Formal Asymmetric Cycloaddition of Activated α,β-Unsaturated Ketones with α-Diazomethylphosphonate Mediated by a Chiral Silver SPINOL Phosphate Catalyst

Article abstract of DOI:10.1021/acs.orglett.8b03436

An efficient method for preparing highly functionalized chiral nonspiro-phosphonylpyrazolines via an asymmetric formal 1,3-dipolar cycloaddition reaction of α-diazomethylphosphonate with activated, acyclic α,β-unsaturated ketones, bearing an additional nitrile electron-withdrawing group, has been developed, utilizing an in situ generated chiral silver phosphate catalyst, affording excellent stereoselectivities (up to 98% ee, 99:1 dr) and yields (up to 95%). A stepwise mechanism is proposed based upon density functional M11 calculations.

Full text of DOI:10.1021/acs.orglett.8b03436

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Formal Asymmetric Cycloaddition of Activated α,β-Unsaturated
Ketones with α‑Diazomethylphosphonate Mediated by a Chiral
Silver SPINOL Phosphate Catalyst
Bo Zheng,∥,† Haohua Chen,∥,‡ Lei Zhu,‡ Xiqiang Hou,† Yan Wang,† Yu Lan,*,‡,§ and Yungui Peng*,†
†Key Laboratory of Applied Chemistry of Chongqing Municipality, School of Chemistry and Chemical Engineering, Southwest
University, Chongqing 400715, P. R. China
‡School of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400030, P. R. China
§College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, P. R. China
S
* Supporting Information
ABSTRACT: An efficient method for preparing highly function-
alized chiral nonspiro-phosphonylpyrazolines via an asymmetric
formal 1,3-dipolar cycloaddition reaction of α-diazomethyl-
phosphonate with activated, acyclic α,β-unsaturated ketones,
bearing an additional nitrile electron-withdrawing group, has
been developed, utilizing an in situ generated chiral silver
phosphate catalyst, affording excellent stereoselectivities (up to
98% ee, 99:1 dr) and yields (up to 95%). A stepwise mechanism is proposed based upon density functional M11 calculations.
Considering that the cycloaddition reaction of SGR with β-
monosubstituted dipolarophiles results in two tautomers,6e we
designed α,β-disubstituted unsaturated ketone 1a containing a
cyano group as the test dipolarophile for the asymmetric
version of the reaction with SGR (2). The Smietana group has
reported the reaction of in situ generated α,β-unsaturated
nitriles with the SGR anion to achieve the aromatization of
phosphonylpyrazoles via a pyrazoline intermediate, revealing
that the cyano group on the pyrazoline ring was easily
eliminated under strong basic conditions.8a This demonstrated
that elimination of the cyano group must be avoided to realize
the asymmetric synthesis of phosphonyl pyrazolines via the
reaction of 1a and SGR (2). We surmised that aromatization
could be inhibited by performing the devised reaction in a
weak base medium. After preliminary trials, we were delighted
to find that the catalyst formed in situ from the chiral
phosphoric acid (CPA)9 (Figure 1) and silver carbonate
smoothly promoted the reaction to afford the desired
phosphonylpyrazoline. The results are shown in Table 1.
Chiral silver phosphates produced by 1,1′-spirobiindane-
7,7′-diol (SPINOL)-based chiral phosphoric acids (SPAs, I)
showed efficient catalytic performance. 6,6′-Diphenyl-substi-
tuted SPA Ia gave cycloaddition product 3aa in 14% yield with
26% ee and 99:1 dr (Table 1, entry 1). Inspired by this result,
we synthesized a series of SPAs and screened them in the
model reaction. Introducing a methyl group at the para-
position of the 6,6′-diphenyl substituents (Ib) led to a
significant increase in the yield and stereoselectivity (65%
yield, 55% ee, and 99:1 dr) (Table 1, entry 2). However, when
yrazoline is a privileged five-membered heterocycle found
1−5
Pin many biologically active molecules
that have shown
anticancer,2 anticonvulsant,3 antimicrobial,4 and immunosup-
pressive activities.5 Therefore, much research effort has been
directed toward the development of synthetic methods for
successfully constructing various pyrazoline derivatives.6
Despite these achievements, developing protocols for the
stereocontrolled formation of highly functionalized pyrazolines
remains challenging and desirable. Furthermore, the biological
activity of azaheterocyclic compounds can be enhanced
significantly when a phosphonate functional group is
introduced.7 Therefore, the bioactivities of pyrazolines with
incorporated phosphonates are anticipated. Several researchers
have reported that reactions of diazophosphonates with
olefinic dipolarophiles afford phosphonylpyrazoles via pyrazo-
line intermediates, with subsequent immediate aromatization
in the presence of a strong base affording the more stable
pyrazoles.8 Only limited success was documented in the
synthesis of racemic phosphonylpyrazolines.6e,8d From a
structure−activity relationship (SAR) viewpoint, the config-
uration of stereogenic centers on the phosphonylpyrazoline
core could significantly influence its biological activity.
Therefore, the development of new approaches to the
synthesis of chiral variants phosphonylpyrazoline is important
for biological activity screening. In this respect, our group has
achieved the synthesis of chiral spiro-phosphonylpyrazoline
oxindoles,6k,l while chiral nonspiro-phosphonylpyrazolines
derived from acyclic olefins have yet to be exploited. In this
study, we aimed to prepare chiral nonspiro-phosphonylpyrazo-
lines for the first time through the asymmetric catalytic 1,3-
dipolar cycloaddition reaction of Seyferth−Gilbert reagent
(SGR) to acyclic substituted α,β-unsaturated ketones.
Received: October 27, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b03436
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(Table 1, entry 8). To further increase the steric hindrance of
the 6,6′-disubstituents, SPAs Ii and Ij were synthesized and
evaluated. Although Ii resulted in only a 44% yield with 59% ee
and 99:1 dr (Table 1, entry 9), Ij afforded the highest
enantioselectivity (85% ee; Table 1, entry 10). The silver
phosphate from 1,10-binaphthol (BINOL)-based phosphoric
acid (II) was also applied to this 1,3-dipolar cycloaddition
reaction, but afforded 3aa in only 20% yield with 14% ee and
99:1 dr (Table 1, entry 11). From these results, the silver
phosphate from Ij was identified as the best catalyst. The effect
of changing the ratio of SPA Ij to silver carbonate was
investigated (Table 1, entries 10, 12−14). The results showed
that a 1:1 ratio was the best choice, with decreased reactivity
observed if either silver carbonate or SPA Ij were in excess
(Table 1, entries 10 and 12−14). Other reaction parameters,
including the concentration (Table 1, entries 10 and 15−17),
the ratio of 1a and 2, catalyst loading, temperature, and
solvent, were also optimized (see Supporting Information, SI).
The best results were achieved when the reaction proceeded at
−30 °C in dichloromethane (DCM) at a concentration of 0.2
M with 10 mol % Ij and 10 mol % silver carbonate.
With optimized conditions in hand, we further evaluated the
substrate scope by employing a series of substituted chalcones
in the cycloaddition reaction. The results are summarized in
Table 2. The reaction tolerated a broad range of substrates,
with both electron-donating and electron-withdrawing sub-
stituents on phenyl ring R2 affording good results (Table 2,
entries 1−10). The substituent pattern on ring R2 had a small
effect on the yield and enantioselectivity. Substituents at the 2-
position afforded slightly lower yields and enantioselectivities
Figure 1. Chiral phosphoric acids screened in this study.
Table 1. Screening the Chiral Phosphoric Acids and
Optimization the Reaction Conditions
a
b
cd
,
d
entry
CPA
ratio (x/y)
yield (%)
dr
ee (%)
1
2
3
4
5
6
7
8
Ia
Ib
Ic
Id
Ie
If
Ig
Ih
Ii
Ij
II
Ij
Ij
Ij
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:0.5
1:1.25
1:1.5
1:1
1:1
1:1
14
65
10
60
55
55
55
78
44
60
20
39
50
50
66
85
86
99:1
99:1
99:1
98:2
97:3
99:1
98:2
99:1
99:1
99:1
99:1
99:1
99:1
99:1
98:2
99:1
99:1
26
55
33
60
59
54
57
80
59
85
14
77
84
77
90
92
92
9
10
11
12
13
14
15
16
17
Table 2. Scope of Substituted Chalcones Applied to the
Asymmetric Cycloaddition Reaction
e
Ij
Ij
Ij
f
g
a
Conditions: Unless otherwise noted, all reactions were conducted
a
b
c d
,
d
entry
R1, R2
t (d) yield (%)
dr
ee (%)
with 1a (0.15 mmol), 2 (0.1 mmol), CPA (10 mol %), and Ag2CO3
b
(10 mol %) in DCM (1.0 mL) at −30 °C for 3 days. Isolated yield.
1
2
3
4
5
6
7
8
Ph, Ph
3
3
2
4
3
2
4
2.5
3
4
5
3.5
4
4
3
2
2
2
3aa, 85
3ab, 83
3ac, 94
3ad, 96
3ae, 92
3af, 95
3ag, 94
3ah, 90
3ai, 76
3aj, 91
3ak, 64
3ba, 92
3ca, 91
3da, 86
3ea, 73
3bd, 94
3bf, 95
3bh, 94
99:1
97:3
99:1
98:2
98:2
98:2
99:1
99:1
99:1
99:1
98:2
91:9
99:1
99:1
97:3
99:1
92:8
91:9
92
82
90
91
87
93
89
94
89
89
82
89
90
91
83
90
95
94
c
Ratio of major diastereomer to the total of the other three
Ph, 2-FC6H4
Ph, 3-FC6H4
Ph, 4-FC6H4
Ph, 3-ClC6H4
Ph, 4-ClC6H4
Ph, 3-BrC6H4
Ph, 4-BrC6H4
Ph, 4-CF3C6H4
Ph, 4-MeC6H4
Ph, 2-naphthyl
2-MeC6H4, Ph
3-MeC6H4, Ph
4-MeC6H4, Ph
4-ClC6H4, Ph
2-MeC6H4, 4-FC6H4
2-MeC6H4, 4-ClC6H4
2-MeC6H4, 4-BrC6H4
d
e
diastereomers. Determined by chiral HPLC. 0.15 M of the reaction
concentration. 0.2 M of the reaction concentration. 0.25 M of the
reaction concentration.
f
g
electron-withdrawing substituents (−CF3) were introduced in
the same positions (Ic), inferior results were obtained
compared with those of Ib (10% yield, 33% ee, and 99:1 dr;
Table 1, entry 3). These results demonstrated that electron-
withdrawing groups on the 6,6′-diphenyl substituents were
detrimental to catalytic performance. Next, electron-donating
para-substituents with increasing steric hindrance on the 6,6′-
diphenyl rings were explored, affording SPAs Id−Ih that were
applied in the model reaction. The silver phosphate produced
by para-ethyl-substituted Id gave 3aa in 60% yield with 60% ee
and 98:2 dr (Table 1, entry 4), while para-propyl (Ie, 55%
yield, 59% ee, 97:3 dr), para-isopropyl (If, 55% yield, 54% ee,
99:1 dr), and para-butyl (Ig, 55% yield, 57% ee, 98:2 dr)
substituents afforded similar results (Table 1, entries 5−7).
Introducing the more sterically hindered tert-butyl group into
the SPA (Ih) resulted in substantially improved catalytic
performance, affording a 78% yield with 80% ee and 99:1 dr
9
10
11
12
13
14
15
16
17
18
a
Conditions: Unless otherwise noted, all reactions were conducted
with 1 (0.3 mmol), 2 (0.2 mmol), Ij (10 mol %), and Ag2CO3 (10
mol %) in DCM (1.0 mL) at −30 °C. Isolated yield. Ratio of major
diastereomer to the total of the other three diastereomers.
b
c
d
Determined by chiral HPLC.
B
DOI: 10.1021/acs.orglett.8b03436
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(Table 2, entries 2−4). Meanwhile, a β-naphthyl α,β-
unsaturated ketone reacted smoothly to give the expected
product only in 64% yield with 82% ee and 98:2 dr (Table 2,
entry 11). α,β-Unsaturated ketones with various substituents
on the other phenyl ring (R1) attached to the carbonyl group
were also tolerated in the reaction. In general, electron-
donating substituents gave better results than electron-
withdrawing groups (Table 2, entries 12−15). In contrast to
R2 phenyl ring substituents, substituents at the 2-position of
the R1 phenyl ring afforded almost the same good results
(Table 2, entries 12−14).
99:1 dr (Table 3, entry 4). Therefore, a series of α,β-
unsaturated ketones bearing a Bu group attached to the
t
carbonyl were synthesized and subjected to this cycloaddition
reaction. Good-to-excellent results were achieved by most of
these substrates (Table 3, entries 5−21). Substrates bearing
both electron-withdrawing and -donating substituents on the
benzene ring (R2) reacted smoothly to give the desired
products with excellent enantioselectivities (84−97% ee) and
good diastereoselectivities (90:10−99:1 dr; Table 3, entries 5−
20). α,β-Unsaturated ketones bearing electron-withdrawing
groups (Table 3, entries 8−20) gave slightly higher yields than
those bearing electron-donating groups (Table 3, entries 5−7).
A β-naphthyl-substituted feedstock also achieved excellent
results (Table 3, entry 21). To our delight, substrates in which
R1 and R2 were both aliphatic groups reacted well, affording
the expected products with excellent yields and stereo-
selectivities (Table 3, entries 22−24).
To demonstrate the efficiency of this method, we next
explored extending the substrate scope to other α,β-
unsaturated ketones bearing an alkyl group (R1) attached to
the carbonyl group, with the results summarized in Table 3.
Table 3. Selected Other Substituted α,β-Unsaturated
Ketones Applied to the Asymmetric Cycloaddition Reaction
To test the synthetic potential of the present approach, in
the case of 1bd, a mmol scale was successfully conducted and
gave the desired product 3bd (98% yield) with 92% ee, >99:1
dr.
The absolute configuration of the products was unambigu-
ously determined to be (4R, 5R) by X-ray crystallographic
analysis of 3ig (Figure 2; CCDC 1474349), with other product
configurations deduced from this result.
a
b
cd
,
d
entry
R1, R2
Me, Ph
Et, Ph
iPr, Ph
t (d)
yield (%)
dr
ee (%)
1
2
3
6
6
4
3fa, 36
3ga, 37
3ha, 61
3ia, 80
3ib, 74
3ic, 70
3id, 53
3ie, 93
3if, 95
3ig, 93
3ih, 81
3ii, 93
3ij, 94
3ik, 83
3il, 94
3im, 93
3in, 92
3io, 95
3ip, 89
3iq, 94
3ir, 94
3is, 92
3it, 92
3iu, 88
97:3
99:1
98:2
99:1
90:10
99:1
99:1
95:5
99:1
98:2
99:1
99:1
99:1
91:9
99:1
99:1
99:1
98:2
94:6
98:2
99:1
99:1
99:1
99:1
53
73
77
92
84
97
93
93
96
93
94
93
94
86
92
96
93
96
92
96
91
97
98
97
4
tBu, Ph
4
5
tBu, 3-MeC6H4
tBu, 4-MeC6H4
tBu, 4-MeOC6H4
tBu, 3-NO2C6H4
tBu, 4-NO2C6H4
tBu, 3-BrC6H4
tBu, 4-BrC6H4
tBu, 3-ClC6H4
tBu, 4-ClC6H4
tBu, 2-FC6H4
tBu, 3-FC6H4
tBu, 4-FC6H4
tBu, 4-CF3C6H4
tBu, 3,4-Cl2C6H3
tBu, 3,5-F2C6H3
tBu, 3,4-F2C6H3
tBu, 2-naphthyl
5
6
5
7
7
8
2
9
3
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
4
4
3
Figure 2. X-ray crystal structure of product 3ig.
3
6
4.5
4
When varying the ratio of SPA (Ij) to silver carbonate in the
model reaction from 1:0.5 to 1:1.5, it was found that the
enantioselectivities varied slightly (77−85% ee), while
dramatically different reactivities were observed (39−60%
yields Table 1, entries 10 and 12−14). These results inspired
us to explore the actual in situ formed catalyst and determine
how it induced enantioselectivity. Density functional M1110
with a standard 6-311+G(d,p) basis set (SDD11 basis set for
Ag) was employed to gain insight into the mechanism and the
origin of enantioselectivity of this asymmetric 1,3-dipolar
cycloaddition (Figure 3). Deprotonation of the diazophosph-
onate 2 by the silver phosphate catalyst would reversibly yield
diazomethyl silver complex 6. The theoretical study showed
that the [3 + 2] cycloaddition could take place via a stepwise
process (for detailed mechanistic studies, see SI). Intermo-
lecular nucleophilic addition of the carbon atom of the
diazomethane moiety in complex 6 to the si face of the β-
carbon atom in α,β-unsaturated ketone 1, activated by silver
bicarbonate salt via transition state 7-ts-RR, leads to generation
of a new C−C bond in intermediate 8-RR. The calculated
activation free energy of this step was only 5.0 kcal/mol.
Generation of intermediate 8-RR was exergonic (12.3 kcal/
mol), which indicated an irreversible process. Therefore, the
enantioselectivity would be controlled by this step. Subsequent
3
2.5
3
2.5
3
n
tBu, Bu
2
i
tBu, Bu
1.5
1.5
tBu, n-C5H11
a
Conditions: Unless otherwise noted, all reactions were conducted
with 1 (0.3 mmol), 2 (0.2 mmol), Ij (10 mol %), and Ag2CO3 (10
mol %) in DCM (1.0 mL) at −30 °C. Isolated yield. Ratio of major
diastereomer to the total of the other three diastereomers.
b
c
d
Determined by chiral HPLC.
When R1 was a methyl or ethyl group, low reactivity was
observed, with only 36% and 37% yields with moderate
enantioselectivities obtained after reacting for 6 days (Table 3,
entries 1 and 2). Improved reactivity and enantioselectivity
were observed as the size of the R1 substituents was increased
(Table 3, entries 3 and 4). When the R1 group was Bu, the
t
product was obtained in a pleasing 80% yield, with 92% ee and
C
DOI: 10.1021/acs.orglett.8b03436
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b03436.
Experimental procedures and detailed characterization
data of all new compounds (PDF)
Accession Codes
CCDC 1474349 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: lanyu@cqu.edu.cn.
*E-mail: pengyungui@hotmail.com; pyg@swu.edu.cn.
ORCID
Yu Lan: 0000-0002-2328-0020
Yungui Peng: 0000-0002-5815-1261
Author Contributions
∥B.Z. and H.C. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for financial support from the National Science
Foundation of China (21472151, 21822303, and 21772020),
the Specialized Research Fund for the Doctoral Program of
Higher Education (20110182110006), and Fundamental
Research Funds for the Central Universities (Chongqing
University; 2018CDXZ0002 and 2018CDPTCG0001/4).
Figure 3. Free energy profiles for silver catalyzed asymmetric 1,3-
dipolar cycloaddition. The values given by kcal/mol represent the
relative free energies calculated by M11 method in dichloromethane
solvent
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■
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DOI: 10.1021/acs.orglett.8b03436
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