Redox Self-Adaptation of a Nitrene Transfer Catalyst to the Substrate Needs

Article abstract of DOI:10.1002/anie.201612472

The development of iron catalysts for carbon–heteroatom bond formation, which has attracted strong interest in the context of green chemistry and nitrene transfer, has emerged as the most promising way to versatile amine synthetic processes. A diiron system was previously developed that proved efficient in catalytic sulfimidations and aziridinations thanks to an Fe^IIIFe^IV active species. To deal with more demanding benzylic and aliphatic substrates, the catalyst was found to activate itself to a Fe^IIIFe^IVL^. active species able to catalyze aliphatic amination. Extensive DFT calculations show that this activation event drastically enhances the electron affinity of the active species to match the substrates requirements. Overall this process consists in a redox self-adaptation of the catalyst to the substrate needs.

Full text of DOI:10.1002/anie.201612472

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201612472
German Edition: DOI: 10.1002/ange.201612472
Iron Catalysis
Redox Self-Adaptation of a Nitrene Transfer Catalyst to the Substrate
Needs
Eric Gourꢀ, Dhurairajan Senthilnathan, Guillaume Coin, Florian Albrieux, Frꢀdꢀric Avenier,
Patrick Dubourdeaux, Colette Lebrun, Pascale Maldivi,* and Jean-Marc Latour*
Abstract: The development of iron catalysts for carbon–
heteroatom bond formation, which has attracted strong interest
in the context of green chemistry and nitrene transfer, has
emerged as the most promising way to versatile amine synthetic
processes. A diiron system was previously developed that
proved efficient in catalytic sulfimidations and aziridinations
thanks to an FeIIIFeIV active species. To deal with more
demanding benzylic and aliphatic substrates, the catalyst was
found to activate itself to a FeIIIFeIVLC active species able to
catalyze aliphatic amination. Extensive DFT calculations show
that this activation event drastically enhances the electron
affinity of the active species to match the substrates require-
ments. Overall this process consists in a redox self-adaptation
of the catalyst to the substrate needs.
thanks to Fe ligands of low denticity that promote trigonal or
tetrahedral environments to the metal.[6] However, this
strategy was detrimental to the activity. More active five-
and six-coordinated iron imido species were reported but
could rarely be isolated,[5a,7] and were at best detected by mass
spectrometry techniques.[7e,8]
We recently reported that the diiron(III,II) complex 1-H
(Scheme 1) is able to mediate the intramolecular nitrene
insertion from an iodinane (ArI = NTs, Ar= phenyl or 2-
ꢀ
tertbutylsulfonylphenyl, Ts = tosyl) into the ortho C H
position of the dangling benzyl group of the ligand,[9] leading
N
itrene transfer reactions have emerged as the most
promising synthetic route to amine derivatives[1] that are
important products or functional intermediates of biological
and pharmaceutical relevance.[2] Several efficient catalytic
systems have been reported but most rely on noble metals,
especially rhodium.[3] Replacement of noble metal catalysts
by more ecologically and economically friendly first-row
transition metal analogues has fueled a considerable amount
of work in the past decade. In particular, the search of iron
catalysts for many coupling reactions has been intensely
developed.[4] A few active systems were reported for catalytic
amination of sulfides and olefins only, and their mechanism is
unknown although high valent Fe-nitrene (or imido) active
species were postulated.[5] On the other hand, several high-
Scheme 1. Top: Intramolecular amination mediated by the diiron(III,II)
complex 1-H [FeIIIFeII(CH3CN)(mpdp)(l-Bn)]2+ leading to the diferric
tosylanilinate [FeIIIFeIII(mpdp)(l-BnNTs)]2+ dication (mpdp2ꢀ =m-phe-
nylenedipropionate). Both compounds were isolated as tetraphenylbo-
rate[11] and perchlorate[12] salts, respectively.
valent iron–imido (Fe NR) complexes could be isolated
to a
diferric tosylanilinate [FeIIIFeIII(mpdp)(l-BnNTs)]-
=
(ClO4)2. When the ortho benzylic positions of the ligand are
protected by chlorine atoms, 1-Cl (Scheme 1) catalyzes
efficient intermolecular nitrene transfers to thioanisole (Ph-
S-Me) in mild conditions.[7g] In-depth mechanistic studies[8]
allowed us to show that these catalytic nitrene transfers
involve an active diiron(III,IV) tosylimido complex [FeIIIFeIV-
[*] Dr. E. Gourꢀ, G. Coin, Dr. F. Avenier, P. Dubourdeaux, Dr. J.-M. Latour
Univ. Grenoble Alpes, CNRS UMR 5249, CEA, LCBM/pmb
38000 Grenoble (France)
E-mail: jean-marc.latour@cea.fr
Dr. D. Senthilnathan, C. Lebrun, Dr. P. Maldivi
Univ. Grenoble Alpes, CNRS UMR 5819, CEA, SyMMES
38000 Grenoble (France)
=
(l-BnCl2)(mpdp)( NTs)] (Scheme 2A; 3), which was evi-
denced by Desorption ElectroSpray Ionization Mass Spec-
trometry (DESI-MS).[10] In the present work we demonstrate
that, when opposed to more challenging substrates such as
ethylbenzene or cyclohexane, the catalysis involves a new
E-mail: pascale.maldivi@cea.fr
Dr. D. Senthilnathan
Center for Computational Chemistry, CRD
PRIST University Vallam, Thanjavur, Tamilnadu (India)
III
V
=
ꢀ
formally diiron(III,V) imido amido species [Fe Fe ( NTs)
(NHTs)], 7 (Scheme 2B), which is unprecedented. This new
catalytic cycle is thus one oxidation state unit higher than the
former one, matching the more difficult oxidation of the
substrate. Extensive DFT calculations show indeed that this
new active species 7 has a stronger electron affinity than 3 by
ca 15 kcalmolꢀ1. These results thus point to a self-adaptation
of the catalyst to the oxidizability of the substrate.
Dr. F. Albrieux
CCSM UMR 5246 CNRS—Univ. Claude Bernard
Lyon 1 (France)
Dr. F. Avenier
LCBB, ICMMO (UMR 8182), Univ. Paris Sud, Universitꢀ Paris Saclay
91405 Orsay (France)
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201612472.
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unassigned patterns at 634.5
and 795.5. The mass value, the
isotopic profile and the shift of
0.5 mass unit induced by the
use of ArI 15NTs concur to
=
show (Supporting Informa-
tion, Figure S3) that the pat-
tern at m/z 794–800 is due to
6-Cl2+, the iodinane adduct of
4-Cl2+,
(mpdp)(NHTs)
[FeIIIFeIII(l-BnCl2)-
=
(ArI
NTs)]2+. A similar analysis of
the pattern at m/z 632–638
(Supporting Information, Fig-
ure S2, inset and Figure S4)
associates it to species 7-Cl2+
best formulated as the diiron-
(III,V) imido amido species
III
V
=
[Fe Fe (l-BnCl2)(mpdp)(
2+
ꢀ
NTs)( NHTs)] ,
expected species resulting
from the 2-electron-oxidative
the
addition of tosylnitrene to 4-
Cl.
To further characterize 7
and prove its ability to trans-
fer the NTs group to various
substrates, we first generated
Scheme 2. Species occurring during nitrene transfers mediated by 1-Cl as detected by DESI-MS experi-
ments. The label “-Cl” was omitted for simplicity and because corresponding species are formed from 1-H.
Species 7 exists under two tautomeric forms evidenced by ESI-MS experiments and DFT calculations, see
Figure 1 and the Supporting Information, Figure S13.
quantitatively 4-Cl or 4-H and
investigated their reactivity in
=
The catalytic ability of the 1-Cl/PhI NTs system was
benchmark reactions, respectively the intermolecular nitrene
transfer to thioanisole and in the intramolecular reaction with
the pendant benzyl group of the ligand. Moreover, to trace
the origin of the transferred NTs group, we used different
labelings of the imido and amido groups. In a first step, 4-Cl
(4-H) was generated by reaction of 1-Cl (1-H) with one equiv
assayed in the amination of substituted ethylbenzenes and
THF. In a typical reaction 1-Cl, PhI NTs and the substrate in
0.05:1:100 ratio were reacted for 5 h in dichloromethane at
room temperature. The tosylamines C6H5CH(NHTs)CH3 and
(C4H7O)(NHTs) were isolated in circa 50% yields (vs. PhI
=
=
=
NTs; see the Supporting Information, Table S1). The UV/Vis
spectra recorded during catalytic amination of ethylbenzene
(Supporting Information, Figure S1a) are dominated by an
intense absorption at 482 nm, which is associated to the
diferric amido derivative 4-Cl (Scheme 2).[7g,8] By contrast,
corresponding spectra recorded during catalytic amination of
thioanisole exhibit an absorption at 580 nm (Supporting
Information, Figure S1b) indicating that a mixed-valent
FeIIIFeII species 1-Cl/2-Cl is the resting species during
catalysis. These observations indicate that the amination of
ethylbenzene or cyclohexane does not follow the catalytic
cycle A (Scheme 2). 4-Cl is formed by HC abstraction by 3-Cl
from an external HC donor (such as dihydroanthracene or
diphenylhydrazine, H2-DPH) or from a benzylic position of
the ligands.[7g,8] It is not intrinsically capable of nitrene
transfer, and therefore a second catalytic cycle must exist
ArI NTs in the presence of one equiv DHA. In the
intermolecular transfer experiment, 4-Cl was treated by
10 equivalents PhSMe and then one equiv of ArI 15NTs,
=
and the produced sulfylimine PhS(NTs)Me was isolated and
analyzed by ESI-MS (Supporting Information, Figure S5).
The isotopic distribution in the 294–299 range can be
reproduced taking into account a 2:98 mixture of 14N/15N
isotopomers. This experiment shows that the NTs group
transferred to the substrate originates essentially from the
second introduced iodinane. In the intramolecular transfer
experiment, 4-H was treated by one equiv of ArI 15NTs and
=
the product of the reaction was analyzed by ESI-MS.
Examination of the pattern at 513–517 (Supporting Informa-
tion, Figure S6) corresponding to the species [FeIIIFeIII-
(mpdp)(l-BnNTs)]2+ (Scheme 1) revealed the insertion of
either 14NTs or 15NTs groups in comparable amounts (42 vs.
58%, respectively). These apparently conflicting results
between inter- and intramolecular NTs transfers can be
easily reconciled by considering the widely different rates of
the two reactions and the occurrence of a tautomeric
equilibrium within 7 (Scheme 2).
=
involving its activation by PhI NTs.
Apart from the known patterns at 464, 494, 549, and 710.5,
(corresponding, respectively to 1-Cl2+, the protonated iodi-
+
2+
2+
2+
=
nane [ArI NTs,H] , the mixture 3-Cl /4-Cl , and 2-Cl
(Scheme 2A),[7g] the DESI-MS spectrum recorded immedi-
=
ately after mixing 1-Cl and ArI NTs at room temperature in
Indeed, whereas an imido group can be transferred to
substrates, an amido group cannot. Hence, to be transferred
acetonitrile (Supporting Information, Figure S2) revealed
2
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the amido group in 7 must have been transformed into an
Examination of energy differences at various computational
imido group, and the simplest way to achieve this trans-
formation is a tautomeric equilibrium between the imido and
amido groups as shown for oxo/hydroxo derivatives.[13] With
such a process going on, the eventual labeling of the product
in the two reactions is dependent on the kinetic competition
between the proton exchange and the nitrene transfer.
Indeed, as shown by UV/Vis experiments the intramolecular
nitrene transfer to the benzyl group occurs on the minute time
scale, slow enough to allow for the tautomeric equilibrium to
be fully established explaining that both 14NTs and 15NTs
groups are transferred equally. By contrast, the nitrene
transfer to thioanisole occurs in the millisecond time scale,
faster than the proton exchange, explaining that only the
labeled nitrene 15NTs group is transferred since the tauto-
meric equilibrium has not yet progressed.
levels pointed to only two low energy geometries, one with
NTs trans vs. the phenoxide group (denoted OPh below) and
NHTs cis (denoted below Nt_NHc) and one with NHTs trans
and NTs cis (denoted below NHt_Nc). The energy differences
between these two isomers are within a 0.3 eV range,
depending on the theoretical level (Supporting Information,
Tables S6, S7). This value can be considered as a reasonable
uncertainty at this level of computation, thus both isomers
must be considered as isoenergetic. Their geometries are
represented on Figure 1.
ꢀ
As expected FeB N distances (from NTs and NHTs) vary
slightly for the different electronic configurations but cluster
around a representative value (Supporting Information,
ꢀ
Tables S4, S5). For both isomers the FeB NTs distance is
about 1.77 ꢀ, similar to the values already obtained for the
mixed-valent species 3 (1.8 ꢀ) and to the distance found by
EXAFS by Klinker et al.[7c] for the [(N4Py)FeIV(NTs)] com-
To substantiate this mechanistic scheme, we reasoned that
slowing down the intermolecular transfer to thioether sub-
strates should allow for increased transfer of the 14NTs group.
This can be achieved by using less reactive sulfides and/or
performing the reaction at lower temperature. Indeed both
factors lead to an increased content of the 14NTs isotopomer
attesting the progress of the tautomeric equilibrium (Sup-
porting Information, Table S2 and Figures S7, S8). We
obtained the ultimate confirmation for the latter by evidenc-
ing a kinetic influence of H/D substitution within 4-Cl
ꢀ
plex (1.73 ꢀ). The FeB NHTs distances are longer (1.89 to
1.97 ꢀ), and perfectly consistent with the ones reported for
[15]
FeIII NHTs complexes
and that found in the binuclear
ꢀ
complex 4 (1.94 to 1.97 ꢀ).[8]
Examination of the spin densities of both Fe ions and NTs,
NHTs, and OPh ligands (Table 1; Supporting Information,
Tables S6, S7) reveal remarkable features. First, the spin
densities for FeA are always between 4 and 4.13, typical of
III
III
III
III
ꢀ
[Fe Fe (-NHTs)]. Reacting D-labeled 4-Cl [Fe Fe (
NDTs)] with ArI 15NTs in the presence of PhSPh(NO2) at
=
Table 1: Spin densities of FeB, NTs, NHTs, and OPh groups from SP
B3LYP in the gas phase for NHt_Nc and Nt_NHc isomers, in the low
energy spin configuration identified through total spin projection
Sz=5.[a]
ꢀ308C as before led to a diminished 14NTs transfer, as
expected (Supporting Information, Figure S8). This weak
effect is significant and reproducible and consistent with very
recent observations made on oxo/hydroxo exchange.[14] All
the above observations thus establish that aliphatic amina-
tions involve a second catalytic cycle (Scheme 2B), of which
the active species 7 is generated by activation of 4 and exists
as an equilibrium of two tautomers.
FeA FeB NTs NHTs Oph
Electronic configuration
ꢀ
NHt_Nc 4.08 2.72 1.09 0.24 0.85
Nt_NHc 4.13 3.23 1.05 0.59
FeIII, FeIV COPh, NTs
ꢀ0.01 FeIII, FeIV OPh, CNTs
ꢀ
[a] Other hybrid functionals (B3LYP*, PBE0, M06, TPSSh) or solvent
phase calculations give very similar results within a 0.2 eV energy range
(see the Supporting Information).
In the absence of any possible spectroscopic character-
ization of 7 due to its transient nature, we relied on DFT
theoretical calculations to get insights into its molecular and
electronic structures. We followed a similar protocol as in our
previous study of 3 and 4,[8] exploring several initial structures
and spin state configurations (see the Supporting Information
for more details). The most reasonable model involves
binding of the two N(H)Ts groups on the same Fe ion (FeB,
Figure 1), concurring with the tautomeric equilibrium.
a high spin FeIII ion, and similar to those found in complexes 3
and 4, showing that FeA is not affected by the redox change
from 4 to 7. Second, by contrast, the FeB ion exhibits much
lower values close or lower than 3. Third, in all cases, a full
radical character is present on OPh and/or NTs ligands.
Thorough examinations of the Kohn–Sham orbitals combined
with these spin density values lead to a consistent picture of
the electronic structures of both tautomers. For the NHt_Nc
isomer, the LUMO is localized on an OPh p orbital
supporting the oxidation of the phenoxide (Supporting
Information, Figure S9, S10). In contrast, for Nt_NHc
isomer the LUMO is localized on an imido p orbital
(Supporting Information, Figures S11, S12). Finally, in both
cases, FeB is a high spin d4 FeIV ion, although its spin density
values are highly depleted compared to an expected value
close to 4, owing to strong donation effects. Hence a clear
picture emerges for the activated species 7. Its generation by
the two-electron oxidation of 4 occurs through a one-electron
oxidation of FeB to give an FeIV ion and a one-electron
oxidation of one ligand. Owing to the strong trans effect of the
Figure 1. Optimized geometries (top) of the two lowest energy config-
urations for the two models of 7 Nt-NHc (left) and NHt-Nc (right). H
atoms are not represented except the H in the NHTs ligand (in ball-
and-stick mode).
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imido ligand, either the phenoxide (NHt_Nc tautomer) or the
NTs group (Nt_NHc tautomer) is oxidized, leading, respec-
of BDE (NH). These values have been computed for 7 and 3
and compared to those of the best documented related system
[(N4Py)FeIV(NTs)],[7c,16,17] (see the Supporting Information
and Table S10). A notable increase in EAs is observed for 7
(142.7 kcalmolꢀ1) compared to 3 (127.5 kcalmolꢀ1) and to
[(N4Py)FeIV(NTs)] (129.6 kcalmolꢀ1). This increase is con-
sistent with the higher oxidation state of 7. By contrast, the
magnitude of BDE (NH) is found similar for 3 (99.7 kcal
molꢀ1) and 7 (95.9 kcalmolꢀ1) and higher than [(N4Py)FeIV-
(NTs)] (89.0 kcalmolꢀ1). This supports that EA is the major
factor contributing to the higher activity of the present
system.
Along with this high activity, the most salient feature is
that the active species involved is dependent on the oxidiz-
ability of the substrate: an easy substrate is aminated through
cycle A, whereas a more demanding one triggers activation of
cycle B relying on a more electrophilic active species. This
process is akin to a redox induced fit, which is unprecedented
to the best of our knowledge. The fact that a single Fe ion is
involved in the redox changes opens the way to mononuclear
catalysts operating along the same line.
tively, to the formal electronic configurations FeIIIFeIV COPh
ꢀ
and FeIIIFeIV CNTs.
ꢀ
In line with the fast tautomeric equilibrium, our optimized
geometries show a short distance (2.45–2.50 ꢀ) between H
from the amido group and N from the imido group within
both tautomers (Figure 1; Supporting Information, Fig-
ure S13). Using this distance as a reaction coordinate, we
carried out a computational modeling of the possible
mechanism for this exchange. It allowed us to characterize
a transition state with an activation free energy of 23.3 kcal
molꢀ1 (see the Supporting Information for details). We
compared this barrier to the only known computed value of
activation energy of a Fe-based NTs transfer to dimethylsul-
fide provided by de Visser et al. (ca. 18 kcalmolꢀ1 at the same
computational level).[16] This comparison shows that both
activation energies (tautomerism and NTs transfer to organic
sulfide) are close, although the NTs transfer activation energy
is slightly lower than that of the tautomeric equilibrium.
These computations are in full agreement with experimental
observations that both reactions have competitive rates. To
sum up these computational investigations on 7, the two-
electron oxidative addition of NTs to 4 leading to 7
corresponds to a change from a FeIIIFeIII binuclear core to
Acknowledgements
a FeIIIFeIV LC (L being one of the ligand on FeB), and not to
J.M.L. and P.M. thank the French National Agency for
Research (ANR) “programme Labex” (ARCANE project
no. ANR-11-LABX-003) for partial funding. This work was
performed using HPC resources from GENCI-CINES (Grant
2015-c2016087648).
ꢀ
any possible FeIVFeIV or FeIIIFeV valence localizations. More-
over, the two resulting tautomers are very close in energy, and
can easily exchange the amide proton competitively with NTs
intermolecular transfer, consistently with the experimentally
observed equilibria.
We have shown that the present diiron system is able to
catalyze very efficient nitrene transfers to a variety of
substrates ranging from thioethers and aziridines[7g,9] to
more demanding aliphatic substrates. This compares favor-
ably with the most active non-heme iron systems reported so
far.[5,7e] This strong activity is based on its peculiar ability to be
activated twice for nitrene transfer. Indeed, reaction of 1-Cl
Conflict of interest
The authors declare no conflict of interest.
Keywords: amination · homogeneous catalysis · iron · nitrene ·
tautomerism
=
with PhI NTs involves two independent but connected cycles
based on the same sequence of reactions: binding of the
iodinane to the catalyst (1!2 and 4!6), intramolecular two-
electron rearrangement with PhI elimination generating an
active high-valent intermediate (2!3 and 6!7) and nitrene
transfer to the substrate regenerating the catalyst (3!1 and
7!4). The two cycles are connected by deactivation of 3,
a FeIIIFeIV active state, through HC abstraction to the
corresponding amido derivative 4 that is reactivated in cycle
B, which is similar to A but one oxidation state higher with an
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ꢀ
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Manuscript received: December 23, 2016
Final Article published: && &&, &&&&
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Iron Catalysis
Self-adaptive catalyst: An efficient diiron
catalyst mediates nitrene transfer to
sulfides through an FeIV active state but
self-activates to FeV when facing aliphatic
substrates that are harder to oxidize.
E. Gourꢁ, D. Senthilnathan, G. Coin,
F. Albrieux, F. Avenier, P. Dubourdeaux,
C. Lebrun, P. Maldivi,*
J.-M. Latour*
&&&& — &&&&
Redox Self-Adaptation of a Nitrene
Transfer Catalyst to the Substrate Needs
6
www.angewandte.org
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
These are not the final page numbers!