Direct access to α-sulfenylated amides/esters: Via sequential oxidative sulfenylation and C-C bond cleavage of 3-oxobutyric amides/esters

Article abstract of DOI:10.1039/c7cc09026a

An efficient, environmentally benign and unprecedented synthesis of various α-sulfenylated amides/esters has been developed under oxygen atmosphere. The reaction shows good functional group tolerance and excellent chemo/regioselectivity. All the desired products were obtained in moderate to excellent yields, even on the gram scale. Practically, the related α-thiol Weinreb amide can be readily transferred to a series of prospective compounds, and selenium atom can be introduced to the α-sites of the amides in high yields.

Full text of DOI:10.1039/c7cc09026a

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
first methanofullerene derivatives of Sc N@D5h-C80
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Direct Access to α-Sulfenylated Amides/Esters via Sequential
Oxidative Sulfenylation and C-C Bond Cleavage of 3-Oxobutyric
Amides/Esters
Received 00th January 20xx,
Accepted 00th January 20xx
Yi Jiang, a,c Jie-dan Deng, b,c Hui-hong Wang, a Jiao-xia Zou, a Yong-qiang Wang, a Jin-hong Chen, a
Long-qing Zhu, a Hong-hua Zhang, a Xue Peng, *a Zhen Wang *a,b
DOI: 10.1039/x0xx00000x
www.rsc.org/
An efficient, environmentally benign and unprecedented synthesis
of various α-sulfenylated amides/esters has been developed
under oxygen atmosphere. The reaction shows a good functional
group tolerance and excellent chemo/regioselectivity, all the
desired products were obtained in moderate to excellent yields,
even in gram scale. Practically, the related α-thiol Weinreb amide
could be readily transferred to a series of prospective compounds,
and selenium atom also can be introduced to α-site of the amides
in high yields.
Carbonyl is one of the most ubiquitous and fundamentally
important chemical groups, which was widely distributed in
diverse bioactive natural products1, pharmaceutical molecules2
as well as functional materials3. Furthermore, as an important
precursor, carbonyl serves as a versatile synthetic handle to
many other valuable functional groups4. For its tremendously
broad utilities described above, the functionalization of
carbonyl compounds has garnered much attention5. Among of
them, owing to the lower acidity of α-site of the
amides/esters, the α-C–H bond functionalization of amides or
esters is still a hotspot and remains a significant challenge6.
During the last decades, several pioneering studies of α-C–H
functionalization of 1,3-dicarbonyl compounds have been
described by Lei7a, Jiao7b, Bolm7c and Peng7d via silver/copper-
catalyzed selective C–C bond cleavage (Scheme 1a), which are
of significant interest to chemists due to their broad
applications in organic synthesis8 and biochemistry9. Recently,
Maulide and co-workers elegantly introduced nitrogen,
oxygen, carbon atom to α-site of the amides using
trifluoromethanesulfonic anhydride (Tf2O) as a strong albeit
chemoselective electrophile (Scheme 1b)10. To the best of our
knowledge, however, with respect to various C–heteroatom
Scheme 1. Methods for synthesis of α-hetero atom substituted amides/esters.
bond formation methods, even though the regioselective α-C–
H sulfenylation is extremely special and critical11, the metal-
free access to construction of C–S bonds through selective C–C
cleavage of 3-oxobutyric derivatives has not been reported
yet12. Additionally, traditional methods for synthesis of α-
sulfenylated amides/esters generally employed nucleophilic
substitution reactions of halogenated carbonyl compounds or
condensation reactions of α-sulfenylated carboxylic acids
(Scheme 1c)13. These methodologies, however, often caused
equal equivalence wastes and suffered from multi-step
preparation of the coupling precursors. Herein, we report a
metal-free protocol to α-sulfenylated amides/esters from
commercially available thiols and 3-oxobutyric amides/esters.
Inspired by the retro-Aldol reactions in biochemical
pathways14, our initial study commenced with thiophenol 1a
and N,N-dimethyl-3-oxobutanamide 2a (see the Supporting
Information table S1). After judicious evaluation of the
reaction parameters, the desired product 3aa could be
obtained in excellent yield (>95%), when NaOH
a.School of Pharmacy, Lanzhou University, West Donggang Road. No. 199, Lanzhou
730000, China. E-mail: pengx13@lzu.edu.cn. E-mail: zhenw@lzu.edu.cn
b.Institution State Key Laboratory of Applied Organic Chemistry, College of
Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China.
c. These authors contributed equally. E-mail: zhenw@lzu.edu.cn
†Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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methoxyl or aryl substitutions on the nitrogen atom of 3-oxo-
butanamides were compatible under the standard reaction
conditions (3ab-3af). Noteworthy, the α-sulfenylated Weinreb
amide 3ae was a readily available synthon that could be
transferred into many other useful α-sulfenylated compounds
easily. It is noteworthy that the secondary or primary 3-oxo-
butanamides are equally applicable in this case (3ag-3ai). To
the best of our knowledge, this is the first example to realize
such kinds of transformation for mono- or none substituted
amides. Encouraged by these successful examples, the
applicability of benzeneselenol 1a' was tested in this protocol
with diverse 3-oxo-butanamides subsequently. As expected,
no matter the acyclic or cyclic alkyl substituents on the N atom
of 3-oxo-butanamides were well tolerated (3a'b, 94%; 3a'j
,
87%), as well as aryl substituent (3a'f, 86%). Logically, the
secondary and primary 3-oxo-butanamides were checked at
last, and good performances were shown as thiophenol (3a'g-
3a'i, 77%-87%).
Scheme 2. Scope of thiols to α-sulfenylated amidesa. aStandard conditions. bThe
reaction temperature was 80 oC.
(1.5 equivalence) was used as the base in combination with O2
as the oxidant in CH3CN at room temperature over a period of
18 h. A series of control experiments demonstrated that
butyronitrile was the best among the screened solvents;
several organic bases as well as weaker inorganic bases
checked shown worse results. While switching butyronitrile to
the more commonly used acetonitrile and lowing the
o
temperature to 80 C led to a slight increase of the yield. With
this in mind, we lowered the reaction temperature in different
gradients and a high yield was detected when the reaction was
operated at room temperature (62%).To our delight, when the
concentration of substrates was increased to 0.2 mmol/mL
and the reaction time was prolonged to 18 h, the yield of the
desired product was increased dramatically to 83%.
Subsequently, nearly a quantitative yield was obtained by
increasing the amount of 2a to 1.2 equivalence. In contrast,
when decreasing the amount of NaOH to 1.2 equivalence, the
yield was slightly reduced at the same time. Additionally,
oxygen was found to be indispensable to the success of this
transformation and a reduced yield was got when oxygen was
replaced with air (84%).
Scheme 3. Scope of 3-oxo-butanamidesa. aStandard conditions. bThe reaction
temperature was 80 oC.
To make the substrates scope broader, as a consequence, a
series of 3-oxobutyric derivatives, such as methyl-3-
oxobutanoate, 3-oxobutanal, and pentane-2,4-dione etc. were
investigated. Unfortunately, no corresponding products were
obtained except for 2-alkyl substituted-3-oxobutanoate esters.
Keep this in mind, diverse thiols were tested for preparation of
α-sulfenylated esters (Scheme 4), which were found owning
With the optimized conditions in hand, we first explored the
scope of the reaction with respect to aryl thiols 1a-1m with
N,N-dimethyl-3-oxobutanamide 2a (Scheme 2). Aryl thiols
bearing electron withdrawing halo groups or electron-donating
groups at the para- (1a-1g), meta- (1k, 1l), as well as sterically
hindered ortho-position (1h-1j), were able to react with 2a to
similar substrates scope as the amides' (5aa-5ma, 71%-85%).
Surprisingly, unlike the amides, the pyridine or
afford the corresponding products 3aa–3ma in excellent
benzo[d]thiazole substituted thiols could undergo this
yields. Interestingly, when 2-aminobenzenethiol 1m was used,
a successive ammonolysis proceeded and cyclization product
3ma was formed solely in 81% yield. Moreover, the
heteroaromatic thiol 1n and alkyl thiol 1o were also suitable in
this scenario, as for the transformation for 1o, which hasn’t
been realized in the related reports. Unexpectedly, when
another benzeneselenol 1a' was tested, a high yield was
observed (3a'a, 94%).
transformation successfully with good yields (5na, 81%; 5oa
,
82%). Subsequently, a sterically hindered ester with an allyl
substituent at crowded α-position was checked later, and a
moderate yield was observed in this case (5ab, 74%), whereas,
such transformations were absolutely suppressed for thiols
with
3-oxo-butanamides.
As
for
ethyl
2-
oxocyclopentanecarboxylate 4c, an acid product was obtained
through the successive ring-opening which was transformed
into 5ac by further esterification.
Afterwards, various 3-oxo-butanamides were checked
further (Scheme 3) (2a', 2b-2j). We found that all the alkyl,
2 | J. Name., 2012, 00, 1-3
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together with 89%/83% yield of the desired product (eq 8),
which revealed that hydroxyl is a nucleophile reagent for the
elimination of acetoxy group.
Scheme 5. Transformation of α-sulfenylated Weinreb amidea.
Scheme 4. Scope of thiols and estersa
.
aReaction conditions: considering the
4 was used as 0.5 mmol, NaOH (0.80 mmol), 1 (0.20 mmol),
On the basis of the previous work7c,17 and the above
experimental results, a plausible mechanism for this reaction
was proposed in Scheme 6: (1) Thiol/Benzeneselenol was
hydrolysis, esters
CH3CN (1.0 mL), O2 (1 atm), room temperature, 18 h. See Supporting Information
table S2.
oxidized into dimer under O2 atmosphere15. (2) Enolate
attacked the dimer with cleavage the S-S/Se-Se bond to afford
intermediate together with generating one molecule of R-
A
The utility of the present strategy was demonstrated in the
transformation of the α-sulfenylated Weinreb amide 3ae
(Scheme 5). Treating 3ae with KHMDS and allyl iodide afforded
the α-alkylated product Y1 in 91% yield. In addition, attacked
by ethylmagnesium bromide, α-sulfenylated ketone Y2 was
obtained in an excellent yield of 94%, which could be further
transferred with the same procedure to afford the α-
sulfenylated tertiary alcohol Y3. Meanwhile, after getting α-
sulfenylated aldehyde Y4 by reduction reaction with diisobutyl
aluminium hydride (DIBAL-H), the valuable α-sulfenylated
primary alcohol Y5 was obtained by subsequent reduction
reaction.
B
SH/R-SeH, which could be oxidized to dimer and participated
in the next reaction cycle7c. (3) The most electron deficient
carbonyl group of intermediate
nucleophilic species (OH-) to afford intermediate
Intermediate decomposed to afford the final product
B
was attacked by the
(4)
, with
C
.
C
3
the release of one molecule of carboxylic anion
simultaneously.
To gain insight of the detailed mechanism, several
controlled experiments were carried out (see the Supporting
Information Scheme S2). According to the literatures reported,
thiols/benzeneselenol could be oxidized to dimer in air15, thus
we hypothesized which may be the important intermediate. To
test this, disulfide 1aa/diselenide 1a'a' was directly used as the
sulfenylated/selenenylated reagent to the reaction without
oxygen, the desired product 3af/3a'f was truly obtained in
94% and 88% yield (eq 2). Furthermore, when the radical
inhibitor such as butylated hydroxytoluene (BHT) or 2,2,6,6-
tetramethyl-1-piperidinyloxy (TEMPO) was added to the
present reaction conditions, the yield did not decrease,
indicating that this transformation did not proceed through a
radical pathway (eq 3). As we treated 2f under the standard
reaction conditions without thiol/benzeneselenol, no acetyl
eliminated product was observed (eq 4), using amide 2ff
instead of 2f to conduct this reaction with strong bases like
potassium bis(trimethylsilyl)amide (KHMDS) and potassium
tert-butoxide (t-BuOK) provided no reaction (eq 5). Whereas,
Scheme 6. A proposed mechanism.
In conclusion, inspired by the retro-Aldol reactions in
biochemical pathways, we have developed the first example
for preparation of α-sulfenylated amides/esters under metal-
free conditions in good to excellent yields, even in gram scale.
In this transformation no additives were needed, which is step
concisely and easy to be handled. As a result, various kinds of
α-sulfenylated amides (tertiary, secondary and primary
amides) and esters were prepared with exclusive
regioselectivity and high functional group tolerance.
Practically, the prepared α-sulfenylated Weinreb amide could
be transferred into a series of useful compounds by some
classic methods, and selenium atom also can be introduced to
α-site of various amides in high yields. Ongoing studies on the
application of this strategy to other systems and late stage
modification of pharmaceutical molecules are currently in
progress in our laboratory. We believe that the robust nature
when we synthesized 3af'
it in this transformation without thiol/benzeneselenol, the
final product 3af 3a'f was afforded smoothly in 89% and 84%
/
3a'f' by the known route16 and used
/
yield (eq 7), this result indicated that the elimination of
acetoxy proceeded after sulfuration/selenylation. When using
2f' as substrate, 85%/78% of benzoic acid can be obtained
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Dedicated to Prof. Ang Li on his 35th birthday. We thank
Prof. Xingang Zhang and Yongmin Liang for helpful discussions.
Financial support was provided by the Recruitment Program of
Global Experts (1000 Talents Plan) and Fundamental Research
Funds for the Central Universities (lzujbky-2016-153).
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