Powerful bispyridinylidene organic reducing agents with iminophosphorano π-donor substituents

Article abstract of DOI:10.1002/chem.201500809

Four members of a new family of powerful bispyridinylidene organic reducing agents have been prepared, which exploit iminophosphorano (-N=PR₃; R=Ph, Cy) π-donor substituents. Electrochemical studies show exceptionally high oxidation potentials, ranging from 1.30 to 1.51 V versus SCE. These new reductants were shown to effectively convert 1-bromonaphthalene to naphthalene under mild reaction conditions. From the redox potentials, substituent constants (σ_p⁺) for the iminophosphorano groups Ph₃P=N- (-1.82) and Cy₃P=N- (-2.21) were determined, demonstrating their superior π-donating properties compared to traditional amino substituents. Extra push: Iminophosphorano groups are utilized to significantly increase the reducing power in a new class of neutral, ground state, organic reducing agents (2; see scheme). In comparative reductions involving reductants of type 1 and 2, the latter allow for shorter reaction times and milder conditions. The superb π-donating properties of the studied iminophosphorano groups were quantified by the determination of their Hammett substituent constants.

Full text of DOI:10.1002/chem.201500809

DOI: 10.1002/chem.201500809
Communication
&
Substituent Effects
Powerful Bispyridinylidene Organic Reducing Agents with
Iminophosphorano p-Donor Substituents
Samuel S. Hanson, Nicholas A. Richard, and C. Adam Dyker*[a]
Abstract: Four members of a new family of powerful bis-
pyridinylidene organic reducing agents have been pre-
pared, which exploit iminophosphorano (ÀN=PR3; R=Ph,
Cy) p-donor substituents. Electrochemical studies show
exceptionally high oxidation potentials, ranging from 1.30
to 1.51 V versus SCE. These new reductants were shown
to effectively convert 1-bromonaphthalene to naphtha-
lene under mild reaction conditions. From the redox po-
tentials, substituent constants (sp+) for the iminophos-
phorano groups Ph3P=NÀ (À1.82) and Cy3P=NÀ (À2.21)
were determined, demonstrating their superior p-donat-
ing properties compared to traditional amino substitu-
ents.
reductions involve the use of excess reducing agent and re-
quire longer reaction times (typically 72 h).[3]
Organic super electron donors,[1–4] as introduced by Murphy
and co-workers,[5–7] are a class of ground-state, neutral organic
molecules having exceptionally positive oxidation potentials.
These electron donors are soluble,[8] tunable and should com-
plement traditional heterogeneous metal-based reducing
agents in that they can offer alternate reaction conditions (in-
cluding the absence of metallic byproducts) or unique selectiv-
ity.[9] The most powerful and intensively studied organic reduc-
ing agents are compounds A (À1.20 V vs. SCE),[5,10–12] and bis-
pyridinylidenes Ba (À1.24 V vs. SCE) and Ca (À1.27 V vs.
SCE).[2,3,7,13–19] These ground-state electron donors have been
used in the reduction of organic substrates such as aryl hal-
ides,[5,7,13,20] sulfones and arenesulfonamides,[7,10] Weinreb
amides,[16] acyloin derivatives,[17] triflates and triflamides.[18]
In certain reductions, however, these reagents are ineffective
even at elevated temperatures, providing trace amounts of the
reduced products or complete recovery of unreacted starting
materials.[3,10,15,21] The desire to reduce these challenging sub-
strates has led Murphy and co-workers to utilize photoexcita-
tion to boost the reducing power of the reagents. Indeed,
upon photoactivation, Ba is able to reduce difficult substrates
such as activated benzenes, N,N-dialkyl arenesulfonamides,
benzylic esters and ethers, benzyl malonates and cyanoace-
tates,[15,19,21,22] which could not be reduced by Ba in the
ground state. Unfortunately, the majority of the photoassisted
To fully realize the utility of organic reducing agents, stron-
ger electron donors are required, which will not only allow for
more convenient reaction conditions (no photoexcitation, stoi-
chiometric reductions, mild temperatures, shorter reaction
times) but should also open the doorway to the reduction of
more difficult substrates. Here we report on the facile prepara-
tion and characterization of more powerful neutral ground-
state bispyridinylidene donors by exploiting iminophosphora-
no (ÀN=PR3; R=Ph, Cy) substituents. The utility of these new
reagents is also highlighted by the mild reduction of 1-bromo-
naphthalene.
Braunschweig et al.[23] have recently prepared diborene D
with a one-electron redox event occurring at À1.95 versus
Fc+/Fc [À1.39 V vs. SCE]. Nevertheless, new scaffolds that
should be stronger electron donors are not obvious. The syn-
thesis and characterization of derivatives of Ba/Ca has led to
the conclusion that significantly more powerful donors cannot
be attained by modifying either the bridge between the endo-
cyclic N-centers, or the exocyclic 4,4’-amino substituents.[14] Ul-
timately, this led Murphy to prepare tricyclic species E, with
a record redox potential of À1.50 V versus SCE.[24] Undesirably
this compound requires six synthetic steps, and is finally pre-
pared by a reduction with Na/Hg amalgam which complicates
its use as an in situ-generated reductant (in contrast to the
simple two-step synthesis of Ba which ends with a convenient
deprotonation). Interestingly, Clennan et al. have shown that
the nature of the 4,4’-substituent does play a significant role in
the reduction potential of a family of 4,4’-disubstituted-2,2’-bi-
pyridinium ions (C2+ a–e),[25,26] with redox potentials spanning
1 V and being most negative for stronger p-donor groups
(OMe, NMe2). This leads to the conclusion that incorporating
[a] S. S. Hanson, N. A. Richard, Prof. Dr. C. A. Dyker
Department of Chemistry, University of New Brunswick
Fredericton, New Brunswick, E3B 5A3 (Canada)
E-mail: cadyker@unb.ca
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201500809.
Chem. Eur. J. 2015, 21, 8052 – 8055
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Communication
substituents more electron donating than amino groups into
the 4,4’-positions, should lead to more powerful bispyridinyli-
dene reducing agents.
a correlation between the NÀCH3 protons and those of the CH
group adjacent to the double bond, indicative of an E arrange-
ment, only in the case of the minor isomer. This is consistent
with the closer values of the 31P{1H} NMR chemical shifts for 4a
(À4.2 ppm) and 4b (15.5 ppm), with fixed Z arrangements, to
those of the major isomers of 5a (À4.3 ppm) and 5b
(15.8 ppm), respectively. It is also in agreement with recent
theoretical studies on isomers of Ca–e which predict the
Z isomer to be more stable in all cases.[26]
Although tables of substituent constants[27,28] indicate that
amino groups are the premier p-donor groups, we were aware
of some theoretical and experimental evidence that N-bound
iminophosphorano groups (R3P=NÀ)[29–33] should be regarded
as stronger electron-donating substituents. Indeed, calculations
by Schoeller have shown a greater stabilization of the singlet
state of the corresponding phosphanylnitrenes (R2PÀN) for R=
H3P=NÀ over R=H2NÀ.[34,35] In addition, whereas diamino-
Although the propylene-bridged compounds 4a and 4b
1
were formed quantitatively in solution, as indicated by H and
chloroboranes [(R2N)2BCl] are covalent species with discrete BÀ 31P{1H} NMR monitoring of their reaction mixtures (see the Sup-
Cl bonds,[36] the isolation of the chloride salt of a dicoordinate
borinium cation [tBu3P=N)2B][Cl] by Stephan et al.[29] demon-
strates that the iminophosphorano functionalities, but not
amino groups, are capable of electronically satisfying the
boron center such that it does not require a bond with chlor-
ine.
porting Information), H, and 31P{1H} NMR spectra of the isolat-
ed solids showed only broadened out and nearly indiscernible
resonances. We attribute this to the formation of small
amounts of radical species, likely the respective radical cations,
by partial oxidation of the sample.[38] Similar observations in
NMR spectra have been made previously for Bb[39] and E.[24]
To further support our assignments, the compounds were
oxidized to their respective dichloride salts by their treatment
with hexachloroethane (Scheme 2). Pleasingly, even isolated
samples of 4a and 4b gave excellent isolated yields of the di-
chloride salts of the dications 4a2+ (95%) and 4b2+ (72%), re-
spectively, indicating a high purity of the isolated electron
donors despite the poorly resolved NMR spectra. Moreover,
along with high isolated yields (84 and 81%, respectively), the
single 31P{1H} NMR resonances in each case for 5a2+ and 5b2+
at 16.5 and 37.7 ppm, respectively, match closely with the sig-
nals for the similarly substituted propylene-bridged derivatives,
4a2+ (16.8 ppm) and 4b2+ (37.8 ppm), thus confirming that
5a, and 5b exists as two stereoisomers and not two separate
species.
1
We therefore sought to prepare 4,4’-iminophosphorano-sub-
stituted bispyridinylidenes 4a, 4b, 5a, and 5b, featuring aryl
(Ph) and alkyl groups (Cy) on phosphorus (Scheme 1). The imi-
nophosphorano group is easily introduced by the treatment of
4-aminopyridine with the corresponding dihalophosphorane
(Ph3PBr2 or Cy3PCl2, generated in situ) to form the iminophos-
phorano-substituted pyridines 1.[37] Subsequently, in a proce-
dure that parallels Murphy’s original protocol,[7,13] the imino-
phosphorano-substituted pyridines 1 were alkylated with 1,3-
diiodopropane or iodomethane to obtain the pyridinium ions
2 and 3, respectively (Scheme 1). Deprotonation of the C2 po-
sitions of these cations with potassium bis(trimethylsilyl)amide
(KHMDS) cleanly afforded the desired compounds. After work-
up, 4a, 4b, 5a, and 5b were isolated in 82, 86, 34, and 83%
yields, respectively, where the low yield of 5a is attributed to
its low solubility in toluene. Despite their sensitivity to air and
moisture, the compounds have a half-life of one year when
stored in an inert atmosphere glovebox.
Interestingly, although only one isomer of Ca was report-
ed,[13] we observed a mixture of diastereoisomers for 5a (1:
2.2) and 5b (1:3.9), with 31P{1H} NMR shifts at À1.5 and
À4.3 ppm (5a) and 18.7 and 15.8 ppm (5b), corresponding to
the minor and major isomers, respectively. In each case,
a NOESY experiment (see the Supporting Information) showed
Scheme 2. Chemical oxidation of organic electron donors 4a,b and 5a,b.
In order to determine the strength of these new organic
electron donors 4a, 4b, 5a, and 5b, and compare them to re-
lated compounds (Table 1), cyclic voltammetric studies (see the
Supporting Information) were carried out on their respective
dications under the same conditions as used by Murphy et al.
Dichlorides of 5a2+ and 5b2+ exhibit a single, reversible two-
electron processes at À1.34 and À1.51 V versus SCE, respec-
tively. The propylene-bridged salts of 4a2+ and 4b2+ exhibit
two distinct but reversible one-electron redox processes at
À1.36 and À1.23 V versus SCE and À1.50 and À1.39 V versus
Scheme 1. Synthesis of iminophosphorano-substituted organic electron
donors 4a,b and 5a,b.
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Communication
SCE, respectively, implying the formation of intermediate radi-
cal cations under the conditions investigated. It is worth
noting that 5b is the strongest neutral organic electron donor
yet reported, being 10 mV more reducing than tricyclic E.
Table 1. Redox potentials of selected electron donors[a] and their corre-
sponding substituent constants.
+
Donor Substituents ÀE11/2 ÀE2
ÀE1/2
sp
1/2
[V]
[V]
[V]
4b
5b
4a
5a
Ba
Ca
Cb
Cf
ÀN=PCy3
ÀN=PCy3
ÀN=PPh3
ÀN=PPh3
ÀNMe2
1.50
–
1.36
–
–
–
–
–
–
1.39
–
1.23
–
–
–
–
–
–
1.45
1.51
1.30
1.34
1.24
À2.21
À2.21
À1.82 (À1.65)[27]
À1.82 (À1.65)[27]
À1.70[27]
+
Figure 1. Plot of Hammett’s sp constants (labels correspond to the sub-
stituents) against the reduction potential of bipyridinium ions Ca2+–Cb2+
,
Cf2+–Ch2+
.
ÀNMe2
1.29 (1.27)[14] À1.70[27]
ÀOMe
0.94
1.35
0.83
1.19
À0.78[27]
À1.80[28]
À0.60[27]
À1.51[28]
ÀN(CH2)4
ÀSMe
represents the strongest neutral p-donating substituent yet re-
ported.
Cg
Ch
morpholino
–
–
Although the redox potentials indicate that 4a, 4b, 5a, and
5b should be strong reducing agents, their use in this capacity
required validation. Although these donors are isolable, which
may be required for some reduction reactions, we also recog-
nized that it is most convenient to generate and use these
donors in situ from their more easily handled precursors. We
therefore tested derivatives 5a and 5b, generated from
KHMDS and 3a or 3b, respectively, in the reduction of 1-bro-
monaphthalene (Scheme 3, Table 2) under various conditions
of time and temperature. The outcome was compared to that
of similar reductions featuring donor Ca, generated in situ
from KHMDS and N-methyl-4-(dimethlyamino)pyridinium
iodide (6).
[a] Scan rate of 50 mVsÀ1, 0.1m [Bu4N][PF6] in DMF as supporting electro-
lyte, at room temperature under inert atmosphere. E1/2 values correspond
to the two-electron neutral/dication couple and were determined directly
or calculated from the two single-electron processes. Literature values are
in parenthesis.
From the electrochemical data, it can be seen that the
methyl derivatives Ca, 5a and 5b are consistently stronger
donors than the identically substituted propylene-bridged de-
rivatives Ba, 4a and 4b, respectively, by 40–60 mV for the
two-electron process. More importantly, substituting the dime-
thylamino group in Ba and Ca by a Ph3P=NÀ group in 4a
and 5a, respectively, leads to an increase in oxidation potential
of 50–60 mV. More dramatically, changing from triphenyl (4a,
5a) to tricyclohexyliminophosphorano (Cy3P=NÀ) groups (4b,
5b) gave an increase of 150–170 mV, which is 210–220 mV
stronger than the respective dimethylamino-substituted Ba
and Ca. It is worth noting that even small improvements in ox-
idation potential for these two-electron donors result in large
changes in the equilibrium constant for a given reduction reac-
tion. Thus, a room temperature reduction with 5a and 5b is
expected to be 2400 and 1.36”109 times more favorable, re-
spectively, than the same reduction employing Ba.
The results of cyclic voltammetric studies clearly demon-
strate the superior electron-donating ability of the iminophos-
phorano substituents over the amino groups. This is in con-
trast to previous reports on the substituent constant (sp+) for
Ph3P=NÀ which showed it to be a weaker donor than NMe2
(À1.65 vs. À1.70, respectively).[27,40,41] We thus sought to deter-
mine substituent constants based on the redox potentials for
a family of derivatives of C2+, which were chosen over B2+ be-
cause of the single two-electron redox event in the former. We
prepared derivatives Ca2+(X=NMe2), Cb2+ (X=OMe), Cf2+
(X=N(CH2)4, pyrollidinyl), Cg2+ (X=SMe), and Ch2+ (X=
O(CH2CH2)2N, morpholino) and plotted their redox potentials
versus the known substituent constants for X (Figure 1,
Table 1), which allowed us to determine the substituent con-
stants for Ph3P=NÀ (sp+ =À1.82) and Cy3P=NÀ (sp+ =À2.21)
Scheme 3. Scheme for the reduction of 1-bromonaphthalene by in situ gen-
erated organic electron donors.
Donor A has been used to reduce 1-bromonaphthalene to
naphthalene in 86% yield at elevated temperatures (Table 2),[5]
but no other organic electron donor has been reported to
effect this reduction. Because of the long reaction time report-
ed with donor A, and simple work-up procedure, we chose
this process to test if our more powerful donors could give im-
proved yields under milder conditions or shorter reaction
times, as generally predicted by Marcus theory. Gratifyingly, 1-
bromonaphthalene was reduced to naphthalene by 5a in an
isolated yield of 94% after only 2 h at 1008C. Under similar re-
action conditions, dimethylamino-derived bispyridinylidene Ca
gave the reduced product in 86%, with 14% unreacted 1-bro-
monaphthalene recovered. After 4 h at 508C, donors Ca and
5a showed only 15 and 35% conversions, respectively; howev-
er, tricyclohexyliminophosphorano-based donor 5b, which is
220 and 170 mV stronger than Ca and 5a, respectively, was
able to reduce 1-bromonaphthalene to naphthalene in 94%
isolated yield under the same conditions. Remarkably, 5b also
effectively reduced 1-bromonaphthalene at room temperature
based on their redox potentials. To our knowledge, Cy3P=NÀ in a yield of 93% after 20 h. Monitoring the reductions by
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Chem. Eur. J. 2015, 21, 8052 – 8055
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Communication
31P{1H} NMR spectroscopy showed that the mixtures were com-
plex, but dications 5a2+ and 5b2+ were identified as the most
abundant single product. Nevertheless, these experiments
clearly demonstrate the feasibility of these new reagents as
powerful organic reducing agents, and the shorter reaction
times and milder conditions highlight the importance of their
increased oxidation potentials.
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A
–
1.5
2
2
2
2
110
100
50
100
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18
2
4
2
4
86[5]
84[b]
14[b]
94[a]
34[b]
94[a]
93[b]
Ca
Ca
5a
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5b
5b
ÀNMe2
ÀNMe2
ÀN=PPh3
ÀN=PPh3
ÀN=PCy3
ÀN=PCy3
2
2
50
RT[c]
4
20
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In summary, we have described four members of a new
class of strong organic reducing agents featuring iminophos-
phorano substituents. These compounds are easy to prepare
and are amongst the best organic donors to date, as evi-
denced by their redox potentials as well as their use in the
high-yielding and mild reduction of 1-bromonaphthalene. Fur-
ther exploration of their reduction chemistry is underway. We
note that the strength of these donors is very sensitive to the
R group on phosphorus, which opens up an accessible means
for further tuning the redox potential of these iminophosphor-
ano-substituted bispyridinylidenes, Moreover, the determina-
tion of the substituent constants for Ph3P=NÀ and Cy3P=NÀ
should encourage more general exploitation of these powerful
p-donor substituents.
Acknowledgements
We are grateful for the support of NSERC of Canada, the
Canada Foundation for Innovation, the New Brunswick Innova-
tion Foundation, and the Harrison McCain Foundation. S.S.H. is
grateful to the Government of Akwa Ibom State, Nigeria for fi-
nancial support. We thank Dr. Larry Calhoun and P. M. Bradley
Scott for their assistance with NMR spectroscopy and reduction
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Keywords: electron transfer · iminophosphorane · organic
reductant · reduction · substituent effects
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Received: February 27, 2015
Published online on April 15, 2015
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8055
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim