Chiral thiourea-phosphine organocatalysts in the asymmetric aza-Morita-Baylis-Hillman reaction

Article abstract of DOI:10.1002/adsc.200700155

A new kind of bifunctional (thio)urea-phosphine catalyst was synthesized and applied to the aza-Morita-Baylis-Hillman reaction of N-sulfonated imines with methyl vinyl ketone, phenyl vinyl ketone, ethyl vinyl ketone or acrolein. Moderate toexcellent ee an

Full text of DOI:10.1002/adsc.200700155

FULL PAPERS
DOI: 10.1002/adsc.200700155
Chiral Thiourea-Phosphine Organocatalysts in the Asymmetric
Aza-Morita–Baylis–Hillman Reaction
Yong-Ling Shia and Min Shia,*
a
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 : (+86)-21-6416-6128; e-mail: mshi@mail.sioc.ac.cn
Received: March 25, 2007; Published online: September 11, 2007
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: A new kind of bifunctional (thio)urea- excellent ee and yields of the products were obtained
phosphine catalyst was synthesized and applied to under mild reaction conditions.
the aza-Morita–Baylis–Hillman reaction of N-sulfo-
nated imines with methyl vinyl ketone, phenyl vinyl Keywords: asymmetric catalysis; Morita–Baylis–Hill-
ketone, ethyl vinyl ketone or acrolein. Moderate to man reaction; NMR spectroscopy; organic catalysis;
thiourea-phosphine organocatalysts
Introduction
The Morita–Baylis–Hillman reaction, one of the most
important methods for converting simple starting ma-
terials to densely functionalized products in a catalytic
and atom economic way, has undergone remarkable
progress since the first report in a patent in 1972.[1]
Due to the great potential of the products for further
transformation also the superior mild reaction condi-
tions, the development of a suitable asymmetric ver-
sion of this reaction has attracted considerable inter-
est in recent years. Various chiral catalysts and cocata-
lysts were developed and, very recently, a chiral
medium for this reaction was also reported.[2] Among
Figure 1. Some representative BINOL-derived bifunctional
phosphines for the aza-Baylis–Hillman reaction.
these successful chiral catalysts, the chiral BINOL-de-
rived bifunctional phoshpines developed by us and
Sasai et al. showed excellent asymmetric induction derivatives as general acids has been demonstrated by
for the aza-Morita–Baylis–Hillman reaction (Fig- several groups.[3] Recently, thioureas have also been
ure 1).[2e,k,l,n,q,r]
applied to the asymmetric Morita–Baylis–Hillman re-
The hydrogen bonding between the phenol group actions.[2m,o,t] However, the synthesis and application
and the intermediate was thought to be essential to of bifunctional (thio)urea-phosphine organocatalyst
achieve good enantioselectivity. We further envi- has never been reported. Herein, we wish to report
sioned that replacing the phenol group with a (thio)- the synthesis and application of bifunctional chiral
urea group might also give similar catalytic activity (thio)urea-phosphine organocatalysts in the asymmet-
and good asymmetric induction ability. As is well ric aza-Morita–Baylis–Hillman reaction.
known, the steric and electronic nature of the (thio)-
urea group can be easily tuned by reacting the corre-
sponding amine with different iso
far, various (thio)urea derivatives have been success-
A
fully used for a variety of diastereo- and enantioselec- As shown in Scheme 1, we first synthesized three cat-
tive reactions and the versatility of these (thio)urea alysts by condensation of the chiral aminophosphine[4]
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Therefore, we analyzed the long-stored 1a by
1H NMR spectroscopy and found that it contained a
small amount of 4-methylbenzenesulfonamide and
benzoic acid. Then we used the fresh-prepared 1a as
the starting material and added some 4-methylbenz-
enesulfonamide or benzoic acid to the reaction
system and found that benzoic acid could accelerate
the reaction rate and improve the ee of product 3a.
Using 10 mol% of benzoic acid as additive, various
solvents were first examined for the reaction of fresh-
prepared 1a and 2a catalyzed by UP1. The results are
presented in Table 1 and dichloromethane was found
to be the best solvent in terms of both yield and ee of
Scheme 1. The synthesis of bifunctional (thio)urea-phos-
phines.
with iso(thio)cyanate in tetrahydrofuran (THF) at 3a (Table 1, entry 3).
A
608C.
Using dichloromethane as the solvent, the loading
Then the catalytic activity of UP1 was tested using of benzoic acid was also examined from the range of
the reaction of N-benzylidene-4-methylbenzenesulfon- 2.0 mol% to 50 mol% (Table 2). It was found that
amide (1a) and methyl vinyl ketone (2a) as a model. using 5.0 mol% of benzoic acid could give the highest
Initially, we used long-stored 1a as the substrate and a ee (Table 2, entry 2).
moderate yield and ee of the corresponding aza-
Next, various additives with different steric hin-
Morita-Baylis–Hillman adduct 3a was obtained. How- drance and acidic strength were examined, and the re-
ever, when we used fresh-prepared 1a to repeat the sults are shown in Table 3. It was found that the acidi-
same reaction under the same conditions, product 3a ties of the additives could significantly affect the yield
was formed in rather low yield and ee (Scheme 2).
and the ee of the product. Additives with weaker
acidity or stronger acidity all resulted in lower yields
and ee (Table 3, entries 1–7). Only those additives
with similar acidity to benzoic acid (pKa =4.20) could
give satisfactory results (Table 3, entries 8–12).[5] Of
the additive examined, benzoic acid could give the
highest ee and yield.
Using 5mol% benzoic acid as the additive, the cat-
alytic activity of UP2 and UP3 was also examined. As
can be seen from Scheme 3, UP2 and UP3 were both
less effective than UP1. We further performed the re-
action between 1a and 2a at 08C using UP1 as the
catalyst and 5mol% benzoic acid as the additive. It
Scheme 2. The reactions of 1a and 2a catalyzed by UP1.
Table 1. Screening of the solvents for the reaction of 1a and 2a catalyzed by UP1 and benzoic acid.
[a]
Isolated yields.
Determined by chiral HPLC.
[b]
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Table 2. Screening of benzoic acid loading for the reaction of 1a and 2a catalyzed by UP1.
[a]
Isolated yields.
Determined by chiral HPLC.
[b]
Table 3. Screening of the additives for the reaction of 1a and 2a catalyzed by UP1.
[a]
Isolated yields.
Determined by chiral HPLC.
[b]
was found that the reaction rate decreased significant- was obtained in 95% yield and 92% ee. Thus we es-
ly and the enantioselectivity did not change obviously. tablished the optimized reaction conditions for this
After 72 h, imine 1a was consumed and product 3a reaction: using 10 mol% UP1 as the catalyst and 5
Adv. Synth. Catal. 2007, 349, 2129 – 2135ꢁ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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sponding aza-Morita–Baylis–Hillman products in
moderate yields and ee under the same reaction con-
ditions (Table 4). The absolute configurations of all
these products shown in Table 1, Table 2, Table 3,
Table 4 and Table 5were assigned by comparison of
their signs of specific rotation and HPLC spectra with
those reported ones.[2i,k]
In order to clarify if the thiourea group in UP1 was
essential to give good asymmetric induction. We pre-
pared CP1, which has the same skeleton with UP1
and lacks the thiourea group. The catalytic activity of
Scheme 3. The reaction of 1a and 2a catalyzed by UP2 or
UP3.
mol% benzoic acid as the additive to perform the re- CP1 was examined under the same optimized reac-
action at room temperature. tion conditions. As can be seen from Scheme 4, the
Under the optimized reaction conditions, several catalytic activity of CP1 was significantly lower than
other imines could also react with methyl vinyl that of UP1. Even after a prolonged reaction time,
ketone to give the products in high yields and moder- imine 1a could not be consumed completely and the
ate to excellent ee (Table 4). Various substituents at yield of 3a was only 62%. Furthermore, the asymmet-
o-, m- or p-position in the aromatic rings of the ric induction ability of CP1 was even lower and just
imines were tolerated (Table 4, entries 1–8 and 10). 13% ee of 3a was obtained. So introducing a thiourea
Besides imines of aryl aldehydes, a cinnamoyl N-sul- group to the catalyst was pivotal to accelerate the re-
fonated imine was also a suitable electrophile for this action rate and improve the enantioselectivity of the
reaction to give the corresponding aza-Morita– product.
Baylis–Hillman product in moderate ee and yield
To gain more mechanistic insight into this reaction,
(Table 4, entry 9). It should be noted here that the ee a rational mechanism was proposed upon 31P NMR
of the product was independent of the reaction time. spectroscopic investigations (see the Supporting Infor-
For the reaction of 1a with 2a under the above opti- mation).
mized reaction conditions, the product of 3a could be
According to the above observation and earlier re-
obtained in 45% yield and 88% ee within 1 hour. ports,[2i,j,k,p,q,r] we proposed a detailed mechanism
Thus, the racemization of the product or autocatalysis shown in Scheme 5for this reaction. We believe that
was not involved in this catalytic system.[6,7]
UP1 acted as a bifunctional organocatalyst in this re-
Acrolein, ethyl vinyl ketone and phenyl vinyl action. The phosphine served as a nucleophile to ini-
ketone could also react with 1a to give the corre- tiate the reaction and the thiourea group served as a
Table 4. The reaction of other N-tosylaldimines 1 with 2a catalyzed by UP1 and benzoic acid.
[a]
Isolated yields.
Determined by chiral HPLC.
[b]
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Table 5. The reaction of 1a with other activated alkenes 2 catalyzed by UP1 and benzoic acid.
[a]
Isolated yields.
Determined by chiral HPLC.
[b]
hydrogen-bonding donor to stabilize the in situ gener-
ated intermediate. First, Micheal addition of UP1 to
MVK afforded enolate intermediate A, which might
be deprotonated by benzoic acid to form intermediate
B. Then intermediate A underwent a Mannich reac-
tion with imine 1a to give the hydrogen-bonding sta-
bilized intermediates C and D. However, as shown in
the Newman projections E and F, the steric repulsion
between the C(O)Me group with the phenyl group
and the phenyl group with two phenyl groups in the
phosphorus atom suggest that intermediate C is more
stable than D in this relatively stabilized transition
state. Subsequently intermediate C underwent proton
Scheme 4. The reaction of 1a with 2a catalyzed by CP1 and
benzoic acid.
Scheme 5. A plausible reaction mechanism.
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transfer and elimination to give the (S)-enriched aza- Experimental Section
Morita–Baylis–Hillman product. Intermediate
B
might be in a very rapid equilibrium with intermedi- General Remarks
ate A. The 31P NMR signal at approximately 27 ppm
Unless otherwise stated, all reactions were carried out under
argon atmosphere. All solvents were purified by distillation.
Other commercially available reagents were used without
further purification. N-Tosylimines,[9] 2’-diphenylphosphanyl-
[1,1’]binaphthalenyl-2-ylamine[4] and CP1[10] were prepared
according to the literature. All reactions were monitored by
TLC. Flash column chromatography was carried out at in-
creased pressure. Infrared spectra were measured on a
Perkin-Elmer 983 spectrometer. 1H NMR, 31P NMR and
13C NMR spectra were recorded on a Varian Mercury vx
300 NMR spectrometer in CDCl3 using tetramethylsilane as
the internal standard. Mass and HR-MS spectra were re-
corded with a Finnigan MA+ mass spectrometer or an Ion
Spec 4.7 Tesla FTMS mass spectrometer. Melting points
were obtained by means of a micro melting point apparatus
and are uncorrected.
might correspond to the average value of intermedi-
ate B and intermediate A (see the Supporting Infor-
mation). Intermediate A and intermediate B were all
stabilized by hydrogen bonding and they might be
more stable under the reaction conditions. So it was
inclined to form intermediate A and intermediate B
in their equilibrium with free UP1, MVK and benzoic
acid in the solution. As this can be seen from the
31P NMR spectroscopic investigations in the Support-
ing Information, when 5mol% benzoic acid (half
amount to UP1) was added, the two signals were
nearly in the same strength. When the amount of ben-
zoic acid was increased to 20 mol% (double amount
to UP1), the signal corresponding to free UP1 could
hardly be detected (see the Supporting Information).
The dramatic effect of benzoic acid to accelerate the
reaction rate and improve the ee of the product can
be explained as follows. First, benzoic acid could pro-
vide a proton to form intermediate B and the anion
could act as a hydrogen-bonding accepter to give fur-
ther stabilization. Second, benzoic acid as the proton
source could accelerate the proton transfer step for
Typical Procedure for the Synthesis of Bifunctional
Chiral Phospine-Thiourea UP1
To a solution of (R)-2’-diphenylphosphanyl-[1,1’]binaphtha-
lenyl-2-ylamine (454 mg, 1.0 mmol) in THF (1.0 mL) was
added phenyl isothiocyanate (144 mL, 1.2 mmol) at room
temperature. The reaction mixture was stirred at 608C for
the formation of the product and suppress some re- 153 h. After the reaction was completed, the solvent was re-
moved under reduced pressure and the residue was purified
by a flash column chromatography (SiO2, eluent: EtOAc/pe-
troleum ether=1/8) to afford (R)-1-(’-diphenylphosphanyl-
versible steps which might cause some loss of the
enantioselectivity.[6,8] Too much benzoic acid or other
stronger acid would disfavor the formation of inter-
mediate A in its equilibrium with intermediate B. A
weaker acid was not so efficient as a proton source to
generate a similar enol intermediate and accelerate
the reaction rate and thus poorer yield and enantiose-
lectivity were observed.
[1,1’]binaphthalenyl-2-yl)-3-phenylthiourea (UP1): as
a
white solid; yield: 412 mg (70%); mp 122–1248C; [a]2D0:
+193.2 (c 1.00, CHCl3); IR (KBr): n=3339, 3164, 3053,
2924, 1594, 1533, 1497, 1433, 1329, 1309, 1265, 1238, 1187,
1
817, 743, 696 cmÀ1; H NMR (300 MHz, CDCl3, TMS): d=
6.58 (2H, d, J=5.4 Hz), 6.70 (1H, d, J=8.4 Hz), 6.89–7.39
(18H, m), 7.50–7.56 (1H, m), 7.83 (1H, d, J=8.1 Hz), 7.93
(2H, d, J=8.1 Hz), 8.00 (1H, d, J=8.4 Hz), 8.18 (1H, d, J=
8.4 Hz): 31P NMR (121.45MHz, CDCl 3, 85 % HPO4): d=
Conclusions
3
À13.0; MS (MALDI): m/Z=589 (M+ +1); HR-MS
(MALDI): m/z=589.1880, calcd. for C39H30N2PS+: 589.1862.
(R)-1-(3,5-Bis-trifluoromethyl-phenyl)-3-(2’-diphenyl-
phosphanyl-[1,1’]binaphthalenyl-2-yl)-urea (UP2): color-
less solid; mp 90–928C; [a]20: +363.9 (c 1.00, CHCl3); IR
(KBr): n=3358, 3053, 1534,D 1478, 1382, 1278, 1252, 1178,
In conclusion, we have synthesized a new kind of bi-
functional (thio)urea-phosphine organocatalyst. In the
aza-Morita–Baylis–Hillman reaction of N-sulfonated
imines with MVK, PVK, EVK or acrolein under the
catalysis of UP1 and benzoic acid in dichoromethane
at room temperature, 67–97% ee and 61–98% yields
of the corresponding adducts were obtained. A ra-
1
1133, 742, 696 cmÀ1; H NMR (300 MHz, CDCl3, TMS): d=
6.74 (1H, d, J=8.1 Hz), 6.97–7.30 (13H, m), 7.38 (1H, d,
J=3.9 Hz, NH), 7.39–7.56 (4H, m), 7.62 (2H, s), 7.67 (1H,
tional mechanism was proposed based upon 31P NMR d, J=8.1 Hz), 7.90–7.95(3H, m), 8.08 (1H, d, J=8.4 Hz),
8.22 (1H, d, J=3.9 Hz, NH); 31P NMR (121.45MHz, CDCl 3,
spectroscopic investigations (see Supporting Informa-
tion). Efforts are in progress to explore other applica-
tions of these new organocatalysts.
85% H3PO4): d=À12.9; MS (ESI): m/z=725(M + +1); HR-
MS (ESI): m/z=725.1617, calcd. for C41H28N2F6PS+:
725.1609.
(R)-1-(2’-Diphenylphosphanyl-[1,1’]binaphthalenyl-2-
yl)-3-phenylurea (UP3): white solid; mp 152–1548C, [a]2D0:
+89.7 (c 0.70, CHCl3); IR (KBr): n=3382, 3338, 3051, 1662,
1596, 1549, 1524, 1497, 1433, 1311, 1284, 1244, 743, 695 cmÀ1.
1H NMR (300 MHz, CDCl3, TMS): d=5.96 (1H, s, NH),
6.01 (1H, s, NH), 6.68 (2H, d, J=7.2 Hz), 6.76 (1H, d, J=
8.4 Hz), 6.94–7.54 (19H, m), 7.84–7.93 (3H, m), 8.00 (1H, d,
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J=9.3 Hz), 8.32 (1H, d, J=9.3 Hz); 31P NMR (121.45MHz,
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CDCl3, 85 % HPO4): d=À13.7; MS (ESI): m/z=573 (M+ +
3
1); HR-MS (ESI): m/z=573.2085, calcd. for C39H30N2PO+:
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Typical Procedure for UP1 and Benzoic Acid-
Catalyzed Aza-Morita-Baylis–Hillman Reaction of N-
Sulfonated Imine 1a with MVK
To a solution of imine 1a (65mg, 0.25mmol), UP1 (15mg,
0.025mmol) and benzoic acid (0.15mg, 0.0125mmol) in di-
chloromethane (1.0 mL) was added methyl vinyl ketone
(42 mL, 0.5mmol) at room temperature. Then reaction mix-
ture was stirred at room temperature for 10 h. After the re-
action was completed, the solvent was removed under re-
duced pressure and the residue was purified by a flash
column chromatography (SiO2, eluent: EtOAc/petroleum
ether=1/5) to afford 4-methyl-N-(2-methylene-3-oxo-1-phe-
nylbutyl)benzenesulfonamide (3a) as a colorless solid; yield:
80 mg (97%); mp 138–1428C; [a]2D0: +32.4 (c 1.00, CHCl3),
1H NMR (CDCl3, TMS, 300 MHz): d=2.16 (3H, s, Me),
2.41 (3H, s, Me), 5.26 (1H, d, J=8.4 Hz, CH), 5.64 (1H, d,
J=8.4 Hz, NH), 6.10 (1H, s), 6.11 (1H, s), 7.01–7.11 (2H,
m, Ar), 7.18–7.26 (5H, m, Ar), 7.66 (2H, d, J=7.8 Hz, Ar);
HPLC (AD column; l = 254 nm; eluent: hexane/2-propa-
nol=80/20; flow rate: 0.7 mLminÀ1):
minor =13.04 min; ee%=91%.
tmajor =11.46 min,
t
1
The H NMR spectroscopic data are consistent with those
reported in the literature[2k] and the absolute configuration
of 3a was assigned by comparison of the sign of specific ro-
tation and HPLC spectra with the reported one.[2k]
Supporting Information
Aza-Morita–Baylis–Hillman reaction products, experimental
details, 31P NMR spectroscopic investigations in Figures S2–
S11, and chiral HPLC traces of the compounds shown in
Table 3, Table 4 and Table 5as well as Scheme 2, Scheme 3
and and Scheme 4.
[3] For reviews, see: a) M. S. Taylor, E. N. Jacobsen,
Angew. Chem. Int. Ed. 2006, 45, 1520–1543; b) S. J.
Connon, Chem. Eur. J. 2006, 12, 5418–5427.
[4] a) K. Sumi, T. Ikariya, R. Noyori, Can. J. Chem. 2000,
78, 697–703; b) P. N. M. Botman, O. David, A. Amore,
J. Dinkelaar, M. T. Vlaar, K. Goubitz, J. Fraanje, H.
Schenk, H. Hiemstra, J. H. van Maarseveen, Angew.
Chem. Int. Ed. 2004, 43, 3471–3473.
Acknowledgements
[5] All pKa values are for the solvent water: J. A. Dean,
Langeꢀs Handbook of Chemistry, 15th edn., McGraw-
Hill Inc, New York, 1998.
We thank the Shanghai Municipal Committee of Science and
Technology (04JC14083, 06XD14005), Chinese Academy of
Sciences (KGCX2-210-01), and the National Natural Science
Foundation of China (20472096, 203900502, and 20672127)
and Cheung Kong Scholar program for the financial support.
[6] P. Buskens, J. Klankermayer, W. Leitner, J. Am. Chem.
Soc. 2005, 127, 16762–16763.
[7] M. Mauksch, S. B. Tsogoeva, I. M. Martynova, S. Wei,
Angew. Chem. Int. Ed. 2007, 46, 393–396.
[8] a) K. E. Price, S. J. Broadwater, H. M. Jung, D. T.
McQuade, Org. Lett. 2005, 7, 147–150; b) V. K. Aggar-
wal, S. Y. Fulford, G. C. Lloyd-Jones, Angew. Chem.
Int. Ed. 2005, 44, 1706–1708; c) K. E. Price, S. J. Broad-
water, B. J. Walker, D. T. McQuade, J. Org. Chem.
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