-space electronic coupling between the tetramethylallyl and joined dianisylmethyl carbonyl subunits in 6^?+.

Full text of DOI:10.1016/j.tetlet.2005.02.109

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 2663–2667
Chemical capture of an unprecedented oxatetramethyleneethane
radical cation with a through-space electronic coupling
Hiroshi Ikeda,* Futoshi Tanaka and Chizuko Kabuto
Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
Received 17 January 2005; revised 7 February 2005; accepted 10 February 2005
Abstract—A photoinduced electron-transfer reaction of 2,2-dianisyl-4-isopropylidene-3,3-dimethylcyclobutanone (5) in acetonitrile
containing molecular oxygen or water gave 4,40-dimethoxybenzophenone (7) and 2,2-dianisyl-4-isopropylidene-5,5-dimethyl-
dihydrofuran-3-one (8), demonstrating the chemical capture of an unprecedented oxatetramethyleneethane-type radical cation inter-
mediate (6Å+). A density functional theory calculation suggests through-space electronic coupling between the tetramethylallyl and
joined dianisylmethyl carbonyl subunits in 6Å+.
Ó 2005 Published by Elsevier Ltd.
Recently, we reported on a novel rearrangement of
2,2-dianisyl-3,3-dimethyl-4-methylenecyclobutanone (1,
Scheme 1a) to give 2,2-dianisyl-4-isopropylidenecyclob-
utanone (2), triggered by a photoinduced electron-trans-
fer (PET) reaction using p-chloranil (CA, Scheme 1b).1
The reaction proceeded irreversibly via an unprece-
cation 3Å+ with ylophilic and nucleophilic trapping re-
agents, such as molecular oxygen2 and methanol3 or
water,3 respectively. This may be attributed to low sta-
tionary concentration of 3Å+, which is caused by irrevers-
ibility between 1 and 2; 2 did not undergo C-2–C-3 bond
cleavage, but underwent C-1–C-2 bond cleavage to give
CA-adduct 4 under the CA sensitized PET conditions.
Another OTMEÅ+-type intermediate 6Å+ can be efficiently
generated from the radical cation of 2,2-dianisyl-4-
isopropylidene-3,3-dimethylcyclobutanone (5, Scheme 1c)
dented
oxatetramethyleneethane
radical
cation
(OTMEÅ+, Scheme 1b) derivative, 3Å+, which is a new
´
category of non-Kekule molecules. Unfortunately, how-
ever, we did not succeed in capturing a distonic radical
(a)
(b)
O
CN CN
5
3
2
+
+
CN
3
2
4
1
Cl
Cl
Cl
Cl
CA*
CA*
Ar
O
4
Cl
1
Ar
Ar
Ar O
Ar
•
O
O
O
O
•
Cl
Cl
Cl
OH
Ar
Ar
CN
O
CA
Ar
OTME•+
TCA
2
1
3•+
4
(c)
•
+
3
5
3
2
4
1
O
Sens.*
4
1
O
+
Ar
Ar
O2 or H2O
Ar
Ar
O
O
O
Ar
2
Ar
Ar
Ar
6•+
5
7
8
Scheme 1. (a) PET reactions of 1 and 2. (b) The parent OTME+ and sensitizers. (c) PET reactions of 5 in acetonitrile. Ar = 4-MeOC6H4.
Keywords: Photochemistry; Electron transfer; Oxatetramethyleneethane; Methylenecyclobutanone; Radical cation; Electronic coupling.
*
Corresponding author. Tel.: +81 22 217 6554; fax: +81 22 217 6557; e-mail: ikeda@org.chem.tohoku.ac.jp
0040-4039/$ - see front matter Ó 2005 Published by Elsevier Ltd.
doi:10.1016/j.tetlet.2005.02.109

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H. Ikeda et al. / Tetrahedron Letters 46 (2005) 2663–2667
by constructing a reversible (degenerate) rearrangement
system. In addition, a steric repulsion between the gem-
inal dianisyl and dimethyl groups at C-2 and C-3 of 6Å+
can extend the lifetime of 6Å+ compared with that of 3Å+.
Expecting to capture an OTMEÅ+-type intermediate, in
this work, we studied PET reactions of 5 in acetonitrile
containing molecular oxygen or water, and found that
6Å+ was captured by molecular oxygen and water at
the same C-2 position to give 4,40-dimethoxybenzophe-
none (7) and 2,2-dianisyl-4-isopropylidene-5,5-dimethyl-
dihydrofuran-3-one (8). Here, we briefly report on the
chemical capture of 6Å+ and its unique electronic
structure using product analyses and a density func-
tional theory (DFT) calculation.
CA*
O2
CA*
H2O
7
5
8
TCA*
O2 or H2O
Scheme 2. PET reactions of 5 in acetonitrile.
irradiation for 60 min in degassed aqueous [2% (v/v)]
acetonitrile, 8,8 and 2,3,5,6-tetrachlorohydroquinone
(CAH2) were formed in 68% and 50% yields, respec-
tively, at 68% conversion. The structure of 8 was deter-
mined by X-ray crystallography (Fig. 1b).6 The
formation of 7 and 8 occurred concurrently in the pres-
ence of both molecular oxygen and water. In acetonitrile
containing methanol [2% (v/v)], no identifiable product
Ketone 5 was prepared in 73% yield by the [2 + 2] cyclo-
addition of dianisylketene and 2,4-dimethylpentan-2,3-
diene (1,1,3,3-tetramethylallene) using known proce-
dures.1,4,5 The X-ray crystallographic analysis of 56
was observed. Changing the sensitizer to 1,2,9,10-tetra-
red
1=2
cyanoanthracene (TCA, Scheme 1b, E ¼ ꢁ0:43 V)9 5
gave 7 in degassed aqueous [4% (v/v)] acetonitrile in
40% yield at 40% conversion or in dry acetonitrile under
oxygen in moderate yield. In addition, 8 was not con-
sumed in a similar PET reaction in degassed aqueous
[4% (v/v)] acetonitrile for 30 min.
˚
reveals that the C-2–C-3 bond of 5 is 1.62 A long (Fig.
1a). In comparison with a normal C(sp3)–C(sp3) bond
˚
(1.54 A), the C-2–C-3 bond is lengthened by steric repul-
sion between the geminal dianisyl and dimethyl groups
at C-2 and C-3, respectively. Therefore, 5 readily cleaves
to 6Å+ on oxidation by PET.1b On irradiation (2 kW xe-
The product analyses suggest that two electronic struc-
tures contribute to the reactivity of the OTMEÅ+ deriva-
tive 6Å+; type A (6-AÅ+) with the dianisylmethyl radical
and the tetramethylallyl cation subunits, and type B
(6-BÅ+), the counterpart of type A (Scheme 3). Note that
ylophilic and nucleophilic additions of molecular oxy-
gen and water, respectively, are suggested to take place
in the same C-2 position of 6Å+. Therefore, 6Å+ does
not behave as a simple distonic radical cation like 3Å+,
but behaves just like a non-distonic radical cation with
two equivalent resonance structures, types A and B. Effi-
cient electronic coupling10 probably operates in 6Å+ (vide
infra).
non lamp, k > 440 nm)7 for 30 min at 20 °C in degassed
red
dry acetonitrile with CA (E ¼ ꢀ0:00 V vs SCE in ace-
1=2
ox
ap
tonitrile), considerable ketone 5 (E ¼ þ1:43 V) was
consumed (30%), but no identifiable product was ob-
served (Table 1). Under similar PET conditions, but
with an oxygen atmosphere, 7 was formed in 59% yield
at 62% conversion (Scheme 2). By contrast, on similar
A plausible reaction mechanism for the PET reactions of
5 is depicted in Scheme 3. The 5Å+ initially formed as a
free radical ion (or in a solvent-separated radical ion
pair) undergoes C-2–C-3 bond cleavage to give the
OTMEÅ+ derivative 6Å+. The ylophilic addition of molec-
ular oxygen11 to the C-2 site of 6Å+ (cf. 6-AÅ+) gives ke-
tone 7 via radical 11Å. Conversely, the nucleophilic
addition of water to the C-2 site of 6Å+ (cf. 6-BÅ+) fol-
Figure 1. The ORTEP drawings of (a) 5 and (b) 8. Hydrogen atoms
are omitted for clarity.
Table 1. PET reactions of 5 in acetonitrilea
Sens.
Atmosphere
H2O% (v/v)
Time (min)
Conv.b
Yieldsb
7
8
CA
Degassed
Oxygen
0
0
2
2
30
20
60
20
30
62
68c
80
0
59
0
0
0
Degassed
Oxygen
68
23
57
d
TCA
Degassed
Oxygen
Degassed
0
0
4
30
60
60
5
40
40
0
0
0
25
40
a [5] = [CA] = 0.01 M, [TCA] = 0.002 M.
b The conversions and yields were determined by 1H NMR analysis and are given in %.
c CAH2 was isolated in 50% yield.
d A trace amount (<2%) of 7 was detected.

H. Ikeda et al. / Tetrahedron Letters 46 (2005) 2663–2667
2665
Sens.*
5•+
5
ylophilic
nucleophilic
addition of
O2 at C-2
addition of
H2O at C-2
6•+
7
+
•
•
O2
Ar
HO
Ar
O
O
Sens. = TCA
Ar
Ar
11•
12•
Sens. = CA
TCA*
CAH2
8
12+
Figure 2. Definition of three subunits in the framework of 6Å+
,
calculated dihedral angles (h and x), and the sum of the partial spin
and charge densities (Rq and Rq) on the optimized structure (hydrogen
atoms are omitted for clarity) obtained from UB3LYP/6-31G
calculations.
3
5
•
+
•
4
1
Ar
Ar
+
•O
Ar
•
O
O
O
2
Ar
6-A•+
Ar
Ar
13••
6-B•+
(h = 119°), whereas the carbonyl and dianisylmethyl
subunits are almost in the same plane (x = 13°). The
resulting bisected conformation favors a through-space
electronic coupling16 between the tetramethylallyl and
jointed dianisylmethyl carbonyl subunits. Note that
the p-orbitals at the C-3 and C-5 are in phase with that
at the C-2 and the carbonyl oxygen, as exemplified by
the SOMO (a) of 6Å+ (Fig. 3). Needless to say, no similar
electronic coupling is observed for the parent planar
OTMEÅ+ and slightly bisected 3Å+ (for the C-1–C-4),1a
but may be observed for the perpendicular tetramethyle-
neethane radical cation.17 Obviously, the deviation from
the planar structure of 6Å+ is due to steric hindrance be-
tween the tetramethylallyl and dianisylmethyl subunits.
Scheme 3. A plausible reaction mechanism for the PET reactions of 5.
lowed by deprotonation, gives radical 12Å and CAHÅ or
TCAHÅ. When the sensitizer is CA, 8 is given by a
secondary electron transfer (with CAHÅ)-cyclization
sequence via 12+. Another pathway via 13ÅÅ, which is
formed by hydrogen abstraction in 12Å by CAHÅ, may
also be possible in the formation of 8. Ketone 7 ob-
served in the PET reaction with TCA in degassed aque-
ous acetonitrile must be formed from 12Å, as 7 was not
formed from 8 under similar PET conditions. The differ-
ence in the reactivity of 12Å between the CA- and TCA-
sensitization may be ascribed to lower efficiency of the
secondary electron transfer from 12Å to TCAHÅ com-
pared with CAHÅ. Unfortunately, we have no details
on the fragmentation of 11Å to 7 because we could not
identify any other product(s) except 7 in the CA-sensi-
tized PET reactions of 5 under oxygen or in the TCA-
sensitized PET reactions of 5. However, the formation
of 7 and 8 and mechanistic conjecture strongly suggests
the chemical capture of 6Å+ with ylophilic molecular oxy-
gen and nucleophilic water at the same C-2 position. An
alternative explanation is a distonic radical cation model
that involves either type A (6-AÅ+) or B (6-BÅ+) as a real
intermediate. For example, ylophilic and nucleophilic
addition of molecular oxygen and water to the C-2 car-
bon of 6-AÅ+ would give 7 and 8, respectively. However,
this pathway seems unlikely because it is impossible to
explain the formation of 7 in the PET reaction of 5 with
TCA in degassed aqueous acetonitrile. Note that 8 did
not give 7 under similar PET conditions.
In conclusion, we succeeded in the chemical capture of
the OTMEÅ+ derivative 6Å+, which is suggested to be
trapped with molecular oxygen and water at the same
carbon, C-2. DFT calculations demonstrate the efficient
through-space electronic coupling in 6Å+, whose elec-
tronic structure can be depicted as a resonance hybrid
of two equivalent resonance structures: types A and B.
This work provides a new perspective on radical ions
´
and non-Kekule chemistry, and will affect the related
fields of reactive intermediate and organic photochemis-
try. Further studies are now in progress, and will be
published elsewhere.18
Moreover, the UB3LYP/6-31G calculation13 suggests
electronic coupling in 6Å+. Figure 2 defines three subunits
in the framework of 6Å+, the calculated dihedral angles (h
and x), and the sum of the partial spin and charge den-
sities (Rq and Rq); there was no significant difference in
Rq or Rq between the dianisylmethyl and tetramethyl-
allyl subunits. The spin and charge are delocalized all
over the molecule. The tetramethylallyl and carbonyl
planes are largely twisted with respect to each other
Figure 3. (a) Representation of the SOMO (a) of 6Å+. A similar figure
was obtained for the SOMO (b). (b) Conceptual representation,
indicating the electronic coupling of the two terminal allylic carbons
(C-3 and C-5), dianisylmethyl carbon (C-2), and carbonyl oxygen.

2666
H. Ikeda et al. / Tetrahedron Letters 46 (2005) 2663–2667
(relative intensity): 351 (21), 350 (85, M+), 268 (30), 254
Acknowledgements
(47), 242 (45), 228 (10), 227 (66), 226 (100, Ar2CÅ+), 211
1
(27); H NMR (200 MHz, CDCl3) dppm 1.27 (s, 6H), 1.86
This paper is dedicated to Emeritus Professor T. Miy-
ashi on the occasion of his retirement from Tohoku Uni-
versity. H.I. and F.T. gratefully acknowledge financial
support by a Grant-in-Aid for Scientific Research on
Priority Areas (No. 417) from the Ministry of Educa-
tion, Culture, Sports, Science, and Technology of Japan,
and a Junior Research Fellowship from the Japan Soci-
ety for the Promotion of Science, respectively, and thank
Professor M. Ueda (Tohoku University) for his gener-
ous support.
(s, 3H), 2.23 (s, 3H), 3.77 (s, 6H), 6.81 (AA0BB0,
J = 9.0 Hz, 4H), 7.32 (AA0BB0, J = 9.0 Hz, 4H); 13C
NMR (50 MHz, CDCl3) dppm 20.26, 22.23, 26.49 (2C),
46.77, 55.12 (2C), 76.26, 113.33 (4C), 129.26 (4C), 133.49
(2C), 144.37, 147.67, 157.97 (2C), 203.05; Anal. Calcd for
C23H26O3: C, 78.83; H, 7.48. Found: C, 78.96; H, 7.51.
6. Crystallographic data for 5 and 8 have been deposited with
the Cambridge Crystallographic Data Centre (CCDC) as
supplementary publication nos. CCDC 252673 (5) and
241921 (8). These data can be obtained free of charge via
www.ccdc.cam.ac.uk/data_request/cif, by emailing data_r-
equest@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12, Union Road, Cam-
bridge CB2 1EZ, UK; fax: +44 1223 336033.
Supplementary data
7. The procedure used for the PET reactions of 5 is as
follows. An acetonitrile solution (5 mL) containing 5
(17.5 mg, 0.05 mmol), CA (12.3 mg, 0.05 mmol) or TCA
(2.8 mg, 0.01 mmol), and water (0, 0.1, or 0.2 mL) in a
Pyrex test tube (diameter 1.5 cm) was degassed with five
freeze (ꢁ196 °C)–pump (10ꢁ2 Torr)–thaw (ambient tem-
perature) cycles and then sealed at 10ꢁ2 Torr. An oxygen-
saturated solution was prepared by successive bubbling
with oxygen for 10 min and then sealed under an oxygen
atmosphere. The sample solution was irradiated through a
cutoff filter (k > 440 nm) with a 2 kW xenon lamp at
20 1 °C. After evaporation in vacuo, the product yields
were determined by 1H NMR analysis with 1,1,2,2-
tetrachloroethane as the internal standard for integration.
8. The procedure for isolating 8 is as follows. An acetonitrile
solution (33 mL) containing 5 (231 mg, 0.66 mol), CA
(391 mg, 1.32 mmol), and water (1 mL) in a large cylin-
drical quartz cell (diameter 2.0 cm) was irradiated through
a cutoff filter (k > 440 nm) with a 2 kW xenon lamp at
20 1 °C under an argon atmosphere for 3 h. After
evaporation in vacuo, column chromatography followed
by recrystallization from ethanol gave 110 mg (0.30 mmol,
46% yield) of 8. Selected data for 8: mp 121.0–121.5 °C
(colorless prisms); IR (KBr) 1703 cmꢁ1; MS (EI, 70 eV) m/
z (relative intensity): 367 (5), 366 (20, M+), 243 (11), 135
Estimation of free energy changes for electron transfer
from 5 to the excited state of sensitizers, X-ray crystal-
lography for 5 and 8, and the calculation results of 6Å+
(PDF). These material are available free of charge via
the Internet at http://www.sciencedirect.com. Supplem-
etary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2005.02.109.
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(25), 124 (73), 96 (100, (CH3)2CCC(CH3)2Å+), 81 (30); H
1
NMR (600 MHz, CDCl3) dppm 1.52 (s, 6H), 1.98 (s, 3H),
2.32 (s, 3H), 3.77 (s, 6H), 6.82 (AA0BB0, J = 8.7 Hz, 4H),
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is calculated to be ꢂ+0.87 eV, using E ðCAÞ ¼ ꢀ0:00 V
1=2
12
red
1=2
and E ðO2Þ ¼ ꢁ0:87 V (vs SCE in acetonitrile).
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ethanol); IR (KBr) 1717 cmꢁ1; MS (EI, 70 eV) m/z

H. Ikeda et al. / Tetrahedron Letters 46 (2005) 2663–2667
2667
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18. Further product analysis using GC–MS are needed for the
complete elucidation of the reaction mechanism. We
are now studying this and will publish the results
separately.
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