Cyclophanes, XLVI oxidative carbon - carbon bond cleavage of a [2.2]paracyclophane derivative - efficient intramolecular trapping of the radical cation

Article abstract of DOI:10.1002/1099-0690(200008)2000:15<2711::AID-EJOC2711>3.0.CO;2-B

4-(2,3,4,5-Tetraphenyl)phenyl[2.2]paracyclophane (3) has been prepared by cycloaddition of tetracyclone (2) to 4-ethynyl[2.2]paracyclophane (1). On treatment with FeCl₃ or AlCl₃ or NOBF₄ in nitromethane, 3 undergoes C-C bond cleavage by an electron transfer process to provide the double benzyl radical cation 10. The phenyl groups of the aryl substituent are ideally oriented for intramolecular trapping and, in the presence of the Lewis acids, ring closure to the new phane system 5 takes place in good yield (65%). In the presence of NOBF₄, the half-cyclized aldehyde 6 (66%) is produced. For comparison, [2.2]paracyclophane (7) was also treated with the latter one-electron oxidant, providing bibenzyl-4,4'-dicarbaldehyde (8, 30%) and its monooxime 9 (35%). The structures of the new phane systems 3 and 5 have been determined by X-ray structural analysis, and the mechanisms leading to these products are discussed.

Full text of DOI:10.1002/1099-0690(200008)2000:15<2711::AID-EJOC2711>3.0.CO;2-B

FULL PAPER
Cyclophanes, XLVI[°]
Oxidative Carbon؊Carbon Bond Cleavage of a [2.2]Paracyclophane Derivative
؊ Efficient Intramolecular Trapping of the Radical Cation
Sethuraman Sankararaman,[a] Henning Hopf,*[b] Ina Dix,[b] and Peter G. Jones[c]
Keywords: Aromatic polycycles / Cycloadditions / Phanes / Radical ions / Redox chemistry
4-(2,3,4,5-Tetraphenyl)phenyl[2.2]paracyclophane (3) has
system 5 takes place in good yield (65%). In the presence of
been prepared by cycloaddition of tetracyclone (2) to 4-ethy- NOBF4, the half-cyclized aldehyde 6 (66%) is produced. For
nyl[2.2]paracyclophane (1). On treatment with FeCl3 or AlCl3
or NOBF4 in nitromethane, 3 undergoes C−C bond cleavage
by an electron transfer process to provide the double benzyl
comparison, [2.2]paracyclophane (7) was also treated with
the latter one-electron oxidant, providing bibenzyl-4,4Ј-di-
carbaldehyde (8, 30%) and its monooxime 9 (35%). The
radical cation 10. The phenyl groups of the aryl substituent structures of the new phane systems 3 and 5 have been de-
are ideally oriented for intramolecular trapping and, in the
presence of the Lewis acids, ring closure to the new phane
termined by X-ray structural analysis, and the mechanisms
leading to these products are discussed.
Introduction
A system containing phenyl groups well positioned for
such an intramolecular trapping is 4-(2,3,4,5-tetra-
phenyl)phenyl[2.2]paracyclophane (3, Scheme 1), which is
also an attractive precursor for the preparation (by cyclode-
hydrogenation) of the [2.2]paracyclophane system 4, in
which one ‘‘deck’’ consists of an extended polycyclic aro-
matic hydrocarbon (PAH). We have prepared several PAH
phanes previously[6Ϫ8] and are presently interested in ex-
tending the size and complexity of the PAH subunit. Al-
though attempts to achieve this latter goal have not yet been
realised for 3 (see below), its chemistry with regard to oxid-
ative cleavage of the cyclophane bridge and the intramolec-
ular trapping of the resulting reactive benzylic interme-
diates have been studied successfully. Here we report on the
synthesis, structural characterization and several bridge
cleavage reactions of 3.
Oxidative cleavage of the central CϪC bond of 1,2-
diarylethanes to give a benzyl radical and a benzyl cation
(and subsequent products) is a well-known reaction that
can be achieved by a variety of conditions involving chem-
ical, electrochemical and photochemical activation.[2]
[2.2]Paracyclophane is a special case of such a system in
which the cleavage of the ethano bridge is thermodyn-
amically substantially more favourable, due to ring opening
and the release of associated strain.[3] The cleavage of the
ethano bridge in this hydrocarbon can be accomplished by
chemical (electrophilic), thermal and photochemical reac-
tions, or under the oxidative conditions mentioned
above.[4,5] Aryl-substituted derivatives of [2.2]paracyclo-
phane, with strong nonbonded steric interaction with the
ethano bridge (and hence still higher strain energy), are po-
tential candidates for study of the oxidative cleavage of the
ethano bridge in greater detail, because the resulting benzyl
radical/benzyl cation pair could in principle be trapped in-
Results and Discussion
Cycloaddition of ethynylparacyclophane (1) and tetra-
tramolecularly by substitution reactions in the neigh- cyclone (2) in diphenyl ether at 200Ϫ220 °C proceeded
bouring aryl substituent(s).
smoothly within 2 h to yield the desired starting material
for the study; namely 2,3,4,5-tetraphenylphenyl[2.2]paracy-
clophane (3), in 85% yield (Scheme 1). In the 1H NMR
spectrum of 3, the signal of the lone aromatic hydrogen
atom on the phenyl ring directly attached to the phane ap-
pears at δ ϭ 7.87. From the X-ray structure (Figure 1) and
the energy-minimized structure calculated using semi-em-
pirical methods (PM3), it is clear that this hydrogen sub-
stituent is located between the two π planes of the phane
moiety and hence is relatively more deshielded. It is also of
importance that one of the phenyl rings lies just above the
ethano bridge, the shortest distance between the bridging
[°]
Part XLV: Ref.[1]
[a]
Department of Chemistry, Indian Institute of Technology,
Madras 600036, India
Fax: (internat.) ϩ 91-44/235-0509
E-mail: sanka@acer.iitm.ernet.in
[b]
Institut für Organische Chemie, Technische Universität
Braunschweig,
Hagenring 30, 38106 Braunschweig, Germany
Fax: (internat.) ϩ 49-(0)531/391-5388
E-mail: h. hopf@tu-bs.de
[c]
Institut für Anorganische und Analytische Chemie, Technische
Universität Braunschweig,
Hagenring 30, 38106 Braunschweig, Germany
Fax: (internat.) ϩ 49-(0)531/391-5382
E-mail: p.jones@tu-bs.de
˚
carbon atom and the ring being ca. 3.7 A. This is crucial
Eur. J. Org. Chem. 2000, 2711Ϫ2716
WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2000
1434Ϫ193X/00/0815Ϫ2711 $ 17.50ϩ.50/0
2711

S. Sankararaman, H. Hopf, I. Dix, P. G. Jones
FULL PAPER
Scheme 1. Preparation of the cyclophane 3 and its bridge cleavage reactions
for the discussion of the reactivity of this molecule during sulted in the formation of an interesting new cyclophane
oxidative cleavage (see below). product. Thus, treatment of 3 with excess FeCl3 in nitro-
The attempted preparation of pentaphenylphenyl[2.2]- methane/dichloromethane at room temperature yielded a
paracyclophane (3 with a pentaphenylphenyl substituent) mixture of products from which the major one was isolated
from the reaction of phenylethynyl[2.2]paracyclophane with in 65% yield by chromatographic separation and identified
tetracyclone (2) under the above conditions did not yield as 5 (Scheme 1) using spectroscopic data and single-crystal
any tractable product.
X-ray crystallography. The molecular structure shown in
The geometries of the cyclophane fragments in 3 are very Figure 2 clearly indicates that one ethano bridge of the ori-
similar to the skeleton of the parent compound [2.2]paracy- ginal cyclophane ring has been cleaved, leading to the
clophane. The interplanar angles in 3 between the planar formation of a tribenzocycloheptatriene ring and a 16-
ring systems are optimized so as to avoid steric repulsion membered ring phane resulting from the intramolecular
between π-systems. The values are as follows (the carbon capture of the reactive intermediates of the CϪC frag-
atoms of the respective planes are given in brackets): mentation (see below).
50.87(7)° [the plane defined by the four coplanar carbon
In compound 5, the bond lengths of the single bonds in
the cyclic fragment C1 to C28 are in the range of the re-
spective standard bond lengths (these are given in brackets).
C(sp3)ϪC(sp3) bonds: 153.8(3) pm [153.9 pm] and
C(sp3)ϪC(sp2) bonds: 150.7(3) (C9ϪC10) to 152.0(3) pm
(C1ϪC45) [151.0 pm]. The C(sp2)ϪC(sp2) bonds C26ϪC41
(149.4(3) pm) and C12ϪC24 (149.1(3) pm) are slightly
atoms C4, C5, C7, C8 and C23 to C28], 53.87(5)° [C23 to
C28 and C29 to C34], 50.77(7)° [C29 to C34 and C35 to
C40], and 61.76(5)° [C35 to C40 and C41 to C46].
Reaction of 3 with Anhydrous FeCl3
The original objective of undertaking the synthesis of 3 elongated in comparison with the standard bond length for
was to carry out cyclodehydrogenation under Scholl reac- conjugated aromatic systems [147.0 pm] due to the torsion
tion[9] conditions using a suitable Lewis acid and also to of the aromatic systems and the consequent hindered con-
perform a photocyclization[10] of the stilbene-to-phen- jugation. The interplanar angles of the cyclic fragment C1
anthrene type to prepare the planarized derivative 4. While to C28 indicate no interaction between the π-systems of the
the latter methodology, tried under various reaction condi- involved aromatic rings: 45.74(8)° [C2 to C7 and C10 to
tions, failed completely, the Scholl reaction with FeCl3 re- C15], 42.07(8)° [C10 to C15 and C23 to C28], 42.75(7)°
2712
Eur. J. Org. Chem. 2000, 2711Ϫ2716

Cyclophanes, XLVI
FULL PAPER
Scheme 2. Mechanism of the bridge cleavage of 3 and [2.2]paracyclophane (7)
atoms C12, C13, C17 and C18 are coplanar, with a mean
deviation of the plane of 0.53 pm. The other three carbon
atoms are bent out of the plane by 72.8(3) (C16), 83.3(3)
(C23) and 78.0(3) pm (C24).
In order to study further the viability of an electron
transfer mechanism for the CϪC bond cleavage, compound
3 was deliberately oxidised with NOBF4, a well-known
strong, one-electron oxidant (E1/2 ϭ 0.98 V vs. Cp2Fe in
CH3NO2),[11] known to form charge transfer complexes and
undergo electron transfer reactions with aromatic hydrocar-
bons.[11] Thus, oxidation of 3 with NOBF4 resulted in the
formation of yet another product, identified as
6
(Scheme 1). The 1H NMR spectrum of 6 shows an aldehyde
signal at δ ϭ 9.81 and an AB-quadruplet at δ ϭ 3.96 and
3.66 (J ϭ 12.6 Hz) characteristic of the methylene hydrogen
atoms of the cycloheptatriene moiety (cf. the 1H NMR
spectrum of 5, the structure of which was unambiguously
established by the X-ray crystallographic data). The oxidat-
ive cleavage with ceric ammonium nitrate (another well-
known electron transfer oxidant) of the ethano bridge in
parent [2.2]paracyclophane (7) has been studied previously
Figure 1. Single-crystal X-ray structure of 3
[C23 to C28 and C41 to C46], 77.23(7)° [C2 to C7 and C23 by Adam and co-workers.[5] For comparison, we oxidized 7
to C28] and 65.09(7)° [C10 to C15 and C41 to C46]. The with NOBF4 and isolated two major products, identified
seven-membered ring in 5 adopts a boat conformation. The as bibenzyl-4,4Ј-dicarbaldehyde (8) and the corresponding
Eur. J. Org. Chem. 2000, 2711Ϫ2716
2713

S. Sankararaman, H. Hopf, I. Dix, P. G. Jones
FULL PAPER
monooxime 9 (Scheme 1). The formation of these products nitroso derivative 12, which, after cyclization to the seven-
with NOBF4 is highly reminiscent of the products of oxida- membered ring, could tautomerize to the corresponding ox-
tion by ceric ammonium nitrate.[5]
ime. This, in turn, would be expected to hydrolyze to the
aldehyde 6 during workup or chromatographic purification.
In the case of NOBF4 oxidation, even though the initial
intramolecular cyclization leading to the cycloheptatriene
ring is still very efficient, the trapping of the benzyl radical
by nitric oxide effectively competes with the second intram-
olecular cyclization, leading to the formation of the alde-
hyde 6 in good yield (66%).
In the case of the parent paracyclophane 7, the oxidation
with NOBF4 leads to the double benzylic radical cation 13.
Since there is now no intramolecular trapping group, the
radical part of 13 reacts with nitric oxide, whereas the cat-
ionic centre is intercepted by nitromethane.[12] The resulting
intermediate 14 can subsequently fragment to nitrosome-
thane and 15, from which the isolated products 9 and 8
would be produced during workup (Scheme 2).
Mechanism
The cleavage, upon one-electron oxidation, of the CϪC
bond of the ethano bridge of [2.2]paracyclophane (7) to
yield a benzyl radical/benzyl cation intermediate has been
well established by chemical and electrochemical oxidation
methods.[4,5] This radical ion intermediate is normally
trapped by external nucleophiles, oxygen etc. The mechan-
ism of formation of 5 and 6 is depicted in Scheme 2. Oxida-
tion of 2 with either FeCl3 or NOBF4 leads to the formation
of the radical cation, which readily undergoes CϪC bond
cleavage, leading to the intermediate double benzylic radical
cation 10.
Conclusions
Oxidation of the phenyl-substituted paracyclophane de-
rivative 3 using FeCl3 and NOBF4 results in the cleavage
of the CϪC bridge; the resulting radical cation is trapped
intramolecularly very efficiently, leading in the former case
to the formation of a novel cyclophane 5 and, in the latter,
the aldehyde 6. The success of the intramolecular trapping
is due to the proximity of one phenyl substituent lying just
above the bridge of the cyclophane moiety, as shown by the
X-ray crystal structure and the calculated structure of the
starting material 3.
Experimental Section
The spectroscopic methods used for structure elucidation have been
described in the previous papers of this series.[1] Nitromethane was
fractionally distilled and stored over freshly activated molecular
Figure 2. Single-crystal X-ray structure of 5
˚
sieves (type 4 A). Dichloromethane was distilled from CaH2 un-
With its eclipsed 1,2-diphenylethane conformation, this
intermediate reduces its internal strain by rotation about
the remaining ethano bridge, thus creating a more ‘‘open’’
structure, 11. In this species, both reactive centres Ϫ the
benzyl radical and the benzyl cation Ϫ face phenyl substitu-
ents, which can now fulfil their role as internal traps. Intra-
molecular FriedelϪCrafts alkylation forms the seven-mem-
bered ring, while the benzyl radical attacks the other outer
der nitrogen.
4-(2,3,4,5-Tetraphenyl)phenyl[2.2]paracyclophane (3): A mixture of
4-ethynyl[2.2]paracyclophane[13] (1, 1.16 g, 5 mmol) and tetracyc-
lone (2, 1.92 g, 5 mmol) in diphenyl ether (10 mL) was heated at
200Ϫ220 °C for 2 h under nitrogen. The dark purple colour of the
tetracyclone faded gradually and evolution of gas was observed.
The reaction mixture was cooled to room temp. and then poured
into 100 mL of methanol. The suspension was refluxed for 10 min,
phenyl group of the p-terphenyl intermediate 11. Alternat- cooled to room temp., and the tan-coloured solid was removed by
filtration and washed several times with methanol. The yield of the
crude product was 2.5 g (86%), free of starting materials and di-
phenyl ether as shown by TLC analysis and the 1H NMR spectrum.
A small portion of the solid was dissolved in CH2Cl2 and treated
with activated charcoal, and the solution was filtered and pentane
added until slight turbidity appeared. The solution was cooled in
a freezer (Ϫ25 °C) to yield analytically pure product as a colourless
amorphous solid. When the above solution was very slowly concen-
trated from a loosely stoppered round bottomed flask, single crys-
tals suitable for X-ray analysis were obtained (see below). Ϫ M.p.
ively, it is possible that, after formation of the cycloheptatri-
ene ring, the intermediate benzyl radical is oxidized a se-
cond time by excess oxidant to provide a further benzyl
cation intermediate, which can lead to 5 by a second
FriedelϪCrafts alkylation. In either case, the high yield of
this new cyclophane system indicates that both ring-for-
ming processes occur efficiently. When NOBF4 is used as
the oxidant, generation of the radical cation is accompanied
by nitric oxide formation. Radical combination between the
benzyl radical part of 10 and nitric oxide could lead to 310Ϫ315 °C (dec.). Ϫ IR (KBr): ν˜ ϭ 2928 cmϪ1, 1599, 697. Ϫ UV/
2714 Eur. J. Org. Chem. 2000, 2711Ϫ2716

Cyclophanes, XLVI
FULL PAPER
Vis (CHCl3): λmax (log ε) ϭ 252 nm (4.60) 276 (sh, 4.39). Ϫ 1H tion initially turned wine-red, then brown. After stirring at room
NMR (CDCl3, 400 MHz): δ ϭ 2.33 (m, 1 H), 2.67 (m, 1 H), 2.80 temp. overnight, the reaction mixture was quenched with water and
(m, 1 H), 3.0 (m, 5 H), 6.01 (d, J ϭ 7.8 Hz, 1 H), 6.30 (dd, J ϭ the organic layer was dried with MgSO4. Removal of the solvent
1.85 and 7.6 Hz, 1 H), 6.41 (d, J ϭ 7.6 Hz, 1 H), 6.52 (d, J ϭ under reduced pressure yielded a tan-coloured solid. Column chro-
1.7 Hz, 1 H), 6.54 (d, J ϭ 1.7 Hz, 1 H), 6.61 (m, 2 H), 6.63 (d, J ϭ matographic separation on silica gel with pentane and dichlorome-
1.8 Hz, 1 H), 6.70Ϫ7.05 (m, 15 H), 7.25 (m, 4 H), 7.87 (s, 1 H). Ϫ thane (1:1) yielded the major product as a pale yellow solid (0.12 g,
13C NMR (CDCl3, 100 MHz): δ ϭ 34.15 (t), 35.26 (t), 35.3 (t), 35.4
66%); m.p. 225Ϫ226 °C. Ϫ IR (KBr): ν˜ ϭ 2925 cmϪ1, 1698 (Cϭ
(t), 125.04 (d), 125.1 (d), 125.5 (d), 126.16 (d), 126.2 (d), 126.4 (d), O), 1604, 701. Ϫ UV/Vis (CHCl3): λmax (log ε) ϭ 244 nm (sh)
1
126.6 (d), 126.8 (d), 127.8 (d), 129.6 (d), 129.9 (d), 131.36 (d), 131.4
(4.59), 262 (4.82), 284 (sh, 4.54). Ϫ H NMR (CDCl3, 400 MHz):
(d), 131.6 (d), 131.7 (d), 131.8 (d), 131.9 (d), 132.1 (d), 132.5 (d), δ ϭ 2.97 (m, 4 H), 3.66 and 3.96 (AB-q, J ϭ 12.6 Hz, 2 H),
132.9 (d), 133.3 (d), 134.1 (d), 138.5 (s) 138.6 (s), 139.02 (s), 139.3
(s), 140.05 (s), 140.1 (s), 140.2 (s), 140.56 (s), 140.6 (s), 140.7 (s),
140.8 (s), 141.2 (s),142.2 (s). Ϫ MS (EI, 70 eV); m/z (%): 590 (8)
6.60Ϫ6.75 (m, 4 H), 6.85Ϫ7.10 (m, 12 H), 7.20 (m, 5 H), 7.42 (d,
J ϭ 1.4 Hz, 1 H), 7.52 (s, 1 H), 7.29 and 7.72 (AAЈBBЈ-m, J ϭ
8.0 Hz, 4H ), 9.81 (s, 1 H). Ϫ 13C NMR (CDCl3, 100 MHz): δ ϭ
[Mϩ ϩ 2] , 589 (36) [Mϩ ϩ 1], 588 (68) [Mϩ] , 485 (10), 484 (54), 36.99 (t), 38.1 (t), 40.2 (t), 124.4 (d), 125.3 (d), 125.4 (d), 125.6 (d),
483 (100). Ϫ C46H36 (588.75): calcd. C 93.87, H 6.12; found C 93.74
H 6.13.
126.3 (d), 126.6 (d), 126.7 (d), 126.8 (d), 126.9 (d), 127.0 (d), 127.5
(d), 127.76 (d), 129.1 (d), 129.3 (d), 129.8 (d), 129.9 (d), 130.7 (d),
131.3 (d), 131.5 (d), 131.7 (d), 132.57 (d), 132.8 (d), 134.5 (s), 135.5
(s), 137.2 (s), 137.8 (s), 138.8 (s), 138.9 (s), 139.6 (s), 139.9 (s), 140.8
(s), 140.9 (s), 141.26 (s), 141.3 (s), 141.8 (s), 143.5 (s), 149.0 (s),191.9
(d). Ϫ MS (EI, 70 eV); m/z (%): 604 (16) [Mϩ ϩ 2], 603 (50) [Mϩ
ϩ 1], 602 (100) [Mϩ], 485 (6), 484 (30), 483 (66). Ϫ HRMS; m/z:
calcd. for C46H34O 602.26096; found 602.2603. Ϫ A very minor
product (5 mg) obtained as a yellow solid was identified as 6-[2-(4-
hydroxymethylphenyl)ethyl]-1,2,3-triphenyl-9H-tribenzo[a,c,e]-
cycloheptatriene from the following spectroscopic data. Ϫ 1H
NMR (CDCl3, 400 MHz): δ ϭ 2.90 (m, 4 H), 3.6 (d, J ϭ 11 Hz, 1
H), 3.9 (d, J ϭ 11 Hz, 1 H), 4.5 (s, 2 H), 6.60 (m, 5 H), 6.9 (m, 14
H), 7.15 (m, 5 H), 7.45 (m, 3 H), 7.55 (s, 1 H). Ϫ MS (EI, 70 eV);
m/z (%): 605 (14) [Mϩ ϩ 1], 604 (28) [Mϩ], 600 (52), 599 (100), 484
(40), 483 (92).
6,3-[2](1,2-Diphenyl-9H-tribenzo[a,c,e]cycloheptatrienyleno)[1](1,4-
phen)yleno(1,3-phenyleno)phane (5): To a stirred solution of anhyd-
rous FeCl3 (0.48 g, 3 mmol) in dry nitromethane (2 mL) under ni-
trogen was added dropwise
a solution of 4-(2,3,4,5-tetrap-
henyl)phenyl[2.2]paracyclophane (3, 0.15 g, 0.25 mmol) in di-
chloromethane (100 mL). The mixture was stirred at room temp.
for 36 h, and water (100 mL) and saturated aqueous NaHSO3
(25 mL) were added. The organic layer was separated, washed sev-
eral times with water, dried with MgSO4, and the solvent was re-
moved under reduced pressure to yield a pale brown solid con-
sisting of a mixture of products as shown by TLC and crude NMR
analysis. Column chromatographic purification on silica gel with
pentane/dichloromethane (1:1) yielded product 5 as a colourless
solid (97 mg, 65%). Recrystallization by slow diffusion of pentane
into a dichloromethane solution of 5 provided single crystals suit-
able for X-ray crystallography (see below). The preparation of 5
was also accomplished using AlCl3 as follows, albeit in lower yield.
To a solution of AlCl3 (1 g) and NaCl (100 mg) (ground together
as a fine powder prior to dissolution) in nitromethane (20 mL) was
added a solution of 3 (0.1 g) in dichloromethane (50 mL) and the
resulting mixture was stirred under nitrogen for 24 h. The solution
turned dark green. Workup by addition of 1  aqueous HCl
(50 mL) followed by extraction with CH2Cl2 yielded a solid after
solvent removal. On chromatographic separation as described
above, 5 was isolated (30 mg, 30%) as a colourless solid; m.p. 278
°C. Ϫ IR (KBr): ν˜ ϭ 3022 cmϪ1, 2919, 1599, 1433, 700. Ϫ UV/Vis
(CHCl3): λmax (log ε) ϭ 244 nm (4.43), 266 (4.61), 296 (sh, 4.34).
Ϫ 1H NMR (CDCl3, 400 MHz): δ ϭ 2.65 (m, 1 H), 3.03 (m, 2 H),
3.30 (m, 1 H), 3.60 (d, J ϭ 12.6 Hz, 1 H), 3.85 (d, J ϭ 12.6 Hz, 1
H), 4.0 and 4.1 (AB-q, J ϭ 15.5 Hz, 2 H), 6.25 (m, 1 H), 6.60 (m,
2 H), 6.70Ϫ7.0 (m, 13 H), 7.15 (m, 8 H),7.47 (s, 2 H). Ϫ 13C NMR
(CDCl3, 100 MHz): δ ϭ 36.7 (t), 37.14 (t), 40.0 (t), 40.6 (t), 124.45
(d), 125.4 (d), 125.69 (d), 125.9 (d), 126.0 (d), 126.34 (d), 126.37
(d), 126.4 (d), 126.6 (d), 126.7 (d), 127.04 (d), 127.2 (d), 127.4 (d),
127.9 (d), 128.5 (d), 129.9 (d), 130.6 (d), 130.8 (d), 131.0 (d), 131.3
(d), 131.5 (d), 131.7 (d), 132.7 (d), 132.8 (d), 134.05 (d), 134.6 (d),
135.3 (s), 136.4 (s), 137.7 (s), 138.12 (s), 139.1 (s), 139.28 (s), 139.4
(s), 139.95 (s), 140.75 (s), 140.76 (s), 140.8 (s), 143.1 (s), 143.3 (s).
Ϫ MS (EI. 70 eV); m/z (%): 588 (18) [Mϩ ϩ 2], 587 (48) [Mϩ ϩ 1],
586 (100) [Mϩ]. Ϫ HRMS; m/z: calcd. for C46H34 586.26605; found
586.2651. Ϫ C46H34 (586.73): calcd. C 94.19, H 5.80; found C
94.37, H 5.61.
Oxidation of [2.2]Paracyclophane (7) with NOBF4: The oxidation
of 7 (0.21 g, 1 mmol) with NOBF4 (0.34 g, 3 mmol) was carried out
as described in the case of 3. After stirring at room temp. for 48 h,
the starting material had disappeared. Chromatographic separation
of the crude product (0.23 g) on silica gel with CH2Cl2 and pentane
(1:1) yielded bibenzyl-4,4Ј-dicarbaldehyde (8, 75 mg, 30%). Sub-
sequent elution with CH2Cl2 provided the corresponding monoox-
ime 9 (90 mg, 35%). Both compounds were identified by IR, NMR
and MS analysis.
Bibenzyl-4,4Ј-dicarbaldehyde (8):[14] 1H NMR (CDCl3, 400 MHz):
δ ϭ 3.06 (s, 4 H), 7.30 (d, J ϭ 8.1 Hz, 4 H), 7.80 (d, J ϭ 8.1 Hz,
4 H), 9.99 (s, 2 H). Ϫ 13C NMR (CDCl3, 100 MHz): δ ϭ 37.42 (t),
129.13 (d), 129.9 (d), 134.74 (s), 148.16 (s),191.8 (d). Ϫ MS (EI,
70 eV); m/z (%): 239 (10), 238 (50), 119 (92), 91 (100).
Bibenzyl-4,4Ј-dicarbaldehyde Monooxime (9): 1H NMR (CDCl3,
400 MHz): δ ϭ 2.95 (br. s, 1 H), 2.98 (AAЈBBЈ-m, 4 H), 7.15 (d,
J ϭ 8.1 Hz, 2 H), 7.29 (d, J ϭ 8.1 Hz, 2 H), 7.48 (d, J ϭ 8.2 Hz,
2 H), 7.79 (d, J ϭ 8.2 Hz, 2 H), 8.12 (s, 1 H), 9.96 (s, 1 H). Ϫ 13C
NMR (CDCl3, 100 MHz): δ ϭ 37.13 (t), 37.68 (t), 127.08 (d),
128.86 (d), 129.16 (d), 129.92 (d), 130.0 (s), 134.6 (s), 143.0 (s),
148.6 (s), 150.1 (d), 192.0 (d). Ϫ MS (EI, 70 eV); m/z (%): 254 (4),
253 (24), 236 (14), 135 (10), 134 (100).
X-ray Crystallography of 3 and 5: A summary of crystal data, data
collection and refinement parameters is given in Table 1. For the
structure determination of 3 and 5, a cut tablet was mounted on a
glass fibre in inert oil and transferred to the cold gas stream of a
Bruker SMART 1000 CCD diffractometer fitted with a Siemens
6-[2-(4-Formylphenyl)ethyl]-1,2,3-triphenyl-9H-tribenzo[a,c,e]- LT-3 low-temperature attachment. Data were collected with ω-
cycloheptatriene (6): A solution of 3 (0.2 g, 0.3 mmol) in dry
CH2Cl2 (100 mL) was added dropwise under nitrogen to a stirred
solution of NOBF4 (0.23 g, 2 mmol) in dry nitromethane. The solu-
scans using graphite-monochromated Mo-Kα radiation (λ
0.71073 A). An absorption correction using SADABS was applied
in case of compound 5. All unique data were used for calculations
ϭ
˚
Eur. J. Org. Chem. 2000, 2711Ϫ2716
2715

S. Sankararaman, H. Hopf, I. Dix, P. G. Jones
FULL PAPER
(program SHELXL-97).[15] Both structures were solved by direct
methods and refined anisotropically by full-matrix, least-squares
on F2. Hydrogen atoms were refined with a riding model. Com-
pound 5 crystallizes with one molecule of dichloromethane. The
solvent is severely disordered and badly resolved. Three alternative
positions were refined isotropically without H atoms; the composi-
tion (and related parameters) should be regarded as tentative.
[1]
[2]
S. Sankararaman, H. Hopf, I. Dix, P. G. Jones, Eur. J. Org.
Chem., 2000, 2703Ϫ2709, preceding paper.
L. W. Reichel, G. W. Griffin, A. J. Muller, P. K. Das, S. N. Ege,
Can. J. Chem. 1984, 62, 424Ϫ436; S. Sankararaman, J. K. Ko-
chi, J. Chem. Soc., Chem. Commun. 1989, 1800Ϫ1802; E. Baci-
occhi, Acta Chem. Scand. 1990, 44, 645Ϫ652; for reviews on
the chemistry of radical cations see P. Maslak, Top. Curr.
Chem. 1993, 168, 1Ϫ46; M. Schmittel, A. Burghart, Angew.
Chem. 1997, 109, 2658Ϫ2699; Angew. Chem. Int. Ed. Engl.
1997, 36, 2550Ϫ2589.
Table 1. Summary of crystal data, data collection, and refinement
parameters for 3 and 5
[3]
The experimental strain energy of 1 is 31Ϫ33 kcal molϪ1. For
a discussion of strain release effects in [2n]cyclophanes see: H.
Hopf, C. Marquard, in Strain and its Implications in Organic
Chemistry (Eds.: A. de Meijere, S. Blechert), NATO ASI series,
Kluwer Academic Publishers, Dordrecht, 1988, vol.273, pp.
297Ϫ332. See also: R. H. Boyd, Tetrahedron 1966, 22,
119Ϫ122.
R. C. Helgeson, D. J. Cram, J. Am. Chem. Soc. 1966, 88,
509Ϫ515; H. J. Reich, D. J. Cram, J. Am. Chem. Soc. 1969, 91,
3517Ϫ3526; T. Shono, A. Ikeda, J. Hayashi, S. Hakozaki, J.
Am. Chem. Soc. 1975, 97, 4261Ϫ4264; T. Sato, K. Torizuka,
M. Shimizu, Y. Kurihara, N. Yoda, Bull. Chem. Soc. Jpn. 1979,
52, 2420Ϫ2423; H. Hopf, J. Kleinschroth, Angew. Chem. 1982,
94, 485Ϫ496; Angew. Chem. Int. Ed. Engl. 1982, 21, 469Ϫ480.
W. Adam, M. A. Miranda, F. Mojarrad, H. Sheikh, Chem. Ber.
1994, 127, 875Ϫ879.
H. Hopf, C. Mlynek, S. El-Tamany, L. Ernst, J. Am. Chem.
Soc. 1985, 107, 6620Ϫ6627.
H. Hopf, M. Zander, J. Hucker, Z. Naturforsch. 1985, 40a,
1316Ϫ1318.
M. Rabinovitz, R. Frim, H. Hopf, J. Hucker, Angew. Chem.
1987, 99, 243Ϫ245; Angew. Chem. Int. Ed. Engl. 1987, 26,
232Ϫ234.
R. Scholl, C. Seer, R. Weitzenböck, Ber. Deutsch. Chem. Ges.
1910, 43, 2202Ϫ2209; C. D. Nenitzescu, A. T. Balaban, in
FriedelϪCrafts and Related Reactions (Ed.: G. A. Olah), Inter-
science, New York, N. Y., 1964, vol. II, pp. 979Ϫ1047; S.
Hagen, H. Hopf, Top. Curr. Chem. 1998, 196, 45Ϫ89.
For reviews on this photocyclization see: F. B. Mallory, C. N.
Mallory, Org. React. 1984, 30, 1Ϫ456; W. H. Laarhoven, W. J.
C. Prinsen, Top. Curr. Chem. 1984, 125, 63Ϫ130; W. H. Laar-
hoven, Org. Photochem. 1987, 9, 129; H. Meier, Angew. Chem.
1992, 104, 1425Ϫ1446; Angew. Chem. Int. Ed. Engl. 1992, 31,
1399Ϫ1420.
T. M. Bockman, Z. J. Karpinski, S. Sankararaman, J. K. Ko-
chi, J. Am. Chem. Soc. 1992, 114, 1970Ϫ1985.
H. B. Hass, M. L. Bender, J. Am. Chem. Soc. 1949, 71,
1767Ϫ1769.
S. Hentschel, Ph.d. Dissertation, University of Braunschweig,
1989. Prepared from 4-formyl[2.2]paracyclophane, (bromome-
thyl)triphenylphosphonium bromide and potassium tert-butox-
ide in THF; yield 70Ϫ75%.
B. Thulin, O. Wenneström, I. Somfai, B. Chmielarz, Acta.
Chem. Scand. 1977, B31, 135Ϫ140.
Compound
3
5·CH2Cl2
Formula
C46H36
588.75
colourless tablet
0.24 ϫ 0.23 ϫ 0.09
monoclinic
P21/c
C47H36Cl2
671.66
colourless tablet
0.43 ϫ 0.21 ϫ 0.12
monoclinic
P21/c
Mr
Crystal habit
Crystal size (mm)
Crystal system
Space group
[4]
˚
a (A)
10.6882(14)
25.388(3)
11.8085(14)
98.781(6)
3166.7
4
11.2468(10)
11.0238(10)
28.040(2)
92.989(10)
3471.8
˚
b (A)
˚
c (A)
β (°)
3
˚
V (A )
[5]
[6]
[7]
[8]
Z
4
Dx (Mg mϪ3
)
1.235
1.285
µ (mmϪ1
)
0.070
0.221
Transmissions
F(000)
Ϫ
0.876Ϫ1.000
976
1248
T (°C)
2θmax
Ϫ130
Ϫ130
51
57
Refl. measured
Refl. unique
Rint
29360
36748
6489
8809
[9]
0.107
0.044
Parameters
Restraints
wR (F2, all refl.)
R [F, Ͼ 4σ(F)]
S
415
448
445
24
0.117
0.173
0.048
0.058
0.89
0.98
[10]
Ϫ3
˚
max. ∆ρ (e A
)
0.25
0.83
Crystallographic data (excluding structure factors) for the struc-
ture(s) included in this paper have been deposited with the Cam-
bridge Crystallographic Data Centre as supplementary publication
nos. CCDC-135530 (3) and -135531 (5). Copies of the data can be
obtained free of charge on application to CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK [Fax: (internat.) ϩ 44-1223/336-033; E-
mail: deposit@ccdc.cam.ac.uk].
[11]
[12]
[13]
[14]
[15]
Acknowledgments
One of us (S. S.) thanks the Alexander von Humboldt Foundation
for a fellowship and the Indian Institute of Technology, Madras,
India for sabbatical leave. The support of this work by the Fonds
der Chemischen Industrie is gratefully acknowledged.
G. M. Sheldrick, SHELXL-97, Program for crystal structure
refinement; University of Göttingen, 1997.
Received March 9, 2000
[O00113]
2716
Eur. J. Org. Chem. 2000, 2711Ϫ2716