Asymmetric aza-Morita-Baylis-Hillman reaction of N-sulfonated imines with activated olefins catalyzed by chiral phosphine Lewis bases bearing multiple phenol groups

Article abstract of DOI:10.1002/adsc.200505476

In the aza-Morita-Baylis-Hillman reaction of N-sulfonated imines (N-arylmethylidene-4-methyl-benzenesulfonamides) with methyl vinyl ketone (MVK), ethyl vinyl ketone (EVK), and acrolein, we found that, through the use of catalytic amounts of chiral phosphi

Full text of DOI:10.1002/adsc.200505476

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DOI: 10.1002/adsc.200505476
ꢀ
Asymmetric Aza-Morita–Baylis Hillman Reaction of
N-Sulfonated Imines with Activated Olefins Catalyzed by Chiral
Phosphine Lewis Bases Bearing Multiple Phenol Groups
Ying-Hao Liu, Lian-Hui Chen, Min Shi*
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of
Sciences, 354 Fenglin Lu, Shanghai 200032, Peopleꢀs Republic of China
Fax: (þ86)-21-6416-6128, mshi@pub.sioc.ac.cn.
Received: December 15, 2005; Accepted: March 2, 2006
Supporting Information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
ꢀ
Abstract: In the aza-Morita–Baylis Hillman reaction is stabilized by intramolecular hydrogen bonding with
of N-sulfonated imines (N-arylmethylidene-4-methyl- phenol groups, was observed by 31P NMR spectrosco-
benzenesulfonamides) with methyl vinyl ketone py. Thus, the most effective bifunctional LBBA
(MVK), ethyl vinyl ketone (EVK), and acrolein, we (Lewis base and Brønsted acid) phosphine-Lewis
found that, through the use of catalytic amounts of base promoter system reported to date has been iden-
chiral phosphine Lewis bases bearing multiple phenol tified for such catalytic, asymmetric aza-Morita–Bay-
ꢀ
groups, the corresponding adducts could be obtained lis Hillman reactions.
in good yields with >90% ee at ꢀ208C or at room
temperature in THF. The mechanism of this process Keywords: asymmetric catalysis; aza-Morita–Bay-
was investigated by 31P NMR spectroscopic analysis. lis Hillman reaction; chiral phosphine; Lewis base;
ꢀ
The phenoxide or the key enolate intermediate, which organocatalysts; phenol groups; N-sulfonated imines
Introduction
tions, we also disclosed that a phenolic hydroxy group
played a key role in this LBBA (Lewis base and Brønst-
Recently, the design and synthesis of multifunctional or- ed acid) bifunctional organocatalyst, with intramolecu-
ꢀ
ganocatalysts for the aza-Morita–Baylis Hillman (aza- lar hydrogen bonding affording the corresponding aza-
MBH) reaction have received much attention,[1] and MBH adduct in high ee.[2b, f] On the basis of these results,
several excellent reaction systems using chiral nitrogen we envisioned that chiral phosphine Lewis base, such as
and phosphine Lewis bases as such multifunctional or- chiral phosphinyl BINOL,[2b, f] bearing multiple phenol
ganocatalysts to achieve high enantioselectivities in groups might accelerate the reaction rates and over-
aza-MBH reactions have been reported.[2] However, to come the drawback of the limited substrates in the cata-
the best of our knowledge, asymmetric aza-MBH reac- lytic, asymmetric aza-MBH reaction since we believe
tions still suffer from slow reaction rates and limited sub- these possibly hydrogen bond donating groups can sig-
strate applicability because less electrophilic N-tosylal- nificantly stabilize the key phosphonium enolate and
dimines bearing strongly electron-donating groups on produce the corresponding adducts in good yields and
the aromatic ring do not react with high reaction rate high ee (Figure 1).
or afford high ee. Therefore, the development of better
Herein, we report the synthesis of chiral phosphine
multifunctional organocatalysts for catalytic, asymmet- Lewis bases (CPLBs) bearing multiple phenol groups
ric aza-MBH reactions is a very attractive and competi- and their application in catalytic, asymmetric aza-
tive field of research. Previously, Hine and Kelly report- MBH reactions. These new chiral phosphine Lewis bas-
ed that double hydrogen bonding can accelerate the re- es are more effectivethan thosepreviously reported[2f] in
action of phenyl glycidyl ether with diethylamine and such reactions. The phenoxide or the key enolate inter-
some Diels–Alder reactions by 1,8-biphenylenediol, re- mediates of these reactions were observed by 31P
spectively.[3] In our previous report of chiral phosphine NMR spectroscopy.
Lewis base [(R)-2’-diphenylphosphanyl-[1,1’]binaph-
thalenyl-2-ol]-catalyzed asymmetric aza-MBH reac-
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to provide compound CPLB1-10 in 90% yield. Removal
of the MOM group with aqueous HCl in methanol at
608C produced the corresponding phosphine oxide
compound CPLB1-11 in 90% yield. Reduction of phos-
phoryl group of CPLB1-11 by HSiCl3 and Et3N in tol-
uene at 1208C (oil bath) afforded the desired compound
CPLB1 in 83% yield (Scheme 1).[4] Using similar syn-
thetic procedures, a variety of CPLBs (Scheme 2) was
synthesized in good yields. Their syntheses procedures
and spectroscopic data are summarized in the Support-
ing Information.
With p-chlorobenzylidene-4-methylbenzenesulfon-
amide (1e) as substrate, the catalytic asymmetric aza-
MBH reaction with methyl vinyl ketone (MVK) was ex-
amined by use of 10 mol % of these chiral phoshine
Lewis bases in THF at room temperature (258C) for
12 h. The results are summarized in Scheme 2. The cor-
responding aza-MBH adduct 2e was obtained in >90%
yield and >90% ee with chiral phoshine Lewis bases
bearing multiple phenol groups such as CPLB1,
CPLB2, CPLB3 and CPLB4 (bearing one phenolic hy-
droxy group and one aliphatic hydroxy group), all of
which are much improved compared to our previously
reported chiral phosphine Lewis base CPLB5 under
identical conditions (Scheme 2).[2f,5] It should be noted
that CPLB2, a diastereomeric isomer of CPLB1, pro-
duced adduct 2e in slightly lower yield and ee compared
Figure 1. A plausible hydrogen bonding structure in chiral
phosphine Lewis bases bearing multiple phenol groups.
Results and Discussion
The synthesis of the chiral phosphine Lewis base to CPLB1. This result suggests that the chirality of the
(CPLB) having multiple phenol groups, CPLB1, is second binaphthol moiety do not influence the ee of
shown in Scheme 1. First, compound CPLB1-1 was syn- the products significantly. In addition, Lewis base cata-
thesized from MOM-protected (R)-BINOL by treat- lyst CPLB4 having one phenolic hydroxy group and
ment with butyllithium in anhydrous tetrahydrofuran one aliphatic hydroxy group, and CPLB3 having two
(THF) at room temperature, followed by N,N-dimethyl- phenol groups and two diphenylphosphinyl moieties
formamide (DMF) at 08C. The aldehyde group in also afforded 2e in higher yield and ee than the original
CPLB1-1 was converted to the corresponding methyl chiral phosphine Lewis base CPLB5 under identical
ester by oxidation of CPLB1-1 with iodine in methanol conditions. The best result was given by Lewis base cat-
in the presence of potassium hydroxide at room temper- alyst (R,R)-CPLB1. All these results suggest that chiral
ature for 2 days to afford compound CPLB1-2 in 90% phosphine Lewis bases bearing hydroxy groups can in-
yield. Deprotection of the MOM-protecting group deed improve the yield and ee of aza-MBH reactions.
with aqueous HCl solution in methanol at 608C pro-
Using (R,R)-CPLB1 as chiral Lewis base promoter,
duced compound CPLB1-3 in 98% yield. Compound the catalytic asymmetric aza-MBH reaction of a variety
CPLB1-4 was obtained in 90% yield by the reaction of of N-sulfonated aldimines 1 with MVK was examined in
CPLB1-3 with Tf2O in the presence of pyridine in di- THF at ꢀ208C.[2f] The results are summarized in Ta-
chloromethane at 08C. The coupling reaction of ble 1. For all of the substrates, >90% ee was achieved
CPLB1-4 with diphenylphosphine oxide afforded com- in good to high yield (Table 1, entries 1–13). In several
pound CPLB1-5 in 80% yield. The phenol group was cases, 95–96% ee was realized (Table 1, entries 2–6,
then protected by MOMCl to give compound CPLB1- and 11). For N-sulfonated aldimines bearing a strongly
6, which was treated with LiAlH4 in THF to afford alco- electron-donating group on the aromatic ring, such as
hol CPLB1-7 in 56% yield (Scheme 1). Alternatively, p-methoxybenzylidene-4-methylbenzenesulfonamide
reduction of CPLB1-1 with NaBH4 in methanol at 08C (1c), 95% ee was achieved in 70% yield using the stand-
produced alcohol CPLB1-8 in 80% yield, and this could ard conditions (Table 1, entry 3).
be transformed into bromide CPLB1-9 by treatment
Under these optimized reaction conditions (ꢀ208C in
with methanesulfonyl chloride (MsCl) in the presence THF), we next examined the aza-MBH reaction of N-to-
of Et3N in toluene/EtOAc followed by LiBr in DMF sylated aldimines 1 with ethyl vinyl ketone (EVK), a less
(Scheme 1). The etherification of CPLB1-7 by reactive Michael acceptor in aza-MBH reactions. The
CPLB1-9 was achieved by treatment of CPLB1–7 results of these reactions are summarized in Table 2.
with NaH in THF followed by addition of CPLB1–9 The corresponding aza-MBH adducts 3a–d were ob-
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Asymmetric Aza-Morita–Baylis Hillman Reaction of N-Sulfonated Imines
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Scheme 1. Synthesis of chiral CPLB1.
tained in >90% ee and good yields (Table 2, entries 1– methylbenzenesulfonamide (1f) as substrate, tempera-
4). ture effects were examined in THF. We found that the
Moreover, we also examined the aza-MBH reaction corresponding aza-MBH reaction adduct 3e was ob-
of N-tosylated aldimines 1 with acrolein in the presence tained in higher yield and ee at room temperature
of CPLB1. In this type of aza-MBH reaction, THF is the (258C) (Table 2, entries 5–7). At ꢀ208C, the achieved
best solvent and the reaction temperature can signifi- ee decreased dramatically under otherwise identical
cantly affect the ee and yield according to our previous conditions (Table 2, entries 5–7). We believe that, in
investigations.[2f] Using N-(4-bromobenzylidene)-4- this case, the equilibrium of the Baylis–Hillman reaction
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Scheme 2. Asymmetric aza-MBH reaction with chiral phosphine Lewis bases bearing multi-phenolic hydroxy groups in THF at
258C.
leans toward the reaction product at higher temperature
In order to gain mechanistic insight into these reac-
whereas, at lower temperature, the equilibrium leans to- tions, we carried out a 31P NMR spectroscopic analysis
ward the opposite way. Thus, we carried out the aza- of CPLB1 in the absence and presence of MVK, as we
MBH reactions ofother N-tosylated aldimines 1 with ac- did previously with CPLB5.[2f] We found that the 31P
rolein at room temperature (258C). These results are NMR (CDCl3, 85% H3PO4) spectroscopic data of
summarized in Table 2 (entries 8 and 9). The corre- CPLB1 showed a signal at ꢀ12.07 ppm (Figure S1 in
sponding adducts 3f and 3g were obtained in 99% and Supporting Information), but the 31P NMR spectroscop-
85% ee, respectively, in good yields.
ic data of CPLB1 with MVK (molar ratio¼1:5) showed
A control experiment was carried out using CPLB6 a new signal at þ26.36 ppm, which is believed to corre-
(Scheme 3), which is partially O-methylated and synthe- spond to the phosphonium phenoxide, which is in an
sized in a manner analogous to that shown in Scheme 1 equilibrium with the corresponding in situ formed phos-
(see Supporting Information), in the aza-MBH reaction phonium enolate, an active species in the aza-MBH re-
of 1e with MVK. The corresponding adduct 2e was action stabilized by intramolecular hydrogen bonding
formed in 55% yield with 88% ee under the standard (Figure S2).[2f,6] However, in the 31P NMR spectroscopic
conditions (Scheme 3). This result suggests that the data of CPLB6 under the same conditions, no such sig-
two phenolic hydroxy groups on the second binaphtha- nal could be observed in the presence of MVK (Figur-
lene moiety indeed play a significant role in improving es S3and S4). In the 31P NMR spectrum of CPLB1,
yields and enantioselectivities of the aza-MBH reaction. the fact that the new signal appeared in the positive re-
Another control experiment was carried out using gion upon the addition of MVK indicated that a posi-
10 mol % of CPLB5 in conjunction with 10 mol % tively charged phosphorus species was formed. A signif-
(R)-BINOL in the aza-MBH reaction of 1e with MVK icant feature in the 31P NMR spectrum of CPLB1 with
under the standard conditions (Scheme 4). We found the addition of MVK is the exclusive appearance of
that the corresponding adduct 2e was formed in 89% the new signal at þ26.36 ppm because in the 31P NMR
yield and 84% ee. These results are similar to those using spectrum of the original chiral phosphine Lewis base
CPLB5 as a sole chiral Lewis base promoter.[2f]
CPLB5, the new signal appeared in the positive field
These control experiments suggest that the multiple upon addition of MVK, along with the signal in the neg-
phenol groups in CPLB1 do indeed play a significant ative field (free phosphine ligand) in a ratio of 1:1,
role in the aza-MBH reactions to give the desired ad- which indicated that the formed phosphonium phenox-
ducts in higher yield and ee.
ide is in an equilibrium with the free chiral phosphine
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Table 1. Asymmetric aza-MBH reaction of N-tosylaldimines with MVK in the pre-
sence of chiral phosphine Lewis base CPLB1.
[a]
Yield of isolated product
[b]
Determined by chiral HPLC.
[c]
Determined by the sign of specific rotation.[2f]
Lewis base CPLB5 as well.[2f] In the case of CPLB1, only tion of MVK,[2f] presumably due to the steric hindrance
a trace of the free phosphine ligand signal was observed, in CPLB6.
indicating that an interaction (hydrogen bonding) be-
tween multiple phenol groups and the oxygen atom of
MVK does indeed exist, which strongly stabilizes the Conclusion
formed phosphonium phenoxide, which is in an equili-
brium with the corresponding in situ formed phosphoni- In conclusion, we found that in the aza-MBH reaction
um enolate stabilized by intramolecular hydrogen bond- of N-sulfonated aldimines 1 with MVK using CPLB1,
ing, and drives the equilibrium largely to the formation bearing multiple phenol groups as a chiral phosphine
of the phosphonium phenoxide or enolate intermediate Lewis base promoter, the corresponding adducts can
stabilized by intramolecular hydrogen bonding.[2f] We be obtained in >90% ee and good to high yields at
believe that this is the key factor for why these chiral ꢀ208C or room temperature (25 8C) in THF for
phosphine Lewis bases bearing multiple phenol groups most of the substrates using MVK, EVK, or acrolein
are more effective catalysts than CPLB5. as a Michael acceptor. The multiple phenol groups
For chiral phosphine Lewis base CPLB6, no spectro- played a key role in these reactions and in order to
scopic alteration was observed upon the addition of achieve high enantioselectivities and yields. Efforts
MVK, indicating that one phenolic hydroxy group is are underway to elucidate additional mechanistic de-
not enough to significantly interact with the oxygen tails of this process and the key factors of chiral Lewis
atom of MVK through hydrogen bonding, although bases in aza-MBH reactions and to disclose their
the new signal was observed with CPLB5 upon the addi- scope and limitations.
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Table 2. Asymmetric aza-MBH reaction of N-tosylaldimines with EVK and acrolein in
the presence of chiral phosphine Lewis base CPLB1.
[a]
[c]
[b]
Isolated yield Determined by chiral HPLC.
Determined by the sign of specific rotation.[2f]
Scheme 3. Asymmetric aza-MBH reaction of N-tosylated aldimine 1e with MVK catalyzed by CPLB6 (10 mol %).
Scheme 4. Asymmetric aza-MBH reaction of N-tosylated aldimine 1e with MVK catalyzed by CPLB5 (10 mol %) and (R)-
BINOL (10 mol %), respectively.
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Experimental Section
Acknowledgements
We thank the State Key Project of Basic Research (Project 973)
(No. G2000048007), Shanghai Municipal Committee of Science
and Technology (04JC14083), Chinese Academy of Sciences
(KGCX2-210-01), and the National Natural Science Founda-
tion of China for financial support (20472096, 203900502, and
20272069).
General Remarks
Melting points were obtained with a Yanagimoto micro melt-
ing point apparatus and are uncorrected. Unlessotherwise stat-
ed, all reactions were carried out under an argon atmosphere.
All solvents were purified by distillation. Infrared spectra
were measured on a Perkin-Elmer 983spectrometer. 1H
NMR spectra were recorded on a Bruker AM-300 spectrome-
ter as a solution in CDCl3 with tetramethylsilane (TMS) as an
internal standard; J values are in Hz. Mass spectra were record-
ed with an HP-5989 instrument and HR-MS were measured by
a Finnigan MAþmass spectrometer. N-Sulfonated imines 1
were prepared according to the literature.[7] All of the solid
compounds reported in this paper gave satisfactory CHN mi-
croanalyses with a Carlo-Erba 1106 analyzer. 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
References and Notes
ꢀ
[1] For reviews on the Morita–Baylis Hillman reaction, see:
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S. Hatakeyama, J. Synth. Org. Chem. Japan 2002, 60, 2–
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2001, 12, 54–64; g) P. Langer, Angew. Chem. Int. Ed.
2000, 39, 3049–3051.
ꢀ
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/2-propanol mixture; flow rate, 0.7 mL
minꢀ1; detection, 254 nm or 220 nm light) and the absolute
configuration of the major enantiomer was assigned according
to the sign of the specific rotation.[2f]
[2] a) M. Shi, Y.-M. Xu, Angew. Chem. Int. Ed. 2002, 41,
4507–4510; b) M. Shi, L. H. Chen, Chem. Commun.
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Typical Reaction Procedure for CPLB-Catalyzed Aza-
Baylis–Hillman Reaction of N-Sulfonated Imines with
MVK
A 10-mL Schlenk tube containing N-(benzylidene)-4-chloro-
benzenesulfonamide (1e) (0.5 mmol) and 3-(2’-diphenylphos-
phanyl-2-hydroxy-[1,1’]binaphthalenyl-3-ylmethoxymethyl)-
[1,1’]bi naphthalenyl-2,2’-diol CPLB1 (0.05 mmol) was de-
gassed and the reaction vessel was protected under an argon at-
mosphere. Then, THF (1.0 mL) was added. After the reaction
mixture was cooled to ꢀ308C, methyl vinyl ketone (MVK)
(1.5 mmol) was added into the Schlenk tube. The reaction mix-
ture was stirred at ꢀ208C for 24–48 hours. The solvent was re-
moved under reduced pressure and the residue was purified by
flash column chromatography (SiO2, eluent: EtOAc/petrole-
um ether¼1/5) to yield the corresponding aza-Baylis–Hillman
adduct as a colorless solid, which was immediately subjected to
the chiral HPLC for the analysis of the achieved enantiomeric
excess. For microanalysis, all these products were recrystallized
from acetone and n-hexane.
[6] The 31P NMR signal of phosphine oxide CPLB1–11 is at
þ32.44 ppm (see Supporting Information). The 31P NMR
signal of the positively charged phosphine (Pþ) is in the
range of þ10 to þ30 ppm, see: M. M. Kayser, K. L.
Hatt, D. L. Hopper, Can. J. Chem. 1991, 69, 1929–1939.
[7] a) B. E. Love, P. S. Raje, T. C. Williams II, Synlett. 1994,
493–495; b) J. H. Wynne, S. E. Price, J. R. Rorer, W. Sta-
lick, Synth. Commun. 2003, 33, 341–352.
Supporting Information Available
1
13C and H NMR spectroscopic and analytical data for chiral
phosphine Lewis bases CPLB1–CPLB4 and CPLB6, aza-
ꢀ
Morita–Baylis Hillman reaction products, experimental de-
tails, Figures S1–S4, and chiral HPLC traces of the compounds
shown in Tables 1 and 2 and Schemes 2 and 3are presented in
the Supporting Information.
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