Efficient synthesis of benzene and planar cyclooctatetraene fully annelated with bicyclo[2.1.1]hex-2-ene [5]

Full text of DOI:10.1021/ja003512m

1768
J. Am. Chem. Soc. 2001, 123, 1768-1769
Scheme 1a
Efficient Synthesis of Benzene and Planar
Cyclooctatetraene Fully Annelated with
Bicyclo[2.1.1]hex-2-ene
Akira Matsuura and Koichi Komatsu*
Institute for Chemical Research, Kyoto UniVersity
Uji, Kyoto 611-0011, Japan
ReceiVed September 27, 2000
a (a) 2,4,6-triisopropylbenzenesulfonylhydrazide, Et2O, rt, 77%. (b)
t-BuLi, THF, -78 f 0 °C. (c) n-Bu3SnCl, -78 °C, 88%. (d) I2, CCl4, rt,
78%. (e) t-BuOK, THF, rt, 94%. (f) n-BuLi, THF, -78 °C. (g) CuI, -78
f -45 °C. (h) CuCl2, -78 °C f rt. (i) CuI, -78 °C f rt.
Recently, the phenomenon of bond alternation (or localization)
in benzene, which is associated with the Mills-Nixon effect,1
has become the focus of revisited attention, and such a bond-
alternated benzene, that is, 1,3,5-cyclohexatriene, has been not
only a subject of considerable theoretical investigation2 but also
a synthetic target for organic chemists.
in THF followed by the sequential addition of CuI and CuCl2
successfully afforded benzene 2 in 43% yield.7 The overall yield
of 2 from the starting ketone was 21%, and the availability of 2
in a larger quantity should allow thorough exploration of the
chemistry of this bond-alternated benzene 2.
Star-phenylene 1 has been reported to undergo reactions
characteristic of olefin rather than of benzene, that is, hydrogena-
tion,8 epoxidation,9 and cyclopropanation,9 which could be
ascribable to significant bond localization. Although the degree
of bond alternation in 2 is much smaller than in 1 (∆R ) 0.159
Å),3a both epoxidation and cyclopropanation of 2 do, in fact, take
place readily. As shown in Scheme 2, reaction of 2 with
m-chloroperbenzoic acid afforded all-cis-trisepoxide 5 in a
quantitative yield, and a modified Simmons-Smith reaction gave
all-cis-triscyclopropane 6 in 75% yield. However, 2 is inert to
catalytic hydrogenation and diimide reduction, presumably for
steric reasons.10
For examples of the compounds that possess cyclohexatriene
motifs, several “phenylenes” such as 1 have been synthesized,3
and X-ray analysis has revealed the manifestation of a pronounced
bond alternation (∆R ) Rendo - Rexo ) 0.1∼0.18 Å) in the central
benzene ring. On the other hand, Siegel et al. have reported the
synthesis of tris(bicyclo[2.1.1]hexeno)benzene (2),4 as the first
example of mononuclear benzenoid hydrocarbon with a cyclo-
hexatriene-like geometry (∆R ) 0.089 Å).5 Compared with such
phenylenes as 1, in which the central benzene ring is embedded
within extensively delocalized π-systems, electronic perturbation
of the central benzene ring would be much less significant in 2
with no annelation of π-systems. Thus, compound 2 may be
considered to be a more appropriate model for probing the nature
of the “cyclohexatriene” itself. However, its yield is extremely
low (<1% in the final step), which hampers further scrutiny of
this fascinating molecule. Here we report an efficient synthesis
of 2, as well as the first synthesis of a novel cyclooctatetraene
(COT) derivative 3 with a completely planar cyclic 8π-system.
Scheme 2a
a (a) m-CPBA, CH2Cl2, rt. (b) ZnEt2/CH2I2, ClCH2CH2Cl, rt.
On the other hand, when the cyclotrimerization reaction was
conducted without CuCl2 (Scheme 1), COT 3 was obtained as
an orange solid in 21% yield, together with a mixture of benzene
2 and the dimer 4 (34 and 5%, respectively). Considering the
observed effect of the annelation with bicyclo[2.1.1]hexene in 2
to localize the double bond to the position exo with respect to
the annelation, the COT 3 was expected to possess a planar eight-
membered ring instead of the usual tub-shaped one.11,12 The
anticipated D4h symmetric structure is reflected in the simplicity
An efficient route to 2 was established by cyclotrimerization
of the organometals derived from 2,3-diiodobicyclo[2.1.1]hex-
2-ene, which was synthesized from bicyclo[2.1.1]hexan-2-one6
(Scheme 1). Treatment of the diiodoolefin with n-butyllithium
* Correspondence to: Koichi Komatsu. Telephone: (81) 774-38-3172;
Fax: (81) 774-38-3178. E-mail: komatsu@scl.kyoto-u.ac.jp.
(1) Mills, W. H.; Nixon, I. G. J. Chem Soc. 1930, 2510. However, it has
been pointed out by Siegel that strain-induced bond alternation should not be
interpreted in terms of the “Mills-Nixon effect”. See Siegel, J. S. Angew.
Chem., Int. Ed. Engl. 1994, 33, 1721.
1
of the H and 13C NMR (CDCl3) spectra:13 1H NMR (Figure 1)
(2) (a) Baldridge, K. K.; Siegel, J. S. J. Am. Chem. Soc. 1992, 114, 9583.
(b) Stanger, A. J. Am. Chem. Soc. 1991, 113, 8277. (c) Stanger, A. J. Am.
Chem. Soc. 1998, 120, 12034. (d) Faust, R.; Glendening, E. D.; Streitwieser,
A.; Vollhardt, K. P. C. J. Am. Chem. Soc. 1992, 114, 8263. (e) Shurki, A.;
Shaik, S. Angew. Chem., Int. Ed. Engl. 1997, 36, 2205. (f) Boese, R.; Bla¨ser,
D.; Billups, W. E.; Haley, M. M.; Maulitz, A. H.; Mohler, D. L.; Vollhardt,
K. P. C. Angew. Chem., Int. Ed. Engl. 1994, 33, 313.
(3) (a) Diercks, R.; Vollhardt, K. P. C. J. Am. Chem. Soc. 1986, 108, 3150.
(b) Diercks, R.; Vollhardt, K. P. C. Angew. Chem., Int. Ed. Engl. 1986, 25,
266. (c) Boese, R.; Matzger, A. J.; Mohler, D. L.; Vollhardt, K. P. C. Angew.
Chem., Int. Ed. Engl. 1995, 34, 1478. (d) Eickmeier, C.; Junga, H.; Matzger,
A. J.; Scherhag, F.; Shim, M.; Vollhardt, K. P. C. Angew. Chem., Int. Ed.
Engl. 1997, 36, 2103. (e) Eickmeier, C.; Holmes, D.; Junga, H.; Matzger, A.
J.; Scherhag, F.; Shim, M.; Vollhardt, K. P. C. Angew. Chem., Int. Ed. 1999,
38, 800.
(6) Bond, F. T.; Jones, H. L.; Scerbo, L. Org. Photochem. Synth. 1971, 1,
33.
(7) The spectral data are in agreement with those reported: ref 4. However,
a DEPT experiment revealed that the previously assigned bridgehead and
methylene carbons should be reversed.
(8) Mohler, D. L.; Vollhardt, K. P. C.; Wolff, S. Angew. Chem., Int. Ed.
Engl. 1990, 29, 1151.
(9) Mohler, D. L.; Vollhardt, K. P. C.; Wolff, S. Angew. Chem., Int. Ed.
Engl. 1995, 34, 563.
(10) The 2,3,6,7,10,11-hexakis(trimethylsilyl) substitution of 1 also makes
the central ring inert to catalytic hydrogenation even under forcing conditions.
See ref 8.
(11) Computational prediction for the planar structure of 3 has been recently
performed by Baldridge and Siegel, see: Baldridge, K. K.; Siegel, J. S. J.
Am. Chem. Soc. 2001, 123, 1755-1759.
(12) As to the COT derivative with a planar π-system, only perfluorocy-
clobutene-annelated derivative has been reported, see: Einstein, F. W. B.;
Willis, A. C. J. Chem. Soc., Chem. Commun. 1981, 526.
(4) Frank, N. L.; Baldridge, K. K.; Siegel, J. S. J. Am. Chem. Soc. 1995,
117, 2102.
(5) Bu¨rgi, H.-B.; Baldridge, K. K.; Hardcastle, K.; Frank, N. L.; Gantzel,
P.; Siegel, J. S.; Ziller, J. Angew. Chem., Int. Ed. Engl. 1995, 34, 1454.
10.1021/ja003512m CCC: $20.00 © 2001 American Chemical Society
Published on Web 02/02/2001

Communications to the Editor
J. Am. Chem. Soc., Vol. 123, No. 8, 2001 1769
Table 1. Calculated Aromatic Stabilization Energies (ASE,
kcal/mol), Magnetic Susceptibility Exaltations (Λ, ppm cgs), and
the Nucleus Independent Chemical Shifts (NICS, ppm) at the Ring
Centersa
compd
benzene
2
planar COT (D4h)
3
ASEb
Λc
NICSd
-34.1
-34.0
2.9
-16.2
-8.4
61.1
-9.7
-8.0
27.2
10.6
4.1
17.2
a B3LYP/6-31G* geometries were employed. b B3LYP/6-311+G**.17
c CSGT-HF/6-31+G**.17 d GIAO-HF/6-31+G**.
Figure 1. 1H NMR (300 MHz) spectrum of 3 in CDCl3.
ticity and antiaromaticity. Since the bridgehead hydrogens in 2
and 3 are rigidly fixed in the same plane of π-systems, they are
1
most susceptible to the effect of the ring current, and their H
NMR chemical shift is the most useful indicator for characterizing
aromaticity/antiaromaticity. Surprisingly, the difference in 1H
NMR chemical shifts of bridgehead protons between 2 and 3 was
found to be only 0.18 ppm (δ 3.21 for 2 in CDCl3), despite the
change in the π-system from formally aromatic 6π to formally
antiaromatic 8π.
To evaluate the aromaticity of 2 and the antiaromaticity of 3,
the nucleus independent chemical shifts (NICS),16 the magnetic
susceptibility exaltations (Λ), and the aromatic stabilization
energies (ASE) were calculated for 2 and 3 as well as for benzene
and the D4h planar COT (Table 1).17 To compensate for the strain,
reference molecules used to estimate the ASE were optimized
under appropriate constraints.18
Figure 2. ORTEP drawing of 3 (50% probability).
δ 3.03 (t, J3 ) 2.7 Hz, 8H, HA), 1.71 (m, 8H, HB), 1.13 (dd,
AB
J3 ) 3.6 Hz, J3 ) 1.5 Hz, 8H, HC); 13C NMR δ 132.25
CC′
BC
(olefinic), 47.19 (bridgehead), 37.11 (methylene).
The molecular structure of 3 was determined by X-ray
crystallography (Figure 2).14 As expected, the central eight-
membered ring is planar, and all internal bond angles of the COT
ring are almost 135° (134.8-135.2°, average 135.0(1)°). The bond
lengths are alternately shorter (average 1.331(1) Å) and longer
(average 1.500(1) Å) with the shorter bonds exocyclic to
bicycloannelation. Thus, compound 3 is proven to be the first
non-benzoannelated hydrocarbon with a completely planar COT
ring.
The UV-vis spectrum of 3 in CH2Cl2 exhibited the longest
wavelength absorption at 459 nm (ꢀ 130). This value is red-shifted
by 177 nm compared with that of tetrakis(bicyclo[2.2.2]octeno)-
COT (7)15 (λmax ) 282 nm, ꢀ 800) with the tub-shaped COT ring,
indicating that planarization of the COT ring in 3 causes a
substantial decrease in the HOMO-LUMO gap.
The remarkable influence of planarization on the electronic
structure of COT was also manifested by the oxidation potential
of 3 as measured by cyclic voltammetry in CH2Cl2. COT 3
exhibited a well-defined reversible oxidation wave at +0.07 V
versus Fc/Fc+ and an irreversible one at +0.76 V. These potentials
are lower by ∼0.4 V than corresponding values in 7, which is
evidently due to planarization that raises the HOMO of COT.
In view of the cyclohexatriene-like geometry of the benzene
ring in 2 and the planar geometry of the COT ring in 3, the most
interesting aspect of these molecules appears to be their aroma-
The large negative NICS value of 2 (-8.0), comparable to that
of D6h benzene itself (-9.7), indicates that 2 retains a substantial
degree of aromaticity, which is consistent with the previous results
of theoretical calculations for hypothetical cyclohexatriene.16,19
The considerable aromatic character of 2 was also confirmed by
the negative Λ value (-8.4) and the large ASE (34.0 kcal/mol).
On the other hand, although the destabilization energy of 3
slightly exceeds that of the D4h planar COT, the NICS value (10.2)
as well as the Λ value (17.2) is considerably reduced, compared
with those of the D4h planar COT. Such a reduction of antiaro-
maticity of 3 might appear to be attributed to an enhanced bond
alternation (∆R ) 0.161 Å) relative to that in the D4h planar COT
(∆R ) 0.132 Å). However, the NICS value of the hypothetical
molecule, tetrakis(cyclobuteno)COT, which has the same degree
of bond alternation (∆R ) 1.491 Å - 1.336 Å ) 0.155 Å) as 3,
reveals the considerable retention of antiaromaticity (NICS: 20.9).
Thus, electronic interaction between 1,3-bridged cyclobutane
subunits in the bicyclohexene frameworks and the planar COT
ring must be responsible for the change in the inherent magnetic
properties and the reduced antiaromaticity of the present 8π
electronic system 3.
Acknowledgment. This work was supported by a Grant-in Aid for
Scientific Research on Priority Areas (A) (No. 10146102 and No.
12042240) from the Ministry of Education, Science, Sports and Culture,
Japan.
1
(13) The H and 13C NMR signals of 3 were assigned on the basis of the
1
DEPT experiment, the CH-COSY spectrum, and the H coupling patterns.
(14) Crystallographic data for C24H24 (3): The space group is P21/n,
monoclinic, with unit-cell demensions a ) 9.9040(10) Å, b ) 8.7204(9) Å,
Supporting Information Available: Experimental and calculation
details and X-ray crystallographic data of 3 (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
c ) 10.5525(11) Å, â ) 114.719(2)°, V ) 827.87(15) Å3, Z ) 2, Dcalc
)
1.253 Mg/m3. Intensity data were collected at 300 K on a Bruker SMART
APEX diffractometer with Mo KR radiation and graphite monochromator.
The structure was solved by direct methods and refined by the full-matrix
least-squares on F2 (SHELXTL). A total of 6948 reflections were measured
and 2574 were independent. Final R1 ) 0.0588, wR2 ) 0.1356 (I > 2σ(I)),
and GOF ) 0.908 (for all data, R1 ) 0.0886, wR2 ) 0.1451). Full details are
described in the Supporting Information.
JA003512M
(16) Schleyer, P. v. R.; Maerker, C.; Dransfeld, A.; Jiao, H.; Hommes, N.
v. E. J. Am. Chem. Soc. 1996, 118, 6317.
(17) Calculations were performed using Gaussian 98 program.
(18) See Supporting Information for details.
(15) Komatsu, K.; Nishinaga, T.; Aonuma, S.; Hirosawa, C.; Takeuchi,
K.; Lindner, H. J.; Richter, J. Tetrahedron Lett. 1991, 32, 6767.
(19) Fleischer, U.; Kutzelnigg, W.; Lazzeretti, P.; Mu¨hlenkamp, V. J. Am.
Chem. Soc. 1994, 116, 5298.