Evidence for Homo-Conjugation between Two Revolving 14π-Electron Systems in 10b-Methyl-10c-[2-(10b,10c-dimethyl-10b,10c-dihydropyrenyl)]-10b, 10c-dihydropyrene

Article abstract of DOI:10.1021/ja0382260

Ring contraction followed by an elimination reaction on anti-9-methyl-18-trans-2-(10b,10c-dimethyl-10b,10c-dihydropyrenyl)-2,11-dithia[3,3]metacyclophane gave the desired compound 10b-methyl-10c-[2-(10b,10c-dimethyl-10b,10c-dihydropyrenyl)]-10b,10c-dihydropyrene. ¹H NMR spectroscopic analysis indicated a ring current effect over a considerable distance from the macro-molecular plane of each of the aromatic rings with the two π systems freely rotating. Bathochromic shifts and peak broadening in its electronic spectrum clearly supports the presence of through-space π?π interactions between the two aromatic rings. This should serve as a good model to verify homo-conjugation effect in such a novel system where the two π systems are freely revolving. Copyright

Full text of DOI:10.1021/ja0382260

Published on Web 11/04/2003
Evidence for Homo-Conjugation between Two Revolving 14π-Electron
Systems in 10b-Methyl-10c-[2-(10b,10c-dimethyl-10b,10c-dihydropyrenyl)]-
10b,10c-dihydropyrene
Jianping Jiang and Yee-Hing Lai*
Department of Chemistry, National UniVersity of Singapore, 3 Science DriVe 3, Singapore 117543
Received August 29, 2003; E-mail: chmlaiyh@nus.edu.sg
Two commonly encountered transannular π-π interactions are
represented by the face-to-face interactions1 in paracyclophane 1
and spiro(homo)-conjugation2 in spiro-tetraene 2. The first involves
two parallel π systems, while the second has two perpendicular π
systems. The spiro-interactions in particular have been employed
recently in the design of organic materials.3 Spectroscopic studies
of 3, however, indicated that its benzene ring is freely rotating and
there is little or no interaction between the two π systems in 3. It
is known that there is a correlation between spiro(homo)-conjugation
and HOMO-LUMO of the molecule.4 Results from theoretical
calculations5 indicate a large decrease in the HOMO-LUMO
separation going from benzene to 10,10c-dimethyl-10b,10c-dihy-
dropyrene 4. We thus wish to report the synthesis of dihydropy-
renyldihydropyrene 5 and spectroscopic evidence for a homo-
conjugation effect between the two π systems in 5.
yield of a mixture of anti-6 and syn-6. These were obtained in a
ratio of 95:5 based on 1H NMR spectral analysis. Isolation of anti-6
could be achieved by either chromatography or fractional recrys-
tallization. Its anti stereochemistry was assigned on the basis of its
1
low-temperature (-40 °C) H NMR spectrum where the internal
methyl protons in the cyclophane moiety were significantly shielded
1
and appeared as a singlet at δ 1.64. A dynamic H NMR study11
was carried out by probing H1′ and H3′ (δ 7.90 and 8.75,
respectively at -40 °C) of the dihydropyrene moiety. The free
energy of activation (∆Gq )11 for rotation about the aryl-aryl bond
c
in anti-6 was thus estimated to be 53.5 kJ mol-1
.
Treatment of anti-6 with LDA followed by quenching the
intermediates with methyl iodide resulted in a mixture of isomers
of m-cyclophane 8. Remethylation13 of 8 with (CH3O)2CHBF4
afforded the salt 9 isolated as a brown solid in about 95% yield. A
Hofmann elimination of 9 carried out with t-BuOK in THF gave
the cyclophanediene 10 which underwent valence tautomerization
to afford the desired product 5 in a 31% yield.
The dithiacyclophane route to a cyclophanediene, and hence the
dihydropyrene, is considered the most efficient.6 Thus, the dithia-
cyclophane 6 is an appropriate precursor to 5. We reported earlier
an arylation7 reaction of dihydropyrene which proved to be a
successful route to 6. A coupling reaction7 of dihydropyrene 46,8
with the diazonium salt derived from 2,6-dicyanoaniline9 gave
2-aryldihydropyrene 7a in 32% yield after chromatography.10
Reduction of 7a with DIBAL at room temperature afforded the
dialdehyde 7b. Reduction of 7b with NaBH4 gave a 98% yield of
diol 7c. Treatment of 7c with PBr3 at room temperature afforded
the desired dibromide 7d. It was evident from their 1H NMR spectra
that atropisomers of 7b-7d existed in solution.7 Surprisingly, a
dynamic 1H NMR study of 7d indicated no coalescence of its pair
of methylene proton signals up to a temperature of 110 °C,
Compound 5 was isolated as thick, yellow-green oil, and its
structure was supported by a molecular ion observed at m/z 448
and a base peak at m/z 202 (corresponding to pyrene) in its mass
spectrum. The chemical shift (δ -4.14) of the methyl protons at
C10b in 5 is very similar to that (δ -4.25)6b of the parent 4 thus
indicating that 5 sustains a ring current essentially identical to that
in 4. The diatropicity of the dihyropyrene ring is sensitive to
geometric deviation from planarity of the macro-ring.14 The above
observation suggests that despite the introduction of a spatially
demanding 2-dihydropyrenyl group at C10c, the molecular structure
of 5 is expected to show no appreciable changes in its overall
geometry of its dihydropyrene moieties. The aromatic protons in 5
(Table 1) could be readily assigned by its 1H NMR (Figure 1) and
COSY spectra. It is evident that the “internal” dihydropyrenyl ring
is freely rotating. There is appreciable downfield shift of H1,3, being
in the deshielding zone of the internal dihydropyrenyl ring. On the
other hand, the series of protons H1′-H10′ are strongly shielded,15
even for H7′ being furthest from the molecular plane of the opposite
dihydropyrene. The ring current of each dihydropyrene ring in 5
corresponding to a conformational energy barrier (∆Gq )11 of >77
c
kJ mol-1 for unrestricted rotation about the aryl-aryl single bond
in 7d.
A coupling reaction between 2,6-bis(mercaptomethyl)toluene6b
and the dibromide 7d under high-dilution conditions12 gave an 18%
9
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J. AM. CHEM. SOC. 2003, 125, 14296-14297
10.1021/ja0382260 CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Chemical Shifts of Protons in 5
those going from 4 to 11 and 12 involving linear conjugation. The
fine structures at 580-650 nm (Figure 2) are also collectively
shifted by about 8 nm to the longer wavelength going from 4 to 5.
A comparison with the electronic spectral data of 317 further adds
evidence that the dihydropyrene unit in 5 interacts with greater effect
than the benzene in 3, validating our prediction that the relatively
small HOMO-LUMO gap of the two identical dihydropyrene
moieties in 5 optimizes the opportunity for observing homocon-
jugation that earlier reported systems failed to reveal.
CH at C10b
3
H1,3
H2
H4,10
H5,9
H6,8
H7
-4.14
9.01
8.42
8.75
8.61
8.97
8.63
CH at C10b′,10c′
H1′,3′
4.05
H2′
H4′,10′
H5′,9′
H6′,8′
H7′
3
-5.16, -4.86
-
7.48
8.12
8.23
7.79
The above observations strongly suggest through-space homo-
conjugation between the two macrocyclic 14π systems in 5. The
behavior of a spiro(homo)-conjugation effect in an electronic
spectrum could be a shift of the absorption maxima to longer or
shorter wavelength.19 Thus, the homo-conjugation effect in 5
observed in our work corresponds to a class I spiro(homo)-
conjugation19 with its absorption maxima shifted to longer wave-
length compared to those of the reference compound 4.
Figure 1. 1H NMR spectrum of the aromatic protons in 5.
Acknowledgment. Financial support for the work was provided
by the National University of Singapore.
Supporting Information Available: Experimental details and
analytical data for compounds 5-8. This material is available free of
charge via the Internet at http://pubs.acs.org.
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compd
R
p
â
â′
317
418
645.5 (2.51)
641 (2.52)
648 (2.78)
648 (2.51)
(tail to700)
481 (3.81)
463 (3.78)
488 (4.06)
493 (4.18)
577 (4.20)
383 (4.66)
377 (4.57)
416 (4.04)
390 (4.52)
400 (4.12)
341 (4.93)
337.5 (4.96)
341 (5.02)
348 (5.15)
368 (4.72)
5
1118
1218
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