The Pt-catalyzed ethylene hydroamination by aniline: a computational investigation of the catalytic cycle.

TL;DR: A full QM DFT study without system simplification and with the inclusion of solvation effects in anilines as solvent has addressed the addition of aniline to ethylene catalyzed by PtBr(2)/Br(-.

Abstract: A full QM DFT study without system simplification and with the inclusion of solvation effects in aniline as solvent has addressed the addition of aniline to ethylene catalyzed by PtBr2/Br−. The resting state of the catalytic cycle is the [PtBr3(C2H4)]− complex (II). A cycle involving aniline activation by N−H oxidative addition was found energetically prohibitive. The operating cycle involves ethylene activation followed by nucleophilic addition of aniline to the coordinated ethylene, intramolecular transfer of the ammonium proton to the metal center to generate a 5-coordinate (16-electron) PtIV−H intermediate, and final reductive elimination of the PhNHEt product. Several low-energy ethylene complexes, namely trans- and cis-PtBr2(C2H4)(PhNH2) (IV and V) and trans- and cis-PtBr2(C2H4)2 (VII and VIII) are susceptible to aniline nucleophilic addition to generate zwitterionic intermediates. However, only [PtBr3CH2CH2NH2Ph]− (IX) derived from PhNH2 addition to II is the productive intermediate. It easily tran...

Summary

Introduction

• Hydroamination, the direct formation of a new C-N bond by addition of an N-H bond across an unsaturated CC bond, currently attracts much interest in academia and industry.
• The intermolecular version of this process is still a great challenge, especially for non-activated olefins.
• The relative free energy of all species in solution under catalytic conditions has also been assessed from combined experimental and computational studies, showing that 2 is the most stable complex under catalytic conditions.

Computational Details

• All geometry optimizations were carried out by the DFT approach with the Gaussian03 suite of programs54 using the B3LYP functional.
• 55-57 In spite of a few problems pointed out for this functional,58 it was selected because it remains one of the most popular functionals used in transition metal computational chemistry.
• The geometry optimizations were carried out on isolated molecules in the gas phase.
• The approach of optimizing the geometries in the gas phase and then keeping the geometry frozen for the C-PCM calculation was preferred because of the known convergence problems of the geometry optimization in the presence of the C-PCM, especially for molecules containing weak interaction (i.e. hydrogen bonds).
• The size of the present computational investigation (~ 240 optimizations) imposed a too heavy burden under the more rigorous approach.

Results

• As is well appreciated, the appropriate description of the reaction energetics for a catalytic reaction must consider solvent effects.
• Views of all the optimized geometries are given in the Supporting Information , although the most relevant structures will also be shown within the main text.
• Since systems XX-XXII are formally 5-coordinate PtIV complexes, a reasonable question is whether they may be stabilized by addition of one of the several available ligands (Br-, PhNH2, C2H4) or by chelation of the aminoalkyl ligand to yield 18-electron octahedral products (E in Scheme 3).
• The energy of these systems is relatively low on the GCPCM scale, unless H and the two most strongly trans directing ligands H and alkyl are unfavorably located trans to each other (i.e. XXXVII, XXXIX, XLI and XLI’).
• Finally, pathway (b2) was checked for the lowest energy zwitterionic intermediate XIII, but also from the related system XI.

Discussion

• At the chosen level of theory, the computational investigation of the PtBr2/Br-–catalyzed aniline addition to ethylene rules out an amine activation pathway and validates the olefin activation pathway.
• Other 5-coordinate PtIV-H complexes, accessible from the other zwitterionic intermediates are located at higher energy and do not provide suitable pathways for product liberation.
• Of the two possible ways to liberate the hydroamination product, namely reductive elimination from a PtIV hydrido-aminoalkyl derivative and protonolysis of a PtII aminoalkyl derivative, the first one seems preferred for this catalytic cycle.
• This value appears in reasonable agreement with the experimental evidence.
• 32 Scheme 5. Competition between the Pt and C atoms in the Pt-C bond for proton capture.

Figures

Figure 8. Optimized geometries and relative GCPCM in kcal mol-1 at 298.15 K (and at 423.15 K in parentheses) for relevant structures along the C-H reductive elimination from XX.

Figure 10. Relevant geometries of transition states along C-protonation pathways, with their relative GCPCM in kcal mol-1 at 298.15 K (and at 423.15 K in parentheses). In brackets….

Figure 6. Optimized geometries and relative GCPCM in kcal mol-1 at 298.15 K (and at 423.15 K in parentheses) for the lowest energy aminoethyl complex, XXXII, and for the stationary points along the deprotonation of XIII by aniline. The values in brackets correspond to GCPCMcor (see text).

Figure 3. Optimized geometries and GCPCM in kcal mol-1 at 298.15 K (and at 423.15 K in parentheses) for systems XIII·PhNH2 (a and b). The values in brackets correspond to GCPCMcor (see text).

Figure 9. Optimized geometry of the reductive elimination transition states from systems XXV, XXV’ and XXXVI, with relative GCPCM in kcal mol-1 at 298.15 K (and at 423.15 K in parentheses). The values in brackets correspond to GCPCMcor (see text).

Figure 4. Optimized geometries and GCPCM in kcal mol-1 at 298.15 K (and at 423.15 K in parentheses) for the lowest energy 5-coordinate Pt(IV) hydride complex XX and for the transition state TS(IX-XX) leading to it.

Figure 11. Lowest-energy pathway for PtBr2/Br-–catalyzed addition of aniline to ethylene. Values are GCPCM relative to I in kcal mol-1 at 298.15 K (and at 423.15 K in parentheses)

Figure 7. Optimized geometries and relative GCPCM in kcal mol-1 at 298.15 K (and at 423.15 K in parentheses) for the lowest energy platina(IV)cyclic derivatives XXXVI and XLIII.

Figure 5. Optimized geometries and GCPCM in kcal mol-1 at 298.15 K (and at 423.15 K in parentheses) for 6-coordinate PtIV hydride complexes.

Full text

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The Pt-Catalyzed Ethylene Hydroamination by Aniline:
A Computational Investigation of the Catalytic Cycle
Pavel Dub, Rinaldo Poli
To cite this version:
Pavel Dub, Rinaldo Poli. The Pt-Catalyzed Ethylene Hydroamination by Aniline: A Computational
Investigation of the Catalytic Cycle. Journal of the American Chemical Society, American Chemical
Society, 2010, 132 (39), pp.13799-13812. �10.1021/ja1051654�. �hal-03177581�

1
The Pt-catalyzed ethylene hydroamination by aniline: a Computational
Investigation of the Catalytic Cycle
Pavel A. Dub
a
and Rinaldo Poli*
,a,b
a
CNRS; LCC (Laboratoire de Chimie de Coordination); Université de Toulouse; UPS, INP; F-
31077 Toulouse, France ; 205, route de Narbonne, F-31077 Toulouse, France; Fax: (+) 33-
561553003; E-mail: poli@lcc-toulouse.fr
b
Institut Universitaire de France, 103, bd Saint-Michel, 75005 Paris, France

2
Summary
A full QM DFT study without system simplification and with the inclusion of solvation effects in
aniline as solvent has addressed the addition of aniline to ethylene catalyzed by PtBr
2
/Br
-
. The
resting state of the catalytic cycle is the [PtBr
3
(C
2
H
4
)]
-
complex (II). A cycle involving aniline
activation by N-H oxidative addition was found energetically prohibitive. The operating cycle
involves ethylene activation followed by nucleophilic addition of aniline to the coordinated
ethylene, intramolecular transfer of the ammonium proton to the metal center to generate a 5-
coordinate (16- electron) Pt
IV
-H intermediate, and final reductive elimination of the PhNHEt
product. Several low energy ethylene complexes, namely trans- and cis-PtBr
2
(C
2
H
4
)(PhNH
2
)
(IV and V) and trans- and cis-PtBr
2
(C
2
H
4
)
2
(VII and VIII) are susceptible to aniline
nucleophilic addition to generate zwitterionic intermediates. However, only
[PtBr
3
CH
2
CH
2
NH
2
Ph]
-
(IX) derived from PhNH
2
addition to II is the productive intermediate. It
easily transfers a proton to the Pt atom to yield [PtHBr
3
(CH
2
CH
2
NHPh)]
-
(XX), which leads to
rate-determining C-H reductive elimination through transition state TS(XX-L) with formation of
the -complex [PtBr
3
(k
2
:C,H-HCH
2
CH
2
NHPh)]
-
(L), from which the product can be liberated
via ligand substitution by a new C
2
H
4
molecule to regenerate II. Saturated (18-electron) Pt
IV
hydride complexes obtained by ligand addition or by chelation of the aminoalkyl ligand liberate
the product through higher energy pathways. Other pathways starting from the zwitterionic
intermediates were also explored (intermolecular N deprotonation followed by C protonation or
chelation to produce platina(II)azacyclobutane derivatives; intramolecular proton transfer from N
to C, either direct or assisted by an external aniline molecule) but all gave higher-energy
intermediates or led to the same rate determining TS(XX-L).
Keywords: platinum, homogeneous catalysis, hydroamination, non-activated olefins, DFT
calculations

3
Introduction
Hydroamination, the direct formation of a new C-N bond by addition of an N-H bond
across an unsaturated CC bond, currently attracts much interest in academia and industry.
1-3
The
intermolecular version of this process is still a great challenge, especially for non-activated
olefins. Seminal work by Coulson showed that ethylene could be hydroaminated by a few highly
basic secondary amines under forcing conditions with RhCl
3
(or IrCl
3
) as catalyst.
4, 5
More
recently, this system was found effective also for less basic amines such as aniline when
modified by the addition of n-Bu
4
PI/I
2
.
6
Other relevant results for the intermolecular
hydroamination of ethylene and other non activated olefins comprise the use of lanthanides,
7, 8
Fe,
9
Ru,
10-12
Rh,
13
Ag,
14
Au,
15-18
Pd,
19, 20
and notably Pt.
21-23
Investigations initiated in our team
by J.-J. Brunet have shown that PtBr
2
, in the presence of nBu
4
PX (X = halide) as activator, is one
of the most performing catalyst so far reported for the hydroamination of ethylene by weakly
basic amines such as aniline and 2-chloroaniline (highest activity for X = Br; TON > 150 after
10 h at 150°C with 0.3 mol % of Pt- precursor).
24-27
Without a clear mechanistic understanding,
however, it is difficult to imagine how to further improve the process efficiency for its potential
application in bulk chemical manufacture.
Two alternative mechanisms are discussed in the literature, one starting with amine
activation by N-H oxidative addition and the other one based on amine nucleophilic addition to a
coordinated olefin. The amine activation mechanism is mostly proposed for Rh- or Ir-based
catalytic systems,
28, 29
whereas the olefin activation mechanism seems adopted by catalysts based
on group 10
30
and 11 metals.
31
Senn and coworkers reported a computational study of the model
NH
3
addition to ethylene catalyzed by the [MCl(PH
3
)(C
2
H
4
)]
z+
complexes of Group 9 (z = 0) and
10 (z = 1) metals.
32
For the group 10 metals, for which only the olefin activation pathway has
been explored, they found that the NH
3
nucleophilic addition is thermodynamically and
kinetically favourable and that the cleavage step is rate-determining (barrier of 34.9 kcal mol
-1

4
for Pt). On the other hand, Tsipis and Kefalidis, using the “Pt
0
” model complex Pt(C
2
H
4
)(PH
3
),
explored only the amine activation pathway, finding the reaction to be limited by the product
reductive elimination step from the Pt
II
amido hydrido intermediate (barrier of 39.7 kcal mol
-1
).
33
Other computational studies (e.g. on gold catalysis for diene hydroamination
31
or palladium
catalysis for the intermolecular hydroamination of vinylarenes
34
and for the asymmetric
intramolecular hydroamination of aminoalkenes
35
) have also explored solely the olefin activation
mechanism. To the best of our knowledge, with the exception of the above mentioned study by
Tsipis and Kefalidis and a study on iridium reported only in a Ph.D. thesis,
36
studies of the N-H
activation pathway have only been reported for alkaline-earths,
37
early transition metals
38-41
and
the lanthanides.
42-48
On the basis of known chemical transformations for related systems and on conventional
wisdom, the Brunet Pt-based system was proposed to follow the ethylene activation pathway as
shown in Scheme 1.
26
However, whether the proton transfer from N to C from the zwitterionic
intermediate occurs directly or via a Pt-hydride intermediate remained open to debate. The
proton transfer process was considered as more facile from the anionic tribromo species
[PtBr
3
(CH
2
CH
2
NH
2
Ph)]
-
because of the anticipated increased basicity, a hypothesis consistent
with the observed activity enhancement when using a moderate excess amount of bromide
salts.
24
+CH
2
=CH
2
-Br
-
+ArNH
2
-Br
-
[PtBr
4
]
2-
+ArNH
2
[(ArNH
2
)PtBr
2
CH
2
CH
2
-NH
2
Ar]
+ArNH
2
ArNHCH
2
CH
3
H
+Br
-
-ArNH
2
[PtBr
2
] + 2 Br
-
[PtBr
3
]
-
+ Br
-
[PtBr
3
(C
2
H
4
)]
-
[PtBr
3
CH
2
CH
2
-NH
2
Ar]
-
[PtBr
3
CH
2
CH
2
-NHAr]
-
[PtBr
2
(ArNH
2
)(C
2
H
4
)]

Frequently asked questions

Q1. What are the contributions in "The pt-catalyzed ethylene hydroamination by aniline: a computational investigation of the catalytic cycle" ?

In this paper, the Pt-catalyzed Ethylene Hydroamination by Aniline: A Computational Investigation of the Catalytic Cycle Pavel Dub, Rinaldo Poli Hydroamination, the direct formation of a new C-N bond by addition of an N-H bond across an unsaturated CC bond, currently attracts much interest in academia and industry.