Tandem Pd/Au-catalyzed route to α-sulfenylated carbonyl compounds from terminal propargylic alcohols and thiols

Article abstract of DOI:10.1002/chem.201304111

An efficient and highly atom-economical tandem Pd/Au-catalyzed route to α-sulfenylated carbonyl compounds from terminal propargylic alcohols and thiols has been developed. This one-step procedure has a wide substrate scope with respect to substituents at

Full text of DOI:10.1002/chem.201304111

DOI: 10.1002/chem.201304111
Communication
&
Organic Synthesis
Tandem Pd/Au-Catalyzed Route to a-Sulfenylated Carbonyl
Compounds from Terminal Propargylic Alcohols and Thiols
Srijit Biswas, Rahul A. Watile, and Joseph S. M. Samec*[a]
The traditional synthesis of a-sulfenylated carbonyl com-
Abstract: An efficient and highly atom-economical
pounds involves substitution of the corresponding a-halogen-
tandem Pd/Au-catalyzed route to a-sulfenylated carbonyl
ated intermediate by sulphide anions (Scheme 1, route A).[2] Al-
compounds from terminal propargylic alcohols and thiols
ternatively, the reaction between a parent carbonyl compound
has been developed. This one-step procedure has a wide
or preformed enolate with sulfenylating agents, such as disul-
substrate scope with respect to substituents at the a-posi-
fides, N-(phenylsulfanyl)succinimides, or sulfenyl chlorides also
tion of the alcohol. Both aromatic and aliphatic thiols gen-
produces the a-sulfenylated carbonyl compounds (Scheme 1,
erated the a-sulfenylated carbonyl products in good to
route B).[3] These traditional synthetic methods are associated
excellent yields. A mechanism is proposed in which the re-
with limitations including: 1) use of toxic and difficult to
action proceeds through a Pd-catalyzed regioselective hy-
handle intermediates, 2) multiple reaction steps, 3) low-atom
drothiolation at the terminal triple bond of the propargyl
efficiency with a stoichiometric use of reagents and formation
alcohol followed by an Au-catalyzed 1,2-hydride migra-
of chemical waste. Asymmetric versions of stoichiometric elec-
tion.
trophilic a-sulfenylations have been reported by using a chiral
auxiliary,[4] an organocatalyst,[5] or chiral complexes of titani-
um(IV) or nickel(II).[6] The lack of a mild, efficient, and atom-
a-Sulfenylated carbonyl compounds belong to an important
class of compounds. The structural motif is often found in
agrochemicals and pharmaceuticals (Figure 1).[1] From a syn-
thetic perspective, these types of compounds have frequently
been used as valuable starting materials or reactive intermedi-
ates in a variety of organic transformations.[2]
economical route to synthesize a-sulfenylated carbonyl com-
pounds motivated further studies.
Scheme 1. Traditional synthesis of a-sulfenylated carbonyl compounds.
Recently, a gold(I)-catalyzed route to a-sulfenylated carbonyl
compounds from propargylic alcohols and aryl thiols has been
reported by our group (Scheme 2A).[7] The reaction was found
to be highly atom efficient in which different primary and sec-
ondary propargylic alcohols with internal carbon–carbon triple
Figure 1. Biologically active molecules containing an a-sulfenylated carbonyl
structural motif.
[a] Dr. S. Biswas, R. A. Watile, Prof. Dr. J. S. M. Samec
Department of Chemistry, BMC
Uppsala University, Box 576
75123, Uppsala (Sweden)
Fax: (+46)18 471 38 18
E-mail: joseph.samec@kemi.uu.se
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201304111.
Scheme 2. Tandem Pd/Au catalysis.
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bonds reacted with aromatic thiols to generate the corre-
sponding a-sulfenylated aldehydes and ketones. The transfor-
mation proceeded by two sequential reaction steps; the first
step consisted of a gold-catalyzed regioselective hydrothiola-
tion reaction[8] forming a sulfenylated allylic alcohol as the re-
active intermediate that underwent an isomerization through
a gold-catalyzed 1,2-hydride migration to generate the a-sulfe-
nylated carbonyl product. When propargylic alcohols contain-
ing a terminal carbon–carbon triple bond were employed
under similar reaction conditions, the regioselectivity and che-
moselectivity of the initial gold-catalyzed[9] hydrothiolation re-
action was completely lost resulting in multiple thiol attack to
the carbon–carbon triple bond. The methodology was also lim-
ited to propargylic alcohols with hydrogen or aliphatic sub-
stituents at the R1-position. Furthermore, only aromatic thiols
were found to be reactive under the reported reaction condi-
tions.
ning the reaction for 8 h at 808C in nitromethane solvent
(Table 1, entry 1). The yield could be increased to 99% by
using 1.2 equivalents of 2a with respect to 1a (entry 2). Lower-
ing the reaction temperature to 608C reduced the yield of 3a
to 69% (entry 3). By using toluene and 1,2-dichloroethane as
solvents produced the product 3a in 96 and 80% yields (en-
tries 4 and 5, respectively), whereas acetonitrile gave 72% for-
mation of 3a (entry 6). Ethanol was also screened as the sol-
vent for which a 51% yield of 3a was observed (entry 7). Non-
regioselective and multiple thio-attack at the CÀC triple bond
was observed by using only AuCl catalyst leading to a mixture
of sulfenylated alcohols (entry 9), whereas exclusive formation
of the intermediate sulfenylated allylic alcohol was observed
using only Pd(OAc)2 as the catalyst through a regioselective hy-
drothiolation reaction (entry 10; see Scheme 4 for details). A
28% product formation was observed upon replacing AuCl by
CuI (entry 11).
We herein report a complementary method to synthesize a-
sulfenylated carbonyl compounds by using terminal propargyl-
ic alcohols and thiols in which the limitations of the previous
report,[7] such as 1) the problems of regioselectivity in the hy-
drothiolation step for terminal propargylic alcohols, 2) wrong
reactivity for propargylic alcohols with aromatic groups at the
R1-position, and 3) lack of reactivity for aliphatic thiols, were
successfully resolved employing a tandem Pd/Au[10] catalytic[11]
system (Scheme 2B). These results significantly enhance the
scope of the methodology in terms of diversity and thereby
complement the previous report.[7]
The optimized reaction conditions were applied to a variety
of terminal propargylic alcohols and aryl thiols (Table 2). Pri-
mary propargylic alcohol reacted with thiophenol to produce
the corresponding a-sulfenylated aldehyde 3b in a quantitative
yield. Secondary propargylic alcohols with different aliphatic
groups, such as ethyl, iso-butyl, and n-butyl, at the R1-position
generated the corresponding a-sulfenylated ketones (3c–e),
respectively, in excellent yields. Aromatic thiols with different
electron-donating or -withdrawing substituents, such as p-
chloro, p-bromo, m,p-dichloro, p-fluoro, p-methoxy, and p-iso-
propyl, at the aromatic ring also reacted smoothly to generate
the desired products (3 f–k) in 94–98% yields.
To optimize the reaction parameters, propargyl alcohol 1a
and thiophenol 2a were chosen (Table 1). Different solvents
were screened over different reaction temperatures. An 82%
yield for the formation of product 3a was observed after run-
A drawback with the previously reported gold(I)-catalyzed
route was the restriction to aryl thiols.[7] The combination of
Table 2. a-Sulfenylated carbonyl compounds from terminal propargylic
alcohols and aryl thiols.[a,b]
Table 1. Optimization of reaction parameters.[a]
Entry
Solvent
T [8C]
80
t [h]
Yield [%][b]
1[c]
2
3
4
5
6
7
8
MeNO2
MeNO2
MeNO2
PhMe
1,2-DCE[d]
AcCN
8
82
99
69
96
80
72
51
90
trace[f]
0[h]
28
80
60
80
80
80
80
80
80
80
80
8
8
8
8
8
8
6
8
8
8
EtOH
MeNO2
MeNO2
MeNO2
MeNO2
9[e]
10[g]
11[h]
[a] Reaction conditions: 1a (1 mmol), 2a (1.2 mmol), Pd(OAc)2 (2 mol%),
and AuCl (2 mol%) were treated in 2 mL solvent. [b] Isolated yield.
[c] 1 mmol of 2a was used. [d] 1,2-Dichloroethane. [e] Only AuCl was
used as the catalyst. [f] Nonregioselective multiple attack of thiol to the
triple bond was observed. [g] Only Pd(OAc)2 was used as the catalyst.
[h] Exclusive formation of intermediate sulfenylated allylic alcohol was ob-
served by regioselective hydrothiolation reaction (see Scheme 4).
[i] 5 mol% CuI was used in place of AuCl.
[a] Reaction conditions: 1 (1 mmol), 2 (1.2 mmol), Pd(OAc)2 (2 mol%), and
AuCl (2 mol%) were treated in nitromethane (2 mL). [b] Isolated yield.
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Pd/Au was found to overcome this limitation. Different aliphat-
ic thiols were found to be reactive under the present reaction
conditions to generate the a-sulfenylated ketones in good to
excellent yields (Table 3). Cyclohexyl thiol reacted with alcohol
1a to produce product 3l in an 84% yield, whereas 80 and
81% yields were obtained utilizing cyclopenyl thiol and benzyl
thiol, respectively. This outcome was in sharp contrast to the
previously reported gold-catalyzed method[7] in which no reac-
tivity was observed for aliphatic thiols.
terminal propargylic alcohol 1a under a similar protocol to
generate the a-selenium substituted ketone 3t in a 41%
yield.[12b]
Another class of challenging substrates was propargylic alco-
hols containing an aromatic group in the benzylic position (R1)
of the alcohol. Previously, these substrates gave poor reactivity
in the a-sulfenylation reaction due to a competing direct sub-
stitution reaction of the activated hydroxyl group. Gratifyingly,
the combination of Pd/Au catalysis overcame the problem of
chemoselectivity in favor of the a-sulfenylation reaction. Pro-
pargyl alcohol with a phenyl group at the R1-position pro-
duced the desired a-sulfenylated product 3o in a 98% yield
under the present reaction conditions. Propargyl alcohol with
an electron-withdrawing chloro substituent at the para-posi-
tion of the phenyl ring generated the desired product 3p in
81% yield. Also, propargyl alcohol with an electron-donating
methoxy group at the para-position of the phenyl ring, that is
expected to promote the direct substitution reaction, generat-
ed 3q in a 69% yield. Diversity of the substrate scope of the
present method was demonstrated when cyclohexyl (aliphatic)
thiol reacted with 1-phenylprop-2-yn-1-ol containing a phenyl
substituent at the R1-position to generate the desired product
3r in 83% yield.[7]
Scheme 3. Application to a cysteine derivative and selenol.
To understand the mechanism,[13] control experiments were
performed by using alcohol 1a and thiophenol 2a (Scheme 4).
Alcohol 1a was treated with thiophenol 2a in the presence of
Pd(OAc)2 which produced the sulfenylated allylic alcohol 4
(Scheme 4A). Compound 4 was treated with AuCl catalyst in
a separate experiment that generated the product 3a in
a quantitative yield (Scheme 4B).
Table 3. Extension of the substrate scope to aliphatic thiols and benzylic
alcohols with terminal triple bonds.[a,b]
Scheme 4. Control experiments to support the mechanistic proposal.
The necessity to apply tandem Pd/Au catalysis in the case of
terminal propargylic alcohols is explained by the initial Pd-
mediated regioselective hydrothiolation step. As mentioned
earlier, using only Au salt as the catalyst gave a nonregioselec-
tive reaction with multiple attacks of the thiol to the terminal
triple bond of the propargylic alcohol. However, using a mix-
ture of Pd and Au catalysts, Pd initially promoted the regiose-
lective attack of the thiol at the b-position of the terminal
propargylic alcohol to produce exclusively the desired sulfen-
ylated allylic alcohol intermediate.[14] Noteworthy in this re-
spect was that the high degree of regioselectivity and chemo-
selectivity by the Pd catalyst was also observed in the presence
of Au salt. Thereby, sequential addition of the catalysts was
not required. On using only Pd catalyst, the reaction stopped
after the formation of the sulfenylated allylic alcohol intermedi-
ate. No further isomerization of this intermediate leading to
the final product was observed. This intermediate was convert-
ed to the desired product by the Au-catalyzed[15] isomerization
To further explore the present synthetic protocol, a protected
cysteine derivative was employed as the sulphenylating re-
agent (Scheme 3A). The reaction between the propargylic alco-
hol 1a and N-(tert-butoxycarbonyl) l-cysteine methyl ester 2k
generated the substituted cysteine derivative 3s in a 44%
yield after 24 h.[12a] Additionally, applicability of the synthetic
protocol was shown when the thiol was successfully replaced
by selenol (Scheme 3B). Benzeneselenol 2l reacted with the
of the intermediate through
(Scheme 5).
a
1,2-hydride migration
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In conclusion, a highly atom efficient transformation of ter-
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Experimental Section
Representative experimental procedure for the synthesis of
3-(phenylthio)butan-2-one (3a)
Pd(OAc)2 (5 mg, 2 mol%) and AuCl (5 mg, 2 mol%) were weighed
and transferred to a 5 mL microwave vial containing a small
magnet in a glove box under a nitrogen atmosphere. The vial was
capped tightly and was taken out from the glove-box. 2.0 mL of
dry nitromethane solvent followed by alcohol 1a (78 mL, 1 mmol)
were added and stirred at room temperature for 5 min. Benzene-
thiol 2a (123 mL, 1.2 mmol) was added and the reaction mixture
was heated at 808C (oil bath) for 8 h. After completion of the reac-
tion (by TLC analysis), nitromethane was evaporated under re-
duced pressure and the residue was purified by silica-gel (100–
200 mess) column chromatography using 3% (v/v) ethyl acetate/
pentane solution to afford the desired product 3a as a pale-yellow
oil (178 mg, 0.99 mmol, 99%). 1H NMR (400 MHz, CDCl3): d=1.43
(d, J=7.2 Hz, 3H; H-4), 2.29 (s, 3H; H-1), 3.80 (q, J=7.2 Hz, 1H; H-
3), 7.27–7.31 (m, 3H; H-arom), 7.32–7.40 ppm (m, 2H; H-arom);
13C NMR (100 MHz, CDCl3): d=16.0, 26.3, 52.0, 128.0, 129.0, 132.6,
132.6, 132.7, 205.5 ppm.
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Acknowledgements
This research was funded by the Swedish Research Council,
Stiftelsen Olle Engkvist Byggmꢁstare, Wenner Gren Foundation,
and Swedish Institute.
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the reaction mixture.
Keywords: atom efficiency · gold · palladium · sulfenylated
carbonyl · tandem catalysis
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Received: October 21, 2013
Published online on January 29, 2014
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