Synthesis of phosphorus amidite ligand and investigation of its flexibility impact on rhodium-catalyzed hydroformylation of 1-octene

Article abstract of DOI:10.1039/c6ra09890h

A new flexible 2,2′-bis((dipyrrolylphosphinooxy)methyl)-1,1′-(±)-biphenyl ligand (L1) was synthesized, characterized and employed in the hydroformylation of 1-octene. In order to investigate the influence of ligand flexibility on the catalytic performance, its analogue 2,2′-bis (dipyrrolylphosphinooxy)-1,1′-(±)-biphenyl (L3) was also applied in the hydroformylation of 1-octene. With the presence of L1, the aldehyde selectivity was approximately 10% higher than that with the relevant less flexible ligand L3. Theoretical calculation indicated that the olefin-insertion into the Rh-H bond of intermediate III and the CO-insertion into Rh-alkyl bond of intermediate V were favorable in terms of thermodynamics, which were vital to the generation of aldehyde. Meanwhile, the ligand flexibility was indeed improved by adding a methylene between benzene and oxygen atom that connected with P, as the calculation showed the variation of the bite angle ∠P-Rh-P of the intermediates I-VI was 8.7° in L1-system, as for L3-system, the variation was 14.0°. This structural feature might make the olefin-insertion and the CO-insertion occur more easily in the L1-system and resulted in higher aldehyde selectivity.

Full text of DOI:10.1039/c6ra09890h

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Synthesis of phosphorus amidite ligand and
investigation of its flexibility impact on rhodium-
catalyzed hydroformylation of 1-octene†
Cite this: RSC Adv., 2016, 6, 53012
Received 16th April 2016
Accepted 23rd May 2016
a
a
a
a
b
a
a
Weibiao Han, Song Qin, Xiao Shu, Qianhui Wu, Bin Xu, Ruixiang Li, Xueli Zheng*
a
and Hua Chen*
DOI: 10.1039/c6ra09890h
www.rsc.org/advances
0
0
3–6
A new flexible 2,2 -bis((dipyrrolylphosphinooxy)methyl)-1,1 -(ꢀ)-biph- hydroformylation. Ligands possessing strong p-acceptor were
7,8
enyl ligand (L1) was synthesized, characterized and employed in the found to be effective in terms of regioselectivity and activity. For
hydroformylation of 1-octene. In order to investigate the influence of example, van Leeuwen has developed a novel bidentate ligand,
ligand flexibility on the catalytic performance, its analogue 2,2 -bis 1,1 -biphenyl-2,2 -diyl-bis(dipyrrolylphosphoramidite), which led
dipyrrolylphosphinooxy)-1,1 -(ꢀ)-biphenyl (L3) was also applied in the to high selectivity and activity in Rh-catalyzed hydroformylation
hydroformylation of 1-octene. With the presence of L1, the aldehyde of 1-octene due to bearing the electron-withdrawing pyrrolyl
0
0
0
0
(
9
selectivity was approximately 10% higher than that with the relevant substituents. Zhang and co-workers reported that its triphos-
10–12
13
less flexible ligand L3. Theoretical calculation indicated that the olefin- phorus
and tetraphosphorus analogue capable of forming
insertion into the Rh–H bond of intermediate III and the CO-insertion multiple chelating modes with Rh, could provide better regiose-
into Rh–alkyl bond of intermediate V were favorable in terms of lectivity in isomerization-hydroformylation of internal olens
thermodynamics, which were vital to the generation of aldehyde. with the presence of Rh(acac)(CO) (n : i values could reach up to
2
Meanwhile, the ligand flexibility was indeed improved by adding 47.0 and 51.7 for 2-octene with triphosphorus and tetraphos-
a methylene between benzene and oxygen atom that connected with phorus ligands, respectively).
P, as the calculation showed the variation of the bite angle :P–Rh–P
van Leeuwen found that the natural bite angle :P–Rh–P in
ꢁ
of the intermediates I–VI was 8.7 in L1-system, as for L3-system, the L2Rh(CO)H(alkene) (L ¼ ligand) had close relationship with
ꢁ
14
variation was 14.0 . This structural feature might make the olefin- activity and selectivity. A better regioselectivity towards the
insertion and the CO-insertion occur more easily in the L1-system linear aldehyde could be obtained when increasing the natural
6,15
and resulted in higher aldehyde selectivity.
bite angle to a certain degree.
Although the complex (L)
2Rh(CO)H(alkene) has never been captured directly, many
research revealed that wide-bite-angle diphosphite ligands,
such as BISBI-based bidentate ligands, the xanthenes family of
Introduction
16
ligands, diary bicycle [2.2.n] compounds and modied
Hydroformylation of olens remains one of the most important
chemical reactions. It has been widely explored under the catal-
ysis of rhodium catalysts for the production of aldehydes. Mostly,
the aldehydes could be used to make other chemical products
17
biphenyl-based bisphosphites ligands, favored the formation
6
of linear alkyl rhodium species and thus the linear aldehyde
1
18
was preferred. Dieter Vogt et al. studied a series of sterically
demanding xanthenes-based diphosphites ligands and found
that more exible ligands exhibited high activities as well as
good selectivities to linear aldehyde in the Rh-catalyzed hydro-
formylation of 1-olen. The crystal structure revealed that less
2
such as plasticisers, pharmaceuticals and lubricants. Recently,
extensive research has been devoted to ligand modication, as
the stereoelectronic properties of ligands have a remarkable
inuence on the catalytic performance in the rhodium-catalyzed

exible ligand aroused weak interaction between the O atom of
xanthene and Rh center, thus the hydroformylation result was
inferior to that with the more exible ligand.
Inspired by the above reports, the electron-withdrawing
pyrrolyl substituent would improve the p-acceptor ability of
the phosphorus ligand. Herein, we attempt to synthesize
a
Key Lab of Green Chemistry and Technology, Ministry of Education, College of
Chemistry, Sichuan University, Chengdu 610064, China. E-mail: scuhchen@163.
com; 14359385@qq.com
b
Key Laboratory of Green Chemistry of Sichuan Institutes of Higher Education, Zigong
6
43000, China
1
13
31
†
Electronic supplementary information (ESI) available: H, C and P NMR pyrrolyl-substituted diphosphine ligands with biphenyl as the
spectra; the calculated bite angle of the major intermediates; Table 3: backbone, and introduce –CH
– between benzene ring and
2
optimization of reaction conditions for the hydroformylation of 1-octenewith
oxygen atom to adjust the exibility (L1, Fig. 1), we envision that
Rh/L2. See DOI: 10.1039/c6ra09890h
53012 | RSC Adv., 2016, 6, 53012–53016
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this ligand could serve as an effective ligand for the hydro-
formylation of 1-octene. DFT calculation revealed that the
introduction of –CH – modied the exibility of the ligand,
2
inuenced the bite angle :P–Rh–P and favored the insertion of
olen and CO, and consequently, increased the selectivity to
aldehyde.
Experimental
Synthesis of backbones and ligands
The synthesis of ligands L1 and L2 were realized as shown in
Scheme 1. Starting from the oxidation of phenanthrene (A) by
21
0
H O , reduction of 2,2 -biphenyldimenthanoic acid (B) by
2
2
Scheme 1 The synthesis of biphosphane ligands L1 and L2.
sodium borohydride 22, and the reaction of diol (C) with
chlorodiarylphosphine afforded the desired phosphane ligands
L1 and L2. The two compounds were unambiguous proven by
10
hydroformylation, increasing the local phosphine concentra-
tion around the rhodium atom could enhance the coordination
between P and Rh to form complex E (Scheme 2), which would
1
13
31
means of H, C and P NMR, as well as mass spectrometry.
19
benet the reaction activity and the selectivity to aldehyde.
Results and discussion
Hydroformylation of 1-octene
However, more excess ligand would block the coordination site
and the coordination of 1-octene with rhodium active species
would become difficult. Therefore, further increasing the
ligand/metal ratio would cause its activity to drop. Decreasing
the ligand concentration might form multiple carbonyls
complex F and G (Scheme 2), which was not conductive to the
formation of aldehyde.
Hydroformylation of 1-octene with the new ligands L1 and L2
was then investigated, respectively. The catalytic condition was
explored by using L1 as the benchmark ligand. The rhodium
catalyst was prepared in situ by mixing L1 with Rh(acac)(CO)
acac ¼ acetylacetonate) in toluene. The reaction proceeded in
toluene and the products were analyzed by gas chromatography.
2
(
The effects of temperature and CO/H pressure on the Rh-
2
The Rh(acac)(CO) concentration was 1.59 mM. The effects of
2
catalyzed hydroformylation reaction were also tested. To our
delight, the aldehyde selectivity was comparably higher at
lower temperature. An obvious decrease in both n/i and alde-
hyde selectivity were observed when the reaction temperature
ligand/metal ratio on hydroformylation of 1-octene were
studied and the results were shown in Table 1 (entries 1–6).
The linear/branched (n/i) ratio increased from 2.7 to 10.6 and
the aldehyde selectivity increased from 16.4 to 89.3 when the
ligand/metal ratio was increased from 0 to 15 : 1. However,
a decrease in the isomerization (22.6 to 10.0) and hydrogenation
ꢁ
was increased from 60 to 120 C (Table 1, entries 7–10). It
should be noted that the aldehyde selectivity was 91.9% and
the n/i ratio was 10.8 when the hydroformylation was con-
(61.0 to 0.7) was detected (Table 1, entries 1–4). Further
ꢁ
ducted at 80 C (Table 1, entry 8), and both the olen isom-
increasing the ligand/metal ratio did not improve the regiose-
lectivity, but resulted in a little drop of conversion (Table 1,
entries 4–6). According to ligand dissociation in Rh-catalyzed
erization and hydrogenation products (octane) increased. It is
well known that hydroformylation at higher temperature
might lead to more olen isomerization and hydrogenation. A
high regioselectivity (n/i ratio 9.7) was obtained under a CO/H
2
pressure of 0.5 MPa, however, a signicant amount of isom-
erization was observed under this pressure, indicating that low
CO/H2 pressure could in turn facilitate the olen isomer-
isation. Increasing the CO/H2 pressure from 0.5 to 3 MPa
(
Table 1, entries 11–14) did not obviously improve the regio-
selectivity but increase the aldehyde selectivity to 95.2%. A
similar tendency was observed when L1 was replaced by L2
(
Table 3) (see ESI†). In order to explore the impact of exibility
and the electronic properties on the catalytic performance,
ligands L2–L5 were employed in the hydroformylation of
1
-octene with the presence of Rh(acac)(CO) , and the results
2
were summarized in Table 2. All the ligands demonstrated
good reaction activities and moderate regioselectivities.
However, the catalytic performance of Rh/L1 was dramatically
superior to Rh/L2, that is, under optimized reaction condition,
the aldehyde selectivity and the n/i ratio could reach up to
9
1.9% and 10.8, respectively, probably as a result of the easy
Fig. 1 Ligand L1–L5.
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a
Table 1 Optimization of reaction conditions for the hydroformylation of 1-octenewith Rh/L1
ꢁ
b
c
d
e
f
Entry
P/Rh
T ( C)
P (MPa)
Conv. (%)
Aldehyde (%)
Oct. (%)
Iso. (%)
n/i
1
2
3
4
5
6
7
8
9
1
1
1
1
1
0
5
80
80
80
80
80
80
60
80
100
120
70
70
70
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
0.5
1.0
2.0
3.0
78.0
97.8
97.2
87.5
71.3
63.3
26.9
96.5
98.2
98.4
97.2
98.1
97.6
97.8
16.4
64.9
85.2
89.3
90.0
88.5
94.1
91.9
86.4
82.0
80.4
89.5
94.1
95.2
61.0
17.6
1.8
0.7
1.0
1.0
3.5
3.3
8.1
8.2
3.5
1.5
0.5
0.4
22.6
17.5
13.0
10.0
9.0
10.5
2.4
4.8
5.5
9.8
16.1
9.0
5.4
2.7
2.8
9.0
10.6
10.8
10.7
10.5
10.8
8.3
5.5
9.7
9.4
9.2
10
15
20
25
10
10
10
10
5
0
1
2
3
4
5
5
5
70
4.4
8.4
a
c
b
Reaction conditions: S/C ratio of entry 1–6 is 2000 and others are 1000, [Rh] ¼ 1.60 mM, toluene 3 ml, 1 h. The conversion of 1-octene.
d
e
f
Selectivity for aldehyde in all product. Selectivity for octane in all product. Selectivity for 2-octene in all product. Molar ratio of linear to
branched aldehyde. The products were analyzed by GC.
afforded lower n/i and higher aldehyde selectivity compared
with L4.
Mechanism by DFT calculation and
discussion
Scheme 2 Ligand dissociation in Rh-catalyzed hydroformylation.
To obtain insight information about the inuence of ligands
with different exibility on the present Rh-catalyzed hydro-
formylation process, the DFT calculation with the emphasis on
the geometric properties of the various active catalytic species of
carbon monoxide dissociation and stronger alkene coordina-
2
0–22
tion
owing to bearing the electron withdrawing pyrrolyl. In
L–Rh(CO) H (L ¼ L1 or L3) was conducted at the M06/6-31G**,
2
addition, L3 also possessed of pyrrolyl group but afforded
higher regioselectivity along with lower aldehyde selectivity
compared with L1 (Table 2), that is, the n/i ratio was 93.9 when
L3 was used as the ligand, correspondingly, and 10.8 was
afforded by L1. But it should be noted that when L1 was used,
the aldehyde selectivity is about 10.0% higher than that with
L3 (Table 2,† entry 1 vs. entry 3). The difference between L1 and
L3 is that –CH – was inserted between the benzene ring and
2
the oxygen atom in L1, for the aim of increasing the exibility
of the ligand and improving the regioselectivity and activity,
23
LanL2DZ level. M06 method combined with the Lanl2DZ basis
24
25
set for Ph and 6-31G** basis set for the rest atoms was
employed in the gas-phase simulation. The optimization on the
3
similar compounds BISBI–Rh(PPh )(CO)H whose crystal struc-
26
ture parameters are available in the reported literature, was
performed to test the reliability of this method. The calculation
shows that the derivation in the length of the key bonds is less
˚
than 0.05 A, indicating that M06 method is suitable for the
present system. To reduce computational cost, the octene was
replaced by propene as the substrate for the present calculation.
DFT calculation predicted that the catalytic process starts
although the addition of –CH – did not benet the regiose-
2
lectivity for 1-octene hydroformylation. Similarly, L2 also
from the formation of active species of L–Rh(CO)
2
H. The bite
a
Table 2 Hydroformylation of 1-octene with Rh/ligands
b
c
d
e
f
Entry
Ligand
Con. (%)
Aldehyde yield (%)
Oct. (%)
Iso. (%)
n/i
1
2
3
4
5
L1
L2
L3
L4
L5
96.5
89.7
97.4
78.0
79.8
91.9
70.0
80.9
51.9
90.0
3.3
21.7
11.5
27.6
5.4
4.8
8.3
7.6
20.5
4.6
10.8
2.6
93.9
7.4
61.0
a
ꢁ
b
Reaction conditions: S/C ¼ 1000, T ¼ 80 C, L/Rh ¼ 10, P ¼ 1.5 MPa, n(CO)/n(H
2
) ¼ 1, [Rh] ¼ 1.6 mM, toluene 3 ml, t ¼ 1 h. The conversion of 1-
c
d
e
f
octene. Selectivity for aldehyde in all product. Selectivity for octane in all product. Selectivity for 2-octene in all product. Molar ratio of linear to
branched aldehyde. The products were analyzed by GC.
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ꢁ
angle of P–Rh–P is predicted to be 93.5 for L1–Rh(CO)
2
H, while
ꢁ
that of L3–Rh(CO) H is supposed to be 114.9 . L–Rh(CO) H
2
2
complex was reported to be a mixture of the equatorial-apical
20,26
(ea) and diequatorial (ee) isomers.
The equilibrium compo-
sition of the isomers depended on the bite angle of P–Rh–P. In
ꢁ
L3–Rh(CO)
2
H, :P–Rh–P 114.9 might make ee isomer
15
predominate which causes linear aldehyde readily to generate.
Nevertheless, :P–Rh–P 93.5 might make ea isomer predomi-
ꢁ
2
nate in L1–Rh(CO) H and result in lower regioselectivity
towards linear aldehyde (the n/i ratio for Rh/L3 is 93.9 and Rh/
L1 is 10.8, Table 3†). In general, the temperature would also
inuence the equilibrium composition and ee isomer is
favourable at high temperature (high n/i ratio). Nevertheless, an Scheme 3 The geometric evolution of the ligand–Rh intermediates.
opposite trend to the Rh/L1 catalyst system was observed at
a lower temperature (Table 1, entry 7–10). The electron-
withdrawing ligand L1 might exert a strong chelate effect on Rh/L1 system and 43.8 kJ mol for Rh/L3 system, respectively.
ꢂ1
Rh atom and could stabilize the catalytic active species (the ee As a result, the insertion of olen and CO step could be
27,28
isomers),
which expresses a good linear aldehyde selectivity easier to occur in the Rh/L1 system than Rh/L3 system in
at a lower temperature. On the other hand, higher temperatures thermodynamics. The above two aspects might be the reason
might cause dissociation of the ligand in Rh-catalyzed hydro- for higher aldehyde selectivity which was afforded by Rh/L1
formylation, which is unfavourable the regioselectivity.
The evolution of the calculated bite angle of the major
compared to Rh/L3 system.
In addition, the regioselectivity was lower at higher CO/H
2
intermediates from I to V is depicted in Fig. 2. As shown in this pressure (Table 1, entries 11–14). It could be explained by the
gure, the geometric evolution from I to V involves in the penta- reason that the phosphorus atoms in the complex IV could be
coordinated and tetra-coordinated Rh species, and the corre- replaced by CO, which was against the generation of linear

8
,21
sponding bite angle of P–Rh–P in compounds (I–V) varies aldehyde with the concentration of CO increasing.
during the entire reaction. For L1–Rh compounds, the bite
ꢁ
angle of P–Rh–P relatively smoothly changes less than 10 . In
Conclusion
contrast, this bite angle of L3–Rh system drastically frustrates in
ꢁ
the range of 14.0 as compared with L1–Rh system. L1–Rh
On the basis of the unique electronic properties of N-pyrrolyl-
phosphorus ligand, –CH – moiety was introduced to modify the
exibility of the ligand. L1 was synthesized and applied to Rh-
compounds seem more exible, which might facilitate the
dissociation and coordination processes, and therefore improve
the reaction rate of the hydroformylation. It should be noted
that the step from III to IV represents the insertion of olen into
Rh–H bond (shown in Scheme 3), and transform of V to VI
corresponds to the insertion step of CO. The above two steps
are very vital for the formation of the aldehyde. M06 calculation
predicts that the DH of the olen-migration step to be ꢂ43.4 kJ
2

catalyzed hydroformylation of 1-octene. Under optimized reac-
ꢁ
tion condition (80 C, 1.5 MPa, L/Rh ¼ 10, S/C ¼ 1000, 1 h), the
aldehyde selectivity and the n/i ratio could reach up to 91.9%
and 10.8, respectively. The results revealed that the increase of
exibility was benecial to the selectivity towards aldehyde. And

calculation showed the olen-insertion into the Rh–H bond of
intermediate III and CO-insertion into Rh–alkyl bond of inter-
mediate V could be facilitated by the more exible ligand L1 and
thus higher aldehyde selectivity was afforded by Rh/L1
compared to Rh/L3 system.
ꢂ1
ꢂ1
mol for Rh/L1 system and ꢂ13.2 kJ mol for Rh/L3 system,
ꢂ1
and the DH of the CO-insertion step to be ꢂ27.3 kJ mol for
Acknowledgements
The authors thank the nancial supports from the National
Natural Science Foundation of China (201202108), from the
Opening Project of Key Laboratory of Green Chemistry of
Sichuan Institutes of Higher Education (LZJ1402) and from the
Sichuan university outstanding scholars research fund
(2015SCU04A05).
Notes and references
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Fig. 2 The evolution of the calculated bite angle of the major inter-
mediates from I to V.
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