Diazaphosphinanes as hydride, hydrogen atom, proton or electron donors under transition-metal-free conditions: Thermodynamics, kinetics, and synthetic applications

Article abstract of DOI:10.1039/c9sc05883d

Exploration of new hydrogen donors is in large demand in hydrogenation chemistry. Herein, we developed a new 1,3,2-diazaphosphinane 1a, which can serve as a hydride, hydrogen atom or proton donor without transition-metal mediation. The thermodynamics and kinetics of these three pathways of 1a, together with those of its analog 1b, were investigated in acetonitrile. It is noteworthy that, the reduction potentials (E_red) of the phosphenium cations 1a-[P]⁺ and 1b-[P]⁺ are extremely low, being-1.94 and-2.39 V (vs. Fc^+/0), respectively, enabling corresponding phosphinyl radicals to function as neutral super-electron-donors. Kinetic studies revealed an extraordinarily large kinetic isotope effect KIE(1a) of 31.3 for the hydrogen atom transfer from 1a to the 2,4,6-tri-(tert-butyl)-phenoxyl radical, implying a tunneling effect. Furthermore, successful applications of these diverse P-H bond energetic parameters in organic syntheses were exemplified, shedding light on more exploitations of these versatile and powerful diazaphosphinane reagents in organic chemistry.

Full text of DOI:10.1039/c9sc05883d

View Article Online
View Journal
Chemical
Science
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: J. Zhang, J. Yang
and J. Cheng, Chem. Sci., 2020, DOI: 10.1039/C9SC05883D.
Volume
Number
9
1
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
7
January 2018
Pages 1-268
Chemical
Science
Accepted Manuscripts are published online shortly after acceptance,
before technical editing, formatting and proof reading. Using this free
service, authors can make their results available to the community, in
citable form, before we publish the edited article. We will replace this
Accepted Manuscript with the edited and formatted Advance Article as
soon as it is available.
rsc.li/chemical-science
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes to the
text and/or graphics, which may alter content. The journal’s standard
Terms & Conditions and the Ethical guidelines still apply. In no event
shall the Royal Society of Chemistry be held responsible for any errors
or omissions in this Accepted Manuscript or any consequences arising
from the use of any information it contains.
ISSN 2041-6539
EDGE ARTICLE
Xinjing Tang et al.
Caged circular siRNAs for photomodulation of gene
expression in cells and mice
rsc.li/chemical-science

Please do not adjust margins
Page 1 of 8
Chemical Science
View Article Online
DOI: 10.1039/C9SC05883D
ARTICLE
Diazaphosphinanes as Hydride, Hydrogen-atom, Proton or
Electron Donors under Transition-metal-free Conditions:
Thermodynamics, Kinetics, and Synthetic Applications
Received 00th January 20xx,
Accepted 00th January 20xx
Jingjing Zhang,a Jin-Dong Yang,a* Jin-Pei Chenga,b
*
DOI: 10.1039/x0xx00000x
Exploration of new hydrogen donors is in large demand for hydrogenation chemistry. Herein, we developed a new 1,3,2-
diazaphosphinane 1a, which can serve as a hydride, hydrogen-atom or proton donor without transition-metal mediation.
The thermodynamics and kinetics of these three pathways of 1a, together with those of its analog 1b, were investigated in
acetonitrile. Noteworthily, the reduction potentials (Ered) of the phosphenium cations 1a-[P]+ and 1b-[P]+ are extremely
low, as being -1.94 and -2.39 V (vs. Fc+/0), respectively, enabling corresponding phosphinyl radicals to function as neutral
super-electron-donors. Kinetic studies revealed an extraordinarily large kinetic isotope effect KIE(1a) of 31.3 for the
hydrogen-atom transfer from 1a to 2,4,6-tri-(tert-butyl)-phenoxyl radical, implying a tunneling effect. Furthermore,
successful applications of these diverse P-H bond energetic parameters in organic syntheses were exemplified, shedding
light on more exploitations of these versatile and powerful diazaphosphinane reagents in organic chemistry.
electronegativity of the
X and H fragments. Nevertheless,
Introduction
Waymouth et al. very recently reported an excellent new example
of such X-H bond where the N-H bond of the phenylazopyridine
ligand in Co(I) complex can serve as either a H-, H• or a H+ donor
(Scheme 1b).13 It is noted, however, that the diverse reactivity of
this N-H bond may have to require
communication between the azopyridine ligand and cobalt within
Hydrogen transfer plays a very important role in chemical science
and related fields. This process occurs through cleavage of the
targeted R-H bond in three possible pathways (Scheme 1), i.e.,
hydride (H-)1, hydrogen-atom (H•)2 and proton (H+)3 transfers, which,
from thermodynamic aspect, depends on the R-H bond strength
and the nature of the atom bound to the transferred hydrogen.
Accordingly, knowledge of the relevant thermodynamics of
a latent electronic
the five-membered ring, so it may be viewed as a quasi-M-H
hydricity ΔGH-,1b,
acidity pKa
bond-dissociation free-energy BDFE5 and
1c,
4
transition-metal
(a) For M-H bonds (
system)
hydride transfer (HT)
3c, 6
as well as the kinetics7 relevant to these processes
GH-
M
M
M
H
H
would facilitate rational exploitations of new transformations. In
recent years, considerable attention has been paid on developing
new hydrogen sources with versatile hydrogen donor reactivities,
with particular interests in transition-metal hydrides M-H1c, such as
hydrogenase enzyme analogs8 and hydrogenation catalysts9, and
main-group organic hydrides X-H,1b such as nicotinamide coenzyme
models10 and Hantzsch esters11, and so on.
pKa
proton transfer (PT)
M-H
BDE
hydrogen-atom transfer (HAT)
H
system.
(b) For cobalt coordinated N-H bonds (
transition-metal
system)
2
Cp
Co
N
N
Ph
Ph
Ph
HT
GH- = 88.7 kcal/mol
It is well known that metal hydrides (M-H) could serve as H-, H•
H
N
and H+ donors (Scheme 1a).1c, 12 Their diverse hydrogen reactivities
originate from the readily-modulated polarity of M-H bonds
through electronic communication between the metal centers and
coordinated ligands. Compared to an M-H system, the X-H hydride
manifested some advantages in mild reaction conditions, good
functional-group compatibility, easy modification, etc. Integration
of these three dissociation possibilities into one X-H covalent bond
is of a substantial challenge, due to the great disparity in their
Cp
Co
Cp
Co
N
N
N
PT
N
Ph
12.1
=
pKa
H
H
N
N
H
Cp
Co
N
HAT
67.7
BDFE =
kcal/mol
N
N
transition-metal-free
(c) For C-H bonds in triarylmethanes (
HT
system)
~ 100
GH- =
kcal/mol
Ph3C
Ph3C
Ph3C
H
PT
H
H
H
Ph3C-
~ 40
pKa
=
HAT
~ 80
BDFE =
kcal/mol
Scheme 1. Possible pathways of hydrogen transfers for transition-metal
and transition-metal-free systems as well as corresponding
thermodynamic driving forces in acetonitrile.
a. Center of Basic Molecular Science, Department of Chemistry, Tsinghua University,
Beijing 100084, China.
b.State Key Laboratory of Elemento-organic Chemistry, College of Chemistry,
Nankai University, Tianjin 300071, China.
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of syntheses and
experimental determinations]. See DOI: 10.1039/x0xx00000x
Please do not adjust margins

Please do not adjust margins
Chemical Science
Page 2 of 8
ARTICLE
Journal Name
For a true transition-metal-free system, till present only the atmosphere, 1a is stable enough for over 12 hours in
View Article Online
triarylmethane analogs, where exist steric as well as resonance
stabilization effects on the incipient radical or charges, were
DOI: 10.1039/C9SC05883D
tetrahydrofuran, acetonitrile, alcohols, toluene, dimethylsulfoxide
and chloroform. On the other hand, 1b is stable in these solvents as
reported to have the α-C-H bond-cleavage energies determined for
well except in alcohols and chlorinated hydrocarbons, where it
all the three pathways (Scheme 1c).14 However, their poor
decomposed rapidly to yield phosphonites (RO)2PH19 or 2-chloro-
hydrogen donability and severe steric hindrance make these
1,3,2-diazaphosphinane [P]-Cl (Scheme 3), showing that 1b is more
dissociation processes not too much useful for synthesis. This,
air- and moisture-sensitive than 1a.
instead, stimulated our interest to find out more useful transition-
metal-free X-H systems that promise both the desired energetic
measurement and new synthetic applications.
Recently, diazaphospholenium hydrides (P-H) have attracted
substantial research activities due to their super hydricity endowed
by
the
unique
diazaphospholene
skeleton.15
Many
diazaphospholenes were developed and successfully used in various
hydridic reductions.16 However, the superior hydricity made their
other promising applications, such as serving as hydrogen-atom and
proton donors, as well as the precursors of the strong electron
donors,
greatly
overlooked.
Considering
the
similar
electronegativity of the hydrogen (χAR = 2.20) and phosphorus (χAR
=
2.06) atoms,15b we reckoned that the P-H bond, beside as a good
hydride donor,17 may have the potential to release proton and
hydrogen-atom18 as well, by fine-tuning its electrical properties of
the P fragment. And indeed lucky enough, this was realized in the
present work.
Herein, we reported a new N-heterocyclic phosphine 1a, which
could act as a H-, H• and H+ donor (Scheme 2). Thermodynamics and
kinetics pertinent to these three processes were examined in
details. For comparison, 1b was investigated as well. Based on the
P-H bond energetic studies, we also exploited their versatile
applications in the reduction of pyridines, synthesis of
bisphosphines, H/D exchange, and activation of carbon-halogen
bonds as original examples.
Figure 1. The crystal structure of 1a (50% probability thermal
ellipsoids).20 Selected bond lengths (in Å): P1-H1 1.42(2), P1-N1
1.747(6), N1-C1 1.329(9), N1-C7 1.440(8), C1-C6 1.413(8).
tBu
N
H
P
CH2Cl2
ROH
[P]-Cl
(RO)2PH
or CHCl3
N
tBu
1b
Scheme 3. Decomposition of 1b in alcohols and chlorinated
hydrocarbons. [P] = (CH2)3(NtBu)2P, R = Me or Et.
Redox property. The oxidation potentials of 1a and 1b were
examined by cyclic voltammetry (CV), exhibiting
reversible oxidation peak of 1a to 1a•+ at 0.23 V (vs. Fc+/0, Figure
S1a) and an irreversible oxidation wave of 1b at 0.47 V (vs. Fc+/0
a partially
+
+
A
1a-[P]
H
A
tBu
iPr
-
GH
N
N
,
N
N
-
+
B
H
P
P
H +
1a-[P]
1a-[P]
H
H
B
Figure S1b) in acetonitrile. The low oxidation potentials of 1a and
1b indicate their good reducing capacity. It is interesting to find that
1a is a better electron donor, in spite of being a far poorer hydride
donor than 1b (vide infra). Besides, the redox behaviors of the
respective phosphinyl radicals (1a-[P]• and 1b-[P]•) were also
examined through the reduction of the corresponding
phosphenium cations (1a-[P]+21 and 1b-[P]+), giving the CV peaks at
-1.94 V and -2.39 V (vs. Fc+/0) in acetonitrile, respectively (Figures
S1c and 1d). The extremely low potentials, especially for 1b-[P]+,
indicate the very high reducing capacity of the corresponding 1a-
[P]• and 1b-[P]• radicals. This suggests that they both have a very
promising potential to serve as a super electron donor in organic
synthesis to reduce aryl halides.22 And indeed, this was verified in
the tentative application of the present work in radical
hydrodebromination of bromobenzene (see Application part, vide
infra).
pKa
tBu
iPr
.
+ C
C
1b
1a
BDFE
Scheme 2. P-H reagents 1a and 1b serving as diverse hydrogen donors.
A+, B- and C• are the hydride, proton and hydrogen-atom acceptors,
respectively.
Results and Discussion
Synthesis. Substrate 1a was prepared via H/Cl exchange between
the corresponding phosphoric chloride and LiAlH4 in THF according
to literature15, 17 (see Supporting Information SI for details). LiAlD4
was used for deriving the deuterated substrates 1a-D and 1b-D. The
crystal structure of 1a was identified as in Figure 1 that features an
envelope configuration with the P atom standing out of the
naphthalene plane and the P-H bond adopting a flagpole position,
similar to the five-membered ring analogs disclosed previously.15a
And the P-H distance of 1.42(2) Å is slightly shorter than the five-
membered ring analog.15 31P NMR of 1a in CD3CN shows a dt peak
at 25.66 ppm that is split by the hydrogens on the P atom and
isopropyl groups. It moves to the higher magnetic field in
comparison with other N-heterocyclic phosphines.17 Under inert
Thermodynamic driving force and reactivity. Hydride Transfer (HT).
Hydride transfer from 1a or 1b17 to N-methylacridinium iodide A1+
yielded equimolar products (Figure S3). To exclude the suspected
interruption of the hydridic isopropyl α-C-H, the deuterated
substrate 1a-D was employed, and over 95% deuterated A1D was
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 3 of 8
Chemical Science
Please do not adjust margins
Journal Name
ARTICLE
derived (Eq. 1 and Figure S4). This confirmed that the P-H hydride of protonated 1a (1aH+, Scheme 4). Based on the doublVeitewpeAratikcleoOfntlhinee
31
DOI: 10.1039/C9SC05883D
1a is a stronger donor than that of the isopropyl α-C-H.
phosphorus atom in P NMR (if the extraneous proton connected
to the P atom, a triplet 31P peak would be observed otherwise) and
the unsymmetrical peaks of naphthyl and isopropyl hydrogens in 1H
NMR (Figure S10), the protonated site was thus assigned to the N
atom (Scheme 4). We thought that their dissimilarity in reactivity
between 1a and some reported diazaphospholenium hydrides
should originate from the relatively weak hydricity of 1a (ΔGH- =
62.2 kcal/mol). As a consequence, it makes the H2-release unable to
compete with its protonation even under the condition of heating
(80 0C for 5 hours). The moderate hydricity of 1a endows it with
good tolerance to even strong acids, avoiding direct elimination of
dihydrogen. This in turn offers a possibility for 1a to act as a hydride
donor in acid-catalyzed reduction. Lots of examples in this
connection1a can be found especially in hydrogenation of imines by
hydride donors with comparable hydricity to 1a (~ 60-70 kcal/mol,
like Hantzsch ester,23 benzothiazoline24 and cyclohexadiene25)
under the catalysis of Brønsted acids (for examples BINOL–
phosphoric acid, trifluoroacetic acid and Tf2NH).
H
Me
Me
H
N
N
Me
Me
Me
N
Me
N
I
N
CH3CN
20 o
+
(1)
P
D
P
I
C
N
H
D
iPr
iPr
> 95% D
1a-D
A1+
A1D
The hydricity (ΔGH-, defined by Eq. 2) of 1a and 1b in
acetonitrile was determined by measuring their equilibrium
constants with the acceptors of known hydricities (Eq. 3). For
example, the measured hydride transfer equilibrium constant
Keq(1a) of 0.48 (equivalent to 0.43 kcal/mol) between 1a and
phenanthridinuim trifluoromethanesulfonate A2+ (ΔGH-(A2H) = 61.4
kcal/mol)1b provided the hydricity ΔGH- of 1a to be 61.8 kcal/mol
(see Figure S5 in SI for details) and other equilibria (in Figure S6 and
S7) verified this value, giving a mean hydricity of 62.2 ± 1.0
kcal/mol. This finds 1a to be a moderate hydride donor, close to the
hydride-donating ability of the NADH analogs.1b Thus, it should be
capable of reducing the commonly-used hydride acceptors, such as
N-methylacridinium cation (ΔGH-
=
76 kcal/mol), 9-
iPr
iPr
iPr
phenylxanthylium cation (89 kcal/mol) and trityl cation (99
kcal/mol). Similarly, the much lower equilibrium constant Keq(1b) of
1.6 × 10-3 (equivalent to 3.8 kcal/mol, Eq. 5 and Figure S8)
established between 1b and benzimidazolium perchlorate A3+ (ΔGH-
(A3H) = 45.0 kcal/mol)4b rendered ΔGH-(1b) to be 48.8 ± 1.0
kcal/mol, which is comparable to that of the commonly used good
hydride donor 2,3-dihydrobenzo[d]imidazoles.1b The good hydridic
reactivity of 1b was illustrated in our recent work in reduction of
various unsaturated compounds.17 It is interesting to note that 1b is
about 13 kcal/mol stronger than 1a in hydricity, but is 0.24 V
weaker in electron-donating ability.
N
N
H
P
N
N
N
N
H
BF4
Pyridine
CH3CN
H
P
H BF4
H
P
CH3CN
iPr
iPr
iPr
1aH+
1a
1a
Scheme 4 Protonation of 1a by HBF4, and recovery of it by adding
pyridine in CD3CN.
The kinetics of the hydride transfer from 1a to A1+ was
conducted by following the decay of the absorption of A1+ in CH3CN
at 430 nm with stopped-flow spectrophotometer (Figure 2, see SI
for details). The second-order rate constant kHT(1a) was found to be
5.94 M-1 s-1 (Inset in Figure 2 and Table S1). Moreover, the
nucleophilicity of 1a can be estimated by the second-order rate
constant of the reaction of 1a with A1+. Because 1a has a similar
structure with 1b, based on the nucleophile-specific sensitivity
parameter sN of 1b (0.52) and the electrophilicity of A1+ in our
recent work,17 the nucleophilicity N of 1a is estimated as 8.64.
Similarly, the kinetics of 1a-D with A1+ kHT(1a-D) was determined to
be 2.34 M-1 s-1 (Table S2). These gave a primary kinetic isotope
effect KIE [kHT(1a)/kHT(1a-D)] of 2.5. A comparison with the
previously determined kHT(1b) of 5.41 × 102 M-1 s-1 for the reaction
of 1b with A1+ in our earlier work17 indicates that 1b is about 100
times more reactive than 1a. This is exactly in accordance with the
order of their hydricities (ΔGH-) of 1a vs. 1b found here.
GH-
CH3CN
P H
(2)
(3)
H
P
1
AH
)
Grxn = -RTlnKeq = GH-( ) - GH-(
Me
iPr
iPr
Me
N
N
N
N
N
N
1a 0.48
( ) =
Keq
(4)
(5)
P
H
+
P
+
iPr
iPr
A2+
Me
1a-[P]+
1a
A2H
tBu
tBu
N
P
Me
N
1b 1.6*10-3
( ) =
N
Keq
N
P
H
+
+
N
N
N
N
tBu
Me
A3+
tBu
Me
A3H
1b-[P]+
1b
The hydricity can be used to rationalize the disparate reactivity
between 1a and some reported diazaphospholenium hydrides. The
latter compounds were reported to react with strong acid HBF4 to
give H2 and the corresponding phosphenium cations.15b However,
when 1a was treated with HBF4 (or HOTf), no gaseous product was
generated. Instead, a new phosphorus species with a doublet peak
at about 80 ppm was detected in 31P NMR spectrum (Figure S9a and
S9b). To identify this newly formed species, a base pyridine was
added to the reaction mixture, which rendered 1a as the primary
product with a small amount of unidentified impurities (Figure S9c).
Consequently, this mysterious P species was speculated to be
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins

Please do not adjust margins
Chemical Science
Page 4 of 8
ARTICLE
Journal Name
i.e. the ET path between 1a and the O• radical (Ered =V-i0ew.7A0rtiVcl5ebO) nclainne
DOI: 10.1039/C9SC05883D
also be excluded by the similar strategy on the basis of their
respective thermodynamics determined, that indicates the ET from
1a to O• is remarkably uphill (ΔGET = 21.4 kcal/mol, Eq. 9).13, 29 It is
thus safe to conclude that the initial HAT should be the rate-
determining step (RDS) for the reaction expressed in Eq. 6.
Figure 2. Monoexponential decay of the absorbance Abs (at 430 nm)
with the time t (s) for the reaction of 1a (3.03 × 10-3 M) with A1+ (5.00 ×
10-5 M) in CH3CN at 20 oC. Inset: Correlation of kobs with [1a].
Hydrogen-atom Transfer (HAT). To examine the possibility of a
HAT reaction from diazaphosphinane substrates 1, the reactions of
1a and 1b with the 2,4,6-tri-tert-butyl-phenoxyl radical O• were
conducted, and yielded 2,4,6-tri-tert-butylphenol OH and the
corresponding phosphorus species (Eq. 6) without generation of
bisphosphines (compared to the reaction of 1a or 1b with AIBN,
Table 1). Stoichiometric study found a [1]/[O•] ratio of 1:2,
consistent with the product analysis (Figure S11 and S12).
Proposed reaction mechanism:
O
H
P
OH
iPr
P
iPr
iPr
iPr
tBu
tBu
N
N
N
N
tBu
tBu
3.8
kcal/mol
HAT
tBu
tBu
O
1a
1a-[P]
BDFE = 80.9 kcal/mol
BDFE = 77.1 kcal/mol
O
Ar
N
O
P
O
iPr
N
P
iPr
iPr
P
iPr
iPr
iPr
tBu
tBu
tBu
tBu
N
N
N
N
O
-28.6 kcal/mol
H
O
iPr
iPr tBu
tBu
tBu
ET
tBu
tBu
N
N
tBu
tBu
N
N
toluene
20 o
P
O
tBu
+
1a-[P]+
Radical coupling
+
H
(6)
2
P
1a-[P]
C
O
tBu
iPr
iPr tBu
tBu
OH
1a 1b
or
O
To
evaluate
the
hydrogen-atom
donability
of
Scheme 6. The possible mechanism for the reaction of 1a with O•. Ar =
2,4,6-tri-tert-butyl-phenyl group.
diazaphosphinanes (i.e., P-H BDFE), the thermodynamic cycle was
applied on the basis of the ΔGH- and Ered([P]+) in hand (Scheme 5
and Eq. 7; FEox(H-) is -26.0 kcal/mol1c in acetonitrile vs. Fc0/+).26
Substituting the known values into Eq. 726a, 27 gives the P-H BDFEs of
80.9 ± 2.0 and 77.9 ± 2.0 kcal/mol for 1a and 1b, respectively, being
comparable with those of phenol and thiophenol antioxidants.
Therefore, they could transfer hydrogen-atom smoothly to
acceptors with X-H BDFEs around or larger than 78 kcal/mol, such
as 2,4,6-tri-tert-butyl-phenoxyl O• (77.1 kcal/mol), CN(CH3)2C•
PT
(8)
(9)
OH
O
1a-[P]
1a
1a
1a
O
O
ET
21.4
kcal/mol
The rate constants of this hydrogen-atom transfers from 1a and
1a-D to radical O• were measured to be kHAT(1a) of 0.70 M-1 s-1 and
kHAT(1a-D) of 2.24 × 10-2 M-1 s-1, respectively (Tables S3 and 4). This
intermediately points out an extremely large KIE(1a) of 31.3,
implying a tunneling effect in this HAT process. Such a large KIE has
been found in biochemical30 and transition-metal systems31, but
seldom observed in organic HAT processes. Similarly, the rate of the
analogous HAT of 1b kHAT(1b) was determined as 0.74 M-1 s-1 (Table
S5). The quite close kHAT for 1a and 1b agrees well with their
comparable P-H BDFEs (80.9 vs. 77.9 kcal/mol, vide supra), verifying
their similar HAT reactivity. However, the same kinetic study for 1b-
D gave a kHAT(1b-D) of 0.12 M-1 s-1 (Table S6), rendering a primary
KIE(1b) of 6.2 with no tunneling phenomenon detected.
(about 88 kcal/mol) and phenyl radicals (104.7 kcal/mol).5b,
28
Compared to 1b, though the fused aryl groups attenuate the
hydricity of 1a by 13 kcal/mol thermodynamically, they show
negligible effect on the HAT reactivity.
BDFE
[P]
H
1a 1b
or
-
]
)
-
GH
(H
ox
E
-
+
)
([P]
red
E
[
-F
[P]
H
To understand this discrepancy, the temperature-dependent
kinetics were further performed (Tables S7-10). The ratio of
Arrhenius pre-factors [A(1a)/A(1a-D)] was obtained as 0.021, and
the activation energy difference [Ea(1a) - Ea(1a-D)] was -5.53
kcal/mol (Figure S2 and Table S11). These kinetic characteristics
Scheme 5. Thermodynamic cycle for deriving the P-H BDFEs of 1a and
1b.
BDFE = ΔGH- – F(Ered([P]+) – Eox(H-))
= ΔGH- – 23.06Ered([P]+) – 26.0
(kcal/mol)
(7)
The reaction mechanism of 1a with radical O• (Eq. 6) was [A(1a)/A(1a-D) << 1, and Ea(1a) - Ea(1a-D) << 0] are in fact in line
proposed in Scheme 6 based on the derived thermodynamic data with the tunneling model illustrated by Klinman,30a which features a
(the reaction of 1b with O• is similar). Since hydrogen-atom transfer “through-the-barrier” transition state near the top of the classical
from 1a to O• is slightly endothermic (~ 4 kcal/mol), it implies the barrier. In the 1b system, on the other hand, the normal primary KIE
possibility for a reversible HAT. This reversible HAT could be further of 6.2, A(1b)/A(1b-D) of 3.39 and [Ea(1b) - Ea(1b-D)] of -0.38
driven to the far right by the largely exothermic electron transfer ET kcal/mol all support a classical “over-the-barrier” transition state. A
of 1a-[P]• radical (Eox(1a-[P]•) = -1.94 V) or a follow-up radical more comprehensive investigation of the tunneling issue is
coupling. The observed stoichiometric ratio of 1:2 for the reactants presently underway.
[1] vs. [O•] is in exact accordance with the HAT mechanism
Proton Transfer (PT). It’s known that a good hydride donor
proposed. On the other hand, a suspected proton transfer between usually exhibits poor acidity. This explains why the acidity of widely-
1a and the O• radical can be easily ruled out due to the very weak used X-H type hydride donors, e.g. 2,3-dihydrobenzo[d]-imidazoles,
acidity of 1a (vide infra) and strong acidity (pKa of ~ -3)5b reported have never been determined in the absence of metal-coordination.
for the phenol radical cation (Eq. 8). The other suspected possibility, The hydricity of the present N-heterocyclic phosphines is also quite
4 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 5 of 8
Chemical Science
Please do not adjust margins
Journal Name
ARTICLE
good (vide supra), hence investigation of their acidic reactivity “protic” solvent as illustrated here may serve as aVniewexAartmiclepOlenlionef
15a
DOI: 10.1039/C9SC05883D
cannot be expected too straightforward. Nevertheless, as learned umpolung which was also proposed by Gudat et al. for P-H bond.
from the initial failures in deprotonating 1a or 1b with some
ordinary strong bases, such as 1,8-diazabicyclo[5,4,0]-7-undecene
Table 1. Applications of 1a and 1b in organic syntheses.
Type
Product
CN
Yield
Reactant
Condition
1a-[P]+
Entry
(1)
(DBU, pKa
pyrimido[1,2-a]pyrimidine (TBD, pKa
=
24.34 in CH3CN), 1,3,4,6,7,8-hexahydro-2H-
26.03) and (tert-
(15 mol%),
=
CN
40%[a]
HT
HBpin (1.5 eq.)
CH3CN, 80 oC, 36 h
butylimino)tris(pyrrolidino)-phosphorane (BTPP, pKa = 28.42)28 (see
SI for details), we finally found that the H/D exchange of the 1a P-H
N
N
Bpin
iPr
iPr
t
proton in the presence of stoichiometric BuOK in CD3CN (Eq. 10
N
N
N
N
(2)
Quant.[b]
tBuOK, CD3CN
20 oC, 10 min.
PT
D
P
H
P
and Figure S13) was accomplished in about 10 minutes at room
temperature,32 whereas the same operation for 1a-D in CH3CN
finished in 8 hours at room temperature (Eq. 11 and Figure S14).
The reversible H/D exchange of 1a in acetonitrile under the
iPr
iPr
1a-D
1a
iPr
iPr
N
N
AIBN (1.5 eq.),C6D6
80 oC, 3 h
H
> 90%[b]
(3)
(4)
t
P
HAT
P
promotion of BuOK suggested that the acidity of 1a reaches the
N
N
2
iPr
iPr
measurable acidity limitation in acetonitrile (see SI for details).33 As
for 1b with much stronger hydricity, its acidity is, unfortunately, too
weak to examine in solution by any existing experiment. However,
it is worth noting that the acidity of a P-H bond can be significantly
enhanced upon activation by coordination with transition metals34
or other Lewis acids.35
1
1-[P2]
Br
1b
(1.5 eq.),
HAT
&
ET
> 90%[c]
AIBN 15 mol%
toluene-d8, 90 oC, 5 h
[a] Isolated yield. [b] NMR yields determined by the amount of phosphorus species. [c] NMR
yeilds with 1,3,5-trimethoxybenzene as the internal standard.
Since bisphosphines can easily decompose into phosphinyl
radicals,38 they could serve as radical reservoirs for relevant studies
and applications. Based on the thermodynamic analysis, CN(CH3)2C•
(C-H BDFE ≈ 85 kcal/mol), generated from homolysis of azo-bis-
isobutyronitrile (AIBN), could abstract the hydrogen-atom from 1a
(P-H BDFE = 80.9 kcal/mol). Then, the generated phosphinyl radical
1a-[P]• coupled quickly with each other to yield bisphosphines
(entry 3 in Table 1, Figure S16 for 1a and Figure S17 for 1b). The
targeted dimers39 are readily extracted from reaction mixtures in a
high yield. This transformation, together with Gudat’s
photochemical dehydrocoupling,18 offers an easy access to
bisphosphines, avoiding complicated and harsh operations and
workups required in other synthetic processes using metals
iPr
iPr
N
N
N
N
t
(1.2 equiv.)
BuOK
H
P
(10)
CD3CN
D
P
10 minutes
iPr
iPr
1a
1a-D
iPr
iPr
tBuOK(2.0 equiv.)
8 hours
N
N
N
N
CH3CN
D
P
(11)
H
P
iPr
iPr
1a-D
1a
Applications in syntheses. Based on the above thermodynamic and
kinetic investigations, the potential of applying 1a and 1b in organic
syntheses, encompassing the diverse hydrogen (H+, H• and H-) or
electron transfer, was preliminarily explored and showcased herein.
sodium38d and magnesium38a-c
Furthermore, we attempted to design
.
Catalytic reduction of pyridines is
a prevail strategy for
a
new radical
synthesis of dihydropyridines. Recently, Kinjo et. al.16g and Speed et.
al.36 exploited 1,3,2-diazaphosphenium-catalyzed hydroboration of
pyridines with good regio- and chemo-selectivity.16g However, in
their system, the substrate 3-CN-pyridine gave a mixture of three
dihydropyridine isomers. To our delight, we found that under the
catalysis of 15 mol% our 1a-[P]+,37 the sole 1,4-hydroborated isomer
of this reaction was selectively produced in moderate yield 40%
(entry 1 in Table 1 and Figure S15). The good region-selectivity may
be ascribed to the steric factors of isopropyl group on the nitrogen
atom of 1a based on Kinjo’s DFT calculations.16g This good regio-
selectivity exhibits the catalytic potential of 1a in the reduction of
unsaturated compounds.
transformation by an integrated use of hydrogen-atom and electron
transfers from N-heterocyclic phosphines. This was inspired by the
reactions of AIBN with 1a and 1b, where phosphinyl radicals can be
easily generated from their P-H precursors through hydrogen-atom
release. The very negative oxidation potentials of phosphinyl
radicals imply their competency in the reduction of challenging
substrates. If a substrate in the reaction system is sufficiently
oxidative, its single-electron capture from phosphinyl radicals may
be able to compete with radical coupling. In such cases, subsequent
transformations may be triggered. Based on the redox potentials of
1b-[P]• (Eox = -2.39 V vs. Fc+/0, the same with the Ered(1b-[P]+)) and
bromobenzene (Ered
=
-2.8 V)40, hydrodehalogenation of
Moreover, the acidic character of 1a (vide supra) offers a more
economic way for the synthesis of deuterated reagent 1a-D (entry 2
in Table 1) by employing the much cheaper deuterated solvent
CD3CN instead of expensive LiAlD4 in almost quantitative yield (see
SI for details about the synthetic method). Meanwhile, it also
provides a new approach for isotope labeling of many other
hydridic species. Generation of the hydridic hydrogen from a
bromobenzene by 1b was performed in toluene with 15 mol% AIBN
as the initiator (entry 4 in Table 1, Figure S18). After 5 hours, the
expected reduction product benzene was obtained in over 90%
yield. Due to the poor reduction ability of 1a-[P]•, the reaction
could not proceed when 1a was used.41 Therefore, it could be
anticipated that these N-heterocyclic phosphines may open a
promising avenue for the development of super electron donors.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
Please do not adjust margins

Please do not adjust margins
Chemical Science
Page 6 of 8
ARTICLE
Journal Name
Ramirez, K. M. Light and M. T. Kieber-Emmons,VJie.wAAmrti.cleCOhnelmine.
Conclusions
Soc., 2017, 139, 18448-18451.
DOI: 10.1039/C9SC05883D
Hydrogen and electron transfers from phosphines 1a and 1b
6. (a) Z. Li, X. Li and J.-P. Cheng, J. Org. Chem., 2017, 82, 9675-
9681; (b) C. Mao, Z. Wang, Z. Wang, P. Ji and J.-P. Cheng, J. Am.
Chem. Soc., 2016, 138, 5523-5526.
7. (a) D. Richter and H. Mayr, Angew. Chem. Int. Ed., 2009, 48,
1958-1961; (b) V. Morozova, P. Mayer and G. Berionni, Angew.
Chem. Int. Ed., 2015, 54, 14508-14512; (c) M. Horn, L. H.
Schappele, G. Lang-Wittkowski, H. Mayr and A. R. Ofial, Chem.
Eur. J., 2013, 19, 249-263; (d) H. Mayr, G. Lang and A. R. Ofial, J.
Am. Chem. Soc., 2002, 124, 4076-4083.
8. (a) R. H. Morris, Chem. Rev., 2016, 116, 8588-8654; (b) M. E.
Ahmed, S. Chattopadhyay, L. Wang, D. Brazzolotto, D. Pramanik,
D. Aldakov, J. Fize, A. Morozan, M. Gennari, C. Duboc, A. Dey
and V. Artero, Angew. Chem. Int. Ed., 2018, 57, 16001-16004.
9. (a) R. Zhong, Z. Wei, W. Zhang, S. Liu and Q. Liu, Chem, 2019, 5,
1552-1566; (b) W. Ai, R. Zhong, X. Liu and Q. Liu, Chem. Rev.,
2019, 119, 2876-2953.
10. (a) Y. Zu, R. J. Shannon and J. Hirst, J. Am. Chem. Soc., 2003, 125,
6020-6021; (b) A. Bucci, S. Dunn, G. Bellachioma, G. Menendez
Rodriguez, C. Zuccaccia, C. Nervi and A. Macchioni, ACS Catal.,
2017, 7, 7788-7796.
were thermodynamically and kinetically investigated in acetonitrile.
The P-H bond of 1a can participate in reactions involving formal
transfer of a hydride/hydrogen-atom/proton to a substrate under
respective conditions in the absence of the usually required
transition-metal- or Lewis acid-mediation. It was observed that 1a is
a moderate hydride and hydrogen-atom donor, but a poor Brønsted
acid. For comparison, 1b is a good hydride donor and moderate
hydrogen-atom donor; but did not show a detectable acidic
reactivity even under strongest basic solution. As regard to electron
donability, 1a (Eox(1a) = 0.23 V) is superior to 1b (Eox(1b) = 0.47 V),
contrasting to the order of their hydridic reactivity. Their
corresponding phosphinyl radicals are very strong electron donors
as reflected by their extremely negative redox potentials. Kinetic
studies revealed a tunneling contribution (KIE = 31.3) to the
hydrogen-atom transfer from 1a to the phenoxy radical. Based on
the derived energetic parameters, their synthetic applications were
tentatively exploited. Preliminary results from the synthetic
attempts by using the new reagents 1a and 1b confirmed their
competence as effective hydrogen and electron donors in various
redox transformations. The effectiveness of applying the relevant
physicochemical parameters in analysis and design of hydrogen
transfer reactions was also convinced.
11. (a) Q.-A. Chen, M.-W. Chen, C.-B. Yu, L. Shi, D.-S. Wang, Y. Yang
and Y.-G. Zhou, J. Am. Chem. Soc., 2011, 133, 16432-16435; (b)
V. N. Wakchaure, P. S. J. Kaib, M. Leutzsch and B. List, Angew.
Chem. Int. Ed., 2015, 54, 11852-11856.
12. (a) D. E. Berning, B. C. Noll and D. L. DuBois, J. Am. Chem. Soc.,
1999, 121, 11432-11447; (b) J. R. Norton and J. Sowa, Chem.
Rev., 2016, 116, 8315-8317.
Conflicts of interest
There are no conflicts to declare.
13. E. A. McLoughlin, K. M. Waldie, S. Ramakrishnan and R. M.
Waymouth, J. Am. Chem. Soc., 2018, 140, 13233-13241.
14. (a) W. S. Matthews, J. E. Bares, J. E. Bartmess, F. G. Bordwell, F.
J. Cornforth, G. E. Drucker, Z. Margolin, R. J. McCallum, G. J.
McCollum and N. R. Vanier, J. Am. Chem. Soc., 1975, 97, 7006-
7014; (b) X.-M. Zhang, J. W. Bruno and E. Enyinnaya, J. Org.
Chem., 1998, 63, 4671-4678; (c) M. Finn, R. Friedline, N. K.
Suleman, C. J. Wohl and J. M. Tanko, J. Am. Chem. Soc., 2004,
126, 7578-7584; (d) K. Xia, G.-B. Shen and X.-Q. Zhu, Org.
Biomol. Chem., 2015, 13, 6255-6268.
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).
15. (a) D. Gudat, A. Haghverdi and M. Nieger, Angew. Chem. Int.
Ed., 2000, 39, 3084-3086; (b) S. Burck, D. Gudat, M. Nieger and
W.-W. Du Mont, J. Am. Chem. Soc., 2006, 128, 3946-3955.
16. (a) C. C. Chong, B. Rao and R. Kinjo, ACS Catal., 2017, 7, 5814-
5819; (b) M. R. Adams, C. H. Tien, B. S. N. Huchenski, M. J.
Ferguson and A. W. H. Speed, Angew. Chem. Int. Ed., 2017, 56,
6268-6271; (c) C. C. Chong, H. Hirao and R. Kinjo, Angew. Chem.
Int. Ed., 2014, 53, 3342-3346; (d) C. C. Chong, H. Hirao and R.
Kinjo, Angew. Chem. Int. Ed., 2015, 54, 190-194; (e) C. C. Chong
and R. Kinjo, Angew. Chem. Int. Ed., 2015, 54, 12116-12120; (f)
A. M. R., T. Chieh‐Hung, M. Robert and S. A. W. H., Angew.
Chem. Int. Ed., 2017, 56, 16660-16663; (g) B. Rao, C. C. Chong
and R. Kinjo, J. Am. Chem. Soc., 2018, 140, 652-656; (h) T.
Lundrigan, E. N. Welsh, T. Hynes, C.-H. Tien, M. R. Adams, K. R.
Roy, K. N. Robertson and A. W. H. Speed, J. Am. Chem. Soc.,
2019, 141, 14083-14088.
Notes and references
1. (a) J.-D. Yang, J. Xue and J.-P. Cheng, Chem. Soc. Rev., 2019, 48,
2913-2926; (b) S. Ilic, A. Alherz, C. B. Musgrave and K. D. Glusac,
Chem. Soc. Rev., 2018, 47, 2809-2836; (c) E. S. Wiedner, M. B.
Chambers, C. L. Pitman, R. M. Bullock, A. J. M. Miller and A. M.
Appel, Chem. Rev., 2016, 116, 8655-8692.
2. (a) A. Wu and J. M. Mayer, J. Am. Chem. Soc., 2008, 130, 14745-
14754; (b) T. Matsuo and J. M. Mayer, Inorg. Chem., 2005, 44,
2150-2158; (c) J. M. Mayer, Acc. Chem. Res., 2011, 44, 36-46.
3. (a) Z. Wang, F. Gao, P. Ji and J.-P. Cheng, Chem. Sci., 2018, 9,
3538-3543; (b) K. Kaupmees, N. Tolstoluzhsky, S. Raja, M.
Rueping and I. Leito, Angew. Chem. Int. Ed., 2013, 52, 11569-
11572; (c) J.-D. Yang, P. Ji, X.-S. Xue and J.-P. Cheng, J. Am.
Chem. Soc., 2018, 140, 8611-8623.
4. (a) S. Ilic, U. Pandey Kadel, Y. Basdogan, J. A. Keith and K. D.
Glusac, J. Am. Chem. Soc., 2018, 140, 4569-4579; (b) X.-Q. Zhu,
M.-T. Zhang, A. Yu, C.-H. Wang and J.-P. Cheng, J. Am. Chem.
Soc., 2008, 130, 2501-2516.
5. (a) M. J. Bezdek and P. J. Chirik, Angew. Chem. Int. Ed., 2018, 57,
2224-2228; (b) J. J. Warren, T. A. Tronic and J. M. Mayer, Chem.
Rev., 2010, 110, 6961-7001; (c) G. Ali, P. E. VanNatta, D. A.
17. J. Zhang, J.-D. Yang and J.-P. Cheng, Angew. Chem. Int. Ed.,
2019, 58, 5983-5987.
18. O. Puntigam, L. Könczöl, L. Nyulászi and D. Gudat, Angew.
Chem. Int. Ed., 2015, 54, 11567-11571.
19. R. B. King and P. M. Sundaram, J. Org. Chem., 1984, 49, 1784-
1789.
6 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 7 of 8
Chemical Science
Please do not adjust margins
Journal Name
ARTICLE
20. Single crystal of 1a was obtained by volatilization of solution of
1a in hexane at -30 oC. Due to the air- and moisture- sensitive of
Majhi, A. W. Kyri, A. Schmer, G. Schnakenburg and R. Streubel,
View Article Online
Chem. Eur. J., 2016, 22, 15413-15419.
DOI: 10.1039/C9SC05883D
this compound, the single crystal could not be fine sealed when 35. (a) A. Longeau and P. Knochel, Tetrahedron Lett., 1996, 37,
X-ray analyzing in the cool air and the minor compound 1a
6099-6102; (b) M. Blum, J. Kappler, S. H. Schlindwein, M. Nieger
(about 10%) was oxidized. And in case of the 10% by product,
and D. Gudat, Dalton Trans., 2018, 47, 112-119.
the obtained crystals are of insufficient qualities despite several 36. T. Hynes, E. N. Welsh, R. McDonald, M. J. Ferguson and A. W. H.
attempts. Thus the quality of the acquired diffraction data was Speed, Organometallics, 2018, 37, 841-844.
slightly below the average level (see the attached CIF file for 37. Although phosphenium triflate 1a-[P]+ (as hydride acceptor) is
details). Despite that, the present X-ray diffraction data
confirmed the chemical structure of 1a unambiguously.
21. H. A. Spinney, I. Korobkov, G. A. DiLabio, G. P. A. Yap and D. S.
Richeson, Organometallics, 2007, 26, 4972-4982.
used as a catalyst in the hydride transfer reaction (entry 1 in
Table 1), generation of P-H 1a from 1a-[P]+ is the initial step of
the catalytic cycle. Then, the pyridine is reduced by in-situ
generated 1a. The hydride donor 1a and acceptor 1a-[P]+
22. (a) S. S. Hanson, E. Doni, K. T. Traboulsee, G. Coulthard, J. A.
Murphy and C. A. Dyker, Angew. Chem. Int. Ed., 2015, 54,
11236-11239; (b) J. A. Murphy, T. A. Khan, S.-z. Zhou, D. W.
coexist
in
the
reaction
system.
Their
hydride-
donating/accepting abilities (hydricity) are crucial in the
catalytic cycle.
Thomson and M. Mahesh, Angew. Chem. Int. Ed., 2005, 44, 38. (a) O. Puntigam, D. Förster, N. A. Giffin, S. Burck, J. Bender, F.
1356-1360; (c) J. A. Murphy, S.-z. Zhou, D. W. Thomson, F.
Schoenebeck, M. Mahesh, S. R. Park, T. Tuttle and L. E. A.
Berlouis, Angew. Chem. Int. Ed., 2007, 46, 5178-5183; (d) L.
Zhang and L. Jiao, Chem. Sci., 2018, 9, 2711-2722.
Ehret, A. D. Hendsbee, M. Nieger, J. D. Masuda and D. Gudat,
Eur. J. Inorg. Chem., 2013, 2013, 2041-2050; (b) R. Edge, R. J.
Less, E. J. L. McInnes, K. Müther, V. Naseri, J. M. Rawson and D.
S. Wright, Chem. Commun., 2009, 1691-1693; (c) D. Förster, H.
Dilger, F. Ehret, M. Nieger and D. Gudat, Eur. J. Inorg. Chem.,
2012, 2012, 3989-3994; (d) N. A. Giffin, A. D. Hendsbee, T. L.
Roemmele, M. D. Lumsden, C. C. Pye and J. D. Masuda, Inorg.
Chem., 2012, 51, 11837-11850; (e) J.-P. Bezombes, K. B.
Borisenko, P. B. Hitchcock, M. F. Lappert, J. E. Nycz, D. W. H.
Rankin and H. E. Robertson, Dalton Trans., 2004, 1980-1988; (f)
A. Dumitrescu, V. L. Rudzevich, V. D. Romanenko, A. Mari, W.
W. Schoeller, D. Bourissou and G. Bertrand, Inorg. Chem., 2004,
43, 6546-6548; (g) M. Blum, O. Puntigam, S. Plebst, F. Ehret, J.
Bender, M. Nieger and D. Gudat, Dalton Trans., 2016, 45, 1987-
1997.
23. (a) S. Hoffmann, A. M. Seayad and B. List, Angew. Chem. Int. Ed.,
2005, 44, 7424-7427; (b) R. I. Storer, D. E. Carrera, Y. Ni and D.
W. C. MacMillan, J. Am. Chem. Soc., 2006, 128, 84-86; (c) Q.-A.
Chen, K. Gao, Y. Duan, Z.-S. Ye, L. Shi, Y. Yang and Y.-G. Zhou, J.
Am. Chem. Soc., 2012, 134, 2442-2448.
24. C. Zhu, K. Saito, M. Yamanaka and T. Akiyama, Acc. Chem. Res.,
2015, 48, 388-398.
25. (a) R. P. Thummel, W. E. Cravey and D. B. Cantu, J. Org. Chem.,
1980, 45, 1633-1637; (b) I. Chatterjee and M. Oestreich, Org.
Lett., 2016, 18, 2463-2466.
26. (a) V. D. Parker, K. L. Handoo, F. Roness and M. Tilset, J. Am.
Chem. Soc., 1991, 113, 7493-7498; (b) M. Tilset and V. D. Parker, 39. E. E. Nifantiev, N. S. Vyazankin, S. F. Sorokina, L. A. Vorobieva,
J. Am. Chem. Soc, 1989, 111, 6711-6717.
27. F. G. Bordwell, J. P. Cheng and J. A. Harrelson, J. Am. Chem. Soc.,
1988, 110, 1229-1231.
O. A. Vyazankina, D. A. Bravo-Zhivotovsky and A. R. Bekker, J.
Organomet. Chem., 1984, 277, 211-225.
40. (a) C. Costentin, M. Robert and J.-M. Savéant, J. Am. Chem. Soc,
2004, 126, 16051-16057; (b) L. Pause, M. Robert and J.-M.
Savéant, J. Am. Chem. Soc., 1999, 121, 7158-7159.
28. J.-P. Cheng, et al., Internet Bond-energy Databank (iBonD)
Home Page: http://ibond.nankai.edu.cn, accessed at Oct. 2019.
29. (a) The energetics of electron transfer (ET) from 1a (Eox = 0.23 V) 41. DFT calculations showed 1a-[P]• and 1b-[P]• should have a
to the O• radical (Ered = -0.70 V) shows that its thermodynamic
driving force is remarkably uphill (ΔGET = 21.4 kcal/mol),
rendering its equilibrium constant close to 0. Such an
unfavorable thermodynamic driving force prohibits electron
transfer to be a feasible path; (b) J.-P. Cheng, Y. Lu, X. Zhu and L.
Mu, J. Org. Chem., 1998, 63, 6108-6114.
comparable ability (with an energy difference of 1.3 kcal/mol,
see SI for details) in abstracting bromine atom. This failed to
explain the disparate yields of <10% for 1a-[P]• and 90% for 1b-
[P]•. Besides, according to the redox potentials of 1b-[P]• (Eox = -
2.39 V vs. Fc in MeCN) and bromobenzene (Ered = -2.8 V), the
electron transfer from 1b-[P]• to bromobenzene is a feasible
reversible process, while that for 1a-[P]• (Eox = -1.94 V) is
thermodynamically prohibited. These findings preferentially
support an ET-initiated mechanism rather than a direct bromine
abstraction.
30. (a) J. P. Klinman and A. R. Offenbacher, Acc. Chem. Res., 2018,
51, 1966-1974; (b) J. C. Price, E. W. Barr, T. E. Glass, C. Krebs and
J. M. Bollinger, J. Am. Chem. Soc., 2003, 125, 13008-13009; (c) J.
P. Layfield and S. Hammes-Schiffer, Chem. Rev., 2014, 114,
3466-3494; (d) M. A. Ehudin, D. A. Quist and K. D. Karlin, J. Am.
Chem. Soc., 2019, 141, 12558-12569.
31. (a) E. J. Klinker, S. Shaik, H. Hirao and L. Que Jr, Angew. Chem.
Int. Ed., 2009, 48, 1291-1295; (b) S. H. Bae, X.-X. Li, M. S. Seo, Y.-
M. Lee, S. Fukuzumi and W. Nam, J. Am. Chem. Soc., 2019, 141,
7675-7679.
32. When the same reaction was performed in MeOH and toluene
solutions (with other conditions identical), none of the H/D
exchange was observed.
33. A. Kütt, S. Selberg, I. Kaljurand, S. Tshepelevitsh, A. Heering, A.
Darnell, K. Kaupmees, M. Piirsalu and I. Leito, Tetrahedron Lett.,
2018, 59, 3738-3748.
34. (a) J.-J. Brunet, R. Chauvin, O. Diallo, B. Donnadieu, J. Jaffart and
D. Neibecker, J. Organomet. Chem., 1998, 570, 195-200; (b) P. K.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
Please do not adjust margins

Please do not adjust margins
Chemical Science
Page 8 of 8
View Article Online
DOI: 10.1039/C9SC05883D
ARTICLE
R
N
P
62.2
48.8
kcal
kcal)
(
+
H
N
hydride transfer
R
iPr
R
P
R
R
N
P
N
R
N
N
N
N
electron transfer
H
H/D exchange
P
H
H
+
+
H
proton transfer
0.23 0.47
)
Eox
=
(
iPr
1a
1b
V vs. Fc+/0
(
)
R
P
R
80.9
77.9
kcal
kcal)
N
N
R = iPr or tBu
(
hydrogen-atom
transfer
Tunneling!
-1.94 -2.39
) V
Eox
=
(
Electron donor
“3-in-1” diazaphospholenium hydride reagent: A new 1,3,2-diazaphosphinane, which can serve as a formal hydride, hydrogen-atom or proton donor
without transition-metal mediation was exploited thermodynamically and kinetically. Kinetic studies imply a tunneling effect for the hydrogen-atom transfer
process. The extremely low oxidation potentials of corresponding phosphinyl radicals enable them to serve as super electron donors in organic syntheses.
And, the very promising potentials in versatile syntheses have been successfully demonstrated.
Please do not adjust margins