Efficient One-Pot Multifunctionalization of Alkynes en Route to α-Alkoxyketones, α-Thioketones, and α-Thio Thioketals by using an Umpolung Strategy

Article abstract of DOI:10.1002/chem.201703087

The use of polarized synthons is a highly desirable strategy, as it generally enables unprecedented retrosynthetic disconnections for the synthesis of unique substances. Herein, a new approach for α-oxygenated ketones, α-thioketones and α-thio thioketals via an intermolecular umpolung reaction between nucleophiles (alcohols/thioalcohols) and N-alkenoxypyridinium salts, which were generated from the corresponding alkynes, has been developed for the first time. The reactions proceed with good substrate scope and excellent functional group tolerance in one-pot manner. Applications of the products, α-oxygenated ketones, to the synthesis of other valuable synthetic moieties has also been successfully achieved.

Full text of DOI:10.1002/chem.201703087

A Journal of
Accepted Article
Title: Efficient One-pot Multifunctionalization of Alkynes en Route to
α- Alkoxyketones, α-Thioketones and α-Thio Thioketals via an
Umpolung Strategy
Authors: Zhou Xu, Rongliang Zhai, Ting Liang, and Liming Zhang
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To be cited as: Chem. Eur. J. 10.1002/chem.201703087
Link to VoR: http://dx.doi.org/10.1002/chem.201703087
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Efficient One-pot Multifunctionalization of Alkynes en Route to α-
Alkoxyketones, α-Thioketones and α-Thio Thioketals via an
Umpolung Strategy
Xu Zhou,*[a] Zhai Rongliang,[a] Liang Ting,[a] Zhang Liming[b]
Abstract: The usage of polarized synthons is a highly desirable
strategy, as it generally enables unprecedented retrosynthetic
disconnections for the synthesis of unique substances. Herein, a
novel approach for α-oxygenated ketones, α-thio ketones and α-thio
thioketals via intermolecular umpolung reaction between
nucleophiles-alcohols/thioalcohols and N-alkenoxypyridinium salts
which generated from the corresponding alkynes has been
developed for the first time. The reactions proceed with good
substrates scope and excellent functional groups tolerance in one
pot manner. Applications of the products, α-oxygenated ketones, to
the synthesis of other valuable synthetic moieties have also been
successfully achieved.
synthesis of α-alkoxyketones, one of the drawbacks is that most
of them must use the corresponding ketones as the starting
materials which limit the reaction scopes and the substrates are
always narrow with poor functional group tolerance.[18-24] The
most effective way to synthesize α-oxygenated ketones, should
be the use of alcohols as nucleophiles directly (Figure 1, a). The
direct displacement of a halogen from α-haloketones with
alkoxydes seems attractively and effectively, however, it has
narrow applications due to side reactions, such as carbonyl
addition, Favorskii rearrangement et.al often occur under the
condition. To overcome the above problems, instead of α-halo
ketones as the starting materials, Kimpe et.al used α-halo imines
which are less reactive due to the weaker electronegativity of
nitrogen with respect to oxygen to achieve the α-alkoxyketones
under basic conditions using alcohols as the nucleophiles.[25]
Newman group reported that alcoholysis of α-diazoketones
under Lewis acid conditions could afford α-alkoxyketones.[26]
Moriarty group reported the synthesis of α-methoxy ketones via
the hypervalent iodine oxidation of silyl-vinyl ethers.[27] However,
the requirement for harsh reaction conditions, such as pre-
prepared special reagents, Schlenk technique, extremely low
reaction temperature, hazardous substrates, tedious work-up
and et.al, hindered the uitilities of those methods.[28-29]
Gold-catalyzed reactions have been intensively investigated in
the past decade.[1-9] In 2016, we reported the first example of
gold catalyzed synthesis of N-alkenoxypyridinium salts which
behave as a surrogate of highly reactive acylcarbenium cation
and were used to synthesize benzene-fused cyclic ketones via
Friedel-Crafts-type intramolecular umpolung reaction.[10] Hashmi
reported earlier that the N-alkenoxypyridinium intermediates
generated from protonation of electron-rich phenoxyethynes
were likewise electrophilic.[11] Although the umpolung of carbonyl
centers is a well-known strategy, polarity reversal at the α-
position of a carbonyl group is much rarer.[12-16] We believe that
the N-alkenoxypyridinium salts which show unconventional
umpolung activity should be distinctive building blocks for other
important chemical transformations. Efficiency synthesis of
these N-alkenoxypyridinium salts using commercial available
catalyst would supply a wide usage of these valuable build
blocks. On the other hand, in view of the potential advantages of
umpolung reactions, it is of great significance to explore new
transformations based on N-alkenoxypyridinium salts formation
that evolve from alkynes. Studies on intermolecular umpolung
reactions
concerning
the
above
electrophilic
N-
alkenoxypyridinium salts will be attractive and challenging.
α-Alkoxyketones, α-thio ketones or α-thio thioketals not only
represents an important class of carbonyl-containing scaffolds
which have wide applications in organic synthesis, but also are
common structures found in natural products, drugs and
flavor.[17] Although many methods have been developed for the
Figure 1. α-Alkoxy ketones and their thio counterparts: prior arts and design.
As an important extension of umpolung reactivity, we supposed
that the α-oxygenated ketones or α-thio ketones could be
obtained via the reaction of electrophilic N-alkenoxypyridinium
salts and nucleophiles-alcohols or thioalcohols (Figure 1, b).
Herein, we report our preliminary results to achieve enolate
umpolung conveniently and broaden its utility from
intramolecular to intermolecular reaction for the first time which
can supply an efficient method for the synthesis of variously
multifunctionalized α-oxygenated ketones, α-thio ketones and α-
thio thioketals by two or three steps in one pot.
[a]
[b]
Prof. Dr. Z. Xu, R.-L. Zhai, T. Liang
Jiangsu Key Laboratory of New Drug Research and Clinical
Pharmacy
Xuzhou Medical University
Tongshan Road 209, Xuzhou, 221004 (P.R. China)
E-mail: xuzhou@xzhmu.edu.cn
Prof. Dr. L.-M. Zhang
Department of Chemistry & Biochemistry
University of California, Santa Barbara
California, 93106 (United States)
At the outset of our investigation, 1-undecyne 1 was chosen as
the model substrate to examine the validity of our concept and
Supporting information for this article is given via a link at the end of
the document.
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i
the reaction parameters such as gold catalysts, acids, solvents
and temperatures were screened. Considering the second step
reaction is a SN2 type, increasing the concentration of the
nucleophile might be helpful for the reaction. Indeed, when
ethanol was used not only as nucleophile but also as the
cosolvent, the reaction could proceed smoothly. As such, after
performing a series of experiments to optimize the reaction
conditions (see the Supporting Information for details), we found
that the best conditions for this two steps one-pot process was
using 2.5% PPh3AuNTf2 in hexafluoroisopropanol (0.2 ml, HFIP)
at room temperature and required heating at 90 ˚C for 5 h with
EtOH as the cosolvent (VHFIP/VEtOH = 0.2 ml/0.8 ml) for the
second step with an over-all yield of 73% (Scheme 1).
sterically hindered PrOH and cyclohexanol, the reactions finish
in 12 h with acceptable overall yield of 45% and 41%,
respectively (8-9). To our delight, unsaturated alcohol could also
work for this transformation. The double bond in the alcohol is
compatible with the remained gold catalyst (10). Generally,
when thiols were used as the nucleophiles, they gave better
yields of addition products than alcohols. Similar with alcohols,
the sterically bulk of the thiols affect the activity of the reaction
and the yield of the corresponding α-thioketones (11-14).
Notably, the reaction temperature for the synthesis of α-
thioketones is much lower than that of α-alkoxyketones. These
examples illustrate that various alcohols or thiols can react
smoothly with 4 to afford α-alkoxy or α-thio ketones with
moderate to good yields in a one-pot manner.
We subsequently turned to explore the scope of the alkynes with
ethanol as the nucleophile. Pleasingly, as revealed in Table 2,
we found a broad range of alkynes possessing various
Table 2. Scope of the intermolecular umpolung reaction using ethanol and
alkynes. [a][b]
Scheme 1. Model reaction employed for reaction conditions optimization.
With the optimized conditions in hand, the chemistry was
extended to the synthesis of others α-alkoxyketones and the
activities of different nucleophiles were evaluated (Table 1). The
straight chain alcohols, such as methanol and n-pentanol, lead
to similar results to that of ethanol (6-7). However, it is
apparently that the size of the nucleophile affects the activity of
the reaction. Methanol is the smallest alcohol and most reactive
and its reaction proceeds smoothly at 60oC, while the reaction
temperature for other alcohols is 90oC. Nevertheless, for the
Table 1. Synthesis of varies α-alkoxyketones and α-thio ketones by different
nucleophiles.[a][b]
[a] The reaction conditions: 1) 1 (0.2 mmol), Pyridine N-oxide 2 (1.2 equiv), 3
(1.1 equiv), PPh3AuNTf2 (2.5 mol%), HFIP (0.2 ml), rt; 2) Alcohols (0.8 ml),
60-90°C or thiol (10 equiv.), 50oC or HFIP/thiols = 0.2 ml/0.8 ml, 40oC. [b]
The figures in the brackets are the over-all isolated yields for two steps.
[a] The reactions were carried in sealed vials in one pot. 1) alkyne 1 (0.2
mmol), 2 (1.2 equiv), 3 (1.1 equiv), PPh3AuNTf2 (2.5 mol%), HFIP (0.2 ml),
rt; 2) EtOH (0.8 ml), 90°C. [b] The numbers in the brackets are the over-all
isolated yields for two steps.
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functional groups readily participate in this two-steps one-pot
reaction with moderate to excellent yields. With 5-chloropent-1-
yne as the substrate, 15 was obtained with 52% isolated yield.
Under this acidic reaction conditions, ethanol does not displace
the terminal chloride, demonstrating substantially higher
electrophilicity of the N-alkenoxypyridinium moiety. Other
functional groups, such as amide, hydroxyl, ester and sulfonyl,
are compatible with the transformation and the corresponding
products were obtained with good yields (16-21). When N-(4-
pentynyl)phthalimide was used, the N-alkenoxypyridinium salt
could be formed with 98% NMR yield, and then reacted with
ethanol to give the valuable α-ethanol-β’-amide ketone 16 in
85% yield. In the case of 1,9-decadiyne, interesting dicarbonyl
compound 22 could be obtained. For substrates with shorter or
cyclic alkyl groups, the reactions proceeded cleanly to afford 23
and 24 in good yields. For the alkynes containing aromatic rings,
although we reported in our previous study[10] that the
corresponding N-alkenoxypyridinium intermediates would
undergo the intramolecular Friedel-Crafts reaction in the
absence of additional nucleophiles. The ethanol attacks
compete well to afford α-ethanol ketones, which are
contaminated only with a small amount of the cyclization side
products. For examples, both 4-phenyl-1-butyne and 5-phenyl-1-
pentyne underwent the two steps reactions in one-pot smoothly
with yield of 60% and 64%, respectively (25 and 27).
Considering the convenience of the operation, we continued
carrying out the reaction using HFIP and EtOH as the cosolvent
though sacrificing approximately 5% yield. Aryl ether alkyne
substrates also afford fairly good yield of the α-ethanol ketones
and aryl chloride is tolerance (29-31).
different active sites under the reaction conditions. By using
large excess of thiols or under higher reaction temperature
(Condition C), the one-pot process permitted the preparation of
α-thio thioketals 44-52 in high yields. Generally, if the α-position
or β-position of the triple bond in the alkyne substrates located
with electron-withdrawing atom, such as O or S, the α-thio
thioketals could be formed much easier even at lower
Table 3. Scope of the intermolecular umpolung reaction using thiols and
alkynes. [a][b]
Unlike the alcohols, when thiols were used as nucleophiles,
some new interesting phenomena were observed. If the reaction
temperature enhanced or the amount of the thiols increased, the
products are not the α-thioketones, but α-thio thioketals which
indicated that the reaction is tunable by adjusting the reaction
conditions (Table 3, 32-36 vs 44-48). Interestingly, in the case of
4-pentyn-2-ol as the substrate, although N-alkenoxypyridinium
salt could be formed with 96% yield in 5 h, nucleophilic attack at
the electrophilic α-carbon by the tethered hydroxyl group
outcompeted the intermolecular umploung reaction to happen
and thus the following intermolecular umploung reaction couldn’t
proceed when ethanol was used as the nucleophile at 90 oC.
Much to our delight, when n-propylmercaptan (10 equiv.) was
used as the nucleophile, the intermolecular umploung reaction
could proceed smoothly and the α-thio ketone 37 could be
achieved with 54% yield at 50 oC. For product 38, it needs to
point out that, although the corresponding N-alkenoxypyridinium
salt could be formed smoothly with excellent yield; however, it
underwent elimination to release the corresponding phenol due
to the electron-deficient nature of the benzene ring at the
temperature (i.e., 90oC) required for the reaction with ethanol.
Fortunately, isopropyl mercaptan could make the reaction
happen at 35oC and α-thio ketone 38 was achieved with 66%
yield. Other terminal alkynes were also tested and the results
indicated that all of the reactions were efficient, affording
compounds 39-41 in good yields. With 5-chloropent-1-yne as the
substrate, although N-alkenoxypyridinium salt could be formed
with 95% yield, the following umpolung reaction with active
nucleophiles, such as ethyl mercaptan or n-propylmercaptan,
was rather complexed due to likely the interference of the C-Cl
bond. When steric hindered isopropyl mercaptan was used as
the nucleophile, compound 42 was formed successfully,
indicating that the isopropyl mercaptan could differentiate the
[a] The reaction conditions: 1) alkyne 1 (0.2 mmol), 2 (1.2 equiv), 3 (1.1 equiv),
PPh3AuNTf2 (2.5 mol%), HFIP (0.2 ml), rt; 2) Condition A (32-33, 36, 38 and
41), Condition B (entries 34-35, 37, 39-40 and 42-43) or C (44-52); [b] The
numbers in the brackets are the over-all isolated yields for two steps.
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temperature (49-50). Otherwise, higher reaction temperature
and/or steric smaller thiols required in order to form α-thio
thioketals. With but-3-yn-1-yl(2-chlorophenyl)sulfane as the
substrate, although the corresponding N-alkenoxypyridinium salt
was formed in 99% NMR yield, the umpolung reaction couldn’t
proceed when ethanol was used as the nucleophile under the
typical reaction condition as described in Table 2, but hydrolysis
of the N-alkenoxypyridinium salt underwent smoothly giving the
corresponding methyl ketone as the major product. Instead of
ethanol, n-propylmercaptan could make the umpolung reaction
proceed successfully and α-thio thioketal 49 was isolated with
good yield and no methyl ketone was observed. In addition, the
size of the thiols also affects the activity of the reaction. Ethyl
mercaptan gave much better yields of the α-thio thioketals than
that of propyl mercaptan (44 vs 52, 48 vs 51).
Scheme 3. Chemical transformations. (a) Ester bond cleavage of 15. (b)
Transformation of 15 to cyclopropane compound 56. (c) Reduction of 5 to 57.
In summary, we established the first construction of unique α-
oxygenated ketones, α-thio ketones and α-thio thioketals using
alkynes as the starting material via intermolecular umploung
reaction in one pot manner. This method broadens the gold (I)-
enabled umpolung reactivity from intramolecular scenarios to the
challenging intermolecular mode. Our studies also indicate that
the method for the synthesis of α-thio ketones or α-thio
thioketals is controllable and tunable by simply adjusting the
reaction conditions. The mild and operationally procedure makes
the method much practical useful. Importantly, applications of
the products, α-oxygenated ketones, to the synthesis of other
valuable synthetic moieties have also been successfully
achieved. Further development and application of this protocol is
underway in our laboratory.
In order to understand the formation of α-thio thioketals, some
control experiments were carried out as depicted in Scheme 2.
The isolated N-alkenoxypyridinium salt 4 could be converted to
1
32 with a small amount of 44 (6% H NMR yield) as the minor
product under condition A, indicating that gold catalyst didn’t
affect the thioketal formation step (Scheme 2, a). With pyridine
as the additive, the formation of 32 was hindered, and the
hydrolysis of the N-alkenoxypyridinium salt increased
significantly, giving the corresponding methyl ketone 53 as the
byproduct (Scheme 2, b). The result demonstrated that it was
the acid which promoted the formation of α-thio thioketals. In
addition, the thioketal 44 could be formed from 32 under
conditions C (Scheme 2, c). Interestingly, with 2-methylbut-1-en-
3-yne 54 as the starting material, 43 could be obtained in 64%
isolated yield in one pot manner (Scheme 2, d).
Acknowledgements
The authors thank the Natural Science Foundation of China
(Grant No. 21602191) and the Natural Science Foundation of
the Jiangsu Higher Education Institutions of China (Grant No.
15KJB150029). The work is sponsored by Qing Lan Project of
Jiangsu Province. Dr. Xu and Mr. Zhai contributed equally to this
work.
Keywords: alkynes • umpolung • α-alkoxyketones • α-thio
ketones • α-thio thioketals
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COMMUNICATION
A novel and broadly applicable
Xu Zhou,* Zhai Rongliang, Liang Ting,
Zhang Liming
method for the synthesis of unique α-
oxygenated ketones, α-thio ketones
and α-thio thioketals via umpolung
reaction of nucleophiles-alcohols/
thioalcohols and N-alkenoxypyridinium
salts generated from alkynes in one-
pot manner was reported for the first
time. The reaction is controllable and
tunable with good substrates scope
(up to 46 examples) and excellent
functional groups tolerance.
Page No. – Page No.
Efficient One-pot
Multifunctionalization of Alkynes en
Route to α- Alkoxyketones, α-
Thioketones and α-Thio Thioketals
via an Umpolung Strategy
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