Cationic acetylacetonate palladium complexes/boron trifluoride etherate catalyst systems for hydroamination of vinylarenes using arylamines

Article abstract of DOI:10.1016/j.catcom.2017.02.004

A simple and solvent-free protocol using cationic acetylacetonate palladium complexes with mono-/bidentate phosphine ligands activated with BF₃·OEt₂as in situ-formed catalyst for hydroamination of vinylarenes with arylamines have bee

Full text of DOI:10.1016/j.catcom.2017.02.004

Accepted Manuscript
Cationic acetylacetonate palladium complexes/boron trifluoride
etherate catalyst systems for hydroamination of vinylarenes using
arylamines
D.S. Suslov, M.V. Bykov, M.V. Pakhomova, P.A. Abramov, I.A.
Ushakov, V.S. Tkach
PII:
DOI:
Reference:
S1566-7367(17)30049-3
doi: 10.1016/j.catcom.2017.02.004
CATCOM 4932
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
23 December 2016
3 February 2017
4 February 2017
Please cite this article as: D.S. Suslov, M.V. Bykov, M.V. Pakhomova, P.A. Abramov, I.A.
Ushakov, V.S. Tkach , Cationic acetylacetonate palladium complexes/boron trifluoride
etherate catalyst systems for hydroamination of vinylarenes using arylamines. The address
for the corresponding author was captured as affiliation for all authors. Please check if
appropriate. Catcom(2017), doi: 10.1016/j.catcom.2017.02.004
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ACCEPTED MANUSCRIPT
Cationic Acetylacetonate Palladium Complexes/Boron Trifluoride Etherate
Catalyst Systems for Hydroamination of Vinylarenes Using Arylamines
D.S. Suslova,†, M.V. Bykova, M.V. Pakhomovaa, P.A. Abramovb,c, I.A. Ushakovd, V.S. Tkacha
aIrkutsk State University, ul. K.Marksa, 1, Irkutsk, Russia, 664003
bNikolaev Institute of Inorganic Chemistry SB RAS, pr-kt Akad. Lavrentieva, 3,
Novosibirsk, Russia, 630090
cNovosibirsk State University,ul. Pirogova, 2, Novosibirsk, Russia, 630090
dNational Research Irkutsk State Technical University, Lermontov St, 83, Irkutsk,
Russia, 664074
†Correspondence
should
be
addressed:
e-mail:
suslov@chem.isu.ru;
suslov.dmitry@gmail.com.

ACCEPTED MANUSCRIPT
Abstract
A simple and solvent-free protocol using cationic acetylacetonate palladium
complexes with mono-/bidentate phosphine ligands activated with BF3·OEt2 as in situ-
formed catalyst for hydroamination of vinylarenes with arylamines have been developed.
Excellent catalytic activities were obtained using [Pd(acac)(PPh3)2][BF4]/BF3·OEt2/i-PrOH
catalyst system with the addition of a small amount of palladium (0.2-0.05 mol%) to the
reaction mixture. Furthermore, the novel unexpected diphosphine-bridged palladium
complexes have been prepared and characterized: [Pd(acac)(dpppt)]2[BF4]2 and
[Pd(acac)(dpph)]2[BF4]2 (dpppt – 1,5-bis(diphenylphosphino)pentane, dpph – 1,6-
bis(diphenylphosphino)hexane)
Keywords
Palladium; acetylacetonate; aniline; hydroamination; styrene

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Introduction
Transition-metal-catalyzed hydroamination of olefins is an efficient route to alkyl-
and benzylamines from readily available reagents [1–3]. Compared to the significant
progress in the intramolecular hydroamination of alkenes by rare earth metals, group 4
metals, and main-group metal complexes, the intermolecular hydroamination reactions are
less explored [1,4]. One promising approach to the hydroamination of olefins, reported by
Hartwig and coworkers [5–7], is the addition of anilines to vinylarenes catalyzed by a
combination of Pd(OCOCF3)2, phosphine (PPh3 or dppf), and an acid with weakly
coordinating anion such as TfOH. Further Pd-catalyzed hydroamination with modified
reaction conditions was reported by Takahashi, Lin and coworkers [8]. In contrast to Rh-
and Ru-catalyzed intermolecular hydroaminations of vinylarenes [1] palladium-catalyzed
transformations exhibit high Markovnikov selectivity and high yields, but suffers from
large palladium loadings. Mechanistic and DFT studies indicated that these transformations
involve an insertion of the styrene double bond into a Pd−H species, followed by an
external nucleophilic attack of the aniline to a η3-benzyl complex [6,9] (Scheme 1).
Scheme 1. Pd(II)-catalyzed hydroamination of styrene ([BF4] as weakly coordinating
anion) and catalysts used in this work (below dashed line).

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Recently, our group showed that acetylacetonate-based cationic palladium
complexes with di-/monophosphine ligands, [Pd(acac)(P^P)]BF4 or [Pd(acac)(PR3)2]BF4,
can be used as very efficient precursors (in combination with BF3·OEt2) for the selective
dimerization of styrene [10] and telomerization of 1,3-dienes with diethylamine [11]. In
the case of styrene dimerization, one can assume that in these catalyst systems Pd−H
species are generated in situ as well [12]. Herein, we report our recent study on the
acetylacetonate cationic palladium complexes catalyzed intermolecular hydroamination of
styrenes with anilines.
Results and discussion
Cationic acetylacetonate monophosphine/diphosphine-ligated palladium complexes
1-8 were prepared combining the corresponding [Pd(acac)(MeCN)2]BF4 with an slight
excess of phosphine ligand in CH2Cl2 or Et2O at room temperature. This approach requires
complex [Pd(acac)(MeCN)2]BF4, which can be easily prepared starting from Pd(acac)2,
MeCN, and BF3·OEt2 [12]. Syntheses of 1-6 using the same or other methodologies have
been already reported [11,13–16]. The structures of new compounds 7 and 8 have been
confirmed by the usual techniques (NMR, FTIR, EA). Instead of the expected monocationic
complexes their dicationic μ-bonded analogues were obtained (Scheme 2). Single crystals
of 8 suitable for X-ray crystallographic analysis were prepared from a Et2O solution of 8
diffused by toluene at 20°C. The crystal structure, selected bond lengths, and angles of the
complex 8 are presented in Figure 1. The palladium atom has a distorted square-planar
coordination environment, created by the two phosphorus atoms and the two oxygen
atoms of the acac ligand. The crystallographic data, and structure refinement summary
31
data for 8 are listed in Table S1 [17] (see Electronic Supplementary Information). The P
NMR chemical shifts of complexes 7 and 8 are 19.9 ppm and 28.1 ppm assuming similar
structural features for complex 7 (literature data for binuclear palladium complexes with
μ-bonded diphosphines: 31.51 ppm (dpppt) [18], 13.5 ppm (dpph) [19]). For comparison,
31
in P^P-chelate monocationic 4, 5, and 6 the P NMR chemical shifts are 37.3, 40.3, and 43.5
ppm respectively [11].
Scheme 2. Syntheses of Acetylacetonate Palladium Complexes 7 and 8.

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Figure 1. ORTEP drawing of 8 (all hydrogen atoms, anion, and solvent molecules are
omitted for clarity). Selected Bond Distances (Å) and Angles (°): Pd1-O1 = 2.027 (2), Pd1-
O2 = 2.059 (2), Pd1i-P2 = 2.2714 (8), Pd1—P2i = 2.2714 (8), Pd1-P1 = 2.2551 (8), O1-Pd1-
O2 = 90.23 (9), P2-Pd1-P1 = 94.97 (3). Symmetry code(s): (i) -x+1, -y+1, -z+1.
Based on the results of Hartwig et al. [5], we investigated the hydroamination
reaction using complexes 1, 2, and 6 by employing similar reaction conditions (Table 1,
entries 1–10). The reaction of aniline with styrene occurred to give the Markovnikov
addition product in high selectivity after 20h at 100°C when catalyzed by a mixture of 6
and [H·OEt2][BF4] co-catalyst (entry 1). No reaction was observed without acid co-catalyst.
We investigated several more polar solvents for this process and found that reaction
conducted in the presence of nitromethane did not occur, but reactions in the presence of i-
PrOH were slightly faster than those conducted with toluene. The same trend was observed
in [20]: reactions of the η3-allyl and η3-benzyl palladium complexes with aniline were faster
in DMSO-d8 than in THF-d8/CD2Cl2 mixture. When simple PPh3 ligand was used (entry 7),
the reaction did give the desired product, but unfortunately with lower selectivity. Very
interestingly, the sterically bulkier ligand PCy3 exhibited high selectivity (99%) to 1,3-
diphenylbuten-1-ne, but hydroamination products were not found in the reaction mixture.
Electronic effects, not ligand bulkiness, must account for this difference in selectivity
because the hydroamination product is formed by external nucleophilic attack of amine on
the η3 complex [9,20]. Thus the PCy3 ligand basicity can dramatically change the degree of
positive charge at the site of nucleophilic attack.

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Table 1 Optimization studies
Entry
Complex nPd,
μmol
20
[B]0:[Pd]0 Co-catalyst
Solvent
TON[a]
Selectivity[b]
[%]
99
1[c]
dppf
10
10
10
10
10
10
10
10
10
10
10
10
20
30
40
50
20
20
20
20
10
50
100
20
[H·OEt2][BF4] Toluene
[H·OEt2][BF4] MeNO2
[H·OEt2][BF4] DMF
32
2[c]
dppf
20
20
20
20
20
20
10
5
52.5
12
0
3[c]
dppf
99
4[c]
dppf
[H·OEt2][BF4] 1,2,4-TCB 9
99
5[c]
dppf
[H·OEt2][BF4] i-PrOH
[H·OEt2][BF4] Toluene
[H·OEt2][BF4] Toluene
[H·OEt2][BF4] i-PrOH
[H·OEt2][BF4] i-PrOH
[H·OEt2][BF4] i-PrOH
42
99
6[c]
2PCy3
2PPh3
dppf
74
0
7[c]
38
94
8[d]
45
99
9[e]
dppf
36
99
10[f]
11[c]
12[c]
13[c]
14[c]
15[c]
16[c]
17[g]
18[h]
19[i]
20[j]
21[i]
22[i]
23[i]
24[k]
dppf
2
24
99
2PPh3
2PPh3
2PPh3
2PPh3
2PPh3
2PPh3
2PPh3
2PPh3
2PPh3
2PPh3
2PPh3
2PPh3
2PPh3
2PPh3
20
20
20
20
20
20
16
8
BF3·OEt2
BF3·OEt2
BF3·OEt2
BF3·OEt2
BF3·OEt2
BF3·OEt2
BF3·OEt2
BF3·OEt2
BF3·OEt2
BF3·OEt2
BF3·OEt2
BF3·OEt2
BF3·OEt2
BF3·OEt2
Toluene
i-PrOH
i-PrOH
i-PrOH
i-PrOH
i-PrOH
—
20
91
41
92
43
83
50
40
49
31
48
12
[l]
[l]
[l]
[l]
[l]
[l]
[l]
[l]
242
430
550
780
520
780
850
298
59
—
80
4
—
81
2
—
84
4
—
81
4
—
48
4
—
43
4
—
93
[a] molSt/molPd. Reaction conditions: 20 h, t = 100°C, V0 = 1.5 mL [b] Towards N-(1-phenylethyl)aniline. The
crude products were analyzed by GC-FID. 1,3-diphenylbut-1-ene was detected as main by-product (GC-MS).
[Pd]0:[St]0:[An]0 = 1:75:50, [d] [Pd]0:[St]0:[An]0 = 1:150:100 [e] [Pd]0:[St]0:[An]0 = 1:300:200
[c]
[f] [Pd]0:[St]0:[An]0 = 1:600:400 [g] [Pd]0:[St]0:[An]0 = 1:250:250 [h] [Pd]0:[St]0:[An]0 = 1:500:500
[j]
[i] [Pd]0:[St]0:[An]0 = 1:1000:1000 [Pd]0:[St]0:[An]0 = 1:2000:2000 [k] [Pd]0:[St]0:[An]0 = 1:350:500
[l] As 20%vol. solution in i-PrOH

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When BF3·OEt2 was applied as co-catalyst in the reaction, TON was substantially
better in i-PrOH than in toluene (entries 11 and 12) due to formation of Brønsted acid
according to eq. (1).
To investigate the effect of the amount of co-catalyst, a set of runs was carried out
with the B/Pd ratios from 10 to 50 at 100°C. As one may see in Table 1 (entries 12-16), the
selectivity to N-(1-phenylethyl)aniline decreased from 92 to 12 %, while the B/Pd molar
ratio changed from 10 to 50, and formation of styrene dimers as well as higher oligomer
were detected. In contrast to [H·OEt2]BF4, usage of BF3·OEt2/6.5i-PrOH as co-catalyst
allows to improve catalyst productivity (to the best of our knowledge the highest catalyst
turnover number ever reported for this reaction was achieved) (entries 18-23). Moreover,
this protocol does not require solvents (5–10%vol. of i-PrOH is added as catalyst system
component) and the reaction selectivity can be increased by aniline excess in the reaction
medium (entry 24).
With the optimized reaction conditions, the scope of substrates was examined. As
shown in Table 2, the aniline with methyl at the ortho positions afforded the corresponding
amines, but selectivity decrease with increasing steric hindrance in anilines (entries 1-4).
For the para-substituted styrenes, the electron-rich substrates gave slightly lower
selectivity values to generate products (entries 5-7).
In general, the phosphine ligand has a profound influence on both the activity and
selectivity of transition-metal catalyst systems. The influence of the phosphine on the
conversion and selectivity of (1–8)/BF3·OEt2/i-PrOH catalyst system is shown in Table 3.
When complexes 1 and 4 with PPh3 and dppp ligands were used to catalyze the reaction,
good selectivities were obtained (entries 2, 4). However, 2, 5, 7, and 8 with PCy3 and
bis(diphenylphosphino)alkane ligands exhibited poor selectivity, which decrease with
increasing ligand basicity (entries 1, 5–7). Finally, compared with the above complexes, 6
with the dppf ligand presented the best selectivity (99%) to afford amine with good TON
(860 molSt/molPd, entry 8). Complexes with dppm and nitrogen ligands exhibited
practically no activity under tested reaction conditions (entries 3, 9–12).

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Table 2
Scope of the substrates
Entry
Aniline
Vinylarene TON[a] Selectivity[b]
[%]
1
2
3
4
5
430
430
345
—
80
53
30
—
45
480
6
7
455
290
—
88
44
—
8
[a]
molSt/molPd
.
Reaction conditions:{[Pd(acac)(PPh3)2]BF4}0:[St]0:[An]0:[B]0= 1:500:500:20, co-catalyst:
[b]
BF3·OEt2 as 20%vol. solution in i-PrOH, 20 h, t = 100°C.
Towards N-(1-arylethyl)anilines. The crude
products were analyzed by GC-FID. 1,3-diarylbut-1-ene were detected as main by-products (GC-MS).

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Table 3
Ligand screening
Entry
Complex
nPd,
μmol
4
[B]0:[Pd]0 TON[a]
Selectivity[b]
[%]
0
1
2
20
20
20
20
20
20
20
20
20
20
20
20
990
2
1
4
560
81
—
86
77
58
28
99
—
—
—
—
3
3
4
traces
440
4
4
4
5
5
4
340
6
7
4
850
7
8
4
750
8
6
4
860
9
[Pd(acac)(bipy)]BF4
[Pd(acac)(MeCN)2]BF4
[Pd(acac)(morpholine)2]BF4
4
traces
traces
traces
traces
10
11
12
4
4
[c]
[Pd(acac)(N^N)]BF4
4
[a] molSt/molPd. Reaction conditions: [Pd]0:[St]0:[An]0 = 1:1000:1000, 20 h, t = 100°C, co-catalyst: BF3·OEt2 as
20%vol. solution in i-PrOH . [b] Towards N-(1-phenylethyl)aniline. The crude products were analyzed by GC-
FID. 1,3-diphenylbut-1-ene was detected as main by-product (GC-MS). [c] N^N = (2,6-iPr2Ph)2-DAB-Me2, DAB =
1,4-diazabutadiene.
Although we have not carried out detailed mechanistic studies of the interaction of
1–8 and BF3·OEt2/i-PrOH in the presence of styrene and aniline, we favor a mechanism that
involves a palladium hydride species (Scheme 1) [5,6,9,20]. We assume that in the case of
cationic acetylacetonate palladium complexes as precursors, the first step of the catalytic
reaction involves BF3-promoted transformation of κ2-O,O-acetylacetonate A to η3-oxoallyl
intermediate B , which leads to the formation of π-complex C with γ-bonded
acetylacetonate ligand [12] (eq. 2).

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Insertion of styrene into Pd-C bond in C followed by β-hydride elimination leads to a
coordinately unsaturated [Pd-H]+ species. The product is then formed by external
nucleophilic attack of amine on the η3-complex (Scheme 1). Most likely, on that step
formation of inactive Pd(0) species can occur via reductive elimination [9]. Regeneration of
a palladium hydride by protonation of Pd(0) by H[BF3·i-PrO] (or the corresponding
ammonium salt ) and reinsertion of vinylarene would regenerate the η3-aryl intermediate.
Conclusions
We have described a Pd-catalyzed intermolecular selective hydroamination of
styrenes using cationic acetylacetonate palladium complexes with mono-/bidentate
phosphine ligands activated by BF3·OEt2/i-PrOH. Compared to known catalysts
[Pd(acac)(PPh3)2][BF4]/BF3·OEt2/i-PrOH catalyst system showed higher activity and
productivity. However, influence of the ligand structure on the catalyst selectivity still
needs to be studied. The related investigation is in progress in our laboratory.
Acknowledgments
The reported study was funded by RFBR, according to the research project No. 31
16-33-60115\15 mol_а_dk.
Supporting Information. Electronic Supplementary Information available:
Experimental procedures, spectral data, and crystallographic data for 8 (in CIF format).
Author Contributions
Chemical Experiments: BMV, SDS, PMV; XRD Experiments: APA; NMR Experiments:
UIA; Writing – original draft: SDS; Writing – review & editing: BMV, SDS, APA, TVS.
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Highlights
Novel Pd-catalyst for hydroamination of vinylarenes with arylamines

 A simple and solvent-free protocol have been developed
 Diphosphine-bridged Pd complexes have been prepared and characterized