Organic Letters
Letter
1
9
versus SCE) might oxidize P-substrate. But the single-
Scheme 4. Substrate Scope for Coupling of Aryl Halides
with H-Phosphine Oxides or H-Phosphites
•
−
a
electron-reduced species RB is a relatively weak reductant
•
−
19
[
E(RB/RB ) = −0.78 V versus SCE], which cannot reduce
Ni(II) into Ni(0). This result is in agreement with our
proposed mechanism (Scheme 2).
18
To further probe the mechanism, we conducted the
following control experiments. The reaction is highly sensitive
to O and the inhibition of the reactivity observed in air
2
implied a radical mechanism (Scheme 3). When 2.0 equiv of
Scheme 3. Mechanistic Investigations
1
,1-diphenylethylene was added to the standard reaction
system, 3aa was obtained in 10% yield. The presence of
2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO, 2 equiv) as a
(
radical trap shut down the reaction completely. Substrates 1a
and 2a were treated with TEMPO (2 equiv) or 1,1-
diphenylethylene (2 equiv) under our optimized reaction
conditions for 1 h, and the resulting mixture was characterized
by electrospray ionization mass spectrometry (ESI-MS). The
Figure S4) are assigned to be [2,2,6,6-tetramethylpiperidin-1-
+
yl diphenylphosphinate + H] (calculated m/z = 358.1930),
+
[(2,2-diphenylvinyl)diphenylphosphine oxide + H] (calcu-
lated m/z = 381.1403) and [(2,2-diphenylethyl)diphenylphos-
+
phine oxide + H] (calculated m/z = 383.1559) ions,
respectively. The above observations clearly indicate the
generation of phosphorus-centered radical involved in this
metallaphotoredox manifold.
a
1
(0.2 mmol, 1 equiv), 2 (0.4 mmol, 2 equiv), NiBr (0.02 mmol, 10
2
mol %), dtbbpy (0.024 mmol, 12 mol %), t-BuNH(i-Pr) (0.4 mmol,
2.0 equiv), TXO (20 mol %), 2 mL MeCN, N , irradiation under a 45
The generality of this dual photocatalysis for the
2
2
b
construction of the C(sp )−P(O) bond was investigated
W CFL for 24 h, isolated yields. Using aryl chloride as the substrate.
c
Using aryl iodide as the substrate.
using the optimized reaction conditions (Scheme 4). Various
aryl bromides bearing electron-withdrawing acetyl, carboxylic
ester, trifluoromethyl, and cyano groups at the para-position
reacted with diphenylphosphine oxide 2a to deliver the
coupled triarylphosphine oxides 3aa−3ea in 87−92% yields.
chloroacetophenone and 2-chloro-6-(trifluoromethyl)pyridine,
did also participate successfully in this dual catalytic system,
giving 3aa and 3ta in moderate yields. 5-Bromobenzo[d][1,3]-
dioxole was unsuitable substrates for this conversion. Aryl
iodides couple with higher levels of efficiency than the
corresponding aryl bromides. The P-arylation using 4-
iodopyridine and 5-iodobenzo[d][1,3]dioxole proceeded
efficiently, giving 3va (62%) and 3wa (69%), respectively.
We further examined the scope of H-phosphine oxide and
H-phosphite substrates for the photoinduced phosphonylation
of 1a under the optimized reaction conditions (Scheme 4). Di-
p-tolylphosphine oxide 2b and bis(3,5-dimethylphenyl)-
phosphine oxide 2c reacted with 1a to provide triarylphos-
phine oxides 3ab (92%) and 3ac (89%) in excellent yields. As
we expected, commercially available dialkyl phosphites reacted
smoothly with 1a to afford the corresponding products 3ad−
3ag in good isolated yields. Dibenzo[c,e][1,2]oxaphosphinine
4
-Bromo-1,1′-biphenyl (1f) is also suitable for this trans-
formation, providing 3fa in good yield. As for aryl bromides
with electron-donating substituents such as 1-bromo-4-
methylbenzene and 1-bromo-4-methoxybenzene, the expected
C−P bond formation products were scarcely observed.
However, 1-iodo-4-methylbenzene or 1-iodo-4-methoxyben-
zene reacted readily with 2a to give arylphosphine oxides in
6
6% (3ga) and 63% (3ha) yields, respectively. Aryl bromides
bearing substituents at the 3-, or 3,5-, 3,4-positions of phenyl
rings afforded 3ia−3ma in 77−83% yields. The reaction is
sensitive to steric hindrance as ortho-substituted aryl bromides
resulted in a consistent decrease in yields of 51−57% (3na-
3
qa). 2-Bromonaphthalene could also deliver the expected
product 3ra with a good yield of 65%. Heteroaryl bromides
including 1-(6-bromopyridin-2-yl)ethan-1-one (2s), 2-bromo-
2
6-oxide underwent C(sp )-P cross coupling reaction with 1a to
6-(trifluoromethyl)pyridine (2t), 3-bromoquinoline (2u), and
4-bromopyridine (2v) were also well tolerated, affording 3sa−
3va in 30−79% yields. Notably, aryl chlorides, such as 4-
deliver the arylphosphinate product of 3ah in 61% yield. When
the P(O)−C(sp ) coupling reactions were performed in
dimethylacetamide under blue LEDs at higher temperature
2
C
Org. Lett. XXXX, XXX, XXX−XXX
Zhu, Da-Liang
