Understanding the effect of α-cationic phosphines and group 15 analogues on &phi-acid catalysis

Article abstract of DOI:10.1021/acs.organomet.6b00859

The factors responsible for the experimentally observed acceleration of ?-Acid-catalyzed reactions induced by ?-cationic phosphines and group 15 analogues have been computationally explored within the density functional theory framework. To this end, the

Full text of DOI:10.1021/acs.organomet.6b00859

Article
pubs.acs.org/Organometallics
Understanding the Effect of α‑Cationic Phosphines and Group 15
Analogues on π‑Acid Catalysis
Yago García-Rodeja and Israel Fernandez*
́
́
Departamento de Química Organica I, Facultad de Ciencias Químicas, Universidad Complutense, 28040 Madrid, Spain
S
* Supporting Information
ABSTRACT: The factors responsible for the experimentally observed
acceleration of π-acid-catalyzed reactions induced by α-cationic phosphines
and group 15 analogues have been computationally explored within the density
functional theory framework. To this end, the gold(I)-catalyzed hydroarylation
reactions of phenylacetylene and mesitylene involving both neutral and
cationic ligands (L) have been quantitatively analyzed in detail using the combination of the activation strain model of reactivity
and the energy decomposition analysis methods. It is found that the activating effect of the cationic ligand finds its origin in the
much stronger interaction between the deformed reactants along the entire reaction coordinate, which in turn derives from the
much higher π(acetylene) → σ*(Au−L) interaction in the initially formed acetylene-gold(I) π-complex.
properties of the ligands such as their Tolman electronic
parameters (TEPs)7 and the corresponding oxidation potentials
(Ep(ox)).3 Whereas one should be cautious when using the
former values, as the CO stretching frequencies do not
unequivocally reflect the donicity of ancillary ligands,8 the
latter values were suggested to be a more reliable parameter to
rank the electronic properties (i.e., σ-donor/π-acceptor ability)
of the ligands. Despite that, the intrinsic physical factors
governing the observed enhanced catalytic activity induced by
this family of ligands are so far not fully understood.
On the other hand, the relatively recent introduction of
the so-called activation strain model (ASM) of reactivity,9 in
combination with the energy decomposition analysis (EDA)
method,10 has allowed us to gain a deeper quantitative
understanding of those factors controlling different fundamental
processes in both organic (for instance, SN2 and E2 reactions11
and pericyclic reactions)12 and organometallic transformations.13
For this reason, we decided herein to apply the combination of
the ASM and EDA methods to assess the influence of these
cationic ligands on π-acid catalysis in a quantitative way. To this
end, we have selected the gold(I)-catalyzed hydroaryla-
tion reaction of phenylacetylene with mesitylene depicted in
Scheme 1, which was also experimentally explored by Alcarazo
and co-workers.5 In addition to α-cationic phosphines, the effect
INTRODUCTION
■
The design and preparation of new ligands arguably play a key
role in the development of known and new catalytic reactions.
This is mainly due to the fact that ligands are crucial for
controlling the reactivity and even the selectivity (from regio-
or chemoselectivity to enantioselectivity) of transition metal
catalyzed transformations.1,2 In this sense, α-cationic phosphines
(and related arsines) have attracted considerable attention
recently because the attachment of a positively charged moiety
directly to the coordinating atom significantly increases the
ability of these ligands to accept electronic density from the
transition metal fragment.3 As a result, it was observed that
these novel charged ligands lead to a significant enhancement
of the activity of π-acid catalysts.
In this particular field of ligand development, Alcarazo and
co-workers3 have prepared a series of monocationic phos-
phines containing cyclopropenium,4 pyridinium,5 imidazolium,
or amidinium substituents6 (see Chart 1). The performance of
Chart 1. Representative α-Cationic Phosphines Prepared
Experimentally
Scheme 1. Gold(I)-Catalyzed Hydroarylation Reaction
Considered in This Study
these ligands was tested in several catalytic processes requiring
high Lewis acidity at the metal center such as gold(I)- or Pt(II)-
catalyzed hydroarylation reactions. In general, it was found that
these species are able to induce a significant acceleration of the
process as compared to their neutral counterparts (i.e., PPh3 or
P(OPh)3).
The enhanced catalytic activity induced by these cationic
species has been qualitatively related to different electronic
Received: November 16, 2016
© XXXX American Chemical Society
A
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with the ADF 2014 program package.26 Scalar relativistic effects were
incorporated by applying the zeroth-order regular approximation
(ZORA).27 This level is therefore denoted ZORA-BP86-D3/TZ2P//
B3LYP/def2-SVP.
of strongly related arsines (also prepared experimentally very
recently)14 and unexplored group 15 analogues on the process
shall be considered as well.
COMPUTATIONAL DETAILS
■
RESULTS AND DISCUSSION
■
All the calculations reported in this paper were obtained with the
GAUSSIAN 09 suite of programs.15 All reactants, transition structures,
and reaction products were optimized using the B3LYP functional16
using the double-ζ quality def2-SVP basis sets17 for all atoms, which
include a pseudopotential for gold. All stationary points were
characterized by frequency calculations.18 Reactants and products
have positive definite Hessian matrices, whereas transition structures
(TSs) show only one negative eigenvalue in their diagonalized force
constant matrices,19 and their associated eigenvectors were confirmed
to correspond to the motion along the reaction coordinate under
consideration using the intrinsic reaction coordinate (IRC) method.20
Single-point calculations were performed to refine the computed
energies on the optimized gas-phase geometries using the B3LYP
functional in conjunction with the D3 dispersion correction sug-
gested by Grimme et al.21 and the triple-ζ quality plus polarization
def2-TZVP basis set17 for all atoms. Solvent effects (solvent =
dichloroethane) were also taken into account in the single-point
calculations using the polarizable continuum model (PCM).22 This
level is thus denoted PCM(dichloroethane)-B3LYP-D3/def2-TZVP//
B3LYP/def2-SVP.
The effect of the α-cationic substituent directly attached to the
−PPh2 moiety on the gold(I)-catalyzed hydroarylation reaction
of phenylacetylene with mesitylene was studied first. As pre-
viously reported for strongly related processes,28 this trans-
formation involves the initial formation of a π-complex (readily
formed upon coordination of the triple bond of phenyl-
acetylene to the gold(I) catalyst) followed by the nucleophilic
attack of mesitylene.29 The final alkene is then produced
through a Au−C bond protonolysis reaction, a process that is
typically mediated by the counteranion present in the initial
gold(I) precatalyst (Figure 1).28 In most cases, the nucleophilic
Activation Strain Analyses of Reaction Profiles. The activation
strain model of reactivity, also known as the distortion/interaction
model,23 is a fragment approach to understanding chemical reactions,
in which the height of reaction barriers is described and understood
in terms of the original reactants.9 Thus, the potential energy surface
ΔE(ζ) is decomposed, along the reaction coordinate ζ, into the strain
ΔEstrain(ζ) associated with deforming the individual reactants plus the
actual interaction ΔEint(ζ) between the deformed reactants (eq 1):
ΔE(ζ) = ΔEstrain(ζ) + ΔEint(ζ)
(1)
Herein, the reaction coordinate ζ is defined as the projection of the
IRC on the forming C···C distance between the reactive carbon atom
of the initially formed phenylacetylene-gold(I) π-complex and the
carbon atom of mesitylene. This reaction coordinate ζ undergoes a
well-defined change in the course of the reaction from ∞ to the
equilibrium C···C distance in the corresponding transition structures.
The strain ΔEstrain(ζ) is determined by the rigidity of the reactants
and on the extent to which groups must reorganize in a particular
reaction mechanism, whereas the interaction ΔEint(ζ) between the
reactants depends on their electronic structure and on how they are
mutually oriented as they approach each other. It is the interplay
between ΔEstrain(ζ) and ΔEint(ζ) that determines if and at which point
along ζ a barrier arises, namely, at the point where dΔEstrain(ζ)/dζ =
−dΔEint(ζ)/dζ.
Figure 1. Computed reaction profile for the initial step of the studied
gold(I)-catalyzed hydroarylation reaction between phenylacetylene
and mesitylene. Relative free energies (computed at 298 K) are given
in kcal/mol. Plain values refer to B3LYP/def2-SVP calculations,
whereas values within parentheses were computed at the PCM-
(dichloroethane)-B3LYP-D3/def2-TZVP//B3LYP/def2-SVP level.
Furthermore, the interaction energy can be further decomposed by
means of the so-called energy decomposition analysis10 method into
the following meaningful terms (eq 2):
addition reaction constitutes the rate-limiting step of the entire
transformation,28 and for this reason, herein we only focused
on this particular reaction step.
ΔEint(ζ) = ΔEelstat(ζ) + ΔEPauli(ζ) + ΔEorb(ζ) + ΔEdisp(ζ)
As readily seen in Figure 1, the replacement of the parent
neutral ligand PPh3 (L1) or P(OPh)3 (L2) by a α-cationic
phosphine (L3−L6) clearly leads to a remarkable decrease of
the corresponding activation barrier (from 26.9 kcal/mol for L1
up to ca. 17.0 kcal/mol for L4 and L6), which nicely agrees
with the acceleration induced by the α-cationic ligand experi-
mentally observed. Furthermore, the computed endergonicity
of the transformation systematically decreases in the presence
of the cationic ligands L3−L6 as well.
Closer inspection of the optimized geometries of the
corresponding transition states TS1−6 indicates that the
α-cationic phosphine ligands L3−L6 lead to earlier transition
states (i.e., the forming C···C distances become longer) than
neutral ligands (see Figure 2). Interestingly, it is found that
these late transition states are associated with higher activation
(2)
The term ΔEelstat corresponds to the classical electrostatic inter-
action between the unperturbed charge distributions of the deformed
reactants and is usually attractive. The Pauli repulsion ΔEPauli
comprises the destabilizing interactions between occupied orbitals
and is responsible for any steric repulsion. The orbital interaction
ΔEorb accounts for electron-pair bonding, charge transfer (interaction
between occupied orbitals on one moiety with unoccupied orbitals
on the other, including HOMO−LUMO interactions), and polar-
ization (empty-occupied orbital mixing on one fragment due to the
presence of another fragment). Finally, the ΔEdisp term takes into
account the interactions that are due to dispersion forces. The EDA
calculations reported herein were carried out at the dispersion-
corrected BP8624-D321 level in combination with the frozen core
TZ2P25 basis sets using the optimized B3LYP/def2-SVP geometries
B
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Figure 4. Comparative activation strain diagrams for the gold(I)-
catalyzed hydroarylation reactions involving PPh3 (L1, solid lines) and
cationic L6 (dashed lines) as ligands along the reaction coordinate
projected onto the forming C···C bond distance. All data have been
computed at the PCM(dichloroethane)-B3LYP-D3/def2-TZVP//
B3LYP/def2-SVP level.
Figure 2. Fully optimized transition states TS1−6 associated with the
C···C bond forming reaction in the considered gold(I)-hydroarylation
reactions. Bond distances, computed at the B3LYP/def2-SVP level, are
given in angstroms.
interaction energy between the deformed reactants, measured
by the ΔEint term, remains nearly constant at the beginning of
the reaction and then inverts at a certain point along the reac-
tion coordinate (i.e., at forming C···C distances of ca. 2.5 Å),
where it becomes more and more stabilizing as one approaches
the corresponding transition-state region. At variance, the strain
energy, measured by the ΔEstrain term, becomes more and more
destabilizing from the initial stages of the reaction up to the
transition state. Interestingly, the energy required to deform
the reactants is nearly identical for both processes, particularly
at the transition-state region where the height of the barrier
is determined. For instance, at the same C···C bond forming
distance of 2.2 Å, a value of ΔEstrain = 18.4 kcal/mol was
computed for the process involving PPh3, whereas a rather
similar value of ΔEstrain = 17.9 kcal/mol was computed for
the process involving L6 (Figure 4). Therefore, it can be safely
concluded that the energy required to deform the reactants
from their equilibrium geometries is not at all decisive for
the observed enhanced reactivity of the catalyst bearing the
α-cationic phosphine ligand.
barriers and more endergonic transformations than earlier
transition states. This finding, which nicely agrees with the
Hammond−Leffer postulate,30 becomes evident from the very
good linear relationships (correlation coefficients R2 ≈ 1.0)
observed when plotting the C···C bond forming distance in the
transition states TS1−6 versus the corresponding computed
reaction and activation energies (Figure 3).
In contrast, significant differences are observed when compar-
ing the interaction energies between the deformed reactants.
Indeed, the ΔEint term is clearly more stabilizing for the process
involving the cationic phosphine along the entire reaction
coordinate. For instance, at the same C···C forming distance
of 2.2 Å, a value of ΔEint = −12.9 kcal/mol was computed for
the process involving PPh3, whereas a higher value (i.e., more
stabilizing) of ΔEint = −16.5 kcal/mol was computed for the
process involving L6 (Figure 4). This stronger interaction
between the deformed reactants from the very beginning of the
transformation constitutes therefore the main factor controlling
the lower activation barrier of the processes involving the
α-cationic phosphines.
Further quantitative insight into the different contributions
to the total interaction energy can be gained by means of the
EDA method.10 As graphically shown in Figure 5, the orbital
attractions between the deformed reactants (measured by
the ΔEorb term and dominated mainly by the π(mesitylene) →
π*(gold-complex) interaction) constitute the differential con-
tribution when comparing the reactions involving the neutral
Figure 3. Plot of the computed activation (black circles) and reaction
(white circles) energies versus the C···C bond forming distances of
transition states TS1−6. All data have been computed at the B3LYP/
def2-SVP level.
The activation strain model9 of reactivity was applied next
to gain more quantitative insight into the physical factors
controlling the observed acceleration of the process induced
by the α-cationic ligand. To this end, we compared the process
involving the parent neutral phosphine L1 (PPh3) with the
hydroarylation reaction involving the cationic pyridinium ligand
L6. Figure 4 shows the computed activation strain diagrams
(ASDs) for both transformations from the initial stages of
the processes up to the corresponding transition states. Both
reactions exhibit quite similar ASDs. Thus, in both cases the
C
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Figure 6. Plot of the deformation densities Δρ of the pairwise orbital
interactions present in the [(Ph3P)Au+]−phenylacetylene complex
and associated stabilization energies ΔE(ρ). The color code of the
charge flow is red → blue. Data computed at the ZORA-BP86-D3/
TZ2P//B3LYP/def2-SVP level (isosurface value of 0.002 au).
Figure 5. Decomposition of the interaction energy for the gold(I)-
catalyzed hydroarylation reactions involving PPh3 (L1, solid lines) and
cationic L6 (dashed lines) as ligands along the reaction coordinate
projected onto the forming C···C bond distance. All data have been
computed at the ZORA-BP86-D3/TZ2P//B3LYP/def2-SVP level.
Table 1. Computed NOCV Stabilization Energies ΔE(ρ)
(in kcal/mol) Present in the Initial [(L)Auq+]-Phenylacetylene
a
Complexes
b
L
ΔE(ρ1)
ΔE(ρ2)
LUMO/eV
ΔGa
L1
L2
L3
L4
L5
L6
−37.9
−44.4
−55.5
−59.9
−56.3
−59.2
−11.2
−11.2
−10.8
−10.5
−10.6
−10.3
−6.40
−6.82
−8.94
−9.25
−9.04
−9.45
26.9
24.5
18.5
16.7
18.1
17.0
PPh3 ligand and the cationic ligand L6 along the reaction
coordinate. Thus, whereas the rest of the contributions
(namely, Pauli repulsion and electrostatic and dispersion
attractions) are practically identical for both transformations,
the orbital term is clearly more stabilizing for the process
involving L6. Indeed, the total interaction energy between the
deformed reactants parallels the shape of the orbital interaction
curve. For instance, at the same C···C forming distance of
2.2 Å, the difference between the orbital attractions for both
processes ΔΔEint = 8.7 kcal/mol roughly matches the difference
in the total interaction energy, ΔΔEint = 10.8 kcal/mol.
Therefore, it can be concluded that the acceleration induced
by the α-cationic phosphine ligand finds its origin mainly in
the stronger orbital interaction between the reactants along the
entire reaction coordinate.
a
All data have been computed at the ZORA-BP86-D3/TZ2P//
B3LYP/def2-SVP level. Activation energies (in kcal/mol) were
computed at the B3LYP/def2-SVP level.
b
The above finding strongly suggests that the α-cationic
ligand significantly increases the π-acceptor ability of the initial
acetylene-Au(I) complex. To quantitatively support this hypo-
thesis, the main orbital contributions present in the different
acetylene-Au(I) π-complexes considered in this study were
analyzed by means of the NOCV (natural orbital for chemical
valence) extension of the EDA method.31 Thus, the EDA-
NOCV approach, which provides pairwise energy contributions
for each pair of interacting orbitals to the total bond energy,
indicates that two main molecular orbital interactions are
present in the initially formed acetylene-Au(I) complexes,
namely, the donation from the π-molecular orbital of the
acetylene fragment to the vacant Au−P antibonding orbital of
the [AuL]+ moiety (denoted as ρ1) and the back-donation (ρ2)
from a doubly occupied atomic orbital located at the transition
metal to the π*-molecular orbital of the acetylene fragment
(see Figure 6 for L = PPh3, charge flow is red → blue).
Not surprisingly, the direct donation from the acetylene frag-
ment is systematically much stronger than the back-donation
interaction (ΔE(ρ1) > ΔE(ρ2)) regardless of the ligand L
attached to the transition metal (Table 1). Moreover, whereas
this back-donation remains almost constant for all species, the
ρ1 interaction strongly depends on the nature of the phosphine
ligand. Indeed, this interaction becomes clearly much stronger
for those complexes having α-cationic phosphine ligands in
the order L1 < L2 ≪ L3 < L5 < L4 ≈ L6, which, strikingly,
Figure 7. Plot of computed NOCV π(acetylene) → σ*(Au−P)
interaction, ΔE(ρ1), in the initial [(L)Au+]−phenylacetylene com-
plexes versus the computed activation barriers (ΔGa).
matches the computed activation barriers for the reactions
involving these ligands. For this reason, a very good linear
relationship (correlation coefficient of 0.998, Figure 7) was
found when plotting the computed barriers versus the energies
associated with the π(acetylene) → σ*(Au−P) interaction,
ΔE(ρ1), which, therefore, can be used as a reliable, quantitative
measure of the activating effect of the ligand on this Au(I)-
catalyzed hydroarylation reaction.
The existence of this linear correlation therefore indicates
that the activation barrier of this type of transformations can
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be easily predicted by simply computing the direct electronic
donation in the initially formed π-complex, even for hitherto
unknown ligands. Furthermore, from the data in Table 1, it
becomes clear that the LUMO values of these complexes follow
a similar trend to that of the ΔE(ρ1) values and, therefore, may
be used as well to assess the π-acceptor ability of these species,
as previously suggested.3b However, in view of the minor
anomalies in the LUMO values observed when comparing
ligands L4 and L6 (i.e., L6 leads to a slightly higher activation
barrier even though the corresponding LUMO is more
stabilized), the ΔE(ρ1) values seem to be a more accurate
parameter to quantify the effect of the ligand on the π-acceptor
ability of the complex.
Table 2. Computed Free Activation (ΔGa) and Reaction
(ΔGR) Energies (in kcal/mol) for the Gold(I)-Catalyzed
a
Hydroarylation Reaction Involving Ligands L1 and L6
To complete this study, we also explored the effect of the
group 15 coordinating atom present in the ligand L on π-acid
catalysis. Quite recently, Alcarazo and co-workers prepared a
series of α-cationic arsines, which were also found to increase
the activity of π-acid catalysts in strongly related transformations.14
Thus, a similar acceleration to that observed when using phosphines
was found in the Pt(II)-catalyzed cycloisomerization of enynes
(leading to trisubstituted cyclopropanes) when replacing a phenyl
group in the parent AsPh3 ligand by an α-cationic substituent.
In order to understand the effect of the experimentally pre-
pared arsenic ligands and make predictions about the influence
of heavier group 15 analogues on this type of transformations,
we focused again on the gold(I)-hydroarylation reaction
involving phenylacetylene and mesitylene considered above
by selecting the parent L1 (EPh3, E = P, As, Sb, and Bi) and
cationic L6 ligands (i.e., having the methylpyridium moiety,
Scheme 2).
a
Plain values refer to B3LYP/def2-SVP calculations, whereas values
within parentheses were computed at the PCM(dichloroethane)-
B3LYP-D3/def2-TZVP//B3LYP/def2-SVP level.
the group 15 element containing L6 ligands finds its origin also
in the stronger interactions between the deformed reactants
along the entire reaction coordinate. To this end, we selected
the hydroarylation reaction involving SbPh3 and its correspond-
ing pyridinium ligand L6. As readily seen in Figure 8, it is
Scheme 2. Gold(I)-Catalyzed Hydroarylation Reactions
Involving Group 15 Element Containing Ligands
Figure 8. Comparative activation strain diagrams for the gold(I)-
catalyzed hydroarylation reactions involving SbPh3 (L1, solid lines)
and cationic L6-Sb (dashed lines) as ligands along the reaction
coordinate projected onto the forming C···C bond distance. All data
have been computed at the PCM(dichloroethane)-B3LYP-D3/def2-
TZVP//B3LYP/def2-SVP level.
From the data gathered in Table 2, it is clear that the
involvement of a heavier group 15 atom in the ligand does not
alter the scenario described above. Thus, the processes with the
α-cationic ligand become kinetically more favorable and less
endergonic than the corresponding transformations with the
neutral EPh3 ligand regardless of the group 15 E atom. Once
again, it is found that the reactions involving the cationic L6
ligands proceed through earlier transition states (C···C forming
distances in the range of 2.174−2.199 Å) than those processes
involving the respective neutral ligands L1 (average C···C
forming distances of 2.032 Å, see Figure S1 in the Supporting
Information). Moreover, our calculations also predict that no
significant further acceleration will be expected when replacing
the phosphorus or arsenic atom by their heavier group 15
congeners in the reaction involving either neutral or cationic
ligands (the computed activation barrier differences, ΔΔGa, are
only ca. 1−2 kcal/mol; see Table 2).
confirmed that the strain energy is not at all responsible for
the computed different barrier energies since the ΔEstrain curves
for both processes are nearly identical along the entire reaction
coordinate. Once again, the stronger interaction between the
deformed reactants computed for the entire pathway involving
the cationic ligand constitutes the differential decisive factor
responsible for the acceleration of this transformation. As
expected, this stronger interaction can be ascribed to the much
higher π-acceptor ability of the corresponding initial gold(I)-
acetylene π-complex. This is again reflected in the much higher
NOCV-ΔE(ρ1) value (i.e., donation from the acetylene to the
gold(I) moiety) computed for the π-complex having the L6-Sb
ligand as compared to the neutral L1 (SbPh3) ligand (Figure 9).
We have also applied the ASM approach to confirm that the
lower activation barrier computed for the processes involving
E
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support was provided by the Spanish Ministerio de
́
Economia y Competitividad (MINECO) and FEDER (Projects
CTQ2013-44303-P and CTQ2014-51912-REDC). Y.G.-R.
acknowledges the MINECO for a FPI grant.
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■
20760.
From the computational study reported herein, the following
conclusions can be drawn: (i) the replacement of a phenyl
group in the EPh3 (E = group 15 atom) ligand by a α-cationic
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/acs.organomet.6b00859.
■
́
dez, I.; Cossío, F. P.;
dez,
S
́
́
́
Figure S1, Cartesian coordinates (in Å), and total free
energies (in au) of all the stationary points discussed in
the text (PDF)
́
́
Optimized structure (XYZ)
́
̀
́
AUTHOR INFORMATION
Corresponding Author
■
(13) (a) van Zeist, W.-J.; Bickelhaupt, F. M. Dalton Trans. 2011, 40,
3028−3038. (b) Wolters, L. P.; Bickelhaupt, F. M. ChemistryOpen
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*E-mail (I. Fernan
́
dez): israel@quim.ucm.es.
ORCID
́
J. Am. Chem. Soc. 2014, 136, 4575−4583. (d) Fernandez, I.; Wolters,
Israel Fernandez: 0000-0002-0186-9774
L. P.; Bickelhaupt, F. M. J. Comput. Chem. 2014, 35, 2140−2145.
́
F
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