Chiral phosphine-catalyzed enantioselective construction of γ-butenolides through substitution of morita-baylis-hillman acetates with 2-trimethylsilyloxy furan

Article abstract of DOI:10.1021/ja802422d

Chiral multifunctional phosphine (R)-N-(2′-diphenylphosphanyl-[1,1-]binaphthalenyl-2-yl)methanesulfonamide L2 or (R)-N-(2′-diphenylphosphanyl-[1,1-]binaphthalenyl-2-yl)acetamide L3 is an efficient catalyst in the allylic substitutions of MBH acetates 1 with 2-trimethylsilyloxy furan 2 to provide γ-butenolides 3 in good to excellent yields and enantiomeric excesses in the presence of water. Copyright

Full text of DOI:10.1021/ja802422d

Published on Web 05/15/2008
Chiral Phosphine-Catalyzed Enantioselective Construction of γ-Butenolides
Through Substitution of Morita-Baylis-Hillman Acetates with
2-Trimethylsilyloxy Furan
Ying-Qing Jiang, Yong-Ling Shi, and Min Shi*
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of
Sciences, 354 Fenglin Lu, Shanghai 200032, China
Received April 3, 2008; E-mail: mshi@mail.sioc.ac.cn
The efficient synthesis of highly functionalized γ-butenolides
remains an important challenge in organic chemistry.1 Recently,
an important finding by Krische’s group indicates that, upon
exposure of Morita-Baylis-Hillman (MBH) acetates to substo-
ichiometric amounts of triphenylphosphane (20 mol %) in the
presence of 2-trimethylsilyloxy furan, regiospecific allylic substitu-
tion occurs to provide the γ-butenolides in good to excellent yields,
high regioselectivities, and diastereoselectivities along with a chiral
auxiliary approach.2 Inspired by this elegant work, we report herein
the first catalytic asymmetric version of this reaction with chiral
multifunctional phosphines, (R)-N-(2′-diphenylphosphanyl-[1,1′]bi-
naphthalenyl-2-yl)methanesulfonamide L2 and (R)-N-(2′-diphe-
nylphosphanyl-[1,1′]binaphthalenyl-2-yl)acetamide L3,3 as cata-
lysts.4
Figure 1. Chiral phosphines for asymmetric allylic substitutions.
Table 1. Chiral Phosphines Catalyzed Allylic Substitution of MBH
Acetate 1a with 2-Trimethylsilyloxy Furan 2 in Different Solvents
Initial examination was carried out by using Morita-Baylis-
Hillman acetate 1a (1.0 equiv) and 2-trimethylsilyloxy furan 2 (2.0
equiv) as the substrates in the presence of chiral phosphines L1-L5
(20 mol %) in THF (Figure 1). The results are summarized in Table
1. We found that the corresponding syn-γ-butenolide 3a was
produced in good chemical yields (81 and 74%) and enantioselec-
tivities (70 and 61% ee) after the reaction was conducted for 57
and 48 h by using multifunctional phosphines L2 and L3. Under
the above conditions, the multifunctional chiral phosphine L1 was
proven to not be an efficient catalyst (Table 1, entries 1-3). Also,
the use of chiral phosphines L4 and L5 afforded syn-γ-butenolide
3a in poor yields and ee’s (Table 1, entries 4 and 5). These
observations suggest that an active amide proton of the catalyst is
crucial for this asymmetric reaction (Table 1, entries 2 and 3). The
examination of solvent effects using catalyst L2 revealed that the
protic solvent, methanol, can significantly facilitate the reaction to
produce the corresponding adduct 3a in 55% yield and 90% ee
within a reaction period of 48 h (Table 1, entries 6-10). However,
the bulky protic solvent, tert-butanol, did not perform well for this
reaction (Table 1, entry 11). We then envisaged that addition of
water would further improve this asymmetric reaction. We thus
carried out the L2-catalyzed reaction in toluene together with
various amounts of water under the standard conditions.5,6 As can
be seen in Table 2, the addition of 1.0 and 3.0 equiv of water to
the reaction system resulted in the corresponding syn-γ-butenolide
3a in much better outcomes: 94% yield and 81% ee for 1.0 equiv,
94% yield and 88% ee for 3.0 equiv, respectively (Table 2, entries
1-3). Increasing the amounts of water to 6.0 equiv afforded 3a in
94% ee but in a lower yield of 46% (Table 2, entry 4). Using 2.5
equiv of 2 along with 6.0 equiv of water provided 3a in 94% yield
and 94% ee (Table 2, entry 5). Further increase of water to 10
equiv did not improve the results (Table 2, entry 6).
entry
catalyst
solvent
time (h)
yield (%)a
ee (%)b
1
2
3
4
5
6
7
8
L1
L2
L3
L4
L5
L2
L2
L2
L2
L2
L2
THF
57
57
48
48
72
48
48
48
48
48
48
50
81
74
40
37
70
69
60
55
85
93
33
70
61
1
-6
75
0
19
90
55
48
CH2Cl2
DMF
MeCN
MeOH
PhMe
9
10
11
Me3COH
a Isolated yields. b Determined by chiral HPLC.
L3 (20 mol %) as a catalyst afforded 3a in 98% yield and 94% ee
within shorter reaction time (36 h), and similar results were obtained
in the presence of L3 (10 mol %) under identical conditions (Table
2, entries 8 and 9). Other chiral phopshines L1, L4, and L5 are
not as effective as L2 and L3, suggesting that the combination of
an active amide proton of the catalyst and an extra proton source
(H2O) is necessary for the present success (Table 2, entries 7, 10,
and 11). In fact, it was also found that, when PPh3 was used as a
catalyst, water did not show an obvious effect on chemical yield.
We next examined the generality of this reaction using a variety
of MBH acetates 1 derived from Michael acceptors such as methyl
vinyl ketone (MVK), ethyl vinyl ketone (EVK), and methyl acrylate.
The results are indicated in Table 3. The corresponding γ-buteno-
lides 3 were obtained in high yields and ee’s whether they have
electron-donating or electron-withdrawing substituents on their
benzene rings (Table 3, entries 1-7, 9, and 10). As for the aliphatic
MBH acetate, the corresponding product 3i was obtained in 60%
yield and 71% ee in the presence of 25 mol % of L3 (Table 3,
entry 8). Using an acrylate-derived MBH acetate as the substrate
afforded the corresponding product 3l in 45% yield and 84% ee
under the standard conditions (Table 3, entry 11).
Under these optimal conditions, we continued screening chiral
phosphines L1 and L3-L5 for this reaction, and the results are
given in the entries 7-11 of Table 2. We found that using phosphine
9
7202 J. AM. CHEM. SOC. 2008, 130, 7202–7203
10.1021/ja802422d CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1. A Plausible Reaction Mechanism
Table 2. Screening of Water Loading on the Allylic Substitution of
MBH Acetate 1a with 2 To Form γ-Butenolide 3a in the Presence
of L
L (20 mol %)
1a
+
2 3a
(1.0 equiv)
(x equiv)PhMe, H2O, 48 h, rt(dr>95:5)
entry
catalyst
H2O (equiv)
2 (equiv)
time (h)
yield (%)a
ee (%)b
1
2
3
4
5
6
7
L2
L2
L2
L2
L2
L2
L1
L3
L3
L4
L5
0.1
1
3
6
6
10
6
6
6
6
2.0
2.0
2.0
2.0
2.5
2.5
2.5
2.5
2.5
2.5
2.5
48
48
48
48
48
48
48
36
36
48
72
91
94
94
46
94
60
50
98
94
57
25
65
81
88
94
94
94
25
94
94
8
9c
10
11
29
-55
6
to optically active γ-butenolides 3 under mild conditions. Good to
excellent yields and ee’s have been achieved by using water as a
coadditive. Further efforts are in progress regarding the scope and
mechanistic details.
a Isolated yields. b Determined by chiral HPLC. c 10 mol% of catalyst
was added.
Table 3. Chiral Phosphine L3-Catalyzed Allylic Substitution of
Various MBH Acetates 1 with 2
Acknowledgment. We thank the Shanghai Municipal Com-
mittee of Science and Technology (04JC14083, 06XD14005),
Chinese Academy of Sciences, and the National Natural Science
Foundation of China for financial support (20472096, 20672127,
and 20732008).
Supporting Information Available: 13C and 1H NMR spectroscopic
and analytic data for 3 and X-ray crystal data of 3d as well as chiral
HPLC traces. This material is available free of charge via the Internet
at http://pubs.acs.org.
absolute
configuration
entry
R1
R2
time (h)
yield (%)a
ee (%)b
1
2
3
4
5
6
p-MeC6H4
m-MeC6H4
p-BrC6H4
p-ClC6H4
m-ClC6H4
o-ClC6H4
p-NO2C6H4 Me
C3H7
C6H5
Me
Me
Me
Me
Me
Me
36
36
24
24
24
24
24
96
72
72
72
3b: 94
3c: 81
3d: 85
3e: 89
3f: 95
3g: 89
3h: 98
3i: 60
3j: 98
3k: 85
3l: 45
96
95
95
95
94
94
91
71
91
91
84
S, R
S, R
S, R
S, R
S, R
S, R
S, R
S, R
S, R
S, R
-
References
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data of 3d are presented in the Supporting Information.8
A plausible mechanism for this asymmetric reaction is outlined
in Scheme 1. As proposed by Krische,2 the treatment of MBH
acetate 1 with L3 produces an electrophile-nucleophile ion pair,
the enone intermediate A; this intermediate is stabilized by an
intramolecular H-bonding.9 The endo-selective Diels-Alder cy-
cloaddition of the siloxy furan ate complex with enone A affords
intermediate B followed by subsequent Grob-type fragmentation
to give γ-butenolide 3 (Scheme 1). This Diels-Alder mechanism
has been originally proposed by Krische and co-worker.2 Although
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mediate B.10
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(9) This active intermediate could be clearly observed in the 31P NMR spectrum
(see the Supporting Information).
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257.
In conclusion, we have established an efficient multifunctional
chiral phosphine L2 or L3-catalyzed allylic substitutions of MBH
acetates 1 with 2-trimethylsilyloxy furan 2 to provide an easy access
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J. AM. CHEM. SOC. VOL. 130, NO. 23, 2008 7203