Tandem platinum dichloride catalysis and thermal reaction of enynes: Versatile synthetic platform based on bicyclo[4.1.0]hept-2-enes

Article abstract of DOI:10.1002/adsc.201100694

A tandem platinum dichloride-catalyzed cycloisomerization and rearrangement reaction of enynes leads to the formation of 2,4-pentadienals, 1-aryl-tetrahydro-1H-indenes, or 3-methylene-4-vinylcyclohex-1-enes; depending upon the substituent(s) and tether group of the enynes, the platinum dichloride-catalyzed cycloisomerization products, bicyclo-[4.1.0]hept-2-enes, undergo a Claisen rearrangement, a 1,5-hydrogen shift, or a vinylcyclopropane rearrangement.

Full text of DOI:10.1002/adsc.201100694

FULL PAPERS
DOI: 10.1002/adsc.201100694
Tandem Platinum Dichloride Catalysis and Thermal
Reaction of Enynes: Versatile Synthetic Platform Based on
Bicyclo[4.1.0]hept-2-enes
ACHTUNGTRENNUNG
Sun Young Kim,a Yujung Park,a Sori Son,a and Young Keun Chunga,*
a
Intelligent Textile System Research Center, Department of Chemistry, College of Natural Sciences, Seoul National
University, Seoul 151-747, Korea
Fax : (+82)-2-889-0310; e-mail: ykchung@snu.ac.kr
Received: September 2, 2011; Revised: October 10, 2011; Published online: January 13, 2012
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201100694.
Abstract: A tandem platinum dichloride-catalyzed
ACHTUNGTREN[NUGN 4.1.0]hept-2-enes, undergo a Claisen rearrangement,
cycloisomerization and rearrangement reaction of a 1,5-hydrogen shift, or a vinylcyclopropane rear-
enynes leads to the formation of 2,4-pentadienals, 1- rangement.
aryl-tetrahydro-1H-indenes, or 3-methylene-4-vinyl-
cyclohex-1-enes; depending upon the substituent(s) Keywords: 1,3-alkyl migration; cycloisomerization;
and tether group of the enynes, the platinum dichlor- homogeneous catalysis; 1,5-hydrogen migration;
ide-catalyzed cycloisomerization products, bicyclo- olefin isomerization; platinum; vinylcyclopropanes
Introduction
rangement to create synthetically useful heterocyclic
molecules, such as bicyclo[3.2.2]nonadienes, and
AHCTUNGTRENNUNG
Transition metal-catalyzed cycloisomerization reac- 1,6,7,9a-tetrahydrocyclohepta[c]pyrans and -pyridines
tions can provide a variety of cyclic scaffolds from (Scheme 1).
readily available starting materials that are not easily
obtained by conventional synthetic methods.[1] catalyzed bicyclizations of enynes leading to bicyclo-
Bicyclo[4.1.0]hept-2-ene derivatives (a) are one class [4.1.0]hept-2-ene and a range of subsequent rear-
of the products which can be easily obtained by tran- rangements, depending upon the substituents
From these previous studies, we envisioned PtCl2-
A
ACHTUNGTRENNUNG
sition metal-catalyzed cycloisomerization of 1,6- (Scheme 2). Finally, we found that bicycloACTHNUGRTENUNG[4.1.0]hept-
enynes,[2] typically utilizing Pt(II) and Au(I) as the 2-ene derivatives can selectively undergo a Claisen re-
catalyst. Recently, a restricted range of enantioselec- arrangement, a [1,5]hydrogen shift, or a vinylcyclo-
tive processes also has been reported.[3] The unique propane rearrangement depending upon the substitu-
structure of bicycloACTHNUTRGENUGN[4.1.0]hept-2-ene derivatives can ent(s) and tether groups in the enyne substrates.
give rise to an array of interesting characteristic trans- Here, we reported a tandem PtCl2-catalyzed cycloiso-
formations, but their use in the synthesis of more merization and rearrangement reaction of enynes to
complex compounds is still rare (Scheme 1).[4] Echa- produce 2,4-pentadienals, 1-aryltetrahydro-1H-in-
varren et al.[4d,e] reported an oxidative ring opening of denes, or 3-methylene-4-vinylcyclohex-1-enes.
3-oxabicycloACHTUNGTRENNUNG[4.1.0]hept-4-enes giving oxepanes, dihy-
drofurans or 3,4-dihydro-2H-chromenes. Very recent-
ly, this transformation has been used in the synthesis Results and Discussion
of GSK1360707F.[5]
Under these circumstances, we have been exploring Synthesis of 2,4-Pentadienals from Allyl Propargyl
the possible utilization of bicycloACHTNUGTREN[NGNU 4.1.0]heptenes in or- Ethers
ganic synthesis further. A few years ago, we had used
cyclopropyl-substituted bicycloACHTNUGRTNE[NUG 4.1.0]hept-2-enes in a We initially studied a PtCl2-catalyzed isomerization of
carbonylative [3+3+1]cycloaddition reaction.[4a] Re- oxygen-tethered enynes. We selected 1 as the basic
cently, we reported[4b,c] on a PtCl2-catalyzed cycloiso- substrate, because the rearrangement occurred only in
merization of dienynes and a subsequent Cope rear- the presence of an aryl substituent at the internal
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Scheme 1. Transition metal-catalyzed cycloisomerization and consecutive reactions.
Scheme 2. Platinum-catalyzed cycloisomerization and consecutive reactions.
carbon of an alkene group. Three years ago, we com-
municated the formation of 2,4-pentadienal from
bicycloACHTUNGTRENNUNG
[4.1.0]hept-2-ene.[4a] However, only one exam-
ple was reported at that time.
When 1 was treated with 2.5 mol% PtCl2 in toluene
at 1108C for 1 h, a mixture of 1a, 1b, and 1c was ob-
tained in 44%, 5%, and 10% yields, respectively. A
Claisen rearrangement and a consecutive hydrogen
shift of 1a took place with 2,4-dienals 1c being ob-
tained (Scheme 3).
We investigated the best conditions for obtaining 1c
using 1 as a model substrate (Table 1). When a reac-
tion was carried out in toluene using 2.5 mol% PtCl2
at 1108C (entries 1–3), the yields and distribution of
the products varied depending on the reaction time
and temperature. As the reaction time increased, the
Scheme 3. PtCl2-catalyzed cycloisomerization and sequential
yield of 1c increased. However, a prolonged reaction rearrangements.
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Tandem Platinum Dichloride Catalysis and Thermal Reaction of Enynes
Table 1. PtCl2-catalyzed cycloisomerization of oxygen-teth-
We next investigated the PtCl2-catalyzed cycloiso-
merization of a variety of oxygen-tethered enynes and
a subsequent rearrangement (Table 2).
ered enynes followed by sequential rearrangements.[a]
Entry
Catalyst
[mol%]
Temp.
[8C]
Time
[h]
Yield [%]
The reaction time varied depending on the sub-
strate. The reaction afforded the corresponding prod-
ucts in reasonable to high yields with a high prefer-
ence for the (2E,4E)-stereoisomer. The diastereo-
meric ratio of each compound was determined by the
1H NMR spectrum of the crude product. In the case
of entry 2, the (2Z,4E)-stereoisomer 2c, having a dif-
ferent geometry at C-2, was obtained in 5% yield. In-
terestingly, only (2E,4E)-stereoisomers were observed
for entries 4 and 6. The stereoselectivity was highly
sensitive to the steric bulkiness (entries 1 and 2 vs. 3
and 5), but rather insensitive to the electronic effect
of the aryl group (entries 1 vs. 9 and 10). An enyne
bearing a 4-MeOC6H4 instead of a phenyl group gave
a poor yield (21%) at 1108C. However, when the re-
N
1a
1b
1c
1
2
3
4
5
2.5
2.5
2.5
1.3
1.3
110
110
110
90
1
4
21
10
4
44
–
–
15
3
5
4
–
–
–
10
68
11
32
110
81
(74[b])
64
6[c]
1.3
130
1
–
–
[a]
1
Yields and isomeric ratio determined by H NMR versus
methyl benzoate as an internal standard.
Isolated yields.
[b]
[c]
Xylene was used as a solvent.
time (21 h; entry 3) was detrimental; probably the al- action was conducted at 1408C, the yield increased to
dehyde 1c was decomposed by heat. Varying the 71%. A substrate having 4-CF3C6H4 gave a higher
amount of catalyst had an effect on the yield, which yield (77%) at 1108C. The structure of the aldehyde
could be reduced to 1.3 mol%. The best yield (81% produced was confirmed by X-ray diffraction analysis
by NMR; 74% isolated yield) was obtained when the of 4c (Figure 1).
reaction was carried out in the presence of 1.3 mol%
of PtCl2 in toluene at 1108C for 4 h. Only a small
amount of 1a (3%) was observed.
Table 2. PtCl2-catalyzed cycloisomerization of a variety of O-tethered enynes and subsequent thermal rearrangements.[a]
Entry
Substrate
Time [h]
Product
Yield [%][b] (4E/4Z)
1
1a
4
1c
74 (14/1)
2[c]
3
2a (R1 =Ph)
3a (R1 =Me)
2
4
3
2
2c
3c
4c
5c
78 (7.7/1)
79 (6/1)
77
4[d]
5
4a (R1 =cyclopropyl)
5a (R1 =vinyl)
58 (6/1)
6
6a
6
6c
48
7
8
7a (R2 =p-CH3OC6H4)
8a (R2 =p-CF3C6H4)
24
24
7c
8c
71 (10/1)
77 (12/1)
[a]
Reaction conditions: 0.3 mmol of substrate and 1.3 mol% PtCl2 were reacted in 6 mL toluene at 1108C.
Isolated yield.
2c was obtained in 5% yield.
In xylene at 1408C.
[b]
[c]
[d]
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Synthesis of 1-Aryltetrahydro-1H-indenes from
Enynes
We next studied a PtCl2-catalyzed reaction of oxygen-
tethered enynes bearing an aryl group at the terminal
position of the alkene. When compound 9 was treated
with 1.3 mol% PtCl2 in toluene at 808C for 2 h, a cy-
clopropanated product 9a instead of a dienal was iso-
lated in 69% yield. Heating the isolated 9a in toluene
at 1108C for 12 h gave 9-phenyloctahydroindeno
c]pyran, 9d, in 92% yield [Eq. (1)].
ACHTUNGTRENNUNG[2,1-
As expected, the vinylcyclopropane rearrangement
of 9a to 9d needs a high reaction temperature and a
long reaction time. However, the reaction tempera-
ture was much lower than those for the thermal rear-
Figure 1. X-ray crystallography of 4c.
Table 3. A consecutive reaction of PtCl2-catalyzed cycloisomerization and vinylcyclopropane rearrangement.[a]
Entry
Substrate
Time [h]
Product
Yield [%][b]
X
Ar
R
1
2
3
4
5
9
O
NTs
O
NTs
NTs
Ph
Ph
Ph
H
H
Me
H
H
18
24
24
24
24
9d
80
80
34
68
74
10
11
12
13
10d
11d
12d
13d
p-CH3OC6H4
p-CH3C6H4
R1
R2
R3
6
7
8
9
14
15
16
17
H
H
Ph
H
H
Me
Me
H
22
24
18
3
14d
15d
16d
17d
59[c]
36[d]
57
Me
Ph
Ph
H
48[e]
[a]
Reaction conditions: 0.3 mmol of substrate and 5 mol% PtCl2 were reacted in 6 mL toluene at 1208C.
Isolated yield.
Isolated as a mixture with 14a in 10% yield.
Isolated as a mixture with 15a in 36% yield.
Isolated as a mixture with 17a in 19% yield.
[b]
[c]
[d]
[e]
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Tandem Platinum Dichloride Catalysis and Thermal Reaction of Enynes
rangement of unactivated vinylcyclopropanes to cy-
clopentenes.[6] Compared to the formation of 1c from
1, the formation of 9d from 9 shows the importance
of the position of a substituent in the enyne substrate.
Thus substituents affect both the direction of the
transformations and their activation barriers.
We investigated a consecutive sequence of PtCl2-
catalyzed cycloisomerization and thermal reaction in
a one-pot reaction. When the same reaction was car-
ried out at 1208C, the total yield was improved to
80%. We further screened the consecutive reactions
of PtCl2-catalyzed cycloisomerization and vinylcyclo-
propane rearrangement with a variety of dienynes
having a phenyl group at the terminal carbon of an
alkene moiety. The results are compiled in Table 3.
Figure 2. X-ray crystal structure of 16d.
prerequisite for the generation of the cyclopentene
Compounds having a cyclopenta[c]pyridine or a cy- products.[7]
clopenta[c]pyran skeleton were obtained in reasona-
ble to high yields. However, a dienyne bearing a gem-
disubstituted alkene moiety (entry 3) afforded a Synthesis of 3-Methylene-4-vinylcyclohex-1-enes
rather lower yield (34%). Dienynes (entries 2, 4, and
5: 67–80%) bearing a cyclic olefin gave higher yields We next investigated a PtCl2-catalyzed reaction of O-
than those having an acyclic olefin (entries 6–9: 36– tethered dienynes bearing a methyl group at the ole-
59%). The structure of the product was confirmed by finic terminal carbon. We expected that this type of
X-ray diffraction analysis of 16d (Figure 2).
the substrate would be transformed to the 2,4-dienal
However, dienynes bearing no phenyl group at the of Eq. (1), but when substrate 20 was reacted with
terminal carbon of an alkene group gave only cyclo- 1.3 mol% of PtCl2 in xylene at 1408C for 24 h, a
propanated products in high yields (18 and 19: [Eq. methylene-4-vinylcyclohexene derivative, 20e, was iso-
(2)]). Thus, it seems that the presence of an aryl sub- lated in 65% yield [Eq. (3)]. Thus, a subtle change in
stituent at the terminal carbon of an alkene group is a the substitution leads to a quite different reaction
product. The product was derived from a cycloisome-
rization followed by a [1,5]hydrogen shift.[8]
The scope of the reaction was further studied with
a variety of dienynes having a methyl group at the
olefinic terminal carbon (Table 4). The reaction tem-
perature was highly dependent upon the substrate
itself. In the case of 26, a reaction temperature of
808C was chosen. In some cases, a reaction tempera-
ture of 1408C was chosen, in order to enhance the
thermal reaction. When significant decomposition of
the products was observed at 1408C, 1008C was
chosen as the reaction temperature. The yield found
to be highly dependent upon the substituent at the al-
keneꢁs inner carbon and on the concentration of the
catalyst. When the substrate (21) in entry 2 was react-
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Sun Young Kim et al.
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Table 4. A consecutive reaction of PtCl2-catalyzed cycloisomerization and [1,5]-hydrogen shift.[a]
Entry
Substrate
Time [h] Temp. [8C]
Product
Yield [%][b]
X
R1
R2
1
2
3
4
5
20[c]
21
22
23
24
O
O
Ph
H
H
Me
H
H
H
24
48
24
24
48
140
140
140
140
100
20e 65
21e 57[e]
22e 78
23e 69
24e 63
NTs Ph
NTs p-CH3OC6H4
NTs
H
X
R1
R2 R3
6
7
8
9
10
11
25
NTs
H
H
H
H
Ph
Me Ph
Me
24
48
8
24
24
8
140
80
140
100
140
140
25e 83
26e 87
27e 65[f]
28e 48
29e 41
30e 58
26[d] NTs
27
28
NTs
NTs Me
O
O
H
Me Ph
Ph Ph
H
29
H
Ph
30[c]
Ph
12
31[d]
20
140
31e 41[g]
[a]
Reaction conditions: 0.3 mmol of substrate and 5 mol% PtCl2 were reacted in 6 mL xylene at the presented temperature.
Isolated yield.
[b]
[c]
[d]
[e]
[f]
1.3 mol% PtCl2 was used.
10 mol% PtCl2 was used.
33% of 21 was recovered.
Isolated as a mixture with 27a in 9% yield.
9% of 31 was recovered.
[g]
ed in the presence of 1.3 mol% PtCl2, no reaction was
observed, but 57% of the expected product (21e) with
a 33% recovery of 21 was isolated in the presence of
5.0 mol% of PtCl2. In the presence of 10 mol% of
PtCl2, all of the reactants were decomposed, that is,
no isolable compound was observed. Interestingly, for
oxygen-tethered substrates (20 and 30), a higher yield
was observed when 1.3 mol% PtCl2 was used instead
of 5.0 mol% PtCl2 (entries 1 and 11). Higher yields (> acted at 1408C over a 4-day of a reaction period,
80%) were observed with enynes having an aryl compound 32a was isolated in 78% with a trace
group at the alkeneꢁs inner position (entries 6 and 7). amount of 32e.
This observation is consistent with the acceleration of
Thus it seems that the electronic effect of the sub-
1,5-homodienyl hydrogen migrations due to electron- stituent at the alkeneꢁs inner position has a profound
withdrawing aryl substituents.[9] Dienyne substrates effect on the yield of the reaction. When a dienyne
(entries 2, 5, and 8) without a substituent at the al- (31) having an ethyl group instead of the methyl at
keneꢁs inner position gave the expected product in the olefinic terminal carbon was used as a substrate
reasonable yields (57–65%).
(entry 12), the expected reaction occurred, but the
However, when a substrate bearing a methyl group yield decreased to 41%. Oxygen-tethered dienynes
at the alkeneꢁs inner position (32 in [Eq. (4)]) was re- with a methyl group at the olefinic terminal carbon
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Tandem Platinum Dichloride Catalysis and Thermal Reaction of Enynes
(entries 1, 2, 10 and 11) gave the expected products at Acknowledgements
slightly lower yields (41–65%) than those of N-Ts-
tethered dienynes. It has been well documented[6a]
that a [1,5]hydrogen shift pathway competes with a
vinylcyclopropane rearrangement when the 2-cis sub-
stituted groups at vinylcyclopropane contain at least
one hydrogen atom at the a-position. However, our
study shows that the 1,5-hydrogen shift is unique to
dienyne substrates having a methyl group at the ter-
minal carbon of the alkene moiety and is much pre-
ferred when dienynes have an aryl group at the inner
position of an alkene.
This work was supported by the National Research Founda-
tion of Korea (NRF) (2010-0029663) and the Basic Science
Research Program through the NRF funded by the Ministry
of Education, Science and Technology (R11-2005-065). SS is
grateful for the BK21 fellowship.
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Conclusions
We have demonstrated two reaction pathways leading
to compounds which are difficult to prepare by con-
ventional synthetic pathways: that is, two new rear-
rangements of bicycloACTHNUGRTENUNG[4.1.0]hept-2-enes to 2,4-penta-
dienals, 1-aryltetrahydro-1H-indendes, or 3-methyl-
ene-4-vinylcyclohex-1-enes which are found as sub-
structures in natural products having a diverse range
of biological activities.[10] The reaction pathways are
highly sensitive to the identity and position of (a) sub-
stituent(s) at the alkene moiety. Thus we can predict
the characteristics of a reaction product based on our
studies. Readily available enynes are used as starting
materials for the reaction. Moreover, the experimen-
tal simplicity is noteworthy. Further studies on the ap-
plication of bicycloACHTNUGRTENUNG[4.1.0]heptenes as molecular scaf-
folds and a comprehensive experimental and compu-
tational study will be reported in due course.
Experimental Section
General Procedure for the Tandem PtCl2-Catalyzed
Cycloisomerization and Thermal Reactions
PtCl2 and toluene (or xylene, 2 mL) were added to a
Schlenk flask equipped with a stirring bar and capped with a
rubber septum. The substrate (0.3 mmol) and toluene (or
xylene, 4 mL) were then added to the flask and heated. The
resulting mixture was reacted until the substrate was com-
pletely consumed (reaction monitored by TLC). The prod-
uct was purified by flash chromatography on a silica gel
column by eluting with n-hexane/ethyl acetate.
Supporting Information
Characterization data (1H and 13C NMR spectra, and HR-
MS) for substrates (1–32) and products (a, b, c, d, and e),
and crystal data of 4c (CCDC 778494) and 16d (CCDC
778495) (these data can also be obtained free of charge
from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif) are available in the
Supporting Information.
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