Convenient Synthesis of the Water-Soluble Ligand Hexasodium Tris(4-phosphonatophenyl)phosphine

Full text of DOI:10.1021/jo000371y

J . Org. Chem. 2000, 65, 5059-5062
5059
Knight, et al., previously reported the synthesis of
4-Ph2PC6H4PO3Na2 (triphenylphosphine monophospho-
nate, TPPMP) from 4-Ph2PC6H4Br.7 Metal-halogen ex-
change with n-butyllithium followed by subsequent re-
action of the aryllithium species with diethyl chlorophos-
phate gave the intermediate phosphonate ester 4-Ph2-
PC6H4PO3Et2. Transesterification with BrSiMe3,8 fol-
lowed by hydrolysis and neutralization with NaOH gave
the desired compound. The phosphonate ester has also
been prepared by the Pd-catalyzed reaction of 4-PPh2C6H4-
Br and diethyl phosphite.9 In our hands, neither of these
strategies was satisfactory for the preparation of the
corresponding tris-phosphonate compound, as they gave
mixtures of products which were difficult to purify.
Con ven ien t Syn th esis of th e Wa ter -Solu ble
Liga n d Hexa sod iu m
Tr is(4-p h osp h on a top h en yl)p h osp h in e
Walter J . Dressick,† Clifford George,‡
Susan L. Brandow,† Terence L. Schull,*,† and
D. Andrew Knight§
Center for Biomolecular Science and Engineering,
Code 6950, Naval Research Laboratory,
4555 Overlook Avenue SW, Washington DC, 20375,
Laboratory for the Structure of Matter, Code 6030,
Naval Research Laboratory, 4555 Overlook Avenue SW,
Washington DC 20375, and Department of Chemistry,
The George Washington University, 725 21st Street NW,
Washington DC 20052
Nucleophilic aromatic substitution of fluoroarylsul-
fonates by phosphine or primary or secondary phosphines
in the superbasic medium KOH/DMSO has been shown
to be a flexible and efficient route to secondary and
tertiary phosphines with sulfonated aromatic substitu-
ents.10 Similarly, it has been reported that the triphen-
ylphosphine diphosphonates PhP(4-C6H4PO3Na2)2 and
PhP(3-C6H4PO3Na2)2 can be prepared by nucleophilic
aromatic substitution of 4-FC6H4P(O)(NEt2)2 or 3-FC6H4P-
(O)(NEt2)2 by PhPLi2, followed by acid hydrolysis of the
resulting arylphosphine-phosphonodiamide and neu-
tralization of the free phosphonic acid with NaOH.11
From these reports, it was reasonable to assume that
P(4-C6H4PO3Na2)3 (triphenylphosphine triphosphonate,
TPPTP) could be prepared from nucleophilic aromatic
substitution of the appropriate aryl fluoride by PH3. We
were dissuaded, however, by the toxic and pyrophoric
properties of phosphine gas.
tls@cbmse.nrl.navy.mil
Received March 15, 2000
Over the last 25 years, there has been considerable
interest in aqueous and biphasic homogeneous transition
metal catalysis. The most frequently used ligands in the
metal complexes employed for these reactions are func-
tionalized triarylphosphines.1 Triarylphosphines are suf-
ficiently good σ-donors and π-acceptors to stabilize syn-
thetically useful transition metal species, yet, compared
to alkylphosphines, are relatively resistant to oxidation
by adventitious oxygen. This is an important factor for
aqueous catalytic reactions, given the difficulty in remov-
ing oxygen from aqueous media.
A wide variety of cationic, anionic, and nonionic
hydrophilic functional groups have been utilized to im-
part water solubility to triarylphosphines. Sulfonated
phosphine ligands such as P(3-C6H4SO3Na)3 (triphen-
ylphosphine trisulfonate, TPPTS) were demonstrated to
be effective in the biphasic hydroformylation reaction
commercialized by Rhone-Poulenc in the mid-1970s and
remain the most common.2 However, we have instead
focused on the synthesis and reactivity of phosphonate-
functionalized phosphine ligands. Phosphonate groups
and their corresponding salts also impart a high degree
of water solubility to these ligands and offer a further
advantage in being an excellent functionality for the
synthesis of hybrid inorganic-organometallic materials.
These materials have found broad application in the
molecular fabrication of materials, including supported
catalysts,3 chemical sensors,4 electroluminescent materi-
als,5 and nonlinear optical materials.6
It is known that phosphide anions can be generated
directly from red phosphorus by the action of alkali
metals in liquid ammonia.12 The reduction is believed to
proceed via a diphosphide anion, [P-P]4-, which, in the
absence of a proton source more acidic than ammonia, is
resistant to further reduction.13 Addition of alkyl halides
gives tetraalkyldiphosphines, R2P-PR2, along with small
amounts of R3P.14 When the reduction is carried out by
the slow addition of 1 molar equivalent of a proton source
such as t-BuOH to a 1:3 molar mixture of red phosphorus
and lithium, fission of the P-P bond of the intermediate
diphosphide is facilitated. Subsequent addition of 2
equivalents of RX gives dialkylphosphines R2PH in good
yields (eq 1).15 These results suggested that phosphonate-
(7) Schull, T. L.; Fettinger, J . C.; Knight, D. A. Inorg. Chem. 1996,
35, 6717.
(8) McKenna, C. E.; Higa, T.; Cheung, N. H.; McKenna, M. C.
Tetrahedron Lett. 1977, 18, 155.
(9) Villemin, D.; J affres, P. A.; Nechab, B.; Courivaud, F. Tetrahe-
dron Lett. 1997, 38, 6581. Machnitzki, P.; Nickel, T.; Stelzer, O.;
Landgrafe, C. Eur. J . Inorg. Chem. 1998, 1029.
(10) Bitterer, F.; Herd, O.; Hessler, A.; Ku¨hnel, M.; Rettig, K.;
Stelzer, O.; Sheldrick, W. S.; Nagel, S.; Ro¨sch, N. Inorg. Chem. 1996,
35, 4103. Herd, O.; Hessler, A.; Langhans, K. P.; Stelzer, O. J .
Organomet. Chem. 1994, 475, 99.
(11) Kant, M.; Bischoff, S. Z. Anorg. Allg. Chem. 1999, 625, 707.
(12) Royen, P. Z. Anorg. Allg. Chem. 1938, 235, 324. Royen, P.;
Zschaage, W. Z. Naturforsch. 1953, 8B, 777.
(13) Evers, C. E. J . Am. Chem. Soc. 1951, 73, 2038.
(14) Bogolyubov, G. M.; Petrov, A. A. Doklady Akad. Nauk. SSSR
(Engl. Trans.) 1967, 173, 329.
† Center for Biomolecular Science and Engineering.
‡ Laboratory for the Structure of Matter.
§ The George Washington University.
(1) Stelzer, O. In Aqueous-Phase Organometallic Catalysis: Concepts
and Applications; Cornils, B., Herrman, W. A., Eds.; Wiley-VCH:
Weinheim, 1998; Chapter 3.
(2) Cornils, B.; Kuntz, E. G. J . Organomet. Chem. 1995, 502, 177.
(3) Dokoutchaev, A.; Krishnan, V. V.; Thompson, M. E.; Balasubra-
manian, M. J . Mol. Struct. 1998, 470, 191. Tang, X.; Yao, N.;
Thompson, M. E. Supramol. Science 1997, 4, 35.
(4) Brousseau, L. C.; Aurentz, D. J .; Benesi, A. J .; Mallouk, T. E.
Anal. Chem. 1997, 69, 688. Alberti, G.; Cherubini, F.; Palombiri, R.
Sensors Actuators B 1995, 24-25, 270.
(5) Katz, H. E.; Bent, S. F.; Wilson, W. L.; Schilling, M. L.; Ungashe,
S. B. J . Am. Chem. Soc. 1994, 116, 6631.
(6) Katz, H. E.; Wilson, W. L.; Scheller, G. J . Am. Chem. Soc. 1994,
116, 6636.
(15) Arbuzova, S. N.; Brandsma, L.; Gusarova, N. K.; Trofimov, B.
A. Recl. Trav. Chim. Pays-Bas 1994, 113, 575.
10.1021/jo000371y CCC: $19.00 © 2000 American Chemical Society
Published on Web 07/07/2000

5060 J . Org. Chem., Vol. 65, No. 16, 2000
Notes
Sch em e 1
functionalized arylphosphines might also be prepared
from red phosphorus, rather than phosphine gas.
Resu lts a n d Discu ssion
We envisioned a synthesis of TPPTP using a 1:3:1
molar ratio of P/Li/t-BuOH in liquid ammonia/THF to
generate “HPLi2” and subsequent reaction of this species
with 2 equivalents of 4-FC6H4P(O)(NMe2)2 (1) to give the
secondary phosphine 2 (Scheme 1). In situ treatment of
this intermediate with a base such as n-BuLi, followed
by an additional equivalent of 4-FC6H4P(O)(NMe2)2 was
expected to give the desired tertiary phosphine 3. Acid
hydrolysis would furnish 4a , and neutralization with
base would give the salt 4b.
reaction with the aryl fluoride. The net reaction is
presented in eq 2.
Carrying out the first step of the reaction sequence
gave a deep red solution, which was presumed to be the
lithium salt of 2. However, analysis of the reaction
mixture by 31P NMR showed little or no formation of
secondary phosphine 2, or the corresponding lithium salt.
Instead, the tertiary phosphine 3 was observed. Also
present was unreacted 1, which was identified by 31P and
19F NMR spectroscopies. When the reaction was repeated
using a stoichiometry of 2:3 for HPLi2 and aryl fluoride
1, the aryl fluoride was completely consumed, with
phosphine 3 being the major product.
The nature of the arylphosphonate used as a precursor
had a significant influence on the course of the reaction.
Use of the diethyl ester of 4-fluorophenylphosphonic acid
resulted in greatly decreased yields of the tertiary
phosphine P(4-C6H4PO3Et2)3 (5). The principle product
was instead the monodealkylated ester, 4-FC6H4P(O)-
1
(OH)(OEt), identified by 31P, 19F, and H NMR and mass
spectral analysis. In contrast, the precursor 1 appeared
to be relatively resistant to cleavage by phosphide anions,
yet intermediate 3 was easily hydrolyzed to the free
phosphonic acid by refluxing in deoxygenated 2.4 M
hydrochloric acid. Use of the disodium salt of 4-fluo-
rophenylphosphonic acid gave no reaction, presumably
due to lack of solubility in the reaction medium. The bulk
of the starting material was recovered unchanged.
The water solubility of TPPTP was determined to be
approximately 550 mg/mL. This was unexpectedly low.
In comparison, the monophosphonate Ph2P(4-C6H4PO3-
Na2) has a water solubility of 400 mg/mL7 and the
diphosphonate PhP(4-C6H4PO3Na2)2 is reported to have
a water solubility of 1000 mg/mL.11 The reason for the
relatively low water solubility of the triphosphonate may
be due to the fact that the molecule is already highly
solvated in the solid state, with extensive hydrogen
bonding in the crystal matrix.19
From the crystal structure, an empirical formula of
C18H12Na6O9P4‚27H2O was determined. The TPPTP mol-
ecules stack along a C3-axis defined by the central
phosphorus atoms. Surrounding the molecules is a lattice
of sodium atoms and water molecules (Figure 1). The
water molecules form an extensive hydrogen-bonding
network linking the sodium atoms, phosphonate oxygens,
and lattice water. From the 18 water molecules in the
unit cell, 33 hydrogen bonds are formed. The sodium
atoms are hexacoordinate, with only one sodium atom
in the asymmetric unit having a phosphonate oxygen in
its coordination sphere, i.e., Na(4)-O-P(2). The rest of
the coordination sites are occupied by water, a fact that
may contribute the relatively low water solubility, since
a favorable enthalpy of mixing from ion solvation would
not be realized. Interestingly, the solubility of 4b in
methanol displays an inverse temperature dependence,
The exact nature of the phosphide species is not known.
The species HPLi2 would seem unlikely to exist as such
in liquid ammonia, since the pKa of HP2- is estimated to
be about 42, well above the pKa of ammonia (ca. 35).16 It
has been suggested that HPLi2 is an equilibrium mixture
of H2P- and MNH2 (for M ) Li or Na).17 To investigate
this, the liquid ammonia was allowed to evaporate from
a preparation of P/Li/t-BuOH in a 1:3:1 molar ratio in
NH3/THF. A gray suspension in a pale yellow solution
was obtained. Analysis of the THF-soluble fraction by
31P{1H} NMR showed a 1:2:1 triplet at -281 ppm, J PLi
)
38 Hz, which is similar to the coupling constant observed
for a cyclic H2PLi dimer in ether and THF solutions.18
We also observed a small set of doublets at -167 and
-266 ppm, J PP ) 225 Hz, presumed to be either a mixed
dimer or diphosphide species. In the presence of an
additional molar equivalent of t-BuOH, the spectrum
consisted of essentially one singlet at -282 ppm; the pair
of doublets was barely discernible in the baseline. Fur-
ther addition of t-BuOH (excess) gave a homogeneous
solution and the only signal observed was a singlet at
-244 ppm, corresponding to PH3.
The fact that little of the secondary phosphine 2 is
observed may be attributable to the relative pKas of the
reaction intermediates. In contrast to alkyl substitution,
progressive aryl substitution of PH3 (pKa ) 29) to give
PhPH2 (pKa ) 25) and Ph2PH (pKa ) 22) results in an
increase in the acidity of the phosphine as substitution
increases.16 Thus, as arylated phosphine intermediates
are formed, they are metalated by more basic species
such as LiPH2 or LiNH2, and quickly undergo further
(16) Issleib, K.; Ku¨mmel, R. J . Organomet. Chem. 1965, 3, 84.
(17) Trofimov, B.; Gusarova, N.; Brandsma, L. Main Group Chem.
News 1996, 4, 18.
(18) Zschunke, A.; Riemer, M.; Schmidt, H.; Issleib, K. Phosphorus
Sulfur 1983, 17, 237.
(19) A detailed description of the crystal structure will be published
elsewhere.

Notes
J . Org. Chem., Vol. 65, No. 16, 2000 5061
prepared by known methods.22 Flash chromatography was
performed using silica gel (70-230 mesh, 60 Å pore size) under
nitrogen pressure. Thin-layer chromatography was done on silica
gel plates (250 µm thickness, Merck) with fluorescent indicator.
Components were visualized with UV light or 5% ethanolic
phosphomolybic acid.
N,N,N′,N′-Tet r a m et h yl-4-flu or op h en ylp h osp h on od ia -
m id e (1). n-Butyllithium (69 mL of a 2.5 M solution in hexanes,
0.17 mol) was cooled to -78 °C (dry ice/acetone), and a solution
of 1-bromo-4-fluorobenzene (19.7 mL, 0.173 mol) in THF (100
mL) was added dropwise with stirring. The resulting light-yellow
suspension was then added slowly via cannula to a stirred
solution of (Me2N)2P(O)Cl (25 mL, 0.17 mol) in THF (150 mL)
at -78 °C. Once the addition was complete, the cooling bath was
removed and the reaction mixture was allowed to warm to room
temperature. The solvent was removed, and the residue was
taken up in dichloromethane and washed with water. The
organic phase was dried (MgSO4), filtered, and concentrated by
rotary evaporation to give a light yellow oil. The crude product
was vacuum distilled, collecting the fraction at bp 104-106 °C/
0.2 mmHg to give 21.25 g of product (54% yield). 31P{1H} NMR
(CDCl3): δ 29.3 (s). 19F{1H} NMR (CDCl3) δ -108.6 (s). 13C{1H}
NMR (CDCl3) δ 162.4 (dd, J ) 251, 3.3 Hz), 134.1 (t, J ) 8.6,
8.9 Hz), 125.9 (dd, J ) 3.8, 155.5 Hz), 115.1 (dd, J ) 15.1, 15.0
F igu r e 1. The figure shows the two crystallographically
unique triarylphosphine rows, completed by symmetry opera-
tions. The hydrogens have been omitted for clarity. The
asymmetric unit consists of four sodium ions, 18 water
molecules, and two independent [triarylphosphine]/3 moieties
in which the central phosphorus atoms lie on a 3-fold axis of
symmetry. The coordination spheres of the sodium ions have
been completed with symmetry-generated oxygens. The so-
dium and phosphorus atoms are labeled; the oxygen atoms are
light gray spheres. Unbonded oxygens represent hydrogen-
bonded lattice water.
2
Hz), 35.8 (d, J CP ) 3.9 Hz). 1H NMR (CDCl3): δ 7.76 (m, 2H,
C6H4), 7.14 (m, 2H, C6H4), 2.64 (d, 4J CP ) 10.1 Hz, 12H, PNMe2).
IR (thin film), cm-1: 1290 (vs), 1205 (vs, broad), 1159 (vs), 1113
(vs).
Tr is[4-(N,N,N′,N′-tetr am eth ylph osph on odiam ido)ph en yl]-
p h osp h in e (3). To a suspension of red phosphorus (0.469 g, 15.1
mmol) in liquid ammonia (ca. 100 mL) was introduced lithium
metal (0.315 g, 45.4 mmol). To the resulting deep blue mixture
was added dropwise a solution of tert-butyl alcohol (1.122 g, 15.14
mmol) in THF (20 mL) over the course of 1 h. A yellow-orange
suspension resulted, to which a solution of 1 (5.222 g, 22.70
mmol) in THF (20 mL) was added dropwise over several minutes.
The reaction mixture was stirred overnight at room temperature,
allowing the ammonia to evaporate. The suspension was filtered
under nitrogen, and the filtrate was concentrated under oil-pump
vacuum to give a sticky yellow foam. Trituration with ether-
hexane gave a yellow-tinged powder (5.08 g). The product is
sufficiently pure to be used directly for hydrolysis to the acid or
may be further purified by chromatography on silica gel using
CHCl3 as the eluant, followed by 2% MeOH-CHCl3, and
collecting the component at Rf 0.40 (10% MeOH-CHCl3).
31P{1H} NMR (CDCl3): δ 27.9 (s, PO), -4.8 (s, P). 1H NMR
(CDCl3): δ 7.73 (m, 6H, C6H4), 7.36 (m, 6H, C6H4), 2.65 (d,
4J HP ) 10.0 Hz, 18H, PNMe2).
Tr is[4-(O,O′-d iet h ylp h osp h on o)p h en yl]p h osp h in e (5).
The above procedure was used with 4-FC6H4PO3Et2 as the
substrate. Red phosphorus (0.123 g, 3.97 mol) and lithium (0.083
g, 12 mmol) were stirred in liquid NH3 (50 mL) and treated with
a solution of t-BuOH (0.19 mL, 2.0 mmol) in THF (5 mL). A
solution of 4-FC6H4PO3Et2 (1.392 g, 6.000 mmol) in THF (15 mL)
was then added dropwise. After overnight stirring of the reaction
mixture, the reddish-brown solution was worked up by addition
of 20% aqueous NH4Cl (30 mL) and ether (50 mL). The phases
were separated, and the aqueous phase was extracted with an
additional portion of ether. The combined ether phases were
dried (MgSO4) and concentrated by rotary evaporation and then
placed under oil-pump vacuum to give 0.630 g of crude product.
This was chromatographed on a column (20 mm) of silica gel
(40 g) using 4% MeOH-Et2O as the eluant. The first component,
Rf 0.51 (8% MeOH-Et2O), was collected to give 0.200 g of a white
solid (∼16% yield). This crystallized from hexane-acetone as
white needles. The compound was identified as 4-FC6H4P(O)-
(OH)(OEt) from 31P, 19F, 13C, and 1H NMR and the mass
spectrum. The eluant was changed to 8% MeOH-Et2O, and a
second component, Rf 0.17, eluted. It was isolated as a clear,
slightly yellow oil, 0.112 g (∼3% yield). 31P{1H} NMR (CDCl3):
δ 18.6 (s, PO), -4.1 (s, P). 13C{1H} NMR (CDCl3): δ 140.6 (dd,
J CP ) 16.1, 3.1 Hz), 133.3 (dd, J CP ) 19.9, 19.5 Hz), 131.5 (dd,
J CP ) 10.4, 9.5 Hz), 129.3 (d, J CP ) 188.8 Hz), 62.0 (d, J CP ) 5.4
forming an insoluble mass at reflux, and redissolving
upon cooling. The neutral phosphine 3 has appreciable
water solubility as well.20
In summary, we have found that the phosphonate-
functionalized phosphine TPPTP may be prepared di-
rectly from red phosphorus by reduction of the phospho-
rus with lithium metal in liquid ammonia and reaction
of the resulting phosphide anion with N,N,N′,N′-tetra-
methyl-4-fluorophenylphosphonodiamide to give inter-
mediate 3, which can be converted to TPPTP by subse-
quent acid hydrolysis and neutralization with NaOH. In
this procedure, the need to generate or handle phosphine
gas is avoided. The method is comparable in yield with
the SRN1 reaction of phosphide anions with aryl halides
described by Rossi et al.,21 but does not require photo-
stimulation and does not incur phosphine oxide forma-
tion.
Exp er im en ta l Section
Gen er a l. All reactions were conducted under dry, prepurified
nitrogen using standard Schlenk line techniques when appropri-
ate. NMR spectra were referenced to internal TMS or 2,2,3,3-
d4-3-trimethylsilylpropionic acid (TSP) for 1H spectra, TSP or
solvent for 13C spectra, external H3PO4 for 31P spectra, and
external PhCF3 for 19F spectra. Mass spectra were acquired
using electrospray ionization techniques by Dr. J ohn Callahan
of the Chemistry Division, Code 6100, Naval Research Labora-
tory, Washington DC. The following solvents and reagents were
obtained from Aldrich Chemical Co. and used as is (purity and
grade): red phosphorus (99%), lithium (99.9%), tert-butyl alcohol
(99.5%, anhydrous), tetrahydrofuran (99.9%, anhydrous), metha-
nol (99.8+%), chloroform (99+%, anhydrous), ether (99+%,
anhydrous), hexane (95+%), sodium hydroxide (99.99%), n-
butyllithium (2.5 M in hexanes), and 1-bromo-4-fluorobenzene
(99%). N,N,N′,N′-Tetramethylphosphonodiamidic chloride (Flu-
ka, > 95%) and liquid ammonia (Matheson, 99.99%, anhydrous)
were used as received. Diethyl 4-fluorophenylphosphonate was
(20) Crude 3 was found to dissolve readily in 5-10 volumes of water.
(21) Bonancini, R. E.; Alonso, R. A.; Rossi, R. A. J . Organomet. Chem.
1984, 270, 177.
(22) Sasse, K. In Methoden der Organischen Chemie, Band E1,
Organische Phosphorverbindungen I; Thieme-Verlag: Stuttgart, 1982;
p 498.

5062 J . Org. Chem., Vol. 65, No. 16, 2000
Notes
Hz), 16.1 (d, J CP ) 5.8 Hz). 1H NMR (CDCl3): δ 7.81 (m, 6H,
C6H4), 7.40 (m, 6H, C6H4), 4.16 (m, 12H, OCH2), 1.35 (m, 18H,
CH3). IR (neat film), cm-1: 1257 (vs), 1040 (vs, broad). ESMS:
671.1 [M + H]+ (100%).
(D2O): δ 11.4 (s, PO), -7.2 (s, P); 1H NMR (D2O): δ 7.74 (m,
6H, C6H4), 7.44 (m, 6H, C6H4). 13C{1H} NMR (D2O): δ 144.5 (d,
J CP ) 268 Hz), 138.9 (d, J CP ) 9.2 Hz), 135.6 (dd, J CP ) 30.5,
9.5 Hz), 133.2 (apparent triplet; J CP ) 13.1, 12.5 Hz). IR (KBr
disk), cm-1: 1073 (vs, broad), 970 (vs). ESMS: 261.1 [M - 5Na
+ 3H]2- (100%), 166.4 [M - 6Na + 3H]3- (40%). The efflorescent
nature of the material precluded obtaining a satisfactory el-
emental analysis. The identity of the product was confirmed by
single-crystal X-ray diffraction, and the purity was established
Tr is(4-p h osp h on a top h en yl)p h osp h in e Hexa sod iu m Sa lt
(TP P TP ) (4b). Starting from 21.25 g (92.39 mmol) of 1, using
the above procedure, the reaction was worked up by addition of
deoxygenated ether and water to the organic phase remaining
after overnight evaporation of the ammonia. The aqueous phase
was separated and sparged vigorously with nitrogen to remove
volatile materials. The mixture was then acidified with deoxy-
genated hydrochloric acid to a pH of 1, resulting in the formation
of a viscous brown oil. The mixture was heated and stirred
vigorously under a nitrogen atmosphere until a suspension was
obtained. The reaction mixture was filtered to give 4.80 g of
phosphonic acid 4a as a light tan powder. Cooling the filtrate
at 5 °C for 48 h gave another 5.49 g of 4a as an off-white powder,
for a total recovery of 10.29 g. Both samples were nearly pure
by 31P, 1H, and 13C NMR spectroscopies. To obtain salt 4b, a
portion of crude 4a was dissolved in methanol and neutralized
with the stoichiometric amount of 50% aqueous NaOH. The
solvent was removed by rotary evaporation, and the resulting
material was crystallized from warm, deoxygenated water
layered with small amount of ethanol. Colorless needles were
obtained, which turned white upon drying. 31P{1H} NMR
1
by 31P, 13C, and H NMR analyses (see the Supporting Informa-
tion).
Ack n ow led gm en t. We thank Dr. J ohn Callahan for
providing electrospray mass spectral data and analysis
and Mr. J ohn Mecholsky for assistance in acquiring the
NMR data. We also thank the National Research
Council for a postdoctoral fellowship (T.L.S.).
Su p p or tin g In for m a tion Ava ila ble: NMR data for com-
pounds 1 (31P, 13C, 1H), 3 (31P, 1H), 4b (31P, 13C, 1H), and 5
(31P, 13C, 1H); X-ray crystallographic data for 4b. This material
is available free of charge via the Internet at http://pubs.acs.org.
J O000371Y