Ligand-enforced intimacy between a gold cation and a carbenium ion: impact on stability and reactivity

Article abstract of DOI:10.1039/d0sc05777k

Controlling the reactivity of transition metal complexes by positioning non-innocent functionalities around the catalytic pocket is a concept that has led to significant advances in catalysis. Here we describe our efforts toward the synthesis of dicationi

Full text of DOI:10.1039/d0sc05777k

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Ligand-enforced intimacy between a gold cation
and a carbenium ion: impact on stability and
reactivity†
Cite this: Chem. Sci., 2021, 12, 3929
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
Elishua D. Litle, Lewis C. Wilkins and François P. Gabbaı
*
¨
Controlling the reactivity of transition metal complexes by positioning non-innocent functionalities around
the catalytic pocket is a concept that has led to significant advances in catalysis. Here we describe our
2 6 4
efforts toward the synthesis of dicationic phosphine gold complexes of general formula [(o-Ph P(C H )
2+
Carb)Au(tht)] decorated by a carbenium moiety (Carb) positioned in the immediate vicinity of the gold
center. While the most acidic examples of such compounds have limited stability, the dicationic
+
+
N
+
N
complexes with Carb ¼ 9-N-methylacridinium and Carb ¼ [C(Ar )
2
]
(Ar ¼ p-(C
6 4 2
H )NMe ) are active
as catalysts for the cycloisomerization of N-propargyl-4-fluorobenzamide, a substrate chosen to
N
2+
2 6 4 2
benchmark reactivity. The dicationic complex [(o-Ph P(C H )C(Ar ) )Au(tht)] , which also promotes
hydroarylation and enyne cyclization reactions, displays a higher catalytic activity than its acridinium
analog, indicating that the electrophilic reactivity of these complexes scales with the Lewis acidity of the
carbenium moiety. These results support the role of the carbenium unit as a non-innocent functionality
which can readily enhance the activity of the adjacent metal center. Finally, we also describe our efforts
Received 19th October 2020
Accepted 8th January 2021
2 6 4
toward the generation and isolation of free g-cationic phosphines of general formula [(o-Ph P(C H )
+
+
N
+
N +
Carb)] . While cyclization into phosphonium species is observed for Carb ¼ [C(Ar )
2
] , [C(Ph)(Ar )] , and
DOI: 10.1039/d0sc05777k
+
9
-xanthylium, [(o-Ph
2
P(C
6
H
4
)-9-N-methylacridinium)] can be isolated as an air stable, biphilic derivative
rsc.li/chemical-science
with uncompromised Lewis acidic and basic properties.
accentuating electron-density depletion at the pnictogen
atom. These effects, the signicance of which depends on the
Introduction
20
With the control of catalytic reactivity as an ultimate goal,
efforts towards the design of new ligand scaffolds have
remained a central thrust of modern coordination chemistry
and one that has recently fueled numerous advances in the
1,2
chemistry of phosphine-based ligands. While the coordina-
tion chemistry of phosphine ligands has traditionally empha-
sized their donor properties, recent efforts have explored the
induction of p-acidity by incorporation of electron withdrawing
3–8
substituents. This strategy, which rests on an energy lowering
3
of phosphorus-centered s*-orbitals, has been revitalized by the
9–12
13,14
15
16
17
advent of cationic phosphines
such as A,
B, C, and D
(Fig. 1). These phosphines bear a carbenium ion directly con-
nected to the phosphorus center, setting the stage for an
inductive depletion of electron density at the phosphorus
1
8,19
center.
At the same time the carbenium ion can also engage
p-interaction, further
the phosphorus lone pair in
a
Department of Chemistry, Texas A&M University, College Station, TX 77843, USA.
E-mail: francois@tamu.edu
Fig. 1 Top: the carbenium resonance form of arbitrarily selected
examples of a-cationic phosphines. Inset: possible electronic effects
at play in a- and g-cationic phosphines. Bottom: recently isolated
examples of complexes featuring pendent carbenium units and dica-
tionic complexes targeted in this study.
†
Electronic supplementary information (ESI) available: Additional experimental
and computational details. CCDC 2039023, 2039025, 2039028,
039030–2039034, 2039036 and 2039040–2039043. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/d0sc05777k
2
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nature of the phosphorus and carbenium ion substituents,
elevate the p-acidity of the phosphorus atom, leading to notable
9
–11,17,21–27
benets in late transition metal catalysis.
Stimulated
by these advances, we have recently become interested in
a variant of the above-mentioned a-phosphine–carbenium
motif in which an ortho-phenylene linker is inserted between
21
the two moieties (E, Fig. 2). In these phosphines, which we
refer to as g-cationic phosphines, the carbenium ion and the
phosphorus atom lack a direct connection and are thus partly
insulated from one another. However, the carbenium ion is
2
8–32
positioned to act as a Z-type ligand,
capable of interacting
+
33–35
directly with the metal center through a M / C interaction
36
as in complexes of type E[M]
.
Interactions are also possible
between the carbenium unit and metal-bound anionic ligands,
37
as evoked by Gianetti for complexes of type F. Because such g-
cationic phosphines are prone to phosphonium ion formation
via “coordination” of the phosphorus atom to the carbenium
38
center, we have developed a strategy that provides direct
+
+
39
access to the gold complexes [1-AuCl] and [2-AuCl] (Fig. 1).
+
We found that [1-AuCl] efficiently catalyze reactions such as the
cyclo-isomerization of propargyl amide without the addition of
any activators. We proposed that the emergence of catalytic
activity for these gold chloride derivatives results from the Scheme 1 Top: synthesis of carbinols 3-OH and 4-OH. Bottom:
+
synthesis of [1]OTf, [2]OTf, [3]OTf and [4]OTf by dehydroxylation of the
corresponding carbinols. (a) TMSOTf, CH Cl , RT.
2 2
formation of an Au / C interaction that hardens the gold
center and enhances its electrophilic reactivity. Because these
complexes possess an Au–Cl bond, we have now decided to
target dicationic derivatives of type G by chloride anion
abstraction. We speculated that the convergence of two cationic
moieties at the core of these complexes may further enhance
their electrophilic reactivity. In this article, we investigate this
possibility by describing a series of such dicationic complexes
as well as their evaluation as carbophilic catalysts. We also
describe the isolation of an uncomplexed g-cationic phosphine
of type E.
target complexes featuring carbenium ions stabilized by the p-
dimethylaminophenyl substituent.
To this end, 1-lithio-2-diphenylphosphino-benzene was
0
allowed to react with 4,4 -bis(dimethylamino)benzophenone or
4
-(dimethylamino)benzophenone leading to the formation of
the phosphinocarbinols 3-OH and 4-OH, respectively (Scheme
). Aer characterization of 3-OH and 4-OH including by X-ray
1
43
diffraction (see ESI†), we investigated their dehydroxylation
reactions as a means to access the phosphine carbenium
39
cations. Carbinols 1-OH and 2-OH, were also included in these
studies. Aer investigating a few dehydrating agents, we found
Results and discussion
Synthesis and reactivity of cationic phosphine ligands
that treatment of 1-OH with trimethylsilyl triate (TMSOTf) in
+
40
CH
2
Cl
2
afforded the phosphine acridinium [1] (referred to as
Given that the pKR values of the carbenium ions present in [1-
+
+
+
EliPhos) as a triate salt (Scheme 1). Conversion of 1-OH into
AuCl] and [2-AuCl] differ by 9 orders of magnitude (Fig. 2), we
reasoned that our understanding of these systems would
benet from the synthesis of additional derivatives with car-
+
31
[
1] is accompanied with the appearance of a P NMR reso-
nance at ꢀ14.3 ppm, signicantly downeld from that of 1-OH
1
13
+
(
ꢀ19.9 ppm) (Scheme 2). The H and C NMR spectra of [1] is
R
benium moieties of intermediate pK values. With this in mind
+
44–47
N +
consistent with the presence of an acridinium unit.
In
and guided by the known pK
R
+
values of [Ph
2
CAr ] and
N
+
N
41
particular, the nitrogen-bound methyl resonance at 4.81 ppm is
[
PhC(Ar )
2
]
(Ar ¼ p-(C
H )NMe ) (Fig. 2), we decided to
6 4 2
shied downeld with respect to that in the carbinol precursor
1
3
1
-OH (3.64 ppm). The carbenium centre gives rise to a C NMR
signal at 162.6 ppm which appears as a doublet (J ¼ 6.3 Hz),
which we presume arises from coupling to the neighboring
phosphorus atom. Salt [1]OTf is air stable but should be stored
in the dark to prevent oxidation. The crystal structure of this
43
cationic phosphine has been determined (Fig. 3). The struc-
+
ture of [1] is reminiscent of that determined for boron-based
ambiphilic derivatives, most of which exhibit signicant P /
4
8–51
C bonding.
Despite a phosphorus–carbenium separation of
Fig. 2 pK
R
values of selected carbenium ions. The resonance struc-
+
+
41,42
3
.11 A in [1] , NBO analysis at the optimized geometry (see ESI†)
˚
tures shown are those corresponding to the carbenium ion.
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52
cyclization into phosphonium borates. A parallel also exists
with the work of Romero-Nieto on the synthesis phospho-
53
nium-containing heterocycles by cyclization. Hydrogen atom
migration can be conrmed by the appearance of an aliphatic
1
signal in the H NMR spectrum of the compounds. A similar
reaction was observed upon dehydroxylation and heating of o-
(
2 6 4 2
Ph P)C H (CPh (OH)) which also affords a cyclic phospho-
38
nium cations upon dehydroxylation. Bimolecular analogs of
such reactions involving phosphines and tritylium cations are
also known.
54
Fig. 3 ORTEP representation of the structure of [1]OTf. Hydrogen
ꢀ
atoms and OTf counterion omitted for clarity. Thermal ellipsoids
drawn at 50% probability and phenyl groups drawn as thin lines.
Cationic gold complexes
+
+
Since we were unable to isolate [3] and [4] in their uncyclized
phosphine carbenium form, we decided to access their corre-
sponding gold complexes by reaction of the carbinols with (tht)
does not show any notable P / C bonding thus indicating that
the donor properties of the phosphorus atom are uncompro-
mised. This conclusion is supported experimentally by the
39
AuCl, followed by treatment with 1 equiv. of HBF
4
.
This
+
method afforded the target monocationic complexes [3-AuCl]
and [4-AuCl] as air stable tetrauoroborate salts that are blue/
+
observation that [1]OTf reacts with (tht)AuCl to afford [1-AuCl] .
+
Encouraged by the successful synthesis of [1]OTf, carbinols
green and red, respectively (Scheme 2). The formation of these
complexes is easily followed by NMR spectroscopy which shows
2
2 2
-OH, 3-OH and 4-OH were also treated with TMSOTf in CH Cl .
3
1
A rapid screening of the product of these reactions by P NMR
spectroscopy were inconsistent with the formation of the tar-
geted cationic phosphines. Indeed, the products of these reac-
31
a shi of the P NMR resonance from ꢀ15.9 to 25.9 ppm upon
+
conversion of 3-OH into [3-AuCl] and from ꢀ16.0 to 25.8 ppm
+
upon conversion of 4-OH into [4-AuCl] . The presence of the
3
1
tions, referred to as [2]OTf, [3]OTf and [4]OTf give rise to
NMR resonances at 8.6 ppm for [2]OTf, 3.4 ppm for [3]OTf, and
.9 ppm for [4]OTf, consistent with the presence of phospho-
P
3
nium centers and indicative of cyclization reactions as depicted
in Scheme 1. The formation of these cyclized phosphonium
43
species has been conrmed by X-ray analysis (see ESI†).
Formation of a six membered phosphonium ring is accompa-
nied by a hydrogen atom migration from the aromatic CH group
attacked by the phopshine to the methylium center. These
rearrangements are reminiscent of those recently described by
Takaya for related ortho-phenylene phosphinoboranes and their Scheme 2 Top: synthesis of the gold complexes [3-AuCl]BF
4
and [4-
AuCl]BF
4
.
Fig. 4 ORTEP representations of the structures of [3-AuCl][BF
4
] (A) and [4-AuCl][BF
4
] (B), with a close up of the torsion angle (C and D).
ꢀ
Hydrogen atoms and BF
Selected bond lengths ( A˚ ) and angles ( ) for [3-AuCl][BF
4
counterions omitted for clarity. Thermal ellipsoids drawn at 50% probability and phenyl groups drawn as thin lines.
ꢁ
4
]: C(19)–C(2) 1.500(4), C(19)–C(20) 1.413(5), C(19)–C(26) 1.427(5); C(2)–C(19)–C(20)
ꢁ
1
17.2(3), C(20)–C(19)–C(26) 125.2(3), C(26)–C(19)–C(2) 117.6(3). Selected bond lengths ( A˚ ) and angles ( ) for [4-AuCl][BF
4
]: C(19)–C(2) 1.494(11),
C(19)–C(20) 1.365(10), C(19)–C(26) 1.473(7); C(2)–C(19)–C(20) 119.0(5), C(20)–C(19)–C(26) 124.8(6), C(26)–C(19)–C(2) 116.1(6). Graphical
+
+
output of the AIM analysis (E and F) showing the bond critical point between Au1 and C19 at the optimized geometry of [3-AuCl] and [4-AuCl] .
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1
ꢀ2
dimethyl amine protons gives rise to a H NMR resonance at the gold atom to the carbenium center are equal to 1.31 ꢂ 10
3
+
ꢀ2
3
2
.88 ppm for both 3-OH and 4-OH. These resonances shi e per bohr for [3-AuCl] and 1.32 ꢂ 10 e per bohr for [4-
+ +
downeld upon dehydroxylation to 3.21 ppm for [3-AuCl] and AuCl] (Fig. 4). These values are about an order of magnitude
+
3
.42 and 3.51 ppm in the case of [4-AuCl] .
The main distinguishing feature among the four gold chlo- / Ccarbenium interactions in [3-AuCl] and [4-AuCl] are weak.
ride complexes considered in this study is the nature of the This conclusion is supported by the results of natural bond
carbenium unit. The pK of the four parent carbenium deriv- orbital calculations that show only weak donor–acceptor
atives shown in Fig. 2 suggests that the carbenium unit of these bonding (E < 1 kcal) between the gold atom and the adjacent
complexes falls in the following Lewis acidity order: [1-AuCl] < carbenium units (see ESI†).
Given the metallobasic
character of gold(I), one could anticipate that a more Lewis
acidic carbenium center would result in a stronger Au / C
lower than those of covalent Au–C bonds, indicating that the Au
+
+
+
R
2
+
+
+
+
41,42
[
3-AuCl] < [4-AuCl] < [2-AuCl] .
55
Dicationic complexes
+
Cationic gold complexes of type H have been previously char-
interaction. However, the crystal structure of [3-AuCl][BF
4
] and
], and those previously determined for [1-AuCl]
] and [2-AuCl][BF ] do not indicate the existence of such
56–58
59–61
43
acterized,
and also implicated as carbophilic catalysts.
[
[
4-AuCl][BF
BF
4
The anking aryl ring in these structures provides stability to
the cationic gold center, facilitating their isolation and
deployment in catalysis. It occurred to us that chloride anion
], [2-AuCl][BF ], [3-AuCl][BF ] and
4 4 4
4-AuCl][BF ] could provide access to the dicationic analogues of
4
4
39
+
a trend. Indeed, the Au–C distances observed for [3-AuCl]
] (3.250(6) A) are both longer
than that in [1-AuCl][BF ] (3.168(9) A) which possesses a less
acidic carbenium unit. A discrepancy also exists between [3-
AuCl][BF ] and [4-AuCl][BF ] since the less Lewis acidic carbe-
] forms a Au–Ccarbenium contact that is
shorter than that in [3-AuCl][BF ]. The lack of correlation
4
˚
˚
[BF
4
] (3.209(3) A) and [4-AuCl][BF
4
˚
abstraction from [1-AuCl][BF
4
[
4
complexes of type H. To test this idea, a solution of [1-AuCl][BF4]
4
4
2 2 4
in CH Cl was treated with AgBF in the presence of tetrahy-
drothiophene (tht) (Scheme 3). This reaction proceeded
nium unit of [3-AuCl][BF
4
between the Au–Ccarbenium distance and the Lewis acidity of the
carbenium unit indicates that Au / Ccarbenium interactions, if
present, are too weak or too sterically congested to impact the
structures of these derivatives. Elaborating on the inuence of
steric congestion, we assign the larger Au–Ccarbenium distances
in [3-AuCl][BF ] and [4-AuCl][BF ] to the greater steric demand
4
4
of their carbenium moieties which are not planarized by
incorporation in a six-membered ring as in the case of [1-AuCl]
[
BF
benium moieties in [3-AuCl][BF
fests in the large value of a , dened as the dihedral angle
4
] and [2-AuCl][BF
4
]. The greater encumbrance of the car-
4
] and [4-AuCl][BF ] also mani-
4
d
existing between the Au1–P1–C1 plane and the C1–C2–C19
plane (Fig. 4). Indeed the a values, which are listed in Table 1,
d
ꢁ
are notably larger for [3-AuCl][BF ] (43.1(3) ) and [4-AuCl][BF ]
4
4
ꢁ
ꢁ
(
(
34.5(6) ) than for [1-AuCl][BF
1.36(18) ).
4
] (10.0(9) ) and [2-AuCl][BF
4
]
ꢁ
A computational investigation of [3-AuCl][BF
4
] and [4-AuCl]
[
BF ] leads to results that are consistent with the weakness of
4
the Au / Ccarbenium interactions present in these structures.
Indeed, an atoms-in-molecules (AIM) analysis shows that the
electron density (r(r)) at the critical point of the path connecting
Table 1 Compilation of key spectroscopic and structural features of
the gold complexes discussed in this study. [1-AuCl][BF
4
] and [1-AuCl]
39
[
4
BF ] were described in an earlier publication
3
1
+
ꢁ
˚
d( P)
Au–C (A)
a
d
( )
ꢁ
ꢁ
[
[
[
[
[
[
[
[
1-AuCl][BF
1-Au(tht)][BF
2-AuCl][BF
2-Au(tht)][BF
3-AuCl][BF
3-Au(tht)][BF
4-AuCl][BF
4-Au(tht)][BF
4
]
22.4
27.5
24.5
28.9
25.9
29.2
25.8
29.5
3.168(9)
3.320(4)
3.131(3)
4.381(3)
3.209(3)
3.290(13)
3.250(6)
3.376(10)
10.0(9)
47.4(4)
4
4
4
4
]
]
]
]
2
2
2
2
ꢁ
4
]
1.36(18)
ꢁ
91.8(3)
ꢁ
4
]
43.1(3)
ꢁ
28.4(9)
ꢁ
4
]
34.5(6)
ꢁ
37.2(9)
Scheme 3 Synthesis of the dicationic complexes.
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smoothly to afford [1-Au(tht)][BF
4
]
2
which could be isolated as the gold center and the carbenium ion (see Table 1). This
an orange crystalline solid that slowly decomposes under increase, which may reect increased electrostatic repulsions
ambient conditions, presumably because of liberation of the tht between the cationic gold moiety and the carbenium ion, only
6
2
˚
ligand as observed for other thioether–gold complexes. While amounts to ꢃ0.1–0.2 A in the case of [1-Au(tht)][BF ] , [3-
4
2
1
4 2 4 2
the H NMR spectrum indicates an intact ligand backbone and Au(tht)][BF ] , and [4-Au(tht)][BF ] (Table 1). By contrast,
the presence of a tht ligand, the P NMR resonance of [1- conversion of [2-AuCl] into [2-Au(tht)] is accompanied by
3
1
+
2+
+
˚
Au(tht)][BF
ical shi of 22.4 ppm measured for [1-AuCl][BF
the pK trend presented in Fig. 2, we became eager to establish 91.8(3) [2-Au(tht)] . We assign the unique structure change
whether dicationic complexes could also be obtained starting seen upon formation of [2-Au(tht)] to the lower pKR
4
]
2
appears at 27.5 ppm, downeld from the chem- a 1.2 A increase of the Au–C distance. At the same time, the
ꢁ
+
4
]. Going back to dihedral angle of a , increases from 1.36(18) in [2-AuCl] to
d
2+
ꢁ
+
R
2
+
+
of the
from the more electron-decient complexes [2-AuCl][BF ], [3- xanthylium unit and the resulting increased electrostatic
4
AuCl][BF ], and [4-AuCl][BF ]. Gratifyingly, we observed forma- destabilization occurring at the core of these complexes.
4
4
tion of the corresponding dicationic complexes [2-Au(tht)]
BF , [3-Au(tht)][BF , and [4-Au(tht)][BF (Scheme 3). As in
the case of [1-Au(tht)][BF , these new complexes show diag-
nostic NMR resonances that conrm the presence of a tht
[
4
]
2
4
]
2
4 2
]
Reactivity studies
4 2
]
Having veried that the dicationic complexes are indeed
accessible, we became eager to assess their performance in
carbophilic catalysis. To this end, we selected the cyclo-
3
1
ligand bound to the gold atom. The P NMR chemical shi of
the dicationic complexes (28.9 ppm for [2-Au(tht)][BF ] ,
4
2
isomerization of propargyl amide 5 as a benchmark reac-
2
9.2 ppm for [3-Au(tht)][BF ] , and 29.5 ppm for [4-Au(tht)]
4 2
63–65
tion.
We decided to investigate this reaction at room
[
BF ] ) are also slightly shied downeld when compared to
4 2
temperature with a 2% loading of catalyst in CD Cl which was
dried to avoid a possible interference of water with the carbe-
nium ion. To establish a baseline, we rst tested the activity of
2
2
those of their gold chloride precursors (24.5 ppm for [2-AuCl]
[
[
BF
BF
4
], 25.9 ppm for [3-AuCl][BF
4
], and 25.8 ppm for [4-AuCl]
4
]) (Table 1). Attempts to isolate the acetonitrile (MeCN)
+
+
+
the four gold chloride complexes [1-AuCl] , [2-AuCl] , [3-AuCl] ,
complexes were also investigated. However, we only obtained
evidence for the formation of [1-Au(MeCN)][BF ] but its isola-
4 2
tion was irreproducible.
+
and [4-AuCl] in the absence of an activator (entries 1–4, Table
+
+
2). While [1-AuCl] and [3-AuCl] gave hardly any conversion
+
+
(entries 1 and 3), [2-AuCl] and [4-AuCl] displayed moderate
The crystal structures of the tetrauoroborate salts of these
activity with conversion aer 20 minutes of 10% and 7%,
43
dications have been determined crystallographically (Fig. 5).
In all cases, we observe an increase in the separation between
+
respectively (entries 2 and 4). The higher activity of [2-AuCl]
+
and [4-AuCl] can be explained based on the greater acidity of
Fig. 5 ORTEP representations of the structures of [1-Au(tht)][BF
a close up showing the torsion angle a
probability and phenyl groups drawn as thin lines. Selected bond lengths ( A˚ ) and angles ( ) for [1-Au(tht)][BF
4
]
2
(A), [2-Au(tht)][BF
4
]
2
(B), [3-Au(tht)][BF
4
]
2
(C), and [4-Au(tht)][BF
4
]
2
(D), with
ꢀ
d
(E–H). Hydrogen atoms and BF
4
counterions omitted for clarity. Thermal ellipsoids drawn at 50%
ꢁ
4
]
2
: Au(1)–(P1) 2.2699(11), Au(1)–S(1)
2
.3352(12), C(19)–C(2) 1.500(6), C(19)–C(20) 1.395(6), C(19)–C(26) 1.406(6); C(2)–C(19)–C(20) 122.1(4), C(20)–C(19)–C(26) 119.6(4), C(26)–
ꢁ
4 2
C(19)–C(2) 118.3(4). Selected bond lengths ( A˚ ) and angles ( ) for [2-Au(tht)][BF ] : Au(1)–(P1) 2.2710(9), Au(1)–S(1) 2.3315(9), C(19)–C(2) 1.479(5),
C(19)–C(20) 1.414(5), C(19)–C(26) 1.415(4); C(2)–C(19)–C(20) 121.5(3), C(20)–C(19)–C(26) 118.2(3), C(26)–C(19)–C(2) 120.2(3). Selected bond
ꢁ
lengths ( A˚ ) and angles ( ) for [3-Au(tht)][BF
1
4
]
2
: Au(1)–(P1) 2.251(5), Au(1)–S(1) 2.286(5), C(19)–C(2) 1.507(19), C(19)–C(20) 1.43(2), C(19)–C(26)
ꢁ
.43(2); C(2)–C(19)–C(20) 116.3(15), C(20)–C(19)–C(26) 127.5(16), C(26)–C(19)–C(2) 116.2(15). Selected bond lengths ( A˚ ) and angles ( ) for [4-
Au(tht)][BF
4
2
] : Au(1)–(P1) 2.281(2), Au(1)–S(1) 2.334(2), C(19)–C(2) 1.477(14), C(19)–C(20) 1.378(14), C(19)–C(26) 1.469(13); C(2)–C(19)–C(20)
1
20.3(9), C(20)–C(19)–C(26) 121.6(9), C(26)–C(19)–C(2) 117.2(9).
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Table 2 Catalytic activity of the complexes in the reaction shown in
1
the Scheme 4. The conversion was measured by H NMR spectros-
2 2
copy, in situ, using CD Cl as a solvent
Catalyst
ꢀ
Entry (as BF
4
salts) Mol% AgX, mol% Time (min) Conv. (%)
+
1
2
3
4
5
6
7
8
9
1
1
1
[1-AuCl]+
[2-AuCl]+
[3-AuCl]
2
2
2
2
2
2
2
2
0
0
0
0
20
20
20
20
20
20
20
20
0
10
2
+
[4-AuCl]+
7
[1-AuCl]+
[3-AuCl]+
[3-AuCl]
AgBF
AgBF
AgNTf
0
0
0
0
4
, 4
, 4
83
>98
>98
85
>98
39
69
14
4
2
, 4
2
+
+
+
+
[1-Au(tht)]2
Scheme 4 Propargylamide cyclization used as a benchmark reaction.
The inset shows a possible working model for substrate activation.
[3-Au(tht)]
2
20
2
0
1
2
[1-Au(tht)]2
0.5
0.5
0.5
180
180
180
[3-Au(tht)]
Ph
3
AuCl
4
AgBF , 1
notion that the reactivity of the metal center in these dicationic
species is inuenced by the acidity of the adjacent carbenium
unit.
To better illustrate this effect, we repeated the experiment
with a catalyst loading of 0.5% and monitored its progress by H
NMR spectroscopy, using [1-Au(tht)][BF ] and [3-Au(tht)][BF ]
4 2 4 2
the carbenium centres present in these structure and their
ability to enhance the electrophilic character of the gold centre.
This observation is aligned with the conclusion of our previous
1
+
+
as pre-catalysts (entries 10 and 11). This lower loading was
selected to slow down the reaction such that its progress could
be more easily monitored. The results of these experiments,
presented in Fig. 6, unambiguously illustrates the superiority of
report on the catalytic properties of [1-AuCl] and [2-AuCl] in
39
the same reaction, but at a higher temperature.
Next, we became eager to test the effect induced by the
addition of an activator capable of engaging the gold-bound
chloride ligand. To this end, the experiments in entries 1–4
2
+
[
3-Au(L)] . We will also note that the reaction is initially cata-
lyzed by Ph PAuCl/AgBF (entry 12). However, aer a few
3
4
were repeated in the presence of 2 mol% AgBF . Treatment of [2-
4
+
+
minutes, the reaction rate slows down dramatically, suggesting
catalyst decomposition.
Given that [3-Au(tht)][BF ] emerged as the best catalyst, we
4 2
decided to also test its activity in the hydroarylative cyclization
AuCl] and [4-AuCl] with AgBF4 resulted in the immediate
production of a black precipitate indicating decomposition of
the catalyst. We propose that the elevated acidity of the carbe-
nium units present in these complexes jeopardizes the stability
of the dications. In fact, addition of the reaction substrate to [2-
of the propargyl ether 7 (ref. 66) as well as the cyclization of
67
1
2
+
2+
enyne 9 (Scheme 5). H NMR monitoring showed that these
reactions proceeded smoothly in 1–2 hours when carried out in
Au(tht)] and [4-Au(tht)] also leads to decomposition as
evinced by the formation of a dark residue. This decomposition
N
+
suggests that the acidity of the 9-xanthylium and [C(Ph)(Ar )]
carbenium units present in these complexes might be excessive.
A different observation was made in the case of complexes [1-
+
+
AuCl] and [3-AuCl] which underwent smooth activation with
1
AgBF
3% conversion aer 20 minutes with [1-AuCl] /AgBF
The reaction was almost complete at the same time point when
4
. H NMR monitoring showed that the reaction reached
+
8
4
(entry 5).
+
+
[
3-AuCl] /AgBF
4
or [3-AuCl] /AgNTf
2
was employed (entry 6 and
7). We also observed that AgBF4 (10 mol%) alone did not
promote this reaction. These results lead us to propose that the
active catalysts are dicationic species resulting from abstraction
of the gold-bound chloride ligand and resembling the tht
2
+
2+
adducts [1-Au(tht)] and [3-Au(tht)] . In fact, these dicationic
tht adducts can also be used as catalysts for these reactions
(
entries 8 and 9). Moreover, the activity of these two catalysts
+
very closely matches those obtained with [1-AuCl] /AgBF
4
and
+
[3-AuCl] /AgBF suggesting that the active species are indeed
4
dications as in the working model depicted in Scheme 4. We
2
+
also note that the greater activity of [3-Au(L)] correlates with
N +
the greater electron deciency of the [CAr
2
] moiety when
compared to the 9-N-methyl-acridinium moiety present in [1-
Au(L)] . This difference in activity allows us to introduce the
Fig. 6 Conversion of propargyl amide 5 into 6 as a function of time
with different catalysts.
2
+
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Acknowledgements
Acknowledgement is made to the Donors of the American
Chemical Society Petroleum Research Fund for partial support
of this research (61541-ND3). We also acknowledge support
from the National Science Foundation (CHE-1856453), the
Welch Foundation (A-1423), and Texas A&M University (Arthur
E. Martell Chair of Chemistry).
Scheme
5
Intramolecular hydroarylation and enyne cyclization Notes and references
4 2
reactions catalyzed by [3-Au(tht)][BF
] .
1
2
3
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Conflicts of interest
There are no conicts to declare.
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3936 | Chem. Sci., 2021, 12, 3929–3936
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