Ferrocenyl-derived electrophilic phosphonium cations (EPCs) as Lewis acid catalysts

Article abstract of DOI:10.1039/c6dt00105j

Oxidation of diphenylphosphinoferrocene and 1,1′-bis(diphenylphosphino)ferrocene with XeF₂, resulted in the formation of CpFe(η⁵-C₅H₄PF₂Ph₂) 1 and Fe(η⁵-C₅H₄PF₂Ph₂)₂2 respectively. Subsequent reactions with [SiEt₃][B(C₆F₅)₄] yielded [CpFe(η⁵-C₅H₄PFPh₂)][B(C₆F₅)₄] 3 and [Fe(η⁵-C₅H₄PFPh₂)₂] [B(C₆F₅)₄]₂4. PhP(η⁵-C₅H₄)₂Fe 5 was prepared, converted to [PhMeP(η⁵-C₅H₄)₂Fe][O₃SCF₃] 6 and then to [PhMeP(η⁵-C₅H₄)₂Fe][B(C₆F₅)₄] 7. The ability of the salts 3, 4 and 7 to catalyze Friedel-Crafts dimerization of 1,1-diphenylethylene, dehydrocoupling of phenol and triethylsilane, deoxygenation of acetophenone and hydrodefluorination of 1-fluoropentane were probed. While compound 7 proved to be ineffective, compounds 3 and 4 were useful Lewis acid catalysts. Compounds 3 and 4 were shown to catalyze the deoxygenation of a series of ketones.

Full text of DOI:10.1039/c6dt00105j

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Ferrocenyl-derived electrophilic phosphonium
cations (EPCs) as Lewis acid catalysts†
Cite this: DOI: 10.1039/c6dt00105j
Ian Mallov and Douglas W. Stephan*
Oxidation of diphenylphosphinoferrocene and 1,1’-bis(diphenylphosphino)ferrocene with XeF2, resulted in
the formation of CpFe(η5-C5H4PF2Ph2) 1 and Fe(η5-C5H4PF2Ph2)2 2 respectively. Subsequent reactions
with [SiEt3][B(C6F5)4] yielded [CpFe(η5-C5H4PFPh2)][B(C6F5)4] 3 and [Fe(η5-C5H4PFPh2)2] [B(C6F5)4]2 4.
PhP(η5-C5H4)2Fe 5 was prepared, converted to [PhMeP(η5-C5H4)2Fe][O3SCF3] 6 and then to [PhMeP(η5-
C5H4)2Fe][B(C6F5)4] 7. The ability of the salts 3, 4 and 7 to catalyze Friedel–Crafts dimerization of 1,1-
diphenylethylene, dehydrocoupling of phenol and triethylsilane, deoxygenation of acetophenone and
hydrodefluorination of 1-fluoropentane were probed. While compound 7 proved to be ineffective, com-
pounds 3 and 4 were useful Lewis acid catalysts. Compounds 3 and 4 were shown to catalyze the deoxy-
genation of a series of ketones.
Received 9th January 2016,
Accepted 16th February 2016
DOI: 10.1039/c6dt00105j
www.rsc.org/dalton
acid.23 Gabbai and coworkers have exploited the synergistic
action of tetracoordinate P(V) phosphonium cations with a
Introduction
Main group catalysts have emerged as potentially green B-Lewis acid to capture fluoride ions in sensor applications.24
alternatives to precious metals species.1 Indeed main group Moreover, phosphonium cations have been used to mediate
Lewis acids have been widely used as initiators and catalysts in addition to polar unsaturates.25 Indeed, the classic Wittig reac-
organic transformations.2–7 In addition, the advent of the tions in which P-based ylides react with ketones are driven by
concept of frustrated Lewis pairs (FLPs) from the discovery the electrophilic nature of the P centre.26,27
8,9
that such species can activate H2
among other small mole-
Our group has recently demonstrated that electrophilic
cules has spurred main group catalyst development over the phosphonium cations (EPCs) are strong Lewis acids and can
last 10 years.10–14 While a variety of main group Lewis acids catalyze transformations such as hydrodefluorination of
have been exploited for FLP chemistry and catalysis, the fluoroalkanes,28–31 hydrosilylation of olefins,32,33 ketones and
majority of systems studied to date have been based on group imines,34 transfer hydrogenation33 and direct hydrogenation
13 (B, Al) or 14 (C, Si) sites.
of alkenes35 and deoxygenation of ketones.36 In these catalysts,
In seeking to broaden the range of Lewis acids,15 we noted the Lewis acidity resides in the σ* orbital and is significantly
the interesting findings from several research groups that have enhanced by the inclusion of electron-withdrawing substitu-
explored the Lewis acidity of electron-deficient phosphorus tents such as C6F5 on the phosphorus atom (Fig. 1A).28 Alter-
compounds.16 For example, dicoordinate P(III) phosphenium
cations have been shown to exhibit Lewis acid behavior,
forming adducts with N-donors.17,18 Similarly, phosphoranes
form Lewis adducts with N-trimethylsilyl imidazole and pyra-
zole derivatives.19 In addition, phosphenium cations have been
shown to activate C–C/H and P–P bonds,20–22 In our own efforts,
we have described the reactions of a triphosphabenzene deriva-
tive with H2 via an intramolecular FLP-type mechanism. Intrigu-
ingly, the computational studies reveal that this results from the
action of a carbanion as the base and a P(III) centre as a Lewis
Department of Chemistry, University of Toronto, 80 St. George Street, Toronto,
Ontario, Canada M5S 3H6. E-mail: dstephan@chem.utoronto.ca
†Electronic supplementary information (ESI) available: Preparative details and
X-ray data. CCDC 1440505–1440508. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c6dt00105j
Fig. 1 Electrophilic phosphonium cation scaffold types.
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natively, incorporation of a neutral donor such as a carbene solution of phenylphosphinoferrocene (74 mg, 0.20 mmol) in
generates dications that offer Lewis acidity with a diminished CH2Cl2 (1 mL). Effervescence was immediately observed. After
need for strongly electron-withdrawing groups (Fig. 1B).31 In stirring 2 h volatiles were removed in vacuo resulting in an
an effort to develop alternative strategies to new P-based Lewis orange solid in quantitative yield (81 mg, yield >99%). X-ray
acids, we have explored simple design concepts that exploit quality crystals were obtained by cooling a saturated solution
1
commercially-available Lewis bases and apply our protocols for of the product in CH2Cl2 to −35 °C. H NMR (CD2Cl2, [ppm]):
conversion to Lewis acids. These protocols involve formal oxi- 3.94 (s, 5H, Cp), 4.50 (s, 2H, Cp CHβ), 4.83 (2H, s, Cp CHα),
dative addition of X2 (X = F, Cl, Br) to the P(III) centre followed 7.44–7.50 (m, 6H, p-Ph, m-Ph), 7.96–8.08 (m. 4H, o-Ph) 13C{1H}
2
by abstraction of X− to yield the phosphonium cation. A comp- NMR (CD2Cl2, [ppm]): 70.2 (Cp Cβ), 72.8 (d, Cp Cα, JPC
=
2
lementary strategy to generate diphosphonium dications 15 Hz), 76.0 (br m, Cp Cipso), 128.9 (d, Ph, Cα, JPC = 17 Hz),
involves the incorporation of proximal phosphonium centres 131.8 (s, Ph, Cβ) 134.2 (m). Resonances for ipso-carbon atoms on
linked by an organic moiety (Fig. 1C and D).29–31 In this case, phenyl, Cp moieties were not observed. 19F{1H} NMR (CD2Cl2,
1
the enhanced Lewis acidity is thought to result from the induc- [ppm]): −42.7 (d, JPF = 656 Hz) 31P{1H} NMR (CD2Cl2, [ppm]):
tive effect of a proximal positive charge rather than by a co- −50.1 (t, 1JPF = 656 Hz); Elemental Analysis for C22H19F2P: calcd:
operative effect on the substrate.5 In a similar effort, we have C: 64.7, H 4.7, found: C: 63.1, H: 4.9;43 HRMS (ESI-QTOF+): m/z
most recently described the utility of pyridinium–phos- 387.0596 ([CpFe(η5-C5H4POPh2)] (calc.: 387.0601).
phonium dications (Fig. 1E)37 as Lewis acid catalysts for a
variety of reactions.
Synthesis of Fe(η5-C5H4PF2Ph2)2 2. A solution of XeF2
(68 mg, 0.40 mmol) in CH2Cl2 (1 mL) was added dropwise to a
We have noted the previous work of Oestreich, who solution of 1,1′-bis(diphenylphosphino)ferrocene (111 mg,
described the synthesis of ferrocenylsilylium cations which are 0.20 mmol) in CH2Cl2 (1 mL) Effervescence was immediately
active Lewis acid catalysts in
a range of acid-catalyzed observed. After stirring for 1–2 min. a yellow solid precipitated
transformations.38–40 Prompted by this precedent, in the from the solution. This was filtered and washed with pentane
present manuscript we describe the preparation of Lewis (2 × 1 mL) and dried in vacuo to give 118 mg of product (94%
acidic ferrocenylphosphonium cations. The utility of these yield). 1H NMR (CD2Cl2, [ppm]): 4.10 (m, 4H, Cp, Hα), 4.62 (m,
species in Friedel–Crafts dimerization, dehydrocoupling, 4H, Cp CHβ), 7.47 (m, 12H, p-Ph, m-Ph), 8.00 (m, 8H, o-Ph)
3
hydrodefluorination and ketone deoxygenation is reported.
13C{1H} NMR (CD2Cl2, [ppm]): 74.6 (dt, Cp Cβ, JPC = 15 Hz,
4JFC = 2 Hz), 77.2 (dt, Cp Cα, JPC = 18 Hz, JFC = 10 Hz), 129.0
2
3
2
4
(td, i-Ph, JPC = 17 Hz, JFC = 2 Hz), 132.1 (s, i-Ph) 134.6 (dt,
m-Ph, 2JCP = 13 Hz, 2JCF = 10 Hz), 138.5 (td, i-Ph, 1JPC = 182 Hz,
2JFC = 28 Hz) Resonances for ipso carbon atoms on Cp moieties
Experimental section
General remarks
were not observed. 19F{1H} NMR (CD2Cl2, [ppm]): −42.8 (d,
All reactions were carried out under an atmosphere of dry, 1JPF = 662 Hz) 31P{1H} NMR (CD2Cl2, [ppm]): −51.5 (t, JPF
=
1
oxygen-free nitrogen using standard Schlenk techniques or a 662 Hz); Elemental Analysis for C34H28F4P2: calcd: C 64.8, H
nitrogen-filled glove box (MBRAUN). All solvents (including 4.5, found: C: 64.9, H: 4.8; HRMS (ESI-QTOF+): m/z 587.0987
deuterated solvents) were dried and stored over molecular ([Fe(η5-C5H4P(OH)Ph2) (η5-C5H4POPh2)]) (calc.: 587.0992).
sieves under
a
nitrogen atmosphere before use. Fe(η5-
Synthesis of [CpFe(η5-C5H4PFPh2)][B(C6F5)4] 3. To a solu-
C5H4PPh2)2, XeF2, HSiEt3, 1,1-diphenylethylene, PhOH, tion of 41 mg (0.10 mmol) 1 in toluene was added 93 mg
MeO3SCF3, and all ketones were commercially available and freshly prepared [Et3Si][B(C6F5)4]2(C7H8) (0.095 mmol). Stirring
used as received. [SiEt3][B(C6F5)4]41 and Fe(η5-C5H4)2PPh 5 42 for 3 h resulted in precipitation of a dark yellow oil. Washing
were prepared according to literature known methods. 1H, 13C, with toluene (1 × 0.5 mL) and pentane (3 × 0.5 mL) and drying
11B and 31P NMR spectra were recorded on a Bruker Avance III in vacuo resulted in 90 mg of 3 as a dull orange solid (84%
or a Bruker Avance 500 spectrometer. A Perkin-Elmer analyser yield). X-ray quality crystals were obtained by slow diffusion of
was used for carbon, hydrogen and nitrogen elemental ana- n-pentane into a saturated solution of 4 in dichloromethane at
1
lyses. H NMR data, referenced to external Me4Si, are reported −35 °C, 1H NMR (CD2Cl2, [ppm]): 4.50 (s, 5H,Cp), 4.64 (m, 2H,
as follows: chemical shift (δ/ppm), coupling constant (Hz), Cp, Hα), 5.07 (m, 2H, Cp, Hβ), 7.79–7.85 (m, 8H, o-Ph, m-Ph),
normalized integrals. 13C{1H} NMR chemical shifts (δ/ppm) 7.99 (t, 2H, p-Ph, 3JHH = 7 Hz) 11B NMR (CD2Cl2, [ppm]): −16.7
are referenced to external Me4Si. Assignments of individual 13C{1H} NMR (CD2Cl2, [ppm]): 72.3 (s, Cp), 75.2 (d, Cp Cα, 2JPC
3
resonances were done using 2D NMR techniques (HMBC, = 17 Hz), 77.9 (d, Cp Cβ, JPC = 13 Hz), 131.3 (d, Ph, JPC = 14
HSQC, HH-COSY) when necessary. High-resolution mass Hz), 133.5 (d, Ph, JPC = 13 Hz), 138.5 (s, Ph) signals for ipso-
spectra (HRMS) were obtained on an Agilent 6538 Q-TOF (ESI) carbon atom not observed. 19F{1H} NMR (CD2Cl2, [ppm]):
1
or a JEOL AccuTOF (DART) mass spectrometer. Elemental ana- −112.5 (d, JPF = 998 Hz). 31P{1H} NMR (CD2Cl2, [ppm]): 102.9
1
lyses were performed at the University of Toronto employing a (d, JPF = 998 Hz) Elemental Analysis for C46H19BF21PFe: calcd:
Perkin Elmer 2400 Series II CHNS Analyser. Catalytic trials C: 51.5, H: 1.8, found: C: 51.0, H: 1.5; HRMS (ESI-QTOF+): m/z
were performed in 5 mm NMR tubes using 0.75 mL of CD2Cl2.
387.0596 ([CpFe(η5-C5H4POPh2)] (calc.: 387.0601).
Synthesis of CpFe(η5-C5H4PF2Ph2) 1. A solution of XeF2
Synthesis of [Fe(η5-C5H4PFPh2)2][B(C6F5)4]2 4. To a solution
(34 mg, 0.20 mmol) in CH2Cl2 (1 mL) was added dropwise to a of 63 mg (0.10 mmol) 2 in toluene was added 186 mg freshly
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prepared [Et3Si][B(C6F5)4]2(C7H8) (0.19 mmol). Stirring for 3 h
resulted in precipitation of a dark yellow oil. Washing with
toluene (1 × 0.5 mL) and pentane (3 × 0.5 mL) and drying in
vacuo resulted in 156 mg of 4 as a dull orange solid (80%
yield). X-ray quality crystals were obtained by slow diffusion of
n-pentane into a saturated solution of 4 in dichloromethane at
1
−35 °C, H NMR (CD2Cl2, [ppm]): 4.51 (m, 4H, Cp CHα), 5.19
(m, 4H, Cp CHβ,), 7.47 (m, 12H, p-Ph, m-Ph), 7.72 (m, 4H, Ph),
7.79 (m, 4H, Ph), 8.04 (m, 2H, p-Ph) 11B NMR (CD2Cl2, [ppm]):
3
−16.7 13C{1H} NMR (CD2Cl2, [ppm]): 74.6 (dt, Cp Cβ, JPC
=
4
2
3
15 Hz, JFC = 2 Hz), 77.2 (dt, Cp Cα, JPC = 18 Hz, JFC = 10 Hz),
2
4
Scheme 1 Syntheses of ferrocenyl-phosphonium cation salts 3 and 4.
129.0 (td, i-Ph, JPC = 17 Hz, JFC = 2 Hz), 132.1 (s, i-Ph) 134.6
(dt, m-Ph, JCP = 13 Hz, JCF = 10 Hz), 139.2 (s, o-Ph) 19F{1H}
2
2
NMR (CD2Cl2, [ppm]): −42.8 (d, JPF = 662 Hz). 31P{1H} NMR
1
associated doublet resonance in the 19F NMR spectrum
appeared at −42.7 ppm. (1JPF = 656 Hz). The corresponding
31P{1H} and 19F NMR spectra for 2 were very similar, showing a
triplet at −51.5 ppm and a doublet at −42.8 ppm, respectively
(1JPF = 662 Hz). Interestingly, the P–F couplings in 1 and 2 are
significantly smaller than those seen in Fe(η5-C5H4PF4)2
(817 Hz, 977 Hz).48
1
(CD2Cl2, [ppm]): −51.5 (t, JPF = 662 Hz); Elemental Analysis
for C82H28B2F42P2Fe: calcd: C: 50.5, H: 1.5, found: C: 51.0, H:
1.5; HRMS (ESI-QTOF+): m/z 587.0987 ([Fe(η5-C5H4P(OH)Ph2)
(η5-C5H4POPh2)]) (calc.: 587.0992).
Synthesis of [PhMeP(η5-C5H4)2Fe][B(C6F5)4] 7. To a solution
of Fe(η-C5H4)2PPh in toluene was added
a solution of
MeO3SCF3 in toluene. Successive precipitations yielded 0.050 g
1
Clear differences between the two compounds were only
of the product (80% yield). H NMR (CD2Cl2, [ppm]): 2.39 (d,
1
2
observed in the H NMR spectra, where the higher symmetry
3H, CH3, JPH = 14 Hz), 4.50 (m, 2H, Cp), 4.80 (m, 2H, Cp),
of 2 results in only two distinct resonances for the Cp protons,
in a 1 : 1 ratio at 4.10 ppm and 4.62 ppm, while 1 exhibits
three resonances in the proton spectrum due to the inequiva-
lence of the Cp rings, with the five protons of the unsubsti-
tuted Cp moiety appearing at 3.94 ppm, and the two
resonances assigned to the β and α protons of the substituted
Cp ring at 4.50 ppm and 4.83 ppm.
A solid-state structural X-ray diffraction analysis of crystals
of 1 (Fig. 2) confirmed a pseudo-trigonal bipyramidal geometry
at phosphorus with the two fluorine atoms occupying the axial
positions with a F–P–F: 177.0(1)° and P–F bond lengths aver-
aging 1.677(5) Å, typical of aryl fluorophosphoranes.28
5.06 (m, 2H, Cp), 5.11 (m, 2H, Cp), 7.79–7.85 (m, 4H, o-Ph,
3
m-Ph), 7.99 (t, 1H, p-Ph, JHH = 7 Hz) 11B{1H}: −16.7 13C {1H}
1
NMR (CD2Cl2, [ppm]): 12.1 (d, Me, JCP = 221 Hz), 72.3 (s, Cp),
2
3
75.2 (d, Cp, JPC = 17 Hz), 77.9 (d, Cp, JPC = 13 Hz), 131.3 (d,
Ph, JPC = 14 Hz), 133.5 (d, Ph, JPC = 13 Hz), 138.5 (s, Ph) signals
for ipso carbon atom not observed. 31P{1H} NMR (CD2Cl2,
[ppm]): 35.6. Elemental Analysis for C41H16BF20P: calcd: C:
49.9 H: 1.6 found: C: 50.2, H: 1.9; HRMS (ESI-QTOF+): m/z
307.0343 (Ph(O)P(η5-C5H4)2Fe) (calc.: 307.0339).
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 diffracto-
meter using graphite monochromated Mo-Kα radiation
(0.71073 Å). The temperature was maintained at 150(2) K using
an Oxford cryo-stream cooler for both, initial indexing and full
data collection. Data were collected using Bruker APEX-2 soft-
ware and processed using SHELX and Olex2 and an absorption
correction applied using multi-scan within the APEX-2
program. All structures were solved by direct methods within
the SHELXTL package42 and refined with Olex2.44,45
Subsequent reaction of 1 and 2 using one or two equiva-
lents of [SiEt3][B(C6F5)4], respectively, resulted in abstraction of
fluoride ion from the difluorophosphoranes yielding the
B(C6F5)4 salts of the ferrocenyldiphenylfluorophosphonium
Results and discussion
Oxidation of diphenylphosphinoferrocene and 1,1′-bis(diphe-
nylphosphino)ferrocene (dppf)46,47 using one or two equiva-
lents of xenon difluoride, respectively, resulted in selective
oxidation of the P(III) centres in the ferrocene derivatives, yield-
ing quantitative conversion to ferrocenyl mono- and bis-diphe-
nyldifluorphosphoranes, CpFe(η5-C5H4PF2Ph2) 1 and Fe(η5-
C5H4PF2Ph2)2 2 respectively (Scheme 1). The 31P{1H} NMR
Fig. 2 POV-ray depiction of 1. C: black. P: orange, F: yellow green. Fe:
spectrum for 1 exhibited a triplet at −50.1 ppm, while the red brown. Hydrogen atoms have been omitted for clarity.
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cation, [CpFe(η5-C5H4PFPh2)][B(C6F5)4] 3 and ferrocenylbis-
(diphenylfluorophosphonium) dication, [Fe(η5-C5H4PFPh2)2]
[B(C6F5)4]2 4, respectively (Scheme 1). The 31P and 19F NMR
spectra of 3 and 4 are very similar, with 31P doublet resonances
appearing at 102.8 and 102.9 ppm and doublets in the 19F
NMR spectra at −112.4 ppm for 3 and at −119.6 ppm for 4.
1JPF coupling constants were characteristic of fluorophospho-
nium cations at 998 Hz for 3 and 1002 Hz for 4. Signals In the
1H NMR spectra of 3 and 4 were shifted downfield compared
to those of the phosphorane precursors.
Crystallographic studies of single crystals of 3 and 4 con-
firmed the proposed formulations (Fig. 3). In 3 and 4, the
fluorophosphonium centers are pseudo-tetrahedral. In 3 the
P–F bond is 1.574(3) Å, while in 4 the P–F bonds are shorter,
averaging 1.545(2) Å. In both cases these observations are
typical of fluorophosphonium cations. Unsurprisingly, the
phosphonium moieties in 4 are oriented so as to maximize the
distance between the cationic centres in the solid state.
Attempts to synthesize a fluorophosphonium cation derived
from a phosphinoferrocenophane were undertaken. To this
end, the species PhP(η5-C5H4)2Fe 5 was prepared following
slightly modified literature methods.49 Addition of a solution
of XeF2 to solutions of 5 in dichloromethane rapidly yielded
an insoluble intractable dark solid, suggesting the possibility
of polymeric products. Similarly, attempts to form the fluoro-
phosphonium cation directly by fluoronium ion transfer from
Selectfluor or fluoropyridines were also unsuccessful. Interest-
ingly, Manners has noted that the chlorophosphine analogue
ClP(C5H4)2Fe was unstable at temperatures above −20 °C.49
Nonetheless, compound 5 could be methylated upon addition
of methyl triflate generating the triflate salt [PhMeP(η5-
C5H4)2Fe][O3SCF3] 6 (Scheme 2). Although this species was not
isolated, treatment of the solution of 6 with one equivalent of
[SiEt3][B(C6F5)4] effected anion exchange providing access to
the salt [PhMeP(η5-C5H4)2Fe][B(C6F5)4] 7 (Scheme 2). The for-
mulation of 7 and the quaternization of the phosphorus centre
was confirmed crystallographically (Fig. 4). The metric para-
meters are unexceptional.
Fig. 3 POV-ray depiction of cations of (a) 3 and (b) 4. C: black. P:
orange, F: yellow green. Fe: red brown. Hydrogen atoms and the anions
have been omitted for clarity.
Fig. 4 POV-ray depiction of cation of 7. C: black. P: orange. Fe: red
brown. Hydrogen atoms and the anion have been omitted for clarity.
Oxidation of the Fe(II) centre of 3 or 4 was investigated. probed (Table 1). Consistent with the observed reactivity of
Compound 3 exhibited a quasi-reversible oxidation wave (see established EPCs of types A, B, C, D, and E, dication 4 was
ESI†) and efforts to generate the nominally Fe(III) species by shown to quantitatively catalyze Friedel–Crafts dimerization of
the addition of AgX (X = F, Cl) did exhibit the color change 1,1-diphenylethylene, dehydrocoupling of phenol and triethyl-
characteristic of Fe oxidation from light orange to a deep tur- silane, and deoxygenation of acetophenone in the presence of
quoise. However, all efforts to isolate the oxidized species were 2.1 equivalents of HSiEt3 as a hydride source. Compound 4
fraught with difficulty and ultimately unsuccessful.
also mediated the hydrodefluorination of 1-fluoropentane in
Having generated several ferrocene-derived phosphonium the presence of HSiEt3 in 55% yield after 7 days at 25 °C. In
cations, the catalytic activities of the salts 3, 4 and 7 were contrast, the monocation 3 was less reactive, requiring heating
to 50 °C to catalyze 10% dimerization of diphenylethylene.
Nonetheless, 3 did catalyze the dehydrocoupling of phenol
and triethylsilane (97%) albeit over the course of 7 days. Simi-
larly heating 3 for 3 days only catalyzed the deoxygenation of
acetophenone to 27% yield. At the same temperature 3 exhibited
essentially no ability to mediate hydrodefluorination converting
less than 1% of 1-fluoropentane to pentane. Interestingly,
neither 3 nor 4 catalyzed the hydrosilylation of olefins with
Scheme 2 Synthesis of 7.
either HSiEt3 or PhSiH3, in contrast to other EPCs. In stark
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Table 1 Lewis acid catalysis by 3, 4 and 7
Table 2 Catalytic deoxygenation of ketones mediated by 3 and 4
3 10% (96 h, 50 °C)
4 >99% (48 h, 25 °C)
7 0% (72 h, 50 °C)
3 97% (7 d, 50 °C)
4 >99% (24 h, 25 °C)
7 0% (72 h, 50 °C)
Substrate
Cat T [°C] t (d) Conv. (%) Product
3
4
50
25
7
1
27a
>99
3 27% (96 h, 50 °C)
4 >99% (24 h, 25 °C)
7 0% (72 h, 50 °C)
3
4
50
25
7
5
>99
>99
3 <1% (96 h, 50 °C)
4 55% (24 h, 25 °C)
7 0% (72 h, 50 °C)
3
4
50
25
7
2
>99
>99
3
4
50
25
7
2
95
91
contrast to 3 and 4, compound 7 showed neither catalytic nor
stoichiometric activity in any of these transformations. This is
consistent with our previous observations that electron-with-
drawing substituents are essential to lower the energy of the σ*
LUMO enhancing the electron acceptor properties of such
phosphonium cations.28 In addition, ferrocene is compara-
tively sterically demanding and may inhibit access to the σ*
LUMO on P.
3
4
50
25
5
2
99
98b
3
4
50
25
5
2
>99
>99
To further probe the reactivity of 3 and 4 in deoxygenation
of ketones, a broader range of substrates was investigated
(Table 2). Catalyst 4 proved capable of carrying out deoxygena-
tion of a range of substrates at ambient temperatures over the
course of 24–120 h. For example, acetophenone is deoxyge-
nated to ethyl benzene during a 1 day treatment with Et3SiH
and the catalyst 4. In contrast, the analogous use of 3 as the
catalyst is much less efficient in this case, giving a 27% yield
of ethylbenzene together with PhCH2CH2SiEt3 after 4 days at
50 °C. Benzophenone is deoxygenated to diphenylmethane in
the presence of silane and either 3 or 4 although 3 requires
7 days at 50 °C for complete conversion whereas 4 is done after
5 days at 25 °C. The para-substituted aromatic ketones PhC(O)-
C6H4X (X = F, Br, OMe) and 2,4,6-iPrPhC(O)CH3 are reduced
using either 3 and 4 as the catalyst (Table 2). The ketone
MeOC6H4C(O)CH2C6H4OMe was effectively reduced under
similar conditions. Aliphatic ketones PhC(O)C6H11 and CH3C-
(O)CH(CH3)2 were also deoxygenated, although for the latter, a
reduced yield of the deoxygenated product was obtained. This
results from competing elimination pathways giving small
amounts of alkene products.36 PhC(O)CF3 underwent hydro-
silylation in the presence of catalyst 4 at 50 °C, however only
the hydrosilylation product was obtained. In contrast, the
corresponding reaction with 3 gave minimal conversion.
Comparing these activities to those previously reported, we
note that the phosphonium dications of types B, C, and D
have shown to be active catalysts for ketone deoxygenation
under similar conditions.31,36 In general, the dicationic cata-
lysts are significantly more active than 3 or 4, effecting the
catalysis at ambient temperatures.31,36 Interestingly, dicationic
catalysts of type C with bridges of one or two carbon atoms
3
4
50
25
7
4
7
92
3
4
50
25
5
4
74
76
3
4
50
25
7
5
>10
89
3
4
50
25
7
4
60
91
Conversion determined according to 1H NMR spectroscopy.
a Significant quantities of PhCH2CH2SiEt3 derived from the
hydrosilylation of styrene were observed. b 80% isolated yield obtained
for this product.
proved generally more active in catalysis than 4. On the other
hand, type C cations with three to five carbon linkers are
much less active, suggesting that enhanced reactivity results
from the enforced proximity of the two phosphonium centres.
In contrast, this is not the case for 4 where the cationic centres
on each Cp-ring are oriented so as to maximize the separation
of the charged centres. Nonetheless, the presence of the
second phosphonium centre in 4 inductively enhances the
Lewis acidity of the other, although it is unlikely that these
centres act in a cooperative fashion. As a result 4 exhibits cata-
lytic activity that is certainly above that of the [Ph3PF][B(C6F5)4]
but somewhat less than that of diphosphonium dications
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derived from Ph2P(CH2)nPPh2 (n = 1, 2), where the proximity of 16 J. M. Bayne and D. W. Stephan, Chem. Soc. Rev., 2016, 45,
the cationic centres is imposed.
765–774.
17 D. Gudat, A. Haghverdi, H. Hupfer and M. Nieger, Chem. –
Eur. J., 2000, 6, 3414–3417.
Conclusions
The species [CpFe(η5-C5H4PFPh2)][B(C6F5)4]
18 A. L. Brazeau, C. A. Caputo, C. D. Martin, N. D. Jones and
P. J. Ragogna, Dalton Trans., 2010, 39, 11069–11073.
19 M. Well, P. G. Jones and R. Schmutzler, J. Fluorine Chem.,
1991, 53, 261–275.
20 C. A. Dyker and N. Burford, Chem. – Asian J., 2008, 3, 28–
36.
3
and [Fe(η5-
C5H4PFPh2)2] [B(C6F5)4]2 4, are easily prepared from commer-
cially or readily available phosphine precursors by oxidation
with XeF2 and subsequent fluoride abstraction. These Lewis
acids are effective catalysts for Friedel–Crafts dimerization of
1,1-diphenylethylene, dehydrocoupling of phenol and triethyl-
silane, hydrodefluorination of 1-fluoropentane and the deoxy-
genation of a series of ketones. The present systems show
activities that are comparable to several of the EPC systems we
have previously reported, although the monocation 3 showed
lower activity than the dication 4. This further indicates the
enhanced Lewis acidity that results from a dicationic Lewis
acidic phosphonium cation. We are continuing to explore and
develop the range of EPCs that are effective Lewis acid catalysts
for a broadening range of useful organic transformations.
21 M. H. Holthausen, A. Hepp and J. J. Weigand, Chem. – Eur.
J., 2013, 19, 9895–9907.
22 M. H. Holthausen, K. O. Feldmann, S. Schulz, A. Hepp and
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23 L. E. Longobardi, C. A. Russell, M. Green, N. S. Townsend,
K. Wang, A. J. Holmes, S. B. Duckett, J. E. McGrady and
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24 T. W. Hudnall, Y. M. Kim, M. W. P. Bebbington,
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25 O. Sereda, S. Tabassum and R. Wilhelm, in Top Curr.
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26 G. Wittig and U. Schöllkopf, Chem. Ber., 1954, 87, 1318–
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Acknowledgements
27 G. Wittig and U. Schoellkopf, Org. Synth., 1960, 40, 66–68.
28 C. B. Caputo, L. J. Hounjet, R. Dobrovetsky and
D. W. Stephan, Science, 2013, 341, 1374–1377.
29 M. H. Holthausen, R. R. Hiranandani and D. W. Stephan,
Chem. Sci., 2015, 6, 2016–2021.
30 M. H. Holthausen, J. M. Bayne, I. Mallov, R. Dobrovetsky
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7301.
D.W.S gratefully acknowledges the financial support of the
NSERC of Canada and the award of a Canada Research Chair.
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