Reactions of elemental phosphorus and phosphine with electrophiles in superbasic systems: XVI. Phosphorylation of benzyl chloride with elemental phosphorus and phosphine

Article abstract of DOI:10.1007/s11176-005-0298-7

The major product of the reaction of benzyl chloride with red phosphorus in the system concentrated aqueous KOH-dioxane-phase-transfer catalyst (43-95°C, Ar) is tribenzylphosphine oxide (yield up to 61%). Under similar conditions, phosphorylation of benzy

Full text of DOI:10.1007/s11176-005-0298-7

Russian Journal of General Chemistry, Vol. 75, No. 5, 2005, pp. 684 688. Translated from Zhurnal Obshchei Khimii, Vol. 75, No. 5, 2005,
pp. 724 728.
Original Russian Text Copyright
2005 by Trofimov, Gusarova, Malysheva, Shaikhudinova, Belogorlova, Kazantseva, Sukhov, Plotnikova.
Reactions of Elemental Phosphorus and Phosphine
with Electrophiles in Superbasic Systems:
XVI.1 Phosphorylation of Benzyl Chloride
with Elemental Phosphorus and Phosphine
B. A. Trofimov*, N. K. Gusarova*, S. F. Malysheva*, S. I. Shaikhudinova**,
N. A. Belogorlova*, T. I. Kazantseva**, B. G. Sukhov*, and G. V. Plotnikova*
* Favorskii Irkutsk Institute of Chemistry, Siberian Division, Russian Academy of Sciences, Irkutsk, Russia
** Irkutsk State University, Irkutsk, Russia
Received November 28, 2003
Abstract The major product of the reaction of benzyl chloride with red phosphorus in the system con-
centrated aqueous KOH dioxane phase-transfer catalyst (43 95 C, Ar) is tribenzylphosphine oxide (yield up
to 61%). Under similar conditions, phosphorylation of benzyl chloride with white phosphorus occurs dif-
ferently, yielding dibenzylphosphine oxide as major product. Conditions are found for preparative synthesis
of dibenzylphosphine from phosphine and benzyl chloride in the system KOH DMSO.
It was reported previously that red phosphorus
reacts with benzyl chloride in superbasic media of the
type alkali metal hydroxide nonhydroxyl solvent
conditions (90 95 C, 3 h, molar ratio red phosphorus:
benzyl chloride = 5 : 1, system concentrated aqueous
KOH dioxane benzyltriethylammonium
chloride).
(DMSO) or under conditions of phase-transfer catal- Dibenzylphosphine oxide II was isolated as by-prod-
ysis at elevated temperatures (75 100 C) to give as
major product tribenzylphosphine oxide (yield up to
30%; here and hereinafter, yield based on the amount
of benzyl chloride taken) and also minor amounts
of tribenzylphosphine and tetrabenzylphosphonium
chloride [2, 3]. Phosphorylation of benzyl chloride
with the system PH3 KOH DMSO H2O at elevated
pressure yields benzylphosphine [4], whereas the same
reaction performed at atmospheric pressure gave di-
benzylphosphine oxide in 50% yield, with dibenzyl-
phosphine detected as intermediate by 31P NMR [5].
uct (yield 12%, see table, run no. 1). The conversion
of red phosphorus in the process is quantitative.
H
KOH/H2O
Pn + PhCH2Cl
(PhCH2)3P=O + (PhCH2)2P
(1)
dioxane
O
I
II
When benzyl chloride is phosphorylated with the
same agents at a lower temperature (at 60 65 C and
especially at 43 45 C), the conversion of red phos-
phorus noticeably decreases (to 63 and 37%, respec-
tively); the yield of the reaction products decreased
also. In these experiments, along with phosphine
oxides I and II, we also isolated benzylphosphinic
acid (by treatment of the aqueous layer with HCl; see
table, run nos. 2 and 3).
To extend the possibilities of preparing organo-
phosphorus compounds from elemental phosphorus
and benzyl chloride and to obtain additional informa-
tion on the comparative reactivity of red and white
phosphorus in reactions with electrophiles (these
reactions attract growing researchers’ attention as a
new convenient route to organic phosphines and phos-
phine oxides [6 11]), we continued studying phos-
phorylation of benzyl chloride with elemental phos-
phorus and phosphine in the presence of a strong base.
With white phosphorus, as expected [10, 11], ben-
zyl chloride reacts more actively than with red phos-
phorus: The conversion of white phosphorus and the
total yield of phosphine oxides I and II and of potas-
sium salts of benzylphosphonic and dibenzylphos-
phinic acids (identified in the form of the correspond-
ing acids III and IV) are virtually quantitative, even
at mild reaction conditions (43 45 and 60 65 C).
The major product of phosphorylation of benzyl chlo-
ride with white phosphorus is secondary phosphine
We found that tribenzylphosphine oxide I can be
prepared in a yield of up to 61% under relatively mild
1
For communication XV, see [1].
1070-3632/05/7505-0684 2005 Pleiades Publishing, Inc.

REACTIONS OF ELEMENTAL PHOSPHORUS AND PHOSPHINE... : XVI.
685
Phosphorylation of benzyl chloride with elemental phosphorus under the conditions of phase-transfer catalysisa
Yield of compounds, %b
Run
Elemental
PhCH2Cl, Temperature,
Phosphorus
no. phosphorus, mmol
mmol
C
conversion, %
I
II
IIIc
IVc
d
d
d
d
1
2
3
Pn, 100
Pn, 33
Pn, 33
P4, 32.5
P4, 36
P4, 31.7
P4, 29
20
90 95
60 65
43 45
90 95
60 65
43 45
60 65
61
46
29
23
35
12
12
1
36
48
46
39
100
63
37
100
100
100
100
6.6
6.5
6.5
7.2
6.3
6.5
11
3
6
10
24
17
4e
5e
6e
7e,f
34
3
2
27
Traces
12
a
f
In run no. 1, we took 625 mmol of KOH, 15 ml of H O, 0.5 g of benzyltriethylammonium chloride, and 40 ml of dioxane; in
2
run nos. 2 7, 208 mmol of KOH, 5 ml of H O, 0.17 g of benzyltriethylammonium chloride, and 20 ml of dioxane; heating time
2
b
c
3 h. Based on the amount of benzyl chloride taken. Acids III and IV were isolated from the reaction mixture after acidification
of the aqueous layer with HCl. Is not formed. In run nos. 4 7, the product yields were calculated from the P NMR spectra.
Benzene (20 ml) was taken instead of dioxane.
d
e
31
oxide II (see table, run nos. 4 6). Similar results were actions of red and white phosphorus with 4-methoxy-
obtained previously in a comparative study of the re-
benzyl chloride [12].
O
O
KOH/H2O
P4 + PhCH2Cl
I + II + PhCH2P(OK)2 + (PhCH2)2POK
dioxane
(2)
HCl/H2O
O
O
PhCH2P(OH)2 + (PhCH2)2POH
III IV
When reaction (2) is performed in benzene (one of
the best solvents for white phosphorus), the phosphor-
ylation selectivity increases. Under these conditions,
virtually no tertiary phosphine oxide I is formed, but
the total yield of II IV somewhat decreases (to 68%;
see table, run no. 7, cf. run no. 5).
phosphorylation of organyl halides [1, 2, 12] and
weakly electrophilic alkenes [6, 8, 13].
The predominant formation of secondary phosphine
oxide II in reaction (2) may be due to easier cleavage
of the P P bonds in a strained tetrahedral molecule of
white phosphorus, compared to a macromolecule of
red phosphorus, and hence to higher concentration of
polyphosphinite and phosphinte ions in the case of
phosphorylation of benzyl chloride with white phos-
phorus. This conclusion is also consistent with exclu-
sive formation of the mono- and diphosphorylation
products in the reaction of benzyl chloride with white
phosphorus in benzene, i.e., a higher concentration of
white phosphorus in this solvent also promotes faster
cleavage of the P P bonds and results in higher con-
centration of phosphinite anions.
Despite the fact that all the steps of experiments on
phosphorylation of benzyl chloride with elemental
phosphorus were performed in an inert atmosphere
(Ar), the major products were four-coordinate phos-
phorus compounds: phosphine oxides I and II and
potassium salts of benzyl- and dibenzylphosphinic
acids. The corresponding benzyl-, dibenzyl-, and tri-
benzylphosphines were detected in the reaction mix-
tures by 31P NMR, but as minor products. This fact
suggests that reactions (1) and (2) mainly involve not
polyphosphide but polyphosphinite anions generated
from elemental phosphorus in the presence of a base,
by analogy with the previously reported schemes of
Phosphine generated together with hydrogen from
red phosphorus and KOH in the water toluene solvent
system does not noticeably react with benzyl chloride
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 75 No. 5 2005

686
TROFIMOV et al.
under the conditions of reactions (1) and (2). At the
same time, phosphorylation of benzyl chloride with
the system phosphine KOH DMSO in the presence
of a small amount of water allows, under appropriate
conditions, preparation of dibenzylphosphine V and
dibenzylphosphine oxide II in 32 and 22% yields,
respectively. The reaction is performed by vigorously
passing phosphine through a heated (40 45 C) KOH
DMSO H2O suspension with simultaneous slow
addition of benzyl chloride. Phosphine V was isolated
from the reaction mixture by extraction with hexane
followed by vacuum fractionation:
were dried over potassium carbonate, and the solvent
was removed. The 31P NMR spectrum of the residue
(1.42 g) contained the signals of I (40.1 ppm) and II
1
(35.5 ppm, JPH 468 Hz) in 91 : 9 ratio. The product
was washed with benzene (3 2 ml), and the residue
was reprecipitated from chloroform into ether and
vacuum-dried to obtain 1.26 g of phosphine oxide I,
mp 216 217 C (mp 212 213 C [2]). IR spectrum, ,
1
1
cm : 1190 (P=O). H NMR spectrum (CDCl3),
,
2
ppm: 3.03 d (6H, CH2, JPH 13.6 Hz), 7.27 m (15H,
Ph). Found, %: C 78.87; H 6.57; P 9.63. C21H21OP.
Calculated, %: C 78.73; H 6.61; P 9.67.
PhCH2Cl
The benzene extracts were combined, the solvent
was removed, and the residue was reprecipitated from
benzene into hexane and vacuum-dried to obtain
0.14 g of II, mp 106 107 C (hexane) (mp 109 C
(3)
PH3
(PhCH2)2PH + II.
KOH/DMSO
V
The key step of reaction (3) is, apparently, the
formation of phosphide anions from phosphine under
the action of a superstrong base (KOH DMSO):
1
[21]). IR spectrum, , cm : 2320 (PH), 1190 (P=O).
1H NMR spectrum (CDCl3), , ppm: 3.16 d.d (4H,
2
1
CH2, JPH 15.3 Hz), 6.96 d.t (1H, PH, JPH 468 Hz),
7.27 m (10H, Ph). Found, %: C 72.87; H 6.57; P
13.63. C14H15OP. Calculated, %: C 73.03; H 6.57;
P 13.45.
PhCH2Cl
KOH/DMSO
PH3
PH2
V.
(4)
KOH/DMSO
The subsequent oxidation of V in the presence of
DMSO or traces of air yields phosphine oxide II,
which is prone to further oxidation and readily trans-
forms into dibenzylphosphinic acid IV in air.
The aqueous phase was acidified with HCl to pH 5
and extracted with chloroform; the chloroform extracts
were dried over magnesium sulfate, and the solvent
was distilled off. The white crystalline residue (0.19 g)
Thus, direct reactions of benzyl chloride with ele-
mental phosphorus and phosphine in the presence of
strong bases allow easy formation of the C P bond
and preparation of promising organophosphorus com-
pounds. In particular, tribenzylphosphine oxide was
successfully used in the Horner Wittig reaction for
the synthesis of substituted alkenes of various struc-
tures [10, 14 17]. This phosphine oxide also proved
to be an effective fireproofing agent for polyvinyl
chloride plastisols [18, 19].
1
consisted, according to the H and 31P NMR spectra,
of 20% I and 80% II. Pure compounds I (0.04 g) and
II (0.14 g) were isolated as described above. Yields
of phosphine oxides I and II 1.30 (61%) and 0.28 g
(12%), respectively.
Benzylphosphinic acid III (see table, run no. 2).
Water (15 ml) was added dropwise with stirring to a
mixture of 1.03 g of red phosphorus, 0.83 g of benzyl
chloride, 0.17 g of benzyltriethylammonium chloride,
and 11.7 g of KOH in 20 ml of dioxane. The mixture
was heated for 3 h at 60 65 C. After cooling, the
aqueous-dioxane layer was analyzed by 31P NMR
EXPERIMENTAL
spectroscopy. The 31P NMR spectrum ( , ppm) con-
The IR spectra were recorded on a Specord IR-75
P
device. The H and 31P NMR spectra were taken on
1
tained the following signals (in parentheses are rela-
tive intensities, % of the total) belonging, apparently,
to benzylphosphine, phosphine V, benzylphosphine
oxide, potassium benzylphosphonate and benzylphos-
phinate, and phosphine oxides II and I, respectively:
122.06 t (1JPH 193 Hz, 1.5%), 47.78 d (1JPH
195 Hz, 8%), 12.70 t (1JPH 475 Hz, 5%), 31.1 d (1JPH
521 Hz, 3.5%), 33.90 s (8%), 35.5 d (1JPH 468 Hz,
18%), and 43.68 s (56%). This mixture was diluted
with water, and the unchanged phosphorus was fil-
tered off and dried in air; 0.37 g of phosphorus was
recovered (63% conversion). The filtrate was extracted
with chloroform, the chloroform extracts were dried
over potassium carbonate, and the solvent was dis-
a Bruker DPX-400 spectrometer, internal reference
HMDS, solvent CDCl3. Phosphine in the form of
a mixture with hydrogen was generated according to
[20]. All the experiments were performed in an inert
atmosphere (Ar).
Tribenzylphosphine oxide I and dibenzylphos-
phine oxide II (see table, run no. 1). Water (15 ml)
was added dropwise with stirring to a mixture of 3.1 g
of red phosphorus, 2.53 g of benzyl chloride, 0.5 g of
benzyltriethylammonium chloride, and 35 g of KOH
in 40 ml of dioxane. The mixture was heated for 3 h
at 90 95 C and, after cooling, diluted with water and
extracted with chloroform. The chloroform extracts
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REACTIONS OF ELEMENTAL PHOSPHORUS AND PHOSPHINE... : XVI.
687
1
tilled off. The residue (0.45 g), according to the 31P
NMR spectrum, contained phosphine oxides I and II
in a 78 : 22 ratio. These products were isolated as
described above; yields 0.32 (46%) and 0.09 g (12%),
respectively. The aqueous layer was acidified with
HCl to pH 5 and extracted with chloroform, the ex-
tracts were dried over magnesium sulfate, and the
solvent was distilled off to obtain 0.12 g (11%) of
acid III, mp 165 166 C (hot water) (mp 165 166 C
128 130 C (1 mm Hg). H NMR spectrum (CDCl3),
2
, ppm: 2.96 d (4H, CH2P, JPH 2.8 Hz), 7.25 7.49 m
(10H, C6H5). 31P NMR spectrum (CDCl3):
47.9 ppm (1JPH 196 Hz). The lower layer was diP-
luted with water and extracted with chloroform, the
extracts were washed with water and dried over potas-
sium hydroxide, and the solvent was distilled off.
Phosphine oxide II was obtained; yield 2.55 g (22%).
1
[22]). H NMR spectrum (D2O), , ppm: 3.59 d (2H,
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CH2, JPH 18.2 Hz), 7.75 m (5H, Ph), 13.19 s (2H,
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