Environmentally benign strategy for arylation of nitronyl nitroxide using a non-Transition metal nucleophile

Article abstract of DOI:10.1021/acs.orglett.9b04655

We have developed a method for the arylation of nitronyl nitroxide without using its transition metal complex as a nucleophile. Various nitronyl nitroxide-substituted I -electronic compounds can be obtained from the parent nitronyl nitroxide and the corresponding aryl iodides using a combination of zero-valent palladium catalysts and a 2-dicyclohexylphosphino-2′,4′,6′-Triisopropylbiphenyl ligand in the presence of sodium tert-butoxide. The utility of the method has been demonstrated by the direct synthesis of open-shell compounds with giant I -electronic systems, such as 10P.

Full text of DOI:10.1021/acs.orglett.9b04655

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Letter
Environmentally Benign Strategy for Arylation of Nitronyl Nitroxide
Using a Non-Transition Metal Nucleophile
Shuichi Suzuki,* Fumiya Nakamura, and Takeshi Naota*
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ABSTRACT: We have developed a method for the arylation of
nitronyl nitroxide without using its transition metal complex as a
nucleophile. Various nitronyl nitroxide-substituted π-electronic
compounds can be obtained from the parent nitronyl nitroxide and
the corresponding aryl iodides using a combination of zero-valent
palladium catalysts and a 2-dicyclohexylphosphino-2′,4′,6′-triiso-
propylbiphenyl ligand in the presence of sodium tert-butoxide. The
utility of the method has been demonstrated by the direct synthesis
of open-shell compounds with giant π-electronic systems, such as
10P.
functional NN-Ar derivatives, such as diradicals with strong
ferromagnetic interactions3c,9 and graphene-like π-electronic
systems substituted with NN moieties.10 Most of these
derivatives could not be obtained by the previous condensation
method due to the difficulty in synthesizing the starting
aldehydes and the high reactivity of the aromatic functionalities
toward the oxidation process.3c,9 These methods require
transition metal NN complexes (NN-M) as the NN anion
equivalent, because the NN-Ms are moderately reactive but
sufficiently robust toward the degradation during these
catalytic processes, probably due to stabilization of the C−M
bond by the specific d−π conjugation. The use of NN anions
of alkaline and alkaline-earth metals for a family of the coupling
reactions11 may help establish an environmentally benign
strategy due to the lack of a need for transition metals and the
subsequent wasting process. However, due to the extraordi-
narily low stability of these “naked” radical anion species, our
trials using stoichiometric lithium and magnesium NN species
always gave unsatisfactory results.5b,12,13 Hence, we adopted a
method for the “in situ” generation of the naked NN anions
based on a strategy that includes (1) the equilibrated
deprotonation of NN by a bulky base such as NaOtBu and
(2) the activation of the NN anion in nonpolar solvents such
as toluene. In this system, the radical anion species is well
protected from inevitable decomposition or disproportiona-
tion. The method thus provides a direct and green method for
the versatile synthesis of the NN-Ar that proceeds with high
efficiency under the mild conditions (Scheme 1c).
table organic radicals are promising components for
Sintriguing electronic and spintronic devices because of
their structural and electronic flexibility and processability.1
Nitronyl nitroxide derivatives having aromatic functionalities
with extended π-electronic systems (NN-Ar) have been
utilized for organic spin sources, and their properties have
been investigated in light of negative magnetoresistance,2 high-
spin molecules,3 and hybrid ferrimagnets.4 Generally, NN-Ar
derivatives are synthesized by condensation reactions of
various aromatic aldehydes with N,N′-dihydroxy-2,3-dimethyl-
butane-2,3-diamine (Scheme 1a).5 Recently, the Okada group
and our group have developed synthetic methods for NN-Ar
derivatives from aryl halides and transition metal complexes
(Au,6 Cu,7 and Zn8) of NN radical anions (Scheme 1b). These
coupling methods serve as powerful tools for the syntheses of
Scheme 1. Synthetic Methods for NN-Ars
Received: December 27, 2019
https://dx.doi.org/10.1021/acs.orglett.9b04655
© XXXX American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
A

Organic Letters
pubs.acs.org/OrgLett
Letter
We initially examined various Li, Na, K, and Cs bases and
solvents to confirm their suitability for the coupling reaction of
4-iodoacetophenone (1a) with NN-H in the presence of a
tetrakis(triphenylphosphine)palladium(0) [Pd(PPh3)4] cata-
lyst (10 mol %) for 1−2 h at 70 °C (Table 1). All trials in
Table 2. Catalytic Activity of Various Palladium Complexes
and Ligands for the Reaction of 1a with NN
Table 1. Cross-Coupling Reaction of 1a with NN-H
a
entry
catalyst
Pd(PPh3)4
additive
yield (%)
1
2
3
4
5
6
7
8
none
63
62
60
65
79
83
73
7
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd2(dba)3·CHCl3
Pd2(dba)3·CHCl3
PdCl2
Pd(PPh3)2Cl2
Pd(OAc)2
Pd-PEPPSI-iPr
none
dppe (10 mol %)
dcype (10 mol %)
BINAP (10 mol %)
dppf (10 mol %)
XPhos (20 mol %)
SPhos (20 mol %)
none
XPhos (20 mol %)
dppf (10 mol %)
none
a
entry
base
solvent
yield (%)
b
1
2
3
4
5
6
7
8
NaOMe
NaOtBu
KOtBu
LiHMDS
NaOMe
NaHMDS
NaOtBu
KOtBu
MeOH
tBuOH
tBuOH
THF
THF
THF
0
b
0
b
0
9
31
b
b
0
10
11
12
13
14
0
b
b
trace
b
trace
b
0
XPhos (20 mol %)
none
trace
b
b
THF
THF
trace
trace
0
0
b
b
none
b
a
b
9
LiHMDS
NaH
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
trace
Isolated yield based on NN-H. Monitored by TLC.
10
11
12
13
14
15
16
21
b
NaOMe
NaHMDS
NaOtBu
NaOtBu
KOtBu
0
respectively). Tris(dibenzylideneacetone)dipalladium(0)−
chloroform adduct [Pd2(dba)3·CHCl3] and various divalent
palladium complexes, PdCl2−bis(triphenylphosphine) [Pd-
(PPh3)2Cl2], PdCl2−[1,1′-bis(diphenylphosphino)ferrocene
(dppf)], [Pd(OAc)2]−[2-dicyclohexylphosphino-2′,4′,6′-trii-
sopropylbiphenyl (Xphos)], and PdCl2−[1,3-bis(2,6-
diisopropylphenyl)imidazol-2-ylidene](3-chloropyridyl) [Pd-
PEPPSI-iPr], showed almost no catalytic activity (entries 8,
10−13, respectively). It is noteworthy that the yield of the
corresponding NN-Ar in this catalytic process (entry 6) was
higher than those obtained upon similar treatment with
transition metal NN nucleophiles such as NN-Au(PPh3)6 and
NN-ZnCl8 (Tables S1 and S2).
The acceleration effect of XPhos was also observed by time-
dependent changes in the concentration of the starting
material NN-H, which can be monitored at λmax maxima of
549 nm by means of UV−vis spectrometry (Figure S1). The
initial rate constants kobs (kobs = −d[NN-H]/dt) were
determined to be 5.50 × 10−4 s−1 for the Pd(PPh3)4 (10
mol %)−XPhos (20 mol %) catalyst and 3.42 × 10−4 s−1 for
the Pd(PPh3)4 catalyst (10 mol %) (Figure 1), which indicated
that the reaction with XPhos proceeds 1.6 times faster than
that without the ligand. The linear time dependence of the
concentration of NN-H in the reaction with the Pd(PPh3)4−
XPhos catalyst (Figure 1a and Figure S1) indicated that the
addition effect of XPhos is ascribed to the acceleration of the
rate-determining step in the catalytic process instead of the
inhibition of the deactivation of the catalytically active species.
To gain information about the reaction mechanism, we
monitored the coupling reaction of NN-H and 1a under the
optimized condition by means of electron spin resonance
(ESR) spectroscopy (Figure 1b). The 10-line ESR signal for
NN-H [|aN| (hyperfine coupling of 14N nuclei) = 0.74 mT, and
|aH| (hyperfine coupling of 1H nucleus) = 0.36 mT],6 observed
at the initial stage of the reaction, was gradually converted into
a five-line signal (|aN| = 0.75 mT) that was assigned to 1P. In
spite of the lack of detection of ESR signals assigned to NN-Na
during the catalytic reaction, the formation of a stoichiometric
29
63
40
53
c
b
Cs2CO3
0
a
b
c
Isolated yield based on NN-H. Monitored by TLC. With 5 mol %
Pd(PPh3)4.
the search for suitable conditions for generating the NN radical
anion using a typical combination of base and soluble solvents,
such as NaOMe and MeOH, NaOtBu and tBuOH, LiHMDS
and THF, and NaOtBu and THF, resulted in no reaction or
gave trace amounts of the desired compound. This was
probably due to fast decomposition or disproportionation of
the unstable NN radical anions (entries 1−8). Notably,
NaOtBu and KOtBu provided good results when used in
combination with toluene (entries 13−15). The successful
results can be explained by the low solubility of NaOtBu and
KOtBu in toluene, which keeps the concentration of the NN
anion low during the catalytic process. This specific
equilibration would successfully inhibit the inevitable dis-
proportionation of NN in this particular system. Cs2CO3
afforded a trace amount of the product probably due to its
weak basicity for the generation of the radical anion (entry 16).
The catalytic activity of various palladium complexes was
examined for the coupling reaction of 1a with NN-H (1 equiv)
in the presence of NaOtBu (1 equiv) in toluene (Table 2).
Pd(PPh3)4 was the best catalyst among the catalysts examined
(entries 1−7). The addition of 1,1′-bis(diphenylphosphino)-
ferrocene (dppf), 2-dicyclohexylphosphino-2′,4′,6′-triisopro-
pylbiphenyl (XPhos), or 2-dicyclohexylphosphino-2′,6′-dime-
thoxybiphenyl (SPhos) effectively increased the product yield
(entry 5, 6, or 7, respectively), while the addition of other
phosphines, 1,2-bis(diphenylphosphino)ethane (dppe), 1,2-
bis(dicyclohexylphosphino)ethane (dcype), or 2,2′-bis-
(diphenylphosphino)-1,1′-binaphthyl (BINAP), did not lead
to any significant additive (or ligand) effect (entries 1−4,
B
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pubs.acs.org/OrgLett
Letter
Table 3. Cross-Coupling Reaction of Various Ar−X
Substrates
Figure 1. Time-dependent changes in (a) absorbance at 549 nm and
(b) ESR spectra during the reaction of 1a (1.32 × 10−2 M) and NN-
H (1 equiv) with the Pd(PPh3)4 (10 mol %)−XPhos (20 mol %)
catalyst in the presence of NaOtBu (1 equiv) in toluene at 70 °C.
Filled and empty squares denote the reactions with and without
XPhos as an additive, respectively. The red and blue solid lines show
the fitting lines by the first-order equation. The observed rate
constants (kobs) of the reaction with and without the additive were
estimated to be 5.50 × 10−4 and 3.42 × 10−4 s−1, respectively.
amount of NN-D from the reaction of NN-H with NaOtBu
was confirmed by the ESR analysis of the control reaction in
CD3OD and toluene (Figure S2), which strongly suggests that
the NN-Na intermediate generated from NN-H and NaOtBu
is consumed immediately upon the subsequent reaction with
the ArPdILn species during this catalytic coupling reaction.
This mechanistic assumption rationally explains the exper-
imental result showing that the present coupling reaction using
NN-Na as the nucleophile proceeded specifically when
NaOtBu and toluene were used as a combination of base
and solvent (Scheme S1). NN-H undergoes equilibrated
proton abstraction with the bulky NaOtBu to give a trace
amount of NN-Na. NN-Na acts as an efficient nucleophile for
this coupling reaction, undergoing rapid transmetalation with
the ArPdILn species formed by the oxidative addition of the
catalytically active PdLn species to Ar−I. The reaction would
proceed smoothly without any degradation of the unstable
NN-Na, despite the low concentration and low stability of this
intermediate, because highly diluted NN-Na is protected from
self-degradation but sufficiently activated as an effective
nucleophile in a noncoordinating solvent such as toluene.
The unsatisfactory results obtained when using NaH and
NaHMDS (entries 10 and 12, respectively, of Table 1) would
be caused by the stoichiometric formation of NN-Na and its
subsequent rapid self-degradation. The acceleration effect of
dppf and Xphos (entries 5 and 6, respectively, of Table 2) can
be explained by the strong electron-donating nature of these
ligands,14 which would accelerate the rate-determining step for
the oxidative addition of PdLn to Ar−I.
Table 3 summarizes the representative results for the
coupling reaction using catalytic amounts of Pd(PPh3)4 and
XPhos as the optimal condition.15 Various aryl iodides could
be efficiently converted into the corresponding NN-Ar
derivatives (entries 4, 5, 7−10, 12, and 14−17), while a
similar treatment of aryl bromides and triflate gave
unsatisfactory results (entries 1−3, 6, 11, and 13). Phenyl
iodides with various electron-donating and electron-with-
drawing groups (4a−7a) could be converted into the desired
NN derivatives (4P−7P, respectively) (entries 7−10,
respectively). π-Extended NNs (8P−10P) and heteroaromatic
NNs (11P and 12P) except for pyridine derivative 13P could
also be obtained with high efficiency (entries 12 and 14−18).
The NN functionality in the NN-Ar products was confirmed
a
b
entry
substrate (Ar-X)
product (NN-Ar)
yield (%)
c
1
2
3
4
5
6
7
8
1b
2b
2c
2a
3a
4b
4a
5a
6a
7a
8b
8a
9b
9a
10a
11a
12a
13a
1P
2P
2P
2P
3P
4P
4P
5P
6P
7P
8P
8P
9P
9P
10P
11P
12P
trace
trace
trace
c
c
78
76
trace
c
84
81
73
72
9
10
11
12
13
14
15
16
17
c
trace
78
trace
c
60
64
62
61
c
18
13P
trace
a
b
For a, X = I. For b, X = Br. For c, X = OTf. Isolated yield based on
c
NN-H. Detected by TLC.
by single-crystal XRD analysis of the 10P crystal obtained by
recrystallization from a dichloromethane/hexane mixture,16 as
shown in Figure 2. This catalytic reaction allows for the facile
and versatile syntheses of NN-Ar compounds with giant π-
electronic systems, which have been recognized as candidates
Figure 2. (a) Top and (b) side ORTEP views of 10P (50%
probabilities). Hydrogen atoms have been omitted for the sake of
clarity.
C
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Organic Letters
pubs.acs.org/OrgLett
Letter
Open-Shell Graphene Fragments. Nat. Chem. 2011, 3, 197. (e) Ratera,
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for molecular graphene nanoribbons functionalized with
magnetic edges.10
We developed a new direct functionalization of NN-H to
NN-Ar derivatives using catalytic amounts of palladium
complexes. The method can be applied to various π-electronic
systems, including benzene moieties, heteroaromatic moieties,
and fused benzene species. We also demonstrated that this
method is a simpler alternative to the conventional multistep
methods using stoichiometric amounts of transition metal
sources for the preparation of new open-shell molecules.
(3) (a) Hiraoka, S.; Okamoto, T.; Kozaki, M.; Shiomi, D.; Sato, K.;
Takui, T.; Okada, K. A Stable Radical-Substituted Radical Cation with
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5,10-Diphenyl-5,10-dihydrophenazine Radical Cation. J. Am. Chem.
Soc. 2004, 126, 58. (b) Kuratsu, M.; Suzuki, S.; Kozaki, M.; Shiomi,
D.; Sato, K.; Takui, T.; Kanzawa, T.; Hosokoshi, Y.; Lan, X.-Z.;
Miyazaki, Y.; Inaba, A.; Okada, K. (Nitronyl Nitroxide)-Substituted
Trioxytriphenylamine Radical Cation Tetrachlorogallate Salt: A 2p-
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Junghoefer, T.; Giangrisostomi, E.; Ovsyannikov, R.; Pink, M.; Rajca,
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Ground State Diradical. J. Am. Chem. Soc. 2019, 141, 4764.
(4) Masuda, Y.; Kuratsu, M.; Suzuki, S.; Kozaki, M.; Shiomi, D.;
Sato, K.; Takui, T.; Hosokoshi, Y.; Lan, X.-Z.; Miyazaki, Y.; Inaba, A.;
Okada, K. A New Ferrimagnet Based on a Radical-Substituted Radical
Cation Salt. J. Am. Chem. Soc. 2009, 131, 4670.
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Nitronyl Nitroxides, a New Class of Stable Radicals. J. Am. Chem. Soc.
1968, 90, 1078. (b) Ullman, E. F.; Osiecki, J. H.; Boocock, D. G. B.;
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and Biradicals as Possible Small Molecule Spin Labels. J. Am. Chem.
Soc. 1972, 94, 7049.
(6) (a) Tanimoto, R.; Suzuki, S.; Kozaki, M.; Okada, K. Nitronyl
Nitroxide as a Coupling Partner: Pd-Mediated Cross-coupling of
(Nitronyl nitroxide-2-ido)(triphenylphosphine)gold(I) with Aryl
Halides. Chem. Lett. 2014, 43, 678. (b) Nishinaga, T.; Kanzaki, Y.;
Shiomi, D.; Matsuda, K.; Suzuki, S.; Okada, K. Radical Cation π-
Dimers of Conjugated Oligomers as Molecular Wires: An Analysis
Based on Nitronyl Nitroxide Spin Labels. Chem. - Eur. J. 2018, 24,
11717. (c) Tahara, T.; Suzuki, S.; Kozaki, M.; Shiomi, D.; Sugisaki,
K.; Sato, K.; Takui, T.; Miyake, Y.; Hosokoshi, Y.; Nojiri, H.; Okada,
K. Triplet Diradical-Cation Salts Consisting of the Phenothiazine
Radical Cation and a Nitronyl Nitroxide. Chem. - Eur. J. 2019, 25,
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ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.9b04655.
Experimental procedure, UV-vis spectra of NN-H and
1P, effects of phosphine ligand for other coupling
reaction, ESR spectra of NN-H in CD3OD−toluene,
proposed reaction mechanism, and ESR spectra of 10P
and 11P (PDF)
Accession Codes
CCDC 1973888 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
■
Shuichi Suzuki − Department of Chemistry, Graduate School of
Engineering Science, Osaka University, Toyonaka, Osaka 560-
8531, Japan; orcid.org/0000-0001-6294-9084;
Email: suzuki-s@chem.es.osaka-u.ac.jp
Takeshi Naota − Department of Chemistry, Graduate School of
Engineering Science, Osaka University, Toyonaka, Osaka 560-
8531, Japan; orcid.org/0000-0001-5348-3390;
Email: naota@chem.es.osaka-u.ac.jp
(7) Yamada, K.; Zhang, X.; Tanimoto, R.; Suzuki, S.; Kozaki, M.;
Tanaka, R.; Okada, K. Radical Metalloids with N-Heterocyclic
Carbene and Phenanthroline Ligands: Synthesis, Properties, and
Cross-Coupling Reaction of [(Nitronyl Nitroxide)-2-ido]metal
Complexes with Aryl Halides. Bull. Chem. Soc. Jpn. 2018, 91, 1150.
(8) Suzuki, S.; Nakamura, F.; Naota, T. A Direct Synthetic Method
for (Nitronyl Nitroxide)-Substituted π-Electronic Compounds via a
Palladium-Catalyzed Cross-Coupling Reaction with a Zinc Complex.
Mater. Chem. Front. 2018, 2, 591.
Author
Fumiya Nakamura − Department of Chemistry, Graduate
School of Engineering Science, Osaka University, Toyonaka,
Osaka 560-8531, Japan
(9) Haraguchi, M.; Tretyakov, E.; Gritsan, N.; Romanenko, G.;
Gorbunov, D.; Bogomyakov, A.; Maryunina, K.; Suzuki, S.; Kozaki,
M.; Shiomi, D.; Sato, K.; Takui, T.; Nishihara, S.; Inoue, K.; Okada,
K. (Azulene-1,3-diyl)-bis(nitronyl nitroxide) and (Azulene-1,3-diyl)-
bis(iminonitroxide) and Their Copper Complexes. Chem. - Asian J.
2017, 12, 2929.
(10) Slota, M.; Keerthi, A.; Myers, W. K.; Tretyakov, E.;
Baumgarten, M.; Ardavan, A.; Sadeghi, H.; Lambert, C. J.; Narita,
A.; Mullen, K.; Bogani, L. Magnetic Edge States and Coherent
̈
Manipulation of Graphene Nanoribbons. Nature 2018, 557, 691.
(11) For example: (a) Murahashi, S.-I. Palladium-Catalyzed Cross-
Coupling Reaction of Organic Halides with Grignard Reagents,
Organolithium Compounds and Heteroatom nucleophiles. J. Organo-
met. Chem. 2002, 653, 27. (b) Murahashi, S.-I.; Tanba, Y.; Yamamura,
M.; Moritani, I. The Reactions of ortho-Carbon σ-Bonded Aromatics-
Palladium Complexes with Alkyllithium. Selective Syntheses of ortho-
Alkyl substituted Aromatic Compounds. Tetrahedron Lett. 1974, 15,
3749. (c) Yamamura, M.; Moritani, I.; Murahashi, S.-I. The Reaction
of σ-Vinylpalladium Complexes with Alkyllithiums. Sereospecific
Syntheses of Olefins from Vinyl Halides and Alkyllithiums. J.
Complete contact information is available at:
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by a Grant-in-Aid for Scientific
Research [JSPS KAKENHI Grants JP26102005 (S.S.),
JP17K05783 (S.S.), and JP16H06516 (T.N.)].
REFERENCES
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(13) Suzuki−Miyaura-type and Migita−Kosugi−Stille-type coupling
reactions also gave unsatisfying results because NN radical derivatives
decomposed under these reaction conditions. See ref 6a.
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(16) CCDC 1973888 (10P) contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.ccdc.
cam.ac.uk/data_request/cif.
E
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