First isolation and characterization of 1,2-oxaphosphetanes with three phenyl groups at the phosphorus atom in typical Wittig reaction using cyclopropylidenetriphenylphosphorane

Article abstract of DOI:10.1016/j.tetlet.2005.10.086

1,2-Oxaphosphetanes bearing three phenyl groups directly bound to the phosphorus atom were successfully isolated for the first time as stable crystals in the typical Wittig reaction of cyclopropylidenetriphenylphosphorane with activated carbonyl compounds. X-ray analysis of the oxaphosphetane showed that the phosphorus atom is at the center of a slightly distorted trigonal bipyramidal structure. Thermal decomposition of these oxaphosphetanes was carried out to give the starting carbonyl compounds and Wittig reaction products, olefins.

Full text of DOI:10.1016/j.tetlet.2005.10.086

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 8949–8952
First isolation and characterization of 1,2-oxaphosphetanes
with three phenyl groups at the phosphorus atom in typical
Wittig reaction using cyclopropylidenetriphenylphosphorane
Masashi Hamaguchi,* Yuji Iyama, Eiko Mochizuki and Takumi Oshima
Department of Applied Chemistry, Graduate School of Engineering, Osaka University, Yamadaoka, Suita 565-0871, Japan
Received 7 September 2005; revised 5 October 2005; accepted 14 October 2005
Available online 2 November 2005
Abstract—1,2-Oxaphosphetanes bearing three phenyl groups directly bound to the phosphorus atom were successfully isolated for
the first time as stable crystals in the typical Wittig reaction of cyclopropylidenetriphenylphosphorane with activated carbonyl com-
pounds. X-ray analysis of the oxaphosphetane showed that the phosphorus atom is at the center of a slightly distorted trigonal bipy-
ramidal structure. Thermal decomposition of these oxaphosphetanes was carried out to give the starting carbonyl compounds and
Wittig reaction products, olefins.
Ó 2005 Elsevier Ltd. All rights reserved.
R1
P
The Wittig reaction is one of the most powerful methods
for the preparation of carbon–carbon double bonds and
is used as a key step in many natural product syntheses
due to stereoselective formation of the double bond.1
Because of its synthetic advantages, the stereoselectivity
and mechanism of the Wittig reaction have long been
studied. Earlier betaines 3 were believed to be the main
intermediates in typical Wittig reactions (Scheme 1).
R2
R3
R4
R1
R1
R2
R3
R4
O
R13
P
R2
1
3
5
+
+
R3
O
R13P O
R1
P
R2
R3
R4
R4
R1
R1
In 1973, however, Vedejs and co-workers succeeded in
detection of only 1,2-oxaphosphetanes 4 at low temper-
ature by NMR spectroscopy during typical Wittig reac-
tions and observed that these intermediates decompose
readily upon warming to room temperature into alkenes
5 and phosphine oxides 6.2 On the other hand, several
stabilized, isolable oxaphosphetanes have been reported
along with their X-ray structures.3–10 Most of the previ-
ously reported stable oxaphosphetane structures contain
fluorine-bearing or bicyclic phosphole-type ligands
either at the phosphorus position or at the 4 position
in the oxaphosphetane ring. Recently, Berger and co-
workers studied Wittig reaction using 2-furylphenyl-
6
2
O
4
Scheme 1.
phosphoranes 7 and found that 2-furyl groups on the
phosphorus atom increase thermal stabilities of
oxaphosphetanes and succeeded in isolation and deter-
mination of X-ray structure of tris(2-furyl) substituted
oxaphosphetane, the stability of which is attributed to
the electron-withdrawing properties of the 2-furyl
group.11,12
Although triphenylphosphoranes are usually used as the
most common reagents for typical Wittig reaction, only
a few stable P,P,P-triphenyloxaphosphetanes have been
isolated. However, a few isolated oxaphosphetanes
reported are unusual oxaphosphetanes 813 and 9,14
Keywords: 1,2-Oxaphosphetanes; Wittig reaction; X-ray crystallo-
graphic analysis; Cyclopropylidenetriphenylphosphorane; Trigonal
bipyramidal structure.
*
Corresponding author. Tel./fax: +81 6 6879 4593; e-mail: hamaro@
chem.eng.osaka-u.ac.jp
0040-4039/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.10.086

8950
M. Hamaguchi et al. / Tetrahedron Letters 46(2005) 8949–8952
P
O
n
3-n
7
F3C
O
F3C
F3C
F3C
Ph3P
O
Ph
Ph
Ph
EtO
Ph
Ph
Ph
EtO
8
9
Scheme 2.
which were not determined by X-ray analysis, as shown
in Scheme 2.
Figure 1. ORTEP drawing of 13a.
an apical equatorial position and is almost coplanar,
as its dihedral angle is 1.0°. 31P NMR spectrum of 13a
was observed around at ꢀ46.2 ppm, which is 31P chem-
ical shift regions typical for pentavalent phosphorus spe-
cies. 13C NMR spectrum of 13a in CDCl3 showed
equivalency of three phenyl groups, indicating fast rota-
tion of the groups attached to the phosphorus atom in
solution.16 At the present time, it is vague how methyl
p-nitrobenzoate 14a was formed.
We would like to report that for the first time we suc-
ceeded in isolation and determination of X-ray structure
of stable typical Wittig reaction intermediates P,P,P-tri-
phenyloxaphosphetanes 13 in the reaction of cyclopro-
pylidenetriphenylphosphorane 11 with dicarbonyl
compounds 12.
To a suspension of triphenylcyclopropylphosphonium
bromide 10 (3.84 g, 10 mmol) in THF at ꢀ12 °C was
added 30% potassium hydride (2.02 g, 15 mmol) and stir-
red for 30 min, then stirred for 20 h at room temperature.
To the resultant yellow solution was added 2.09 g of
methyl p-nitrobenzoylformate 12a and stirred at room
temperature for 2 h. The reaction mixture was poured
into methanol. The reaction mixture was chromato-
graphed over silica gel using benzene and ethyl acetate
as eluents to give an 1,2-oxaphosphetane 13a in 38 % yield
along with methyl p-nitrobenzoate 14a (5%) and cyclo-
propyldiphenylphosphine oxide 14 (38%) (Scheme 3).
The reaction of 11 with methyl bromobenzoylformate
12b gave the oxaphosphetane 13b (15%), methyl p-bro-
mobenzoate 14b (8%), and cyclopropyldiphenylphos-
phine oxide 15 (20%).
During recrystallization of 13a at room temperature
over a month, 13a decomposed to yellow phosphonium
salt 16a, almost quantitatively. The structure of the salt
16a was determined by X-ray analysis. The formation of
the salt 16a suggests that the oxaphosphetane slowly dis-
sociated at room temperature into the starting materials,
the phosphorane 11 and the ketoester 12a. The X-ray
structure of the starting cyclopropylidenephosphorane
11 reported that the angle between the bond P–C and
cyclopropyl ring is very close to that of the oxaphosphe-
tane 13a (Scheme 5).17
The structure of the oxaphosphetane 13a was deter-
mined by X-ray analysis as shown in Figure 1.15 The
phosphorus atom is at the center of a slightly distorted
trigonal bipyramidal structure, the angle between the
two apical positions and the phosphorus atom being
166.6°. The four-membered oxaphosphetane ring is in
Ph3P
Br
10
p-XC6H4
O
THF
KH
-10 oC
O
OCH3
Ph
P
Ph
Ph
12a, b
O
H
C6H4X-p
O
p-XC6H4COOCH3
+
Ph3P
+
Ph2P
O
r.t. in THF
11
CH3O
13a, b
14a,b
15
a; X = NO2
b; X = Br
Scheme 3.

M. Hamaguchi et al. / Tetrahedron Letters 46(2005) 8949–8952
8951
Table 1. Ratios of methyl phenyloxoacetates 12a,b and olefins 17a,b
on the thermolysis of oxaphosphetanes 13a,b at several temperatures
The reaction of 11 with methyl benzoylformate, methyl
p-toluoylformate, methyl p-anisoylformate was carried
out, but in most cases, the starting materials were
recovered.
Temperature (°C)
12/17 ratioa (total yield/%)a
13a
13b
80
90
100
68/32 (73)
60/40 (77)
37/63 (61)
53/47 (89)
49/51 (97)
45/55 (93)
The reaction of 11 with p,p0-dichlorobenzil 12c also gave
the corresponding oxaphosphetane 13c. However,
oxaphosphetanes were not obtained from benzil and a-
furil.
a Determined by 1H NMR.
The reason for isolation of P,P,P-triphenyloxaphosphe-
tanes 13 seem to be attributed to high transition state
activation energy for process to olefins due to the prod-
uct being highly strained methylenecyclopropane 17.
In conclusion, we have shown that, for the first time, we
succeeded in isolation of stable 1,2-oxaphosphetanes 13
bearing three phenyl groups directly bound to the phos-
phorus atom using cyclopropylidenetriphenyl–phospho-
rane 11 in Wittig reaction with some dicarbonyl
compounds 12a–c, and in determination of X-ray struc-
ture of 13a.
Quantitative dissociation of 13a to the starting materials
11 and 12a on standing at room temperature prompts us
to study the thermolysis of the P,P,P-triphenyloxaphos-
phetanes 13a,b. Thermolysis of the oxaphosphetane
13a,b was carried out at several temperatures to give
the starting oxophenylacetates 12a,b, cyclopropyldiphen-
ylphosphine oxide 15, Wittig reaction products,
cyclopropylidenephenylacetate derivatives 17a,b, and
triphenylphosphine oxide as shown in Scheme 4 (Table
1).
References and notes
1. (a) Nicolaou, K. C.; Ha¨rter, M. W.; Gunzer, J. L.; Nadin,
A. Liebigs Ann./Recueil 1997, 1283; (b) Maryanoff, B. E.;
Reitz, A. B. Chem. Rev. 1989, 89, 863.
2. (a) Vedejs, E.; Snoble, K. A. J. J. Am. Chem. Soc. 1973,
95, 5778; (b) Vedejs, E.; Meier, G. P.; Snoble, K. A. J. J.
Am. Chem. Soc. 1981, 103, 2823; (c) Maryanoff, B. E.;
Reitz, A. B.; Mutter, M. S.; Inners, R. R.; Almond, H. R.,
Jr.; Whittle, R. R.; Olofson, R. A. J. Am. Chem. Soc. 1986,
108, 7664.
3. Ul-Haque, M.; Caughlan, C. N.; Ramirez, F.; Pilot, J. F.;
Smith, C. P. J. Am. Chem. Soc. 1971, 93, 5229.
4. Kawashima, T.; Kato, K.; Okazaki, R. J. Am. Chem. Soc.
1992, 114, 4008.
Ph3P
Ph3P
C6H4NO2-p
11
O
+
COOCH3
p-NO2C6H4COCO2CH3
13a
5. (a) Kawashima, T.; Kato, K.; Okazaki, R. Angew. Chem.,
Int. Ed. Engl. 1993, 105, 941; (b) Kawashima, T.; Kato,
K.; Okazaki, R. Angew. Chem., Int. Ed. Engl. 1993, 32,
869.
6. Kawashima, T.; Takami, H.; Okazaki, R. J. Am. Chem.
Soc. 1994, 116, 4509.
12a
H2O
H
p-NO2C6H4COCO2
Ph3P
7. Kawashima, T.; Okazaki, R.; Okazaki, R. Angew. Chem.,
Int. Ed. 1997, 36, 2500.
8. Bestmann, H. J.; Roth, K.; Wilhelm, E.; Bo¨me, R.;
Burzlaff, H. Angew. Chem., Int. Ed. Engl. 1979, 18, 876.
9. Aly, H. A. E.; Barlow, J. H.; Russell, D. R.; Smith, D. J.
H.; Swindles, M.; Trippett, S. J. Chem. Soc., Chem.
Commun. 1976, 449.
16a
Scheme 4.
10. Kojima, S.; Sugino, M.; Matsukawa, S.; Nakamoto, M.;
Akiba, K. J. Am. Chem. Soc. 2002, 124, 7674.
11. Appel, M.; Blaurock, S.; Berger, S. Eur. J. Org. Chem.
2002, 1143.
12. Andersen, N. G.; Keay, B. A. Chem. Rev. 2001, 101, 997.
13. Birum, G. H.; Matthews, C. N. J. Chem. Soc., Chem.
Commun. 1967, 137.
121o
Ph
∆
Ph
Ph
P
R3
O
+
Ph3P O
O
O
R3
OCH3
H3CO
14. Saalflank, R. W.; Paul, W.; Liebenow, H. Angew. Chem.,
Int. Ed. Engl. 1980, 19, 713.
13a,b
17a,b
15. Crystallographic data for the compound 13a have been
deposited with the Cambridge Crystallographic Data
Centre as supplementary publication number CCDC
281955. Copies of the data can be obtained, free of
charge, on application to CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK (fax: +44 (0)1223 336033 or
e-mail: deposit@ccdc.cam.ac.uk).
∆
R3
O
122o
H2O
O
Ph
P
Ph
+
O
Ph3P
- C6H6
H3CO
16. Compound 13a: 13C NMR(BCM) in CDCl3: 174.2
(d, J = 5.0 Hz), 148.8 (d, J = 6.7 Hz), 147.1 (s), 137.8 (d,
J = 96.7 Hz), 132.7 (d, J = 8.9 Hz), 129.1 (s), 127.4 (d,
J = 12.3 Hz), 127.2 (s), 122.7 (s), 73.7 (d, J = 3.5 Hz), 52.3
11
12a,b
15
Scheme 5.

8952
M. Hamaguchi et al. / Tetrahedron Letters 46(2005) 8949–8952
(s), 49.0 (d, J = 123.5 Hz), 8.5 (d, J = 3.4 Hz); 31P NMR: d
Compound 13c: 13C NMR(BCM) in CDCl3: 201.4 (d,
J = 1.7 Hz), 140.6 (d, J = 9.5 Hz), 138.3 (d, J = 96.1 Hz),
137.9 (s), 134.1 (s), 133.0 (s), 132.8 (d, J = 8.9 Hz), 131.2
(s), 128.9 (d, J = 2.2 Hz), 128.5 (s), 127.6 (s), 127.3 (d,
J = 11.7 Hz), 127.0 (s), 77.2 (s), 48.7 (d, J = 121.3 Hz), 7.9
(d, J = 5.6 Hz); 31P NMR: d ꢀ46.9.
ꢀ46.2.
Compound 13b: 13C NMR(BCM) in CDCl3: 174.8 (d,
J = 4.5 Hz), 140.3 (d, J = 7.3 Hz), 148.2 (d, J = 96.1 Hz),
132.8 (d, J = 8.9 Hz), 130.8 (s), 128.9 (d, J = 2.2 Hz),
127.9 (s), 127.3 (d, J = 12.3 Hz), 121.3 (s), 73.7 (d,
J = 3.5 Hz), 52.3 (s), 48.6 (d, J = 122.4 Hz), 8.6 (s); 31P
NMR: d ꢀ46.2.
17. Schmidbaur, H. S.; Schier, A.; Milewski-Mahrla, B.;
Schubert, U. Chem. Ber. 1982, 115, 722.