10.1002/anie.201804159
Angewandte Chemie International Edition
COMMUNICATION
a.
of I(I) to I(III) with facile disproportionation of the initially
formed I(III) reagents, efficient access to I(V) is accomplished.
The I(V) reagent generated in this protocol is functionally
analogous to DMP, and provides a platform to couple O2
reduction with oxidation of alcohols, diols, and amines. In situ
generation of iodylbenzene intermediates supports aerobic
oxidation catalysis and we have demonstrated that efficient
catalysis requires access to the I(V) oxidation state. These
studies add to the growing field of aerobically generated
oxidants that underpin development of sustainable oxidation
chemistry. We anticipate that development of new strategies
to couple O2 reduction to iodobenzene oxidation without the
intermediacy of reactive aldehyde autoxidation intermediates
will engender broadly applicable hypervalent-iodine-
catalyzed aerobic oxidation chemistry.
O2, RCHO
subox
subred
O
O
O
O
I
I
I
O
t-Bu
S
S
S
t-Bu
2a
t-Bu
O
O
O
O
1a
3a
disproportionation
b.
O
O
AcOH
CDCl3
O
O
I
I
I
+
t-Bu
O
S
S
S
t-Bu
t-Bu
O
O
O
O
2a
3a
1a
O
I
AcOH
CDCl3
I
I
O
O
+
Acknowledgements
Me
Me
Me
We thank Texas A&M University and the Welch Foundation
(A-1907) for financial support. Computations were carried out
at the Supercomputing Facility and the Laboratory for
Molecular Simulation at Texas A&M University.
2e
3e
1e
Scheme 5. a. Proposed catalytic cycle for iodylbenzene-catalyzed
aerobic oxidation. b. Disproportionation of both 2a and 2e are
thermodynamically preferred. Experimentally, 2a is observed to
participate in rapid disproportionation whereas 2e is indefinitely stable.
Keywords: aerobic oxidation • hypervalent iodine •
sustainable • autoxidation
1H NMR revealed the consumption of aryliodide 1a and
evolution of iodylbenzene 3a without the intermediacy of
observable iodosylbenzene 2a (Figure S3). In addition,
subjecting a solution of independently prepared 2a to AcOH,
which is generated during autoxidation, results in the
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1
immediate disproportionation to 1a and 3a (Figure 5b; for H
NMR spectrum, see Figure S4). Density Functional Theory
(DFT) calculations — carried out with the M06-2X functional26
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There are two reports of aerobic oxidation to generate
iodylbenzenes. a) Aerobically generated I(V) derivatives were
proposed in alcohol oxidation (R. Mu, Z. Liu, Z. Yang, Z. Liu, L.
Wu, Z.-L. Liu, Adv. Synth. Catal. 2005, 347, 1333–1336) but the
active catalyst was subsequently reassigned as Br2 (M. Uyanik, R.
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aerobic oxidation of PhI to PhIO2 has been reported: W. P.
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1125–1126. The iodylbenzene obtained in this procedure is not a
useful oxidant in synthetic chemistry.
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(basis sets: LANL2DZ27 for I, 6-31G(d,p)28 for other atoms)
29
and the SMD solvation model
—
indicate that
disproportionation of 2a to generate 1a and 3a is
thermodynamically favored (ΔG –8.8 kcal/mol; see
=
[5]
[6]
Supporting Information for computational details). In contrast
to 2a, iodosylbenzene 2e does not participate in facile
disproportionation: in situ monitoring of the oxidation of 1e by
1H NMR spectroscopy indicates that initial oxidation affords
I(III)-derivative 2e, which subsequently converts to I(V)
derivative 3e slowly (Figure S5). Despite being
[7]
thermodynamically favorable (calculated ΔG
=
–18.1
kcal/mol), treatment of independently synthesized 2e with
AcOH resulted in no observed disproportionation (Figure 5b).
While independently prepared 3e effects oxidation of
cyclohexanol in 66% yield, 1e (which generates 2e, an
[8]
iodosylbenzene
derivative
that
does
not
rapidly
disproportionate) is an ineffective oxidation catalyst (24%
yield for oxidation, for comparison, Co-catalyzed background
provides 15% yield). These data indicate that when facile
disproportionation of I(III) is not available, the efficiency of
aerobic oxidation catalysis is significantly compromised.
In conclusion, here we have reported the first
aerobically generated I(V) reagents that participate in
substrate oxidation chemistry. By coupling aerobic oxidation
[9]
[10]
C. Hartman, V. Meyer, Chem. Ber. 1893, 26, 1727–1732.
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