Isotopic perturbations in aromatic character and new closely related conformers found in [16]- and [18]annulene [6]

Full text of DOI:10.1021/ja993604f

722
J. Am. Chem. Soc. 2000, 122, 722-723
Scheme 1
Isotopic Perturbations in Aromatic Character and
New Closely Related Conformers Found in [16]- and
[18]Annulene
Cheryl D. Stevenson* and Todd L. Kurth
Department of Chemistry, Illinois State UniVersity
Normal, Illinois, 61701-4160
ReceiVed October 7, 1999
ReVised Manuscript ReceiVed December 9, 1999
No series of compounds have contributed more to the study
of aromaticity than have the annulenes. Considering only the
number of π-electrons, the Hu¨ckel 4n + 2 rule successfully
predicts the aromatic character, and other properties of many of
these monocyclic systems, despite the fact that this theory was
developed without consideration of the effects of attached pro-
tons.1 However, the importance of this electron count rule has
been challenged.2 It has been theorized that the σ-framework
imposes the relative delocalization and thus aromaticity of a
particular system. Here, we present evidence for the existence of
previously unobserved, nearly degenerate isomeric forms of [16]-
annulene and [18]annulene that differ in the spatial arrangement
of the internal hydrogens and their internal C-CdC bond angles.
The existence of these new isomers supports the relatively new
theories coming from Shaik’s and Jug’s groups,2 and it explains
low-temperature chemical shift dependence, of the [16]- and [18]-
annulene 1H NMR spectra, that are not explained by the previously
studied exchange mechanisms described in Schemes 1 and 2.3
The first (smallest) thermodynamically stable annulenes (at
ambient temperatures) with internal protons are [16]- and [18]-
annulene. Unlike the [6]- and [8]annulene systems, these two
larger systems are effected by the steric interactions of internal
protons. Indeed, recent 1H NMR results suggest that these steric
interactions, which have a profound effect on the planarity of
the [16]annulene system, can be attenuated by replacing the inter-
nal protons with deuteriums.3 Specifically, deuteriation results in
increased paratropic shifts of the low temperature 1H NMR reso-
nances of [16]annulene-d15 relative to those of [16]annulene-h16.4a
At temperatures above 163 K, consideration of the intramo-
lecular proton exchange between the possible conformations
Scheme 2
Scheme 3
perturbation in a neutral, formally aromatic, (4n + 2) π-electron
system, which also contains internal protons ([18]annulene).
It was found that [18]annulene-d18 can be synthesized using
the synthetic scheme described by Sondheimer5 but replacing the
tert-butyl alcohol in the second step with tert-butyl alcohol-d and
the H2 in the third step with D2 (Scheme 3).
1
(Scheme 1) is required for the simulation of the 400 MHz H
NMR spectra.3 This exchange accounts for the broadening and
ultimate coalescence of resonances as the temperature is raised.
Since the C-D bond is known to be shorter than the C-H
bond, the increased paratropic shifts of the deuteriated [16]annu-
lene has been accounted for in terms of the increased planarity
and consequent increased paratropic (anti-aromatic) ring current
(π-delocalization).4 However, Shaik and Jug with their collabora-
tors2 have presented evidence indicating that delocalization of π
electrons, and bond equalization, is enforced by the σ framework
(C-CdC bond angles). Consequently, it is also possible that
σ-framework changes, which occur upon deuteriation, may
contribute to the increased level of delocalization. To further
examine this effect, we sought to observe the analogous isotopic
1
The 400 MHz H NMR spectrum of [18]annulene, in CDCl3
at 213 K, is characterized by a quartet at δ 9.17 and a pentet at
δ -2.96 for the 12 external and 6 internal protons, respectively.
The analogous spectrum of [18]annulene-d17 (plus further protic
impurity, e.g., [18]annulene-d16) reveals increased diatropic shifts
of both the internal protons and external shifts due to deuteriation.
The external protons are shifted downfield by 0.03 ppm, and the
internal protons are shifted upfield by 0.12 ppm (Figure 1). NMR
spectra of mixtures of the perprotiated and deuteriated materials
in THF-d8/ethanol-d8 allow the perprotiated system to serve as
an internal reference, and these spectra confirm the data presented
in Figure 1. The shorter internal C-D bonds in the [18]annulene-
d17 system allow relaxation of the σ framework and consequently
more σ enforced delocalization of the π-electrons.
(1) Garrat, P. J. Aromaticity, Wiley: New York, 1986. (b) Minkin, V. I.;
Glukhovtstev, M. N.; Simkin, B. Y. Aromaticity and Anti-aromaticity,
Wiley: New York, 1994.
(2) Shaik, S. S.; Hiberty, P. C.; Ohanessian, G.; Lefour, J.-M.; J. Phys.
Chem. 1988, 92, 5086. (b) Jug. K.; Koster, A. M. J. Am. Chem. Soc. 1990,
112, 6772.
Similar to [16]annulene, [18]annulene is fluxional at more
elevated temperatures.3 In the spectra of the [16]- and [18]-
annulenes, thermal dependence of the chemical shifts continues
well below the temperature where resolution is maximized, and
(3) Oth, J. F. M. Pure Appl. Chem. 1971, 25, 573.
(4) Stevenson, C. D.; Kurth, T. L. J. Am. Chem. Soc. 1999, 121, 1623. (b)
For naphthalene, X-ray crystallography reveals a reduction of the C-L (L )
H or D) bond length from 1.085 (C-H) to 1.073 (C-D) Å see: Berger, S.;
Kunzer, H. Tetrahedron 1983, 39, 1327. (c) For another cases of isotopic
bond length reduction, see: Bartell, L. S.; Roth, E. A.; Hollowell, C. D. J.
Chem. Phys. 1965, 42, 2683 and Melander, L.; Saunders: W. H. Reaction
Rates of Isotopic Molecules; Wiley: New York, 1980; pp 189-197.
(5) Sondheimer, F.; Wolovsky, R. J. Am. Chem. Soc. 1962, 84, 260. (b)
Sondheimer, F.; Wolovsky, R.; Amiel, Y. J. Am. Chem. Soc. 1962, 84, 274.
10.1021/ja993604f CCC: $19.00 © 2000 American Chemical Society
Published on Web 01/15/2000

Communications to the Editor
J. Am. Chem. Soc., Vol. 122, No. 4, 2000 723
Figure 1. Overlaid 400 MHz 1H NMR spectra of [18]annulene and [18]-
annulene-d17 (plus further protic impurity, e.g., [18]annulene-d16), both
in CDCl3 at 213 K. The [18]annulene spectrum is characterized by a
quartet at δ 9.17 and a pentet at δ -2.96 for the 12 external and 6 internal
protons, respectively. In the spectrum of [18]annulene-d17, singlets at δ
9.20 and -3.084 account for external and internal protons, respectively.
Figure 3. The conformers (A, B, C, and D) for [16]annulene that were
proposed by Hernando are shown (ref 6). The plus signs indicate that
the proton extends forward from the plane of the molecule, while a minus
signifies that it extents back into the page. The zero indicates that the
proton is in the plane of the ring (the plane containing the largest number
of carbon atoms). According to Hernando, the most stable isomer of [16]-
annulene does not exist in conformations with protons lying in the plane
of the ring. The analogous conformers for most table isomer of [18]-
annulene are also shown (E, F, and G).
rangements of the internal protons (Figure 3), that would account
for the structures involved in the proposed exchange. Confirming
the intuitive observation, it was found that Hernando et al.,6 using
semiemperical calculations, had already predicted the existence
of these isomers of [16]annulene. The aforementioned authors
further indicate that the two isomers have slightly different
energies, the most stable of each pair having relatively more ideal
bond angles.6 As the temperature of the [16]annulene system is
lowered, the energetically allowed geometry of the σ-framework
approaches ideality (120° C-CdC bond angles). The conse-
quence of this is increased enforcement of π-delocalization2 and
1
augmented paratropicity of the annulene system. From the H
NMR data, it is apparent that in both the [16]- and [18]annulenes,
there must exist such lower-energy structures having σ-frame-
works that enforce greater π-delocalization and thus greater ring
current (paratropic in the 4n π-electron system and diatropic in
the 4n + 2 π-electron system).
Paratropicity and diatropicity in the [16]annulene and [18]annu-
lene systems is quite sensitive to the steric interactions involving
their respective internal protons. Alterations in these steric
interactions via isotopic substitution or via changes in the stacking
of the internal protons in the planes above and below the plane
of the ring perturbs the aromaticity and antiaromaticiy (measured
in terms of NMR chemical shift) exhibited by these two annulenes.
Presumably this will prove to be the case in all but the largest
annulenes that have internal hydrogens. Work is in progress to
observe these differences via other analytical methods.
Figure 2. Plot of the chemical shift values (ppm) of the external proton
resonances for the [18]annulene and [18]annulene-d17 systems vs tem-
perature (K). The protiated and deuteriated data were taken from spectra
of a mixed sample (deuteriated and protiated materials) in THF-d8/ethanol-
d8 as well as data from a pure perprotiated sample in CDCl3.
computer simulation no longer predicts chemical shift thermal
dependency (Figure 2). At the working temperatures of the solvent
(down to 153 K in a THF-d8/ethanol-d8 mixture), the exchange
responsible for this chemical shift movement does not even begin
to become slow on the NMR time scale. There clearly exists an
exchange in both of these annulene systems that is not accounted
for by the reactions shown in Schemes 1 and 2. The new exchange
mechanisms must involve very similar molecular structures and
very low energy barriers to exchange.
Acknowledgment. We thank the National Science Foundation (Grant
CHE-9617066) for support of this work.
Supporting Information Available: 1H NMR of [18]annulene at 263,
243, and 213 K (PDF). This material is available free of charge via the
Internet at http://pubs.acs.org.
JA993604F
By observing simple models, it became apparent that the [16]-
annulene and [18]annulene systems can have two distinct ar-
(6) Hernando, J. M.; Quirante, J. J.; Enr´ıquez, F. Collect. Czech. Chem.
Commun. 1992, 57, 1.