The electronic and steric effects of neutral and ionic phosphines on Ir(I)-complex catalyzed hydroaminomethylation of olefins

Article abstract of DOI:10.1016/j.mcat.2020.110843

The electronic and steric effects of a series of neutral and ionic (mono-/di-)phosphines on the performance of Ir(I)-complex catalysts for the hydroaminomethylation of olefins were systematically investigated by means of ¹J³¹P-⁷

Full text of DOI:10.1016/j.mcat.2020.110843

MolecularCatalysis485(2020)110843
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Molecular Catalysis
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The electronic and steric effects of neutral and ionic phosphines on Ir(I)-
complex catalyzed hydroaminomethylation of olefins
Huan Liu, Da Yang, Yixuan Yao, Yongqiang Xu, Hongyan Shang*, Xufeng Lin*
Department of Chemistry, College of Science, China University of Petroleum (East China), Qingdao, Shandong 266580, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
The electronic and steric effects of a series of neutral and ionic (mono-/di-)phosphines on the performance of Ir
(I)-complex catalysts for the hydroaminomethylation of olefins were systematically investigated by means of
1J31P-77Se coupling constant measurement, single-crystal X-ray diffraction, and high pressure in situ FTIR
spectroscopy. It was found that the neutral mono-phosphines of L1 with the moderate π-accepting nature
(1J31P-77Se of 753 Hz) and relatively less steric hindrance was able to spur the activities of the corresponding Ir-
catalysts. In addition, the catalytic performances over the Ir(I)- and Rh(I)-precursors was compared under the
same reaction conditions. The advantages of Ir(I)-catalyst over Rh(I)-catalyst for this tandem reaction were also
discussed in detail.
Hydroaminomethylation
Iridium catalyst
Neutral mono-phosphine
Green homogeneous catalysis
1. Introduction
containing ligands [30–32]. Accordingly, the competitive coordination
of organic amines to the Ir(I)-center and the isomerization of olefin are
As important bulk and fine chemicals in agrochemical and phar-
maceutical industries, amines are widely used for synthesizing various
compounds like biologically active molecules, dyes, solvents and
functional materials. Around one million tons of amine are produced
annually [1,2]. Many types of reaction have been explored to produce
amines up to date, such as hydroamination of alkenes or alkynes [3–5],
nucleophilic substitution of alkyl halides [6–10], and cross coupling
reactions [11–13], etc. Among the established processes, hydro-
aminomethylation features with high atomic economy, and therefore is
one of the most effective protocols for the synthesis of valuable amines
from olefins. A typical hydroaminomethylation contains three-steps
(Scheme 1), i.e., the hydroformylation of an olefin to an aldehyde, the
subsequent condensation of the aldehyde with an amine to an imine or
enamine, followed by a hydrogenation step to give the desired amine
[14–16]. After the discovery of hydroaminomethylation by Reppe in
1949 [17], a number of transition-metal catalysts used for this method
have been studied including Rh [18–22] and Ru [23–27] complexes. In
addition, the dual metallic Rh/Ir catalysts were found to be more effi-
cient for the hydroaminomethylation of olefins due to the individual
responsibilities of the Rh-catalyst for hydroformylation and the Ir-cat-
alyst for condensation of aldehyde with amine and the subsequent
hydrogenation [28,29].
supposed to be suppressed over the Ir(I)-catalysts, making the hydro-
aminomethylation pathway becoming dominant. However, the reports
about Ir-complex catalyzed hydroaminomethylation are sparse in the
literature [33]. An ionic diphosphine-based Ir-catalyst for hydro-
aminomethylation of olefins with H2O as the hydrogen source has been
described by Liu group [33] (Scheme 2). The hydrogenation of olefins
could be effectively depressed by water-gas shift reaction. Moreover, Ir-
catalyzed hydroformylation with hydrogen was milder than water
[34–38], and the hydrogenation of olefins can also be effectively in-
hibited by adjusting the syngas (CO/H2) composition [39]. Herein, we
reported Ir-catalyzed hydroaminomethylation with hydrogen by
screening the suitable phosphines in milder conditions.
In order to better understand the electronic and steric effects on the
performance of Ir-complex catalysts for the hydroaminomethylation of
olefins, a series of neutral and ionic (mono-/di-)phosphines (L1-L4
showed in Scheme 1) were explicitly selected (Scheme 2). Then the
difference between the Ir(I)-catalysis and the Rh(I)-catalysis for hy-
droformylation was compared under the same reaction conditions.
Since the positively charged quaternary ammoniums (such as imida-
zolium) are the most intensive electron-withdrawing moieties in com-
parison with eCF3, eNO2, eCOR etc., the imidazolyl- and imidazolium-
tailed phosphines with skeleton similarity (L1 and L2; L3 and L4) were
selected in order to emphasize the different electronic effect without
interference of steric hindrance. On the other hand, the pair of mono-
Compared to the Rh(I) ion, the Ir(I) ion presents a much weaker
Lewis acidity, and thus it prefers soft phosphines over hard N-
⁎ Corresponding authors.
E-mail addresses: catagroupsh@163.com (H. Shang), hatrick2009@upc.edu.cn (X. Lin).
https://doi.org/10.1016/j.mcat.2020.110843
Received 9 December 2019; Received in revised form 7 February 2020; Accepted 14 February 2020
2468-8231/©2020ElsevierB.V.Allrightsreserved.

H. Liu, et al.
MolecularCatalysis485(2020)110843
Scheme
1. Tandem
hydro-
aminomethylation of olefins.
Scheme 2. Ir-catalyzed hydroaminomethylation of olefins.
case of Ir(CO)(PPh3)2Cl [Ir-P and Ir-CO distances of 2.330(1) Å and
1.791(13), respectively, and νco 1953 cm-1] [44], Ir-L2, Ir-L3 and Ir-L4
were all featured with a much shorter Ir-P [2.291∼2.317 Å] and a
longer Ir-CO bond distance (Ir-CO 1.815∼1.845 Å), accompanied by
blue-shifted CO vibrations frequency (νco of 1981∼1996 cm-1, Table 1).
These features indicated that the enhanced π-accepting ability of L2-L4
over that in PPh3 facilitated the more intensive π-backdonation inter-
action between the Ir(I)-center and the phosphine, leading to more
consolidated Ir-P linkages. To further explore the influence of the steric
effect in the Ir-complexes, the topographic steric maps (Fig. 2) of these
complexes was drawn with the web-page application developed by
Poater et al. [45–48]. Consistently, the %VBur of Ir-L3 was equal to Ir-
L4, and both greater than the %VBur of Ir-L2. Therefore, the bi-phos-
phines based Ir-complexes (Ir-L3 and Ir-L4) had similar steric hin-
drance, the regularity was also applicable to the mon-phosphines
(based Ir-complexes Ir-L1 and Ir-L2). In comparison to Ir-L3 and Ir-L4
wherein L3 (1J31P-77Se =751 Hz) and L4 (1J31P-77Se =782 Hz) pos-
sessed a similar steric effect, the two Ir-P distances in Ir-L4 [2.291(1),
2.309(1) Å] were much shorter than those in Ir-L3 [Ir-P 2.317(1),
2.310(1) Å]. In comparison to Ir-L2 and Ir-L4 wherein L2 (1J31P-77Se
=780 Hz) and L4 (1J31P-77Se =782 Hz) possessed an similar electronic
effect but only differ in the steric hindrance, the Ir-P distances in Ir-L4
were also shorter than those in Ir-L2 due to the chelating effect of the
diphosphine of L4. As a whole, Ir-L4 was the most stable complex due
to the facilitated electronic and steric effects of L4. Concerning Ir-L3
phosphine and diphosphine (L1 and L3; L2 and L4) that differentiated
in terms of steric effect were investigated. The means of the 1J31P-77Se
measurement, the single-crystal X-ray diffraction (XRD) and the high
pressure in situ Fourier transform infrared (FTIR) spectroscopy were
utilized to elucidate the electronic and steric effects of the phosphine on
the performance of Ir(I)-catalyst.
2. Results and discussion
The experimental sections can be found in the Supplementary in-
formation.
2.1. The electronic and steric effects of phosphines in the Ir-complexes
In order to understand the electronic effect of the selected phos-
phines, the values of the 1J31P-77Se coupling constant of the corre-
sponding phosphines were measured by the 31P NMR spectroscopy. It is
well known that 1J31P-77Se value can be used to evaluate the π-acceptor
ability of a phosphine [40,41]. An increased 1J31P-77Se value indicates
an increase in the character of π-acceptor ability (i.e., less σ-donor
ability). L1-L4 were obtained respectively via the replacement of one
phenyl ring of PPh3 by the imidazolyl- or imidazolium-groups.The
measurement of 1J31P-77Se indicated that the π-accepting ability of
these phosphines had the order of L2 (780 Hz) ∼ L4 (782 Hz) > L1
(753 Hz) ∼ L3 (751 Hz) > PPh3 (729 Hz) [42].
wherein L3 possessed
a
moderate π-accepting nature (1J31P-77Se
Fig. 1 presents the molecular structures of the Ir-complexes (Ir-L2,
Ir-L3 and Ir-L4) [36,42] deduced from the single-crystal XRD (see
Table 1 for detailed structural parameters as well). These complexes
may serve as pre-catalysts for the investigated hydroaminomethylation.
The structures can further help elucidate the electronic and steric ef-
fects of the involved phosphines on these complexes. As shown in Fig. 1,
all these Ir(I)-complexes had a typical square-planar geometry, in which
the four-coordinated Ir(I)(5d8)-center was bound to two phosphine
fragments in trans-position, one CO, and one Cl- anion. Compared to the
=751 Hz), it is noticeable that the Ir-Cl distance [2.384(1) Å] is dis-
tinctively lengthened compared to that in Ir-L2 [2.344(2)] and in Ir-L4
[2.374(1)], while all these complexes had a close Ir-P distance. The
loose Ir-Cl linkage in Ir-L3 could facilitate the Ir-Cl cleavage to make a
vacancy on the Ir center for the accommodation of other molecules like
substrate. In addition, the unimpaired Ir-P linkages in Ir-L3 can guar-
antee its stability.
2

H. Liu, et al.
MolecularCatalysis485(2020)110843
Fig. 1. The molecular models of Ir-L2, Ir-L3 and Ir-L4 deduced from the single crystal XRD results (the hydrogen atoms and the solvent molecules are omitted for
clarity).
the yield of amine for hydroaminomethylation of olefin was the highest,
but the formation of a large number of imines was also observed (Entry
4). The N-methylaniline was found to be the best amine source in terms
of chemoselectivity without the production of imines (Entry 5). When
the amine source was replaced by piperidine, the tandem reaction was
stopped completely (Entry 6). Since Ir-complexes were catalytically
active for hydrogenation of olefins, a large number of hydrogenation
products were observed in this reaction (Entries 1–5). The reduction in
the partial pressure of H2 efficiently improved the chemoselectivity of
the amine product (Entries 7 and 8).
Table 1
Selected bond distances and bond angles in M-complexes studied in this work.
Ir-complex
Selected bond length/Ǻ
ν (CO)
/cm−1
M-P1 M-P2 M-Cl
M-CCO
Ir-L2 [42]
Ir-L3 [42]
Ir-L4 [42]
Rh-L4 [43]
Ir(CO)(PPh3)2Cl
[44]
2.319(2) 2.309(2) 2.344(2) 1.845(110) 1985
2.317(1) 2.310(2) 2.384(1) 1.815(57)
2.291(1) 2.309(1) 2.374(1) 1.825(50)
2.310(4) 2.299(4) 2.370(4) 1.830(50)
1981
1996
–
2.330(1)
–
2.382(3) 1.791(13)
1953
Then the electronic and steric effects of the phosphines on the
performance of Ir(I)-catalyst were investigated, and the results are
shown in Table 3. It can be seen that the neutral imidazolyl-tailed
phosphines (L1 and L3) universally corresponded to a much higher
conversion of 1-octene and better selectivity to the target amines than
those over the ionic imidazolium-tailed counterparts (Entries 2 vs 1; 4 vs
3). For example, over the neutral mono-phosphine of L1, the yield of
the two kinds of amines [N-methyl-N-(2-methyloctyl)aniline and N-
methyl-N-nonylaniline] reached 85 % (see GC-Mass analysis results for
the product distribution in ESI) along with the low selectivity of 5% to
the by-products of iso-octenes, whereas over L2 only 39 % yield was
obtained along with 10 % selectivity to iso-octenes (Entries 1 vs 2).
Comparison between L1 and L3 that had a same electronic effect
showed that the mono-phosphine of L1 with less steric hindrance led to
2.2. Hydroaminomethylation of olefins over Ir(I)-catalyst
In the early stage of this work, different solvents, amine sources and
the pressure ratios of CO/H2 were examined under a given set of con-
ditions with 1-octene as the model olefin (Table 2). In general, the
experiments were carried out at 120 °C, under 4.0 MPa pressure of
syngas, with [Ir(COD)Cl]2 as the iridium precursors, catalyst to sub-
strate ratio of 1/100, and L1 as the ligand. This ligand was previously
used for Ir-catalysed hydroformylation of olefins with high activity
[36]. The solvent had a significant influence on the chemoselectivity of
the reaction with the aniline as the amine sources (Entries 1–4). In
particular, when N-methylpyrrolidone (NMP) was used as the solvent,
Fig. 2. Topographic steric maps of the Ir-complexes studied in this work [45–48].
3

H. Liu, et al.
MolecularCatalysis485(2020)110843
Table 2
Ir-catalyzed hydroaminomethylation of 1-octene with N-methylaniline under different conditionsa.
Entry
Amine
Sol.
Pressure (CO/H2)
Conv. (%)
Sel.b (%)
Amines
47
Yield (%)
L/Bb
Aldehydes
–
Isomers
6
Octane
30
Imines
17
1
2
3
4
5
6
MeOH
Tol.
1/1
1/1
1/1
1/1
1/1
1/1
97
95
97
96
95
10
46
28
30
51
61
–
50:50
52:48
41:59
61:38
64:36
–
29
31
53
64
–
–
–
–
–
–
9
8
5
8
3
38
33
15
28
7
24
28
27
–
THF
NMP
NMP
NMP
–
7
8
NMP
NMP
3/1
5/1
98
98
80
87
–
7
5
13
4
–
–
78
85
72:28
72:28
4
a
[Ir(COD)Cl]2 0.025 mmol (Ir 1.0 mol %), ligand L1 0.05 mmol, P/Ir = 1 (molar ratio), 1-octene 5.0 mmol, amine 7.5 mmol, NMP 2 mL, CO/H2 4.0 MPa, 7 h,
temperature 120 °C.
b
Determined by GC.
a higher conversion of 1-octene and the better yield to the target amines
(Entry 1 vs 3). A similar result was observed by the comparison between
L2 and L4 that only differenced in their steric hindrances (Entries 2 vs
4). Without the involvement of any phosphine, the low yield of the
amine products (35 %) was obtained over [Ir(COD)Cl]2 (Entry 5). When
the complexes of Ir-L1 (or Ir-L3) was used to replace the mixture of [Ir
(COD)Cl]2 and L1 (or L3) as the pre-catalyst, the very similar conver-
sion of 1-octene and the yields of the amine products were obtained
under the same conditions, indicating that the in situ formed Ir-L1 (or
Ir-L3) exhibited a same activity as the as-synthesized one (Entry 6 vs 1;
Entry 7 vs 3). In contrast, the commercial PPh3 as a typical electron-rich
σ-donor ligand resulted in much lower activity towards this tandem
reaction than L1 (Entry 8 vs 1). Consistent to the results presented in
Fig. 1 and Table 1, the enhanced π-accepting ability for L2 and L4
(1J31P-77Se ∼780 Hz) rendered the Ir-L2 and Ir-L4 complexes better
stability with indication of the more consolidated Ir-P and Ir-Cl linkages
due to π-backdonation interaction. Especially for L4 with the intensive
π-accepting ability as well as a favorable chelating steric effect, the
corresponding Ir-L4 was the most stable complex, which corresponded
to the most inert activity due to too sluggish dissociation of the Ir-Cl
bond to make a room for the activation of the olefin substrate and then
the migration of CO (see Entry 4 in Table 3). As for Ir-L3, firstly, the
exclusively loose Ir-Cl linkage facilitates the Ir-Cl cleavage to make a
vacancy for the accommodation of the other molecule (like substrate).
Table 3
Ir-catalyzed hydroaminomethylation of 1-octene with N-methylaniline in syngas with the presence of different ligandsa.
Entry
Ligand
Conv. (%)b
Sel. (%)b
Yield (%)
L/Bb
Amines
Imines
Nonanals
Iso-octenes
Octane
1
L1
98
47
80
43
56
99
82
64
95
99
96
45
87
83
79
69
63
86
77
73
18
–
–
4
5
4
85
39
63
30
35
85
63
47
17
–
72:28
79:21
83:17
81:19
70:30
70:30
83:17
76:24
60:40
66:34
61:39
65:35
2
L2
–
–
10
11
19
13
6
7
3
L3
–
2
8
4
L4
–
–
12
10
4
5
–
–
14
4
6
Ir-L1
Ir-L3
PPh3
L1
–
7
–
3
12
13
28
14
5
8
8
–
6
8
9c
10d
11e
44
–
10
84
90
7
–
L1
2
L1
–
–
5
–
c
12
L4
17
35
41
–
8
a
[Ir(COD)Cl]2 0.025 mmol (Ir 1.0 mol%), P/Ir = 1 (molar ratio), 1-octene 5.0 mmol, N-methylaniline 7.5 mmol, NMP 2 mL (solvent), CO/H2 = 5/1 (4 MPa), 7 h,
temperature 120 °C.
b
c
d
e
The selectivity to product and the ratio of linear product to branched one (L/B) were determined by GC and.GC–MS.
[Rh(COD)Cl]2 (0.025 mmol) in place of [Ir(COD)Cl]2.
[Rh(COD)Cl]2 (0.025 mmol) in place of [Ir(COD)Cl]2, but without N-methylaniline.
[Ir(COD)Cl]2 (0.025 mmol) without N-methylaniline.
4

H. Liu, et al.
MolecularCatalysis485(2020)110843
Secondly, the solid Ir-P linkages, which was due to the π-backdonation
from L3 with the moderate π-accepting ability (1J31P-77Se =751 Hz),
can guarantee the stabilization of Ir-center against deactivation. War-
ranted by these two factors, L3-based Ir(I)-catalyst exhibited a much
higher activity towards the hydroaminomethylation of 1-octene than
the L2 and L4-based ones (Entries 3 vs 2 and 4 in Table 3). Reasonably,
the neutral phosphines of L1 with a same moderate π-accepting ability
as L3 (1J31P-77Se ∼ 750 Hz) but without the chelating steric effect was
responsible for the best performance for this reaction (see Entry 1 in
Table 3)
However, over the L1-based Rh-catalyst, the low yields (17 %) of
the target amines (Conv.1-octene 95 %, Sel.amines 18 %) were obtained
accompanied by the formation of the imine by-products and the higher
selectivity to nonanals and iso-octenes (Entry 9: Sel.imines 44 %,
Sel.nonanals 10 %; Sel.iso-octenes 28 %) (see the results of GC-Mass analyses
for the product distributions provided in ESI). Evidently, over L1-based
Rh-catalyst, the overall yield of the hydroformylation products in-
cluding the aldehydes, the imines and the target amines was only 68 %
(Entry 9 in Table 3). Without the presence of N-methylaniline the yield
of the hydroformylation products (nonanals) was increased to 83 %
under the same conditions (Entry 10 in Table 3). Over L1-based Ir-
catalyst, the total yields of the hydroformylation products including the
aldehydes and the target amines was 91 % (Entry 1 in Table 3), which
was quite close to the case where N-methylaniline was absent (Entry 11
in Table 3). Moreover, much higher selectivities to imines and nonanals
were observed over the Rh(I)-catalyst compared to those over the Ir(I)-
base one (Entry 9 vs 1 in Table 3). It is also noticeable that only two
kinds of target amine (derived from 1-octene) were obtained over L1-
based Ir-catalyst whereas four kinds of the amines (derived from 1-
octene, 2-octene and 3-octene) were found in the Rh-system (see the
results of GC-Mass analyses for the product distributions provided in
ESI). Although the Ir-L4 and Rh-L4 complexes had similar structural
characteristics (the IreP bond distances, IreCO bond distances and
IreCl bond distances can be seen in Table 1), the catalytic performance
of Ir-L4 (yield of the hydroformylation products of 30 %) was obviously
better than that of Rh-L4 (yield of the hydroformylation products of 27
%, see Entry 4 vs 12 in Table 3). These results revealed that L1-based Ir-
catalyst was obviously advantageous over the corresponding Rh-cata-
lyst for hydroaminomethylation in the following aspects. (1) The pre-
sence of N-methylaniline as a hard N-containing ligand deteriorated the
activity of the corresponding Rh(I)-catalyst towards hydroformylation
(Entry 9 vs 10 in Table 3), but exhibited negligible effect on the softer Ir
(I)-center (Entry 1 vs 11 in Table 3). (2) L1-based Ir-catalyst gave more
rise to condensation of the aldehydes with N-methylaniline and the
subsequent hydrogenation of the imines with indication of less presence
of aldehydes and no formation of the imines in the final product dis-
tributions (Entry 1 vs 9 in Table 3). (3) The isomerization of 1-octene
was greatly depressed leading to much higher selectivities to the target
amines with an L/B of 2.6 (Entry 1 vs 9 in Table 3), due to the relatively
weaker Lewis acidity of Ir(I)-ion than Rh(I).
Fig. 3. The evolution profiles of the 1-octene conversion and product yield
depending on the reaction time. Catalyst: L1-based Ir-catalyst ([Ir(COD)Cl]2 of
0.025 mmol. Other conditions: L1 of 0.025 mmol, 1-octene of 5.0 mmol, N-
methylaniline of 8.0 mmol, NMP of 2 mL, CO/H2 =5:1, reaction pressure of
4.0 MPa and reaction temperature at 120 °C).
The evolution of L1-[Ir(COD)Cl]2 catalyzed hydroaminomethyla-
tion of 1-octene with N-methylaniline in NMP was recorded under the
optimized reaction conditions as shown in Fig. 3. The yields of the
target amines as well as the conversion of 1-octene increased steadily
with the reaction time. In the whole process, the aldehydes (nonanal
and 2-methyloctanal) always had a yield of less than 6%. Evidently, the
hydroformylation of 1-octene was the rate-controlling step in this
tandem hydroaminomethylation.
Fig. 4. The high-pressure in situ FTIR spectra recorded from 30 to 120 °C after
mixing [Ir(COD)Cl]2, L1 (or L2), 1-octene, N-methylaniline and NMP sequen-
tially. The mixture was pressured by 1.5 MPa syngas (CO/H2 volume
ratio = 5:1).
The results in Fig. 3 suggested that all the effects that facilitated the
first-step hydroformylation can give rise to the overall hydro-
aminomethylation. Herein, the high-pressure in situ FTIR spectroscopy
was used to monitor the formation and stability of the active iridium-
hydride (Ir-H) species responsible for the homogenous hydroformyla-
tion of olefins [49]. The continuous time-dependent highly pressure in
situ FTIR spectra were recorded over the L1- and L2-based Ir-catalysts
for hydroaminomethylation of 1-octene respectively (Fig. 4). Over the
L1-based Ir-catalyst (Fig. 4A), the strong features at 2180 and
2115 cm−1 assigning to gaseous CO were always observed. In the
overall monitoring process when the temperature was increased from
5

H. Liu, et al.
MolecularCatalysis485(2020)110843
30 to 120 °C, an absorption peak at 2083 cm−1 appeared at 110 °C after
holding 1 min. This peak can be identified as an active Ir-H (iridium
hydride) species according to the previously reported work [33,50].
This feature presented at 120 °C for 30 min. At the same time the ca.
1972 cm−1 peak was also observed, which can be attributed to an
IreCO intermediate. This intermediate can be accounted for the com-
plexation of [Ir(COD)Cl]2 with CO. Along with the appearance of the
2083 cm−1 peak (Ir-H species), the characteristic vibration of CeN
bond at 1352 cm−1 grew stronger and stronger, which was assigned to
the products of amine (see the standard FTIR spectrum of N-methyl-N-
pentylaniline in S.Fig. 8 in ESI). This peak was partially overlapped
with the CeN vibrations in N-methylaniline and NMP. In contrast, over
L2-based Ir-catalyst, the appearance of the Ir-H vibration (see the
2103 cm−1 peak in Fig. 4B) occurred at 120 °C after holding 10 min,
which implied that L2-based Ir-catalyst was relatively inert compared
to over L1-based one.
In contrast, the internal aliphatic olefins like 2-octene and cyclooctene,
having increased steric hindrance, corresponded to much lower amine
yields (Entries 4 and 5). When a series of styrene derivatives were ap-
plied to repeat the reactions at a prolonged time of 12 h, the good to
excellent yields were obtained with branched amines as the major
products (Entries 6–11). For example, styrene gave a 80 % yield of the
target amines whereas styrene derivatives with para-positioned F-/Cl-
substituents afforded 98 %∼99 % yields of the products (Entries 6, 9
and 10). Unfortunately, the uses of the aliphatic primary/secondary
amines instead of N-methylaniline universally led to dramatically re-
duced yields of the products (see S.Table 1 in SI). Only when the con-
centration of Ir-catalyst was increased to 2.0 mol %, the hydro-
aminomethylation of 1-octene with N-methyl-1-phenylmethanamine
gave an improved product yield of up to 88 % (Entry 13).
3. Conclusions
It has been reported in Moser’s pioneering work [51] that the
electron-withdrawing effect of the substituents (R) in the active RhH
(CO)3(PR3) complexes could dramatically influence the Rh-H infrared
vibrational frequency within the range of 2018–2050 cm−1. For ex-
ample, when R= eCF3, RhH(CO)3(PR3) presents a quite blue-shifted
Rh-H feature of 2050 cm−1. Consistent with Moser’s observation, the
dramatic blue-shifted Ir-H vibration of 2103 cm−1 was found in Fig. 3B
due to the intensive electron-withdrawing effect of the positive-charged
imidazolium in L2 in comparison to the observed Ir-H species at
2083 cm−1 over L1-based system (Fig. 4A). At the same time, the CO-
vibrations in the ionic phosphine based Ir-complexes/intermediates
also blue-shifted to the higher frequency range (Fig. 4-B).
The electronic and steric effects of a series of neutral and ionic
(mono-/di-) phosphines (L-1L4) were studied in the Ir(I)-complex cat-
alyzed hydroaminomethylation of olefins. The Ir-complex with the
moderate and the less sterically hindrance ligands had the highest ac-
tivity for the tandem hydroaminomethylation of olefins. It was found
that L1 with a moderate π-accepting ability (1J31P-77Se of ∼750 Hz)
corresponded to the most efficient hydroaminomethylation in the four
ligands examined. L3 presented a relatively lower reaction rate than L1
due to a larger steric hindrance and the potential chelating effect as a
diphosphine, even though these two ligands had the same 1J31P-77Se of
751 Hz. The molecular structure of Ir-L3 supported that L3 was able to
consolidate the Ir-P linkage along with a weakened Ir-Cl linkage. These
structural characters led to a better performance of the L3-based Ir-
catalyst for hydroaminomethylation than those of the L2- and L4-based
ones. The in situ FTIR analysis proved that the presence of L1 could
The scope of hydroaminomethylation catalyzed by L1-based Ir-
catalyst was explored as shown in Table 4. It can be seen that good
product yields (85 %∼87 %) can be obtained for the linear terminal
aliphatic olefins like 1-heptene, 1-octene and 1-dodecene (Entries 1–3).
Table 4
Generality of L1-[Ir(COD)Cl]2 catalytic system for hydroaminomethylation of olefins with aminesa.
a[Ir(COD)Cl]2 0.025 mmol (Ir 1.0 mol %), L1 0.025 mmol, P/Ir = 1 (molar ratio), alkene 5.0 mmol, N-
methylaniline 7.5 mmol, NMP 2 ml (solvent), CO/H2 = 5/1 (4 MPa), 7 h, temperature 120 °C; b Determined
by GC and GC–MS; c CO/H2 = 3/1 (4.0 MPa), 12 h; d [Ir(COD)Cl]2 0.05 mmol (Ir 2.0 mol %), L1 0.05 mmol.
6

H. Liu, et al.
MolecularCatalysis485(2020)110843
greatly facilitate the formation and stability of the active Ir-H species
responsible for hydroformylation (as the rate-controlling step for hy-
droaminomethylation). Moreover, compared to the Rh(I)-complexes,
the Ir(I)-complexes were more advantageous for the catalytic tandem
hydroaminomethylation under the same reaction conditions, which can
be due to three reasons. First, as a much softer Lewis acidic centre, Ir(I)-
ion preferred phosphines over hard N-containing ligands. Hence, the
available organic amines as raw materials in hydroaminomethylation
had negligible influence on the performance of the Ir(I)-catalyst.
Second, the isomerization of the olefins were greatly depressed over the
weaker Lewis acidic Ir(I)-centre. Third, the Ir(I)-catalyst was more ac-
tive for hydrogenation of the imines than the corresponding Rh(I)-
catalyst.
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CRediT authorship contribution statement
Huan Liu: Conceptualization, Investigation, Formal analysis, Data
curation, Writing - original draft. Da Yang: Investigation, Formal
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analysis, Data curation, Writing
- original draft. Yixuan Yao:
Investigation, Formal analysis, Data curation. Yongqiang Xu: Formal
analysis, Resources. Hongyan Shang: Supervision, Funding acquisi-
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Declaration of Competing Interest
[21] C. Chen, S. Jin, Z. Zhang, B. Wei, H. Wang, K. Zhang, H. Lv, X.-Q. Dong, X. Zhang,
Rhodium/yanphos-catalyzed asymmetric interrupted intramolecular hydro-
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9017–9020, https://doi.org/10.1021/jacs.6b03596.
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
[22] P. Kalck, M. Urrutigoïty, Tandem hydroaminomethylation reaction to synthesize
amines from alkenes, Chem. Rev. 118 (2018) 3833–3861, https://doi.org/10.1021/
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Acknowledgements
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Support from the National Natural Science Foundation of China
(21576291), Shandong Province Natural Science Foundation
(ZR2014BM002), and the Fundamental Research Funds for the Central
Universities (17CX02067) is gratefully acknowledged.
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Appendix A. Supplementary data
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Supplementary material related to this article can be found, in the
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