Cyclopropane Intermediates from Insertion Reactions of Platinum-Carbenes: A Route to Heterospiranes

Article abstract of DOI:10.1055/s-0036-1591489

Heteroaromatic-anchored enynals with a pendent alkene group were successfully cyclized through a Huisgen-type [3+2] cycloaddition to give a tetracyclic Pt-carbene complex that underwent insertion into the C-H bond in the β-position to give fused cyclopropanes that are otherwise inaccessible. On heating, the cyclopropanes smoothly rearranged to form the corresponding heterospiranes with excellent levels of stereoselectivity and high yields.

Full text of DOI:10.1055/s-0036-1591489

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© Georg Thieme Verlag Stuttgart · New York
2017, 28, A–E
letter
A
en
K. Kim et al.
Letter
Synlett
Cyclopropane Intermediates from Insertion Reactions of
Platinum–Carbenes: A Route to Heterospiranes
Kiseong Kim
F
F
F
H
Soyung Kim
O
Pt(II)
O
O
Chang Ho Oh*
R
N
R
N
R
N
R''
R''
R''
H
R'
Pt(II)
Pt(II)
B
A
1
R'
R'
Department of Chemistry and Research Institute
of Natural Science, Hanyang University, 222,
Wangsimni-ro, Seongdong-gu, Seoul 04763,
Republic of Korea
R = Ph, 2-Naphthyl, OBn, Piperidinyl
R', R'' =
OTBS, H
CO2Et, CO2Et
changho@hanyang.ac.kr
F
F
O
R
R
N
N
R''
R'
O
R''
R'
2
3
3 examples
5 examples
up to 94% yield
up to 78% yield
Received: 30.07.2017
Accepted after revision: 14.09.2017
Published online: 11.10.2017
the cyclopropanes 2, which rearranged to form spiranes 3.9c
Next, we extended the Pt-catalyzed cyclization to the cy-
cloalkane-anchored enynals 4. The cycloalkane-based
enynals were catalytically converted into the corresponding
tricyclic spiranes 5 in good to excellent yields [Scheme
1(b)].9f With this successful result, we extended our proto-
col to the intramolecular Pt-catalyzed spirocyclization of N-
fused enynal systems to synthesize biologically interesting
carbo- and heterospiranes.5,6 Furthermore, we designed an
enynal system that contained a cyclohexyl group on a pen-
dent alkene, which we used to obtain polycyclic spiranes.
DOI: 10.1055/s-0036-1591489; Art ID: st-2017-u0595-l
Abstract Heteroaromatic-anchored enynals with a pendent alkene
group were successfully cyclized through a Huisgen-type [3+2] cycload-
dition to give a tetracyclic Pt–carbene complex that underwent inser-
tion into the C–H bond in the β-position to give fused cyclopropanes
that are otherwise inaccessible. On heating, the cyclopropanes smooth-
ly rearranged to form the corresponding heterospiranes with excellent
levels of stereoselectivity and high yields.
Key words cyclization, platinum catalysis, pyrylium ions, carbenes,
heterospiranes
a) Pt-catalyzed reaction of aromatic substrate
PTSA
"Pt(II)"
O
Spirocyclic compounds have garnered a considerable
amount of recent attention due to their biological impor-
tance and their widespread occurrence in nature.1 Spirocy-
clic compounds consist of two or more cyclic rings linked
together by a common carbon.2 The importance of spirocy-
cles has also been illustrated by their use as valuable syn-
thetic intermediates in total syntheses of natural products.3
Spirocycles are also widely used in materials science.4 De-
spite the utility of spirocycles, the efficient formation of the
spiro quaternary carbon remains an important challenge in
organic synthesis.5 Heterospiranes are spiro compounds
from biological sources that contain heteroatoms (N, O, or
S) in the spirane rings or elsewhere.6
O
+
O
1
3
2
b) Pt-catalyzed reaction of nonaromatic substrate
O
"Pt(II)"
O
O
4
5
c) Pt-catalyzed reaction of N-heterocyclic substrate
This work
O
"Pt(II)"
O
+
N
N
N
O
6
We have long been interested in developing methods for
synthesizing a variety of polycyclic compounds by using
rhodium,7 gold,8 or platinum catalysts.9 The aromatic-an-
chored enynals 1 have been shown to generate Pt–carbene
intermediates via a platinum-bound pyrylium and through
a Huisgen-type [3+2] cyclization [Scheme 1(a)].9 The Pt–
carbene intermediate was expected to furnish a [6,7,6]-tri-
cyclic (±)-faveline derivative under our initial condi-
tions.9c,9d However, the Pt–carbene intermediate generated
8
7
Scheme 1 Platinum-catalyzed cyclizations to form spiranes
On the basis of these previous reaction trends, spirocy-
cles 3 should be obtained via the cyclopropanes 2 (Scheme
2). We therefore examined the cyclization of enynal 1a in
the presence of a platinum catalyst under our previous re-
action conditions,9c and we isolated cyclopropane 2a in 91%
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–E

B
K. Kim et al.
Letter
Synlett
corresponding cyclopropanes 2c and 2d in 84% yield. When
the products 2b and 2c were refluxed in benzene in the
presence of PTSA, the corresponding spiro compounds 3b
and 3c were obtained in good to excellent yields with reten-
tion of stereochemistry. The oxygen-containing OAc func-
tional groups did not interfere with the reaction, but was
eliminated during PTSA-catalyzed rearrangement. Note
that products 2a–c and 3a–c were isolated with high levels
of stereoselectivity, although these reactions generated
multiple stereogenic centers that led to the formation of the
several isomeric products.
PTSA (0.1 equiv)
benzene
PtCl2 (5mol%)
O
O
100 °C, 2 h
toluene,
80 °C, 1 h
2
1
O
3
O
O
MeO
MeO
MeO
O
1a
3a (88%)
2a (92%)
Unfortunately, the rearrangement reaction of the 5-me-
thoxy-substituted substrate 2d was messy, and we could
not establish optimal condition for this reaction. Consequent-
ly, we further studied the formation of 5-methoxy spiranes.
We attempted to accomplish isomerization of cyclopro-
panes 2d–g, because we suspected that the OAc and OMe
groups might interact. The present method worked for 1d–
g and gave the corresponding cyclopropanes 2d–g without
any problems, but longer reaction times at the given tem-
perature led to decomposition in all cases. These phenome-
na might be understood in terms of an electron-donating
effect of the methoxy group, where the electron-rich me-
thoxy group mismatched with the electron-rich carbon gen-
erated during rearrangement to the spirocycle 3d (Scheme
3). The cyclopropanes 2e–g showed similar phenomena.
Note that 1a, which has a methoxy substituent in the meta-
position to the benzylic hydrogen in 2a rearranged to the
corresponding spiro compound 3a.
O
O
OAc
O
OAc
1b
3b (69%)
2b (59%)
O
O
F
F
OAc
OAc
1c
2c (84%)
F
O
3c (86%)
MeO
MeO
MeO
MeO
O
O
OAc
OAc
OAc
1d
2d (84%)
O
O
OAc
1e
MeO
2e (83%)
O
PTSA
O
MeO
4f
MeO
MeO
MeO
MeO
O
O
O
2a
OTBS
MeO
OTBS
1f
2f (75%)
O
MeO
O
O
CO2Et
CO2Et
3a (88%)
CO2Et
CO2Et
1g
2g (29%)
MeO
Scheme 2 Platinum-catalyzed cyclization to give products 2 and 3
OAc
OAc
MeO
O
PTSA
O
4k
yield and, subsequently, spiro compound 3a in 88% yield
through rearrangement with a catalytic amount of 4-tolue-
nesulfonic acid (PTSA) in refluxing benzene. To extend the
substrate scope, we selected substrates 1b–d, which con-
tained a spirocycle-like cyclohexyl group and an oxygen-
containing functional group (OAc) on the pendent alkene.
The 4-fluoro-substituted substrate 1c and 4-methoxy-sub-
stituted substrate 1d, containing an electron-withdrawing
and an electron-donating group, respectively, afforded the
MeO
OAc
2d
unstable!!
O
MeO
O
OAc
3d
Scheme 3 Possible mechanism for the formation of spiranes 3
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–E

C
K. Kim et al.
Letter
Synlett
Finally, we applied our previous work to a typical N-het-
erocyclic system, 2,6-dichloro-5-fluoronicotinonitrile (9;
CFN). First, we prepared pyridines 6 in three steps as start-
ing materials for the CFN system. The advantage of this
method was that we could freely change the R1 group to give
various N-containing compounds substituted with piperi-
din-1-yl, phenyl, 2-naphthyl, or benzyloxy groups (Scheme 4).
We then used a variety of substrates to explore the
scope and limitations of this reaction. Most of the sub-
strates gave the corresponding products 710 and 811 in good
to excellent yields, but compound 6b, which contained a 2-
naphthyl group, gave products 7b and 8b in reduced yields
(Scheme 5). On the other hand, when substrate 6c was
F
O
O
R5
F
R1
N
N
F
CN
Cl
F
R4
R3
F
CN
Cl
DIBAL-H, toluene
–78 °C r.t.
O
PtCl2(PPh3)2 (10 mol%)
R5
R2
R
N
Cl
7
R1
N
R
N
7: toluene, 120 °C, 4 h
8: xylene, 150 °C, 4 h
R4
R3
Cl
N
9
F
10 (70%)
11 (80%)
R5
R4
R3
R2
R2
6
R1
R =
N
+
O
,
,
O
8
F
Sonogashira
Reaction
6
,
O
R
N
6
F
F
R'
O
O
12
6
R'
R'= OTBS, CO2Et
N
N
OTBS
OTBS
Scheme 4 Synthesis of starting material for the CFN system
OTBS
6b
7b (34%)
F
Table 1 Optimization of the Platinum-Catalyzed Reaction of Enynal 6aa
N
F
O
O
8b (23%)
F
Ph
N
N
O
PtCl2(PPh3)2
CO2Et
CO2Et
F
F
7a
O
Ph
N
O
F
CO2Et
CO2Et
O
N
O
N
CO2Et
CO2Et
6a
CO2Et
CO2Et
Ph
CO2Et
CO2Et
6c
7c (81%)
O
8a
F
O
N
Entry
Solvent
Temp (°C)
Time (h)
Product
Yieldb (%)
94
CO2Et
CO2Et
O
1
2
toluene
toluene
120
120
4
7a
8c (73%)
F
F
20
7a
8a
35
8
O
O
N
N
N
N
3
4
5
DCE
80
120
150
12
8
7a
7a
8a
43
63
78
OTBS
OTBS
1,4-dioxane
xylene
6d
7d (0%)
F
4
a Reaction conditions: 6a (0.40 mmol), PtCl2(PPh3)2 (10 mol%) , under Ar.
N
N
b Isolated yield.
O
OTBS
8d (51%)
We then examined the reaction of substrate 6a in the
presence of a Pt catalyst under a variety of conditions (Table
1). First, the reaction of 6a with PtCl2(PPh3)2 as the catalyst
proceeded smoothly in refluxing toluene to furnish product
7a in 94% yield (Table 1, entry 1). A longer reaction time un-
der the same conditions gave cyclopropane 7a and spiro
compound 8a in 35 and 38% yield, respectively (entry 2).
DCE, and 1,4-dioxane, could also be used as solvents for the
reaction (entries 3–4). Surprisingly, when 6a was treated
with PtCl2(PPh3)2 at 150 °C for four hours in xylene, the re-
action gave spiro compound 8a in 78% yield (entry 5).
F
F
O
O
N
N
N
N
N
CO2Et
CO2Et
CO2Et
CO2Et
6e
7e (0%)
F
N
CO2Et
CO2Et
O
8e (73%)a
Scheme 5 Platinum-catalyzed cyclization of CFN systems. Reaction
conditions: 6 (0.40 mmol) PtCl2(PPh3) (10 mol%); (for product 7): tolu-
ene, 120 °C, 4 h, under Ar; or (for product 8) xylene, 150 °C, 4 h, under
Ar. Isolated yields are reported.
a Hexane was used instead of xylene.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–E

D
K. Kim et al.
Letter
Synlett
treated under the same condition, product 7c was obtained
in an excellent yield. When substrate 6c was treated with
PtCl2(PPh3)2 in refluxing xylene (150 °C) for four hours, the
spiro compound 8c was obtained in 73% yield. Unfortunate-
ly, substrates 6d and 6e, which contain a piperidin-1-yl
group, did not form the corresponding cyclopropane com-
pound; instead, the spiranes 8d and 8e were obtained in 51
and 73% yield, respectively. Interestingly, the OTBS group of
substrate 8d was not eliminated during the Pt-catalyzed
isomerization. The product 8e was formed in a high yield
on changing the solvent to hexane (Scheme 5).
In our proposed mechanism, intermediate A undergoes
[3+2] cycloaddition to the pendent double bond to form the
Pt–carbene complex B. The Pt–carbene complex B inserts
into the tertiary C–H bond to form the cyclopropane inter-
mediate 7, which then isomerizes to the corresponding spi-
rane 8 on heating or under PTSA-catalyzed conditions
(Scheme 6).9c,9f
W.; Copp, B. R.; Keyzers, R. A.; Munro, M. H. G.; Prinsep, M. R.
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C.-H.; Hsu, C.-W. Eur. J. Org. Chem. 2016, 589. (c) Kotha, S.; Deb,
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Catal. 2015, 357, 3880. (f) Mostinski, Y.; Lankri, D.;
Tsvelikhovsky, D. Synthesis 2017, 49, 2361.
O
H
R
O+
-
Pt(II)
R
O
R
N
N
N
Pt(II)
Pt(II)
6
A
B
(6) (a) Undheim, K. Synthesis 2014, 46, 1957. (b) Undheim, K. Syn-
thesis 2015, 47, 2497. (c) Undheim, K. Synthesis 2017, 49, 705.
(d) Brandi, A.; Cicchi, S.; Cordero, F. M.; Goti, A. Chem. Rev. 2003,
103, 1213.
(7) (a) Shin, S.; Gupta, A. K.; Rhim, C. Y.; Oh, C. H. Chem. Commun.
2005, 4429. (b) Oh, C. H.; Reddy, K. V. Bull. Korean Chem. Soc.
2007, 28, 1927.
R
R
O
Isomerization
N
N
O
7
8
Scheme 6 Proposed mechanism for the formation of 7 and 8
(8) (a) Gupta, A. K.; Rhim, C. Y.; Oh, C. H.; Mane, R. S.; Han, S.-H.
Green Chem. 2006, 8, 25. (b) Oh, C. H.; Kim, A.; Park, W.; Park, D.
I.; Kim, N. Synlett 2006, 17, 2781. (c) Oh, C. H.; Kim, A. New J.
Chem. 2007, 31, 1719. (d) Oh, C. H.; Kim, A. Synlett 2008, 777.
(e) Oh, C. H.; Lee, S. J.; Lee, J. H.; Na, Y. J. Chem. Commun. 2008,
5794. (f) Oh, C. H.; Karmakar, S. J. Org. Chem. 2009, 74, 370.
(g) Karmakar, S.; Kim, A.; Oh, C. H. Synthesis 2009, 194. (h) Oh, C.
H.; Karmakar, S.; Park, H. S.; Ahn, Y. C.; Kim, J. W. J. Am. Chem.
Soc. 2010, 132, 1792. (i) Oh, C. H.; Piao, L.; Kim, J. H. Synthesis
2013, 45, 174. (j) Oh, C. H.; Kim, J. H.; Oh, B. K.; Park, J. R.; Lee, J.
H. Chem. Eur. J. 2013, 19, 2592. (k) Oh, C. H.; Kim, J. H.; Piao, L.;
Yu, J.; Kim, S. Y. Chem. Eur. J. 2013, 19, 10501. (l) Lee, Y. J.; Heo,
H. G.; Oh, C. H. Tetrahedron 2016, 72, 6113.
The Pt-catalyzed cycloisomerization of aromatic and N-
heterocyclic aldehydes substituted with a enynyl group at
the 2-position gave the corresponding spirocycles isolated
in high to excellent yields via cyclopropane intermediates.
This reaction is a valuable choice for the synthesis of certain
spirocyclic compounds.
Funding Information
Financial support was provided by a grant from the National Research
(9) (a) Oh, C. H.; Reddy, V. R.; Kim, A.; Rhim, C. Y. Tetrahedron Lett.
2006, 47, 5307. (b) Oh, C. H.; Lee, J. H.; Lee, S. J.; Kim, J. I.; Hong,
C. S. Angew. Chem. Int. Ed. 2008, 47, 7505. (c) Oh, C. H.; Lee, J. H.;
Lee, S. M.; Yi, H. J.; Hong, C. S. Chem. Eur. J. 2009, 15, 71. (d) Oh,
C. H.; Lee, S. M.; Hong, C. S. Org. Lett. 2010, 12, 1308. (e) Oh, C.
H.; Yi, H. J.; Lee, J. H.; Lim, D. H. Chem. Commun. 2010, 46, 3007.
(f) Oh, C. H.; Tak, S. Y.; Lee, J. H.; Piao, L. Bull. Korean Chem. Soc.
2011, 32, 2978. (g) Kim, J. H.; Ray, D.; Hong, C. S.; Han, J. W.; Oh,
C. H. Chem. Commun. 2013, 49, 5690. (h) Han, J. W.; Lee, J. H.;
Oh, C. H. Synlett 2013, 24, 1433.
Foundation of Korea (NRF–2014R1A5A1011165).
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Supporting Information
Supporting information for this article (copies of 1H and 13C NMR
spectra for compounds 2b–g, 3b–c, 7a–c, 8a–e) is available online at
https://doi.org/10.1055/s-0036-1591489.
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(10) Heterocyclopropane 7a; Typical Procedure
References and Notes
A new 5 mL test tube was charged with the 2-alkynylnicotinal-
dehyde 6a (0.40 mmol), PtCl2(PPh3)2 (11 mg, 0.04 mmol), and
anhyd toluene (1.5 mL) at 0 °C under argon. The mixture was
stirred for 4 h in a preheated oil bath (120 °C). When the reac-
(1) (a) Müller, G.; Berkenbosch, T.; Benningshof, J. C. J.; Stumpfe, D.;
Bajorath, J. Chem. Eur. J. 2017, 23, 703. (b) Zheng, Y.; Tice, C. M.;
Singh, S. B. Bioorg. Med. Chem. Lett. 2014, 24, 3673. (c) Blunt, J.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–E

E
K. Kim et al.
Letter
Synlett
tion was complete (TLC), the solvent was removed under
vacuum, and the crude product was purified by flash column
chromatography (silica gel, hexane–EtOAc) to give a yellow oil;
yield: 169 mg (94%); Rf = 0.41 (EtOAc–hexane, 1:4). IR (NaCl):
anhyd xylene (1.5 mL) at 0 °C under argon. The mixture was
stirred for 1 h in a preheated oil bath (150 °C) until the reaction
was complete (TLC). The solvent was removed under vacuum,
and the crude product was purified by flash column chromatog-
raphy (silica gel, hexane–EtOAc) to give a yellow oil; yield: 140
mg (78%); Rf = 0.33 (EtOAc–hexane, 1:4). IR (NaCl): 2981, 1731,
2980, 1732, 1440, 1244 cm–1 1H NMR (400 MHz, CDCl3): δ =
.
7.92 (d, J = 8.0 Hz, 2 H), 7.49–7.46 (m, 2 H), 7.42 (d, J = 7.2 Hz, 1
H), 7.24 (d, J = 10.0 Hz, 1 H), 5.13 (d, J = 5.6 Hz, 1 H), 4.30–4.16
(m, 4 H), 3.12 (d, J = 15.2 Hz, 1 H), 2.45 (s, 1 H), 2.30 (d, J = 14.4
Hz, 2 H), 2.18–2.11 (m, 1 H), 1.93–1.78 (m, 3 H), 1.42 (d, J = 11.6
Hz, 1 H), 1.30 (t, J = 7.2 Hz, 3 H), 1.26 (t, J = 7.2 Hz, 3 H). 13C NMR
(100 MHz, CDCl3): δ = 171.5, 170.8, 157.2, 154.6, 148.8, 148.7,
144.8, 144.7, 135.7, 135.6, 128.9, 128.8, 128.8, 128.5, 118.3,
118.1, 73.8, 61.9, 61.7, 61.7, 53.3, 33.7, 31.4, 30.7, 26.5, 22.8,
21.6, 14.2, 14.1. HRMS (ESI): m/z [M + Na]+ calcd for C26H26FNN-
aO5: 474.1688; found: 474.1691.
1441, 1249, 1249 cm–1 1H NMR (400 MHz, CDCl3): δ = 7.96 (d,
.
J = 8.4 Hz, 2 H), 7.49–7.39 (m, 3 H), 7.13 (d, J = 11.2 Hz, 1 H), 6.52
(d, J = 9.2 Hz, 1 H) , 6.15 (d, J = 9.6 Hz, 1 H), 4.27–4.19 (m, 4 H),
3.36 (d, J = 16.0 Hz, 1 H), 3.10 (d, J = 16.0 Hz, 1 H), 2.92 (ABq, Δδ
= 38.8 Hz, J = 15.2 Hz, 2 H), 2.44–2.28 (m, 2 H), 1.91–1.85 (m, 2
H), 1.26 (t, J = 7.2 Hz, 6 H). 13C NMR (100 MHz, CDCl3): δ = 207.0,
170.4, 170.1, 158.0, 155.4, 149.6, 149.5, 143.8, 143.7, 135.4,
135.4, 132.7, 132.6, 129.2, 129.1, 128.9, 128.9, 128.8, 128.8,
128.6, 127.9, 127.8, 126.3, 121.1, 120.9, 62.2, 62.2, 57.5, 50.6,
42.5, 38.4, 32.0, 26.0, 14.1. HRMS (ESI): m/z [M + Na]+calcd for
(11) Spiro(cyclohexane-1,7′-quinoline) 8a; Typical Procedure
A new 5 mL test tube was charged with the 2-alkynylnicotinal-
dehyde 6a (0.40 mmol), PtCl2(PPh3)2 (11 mg, 0.04 mmol), and
C
26H26FNNaO5: 474.1688; found: 474.1695.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–E