A convergent approach to polycyclic aromatic hydrocarbons

Article abstract of DOI:10.1039/c1cc15095b

A new concise route to Polycyclic Aromatic Hydrocarbons (PAHs) through radical addition and cyclisation of xanthates is described. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c1cc15095b

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A convergent approach to polycyclic aromatic hydrocarbonswz
¨
Raphael F. Guignard* and Samir Z. Zard*
Received 16th August 2011, Accepted 5th October 2011
DOI: 10.1039/c1cc15095b
A new concise route to Polycyclic Aromatic Hydrocarbons
(PAHs) through radical addition and cyclisation of xanthates
is described.
Polycyclic Aromatic Hydrocarbons (PAHs) form a wide class
of substances.1 Their importance lies in their use as ligands in
transition metal-catalysed reactions,2 as p-conjugated func-
tional materials,3 and as tools for the study of the correlation
between structure and carcinogenic activity.4 Some PAHs are
indeed known to be potent carcinogens. Very few of these
compounds are commercially available and convenient methods
to prepare them are relatively scarce.1 These are mainly based
on photocycloadditions,5 Diels–Alder cycloadditions,6
Friedel–Crafts reactions7 and transition metal catalysed trans-
formations, especially palladium mediated aryl–aryl couplings.8
As for radical based processes, only the Pschorr reaction has
proved of any significance.9
Scheme 2 Formation of oxygen-bridged tetralones.
In one case, norbornene could be used as a starting olefin to
give polycyclic ketone 1 in a 33% yield over two steps.12
This encouraging result allowed us to contemplate a possible
route to PAHs involving radical additions to alkenes such as 2,
as these are easily derived from the cycloadditions of benzynes
with furans.13 The main potential complication concerned the
Smiles-type rearrangement of the intermediate adduct radicals,
as observed by Hodgson and co-workers in related structures.14
We were thus pleased to find that the reaction of phenacyl type
xanthates with alkene 2 furnished indeed the expected poly-
cyclic compounds in acceptable overall yields, as shown in
Scheme 2.
Our laboratory has developed a concise and efficient route
to 1-tetralones which has allowed us to synthesize a very wide
range of such compounds in a convergent manner and in
synthetically useful yields (Scheme 1).10 This method is based
on xanthate chemistry,11 which enables the intermolecular
addition of a stabilised acetophenone radical to an unactivated
olefin followed by a radical cyclisation onto the aromatic ring.
We then proceeded to induce the aromatisation of compound
4a by refluxing it in ethanol under acidic conditions. We were
surprised to isolate ethyl ether 5a instead of the expected
phenol (R0 = H). Analogous ethers could be obtained as
indicated by the examples displayed in Scheme 3. It is
Scheme 1 A convergent route to 1-tetralones.
`
Laboratoire de Synthese Organique, CNRS UMR 7652,
Ecole Polytechnique, F-91128 Palaiseau, France.
E-mail: zard@poly.polytechnique.fr; Fax: +33 169335972;
Tel: +33 169335971
w This paper is dedicated with respect and admiration to Professor
Mario D. Bachi.
z Electronic supplementary information (ESI) available. See DOI:
10.1039/c1cc15095b
Scheme 3 Formation of polycyclic aromatic hydrocarbons.
Chem. Commun., 2011, 47, 12185–12187 12185
c
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Scheme 6 Access to amino substituted PAHs.
Scheme 4 Further variations in the synthesis of PAHs.
indeed furnished compounds 9a–b, which proved to be highly
fluorescent and rather unstable, possibly explaining the low
yields observed (Scheme 6). Acetylation of the amino group in
9a with acetic anhydride directly after the aromatisation step
did not improve the yield of the expected acetamide 10, which
also proved to be relatively unstable.
noteworthy that trifluoroethyl ethers could be readily
prepared by this procedure (5d–f).
Primary and secondary alcohols were readily incorporated
into PAH skeletons. Not unexpectedly, tertiary alcohols did
not give satisfactory results as they dehydrated in the acidic
medium before any reaction took place.
The present new route to substituted PAH derivatives, while
still imperfect in terms of yields, provides nevertheless an
expedient access to structures that would be very tedious to
prepare by more traditional approaches. A better delineation
of the scope and limitations of the method and possible
extensions to related polycyclic heteroaromatic compounds
are in progress.
Functionalisation of the ketone group in intermediates 4
may be used to further widen the range of accessible PAHs.
Thus, addition of MeLi or PhLi to 4a–b delivered compounds
6a–c, which were then converted into 7a–b by dehydration/
aromatisation under acidic conditions in moderate overall
yield (Scheme 4).
Curiously, and for reasons not yet clear, the same sequence
could not be completed starting from 6c–d. Instead, a retro
Diels–Alder reaction apparently took place leading to
compounds 8a–b in good yield (Scheme 5).
We thank Ecole Polytechnique for a scholarship to one of us
(R. F. G.).
Notes and references
The same outcome was observed when compound 4a was
treated with PBr3 in refluxing toluene. Only naphthalene 8c
was isolated. No trace of the expected substituted anthracenes
was observed in any of these examples. Furthermore, attempts
to introduce vinyl and alkynyl groups met with complete
failure. Only a complex mixture was obtained upon attempted
aromatisation with acid.
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Scheme 5 Some unexpected transformations.
c
12186 Chem. Commun., 2011, 47, 12185–12187
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Chem. Commun., 2011, 47, 12185–12187 12187