Competitive [4+2] cycloadditions in equimolar mixtures of 1-arylphosphole oxides

Article abstract of DOI:10.1002/hc.1096

Four different [4+2] cycloadditions were observed to take place in the equimolar mixtures of two different arylphosphole oxides 2a + 2b or 2b + 2c. In addition to the phosphole oxide dimers 3a-a and 3b-b, or 3b-b and 3c-c, the crossed cycloadducts 3a-b and 3b-a, or 3b-c and 3c-b were also formed in considerable portions.

Full text of DOI:10.1002/hc.1096

Heteroatom Chemistry
Volume 12, Number 7, 2001
Competitive [4 + 2] Cycloadditions in
Equimolar Mixtures of 1-Arylphosphole Oxides
Gyo¨rgy Keglevich,1 Tungalag Chuluunbaatar,1 Bea´ta Dajka,1
Bat-Amgalan Namkhainyambuu,1 Krisztina Luda´nyi,2
00
and La´szlo´ Toke3
1Department of Organic Chemical Technology, Budapest University of Technology and Economics,
1521 Budapest, Hungary
2Hungarian Academy of Sciences, Chemical Research Center, 1525 Budapest, Hungary
3Research Group of the Hungarian Academy of Sciences, Department of Organic Chemical
Technology, Budapest University of Technology and Economics, 1521 Budapest, Hungary
Received 19 October 2000
the P O groups are directed toward the centre of
ABSTRACT: Four different [4 + 2] cycloadditions
the molecule.
were observed to take place in the equimolar mix-
tures of two different arylphosphole oxides 2a + 2b
or 2b + 2c. In addition to the phosphole oxide dimers
3a-a and 3b-b, or 3b-b and 3c-c, the crossed cy-
cloadducts 3a-b and 3b-a, or 3b-c and 3c-b were also
C
formed in considerable portions. 2001 John Wiley &
Sons, Inc. Heteroatom Chem 12:633–635, 2001
The phosphole oxides can be generated from
phospholes by oxidation or from 3,4-dibromo-
tetrahydrophosphole oxides by the elimination of
two molecules of hydrogen bromide [2].
The [4 + 2] cycloaddition of two molecules of phos-
phole oxides to afford phosphole oxide dimers has
been known for many years [1,2]. The advantage
of the cyclodimerization giving an easy access to
phosphanorbornenes useful in fragmentation reac-
tions [2–4] is that it takes place in a regio- and
stereospecific manner [5]. The prefered isomer is
the one in which the phosphole rings are joined in
the endo fusion and in which the oxygen atoms of
In some cases, especially those in which a
sterically demanding trialkylphenyl substituent is
bonded to the phosphorus atom, it was possible to
detect the phosphole oxide intermediates in the re-
action medium [6–8]. In the presence of dienophiles
such as N-phenylmaleimide, the phosphole oxides
were efficiently trapped [2].
In this article, we describe our results on the
outcome of cycloadditions as applied to equimo-
lar mixtures of two different of arylphosphole
oxides.
Dedicated to Dr Imre Petneha´zy on the occasion of his sixtieth
birthday.
In the first experiment, an equimolar mixture
of 1-phenyl-3,4-dimethylphosphole oxide (2a) and
1-(2,4,6-tri-tert-butylphenyl-)3-methylphosphole ox-
ide (2b), both generated from the corresponding
Correspondence to: Gyo¨rgy Keglevich; e-mail: keglevich@oct.
bme.hu.
Contract grant sponsor: OTKA.
Contract grant number: T 029039.
2001 John Wiley & Sons, Inc.
c
633

634 Keglevich et al.
phospholes (1a and 1b, respectively) by hydrogen
peroxide oxidation, were reacted at 0 C for 1 h. The
reaction mixture obtained after removing the excess
of the peroxide and after flash column chromatog-
raphy was analyzed by 31P NMR spectroscopy and
FAB-MS (Table 1). It was easy to recognize that the
mixture of four cycloadducts 3a-a, 3b-b, 3a-b, and
3b-a had been formed (Scheme 1). Products 3a-a
(25%) and 3b-b (22%) were the dimers of phosphole
oxides 2a and 2b, respectively. The other two com-
ponents were mixed cycloadducts 3a-b (25%) and
3b-a (28%). In 3a-b, phenylphosphole oxide 2a func-
tioned as the diene, while in 3b-a, phosphole oxide
2a served as the dienophile. It can be seen that the
relative proportions of dimers 3a-a and 3b-b were
comparable with those of the mixed cycloadducts
3a-b and 3b-a. Attempts to isolate the mixed cy-
cloadducts 3a-b and 3b-a from the reaction mixture
by column chromatography were not succesful, only
a partial enrichment in both these could be achieved.
The dimer with a tri-tert-butylphenyl substituent on
the phosphorus atoms (3b-b) could, however, be ob-
tained in a pure form.
At 0 C neither the phenylphosphole oxide 2a nor
the tri-tert-butylphenyl derivative 2b discriminated
between the two phosphole oxides 2a and 2b present
in the reaction mixture. In an attempt to effect a
crossed cycloaddition at 15 C, practically no mixed
“dimers” could be detected in the reaction mixture.
According to this, the selection process seems to be
very sensitive to the reaction temperature.
In the second experiment, an equimolar mixture
of 2,4,6-tri-tert-butylphosphole oxide (2b) and the
tri-isopropylphenyl derivative 2c, both generated in
situ by oxidation of the corresponding phospholes 1b
and 1c, respectively, were reacted at 0 C for 1.5 h. The
SCHEME 1 The composition of the reaction mixture was
31
analyzed on the basis of relative P NMR intensities (esti-
mated error 2.5%).
TABLE 1 31P NMR and FAB-MS Data of Phosphole Oxide
Cycloadducts 3
31
P NMR (CDCl )
3
31P NMR and FAB-MS analyses of the reaction mix-
ture (Table 1) revealed that the comparable stability
of the phosphole oxides 2b and 2c had resulted in
a less efficient mixed Diels–Alder reaction: 16% of
3b-c and 11% of 3c-b were formed in addition to 35%
of dimer 3b-b and 38% of dimer 3c-c (Scheme 2). It
is noteworthy that in this case, the self-dimerization
was favored to a major extent.
It can be concluded that all possible Diels–Alder
cycloadducts are formed in the equimolar mixtures
of two different arylphosphole oxides. This is the
first case in which mixed phosphole oxide “dimers”
are described. Extension of the above experiments to
FAB-MS
[M + H]
3
Comp.
JPP (Hz)
P
P
1
8
3a-a
3b-b
52.9
56.8
74.4
84.2
38.7
39.8
409
717
(57.1)a
57.0
(84.1)a
80.6
(39.6)a
38.0
3c-c
633
(56.4)b
56.3
(80.1)b
74.3
(38.1)b
37.6
3a-b
3b-a
3b-c
3c-b
563c
563c
675
54.0
57.0
57.1
83.8
84.7
79.8
40.2
37.7
39.7
675
aFrom Ref. [8].
bFrom Ref. [7].
cHR-FAB, [M + H]f+ound = 563.3101; C35H49O2P2 requires 563.3208.

Competitive [4 + 2] Cycloadditions in Equimolar Mixtures of 1-Arylphosphole Oxides 635
General Procedure for the Preparation
of the Mixture of Cycloadducts
To the mixture of 0.53 mmol of phosphole 1a or 1b
and 0.18 g (0.53 mmol) of phosphole 1c in 30 ml of
chloroform was added 1.5 ml ( 13 mmol) of 30%
hydrogen peroxide at 0 C. After a 1 h of stirring, 3
ml of water was added and the mixture stirred for
an additional 5 min. The extraction was repeated
with another 3 ml portion of water, and then the or-
ganic phase was separated and dried (Na2SO4). The
oil obtained after evaporation was purified by flash
column chromatography (silica gel, 3% methanol in
chloroform) to give the mixtures in practically quan-
titative yields, as shown in Schemes 1 and 2 and as
characterized in Table 1.
Repeated column chromatography (as above) af-
forded 3b-b (first experiment) and 3c-c (second ex-
periment) in an efficiency of 21% and 32%, respec-
tively. The content of the mixed dimers 3a-b and
3b-a (first experiment), as well as that of products
3b-c and 3c-b (second experiment) could be enric-
hed to an extent of 32–53%.
ACKNOWLEDGMENT
T. C. is grateful for the support from the Ministry of
Education.
REFERENCES
SCHEME 2 The composition of the reaction mixture was
analyzed on the basis of relative P NMR intensities (esti-
mated error 2.5%).
31
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Westheimer, F. H. J Am Chem Soc 1967, 89, 3919.
[2] Quin, L. D. Rev Heteroat Chem 1990, 3, 39.
[3] Keglevich, Gy.; Luda´nyi, K.; Quin L. D. Heteroatom
Chem 1997, 8, 135.
00
three or more phosphole oxides may be a real com-
binatorial technique giving valuable information on
the reactivity of the different phosphole oxides.
[4] Keglevich, Gy.; Trecska, M.; Nagy, Z.; Toke, L. Het-
eroatom Chem 2001, 12, 6.
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EXPERIMENTAL
00
The 31P NMR spectra were obtained on a Bruker
DRX-500 instrument at 202.4 MHz. The FAB-MS
measurements were performed on a ZAB-2SEQ
spectrometer.
The arylphospholes 1b and 1c were prepared as
described earlier [7,9].
M.; Kalgutkar, R.; Lahti, P. M. J Organomet Chem
1997, 532, 109.
[8] Keglevich, Gy.; Bo¨cskei, Zs.; Harmat, V.; Toke L. Het-
00
eroatom Chem 1997, 8, 527.
00
[9] Keglevich, Gy.; Bo¨cskei, Zs.; Keseru, Gy. M.;
´
Ujsza´szy, K.; Quin, L. D. J Am Chem Soc 1997, 119,
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