Electrophilic phosphonium cations (EPCs) with perchlorinated-aryl substituents: Towards air-stable phosphorus-based Lewis acid catalysts

Article abstract of DOI:10.1039/c6dt01339b

A series of phosphines incorporating (C₆Cl₅) substituents, Ph₂P(C₆Cl₅) 1, PhP(C₆Cl₅)₂2, P(C₆Cl₅)₃3 and (C₆F₅)P(C₆Cl₅)₂4 were prepared. In the case of 1, 2 and 4, these were converted to the corresponding aryl-difluorophosphoranes 5-7via reaction with XeF₂, whereas reaction of 3 with XeF₂ afforded only an inseparable mixture of products. The compounds 5-7 were converted to the fluorophosphonium cations 8-10, whereas the reaction of 3 with Selectfluor afforded (C₆Cl₅)₂POF and (C₆Cl₅)₂. The fluorophosphonium salts showed evidence of improved air stability as well as Lewis acid catalytic activity in hydrodefluorination, hydrosilylation, deoxygenation and dehydrocoupling chemistry.

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ARTICLE TYPE
Electrophilic Phosphonium Cations (EPCs) with Perchlorinated-Aryl
Substituents: Towards Air-Stable Phosphorus-based Lewis Acid Catalysts
Shawn Postle‡, Vitali Podgorny‡ and Douglas W. Stephan*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
The series of phosphines incorporating (C6Cl5) substituents, Ph2P(C6Cl5) 1, PhP(C6Cl5)2 2, P(C6Cl5)3 3 and (C6F5)P(C6Cl5)2 4 were prepared.
In the case of 1, 2 and 4, these were converted to the corresponding aryl-difluorophosphoranes 5-7 via reaction with XeF2, whereas
reaction of 3 with XeF2 afforded only an inseparable mixture of products. The compounds 5-7 were converted to the
fluorophosphonium cations 8-10, whereas the reaction of 3 with Selectfluor afforded (C6Cl5)2POF and (C6Cl5)2. The fluorophosphonium
10 salts showed evidence of improved air stability as well as Lewis acid catalytic activity in hydrodefluorination, hydrosilylation,
deoxygenation
and
dehydrocoupling
chemistry.
clearly be favourable to develop analogs that exhibit similar
50 Lewis acidic reactivity, but enhanced tolerance to moisture. It is
interesting to note that similar moisture tolerance issues occur
with the electrophilic boron based Lewis acid B(C6F5)3. We noted
the very creative approach developed by Ashley and coworkers12
who investigated the family of closely related boranes that
55 incorporated perchlorophenyl substituents. While these boranes
retained comparable Lewis acidity, the presence of the
perchlorophenyl substituents offered additional steric protection
to the boron centre and thus provided a significant increase in
Introduction
Our initial work which precipitated the concept of frustrated
Lewis pairs (FLPs) is now ten years old.1 In the intervening period,
15 this area has seen dramatic growth.2 More recently, applications
of the concept to a broadening range of chemical systems has
been reported. For example, systems that activate H2 in the
enzyme hydrogenase3 have been described as FLP-like, and
heterogeneous catalysts for CO2 reduction4 have been similarly
20 described. Despite the ubiquity of Lewis acids of varying
strengths within main group chemistry, it is only recently that
this broader range of Lewis acids has been exploited for FLP
chemistry.5 We, and subsequently others, have reported the use
of borenium cations in FLP catalysis.6 Most recently, Krempner
25 has described a very nice study in which strong bases are paired
with weak Lewis acids to perform FLP chemistry.7 The use of
Lewis acids based on elements other than B, including Al, Si and
C have also emerged and been reviewed.5a
air-stability.
A further related study on the impact of
60 perchlorophenyl substituents in borane has recently been
authored by Wildgoose and coworkers.13 Herein, we apply this
strategy to fluorophosphonium cations and probe the impact of
these substituents on air-stability and catalytic reactivity.
Among the variety of Lewis acids under examination, we
30 have focused attention on electrophilic phosphonium cations
(EPCs). We initially exploited electron deficient substituents as
an obvious approach to generate Lewis acidic phosphonium
cations.8 Subsequently, we found that the introduction of
additional positively charged centers enhanced Lewis acidity
35 (Figure 1).9 Indeed this latter observation proved to be
reminiscent of the strategy developed by Gabbai in his design of
fluoride ion sensors.10 We have used such Lewis acidic
phosphonium cations in a variety of catalytic transformations.
These include hydrodefluorination of fluoroalkanes, the
40 hydrosilylation of ketones, imines, nitriles and olefins;
dehydrocoupling of amines, thiols, or alcohols with silane;
transfer-hydrogenation reactions; FLP hydrogenations of olefins
65
Figure 1. Examples of Electrophilic Phosphonium Cations.
and the activation of C-F bonds in Friedel-Crafts chemistry.8, 9b, 9c,
11
Experimental Section
45
A
general feature of these EPCs, specifically
fluorophosphonium cations such as [(C6F5)3PF]+ is the sensitivity
to moisture, which leads to catalyst degradation. While these
catalysts are effective under rigorously dry conditions, it would
General Considerations All manipulations were performed in an
MBraun MB Unilab glovebox or using standard Schlenk
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DOI: 10.1039/C6DT01339B
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1
techniques under an inert atmosphere of anhydrous N2. All
glassware was oven-dried and cooled under vacuum before use.
Dry, oxygen-free solvents (CH2Cl2, Et2O, toluene and n-pentane)
C6D6): δ 137.1 (d, JPC = 19 Hz, o-C6Cl5), 136.0 (d, JPC = 36 Hz, i-
4
3
C6Cl5), 135.3 (d, JPC = 1 Hz, p-C6Cl5), 133.4 (d, JPC = 1 Hz, m-
1
C6H5), 131.8 (d, JPC = 15 Hz, i-C6H5), 130.8 (s, p-C6H5), 128.9 (d,
were prepared using an Innovative Technologies solvent 65 JPC = 9 Hz, o-C6H5), 128.5 (d, JPC = 14 Hz, m-C6H5) ppm. 31P{1H}
2
3
5
purification system. CD2Cl2 and CD3CN (Aldrich) were
deoxygenated, distilled over CaH2, then stored over 4 Å
molecular sieves before use. C6D6 and C6D5Br (Aldrich) were
deoxygenated and stored over 4 Å molecular sieves before use.
NMR (243 MHz, C6D6): δ 15.1 (s) ppm. Anal. Calcd. for PC18H5Cl10:
C: 35.63, H: 0.83. Found: C: 35.29%, H: 0.92%. HRMS (DART
Ionization) m/z: [M+H]+ Calcd. for C18H5Cl10P: 602.70924, Found:
602.70831.
Commercial reagents were purchased from Sigma-Aldrich, Strem 70 3: Yield: (147 mg, 21%) as a white solid. 31P{1H} NMR (162 MHz,
10 Chemicals, Apollo Scientific, TCI Chemicals or Alfa Aesar, and
were used without further purification unless indicated
otherwise. [Et3Si][B(C6F5)4]·(C7H8) was prepared by the reported
procedure.14 NMR spectra were obtained on a Bruker AvanceIII-
C6D6): δ 19.1 (s) ppm. Anal. Calcd. for PC18Cl18: C: 27.76. Found:
C: % 25.87. HRMS (EI-TOF) m/z: [M+H]+ Calcd. for C18HCl15P:
778.50553, Found: 778.50594.
4: Yield: (261 mg, 42% yield) as a white powder. 13C NMR (125
400 MHz spectrometer, Varian NMR system 400 MHz 75 MHz, C6D6): δ 147.9 (br d, 1JFC = 245 Hz, o-C6F5), 143.2 (br d, 1JFC
=
=
15 spectrometer, Agilent DD2-500 MHz spectrometer, or Agilent
DD2-600 MHz spectrometer. 1H NMR data, referenced to
external Me4Si, are reported as follows: chemical shift (δ),
multiplicity (s = singlet, d = doublet, t = triplet, q = quartet, quin
258 Hz, p-C6F5), 137.9 (br d, 1JFC = 248 Hz, m-C6F5), 137.1 (d, 2JPC
21 Hz, o-C6Cl5), 136.4 (s, p-C6Cl5), 133.6 (s, m-C6Cl5), 132.4 (d, 1JPC
= 34 Hz, i-C6Cl5), 108.6 (br s, i-C6F5) ppm. 19F{1H} NMR (376 MHz,
3
C6D6): δ -129.2 to -129.6 (m, 2F, o-C6F5), -147.5 (tt, JFF = 22 Hz,
5
= quintet, m = multiplet, br = broad), coupling constant (Hz), 80 JFF = 5 Hz, 1F, p-C6F5), -159.6 to -159.9 (m, 2F, m-C6F5) ppm.
20 normalized integrals. 13C{1H} NMR chemical shifts (δ) are
referenced to external Me4Si. Assignments of individual
resonances were done using 2D NMR techniques (HMBC, HSQC,
HH-COSY) when necessary. High-resolution mass spectra (HRMS)
were obtained on an Agilent 6538 Q-TOF (ESI) or a JEOL AccuTOF
25 (DART) mass spectrometer. Elemental analyses were performed
at the University of Toronto employing a Perkin Elmer 2400
Series II CHNS Analyser.
31P{1H} NMR (162 MHz, C6D6): δ -17.8 (t, JPF = 40 Hz) ppm. Anal.
Calcd. for PC18F5Cl10: C: 31.03. Found: C: 31.19%. HRMS (DART
Ionization) m/z: [M+H]+ Calcd. for C18F5Cl10P: 692.66213, Found:
692.66280.
3
85
Synthesis of Ph2PF2(C6Cl5) 5, PhPF2(C6Cl5)2 6, (C6F5)PF2(C6Cl5)2 7:
These compounds were prepared in a similar fashion and thus
only one preparation is detailed. In a cold well, a 20 mL vial was
charged with 1 (257 mg, 0.59 mmol), CH2Cl2 (4 mL) and a
Synthesis of Ph2P(C6Cl5) 1, PhP(C6Cl5)2 2, P(C6Cl5)3 3, 90 magnetic stir bar, forming a light yellow solution. To the stirring
30 (C6F5)P(C6Cl5)2 4: These compounds were prepared in a similar
fashion and thus only one preparation is detailed. A 100 mL
Schlenk flask was charged with C6Cl6 (318 mg, 1.12 mmol), Et2O
(30 mL) and a magnetic stir bar, generating a white slurry. The
solution, a solution of XeF2 (100 mg, 0.59 mmol) in CH2Cl2 (3 mL)
was quickly added. The effervescent solution lightened as it was
left to stir for 2 hr. The solvent was reduced and the solution
cooled to -35 °C to produce a white precipitate. The precipitate
reaction flask was cooled to –15 °C in a dry ice/acetone bath. A 95 was collected by filtration and washed with n-pentane (2 x 2 mL)
35 hexane solution of 2.5 M n-BuLi (0.44 mL, 1.12 mmol) was added
dropwise to the stirring solution, which gradually turned the
slurry to a clear, light yellow solution. The solution was then
cooled to –78 °C, before a solution of Ph2PCl (247 mg, 1.12
yielding a white solid. The CH2Cl2 was removed in vacuo yielding
5 (230 mg, 83%) as a yellow solid. Recrystallization of 5 from
vapour diffusion of n-pentane into a solution of the compound in
dichloromethane yielded x-ray quality crystals.
1
mmol) in Et2O (3 mL) was added to the reaction flask in a 100 5: Yield: (321 mg, 98 %). H NMR (500 MHz, C6D6): δ 8.17 – 8.10
40 dropwise fashion. The stirred solution was warmed to room
temperature overnight. The solvent was removed in vacuo
leaving an off-white solid. The product was dissolved in CH2Cl2 (6
mL) and filtered through Celite. The solvent was reduced and the
(m, 4H, m-C6H5), 7.08 – 7.01 (m, 6H, o-, p-C6H5) ppm. 13C NMR
1
(125 MHz, C6D6): δ 140.1 (dt, JPC = 217 Hz, 2JFC = 42 Hz, i-C6Cl5),
1
2
2
137.5 (dt, JPC = 181 Hz, JFC = 24 Hz, i-C6H5), 135.0 (dt, JPC = 13
Hz, 3JFC = 10 Hz, o-C6H5), 134.5 (dt, 2JPC = 3 Hz, 3JFC = 2 Hz, o-C6Cl5),
solution was cooled to -35 °C to produce a white precipitate, 105 133.5 (d, JPC = 17 Hz, p-C6Cl5), 132.6 (dt, JPC = 3 Hz, 4JFC = 3 Hz,
4
3
4
5
3
45 which was collected by filtration. The white filtrate was then
washed with n-pentane (2 x 2 mL) to yield 1 (340 mg, 70%) as a
white solid. The recrystallization of 1 and 4 from vapour diffusion
m-C6Cl5), 131.7 (dt, JPC = 4 Hz, JFC = 1 Hz, p-C6H5), 128.6 (dt, JPC
= 13 Hz, 4JPC = 10 Hz, m-C6H5) ppm. 19F{1H} NMR (376 MHz, C6D6):
1
δ -41.8 (d, JPF = 716 Hz, PF2) ppm. 31P{1H} NMR (162 MHz, C6D6):
1
of n-pentane into
a
solution of the compound in
δ -50.7 (t, JPF = 715 Hz) ppm. Anal. Calcd. for PC18H10Cl5F2: C:
dichloromethane yielded x-ray quality crystals.
50 1: Yield: (338 mg, 70%). H NMR (500 MHz, C6D6): δ 7.33 – 7.39
110 45.76, H: 2.13. Found: C: 45.24%, H: 2.00%.
1
6: Yield: (123 mg, 76%) as an orange solid. 1H NMR (500 MHz,
C6D6): δ 8.11 – 8.04 (m, 2H, m-C6H5), 7.16 – 7.08 (m, 3H, o-, p-
(m, 4H, m-C6H5), 7.08–7.03 (m, 6H, o-, p-C6H5) ppm.
13C NMR (125 MHz, C6D6): δ 139.7 (d, 2JPC = 18 Hz, o-C6Cl5), 136.4
C6H5) ppm. 13C NMR (125 MHz, C6D6): δ 137.7 (dt, JPC = 210 Hz,
1
(s, p-C6Cl5), 136.0 (d, 1JPC = 24 Hz, i-C6Cl5), 134.3 (d, 1JPC = 15 Hz, i-
2JFC = 32 Hz, i-C6Cl5), 136.7–136.6 (m, o-C6Cl5), 135.8 (dt, 2JPC = 14
2
3
1
2
C6H5), 133.3 (s, m-C6Cl5), 132.9 (d, JPC = 21 Hz, o-C6H5), 129.1 (s, 115 Hz, JFC = 10 Hz, o-C6H5), 135.4 (dt, JPC = 187 Hz, JFC = 23 Hz, i-
3
4
55 p-C6H5), 129.0 (d, JPC = 6 Hz, m-C6H5) ppm. 31P{1H} NMR (162
C6H5), 134.3 (d, JPC = 18 Hz, p-C6Cl5), 134.0 – 133.8 (m, m-C6Cl5),
4
3
MHz, C6D6): δ 10.7 (s) ppm. Anal. Calcd. for PC18H10Cl5: C: 49.76,
H: 2.32. Found: C: 49.82%, H: 1.99%. HRMS (DART Ionization)
m/z: [M+H]+ Calcd. for C18H10Cl5P: 432.90410, Found: 432.90360.
133.0 (br d, JPC = 4 Hz, p-C6H5), 129.3 (dm, JPC = 18 Hz, m-C6H5)
ppm. 19F{1H} NMR (470 MHz, C6D6): δ -28.9 (d, JPF = 747 Hz, PF2)
1
ppm. 31P{1H} NMR (202 MHz, C6D6): δ -44.9 (t, 1JPF = 747 Hz) ppm.
2: Yield: (421 mg, 36%) as a pale yellow solid. 1H NMR (500 MHz, 120 Anal. Calcd. for PC18H5Cl10F2: C: 33.53, H: 0.78. Found: C: 34.16%,
3
3
60 C6D6): δ 7.44 (apparent triplet, JPH = 7 Hz, JHH = 7 Hz, 2H, m-
C6H5), 6.98 – 7.07 (m, 4H, o-, p-C6H5) ppm. 13C NMR (125 MHz,
H: 0.86%.
2
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7: white solid (192 mg, 80% yield). 13C NMR (125 MHz, C6D5Br): δ
146.5 (br d, JFC = 254 Hz, o-C6F5), 143.9 (br d, JFC = 259 Hz, p-
10: (229 mg, 74% yield) as an off-white solid. 13C NMR (125 MHz,
1
1
1
CDCl3): δ 150.6 (br d, 1JFC = 277 Hz, P(o-C6F5)), 148.3 (br d, JFC
=
1
4
4
C6F5), 137.9 (br dm, JFC = 255 Hz, m-C6F5), 136.91 (d, JPC = 4 Hz,
241 Hz, B(o-C6F5)), 147.1 (d, JPC = 3 Hz, P(p-C6Cl5)), 139.4 (br d,
2
1
1
p-C6Cl5), 135.0 (br d, JPC = 264 Hz, o-C6Cl5), 134.5 (dt, JPC = 217
1JFC = 269 Hz, P(p-C6F5)), 138.3 (br d, JFC = 241 Hz, B(p-C6F5)),
2
3
3
2
5
Hz, JFC = 29 Hz, i-C6Cl5), 134.5 (dm, JPC = 18 Hz, m-C6Cl5), 113.0 65 137.7 (d, JPC = 15 Hz, P(m-C6Cl5)), 136.7 (d, JPC = 6 Hz, P(o-
(br dm, JPC = 201 Hz, i-C6F5) ppm. 19F{1H} NMR (376 MHz,
C6Cl5)), 136.4 (br d, JFC = 235 Hz, B(m-C6F5)), 134.9 (br d, 1JFC
=
1
1
1
1
C6D5Br): δ -10.9 (dm, JPF = 756 Hz, PF2), -129.2 to -129.6 (m, 2F,
260 Hz, P(m-C6F5)), 124.0 (br s, B(i-C6F5)), 116.3 (dd, JPC = 144
o-C6F5), -145.9 (t, 3JFF = 22 Hz, 1F, p-C6F5), -158.5 to -158.7 (m, 2F,
Hz, JFC = 10 Hz, P(i-C6Cl5)), 95.8 (br d, 1JPC = 137 Hz, P(i-C6F5))
2
m-C6F5) ppm. 31P{1H} NMR (162 MHz, C6D5Br): δ -41.2 (t, JPF
=
ppm. 11B NMR (128 MHz, CD2Cl2): -16.6 (s) ppm. 19F{1H} NMR
1
1
3
10 757 Hz) ppm. Anal. Calcd. for PC18Cl10F7: C: 29.43. Found: C: 70 (470 MHz, C6H5Br): δ -117.0 (dd, JPF = 1030 Hz, JFF = 26 Hz, 1F,
28.52%.
PF), -123.4 (br s, P(o-C6F5)), -124.7 (m, 1F, P(p-C6F5)), -126.8 (m,
1F, P(o-C6F5)), -132.2 (m/br, 8F, B(o-C6F5)), -150.4 (br s, P(m-
C6F5)), -162.4 (t, 3JFF = 21 Hz, 4F, B(p-C6F5)), -166.4 (m/br, 8F, B(p-
Synthesis of [Ph2PF(C6Cl5)][B(C6F5)4] 8, [PhPF(C6Cl5)2][B(C6F5)4] 9,
[(C6F5)PF(C6Cl5)2][B(C6F5)4] 10: These compounds were prepared
in a similar fashion and thus only one preparation is detailed. A
15 20 mL vial was charged with [Et3Si][B(C6F5)4] (682 mg, 0.77
mmol), toluene (10 mL) and a magnetic stir bar, forming a white
slurry. To the stirring slurry, a solution of 5 (382 mg, 0.81 mmol)
in toluene (5 mL) was added. The solution was allowed to stir
overnight at room temperature, resulting in a dark orange
20 solution. When the reaction mixture was allowed to settle, a
dark orange oil collected at the bottom of the vial leaving a clear
supernatant. After decanting the toluene from the oil, additional
toluene (2 x 3 mL) containing a few drops of CH2Cl2 was added to
wash the oil. After decanting the toluene washes, the oil was
25 washed with n-pentane (3 x 4 mL) before removing the solid in
vacuo resulting in 8 (697 mg, 80 %) as a fluffy white solid
1
C6F5)) ppm. 31P{1H} NMR (162 MHz, C6D5Br): δ 71.0 (d, JPF = 1030
75 Hz) ppm. Anal. Calcd. for PC42Cl10F26B: C: 36.17. Found: C: 37.21%
HRMS (DART Ionization) m/z: [M]+ Calcd. for C18F6Cl10P:
710.65271, Found 710.65339.
Synthesis of FPO(C6Cl5)2 11: A 20 mL vial was charged with 4 (78
80 mg, 0.10 mmol), MeCN (3 mL) and a magnetic stir bar. A solution
of
1-chloromethyl-4-fluoro-1,4-diazonia-bicyclo-[2.2.2]octane
bis(tetrafluoroborate) in MeCN was added. The solution briefly
turns dark purple as a black precipitate is formed before
returning to a pale green colour. The supernatant is decanted
85 and the solvent is removed in vacuo yielding 11 (24 mg, 43 %) as
a yellow solid. 19F NMR (376 MHz, CH2Cl2): δ –54.3 (d, 1JPF = 1065
1
Hz). 31P{1H} NMR (162 MHz, CH2Cl2): δ 21.2 (d, JPF = 1065 Hz).
8: Yield: (697 mg, 80 %). 1H NMR (500 MHz, CDCl3): δ 8.09 – 8.04
(m, 1H, p-C6H5), 7.88 – 7.78 (m, 4H, o-, m-C6H5) ppm. 13C NMR
HRMS (EI-TOF) m/z: [M]+ Calcd. for C12Cl10FPO: 564.65753,
Found: 564.65713.
1
(125 MHz, CDCl3): δ 148.3 (br d, JFC = 241 Hz, B(o-C6F5)), 145.3
90
4
4
5
30 (d, JPC = 3 Hz, P(p-C6Cl5)), 139.5 (dd, JPC = 2 Hz, JFC = 2 Hz, P(p-
C6H5)), 138.3 (br d, 1JFC = 238 Hz, B(p-C6F5)), 137.7 (dd, 2JPC = 6 Hz,
X-ray Diffraction Studies: Crystals were coated in paratone oil
and mounted in a cryo-loop. Data were collected on a Bruker
APEX2 X-ray diffractometer using graphite monochromated Mo-
Kα radiation (0.71073 Å). The temperature was maintained at
95 150(2) K using an Oxford cryo-stream cooler for both, initial
indexing and full data collection. Data were collected using
Bruker APEX-2 software and processed using SHELX and Olex2 an
absorption correction applied using multi-scan within the APEX-2
program. All structures were solved by direct methods within
100 the SHELXTL package15 and refined with Olex2.16
3JFC = 1 Hz, P(o-C6Cl5)), 137.1 (d, JPC = 12 Hz, P(m-C6Cl5)), 136.3
3
(br d, 1JFC = 235 Hz, B(m-C6F5)), 133.5 (dd, 2JPC = 14 Hz, 3JFC = 1 Hz,
3
P(o-C6H5)), 131.5 (d, JPC = 15 Hz, P(m-C6H5)), 123.8 (br s, B(i-
1
2
35 C6F5)), 116.1 (dd, JPC = 112 Hz, JFC = 14 Hz, P(i-C6H5)), 115.9 (dd,
1JPC = 121 Hz, JFC = 11 Hz, P(i-C6Cl5)) ppm. 11B NMR (128 MHz,
2
C6D5Br): -16.5 (s) ppm. 19F{1H} NMR (376 MHz, C6D5Br): δ -116.0
(d, 1JPF = 1010 Hz, PF2), -132.0 (m/br, 8F, B(o-C6F5)), -162.3 (t, 3JFF
= 21 Hz, 4F, B(p-C6F5)), -166.2 (m/br, 8F, B(m-C6F5)) ppm. 31P{1H}
1
40 NMR (162 MHz, C6D5Br): δ 89.5 (d, JPF = 1009 Hz) ppm. Anal.
Calcd. for PC42H10Cl5F21B: C: 44.54, H: 0.89. Found: C: 45.15%, H:
0.94%. HRMS (DART Ionization) m/z: [M]+ Calcd. for C18H10Cl5PF:
450.89468, Found 450.89445.
Results and Discussion
A
series of mixed phenyl/pentachlorophenyl substituted
1
9: Yield: (320 mg, 90 %) as an off white solid. H NMR (500 MHz,
phosphines were synthesized via
a modified literature
45 CDCl3): δ 8.13 – 8.08 (m, 1H, p-C6H5), 7.98 – 7.76 (m, 4H, o-, m-
procedure17. Lithiation of C6Cl6 in Et2O followed by the addition
1
C6H5) ppm. 13C NMR (125 MHz, CDCl3): δ 148.3 (br d, JFC = 240
105 of Ph2PCl, PhPCl2, or PBr3 yields Ph2P(C6Cl5) 1, PhP(C6Cl5)2 2, or
4
Hz, B(o-C6F5)), 145.5 (dm, JPC = 3 Hz, P(p-C6Cl5)), 140.5 (m, P(p-
1
P(C6Cl5)3 3, respectively, in moderate yields (Scheme 1). The H
C6H5)), 138.3 (br d, 1JFC = 239 Hz, B(p-C6F5)), 137.2 (d, 3JPC = 13 Hz,
NMR data for 1 and 2 shows the expected phenyl proton
resonances while the 31P NMR data for 1, 2 and 3 show singlets
at 10.7, 15.1 and 19.1 ppm respectively. This trend of downfield
110 shifting due to increasing perchlorophenyl substitution stands in
contrast to the analogous series of pentafluorophenyl-
substituted phosphines. The present contrasting observation
appears to result from the greater steric bulk of the ortho-
chlorine atoms in comparison to fluorine substituents, thus
115 increasing the distortion of the phosphorus coordination sphere
towards planarity at phosphorus. Mass spectral and elemental
analysis data for compounds 1-3 were also consistent with
formulations. In addition, a solid-state structure of 1 was
obtained by X-ray diffraction (Figure 2).
2
1
P(m-C6Cl5)), 136.7 (d, JPC = 6 Hz, P(o-C6Cl5)), 136.3 (br d, JFC
=
2
50 239 Hz, B(m-C6F5)), 133.1 (d, JPC = 14 Hz, P(o-C6H5)), 132.2 (d,
3JPC = 17 Hz, P(m-C6H5)), 124.1 (br s, B(i-C6F5)), 118.3 (dd, JPC
=
1
2
1
2
131 Hz, JFC = 11 Hz, P(i-C6H5)), 116.0 (dd, JPC = 114 Hz, JFC = 13
Hz, P(i-C6Cl5)) ppm. 11B NMR (128 MHz, CD2Cl2): -16.7 (s) ppm.
19F{1H} NMR (470 MHz, C6H5Br): δ -125.6 (d, 1JPF = 1009 Hz, PF2), -
55 138.9 (m/br, 8F, B(o-C6F5)), -169.2 (m/br, 4F, B(p-C6F5)), -173.1
(m/br, 8F, B(m-C6F5)) ppm. 31P{1H} NMR (162 MHz, CD2Cl2): δ
1
84.4 (d, JPF = 1009 Hz) ppm. Anal. Calcd. for PC42H5Cl10F21B: C:
38.66. H: 0.39. Found: C: 42.17%, H: 0.87%. HRMS (DART
Ionization) m/z: [M]+ Calcd. for C18H5Cl10PF: 620.69982, Found
60 620.69896.
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41.8 ppm with
a
1JPF coupling constant of 716 Hz.
In addition, Br2P(C6F5) was synthesized by literature procedure18
Correspondingly, the 31P NMR spectrum of 5 was seen as a
and reacted with two equivalents of LiC6Cl5 in Et2O at -78 °C. 30 triplet at -50.7 ppm, consistent with the presence of two
Following workup, was isolated as a white powder (Scheme 1).
The 19F NMR data for 4 displayed 3 resonances, consistent with
the presence of a pentafluorophenyl group, while the 31P NMR
spectrum exhibited a triplet at -17.8 ppm. These data are
chemically equivalent fluorine atoms on phosphorous.
Collectively these data inferred the formulation of 5 as the
difluorophosphorane species Ph2PF2(C6Cl5). Additionally, the
solid-state structure of 5 was confirmed by X-ray crystallography
5
consistent with the formulation of 4 as (C6F5)P(C6Cl5)2 and this 35 (Figure 4). In a similar fashion treatment of 2 with XeF2 yields the
confirmed by a solid state structure obtained by single crystal X-
10 ray diffraction studies (Figure 3).
corresponding species PhPF2(C6Cl5)2 6 as a white powder in
moderate yield. The 19F and 31P NMR data for 6 are analogous to
that of 5 (Scheme 2).
40
Treatment of 3 with XeF2 yielded an inseparable mixture of
two products that exhibited 31P NMR signals. A minor product
displayed NMR features consistent with those seen for 5 and 6,
suggesting the formation of the difluorophosphorane,
PF2(C6Cl5)3. However, the major product displayed two
45 resonances in the 19F NMR spectrum: a doublet of doublets and
a doublet of triplets in a 2:1 ratio, while the corresponding 31P
NMR data showed a doublet of triplets. These data are
consistent with the formation of PF3(C6Cl5)2. The inability to
isolate PF2(C6Cl5)3 is attributed to the steric congestion
50 prompting degradation. Indeed, despite multiple trials with
varied conditions all efforts to separate the two products were
unsuccessful. The formation of the major product suggests the
loss of a (C6Cl5) ring from the starting phosphine. This view was
supported by the isolation of crystals suitable for X-ray
55 diffraction from the reaction mixture. These crystals were
subsequently confirmed to be decachlorobiphenyl, C12Cl10
(Figure 5). The presence of this by-product suggests that the
oxidation of 3 proceeds via a radical mechanism at least to some
degree and that loss of the radical (C6Cl5) leads to radical
60 coupling.
Figure 2. POV-ray depiction of 1; C: black, P: orange, Cl: green, H-atoms
15 were omitted for clarity.
Figure 3. POV-ray depiction of 4; C: black, P: orange, Cl: green, F: pink.
Cl
Cl
Cl
Cl
Cl
n-BuLi
P(C6Cl5)3
PCl3
3
Cl
n-BuLi
RPX2
Ph2PCl
n-BuLi
RP(C6Cl5)2
R = Ph, X = Cl
Ph2P(C6Cl5)
Figure 5. POV-ray depiction of decachlorobiphenyl; C: black Cl: green.
2:
4: R = (C6F5), X = Br
1
In contrast, reaction of 4 with XeF2 in CH2Cl2 generates a light
65 yellow solution. Upon reducing the solvent volume and cooling
compound 7 could be obtained as a white powder in excellent
yield. The 19F NMR spectrum of 7 displays a distinctive doublet in
addition to (C6F5) resonances. A triplet is observed by 31P NMR at
-41.2 ppm and these data lead to the formulation of 7 as the
70 difluorophosphorane (C6F5)PF2(C6Cl5)2 (Scheme 2).
20 Scheme 1. Synthesis of 1-4.
Figure 4. POV-ray depiction of 5; C: black, P: orange, Cl: green, F: pink,
hydrogen atoms have been omitted for clarity.
25
Treatment of 1 with an equimolar amount of XeF2 in
CH2Cl2 cleanly generates 5 as a white powder upon workup, in
excellent yield. The 19F NMR data for 5 displayed a doublet at -
4
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1
2 or 4
-Xe
XeF2
XeF2 -Xe
Cl
Cl
Cl
F
P
Cl
Cl
F
P
F
R
Ph
Ph
Cl
Cl
Cl
Cl
Cl
F
Cl
Cl
Cl
Cl
5
Cl
[Et3Si]
[B(C6F5)4]
[Et3Si]
6
: R = Ph
7:
[B(C6F5)4]
R = (C6F5)
R
Cl
Cl
F
Cl
Cl
Cl
Figure 6. POV-ray depiction of 9; C: black, P: orange, Cl: green, B: yellow-
green, F: pink, hydrogen atoms have been omitted for clarity.
Ph
Ph
Cl
Cl
Cl
P
Cl
Cl
Cl
P
F
5
With the difluorophosphoranes 5-7 in hand, the compound
5 was added to a stirring slurry of [Et3Si][B(C6F5)4] in toluene,
affording a dark orange solution over the course of 1 h. A dark
amber coloured oil collected at the bottom of the flask, leaving a
clear supernatant. Separation of the oil by decantation of the
Cl
Cl
Cl
[B(C6F5)4]
8
Cl
[B(C6F5)4]
9
: R = Ph
10: R = (C6F5)
40
Scheme 2. Synthesis of 5-10.
10 supernatant and subsequent washing of the oil with toluene and
pentane afforded 8 as a powdery white solid in excellent yield.
The 19F NMR data for 8 display a doublet with P-F coupling of
1009 Hz and a set of three resonances consistent with the
presence of the perfluorophenyl groups of the counteranion
15 [B(C6F5)4]-. The corresponding 31P NMR spectrum exhibits a
doublet centred at 89.5 ppm. Collectively these data lead to the
formulation of 8 as [Ph2PF(C6Cl5)][B(C6F5)4] (Scheme 2).
Similarly, the reaction of [Et3Si][B(C6F5)4] with 6 yields 9 as an
off-white powder with analogous NMR parameters leading to
20 the formulation of 9 as [PhPF(C6Cl5)2][B(C6F5)4]. In this case the
corresponding formulation was confirmed via X-ray
crystallography (Figure 6).
As the difluorophosphorane PF2(C6Cl5)3 was not obtained, an
alternate synthetic protocol targeting the fluorophosphonium
45 cation was undertaken. The perchlorinated phosphine 3 was
reacted with Selectfluor in a solution of MeCN. This leads to the
generation and subsequent isolation of a species 11 as a yellow
solid. NMR data revealed a single doublet in both 19F and 31P
spectra, with the expected P-F coupling of 1066 Hz. No peak was
50 found in the 19F NMR spectrum attributable to a counteranion.
Mass spectral data for CH2Cl2 extract showed a [M]+ ion of
564.65713 m/z consistent with the formulation of 11 as
FOP(C6Cl5)2. In addition, crystals of decachlorobiphenyl were also
isolated from this reaction suggesting complex redox chemistry
55 although the source of oxygen in the phosphineoxide remains
unknown.
Subsequent treatment of
7 with one equivalent of
[SiEt3][B(C6F5)4] in toluene yielded a dark amber oily suspension.
25 After workup, 10 was obtained as an off-white solid. The 19F
NMR spectrum of 10 displays three resonances attributable to a
With the fluorophosphonium cations 8-10 in hand, the
stability of these compounds to atmospheric mositure was
qualitatively evaluated. Samples of each species were prepared
60 in bromobenzene, exposed to air for successively longer
intervals and the solution was monitored by 19F and 31P NMR
spectroscopies. Compound 8 proved to be somewhat robust to
air, showing the onset of oxidation after 21 h. In contrast, the
solution of compound 9 was found to be significantly more
65 robust exhibiting no evidence of degradation after 48 h of
exposure to air. Indeed, solid samples of 9 could be manipulated
in the air and solutions prepared using unpurified reagent-grade
bromobenzene as purchased with no observed evidence of
degradation of 9. A bromobenzene solution of 10 was less stable
-
B(C6F5)4 anion, a doublet consistent with a P-F moiety and four
resonances attributed to
a pentafluorophenyl substituent.
Variable temperature 19F NMR studies reveal coalescence of the
30 ortho-fluorine signals and sharpening of the meta-fluorine peaks
at higher temperatures. The inequivalence of the fluorine atoms
of the (C6F5) ring infer hindered rotation about the P-C bond,
presumably
a result of the enforced pseudo-tetrahedral
geometry about P in the cation and the steric demands of the
35 (C6Cl5) substituents. The 31P NMR spectrum of 10 displays a
doublet, with the expected coupling to fluorine. These data are
consistent with the identity of 10 as [(C6F5)PF(C6Cl5)2][B(C6F5)4]
(Scheme 2).
70 than
9
showing minor formation of HC6F5 by 19F NMR
spectroscopy after 4 h. As a point of comparison, the analogous
treatment of the perfluorinated fluorophosphonium cation salt
[PF(C6F5)3][B(C6F5)4] led to evidence of degradation after 1
minute of exposure to air, with full decomposition occurring
75 within 1 h. These observations suggest the presence of the
perchlorophenyl substituents does enhance the stability of
related fluorophosphonium cations, presumably a result of the
steric protection of the P-F fragment. This view is consistent with
data reported by Ashley12 who used perchlorophenyl
80 substituents to generate air-stable boranes.
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40 Conclusions
This report has provided synthetic protocols for the syntheses of
several phosphines that incorporate perchlorinated phenyl
substituents, and the conversion of these species to the
corresponding difluorophosphoranes and fluorophosphonium
45 cations. These compounds exhibit improved stability in the air
and prove to be effective catalysts in a series of prototypical
reactions of fluorophosphonium cations. This stability is
attributed to the steric protection associated with the
perchlorinated aryl substituents. This strategy provides some
50 improvement in air-stability of the fluorophosphonium cation,
suggesting that steric protection of the P-F bond is a viable
approach. Nonetheless, the steric protection comes at the cost
of lower reactivity. We are continuing to explore other strategies
targeting the syntheses of active, air-stable phosphonium Lewis
55 acid catalysts. The results of these endeavours will be reported
in due course.
Acknowledgements
Scheme 3. Lewis acid catalyzed reactions.
D.W.S gratefully acknowledges the financial support of the
NSERC of Canada and the award of a Canada Research Chair.
The utility of 8-10 as Lewis acid catalysts was also evaluated in a
series of reactions that are readily mediated by
[PF(C6F5)3][B(C6F5)4] (Scheme 4). Using 5 mol% of compound 8 as
5
60 Notes and references
a
a
catalyst, the dimerization of 1,1-diphenylethylene was
complete in 18 h at room temperature. Similarly, 5 mol% of 8
catalyzed the complete hydrodefluorination of 1-
Department of Chemistry, University of Toronto, 80 St.George St.,
Toronto, Ontario, M5S 3H6. E-mail: dstephan@chem.utoronto.ca
† Electronic Supplementary Information (ESI) available: Crystrallographic
data have been deposited in CCDC#: 1471988-1471992. Experimental
65 details
DOI: 10.1039/b000000x/
‡These authors have contributed equally to this publication.
10 fluoroadamantane in the presence of Et3SiH in 3.5 h. In addition,
the dehydrocoupling of phenol and Et3SiH was completed in 24 h
at 140 °C affording PhOSiEt3. As well, 5 mol% of 8 mediated the
deoxygenation of benzophenone to give complete conversion to
Ph2CH2 in 17 h at 140 °C. Additionally, 5 mol% of 8 was able to
15 catalyze the hydrosilylation of α-methylstyrene with Et3SiH in 48
h at 140 °C.
Compound 9 was found to be a more active catalyst than 8 in
analogous testing, again using 5 mol% catalyst loadings. The
dimerization of 1,1-diphenylethylene reached completion within
20 6 h. The hydrodefluorination of 1-fluoroadamantane in the
presence of Et3SiH was complete in 10 minutes, benzophenone
was fully deoxygenated in 40 h, while the dehydrocoupling of
phenol and Et3SiH was complete in 72 h at room temperature.
Compound
25 methylstyrene within 32 h at 120 °C.
Compound 10 was also screened as a catalyst under similar
conditions using 5 mol% catalyst loadings. As expected, the
presence of the (C6F5) substituent in 10 enhances its activity in
Lewis acid catalysis. Indeed, it proved to be much more reactive
and
spectral
data
have
been
deposited.
See
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also catalyzed the hydrosilylation of α-
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1
h
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