2
S. DING ET AL.
involved the intramolecular addition of radicals to the unsaturated bond, followed by
the intramolecular radical cyclization to afford the skeleton. Among these strategies,
various compounds can be converted to their corresponding alkyl radicals under specific
conditions. Liu et al. summarized an organocatalytic method using [bis(pivaloyloxy)io-
[
3]
do]benzene as the alkyl precursor with calcium carbonate. Zhu et al. provided effi-
cient access to alkyl radicals using aliphatic carboxylic acids under visible light
[
4]
photocatalysis conditions.
FeCl2,
Cheng group reported a catalytic reaction system of
[
5a]
[5b]
moreover, Liu and coworkers developed a similar CuBr system
using di-
tert-butyl peroxide (DTBP) and dicumyl peroxide (DCP) as methyl radical precursor
respectively. And then, Liu group invented a strategy using catalytic amounts of IrCl3
with DTBP or DCP as radical initiators. Under this condition, phenylethane and
[
6]
cumene can be transformed into their corresponding alkyl radicals. Almost at the
same time, Duan and his coworkers found another way making use of Cu O together
2
[
7]
with tert-butyl peroxybenzoate (TBPB) to obtain the benzylic radical. Compared with
active C-H bonds in the benzyl position, Liu group developed an efficient and conveni-
[
8]
ent Cu O catalyzed path using simple alkanes as the starting materials. For the first
2
time, Li and coworkers provided a FeCl catalyzed system to various ethers that can be
3
[
9]
served as alkyl radicals. Furthermore, Duan and Guo group explored using stoichio-
metric amounts of tert-butyl hydroperoxide to obtain a-hydroxycarbon radicals and
[
10]
then to complete the cyclization reaction.
1,3-dicarbonyl compounds can be trans-
formed into their corresponding radicals by AgNO and potassium persulfate, which
3
[
11]
was invented by Duan group.
The above methods have some disadvantages, such as
long reaction time and high reaction temperatures. However, alkyl-boron compounds
for this synthesis have not been established. Herein, we report a novel tandem oxidative
cyclization reaction of acrylamides with alkyl trifluoroborates (Figure 1). This approach
would provide a new entry to more valuable high-efficiency 3,3-disubstituted oxindoles.
Results and discussion
Based on our earlier studies, we devised and developed a rapid and highly efficient
method for radical formation using potassium alkyl trifluoroborates as a radical precur-
[
12]
sor, that conducts under relatively mild conditions using silver(I) as the oxidant.
The
generated alkyl radicals can be trapped immediately with TEMPO, providing critical evi-
dence for the existence of the radical intermediate. For making this method useful, we
must find its application. Currently, the successful application of this new radical forma-
[
13]
tion strategy is in the boron-selective oxidative cross-coupling reaction
using arylbor-
onic acids and alkyl trifluoroborates as coupling partners. Encouraged by the
preliminary results, we explored meaningful application from the synthetic point. Aryl-
isonitrile is another well-established radical acceptor, which is isoelectronic with carbon
monoxide, and they can undergo insertion reaction to provide nitrogen-containing het-
erocycles that are important substrate present in many natural products with varying
[
14]
biological activities.
Our ongoing interest in the construction of diversely functional-
ized oxindoles prompted us to develop an effective method for these valuable
heterocycles.