10.1002/ejoc.202001329
European Journal of Organic Chemistry
COMMUNICATION
benzylic radical intermediate B undergoes dimerization reaction
to give a diamine product. The concomitantly formed IrII complex
(E1/2 (IrIII/IrII) = -1.51 V vs. SCE in CH3CN)[6k] would be oxidized
by protonated aldehyde C to regenerate the IrIII complex 4.
Though the co-products derived from pivalaldehyde have not
been identified, the resulting a-oxy radical D is assumed to
follow similar reaction pathways to those observed in the case of
aldehyde 18. It should be noted that the addition of external
bases completely blocked the reaction, suggesting that our
reaction requires mildly acidic conditions. In the preceding
example, it is assumed that Brønsted or Lewis acidic activation
of aldehydes or imines is operative in the reductive dimerization
reaction of carbonyl derivatives.[8,9] In our case, thiobenzoic acid
(pKa = 3.2)[18] or its ammonium salt activates the acceptor
aldehyde by protonation to lower the high cathodic peak
potential of pivalaldehyde,[19,20] which would facilitate SET from
the Ir(II) complex via a proton-coupled electron transfer process.
Keywords: HAT catalyst • thiobenzoic acid • photoredox
catalyst • oxidative coupling • redox neutral coupling
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O
D
IrII
Ph
S
SET
IrIII
O
HAT
cycle
SET
cycle
SET
Ir-cat. 4
Ph
SH
O
HAT
H
*
hv
IrIII
Ph
S
Ar
NMe2
A
NMe2
Ar
Ar
Ar
NMe2
[6]
NMe2
B
Figure 1. Proposed catalytic cycle.
In summary, oxidative homocoupling reaction of
benzylamines and benzyl alcohols proceeds under photo-
irradiation conditions in the presence of a combination of
photoredox catalyst and thiobenzoic acid. Switching of the
electron acceptor led to the development of redox-neutral
coupling reactions. We propose that protonation of the electron-
accepting aldehydes occurs to enable smooth SET reduction,
leading to the production of various symmetrical diamines and
diols in good yields. Further studies are ongoing in our
laboratory.
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Acknowledgements
J. Rong, P. H. Seeverger, K. Gilmoure, Org. Lett. 2018, 2, 4081-4085.
This work was supported by grants from the University of
Shizuoka and Basis for Supporting Innovative Drug Discovering
and Life Science Research (BINDS) from Japan Agency for
Medical Research and Development (AMED).
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4
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