COMMUNICATION
Synthesis and structural characterization of a fused bispyrone and
preparation of the first metal bispyrylium complexesw
Kristopher V. Waynant, James D. White* and Lev Zakharov
Received 7th May 2010, Accepted 7th June 2010
First published as an Advance Article on the web 22nd June 2010
DOI: 10.1039/c0cc01291b
Dehydroacetic acid reacts with acetic anhydride in the presence of
perchloric acid to give a salt which upon careful neutralization
affords a fused bis-c-pyrone. The bis-c-pyrone forms stable metal
complexes with Li, Mg, Cu(II), Ni(II), Zn, Fe(II), Co(II) and Ba
perchlorates whose crystal structures reveal that the ligand
possesses significant bispyrylium character while the metal is
coordinated at the negatively charged edge of the structure.
salt were not reported. A later investigation encountered similar
problems, although substituted derivatives of 1 were apparently
prepared in stable form.6 Recently, Hsung et al. reported that
the reaction of 3 with acetic anhydride in the presence of
perchloric acid led directly to fused a,g-bispyrone 5.7
Our reinvestigation of the work of Praill and Whitear has
established that the reaction of 3 with acetic anhydride in 60%
perchloric acid yields crystalline monoperchlorate 6 [dH (DMSO)
2.29 (s, 6H), 6.22 (s, 1H), 6.58 (s, 1H) 8.90 (br s, 1H); dC 19.4,
20.4, 98.9, 104.9, 115.3, 157.4, 164.6, 167.7, 169.7, 174.0]
whose structure was established by X-ray crystallography
(Fig. 2). The crystal structure of 6 shows that only one of
the rings is in the pyrylium form and that an intramolecular
hydrogen bond bridges the hydroxyl group of the pyrylium
ring and the adjacent pyrone carbonyl group. Eiden and
Schweiger reported a similar internal hydrogen bond in the
perchlorate salt of a dibenzo(bis)-g-pyrone.8 Careful neutrali-
zation of 6 with base as noted by Biglino et al.6 afforded
crystalline bispyrone 4 whose structure was also confirmed by
Pyrone–pyrylium resonance has been studied extensively in the
context of 4-pyranones (g-pyrones)1 and while g-pyrone itself
shows little aromatic character its exposure to metal salts
or electrophilic reagents generates species that exhibit true
benzenoid-like, i.e. pyrylium, aromaticity.2
By contrast, little is known about fused g-pyrones
such as bispyrone 1 and whether this structure can exhibit
naphthalenoid-type stabilization as expressed in resonance
hybrid 2 (Fig. 1). More generally, the question arises whether
a higher order array of fused g-pyrones in parallel orientation
as in 2 will possess sufficient aromatic stabilization to over-
come charge repulsion along the edges of the array. DFT
calculations (B3LYP/6-31Gld) predict that while bispyrylium
resonance hybrid 2 will make a negligible contribution to
the structure,3 this charge-separated form will be stabilized
if electron density along the negatively charged edge of
the bispyrone is dispersed through metal complexation. We
have now confirmed this expectation by preparing metal
complexes that are stable and easily characterized by X-ray
crystallography.
1
X-ray analysis. Spectroscopic properties of 4 (IR, H NMR,
13C NMR) are consistent with its bispyrone formulation and a
calculated dipole moment (m) of 7.53 D for 4 suggests there is a
negligible contribution from a bispyrylium structure corres-
ponding to 2 (m = 3.72 D for g-pyrone itself).9 Rearrangement
of bispyrone 4 to a,g-bispyrone 5 occurs readily upon extended
exposure to water or basic conditions.5,6
When 4 was exposed to various divalent metal perchlorates
in methanol the highly crystalline metal complexes 7 shown in
Fig. 3 were produced. The structures of these complexes were
determined by X-ray crystallography which established that
the organic ligand has a symmetric bispyrylium form. In these
structures each pyrylium ring shares a half positive charge with
its neighbor. Thus, treatment of 4 with anhydrous Mg(ClO4)2
produced complex 7 (M = Mg) in which the Mg(II) ion
symmetrically bridges each pair of alkoxide oxygens as shown
An attempt to synthesize 1 along lines used to prepare its
sulfur analogue4 was unsuccessful and to our knowledge this
parent bispyrone remains unknown. However, in 1961 Praill
and Whitear reported that when dehydroacetic acid (3) was
condensed with acetic anhydride in the presence of perchloric
acid a compound believed to be a perchlorate salt of
a bispyrone (4) was formed.5 Few structural details were
provided and attempts to isolate a neutral species from the
Fig. 1 Bispyrone–bispyrylium resonance.
Department of Chemistry, Oregon State University, Corvallis, OR,
USA. E-mail: james.white@oregonstate.edu; Fax: +1 541-737-2660
w Electronic supplementary information (ESI) available: Experimental
procedures as well as spectral and X-ray data are included for all
compounds. CCDC 774941–774951. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c0cc01291b
Fig. 2 Conversion of dehydroacetic acid (3) to monoperchlorate 6
and to fused bispyrone 4.
ꢀc
This journal is The Royal Society of Chemistry 2010
5304 | Chem. Commun., 2010, 46, 5304–5306
Waynant, Kristopher V.
