Synthesis of rac-ɑ-aryl propionaldehydes via branched-selective hydroformylation of terminal arylalkenes using water-soluble Rh-PNP catalyst

Article abstract of DOI:10.1016/j.cclet.2021.07.068

This work detailed the preparation of a class of water-soluble PNP ligands that differed by the nature of the substitute on phenyl ring of ligands. These ligands were incorporated into water-soluble rhodium-PNP complex catalysts that were used to regioselective hydroformylation of a series of terminal arylalkenes, providing efficient access to rac-α-aryl propionaldehydes in good to excellent yield (up to 97%) and branched-regioselectivity (up to 40:1 b/l ratio). Furthermore, gram-scale and diverse synthetic transformation demonstrated synthetic application of this methodology for non-steroidal antiinflammatory drugs.

Full text of DOI:10.1016/j.cclet.2021.07.068

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Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
Communication
Synthesis of rac-ɑ-aryl propionaldehydes via branched-selective
hydroformylation of terminal arylalkenes using water-soluble Rh-PNP
catalyst
Peng Gaoa, Miaolin Keb,c, Tong Rub,c, Guanfeng Liangb,c,∗, Fen-Er Chena,b,c,∗
a Department of Chemistry, Sichuan University, Chengdu 610064, China
b Department of Chemistry, Fudan University, Shanghai 200433, China
c Shanghai Engineering Center of Industrial Asymmetric Catalysis for Chiral Drugs, Fudan University, Shanghai 200433, China
a r t i c l e i n f o
a b s t r a c t
Article history:
This work detailed the preparation of a class of water-soluble PNP ligands that differed by the nature of
the substitute on phenyl ring of ligands. These ligands were incorporated into water-soluble rhodium-PNP
complex catalysts that were used to regioselective hydroformylation of a series of terminal arylalkenes,
providing efficient access to rac-α-aryl propionaldehydes in good to excellent yield (up to 97%) and
branched-regioselectivity (up to 40:1 b/l ratio). Furthermore, gram-scale and diverse synthetic transfor-
mation demonstrated synthetic application of this methodology for non-steroidal antiinflammatory drugs.
© 2021 Published by Elsevier B.V. on behalf of Chinese Chemical Society and Institute of Materia
Medica, Chinese Academy of Medical Sciences.
Received 27 May 2021
Revised 16 July 2021
Accepted 30 July 2021
Available online xxx
Keywords:
Aryl propionaldehydes
Hydroformylation
Water-soluble PNP ligands
Regioselectivity
Rhodium
rac-ɑ-Arylaldehydes have been extensively used as impor-
tant synthetic precursors for the profen family (ɑ-arylpropionic
acid derivatives), a class of non-steroidal anti-inflammatory drugs
(NSAIDs) in pharmaceutical chemistry [1-5]. At present, more than
20 profen NSAIDs has been used in clinical for treating the heat of
inflammation, pain, swelling and fever [6,7]. Some representative
examples are illustrated in Fig. 1, such as ibuprofen (1) [8], keto-
profen (2) [9], flurbiprofen (3) [10], naproxen (4) [11] and loxopro-
fen (5) [12]. Undoubtedly, the development of efficient approaches
towards rac-ɑ-arylaldehydes is always a popular topic in both
academic and industrial laboratories [13-17]. Traditional methods
for synthesis of rac-ɑ-arylaldehydes include the rearrangement via
Darzen’s glycidic esters [18,19] and Corey-Chaykovsk’s epoxides
[20] from aryl methylketones (Schemes 1a and b). However, these
processes suffer from several drawbacks such as more compli-
cated synthetic operations, harsh reaction conditions, and func-
tional group incompatibility. Attractive alternatives have been de-
veloped using 1,1-arylmethyl alkenes as feedstocks (Scheme 1c)
[21-23], including Che’s Ru(IV) porphyrin-catalyzed aerobic oxida-
tion via an epoxidation/isomerization tandem pathway [21], and
Lahiri’s catalytic PhIO oxidation by applying iron(II) dipic catalyst,
in situ formed from Fe(BF4)2·6H2O and pyridine-2,6-dicarboxylic
acid [22], and Lei’s anti-markovnikov oxygenation with H2O as oxi-
dant by a photoredox-metal dual catalysis [23]. However, protocols
currently lack broad substrate scope due to the difficulties for ac-
cess to 1,1-arylmethyl alkenes as special substrates.
The transition metal-catalyzed hydroformylation of terminal
arylalkenes to arylaldehydes could provide an alternative, however
typically only a small percent of the β-arylaldehyde products was
formed alongside the ɑ-arylaldehyde products. There are remark-
able exception of Rh phosphine catalysts [24-35] including Wink’s
a cationic bis(dioxaphospholane)rhodium complex [24], Marchetti’s
Rh/pyridylphosphines pydiphos P-oxide complex [25], Amer’s
Rh/amino phosphine complex [26], Alper’s Rh/diphenylphosphino-
yl)phenylmethanol catalysis [27], Börner’s Rh/diphosphate catalysis
[28], Lerous’s Rh/dibenzophosphole catalysis etc. [29], which form
the β-aldehyde products with a good level of selectivity, but only
for very limited substrates (Scheme 1d). Recent branched-selective
hydroformylation of terminal arylalkenes has been reported using
PPh3-modified Fe as catalyst [30], but the limited success has been
achieved toward this hydroformylation (Scheme 1d). Hydroformy-
lations of terminal olefins were independently reported by Nozaki’s
group and Clarke’s group, but the two methods were limited to the
preparation of ɑ-alkylaldehyde products [31, 32]. Apart from this
class of homogeneous catalysis, significant breakthrough have been
achieved toward synthesis of β-arylaldehydes via biphasic Rh-
∗
Corresponding authors.
E-mail addresses: lianggfeng@fudan.edu.cn (G. Liang), rfchen@fudan.edu.cn (F.-E.
Chen).
https://doi.org/10.1016/j.cclet.2021.07.068
1001-8417/© 2021 Published by Elsevier B.V. on behalf of Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences.
Please cite this article as: P. Gao, M. Ke, T. Ru et al., Synthesis of rac-ɑ-aryl propionaldehydes via branched-selective hydroformylation of
terminal arylalkenes using water-soluble Rh-PNP catalyst, Chinese Chemical Letters, https://doi.org/10.1016/j.cclet.2021.07.068

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Fig. 1. Representative examples for synthesis of rac-profen family (1-5) from the
corresponding rac-α-aryl propionaldehydes.
Fig. 2. Synthesis of water-soluble PNP ligands (L1-L11). (i) TEA, THF, r.t., 20-24 h;
(ii) NaI, acetone, reflux, 20-30 h, or LiOH, THF/H2O, 4-20 h; (iii) NaOH, THF/H2O,
0.5-1 h.
atom economy [39]. Herein, we report the preparation of a series
of novel water-soluble PNP ligands and their applications to the
Rh-catalyzed branched-selective hydroformylation of terminal ary-
lalkenes for the synthesis of rac-ɑ-aryl propionaldehydes in aque-
ous biphasic catalytic system.
The known water-soluble PNP ligands L5 and L11 and new lig-
ands L1-L4 and L6-L10 were prepared in high to excellent yields
from bis(2-diphenylphosphinoethyl)amine hydrochloride (6) and
appropriate carbonyl halides, and diphenic anhydride (9) and 2-
sulfobenzoicanhydride (10) following a modified procedure devel-
oped by Whiteside and co-workers (Fig. 2) [40].
With this library of water-soluble PNP ligands in hand, we be-
gan our studies with an examination of their catalytic performance
in Rh-catalyzed branched-selective hydroformylation of styrene 11a
as the benchmark substrate. As described in Table 1, all styrene
hydrorformylation reactions were completed in 24 h toluene/H2O
(1:1) solvent at 3.0 MPa of syngas and 60 °C in the presence
of 0.05 mol% [Rh(COD)Cl]2 and 0.6 mol% ligand (Rh/L = 1:6). In
all the excellent chemoselectivities were observed, and no hydro-
genation product was detected via 1H NMR analysis. The carbon-
ated PNP ligands L1-L6 were first examined. The ligands L1 and
L2 bearing COONa at benzene’s ortho- or meta-position, gave the
branched aldehyde 12a in moderate yields with good regioselectiv-
ities (b/l = 8.5:1 and 8.4:1, entries 1 and 2), respectively. When the
ligand L3 substituted with meta-OMe and meta-COONa groups on
phenyl ring was employed, poor yield of 12a was obtained, how-
ever, the b/l ratio in this instance was still exceptional (8.4:1, entry
3). Reaction conducted with the ligand L4 possessing COONa group
at para-position of phenyl ring, delivered branched aldehyde 12a in
only 42% yield and with lower regioselectivity (b/l = 6.5:1, entry 4).
Scheme 1. Strategies to access rac-ɑ-arylaldehydes.
catalyzed hydroformylation of terminal olefin using water-soluble
poly(4-pentenoic acid (PPA)/bis(2-diphenylphosphino)ethyl) (DP-
PEA) or tri-meta-sulfonatophenylphosphine (TPPTS), or phenyl-β-
D-glucopyranoside catalyst as ligands (Scheme 1e) [36-38]. Despite
the obtained achievements in this field, the developing of a prac-
tical and efficient approach to access the rac-ɑ-aryl propionaldehy-
des is still highly desirable from the perspective of green, step and
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Table 1
Table 2
Ligands screening for the Rh-catalyzed branched-selective hydroformylation of
styrene.a
Optimization of the Rh-catalyzed hydroformylation of styrene using L11 as the lig-
and.a
Entry
Ligand
t (h)
Yield of 11a (%)b
b/l (12a/13a)c
Entry
Rh/L
T (°C)
P
Solvent
Yield of
12a (%)b
b/l
(MPa)
(12a/13a)c
1
L1
24
24
24
24
24
24
24
24
24
24
24
24
24
24
12
12
12
60
49
21
42
75
84
26
76
23
44
92
74
85
56
91
72
74
8.5:1
8.4:1
8.4:1
6.5:1
3.0:1
5.4:1
3.8:1
3.2:1
10.1:1
4.8:1
11.3:1
2.8:1
5.9:1
10.4:1
10.1:1
10.0:1
2.8:1
1
2d
1/6
1/6
1/6
1/6
1/6
1/3
1/8
1/6
1/6
1/6
1/6
1/6
1/6
1/6
1/6
60
25
50
70
80
60
60
60
60
60
60
60
60
60
60
3.0
3.0
3.0
3.0
3.0
3.0
3.0
2.0
4.0
4.0
4.0
4.0
4.0
4.0
3.0
Tol./H2O
91
8
10.1:1
11.2:1
11.1:1
2.1:1
1.6:1
3.7:1
9.7:1
7.9:1
11.3:1
7.9:1
7.5:1
8.2:1
4.0:1
5.1:1
8:1
2
L2
Tol./H2O
3
L3
3
Tol./H2O
70
68
60
78
47
88
92
89
23
19
80
71
88
4
L4
4
Tol./H2O
5
L5
5
Tol./H2O
6
L6
6
Tol./H2O
7
L7
7
Tol./H2O
8
L8
8
Tol./H2O
9
L9
9
Tol./H2O
10
11
12
13d
14e
15f
16g
17
L10
L11
TPPTS
L11
L11
L11
L11
-
10
11
12
13
14
DCM/H2O
MTBE/H2O
Et2O/H2O
hexane/H2O
c-hexane/H2O
Tol./H2O
e
15
a
Reaction conditions: substrate (3.0 mmol), [Rh(COD)Cl]2 (0.05mol %), L11, or-
ganic solvent (15 mL)/H2O (15 mL), syngas (CO/H2 = 1:1), SDBS (0.86 mol %).
a
Reaction conditions: Styrene 11a (3.0 mmol), [Rh(COD)Cl]2 (0.05 mol%), lig-
and (0.6 mol%), toluene (15 mL), H2O (15 mL), syngas (CO/H2 = 1:1, 3.0 MPa),
24 h.
b
GC yield.
c
Determined by GC.
d
b
10% conversion.
GC yield.
e
c
The second recycle.
Determined by GC.
d
Rh(acac)(CO)2 (0.1 mol%).
e
RhCl3 (0.1 mol%).
f
0.86 mol% sodium dodecylbenzenesulfonate (SDBS) was used.
g
Absence of SDBS.
activity and regioselectivity of the Rh-catalyzed styrene hydro-
formylation was investigated. Notably, the activity and regioselec-
tivity of the reaction had obvious temperature effect. Lowering
the temperature from 60 °C to 25 °C under standard condition
(toluene/H2O (1:1), 3.0 MPa CO/H2 (1:1), 0.86 mol% SDBS) dramat-
ically reduced the yield to 8% within 12 h (entry 1 vs. 2). Perform-
ing the reaction at 50 °C revealed an improved yield of 70% with
good regioselectivity (b/l = 11.1:1, entry 3) with full conversion in
12 h. At temperature over 60 °C, the regioselectivity and yield for
the branched 12a was also significantly reduced (68%, b/l = 2.1:1
at 70 °C; 60%, b/l = 1.6:1 at 80 °C; entries 4 and 5). Increasing
or decreasing the molar ratio of Rh/ligand L11 distinctly impaired
the yield and regioselectivity for this transformation. A decrease
of the Rh ligand L11 molar ratio from 1:6 to 1:3 led to low re-
gioselectivity (b/l = 3.7:1, entry 1 vs. 6). Moreover, further increas-
ing the molar ratio of Rh/ligand L11 to 1:8 produced a significant
rise in regioselectivity (b/l = 9.7:1), but with poor yield (47%, en-
try 7). Varying pressure of syngas (CO/H2 = 1:1) did not retard the
rate but did alter the selectivity towards branched-selective prod-
uct. Reducing the syngas pressure to 2.0 MPa resulted in a decrease
in branched aldehyde 12a from b/l = 10.9:1 to 7.9:1 (entry 1 vs. 8).
A slightly increase in yield and regioselectivity was observed when
4.0 MPa syngas pressure was used for this transformation (entry
9). In addition, the use of other organic/aqueous biphasic systems,
suh as DCM/H2O, MTBE/H2O, hexane/H2O, cyclohexane/H2O, was
found to be less effective than toluene/H2O (entries 10-14). After
reaction conditions were screened, the Rh-catalyzed hydroformy-
lation reaction was performed using L11 as a ligand under 4.0
MPa of syngas (CO/H2, 1:1) at 60 °C in toluene/H2O (1:1) bipha-
sic system in the presence of 0.86 mol% SDBS. Optimized conver-
sion, yield for branched aldehyde 12a, and regioselectivity were
achieved under this reaction conditions. The water-soluble Rh/L11
catalyst can be readily recycled by simple phase separation and
still gave 88% yield of 12a in the second run (entry 15).
The dicarboxylated PNP ligand L5 gave better yield (75%, entry 5)
compared with results of monocarboxylated PNP ligands (L1-L4),
but with poor regioselectivity (b/l = 3.0:1). Moreover, the mono-
carboxylated PNP ligand L6 with a biphenyl ring resulted in much
higher yield (84%), albeit with a ratio of 12a/13a of 5.4:1 (entry
6). The sulfonated PNP ligand L7 with meta-SO3Na gave poor yield
and regioselectivity (26%, b/l = 3.8:1, entry 7) under identical con-
ditions. The ligand L8 bearing meta-SO3Na and para-MeO groups
of phenyl ring was shown to give superior yield of 12a, but much
worse regioselectivity (12a/13a = 3.2:1) was obtained (entry8). A
change of the position of SO3Na group on ligand L7 from meta- to
para-position resulted in similar yield (entry 9), but high regiose-
lectivity compared with the results of the corresponding sulfonated
PNP ligand L7-L10. The use of ligand L11 with ortho-SO3Na group
on phenyl ring afforded branched aldehyde 12a in 92% yield with
high regioselectivity (b/l = 11.3:1, entry 11). The well-known TPPTS
only gave 74% yield of 12a, with poor regioselectivity (b/l = 2.8:1,
entry 12) under identical conditions. Other common Rh catalysts
for hydroformylation, such as Rh(acac)(CO)2) and RhCl3, low re-
gioselectivities and reactivities were respectively obtained (entries
13 and 14). Addition of SDBS (0.86 mol%) as the surfactant could
short reaction time to 12 h, and provide branched 12a in 91% yield
and a 10.1:1 ratio of b/l (entry 15). Branched product was obtained
in 72% yield with 10.0:1 ratio of b/l in absence of SDBS (Table 1,
entry 16). The control experiment of absence of ligand was also
carried out to give the product in 74% yield with 2.8:1 ratio b/l (en-
try 17). These results revealed the crucial role of these two types
of ligand structures in controlling of yield and regioselectivity for
this Rh-catalyzed hydroformylation reaction.
With the best ligand L11, a series of Rh-catalyzed styrene hy-
droformylation parameters were examined, including temperature,
Rh/ ligand L11 molar ratio, organic/aqueous biphasic system, and
CO/H2 pressure (Table 2). First, the temperature impacted on the
3

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Scheme 2. Scope for the hydroformylation catalyzed by Rh/L11. Reaction conditions: Substrate (0.5 mmol), [Rh(COD)Cl]2 (0.2 mol %), L11 (2.4 mol%), toluene (1 mL), H2O (1
mL), 60°C, 18-48 h, syngas (CO/H2 = 1), SDBS (0.86 mol%), all yields were isolated yield.
With the optimized reaction conditions, the suitability of this
protocol was investigated with a range of terminal arylalkenes. As
shown in Scheme 2, the mono-substituted phenyl terminal alkenes
bearing electron-donating (methyl (11b-11d), iso-butyl (11e), tert-
butyl (11f), hydroxyl (11g), methoxy (11h)), electron-withdrawing
(fluro (11i-11k), chloro (11l-11n), bromo (11o-11q), and nitro (11r))
groups at any of the position of phenyl ring were hydroformy-
lated efficiently to give the branched aldehydes (12b-12r) in 83%-
94% yield with b/l = 6.9:1-26:4) with full conversion within 24-48
h. Generally, the mono-substituted phenyl terminal alkenes bear-
ing electron-withdrawing groups exhibited higher reactivity and
regioselectivity than their electron-donating groups (11i-11r vs.
11b-11h). Moreover, several sensitive functional groups including
chloro (11l-11n), bromo (11o-11q), and nitro (11r) were well tol-
Scheme 3. Gram-scale synthesis of rac-naproxen (4).
erated, no hydrogenolysis or hydrogenation products was detected
by GC analysis. Similarly, disubstituted phenyl termial alkenes with
electron-donating (dimethyl 11s, dimethoxy 11t) groups were also
mance in biphasic aqueous hydroformylation of vinyl arenes. A
smoothly hydroformylated into the branched products 12s-12t in
benzenesulfonates-containing PNP ligand L11 gave the most effi-
86% and 90% yield with good regioselectivity (b/l = 8.1:1 and 9.1:1)
cient Rh complex catalyst, affording high regioselectivities towards
resepectively. In addition to phenyl terminal alkenes, 2-naphthyl
ɑ-aryl propionaldehydes from the branched-selective hydroformy-
terminal alkene (11u) was also a suitable substrate for this hydro-
lation of various terminal arylalkenes. The exploration of substrate
formylation reaction, delivering the branched product 12u in 89%
scope showed that the vinyl arenes with electron-withdrawing
yield with b/l = 15.1:1. The heterocylcic olefin bearing thienyl ring
groups were usually more branched selective than those bear-
also smoothly transformed to the corressponding product 12v in
ing electron-donating groups. Finally we demonstrated an aqueous
65% yield along with low selectivity (b/l = 3.3:1).
route to the synthesis of anti-inflammatory drugs rac-naproxen in
To further evaluate the efficiency and synthetic utility of
an efficient and green manner.
this branched selective hydroformylation, we carried out the
scale-up experiment using
6 mmol (1.10 g) of 6-methoxy-2-
vinylnaphthalene 11w [41], which could be easily prepared in
two steps with an overall yield of 50% starting from commer-
cially available 6-methoxy-2-acetonaphthalene (14). The branched-
selectivehydroformylation of 11w could be achieved using 0.2 mol%
of the Rh/L11 catalyst within 24 h under standard reaction condi-
tions (4.0 MP of CO/H2 (1:1)) at 60 °C in tulene/H2O (1:1) to give
the aldehyde 12w in 97% yield with b/l = 40:1. Oxidation of crude
product 12w under Pinnick reaction conditions (NaClO2, KHPO2,
2-methyl-2-butene, t-BuOH, H2O, 0 °C to r.t., 1 h) provided rac-
naproxen (4) in 90% isolated yield (Scheme 3) [42].
Declaration of competing interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared to
influence the work reported in this paper.
Acknowledgments
We are grateful for financial support from the National Natu-
ral Science Foundation of China (No. 21902032). G. Liang acknowl-
edges the funding support from Fudan University.
In conclusion, we prepared
a series of new water-soluble
PNP rhodium catalysts and screened their catalytic perfor-
4

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