Intramolecular aromatic 1,5-hydrogen transfer in preparation of
oxacyclic naphthalic anhydride via unusual Pschorr cyclisation
Xuhong Qian,* Jingnan Cui and Rong Zhang
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116012, China
Received (in Cambridge, UK) 20th September 2001, Accepted 1st November 2001
First published as an Advance Article on the web 23rd November 2001
Unusual Pschorr cyclisation via naphthyl radical-induced
intramolecular aromatic 1,5-hydrogen transfer using the
corresponding diazonium salts as starting materials gave
five- and six-membered oxacyclic-fused naphthalic anhy-
dride isomers.
The diazonium salts 1 and 2 were prepared from 4-bromo-
1,8-naphthalic anhydride according to the procedure published
1
earlier7 and all intermediates were characterized by H-NMR,
MS, IR and elemental analysis.
Pschorr cyclisation of 1 initiated by CuSO4, CuO, or
ferrocene produced two isomeric fluorescent compounds 3 and
4 in over 9+1 ratio (Table 1, entry 1–3). The most plausible
explanation for the 9+1 ratio of the isomers is that the initially
formed radical A rearranged itself to radical B by 1,5-hydrogen
transfer and that these two radicals in some equilibrium were
then converted to the products 3 and 4 (Scheme 1). It has been
demonstrated that Pschorr cyclisation and Sandmeyer iodode-
diazoniation of benzophenone derivatives gave cyclized iso-
mers in nearly equal amounts and some iodonized isomers,
respectively.5 However, in our case, the ratio of the cyclized
isomers was far from 1+1 and the iodonized isomers were
hardly found. The iododediazoniation of diazonium salts 1 and
2 gave only iodonized compounds 5 and 6, respectively (entry
6 and 7). It seems that the 1,5-hydrogen transfer rate between A
and B is slower than that between benzophenone radicals
according to the literature.5 Under similar reaction conditions to
entry 1, diazonium salt 2 gave also 3 and 4 in the ratio of 7+93
(entry 5). This suggests that radical B could be also rearranged
into radical A and the rate of formation of B from A is faster
than that of A from B. This indicates that radical B is more
stable than radical A. Even so, the structure of major product
and the ratio of isomers still mainly depended on the starting
compounds, as the rate of cyclisation to five- and six-membered
rings is obviously faster than that of hydrogen transfer. Beyond
what we expected, the mode of radical generation had obvious
effects on hydrogen transfer, as various initiation methods
Pschorr cyclisation has been widely used in the preparation of
polycyclic compounds1 for over a century.2 Many applications
of this reaction can be found in the pharmaceutical and dye
industries. Normally, the reaction was accomplished in a three-
step mechanism, namely, the diazotization of an o-amino group
to a diazonium salt; the dediazoniation of the diazonium salt to
a radical intermediate; and the closing of the ring. The
rearrangements via radical-induced aromatic hydrogen migra-
tion to produce isomers are rarely found in Pschorr cyclisation
because aromatic compounds are usually poor hydrogen donors.
In contrast, radical-induced hydrogen migration is commonly
seen when the migrating hydrogen originates from an aliphatic
carbon or from a heteroatom.3,4 After a century of diazonium
chemistry, phenyl radical-induced aromatic 1,5-hydrogen mi-
gration of the benzophenone derivatives was the only example
of such isomerization through Pschorr cyclisation (giving two
five-membered ring isomers) and Sandmeyer reaction.5,6 De-
spite the lack of reports for similar isomerizations, it has been
speculated that aromatic 1,5-hydrogen transfer might be a
general phenomenon in aromatic radical transformations.
Recently, we found five- and six-membered oxacyclic isomers
during preparation of benzoxanthene derivatives used as
fluorescent probes. We report herein the naphthyl radical-
induced aromatic 1,5-hydrogen transfer in Pschorr cyclisation
of the corresponding diazonium salts.
Scheme 1
2656
Chem. Commun., 2001, 2656–2657
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b108874m