Journal of the American Chemical Society
Communication
,
a b
Scheme 1. Substrate Scope of Hydroxylation Catalyzed by
PSP-95C-Ni (bpy)
Scheme 2. Catalytic C−N Bond Formation
II
,
a b
a
Reaction conditions: PSP-95C-bpy (0.44 μmol, in 16 mL of 75 mM/
L Tris-HCl buffer, pH 8.8), NiSO (1 μmol), aryl halides (50 μmol),
4
and 100 equiv of pyrrolidine or imidazole were mixed in H O/DMF
2
b
(
v/v = 4/1, 20 mL) under Ar. Reaction time was 24 h.
transient absorption spectroscopic (TAS) measurements were
conducted. First, TAS of both PSP-95C-bpy alone and the
II
reaction mixture containing PSP-95C-Ni (bpy), 1a, and
DIPEA (Figure 2a,b) exhibited a ground-state bleach at 380
nm and a new peak at 430 nm upon 430 nm laser irradiation,
thereby pointing to the formation of a triplet excited state
(
S5) and the positive outcome of direct excitation reaction
(
Table 1, entry 11) suggest that triplet photosensitization of
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13
the Ni complex by PSP* is likely in operation. Moreover,
the lifetime (τ) of PSP-95C-bpy* remained almost unchanged
II
(
Figure S6). However, kinetic traces showed that the triplet
a
lifetime dropped from 158 to 149 μs when 5 mM 1a was
added, and a further dramatic decrease to 96 μs was observed
Reaction conditions: PSP-95C-bpy (0.044 μmol, in 1.9 mL of 75
mM/L Tris-HCl buffer, pH 8.8), NiSO (0.1 μmol), aryl halides (10
4
(
Figure 2c). This led us to carefully examine the quenching
μmol), and DIPEA (15 μmol) were mixed in H O/DMF (v/v = 19/1,
2
b
1
effect with respect to substrate concentration, which revealed a
clear linear correlation with the excited-state lifetime. The
2
.0 mL) under Ar. Yield was determined in triplicate by H NMR
analysis. Tris-HCl buffer (pH = 8.8)/DMF = 4:1 (2 mL) was used.
c
d
Reaction time was 36 h.
quenching rate constant (k ) extracted from the Stern−Volmer
q
4
−1 −1
plot versus the 1a concentration was 8.5 × 10 M
s
for
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3
−1 −1
PSP-95C-Ni (bpy) and 9.3 × 10 M
thiophene, and quinolone were also uneventfully accommo-
This result established the occurrence of a direct interaction
between the excited state of the photosensitizer and the
ground-state substrate 1a. Collectively, a plausible mechanistic
scenario is depicted in Figure 2e. The Bp chromophore of
PSP-95C enters into an excited triplet state under irradiation,
which acts as a position-defined antenna to raise the electron
dated, albeit with somewhat compromised reactivity (2k−2n,
4
0−60% yield). Notably, the entry of alcohol 2j showed the
applicability of this photosensitizer metalloenzyme also for
aliphatic bromides with no background base-promoted
reaction. It paves a novel access to hydrolyze aliphatic halides
under very mild photobiocatalytic conditions. Apart from
bromide substrates, this system also holds fidelity for relevant
chloride and iodide compounds. The latter are challenging
substrates, probably due to decreased aqueous solubility (2p,
II
spin of Ni aryl oxide to the triplet excited state, thereby
effectively driving the reductive elimination step to form a C−
O bond. On the other hand, the reductive elimination from a
17
III
2
q, 2s).
Ni intermediate, particular via the recently unveiled photo-
I
III
16
H O was the coupling partner for this formal hydrolytic
2
initiated/sustained Ni /Ni cycle, could not be ruled out at
18
this stage.
As triplet energy transfer is strongly distance-dependent,
1
8
2
18
19
O from H O (Figure S3). Despite this kinetically favorable
process in aqueous media, coupling with nitrogen nucleophile
was still possible. For instance, aniline derivative 3 could be
successfully isolated in 57% yield by subjecting pyrroline to the
reaction mixture devoid of DIPEA (Scheme 2a). Analogously,
C−N cross-coupling took place smoothly between bromide 1a
and imidazole to furnish product 4 with 39% yield (Scheme
the spatial situations of photosensitizer/metal catalyst would
likely correlate their synergism and consequently the enzymatic
performance. Accordingly, cysteine mutant was introduced
PSP-147C, PSP-151C, and PSP-186C (Table S5). The
distances from the corresponding Ni center to the
chromophore are 1.98, 1.19, 0.6, 1.02, and 1.67 nm,
respectively (Figure 3a). Interestingly, a profound impact of
evident (Figure 3b and Table S6). Among them, the 95C
mutant gave the optimal catalytic performance for the phenol
2
b).
Different mechanistic scenarios have been proposed for the
organophotoredox/nickel dual catalysis that accomplishes the
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10a,b,16
reductive elimination via Ni species
or electronically
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17
excited Ni species. To gain more mechanistic insights,
6
19
J. Am. Chem. Soc. 2021, 143, 617−622