Diatropicity of dehydrobenzo[14]annulenes: Comparative analysis of the bond-fixing ability of benzene on the parent 3,4,7,8,9,10,13,14-octadehydro[14]annulene

Article abstract of DOI:10.1021/ol016764g

(matrix presented) We report the synthesis of 3,4,7,8,9,10,13,14-octadehydro[14]annulene (1) and detail a comparative aromaticity study with its benzannelated derivatives (e.g., 2 and 3).

Full text of DOI:10.1021/ol016764g

ORGANIC
LETTERS
2001
Vol. 3, No. 22
3599-3601
Diatropicity of
Dehydrobenzo[14]annulenes:
Comparative Analysis of the
Bond-Fixing Ability of Benzene on the
Parent
3,4,7,8,9,10,13,14-Octadehydro[14]annulene
A. J. Boydston and Michael M. Haley*
Department of Chemistry, UniVersity of Oregon, Eugene, Oregon 97403-1253
haley@oregon.uoregon.edu
Received September 17, 2001
ABSTRACT
We report the synthesis of 3,4,7,8,9,10,13,14-octadehydro[14]annulene (1) and detail a comparative aromaticity study with its benzannelated
derivatives (e.g., 2 and 3).
An intensely debated area of dehydrobenzoannulene (DBA)
chemistry is the extent of the delocalization in the macro-
cyclic ring.1 While support of the diatropicity and para-
tropicity of DBAs has been presented,2 skepticism still
exists.3 Our group has shown, qualitatively, that despite their
large size and extensive benzannelation DBAs possess weak
but discernible induced ring currents.2a,c,d While work has
gone into utilizing molecular NMR probes to measure the
aromaticity of DBAs4 and other fused ring systems,5 Bunz
et al. recently turned the tables and used octadehydro[14]-
annulenes as a tool for making indirect, qualitative measure-
ments of the aromaticity of ring systems fused to the
[14]annulene skeleton.6 Their arguments relied on the notion
that macrocycle 1, an unknown molecule, is an aromatic
compound capable of ring current competition. Thus, the
bond-fixing ability of the “study” ring systems could be
determined qualitatively by the change in chemical shifts of
the alkene protons going from the precyclized polyynes to
cyclized annulenes. This raised the question of what are the
observable effects of stepping down the aromaticity of the
parent octadehydro[14]annulene via benzannelation? We
expected to observe a distinguishable trend when additional
ring systems were attached to 1. In doing so, we hoped to
find further support for the hypothesis that 1 and its DBA
derivatives do in fact possess diatropic ring currents and to
(1) (a) Balaban, A. T.; Banciu, M.; Ciorba, V. Annulenes, Benzo-,
Hetero-, Homo- DeriVatiVes and their Valence Isomers, Vol. 1-3; CRC
Press: Boca Raton, 1987. (b) Garratt, P. J. Aromaticity; Wiley: New York,
1986. (c) Minkin, V. I.; Glukhovtsev, M. N.; Simkin, B. Ya. Aromaticity
and Antiaromaticity; Wiley: New York, 1994.
(2) (a) Kimball, D. B.; Wan, W. B.; Haley, M. M. Tetrahedron Lett.
1998, 39, 6795-6798. (b) Matzger, A. J.; Vollhardt, K. P. C. Tetrahedron
Lett. 1998, 39, 6791-6794. (c) Bell, M. L.; Chiechi, R. C.; Johnson, C.
A.; Kimball, D. B.; Matzger, A. J.; Wan, W. B.; Weakley, T. J. R.; Haley,
M. M. Tetrahedron 2001, 57, 3507-3520. (d) Wan, W. B.; Chiechi, R. C.;
Weakley, T. J. R.; Haley, M. M. Eur. J. Org. Chem. 2001, 3485-3490.
(3) Juse´lius, J.; Sundholm, D. Phys. Chem. Chem. Phys. 2001, 3, 2433-
2437.
(4) Kimball, D. B.; Haley, M. M.; Mitchell, R. H.; Ward, T. R. Org.
Lett. 2001, 3, 1709-1711.
(5) Mitchell, R. H. Chem. ReV. 2001, 101, 1301-1315.
(6) (a) Laskoski, M.; Steffen, W.; Smith, M. D.; Bunz, U. H. F. Chem.
Commun. 2001, 691-692. (b) Laskoski, M.; Smith, M. D.; Morton, J. G.
M.; Bunz, U. H. F. J. Org. Chem. 2001, 66, 5174-5181.
10.1021/ol016764g CCC: $20.00 © 2001 American Chemical Society
Published on Web 10/05/2001

show that 1 is sensitive enough to be used as a probe for
aromaticity. To do this, we prepared 1 and compared its
proton NMR data with those for a series of benzannelated
derivatives, both new (2, 3) as well as known (4,7 5,8 69).
The straightforward synthetic route to the parent annulene
1 is shown in Scheme 1. Selective deprotection of enediyne
The synthesis of 3 (Scheme 3) required iodoarene 122c,11
and enediyne 7, which were subjected to an in situ depro-
Scheme 3a
Scheme 1a
a Reagents: (a) 7, K2CO3, MeOH, PdCl2(PPh3)2, CuI, Et3N; (b)
Bu4NF, THF, MeOH; (c) Cu(OAc)2, MeCN, reflux.
a Reagents: (a) K2CO3, MeOH, THF; (b) (Z)-1,2-dichloroethene,
Pd(PPh3)4, CuI, PrNH2, THF; (c) Bu4NF, THF, MeOH; (d)
Cu(OAc)2, MeCN, reflux. (Th ) 1,1,2-trimethylpropyl-).
tection/alkynylation procedure2c,11 to yield polyyne 13.
Stepwise deprotection and cyclization gave unsymmetrical
annulene 3, which also could be handled only in dilute
solutions. Concentrated solutions or solid samples of 1-3
rapidly polymerized to furnish dark, intractable materials.
This instability precluded complete removal of the residual
trialkylfluorosilane impurities.
With solutions of 1-3 in hand, we examined the chemical
shifts of the alkene and arene protons of all six octadehydro-
[14]annulenes (Figure 1). The average of the chemical shift
7,10 followed by Pd/Cu cross-coupling with (Z)-1,2-di-
chloroethene, gave R,ω-polyyne 8 in 61% yield. Stepwise
deprotection and cyclization furnished 3,4,7,8,9,10,13,14-
octadehydro[14]annulene (1), which proved to be stable only
in dilute solutions.
As shown in Scheme 2, monobenzoannulene 2 was
synthesized from known diethynylbenzene 92c,11 via Pd/Cu
Scheme 2a
a Reagents: (a) K2CO3, MeOH, THF; (b) (Z)-(4-chloro-3-buten-
1-ynyl)trimethylsilane, Pd(PPh3)4, CuI, PrNH2, THF; (c) Bu4NF,
THF, MeOH; (d) Cu(OAc)2, MeCN, reflux.
cross-coupling with (Z)-(4-chloro-3-buten-1-ynyl)trimethyl-
silane12 to furnish intermediate aryl enediyne 10. Repeated
deprotection and cross-coupling gave R,ω-polyyne 11 in 48%
yield from 9. Deprotection and cyclization resulted in
formation of unstable benzocyclyne 2.
(7) Boydston, A. J.; Bondarenko, L.; Dix, I.; Weakley, T. J. R.; Hopf,
H.; Haley, M. M. Angew. Chem., Int. Ed. 2001, 40, 2986-2989.
(8) Blanchette, H. S.; Brand, S. C.; Naruse, H.; Weakley, T. J. R.; Haley,
M. M. Tetrahedron 2000, 56, 9581-9588.
(9) Baldwin, K. P.; Matzger, A. J.; Scheiman, D. A.; Tessier, C. A.;
Vollhardt, K. P. C.; Youngs, W. J. Synlett 1995, 1215-1218.
(10) Bharucha, K. N.; Marsh, R. M.; Minto, R. E.; Bergman, R. G. J.
Am. Chem. Soc. 1992, 114, 3120-3121.
Figure 1. NMR chemical shifts of the alkene and selected arene
protons for 1-6.
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Org. Lett., Vol. 3, No. 22, 2001

differences of the alkene protons show that in going from 1
to an annulene with one fused arene ring (2, 4) results in an
average upfield shift (∆δ) of 0.67 ppm (Table 1). Attachment
the ∆δ trend should be observable only in the aromatic
(cyclized) target molecules, not in the nonaromatic polyynes.
We were pleased to realize that the chemical shifts of each
set of alkene protons in the polyynes were essentially the
same, the range being only 0.04-0.12 ppm going from zero
to two benzene rings.
Table 1. Chemical Shift Difference (ppm) of the Alkene
Another interesting comparison is that of annulene 1 with
the 1,2-dihydro analogue 14 reported by Schreiber’s group.14
In the latter compound, the aromaticity is interrupted and
the system is sufficiently “bond-fixed”. If octadehydro[14]-
annulene possesses a diatropic ring current, then this should
manifest itself in a significant downfield shift of the 5,6-
alkene protons relative to those of 14. This is in fact what is
observed. The alkene protons of 14 appear at 6.17 and 5.79
ppm and thus are close to the values of the precyclized
compounds in Table 2, whereas in aromatic macrocycle 1
the analogous protons appear at 7.92 and 7.39 ppm. The large
difference in chemical shifts cannot be ascribed to the
anisotropy of the double bond; therefore, the difference must
be a direct result of a diatropic ring current.
Protons upon Successive Benzannelation
protons
1 f 2
1 f 4
2 f 3
2 f 5
4 f 3
H1
H2
H5
Η6
-0.61
-0.74
-0.68
-0.65
-0.44
-0.31
-0.51
-0.66
-0.23
-0.37
-0.40
-0.36
of a second benzene moiety gives a smaller upfield shift (∆δ)
of 0.38 ppm. One adjustment that is apparent from our data
is a +0.15 ppm correction when the alkene proton is on a
carbon γ to a benzene ring (e.g., H5 in going from 2 f 3).
It appears that this is the amount of change caused by steric
deshielding of the nearby arene proton. It is interesting to
note that the average change upon annelation of the second
benzene ring is approximately half that of affixing the first.
It is known that fusion of one benzene reduces the aroma-
ticity of an annulene circuit by about one-half.5,13 It follows
that fusion of the second arene reduces the remaining ring
current again by half. The mean ∆δ values corroborate this
notion.
A subtler trend can be extracted from examination of the
arene protons. The sensitivity of these protons is much less
than that shown for the alkene protons, as expected, but a
consistent upfield shift is still observed for certain arene
protons as the degree of benzannelation is increased. The
protons ortho to the diacetylene bridge are compared, as these
are the most consistent with regard to regiochemistry and
do not experience steric deshielding effects. The ∆δ values
(with respect to monobenzo 2) are 0.23 ppm for 5 and 0.25
ppm for 3. When a third benzene ring is added (6), the ∆δ
value decreases to 0.19 ppm.
These data, although qualitative, provide strong support
for the aromaticity of 1 and thus uphold the idea of using
dehydroannulenes as a means for evaluating the relative
aromaticity of fused ring systems. The narrow ranges of the
alkene chemical shift differences (typically 0.10-0.15 ppm),
which are much less than the average ∆δ values themselves,
support the contention that the alkene protons of the parent
macrocycle are sensitive enough to be used as a qualitative
probe for relative aromaticity. Furthermore, the effects of
the step-down in aromaticity by increasing benzannelation
are clearly a result of competing ring currents in the annulenic
systems. We are currently working toward further under-
standing the aromaticity of dehydro- and dehydrobenzo-
annulenes both theoretically and experimentally.
We considered the possibility of the ∆δ trend to be simply
a result of aryl substitution on the conjugated backbone. It
was necessary, then, to compare the alkene chemical shifts
of the precyclized polyynes (Table 2). If the results we
Acknowledgment. We thank the Petroleum Research
Fund, administered by the American Chemical Society, and
The Camille and Henry Dreyfus Foundation (Teacher-
Scholar Award 1998-2003 to M.M.H.) for financial
support.
Table 2. Chemical Shifts (ppm) of the Alkene Protons in
Pre-Cyclized Compounds
protons
8
11
13
pre-4a
pre-5b
H1
H5
H6
6.07
6.05
5.90
6.11
6.11
5.91
6.11
6.15
5.95
6.17
5.94
Supporting Information Available: Selected spectro-
scopic data for compounds 1-3, 8, 10, 11, and 13. This
material is available free of charge via the Internet at
http://pubs.acs.org.
a Reference 7. b Reference 8.
observed in comparing the DBAs to the parent compound
were an outcome of diatropicity in the annulene circuit, then
OL016764G
(11) Haley, M. M.; Bell, M. L.; English, J. J.; Johnson, C. A.; Weakley,
T. J. R. J. Am. Chem. Soc. 1997, 119, 2956-2957.
(12) John, J. A.; Tour, J. M. Tetrahedron 1997, 53, 15515-15534.
(13) Mitchell, R. H. Isr. J. Chem. 1980, 20, 294-299.
(14) Elbaum, D.; Toan, N.; Jorgensen, W.; Schreiber, S. Tetrahedron
1994, 50, 1503-1518.
Org. Lett., Vol. 3, No. 22, 2001
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