Regioselective cycloadditions of phosphonyl nitrile oxides with vinylphosphonate and phosphaalkyne

Article abstract of DOI:10.1080/10426500902917651

1,3-Dipolar cycloaddition reactions are important synthetic manipulations allowing the construction of five-membered heterocycles. In this article, we report the cycloaddition of phosphonyl nitrile oxides with vinylphosphonate and phosphaalkyne to form th

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Phosphorus, Sulfur, and Silicon and the
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Regioselective Cycloadditions of
Phosphonyl Nitrile Oxides with
Vinylphosphonate and Phosphaalkyne
Li-li Shen a b , Yong Ye a c , Yong Luo c & Lun-zu Liu c
a Department of Chemistry, Key Laboratory of Chemical Biology and
Organic Chemistry , Zhengzhou University , Zhengzhou, China
b Department of Chemistry , Henan Institute of Education ,
Zhengzhou, China
c State Key Laboratory of Elemento-Organic Chemistry , Nankai
University , Tianjin, China
Published online: 24 Feb 2010.
To cite this article: Li-li Shen , Yong Ye , Yong Luo & Lun-zu Liu (2010) Regioselective Cycloadditions
of Phosphonyl Nitrile Oxides with Vinylphosphonate and Phosphaalkyne, Phosphorus, Sulfur, and
Silicon and the Related Elements, 185:3, 680-687, DOI: 10.1080/10426500902917651
To link to this article: http://dx.doi.org/10.1080/10426500902917651
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Phosphorus, Sulfur, and Silicon, 185:680–687, 2010
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500902917651
REGIOSELECTIVE CYCLOADDITIONS OF PHOSPHONYL
NITRILE OXIDES WITH VINYLPHOSPHONATE
AND PHOSPHAALKYNE
Li-li Shen,1,2 Yong Ye,1,3 Yong Luo,3
and Lun-zu Liu3
1Department of Chemistry, Key Laboratory of Chemical Biology and Organic
Chemistry, Zhengzhou University, Zhengzhou, China
2Department of Chemistry, Henan Institute of Education, Zhengzhou, China
3State Key Laboratory of Elemento-Organic Chemistry, Nankai University,
Tianjin, China
1,3-Dipolar cycloaddition reactions are important synthetic manipulations allowing the con-
struction of five-membered heterocycles. In this article, we report the cycloaddition of phos-
phonyl nitrile oxides with vinylphosphonate and phosphaalkyne to form the unexpected 2:1
cycloaddition product with excellent levels of regiocontrol product. The structures of title
compounds were confirmed by 1H NMR, 31P NMR, MS, and IR. The mechanism of the
cycloaddition was explored using the density functional theory (DFT) method.
Keywords Cycloaddition; dihydroisoxazoles; nitrile oxides
INTRODUCTION
Cycloaddition reactions are important synthetic manipulations for construction of
the five-member 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 versatile intermediates for the syntheses of natural products and biologically active
compounds.3 Intensive investigations have been undertaken on 1,3-dipolar cycloaddition
reactions between nitrile oxide and electron-deficient alkenes, and in most cases 4,5-
dihydroisoxazoles were obtained. Although there are several examples of the 1,3-dipolar
cycloaddition of electron-deficient nitrile oxide with unsaturated alkenes, the scope and
generality of the nitrile oxides remain insufficient. In this article, we report the cycload-
dition of phosphonyl nitrile oxides and the formation of an unexpected 2:1 cycloaddition
product. It provides a direct route to obtain triphosphonyl-substituted dihydroisoxazolyl
dihydroisoxazoles and diphosphonyl-substituted dihydroisoxazolyl dihydroisoxazoles with
Received 16 January 2009; accepted 23 March 2009.
The authors would like to thank financial supports from NNSFC (Nos. 20602032, 20972143).
Address correspondence to Yong Ye, Department of Chemistry, Key Laboratory of Chemical Biology and
Organic Chemistry, Zhengzhou University, Zhengzhou 450052, China. E-mail: yeyong03@mail.nankai.edu.cn
680

REGIOSELECTIVE CYCLOADDITIONS OF PHOSPHONYL NITRILE OXIDES
681
excellent regiocontrol product. We believe that this strategy provides a potentially facile
route to a wide range of dihydroisoxazolyl dihydroisoxazoles–based targets.
RESULTS AND DISCUSSION
Cycloaddition of Phosphonyl Nitrile Oxides with Phosphaalkyne
The requisite nitrile oxides 1 were prepared according to the literature from the
corresponding hydroximoyl bromides via base-induced dehydrobromidination.4–6 We first
investigated the 1,3-dipolar cycloaddition between nitrile oxide and phosphaalkyne 2, as
shown in Scheme 1. In our initial attempts, we failed to isolate nitrile oxide 1. Therefore,
without purification, nitrile oxide was directly used for the cycloaddition reaction. Tert-butyl
phosphaalkyne 2 can react with nitrile oxides 1 to give heteocyclic compounds. However,
we found that an unexpected 2:1 cycloaddition product 4 is achieved when threefold excess
of tert-butyl phosphaalkyne reacts with phosphonyl nitrile oxides. Its regio isomer 4ꢁ is not
obtained. This is because of the steric bulk of the tert-butyl on phosphaalkyne 2 and the
highly reactive dipole 1, so only one regio isomer is obtained.
O
O
N
RO
RO
C+=N O-
+
P
tBu
(RO)2P
C
P
O
P
tBu
3
2
1
+1
O
O
N
N
O
P
(RO)2P
But
O
O
(RO)2P
O
N
RO
O
P
P
tBu
+
RO
(RO)2P
(RO)2P
O
O
P
N
But
N
O
4'
3'
4
a-b: R=iPr; nBu
Scheme 1 Cycloaddition of phosphonyl nitrile oxides with phosphaalkyne.
When onefold excess of dibutoxyphosphonyl nitrile oxides 1b was added, a 1:1
1
cycloaddition product 3b was observed from H NMR and 31P NMR spectra. From 31P
NMR (Figure 1), a doublet at 86.3 ppm is observed, which indicates a typical bivalent
phosphorus chemical displacement.7 It is a chemical displacement of phosphorus atom in
isoxazole. Another doublet at 7.5 ppm is the chemical displacement of phosphorus atom
in phosphonate. From 31P NMR, a coupling constant of 67.7 Hz can also be observed. In
the case of a reverse dipole orientation as represented by the structure 3ꢁ, the αP resonances
would be expected at a considerably lower field (δ = ca. 160–180).8
1
In addition, from the integration of the hydrogen in H NMR, a 1:1 cycloaddition
product is also observed. The 1:1 cycloaddition product 3 could not be isolated. After

682
L.-L. SHEN ET AL.
Figure 1 31P NMR of compound 3.
another one equivalent of phosphonyl nitrile oxide was added to the reaction, the expected
product 4 is formed. We can draw the conclusion that the cycloaddition reaction is completed
in two steps. First, a 1:1 cycloaddition product 3 is formed. Second, another phosphonyl
nitrile oxide 1 adds to the P C double bond of 3, so forming a 2:1 product 4.
1
The structure of the 2:1 cycloaddition product 4a is also confirmed by H and 31P
NMR. Figure 2 shows a 31P spectrum of compound 4a. Since the middle-group phosphorus
(βP) is attached to two chemically different phosphorus atoms (αP), spin–spin splitting
into a 1:2:1 triplet is to be expected in δ 16.1–17.6. Coupling constants of 56.1 Hz can
be observed. Likewise, the attachment of the two end-group phosphorus atoms to the lone
middle-group phosphorus gives spin–spin splitting into 1:1 doublet at δ 1.05–1.75.
Cycloaddition of Phosphonyl Nitrile Oxides with Alkene
We also examined the scope of phosphonyl nitrile oxides with an electron-deficient
alkene. With acrylonitrile, diisopropoxy phosphonyl nitrile oxides 1 showed analogous
Figure 2 31P NMR of compound 4a.

REGIOSELECTIVE CYCLOADDITIONS OF PHOSPHONYL NITRILE OXIDES
683
O
i
(PrO) P
2
O
O
N
(iPrO)2P
CH =CH CN
+
-
N
CN
C =N
O
+
2
O
i
(Pr O) P
2
O
5
1
Scheme 2 Cycloaddition of phosphonyl nitrile oxides with acrylonitrile.
behavior. Nitrile oxide 1 reacted regioselectively with acrylonitrile to give an unexpected
2:1 product 5 (shown in Scheme 2).
When reacted with trans-vinylphosphonate, phosphonyl nitrile oxides 1 also showed
analogous behavior (shown in Scheme 3). The vinylphosphonate 6 was prepared according
to the procedures we previously described.2h When diisopropoxy phosphonyl nitrile ox-
ides 1 reacted with β-substituted vinylphosphonate 6, only the regioselective 2:1 addition
product 7 was obtained. Meanwhile, when excessive β-substitute vinylphosphonate 6 was
added, only the 2:1 cycloadduct was obtained.
O
O
(PriO)2P
O
O
O
N
(EtO)2P
P(OEt)2
(iPrO)2P
N
C+=N O-
+
O
R
(PriO)2P
R
O
6
7
1
A-F: R= C6H5, C6H13, p-O2NC6H4, CO2CH3, 2,4-Cl2C6H3, m-O2NC6H4
Scheme 3 Cycloaddition of phosphonyl nitrile oxides with vinylphosphonate.
The Mechanism of Cycloaddition
The detailed mechanism has been explored using the density functional theory (DFT)
method. Each structure was fully optimized with the B3LYP9–13 method using the 6-31G∗
basis set for C, O, and H atoms (Figure 3). Harmonic vibrational frequencies were calculated
for each structure. The bond orders reported were Wiberg bond calculated by means of
natural bond orbitals (NBO). The charges reported were the Mulliken atomic charges. All
calculations were carried out with the Gaussian 03 program package. All relative energies
discussed within this context are free energies at 298 K. In order to make the calculations
easier, we used methyl instead of isopropyl.
A plausible reaction pathway has been proposed to account for the high reactivity
and selectivity of diisopropoxy phosphonyl nitrile oxide. First, a regiospecific 1:1 product
was formed. Second, a highly reactive diisopropoxy phosphonyl nitrile oxide can continue
reacting with the C N double bond and give the corresponding cycloadduct. From Figure 3

684
L.-L. SHEN ET AL.
Figure 3 Potential energy surface of the reaction with the B3LYP method. All energies are electronic energies
without ZPE (zero point energy) corrections with respect to the reactants. ts1 and ts2 stand for the transition state
of the first step and second step.
we can see that the free energy barriers for the first and second steps are 11.7 and −14.6
kcal/mol, respectively. The free energy of the second step is quite low. So the cycloaddition
stopped in this step.
CONCLUSIONS
In conclusion, the [3+2] cycloaddition reaction of nitrile oxides with alkynes and
alkenes provides a direct route to triphosphonyl-substituted dihydroisoxazolyl dihydroisox-
azoles and diphosphonyl-substituted dihydroisoxazolyl dihydroisoxazoles with excellent
levels of regiocontrol product. Additionally, an unexpected 2:1 cycloaddition takes place in
the presence of Et3N when phosphonyl hydroximoyl bromides are employed to furnish the
dihydroisoxazoles. We believe that this strategy provides a potentially facile route to a wide
range of dihydroisoxazolyl dihydroisoxazoles–based targets. The DFT studies of the reac-
tions between nitrile oxides and acrylonitrile reveal that the following mechanism is quite
possible: It starts as a normal 1,3-dipolar cycloaddition reaction to produce a regiospecific
1:1 product, then highly reactive diisopropanyl phosphonyl nitrile oxide continue to react
with the regiospecific 1:1 product and give the corresponding cycloadduct. Further study
is underway to expand the scope of this methodology, as well as to ascertain mechanistic
details of the cycloaddition process.
EXPERIMENTAL
Elemental analyses were carried on a Yanaco CHN Corder MT-3 apparatus. 1H, 13C,
and 31P NMR spectra were measured by using a Bruker 300 spectrometer with TMS and

REGIOSELECTIVE CYCLOADDITIONS OF PHOSPHONYL NITRILE OXIDES
685
85% H3PO4 as the internal and external references, respectively, and with CDCl3 as the
solvent. IR spectra were recorded as KBr pellets on a Bruck spectrometer. Mass spectra
were acquired in positive ion mode using a Bruker Esquire-LCTM ion trap spectrometer
equipped with a gas nebulizer probe, capable of analyzing ions up to m/z 20000. Solvents
used were purified and dried by standard procedures.
Procedure for the Synthesis of 4
To a rapidly stirred solution of tert-butyl phosphaacetylene 2 (0.2 mL, 1 M in
Me3SiOSiMe3) and Et3N (0.15 g) in dry ether (10 mL) under N2, a solution of com-
pound 1 (0.288 g, 1 mmol) in dry ether (6 mL) was added dropwise at −10◦C. The mixture
was stirred at room temperature for 24 h. Then, the reaction mixture was filtered to remove
triethylamine hydrobromides, and the solvent was evaporated in vacuum. The residue was
chromatographed on a silica gel column with petroleum ether:ethyl acetate 1:1 (V:V) to
give pure 4a–b as an oil.
1
Similar to the synthesis of 4, compound 3 was 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:
86.3 (d, J = 67.7 Hz, αP), 7.5 (d, J = 67.7 Hz, βP).
4a: Yield 50.4%, anal. calcd. found (calcd): C 44.27 (44.36), H 7.32 (7.25), N 5.44
(5.44). 1H NMR: 4.78 (m, 4H, CHMe2), 1.26–1.33 (t, 24H, CHMe2), 1.03 (s, 9H, Me); 13
C
P
NMR: 20.1 CH3), 50.0 (CH), 26.3 (MeC), 36.6 (Me3C), 130.0 (P C), 156.0 (C N); 31
NMR: 1.4 (αP), 16.8 (βP).
4b: Yield 76.4%, anal. calcd. found (calcd): C 48.46 (48.42), H 7.92 (7.95), N 4.88
(4.91). 1H 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: 1.7 (αP), 17.6 (βP).
General Procedure for the Synthesis of 5 and 7
To stirred solution of compound 1 (0.432 g, 1.5 mmol) and alkene (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 30 min, the solution was warmed to ambient temperature. The
reaction mixture was stirred continuously for 3 days. After it was filtered, the solvent
was evaporated in vacuo. The residue was chromatographed on a silica gel column with
petroleum ether:ethyl acetate (1:3) to give pure 5 or 7 as an oil.
5: Yield 22%, anal. calcd. found(calcd): C 43.64 (43.69), H 6.63 (6.68), N 8.95 (8.99),
1H NMR: 5.03–5.12 (dd, J = 35 Hz, 1H, CHCN), 4.53–4.61 (m, 4H, OCHMe2), 3.53–3.59
(t, J = 7.8 Hz, 2H, CH2CHCN), 0.99–1.23 (m, 24H, OCHMe2); 13C NMR: 22.0 (CH3CH),
50.0 (CH3CH), 70.0 (CH2), 128.0 (CP), 131.0 (CH), 148.0 (PC N), 151.0 (CN); 31P NMR:
0.6 (αP), −3.4 (βP).
7a: Yield 19.6%, anal. calcd. found(calcd): C 47.72 (47.71), H 6.79 (6.93), N 4.29
1
(4.28), H NMR: 7.32 (m,5H), 5.89–6.03 (dd, J = 30 Hz, 1H), 4.69 (m,4H), 4.12–4.22
(m,5H), 1.20–1.39 (m, 30H), 13C NMR: 15.0 (CH2CH3), 22.3 (CH3), 22.6 (CH3), 30.6
(CH), 49.0 (CPh), 50.0 (CH), 56.0 (CH2CH3), 82.0 (CHP), 92.0 (PC N), 160.0 (PC N),
126.2, 128.5, 129.6, 143.7 (Carom); 31P NMR: 0.9 (αP), −3.2 (βP), 18.5 (γ P).
7b: Yield 31.4%, anal. calcd. found(calcd): C 46.98 (47.13), H 8.13 (8.06), N 4.28
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), 13C NMR: 14.2 (CH2CH3), 20.3 (CH2), 22.3 (CH2), 22.6 (CH3), 27.4 (CH2),

686
L.-L. SHEN ET AL.
30.4 (CH2), 32.4(CH2), 62.3 (CH2CH3), 80.1 (CHO), 82.5 (PC O), 92.4 (PCN), 160.0
(PC N); 31P NMR: 1.1 (αP), −3.1 (βP), 19.2 (γ P).
7c: Yield 56.1%, anal. calcd. found(calcd): C 44.59 (44.64), H 6.35 (6.34), N 5.98
(6.01), 1H NMR: 8.19–8.23 (d, J = 15 Hz, 2H), 7.51–7.55 (d, J = 22 Hz, 2H), 6.06 (dd, J =
35 Hz, 1H), 4.83 (m, 4H), 4.21 (m, 5H), 1.23–1.37 (m, 30H), 13C NMR: 14.1 (CH2CH3),
22.4 (CH3), 22.5 (CH3), 30.5 (CH), 62.4 (CH2CH3), 71.5 (CH), 83.3 (CHP), 92.0 (PC O),
123.2, 127.5, 145.7, 154.6 (Carom), 162.0 (PC N); 31P NMR: 0.5 (αP), −3.3 (βP), 17.7
(γ P).
7d: Yield 78.8%, anal. calcd. found(calcd): C 41.49 (41.51), H 6.80 (6.81), N 4.38
1
(4.40), H NMR: 5.31–5.45 (dd, J = 18 Hz, 1H), 4.77 (m, 4H), 4.13–4.20 (m, 5H),
3.75–3.77 (s, 3H), 1.27–1.37 (m, 30H),13C NMR: 14.5 (CH2CH3), 22.4 (CH3), 32.5 (CH),
50.7 (OCH3), 62.7 (CH2CH3), 72.1 (CHO), 79.2 (PCHO), 88.6 (PC N), 162.5 (PC N),
175.4 (C O); 31P NMR: 0.03 (αP), −3.3 (βP), 17.7 (γ P); IR:3268 2957 1739 1649 1579
1490 1439 1204 924 769cm−1; ESI-MS: 637.3 [M+H]+.
7e: Yield 56.1%, anal. calcd. found(calcd): C 43.1 (43.16), H 5.96 (5.99), N 3.85
(3.87), 1H NMR: 7.24–7.37 (m, 3H), 6.19–6.32 (dd, J = 27 Hz, 1H), 4.67–4.87 (m, 4H),
4.15–4.31 (m, 5H), 1.20–1.39 (m, 30H), 13C NMR: 14.6 (CH2CH3), 22.3 (CH3), 22.5
(CH3), 61.7 (CH2CH3), 72.2 (CHO), 82.8 (PCHO), 91.6 (PC N), 162.0 (PC N), 126.6,
128.8, 131.8, 132.7, 147.4 (Carom); 31P NMR: 0.9 (αP), −3.2 (βP), 18.5 (γ P).
7f: Yield 35.3%, anal. calcd. found(calcd): C 44.65 (44.64), H 6.37 (6.34), N 6.03
(6.01),1H NMR: 8.14–8.18 (d, J = 27 Hz, 2H), 7.54–7.66 (dd, J = 24 Hz, 2H), 5.95–6.09
(dd, J = 22Hz, 1H), 4.69 (m, 4H), 4.14–4.36 (m, 5H), 1.21–1.35 (m, 30H), 13C NMR:
14.7 (CH2CH3), 22.3 (CH3), 22.5 (CH3), 30.2 (CH), 62.7 (CH2CH3), 72.0 (CHP), 85.4
(PCH O), 90.3 (PCO), 161.0 (PC N), 120.2, 122.5, 127.6, 129.3, 147.7, 149.2 (Carom);
31P NMR: 0.5 (αP), −3.4 (βP), 17.6 (γ P); ESI-MS: 700.5 [M+H]+.
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