negatively (entries 10À15, Table 1). The screening of
solvents demonstrated that chlorobenzene was superior
to other solvents such as toluene and xylene, protic solvent
i-propanol, ether solvent dioxane, basic solvent pyridine,
and so on (entries 16À22, Table 1). As to the oxidant,
TBHP provided the best yield (entry 23À25, Table 1).
Lowering the reaction temperature led to a relatively lower
yield (entry 26, Table 1) and so did a shorter reaction time
(entry 27, Table 1). However, a prolonged reaction time
did not increase the yield either (entry 28, Table 1). Con-
sequently, the reaction was carried outwith5 mol % RhCl3
as the catalyst, 10 mol % PPh3 as the ligand, and 2 equiv of
TBHP as the oxidant in chlorobenzene at 160 °C for 24 h.
With the optimized conditions in hand, the scope of the
reaction was investigated. The results are listed in Table 2.
To our delight, the reaction can serve as a really general
protocol to the syntheses of various substituted xanthones,
affording moderate to excellent yields bearing both elec-
tron-donating and electron-withdrawing substituents.
More importantly, this strategy showed an excellent toler-
ance to diverse catalytically reactive substituent groups
such as aryl halides, amide, ketone, ester, and a cyano
group (entries 2À17, Table 2). Generally speaking, the
aryloxy parts with electron-donating groups were rela-
tively more reactive than those with electron-withdrawing
ones, and hence gave relatively higher yields. However,
substituents at the ortho position of the aryloxy group
reduced the yield, possibly due to steric effect (entries
21À24, Table2). Ifthesubstituentwas atthe metaposition,
xanthones were obtained as isomers in some cases. For
example, when 2-(m-bromophenoxy) benzaldehyde was
used, we obtained a 1:1 mixture of 3-bromo-9H-xanthen-
9-one and 1-bromo-9H-xanthen-9-one (entry 7, Table 2).
However, for 2-(m-chlorophenoxy) benzaldehyde, the
ratio of 3-chloro-9H-xanthen-9-one to 1-chloro-9H-
xanthen-9-one became 2:1 (entry 6, Table 2). When it
was switched to 2-(m-trifluoromethylphenoxy) benzalde-
hyde, only 3-trifluoromethyl-9H-xanthen-9-one was
obtained (entry 10, Table 2). Such differences could be
attributed to the distinction between the electron-with-
drawing capacity of these three substituents.
A tentative mechanism for the reaction is proposed in
Scheme 2. A sequence of an oxidative addition of the
Scheme 2. Proposed Mechanism
aldehyde CÀH bond, an oxidative dehydrogenation, and
finally a reductive elimination gave the desired product.
In this process, the stability of the five-membered ring
intermediate A has prevented decarbonylation, generat-
ing the sebsequent oxidative dehydrogenation product
predominantly.
In summary, we have developed a new way to con-
struct xanthone skeletons from aldehydes directly. It
does not require any preactivation of the aldehyde
group. In addition, the reaction can tolerate diverse
functional groups and can be applied to obtain a rather
wide range of xanthone derivatives. In this sense, it
is a useful complementary method for synthesizing
xanthones.
Acknowledgment. We are grateful to the Canada
Research Chair Foundation (to C.J.L.), the CFI, FQRNT
Center for Green Chemistry and Catalysis, NSERC,
and McGill University for support of our research. P.W.
also thanks the China Scholarship Council for financial
support.
Supporting Information Available. Typical experimen-
tal procedure and characterization data for all products.
This material is available free of charge via the Internet at
Disubstituted ethers could also proceed via the CDC
reaction under the optimized conditions, affording a struc-
ture with double xanthone skeletons (entry 25, Table 2). In
addition, a free hydroxyl group could also be tolerated in
this reaction (entry 27, Table 2).
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 3, 2012
905