Palladium-catalyzed acetoxylation of cyclic allyl phosphonates in the presence of isopentyl nitrite and using molecular oxygen as final oxidant

Article abstract of DOI:10.1016/S0040-4020(00)00172-1

The palladium-catalyzed acetoxylation of cyclic dialkyl allyl phosphonates is effected using palladium chloride as catalyst, in the presence of isopentyl nitrite in acetic acid under an oxygen atmosphere. The proposed mechanism for the reaction involves a

Full text of DOI:10.1016/S0040-4020(00)00172-1

TETRAHEDRON
Pergamon
Tetrahedron 56 (2000) 2693±2697
Palladium-Catalyzed Acetoxylation of Cyclic Allyl Phosphonates
in the Presence of Isopentyl Nitrite and Using Molecular Oxygen
as Final Oxidant
*
Mireille Attolini, Gilbert Peiffer and Michel Maffei
   à Â
Laboratoire des Organo-Phosphores (E.S.A. 6009 du C.N.R.S.), B.P. 552, Faculte de Saint Jerome, Avenue Escadrille Normandie Niemen,
13397 Marseille Cedex 20, France
Received 2 November 1999; accepted 24 February 2000
AbstractÐThe palladium-catalyzed acetoxylation of cyclic dialkyl allyl phosphonates is effected using palladium chloride as catalyst, in the
presence of isopentyl nitrite in acetic acid under an oxygen atmosphere. The proposed mechanism for the reaction involves a palladium
nitro±nitroso redox couple. q 2000 Elsevier Science Ltd. All rights reserved.
Introduction
synthesis of antiviral nucleosides5 and optically active
cyclopropyl ketones.6
The catalytic acetoxylation of alkenes is a useful and
straightforward method for the preparation of allyl
acetates,1 which have become important synthetic inter-
mediates.2 We have recently described the Pd-catalyzed
acetoxylation of dialkyl allyl phosphonates with benzo-
quinone and manganese dioxide as reoxidants,3 which
allows an easy, one step synthesis of dialkyl 3-acetoxy-1-
alkenyl phosphonates. We further illustrated the synthetic
utility of these products through the preparation of pre-
cursors of biologically active phosphono amino acids.4
Dialkyl 3-acetoxy-1-alkenyl phosphonates can also be
used to prepare the corresponding allyl alcohols which
have, in turn, been used as starting materials for the
We became interested in extending the scope of the acetoxyla-
tion reaction to cyclic allyl phosphonates 1±3 (Scheme 1)
since the corresponding products, in light of the references
cited above, would lead to useful synthetic intermediates.
When 1a was subjected to the standard acetoxylation con-
ditions (i.e. 5% Pd(OAc)2, 20% benzoquinone, 110% MnO2,
acetic acid, 658C), only ca. 30% of starting material was
consumed after 65 h, and the expected acetate 4a was
formed with 48% selectivity, beside several by-products,
as evidenced by 31P NMR analysis of the crude product.
The same reaction performed at 908C led to a complete
Scheme 1.
Keywords: palladium-catalyzed acetoxylation; cyclic dialkyl allyl phosphonates; isopentyl nitrite; 3-acetoxy-1-alkenyl phosphonates.
* Corresponding author. Fax: 133-4-91-28-80-30; e-mail: michel.maffei@lop.u-3mrs.fr
0040±4020/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020(00)00172-1

2694
M. Attolini et al. / Tetrahedron 56 (2000) 2693±2697
Table 1. Palladium-catalyzed acetoxylation of cyclic allyl phosphonates 1±3 in the presence of 5% Pd(OAc)2, 20% benzoquinone, and 110% MnO2, in acetic
acid
Compound
Reaction time (h)
Temperature (8C)
Conversion (%)
Selectivity (%)a
1a
1a
2a
3a
65
20
70
40
65
90
65
65
30
100
38
48
34
84
40
20
a For the acetoxylation product, measured by 31P NMR of the crude.
conversion after 20 h, but the selectivity was lower (34%).
Similar results were obtained with cyclopentenyl and cyclo-
heptenyl phosphonates 2a and 3a, respectively (Table 1).
however), from which the acetoxyphosphonates are easily
separated. In one case (Table 2, entry 3), the by product
could be isolated in a pure state. Careful examination of
its NMR data as well as its mass spectrum ([M1]ˆ291)
and its infrared spectrum (nˆ1540 and 1395 cm21) led us
to assign its structure as the 3-nitro derivative 7.
The use of benzoquinone alone (without MnO2) did not
improve the conversion and/or selectivity.
We thought that these low yields might result from an
inef®cient reoxidation of palladium. In order to check this
assumption, we employed the conditions described by Tsuji
(i.e. 10 mol% PdCl2, 150 mol% isopentyl nitrite, acetic acid,
oxygen atmosphere) for the acetoxylation of b,g-unsatured
esters,7 in which isopentyl nitrite acts as the reoxidizing
reagent. In this case, we were pleased to note that allyl
phosphonates 1±3 were totally converted (Table 2), and
the expected acetates 4±6 were obtained with moderate to
good yields. It should be noted that the reactions started
after a lag period of ca. 1.5 h. The use of palladium acetate
instead of PdCl2 gave less satisfactory results since the
reaction was incomplete in all cases.
In the case of the cycloheptenyl phosphonates, the low
yields are attributed to the additional formation of (cyclo-
hepten-1-yl) phosphonates, resulting from an isomeri-
²
zation8of the starting material. These were found to be
totally unreactive towards acetoxylation.
In order to assess the in¯uence of both nitrite and oxygen on
the reaction outcome, we carried out the experiments
described below.
Although cyclopentenyl phosphonate 2a affords the
expected acetate 5a as a single product, the acetoxylation
of the six- (1a, 1b and 1c) and seven- (3a and 3c) membered
ring analogues still leads to side products (in lower amounts
First, it appears that the presence of oxygen during the
reaction is essential: thus, the acetoxylation of dimethyl
cyclohexen-2-yl phosphonate 1b is complete after 14 h
under the conditions listed above (i.e. 10 mol% PdCl2,
150 mol% isopentyl nitrite, acetic acid, oxygen atmosphere,
658C), whereas the same reaction run in air needs 25 h to go
to completion. Interestingly, in this last case, the lag period
was increased to 3 h.
Table 2. Palladium-catalyzed acetoxylation of cyclic allyl phosphonates 1±
3 in the presence of 10 mol% PdCl2 and 150 mol% isopentyl nitrite in acetic
acid under oxygen, 658C
Entry
Substrate Selectivity (%)a Reaction time (h) Yield (%)b
1
2
3
4
5
6
1a
1b
1c
2a
3a
3c
82
75
70
100
48
42
14
21
19
20
17
24
70
69
65
75
43
34
On the other hand, the in¯uence of the amount of isopentyl
nitrite on the reaction has been evaluated, again with 1b as
substrate (Table 3).
These experiments clearly show that only a small amount of
nitrite is suf®cient to drive the reaction to completion with-
out altering appreciably the yields and reaction times. Thus,
the use of only 20 mol% of isopentyl nitrite with respect to
the substrate is suf®cient to drive the reaction to completion.
a For the acetoxylation product, measured by 31P NMR of the crude.
b Of pure product.
Table 3. In¯uence of the amount of isopentyl nitrite on the Pd-catalyzed
acetoxylation of 1b (conditions: 10 mol% PdCl2 in acetic acid under
oxygen, 658C, reaction time: 20 h)
These results led us to the conclusion that isopentyl nitrite
may produce a palladium species easily oxidised by oxygen,
resulting in an ef®cient recycling of Pd(0).
Isopentyl nitrite (mol%) Conversion (%) Selectivity (%)a Yield (%)b
500
150
50
20
0
100
100
100
100
0
70
75
80
80
±
60
69
74
72
±
²
This assumption was deduced from the examination of 1H and 31P NMR
spectra of the crude product, which display several signals in agreement
with an a,b-unsaturated phosphonate structure. However, the presence of
dienyl phosphonates resulting from a palladium-catalyzed elimination from
the 3-acetoxy-1-alkenyl phosphonates 6a and 6c under the reaction con-
ditions9 cannot be excluded.
a For the acetoxylation product, measured by 31P NMR of the crude.
b Of pure product.

M. Attolini et al. / Tetrahedron 56 (2000) 2693±2697
2695
Scheme 2.
10
Ê
Akermark et al. have reported the catalytic acetoxylation
of cyclohexene with iron nitrate as reoxidant, and proposed
a mechanism which relies on a Pd±NO species, which can
be further oxidised to an active Pd±NO2 complex by molec-
ular oxygen. Similar trends were put forward to explain the
palladium-catalyzed oxidation of ole®ns,11 the epoxidation
of cyclic alkenes,12 and some other metal-catalyzed
reactions.13 Our reactions may proceed through a similar
pathway, as outlined in Scheme 2. Thus, a palladium nitroso
complex A might be formed and further oxidised to a palla-
dium nitro species B responsible for the acetoxylation.
group coordinated to the metal, may account for the higher
reactivity.
In conclusion, we have shown that the palladium-catalyzed
acetoxylation of cyclic allyl phosphonates can be performed
ef®ciently through the use of isopentyl nitrite as reoxidant,
the reaction being run under oxygen. The use of only
20 mol% of nitrite (with respect to the substrate) is
suf®cient, indicating that the reaction may proceed via a
nitro±nitrosyl redox couple with oxygen being the ®nal
oxidant. This procedure is attractive since it allows an
easy dioxygen activation15 and enables minimization of
by-products, which is satisfactory from an environmental
and economical point of view. We are currently applying
this procedure to other organophosphorus substrates.
Following this model, the formation of 7 may be rationa-
lized through an internal migration of the nitro group in
complex C. A similar trend was already described in a
related system.11a
The superiority of isopentyl nitrite over the benzoquinone/
manganese dioxide system as reoxidant deserves some
Experimental
14
comments. As demonstrated by Backvall, the (p-allyl)
È
All solvents were puri®ed according to reported procedures,
and reagents were used as received. 1H NMR and 13C NMR
spectra were recorded in CDCl3, on a Bruker AC 200 spec-
trometer working at 200.00 MHz and 50.16 MHz, respec-
tively (the usual abbreviations are used: s: singulet, d:
doublet, t: triplet, q: quadruplet, qt: quintuplet, o: octuplet,
m: multiplet). Tetramethylsilane was used as internal
standard.31P NMR spectra were recorded on a Bruker AC
100 spectrometer at 40.54 MHz, using 85% H3PO4 as
external standard. All chemical shifts are given in ppm.
Infrared spectra (n in cm21) were recorded as thin ®lms
on a Hewlett-Packard Paragon 1000. Elemental analyses
palladium complex involved in the closely related palla-
dium-catalyzed 1,4-diacetoxylation of conjugated dienes
incorporates the benzoquinone directly coordinated to the
metal. We can reasonably assume that the intermediate for
the acetoxylation of cyclic allyl phosphonates using the
benzoquinone/MnO2 system has a similar structure. The
formation of this (p-allyl) palladium complex from a cyclic
allyl phosphonate may be prevented due to the steric
congestion created by the presence of benzoquinone. On
the other hand, the easier formation of the (p-allyl)
palladium complex C, incorporating only the smaller NO2

2696
M. Attolini et al. / Tetrahedron 56 (2000) 2693±2697
Â
were carried out at the Microanalytical Center, Faculte de
H, 7.31; P, 11.80. C11H19O5P requires: C, 50.38; H, 7.25;
P, 11.83%]. IR (thin ®lm): n 1734 (CvO); 1617 (CvC);
1241 (PvO); 1163 (C±O); 1024 (P±O±C). 1H NMR: 1.3 (t,
 Ã
Saint Jerome, Marseille. The starting cyclic allyl phospho-
nates were prepared by a Michaelis Becker16 reaction from
the corresponding dialkyl phosphites and the cyclic allyl
bromides, which were in turn obtained via radical bromi-
nation of the cyclic alkenes with N-bromo succinimide and
benzoyl peroxide in CCl4.
3
CH3CH2O, JHHˆ7.1 Hz); 1.8±2.7 (m, cyclic CH2); 2.0 (s,
CH3C(O)); 4.1 (qt, CH3CH2O, 3JHHˆ3JPHˆ7.1 Hz); 5.6 (m,
3
CH±O); 6.5 (d, CHvC, JPHˆ17.9 Hz). 13C NMR: 16.2
3
(2d, CH3CH2O, JPCˆ7.0 Hz); 20.8 (s, CH3C(O)); 30.5 (d,
CH2, 2JPCˆ10.4 Hz); 31.8 (d, CH2, JPCˆ12.8 Hz); 62.3 and
3
63.7(d, CH3CH2O, 2JPCˆ5.8 Hz and 2JPCˆ5.8 Hz); 80.0 (d,
General procedure for the palladium-catalyzed acet-
oxylation of cyclic allyl phosphonates
3
1
CH±O, JPCˆ24.1 Hz); 137.2 (d, P±Cv, JPCˆ186.9 Hz);
143.1 (d, CHvC, JPCˆ13.8 Hz); 170.7 (s, CvO). 31P
2
NMR: 13.7.
The reactions were run in a two necked round-bottomed
¯ask ®tted with a rubber septum and a condensor which
was connected to a gas burette for the oxygen supply. In
the ¯ask were placed successively palladium chloride
(17.7 mg, 0.1 mmol), potassium acetate (196 mg, 2 mmol),
and the allyl phosphonate (1 mmol) in acetic acid (2 ml).
The system was purged with oxygen by three vacuum/
¯ushing cycles. Then isopentyl nitrite (200 ml, 1.5 mmol)
was added via syringe, and the mixture was stirred at 608C.
The reaction was monitored by gas chromatography, or
more simply, by measuring the oxygen uptake. After the
end of the reaction, the mixture was cooled, diluted with
ether (2£15 ml) and ®ltered over a short pad of Celite. The
®ltrate was washed with 10% Na2CO3, brine, and dried over
MgSO4. The residue obtained after ®ltration and removal of
solvent was puri®ed by ¯ash chromatography over silicagel
with ethyl acetate. 1H and 13C NMR data for acetoxy phos-
phonate 4b were in full accordance with those already
reported,17 and 31P NMR gives dˆ19.7 ppm.
Diethyl (3-acetoxy cyclohepten-1-yl)phosphonate 6a.
Colorless oil, Rf: 0.38 (ethyl acetate). [Found: C, 53.73;
H, 7.90; P, 10.64. C13H23O5P requires: C, 53.79; H, 7.93;
P, 10.69%]. IR (thin ®lm): n 1738 (CvO); 1645 (CvC);
1
1242 (PvO); 1163 (C±O); 1026 (P±O±C). H NMR: 1.2
(2t, CH3CH2O, 3JHHˆ7.0 Hz); 1.4±2.4 (m, cyclic CH2); 2.0
(s, CH3C(O)); 4.0 (2 qt, CH3CH2O, 3JHHˆ3JPHˆ7.1 Hz); 5.4
(m, CH±O); 6.7 (d, CHvC, 3JPHˆ24.6 Hz). 13C NMR: 16.5
3
(d, CH3CH2O, JPCˆ6.5 Hz); 21.3 (s, CH3C(O); 26.0 (d,
3
CH2, JPCˆ6.9 Hz); 27.8 (s, CH2); 28.3 (d, CH2,
4
2JPCˆ8.9 Hz); 32.3 (d, CH2, JPCˆ2.4 Hz); 62.5 (d,
3
2
CH3CH2O, JPCˆ5.8 Hz); 74.8 (d, CH±O, JPCˆ27.4 Hz);
1
131.8 (d, P±Cv, JPCˆ179.8 Hz); 150.9 (d, CHvC,
2JPCˆ10.5 Hz); 176.7 (s, CvO). 31P NMR: 19.5.
Di-isopropyl (3-acetoxy cyclohepten-1-yl)phosphonate
6c. Slightly yellow oil, Rf: 0.54 (ethyl acetate). [Found: C,
56.64; H, 8.55; P, 9.79. C15H27O5P requires: C, 56.60; H,
8.49; P, 9.75%]. IR (thin ®lm): n 1738 (CvO); 1615
(CvC); 1242 (PvO); 1168 (C±O); 1035 (P±O±C).1H
Diethyl (3-acetoxy cyclohexen-1-yl)phosphonate 4a. Color-
less oil, Rf: 0.30 (ethyl acetate). [Found: C, 52.11; H, 7.56;
P, 11.31. C12H21O5P requires: C, 52.17; H, 7.61; P, 11.23%].
IR (thin ®lm): n 1734 (CvO); 1641 (CvC); 1239 (PvO);
NMR: 1.3 (d, (CH3)CH±, 3JHHˆ6.1 Hz); 1.4±2.3 (m, cyclic
3
CH2); 2.0 (s, CH3C(O)); 4.5 (o, (CH3)CH±, JHH
ˆ
3
3JPHˆ6.2 Hz); 5.4 (m, CH±O); 6.6 (d, CHvC, JPHˆ
1
1164 (C±O); 1022 (P±O±C). H NMR: 1.3 (t, CH3CH2O,
3JHHˆ7.1 Hz); 1.5±2.2 (m, cyclic CH2); 2.0 (s, CH3C(O));
24.9 Hz). 13C NMR: 21.2 (s, CH3C(O)); 23.7 (d,
3
3
4.0 (qt, CH3CH2O, 3JHHˆ3JPHˆ7.1 Hz); 5.3 (m, CH±O); 6.5
(CH3)CH±, JPCˆ4.6 Hz);26.7 (d, CH2, JPCˆ7.3 Hz);
2
3
(d, CHvC, JPHˆ21.8 Hz). 13C NMR: 16.3 (d, CH3CH2O,
27.6 (s, CH2); 28.4 (d, CH2, JPCˆ10.1 Hz); 32.1 (s, CH2);
2
3
3JPCˆ6.5 Hz); 19.0 (d, CH2, JPCˆ11.2 Hz); 21.0 (s,
72.0 (d, (CH3)CH±, JPCˆ5.9 Hz); 74.3 (d, CH±O,
3JPCˆ27.0 Hz); 134.3 (d, P±Cv, JPCˆ176.1 Hz); 148.8
1
2
CH3C(O)); 24.2 (d, CH2, JPCˆ7.9 Hz); 27.4 (s, CH2);
2
31
2
(d, P±CvC, JPCˆ10.9 Hz); 170.2 (s, CvO). P NMR:
61.9 (d, CH3CH2O, JPCˆ5.7 Hz); 67.5 (d, CH±O,
1
3JPCˆ20.3 Hz); 132.6 (d, P±Cv, JPCˆ178.4 Hz); 139.0
17.1.
(d, CHvC, 2JPCˆ8.8 Hz); 170.0 (s, CvO). 31 P NMR: 17.1.
Di-isopropyl (3-nitro cyclohexen-1-yl)phosphonate 7 was
isolated during puri®cation of 4c. Yellow oil, Rf: 0.38
(ethyl acetate). IR (thin ®lm): n 1630 (CvC); 1540 and
1395 (NO2); 1258 (PvO); 1014 (P±O±C).1H NMR: 1.3
Di-isopropyl (3-acetoxy cyclohexen-1-yl)phosphonate 4c.
Slightly yellow oil, Rf: 0.34 (ethyl acetate). [Found: C,
55.29; H, 8.18; P, 10.27. C14H25O5P requires: C, 55.26; H,
8.22; P, 10.20%]. IR (thin ®lm): n 1741 (CvO); 1615
3
(d, (CH3)2CH±, JHHˆ6.9 Hz); 1.7±2.4 (m, cyclic CH2);
3
4.7 (o, (CH3)2CH±O, JHHˆ3JPHˆ6.3 Hz); 5.5 (m, CH±
1
(CvC); 1236 (PvO); 1161 (C±O); 1022 (P±O±C). H
3
NO2); 6.6 (d, CHvC, JPHˆ19.6 Hz).13C NMR: 18.5 (d,
3
NMR: 1.3 (d, (CH3)2CH±, JHHˆ6.3 Hz); 1.6±2.3 (m,
3
3
CH2, JPCˆ10.1 Hz); 24.0 (s, (CH3)2CH), JPCˆ4.3 Hz);
cyclic CH2); 2.0 (s, CH3C(O)); 4.7 (2o, (CH3)2CH±O,
3JHHˆ3JPHˆ6.2 Hz); 5.3 (m, CH±O); 6.6 (d, CHvC,
2
24.3 (d, CH2, JPCˆ7.6 Hz); 26.1 (s, CH2); 70.9 (d, CH±
O±P, 2JPCˆ5.4 Hz); 76.4(d, CH±NO2, JPCˆ21.2 Hz);133.4
3
3
3JPHˆ21.9 Hz).13C NMR: 19.2 (d, CH2, JPCˆ10.3 Hz);
(d,
CHvC,
2JPCˆ10.2 Hz);
137.9(d,
P±Cv,
21.0 (s, CH3C(O)); 23.8 and 24.0 (d, (CH32)2CH,
1JPCˆ178.9 Hz). 31 P NMR: 13.6.
3
3JPCˆ4.1 Hz and JPCˆ3.9 Hz); 24.2 (d, CH2, JPCˆ
3
8.6 Hz); 27.5 (s, CH2); 67.6 (d, CH±O, JPCˆ20.3 Hz);
2
70.6 (d, CH±O±P, JPCˆ5.6 Hz); 133.8 (d, P±Cv,
2
1JPCˆ179.0 Hz); 138.4 (d, CHvC, JPCˆ9.1 Hz); 170.3
Acknowledgements
Financial support from Centre National de la Recherche
(s, CvO). 31P NMR: 14.9.
Â
Scienti®que and Conseil Regional PACA as a grant to
M.A. is gratefully acknowledged. We also acknowledge
Diethyl (3-acetoxy cyclopenten-1-yl)phosphonate 5a.
Colorless oil, Rf: 0.47 (ethyl acetate). [Found: C, 50.31;

M. Attolini et al. / Tetrahedron 56 (2000) 2693±2697
2697
phosphine oxide, see: Attolini, M.; Principato, B.; Peiffer, G.;
 Â
®nancial support from `Societe de Secours des Amis des
Sciences'.
Maffei, M. Phosphorus, Sulfur, Silicon 1998, 142, 229±238.
Ê
10. Larsson, E. M.; Akermark, B. Tetrahedron Lett. 1993, 34,
2523±2526.
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8. A similar isomerization process was reported for the acetoxyl-
ation of b,g-unsaturated esters, see Ref. 7.
È
17. Ohler, E.; Zbiral, E. Chem. Ber. 1991, 124, 175±184.
9. We have already reported an analoguous reaction during the
palladium-catalyzed acetoxylation of (2-butenyl)-tert-butyl-phenyl