Conjugate reduction and reductive aldol cyclization of α,β- unsaturated thioesters catalyzed by (BDP)CuH

Article abstract of DOI:10.1039/c1ob05352c

A conjugate reduction of α,β-unsaturated thioesters catalyzed by copper hydride using PMHS as stoichiometric reductant has been developed. 1,2-Bis(diphenylphosphino)benzene (BDP) was the most effective ligand for this reduction. Saturated thioesters could be produced in excellent yields when the substituent on the thiol is not sterically-demanding. This protocol was applied to induce the reductive aldol cyclization of keto-enethioates, which could offer β-hydroxythioesters in moderate to good yields.

Full text of DOI:10.1039/c1ob05352c

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PAPER
Conjugate reduction and reductive aldol cyclization of a,b-unsaturated
thioesters catalyzed by (BDP)CuH†
Ninglin Li,a Jun Ou,a Michel Mieschb and Pauline Chiu*a
Received 7th March 2011, Accepted 14th June 2011
DOI: 10.1039/c1ob05352c
A conjugate reduction of a,b-unsaturated thioesters catalyzed by copper hydride using PMHS as
stoichiometric reductant has been developed. 1,2-Bis(diphenylphosphino)benzene (BDP) was the most
effective ligand for this reduction. Saturated thioesters could be produced in excellent yields when the
substituent on the thiol is not sterically-demanding. This protocol was applied to induce the reductive
aldol cyclization of keto-enethioates, which could offer b-hydroxythioesters in moderate to good yields.
reported. One reason is probably because sulfur-rich compounds
Introduction
are well-known poisons of many metal catalysts.
The thioester is a versatile functional group in organic chemistry,
Conjugate reductions mediated by copper hydrides have been
reported for many electron-deficient olefins,14 including enones,15
enoates,16 nitroalkenes,17 2-alkenylheteroarenes,18 and unsaturated
sulfones,19 nitriles,20 phosphonates.21 Due to the paucity of exam-
ples of reductions of unsaturated thioesters, and in line with our
interest in reductions mediated by copper hydride,22 we undertook
a study examining the reduction of enethioates. Herein we
report that (BDP)CuH catalyzes the conjugate reduction of a,b-
unsaturated thioesters, as well as the reductive aldol cyclization of
these substrates.
undergoing a range of reactions effectively as an electrophile and
as a nucleophile. Mediated by soft metal salts such as Hg(II), Ag(I),
Cu(I), and Cu(II),1 thioesters are acylating agents for alcohols and
amines in peptide synthesis.2 They can be selectively reduced by
silane in the presence of palladium using the Fukuyama reduction
to yield aldehydes,3 and alkylated by palladium-catalyzed coupling
with organozinc4 and organoindium reagents,5 organostannanes6
and boronic acids7 to produce ketones. As precursors for carbon
nucleophiles, thioesters have been used extensively as enol ether
precursors in chiral crossed aldol reactions8 and enol precursors
in direct aldol reactions, both in the laboratory9 and in nature.10
The generation of thioester enolate derivatives for carbon–carbon
bond formations have been realized also from a,b-unsaturated
thioesters by conjugate addition11 and Morita–Baylis–Hillman
reactions.12
While the reductive generation of enolates from unsaturated
ketones and esters has seen enormous success for catalytic diastere-
oselective and enantioselective carbon–carbon bond formation,13
the corresponding reductive generation of enolates from unsatu-
rated thioesters has never been reported. In fact, upon searching
the literature, there have only been a handful of examples of
even simple reductions of a,b-unsaturated thioesters. Whereas the
reduction of unsaturated thioester derivatives, such as crotonyl
ACP, is a transformation in the fatty acid biosynthesis pathway,
a general, bench-top corollary of this reduction has yet to be
Results and discussion
Using a,b-unsaturated thioester 1a as a model substrate for
examining the conjugate reduction, we were initially surprised that
the treatment with a stoichiometric amount of Stryker’s reagent
([Ph3PCuH]6) afforded the saturated thioester 2a in a very low
yield (3–22%), along with numerous by-products. The reduction
using a catalytic amount of [Ph3PCuH]6 and silane utterly failed
to proceed (Table 1, entry 1). This result was initially unexpected,
because both enones and enoates, i.e. olefins conjugated with more
and less electron-withdrawing groups respectively, underwent
reductions with stoichiometric or catalytic amounts of Stryker’s
reagent readily and without incident. The affinity of copper for
sulfur could account for the anomalous behaviour of Stryker’s
reagent with the enethioate.
Recent studies have demonstrated the profound effect of ligands
on the chemistry of the resultant copper hydride, including
the modulation of reactivity and chemoselectivity.23,24 Thus we
undertook a screening of ligands to find one which would
minimize the copper–sulfur interaction and would promote the
desired catalytic reduction to furnish 2a in high yield. The copper
hydride catalysts were prepared in situ using Cu(OAc)2–H2O
(3) as the copper source. The results are summarized in
Table 1.
aDepartment of Chemistry, The University of Hong Kong, Pokfulam Road,
Hong Kong, P. R. China. E-mail: pchiu@hku.hk; Fax: +852 28571586;
Tel: +852 28598949
bUniversite´ de Strasbourg, Institut de Chimie, UMR 7177, Laboratoire de
Chimie Organique Synthe´tique, 1 rue Blaise Pascal, BP 296/R8, 67008
Strasbourg, France
† Electronic supplementary information (ESI) available: Detailed experi-
1
mental procedures and H and 13C NMR spectra of 1a–1q, 2a–2p, 4a–c.
See DOI: 10.1039/c1ob05352c
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Table 1 Screening of ligands
when Ph2SiD2 is used as the stoichiometric reductant (Scheme 1).
Presumably, the copper enolate formed from reduction is subse-
quently quenched by tBuOH. The CuOtBu thus formed undergoes
metathesis with silane to regenerate the copper hydride.
Entry
Ligand
Conversion (%)a
Yield of 2a (%)b
1
2
3
4
5
6
Ph3P
dppm
dppe
dppf
BDP
0
0
82
95
100
70
0
0
78
90
92
65
Scheme 1 Reduction of 1a using Ph2SiD2.
rac-BINAP
The scope of the reaction was examined by subjecting a
range of a,b-unsaturated thioesters to (BDP)CuH-catalyzed
conjugate reduction, and the results are shown in Table 3.
The unsaturated thioester substrates were synthesized by three
methods: the esterification of unsaturated acids using EDCI and
alkanethiols, the Wittig reaction of aldehydes with S-ethyl 2-
(triphenylphosphoranylidene)ethanethioate,25 and zinc-activated
acylation of alkanethiols with acid chlorides.26
a Conversion was determined by the 1H NMR spectrum of the crude
reaction mixture. b Isolated yield.
Table 2 Optimization of the conjugate reduction of 1a
Unsaturated thioesters of unhindered alkanethiols, such as 1a
(R1 = Et) or 1b (R1 = Bu) were reduced readily (Table 3, entries
n
1, 2). However, as R1 increased in length (1c, R1
=
nC12H25)27
Entry
BDP (mol%)
M
t (h)
Yield of 2a (Recovered 1a)
t
and steric hindrance (1d,e R1 = Bu, Ph), the reduction became
sluggish (Table 3, entries 3, 4). Thus even with this highly
active copper hydride, some unsaturated thioesters remain quite
unreactive toward reduction. Therefore, this reaction should be
chemoselective for the reduction of ethyl or butyl enethioates in the
presence of phenyl enethioates. The monoreduction of substrates
1f and 1j demonstrate that the (BDP)CuH-catalyzed reduction is
chemoselective for electron-deficient alkenes (Table 3, entry 6, 10).
Unsaturated thioesters (1g) with a-substituents (R3 π H) are
reduced in good yield, but require a higher loading of copper
catalyst (Table 3, entry 7). However, thioesters 1h and 1i derived
from cyclohex-1-ene carboxylic acid underwent reaction very
slowly. b-Substitution (1j) is also tolerated, albeit using a higher
catalyst loading as well.
1
2
3
10
10
5
5
5
1.0
0.5
1.0
0.5
1.0
1.0
4
7
7
9
12
12
91% (0)
92% (0)
90% (0)
88% (0)
83% (12)
78% (5)
4
5a
6b
5
a 5 mol% of 3 used. b 2.0 equiv of PMHS was used.
As the copper hydride of the monodentate triphenylphosphine
failed to induce catalytic reduction, a range of bisphosphines were
examined as ligands. The activities of the catalysts were compared
by conducting the reduction with 10 mol% of each of 3 and the
t
ligand, 3 equivalents of PMHS, and in the presence of BuOH
to increase the reaction rate. Bis(diphenylphosphino)methane
(dppm) produced a very unreactive copper hydride (Table 1,
entry 2). However, the use of bis(diphenylphosphino)ethane
(dppe) and bis(diphenylphosphino)ferrocene (dppf) offered
significantly higher conversion rates over dppm. 1,2-
Bis(diphenylphosphino)benzene (BDP) afforded the copper
hydride with the highest activity among the ligands tried,24
resulting in complete conversion and the highest yield of 2a. On
the other hand, rac-BINAP exhibited an inferior activity.
Using BDP as the ligand, we examined the optimization of the
reaction conditions (Table 2). Using 10 mol% 3 and 10% BDP, the
reaction at 1.0 M concentration is complete in 4 h in good yield
(Table 2, entry 1). Lowering the concentration of the reaction
to 0.5 M, or the amount of BDP to 5 mol% results in complete
conversion in about 7 h (Table 2, entries 2–3). The lowering of both
the concentration of the reaction and the amount of BDP used
still resulted in full conversion over a longer reaction time (Table 2,
entry 4), but limiting 3 to 5 mol% resulted in incomplete conversion
even after reaction overnight (Table 2, entry 5). Reducing the
amount of PMHS to 2 equivalents also resulted in incomplete
conversion (Table 2, entry 6).
Thioesters of cinnamic acid derivatives (1k–1o) are stabilized
by the extended conjugated system. Nevertheless, they were
successfully reduced, but the reactions generally required longer
reaction times, additional copper and/or PMHS (Table 3, entries
11–13). Electron-withdrawing substituents on the aromatic ring
greatly increase the reactivity of these substrates (Table 3, entries
15, 16).
The (BDP)CuH-catalyzed reduction using PMHS as stoichio-
metric reductant is a much more effective method for the reduction
of unsaturated thioesters compared with other protocols.28 More-
over, the copper-catalyzed reduction also permits reductive aldol
cyclizations if tBuOH was omitted to prevent the quenching of the
enolate.13,29 But to be able to continue to an aldol reaction, the
rate of the intramolecular aldol reaction of the copper thioester
enolate must be faster than transmetallation with the silane.
t
Indeed, in the presence of BuOH, the simple reduction of
keto-enethioate 1p occurred to solely generate 2p in good yield
(Scheme 2). Gratifyingly, however, in the absence of tBuOH, a sin-
gle diastereomeric b-hydroxythioester 4a having three contiguous
stereocentres was obtained from reductive aldol cyclization as the
major product (Scheme 2). In the reduction of 1q, hydroxythioester
4b and its b-lactone derivative 4c could be obtained as major
products even in the presence of a proton source (Scheme 2).
The reduction occurs by the 1,4-addition of copper hydride
to the conjugated system, which is clearly shown by deuteration
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Table 3 Conjugate reduction of unsaturated thioesters 1a–o
Entry
Substrate
Product
3 (mol%)
t (h)
Yield of 2 (Recovered 1)
1a
2a
3
1a, R1 = Et
2a, R1 = Et
10
10
10
10
20
10
4
5
12
12
12
6
91%
90%
60% (18%)
52% (34%)
8% (82%)
87%
1b, R1 = nBu
1c, R1 = nC12H25
1d, R1 = tBu
1e, R1 = Ph
2b, R1 = nBu
2c, R1 = nC12H25
2d, R1 = tBu
2e, R1 = Ph
4
5
6a
7
20
12
97%
8
9
10
1h, R1 = Et
1i, R1 = nBu
2h, R1 = Et
2i, R1 = nBu
10
10
20
12
12
12
25% (69%)
22% (68%)
88% (4%)
11a
12a
13
1k, Ar = Ph
2k, Ar = Ph
10
20
20
20
10
10
10
12
12
12
12
5
4
12
82% (12)
85% (6%)
90%
86% (3%)
81% (4%)
90%
1k
2k
1k
2k
14
1l, Ar = o-Cl-C6H4
1m, Ar = m-NO2-C6H4
1n, Ar = p-NO2-C6H4
1o, Ar = m-MeO-C6H4
2l, Ar = o-Cl-C6H4
2m, Ar = m-NO2-C6H4
2n, Ar = p-NO2-C6H4
2o, Ar = m-MeO-C6H4
15a,b
16b
17
86% (3%)
a 3.0 equiv. of PMHS used. b 0.5 M in PhMe.
This is probably due to the facility for cyclization to form a five-
membered ring. Without BuOH, aldol cyclization was the only
catalysis by (BDP)CuH with PMHS as stoichiometric reductant.
Under similar conditions but by omitting the proton source,
intramolecular reductive aldol cyclizations of keto-enethioates
were effected to afford the corresponding b-hydroxythioesters in
moderate to good yield. We are continuing our work to examine
the enantioselective variants of this reduction.
t
observed outcome. At a higher catalyst loading, hydroxythioester
4b was produced in 81% yield as a single diastereomer. The relative
structures of 4a–c were determined by NOESY 2D-NMR, and
the stereochemical outcomes are similar to those obtained from
the copper-mediated reductive aldol cyclizations of the analogous
enones and enoates.22
Experimental section
Typical procedure for reductions catalyzed by (BDP)CuH
Conclusion
A solution of 3 (39.4 mg, 0.197 mmol) and BDP (89.1 mg, 0.199
mmol) in 1.0 mL PhMe was stirred at room temperature for
5 min. PMHS (360 mL, 6.0 mmol) was added and the reaction
In summary, a range of a,b-unsaturated thioesters could be
reduced in a conjugate fashion, in good to excellent yields under
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Acknowledgements
This work was supported by the University of Hong Kong URC
Seed Funding 2007, the Research Grant Council of Hong Kong
SAR, P. R. China (GRF HKU 7017/09P, HKU1/CRF/08),
and the France/Hong Kong PROCORE-RGC Joint Research
Scheme 2008–2009 (F-HK30/07T).
Notes and references
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Scheme 2 Reductive aldol cyclization catalyzed by (BDP)CuH.
mixture became greenish-yellow. Thioester 1a (440.5 mg, 1.999
t
mmol) in 1.0 mL PhMe and BuOH (380 mL, 3.97 mmol) were
added sequentially. The reaction was monitored by TLC and
quenched by the addition of saturated aqueous NH4Cl solution.
The reaction mixture was filtered through a pad of silica gel.
The filtrate was extracted with EtOAc (3 ¥ 10 mL), dried over
anhydrous MgSO4 and concentrated. The residue was purified
by flash chromatography using 1.5% EtOAc in hexane to afford
2a (403.8 mg, 91%) as a pale yellow oil. 2a: Rf (5% EtOAc in
hexane): 0.56; IR (CH2Cl2): 3035, 2938, 2857, 1684 (C O), 1449,
1
1264 cm-1; H NMR (400 MHz, CDCl3): d 7.25–7.29 (m, 2H),
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7.16–7.20 (m, 3H), 2.87 (q, J = 7.4 Hz, 2H), 2.62 (t, J = 7.5 Hz, 2H),
2.56 (m, 2H), 1.63–1.73 (m, 4H), 1.24 (t, J = 7.4 Hz, 3H) ppm; 13
C
NMR (100 MHz, CDCl3): d 199.6, 142.1, 128.42, 128.37, 125.8,
43.9, 35.6, 30.7, 25.3, 23.3, 14.8 ppm; LRMS (ESI): m/z 245 ([M+
+ Na]+, 32), 161 (100), 162 (13); HRMS (ESI): calcd for C13H18OS
([M+ + Na]+), 245.0976, found 245.0983.
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Typical procedure for reductive aldol reactions
A solution of 3 (23.4 mg, 0.119 mmol) and BDP (26.6 mg, 0.0596
mmol) in 2.0 mL PhMe was stirred for 5 min. PMHS (90 mL, 1.5
mmol) was added and the reaction mixture turned greenish yellow.
Thioester 1p (93.1 mg, 0.298 mmol) in 1.0 mL PhMe was added.
The reaction was monitored by TLC and quenched by the addition
of saturated aqueous NH4Cl solution. The reaction mixture was
filtered through a pad of silica gel. The filtrate was extracted
with EtOAc (3 ¥ 10 mL), dried over anhydrous MgSO4 and
concentrated. The residue was purified by flash chromatography
using 10% EtOAc in hexane to afford 4a (53.9 mg, 57%) as a pale
yellow oil and 2p (21.5 mg, 23%). 4a: Rf (10% EtOAc in hexane):
0.53; IR (CH2Cl2): 3468, 2937, 2870, 1695 (thioester C O), 1655
1
(ester C O), 1456, 1236 cm-1; H NMR (500 MHz, toluene-d8,
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1H), 2.70 (dq, J = 7.4, 0.7 Hz, 2H), 2.32–2.08 (m, 2H), 1.98–1.92
(m, 2H), 1.71–1.67 (m, 1H), 1.60–1.53 (m, 4H), 1.52–1.32 (m, 4H),
1.31–1.22 (m, 1H), 1.06 (t, J = 7.4 Hz, 3H), 0.98 (m, J = 7.1 Hz,
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73.3, 60.5, 55.4, 51.8, 35.7, 31.6, 31.0, 26.1, 23.7, 23.5, 23.2, 20.9,
14.8, 14.2 ppm; LRMS (EI, 20 eV): m/z 253 (M+ - C2H4, 11),
179 (54), 135 (100); HRMS (EI, 20 eV): calcd for C14H21O4 (M+ -
C2H4), 253.1434, found 253.1441.
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