Phosphane-catalyzed umpolung addition reaction of nucleophiles to ethyl 2-methyl-2,3-butadienoate

Article abstract of DOI:10.1002/ejoc.201100095

The phosphane-catalyzed umpolung addition of various nucleophiles to ethyl 2-methyl-2,3-butadienoate is described. Oxygen, nitrogen, and carbon nucleophiles smoothly reacted with ethyl 2-methyl-2,3-butadienoate to give the corresponding umpolung addition

Full text of DOI:10.1002/ejoc.201100095

FULL PAPER
DOI: 10.1002/ejoc.201100095
Phosphane-Catalyzed Umpolung Addition Reaction of Nucleophiles to Ethyl
2-Methyl-2,3-butadienoate
Xiao-Yang Guan,[a] Yin Wei,*[a] and Min Shi*[a]
Keywords: Phosphanes / Umpolung / Addition reactions / Nucleophiles / Allenes
The phosphane-catalyzed umpolung addition of various nu- ing umpolung addition products in good to excellent yields
cleophiles to ethyl 2-methyl-2,3-butadienoate is described.
Oxygen, nitrogen, and carbon nucleophiles smoothly reacted
with ethyl 2-methyl-2,3-butadienoate to give the correspond-
by a similar reaction mechanism. For sulfur nucleophiles, the
addition reactions with ethyl 2-methyl-2,3-butadienoate pro-
ceeded by a different mechanism.
tions arise from the difference in the electronic properties of
the two connected carbon–carbon double bonds. The α,β-
double bond is electron-deficient whereas the β,γ-double
bond is relatively electron-rich.[7,8]
Introduction
Reactivity inversion (umpolung) plays an important role in
modern organic synthesis.[1] In recent years, the umpolung
addition reaction between nucleophiles and 2,3-buta-
According to Lu and co-workers,[2a,3b] allenoates with a
dienoates or 2-butynoates has been developed;[2–6] for ex- substituent at the α-position, such as ethyl 2-methyl-2,3-buta-
ample, the nucleophilic addition of various nucleophiles to dienoates,[9] also gave the inverse addition products with di-
allenes bearing an electron-withdrawing group (EWG) can methyl malonate as a nucleophile in benzene in the presence
be divided into two categories. The first is nucleophilic ad- of tributylphosphane (PBu3) [Scheme 1, Equation (3)]. In
dition at the α,β-double bonds to give the corresponding contrast, under our reaction conditions, nucleophilic addition
Michael-type adducts [Scheme 1, Equation (1)].[7] In the occurs at the 2-methyl group of ethyl 2-methyl-2,3-butadieno-
second, nucleophiles add inversely to the β,γ-double bonds ate [Scheme 1, Equation (4)]. Herein we report the details of
in the presence of phosphane catalysts to give the umpolung this unexpected phosphane-catalyzed umpolung addition re-
addition products [Scheme 1, Equation (2)]. The two reac- action of nucleophiles to ethyl 2-methyl-2,3-butadienoate.
Scheme 1. Different kinds of nucleophilic addition to allenoates.
[a] State Key Laboratory of Organometallic Chemistry, Shanghai
Results and Discussion
Institute of Organic Chemistry, Chinese Academy of Sciences,
354 Fenglin Road, Shanghai 200032, China
Initially, the reaction of ethyl 2-methyl-2,3-butadienoate
(1) with 4-methoxyphenol (2a) was conducted in the pres-
ence of PPh3 and THF at 60 °C, which afforded an unex-
pected new product, 3a [(E)/(Z) = 2.5:1], in 82% yield
Fax: +86-21-64166128
E-mail: weiyin@mail.sioc.ac.cn
mshi@mail.sioc.ac.cn
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201100095.
Eur. J. Org. Chem. 2011, 2673–2677
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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X.-Y. Guan, Y. Wei, M. Shi
FULL PAPER
(Table 1, Entry 1). A series of analytical measurements on just giving traces of products. Therefore, it was necessary to
compound 3a showed that the nucleophilic addition reac- reexamine the reaction conditions, focusing especially on
tion between 1 and 2a occurs at the α-methyl group of ethyl the phosphane catalysts. Trimethylphosphane (PMe3),
2-methyl-2,3-butadienoate (1) rather than at the γ-position which is a stronger nucleophilic phosphane than PBu3, did
of 1. Decreasing the reaction temperature to room tempera- not catalyze the reaction of 1 with phenols 2 bearing elec-
ture slightly improved the stereoselectivity of the product tron-withdrawing groups on the benzene rings. Analysis of
3a, but the yield was sacrificed (Table 1, Entry 2). The po- the 1H NMR spectroscopic data of the crude product
tential of several commonly used phosphanes as catalysts showed the product derived from self-cycloaddition of 1 in
was assessed in the nucleophilic addition reaction between the presence of the phosphane catalyst. The poor yields
1 and 2a. The results are summarized in Table 1. More nu- from the reactions of 1 with 2 having electron-withdrawing
cleophilic phosphanes catalyzed this reaction to give the groups may be due to a rather slow reaction; meanwhile, 1
product 3a with higher stereoselectivities. Of these catalysts, readily undergoes self-cycloaddition in the presence of
dimethyl(phenyl)phosphane (PPhMe2) gave 3a with the strong nucleophilic phosphanes such as PBu3 and PMe3
highest stereoselectivity but in a low yield (Table 1, En- (see the Supporting Information). Thus, we switched to the
try 7). PBu3 catalyzed this reaction to give 3a in excellent less nucleophilic phosphane PPh3 as the catalyst in this re-
yield and with satisfactory stereoselectivity in THF action and found that satisfactory results could be ob-
(Table 1, Entry 8). The solvent effects were examined by tained. As outlined in Table 2, phenols 2 bearing electron-
using PBu3 as the catalyst. tert-Butyl methyl ether (MTBE) withdrawing groups on the benzene rings or naphthols re-
was also a suitable solvent, giving 3a in 80% yield with acted with 1 smoothly to afford the corresponding products
(E)/(Z) = 16.1:1 (Table 1, Entry 13), but the reaction did in good to excellent yields with acceptable stereoselectivities
not take place effectively in acetonitrile or 1,2-dichloroe- (Table 2, Entries 10–16).
thane (DCE) (Table 1, Entries 11 and 12).
Table 2. Scope of the reactions of ethyl 2-methyl-2,3-butadienoate
(1) with phenols 2.[a]
Table 1. Catalyst and solvent screening for the reaction of ethyl 2-
methyl-2,3-butadienoate (1) with 4-methoxyphenol (2a).[a]
Entry
R
Product
Yield [%][b]
(E)/(Z)[c]
1
2
3
4
5
6
7
8
9
4-tBu
4-NH2
4-Ac(CH2)2
3-Me
3-MeO
2-Me
2-MeO
2-Bn
H
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
3p
3q
98
98
90
88
95
89
93
94
95
98
82
89
98
98
97
98
16.3:1
18.1:1
14.2:1
11.4:1
11.8:1
9.8:1
16.2:1
8.0:1
13.9:1
6.3:1
11.1:1
8.7:1
7.8:1
8.8:1
7.4:1
Entry
Solvent
Cat.
Yield [%][b]
(E)/(Z)[c]
1
2[d]
3
4
5
6
7
8
9
10
11
12
13
THF
THF
THF
THF
THF
THF
THF
THF
toluene
dioxane
CH3CN
DCE
PPh3
PPh3
82
32
50
89
57
18
44
98
23
64
trace
8
2.5:1
4.7:1
3.7:1
4.9:1
1.8:1
24.5:1
32.1:1
14.1:1
2.9:1
8.4:1
–
P(p-FC6H4)3
P(p-MeC6H4)3
P(p-MeOC6H4)3
PPh2Me
PPhMe2
PBu3
10[d]
11[d]
12[d]
13[d]
14[d]
15[d]
16[d]
4-Cl
4-NO2
4-CN
3-Cl
PBu3
PBu3
PBu3
PBu3
2-Br
11.6:1
16.1:1
[e]
MTBE
PBu3
80
[f]
5.7:1
[a] All the reactions were performed with 1 (0.2 mmol) and 2a
[a] All the reactions were performed with 1 (0.4 mmol) and 2
(0.1 mmol) in 1.0 mL of solvent. [b] Isolated yields. [c] Determined
(0.2 mmol) in 3.0 mL of THF. [b] Isolated yields. [c] Determined
1
by H NMR spectroscopy. [d] At r.t.
1
by H NMR spectroscopy. [d] Catalyzed by 20 mol-% of PPh3. [e]
1-Naphthol as the substrate. [f] 2-Naphthol as the substrate.
Having identified the optimal reaction conditions, we
next set out to examine the scope and limitations of this
We next examined the reactions of 1 with sulfur nucleo-
reaction by using various phenols 2 with different substitu- philes in the presence of PBu3. Under otherwise identical
ents on the benzene rings. The results are summarized in conditions, 1 reacted with thiophenols 4 smoothly to give
Table 2. As shown in Table 2, when phenols 2 bearing elec- the corresponding products 5 with different structures to 3
tron-donating groups at the ortho, meta, or para position of (Table 3). It is clear that product 5 is formed by nucleophilic
the benzene rings were employed, the reactions proceeded addition of thiophenols 4 to the α,β-double bond of 1
smoothly to give 3 in good to excellent yields with good through a Michael-type addition reaction. To clarify the re-
stereoselectivities (Table 2, Entries 1–9). However, phenols action pathway, we performed some control experiments to
2 with electron-withdrawing groups on the benzene rings further investigate the reaction mechanism (Scheme 2).
were not suitable substrates under the standard conditions, Thiophenol 4c also reacted with 1 in the absence of PBu3
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Eur. J. Org. Chem. 2011, 2673–2677

Addition of Nucleophiles to Ethyl 2-Methyl-2,3-butadienoate
Scheme 2. Control experiments.
to give two adducts 5c and 5e (5c/5e = 1:2.1) in 44% total Table 4. Scope of the reactions of ethyl 2-methyl-2,3-butadienoate
(1) with Ts-protected anilines 6.[a]
yield along with some unidentified byproducts. However, 5e
could not be transformed into 5c under the standard reac-
tion conditions, which suggests that PBu3 is crucial in this
reaction.
Table 3. Scope of the reactions of ethyl 2-methyl-2,3-butadienoate
(1) with thiophenols 4.[a]
Entry
R
Product
Yield [%][b]
(E)/(Z)[c]
1
2
3
4
5
H
4-MeO
4-Me
3-Me
2-Me
3,5-(MeO)2
H
2-Me
3,5-(MeO)2
4-Cl
7a
7b
7c
7d
7e
7f
79
81
71
62
47
35
99
98
98
98
10.5:1
10.5:1
11.8:1
12.5:1
8.0:1
15.4:1
1.6:1
1.4:1
1.6:1
1.8:1
6
Entry
R
Yield [%][b]
7[d]
8[d]
9[d]
10[d]
7a
7e
7f
1
2
3
4
H
4-Me
4-Cl
5a, 94
5b, 58
5c, 76
5d, 81
7g
2,4-Cl2
[a] All the reactions were performed with 1 (0.4 mmol) and 6
(0.2 mmol) in 3.0 mL of THF. [b] Isolated yields. [c] Determined
[a] All the reactions were performed with 1 (0.4 mmol) and 4
(0.2 mmol) in 3.0 mL of THF. [b] Isolated yields.
1
by H NMR spectroscopy. [d] Catalyzed by 20 mol-% of PPh3 for
72 h.
We also examined the reactions of 1 with nitrogen nu-
cleophiles 6 in the presence of PBu3. The results are sum-
marized in Table 4. Tosyl-protected anilines with electron-
donating groups at the ortho, meta, or para position of the
aniline benzene ring underwent the nucleophilic addition
reaction in the presence of PBu3 at room temperature in-
stead of 60 °C to give the corresponding products 7 in mod-
erate to good yields with high stereoselectivities (Table 4,
Entries 1–6). We also confirmed that products 7 could de-
compose to the starting materials at 60 °C in the presence
of PBu3. However, Ts-protected anilines with electron-with-
drawing groups did not react with 1 in the presence of
PBu3, similarly to the reaction of 1 with phenols bearing
electron-withdrawing groups. PPh3 was able to solve this
substrate limitation again to afford the products 7 from the
reaction between Ts-protected anilines bearing either elec-
tron-donating or -withdrawing groups and 1 in excellent
yields at room temperature, although the reaction rates were
relatively low and the stereoselectivities poor (Table 4, En-
tries 7–10).
desired product was not formed in THF under the standard
conditions.
Table 5. Scope of the reactions of ethyl 2-methyl-2,3-butadienoate
(1) with cyclic 1,3-diones 8.[a]
[a] All the reactions were performed with 1 (0.4 mmol) and 8
Some carbon nucleophiles were also examined in this re-
action. The results of these experiments are shown in
Table 5. As can be seen from Table 5, cyclic 1,3-diones 8
also reacted with 1 smoothly to give the corresponding ad-
(0.2 mmol) in 3.0 mL of THF. [b] Isolated yields. [c] Determined
1
by H NMR spectroscopy. [d] At 60 °C.
Other allenoates, such as ethyl 2-ethyl-2,3-butadienoate,
ducts 9 with enolization of one of the carbonyl groups in ethyl 2-benzyl-2,3-butadienoate, and ethyl 2-methyl-2,3-
high yields with good stereoselectivities (Table 5). Dimethyl pentadienoate, were also examined in this interesting phos-
malonate was also examined in this reaction. However, the phane-catalyzed umpolung addition reaction. However,
Eur. J. Org. Chem. 2011, 2673–2677
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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X.-Y. Guan, Y. Wei, M. Shi
FULL PAPER
none of the desired products were formed under the stan- nucleophiles and solvents can lead to different results. In
dard reaction conditions with nucleophiles that are suitable polar solvents such as THF (our work), intermediate A is
for ethyl 2-methyl-2,3-butadienoate.
inclined to be transformed into intermediate D. However,
On the basis of the above results and previous reports in in nonpolar solvents such as benzene (Lu and co-workers),
the literature,[3b,9t,10] a plausible mechanism for the phos- intermediate A predominately exists in the reaction system
phane-catalyzed umpolung addition reactions between (Scheme 3).
ethyl 2-methyl-2,3-butadienoate (1) and different kinds of
nucleophiles is proposed (Scheme 3). With oxygen, nitro-
gen, or carbon nucleophiles, PBu3 or PPh3 as nucleophilic
initiator reacts with ethyl 2-methyl-2,3-butadienoate to pro-
Conclusion
We have developed a novel phosphane-catalyzed umpo-
lung addition reaction for oxygen, nitrogen, and carbon nu-
cleophiles with ethyl 2-methyl-2,3-butadienoate in which
nucleophiles attack the 2-methyl group of ethyl 2-methyl-
2,3-butadienoate. These addition reactions afforded the cor-
responding adducts in good to excellent yields with good
to high stereoselectivities. As for sulfur nucleophiles, they
added to the β-carbon atom of allenoates to give different
adducts. Research is in progress to elucidate further mech-
anistic details of these reactions and to explore the scope
and limitations of allenoates.
duce the zwitterionic intermediate A, which can isomerize
to intermediate B. Intermediate B can be further trans-
formed into intermediate D, presumably via intermediate C
by stepwise proton transfers. Intermediate D can depro-
tonate the pronucleophile employed to generate intermedi-
ate E and the corresponding nucleophilic anion. The nucle-
ophilic anion undergoes subsequent addition to intermedi-
ate E to afford intermediate F. Subsequent elimination of
the phosphane from F furnishes the corresponding product
and regenerates the phosphane catalyst. With sulfur nucleo-
philes, the reaction proceeds in a different way. The zwitter-
ionic intermediate A deprotonates the nucleophile to form
the ion pair G. The nucleophilic anion in G then undergoes
a subsequent conjugate addition to the α,β-double bond of
1 to afford the other ion pair H. Protonation of the anion
partner in H results in the formation of the corresponding
product and regenerates the ion pair G to complete the
catalytic cycle. The reason for the difference between our
results and those of Lu and co-workers is presumably due
Experimental Section
General: 1H and 13C NMR spectra were recorded at 400 and
100 MHz, respectively. MS and HRMS was performed by using
the ESI method. The organic solvents were dried by standard meth-
ods if it was necessary. Satisfactory CHN microanalyses were ob-
tained with an analyzer. Commercially obtained reagents were used
to the nucleophiles employed and solvent polarity. Different without further purification. All the reactions were monitored by
Scheme 3. Plausible mechanism for the phosphane-catalyzed umpolung addition of nucleophiles to ethyl 2-methyl-2,3-butadienoate.
2676 Eur. J. Org. Chem. 2011, 2673–2677
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Addition of Nucleophiles to Ethyl 2-Methyl-2,3-butadienoate
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was carried out by using silica gel at increased pressure.
General Procedure for the PBu3-Catalyzed Reaction of Ethyl 2-
Methyl-2,3-butadienoate (1) with Phenols 2: Ethyl 2-methyl-2,3-but-
adienoate (1; 50 mg, 0.4 mmol), phenols (0.2 mmol), and PBu3
(8 mg, 0.04 mmol) were stirred in THF (3.0 mL) under argon in a
10 mL Schlenk tube. After stirring the reaction mixture at 60 °C
for 12 h, the solvent was removed under reduced pressure and the
residue was purified by flash column chromatography (SiO2;
EtOAc/petroleum ether, 1:30) to yield the corresponding product
3.
Compound (E)-3a: Colorless oil (23 mg, 92% yield, 0.1 mmol scale).
1H NMR (CDCl3, 300 MHz, TMS): δ = 1.29 (t, J = 7.2 Hz, 3 H,
CH3), 1.93 (d, J = 7.5 Hz, 3 H, CH3), 3.77 (s, 3 H, OCH3), 4.22
(q, J = 7.2 Hz, 2 H, OCH2), 4.73 (s, 2 H, OCH2), 6.80–6.85 (m, 2
H, Ar), 6.88–6.92 (m, 2 H, Ar), 7.19 (q, J = 7.5 Hz, 1 H, =CH)
ppm. 13C NMR (CDCl3, 75 MHz, TMS): δ = 14.2, 14.7, 55.7, 60.7,
62.6, 114.5, 116.1, 129.2, 144.2, 152.9, 154.0, 166.7 ppm. IR
(CH Cl ): ν = 2982, 2939, 2907, 2834, 1710, 1506, 1282, 1222, 1139,
˜
2
2
1036, 1012, 824, 732 cm–1. MS (EI): m/z (%) = 250 (13.6) [M]+, 124
(100.0), 123 (25.9), 109 (25.3), 95 (8.4), 53 (8.1), 205 (6.9), 54 (6.7),
41 (4.7). HRMS (EI): calcd. for C14H18O4 [M]+ 250.1205; found
250.1206.
Supporting Information (see footnote on the first page of this arti-
cle): Spectroscopic data of all new compounds shown in Tables 1–
5, detailed descriptions of experimental procedures.
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Acknowledgments
We thank the Shanghai Municipal Committee of Science and Tech-
nology (08dj1400100-2), the National Basic Research Program of
China (973-2010CB833302), and the National Natural Science
Foundation of China for financial support (21072206, 20872162,
20672127, 20821002, 20732008, and 20702059).
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Received: January 22, 2011
Published Online: March 29, 2011
Eur. J. Org. Chem. 2011, 2673–2677
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
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