Synthesis of H-phosphinates by the UV light - Mediated fragmentation- related phosphorylation using simple P-heterocycles

Article abstract of DOI:10.1080/00397910500297404

Photolysis of aryl-substituted 2,5-dihydrophosphole oxides (5a-e and 8) in the presence of methanol afforded methyl aryl-H-phosphinates (2a-e) in good yields. In the case of 1-ethyl-, cyclohexyl-, or ethoxy-2,5-dihydrophosphole oxides, the reaction was mu

Full text of DOI:10.1080/00397910500297404

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Synthesis of H‐Phosphinates
by the UV Light–Mediated
Fragmentation‐Related
Phosphorylation Using Simple
P‐Heterocycles
b
a
b
Helga Szelke , János Kovács & György Keglevich
a
Research Group of the Hungarian Academy of
Sciences at the Department of Organic Chemical
Technology , Budapest University of Technology and
Economics , Budapest, Hungary
b
Department of Organic Chemical Technology ,
Budapest University of Technology and Economics ,
Budapest, Hungary
Published online: 22 Aug 2006.
To cite this article: Helga Szelke , János Kovács & György Keglevich (2005) Synthesis
of H‐Phosphinates by the UV Light–Mediated Fragmentation‐Related Phosphorylation
Using Simple P‐Heterocycles, Synthetic Communications: An International Journal for
Rapid Communication of Synthetic Organic Chemistry, 35:22, 2927-2934
To link to this article: http://dx.doi.org/10.1080/00397910500297404
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Synthetic Communications , 35: 2927–2934, 2005
Copyright # Taylor & Francis, Inc.
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910500297404
Synthesis of H-Phosphinates by the UV
Light–Mediated Fragmentation-Related
Phosphorylation Using Simple
P-Heterocycles
Helga Szelke
Research Group of the Hungarian Academy of Sciences at the
Department of Organic Chemical Technology, Budapest University of
Technology and Economics, Budapest, Hungary
J a´ nos Kov a´ cs and Gy o¨ rgy Keglevich
Department of Organic Chemical Technology, Budapest University of
Technology and Economics, Budapest, Hungary
Abstract: Photolysis of aryl-substituted 2,5-dihydrophosphole oxides (5a–e and 8) in
the presence of methanol afforded methyl aryl-H-phosphinates (2a–e) in good yields.
In the case of 1-ethyl-, cyclohexyl-, or ethoxy-2,5-dihydrophosphole oxides, the
reaction was much slower (5f and 5h) or did not take place at all (5g). In such
instances, the presence of an additional skeletal methyl group (7) or the use of the
more strained 7-phosphanorbornene derivatives (6) promoted the fragmentation-related
phosphorylations. Furthermore, the effect of the ring saturation in 8 and the possible
extensions to 2-phosphabicyclo[3.1.0]hexanes (10a and 10f), a 1,2-dihydrophosphinine
oxide (11), and a 1,2,3,4,5,6-hexahydrophosphinine oxide (12) were also investigated.
Model compounds with P-phenyl substituent that are of sufficient ring strain (8, 10a,
and 11) could be utilized well.
Keywords: Photolysis, fragmentation, P-heterocycles, H-phosphinates
Received in the UK March 21, 2005
Address correspondence to Gy o¨ rgy Keglevich, Department of Organic Chemical
Technology, Budapest University of Technology and Economics, H-1521 Budapest,
Hungary. E-mail: keglevich@mail.bme.hu
2
927

2
928
INTRODUCTION
-Phosphanorbornene derivatives, such as 7-oxides and 7-sulfides, are useful
H. Szelke, J. Kov a´ cs, and G. Keglevich
7
in the synthesis of H-phosphinates and H-thiophosphinates. Irradiation of the
bridged precursors in the presence of an alcohol led to the formation of the
[
1–4]
corresponding phosphorylated products [Scheme 1(I)].
methods made available mainly phenyl-H-(thio)phosphinates; later the
The early
[5,6]
method was extended to aryl-H-phosphinates.
It is noteworthy that simple 1-phenyl-2,5-dihydrophosphole oxides could
also be used in the UV light–induced fragmentation-related phosphorylation
[
7,8]
of alcohols [Scheme 1(II)].
Although this is an excellent way for the
synthesis of phenyl-H-phosphinates, the scope of the method is rather
limited. We wished to study the possible extensions in detail.
Tomioka et al. suggested an elimination–addition mechanism involving a
[
7,8]
phosphenite/phosphinidene oxide [PhP(O)] intermediate.
proved that the real intermediate is, as a matter of fact, an adduct formed by
Quin et al. later
[9]
the addition of the alcohol on the P 55 O group.
RESULTS AND DISCUSSION
We aimed to synthesize aryl-H-phosphinates; a few aryl-2,5-dihydro-1H-
[
10–12]
phosphole oxides (5b–e)
were irradiated at 254 nm in acetonitrile, in
the presence of methanol. As a comparison, the reaction of the phenyl-
dihydro-1H-phosphole oxide (5a) was also carried out. The corresponding
H-phosphinates (2a–e) were obtained in good (68–93%) yields after flash
column chromatography (Scheme 2).
Scheme 1. Refs. [1–4,7,8]

Synthesis of H-Phosphinates
2929
Scheme 2.
3
The products (2a–e) were identified by P NMR chemical shifts, J
1
1
PH
couplings, and FAB-MS (Table 1).
The 2,4,6-trialkylphenyl products (2d and 2e) were obtained as the
mixture of a major and minor isomers that was confirmed by GC-MS. The
formation of isomers may be the consequence of the migration of the alkyl
groups in the aromatic ring under photochemical conditions.
Aryl-H-phosphinates 2a, 2b, and 2e were described earlier in the litera-
[5,9,13]
ture. The new products 2c and 2d were fully characterized (see Table 1).
We tried to extend this method to the photolysis of the 1-cyclohexyl- and
[
14]
[15]
the 1-ethyl-dihydro-1H-phosphole oxides (5f and 5g ) in the presence of
methanol. In the first case, the reaction was rather slow (Scheme 3), whereas in
the second instance, 5g was regenerated unchanged. Consequently, ethyl-H-
phosphinate 2g had to be prepared by the irradiation of the mixture of 7-phos-
phanorbornene 6 and methanol (Scheme 4).
[
16,17]
Both H-phosphinates (2f and 2g) are known compounds.
chemical shifts around 45 ppm confirm their structure.
The dP
Unreactivity of 5g
was in accord with earlier observations on 1-alkyl-dihydro-1H-phosphole
[18]
[19]
oxides.
For the characterization of products 2f and 2g, see Table 1.
An extension for the preparation of the ethyl methyl-H-phosphonate (2h)
by the photolysis of the 1-ethoxy-dihydrophosphole oxide (5h) was not suc-
cessful because the reaction was reluctant and hence rather slow. It was
observed, however, that the model compound with an extra methyl group
[
20]
(7)
was more reactive in fragmentation-related phosphorylation leading
to H-phosphonate 2h within a reasonable reaction time (Scheme 5,

2930
H. Szelke, J. Kov a´ cs, and G. Keglevich
Table 1. Identification/characterization of the H-phosphinates prepared
3
1
3
P NMR (CDCl )
Starting
product
FAB-MS
[M þ H]
1
þ
P-cycle
dP (lit.)
JPH (lit.) in Hz
Ref.
5
5
5
5
5
5
6
7
a (8, 10a, 11)
b
2a
2b
27.7 (26.8)
28.0
28.5
566.0 (568)
565.5
560.2
157
171
171
199
283
163
109
125
[9]
[13]
a
b
c
2c
2d
c
d
27.4
555.4
d
e
2e
2f
27.3 (26.9)
46.0 (46.7)
44.3
539.5 (551.0)
524.9 (521)
534.1
[5]
f
[16]
[17]
[21]
(10g)
2g
2h
9.0
697.1
a13
C NMR d (CDCl
ꢀ
3
) 21.7 (C
) , 131.0 (J ¼ 12.3, C
4
0
–Me), 52.0 (J ¼ 6.6, OCH
3
), 125.9 (J ¼ 134.4, C
1
0
),
–Me), 51.5
ꢀ
[13]
), [lit., 21.8 (C
1
29.5 (J ¼ 14.3, C
0
0
) , 144.0 (C
0
0
3
2
4
4
)ꢀ, 131.0 (J ¼ 12.0, C
) ,
ꢀ
(
J ¼ 7.1, OCH
b13
3
), 126.1 (J ¼ 134.0, C
1
0
), 129.5 (J ¼ 14.2, C
3
0
2
0
ꢀ
1
43.9 (C
4
0
)]. May be reversed.
C NMR d (CDCl
) , 127.2 (J ¼ 130.7, C
3
) 20.1 (J ¼ 5.4, C
2
0
–Me), 52.4 (J ¼ 6.9, OCH
3
), 125.0 (J ¼ 14.5,
ꢀ
ꢀ
ꢀ
0
6
C5
0
0
), 131.1 (J ¼ 10.8, C ) , 131.9 (J ¼ 12.1, C
0
) , 133.0 (C ),
0
4
1
3
1
); H NMR d (CDCl
1
41.0 (J ¼ 11.5, C
2
0
3
) 2.56 (s, 3H, C
2
0
–CH
3
), 3.80 (d, JPH ¼ 10.5,
3
H, OCH
3
), 7.30–7.85 (m, 4H, Ar–H), 7.60 (d, JPH ¼ 559.7, P–H); HR-FAB:
þ
ꢀ
(
M þ H)
¼ 171.0553; C H PO requires 171.0575. May be reversed.
found
8
12
2
c
As the mixture of a major (67%, see above) and two minor isomers [d
P
30.7 (20%)
0
and 28.8 (13%)]. For the major isomer C NMR d (CDCl ) 19.8 (C –Me), 20.9
3 4
1
3
(
J ¼ 8.3, C
0
–Me) 52.4 (J ¼ 6.4, OCH ), 128.3 (J ¼ 131.0, C
0
1
), 130.2 (J ¼ 12.3,
2
3
þ
C
C
3
0
), 142.0 (J ¼ 11.5, C
2
0
), 142.9 (C
4
0
); HR-FAB: (M þ H)found ¼ 199.0870;
10
d
H16PO
2
requires 199.0888.
As the mixture of a major (51%, see above) and three minor isomers [d 34.2 (23%)
P
and 30.6 (15%) and 26.4 (11%)].
[
21]
Table 1). H-Phosphonate 2h was described in the literature.
Its d che-
P
1
mical shift and J coupling (see Table 1) fit the data (d 5–9 and J
1
PH
P
PH
[18]
6
82–696 Hz) reported for analogous derivatives.
Although 2-methyl-1-phenyl-dihydro-1H-phosphole oxide 8
[22]
was as
efficient in the synthesis of H-phosphinate 2a as the 3-methyl isomer (5a),
Scheme 3.

Synthesis of H-Phosphinates
2931
Scheme 4.
Scheme 5.
[
22]
photolysis of the 1,2,3,4-tetrahydro derivative (9)
methanol was much slower (Scheme 6).
It was interesting to find that P-phenyl 2-phosphabicyclo[3.1.0]hexane
in the presence of
[23]
oxide (10a) could be used as well as dihydrophosphole oxide 5a for the
preparation of H-phoshinate 2a. At the same time, reaction of the P-ethyl
[
15]
phosphabicyclohexane (10g)
From this experiment it followed that the compound of type 10 is of
was reluctant (Scheme 7).
[
24]
significant ring strain, as was also observed earlier.
Finally, we found that 1,2-dihydrophosphinine oxide 11
suitable precursor in the preparation of H-phosphinate 2a (Scheme 8). At
[25]
was also a
[26]
the same time, 1,2,3,4,5,6-hexahydrophosphinine oxide 12
going photolysis in the presence of methanol. This is obviously due to the lack
resisted under-
of ring strain. No fragmentation of the suitable P-cycles (e.g., 5, 7, 8, 10, and
1
1) could be observed in the absence of methanol, confirming the addition–
elimination mechanism involving intermediate 14 with a pentavalent
pentacoordinated phosphorus atom (Scheme 9).
To summarize our results, the fragmentation-related phosphorylations
were extended to simple precursors, such as aryl-2,5-dihydrophosphole
Scheme 6.

2932
H. Szelke, J. Kov a´ cs, and G. Keglevich
Scheme 7.
oxides, 2-phosphabicyclo[3.1.0]hexane 2-oxides, and a 1,2-dihydrophosphinine
oxide. The roles of structure and substitution pattern were evaluated.
EXPERIMENTAL
General
3
1
13
1
The P, C, and H NMR spectra were obtained on a Bruker DRX-500 spec-
trometer operating at 202.4, 125.7, and 500 MHz, respectively. Chemical
shifts are downfield relative to 85% H PO or TMS. The couplings are
3
4
given in Hz. Mass spectrometry was performed on a ZAB-2SEQ instrument.
The starting P-heterocycles (5a–h, 6–12) were synthesized as described
[10–12,14,15,20,22–26]
earlier.
General Procedure for the Fragmentation-Related Phosphorylation
of Methanol using a Variety of P-Heterocycles (5–11)
A solution of 0.52 mmol of the P-hetrocycle (5–11) in 45 ml of acetonitrile
and 4.0 ml of methanol was irradiated by a mercury lamp (125 W) in a
quartz reactor for the time shown in Schemes 2–8. Solvent was evaporated
and the obtained crude product was purified by flash column chromatography
Scheme 8.

Synthesis of H-Phosphinates
2933
Scheme 9.
(
silica gel, 3% methanol in chloroform) to afford H-phosphinates 2a–h
Table 1).
(
ACKNOWLEDGMENT
The authors are grateful for the support from the National Scientific Research
Fund (OTKA No. T 042479).
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