3-Cyanoallyl boronates are versatile building blocks in the synthesis of polysubstituted thiophenes

Article abstract of DOI:10.1039/c7sc00831g

We report the preparation of hitherto unprecedented 3-cyanoallyl boronates using condensation of the parent α-boryl aldehyde and nitriles. The resulting allyl boronates have been used to generate a wide range of borylated thiophenes, which represent a valuable class of heterocycles in modern drug discovery. Subsequent Suzuki-Miyaura cross-coupling enabled the synthesis of pharmaceutically important 3,5-disubstituted aminothiophenes. Moreover, late stage functionalization gave access to borylated bromothiophene and thieno[2,3-b]pyridines.

Full text of DOI:10.1039/c7sc00831g

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DOI: 10.1039/C7SC00831G
The Knoevenagel condensation between α-MIDA boryl acetaldehyde and active methylene
compounds enables synthesis of novel electron-poor allylboronates and facilitates access to
synthetically challenging thiophene derivatives.

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DOI: 10.1039/C7SC00831G
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3-Cyanoallyl Boronates Are Versatile Building Blocks in the
Synthesis of Polysubstituted Thiophenes
Wenjie Shao, Sherif J. Kaldas and Andrei K. Yudin*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
We report the preparation of hitherto unprecedented 3-cyanoallyl annulation from allylboronates has not received attention in
boronates using condensation of the parent α-boryl aldehyde and synthesis.
nitriles. The resulting allyl boronates have been used to generate a
wide range of borylated thiophenes, which represent a valuable
class of heterocycles in modern drug discovery. Subsequent
Suzuki-Miyaura cross-coupling enabled the synthesis of
pharmaceutically important 3,5-disubstituted aminothiophenes.
Moreover, late stage functionalization gave access to borylated
bromothiophene and thieno[2,3-b]pyridines.
The
boron
motif
found
in
Scheme 1. Contrasting typical reactivity of allyl boronates in allylation reactions with
synthesis of heterocycles
Allylboronates1 are among the most widely used building
blocks in organic synthesis and are commonly employed in
drug discovery. Despite the widespread application of
allylboron reagents in chemical synthesis, their use has been
largely limited to the corresponding allylation reactions, which
typically utilize electron-rich allylboranes or boronates. In
contrast, the preparation and application of electron-poor
allylboronates has not received enough attention in organic
synthesis to date. Most of the electron-withdrawing groups
have been limited to the halogens2 or their position was
restricted at C-2.3 We have come across a single example4
wherein a C-3 amide-containing allylboronate was isolated as a
byproduct in 35% yield. Attempts were made to access the C-3
ester-containing electron-poor allylboronates, but both5 failed
to give desired products. We wondered whether the C-3
substituted electron-poor allylboronates could be generally
obtained from amphoteric α-(MIDA)boryl aldehydes,6 as
MIDA-protected boron species showed enhanced stability7
that allow quick and facile access to previously inaccessible
boron compounds.6f If such allylboronates were accessible, the
four-carbon unit arising from these allylboronate species could
serve as the foundation in the synthesis of heterocycles
(Scheme 1). To the best of our knowledge, heterocycle
the borylated heterocycle derivatives could enable late-stage
cross-coupling with suitable partners or provide a handle for
site-selective appendage of a boron-containing heterocycle
using recently described protocols.6d
Results and discussion
We commenced our study with the preparation of
substituted 3-cyanoallyl boronates by reacting the α-boryl
aldehyde6c with nitrile derivatives. Malononitrile (3a) and ethyl
cyanoacetate
(3b
)
were first examined. We have
a.Davenport Research Laboratories, Department of Chemistry, University of
Toronto, 80 St. George St., Toronto, ON M5S 3H6, Canada. E-mail address:
ayudin@chem.utoronto.ca
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
Table 1. Synthesis of substituted 3-cyanoallyl lboronates
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determined that the highest yields were obtained when the restricted by the presence of directing groups and functional
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DOI: 10.1039/C7SC00831G
Knoevenagel condensation was carried out in acetonitrile with group tolerance. Borylation of aminothiophenes, on the other
diethylamine as the base (Table 1). Other organic bases, such hand, is an appealing alternative to these methods that has
as imidazole, triethylamine, morphiline, and piperidine gave
low yields or no products. Under the optimized reaction
conditions, the 3-cyanoallyl boronates were isolated in
excellent yields (91% for 3a and 93% for 3b). Benzyl amide
group was also tested and the reaction was clean, giving
compound 3c in 84% yield after isolation. Cyanoacetamide was
well tolerated (3d), showing that the presence of the -NH2
group does not negatively affect the reaction outcome. For
compounds 3b, 3c and 3d, only one isomer was obtained from
the reaction mixture and the geometry of the double bond
was determined by NOESY experiments. However, the
condensation reaction with cyanoacetic acid (R = COOH)
resulted in complex mixture. The resulting allylboronates,
bearing two electron-withdrawing groups at the terminus,
have not been reported in the literature and would be difficult
to access by other methods. With the successful preparation of
these 3-cyanoallyl boronates, we pursued their application in
the synthesis of polysubstituted borylated thiophenes.
Polysubstituted thiophenes received attention as valuable
building blocks in organic synthesis.8 They have been widely
used in the pharmaceutical industry,9 dye chemistry,10 and as
functional materials.11 In modern drug discovery,
polysubstituted thiophenes are important because they
constitute a bioisosteric replacement12 for the phenyl ring.
Thiophene-containing derivatives are often characterized by
reduced toxicity and better pharmacokinetic properties.13
Substituted 2-aminothiophenes are one of the most important
thiophene categories14 as they are common structures in the
FDA approved drugs (Figure 1).
Figure 1. Examples of 2-aminothiophene-containing pharmaceutical drugs
Depending on the functional groups tolerance and the
stability of the products, borylated thiophenes are commonly
synthesized through halogen-metal exchange15 on the
preformed halothiophenes. Recent C-H activation16 with
iridium catalysts showed considerable advantages, but is still
Table 2. Synthesis of borylated thiophenes
received limited attention. This can partially be attributed to
the protodeborylation reaction of the resulting products16a,17
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as well as to the detrimental effect of the amino group. To the
best of our knowledge, the reported examples only describe
tertiary amine or amide-containing thiophenes.18 Therefore, a
general method for making borylated aminothiophenes is
highly desirable.
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DOI: 10.1039/C7SC00831G
Our 3-cyanoallyl boron species could be readily converted
to borylated thiophenes in the presence of elemental sulfur
(the Gewald reaction). When treated with sulfur and Et2NH in
THF, compounds 3a and 3b could be smoothly transformed to
borylated thiophenes. We have also developed a more
convenient one-pot process (Table 2). The reaction was best
done at a relatively low concentration, no more than 0.05M, to
suppress the dimerization of the 3-cyanoallyl boronates
intermediates.19 A variety of nitrile compounds were tested
and the reactions were generally good with isolated yields
varying from 33% to 91%. The electron density of the aromatic
ring has a small effect on the reaction, as electron-rich phenyl
Table 3. Suzuki-Miyaura-cross-coupling of amonothiophenes
It is known that 2-heteroaryl boronates are unfavorable
substrates for Suzuki-Miyaura coupling due to the tendency of
protodeborylation16a, and the cross-coupling of borylated
aminothiophenes could be even more challenging because of
the interfering adjacent -NH2 group. To achieve reasonable
yields of the cross-coupling reaction, an extensive screen of
reaction conditions was carried out. For most of the screened
(4l) gave lower yield (80%) while electron-poor (4e, 4f) phenyl
examples gave better yields (91% and 89%). Heteroaryl nitriles
such as thiophene (4g), furane (4i), pyrrole (4k) were also
suitable and some susceptible groups were well tolerated (4d
,
4h). It is noteworthy that all the products (4a to 4l) are new
and many of them cannot be easily made using alternative
methods. Larger scale synthesis was also desirable. With that
in mind, compound 4c was obtained in 87% on a 0.5g scale.
The reaction was fast and TLC showed the complete
consumption of the α-boryl aldehyde component in 3 hours.
However, the sulfonyl example (4j) turned out to be more
difficult to synthesize, requiring longer reaction time (24 h)
and excess amount of (phenylsulfonyl)acetonitrile. The low
yield of 4j was attributed to the dimerization of the
allylboronate intermediate. Most of the borylated
aminothiophenes are partially water-soluble, thus aqueous
work-up should be avoided to achieve highest yields.
Additionally, we were pleased that no C-B bond scission was
observed under the standard reaction conditions and the
products are stable under ambient conditions.
We sought to demonstrate our borylated aminothiophenes
as intermediates to access other synthetically challenging
borylated thiophene derivatives. A recent study showed that
borylated bromothiophenes are promising monomers in
material science due to their ability to polymerize under
suitable Suzuki-Miyaura conditions.20 Our method offers a
convenient route to previously inaccessible bromothiophene
derivatives. Compound 4d was reacted with t-BuONO and
CuBr2 in acetonitrile to generate borylated bromothiophenes
5a (Scheme 2), which was difficult to synthesize by alternative
methods21, in 63% yield.
Scheme 2. Synthesis of borylated bromothiophenes
Table 4. Synthesis of borylated thieno[2,3-b]pyridines
conditions, mainly protodeborylation product was observed.
Fortunately, it was found that with 0.1 equiv RuPhos Pd G3 as
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Acids: Preparation, Applications in Organic Synthesis and
the catalyst and 3.0 euiqv Na2CO3 as the base, the desired
products 6a and 6b were obtained in 78% and 71% yields
(Table 3). Though 3,5-disubstituted aminothiophenes can be
generally prepared by Gewald reaction from suitable
substituted acetaldehydes, our Suzuki approach gave access to
3,5-disubstituted variants, for which the corresponding
View Article Online
Medicine, Wiley-VCH, Weinheim, 2005D; O(eI:)1M0.1.0Y3u9s/,CJ7.SCC.00831G
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Our borylated thiophenes also proved to be useful building
blocks in the synthesis of borylated bisheterocycle. Recent
studies23 showed that substituted thieno[2,3-b]pyridine is an
important motif in the medicinal chemistry. Borylation of
thieno[2,3-b]pyridines has been underexplored and to the best
of our knowledge, there are no previous publications covering
this topic. We wondered if the borylated thieno[2,3-
b]pyridines could be made by condensing borylated
aminothiophene with ketones. We initiated our study by
treating 4c with cyclohexanone, using trimethylclhorosilane24
as the Lewis acid (Table 4). The cyclization reaction was
finished in 1 hour and 8a was isolated in excellent yield (95%).
Several ketones were examined and it was found that the
reaction was influenced by the steric effect. Hindered ketones
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(
8c
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7b) gave excellent
are relatively lower while cyclic ketones (7a
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yields, regardless of the ring size. No protodeborylation was
observed during the reaction progress for all borylated starting
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In summary, we have successfully developed a series of stable,
previously inaccessible 3-cyanoallyl boronates. These
compounds have allowed us to generate a series of borylated
thiophenes in good to excellent yields. The utility of the
resulting thiophene products as key intermediates toward
synthetically challenging borylated bromothiophene and
thieno[2,3-b]pyridines has been demonstrated. The successful
cross-coupling of borylated aminothiophenes gave access to
3,5-disubstituted aminothiophenes, which are of interest in
medicinal chemistry. Further applications of electron-poor
allylboronates in synthesis are now enabled and are under
intense investigation in our laboratory.
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Acknowledgements
We thank the Natural Science and Engineering Research
Council (NSERC) for funding. S.J.K. thanks Ontario Graduate
Scholarship for funding. Dr. Milan Bergeron-Brlek is also
thanked for discussion.
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