82
L. Chen et al. / Catalysis Communications 65 (2015) 81–84
Tf
O
mo
l
%
Pd PPh
1
0
(
3)4 (
)
re .
f 6
[ ]
s
F
,
H N RT
C 3C
,
C
TM
S
83
%
a
1
a
a
2
u
mo
l
%
I 2
C
0
(
)
a
2
re .
f 26
1
[
]
s
re ux
N
fl
3C
,
%
F
,
H
C
C
81
u
mo
l
%
PPh3A l 1
C
0
(
)
a
2
a
1
s wor
thi
k
3C 60 oC
s
F
,
H
N
C
C
,
88
%
Scheme 1. Overview of cyclotrimerization of benzyne using Kobayashi's precursor.
residue was then purified by flash column chromatography on silica gel
(eluent = hexanes/ethyl acetate 40:1 v/v) to give the desired
triphenylene 2.
Au(I)-catalyzed cyclotrimerization of arynes was neither air nor moisture
sensitive, which only decreased the yields slightly.
Based on the experimental results, a plausible mechanism [34] for
the formation of triphenylenes, which is similar to that of Pd(0)-cata-
lyzed cyclotrimerization [35], is proposed as shown in Scheme 2.
Benzyne 3, which is generated by fluoride-induced elimination of
benzyne precursor 1a, interacts with a cationic gold(I) complex
([PPh3Au+] was probably formed in the presence of an excess of
CsF) to form complex 4. Subsequent cyclometallation of two
molecules of 4 gives the organogold intermediate 5, followed by
one more benzyne insertion to produce complex 6, which liberates
the cyclotrimerization product triphenylene 2a and the cationic
gold(I) complex by demetallation.
3. Results and discussion
The catalytic activity of AuCl3 (10 mol%) was first tested for the
reaction of benzyne precursor 1a in the presence of CsF in DCE at
room temperature (Table 1, entry 1) [31], and the desired triphenylene
2a was not detected even when the reaction temperature was set to
60 °C (entry 2). To our delight, switching the solvent from DCE to
CH3CN led to the formation of cyclotrimerization product 2a in 76%
yield (entry 3) [32]. We then turned to screen other Au/Ag/Pt catalysts,
and PPh3AuCl was proved to be the optimum catalyst (92% yield, entry
8). When the catalytic amount of PPh3AuCl was decreased to 5 mol%,
the yield of 2a was significantly decreased to 66% (entry 11). When
the reaction was carried out at RT or 40 °C, the yield of 2a was also
decreased (entries 12, 13). Switching solvent from CH3CN to THF did
not increase the yield of triphenylene (entry 14). Thus, we concluded
that the optimized combination for the cyclotrimerization of benzyne
was to use 10 mol% of PPh3AuCl as the catalyst, 2 equiv. of CsF as the
fluoride ion, CH3CN as the solvent, and the reaction was set at 60 °C
(entry 8).
Table 1
Gold-, silver- and platinum-catalyzed cyclotrimerization of benzyne.a
Entry
Catalyst
Solvent
Temperature
Yield (%)b
To investigate the scope of the cyclotrimerization protocol, we applied
the optimum reaction conditions to a variety of Kobayashi's aryne precur-
sors 1 and the results are illustrated in Table 2. Benzyne precursor 1a, 4-
methoxy-, 4-methyl-, 4-tert-butyl-, 4-trifluoromethoxy-, and 3,6-di-
methyl-substituted aryne precursors 1b–1f and napthalyne precursor
1g were all well tolerated for the cyclotrimerization to give the
corresponding triphenylenes in 45–88% yields. For unsymmetrical
aryne precursors 1b–1e and 1g, the consistent 1:3 ratio of triphenylene
regioisomers, which was in good agreement with the experimental
results of Pd(0)-catalyzed cyclotrimerization of arynes [33], strongly
demonstrated that these reactions involved an aryne pathway. It is note-
worthy that the cyclotrimerization of 2c, 2e and 3g also was achieved
smoothly at room temperature, and the yields were even higher than
those at 60 °C. We also tried the cyclotrimerization of 1 under optimum
conditions with addition of a small amount of water (~5 equiv.,
0.05 mL), and the reaction gave the desired product in 80% yield. We
then tried Pd(PPh3)4-catalyzed cyclotrimerization [6] of 1 with addition
of the same amount of water, however, no desired product was
detected. In comparison with Pd(0)-catalyzed reaction conditions, the
1
2
3
4
AuCl3
AuCl3
AuCl3
AuCl
PPh3AuSbF6
AgSbF6
AgSbF6
PPh3AuCl
PtCl2
DCE
DCE
RT
–
60 °C
60 °C
60 °C
60 °C
60 °C
60 °C
60 °C
60 °C
60 °C
60 °C
40 °C
RT
–
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
THF
76
14
50
–
5
6
7c
8
Trace
92 (88)d
–
9
10
11e
12
13
14
PtCl4
12
66
78
46
PPh3AuCl
PPh3AuCl
PPh3AuCl
PPh3AuCl
60 °C
40
The significance of bold entries in table 1 shows the optimum conditions.
a
The reaction of 1a (0.20 mmol) was carried out in the presence of 10 mol% of catalysts
and CsF (0.40 mmol) in the corresponding solvent (1.0 mL) for 6 h.
b
Yield was determined by 1H NMR analysis of crude products using an internal
standard.
c
PPh3 (10 mol%) was added.
Isolated yield on 0.6 mmol scale in parentheses.
5 mol% of PPh3AuCl was used.
d
e