Tropone is a mere ketone for cycloadditions to ketenes

Article abstract of DOI:10.1002/hlca.200590121

Tropone (1) reacts with ketenes 2 to yield [8 + 2] cycloadducts, the γ-lactones 3. The concerted [8 + 2] cycloaddition path is formally symmetry-allowed, but we established that it is unfavorable. Careful low-temperature NMR (¹H, ¹³C, and ¹⁹F) spectroscopies of the reaction of diphenyl ketene (2b) or bis(trifluoromethyl) ketene (2c) with tropone (1) allowed the direct detection of a β-lactone intermediates 5b,c and novel norcaradiene species 6b,c in head-to-head configurations. The [2 + 2] cycloadducts 5b,c equilibrated with the norcaradienes 6b,c. The β-lactones 5b and 5c were converted to the γ-lactones 3b and 3c, respectively, in quantitative yields. The DFT calculations showed that the concerted [8 + 2] cycloaddition is unfavorable. The first step of the calculated reaction 1 + 2c is a cycloaddition which leads to a dioxetane intermediate. This initial [2 + 2] cycloadduct is isomerized to the β-lactone 5c via the first zwitterionic intermediate. The β-lactone 5c is further isomerized to the product γ-lactone 3c via the second zwitterion intermediate. Thus, 3c is not formed via the well-established two-step mechanism including zwitterionic intermediates but via a five-step mechanism composed of a [2 + 2] cycloaddition and subsequent isomerization (Scheme 12).

Full text of DOI:10.1002/hlca.200590121

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Tropone Is a Mere Ketone for Cycloadditions to Ketenes
by Junko Okamotoa), Shinichi Yamabe*b), Tsutomu Minatoc), Toshio Hasegawaa),
and Takahisa Machiguchi*a)
a) Department of Chemistry, Faculty of Science, Saitama University, Sakura-ku, Saitama 338-8570, Japan
(e-mail: machiguc@chem.saitama-u.ac.jp)
b) Department of Chemistry, Nara University of Education, Takabatake-cho, Nara 630-8528, Japan
(e-mail: yamabes@nara-edu.ac.jp)
c) Institute for Natural Science, Nara University, 1500 Misasagi-cho, Nara 631-8502, Japan
Dedicated to Professor Rolf Huisgen on the occasion of his 85th birthday
Tropone (1) reacts with ketenes 2 to yield [8 ‡ 2] cycloadducts, the g-lactones 3. The concerted [8 ‡ 2]
cycloaddition path is formally symmetry-allowed, but we established that it is unfavorable. Careful low-
temperature NMR (1H, 13C, and 19F) spectroscopies of the reaction of diphenyl ketene (2b) or bis(trifluoro-
methyl) ketene (2c) with tropone (1) allowed the direct detection of a b-lactone intermediates 5b,c and novel
norcaradiene species 6b,c in head-to-head configurations. The [2 ‡ 2] cycloadducts 5b,c equilibrated with the
norcaradienes 6b,c. The b-lactones 5b and 5c were converted to the g-lactones 3b and 3c, respectively, in
quantitative yields. The DFT calculations showed that the concerted [8 ‡ 2] cycloaddition is unfavorable. The
first step of the calculated reaction 1 ‡ 2c is a cycloaddition which leads to a dioxetane intermediate. This initial
[2 ‡ 2] cycloadduct is isomerized to the b-lactone 5c via the first zwitterionic intermediate. The b-lactone 5c is
further isomerized to the product g-lactone 3c via the second zwitterion intermediate. Thus, 3c is not formed via
the well-established two-step mechanism including zwitterionic intermediates but via a five-step mechanism
composed of a [2 ‡ 2] cycloaddition and subsequent isomerization (Scheme 12).
1. Introduction. ± Cycloadditions1) occur concertedly when the orbital overlap is in-
phase, i.e., is symmetry-allowed [4]. Tropone (1) is a trienone with 8p electrons, and the
reaction between 1 and a ketene 2 [5] (for excellent reviews, see [6]) as shown in
Scheme 1 is a symmetry-allowed [8 ‡ 2]CˆC cycloaddition. The term [8 ‡ 2]CˆC denotes
a head-to-head addition path where the CˆC bond of 2 provides two electrons. For
R ˆ Ph, the product 3b was identified first by Gompper and co-workers [7][8]. In the
first work on the reaction between 1 and 2b, structure I was assigned to the [2 ‡ 2]
cycloadduct through the C(2)ÀC(3) bond of 1 [9]. Later, this structural assignment
was corrected [7].
While the frontier-molecular-orbital (FMO) theory predicts the concerted addition
path, the kinetic measurement suggested that the reaction between 1 and diphenyl
ketene (2b) proceeds in a stepwise process including a zwitterionic intermediate 4b
(Scheme 2) [8]. Thus, there is a dilemma concerning the reaction mechanism: either the
symmetry-allowed concerted cycloaddition (Scheme 1) or the stepwise path including a
zwitterionic intermediate (Scheme 2) operates.
1
)
Professor Huisgen made an extremely brilliant achievement on cycloadditions including those of ketenes.
For his autobiography, see [1]. For his representative work on ketene cycloadditions, see [2]. For reviews
on cycloadditions, see, e.g., [3].
ꢀ 2005 Verlag Helvetica Chimica Acta AG, Zürich

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Scheme 1
Scheme 2
Ketenes 2 usually react with alkenes and conjugated compounds across their CˆC
bonds [6][10]. A typical example is the ketene ± cyclopentadiene (A) reaction to form
a cyclobutanone B (Scheme 3,a2)). We have demonstrated that the carbonyl group of
the ketene adds to A in a Diels ± Alder reaction and that the [4 ‡ 2] cycloadduct C is
converted to the cyclobutanone B in a [3,3] sigmatropic rearrangement (Scheme 3, b)
[13][14].
In our recent work, the CˆO group of ketene 2c has been found to be the reaction
center even for the activated alkenes D [15] (Scheme 4). A zwitterion F is generated
not at the initial stage of the ketene ± alkene reaction but by the isomerization of the
[2 ‡ 2]CˆO cycloadduct, the a-methyleneoxetane E, to the product cyclobutanone G.
These findings seem to be important for us to recognize that the carbonyl groups of
ketenes are not mere substituents but are reaction centers in many ketene cyclo-
additions. Ketenes are electrophiles, and their lowest unoccupied MOs (LUMOs),
p*CˆO, are FMOs. Thus, it is natural that carbonyl groups of ketenes are reaction
centers toward nucleophiles. Here, the dilemma is expected to be related to the ketene
CˆO reaction with tropone. In other words, the tropone ± ketene reaction is thought to
be composed of a cycloaddition and a subsequent isomerization. What kind of
cycloaddition and isomerization are involved?
To examine the reaction mechanism of Scheme 1, we conducted low-temperature
NMR and Fourier-transform (FT) IR experiments. The two weight-measurable ketenes
2
)
For ketene ± cyclopentadiene reactions, see, e.g., [11]. For ketene ± open-chain-1,3-diene reactions, see, e.g.,
[12].

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Scheme 3. a) Reaction between Ketene 2 and Cyclopentadiene A (see [11][12]). b) Reaction Mechanism
Composed of Two Symmetry-Allowed Processes (see [14])
Scheme 4. Reaction between 2c and an Activated Alkene, Ethyl Vinyl Ether D, Involving a Neutral Intermediate
E and a Zwitterion F [15]
2b and 2c were chosen for quantitative experiments. DFT Calculations were carried out
to elucidate the reaction mechanism. We would like to show that the tropone ± ketene
reaction is essentially a ketone ± ketene cycloaddition leading to novel intermediates, b-
lactones with cycloheptatriene and norcaradiene structures.
2. Results of Model Calculations and the Orbital-Symmetry Rule. ± The first step is
to check the symmetry rule in the tropone ± ketene reaction. FMO Interactions
between 1 and 2a are examined. Since the ketene is obviously an electrophile, Scheme 5
shows its lumo and (lu ‡ 1)mo, FMOs for a nucleophile. It is noteworthy that the (lu ‡
1)mo is a typical vacant orbital in cummulenes but the lumo has a shape localized at the
carbonyl bond, i.e., p*CˆO in the ketene plane.
There are many orientations to make effective HOMO ! lumo and HOMO !
(lu ‡ 1)mo charge transfers (CTs). Since the [8 ‡ 2]CˆC addition in Scheme 1 appears
not to take the best FMO interaction, systematic analyses of possible cycloaddition
paths are required. Scheme 6 show these models between tropone (1) and the parent
ketene (2a; R ˆ H). Nine addition paths are exhibited, the carbonyl C-atom
(>C(3)ˆC(2)ˆO(9)) of 2a is the charge-accepting center in all models. Transition
state 1 obtained from 2a (ˆ TS1a) uses the HOMO ! (lu ‡ 1)mo CT interaction. The
other eight TSs are based on the HOMO ! lumo CT interaction. Symmetry-forbidden
[2 ‡ 2] paths, TS2a, TS3a, TS7a, and TS9a, require the homo ! LUMO or homo !

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Scheme 5. Frontier-Orbital (Charge-Transfer, CT) Interactions for the Symmetry-Allowed [8 ‡ 2]CˆC Cyclo-
addition
a) An arrow and a broken line indicate the interaction leading to the [8 ‡ 2]CˆC cycloaddition TS1a, in Fig. 1.
The atom numbering follows the IUPAC nomenclature for the product 3a (except for the H-atoms).
(LU ‡ 1)MO back CT [16]. In view of the HOMO shape given in Scheme 5, the active
site of tropone is the O-atom. Thus, one may predict that TS1a ([8 ‡ 2]CˆC) and TS4a
([8 ‡ 2]CˆO) are likely paths. If another TS has a smaller activation energy than TS1a, a
subsequent isomerization will be needed to give the product 3a. Among the nine
orientations, three reaction paths are adopted since the O(1) ´´´ C(2) linkage is
primarily formed through effective HOMO ! lumo and HOMO ! (lu ‡ 1)mo CTs.
Fig. 1 shows three favorable TS geometries (Ea < 27kcal/mol). Five other TSs (TS5a,
TS6a, TS7a, TS8a, and TS9a) were ruled out on the basis of the activation energies
(30 ± 50 kcal/mol) by preliminary calculations. TS4a is of the smallest activation energy
(Ea ˆ 17.0 kcal/mol). However, the cycloadduct of TS4a has no reaction channel
toward 3a. TS4a is ruled out by the absence of the subsequent isomerization path.
The calculated activation energies are 26.5 kcal/mol for TS3a, 24.6 kcal/mol for
TS1a, and 21.5 kcal/mol for TS2a. Noteworthy is that the ꢁsymmetry-forbiddenꢂ
[2 ‡ 2]CˆC cycloaddition is more favorable than the ꢁsymmetry-allowedꢂ [8 ‡ 2]CˆC one.
Formally, the g-lactone 3a is obtained by the concerted [8 ‡ 2]CˆC addition via TS1a.
However, the calculation showed that the concerted path is unfavorable and a stepwise
path involving some other cycloadducts is predicted. Although TS2a with the smallest
activation energy appears to be likely, it would suffer steric crowd in the case of the
substituted ketenes 2b (R ˆ Ph) and 2c (R ˆ CF3). TS3a with the largest activation
energy is almost free from steric crowd. Through the model calculations (Fig. 1), the
tropone ± ketene reaction is thought to involve various steps toward the final product,
the g-lactone 3. That is, the reaction would be different from many other tropone
reactions [17]. In Sect. 3, the experimental search for intermediates will be described.

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Scheme 6. Nine Cycloaddition Models for Transition-State Searches
a) Arrows show dominant CT interactions, and broken lines (a ± i) indicate minor ones. TS ˆ transition state; ꢁaꢂ
in TS1a ± TS9a means that the parent ketene (2a; R ˆ H, represented by and ).
3. Experimental Examination. ± We traced the reaction of tropone (1) with diphenyl
ketene (2b) [18] in CH2Cl2 at room temperature, which gave the g-lactone 3b [7]
(Scheme 7). Similarly to bis(trifluoromethyl) ketene (2c) [19], the same type of [8 ‡ 2]
cycloadduct, the g-lactone, 3c was obtained exclusively.
We attempted to detect transient reaction intermediates to clarify the reaction
mechanism. Choosing experimental conditions (Scheme 8) precisely, we succeeded in
detecting the transient intermediates 5b and 5c in the reactions of tropone (1) with
ketenes 2b and 2c, respectively, by low-temperature NMR (1H and 19F) monitoring, as
exemplified by Fig. 2 for the reaction 1 ‡ 2c ! 5c > 6c ! 3c. Surprisingly, the concen-
trations of the head-to-tail type [2 ‡ 2]-cycloadducts, the b-lactones 5c > 6c, exceeded
more than 80% of the total amount of the reaction mixture at a certain stage (08, 1 h;
see Fig. 2). The cycloheptatriene spiro b-lactones 5b and 5c equilibrated with their

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Fig. 1. Three transition states of cycloadditions between tropone (1) and the parent ketene (2a; R ˆ H).
³
Geometries were optimized with B3LYP/6-31G* calculations. Distances are in Š. nÄ is the sole imaginary
frequency, which verifies that the obtained geometry is that of the TS.
Scheme 7. Reactions of Tropone (1) with Diphenyl Ketene (2b) or Bis(trifluoromethyl) Ketene (2c) Giving the
[8 ‡ 2] Cycloadducts 3b and 3c
a) 1 (1 equiv.), 2 (1.1 equiv.), CH2Cl2, r.t., 2 h; 3b: 96% (isolated yield); 3c: 99% (isolated yield).
norcaradiene3) spiro b-lactones 6b and 6c, respectively (Scheme 8). At temperatures
above À 408 for 5b and 08 for 5c, the b-lactones were converted quantitatively to the
final g-lactones 3b and 3c, respectively.
The detection of the initially formed intermediates 5b and 5c established the
reaction scheme 1 ‡ 2 ! 5 ! 3. Apparently, the conventional two-step formation of the
final [8 ‡ 2] cycloadduct 3b via the zwitterion 4b in the reaction of 2b and 1 according
to Scheme 2 does not take place; 4b was not detected by the low-temperature NMR
monitoring. On the other hand, in the reaction 1 ‡ 2c, a zwitterion 4c was observed in
trace amounts (see Exper. Part) at À 708; but its 1H- and 19F-NMR signals ((CF3)2Cˆ,
not (CF3)2C < ) disappeared on warming to À 658; the signals due to 5c were generated
gradually. The intermediate 4c will be discussed in Sect. 4.
1
The low-temperature H-NMR spectra of the metastable intermediate 5b (t1/2
(À408) ca. 3 h) allowed us to solve the spin networks of the H-atoms of the structure.
Characteristic 1H-NMR patterns in the region of olefinic protons and 13C-NMR signals
were assigned to a cycloheptatriene moiety of 5b and 5c. The 13C-NMR signal of the
spiro atom C(4) was shifted downfield due to its direct linkage with O(1). The
3
)
For typical studies and reviews on norcaradiene, see [20].

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Scheme 8. Experimental Detection of the New Intermediates 5b,c and 6b,c in the Reaction between Tropone (1)
and Ketene 2b or 2c
a) Equimolar amounts of 1 and 2 were mixed with CD2Cl2 (or (D8)toluene) in an NMR tube. b) NMR detection
of the transient reaction intermediates (for details, see Exper. Part): i) For 1 ‡ 2b ! 5b/6b; after 30 min at À 508,
maximum concentration of 5b/6b, 4 mol-%; for 1 ‡ 2c ! 5c/6c, after 1 h, at 08 maximum concentration of 5c/6c,
83 mol-%. ii) For 5b ! 3b, after 30 min at À 208, 100 mol-%; for 5c ! 3c: after 2 h at 208, 100 mol-%.
intermediates 5b and 5c must, therefore, be head-to-head [2 ‡ 2] cycloadducts, i.e., b-
lactones which is confirmed by the spectral data. The structure of the cycloadduct 5b
was supported by comparison of its 1H- and 19F-NMR data with those of 5c. The FT-IR
spectra of the intermediates 5b and 5c showed the CˆO stretching vibrations at 1832
and 1848 cmÀ1, respectively, typical for b-lactones. In the 13C-NMR spectra
(100.6 MHz) of all the intermediates, the individual C-atoms were present. The 19F-
NMR spectra of 5c and 3c showed the presence of a > C(CF3)2 group (not ˆC(CF3)2).
The detailed analysis of the NMR data was accomplished by the following techniques:
resolution-enhanced 1H-, 13C-, and 19F-NMR spectra with Gaussian and sine-bell wind
1
1
functions, separately [21], H,1H-COSY [22], H,1H-COLOC [23], and 13C,1H-COSY
[24]. Thus, the presence of isomeric cycloheptatriene (5b-1 and 5b-2 or 5c-1 and 5c-2)
and norcaradiene structures (6b-1 or 6c-1 and 6c-2) was established (see Scheme 9 and

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1
Fig. 2. Variable-temperature H-NMR monitoring of the reaction between tropone (1) and bis(trifluoromethyl)
ketene (2c) in CD2Cl2. Molar percentages for intermediate 5c take into account the total amount of equilibrium
mixture, i.e., cycloheptatrienes 5c-1 and 5c-2 and norcaradienes 6c-1 and 6c-2 (see below)
Figs. 3 ± 6); in addition to the equilibrations between the cycloheptatriene and
norcaradiene structures, the conformational equilibria shown in Scheme 9 were
observed. The composite-decoupled 13C-NMR spectrum of the b-lactones 5c and 6c
at À 458 consisted of 28 sharp signals due to the individual seven-membered-ring C-
atoms of the four isomers, which coalesced to 16 signals at higher temperature (08) as a
1
consequence of rapid interconversion of conformations. The H-NMR spectra also
showed similar changes. Thus, the b-lactones 5 and 6 possess a time-averaged plane of
symmetry at higher temperatures, which no longer exists below À 208. The following
ratios were determined: 5b-1/5b-2/6b-1/6b-2 2.5 :0.8 :0.7:0% at À 508 (30 min) or 5c-1/
5c-2/6c-1/6c-2 45 :24 :11:3% at 08 (1 h). At elevated temperatures, the cyclohepta-
triene key intermediates 5b and 5c existed preferentially.
Asao et al. reported a trace (5%) amount of the heptafulvene by-product in the
reaction between 2-methoxytropone (1-OMe) and 2b, the by-product arising from
decarboxylation of the b-lactone 5d (Scheme 10,a) [25]. Thus, the decarboxylation
from 5b was investigated; however, even at high temperature, no CO2 elimination was
observed in the reaction of 1 with 2b (Scheme 10,b). The absence of decarboxylation in
the reactions 1 ‡ 2b and 1 ‡ 2c will be discussed in Sect. 4.

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Scheme 9. Equilibria of Isomeric Cycloheptatriene and Norcaradiene Intermediates 5 and 6, respectively
a) Isomer 6b-2 is unstable owing to steric congestion between the Ph groups and the cyclohexadiene plane.
Fig. 3. 1H-NMR Spectrum (400 Mz) of the [2 ‡ 2]-intermediate b-lactones (5b-1 > 5b-2 > 6b-1) generated from
the reaction between tropone (1) and diphenyl ketene (2b). H-5, H-6,9, etc., are short forms of HÀC(5), HÀC(6)
4
and HÀC(9), etc. )
4. Computational Search for Reaction Paths. ± The direct formation of 5b and 5c
via the [2 ‡ 2]CˆC cycloaddition (TS2) between 1 and 2b (R ˆ Ph) or 2c (R ˆ CF3) has
been predicted to be unlikely if the ketene substituents are bulky (R ˆ Ph and CF3).
For the reaction 1 ‡ 2c, Fig. 7 shows the geometries of stationary points, intermediates,

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Fig. 5. 1H-NMR Spectrum (400 MHz) of [2 ‡ 2]-intermediate b-lactones (5c-1 > 5c-2 > 6c-1 > 6c-2) generated
from the reaction between tropone (1) and bis(trifluoromethyl) ketene (2c). H-5, H-5,10, etc., are short forms of
HÀC(5), HÀC(5) and HÀC(10), etc.4)
and transition states. The first step is dioxetane formation via TS3c that corresponds to
TS3a in Fig. 1. The dioxetane intermediate 7c suffers ring strain by the exocyclic
CˆC bond and undergoes ring cleavage (TS10c) to give the first zwitterionic
intermediate 4c. TS10c could not be determined in the present calculations. The
small geometry changes in intermediate 7c gave intermediate 4c since 7c is unstable
relative to the reactants (‡4.9 kcal/mol) and 4c is stable relative to them (À 7.3 kcal/
mol). Therefore, TS10c would be located on a very narrow region of the potential-
energy surface. In 4c, the connecting CÀO bond distance is 1.526 Š. Rotation around
this bond would lead to the cycloheptatriene b-lactone intermediate 5c-2; in spite of
many attempts, the ring closure TS11c could not be obtained. On ring closure, steric
repulsion between a CF3 substituent and the p-electron density of the tropone moiety
must be avoided. The path would stay in an extremely limited region of the potential
surface. From the b-lactone intermediate 5c-2, the second zwitterionic intermediate 8c
is generated via the ring-opening TS12c. The species 8c seems to be more stable than
the first zwitterion 4c, because the former involves an O ´´´ H H-bond. The zwitterion
4
)
For convenience, the b-lactones 6 are numbered similarly to the b-lactones 5; for systematic names, see
Exper. Part.

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Scheme 10. Examination of Decarboxylation from the b-Lactone Intermediate (a) see [25]; b) the present work)
8c is isomerized to the final g-lactone 3c via TS13c. The intermediacy of the two
zwitterions 4c and 8c are needed to avoid the symmetry-forbidden concerted [1,3]
rearrangements 7c ! 5c-2 and 5c-2 ! 3c. The four-membered neutral intermediates 7c
and 5c-2 are needed to reach the sterically congested product 3c. In forming those
neutral species, tropone (1) acts as if it were a simple ketone. The conjugated triene
moiety in the seven-membered ring of 1 is unreactive in the initial stage, i.e., in the
cycloaddition.
Fig. 8 shows the energy diagram of the reaction 1 ‡ 2c. Although TS11c could not be
obtained, it is not a rate-determining step because a CÀC bond is newly formed, and no
covalent bonds are cleaved. The initial cycloadduct 7c is unstable relative to the
reactants 1 ‡ 2c. In fact, the transient species 7c was not detected experimentally. The
stability order of the intermediates and the product is 3c > 8c > 5c-2 > 4c > 7c. The g-
lactone 3c is the destination as the most-stable species in the present multi-step
reaction. The zwitterion 4c was experimentally observed slightly prior to 5c. The second
zwitterion 8c should be detected in view of its stability, but as it is in a highly excited
vibrational state after TS12c, it is transformed rapidly to the product 3c passing through
the low-energy barrier of TS13c.
In Scheme 9 (see above), the equilibria of the four isomers 5c-1, 5c-2, 6c-1, and 6c-2
are illustrated. The coexistence of 5c-1 and 5c-2 is due to the puckering motion of the
cycloheptatriene moiety. The conformers 5c-1 and 5c-2 have almost the same stability;
the latter is only ‡ 1.50 kcal/mol less stable than the former. However, the
norcaradiene conformers 6c-1 and 6c-2 are ‡ 5.98 kcal/mol and ‡ 7.82 kcal/mol less
stable than 5c-1, respectively. Apparently, the equilibria in Scheme 9 are curious in view
of these energy differences. They may be explicable in terms of molecular interactions

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Fig. 7. Geometries of intermediates, transition states, and the product 3c of the reaction between bis(trifluoro-
methyl) ketene (2c) and tropone (1) optimized by RB3LYP/6-31G* SCRF ˆ dipole. TS ˆ transition state; c in
TS3c and TS10c ± TS13c means that the ketene 2c is used. Distances in Š.

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Fig. 8. Energy changes in kcal/mol along the stepwise path corresponding to Fig. 7 for the reaction 1 ‡ 2c ! 3c
as follows. The b-lactones 5c and 6c have large dipole moments (5 ± 6 D), and their
antiparallel orientation brings about a permanent dipole ± dipole stabilization. At this
orientation, the 4p-electron system of the norcaradiene 6c may overlap with the 6p-
electron system of the cycloheptatriene 5c in the symmetry-allowed [6 ‡ 4] orbital
interaction (Fig. 9).
In Scheme 10 (see above), the nonoccurrence of decarboxylation from b-lactones 5
(! heptafulvene ‡ CO2) is illustrated. The lack of decarboxylation from 5 is also
explicable in terms of this dipole ± dipole interaction. The interaction fixes the
cycloheptatriene ring thus blocking the geometric conversion to the heptafulvene
skeleton. If the exocyclic CˆC bond were formed, it would reinforce the flattening of
the seven-membered ring. The formation would make the HOMO coefficient very
small in the ring, and consequently the (HOMO ! lumo) interaction shown in Fig. 9
would be decreased.
In the case of the reaction of diphenyl ketene (2b) and tropone (1), presence or
absence of the two zwitterions 4b and 8b is examined. Only the second zwitterion 8b is
obtained (see Fig. 10); the absence of 4b is in accord with the experimental result. In
the reaction 1 ‡ 2b, the b-lactone 5b would be formed by the concerted [2 ‡ 2]CˆC
cycloaddition corresponding to TS2a in Fig. 1. In fact, the geometry of TS2b is
successfully obtained and shown in Fig. 11.
Through IRC calculations, TS2b is confirmed to be that of a [2 ‡ 2]CˆC cyclo-
addition. Indeed, the Ph group of 2b may avoid the steric crowd flexibly (see 2b ‡ D !

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Fig. 9. Antiparallel orientation of 5c-2 and 6c-2 inducing the homo ! LUMO and HOMO ! lumo charge-
transfer interactions in the [6 ‡ 4] symmetry-allowed orbital overlap. Note that the terminal C ´´´ C distances
(2.8753 and 2.4535 Š) are similar, which is suitable for the interactions
Fig. 10. Second zwitterion intermediate 8b in the reaction between tropone (1) and diphenyl ketene (2b). The first
zwitterion intermediate 4b is not obtained.
H in Scheme 11) which is crucially different from the CF3 group of 2c (see 2c ‡ D ! E);
the difference has been reported in our recent work [15]. The second zwitterion 8b
needs to intervene between the b-lactone 5b and the product g-lactone 3b. However,
the DFT calculations demonstrate that the b-lactone 5 is the key intermediate,
irrespective of the reactant ketene 2a, 2b, or 2c.

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Fig. 11. [2 ‡ 2]CˆC Cycloaddition (TS2b) geometry com-
posed of tropone (1) and diphenyl ketene (2b). TS2b
affords the b-lactone 5b.
Scheme 11. Difference of Steric Crowd in the Formation of Cycloadducts from 2b and 2c. Cyclobutanone H is
formed from 2b and D; an increase of the steric crowd in the course of the cycloaddition of 2c and D leads to an
a-methyleneoxetane E
5. Concluding Remarks. ± In this work, reactions between tropone (1) and ketenes 2
were investigated theoretically and experimentally. Although symmetry-allowed, the
concerted [8 ‡ 2] cycloaddition leading directly to the g-lactone products 3 is
unfavorable. Novel [2 ‡ 2] cycloadducts, the b-lactones 5 were found as key
intermediates. Thus, tropone (1) acts as a mere ketone for ketenes, which represents
an unprecedented reaction in tropone chemistry [17]. As illustrated in Scheme 2, the
formation of a zwitterionic intermediate has been suggested by kinetics [8]. But the
intermediate species is the b-lactone 5, and the zwitterion is a transient species in the
course of the isomerization 5 ! 3. Experimental low-temperature NMR analyses were
consistent with the computational data. The b-lactones 5 and 6 with the isomeric
cycloheptatriene and norcaradiene structures were unequivocally detected. Scheme 12
summarizes the presented results.

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Helvetica Chimica Acta ± Vol. 88 (2005)
Scheme 12. New Mechanisms for the Cycloaddition between Tropone (1) and Ketenes 2 Established in This
Work (revision of Schemes 1 and 2)
Experimental Part
General. Diphenyl ketene (2b) [26], bis(trifluoromethyl) ketene (2c) [18], and tropone (1) [27][28] were
prepared according to literature methods. Freshly distilled 2b, 2c, and 1 were used for the reactions. The solvents
used for the reactions were freshly distilled under N2 from appropriate drying agents, and all acids were removed
carefully [29]. All reactions and their NMR monitorings were performed under either an inert atmosphere or
high vacuum (<10À4 Torr); solns. were degassed by three successive freeze-pump-thaw cycles at 10À4 Torr. NMR
Tubes for sealed-tube experiments were flame-dried under vacuum immediately prior to the experiments.
Column chromatography (CC): Merck silica gel 60 (70 ± 230 mesh); elution with CH2Cl2. M.p.: Büchi 511
apparatus; uncorrected. UV/VIS Spectra (EtOH): Hitachi U-3300 spectrometer; 1-cm quartz cell; lmax in nm.
IR Spectra: Hitachi 260-50 grating spectrometer for routine spectra; Jeol JIR-100 FT-IR instrument for FT
spectra (CH2Cl2); nÄ in cmÀ1. NMR Spectra: Bruker AM-400 (400 (1H), 100.6 (13C), and 376.5 MHz (19F)
spectrometer; CD2Cl2 solns., unless otherwise specified; chemical shifts d in ppm rel. to SiMe4 (ˆ0 ppm) for 1H
and 13C, and C6F6 (ˆ À 162.9 ppm) for 19F; J values in Hz; assignments by 1H{1H} homonuclear decoupling,
13C{1H} heteronuclear decoupling, and 2D experiments. MS: Jeol DM-303 double focusing spectrometer; in m/z
(rel. %). Elemental analyses were performed at the microanalytical laboratory, of Daikin Industries, Osaka,
Japan.
Method of Calculation. Geometry optimizations were carried out by the DFT method. Vibrational analyses
of the method were also performed to check whether the obtained geometries are at stable points
or at saddle points. The DFT method was Beckeꢂs three-parameter hybrid functional [30] that consists of

Helvetica Chimica Acta ± Vol. 88 (2005)
1537
HF ‡ nonlocal exchange and correlation parts (B3LYP/6-31G* SCRF ˆ dipole). SCRF ˆ dipole means the
solvent effect of the Onsager reaction field model [31]. The self-consistent reaction-field method used the
dielectric constant e ˆ 9.08 of the solvent CH2Cl2. Reaction paths were traced by the use of the IRC (intrinsic
reaction coordinate) method [32]. All the calculations were carried out with GAUSSIAN 98 [33] installed at a
Compaq ES40 computer (Information Processing Center, Nara University of Education, and Computer Center
of Nara University).
Reaction of Tropone (1) with Diphenyl Ketene (2b): 3,3a-Dihydro-3,3-diphenyl-2H-cyclohepta[b]furan-2-
one (3b). In a typical experiment, a soln. of 1 (1.98 g, 18.7mmol) in CH 2Cl2 (10 ml) was added dropwise at 08 to
a soln. of 2b (3.86 g, 19.9 mmol) in CH2Cl2 (30 ml). The mixture was stirred at r.t. for 2 h until the reaction was
complete. Solvent removal gave a residue which was purified by a short CC followed by recrystallization from
1
EtOH: 3b (5.40 g, 96%) (IR and H-NMR: identical with those of an authentic sample obtained according to
[7][9] (in benzene at r.t. overnight)). Colorless prisms. M.p. and mixed m.p. 108 ± 1098 ([8]: m.p. 108 ± 1098).
UV/VIS: 280 (3.39). IR (KBr): 1800vs, 1640s, 1122vs, 1102vs, 699vs. 1H-NMR (CDCl3): 7.41 ± 7.25 (complexm,
10 arom. H); 6.55 (dd, J ˆ 11.1, 6.3, HÀC(7)); 6.44 (dd, J ˆ 11.1, 5.8, HÀC(6)); 6.09 (dd, J ˆ 6.3, 1.4, HÀC(8));
6.01 (ddd, J ˆ 9.7, 5.8, 1.7, HÀC(5)); 4.54 (dd, J ˆ 9.7, 4.3, HÀC(4)); 3.65 (ddd, J ˆ 4.3, 1.7, 1.4, HÀC(3a)).
13C-NMR (CDCl3): 174.95 (s, C(2)); 142.81 (s, Ph); 141.29 (s, Ph); 138.90 (s, C(8a)); 129.07( d, 2 C, Ph); 128.84
(d, 2 C, Ph); 128.77 (d, 2 C, Ph); 128.19 (d, Ph); 127.69 (d, Ph); 127.61 (d, C(7)); 127.52 (d, C(6)); 127.49 (d, 2 C,
Ph); 126.17( d, C(5)); 120.09 (d, C(4)); 100.56 (d, C(8)); 59.39 (s, C(3)); 48.39 (d, C(3a)). EI-MS (70 eV): 300
‡
‡
(8, M ), 194 (100, [M À C7H6O] ), 166 (54), 165 (48), 106 (12), 78 (4), 77 (4).
Reaction of Tropone (1) with Bis(trifluoromethyl) Ketene (2c). 3,3a-Dihydro-3,3-bis(trifluoromethyl)-2H-
cyclohepta[b]furan-2-one (3c). As described above for 3b, with 1 (2.01 g, 19.0 mmol) and 2c (3.59 g, 20.2 mmol)
in CH2Cl2 (40 ml) at r.t. for 2 h: 3c (5.34 g, 99%), after distillation under vacuum (6 Torr). An anal. sample was
obtained by further bulb-to-bulb distillation. Colorless oil. B.p. 58 ± 598/6 Torr. UV/VIS: 277 (3.55). IR (neat):
1823vs, 1659s, 1324s, 1289vs, 1262s, 1218vs, 1178s, 1068s, 999s, 809s, 696s. 1H-NMR (CDCl3): 6.55 (ddd, J ˆ 9.6,
6.1, 2.2, HÀC(7)); 6.52 (ddd, J ˆ 9.6, 6.2, 2.2, HÀC(6)); 6.33 (ddt, J ˆ 11.8, 6.6, 1.7, HÀC(5)); 6.11 (ddt, J ˆ 6.1,
2.2, 1.7, HÀC(8)); 5.43 (ddq, J ˆ 11.8, 4.6, 2.2, HÀC(4)); 3.38 (dt, J ˆ 4.6, 1.7, HÀC(3a)). 13C-NMR (CDCl3):
162.10 (s with q fine structure 3J(C,F) ˆ 1.4, C(2)ˆO); 139.89 (s, C(8a)); 128.58 (d, C(5)); 128.55 (d, C(7));
1
3
127.69 (d, C(6)); 121.93 (s with qq fine structure, J(C,F) ˆ 285.1, J(C,F) ˆ 2.7, 1 CF3); 121.43 (s with qq fine
1
4
structure, J(C,F) ˆ 285.1, 3J(C,F) ˆ 1.7, 1 CF3); 114.62 (d with q fine structure, J(C,F) ˆ 3.6, C(4)); 102.86 (d,
C(8)); 60.11 (s with sept. fine structure, 2J(C,F) ˆ 30.7, C(3)); 41.25 (d with q fine structure, 3J(C,F) ˆ 2.2,
C(3a)). 19F-NMR (CDCl3): À 70.82 (q, J(F,F) ˆ 10.1, CF3); À 63.75 (q, J(F,F) ˆ 10.1, CF3). EI-MS (70 eV):
4
4
‡
284 (100, M ), 283 (7), 215 (72), 106 (21), 78 (54), 77 (10). Anal. calc. for C11H6F6O2 (284.16): C 46.50, H 2.13, F
40.11; found: C 46.22, H 2.19, F 40.39.
Low-Temperature NMR Spectroscopic Monitoring of the Reaction 1 ‡ 2: General Procedure. Tropone (1;
0.14 mmol), ketene 2 (0.14 mmol), and CD2Cl2 (or (D8)toluene; 0.40 ml) were vacuum-transferred into an
NMR tube. The temp. of the mixture was regulated in an NMR probe. The monitoring of the reactions was
performed periodically at various temp. between À 908 and 208. The 13C-NMR measurements were performed
at a probe temp. of À 908 (below À 908, the reaction does not proceed).
Monitoring of the Reaction 1 ‡ 2b: 3,3-Diphenyl-1-oxaspiro[3.6]deca-5,7,9-trien-2-one (5b)/3',3'-Diphenyl-
spiro[bicyclo[4.1.0]hepta-2,4-diene-7,2'-oxetan]-4'-one (6b). According to the General Procedure, with 1 (15 mg)
and 2b (28 mg) and 1H-NMR spectroscopy. At the initial stages of the reaction at À 608, a very minor amount of
5b/6b was observed. After 4 h at À 558, 5b/6b (ca. 1 mol-% of the total amount of the mixture) was clearly
detected. At À 508, mixture contained the highest concentration of 5b/6b (ca. 4 mol-%). Disappearance of the
signals due to 5b/6b was observed after 30 min at À 208, while the product 3b was clearly detected. The absence
1
of any product other than 3b was established by the H-NMR at r.t.
1
Data of 5b/6b at À 508: FT-IR: 1832vs. H-NMR: 5b-1 (2.5 mol-%): 6.58 (dd, J ˆ 7.0, 5.2, HÀC(9)); 6.42
(dd, J ˆ 10.3, 5.6, HÀC(7)); 6.38 (dd, J ˆ 10.3, 5.2, HÀC(8)); 6.10 (dd, J ˆ 8.7, 5.6, HÀC(6)); 5.81 (d, J ˆ 7.0,
HÀC(10)); 5.60 (d, J ˆ 8.7, HÀC(5)); 5b-2 (0.8 mol-%): 6.73 (d, J ˆ 5.6, HÀC(7), HÀC(8)); 6.60 (dd, J ˆ 10.4,
5.6, HÀC(6), HÀC(9)); 5.91 (d, J ˆ 10.4, HÀC(5), HÀC(10)); 6b-1 (0.7mol-%): 6.09 ( d, J ˆ 8.7, HÀC(7),
HÀC(8)); 5.57( dd, J ˆ 8.7, 5.8, HÀC(6), HÀC(9)); 2.81 (d, J ˆ 5.8, HÀC(5), HÀC(10)); 6b-2 (0 mol-%).
13C-NMR: 5b-1: 167.51 (s, CˆO); 136.69 (d, C(7), C(8)); 131.93 (d, C(6)); 131.92 (d, C(9)); 127.43 (d, C(5));
127.33 (d, C(10)); 56.83 (s, C(3)); 54.46 (s, C(4)); 5b-2: 167.63 (s, CˆO); 136.66 (d, C(7), C(8)); 132.47 (d, C(6),
C(7)); 129.40 (d, C(5), C(10)); 56.54 (s, C(3)); 53.37( s, C(4)); 6b-1: 165.02 (s, CˆO); 121.98 (d, C(7 ), C(8));
121.67( d, C(6), C(9)); 58.92 (s, C(3)); 47.23 (s, C(4)); 42.01 (d, C(5), C(10)).
Monitoring of the 1 ‡ 2c Reaction: 1-{[3,3,3-Trifluoro-1-hydroxy-2-(trifluoromethyl)prop-1-enyl]oxy}cy-
clohepta-2,4,6-trien-1-ylium Inner Salt (4c) and 3,3-Bis(trifluoromethyl)-1-oxaspiro[3.6]deca-5,7,9-trien-2-one

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Helvetica Chimica Acta ± Vol. 88 (2005)
(5c)/3',3'-Bis(trifluoromethyl)spiro[bicyclo[4.1.0]hepta-2,4-diene-7,2'-oxetan]-4'-one (6c). According to the
General Procedure, with 1 (15 mg) and 2c (25 mg) and 1H- and 19F-NMR spectroscopies (results in Fig. 2). At
À 708, a trace amount of 4c was detected (<1 mol-% of the total amount of the mixture). At À 658, the signals
of 4c had disappeared, while the formation of 5c/6c was detected. At À 408, the signals due of 5c/6c reached
6 mol-% of the total amount of the mixture. At À 208 (3 h), the formation of 5c/6c (32 mol-%) was clearly
established. Instead of increasing the signals of 5c/6c at 08, the mixture contained the highest concentration of
5c/6c (83 mol-% of the total amount of the mixture). Remarkably, increase of the signals of 3c was detected at
208, while complete disappearance of the signals of 5c/6c was observed after 2 h at 208. The absence of any
1
product other than 3c was established by the H-NMR at r.t.
Data of 4c at À 708 ((D8)toluene): 1H-NMR: 6.40; 6.56; 7.46. 13C-NMR (at À 908): 179.11; 158.74; 155.37;
149.99. 19F-NMR: À 51.91 (br. s).
Data of 5c/6c at À 208: FT-IR: 1848vs. 19F-NMR: À 63.19 (br. s); À 63.42 (br. s). 1H-NMR: 5c-1 (26 mol-
%): 6.71 (dd, J ˆ 5.0, 2.0, HÀC(8)); 6.70 (dd, J ˆ 4.8, 2.0, HÀC(7)); 6.48 (dd, J ˆ 9.2, 5.0, HÀC(9)); 6.22 (dd, J ˆ
9.0, 4.8, HÀC(6)); 5.66 (dd, J ˆ 9.0, 2.0, HÀC(5)); 5.58 (dd, J ˆ 9.2, 2.0, HÀC(10)). 5c-2 (17mol-%): 6.97( dd,
J ˆ 6.7, 3.3, HÀC(7), HÀC(8)); 6.54 (br. d, J ˆ 9.8, HÀC(6), HÀC(9)); 6.51 (m, HÀC(9), HÀC(6)); 5.30 (br.
m, HÀC(5), HÀC(10)); 6c-1 (11 mol-%): 6.45 (m, HÀC(8)); 6.43 (d, J ˆ 10.4, HÀC(7)); 5.65 (dd, J ˆ 10.4, 5.9,
HÀC(6)); 5.57( dd, J ˆ 10.1, 5.9, HÀC(9)); 2.68 (br., HÀC(10)); 2.53 (t, J ˆ 5.9, HÀC(5)); 6c-2 (8 mol-%): 6.51
(m, HÀC(7), HÀC(8)); 5.30 (br., HÀC(6), HÀC(9)); 2.73 (br., HÀC(10)); 2.58 (t, J ˆ 6.1, HÀC(5)).
13C-NMR: 5c-1: 158.23 (s, CˆO); 124.37( d, C(9)); 122.26 (d, C(6)); 121.73 (d, C(7), C(8)); 121.41 (q, J(C,F) ˆ
286.9, CF3); 118.99 (d, C(5)); 117.18 (d, C(10)); 60.82 (s with sept. fine structure C(3)); 52.83 (s, C(4)); 5c-2:
160.24 (s, CˆO); 134.56 (d, C(8)); 134.05 (d, C(7)); 124.68 (d, C(6), C(9)); 121.53 (q, J(C,F) ˆ 288.4, CF3);
102.53 (d, C(5), C(10)); 61.22 (s with sept. fine structure, C(3)); 53.11 (s, C(4)); 6c-1: 154.65 (s, CˆO); 126.85 (d,
C(7), C(8)); 122.28 (q, J(C,F) ˆ 289.1, CF3); 118.51 (d, C(6)); 117.04 (C(9)); 62.59 (s with sept. fine structure,
C(3)); 40.80 (d, C(10)); 40.46 (d, C(5)); 36.36 (s, C(4)); 6c-2: 154.83 (s, CˆO); 124.68 (d, C(7), C(8)); 123.37 (q,
J(C,F) ˆ 289.2, CF3); 102.60 (d, C(6), C(9)); 59.38 (s with sept. fine structure, C(3)); 36.69 (s, C(4)); 36.62 (d,
C(10)); 36.36 (d, C(5)). 19F-NMR (08): À 62.84 (s, 6c-2); À 63.10 (s, 6c-1); À 63.40 (s, 5c-1); À 63.59 (s, 5c-2).
REFERENCES
[1] R. Huisgen, in ꢁProfiles, Pathways, and Dreams: Autobiographies of Eminent Chemistsꢂ, Ed. J. I. Seeman,
American Chemical Society, Washington, DC, 1994.
[2] R. Huisgen, L. A. Feiler, G. Binsch, Angew. Chem. 1964, 76, 892; P. Otto, L. A. Feiler, R. Huisgen, Angew.
Chem., Int. Ed. 1968, 7, 737; R. Huisgen, L. A. Feiler, P. Otto, Tetrahedron Lett. 1968, 4485; R. Huisgen, P.
Otto, J. Am. Chem. Soc. 1968, 90, 5342; R. Huisgen, L. A. Feiler, P. Otto, Chem. Ber. 1969, 102, 3444; R.
Huisgen, L. A. Feiler, G. Binsch, Chem. Ber. 1969, 102, 3460; R. Huisgen, P. Otto, Chem. Ber. 1969, 102,
3475; R. Huisgen, P. Otto, J. Am. Chem. Soc. 1969, 91, 5922; R. Huisgen, H. Mayr, Tetrahedron Lett. 1975,
2965; R. Huisgen, H. Mayr, Tetrahedron Lett. 1975, 2969; R. Huisgen, Pure Appl. Chem. 1980, 52, 2283; G.
Swieton, J. Von Jouanne, H. Kelm, R. Huisgen, J. Chem. Soc., Perkin Trans. 2 1983, 37 .
[3] R. Huisgen, Chem. Pharm. Bull. 2000, 48, 757; R. Huisgen, Pure Appl. Chem. 1989, 61, 613; R. Huisgen, in
ꢁ1,3-Dipolar Cycloaddition Chemistryꢂ, Ed. A. Padwa, Wiley, New York, 1984, Vol. 1, p. 1; R. Huisgen, Acc.
Chem. Res. 1977, 10, 117; R. Huisgen, Angew. Chem., Int. Ed. 1968, 7, 321.
[4] R. B. Woodward, R. Hoffmann, Angew. Chem., Int. Ed. 1969, 8, 781; R. B. Woodward, R. Hoffmann, ꢁThe
Conservation of Orbital Symmetryꢂ, VCH, Weinheim, 1970; I. Fleming, ꢁFrontier Orbitals and Organic
Chemical Reactionsꢂ, Wiley, New York, 1976.
[5] ꢂThe Chemistry of Ketenes, Allenes and Related Compoundsꢂ, ꢁThe Chemistry of Functional Groupsꢂ, Ed.
S. Patai, Wiley, Chichester, 1980, 2 vols.
[6] J. A. Hyatt, P. W. Raynolds, Org. React. 1994, 45, 159; T. T. Tidwell, ꢁKetenesꢂ, Wiley, New York, 1995.
[7] R. Gompper, A. Studeneer, W. Elser, Tetrahedron Lett. 1968, 1019.
[8] R. Gompper, Angew. Chem., Int. Ed. 1969, 8, 312.
[9] C. Jutz, I. Rommel, I. Lengyel, J. Feeney, Tetrahedron 1966, 22, 1809.
[10] H. Ulrich, ꢁCycloaddition Reactions of Heterocumulenesꢂ, Academic Press, New York, 1967, Chapt. 2,
p. 38; J. D. Roberts, C. M. Sharts, Org. React. 1962, 12, 1; L. Ghosez, M. J. OꢂDonnell, in ꢁPericyclic
Reactionsꢂ, Eds. A. P. Marchand, R. E. Lehr, Academic Press, Orlando, 1977, Vol. 2, p. 79; L. Ghosez, J.
Marchand-Brynaert, in ꢁComprehensive Organic Synthesisꢂ, Ed. L. A. Paquette, Pergamon, Oxford, 1991,
Vol. 5, p. 85.

Helvetica Chimica Acta ± Vol. 88 (2005)
1539
[11] H. Staudinger, E. Suter, Ber. Dtsch. Chem. Ges. 1920, 53B, 1092 (Chem. Abstr. 1920, 14, 3423); E. H.
Farmer, M. O. Farooq, J. Chem. Soc. 1938, 1925; H. L. Dryden Jr., B. E. Burgert, J. Am. Chem. Soc. 1955,
77, 5633; H. Mayr, U. W. Heigl, J. Chem. Soc., Chem. Commun. 1987, 1804.
[12] M. O. Farooq, T. A. Vahidy, S. M. Husain, Bull. Soc. Chim. Fr. 1958, 830; E. Vogel, K. Müller, Liebigs Ann.
Chem. 1958, 615, 29; J. C. Martin, G. P. Gott, V. W. Goodlett, R. H. Hasek, J. Org. Chem. 1965, 30, 4175.
[13] T. Machiguchi, T. Hasegawa, A. Ishiwata, S. Terashima, S. Yamabe, T. Minato, J. Am. Chem. Soc. 1999, 121,
4771.
[14] S. Yamabe, T. Dai, T. Minato, T. Machiguchi, T. Hasegawa, J. Am. Chem. Soc. 1996, 118, 6518.
[15] T. Machiguchi, J. Okamoto, J. Takachi, T. Hasegawa, S. Yamabe, T. Minato, J. Am. Chem. Soc. 2003, 125,
14446.
[16] S. Yamabe, T. Minato, Y. Osamura, J. Chem. Soc., Chem. Commun. 1993, 450.
[17] D. Lloyd, ꢁThe Chemistry of Conjugated Cyclic Compoundsꢂ, Wiley, New York, 1990, p. 1; T. Asao, M. Oda,
in ꢁCarbocyclische p-Elektronen-Systeme: Houben-Weyl, Methoden der organischen Chemieꢂ, Eds. E.
Müller and O. Bayer, Georg Thieme Verlag, Stuttgart, 1986, Vol. 5/2c, p. 49, 710 ± 780; D. Lloyd, ꢁNon-
benzenoid Conjugated Carbocyclic Compoundsꢂ, Elsevier, Amsterdam, 1984, p. 1; T. Nozoe, in ꢁNon-
benzenoid Aromatic Compoundsꢂ, Ed. M. Kotake, ꢁComprehensive Organic Chemistryꢂ, Asakura-Shoten,
Tokyo, 1973, Vol. 13; F. Pietra, Chem. Rev. 1973, 73, 293.
[18] H. Staudinger, Chem. Ber. 1905, 38, 1735.
[19] D. C. England, C. G. Krespan, J. Am. Chem. Soc. 1965, 87, 4019; D. C. England, C. G. Krespan, J. Am.
Chem. Soc. 1966, 88, 5582.
[20] E. Ciganek, J. Am. Chem. Soc. 1965, 87, 652; G. Maier, Angew. Chem., Int. Ed. 1967, 6, 402; J. D. Roberts,
G. E. Hall, J. Am. Chem. Soc. 1971, 93, 2203; E. Ciganek, J. Am. Chem. Soc. 1971, 93, 2207; K. Takeuchi, T.
Kitagawa, T. Toyama, K. Okamoto, J. Chem. Soc., Chem. Commun. 1982, 313; K. Takeuchi, H. Fujimoto, T.
Kitagawa, H. Fujii, K. Okamoto, J. Chem. Soc., Perkin Trans. 2 1984, 461; S. Kohmoto, T. Kawatsuji, K.
Ogata, M. Yamamoto, K. Yamada, J. Am. Chem. Soc. 1991, 113, 5476.
[21] A. E. Derome, in ꢁModern NMR Techniques for Chemistry Researchꢂ, Ed. J. E. Baldwin, Organic
Chemistry Series 6, Pergamon, Oxford, 1987; H. Friebolin, ꢁBasic One- and Two-Dimensional NMR
Spectroscopy (Engl. Transl.)ꢂ, 2nd edn., VCH, New York, 1993.
[22] W. P. Aue, E. Bartholdi, R. R. Ernst, J. Chem. Phys. 1976, 64, 2229; K. Nagayama, A. Kumar, K. Wuethrich,
R. R. Ernst, J. Magn. Reson. 1980, 40, 321.
[23] A. Bax, R. Freeman, J. Magn. Reson. 1981, 44, 542.
[24] A. A. Maudsley, R. R. Ernst, Chem. Phys. Lett. 1977, 50, 368; R. Freeman, G. A. Morris, J. Chem. Soc.,
Chem. Commun. 1978, 684.
[25] T. Asao, N. Morita, N. Iwagame, Bull. Chem. Soc. Jpn. 1974, 47, 773.
[26] L. I. Smith, H. H. Hoehn, ꢁOrganic Synthesisꢂ, Wiley, New York, 1955, Coll. Vol. 3, p. 356.
[27] T. Machiguchi, Synth. Commun. 1982, 12, 1021.
[28] T. Machiguchi, Tetrahedron 1995, 51, 1133.
[29] D. D. Perrin, W. L. F. Armarego, D. R. Perrin, ꢁPurification of Laboratory Chemicalsꢂ, 3rd edn., Pergamon,
Oxford, 1988.
[30] A. D. Becke, J. Chem. Phys. 1993, 98, 5648.
[31] L. Onsager, J. Am. Chem. Soc. 1936, 58, 1486.
[32] K. Fukui, J. Phys. Chem. 1970, 74, 4161; C. Gonzalez, H. B. Schlegel, J. Phys. Chem. 1990, 94, 5523.
[33] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, V. G. Zakrzewski,
J. A. Montgomery Jr., R. E. Stratmann, J. C. Burant, S. Dapprich, J. M. Millam, A. D. Daniels, K. N. Kudin,
M. C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S.
Clifford, J. Ochterski, G. A. Petersson, P. Y. Ayala, Q. Cui, K. Morokuma, D. K. Malick, A. D. Rabuck, K.
Raghavachari, J. B. Foresman, J. Cioslowski, J. V. Ortiz, A. G. Baboul, B. B. Stefanov, G. Liu, A. Liashenko,
P. Piskorz, I. Komaromi, R. Gomperts, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A.
Nanayakkara, C. Gonzalez, M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen, M. W. Wong, J. L.
Andres, C. Gonzalez, M. Head-Gordon, E. S. Replogle, J. A. Pople, ꢁGAUSSIAN 98, Revision A.7ꢂ,
Gaussian, Inc., Pittsburgh, PA, 1998.
Received February 14, 2005