Direct Access to π-Extended Phosphindolium Salts by Simple Proton-Induced Cyclization of (o-Alkynylphenyl)phosphanes

Article abstract of DOI:10.1002/chem.201700889

A detailed synthetic and mechanistic study for the synthesis of phosphindolium salts from easy accessible (o-alkynylphenyl)phosphanes is reported. Mechanistic investigation indicates a fast protonation at phosphorus as evidenced by the isolation of the phosphonium intermediate, followed by a protophosphonylation reaction across the alkyne moiety. DFT calculations support our mechanistic proposal and indicate a reaction highly exergonic compared to our recently reported phosphinoauration. Photophysical measurements recorded fluorescence quantum yields up to 97 % in solution for the phosphindolium core and fluorescence was observed in the solid state.

Full text of DOI:10.1002/chem.201700889

A Journal of
Accepted Article
Title: Direct Access to pi-Extended Phosphindolium Salts by Simple
Proton-Induced Cyclization of (o Alkynylphenyl)phosphanes
Authors: Sebastian Arndt, Max M. Hansmann, Frank Rominger,
Matthias Rudolph, and A. Stephen K. Hashmi
This manuscript has been accepted after peer review and appears as an
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To be cited as: Chem. Eur. J. 10.1002/chem.201700889
Link to VoR: http://dx.doi.org/10.1002/chem.201700889
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10.1002/chem.201700889
Chemistry - A European Journal
COMMUNICATION
Direct Access to -Extended Phosphindolium Salts by Simple
Proton-Induced Cyclization of (o-Alkynylphenyl)phosphanes
Sebastian Arndt,[a] Max M. Hansmann,[a] Frank Rominger,[a] Matthias Rudolph[a] and A. Stephen K.
Hashmi*[a,b]
Abstract: A detailed synthetic and mechanistic study for the synthesis
of phosphindolium salts from easy accessible
an intramolecular fashion (Scheme 1a). In Pietrusiewicz’s work
only one derivative was synthesized in low yields and a large
excess of the dihalide precursor was necessary.
(o-alkynylphenyl)phosphanes is reported. Mechanistic investigation
indicates a fast protonation at phosphorus as evidenced by the
(a) Light-induced cascade cyclization by Yamaguchi (2008):
isolation of the phosphonium intermediate, followed by
a
PPh2
Ph Ph
protophosphonylation reaction across the alkyne moiety. DFT
calculations support our mechanistic proposal and indicate a reaction
P
h
CH2Cl2, 1 h
B
highly
exergonic
compared
to
our
recently
reported
recorded
61%
Mes Mes
Mes2B
phosphinoauration.
Photophysical
measurements
(b) Our recent cyclization approach (2016):
fluorescence quantum yields up to 97% in solution for the
phosphindolium core and fluorescence was observed in the solid state.
R2
R2
P
X
R2
protodeauration
HX
R2
P
2
R
P
X
R1
LAuX
R1
2
R
-[LAuX]
Au
H
R1
L
In the past decade, phosphorus-containing -extended
phospholes[1,2]
and
initial goal
systems
such
as
non-fused
Scheme 1. Synthetic approaches to phosphindolium derivatives.
benzophospholes[3-6] have attracted extensive interest as
materials for organic electronics, reasoned by their unique
photophysical and electronical properties. Phospholes have been
widely explored in the recent years and research has led to
phosphole-cored dendrimers, phosphole-based polymers, fused
phosphole-derivatives and other phosphole congeners.[1-7] As part
of our research, we are specifically interested in phosphindolium
salts. The limited reports on this substrate class[6,8–10] can be
rationalized by the common synthetic strategies towards
phosphindole systems. Most of the strategies towards the
phosphindole core are either based on the prior formation of a
phosphindole bearing a trivalent phosphorous atom,[3,4,8,9,11,12] or
on the direct formation of a phosphindole oxide.[13] Due to the
reduced stability of the trivalent phosphindole core these species
are most often directly oxidized to their more stable
Recently, we reported the intramolecular phosphino-auration
of alkynes starting from [IPr-Au]NTf2 and ortho-phosphane
tolanes.[17] After a prior coordination of the phosphane to the
cationic gold fragment, cyclization afforded [(phosphindolium)-Au-
-
IPr]+NTf2 as product. A protodeauration experiment revealed that
substitution of the [IPr-Au]+ by a proton was possible. By using
strong acids in combination with weakly coordinating counterions,
we envisioned a catalytic process affording the protonated
phosphindolium salt (Scheme 1b). Our investigation finally led to
a highly efficient gold-free process, which is presented here.
Initial experiments were conducted by using catalytic amounts
of IPrAuNTf2 (5 mol%) in the presence of 1 equivalent of HNTf2.
Indeed, full conversion (no side reactions) was observed by 31P
NMR (Table 1, entry 1). In order to accelerate the reaction, the
considerably larger IPr* ligand[18] was applied (entry 2). In contrast
to our prior findings concerning the stoichiometric reaction,[17] the
catalytic reaction rate turned out to be independent of the catalyst
used. Based on this result, we performed the reaction in the
absence of any gold catalyst and 2a was isolated in 93%
(entry 3).[19]
benzo[b]phosphole
oxide
derivatives
before
isolation.
Transformation to the salts is challenging[6,8–10] which might be
attributed to the partial conjugation of the phosphorus electron
pair.[8] Based on the low efficiency of post functionalized
approaches towards the phosphindolium salts, alternatives in
which the compounds are directly formed in one synthetic step
are highly attractive. So far, direct syntheses were only described
by Pietrusiewicz[14] and Yamaguchi.[15,16] Yamaguchi et al.
successfully demonstrated the formation of the borate-bridged 2-
benzophosphindolium structure that was triggered by a borane in
Table 1. Cyclization in the presence of catalytic amounts of gold.
Ph
P
-
5 mol% [Au]
NTf2
Ph
Ph
Ph
HNTf2 (1 equiv.)
P
O
solvent (0.043 M)
t, T
[a]
M. Sc. Sebastian Arndt, Dr. M. M. Hansmann, Dr. F. Rominger,[+] Dr.
M. Rudolph, Prof. Dr. A. S. K. Hashmi
Organisch-Chemisches Institut
H
O
1a
2a
Universität Heidelberg
Im Neuenheimer Feld 270, 69120 Heidelberg (Germany)
Fax: (+49)-6221-54-4205
E-mail: hashmi@hashmi.de
Homepage: http://www.hashmi.de
Chemistry Department, Faculty of Science, King Abdulaziz University
(KAU), Jeddah 21589, Saudi Arabia.
Crystallographic investigation
Entry
Solvent
[Au]
Acid
T/°C
60
t
Yield
1
2
3
CH2Cl2
CH2Cl2
CH2Cl2
IPrAuNTf2
IPr*AuNTf2
none
HNTf2
HNTf2
HNTf2
2 d
2 d
2 d
99%
93%
93%
60
[b]
[+]
60
Supporting information for this article is given via a link at the end of
the document. MMH thanks the Fonds and the Studienstiftung.
This article is protected by copyright. All rights reserved.

10.1002/chem.201700889
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COMMUNICATION
We next evaluated the general applicability of this methodology.
Various phenylacetylene precursors 1a-1i were reacted in the
presence of HNTf2 as acid (Scheme 2). 1b bearing the less
electron-rich methyl substituent showed no conversion to the final
product under the same conditions. Instead protonation of the
phosphane was observed [31P NMR 3.32 ppm; 1H NMR 8.86 ppm
(d, J = 520.3 Hz)] and could be verified by an X-ray analysis of 3b
(Fig. 1). Fortunately, upon heating to 150 °C in bromobenzene,
3b could be further cyclized to the final product 2b (99%). Under
the elevated temperatures, all of the applied tolane derivatives 1c-
1f could be transferred to the corresponding phosphindolium
triflimides 2c-2f in excellent yields no matter if electron-neutral
(2c), electron-deficient (2d), electron-donating (2f) or sterically
bulky arene moieties were attached (2e). Even a tert-butyl-
substituted alkyne afforded the cyclization product 2g in moderate
yield. Then different aromatic backbones were investigated.
Besides a dimethyl-substituted benzene backbone that delivered
2i in moderate yield, even a thiophene backbone gave rise to the
phospholothiophenium scaffold (2h) in excellent yield.
Interestingly, we also investigated the substitution of phosphorus
by nitrogen (substrate 1j). In agreement with reports by Bertrand
et al., only protonation of the aniline was observed not leading to
the desired cyclization product.[20] It should be highlighted that in
order to affect the cyclization with the aniline derivatives a cationic
gold catalyst is necessary, in stark contrast to our findings with
the here described phosphorus system.
(Fig. 3).[2] Depending on the number of methoxy groups bound to
the 2-phenyl substituent, a red-shift of absorption was observed
for 2a, 2e, 5a relative to those structures without methoxy groups
(2b, 5b; Table 3).
Ph
P
Ph
P
Ph
H
Ph
condition [a-e]
P
Ph
Ph
R1
R1
O
R1
E
2a-2i
1a-1i
3a-3i
CF3
[b]
[b]
[b]
2a 99%[a]
2b
2c
2d 94%
99%
96%
[b]
(1 mmol)
2e 89%
Ph
Ph
Ph
Ph
O
P
P
O
O
O
S
[b]
2g
60%
H
H
2f 99%[c]
[d]
[b]
2h 99%
2i
40%
Ph
H
N
P
Ph
1j unspecific[b]
[e]
3b 99%
Scheme 2. Substrate scope. Conditions: [a] CH2Cl2 (0.20 M), 1.00 eq. HNTf2,
60 °C, 2 d (sealed vial); [b] C6H5Br (0.04 M), 1.00 eq. HNTf2, 150 °C, 1 d – the
yiled of 2i is reduced due to losses during the recrystallization; [c] C6H5Br
(0.04 M), 1.00 eq. HNTf2, 90 °C, 1 d; [d] C6H5Br (0.04 M), excess HCl (in
dioxane), 150 °C, 1 d; [e] CH2Cl2 (0.04 M), 1.00 eq. HNTf2, -20 °C, 5 min.
Counter-anions are omitted (NTf2- or Cl-).
As a next step, we investigated the scope of acidic promoters.
For some strong acids such as HOTf or HBF4 undesired
degradation pathways were observed (Table 2, entries 1, 2). For
this reason, acids with lower acidity were selected. Indeed, clean
reactions were observed by using hydrochloric acid (entry 3),
trifluoroacetic acid (HTFA, entry 4) and monochloracetic acid
(HMCA, entry 5). In the case of HCl, polymerization was observed
which was prevented at low temperatures. In situ NMR
experiments showed that with 1a a cyclization reaction can be
even performed at -10 to 0 °C. Phenol as well as methanol
showed an unspecific reaction (entry 7, 8). Surprisingly, a
different product (4) was found, if water was present in the
reaction media. An aqueous solution of acetic acid (entry 6)
afforded the phosphane oxide product 4 that is most probably
Figure 1. Solid-state molecular structures of 2e (left) and 3b (right).[21] Counter-
anions and co-crystallized solvent molecules are omitted. Selected bond
distances [Å] for 2e: P1-C11 1.781(4), P1-C41 1.783(4), P1-C31 1.788(4), P1-
C2 1.819(4), C2-C3 1.333(6), C3-C12 1.474(6), C3-H3 0.9500, C11-C12
1.409(6). For 3: P1-C31 1.781(4), P1-C12 1.787(3), P1-C41 1.788(4), P1-H1
1.29(4), C10-C20 1.205(5), C10-C11 1.433(5), C11-C12 1.412(5), C20-C21
1.432(5).
formed via
accordance to reports of Winter et al.[22] Reactions conducted with
HNTf2/water mixture (entry 9) confirmed this reactivity,
a hydroxyphosphorane intermediate in good
Table 2. Acid scope.
Ph
a
P
O
A-
Ph
Ph
P
Ph
Ph
HA (excess)
conditions
highlighting the importance of anhydrous conditions.
Nevertheless, the isolated phosphindolium product 2a showed no
decomposition or rearrangement into 4 over one month in wet
chloroform.
P
Ph H
O
O
H
H
O
2a
4
1a
Besides the use of a proton as electrophile we were curious if
boranes can also affect the cyclization step, similar to the
reactivity observed by frustrated Lewis pairs.[15,16,23] When
B(C6F5)3 was reacted with 1a, b and 1d the corresponding desired
products were cleanly formed in less than 30 min at rt and the
structure clearly established by X-ray diffraction (Scheme 3,
Fig. 2).
Entry
Solvent
C6H5Br
C6H5Br
CH2Cl2
C6H5Br
C6H5Br
C6H5Br
C6H5Br
acid
HOTf
HBF4
HCl
T/°C
150
150
t
Yield
1
2
3
4
5
6
7
1 d
1 d
1 d
3 h
2.5
1 d
1 d
87%[a]
75%[a]
100%
100%
97%
-78…rt
150
HTFA
HMCA
AcOH
PhOH
We investigated the optical and photophysical properties of
the 2-phenyl phosphindolium structure in solution (all compounds
are highly soluble in CH2Cl2, 0.74 - 1.5 mol/l). For all tested
analogues (2a, 2b, 2e, 2h, 5a, 5b), the UV-vis absorption spectra
showed an intense absorption band in the UV and violet region
150
150
100% (4)
Unspecific
140
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8
C6H5Br
MeOH
80
7 h
1 d
Unspecific
5a
5b
334 (4.39)
336 (4.65)
471 (76.4%)
464 (96.9%)
72933
78125
9
C6H5Br
H2O/HNTf2 (10 mol%)
100
99% (4)
Notes: [a] small impurities detected.
Notes: [a] excited at λabs; [b] fluorescence quantum yields in solution.
Ph
A blue-shift for λabs was observed for 5a and 5b with the borate
as substituent in 3-position of the phosphindolium core, while the
Ph
P
Ph
Ph
P
1.00 eq. B(C6F5)3
CH2Cl2 (0.04 M), rt, 30min
R
phospholium core structure 2h showed a red-shifted λabs
.
Fluorescence spectra of 2a, 2b, 2e, 2h, 5a and 5b showed a
strong emission in the visible region (Fig. 3). In general, a large
Stokes shift was determined [86 nm (2a), 89 nm (2b), 118 nm (2e,
h), 137 nm (5a) and 128 nm (5b)]. According to the spectra
measured (Fig. 3), the 2-substituent had a greater influence on
the emission than the 3-substituent. The phosphindolium salts
showed ΦF in the range from 70% (2h) to 97% (5b) [88% (2a),
84% (2b), 83% (2e), 76% (5a), 83% in average]. The values are
comparable to those reported for phosphindole oxide
structures.[12]
B(C6F5)3
R
5a R = OMe (99%)
5b R = Me (50%)
R = CF3 (99%)
1a,b,d
5d
Scheme 3. Tris(pentafluorophenyl)borane-induced rearrangements. The yield
of 5b is reduced due to losses during the recrystallization.
DFT calculations were performed in order to investigate the
mechanism for the protophosphonylation reaction. First the
thermodynamics of the cyclization transformation was calculated
at the M06-2X-D3-SMD(C6H5Br)/cc-pVDZ level of theory as a
function of the electrophile employed (AuCl, BMe3, H+)
(Scheme 4). Interestingly, due to the strong affinity of gold and
boron to phosphorus the calculations predict only a slightly
electronic stabilization of the cyclization product over the starting
material [AuCl: E = -0.9 kcal/mol; BMe3:E = -4.3 kcal/mol]. In
these two cases it seems important that steric bulk weakens the
P-Au/P-B interaction, thereby favoring the cyclization as
demonstrated above with the sterically demanding borane
B(C6F5)3. Importantly, in contrast to Au and B the cyclization with
H+ is electronically strongly favored [H+: E = -36.0 kcal/mol]. It
seems likely that reversible protonation allows the cyclization to
proceed.
Figure 2. Solid-state molecular structure of 5a.[21] Hydrogen atoms are omitted.
Selected bond distances in [Å]: P1-C11 1.764(3), P1-C2 1.796(3), B1-C3
1.644(4), C2-C3 1.368(4), C3-C12 1.505(4), C11-C12 1.405(4).
Ph
Ph
X
Ph
Ph
P
Figure 3. Fluorescence in solution of 2h, 2e, 2a, 2b, 5a and 5b (from left to
P
CH
3
right).
300
400
500
600
700
X
CH3
2a
2b
2e
2h
5a
5b
2a
2b
2e
5a
5b
2h
fluorescence
E(X) = AuCl (-0.9 kcal/mol)
absorption
1
1
BMe3 (-4.3kcal/mol)
H
(-36.0kcal/mol)
Scheme 4. DFT calculated electronic energy differences (E) for the cyclization
reaction with different electrophiles at the M06-2X-D3-SMD(C6H5Br)/cc-pVDZ
level of theory.
Mechanistically, we analyzed four different reaction pathways
(Scheme 5): Pathway I involves the direct cyclization of the ortho-
alkynylphosphane to form zwitterionic compound A, according to
the hydrolysis mechanism proposed by Winter et al.,[8] followed
by protonation to give the desired cyclization product. DFT
optimization of compound A (R = Ph) at the M06-2X-D3-
SMD(C6H5Br)/cc-pVDZ level of theory predictG = +23.7
0
0
300
400
500
600
700
Wavelength/nm
Figure 4. UV/Vis absorption spectra (solid lines) and emission spectra (dotted
lines) in CH2Cl2.
Table 3. Optical and photophysical data in CH2Cl2.
kcal/mol
[for
R = Me;
B3LYP-D3BJ/cc-pVDZ
Compound
λabs/nm (log ε)
390 (4.13)
374 (3.82)
406 (4.45)
417 (3.92)
λema/nm (ΦF
)
Stokes shift/nm
116279
b
G = +29.2 kcal/mol[17]] to form A. This high value is not in
agreement with the experimental results which disfavors
mechanism I. More reasonably, there is an equilibrium between 1
and the P-protonated phosphonium C which we could identify by
X-ray diffraction (see above). In analogy to our recently described
gold mediated cyclization, H+ (as the free acid, protonated solvent
or compound C) could engage in the electrophilic activation of the
alkyne moiety followed by a concerted nucleophilic attack of
2a
2b
2e
2h
476 (88.1%)
463 (83.7%)
524 (82.5%)
535 (69.8%)
112360
84746
84746
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10.1002/chem.201700889
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COMMUNICATION
phosphorus (path II). Alternatively, we envisioned either a
cyclization of the alkyne onto the cationic phosphorus center,
followed by a 1,3-shift (path III), or first a (intra-molecular)
protonation of the alkyne (E) to form vinyl cation F followed by
cyclization (path IV). Pathway III involving a 1,3-hydride migration
seems unlikely as we could not locate compound D as a stable
minimum on the potential surface. Mechanism II involving a
concerted protonation/cyclization event or mechanism IV
involving a stepwise protonation/cyclization seem most likely and
are in agreement with the observed rate acceleration with electron
donating groups on the aryl group.
Experimental Section
General Procedure for the Synthesis of Phosphindolium Salts: Under
an atmosphere of argon the corresponding 2-phosphane tolane, and the
given amount of acid were dissolved in 0.6 mL solvent, the mixture was
transferred to a Young-NMR tube and heated to the described temperature.
The reactions were monitored using 31P and 1H NMR spectroscopy. Upon
complete conversion, the crude product was washed using diethyl ether or
precipitated in solvent/pentane.
Keywords: phosphorus compounds • Brønsted-acid • -
extended systems • rearrangement • phospholes
Ph
P
Ph
Ph
Ph
H+
[1]
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(I)
P
R
R
1
A
Ph
Ph
Ph
(II)
Ph
P
P
H+
R
R
H
H
solvent/SM
2
B
Ph
P
Ph
H
Ph
H
Ph
(III)
P
[2]
[3]
R
1,3-
migration
R
C
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2009, 4, 1729–1740.
D
Ph
P
Ph
H
Ph
Ph
[4]
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H
(IV)
P
Ph
cyclization
H
R
E
F
[6]
[7]
Scheme 5. Mechanistic pathways for the protophosphonylation reaction.
Interestingly, we could detect different reaction pathways by
subjecting the cyclization product 2 to two different bases
(Scheme 6). While 2 is cleaved with KOH to phosphane oxide
product 4, a strong base such as LiHMDS under inert conditions
affords starting material 1. It seems likely that in the first case OH-
attacks at phosphorus followed by opening and protonation, while
in the second case with the more bulky and more nucleophilic
base the vinyl proton can be deprotonated leading to zwitterionic-
intermediate A which undergoes a rapid ring opening to 1.
a) G. Märkl, Z. Naturforschung B, 1962, 84–85; b) O. Berger, J.-L.
Montchamp, Beilstein J. Org. Chem. 2014, 10, 732–740.
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Ph
O
H
Ph
Ph
Ph
P
LiHMDS
KOH
P
2a
1a
Ar
CH2Cl2
-78°C
EtOH
reflux
1 d
Ar
H
4
A
Scheme 6. Ring-opening reactions of phosphindolium 2 mediated by KOH and
LiHMDS.
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In conclusion, in this study an acid-promoted, intramolecular
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stable and structurally valuable phosphindolium salts. This atom-
economic transformation was found to proceed under mild
reaction conditions cleanly affording the corresponding cyclization
products in up to quantitative yield. The simple and direct access
to the phosphindolium salts combined with their valuable
properties, such as fluorescence, stability and good solubility,
offers an attractive alternative strategy to common methodologies
that are based on the post-functionalization of trigonal
phosphindole systems.
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10.1002/chem.201700889
Chemistry - A European Journal
COMMUNICATION
COMMUNICATION
Sebastian Arndt,[a] Max M. Hansmann,[a]
Frank Rominger,[a] Matthias Rudolph[a]
and A. Stephen K. Hashmi*[a,b]
Page No. – Page No.
Direct Access to -Extended
Phosphindolium Salts by Simple
Proton-Induced Cyclization of
(o-Alkynylphenyl)phosphanes
The acid-mediated cyclization of 2-phosphane tolanes to phosphindolium salts is
described. The process allows for a general metal-free and extraordinary simple
protocol towards the target structures. The rearrangement proceeds in complete
atom economy without the need for further purification.
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