Palladium-catalyzed intramolecular carboesterification of olefins

Article abstract of DOI:10.1002/anie.200905478

One catalyst three bonds: The title reaction between propiolic acids and unactivated olefins (see scheme; O red, Cl green) results in vicinal functionalization of the olefin, with the formation of new C-C and C-O bonds. Structurally complex 6,7,5-tricyclic ring systems are formed in a single step by this cascade chloropalladation and formal [3+2] cycloaddition.

Full text of DOI:10.1002/anie.200905478

Communications
DOI: 10.1002/anie.200905478
Cascade Reactions
Palladium-Catalyzed Intramolecular Carboesterification of Olefins**
Yang Li, Katherine J. Jardine, Runyu Tan, Datong Song,* and Vy M. Dong*
Palladium-catalyzed olefin difunctionalization is an attractive
strategy for converting simple alkenes into diverse and
valuable synthetic products.[1] For example, palladium-cata-
lyzed diamination,[2] aminooxygenation,[3] aminohalogena-
tion,[4] carboamination,[5] carboetherification,[5] and diacetox-
ylation[6] of unactivated alkenes have been achieved. Poly-
cyclic motifs are commonly found in natural products and
medicinal targets.[7] Therefore, developing new methods for
constructing rings from simple alkenes represents an impor-
tant goal. Most palladium-catalyzed cycloadditions involve
strained rings (e.g., trimethylenecyclopropanes) or require
highly activated olefins (e.g., Michael acceptors).[8] Herein,
we report a novel palladium-catalyzed formal [3+2] cyclo-
addition between propiolic acids[9] and unactivated alkenes.
Figure 1. Proposed [3+2] cycloaddition of propiolic acids.
This intramolecular carboesterification results in difunction-
À
À
alization of an alkene to form C C and C O bonds, thereby
generating a fused ring system.
Table 1: Palladium-catalyzed cycloaddition of propiolic acid to olefin.[a]
Our proposed [3+2] cycloaddition is based on the unique
combination of three steps: 1) trans chloropalladation, 2) syn
oxypalladation, and 3) reductive elimination (Figure 1). Both
cis and trans chloropalladation of alkynes are well prece-
dented.[10] Halopalladation of propiolic acids, however, has
not been investigated. We envisioned that chloropalladation
of a propiolic acid, accompanied by ligand substitution, could
generate the novel palladium–carboxylate intermediate II.[11]
On the basis of mechanistic studies on carboetherification
reported by Wolfe,[12] we proposed that II would undergo
Entry
[Pd] [mol%]
Solvent
[ClÀ] source (equiv)
Yield [%][b]
1
2
3
4
5
6
7
0
1
1
1
1
1
1
MeCN
MeCN
MeCN
MeCN
HOAc
HOAc
MeCN
–
–
0
48
76
83
50
78
81
nBu4NCl (1)
LiCl (3)
LiCl (6)
LiCl (12)
LiCl (12)
À
syn oxypalladation to form the palladacycle III. A C C bond-
forming reductive elimination would produce the lactone IV.
Finally, oxidation of the Pd0 species with CuCl2 as the terminal
oxidant would regenerate the active PdII catalyst.[13]
[a] 0.05m on 0.1 mmol scale, 15 h. [b] NMR yield determined using 1,3,5-
trimethoxybenzene as an internal standard.
Initial studies began with cyclization of propiolic acid 1a
to afford a 6,7,5-tricyclic product 2a. As shown in Table 1, in
the absence of a catalyst, no reaction was observed. To
achieve the desired trans chloropalladation of 1a, we inves-
tigated reaction conditions reported by Lu and co-workers in
the trans chloropalladation of propargylic esters;[14] they
demonstrated that cascade reactions initiated by chloropalla-
dation of an alkyne benefit from the use of polar solvents such
as MeCN and AcOH.[15] In accordance with these results,
using 1 mol% of [PdCl2(MeCN)2] and three equivalents of
CuCl2 in MeCN, we observed a 48% conversion of 1a into 2a
(Table 1, entry 2). The reaction efficiency depends on the
chloride source and concentration. When nBu4NCl was added
in addition to CuCl2, the product yield increased to 76%
(Table 1, entry 3). Changing the chloride source to LiCl
additionally improved the yield to 83% (Table 1, entry 4).
The reaction was also found to proceed in AcOH, although
higher loadings of LiCl were required (Table 1, entries 5 and
6). Increasing the amount of LiCl to more than three
equivalents in MeCN did not improve the conversion because
of the limited solubility of LiCl in MeCN (Table 1, entry 7).
[*] K. J. Jardine, R. Tan,[+] Prof. D. Song, Prof. V. M. Dong
Department of Chemistry, University of Toronto
80 St. George Street, Toronto, ON, M5S 3H6 (Canada)
E-mail: dsong@chem.utoronto.ca
vdong@chem.utoronto.ca
Dr. Y. Li
Department of Chemistry, Massachusetts Institute of Technology
77 Massachusetts Avenue, Cambridge, MA 02139-4307 (USA)
[+] To whom correspondence about crystallographic data should be
addressed.
[**] We thank the University of Toronto, Canadian Foundation for
Innovation, Ontario Ministry of Research and Innovation, and
Natural Sciences and Engineering Research Council (NSERC) of
Canada for funding. K.J.J. and R.T are grateful for NSERC and
Ontario Graduate fellowships, respectively. Dr. Xiaodan Zhao and
Prof. Shannon Stahl are thanked for helpful discussions.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200905478.
9690
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Angewandte
Chemie
The structure of 2a was confirmed by X-ray crystallo-
graphic analysis (see the Supporting Information for details).
Notably, this polycyclic framework makes up the core of a
family of natural products having anti-HIV activity.[16] In
addition, the vinylchloride functionality provides a handle for
additional synthetic manipulations on a tetra-substituted
alkene.
Electronic and steric effects of the aromatic ring were
examined (Table 2, entries 2–8). Both electron-withdrawing
groups (Table 2, entry 4) and weakly electron-donating
groups (Table 2, entry 2) para to the propiolic acid group
were well-tolerated, resulting in 88% and 86% yields,
respectively. However, a strongly electron-donating methoxy
group at this position resulted in formation of the corre-
sponding product 2c in 61% yield (Table 2, entry 3). Increas-
ing steric demand ortho to the allyl ether group (by
substitution with a methyl or a methoxy group) gave high
yields of 2e and 2 f (Table 2, entries 5 and 6). In contrast,
increasing the steric demand ortho to the propiolic acid group
(Table 2, entries 7 and 8) disfavored the desired cyclization
and resulted in moderate yields of both 2g and 2h, at elevated
temperature (808C). These results suggest chloropalladation
is sensitive to steric bulk at the b-position of the propiolic acid
derivative.
Replacing the ether oxygen atom with a methylene group
resulted in a 71% yield of 2i (Table 2, entry 9). The
introduction of a phenyl substituent on this carbon atom
gave a 2.5:1 ratio of diastereoisomers in 52% overall yield,
with the major product having 1,3-cis stereochemistry
(Table 2, entry 10). In contrast, a soft thioether group inhibits
the desired reaction completely (Table 2, entry 11), presum-
ably because of coordination to the palladium catalyst.
This methodology can be extended to include 1,2-disub-
stituted olefins as coupling partners. Cyclization of the
substrate (E)-3 under standard reaction conditions resulted
in the formation of a 3:1 mixture of trans-4 to cis-4 in 69%
overall yield [Eq. (1)]. The products do not epimerize under
Table 2: Palladium-catalyzed cycloaddition of propiolic acid to olefin.[a]
Entry Substrate
Product
X
Yield[b]
[%]
1
1a
2a
82[c]
the reaction conditions, therfore, we believe that olefin
isomerization prior to cyclization is responsible for the
formation of the minor diastereomer.[12] X-ray crystallo-
graphic analysis of both diastereoisomers unambiguously
confirmed that trans-4 is the major product (see the Support-
ing Information).
2
3
4
1b
1c
1d
2b Me
86
2c MeO 61[d]
2d
F
88
In summary, a palladium-catalyzed intramolecular formal
[3+2] cycloaddition has been achieved using unactivated
alkenes. The reaction proceeds efficiently in the presence of
air and moisture at low catalyst loadings. Moderate diaste-
reoselectivity can be achieved with 1,2-disubstituted olefins.
Future work will focus on expanding the scope and elucidat-
ing the mechanism of this unique carboesterification.[17]
5
6
1e
1 f
2e Me
2 f MeO 90
85
7
8
1g
1h
2g CH2
71[e]
Experimental Section
A
solution of [PdCl2(MeCN)2] (0.52 mg in 0.4 mL MeCN,
0.002 mmol) was added to a solution of the propiolic acid derivative
(0.2 mmol), LiCl (26 mg, 0.6 mmol, 3 equiv), and CuCl2 (80 mg,
0.6 mmol, 3equiv) in acetonitrile (3.6 mL). The mixture was heated at
508C for 14 to 20 h. The resulting solution was concentrated in vacuo
and the lactone product was isolated after flash column chromatog-
raphy on silica gel using diethyl ether/hexanes (1:1) as the eluent.
2h
54[e]
Received: September 30, 2009
Published online: November 24, 2009
9
10
11
1i
1j
1k
2i CH2
71
2j CHPh 52[f]
2k
S
0
[a] Reaction conditions: 0.2 mmol scale, [PdCl2(MeCN)2] (1 mol%) , LiCl
(3 equiv), CuCl2 (3 equiv), 0.05m in MeCN, 508C. [b] Yield of isolated
product. [c] 1.0 mmol scale. [d] Used 2.0 mol% of [Pd], RT. [e] Run at
808C. [f] The d.r.=2.5:1 as determined by NMR spectroscopy.
Keywords: cascade reaction · cycloaddition · lactones ·
palladium
.
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Communications
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À
could then undergo (CuCl2 induced) C O bond-forming reduc-
tive elimination.
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