Nonplanar aromatic compounds. 5. A strategy for the synthesis of cis-10b,10c-dimethyl-10b,10c-dihydropyrenes. First crystal structure of a cis-10b,10c-dimethyl-10b,10c-dihydropyrene

Article abstract of DOI:10.1021/ja002901d

Through the use of a potentially removable tether, a heavily substituted 10b,10c-dimethyl-10b,10c-dihydropyrene (DMDHP), 20, was synthesized exclusively as the cis-isomer. It exists as the major component (20:1) in an equilibrium with its valence isomer s

Full text of DOI:10.1021/ja002901d

4704
J. Am. Chem. Soc. 2001, 123, 4704-4708
Nonplanar Aromatic Compounds. 5.1 A Strategy for the Synthesis of
cis-10b,10c-Dimethyl-10b,10c-dihydropyrenes. First Crystal Structure
of a cis-10b,10c-Dimethyl-10b,10c-dihydropyrene
Graham J. Bodwell,*,† John N. Bridson, Shu-Ling Chen, and Raymond A. Poirier
Contribution from the Department of Chemistry, Memorial UniVersity of Newfoundland,
St. John’s, NF, Canada A1B 3X7
ReceiVed August 4, 2000
Abstract: Through the use of a potentially removable tether, a heavily substituted 10b,10c-dimethyl-10b,10c-
dihydropyrene (DMDHP), 20, was synthesized exclusively as the cis-isomer. It exists as the major component
(20:1) in an equilibrium with its valence isomer syn-[2.2]metacyclophanediene 19. An X-ray crystal structure
determination of 20, a cis-(2,7)-10b,10c-dihydropyrenophane, provided the first experimental measurements
of the cis-DMDHP skeleton. The observed bond alternation in the [14]annulene was found to be larger than
that of the corresponding trans-DMDHP framework. Prior MMPI calculations, on which previous discussion
of the structure of the cis-DMDHP system had been based, are in very good agreement with the experimental
results. Our own DFT calculations predict a more symmetric and more bond equalized structure than was
observed in 20.
Introduction
Scheme 1
By virtue of its 14π electron perimeter, the 10b,10c-
dihydropyrene framework has been the subject of considerable
experimental and theoretical interest.2,3 The best studied mem-
bers of this class of compounds are the trans-10b,10c-dimethyl-
10b,10c-dihydropyrenes (trans-DMDHPs), the internal methyl
groups of which have been used to great effect as probes for
aromaticity.3 The most widely used synthetic approach to 10b,-
10c-dihydropyrenes is the “cyclophane route”,4 which involves
the synthesis of a 2,11-dithia[3.3]metacyclophane 1, its conver-
sion to a [2.2]metacyclophane-1,9-diene 2, and the thermally
and/or photochemically driven valence isomerization thereof to
the corresponding dihydropyrene 3 (Scheme 1). In most cases,
the dihydropyrene is the thermodynamically favored isomer.2,3
The majority of known DMDHPs are trans isomers, and this
has its origin in the general preference for the anti conformation
of their progenitors, the [2.2]metacyclophane-1,9-dienes 2.
Owing to the difficulty of preparing synthetically useful amounts
of the corresponding syn-[2.2]metacyclophane-1,9-dienes, the
corresponding cis-DMDHPs are comparatively rare entities.5
Structural details of the trans-DMDHP skeleton have been
measured crystallographically,3c,6,7 and these are in good agree-
ment with those generated from computational studies,3a,7,8
provided that electron correlation9 is employed. Important
structural features of the trans-DMDHP system are the high
degree of bond equalization and relative planarity of the [14]-
annulene unit.
† E-mail: gbodwell@mun.ca. Fax: (709)-737-3702.
(1) Paper 4 in this series: Bodwell, G. J.; Fleming, J. J.; Miller, D. O.
Tetrahedron. 2001, 57, 3579-3587.
(2) (a) Mitchell, R. H. In AdVances in Theoretically Interesting Molecules;
Thummel, R. P., Ed.; JAI Press: London, 1989; Vol. 1, pp 135-199. (b)
Mitchell, R. H. Eur. J. Org. Chem. 1999, 2695-2703. (c) Mitchell, R. H.;
Vinod, T. K.; Bodwell, G. J.; Weerawarna, K. S.; Anker, W.; Williams, R.
V.; Bushnell, G. W. Pure Appl. Chem. 1986, 58, 15-24.
(3) (a) Mitchell, R. H.; Chen, Y.; Iyer, V. S.; Lau, D. Y. K.; Baldridge,
K. K.; Siegel, J. S. J. Am. Chem. Soc. 1996, 118, 2907-2911. (b) Mitchell,
R. H.; Iyer, V. S. J. Am. Chem. Soc. 1996, 118, 2903-2906. (c) Mitchell,
R. H.; Iyer, V. S.; Khalifa, N.; Mahadevan, R.; Venugopalan, S.;
Weerawarna, S. A.; Zhou, P. J. Am. Chem. Soc. 1995, 117, 1514-1532.
(4) Mitchell, R. H. Heterocycles 1978, 11, 563-586.
(5) (a) Mitchell, R. H.; Chaudhary, M.; Kamada, T.; Slowey, P. D.;
Williams, R. V. Tetrahedron 1986, 42, 1741-1744. (b) Mitchell, R. H.;
Mahadevan, R. Tetrahedron Lett. 1981, 22, 5131-5134. (c) Kamp, D.;
Boekelheide, V. J. Org. Chem. 1978, 43, 3475-3477. (d) Mitchell, R. H.;
Boekelheide, V. J. Am. Chem. Soc. 1974, 96, 1547-1557. (e) Mitchell, R.
H.; Boekelheide, V. Chem. Commun. 1970, 1555-1557.
Until now, there has been no X-ray crystallographic structure
determination of a cis-DMDHP10 and all previous discussion
(6) (a) Hanson, A. W. Acta Crystallogr. 1967, 23, 476-481. (b) Hanson,
A. W. Acta Crystallogr. 1965, 18, 599-604.
(7) Williams, R. V.; Edwards, W. D.; Vij, A.; Tolbert, R. W.; Mitchell,
R. H. J. Org. Chem. 1998, 63, 3125-3127.
(8) (a) Laali, K. K.; Bolvig, S.; Raeker, T. J.; Mitchell, R. H. J. Chem.
Soc., Perkin Trans 2 1996, 2635. (b) Mitchell, R. H.; Iyer, V. S.; Mahadevan,
R.; Venugopalan, S.; Zhou, P. J. Org. Chem. 1996, 61, 5116-5120.
(9) Borden, W. T.; Davidson, E. R. Acc. Chem. Res. 1996, 29, 67-75.
10.1021/ja002901d CCC: $20.00 © 2001 American Chemical Society
Published on Web 04/27/2001

Nonplanar Aromatic Compounds. 5
J. Am. Chem. Soc., Vol. 123, No. 20, 2001 4705
Scheme 2
Scheme 3a
of their structures has been based on computational work. A
bowl, or saucer, shaped [14]annulene with greater bond alterna-
tion than that of the trans-DMDHP system has been calculated.11
Thus the original goals of this work were the development of
a reliable synthetic route to cis-DMDHPs and the X-ray
crystallographic characterization of the cis-DMDHP skeleton.
We now present our initial findings.
a Key: (a) Br(CH2)10Br, K2CO3, DMF, 80 °C; (b) 1,3,5-trioxane,
Synthesis of cis-20. The chief problem with existing synthetic
routes to syn-[2.2]metacyclophane-1,9-dienes is the strong
preference for the adoption of the anti conformation during the
formation of the 2,11-dithiacyclophane 1 (in the case of
internally substituted systems5,12) or the subsequent bridge
contraction to a [2.2]metacyclophane.13 To circumvent this
problem, the use of a temporary third bridge, a tether, in the
cyclophane system was envisaged (Scheme 2). An important
consideration was that the tether be short enough to ensure that
dithiacyclophane 4 forms and remains exclusively in the syn
conformation but long enough to permit valence isomerization
of syn-5 to cis-6, during which the attachment points of the
tether move away from one another substantially. In related
work,14 we observed that a 13-atom tether between the 5 and
13 positions of the [2.2]metacyclophane skeleton is long enough
to allow syn/anti isomerism at room temperature, but a 12-atom
tether is not. Thus a 12-atom tether was chosen for initial
investigation.
35% HBr/HOAc, reflux.
Scheme 4a
For the sake of synthetic simplicity, 2,4,6-trimethylphenol 7
was chosen as the starting material, despite the presence of
extraneous methyl groups. The original synthetic plan (Scheme
3) commenced with the alkylation of 7 to afford 8 (74%).
Attempted bromomethylation gave only 8% of the desired
tetrabromide 10, extensive tether cleavage having occurred
during this reaction. Indeed the major product was the dibromide
9 (86%). Reversing the order of these reactions proved to be
no more successful. Bromomethylation of 7 gave dibromide 9
in 99% yield, but attempted introduction of the tether to give
10 afforded only intractable material. It had been hoped that
the hindered nature of both the benzylic bromides and the
hydroxy group of 9 would disfavor self-alkylation reactions,
but this was clearly not the case.
The successful route (Scheme 4) involved the protection of
the benzylic bromides prior to the introduction of the tether.
Dibromide 9 was first treated with potassium acetate to provide
diacetate 11 (89%) and the tether could now be installed
satisfactorily (69%). LiAlH4 reduction of the resulting tetraac-
etate 12 provided tetraol 13 (77%) and reaction with PBr3 then
furnished tetrabromide 10 (92%).
a Key: (a) KOAc, CH3CN, reflux, 22 h; (b) Br(CH2)10Br, K2CO3,
DMF, 80 °C, 19 h; (c) LiAlH4, THF, r.t., 21 h; (d) PBr3, CH2Cl2, r.t.,
14 h; (e) KSAc, CH3CN, reflux, 23 h; (f) KOH, DMF, 60 °C, 4 h; (g)
I2, pyridine, CH2Cl2/EtOH, r.t., 5 d; (h) (Me2N)3P, benzene, reflux, 31
h; (i) (MeO)2CHBF4, CH2Cl2, 0 °C to rt, 11-12 h; (j) t-BuOK, THF,
rt, 3 h; (k) t-BuOK, 1:1 THF:t-BuOH, 0 °C to rt, 1 h.
At this point, the crucial step of ring closure of 10 to give
dithiacyclophane 17 had to be addressed. Attempts to ac-
complish this transformation using Na2S/Al2O3,15 which we had
found to be a very effective reagent in related systems,1,14,16,17
afforded none of the desired product. This is presumably due
to the congested environment of the benzylic bromides in 10.
To negotiate this obstacle, the oxidative coupling of thiols to
disulfides,18 which can be converted into thioethers,19 was then
considered. This approach was attractive because the closure
(10) Repeated attempts to grow crystals of the parent cis-10b,10c-
dimethyl-10b,10c-dihydropyrene have consistently failed to afford crystals
of sufficient quality for X-ray crystal structure determination. Mitchell, R.
H. Private communication.
(11) Allinger, N, L.; Sprague, J. T. J. Am. Chem. Soc. 1973, 95, 3893-
3907.
(12) Mitchell, R. H.; Boekelheide, V. Tetrahedron Lett. 1970, 1197-
1202.
(13) Mitchell, R. H.; Vinod, T. K.; Bushnell, G. W. J. Am. Chem. Soc.
1990, 112, 3487-3497.
(14) Bodwell, G. J.; Houghton, T. J.; Kennedy, J. W. J.; Mannion, M.
R. Angew. Chem., Int. Ed. Engl. 1996, 35, 2121-2123.
(15) Bodwell, G. J.; Houghton, T. J.; Koury, H. E.; Yarlagadda, B. Synlett
1995, 751-752.
(16) Bodwell, G. J.; Houghton, T. J.; Kennedy, J. W. J.; Mannion, M.
R. Angew. Chem., Int. Ed. Engl. 1996, 35, 1320-1321.
(17) Bodwell, G. J.; Bridson, J. N.; Houghton, T. J.; Yarlagadda, B.
Tetrahedron Lett. 1997, 38, 7475-7478.

4706 J. Am. Chem. Soc., Vol. 123, No. 20, 2001
Bodwell et al.
of the bridges involves reaction at homobenzylic sulfur atoms,
as opposed to sterically encumbered benzylic carbon atoms. In
addition, there is precedent for the use of this methodology in
the synthesis of metacyclophanes.20
The required tetrathiol 15 was prepared from tetrabromide
10 by treatment with potassium thioacetate to afford 14 (95%)
and hydrolysis of 14 with KOH/DMF (97%). Reaction of 15
with iodine and pyridine under high dilution conditions gave
tetrathiacyclophane 16 (31%) along with a dimeric product in
28% yield.21 Desulfurization of 16 with HMPT then afforded
the desired dithiacyclophane 17 (21%). No other nonpolar
products were obtained from this reaction.
All that remained now was the bridge contraction and
formation of the double bonds. Fortunately the standard protocol
did not present any problems. Methylation of the sulfur atoms
of 17 with (MeO)2CHBF4 (Borch reagent) followed by Stevens
rearrangement afforded the ring-contracted products 18 (95%)
as a mixture of isomers. Finally, after 2-fold S-methylation of
18 with Borch reagent, Hofmann elimination led to the
formation of a ca. 1:20 mixture (1H NMR) of the cyclophane-
diene 19 and its valence isomeric cis-DMDHP 20 in a combined
yield of 70% from 18. The overall combined yield of 19 and
20 for the 13-step sequence starting from 7 was 1.7%, the
majority of the losses having been suffered during the conversion
of 15 to 17 (7% yield over two steps).
Figure 1. ORTEP representation of cis-20 in the crystal.
cis-DMDHP moiety. Analysis of the chemical shifts of the
external aromatic protons leads to the same conclusion.
Discussion
The external methyl protons of 20 (δ 3.02) are deshielded
by 1.05 ppm from those of cyclophanediene 19 (δ 1.93). The
bridge protons of 20 appear as a series of 4H multiplets centered
at δ 4.11, 1.17, -0.09, -0.39, and -0.83. Although the
observation of high field shifted protons was fully expected, it
is the first instance of NMR active nuclei being held directly
under the concave face of a cis-DMDHP. Interestingly, the
chemical shifts of the bridge protons of 20 are very similar to
those of the [n](2,7)pyrenophane with the same tether, 1,12-
dioxa[12](2,7)pyrenophane 24: δ 4.31 (4H), 1.41 (4H), -0.14
(8H), -0.63 (4H).24
X-ray Crystal Structure of 20 and DFT-Calculated
Structure of cis-21. Prior to this work, all structural data for
the cis-DMDHP skeleton was derived from MMPI calculations.
Fortunately, slow evaporation of a deep emerald green solution
of 19 and 20 in dichloromethane/heptane afforded dark red-
green birefringent crystals of 20, which were of suitable quality
for an X-ray crystal structure determination. The crystal structure
(Figure 1) was solved with ease, but a disorder problem of some
kind in the tether was evident. This did not appear to be a case
of two alternate conformations, but rather a high degree of
“looseness”, which could not be modeled with any degree of
confidence. For comparison purposes, the structure of cis-21
was optimized at the B3LYP/6-31G(d) level of theory using
Gaussian 94.25 Selected structural data are presented in Figure
2 and Tables 1-4 along with previously calculated (MMPI)
data for cis-21.
1H NMR Spectrum of 20. The H NMR spectrum of 20
1
exhibits singlets at δ 8.66 and -1.78 for the external protons
and internal methyl protons, respectively. By comparison, the
corresponding protons of the parent cis-DMDHP 21 are
observed at δ 8.74 and -2.06, respectively.5d,e The chemical
shift difference between the internal methyl protons of 20 and
those of cis-21 is ≈0.3 ppm, which is much the same as that
(≈0.4 ppm) between those of the parent trans-DMDHP trans-
2122 and 22,23 the closest known trans-DMDHP analogue of
20. In light of the sensitivity of the internal methyl protons’
chemical shifts to physical changes in the dihydropyrene
skeleton,2 these observations suggest that the presence of the
bridge in 20 does not significantly affect the geometry of the
(18) Block, E. Reactions of Organosulfur Compounds; Academic
Press: New York, 1978.
(19) Harpp, D. N.; Gleason, J. G. J. Am. Chem. Soc. 1971, 93, 2437-
2445.
(20) (a) Raasch, M. S. J. Org. Chem. 1979, 44, 2629-2632. (b) Wong,
D. T.-M.; Marvel, C. S. J. Polym. Sci., Polym. Chem. 1976, 14, 1637-
1644. (c) Tam, T.-F.; Wong, P.-C.; Siu, T.-W.; Chan, T.-L. J. Org. Chem.
1976, 41, 1289-1291. (d) Houk, J.; Whitesides, G. M. Tetrahedron 1989,
45, 91-102.
(21) A similar byproduct was obtained in the oxidative self-coupling of
a lower homologue of 15, the structure of which was determined
crystallographically. By analogy, structure 23 is assigned to the dimer of
16.
The prediction of a bowl-shaped [14]annulene moiety in the
cis-DMDHP skeleton by the MMPI and DFT calculations is
(24) Bodwell, G. J.; Bridson, J. N.; Kennedy, J. W. J.; Mannion, M. R.
Unpublished results. Manuscript in preparation.
(25) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.;
Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T.; Petersson, G.
A.; Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski,
V. G.; Ortia, J. V.; Foresman, J. B.; Peng, C. Y.; Ayala, P. Y.; Chen, W.;
Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.;
Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. P.; Head-
Gordon, M.; Gonzalez, C.; Pople, J. A. Gaussian 94, Revision B.3; Gaussian
Inc.: Pittsburgh, PA, 1995.
(22) Renfroe, H. B.; Gurney, J. A.; Hall, L. A. R. J. Org. Chem. 1972,
37, 3045-3052.
(23) (a) Mitchell, R. H.; Yan, J. S. H.; Dingle, T. W. J. Am. Chem. Soc.
1982, 104, 2551-2559. (b) Boekelheide, V.; Phillips, J. B. J. Am. Chem.
Soc. 1967, 89, 1695-1704.

Nonplanar Aromatic Compounds. 5
J. Am. Chem. Soc., Vol. 123, No. 20, 2001 4707
Table 2. Bond Anglesa
20
cis-21
angleb
X-ray
MMPI
B3LYP/6-31G(d)
C(4)-C(15)-C(14)
C(4)-C(15)-C(16)
C(4)-C(15)-C(17)
C(14)-C(15)-C(16)
C(14)-C(15)-C(17)
C(16)-C(15)-C(17)
C(7)-C(16)-C(11)
C(7)-C(16)-C(15)
C(7)-C(16)-C(18)
C(11)-C(16)-C(15)
C(11)-C(16)-C(18)
C(15)-C(16)-C(18)
111.9(2)
114.6(2)
102.0(2)
113.3(2)
102.7(2)
111.2(2)
111.9(2)
113.5(2)
102.2(2)
115.0(2)
102.1(2)
110.7(2)
109.1
113.7
104.0
113.7
104.0
112.9
109.1
113.7
104.0
113.7
104.0
112.9
111.0
114.0
102.8
114.0
102.8
111.2
111.0
114.0
102.8
114.0
102.8
111.2
a Bond angles in degrees. b Numbering as shown in Figure 2.
Table 3. Observed and Calculated Torsion Anglesa
20
cis-21
bondb,c
X-ray
-3.2(4)
MMPI B3LYP/6-31G(d)
C(1)-C(2)-C(3)-C(4)
C(2)-C(3)-C(4)-C(5)
C(3)-C(4)-C(5)-C(6)
C(4)-C(5)-C(6)-C(7)
C(5)-C(6)-C(7)-C(8)
C(6)-C(7)-C(68)-C(9)
C(7)-C(8)-C(9)-C(10)
C(8)-C(9)-C(10)-C(11)
7
4.7
-167.7
160.1
0.0
-173.3(2) -176
162.1(2)
160
-3
-2.5(4)
-155.4(3) -155
-160.1
167.7
4.7
161.5(2)
5.4(4)
164
3
7
-1.9(4)
4.7
C(9)-C(10)-C(11)-C(12) -174.2(2) -176
-167.7
160.1
0.0
C(10)-C(11)-C(12)-C(13)
C(11)-C(12)-C(13)-C(14)
162.3(3)
-3.6(4)
160
-3
Figure 2. Bond lengths (Å) and torsion angles (deg) in cis-20.
Table 1. Measured and Calculated Dihydropyene Bond Lengthsa
C(12)-C(13)-C(14)-C(1) -154.1(3) -155
-160.1
167.7
4.7
C(13)-C(14)-C(1)-C(2)
C(14)-C(1)-C(2)-C(3)
C(17)-C(15)-C(16)-C(18)
162.0(2)
5.4(4)
-6.8(2)
164
3
8
20
cis-21
0.0
bondb
X-ray
MMPI
B3LYP/6-31G(d)
a Bond angles in degrees. b The sign is positive if, when looking
from atom 2 to atom 3, a clockwise motion of atom 1 superimposes it
on atom 4. c Numbering as shown in Figure 2.
C(1)-C(2)
1.393(4)
1.383(3)
1.396(3)
1.385(4)
1.379(4)
1.377(4)
1.403(3)
1.389(4)
1.401(4)
1.383(3)
1.392(4)
1.384(4)
1.374(4)
1.405(3)
1.530(3)
1.516(3)
1.525(3)
1.519(3)
1.610(3)
1.567(3)
1.574(3)
1.398
1.398
1.402
1.405
1.389
1.405
1.402
1.398
1.398
1.402
1.405
1.389
1.405
1.402
1.519
1.519
1.519
1.519
1.572
1.555
1.555
1.398
1.398
1.397
1.398
1.392
1.398
1.397
1.398
1.398
1.397
1.398
1.392
1.398
1.397
1.534
1.534
1.534
1.534
1.630
1.571
1.571
C(2)-C(3)
C(3)-C(4)
C(4)-C(5)
the ethano unit of the trans-DMDHP skeleton is held in a
staggered conformation and the [14]annulene moiety is not far
from being perfectly planar.7 Thus it is not surprising that the
unusual structural features observed in the crystal structure of
20 are all associated with the ethano unit.
The crystal structure of 20 revealed that the central ethano
unit is not fully eclipsed, the C(17)-C(15)-C(16)-C(18)
torsion angle being 6.8°. This is very close to the angle predicted
by MMPI calculations (8°). On the other hand, the DFT
calculations predict a symmetric structure for cis-21 containing
two planes of symmetry and a corresponding torsion angle of
0°. To verify that this structure was a true energy minimum,
frequencies were calculated at the B3LYP/6-31G(d) level of
theory and no imaginary frequencies were found.
In the crystal structure of 20, the bond between the two central
sp3 carbon atoms {C(15)-C(16)} is quite long (1.610 Å). This
is significantly longer than the MMPI-calculated value (1.572
Å) and slightly shorter than the DFT-calculated value (1.630
Å). The bonds between the quaternary sp3 carbons and their
attached internal methyl carbon atoms (1.567 and 1.564 Å) are
also observed to be somewhat elongated and quite close to their
calculated values (MMPI, 1.555 Å; DFT, 1.571 Å).
The bond angles about the central carbon atoms of 20 are all
distorted from the tetrahedral angle. For C(15), the angles C(4)-
C(15)-C(17) (102.0°) and C(14)C(15)-C(17) (102.7°) are
substantially compressed and the remaining four angles are in
the range 111.2-114.6° (Table 2). A similar situation exists
around C(16), where the angles C(7)C(16)-C(18) (102.2°) and
C(5)-C(6)
C(6)-C(7)
C(7)-C(8)
C(8)-C(9)
C(9)-C(10)
C(10)-C(11)
C(11)-C(12)
C(12)-C(13)
C(13)-C(14)
C(14)-C(1)
C(4)-C(15)
C(7)-C(16)
C(11)-C(16)
C(14)-C(15)
C(15)-C(16)
C(15)-C(17)
C(16)-C(18)
a Bond lengths in angstroms. b Numbering as shown in Figure 2.
confirmed by the crystallographic results. The origin of the
nonplanarity of the [14]annulene system can be traced back to
the geometric requirements of the central ethano moiety. A
planar [14]annulene skeleton would force the internal methyl
groups to well within van der Waals contact and impose severe
angle strain about the two central carbon atoms. Adoption of a
bowl shape relieves these forms of strain but still leaves the
ethano moiety in an eclipsed conformation. Torsional strain in
the ethano unit can be relieved by rotation about the central
C-C bond, but it is counterbalanced by an increase in torsional
strain of the peripheral [14]annulene system. By comparison,

4708 J. Am. Chem. Soc., Vol. 123, No. 20, 2001
Table 4. Structural Data for 20, cis-21, and trans-21
Bodwell et al.
20
cis-21
trans-21
MMPI
X-ray
MMPI
X-raya
AM1/CI4
range
dh
1.374-1.405
1.389-1.405
1.392-1.398
1.397
1.389-1.398
1.388-1.397
1.393
1.392
0.003
0.003
0.004
0.005
2.5
1.400-1.405
1.393-1.397
1.389
0.008
0.016
11.2
1.400
0.003
0.011
11
1.402
0.002
0.004
6
1.396
0.001
0.003
n.a.
∆h dh
∆ dhmax
τj
0.001
0.005
10.5
2.7
τmax
25.9
25
19.9
4.1
5.0
11
n.a.
ref
2a
7
2a
7
a The unit cell contains two crystallographicaly independent molecules.
C(11)C(16)-C(18) (102.1°) are compressed and the other four
angles lie in the range 110.7-115.0°. These distortions are
consistent with a steric repulsion between the nearly eclipsed
internal methyl groups. Both the MMPI and DFT calculations
predict this distortion, although the DFT results are much closer
to the observed data.
deviation from coplanarity (τmax) is 25.9°. This is in excellent
agreement with the MMPI calculations (Tables 3 and 4). The
DFT calculations afford lower numbers (a less distorted annu-
lene), primarily due to the prediction of a symmetric structure
containing a fully eclipsed central ethano unit. As mentioned
earlier, rotation about the central bond causes a twist in the [14]-
annulene system, and this necessarily results in an increase in
Another consequence of the proximity of the internal methyl
groups is that all of their attached hydrogens were located from
a difference map at an early stage of the refinement, which
suggests a higher than normal barrier to rotation at room
temperature in the solid state. However, the signals of the
internal methyl groups are no broader than those of the external
τj and τmax
.
Having established a viable synthetic route to the cis-DMDHP
skeleton, we are now in the process of investigating whether
the bridge of 20 and its homologues can be cleaved to afford
free cis-DMDHPs and how the length of the tether affects the
valence isomerization between a tethered syn-[2.2]metacyclo-
phanediene and the corresponding cis-dimethyldihydro-
pyrenophane.
1
methyl groups in the ambient temperature H NMR spectrum
and only slightly broader at -90 °C. This suggests that rotation
is not significantly restricted in solution, even at low temper-
ature.
A particularly unusual feature of the crystal structure was
the set of four independent regions of lower than expected
electron density observed on the final difference map. These
were located beneath the four bonds C(3)-C(4), C(7)-C(8),
C(10)-C(11), and C(14)-C(1). The origin of this phenomenon
is unclear.
Conclusions
Dimethyldihydropyrenophane 20 was synthesized exclusively
as its cis isomer by using a 12-atom tether to ensure formation
and maintenance of only the syn conformation of the metacy-
clophanes that preceded it. A crystal structure determination of
20 revealed that the cis-DMDHP skeleton is indeed less bond
equalized and more distorted from planarity than the corre-
sponding trans-DMDHP skeleton. The structure predicted for
cis-21 by the MMPI calculations is in excellent agreement with
the observed cis-DMDHP moiety of 20, right down to the twist
of the central ethano unit. DFT calculations predict a more
symmetric and more bond equalized cis-DMDHP structure.
Structural features of the peripheral [14]annulene systems of
both cis- and trans-DMDHPs have been used to comment on
the aromatic character of the system.2 Key structural parameters
associated with the [14]annulene moiety of 20 are summarized
in Table 4 along with the corresponding data for cis-21 and
trans-21. The bond lengths of 20 range from 1.374 to 1.405 Å,
and their average length (dh) is 1.389 Å. The computational
methods tend to overestimate the bond lengths of both the cis-
and trans-DMDHP skeleton. The average deviation from dh (∆h
dh) is 0.008 Å, and the largest deviation from dh (∆ dhmax) is 0.016
Å. Both the MMPI and our DFT calculations predict a
significantly more bond equalized [14]annulene system than was
observed in 20, especially the DFT calculations. The actual
degree of bond equalization in the cis-DMDHP skeleton of 20
is considerably lower than that observed in the crystal structure
of trans-21, which is what was predicted by MMPI calculations
on the two systems.
Acknowledgment. Financial support of this work from the
Natural Sciences and Engineering Research Council of Canada
(NSERC) and Xerox Research Centre of Canada (XRCC) is
gratefully acknowledged.
Supporting Information Available: Full experimetal details
1
and characterization data for all new compounds. H NMR
spectra for 8-20 and 23. 13C NMR spectra for all 8-17, 19,
20, and 23. X-ray structural data for 20. This material is
available free of charge via the Internet at http://pubs.acs.org.
The average deviation of the torsional angles from 0 or 180°
(τj) (a coplanar arrangement) is 11.2°, and the maximum
JA002901D