Mono- and diylide-substituted phosphines (YPhos): Impact of the ligand properties on the catalytic activity in gold(i)-catalysed hydroaminations

Article abstract of DOI:10.1039/c9cy01861a

Understanding the impact of ligand properties on their performance in catalysis is seminal for future ligand design and catalyst improvement. In this work, the influence of the steric and electronic properties on the efficiency of a series of mono- and di

Full text of DOI:10.1039/c9cy01861a

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Mono- and diylide-substituted phosphines
(YPhos): impact of the ligand properties on the
catalytic activity in goldIJI)-catalysed
hydroaminations†
Cite this: Catal. Sci. Technol., 2019,
9, 6808
Christopher Schwarz, Jens Handelmann, Daniel M. Baier,
*
Alina Ouissa and Viktoria H. Gessner
Understanding the impact of ligand properties on their performance in catalysis is seminal for future ligand
design and catalyst improvement. In this work, the influence of the steric and electronic properties on the
efficiency of a series of mono- and diylide-functionalized phosphines of type YCNPR2 and (YCN)2PR (with
YCN = Ph3PC(CN) and R = Ph, Cy or tBu) on their efficiency in goldIJI) catalyzed haydroaminations is
studied. The diylidephosphines are particularly electron-rich and sterically encumbering but led to lower
conversions than expected based on their donor strength. Systematic analysis of the relation between the
donor strength and the catalytic activity revealed a linear correlation for the monoylide-functionalized
phosphines. Thus, the most electron-rich phosphine YCNPtBu2 gives rise to the most active catalyst with
turnover numbers greater than 5000. In contrast, no correlation was found for the diylide compounds due
to steric congestions which overcompensated the beneficial electronic properties. Accordingly, higher
temperatures had a stronger impact on the diylide phosphine-based catalysts, thus leading to similar
activities of YCNPtBu2 and (YCN)2PCy at 50 °C. Overall, these results provide valuable information for future
ligand design and the general impact of steric and electronic properties on the catalytic activity.
Received 13th September 2019,
Accepted 11th October 2019
DOI: 10.1039/c9cy01861a
rsc.li/catalysis
ligand design to the somewhat different requirements in gold
catalysis and thus led to a further improvement in the catalyst
Introduction
Buchwald-type biarylphosphines are amongst the most
important ligands in homogenous catalysis.1 Their success
can be attributed to their electronic and steric properties,
which allow the stabilization of low-coordinate as well as
cationic complexes which are often the active species in
catalytic transformations. In general, dialkylbiarylphosphines
are usually electron-rich and sterically demanding and thus
also allow for the stabilization of reactive metal species.
Thereby, steric protection is further supported by a secondary
interaction between the π system of the lateral arene with the
metal centre (Fig. 1).2 This unique architecture was found to
be particularly beneficial in palladiumIJ0) catalysed coupling
reactions3 but later also successfully applied to goldIJI)
catalysis.4 While first experiments focused on biaryl
phosphines that were also employed in Pd-catalysed
reactions,5 more recent studies addressed the adaption of the
activities and productivities.6 The customized ligand design
was necessary to decrease the catalyst loading below 0.5
mol%, which is required to make gold catalysis more
attractive for medium- or large-scale applications. Detailed
mechanistic studies by Xu and coworkers revealed that
Faculty of Chemistry and Biochemistry, Chair of Inorganic Chemistry II, Ruhr
University Bochum, Universitätsstr. 150, 44801 Bochum, Germany.
E-mail: viktoria.gessner@rub.de
† Electronic supplementary information (ESI) available: CCDC 1943517–1943526.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c9cy01861a
Fig.
phosphines.
1 Cationic gold complexes with biaryl- and ylide-substituted
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biarylphosphines are particularly suited for AuIJI)-catalysed
reactions such as the hydroamination of alkynes, in which
the protodeauration step is rate-determining and catalyst
deactivation a serious limiting factor.7 Here, the biaryl moiety
on the phosphine ligand was found to slow down the decay
of the cationic gold species and thus improved catalyst
stability, while the high donor strength facilitated the rate-
limiting prododeauration step. Recently, this molecular
design was successfully applied to other phosphines8 as well
as N-heterocyclic carbenes.9
Recently, our group reported on the synthesis of ylide-
substituted phosphines (YPhos) and their application in gold-
catalyzed hydroamination reactions as well as palladium
catalyzed Buchwald–Hartwig aminations and α-arylations.10,11
The molecular architecture of the triphenylphosphonium-
substituted YPhos ligands is somewhat reminiscent to
biarylphosphines due to the possible interaction of the metal
with one of the phosphonium bound phenyl groups (Fig. 1).
However, due to the carbanionic centre next to the
phosphorus atom, these ligands possess an increased donor
capacity, which – depending on the substitution pattern –
may reach or even surpass that of N-heterocyclic carbenes.10
Given the efficiency of mono-ylide substituted phosphines, we
now became interested in the impact of two ylide moieties
on the ligand synthesis and their efficiency in catalytic
applications. Owing to their enhanced donor properties as
well as increased steric demand, we expected these ligands to
be particularly suited for the stabilization of the cationic AuIJI)
species and the production of highly effective catalysts for
hydroamination reactions. In order to obtain systematic
information about the impact of the ligand design on the
catalytic activity we addressed a comparison of the electronic
and steric properties of a series of mono-, di- and triylide-
substituted phosphines.
Scheme
phosphines.
1
Synthesis of the mono-, di- and tri-ylide substituted
The diylide-substituted phosphines were prepared in
a
similar way by reaction of YCN–K with the corresponding
dichloro-phosphines. Thus, (YCN)2PPh (4), (YCN)2PCy (5) and
(YCN)2PtBu (6) could be obtained after work-up as colourless
solids in good yields between 66 and 77%. Overall, the
described reaction protocol provides facile access to a series
of ligands by simple salt metatheses in the last reaction step.
This versatility is certainly an advantage to many other ligand
platforms.
Unfortunately, employment of the same reaction protocol
as outlined in Scheme 1 with phosphorus trichloride did not
result in the selective formation of the desired tri-ylide
substituted phosphine (YCN)3P (7). Likewise, attempts to
introduce the three ylide groups in a step-wise fashion via
formation of the chlorophosphines YCNPCl2 and (YCN)2PCl
proofed to be unsuccessful. Only small amounts of 7 could
be isolated in form of yellow crystals, which confirmed the
identity of 7 (Fig. 2). However, this reaction protocol did not
deliver sufficient amounts of the triylidephosphine for
further investigations.
Results and discussion
Ligand synthesis and characterization
All mono- and diylidephosphine ligands were
characterized by NMR and IR spectroscopy as well as
elemental and single-crystal X-ray diffraction analysis.15
Particularly, the NMR data of these ligand systems are highly
meaningful (Table 1). The mono-ylide-substituted phosphines
To access a series of mono-, di- and tri-ylide substituted
phosphines for systematic studies we chose a synthetic route
via the metallated ylide YCN–M (M = Na or K).12,13 We
hypothesized that the cyanido group in the ylide backbone
would be sufficiently small to allow for multiple ylide-
2
exhibit two doublets with JPP coupling constants between
135 and 150 Hz. In contrast, the diylide-functionalized
compounds Y2PR show AX2 coupling patterns with slightly
larger coupling constants than their monoylide analogues.
This is probably due to the increased steric bulk, which
results in larger P–C–P angles and thus larger coupling
constants. For example, in case of the phenylphosphines 1
and 4 an increase of 13.4 Hz was observed. It is also worth
mentioning, that the introduction of the second ylide moiety
is also accompanied by a high-field shift of the signal of the
phosphine phosphorus atom. For example, the signal of the
phosphine moiety appears at δP 17.0 ppm in YCNPtBu2 (3),
while it is shifted to δP −24.4 ppm in (YCN)2PtBu (6).
functionalisations at
a
single phosphorus centre.14
Furthermore, CN–M is readily available from the
Y
corresponding phosphonium salt via step-wise deprotonation
with potassium or sodium hexamethyldisilazide. Thus, the
corresponding phosphines are accessible in only few reaction
steps from readily available compounds. The synthesis of the
mono-ylide substituted phosphines 1 and 2 was previously
reported via reaction of YCN–M with one equivalent of the
corresponding chlorophosphine (Scheme 1).10 Analogous
treatment of YCN–Na with di-tert-butylchlorophosphine, tBu2-
PCl, selectively produced the ylide-substituted phosphine
Y
CNPtBu2 (3) as evidenced by 31P{1H} NMR spectroscopy.
The molecular structures of 3, 5 and 7 are shown in Fig. 2,
those of the other ligands are provided in the ESI.†
Work-up delivered the ligand as colorless solid in 46% yield.
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Determination of the steric and electronic ligand properties
To estimate the donor strengths of the phosphines 1–6 we
set out to measure the corresponding Tolman electronic
parameter (TEP). Since the classical procedure via synthesis
of the LNiIJCO)3 complexes was found to be no reliable
method for the bulky YPhos ligands due to the often
observed displacement of two CO ligands,16 the
[(L)RhIJacac)CO] complexes (acac = acetylacetonate) were used
as alternative.17 The TEP value can be calculated from the
linear relationship of the CO stretching frequencies in both
complexes. In case of the diylidephosphine (YCN)2PCy, we
were also able to isolate the rhodium complex in crystalline
form to verify the identity of the complex. X-ray
crystallography (Fig. 3) unambiguously proved the formation
of the expected rhodium complex with one YPhos and one
CO ligand coordinating to the metal. The TEP values
obtained by IR spectroscopy are given in Table 2.
Unfortunately, attempts to determine the TEP value of the
bulkiest ligand 6 by this method repeatedly failed. Thus, the
corresponding [(L)IrIJCO)2Cl] complex of 6 was synthesized
and the TEP value determined according to literature
reports.18 The obtained data clearly reflect the expected
trends in the donor strengths, i.e. YPPh2 < YPCy2 < YPtBu2
and YPR2 < Y2PR. Thus, (YCN)2PtBu is the strongest donor in
the ligand series. With a TEP value of 2054.7 cm−1, the donor
Fig. 2 Molecular structures of YCNPtBu2 (3), (YCN)2PCy (5) and (YCN)3P
(7). Thermal ellipsoids drawn at the 50% probability level. Structures of
the other ligands are given in the SI, important bond lengths and
angles in Table 1.
Interestingly, in all structures of the diylidephosphines and
even in that of (YCN)3P, the cyano moieties point to the same
side of the molecule, that is the opposite side of the
phosphine lone pair. This is particularly noteworthy
for the triylide-substituted ligand, where all bulky
triphenylphosphonium moieties still prefer to arrange on the
same side. We assume that this arrangement is due to
electronic effects and the stabilization of the lone pair by
interaction with antibonding CN orbitals or the minimization
of repulsion between the phosphine lone pair and the π
orbitals of the CN moiety. The PIJ1)–C bonds are all longer
than the corresponding P–C bonds to the phosphonium
moieties and only change slightly within the whole ligand
series. The same holds true for the C–CCN and C–N bonds in
the ligand backbone. It is also noteworthy, that the P–C–P
angles are similar in all structures despite the different steric
demand of the ligands. This suggests that despite the bulk of
the PPh3 groups the ligands are not overcrowded.
capacity of
6 is somewhat larger than that of PtBu3
(TEPIJPtBu3) = 2056.1 cm−1).
Next, the steric properties of the ligands were studied by
determination of their buried volume (% Vbur). To this end,
the goldIJI) chloride complexes of all ligands were prepared by
treatment of the phosphines with (THT)AuCl. All complexes
could be isolated as colourless solids in good yields higher
than 80%, except for 6·AuCl which was only isolable in 33%
yield. In the 31P{1H} NMR spectra, all complexes exhibit a
distinct down-field shift of the signal of the phosphine
phosphorus atom as expected for metal coordination. Single
crystals were grown to determine the structure and the
buried volume of the ligands. The structures of the AuCl
complexes with 3 and 5 as ligands are exemplarily depicted
in Fig. 4, the others are given in the ESI.† In the molecular
structures of the gold complexes, the phosphines retain the
Table 1 Comparison of the most important NMR and crystallographic data for the YPhos ligands 1–6 (bond lengths are given in Å, angles in °)
Ligand
δ(31PIJ1)) [ppm], 2JPP [Hz]
C–PIJ1)
C–CCN
C–N
P–C–P
1
−16.3, 146.8
−11.3, 135.7
17.0, 149.7
−26.4, 160.2
1.800(1)
1.819(1)
1.813(2)
1.809(3)
1.792(3)
1.809(2)
1.818(2)
1.808(2)
1.814(2)
1.407(2)
1.398(2)
1.404(3)
1.407(4)
1.412(4)
1.404(3)
1.392(3)
1.410(3)
1.400(3)
1.160(2)
1.161(2)
1.163(3)
1.157(4)
1.160(4)
1.163(3)
1.160(3)
1.158(3)
1.161(3)
118.1(1)
119.4(1)
119.7(1)
115.2(2)
120.1(2)
120.3(1)
118.8(1)
119.5(1)
114.1(1)
2
3a
4
5
6
−31.2, 140.5
−24.4, 161.2
a
Two molecules in the asymmetric unit; values are average values.
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Fig. 4 Molecular structures of the gold complexes of YCNPtBu2 (3)
and (YCN)2PCy (5). Thermal ellipsoids drawn at the 50% probability
level.
Fig. 3 Molecular structure of [5·RhIJCO)acac].
experiments were conducted at room temperature to
examine, whether the catalysts are already active at these
mild conditions. The conversion over time plots are depicted
in Fig. 5 (top). To our delight, all gold complexes exhibited a
conformation observed in the free ligands with the CN
moieties pointing away from the phosphine lone pair. Thus,
the metal is always shielded by one or two PPh3 moieties. All
ligands – particularly the diylide-substituted compounds –
are extremely bulky showing a buried volume between 42 (for
Y2PCy) and 63% (Y2PtBu). The diylidephosphines are thus
more encumbering than classical phosphine ligands such as
PtBu3 (% Vbur = 38.1 (ref. 20)) or PAd3 (% Vbur = 40.5 (ref. 19))
and in the range of biaryl phosphines (e.g. (%
VburIJCyJohnPhos) = 46.7 (ref. 20)). Hence, they should be
capable to efficiently sterically protect a cationic gold species
formed after halide abstraction. Overall, the analysis of the
steric and electronic properties of the YPhos ligands showed
that the ylide substituent YCN can be regarded as a slightly
more electron-releasing and considerably more sterically
demanding tert-butyl group.
Catalytic activity
With the AuCl complexes in hand we turned our attention
towards catalytic applications. As a preliminary test reaction
to determine the relative activities of 1–6 we compared their
performance in the hydroamination of phenyl acetylene with
aniline. The performance was tested with NaBArF for halide
4
abstraction and a low catalyst loading of only 0.1 mol%. First
Table 2 Comparison of the IR data for the YPhos ligands 1–6, their
calculated TEP values and buried volume (% V)
νCOIJRh)a [cm−1
]
TEPb [cm−1
]
% Vbur
c
Ligand
1
2
3
4
5
6
1973.8
1958.3
1954.1
1963.6
1955.0
—
2066.7
2057.8
2055.4
2060.9
2055.9
2054.7d
44.3
45.4
50.8
57.2
55.2
62.7
a
CO stretching frequency in the [(L)RhIJCO)acac] complexes.
b
Fig. 5 Conversion over time plots of the hydroamination of phenyl
acetylene with aniline at (top) room temperature and (bottom) 50 °C.
NMR yields determined by direct integration of the peak for the imine
product with respect to the peak for the alkyne starting material.
Values are average values of at least two runs.
Determined by the relationship between νCO for [(L)NiIJCO)3] and
[(L)RhIJacac)IJCO)].19 Calculated using the SambVca 2.0 program for
c
the LAuCl complexes with a P–M distance of 2.28 Å including H
d
atoms.20
Determined by the relationship between νCO for
[(L)NiIJCO)3] and [(L)IrIJCO)2Cl].18
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remarkable activity already at room temperature with only 0.1
mol% of catalyst loading. Typically, higher catalyst loadings
as well as harsher reaction conditions are required in gold
catalysis to obtain high conversions in hydroamination
reactions.21 However, all catalysts based on L1–L6 delivered
more than 80% conversion after 48 h. The highest activity
was observed with the most electron-rich mono-ylide
substituted phosphine YCNPtBu2, which delivers 98% of
product after 24 h. With 0.5 mol% of catalyst loading the
reaction was complete within only 3 h reaction time, which is
certainly a remarkable activity for room temperature AuIJI)
catalysis and has rarely been observed for any other catalytic
system.
The monoylide phosphines YCNPR2 show a clear increase in
activity with increasing donor strength, i.e. YCNPPh2 < YCNPCy2
< YCNPtBu2. This is expected, since previous studies have
shown that more electron-rich ligands usually facilitate the
rate-limiting protodeauration step and give way to more active
hydroamination catalysts.6 However, this contrasts the
observations for the diylide phosphines. For example, the most
electron-rich diylidephosphine (YCN)2PtBu gave lower yields
than its monoylide-substituted analogue and comparable
yields than YCNPPh2. In fact, all three diylidephosphines
showed lower activities than expected based on their donor
strength. This indicated that either the beneficial electronic
effects of the diylide systems are overcompensated by steric
crowding or that these ligands become too electron-rich, thus
resulting in a less effective alkyne activation and hence in a
shift of the rate-limiting step to an earlier point in the catalytic
cycle. However, since for example (YCN)2PCy is according to its
TEP value (Table 2) a similar strong donor than YCNPtBu2 we
believe that steric hinderance is the main factor leading to
different activation enthalpies for the gold mono- and
diylidephosphine-catalysed transformations. Accordingly, we
turned our attention toward catalysis studies at 50 °C to
examine whether at those temperatures the diylide systems
perform superior to their monoylide congeners.
is much more active at 50 °C and gives – in contrast to
YCNPPh2 – full conversion within 5 h. Likewise, (YCN)2Cy and
(YCN)2Ph now perform superior than their mono-ylide
congeners at higher temperatures. This observation supports
our hypothesis that effective room-temperature catalysis with
the diylide phosphines is hampered by steric effects. Spatial
limitations are presumably reached in case of the tert-butyl
system. Here, the monoylide-substituted phosphine is still
more active than its diylide analogue at 50 °C.
To explain the different activities of our catalysts we
looked for any structure–activity relationship as guideline for
future ligand design. Since the rate-limiting protodeauration
step is facilitated by more electron-rich ligands,6 one could
expect a dependency between the TEP value and the catalytic
activity. Due to the already noted different performance of
the mono and diylide phosphines at room temperature no
correlation between the TEP and the catalytic performance
exists for the whole ligand set. However, a linear correlation
between the TEP value and the initial reaction rates22 is
observed for the monoylide phosphines where presumably
no or only little steric hindrance is present (green and blue
lines in Fig. 6, top, Table S4, Fig. S3†). In contrast, no such
At 50 °C, the catalysis becomes in general faster (Fig. 5,
bottom). However, this particularly accounts for the diylide-
phosphines. While YCNPtBu2 still yields the most active catalyst,
(YCN)2PCy now shows a similar activity, giving more than 95%
yield after 3 h reaction time with 0.1 mol% catalyst loading.
The gold catalysts based on YCNPCy2 (2) and (YCN)2PtBu (6) are
somewhat slower, but also allow for full conversion to the
imine within 5 h reaction time. For comparison, PPh3 only
delivers approx. 30% conversion after 24 h under the same
reaction conditions.10 At higher temperatures (70 °C), the
reaction with 6 as catalyst is also completed within less than 3
h reaction time. Overall, YCNPtBu2 and (YCN)2PCy are the most
active catalysts and comparable to CyJohnPhos under these
reaction conditions (see the ESI,† Table S1).
It has to be noted that the temperature dependency of the
catalytic activity is more pronounced for the diylide-
substituted phosphines than for their monoylide analogues.
For example, the most demanding phosphine (YCN)2PtBu (6),
which performed equally than YCNPPh2 at room temperature,
Fig.
6 Correlation between the TEP value and the catalytic
performance for the mono and diylide-substituted phosphines at rt
and 50 °C: (top) with the initial reaction rate (in the first 30 min at 50
°C and in the 1st h at RT) and (below) the yield of the reaction after 3 h
(50 °C) and 24 h (RT).
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correlation is seen for the diylide-substituted phosphines
(orange and yellow line). Here, at first an increase in activity
with an increase of donor strength is observed when going
from (YCN)2PPh to (YCN)2PCy, but a clear drop in activity is
found when moving to (YCN)2PtBu. This further supports that
steric limits are reached with (YCN)2PtBu.
correlates with the donor strength of the YPhos ligands, but
that steric congestions might lead to restrictions in the
performance.
Catalyst productivity and scope of catalytic applications
The same observations can be made when correlating the
TEP with the yields obtained after 3 and 24 h, respectively
(Fig. 6, bottom). This suggests that besides facilitating the rate-
limiting protodeauration step (kinetic effect), the stronger
donors also stabilize the active cationic gold species more
efficiently and thus lead to higher yields (thermodynamic
effect). This is well in line with studies by Xu and coworkers
showing that the protodeauration determines the reaction rate
of the hydroamination reaction and that catalyst deactivation
significantly affects the final yield of the reaction.7 Both
processes are positively influenced by strong donor ligands.
The correlation between the TEP and the yield at different
temperatures (black arrow, Fig. 6) nicely illustrates the
stronger impact of the temperature on the activity for the
diylidephosphine-based catalysts (cf. yellow line and orange
line). This supports the conclusion that steric crowding in
the diylide ligands hampers fast catalysis at room
temperature. It should be noted that there is neither a direct
correlation between the yield/rate and the buried volume nor
a correlation between the difference between the observed
and expected rate (calculated based on the linear regression
obtained for the monoylide phosphines) and the % Vbur (see
Fig. S1 and S2†). This is of course expected, since on the one
hand steric protection is required for the stabilization of
LAu+, but on the other hand seems to increase the activation
enthalpy of the rate-limiting step. The fact that we still
observe a correlation between the TEP and the reaction rates
indicates that the performance of the complexes based on
the “smaller” monoylide phosphines is predominately
determined by the different electronic effects. This is also
supported by the fact that the results for the smallest
diylidephosphine (YCN)2PCy best fit into the linear
regressions obtained for the monoylide catalysts (orange and
yellow diamond in Fig. 6). Overall, the findings clearly show
that the catalytic activity in the hydroamination nicely
Motivated by the excellent performance of the gold
complexes based on YCNPtBu2 and (YCN)2PCy we examined
their actual productivity. Overall, the catalysts provide TOFs
of more than 500 h−1 with only 0.1 mol% of catalyst loading.
A further decrease of the catalyst loading to 0.05 mol% still
gave good yields of greater than 85% within 5 h when using
YCNPtBu2 and (YCN)2PCy as ligands (Table 3). Without further
optimization of the reaction conditions (e.g. impact of
additives or further increase of temperature) these ligands
allowed for turnover numbers of 5000 and more at 50 °C.
Thus, the ylide and diylidephosphines render remarkably
active catalysts, which are amongst the very few catalytic
systems that reach TON of 5000 in AuIJI) catalysis. Notable
examples include, catalysts
based on specifically
functionalized biarylphosphines,6 a carbanionic phosphine
decorated with a chlorinated carba-closo-dodecaborate23 or
specifically designed NHC ligands.9a The activity of our
YPhos-based catalysts is certainly remarkable particularly due
to their facile synthesis.
Next, we addressed the substrate scope of the amination
reaction with both ligands YCNPtBu2 (L3) and (YCN)2PCy (L5).
Fortunately, high conversions were observed with both gold
complexes at 50 or 80 °C for a series of different amines and
alkynes (Fig. 7). Electron-rich as well as electron-poor aryl
amines as well as secondary amines could be coupled as well
as internal alkynes. In general, both phosphines performed
equally well, albeit in case of the secondary amine (N-methyl
aniline) L5 gave only poor yields, presumably due to steric
congestions. Accordingly. the amination with sterically even
more demanding amines (e.g. H2N-Mes, Mes
= 1,3,5-
trimethylphenyl) failed, thus further supporting that steric
limitations are reached with these ligands. It is also
interesting to note that in case of the amination of methyl
phenyl acetylene to 10ea both ligands prefer the formation of
the opposite regioisomers. While L3 results in the preferred
Table 3 Performance of the ligands in the hydroamination of phenylacetylene with anilinea
Entry
Ligand
Mol% LAuCl
Reaction time [h]
Yield [%]
TON
1
2
3
4
5
6
7
8
9
YCNPCy2
0.05
0.05
0.05
0.01
0.01
0.01
0.1b
0.1b
0.1c
5
5
5
48
72
48
72
72
48
68
90
87
58
62
48
92
73
49
1360
1800
1740
5800
6200
4800
7360
5840
3920
YCNPtBu2
(YCN)2PCy
YCNPtBu2
YCNPtBu2
(YCN)2PCy
YCNPtBu2
(YCN)2PCy
(YCN)2PtBu
a
b
5.0 mmol phenylacetylene, 5.05 mmol aniline, 50 °C, NMR yields. After 2 h reaction time with 0.1 mol% catalyst loading a further portion
c
of reagents was added. This was repeated 6 times. Reaction temperature: 70 °C; 6 portions of reagents were added after 1 h reaction time
each to 0.1 mol% catalyst.
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L5 provide highly productive catalysts, which give rise to
turnover numbers greater than 5000.
Overall, these studies demonstrate the beneficial effect of
strong donor properties on the activity of goldIJI) catalysts in
alkyne hydroaminations. The increase of activity appears to
be almost linear to the TEP value of the YPhos ligands. This
suggests that the catalytic efficiency can be reliably estimated
based on the donor strength of a ligand. This predictability
however is complicated by steric congestions (as well as other
stabilizing effects on the catalytically active goldIJI) species).
Our data implicate that a buried volume greater than 55%
can lead to detrimental steric encumbrance which might lead
to an overcompensation of beneficial electronic effects at
mild conditions. Since phosphines are ubiquitous ligands in
homogenous catalysis, we are currently studying the impact
of the steric and electronic properties of our ligands on their
catalytic efficiency to further facilitate parametrization and
ligand design in the future.
Conflicts of interest
Fig. 7 Scope of the amination of alkynes with YCNPtBu2·AuCl and
(YCN)2PCy·AuCl as precatalysts. NMR yields (yields in brackets are
isolated yields). Reaction conditions are given in brackets as follows
(mol% catalyst/temperature/reaction time). [a] 2.6 : 1 mixture of the E
and Z isomers. [b] 1.5 : 1 mixture of both isomers. [c] 1 : 1.5 mixture of
both isomers (N,1-diphenylpropan-1-imine: N,1-diphenylpropan-2-
imine).
The authors have filed patent WO2019030304 covering the
YPhos ligands discussed, which is held by UMICORE and
products will be made commercially available.
Acknowledgements
Funded by the Deutsche Forschungsgemeinschaft (DFG,
German Research Foundation) under Germany's Excellence
formation of the Markovnikov product, L5 favored the anti-
Markovnikov product, albeit with only low selectivities (1.5 :
1). Nonetheless, this suggests that tailoring of the steric
properties of these ligands can also allow for the
regioselective amination reactions of unsymmetrical alkynes.
We also attempted the amination with alkyl amines.
However, these compounds showed no conversion under the
reaction conditions.
Strategy-EXC-2033-Projektnummer
390677874
and
the
European Research Council (Starting Grant: YlideLigands
677749). We thank UMICORE for donating chemicals.
Notes and references
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In summary, we have prepared a series of mono and diylide-
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This journal is © The Royal Society of Chemistry 2019
Catal. Sci. Technol., 2019, 9, 6808–6815 | 6815