Ortho-Acidic aromatic thiols as efficient catalysts of intramolecular Morita-Baylis-Hillman and Rauhut-Currier reactions

Article abstract of DOI:10.1016/j.tetlet.2010.11.077

ortho-Mercaptobenzoic acid and ortho-mercaptophenols were discovered as efficient thiol catalysts of both the intramolecular Morita-Baylis-Hillman (MBH) and Rauhut-Currier (RC) reaction. High reaction rates were achieved under mildly basic, aqueous condit

Full text of DOI:10.1016/j.tetlet.2010.11.077

Tetrahedron Letters 52 (2011) 2148–2151
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ortho-Acidic aromatic thiols as efficient catalysts of intramolecular
Morita–Baylis–Hillman and Rauhut–Currier reactions
⇑
Philipp S. Selig, Scott J. Miller
Department of Chemistry, Yale University, PO Box 208107, New Haven, CT 06520-8107, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 18 November 2010
ortho-Mercaptobenzoic acid and ortho-mercaptophenols were discovered as efficient thiol catalysts of
both the intramolecular Morita–Baylis–Hillman (MBH) and Rauhut–Currier (RC) reaction. High reaction
rates were achieved under mildly basic, aqueous conditions. The unprecedented catalytic activity of these
protic nucleophiles could originate from a Brønsted acid induced destabilization of intermediate thio-
ethers and thus represent a unique mechanism of multifunctional Lewis base catalysis.
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Morita–Baylis–Hillman reaction
Rauhut–Currier reaction
Lewis base catalysis
Organocatalysis
Thiols
The Morita–Baylis–Hillman (MBH) reaction1 represents one of
the most well known reactions in the field of Lewis base induced
organocatalysis. As a prototypical C,C-bond formation following
developed for cysteine catalysis proved unsuitable, we were
delighted to find that the desired formation of product 2a could
indeed be realized in basic, aqueous acetonitrile by certain
ortho-acidic aromatic thiols 3a–c (see Table 1).
2
nucleophilic enone activation, the MBH reaction has been the tar-
get of extensive investigations. Likewise, numerous synthetically
useful transformations, some of which are referred to as ‘atypical’
NMR studies on the initial catalyst, ortho-mercaptobenzoic acid
(thiosalicylic acid, 3a) clearly suggested the involvement of both a
relatively short-lived Michael intermediate Int1 as well as up to
four diastereomeric Michael/aldol intermediates Int2, which were
formed in a highly time-dependent fashion (see Supplementary
data). ortho-Mercaptophenols (ortho-hydroxythiophenols, 3b/c) re-
acted similarly, yet much faster, and gentle heating of the reaction
mixture led to a further dramatic increase in reaction rates (Table
1, entries 1/2, 3/4, 7/8). Formation of the desired MBH product 2a
was, however, strictly dependent on the presence of small amounts
of both external base and water in all instances (Table 1, entry 6).
Base concentration proved to be most critical. Reactions were slug-
3
MBH reactions, have been developed on its basis. Mechanistically,
the MBH reaction was shown to be remarkably complex, and many
recent studies strongly indicate the crucial importance of a proton
4
transfer in the rate-determining step. However, other related pro-
cesses reveal quite different reaction kinetics.5 In this respect,
intramolecular MBH cyclizations constitute a special case, with a
relatively easy C,C-bond formation but tight steric constraints to
the conformation of the intermediate structures involved in the
6
proton transfer step. Consequently, intramolecular MBH reactions
differ considerably from their intermolecular counterparts, and bi-
7
or multifunctional catalyst systems are often required. The Lewis
gish at c(K
2
CO
3
) <0.1 M, and base-induced side-reactions and prod-
CO ) >0.3 M. On the
basic site of these catalysts is generally provided by a tertiary
uct decomposition became dominant at c(K
2
3
1
3
amine (NR ), imine (R@N–R) or phosphine (PR
3
). In sharp contrast,
2
protic nucleophiles like secondary amines (HNR ) or thiols (RSH)
are often of extremely low catalytic activity in MBH reactions,
and the catalytic cycles commonly stall after a formal Michael or
tandem Michael/aldol reaction, leading to stable catalyst-incorpo-
rating adducts Int1/Int2 (see Fig. 1).8
O
O
O
OH
[
catalyst]
R-SH]
[
Ph
Ph
1a
2a
Following our recent report of a catalytically active cysteine-
O
OH
O
O
derivative in the closely related intramolecular Rauhut–Currier
9
(
RC) reaction, we decided to re-examine the thiol-catalyzed
Ph
RS
Ph
RS
MBH reaction of enone 1a as a test-system. While the conditions
Int1
Int2
⇑
Corresponding author. Tel.: +1 203 432 9885; fax: +1 203 432 4900.
E-mail address: scott.miller@yale.edu (S.J. Miller).
Figure 1. Intramolecular MBH cyclization and putative catalyst–substrate inter-
mediates Int1/Int2.
0
040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.11.077

P. S. Selig, S. J. Miller / Tetrahedron Letters 52 (2011) 2148–2151
2149
Table 1
aliphatic enones 1a/b/d gave products 2a/b/d in high yields and
short reaction times (Table 2, entries 1/2/4). The formation of the
cyclopentenyl-product 2d was exceptionally fast, thus making 3c
one of the most active catalysts for intramolecular MBH reactions
Thiol-mediated MBH reaction via stable Michael and Michael/aldol intermediates
3
3
3
3
a X = CO H Y = H
2
Y
X
b X = OH
c X = OH
d X = H
Y = H
Y = OH
Y = OH
of enones known to date. In contrast,
a,b-unsaturated ester 1c
O
O
O
OH
was completely unreactive toward nucleophilic attack (Table 2, en-
try 3). This was also indicated by the absence of any E,Z-isomeriza-
tion in enoate (Z)-1c, a reaction, which was almost instantaneous
for enone (Z)-1b. Formation of a seven-membered ring failed due
to a rapid, base-catalyzed intramolecular Michael-reaction of sub-
strate 1e giving the cyclopentyl carboxaldehyde 4 as the only prod-
SH
Ph
Ph
Ph
5
vol% K CO3 (0.2 M in H O)
2
2
(
MeCN)
1a
2
a
O
O
O
OH
O
OH
O
OH
1
1
uct (Table 2, entry 5).
Ph
Ph
ArS
Ph
We next turned our attention to the intramolecular RC reac-
tion.1 For protic nucleophiles, previous studies on the RC reaction
suggested an analogous mechanism as outlined in Table 1 for MBH
reactions, with tandem-Michael/Michael catalyst adducts as the
main products, but generally no catalytic turnover through catalyst
2
ArS
ArS
ArS
Int1
Int2tc
Int2tt
Int2ct/cc
1
3
elimination. A small array of RC substrates 5a–f were investi-
gated and catalysts 3a–c were indeed moderately active also in
Entry
Cat.
mol %
N
t (h)
Yield 2aa (%)
Othera (%)
1
4
these cyclizations (see Table 3).
1
2
3
4
5
6
7
8
9
3a
3a
3b
3b
—
3b
3c
3c
3d
100
rt
18
6.0
48
1.0
18
1.0
16
(trace)
53
90 Int2tc
15 Int2tt
—
20
10
1
0
70 °C
rt
70 °C
70 °C
70 °C
rt
In general, all RC reactions proceeded more slowly than the re-
lated MBH reactions, and yields remained limited to <75%. Only the
most active catalyst 3c still gave practical conversions for reactions
leading to six-membered rings 6a–c (Table 3, entries 1-4). Reac-
tions involving aromatic enones 5a/e could be further accelerated
thermally (120 °C, sealed tube; Table 3, entries 1/2, 6/7). Aliphatic
bis-enone 5b reacted sluggishly, and yields were diminished due to
66/93
83/97
0
—
b
c
d
Int2e
5–100
0
5
5
95
98
0
—
—
70 °C
70 °C
60.1
1.0
5–100
73 Int2tc
3 Int2tt
1
the formation of considerable amounts of secondary aldol-conden-
a
b
c
13b,15
Yield of pure product after flash-chromatography/after aqueous workup.
Trace amounts of Int2-type 1,3-diols (ArS = OH) formed.
Reaction was performed in the absence K
sation by-products (Table 3, entry 3).
Mixed bis-enone 5c
exhibited intermediate reactivity and fair regioselectivity (Table
3, entry 4). Enone-enoate 5d did not undergo productive catalyst
elimination at all (Table 3, entry 5).
2
CO
3 2
in MeCN/H O.
d
Sub-stoichiometric amounts of 2a and 1a formed only after flash-
chromatography.
e
Mixture of all four diastereoisomers, as well as S,S- and S,O-acetals.
Interestingly, phenolic catalyst 3c performed rather poorly in
RC reactions leading to five-membered rings despite its high
activity. Cyclopentenes 6e/f contained considerable amounts of
inseparable, double-bond migrated side-products 7e/f that were
other hand, the relative amount of aqueous base was of little
importance, and the reaction proceeded well with any amount be-
tween 2.5 and 15 vol %, where the solution became biphasic,
regardless of the substrate concentration [c(1a) = 0.1–0.5 M]. No
background reaction was observed in the absence of a thiol catalyst
1
6
formed as single diastereoisomers, presumably through a trans-
annular, enolate-induced elimination reaction (Table 3, entries 9/
11). In the cyclohexenes 6a–c, double-bond migrated products
were merely present in trace amounts (<5%). Additionally, the open
(
Table 1, entry 5). Basic aprotic conditions (K
intermolecular aldol-condensations but no MBH product.
Under optimized conditions (5 vol % 0.2 M aq K CO in MeCN),
2 3 3
CO in CHCl ) led to
2
3
Table 2
a
excellent yields of clean, crystalline product 2a could be obtained
using as little as 5 mol % catalyst within 5 min (Table 1, entry 8),
all without the need for chromatographic purification, which is
known to effect noticeable decomposition of the desired product
Thiol-catalyzed intramolecular MBH reactions
O
O
OH
n
O
Table 1, entries 3/4).7b Most importantly, para-mercaptophenol
3d), lacking the crucial proton donor in the ortho-position was en-
X
X
(
(
cat. 3b/c
n
tirely ineffective as a catalyst and gave no product even after pro-
longed heating (Table 1, entry 9). In all cases, the trans–cis
diastereoisomer Int2tc was initially formed as the kinetic product,
followed by the all-trans isomer Int2tt as the thermodynamic
product, and eventually followed by the formation of product 2a
1a-e
2a/b/d
Entry
s.m.
Prod.
X
n
Cat.
Time
Yieldb (%)
1
2
3
4
5
1a
1b
1c
1d
1e
2a
2b
—
2d
—
Ph
2
3b
3b
3c
3c
3c
1 h
2 h
2 h
90 s
1 h
71/94
65/90
Me
OEt
Ph
2
2
1
3
c
0
(
see Supplementary data). Other intermediates (Int2ct/cc) were
73/95
d
not isolable, and their relative amounts, as measured by NMR-
monitoring of the reaction mixture, never exceeded 10%, likely
due to their higher lability toward either productive catalyst
elimination and/or unproductive reverse reactions. Importantly,
the efficient trapping of free thiol catalysts in these thioether
intermediates also provides a convenient mode of protecting the
otherwise quite air-sensitive thiols from oxidation to catalytically
Ph
0
a
Reactions were performed with 5 mol % catalyst at 70 °C in MeCN (0.1–0.3 M)
containing 5 vol % aq K CO (0.2 M, H O).
2
3
2
b
c
Yield of pure product after flash-chromatography/after aqueous workup.
No conversion.
Michael-cyclization product 4 was obtained in 77% yield (74% yield without
d
catalyst).
1
0
inactive disulfides.
O
After a small substrate-screen (1a–e), we were pleased to find
that our catalytic system was somewhat general with regard to
enone substitution and ring-size (see Table 2). Both aromatic and
Ph
O
4

2
150
P. S. Selig, S. J. Miller / Tetrahedron Letters 52 (2011) 2148–2151
Table 3
Table 4
Thiol-catalyzed intramolecular RC reactionsa
Investigation of chiral hydroxy-thiol catalysts
a
SH
O
O
O
O
R2
O
O
O
OH
OH
OH
SH
R2
R2
O
OH
O
Ph
R1
R1
R1
Ph
Ph
cat. 3a-c
n
n
SH
5a-f
6a-c/e/f
7e/f
(R)-9
Entry
(R)-10
(S)-2a
(S)-6a
Product
Cat.
t (h)
T (°C)
Yieldb (%)
eec (%)
1
2
3
4
5
6
7
8
9
0
(R)-2a
(S)-2a
(S)-2a
(S)-2a
(S)-2a
(S)-9
(R)-9
(R)-9
(R)-9
(R)-9
(R)-10
(R)-9
(R)-9
(R)-9
(R)-9
0.25
18
0.5
0.25
0.1
16
1.5
1.0
20
70
20
45
70
100
70
70
70
80
120
81
76
90
89
93
0
66
74
50
45
43
24
35
42
34
—
32
<2
12
16
R1
R2
Yieldb (%) (6/7)
Entry
s.m.
Prod.
n
Cat., t (h)
1
2
3
4
5
6
7
8
9
5a
5a
5b
5c
5d
5e
5e
5e
5e
5f
6a
6a
6b
Ph
Ph
Me
Ph
Ph
Ph
Ph
Ph
Ph
Me
Me
Ph
Ph
Me
Me
OEt
Ph
Ph
Ph
Ph
Me
Me
2
2
2
2
2
1
1
1
1
1
1
3c, A, 4.0
3c, B, 0.5
3c, A, 40
3c, A, 4.5
3c, B, 4.5
3a, A, 18
3a, B, 1.0
3b, A, 0.5
3c, A, 0.5
3a, A, 16
3c, A, 0.5
75 (>30/1)
75 (20/1)
20 (>30/1)
—
c
c
6c
57 (>30/1)
(R/S)-2bd
—
0 —
rac-2d
(S)-6a
(S)-6a
6e/7e
6e/7e
6e/7e
6e/7e
6f/7f
52 (30/1)
63 (12/1)
44 (9/1)
35 (4/1)
75 (>30/1)
1
2.5
a
Reactions were performed with 5 mol % catalyst in MeCN (0.1–0.2 M) con-
taining 5 vol % aq K CO (0.2 M, in H O).
1
1
0
1
2
3
2
d
d
b
5f
6f/7f
24 (6/1)
Yield of isolated product after flash-chromatography.
Determined by chiral HPLC.
Absolute stereochemistry not determined.
c
a
Reactions were performed with 5–10 mol % catalyst in MeCN containing 5–
0 vol % aq K CO (0.2 M in H O); method A: 70 °C, under N ; method B: 120 °C,
sealed tube.
d
1
2
3
2
2
b
Yield of product(s) after flash-chromatography.
c
Contained ꢀ20% inseparable regioisomeric product (R = Me, R2 = Ph).
1
activity of (R)-10 (Table 4, entry 6) further emphasizes the crucial
importance of a protic substituent in the ortho-position to the
nucleophilic thiol (see Supplementary data).
d
Contained approx. 50% open-chain decomposition product 8.
O
To account for this critical structure–activity relationship, we
speculate that there is an interaction of the protic ortho-substitu-
ent and the thioether moiety in catalyst adducts Int1 and Int2,
which is shown for adducts of ortho-mercaptophenol (3b) in
Figure 2 below.
8
Intermediates such as (I) are assumed to be present at least par-
tially in a 1,4-zwitterionic form, enabled by intramolecular hydro-
gen bonding (II) or by a complete proton transfer (III). The
formation of a cationic b-sulfonium substituent would then in-
crease the leaving group tendency of the sulfur nucleophile in
the rate-limiting catalyst elimination step. Moreover, sulfonium-
O
chain product 8 was a third major impurity in product 6f (entry
1), possibly arising from further fragmentation of 7f. As in the
MBH reaction, catalyst activity markedly decreased in the order
c > 3b > 3a. For RC cyclizations leading to five-membered rings,
however, both yields and selectivities increased in the reverse or-
der 3c < 3b < 3a as destructive side-reactions were suppressed.
Thus, carboxylic acid 3a was found to be the optimal catalyst for
RC cyclizations leading to five-membered rings (Table 3, entries
–11), while phenol 3c remained superior for the formation of
six-membered rings (Table 3, entries 1–4).
In order to test for prospects of enantioselctive catalysts, we
conducted a brief study on the axially-chiral BINOL derived mer-
captophenols 9/10 (see Table 4). Bis-ortho-acidic catalyst 9 was
1
phenolate zwitterions are assumed to exhibit increased
a-proton
3
acidity due to the electrostatic stabilization of the formed enolate
2
0
(
IV). 1,5-Zwitterionic sulfonium-enolates (IV) closely resemble
the corresponding 1,5-zwitterions in the classical MBH reaction
of tertiary amine and phosphine catalysts, and can thus be consid-
ered as unstable, reactive tautomers of the otherwise unreactive
thioethers (I). It is unclear whether this mode of intermediate acti-
vation mainly relies on intramolecular hydrogen bonding (II) or the
electrostatic stabilization of the protonated sulfonium-intermedi-
ate by the proximal phenolate anion (III). In any case, however,
the extent of thioether protonation should roughly correspond
with the electron density on the sulfur atom. Thus, the most elec-
tron-rich catalyst 3c would form the strongest leaving group,
which is in fact also seen experimentally. The outlined mode of
activating stable Lewis base adducts via the secondary interaction
6
1
7
fully active in all MBH and RC reactions. The related structure
0, however, closely resembling the well known catalyst class
of BINOL derived bifunctional hydroxyphosphines, remained en-
tirely inactive (Table 4, entry 6).
1
8
1
19
Under established conditions, chiral catalysts (S)- and (R)-9
afforded high yields and moderate asymmetric inductions in the
MBH reactions to cyclohexenes 2a/b (Table 4, entries 1/4/7), but
gave only low enantioselectivities in the corresponding RC reaction
to 6a (Table 4, entries 9/10). Cyclopentene product 2d was ob-
tained in good yield, yet completely racemic (Table 4, entry 8).
The obtained enantioselectivities remained largely unaffected by
the amount of water and base added, catalyst loading and sub-
strate concentration but were markedly influenced by the reaction
temperature (Table 4, entries 2–5). Interestingly, both increasing
and decreasing the temperature from the established value of
O
S
O
S
O
S
Ph
S
H
H
H
Ph
Ph
H
Ph
H
O
H
δ+
HO
O
O
HO
δ−
II
I
III
IV
Figure 2. Proposed mode of activation of thioether intermediates Int1/Int2 by the
acidic ortho-substituent.
7
0 °C resulted in lower ee values. The complete absence of catalytic

P. S. Selig, S. J. Miller / Tetrahedron Letters 52 (2011) 2148–2151
2151
of a hydrogen bond or proton donor with the nucleophilic moiety
itself represents an intriguing approach in the design of multifunc-
tional MBH catalysts. The asymmetric induction of chiral catalyst 9
could stem from hydrogen-bond interactions with the secondary
4. (a) Robiette, R.; Aggarwal, V. K.; Harvey, J. N. J. Am. Chem. Soc. 2007, 129,
5513–15525; (b) Aggarwal, V. K.; Fulford, S. Y.; Lloyd-Jones, G. C. Angew.
1
Chem., Int. Ed. 2005, 44, 1706–1708; (c) Price, K. E.; Broadwater, S. J.; Walker, B.
J.; McQuade, D. T. J. Org. Chem. 2005, 70, 3980–3987; (d) Price, K. E.;
Broadwater, S. J.; Jung, H. M.; McQuade, D. T. Org. Lett. 2005, 7, 147–150; (e)
Hill, J. S.; Isaacs, N. S. J. Phys. Org. Chem. 1990, 3, 285–288.
4
a,19,21
phenolic OH-group.
5
.
.
(a) Saunders, L. B.; Cowen, B. J.; Miller, S. J. Org. Lett. 2010, 12, 4800–4803; (b)
Yukawa, T.; Seelig, B.; Xu, Y.; Morimoto, H.; Matsunaga, S.; Berkessel, A.;
Shibasaki, M. J. Am. Chem. Soc. 2010, 132, 11988–11992.
For a different interpretation of the intramolecular MBH reaction, see: Teng,
W.-D.; Huang, R.; Kwong, C. K.-W.; Shi, M.; Toy, P. H. J. Org. Chem. 2005, 71,
In conclusion, we have discovered the catalytic activity of cer-
tain ortho-acidic aromatic thiols 3a–c in the intramolecular MBH
and RC reaction. These catalysts are cheap, readily available, easily
handled, and thus provide a useful alternative to tertiary amines
and phosphines, especially in MBH cyclizations. The reactions are
best described as a series of highly reversible equilibria between
different diastereomeric thioether intermediates Int1/2, which
ultimately lead to products through catalyst elimination. Catalytic
turnover may be enabled through a Brønsted acid induced increase
of sulfur nucleophugality via S-protonation. The chiral mercap-
tophenol 9 afforded reactions with low to moderate enantioselec-
tivities, presumably under thermodynamic control. Further
investigations of this class of Lewis base catalysts are currently
in progress.
6
368–371.
7. Examples: (a) Seidel, F.; Gladysz, J. A. Synlett 2007, 986–988; (b) Aroyan, C. E.;
Vasbinder, M. M.; Miller, S. J. Org. Lett. 2005, 7, 3849–3851; (c) Yagi, K.;
Turitani, T.; Shinokubo, H.; Oshima, K. Org. Lett. 2002, 4, 3111–3114.
8.
(a) Richards, E. L.; Murphy, P. J.; Dinon, F.; Fratucello, S.; Brown, P. M.; Gelbrich,
T.; Hursthouse, M. B. Tetrahedron 2001, 57, 7771–7784; (b) Black, G. P.; Dinon,
F.; Fratucello, S.; Murphy, P. J.; Nielsen, M.; Williams, H. L.; Walshe, N. D. A.
Tetrahedron Lett. 1997, 38, 8561–8564.
9.
(a) Aroyan, C. E.; Miller, S. J. J. Am. Chem. Soc. 2007, 129, 256–257; (b) Aroyan, C.
E.; Dermenci, A.; Miller, S. J. J. Org. Chem. 2010, 75, 5784–5796.
10. In the absence of any enone, catalyst 3c oxidized rapidly (<1 h), even at rt, and
in a neutral CD CN solution. However, in reaction mixtures containing a
trapping’ enone, 3c retained its catalytical activity over several hours (see
3
‘
Table 3, entry 1), in spite of basic conditions, higher temperatures, and the use
of standard, non-degassed solvents.
1
1
1
1. For asymmetric variants of this reaction, see: (a) Hayashi, Y.; Gotoh, H.;
Acknowledgments
Tamura, T.; Yamaguchi, H.; Masui, R.; Shoji, M. J. Am. Chem. Soc. 2005, 127,
16028–16029; (b) Hechavarria Fonseca, M. T.; List, B. Angew. Chem., Int. Ed.
Support of this work by the National Science Foundation (CHE-
2004, 43, 3958–3960.
2. For a recent review and current developments, see: (a) Aroyan, C. E.; Dermenci,
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Tetrahedron Lett. 2002, 43, 8707–8710.
0
848224) and the German Academic Exchange Service (DAAD,
postdoctoral fellowship for P.S.) is gratefully acknowledged.
Supplementary data
Experimental procedures, negative controls, spectral data and
graphical representations of optimization data. Supplementary
data associated with this article can be found, in the online version,
at doi:10.1016/j.tetlet.2010.11.077.
14. For the corresponding reactions catalyzed by tertiary phosphines, see: (a)
Wang, L.-C.; Luis, A. L.; Agapiou, K.; Jang, H.-Y.; Krische, M. J. J. Am. Chem. Soc.
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