Gold-catalyzed 6-exo-dig cycloisomerization: A versatile approach to functionalized phenanthrenes

Article abstract of DOI:10.1002/asia.201400034

A novel gold-catalyzed 6-exo-dig cycloisomerization 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. Golden cat: A novel gold-catalyzed 6-exo-dig cycloisomerization 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.

Full text of DOI:10.1002/asia.201400034

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
E-mail: longwuye@xmu.edu.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201400034.
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Long-Wu Ye et al.
Table 1. Optimization of reaction conditions.[a]
Table 2. Study of the reaction scope.[a]
Entry
1
Substrate
1
Product
2
Yield [%]
Entry
Metal catalyst
Acid [equiv]
Yield [%][b]
2a
3a
94
1[c]
2
3
4
5
6
7
8
[Et3PAuNTf2]
[Ph3PAuNTf2]
[Cy-JohnPhosAuNTf2]
[XPhosAuNTf2]
[BrettPhosAuNTf2]
[IPrAuNTf2]
–
–
–
–
–
–
–
–
26
57
42
17
10
56
59
87
14
8
<5
89
92
96
94
40
<5
28
46
40
41
35
25
7
<5
<5
<5
6
1a
2a
(86)[b]
2
3
1b
1c
2b
2c
91
[(4-CF3C6H4)3PAuNTf2]
[(C6F5)3PAuNTf2]
9[d]
10[f]
11[g]
12
13[h]
14[h]
15[h]
16[h,i]
AuIII[e]
–
–
–
96
85
PtCl2 (5 mol%)
AgNTf2 (5 mol%)
T
CF3CO2H (0.5)
MsOH (0.5)
HNTf2 (0.5)
HNTf2 (1.0)
HNTf2 (0.2)
4
1d
1e
1 f
1g
1h
2d
2e
2 f
2g
2h
T
5
N
<1
<1
<5
ACHTUNGTRENNUNG
80
ACHTUNGTRENNUNG
5[c]
6[c]
7[c]
8[c,f]
(26)[d]
(28)[e]
[a] [1a]=0.05m; DCE: 1, 2-dichloroethane. [b] Estimated by 1H NMR
spectroscopy using diethyl phthalate as internal reference. [c] 60% of 1a
did not react. [d] 70% of 1a did not react. [e] Dichloro(2-picolinato)gold-
ACHTUNGTRENNUNG(III). [f] 808C, 80% of 1a did not react. [g] 808C, 95% of 1a did not
react. [h] Reaction time=5 h. [i] 55% of 1a did not react.
83
93
80
Under the optimal reaction conditions, various propargyl-
biphenyls 1 were tested to examine the generality of the cur-
rent reaction. As shown in Table 2, the transformation could
be applied to a series of propargylbiphenyls 1a–k. The cor-
responding phenanthrenes 2a–k were obtained in good to
excellent yields (80–96%). In particular, this gold-catalyzed
cycloisomerization chemistry could also work well for the
electron-deficient biaryl substrates 1e–i when the tempera-
ture was raised to 808C, thus affording the desired phenan-
threnes 2e–i in good to excellent yields (Table 2, entries 5–
9). By contrast, cycloisomerization of the electron-deficient
biaryl substrates occurred with low efficiency under Kimꢀs
reaction conditions (Table 2, entry 5), highlighting the
unique feature of gold catalysis. In addition, such gold-cata-
lyzed cycloisomerization reactions were also operable for
naphthyl-substituted biaryl substrates 1j and 1k to give the
corresponding 5-methylchrysene 2j and 6-methylbenzo[c]-
phenanthrene 2k in excellent yields (Table 2, entries 10–11).
It should be mentioned that only one regioisomer was ob-
tained during this gold-catalyzed hydroarylation reaction. In
general, the reaction favored cyclization to the less hindered
position (Table 2, entry 3 and entry 9). However, in case of
substrate 1j, the reaction afforded 5-methylchrysene 2j with
more favorable cyclization to the more hindered position,
and the formation of 6-methyltetraphene was not observed
(Table 2, entry 10). Of note, the reaction could also be per-
formed in the presence of 1 mol% gold catalyst without sig-
nificant loss in yield (Table 2, entry 1).
9[c]
1i
1j
2i
2j
95
95
10
11
1k
2k
90
[a Reactions were carried out in vials; [1]=0.05m; isolated yields are re-
ported. [b] [1a]=0.10m; reaction time=20 h. [c] 808C, 5 h. [d] 10 mol%
InCl3 was used, 808C, 5 h, 11% of 1e did not react. [e] 10 mol% InCl3
and 0.5 equiv of HNTf2 were used, 808C, 5 h, 42% of 1e did not react.
[f] Reaction time=48 h.
To further explore the reaction scope, the gold-catalyzed
cycloisomerization reaction was extended to other biaryl
systems. As can be seen from the results compiled in
Table 3, the reaction was also very efficient. Various substi-
tuted aryl derivatives (1i–p) could be converted into the ex-
pected compounds (2i–p) in yields ranging from 83 to 95%
(Table 3, entries 1–5). Similar to its analogue 1k, propargyl-
biphenyl 1q was compatible with this catalysis, giving the
desired compound 5-methylbenzo[c]phenanthrene 2q in
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Table 3. Study of the reaction scope.[a]
Entry
1
Substrate
1
Product
2
Yield [%]
83
1l
2l
2
1m
1n
1o
1p
2m
2n
2o
2p
94
91
92
95
3[b]
4[b]
5[b]
Scheme 2. Synthetic applications.
6
1q
2q
85
7[b]
1r
1s
1t
2r
2s
2t
43
95
70
8
9
Scheme 3. Plausible reaction mechanism.
[a] Reactions were carried out in vials; [1]=0.05m; isolated yields are re-
ported. [b] 808C, 5 h.
yields, which may find practical applications in material sci-
ence.
Two plausible mechanisms[12] to rationalize the formation
of phenanthrene 2a are proposed (Scheme 3). Path a is gen-
erally considered via a gold-catalyzed 6-exo-dig hydroaryla-
tion, followed by an exo-endo double-bond migration.[13]
Another pathway, path b, may involve a gold-catalyzed hy-
dration and subsequent acid- or gold-catalyzed cyclodehy-
dration.
85% yield (Table 3, entry 6 vs. Table 2, entry 11). It is note-
worthy that for the alkyl-substituted internal alkyne sub-
strate 1r, the resulting product 2r was formed only in 43%
yield (Table 3, entry 7), which was similar to that observed
for Kimꢀs chemistry (48% yield).[8] Furthermore, the use of
1s bearing an ester substituent also resulted in the forma-
tion of the desired phenanthrene 2s in excellent yield
(Table 3, entry 8). However, in case of phenyl-substituted in-
ternal alkyne substrate 1t, the reaction only afforded
To probe the feasibility of the above two paths, we first
prepared the methyl ketone compound 3a.[14] Treatment of
compound 3a with the gold catalyst did not afford the corre-
sponding compound 2a (Table 4, entry 1). Moreover, cycli-
zation of 1a in the presence of only gold catalyst could fur-
nish 2a in 87% yield (Table 1, entry 8). These results indi-
cated that path a should be the main pathway of the reac-
tion, although the formation of intermediate A could not be
dibenzoACHTUNGTRENNUNG[a,c]cycloheptene 2t in 70% yield, and no desired
phenanthrene was observed (Table 3, entry 9), which was
distinctively different from Kimꢀs In-catalyzed cycloisomeri-
zation.[8]
The obtained phenanthrenes are potentially useful in or-
ganic synthesis and will constitute valuable precursors for
the construction of versatile phenanthrene derivatives, as
outlined in Scheme 2.[11] In addition, the phenanthrene 2g
could be subjected to Suzuki cross-coupling with boronic
acids to generate more conjugated systems 4 and 5 in good
1
detected by monitoring the cyclization reaction by H NMR
spectroscopy under the optimized reaction conditions
(Table 1, entry 8). Notably, similar to our previously report-
ed results,[9a] part of 2a might be generated via path b involv-
ing a gold-catalyzed hydration and subsequent acid-cata-
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Table 4. Experiments carried out to examine the reaction mechanism.
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Entry
Catalyst
Yield
1
2
3
5 mol% [(C6F5)3PAuNTf2]
HNTf2 (0.5 equiv)
5 mol% [(C6F5)3PAuNTf2]/
HNTf2 (0.5 equiv)
<5% (>95% 3a)
42% (50% 3a)
45% (50% 3a)
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also explain the role of acid here to accelerate the minor
pathway.
In conclusion, we have developed a practical and general
method for the synthesis of various functionalized phenan-
threnes in mostly good to excellent yields from readily avail-
able o-propargylbiaryls that exhibits a much broader sub-
strate scope and works under milder reaction conditions in
comparison with Kimꢀs chemistry based on indium catalysis.
Mechanistic studies revealed that this reaction presumably
proceeds via a gold-catalyzed 6-exo-dig cycloisomerization
and subsequent exo-endo double-bond migration. Further
related investigations on the gold-catalyzed 6-exo-dig cyclo-
isomerization are under way.
Experimental Section
Typical procedure for gold-catalyzed 6-exo-dig cycloisomerization of o-
propargylbiaryls
HNTf2 (3.0 mL, 0.05m in 1,2-dichloroethane) and [(C6F5)3PAuNTf2]
(15.1 mg, 0.015 mmol) were added to a solution of o-propargylbiaryl
1 (0.30 mmol) in 1,2-dichloroethane (3.0 mL) at room temperature. The
reaction mixture was stirred at room temperature and the progress of the
reaction was monitored by thin-layer chromatography. Typically, the re-
action was complete after 5 h. Upon completion, the reaction mixture
was diluted with CH2Cl2 (30 mL) and washed with saturated aqueous
NaHCO3 (2ꢂ15 mL). The resulting solution was extracted again with
CH2Cl2 (30 mL), and the combined organic layers were dried over
MgSO4. The mixture was then concentrated in vacuo, and the residue
was purified by chromatography on silica gel (hexanes/ethyl acetate) to
afford the desired product 2.
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Acknowledgements
We are grateful for financial support from the National Natural Science
Foundation of China (No. 21102119 and 21272191), NFFTBS (No.
J1310024), and the Program for Changjiang Scholars and Innovative Re-
search Team in University.
Keywords: cycloisomerization
·
gold
·
homogeneous
À
of C H Transformations (Ed.: G. Dyker), Wiley-VCH, Weinheim,
catalysis · hydroarylation · phenanthrenes
Germany, 2005.
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Received: January 7, 2014
Published online: March 12, 2014
Chem. Asian J. 2014, 9, 1525 – 1529
1529
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