Efficient syntheses of (thio)phosphonylated isobenzofurans by tandem nucleophilic addition and regioselective 5-exo-dig addition to carbon-carbon triple bond: Cooperative effect to 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)

Article abstract of DOI:10.1002/adsc.200800484

The tandem nucleophilic addition-cyclization reaction of o-alkynylbenzaldehydes or o-alkynylacetophenones 2 with dialkyl phosphites or dialkyl phosphonothioates 1 took place very smoothly in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in THF at room temperature. In all cases, the reaction proceeded in a regioselective manner leading to the 5-exo-dig products 3 in excellent yields. The phenomenon of a 1,5-sigmatropic hydrogen shift or a 1,5-sigmatropic methyl shift was observed in this reaction depending on the different substituent groups such as R³ in the o-alkynylbenzaldehyde or o-alkynylacetophenone 2 substrates.

Full text of DOI:10.1002/adsc.200800484

FULL PAPERS
DOI: 10.1002/adsc.200800484
Efficient Syntheses of (Thio)phosphonylated Isobenzofurans by
Tandem Nucleophilic Addition and Regioselective 5-exo-dig
Addition to Carbon-Carbon Triple Bond: Cooperative Effect to
1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU)
G
Fei Wang,a Yadan Wang,a Lingchao Cai,a Zhiwei Miao,a,b,* and Ruyu Chena
a
State Key Laboratory and Institute of Elemento-Organic Chemistry, Nankai University, Tianjin 300071, Peopleꢀs Republic
of China
Fax : (+86)-22-2350-2351; e-mail: miaozhiwei@nankai.edu.cn
Key Laboratory of Bioorganic Phosphorus Chemistry & Chemical Biology (Ministry of Education), Tsinghua University,
b
Beijing 100084, Peopleꢀs Republic of China
Received: August 2, 2008; Published online: November 17, 2008
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200800484.
Abstract: The tandem nucleophilic addition-cycliza- or a 1,5-sigmatropic methyl shift was observed in this
tion reaction of o-alkynylbenzaldehydes or o-alkyn- reaction depending on the different substituent
ACHTUNGTRENNUNG
ylacetophenones 2 with dialkyl phosphites or dialkyl groups such as R3 in the o-alkynylbenzaldehyde or o-
phosphonothioates 1 took place very smoothly in alkynyl
the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene
(DBU) in THF at room temperature. In all cases, the
reaction proceeded in a regioselective manner lead- Keywords: cyclization reaction; 1,8-diazabicyclo-
ing to the 5-exo-dig products 3 in excellent yields. [5.4.0]undec-7-ene (DBU); regioselective 5-exo-dig
The phenomenon of a 1,5-sigmatropic hydrogen shift addition; tandem nucleophilic addition
ACHTUNGTNERaNUNG cetophenone 2 substrates.
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
Introduction
example, indoles,[7] benzo[b]furans,[8] isoquinolines,[9]
3-alkylidenephthalides vs. 3-substituted isocoumar-
Organophosphorus compounds continue to receive ins,[10] 1-alkylideneisobenzofurans vs. 3-substituted
wide-spread attention due to their ubiquity in biologi- 1H-2-benzopyrans,[11] and 3-alkylideneisoindoline-1-
cal systems.[1] Isobenzofuran derivatives are useful ones vs. 3-substituted isoquinolin-1-ones.[12] Further-
compounds as building blocks of bioactive compounds more, Baldwinꢀs rule predicts that both 5-exo-dig and
and functional materials.[2] In addition, they have re- 6-endo-dig cyclizations are favorable, making selective
active diene moieties and can be used for Diels– synthesis difficult in practice (Scheme 1). Therefore,
Alder reactions.[3] Intramolecular ring closure reac- much attention has been paid to the development of
tions, which can be carried out between the nucleo- a highly regioselective cyclization route.[13] Herein we
philic part and carbon-carbon multiple bond in the report a tandem nucleophile attack-intramolecular
same molecule, are one of the useful methods for con- cyclization reaction of o-alkynylbenzaldehyde or o-al-
structing cyclic compounds.[4] Carbonyl groups are of
particular interest, since tandem reactions with nucle-
ophiles can occur at this function, broadening the
structural panel of the products formed.[5] It is well
known that in nucleophile attack processes of acety-
lenic aldehydes, which have a triple bond as the coun-
terpart, both the ꢁ6-endo-digꢀ mode and ꢁ5-exo-digꢀ
modes are allowed by Baldwinꢀs rule (Scheme 1).[6] In
fact, both cyclized products for the 2-ethynylphenyl
derivatives have been reported in the literature, for Scheme 1.
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Fei Wang et al.
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kynylacetophenone derivatives with a cooperative reaction, we systematically examined the relationship
effect from DBU (1,8-diazabicyclo[5.4.0]undec-7-ene) between regioselectivity and solvent. The simple reac-
that affords (thio)phosphonylated isobenzofuran de- tion at a ratio 2:1:n-BuLi of 1:1:1 in dichloromethane
AHCTUNGTRENNUNG
rivatives in a highly regioselective manner.
or toluene did not result in thermal cyclization
(Table 1, entries 4 and 5).
The transition metal-catalyzed synthesis of various
heterocycles via cyclization of alkynes possessing a
nucleophile in proximity to the triple bond is one of
Results and Discussion
A preliminary experiment was conducted using di- the most important processes in organic synthesis.[14]
methyl phosphite 1a (0.5 mmol) which was reacted We examined the reaction of our substrates in the
with o-alkynylbenzaldehyde 2 (0.5 mmol) in the pres- presence of a variety of metal catalysts. Cyclizations
ence of n-butyllithium (n-BuLi) (0.5 mmol) in THF at were observed in the presence of all kinds of catalysts,
À508C (Table 1, entry 3). In this reaction, dimethyl whereby CuI, Pd[P
ACHTUNTGRENGU(N Ph3)]2Cl2 and PdACHTUNGTRENN(GUN OAc)2 catalyzed
phosphite 1a was deprotonated by n-BuLi and then the 5-exo cyclization to give the isobenzofuran deriva-
attacked the o-alkynylbenzaldehyde 2, generating 5- tive 3a in nearly identical yields (Table 1, entries 6–8).
exo-dig isobenzofuran derivatives 3a and 4a in 88%
As the metal catalysts did not affect the cyclization,
and 12% yields, respectively, as determined by we next focused our attention on the role of the sol-
31P NMR. The result suggests that the reaction time is ventꢀs acidity or basicity in promoting the cyclization
vital, as both 3a and 5a will be generated if the reac- modes of 2. Cyclization was not observed at all in
tion time is below half an hour (Table 1, entries 1 and acids such as CF3COOH, and the only isolated prod-
2). In order to achieve high yields of this cyclization ucts were 5a and starting material 2 (Table 1,
Table 1. Optimization of reaction conditions.[a]
Entry
Base[b]
Solvent
Time [h]
Yield [%][c]
3a
4a
5a
1
2
3
4
5
6
7
8
BuLi
BuLi
BuLi
BuLi
BuLi
BuLi
BuLi
BuLi
THF[d]
THF[d]
THF[d]
CH2Cl2
Toluene[d]
THF[e]
THF[f]
THF[g]
THF
THF
THF
THF
THF
1/4
1/2
20
20
20
20
20
20
20
20
20
20
20
20
20
20
–
–
–
12
–
–
12
14
13
10
–
–
–
–
–
–
–
>99
70
–
–
–
6
–
–
–
>99
92
–
>99
–
30
88
–
[d]
–
84
86
87
90
–
8
–
–
9
BuLi
10
11
12
13
14
15
16
NaOCH3
K2CO3
Pyridine
Et3N
DBU
DMAP
CF3COOH
THF
THF
THF
>99
–
–
>99
10
[a]
Unless otherwise noted all the reactions were performed with 0.5 mmol of 2, 1.0 mmol dimethyl phosphite 1a and
1.0 mmol base in 8 mL solvent at room temperature.
[b]
[c]
[d]
[e]
[f]
When n-BuLi was used, the reactions were all performed below À508C.
The yields of products were determined from a 31P NMR spectrum of the crude mixture.
All reactions were performed with 0.5 mmol of 2, 0.5 mmol dimethyl phosphite 1a and 0.5 mmol base in 8 mL solvent.
5 mol% of CuI was added.
5 mol% of Pd
ACHTUNGTRENNUNG
[g]
5 mol% of PdACHTUNGTRENNUNG
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Efficient Syntheses of (Thio)phosphonylated Isobenzofurans
FULL PAPERS
entry 16). In contrast, nitrogen-containing basic cata- phenylethynyl)phenyl]
ACHTUNGTRENNUNG
lysts, such as 4-(N,N-dimethylamino)pyridine 5a, pyridine proved ineffective in the cyclization reac-
(DMAP) and triethylamine, gave only dimethyl [2-(2- tion (Table 1, entry 12). More strongly basic catalysts,
such as K2CO3, induced the regioselectivity, giving di-
methyl 3-benzyl-2H-isobenzofuran-1-ylphosphonate
3a selectively, together with a large amount of 5a in
92% yield (Table 1, entry 11). Sodium methoxide
could only induce product 5a in nearly quantitative
yield. DBU could certainly catalyze the cyclization re-
action and produced 3a with 99% yield. Thus, the se-
lective syntheses of the 5-exo-dig isobenzofuran deriv-
ative 3a from o-alkynylbenzaldehyde 2 and dialkyl
phosphite 1 were achieved through the use of DBU
and THF as solvent, respectively.
In an extension of this work, we also examined the
base-induced reaction in order to evaluate the mode
of internal cyclization. We found that the base-in-
duced cyclization (n-BuLi/THF) of 2 afforded product
3a in moderate yield with by-products 5a and 6 in the
same reaction. The interesting result is that when the
ratio of 2:1:n-BuLi was changed to 1:4:4, the only iso-
lated product was the butylated compound 6 (yield
87%) on the phosphorus atom in place of the cycliza-
tion. The structures of 5a and 6 were assigned on the
basis of a detailed NMR analysis and firmly estab-
lished by an X-ray crystallographic study (Figure 1
and Figure 2).[15]
Figure 1. ORTEP diagram of 5a.
The formation of 3a was followed by 31P NMR
spectroscopy as shown in Figure 3. The starting mate-
rial dimethyl phosphite 1 in THF showed a 31P NMR
signal at 10.61 ppm. After DBU (0.153 g, 1 mmol) had
been added to the solution of 1, a new single peak at
6.56 ppm appeared and was assigned as the deproto-
nation product of dimethyl phosphate 1. When o-al-
kynylbenzaldehyde 2 was added to the mixture, the
expected nucleophilic addition product was produced
(single peaks at 24.68 ppm) in five minutes. As time
passed, the 31P NMR signals of the starting material
disappeared gradually and the signals of 3a (single
peaks at 1.35 ppm) increased. There is one intermedi-
ate that appeared during the synthesis of 3a. The sig-
Figure 2. ORTEP diagram of 6.
Figure 3. 31P NMR stack spectra for the synthesis of 3a (ppm).
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Fei Wang et al.
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Scheme 2.
nals at dP =20.22 ppm may belong to intermediate 7 thioate 1 exhibited low reactivity and required a
(Scheme 2). DBU accelerates the reaction via forma- longer reaction time, and achieve complete conver-
tion of the intermediate 7, and activates the carbon- sion after 32 h (Table 2). When o-alkynylacetophe-
carbon triple bond. The reaction was almost complet- none 2 was used in this nucleophilic addition cycliza-
ed after 22 h according to the 31P NMR spectra tion reaction, the 1,5-sigmatropic methyl shift prod-
(Figure 3).
ucts 3h and 3i were obtained in moderate yield re-
Based on 31P NMR stack spectra (Figure 3), a plau- spectively (Table 2, entry 8 and 9).[17] Compared with
sible mechanism for the cooperative effect of DBU the 13C NMR of 3a, there are also double peaks with
on the cyclization reaction of 2 to 3a is outlined in chemical shifts at about 90.55 ppm representing
(OC2H5)2 in the 13C NMR
Scheme 2. The present intramolecular cyclization is OCPACTHUNRTGENNUG(OCH3)2 and OCPACHUTNGTRENNUGN
most probably initiated by the generation of a phos- spectra of 3h and 3i with the coupling constants of
phonate anion intermediate via deprotonation of di- about 620 Hz, which can represent proof of the 1,5-
ACHTUNGTRENNUNGalkyl phosphite 1 by DBU. In the meantime, the 5- sigmatropic methyl shift during the reaction. It was
exo cyclization of the oxygen anion to the triple bond found that the use of o-alkynylbenzaldehyde led to
would be assisted by the conjugate base of DBU. This much better yields than o-alkynylacetophenone. The
is followed by a 1,5-sigmatropic hydrogen shift to reaction of o-alkynylbenzaldehyde 2c, bearing a butyl
afford the alkylidenephthalan derivative 3a.[16]
group as R3, with dimethyl phosphite or diethyl phos-
In order to prove the 1,5-sigmatropic hydrogen phite 1, proceeded smoothly to give 3’a or 3’b in good
1
shift to generate the alkylidenephthalan derivative 3a, yields (Table 2 entry 10 and 11). In the H NMR spec-
2D NMR HSQC was employed to verify the struc- trum of 3’a the single peak with a chemical shift of
ture. The cross-peak in the HSQC spectrum of 3a 5.28 ppm represents the existence of an ethene
À
À
(Figure 4) between H PhCH2 and C PhCH2 atoms proton, which can prove that there is no 1,5-sigma-
confirmed the occurrence of the 1,5-sigmatropic hy- tropic hydrogen shift during the reaction. Similarly,
drogen shift from 9 to 3 in Scheme 2.
the trimethylsilyl-substituted 2e also cyclized in mod-
Under the optimized conditions and in the presence erate yield. The non-substituted alkyne substrate 2f
of DBU, the 5-exo-dig cyclization reaction of o-alky- can also react with dimethyl phosphite 1 with the aid
nylbenzaldehyde 2 with dialkyl phosphite or dialkyl of DBU in good yield. The reaction of o-alkynylace-
phosphonothioate 1 exhibits a broad scope. Excellent tophenone 2d, bearing a butyl group as R3, with di-
regioselectivities were obtained and provided good to methyl phosphite 1, only gave a 50% yield of 3’c
excellent yields of the desired product 3 for a variety (Table 2 entry 12). An interesting result is that in the
of phosphites and thiophosphites. Dialkyl phosphono- reactions of o-alkynylbenzaldehyde or o-alkynylaceto-
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Efficient Syntheses of (Thio)phosphonylated Isobenzofurans
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Figure 4. The HSQC spectrum of 3a.
Table 2. Reactions of dialkyl (thio)phosphite 1 with o-alkynylbenzaldehyde or o-alkynylacetophenone 2.
Entry[a]
Substrate 1
Substrate 2
R2
R3
Temperature [8C]
Product
Yield [%][b]
1
2
3
4
5
6
7
8
H(O)P
H(O)P
H(O)P
H(O)P
H(O)P
U
2a
2a
2a
2a
2a
2a
2a
2b
2b
2c
2c
2d
2e
2f
H
H
H
H
H
H
H
CH3
CH3
H
H
CH3
H
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
r.t.
40
40
40
40
30
35
40
40
40
40
60
r.t.
r.t.
3a
3b
3c
3d
3e
3f
3g
3h
3i
3’a
3’b
3’c
3’d
3’e
98
96
94
91
92
72
61
65
60
72
64
50
54
83
[c]
H(S)P
H(S)P
N
[c]
H(O)P
H(O)P
H(O)P
H(O)P
H(O)P
H(O)P
H(O)P
9
Ph
10
11
12
13
14
C4H9
C4H9
C4H9
Me3Si
H
H
[a]
Unless otherwise noted the reactions performed with 0.5 mmol of 2, 1.0 mmol 1, 1.0 mmol DBU in 8 mL THF for 20 h.
Isolated yields.
Reaction time is 32 h.
[b]
[c]
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Fei Wang et al.
FULL PAPERS
References
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Conclusions
In conclusion, we have described a general method
for the efficient synthesis of isobenzofuran phospho-
nate or isobenzofuran thiophosphonate derivatives
via cooperative DBU intramolecular cyclization of o-
alkynylbenzaldehydes or o-alkynylacetophenones and
dialkyl phosphite or dialkyl phosphonothioate. The
yields are essentially quantitative or very high in most
cases, and the regioselectivity was always 100% favor-
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benzaldehydes or o-alkynylacetophenones 2, bearing
different substituent groups as R3, with dimethyl
phosphite or diethyl phosphite 1, will furnish dialkyl
1,3-dihydroisobenzofuran-1-ylphosphonates 3 or di-
ACHTUNGTRENNUNGalkyl isobenzofuran-1-ylphosphonates 3’, respectively.
Experimental Section
General Comments
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The spectroscopic data of all compounds are given in the
Supporting Information.
General Procedure for the Synthesis of
(Thio)phosphonylated Isobenzofurans 3
o-Alkynylbenzaldehyde
or
o-alkynylacetophenone
2
(0.5 mmol) in THF (4 mL) was added dropwise to a stirred
mixture of dialkyl phosphite or dialkyl phosphonothioate 1
(1.0 mmol) and DBU (0.153 g, 1.0 mmol) in THF (4 mL) at
room temperature with TLC (silica gel) monitoring. After
20 h stirring at 40–608C (Table 2), the mixture was cooled to
room temperature and worked-up with water (5 mL) below
08C. The resulting mixture was then extracted by AcOEt
and dried with anhydrous sodium sulfate. After concentra-
tion, the residue was purified by CC [silica gel, AcOEt/pe-
troleum ether (b.p. 60–908C), 1:3] to afford the product 3.
Acknowledgements
We thank the Committee of Science and Technology of Tian-
jin (07JCZDJC04800), the Research Foundation for the Doc-
toral Program of Higher Education of China (20070055042)
and Nankai University (J02044 to Z. W. Miao) for financial
support.
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These data can be obtained free of charge from The
Cambridge
Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif or on request to
Cambridge Crystallographic Data Center, 12 Union
Road, Cambridge CB2 1EZ, U. K. Crystal data for 5a:
empirical formula: C17O17O4P; unit cell parameters: a=
7.599(4) ꢇ, b=7.866(4) ꢇ, c=29.288(16) ꢇ, a=908,
b=93.847(9)8, g=908; space group P2(1)/C. Crystal
data for 6a: empirical formula: C46O58O4P2; unit cell
parameters: a=16.0384(3) ꢇ, b=16.5568(9) ꢇ, c=
17.18740(10) ꢇ, a=74.923(14)8, b=74.172(14)8, g=
79.585(17)8; space group P-1.
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[15] CCDC 685563 (5a) and CCDC 685564 (6a) contain the
supplementary crystallographic data for this paper.
Adv. Synth. Catal. 2008, 350, 2733 – 2739
ꢂ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
2739