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1.1 equiv of NBS in heptane (Scheme 6b). Upon completion, the
insoluble succinimide was filtered off and the filtrate was
concentrated to give tetrahydrofuroindoline 7e in 95% yield
with >95% purity based on the 1H NMR study. Pure succinimide
(confirmed by 1H NMR) was recovered in 97% yield.
Figure 1 shows the progress in product conversion from 8a to
9a with respect to time for a catalyzed vs an un-catalyzed
reaction. In the presence of catalyst, the reaction proceeds
slowly at the beginning (also known as the induction period)
followed by acceleration at the later stage. The reaction follows
a zero-order kinetics after the initial induction period. The graph
describes how the presence of catalyst is important for the
reaction to take place.
In summary, we have designed a new phase-transfer catalyst,
the alkoxy-amide which has been used effectively for
bromocyclizations and brominations. The sulfonyl and alkoxy
groups act cooperatively for the capture and release of
electrophilic bromonium ion from brominating agent to
substrates. The reaction uses a green solvent, mild and
operationally simple procedure. The protocol also provides easy
isolation of the bromine carrier succinimide by simple filtration.
Finally, the role of the catalyst is also supported by NMR
experiments. Further application of this alkoxyamide catalysis
for the synthesis of enantioenriched compounds are underway.
M.S.M. gratefully acknowledges SERB, Department of Science
and Technology, New Delhi, India (Sanction No.
CRG/2018/000317). H.M. thanks U.G.C. India and M.R.S. thanks
IIT Kharagpur for fellowship.
DOI: 10.1039/D0CC04673F
52, 2288.
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Conflicts of interest
There are no conflicts to declare.
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