Syntheses of Phosphonium Salts from Phosphines and Carbenium: Efficient CO2 Fixation and Phase-Transfer Catalysts

Article abstract of DOI:10.1002/ejoc.202000221

A new library of phosphonium salts has been synthesized from the reaction of tris-(2,6-dimethoxyphenyl)carbenium with different phosphines. The resultant phosphoninum salts work as efficient phase-transfer catalysts for the alkylation of 1,3-dicarbonyl co

Full text of DOI:10.1002/ejoc.202000221

A Journal of
Accepted Article
Title: Syntheses of Phosphonium Salts from Phosphines and
Carbenium: Efficient CO2 Fixation and Phase-Transfer Catalysts
Authors: Aslam C Shaikh, Jose M Veleta, Jan Bloch, Hannah J.
Goodman, and Thomas L. Gianetti
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10.1002/ejoc.202000221
European Journal of Organic Chemistry
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Syntheses of Phosphonium Salts from Phosphines and
Carbenium: Efficient CO2 Fixation and Phase-Transfer Catalysts
Aslam C. Shaikh,[a] José M. Veleta,[a] Jan Bloch,[b] Hannah J. Goodman,[a] and Thomas L. Gianetti,*[a]
Abstract: A new library of phosphonium salts has been synthesized
from the reaction of tris-(2,6-dimethoxyphenyl)carbenium with
different phosphines. The resultant phosphoninum salts work as
efficient phase transfer catalysts for the alkylation of 1,3-dicarbonyl
that small phosphines bind these moieties at the carbon cation
center (compound A, Scheme 1a), while sterically demanding
phosphines undergo nucleophilic aromatic substitution (SNAr) at
the para position (compounds B - C, Scheme 1a). They also
showed that the isomerization of cyclohexadienyl isomer B to the
phenyl isomer C is Lewis base-catalyzed. This phosphine–trityl
interaction was further explored by Stephan in 2014 and Berionni
in 2017 with the design of new Frustrated Lewis Pairs (FLP), in
compounds. One compound,
a bifunctional phosphonium salt
prepared from tris-(2,6-dimethoxyphenyl)carbenium and an
aminophosphine, was also found to be an efficient catalyst for CO2
fixation reactions with epoxides under mild conditions.
which tritylium ions act as Lewis acids and phosphines act as
[ 13 ],[ 14 ]
Lewis bases.
However, other than FLP reactivity, trityl
phosphonium compounds remain largely unexplored for
applications in organic synthesis. To the best of our knowledge,
the reactivity between derivatives of trityl cation and phosphines
to form novel quaternary phosphonium ions has not yet been
reported (Scheme 1b). Of the compounds that make up the triaryl
carbenium family, tris-(2,6-dimethoxyphenyl)carbenium[ 15 ] has
received increasing interest since it was first published in
1964.[10,16] Notably, Lacour et al. reported the direct coupling of
this triaryl carbenium ion with indoles and anilines that proceed
via nucleophilic attack at the para position,[17] similarly to the
reactivity observed in isomer B (Scheme 1a).
Introduction
Quaternary phosphonium salts[ 1 ] are used in a variety of
organic transformations. They are used as Wittig reagents, Lewis
acid organocatalysts, ionic liquids, and anti-cancer agents.[ 2 ]
Further applications include asymmetric catalysis, drug delivery,
and phase-transfer catalysis precursors.[
Phase-transfer
3
]
catalysis (PTC) is a process widely used in laboratory synthesis
and in industry. Recent developments in PTC involve tetraamino-
phosphonium salts, along with other cationic systems
(imidazolium, triazolium, cyclopropenium, etc.).[4] Most practical
syntheses of quaternary phosphonium salts are limited to either
phosphine arylation/alkylation[5] or coupling reactions of an aryl
Herein,
we
report
the
reaction
of
tris-(2,6-
dimethoxyphenyl)carbenium with alkyl/aryl phosphines (PMe3,
PPhMe2, PPh2R, with R = CH3, (CH2)2NH2)). This results almost
exclusively in SNAr at the para position of the carbenium moiety,
leading to the formation of cyclohexadiene isomer B′ regardless
of the phosphine used (Scheme 1b). Aromaticity is rapidly
reformed in the isomer C′ by amine-catalyzed tautomerization.11a
Both phosphonium isomers B′ and C′ were found to be efficient
phase transfer catalysts. We also report the synthesis of a
bifunctional aminoethyl–phosphonium cation 5 and its application
as a catalyst for CO2 fixation.
halide (alkyl/Ar-X) with
a triarylphosphine (Ar3P). Another
extensively studied transformation that uses bifunctional
quaternary ammonium and phosphonium salts is the synthesis of
cyclic carbonates using epoxides and CO2.[6] These processes
have also been studied using KI−tetraethylene glycol complexes,
transition metals, and cationic polymer systems.[7] Considering
their diverse range of applications, it is desirable to develop novel
synthetic approaches to quaternary phosphonium salts.
Trityl[8] cation and its analogues have played significant roles in
a variety of stoichiometric and catalytic reactions,[9] in material
science, and in cell-imaging studies.[10] Several groups have used
the Lewis acidity of these carbeniums to form quaternary
phosphonium adducts (Scheme 1a),[11] and in 2006 Stephan and
coworkers reported their crystal structures.[ 12] Insight into the
reactivity of these compounds was provided from Bidan and
Genies’ conclusion that the steric demands of the phosphine
determine the course of the reaction.[10d] They further determined
[a]
Dr. A. C. Shaikh, J. M. Veleta, H. J. Goodman, Prof. T. L. Gianetti
Department of Chemistry and Biochemistry
University of Arizona
1306 E. University Blvd., Tucson, AZ 85719, USA
E-mail: tgianetti@email.arizona.edu.
https://www.gianettigroup.com/
[b]
J. Bloch
Department of Chemistry and Applied Biosciences
ETH Zürich
Scheme 1. Reactivity of phosphine towards trityl cation.
Vladimir-Prelog-Weg 1, 8093 Zürich, Switzerland
Supporting information for this article is given via a link at the end of
the document. CCDC 1984242 contain the supplementary
crystallographic data for this paper
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10.1002/ejoc.202000221
European Journal of Organic Chemistry
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Results and Discussion
Synthesis and characterization of phosphonium salts: The
addition of trimethylphosphine to
a
solution of tris(2,6-
dimethoxyphenyl)methylium triflate in CH2Cl2 at room
temperature yields a colorless, crystalline, air-stable mixture of
phosphonium salts 2a and 2b in 92% yield (Scheme 2i). The 31
P
NMR spectrum indicates the formation of two phosphonium
species in 4:1 ratio (Figure S1). The major isomer 2a has a 31P
chemical shift of 35 ppm, while the minor isomer 2b resonates at
56 ppm. The {1H}31P NMR spectrum for 2a, reveals that the
phosphorous is strongly coupled to several hydrogens, while the
1
phosphorous in 2b possesses much weaker coupling. H NMR
spectroscopy shows that the minor isomer 2b has C3 symmetry
(Figure S2), based on comparison to the D3 symmetry of 1. The
loss of the C2 rotation axis is shown by the presence of two
asymmetrical methoxy groups presenting two distinct proton
signals. The retention of the principal C3 axis of rotation suggests
that the phosphorous is bound to the central carbon of the
carbenium scaffold. On the other hand, the 1H NMR spectrum of
2a shows Cs symmetry, suggesting that PMe3 attacked another
position of carbenium 1. The 2 sets of correlated protons in a 2:1
ratio at 4.30 ppm (dd, 3JP-H = [4-6] Hz, 3JH-H = 5.4 Hz, 2H) and 3.75
Scheme 2. Synthesis of phosphonium salts.a Unless otherwise noted, all
reactions were carried out on a 1 mmol scale using 1, 1.2 mmol phosphine in 5
mL of CH2Cl2 for 5 min at rt.b Isolated yields are given; (X = BF4, OTf).
2
3
ppm (dt, JP-H = 25.2 Hz, JH-H = 5.4 Hz, 1H) suggest that PMe3
attacked at the para position of one aromatic ring. Thermal
treatment and variable temperature NMR spectroscopy analysis
did not show interconversion between 2a–2b or affect their
relative ratio. Only thermal decomposition and formation of an
untraceable mixture of products was observed after several hours.
Interestingly, this reactivity is different from prior reports of
interactions between PMe3 and trityl cation,[11] which provided
exclusive formation of the symmetric isomer A (vide supra). DFT
calculations of 1 and trityl cation show that, even though 1 has a
larger number of electron donating groups, the lowest unoccupied
molecular orbital (LUMO) of both molecules has a larger
distribution of the wavefunction on the central carbon (Figure 1).
This suggests that differences in reactivity are mostly controlled
by steric hindrance imposed by the methoxy groups. The upfield
31P NMR chemical shift of 2b compared to 2a (56 ppm and 35
ppm, respectively, Figure S1) supports this claim of larger positive
charge at the central carbon. It is conceivable, then, that
increasing the cone angle and steric bulk of the phosphine should
result in quantitative formation of the para product. Unlike the trityl
cation, treatment of tris(2,6-dimethoxyphenyl)methylium cation
with triphenylphosphine failed to yield the phosphonium salt. Free
PPh3 and 1 were observed in solution, which can be rationalized
by the fact that 1 is less Lewis acidic than the unsubstituted trityl
cation. However, using sterically demanding and stronger s-
donor phosphines (diphenylmethyl or phenyldimethyl) led to the
formation of 3a and 4a (Scheme 2ii-iii). These products were
crystallized from CH2Cl2/hexanes and isolated in good yield (90%
and 94%, respectively). The 1H NMR spectrum of complexes 3a-
4a indicates Cs symmetry with 2 sets of correlated protons in a
2:1 ratio at 4.66 ppm (t, 3JH-H = 5.4 Hz, 2H) and 4.35 ppm (dt, 2JP-
H = 25.3 Hz, 3JH-H = 5.3 Hz, 1H) for 3a (Figure S4) and at 4.45 ppm
Figure 1. Molecular orbitals of (a) triphenylcarbenium and (b) tris(2,6-
dimethoxyphenyl)methylium cations. Contribution of the carbocation center for
each MO was obtained from the normalized wavefunction after geometry
optimization and frequency calculations. Contour surface diagrams of the MOs
are shown at a 0.04 isovalue.
Interestingly, conversion to the cyclohexadiene moiety was not
quantitative when PPhMe2 was used. Careful analysis of both 1H
and 31P NMR spectra reveals the formation of about 15% of a
minor isomer 4b (Figure S7 and S9). The 31P NMR spectrum of
this minor phosphonium isomer resonates at 21 ppm, 5 ppm
upfield from 4a. The slight upfield shift suggests that 4b has a
more electropositive phosphonium moiety. In the 1H NMR
spectrum, isomer 4b has a singlet at 6.53 ppm (1H) and a doublet
at 6.81 ppm (d, 3JP-H = 14.8 Hz, 2H) assigned to the central sp3-
hybridized methyl proton and the two aromatic protons ortho to
the phosphonium group. These values are consistent with
previously reported values for HC(Ar)3 (6.38 ppm for methine, Ar
= 2,6-dimethoxyphenyl)[17] and for HC(p-iPr3PC6H4)Ph2) (5.69
ppm methine).[11] These results support proton transfer from the
3
2
3
(t, JH-H = 5.5 Hz, 2H) and 4.97 ppm (dt, JP-H = 24.3 Hz, JH-H
=
5.2 Hz, 1H) for 4a (Figure S6). Both 31P NMR signals for these
phosphoniums are characteristic of the formation of the
cyclohexadiene scaffold as observed in 2a (Figure S6, S9).
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10.1002/ejoc.202000221
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para position to the carbon center along with aromatization of the
ring, which is similar to the isomerization observed during the
reaction between trityl cation and PiPr3 (scheme 1, compound
C).[11]
resonates at 31.18 ppm (Figure S15, S19). This contrasts with the
doublet observed for the carbon bound to the methine proton in
2a-4a (d = 35.04 ppm for 2a, 86.41 ppm for 3a, and 41.43 ppm for
4a), supporting isomerization.
The major phosphonium isomer 2a was recrystallized from
CH2Cl2/hexanes. Its structure was confirmed by single crystal X-
ray diffraction, which showed the formation of a dissymmetric
phosphonium salt from phosphine attack at the less sterically
hindered para position of 1 (Figure 2). The alternating short/long
C-C bond distances of the ring and the short C4-C7 bond distance
support the presence of a dearomatized cyclohexadiene system.
The P1-C1 bond distance of 1.835 Å is within the normal range
Two pathways can be proposed for hydrogen atom transfer and
aromatization of the cyclohexadiene moiety: 1) intramolecular
[1,5] sigmatropic shift, or 2) base-assisted tautomerization.
Thermal treatment of cycohexadiene isomers 3a or 4a did not
lead to isomerization and hydrogen transfer to form 3b and 4b.
Additionally, PPh2Me and PPh2R (R = CH2CH2NH2) have similar
[ 19 ]
cone angle and s-donor ability.
Therefore, quantitative
isomerization observed in 5 cannot be rationalized by electronic
and steric properties of the phosphine. Furthermore, when the
cyclohexadienyl isomers 3a and 4a were treated with 1.0
equivalents of nPrNH2, immediate formation of the aryl isomers 3b
and 4b was observed (see Scheme 3 and Figure S10-S17). This
results suggests that isomerization occurs via base-assisted
tautomerization, which is consistent with the work of Bidan and
Genies on trityl cation.[10d] The stronger basicity of PPhMe2 (pKa
= 6.50) relative to PPh2Me (pKa = 4.57)[18, 20 ] explains the
formation of minor isomer 4b from PPhMe2, while the amine group
of PPh2CH2CH2NH2 promotes quantitative tautomerization to
form 5.
]
for phosphonium–carbon bonds (1.8–1.9 Å).[ 18 The steric
hindrance imposed by the methoxy groups is evident by the
distortion observed around the C7=C4 double bond, and a
dihedral angle of about 17˚ is observed.
Figure 2. Ortep diagram of 2a. Hydrogen atoms and solvent molecules have
been omitted for clarity. Selected bond distances (A): P1–C1 = 1.8348(16); C1–
C2 = 1.498(2); C2–C3 = 1.342(2); C3–C4 = 1.482(2); C4–C5 = 1.484(2); C5–
C6 = 1.339(2); C6–C1 = 1.497(2); C4–C7 = 1.357(2). Selected angles (˚): C5-
C4-C7-C8 = 17.1.
Scheme 3. Base-catalyzed tautomerization of 3a-4a to 3b-4b.
The phosphonium salts 2-5 were investigated as catalysts for
phase-transfer alkylation.[21] The reaction between β-ketoester
(6a) and alkyl bromide (7) in the presence of base and the
phosphonium salts (1 mol%) in toluene/H2O (1:1 mixture) yielded
the desired product 8 (See Table 1 and SI). Of the phosphonium
salts screened, 5 gave the highest yield (8a in 98% yield). The
scope of the phase-transfer alkylation reaction was further
investigated under optimized reaction conditions (Table 1).
Several β-ketoesters were screened that produced alkylation
products 8b-8h in good to excellent yield (81-98%). This reaction
was performed on a gram scale to produce 8a, which was isolated
in 92% yield.
Seeking inspiration from the formation of 2-4, a bifunctional
phosphonium salt (5) was targeted and synthesized by reacting 1
with 2-(diphenylphosphenyl)ethan-1-amine at rt for
5 min
(Scheme 2iv). Crystallization from CH2Cl2/hexanes afforded pale
pink microcrystalline material in 87% yield. The 1H and 31P NMR
spectra of 5 show characteristic signals for the triaryl moieties
(with two pairs of methoxies) and for the P(Ph2)CH2CH2NH2 unit
(31P at 23 ppm, two methylene sets coupled to 31P, and a broad
singlet for NH2) in a 2:1 ratio. These data suggest low Cs
symmetry, supporting phosphine attack at the para position
(Figure S18-S24). However, the 1H and {1H}13C NMR spectra are
not consistent with the signature observed for the phosphonium
salts 2a, 3a and 4a. Instead, the 1H NMR spectra of 5 resembles
that of the minor isomer 4b (Scheme 2iii) and compound C
(Scheme 1), which both undergo hydrogen transfer and
aromatization. The proton beta to the phosphorous moiety
resonates at 7.06 ppm (compared to 4-5 ppm for 2-4) and only
exhibits coupling to the phosphorous (d, 3JP-H= 8.2 Hz, 2H), which
supports the presence of an aromatic, not olefinic, proton. The
methine proton resonates at 6.49 ppm as a singlet with no 31P or
1H coupling (compared to the doublet of triplets between 3.5–4.5
ppm for 2-4), further supporting hydrogen atom transfer from the
cyclohexadiene moiety to the central carbon (see ESI). The
{1H}13C NMR and 1H–13C HSQC spectroscopy analysis of 5
reveals that the methine proton is bound to a singlet carbon that
Bifunctional catalyst, species containing both a cationic and
protic fucntionality, are known to catalyse the chemical fixation of
carbon dioxide (CO2) using epoxide for production of cyclic
22
]
carbonates under
1
atm of CO2.[7],[
The bifunctional
phosphonium 5 was found to be a very efficient catalyst for this
transformation. After optimizating reaction conditions,[23] 1 mol%
of 5 and 10 mol% of NaI under CO2 in chlorobenzene at 100 °C
for 12 h yielded formation of the desired cyclic carbonate in 92%
yield with >99% conversion. A series of substituted epoxides (9b–
9g) performed efficient CO2 fixation to yield the corresponding
desired carbonates (10b–10g) in good to excellent yields (83–
92%, Table 2). The reactions with 1,2-disubstituted epoxide 9h
proceeded slowly to give cyclic carbonate 10h. Raising the
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10.1002/ejoc.202000221
European Journal of Organic Chemistry
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temperature to 120 °C did not significantly alter the rate of the
reaction.
It is established in relevant literature that the bifunctional nature
of catalysts is required to promote CO2 fixation with epoxides.[7,22]
It has been proposed that, on one hand, epoxide acitivation and
CO2 insertion are both facilitated by hydrogen bonding to the
protic part of the catalyst.[22a-b] On the other hand, the cationic
Table 1. Substrate scope for phase-transfer alkylation reactiona,b
nature of the catalyst promotes ring opening by bringing the halide
[ 25 ]
anion in close proximity to the activated epoxide.
Under
optimized conditions, abscence of catalyst, or when compounds
3b, 4b, or PPh3 are used instead of 5, little to no conversion was
observed, supporting the need for a hydrogen bonding interaction
(entry 2-5, Table S4). Furthermore, no conversion was observed
n
in this reaction with PrNH2 or PPh2(CH2)2NH2 alone (entry 6-7,
Table S4), or with 5 (that contains a triflate counterion) in the
absence of NaI (Entry 8, Table S4). Trace amounts of product
i
were formed when 1 mol% of 3b and 1 equiv. PrNH2 was used
(entry 9, Table S4). Finally, by monitoring the 1H NMR spectrum
of the reaction between the epoxide 9 and either 3b or 5 in C6D5Br,
in the presence of NaI, revealed that ring opening only ocurred
with the bifunctional molecule 5 (Figure S25-S26). These
observations are consistent with the need for an ion pair in close
proximity to the hydrogen bond activated epoxide.
aUnless otherwise noted, all reactions were carried out on a 0.50 mmol scale
using 6, 0.55 mmol 7, cat 5 (1 mol%), 1.1 equiv. K2CO3, 2 mL toluene:H2O
(1:1) for 6 h. See Supporting Information for details.b Isolated yields are given.
Table 2. Substrate scope for CO2 fixation.a,b
Scheme 4. A plausible mechanism for CO2 fixation reaction.
Based on our mechanistic studies and previous reports,[7, 22, 25]
we propose a plausible catalytic cycle for the CO2 fixation in
Scheme 4. The epoxide (9) is activated via hydrogen bonding with
the amino group of aminoethyl–phosphonium cation (intermediate
A). The activated epoxide intermediate A then undergoes
nucleophilic attack from an iodide anion to form intermediate B.
The reactive alkoxide in intermediate B attacks CO2 to yield
intermediate C, which affords cyclic carbonate (10) upon
intramolecular ring closure.
aUnless otherwise noted, all reactions were carried out on a 1.0 mmol scale
using 9, 0.01 mmol NaI, 1 mol% cat 5, 2.7 mL chlorobenzene at 100 °C for 12
h (See ESI).b Isolated yields are given.
Conclusions
Frustrated Lewis pairs (FLPs) are well known to promote a large
variety of organic transformations. [24] However, no catalytic CO2
fixation was observed when the triaryl cation 1 was used in the
presence of PPh3, thus ruling out the involvement of an FLP-type
mechanism (entry 1, Table S4).
A
variety of phosphines were reacted with tris-(2,6-
dimethoxyphenyl)carbenium to yield novel phosphonium salts,
which were characterized by H NMR, 31P NMR, H-13C HSQC,
1
1
and {1H}13C NMR spectroscopy, along with single crystal X-ray
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CH2Cl2 solution to yield pure 3a as light purple microcrystalline
material (0.690 g, 96% yield).
crystallography. The steric hindrance imposed by the methoxy
groups was rationalized as the cause for differences in reactivity
between trityl cation and tris-(2,6-dimethoxyphenyl). These new
phosphonium salts were found to be efficient catalysts for phase-
I.3 Reaction between tris(2,6-dimethoxyphenyl)methylium
trifluoro-methanesulfonate (1) and PPhMe2: In a glove box,
dimethylphenylphosphane (222 mg, 1.2 mmol, 1.2 equiv) was
added to a stirred solution of 1 (501 mg, 1.0 mmol, 1.0 equiv) in
CH2Cl2 (10 mL). The solution was stirred at ambient temperature
for 5 min. After addition of the amine precursor, the reaction
mixture color immediately changed violet to purple. The product
was precipitated by addition of hexanes, filtered, and washed with
hexanes. Finally, it was precipitated by adding twofold of hexanes
to the CH2Cl2 solution, and further crystalized by slow diffusion of
a hexane layer into a DCM solution containing the product,
yielding 4a/4b as colourless microcrystalline powder (0.610 g,
94% yield).
transfer alkylation reactions. Finally, the synthesis of
a
bifunctional phosphonium salt was reported as well as its catalytic
application in the synthesis of industrially significant cyclic
carbonates by CO2 fixation. Because of the efficiency of this
catalyst system, it is worth further investigation for application to
other synthetic aims.
Experimental Section
General considerations
I.4 Representative procedure for amine mediated conversion
of 3a/4a to 3b/4b: To a stirred solution of 3a (200 mg, 0.28 mmol,
1.0 equiv) in CH2Cl2 (5 mL) was added nPrNH2 (17 mg, 1.0 mmol,
1.0 equiv) and the solution was stirred at ambient temperature for
5 min. After addition of the amine precursor, the reaction mixture
color immediately changed from violet to light red. The product
was precipitated with addition of hexanes, filtered, and washed
with hexanes. Finally, it was crystallized by adding twofold of Et2O
to the CH2Cl2 solution yielding pure 3b as red solid (0.192 g, 96%
yield).
The syntheses of phosphonium salts were carried out in oven
dried vials or reaction vessels with magnetic stirring inside a N2
filled glove box. Solvents were dried by passing them through an
alumina column on a solvent purification system, then stored
under molecular sieves. Dried solvents and liquid reagents were
transferred by oven-dried syringes or hypodermic syringes. Most
of the catalytic experiments were performed on the bench and
monitored by analytical thin layer chromatography (TLC). TLC
was performed on pre-coated silica gel plates. After elution, plates
was visualized under UV light at 254 nm. Further visualization was
achieved by staining with p-anisaldehyde solution and heating.
Solvents were removed in vacuo and heated with a water bath at
35 °C. Silica gel finer than 200 mesh was used for flash column
chromatography. Columns were packed as slurry of silica gel in
hexanes and flushed with the appropriate solvent mixture prior to
use. The compounds were loaded neat or as a concentrated
solution using the appropriate solvent system. The elution was
assisted by applying pressure with an air pump. All chemicals and
solvents were purchased from Sigma Aldrich, Fisher Scientific,
Oakwood or VWR. Commercially obtained reagents were used
without further purification. Compound 1 was prepared according
to Laursen’s report. [26]
I.5 Reaction between tris(2,6-dimethoxyphenyl)methylium
trifluoro-methanesulfonate (1) and NH2CH2CH2PPh2: In a
glove box, 2-(diphenylphosphaneyl)ethan-1-amine (229 mg, 1
mmol, 1.0 equiv) was added to a stirred solution of 1 (570 mg, 1
mmol, 1.0 equiv) in CH2Cl2 (10 mL) and stirred at ambient
temperature for 5 min. After addition of the amine precursor, the
reaction mixture color immediately changed from purple to wine
red. The product was precipitated by addition of Et2O, filtered, and
washed with Et2O. Finally, it was crystallized by adding twofold of
diethyl ether to the acetonitrile solution, yielding pure compound
5 as pink powder (0.840 g, 87% yield).
II.1 Representative Procedure for phase-transfer alkylation
reaction: To a stirred solution of 6a (0.5 mmol, 1.0 equiv) and cat.
5 (1 mol%) in toluene (1 mL) and aq. K2CO3 solution (1 ml, 1M)
was added p-iPr benzyl bromide (1.1 equiv). The reaction mixture
was stirred for 12 h and monitored by TLC. After complete
conversion, the residue was diluted with water (5 ml) and
extracted with EtOAc (10 mL × 3). The combined organic layers
were dried over Na2SO4, filtered, and concentrated in vacuo. The
crude reaction mixture was purified by flash chromatography on
silica gel using Hexane/EtOAc in a 4:1 ratio.
I.
Synthetic procedures for synthesis of 2-5
I.1 Reaction between tris(2,6-dimethoxyphenyl)methylium
trifluoro- methanesulfonate (1) and PMe3: To a stirred
solution of 1 (500 mg, 1.0 mmol, 1.0 equiv) in CH2Cl2 (5 mL) was
added trimethylphosphine (91 mg, 1.2 mmol, 1.2 equiv). The
solution was stirred at rt for 5 min. After addition of the amine
precursor, the reaction mixture immediately changed color from
violet to colorless. The product was precipitated by addition of
Et2O (10 mL), filtered off, and washed with Et2O (2 x 5 mL). Finally,
the product was crystallized from layering diethyl ether on the
product dissolved in acetonitrile to yield pure 2a/2b as colourless
crystals (0.595 g, 92% yield).
II. 2 Representative procedure for CO2 fixation reaction: In a
25 mL Schlenk flask, a mixture of styrene oxide 9a (120 mg, 1
mmol), catalyst 5 (8 mg, 0.01 mmol, 1 mol %), sodium iodide (17.0
mg, 0.10 mmol, 10 mol %), and chlorobenzene (2.7 ml, 0.3 M)
was heated at 100 °C for 12 h under a CO2 atmosphere (1 atm,
using a balloon). After cooling to room temperature, the reaction
mixture was purified by flash column chromatography on silica gel
(hexane/EtOAc = 10:1−3:1 as the eluent) to afford cyclic
carbonate 10a (142 mg, 92% yield, 4.95 mmol; Rf = 0.24 in
hexane/EtOAc = 3:1).
I.2 Reaction between tris(2,6-dimethoxyphenyl)methylium
trifluoro-methanesulfonate (1) and PPh2Me: To a stirred
solution of 1 (501 mg, 1.0 mmol, 1.0 equiv) in CH2Cl2 (10 mL) was
added methyldiphenylphosphine (222 mg, 1.2 mmol, 1.2 equiv).
The solution was stirred at rt for 5 min. After addition of the amine
precursor, the reaction mixture color immediately changed from
deep to pale purple. The product was precipitated by addition of
hexanes (5 mL), filtered, and washed with hexanes (2 x 5 mL).
Finally, the product was crystallized by layering hexanes over a
Acknowledgements
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10.1002/ejoc.202000221
European Journal of Organic Chemistry
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Support has been provided by the Donors of the American
Chemical Society Petroleum Research Fund (59631-DNI3).
Keywords: triarylcarbenium • phosphonium • bifunctional catalyst
•CO2 fixation • phase-transfer catalysis
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This article is protected by copyright. All rights reserved.

10.1002/ejoc.202000221
European Journal of Organic Chemistry
FULL PAPER
Entry for the Table of Contents
FULL PAPER
Phosphonium Salts
Aslam C. Shaikh, José M. Veleta, Jan
Bloch, Hannah J. Goodman, and Thomas
L. Gianetti,*
Page No. – Page No.
A new library of phosphonium salts has been synthesized from the reaction tris-(2,6-
dimethoxyphenyl)carbenium with different phosphines. All of these phosphoninum
salts were efficient phase transfer catalysts for alkylation of 1,3 dicarbonyl
compounds. One compound, a bifunctional phosphonium salt prepared from tris-(2,6-
dimethoxyphenyl)carbenium and an aminophosphine, was found to be an efficient
catalyst for CO2 fixation reactions with epoxides under mild conditions.
Syntheses of Phosphonium salt from
Phosphines and carbenium: Efficient
CO2 Fixation and Phase-Transfer
Catalysts
This article is protected by copyright. All rights reserved.