Exploiting the π-acceptor properties of carbene-stabilized phosphorus centered trications [L3P] 3+: Applications in Pt(II) catalysis

Article abstract of DOI:10.1021/ja306947m

Reaction of tris(dimethylaminocyclopropenium) substituted phosphine 1 with K₂PtCl₄ afforded the bench stable complex 3 which upon treatment with Ag[CB₁₁H₆Cl₆] turned out to be an excellent catalyst for the transformation of a variety of ortho-biaryl substituted alkynes into polycyclic homo- and heteroarenes of different size, shape, and curvature through a 6-endo-dig cyclization. This constitutes the first example ever reported of using a P₁-centered trication as ligand in catalysis. The strong π-acceptor character of 1 that derives from its three positive charges substantially increases the intrinsic π-acidity of Pt in complex 1?PtCl₂ and dramatically enhances its ability to activate π-systems toward nucleophilic attack. As a consequence, a remarkable acceleration of the model transformation is observed when compared with other classical π-acceptor ligands such as P(OPh)₃ or P(C ₆F₅)₃. Moreover, the employment of 1 as ligand also expands the scope of this reaction to previously inaccessible substitution patterns. Kinetic studies and deuterium labeling experiments as well as density functional theory (DFT) calculations were performed in order to explain these findings.

Full text of DOI:10.1021/ja306947m

Article
pubs.acs.org/JACS
Exploiting the π‑Acceptor Properties of Carbene-Stabilized
3+
Phosphorus Centered Trications [L3P] : Applications in Pt(II)
Catalysis
Javier Carreras, Mahendra Patil, Walter Thiel, and Manuel Alcarazo*
Max-Planck-Institut fur Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mulheim an der Ruhr, Germany
̈
̈
S
* Supporting Information
ABSTRACT: Reaction of tris(dimethylaminocyclopropenium) substituted
phosphine 1 with K2PtCl4 afforded the bench stable complex 3 which upon
treatment with Ag[CB11H6Cl6] turned out to be an excellent catalyst for the
transformation of a variety of ortho-biaryl substituted alkynes into polycyclic
homo- and heteroarenes of different size, shape, and curvature through a
6-endo-dig cyclization. This constitutes the first example ever reported of using
a P1-centered trication as ligand in catalysis. The strong π-acceptor character
of 1 that derives from its three positive charges substantially increases
the intrinsic π-acidity of Pt in complex 1·PtCl2 and dramatically enhances its
ability to activate π-systems toward nucleophilic attack. As a consequence,
a remarkable acceleration of the model transformation is observed when
compared with other classical π-acceptor ligands such as P(OPh)3 or P(C6F5)3.
Moreover, the employment of 1 as ligand also expands the scope of this
reaction to previously inaccessible substitution patterns. Kinetic studies and deuterium labeling experiments as well as density
functional theory (DFT) calculations were performed in order to explain these findings.
a ligand for π-acidic metals such as Pt. Thus, when 1 is treated
with K2PtCl4 in acetonitrile, the bench stable complex 3 is
formed (Scheme 1). More interestingly, charge decomposition
1. INTRODUCTION
π-Acid catalysis, mainly with Au(I) and Pt(II) based species,
has emerged in the past decade as one of the most efficient
tools for the promotion of rearrangements in unsaturated
organic substrates, which provide an exquisite entry for the
synthesis of intricate skeletons that may be otherwise difficult to
prepare.1 The generally accepted mechanism that governs most
of these transformations involves three main steps: (i)
coordination of the π-acid metal to the alkyne or allene moiety
present in the starting material, (ii) nucleophilic intra- or
intermolecular attack to the thus activated substrate forming a
vinyl-metal species, and (iii) protodemetalation of the vinyl
intermediate with concomitant regeneration of the active
catalyst.2 It seems reasonable to expect that the first two of
these steps may be accelerated by strong π-acceptor ancillary
ligands, which should increase the Lewis acidity of the metal
center they coordinate. In striking contrast, potent σ-donating
ligands will weaken the M−C bond in the vinyl intermediates
by their trans influence and thus facilitate the third
protodemetalation step. Hence, the selection of the most
appropriate ligand for a particular transformation of this type
can only be done on the basis of an in-depth understanding of
the nature of the rate-determining step.3
Scheme 1. Synthesis of [L3P]+3 Cation 1, its Pt Complex 3,
and Molecular Structure of 3 in the Solid State
analysis of the metal−ligand interaction in 3 gave the surprising
result that the total L→M σ-donation (0.31 e) is lower than the
M→L π-back-donation (0.43 e) into the very low-lying LUMO
of 1, which must hence be regarded as the main interaction in
3. This unconventional situation in which the P-ligand removes
net electron density from the metal suggests that compound 1
increases the natural π-acidity of Pt(II) centers. It should thus
accelerate known reactions, or even permit new reactions, in
Very recently, we reported the synthesis of the first ever
isolated carbene-stabilized P1-centered trication [L3P]3+ (L =
2,3-dialkylamino-cyclopropenyl-1-ylidene) 1 by reaction of
the 1-chloro-2,3-dimethylamino)cyclopropenium salt 2 and
P(SiMe3)3.4 Despite the three positive charges on the groups
directly attached to the P atom, this compound can still serve as
Received: July 16, 2012
© XXXX American Chemical Society
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which either the coordination of the substrate or the
nucleophilic attack to the already activated substrate is the
rate-determining step.5
A plot of the obtained rate constants kobs versus [Ph3P·PtCl2]
provided a straight line with a slope of 0.01 indicating an overall
first order in Pt during the process (Supporting Information,
Figure S3).
2.3. Deuterium Labeling Experiments. The nature of the
intermediates involved in this catalytic cycle was also inves-
tigated by deuterium labeling experiments. The fact that no
H/D scrambling occurs during the crossover cycloisomeriza-
tion depicted in Scheme 3 strongly supports the intramolecular
Herein, we report the first practical application of cation 1 as
ligand in a catalytic process, the Pt-catalyzed cyclization of
ortho-biaryl substituted alkynes into polycyclic homo- and
heteroarenes, and describe the advantages derived from its
employment in terms of higher reactivity and broader substrate
scope. Moreover, we also present kinetic, isotope labeling, and
DFT computational studies on the reaction mechanism
governing this transformation in order to precisely ascertain
the role of ligand 1.6
Scheme 3. Deuterium Labeling Crossover Experiment
2. RESULTS AND DISCUSSION
2.1. Selection of a Model Reaction. The Pt-catalyzed
6-endo-dig cyclization of 2-ethinyl-1,1′-binaphtalene 4 into
pentahelicene 5, first reported by Furstner and co-workers,7 was
̈
chosen as model reaction for two main reasons (Scheme 2): (a)
Scheme 2. Cyclization of 2-Ethynyl-1,1′-binaphthalene 4
into Pentahelicene 5 as Model Reaction
nature of any occurring proton shift. In addition, the small
measured isotope effects suggest that none of the plausible
transition states involving C−H bond cleavage are likely to be
rate-determining (Supporting Information, Figure S3). Instead,
the observed effect is probably derived from the hybridization
change that must take place during an earlier cyclization step.
2.4. DFT Calculations on the Reaction Mechanism.
The reaction mechanism was explored by means of hybrid DFT
calculations. Geometry optimizations were performed using the
B3LYP functional with the LANL2DZ effective core potential
and the associated double-ζ basis set for Pt and the 6-31G*
basis set for all other atoms (B3LYP-I level). Energies were
determined from single-point calculations, in which the
6-31G* basis was replaced by the larger 6-31+G** basis
whereas LANL2DZ was replaced by LANL2TZ(f) for Pt
(B3LYP-II level). The reported free energies (Scheme 4) were
obtained from the B3LYP-II energies by adding thermal and
entropic corrections (B3LYP-I level) as well as dispersion and
continuum solvation corrections (see Supporting Information
for details). All calculations were done with the Gaussian09 suite
of quantum chemical programs.11
We investigated the cycloisomerization of 4 into 5 using
PPh3 and the cation 1 as ancillary ligands.12 As can be seen
from Scheme 4, the free energy of coordination of alkyne 4 is
computed to be −2.3 and −7.9 kcal/mol for Ph3P·PtCl2 and
1·PtCl2, respectively, indicating exergonic formation of the
intermediate complexes Ia and Ib. The higher thermodynamic
stability of Ib can be rationalized by inspection of the frontier
orbitals of the actual catalysts (Figure 1). The LUMO has a
similar shape in both cases (σ* P−Pt), but it lies much lower in
the 1·PtCl2 complex and will therefore confer a much stronger
Lewis acidity to this catalyst, thus favoring the coordination of
the alkyne to Pt. This facile coordination is fully consistent with
the observed pseudo-zero-order kinetics in substrate.
The interest in polycyclic homo- and heteroarenes has been
refueled in the last years due to their unique optoelectronic
properties and their potential applications in organic electronic
devices. The chosen cyclization is a very attractive entry for
the preparation of these carbon-rich materials.8 However, under
the previously described conditions, quite long reaction times
are necessary. Additionally, the development of a more reactive
catalyst that broadens the scope of this transformation in terms
of new substitution patterns is still necessary.9(b) Due to the
relatively weak nucleophile that is employed (an aromatic ring),
the nucleophilic attack is expected to be the rate-determining
step for this reaction.10 Hence, the use of strong π-acceptor
ancillary ligands such as 1 should facilitate this transformation.
Finally, it is likely that under the selected reaction conditions,
with high substrate concentration and noncoordinating
solvents, the coordination of the substrate to Pt is not a rate-
relevant step.
2.2. Kinetic Studies. In order to prove these assumptions,
kinetic as well as computational studies were carried out to
identify the rate-determining step for the catalytic reaction.
Thus, we started by examining the order of this transformation
in substrate. By monitoring by GC the reaction of 4 (50 mM),
PPh3 (2.5 mM), and PtCl2 (2.5 mM) in dichloroethane at
80 °C, the cyclization was found to follow pseudo-zero-
order kinetics in 4. The measurements gave a linear plot of
(Δ[5]/Δt)/[4]0; kobs = 0.0274 h−1, R2 = 0.996 (Supporting
Information, Figure S1), indicating (as expected) that
coordination of 4 to the Pt center is not the rate-determining
step.
The order in Pt was evaluated by measuring the rate constant
for the reaction at different catalyst loadings (1−2.5 mM)
keeping constant the concentration of alkyne 4 at 50 mM and
the temperature at 80 °C (Supporting Information, Figure S2).
The next step along the reaction coordinate involves the
nucleophilic attack of the arene ring to the activated alkyne
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Scheme 4. Free Energy Profiles (kcal/mol) for the Cyclization of 2-Ethynyl-1,1′-binaphthalene 4 into Pentahelicene 5
Figure 1. Left: LUMO of Ph3P·PtCl2, E = −3.28 eV. Right: LUMO of
1·PtCl2, E = −11.69 eV.
Figure 3. Ligand effect on the Pt-catalyzed cyclization of 2-ethynyl-
1,1′-binaphthalene 4 into pentahelicene 5. (a) Conditions for L =
PPh3, P(OPh)3, P(C6F5)3: 4 (0.05 M), PtCl2 (5 mol %), and ligand
(5 mol %), Cl(CH2)2Cl, 80 °C. (b) Conditions for precatalyst 3: 4
(0.05 M), 3 (5 mol %), AgClO4 or Ag[CB11H6Cl6] (5 mol %),
Cl(CH2)2Cl, 80 °C.
to form the cyclopropyl intermediates IIa/b through transi-
tion states TS1a/b, respectively. Our calculations indicate that,
regardless of the ancillary ligand used, these are the highest-
energy transition states for both catalytic cycles (rate-deter-
mining in the framework of the energetic span model, see page
S36 in the Supporting Information for a detailed discussion).
Importantly, TS1b lies no less than 15 kcal/mol lower than
TS1a, suggesting that the use of 1 as ligand significantly fosters
Figure 2. Left: calculated transition state TS1a. Selected distances (Å):
C(1)−C(2) 1.30, C(1)−C(5) 2.07, C(1)−C(6) 2.30. Right: calculated
transition state TS1b. Selected distances (Å): C(1)−C(2) 1.26, C(1)−
C(5) 2.81, C(1)−C(6) 2.86. Hydrogen atoms on the ligands have
been omitted for clarity.
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Table 1. Scope of the Pt-Catalyzed Cycloisomerization of ortho-Alkynylated Biaryls Employing 1 as Ligand
a
Reaction initiated with a catalytic charge of 2.5 mol %. After 45 min a second portion of catalysts 2.5 mol % was added.
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the whole process. The beneficial stabilizing effect of 1 can be
easily rationalized by comparing the key C−C distances in
transition states TS1a and TS1b (Figure 2). The calculated
C1−C5 and C1−C6 distances are, respectively, 2.07 and 2.30 Å
in TS1a, but much longer in TS1b (2.81 and 2.86 Å) reflecting
an earlier and therefore energetically lower transition state when
1 is used as ligand.
The subsequent opening of the cyclopropane ring requires
moderate activation that does not depend much on the nature
of the ancillary ligand (barriers of 9−10 kcal/mol) and leads to
intermediates IIIa/b, which evolve via a 1,2-H shift to form the
Pt-carbenes IVa/b through transition states TS3a/b. The two
intermediates IVa/b are 22−27 kcal/mol more stable than their
precursors IIIa/b, and the energies of the following transition
states TS4a/b are much lower than those of TS3a/b, respectively.
Thus, the formation of the Pt-carbenes IVa/b is likely to be
irreversible. Finally, a second 1,2-H shift leads to the penta-
helicene-Pt π-complexes Va/b, which after dissociation regener-
ate the active catalysts and the desired product 5. The two
proposed intramolecular proton shifts are in agreement with
the observations from deuterium labeling experiments.
2.5. Ligand Effect. The whole set of kinetic and calculated
data suggests that (i) the rate-determining transition state must
be TS1, and (ii) this step must be facilitated by the use of
electron-poor ancillary ligands. Accordingly, we chose for our
studies of ligand effects a series of phosphanes such as PPh3,
P(OPh)3, P(C6F5)3 and precatalyst 3 in combination with a
silver salt.13 As can be seen from Figure 3 the theoretical pre-
dictions were confirmed, and both P(OPh)3 and P(C6F5)3
performed better than PPh3 in terms of reactivity. Interestingly,
the mixture of 3 and AgClO4 produces a much faster initial
reaction surpassing any of the other π-acceptor ligands. How-
ever, the catalytic species rapidly decays and complete con-
version of the substrate was not observed after two hours. To
overcome this problem, the less coordinating and more soluble
carborane-based anion [CB11H6Cl6]− was evaluated.14 We were
glad to see that this new catalytic mixture was vastly superior to
any other described, and complete conversion of the model
substrate to pentahelicene 5 was achieved in less than 20 min
under the standard reaction conditions.
With this optimized catalyst in hand we explored the scope
of this methodology in detail. To this end a representative set
of biaryl substituted alkynes was prepared and submitted to the
newly developed procedure. Particular attention was paid to
investigating the compatibility of the novel catalysts with dif-
ferent functional groups. The examples compiled in Table 1
clearly demonstrate the generality of this catalytic system for
the synthesis of substituted phenanthrenes 11a−g, benzophe-
nanthrenes 11 h−n, benzotriphenylenes 11o−p, substituted
benzofuranes 11q−t, and thiophenes 11u−v. Moreover, the
functional group compatibility is outstanding. Ethers, free or
silylated alcohols, esters, halogen substituents, silyl and tri-
fluoromethyl groups, thiophenes, and furanes were well tole-
rated, and the cyclized products were obtained in few minutes
with very good to excellent yields.6 Only compounds contain-
ing strongly coordinating functionalities such as thioethers 10g
were found to interfere with the activity of the catalytic system
(Table 1, entry 7). Also remarkable is the pronounced selec-
tivity for the 6-endo-dig cyclization over the also conceivable
5-endo-dig mode. Single crystals suitable for X-ray analysis
were grown for various representative products, and their struc-
tures are shown in Figure 4 (11c, 11p) and in the Supporting
Information (11o, 11t).
Figure 4. Molecular structures of compounds 11p (left) and 11c
(right). Anisotropic displacement parameters shown at 50%
probability level and hydrogen atoms omitted for clarity.
2.6. Conclusions. We report the first application of a
P1-centered trication as ligand in catalysis. Due to its high
π-acceptor character, the trication 1 substantially enhances the
intrinsic π-acidity of Pt and hence its ability to activate alkynes
toward nucleophilic attack. Specifically, upon treatment with
Ag[CB11H6Cl6], complex 3 turns out to be an extremely reac-
tive catalyst for the hydroarylation of ortho-alkynylated biaryls,
which clearly surpasses other known catalytic systems not only in
terms of reactivity but also regarding functional group tolerance.
Moreover, the nonspecific mode of action of 1, a simple
reinforcement of metal-to-ligand back-donation, makes us think
that the impact of this study may be limited neither to the
particular transformation chosen as model reaction, nor to
Pt(II) catalysis. Whether these or related P1-centered mono- or
polycations could find application as ligands in other metal-
catalyzed transformations is currently under investigation in our
laboratories and will be reported in due course.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures including characterization data for all
new compounds, additional deuterium labeling experiments,
CIF files for 11c, 11p, 11o, and 11t, computational procedures,
further computational results (structural parameters, charge
distributions, and optimized Cartesian coordinates) as well as
complete ref 11. NMR spectra. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
alcarazo@mpi-muelheim.mpg.de
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Dedicated to Professor Alois Furstner on the occasion of his
■
̈
50th birthday. Generous financial support from the European
Research Council (ERC Starting Grant), the Deutsche
Forschungsgemeinschaft, and the Fonds der Chemischen
Industrie is gratefully acknowledged. We also thank the
NMR, X-ray, and chromatography departments of our Institute
for excellent support. J.C. thanks the Spanish Ministry of
Education for support.
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