An efficient asymmetric hydrophosphonylation of unsaturated amides catalyzed by rare-earth metal amides [(Me3Si)2N]3RE(μ-Cl)Li(THF)3 with phenoxy-functionalized chiral prolinols

Article abstract of DOI:10.1039/c7ra00468k

An asymmetric hydrophosphonylation reaction of diethyl phosphite with α,β-unsaturated amides catalyzed by [(Me₃Si)₂N]₃RE(μ-Cl)Li(THF)₃ (RE = Sc (1), Y (2), La (3), Yb (4) and Lu (5)) with H₂Lⁿ ((S)-2,4-R₂-6-[[2-(hydroxydiphenylmethyl)pyrrolidin-1-yl]methyl]phenol) (R = ^tBu (H₂L¹); R = 1-cumyl (H₂L²) and R = 1-adm (H₂L³)) was disclosed. The effects of different central metals and proligands on the addition reaction were tested and it was found that the combination of Sc complex 1 and ligand H₂L² gave the best results. An excellent chemical yield (up to 99%) and good to high enantioselectivities (varied from 73 to 89%) were achieved with a relatively broad scope of the unsaturated amides. The active species in the current system was also discussed.

Full text of DOI:10.1039/c7ra00468k

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PAPER
An efficient asymmetric hydrophosphonylation of
unsaturated amides catalyzed by rare-earth metal
amides [(Me3Si)2N]3RE(m-Cl)Li(THF)3 with phenoxy-
functionalized chiral prolinols†
Cite this: RSC Adv., 2017, 7, 19306
*
*
and Yingming Yao
Zenghui Fei, Chao Zeng, Chengrong Lu, Bei Zhao
An asymmetric hydrophosphonylation reaction of diethyl phosphite with a,b-unsaturated amides catalyzed
by [(Me3Si)2N]3RE(m-Cl)Li(THF)3 (RE ¼ Sc (1), Y (2), La (3), Yb (4) and Lu (5)) with H2Ln ((S)-2,4-R2-6-[[2-
(hydroxydiphenylmethyl)pyrrolidin-1-yl]methyl]phenol) (R ¼ tBu (H2L1); R ¼ 1-cumyl (H2L2) and R ¼ 1-
adm (H2L3)) was disclosed. The effects of different central metals and proligands on the addition reaction
were tested and it was found that the combination of Sc complex 1 and ligand H2L2 gave the best
results. An excellent chemical yield (up to 99%) and good to high enantioselectivities (varied from 73 to
89%) were achieved with a relatively broad scope of the unsaturated amides. The active species in the
current system was also discussed.
Received 12th January 2017
Accepted 12th March 2017
DOI: 10.1039/c7ra00468k
rsc.li/rsc-advances
conjugate addition reactions of phosphite to a,b-unsaturated
Introduction
,
N-acylpyrroles.4c–e 7 Subsequently, his group continued to
develop the phospha-Michael reaction of dialkyl phosphine
oxide with various a,b-unsaturated substrates, such as N-
acylpyrroles, N-acylated oxazolidinones and b,b-disubstituted
a,b-unsaturated carbonyl compounds. Most of them gave
good yields and high enantioselectivities. In 2011, a palla-
dium-catalyzed asymmetric addition of diarylphosphines to
a,b-unsaturated N-acylpyrroles was explored by Duan and his
results were as good as those previously reported.4f Ishihara
conducted a 1,4-addition reaction to an a,b-unsaturated
amide with chiral magnesium(II)binaphtholates in 2013,
which gave an 80% yield and 94% enantioselectivity.4g
However, the diversity of metal-based catalysts used in the
phospha-Michael reaction of a,b-unsaturated amide still
needs to be developed.
Our group has been devoted to studying lanthanide
chemistry for several years.8 Recently, we turned to investi-
gate the enantioselective synthesis of some valuable target
molecules using rare-earth metal catalysts. Fortunately,
a series of rare-earth metal complexes together with phenoxy-
functionalized chiral prolinols were proven to be highly effi-
cient catalysts in the epoxidation of a,b-unsaturated ketones9
and also effective in the Michael addition of malonates to
unsaturated ketones.10 All these wonderful results encour-
aged us to seek for new applications of the current catalytic
system. Since the asymmetric phospha-Michael reaction of
a,b-unsaturated amide is still challenging, herein, we report
this novel transformation catalyzed using rare-earth amides
[(Me3Si)2N]3RE(m-Cl)Li(THF)3 with phenoxy-functionalized
chiral prolinols.
In recent years, the catalytic asymmetric synthesis of optically
active phosphonates has attracted increasing attention due to
their important applications in biology and medical science.1
The asymmetric conjugate addition reactions of phosphite to
a,b-unsaturated carbonyl compounds, including aldehydes,2
ketones,3 amides4 and esters,3i,4g,5 promoted by metal-based
catalysts are regarded as simple and efficient methods for
C–P bond formation. Among the investigations, the phospha-
Michael reaction of a,b-unsaturated amides is quite limited,
with only a few reports focused on the subject. As early as
2001, Quirion realized the synthesis of amidophosphonates,
which was the rst case of a phospha-Michael reaction of an
a,b-unsaturated amide.4a Thirteen years later, Leung dis-
closed the rst asymmetric phospha-Michael addition of
diarylphosphines to N-enoyl phthalimides catalyzed by
a chiral palladacycle catalyst.4b Soon aer, they achieved the
rst catalytic asymmetric phospha-Michael addition of
Ph2PH to 4-oxo-enamides.6 Almost at the same period, Wang
successfully employed a dinuclear zinc catalyst to catalyze the
College of Chemistry, Chemical Engineering and Materials Science, Soochow
University, Key Laboratory of Organic Synthesis of Jiangsu Province, Dushu Lake
Campus, Suzhou 215123, P. R. China. E-mail: zhaobei@suda.edu.cn; yaoym@suda.
edu.cn; Fax: +86 512 65880305; Tel: +86 512 65880305
† Electronic supplementary information (ESI) available: General procedures for
the preparation of the complexes and the catalytic reaction; characterization
data, including 1H and 13C NMR spectra, HPLC chromatograms, HRMS,
crystallographic data for compounds 8 and 7a and gures depicting solid state
structures. CCDC 1526584 and 1536158. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c7ra00468k
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an acceptable yield, considering the good enantiomeric excess
achieved (Table 1, entries 4, and 6–7).
Results and discussion
The effects of the center rare-earth metals of the amides were
next to be explored. As expected, the radius of the center metals
signicantly inuenced both the yield and enantioselectivity,
that is, the outcome of the transformation improved when the
radius of center metals decreased. When scandium amide 1 was
employed as the catalyst in the model reaction, the ee value
reached 83%, while the yield was maintained at 80% (Table 1,
entries 7–11). As we know, solvent oen plays an important role
in homogenous reactions, and some commonly used solvents
were screened subsequently. Fortunately, the reaction gave an
excellent yield and high enantioselectivity in 1,4-dioxane at
20 ꢀC (Table 1, entries 11–16). Finally, the ratio of scandium
amide [(Me3Si)2N]3Sc(m-Cl)Li(THF)3 to the proligand was tested
again in 1,4-dioxane and the results showed that the model
reaction proceeded better with a 1 : 2 molar ratio of the Sc
complex to the proligand H2L2 at 20 ꢀC (Table 1, entries 16–18).
The effect of the ester moiety in different phosphites was then
studied since it may have had an inuence on the level of the
asymmetric induction.12 Dimethyl phosphite gave the target
product in 69% yield and 75% ee, while diisopropyl phosphite
gave a 21% yield and 81% ee. As to diphenyl phosphite, only
a trace amount of the corresponding adduct was observed. As a
result, the aliphatic ester moiety matched better in the enan-
tioselective transformation in the present case, particularly
diethyl phosphite (Table 1, entries 18–21).
First, to test the reactivity and selectivity of our catalytic
systems, we prepared ve rare-earth metal amides [(Me3Si)2-
N]3RE(m-Cl)Li(THF)3 (RE ¼ Sc (1), Y (2), La (3), Yb (4) and Lu (5))
according to the previous study.11 Then, three phenoxy-
functionalized chiral prolinols with bulky groups ((S)-2,4-di-
R2-6-[[2-(hydroxydiphenylmethyl)pyrrolidin-1-yl]methyl]phenol)
(R ¼ tBu (H2L1); R ¼ cumyl (H2L2) and R ¼ 1-adm (H2L3)) were
synthesized as previously reported.9 With the ytterbium amide 4
in hand, the model reaction of N-benzyl unsaturated amide 6
with HPO(OEt)2 was carried out in the presence of the proligand
H2L1. To our delight, the yields of the corresponding 1,4-
adducts were satisfying and the enantiomeric excess was
moderate, from 47% to 59% (Table 1, entries 1–3). The steric
effect of the different proligands was investigated and the bulky
cumyl groups in proligand H2L2 revealed a slight ascendant in
controlling the enantioselectivity (Table 1, entries 2, and 4–5). A
relatively low temperature is usually helpful to improve the
enantioselectivity of the asymmetric reaction. Attempts to lower
the reaction temperature in our case have a positive effect on
the enantioselectivity. Although the yields dramatically dropped
from 99% to 72% when the temperature was decreased, it was
Table 1 Optimization of the reaction conditionsa
Thus, various substrates were investigated using 10 mol% of
Sc catalyst 1 in 1,4-dioxane at 20 ꢀC in the presence of 20 mol%
of proligand H2L2, and the results are summarized in Scheme 1.
As we can see, almost all of the substrates participated in the
reaction and gave excellent yields and good to high enantiose-
lectivities under the optimal conditions. Whatever the substrate
was, a,b-unsaturated secondary amides (7a–7p) or tertiary
amides (7q–7t) bearing either aliphatic or aromatic N-substit-
uents, the yields were excellent, from 90% to 99%, except in two
cases (7b and 7c). Perhaps the exible n-butyl and i-propyl
groups attached to the nitrogen atom in the molecule are rela-
tively crowded and blocked the interaction between the catalyst
and the substrate, which also affected the enantioselectivity
(only 74% ee for 7b and 64% ee for 7c). In addition, there was no
evidence that the electronic effect of the groups on the phenyl
rings had an signicant inuence on the results. Moreover,
we successfully realized the asymmetric hydrophosphonylation
of unsaturated amide with an aliphatic acyl group in our
system (7l).
To further investigate the possible active species in the
current catalytic system based on the optimal molar ratio of the
Sc amide 1 to proligand H2L2, crystals of 8 were obtained from
the reaction of the Sc-based precatalyst 1 with 2 equiv. of pro-
ligand H2L2 in THF. The NMR spectra and elemental analysis
results of crystal 8 were informative. One peak at 8.86 was
assigned to the hydrogen of the hydroxyl group and was
observed in the proton NMR spectrum; the chemical shis at
1.64 and 1.36 were ascribed to the hydrogens in the C(CH3)2
group the peaks at 3.88 and 3.73 were ascribed to hydrogens of
the methine in group NCH2Ar. These observations prove the
T
Yieldb
(%)
eec
(%)
Entry
Cat.
x
Ligand
(ꢀC)
Solvent
1
2
3
4
5
6
7
8
Yb
Yb
Yb
Yb
Yb
Yb
Yb
La
Y
Lu
Sc
Sc
Sc
Sc
Sc
Sc
Sc
Sc
Sc
Sc
Sc
10
15
20
15
15
15
15
15
15
15
15
15
15
15
15
15
10
20
20
20
20
H2L1
H2L1
H2L1
H2L2
H2L3
H2L2
H2L2
H2L2
H2L2
H2L2
H2L2
H2L2
H2L2
H2L2
H2L2
H2L2
H2L2
H2L2
H2L2
H2L2
H2L2
25
25
25
25
25
0
ꢁ20
ꢁ20
ꢁ20
ꢁ20
ꢁ20
ꢁ20
ꢁ20
ꢁ20
ꢁ20
20
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
Tol
DME
Et2O
CH3CN
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
99
96
99
99
99
99
72
10
69
65
80
22
9
10
99
99
92
99
69
21
Trace
47
59
58
59
58
61
64
70
64
63
83
80
82
80
57
81
78
85
75
81
—
9
10
11
12
13
14
15
16
17
18
19d
20e
21f
20
20
20
20
20
a
Reactions were performed with unsaturated amide (0.5 mmol),
HPO(OEt)2 (1.0 mmol) in 2 mL solvent. HPLC yield. Determined by
chiral HPLC analysis. The reaction was conducted with dimethyl
phosphite. The reaction was conducted with diisopropyl phosphite.
b
c
d
e
f
The reaction was conducted with diphenyl phosphite.
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Scheme 1 The asymmetric hydrophosphonylation of unsaturated amides. Reactions were performed with the substrate (0.5 mmol), HPO(OEt)2
(1.0 mmol) in 2 mL of 1,4-dioxane at 20 ꢀC. The ee values were determined by chiral HPLC. The isolated yields are shown.
existence of the ligand in the crystal. Furthermore, the obtained in the abovementioned reaction were not good
elemental analysis shows that the ratio of Sc : L2 was 1 : 2, and enough for the X-ray diffraction analysis and only the skeleton
the LiCl component was not observed. However, the crystals of 8 of the molecular structure could be presumed. Fortunately,
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Table
2
The effect of additives on the asymmetric hydro-
phosphonylation of unsaturated amidesa
Entry
Additive
Yieldb (%)
eec (%)
1d
2
—
74
98
98
98
96
81
73
98
79
85
78
79
77
76
77
75
HNSiMe3
HNSiMe3
DBU
3e
4
Scheme 2 Synthesis of complex 8.
5
6
7
8
Et3N
TMEDA
Cy2NMe
DABCO
a
Reactions were performed with unsaturated amide (0.5 mmol),
HPO(OEt)2 (1.0 mmol), complex 8 (10 mol%), LiCl (10 mol%), additive
b
(20 mol%) in
2
mL of 1,4-dioxane at 20 ꢀC.
HPLC yield.
c
d
Determined by chiral HPLC analysis. Reaction was performed
without LiCl and HNSiMe3. e Reaction was performed without LiCl.
the base is helpful to the transformation; however, all the
results are not as good as those achieved using aminosilane
(Table 2, entries 4–8). Therefore, we came to the conclusion
that the bases are benecial for the asymmetric hydro-
phosphonylation of unsaturated amides and aminosilane is
optimal. Because complex 8 was recrystallized in a mixed
solvent of toluene and hexane, the lithium chloride molecule
was removed aer work up due to its poor solubility in toluene;
it was almost impossible for us to gain the expected crystals
bearing LiCl in the crystal cell with the current situation. A
detailed study of the mechanism is underway in our laboratory.
Furthermore, the outcome of the model reaction on a gram
scale was satisfactory with an appropriate prolonging of the
reaction time (Scheme 3). The absolute conguration of the
Fig. 1 The molecular structure of 8 showing 20% probability ellip-
soids. Hydrogen atoms are omitted for clarity. Selected bond lengths
(A˚) and bond angles (ꢀ): Sc(1)–O(1) 2.0350(16), Sc(1)–O(2) 1.9518(17),
Sc(1)–O(3) 2.0141(16), Sc(1)–O(4) 1.9833(16), Sc(1)–N(2) 2.3539(19),
O(2)–Sc(1)–O(4) 108.05(7), O(2)–Sc(1)–O(3) 115.27(7), O(4)–Sc(1)–
O(3) 132.87(7), O(2)–Sc(1)–O(1) 97.92(7), O(4)–Sc(1)–O(1) 99.48(7),
O(2)–Sc(1)–O(1) 92.29(6), O(2)–Sc(1)–N(2) 99.29(7), O(4)–Sc(1)–N(2)
74.83(7), O(3)–Sc(1)–N(2) 80.64(7), O(1)–Sc(1)–N(2) 162.78(7).
a colourless crystal of 8 suitable for the X-ray diffraction analysis
could also be obtained by treating tris((trimethylsilyl)methyl)
scandium with 2 equiv. of proligand H2L2 in hexane (Scheme 2).
Finally, the denite solid structure of 8 [L2Sc(L2H)] was deter-
mined as expected and is depicted in Fig. 1. The scandium
complex has a mononuclear structure with two chiral ligands in
the unit cell. One hydrogen atom of the hydroxyl group in one of
the chiral prolinolates remains to meet the requirement of the
trivalence of the central metal.
Subsequently, complex 8 was added to the model reaction of
the asymmetric hydrophosphonylation of N-benzyl unsaturated
amide and the target molecule was obtained in 74% yield and
79% ee (Table 2, entry 1). Taking into consideration the differ-
ence between the catalyst system generated in situ and the
abovementioned structure of complex 8, 10 mol% of lithium
chloride and 20 mol% of HNSiMe3 were deliberately added to
the reaction. The outcome was satisfactory with a 98% yield
and 85% ee, almost the same as the in situ catalyst system (Table
2, entry 2). The following test without the addition of lithium
chloride indicates that lithium chloride was essential.
Numerous additives such as DBU, Et3N, TMEDA, Cy2NMe, and
DABCO were added to make clear whether the aminosilane
HNSiMe3 was essential in the reaction. The results revealed that
Scheme 3 The amplification reaction of the preparation of compound
7a and its optical rotation analysis.
Scheme 4 The molecular structure of compound 7a.
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target phosphonate 7a was determined by a combination of
single crystal diffraction, chiral HPLC and optical rotation
analysis (Schemes 3 and 4).
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Conclusion
In summary, the asymmetric hydrophosphonylation of a,b-
unsaturated amides was successfully catalyzed by rare-earth
metal amides with phenoxy-functionalized chiral prolinols.
Aer careful screening, 10 mol% of the Sc complex [(Me3Si)2-
N]3Sc(m-Cl)Li(THF)3 together with the proligand H2L2 ((S)-2,4-
dicumyl-6-[[2-(hydroxydiphenylmethyl)pyrrolidin-1-yl]methyl]
phenol) in a 1 : 2 molar ratio were found to be the optimal
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ꢀ
catalytic conditions in 1,4-dioxane at 20 C. Substrates bearing
various groups were examined in this catalytic system,
including secondary amides and tertiary amides, which gave
excellent yields (up to 99%) and good to high enantioselectiv-
ities (73–89%). The absolute conguration of the target phos-
phonate was determined. During the exploration of the denite
structure of the actual catalyst, complex 8 was obtained. It was
proven to be an efficient catalyst in the presence of 1 equiv. LiCl
and 2 equiv. base in the hydrophosphonylation reaction. We are
still on the way to deduce the clear catalytic cycle occurring in
the reaction.
Acknowledgements
We gratefully acknowledge nancial support from the National
Natural Science Foundation of China (Grant No. 21572151,
21372172), PAPD and the Qing Lan Project.
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