Generation and reaction of a phenyl-substituted cyclopentadienyl cation annelated with two homoadamantene frameworks

Article abstract of DOI:10.1021/jo800340g

(Chemical Equation Presented) The generation of the title cation 2 ⁺ and its reaction under solvolytic and non-nucleophilic conditions were investigated. When the precursor chlorocyclopentadiene 5 was reacted with silica gel that contained water or with anhydrous MeOH, the corresponding 5-hydroxy- and 5-methoxycyclopentadienes (7 and 6) were produced in 68 and 81% yields, respectively. This indicates that 2⁺ is formed as an intermediate under solvolytic conditions and persists without any rearrangement of the homoadamantane frameworks, at least during the period before capture by the nucleophile. On the other hand, the abstraction of a chloride ion from 5 by Ag⁺ in the absence of a nucleophile at -78°C resulted in the quantitative formation of allyl cation 8⁺, incorporated in a bicyclo[3.1.0]hexane framework, via the Wagner-Meerwein rearrangement of a homoadamantane framework. Cation 8⁺ was isolated as the SbF ₆ salt, and its structure was determined by X-ray crystallography. Quenching this cation with MeOH afforded a methyl ether 14, with a cyclopentadiene structure retained but one of the homoadamantane frameworks had undergone a structural change by a further Wagner-Meerwein rearrangement.

Full text of DOI:10.1021/jo800340g

Generation and Reaction of a Phenyl-Substituted Cyclopentadienyl
Cation Annelated with Two Homoadamantene Frameworks
Kohei Ogawa,† Shinya Minegishi,† Koichi Komatsu,†,‡ and Toshikazu Kitagawa*,§
Institute for Chemical Research, Kyoto UniVersity, Uji, Kyoto 611-0011, Japan, Department of
EnVironmental and Biotechnological Frontier Engineering, Fukui UniVersity of Technology, Gakuen, Fukui
910-8505, Japan, and Department of Chemistry for Materials, Graduate School of Engineering,
Mie UniVersity, Tsu, Mie 514-8507, Japan
kitagawa@chem.mie-u.ac.jp
ReceiVed February 10, 2008
The generation of the title cation 2+ and its reaction under solvolytic and non-nucleophilic conditions
were investigated. When the precursor chlorocyclopentadiene 5 was reacted with silica gel that contained
water or with anhydrous MeOH, the corresponding 5-hydroxy- and 5-methoxycyclopentadienes (7 and
6) were produced in 68 and 81% yields, respectively. This indicates that 2+ is formed as an intermediate
under solvolytic conditions and persists without any rearrangement of the homoadamantane frameworks,
at least during the period before capture by the nucleophile. On the other hand, the abstraction of a
chloride ion from 5 by Ag+ in the absence of a nucleophile at -78 °C resulted in the quantitative formation
of allyl cation 8+, incorporated in a bicyclo[3.1.0]hexane framework, via the Wagner-Meerwein
rearrangement of a homoadamantane framework. Cation 8+ was isolated as the SbF6- salt, and its structure
was determined by X-ray crystallography. Quenching this cation with MeOH afforded a methyl ether
14, with a cyclopentadiene structure retained but one of the homoadamantane frameworks had undergone
a structural change by a further Wagner-Meerwein rearrangement.
+
Introduction
4π systems. For example, the pKR value of the parent Cp
cation, C5H5+, was determined to be -40 by an electrochemical
method (eq 1).4 This value is remarkably low compared to that
The stability of a cyclopentadienyl (Cp) system is greatly
dependent on its oxidation state, i.e., anion (aromatic), radical
(nonaromatic), or cation (antiaromatic). A wide variety of
theoretical and experimental approaches have been used in the
investigations of these species.1 In contrast to Cp anions,
however, experimental studies of Cp radicals and cations are
quite limited due to their thermal instability. In particular, Cp
cations are highly labile species, as shown by the thermodynamic
criteria described below, except for cations with several strongly
electron-donating substituents2 and those stabilized as cobalt
complexes.3 Several investigations have revealed the antiaro-
matic resonance destabilization of Cp cations due to the cyclic
5
+
of the unsubstituted allyl cation (pKR ≈ -21). 5-Iodocyclo-
pentadiene does not form the Cp cation, even with the assistance
of silver ion (eq 2).6 The rate of its solvolysis is at least 105
times slower than that of cyclopentyl iodide, the corresponding
saturated analogue. Furthermore, the hydride affinity, as esti-
mated by gas-phase ionization potential measurements, indicates
that the Cp cation is destabilized by 1.37 eV (31.6 kcal/mol)
with respect to the cyclopentenyl cation (eq 3).7
ESR and IR spectroscopy data, collected at low temperatures,
demonstrated that several pentasubstituted Cp cation derivatives
8
such as C5H5+, C5Cl5
,
9,10 and C5(i-Pr)5 11 have C5 or higher
+
+
symmetry and a triplet ground state. On the other hand, pentaaryl
† Kyoto University.
‡ Fukui University of Technology.
§ Mie University.
(3) Dunn, J. A.; Hunks, W. J.; Ruffolo, R.; Rigby, S. S.; Brook, M. A.;
McGlinchey, M. J. Organometallics 1999, 18, 3372.
(1) (a) Harder, S. Coord. Chem. ReV. 1998, 176, 17; erratum 2000, 199,
331. (b) Jutzi, P.; Burford, N. Chem. ReV. 1999, 99, 969. (c) Allen, A. D.; Tidwell,
T. T. Chem. ReV. 2001, 101, 1333. (d) Wiberg, K. B. Chem. ReV. 2001, 101,
1317.
(4) Breslow, R.; Mazur, S. J. Am. Chem. Soc. 1973, 95, 584.
(5) Breslow, R. Pure Appl. Chem. 1971, 28, 111.
(6) Breslow, R.; Hoffman, J. M., Jr. J. Am. Chem. Soc. 1972, 94, 2110.
(7) Lossing, F. P.; Traeger, J. C. J. Am. Chem. Soc. 1975, 97, 1579.
(2) Gompper, R.; Glo¨ckner, H. Angew. Chem., Int. Ed. Engl. 1984, 23, 53.
5248 J. Org. Chem. 2008, 73, 5248–5254
10.1021/jo800340g CCC: $40.75 2008 American Chemical Society
Published on Web 06/18/2008

Reaction of a Phenyl-Substituted Cyclopentadienyl Cation
SCHEME 1. Possible Charge Distribution in Cp Cations
SCHEME 2. Synthesis of Chlorocyclopentadiene 5
derivatives have an excited triplet state and persist as long-lived
species at temperatures below -40 °C due to the substantial
delocalization of the positive charge.12 Trapping experiments
for several derivatives under solvolytic conditions have been
reported.13 Reactions with nucleophiles, such as alcohols or
amines, afforded the corresponding substitution products, sug-
gesting the formation of a Cp cation as an intermediate.
However, when a solution of the pentamethyl Cp cation,
prepared at low temperature, is warmed and such nucleophiles
are not present, polymers resulting from tetramethylpentafulvene
generated by deprotonation are formed.13a These facts make
investigations of the reaction behavior of Cp cation very difficult.
More recently, the attempted hydride abstraction of pentam-
substitution with alkyl groups. Our attempted generation of the
cation corresponding to 1• at -78 °C, however, resulted in the
formation of a complex mixture of unidentified compounds due
to rearrangements initiated by methyl migration from the tert-
butyl group to the five-membered ring.18 We now wish to report
on the generation and a new reaction of a phenyl-substituted
Cp cation 2+, in which a phenyl group has been introduced
instead of a tert-butyl group to avoid alkyl migration.
-
ethylcyclopentadiene was reported to give a stable B(C6F5)4
14
+
salt of C5Me5
,
but it was subsequently proved to be the
pentamethylcyclopentenyl cation.15
We recently reported on the isolation and X-ray structure of
the cyclopentadienyl radical 1•, annelated with two homoada-
mantene units.16 Structural modification with rigid σ frameworks
has been found to effectively stabilize the cyclic π-system by
(i) σ-π conjugation, i.e. C-C hyperconjugation, (ii) Bredt’s
rule protection, which avoids the elimination of hydrogen to
form a bridgehead olefin, and (iii) steric protection by the
bulkiness of the annelated structures.17 When the cyclic
π-system has a positive charge, additional stabilization would
be expected due to inductive electron donation as a result of
Structure 2+ does not specify the degree of charge delocal-
ization and potentially means one of the three possibilities
indicated in Scheme 1, i.e., fully delocalized Cp cation (a), allyl-
localized Cp cation (b), and localized Cp cation (c).
Results and Discussion
Synthesis of Chlorocyclopentadiene 5. Scheme 2 shows the
synthetic route for preparing the homoadamantene-fused cy-
clopentadienes. The lithiation of 4-bromo-4-homoadamantene16
followed by reaction with 0.5 equiv of benzoyl chloride afforded
the tertiary alcohol 3. The acid-catalyzed cyclization16,19 of this
alcohol using p-toluenesulfonic acid, followed by recrystalli-
zation from hexane, gave colorless crystals of cyclopentadiene
4 in 56% yield (based on 4-bromo-4-homoadamantene). The
structure of 4 was determined by NMR, X-ray crystallography,
and elemental analysis. The position of the hydrogen in the five-
membered ring was confirmed by single-crystal X-ray analysis
(see the Supporting Information).
Treatment of 4 with an equimolar amount of N-chlorosuc-
cinimide in CH2Cl2 gave 5 as yellow crystals in 88% yield.
The unsymmetrical structure of 5 was confirmed by NMR,
elemental analysis, and X-ray crystallography (Figure 1a). A
space-filling model (Figure 1b) shows that the bulky homoada-
mantane frameworks of 5 completely protects the backside of
(8) Saunders, M.; Berger, R.; Jaffe, A.; McBride, J. M.; O’Neill, J.; Breslow,
R.; Hoffman, J. M., Jr.; Perchonock, C.; Wasserman, E.; Hutton, R. S.; Kuck,
V. J. J. Am. Chem. Soc. 1973, 95, 3017.
(9) Breslow, R.; Hill, R.; Wasserman, E. J. Am. Chem. Soc. 1964, 86, 5349.
(10) Vancˇik, H.; Novak, I.; Ki{emet, D. J. Phys. Chem. A 1997, 101, 1523.
(11) Sitzmann, H.; Bock, H.; Boese, R.; Dezember, T.; Havlas, Z.; Kaim,
W.; Moscherosch, M.; Zanathy, L. J. Am. Chem. Soc. 1993, 115, 12003.
(12) (a) Breslow, R.; Chang, H. W.; Yager, W. A. J. Am. Chem. Soc. 1963,
85, 2033. (b) Breslow, R.; Chang, H. W.; Hill, R.; Wasserman, E. J. Am. Chem.
Soc. 1967, 89, 1112.
(13) (a) Jutzi, P.; Mix, A. Chem. Ber. 1992, 125, 951. (b) Allen, A. D.;
Sumonja, M.; Tidwell, T. T. J. Am. Chem. Soc. 1997, 119, 2371. (c) Allen,
A. D.; Tidwell, T. T. J. Org. Chem. 2001, 66, 7696. (d) de Meijere, A.;
Kozhushkov, S. I.; Fokin, A. A.; Emme, I.; Redlich, S.; Schreiner, P. R. Pure
Appl. Chem. 2003, 75, 549.
(14) Lambert, J. B.; Lin, L.; Rassolov, V. Angew. Chem., Int. Ed. 2002, 41,
1429.
(15) (a) Otto, M.; Scheschkewitz, D.; Kato, T.; Midland, M. M.; Lambert,
J. B.; Bertrand, G. Angew. Chem., Int. Ed. 2002, 41, 2275. (b) Jones, J. N.;
Cowley, A. H.; Macdonald, C. L. B. Chem. Commun. 2002, 1520. See
also: Mu¨ller, T. Angew. Chem., Int. Ed. 2002, 41, 2276. Lambert J. B. Angew.
Chem., Int. Ed. 2002 41, 2278.
(16) Kitagawa, T.; Ogawa, K.; Komatsu, K. J. Am. Chem. Soc. 2004, 126,
9930.
(18) Ogawa, K.; Kitagawa, T.; Komatsu, K. Unpublished results.
(19) (a) Threlkel, R. S.; Bercaw, J. E. J. Organomet. Chem. 1977, 136, 1.
(b) Erker, G.; Schamberger, J.; van der Zeijden, A. A. H.; Dehnicke, S.; Kru¨ger,
C.; Goddard, R.; Nolte, M. J. Organomet. Chem. 1993, 459, 107.
(17) For example, see: Nishinaga, T.; Wakamiya, A.; Yamazaki, D.; Komatsu,
K. J. Am. Chem. Soc. 2004, 126, 3163, and references therein.
J. Org. Chem. Vol. 73, No. 14, 2008 5249

Ogawa et al.
FIGURE 1. X-ray structures of 5. The chlorine atom and the adjacent
carbon atom (CR) are shown in green and purple, respectively. The Cp
ring is indicated in blue. (a) ORTEP drawing. Hydrogen atoms are
omitted for clarity. Ellipsoids are drawn at the 50% probability level.
(b) Space-filling model viewed from the same direction. The carbon
and hydrogen atoms of the phenyl groups are shown in dark colors.
Note that the backside of the CR-Cl bond is hidden by the homoada-
mantane frameworks.
FIGURE 2. Structures of singlet and triplet states of 2+ optimized at
the B3LYP/6-31+G(d,p) and the UB3LYP/6-31+G(d,p) levels, re-
spectively. Selected bond lengths are indicated, together with the NICS
values calculated at the (U)B3LYP/6-311+G(d,p)//(U)B3LYP/6-
31+G(d,p) level in brackets.
SCHEME 4. Formation of Cation 8+
SCHEME 3. Solvolytic Substitution of
Chlorocyclopentadiene 5
the CR-Cl bond from attack by nucleophiles. As a result, SN2
type reactions at that bond are highly unlikely.
for singlet and triplet states are +32.0 and -0.8, exhibiting anti-
and nonaromatic characteristics, respectively. The optimized
structure of singlet 2+ shows a pronounced bond alternation in
the five-membered ring and indicates that the cation has an allyl-
like moiety at C2-C1-C5 with a C3dC4 double bond, whereas
that of triplet 2+ possesses less bond alternation [∆R(C-C) e
0.014 Å], as shown in Figure 2. These results, as well as the
absence of triplet signals in the ESR spectra of 2+ (vide infra),
suggest that 2+ exists as a singlet state, which would be best
illustrated as below:
Reaction of Chloride 5 under Solvolytic Conditions.
Chloride 5 was spontaneously converted to methyl ether 6 upon
allowing it to stand in methanol at room temperature (Scheme
3). After 24 h, 6 was obtained in 81% isolated yield. Cyclo-
pentadienol 7 was also obtained as colorless crystals in 68%
isolated yield by passing chloride 5 through a silica gel column
with CH2Cl2-hexane as the eluent. The structures of 6 and 7
were verified by NMR spectroscopy and elemental analysis. The
position of the methoxy group in the five-membered ring of 6
was unambiguously confirmed by X-ray crystallography (see
the Supporting Information). These findings suggest that the
Cp cation 2+ was generated from 5 under solvolytic conditions
and then reacted with a solvent molecule to afford the corre-
sponding methyl ether 6 or alcohol 7.20 It is also demonstrated
that the cation 2+ does not undergo rearrangement at room
temperature, at least during the period prior to the nucleophilic
attack.
The rate of methanolysis of 5 was determined by the
conductometric method. The pseudo-first-order rate constant
observed at 25 ( 0.1 °C, k1 ) 1.33 × 10-4 s-1, is over 2 orders
of magnitude greater than that of tert-butyl chloride under the
same conditions (k1 ) 7.53 × 10-7 s-1).21 This indicates that
annelation with two homoadamantene frameworks makes the
formation of cation 2+ easier than that of the tert-butyl cation.
Theoretical Calculations of Cp Cation 2+. Theoretical
calculations at the (U)MP2/6-311+G(d,p)//(U)B3LYP/6-
31+G(d,p) level predicted that Cp cation 2+ has a singlet ground
state, with the energy of the corresponding triplet state being
3.0 kcal/mol higher in energy. The nucleus-independent chemi-
cal shift (NICS) values in the center of the five-membered ring
Generation and Reaction of the Cation 2+ in the
Absence of Nucleophiles. In an attempt to directly observe 2+,
the abstraction of the chloride ion from 5 with Ag+ was
conducted under weakly nucleophilic conditions. When chloride
5 was treated with Ag+(C6H6)3B(C6F5)4 22 in vacuum-degassed
-
CD2Cl2 at -78 °C, the color of the solution changed from
yellow to intense reddish orange over a period of 30 min.
1
Monitoring this reaction by H and 13C NMR spectra at -80
°C showed an almost quantitative conversion of 5 to a new,
C1-symmetrical carbocation 8+, in which the structure of one
of the homoadamantane frameworks changed by a skeletal
rearrangement (Scheme 4). Treatment of 5 with AgSbF6 under
-
the same conditions afforded the SbF6 salt of the identical
cation 8+, which was isolated as reddish orange crystals. The
reaction of alcohol 7 with B(C6F5)3 in CD2Cl2 at -78 °C also
(20) Although the SN2 process is unlikely, some SN2′-like nucleophilic
assistance of the solvent molecule upon ionization might be possible. We
appreciate a comment of one of the reviewers that the solvolysis process would
(22) Ogawa, K.; Kitagawa, T.; Ishida, S.; Komatsu, K. Organometallics 2005,
24, 4842.
be further elucidated by examining the product obtained from the reaction in
-
the presence of an added strong nucleophile, such as N3
.
(23) The assignment of this signal is based on DEPT and H-C COSY
measurements and the result of GIAO calculations (Figure 3).
(21) Fainberg, A. H.; Winstein, S. J. Am. Chem. Soc. 1956, 78, 2770.
5250 J. Org. Chem. Vol. 73, No. 14, 2008

Reaction of a Phenyl-Substituted Cyclopentadienyl Cation
SCHEME 5. Generation and Rearrangement of 11+
FIGURE 3. (a) ORTEP drawing of 8+SbF6-. Hydrogen atoms are
omitted for clarity. Thermal ellipsoids are drawn at the 50% probability
level. Selected bond lengths (Å): C1-C2 ) 1.404(5), C2-C3 )
1.398(5), C3-C4 ) 1.450(5), C4-C5 ) 1.490(5), C1-C5 ) 1.478(5),
C4-C6 ) 1.580(5), C5-C6 ) 1.539(5). (b) 13C NMR chemical shifts
(δ) of 8+SbF6- in CD2Cl2 at ambient temperature. Values obtained by
the GIAO method [B3LYP/6-311+G(d,p)//B3LYP/6-31+G(d,p)] are
shown in parentheses.
HOMOfLUMO (460 nm, 2.70 eV) and HOMO–2fLUMO
(371 nm, 3.34 eV) transitions with oscillator strengths f )
0.1468 and 0.1693, respectively, based on theoretical calcula-
tions [TD-B3LYP/6-311+G(d,p)//B3LYP/6-31+G(d,p)]. The
longest wavelength absorption is red-shifted compared with that
of the related cyclopentenyl cation 9+ (λmax 394 nm in 96%
H2SO424) due to the σ-π conjugation between the cationic p
orbitals and the cyclopropane ring.
It has been reported that bicyclo[3.1.0]hexenyl cation deriva-
tives can be generated by irradiation of the corresponding
benzenium ion in FSO3H at -78 °C (Scheme 5).25 They are
thermally unstable and readily isomerize back to the benzenium
ion via cleavage of the cyclopropane ring. The dimethylmeth-
ylene carbon of the heptamethylbicyclo[3.1.0]hexenyl cation
(11+), formed from heptamethylbenzenium ion 10+, was
reported to undergo migration around the five-membered ring,
which can be observed by NMR at temperatures above
-90 °C.25b,c In contrast, cation 8+ does not undergo decomposi-
tion or isomerization to a benzenium ion even at room
temperature for over 3 months in solution in a tube sealed under
vacuum. The ring-opening reaction is suppressed because the
cyclopropane ring is incorporated into a rigid polycyclic
σ-framework.
FIGURE 4. UV-vis spectrum of 8+SbF6 in CH2Cl2 at room
-
temperature.
afforded the same cation 8+ quantitatively, which was observed
by NMR spectroscopy.
The molecular structure of 8+SbF6- was determined by X-ray
crystallography (Figure 3a), which revealed 8+ to be a
bicyclo[3.1.0]hexenyl cation. To the best of our knowledge,
-
8+SbF6 is the first bicyclo[3.1.0]hexenyl cation salt to be
analyzed by X-ray crystallography. The phenyl ring is twisted
by 38.8°, and the cyclopropane ring is fixed at an angle of 102.8°
with respect to the planar five-membered ring. The five
membered ring is nearly planar (the sum of the internal angles
is 539.9°). The C1-C2 [1.404(5) Å] and C2-C3 [1.398(5) Å]
bond lengths in the allyl cation moiety are comparable to those
of the pentamethylcyclopentenyl cation [1.396(3) and 1.385(3)
Å].15b
A possible mechanism for the formation of 8+ is shown in
Scheme 6. The Cp cation 2+, formed by abstraction of a chloride
or a hydroxide ion from 5 and 7, respectively, is unstable due
to the absence of aromatic stabilization. As a result, a
Wagner-Meerwein rearrangement readily takes place in the
homoadamantane framework to give a secondary cyclopenta-
dienylmethyl cation 12+ (path a),26 followed by cyclopropana-
tion to form a more stable cation, 8+. This rearrangement does
not appear to be concerted with the formation 2+, since no
rearranged products such as 14 were detected under solvolytic
conditions. DFT calculations [MP2/6-311+G(d,p)//B3LYP/6-
31+G(d,p)] predict that cation 8+ is 24.2 kcal/mol more stable
Two bonds of the cyclopropane ring external to the five
membered ring, C4-C6 and C5-C6, are significantly elongated,
the former [1.580(5) Å] being much longer than the other
[1.539(5) Å]. The positive charge on the allyl cation moiety
(C1-C2-C3) is not equally distributed over the C1 and C3
carbons, as indicated by the 13C NMR chemical shifts (241.2
and 210.5 ppm, respectively) (Figure 3b). These findings
demonstrates more effective σ-π conjugation between the 2p
orbital of the C3 carbon and the C4-C6 σ bond. The signal for
the C6 atom was at a considerably low field (δ 119.623), which
indicates the distribution of positive charge on this carbon
through hyperconjugation.
(24) Deno, N. C.; Houser, J. J. J. Am. Chem. Soc. 1964, 86, 1741.
(25) (a) Childs, R. F.; Sakai, M.; Winstein, S. J. Am. Chem. Soc. 1968, 90,
7144. (b) Childs, R. F.; Winstein, S. J. Am. Chem. Soc. 1968, 90, 7146. (c)
Childs, R. F.; Winstein, S. J. Am. Chem. Soc. 1974, 96, 6409.
(26) The 2-homoadamantyl cation, generated under the solvolysis conditions,
has been reported to readily undergo a Wagner-Meerwein rearrangement: (a)
Kitagawa, T.; Okazaki, T.; Komatsu, K.; Takeuchi, K. J. Org. Chem. 1993, 58,
7891. (b) Okazaki, T.; Kitagawa, T.; Takeuchi, K. J. Phys. Org. Chem. 1994, 7,
The UV-vis spectrum of 8+SbF6 (Figure 4), measured in
CH2Cl2 at room temperature, shows absorption maxima at 459
(ꢀ 7170) and 373 (ꢀ 8090) nm, which were assigned to
-
485. Although 12+ might be higher in energy than 2+, the equilibrium 2+
a
12+ can be shifted to the right if 12+ is consumed by conversion to 8+, thus
enabling overall transformation from 2+ to 8+.
J. Org. Chem. Vol. 73, No. 14, 2008 5251

Ogawa et al.
SCHEME 6. Mechanism for the Formation of 8+
FIGURE 5. Pictorial presentation of HOMO and LUMO in 8+
calculated at the B3LYP/6-31+G(d,p) level.
considerably large coefficient in LUMO (Figure 5), supporting
this assumption. It should be noted that 14 was not formed by
the methanolysis of 5 (Scheme 3), confirming that the inter-
mediate 2+ underwent a nucleophilic attack without rearranging
to 8+ under solvolytic conditions.
SCHEME 7. Trapping of 2+ with Cycloheptatriene
SCHEME 8. Reaction of 8+ with MeOH
Experimental Section
1,1-Bis(4-homoadamanten-4-yl)-1-phenylmethanol (3). To a
stirred solution of 4-bromo-4-homoadamantene16 (2.85 g, 12.5
mmol) in ethyl ether (100 mL), cooled at 0 °C, was added a solution
of tert-butyllithium (1.60 M) in pentane (15.0 mL, 24.0 mmol)
dropwise over 15 min. After the mixture was stirred at 0 °C for 15
min, benzoyl chloride (0.72 mL, 6.2 mmol) was added at 0 °C
over a 5 min period. The mixture was gradually warmed to room
temperature over 1 h and then stirred for an additional 13 h. The
reaction mixture was quenched by adding water (100 mL) and
extracted with ethyl ether, and the organic layer was washed with
10% NaCl and dried (MgSO4). Evaporation of the solvent gave a
pale yellow oil (2.61 g). This oil was analyzed by 1H and 13C NMR,
which confirmed 3 as the major product, and was used in the next
step without further purification: 1H NMR (300 MHz, C6D6) δ 7.57
(dd, J ) 8.4, 1.2 Hz, 2H), 7.27-7.04 (m, 3H), 5.96 (dd, J ) 9.2,
1.7 Hz, 2H), 2.63 (br s, 2H), 2.23 (br s, 2H), 2.02 (br s, 4H),
1.84-1.58 (m, 20H); 13C NMR (75 MHz, C6D6) δ 152.7, 144.6,
135.5, 128.5, 127.9, 127.2, 87.3, 37.7, 35.5, 35.1, 35.0, 34.6, 33.8,
32.5, 30.41, 30.35.
Cyclopentadiene 4. To a solution of crude 3 (2.61 g) in ethyl
ether (100 mL), cooled to 0 °C, was added dropwise a solution of
p-toluenesulfonic acid monohydrate (0.517 g, 2.72 mmol) in ethyl
ether (20 mL). The mixture was gradually warmed to room
temperature over 6 h and stirred for an additional 6 h. The mixture
was quenched by adding H2O and extracted with ethyl ether. The
organic layer was washed with 5% NaHCO3 and 10% NaCl and
dried (MgSO4). The solvent was evaporated, and the residue was
purified by chromatography on SiO2 followed by recrystallization
from hexane to give 4 (1.35 g, 56% based on 4-bromo-4-
homoadamantene) as colorless crystals: mp 187-188 °C; 1H NMR
(300 MHz, C6D6) δ 7.26 (d, J ) 4.8 Hz, 4H), 7.12-7.05 (m, 1H),
3.58 (s, 1H), 3.39 (t, J ) 5.3 Hz, 1H), 3.20 (br s, 1H), 2.96 (t, J )
5.4 Hz, 1H), 2.16 (t, J ) 5.4 Hz, 1H), 2.11-1.33 (m, 23H), 0.98
(d, J ) 13.8 Hz, 1H); 13C NMR (75 MHz, C6D6) δ 150.8, 149.1,
145.8, 138.6, 138.4, 129.5 (2C), 128.9 (2C), 126.0, 64.1, 43.2, 42.3,
38.0, 37.6 (2C), 37.1, 36.7, 36.6, 34.5, 32.4, 31.3, 31.1, 30.1 (2C),
29.1, 29.0, 28.6, 27.7; IR (KBr, cm-1) 2894, 2842, 1595, 1560,
1488, 1440, 1352, 1085, 947, 779, 754, 700. Anal. Calcd for C29H34:
C, 91.04; H, 8.96. Found: C, 91.18; H, 8.99.
than singlet 2+. Unlike the case of the pentamethyl Cp cation,13a
2+ cannot be transformed into fulvene 13 (path b), a highly
strained bridgehead olefin. Thus, the formation of 8+ can be
taken not only as evidence of the generation of the Cp cation
2+ but also as a result of a novel rearrangement of the alkyl-
substituted Cp cation.
Trapping and Attempted ESR Measurement of 2+. To
confirm the formation of 2+ prior to rearrangement, 5 was
treated with AgSbF6 in CH2Cl2 at -100 °C for 10 min, and the
subsequent addition of excess cycloheptatriene as a hydride
donor at -100 °C afforded a mixture of the original cyclopen-
tadiene 4 and unchanged 5 (87:13), which strongly supports
the formation of singlet 2+ as an intermediate (Scheme 7).
The ESR spectral measurement conducted for the product of
the chloride ion abstraction of 5 with Ag+(C6H6)3B(C6F5)4- or
with SbF5 in frozen CH2Cl2 at -100 °C showed a single line at
approximately g ) 2.0. In both cases, the signal for a ∆m ) 2
transition and the xy- and z-components of the ∆m ) 1
transition, expected for a triplet biradical, were not observed
even at -223 °C. The observed ESR signal is most probably
due to a residual amount of paramagnetic species.
Reaction of 8+ with MeOH. On quenching a CH2Cl2
solution of 8+SbF6- with MeOH, another rearrangement of the
polycycloalkane framework took place to give an ether 14 in
83% isolated yield as colorless crystals (Scheme 8). The
molecular structure of 14 was confirmed by X-ray crystal-
lography and spectral data (see the Supporting Information).
The formation of 14 can be rationalized by assuming a
nucleophilic attack of MeOH on the cyclopropane carbon C6
of 8+. This is consistent with the significant positive charge
located on this carbon, as suggested by the X-ray structure and
the unusual low field shift of this carbon in 13C NMR (Figure
3b). Theoretical calculations indicated that this carbon has a
Chlorocyclopentadiene 5. N-Chlorosuccinimide (95.2 mg, 0.71
mmol) was added to a solution of cyclopentadiene 4 (0.267 g, 0.70
mmol) in CH2Cl2 (10 mL) under argon at room temperature. The
mixture was stirred for 14 h and quenched with water (100 mL).
The mixture was extracted with CH2Cl2, and the organic layer was
washed with 10% NaCl and dried (MgSO4). The solvent was
evaporated, and the residue was recrystallized from CH2Cl2-MeCN
5252 J. Org. Chem. Vol. 73, No. 14, 2008

Reaction of a Phenyl-Substituted Cyclopentadienyl Cation
to afford 5 (0.256 g, 88%) as pale yellow crystals: mp 153-154
°C; 1H NMR (300 MHz, C6D6) δ 7.60 (d, J ) 7.5 Hz, 2H),
7.31-7.23 (m, 2H), 3.16 (br s, 1H), 2.94-2.82 (m, 2H), 2.77 (br
s, 1H), 2.64 (d, J ) 13.8 Hz, 1H), 2.37 (t, J ) 5.9 Hz, 1H),
1.94-1.38 (m, 20H), 1.32 (d, J ) 12.3 Hz, 1H), 1.11 (d, J ) 14.4
Hz, 1H), one of the phenyl proton signals was overlapped with the
solvent peak; 13C NMR (75 MHz, C6D6) δ 150.6, 147.7, 146.0,
140.2, 136.7, 130.9 (2C), 128.8 (2C), 127.6, 89.0, 44.7, 38.3, 37.8,
37.2, 36.4, 36.3, 36.13, 36.07, 33.8, 32.7, 31.6, 30.9 (2C), 30.2,
28.6, 28.5, 28.2, 27.8; IR (KBr, cm-1) 2906, 2848, 1489, 1465,
1440, 1084, 946, 823, 781, 695. Anal. Calcd for C29H33Cl: C, 83.52;
H, 7.98. Found: C, 83.59; H, 7.95.
(B) Preparation with Chloride 5 and Ag+(C6H6)3B(C6F5)4-. An
NMR sample tube containing chloride 5 (23.5 mg, 0.056 mmol)
and Ag+(C6H6)3B(C6F5)4- 22 (58.4 mg, 0.057 mmol) was evacuated
with a vacuum line. CD2Cl2 (0.75 mL), dried over CaH2 and
degassed by repeated freeze-pump-thaw cycles, was vacuum-
transferred to the NMR sample tube, which was then sealed under
1
vacuum. The reaction mixture was monitored at -80 °C by H
and 13C NMR, which showed the quantitative formation of cation
8+.
(C) Preparation with Cyclopentadienol 7 and B(C6F5)3. An
NMR sample tube containing alcohol 7 (38.2 mg, 0.096 mmol)
and B(C6F5)3 (51.5 mg, 0.101 mmol) was evacuated with a vacuum
line. CD2Cl2 (0.75 mL), dried over CaH2 and degassed by repeated
freeze-pump-thaw cycles, was vacuum-transferred to an NMR
sample tube, which was then sealed under vacuum. The reaction
mixture was stirred at -78 °C for 30 min and monitored at -90
°C by 1H and 13C NMR, which showed the quantitative formation
of the cation 8+.
Methoxycyclopentadiene 6. A solution of chlorocyclopentadiene
5 (14.1 mg, 0.034 mmol) in MeOH (2 mL) was stirred under N2
for 24 h. Water (100 mL) was added, and the mixture was extracted
with CH2Cl2. The organic layer was washed with 5% NaHCO3 and
10% NaCl and dried (MgSO4). The solvent was evaporated, and
the residue purified by column chromatography on SiO2 using
CH2Cl-hexane as the eluent to afford 6 (11.4 mg, 81%) as pale
-
MeOH Quenching of 8+SbF6 to Form 14. Chloride 5 (35.6
1
yellow crystals (prisms): mp 179-180 °C; H NMR (300 MHz,
mg, 0.085 mmol) was allowed to react with AgSbF6 (31.4 mg, 0.091
mmol) in dry CH2Cl2 (1.5 mL) for 10 min at -78 °C and for a
further 30 min at room temperature. The solution was cooled again
to -78 °C, and dry MeOH (1.0 mL) was added dropwise over 5
min. The mixture was warmed to room temperature, stirred for 30
min, and filtered through a PTFE membrane filter to remove the
AgCl. Water (50 mL) was added, and the mixture was extracted
with CH2Cl2. The organic layer was dried (MgSO4) and evaporated.
The residue was purified by chromatography on SiO2 to give 14
(29.2 mg, 83% based on 5) as colorless crystals: mp 139-140 °C;
1H NMR (300 MHz, C6D6) δ 7.34 (d, J ) 8.3 Hz, 2H), 7.27 (t, J
) 7.5 Hz, 2H), 3.29 (dt, J ) 12.6, 2.6 Hz, 1H), 3.12-2.97 (m,
4H), 2.55 (s, 3H), 2.14-1.36 (m, 23H), one of the phenyl proton
signals was overlapped with the solvent peak; 13C NMR (75 MHz,
C6D6) δ 148.5, 147.1, 145.5, 143.2, 140.5, 130.5, 128.0, 126.2,
82.2, 65.5, 55.8, 42.4, 41.9, 39.7, 38.2, 38.0, 37.8, 37.5, 37.0, 36.5,
C6D6) δ 7.55 (d, J ) 8.0 Hz, 2H), 7.28 (t, J ) 7.8 Hz, 2H), 7.09
(d, J ) 7.5 Hz, 1H), 3.32-3.24 (m, 1H), 3.27 (s, 3H), 3.10-2.94
(m, 2H), 2.82 (t, J ) 5.7 Hz, 1H), 2.32 (t, J ) 5.6 Hz, 1H), 2.21
(d, J ) 13.5 Hz, 1H), 2.04-1.44 (m, 20H), 1.39 (d, J ) 12.0 Hz,
1H), 1.16 (d, J ) 13.5 Hz, 1H); 13C NMR (75 MHz, C6D6) 152.7,
147.9, 142.6, 137.6, 135.5, 128.9 (2C), 128.8 (2C), 126.8, 95.8,
49.9, 44.2, 38.6, 38.5, 37.8, 37.6, 36.3, 36.1, 34.0, 32.7, 32.6, 31.5,
31.02, 30.99, 30.95, 29.3, 28.5, 28.3, 28.2; IR (KBr, cm-1) 2898,
2843, 1598, 1489, 1464, 1442, 1073, 942, 762, 700. Anal. Calcd
for C30H36O: C, 87.33; H, 8.79. Found: C, 87.08; H, 8.79.
Cyclopentadienol 7. Crude chloride 5, prepared from cyclo-
pentadiene 4 (0.372 g, 0.97 mmol) and N-chlorosuccinimide (0.142
g, 1.06 mmol) by the method described above, was passed through
a silica gel column using hexane-CH2Cl2 as an eluent. The solvent
was removed by evaporation, and the residue was recrystallized
from CH2Cl2-MeCN (1:1) to afford 7 (0.261 g, 67% based on 4)
as colorless crystals: mp 192-193 °C; 1H NMR (300 MHz, C6D6)
δ 7.59 (d, J ) 8.1 Hz, 2H), 7.26 (t, J ) 7.5 Hz, 2H), 7.11 (t, J )
7.4 Hz, 1H), 3.13 (t, J ) 5.3 Hz, 1H), 3.05 (br s, 1H), 2.91 (d, J
) 13.2 Hz, 1H), 2.78 (t, J ) 5.6 Hz, 1H), 2.46-2.28 (m, 2H),
2.04-1.45 (m, 20H), 1.38 (d, J ) 10.5 Hz, 1H), 1.32 (s, 1H), 1.13
(d, J ) 14.1 Hz, 1H); 13C NMR (75 MHz, C6D6) δ 150.0, 147.7,
145.3, 140.8, 137.6, 129.5 (2C), 126.9, 91.7, 44.2, 38.3, 38.0, 37.9,
37.0, 36.6, 36.2, 33.2, 32.8, 32.6, 31.33, 31.27, 30.9, 30.8, 29.0,
28.5, 28.3, 28.2, one of the phenyl carbon signals was overlapped
with the solvent peak; IR (KBr, cm-1) 2903, 2841, 1491, 1440,
1348, 991, 943, 701. Anal. Calcd for C29H34O: C, 87.39; H, 8.60.
Found: C, 87.30; H, 8.65.
34.9, 32.2, 31.8, 30.82, 30.77, 29.6, 29.0, 28.4; IR (KBr, cm-1
)
2907, 2876, 2843, 1493, 1441, 1092, 1077, 1064, 959, 934, 799,
754, 744, 700. HRMS (EI+) m/z calcd for C30H36O (M+) 412.2766,
found 412.2774.
X-ray Crystal Structure Analysis. Intensity data were collected
at 100 K on a Bruker SMART APEX diffractometer with Mo KR
radiation (λ ) 0.71073 Å) and graphite monochromator. The
structures were solved by direct method (SHELXTL) and refined
by the full-matrix least-squares on F2 (SHELXL-97). All non-
hydrogen atoms were refined anisotropically, and hydrogen atoms
were placed using AFIX instructions and refined isotropically.
4: C29H34; FW ) 382.56, crystal size 0.10 × 0.10 × 0.10 mm3,
monoclinic, P2(1)/n, a ) 6.3533(12) Å, b ) 17.527(3) Å, c )
18.820(4) Å, ꢀ ) 96.512(4)°, V ) 2082.2(7) Å3, Z ) 4, Dc )
1.220 g cm-3. The refinement converged to R1 ) 0.0528, wR2 )
0.1067 (I > 2σ(I)), GOF ) 1.013.
5: C29H33Cl; FW ) 417.00, crystal size 0.20 × 0.20 × 0.20
mm3, triclinic, P-1, a ) 6.5347(13) Å, b ) 10.784(2) Å, c )
15.777(3) Å, R ) 74.790(4)°, ꢀ ) 82.116(4)°, γ ) 81.619(4)°, V
) 1055.7(4) Å3, Z ) 2, Dc ) 1.312 g cm-3. The refinement
converged to R1 ) 0.0406, wR2 ) 0.0869 (I > 2σ(I)), GOF ) 1.082.
6: C30H36O; FW ) 412.59, crystal size 0.20 × 0.20 × 0.20 mm3,
monoclinic, C2/c, a ) 11.0557(16) Å, b ) 17.650(3) Å, c )
22.609(3) Å, ꢀ ) 97.847(3)°, V ) 4370.6(11) Å3, Z ) 8, Dc )
1.254 g cm-3. The refinement converged to R1 ) 0.0429, wR2 )
0.0857 (I > 2σ(I)), GOF ) 1.012.
Preparation and Characterization of Cation 8+. (A)
Preparation with Chloride 5 and AgSbF6. An NMR sample tube
containing chloride 5 (16.6 mg, 0.040 mmol) and AgSbF6 (14.4
mg, 0.042 mmol) was evacuated with a vacuum line. CD2Cl2 (0.75
mL), dried over CaH2 and degassed by repeated freeze-pump-thaw
cycles, was vacuum-transferred to the NMR sample tube, which
was then sealed under vacuum. The reaction mixture was warmed
1
to room temperature and monitored by H and 13C NMR, which
showed the quantitative formation of cation 8+. Reddish orange
-
crystals of 8+SbF6 suitable for X-ray analysis were obtained by
allowing the reaction mixture to stand at room temperature for 1
1
week: mp 240 °C dec; H NMR (300 MHz, CD2Cl2, rt) δ 7.72 (t,
J ) 7.4 Hz, 1H), 7.64 (t, J ) 7.2 Hz, 2H), 7.43 (d, J ) 6.9 Hz,
2H), 4.15 (d, J ) 5.4 Hz, 1H), 3.54 (t, J ) 5.6 Hz, 1H), 3.30 (t, J
) 5.7 Hz, 1H), 3.13 (br s, 1H), 2.50 (dtd, J ) 18.5, 6.5, 1.7 Hz,
1H), 2.40-1.55 (m, 22H), 1.42 (d, J ) 13.2 Hz, 1H); 13C NMR
(75 MHz, CD2Cl2, rt) δ 241.2, 210.5, 155.4, 135.4, 132.6, 130.3
(2C), 129.9 (2C), 119.6, 68.2, 61.5, 40.0, 38.03, 37.99, 36.5, 35.0,
34.3, 33.2 (2C), 32.4, 32.3, 30.8, 30.2, 28.8, 28.3, 27.3, 26.7, 26.5;
IR (KBr, cm-1) 2903, 1456, 1449, 1437, 1232, 937, 657; UV-vis
(CH2Cl2) λmax (ꢀ) 327 (6720), 373 (8090), 459 (7170).
8+SbF6-: C29H33F6Sb; FW ) 617.30, crystal size 0.10 × 0.10
× 0.10 mm3, monoclinic, P2(1)/n, a ) 10.0120(13) Å, b )
21.486(3) Å, c ) 11.6831(16) Å, ꢀ ) 100.848(2)°, V ) 2468.3(6)
Å3, Z ) 4, Dc ) 1.661 g cm-3. The refinement converged to R1 )
0.0378, wR2 ) 0.0726 (I > 2σ(I)), GOF ) 1.061.
14: C30H36O; FW ) 412.59, crystal size 0.10 × 0.10 × 0.10
mm3, monoclinic, P2(1)/c, a ) 16.001(2) Å, b ) 9.9744(14) Å, c
) 14.124(2) Å, ꢀ ) 95.655(3)°, V ) 2243.3(6) Å3, Z ) 4, Dc )
J. Org. Chem. Vol. 73, No. 14, 2008 5253

Ogawa et al.
1.222 g cm-3. The refinement converged to R1 ) 0.0441, wR2 )
0.0798 (I > 2σ(I)), GOF ) 1.012.
which gave optimized structures similar to those obtained by
B3LYP/6-31+G(d,p), to reduce computational time. The absence
of a negative frequency was confirmed for all structures.
Kinetic Measurement for the Methanolysis of 5. The reaction
was conducted at 25.0 ( 0.1 °C using a 1.9 × 10-4 M solution of
5 in MeOH in the presence of 2.3 × 10-4 M 2,6-lutidine. The
progress of the reaction, monitored by conductometry, was fitted
to the first-order rate equation, yielding a rate constant of k1 )
Quenching of 2+ with Cycloheptatriene. To a stirred solution
of chlorocyclopentadiene 5 (21.4 mg, 0.051 mmol) in CH2Cl2 (3
mL), cooled at -100 °C under argon, was added AgSbF6 (21.3
mg, 0.062 mmol), and the mixture was stirred at -100 °C for 10
min. Cycloheptatriene (0.18 mg, 2.0 mmol) was added, and the
temperature was raised to -85 °C over a period of 1.5 h and then
to room temperature. Water (10 mL) was added, and the mixture
was extracted with CH2Cl2. The organic layer was washed with
10% NaCl, dried (MgSO4), and evaporated to dryness. Analysis
by 1H NMR indicated that the residue (24 mg) consisted of
cyclopentadiene 4 and unchanged 5 in a molar ratio of 87:13.
1.33 × 10-4 s-1
.
Calculations. Molecular structures of singlet 2+, triplet 2+, and
8+ were optimized by DFT calculations at the (U)B3LYP/6-
31+G(d,p) levels, using the Gaussian-98 program.27 The energies
were computed at the (U)MP2/6-311+G(d,p) level with the single-
point calculations using the (U)B3LYP/6-31+G(d,p) geometries.
Frequency calculations were performed at the 6-31G(d) level,
(27) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann,
R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.;
Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.;
Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson,
G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Salvador, P.; Dannenberg, J. J.;
Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski,
J.; Ortiz, J. V.; Baboul, A. G.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,
P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham,
M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson,
B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-Gordon, M.;
Replogle, E. S.; Pople, J. A.; Gaussian 98, ReVision A.11; Gaussian, Inc.:
Pittsburgh, PA, 2001.
Acknowledgment. This work was supported by a Grant-in-
Aid for Scientific Research from the Ministry of Education,
Culture, Sports, Science and Technology, Japan.
Supporting Information Available: 1H and 13C NMR
spectra of new compounds and crystallographic data (table and
CIF file) for 4, 5, 6, 8+, and 14. This material is available free
of charge via the Internet at http://pubs.acs.org.
JO800340G
5254 J. Org. Chem. Vol. 73, No. 14, 2008