acid.13 The oxidative coupling of benzil derivatives was
previously demonstrated using highly toxic and relatively
expensive thallium and vanadium reagents.14 The closure
of the central ring can be reductively accomplished by
potassium graphite15 or under drastic Scholl-type condi-
tions using aluminum chloride.16 The scope of these
methods is quite limited, and only a few derivatives could
be obtained in acceptable yields. The initial biaryl forma-
tion by a Suzuki coupling followed by an electrophilic
cyclization of a glyoxylic moiety was successfully dem-
onstrated by Yoshikawa and co-workers.17 However,
the method requires a donor functionality in position 3
of the phenanthrenequinone. The formation of six-
membered rings by oxidative arylꢀaryl coupling can be
accomplished by many methods18 and usually involves no
electron-withdrawing moiety in the tether.19,20 More so-
phisticated reagents are extremely powerful and can use
fluoroarenes as substrates.21 For the oxidative coupling
reaction, MoCl5 is a versatile and readily available
reagent.20 Because of the fast coupling process several
labile moieties are tolerated.22 The performance of this
reagent can be enhanced if Lewis acids are added for
intercepting hydrogen chloride formed in the course of
the reaction.23 The oxidative power of MoCl5 is often
compared to the commonly used hypervalent iodine
reagents.20 In several examples MoCl5 exhibits a better
performance.24 The molybdenum salts generated during
the transformation can direct the stereoselectivity of the
oxidative coupling25 or induce subsequent reductions by a
redox procedure.26
Scheme 1. Synthesis of Starting Materials
Here, we report an alternative and modular approach to
highly functionalized phenanthrenequinones which is
complementary to the existing methods. The key step
employs a MoCl5/TiCl4 mixture as an efficient oxidative
coupling reagent to yield 9,10-phenanthrenequinones. The
modular synthesis of the starting materials is based on the
method of Vasilevsky and co-workers27 and commences
with the formation of tolanes 3 (Scheme 1). Here, standard
conditions for the Sonogashira coupling (Pd(PPh3)2Cl2,
CuI, amine, DMF, 81 °C, 16 h) provided these intermedi-
ates in excellent yields up to 96%. When two iodo sub-
stituents are present in the component 1, the less hindered
leaving functionality cleanly undergoes the coupling pro-
cess. Installation of the 1,2-dicarbonyl moiety was best
accomplished by oxidation using KMnO4 at room tem-
perature using a pH buffer system based on MgSO4/
NaHCO3 in an aqueous acetone solution.28 Without the
buffered reagent mixture,27 the oxidation was unreliable
and labile substrates were mostly decomposed. The elabo-
rated protocol is easy to perform and provides very good
yields for the benzil derivatives 4 in the range of 83 and
97%. The next step, the oxidative cyclization reaction, was
not observed with KMnO4 since this particular oxidant is
not electrophilic enough.18 For details, see the Supporting
Information.
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The oxidative cyclization of the benzil substrates 4 to the
9,10-phenanthrenequinones 5 is very efficiently performed
by the MoCl5/TiCl4 mixture (Scheme 2). In the protocol,
TiCl4 does not only bind the formed hydrogen chloride but
also induces a reactive conformation by complex forma-
tion with the 1,2-dicarbonyl moiety. Therefore, the TiCl4 is
€
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