Phosphane-catalyzed [3+2] annulation of allenoates with aldehydes: A simple and efficient synthesis of 2-alkylidenetetrahydrofurans

Article abstract of DOI:10.1002/chem.200901276

A phosphane-catalyzed [3 +2] annulation of γ-methyl allenoates with aromatic aldehydes has been reported. This annulation provides a convergent and efficient synthesis of 2-alkylidenetetrahydrofurans, which are versatile synthetic building blocks for a vast array of 5-membered oxygenated heterocycle derivatives. The EIZ isomers of 2-(ethoxycarbonylmethylene) tetrahydrofurans, was separated by column chromatography on silica gel and their structural assignments were confirmed by H and C NMR spectroscopy data and X-ray crystallographic analysis. The nature of the substituent, as well as the reaction conditions, has a significant influence on the reactivity patterns of γ-substituted allenoates with aldehydes. The phosphorus ylide, generated from the phosphonium dienolate 8 by an overall 1,4-hydrogen shift, is believed to be the key intermediate responsible for the [3 + 2] annulation and other transformations of γ-methyl allenoates with aldehydes.

Full text of DOI:10.1002/chem.200901276

DOI: 10.1002/chem.200901276
Phosphane-Catalyzed [3+2] Annulation of Allenoates with Aldehydes:
A Simple and Efficient Synthesis of 2-Alkylidenetetrahydrofurans
Silong Xu, Lili Zhou, Renqin Ma, Haibin Song, and Zhengjie He*[a]
Five-membered oxygen-containing heterocycles are im-
portant structural components in a diverse range of natural-
ly occurring and pharmacologically active molecules.[1] Their
widespread occurrence in the structures of natural or artifi-
cial bioactive substances has stimulated considerable interest
in the development of new, efficient preparation methods.[2]
Among numerous known synthetic methods, the convergent
terns of allenoates with aldehydes, olefins, and imines has
been well rationalized.[8b,9] Under the nucleophilic catalysis
of a phosphane, activated olefins and imines undergo pre-
dominant a addition to the nonsubstituted allenoate (R’=
H), leading to [3+2] cycloaddition products (Scheme 1,
pathway A). In sharp contrast, aldehydes undergo exclusive
g addition to the allenoate, resulting in the formation of a
cyclic adduct, for example, 1,3-dioxan-4-ylidene,[8c] rather
than the normal [3+2] cycloaddition product (Scheme 1,
pathway B). It is also understood that a substituent, (e.g.,
methyl) at the a carbon of allenoates can alter the inherent
reactivity pattern of nonsubstituted allenoates. For example,
under the catalysis of nucleophilic phosphanes, both activat-
ed olefins and imines can exclusively undergo g addition to
a-methyl allenoates, resulting in [4+2] annulation reactions
(Scheme 1, pathway C);[7n–q] for aldehydes no such reaction,
with a-substituted allenoates, has been reported in the liter-
ature.[10]
Intrigued by these elegant studies, especially from the
Kwon group, we suspected that the introduction of a sub-
stituent at the g carbon of an allenoate may be able to alter
the normal regioselectivity of g addition of aldehydes to al-
lenoates. Although it is known that g-substituted allenoates
still retain similar reactivity patterns with activated olefins
and imines to those of nonsubstituted allenoates.[7b,j,k] To
evaluate this hypothesis, we began our investigation with al-
lenoates bearing a small substituent like methyl (2a) or a
bulky substituent like phenyl (2b) or tertiary butyl (2c) at
the g carbon. The preliminary experimental results showed
that in the presence of PPh3 (20 mol%) the reaction of g-
methyl allenoate (2a) and o-chlorobenzaldehyde (1a) pro-
ceeded smoothly to give the new products (3a, 4a, and 5a)
in appreciable yields (Scheme 2). Under similar conditions,
however, neither g-phenyl nor g-tert-butyl allenoates afford-
ed any new products. Clearly 3a, 4a, and 5a were formed
by unprecedented reaction pathways. The tetrahydrofuran
derivative 3a is indeed the product of a [3+2] annulation,
resulting from the incorporation of three carbons of the alle-
noate with the carbonyl of the aldehyde; the g-methyl of 2a
is directly involved in the carbon–carbon bond-forming
À
À
annulation, which features both C O and C C bond forma-
tion in one step, is one of the promising strategies to con-
struct oxygen-containing heterocycles from simple and
stable starting materials. Previously, only a few such exam-
ples were reported.[3] Herein, we report a phosphane-cata-
lyzed [3+2] annulation of g-methyl allenoates with aromatic
aldehydes.[4] This annulation provides a convergent and effi-
cient synthesis of 2-alkylidenetetrahydrofurans, which are
versatile synthetic building blocks for a vast array of 5-mem-
bered oxygenated heterocycle derivatives.[5]
Recently, phosphane-catalyzed cycloaddition reactions of
allenes have been widely applied in the construction of a va-
riety of carbo- and heterocycles.[6] Among them, [3+2] and
[4+2] cycloadditions of allenoates with electron-deficient
olefins or imines are especially attractive because they pro-
vide metal-free and highly atom economic strategies to
build five- and six-membered ring systems.[7] However, alde-
hydes as electrophiles in reactions with allenoates show dis-
tinctive reactivity patterns relative to electron-deficient ole-
fins and imines.[8] As a result, the corresponding [3+2] and
[4+2] annulations of allenoates with aldehydes have not
been developed to the same extent as annulations with acti-
vated olefins or imines.
On the basis of experimental and theoretical studies by
Kwon and co-workers, the difference in the reactivity pat-
[a] S. Xu, L. Zhou, R. Ma, Prof. Dr. H. Song, Prof. Dr. Z. He
The State Key Laboratory of Elemento-Organic Chemistry
and Department of Chemistry, Nankai University
94 Weijin Road, Tianjin 300071 (China)
Fax : (+86)22-23501520
E-mail: zhengjiehe@nankai.edu.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200901276.
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COMMUNICATION
acyclic 5a. Carrying out the re-
action in xylene at reflux exclu-
sively afforded tetrahydrofuran
3a. Excessive loadings of 2a
(about 2.0 equiv) led to the for-
mation of 3a in yields of over
90%.
The catalyst loading was also
optimized; a catalyst loading of
5–10 mol% afforded
a high
yield. Decreasing the catalyst
loading further resulted in a sig-
nificant decrease in the yield.
Thus, the optimal reaction con-
ditions for 2a can be summar-
ized as follows: tris(p-fluoro-
phenyl)phosphane catalyst (5–
10 mol% relative to the alde-
hyde) in xylene at 1008C or at
reflux.
Scheme 1. Distinctive reactivity patterns of allenoates with activated olefins, imines, and aldehydes.
Under the optimized condi-
tions, a series of 2-(ethoxycar-
bonylmethylene)tetrahydrofurans, 3, were prepared in mod-
erate to excellent yields (Table 1). The E/Z isomers of 3
were separable by column chromatography on silica gel and
their structural assignments were confirmed by 1H and
13C NMR spectroscopy data[11] and X-ray crystallographic
Table 1. Synthesis of 3 from 2a and aldehydes.[a]
Scheme 2. Phosphane-catalyzed [3+2] annulation of g-methyl allenoate
2a.
steps. Therefore, the presence of a g-methyl group changed
the normal regioselectivity of addition of the aldehyde to
the nonsubstituted allenoates, but did not lead to the antici-
pated a-addition selectivity.
With this encouraging result, we continued to use the re-
action of 2a with 1a as a model to optimize the reaction
conditions in the hope of improving efficiency and selectivi-
ty, especially for the formation of tetrahydrofuran 3a (for
details, see the Supporting Information). A number of readi-
ly available tertiary phosphanes from trialkylphosphanes to
triarylphosphanes were screened first, and the relatively
electron-deficient tris(p-fluorophenyl)phosphane emerged
as the best catalyst.
A solvent survey revealed that the solvent had a remark-
able effect on the reaction yield: xylene was the most suita-
ble solvent for the reaction, although benzene and chloro-
form generally gave comparable results; solvents such as
1,4-dioxane, THF, DMF, and hexane were detrimental to
the reaction and gave either a trace amount or none of the
desired products.
The reaction temperature was found to have a dramatic
influence on the chemoselectivity. Elevation of the tempera-
ture gradually suppressed the formation of dioxane 4a and
Entry Ar
2a
A
t
Prod. Yield E/Z
[%][b] ratio[c]
1
2-ClC6H4 (1a)
2.4
2.5
1.8
1.8
1.8
1.5
1.8
1.8
1.8
2.0
2.0
18 3a
40 3b
24 3c
27 3d
22 3e
14 3 f
94
85
81
83
98
90
83
84
76
91
81
89
87
62
31
37
81
72
53
4:1
2:1
4:1
2:1
6:1
2
3
4-ClC6H4 (1b)
2-FC6H4 (1c)
4
3-FC6H4 (1d)
5
6
7
8
2,4-Cl2C6H3 (1e)
2-NO2C6H4 (1 f)
3-NO2C6H4 (1g)
4-NO2C6H4 (1h)
2-CF3C6H4 (1i)
2-CNC6H4 (1j)
4-CF3C6H4 (1k)
3-CH3O-2-NO2C6H3 (1l) 1.5
5-Cl-2-NO2C6H3 (1m)
C6H5 (1n)
1.5:1
1:1
6
5
3g
3h
5:1
9
12 3i
12 3j
17 3k
13 3l
17 3m
46 3n
48 3o
48 3p
20:1[d]
9:1
10
11
12
13
14
15
16
17
18
19
4:1
4:1
1.5:1
1.5:1
2:1
1.5
2.5
3.0
3.0
1.8
2.0
2.0
4-CH3C6H4 (1o)
2-CH3OC6H4 (1p)
3-pyridyl (1q)
2-furyl (1r)
20:1[d]
20:1[d]
2:1
5
3q
24 3r
24 3 s
2-thiofuryl (1 s)
2:1
[a] Reaction conditions: catalyst loading: 10mol%, except for entries 6,
9, 11, and 12: 5 mol%; reaction temperature: reflux, except for entries 2,
6, 14–16, 18, and 19: 1008C. [b] Yield of the isolated products based on
the aldehyde. [c] Determined on the basis of isolated products. [d] Based
on the 1H NMR spectroscopy analysis of the crude product (only the E
isomer was isolated).
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Z. He et al.
analysis (for (E)-3l). Benzaldehydes bearing electron-with-
drawing groups were more reactive and resulted in higher
yields in shorter reaction times (Table 1, entries 6–13). Halo
benzaldehydes also showed good reactivity, giving 3 a–e in
good to excellent yields (Table 1, entries 1–5). Heteroaro-
matic aldehydes, namely, 3-pyridylaldehyde, 2-furylaldehyde,
and 2-thiofurylaldehyde were also effective, readily afford-
ing the corresponding tetrahydrofuran derivatives in moder-
ate to good yields (Table 1, entries 17–19). Relatively elec-
tron-rich aromatic aldehydes underwent slower reactions,
giving the normal annulation products in poor yields even
after elongated reaction times (Table 1, entries 15 and 16).
Aliphatic aldehydes, such as cinnamaldehyde and n-propy-
laldehyde, were completely unreactive, giving no desired
product.
To expand the scope of the [3+2] annulation, several
structurally similar allenoates with different substituents at
the a and/or g carbon were investigated. a,g-Dimethyl alle-
noate (2d) possessed similar reactivity to that of 2a. Under
the catalysis of (p-FC6H4)3P (10 mol%), compound 2d read-
ily underwent the [3+2] annulation with representative aro-
matic aldehydes, giving tetrahydrofurans 3 in moderate
yields, but only in the protic solvent ethanol (Table 2).
Scheme 3. Effect of substitution at the g carbon of the allenoate.
investigated with 1a. None of the usual [3+2] annulation
product was detected, but a small amount of olefination
product 7 was isolated. The structural identification of 7, to-
gether with detection of the byproduct phosphane oxide, im-
plied a stoichiometric phosphane-mediated olefination be-
tween the allenoate 2 f and 1a. Use of one equivalent of (4-
FC6H4)3P led to the formation of 7 in 80% yield under oth-
erwise identical conditions (Scheme 4).
Scheme 4. Phosphane-mediated olefination of 2 f.
To elucidate the diverse reactivity patterns of g-substitut-
ed allenoates with aldehydes, deuterium-labeling experi-
ments were conducted. Under the optimal conditions for 2a,
g-methyl-deuterated allenoate ([D3]2a; purity >99%) was
treated with p-chlorobenzaldehyde (1b), and the deuterated
[3+2] annulation product [D3]3b (E isomer only) was iso-
lated in 59% yield with deuterium incorporations as shown
in Scheme 5. Interestingly, with the addition of D2O
Table 2. The [3+2] annulation of 2d with representative aldehydes.[a]
Entry
Ar
t [h]
Prod.
Yield [%][b]
E/Z ratio[c]
1
2
3
4
5
2-ClC6H4 (1a)
3-NO2C6H4 (1g)
C6H5 (1n)
4-CH3C6H4 (1o)
3-pyridyl (1q)
23
35
52
60
24
3t
63
68
80
57
51
3:1
1:1
5:1
3:1
3u
3v
3w
3x
E only
[a] Reaction conditions: 1 (0.5 mmol), 2d (0.75 mmol), and PACHTUNGTRNEG(UN 4-FC6H4)3
(0.05 mmol) were stirred in ethanol (2 mL) at room temperature under
an N2 atmosphere. [b] Yield of the isolated product based on the alde-
hyde. [c] Determined on the basis of isolated products.
Scheme 5. Deuterium-labeling experiment of [D3]2a.
Seemingly, the presence of an a-methyl substituent does not
significantly affect the [3+2] annulation reactivity pattern
of g-methyl allenoate.
(1.5 equiv) under otherwise identical conditions, the nondeu-
terated allenoate 2a also afforded [D3]3b (E isomer) with
comparable deuterium incorporations (Scheme 6). These re-
On the contrary, the substituent at the g carbon of the al-
lenoate plays a critical role in the occurrence of the [3+2]
annulation with aldehydes. As shown in the preliminary re-
sults, bulky substituents like phenyl and tert-butyl at the g
carbon completely prevent allenoates from reacting with al-
dehydes. Compared with 2a, g-ethyl allenoate (2e) has a sig-
nificantly decreased reactivity. Under the catalysis of (4-
FC6H4)3P (10 mol%) at room temperature, the normal [3+
2] annulation product 3y was formed in a low yield (19%)
along with the byproduct ethyl (E,E)-2,4-hexadienoate 6
(40%), which competitively formed by isomerization of the
allenoate itself (Scheme 3). Increasing the reaction tempera-
ture proved to be disadvantageous to the formation of the
cycloaddition product 3y. g-Benzyl allenoate (2 f) was also
Scheme 6. Annulation of 2a in the presence of D2O.
sults clearly imply that proton transfer occurs during the re-
action at the a, g, and d carbon atoms of the allenoate, and
that water is most likely to be involved in such proton trans-
fers.
8700
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Phosphane-Catalyzed Annulation of Allenoates
COMMUNICATION
As depicted in Scheme 7, the formation of an intermedi-
ate, 9, that is believed to be the key intermediate responsi-
ble for the formation of the products 3, 4, 5, as well as, 6
The isolation of 7 in high yield provides solid evidence for
the existence of ylide 9b.
When the substituent R’’ in 9 is the much less conjugative
hydrogen, the allylic carbanion 9a is supposedly the favored
form of the phosphorus ylide 9 and tends to undergo an ad-
dition to aryl aldehydes to form the intermediate 10. At ele-
vated temperatures, intermediate 10 preferentially under-
goes a double-bond migration, giving the intermediate 11,
which can cyclize through an intramolecular Michael addi-
tion with subsequent elimination of the phosphane to afford
tetrahydrofuran 3. The double-bond migration is believed to
be thermodynamically driven and responsible for the sup-
pression of the yields of 4 and 5 at higher temperature. On
the other hand, at room temperature intermediate 10 com-
petitively undergoes addition to a molecule of aldehyde or a
proton transfer, leading to the formation of intermediates 13
and 15, respectively. A 6-exo cyclization[8c] of intermediate
13, followed by consecutive proton transfers and final elimi-
nation of the phosphane produces dioxane 4. Phosphonium
dienolate 15 undergoes a series of prototropic shifts fol-
lowed by regeneration of the phosphane to give acyclic
product 5.[6a,13]
In conclusion, the phosphane-catalyzed [3+2] annulation
of g-methyl allenoates with aldehydes was demonstrated.
This reaction provides an efficient synthetic approach to
access the synthetically versatile 2-(ethoxycarbonylmethyle-
ne)tetrahydrofurans 3 from simple and stable starting mate-
rials. It also represents an unprecedented reactivity pattern
of allenoates with aldehydes under nucleophilic phosphane
catalysis. The phosphorus ylide 9, generated from the phos-
phonium dienolate 8 by an overall 1,4-hydrogen shift, is be-
lieved to be the key intermediate responsible for the [3+2]
annulation and other transformations of g-methyl allenoates
with aldehydes. The nature of the g substituent, as well as
the reaction conditions, has a significant influence on the re-
activity patterns of g-substituted allenoates with aldehydes.
Further efforts in our laboratory will be directed toward
other transformations, such as the formation of dioxane 4,
acyclic product 5, and olefination product 7. Results from
such efforts will be published in due course.
Scheme 7. Proposed mechanism for the formation of 3, 4, and 5.
and 7 can be rationalized. The attack of the phosphane cata-
lyst at the b carbon of allenoate 2 initially forms the zwitter-
ionic intermediate 8, which exists in two resonance forms 8a
and 8b. Intermediate 8 undergoes a reversible, overall 1,4-
hydrogen shift, giving the phosphorus ylide 9, which also
presumably exists in two resonance forms 9a and 9b. On
the basis of the deuterium-labeling study, it is believed that
this 1,4-hydrogen shift is most likely to be a stepwise process
aided by water.[9b,12] Previously, similar 1,4-hydrogen shifts
were also proposed in the phosphane-catalyzed reactions of
alkynoates and allenoates by Trost and Kazmaier[13] and oth-
ers.[6a,7q] As the substituent R’’ in 9 changes from hydrogen
to the more conjugative phenyl, the more stable resonance
form 9b represents the major contributor of the intermedi-
ate 9. As a consequence, the reactivity pattern of the g-sub-
stituted allenoate with aldehydes shifts from the [3+2] an-
nulation to isomerization and olefination. Ylide 9b gives
rise to the isomerization product 6 after a series of proto-
tropic shifts and subsequent elimination of the phospha-
ne.[6a,13] When 9b is intercepted by one equivalent of alde-
hyde in a Wittig olefination process, the diene 7 is formed.
Experimental Section
Typical procedure for the phosphane-catalyzed [3+2] annulation of 2a
with aldehydes (Table 1): A solution of 1a (70 mg, 0.5 mmol) and tris(p-
fluorophenyl)phosphane (16 mg, 0.05 mmol) in xylene (1.0 mL) was
heated to reflux under an atmosphere of nitrogen before a solution of 2a
(151 mg, 1.2 mmol) in xylene (1.0 mL) was slowly added by means of a
syringe over 10 min. The resulting reaction mixture was stirred at reflux
for 18 h. After this time 1a was completely consumed, as monitored by
TLC. Then the solvent was removed on a rotary evaporator under re-
duced pressure and the residue was subjected to column chromatography
on silica gel (gradient eluant: petroleum ether/ethyl acetate 20:1–5:1) to
give cycloadducts (E)-3a (the earlier fraction; 96 mg, 72%) and (Z)-3a
(the later fraction; 29 mg, 22%) in 94% combined yield. The identity
and purity of the product was confirmed by 1H and 13C NMR spectro-
scopic analysis.
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Z. He et al.
Typical procedure for the phosphane-catalyzed [3+2] annulation of 2d
with aldehydes (Table 2): Compound 2d (105 mg, 0.75 mmol) was added
by means of a microsyringe over 5 min to a stirred solution of 1a (70 mg,
0.5 mmol) and tris(p-fluorophenyl)phosphane (16 mg, 0.05 mmol) in etha-
nol (2.0 mL) at room temperature under an atmosphere of nitrogen. The
resulting reaction mixture was stirred at room temperature for 23 h until
1a was completely consumed, as monitored by TLC. Then the solvent
was removed on a rotary evaporator under reduced pressure and the resi-
due was subjected to column chromatography on silica gel (gradient
eluant: petroleum ether/ethyl acetate 20:1–5:1) to give cycloadducts (E)-
3t (the earlier fraction; 66 mg, 47%) and (Z)-3t (the later fraction;
22 mg, 16%) in 63% combined yield.
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[11] The E/Z configuration of the double bond in 3 was determined by
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and references therein. The diagnostic feature is that the chemical
shift (recorded in CDCl3) of the vinylic proton for (E)-3 is about
d=0.5 ppm downfield from that of their counterpart (Z)-3.
[12] In the phosphane-catalyzed [3+2] cycloaddition of allenoates with
activated olefins, a stepwise mechanism and the involvement of
water in the proton transfer have been confirmed experimentally
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Huang, S. Liu, Y. Li, Z.-X. Yu, J. Am. Chem. Soc. 2007, 129, 3470–
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X-ray crystallographic analysis: CCDC-730034 ((E)-3l) contains the sup-
plementary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif. For an ORTEP representation
of the structure, see the Supporting Information. X-ray crystal diffraction
data were collected on a Nonius Kappa CCD diffractometer with MoKa
radiation (l = 0.7107 ꢁ).
Acknowledgements
Financial support from National Natural Science Foundation of China
(Grant nos. 20872063; 20672057) is gratefully acknowledged.
Keywords: aldehydes
·
allenes
·
cycloaddition
·
organocatalysis · phosphanes
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[4] In the preparation of this manuscript, a single case of a PPh3-cata-
lyzed [3+2] annulation of ethyl g-methylallenoate with 6-bromopi-
colinaldehyde was reported, with the Z configuration incorrectly as-
signed to the product according to the reported NMR spectroscopy
Received: May 13, 2009
Published online: August 19, 2009
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