Synthesis of Diverse Boron-Handled N-Heterocycles via Radical Borylative Cyclization of N-Allylcyanamides

Article abstract of DOI:10.1021/acs.orglett.8b03303

A synthetic method based on radical borylation/cyclization cascades of N-allylcyanamides was developed to construct diverse boron-substituted N-heterocycles. In the reaction process, the N-heterocyclic carbene-boryl radical underwent a chemo- and regiosel

Full text of DOI:10.1021/acs.orglett.8b03303

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Synthesis of Diverse Boron-Handled N‑Heterocycles via Radical
Borylative Cyclization of N‑Allylcyanamides
Ji-Kang Jin,† Feng-Lian Zhang,† Qiang Zhao,† Jun-An Lu,† and Yi-Feng Wang*,†,‡
†Hefei National Laboratory for Physical Sciences at the Microscale, Center for Excellence in Molecular Synthesis of CAS, and
Department of Chemistry, University of Science and Technology of China, Hefei 230026, China
‡State Key Laboratory of Elemento-Organic Chemistry, Nankai University, Tianjin 300071, China
S
* Supporting Information
ABSTRACT: A synthetic method based on radical borylation/cyclization cascades of N-allylcyanamides was developed to
construct diverse boron-substituted N-heterocycles. In the reaction process, the N-heterocyclic carbene−boryl radical
underwent a chemo- and regioselective addition to the alkene moiety, followed by cyclization with the N-cyano group. The
resulting amide−iminyl radical intermediates underwent further reactions to afford various boron-tethered N-heterocyclic
molecules. Further transformations to access synthetically useful building blocks were also demonstrated.
-Heterocycles are prevalent in numerous natural products,
pharmaceuticals, agricultural chemicals, and functional
Radical cascade reactions are a powerful tool to construct N-
heterocyclic skeletons.5 Although many radical cascades and
methods have been developed, the synthesis of diversified
boron-handled N-heterocycles from readily available starting
materials is rare. Recently, our group has reported some
intriguing borylative radical cascades of 1,6-enynes and 1,6-
dienes to construct boron-substituted cyclic molecules.6 In these
studies, we have investigated the reactions of N-heterocyclic
carbene (NHC)−boryl radicals with alkenes and alkynes.
Despite this, the reactivity and selectivity of NHC−boryl
radicals with a complex system containing multiple different
radical acceptors still remain elusive. Such research would be of
great value to enrich boryl radical chemistry. More importantly,
new findings may lead to the discovery of unprecedented
synthetic approaches that may afford structurally novel and
useful boron-substituted frameworks. With this in mind, we
became interested in studying radical borylative cascades of N-
allylcyanamides, from which diverse valuable boron-substituted
N-heterocycles would be constructed. The challenge that needs
to be addressed is how to achieve a chemoselective initial radical
addition reaction between the alkene and the N−CN motifs
because both functional groups may react with the NHC−boryl
radical.
N
materials.1 Among various N-heterocyclic skeletons, cyclic
amidines and quinazolinones have been recognized as privileged
pharmacophores in various drug molecules and candidates,
bioactive molecules, and lead compounds (Figure 1).2 Thus,
Figure 1. Bioactive molecules containing cyclic amidines and
quinazolinones.
NHC−boryl radicals have shown versatile utility in various
organic transformations.7 For example, Curran has recently
reported an NHC−boryl radical-promoted reductive decyana-
tion reaction of organic nitriles.8 In this transformation, the
NHC−boryl radical attacked the cyano group to generate an
iminyl radical intermediate, followed by a fragmentation and
synthetic methods that enable their diverse synthesis and
variable structure modifications are highly desirable in drug
discovery and development. In this context, installation of a
boron handle onto the cyclic framework represents a promising
strategy as the versatile C−B bond can be converted to a wide
range of functional groups of interest.3 However, synthesis of
these boron-substituted molecules in a chemo-, regio-, and
stereocontrolled manner has been challenging.4
Received: October 16, 2018
© XXXX American Chemical Society
A
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a
reduction to give a decyanated product. On this basis, we
envisioned that an analogous addition reaction of an NHC−
boryl radical to the N−CN group of N-allylcyanamides would
also occur to give amide−iminyl radical intermediate I (Scheme
1), which would then lead to either decyanation or cyclization
Table 1. Optimization of Reaction Conditions
Scheme 1. Substrate-Controlled Chemodivergent Reactions
initiator
(x mol %)
5b yield
b
3b yield
b
entry
RSH (y mol %)
(%)
(%)
c
1
2
3
4
5
C9H19C(CH3)2SH (20)
C9H19C(CH3)2SH (5)
C9H19C(CH3)2SH (10)
C9H19C(CH3)2SH (20)
PhSH (20)
ND
47
75
ND
7
trace
trace
12
d
AIBN (20)
AIBN (20)
AIBN (20)
AIBN (20)
TBHN (20)
e
85 (81)
54
f
6
C9H19C(CH3)2SH (20)
73
trace
a
Unless otherwise noted, the reactions were performed on a 0.2−0.3
mmol scale of 1b, 2 (1.2 equiv), initiator (x mol %), and RSH (y mol
%) in CH3CN (2 mL) at 80 °C under a N2 atmosphere. NMR yield
using tetrachloroethane as an internal standard. 1b was recovered in
94% yield. 1b was recovered in 27% yield. Isolated yield. The
reaction was performed at 60 °C.
b
c
d
e
f
with the pendant alkene tether. To test the reactivity, we
examined the reaction of N-allylcyanamide 1a and 1,3-
dimethylimidazol-2-ylidene borane (2) in the presence of
AIBN as the radical initiator. As a result, the reductive
decyanation product 3a was isolated in 34% yield along with
the formation of NHC−borane nitriles 4a and 4b in 15 and 11%
yields, respectively. Meanwhile, 1a was recovered in 60% yield.
However, no cyclized product was detected at all.
Next, we attempted a cascade process that would initiate from
the addition to the alkene moiety rather than the N−CN group.
For this purpose, the choice of a suitable R group in N-
allylcyanamide 1 that can render the alkene addition more
favorable is crucial. Our previous studies revealed that an aryl
group attached at the alkene end could promote NHC−boryl
radical addition.6 However, the reaction preference when a N−
CN group was present in the substrate was unclear. To figure out
the reactivity difference, the reaction of 1b was examined. In the
event, a boron-handled cyclic amidine 5b was isolated in 16%
yield as a single diastereomer with 3,4-trans stereochemistry.
This stereochemistry was secured by X-ray crystallographic
analysis.9 Meanwhile, the reductive decyanation product 3b was
also formed in 14% yield. The process to form 5b provides a new
strategy to construct boron-tethered N-H cyclic amidines, which
are potentially useful in medicinal chemistry but are difficult to
access by the existing methods. This prompted us to optimize
the reaction conditions further.
First, a control experiment was conducted in the absence of a
radical initiator (Table 1, entry 1). As a result, no reaction
occurred, and 1b was fully recovered, indicating the radical
reaction nature of this cascade. The following optimization
revealed that the addition of RSH as a polar reversal catalyst10
could significantly affect the product distribution and yield. For
example, when 5 mol % of tert-dodecanethiol was added as a
catalyst, the yield of 5b was improved to 47% yield and the
formation of 3b was suppressed (entry 2). Increasing the loading
of RSH catalyst led to further improvement of the yield of
product 5b (entries 3 and 4). Eventually, 5b was isolated in 81%
yield with the use of 20 mol % of tert-dodecanethiol (entry 4).
Switching the thiol catalyst to PhSH resulted in a diminished
product yield (entry 5). Using di-tert-butyl hyponitrite (TBHN)
as the radical initiator also afforded product 5b in a good yield
(entry 6). Other NHC−boryl radical precursors bearing varied
substituents on the NHC moiety gave the desired cyclized
products in good yields and exclusive 3,4-trans stereo-
selectivity.11 It should be mentioned that when 1a was subjected
to the optimized reaction conditions, the decyanation product
3a was still obtained as the sole product, and no cyclized product
was detected.
With the optimized reaction conditions in hand, we next
investigated the scope and generality of this radical borylative
cascades for the construction of borylated cyclic amidines 5
(Table 2). In general, N-allylcyanamides bearing an aryl group at
the alkene end afforded the desired boron-substituted cyclic
amidines 5 in good yields. Only a trans-diastereomer was
obtained in all cases. A gram scale reaction of 1b gave 5b in 76%
isolated yield. Various functional groups on the aryl ring were
compatible (for 5c−5f). Moreover, both naphthalene (for 5g)
and pyridine (for 5h) motifs could be installed. Importantly,
when the R1 substituent was changed to tert-butyloxycarbonyl
(for 5i), benzyl (for 5j), and phenyl (for 5k) groups, good
selectivity and efficiency were maintained. A range of N-aryl-
substituted substrates bearing various functional groups were
converted to borylated cyclic products in good yields (for 5l−
5o). In particular, heteroaryls, such as pyrimidine (for 5p) and
carbazole (for 5q), were successfully incorporated. The present
method also allowed for the construction of a boron-handled
bicyclic amidine 5r through the trapping of the resulting amide−
iminyl radical intermediate with an additional intramolecular
alkene tether. When an aryl group was replaced, a methyl group
as the substituent R (for 1s) was not viable to afford cyclized
product, and the corresponding decynated product 3s was
formed in 9% yield. Meanwhile, 1s was recovered in 87% yield
without any E/Z isomerization, indicating that the addition of an
NHC−boryl radical to a methyl-substituted alkene may not
occur under the present reaction conditions. Moreover, the
cascade of N-homoallylcyanamides 1t to construct boron-
tethered 2-iminopiperidine 5t was unsuccessful, and the
decyanated product 3t was formed instead.
N-Acylcyanamides have been found to be competent
substrates in radical cascades to construct quinazolinones12
since the pioneering work of Malacria and Courillon.12a Despite
the fact that various functionalized products have been prepared,
the synthesis of versatile boron-substituted ones has not yet
been documented. To expand the scope and generality of the
present method, we next investigated reactions of N-benzoyl-
protected allylcyanamides 6 (Scheme 2). The reaction
B
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proceeded smoothly in the presence of 1.4 equiv of AIBN,
affording boron-substituted quinazolinones 7 in moderate yields
with the tolerance of a variety of functional groups.
Table 2. Scope of Radical Borylative Cyclizations of N-
Allylcyanamides
a
Based on our previous mechanistic studies on the NHC−
boryl radical reaction with alkenes,6 we proposed plausible
reaction pathways as depicted in Scheme 3 for the formation of
Scheme 3. Proposed Reaction Pathways for Cascade
Products
cascade products. Initially, the NHC−boryl radical (II) is
generated by hydrogen atom abstraction from 2 in the presence
of AIBN as the radical initiator.13 Boryl radical II then undergoes
chemo- and regioselective addition to the phenyl-substituted
alkene moiety, affording a resonance-stabilized radical inter-
mediate III, which then cyclizes with the intramolecular N−CN
group to form amide−iminyl radical intermediate IV. For the
reaction of 1b−1q, hydrogen atom transfer from the thiol
catalyst (RSH) occurs to give products 5b−5q with the
generation of a sulfur radical (RS•), which in turn abstracts a
hydrogen atom from 2 to regenerate RSH and boryl radical II.10
As shown in Table 1, the RSH catalyst has played an important
role in the product distribution and yield. When an RSH catalyst
was present, the hydrogen atom transfer (IV to 5b) was
dramatically enhanced because of the matching polarity between
the nucleophilic amide−iminyl radical IV and the thiol
catalyst.14 In contrast, such a polarity effect was not obvious
for electrophilic aminyl radical I-c. As a consequence, the radical
chain process to afford product 5b was facilitated, and the
pathway to form 3b was inhibited. When another allyl group is
installed on the nitrogen atom (for 1r), the resulting amide−
iminyl radical IV can be trapped to assemble a bicyclic
framework. In this domino process, the propagation of a radical
chain reaction is also achieved using a thiol catalyst. In addition,
for the reaction of 6, the radical intermediate IV is capable of
performing aromatic homolytic substitutions15 with intra-
molecular N-benzoyl groups, leading to boron-substituted
quinazolinones 7. In this case, an excess amount of AIBN is
required to facilitate the rearomatization step.12c,e,16
a
Unless otherwise noted, the reactions were performed on a 0.2−0.4
mmol scale of 1, 2 (1.2 equiv), AIBN (20 mol %), and
C9H19C(CH3)2SH (20 mol %) in CH3CN (4 mL) at 80 °C under
a N2 atmosphere. Isolated yield of a gram scale reaction of 1b. 1s
b
c
d
was recovered in 87% yield. 1t was recovered in 57% yield.
a
Scheme 2. Synthesis of Boron-Substituted Quinazolinones
a
Unless otherwise noted, the reactions were performed on a 0.3−0.4
The synthetic utility of borylated N-heterocycles was
demonstrated (Scheme 4). The NHC−borane-tethered N-H
amidine 5n was treated with tosyl chloride in the presence of
NaH as the base, giving N-tosyl-protected product 8. The
mmol scale of 6, 2 (1.2 equiv), and AIBN (1.4 equiv, added in several
portions) in CH3CN (3 mL) at 80 °C under a N2 atmosphere.
C
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a
ORCID
Scheme 4. Transformations of Borylated Products
Yi-Feng Wang: 0000-0003-1360-8931
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the NSFC (21672195 and
21702201), the Fundamental Research Funds for the Central
Universities (WK2060190082), Recruitment Program of Global
Experts, and University of Science and Technology of China. F.-
L.Z. is grateful for the grant from the China Postdoctoral Science
Foundation (2016M602014 and 2018T110620). We thank
Prof. Shunsuke Chiba (Nanyang Technological University,
Singapore) for valuable suggestions.
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Corresponding Author
*E-mail: yfwangzj@ustc.edu.cn.
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