Gold(I)-catalyzed cycloisomerization of enynes containing cyclopropenes

Article abstract of DOI:10.1002/anie.201002673

Goldenyne: Gold-catalyzed cycloisomerization reactions of propargyl cyclopropenes afford benzene derivatives in a highly efficient manner. The reaction either proceeds through a mechanism with or without double and triple bond cleavage, depending on the substituents (see scheme). Copyright

Full text of DOI:10.1002/anie.201002673

Angewandte
Chemie
DOI: 10.1002/anie.201002673
Gold Catalysis
Gold(I)-Catalyzed Cycloisomerization of Enynes Containing
Cyclopropenes**
Changkun Li, Yi Zeng, Hang Zhang, Jiajie Feng, Yan Zhang, and Jianbo Wang*
Transition-metal-catalyzed cycloisomerizations of enynes
have recently been extensively studied. In particular, gold
catalysts have been shown to be highly efficient in this type of
reactions, which provide rapid and atom economical access to
a variety of cyclic structural motifs.[1,2] Previous investigations
have demonstrated that the reaction pathway is highly
substrate-dependent. In the absence of external nucleophiles,
the alkene moiety usually acts as a nucleophile to attack the
gold-activated alkyne moiety and trigger skeletal rearrange-
ment. In the case of 1,5-enyne systems, gold-catalyzed
reactions lead to the formation of [3.1.0] bicyclic compounds,
cyclopropene makes it highly reactive toward transition metal
catalysis. Recently, gold-catalyzed reactions of cyclopropenes
have been reported.[7] These reports show that gold com-
plexes can efficiently interact with the double bond of
cyclopropene to trigger ring-opening of the cyclopropene.
Inspired by these findings, we became interested in the gold-
catalyzed reaction of a system that contained both triple-bond
and cyclopropene moieties, such as the propargyl cyclopro-
pene shown in Scheme 1c. As triple bonds and cyclopropenes
are supposed to have similar reactivities toward gold com-
plexes, an intriguing question would be which unsaturated
bond is preferentially activated by a gold catalyst. Further-
more, the propargyl cyclopropene can be considered as a 1,5-
enyne system. Thus, we were also intrigued as to whether it
reacts in a similar manner to conventional 1,5-enynes. Herein,
we report a highly efficient gold-catalyzed rearrangement of
propargyl cyclopropenes, which affords benzene derivatives.
The triple bond is preferentially activated by gold catalyst and
the reaction may proceed through a novel mechanism
involving multiple alkyl migrations.
presumably through
a cyclopropylcarbene intermediate
(Scheme 1a).[3] When there is a siloxy substituent at the
terminal position of the alkyne moiety, the gold-catalyzed
reaction gives cyclohexadienes through a mechanism involv-
ing a series of alkyl migrations (Scheme 1b).[4]
At the outset, we examined the reactivity of 3-hydroxy
substituted propargyl cyclopropene 1a with transition metal
catalysts (Table 1). To our delight, with 1 mol% of AuCl or
AuCl3, an efficient reaction occurred in 5 minutes to afford
phenol derivative 2a in high yield (Table 1, entries 1 and 2).
AgOTf also catalyzed this transformation, but the reaction
took much longer to complete (Table 1, entry 3). The yield
was improved by using [Au(PPh3)Cl]/AgOTf (Table 1,
entry 4). PtCl2 also gave high yield of 2a, but it took 6 hours
for the reaction to complete (Table 1, entry 5). We also
examined In(OTf)3 and HOTf; the former afforded trace
amount of product and the latter gave no desired product.
Scheme 1. Gold-catalyzed cycloisomerizations of 1,5-enynes. TIP-
S=triisopropylsilyl.
On the other hand, cyclopropenes have attracted consid-
erable attention from the synthetic community as a result of
their diverse reactivity.[5] The high steric ring strain of
cyclopropenes give them a comparable character to alkynes;[6]
in particular, the high p-density of the double bond in
Table 1: Catalytic cycloisomerization of 1a.[a]
[*] C. Li, Y. Zeng, H. Zhang, J. Feng, Dr. Y. Zhang, Prof. Dr. J. Wang
Beijing National Laboratory of Molecular Sciences (BNLMS) and
Key Laboratory of Bioorganic Chemistry and Molecular Engineering
of the Ministry of Education, College of Chemistry
Peking University, Beijing 100871 (China)
Entry
Catalyst
Reaction time [min]
Yield [%][b]
Fax: (+86)10-6275-1708
1
2
3
4
5
6
7
AuCl
AuCl3
AgOTf
[Au(PPh3)Cl]/AgOTf
PtCl2
5
5
120
5
360
120
120
88
82
86
96
94
<5
0
E-mail: wangjb@pku.edu.cn
Homepage: http://www.chem.pku.edu.cn/physicalorganic/
home.htm
[**] The project is supported by the Natural Science Foundation of
China (Grant No. 20902005, 20832002, 20772003, 20821062) and
the National Basic Research Program of China (973 Program, Grant
No. 2009CB825300).
In(OTf)3
HOTf
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201002673.
[a] All reactions were carried out using 50 mg of 1a in 3 mL CH2Cl2.
[b] Yield of isolated product. Tf=trifluoromethanesulfonyl.
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Next, we prepared a series of secondary and tertiary
derivatives (2p and 2q) were isolated in high yields
(Scheme 2). The generation of products 2p’ and 2q’ indicates
a complete cleavage of the cyclopropene double bond and the
alkyne triple bond similar to the siloxy-substituted 1,5-enyne
system reported by Kozmin and co-workers.[4]
propargyl alcohols that bear cyclopropene moieties (1b–o).
All of these substrates underwent the cycloisomerization
reaction to give high yields of phenol derivatives (Table 2).
Table 2: Gold(I)-catalyzed cycloisomerization of 1b–o.[a]
Entry
R1
R2
R3
R4
Yield [%][b]
Scheme 2. Gold-catalyzed reaction of 1p and 1q.
1
2
3
4
5
6
7
8
Me
nBu
CH2 CH
PhC C
Ph
CN
H
H
H
Ph
Ph
Ph
Ph
Ph
Ph
Ph
OH
OH
OH
OH
OH
OTMS
OAc
OH
OH
OH
OH
OH
OH
H
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
nBu
Ph
2b, 97
2c, 96
2d, 95
2e, 82
2 f, 90
2g, 89[c]
2h, 96
2i, 97
2j, 96
2k, 95
2l, 91
2m, 71
2n, 74
2o, 97
=
ꢀ
We further observed that substituents in the cyclopropene
moiety have a crucial effect on the reaction pathway. As
shown in Scheme 3, the gold-catalyzed reaction of 3a–c, in
which the substituents on cyclopropene were n-butyl groups,
afforded exclusively symmetric phenol derivatives 4a–c,
which were formed through a double-cleavage process. A
comparison of these results with those in Table 2 suggests that
the R2 substituent at the terminal position of alkyne has a less
significant effect on the switch of reaction pathway.
CO2Et
p-O2NC6H4
p-F3CC6H4
p-MeOC6H4
p-Me2NC6H4
nBu
9
10
11
12
13
14[d]
H
H
H
ꢀ
nBuC C
H
H
[a] All reactions were carried out using 100 mg of 1b–o and 3 mL of
CH2Cl2. [b] Yield of isolated product. [c] Phenol was isolated after column
chromatography. [d] Reaction was carried out with 2 mol% of the
catalyst.
For the tertiary propargyl alcohols, the products were isolated
with an R1 migration onto the adjacent alkyne carbon. Alkyl,
alkenyl, alkynyl, aryl, and cyano groups all migrated success-
fully (Table 2, entries 1–6, 13).[8] The products were fully
characterized by spectroscopy and for one of the products
(2b), the structure was further confirmed by single-crystal
X-ray analysis.[9] Interestingly, for the acetate ester 1h, the
1,2-acetoxy migration product was not observed (Table 2,
entry 7).[2,10] As the siloxy substituent in the alkyne dramat-
ically altered the reaction pathway in the 1,5-enyne system
reported by Kozmin and co-workers,[4] we then examined
substrates with substituents other than a phenyl group at the
terminal position of the alkyne moiety. Introducing either
electron-withdrawing or electron-donating groups onto the
phenyl substituent of R2 did not affect the reactions (Table 2,
entries 9–12). With an electron-withdrawing ester substituent,
2i was isolated in high yield (Table 2, entry 8). In the case of
1n, in which R1 was alkynyl group and R2 was alkyl group, the
reaction also afforded the expected cycloisomerization prod-
uct 2n as the main product, together with small amount of
furan side-product, which was derived from the secondary
gold-catalyzed reaction of product 2n. Finally, it is worthy of
note that the reaction occurs with equally high efficiency
when both R1 and R3 are H, to give 1,2-diphenylbenzene 2o
(Table 2, entry 14).
Scheme 3. Gold-catalyzed reaction of 3a–c.
The importance of the cyclopropene substituents on the
reaction pathway is further demonstrated by the reaction of
5a–d (Scheme 4). In substrates 5a–d, the substituents on the
double bond of the cyclopropene moiety are unsymmetrical,
with one being nBu and other being TMS (trimethylsilyl). The
gold-catalyzed reactions of 5a–d all afforded the double-
cleavage products exclusively. The two diastereoisomers in
the substrates were both converted into the same phenol
derivatives. It is worthy of note that the TMS group was
positioned between R1 and R2 in all of those cases. This result
is consistent with the b-cation-stabilizing effect of silicon for
To our surprise, when propargyl alcohols 1p and 1q were
subjected to the same reaction conditions, a mixture of
symmetrical (2p’ and 2q’) and unsymmetrical benzene
Scheme 4. Gold-catalyzed reaction of 5a–d.
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carbon cations (see A, Scheme 5). The structures of 6b–d
were confirmed by NOESY experiments.
A possible mechanism to account for the above experi-
ment results is shown in Scheme 5. We assumed that the triple
reactive sites would be too crowded to form the intermediates
C, D, E, and G. As a result, the reaction will follow path a. In
contrast, when smaller hydrogen or alkyl groups were on the
terminal alkyne, the reaction partially followed path b, as
shown in the reaction of 1p and 1q. When the cyclopropene
double bond was substituted by relatively smaller n-butyl
groups, path b was favored, regardless of the substituents on
the alkyne.
An intriguing question in the mechanism is the reactivity
of cyclopropene toward gold complexes as compared with
that of an alkyne. We have previously reported that cyclo-
propene 7 rearranges to indene 8 when catalyzed by [Au-
(PPh3)Cl]/AgOTf (2 mol%) in dichloromethane at room
temperature (Scheme 6);[7c] vinyl gold carbene species 10
was suggested as the intermediate. The reaction conditions
were identical to the gold-catalyzed reaction of propargyl
cyclopropene, but it took longer (30 min) with slightly higher
catalyst loading.
Scheme 6. Gold-catalyzed reaction of cyclopropene.
Scheme 5. Mechanistic rationale.
bond in the propargyl cyclopropene was preferentially
complexed to the gold catalyst. The p-electron of the
cyclopropene then attacks the gold-activated alkyne as a
nucleophile in a 5-endo-dig manner to form a bicyclo-
[3.1.0]hexene intermediate A, from which two pathways are
possible, thus leading to two regioisomeric products. For
path a, back-donation of the electron from the gold center
leads to ring enlargement and the formation of six-membered
gold carbene intermediate B,[11] which undergoes 1,2-shift of
an R1 group to afford the phenol product. In this pathway,
there is no complete disconnection of double bond and triple
bond. In path b, back-donation of the electron from the gold
center leads to the formation of bicyclo[1.1.0]butane inter-
mediate C.[12] From intermediate C, three consecutive 1,2-
alkyl shifts via carbocation species D, E, and F occur, thus
affording Dewar-benzene-type intermediate G.[13] Through
the three consecutive 1,2-alkyl shifts, the cleavage of both
double and triple bonds are complete.[14] Subsequently, ring
opening of intermediate G leads to the formation of
intermediate H,[15,16] from which a 1,2-shift generates the
final product 2’.
To gain further insights into the substituent effects, we
synthesized cyclopropene derivatives 11–18 and investigated
their reactions under gold catalysis. Compounds 11–13
showed no reaction under identical conditions, which suggests
that the cyclopropene moiety has a relatively low reactivity
toward gold complex. These results are in agreement with the
mechanism proposed in Scheme 5, in which the activation of a
triple bond triggers the cycloisomerization process.[17]
Cyclopropene derivatives 14 and 15 are 1,6-enyne sys-
tems. We observed that the gold-catalyzed reactions of 14 and
15 proceeded through similar mechanisms, initiated by the 5-
exo-dig attack of the gold-catalyzed triple bond by a cyclo-
The dramatic effect of substituents on the switch of
reaction pathway may be due to steric effects of the
substituent. When there are two phenyl groups on the
double bond and one aryl group in the terminal alkyne, the
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propene to generate gold–carbene 22 (Scheme 7). The
reaction affords tricyclic products 19 and 20. The structure
of 19 was confirmed by X-ray crystallographic analysis.[9]
Scheme 8. Gold-catalyzed reaction of 1,7-ene-cyclopropene. DCE=di-
chloroethane, Ts=4-toluenesulfonyl.
those reported in the literature, it can be concluded that
alkyne groups is more effectively activated by gold catalyst
than cyclopropenes.
Received: May 4, 2010
Revised: June 9, 2010
Published online: July 29, 2010
Scheme 7. Gold-catalyzed reaction of 1,6-enyne systems.
Keywords: cycloisomerization · cyclopropenes · enynes · gold ·
.
phenols
Cyclopropene derivative 16 is a 1,7-enyne system; its gold-
catalyzed reaction proceeded in a similar manner to conven-
tional 1,7-enyne systems, that is, through 1,6-exo-dig attack to
afford 23 in 30% yield with catalytic [Au(PPh3)Cl]/AgOTf
[Eq. (1)]. Interestingly, gold(I) complex 24 was highly effi-
cient for this reaction, which afforded 23 in 88% yield.
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