COMMUNICATION
DOI: 10.1002/asia.201400034
Gold-Catalyzed 6-Exo-Dig Cycloisomerization: A Versatile Approach to
Functionalized Phenanthrenes
Chao Shu, Long Li, Cheng-Bin Chen, Hong-Cheng Shen, and Long-Wu Ye*[a]
Abstract: A novel gold-catalyzed 6-exo-dig cycloisomeriza-
tion of o-propargylbiaryls has been developed that provides
ready access to functionalized phenanthrenes in largely
good to excellent yields. Notable features of this method are
readily available starting materials, mild reaction conditions,
and broad substrate scope.
Phenanthrene is an important organic structural motif in
materials science because of its versatile physicochemical
properties.[1] In addition, it is also found in a very large
number of bioactive natural and non-natural molecules.[2]
Scheme 1. Synthesis design for the formation of phenanthrenes 2 through
gold-catalyzed 6-exo-dig cycloisomerization of o-propargylbiaryls 1.
Therefore, the development of methods that allow for its
synthesis is of prime importance, and various synthetic ap-
proaches to phenanthrene compounds have been established
over the past decade.[3] Among the different methods devel-
oped so far, those based on transition metal catalysis are of
high value as they generally allow an efficient and selective
access to functionalized phenanthrenes under rather mild re-
action conditions.[4]
tion that enables the efficient and practical preparation of
versatile phenanthrene derivatives in generally good to ex-
cellent yields under mild reaction conditions. Importantly, in
comparision to Kimꢀs chemistry based on indium catalysis,
which only worked well for electron-rich biaryl substrates,
this gold-catalyzed process offers a much broader substrate
scope.
The recent rapid advances in gold-catalyzed hydroaryla-
tion reactions offer an easy access to an incredible variety of
functionalized cyclic compounds.[5,6] It is surprising, howev-
er, that reports on gold-catalyzed 6-exo-dig hydroarylation
are rare.[7,5a] Recently, Kim and co-workers reported an ele-
gant protocol for the synthesis of functionalized phenan-
threnes involving an InIII-catalyzed 6-exo-dig cycloisomeriza-
tion.[8] In our recent study toward a gold-catalyzed cycloiso-
merization reaction,[9] we reported a gold-catalyzed 6-exo-
dig cycloisomerization of o-alkynyldiarylmethanes to pre-
pare versatile anthracenes.[9a] Inspired by these results, we
envisioned that the synthesis of phenanthrenes 2 might be
accessed through the gold-catalyzed 6-exo-dig cycloisomeri-
Propargylbiphenyl 1a was chosen as a model substrate for
our initial study, and some of the results are listed in
Table 1. The catalytic potential of the gold complex
[Et3PAuNTf2] was first evaluated given its previously report-
ed high activity for the conversion of o-alkynyldiarylme-
thanes into anthracenes.[9a] To our delight, upon exposure of
1a to 5 mol% [Et3PAuNTf2] at room temperature, the reac-
tion could indeed give the desired phenanthrene 2a, albeit
in low yield after 24 h (Table 1, entry 1). Next, a variety of
gold catalysts were investigated (Table 1, entries 1–9), and
[(C6F5)3PAuNTf2] was found to be the best among them
(Table 1, entry 8). Notably, similar to our previously report-
ed protocol,[9a] the main byproduct here was methyl ketone
3a formed by a gold-catalyzed hydration reaction. In addi-
tion, PtCl2 and AgNTf2 only led to poor results (Table 1, en-
tries 10–11). Gratifyingly, the use of strong acids such as
MsOH and HNTf2 could substantially accelerate the reac-
tion (5 h vs. 24 h) and further reduce the formation of 3a
(Table 1, entries 13–15), and the reaction yield could be im-
proved to 96% in the presence of 0.5 equiv of HNTf2
(Table 1, entry 14). It is noteworthy that without the use of
gold catalyst, no desired product 2a was observed under the
acidic reaction conditions.
zation reaction of o-propargylbiaryls
1
(Scheme 1).[10]
Herein, we describe the realization of such a cycloisomeriza-
[a] C. Shu, L. Li, C.-B. Chen, H.-C. Shen, Prof. Dr. L.-W. Ye
State Key Laboratory for Physical Chemistry of Solid Surfaces
The Key Laboratory for Chemical Biology of Fujian Province and
Department of Chemistry, College of Chemistry and Chemical Engi-
neering
Xiamen University
Xiamen, Fujian 361005 (P.R. China)
Fax : (+86)592-218-5833
Supporting information for this article is available on the WWW
Chem. Asian J. 2014, 9, 1525 – 1529
1525
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim