Dendritic chiral phosphine lewis bases-catalyzed asymmetric aza-Morita-Baylis-Hillman reaction of N-sulfonated imines with activated olefins

Article abstract of DOI:10.1002/adsc.200700338

A series of polyether dendritic chiral phosphine Lewis bases was synthesized, and successfully applied to the asymmetric aza-Morita - Baylis - Hill -man reaction of N - sulfonated imines (N - arylmethyl -idene-4- methylbenzenesulfonamides) with methyl vin

Full text of DOI:10.1002/adsc.200700338

FULL PAPERS
DOI: 10.1002/adsc.200700338
Dendritic Chiral Phosphine Lewis Bases-Catalyzed Asymmetric
Aza-Morita–Baylis–Hillman Reaction of N-Sulfonated Imines
with Activated Olefins
Ying-hao Liua and Min Shia,*
a
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of
Sciences, Shanghai 200032, Peopleꢀs Republic of China
Fax : (+86)-21-6416-6128; e-mail: mshi@mail.sioc.ac.cn
Received: July 10, 2007; Revised: October 8, 2007; Published online: December 14, 2007
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: A series of polyether dendritic chiral phos- phosphine Lewis bases. In addition, the dendrimer-
phine Lewis bases was synthesized, and successfully supported chiral phosphine Lewis bases can be easily
applied to the asymmetric aza-Morita–Baylis–Hill- recovered and reused.
man reaction of N-sulfonated imines (N-arylmethyl-
idene-4-methylbenzenesulfonamides) with methyl
vinyl ketone (MVK), ethyl vinyl ketone (EVK), and Keywords: activated olefins; asymmetric catalysis;
acrolein to give the adducts in good to excellent aza-Morita–Baylis–Hillman reaction; dendritic chiral
yields along with up to 97% ee, which are more ef- phosphine Lewis bases; N-sulfonated imines; orga-
fective than our previously reported original chiral nocatalysts
Introduction
base system is that it is a “structure-tunable” organo-
catalyst,[2h,3] and through modification of the catalyst
The asymmetric Morita–Baylis–Hillman (MBH) or framework, several excellent results have been report-
aza-Morita–Baylis–Hillman (aza-MBH) reaction is ed by our group.[4] Recently, we have focused on the
one of the most useful and interesting carbon-carbon search for a more effective, useful, recoverable and
bond forming reactions to give enantiomerically en- reusable chiral phosphine Lewis base system for the
riched b-hydroxy carbonyl compounds or b-amino aza-MBH reaction. Dendrimers are highly branched
carbonyl compounds bearing an a-alkylidene group and well-defined macromolecules with controllable
with enormous potential synthetic utility under mild structure, which offer a unique tool for fine-tuning
reaction conditions.[1] Over the last decade, great the catalytic activity through the microenvironment.[5]
progress has been made in catalytic asymmetric ver- To the best of our knowledge, in the field of asym-
sions of these reactions and several excellent multi- metric aza-MBH reactions, a dendritic organocatalytic
functional organocatalysts for aza-MBH to achieve system has not been investigated so far. Therefore, we
high enantioselectivities have been reported.[2] How- attempted to anchor chiral phosphine Lewis bases
ever, the asymmetric aza-MBH reaction is often ham- onto a dendrimer supporter and investigate their cata-
pered by a low reaction rate and limited substrate ap- lytic activity in aza-MBH reactions. Herein, we report
plicability because it is still highly sensitive to the sub- the synthesis of chiral phosphine Lewis bases support-
stitution at both imine and Michael acceptor. There- ed by a polyether dendrimer and their application in
fore, the design and synthesis of efficient organocata- catalytic, asymmetric aza-MBH reactions. Compared
lysts for the aza-MBH reaction is still a challenge for to the bifunctional chiral phosphine Lewis base
organic chemists. In our previous report, we disclosed core,[2b,h] a positive dendrimer effect is observed along
that chiral phosphine Lewis base [(R)-2’-diphenyl- with a higher ee of the aza-MBH products and these
phosphanyl-[1,1’]binaphthalenyl-2-ol]
[(R)-PLB8 organocatalysts can be easily recovered and reused.
shown below in Scheme 3] having a phenolic hydroxy
group is an effective chiral phosphine Lewis base for
the catalytic, asymmetric aza-MBH reaction.[2b,h] The
significant advantage in this chiral phosphine Lewis
122
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Scheme 1. Synthesis of polyether dendrimer supported chiral Lewis bases (R)-DPLBs.
Results and Discussion
To investigate their catalytic activities in asymmet-
ric aza-MBH reaction with these polyether dendritic
The synthetic route of the chiral phosphine Lewis chiral phosphine Lewis bases [(R)-DPLB1-7], an ini-
bases supported by polyether dendrimers is shown in tial examination was carried out using p-chlorobenzyl-
Scheme 1. The synthesis of MOM-protected chiral idene-4-methylbenzenesulfonamide 1e and methyl
phosphine Lewis base (R)-PLB-1 bearing one aliphat- vinyl ketone (MVK) as the substrates in the presence
ic hydroxy group has been reported in our previous of 10mol% of these ( R)-DPLBs in THF at room
paper.[4] After treatment of (R)-PLB-1 with NaH in temperature (258C). The results are summarized in
THF, the brominated polyether dendrimers A, B-1, Scheme 2. As can be seen from Scheme 2, regardless
B-2, D-1, D-2, D-3, and D-4[6] were added to produce of the benzyl group or long carbon chain in the poly-
the corresponding etherified products, which could ether dendritic catalysts the asymmetric aza-MBH re-
give the corresponding precursors of various dendritic action proceeded smoothly to give the adducts in ex-
chiral phosphine Lewis bases (R)-DPLB1-1, (R)- cellent results (up to 99% yield and 94% ee). Nota-
DPLB2-1, (R)-DPLB3-1, and (R)-DPLB4-1 in 76– bly, the second generation of dendritic chiral phos-
88% yields after removal of the MOM protecting phine Lewis base (R)-DPLB3 provided the best result
group with aqueous HCl in methanol. Reduction of in the catalytic activity (99% yield, 93% ee) under
the phosphoryl group of (R)-DPLB1-1, (R)-DPLB2- identical conditions. Although the third generation of
1, (R)-DPLB3-1, and (R)-DPLB4-1 by HSiCl3 and dendritic chiral phosphine Lewis base (R)-DPLB4
Et3N in toluene afforded the corresponding dendritic showed slightly higher enantioselectivity (94% ee), a
chiral phosphine Lewis bases (R)-DPLB1-4 in good huge drop in reactivity was also observed (67% yield)
yields (65–87%). As for the preparation of (R)- under the standard conditions. Therefore, the second
DPLB5-7 (Scheme 1), the corresponding precursors generation of polyether dendritic chiral phosphine
(R)-DPLB5-1, (R)-DPLB6-1, and (R)-DPLB7-1 were Lewis base (R)-DPLB3 was selected for the subse-
directly used for the reduction with HSiCl3 and Et3N quent studies.
in toluene without isolation. The synthetic procedures
Since the catalytic performance, i.e., the activity, se-
are quite simple. Their detailed experimental proce- lectivity, and recyclability of these focal point-func-
dures and the spectroscopic data are summarized in tionalized (active sites on a dendritic wedge) dendritic
the Supporting Information.
catalysts could depend considerably on the microen-
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Ying-hao Liu and Min Shi
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vironment of solvents, we carefully examined the in-
fluence of the employed solvents on this reaction.[5d]
Next, using (R)-DPLB3 as a chiral Lewis base pro-
moter, p-chlorobenzylidene-4-methylbenzenesulfon-
amide 1e and methyl vinyl ketone (MVK) as the sub-
strates, the solvent effect was examined to clarify such
microenvironment effect in dendrimer supported or-
ganocatalysts. The results are summarized in Table 1.
Dendritic organocatalyst (R)-DPLB3 with the benzyl
group-tethered dendrimer showed the best solubility
and activity in THF and no reaction occurred in tert-
pentanol (Table 1, entries 1 and 7). In 1,2-dichloro-
ethane (DCE), toluene, acetonitrile (MeCN), ether or
DMF, 2e was formed in 85–99% yields and 75–91%
ees under identical conditions (Table 1, entries 2–6).
We also found that the corresponding aza-MBH reac-
tion adduct 3e was obtained in higher ee (up to 97%)
along with 98% yield at the lower reaction tempera-
ture (À208C) (Table 1, entries 8 and 9).
To examine the scope and limitations of the aza-
MBH reaction of N-sulfonated aldimines 1 with
MVK by means of the dendritic oganocatalyst (R)-
DPLB3, a series of N-sulfonated aldimines 1 was
evaluated under the optimized reactions, and the re-
sults are summarized in Table 2. In general, regardless
of the electronic nature of the N-sulfonated aldimines,
excellent ees of 2 were attained (Table 2, entries 1–
10), because even for N-sulfonated aldimines 1b and
1c bearing an electron-donating group on the aromat-
ic ring, the reaction also proceeded smoothly to
Scheme 2. Asymmetric aza-Morita–Baylis–Hillman reactions
of N-(4-chlorobenzylidene)-4-methylbenzenesulfonamide 1e
(1.0equiv.) with methyl vinyl ketone (3.0 equivs.) in the
presence of polyether dendrimer-supported chiral phosphine
Lewis base (R)-DPLB (10mol%).
Table 1. Aza-Morita–Baylis–Hillman reactions of N-(arylmethylidene)arylsulfonamide 1e (1.0equiv.) with methyl vinyl
ketone (3.0equivs.) in the presence of polyether dendrimer-supported chiral Lewis base ( R)-DPLB3 (10mol%).
[a]
Yields of isolated products.
Determined by chiral HPLC.
Determined by the sign of specific rotation.
[b]
[c]
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Table 2. Aza-Morita–Baylis–Hillman reactions of N-(arylmethylidene)arylsulfonamide (1.0equiv.) with methyl vinyl ketone
(3.0equivs.) in the presence of polyether dendrimer-supported chiral Lewis base ( R)-DPLB3 (10mol%).
[a]
Yields of isolated products.
Determined by chiral HPLC.
Determined by the sign of specific rotation.
The recovered chiral Lewis base was reused.
[b]
[c]
[d]
afford the aza-MBH reaction products 2b and 2c in firmed by X-ray crystal structure diffraction (see Sup-
good yields along with high ees (Table 2, entries 2 and porting Information),[7] and the catalytic activity dis-
3). Moreover, in several cases, 97% ees were realized appeared completely with these recovered (R)-PLB8
(Table 2, entries 5 and 8). It should be pointed out (Scheme 3). The results using original (R)-PLB8 as a
that the dendritic oganocatalyst (R)-DPLB3 can be catalyst in aza-MBH reaction under similar conditions
recovered by filtration after the reaction was com- are shown in Table 3.[2b,h] As can be seen from Table 3
plete and the product was separated from the reaction clearly, (R)-DPLB3 is more effective than the original
mixtures by washing with the solvent mixture hexane/ chiral phosphine Lewis base (R)-PLB8 under the
ether (8/1) and can be reused in the same reaction to standard conditions. Therefore, we believe that the
give the adduct in similar results (Table 2, entry 11).
large branched polyether carbon chain in dendrimer
To compare with our original chiral phosphine supported organocatalyst (R)-DPLB3 retarded the
Lewis base [(R)-2’-diphenylphosphanyl-[1,1’]binaph- oxidation of phosphorus atom during the reaction and
thalenyl-2-ol] (R)-PLB8, we examined the final prod- therefore, the recovered organocatalyst (R)-DPLB3
uct of the recovered (R)-PLB8 by a similar manner can be reused to give the product in similar result.
after aza-MBH reaction was complete, and found that
most of (R)-PLB8 (>90%) has been oxidized to the
Under these optimized reaction conditions (À208C
in THF), we next examined the aza-MBH reaction of
corresponding phosphine oxide, which has been con- N-tosylated aldimines 1 with ethyl vinyl ketone
(EVK), a less reactive Michael acceptor in aza-MBH
reaction. The results are summarized in Table 4. As
can be seen from Table 4, the corresponding aza-
MBH adducts 3a–3f were obtained in 73–99% yields
along with >94% ee (Table 4, entries 1–6), and in
several cases, 97% ees were attained (Table 4, en-
tries 1, 2, and 5). Moreover, the aza-MBH reaction of
N-tosylated aldimines 1 with acrolein in the presence
of (R)-DPLB3 was also examined under the standard
conditions. We were pleased to found that the corre-
sponding aza-MBH adducts 3g–i were similarly ob-
Scheme 3. Final product of original chiral phosphine Lewis
base (R)-PLB8.
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Ying-hao Liu and Min Shi
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Table 3. Aza-Baylis–Hillman reactions of N-(arylmethylidene)arylsulfonamide 1 (1.0equiv.) with methyl vinyl ketone (3.0
equivs.) in the presence of chiral phosphine Lewis base (R)-PLB8 (10mol%).
[a]
Yields of isolated products.
Determined by chiral HPLC.
[b]
Table 4. Asymmetric aza-Morita–Baylis–Hillman reaction of N-tosyl aldimines with EVK and acrolein in the presence of
polyether dendrimer-supported chiral Lewis base (R)-DPLB3 (10mol%).
[a]
Yields of isolated products.
Determined by chiral HPLC.
Determined by the sign of specific rotation.
[b]
[c]
tained in good yields along with 89–94% ees (Table 4, Conclusions
entries 7–9).
In conclusion, we have synthesized a series of poly-
ether dendritic chiral phosphine Lewis bases and in-
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orless solid, which was immediately subjected to chiral
HPLC for the analysis of the achieved enantiomeric excess.
vestigated their application in asymmetric aza-
Morita–Baylis–Hillman reaction of N-sulfonated
imines with methyl vinyl ketone (MVK), ethyl vinyl
ketone (EVK), and acrolein under mild conditions.
The corresponding aza-MBH adducts can be obtained
in good to excellent yields (up to 99%) and excellent
enantioselectivities (up to 97%). In addition, the den-
drimer-supported chiral phosphine Lewis bases can be
Typical Reaction Procedure for the Recovery and
Reuse of the Catalyst
After reaction, the solvent was removed under reduced
pressure and the residue was washed with the solvent mix-
ture hexane/ether (8/1) (20mL). The product and the re-
easily recovered and reused after reaction. Efforts are maining MVK were extracted from the reaction mixtures
and the dendritic oganocatalyst (R)-DPLB3 can be partially
recovered by filtration in 75% yield, which can be reused in
the same reaction to give the adduct in similar results
(Table 2, entry 11). However, the second recycling would
lower the catalytic activity of the recovered catalyst to give
the product in 50% yield and 89% ee since only about half
of the initial amounts of the catalyst were recovered in the
second recycling (the catalyst was recovered in 48% yield
based on the initial amounts of catalyst during the second
recycling).
underway to examine the effect of other types of den-
drimer supporters in chiral organocatalysts and to elu-
cidate the key factors of dendrimer-supported chiral
Lewis bases in aza-MBH or MBH reactions.
Experimental Section
General Remarks
All solvents were purified by distillation. Unless otherwise
stated, all reactions were carried out under argon atmos-
phere. H NMR spectra were recorded on a Bruker AM-300
Supporting Information
1
1H NMR spectroscopic and analytical data for dendritic
chiral phosphine Lewis bases (R)-DPLB1--7, aza-Morita-
Baylis–Hillman reaction products, experimental details, and
chiral HPLC traces of the compounds shown in Tables 1, 2,
and 3 and Scheme 2 as well as the X-ray crystal data for
Scheme 3 are presented in the Supporting Information.
spectrometer as a solution in CDCl3 with tetramethylsilane
(TMS) as an internal standard; J values are in Hz. Mass
spectra were recorded with an HP-5989 instrument and HR-
MS was measured by a Finnigan MA+ mass spectrometer.
Infrared spectra were measured on a Perkin-Elmer 983
spectrometer. Melting points were obtained with a Yanagi-
moto micromelting point apparatus and are uncorrected. N-
Sulfonated imines 1 were prepared according to the litera-
ture. Commercially obtained reagents were used without
further purification. All reactions were monitored by TLC
with Huanghai GF254 silica gel-coated plates. Flash column
chromatography was carried out using 200–300 mesh silica
gel at increased pressure. The optical purities of the aza-
Morita–Baylis–Hillman adducts were determined by HPLC
analysis using a chiral stationary phase column (column,
Daicel Co. Chiralcel AD, AS, TBB and OJ; eluent: hexane/
Acknowledgements
We thank the Shanghai Municipal Committee of Science and
Technology (04 JC14083, 06XD14005), Chinese Academy of
Sciences (KGCX2–210–01), and the National Natural Science
Foundation of China for financial support (20472096,
203900502, 20672127 and 20732008).
2-propanol mixture; flow rate, 0.7 mLminÀ1
; detection,
254 nm or 220nm) and the absolute configuration of the
major enantiomer was assigned according to the sign of the
specific rotation.
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aza-Morita–Baylis–Hillman Reaction of N-Sulfonated
Imines with MVK.
A 10-mL of Schlenk tube containing p-chlorobenzylidene-4-
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habit: colorless, prismatic; crystal dimensions: 0.492ꢂ
0.403ꢂ0.357 mm; crystal system: orthorhombic; lattice
type: primitive; lattice parameters: a=9.6013(7) ꢃ, b=
14.3110(10) ꢃ, c=17.4804(12) ꢃ, a=908, b=908, g=
908, V=2401.9 (3) ꢃ3; space group: P2(1)2(1)2(1); Z=
4; Dcalc =1.301 gcmÀ3; F000 =984; diffractometer: Rigaku
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