Reaction of 7,7-diphenyl-6-oxabicyclo[3.2.0]hept-1-ene with ROH; controlling factors on the regioselectivity in the nucleophilic addition reaction

Article abstract of DOI:10.1016/S0040-4020(02)00700-7

ROH-induced decomposition of the strained 7,7-diphenyl-6-oxabicyclo[3.2.0]hept-1-ene 1 was investigated in detail. The selective formation of the endo double-bonded alkene 3 was observed for the reaction with H₂O or MeOH. Alternatively, the rea

Full text of DOI:10.1016/S0040-4020(02)00700-7

Tetrahedron 58 (2002) 7043–7047
Reaction of 7,7-diphenyl-6-oxabicyclo[3.2.0]hept-1-ene
with ROH; controlling factors on the regioselectivity
in the nucleophilic addition reactionq
*
Manabu Abe, Takafui Minamoto, Yasunori Ino, Takanori Kawakami and Masatomo Nojima
Department of Materials Chemistry, Graduate School of Engineering, Osaka University, Yamadaoka 2-1, Suita 565-0871, Osaka, Japan
This paper is dedicated to Professor Waldemar Adam on the occasion of his 65th birthday
Received 14 February 2002; accepted 19 April 2002
Abstract—ROH-induced decomposition of the strained 7,7-diphenyl-6-oxabicyclo[3.2.0]hept-1-ene 1 was investigated in detail. The
selective formation of the endo double-bonded alkene 3 was observed for the reaction with H2O or MeOH. Alternatively, the reaction with
hexafluoro-2-propanol or acetic acid exclusively afforded the exo double-bonded alkene 5. The dramatic ROH effect on the product
distribution is rationalized in this study. Strain energy (SE¼41.3 kcal/mol) of 6-oxabicyclo[3.2.0]hept-1-ene was calculated at RHF/6-31Gp
level of theory. q 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction
favored CO-bond cleavage in 2-spiroepoxy-1,3-cyclo-
pentanediyl 1,3-DR (1,3-diradical) generated by the photo-
denitrogenation of diazene DZ, and described some of its
reactivity (Scheme 1):2 (1) the thermal decomposition of 1
led to the allene derivative 2 (78%) via the retro [2þ2] type
of reaction; (2) the methanolysis was found to give
selectively the adduct 3b in high yield (.90%). The
transformations under the mild conditions undoubtedly
originate from the release of strain in compound 1 (vide
infra). In this Symposia-in-Print work, we aim at clarifying
the mechanism for the alcohol-trapping reaction of 1. Why
does the methanolysis occur selectively at C1, although the
bond formation at C2 and/or C3 would also be possible to
give the regioisomeric MeOH-adduct 4b and/or 5b (R¼Me)
(Scheme 2)? This pertinent question was addressed by
investigating the reaction of 1 with a variety of ROH, i.e.
water (H2O), trifluoroethanol (TFE), hexafluoro-2-propanol
(HFIP), and acetic acid (AcOH).
The challenging investigations on the synthesis and
reactivity of strained molecules have produced a number
of important concepts and synthetic methodologies.1 We
have recently reported for the first time the quantitative
formation of the 3-alkylideneoxetane derivative, 7,7-di-
phenyl-6-oxabicyclo[3.2.0]hept-1-ene 1, via the kinetically
Scheme 1. Preparation and reactivity of 7,7-diphenyl-6-oxa-bicyclo[3.2.0]-
hept-1-ene 1.
Keywords: strained molecule; 3-alkylideneoxetane; ring-opening reaction;
regioselectivity; stereoselectivity.
*
Corresponding author. Tel.: þ81-6-6879-7929, fax: þ81-6-6879-7928;
e-mail: abe@ap.chem.eng.osaka-u.ac.jp
Scheme 2. ROH-induced decomposition of 1.
0040–4020/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020(02)00700-7

7044
M. Abe et al. / Tetrahedron 58 (2002) 7043–7047
(entries 3–5). When TFE was used for the reaction (entry
3), the regioisomeric TFE-adduct 5c (R¼CH2CF3) was
obtained as a minor product in a significant amount (25%,
3c/5c¼74/26). It should be noted that one of the possible
stereoisomers was selectively formed, .90/10, as judged by
the NMR spectroscopic analysis. To determine the con-
Figure 1. Strain energy of 6-oxabicyclo[3.2.0]hept-1-ene OBH and its
structural analysis at RHF/6-31Gp level of theory.
1
figuration of the TFE-adduct 5c, the H NMR (600 MHz)
NOE measurements were performed for the isolated adduct.
The NOE measurement between Ha (d 4.42 ppm) and Hb (d
4.47 ppm) protons was difficult due to the closely positioned
chemical shifts. Fortunately, the NOE enhancements (1.4
and 1.0%) between the methylene protons (d 3.40 and
3.65 ppm) of the –CH2CF3 moiety and the alcoholic proton
(d 2.26 ppm) were observed.
2. Results and discussion
Prior to discussing the mechanism for the ROH-induced
decomposition of 1, we would like to indicate briefly the
strain energy (SE) of the parent 6-oxabicyclo[3.2.0]hept-1-
ene OBH and its origin, which have not been reported until
now. The strain energy of OBH, SE¼41.3 kcal/mol, was
computed using the heat of formation (DHf¼18.4 kcal/mol)
calculated by Wiberg’s method3 at RHF/6-31Gp level4 and
Franklin group equivalents5 for the energy (DHf0¼222.9) of
the corresponding unstrained models (Fig. 1).6
The strain energy exceeded the sum of the energies of the
oxetane (SE¼25.7 kcal/mol)1b and cyclopentene (SE¼6.8
kcal/mol)7 by ca. 9 kcal/mol. As can be easily imagined
from the calculated angles u1–3, the deviation from the
additivity role should be due to the expansion of the double-
bond angle u1 (¼1598, deviation from 208) and the torsional
strain (or pyramidalization) u3(¼278, deviation from 08).8
The small but significant NOE-enhancements clearly
suggest the cis-configuration between –OH and
–OCH2CF3 groups; thus, the cis-isomer 5c was stereoselec-
tively formed in the reaction with TFE. Before going to the
appropriate discussion for the notable ROH effects on
the product distribution, we had to confirm perfectly the
structure of 3c, since the spectroscopic data for
the regioisomeric TFE-adduct 4c, which is also one of the
possible candidates of the TFE-adducts, would be quite
similar to those of 3c. The structural assignment of 3c was
unambiguously achieved by the chemical transformation of
3c to ketone 6. A similar transformation was also useful for
confirming the structure of 3b2a (Eq. (1)). The TFE-adduct
4c should be intact under the oxidation conditions.
2.1. ROH-induced decomposition of 7,7-diphenyl-6-oxa-
bicyclo[3.2.0]hept-1-ene (1)
The products and yields (%) of the ROH-induced decompo-
sition of the 3-alkylideneoxetane 1 were summarized in
Table 1. The reaction of 1 with H2O (R¼H) selectively
afforded the diol 3a (¼4a) in 88% (entry 1). It was
impossible to determine the regioselectivity of the nucleo-
philic H2O addition, i.e. C1 vs C2. However, the selective
formation of the MeOH-adduct 3b (entry 2) suggests that
the reaction site is at C1.
ð1Þ
The dramatic ROH effects on the product distributions were
observed for the reactions with TFE, HFIP, and AcOH
Next, we performed the control experiments. The adduct 3c
(R¼CH2CF3) did not convert to the regioisomer 5c under
the reaction conditions, and vice versa. The control
experiments strongly suggest that 3c and 5c are not the
thermodynamically controlled products but the primary
products in the ROH-induced decomposition of 3-alkyl-
ideneoxetane 1.
Table 1. The reactions of 3-alkylideneoxetane 1 with ROH
a
Entry ROH pKa Time (h)
Products and yields (%)b
3/5c
3
4
5d
1e
2e
3e
4
H2O 15.7
MeOH 15.5
24
20
18
,1
,1
3a (88)
–
5a (,5) .95/5
3b (91) 4a (,5) 5b (,5) .95/5
3c (63) 4a (, 5) 5c (25) 74/26
3d (,5) 4a (,5) 5d (92) ,5/95
3e (,5) 4a (,5) 5e (82) ,6/94
TFE
HFIP
AcOH
12.4
9.2
4.8
For the reaction with HFIP or AcOH, the exclusive
formation of the adduct 5d (R¼CH(CF3)2) or 5e (COCH3)
was observed (entries 4 and 5). We could not detect any
trace of 3,4 ,5%. The NOE enhancements (1.0 and 1.2%,
600 MHz) between Ha (for 5d; d 4.43, for 5e; d 4.63) and Hb
5
To a solution of 3-alkylideneoxetane 1 (0.05 M) in benzene (20 mL) was
added a solution of ROH (1.0 M) in benzene (20 mL) at rt (ca. 158C),
except for H2O. H2O was added as a solution in THF.
a
Data taken from Ref. 9, which were measured in H2O at 258C.
Isolated yields, after column chromatography on silica gel. The
b
expression, ,5, means that the compound did not detect under the
isolation conditions.
c
The ratios, 3/5, were normalized to 100%.
cis-5 was selectively formed, cis-5/trans-5¼.90/10.
d
e
Allene 2 was obtained, ca. 5%.

M. Abe et al. / Tetrahedron 58 (2002) 7043–7047
7045
Scheme 3. Mechanism of the reaction of 1 with ROH.
(for 5d; d 4.85, for 5e; d 5.55) again confirmed the cis-
configuration of the substituents, as depicted for 5d,e.
3. Experimental
3.1. General
As shown in Table 1, the product ratios (3/5) and
the reaction time (h) were closely related to the acidity of
ROH, i.e. pKa.9 Namely, the stronger the acidity of ROH,
the faster the reaction was detected, and the higher the
chemical yield of 5 was observed, in spite of lowering
the nucleophilicity of ROH. The clear phenomena suggest
that the formation of the ROH-adduct 5 was initiated
by the acid-induced ring-opening of the oxetane 1 (see,
Scheme 3).
1H and 13C NMR (DEPT) spectra were recorded on JEOL
JNM-EX-270 spectrometer at 270 MHz and 67.8 MHz.
NOE measurements at 600 MHz were performed on
1
Brucker AM-600, UNITY-INOVA600. H NMR chemical
shifts were reported in ppm (d) using the residual CHCl3 (d
7.26) in CDCl3. 13C NMR chemical shifts were reported in
ppm (d ) relative to the internal standard CDCl3 (d 77.0). IR
spectra were recorded on a HORIBA FT-720 spectrometer.
Mass spectrometric data were obtained by using a JEOL
JNS-DX303 mass spectrometer. Elemental analyses were
carried out at the Analytical Center of Osaka University,
Faculty of Engineering.
The plausible mechanism of the ROH-induced decompo-
sition of 3-alkylideneoxetane 1 can be summarized as
shown in Scheme 3. When ROH (R¼H, Me) is a relatively
strong nucleophile (entries 1 and 2), the nucleophilic attack
at C1 position is dominant to give the ROH-adduct 3 via the
protonated intermediate I1, before converting to the allylic
cation I2. Alternatively, when the acidity of the ROH
increases with decreasing the nucleophilicity, R¼
CH(CF3)2, Ac, the allylic cation I2 becomes an important
intermediate for the nucleophile trapping reactions. The
intervention of the hydrogen-bonded intermediate I2 can
reasonably explain the cis-selective formation of the adduct
5 (R¼CH2CF3, CH(CF3)2, Ac), as depicted in the structure
I2. In principle, the formation of the regioisomeric adduct 3
is possible from the allylic cation I2. However, as suggested
by the PM3-calculated heat of formations (DHf),10,11
DDHf¼DHf (5d)2DHf (3d)<215 kcal/mol, the nucleo-
philic attack at C1 carbon in the intermediate I2, which
leads to the adduct 3, is expected to be less energetically
favored process, compared with the attack at C3 carbon
leading to the adduct 5. The regioselective C1–O
bond cleavage, which observed in this study, can be
reasonably explained by the presence of the adjacent
phenyl-rings which stabilize effectively the positive charge
at C1 carbon.
3.2. Preparation of 3-alkylideneoxetane 12
A degassed solution of DZ (0.05 M) in benzene (20 mL)
was irradiated with a high-pressure Hg lamp (500 W)
through a Pyrex filter (.290 nm) at ca. 108C. The
quantitative formation of 3-alkylideneoxetane 1 was
confirmed by the NMR measurement after 3 h.
3.3. General procedure for the reaction of 3-
alkylideneoxetane 1 with ROH
To a solution of 3-alkylideneoxetane 1 (0.05 M) in benzene
(20 mL) was added a solution of ROH (1.0 M) in benzene
(20 mL) at rt (ca. 158C), except for H2O. H2O was added as
a solution in THF. After the completion of the reaction,
which was checked by TLC analyses, the mixture was
washed with sat. NaHCO3 aqueous solution (50 mL£2).
The organic phase was separated and the solvent was
removed under the reduced pressure by using a rotary
evaporator. Separation of the products by using the column
chromatography on silica gel (EtOAc/hexanes as eluent)
afforded the ROH-adduct 3 and/or 5. The yields were listed
in Table 1. The MeOH-adduct 3b (R¼Me) was already
identified in the previous study.2a The spectroscopic data for
new compounds obtained in this study are as follows.
In this paper, we describe the reactivity of the strained
bicyclic-3-alkylideneoxetane 1 in the reaction with ROH.
The regioselectivity observed in this study is quite sensitive
to the acidity and the nucleophilicity of ROH. The origin of
the strain is suggested to be the coexistence of small ring
and unsaturation.
3.3.1. 2-(10-Hydroxy-10,10-diphenylmethyl)cyclopent-2-
enol (3a). White powder: mp 109–1108C; FTIR (KBr)

7046
M. Abe et al. / Tetrahedron 58 (2002) 7043–7047
3255, 2946, 2892, 1442, 1311, 1049 cm21
;
1H NMR
3.4. Oxidation of the TFE-adduct 3c
(270 MHz, CDCl3) d 1.80–1.86 (m, 1H), 2.15–2.58 (m,
3H), 2.96 (br s, 1H, OH), 4.78 (br s, 1H, OH), 4.85–4.87 (m,
1H), 5.34 (br s, 1H), 7.24–7.46 (m, 10H); 13C NMR
(67.8 MHz, CDCl3) d 29.5 (t), 33.9 (t), 78.3 (d), 80.2 (s),
127.0 (4£d), 127.1 (d), 127.3 (2£d), 127.6 (d), 128.0 (2£d),
134.5 (d), 144.8 (s), 145.7 (s), 148.2 (s). Anal. Calcd for
C18H18O2: C, 81.17; H, 6.81. Found: C, 81.38; H, 6.78.
To a solution of PCC (0.19 g, 0.87 mmol) in dry CH2Cl2
(3 mL) was added a solution of 3c (0.15 g, 0.43 mmol) in
dry CH2Cl2 (3 mL) at room temperature under Ar. After
stirring for 2 h, diethyl ether (20 mL) and MgSO4 (5 g) was
added and the mixture was filtrated over Celite. The filtrate
was concentrated under the reduced pressure by using the
rotary evaporator. Column chromatography on silica gel
(30% EtOAc/hexanes, Rf¼0.5) afforded 0.12 g (81%) of
ketone 6. White powder: mp 84–858C; FTIR (KBr) 2927,
1710, 1286, 1168, 1115 cm21; 1H NMR (270 MHz, CDCl3)
d 2.45–2.48 (m, 2H), 2.63–2.68 (m, 2H), 3.52 (q,
3.3.2. 2-[10,10-Diphenyl-10-(2,2,2-trifluoroethoxy)-
methyl]cyclopent-2-enol (3c). Viscous oil; FTIR (liquid
;
film) 3522, 1449, 1277, 1166, 1107 cm21 1H NMR
(600 MHz, CDCl3) d 1.86–1.90 (m, 1H), 2.11–2.17 (m,
1H), 2.21 (br s, 1H), 2.30–2.35 (m,1H), 2.63–2.69 (m, 1H),
3.41–3.65 (m, 2H), 4.71 (d, J¼7.2 Hz, 1H), 5.95 (t, J¼
2.4 Hz, 1H), 7.18–7.53 (m, 10H); 13C NMR (67.8 MHz,
CDCl3) d 30.3 (t), 32.6 (t), 61.8 (–C H2CF3, 2JCF¼34.1 Hz),
77.1 (d), 86.6 (s), 124.2 (–C F3, 1JCF¼276 Hz), 127.8 (4£d),
128.2 (2£d), 129.3 (4£d), 135.0 (d), 140.1 (s), 141.1 (s),
146.4 (s). Anal. Calcd for C20H19F3O2: C, 68.96; H, 5.50.
Found: C, 68.96; H, 5.65.
JHF¼8.5 Hz, 2H), 7.32–7.45 (m, 10H); 13C NMR
2
(67.8 MHz, CDCl3) d 26.2 (t), 36.1 (t), 61.9 (tq, JCF
¼
1
34.1 Hz, –C H2CF3), 84.1 (s), 124.2 (q, JCF¼276.6 Hz,
–CH2CF3), 128.2 (4£d), 128.3 (4£d), 128.7 (2£d), 139.3
(2£s), 148.3 (s), 161.9 (d), 206.0 (s). Anal. Calcd for
C20H17F3O2: C, 69.35; H, 4.95. Found: C, 69.21; H, 5.02.
Acknowledgments
3.3.3. 2-Benzhydrylidene-3-(20,20,20-trifluoroethoxy)cyclo-
pentanol (5c). Viscous oil; FTIR (liquid film) 2561, 3027,
2940, 1599, 1449, 1368, 1285, 1228, 1195, 1104 cm21; 1H
NMR (600 MHz, CDCl3) d 1.82–1.90 (m, 1H), 1.95–2.05
(m, 1H), 2.07–2.15 (m, 2H), 2.26 (d, J¼9.0 Hz, OH), 3.36–
3.43 (m, 1H), 3.61–3.78 (m, 1H), 4.42 (br s, 1H), 4.47 (m,
1H), 7.31–7.44 (m, 10H); 13C NMR (67.8 MHz, CDCl3) d
This research was supported in part by a grant-in-aid for
Scientific Research on Priority Areas ‘Molecular Physical
Chemistry’ from the ministry of Education, Science, Sports,
and Culture of Japan. We thank Mrs T. Muneishi at
Analytical Center of Faculty of Engineering, Osaka
University, for measuring the 600 MHz NMR spectra.
2
28.9 (t), 33.3 (t), 65.4 (–C H2CF3, JCF¼34.1 Hz), 72.5
1
(d), 81.8 (d), 123.8 (–C F3, JCF¼278 Hz), 127.6 (d),
127.9 (d), 128.1 (2£d), 128.3 (2£d), 129.3 (2£d), 129.4
(2£d), 140.3 (s), 140.7 (s), 141.1 (s), 144.2 (s). Anal. Calcd
for C20H19F3O2: C, 68.96; H, 5.50. Found: C, 69.15; H,
5.71.
References
1. (a) Liebman, J. F.; Greenberg, A. Chem. Rev. 1976, 76, 311.
(b) Greenberg, A.; Liebman, J. F. Strained Organic Molecules.
Academic: New York, 1978. (c) Chem. Rev. 1989, 5, 89;
special issue for Strained Organic Compounds.
3.3.4. 2-Benzhydrylidene-3-(20,20,20-trifluoro-10-trifluoro-
methylethoxy)cyclopentanol (5d). Viscous oil; FTIR
(liquid film) 3560, 3060, 3026, 2939, 1491, 1448, 1368,
2. (a) Abe, M.; Adam, W.; Nau, W. M. J. Am. Chem. Soc. 1998,
120, 11304. (b) Abe, M.; Adam, W.; Ino, Y.; Nojima, M. J. Am.
Chem. Soc. 2000, 122, 6508.
1
1284, 1194, 1104 cm21; H NMR (600 MHz, CDCl3) d
1.85–1.90 (m, 1H), 1.95–2.04 (m, 1H), 2.05–2.13 (m, 3H),
3.78 (sept, JF¼6.0 Hz, 1H), 4.45 (m, 1H), 4.85 (m, 1H),
7.09–7.35 (m, 10H); 13C NMR (67.8 MHz, CDCl3) d 29.1
3. Wiberg, K. B. J. Org. Chem. 1985, 50, 5285.
4. The ab initio calculations at RHF/6-31Gp level of theory were
performed with GAUSSIAN 98W: 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.
2
(t), 33.6 (t), 72.6 (d), 72.9 (–CH(CF3)2, JCF¼31.8 Hz),
1
83.8 (d), 124.2 (C F3£2, JCF¼276 Hz), 127.9 (d), 128.1
(4£d), 128.5 (2£d), 129.1 (2£d), 129.3 (d), 140.0 (s), 140.0
(s), 140.6 (s), 145.5 (s). Anal. Calcd for C21H18F6O2: C,
60.58; H, 4.36. Found: C, 60.33; H, 4.56.
3.3.5. 2-Benzhydrylidene-3-acetoxycyclopentanol (5e).
White powder: mp 108–1098C; FTIR (KBr) 3420, 3020,
2870, 1730, 1600, 1240 cm21; 1H NMR (600 MHz, CDCl3)
d 1.84 (s, 3H), 1.85–1.89 (m, 1H), 1.94–1.98 (m, 1H),
2.01–2.06 (m, 1H), 2.07–2.14 (m, 1H), 2.30 (br s, OH),
4.61–4.64 (m, 1H), 5.55 (t, J¼5.6 Hz, 1H), 7.13–7.43
(m, 10H); 13C NMR (67.8 MHz, CDCl3) d 21.5 (q), 30.4
(t), 33.4 (t), 72.9 (d), 76.0 (d), 127.8 (d), 128.2 (d), 128.7
(2£d), 128.8 (2£d), 128.9 (2£d), 129.3 (2£d), 141.1 (2£s),
141.3 (s), 145.0 (s), 170.8 (s); EI MS (m/z, relative intensity)
308 (Mþ, 4), 205 (31), 220 (48), 248 (100); HRMS (EI)
Calcd for C20H20O3 308.1413. Found 308.1418. Anal.
Calcd for C20H20O3: C, 77.90; H, 6.54. Found: C, 77.67; H,
6.45.
5. Franklin, J. L. J. Chem. Phys. 1953, 21, 2029.
6. The strain energies of 3-alkylideneoxetane (SE¼22.6 kcal/
mol), 7-oxabicyclo4.2.0.oct-1-ene (SE¼24.6 kcal/mol), and

M. Abe et al. / Tetrahedron 58 (2002) 7043–7047
7047
5-oxabicyclo2.2.0.hex-1-ene (SE¼87.1 kcal/mol) were calcu-
lated by the same procedure.
114, 774. (e) Staneke, P. O.; Ingemann, S.; Eaton, P.;
Nibbering, N. M. M.; Kass, S. R. J. Am. Chem. Soc. 1994,
116, 6445. (f) Sauers, R. R.; Harris, J. J. Org. Chem. 2001, 66,
7951.
7. Schleyer, P. V. R.; Williams, Jr. J. E.; Blanchard, K. R. J. Am.
Chem. Soc. 1970, 92, 2377.
8. (a) Borden, W. T. Chem. Rev. 1989, 89, 1067. (b) Warner,
P. M. Chem. Rev. 1989, 89, 1095. (c) Chou, P. K.; Kass, S. R.
J. Am. Chem. Soc. 1991, 113, 697. (d) Branan, B. M.; Paquette,
L. A.; Hrovat, D. A.; Borden, W. T. J. Am. Chem. Soc. 1992,
9. DeTar, D. F. J. Am. Chem. Soc. 1982, 104, 7205.
10. The PM3 calculations (Ref. 11) were performed within
SPARTAN’02; Wavefunction, Inc., 2001.
11. Stewart, J. J. P. J. Comput. Chem. 1989, 10, 209.