A Nucleophilicity Scale for the Reactivity of Diazaphospholenium Hydrides: Structural Insights and Synthetic Applications

Article abstract of DOI:10.1002/anie.201901456

Nucleophilicity parameters (N, s_N) of a group of representative diazaphospholenium hydrides were derived by kinetic investigations of their hydride transfer to a series of reference electrophiles with known electrophilicity (E) values, using the Mayr equation log k₂=s_N(N+E). The N scale covers over ten N units, ranging from the most reactive hydride donor (N=25.5) to the least of the scale (N=13.5). This discloses the highest N value ever quantified in terms of Mayr's nucleophilicity scales reported for neutral transition-metal-free hydride donors and implies an exceptional reactivity of this reagent. Even the least reactive hydride donor of this series is still a better hydride donor than those of many other nucleophiles such as the C?H, B?H, Si?H and transition-metal M?H hydride donors. Structure–reactivity analysis reveals that the outstanding hydricity of 2-H-1,3,2-diazaphospholene benefits from the unsaturated skeleton.

Full text of DOI:10.1002/anie.201901456

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Accepted Article
Title: Nucleophilicity Scale for the Reactivity of Diazaphospholenium
Hydrides: Structural Insights and Synthetic Applications
Authors: Jingjing Zhang, Jin-Dong Yang, and Jin-Pei Cheng
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201901456
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10.1002/anie.201901456
Angewandte Chemie International Edition
COMMUNICATION
Nucleophilicity Scale for the Reactivity of Diazaphospholenium
Hydrides: Structural Insights and Synthetic Applications
Jingjing Zhang, Jin-Dong Yang,* and Jin-Pei Cheng*
Abstract: Nucleophilicity parameters (N, sN) of a group of
representative diazaphospholenium hydrides were derived by
kinetic investigations of their hydride transfer to a series of
reference electrophiles with known electrophilicity (E) values,
using the well-established Mayr equation logk2 = sN(N + E). The
achieved N scale covers over ten N units, ranging from the most
reactive hydride donor (N = 25.5) to the least of the scale (N =
13.5). This immediately discloses the highest N value ever
quantified in terms of Mayr’s nucleophilicity scales reported
previously for neutral transition metal-free hydride donors, and
hence, implies an exceptional reactivity of this reagent. Even the
least reactive hydride donor of this series is still a better hydride
donor than those of many other popular types of nucleophiles
such as the C-H, B-H, Si-H and transition metal M-H hydride
donors. Structure–reactivity analysis reveals that 2-H-1,3,2-
diazaphospholene’s outstanding hydricity is benefited from the
unsaturated skeleton.
reagents is, inevitably, hampered by a bottleneck, that is, lack of
quantitative understanding of the structure-reactivity
a
relationship. In this regard, and also, based on our expertise in
this area,[11] we have carried out a systematic kinetic study,
aiming to establish a reactivity scale for the typical group of
diazaphospholenium hydrides illustrated in Figure 1. This scale
can then be used to quantify their hydride-donating abilities and
disclose structural insights behind the data that is meaningful for
guiding further investigation. Hence, here in this work, the first
quantitative measurements of the nucleophilicity (N) parameters
of the P-H hydrides have been conducted, based on the Mayr
equation [eq. (1)].
Contrasting to the well-known protic reactivity of conventional P-
H bonds,[1] 2-H-1,3,2-diazaphospholene 1a (Figure 1) exhibits a
peculiar hydridic propensity, as endowed by its unique
diazaphospholene skeleton.[2] This unexpected P-H bond
philicity leads to an inverted regioselectivity and opens new
applications of phosphines in organic synthesis. Gudat and co-
workers’ seminal work demonstrated that 1a is a sort of super-
hydride,[3] yet with common features of organic reagents,[4] such
as relatively low cost and good solubility in organic media. These
advantages make synthetic applications of the P-H hydrides a
burgeoning field. Consequently, a number of N-heterocyclic
phosphorus hydrides (Figure 1) and their analogues have been
developed in recent years, rendering many originally infeasible
reactions to occur under metal-free conditions, such as
reductions of polar olefins,[5] imines,[5a] ketones[6] and
azocompounds,[7] valorization of CO2,[8] and hydroboration of
pyridines[9]. Moreover, their chiral analogs (like 1g) were also
exploited in catalysis of asymmetric hydride-initiated
transformations with high enantioselectivity.[10] In these
processes, the propensity of P-H species to cleavage their P-H
bonds dominates the feasibility of the reaction.
Figure 1. Commonly used diazaphospholenes 1 with Mes = 2,4,6-Me3C6H2
and Dipp = 2,6-iPr2C6H3.
According to Mayr’s three-parameter equation [eq. (1)],[12]
the hydridic reactivity of these P-H reagents 1a-f can be
described by the nucleophilicity parameters N, together with a
nucleophile-specific sensitivity factor sN, derived from their rate
constants with a series of reference electrophiles of known
eletrophilic parameters (E). In the present work, benzhydrylium
ions[13] and quinone methides[14] were taken as the electrophiles,
whose E vaues and the respective maximum absorptions (λmax
)
are presented in Table 1.
logk2 (20 °C) = sN(N + E)
(1)
-
Table 1. Benzhydrylium ions 2a–c (BF4 salts) and quinone methides 2d-i
employed as reference electrophiles in this work.
Despite the wide applications of this new group of strong
hydrides in syntheses, further extending their utility to find new
synthetic methodologies or to explore new highly efficient P-H
J. Zhang, Dr. J.-D. Yang, Prof. Dr. J.-P. Cheng
Center of Basic Molecular Science, Department of Chemistry,
Tsinghua University, Beijing 100084, China.
J.-D. Yang ORCID: 0000-0001-7351-2152
E-mail: jinpei_cheng@mail.tsinghua.edu.cn
jdyang@mail.tsinghua.edu.cn
Prof. Dr. J.-P. Cheng
State Key Laboratory of Elemento-organic Chemistry, College of
Chemistry, Nankai University, Tianjin 300071, China.
E-mail: chengjp@nankai.edu.cn
Supporting information for this article is given via a link at the end
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Diazaphospholenes 1a-f were prepared according to
values of hydrides 1 are generally in a range of 0.34 ~ 0.68,
smaller than the previously reported values for C-H, B-H and Si-
H hydride donors (sN > 0.6).[17] In fact, such small sN has only
been previously found in extremely reactive nucleophilic
systems,[16] suggesting an insensitivity (i.e., low selectivity) of
highly strong hydride donors to electrophiles. Nevertheless, 1b
gives a somewhat greater sN (0.68), due most likely to a less
steric repulsion of 1b to benzhydrylium ion 2c than to quinone
methides. As a consequence, the hydride transfer from 1b to 2c
should be faster. This leads to a larger slope of the fitted plot of
logk2 vs E for 1b (Table S10). As for the reactions of 1c, the very
high congestion of its bulky Dipp groups makes it even less
sensitive to distinguish the steric difference between
benzhydrylium ions and quinone methides, resulting in a
smallest sN value.
3, 15]
literature[2a,
or our modified procedures (see Supporting
Information, SI). Representative reaction outcomes can be found
in Scheme 1 (for others, see SI). Benzhydrylium ion 2c was
observed to react with 1b-f quantitatively in CH3CN to furnish the
corresponding diarylmethane. The reactions of quinone methide
2d with 1d and 1f also gave the desired diarylmethane in fairly
high yields. Similar results were obtained for the reactions of 2h
with 1a and 1b.
Table 2. Second-order rate constants k2 for the reactions of reagents 1 with
reference electrophiles 2 in CH3CN at 20 °C . Mes = 2,4,6-Me3C6H2 and Dipp =
2,6-iPr2C6H3.
exp
k2calcd/k2
Nucleophiles
N (sN)
k2exp (M-1 s-1)
2e
2f
(1.57±0.06)×104
(8.70±0.19)×103
(1.95±0.06)×103
(4.40±0.13)×102
1.13
0.94
1.03
1.07
25.54
(0.35)
Scheme 1. Isolated yields for the reactions of 1 with reference electrophiles in
CH3CN. Details are shown in SI.
2h
2i
The kinetics of the reactions of 1a-f with benzhydrylium ions
2a-c or quinone methides 2d-i was investigated by following the
disappearance of the UV/Vis absorptions of the reference
electrophiles under pseudo-first-order conditions ([1]0/[2]0 > 10),
using stopped-flow spectrophotometer (Figure 2, for others see
SI). The second-order rate constants k2, i.e., the slopes of the
plots for the first-order rate constants kobs vs the concentrations
of nucleophiles 1, are presented in Table 2. It is worth
mentioning that 1b, 1c and 1e reacted much more rapidly with 2i
than expected (see more details in the SI).
2c
2d
2g
2h
(7.28±0.56)×105
(2.14±0.05)×103
(8.41±0.19)×101
(2.30±0.05)×101
0.54
2.57
1.72
0.51
17.68
(0.68)
2b
(1.25±0.05)×104
1.09
19.85
(0.34)
2c
2d
2c
2d
2e
2h
(3.08±0.17)×103
(3.69±0.02)×102
(2.18±0.09)×104
(1.40±0.06)×103
(4.46±0.04)×102
(1.68±0.06)×101
1.11
1.10
1.06
0.86
0.73
1.02
18.74
(0.47)
2d
(6.21±0.20)×103
0.93
20.93
(0.43)
2g
2h
2a
2b
2c
2d
2e
(5.55±0.18)×102
(1.30±0.05)×102
(2.05±0.08)×103
(8.39±0.31)×102
(1.63±0.04)×102
3.56±0.17
1.05
0.91
1.09
1.19
0.75
1.30
1.05
13.46
(0.52)
1.04±0.04
Figure 2. Monoexponential decay of the absorbance Abs (at 612 nm) with
time for the reaction of 2b (1.10 × 10-5 M) with 1f (3.70 × 10-3 M) in CH3CN at
20 °C. Inset: Correlation of kobs with the concentrations of 1f.
For the purpose of comparison, the nucleophilicity
parameters N of P-H hydrides 1a-e of this work, together with
those of some conventional hydride donors, are drwan in Figure
Plots of logk2 vs E values (Table 1) for the reactions of 1 with
electrophiles 2 were linear, allowing the nucleophile-specific
sensitivity parameters (sN) and the nucleophilic parameters (N)
of hydrides 1 to be evaluated from the slopes and horizontal
4. The
N value of the six-membered N-hetercyclic P-H
compound 1f[15] was also investigated in this work and included
in Figure 4 for the same purpose. It can be seen from the right
side of Figure 4 that the P-H centered N scale covers over ten N
units, ranking as 1a (25.5) >> 1b-e (20.9-17.7) >> 1f (13.5).
Note that most of the P-H hydrides examined, except for 1f, are
much more reactive than those commonly used NaBH4,
dihydropyridines and metal hydrides, with 1a (N = 25.54) as the
intercepts (Figure 3 and Table 2), respectively, by using eq. (1).
exp
As shown in Table 2, experimental rate constants k2
agree
calcd
well with the k2
predicted from eq. (1) using the E, N and sN
values in Tables 1 and 2, within a factor (k2calcd/k2exp) of 3 (note
that factor < 100 is regarded a reasonable agreement[16]). The sN
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10.1002/anie.201901456
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most nucleophilic donor ever quantified by eq. (1). The super
hydricity of 1a was already confirmed by Gudat et al., who found
that 1a can directly transfer a hydride to aldehydes and
diarylketones (within two minutes), and even to alkylketones (E
= -22.30), although rather slowly.[3] The powerfulness of 1a was
also evidenced in other applications such as in catalytic
criterion of E + N > -3. In other words, to expect a hydride-
transfer reaction from P-H reagents to certain electrophiles to
occur, one has to select the electrophile (see left side of Figure
4) with an E value that allows the sum of (E + N) to be larger
than -3.
Due to their superior reactivity, all P-H hydrides investigated
here would be expected to react readily with common oxidants
(e.g., trityl cation and DDQ) and iminium ions. This has been
verified by the experiments demonstrated in Table 3, where
three P-H reagents of different structural features (1a, 1d, and
1f) were allowed to react with representative electrophiles at
20 °C in CH3CN. As predicted by eq. (1), the super-hydride 1a
can react with all electrophiles in Figure 4, including the
extremely inert α,β-unsaturated esters 3e and 3f (E = -22.77 and
-24.52), with quantitative conversion.
[8]
reductions of imines,[5a] azocompounds,[7] pyridines[9a] and CO2
.
According to the nucleophilicity parameters disclosed here, 1a
should be roughly 102 times more reactive than 1d (N = 18.74).
This is obviously due to the aromaticity of the resulted 1a
phosphenium cation.
On the other hand, the aryl groups either on the N atom or in
a benzannulated moiety, like in 1b, 1c and 1e, appeared to
attenuate the P-H nucleophilicity of the corresponding
phospholenes compared to 1a (N = 25.54), depressing their N
values down to 17.68, 19.85, and 20.93, respectively. Indeed, it
is convinced by the observed lower catalytic reactivity of 1b than
1a in imine reduction.[5a] As for the six-membered ring hydride 1f
(N = 13.46), it shows the least nucleophilicity in this series,
which is close to that of NaBH4 but still much more reactive than
the renowned biomimetic and the metal hydrides depicted in
Figure 4. These structure–reactivity analyses indicate that the
incubated aromaticity should be the primary factor to the
exceptional nucleophilicity of 1a.
Figure 3. Plots of logk2 for the reactions of reference electrophiles 2a-i with 1a
and 1c-f against the electrophilicity parameters E (see Table S10 for 1b).
To apply the N parameters to examine the feasibility of a
reaction, one can refer to a recent example specified for
fluoromethylthiolating reagents.[11f] As showed therewith,
a
reaction system with E + N = -3, as estimated from eq. (1)
assuming the sensitivity parameter sN close to unity, can
proceed in a rate around 10-3 M-1 s-1 at 20 °C. This value of -3
can be taken as an empirical criterion for judging whether or not
a non-catalytic reaction can occur spontaneously at room
temperature.[11f] However, in the present work, since the sN
values are in the range of 0.34 ~ 0.68, the original term in Mayr
equation, i.e., sN(E + N), instead of the simplified one E + N,
should be used for predicting the rate constants. Rate constants
between 10-2 ~ 10-1 M-1 s-1 can be expected for electrophiles and
nucleophiles on the same level of Figure 4. Thus, one can
expect that hydrides 1 will be able to react smoothly with a
particular electrophile provided that the reaction can satisfy the
Figure 4. Comparision of nucleophilicity parameters N of 1a-f with those of
other hydride donors and predicting the scope of reactions of 1a-f with carbon-
centered electrophiles.[18]
As observed, the reactions of the relatively weak donor 1d
with α,β-unsaturated esters rendered a sluggish process with a
yield less than 10% after 3 h, which is basically in accordance
with the prediction based on eq. (1) [sN(N + E) = -1.93 for methyl
methacrylate 3e, and -2.72 for methyl cinnamate 3f]. Table 3
also revealed that the electrophiles on a higher position in the
left of Figure 4 (e.g., N-methylacridinium 3a, MacMillan-iminium
3b, 2-benzylidene-indan-1,3-dione 3c, active imine 3d, and 2h)
could react smoothly with 1d (yields > 80%). Also, as can be
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10.1002/anie.201901456
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COMMUNICATION
understood, the 1a (N = 25.54) and 1b (N = 17.68) P-H agents,
which are generated in-situ, were reported to be able to catalyze
the imine reduction by pinacolborane [HB(pin)] in 98% and 25%
yields, respectively.[5a] The different performances of 1f between
the reactions with stronger electrophiles (such as iminium ions,
active imines and quinone methides) and with less electrophilic
α,β-unsaturated esters can be easily understood by following the
same line of argument.
Acknowledgements
We are grateful for the financial grants from National Natural
Science Foundation of China (Nos. 21672124, 21602116,
21772098, 91745101), and Tsinghua University Initiative
Scientific
Research
Program
(Nos.
20131080083,
20141081295).
Surprisingly, the analogous reaction of 1d derivative with a
similar N value as 1b (18.74 vs 17.68) was reported incapable.
This discrepancy in catalytic performance could, however, not be
simply viewed as a collapse of the above mentioned judging-
rule. Instead, it may be used as a handy tool for probing other
undetected factors at work in complex systems. Although
attempts to untangle similar puzzles occasionally appeared in
literature,[5a, 9a] more intensive research in future to identify the
insights behind abnormal observations should be highly
recommended.
Keywords: P-H hydride • diazaphospholene • nucleophilicity
parameter • linear free-energy relationship • structure–reactivity
relationship
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CH3CN at 20 °C . Reaction conditions: 1 (0.2 mmol), nucleophiles (0.2 mmol),
CH3CN or CD3CN (1 mL), 20 °C, 30 min.
[3]
[4]
[5]
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Electrophile
E
1a
1d
1f
Product
-7.15
>90%[a]
>90%[a]
>90%[a]
-8.00
90%[b]
92%[b]
81%[b]
87%[b]
87%[b]
[6]
[7]
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[c]
-11.32
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[8]
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[c]
-15.09
+
>90%[a]
55%[a,f]
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-22.77[g]
-24.52[g]
88%[b]
Quant.[d]
>90%[a]
87%[b]
70%[a,f]
<10%[a,e,f]
0
<10%[a,e,f]
0
[a] Yields were determined by 1H NMR analysis using CH2Br2 as the internal
standard. [b] Isolated yields. [c] Reactions expected, but not tested. [d] From
reference 5b. [e] With some unidentified side products. [f] Reaction time: 3 h.
[g] Estimated values from similar structures in Figure 4.
Internet
Bond-energy
Databank
(iBonD)
Home
Page:
http://ibond.nankai.edu.cn or http://ibond.chem.tsinghua.edu.cn.
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Besides the product analyses, kinetic examinations (k2) of
the reactions of diazaphospholenes 1 with several electrophiles
3 (Table S36) were also performed, which appeared to match
the values predicted by eq. (1).
In summary, the first P-H type nucleophilicity scale in terms
of the Mayr’s N and sN values for the diazaphospholenium ions 1
was established to be in an N range of 13.5 ~ 25.5, with N of
25.5 for 1a as the most nucleophilic hydride donor ever
quantified by the Mayr equation. Successful applications of the
empirical criterion of E + N > -3 verified the good value of these
parameters in rationalizing experimental observations and
predicting feasibility of reactions, adding confidence for their use
[15] R. B. King, P. M. Sundaram, J. Org. Chem. 1984, 49, 1784-1789.
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[18] For a comprehensive database of electrophilicity parameters E and
nucleophilicity
parameters
N
and
sN,
see:
as
a
guiding index in the development of new
http://www.cup.lmu.de/oc/mayr/DBintro.html.
diazaphospholenium-like
reagents
and
the
relevant
transformations.
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Entry for the Table of Contents
Layout 2:
COMMUNICATION
Jingjing Zhang, Jin-Dong
Yang*, Jin-Pei Cheng*
Page No. – Page No.
Nucleophilicity Scale for the
Reactivity of Diazaphospholenium
Hydrides: Structural Insights and
Synthetic Applications
P-H super-hydrides: The unexpected philicity of P-H bonds in diazaphospholenes has made their applications to catalytic reductions a
burgeoning field. Herein, we used a three-parameter kinetic equation to evaluate their nucleophilicity parameters (N), which cover over ten N
units. Kinetic studies imply their much superior hydricity over commonly-used hydrides, with 2-H-1,3,2-diazaphospholene being the strongest
nucleophilic donor ever quantified by Mayr equation. These reactivity parameters are beneficial to the rational design of diazaphospholene-
involved reactions.
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