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Chemical Science
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ARTICLE
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On the other hand, we have recently found that 1a reacted phosphinyl radical, accessed in a catalytic cycle, is believed to be
smoothly with AIBN to produce the bisphosphine [1a-P]2.10 responsible for activating the carbon-bromine bonds through ET.
DOI: 10.1039/D0SC01352H
Considering the facile interconversion between [1a-P]2 and This new reaction may provide a superior approach to remove
phosphinyl radical 1a-[P]•,19 bisphosphine was synthesized halogen from many organohalides in terms of substrate scope,
separately to examine whether it may have functioned as a radical reaction efficiency and chemo-selectivity. Exploitation of other
reservoir during hydrodehalogenation. As depicted in Scheme 5 radical reductions using the strategy presented here is ongoing.
(Eq. 1), no product was detected when [1a-P]2 was mixed with
bromobenzene under the standard conditions. This is presumably
because the relatively strong P-P bond of [1a-P]2 made it not
Conflicts of interest
easily dissociate to render the corresponding phosphinyl radical
1a-[P]• at the reaction temperature (90 oC, see SI for details) Thus,
the possibility of [1a-P]2 as a reaction intermediate was excluded.
Because the solvent toluene could be a common hydrogen donor,
and if so, it has a potential to quench the phenyl radical. To
examine this, deuterium labelling experiments were conducted in
toluene-d8 (Eqs. 2 and 3). The absence of deuterium incorporation
indicated that the hydrogen could not come from toluene.
Furthermore, replacement of 1a with its deuterated counterpart
1a-D resulted in the desired product with a 90% deuterium
abundance (Eq. 4). Hence, 1a is suggested to be the hydrogen
source at last.
There are no conflicts to declare.
Acknowledgements
We are grateful for the financial grants from National Natural Science
Foundation of China (Nos. 21973052, 21933008, 21602116,
91745101), National Science & Technology Fundamental Resource
Investigation Program of China (No. 2018FY201200), and Tsinghua
University Initiative Scientific Research Program (No. 20181080083).
Notes and references
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tBu
Br
90 oC, 12 h
N
no reaction
Me
(1)
(2)
(3)
P
toluene-d8
N
2
tBu
[1a-P]2
Me
H
Br
1a
1a
(1.2 equiv.), AIBN 10 mol%
toluene-d8, 90 oC, 5 h
< 5% D
Me
Me
Me
Me
3e,
yield > 90%
2e
Br
H
< 5% D
(1.2 equiv.), AIBN 10 mol%
toluene-d8, 90 oC, 5 h
2i
3i,
yield > 90%
4. (a) D. Gudat, A. Haghverdi and M. Nieger, Angew. Chem. Int. Ed.,
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and A. W. H. Speed, Angew. Chem. Int. Ed., 2017, 56, 6268-
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tBu
N
D
Br
D
P
90% D
N
1a-D
(1.2 equiv.)
tBu
(4)
AIBN 10 mol%, toluene-d8, 90 oC, 5 h
2m
3m-D,
yield > 90%
Scheme 5. Control experiments with 1H NMR yields (using 1,3,5-
trimethoxybenzene as the internal standard; see SI for details).
Based on these control experiments, the plausible catalytic
mechanism proposed in Scheme 2 can be verified. The in-situ
generated phosphinyl radical preferentially served as a potent
electron donor to activate the bromides. The reaction should be
most likely to proceed through an ET-initiated radical chain
process,20 although a direct bromine abstraction cannot be
completely excluded at the present stage.
6. C. C. Chong, H. Hirao and R. Kinjo, Angew. Chem. Int. Ed., 2015,
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9. J. Zhang, J.-D. Yang and J.-P. Cheng, Angew. Chem. Int. Ed., 2019,
58, 5983-5987.
10. J. Zhang, J.-D. Yang and J.-P. Cheng, Chem. Sci., 2020, DOI:
10.1039/C9SC05883D.
Conclusions
In conclusion, in this work we unlocked unprecedented radical
reactivity of diazaphosphinanes to achieved efficient
hydrodehalogenation and cascade cyclization, which is
distinguished from their well-established hydridic reactivity. The
4 | J. Name., 2012, 00, 1-3
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