Synthesis of unsymmetrically substituted phosphane oxides (R 1R2P(O)H) and phosphinous acids (R1R 2POH)

Article abstract of DOI:10.1002/chem.201402425

This paper describes the synthesis of unsymmetrically substituted phosphinous acids and phosphane oxides featuring at least one electron-withdrawing pentafluoroethyl group. The presence of a diethylamino function as a protecting group allows a selective reaction of RClPNEt ₂ (R=CF₃, C₆F₅, C₆H ₅) with LiC₂F₅. On treatment with para-toluenesulfonic acid the isolated aminophosphanes R(C₂F ₅)PNEt₂ are readily converted into the corresponding phosphinous acids or phosphane oxides, respectively. Investigation of the tautomeric equilibrium between oxide and acid tautomer revealed (CF ₃)(C₂F₅)POH as a stable phosphinous acid, whereas the pentafluorophenyl and phenyl derivatives constitute a solvent dependent equilibrium between the acid and the oxide tautomer.

Full text of DOI:10.1002/chem.201402425

DOI: 10.1002/chem.201402425
Full Paper
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Phosphorus Chemistry
Synthesis of Unsymmetrically Substituted Phosphane Oxides
(R1R2P(O)H) and Phosphinous Acids (R1R2POH)
Nadine Allefeld,[a] Michael Grasse,[a] Nikolai Ignat’ev,[b] and Berthold Hoge*[a]
Abstract: This paper describes the synthesis of unsymmetri-
cally substituted phosphinous acids and phosphane oxides
featuring at least one electron-withdrawing pentafluoroethyl
group. The presence of a diethylamino function as a protect-
ing group allows a selective reaction of RClPNEt2 (R=CF3,
C6F5, C6H5) with LiC2F5. On treatment with para-toluenesul-
fonic acid the isolated aminophosphanes R(C2F5)PNEt2 are
readily converted into the corresponding phosphinous acids
or phosphane oxides, respectively. Investigation of the tauto-
meric equilibrium between oxide and acid tautomer re-
vealed (CF3)(C2F5)POH as a stable phosphinous acid, whereas
the pentafluorophenyl and phenyl derivatives constitute
a solvent dependent equilibrium between the acid and the
oxide tautomer.
Introduction
to date, the only literature-known examples of stable phosphi-
nous acids.[2,3a] All other known diorganyl derivatives of the
composition R2P(O)H favor the phosphane oxide form in the
pure state. With the less electron-withdrawing C5F4N or 2,4-
(CF3)2C6H3 groups[4] a solvent-dependent equilibrium was ob-
served, whereby donor solvents, such as DMF or THF, stabilize
the acid tautomer.
Phosphorus-based ligands are ubiquitous in the synthesis of
catalytically active transition-metal complexes. However, the
widely used triorganophosphanes suffer from several disadvan-
tages, such as sensitivity towards oxidation or hydrolysis. With
regard to this, air and moisture stable secondary phosphane
oxides (SPOs), which are easy to handle, are a conceivable al-
ternative. A tautomeric equilibrium between phosphane oxide
and phosphinous acid is invoked to explain the role of SPOs as
preligands in coordination chemistry.[1]
A greater variety of electronic and steric situations can be
encountered in unsymmetrically substituted phosphane
oxides. In the literature several examples for unsymmetrically
substituted alkyl and aryl SPOs are described. Emmick et al. re-
ported on the treatment of monosubstituted phosphinic esters
RP(O)(OEt)H with Grignard reagents, yielding in SPOs of the
type RR’P(O)H.[7] More recently the synthesis of several alkyl-
and aryl-substituted SPOs starting from a mixed-phenyl-substi-
tuted aminochlorophosphane was performed.[8]
The introduction of electron-withdrawing substituents leads
to the generation of strongly p-acidic phosphinous acid li-
gands, with application in cross-coupling reactions, such as the
Suzuki type reaction.[2–6] From a molecular point of view, per-
fluoroorganyl groups cause a stabilization of the phosphinous
acid tautomer with respect to the phosphane oxide form
[Eq. (1)].
In this paper we present an efficient synthesis of unsymmet-
rically substituted secondary phosphane oxides and phosphi-
nous acids featuring one pentafluoroethyl group.
Results and Discussion
In order to synthesize unsymmetric SPOs with different per-
fluoroorganyl groups, in the first step the aminochlorophos-
phanes RClPNEt2 (R=CF3, C6F5, C6H5) have to be synthesized as
starting materials. Treatment of CF3P(NEt2)2 with two equiva-
lents of hydrogen chloride led to the selective substitution of
one amino group by a chlorine atom [Eq. (2)]. The change in
the electronic nature of the substituents—an electron-donat-
ing amino substituent is substituted by an electron-withdraw-
ing chlorine atom—is reflected in the 31P NMR spectrum, in
which the resonance of CF3P(NEt2)2 at 45.6 ppm is shifted to
98.5 ppm for (CF3)P(Cl)NEt2.
The bis(trifluoromethyl)phosphinous acid ((CF3)2POH) and
bis(pentafluoroethyl)phosphinous acid ((C2F5)2POH) represent,
[a] Dr. N. Allefeld, M. Grasse, Prof. Dr. B. Hoge
Centrum fꢀr Molekulare Materialien
Fakultꢁt fꢀr Chemie, Anorganische Chemie
Universitꢁt Bielefeld
Universitꢁtsstraße 25, 33615 Bielefeld (Germany)
E-mail: b.hoge@uni-bielefeld.de
On the other hand, treatment of the dichlorophosphanes
RPCl2 (R=C6F5, C6H5) with two equivalents of diethylamine se-
lectively affords RP(Cl)NEt2 accordingly to Equation (3).
[b] Dr. N. Ignat’ev
PM-ATI, Merck KGaA
Frankfurter Str. 250, 64293 Darmstadt (Germany)
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ly.[2,11] Similarly CF3(C2F5)PNEt2 was added to a slurry of excess
para-toluenesulfonic acid in 1,6-dibromohexane. The phosphi-
nous acid CF3(C2F5)POH was isolated by means of fractional
condensation as the only volatile compound in 90% yield
[Eq. (5)].
The 31P NMR spectrum in [D]chloroform reveals one multip-
let at 80.7 ppm (Table 2), which compares well with the reso-
nances of (CF3)2POH (d(31P)=77.9)[2] and (C2F5)2POH (d(31P)=
93.0).[11] Based on an A3B3MNX spin system (A, B, M, N=19F;
X=31P) the 31P and 19F NMR spectra were simulated. No spec-
troscopic evidence for a second isomer could be obtained.
Substitution of an electron-withdrawing chlorine atom by an
electron-donating amino group results in an upfield shift of
the 31P NMR signal from 162.0 (C6H5PCl2) to 142.1 ppm
(C6H5P(Cl)NEt2) and from 137.0 (C6F5PBr2) to 106.0 ppm
(C6F5P(Br)NEt2) (Table 1).
Table 2. NMR data of phosphinous acids and phosphane oxides.
Table 1. Selected NMR spectroscopical data of the aminophosphanes.
d(31P) [ppm]
1J(P,H) [Hz]
2J(P,F) [Hz]
d(31P) [ppm]
2J(P,F) [Hz]
CF3(C2F5)POH[a]
C6F5(C2F5)POH[c]
C6F5(C2F5)P(O)H[a]
C6H5(C2F5)POH[c]
C6H5(C2F5)P(O)H[a]
80.7
81.2
À1.9
90.0
17.2
–
–
572
–
519
52/65[b]
65/82[b]
n.d.[d]
86
78/76[b]
[a]
[a]
CF3P(Cl)NEt2
CF3P(Br)NEt2
C6F5P(Br)NEt2
98.5
97.8
106.0
142.1
44.8
85
82
54[c]
[b]
[a]
C6H5P(Cl)NEt2
CF3(C2F5)PNEt2
–
[a]
88[d]
47/59[e]
43/46[e]
71/45[e]
[a] [D]chloroform. [b] diastereotopic fluorine atoms of CF2 group. [c] Et2O.
[d] Not detected.
[a]
C6F5(C2F5)PNEt2
29.9
57.0
[a]
C6H5(C2F5)PNEt2
[a] [D]chloroform. [b] [D6]benzene. [c] 3J(P,F). [d] Coupling to CF3. [e] Dia-
stereotopic fluorine atoms of CF2 group.
The experimentally determined (Figure 1(top)) and the simu-
lated 31P NMR spectra (Figure 1(bottom)) are in excellent
agreement. In addition to the coupling of the phosphorus
3
atom to the CF3 groups (2J(P,F)=83.0 Hz, J(P,F)=14.1 Hz), two
Previous work has shown that pentafluoroethyl lithium
(LiC2F5), generated in situ from nBuLi and C2F5H at À788C,[9]
represents an efficient reagent for the pentafluoroethylation of
chlorophosphanes ClnP(NEt2)3Àn (n=1, 2).[10,11] Treatment of the
aminochlorophosphanes RP(X)NEt2 (R=CF3, C6F5, C6H5; X=Cl,
Br) with LiC2F5 selectively afforded the unsymmetrically substi-
tuted aminophosphanes R(C2F5)PNEt2 [R=CF3, C6F5, C6H5,;
Eq. (4)].
couplings to the diastereotopic fluorine atoms of the CF2
2
group were detected (2J(P,Fa)=52.0 Hz, J(P,Fb)=64.9 Hz).
Figure 1. Experimental (top) and simulated (bottom) 31P NMR spectrum of
(CF3)(C2F5)POH.
The pure compounds were isolated by distillation as color-
less liquids in good yields.
Bis(trifluoromethyl)phosphinous acid ((CF3)2POH) and bis-
(pentafluoroethyl)phosphinous acid ((C2F5)2POH) have been
prepared from the corresponding aminophosphane R2PNEt2
(R=CF3, C2F5) by treatment with para-toluenesulfonic acid,
which results in the replacement of the amino group by a hy-
droxyl group. The isolation of the phosphinous acid was effect-
ed by fractional condensation in 88 and 93% yield, respective-
In accordance to this, the 19F NMR spectrum displays four
sets of signals, one for the CF3 group directly bonded to the
phosphorus atom at À64.0 ppm, and at À81.9 ppm for the CF3
unit of the C2F5 substituent. The CF2 unit causes two resonan-
ces for the diastereotopic fluorine atoms Fa at À126.4 and Fb
at À125.8 ppm (Figure 2).
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tautomer. In the liquid phase
(C2F5)2POH is rearranged into
20% of the oxide tautomer,
whereas (CF3)2POH and CF3-
(C2F5)POH are exclusively known
in the acid tautomer.
The pentafluorophenyl and
phenyl derivatives were synthe-
sized analogously by the expo-
sure of the corresponding ami-
nophosphanes R(C2F5)PNEt2 (R=
C6F5, C6H5) to para-toluenesul-
fonic acid and were isolated as
colorless liquids by distillation
under reduced pressure in a 52
(R=C6F5) and 42% (R=Ph) yield,
respectively [Eq. (6)].
In the neat liquid, the penta-
fluorophenyl derivative exists in
an equilibrium between the
Figure 2. Experimental (top) and simulated (bottom) 19F NMR spectrum of (CF3)(C2F5)POH; I (Fa), II (Fb), III (CF2CF3),
IV (CF3).
The phosphinous acid was further studied by IR spectrosco-
py in the gas phase. Two OH stretching modes were detected
at 3669 and 3596 cmÀ1, respectively, due to the existence of
two rotational isomers in the gas phase. The phosphinous
acids (CF3)2POH and (C2F5)2POH are also present in two rotam-
ers, in a cis and a trans conformation.[2,3] Calculations revealed
that the cis rotamer is more stable by 7.5 ((CF3)2POH) and
6.3 kJmolÀ1 ((C2F5)2POH), respectively (B3LYP/6-311+G(2df,p)).
Correspondingly, the cis rotamer of (CF3)(C2F5)POH is stabilized
by 5.1 kJmolÀ1 with respect to the trans rotamer (Figure 3).
The absence of a PÀH as well as a P=O stretching mode
confirms the exclusive presence of the acid tautomer in the
gas phase at room temperature. This agrees with the observa-
tions for (CF3)2POH and (C2F5)2POH. Both compounds exist in
the solid state as well as in the gas phase solely as the acid
phosphane oxide C6F5(C2F5)P(O)H and the phosphinous acid
tautomer, C6F5(C2F5)POH.
The 31P NMR spectrum of the neat liquid shows two reso-
nances at 84.3 and À0.7 ppm in an integral ratio of 48:52 that
are assigned to the acid and the oxide form, respectively
(Figure 4). The resonance of the oxide tautomer reveals a char-
1
acteristic J(P,H) coupling constant of 582 Hz. In solvents with
high donor numbers,[12] such as diethyl ether, the tautomeric
equilibrium is shifted completely towards the acid tautomer
(Table 3). In solvents with low donor numbers, however, the
equilibrium is shifted towards the oxide tautomer. In the
31P NMR spectrum of the chloroform two signals at 80.6 and
À1.9 ppm in an integral ratio of 42:58 are detected—the por-
tion of the oxide tautomer increased with regard to the pure
sample. A similar behavior was observed for the equilibrium
between C6H5(C2F5)POH and C6H5(C2F5)P(O)H.
Figure 3. Relative zero-point corrected energies of the two rotational iso-
mers of CF3(C2F5)POH and the oxide tautomer CF3(C2F5)P(O)H calculated at
the B3LYP/6-311+G(2df,p) level of theory.
Figure 4. 31P NMR spectrum of C6F5(C2F5)POH/C6F5(C2F5)P(O)H, neat liquid.
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100% in chloroform and only to 40% in the donor solvent di-
ethyl ether.[13]
Table 3. Solvent dependent equilibrium between phosphinous acids and
their corresponding phosphane oxide tautomers.
Solvent
R(C2F5)POH
R(C2F5)P(O)H
Conclusions
R=CF3
CHCl3
neat
100
100
0
0
This work describes an efficient strategy for the synthesis of
phosphinous acids and phosphane oxides featuring one penta-
fluoroethyl group with an additional trifluoromethyl, penta-
fluorophenyl, or phenyl substituent. The differing stabilization
of the phosphinous acid form is evidenced in the tautomeric
equilibrium. Whereas the compound CF3(C2F5)POH exists in so-
lution and the gas phase solely as the acid tautomer, the less
electron-withdrawing pentafluorophenyl-substituted com-
pounds C6F5(C2F5)POH/C6F5(C2F5)P(O)H are involved in tauto-
meric equilibria in solution as well as in the neat liquid phase.
The even less electron-withdrawing phenyl group, which
may be rather regarded as a donor function, clearly favors the
oxide tautomer, as observed exclusively in chloroform.
R=C6F5
neat
CHCl3
Et2O
MeCN
DMF
48
42
100
100
100
52
58
0
0
0
R=C6H5
CHCl3
MeCN
neat
Et2O
DMF
0
7
15
30
87
100
93
85
70
13
The phenyl group is less electron-withdrawing than a per-
fluorophenyl group. Therefore, in chloroform only the oxide
tautomer was detected by NMR spectroscopy. In the more do-
nating solvent diethyl ether a tautomeric ratio of acid to oxide
of 30:70 was detected. This clearly reflects the different induc-
tive effects of a phenyl and a pentafluorophenyl substituent.
For an improved understanding, the energy differences be-
tween the acid and the oxide tautomer were calculated on
a B3LYP/6-311+G(2df,p) level of theory. Stabilization of the
phosphinous acid tautomer decreases in the series CF3 >C2F5 >
C6F5 >C6H5 (Table 4). The phosphinous acid CF3(C2F5)POH is
In general, electron-withdrawing groups stabilize the acid
tautomer. Although the electronic nature of trifluoromethyl
and pentafluoroethyl groups are almost comparable, the stabi-
lization of a phosphinous acid is most efficiently realized by
a trifluoromethyl substituent.
Experimental Section
All chemicals were obtained from commercial sources and used
[14]
without further purification. The phosphanes CF3P(NEt2)2
and
[15]
C6F5PX2 were synthesized according to literature methods. Stan-
dard high-vacuum techniques were employed throughout all prep-
arative procedures. Non-volatile compounds were handled in a dry
N2 atmosphere using Schlenk techniques. NMR spectra were re-
corded on a Bruker Model Avance III 300 spectrometer (31P
111.92 MHz; 19F 282.40 MHz; 13C 75.47 MHz, 1H 300.13 MHz) with
positive shifts being downfield from the external standards (85%
orthophosphoric acid (31P), CCl3F (19F) and TMS (1H)). IR spectra
were recorded on a ALPHA-FT-IR spectrometer (Bruker) using a gas
cell with KBr windows.
Table 4. Calculated energy differences of the tautomeric equilibrium be-
tween phosphinous acid R1R2POH and phosphane oxide tautomers
R1R2P(O)H with CF3, C2F5, C6F5 and C6H5 groups (B3LYP/6-311+G(2df,p)).[a]
R1
R2
DEZP,T [kJmolÀ1
]
CF3
CF3
C2F5
C6F5
C6F5
C6H5
CF3
À25.7
À24.4
À22.8
À13.3
À1.7[b]
8.0
C2F5
C2F5
C2F5
C6F5
C2F5
Synthesis of CF3P(X)NEt2 (X=Cl, Br)
1) The addition of diethylamine (2.2 g, 30.8 mmol) to a solution of
(CF3)PBr2 (4.0 g, 15.4 mmol) in n-hexane at À788C led to the pre-
cipitation of a colorless solid. The reaction mixture was slowly
warmed to room temperature. After filtration the solvent was re-
moved under reduced pressure. Fractional condensation yielded
CF3P(Br)NEt2 (1.8 g, 7.2 mmol, 46%) in the À408C trap as a colorless
liquid. 1H NMR ([D]chloroform, RT): d=0.9 (t, 3J(H,H)=7 Hz, 3H;
CH3), 3.2 ppm (m, 2H; CH2); 31P NMR ([D]chloroform, RT): d=
[a] DEZP,T =EZP,T(acid)ÀEZP,T(oxide). [b] B3LYP/6-311G(2d,p).[13]
more stable than the corresponding phosphane oxide by
24.4 kJmolÀ1. Thus, this compound is less stabilized than
(CF3)2POH (DEZP,T =À25.7 kJmolÀ1), but more than (C2F5)2POH
(DEZP,T =À22.8 kJmolÀ1) with respect to their corresponding
phosphane oxide counterparts.
The phosphinous acid C6F5(C2F5)POH is by 13.3 kJmolÀ1
more stable than the oxide, whereas stabilization of the sym-
metrically substituted (C6F5)2POH amounts only to 1.7 kJmolÀ1.
These calculated energy differences are in agreement with the
experimental findings. In the neat liquid the ratio of phosphi-
nous acid C6F5(C2F5)POH to phosphane oxide C6F5(C2F5)P(O)H is
48:52, in diethyl ether only the acid tautomer is detected. For
comparison, the oxide tautomer (C6F5)2P(O)H is observed to
2
97.8 ppm (quart/m, J(P,F)=82 Hz, P).
2) A solution of CF3P(NEt2)2 (32.3 g, 132.3 mmol) in n-pentane
(300 mL) was treated with gaseous HCl (293.6 mmol) at À788C.
The reaction mixture was slowly warmed to room temperature and
the ammonium salts were removed by filtration. The solvent was
removed under reduced pressure. The product CF3P(Cl)NEt2 (16.5 g,
79.6 mmol, 60%) was separated by vacuum distillation at À208C.
1H NMR ([D]chloroform, RT): d=0.9 (t, 3J(H,H)=7 Hz, 3H; CH3),
3.3 ppm (m, 2H; CH2); 13C{1H} NMR ([D]chloroform, RT): d=13.9 (d,
2
3J(P,C)=5 Hz, CH3), 44.3 (d, J(P,C)=18 Hz, CH2), 122.6 ppm (quart/
1
1
d, J(C,F)=324 Hz, J(P,C)=49 Hz, CF3); 19F NMR ([D]chloroform, RT):
d=À67.3 ppm (d, J(P,F)=86 Hz, CF3); 31P NMR ([D]chloroform, RT):
2
2
d=98.5 ppm (quart, J(P,F)=85 Hz, P).
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Synthesis of C2F5(CF3)P(NEt2): Caution! LiC2F5 is highly reactive
and tends to violently decompose above temperatures of
À508C. A 1.6m solution of n-butyllithium in n-hexane (59.0 mL,
94.4 mmol) was diluted in diethyl ether (200 mL) and degassed at
À788C. Then the resulting solution was stirred for 30 min in an at-
mosphere of pentafluoroethane (123.0 mmol). After the addition of
CF3P(Cl)NEt2 (16.5 g, 79.6 mmol) at À788C, the mixture was
warmed to room temperature and stirred overnight. A precipitate
was filtered off. After evaporation of the solvent, C2F5(CF3)PNEt2
(17.1 g, 61.0 mmol, 77%) was obtained by distillation under re-
duced pressure at 758C as a colorless liquid. 1H NMR ([D]chloro-
at À788C, the mixture was warmed to room temperature and
stirred overnight. The precipitate was filtered off. After evaporation
of the solvent, C6F5(C2F5)PNEt2 (7.2 g, 18.4 mmol, 92%) was ob-
1
tained by vacuum distillation at 438C as a colorless liquid. H NMR
3
([D]chloroform, RT): d=1.1 (t, J(H,H)=7 Hz, 3H; CH3), 3.2 ppm (d/
quart, 3J(P,H)=11 Hz, 2J(H,H)=7 Hz, 2H; CH2); 1H{31P} NMR
([D]chloroform, RT): d=1.1 (t, 3J(H,H)=7 Hz, 3H; CH3), 3.2 ppm
(quart, 2J(H,H)=7 Hz, 2H; CH2); 13C{1H} NMR ([D]chloroform, RT):
d=13.7 (d, 3J(P,C)=4 Hz, CH3), 44.6 ppm (d, 2J(P,C)=19 Hz, CH2);
13C{19F} NMR ([D]chloroform, RT): d=117.9 (d, 1J(P,C)=60 Hz, CF2),
119.8 (d/m, 2J(P,C)=32 Hz, CF3), 137.6 (s, meta-CF), 143.1 (s, para-
CF), 147.5 ppm (d, 2J(P,C)=13 Hz, ortho-CF); 19F NMR ([D]chloro-
3
form, RT): d=1.2 (t, J(H,H)=7 Hz, 3H; CH3), 3.2 ppm (m, 2H; CH2);
13C{1H} NMR ([D6]benzene, RT): d=14.1 (d, 3J(P,C)=3 Hz, CH3),
form, RT): d=À159.9 (m, 2F; meta-CF), À148.2 (t/t, J(F,F)=31 Hz,
3
2
44.2 ppm (d, J(P,C)=21 Hz, CH2); 13C{19F} NMR ([D]chloroform, RT):
4J(F,F)=5 Hz, 1F; para-CF), À126.8 (m, 2F; ortho-CF), À118.4 (d/d/t/
quart, 2J(F,F)=297 Hz, 2J(P,F)=43 Hz, 5J(F,F)=15 Hz, 3J(F,F)=3 Hz,
1F; CFaFbCF3), À116.4 (d/d/t/quart, 2J(F,F)=297 Hz, 2J(P,F)=46 Hz,
1
2
d=119.5 (d, J(P,C)=52 Hz, CF2), 120.1 ppm (d, J(P,C)=25 Hz, CF3);
19F NMR ([D]chloroform, RT): d=À119.0 (d/d/m, 2J(F,F)=305 Hz,
2J(P,F)=59 Hz, 1F; CFaFbCF3), À118.4 (d/d/m, 2J(F,F)=304 Hz,
2J(P,F)=47 Hz, 1F; CFaFbCF3), À82.5 (d/m, 3J(P,F)=18 Hz, 3F;
CF2CF3), À57.3 ppm (d/m, 2J(P,F)=88 Hz, 3F; CF3); 31P NMR
([D]chloroform, RT): d=44.8 ppm (brm, P).
3
3
5J(F,F)=20 Hz, J(F,F)=3 Hz, 1F; CFaFbCF3), À82.2 ppm (d/t, J(P,F)=
23 Hz, 3J(F,F)=3 Hz, 3F; CF2CF3); 19F{31P} NMR ([D]chloroform, RT):
d=À159.9 (m, 2F; meta-CF), À148.2 (t/t, 3J(F,F)=31 Hz, 4J(F,F)=
5 Hz, 1F; para-CF), À126.8 (m, 2F; ortho-CF), À118.4 (d/t/quart,
2J(F,F)=297 Hz, 5J(F,F)=15 Hz, 3J(F,F)=3 Hz, 1F; CFaFbCF3), À116.4
(d/t/quart, 2J(F,F)=297 Hz, 5J(F,F)=20 Hz, 3J(F,F)=3 Hz, 1F;
CFaFbCF3) À82.2 ppm (t, 3J(F,F)=3 Hz, 3F; CF2CF3); 31P NMR
([D]chloroform, RT): d=29.9 ppm (m, P).
Synthesis of C2F5(CF3)POH: para-Toluenesulfonic acid (14.8 g,
86 mmol) was suspended in 1,6-dibromohexane (100 mL). After the
addition of CF3(C2F5)PNEt2 (3.9 g, 13.8 mmol) the reaction mixture
was stirred for 2 d. Fractional condensation at À788C yielded C2F5-
(CF3)POH (3.0 g, 12.5 mmol, 90%) as a colorless liquid. 1H NMR
([D]chloroform, RT): d=4.2 ppm (brs, C2F5(CF3)POH); 13C{19F} NMR
([D]chloroform, RT): d=116.5 (d, 1J(P,C)=55 Hz, CF2), 119.0 (d,
Synthesis of C6F5(C2F5)POH/C6F5(C2F5)P(O)H: A slurry of para-tol-
uenesulfonic acid (4.2 g, 22.1 mmol) in diethyl ether (50 mL) was
treated with C6F5(C2F5)PNEt2 (2.1 g, 5.6 mmol). The yellow mixture
was stirred overnight. Removal of the solvent under reduced pres-
sure and vacuum distillation afforded C6F5(C2F5)POH/C6F5-
(C2F5)P(O)H (1.0 g, 2.9 mmol, 52%) as a colorless oil. 1H NMR
([D]chloroform, RT): d=6.2 (brs, C6F5(C2F5)POH), 8.2 ppm (d,
1J(P,H)=572 Hz, C6F5(C2F5)P(O)H); 13C{19F} NMR (Et2O, RT): d=120.3
1
2J(P,C)=20 Hz, CF2CF3), 127.0 ppm (d, J(P,C)=34 Hz, CF3); 19F NMR
([D]chloroform, RT): d=À126.4 (d/d/quart/quart, 2J(F,F)=308 Hz,
2J(P,F)=52 Hz, 4J(Fa,F)=8 Hz, 3J(F,F)=3 Hz, 1F; CFaFbCF3), À125.9
(d/d/quart/quart, 2J(F,F)=308 Hz, 2J(P,F)=65 Hz, 4J(Fb,F)=10 Hz,
3J(F,F)=3 Hz, 1F; CFaFbCF3), À81.9 (d/d/d/quart, 3J(P,F)=14 Hz,
3J(F,Fa)=3 Hz, 3J(F,Fb)=3 Hz, 5J(F,F)=2 Hz, 3F; CF2CF3), À64.0 ppm
(d/d/d/quart, 2J(P,F)=83 Hz, 4J(F,Fb)=10 Hz, 4J(F,Fa)=8 Hz, 5J(F,F)=
2 Hz, 3F; CF3); 31P NMR ([D]chloroform, RT): d=80.7 ppm (quart/d/
d/quart, 2J(P,F)=83 Hz, 2J(P,Fb)=65 Hz, 2J(P,Fa)=52 Hz, 3J(P,F)=
14 Hz, P); IR (gas phase): n˜ =398 (vw), 423 (w), 457 (w), 547 (w),
628 (w), 748 (w), 857 (w), 966 (w), 1056 (w), 1151 (m), 1165 (m),
1186 (m), 1227 (s), 1336 (w), 3596 (w), 3669 cmÀ1 (vw).
2
(d, J(P,C)=19 Hz, C6F5(CF3CF2)POH), 137.9 (s, C6F5(C2F5)POH (meta-
CF)), 143.9 (s, C6F5(C2F5)POH (para-CF)), 148.1 ppm (d, 2J(P,C)=
12 Hz, C6F5(C2F5)POH (ortho-CF)); 19F NMR ([D]chloroform, RT): d=
À160.3 (brs, meta-CF (acid)), À156.5 (brs, meta-CF (oxide)), À146.9
(brs, para-CF (acid)), À137.8 (brs, para-CF (oxide)), À132.8 (brs,
ortho-CF (acid)), À129.5 (brs, ortho-CF (oxide)), À123.9–128.3 (m,
CF2), À81.5 (brs, CF3 (acid)), À80.4 ppm (brs, CF3 (oxide)); 19F NMR
(Et2O, RT): d=À162.3 (m, 2F; meta-CF (acid)), À149.5 (m, 1F; para-
Synthesis of C6F5P(Br)NEt2:
A sample of C6F5PBr2 (10.3 g,
28.9 mmol) was dissolved in n-hexane (50 mL) and cooled to
À788C. Addition of diethylamine (6.2 mL, 58.9 mmol) led to the
precipitation of a colorless solid. The reaction mixture was slowly
warmed overnight to room temperature and filtered. The solvent
was removed under reduced pressure and vacuum distillation at
1ꢁ10À3 mbar and 508C yielded C6F5P(Br)NEt2 (3.6 g, 10.3 mmol,
36%) as a colorless oil. 1H NMR ([D6]benzene, RT): d=0.9 (t,
3J(H,H)=7 Hz, 3H; CH3), 2.9 ppm (m, 2H; CH2); 13C{1H} NMR
([D6]benzene, RT): d=9.5 (d, 3J(P,C)=7 Hz, CH3), 40.7 ppm (d,
2J(P,C)=18 Hz, CH2); 13C{19F} NMR ([D6]benzene, RT): d=133.1 (s,
meta-CF), 138.1 (s, para-CF), 142.5 ppm (d, 2J(P,C)=14 Hz, ortho-
CF); 19F NMR ([D6]benzene, RT): d=À160.9 (m, 2F; meta-CF),
À150.0 (m, 1F; para-CF), À131.2 ppm (d/m, 3J(P,F)=54 Hz, 2F;
ortho-CF); 19F{31P} NMR ([D6]benzene, RT): d=À160.9 (m, 2F; meta-
CF), À150.0 (m, 1F; para-CF), À131.2 ppm (m, 2F; ortho-CF);
31P NMR ([D6]benzene, RT): d=106.0 ppm (t/m, 3J(P,F)=54 Hz, P);
31P{19F} NMR ([D6]benzene, RT): d=106.0 ppm (quint, 3J(P,H)=
12 Hz, P).
2
CF (acid)), À131.9 (m, 2F; ortho-CF (acid)), À125.8 (d/d/m, J(F,F)=
303 Hz, 2J(P,F)=65 Hz, 1F; C6F5(CF3CFaCFb)POH), À124.4 (d/d/m,
2J(F,F)=303 Hz,
2J(P,F)=82 Hz,
1F;
C6F5(CF3CFaFbCFb)POH),
À81.7 ppm (d, 3J(P,F)=14 Hz, 3F; C6F5(CF3CFaCFb)POH); 31P NMR
([D]chloroform, RT): d=À1.9 (d/t, 1J(P,H)=572 Hz, 2J(P,F)=87 Hz,
C6F5(C2F5)P(O)H), 81.2 ppm (brm, C6F5(C2F5)POH); 31P NMR (Et2O,
RT): d=87.3 ppm (m, C6F5(C2F5)POH); IR (ATR): n˜ =417 (m), 478 (m),
515 (m), 546 (w), 589 (w), 640 (m), 730 (w), 747 (m), 760 (w), 836
(w), 864 (m), 875 (m), 898 (m), 962 (s), 980 (s), 1003 (m), 1092 (s),
1130 (m), 1203 (s), 1248 (m), 1313 (m), 1390 (w), 1402 (w), 1475 (s),
1492 (s), 1519 (m), 1644 (w), 3186 cmÀ1 (w).
Synthesis of C6H5P(Cl)NEt2: Diethylamine (29.3 g, 400.0 mmol) was
added to a solution of PhPCl2 (35.8 g, 200.0 mmol) in n-pentane
(500 mL) at À788C. The reaction mixture was slowly warmed to
room temperature and filtered. The solvent was removed under re-
duced pressure and C6H5P(Cl)NEt2 (22.9 g, 106.5 mmol, 53%) was
obtained by vacuum distillation of the residue as a colorless liquid.
3
Synthesis of C6F5(C2F5)PNEt2: A 1.6m solution of n-butyllithium in
n-hexane (14.4 mL, 23.0 mmol) was diluted in diethyl ether
(100 mL) and degassed at À788C. Then the resulting solution was
stirred for 30 min in an atmosphere of pentafluoroethane
(25.0 mmol). After the addition of C6F5P(Br)NEt2 (7.0 g, 20.1 mmol)
1H NMR ([D]chloroform, RT): d=1.2 (t, J(H,H)=7 Hz, 6H; CH3), 3.2
(m, 4H; CH2), 7.5 (brm, 3H; meta/para-CH), 7.8 ppm (brm, 2H;
ortho-CH); 13C{1H} NMR ([D]chloroform, RT): d=14.2 (d, 3J(P,C)=
6 Hz, CH3), 43.9 (d, 2J(P,C)=13 Hz, CH2), 128.5 (d, 3J(P,C)=4 Hz,
4
2
meta-CH), 129.7 (d, J(P,C)=1 Hz, para-CH), 130.7 (d, J(P,C)=20 Hz,
Chem. Eur. J. 2014, 20, 1 – 7
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Full Paper
ortho-CH), 139.6 ppm (d, J(P,C)=29 Hz, ipso-C); 31P NMR ([D]chloro-
form, RT): d=142.1 ppm (brs, P).
1
Acknowledgements
This work was supported by Merck KGaA (Darmstadt, Germa-
ny) and Solvay (Hannover, Germany) which is gratefully ac-
knowledged. We thank Prof. Dr. Lothar Weber and Dr. Julia
Bader for helpful discussions.
Synthesis of C6H5(C2F5)PNEt2: A sample of C6H5P(Cl)NEt2 (5.8 g,
26.9 mmol) was added to a solution of LiC2F5, generated in situ
from nBuLi (21.9 mL of 1.6m in n-hexane) and pentafluoroethane
(35.0 mmol) in diethyl ether (150 mL) at À788C. The reaction mix-
ture was slowly warmed to room temperature and filtered. The sol-
vent was removed under reduced pressure and vacuum distillation
of the residue at 1ꢁ10À3 mbar yielded C6H5(C2F5)PNEt2 (5.4 g,
Keywords: fluorine · phosphane oxides · phosphinous acids ·
phosphorus · tautomeric equilibrium
1
18.2 mmol, 68%) at 328C as a colorless liquid. H NMR ([D]chloro-
3
3
form, RT): d=1.1 (t, J(H,H)=7 Hz, 6H; CH3), 3.2 (d/quart, J(P,H)=
10 Hz, 3J(H,H)=7 Hz, 4H; CH2), 7.5 (brm, 3H; meta/para-CH),
7.7 ppm (brm, 2H; ortho-CH); 13C{1H} NMR ([D]chloroform, RT): d=
14.1 (d 3J(P,C)=3 Hz, CH3), 45.0 (d, 2J(P,C)=17 Hz, CH2), 128.5 (d,
3J(P,C)=7 Hz, meta-CH), 129.6 (s, para-CH), 132.0 (d/d, 2J(P,C)=
[1] a) N. V. Dubrovina, A. Bçrner, Angew. Chem. 2004, 116, 6007–6010;
Angew. Chem. Int. Ed. 2004, 43, 5883–5886; b) L. Ackermann, Synthesis
2006, 10, 1557–1571.
[2] B. Hoge, P. Garcia, H. Willner, H. Oberhammer, Chem. Eur. J. 2006, 12,
3567–3574.
[3] B. Hoge, J. Bader, H. Beckers, Y. S. Kim, R. Eujen, H. Willner, N. Ignatiev,
Chem. Eur. J. 2009, 15, 3567–3576; B. Hoge, J. Bader, B. Kurscheid, N. Ig-
natyev, E. F. Aust, (Merck Patent GmbH, Darmstadt, Germany), WO
2010/009818A1.
[4] B. Kurscheid, W. Wiebe, B. Neumann, H.-G. Stammler, B. Hoge, Eur. J.
Inorg. Chem. 2011, 5523–5529.
[5] N. Allefeld, B. Neumann, H.-G. Stammler, N. Ignatiev, B. Hoge, 2014, un-
published results.
[6] B. Kurscheid, L. Belkoura, B. Hoge, Organometallics 2012, 31, 1329–
1334.
[7] T. L. Emmick, R. L. Letsinger, J. Am. Chem. Soc. 1968, 90, 3459–3465.
[8] C. Petit, A. Favre-Reguillon, G. Mignani, M. Lemaire, Green Chem. 2010,
12, 326–330.
[9] a) M. Henrich, A. Marhold, A. Kolomeitsev, A. Kadyrov, G.-V. Rçschenthal-
er, J. Barten, (Bayer AG, Leverkusen), DE 10128703, 2001; b) M. F. Ernst,
D. M. Roddick, Inorg. Chem. 1989, 28, 1624–1627; c) M. H. Kçnigsmann,
PhD thesis, University of Bremen, Bremen, 2004.
[10] a) R. G. Peters, B. L. Bennett, R. C. Schnabel, D. M. Roddick, Inorg. Chem.
1997, 36, 5962–5965; b) M. M. Choate, R. G. Baughman, J. E. Phelps,
R. G. Peters, J. Organomet. Chem. 2011, 696, 956–962.
[11] A. V. Zakharov, Y. V. Vishnevskiy, N. Allefeld, J. Bader, B. Kurscheid, S.
Steinhauer, B. Hoge, B. Neumann, H.-G. Stammler, R. J. F. Berger, N. W.
Mitzel, Eur. J. Inorg. Chem. 2013, 3392–3404.
[12] V. Gutmann, Electrochim. Acta 1976, 21, 661–670.
[13] B. Hoge, S. Neufeind, S. Hettel, W. Wiebe, C. Thçsen, J. Organomet.
Chem. 2005, 690, 2382–2387.
1
21 Hz, J=3, ortho-CH), 133.3 ppm (d/d, J(P,C)=9 Hz, J=3 Hz, ipso-
C); 19F NMR ([D]chloroform, RT): d=À119.0 (d/d/quart, 2J(F,F)=
2
3
1
300 Hz, J(P,F)=45 Hz, J(F,F)=3 Hz, J(C,F)=300 Hz, 1F; CFaFbCF3),
À115.7 (d/d, 2J(F,F)=300 Hz, 2J(P,F)=71 Hz, 1J(F,C)=289 Hz, 1F;
CFaFbCF3), À81.4 ppm (d/t, 3J(P,F)=17 Hz, 3J(F,F)=2 Hz, 1J(F,C)=
287 Hz, 3F; CF3); 31P NMR ([D]chloroform, RT): d=57.0 ppm (brm,
P); 31P{1H} NMR ([D]chloroform, RT): d=57.0 ppm (d/d/quart,
2
2J(P,Fb)=71 Hz, J(P,Fa)=45 Hz, 3J(P,F)=17 Hz, P).
Synthesis of C6H5(C2F5)P(O)H:
A sample of C6H5(C2F5)P(NEt2)
(1.77 g, 6.18 mmol) was combined with a slurry of para-toluenesul-
fonic acid (4.7 g, 24.7 mmol) in 1,6-dibromohexane (30 mL). After
12 h the solvent was removed under reduced pressure and C6H5-
(C2F5)P(O)H (0.63 g, 2.58 mmol, 42%) was isolated by vacuum distil-
1
lation at 1ꢁ10À3 mbar and 1008C. H NMR ([D]chloroform, RT): d=
7.6 (m, 2H; meta-CH), 7.7 (m, 1H; para-CH), 7.7 (d/d, 1J(P,H)=
3
518 Hz, J=10 Hz, 1H; PH), 7.8 ppm (d/m, J(P,H)=15 Hz, 2H; ortho-
CH); 13C{1H} NMR ([D]chloroform, RT): d=129.4 (d, 2J(P,C)=14 Hz,
ortho-C), 131.5 (d, 3J(P,C)=13 Hz, meta-C), 135.2 (brs, para-C);
13C{19F} NMR ([D]chloroform, RT): d=111.9 (d, 1J(P,C)=90 Hz, CF2),
2
118.6 ppm (d, J(P,C)=15 Hz, CF3); 19F NMR ([D]chloroform, RT): d=
À129.7 (d/d, 2J(F,F)=324 Hz, 2J(P,F)=76 Hz, 1F; CFaFbCF3), À125.2
(d/d, 2J(F,F)=323 Hz, 2J(P,F)=78 Hz, 1F; CFaFbCF3), À80.1 ppm (s,
3F; CF3); 19F{31P} NMR ([D]chloroform, RT): d=À129.7 (d, 2J(F,F)=
324 Hz, 1F; CFaFbCF3), À125.2 (d, 2J(F,F)=323 Hz, 1F; CFaFbCF3),
À80.1 ppm (s, 3F; CF3); 31P NMR ([D]chloroform, RT): d=17.2 ppm
(d/t/t, 1J(P,H)=519 Hz, 2J(P,F)=77 Hz, 3J(P,H)=14 Hz, P); 31P{1H}
[14] W. Volbach, I. Ruppert, Tetrahedron Lett. 1983, 24, 5509–5512.
[15] D. D. Magnelli, G. Tesi, J. U. Lowe, W. E. McQuistion, Inorg. Chem. 1966,
5, 457–461.
2
NMR ([D]chloroform, RT): d=17.2 ppm (t, J(P,F)=77 Hz, P); 31P{19F}
NMR ([D]chloroform, RT): d=17.2 ppm (d/t, 1J(P,H)=519 Hz,
3J(P,H)=14 Hz, P).
Received: February 28, 2014
Published online on && &&, 0000
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FULL PAPER
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Phosphorus Chemistry
N. Allefeld, M. Grasse, N. Ignat’ev,
B. Hoge*
&& – &&
Synthesis of Unsymmetrically
Substituted Phosphane Oxides
(R1R2P(O)H) and Phosphinous Acids
(R1R2POH)
A tautomeric equilibrium between
phosphane oxide and phosphinous acid
is invoked to explain the role of secon-
dary phosphane oxides (SPOs) as pre-
ligands in coordination chemistry.
and the gas phase solely as the acid
tautomer, the less electron-withdrawing
C6F5(C2F5)POH and C6F5(C2F5)P(O)H are
involved in equilibria in solution as well
as in the neat liquid phase (see figure).
Whereas CF3(C2F5)POH exists in solution
Keeping the balance…
..with electron-withdrawing groups!
The unusual form of a phosphinous
acid can be stabilized with respect
to the corresponding phosphane
oxide tautomer by the influence of
electron-withdrawing groups. In
their Full Paper on page && ff., B.
Hoge et al. describe an efficient
strategy for the synthesis of
phosphinous acids and phosphane
oxides featuring one
pentafluoroethyl group with an
additional trifluoromethyl,
pentafluorophenyl, or phenyl
substituent. The differing
stabilization of the phosphinous
acid form is evidenced in the
tautomeric equilibrium.
Chem. Eur. J. 2014, 20, 1 – 7
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ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers! ÞÞ