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Methylchlorophosphanes and Their Reaction Products.

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Methylchlorophosphanes and Their Reaction Products
By Horst Staendeke and Hanss-Jerg Kleiner'*'
Dedicated to Professor Werner Schukheis on the occosioi~of his 70th birthday
Methyldichlorophosphane and dimethylchlorophosphane can be prepared only with considerable difficulty. The ease with which they yield numerous reaction products is an indication
of the pronounced reactivity of these organohalophosphanes. Possible applications of such
organophosphorus compounds exist inter a h in the fields of plant protection, corrosion
prevention, and flame retardants, as well as in the catalysis of gas phase reactions, for
instance the desulfurization of gas mixtures.
1. Introduction
The large number of publications in the area of organophosphorus chemistry provides a measure for the interest
commanded by this class of compounds. This applies both
to the synthesis of new compounds and to the introduction
of phosphorus-containing functionalities into other molecules in order to modify their chemical or physical properties.
As an example, the hydrolysis-resistah linkage of the
dimethylphosphane oxide group, (CH 3)2P(0)-, to
organic compounds gives rise to a variety of widely differing
effects, such as improved solubility, enhanced surface activity, and antistatic action.
The reduction in flammability of combustible compounds
that can be achieved by incorporation of such groups
has recently become of particular significance. Owing to
the high phosphorus content in the small molecular group,
an adequate retarding effect is provided by incorporation
of relatively small amounts of the phosphorus component.
So slight are the resultant changes in the skeleton of polymeric compounds that their other physical and technological properties are largely retained.
Because of their relatively poor accessibility and their special properties the present article will concentrate on the
methyl compounds. (Surveys of organophosphorus compounds will be found in refs.[" 2].)
The methylchlorophosphanes are colorless liquids that can
bedistilled without decomposition (CH3PCI2:b. p. 82 "C131,
m.p. -8O'C[''], d 2 o = 1.30131, nho= 1.4940'31; (CH3)2PCl:
b.p. 79'CI3I, m.p. -2'C[''], d 2 0 = 1.07[31,nho= 1.4760'31)
and are readily soluble in inert organic solvents; their
pronounced sensitivity to moisture and air [(CH3)2PCI
undergoes spontaneous combustion on admission of air]
requires special apparatus for all operations.
2. Synthesis of Methyldichlorophosphane['1
Preparations starting from methanephosphonous acid,
CH,P(O)(H)OH, and derivatives, from methylphosphane,
[*] Dr. H. Staendeke
Knapsack A G
5033 Knapsack (Germany)
Dr. H . J . Kleiner
Farbwerke Hoechst A G
6230 Frankfur! ( M a i n ) XO (Germany)
Angew. Chem. internat. Edit.
1 Vol. 12
(1973)
No. 11
from tetramethylcyclotetraphosphane, and from organometallic compounds are of little interest because the starting
materials themselves are not readily available.
2.1. Synthesisfrom Methyltrichlorophosphonium
Tetrachloroaluminate[ '1
In contrast, methyldichlorophosphane is readily obtained
by reduction (c.9. with aluminum) of the complex ( I )
formed in the Kinnear-Perren reaction
PC13 + AlCls
+ CH3CI
+
[CH3PC13]a[AIC14]e
(1)
and subsequent addition of an equimolar quantity of alkali
metal chloride.
Partial hydrolysis of the complex ( 1 ) or reaction with
sulfur yields phosphonic dichloride, CH3P(O)CI2,or thiophosphonic dichloride, CH3P(S)CI2,as intermediate which
may be converted into methyldichlorophosphane on treatment with reducing agents (yields ca. 50%) or desulfurating
agents (yields up to 92 O/O) respectively.
2.2. Synthesis by Gas Phase Reactions
2.2.1. Reaction of Phosphorus Halides with Methane[']
The reaction of phosphorus(rr1) chloride and methane at
around 60072 leads to about 20 YOconversion into methyldichlorophosphane with residence times as low as 0.3s.
Apart from oxygen and chlorine, suitable catalysts include
carbon tetrahalided41 and phosgene['].
Methyldichlorophosphane was produced for several years
by this method in the USA.
2.2.2. Reaction of Red Phosphorus with Methyl Halides[']
Above 200 'C red phosphorus reacts with methyl chloride
to give methylchlorophosphanes. Optimum conditions for
the copper-catalyzed reaction exist when the catalyst mass
consists of red phosphorus and copper powder and the
temperature is kept at 350-36O'C. The major product
is methyldichlorophosphane; the yield (based on reacted
methyl chloride) is around 25 %.
877
The high price of red phosphorus and the complicated
preparation of the catalyst/phosphorus mixture would
oppose attempts to run this reaction on an industrial
scale.
In the reaction with sulfur-hydrogen compounds, hydrogen
also acts as a reducing agent whereas with disulfane[131the only reaction occurring is sulfuration to give
(2).
If a mixture of phosphorus vapor and methyl chloride
is passed over activated
at 350 ,C, the main
reaction products are methyldichlorophosphane and
dimethylchlorophosphane. The yield amounts to 70 YO
(again based on reacted methyl chloride).
2.2.3. Reaction Course
The gas phase alkylation reactions probably proceed via
a radical mechanism[*] whose first step is assumed to
consist in homolysis of the alkyl halide (Scheme 1). In
accord with this assumption, the complete range of conceivable phosphorus compounds-(CH3)~P, (CH3)2PCI,
CH3PC12,and PCI3--could be detected. Further evidence
for a radical mechanism comes from the appearance of
numerous by-products containing no phosphorus which
could be formed by recombination reactions (Scheme 2).
/p\
P-( -P
'P'
.1
+CH",+C''
I
I
H3C,
P'
H3C'
+2
H,D
H3C\
\OH
RCH'
I
OH
(3)
/p
P
CH3PC1,
I
1
I
H3C,
+ RCHO
CII,', +CIS
c
4
4
CH3PCl2
A route to tertiary cyclic phosphane oxides such as ( 5 )
and (6) is provided by condensation reactions with
diened ' 51 (Diels-Alder reaction).
,P-CH3
P - -P-c 1
P
''
1+
A new phosphorus-carbon bond can result from a large
number of
Particular importance attaches
to reactions with compounds of type (3) and ( 4 ) bearing
C=O and C=C functions respectively.
C1\
P'
c1'
t
+ 1120,
- 2 HCl
t
P'
c1/
Scheme 1. Gas phaac reaction of red phosphorus with methyl chloride.
CHS'
+
---
CH3Cl * CH4 + CH2C1'
2 CH3'
CH3'
2 CH,:
2 H'
C2H6
CH2:
4. Synthesis of Dimethylchlorophosphane['I
+ H'
C2H4
H2
Scheme 2. By-products devoid of phosphorus formed in the gas phase
reaction of red phosphorus with methyl chloride.
The preparation of compounds with the (CH3)2Pgroup
proves considerably more difficult than that of CH3P
derivatives. Processes such as the reaction of dimethylphosphinous amides, (CH3)2PNR2,with hydrogen chloride or
of dimethylphosphane or tetramethyldiphosphane with
chlorine therefore remain of no significance.
3. Reactions of Methyldichlorophosphane
Owing to the pronounced reactivity of methyldichlorophosphane, many of its products are accessible by simple
reactions (Scheme 3).
+
02,
SOSI:, N ~ 0 4
-
CH3P(0)Clz191
CHsP(O)(H)OH
f
CH3PC12
+ KOII
(HCI-Acceptor)
CH3P(0)(H)OR"01
If methyldichlorophosphane is available as starting compound, it can be subjected to a Kinnear-Perren type reaction:
C H J P C I ~+ AICI,
+ CH3CI
-+
[(CH,)2PCII]"[AlCI,Ie
CH3P(OR)2
* CH3PC14"']
*
CH~P(NRZ)Z
*
CH3P(S)C12
Scheme 3. Reactions of methyldichlorophosphane.
878
4.1. Synthesis from Dimethyldichlorophosphonium
Tetrachloroaluminate
Reductive work-up of the complex yields dimethylchlorophosphane. As in the case of the monomethyl compound,
this complex also furnishes the thiophosphinic chloride
(CH,)2P(S)CI, whose desulfuration (preferably with trialkylphosphanes)affords good yields of dimethylchlorophosphane.
Angrw. Chem. internut. Edit. f Vol. I 2 (1973) / N o . I 1
4.2. Synthesis by Gas Phase Reactions
The low thermal stability of the dimethyl compound precludes its preparation by both the high-temperature reaction
of phosphorus(lI1) chloride with methane and the coppercatalyzed reaction of red phosphorus with methyl chloride.
However, the reaction of phosphorus vapor with methyl
chloride in an active charcoal zone[6] at 350 C yields
a mixture of methylchlorophosphanes containing at least
30 % of dimethylchlorophosphane~’J.
The two components
can be isolated by phase separation[“] since, after conversion into the hydrogen chloride adduct, dimethylchlorophosphane is no longer miscible with methyldichlorophosphane.
5. Reactions of Dimethylchlorophosphane
Dimethylchlorophosphane also readily undergoes a variety
of reactions (Scheme 4).
Scheme 6. Reactions of dimethylphosphane oxide.
6. Applications
6.1. Plant Protection
Numerous biologically active derivatives of phosphonic
and phosphinic acids have been synthesized in connection
with the development of biocidal phosphoric esters. The
difficulties encountered in the preparation of the starting
materials, however, make such products expensive and
thus oppose their widespread use. In addition, a general
observation regarding toxicity to warm-blooded animals
and insecticidal activity must be taken into account, which
will be illustrated here for the transition from phosphoric
esters such as methylparathion ( 7 ) to the corresponding
phosphonic (8) and phosphinic acid derivatives (9) (Table
1)“6l.
Table I . Toxicity (rat) and insecticidal activity (aphid) of methylparathion
( 7 ) and the corresponding phosphonic (8) and phosphinic acid derivatives
(91.
Cpd.
(71
(8)
The reaction of dirnethylchlorophosphane with its hydrolysis product dimethylphosphane
appears highly
complicated (Scheme 5).
(91
R‘
CH30
CH,O
CH3
RZ
CHzO
CH3
CHI
Toxicity
LDX
[mg/kd
Insecticidal activity
Conc.
Mortality
[”/.I
p,]
20
I
I00
o.Oo0 1
0.001
0. I
100
100
I00
Oxidative and hydrolytic degradation of phosphonic and
phosphinic esters, like that of the phosphoric acid derivatives, leads to harmless compounds, the acids.
Theonly insecticide we know to possess a methyl-phosphorus functionality, Colep@ (JO)1*’], is remarkable because
Scheme 5. Reactions of dimethylchlorophosphane with dirnethylphosphane oxide.
The transition from dimethylchlorophosphane to the tertiary phosphanes and phosphane oxides can be accomplished in a variety of ways, e. 9. by reaction with unsaturated compounds or carbonyl compounds in which a new
P-C bond is formed.
it does not fit into the formula scheme ( 1 I ) of the Schrader
for biologically active phosphorus compounds. In
Colep an alkoxyl group is replaced by phenoxyl.
5.1. Reactions of Dimethylphosphane Oxides
6.2. Dyeing
A number of reaction products of dimethylchlorophosphane can be advantageously prepared from dimethylphosphane oxideI2I1; the latter compound is stable to air and
only moderately hygroscopic (m. p. 39-41 ’ ~ C ;b. p.
54 C/1 torr) (Scheme 6).
Anguw. Chum. mrernat. Edit.
Vol. 12 (1973) 1 N o . I I
On dyeing of synthetic fibers or mixed textiles in aqueous
dye baths, addition of organophosphorus compounds[281,
e. g. dimethyl methanephosphonate, improves the uniformity and depth of dyeing.
879
The use of carriers which “open up” the fiber and thus
promote better penetration’of the dye can impair the lightfastness of the dye if residues of certain carrier types remain
in the fiber. However, addition of phosphane
e. g. octyldimethylphosphane oxide, to the dye bath, the
after-treatment bath, or the carrier preparation almost
completely eliminates loss of light-fastness.
When the purification of the waste water from aqueous
dye liquors proves particularly difficult an obvious solution
is to carry out the process in organic solvents (in the
presence of small amounts of water). Work-up of the spent
dye liquor reduces to distillation of the water/solvent mixture. However, numerous commercial dyes are insufficiently soluble in organic solvents. A considerable improvement results on use of organophosphorus compounds[30’,
e. g. decyldimethylphosphane oxide:
1) the ambivalent character of the phosphane oxide (hydrophilic methyl group, lyophilic decyl group) effects ready
emulsifiability of the water in the solvent;
2) the pronounced solubilization property considerably
increases the dye concentration in the organic dye liquor
and thereby permits uniform and deep dyeing with a much
broader range of dyestuffs.
6.3. Corrosion Prevention
An important requirement for lasting protection against
corrosion lies in adequate adhesion of the coating to the
metal surface. Treatment of the metal with a compound
with which it reacts and yet which possesses sufficient
“affinity” for the protective coating should consequently
have a particularly favorable effect.
The compound dimethyl-2-oxopropylphosphaneoxide,
(CH3jzP(0)CH,COCH3, represents a suitable chemisorption agent for treatment of steel, aluminum, copper, and
titanium surfacesc3‘I. A chemisorption layer is presumably
formed with the surface oxide coating, with participation
of the C=O and P=O groups, whose predominantly
lyophilic character ensures good binding with the protective
paint.
6.4. Catalysis
The reaction
2HxS+ SO z
+
3SC2H2O
represents a possible method for removal of sulfur compounds from gaseous mixtures. If the reaction is carried
out in polar aprotic solvents in the presence of phosphane
oxides[32],e. g. trimethylphosphane oxide, the sulfur content can be lowered almost to the detection limit and
crystalline sulfur obtained simultaneously in a purity
exceeding 99.8 Yo.This gas purification reaction has a broad
scope: the content of hydrogen sulfide can vary between
O.ooO1 vol-YOand more than 5 vol-YO.
6.5. Formulation of Detergents
The development of new detergents must take account
of the following properties: stability towards water-har880
deners, powerful washing action, high dissolving power
for calcium soaps, biodegradability, and resistance to hydrolysis. A group of trialkylphosphane oxides[331in which
one alkyl group contains 10-18 C atoms and the other
two not more than 3 C atoms fulfills these and further
stipulations:
thermal stability (important for spray drying of granular
detergents)
low hygroscopicity (to avoid caking of granular detergents)
bacteriostatic activity (important for use as cold detergents).
These favorable properties suggest applications in detergents, soaps, liquid cleansers, and scouring
6.6. Flarneproofir~g[~~.
351
Increasing attention has recently been paid to the reduction
of flammability of flammable materials for a number of
applications (e.g. internal fittings in aircraft and automobiles, building units, textiles for hospitals). Corresponding
legislation has already been introduced, particularly in
the USA. These stipulations are being met by manufacturers in the development of flame resistant products. The
varying combustion characteristics of plastics and textiles
require development of specific flameproofing agents.
The elements chlorine, bromine, nitrogen, phosphorus, and
antimony in the form of suitable compounds, which are
present as additives or chemically bound to the materials
concerned, have proved to be effective for this purpose.
Numerous concepts exist concerning the mode of action
of the flameproofing agents but we shall not go into details
here. Experience has shown phosphorus to be the most
effective agent. Between 1 and 6 % of phosphorus must
be incorporated, depending upon the chemical formulation
of the material, to provide a flame-resistant product; e.g.
cellulose becomes self-extinguishing at a 2.5-4% P content, and nonflammable at 7--8%. It was observed that
the effectiveness as flameproofing agent (based on the phosphorus content) decreases along the series phosphane oxide
> phosphinic acid > phosphonic acid > phosphoric
acid.
An additional advantage of using phosphane oxides and
derivatives of phosphinic and phosphonic acids, apart from
the resistance to hydrolysis, lies in the fact that the molecular group introduced becomes firmly bound to the polymer
and its chemical structure can be modified. This permits
“reactive” incorporation into the matrix of polymers which
has only a slight influence on the other physical and technological properties of the host material even after prolonged
usage. The flameproofing treatment of cellulosic fibers will
be mentioned as an example.
Ignition of cotton is preceded by depolymerization (above
180‘C) and pyrolysis to give combustible cleavage products
whose concentration at 350-C becomes so great that a
flammable mixture is present. The mechanism of the pyrolytic processes is modified by the flameproofing agents
in that they
1) lower the pyrolysis temperature,
2) increase the amounts of non-combustible gaseous cleavAnyew. Chrm. inrrmat. Edit.
Vol. 12 ( 1 9 7 3 ) / No. I I
age products (CO,, H,O) because they exhibit. inter a h ,
a dahydrating action[ I.
3) increase the pyrolysis residue from 10 to about 50%
so that only 30--40% of flammable products are formed
instead of 80%.
Dimethylphosphinic
or methanephosphonic
have been proposed as flameproofing agents
that are linked to the hydroxyl groups of cellulose. Greater
importance attaches to the processes in which an initial
impregnation of the textile with the flameproofing agent
is followed by a condensation reaction with a second component with the aim of fixing the substrate to the
fiberl34.381,
The flameproofing agents THPC [(HOCHJ4P+CI-] and
Pyrovatex@’
[(CH30)2P(0)CH2CH2CONHCHzOH]
used in this process are of particular significance. The
flameproofing achieved by this process is washproof and
hardly impairs the properties of the fiber (feel, tear strength,
wear resistance, crease recovery).
Received: July 27, 1973 [A 970 IE]
German version: Angew. Chem. 85,973 (1973)
[*] Fireproofing agents based on phosphorus evolve phosphoric acid
duringa fire. As adehydratingagent thiscompound favors thedegradation
of organic products to carbon.
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881
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