PtCh-catalyzed tandem triple migration reaction toward (Z)-1,5-dien-2-yl esters

Article abstract of DOI:10.1021/ol8015463

(Figure Presented) A novel method for the selective synthesis of (Z)-1,5-dien-2-yl esters has been developed though Pt(II)-catalyzed tandem 1,2-acyl and 1,2-hydride migration, along with an allyl migration reaction of propargylic carboxylates with electro

Full text of DOI:10.1021/ol8015463

ORGANIC
LETTERS
2008
Vol. 10, No. 17
3919-3922
PtCl2-Catalyzed Tandem Triple Migration
Reaction toward (Z)-1,5-Dien-2-yl Esters
Ke-Gong Ji,† Xing-Zhong Shu,† Jin Chen,† Shu-Chun Zhao,† Zhao-Jing Zheng,†
Li Lu,† Xue-Yuan Liu,† and Yong-Min Liang*,†
State Key Laboratory of Applied Organic Chemistry, Lanzhou UniVersity, Lanzhou
730000, China, and State Key Laboratory of Solid Lubrication, Lanzhou Institute of
Chemical Physics, Chinese Academy of Science, Lanzhou 730000, P.R. China
liangym@lzu.edu.cn
Received July 9, 2008
ABSTRACT
A novel method for the selective synthesis of (Z)-1,5-dien-2-yl esters has been developed though Pt(II)-catalyzed tandem 1,2-acyl and 1,2-
hydride migration, along with an allyl migration reaction of propargylic carboxylates with electronically unbiased internal alkynes. The unusual
selectivity of 1,2-acyloxy migration was realized.
In recent years, transition-metal-catalyzed isomerization of
propargylic carboxylates has led to the development of a
number of atom-economy cycloisomerization and tandem
reactions.1 Particularly interesting is reactivity of these easily
accessible compounds in the context of platinum chloride,
which has been recognized to induce highly selective skeletal
rearrangements and cycloisomerizations and, hence, has
become a widely used catalyst.2 Remarkably, propargylic
carboxylates are versatile substrates in platinum catalysts and
have been transformed into various synthetically valuable
products via 1,3-acyloxy3 and 1,2-acyloxy4 migration (Scheme
1, complex A). In this context, it is relevant that the Sarpong
group has reported the stereoselective Pt(II)-catalyzed pro-
pargylic esters into cycloisomerization products which takes
(3) For selected examples of 1,3-acyloxy migration not cited in the
reviews listed in ref 1, see: (a) Correa, A.; Marion, N.; Fensterbank, L.;
Malacria, M.; Nolan, S. P.; Cavallo, L. Angew. Chem., Int. Ed. 2008, 47,
718. (b) Schwier, T.; Sromek, A. W.; Yap, D. M. L.; Chernyak, D.;
Gevorgyan, V. J. Am. Chem. Soc. 2007, 129, 9868. (c) Barluenga, J.; Riesgo,
L.; Vicente, R.; Lo´pez, L. A.; Toma´s, M. J. Am. Chem. Soc. 2007, 129,
7772. (d) Lemie`re, G.; Gandon, V.; Cariou, K.; Fukuyama, T.; Dhimane,
A.-L.; Fensterbank, L.; Malacria, M. Org. Lett. 2007, 9, 2207. (e) Marion,
N.; Carlqvist, P.; Gealageas, R.; De Fre´mont, P.; Maseras, F.; Nolan, S. P.
Chem. Eur. J. 2007, 13, 6437. (f) Cariou, K.; Mainetti, E.; Fensterbank,
L.; Malacria, M. Tetrahedron 2004, 60, 9745. (g) Brabander, J. K. D.; Liu,
B.; Qian, M. Org. Lett. 2008, 10, 2533, and references therein.
(4) For selected examples of 1,2-acyloxy migration, see: (a) Prasad,
B. A. B.; Yoshimoto, F. K.; Sarpong, R. J. Am. Chem. Soc. 2005, 127,
12468. (b) Pujanauski, B. G.; Prasad, B. A. B.; Sarpong, R. J. Am. Chem.
Soc. 2006, 128, 6786. (c) Motamed, M.; Bunnelle, E. M.; Singaram, S. W.;
Sarpong, R. Org. Lett. 2007, 9, 2167. (d) Smith, C. R.; Bunnelle, E. M.;
Rhodes, A. J.; Sarpong, R. Org. Lett. 2007, 6, 1169. (e) Hardin, A. R.;
Sarpong, R. Org. Lett. 2007, 9, 4547. (f) Soriano, E.; Marco-Contelles, J.
J. Org. Chem. 2007, 72, 1443. (g) Mainetti, E.; Mourie`s, V.; Fensterbank,
L.; Malacria, M.; Marco-Contelles, J. Angew. Chem., Int. Ed. 2002, 41,
2132, and references therein.
† Lanzhou University.
(1) For recent reviews, see: (a) Zhang, L.; Sun, J.; Kozmin, S. A. AdV.
Synth. Catal. 2006, 348, 2271. (b) Marco-Contelles, J.; Soriano, E. Chem.
Eur. J. 2007, 13, 1350. (c) Marion, N.; Nolan, S. P. Angew. Chem., Int.
Ed. 2007, 46, 2750. (d) Fu¨rstner, A.; Davies, P. W. Angew. Chem., Int. Ed.
2007, 46, 3410. (e) Hashmi, A. S. K. Chem. ReV. 2007, 107, 3180. (f)
Correa, A.; Marion, N.; Fensterbank, L.; Malacria, M.; Nolan, S. P.; Cavallo,
L. Angew. Chem., Int. Ed. 2008, 47, 718.
(2) For reviews, see: (a) Aubert, C.; Buisine, O.; Malacria, M. Chem.
ReV. 2002, 102, 813. (b) Me´ndez, M.; Mamane, V.; Fu¨rstner, A.
ChemtractssOrg. Chem. 2003, 16, 397. (c) Lloyd-Jones, G. C. Org. Biomol.
Chem. 2003, 1, 215. For selected examples of skeletal rearrangements, see:
(d) Mamane, V.; Gress, T.; Krause, H.; Fu¨rstner, A. J. Am. Chem. Soc.
2004, 126, 8654. (e) Fu¨rstner, A.; Hannen, P. Chem. Commun. 2004, 2546.
(f) Mamane, V.; Hannen, P.; Fu¨rstner, A. Chem. Eur. J. 2004, 10, 4556.
(g) Fu¨rstner, A.; Davies, P. W.; Gress, T. J. Am. Chem. Soc. 2005, 127,
8244. (h) Fehr, C.; Galindo, J. Angew. Chem., Int. Ed. 2006, 45, 2901. (i)
Fu¨rstner, A.; Hannen, P. Chem. Eur. J. 2006, 12, 3006. (j) Fu¨rstner, A.;
Aissa, C. J. Am. Chem. Soc. 2006, 128, 6306.
(5) For a study with Pt(II) as catalyst, see ref 4e. Propargylic esters
bearing terminal alkynes typically undergo 5-exo-dig cyclization using Ru,
Pt, or Au catalysts. Internal alkynes bearing alkyl or aryl substituents usually
undergo 6-endo-dig cyclization. (a) Miki, K.; Ohe, K.; Uemura, S.
Tetrahedron Lett. 2003, 44, 2019. (b) Miki, K.; Ohe, K.; Uemura, S. J.
Org. Chem. 2003, 68, 8503.
10.1021/ol8015463 CCC: $40.75
Published on Web 08/01/2008
2008 American Chemical Society

(Z)-1,5-dien-2-yl esters 2. Herein, we report a novel Pt(II)-
catalyzed tandem 1,2-acyloxy, 1,2-hydride, and allyl migra-
tion reaction of propargylic carboxylates 1, which contain
electronically unbiased internal alkynes, to (Z)-1,5-dien-2-
yl esters 2. In this reaction, 1,2-acyloxy migration and 1,2-
hydride migration were key steps, and a new carbon-carbon
formation was also achieved.
Scheme 1
.
Proposed Pathways for Pt-Catalyzed Isomerization of
Propargyl Carboxylates
Initially, we started by using 0.3 mmol of 4-(allyloxy)-
1,4-diphenylbut-2-ynyl acetate 1a and 5 mol % of PtCl2 in
toluene at room temperature, and no reaction was observed.
When the temperature was increased to 80 °C, the desired
product (Z)-3-benzoyl-1-phenylhexa-1,5-dien-2-yl acetate
2a11 was formed in 48% yield. However, the reaction was
fairly slow and required 24 h to go to completion (Table 1,
place via path I.4b,f,5 From the previous works, it seems that
propargylic carboxylates normally undergo 1,2-acyloxy
migration with propargylic esters with terminal or electron-
deficient C-C triple bonds, while those with electronically
unbiased internal C-C triple bonds prefer 1,3-acyloxy
migration (via path II).6 Examples of the propargylic
carboxylates undergoing 1,2-acyloxy migration in propargylic
esters with internal C-C triple bonds are limited.4e,6
In the context of our ongoing efforts to develop tandem
reactions,7 we found that Pt carbenoids might be perfect
intermediates in a domino process.8 We envision that
propargylic esters 1 with electronically unbiased internal
alkynes could realize 1,2-acyloxy migration selectively under
Pt(II) catalysis to form alkenyl Pt carbenoid 4,6a which might
induce the migration of neighboring groups when assisted
with the allylic oxygen group (Scheme 2). Then the oxygen
Table 1. Transition-Metal Catalysts for the Transformation of
1a to 2a
T
time yielda
entry catalyst
solvent (°C) (h)
(%)
1b
2
3
4
5
6
7
8
PtCl2 (5 mol %)
PtCl2 (5 mol %)
PtCl2/COD (5 mol %)
PtCl2/CO (5 mol %/1 atm)
PtCl2/CO (10 mol %/1 atm) toluene 80
AuCl (10 mol %)
AuCl3 (10 mol %)
NaAuCl4·2H2O (10 mol %) toluene 80
toluene rt
toluene 80
toluene 80
toluene 80
24
24
12
6
0
48
55
69
78
0
4.5
toluene 80
toluene 80
24
24
24
0
0
Au(PPh3)Cl/AgSbF6
9
(5 mol %/10 mol %)
CuI (10 mol %)
PtCl2/CO (10 mol %/1 atm) DCE
toluene 80
toluene 80
24
24
10
10
0
0
0
0
Scheme 2
.
Design a Pt-Catalyzed Formation of
(Z)-1,5-Dien-2-yl Esters
10c
11
12
80
PtCl2/CO (10 mol %/1 atm) CH3CN 80
a Isolated yield. b 93% of 1a was recovered. c 85% of 1a was recovered.
entries 1 and 2). In an attempt to accelerate this transforma-
tion, the mixture was stirred under an atmosphere of CO (1
heteroatom promotes a 1,2-hydride migration to the Pt
carbenoid moiety in 46,9 to afford allylic oxygen cation
intermediates 5, which promotes an allyl migration10 to afford
(9) For 1,2-hydride migration in Au- and Pt-carbenoid intermediates,
see: (a) Fu¨rstner, A.; Stelzer, F.; Szillat, H. J. Am. Chem. Soc. 2001, 123,
11863. (b) Sromek, A. W.; Rubina, M.; Gevorgyan, V. J. Am. Chem. Soc.
2005, 127, 10500. (c) Fehr, C.; Farris, I.; Sommer, H. Org. Lett. 2006, 8,
1839. (d) Lee, J. H.; Toste, F. D. Angew. Chem. 2007, 119, 930; Angew.
Chem., Int. Ed. 2007, 46, 912. (e) Funami, H.; Kusama, H.; Iwasawa, N.
Angew. Chem. 2007, 119, 927; Angew. Chem., Int. Ed. 2007, 46, 909. (f)
Shi, F.-Q.; Li, X.; Xia, Y.; Zhang, L.; Yu, Zh.-X. J. Am. Chem. Soc. 2007,
129, 15503. (g) Zhang, G.; Catalano, V. J.; Zhang, L. J. Am. Chem. Soc.
2007, 129, 11358.
(6) (a) Li, G.; Zhang, G.; Zhang, L. J. Am. Chem. Soc. 2008, 130, 3740.
(b) Shi, X.; Gorin, D. J.; Toste, F. D. J. Am. Chem. Soc. 2005, 127, 5802.
(c) Amijs, C. H. M.; Lopez-Carrillo, V.; Echavarren, A. M. Org. Lett. 2007,
9, 4021. (d) Blaszykowski, C.; Harrak, Y.; Goncalves, M.-H.; Cloarec, J.-
M.; Dhimane, A.-L.; Fensterbank, L.; Malacria, M. Org. Lett. 2004, 6, 3771.
(7) (a) Shu, X.-Z.; Liu, X.-Y.; Xiao, H.-Q.; Ji, K.-G.; Guo, L.-N.; Qi,
C.-Z; Liang, Y.-M. AdV. Synth. Catal. 2007, 349, 2493. (b) Shu, X.-Z.;
Liu, X.-Y.; Xiao, H.-Q.; Ji, K.-G.; Guo, L.-N.; Qi, C.-Z; Liang, Y.-M. AdV.
Synth. Catal. 2008, 350, 243. (c) Shu, X.-Z.; Liu, X.-Y.; Ji, K.-G.; Xiao,
H.-Q.; Liang, Y.-M. Chem.sEur. J. 2008, 14, 5282. (d) Ji, K.-G; Shen,
Y.-W.; Shu, X.-Z.; Xiao, H.-Q.; Bian, Y.-J.; Liang, Y.-M. AdV. Synth. Catal.
2008, 350, 1275.
(10) For selected examples of Pt-catalyzed allyl migration, see: (a)
Fu¨rstner, A.; Stelzer, F.; Szillat, H. J. Am. Chem. Soc. 2001, 123, 11863.
(b) Fu¨rstner, A.; Davies, P. W. J. Am. Chem. Soc. 2005, 127, 15024. (c)
Fu¨rstner, A.; Szillat, H.; Stelzer, F. J. Am. Chem. Soc. 2000, 122, 6785. (d)
Nakamura, I.; Sato, T.; Terada, M.; Yamamoto, Y. Org. Lett. 2008, 10,
2649. (e) Cariou, K.; Ronan, B.; Mignani, S.; Fensterbank, L.; Malacria,
M. Angew. Chem., Int. Ed. 2007, 46, 1881. (f) Crone, B.; Kirsch, S. F.
Chem. Eur. J. 2008, 14, 3514.
(8) For selected examples of platinum carbenes as reactive intermediates,
see: (a) Me´ndez, M.; Echavarren, A. M. Eur. J. Org. Chem. 2002, 15. (b)
Mechanistic investigation: Martin-Matute, B.; Nevado, C.; Ca´rdenas, D. J.;
Echavarren, A. M. J. Am. Chem. Soc. 2003, 125, 5757.
(11) The stereochemistry of (Z)-1,5-dien-2-yl esters were clearly estab-
lished with NOESY experiments. For details, see the Supporting Informa-
tion.
3920
Org. Lett., Vol. 10, No. 17, 2008

atm), and 6 h was necessary to effect the rearrangement of
1a, providing the desired product 2a in 69% yield (Table 1,
entry 4). To our delight, on increasing the amount of catalyst
to 10 mol %, a 78% yield of 2a was obtained after 4.5 h
(Table 1, entry 5). With other catalysts and solvents, no
desired product 2a was obtained (Table 1, entries 6 and 12).
Thus, the use of PtCl2 (10 mol %) in toluene at 80 °C under
an atmosphere of CO (1 atm) was found to be the most
efficient and used as the standard conditions.
Table 3. PtCl2-Catalyzed Double 1,2-Acyloxy Migration for the
Synthesis of 1,2-Dienyl Acetates 3 from Propargylic
Carboxylates 1a
To study the scope of this formation of (Z)-1,5-dien-2-yl
esters, various representative propargylic carboxylates 1a-j
were then submitted to the above conditions, as depicted in
Table 2. Thus, a tandem 1,2-acyloxy, 1,2-hydride and allyl
Table 2. Pt(II)-Catalyzed Synthesis of (Z)-1,5-Dien-2-yl esters 2
from Propargylic Carboxylates 1a
1
2
yieldb (%)
R, R1, R2
1
2
3
4
5
6
7
8
9
Me, Ph, Ph
1a
1b
1c
1d
1e
1f
1g
1h
1i
2a
2b
2c
2d
2e
2f
78
79
72
68
55
53
55
75
85
78
Me, Ph, p-Me-C6H4
Me, Ph, p-Cl-C6H4
Me, Ph, furyl
Me, Ph, 2-styrene-
Me, p-Cl-C6H4, p-Cl-C6H4
Me, p-Me-C6H4, Ph
Ph, Ph, Ph
a Conditions: 0.3 mmol of 1a with 10 mol % of catalyst in toluene (3.0
mL) under an atmosphere of CO (1 atm) at 80 °C. b Isolated yield.
2g12
2h
2i
m-Br-C6H4, Ph, Ph
3,5-dinitrophenyl, Ph, Ph
10
1j
2j
On the basis of the above observations, we propose the
following plausible mechanisms for this cascade transforma-
tion (Scheme 3). (i) Coordination of the alkynyl and alkenyl
moiety of A to the Pt catalyst gives the complex B. (ii)
According to path I, 1,2-acyloxy migration produces Pt
carbenoid intermediate C (via 5-exo-dig cyclization/ring
a Conditions: 0.3 mmol of 1a with 10 mol % of catalyst in toluene (3.0
mL) under an atmosphere of CO (1 atm) at 80 °C. b Isolated yield.
migration reaction of propargylic carboxylates 1a-j proceeded
smoothly to provide corresponding products 2a-j in moder-
ate to good yields. The reaction works well with aromatic
R2 groups.
Electron-rich aryl groups showed better results than those
with an electron-withdrawing group in this tandemreaction
(entries 2 vs 3). Substrate 1d with a heteroaromatic R2 group
can also afford the desired product 2d in 68% yield. The
derivatives 1g gave the expected 1,5-dien-2-yl esters 2g, but
as a mixture of stereoisomers (Z/E ) 2/1).12 The role of the
nucleophilic acyloxy group was examined with a series of
esters 1h-j, and the desired products 2h-j were smoothly
obtained in 75-85% yield.
Scheme 3. Proposed Mechanism
Furthermore, to expand the scope of this reaction, we also
investigated a range of propargylic carboxylates 1, which
contain aliphatic R1 and R3 groups (Table 3). To our surprise,
only 1,2-dienyl acetates 3 were obtained in moderate to
excellent yields.
(12) From the 1H NMR spectrum, the ratio of the isomers is Z/E )
2/1.
Org. Lett., Vol. 10, No. 17, 2008
3921

opening). (iii) When R1 are aromatic groups and R3 is H,
then the oxygen heteroatom promotes 1,2-hydride migration
to the Pt carbenoid moiety and produces allylic oxygen cation
E through D process. (iv) Allylic oxygen cation E causes
an allyl migration and affords 1,5-dien-2-yl esters F, during
which the Pt anion may capture the allyl cation and
regenerate the catalyst. (v) When R1 and R3 are aliphatic
groups, R1 or R3 do not undergo 1,2-alkyl migration, while
the nucleophilic acyloxy group in G undergoes 1,2-acyloxy
migration and gives 1,2-dienyl acetates I,13 which may be
undergoing a 5-exo-dig cyclization/ring-opening process.14
In summary, a novel method for the selective synthesis
of (Z)-1,5-dien-2-yl esters has been developed through a
Pt(II)-catalyzed tandem triple-migration reaction of propar-
gylic carboxylates with electronically unbiased internal
alkynes. The unusual selectivity of 1,2-acyloxy migration
was realized. A more detailed investigation on the mecha-
nism, as well as the scope of this cascade, is ongoing in our
laboratory.
(13) When R1 and R3 are aliphatic groups, the reaction may involve
double 1,2-acyloxy migrations and account for the 1,3-acyloxy migration.
Alternatively, the reaction may undergo 1,3-acyloxy migration process, and
1,2-dienyl acetates could be formed from 6-endo-dig cyclization/ring
opening intermediate (see below).
Acknowledgment. We thank the NSF (NSF-20621091,
NSF-20732002) for financial support.
Supporting Information Available: Experimental pro-
cedures and full spectroscopic data for all new compounds.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(14) A recent calculation suggests that double 1,2-acyloxy migrations
can also account for the 1,3-acyloxy migration and all these transformations
are reversible. For reference, see ref 3a.
OL8015463
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Org. Lett., Vol. 10, No. 17, 2008