Rhodium-Catalyzed Enantioselective Anti-Markovnikov Hydroformylation of α-Substituted Acryl Acid Derivatives

Article abstract of DOI:10.1021/acs.orglett.9b04624

Rhodium-catalyzed asymmetric anti-Markovnikov hydroformylation of α-substituted acrylates/acrylamides has been developed. By employing the Rh/(S,S)-DTBM-YanPhos complex, a series of β-chiral linear aldehydes were obtained in high yields (up to 94% yield) and high enantioselectivities (up to 96% ee). The utility of this methodology is demonstrated by a gram-scale reaction and a concise synthetic route to chiral ?3-butyrolactone.

Full text of DOI:10.1021/acs.orglett.9b04624

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Letter
Rhodium-Catalyzed Enantioselective Anti-Markovnikov
Hydroformylation of α‑Substituted Acryl Acid Derivatives
Shuailong Li, Zhuangxing Li, Cai You, Xiuxiu Li, Jiaxin Yang, Hui Lv,* and Xumu Zhang*
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ABSTRACT: Rhodium-catalyzed asymmetric anti-Markovnikov hydroformylation of α-substituted acrylates/acrylamides has been
developed. By employing the Rh/(S,S)-DTBM-YanPhos complex, a series of β-chiral linear aldehydes were obtained in high yields
(up to 94% yield) and high enantioselectivities (up to 96% ee). The utility of this methodology is demonstrated by a gram-scale
reaction and a concise synthetic route to chiral γ-butyrolactone.
asymmetric hydroformylation of 1,1-disubstituted olefins has
been far less established.
wing to the efficient formation of chiral aldehydes in an
Oatom-economic manner, enantioselective hydroformyla-
tion has attracted great attention in both academia and
industry, and a great deal of efforts have been made in this
field.1 As a result, a variety of privileged ligands for asymmetric
hydroformylation (AHF) of monosubstituted and 1,2-disub-
stituted alkenes, such as Binaphos,2 bis(diazaphospholane)
(BDP),3 YanPhos,4 Ph-BPE,5 Chiraphite,6 Bobphos,7 and
Bettiphos,8 have been developed, giving α-chiral aldehydes in
good yields with high enantioselectivities, which have greatly
promoted the development of asymmetric hydroformylation.
However, most of these ligands were incapable of catalyzing
1,1-disubstituted alkenes to the corresponding β-chiral linear
aldehydes (as indicated by Keulemans’ empirical rule),9
although they performed very well on AHF of monosub-
stituted and 1,2-disubstituted alkenes. To date, the inves-
tigations on AHF of 1,1-disubstituted alkenes are rather
limited10 due to the low reactivity derived from the steric
hindrance standing on the same side of the double bond and
the low enantioselectivity derived from the inability of the
chiral catalyst to differentiate the two enantiotopic faces of the
substrates.
With respect to asymmetric hydroformylation, our group is
devoted to addressing some long-standing tough problems in
AHF by the development of new chiral ligands. Recently, our
group designed and synthesized a series of (S,S)-YanPhos,
which showed high activity and selectivity on unfunctionalized
1,1-disubstituted alkenes.4g,i Given the very similar stereo-
control model, we believe that (S,S)-YanPhos may also have
good performance in asymmetric hydroformylation of
functionalized 1,1-disubstituted olefins, providing an efficient
approach to 1,4-dicarbonyl compounds, which are widely
occurring in biologically active molecules and active
pharmaceutical ingredients (Figure 1).12 With this thought in
mind, we investigated the asymmetric hydroformylation of α-
substituted acryl acid derivatives.
On the basis of previous studies, 1a was chosen as the
standard substrate to optimize reaction conditions (Table 1).
Initially, several representative ligands in AHF and in the
asymmetric hydrogenation area, such as (S)-BINAP, Xuphos,
(S,S)-Me-Duphos, and Walphos, were tested (Figure 2).
Disappointingly, all of them exhibited low activities in this
transformation and afforded target product 2a with low
enantiomeric excess (ee). When (S)-Binapine was employed,
only a racemic hydrogenation product was generated (entry 5).
In 2011, Buchwald and co-workers reported their pioneering
work in Rh-catalyzed AHF of functionalized 1,1-disubstituted
olefins and successfully synthesized a series of β-chiral linear
aldehydes from α-alkyl acrylates by using a Rh/BenzP*
catalytic system.11 However, the substrate scope is limited,
and only bulky α-branched alkyl-substituted acrylates can
afford target products with high yields and high ee’s (up to
91% yield, 94% ee); hence, the general and efficient
Received: December 23, 2019
https://dx.doi.org/10.1021/acs.orglett.9b04624
© XXXX American Chemical Society
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A

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a
Table 1. Ligand Screening in the Asymmetric Hydroformylation of Ethyl 2-Benzylacrylate (1a)
b
c
d
entry
ligand
(S)-BINAP
Xuphos
(S,S)-Me-Duphos
Walphos
(S)-Binapine
(Rc,Sp)-Duanphos
(S,R)-YanPhos
(S,S)-YanPhos
(S,S)-DTB-YanPhos
(S,S)-DTBM-YanPhos
(S,S)-DTBM-YanPhos
(S,S)-DTBM-YanPhos
(S,S)-DTBM-YanPhos
CO/H2 (bar)
conv. (%) (%)
2a:3a
ee (%)
1
2
3
4
5
6
7
8
5/5
5/5
5/5
5/5
5/5
5/5
5/5
5/5
5/5
11(3)
6(5)
14(3)
17:83
84:16
22:78
28:72
-
31:69
81:19
97:3
97:3
97:3
97:3
97:3
97:3
10(R)
35
53
55
64(13)
>99(0)
>99(28)
55(46)
>99(78)
>99(80)
>99(84)
>99(84)
88(85)
ND
76(R)
20
54
86
90
90
90
90
9
10
11
5/5
2.5/2.5
2.5/2.5
2.5/2.5
e
12
f
g
13
>99(94)(94)
a
Unless otherwise stated, all reactions were performed on a 0.2 mmol scale in 1 mL of toluene with substrate/Rh = 100 at 90 °C for 20 h. All
temperatures in the text were the setting temperatures of the automatic heating control system, and the actual temperatures were changed within
b
c
d
0.5 °C of the setting temperatures. Determined by 1H NMR spectroscopy. Yield of 2a by 1H NMR spectroscopy. Determined by HPLC analysis
after a Wittig reaction. The absolute configuration was assigned by comparing the sign of the optical rotation of (S)-2a with that reported in the
literature. See ref 1 in the Supporting Information. T = 70 °C. L/Rh = 2/1, 80 °C. Isolated yield.
e
f
g
Figure 1. Biologically active compounds containing 1,4-dicarbonyl
moieties.
(Rc,Sp)-Duanphos gave 2a (entry 6, 76% ee) with good
Figure 2. Structures of the evaluated ligands for asymmetric
enantioselectivity along with a mass amount of branched
aldehyde 3a. (S,R)-YanPhos, which proved to be efficient on
AHF of monosubstituted and 1,2-disubstituted alkenes,
afforded 2a in 46% yield with 20% ee as well as a spot of 3a
(entry 7). (S,S)-YanPhos which has excellent performance in
AHF of unfunctionalized 1,1-disubstituted olefins was
evaluated; to our delight, full conversion and moderate ee
(entry 8, >99% conv., 54% ee) were obtained, and the
fomation of 3a was effectually inhibited simultaneously. Then,
more bulky chiral ligands, such as (S,S)-DTB-YanPhos and
(S,S)-DTBM-YanPhos, were tested, and the results indicated
that the enantioselectivity was gradually improved with the
increase of the steric hindrance on the phosphine atom of the
ligand. Thus, the highest ee was obtained (entry 10, 90% ee)
with (S,S)-DTBM-YanPhos, albeit with 3% branched aldehyde
3a. Subsequently, the impact of temperature and the ratio of
ligand to metal was evaluated (for more details on the
hydroformylation of 1a.
condition optimization, see the ligand screening in Supporting
Information), and the best result was obtained when the
reaction was conducted at a lower temperature (80 °C) and
lower ratio of ligand to metal (L/Rh = 2/1), giving 2a with
high isolated yield and excellent ee (entry 13, 94% isolated
yield, 90% ee).
Under the optimized reaction conditions, we explored the
substrate scope and tested the generality of this asymmetric
transformation. In order to probe the effect of the ester group
on this AHF reaction, a series of substrates containing different
ester groups were investigated (Scheme 1). Linear aldehydes
with β-chirality (2a, 2b, 2c, 2d) were obtained with high yields
and excellent ee’s, and a slight trend emerged that the ee value
increased slightly with the increase of ester group steric
B
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Scheme 1. Asymmetric Hydroformylation of α-Substituted
Acrylates and Acrylamides*
Scheme 2. Asymmetric Hydroformylation of α-Substituted
Acrylates*
*
Reaction conditions: 1 (0.2 mmol), Rh(acac)(CO)2 (1 mol %),
(S,S)-DTBM-YanPhos (2 mol %), CO (2.5 bar), H2 (2.5 bar),
toluene (1 mL), 80 °C, 20 h. Enantiomeric excess was determined by
a
HPLC analysis using a chiral stationary phase. Isolated yields. 0.5 mL
of toluene, 48 h.
hindrance. The steric hindrance of benzhydryl is too large,
which might reduce the coordination ability of 1d to rhodium,
and the conversion was thus decreased to some extent. At the
same time, phenyl acrylate and benzyl acrylate were trans-
formed to 2e and 2f smoothly with high yields and good ee,
respectively. However, acrylamides showed lower reactivity
and enantioselectivity (2g). By changing the amide-protecting
group and prolonging the reaction time, the corresponding
aldehydes with β-chirality (2h, 2i) were obtained with
moderate to good yields and ee, respectively.
We then investigated the scope of the α-substituted acrylates
(Scheme 2). When R1 is an aryl group, a set of halogen atoms
on the para-position of the benzene ring were compatible for
this transformation to generated β-chiral aldehydes with good
yield and high ee values (2j, 2k, 2l). For the substrates bearing
an electron-donating group (OMe, t-Bu) on the benzene ring,
the yields were increased slightly no matter the substituent
position (2m, 2n, 2o, 2p). In contrast, with an electron-
withdrawing cyano group at the para-position on the benzene
ring, 2q was formed with a lower yield as partially
hydrogenated byproduct was produced, but good ee was
retained. Replacing R1 with a heteroaryl group (2-furyl), the
reaction worked very well (2r). Substrates containing either a
benzyl ether group or an n-amyl were also successfully
transformed to the corresponding chiral aldehydes (2s, 2t).
Commercially available α-methyl methacrylate and dimethyl
itaconate proceeded smoothly in this reaction, offering the
corresponding aldehydes with high yields and good ee (2u,
2v). It is worth noting that the ee was decreased to some
extent when α-branched acrylate was employed (2w).
Compared with the best catalytic system reported previously
for AHF of α-substituted acrylates,11 this method exhibited
better substrate generality as well as better performance in
yield and in enantioselectivity except for AHF of α-branched
acrylate 2w.
*
Reaction conditions: 1 (0.2 mmol), Rh(acac)(CO)2 (1 mol %),
(S,S)-DTBM-YanPhos (2 mol %), CO (2.5 bar), H2 (2.5 bar),
toluene (1 mL), 80 °C, 20 h. Enantiomeric excess was determined by
HPLC analysis using a chiral stationary phase. Isolated yields. 48 h.
a
With the scope of the transformation established, we sought
to demonstrate the synthetic utility of the current method-
ology. As summarized in Scheme 3, a gram-scale AHF reaction
Scheme 3. Gram-Scale Asymmetric Hydroformylation and
the Synthesis of Chiral γ-Butyrolactone
C
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of 1u was conducted under the optimized condition with 0.05
mol % of catalyst loading, giving the target linear aldehyde
(2u) in 91% yield without compromising the enantioselectiv-
ity. Remarkably, a one-pot hydroformylation/Wittig olefination
sequence13 was also compatible, furnishing the corresponding
olefin 4a with 90% yield and 93% ee. In addition, a concise
synthetic route to γ-benzyl butyrolactone 4b was also enabled
by an efficient AHF of 1c, followed by the reduction with
NaBH4 and intramolecular transesterification reaction.
In summary, a rhodium-catalyzed asymmetric hydroformy-
lation of α-substituted acrylates and acrylamides has been
developed, furnishing the corresponding β-chiral linear
aldehydes with generally high yields and good enantioselectiv-
ities. Furthermore, the broad substrate scope, easy scale-up
with a low catalyst loading, and versatile transformations of
aldehyde guarantee wide application of this method in organic
synthesis. Further investigations on asymmetric hydroformyla-
tion and related cascade reactions are currently ongoing in our
laboratory.
(C17783101), and SZDRC Discipline Construction Program
for financial support.
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* Supporting Information
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The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.9b04624.
General Information; ligand screening in the asymmetric
hydroformylation (AHF) of 1a; general procedure for
AHF; procedures for gram-scale AHF of 1u and the
synthesis of 4a; procedures for gram-scale AHF of 1c
and the synthesis of 4b; references; NMR spectra; and
HPLC spectra (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
Hui Lv − Wuhan University, Wuhan, China;
orcid.org/0000-0003-1378-1945; Email: huilv@
whu.edu.cn
Xumu Zhang − Southern University of Science and
Technology, Shenzhen, China; orcid.org/0000-0001-
5700-0608; Email: zhangxm@sustc.edu.cn
Other Authors
Shuailong Li − Wuhan University, Wuhan, China
Zhuangxing Li − Wuhan University, Wuhan, China
Cai You − Wuhan University, Wuhan, China
Xiuxiu Li − Wuhan University, Wuhan, China
Jiaxin Yang − Wuhan University, Wuhan, China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.9b04624
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for financial support from the National Natural
Science Foundation of China (Grant Nos. 21871212,
21432007, 21672094), the Natural Science Foundation of
Hubei Province (2018CFB430), Science and Technology
I n n o v a t i o n C o m m i t t e e o f S h e n z h e n
(JSGG20170821140353405 and KQTD2015071710315717),
Shenzhen Nobel Prize Scientists Laboratory Project
D
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