Ferrocene Derived Bifunctional Phosphine-Catalyzed Asymmetric Oxa-[4+2] Cycloaddition of α-Substituted Allenones with Enones

Article abstract of DOI:10.1002/chem.201703368

An efficient ferrocene-derived bifunctional phosphine-catalyzed enantioselective oxa-[4+2] cycloaddition of α-substituted allenones with a broad range of enones is investigated for the preparation of stereodefined dihydropyrans in good to excellent yields (up to 99 %) and excellent enantioselectivity (up to 99 % ee). Furthermore, a series of valuable chiral polyheterocyclic frameworks can be efficiently achieved in good yields with excellent enantioselectivities.

Full text of DOI:10.1002/chem.201703368

A Journal of
Accepted Article
Title: Ferrocene Derived Bifunctional Phosphine-Catalyzed Asymmetric
Oxa-[4+2] cycloaddition of ɑ-Substituted Allenones with Enones
Authors: Junliang Zhang, Huamin Wang, and Weike Lu
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10.1002/chem.201703368
Chemistry - A European Journal
COMMUNICATION
Ferrocene Derived Bifunctional Phosphine-Catalyzed Asymmetric
Oxa-[4+2] cycloaddition of ɑ-Substituted Allenones with Enones
Huamin Wang, Weike Lu and Junliang Zhang*[a]
Dedication ((optional))
Abstract: An efficient ferrocene derived bifunctional phosphine-
catalyzed enantioselective oxa-[4+2] cycloaddition of α-substituted
allenones with a broad range of enones is investigated for the
preparation of stereodefined dihydropyrans in good to excellent
yields (up to 99%) and excellent enantioselectivity (up to 99% ee).
asymmetric oxo-[4+2] cycloaddition of α-substituted
allenones with a broad range of enones, furnishing diverse
valuable chiral functionalized 3,4-dihydropyrans in good
yields and high enantioselectivity (Scheme 1c).
Furthermore,
a
series of val-uable chiral polyheterocyclic
frameworks can be efficiently achieved in good yields with excellent
enantioselectivities.
Dihydropyrans and pyrans have attracted wide synthetic
interest due to their rich appearance as important skeletons
of bioactive active molecules and natural products[1].
Among them, trifluoromethylated dihydropyrans are an
important class of trifluoromethylated heterocycles which
emerged in a variety of biologically and pharmaceutically
active molecules, for instance, antimalarial agents III[2] and
NK-1 receptor antagonist IV. [3] Despite much progress has
made in the synthesis of dihydropyrans and pyrans using
metal-free catalysts such as N-heterocyclic carbene[4] and
amine,[5] the methods for the synthesis of optically active
dihydropyrans with a chiral carbon center bearing a CF3
group are very rare[6] and is therefore highly desirable.
Recently, Lu and workers[7a] describe the first example of
generating chiral pyrans via a dipeptide-based bifunc-tional
Scheme 1. Phosphine-Catalyzed Cycloaddition Reactions of α-Substituted
Allenes.
phosphine
allenones with β,γ-unsaturated α
(
P0)-catalyzed annulation reaction between
ketoesters (Scheme 1a).
‑
Subsequently, Lu et al.[7b] extended this protocol through
the introduction of a cyano-group at the α-position of the
chalcone by the use of a much simple L-valine-derived
bifunctional phosphine (P1) (Scheme 1b). As part of our
ongoing interest in the enantioselective synthesis of
enantioenriched trifluoromethylated building blocks,[8,9f-h]
and with a series of bi- or multi-functional phosphine
catalysts[9] in our hand, which were developed by our group
and have demonstrated their good performance in the
enantioselective Rauhut-Currier reactions,[9a-e] allylation
reaction[9f] and asymmetric cycloaddition,[9h,i] we became
interest in the asymmetric phosphine-catalyzed oxo-[4+2]
annulation of perfluoroalkylated ɑ,β-enones and allenones,
if success, a variety of valuable chiral perfluoroalkylated
dihydropyrans will be delivered.[10] We wish to report herein
an efficient ferrocene derived bifunctional phosphine
β-Trifluoromethylated enone 1f and allenone 2a were
selected as the model substrates for screening the reac-tion
conditions (Table 1) and the chiral phosphine catalysts
(Figure 1). Gratifyingly, catalyst P1, used in previous Lu’s
work,[7b] could indeed catalyze the cycloaddition reaction in
toluene at room temperature to furnish the desired
trifluoroalkylated 3,4-dihydropyran in 95% yield with
acceptable ee (Table 1, entry 1). The replacement of the
side iso-propryl group of P1 with the phenyl, structured as
P2, led to a similar ee (Table 1, entry 2). Further
introduction of a phenyl group at the ortho-position of
phenyl in P2 would give a much lower ee (Table 1, entries
3). However, the bulkier 3,5-di-tert-butylphenyl group
located to the phenyl ring would deliver better
enantioselectivity to 90% ee (Table 1, entries 4). In an effort
to further enhance enantioselectivity, we are pleased to find
that ferrocene derived catalyst P5 which could be easily
prepared using our previously developed protocol,[9g] could
improve the ee to 93% of the product in a better yield
(Table 1, entry 5). The variation of the 3,5-bis(trifluoro-
methyl)benzoyl derived amide to tert-butanesulfinamide led
to much lower enantioselectivity, indicating that the amide
moiety plays crucial role in inducing enantioselectivity
(Table 1, entry 6). Employing commercially available chiral
ferrocene phosphine P7 exhibited no catalytic activities
(Table 1, entry 7). Further studies showed that toluene was
catalyst P5
,
which displayed high performance in
H. Wang, W, Lu, Prof. Dr. J. Zhang*
Shanghai Key Laboratory of Green Chemistry and Chemical Processes,
School of Chemistry and Molecular Engineering,
East China Normal University
Shanghai, 200062 (P. R. China)
E-mail: jlzhang@chem.ecnu.edu.cn
http://faculty.ecnu.edu.cn/s/1811/main.jspy
Supporting information for this article is given via a link at the end of the
document.((Please delete this text if not appropriate))
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10.1002/chem.201703368
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the optimal solvent and variation of the reaction
temperature did not lead to obvious improvements (Table 1,
entries 8-14).
Table 2: Investigation the scope of β-perfluoroalkyl enone component[a]
Entry
1
1, R1/Rf
1a, Ph/CF3
Yield [%][b]
3aa, 97
Ee [%][c]
94
2
1b, 4-MeC6H4/CF3
1c, 4-MeOC6H4/CF3
1d, 4-PhC6H4/CF3
1e, 4-FC6H4/CF3
1f, 4-ClC6H4/CF3
1g, 4-BrC6H4/CF3
1h, 4-IC6H4/CF3
1i, 4-O2NC6H4/CF3
1j, 4-NCC6H4/CF3
1k, 4-MeO2SC6H4/CF3
1l, 4-CF3C6H4/CF3
1m, 2-BrC6H4/CF3
1n, 3-BrC6H4/CF3
1o, 2-O2NC6H4/CF3
1p, 3-O2NC6H4/CF3
1q, 3,4-Cl2C6H3/CF3
1r, 1-naphthy/CF3
1s, 2-naphthy/CF3
1t, 2-benzothienyl/CF3
1u, 2-furyl/CF3
3ba, 96
3ca, 95
3da, 95
3ea, 85
3fa, 91
3ga, 91
3ha, 86
3ia, 98
3ja, 93
3ka, 92
3la, 98
3ma, 97
3na, 96
3oa, 98
3pa, 97
3qa, 99
3ra, 86
3sa, 96
3ta, 95
3ua, 96
3va, 98
3wa, 90
3xa, 95
3ya, 97
3aa, 70
92
94
92
93
93
92
91
90
91
90
92
92
94
91
93
94
94
92
95
92
94
93
90
91
94
3
4
5
Figure 1. Phosphine catalysts employed in this study.
Table 1: Optimization Reaction Conditions[a]
6
7
8
9
10
11
12
13
14
15
16
17
18
19[c]
20
21
22
23
24
25
26
Entry
1
Cat
P1
P2
P3
P4
P5
P6
P7
P5
P5
P5
P5
P5
P5
P5
Solvent
toluene
toluene
toluene
toluene
toluene
toluene
toluene
DCM
Yield [%]b]
95
Ee [%][c]
83
2
95
80
3
90
66
4
92
90
5
6[d]
96
93
40
-54
--
7
n.r.
90
1v, 2-thienyl/CF3
1w, 2-pyridyl/CF3
1x, Ph/C2F5
8
88
9
CHCl3
94
92
10
11
12
13[e]
14[f]
THF
80
74
1y, Ph/C3F7
MeCN
acetone
toluene
toluene
40
57
(Z)-1a
50
57
[a] Reactions were performed with
(0.01 mmol) in toluene (1.0 mL) at room temperature. [b] Isolated yield.
[c] 3.0 mmol scale and 2.5 mol% P5
1 (0.2 mmol), 2a (0.24 mmol), and P5
95
93
94
94
.
[a] Reaction conditions: 1f (0.1 mmol), 2a (0.12 mmol), and the catalyst
(0.005 mmol) in the solvent specified (1 mL) at room temperature for 0.5
h. [b] Isolated yield. [c] Determined by HPLC analysis on a chiral
stationary phase. [d] 12 h. [e] at 0 oC for 1 h. [f] at -20 oC for 10 h.
Table 3: Investigation the scope of allenone component[a]
With the optimized reaction conditions in hand, we
examine the scope and limitation of this reaction using
various β-perfluoroalkylated ɑ,β-enone
1, and the results
are listed in Table 2. In general, the reaction could proceed
smoothly to produce the desired adducts in good yields with
high enantioselectivities regardless of the structure of
enones with different aryl or heteroaryl groups (Table 2,
entries 1-23). The absolute configuration of 3ta was
established by single crystal X-ray diffraction analysis[11]
(see ESI). It was particularly gratifying that reducing the
catalyst loading to 2.5 mol%, the reaction still delivers high
yield of 3sa on a 3.0 mmol scale without affecting the
Entry
2, R4/R5
3,Yield [%][b]
3fb, 97
3fc, 98
3fd, 98
3fe, 97
3ff, 60
Ee [%][c]
93
1
2
3
4
5
6
7
8
9
2b, CH3/4-MeC6H4
2c, CH3/4-FC6H4
2d, CH3/4-ClC6H4
2e, CH3 /4-BrC6H4
2f, CH3/3-ClC6H4
2g, CH3/2-benzodioxole
2h, CH3/CH3
93
91
91
91
3fg, 92
3fh, 76
3fi, 73
92
excellent
trifluoromethyl group to C2F5 and C3F7 groups in substrates
were also applicable in the present asymmetric
cycloaddition transformation, furnishing valuable
stereoselectivity.
Finally,
changing
the
93
2i, CH3/ C2H5
99
1
2j, Bn /CH3
3fj, 50
97
[a] Reactions were performed with 1f (0.2mmol),
2
(0.24 mmol), and P5
perfluoroalkylated dihydropyran 3xa-3ya in high ee (Table 2,
24 and 25). The use of Z-isomer of 1a afforded the 3aa with
the same absolute configuration as the reaction of E-isomer
of 1a but in a relatively lower yield (Table 2, entry 26).
Next, we examined the reaction scope with a series of
allenones, and the results are shown in Table 3. Several
electron-withdrawing or -donating substituents were tolerant
at the different positions of the aromatic ring, providing the
(0.01 mmol) in toluene (1.0 mL) at room temperature. [b] Isolated yields.
[c] Determined by HPLC analysis on a chiral stationary phase.
corresponding products in 60-98% yield with excellent
enantioselectivities (Table 3, entries 1-6). The halogens in
the products provide a handle for further transformations. In
addition, allenone
2 with only alkyl substituents (2h-2j) in
general gave higher ees (Table 3, entries 7-9).
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Table 4: Investigation the scope of trifluoromethyl enone
10 mol% of catalyst at -50 oC. This catalyst system could be
also applicable to the reaction of ɑ-cyano-enone, furnishing
the desired product 7ga in 96% yield with 92% ee, a similar
result to Lu’s catalyst.[7b]
component[a]
To demonstrate the synthetic utility of the products, two
transformations of the representative
demonstrated (Scheme 2), Ring opening of 3sa through
treatment with HCl in ethanol provided 1,3,7-triketone 10a
followed by methylation reaction affording the gem-dimethyl
ketone 11a. The chiral trifluoromethylated 3,4-dihydropyran
3sa was treated with NaBH4 in THF for 3 h, giving the
3sa were
,
Entry
1
4, R
4a, Ph
5, Yield [%][b]
5aa, 94
Ee [%][c]
90
2d
3d
4d
4b, 4-CH3C6H5
5ba, 95
5ca, 97
5da, 95
90
91
90
4c, 4-BrC6H5
reduction product 10b in 60% yield as
a
single
4d, 3,4,5-(OMe)3C6H2
diastereomer.
[a] Reactions were performed with 5 (0.2 mmol), 2a (0.24 mmol), and P5 (0.01
mmol) in toluene (1.0 mL) at room temperature for 0.5 h. [b] Isolated yields.
[c] Determined by HPLC analysis on a chiral stationary phase. [d] at 0 o
for 12 h.
C
We next examined the reactivity of 1,1,1-trifluorobut-3-en-
2-ones for the preparation of 6-(trifluoromethyl)
dihydropyrans (Table 4). An array of trifluoromethyl enone,
irrespective of methyl, methoxy, and halogen substituents
on the aromatic ring, reacted well with 2a to produce the
corresponding products in good yields with 90%-91% ee
values (Table 4, entries 1-4).
Table 5: Further investigation the scope of enone bearing without
perfluoroalkyl group[a]
Scheme 2. Further Synthetic Transformations of 3sa
.
In summary, we have developed a highly effective
phosphine-catalyzed asymmetric [4+2] cycloaddition of α-
substituted allenones with a range of perfluoroalkyl enones
to afford a wide variety of CF3-containing dihydropyrans in
high yields with excellent enantioselectivities (90−99% ee).
Meanwhile, a series of benzocyclic ɑ,β-enones have also
been successfully utilized to construct a diverse range of
chiral
enantioselectivities. The method is also applicable to β,γ-
unsaturated ɑ-keto ester and ɑ-cyano-enone. Further
polycyclic
molecules
frameworks
in
high
elaboration of the approach to the syntheses of bioactive
natural products and molecules are under way and will be
reported in due course.
[a] Reactions were performed with 6 (0.2 mmol), 2a (0.24 mmol), and P5 (0.02
mmol) in toluene (1.0 mL) at room temperature. Yields are isolated yield.
Enantiomeric excesses are determined by HPLC analysis on a chiral
stationary phase. [b] at 0 oC. cat -50 oC.
Acknowledgements
We are grateful to 973 Programs (2015CB856600), National
Natural Science Foundation of China (21372084, 21425205,
21672067) and Changjiang Scholars and Innovative Research
Team in University (PCSIRT) for financial supports.
Inspired by the above success, we turned to further
explore the indolinone substituted ɑ,β-enones bearing a
CF3 or Ph group have also been successfully utilized to
construct the chiral cyclic frameworks in excellent
enantioselectivities and good yields (Table 5). The six or
seven-membered benzocyclic ɑ,β-enone substrate were
well tolerated, and the corresponding products were
isolated in good to excellent yields and enantioselectivities.
Importantly, these polyheterocyclic compounds might be
Keywords: chiral phosphines • oxa-cycloaddition reaction •
dihydropyrans • polyheterocyclic • allenones
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H. Wang, W. Lu, J. Zhang*
Page No. – Page No.
Ferrocene Derived
Bifunctional Phosphine-
Catalyzed Asymmetric Oxa-
[4+2] cycloaddition of
ɑ-Substituted Allenones
with Enones
An efficient ferrocene derived bifunctional phosphine-catalyzed enantioselective oxa-
[4+2] cycloaddition of α-substituted allenones with a broad range of perfluoroalkyl
enones is investigated for the preparation of stereodefined fluoroalkylated dihydropyrans
in good to excellent yields (up to 99%) and excellent enantioselectivity (up to 99% ee).
Furthermore, a series of valuable chiral fluoroalkylated polyheterocyclic frameworks can
be efficiently achieved in good yields with excellent enantioselectivities.
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