Evaluation of the Synthetic Scope and the Reaction Pathways of Proton-Coupled Electron Transfer with Redox-Active Guanidines in C?H Activation Processes

Article abstract of DOI:10.1002/chem.202003424

Proton-coupled electron transfer (PCET) is currently intensively studied because of its importance in synthetic chemistry and biology. In recent years it was shown that redox-active guanidines are capable PCET reagents for the selective oxidation of organic molecules. In this work, the scope of their PCET reactivity regarding reactions that involve C?H activation is explored and kinetic studies carried out to disclose the reaction mechanisms. Organic molecules with potential up to 1.2 V vs. ferrocenium/ferrocene are efficiently oxidized. Reactions are initiated by electron transfer, followed by slow proton transfer from an electron-transfer equilibrium.

Full text of DOI:10.1002/chem.202003424

Accepted Article
Accepted Article
Title: Evaluation of the Synthetic Scope and the Reaction Pathways of
Proton-Coupled Electron Transfer with Redox-Active Guanidines
in C-H Activation Processes
Authors: Ute Wild, Petra Walter, Olaf Hübner, Elisabeth Kaifer, and
Hans-Jörg Himmel
This manuscript has been accepted after peer review and appears as an
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To be cited as: Chem. Eur. J. 10.1002/chem.202003424
Link to VoR: https://doi.org/10.1002/chem.202003424
01/2020

10.1002/chem.202003424
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Evaluation of the Synthetic Scope and the Reaction Pathways of Proton-
Coupled Electron Transfer with Redox-Active Guanidines in C-H Activation
Processes
Ute Wild, Petra Walter, Olaf Hübner, Elisabeth Kaifer, Hans-Jörg Himmel*
[a]
U. Wild, P. Walter, Dr. O. Hübner, Dr. E. Kaifer, Prof. H.-J. Himmel
Institut für Anorganische Chemie
Ruprecht-Karls-Universität Heidelberg
Im Neuenheimer Feld 270, 69120 Heidelberg
E-mail: hans-jorg.himmel@aci.uni-heidelberg.de
Supporting information for this article is given via a link at the end of the document.
[14,15]
of charges.
Conceptual work demonstrated how electron
Abstract: Proton-coupled electron transfer (PCET) is currently
intensively studied due to its importance in synthetic chemistry and
biology. In recent years it was shown that redox-active guanidines are
capable PCET reagents for the selective oxidation of organic
molecules. In this work the scope of their PCET reactivity regarding
reactions that involve C−H activation is explored and kinetic studies
carried out to disclose the reaction mechanisms. Organic molecules
with potential up to 1.2 V vs. ferrocenium/ferrocene are efficiently
oxidized. Reactions are initiated by electron transfer, followed by slow
proton transfer from an electron-transfer equilibrium.
transfer could initiate proton movement over large distances.
Hence, first proton shuttles were designed in which the transfer of
several protons within hydrogen-bonds is triggered by electron
transfer. [16,17] Hydrogen-bonding triggered by redox processes
could be employed in sensor devices, and in hydrogen-bonded
molecular shuttles that might be usable as a basis for artificial
molecular machines.
Hence, in theory an extensive, rational and in some areas
biomimetic use of organic PCET reagents in synthetic chemistry
is now possible. However, progress does not only depend on the
detailed knowledge of the mechanisms and the developments of
advanced concepts. Due to the various areas of use of PCET
reactions in modern synthetic chemistry, the availability of a larger
number of PCET compound classes is required. Many
applications still suffer from the liability of the known organic
PCET reagents to side reactions, prohibiting the formation or
diminishing the yield of the desired product. In stoichiometric
reactions, an excess of the PCET reagent is not seldomly required
to obtain good yields. High catalyst loadings are necessary in a
number of catalytic reactions, making them unattractive for large-
scale processes. Some reactions (e.g. Scholl-type aryl-aryl
coupling reactions) need to be carried out in highly acidic media,
that could initiate degradation of the PCET reagent or the organic
substrate. Photochemical applications are still limited due to the
relatively small number of available photoactive PCET reagents.
Finally, frequently employed quinones such as 2,3-dichloro-5,6-
Introduction
Proton-coupled electron transfer is of key importance for the
progress in modern synthetic chemistry,[ 1 , 2 ] as it allows e.g.
selective and green oxidation of organic substrates, including
C−H activation processes, carbon dioxide conversion or water
splitting. In the last decades, important advancements were made
concerning theory, concepts and synthetic applications of PCET,
helping to decode biological processes and paving the grounds
for new applications in synthetic chemistry.[1-
It was
8
]
systematically explored how the leveling effect, diminishing the
effect of derivatisations (by which the redox potential or the pKa
value could be tuned) on the PCET reactivity, could be
circumvented by the design of bidirectional PCET reactions, in
which the electron and proton are transferred to different
molecules or different sites of the same molecule.[
9
, 10 ]
Applications of bidirectional PCET in synthesis were
comprehensively reviewed by Knowles.[ 11 - 13 ] Detailed studies
showed how environmental effects (e.g. hydrogen-bonding, the
solvent polarity or the presence of acids) could be used to enable
or speed up PCET reactions. The accumulation of charges in
photoredox catalytic systems could be avoided by PCET, allowing
the accumulation of oxidative and reductive equivalents instead
dicyano-1,4-benzoquinone
(DDQ)
and
tetrachloro-1,4-
benzoquinone (chloranil, CA), are highly toxic. Therefore, it is
essential to develop new classes of PCET reagents to overcome
limitations concerning stability, reactivity and toxicity of
traditionally applied compounds. Numerous fields in synthetic
1
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chemistry could profit immensely from the developments of such
new PCET reagents.
In the last years, we developed redox-active guanidines as a new
class of capable PCET reagents.[18] We already studied to some
extend
the
PCET
chemistry
of
1,2,4,5-
tetrakis(tetramethylguanidino)-benzene (1). It is readily
accessible from commercially available 1,2,4,5-tetrakisamino-
benzene-tetrahydrochloride,[ 19 ] and a relatively strong electron
donor with a redox potential (E1/2 value) of -0.73 V vs.
ferrocenium/ferrocene for the redox couple 12+/1 in CH3CN
solution.[20] The compound looses two electrons at the same
potential. The free radical monocation 1·+ is unstable towards
disproportionation into 1 and 12+. Hence it is not formed in
mixtures of 1 and 12+. On the other hand, the radical monocationic
form is stable as bridging ligand in several dinuclear late-transition
metal complexes.[ 21 , 22 ] A number of salts of 12+ were fully
[23]
[24]
characterized, including 1(PF6)2
and 1(BF4)2
used in this
work, and 1(I3)2.[19] Also, the nitrogen-rich 1[N(CN)2]2 was
prepared,[25] that melts above 200 °C and decomposes smoothly
at 220 °C, demonstrating its excellent thermal stability. In the
dication 12+, the bond length between the carbons in 1 and 2
positions and that between the carbons in 4 and 5 positions (all
directly bound to guanidino groups) are considerably elongated,
in line with the Lewis structure in Figure 1. The twofold protonated,
reduced form (1+2H)2+ could be re-oxidized to the dication 12+ by
Figure 1. The three redox-active guanidines studied in this work and equations
to illustrate the PCET reactivity of 12+
.
Results and Discussion
dioxygen under mild conditions with
catalyst,[26,27] opening up the possibility to use 1 as redox-catalyst
for the green aerobic oxidation of variety of organic
a copper or cobalt
1) Expansion of the scope of PCET chemistry with 12+.
a
We decided to test first the oxidative coupling of N-ethylcarbazole
to N,N‘-diethyl-3,3‘-bicarbazole (Scheme 1). N-ethylcarbazole
exhibits an Eox value of 1.12 V vs. SCE,[30] translating into a value
of 0.66 V vs. Fc+/Fc.[31] Since previous experiments showed that
substrates with a redox potential of up to 0.77 V vs. Fc+/Fc could
be oxidized,[28] it was clear that the potential is not too high for a
reaction with initial electron transfer. In our experiments, one
equivalent of 1(BF4)2 or 1(ClO4)2 (see SI for synthesis and
characterization of this new salt) was reacted with N-
ethylcarbazole in the presence of 16 equivalents of HBF4·OEt2.
Acetonitrile was chosen as solvent, since the protonated, oxidized
guanidine (1+2H)4+ that forms immediately in the presence of a
strong acid, is not soluble in unpolar solvents such as CH2Cl2.
Indeed, near quantitative conversion (94% and 95%, respectively)
was obtained within 1 h reaction time. Recently, Venkatakrishnan
et al. showed that reaction of DDQ or CA with N-ethylcarbazole
gives quantitative yield (>99%) of the bicarbazole coupling
product in very short time when carried out in CH2Cl2 solution with
molecules.[27] The scope for stoichiometric PCET reactions with
12+ could be greatly extended in the presence of strong acids,[28]
leading in the first place to di-protonation of 12+ to give the
tetracation (1+2H)4+ (see Lewis structure in Figure 1)[29] with ca.
0.7 V higher oxidation potential.
In this work we elaborate on the PCET chemistry of the three
redox-active guanidines 1,2,4,5-tetrakis(tetramethylguanidino)-
benzene (1), 1,4-bis(tetramethylguanidino)-benzene (2) and the
new
compound
1,4-bis(N,N‘-dimethylethylene)guanidino-
benzene (3) shown in Figure 1, in PCET reactions that involve
C−H activation. The conversion was followed by NMR or UV-vis
spectroscopy, and the yields were estimated by NMR signal
integration. Please note that the equations shown in the following
do not account for protonation equilibria that are observed for the
reduced, protonated guanidines arising as products in these
reactions (see SI for NMR spectra of the protonated compounds).
Also, in the presence of strong acids all guanidino groups become
protonated.
32
, 33 ]
an excess of methanesulfonic acid.[
However, two
equivalents of the quinone (fourfold excess) had to be used; the
2
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yield decreased to 79% for CA and 87% for DDQ when only one
equivalent was used (like in our experiments). Moreover, the yield
decreased to 83% when the reaction was carried out in CH3CN
instead of CH2Cl2. Hence the results demonstrate that salts of 12+
are valuable alternatives to toxic CA or DDQ in aryl-aryl coupling
reactions.
filtration. Then the reduced, protonated (1+2H)2+ was re-oxidized
to 12+ by catalytic oxidation with dioxygen (see SI for details).
Scheme 2. Intramolecular oxidative coupling of 3,3’’-dimethoxy-3’,4’-dimethyl-
o-terphenyl to the corresponding dimethoxy-triphenylene. Conditions:
acetonitrile, 1.5 eq. 1(BF4)2, 20 eq. HBF4·OEt2, 10 min at 0 °C, 120 min at r.t.,
76% yield.
2) Further C-H bond cleavage reactions with salts of 12+.
Scheme 1. Oxidative intermolecular aryl-aryl coupling of N-ethylcarbazole to
N,N‘-diethyl-3,3‘-bicarbazole. Conditions: acetonitrile, 1 eq. 1(ClO4)2, 16 eq.
HBF4·OEt2, 15 min at 0 °C, 45 min at r.t., 95% yield.
Having demonstrated the scope of oxidative aryl-aryl coupling
reactions for substrates with potentials of up to 1.2 V vs. Fc+/Fc,
we next focussed on the oxidation of 1-benzyl-1,4-
dihydronicotinamide (BNAH), 10-methyl-9,10-dihydroacridane
(AcrH2) and 9,10-dihydroanthracene (AnH2) to systematically
study PCET reactions without and with addition of a strong acid.
Next we inspected the oxidative coupling of 3,3’’-dimethoxy-3’,4’-
dimethyl-o-terphenyl to 3,10-dimethoxy-6,7-dimethyltriphenylene
with salts of 12+, again in the presence of excess (ca. 20
equivalents) of HBF4·OEt2 (Scheme 2). With 1(BF4)2, a yield of
76% was obtained after 130 min reaction time; use of 1(PF6)2
10-Methyl-9,10-dihydroacridane (AcrH2) exhibits
a
bond
dissociation energy (BDE) of 308 kJ mol−1 in CH3CN solution,[36]
and an Eox value of 0.492 V vs. Fc+/Fc in CH3CN. With a reduction
potential (Ered) of -0.77 V vs. Fc+/Fc for 12+,[20] the energy gap Gel
for electron transfer between AcrH2 and 12+ is 1.26 V or 122 kJ
mol−1, exceeding clearly the assumed limit for conventional
electron transfer at standard conditions of ca. 1 V (96.5 kJ
mol−1).[37] Hence, 12+ is too weak to oxidize AcrH2. Since electron
transfer is supposed to be the first step of the reaction (see
discussion in section 5), the expectation is that no reaction takes
place. Indeed, only traces of N-methylacridinium (AcrH+) are
observed if the reaction is carried out in the absence of an acid.
On the other hand, a high product yield (90%) in 6.5 h reaction
time is obtained upon addition of 7 equivalents of HBF4·OEt2
(Scheme 3). Again, the acid instantly protonates the guanidine to
give the strong oxidant (1+2H)4+ (see Figure 1),[24] decreasing the
energy gap between the AcrH2 electron donor and the acceptor
to ca. 0.5 V, enabling electron-transfer as initial reaction step. The
further excess of acid warrents fast conversion. A small but
significant isotope effect was observed when AcrH2 was replaced
by AcrD2 (see SI), implying that the rate of proton transfer enters
into the overall reaction rate (see discussion below).
resulted in
a slightly lower yield of 71%. This reaction
demonstrates that salts of 12+ could oxidize compounds with
redox potentials of 1.2 V vs. Fc+/Fc, being thus at least similar in
their oxidizing capability to DDQ or CA. The strong acid leads to
double-protonation of 12+ to (1+2H)4+,[29] which is the oxidant in
this reaction. Previous quantum-chemical calculations indicate
that (1+2H)4+ is even a stronger oxidant in PCET reactions than
DDQ or CA.[28] The reaction requires the use of 1.5 equivalents of
12+. This can be explained by the considerably lower redox
potential of the triphenylene product (0.72 V vs. Fc+/Fc in CH2Cl2)
compared with the reactant (1.22 V vs. Fc+/Fc in CH2Cl2 for 3,3’’-
dimethoxy-3’,4’-dimethyl-o-terphenyl), leading to preferred
oxidation of the product by (1+2H)4+ to the radical monocation.
The formation of this radical cation leads to broad signals in the
NMR spectra, and therefore the reaction was quenched before
analysis. Oxidative coupling of 3,3’’-dimethoxy-3’,4’-dimethyl-o-
terphenyl could also be carried out with 1.5 equivalent of DDQ
and an excess of MeSO3H in CH2Cl2, giving quantitatively the
triphenylene coupling product in short time.[ 34 , 35 ] A detailed
analysis showed that this reaction follows a cation-radical
(electron transfer) mechanism rather than an arenium-ion (proton
transfer) mechanism.[35] Similarly, electron transfer between
(1+2H)4+ and 3,3’’-dimethoxy-3’,4’-dimethyl-o-terphenyl is
assumed to proceed proton transfer (see discussion in section 3).
Please note that for both reactions the guanidine PCET reagent
could be recycled from the reaction mixture. For this purpose it
was first separated from the aryl-aryl coupling product by addition
of ether (in which only the coupling product is soluble) and
Scheme 3. Oxidation of 10-methyl-9,10-dihydroacridane (AcrH2). Conditions:
acetonitrile, 1 eq. 1(PF6)2, 7 eq. HBF4·OEt2, 6.5 h at r.t., 90% yield.
3
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The BDE value of 9,10-dihydroanthracene (ca. 326 kJ mol−1) is
higher than that of AcrH2.[36] Although quantum-chemical
calculations (B3LYP/def2-TZVP) predict the reaction of 12+ with
9,10-dihydroanthracene (AnH2) to give anthracene (An) to be still
exothermic (Gibbs free reaction enery of -104.5 kJ mol−1 at r = 38
and -87.7 kJ mol−1 at r = 1), a massive barrier is expected for the
unfavored initial electron transfer step. Therefore, also reaction of
12+ with 9,10-dihydroanthracene requires the presence of an
excess of HBF4·OEt2. With 5 equivalents of HBF4·OEt2 and using
the salt 1(PF6)2, a yield of 78% anthracene is obtained within 3 h
at room temperature (Scheme 4). The guanidine PCET reagent
could be recycled (see SI). When more HBF4·OEt2 is applied, the
rate increases, but the An yield decreases (e.g. 65% yield after
1.5 h with 9 equivalents of HBF4·OEt2). Interestingly, an induction
period with low rate is visible in the conversion vs. time plot (see
SI). A possible explanation is an autocatalytic effect by formation
of the anthracene radical. Indeed, NMR spectra recorded after 2
h reaction time displayed broad signals indicating the presence of
radicals in the reaction mixture. By contrast, the signals in the
NMR spectra recorded after 3 h were again sharp, arguing for
complete conversion of the radical intermediates. For comparison,
kinetic measurements for the reaction between anthracene (An)
Scheme 5. Oxidation of 1-benzyl-1,4-dihydronicotinamide (BNAH). Conditions:
acetonitrile, 1 eq. 1(PF6)2, 24 h at 60 °C, 27% yield.
3) Comparison between the PCET reactivity of 12+ and 22+.
The results assembled in the previous section demonstrate that
salts of 12+ are capable PCET reagents. However, oxidation of
higher potential substrates requires the addition of a strong acid.
Since some substrates, e.g. BNAH, degrade in the presence of
acids, we thought of ways to increase the oxidation power in the
absence of acids. We previously showed that the redox potential
of 1,4-bis(tetramethylguanidino)-benzene (2) is significantly
higher than that of 1 (E1/2 = -0.21 V vs. Fc+/Fc in CH3CN for 2 [40]
and -0.73 V for 1 [20]). Although 22+ is therefore a stronger oxidant
in electron-transfer reactions, it is not neccessarily a stronger
oxidant in PCET reactions, since the thermodynamics of PCET
reactions depends not only on the potentials, but also on the pKa
values of the reduced, protonated species. Generally, an increase
of the potential is accompanied by a decrease of the pKa value of
the protonated, reduced form. This “leveling effect” limits the
possibility of tuning the PCET reactivity by derivatization. In this
section we compare the PCET reactivity of 22+ with that of 12+ by
direct reaction between the oxidized form of one compound with
the reduced, doubly protonated form of the other compound.
Our experiments show that 22+ reacts fast with (1+2H)2+ to give
(2+2H)2+ and 12+. In an acetonitrile solution with concentrations of
1.25·10−2 mol/l for both reactants, quantitative conversion (>99%)
is obtained after 5 min at 25 °C (Scheme 6). On the other hand,
no reaction takes place when 12+ is mixed with (2+2H)2+. From
these experiments one could deduce that 22+ is indeed a stronger
PCET reagent than 12+, at least in the absence of a strong acid.
This result is supported by quantum-chemical calculations
predicting a Gibbs free reaction energy of -84.5 kJ mol−1 at r = 38
and -105.7 kJ mol−1 at r = 1.
and 9,10-dihydrophenanthrene (PhenH2) to give eventually AnH2
[38]
and Phen
showed that fast reaction between the produced
AnH2 and the reactant An generates the reactive radical AnH·.
Scheme 4. Oxidation of 9,10-dihydroanthracene. Conditions: acetonitrile, 1 eq.
1(PF6)2, 5 eq. HBF4·OEt2, 3 h at r.t., 78% yield.
Finally, the reaction of 12+ with 1-benzyl-1,4-dihydronicotinamide
(BNAH) was tested. The addition of a strong acid is not possible
in this reaction due to acid-catalysed hydration of BNAH.[39] The
BDE value of BNAH is 284 kJ mol−1,[36] and the Eox value is 0.259
V vs. Fc+/Fc in CH3CN. The energy gap Gel for electron transfer
between 12+ (Ered = -0.77 V vs. Fc+/Fc) and BNAH of 1.0 V is just
at the assumed limit value for conventional electron transfer at
standard conditions. Hence, reaction between 12+ and BNAH
might be possible at higher temperatures. Indeed, a slow reaction
was observed at a temperature of 60 °C, yielding 27% of the
pyridinium salt after 24 h (Scheme 5). Prolonged reaction times
(2 d) lead to degradation of BNAH.
Scheme 6. Oxidation of (1+2H)2+. Conditions: acetonitrile, 1 eq. 2(PF6)2, 5 min
at r.t., >99% yield.
The fast rate of the reaction between 22+ and (1+2H)2+ gets
evident from the appearance of the green colour characteristic for
4
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12+, motivating further analysis by UV-vis spectroscopy (Figure 2).
For equal concentrations (5.6·10−5 mol/l) of the two reactants, the
reaction is completed after 20 min. The growth of a strong band
at 425 nm signals the formation of 12+. The presence of an
isosbestic point at ca. 357 nm indicates clean conversion, in line
with the NMR experiments at higher concentrations. In further
experiments, we added 0.25 equivalents of HBF4·OEt2 to see if
the reaction rate is influenced by the presence of acid (see SI). A
decrease of the reaction rate upon acid addition was observed.
This could be explained by the protonation of the (1+2H)2+
reactant to give (1+4H)4+, which is less oxidizable. On the other
hand, additional NMR experiments showed that the reaction could
not be reversed by addition of larger quantities of HBF4·OEt2. Also,
addition of HBF4·OEt2 to a 12+ / (2+2H)2+ mixture only leads to the
protonated, oxidized guanidine (1+2H)4+, but not to PCET.
4) PCET reactions with 22+.
The reactions of 22+ with 1-benzyl-1,4-dihydronicotinamide
(BNAH), 10-methyl-9,10-dihydroacridane (AcrH2) and 9,10-
dihydroanthracene (AnH2) were carried out in the absence of a
strong acid. Salts of 22+ react very fast with BNAH; quantitative
yield (>99%) is obtained instantly upon mixing BNAH with 2(PF6)2
together at room temperature in acetonitrile solution (Scheme 7).
With Ered = -0.24 V for 22+, the energy for electron transfer, Gel,
is 0.50 V (48 kJ mol−1) and therefore well below 1 V. Hence the
observation of fast reaction is in line with electron transfer in the
first step.
22+
0.5 min
1.25
2
Scheme 7. Oxidation of BNAH. Conditions: acetonitrile, 1 eq. 2(PF6)2, r.t.,
instantly, >99% yield.
2.75
3.75
5
6
7.5
10
13,5
18
As expected due to its higher potential, reaction of 22+ (applied as
2(PF6)2 salt) with 10-methyl-9,10-dihydroacridane is slower. NMR
experiments showed that a yield of ca. 92% is obtained after 2.5
h at room temperature using 2 equivalents of 2(PF6)2 (Scheme 8).
The reaction is further slowed down if only one equivalent of
2(PF6)2 is applied, yielding 73% ArcH+ in 3 h at room temperature.
Again, this result is in line with electron transfer in the first step,
since Gel is 0.73 V (70 kJ mol−1).
20.75
300
350
400
450
500
550
l
/ nm
Figure 2. UV-vis spectra recorded for the 1:1 reaction between 22+ and (1+2H)2+
The strong band at 426 nm signals formation of 12+ as one reaction product.
.
To analyse the kinetics in more detail, stopped-flow
measurements were conducted in which 22+ was applied in
excess (concentrations of 3.94·10−4 mol/l (ratio 22+:(1+2H)2+
=
10:1), 7.87 10−4 mol/l (20:1), and 1.58 10−3 mol/l (40:1)). The
results are in line with a pseudo-first order reaction only within the
first seconds, hampering an unambigeous identification of the
reaction order. On the other hand, the estimated pseudo-first
Scheme 8. Oxidation of N-methylacridane (AcrH2). Conditions: acetonitrile, 2
eq. 2(PF6)2, 160 min at r.t., 92% yield.
order rate constants increase linearly with the concentration of 22+.
Hence, the results might be in line with a first order in both 22+ and
(1+2H)2+ in the initial reaction period (see SI for details), and the
analysis yielded a second order rate constant kH = 257 ± 27 M−1s−1
at room temperature.
The results in this section clearly show that 22+ is a stronger PCET
oxidant than 12+. Additional NMR experiments indicated
degradation of 22+ to so far unidentified products in the presence
of excess HBF4·OEt2 (see SI). Therefore most reactions reported
in the following were carried out in the absence of acid.
The reaction of 22+ with 9,10-dihydroanthracene is again slower.
A yield of only 6% of anthracene is obtained after 50 h at 60 °C.
The yield could be increased to 29% by addition of ca. 2
equivalents of NH4PF6 (Scheme 9). In difference to the results
obtained for the reaction of 12+ with 9,10-dihydroanthracene in the
presence of acid, there is no induction period, and the conversion
increases almost linearly with time. Quantum chemical
calculations (B3LYP/def2-TZVP) predict the reaction of 22+ with
9,10-dihydroanthracene to be significantly exothermic (Gibbs free
reaction energy of -189.0 kJ mol−1 at r = 38, and -193.4 kJ mol−1
at r = 1). The slow reaction rate could be explained by a
5
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substantial barrier for the initial electron-transfer step. A similar
strong correlation between the potential (Eox or E1/2 value) of the
PCET reagent and the reaction yield was recently reported for
oxidative C-H amination reactions of 9,10-dihydro-9-
heteroanthracenes with quinones as PCET reagents.[41]
when assuming second or first order rate equations. An analysis
by UV-vis spectroscopy is hampered by the close proximity of the
absorptions due to reactants and products. Quantum-chemical
calculations (B3LYP/def2-TZVP) found a Gibbs free reaction
energy of -96.1 kJ mol−1 at r = 38 and -107.8 kJ mol−1 at r = 1.
For comparison, the analogue reaction with 12+ in place for 22+ is
only slightly exergonic (-11.6 kJ mol−1 at r = 38, calculations with
counterions).
Scheme 9. Oxidation of 9,10-dihydroanthracene. Conditions: acetonitrile, 1 eq.
2(PF6)2, 2 eq. NH4PF6, 50 h at 60 °C, 29% yield.
Next, we tested the performance of 22+ in an aryl-aryl coupling
reaction. Reaction of 22+ with 3,3‘‘-4,4‘‘-tetramethoxy-o-terphenyl
(TMTP) at room temperature did not produce the product in the
absence of an acid. The oxidation potential of TMTP is 0.74 V,
and hence the energy for electron transfer, Gel, is 0.98 V, close
to the assumed limit value of 1.0 V. On the other hand, only small
amounts of HBF4·OEt2 are required to initiate fast and clean
reaction. With 12.5 mol% of HBF4·OEt2, a yield of 95% is obtained
within 10 min reaction time at a temperature of 60 °C (Scheme
10). For comparison, the analogue reaction with 12+ in place for
22+ required the use of a large excess (21 eq.) of HBF4·OEt2 to be
completed in relatively short time (45 min at room temperature,
99% yield).[28] In the case of 22+, small amounts of a strong acid
could be applied and do not lead to acid-induced degradation of
22+. However, an excess of the acid (e.g. 4 equivalents and more)
has to be avoided as it leads to quite fast degradation of 22+ (see
section 3 and SI).
Scheme 11. Oxidation of p-dihydrobenzoquinone. Conditions: acetonitrile, 1 eq.
2(PF6)2, 12 min at 55 °C, >99% yield.
5) Kinetic measurements and mechanistic considerations.
The reaction of 22+ with 10-methyl-9,10-dihydroacridane was
examined in more detail to derive information about the reaction
mechanism. Preliminary NMR experiments point to a slower
reaction with ArcD2 in place for AcrH2 (see SI); UV-vis
experiments were conducted for further kinetic analysis. To obtain
pseudo-first order conditions, a ten-, twenty- and forthy-fold
excess of AcrH2 was applied. The formation of N-
methylacridinium is clearly visible from the appearance of the
typical sharp band at 357 nm and a broader one around 420 nm
(with maxima at 395/415/440 nm). The band at 357 nm was
chosen for the analysis. Figure 3a shows the spectra recorded
with a tenfold excess of AcrH2. In the first 5 min, the reaction nicely
follows a pseudo first-order kinetics as seen from the ln(A) vs.
time plot in the inlet. The kobs values derived from such fits were
plotted as a function of the AcrH2 concentration (see Figure 3b).
The slope of this plot gives the second-order rate constant at room
temperature, kH = 3.32 ± 0.16 M−1s−1. For comparison, a ca. 4.5
times larger rate constant (kH = 15 M−1s−1) was previously
published for the reaction between chloranil and AcrH2 (also in
acetonitrile).[42] A similar analysis for AcrD2 yielded a second-oder
rate constant kD = 0.593 ± 0.021 M−1s−1 (see plot of the kobs values
as a function of AcrD2 concentration in Figure 3b and SI for further
details). Thus a quite large kinetic isotope effect (KIE = kH/kD) of
5.6 results, being nevertheless still smaller than the KIE of 8.8
obtained for the reaction between CA and N-methyl-acridane
(AcrH2).[42]
Scheme 10. Oxidation of 3,3‘‘-4,4‘‘-tetramethoxy-o-terphenyl. Conditions:
acetonitrile, 1 eq. 2(PF6)2, 12.5 mol% HBF4·OEt2, 10 min at 60 °C, 95% yield.
Finally, we inspected the reaction of 22+ with p-dihydro-
benzoquinone (Scheme 11). NMR experiments (c
=
2.9·10−2 mol·l−1 for both reactants) showed complete conversion
(>99%) within 12 min at 55 °C. At room temperature, 60 min are
required (98% yield). Fits of the conversion vs. time curves,
obtained for different concentrations of the two reactants, while
keeping the 1:1 stoichiometric ratio, and for experiments at
different temperatures (see SI), gave no satisfactorily results
6
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a)
-1
0,5
0.42 min
2.45
4.45
6.45
8.45
12.47
16.47
20.45
24.45
30.94
42.96
54.96
77.79
-2
0
2
4
t / min
0,0
350
400
450
500
550
l
/ nm
Scheme 11. Suggested mechanism for the oxidation of N-methylacridane with
.
b)
22+
AcrH2
0,010
0,005
0,000
In the UV-vis experiment, a small and broad absorption around
575 nm first increased in intensity, reaching a maximum after ca.
26 min, and then decreased (see difference spectrum in Figure
4 a), belonging to a reaction intermediate. According to the
proposed reaction scheme, the obvious candidates for this
intermediate are the radicals 2·+ and AcrH2·+, formed upon first
electron transfer. A broad, weak absorption around 640 nm was
AcrD2
previously assigned to the radical cation AcrH2 .
·+ [42-44] We did not
observe this band, most likely because the concentrations of
0,000
0,001
0,002
0,003
·+
[AcrH2] or [AcrD2] / M
AcrH2 are too low. To obtain more information on the other
candidate, 2·+, we prepared a 1:1 mixture of 2 (colourless) and
2(PF6)2 (yellow) in CH3CN solution. The reaction turned instantly
deep purple. In the UV-vis spectrum, bands at 575 nm (with a
shoulder at 542 nm) and 404 nm appeared (Figure 4 b), vanishing
within a few hours. TD-DFT calculations (B3LYP/def2-TZVP)
predicted electronic transitions at 543/501 and 365/346 nm for 2·+,
in good agreement with the experimental values (see SI for
details). The band at 575 nm could therefore be assigned to the
radical 2·+. The other band of 2·+ in the visible region at 404 nm is
obscured by the strong, broad bands of AcrH+ in this region
(Figure 4 a). The detection of 2·+ as a reaction intermediate is a
valuable additional evidence for the reaction pathway sketched in
Scheme 11.
Figure 3. a) Selected UV-vis spectra for the reaction between 2(PF6)2 and AcrH2
(10 equivalents) in CH3CN solution. The inlet shows the ln(A) vs. time plot in the
first 4 min of the reaction, from which the pseudo first-order rate constant is
determined. b) Plot of the pseudo first-order rate constants as a function of the
acridane concentration (for an invariant concentration of 2(PF6)2 of ca. 8·10−5
M). See SI for details.
The large KIE clearly shows that the rate constant for the
protonation step is contributing to the overall rate constant. AcrH2
is known to prefer a stepwise e−, H+, e− pathway for hydride
transfer in many reactions. The energy for electron transfer, Gel,
for reaction between 22+ and AcrH2+ of 0.73 V (70 kJ mol−1) is well
within the region of conventional electron transfer under standard
conditions, supporting electron transfer in the first step. Therefore
a stepwise e−, H+, e− pathway, as sketched in Scheme 11, is likely
to be operative here, too. The last step of the proposed
mechanism is a fast second electron transfer, profiting from the
low potential of the AcrH· radical (Eox = -0.46 V vs. SCE, ca. -0.06
V vs. Fc+/Fc).[43] Hence this last step plays no role in the overall
rate. According to the formula kH = kPket/(k-et + kP),[42] the
observation of a large KIE value means that kP (the rate constant
for proton transfer) is much smaller than k-et (the rate constant for
back electron-transfer regenerating the reactants).
7
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a)
*
ArcH+
400
500
600
700
l
/ nm
b)
2
0.5 min
5
10
15
20
25
40
55
70
90
1
0
Figure 5. Visualisation of the solid-state structures of the stable protonation and
redox states of 3. C-H protons omitted. Displacement ellipsoids drawn at the
50% probability level. Colour code: N blue, C grey, P pink, F green. See SI for
details.
400
600
120
150
180
210
1140
According to cyclic voltammetry (Figure 6), the redox properties
of 3 are very similar to those of 2. In both cases two-electron
waves are observed, leading reversibly to the dicationic state. An
E1/2 value of -0.21 V vs. Fc+/Fc (Eox = -0.18 V, Ered = -0.24 V) for
2 compares with an E1/2 value of -0.24 V vs. Fc+/Fc (Eox = -0.20 V,
Ered = -0.27 V) for 3. Although the radical cation 3·+ does not
appear in the CV measurements, it could be generated (similar to
2·+) by mixing neutral 3 and 3(PF6)2 in CH3CN solutions or by
titration of solutions of 3 with 3(PF6)2, and displays strong
absorptions at 566 and 383 nm (see SI). Interestingly,
degradation of 3·+ is much slower than 2·+ (see SI).
400
500
600
700
l
/ nm
Figure 4. a) Difference spectrum (26 minus 78 min reaction time) for the
reaction between 2(PF6)2 and AcrH2 (10 equivalents) in CH3CN solution. The
band assigned to 2·+ is highlighted by an asterisk. b) Decay of the absorptions
due to the radical monocation 2·+, formed instantly upon mixing 2 and 22+ in
acetonitrile solution.
6) Reaction between two bisguanidines.
To test PCET between closely related redox-active guanidines,
we synthesized the new bisguanidine 3 (77% yield) by reaction of
p-phenylenediamine-dihydrochloride with 2-chloro-1,3-dimethyl-
4,5-dihydro-1H-imidazolium chloride (prepared in situ from 1,3-
dimethyl-2-imidazolidinone and oxalyl chloride, see SI for details).
Compound 3 was then oxidized with FcPF6 to give the salt 3(PF6)2
(48% isolated yield), and protonated with NH4PF6 to give
(3+2H)(PF6)2 (72% isolated yield), and these two states
completely characterized. In addition, (3+H)PF6 was synthesized
from a 1:1 mixture of 3 and (3+2H)(PF6)2. The structures in the
solid state of all relevant states are illustrated in Figure 5. Further
experiments showed that 32+ does not degrade in the presence of
larger amounts of a strong acid, in contrast to 22+.
2
50 mA
3
-0,8
-0,6
-0,4
-0,2
E / V vs. Fc+/0
0,0
0,2
0,4
Figure 6. CV curves (CH3CN, Ag/AgCl reference electrode, 0.1 M N(nBu)4(PF6)
as supporting electrolyte, scan speed 100 mV s−1) for the two compounds 2 and
3 measured in oxidation direction. Potentials measured vs. Fc+/Fc.
We then directly compared the PCET reactivity of the two
compounds. Reaction between 22+ and (3+2H)2+ gave almost
8
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quantitative yields of 32+. For example, reaction at 55 °C for a
period of 45 min gave 99% yield (Scheme 12). On the other hand,
no reaction was observed when 32+ and (2+2H)2+ were mixed
together. These results are in line with quantum chemical
calculations (B3LYP/def2-TZVP), predicting the reaction (at r =
1) in Scheme 12 to exhibit a reaction enthalpy at 0 K of -66.7 kJ
mol−1 and a Gibbs free energy at 298 K of -58.2 kJ mol−1.
Conclusion
Proton-coupled electron transfer (PCET) reactions between
redox-active guanidines and a variety of organic compounds in
which C-H bonds are cleaved were systematically evaluated. First
it is demonstrated that substrates with oxidation potentials up to
at least 1.2 V vs. Fc+/0 could be oxidized in the presence of a
strong acid. Then, the two redox-active guanidines 1,2,4,5-
tetrakis(tetramethylguanidino)-benzene
(1)
and
1,4-
bis(tetramethylguanidino)-benzene (2), both applied in their
oxidized, dicationic state (12+ or 22+) are compared in their PCET
reactivity. In the course of our analysis, the reactions with the
three substrates 1-benzyl-1,4-dihydronicotinamide (BNAH), 10-
methyl-9,10-dihydroacridane
(AcrH2)
and
9,10-
dihydroanthracene (AnH2) were studied for both 12+ and 22+. The
results clearly show that 22+ is a significantly stronger oxidant in
PCET reactions. However, in contrast to 12+, it degrades in the
presence of larger quantities of a strong acid. In all PCET
reactions discussed in this work, slow proton transfer is likely to
proceed from an initial electron-transfer equilibrium.
Consequently, a relatively large kinetic isotope effect (KIE) of 5.6
was obtained for the reaction between 22+ and AcrH2.
Scheme 12. Oxidation of (3+2H)2+. Conditions: acetonitrile, 1 eq. 2(PF6)2, 45
min at 55 °C, 99% yield.
From the temperature dependence of the conversion vs. time
plots and assuming a second-order rate law, the activation energy
EA could be estimated from an Arrhenius plot (see Figure 7 and
SI for details). An EA value of 54 ± 8 kJ mol−1 was obtained in this
way. This quite low activation energy might argue for a concerted
e−, H+ transfer; a stepwise e−, H+ transfer would create a highly
unfavorable intermediate (3+2H)3+, and a stepwise H+, e− transfer
an similarly unfavorable intermediate (2+H)3+. The basicity of
GFAs with tetramethylguanidino groups is significantly higher
than that of GFAs with N,N’-dimethylethylene-guanidino
groups.[45] Thus, the redox potential and the pKa value, being the
two decisive factors for the PCET thermodynamics, seem to
operate in the same direction, making 32+ a weaker PCET reagent
than 22+. Hence, the leveling effect oberved for quinones, that
results from opposite trends of potential and pKa value, might not
occur for redox-active guanidines.
Finally, we synthesized a new redox-active guanidine, 1,4-
bis(N,N‘-dimethylethylene-guanidino)-benzene (3) and compared
the PCET reactivity of 32+ and 22+. Compound 32+ has a slightly
lower reduction potential and is a weaker oxidant in PCET
reactions. The comparison between the two closely related
bisguanidines indicates that a ʺleveling effect“ (resulting from
opposite trends of potential and pKa) as observed for quinones,
might not be an issue for redox-active guanidines, allowing the
tuning of the PCET thermodynamics by derivatisations. However,
more work is necessary to confirm this assumption, that would be
very helpful for a directed approach to PCET reactions.
The results presented in this work clearly show that redox-active
guanidinces are potent PCET reagents and real alternatives to
toxic quinones like chloranil or DDQ.
100
-11
80
-12
-13
0,0030
60
0,0032
0,0034
55 °C
45 °C
35 °C
25 °C
Acknowledgements
1/T / K
40
20
0
The authors gratefully acknowledge financial support by the
Deutsche Forschungsgemeinschaft (DFG).
Keywords: proton-coupled electron transfer • guanidine •
oxidation • C-H activation • oxidative coupling
0
50
100
150
t / min
Figure 7. Conversion vs. time plots for the reaction between 22+ and (3+2H)2+
at different temperatures. The second-order k values estimated from each curve
were used in the lnk vs. 1/T plot in the inlet, allowing to estimate the Arrhenius
activation energy.
9
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10
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Entry for the Table of Contents
Redox-active guanidines are used in PCET reactions that involve C-H bond cleavage, disclosing the scope and the mechanism of
PCET reactions with and without addition of a strong acid. Substrates with potentials of up to 1.2 V vs. ferrocenium/ferrocene could
be oxidized in stepwise PCET processes initiated by electron transfer. Experiments with different guanidines show the possibility to
tune the PCET reactivity.
11
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