[1.1]Paracyclophane. Photochemical generation from the corresponding bis(Dewar benzene) derivative and theoretical study of its structure and strain energy

Article abstract of DOI:10.1021/ja9715253

The first generation of [1.1]paracyclophane (1a) and its bis(methoxycarbonyl) derivative (1b) from the corresponding bis(Dewar benzene) precursors, 3a and 3b, has been investigated. Irradiation of 3a in a glassy mixture of ether-isopentane-ethanol at 77 K

Full text of DOI:10.1021/ja9715253

J. Am. Chem. Soc. 1997, 119, 8425-8431
8425
[1.1]Paracyclophane. Photochemical Generation from the
Corresponding Bis(Dewar benzene) Derivative and Theoretical
Study of Its Structure and Strain Energy
Takashi Tsuji,* Masakazu Ohkita, Tomohisa Konno, and Shinya Nishida
Contribution from the DiVision of Chemistry, Graduate School of Science, Hokkaido UniVersity,
Sapporo 060, Japan
ReceiVed May 12, 1997X
Abstract: The first generation of [1.1]paracyclophane (1a) and its bis(methoxycarbonyl) derivative (1b) from the
corresponding bis(Dewar benzene) precursors, 3a and 3b, has been investigated. Irradiation of 3a in a glassy mixture
of ether-isopentane-ethanol at 77 K leads to the formation of species exhibiting absorption extending to 450 nm,
which readily undergoes secondary photolysis to give an isomer showing λmax at 244 nm. On the basis of these
UV/vis spectral observations and the accompanying 1H NMR measurement, the structures of 1a and the corresponding
transannular [4 + 4] adduct (21a) are assigned to the initial and the secondary products, respectively. Compound
3b undergoes similar successive phototransformation into 1b and 21b. [1.1]Paracyclophanes, 1a and 1b, and their
photoisomers, 21a and 21b, are sufficiently stable to permit the measurement of 1H NMR spectra at low temperature,
but are consumed fairly rapidly in solution at ambient temperature, defying their isolation. The results of geometrical
optimization of 1a undertaken at the RHF-SCF, MP2, and B3LYP levels employing the 6-31G* basis set are also
presented. Calculations indicate that 1a is a highly strained molecule, but more stable than the related isomers, 3a
and 21a: the strain energy calculated for 1a is 128.1, 106.5, and 93.6 kcal/mol at the RHF/6-31G*, B3LYP/6-31G*,
and MP2/6-31G* levels, respectively. The closest nonbonding interatomic distance between the aromatic rings in
1a is in a range of 2.36-2.40 Å, and the degree of bending of the benzene rings is comparable to that in [5]-
paracyclophane, much less as compared to that in [4]paracyclophane. Calculations also support strong transannular
electronic interactions between the π-bonds of the aromatic moieties of 1a, which lead to a significantly diminished
HOMO-LUMO gap as compared to that in p-xylene. The preparation of 3a and 3b from diethyl 3,6-
dihydroterephthalate in 15 and 11 steps, respectively, is described.
Introduction
Scheme 1
Benzene prefers a planar configuration. It is apparently not
possible to connect two benzene rings at the 1- and 4-positions
with two methylene bridges to produce [1.1]paracyclophane 1
without severely bending bonds, and its construction is seem-
ingly almost prohibitive. Despite its fascinating structure, in
which two benzene rings are bent and alligned in parallel in
close proximity, and extensive studies on the chemistry of
cyclophanes1 subsequent to the isolation of [2.2]paracyclophane
more than 45 years ago,2 to our knowledge, no report concerning
1 had been published when we embarked on the preparation of
1 several years ago. Recent successful generation of [4]-
paracyclophanes from the corresponding Dewar benzene precur-
sors,3,4 however, suggested that 1 and its derivatives might be
accessible via valence isomerization of the bis(Dewar benzene)
isomers 3 (Scheme 1). The first step of the isomerization (from
3 to 2) is nothing else but the production of [4]paracyclophane
derivative, if a stepwise process is assumed, and the second
step (from 2 to 1) corresponds to the formation of much less
strained, six-carbon-bridged paracyclophane from the corre-
sponding Dewar forms, respectively. Computational analysis
on 1-3, in fact, indicates that the first step will be slightly
endothermic whereas the second step will be highly exothermic,
suggesting that the latter step should be much easier than the
former. Calculations also reveal that the degree of deformation
of aromatic ring in 1 will be comparable to that in [5]-
paracyclophane,5 much less than that in [4]paracyclophane,6
implying that 1 may possibly be isolable unless electronic
interactions between the aromatic rings held in close proximity
significantly destabilize the system.7 In a preliminary account
of this work8 we reported spectroscopic evidence for the
generation of the first [1.1]paracyclophane, the bis(methoxy-
X Abstract published in AdVance ACS Abstracts, August 15, 1997.
(1) (1) For reviews of cyclophanes, see: (a) Cram, D. J.; Cram, J. M.
Acc. Chem. Res. 1971, 4, 204-213. (b) Cyclophanes; Keehn, P. M.,
Rosenfeld, S. M., Eds.; Academic Press: New York, 1983. (c) Cyclophanes
I and II. Topics in Current Chemistry; Vo¨gtle, F., Ed.; Springer-Verlag:
Berlin, 1983; Vols. 113 and 115. (d) Bickelhaupt, F.; de Wolf, W. H. Recl.
TraV. Chim. Pays-Bas 1988, 107, 459-478. (e) Vo¨gtle, F. Cyclophan-
Chemie; B. G. Teubner Verlag: Stuttgart, 1990. (f) Bickelhaupt, F.; de
Wolf, W. H. In AdVances in Strain in Organic Chemistry; Holton, B., Ed.;
JAI Press: Greenwich, CT, 1993; Vol. 3, p 185. (g) Cyclophanes. Topics
in Current Chemistry; Weber, E., Ed.; Springer-Verlag: Berlin, 1994; Vol.
172. (h) Kane, V. V.; de Wolf, W. H.; Bickelhaupt, F. Tetrahedron 1994,
50, 4575-4622.
(4) (a) Tsuji, T.; Nishida, S. J. Chem. Soc., Chem. Commun. 1987, 1189-
1190. (b) Tsuji, T.; Nishida, S. J. Am. Chem. Soc. 1988, 110, 2157-2164.
(c) Tsuji, T.; Nishida, S. J. Am. Chem. Soc. 1989, 111, 368-369. (d) Tsuji,
T.; Nishida, S.; Okuyama, M.; Osawa, E. J. Am. Chem. Soc. 1995, 117,
9804-9813.
(2) (a) Brown, C. J.; Farthing, A. C. Nature 1949, 164, 915-916. (b)
Brown, C. J. J. Chem. Soc. 1953, 3265-3270.
(3) Kostermans, G. B. M.; Bobeldijk, M.; de Wolf, W. H.; Bickelhaupt,
F. J. Am. Chem. Soc. 1987, 109, 2471-2475.
S0002-7863(97)01525-4 CCC: $14.00 © 1997 American Chemical Society

8426 J. Am. Chem. Soc., Vol. 119, No. 36, 1997
Tsuji et al.
Scheme 2a
Scheme 3a
a Reagents and conditions: (i) 70 °C, 4 h; (ii) DBU; (iii) AcOH-
H2O; (iv) PCC.
carbonyl) derivative. In this paper we present details of the
invesitigation including the theoretical study of 1.
Results and Discussion
Preparation of the Bis(Dewar Benzene) Derivatives (3).
On the basis of our experience on the preparation of Dewar
benzene derivatives, tricyclic diketone 6 or 7 was envisioned
as the key intermediate for the preparation of 3. Since those
diketones were unkown then, we first investigated their prepara-
tion. A convenient synthesis of 4,5,6,7-tetrahydro-1-indanone
via the SnCl4-catalyzed condensation of 1-cyclohexenecarbonyl
chloride with trimethylvinylsilane has been reported by Magnus
et al.9 3,6-Dihydroterephthaloyl chloride, however, turned out
to be rather puzzlingly unreactive under these conditions and
provided only an intractable mixture under the more forcing
conditions. After several unsuccessful attempts, 6 and 7 were
eventually synthesized via the [2 + 4] cycloaddition of 2,3-
dimethylenecyclopentyl trimethylsilyl ether (4)10 with 3-nitro-
2-cyclopentenone (5)11 followed by elimination of nitrous acid,
desilylation, and PCC oxidation (Scheme 2). This procedure
was rather laborious, and the preparation of 5, moreover,
required the use of concentrated (g90%) hydrogen peroxide.12
The diketones thus obtained, however, were found to be
relatively stable toward oxidative aromatization and acid- or
base-catalyzed isomerization, and their chemical stability prompted
us to investigate the preparation of 6 via the acid-promoted
intramolecular acylation of unsaturated dicarboxylic acid 12
(Scheme 3).13 Compound 12 was prepared from readily
available 814 via a sequence in which the latter was successively
a Reagents and conditions: (a) LAH; (b) PBr3; (c) AcOt-Bu/LDA;
(d) CF3CO2H; (e) (CF3CO)2O/BF3‚OEt2; (f) hν/C2H2/acetone-CH2Cl2;
(g) HCO2Et/NaOEt; (h) TsN3/Et3N; (i) hν/MeOH; (j) NaOH/H2O, H3O+,
(PhO)2P(O)N3/Et3N, MeSCH2CH2OH; (k) Me2SO4, NaOH/MeOH-
H2O, H3O+, NaOH; (l) MeI/NaHCO3, t-BuOK/DMF; (m) LDA,
PhSeBr; (n) H2O2/C5H5N.
treated with LAH, PBr3, t-BuOAc/LDA, and CF3CO2H. Treat-
ment of the mixed acid anhydride, which was in situ generated
from 12 and (CF3CO)2O, with BF3‚OEt2 cleanly provided 6 in
52% yield, in 20% yield overall from 8.
The photocycloaddition of acetylene to 6 stereoselectively
proceeded to furnish anti-bis(acetylene) adduct 14 together with
several side products. The stereochemical assignment to 14 was
based on NOE experiments, in which the methylene bridge
protons and the â-endo protons of the cyclopentanone rings were
separately irradiated, and was later supported by similar
experiments on 3b. The stereoselective formation of the anti-
adduct 14 probably resulted from the preferential bending of
the central C6 ring toward the cyclobutene ring in the monoad-
duct 13 as suggested by force field calculations.15 Prevailing
steric factors would thus guide the entry of the second acetylene
anti to the first one. The subsequent transformation of 14 into
3a and 3b was carried out in essentially the same manner as
reported for the preparation of [4.2.2]propellatetraene.4,16 Thus,
14 was diazotized in the usual manner17 to give 16 which was
in turn subjected to photolysis in methanol to furnish diesters
17 as a mixture of three stereoisomers, syn,syn-, syn,anti-, and
anti,anti-adducts (with respect to the C6 ring), in a ratio of 0.9:
1.1:1.0. These diesters were separated from each other and their
stereochemistries were assigned. In the following reactions,
however, they were used as a mixture. The diesters 17 were
hydrolyzed to deliver a mixture of carboxylic acids, which were
(5) (a) Allinger, N. L.; Sprague, J. T.; Liljefors, T. J. Am. Chem. Soc.
1974, 96, 5100. (b) Schmidt, H.; Schweig, A.; Thiel, W.; Jones, M., Jr.,
Chem. Ber. 1978, 111, 1958. (c) Carballeira, L.; Casado, J.; Gonza´lez, E.;
Rios, M. A. J. Chem. Phys. 1982, 77, 5655. (d) Rice, J. E.; Lee, T. J.;
Remington, R. B.; Allen, W. D.; Clado, D. A.; Schaefer, H. F., III J. Am.
Chem. Soc. 1987, 109, 2902. (e) Bockisch, F.; Rayez, J.-C.; Liotard, D.;
Dugnay, B. J. Comput. Chem. 1992, 13, 1047. (f) Arnim, M. v.;
Peyerimhoff, S. D. Theor. Chim. Acta 1993, 85, 43-59.
(6) (a) Grimme, S. J. Am. Chem. Soc. 1992, 114, 10542-10547. (b)
Ma, B.; Sulzbach, H. M.; Remington, R. B.; Schaefer, H. F., III J. Am.
Chem. Soc. 1995, 117, 8392-8400.
(7) [5]Paracyclophane and its derivatives so far prepared are not
sufficiently stable to permit their isolation, but persist for a fairly long time
in dilute solution in the absence of air at ambient temperature. Van Es, D.
S.; de Kanter, F. J. J.; de Wolf, W. H.; Bickelhaupt, F. Angew. Chem., Int.
Ed. Engl. 1995, 34, 2553-2555 and references cited therein.
(8) Tsuji, T.; Ohkita, M.; Nishida, S. J. Am. Chem. Soc., 1993, 115,
5284-5285.
(9) Magnus, P.; Cooke, F.; Moerck, R.; Schwindeman, J. J. Org. Chem.
1980, 45, 1046.
(10) Compound 4 was prepared from the trimethylsilyl ether of hept-1-
yn-6-en-3-ol using the method developed by Trost et al. (a) Trost, B. M.;
Lautens, M.; Chan, C.; Jebaratnam, D. J.; Mueller, T. J. Am. Chem. Soc.
1991, 113, 636. (b) Trost, B. M. Angew. Chem., Int. Ed. Engl. 1995, 34,
259-281.
(11) Corey, E. J.; Estreicher, H. Tetrahedron Lett. 1981, 22, 603-606.
(12) (a) Emmons, W. D.; Pagano, A. S. J. Am. Chem. Soc. 1955, 77,
4557. (b) Takamoto, T.; Ikeda, Y.; Tachimori, Y.; Seta, A.; Sudoh, R. J.
Chem. Soc., Chem. Commun. 1978, 350-351.
(13) (a) Bu¨chi, G.; MacLeod, W. D., Jr. J. Am. Chem. Soc. 1962, 84,
3205. (b) Marshall, J. A.; Andersen, N. H.; Schlicher, J. W. J. Org. Chem.
1970, 35, 858. (c) Semmelhack, M. F.; Foos, J. S.; Katz, S. J. Am. Chem.
Soc. 1972, 94, 8637. (d) Groves, J. K. Chem. Soc. ReV. 1972, 1, 73.
(14) (a) Sinnreich, J.; Batzer, H. HelV. Chim. Acta 1979, 62, 156. (b)
Jones, D. G. Ger. Patent 950 285, 1956; Chem. Abstr. 1959, 53, 17926d.
(15) The calculations were performed with Chem3D Plus, V. 3.0;
Cambridge Scientific Computing: Cambridge, MA, 1990.
(16) (a) Tsuji, T.; Komiya, Z.; Nishida, S. Tetrahedron Lett. 1980, 21,
3583-3586. (b) Tsuji, T.; Nishida, S. Tetrahedron Lett. 1983, 24, 3361-
3364.
(17) Regitz, M. In The Chemistry of Diazonium and Diazo Groups; Patai,
S., Ed.; J. Wiley & Sons: Chichester, 1975; Chapter 17.

[1.1]Paracyclophane
J. Am. Chem. Soc., Vol. 119, No. 36, 1997 8427
Figure 2. Difference UV/vis spectra (A) between the absorptions
before and after the irradiation of 3b with 254-nm light in EPA at
77 K and (B) between those before and after further irradiation with
>390-nm light. Spectrum C was obtained by adding spectrum B to
spectrum A.
Figure 1. Difference UV/vis spectra (A) between the absorptions
before and after the irradiation of 3a with 254 nm light in EPA at 77
K and (B) between those before and after further irradiation with >335-
nm light. Spectrum C was obtained by adding spectrum B to spectrum
A.
Scheme 4
converted to the isocyanates18 and then exposed to 2-(meth-
ylthio)ethanol to furnish the carbamates 18. Successive treat-
ment of 18 with dimethyl sulfate and with aqueous NaOH
provided carbamate salt, from which a stereoisomeric mixture
of amines 19 was liberated through acidification and decar-
boxylation.19 The quaternization of 19 with methyl iodide and
treatment of the resultant ammonium salt with t-BuOK in a
DMF/pentane mixture furnished the parent bis(Dewar benzene)
3a in an overall yield of 15% from 17. The extremely simple
1H and 13C NMR spectra of 3a, which consist of only two and
three signals, respectively, were fully consistent with its
structure, highly symmetrical at least on the NMR time scale.
On the other hand, treatment of 17 with LDA followed by
selenenylation with phenylselenenyl bromide afforded a mixture
of the phenyl selenides 20.20 Oxidation of 20 to the selenoxides
and the following elimination of benzeneselenenic acid smoothly
proceeded to give 3b as colorless crystals.
ments, the structures of 1a and 21a were assigned to the initial
and the secondary products, respectively (Scheme 4). It is of
interest that 1a exhibits an absorption band in such a long-
wavelength region.
The bis(methoxycarbonyl) derivative 3b diplayed similar
photochemical behavior. Thus, irradiation of 3b in EPA with
254-nm light at 77 K led to the formation of species which
exhibited absorption with λmax at 256 and 348 nm together with
a weak, broad band (λmax at 405 nm) extending to 480 nm
(Figure 2A). The newly generated species was rapidly con-
verted to the secondary product showing λmax at 267 nm upon
irradiation with >390-nm light with which only the transient
species but not 3b was electronically excited. The observed
spectral changes were attributed to the initial generation of 1b
from 3b and its secondary transformation into 21b, on the basis
Generation of [1.1]Paracyclophanes from the Bis(Dewar
benzene) Derivatives in Rigid Glass at Low Temperature.
Since the degree of deformation calculated for [1.1]paracyclo-
phane suggested that its thermal stability would be marginal at
ambient temperature, the generation of 1 via photolysis of 3
was first investigated under matrix isolation at low temperature.
The parent 3a exhibits only an end absorption extending to ca.
260 nm in its electronic spectrum. Irradiation of a glassy
mixture of 3a in EPA21 with a 254-nm light source at 77 K
resulted in the development of absorption showing λmax at 226,
237, and 290 nm, accompanied by a weak, broad band in the
region of 330-450 nm (Figure 1A). The generated species was
photochemically labile and efficiently consumed when irradiated
with filtered light (>335 nm) (Figure 1B), verifying that the
weak band in the long-wavelength region was not an artifact.
The spectral changes observed upon the secondary photolysis
indicated that the initial photoproduct was transformed into the
species exhibiting λmax at 244 nm (Figure 1C). On the basis of
of the accompanying H NMR measurements (vide infra).22,23
The UV/vis absorption spectra of 1a and 1b remained un-
changed for a few hours at -20 °C, indicating that they are
sufficiently stable to permit H NMR measurement, but had
almost completely decayed after 4 h at room temperature,
defying their isolation at ambient temperature.
Measurement of H NMR Spectra of 1a and 1b. The H
NMR spectrum of 3b consists of two singlets due to HA and
the methoxy protons and two pairs of AX doublets due to vicinal
HB/HC and geminal HD/HE. Thus, 3b is Ci symmetric at least
on the 1H NMR time scale.24 Calculations suggest that the same
symmetry is also held in 1b and 21b. Irradiation of 3b with a
254 nm light source in THF-d8 at -70 °C led to the formation
1
1
1
1
1
these observations and the accompanying H NMR measure-
(18) (a) Shioiri, T.; Ninomiya, K.; Yamada, S. J. Am. Chem. Soc. 1972,
94, 6203-6205. (b) Ninomiya, K.; Shioiri, T.; Yamada, S. Tetrahedron
1974, 30, 2151-2157.
(19) (a) Amaral, M. J. S. A.; Barrett, G. C.; Rydon, H. N.; Willet, J. E.
J. Chem. Soc. C 1966, 807-813. (b) Kunz, H. Chem. Ber. 1976, 109, 3693-
3706.
(22) The UV/vis absorptions initially developed upon the irradiation of
3a and 3b markedly differ in shape from those of [4]paracyclophane and
its methoxycarbonyl derivative, respectively.4 Thus, the possibility that the
observed spectra were due to 2a and 2b, [4]paracyclophane derivatives, is
ruled out.
(20) Reich, H. J.; Wollowitz, S. Org. React. 1993, 44, 1-216.
(21) A 5:5:2 mixture of diethyl ether, isopentane, and ethanol.
(23) To our knowledge, the photochemical transformation of 1a,b into
21a,b represents the first direct formation of benzene p,p-dimer.

8428 J. Am. Chem. Soc., Vol. 119, No. 36, 1997
Tsuji et al.
Table 1. 1H NMR Parameters for 1b, 3b, and 21ba
species HA(d) JAB (Hz) HB/HC(d) JBC (Hz) HD/HE(d) JDE (Hz)
1b
3b
21b
7.60
7.19
7.12
2.5 6.95, 7.14
8.8
2.4
6.4
4.44b
2.38, 2.50
1.95, 2.45
11.7
14.6
6.6
<1
6.45, 6.56
6.03, 6.13
<1
a In THF-d8 at -60 °C. The signals of OCH3 were observed at δ
3.94, 3.65, and 3.61 for 1b, 3b, and 21b, respectively. b The signal of
the other proton could not be identified, probably due to its overlap
with one of the much stronger signals due to the methoxy group of 3b
at δ 3.65, the solvent at δ 3.58, and water at δ 3.30.
Figure 3. Experimental and calculated (in parentheses) 1H NMR
chemical shifts for 1a, 3a, 21a, and 22.
latter to 1a. The observed spectra were, however, extremely
simple and of low relative intensities,28 and could accidentally
be due to unknown products. Therefore, recourse to theoretical
calculations was made to confirm the structural assignments.
Proton chemical shifts were calculated for 1a, 3a, 21a, and 22
by GIAO29 /6-31G* with the RHF/6-31G* geometries by using
the Gaussian 94 program package.30 The experimental and
computed proton chemical shifts summarized in Figure 3 are
in reasonable agreement with each other, supporting the above
assignments.31 Bis(prismane) 22 of the same symmetry is
apparently incompatible with either of the observed spectra. The
formation of the less symmetrical 2a was not detected again. A
signal for the methylene bridge protons of 21a was not
identified, probably owing to overlap with the much stronger
signal due to the residual protons in the deuterated THF at δ
1.75.
of two species in ca. 3% and 6% yields at the expense of 10%
of 3b. Both the newly observed two sets of signals are simple,
as listed in Table 1, and their characteristics unambiguously
indicated that both the generated species also retained the Ci
symmetry.25 Increase in the vicinal coupling between the
olefinic protons HB and HC from a small 2.4 Hz, characteristic
of cyclobutene,26a to 8.8 Hz in the minor product and 6.4 Hz in
the other, moreover, suggested the cleavage of the central bonds
of both the Dewar benzene moieties in both products. None of
the less symmetrical products resulting from the rearrangement
of only one of the Dewar benzenes, e.g., 2b, was detected by
the 1H NMR. When the resultant NMR sample was irradiated
with >390-nm light, the minor product was quantitatively
converted to the major product in close correspondence with
the changes in UV/vis absorption, implying that the former is
responsible for the absorption extending to 480 nm and the latter
for the one showing λmax at 267 nm.
Computational Analysis. The structure of 1a, its thermo-
dynamic stability relative to related compounds, and the strain
energy are of special interest. To gain insights into these points,
theoretical analyses were carried out by ab initio and DFT
quantum mechanical methods.
The observed UV/vis spectral changes and the details of 1H
NMR spectra are reasonably accommodated by postulating the
initial rearrangement of 3b into 1b, which was in turn
transformed into 21b via the transannular [4 + 4] photocy-
cloaddition. The downfield shifts of all the signals due to HA-
HE, coupled with the appearance of substantial coupling between
HA and HB, upon the initial isomerization strongly suggest the
aromatization of the Dewar benzenes. The subsequent upfield
shifts of the methylene proton signals and the decrease in their
mutual coupling from the normal 11.7 Hz to 6.6 Hz, typical of
cyclopropyl derivatives,26b upon secondary photolysis are also
consistent with the proposed transformation. The assignment
of the electronic absorption extending to 480 nm to 1b, which
possesses strongly interacting bent benzene chromophores,
seems reasonable, and the band at 267 nm is also compatible
with that in 21b.27 Thus, we concluded that 1b was successfully
generated photochemically from 3b, but prone to undergo the
secondary photolysis to give 21b.
Similar successive transformation was observed for 3a. Thus,
irradiation of 3a with a 254 nm light source in THF-d8 at -80
°C led to the development of three singlets at δ 3.38, 5.90, and
6.94 at the expense of two singlet signals at δ 2.24 and 6.48
due to 3a. When the resultant mixture was irradiated with
>335-nm light by which 3a was not affected, the singlet signal
at δ 5.90 was enhanced whereas those at δ 3.38 and 6.94
decayed. The former was accordingly assigned to 21a and the
(1) Geometry. The geometrical optimizations of 1a and 21a
were undertaken at the Hartree-Fock (RHF-SCF), second-order
perturbation (MP2) and density functional B3LYP levels
employing the 6-31G* basis set implemented in the Gaussian
94 program package.30 The structures of the related compounds,
2a and 3a, were also optimized at the RHF/6-31G* level and
with semiempirical MNDO Hamiltonian in the MOPAC 5.0
program.32 An initial exploration of the potential energy surface
of 1a with the 3-21G basis set furnished a single energy
minimum corresponding to the D2h symmetric 1a. The opti-
mizations with the 6-31G* basis set were accordingly performed
within the constraint of D2h symmetry. The optimized geo-
metrical parameters for 1a are summarized in Tables 2 and 3.
(28) The sample contained 1a, 21a, and 3a in a ratio of ca. 3:2:300 after
the initial irradiation with 254-nm light. Extended irradiation did not bring
about an appreciable increase in the proportion of (1a + 21a):3a and only
induced the decomposition of the products and 3a.
(29) Wolinski, K.; Hinton, J. F.; Pulay, P. J. Am. Chem. Soc. 1990, 112,
8251.
(30) Gaussian 94 (Revision C.3), M. J. Frisch, G. W. Trucks, H. B.
Schlegel, P. M. W. Gill, B. G. Johnson, M. A. Robb, J. R. Cheeseman, T.
Keith, G. A. Petersson, J. A. Montgomery, K. Rachavachari, M. A. Al-
Laham, V. G. Zackrewski, J. V. Ortiz, J. B. Foresman, J. Cioslowski, B.
B. Stefanov, A. Nanayakkara, M. Challacombe, C. Y. Peng, P. Y. Ayala,
W. Chen, M. W. Wong, J. L. Andres, E. S. Replogle, R. Gomperts, R. L.
Martin, D. J. Fox, J. S. Binkley, D. J. Defrees, J. Baker, J. P. Steward, M.
Head-Gordon, C. Gonzales, J. A. Pople, Gaussian Inc.: Pittsburgh, PA,
1995.
(24) According to calculations (MNDO and RHF/6-31G*), the central
six-membered ring of 3a is significantly bent in a boat form. Interconversion
between the bent conformers would be facile on the NMR time scale at
room temperature. By contrast, the optimized structures of 1a and 21a are
D2h symmetric (vide infra).
(31) It is of interest to compare the 1H NMR spectra of 1a,b with those
of the corresponding [2.2]paracyclophane homologues. The aromatic ring
protons of [2.2]paracyclophane resonate at δ 6.47. The protons of [2.2]-
paracyclophanedicarboxylic ester corresponding to the HA-C of 1b resonate
at δ 7.17 (d, J ) 1.8 Hz), 6.68 (dd, J ) 7.7 and 1.8 Hz), and 6.51 (d, J )
7.7 Hz), respectively. Hopf, H.; Lenich, F. T. Chem. Ber. 1974, 107, 1891-
1902.
(32) Stewart, J. J. P. QCPE Bull. 1989, 9, 10. Hirano, T. JCPE
NewsLetter 1989, 1, 36; Revised as Ver. 5.01 by Toyoda, J. for Apple
Macintosh.
(25) The 1H NMR spectra recorded in this experiment were shown in a
preliminary account of this work.8
(26) (a) Jackman, L. M.; Sternhell, S. Applications of Nuclear Magnetic
Resonance Spectroscopy in Organic Chemistry, 2nd ed.; Pergamon Press:
Oxford, England, 1969. (b) Morris, D. G. The Chemistry of Cyclopropyl
Group; Rappoport, Z., Ed.; J. Wiley & Sons: Chichester, England, 1987;
pp 101-172.
(27) Methyl tetracyclo[4.2.2.22,5.01,6]dodeca-3,7,9,11-tetraene-3-carboxy-
late, which possesses a π-bond system similar to that of 21b, exhibits λmax
at 274 nm.16
(33) (a) Lonsdale, K.; Milledge, H. J.; Krishna Rao, K. W. Proc. R. Soc.,
Ser. A 1960, 255, 82. (b) Hope, H.; Bernstein, J.; Trueblood, K. N. Acta
Crystallogr., Sect. B 1972, 28, 1733.

[1.1]Paracyclophane
J. Am. Chem. Soc., Vol. 119, No. 36, 1997 8429
Table 4. Calculated Energies of 2a, 3a, and 21a Relative to 1a
(kcal/mol)
method
1a
2a
3a
21a
MNDO
RHF/6-31G*
MP2/6-31G*//RHF/6-31G*
B3LYP/6-31G*
0a
0b
0c
0d
53.7
45.1
51.5
13.7
8.7
20.0
26.2
Figure 4. Definition of the deformation angles R-γ , the bond angles
φ1-φ4, the interatomic distances R1-R4, and the bond lengths r1-r3.
a Heat of formation: ∆Hf ) 183.5 kcal/mol. b Total electronic
energy: -536.962707 (b), -538.791204 (c), and -540.563026 au (d).
Table 2. Calculated Distortion Angles and Nonbonding
Interatomic Distances for [1.1]Paracyclophane (1a) and the Related
Compounds
(2) Energy. In Table 4 are listed calculated energy differ-
ences of 1a and the related compounds. Inspection of the table
reveals that 1a is predicted to be the most stable of the calculated
isomers, more stable than 21a, irrespective of the computation
methods and theoretical levels. In the generation of 1a via the
photochemical isomerization of 3a, the accompanying produc-
tion of 21a was unavoidable even at the low conversion of 3a,
since the isomerization of 1a to 21a proceeded in preference to
that of 3a to 1a owing to the preferential absorption of irradiated
light by 1a, coupled with its high photochemical reactivity to
afford 21a. The above calculations suggest that thermolysis
may be superior to photolysis for the preparation of 1a from
3a, provided that the requisite reaction conditions are sufficiently
mild to secure 1a from potential secondary reactions. As
mentioned in the introduction, the calculations also indicate that
the isomerization of 3a to 2a is endothermic whereas that of
2a to 1a is strongly exothermic, suggesting that the latter process
is significantly more facile than the former. Accordingly, it is
not unreasonable that 2a and 2b were not detected in the
photolysates of 3a and 3b, respectively, though a stepwise
mechanism was likely in operation.
method
R
â
(R + â)
[1.1]Paracyclophane (1a)
23.7 26.3 50.0 28.6 2.383 2.982 2.351 2.761
γ
R1
R2
R3
R4
RHF/6-31G*
B3LYP/6-31G* 23.3 27.0 50.3 28.3 2.396 2.995 2.371 2.787
MP2/6-31G*
22.5 27.4 49.9 27.2 2.363 2.938 2.378 2.792
[2.2]Paracyclophanea
exptl
12.6 11.2 23.8
2.778 3.093
[4]Paracyclophaneb
DZPc
29.7 38.2 67.9 34.1
2.374 2.657
2.401 2.773
[5]Paracyclophaneb
RHF/6-31G
23.5 28.7 52.2 27.2
a From ref 33. b From refs 5f and 6a. c Double-ú plus polarization
basis set.
Table 3. Calculated Bond Angles and Bond Lengths for
[1.1]Paracyclophane (1a)
method
r1
r2
r3
φ1
φ2
φ3
φ4
RHF/6-31G*
1.557 1.393 1.390 99.8 118.3 115.1 119.5
B3LYP/6-31G* 1.558 1.406 1.400 100.5 118.6 115.0 118.6
MP2/6-31G* 1.545 1.407 1.401 99.8 118.3 115.3 119.6
Although 1a is the most stable of the calculated isomers, 1a
is still a highly strained molecule. For the evaluation of strain
energy in 1a, 9,10-dihydroanthracene, which possesses two
benzene rings bridged by two methylenes as 1a and yet is strain-
free, serves as an ideal reference compound. The total strain
energy of 1a thus evaluated is 128.1, 93.6, and 106.5 kcal/mol
at the RHF/6-31G*, MP2/6-31G*, and B3LYP/6-31G* levels,
respectively, and is in good agreement with the values evaluated
by using the homodesmic reaction 1, 127.8 kcal/mol at the RHF/
6-31G* level and 90.8 kcal/mol at the MP2/6-31G* level.
For comparison, the data for [2.2]- and [n]paracyclophanes
(n ) 4, 5) from the literature are also listed in Table 2.
The differences between the optimized geometries of 1a are
generally small, ∆(bond length) e 0.014 Å and ∆(bond angle)
e 0.7°. The extent of bond alteration in the aromatic rings is
less than 0.006 Å, small for the significant deformation of the
rings, indicating the retention of cyclic delocalization of
electrons in the bent benzene rings.34 The transannular distance
R1 is in a range of 2.36-2.40 Å, shorter by 1.0 Å than the
interlayer distance in graphite (3.35 Å), suggesting strong
electronic interactions between the π bonds of the opposing
aromatic rings. The calculated R2 is also significantly less than
3.35 Å. Those nonbonding interatomic distances in the MP2/
6-31G* structure are somewhat smaller compared to those in
the RHF/6-31G* and B3LYP/6-31G* structures.
The bending angle R at the MP2/6-31G* level is slightly
smaller, so that the aromatic rings are slightly more flat (∆γ e
1.4°) than those at the RHF/6-31G* and B3LYP/6-31G* levels.
The total deformation angles (R + â), however, agree within
0.4° (50.1 ( 0.2°). The angle (R + â) in 1a is much smaller
than that calculated for [4]paracyclophane,6 even slightly smaller
than that for [5]paracyclophane.5 In accord with the calculation
results, 1a is much superior to the former and is comparable to
the latter in kinetic stability. Thus, it appears that the reactivity
of 1a is primarily determined by the extent of bending of the
aromatic rings and 1a is neither particularly stabilized nor
destabilized by the close stacking of the benzene rings.
C14H12 (1a) + 4C2H6 2p-C6H4Me2 + 2C3H8
(1)
The close face-to-face arrangement of the two bent aromatic
rings in 1a suggests strong electronic interactions between them.
The RHF/6-31G* calculations confirm that the HOMO and
LUMO of 1a are significantly raised and lowered in energy,
respectively, as compared to those of p-xylene as the result of
bending of the benzene rings and of their mutual electronic
coupling. In Figure 5 is depicted a qualitative derivation of
high energy occupied and low energy unoccupied orbitals of
1a from those of planar p-xylene. The distortion of C2V-p-xylene
to the geometry present in 1a leads to the destabilization of
b2(π) and the stabilization of a1(π*) MO while leaving the a2
and b1 MOs virtually unperturbed. The through-space and
through-bond coupling of the resulting b2 orbitals in 1a yields
b1g and b3u MOs and that of the a1 MOs leads to MOs of ag and
a2u symmetries. The a2 and b1 MOs mutually interact respec-
tively, predominantly through space, to yield the b3g/b1u and
au/b2g MOs of 1a. The decreased HOMO-LUMO gap in 1a
(8.83 eV) as compared to that in p-xylene (12.33 eV) manifests
itself in the UV/vis absorption extending to 450 nm. The
physical and chemical consequences of bending of the aromatic
rings and of those orbital interactions in 1a are now under
investigation.
(34) According to recent theoretical calculations, the degree of bond
alteration in [4]paracyclophane, the benzene ring of which is more severely
bent than that of 1a, is still small.6 The insignificant bond alteration in 1a,
therefore, is not surprising. We recently observed that significant diatropicity
is retained in a [4]paracyclophane derivative, suggesting the sustenance of
substantial aromatic ring current in its benzene ring. Okuyama, M.; Tsuji,
T. Angew. Chem., Int. Ed. Engl. 1997, 36, 1085-1087.

8430 J. Am. Chem. Soc., Vol. 119, No. 36, 1997
Tsuji et al.
Figure 5. Derivation of the high energy occupied and low energy unoccupied MOs of [1.1]paracyclophane (1a) from those of planar C2V-p-xylene
and schematic representations of the HOMO, NHOMO (N denotes next), LUMO, and NLUMO of 1a (RHF/6-31G* at the MP2/6-31G*-optimized
geometries).
The thermal stability of 1a was examined in a separate experiment.
A degassed solution of 3a in EPA was irradiated with a 254 nm light
source at 77 K to generate 1a, thawed at -90 °C, and refrozen in a
liquid N2 bath. After recording an UV-vis absorption spectrum, the
glassy mixture was thawed, kept at -20 °C for 1 h, and frozen again.
The spectrum then recorded indicated that 1a remained unchanged.
After 1 h at 20 °C, however, the characteristic bands due to 1a had
largely decayed.
Conclusions
[1.1]Paracylophane and the bis(methoxycarbonyl) derivative
were successfully generated for the first time via the photoi-
somerization of the corresponding bis(Dewar benzene) precur-
sors. They exhibit UV/vis absorptions extending to unusually
long wavelength and undergo unprecedented transannular pho-
tocycloaddition between the benzene rings to give the corre-
sponding p,p-dimer structures, reflecting the strong electronic
interactions between the bent aromatic ring moieties. Theoreti-
cal calculations on [1.1]paracyclophane reveal that the extent
of distortion of the benzene rings is comparable to that in [5]-
paracyclophane and that the shortest transannular interatomic
distance is 2.36-2.40 Å, being shorter by 1.0 Å than the
interlayer distance in graphite. The calculated strain energy of
[1.1]paracyclophane is in a range of 93-128 kcal/mol depending
on the level of theory. The [1.1]paracyclophanes prepared in
this study are, however, thermally labile and persist only below
-20 °C in solution, necessitating their kinetic stabilization for
isolation.
Measurement of the Electronic Absorption Spectrum of 1b. A
glassy solution of 3b (1 mg) in 4.8 mL of EPA in a quartz cuvet at 77
K was prepared as described above for 3a. The solution exhibited λmax
at 235 (absorbance ) 3.37) and 259 (2.29) nm and a shoulder at 316
(0.41) nm. Irradiation of the glass with a low-pressure mercury lamp
led to development of absorption showing λmax at 256, 348, and 405
nm. After 10 min, absorbances at those λmax in the difference spectrum
were 0.73, 0.15, and 0.03, respectively. When the resulting glass was
irradiated with >390-nm light, the developed absorption was almost
completely bleached and the observed spectral changes indicated that
the initial product was transformed into a secondary photoproduct
showing λmax at 267 nm.
The thermal stability of 1b was examined in a separate experiment.
A degassed solution of 1b was prepared as described above and its
decay was monitored spectrometrically. The observed spectral changes
indicated that 1b remained unchanged after 2 h at -20 °C, but had
decayed to half the initial concentration after 6 days at the same
temperature. After 4 h at 20 °C, the absorption due to 1b had almost
completely decayed.
Measurement of the 1H NMR Spectrum of 1a. In a quartz NMR
tube was placed a solution of 3a (ca. 2 mg) in 0.6 mL of THF-d8 to
which a small amount of tetramethylsilane in CCl4 was added. The
solution was degassed by freeze-pump-thaw cycles and then saturated
with argon. The tube was sealed with vaseline and connected to a
mechanical stirrer to be rotated during irradiation. The 1H NMR
spectrum (500 MHz) recorded at -60 °C after irrdiating the sample
for 4 min with a low-presssure mercury lamp at -85 to -80 °C
indicated the formation of 1a and 21a in a ratio of ca. 3:2 in a yield of
ca. 2%. Upon further irradiation with a >335 nm light source, the
former was cleanly transformed into the latter as demonstrated in the
difference spectrum. When the resulting mixture was left to stand for
1 h at room temperature, 21a had completely disappeared.
Experimental Section
Halos (Eiko-sha, Japan) 500-W high-pressure and 120-W low-
pressure mercury lamps were employed as the light sources of
photochemistry. A 500-W high-pressure mercury lamp fitted with a
Corning 0-52 glass filter was used as a >335-nm light source and a
500-W xenon lamp fitted with a Corning 3-73 glass filter as a >390-
nm light source.
Measurement of Electronic Absorption Spectrum of 1a.
A
solution of 3a (1 mg, freshly purified by preparative GLC) in 4.9 mL
of EPA was placed in a quartz cuvet, degassed by freeze-pump-thaw
cycles, and frozen in liquid nitrogen to give a clear, colorless glass.
The mixture exhibited only an end absorption extending to 260 nm
and an absorbance at 220 nm was 1.2. Irradiation of the glass with a
120-W low-pressure mercury lamp led to development of absorption
with λmax at 226, 237, 290, and 377 nm. After 10 min, absorbances at
those λmax in the difference spectrum were 0.65, 0.57, 0.24, and 0.02,
respectively. When the resulting glass was irradiated with >335-nm
light by which only the newly generated species was excited, the
developed absorption was efficiently bleached and the observed spectral
changes indicated that the initial product was transformed into a
secondary product showing λmax at 244 nm.
Measurement of the 1H NMR spectrum of 1b. In a quartz NMR
tube was placed a solution of 3b (5 mg) in 0.6 mL of THF-d8 to which
a small amount of tetramethylsilane in CCl4 was added. The solution
was degassed by freeze-pump-thaw cycles and then saturated with

[1.1]Paracyclophane
J. Am. Chem. Soc., Vol. 119, No. 36, 1997 8431
argon. The tube was sealed with vaseline and connected to a
mechanical stirrer to be rotated during irradiation. The 1H NMR
spectrum (500 MHz) recorded at -60 °C after irradiating the solution
for 15 min at -70 °C with a low-pressure mercury lamp indicated the
formation of 1b and 21b in 2-3% and 5-6% yield, respectively, at
the expense of ca. 10% of 3b. Upon further irradiation with a >390
nm light source, the generated 1b was cleanly transformed into 21b as
demonstrated in the difference spectrum. When the resulting mixture
was left to stand for 1 h at room temperature, 21b had completely
disappeared.
the Ministry of Education, Science, and Culture of Japan. T.T.
thanks the Computer Center of the Institute for Molecular
Science for the use of the NEC HSP computer.
Supporting Information Available: Full experimental
details for the preparation of all the compounds reported in the
paper (13 pages). See any current masthead page for ordering
and Internet access instructions.
Acknowledgment. This research was supported by Grant-
JA9715253
in-Aids for Scientific Research (06453031 and 08454192) from