Catalytic C-H amination driven by intramolecular ligand-to-nitrene one-electron transfer through a rhodium(III) centre

Article abstract of DOI:10.1039/c7cc01840a

Werner type six-coordinate rhodium(iii) complexes coordinated by a planar trianionic ligand and two axial aniline ligands are synthesised. The trianionic ligand behaves as a redox-active ligand to form a ligand radical species upon one-electron oxidation of the complex. The rhodium(iii) complexes catalyse C-H amination of external substrates such as xanthene with tosylazide as the nitrene source. DFT-calculation and kinetic deuterium isotope effects indicate that a di-radical rhodium(iii) complex formed by one-electron transfer from the redox-active ligand to the nitrene group works as a reactive intermediate to induce aliphatic C-H activation.

Full text of DOI:10.1039/c7cc01840a

ChemComm
View Article Online
View Journal
COMMUNICATION
Catalytic C–H amination driven by intramolecular
ligand-to-nitrene one-electron transfer through a
rhodium(III) centre†
Cite this:DOI: 10.1039/c7cc01840a
Received 10th March 2017,
Accepted 10th April 2017
Daiki Fujita,a Hideki Sugimoto, *a Yoshihito Shiota,b Yuma Morimoto,a
Kazunari Yoshizawa*b and Shinobu Itoh*a
DOI: 10.1039/c7cc01840a
rsc.li/chemcomm
Werner type six-coordinate rhodium(III) complexes coordinated by a
planar trianionic ligand and two axial aniline ligands are synthesised.
The trianionic ligand behaves as a redox-active ligand to form a
ligand radical species upon one-electron oxidation of the complex.
The rhodium(III) complexes catalyse C–H amination of external
substrates such as xanthene with tosylazide as the nitrene source.
DFT-calculation and kinetic deuterium isotope effects indicate that a
di-radical rhodium(III) complex formed by one-electron transfer from
the redox-active ligand to the nitrene group works as a reactive
intermediate to induce aliphatic C–H activation.
Development of catalysts consisting of a redox innocent metal and
a redox-active ligand is one of the attractive research subjects
in current synthetic organic chemistry as well as coordination
chemistry, since such a combination enables us to make use of any
metal elements in the periodic table for several types of redox
transformation reactions.1 Typical examples are oxidative addition
of Cl2 and Br2 to a Zr(IV) (d0) centre, aliphatic C–H activation by
an Al(III) complex, and dihydrogen production using an Al(III)
Scheme 1 (a) C–H amination methods based on metal-based redox system
(path I) and ligand-based redox system (path II). (b) van der Vlugt’s intramolecular
amination system using ligand-based redox system. (c) Neutral ligand LH3,
2À
[RhIII(L3À)(AX)2] (1X) and [RhIII(Lꢀ )(TsNꢀÀ)(AX)] (3X) generated by the reaction
of 1X and tosylazide (AX = p-X-C6H4NH2).
complex, where the supporting ligands have been demonstrated combine a redox innocent metal with a redox-active ligand in
to participate in the redox processes.2
the reaction with nitrene, single-electron transfer from the redox-
The C–H amination methodologies have been significantly active ligand to the nitrene might proceed without change of the
evolved in the past decade by the advent of redox-active metal oxidation state of the metal centre (Scheme 1(a), path II). In such a
catalysts that enable formally one or two electron transfer from case, the generated complex will formally be a metal nitrene radical
the metal centre to the nitrene functional group (Scheme 1(a), species with a single bond character supported by a ligand-based
path I).3 Such electron transfer adds multiple bond character radical, thus giving a di-radical complex. Such a nitrene radical
to the metal-nitrogen moiety. On the other hand, if one can complex is expected to have a radical type C(sp3)–H amination
reactivity.4 In fact, van der Vlugt and coworkers reported an
intramolecular cyclization reaction of 4-azidobutylbenzene via
one-electron transfer from a redox-active aminophenol tridentate
a Department of Material and Life Science, Division of Advanced Science and
Biotechnology, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka,
pincer ligand to the generated nitrene moiety of the substrate
Suita, Osaka 565-0871, Japan. E-mail: sugimoto@mls.eng.osaka-u.ac.jp,
(Scheme 1(b)).5 The square planar palladium(II) complex has an
shinobu@mls.eng.osaka-u.ac.jp
b Institute for Materials Chemistry and Engineering and International Research
Center for Molecular System, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka,
819-0395, Japan. E-mail: kazunari@ms.ifoc.kyushu-u.ac.jp
open-shell singlet character comprising ‘‘nitrene radical’’ and
‘‘tridentate ligand radical’’ to undergo intramolecular radical
type C(sp3)–H amination–cyclization reaction without change
of the palladium(II) oxidation state.5 However, intermolecular
C–H amination has yet to be accomplished in the palladium(II)
system.
† Electronic supplementary information (ESI) available: Experimental details for
preparation and DFT calculations (Tables S1–S10, and Fig. S1–S10). CCDC
1535694, 1535696 and 1535697. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c7cc01840a
This journal is ©The Royal Society of Chemistry 2017
Chem. Commun.

View Article Online
Communication
ChemComm
Recently, we have synthesised a series of late transition-metal
complexes [M(L3À)]À (M = Ni2+, Cu2+, and Pd2+) supported by a
planar tetradentate ligand (L3À) consisting of a b-diketiminate
framework with two aminophenol moieties (Scheme 1(c)).6 One-
electron oxidation of the complexes (both chemically and electro-
chemically) gave the corresponding ligand-radical complexes
[M(Lꢀ2À)]2À 6
. We, therefore, envisioned that the redox-active
ligand, L3À, can facilitate formation of a nitrene radical
complex having metal–nitrene single bond character through
ligand to nitrene one-electron transfer when combined with a
redox innocent transition metal ion. In this study, we employed
rhodium(III) ion as the central metal, since most of the Werner
type octahedral rhodium(III) complexes are hardly oxidised to
the higher oxidation states.7 We herein report the first inter-
molecular C(sp3)–H amination with tosylazide catalysed by
[RhIII(L3À)( p-X-C6H4NH2)2] (1X, Scheme 1(c)). DFT calculations
indicate that a triplet nitrene di-radical Rh(III) complex works as
a reactive intermediate in the C–H amination reaction.
Fig. 1 Crystal structures of (a) 1H and (b) [1H]SbF6. Hydrogen atoms and
solvent molecules are omitted for clarity. Selected bond lengths are shown
in the right.
Cyclic voltammogram of 1X in CH2Cl2 was measured in a
range from À1.5 to +1.5 V vs. Fc/Fc+. 1H exhibited two reversible
redox couples at E1/2 = 0.22 and 0.62 V vs. Fc/Fc+ (Fig. 2) and one
irreversible oxidation process at Epa = +1.34 V in a range from
À1.5 to +1.5 V in CH2Cl2 (CV from À1.5 to +1.5 V is give in
Fig. S3, ESI†). The redox potentials are relatively close to those
of the two redox waves that observed for Et4N[CuII(L3À)] (À0.06
and 0.46 V) and Et4N[NiII(L3À)] (0.04 and 0.54 V).6 Furthermore,
The synthetic procedure for 1X is outlined in Scheme 2. First,
[RhI(LH2À)(cod)] (cod = 1,5-cyclooctadiene) was prepared by
treating [RhI(cod)Cl]2 with LH3 in the presence of an equimolar
amount of Et3N in THF. The crystal structural analysis of
[RhI(LH2À)(cod)] showed that the Rh ion is coordinated Àwith the
two nitrogen atoms of mono-deprotonated ligand LH2 at the
b-diketiminate moiety and 1,5-cyclooctadiene in a Z2-p-coordination
mode to adopt a square planer geometry, where the two phenol
groups remain uncoordinated (Scheme 2 and Fig. S1, ESI†). The
overall geometry around the Rh center is close to those of
[RhI{k2(N,N)-(Ar)NC(Me)CHC(Me)N(Ar)}(cod)] (Ar = 2,6-dimethyl-
phenyl) and [RhI{k2(N,N)-(C6F5)NC(Me)CHC(Me)N(C6F5)}(cod)].8
Then, [RhI(LH2À)(cod)] was converted to 1X by treating it with 2
equiv. of p-substituted anilines AX ( p-X-C6H4NH2; 1H (X = H), 1Cl
(X = Cl), and 1OMe (X = OMe)) in the presence of 2 equiv. of Et3N
as a base in THF under air. The obtained complexes were
characterised by 1H NMR and FAB-mass spectra as well as
elemental analysis. 1X was EPR silent and exhibited the sharp
1H NMR signals in the diamagnetic region from 0 to 10 ppm and
no absorption band in a near-IR to IR region (see the Experimental
section in the ESI,† and Fig. S2). As shown in Fig. 1(a), the crystal
structure of 1H reveals that the Rh center adopts a slightly distorted
octahedral geometry comprising a planar tetradentate ligand L3À in
the basal plan and two aniline ligands in the axial positions.
The metrical oxidation states that defined by Brown9 were
À1.68 (ring A) and À1.74 (ring B) for 1H. Above observation
indicates that 1X is best described as a Rh(III) complex coordinated
with the trianionic L3À ligand. Two electron oxidation from Rh(I) to
Rh(III) took place during the treatment under air.
1
OMe and 1Cl gave very similar redox couples at 0.22 and 0.61 V
for 1OMe and 0.26 and 0.67 V for 1Cl (Fig. S4, ESI†). Thus, these
redox couples are safety assigned as the consecutive ligand-
2À
based oxidation from L3À to Lꢀ and to LÀ.
The one electron-oxidised product of 1H was obtained by the
oxidation with AgSbF6, and isolated as an SbF6À salt ([1H]SbF6).
Crystal structure of [1H]SbF6 is shown in Fig. 1(b), which reveals
distinct dimensional changes with respect to one of the amino-
phenolate moieties (ring A). Namely, the diatomic distances of
the corresponding bonds of the aminophenolate moieties
(rings A and B) in 1H are nearly equivalent (Fig. 1(a) and Table
S1, ESI†), whereas ring A and ring B became inequivalent in
[1H]SbF6; C2–C3, C4–C5, N1–C1 and C6–O1 bonds become
shorter, whereas the C1–C2, C3–C4, C5–C6 and C1–C6 are
elongated. The results suggest that ring A gets contribution of
a quinonoid canonical form upon the oxidation. Such a dimen-
sional change of the aminophenolate moiety also makes the
bond length of Rh1–O1 longer from 2.0190(11) Å in 1H to
Fig. 2 Cyclic voltammogram of 1H (1.0 mM) in a CH2Cl2 solution containing
0.10 M nBu4NPF6 (working electrode = GC, counter electrode = Pt, scan
rate = 0.1 V sÀ1).
Scheme 2 Synthetic scheme of 1x.
Chem. Commun.
This journal is ©The Royal Society of Chemistry 2017

View Article Online
ChemComm
Communication
2.0402(19) Å in [1H]SbF6. The metrical oxidation states of ring A sum of [Rh + L + NTs + AH + 2H]+ and [Rh + L + NTs + AH + H]+ in
and ring B changed from À1.68 and À1.74 to À1.03 and À1.49, a 60 : 40 ratio.‡ Thus, the catalytic reaction begins from replace-
respectively, upon the oxidation. The EPR spectrum of [1H]SbF6 ment of one of the two aniline ligands (AH) of 1H with TsN3 to
in CH2Cl2 at 100 K exhibited a typical signal for an organic give [Rh(L)(N3Ts)(AH)] (2H), where Boc2O traps the aniline (AH)
radical and the absorption spectrum had a broad band around giving C6H4NHBoc to enhance the binding of TsN3. The results
2400 nm that is assignable to an inter-valence charge transfer of both entry 2 in Table 1 and the mass measurements suggest
transition (Fig. S2 and S5, ESI†). Furthermore, DFT calculated that this replacement process will be slow and the catalytic
structure (B3LYP/SDD) of [1H]SbF6 had a dianionic radical cycle runs before the second replacement of the remaining
structure of the tetradentate ligand (Fig. S6, the calculation aniline ligand with tosylazide. Then, dissociation of dinitrogen
methods are described in the Experimental section in the ESI†). (N2) from 2H generates a nitrene bound rhodium complex
Collectively, the oxidation product can be best described as the [Rh(L)(NTs)(AH)] (3H) as a key reactive intermediate. Feasibility
RhIII–phenoxyl radical complex [RhIII(Lꢀ2À)], in which one-electron of the mechanism for the formation of 3H from 1H is supported
oxidation takes place at the tetradentate ligand to give Lꢀ2À rather by DFT calculation shown in Fig. S8 (ESI†), where the N2
than at the metal center.
elimination proceeds with DEa = 24.3 kcal molÀ1
.
To examine catalytic activity of 1H in C–H amination reaction,
The active species 3H can be a closed-shell singlet (3H-CSS)
we employed xanthene as a substrate and tosylazide (TsN3) as a like [RhIII(L3À)(NTs)(AH)] or an open-shell singlet di-radical (3H-
nitrene source (experimental procedures are described in the OSS) or a triplet di-radical (3H-T) of [RhIII(Lꢀ )(NꢀÀTs)(AH)].
2À
Experimental section in the ESI†). Thus, TsN3 (35.0 mmol) was The DFT calculation study (B3LYP/SDD) indicated that 3H-OSS
treated with the substrate (35.0 mmol) in toluene (1 mL) in the and 3H-T were close to each other in energy but energetically
presence of Boc2O (di-tert-butyl dicarbonate, 35.0 mmol) and 1H more stable than 3H-CSS by 13.6 and 13.7 kcal molÀ1, respec-
(3.5 mmol, 10 mol%) for 24 h at 100 1C to give N-xanthyl-p- tively. The spin density plots of 3H-OSS and 3H-T are shown in
toluenesulfonamide (A) in a 52% yield together with xanthone Fig. 3(a) and (b), respectively, where one radical delocalizes over
(B) in a 5% yield (Table 1, entry 1). When the reaction was carried the redox-active tetradentate ligand to make it dianion radical
2À
out in the absence of Boc2O, the reaction did not proceed at all structure as Lꢀ and another radical localizes at the nitrene
(entry 2). Addition of an excess amount of Boc2O (5 equiv.) did nitrogen atom in both structures.§ The extent of the radical
not improve the product yields (entry 3). Higher product yields delocalization on the tetradentate ligand is quite close to that of
were obtained, when 1OMe or 1Cl was employed as the catalyst, [1H]+ (one-electron oxidized complex of 1H, Fig. 1b). Therefore,
but the product selectivity [A/(A + B)] was slightly decreased the formation of the diradical species (3H-OSS or 3H-T) involves
(entries 4 and 5). When 9,10-dihydroanthracene was used as the an intramolecular one-electron transfer from the trianionic
substrate under the same conditions as entry 1, anthracene was tetradentate ligand L3À to the nitrene nitrogen atom without
produced in a 14% yield based on the substrate. The result may change of the oxidation state of RhIII center. The calculated
indicate that hydrogen atom abstraction from the sp3 carbon TsN–RhIII bond length are 1.90 Å for 3H-OSS and 1.99 Å for 3H-T,
of the substrate is involved as a key step, but, in the case of respectively, which are significantly longer than the RhIIIQN double
9,10-dihydroanthracene, aromatization from the generated bond length in a tetrahedral complex of [RhIII(NAd)(PhB(CH2PPh2)3)]
organic radical intermediate proceeds much faster than the (1.780(2) Å, Ad = adamantyl).11 Thus, the calculation study suggests
nitrogen rebound process (for detailed mechanistic discussion, that TsN–RhIII bond has a single bond character. The calculated SCF
see Fig. 4).10
energy profile for the C–H amination of xanthene with 3H-T is
To get insight into the C–H amination mechanism, a FAB- displayed in Fig. 4.¶ The optimized structures of the intermediates
mass spectrum (positive mode) of a mixture of 1H and TsN3 in and transition states involved in the pathways are provided in
toluene was measured (Fig. S7, ESI†) to give a peak cluster at Fig. S9 (ESI†). The first step of the amination involves inter-
m/z = 886.3 with an isotope distribution pattern consisting of a molecular H-atom abstraction from xanthene by the nitrene
radical complex of 3H-T to yield an intermediate 4H and the
xanthene radical (À20.1 kcal molÀ1). This step requires a
Table 1 C–H amination of xanthene with tosylazide catalyzed by 1X
transition state TS2 with a barrier of 10.8 kcal molÀ1. Then, a
Entry
Cat.a
Additive
Ab (%)
Bb (%)
Selectivityc (%)
1
2
3
4
5
1H
Boc2O (10 eq.)
None
Boc2O (5 eq.)
Boc2O (10 eq.)
Boc2O (10eq.)
52
0
52
73
72
5
0
6
12
21
91
—
90
86
77
1H
1H
1OMe
1Cl
Fig. 3 Spin density plots of (a) open-shell singlet (3H-OSS) and (b) triplet
(3H-T) of active oxidant. The color in blue represents positive regions and
the color in orange represents negative regions.
a
c
10 mol%. b 1H NMR yield (%). 100 Â A/(A + B).
This journal is ©The Royal Society of Chemistry 2017
Chem. Commun.

View Article Online
Communication
ChemComm
Notes and references
‡ The appearance of the additional protons to [Rh + L + NTs + AH] suggests
that the [Rh(L)(NTs)(AH)] might abstract hydrogen atom from the solvent.
§ The extent of the one radical localization on the nitrene nitrogen atom is
85%. Rh atom has only 5% and 6% spin density in 3H-OSS or 3H-T,
respectively. The rest is on –SO2– group of Ts moiety in the both structure.
¶ The C–H amination profile with 3H-OSS is identical with that with
3H-T.
1 Review and recent examples: B. de Bruin and D. G. H. Hetterscheid,
Eur. J. Inorg. Chem., 2007, 211; J. I. van der Vlugt and J. N. H. Reek,
Angew. Chem., Int. Ed., 2009, 48, 8832; P. J. Chirik and K. Wieghart,
Science, 2010, 327, 794; W. I. Dzik, J. I. van der Vlugt, J. N. H. Reek
and B. de Bruin, Angew. Chem., Int. Ed., 2011, 50, 3356; V. Lyasvskyy
and B. de Bruin, ACS Catal., 2012, 2, 270; J. I. van der Vlugt, Eur.
J. Inorg. Chem., 2012, 363; O. R. Luca and R. H. Crabtree, Chem. Soc.
Rev., 2013, 42, 1440; A. F. Heyduk, Dalton Trans., 2013, 42, 3751;
L. A. Berben, Chem. – Eur. J., 2015, 21, 2734; D. L. J. Broere,
R. Plessius and J. I. van der Vlugt, Chem. Soc. Rev., 2015, 44, 6886;
B. Bagh, D. L. J. Broere, V. Sinha, P. F. Kuijpers, N. P. van Leest, B. de
Bruin, S. Demeshko, M. A. Siegler and J. I. van der Vlugt, J. Am.
Chem. Soc., 2017, 139, 5117; V. Vreeken, M. A. Siegler and J. I. van
der Vlugt, Chem. – Eur. J., DOI: 10.1002/chem.201700358.
2 K. J. Blackmore, J. W. Ziller and A. F. Heyduk, Inorg. Chem., 2005,
44, 5559; T. W. Myers and L. A. Berben, J. Am. Chem. Soc., 2011,
133, 11865; E. J. Thompson and L. A. Berben, Angew. Chem., Int. Ed.,
2015, 54, 11642.
3 Review and recent: H. Lu and X. P. Zhang, Chem. Soc. Rev., 2011,
40, 1899; F. Collet, C. Lescot and P. Dauban, Chem. Soc. Rev., 2011,
40, 1926; C.-M. Che, V. Kar-Yan Lo, C.-Y. Zhou and J.-S. Huang,
Chem. Soc. Rev., 2011, 40, 1950; J. L. Roizen, M. E. Harvey and
J. Du Bois, Acc. Chem. Res., 2012, 45, 911; T. Uchida and T. Katsuki,
Chem. Rec., 2014, 14, 117; D. Intrieri, P. Zardi, A. Caselli and E. Gallo,
Chem. Commun., 2014, 50, 11440; K. Shin, H. Kim and S. Chang,
Acc. Chem. Res., 2015, 48, 1040.
Fig. 4 A computed reaction pathway for the C–H amination of xanthene.
Relative SCF energies with respect to 3H-T are reported in units of kcal molÀ1
.
subsequent radical rebound process takes place via a transition
state TS3 with a low barrier of 1.1 kcal molÀ1 to yield 5H, where
TS3 and 5H are singlet species. When a 1 : 1 mixture of xanthene
(17.5 mmol) and dideuterated-xanthene (xanthene-d2, 17.5 mmol)
were employed under the same conditions as entry 1 of Table 1,
a kinetic isotope effect (KIE) of 5.0 was observed, which was nearly
the same to the calculated KIE value of 5.7 at 375 K (Fig. S10,
experimental procedures are described in the Experimental section
in the ESI†). These results strongly support that the amination
reaction involves a hydrogen atom abstraction process from the sp3
carbon of the substrate.
In summary, Werner type six-coordinate rhodium(III) com-
plexes coordinated with a redox-active tetradentate ligand (L3À
)
4 W. Zhou, B. O. Patrick and K. M. Smith, Chem. Commun., 2014,
50, 9958; A. N. Desnoyer and J. A. Love, Chem. Soc. Rev., 2016, 46, 197.
5 D. L. J. Broere, B. de Bruin, J. N. H. Reek, M. Lutz, S. Dechert and
J. I. van der Vlugt, J. Am. Chem. Soc., 2014, 136, 11574; D. L. J. Broere,
N. P. van Leest, B. de Bruin, M. A. Siegler and J. I. van der Vlugt,
Inorg. Chem., 2016, 55, 8603.
and anilines (AX) axial ligands were found to undergo the
ligand-based oxidation with electrochemical and chemical
methods to give a ligand radical species, which was supported
by spectroscopic, electrochemical, X-ray crystallographic, and
computational studies. The complexes are capable to catalyze
the intermolecular C–H amination reaction of xanthene with
tosylazide. The DFT calculations indicated that one electron
transfer from the tetradentate ligand to the nitrene nitrogen
atom gives a reactive intermediate that possesses a di-radical
character with RhIII–N single bond. The kinetic deuterium
isotope effect (KIE) revealed the C–H amination involved hydro-
gen abstraction from the substrate. Although the substrate
scope was limited, this study expands ligand-based C–H activa-
tion chemistry using redox-non-innocent metal ions.
6 J. Takaichi, K. Ohkubo, H. Sugimoto, M. Nakano, D. Usa,
H. Maekawa, N. Fujieda, N. Nishiwaki, S. Seki, S. Fukuzumi and
S. Itoh, Dalton Trans., 2013, 42, 2438; Y. Morimoto, J. Takaichi,
S. Hanada, K. Ohkubo, H. Sugimoto, N. Fujieda, S. Fukuzumi and
S. Itoh, J. Porphyrins Phthalocyanines, 2015, 19, 377.
7 Only [RhIVF6]2À and [RhIVCl6]2À complexes have been reported as
Werner type rhodium(IV) complex. C. E. Housecroft and A. G. Sharpe,
in Inorganic Chemistry, ed. S. A. Grafos, Barcelona, 4th edn, 2012,
p. 822.
8 P. H. M. Budzelaar, N. N. P. Moonen, R. de Gelder, J. M. M. Smits
and A. W. Gal, Eur. J. Inorg. Chem., 2000, 753; G. Meier, V. Steck,
B. Braun, A. Eißler, R. Herrmann, M. Ahrens, R. Laubenstein and
T. Braun, Eur. J. Inorg. Chem., 2014, 2793.
9 S. N. Brown, Inorg. Chem., 2012, 51, 1251.
This work was supported by a Grants-in-Aid for scientific 10 V. Bagchi, P. Paraskevopoulou, P. Das, L. Chi, Q. Wang, A. Choudhury,
J. S. Mathieson, L. Cronin, D. B. Pardue, T. R. Cundari, G. Mitrikas,
Y. Sanakis and P. Stavropoulos, J. Am. Chem. Soc., 2014, 136, 11362.
11 A. M. Geer, C. Tejel and M. A. Ciriano, Angew. Chem., Int. Ed., 2014,
Research on Innovative Areas (No. 2408, JSPS KAKENHI Grant
Numbers JP24109014 and 23109015) and by CREST, JST
(JPMJCR16P1 and JPMJCR15P5).
53, 5614.
Chem. Commun.
This journal is ©The Royal Society of Chemistry 2017