Science - USA (2022-02-25)

undergo reduction and condensation while main-
taining enantiopurity (Fig. 2B).
In our deracemization process, the photo-
chemical perturbation may proceed through
electron transfer (followed by hydrogen trans-
fer) or energy transfer–induced isomerization.
The oxidation potential of theE- orZ-enamine
intermediate was determined to be 0.73 V ( 23 ),
and Ir(ppy) 3 hasE1/2PC+/PC*=0.31V.Although
photoinduced electron transfer between en-
amine and photocatalyst is possible, this was
discounted because the obtained enantioselec-

tivity seemed uncorrelated with the excited-

state redox potential but rather correlated with

the triplet state energy (ET), as revealed in the

screening of different photocatalysts (Fig. 3A).

High enantioselectivity was generally obtained

by using photocatalysts with triplet-state ener-

gies in the 56 to 59 kcal/mol range, which

is in line with the triplet-state energy of an

enamine intermediate, 54.2 kcal/mol as de-

termined with density functional theory (DFT)

calculation (Fig. 4B). The observed inhibition

effect with typical triplet-state quenchers such

as stilbene or oxygen provides further support

for an energy transfer mechanism (table S1,

entries 5 and 6).

Stoichiometric experiments were conducted

to examine the enamine formation by means

of nuclear magnetic resonance (NMR) spec-

troscopy (Fig. 3D and figs. S15 to S18). The

joint use of a strong acid HNTf 2 and a weak

benzoic acid could significantly enhance the

rate of iminium-enamine tautomerization,

as previously reported (fig. S15) ( 14 ). In the

presence of (S)-1a/HNTf 2 ,(S)-2aselectively

870 25 FEBRUARY 2022•VOL 375 ISSUE 6583 science.orgSCIENCE

Fig. 1. Deracemization strategies.(A) Asymmetric carbonyl transformation. (B) Prior works on photocatalytic deracemization. (C) Deracemization ofa-branched
aldehydes through photochemicalE/Zisomerization of enamine intermediate.

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