Cationic rhodium(I)-dppf complex-catalyzed olefin isomerization/propargyl claisen rearrangement/carbonyl migration cascade

Article abstract of DOI:10.1021/ja9038449

(Chemical Equation Presented) We have established that a cationic rhodium(I)-dppf complex catalyzes isomerizations of allyl propargyl ethers to allenic aldehydes in good yields from room temperature to 40°C. On the other hand, carbonyl migration reactions

Full text of DOI:10.1021/ja9038449

Published on Web 07/17/2009
Cationic Rhodium(I)-dppf Complex-Catalyzed Olefin Isomerization/Propargyl
Claisen Rearrangement/Carbonyl Migration Cascade
Ken Tanaka,* Eri Okazaki, and Yu Shibata
Department of Applied Chemistry, Graduate School of Engineering, Tokyo UniVersity of Agriculture and
Technology, Koganei, Tokyo 184-8588, Japan
Received May 12, 2009; E-mail: tanaka-k@cc.tuat.ac.jp
Table 1. Screening of Reaction Conditions for Rhodium-Catalyzed
Isomerization of Allyl Propargyl Ether 1a
Cascade reactions, which can eliminate a number of reaction steps
and reduce hazardous waste, have attracted much attention in current
organic synthesis.1 It is well-known that transition-metal complexes are
able to activate σ bonds by oxidative addition of those bonds to the
transition metal or by an electrophilic substitution pathway;2 π bonds can
also be activated through the formation of a complex between an
electrophilic transition metal and the π electrons of alkene or alkyne
multiple bonds.3 Sequential activation of these bonds by a single transition-
metal complex would allow the development of a novel cascade reaction.
For such a process, a cationic rhodium(I) complex is a promising catalyst,
as these complexes can catalyze isomerizations4 of alkenes or alkynes
via a π-allyl intermediate generated through cleavage of an allylic C-H
bond,5 hydroacylations of alkenes or alkynes via aldehyde C-H bond
activation,6 and aromatic amino-Claisen rearrangements via π-bond
activation.7
yield (%)a
entry
ligand
temp (°C)
conv. (%)a
2a
3a
1
2
3
4
5
6
7
8
BINAP
H8-BINAP
Segphos
BIPHEP
dppb
dppe
dppf
dppf
rt
rt
rt
rt
rt
rt
rt
80
100
100
100
100
72
10
100
100
39
67
70
67
56
0
27
0
0
0
0
0
5
82
As a target for a cationic Rh(I) complex-catalyzed cascade reaction,
we focused on a combination of olefin isomerization and the propargyl
Claisen rearrangement that has not been realized to date.8 Toste and Sherry
previously reported the novel cationic gold(I)-catalyzed Claisen rearrange-
ment of propargyl vinyl ethers to allenic aldehydes.9,10 If an isomerization
of readily prepared and stable allyl propargyl ethers to propargyl vinyl
ethers followed by the propargyl Claisen rearrangement can be catalyzed
by a single cationic Rh(I) complex, this would be useful from a synthetic
point of view. Furthermore, the product allenic aldehyde might react with
the cationic Rh(I) complex to form another product.11 Here we describe
cationic Rh(I)-dppf complex-catalyzed isomerizations of allyl propargyl
ethers to allenic aldehydes and dienals (Scheme 1).
80
0
a Determined by 1H NMR.
diverse aryl and sterically diverse alkyl groups could be incorporated at
the alkyne terminus (entries 1-5). With respect to substituents at the
propargylic position, not only methyl- but also cycloalkyl- and isopropyl-
substituted tertiary propargyl ethers could participate in this reaction,
affording the corresponding tetrasubstituted allenes in good yields (entries
1-7). Furthermore, tri- and disubstituted allenes could also be obtained
in good yields (entries 9 and 10). It is noteworthy that the reactions of
both tertiary and secondary propargyl ethers proceeded under mild
conditions (room temperature to 40 °C) in comparison with the precedent
gold catalysis, which requires elevated temperatures (75 °C) for tertiary
propargyl vinyl ethers.9 Phenyl instead of methyl substitution of the alkene
moiety was tolerated, although a prolonged reaction time was required
(entry 8).
The scope of the olefin isomerization/Claisen rearrangement/
carbonyl migration cascade was also examined using the same
substrates and rhodium catalyst (Table 3). Interestingly, the isomer-
izations of tertiary propargyl ethers 1a-h proceeded at 80 °C to yield
the corresponding dienals in good yields (entries 1-8), but the
isomerizations were terminated at the stage of the corresponding allenic
aldehydes in the case of secondary and primary propargyl ethers 1i
and 1j (entries 9 and 10).
A possible mechanism for the present isomerization is outlined in
Scheme 2. In the first step, the olefin isomerization of allyl ether 1 proceeds
to give enol ether 5.12 Next, the propargyl Claisen rearrangement proceeds
through activation of the alkyne triple bond of 5,13 affording allenic
aldehyde 2. At elevated temperature, activation of the aldehyde C-H bond
followed by hydrorhodation of the pendant allene furnishes rhodacycle
A. Carbonyl migration then occurs, producing conjugated rhodacycle B.14
ꢀ-Hydride elimination followed by reductive elimination generates dienal
3. When R3 ) H, allenic aldehyde 2 rather than dienal 3 is generated,
We first investigated the reaction of allyl propargyl ether 1a in the
presence of a cationic Rh(I)-BINAP complex (5 mol %). Pleasingly, the
expected olefin isomerization/Claisen rearrangement proceeded at room
temperature to give the corresponding allenic aldehyde 2a, and the carbonyl
migration product, dienal 3a, was also formed (Table 1, entry 1). Thus,
various cationic Rh(I)-bisphosphine complexes were screened in order
to realize the selective formation of 2a. Among the ligands examined
(entries 1-7), the use of dppf gave 2a in the highest yield (entry 7). The
selective formation of 3a could also be realized by increasing the reaction
temperature (80 °C) in the presence of the same Rh(I) catalyst (entry 8).
Thus, we explored the scope of the olefin isomerization/Claisen
rearrangement cascade by using 5 mol % cationic Rh(I)-dppf complex
(Table 2). The products were isolated as the corresponding reduced
alcohols because of the instability of allenic aldehydes. Electronically
Scheme 1
9
10822 J. AM. CHEM. SOC. 2009, 131, 10822–10823
10.1021/ja9038449 CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
Table 2. Cationic Rh(I)-dppf Complex-Catalyzed Isomerizations of
Allyl Propargyl Ethers 1 to Homoallenic Alcohols 4
dienals. Future work will focus on using the cationic Rh(I) complex for
sequential activation of σ and π bonds in cascade reactions.
Acknowledgment. This work was supported partly by Grants-in-
Aid for Scientific Research (20675002, 19028015, and 21 ·906) from
MEXT, Japan. We thank Takasago International Corporation for the gift
of H8-BINAP and Segphos and Umicore for generous support in supplying
a rhodium complex.
1
2
3
4
R
entry
1
R
R , R
time (h)
4
yield (%)a
1
2
3
4
1a
1b
1c
1d
1e
1f
1g
1h
1i
Ph
Me, Me
Me, Me
Me, Me
Me, Me
Me, Me
(CH2)5
i-Pr, Me
Me, Me
Me, H
Me
Me
Me
Me
Me
Me
Me
Ph
23
19
44
17
30
23
15
72
17
72
4a
4b
4c
4d
4e
4f
4g
4h
4i
74
72
71
69
49
70
81
45
72
65
4-MeOC6H4
4-ClC6H4
n-Bu
Cy
b
5
6
7
8
Ph
Ph
Ph
Ph
b
Supporting Information Available: Experimental procedures and
compound characterization data. This material is available free of charge via
the Internet at http://pubs.acs.org.
9
10
Me
Me
c
1j
Ph
H, H
2j
a Isolated yield. b At 40 °C. c Isolated as allenic aldehyde 2j without treatment
with NaBH4.
References
(1) For recent reviews of cascade reactions, see: (a) Chapman, C. J.; Frost, C. G.
Synthesis 2007, 1. (b) Nicolaou, K. C.; Edmonds, D. J.; Bulger, P. G. Angew.
Chem., Int. Ed. 2006, 45, 7134. (c) Wasilke, J.-C.; Obrey, S. J.; Baker,
R. T.; Bazan, G. C. Chem. ReV. 2005, 105, 1001.
Table 3. Cationic Rh(I)-dppf Complex-Catalyzed Isomerizations of
Allyl Propargyl Ethers 1 to Dienals 3
(2) For a recent review of transition-metal-catalyzed C-H bond activation, see:
Kakiuchi, F.; Kochi, T. Synthesis 2008, 3013.
(3) For a recent review of π-electrophilic Lewis acid catalysts, see: Yamamoto, Y. J.
Org. Chem. 2007, 72, 7817.
(4) For a recent review of transition-metal-catalyzed olefin isomerization, see: Tanaka,
K. In ComprehensiVe Organometallic Chemistry III; Crabtree, R. H., Mingos,
D. M. P., Ojima, I., Eds.; Elsevier: Oxford, U.K., 2007; Vol. 10, p 71.
(5) A mechanism via π-allyl intermediates has been proposed for cationic Rh(I)-
bisphosphine complex-catalyzed isomerizations of allylic compounds. Allyl
ethers: (a) Fatig, T.; Soulie´, J.; Lallemand, J.-Y.; Mercier, F.; Mathey, F.
Tetrahedron 2000, 56, 101. (b) Hiroya, K.; Kurihara, Y.; Ogasawara, K. Angew.
Chem., Int. Ed. Engl. 1995, 34, 2287. Allyl amines: (c) Nova, A.; Ujaque, G.;
Albe´niz, A. C.; Espinet, P. Chem.sEur. J. 2008, 14, 3323. (d) Inoue, S.-I.;
Takaya, H.; Tani, K.; Otsuka, S.; Sato, T.; Noyori, R. J. Am. Chem. Soc. 1990,
112, 4897. Allylic and propargylic alcohols: (e) Tanaka, K.; Shoji, T.; Hirano,
M. Eur. J. Org. Chem. 2007, 2687. (f) Tanaka, K.; Fu, G. C. J. Org. Chem.
2001, 66, 8177. (g) Bergens, S.; Bosnich, B. J. Am. Chem. Soc. 1991, 113,
958.
(6) For selected recent examples, see: (a) Coulter, M. M.; Dornan, P. K.; Dong, V. M.
J. Am. Chem. Soc. 2009, 131, 6932. (b) Osborne, J. D.; Randell-Sly, H. E.;
Currie, G. S.; Cowley, A. R.; Willis, M. C. J. Am. Chem. Soc. 2008, 130,
17232. (c) Tanaka, K.; Shibata, Y.; Suda, T.; Hagiwara, Y.; Hirano, M.
Org. Lett. 2007, 9, 1215. (d) Kundu, K.; McCullagh, J. V.; Morehead, A. T.,
Jr. J. Am. Chem. Soc. 2005, 127, 16042. (e) Sato, Y.; Oonishi, Y.; Mori,
M. Angew. Chem., Int. Ed. 2002, 41, 1218. (f) Tanaka, K.; Fu, G. C. J. Am.
Chem. Soc. 2001, 123, 11492, and references therein.
% yield (E/Z)a
1
2
3
4
R
entry
1
R
R , R
time (h)
3
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
1e
1f
1g
1h
1i
Ph
Me, Me
Me, Me
Me, Me
Me, Me
Me, Me
(CH2)5
i-Pr, Me
Me, Me
Me, H
Me
Me
Me
Me
Me
Me
Me
Ph
16
16
16
16
72
16
16
40
15
16
3a
3b
3c
3d
3e
3f
3g
3h
3i
82
76
76
72
4-MeOC6H4
4-ClC6H4
n-Bu
Cy
b
43
Ph
Ph
Ph
Ph
77
67 (4:1)
80 (2:1)
c
Me
Me
0
0
d
10
1j
Ph
H, H
3j
a Isolated yield. b The corresponding allenic aldehyde 2e remained in ∼32% yield.
c Allenic aldehyde 2i was generated in 61% yield. d Allenic aldehyde 2j was generated in
35% yield along with the decarbonylated byproduct.
Scheme 2
(7) (a) Saito, A.; Kanno, A.; Hanzawa, Y. Angew. Chem., Int. Ed. 2007, 46, 3931.
(b) Saito, A.; Oda, S.; Fukaya, H.; Hanzawa, Y. J. Org. Chem. 2009, 74, 1517.
(8) For selected recent examples of sequential olefin isomerization/Claisen rearrange-
ment of di(allyl) ethers, see: (a) Kerrigan, N. J.; Bungard, C. J.; Nelson, S. G.
Tetrahedron 2008, 64, 6863. (b) Trost, B. M.; Zhang, T. Org. Lett. 2006,
8, 6007. (c) Nelson, S. G.; Wang, K. J. Am. Chem. Soc. 2006, 128, 4232.
(d) Nevado, C.; Echavarren, A. M. Tetrahedron 2004, 60, 9735. (e) Schmidt,
B. Synlett 2004, 1541. (f) Nelson, S. G.; Bungard, C. J.; Wang, K. J. Am.
Chem. Soc. 2003, 125, 13000. (g) Le Notre, J.; Brissieux, L.; Semeril, D.;
Bruneau, C.; Dixneuf, P. H. Chem. Commun. 2002, 1772. (h) Ben Ammar,
H.; Le Notre, J.; Salem, M.; Kaddachi, M. T.; Dixneuf, P. H. J. Organomet.
Chem. 2002, 662, 63, and references therein.
(9) Sherry, B. D.; Toste, F. D. J. Am. Chem. Soc. 2004, 126, 15978.
(10) For a Ag(I)-catalyzed propargyl Claisen rearrangement, see: Grissom, J. W.;
Klingberg, D.; Huang, D.; Slattery, B. J. J. Org. Chem. 1997, 62, 603.
(11) A thermal Claisen rearrangement of di(allyl) ethers followed by a Rh(I)-catalyzed
intramolecular hydroacylation has been reported. See: Eilbracht, P.; Gersmeir, A.;
Lennartz, D.; Huber, T. Synthesis 1995, 330.
(12) No reaction was observed upon treatment of 1a with 5 mol % PdCl2 (CH2Cl2, rt,
16 h), which is known to be a very effective catalyst for isomerizations of allyl
ethers to enol ethers through electrophilic double bond activation. Therefore,
electrophilic double bond activation by the cationic Rh(I) complex might not be
involved in this olefin isomerization, although the precise mechanism cannot be
determined at the present stage. For a PdCl2-catalyzed isomerization of allyl ethers,
see: Mereyala, H. B.; Lingannagaru, S. R. Tetrahedron 1997, 53, 17501.
(13) No reaction was observed upon treatment of isolated enol ether 5d in refluxing
CH2Cl2 for 14 h. Furthermore, the reaction of 1h with 5 mol % Rh(I)-(S)-
H8-BINAP in refluxing CH2Cl2 furnished 4h with 15% ee. These results clearly
indicate that the cationic Rh(I) complex indeed catalyzes the propargyl Claisen
rearrangement.
presumably because of reversible hydrorhodation/ꢀ-hydride elimination
through rhodacycle C.
Consistent with this pathway, enol ether 5d and allenic aldehyde
2d were initially observed by 1H NMR spectroscopy at room
temperature (eq 1) and 80 °C (eq 2), respectively.15
(14) (a) Tanaka, K.; Fu, G. C. Chem. Commun. 2002, 684. (b) Yu, R. T.; Rovis,
T. J. Am. Chem. Soc. 2006, 128, 2782. (c) Yu, R. T.; Lee, E. E.; Malik,
G.; Rovis, T. Angew. Chem., Int. Ed. 2009, 48, 2379, and references therein.
(15) Isolated crude 2a (ca. >90% purity) was indeed transformed into 3a in 72% isolated
yield under the same conditions as entry 1, Table 3 [5 mol % Rh(I)–dppf, 80 °C,
16 h].
In conclusion, we have developed the cationic Rh(I)-dppf complex-
catalyzed isomerizations of allyl propargyl ethers to allenic aldehydes and
JA9038449
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