Reactions of elemental phosphorus and phosphines with electrophiles in superbasic systems: XII. Synthesis of unsymmetrical tertiary phosphine oxides from red phosphorus and organyl halides

Article abstract of DOI:10.1023/A:1012349116625

Alkyldibenzyl- and benzyldialkylphosphine oxides were prepared in one stage by direct phosphorylation of a mixture of alkyl bromides and benzyl chloride with red phosphorus under the conditions of phase-transfer catalysis (concentrated aqueous KOH solution-dioxane-benzyltriethylammonium chloride).

Full text of DOI:10.1023/A:1012349116625

Russian Journal of General Chemistry, Vol. 71, No. 5, 2001, pp. 718 720. Translated from Zhurnal Obshchei Khimii, Vol. 71, No. 5, 2001,
pp. 765 767.
Original Russian Text Copyright
2001 by Gusarova, Shaikhudinova, Ivanova, Reutskaya, Albanov, Trofimov.
Reactions of Elemental Phosphorus and Phosphines
with Electrophiles in Superbasic Systems:
XII.1 Synthesis of Unsymmetrical Tertiary Phosphine
Oxides from Red Phosphorus and Organyl Halides
N. K. Gusarova, S. I. Shaikhudinova, N. I. Ivanova, A. M. Reutskaya,
A. I. Albanov, and B. A. Trofimov
Favorskii Irkutsk Institute of Chemistry, Siberian Division, Russian Academy of Sciences, Irkutsk, Russia
Received April 12, 2000
Abstract Alkyldibenzyl- and benzyldialkylphosphine oxides were prepared in one stage by direct phos-
phorylation of a mixture of alkyl bromides and benzyl chloride with red phosphorus under the conditions of
phase-transfer catalysis (concentrated aqueous KOH solution dioxane benzyltriethylammonium chloride).
Red phosphorus reacts with benzyl chloride in
superbasic systems such as alkali metal hydroxide
polar hydroxyl-free solvent (DMSO, hexamethylphos-
phoramide) to form tribenzylphosphine oxide as
the main product (yield up to 65%), and also small
amounts of tribenzylphosphine and tetrabenzylphos-
phonium chloride [2, 3]. Phosphorylation of benzyl
chloride or alkyl bromides with the system red phos-
phorus KOH under the conditions of phase-transfer
catalysis allows preparation of the corresponding
tertiary phosphine oxides in a 50 65% yield [3, 5].
Reactions with alkyl bromides also yield small
amounts of dialkylphosphine oxides and alkyl dial-
kylphosphinates [2, 4].
O
KOH H2O
P + RBr + PhCH2Cl
Ia, Ib II
R3P O + R2PCH2Ph
IIIa, IIIb IVa, IVb
O
+ (PhCH2)2PR + (PhCH2)3P O
Va, Vb VI
R = Et (a), n-Pr (b).
Due to the large difference in boiling points and
solubility, individual phosphine oxides IVa, IVb and
Phosphorylation of alkyl bromides (RBr) and benzyl
chloride with red phosphorus under the conditons of
phase-transfer catalysisa
In this work we studied simultaneous phosphoryla-
tion of alkyl bromides and benzyl chloride with red
phosphorus with the aim of one-stage synthesis of
difficultly accessible unsymmetrical tertiary phosphine
oxides. Previously known methods are multistage and
involve the use of aggressive phosphorus chlorides
[6 8].
Molar ratio Conversion
Yield,c
%
Run
no.
R
RBr :
PhCH2Cl
of PhCH2Cl,b
%
III IV V VI
1
2
3
4
5
Et
Et
Et
n-Pr
n-Pr
2 : 1
3 : 1
4 : 1
3 : 1
1 : 1
72
86
100
84
4
5
6
3
10 13
15 17
14 16
12 16
5
2
3
5
We have found that red phosphorus reacts with
a mixture of ethyl or propyl bromide Ia, Ib with
benzyl chloride II at 65 70 C in a system includ-
ing concentrated KOH solution, dioxane, and phase-
transfer catalyst (benzyltriethylammonium chloride) to
form a mixture of symmetrical and mixed tertiary
phosphine oxides: trialkyl-, benzyldialkyl-, alkyldi-
benzyl-, and tribenzylphosphine oxides III VI. Their
total yield is up to 40% (see table, run nos. 1 4).
d
60
2 14 16
a
0.13 mol of red phosphorus and 0.05 mol of benzyl chloride.
The reaction was carried out at 65 75% C in the system
KOH H O dioxane phase-transfer catalyst (benzyltriethyl-
ammonium bromide). In run no. 2 the conversion of EtBr
was 97%; in run no. 4 the conversion of n-PrBr was 52%;
2
b
in the other experiments the conversion of RBr was not de-
c
termined. Yields of phosphine oxides IV VI are based on
the consumed benzyl chloride. Yield of trialkylphosphine
oxides III is based on the amount of alkyl bromide taken.
Product is not formed.
1
d
For communication XI, see [1].
1070-3632/01/7105-0718$25.00 2001 MAIK Nauka/Interperiodica

REACTIONS OF ELEMENTAL PHOSPHORUS AND PHOSPHINES WITH ELECTROPHILES : XII. 719
Va, Vb can be readily isolated pure. Their yield de-
pends primarily on the ratio of the starting organyl
halides. For example, the main products of phos-
phorylation of equimolar amounts of propyl bromide
and benzyl chloride are tribenzyl- and dibenzylpro-
pylphosphine oxides. Their yields are 16 and 14%,
respectively, while dipropylbenzylphosphine oxide is
formed in a 2% yield, and no tripropylphosphine ox-
ide was found under these conditions (see table, run
no. 5). This fact shows that the rate of the reaction of
red phospohorus with benzyl chloride is higher than
with alkyl bromides. With a 2 3-fold molar excess of
alkyl bromide Ia, Ib relative to benzyl chloride (see
table, run nos. 1, 2, 4) the yield and relative content of
unsymmetrical phosphine oxides IVa, IVb and Va,
Vb in the reaction mixture are increased. These
compounds were isolated pure in a 15 17% yield.
The yield of symmetrical phosphine oxides is low
(2 6%), and the conversion of organyl halides Ia
(or Ib) and II is 52 97% and 72 86%, respectively.
Further increase in the molar ratio EtBr : PhCH2Cl to
4 : 1 is not appropriate, because it practically does not
affect the yield and ratio of phosphine oxides III VI
(see table, run no. 3).
NMR spectra were recorded on a Bruker DPX-400
spectrometer, and the 31P NMR spectra, on a Jeol
FX-90Q spectrometer. The NMR spectra were ob-
tained in CDCl3 against internal HMDS and external
85% phosphoric acid. The GLC analysis was carried
out on a Chrom-4 chromatograph (2400 3-mm
column, 1% polyethylene glycol 20 000 on NaCl,
thermal conductivity detector, helium carrier gas).
Symmetrical phosphine oxides IIIa, IIIb, and VI
were identified spectroscopically using the authentic
samples [2].
Benzyldiethylphosphine oxide IVa and diben-
zylethylphosphine oxide Va (see table, run no. 2).
A solution of 40.0 g of KOH in 27 ml of water was
added dropwise with stirring to a mixture of 4.0 g of
red phosphorus, 16.4 g of ethyl bromide, 6.3 g of
benzyl chloride, and 0.1 g of benzyltriethylammonium
chloride in 40 ml of dioxane. The reaction mixture
was heated at 65 75 C for 4 h and cooled to room
temperature; the organic layer was separated, dried
over potassium carbonate, and fractionated. A fraction
with bp 50 100 C was isolated at atmospheric pres-
sure (8.4 g). According to GLC data, it contained
0.4 g of ethyl bromide (conversion 97%), 1.4 g of
ethanol (20% yield), and 6.6 g of dioxane. The sec-
ond fraction with bp 105 115 C consisted of dioxane.
The residue containing phosphine oxides IIIa Va and
VI in a 20 : 49 : 26 : 5 molar ratio (31P NMR data)
was fractionated in a vacuum. Benzyl chloride, 0.87 g
(conversion 86%), triethylphosphine oxide, 0.35 g
(5%), and 1.3 g (15%) of phosphine oxide IVa were
isolated. Data for IVa: bp 166 170 C/1 mm Hg; pub-
lished data [6]: 148 149 C/0.5 mm Hg. IR spectrum:
Note that under the examined conditions the con-
version of alkyl bromides I is higher than the yield
of the final products III V. This means that alkyl
bromides are evidently involved not only in phos-
phorylation but also in some other transformations,
e.g., in base-catalyzed dehydrobromination or hy-
drolysis. Indeed, in run no. 2 (see table) formation of
ethanol was observed. Furthermore, in the case of
phosphorylation of butyl bromide with red phosphorus
under the conditions of phase-transfer catalysis [2] we
have previously observed the formation of 1-butene
(identified by gas chromatography and mass spec-
trometry) along with tributylphosphine oxide.
1
(P=O) 1160 cm . 1H NMR spectrum, , ppm:
3
1.15 d.t (6H, CH3, JHP 16.4 Hz), 1.65 d.q (4H, CH2,
2
2JPH 11.3 Hz), 3.06 d (2H, CH2, JPH 14.3 Hz),
1
In the H NMR spectra of alkyldibenzylphosphine
7.24 m (5H, C6H5). 31P NMR spectrum: P 48.3 ppm.
Found, %: C 67.37; H 8.74; P 15.60. C11H17OP. Cal-
culated, %: C 67.33; H 8.73; P 15.78.
oxides the signal of methylene protons in the
PhCH2P=O fragment is a multiplet consisting of 6
lines. This is due to the fact that the protons in each
CH2 group are diastereotopic and give an AB quartet:
JAB 14.9 (Va), 14.7 Hz (Vb). Each component of this
system is also split owing to the coupling with phos-
phorus in a doublet with the close values of the cou-
pling constant: JHP 14.9 (Va), 14.7 Hz (Vb). The pro-
tons in two CH2 groups are mutually equivalent. In
The bottom residue was dissolved in 15 ml of ether
and filtered to remove the crystalline residue A; the
ether was removed, and the residue was washed with
hexane. Phosphine oxide Va, 0.96 g (17%), was ob-
1
tained, mp 64 66 C. IR spectrum: (P=O) 1180 cm .
1H NMR spectrum, , ppm: 1.06 d.t (3H, CH3,
1
the 31P-decoupled H NMR spectra of these phos-
2
3JPH 16.7 Hz), 1.56 d.q (2H, CH2, JPH 11.6 Hz),
phine oxides, the multiplet of the CH2 group trans-
forms into an AB quartet.
2
3.07 m (4H, CH2, JPH = JAB = 14.9 Hz), 7.24 m
(10H, C6H5). 31P NMR spectrum:
44.3 ppm.
Found, %: C 73.92; H 7.54; P 10.54. C1P6H19OP. Cal-
culated, %: C 74.42; H 7.36; P 12.02. The crystalline
residue A was washed with ether and dried in a vac-
uum to give 0.1 g (2%) of tribenzylphosphine oxide.
EXPERIMENTAL
The IR spectra were registered on a Specord IR-75
1
spectrometer in KBr pellets or in thin layer. The H
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 5 2001

720
GUSAROVA et al.
NMR spectrum:
Benzyldipropylphosphine oxide IVb and diben-
42.8 ppm. Found, %: C 75.97;
P
zylpropylphosphine oxide Vb (see table, run no. 4).
A solution of 40.0 g of KOH in 27 ml of water was
added dropwise with stirring to a mixture of 4.0 g of
red phosphorus, 18.45 g of propyl bromide, 6.3 g of
benzyl chloride, and 0.1 g of benzyltriethylammonium
chloride. The reaction mixture was kept for 5 h at
70 75 C, cooled to room temperature, and the organic
layer was separated. Propyl bromide (8.8 g, 52% con-
version) and dioxane were distilled off, and the re-
sidue containing phosphine oxides IIIb Vb and VI
in a ratio of 20 : 48 : 30 : 2, respectively (31P NMR
data), was fractionated in a vacuum. Benzyl chloride,
1.0 g (84% conversion), tripropylphosphine oxide,
0.3 g (3%), and phosphine oxide IVb, 1.1 g (12%,
bp 167 171 C/1.5 mm Hg) were obtained. IR spec-
H 7.98; P 11.02. C17H21OP. Calculated, %: C 75.00;
H 7.72; P 11.40. The crystalline precipitate B was
washed with ether and dried in a vacuum to give 0.2 g
(4%) of tribenzylphosphine oxide.
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1
1
trum of IVb: (P=O) 1170 cm . H NMR spectrum,
, ppm: 0.98 m (6H, CH3), 1.58 m (8H, CH2), 3.08 d
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2
(2H, CH2, JPH 14.4 Hz), 7.24 m (5H, C6H5).
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H 9.37; P 13.84.
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1
(P=O) 1180 cm . 1H NMR spectrum, , ppm:
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2
CH2, JPH = JAB = 14.7 Hz), 7.24 (10H, C6H5). 31P
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 5 2001