A study on the reduction of 4-chloro-1,2-dihydrophosphinine oxides by transfer hydrogenation

Article abstract of DOI:10.1080/10426507.2011.608100

Under the conditions of transfer hydrogenation by ammonium formate, the title compounds (6) were converted to a mixture of the corresponding 1,2,3,6-tetrahyidrophosphinine oxide (7), its dechlorinated derivative (8) and the respective 1,2,3,4,5,6-hexahydrophosphinine oxide (9). In the phenyl-substituted instance, the ratios of the components depended on the mode of heating (traditional heating or microwave irradiation), temperature, and the solvent (toluene or [bmim][BF₄]) used. In the ethoxy-substituted series, the dechlorinated 1,2-dihydrophosphinine oxide (10b) was detected as an intermediate. Copyright Taylor and Francis Group, LLC.

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A Study on the Reduction of 4-
Chloro-1,2-Dihydrophosphinine Oxides
by Transfer Hydrogenation
Rita Kovács a , György T. Balogh b , Krisztina Ludányi c , László
Drahos d & György Keglevich a
a Department of Organic Chemistry and Technology , Budapest
University of Technology and Economics , Budapest , Hungary
b Gedeon Richter Plc. , Budapest , Hungary
c Department of Pharmaceutics, Faculty of Pharmacy , Semmelweis
University , Budapest , Hungary
d Hungarian Academy of Sciences , Chemical Research Center ,
Budapest , Hungary
Published online: 06 Dec 2011.
To cite this article: Rita Kovács , György T. Balogh , Krisztina Ludányi , László Drahos & György
Keglevich (2012) A Study on the Reduction of 4-Chloro-1,2-Dihydrophosphinine Oxides by Transfer
Hydrogenation, Phosphorus, Sulfur, and Silicon and the Related Elements, 187:1, 121-127, DOI:
10.1080/10426507.2011.608100
To link to this article: http://dx.doi.org/10.1080/10426507.2011.608100
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Phosphorus, Sulfur, and Silicon, 187:121–127, 2012
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426507.2011.608100
A STUDY ON THE REDUCTION OF 4-CHLORO-1,2-
DIHYDROPHOSPHININE OXIDES BY TRANSFER
HYDROGENATION
Rita Kova´cs,1 Gyo¨rgy T. Balogh,2 Krisztina Luda´nyi,3
La´szlo´ Drahos,4 and Gyo¨rgy Keglevich1
1Department of Organic Chemistry and Technology, Budapest University
of Technology and Economics, Budapest, Hungary
2Gedeon Richter Plc., Budapest, Hungary
3Department of Pharmaceutics, Faculty of Pharmacy, Semmelweis University,
Budapest, Hungary
4Hungarian Academy of Sciences, Chemical Research Center, Budapest, Hungary
GRAPHICAL ABSTRACT
Abstract Under the conditions of transfer hydrogenation by ammonium formate, the title com-
pounds (6) were converted to a mixture of the corresponding 1,2,3,6-tetrahyidrophosphinine
oxide (7), its dechlorinated derivative (8) and the respective 1,2,3,4,5,6-hexahydrophosphinine
oxide (9). In the phenyl-substituted instance, the ratios of the components depended on the
mode of heating (traditional heating or microwave irradiation), temperature, and the sol-
vent (toluene or [bmim][BF4]) used. In the ethoxy-substituted series, the dechlorinated 1,2-
dihydrophosphinine oxide (10b) was detected as an intermediate.
Keywords
Transfer hydrogenation; 1,2-dihydrophosphinine oxide; 1,2,3,6-
tetrahydrophosphinine oxide; 1,2,3,4,5,6-hexahydrophosphinine oxide; dechlorination;
intermediate
INTRODUCTION
Six-membered P-heterocycles called phosphinine derivatives form a representative
group of P-heterocycles.1–3 An attractive approach toward dihydro-, tetrahydro-, and hex-
ahydrophosphinine oxides is based on ring enlargement of easily available 3-phospholene
Received 20 June 2011; accepted 21 July 2011.
The authors are grateful for the OTKA (K83118) support of this work. This work is connected to the
scientific program of the “Development of quality-oriented and harmonized R+D+I strategy and functional
´
model at BME” project. This project is supported by the New Sze´chenyi Plan (Project ID: TAMOP-4.2.1/B-
09/1/KMR-2010-0002).
Address correspondence to Gyo¨rgy Keglevich, Department of Organic Chemistry and Technology, Budapest
University of Technology and Economics, 1521 Budapest, Hungary. E-mail: gkeglevich@mail.bme.hu
121

´
R. KOVACS ET AL.
122
oxides and their subsequent transformations. In the first step, dichlorocarbene is added to the
double-bond of 3-phospholene oxide (1) to afford 3-phosphabicyclo[3.1.0]hexane 3-oxide
(2). Then the cyclopropane ring of the dichlorocarbene adduct 2 is opened up on heating,
resulting in the formation of 1,2-dihydrophosphinine oxide 3 as a mixture of two double-
bond isomers.4,5 Selective reduction of the α,β-double-bond of 1,2-dihydrophosphinine
oxide 3 by dimethyl sulfide-borane led to 1,2,3,6-tetrahydrophosphinine oxides 4,6
while catalytic hydrogenation of 1,2-dihydrophosphinine oxides 3, or even that of
3-phosphabicyclo[3.1.0]hexane oxides 2 provided 1,2,3,4,5,6-hexahydrophosphinine
oxides 5 (Scheme 1).7–9
Scheme 14–8
Catalytic transfer hydrogenation is a useful synthetic method for the reduction of
organic compounds.10,11 Recently, transfer hydrogenations applying ammonium formate
derivatives as the hydrogen source have been carried out in ionic liquids (ILs).12–17 Sub-
strates with C C and C N groups, as well as the carbonyl group of ketones, were reduced
in ILs (e.g. in [bmim][PF6]) in the presence of Pd/C or ruthenium complex catalysts.12–14
The unsaturation of cinnamic acid derivatives was reduced in a few imidazolium ILs using
Pd(OAc)2, Pd/MgLa mixed oxide or Pd/MgAl hydrotalcite catalysts.15,16 The outcome of
the rhodium catalyzed transfer hydrogenation of α,β-unsaturated ketones depended on the
nature of the solvent used. In ILs only the unsaturation was reduced, but in conventional
solvents the carbonyl group was reduced in addition.17
In this paper, the transfer hydrogenation of 1,2-dihydrophosphinine oxides under dif-
ferent conditions, including the use of ILs and/or microwave (MW) irradiation, is discussed.

TRANSFER HYDROGENATION OF 1,2-DIHYDROPHOSPHININE OXIDES
123
RESULTS AND DISCUSSION
In the first approach, 1-phenyl-1,2-dihydrophosphinine oxide 6a was reduced with a
20-fold quantity of ammonium formate in the presence of PdCl2 as the catalyst in different
solvents (Scheme 2). After a 4 h reflux in toluene, the crude mixture contained 22% of
4-chloro-1,2,3,6-tetrahydrophosphinine oxide 7a,6 31% of 1,2,3,6-tetrahydrophosphinine
oxide 8a9, and 47% of 1,2,3,4,5,6-hexahydrophosphinine oxide 9a8 (Table 1/Entry 1).
The components were identified after analyzing the mixture by 31P NMR and mass spec-
trometry, along with LC-MS liquid chromatography-mass spectrometry. Repeated column
chromatography led to a 55%:45% mixture of hexahydrophosphinine oxide 9a and tetrahy-
drophosphinine oxide 8a. The 13C NMR spectrum of this fraction confirmed the presence
of components 8a9 and 9a.8 It is noteworthy that the hexahydrophosphinine oxide (9a) was
formed as only one diastereomer, meaning that its formation by transfer hydrogenation was
diastereoselective. The earlier described catalytic hydrogenation of dihydrophosphinine ox-
ide 6a afforded hexahydrophosphinine oxide 9a as a 77:23 mixture of two diastereomers.8
Scheme 2
Carrying out the reduction in toluene, but under MW conditions, the dechlorinated
tetrahydrophosphinine oxide (8a) was the major (79%) P-heterocycle formed along with
11% of 7a and 5% of 9a. There were also a few unidentified byproducts in a small quantity
(∼5%) in the reaction mixture (Table 1/Entry 2). By replacing toluene with [bmim][BF4],
the relative quantity of the chloro-tetrahydrophosphinine oxide (7a) decreased, but that
of the dehalogenated tetrahydrophosphi◦nine oxide (8a), or the hexahydrophosphinine ox-
ide (9a) increased. On heating at 110 C for 3 h, only 10% of compound 7a and 41%
Table 1 Transfer hydrogenation of 1,2-dihydrophosphinine oxides 6 under different conditions
Composition (%)a
Mode of
heating
Entry
Y
Solvent
PhMe
T (◦C)
t (h)
7
8
9
Other
1
2
3
4
5
6
7
Ph (a)
Ph (a)
Ph (a)
Ph (a)
Ph (a)
Ph (a)
EtO (b)
ꢀ
110
110
110
110
78
4
2
3
2
4
22
11
10
2
9
3
31
79
41
59
44
45
36
47
5
MW
ꢀ
PhMe
5
[bmim][BF4]
[bmim][BF4]
PhH
[bmim][BF4]
PhMe
49
39
47
52
54
MW
ꢀ
ꢀ
130
110
1.5
24
ꢀ
10
aDetermined on the basis of relative 31P NMR intensities.

´
R. KOVACS ET AL.
124
of its dechlorinated form (8a) were formed (Table 1/Entry 3), while MW irradiation at
◦
110 C for 2 h resulted in, 2% of 7a and 39% of hexahydrophosphinine oxide 9a. In the
thermal experiment, the hexahydrophosphinine oxide 9a (49%) dominated, while in the
MW variation the dehalogenated tetrahydrophosphinine oxide 8a (59%) was the major
product (Table 1/Entry 4). Hence, under MW irradiation it was possible to carry out the
reaction so that the dechlorinated tetrahydrophosphinine oxide (8a) was formed as the ma-
jor component (Table 1/Entries 2 and 4). If the dechlorinated tetrahydrophosphinine oxide
(8a) is the target compound, whose preparation is not easy by the other method described,9
the transfer hydrogenation should be carried out under MW irradiation using toluene as
the solvent. Tetrahydrophosphinine oxide 8a may be a valuable intermediate in further
syntheses to be explored in due course. In boiling benzene, the reduction was not more
selective for the formation of the chloro-tetrahydrophosphinine oxide (7a) (Table 1/Entry
5). In IL at 130 ◦C, the reduction did not proceed to afford the hexahydrophosphinine oxide
9a as an almost exclusive product (Table 1/Entry 6). It can be seen that variation of the
temperature in the range of 80 ◦C –100 ◦C does not have much impact on the course of the
transfer hydrogenation.
It is also noteworthy that a large excess, namely a 20-fold molar equivalent of the
ammonium formate, must be used, otherwise, the conversion of the starting material (6a) to
any of the products (7a, 8a, or 9a) is incomplete. In the dihydrophosphinine oxide 6a, there
are three functions that may be reduced, and the reaction sequence could not be influenced
to take place with an exclusive selectivity toward any of the products. There is also a need
to use the PdCl2 catalyst in a quantity of 50%. In the experiments, where only 10% of the
catalyst was used, there was practically no reduction.
The 1-ethoxy-1,2-dihydrophosphinine oxide (6b) was also subjected to transfer hy-
drogenation applying ammonium formate and PdCl2 in a the similar reaction of the phenyl
substituted starting material (6a) and using toluene as the solvent at the boiling point. After
a 24 h of stirring at reflux, a mixture of 4-chloro-1,2,3,6-tetrahydrophosphinine oxide 7b6
(10%), its dechlorinated form 8b (36%) and the 1,2,3,4,5,6-hexahydrophosphinine oxide
9b8 (54%) was obtained (Scheme 2, Table 1/Entry 7). Composition of the mixture was
analyzed by 31P NMR, mass spectrometry and LC-MS after flash column chromatography.
The 13C NMR spectrum obtained on this mixture was also informative as the 13C NMR
data of compounds 7b6 and 9b8 were known.6,8 Repeated column chromatography afforded
tetrahydrophosphinine oxide 8b in a yield of 16% and in a purity of 96%. The structure of
this new compound was proved by 31P, 13C, and 1H NMR, as well as MS methods.
We observed that varying the reaction conditions for the transfer hydrogenation of
dihydrophosphinine oxide 6b did not influence the outcome of the reaction to a significant
extent.
Tetrahydrophosphinine oxides 7 and 8, as well as the dechlorinated 1,2-
dihydrophosphinine oxide (10), may be intermediates toward the fully saturated
hexahydrophosphinine oxide (9) (Scheme 3). Intermediate 10 could only be detected in the
Scheme 3

TRANSFER HYDROGENATION OF 1,2-DIHYDROPHOSPHININE OXIDES
125
ethoxy-substituted model (Y = EtO, 10b) after interrupting the reaction before completion
and analyzing the mixture by 31P NMR (δP (CDCl3) 36.2) and MS ([M+H]+
=
found
173.0733, C8H14O2P requires 173.0731).
In summary, it was found that the transfer hydrogenation of 4-chloro-
1,2-dihydrophosphinine oxides led to a mixture of three products, a chloro-
tetrahydrophosphinine oxide, its dechlorinated form, and the fully saturated hex-
ahydrophosphinine oxide formed diastereoselectively. In the phenyl-substituted case, the
relative proportion of the components depended on the conditions applied. Choosing
appropriate conditions (MW/PhMe/110 ◦C/2 h), the dechlorinated 1-phenyl-1,2,3,6-
tetrahydrophosphinine oxide (8a) could be obtained in a selectivity of 79%. In the
ethoxy-substituted series, the dechlorinated dihydrophosphinine oxide could also be
detected as an intermediate.
EXPERIMENTAL
The 31P and 13C NMR spectra were recorded on a Bruker DRX-500 spectrometer
operating at 202.4 and 125.7 MHz, respectively. The couplings are given in Hz. Mass
spectrometry was performed on a ZAB-2SEQ instrument.
LC-MS experiments were carried out on an Agilent 1200 liquid chromatography
system coupled with a 6120 MSD (Agilent Technologies, Palo Alto, CA, USA). Analysis
was at 40 ◦C on an Kinetex C18 column (5 cm × 2.1 mm, 2.6 µm) (Phenomenex, Torrance,
CA, USA) with a mobile phase flow rate of 0.9 mL/min. Composition of eluent A was 0.1%
(v/v) trifluoroacetic acid in water (pH 1.9), and eluent B was the mixture of acetonitrile and
water in 95:5 (v/v) with 0.1% (v/v) trifluoroacetic acid. A fast linear gradient of 0%–100%
B was applied at a range of 0–4 min, then 100% B held for 3 min. The injection volume
was set at 1 µL, and the sample concentration was uniformly 1.0 mg/mL (using dimethyl
sulfide as the solvent). The chromatographic profile was registered at 240 nm. The Mass
spectrometry defector operating parameters were as follows: positive ionization mode, scan
spectra from m/z 100 to 800, drying gas temperature 350 ◦C, nitrogen flow rate 12 L/min,
◦
nebuliser pressure 60 psi, quadrupole temperature 100 C, capillary voltage 3000 V, and
fragmentor voltage 50 V.
The MW experiments were carried out in a CEM Discover 300 W reactor equipped
with a pressure control system.
The starting 1,2-dihydrophosphinine oxides (6a and 6b) were prepared by our meth-
ods described earlier.18–20
A Typical Procedure for the Transfer Hydrogenation of
1-Phenyl-4-Chloro-3-Methyl-1,2-Dihydrophosphinine Oxide 6a
(Table 1/Entry 1)
To a mixture of 0.15 g (0.63 mmol) of dihydrophosphinine oxide 6a and 60 mg
(0.34 mmol) of PdCl2 in dry toluene (10 mL) was added 0.80 g (12.7 mmol) of ammonium
formate, and the contents of the flask were stirred at the boiling point for 4 h. After
cooling to room temperature, the solids were removed by filtration, and the solvent was
evaporated. The crude product obtained was passed through a 10 cm layer of silica gel
using 3% methanol in chloroform as the eluant. This mixture consisted of 22% of 4-chloro-
tetrahydrophosphinine oxide 7a (31P NMR (CDCl3) δ 27.7, δP 28.2; [M+H]+
=
6
found
241.0552, C12H1535ClOP requires 241.0549), 31% of tetrahydrophosphinine oxide 8a, and
47% of hexahydrophosphinine oxide 9a. Further chromatography (as described above, but

´
R. KOVACS ET AL.
126
on a longer column) of this fraction provided a 55:45 mixture of products 9a and 8a as an
oil.
8a: 31P NMR (CDCl3) δ 29.8, δP9 30.5; 13C NMR (CDCl3) δ 22.5 (J = 5.3, C3), 24.2
(J = 67.4, C6), 26.1 (J = 11.1, Me), 31.9 (J = 66.2, C2), 122.9 (J = 12.6, C4), (δC9 22.1
(J = 4.9, C3), 23.8 (J = 67.4, C6), 25.7 (J = 11.1, Me), 31.4 (J = 66.2, C2), 122.5 (J = 12.6,
C4)); 1H NMR (CDCl3) δ 1.76 (s, 3H, Me), 5.59 (broad signal, 1H, CH=); [M+H]+
=
found
207.0938, C12H16OP requires 207.0939.
9a: 31P NMR (CDCl3) δ 32.7, δP8 33.3; 13C NMR (CDCl3) δ 20.9 (J = 5.7, C5), 24.7
(J = 14.9, Me), 27.4 (J = 66.3, C6), 28.5 (J = 4.7, C3), 35.5 (J = 5.1, C4), 36.6 (J =
64.8, C2), (δC8 22.3 (J = 3.7, C5), 24.1 (J = 14.7, Me), 26.8 (J = 65.3, C6), 31.1 (J = 3.6,
C3), 34.5 (J = 5.2, C4), 35.7 (J = 63.3, C2)); 1H NMR (CDCl3) δ 1.07 (dd, J1 = 6.6, J2 =
2.4, 3H, Me), δH8 1.08 (dd, J = 6, 3H, Me); [M+H]+found = 209.1090, C12H18OP requires
209.1095.
A Typical Procedure for the Transfer Hydrogenation of
1-Ethoxy-4-Chloro-3-Methyl-1,2-Dihydrophosphinine Oxide (6b)
The reaction was carried out as that described for the phenyl substituted starting
material 6a. 0.38 g (1.84 mmol) of 6b and 2.3 g (36.8 mmol) of ammonium formate were
reacted in toluene (25 mL), in the presence of 0.16 g (0.92 mmol) of PdCl2 by stirring the
mixture at the boiling point for 24 h. Flash column chromatography (as above) furnished
a mixture of 10% of 4-chloro-1,2,3,6-tetrahydrophosphinine oxide 7b, 36% of 1,2,3,6-
tetrahydrophosphinine oxide 8b, and 54% of 1,2,3,4,5,6-hexahydrophosphinine oxide 9b
as an oil. Repeated column chromatography gave compound 8b in a yield of 16% in a
purity of 96%.
7b: 31P NMR (CDCl3) δ 45.1, δP6 45.6; 13C NMR (CDCl3) δ 16.4 (J = 3.1, CH2CH3),
22.8 (3J = 11.5, C5–Me), 23.8 (1J = 88.6, C6), 31.4 (1J = 86.6, C2), 32.0 (2J = 6.3, C3),
59.7 (2J = 5.4, CH2CH3), 124.4 (3J = 4.9, C5), 126.3 (J = 13.3, C4), (δC6 16.6 (CH2CH3),
23.2 (3J = 11.0, C5–Me), 24.5 (1J = 89.1, C6), 31.6 (1J = 89.7, C2), 32.3 (2J = 4.9, C3),
60.6 (2J = 5.4, CH2CH3), 124.8 (C5), 126.6 (3J = 12.9, C4)); [M+H]+
= 209.0509,
found
C8H1535ClO2P requires 209.0498.
8b: 31P NMR (CDCl3) δ 48.7; 13C NMR (CDCl3) δ 16.4 (J = 3.9, CH2CH3), 21.9
(J = 89.5, C6), 23.2 (J = 5.9, C3), 25.7 (J = 12.9, C5–Me), 30.2 (J = 88.3, C2), 59.7 (J =
3.3, CH2CH3), 121.7 (J = 13.9, C4), 128.9 (J = 4.7, C5); 1H NMR (CDCl3) δ 1.34 (t, J =
7.0 CH2CH3), 1.75 (s, C5–Me), 4.03–4.17 (m, CH2CH3), 5.26 (s, C4H); [M+H]+
=
found
175.0886, C8H16O2P requires 175.0888.
9b8: 31P NMR (CDCl3) δ 51.8, δP8 50.7; 13C NMR (CDCl3) δ 16.4 (J = 5.5, CH2CH3),
21.6 (J = 3.7, C5), 23.8 (J = 16.3, C3–Me), 26.1 (J = 86.5, C6), 30.2 (J = 3.1, C3), 34.7
(J = 6.2, C4), 35.0 (J = 83.9, C2), 59.7 (J = 6.2, CH2CH3), (δC8 15.5 (J = 5.9, CH2CH3),
21.8 (J = 5.2, C5), 23.1 (J = 16.1, C3–Me), 25.0 (J = 86.4, C6), 30.3 (J = 5.1, C3), 33.7
(J = 6.5, C4), 34.3 (J = 84.3, C2), 58.2 (J = 5.8, CH2CH3)); [M+H]+
= 177.1042,
found
C8H18O2P requires 177.1044.
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