Download A theoretical study on the mechanisms of intermolecular

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Chemistry
• ARTICLES •
September 2014 Vol.57 No.9: 1264–1275
doi: 10.1007/s11426-014-5102-2
A theoretical study on the mechanisms of intermolecular
hydroacylation of aldehyde catalyzed by neutral and
cationic rhodium complexes
WANG Min1, ZHANG Xin1*, CHEN Zhuo1, TANG YanHui2 & LEI Ming1*
1
State Key Laboratory of Chemical Resource Engineering, Institute of Materia Medica, College of Science, Beijing University of Chemical
Technology, Beijing 100029, China
2
College of Materials Science and Engineering, Beijing Institute of Fashion Technology, Beijing 100029, China
Received January 21, 2014; accepted February 7, 2014; published online June 6, 2014
In this paper, we used density functional theory (DFT) computations to study the mechanisms of the hydroacylation reaction of
an aldehyde with an alkene catalyzed by Wilkinson’s catalyst and an organic catalyst 2-amino-3-picoline in cationic and neutral systems. An aldehyde’s hydroacylation includes three stages: the C–H activation to form rhodium hydride (stage I), the alkene insertion into the Rh–H bond to give the Rh-alkyl complex (stage II), and the C–C bond formation (stage III). Possible
pathways for the hydroacylation originated from the trans and cis isomers of the catalytic cycle. In this paper, we discussed the
neutral and cationic pathways. The rate-determining step is the C–H activation step in neutral system but the reductive elimination step in the cationic system. Meanwhile, the alkyl group migration-phosphine ligand coordination pathway is more favorable than the phosphine ligand coordination-alkyl group migration pathway in the C–C formation stage. Furthermore, the
calculated results imply that an electron-withdrawing group may decrease the energy barrier of the C–H activation in the benzaldehyde hydroacylation.
C–H activation, DFT, hydroacylation, metal organic cooperative catalyst, reaction mechanism, rhodium complex
1 Introduction
C–H activation is very useful in synthetic methods [1, 2].
The hydroacylation of aldehyde attracted a lot of attention
in the past decades, because the C–H activation of aldehydes could result in some important ketones via this reaction [3, 4]. However, there is another competitive reaction,
the decarbonylation reaction during the hydroacylation catalyzed by traditional Wilkinson’s catalyst [5]. Some aldehydes, such as 8-quinolinecarboxaldehyde (Scheme 1) can
be used by intramolecular chelation-assisted method to
solve this problem, but this structure is too specific to popularize [6]. Some intramolecular hydroacylation reactions
*Corresponding authors (email: [email protected]; [email protected])
© Science China Press and Springer-Verlag Berlin Heidelberg 2014
can also avoid the decarbonylation process by means of the
alkene coordinating with the metal. The mechanism of the
intramolecular hydroacylation reaction has already been
investigated by both experimental and theoretical works
[7–10].
On the other hand, the development of the intermolecular
hydroacylation met with many challenges in avoiding the
decarbonylation process. Many attempts have been made to
prevent the undesired decarbonylation reaction [11, 12]. In
1988, Milstein’s group [13] discovered that the indenyl
complex [(5-C9H7)Rh(2-C2H4)2] was an active catalyst for
the intermolecular addition of simple aldehydes and ethylene
(Scheme 2). Many aldehyde intermolecular hydroacylation
reactions have been achieved using heteroatoms such as
phosphine [14], oxygen [15, 16], sulfur [17, 18], or nitrogen
atoms [6, 19], which act as a coordination site chelating
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with transition metals of catalysts to stabilize the metal-acyl
intermediate. Suggs [20] found that ketimines could be obtained by the reaction of 2-amino-picoline-derived aldimines and alkenes in the presence of (Ph3P)3RhCl, thus
hydroacylation provides a new ketone synthesis using readily available precursors. Jun et al. [21, 22] employed a new
concept catalyst containing a Wilkinson’s catalyst and an
organic co-catalyst 2-amino-3-picoline (Scheme 3), this
system was named metal-organic cooperative catalysis
(MOCC). It could effectively trigger many reactions such as
alcohol hydroacylation, oxo-ester synthesis, C–C triple
bond cleavage, and aldehyde or ketone hydroacylation [23].
In addition, this MOCC is easily recycled by immobilizing
both metal and organic components. Thus, MOCC avoids
the coordination of the carbonyl group of aldehyde or ketone substrates. This catalysis has been successfully applied
in many kinds of C–H and C–C activation. Recently, based
on MOCC, Breit et al. [24] described a new bifunctional
co-catalyst, 6-((diphenylphosphino)-methyl)-2-aminopicoline, which improved co-catalyst effiency relative to
2-aminopicoline in intermolecular hydroacylation reactions
of alkenes. The pyridine ring was bound to a phosphine
ligand as one ligand of metal center instead of the
standalone co-catalyst (2-amino-3-picoline). Inspired by the
works mentioned so far using MOCC strategy, Douglas et
al. [25] completed intramolecular hydroacylation of disubstituted alkenes to form seven- and six-membered rings
without requiring substrate-embedded chelating groups.
Understanding the mechanism of MOCC is indispensable
to develop catalyst of the hydroacylation of aldehyde and
ketone. The proposed mechanism of catalytic cycle of
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MOCC reactions is mainly divided into three steps: the oxidative addition of the aldimine, alkene insertion, and reductive elimination of the product [26–28]. Herein the catalytic cycle is proposed to involve three portions: the C–H
activation (stage I), the alkene insertion (stage II), and the
C–C formation (stage III). The reaction could proceed in
neutral or cationic systems, the pathway with chlorine ligand in the neutral system is named path A and that without
Cl ligand in the cationic system is named path B (Scheme 4).
2-Amino-3-picoline (1) firstly condenses with an aldehyde
Scheme 1
hyde.
The intramolecular hydroacylation of 8-quinolinecarboxalde-
Scheme 2 The intermolecular hydroacylation of simple aldehydes to
ethylene catalyzed by indenyl complex.
Scheme 3 The intermolecular hydroacylation of aldehydes catalyzed by a
Wilkinson’s catalyst and an organic catalyst 2-amino-3-picoline.
Scheme 4 Proposed catalytic mechanism of aldehyde hydroacylation under neutral and cationic conditions (neutral mechanism in red route, cationic
mechanism in blue route).
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(2) to form aldimine (3) in aldehyde hydroacylation, then
coordinating with Rh center of (PPh3)3RhCl (A) or
(PPh3)3Rh+ (B), 4A or 4B is obtained (4A means intermediate
4 in path A). The first step of the catalytic cycle is the C–H
activation step from 4A/4B to 6A/6B. The alkene coordinates
with the resulting acylrhodium (III) hydride species (6A/6B)
to form 7A/7B. Then the alkene gets inserted into the metalhydride (Rh–H) bond to give Rh-alkyl complexs (8A/8B).
Finally, they undergo a C–C formation step to get 11A/11B,
which finally affords ketimine (12). Then ketimine (12)
hydrolyzes to generate the product ketone, which is the inverse process of formation of aldimine (3). It was reported
by Wang et al. [29, 30] that the process of formation of aldimine (3) involves two steps, which are coupling of aldehyde with 2-amino-3-picoline to form hemiaminal and
hemiaminal dehydration to give aldimine. The mechanism
of C–H activation from 4A/4B to 6A/6B of the aldehyde hydroacylation have been discussed in both experimental and
theoretical studies [31, 32]. In the viewpoint of the theoretical study, Sim et al. [32] described the mechanism of the
C–H activation step and explained possible formation
pathways of the highly stable C–H activation product,
which could be isolated and examined by 1H NMR analysis.
Herein the theoretical calculated results indicate that the
pathway leading to the highly stable C–H activation product
may not be the most favorable pathway in the whole catalytic cycle of the aldehyde hydroacylation. A systemic theoretical study on the common characters of the reaction
mechanism will shed light on the role of components of
metal-organic cooperative catalysis. This DFT study mainly
focused on the details of the mechanism of the aldehyde
hydroacylation in the neutral and cationic systems. And
trans and cis isomers, which were defined according the
relative position between Cl and PH3 ligand, were discussed
in pathways. In order to clarify the electronic effect of aldehyde substrate, the C–H activation step of benzaldehyde
involving different substituent groups in para-position was
also investigated.
2 Computational method
All of calculations were studied at M06/BSI level with
Gaussian 09 program package [33, 34]. BSI was denoted as
LANL2DZ basis set for Rh center and 6-31+G(d,p) basis set
for all the other atoms [35, 36]. The recently developed
M06 functional was chosen, because it includes noncovalent
interactions and can give accurate energies for organotransition-metal systems on the basis of validation and other
studies [37, 38]. The basis sets are reliable and were used in
our previous works [39–41]. In this study, a simplified
model (the triphenylphosphine ligand PPh3 was replaced
with PH3) was used in all pathways (path A-cis, path A-cis′,
path A-trans, path A-trans″, path B-cis, and path B-trans).
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Although –PPh3 is a better electron donor than –PH3, the
simplified model also can give reliable results in organometallic system and it was verified in our previous works
[40, 42]. All the transition states (TSs) were further confirmed by vibrational analysis and characterized by only one
imaginary frequency. Intrinsic reaction coordinate (IRC)
calculations along the whole pathway were performed in
order to unveil more details around transition states and
corresponding intermediates. Atomic polar tensor (APT)
charge was presented describing the charge population [43].
Solvent effects (toluene) were evaluated using polarizable
continuum model (PCM) performing single-point calculations on gas phase optimized geometries [41]. All potential
energies were shown with zero-point vibrational energy
correction (ZPE). Gibbs free energies were calculated at
298.15 K, which were used in the discussion of the mechanistic details unless otherwise stated. All relative energies of
stationary points along the reaction pathway are relative to
3+catalyst if there are no other special descriptions. The
computed entropy contribution for a bimolecular reaction in
a solvent is sometimes overestimated by the procedure used
here [45, 46]. Moreover, Yu et al. [47, 48] have demonstrated that the ideal gas model could overestimate entropic
contribution by 50%–60% in their cyclization reactions. On
the basis of the experimental results, Wang et al. [29] reported a general method to correct the free energy that uses
a scaling factor of 0.5 to the gas phase entropic contributions as a rough estimate. In this paper, we also applied
Wang’s method to correct the free energy profiles. The energies for all stationary points, corrected free energies using
0.5 scaling factor, calculated imaginary frequencies of TSs
and The APT charge of intermediate 4 are listed in Table S1,
S2, S3 and S4 (see the Supporting Information online), respectively.
3
3.1
Results and discussion
Mechanism of neutral system
The cis-trans isomerization of stationary points are found in
experimental studies [31]. Four pathways, path A-cis (denotes the cis pathway of path A), path A-trans (denotes the
trans pathway of path A), path B-cis (denotes the cis pathway of path B), and path B-trans (denotes the trans pathway of path B), are discussed in this part. The optimized
structures of the catalytic cycle of the aldehyde hydroacylation in the neutral system are shown in Figure 1. The atomic
labels of stationary points are shown in Scheme 4. In the
catalytic cycle of aldehyde hydroacylation, square-planar
intermediate 4Acis was generated first with the coordination
of aldimine (3) to the catalytic species A ((PH3)3RhCl). The
C–H activation step owns a barrier of 17.3 kcal/mol (Figure
1), the C1–H1 bonds in intermediate 4Acis (4Acis denotes intermediate 4 the cis pathway of path A) are activated by Rh
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Figure 1 Optimized structures and energy data (free energy, potential energy in parenthesis, unit: kcal/mol) of the catalytic cycle of the aldehyde hydroacylation in neutral system at M06/BSI level. All distances are in angstroms and angle in degree (energies are relative to 3+A).
center via TS4Acis-6Acis to generate intermediate 6Acis in path
A-cis. In this process, Rh–H1 bond length decreased to
1.572 Å in 6Acis from 2.784 Å in 4Acis, and C1–H1 bond
length (1.097 Å) of 4Acis is elongated to 2.488 Å in 6Acis,
which means that the C1–H1 single bond was broken and
Rh–H1 bond was formed. Similarly, the distance between
Rh and C1 was shortened by 0.887 Å from 2.897 Å in 4Acis
to 2.010 Å in 6Acis, which implies that Rh–C1 bond was
formed gradually in this step. Notably, due to the transfer
transition of H1, the equatorial PH3 ligand was oriented to
the opposite position of C1, since the angle between the
phosphine ligand and C1 increased from 95.3° (4Acis) to
171.6° (6Acis). As a result, the saturated six-coordinate
(C1–H–Cl–N2–P–P) octahedron product (6Acis) was generated. The C–H activation was endothermic by 9.1 kcal/mol.
Before the alkene insertion step, one phosphine ligand dissociates from Rh center of 6Acis at first. Then the C=C bond
of tert-butylethylene was parallel to the Rh–H1 bond in the
axial position. The coordination of tert-butylethylene to Rh
center had no obvious effect on the bond lengths of Rh–H1
and Rh–C1 in 7Acis and the distance between Rh and alkene
in 7Acis was 2.692 Å. The alkene coordination step was endothermic by 11.2 kcal/mol. Then the alkene inserted into
the Rh–H bond to generate an unsaturated Rh-alkyl complex with a four-membered ring transition state
(TS7Acis-8Acis), which owned a barrier of 13.3 kcal/mol. In
this step, the axial hydride bent towards the incoming CH2
group to expedite insertion. Simultaneously, C2 turned from
the C1 atom’s opposite position to its neighbour position to
generate 8Acis with a trigonal bipyramidal structure. The
Rh–H1 distance in 8Acis increased to 3.182 Å, meanwhile the
C3–H1 distance decreased to 1.102 Å. And the Rh–C2 distance decreased to 2.116 Å. It indicates that H1 is transferred from Rh center to C3 and Rh–C2 bond was formed in
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the alkene insertion step. While the C1–Rh–C2 angle decreased to 80.8° and the C1–Rh–Cl angle increased to
146.1°. The alkene insertion step was exothermic by 13.5
kcal/mol.
Following the generation of the Rh-alkyl intermediates,
the C–C formation step involved the transfer of C2 of alkyl
group from Rh center to C1 and the PH3 ligand coordinates
with Rh center. Two possible C–C formation pathways are
discussed here. In one pathway, C2 first transfered from Rh
to C1 (8Acis → TS8Acis-9Acis → 9Acis) and then PH3 coordinated with Rh center (9Acis →11Acis), which is called as
the alkyl group migration-phosphine ligand coordination
pathway. In this pathway, the C2 of alkyl ligand bent towards the C1 atom to expedite alkyl group transfer. The
Rh–C2 and Rh–C1 distances increased to 2.376 and 2.308 Å
in 9Acis and the C1–C2 distance decreased to 1.516 Å. This
demonstrates that the C1–C2 bond was formed. Interestingly,
there was a weak interaction between Rh and the hydrogen
of transferred alkyl group (so-called  agostic interaction) in
9Acis with a distorted square-planer structure. With the following coordination of PH3, Rh–C1 bond was broken and
11Acis was formed with a four-coordinate square-planer
structure. In another pathway, PH3 coordinated with Rh
center first (8Acis → 10Acis) and then the alkyl group was
migrated from Rh to C1 (10Acis → TS10Acis-11Acis →
11Acis), which is called as the phosphine ligand coordination-alkyl group migration pathway. A saturated octahedral
intermediate (10Acis) was generated with the coordination of
PH3. In the following alkyl group migration process, the C2
of alkyl ligand bent toward the C1 atom to expedite transfer
and finished the C–C formation step (11Acis).
In the C–C formation step, the energy barrier for the
pathway of the alkyl group migration-phosphine ligand coordination pathway was 14.3 kcal/mol, but was 23.1
kcal/mol for another pathway of the phosphine ligand coordination-alkyl group migration. The PH3 coordination processes were exothermic by 15.8 and 2.0 kcal/mol, respec-
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tively. The former pathway was 13.8 kcal/mol lower than
the latter one, which indicates that the alkyl group migration-phosphine ligand coordination pathway is the more
favorable C–C formation path. Finally, A was regenerated
with the elimination of the ketimine (12).
The energy profiles in the catalytic cycles of aldehyde
hydroacylation in the neutral system are shown in Figure 2.
In the whole catalytic cycle of path A-cis, the energy barriers of the C–H activation step, the alkene insertion step and
the C–C formation step were 17.3, 13.3, and 14.3 kcal/mol,
respectively. It is obvious that the C–H activation step is the
rate-determining step in path A-cis. To evaluate the solvent
effect for toluene, single-point computations were performed at the M06/BSI level using PCM model. The solvation energies and the potential energies are listed in Figure 2
(a is potential energy, and b is solvation energy. The solvation energies and potential energy do not change the trends
presented in the gas-phase studies above.
3.2
Conformers in the neutral system
Each step of the catalytic cycle was investigated by examining all possible and reasonable isomers and conformers of
the various intermediates and transition states (TSs). In the
following sections, the results for each elementary step of
the catalytic cycle in the neutral system, which may generate the conformers, are presented. For the neutral system
(path A), as seen in Figures 1 and 3, intermediate 4A generated by the oxidative addition of aldimine (3) and catalyst
((PH3)3RhCl) (A) have three possible isomers: 4Acis with the
Cl ligand on the opposite side of H1 trans to the PH3 ligand,
4Acis′ with the Cl ligand on the same side of H1 trans to the
PH3 ligand or 4Atrans with the PH3 ligand trans to the PH3
ligand. Similar to 4 A cis , 4 A cis′ and 4 A trans are with a
square-planer structure. The difference in the C1–H1 distance change among three isomers is no more than 0.002 Å
and the Rh–H1 distances are around 2.8 Å, which indicates
Figure 2 Calculated free energies (kcal/mol) and structures for aldehyde hydroacylation in the neutral system at M06/BSI level (energies are relative to
3+A; a is potential energy, and b is solvation energy).
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Figure 3 Optimized structures and energy data (free energy, potential energy in parenthesis, unit: kcal/mol) for the possible isomeric species involved in
the C–H activation step of aldehyde hydroacylation in neutral system. All distances are in angstroms and angle in degree (energies are relative to 3+A).
that the Rh center has a weak effect on the C1–H1 moiety.
Figures 1 and 3 also show the optimized geometries of transition states of the C–H activation (TS4Acis-6Acis,
TS4Acis′-6Acis′, TS4Atrans-6Atrans, TS4Atrans-6Acis″) and their
corresponding products (6Acis, 6Acis′, 6Atrans, 6Acis″), the
neighbouring equatorial in-plane ligands (PH3 in 4Acis and
4Atrans, Cl in 4Acis′) on the same side of C1–H1 bone moved
towards the opposite position of C1 atom with the H1 migration from C1 to Rh. But in the process of path A-trans, the
trans C–H activation (4Atrans→6Atrans), the axial Cl ligand in
4Atrans shifted from the N1 opposite position towards C1 opposite position in the equatorial plane, the pyridine ring was
distorted with the H1 migration along equatorial vertical
direction simultaneously. In this C–H activation step, H1
could be migrated from C1 to Rh center along with the split
of C1–H1 and the formation of Rh–H1. All isomers of 6A
adopted a saturated octahedral structure.
Figure 2 presents the energy profiles of path A-cis, path
A-cis′, path A-trans and path A-trans″. Although a strong
energetic preference of ca. 4.3 kcal/mol was found for the
trans isomer 4Acis′ over the cis isomer 4Acis and 4Atrans, the
C–H activation barriers for four pathways was 17.3, 18.9,
26.2, and 18.3 kcal/mol, respectively. Based on this theoretical study, path A-cis, path A-cis′, and path A-trans″ are
more favorable than path A-trans in the C–H activation step.
In transition state TS4Atrans-6Atrans, it was broken for the
Rh–N2 bond with a destabilizing coordination mode due to
H1 migration in the equatorial vertical plane, which was not
presented in the other transition states. However, the whole
catalytic cycle along path A-trans was still studied for
comparison with path A-cis. As mentioned previously, the
C–H activation step was the rate-determining step of aldehyde hydroacylation for path A-cis.
For path A-trans (purple route in Figure 2), in the alkene
coordination, the PH3 ligand in the equatorial position was
dissociated from Rh center as the 6Atrans was a saturated
rhodium octahedral complex. At the same time, the alkene
coordinated with Rh center in the equatorial position. The
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saturated Rh-alkene product 7Atrans adopted a saturated octahedral geometry. The coordination of ethylene induced a
slight lengthening of the C=C bond from 1.332 Å in free
tert-butylethylene to 1.378 Å for 7Atrans. Furthermore, the
coordination of tert-butylethylene to Rh center induced a
slight shortening of the Rh–H1 bond by 0.007 Å. In the following alkene insertion step, the transition state
TS7Atrans-8Atrans with the energy barrier of 2.0 kcal/mol also
underwent a four-membered ring structure similar to
TS7Acis-8Acis. An energy barrier preference in the alkene
insertion step, of 11.3 kcal/mol, was found for the path
A-trans over the path A-cis because in the path A-cis
Rh–H1 bond was parallel to the tert-butylethylene C=C
bond. Cl moved from C1 opposite position in the equatorial
plane to N2 opposite position in the axial position.
For the C–C formation step, two different mechanisms
were found in the path A-trans similar to path A-cis. In one
mechanism, C2 of alkyl group first transfered from Rh to C1
(8Atrans → TS8Atrans-9Atrans → 9Atrans) and then PH3 coordinated with Rh center (9Atrans →11Atrans). In the other
mechanism, PH3 coordinated with Rh center first (8Atrans →
10Atrans) and then the alkyl group migrated from Rh to C1
(10Atrans → TS10Atrans-11Atrans → 11Atrans). The energy
barrier in the first mechanism was 3.8 kcal/mol lower than
the second mechanism in path A-trans, which indicates that
the alkyl group migration-phosphine ligand coordination
pathway is the more favorable C–C formation pathway.
3.3 Mechanism of the cationic system
Herein the aldehyde hydroacylation reaction in the cationic
system was also investigated (path B). The cationic mecha-
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nism of hydroacylation catalyzed individual metal catalysts
were already being studied in both intramolecular and intermolecular hydroacylation [7]. Castillón et al. [31] reported the intermolecular hydroacylation of alkenes with
aldimine (3) in cationic the rhodium system using in situ
NMR techniques. Figure 4 plots the energy profiles and
related structures along path B. The cis pathway of the cationic system (path B-cis, blue route in Figure 4) was calculated and compared with path A-cis. The cationic catalytic
species (B) could be produced from the neutral one (A) with
the dissociation of Cl ligand, which was trapped by the
AgBF4 in the experiment [31]. Herein the reaction was calculated to be 0.4 kcal/mol for (PPh3)3RhCl + AgBF4 →
[Rh(PPh3)3]BF4 + AgCl. The ionic compound [Rh(PPh3)3]+
BF4 was easily dissociated in the solvent, so the conversion
between A and B should be feasible if AgBF4 was added.
And the nonpolar solvent (toluene) was considered for solvation energy correction in this study. For the C–H activation
step of aldehyde hydroacylation in path B-cis, the rhodium
squared-planar intermediate 4Bcis and its isomer 5Bcis, 6Bcis,
and the transition state (TS5Bcis-6Bcis) were determined. Differening from the squared-planar complex 4Acis, 4Bcis owned
a four-coordinate structure but two N atoms of pyridine ring
(N1 and N2) coordinated with the Rh center. The Rh–C1
distance in 4Bcis (3.264 Å) was longer than that in 4Acis
(2.897 Å). And the Rh–H1 distance in 4Bcis was 4.133 Å,
which is longer than that in 4Acis (2.784 Å). The distance
between Rh and N1 in 4Bcis (2.197 Å) was shorter than that
in 4Acis (3.171 Å). Therefore, after the decoordination and
rotation around the C1–N1 bond in 4Bcis, another isomeric intermediate 5Bcis was characterized in which C1–H1 bond was in
close proximity to the Rh center. The distances of Rh–C1
Figure 4 Calculated relative free energies (kcal/mol) and structures for aldehyde hydroacylation in the cationic system at M06/BSI level (energies are
relative to 3+B; a is potential energy, and b is solvation energy).
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and Rh–H1 bond was reduced to 2.418 and 1.989 Å, respectively and the Rh–N1 distance increased to 2.985 Å in 5Bcis.
This implies that the Rh–N1 bond was broken and there was
an agostic interaction between the Rh center and the H1 atom. Meanwhile, in proceeding from the intermediate 5Bcis to
6Bcis, the PH3 ligand trans to N1 rotated around the Rh center with H1 migration from C1 to Rh and 6Bcis was produced
with a five-coordinate (Cl, N2, P, P and H) and a trigonal
bipyramid structure. The Rh–C1 and Rh–H1 bond lengths in
intermediate 6Bcis were 1.572 and 1.582 Å, respectively. The
Rh–C1 and Rh–H1 bonds were formed in 6Bcis. The C–H
activation in the path B-cis was predicted to be endothermic
by 14.0 kcal/mol.
In the alkene coordination step, tert-butylethylene occupied the equatorial plane and the C=C double bond was
parallel to the Rh–H1 bond, then coordinated with Rh center
forming 7Bcis. As a result, the Rh–H1 bond lengths had no
obvious changes and the average distance between Rh and
alkene in 7Bcis (2.577 Å) were shorter than that of 7Acis
(2.692 Å). The alkene insertion step also underwent a transition state TS7Bcis-8Bcis with a four-membered ring structure
slimilar to that of path A-cis. The distance of Rh–H1 in 8Bcis
increased to 3.323 Å, and the distance of C3–H1 and Rh–C2
in 8Bcis decreased to 1.103 and 2.104 Å, respectively. This
indicates that H1 was transfered to C3 from the rhodium
center and Rh–C2 bond was formed in the alkene insertion
step. Similar to the path A-cis, in this process C2 turned
from the C1 atom’s opposite position to its neighbour position to generate the advantage trigonal bipyramidal intermediate (8Bcis). The C1–Rh–C2 angle decreased to 96.4°
(8Bcis). This step proceeded overcoming an energy barrier of
9.3 kcal/mol.
In the C–C formation step, two different mechanisms for
path B-cis were also discussed. The route of the alkyl group
migration-phosphine ligand coordination pathway (8Bcis →
TS8Bcis-9Bcis → 9Bcis →11Bcis) owned an energy barrier of
12.6 kcal/mol, which was more favorable than that of the
phosphine ligand coordination-alkyl group migration pathway (8Bcis → 10Bcis → TS10Bcis-11Bcis → 11Bcis, 19.7
kcal/mol). The C–C formation product (11Bcis) adopted a
square-planer geometry. The results showed that the energy
barrier of the C–H activation in the path B-cis was 16.1
kcal/mol (Figure 4), which was almost the same with that of
path A-cis. But the energy barrier of the reductive elimination of production was 25.3 kcal/mol, and it needed at least
25.3 kcal/mol to achieve the reaction.
Therefore, in the viewpoint of this theoretical study, the
preference of one mechanism over another depends on the
experimental conditions, which similar to Heck reaction,
also has neutral and cationic mechanisms. This is strongly
related to the conversion of two possible catalytic species
(B in the cationic system, Rh(PH3)3+ and A in the neutral
system, RhCl(PH3)3). If there is no salt such as AgBF4 in the
present case, the reaction may follow the neutral mechanism.
If salt such AgBF4 is added, the conversion between A and
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B is feasible and the reaction may undergo the cationic
mechanism. Meanwhile, the reaction may also have two
competitive pathways without changing the experimental
condition. The cationic mechanism can start from the intermediate 6. The dissociation of a PH3 ligand leads to the
neutral mechanism, and the dissociation of Cl leads to the
cationic mechanism. This also depends on the degree of
difficulty of the dissociation of Cl from 6.
3.4
Conformers of the cationic system
As seen in Figure 5, in the cationic system the product of
the oxidative addition of aldimine (3) and catalyst active
species ((PH3)3Rh+) (B), 4B, have two possible isomers: 4Bcis
with two adjacent PH3 ligand in the plane, or 4Btrans with
PH3 ligand trans to the other PH3 ligand. Both isomers own
a square-planer structure. Interestingly, the Rh planer complex in the intermediate 4Btrans is a destabilizing threecoordinate planer compound, instead of the four-coordinate
configuration in 4Bcis. Hence, the 21.5 kcal/mol preference
was found for the four-coordinate adduct (isomer 4Bcis) over
the three-coordinate adduct (isomer 4Btrans), which showed
that the isomer 4Btrans is extremely unstable. Similar to the
path A-trans, the product (6Btrans) of trans C–H activation
was generated from oxidation adduct (4Btrans) through the
transition state TS4Btrans-6Btrans. The pyridine ring was distorted due to the shift of H1 in the axial position, this geometric structure of transition state was unstable (Figure 5).
The energy barrier of 26.1 kcal/mol was predicted for path
B-trans, which was 10.0 kcal/mol higher than the energy
barrier of path B-cis. The saturated C–H activation adduct
(6Btrans) with a relative energy of 1.7 kcal/mol adopted a
trigonal bipyramidal geometry.
In the alkene coordination step, the C=C double bond
was parallel to the Rh–H1 bond in the intermediate 7Btrans.
Then the H1 atom migrated from Rh center to the C2 atom
and 8Btrans was generated via TS7Btrans- 8Btrans with a barrier
of 14.2 kcal/mol. In the C–C formation step, the alkyl group
in the axial position bent towards the C1 atom. The alkyl
group migration from the axial position was more difficult
than that in the equatorial plane. The barrier of the C–C
formation step was 55.0 kcal/mol, which was 42.4 kcal/mol
higher than that of path B-cis. Thus, the C–C formation step
for path B-trans was difficult compared to that of path B-cis.
6Btrans was used to be dissolved in CD2Cl2 in the experiment
[31]. Herein a comparative study on path B-cis and path
B-trans could explain that only 6Btrans and 7Btrans were observed but the final product was not found by the experiment.
3.5
The conversion of trans and cis intermediates
For the neutral system of the aldehyde hydroacylation, it is
very interesting that only the trans intermediate (6Atrans) was
isolated and examined by 1H NMR analysis [31]. According
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September (2014) Vol.57 No.9
Figure 5 Predicted structural and energetic data (free energy, potential energy in parenthesis, unit: kcal/mol) for the species involved in the C-H activation
step of aldehyde hydroacylation in cationic system. All distances are in angstroms, and angles in degrees (energies are relative to 3+B).
to the energy profile (Figure 2), 6Atrans was 16.7 kcal/mol
and lower than the other isomers of 6A. This may account
for the observation of 6Atrans in the NMR experiment. It
could be understood that the lowest energy of 6Atrans was
due to the trans effect of ligands compared with other cis 6A.
Sim et al. [32] proposed that 6Atrans could be converted from
the cis intermediates (6Acis, 6Acis′, 6Acis″) by subsequent ligand migration steps. However, this conversion between
trans and cis intermediates 6A should be difficult this way
because the energy barriers of the subsequent ligand migration steps were higher than the C–H activation step. On the
other hand, the cis intermediates 6A like 6Acis could generate
4Acis in the inverse process of C–H activation step, then
transforms into 4Atrans via 3+A and path A-trans (6Acis →
4Acis → 3+A → 4Atrans → 6Atrans). It is not a practical
way because of a high energy barrier (26.2 kcal/mol) from
4Atrans to 6Atrans in path A-trans compared with path A-cis.
Therefore, the most possible way leading to 6Atrans observed
by experiment is that of 8Acis from 6Acis, which could
isomerize into 8Atrans via a Berry pseudorotation (6Acis →
8Acis → 8Atrans → 6Atrans). As shown in Figure 6, the intermediates 8_1Acis, 8_1Atrans and corresponding transition
states (TSs) were located in this study. In this process, the
square pyramidal (SPy) intermediate 8_1Acis was generated
from the cis isomer 8Acis first, then the trigonal bipyramidal
(TBP) intermediate 8_1Atrans could be achieved via
TS8_1Acis-8_1Atrans. The energy barrier from 8Acis to 8Atrans
was 11.4 kcal/mol. This accounts for the existence of 6Atrans
in the experiment.
Figure 6 Calculated free energies (unit: kcal/mol) and structures for the
isomer conversion from 8Acis to 8Atrans at M06/BSI level (energies are relative to 3+A; a is potential energy, and b is solvation energy).
Meanwhile, the isomerization between 6Bcis and 6Btrans
was also investigated for the cationic system of the aldehyde hydroacylation. As shown in Figure 7, the intermediate 6_1B was formed from the cis isomer 6Bcis via
TS6Bcis-6_1B with an energy barrier of 6.3 kcal/mol. The
intermediate 6_1B owned a SPy structure because of the
rotation of PH3 ligand. By means of the rotation of hydride
ligand and two PH3 ligands of 6_1B, the trans isomer 6Btrans
was achieved via TS6_1B-6Btrans with an energy barrier of
21.7 kcal/mol. Therefore, the trans isomer 6Btrans could be
obtained from the cis isomer 6Bcis, but it was difficult for the
isomerization in cationic system compared with that of the
neutral system.
Wang M, et al.
Sci China Chem
September (2014) Vol.57 No.9
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Figure 7 Calculated free energies (unit: kcal/mol) and structures for the
isomer conversion from 6Bcis to 6Btrans at M06/BSI level (energies are relative to 3+B; a is potential energy, and b is solvation energy).
3.6
Ligand effect in aldehyde hydroacylation
In order to clarify the electronic effect of substrate aldehyde
in this reaction, the substituent group in the para-position of
benzaldehyde was changed to nitro, trifluoromethyl, chlorine, hydrogen, hydroxyl, methoxyl, amino groups (R =
–NO2, –CF3, –Cl, –H, –OH, –OCH3 and –NH2, corresponding systems are abbreviated in A1, A2, A3, A4, A5, A6 and
A7). Only the C–H activation step in the neutral system in
path A-cis was considered and the potential energies are
reported in this part. The calculated results demonstrate that,
if the benzaldehyde links the electron-withdrawing groups
such as –NO2, –CF3 and –Cl, the potential energy barrier of
C–H activation becomes lower (Figure 8). On the contrary,
if the benzaldehyde with the electron-donating groups such
as –OH, –OCH3 and –NH2, the energy barrier becomes
higher. A1 owns the lowest potential energy barrier in the
C–H actiation step among the seven systems, which is 1.5
kcal/mol lower than that of the A7 system. Meanwhile, the
R substituent group also may affect the APT charges of related atoms. Figure 9 shows the correlation between the
APT charge of C1 in complex 4 and the energy barriers of
C–H activation. The results indicate that the more positive
charge C1 has, the higher the barrier is. In other words, an
increase in the basicity of C1 in complex 4 will make the
hydrogen transfer easier in the C–H activation step of path
A-cis. Although the difference in the energy barrer of C–H
activation is not too large, it is meaningful that the electron-withdrawing group may decrease the energy barrier in
the C–H activation step of benzaldehyde hydroacylation.
4 Conclusions
In summary, the mechanism of aldehyde hydroacylation in
both the neutral and cationic systems was studied by DFT
method in this work. The calculated results indicate that the
rate-determining step in the aldehyde hydroacylation is the
C–H activation step (4 to 6 via TS4-6) in the neutral system
but the reductive elimination step in the cationic system.
Figure 8 The potential energy profile (unit: kcal/mol) of C–H bond activation of different substituent group complexes in path A-cis (the relative
energy is relative to corresponding 4Acis).
Figure 9 Correlation between the atomic charge in complex 4 and the
potential energy barrier of C–H activation. R means coefficient of linear fit
regression.
The pathways originating from the cis and trans isomers of
the oxidative addition product (4Acis′ and 4Atrans), path A-cis′,
path A-trans and path A-trans″ were also discussed. For the
neutral system, the preferred pathway, path A-cis, was
found to originate from the cis isomer (4Acis) of the oxidative addition prduct compared with path A-trans. In the
C–C formation process, an energetic preference was found
for the alkyl group migration-phosphine ligand coordination
mechanism, instead of the phosphine ligand coordinationalkyl group migration mechanism. For the cationic system,
the preferred pathway for the catalytic cycle, path B-cis,
was found to originate from the cis isomer (4Bcis). The alternative pathway originating from the trans isomer of the
oxidative addition prduction (4Btrans), path B-trans was
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found to be less favorable due to the instability of the trans
isomer 4Btrans and significantly higher barriers for the C–H
activation step. Similarly, in the C–C formation step, an
energetic preference was found for the alkyl group migration-phosphine ligand coordination pathway, instead of the
phosphine ligand coordination-alkyl group migration pathway. Furthermore, electronic effects in the C–H activation
step of benzaldehyde are investigated. These results demonstrate that electronic effects play an important role in the
C–H activation step, and that electron-withdrawing group
such as –NO2, –CF3 decrease the energy barriers of benzaldehyde hydroacylation. Meanwhile, the preference of one
mechanism over another depends on the experimental conditions [49]. The reaction may follow the neutral mechanism if no salt is added and the reaction may follow the
cationic mechanism if a salt such as AgBF4 is added.
This work was supported by the National Natural Science Foundation of
China (21373023, 21203006, 21072018).
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