Recycle waste salt as reagent: A one-pot substitution/Krapcho reaction sequence to -fluorinated esters and sulfones

Article abstract of DOI:10.1021/acs.orglett.5b00072

A one-pot tandem substitution/Krapcho reaction is reported for the facile synthesis of α-fluorinated esters and sulfones, which utilizes the byproduct salt formed in the substitution step as an indispensible reagent to facilitate the Krapcho reaction step. This represents the first sustainable tandem reaction that internally recycles the waste salt formed in the upstream step as the reagent for the downstream step.

Full text of DOI:10.1021/acs.orglett.5b00072

Letter
pubs.acs.org/OrgLett
Recycle Waste Salt as Reagent: A One-Pot Substitution/Krapcho
Reaction Sequence to α‑Fluorinated Esters and Sulfones
Feng Zhu,† Peng-Wei Xu,† Feng Zhou,† Cui-Hong Wang,*,† and Jian Zhou*,†,‡
†Shanghai Key Laboratory of Green Chemistry and Chemical Process, Department of Chemistry, East China Normal University,
Shanghai 200062, P. R. China
‡State Key Laboratory of Elemento-organic Chemistry, Nankai University, Tianjin 300071, P. R. China
S
* Supporting Information
ABSTRACT: A “one-pot” tandem substitution/Krapcho
reaction is reported for the facile synthesis of α-fluorinated
esters and sulfones, which utilizes the byproduct salt formed in
the substitution step as an indispensible reagent to facilitate the
Krapcho reaction step. This represents the first sustainable
tandem reaction that internally recycles the waste salt formed
in the upstream step as the reagent for the downstream step.
ecause of the source-intensive nature of organic synthesis, it
reduction sequence which recycled Ph3PO to activate HSiCl3
Bis very important to combine multistep syntheses into one- for the conjugate reduction of enones, along with a Wittig−
asymmetric cyanosilylation reaction which reused Ph3PO as a
necessary Lewis base catalyst for the cyanosilyaltion of enones.6a
The tandem Wittig−conjugate reduction sequence further found
its use in the chemoselective synthesis of α-CF3 γ-keto esters
from the corresponding β-CF3 substituted enones, the selective
conjugate reduction of which proved to be difficult by other
methods.6b Based on these results, we further tried integrating
salt-generating reactions such as substitution and coupling
reactions into tandem sequences to internally reuse byproduct
metal halides. Herein, we wish to report an unprecedented one-
pot tandem substitution−Krapcho reaction that efficiently reuses
the in situ generated metal halide for the dealkoxycarbonylative
synthesis of α-fluorinated esters 6.
The α-fluorinated esters 3 are a type of versatile building block
to various fluorinated compounds.7,8 In 1990, Burton reported a
general method involving an alkylation of phosphorane 2 with
primary alkyl iodides or activated alkyl bromides and the
following hydrolysis (eq 1, Scheme 1),9a which allowed the
synthesis of α-fluorinated esters bearing an alkyl, allyl or benzyl
type substituent at α position in up to 59% yield by a one-pot
operation. There are still other methods developed,9 including
nucleophilic fluorination of α-hydroxy ester derivatives9b and
electrophilic fluorination of trimethylsilyl ketene acetals9c,d or
other ester derivatives,9e,f but the scope of these methods was not
as broad as Burton’s protocol. Therefore, new general and
efficient methods to differently substituted α-fluorinated esters,
and in particular with α-allyl or α-propargyl groups, from easily
available materials in a simple operation, are still highly desirable.
Considering readily available fluoromalonate 4 is highly
reactive toward alkylation,10 along with the fact that the Krapcho
reaction requires using at least 1 equiv of a salt,11 we designed a
pot tandem reactions to reduce waste generation, save time,
labor, energy, and materials, and minimize yield losses associated
with the purification of intermediates.1 In this context, there is an
ever-increasing interest in developing novel tandem reactions
that internally recycle waste produced in the upstream step to
facilitate the downstream steps.2 In 2003, Shibasaki reported the
first example of such a type of tandem reaction, which
ingeniously employed the byproduct Ph3PO from the Wittig
step as an effective additive to improve the reactivity and
enantioselectivity of the downstream asymmetric epoxidation
step.3a Since then, the independent works by Baba,3b Alaimo,3c
Tian,3d,e Wu,3f Toy,3g,h and Coeffard and Greck3i have nicely
demonstrated the possibility of internally recycling byproducts,
and even excess reagents, from the upstream step of a tandem
sequence to serve as a catalyst or a promoter for the downstream
step, given careful design. As the byproduct of many reactions is
essentially a kind of acid or base capable of facilitating certain
reactions, there is ample room for the further development of
such sustainable tandem reactions to effectively improve the
atom-utilization of multistep syntheses because the internal reuse
of byproduct avoids the use of extra substance, which in turn
contributes to the reduction in waste generation.4 Nevertheless,
the corresponding research is still at its infancy, and successful
examples are very limited.2,3 Therefore, it is very important to
design novel tandem sequences to discover new pathways that
internally recycle various wastes and to provide an efficient and
cost-effective synthesis of value-added products from simple
materials by easy manipulation.
We have been engaged in developing tandem reactions for the
synthesis of compounds that are interesting for medicinal
research.5 In particular, we are interested in coupling a reaction
with low atom-efficiency into tandem reactions to internally
utilize its byproduct to benefit the following reaction.6 In this
context, we have developed a tandem Wittig−conjugate
Received: January 8, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b00072
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Letter
optimized the condition for the desired sequence, as shown in
Table 2. The initial alkylation of bromide 1a and fluoromalonate
Scheme 1. Working Model
Table 2. Optimization for Tandem Reaction
entry
base
solvent
isolated yield of 6a (%)
1
2
3
4
5
6
7
8
9
Li2CO3
LiOH
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMF
DMSO
37
49
71
66
74
78
80
75
80
LiOtBu
Na2CO3
NaOH
NaOtBu
NaH
one-pot tandem substitution/Krapcho reaction for the modular
syntheses of α-fluorinated esters from 4 and halides 1 (eq 2,
Scheme 1), with our efforts in selective fluoroalkylation.12 This
sequence allows the reuse of byproduct salt to facilitate the
following Krapcho reaction and offers the premise to synthesize
α-fluorinated esters with various functional groups, owing to the
almost neutral condition of Krapcho reaction. To examine this
hypothesis, we first tried different salts in the dealkoxycarbony-
lation reaction of malonate 5a, using N,N-dimethylacetylamide
(DMA) as the solvent. In the absence of any salt, no reaction
happened even at 120 °C (entry 1, Table 1), which confirmed the
NaH
NaH
4a was run at 25 °C, using 1.2 equiv of base (equal amount to 4a).
The bromide 1a was used in less amount than 4a to avoid
possible side reactions caused by excess halide at high
temperature in the Krapcho step. Until the alkylation finished,
the mixture was heated to 80 °C for the Krapcho reaction. It
proved that base strongly affected the outcome of the sequence.
While both LiCl and LiBr were efficient for the Krapcho reaction,
all reactions mediated by three lithium bases gave product 6a in
lowered yield than the corresponding sodium bases (entries 1−3
vs 4−6). Finally, NaH (60% in mineral) proved to be the best,
giving product 6a in 80% yield (entry 7). It also revealed that
both DMF and dimethyl sulfoxide (DMSO) were suitable for
this sequence (entries 8 and 9) and almost no reaction took place
in toluene or 1,4-dioxane.
On the basis of the above optimization, the scope of this
tandem sequence was examined by running the reaction on a 0.5
mmol scale. The initial alkylation step was run at 25 or 50 °C for
2.5 h until full conversion of bromides 1, and then the mixture
was heated to indicated temperature until the completion of the
following Krapcho reaction. By such a simple and operationally
friendly procedure, a variety of α-fluoroesters 6a−i with various
α-substituents, benzyl type or simple aliphatic ones, could be
obtained in good yield (entries 1−9, Table 3). In addition,
differently substituted allyl or propargyl groups could be
introduced, giving products 6k−v in good yield. Noticeably,
the synthesis of α-propargyl α-fluoroester was unprecedented.
The broad substrate scope and high efficiency achieved by this
simple one-pot sequence was impressive, as it enabled modular
synthesis of α-fluoroesters from readily available materials
without special care except for raising the temperature at the
end of the alkylation step while reusing waste NaBr to facilitate
the Krapcho step. The practicability of our protocol was further
demonstrated by a gram-scale synthesis of product 6k. By this
sequence, a 7.5 mmol experiment readily gave the desired
product 6k in 72% yield (1.12 g), showing the good scalability of
our protocol.
Table 1. Screening of Different Salts
a
entry
salt
temp (°C)
time (h)
conv (%)
1
2
3
4
5
6
7
8
9
10
120
100
100
100
100
100
100
100
100
80
24
4
nr
100
100
82
LiCl
LiBr
NaCl
NaBr
NaI
6
24
8
100
100
78
10
24
24
13
24
KCl
KBr
KI
73
100
100
NaBr
a
1
Determined by H NMR analysis of the crude mixture.
prerequisite role of a salt in the Krapcho reaction of 5a. On the
other hand, all reactions proceeded to some extent at 100 °C if
adding any of the lithium, sodium and potassium halides we tried
(entries 2−9). Finally, LiCl, LiBr, and NaBr proved to be better
than NaCl and potassium halides, allowing the reaction to
complete within hours. This result showed it is possible to use
chlorides or bromides to develop the reaction. NaI and KI also
worked well (entries 6 and 9), suggesting that, on demand, the
more reactive but expensive organic iodides could be used in the
alkylation step. It also turned out that in the presence of 1.0 equiv
of NaBr the reaction worked well at 80 °C, albeit with a longer
reaction time (entry 10).
While several salts could promote the Krapcho step, the
development of the designed sequence was not as trivial as it first
appeared. In principle, the efficiency of the whole sequence
depended on a careful balance of the alkylation and downstream
dealkoxycarbonylation, as the base used in the initial alkylation
must give a suitable salt for the Krapcho step. Accordingly, we
Importantly, multifunctional chloride 1w also worked well to
give α-fluoroester 6w in 60% yield. This result showed chlorides
were viable substrates for this tandem sequence, although the
initial alkylation required higher temperature. It should be noted
that it was difficult to prepare product 6w via electrophilic
B
DOI: 10.1021/acs.orglett.5b00072
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Letter
α-fluoroketone 11 and α-fluoroester 6j in almost 1:1 ratio by
NMR analysis of the crude mixture, which could be separated by
column chromatography.
Table 3. Substrate Scope
a
entry
R
1
4
6
yield (%)
The use of 2-fluoro-2-(phenylsulfonyl)acetate 12 allowed the
synthesis of α-fluorinated sulfones 13 in acceptable yield. As
compound 12 could be readily prepared from phenylsulfonyla-
cetate,15 our protocol provided a new method for the efficient
synthesis of α-fluorinated sulfones 13, versatile building blocks
for the selective fluoroalkylation.16
b
1
2-naphthyl
1-naphthyl
4-ClC6H4
4-BrC6H4
9-anthracyl
2-thienyl
Bn
1a
1b
1c
1d
1e
1f
4a
4a
4a
4a
4a
4a
4a
4a
4a
4b
6a
6b
6c
6d
6e
6f
80
65
78
85
86
79
85
77
76
67
b
2
b
3
b
4
b
5
b
6
d
7
1g
1h
1i
6g
6h
6i
d
8
BnCH2
nC8H17
d
9
10
2-naphthyl
1a
6j
a
b
c
d
Isolated yield. t2 = 80 °C. t2 = 70 °C. t1 = 50 °C.
In conclusion, we have developed a novel one-pot tandem
substitution/Krapcho reaction sequence, which nicely demon-
strated the possibility of internally recycling the in situ generated
waste salt to facilitate the downstream step in a tandem reaction,
given careful design. The thus-developed protocol constitutes a
new efficient modular method for the synthesis of α-florinated
esters and sulfones, versatile building blocks for the selective
fluoroalkylation. The development of new one-pot tandem
sequences that take advantage of the internally generated
byproduct to facilitate the downstream steps is now ongoing in
our laboratory.
fluorination of ester derivatives due to the presence of the ketone
moiety, which constituted an obvious advantage of our protocol.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures and characterization data. This material
is available free of charge via the Internet at http://pubs.acs.org.
To our delight, the SNAr reaction of nitrobenzene 7 and
fluoromalonate 4a could also be coupled into a one-pot tandem
synthesis of α-fluoroester 9 in 58% yield. Recently, Sanford et al.
reported the synthesis of product 9 via a stepwise synthesis
involving SNAr reaction, KOH-mediated decarboxylation, and an
acid esterification.13 Our tandem sequence showed improved
efficiency by avoiding an extra step, two extractions, and the use
of additional materials. However, NaF was not efficient enough
to promote the Krapcho step,14 and the scope of the tandem
SNAr/Krapcho sequence is still under investigation.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: chwang@chem.ecnu.edu.cn.
*E-mail: jzhou@chem.ecnu.edu.cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The financial support from the 973 program (2011CB808600),
NSFC (21222204, 21472049), and Ministry of Education
(NCET-11-0147and PCSIRT) is appreciated.
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C
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