Intramolecular anti-Phosphinoauration of Alkynes: An FLP-Motivated Approach to Stable Aurated Phosphindolium Complexes

Article abstract of DOI:10.1002/chem.201605914

The synthesis of aurated phosphindolium complexes from easy accessible 1,5-alkynylphosphine derivatives has been studied by using gold(I) complexes featuring carbene and phosphine ligands as initiators. Upon formation of the mixed phosphine NHC/phosphine

Full text of DOI:10.1002/chem.201605914

A Journal of
Accepted Article
Title: Intramolecular Anti-Phosphinoauration of Alkynes - An
FLP-Motivated Approach - Synthesis of Stable Aurated
Phosphindolium Complexes
Authors: Sebastian Arndt, Max M. Hansmann, Petr Motloch, Matthias
Rudolph, Frank Rominger, and A. Stephen K. Hashmi
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To be cited as: Chem. Eur. J. 10.1002/chem.201605914
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Intramolecular Anti-Phosphinoauration of Alkynes – An FLP-
Motivated Approach - Synthesis of Stable Aurated
Phosphindolium Complexes
Sebastian Arndt,[a] Max M. Hansmann,*,[a] Petr Motloch,[a] Matthias Rudolph,[a] Frank Rominger,[a] and
A. Stephen K. Hashmi*,[a,b]
addition reactions have been reported,9 which attracted our
curiosity if the elementary step of a phosphinoauration was
Abstract: The synthesis of aurated phosphindolium complexes
from easy accessible 1,5-alkynylphosphine derivatives has been
feasible. As mentioned before, the lack of reports on
studied by using gold(I) complexes featuring carbene and phosphine
phosphines as nucleophiles in gold catalysis might be
ligands as initiators. Upon formation of the mixed phosphine
attributed to the strong affinity of P/Au to engage in the
soft/soft Lewis pair combination Au(I)/PR3 leading to a
catalytically inactive LAuPR3 species. Interestingly, Bertrand
NHC/phosphine phosphine gold species, elevated temperatures
induced the cyclization to give stable aurated phosphindolium salts
which is supported by DFT calculations. The key elementary step that
comprises a yet unknown anti-phosphinoauration of an unactivated et al. reported on the stoichiometric aminoauration reaction
(Scheme 1a),10 however as nitrogen is much less likely to
alkyne is favored if bulky NHC ligands are used which was analyzed
by kinetic measurements. This concept could furthermore be
extended to neutral (phosphindolium)AuCl complexes featuring the
yet unknown phosphindole ligand.
form stable adducts with Au(I) centers than phosphorus, this
process is obviously much easier and fits into the reactivity
observed in the plenty of reported catalytic hydroamination
reactions.11 However, to the best of our knowledge, no report
on any gold catalyzed hydrophosphination reaction,12 clearly
demonstrates the challenges attributed with phosphorus
nucleophiles compared to nitrogen.
In the past decades, numerous applications have been
found for homogeneous gold-catalysts.1 In most cases, the
reactions include the addition of a nucleophile such as
amines,2 alcohols/water,3 or carbon based nucleophiles,4
onto a gold-activated multiple bond as key elementary step.5
However, taking into account the reported manifold of
nucleophiles employed in gold catalysis, phosphines have
been largely unexplored which stands in stark contrast to
their common use as ligands in these transformations. One
reason might be the assumption that in the case of
stoichiometric amounts of phosphine a strong coordination to
the free binding site of the gold catalyst can inhibit the
catalysts activity. So far, only one report by Kuniyasu and
Kambe deals with the gold-mediated cis-addition of
phosphines from RSAuPPh3 complexes.6 But due to the
special electronic nature of the applied electron-deficient
To overcome the problem of strong P/Au coordination, we were
inspired by the trans-phosphinoboration of non-activated alkynes
with frustrated Lewis pairs (FLPs).13 While phosphorus also forms
strong adducts with Lewis acidic boranes, the combination of a
sterically bulky phosphine and borane (FLP) can easily activate
,
15
alkynes in a trans-phosphinoboration reaction (Scheme 1b).14
Based on the analogy of the soft Lewis acidity of boron and gold(I)
we assumed that steric bulkiness for either the phosphine or/and
the gold species might enable a weakening of the Au-P bond and
reinstalling unquenched Lewis acidity and basicity. In the
following work we will analyze this initial idea for an intramolecular
trans-phosphinoauration reaction (Scheme 1c).
alkyne dimethylacetylendicarboxylate (which itself is
a
a) Anti-Aminoauration
powerful electrophile known to be attacked by phosphines) no
general information can be obtained regarding the tolerance
of phosphines in gold catalysis; for example generally cis-
additions are extremely rare which indicates that triggered by
the nature of this specific alkyne an unusual pathway takes
place.6 Therefore, a general study of a phosphinoauration
reaction with electronically non-activated alkynes which are
usually applied in gold-catalyzed reactions is still missing.
Other reports on the connection of a C-P bond by means of
homogeneous gold catalysts are restricted to the addition of
a phosphite onto an imine7 or by reductive elimination from
an intermediate gold(III) species.8 Several phosphine-alkyne
Me
Me
Me
N
N
1 eq. (CAAC)Au+X-
Me
Ph
5 min, RT
Au
CAAC
Ph
b) FLP-activation of alkynes
R'
tBu3P
tBu3P
+
+
B(C6F5)3
R
R'
B(C6F5)3
R
c)
This study
PR2
R2
P
?
R
+
Au-L
[a]
M. Sc. S. Arndt, Dr. M. M. Hansmann, Mgr. Petr Motloch, Dr. M.
Rudolph, Dr. F. Rominger[+], Prof. Dr. A. S. K. Hashmi
Organisch-Chemisches Institut
Au
L
R
Scheme 1. Reported aminoauration reaction (1a) and comparison
between intermolecular frustrated Lewis-pair (1b) and intramolecular gold
mediated activation of alkynes (1c).
Ruprecht-Karls-Universität Heidelberg
Im Neuenheimer Feld 270, 69120 Heidelberg (Germany)
Fax: (+49)-6221-54-4205
E-mail: hashmi@hashmi.de; hansmann@oci.uni-heidelberg.de
Homepage: http://www.hashmi.de
Chemistry Department, Faculty of Science, King Abdulaziz University
(KAU), Jeddah 21589, Saudi Arabia.
Crystallographic investigation
Initial attempts aimed for the synthesis of frustrated Lewis pairs
containing a cationic Lewis acidic gold center in the presence of
a non-coordinating Lewis basic phosphine. However, even the
combination of the bulkiest so far reported NHC ligand (IPr**)16
with tris-o-tolyl phosphine led to the formation of the
[(NHC)Au(PR3)][NTf2] adduct 1 verified by X-ray diffraction
[b]
[+]
Supporting information for this article is given via a link at the end of
the document.
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10.1002/chem.201605914
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(Figure 1). 17 Also the synthesis of bulky phosphine ligands
containing tolane substituents led to the corresponding
structure confirms the trans-phosphinoauration product exhibiting
a stable vinyl gold moiety.19
-
NTf2
O
a)
-
NTf2
Ph Ph
Ph
Ph
P
-
P
Au
-
NTf2
R =
R
NTf2
140 °C, 12 h
R
IPr
R
R
Ph
Ph
P
N
R
N
R
C6D5Br
80% yield
Au
IPr
Au
Au
P
R
R
P
4a
5a
Ph
Ph
o-tolyl
o-tolyl
o-tolyl
b)
-
-
1
2
NTf2
NTf2
Ph
Ph
Ph
1
Ph
P
P
2
Au
Au
IPr
IPr
5a
5a'
-
-
NTf2
NTf2
Ph
Ph
Ph
Ph
P
P
:
Au
IHPr
5a''
Au
Figure 1. Solid state structure of compound 1; and 2. Hydrogen atoms, the
counter-anion (NTf2 ) and co-crystallized solvent molecules are omitted for
clarity.
-
IPr
5a'''
Scheme 3. a) Initial cyclization experiment with 4a; b) mesomeric structures for
the phosphindolium core.
[(R3P)Au(PR3)][NTf2] adduct 2, clearly verifying the high tendency
of phosphines to coordinate to the soft gold(I) center. Even though
the synthesis of Au/P frustrated pairs seemed challenging, we
were curious if this is necessary as phosphine dissociation might
be reversible at elevated temperatures. In order to investigate an
intramolecular gold(I)-mediated phosphino-alkyne addition, we
prepared
ortho-phosphine-substituted
phenylacetylene
derivatives 3 following a procedure of Mathey.18 In our initial
experiment the addition of a stoichiometric amount of IPrAuNTf2
at room temperature led to the quantitative and fast formation of
the Lewis acid/base adducts 4, but in contrast to the Bertrand´s
system, no further conversion was monitored (Scheme 2).
R2
P
2
-
R2
P
R
NTf2
-
Figure 2. Solid-state structure of 5a. Hydrogen atoms, the counter-anion (NTf2 )
IPrAuNTf2
CH2Cl2, rt
R2
Au
and co-crystallized solvent molecules are omitted for clarity. Selected bond
lengths (Å) and angles (°) for 5a: Au1-C2 2.042(6), Au1-C7 2.025(3), C1-C2
1.359(8), C2-C3 1.531(7), C3-C4 1.3900, P1-C1 1.814(6), P1-C4 1.771(5), P1-
C5 1.812(5), P1-C6 1.845(4); C7-Au1-C2 170.18(18), C4-P1-C1 94.2(3), C5-
P1-C1 112.8(2), C4-P1-C5 113.0(2); C4-P1-C1 94.2(3); C4-P1-C6 118.0(2);
C5-P1-C6 106.16(18); C1-C2-C3 112.4(5).
IPr
N
N
R1
R1
4a-i
3a-i
a: R1 = Ph; R2 = Ph
f: R1 = Cy; R2 = Ph
g: R1 = t-Bu; R2 = Ph
h: R1 = n-Pr; R2 = Ph
b: R1 = p-Tol; R2 = Ph
c: R1 = p-C6H4-OMe; R2 = Ph
d: R1 = Mes; R2 = Ph
IPr
Interestingly, the so far unknown phosphinindolium-3-yl ligand
can be described ranging from a phosphoniovinyl gold compound
(5a), carbene λ5-phosphindole (5a’), carbene ylide (5a’’) up to a
mesoionic carbene (5a’’’) (Scheme 3b). Note, in the case of
Bertrand’s indole derived vinyl gold compounds such mesomeric
structures cannot be formulated. The bond length of the C1-C2
bond [1.359(8) Å] is in the upper range for the phospha-indene
type double bond [1.348-1.362 Å], 20 while the P1-C1 bond
distance 1.814(6) Å] is in agreement with a P-C single bond
observed in similar systems [1.775-1.794 Å]20 and significant
longer than for a λ5-ylen structure [1.695 Å].21 The Au-C2 bond
i: R1 = p-C6H4-OMe; R2 = o-Tolyl
e:
R
1 = p-C6H4-CF3; R2 = Ph
Scheme 2. Synthesis of mixed phosphine NHC complexes.
As a next step, phosphine gold complex 4a was heated at
140 °C in bromobenzene-d5 as solvent (40.7 mM) under inert
conditions (Scheme 3). A complete consumption of the starting
material was observed [in situ 31P NMR monitoring from
δ = 38.8 ppm (4a) to δ = 26.5 ppm (5a)] after 12 h and 5a
assigned as phosphindolium could be isolated in 80% yield by
crystallization. The NMR monitoring also revealed that the
rearrangement is irreversible and that the evolving
phosphindolium species was stable against air and moisture.
Crystals of 5a suitable for X-ray analysis were obtained by
recrystallization from CH2Cl2Br/pentane (Figure 2). The molecular
length [2.04 Å] is in
a typical range of other vinyl gold
compounds,19 indicating resonance structure 5a as the major
contribution for the electronic description. 22 This is further
supported by Natural Resonance Theory calculations (NRT)
employing the NBO6.0 program,23 clearly indicating 5a as the
leading resonance structure containing a vinyl gold moiety and a
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Ph
o-Tol
3-center-4-electron bond between C2-Au-C7 (~96%), while ylide
resonance form 5a’’ accounts only to 1-3%.
Ph
o-Tol
P
P
O
O
As a next step, we evaluated the scope of the transformation
with various phenylacetylene derivatives 4a-4m (Scheme 4). All
of the tolane precursors 4a-4e showed clean reactions and the
corresponding products were obtained in high yields no matter if
electron-rich (4b, c), sterically bulky (4d) or electron-poor (4e)
aromatic substituents attached to the triple bond were applied. It
is noteworthy that the rearrangement for the electron-poor aryl
alkyne (4e) proceeded much slower than for the electron-rich
substrates (4b, c), which might be attributed to the stronger
tendency of the electron-rich alkyne to form a gold alkyne complex,
that is necessary for the activation of the substrate. Aliphatic
alkynes turned out to be suitable substrates as well. Cyclic (4f),
sterically bulky (4g) and linear (4h) alkyl groups were all tolerated
and the corresponding vinyl gold compounds (5f-5h) were
obtained in moderate to excellent yields.
S
Au
IPr
5i 66%
Au
IPr
5j 28%
Ph
Ph
P
Ph
O
Ph
Au BrettPhos
P
Au
IPr
5k 33%
5l 73%
-
Scheme 5. Extended substrate scope. Triflimide (NTf2 ) omitted.
Figure 3. Solid state structure of compound 5l (left) and 6 (right). Counter-
anion NTf2- and hydrogen atoms omitted for clarity. Selected bond lengths
(Å) and angles (°) for 5l: P2-Au1 2.3081(9), Au1-C1 2.032(4), C1-C2
1.329(5), C2-C3 1.535(5), C2-P1 1.798(4), P2-Au1-C1 174.8(1), Au1-C1-
-
Scheme 4. Reaction Scope. Triflimide (NTf2 ) as counter-anion (omitted);
C2 125.9(3), C1-C2-P1 124.4(3), C3-C2-P1 107.2(3) ; and
6 Au1-C2
(a) X-ray structure analysis14; (b) reaction time 48 h.
2.042(16), C1-C2 1.36(2), C1-P1 1.795(16), C2-Au-C2’ 180.0, Au1-C2-
C1 127.2(13).
Following the FLP idea of weakening the Au-P bond by
increasing the steric bulk of the phosphine moiety we prepared
ortho-tolyl substituted phosphine derivative 4i that could be
converted to the corresponding aurated phosphindolium 5i in
good yield (see SI for the result of an X-ray analysis). Variations
of the aromatic backbone were also possible but yields for a
dimethyl-substituted benzene backbone (5k) and for a thiophene
backbone (5j) turned out to be significantly lower than for the other
test substrates. Importantly, these transformations are not limited
to internal alkynes featuring a phenyl backbone. While it might be
possible to generate the corresponding gold acetylide with the
phosphine acting as a base, we observed in case of terminal
alkyne featuring an alkyl tether (3l) the generation of the
corresponding 5-exo phosphino-auration product 5l. An X-ray
analysis is in agreement with this assignment (Figure 3; left).
Importantly, it should be noted that while previous transformations
contained an NHC ligand on gold this product bears two
phosphines. Interestingly, when phosphine ligands are employed
both as ligand and as nucleophile there are more options
available as ligand scrambling is possible. This is demonstrated
with adduct 4c* featuring the phosphaalkyne 3c and a bulky
phosphine ligand (PtBu3). Heating this complex in bromobenzene
led to the formation of the phosphindole dimer 6 and dimer 7
which can be explained by ligand scrambling (Scheme 6). The
unique structure of a bisphosphindolium ligated metal could be
clearly verified by X-ray diffraction (Figure 3; right). It should be
noted that in the previous cases with NHC ligands on gold no
ligand scrambling processes were observed, presumably a result
of the stronger carbene-Au bond.
-
Ph Ph
P
-
NTf2
NTf2
Ph Ph
P
MeO
Au
P(tBu)3
120 °C, 14 h
C6H5Br
Au
2
OMe
P
O
Ph
Ph
4c*
6
+
-
NTf2
(tBu)3P Au P(tBu)3
7
Scheme 6. Scrambling: dual role of phosphines - ligand and nucleophile.
After evaluation of the scope with regard to the alkyne fused
ligand, we turned our focus on variations of robust ligand
structures that surrounds the cationic gold center (Scheme 7).
Besides IPr and phosphines (tBuXPhos, PtBu3) ligands that were
used for the upper examples, rearrangements using the sterically
more demanding IPr*24 (3m) and IPr**16 (3n) were conducted.
Both test reactions delivered full conversion accompanied with
high yields (Scheme 7). Despite the steric bulk of the applied
ligands, the cave-like structure of the applied NHCs perfectly fits
to the organic substituent of the product. The result of the X-ray
analysis of compound 5n underlines this assumption (Figure 4).14
Switching to less hindered ligands was also possible which was
demonstrated by using IMes as NHC ligand (5o).
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Ph
phosphinoauration product should give access to neutral
unprecedented gold phosphindole complexes. 25 Upon mixing
(dms)AuCl with ligands 3a-c the gold complexes 4a**-4c** were
instantaneously formed. Interestingly, under forcing conditions
(160 °C) we were able to generate a mixture of starting material
and the desired phosphinoauration products 5a**-5b**. The
starting materials and cyclization products could clearly be
verified by X-ray diffraction (Figure 6; see also SI). Note, that
similar compounds featuring an intact Au-Cl bond were not
prepared with Bertrand’s aminoauration system, but might be very
interesting targets for further catalytic studies.
Ph
-
-
NTf2
NTf2
P
Ph Ph
O
P
Au
R
NHC
140 °C, 14 h
C6H5Br
R'
R
Au
R
R
R'
N
N
O
R
R
R
R
5c 80%, R = CH3, R' = H; (NHC = IPr)
5m 96%, R = Ph, R' = CH3; (NHC= IPr*)
5n 80%, R = 4-tBu-Ph, R' = CH3; (NHC= IPr**)
5o 87%, R = H, R' = CH3; (NHC = IMes)
Scheme 7. Variations of the NHC ligand.
Ph
Ph
P
Ph Ph
P
R
Au
140 °C
Au
Cl
Cl
C6H5Br
or DMF
Ph
Ph
P
R
);
R
:
4a**
R = H (
4b**
); Me (
OMe (4c**)
Au
Cl
R = H (5a**); Me (5b**);
5c**
OMe (
)
Scheme 9. Phosphinoauration leading to neutral gold phosphindole complexes.
Figure 4. Solid state structure of compound 5n. Hydrogen atoms, the counter-
-
anion (NTf2 ) and co-crystallized solvent molecules are omitted for clarity.
In a next step we investigated our initial idea of a frustrated Au/P
Lewis pair system. We were curious if bulkiness could affect a
weakening of the P-Au bond and hence speed up the cyclization
reactions. Therefore, we performed kinetic measurements with
alkyne 3c in the presence of varying NHC ligands in order to
evaluate the effect of ligand size on the transformation (see SI).
Indeed, we found an enhanced reaction rate for the sterically
more demanding IPr* ligand compared to the less hindered IPr
ligand. In the course of this measurement, we also recognized
Figure 5. Solid-state structure of
(NTf2 ) are omitted for clarity. Selected bond lengths (Å) and angles (°) P1-C1
1.808(8), C1-C2 1.335(10), C2-C3 1.483(11), C3-C4 1.389(11), C4-P1 1.814(8).
8. Hydrogen atoms, the counter-anion
-
that
a decent temperature of 60 °C is sufficient for the
rearrangement of substrate 3c, leading to full conversion after
24 h. Noteworthy, with this electron-rich substrate
conversion can even be observed at room temperature.
a slow
A primary experiment concerning the reactivity of the obtained
vinyl gold species was performed with substrate 5c (Scheme 8).
In the presence of stoichiometric amounts of HNTf2 a fast
protodemetallation was observed giving rise to the
phosphindolium 8 accompanied by the released cationic gold
fragment. X-ray diffraction unambiguously established the
structure of 8 (Figure 5).
-
Ph
NTf2
Ph
P
-
Ph
NTf2
Ph
O
HNTf2 (1 equiv.)
P
O
C6D5Br
rt, 1 h
Au
IPr
Figure 6. Solid state structure of compound 4b** (left) and 5b** (right).
Hydrogen atoms omitted for clarity. Selected bond lengths (Å) for 5b** (°): Au1-
Cl1 2.3098(8), Au1-C2 1.999(3), C2-C1 1.374(4), C1-P1 1.802(3), P1-C4
1.785(3), C4-C3 1.408(4), C3-C2 1.487(4), P1-C5 1.801(3).
H
5c
8
+
IPrAuNTf2
Scheme 8. Protodemetalation experiment.
Even though the concept of frustrated Lewis pair chemistry is
valid for large ligands we were curious about small ligands in
which the Au-P dissociation is independent of steric bulk. We
were specifically interested in just AuCl as the resulting
This result prompted us to investigate the trans-
phosphinoauration from
a theoretical point of view. DFT
calculations at the B3LYP-D3BJ/cc-pVDZ level of theory indicate
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that the novel free singlet phosphindole-yne ligand (Singlet/Triplet
gap ∆S/T = 25.5 kcal/mol) is only stable bound to a transition
metal center. Upon dissociation of the metal a low lying transition
state of ca. ∆G‡ = 1 kcal/mol leads back to the thermodynamically
more favorable phosphaalkyne (∆G = -29.2 kcal/mol) (see SI;
Figure S1). Interestingly, the transition state energy for the
cyclization event is similar high (∆G‡ = 29.7 kcal/mol) employing
the Au+-NHC fragment, taking into account the non-favored
dissociation of the P-Au bond (Figure 7). However, in this case
the phosphindole-yne ligand is trapped at the transition metal
therefore leading to a more stable complex (∆G = -3.9 kcal/mol).
calculated to be endergonic by +6.5 kcal/mol at the same level of
theory (B3LYP-D3BJ/cc-pVDZ) (see SI; Figure S3). Taking into
account the polar reaction medium (SMD calculation with
bromobenzene) leads to a similar thermodynamic stability of the
starting material compared to the cyclization product
(∆G = -0.1 kcal/mol) highlighting the importance of the polarity of
the solvent in this transformation.
In summary, we could demonstrate that the elementary step of
an anti-phosphinoauration is feasible leading to a novel class of
phosphindolium ligands. This new elementary step for gold
chemistry might open up new synthetic perspectives also with
regard to catalytic processes. For the best conversion rates it was
found that sterically bulky ligands surrounding the gold center are
beneficially in conclusion with an FLP approach. However, at
elevated temperatures even the cyclization with AuCl takes place
to generate the new phosphindolium ligand framework. The
reaction provides structurally and valuable phosphindolium
complexes in high yields and selectivity.
Acknowledgements
The authors thank Umicore AG & Co. KG for the generous
donation of gold salts. MMH is grateful to the Fonds der
chemischen Industrie for a Chemiefonds scholarship and the
Studienstiftung des deutschen Volkes. PM would like to thank
Nadace Sophia, Karel Dvořák and Verbindung Rupertia for their
support.
Figure 7. Calculated energy profile at the B3LYP-D3BJ/cc-pVDZ level.
Keywords: gold • frustrated Lewis pair • alkynes • phosphines
When calculating the energy profile employing the AuCl precursor
it should be noted that the initial cyclization complex was
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