Hydrophobic encapsulated phosphonium salts - Synthesis of weakly coordinating cations and their application in wittig reactions

Article abstract of DOI:10.1002/chem.201406370

Large and rigid tetraarylphosphonium tetrafluoroborate salts have been synthesized representing weakly coordinating cations with diameters of several nanometers. Divergent dendritic growth by means of thermal Diels-Alder cycloaddition was employed for the construction of the hydrophobic polyphenylene framework up to the third generation. X-ray crystal structure analysis of first-generation phosphonium tetrafluoroborate supported the rigidity of the non-collapsible shell around the phosphorus center and gave insight into solid-state packing and cation-anion distances. Copper(I)-catalyzed azide-alkyne ligation served as reliable method for the preparation of a first-generation triazolylphenyl hybrid phosphonium cation under mild reaction conditions. Furthermore, from the synthesis of triarylbenzylphosphonium bromides, Wittig precursors with unprecedented bulky substituents in the α-position were accessible. Employment of these precursors under Wittig conditions by treatment with base and subsequent reaction with aldehydes preferentially provided (Z)-olefins with bulky polyphenylene substituents. Just add salt: The application of benzylphosphonium bromides as Wittig precursors resulted in the formation of encapsulated phosphorus ylides, which were further reacted with aldehydes to yield novel olefins with bulky substituents. The stereochemical analysis of the obtained alkenes showed the exclusive formation of (Z)-isomers when aromatic aldehydes were employed and the sterically congested polyphenylene (Z)-stilbene derivative (see scheme) is reported.

Full text of DOI:10.1002/chem.201406370

DOI: 10.1002/chem.201406370
Full Paper
&
Phosphonium Salts
Hydrophobic Encapsulated Phosphonium Salts—Synthesis of
Weakly Coordinating Cations and their Application in Wittig
Reactions
Ralf Moritz,[a] Manfred Wagner,[a] Dieter Schollmeyer,[b] Martin Baumgarten,[a] and
Klaus Müllen*[a]
Abstract: Large and rigid tetraarylphosphonium tetrafluoro-
borate salts have been synthesized representing weakly co-
ordinating cations with diameters of several nanometers. Di-
vergent dendritic growth by means of thermal Diels–Alder
cycloaddition was employed for the construction of the hy-
drophobic polyphenylene framework up to the third genera-
tion. X-ray crystal structure analysis of first-generation phos-
phonium tetrafluoroborate supported the rigidity of the
non-collapsible shell around the phosphorus center and
gave insight into solid-state packing and cation–anion dis-
tances. Copper(I)-catalyzed azide–alkyne ligation served as
reliable method for the preparation of a first-generation tria-
zolylphenyl hybrid phosphonium cation under mild reaction
conditions. Furthermore, from the synthesis of triarylbenzyl-
phosphonium bromides, Wittig precursors with unprece-
dented bulky substituents in the a-position were accessible.
Employment of these precursors under Wittig conditions by
treatment with base and subsequent reaction with alde-
hydes preferentially provided (Z)-olefins with bulky poly-
phenylene substituents.
introduction about 60 years ago,[8,9] the Wittig reaction has
become of particular interest with regard to the stereochemis-
try of the initial adduct formation and the resultant (E/Z) olefin
ratio.[10,11] A variety of factors including temperature, solvent,
choice of carbonyl reagent, type of ylide as well as the pres-
ence and concentration of lithium salts were demonstrated to
influence the resulting (E/Z) alkene ratio.[12,13] Detailed mecha-
nistic studies by Vedejs et al.[14] investigated the role of sub-
stituents at phosphorus ylides and attributed the relevant ste-
reochemistry to a kinetically controlled formation of cis- or
trans-oxaphosphetanes. However, among the substituent
effect reports so far, the moiety in the a-position of the ylide
(see Scheme 5 later) was mostly limited to a simple methyl
group or slightly larger alkyl homologues. Hence, we consid-
ered dendronization as a valuable tool for the introduction of
an unprecedentedly large and rigid polyphenylene substituent
at the phosphonium component. By employing this approach,
novel alkenes with markedly large substituents became
accessible.
Introduction
For decades the field of weakly coordinating anions (WCAs)
has been of significant interest for both academia and indus-
try.[1] Their applications range from catalysis,[2] polymerization,[3]
electrochemistry,[4] and ionic liquids[5] to battery technology.[6]
Until recently the corresponding class of compounds, conse-
quently termed weakly coordinating cations (WCCs), has hardly
attracted attention. We report reliable and versatile synthetic
pathways to form polyphenylene dendrimers[7] with a cationic
phosphonium core. The macromolecular size is increased to
control Coulombic interactions between ion pairs. We regard-
ed phosphonium salts as suitable candidates to establish
a new generation of WCCs, hydrophobic salts constituted from
molecularly defined cations with diameters up to about 5 nm.
In addition to synthesis we present a brief investigation of
their stability against alkaline degradation in solution due to
the presence of large, screening polyphenylene dendrons.
The main focus is the application of triarylbenzylphosphoni-
um salts as Wittig reagents, bearing bulky substituents of un-
precedented size in the a-position to the phosphorus. Since its
Results and Discussion
[a] R. Moritz, Dr. M. Wagner, Prof. Dr. M. Baumgarten, Prof. Dr. K. Müllen
Max Planck Institute for Polymer Research
Synthesis of tetraaryl-substituted polyphenylene phosphoni-
um cations
55128 Mainz, Ackermannweg 10 (Germany)
E-mail: muellen@mpip-mainz.mpg.de
Initially, an ethynyl-functionalized tetraphenylphosphonium de-
rivative (4) was synthesized according to Scheme 1.
Grignard formation of (4-bromophenyl)trimethylsilane fol-
lowed by subsequent reaction with PCl3 afforded tris[4-(trime-
thylsilylethynyl)phenyl]phosphine (1). After deprotection of
[b] Dr. D. Schollmeyer
Institute of Organic Chemistry, University of Mainz
55128 Mainz (Germany)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201406370.
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1 using K2CO3 in methanol, tris[4-(ethynyl)phenyl]phosphine
(2) was obtained. The quarternization of 2 as the crucial syn-
thetic step was accomplished with the highly electrophilic tet-
rafluoroborate diazonium salt 3, which was obtained by react-
ing 4-ethynylaniline with tert-butyl nitrite similar to a literature
procedure.[15]
The build-up of the polyphenylene dendrons proceeded by
means of a Diels-Alder reaction between ethynyl-functionalized
phosphonium core 4 and tetraphenylcyclopentadienone,[16]
making sure to remove any oxygen source (air, water, perox-
ides) to prevent the formation of phosphine oxide. Thus,
a first-generation phosphonium tetrafluoroborate (5) was ob-
tained in 78% yield after purification by column chromatogra-
phy and precipitation in hexane. MALDI-TOF mass spectrome-
try (single peak at m/z 1860.1), NMR measurements (for full
characterization see the Supporting Information), and a single-
crystal structure clearly showed the formation of a monodis-
perse first-generation phosphonium cation 5.
The divergent dendritic growth was further exploited in
order to construct even larger and more sterically shielded
phosphonium structures (see Scheme 2, compounds 8 and 11).
The completion of the reactions was monitored by means of
MALDI-TOF spectrometry (m/z 4904.9 showed formation of 8
and m/z 10987.9 of 11 accordingly, a full characterization of
the isolated compounds is provided in Supporting
Information).
Scheme 1. Synthetic route to first-generation dendronized phosphonium
cations: a) Mg, THF, PCl3 (2.0m in CH2Cl2), RT, overnight, 79%; b) K2CO3 in
CH3OH, RT, 3 h, 86%; c) BF3*OEt2, tBuNO2, CH2Cl2, À208C, 90%; d) toluene,
658C, 67%; e) tetraphenylcyclopentadienone or AB2 building block, respec-
tively, o-xylene, 1658C, 12 h, 78% (5), 73% (6); f) TBAF, THF, À208C, 2 h,
Scheme 2. Synthetic route to higher generations of dendronized phosphonium cations: a) tetraphenylcyclopentadienone, o-xylene, 1658C, 12 h, 61% (8),
58% (11); b) AB2 building block, o-xylene, 1658C, 36 h, 67%; c) TBAF, THF, À208C, 2 h, 77%.
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The hydrodynamic radii of the herein reported dendrimers
were calculated from experimental diffusion coefficients (Dexptl
)
measured by DOSY-NMR experiments (CD2Cl2, 298 K) and are
summarized in Table 1.
Table 1. Diffusion coefficients, hydrodynamic radii and diameters of
dendronized phosphonium salts 5, 8 and 11 obtained from DOSY-NMR
experiments (CD2Cl2, 298 K, 500 MHz).
Dexptl [m2 sÀ1
]
Hydrodynamic diameter
radius rH [nm]
[nm]
À
5, [P-G1]+BF4À
4.07”10À10 Æ1.05”10À10 1.2Æ0.2
2.82”10À10 Æ3.44”10À11 1.8Æ0.2
2.4Æ0.4
3.5Æ0.4
4.9Æ0.4
8, [P-G2]+BF4
11, [P-G3]+BF4
À
2.04”10À10 Æ4.7”10À11
2.4Æ0.2
Synthesis of a triazole-rich phosphonium cation
To avoid side products such as phosphine oxides, a dendrimer
growth proceeding at lower temperatures and milder reaction
conditions was needed. Therefore, a dendritic framework, com-
prising phenyl and triazolyl rings, was constructed by reacting
bulky polyphenylene azide 12 (synthesis see Supporting Infor-
mation) with ethynyl-functionalized tetraphenylphosphonium
core 4 in a copper(I)-catalyzed dipolar 1,3-cycloaddition[17]
(Scheme 3). The crude product 13 could be easily purified by
Scheme 3. Synthetic route to first-generation phosphonium salt 13:
a) CH3CN/THF 5:1, CuSO4, Na-ascorbate, NEt3, TBTA, 238C, 12 h, 87%.
Figure 1. Left: Single-crystal structure of G1-phosphonium tetrafluoroborate
5 (solvents are omitted for clarity). Right: space-filling structure of 5.
column chromatography. This was attributed to its enhanced
solubility in organic solvents (CH2Cl2, THF, CH3CN) compared to
recently reported triazole-rich polyphenylene dendrimers.[18]
1H NMR signals of the pure product 13 (87% yield) could be
assigned and were used to confirm complete conversion,
purity and regioselectivity (spectra see Supporting Information)
in combination with MALDI-TOF analysis (m/z 2434.1).
X-ray structure of 5. The central P atom occupies the spatial
position (01/41/8)—this means the four polyphenylene ligands
are equivalent. The angles between the least-square planes of
the phenyl ring with respect to the central ring varied from
57.0 to 63.68.
The impact of dendronization on charge separation in the
solid state can be reflected by the closest distance between
cation and anion. In the single crystal of 5, the distance from
Crystal structure of first-generation (G1) phosphonium tetra-
fluoroborate (5)
À
the boron atom in BF4 to the central cationic phosphorus
À
Yellow needles of 5 (or [P-G1]+BF4 ) were obtained from
atom is 0.57 nm. Thus, the anion undergoes a significant inter-
digitation into the cavities of the hydrophobic polyphenylene
scaffold due to Coulombic attraction. This becomes evident
from the cation radius of 5 (1.40 nm, see Figure 1).
a CH2Cl2/hexane solution by slow evaporation at room temper-
ature. The first-generation phosphonium salt crystallized in the
tetragonal space group I41/a. Figure 1shows the single-crystal
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Chemical and physical properties of hydrophobic phospho-
nium salts
À
At first the chemical stability of 5 (or [P-G1]+BF4 ) compared
to non-dendronized tetraphenylphosphonium (PPh4+BrÀ) with
regard to alkaline decomposition in solution was investigated
in a series of 31P NMR kinetic studies at 25 and 608C. The initial
+
PR4 species (0.1m) was reacted at a tenfold molar excess with
hydroxide ions (0.01m), so that the degradation follows
pseudo-second-order kinetics.[19] The rate constants k, the half-
life times (t1/2) and the activation energies (EA) are reported in
Table 2.
+
Table 2. Kinetic parameters for the alkaline degradation reaction of PR4
XÀobtained from 31P NMR measurements assuming pseudo-second-order;
c0(PR4+)=0.1m; c(OHÀ)=0.01m; a series of experiments was performed
in DMF/D2O (9:1, v/v) over 24 h. Details see Supporting Information.
Phosphonium
salt
T
k
t1/2
[h]
EA
[8C] [LmolÀ1 hÀ1
]
[kJmolÀ1
]
+
PPh4 BrÀ
25
60
0.19Æ0.02
0.84Æ0.08
52.8Æ6.6
11.9Æ1.1
35.0Æ3.0
À
dendrimer [P-G1]+BF4
25
60
0.08Æ0.01
0.52Æ0.02
122.5Æ17.9 45.6Æ3.0
19.4Æ0.9
Scheme 4. Synthetic route to benzylphosphonium bromides 16–18, as well
as intermediates P(PPD)3 (15) and PPD-CHO (19): a) o-xylene, 1558C, 12 h,
100% (PP-CH2Br), 69% (P(PPD)3); 89% (PPD-CHO) b) CH3CN, 608C, 12 h, 90–
96% yield.
It is known from literature that common alkylphosphonium
salts decompose rapidly under basic conditions (half-lives of
minutes) by means of an ylide mechanism yielding phosphine
oxides and hydrocarbons.[20] The obtained data indicate that
base-stability is increased for the phosphonium dendrimer 5
(t1/2 =122.5Æ17.9 h, 258C), in comparison to non-dendronized
PPh4+BrÀ (t1/2 =52.8Æ6.6 h, 258C).
(>90%), easy purification, incorporation of an a-methylene-
group, and the tailor-made arrangement of differently sized li-
gands X (Scheme 4) around the cationic phosphorus center.
The bulky polyphenylene phosphine 15 (P(PPD)3), readily react-
ed with 14 to yield fully dendronized phosphonium bromide
18 ([(PPD)3PCH2(PP)]+ BrÀ, see Scheme 4).
In contrast to that, a neutral solution of dendronized salt 5
(non-degassed o-xylene, stirred in a sealed vial) exhibited tem-
perature stability up to 1208C without any decomposition (for
at least three days). After additional refluxing (1508C) for three
weeks, 31P NMR analysis showed a conversion to phosphine
oxide below 20%. The thermal stability of G1-dendrimer 5 in
the solid state was investigated by means of thermogravimet-
ric analysis (TGA). The temperature range for decomposition
under nitrogen-stream was found to be 350–3608C, but TGA
performed under air-stream essentially provided the identical
result.
The stereochemistry of the Wittig olefination in the presence
of bulky a-substituents was investigated. Therefore, the forma-
tion of bulky, semi-stabilized ylides Ph3P=CH-PP (16*) and
(nBu3)P=CH-PP (17*) and the sterically completely shielded
ylide (PPD)3P=CH-PP (18*) was initiated by treatment of the ac-
cording benzylphosphonium bromide with potassium hexame-
thyldisilazide (KHMDS, salt-free conditions) and LiHMDS (lithi-
um halide present) as bases at À788C (THF or toluene). After
addition of aldehydes (benzaldehyde, hexanal, pivaldehyde
and polyphenylene benzaldehyde derivative PPD-CHO 19) to
the ylide suspensions, the (E/Z) ratio of the obtained alkenes
was determined from the coupling constant of the two vinylic
protons (reaction conditions see Scheme 5, results see Table 3).
By combining increased chemical stability and ion dissocia-
tion,[21] dendritic encapsulated phosphonium salts represent an
effective modulation of essential properties for the develop-
ment of advanced hydroxide exchange membranes (HEMs).[22]
Wittig ylides with bulky substituents around the phospho-
rus
3
Typical JHH-values for the (E)-olefins were observed to be 16–
17 Hz and 11-12.5 Hz for the (Z)-olefins, which is in agreement
with experimental data of unsymmetrical substituted
stilbenes.[23]
The synthesis of novel benzylphosphonium bromides 16–18
with unprecedentedly large polyphenylene substituents in a-
position was achieved by a SN2-type reaction between phos-
phines with first-generation benzylbromide derivative 14. The
advantages of the quarternization step are high product yields
In salt-free Wittig reactions, semi-stabilized ylides usually
react unselectively with aldehydes to yield (E)- and (Z)-alk-
enes.[11a] Our results revealed the exclusive formation of (Z)-ole-
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Scheme 5. Reaction conditions, intermediates and final products of the Wittig reaction between benzylphosphonium bromides 16–18 and aldehydes R-CHO.
entries 1–3, 9–11). Stabilized
Table 3. Results of the Wittig reactions[a] according to Scheme 5.
ylides are generally known to
result in the preferential forma-
Phosphonium
ylide L3P=CHPP
Base
Solvent
Aldehyde
(E/Z) ratio
Lit. (E/Z) ratio
for R’=CH3
Isolated
tion of (E)-alkenes.[11a,14a] Likewise
the Wittig reaction of the ylide
nBu3P=CH-PP (17*) with hexanal
and pivaldehyde predominantly
yielded (E)-alkenes 21 and 22
(see entries 15–18); however,
upon reacting with benzalde-
hyde and PPD-CHO 19, exclu-
sively (Z)-alkenes 20 and 23
were obtained.
yield[b] [%]
[Ph3P=CH-PP] (16*)
1
2
3
4
5
6
7
8
9
L=Ph
L=Ph
L=Ph
L=Ph
L=Ph
L=Ph
L=Ph
L=Ph
L=Ph
L=Ph
L = Ph
LiHMDS
LiHMDS
KHMDS
LiHMDS
KHMDS
LiHMDS
LiHMDS
KHMDS
LiHMDS
LiHMDS
KHMDS
THF
toluene
THF
THF
THF
THF
toluene
THF
THF
toluene
THF
PhCHO
PhCHO
PhCHO
1/99
1/99
1/99
50/50
40/60
37/63
41/59
n.d.
17/83[11a]
84
82
63
78
61
82
80
0[c]
74
70
59
8/92[14]
14/86[11a]
6/94[14a]
n-C5H11CHO
n-C5H11CHO
(CH3)3CCHO
(CH3)3CCHO
(CH3)3CCHO
PPD-CHO
PPD-CHO
PPD-CHO
1/99[11a]
1/99
1/99
1/99
10
11
It is known that cis- or trans-
disubstituted oxaphosphetanes
represent the initially formed in-
termediates for the reaction be-
tween phosphorus ylides and al-
dehydes.[14a] The cis/trans ratio
depends on the size and spatial
arrangement of the substituents
at the phosphorus. Inspired by
the non-planar cycloaddition
mechanism proposed by Vedejs
et al.,[14b] we suggest the most
likely transition state geometries
in Figure 2.[24]
[n-Bu3P=CH-PP (17*)
12
13
14
15
16
17
18
19
20
21
L=nBu
L=nBu
L=nBu
L=nBu
L=nBu
L=nBu
L=nBu
L=nBu
L=nBu
L=nBu
LiHMDS
LiHMDS
KHMDS
LiHMDS
KHMDS
LiHMDS
KHMDS
LiHMDS
LiHMDS
KHMDS
THF
toluene
THF
THF
THF
THF
THF
THF
toluene
THF
PhCHO
PhCHO
PhCHO
n-C5H11CHO
n-C5H11CHO
(CH3)3CCHO
(CH3)3CCHO
PPD-CHO
PPD-CHO
PPD-CHO
1/99
1/99
1/99
72/28
85/15
81/19
71/29
1/99
88
84
76
83
71
69
64
86
76
59
83/17[11a]
67/33[11a]
90/10[11a]
1/99
1/99
sterically shielded ylide [PPD3P=CH-PP] (18*)
22
23
L=PPD
L=PPD
LiHMDS
LiHMDS
THF
THF
PhCHO
n-C5H11CHO
1/99
52/48
62
55
For the cis-configuration 24,
the group R of the aldehyde
aligns in a pseudo-equatorial po-
sition in order to minimize de-
stabilizing steric repulsion from
the bulky PP dendron. For the
[a] Exact reaction procedure is described in Supporting Information. [b] After purification and isolation via
column chromatography. [c] Product 31 was isolated in 86 % yield (structure see reaction scheme in Figure 15
in the Supporting Information). [d] Polyphenylene aldehyde 19 (PPD-CHO) was synthesized to incorporate an
uncommonly large substituent at the aldehyde (Scheme 5). [e] Given that CH3-groups in the a-position were
not found in the literature, the shortest primary alkyl-chain analogue was selected.
fins when aromatic aldehydes (benzaldehyde, PPD-CHO 19) are
reacted with bulky, semi-stabilized ylide Ph3P=CH-PP (16*).
Even the presence of Li+ ions (c=0.05m) and the use of non-
polar toluene as solvent did not affect the (Z)-selectivity (see
case of the trans-orientation 25, however, the group R encoun-
ters even more unfavorable steric interaction with the PP
moiety, since both large groups are arranged in the pseudo-
equatorial plane.
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Acknowledgements
We gratefully thank the Deutsche Forschungsgemein-
schaft(SFB625) for financial support.
Keywords: phosphonium salts · phosphorus · polyphenylene
olefins · weakly coordinating cations · Wittig reactions · ylides
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We have presented the synthesis of rigid, bulky, and hydropho-
bic polyphenylene phosphonium cations by divergent dendrit-
ic growth through a thermal Diels–Alder cycloaddition. The
use of an organic diazonium salt enabled the synthesis of the
functionalized phosphonium core 4, which has not been re-
ported before, and provided the tetrahedral branching unit for
the subsequent synthesis of dendrimers. In addition to that, an
improved SN2-type method for the quarternization reaction of
phosphines was employed to prepare benzylphosphonium
bromides with unprecedented large and rigid a-substituents.
Stability investigations by means of 31P NMR kinetic studies
revealed that the dendritically shielded phosphonium cation
[P-G1]+ (first-generation) undergoes a remarkably slower deg-
radation to phosphine oxide in alkaline solution compared to
the non-dendronized tetraphenylphosphonium cation [PPh4]+.
The application of the benzylphosphonium bromides as
Wittig precursors resulted in the formation of encapsulated
phosphorus ylides, which were further reacted with aldehydes
to yield novel olefins with bulky substituents. The stereochemi-
cal analysis of the obtained alkenes showed the exclusive for-
mation of (Z)-isomers when aromatic aldehydes were em-
ployed. For the first time, we report a successful preparation of
the sterically congested polyphenylene (Z)-stilbene derivative
23. This work will enable further research on its structure[25]
and related rotational barriers.[26] Additionally, conformational
differences between the ground state and excited state may
be investigated, since even less hindered stilbenes are known
to exhibit unusual Stokes Shifts in fluorescence emission spec-
tra.[27] Moreover, the photocyclization of (Z)-stilbene 23 to con-
formationally strained dihydrophenanthrene intermediates[28]
has to be elucidated. Finally, the applicability of organic photo-
isomers for energy storage[29] based on polyphenylene stilbene
derivatives will be discussed in future studies.
Experimental Section
Experimental details are provided in the Supporting Information.
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Received: December 5, 2014
Published online on May 12, 2015
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim