Convenient preparation of tri-tert-butylphosphonium tetrafluoroborate

Article abstract of DOI:10.1055/s-0030-1260113

The versatile tri-tert-butylphosphonium tetrafluoroborate ligand is prepared in a convenient, simple, and high-yielding procedure without the isolation of sensitive intermediates. Georg Thieme Verlag Stuttgart New York.

Full text of DOI:10.1055/s-0030-1260113

PRACTICAL SYNTHETIC PROCEDURES
2369
Convenient Preparation of Tri-tert-butylphosphonium Tetrafluoroborate
C
T
onvenient Pre
a
parationof
n
Tri-tert-buty
g
lphosphonium
u
Tetrafluoro
y
borate Saget,a,b Nicolai Cramer*a
a
Laboratory of Asymmetric Catalysis and Synthesis, Ecole Polytechnique Fédérale de Lausanne,
EPFL SB ISIC LCSA, BCH 4305, 1015 Lausanne, Switzerland
Fax +41(21)6939700; E-mail: nicolai.cramer@epfl.ch
Laboratorium für Organische Chemie, ETH Zürich, HCI H 304, Wolfgang-Pauli-Strasse 10,
b
8
093 Zurich, Switzerland
PSP
Received 28 April 2011; revised 25 May 2011
No204
Abstract: The versatile tri-tert-butylphosphonium tetrafluoroborate ligand is prepared in a convenient, simple, and high-yielding procedure
without the isolation of sensitive intermediates.
Key words: ligands, homogeneous catalysis, phosphorus, transition metals, Grignard reaction
t-BuMgCl (4 equiv)
t-Bu
CuBr⋅Me2S (5 mol%), LiBr (10 mol%), Et2O, hexane
PH+ BF4–
PCl3
t-Bu
then: 3 M aq HBF4
t-Bu
extraction and recrystallization
1
7
5%
Scheme 1 Synthesis of tri-tert-butylphosphonium tetrafluoroborate from inexpensive starting materials without isolation and purification of
pyrophoric material
The introduction of basic and bulky trialkylphosphines, However, tri-tert-butylphosphine has a low melting point
such as tricyclohexylphosphine and tri-tert-butylphos- (30 °C), pyrophoric nature, and high oxidation propensity
phine, has had a tremendous impact in transition-metal ca- in the solid state as well as in solution, hence it requires
1
talysis. According to the carbonyl stretching frequency of careful handling and sophisticated inert gas techniques. A
the Ni(CO) [P(t-Bu) ] complex, tri-tert-butylphosphine is broadly applicable solution to these shortcomings was
3
3
2
a very strong electron-donating ligand. It is also a steri- made by Fu and co-workers, who introduced trialkylphos-
cally very bulky ligand having a large cone angle of 182°.3 phonium salts as replacements for the free phosphines. In
Its electron-rich nature facilitates the oxidative addition of stark contrast, these salts are nonhygroscopic, they have
previously reluctant substrates (e.g., aryl chlorides) in pal- high melting points, and, most importantly, they are inde-
6
ladium-catalyzed reactions even at ambient temperature.4 finitively air-stable compounds. The free phosphine (pK
The steric demand of the phosphine aids the formation of 11.4) is subsequently generated conveniently in situ by
a
7
coordinatively unsaturated species of crucial importance treatment with almost any base, which is usually required
for initiating catalytic processes and its steric bulk en- in stoichiometric amounts, in the individual transition-
5
hances the rate of reductive elimination. Therefore, tri- metal-catalyzed process. Although widely available, tri-
tert-butylphosphine has found widespread application in tert-butylphosphonium tetrafluoroborate (1) has a rela-
8
many transition-metal-catalyzed reactions (Scheme 2).
tively high price. The reported synthesis of the phospho-
nium salt requires the hazardous and equally expensive
free phosphine as a reactant. In contrast to triarylphos-
phines, sterically demanding trialkylphosphines are prob-
lematic to synthesize from phosphorus trichloride and
magnesium or lithium organyls. Poor yields and forma-
tion of dialkylchlorophosphines are often observed. The
addition of copper salts has largely addressed this issue
Stille
Suzuki
Hiyama
(
t-Bu3PH)BF4
0
Ar Pd Cl
Pt-Bu3
Ar Cl
Heck
[
Pd ], base
amination
enolate arylation
direct arylation
9
and good yields can be obtained. Several dialkyl(ar-
Scheme 2 Illustrating the versatility of tri-tert-butylphosphonium
yl)phosphonium salts can be directly isolated by precipi-
tetrafluoroborate in palladium catalysis; the oxidative addition of aryl tation from the crude reaction mixture by addition of
aqueous acids such as tetrafluoroboric acid.10 However,
chlorides enables a myriad of reactions
the most versatile tri-tert-butylphosphonium salts are not
accessible by this method as they possess a relatively high
solubility in water. Hence, their preparation requires a te-
dious isolation and purification procedure of the highly
oxidation-sensitive and pyrophoric free phosphine.
SYNTHESIS 2011, No. 15, pp 2369–2371
x
x.
x
x
.
2
0
1
1
Advanced online publication: 14.07.2011
DOI: 10.1055/s-0030-1260113; Art ID: Z44711SS
Georg Thieme Verlag Stuttgart · New York
©

2
370
T. Saget, N. Cramer
PRACTICAL SYNTHETIC PROCEDURES
Herein we report a procedure addressing these shortcom- Tri-tert-butylphosphonium Tetrafluoroborate (1)
To a dried 500-mL three-neck flask equipped with an addition fun-
nel, an internal thermometer, and a magnetic stirrer bar were added
ings and present a synthesis of tri-tert-butylphosphonium
tetrafluoroborate (1) from inexpensive starting materials
without the need for sophisticated equipment that gives 1
reliably, in good yields, and on a 50-mmol scale
CuBr·Me S (514 mg, 2.50 mmol) and LiBr (434 mg, 5.00 mmol).
2
The reaction vessel was purged with N and hexane (100 mL) was
2
added. PCl (4.37 mL, 50.0 mmol) was added to the suspension and
3
(
Scheme 1). Simple extraction procedures followed by
the reaction was cooled with an ice bath. 2.0 M t-BuMgCl in Et O
2
crystallization avoid any handling of air sensitive and haz- (100 mL, 200 mmol) was added dropwise and during the addition
ardous phosphine. We use advantageously the high solu- of the first 50 mL, the internal temperature was kept below 8 °C.
The flask was warmed with an ambient water bath and the remain-
der of the t-BuMgCl soln was added and the mixture was stirred vig-
orously for 13 h at 23 °C. The mixture was then recooled with an ice
bility of the tri-tert-butylphosphonium salts in aqueous
solution, thus allowing the removal of uncharged and ap-
olar byproducts by extraction with hydrocarbon or ethere-
al solvents. The desired phosphonium salt itself is
bath and 3 M aq HBF soln (175 mL, 525 mmol) was carefully add-
4
ed, keeping the internal temperature below 25 °C. The biphasic
subsequently extracted with dichloromethane. By such mixture was stirred for 15 min and filtered over a pad of Celite. The
treatment, the obtained crude product already has of a pu- layers were separated and the aqueous layer was washed with hex-
1
ane (2 × 100 mL) to remove apolar impurities. The aqueous layer
rity of >95% ( H NMR). The main contamination consists
of di-tert-butylphosphonium tetrafluoroborate. A single
recrystallization from ethanol reduces this impurity to less
than 0.3% and provides 1 as crystalline material. This also
allows the previously unobtained X-ray crystallographic
was then extracted with CH Cl (3 × 200 mL). The combined
2
2
CH Cl layers were dried (MgSO ), filtered, and evaporated in vac-
2
2
4
uo to afford crude tri-tert-butylphosphonium tetrafluoroborate
12.0 g) as a white solid (96% NMR purity). Crystallization (EtOH,
mL/g) afforded analytically pure material (9.76 g) as colorless
(
6
structure of the phosphonium tetrafluoroborate to be re- plates. The mother liquor was concentrated and crystallization
corded (Figure 1).
1
1
(EtOH) afforded additional product (1.06 g) (75% combined yield);
mp 300–302 °C (dec.) (EtOH).
–
1
IR (ATR): 3003, 1474, 1382, 1179, 1052, 1029, 907, 885, 726 cm .
1
1
H NMR (400 MHz, CDCl ): d = 6.08 (d, J = 465 Hz, 1 H), 1.67
3
PH
3
(
d, J = 15.3 Hz, 27 H).
PH
1
3
1
C NMR (100 MHz, CDCl ): d = 37.1 (d, J = 28.8 Hz), 30.1.
3
PC
3
1
P NMR (162 MHz, CDCl ): d = 51.5.
3
Anal. Calcd for C H BF P: C, 49.68; H, 9.73. Found: C, 49.68; H,
1
2
28
4
9
.63.
Acknowledgment
We thank the ETH Zurich (ETH-16 09-3) as well as Prof. E. M. Car-
reira for generous support. The Fonds der Chemischen Industrie is
acknowledged for a Liebig-Fellowship to N.C. We thank Dr. R.
Scopelliti (EPFL) for the X-ray crystallographic analysis of 1.
Figure 1 ORTEP representation of tri-tert-butylphosphonium
tetrafluoroborate (probability ellipsoids at 50%, hydrogen atoms
omitted for clarity)
References
(
(
1) Fu, G. C. Acc. Chem. Res. 2008, 41, 1555.
2) Bartik, T.; Himmler, T. J. Organomet. Chem. 1985, 293,
In summary, we have developed an experimentally simple
and practical protocol for the synthesis tri-tert-butylphos-
phonium tetrafluoroborate. This procedure gives good
yields and high purities in a scalable fashion and requires
only basic equipment and inexpensive chemicals.
343.
(3) Tolman, C. A. Chem. Rev. 1977, 77, 313.
(4) Littke, A. F.; Fu, G. C. Angew. Chem. Int. Ed. 2002, 41,
4176.
(
5) (a) Christmann, U.; Vilar, R. Angew. Chem. Int. Ed. 2005,
4, 366. (b) Hartwig, J. F. Angew. Chem. Int. Ed. 1998, 37,
4
Hexane was dried by passage over activated alumina under an N at-
2
2046. (c) Galardon, E.; Ramdeehul, S.; Brown, J. M.;
Cowley, A.; Hii, K. K.; Jutand, A. Angew. Chem. Int. Ed.
2002, 41, 1760. (d) Stambuli, J. P.; Buehl, M.; Hartwig, J. F.
J. Am. Chem. Soc. 2002, 124, 9346. (e) Stambuli, J. P.;
Incarvito, C. D.; Buehl, M.; Hartwig, J. F. J. Am. Chem. Soc.
mosphere prior to use. Technical grade hexane and CH Cl were
2
2
used for the workup. CuBr·Me S (Acros), LiBr (Fluka), PCl (Flu-
2
3
ka), 2.0 M t-BuMgCl in Et O (TCI) were used as received. 48% aq
2
HBF was purchased from Acros and diluted to 3 M prior to use.
4
The soln was briefly degassed prior to use with three vacuum cycles
2
004, 126, 1184. (f) Yamashita, M.; Hartwig, J. F. J. Am.
[
300 mbar/N ]. The melting point was obtained on a Büchi B-540
2
Chem. Soc. 2004, 126, 5344.
1
apparatus in open capillary tubes. H NMR data were acquired on a
DPX-400 400 MHz spectrometer in CDCl with TMS as reference.
(
6) Netherton, M. R.; Fu, G. C. Org. Lett. 2001, 3, 4295.
3
(7) Rahman, M. M.; Liu, H.-Y.; Eriks, K.; Prock, A.; Giering,
1
3
1
C NMR data were acquired with H-decoupling on a Bruker DPX-
00 100 MHz spectrometer in CDCl , relative to the triplet at d =
7.0 ppm for CDCl . IR data were recorded on a Bruker ALPHA
W. P. Organometallics 1989, 8, 1.
4
7
3
(8) 57 $/g from Strem Chemicals Inc on May 23, 2011.
(9) (a) See supporting information in: Stambuli, J. P.; Stauffer,
S. R.; Shaughnessy, K. H.; Hartwig, J. F. J. Am. Chem. Soc.
3
FT-IR spectrophotometer. Combustion elemental analyses was per-
formed by the analytical facilities of EPFL.
2
2
001, 123, 2677. (b) Maehara, S.; Iwasaki, H. EP 1473297,
004.
Synthesis 2011, No. 15, 2369–2371 © Thieme Stuttgart · New York

PRACTICAL SYNTHETIC PROCEDURES
Tri-tert-butylphosphonium Tetrafluoroborate
2371
(
(
10) Rampf, F.; Militzer, H.-C. EP 1354886, 2003.
11) Crystallographic data for tri-tert-butylphosphonium
tetrafluoroborate C H BF P, M = 290.13, monoclinic,
26253, independent reflections: 6289, R(all) = 0.1017,
wR(gt) = 0.3027. CCDC 822956 contains the supplementary
crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
1
2
28
4
space group P21/c, a = 12.3185(17) Å, b = 23.883(4) Å,
3
c = 16.219(3) Å, b = 90.319(13), V = 4771.6(13) Å , Z = 12,
–
3
Dcalc = 1.212 mg m , T = 100 K, reflections collected:
Synthesis 2011, No. 15, 2369–2371 © Thieme Stuttgart · New York