Fig. 1 ORTEP of the organometallic dehydroannulenes 8 and 9
Scheme 2
yield as sole isolable products. The remainder of these reactions
is black, insoluble, and intractable, a consequence of the partial
oxidative decomposition of the ferrocene nuclei by the CuII
salts, and low yields in the copper-catalyzed homocoupling
reactions of ethynylated ferrocene derivatives have been
observed earlier.10,11 If instead of 1, 1,2-diethynyl-4,5-dibu-
tylbenzene 11 is coupled to 4, we obtain the novel hexadehy-
drobenzo[14]annulene 13 in an overall yield of 53% after Cu-
catalyzed coupling.8 The material is orange–yellow and stable
in dilute solution, but unstable as a solid at ambient temperature.
Surprisingly, this is the first reported hexadehydroben-
zo[14]annulene.
The cycles 8 and 9 are stable in the solid state and crystallize
well from hexane–dichloromethane mixtures to give a specimen
suited for single-crystal X-ray analysis.‡ Fig. 1 shows the
ORTEPs of 8 and 9. The bond angles and bond lengths are in
excellent agreement with the expected values showing distinct
bond alternation between triple, double and single bonds. The
large hydrocarbon ligand is planar, as expected for a [14]-dehy-
droannulene. In the solid state, the diyne units of the cycles do
not show unusual contacts, and we did not observe solid-state
polymerization of these dehydroannulenes.
show that upon ring closure to 10 the downfield shift of the
vinyl protons is relatively small, while the downfield shift in the
1H NMR spectra, when closing the benzo-fused ring (12 ? 13),
is almost twice of these values. Consequently a benzene ring
disturbs the aromaticity of the fused dehydro[14]annulene less
than a ferrocene ring. Both sets of data suggest, that in the
ferrocene case there is a stronger localization of the dehy-
dro[14]annulene than for benzo-fused 13, and thus ferrocene is
more aromatic than benzene by this measure. These arguments
are in line with Mitchell’s findings.2 In future we will report the
electrochemistry, Bergman rearrangement and the products of
the pyrolysis of the ferrocene-fused dehydroannulenes 8–10.
U. H. F. B., W. S. and M. L. thank the National Science
Foundation (CAREER award, CHE 9981765, 2000–2004).
U. H. F. B. is Camille Dreyfus Teacher-Scholar (2000–2004).
Notes and references
suppdata/cc/b0/b009696m/ for crystallographic data in .cif or other
electronic format.
1 B. E. Bursten and R. F. Fenske, Inorg. Chem., 1979, 18, 1760.
2 R. H. Mitchell, Y. Chen, N. Khalifa and P. Zhou, J. Am. Chem. Soc.,
1998, 120, 1785; for other interesting organometallic dehydroannu-
lenes, see: P. R. Sharp, J. Am. Chem. Soc., 2000, 122, 9880; S. S. H.
Mao, F. Q. Liu and T. D. Tilley, J. Am. Chem. Soc., 1998, 120, 1193.
3 P. v. R. Schleyer, B. Kiran, D. V. Simion and T. S. Sorensen, J. Am.
Chem. Soc., 2000, 122, 510.
4 L. T. Scott, M. A. Kirms, H. Günther and H. Puttkammer, J. Am. Chem.
Soc., 1983, 105, 1372.
5 A. J. Matzger and K. P. C. Vollhardt, Tetrahdron Lett., 1998, 39, 6791
and references therein.
6 K. P. Baldwin, A. J. Matzger, D. A. Scheiman, C. A. Tessier, K. P. C.
Vollhardt and W. J. Youngs, Synlett, 1995, 1215; for a general treatise
of cyclic oligophenylacetylenes, see: M. M. Haley, J. J. Pak and S. C.
Brand, Top. Curr. Chem., 1999, 201, 82.
7 For a similar route to important cage-type phenyleneethynylenes, see:
Y. Rubin, T. C. Parker, S. I. Khan, C. L. Holliman and S. W. McElvany,
J. Am. Chem. Soc., 1996, 118, 5308.
8 F. Vögtle and R. Berscheid, Synthesis, 1992, 58; P. Siemsen, R. C.
Livingston and F. Diederich, Angew. Chem., 2000, 39, 2633.
9 For cyclobutatdiene containing dehydro[14]annulenes, see: U. H. F.
Bunz, G. Roidl and R. D. Adams, J. Organomet. Chem., 2000, 600,
56.
10 U. H. F. Bunz, J. Organomet. Chem., 1995, 494, C8; U. H. F. Bunz, G.
Roidl, M. Altmann, V. Enkelmann and K. D. Shimizu, J. Am. Chem.
Soc., 1999, 121, 10719.
Most interesting with respect to ferrocene’s aromaticity are
1
1
the H NMR spectra of 8–10. Comparison of the H NMR
spectrum of Vollhardt’s cyclyne (14)5,6 and that of the benzo–
fused dehydro[14]annulene 13 allows extraction of the relative
aromaticity of ferrocene with respect to benzene. When the 1H
NMR spectrum of 14 is compared to that of 9, a small upfield
shift is observed for Ha (the protons adjacent to the dehy-
dro[14]annulene), suggesting that, if no other effects are
present, ferrocene has a similar, or bigger localizing influence
on the dehydroannulenic core than benzene (Scheme 2). This
measure is clear but relatively indirect. A proton attached to the
reporter annulene would increase the effect, and thus the
compounds 10 and 13 offer a much more direct comparison.
1
The H NMR spectrum of the benzo compound 13 shows two
doublets at d 7.33 and 6.63, while in the 1H NMR spectrum of
10 the same vinylic protons appear at d 6.75 and 6.21,
1
respectively. In the same wake we can compare the H NMR
spectrum of 7 (open, non-aromatic) to that of 10 (closed) and the
1H NMR spectrum of 12 to that of 13. Upon conversion of 7
?10 the annulenic vinyl protons experience a modest down-
3
field shift (Ddvinyl-H = 0.40 and 0.77) and the vinylic JHH
coupling decreases slightly by DJHH = 0.7 Hz, while for the
conversion of 12 ? 13 the shift of the vinyl protons is much
bigger (Ddvinyl-H = 0.78 and 1.32 respectively). Here the
coupling constant decreases by DJHH = 0.8 Hz. These data
11 Z. Yuan, G. Stringer, I. R. Jobe, D. Kreller, K. Scott, L. Koch, N. J.
Taylor and T. B. Marder, J. Organomet. Chem., 1993, 452, 115.
692
Chem. Commun., 2001, 691–692