P-C bond formation via P-H addition of a fluoroaryl phosphinic acid to ketones

Article abstract of DOI:10.1016/j.jfluchem.2010.07.008

The synthesis, structure and reactivity of the fluoroaryl phosphinic acid HF₄C₆-P(O)HOH is reported and compared to a sterically comparable yet non-fluorinated analog with similar size. The fluoroaryl phosphinic acid undergoes revers

Full text of DOI:10.1016/j.jfluchem.2010.07.008

Journal of Fluorine Chemistry 131 (2010) 1025–1031
Contents lists available at ScienceDirect
Journal of Fluorine Chemistry
journal homepage: www.elsevier.com/locate/fluor
P–C bond formation via P–H addition of a fluoroaryl phosphinic acid to ketones
Andreas Orthaber a, Jo¨rg H. Albering b, Ferdinand Belaj a, Rudolf Pietschnig a,
*
a Department of Chemistry, Inorganic Section, University of Graz, Schubertstr. 1, 8010 Graz, Austria
b Institute of Chemistry and Technology of Materials, Graz University of Technology, Stremayrg. 16, 8010 Graz, Austria
A R T I C L E I N F O
A B S T R A C T
Article history:
The synthesis, structure and reactivity of the fluoroaryl phosphinic acid HF4C6–P(O)HOH is reported and
compared to a sterically comparable yet non-fluorinated analog with similar size. The fluoroaryl
phosphinic acid undergoes reversible P–H addition to the carbonyl functionality of ketones under
Received 18 May 2010
Received in revised form 9 July 2010
Accepted 18 July 2010
Available online 23 July 2010
formation of a P–C bond which is retained in the resulting
a
-hydroxy phosphinic acid. The latter shows
an extended 2D hydrogen bonded network in the solid state.
ß 2010 Elsevier B.V. All rights reserved.
Keywords:
Phosphorus
Tautomerism
Hydrogen bonds
Ab initio calculations
Fluorinated ligands
1. Introduction
acids had been previously synthesized using alternative pathways
[15–19]. For non-fluorinated -hydroxy phosphinic acid
a
a
Phosphinic acids are relevant for many fields of chemistry [1],
biology [2] and medicinal chemistry [3]. On the other side, also
phosphorus compounds with electron withdrawing moieties
attracted considerable attention in the last decade. Electron
withdrawing groups are known to influence electronic and optical
properties of the phosphorus containing compounds besides their
reactivity [4–7]. It has been demonstrated that the phosphinic acid
(CF3)2POH is the prevalent tautomeric structure over
(CF3)2P(55O)H, which is in sharp contrast to other known
phosphinic acids [8,9]. Also C2F5 substitution leads to a similar
behavior [10]. By using 1,3 bis(trifluoro)methyl substituted aryl
rings Hoge and co-workers showed that the corresponding
phosphinic anhydrides can be stabilized over the tautomeric
diphosphane monoxides which was attributed to the electronic
nature of the fluorinated groups [11]. In the last years the synthesis
of fluorinated phosphanes aiming at phosphano-borane based
‘‘frustrated’’ Lewis acid/base pairs has flourished enormously
looking at applications as e.g. hydrogen activation and storage
[12,13]. The perfluorinated phosphinic acid (C6F5)2P(55O)H forms
an adduct with acetone in solution, which is however not retained
in the solid state owing to its alleged kinetic instability [14]. By
increasing the Lewis acidity of the carbonyl component the adduct
formation becomes irreversible as is evidenced on going from
obtained by the latter route the isolation and characterization
with X-ray diffraction of a single enantiomer (S) could be achieved
[20]. Applications of this class of compounds range from flame
retardants [21] to asymmetric synthesis [22] and neutral amino
peptidase (NAPN) inhibitors [23].
We investigated the P–H addition of a phosphinic acid to
acetone where the partially fluorinated substituent (HF4C6–)
carries a single hydrogen atom as spectator unit for spectroscopic
purposes. A comparison with the non-fluorinated mesityl phos-
phinic acids shows the influence of the electron withdrawing
substituent on the phosphinic acid, while the steric demand of the
fluorine groups should be only slightly smaller than that of the
methyl groups [24,25]. For the partially fluorinated phosphinic
acid we found a reversible addition equilibrium with acetone
forming the corresponding
a-hydroxy phosphinic acid. Surpris-
ingly the latter is also stable in the solid phase, unlike its previously
reported perfluorinated counterpart.
2. Results and discussion
The partially fluorinated substituent 2,3,5,6-tetrafluorophenyl
(HF4C6–) turned out to be useful to track reactions employing the
single hydrogen atom as
a spectroscopic probe [26]. The
acetone to benzaldehyde [14]. The resulting a-hydroxy phosphinic
corresponding dichlorophosphane 1a which was recently pub-
lished [26] should be a suitable precursor for the synthesis of
phosphinic acid 2a. For HF4C6-compounds the electronic and steric
situation should be very similar to their C6F5 substituted analogs.
On the other hand for the corresponding mesityl (Mes = 2,4,6-
* Corresponding author. Tel.: +43 316 380 5287; fax: +43 316 380 9835.
E-mail address: rudolf.pietschnig@uni-graz.at (R. Pietschnig).
0022-1139/$ – see front matter ß 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2010.07.008

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A. Orthaber et al. / Journal of Fluorine Chemistry 131 (2010) 1025–1031
Table 1
Geometric parameters for the donor–acceptor interactions in compounds 2a, 2b, 4,
5.
˚
˚
˚
D–Hꢀ ꢀ ꢀA
Dꢀ ꢀ ꢀA (A)
D–H (A)
Hꢀ ꢀ ꢀA (A)
D–Hꢀ ꢀ ꢀA (8)
2a
2b
4
O2–H2ꢀ ꢀ ꢀO1
O1–H2ꢀ ꢀ ꢀO2
O2–H2ꢀ ꢀ ꢀO1
O2–H2ꢀ ꢀ ꢀO1
2.485(1)
2.492(1)
2.513(3)
2.509(4)
0.88(3)
0.83(2)
0.88(4)
0.84(5)
1.62(3)
1.66(2)
1.64(4)
1.68(5)
174(3)
172(2)
168(3)
168(5)
5
an orientation in a way that the P55O bond is arranged almost
coplanar with the phenyl ring (O2–P1–C1–C2 10.0(1)8). The quality
of the crystal of 2b allowed the analysis of the hydrogen bond
system which forms a zig-zag arranged linear chain. Also in non-
fluorinated 2b shorter donor acceptor distances than in (4) and (5)
are observed which indicates that the electron withdrawing
properties of the substituent in fluorinated 2a are only in part
responsible for the strong donor–acceptor interaction. Other
structural parameters of 2b show no peculiarities (Fig. 2).
To check how compounds 2a,b behave in solution we
performed NMR measurements in acetone. Based on 31P NMR
measurements the phosphinic acid R–PH(55O)OH is the only
detectable tautomer for fluorinated 2a and non-fluorinated 2b. The
chemical shifts and coupling constants are with 2.2 ppm (d,
1JPH = 630 Hz) (2a) and 24.2 (d, 1JPH = 562 Hz) (2b) in the expected
range for the phosphorane form of phosphinic acids. One
significant difference between 2a and 2b is the reaction behavior
Scheme 1. Synthesis of the phosphinic acids (2a,b) and the acetone adduct (3a)
starting from the corresponding dichlorophosphanes (1a,b). (i) H2O, CDCl3 0 8C
giving 2a and 2b in 99% and 75% yield, respectively. (ii) acetone (yield: 52%). For (a)
R = –C6F4H and (b) R = Mes.
towards acetone. Unexpectedly, we found
a dynamic H/D
trimethyl-phenyl) derivative 2b, the electronic – unlike the steric –
situation at the adjacent phosphorus atom should be very different
from the fluorinated counterparts. In order to explore the balance
of steric and electronic effects for these compounds we prepared
2a and 2b to compare their structure and reactivity.
exchange for fluorinated 2a in acetone–d6 while no such behavior
was observable for non-fluorinated 2b. In the case of 2a the
stepwise exchange of both protons of the –PH(55O)OH functionali-
ty can be monitored with 1H NMR where the presence of the C–H
The fluoroaryl phosphinic acid 2a is obtained in a clean reaction
by controlled hydrolysis of 1a in chloroform solution as an oily
material, which solidifies at 4 8C upon prolonged cooling after
removal of the solvent in almost quantitative yields. In a similar
way, the mesityl analog 2b is obtained in 75% by a clean reaction
from 1b (Scheme 1). The IR data of solid 2a,b show the presence of
the P–H and the P55O functionalities [27]. For both compounds the
tautomeric form present in the solid state is the phosphorane form
2 of the phosphinic acid rather than the phosphane tautomer 20 as
was clearly confirmed by X-ray diffraction on suitable single
crystals.
Compound 2a crystallizes in the monoclinic space group P21/c
as colorless plates. The phosphinic acid group in 2a is oriented to
near coplanarity of the P–H bond relative to the aryl ring (C6–C1–
P1–H1 1.8(7)8) leading to a close contact with an F atom (H1ꢀ ꢀ ꢀF6
˚
˚
˚
2.58(2) A). The P–H and O–H bonds are 1.28(2) A and 0.87(3) A,
respectively, which is in the typical range for these bonds. In
general phosphinic acids form strong hydrogen bonds owing to the
polarity of the P–O unit. In 2a this is further enhanced resulting in
very short donor acceptor distances, which is significantly shorter
than in phenyl phosphinic acid (4) [28], but also shorter than in
[2,4,6-tris(trifluoromethyl)phenyl]-phosphinic acid (5) [29].
A
summary of the donor acceptor geometry parameters is given in
Table 1. The formation of a linear chain is typical for phosphinic
acid with little steric shielding [28], while phosphinic acids with
bulky substituents usually form dimers [30,31]. Short contacts of
neighboring chains via the aromatic hydrogen atom exhibit weak
˚
˚
vdW interactions H4–F2 (2.44(2) A and H4–O1 2.70(2) A). A
graphical representation of the structure of 2a is depicted in Fig. 1
and details of the data acquisition and structure solution are
summarized in Table 2.
Fig. 1. ORTEP [32] drawing of the unit cell content of compound 2a at a probability
level of 50%. Compound 2a forms an infinite chain in a zig-zag pattern along the
˚
crystallographic c-axis (0 0 1). Selected bond lengths (A) and angles (8): P1–O1
1.491(1), P1–O2 1.5370(1), P1–C1 1.809(1), P1–H1 1.28(2), O2–H2 0.87(3), P1–O2–
H2 119(2), O1–P1–C1–C6 121.1(1), O2–P1–C1–C6 111.5(1), C6–C1–P1–H1 1.8(7).
The mesityl substituted analog 2b crystallizes in the monoclinic
space group P21/n as colorless needles. The phosphinic acid shows

A. Orthaber et al. / Journal of Fluorine Chemistry 131 (2010) 1025–1031
1027
Table 2
2
Structure collection and refinement data for compounds 2a, 2b and 3a. Weighting scheme w ¼ 1=½s2ðFo2Þ þ ðaPÞ þ bPꢂ where P ¼ ðFo2 þ 2Fc2Þ=3.
2a
2b
3a
CCDC no.
753926
C6H3F4O2P
214.05
754559
C9H13O2P
184.16
754558
Empirical formula
Formula weight
Temperature (K)
C9H9F4O3P
272.13
100(2)
˚
Wavelength (A)
0.71073
Monoclinic
P21/c
Crystal system
Space group
Monoclinic
Monoclinic
P21/n
P21/c
Unit cell dimensions
˚
a (A)
7.8645(4)
13.2701(7)
7.1937(4)
102.879(2)
12.6132(6)
5.1517(2)
14.6367(7)
110.060(1)
11.8615(5)
18.4348(7)
10.5508(4)
114.714(1)
˚
b (A)
˚
c (A)
(8)
b
3
˚
Volume (A )
731.87(7)
893.39(7)
4
2095.77(14)
8
Z
4
Density (calculated) (Mg/m3)
Absorption coefficient (mmꢁ1
F(0 0 0)
1.943
1.369
0.263
392
1.725
0.314
1164
)
0.411
1164
Absorption correction
Crystal size (mm3)
Semi-empirical from equivalents
0.38 ꢃ 0.30 ꢃ 0.12
2.66–30.00
0.66 ꢃ 0.19 ꢃ 0.13
1.84–30.00
0.20 ꢃ 0.17 ꢃ 0.09
1.89–30.00
u
range for data collection (8)
Index ranges
ꢁ9 ꢄ h ꢄ 11,
ꢁ17 ꢄ h ꢄ 17,
ꢁ7 ꢄ k ꢄ 7,
ꢁ16 < h < 16,
ꢁ25 < k < 25,
ꢁ14 < l < 14
78339
ꢁ18 ꢄ k ꢄ 18,
ꢁ10 ꢄ l ꢄ 9
ꢁ20 ꢄ l ꢄ 20
24158
Reflections collected
8076
Independent reflections
2125 [R(int) = 0.0212]
99.99
2606 [R(int) = 0.0255]
100.00
6113 [R(int) =0.0339]
100.00
Completeness to
u= 30.008 (%)
Max./min. transmission
Refinement method
1.000/0.834
0.9662/0.8455
0.9655/0.9253
Full-matrix least-squares on F2
2125/0/130
Data/restraints/parameters
Goodness-of-fit on F2
2606/0/119
6113/0/323
1.046
1.066
1.004
Final R indices (I > 2
R indices (all data)
s(I))
R1 =0.0252, wR2 = 0.0710
R1 =0.0273, wR2 = 0.0725
R1 = 0.0297, wR2 = 0.0914
R1 = 0.0309, wR2 = 0.0927
R1 = 0.0281, wR2 = 0.0798
R1 = 0.0349, wR2 = 0.0843
Weighting scheme parameters
a
b
0.0397
0.2881
0.055700
0.515700
0.0437
0.8194
Largest
D
/
s
in last cycle
0.000
0.000
0.000
ꢁ3
˚
Largest diff. peak/hole (e A
)
0.463/ꢁ0.341
0.453/ꢁ0.319
0.471/ꢁ0.312
proton turns out to be especially useful. Initially, the P–OH
resonance of 2a can be observed, but already after 5 min integrals
are no longer in agreement with the intensity of the other signals
(e.g. HF4C6–). After 12 h both proton resonances of the phosphinic
acid have disappeared, owing to complete exchange with deuteri-
um. The transfer of the more acidic OH proton to the less acidic P–H
position (H/D scrambling) can be explained by a successive
tautomeric equilibrium involving a sequential change of the formal
oxidation state from P(V) to P(III) and vice versa as summarized in
Scheme 2. As known from the literature electron withdrawing
substituents favor the trivalent P(III) intermediate 20 which explains
why such an exchange is observed for 2a but not for 2b [8,9]. Based
on DFT calculations (B3LYP/6-311G**) the preference for the
phosphorane structure 2 over the phosphane structure 20 can be
quantified and as expected the energy difference is smaller for the
electron-withdrawing HF4C6-substituent (20a ! 2a j
DG
298 = ꢁ1.9
kcal/mol and 20b ! 2b j
DG
298 = ꢁ6.5 kcal/mol).
Moreover, the proton exchange is presumably facilitated via a
dimeric intermediate [33]. To the best of our knowledge no evidence
for P–H/P–D exchange in acetone–d6 at ambient temperature has
beenreportedsofar.BycontrastH/Dexchangeofprimaryphosphanes
inD2Oiswellestablished[34].Weassumethatacetone–d6 actsasthe
deuterium source owing to the
a-acidity of carbonyl compounds
involving the corresponding enol. The fact that H/D exchange only
occurs for 2a but not 2b might point to the higher acidity of 2a which
may lead to protonated acetone–d6 which then on deprotonation
would be able to transfer D+ to the conjugated base of 2a.
Fig. 2. Crystal structure 2b. ORTEP [32] plot at a probability level of 50%. Hydrogen
are omitted for clarity. The molecules show a zig-zag arrangement along the
˚
crystallographic b-axis (0 1 0). Selected distances (A) and angles (8): P1–O2
In the course of the H/D exchange, the initial doublet of the 31
signal of 2a (1JPH 630 Hz) turns into a triplet of constant intensity
P
1.4971(8), P1–O1 1.5531(8), P1–C1 1.802(1), P1–H1 1.35(2), O1–H2 0.83(2), P1–
O1–H2 117(1), O2–P1–C1–C2 10.0(1), O1–P1–C1–C2 119.0(1).

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A. Orthaber et al. / Journal of Fluorine Chemistry 131 (2010) 1025–1031
Scheme 2. Tautomeric equilibrium resulting in H/D scrambling for 2a.
Scheme 3. Equilibrium of the free phosphinic acid (2) its P(III) tautomer (20) and the acetone adduct (3) (with a R = 2,3,5,6-F4C6H– and b R = Mes).
due to the 31P–D coupling (1JPD 92 Hz). The ratio of the coupling
constants is ꢅ6.85, which roughly matches the ratio of the
gyromagnetic constants (42.58:6.54 = 6.51). The aromatic proton
in para position offers the possibility for easy detection of the
aromatic carbon atom signals in HMBC 13C NMR measurements
and additionally the P-bonded Caryl resonance (116.4 ppm) could
be detected in D2O/H2O solution. Detection of these signals by
more simple 13C NMR experiments is often limited by small and
broad signals owing to CF couplings over more than one bond.
In the course of the NMR measurements in acetone–d6, we
observed a broad-unresolved-signal around 46 ppm in the 31P
spectra, but no additional signals in the proton spectra. Hoge and
co-workers described the formation of an acetone adduct of
(C6F5)2P(55O)H in acetone in solution with a 31P chemical shift at
29.2 ppm. However the latter turned out to be kinetically unstable
and upon evaporation of the solvent also the incorporated acetone
molecule was removed and the adduct could not be obtained and
isolated in the solid state [14]. By contrast, we could easily isolate
the analogous adduct 3a in crystalline form from solutions of
phosphinic acid 2a in non-deuterated acetone, which is formed as
depicted in Scheme 3. Attempts to crystallize 2a from any other
organic solvent than acetone failed however. X-ray structure
analysis of these crystals confirms the presence of the phosphinic
acid–acetone adduct (3a) in the solid state (Fig. 3). The acetone
adduct was isolated in 52% yield.
hydrogen bonds (D: Oꢀ ꢀ ꢀH 1.86(2) A), E: Oꢀ ꢀ ꢀH 1.59(2) A). A second
˚
˚
˚
dimeric hydrogen bonded motif (F) of Oꢀ ꢀ ꢀH 1.67(2) A completes the
comb like structure. Weak interactions of fluorine atoms with the
˚
˚
phenyl rings (F6–C11 3.000(1) A, F6–C12 2.934(1) A, F3–C2
˚
˚
2.973(1) A, F3–C3 3.155(1) A) shows stacking between the layers
formed by the H-bonded network. Relevant crystallographic details
for 3a are summarized in Table 2 and a graphical representation of
the 2-dimensional network is depicted in Fig. 4.
The IR spectra of solid 3a show a strong band 1248 cmꢁ1 which
can be assigned to P55O stretching modes based on calculated IR
Compound 3a crystallizes in the monoclinic space group P21/c as
colorless plates. The P–Caryl bond lengths are in the upper range
˚
˚
(1.822(1) A–1.830(1) A) comparable to the benzaldehyde adduct of
˚
˚
phenyl phosphinic acid (1.818(4) A–1.821(3) A) [20] and (2,6-
˚
˚
CF3C6H4)2P(55O)H (1.819(2) A–1.822(2) A) [35]. The phosphorus
˚
˚
oxygen bonds are also in the typical range 1.482(1) A–1.488(1) A
˚
˚
and1.549(1) A–1.554(1) A fordoubleandsinglebonds, respectively.
The aliphatic hydroxyl group shows significantly shorter O–H
˚
˚
distances (0.77(2) A–0.81(2) A) compared to the more acidic
˚
˚
phosphinic OH group (0.89(2) A–0.95(2) A). The packing of the
molecules in the solid state is dominated by the formation of a
hydrogen bonded network. Compound 3a forms an extended 2-
dimensional hydrogen bonded network orthogonal to the crystallo-
graphic b-axis (0 1 0). Hexagonal cycles are formed by six individual
molecules resulting in a honey-comb-like structure (six membered
cycles A and B). Two pairs of molecules form a dimeric unit by two
P55Oꢀ ꢀ ꢀHO–C hydrogen bonded motifs (C) with Oꢀ ꢀ ꢀH distances of
Fig. 3. ORTEP [32] drawing of 3a at a probability level of 50%. The asymmetric unit
contains two independent molecules with nearly the same geometry. Selected bond
˚
lengths (A) and angles (8): P1–O1 1.489(1), P1–O2 1.549(1), P1–C1 1.822(1), P1–C13
1.842(1), C13–O5 1.442(1), C13–C14 1.525(1), C13–C15 1.528(1), O2–H1a 0.89(2),
O5–H3a 0.78(12), O4–P2–C7–C8 33.8(1), O1–P1–C1–C2 23.5(1).
˚
1.82(2) A. These dimers are bridged by further 2 molecules via single

A. Orthaber et al. / Journal of Fluorine Chemistry 131 (2010) 1025–1031
1029
where the steric situation is similar while the electronic situation
differs. The behavior of these phosphinic acids was studied in
solution indicating the exclusive presence of the phosphorane
tautomer 2, or a fast equilibrium on the NMR time scale in which
the latter is dominant over the corresponding phosphane tautomer
20. In the solid state molecules of 2a and 2b are connected via their
phosphorane functionalities to infinite chains via strong hydrogen
bonds. Besides these similarities, their reactivity towards acetone
differs substantially. A characteristic reaction of compound 2a is
the addition of the P–H unit to the carbonyl group of acetone under
formation of 3a, which occurs in solution and persists also in the
solid state. Its mesityl analog 2b does not show a reaction with
acetone at all. Moreover, in solution 2a is also involved in a
dynamic proton exchange with acetone as evidenced by H/D
scambling. Again its mesityl analog 2b does not show similar
behavior. According to our findings, the energetically lower
accessibility of the phosphane tautomer 20a along with the higher
acidity of 2a are likely to be responsible for this different behavior.
In summary the unique reactivity observed in the formation of 3a
allows the facile formation of a P–C bond without involvement of
salt elimination or expensive or sensitive organometallic coupling
reagents. An obvious perspective for future work will be the
control of the reversibility of this addition also exploring the
possibility to induce an enantiomeric excess for the asymmetri-
cally substituted phosphorus center formed during this addition.
Fig. 4. View of 3a along the b-axis (0 1 0) illustrating the comb like structure built of
alternating hexagons A and B. HC6F4– and methyl-groups are omitted for clarity.
data. The strong band at around 1600 cmꢁ1 can be attributed to
different aromatic stretch and methyl bending modes. The
different hydroxyl groups give weak broad bands around
3120 cmꢁ1. The measured bands with the exception of OH modes
can be assigned reasonably based on calculated ones. The deviation
for the OH vibrations may be attributed to the associated hydrogen
bond network present in the experimental sample as compared to
the isolated molecules in vacuo used for calculations.
4. Supplementary information
A solution of crystalline 3a in acetone–d6 shows a singlet at
46.4 ppm in the 31P NMR spectra. Furthermore, minor amounts of
phosphinic acid 2a are observed as well which is likely to be derived
from elimination of acetone from 3a. In the 1H NMR spectra one
signal for the aromatic proton (7.77 ppm) and a doublet for the
methyl groups (1.39 ppm, 16.0 Hz) are observed for 3a. According to
2D-NMR (HMBC/HSQC) measurements the proton signal of these
methylgroupsshowsacorrelationtothequaternarycarbinolcarbon
CCDC-753926 (for 2a), -754559 (for 2b) and -754558 (for 3a)
contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge at www.ccdc.cam.ac.uk/
conts/retrieving.html [or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; Fax:
(internat.) +44 1223 336 033; E-mail: deposit@ccdc.cam. ac.uk].
For compounds 2a,b, 20a,b and 3 output summaries for the DFT
calculations (optimized geometries, total energies (E, in hartree))
have been provided as supplementary material.
atom (d 13
( C) = 70.11 ppm) which proves the connection of the
methyl groups to the latter also in solution. The integrals of the
proton signals of the methyl groups in 3a are smaller than expected,
which can be explained assuming a fast (on the NMR time scale)
exchangeof the non-deuterated acetoneunit intheadditionproduct
with the deuterated acetone used as a solvent. To suspend this
exchange we dissolved 3a in tetrahydrofurane–d8 giving a single
resonance at 34.3 ppm in the 31P NMR spectra, which is at rather
5. Experimental and computational details
5.1. General
Reactions are carried out under ambient atmosphere unless
otherwise noted. NMR measurements were performed on a Varian
UnityInova 400 and a BrukerAvanceIII 300 operating at proton
frequencies of 399.9 MHz and 300.1 MHz, respectively. 1H and 13
C
high field compared to other a-hydroxy phosphinic acids [19]. The
difference of the 31P chemical shifts of 3a in acetone and THF
underlines the significance of solvent effects for this system. Similar
adducts of 2a with other ketones can be formed as well and the
correspondingmethylethylketoneadductshowsa31Pchemicalshift
of 38.1 ppm in chloroform solution.
shifts are given in ppm and referenced to residual solvent signal.
19F and 31P shifts are given in ppm and are referenced externally to
C6H5CF3 (ꢁ63.7 ppm) and H3PO4 (85%), respectively. Dichlor-
ophosphanes 1a, Mes–PCl2, and Mes–PH(55O)OH were synthe-
sized according to published procedures [26,36,37]. Freshly
bought acetone–d6 (0.02% water) was dried over P4O10 and
stored in a Schlenk-tube after distillation. DFT calculations were
performed with the Gaussian program package [38]. The B3LYP
hybrid functional with a 6-311G** basis set was used for all
calculations. All structures are fully optimized and determined to
be local minima by inspection of their harmonic frequencies
having no imaginary frequency. Frequencies were used without
scaling factors for determination of thermal correction at
298.15 K.
An investigation of the thermodynamic balance of the adduct
formation has been performed with computational methods
(B3LYP/6-311G**). The surprising result is that the adduct
formation of 2a and acetone leading to 3a is in fact an endergonic
reaction (DG
298 = 11.1 kcal/mol). Since the calculations refer to the
gas phase, the relevant driving force for the formation of 3a may be
the formation of the extended hydrogen bonded network as found
in the solid state. Nevertheless the parameters (i.e. geometry,
spectroscopic data) calculated for an isolated molecule of 3a in
vacuo are in good agreement with the experimental data measured
for the solid (H-bridged) compound.
5.2. Synthesis of 2a
3. Conclusion
To a solution of 1a (240 mg, 0.96 mmol) in CHCl3 (1.3 ml) ca. 2
equivalents of H2O (40 mg, 2.2 mmol) are added. The resulting
suspension is dried by slow evaporation under ambient conditions.
In summary, we prepared a phosphinic acid with a fluorinated
aryl group (2a) in comparison to its mesityl substituted analog (2b)

1030
A. Orthaber et al. / Journal of Fluorine Chemistry 131 (2010) 1025–1031
The phosphinic acid is insoluble in typical organic solvents, with
the exception of acetone and water. Compound 2a is an oily
material which solidifies upon standing at 4 8C. Yield: 203 mg
n.d. 19F NMR (CDCl3, 282 MHz):
d
= ꢁ128.73 (m), ꢁ136.99 (br. s).
31P NMR (CDCl3, 121 MHz):
d = +38.1 (br. s).
(0.95 mmol, 99%). Mp. 27 8C. 1H NMR (CD3COCD3, 400 MHz):
5.6. Crystallographic details
1
d
= 7.67 (1H, m, aryl), 7.90 (1H, d, JPH = 629.5 Hz), 8.37 (s, 1H,
exchanges rapidly with acetone–d6); 13C NMR (CD3COCD3,
X-ray studies have been carried out with a Bruker Apex-III
diffractometer equipped with a CCD detector. Structures are solved
by direct methods using SHEL-XS and refined with SHEL-XL [39]. In
compounds 2b and 3a the H atoms of the methyl groups were
refined with common isotropic displacement parameters for the H
atoms of the same group and idealized geometry with tetrahedral
angles, enabling rotation around the X–C bond, and C–H distances
1
100 MHz):
d
= 110.61 ppm, 146.00 (dm, JCF 235.7 Hz), 147.53
(dm, 1JCF 259.3 Hz), Caryl–P 116.4 (from freshly prepared D2O/H2O
solution). 31P NMR (CD3COCD3, 162 MHz):
d = 2.2 ppm (d,
1JPH = 630 Hz) after 12 h. 1H was completely exchanged by 2H.
The initial doublet becomes a triplet of constant intensity. 2.2 ppm
(t, 1JPD = 92 Hz). 19F NMR (THF–d8, 282.4 MHz):
d
= ꢁ141.09 (br. s,
4F). IR(KBr [cmꢁ1]): 3104 (w, Carom.–H), 2469 (w, P–H), 2090 (m
br.), 1610 (m arom.), 1484 (str, arom.), 1375 (m), 1246 (str, P55O),
863 (m, P–OH).
of 0.98 A. Hydrogen atoms of the phenyl rings were put at the
˚
˚
external bisector of the C–C–C angle at a C–H distance of 0.95 A and
common isotropic displacement parameters were refined for the H
atoms of the same phenyl group. All other hydrogen atoms in 2a,
2b and 3a have been located on the difference Fourier map and
were refined isotropically without any constraints. In Table 2,
crystal and refinement data for 2a, 2b, and 3a are summarized.
5.3. Synthesis of 2b
To a solution of Mes–PCl2 (0.65 g, 2.9 mmol) in chloroform
2.5 equiv of H2O are added at 0 8C. The mixture is stirred for 1 h.
The organic phase is dried over MgSO4 and the solvent removed in
vacuum yielding 0.40 g pure product (2.2 mmol, 75%). Colorless
crystals suitable for X-ray diffraction are obtained by slow
evaporation of chloroform solutions. Mp: 143 8C. 1H NMR (CDCl3,
Acknowledgements
Financial support by the Austrian Science Fund (FWF) (Grants
P18591-B03 and P20575-N19) and the EU-COST Action CM0802
‘‘PhoSciNet’’ are gratefully acknowledged.
300 MHz):
aryl), 2.57 (6H, s, o-CH3), 2.28 (3H, s, p-CH3). 13C NMR (CDCl3,
75.5 MHz): = 142.64 (s, Cpara) 141.47 (d, 11.7 Hz, Cmeta), 139.11 (d,
12.2 Hz, Cortho), 124.03 (d, 1JPC 138.1 Hz, Cipso), 20.96 (d, 3JPC 8.9 Hz,
m-CH3), 14.21 (s, p-CH3). 31P NMR (CDCl3, 121.5 MHz):
= 24.2 (d,
d
= 8.03 (1H, d, 1JPH = 562.0 Hz), 6.76 (2H, d, 4JPH 4.3 Hz,
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5.5. Addition of 2a to 2-butanone
The solid phosphinic acid 2a (27 mg, 0.13 mmol) is dissolved in
2-butanone (5 ml) and stirred at slightly elevated temperatures for
2 days. Complete conversion into the corresponding
phosphinic acid was checked by 31P NMR spectroscopy. After
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a-hydroxy
1H NMR (CDCl3, 300 MHz):
CH3), 1.35 (d, JPH 17.2 Hz, 3H, CH3), 1.76 (m, 2H, CH2–CH3), 6.75
(br. S, OH), 7.26 (m, 1H, aryl). 13C NMR (CDCl3, 75 MHz):
d
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3
d = 6.6
1
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