Palladium-catalyzed cyclization of cyclopropyl-substituted 1,6-enynes to 57-bicyclic trienes or monocyclic trienes depending upon the leaving group

Article abstract of DOI:10.1055/s-0031-1290465

Palladium-catalyzed cyclization of cyclopropyl-substituted 1,6-enynes proceeded in two ways depending on the presence or absence of a leaving group. In the presence of the leaving group, 5.7-bicyclic trienes were obtained as the sole products; in the absence of the leaving group, monocyclic trienes were isolated.

Full text of DOI:10.1055/s-0031-1290465

LETTER
▌2657
letter
Palladium-Catalyzed Cyclization of Cyclopropyl-Substituted 1,6-Enynes to
5.7-Bicyclic Trienes or Monocyclic Trienes Depending upon the Leaving
Group
5.7-Bicyclic and Monocyclic Trienes
Hye Mi Oh, So Hee Sim, Sang Ick Lee, Jisu Kim, Young Keun Chung*
Intelligent Textile System Research Center and Department of Chemistry, College of Natural Sciences, Seoul National University,
Seoul 151-474, Korea
Fax +82(2)8890310; E-mail: ykchung@snu.ac.kr
Received: 26.07.2012; Accepted after revision: 15.08.2012
In particular, Wender and co-workers6 demonstrated that
Abstract: Palladium-catalyzed cyclization of cyclopropyl-
VCP is a very versatile strained cyclic synthon in the
substituted 1,6-enynes proceeded in two ways depending on the
rhodium-catalyzed [5+2] cyclizations of cyclopropylen-
ynes. However, the reaction products in the transition-
presence or absence of a leaving group. In the presence of the leav-
ing group, 5.7-bicyclic trienes were obtained as the sole products;
in the absence of the leaving group, monocyclic trienes were isolat- metal-catalyzed reaction of cyclopropylen-ynes seem to
ed.
be rather dependent upon the catalyst and the substituent
on the alkyne (Scheme 1). For example, Louie et al.5
Key words: palladium, enynes, vinyl cyclopropanes, seven-
membered rings, bicyclic compounds
found a size effect, due to the substituent on the alkyne, on
the heterocyclic product formed in the Ni(cod)2–SIPr-cat-
alyzed reaction, although employment of Ni(cod)2–ItBu
as a catalytic system solved the selectivity problem.
Three-membered carbocycles are extremely important
and versatile building blocks for organic chemistry.1 The
bonds between the carbon atoms are considerably weaker
than a typical carbon–carbon bond, yielding a reactivity
similar to or greater than that of alkenes. Their unique
structural and electronic properties give rise to an array of
very notable, characteristic transformations. For example,
cyclopropyl groups engage in many rearrangement reac-
tions including the vinylcyclopropane rearrangement and
the divinylcyclopropane–cycloheptadiene rearrange-
ment.2 Cyclopropyl groups also participate in cycloaddi-
tion reactions such as formal [5+2] cycloadditions.3 Vinyl
cyclopropanes (VCPs) are considered to be particularly
suitable C5 building blocks in the formal [5+2] cycloaddi-
tion. Thus, a variety of transition-metal-catalyzed cycliza-
tions have been developed over the past decades.4
Pimm and co-workers reported5 the formation of alk-1-
enyl-(3-aza)bicyclo[3.1.0]hexanes in the palladium-cata-
lyzed cyclization of olefinic propargylic carbonates. Re-
cently, we reported5 that the consecutive reactions of
platinum- and rhodium-catalyzed cycloisomerization of
enynes bearing a cyclopropyl and aryl group in the alkene
and alkyne termini, respectively, afforded azabicyclo-
[3.2.2]nona-2,8-dienes in high yields. While we were
studying the design and development of new metal-
catalyzed cycloaddition reactions as part of an on-going
project,7 we recently discovered Pd-catalyzed reactions of
cyclopropylenynes that selectively afforded 4-methylene-
hexahydroazulenes or 1-[(E)-buta-1,3-dienyl]-2-methylene-
cyclopentanes depending upon the substituent on the al-
kyne moiety. We herein report our results.
R1
R1
ref. 5a
ref. 5b
X
X
Rh cat.
Ni cat.
R1
R
R
1 = alkyl or aromatic
2 = cyclopropyl
R1 = alkyl or aromatic
X
R
2 = cyclopropyl
R2
PtCl2 or
Rh(I) cat.
Pd cat.
R1
X
X
ref. 5c
ref. 5d
R1 = CH2OC(O)OMe
2 = H
R
R
1 = aryl
2 = cyclopropyl
R
Scheme 1 Metal-catalyzed cycloisomerization5
SYNLETT 2012, 23, 2657–2662
Advanced online publication: 21.09.2012
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
DOI: 10.1055/s-0031-1290465; Art ID: ST-2012-U0630-L
© Georg Thieme Verlag Stuttgart · New York

2658
H. M. Oh et al.
LETTER
OR
OMe
R = Me
R = PO(OEt)2
TsN
TsN
TsN
1a 81% (1 h)
11b 40% (3 h)
1
Scheme 2 Palladium-catalyzed cycloisomerization depending upon the leaving group
Initial treatment of olefinic propargylic diethyl phosphate hour. The yield of the reaction was also highly sensitive to
1 with a catalytic amount of [Pd2(dba)3] in dioxane at the reaction solvent (entries 9 and 10). We also screened
90 °C for one day gave bicyclic triene 1a in 17% yield nickel compounds such as Ni(acac)2/Ph3P (1:2) and
with 67% recovery of the reactant (Scheme 2).
Ni(OAc)2/Ph3P (1:2) as potential catalysts. However, no
reaction was observed with the nickel systems. We were
able to establish the following optimized reaction condi-
tions: 5 mol% Pd(OAc)2, 10 mol% Ph3P, in dioxane, at
90 °C for 1 hour.9,10
1
The structure of 1a was confirmed by H and 13C NMR
studies.8 It has a 4-methylene-1,2,3,4,5,6-hexahydro-
azulene skeleton, which can be considered to be an impor-
tant building block because it may participate in a variety
of functionalizations and tandem carbon–carbon bond- We first examined whether phosphate was the best leav-
forming processes. The formation of 1a suggests that the ing group under the optimized reaction conditions (Table
phosphate acts as a leaving group. Encouraged by the ob- 2). Olefinic propargylic compounds (entries 2 and 3) hav-
servation of the formation of 1a, we also screened other ing an acetoxy or carbonate as a leaving group afforded
palladium compounds (Table 1, entries 1–8). Among the expected product 1a in rather low yields (40 and 49%,
them, the best yield (81%) was observed in the presence respectively) under our reaction conditions. In the case of
of 5 mol% Pd(OAc)2/10 mol% Ph3P in dioxane for one the carbonate, when the reaction was carried out in the
Table 1 Reaction of 1 under Various Reaction Conditions
OPO(OEt)2
catalyst
TsN
TsN
1
1a
Entry
Catalyst
Solvent
Temp (°C)
Time (h)
Yield (%)c
1a
2a
Pd2(dba)3
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
DCE
90
90
90
90
25
70
80
100
90
90
70
90
90
24
24
24
24
24
2
17
Pd(PPh3)4
42
3a
Pd(OAc)2
13
4a
Pd(OAc)2/dppp (1:1)
Pd(OAc)2/dppf (1:1)
Pd(OAc)2/dppe (1:1)
Pd(OAc)2/P(o-toly)3 (1:2)
Pd(OAc)2/Ph3P (1:2)
Pd(OAc)2/Ph3P (1:2)
Pd(OAc)2/Ph3P (1:2)
Pd(OAc)2/dppp(1:1), CO (1 atm)
Ni(acac)2/Ph3P (1:2)
Ni(acac)2/Ph3P (1:2)
28
5a
41
6a
71
7a
24
1
23 (52)d
81
8b
9b
22
1
33
10b
11b,e
12b
13b
toluene
THF–MeOH
dioxane
dioxane
39
21
24
24
63
n.r.f
n.r.f
a Reaction conditions: Pd catalyst (10 mol%), enyne (0.22 g, 0.5 mmol), solvent (5 mL).
b Reaction conditions: Pd catalyst (5 mol %), enyne (0.22 g, 0.5 mmol), solvent (5 mL).
c Isolated yield.
d Yields in parenthesis are for the recovered reactant.
e Reaction conditions: enyne (0.22 g, 0.5 mmol), Pd(OAc)2 (5 mol%), THF (5 mL), MeOH (0.5 mL), CO (1 atm).
f n.r. = No reaction
Synlett 2012, 23, 2657–2662
© Georg Thieme Verlag Stuttgart · New York

LETTER
5.7-Bicyclic and Monocyclic Trienes
2659
presence of 5 mol% Pd(OAc)2 and 5 mol% dppp under bearing a phosphate group under our reaction conditions.
CO (1 atm), a higher yield (63%) was observed. The leav- As the steric bulk of the N-tether groups increased
ing group had a dramatic effect on the yield of the reaction (cf. entries 4 vs. 5), the yield decreased. The number of in-
and the best yield (81%) was observed with the phosphate ternal substituents (cf. entries 6 vs. 8) and the identity and
group. Therefore, we examined the scope of substrates position of the substituents (cf. entries 6 vs. 7) affected the
Table 2 Palladium-Catalyzed Cyclization of Cyclopropyl-Substituted 1,6-Enynes to 5.7-Bicyclic Trienesa
Entry
Substrate
Time (h)
Product yield (%)b
OR
TsN
TsN
1
1 R = PO(OEt)2
2 R = COMe
3 R = CO2Me
1
1
1
1a 81
1a 40
1a 63
2
3c
OPO(OEt)2
O2S
N
4
5
4
5
O2S
N
4a 65
4
OPO(OEt)2
O
O
S
Naph
S
N
Naph
N
O
O
5a 35
5
OPO(OEt)2
R
R
TsN
TsN
6
7
8
6 R = Me
7 R = Et
20
4
6a 93
7a 60
8a 43
8 R = gem-dimethyl
3
OPO(OEt)2
TsN
9
20
TsN
9a 50
9
OPO(OEt)2
MeO2C
MeO2C
MeO2C
MeO2C
10
4
10a 47
10
a Reaction conditions: substrate (0.3 mmol), Pd(OAc)2 (5 mol%), Ph3P (10 mol%), dioxane (3 mL), 90 °C.
b Isolated yield.
c Under 1 atm CO.
© Georg Thieme Verlag Stuttgart · New York
Synlett 2012, 23, 2657–2662

2660
H. M. Oh et al.
LETTER
yield of the reaction. A 1,7-enyne was also a good sub-
strate (entry 9) and a carbon-tethered substrate (entry 10)
also underwent the reaction.
OPO(OEt)2
5 mol% Pd(OAc)2
10 mol% Ph3P
TsN
D
TsN
dioxane, 90 °C
We then investigated the reactions of substrates without
having a leaving group to determine whether they gave a
bicyclic triene as a product. When substrate 11 was treated
in dioxane under the standard catalytic conditions, mono-
cyclic triene 1-[(E)-buta-1,3-dienyl]-2-methylene-cyclo-
pentane (11b), was isolated in 40% yield (Scheme 2).
1-D
1a 81% (1.5 h)
Scheme 4 Palladium-catalyzed cycloisomerization of 1-D
Zuo and Louie reported5a a similar reaction in the pres-
ence of Ni(cod)2–NHC. To ascertain whether this rear-
rangement was typical, a range of substrates 12–14 were
prepared and tested (Scheme 3).
OPO(OEt)2
5 mol% Pd(OAc)2
10 mol% Ph3P
TsN
TsN
dioxane, 90 °C
OBn
15
15c 60% (17 h)
5 mol% Pd(OAc)2
10 mol% Ph3P
OBn
TsN
TsN
TsN
Scheme 5 Palladium-catalyzed cycloisomerization of 15
TsN
TsN
TsN
dioxane, 90 °C
A similar reaction product was obtained in the palladium-
catalyzed carbocyclization of aza-enallenes.11 It seems
that the presence of a methyl group blocks the formation
of a seven-membered ring compound.
12b 42% (20 h)
12
13
14
OMe
5 mol% Pd(OAc)2
10 mol% Ph3P
OMe
dioxane, 90 °C
Based on the experimental observations, a plausible reac-
tion mechanism for the formation of a has been proposed
(Scheme 6). In the presence of a palladium catalyst, an
equilibrium mixture of η1-allenic and propargylic inter-
mediate I forms,12 which may be in equilibrium with η3-
propargyl intermediate II.13 Both intermediates can be
transformed into a (π-olefin)(σ-allenyl)palladium inter-
mediate III. Carbon–carbon bond formation followed by
ring-opening of the allyl group leads to intermediate V.
The allyl group then attacks the central carbon of the al-
Ph
Ph
13b 67% (24 h)
OMe
5 mol% Pd(OAc)2
10 mol% Ph3P
OMe
dioxane, 90 °C
14b 65% (1.5 h)
Scheme 3 Palladium-catalyzed cyclization of cyclopropyl-substitut- lene group to form a seven-membered ring and regenerate
ed 1,6-enynes to monocyclic trienes
the catalytic species to reenter the catalytic cycle. When
the final step is blocked, b instead of a is formed.
With substrate 12, containing the cyclopropyl group,
monocyclic triene 12b was isolated in 42% yield. With a
phenyl (13) or methyl group (14) instead of a cyclopropyl
group, monocyclic dienes 13b and 14b were isolated in
67 and 65% yields, respectively. Thus, the formation of
cyclopentane derivatives was quite general for the palla-
dium-catalyzed reaction of enyne substrates not bearing a
leaving group. In the formation of a bicyclic triene from a
substrate, both the leaving group and one of the hydrogen
atoms were lost. To gain some information on the forma-
tion of bicyclic trienes, deuterated compound 1-D was re-
acted under the standard reaction conditions (Scheme 4).
In conclusion, we have demonstrated that palladium-
catalyzed reaction of 1,6-enynes bearing a cyclopropyl
moiety at the alkene affords bicyclic or monocyclic tri-
enes as products depending on the presence or absence of
a leaving group. The bicyclic or monocyclic trienes can
form a class of important building blocks because they
possess three double bonds that may be used for further
derivatization.
Acknowledgment
This work was supported by the National Research Foundation of
When the product of the reaction was isolated, the deute- Korea (NRF) grant, which was funded by the Korea Government
(MEST) (2011-0031444) and the Basic Science Research Program
through the NRF, funded by the Ministry of Education, Science and
Technology (R11-2005-065). H.M.O. thanks the Brain Korea 21 for
fellowships.
rium was not found in the product. This observation sug-
gested that the disappearing hydrogen atom was from the
inner position of an alkene moiety.
Moreover, when a methyl group was introduced into the
same position as that of the deuterium in 1-D, a new cy-
clopentene derivative, 15c, produced by the isomerization
of an allene to a diene, instead of a bicyclic triene, was iso-
lated in 60% yield (Scheme 5).
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.SnuIpofoiprmntgirStanoIuifpog
r
t
iornat
Synlett 2012, 23, 2657–2662
© Georg Thieme Verlag Stuttgart · New York

LETTER
5.7-Bicyclic and Monocyclic Trienes
2661
R1O
Pd
OR1
Pd
OR1
•
R2
X
R2
X
R2
X
I
R1 = PO(OEt)2
R2 = H, D, Me
X = NTs
•
OR1
Pd
X
R2
Pd+
OR1
X
R2
III
II
– R1OH(D)
– Pd2+
•
•
•
Pd+
Pd+
R2
R2
X
X
X
X
R2
R1O
R1O
Pd+
OR1
a
IV
V
when R2 = H or D
•
Pd+
R2
X
R2O
H'
– R1OH(D)
– Pd2+
X
R2
b
when R2 = Me
Scheme 6 A plausible mechanism
Reed, J. W. Angew. Chem. Int. Ed. 2010, 49, 4864. (d) For
recent papers, see: Shimokawa, J.; Harada, T.-A.;
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Synlett 2012, 23, 2657–2662

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H. M. Oh et al.
LETTER
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C
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capped with a rubber septum, dioxane (2 mL), Pd(OAc)2 (5
mol%), and Ph3P (10 mol%) were added under a N2 flow.
Compound 1 (0.3 mmol) was added to the flask under N2.
The mixture was heated at 90 °C and the progress of the
reaction was monitored by TLC. The mixture was cooled to
room temperature, filtered, and the solvent was evaporated
under reduced pressure. The product was purified by flash
chromatography on a silica gel column (n-hexane–ethyl
acetate, 10:1).
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Synlett 2012, 23, 2657–2662
© Georg Thieme Verlag Stuttgart · New York

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