Cycloaddition reaction of phosphonyl nitrile oxides to phosphaacetylene and alkene

Article abstract of DOI:10.1002/hc.20518

Here, we report the cycloaddition reaction of phosphonyl nitrile oxides and the formation of an unexpected 2:1 cycloaddition product. It provides a direct route to triphosphonyl-substituted dihydroisoxazolyl dihydroisoxazoles and diphosphonylsubstituted d

Full text of DOI:10.1002/hc.20518

Heteroatom Chemistry
Volume 20, Number 2, 2009
Cycloaddition Reaction of Phosphonyl Nitrile
Oxides to Phosphaacetylene and Alkene
Yong Ye,1,2 Yong Luo,2 Chun-Fang Wang,3 Dong-Qing Wei3,
Lun-Zu Liu,2 and Yu-Fen Zhao1
1Key Laboratory of Chemical Biology and Organic Chemistry, Department of Chemistry,
Zhengzhou University, Zhengzhou 450052, China
2State Key Laboratory of Elemento-Organic Chemistry, Nankai University, Tianjin 300071, China
3College of Life Science and Technology, Shanghai Jiaotong University, Shanghai 200240, China
Received 26 January 2008; revised 18 June 2008, 28 August 2008
versatile intermediates for the syntheses of natural
ABSTRACT: Here, we report the cycloaddition reac-
products and biologically active compounds [3].
tion of phosphonyl nitrile oxides and the formation
Intensive investigations of 1,3-dipolar cycloaddition
of an unexpected 2:1 cycloaddition product. It pro-
reactions between nitrile oxide and electron-
vides a direct route to triphosphonyl-substituted di-
deficient alkenes have been undertaken, and in
hydroisoxazolyl dihydroisoxazoles and diphosphonyl-
most cases, 4,5-dihydroisoxazoles were obtained.
substituted dihydroisoxazolyl dihydroisoxazoles with
Although there are several examples of 1,3-dipolar
excellent levels of regiocontrol product. Density func-
cycloaddition of an electron-deficient nitrile oxide
tional theory studies of the reactions between nitrile
with an unsaturated alkene, the scope and generality
oxides and acrylonitrile are used to propose a pos-
of nitrile oxides remain insufficient. Here, we report
ꢀ
C
sible reaction mechanism.
2009 Wiley Periodicals,
the cycloaddition of phosphonyl nitrile oxides and
the formation of an unexpected 2:1 cycloaddition
product. This method provides a direct route
to triphosphonyl-substituted dihydroisoxazolyl
dihydroisoxazoles and diphosphonyl-substituted
dihydroisoxazolyl dihydroisoxazoles with excellent
levels of regiocontrol. We believe that this strategy
provides a potentially facile route to a wide range of
dihydroisoxazolyl dihydroisoxazoles–based targets.
Inc. Heteroatom Chem 20:95–100, 2009; Published online
in Wiley InterScience (www.interscience.wiley.com). DOI
10.1002/hc.20518
INTRODUCTION
,3-Dipolar cycloaddition reactions are among the
most important synthetic methods for the prepa-
ration of five-membered ring carbocycles and
heterocycles [1,2]. The reaction between nitrile
oxides and alkenes is of considerable interest in
organic synthesis, as the resulting heterocycles are
RESULTS AND DISCUSSION
Cycloaddition Reaction of Phosphonyl Nitrile
Oxides with Phosphaacetylene
The requisite nitrile oxides 1 were prepared ac-
cording to literature routes starting with the corre-
sponding hydroxymoyl bromides via base-induced
dehydrobromidination [4]. We first investigated
the 1,3-dipolar cycloaddition of nitrile oxide and
Correspondence to: Yong Ye; e-mail: yeyong03@mail.nankai.
edu.cn.
Contract grant sponsor: Natural Science Foundation of China.
Contract grant numbers: 20602032, 20572061, 10376024, and
20373048.
c
ꢀ
2009 Wiley Periodicals, Inc.
95

96 Ye et al.
O
O
N
N
(RO) P
O
(RO) P
O
O
2
2
O
t
t
P
Bu
+
P
Bu
_
t
C =N
O
(RO)2P
C
+
P
Bu
+
_
(RO) P
O
(RO) P
2
2
N
N
O
O
4
3
2
1
a-b: R = iPr; nBu
SCHEME 1 Cycloaddition of phosphonyl nitrile oxides with phosphaacetylene.
phosphaacetylene 2, as shown in Scheme 1. In our
initial attempts, we failed to isolate the pure nitrile
oxide 1. Therefore, unpurified nitrile oxide was di-
rectly used for the cycloaddition reaction. tert-Butyl
phosphaacetylene 2 can react with nitrile oxides 1
to give heterocyclic compounds. However, we found
that when phosphonyl nitrile oxides are treated with
a threefold excess of tert-butyl phosphaacetylene, an
unexpected 2:1 cycloaddition product 3 is achieved.
Its regioisomer 4 is not obtained. This is probably
a consequence of the sterically demanding tert-butyl
groups on phosphaacetylene 2 and the bulky phos-
phonyl groups on the nitrile oxides so that only one
regioisomer is obtained.
When a onefold excess of dibutoxyphosphonyl
nitrile oxides 1 was used, a 1:1 cycloaddition prod-
1
uct 5 was detected in the H and 31P NMR spectra
(the structure is shown in Scheme 2). From 31P NMR
spectroscopy (Fig. 1), a doublet at 85.9–86.7 ppm is
observed, which suggests a typical bivalent phospho-
rus [5] and is consistent with a phosphorus atom in
isoxazole. Another doublet resonance at 7.1–7.9 ppm
is assigned to a phosphorus atom in the phosphonate
group. The 31P NMR coupling constant is 67.7 Hz.
FIGURE 1 31P NMR of compound 5.
In addition, the integrated area of hydrogen in
1
the H NMR spectrum is consistent with a 1:1 cy-
cloaddition product. By this method, we can draw
the conclusion that this cycloaddition reaction is
completed in two steps. In the first step, a 1:1 cy-
cloaddition product 5 is formed. In the second step,
a second phosphonyl nitrile oxide 1 adds to the P C
double bond of 5 to form the 2:1 product 3.
The proposed structure of the 2:1 cycloaddition
product 3a was confirmed by 1H and 31P NMR spec-
troscopy. Figure 2 shows the 31P spectrum of com-
pound 3a. Since the middle-group phosphorus (βP)
is attached to two chemically different phosphorus
atoms (αP), spin–spin coupling leads to a 1:2:1 triplet
between 16.1 and 17.6 ppm. Coupling constants of
56.1 Hz are observed. Likewise, the attachment of
the two end-group phosphorus atoms to the lone
middle-group phosphorus gives spin–spin splitting
into a 1:1 doublet at 1.05–1.75 ppm.
O
(H3CH2CH2CH2CO)2P
N
O
_
_
P
tBu
5
SCHEME 2 Cycloaddition product structure of phosphonyl
nitrile oxides with phosphaacetylene.
Heteroatom Chemistry DOI 10.1002/hc

Cycloaddition Reaction of Phosphonyl Nitrile Oxides to Phosphaacetylene and Alkene 97
When treated with trans-vinylphosphonate,
phosphonyl nitrile oxides 1 also showed analogous
behavior (Scheme 4). Vinylphosphonate 7 was pre-
pared according to the procedures described previ-
ously [2h]. When diisopropoxy phosphonyl nitrile
oxides 1 are treated with β-substituted vinylphos-
phonate 7, only the regioselective 2:1 addition prod-
uct 8 was obtained. Meanwhile, when excessive β-
substituted vinylphosphonate 7 was added, only 2:1
cycloadduct was obtained.
Theoretical Calculations
In the current case, the detailed mechanism has
been explored with the density functional theory
method. Each structure was fully optimized with
the B3LYP (B3LYP calculations give relative ener-
gies of the various structural transitions to be within
∼5 kcal/mol of the actual energies) [7–11] method
by using the 6-31G* basis set for C, O, and H atoms.
Harmonic vibrational frequencies were calculated
for each structure. The bond orders reported were
the Wiberg bond, calculated by means of natural
bond orbitals. The charges reported were the Mul-
liken atomic charges. The structures of ts1 and ts2
are shown in the supporting information. All calcula-
tions were performed with the Gaussian 03 program
package. All relative energies discussed within this
context are free energies at 298 K. To make the cal-
culations easier, we used the methyl group instead
of the propyl group.
FIGURE 2 31P NMR of compound 3a.
A plausible reaction pathway has been proposed
to account for the high reactivity and selectivity of
diisopropoxy phosphonyl nitrile oxide. First, a re-
giospecific 1:1 product was formed. Second, highly
reactive diisopropoxy phosphonyl nitrile oxide can
Cycloaddition of Phosphonyl Nitrile
Oxides with Alkene
We also examined the reaction of phosphonyl nitrile
oxides with electron-deficient alkenes. When reacted
with acrylonitrile, diisopropoxy phosphonyl nitrile
oxides 1 showed analogous behavior. Nitrile oxide 1
reacted regioselectively with acrylonitrile to give an
unexpected 2:1 product 6 (Scheme 3).
continue reacting with C N double bond and give
the corresponding cycloadduct. From Fig. 3, we can
see that the free energy barriers for the first and sec-
ond steps are 11.7 and −14.6 kcal/mol, respectively.
The free energy of the second stepis quite low. So,
the cycloaddition stops at this step.
|å O
(PriO)2P
O
N
O
(iPrO)2P
CH =CH CN
2
C+=N
O
N
CN
+
e
O
-
|e
(PriO)2P
O
6
1
SCHEME 3 Cycloaddition of phosphonyl nitrile oxides with acrylonitrile.
Heteroatom Chemistry DOI 10.1002/hc

98 Ye et al.
_
O
O
(PriO)2P
O
O
O
N
(EtO)2P
P(OEt)2
_
N
(iPrO)2P
+
C+=N
O
O
_
R
(PriO)2P
R
O
8
7
1
A-F: R = C6H5, C6H13, p-O2NC6H4, 2,4-Cl2C6H3, m-O2NC6H4,CO2CH3
SCHEME 4 Cycloaddition of phosphonyl nitrile oxides with β-substituted vinylphosphonate.
FIGURE 3 Potential energy surface of the reaction with the B3LYP method. All energies are electronic energies without zero
point energy corrections with respect to the reactants. ts1 and ts2 stand for the transition state of the first and second steps.
EXPERIMENTAL
General Procedures
was stirred at room temperature for 24 h. Then,
the reaction mixture was filtered to remove triethy-
lamine hydrobromides and the solvent was evapo-
rated in vacuum. The residue was chromatographed
on a silica gel column with petroleum ether/ethyl
acetate (1:1, v/v) to give pure 3a–b as an oil.
Elemental analyses were carried on a Yanaco CHN
Corder MT-3 apparatus. 1H, 13C, and 31P NMR spec-
tra were measured with a Bruker 300 spectrometer
using TMS and 85% H3PO4 as the internal and exter-
nal references, respectively, and CDCl3 as the solvent.
Solvents used were purified and dried by standard
procedures. Compounds 1 and 2 were synthesized
according to refs. [4] and [6], respectively.
[7a-tert-Butyl-5-(diisopropoxy-phosphoryl)-[1,2,
4]oxazaphospholo[4,5-d][1,2,4]oxazaphosphol-3-yl]-
phosphonic Acid Diisopropyl Ester (3a). This
compound was obtained in 50.4% yield as an oil.
Elemental analysis found (calcd): C 44.27 (44.36),
Procedure for the Synthesis of 3
1
H 7.32 (7.25), N 5.44 (5.44). H NMR: 4.78 (m, 4H,
To a rapidly stirred solution of tert-butyl phos-
phaacetylene 2 (0.2 mL, 1 M in Me3SiOSiMe3) and
Et3N (0.15 g) in dry ether (10 mL) under N2, a solu-
tion of compound 1 (0.288 g, 1 mmol) in dry ether
(6 mL) was added dropwise at −10◦C. The mixture
CHMe2), 1.26–1.33 (t, 24H, CHMe2), 1.03 (s, 9H,
Me); 13C NMR: 20.1 (d, CH3), 50 (d, CH), 26.3 (d,
2
3 Jcp = 5.9 Hz, MeC), 36.6 (d, Jcp = 16 Hz, Me3C),
130 (1 Jcp = 30 Hz, P C), 156 (1 Jcp = 45 Hz, C N);
31P NMR: 1.4 (αP), 16.87 (βP).
Heteroatom Chemistry DOI 10.1002/hc

Cycloaddition Reaction of Phosphonyl Nitrile Oxides to Phosphaacetylene and Alkene 99
[7a-tert-Butyl-5-(dibutoxy-phosphoryl)-[1,2,4]-
petroleum ether/ethyl acetate (1:3) to give pure 8a–e
oxazaphospholo[4,5-d][1,2,4]oxazaphosphol-3-yl]-
phosphonic Acid Dibutyl Ester (3b). This com-
pound was obtained in 76.4% yield as an oil.
Elemental analysis found (calcd): C 48.46 (48.42),
as an oil.
[3,7a-Bis(diisopropoxy-phosphoryl)-7-phenyl-7,7a-
dihydro-6H-isoxazolo[2,3-d][1,2,4]oxadiazol-6-yl]-
phosphonic Acid Diethyl Ester (8a). The compound
was obtained in 19.6% yield. Elemental analysis
found (calcd): C 47.72 (47.71), H 6.79 (6.93), N 4.29
1
H 7.92 (7.95), N 4.88 (4.91). H NMR: 4.66 (m, 4H,
CH), 1.67 (m, 8H, CH2), 1.28–1.36 (m, 12H, CH3),
1.08 (s, 9H, CMe3), 0.88–0.95 (t, 12H, Me); 31P NMR:
1.71 (αP), 17.57 (βP).
1
(4.28). H NMR: 7.32 (m, 5H), 5.89–6.03 (dd, 1H),
4.69 (m, 4H), 4.12–4.22 (m, 5H), 1.20–1.39 (m, 30H);
13C NMR: 22.3 (d, CH3), 16 (CH2CH3), 49 (CPh), 50
(d, CH), 56 (CH2CH3), 82 (1 Jcp = 30 Hz, CHP), 92
(1 Jcp = 50 Hz, PC N), 160 (1 Jcp = 37 Hz, PC N),
126–130 (Carom); 31P NMR: 0.89 (αP), −3.25 (βP),
18.53 (γP).
Procedure for the Synthesis of 5
The procedure is similar to the synthesis of 3. When
0.04 mL solution of tert-butyl phosphaacetylene 2
(1 M in Me3SiOSiMe3) was used, compound 5 was
1
obtained as an oil. Yield 43.2%. H NMR: 0.87–0.94
(m, 6H), 1.19–1.44 (m, 13H), 1.67–1.68 (m, 4H), 4.17
(m, 4H); 31P NMR: 85.9–86.7 (d, J = 67.7 Hz, αP),
7.1–7.9 (d, J = 67.7 Hz, βP).
[3,7a-Bis(diisopropoxy-phosphoryl)-7-hexyl-7,7a-
dihydro-6H-isoxazolo[2,3-d][1,2,4]oxadiazol-6-yl]-
phosphonic Acid Diethyl Ester (8b). The compound
was obtained in 31.4% yield. Elemental analysis
found (calcd): C 46.98 (47.13), H 8.13 (8.06), N 4.28
Procedure for the Synthesis of [6-Cyano-3-(diiso-
propoxy-phosphoryl)-6,7-dihydro-isoxazolo[2,3-
d][1,2,4]oxadiazol-7a-yl]phosphonic Acid
Diisopropyl Ester (6)
1
(4.23). H NMR: 4.65–4.75 (m, 5H), 3.99–4.10 (m,
5H), 1.08–1.41 (m, 40H), 0.73–0.76 (m, 3H); 31P
NMR: 1.08 (αP), 3.14 (βP), 19.2 (γP).
To a stirred solution of compound 1 (0.432 g,
1.5 mmol) and acrylonitrile (0.53 g, 10 mmol) in dry
ether (10 mL), a solution of Et3N (0.2 g, 2 mmol) in
dry ether (10 mL) was added dropwise at −10◦C◦fter
half an hour, the solution rises to ambient tempera-
ture. The reaction mixture was stirred continuously
for 3 days. After filtration, the solvent was evaporated
in vacuum. The residue was chromatographed on a
silica gel column with petroleum ether/ethyl acetate
3:1 (v/v) to give pure 6 in 22% yield as an oil.
[3,7a-Bis(diisopropoxy-phosphoryl)-7-(4-nitro-
phenyl)-7,7a-dihydro-6H-isoxazolo[2,3-d][1,2,4]oxa-
diazol-6-yl]phosphonic Acid Diethyl Ester (8c). The
compound was obtained in 56.1% yield. Elemental
analysis found (calcd): C 44.59 (44.64), H 6.35
1
(6.34), N 5.98 (6.01). H NMR: 8.19–8.23 (d, 2H),
7.51–7.55 (d, 2H), 6.06 (dd, 1H), 4.83 (m, 4H), 4.21
(m, 5H), 1.23–1.37 (m, 30H); 31P NMR: 0.53 (αP),
−3.32 (βP), 17.68 (γP).
Elemental analysis found (calcd):
C 43.64
(43.69), H 6.63 (6.68), N 8.95 (8.99). 1H NMR: 5.03–
5.12 (dd, 1H, CHCN), 4.53–4.61 (m, 4H, OCHMe2),
3.53–3.59 (t, 2H, CH2CHCN), 0.99–1.23 (m, 24H,
OCHMe2); 13C NMR: 22 (CH3CH), 50 (d, CH3CH), 70
[7-(2,4-Dichloro-phenyl)-3,7a-bis(diisopropoxy-
phosphoryl)-7,7a-dihydro-6H-isoxazolo[2,3-d][1,2,
4]oxadiazol-6-yl]phosphonic Acid Diethyl Ester (8d).
The compound was obtained in 56.1% yield. Ele-
mental analysis found (calcd): C 43.1 (43.16), H 5.96
(CH2), 128 (1 Jcp = 31 Hz, CP), 131 (CH), 148 (1 Jcp
=
1
42 Hz, PC N), 151 (CN); 31P NMR: 0.57 (αP), −3.37
(5.99), N 3.85 (3.87). H NMR: 7.24–7.37 (m, 3H),
(βP).
6.19–6.32 (dd, 1H), 4.67–4.87 (m, 4H), 4.15–4.31 (m,
5H), 1.20–1.39 (m, 30H); 31P NMR: 0.93 (αP), −3.25
(βP), 18.53 (γP).
General Procedure for the Synthesis of 8
To a stirred solution of compound 1 (0.432 g,
1.5 mmol) and β-substituted vinylphosphonate
(15 mmol) in dry ether (10 mL), a solution of Et3N
(0.2 g, 2 mmol) in dry ether (10 mL) was added
dropwise at −10◦C. After half an hour, the solution
rises to ambient temperature. The reaction mixture
was stirred continuously for 3 days. After filtration,
the solvent was evaporated in vacuum. The residue
was chromatographed on a silica gel column with
[3,7a-Bis(diisopropoxy-phosphoryl)-7-(3-nitro-
phenyl)-7,7a-dihydro-6H-isoxazolo[2,3-d][1,2,4]oxa-
diazol-6-yl]phosphonic Acid Diethyl Ester (8e). The
compound was obtained in 35.3% yield. Elemental
analysis found (calcd): C 44.65 (44.64), H 6.37
1
(6.34), N 6.03 (6.01). H NMR: 8.14–8.18 (d, 2H),
7.54–7.66 (dd, 2H), 5.95–6.09 (dd, 1H), 4.69 (m, 4H),
4.14–4.36 (m, 5H), 1.21–1.35 (m, 30H); 31P NMR:
0.54 (αP), −3.35 (βP), 17.61 (γP).
Heteroatom Chemistry DOI 10.1002/hc

100 Ye et al.
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6-(Diethoxy-phosphoryl)-3,7a-bis(diisopropoxy-
phosphoryl)-7,7a-dihydro-6H-isoxazolo[2,3-d][1,2,
4]oxadiazole-7-carboxylic Acid Methyl Ester (8f).
The compound was obtained in 78.8% yield. Ele-
mental analysis found (calcd): C 47.79 (47.71), H 7.0
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(6.97), N 4.28 (4.31). H NMR: 5.31–5.45 (dd, 1H),
4.77 (m, 4H), 4.13–4.20 (m, 5H), 3.75–3.77 (s, 3H),
1.27–1.37 (m, 30H); 31P NMR: 0.03 (αP), −3.27 (βP),
17.7 (γP).
Isolated yields of 8 are based on substituted
vinylphosphonate.
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Heteroatom Chemistry DOI 10.1002/hc