Stepwise Reversible Oxidation of N-Peralkyl-Substituted NHC-CAAC Derived Triazaalkenes: Isolation of Radical Cations and Dications

Article abstract of DOI:10.1021/acs.orglett.7b02721

Herein, the isolation and characterization of N-peralkyl-substituted NHC-CAAC derived triazaalkenes in three oxidation states, neutral, radical cation, and dication, are reported. Cyclic voltammetry has shown the reversible electronic coupling between the first and second oxidation to be δE_1/2 = 0.50 V. As a proof-of-principle, to demonstrate the electron-rich nature of the triazaalkene, it was shown that it can be used as an electron donor in the reduction of an aryldiazonium salt to the corresponding arene.

Full text of DOI:10.1021/acs.orglett.7b02721

Letter
pubs.acs.org/OrgLett
Stepwise Reversible Oxidation of N‑Peralkyl-Substituted NHC−CAAC
Derived Triazaalkenes: Isolation of Radical Cations and Dications
†
†
†
‡
§
⊥
Debdeep Mandal, Ramapada Dolai, Nicolas Chrysochos, Pankaj Kalita, Ravi Kumar,
†
†
,⊥
Debabrata Dhara, Avijit Maiti, Ramakirushnan Suriya Narayanan, Gopalan Rajaraman,*
,
‡
,†,∥
,†
Carola Schulzke,* Vadapalli Chandrasekhar,*
and Anukul Jana*
†
TIFR Centre for Interdisciplinary Sciences Hyderabad, 21, Brundavan Colony, Narsingi, Hyderabad 500075, India
‡
Institut fu
̈
̈
r Biochemie, Ernst-Moritz-Arndt Universitat Greifswald, Felix-Hausdorff-Straße 4, D-17487 Greifswald, Germany
§
School of Chemical Sciences, National Institute of Science Education and Research, Bhimpur-Padampur, Jatni, Khurda, Bhubaneswar
7
52050, Odisha, India
⊥
Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India
∥Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, India
*
S Supporting Information
ABSTRACT: Herein, the isolation and characterization of N-
peralkyl-substituted NHC−CAAC derived triazaalkenes in
three oxidation states, neutral, radical cation, and dication,
are reported. Cyclic voltammetry has shown the reversible
electronic coupling between the first and second oxidation to
be ΔE1 = 0.50 V. As a proof-of-principle, to demonstrate the
/2
electron-rich nature of the triazaalkene, it was shown that it can be used as an electron donor in the reduction of an
aryldiazonium salt to the corresponding arene.
n recent years, electron-rich alkenes such as I (Scheme 1) have
caught the imagination of the research community,
(Scheme 1), possessing three stable oxidation states, utilizing two
I
different motifs: a benzannulated backbone functionalized N-
10
methyl-substituted NHC and a N-aryl-substituted CAAC.
Scheme 1. Chemical Structures of an Electron-Rich Olefin I,
One of the important features of NHCs has been their
sensitivity to the substituents present, either in the backbone or
on nitrogen. Indeed, remarkable differences in reactivities of
Tetrathiofulvalene II, and NHC−CAAC Heterodimer III
i
(
Dip = 2,6- Pr C H )
2
6
3
11
NHCs have been observed in organocatalysis and in main-
12
group organometallic chemistry upon variation in the ligand
backbone (e.g., saturated, unsaturated, benzannulated) and N-
13
14
substituents (aryl vs alkyl ) as a result of the modulation of the
stereoelectronic factors. Given this background in NHC
chemistry, it is surprising that the reported chemistry of
15
CAAC, thus far, is confined to only N-aryl substitution except
for a single report describing the synthesis of a N-tBu-substituted
CAAC as its lithium triflate adduct. This has motivated us to
study the N-alkyl-substituted CAAC system, which is reported
herein.
particularly in view of their potential applications as electron
16
1
donors in organic reactions. However, in contrast to the classical
2
example of tetrathiofulvalene (TTF), II (Scheme 1), with many
other electron-rich olefins, it has not been possible to isolate
3
We have considered the Me group as the backbone of
radical cations. In view of the potential applications
Me
iPr
unsaturated N-alkyl-substituted NHC 1 and 1 along with N-
demonstrated by the tetrathiofulvalene family of compounds in
iPr
4
5
isopropyl-substituted cyclic iminium salt 2 as the building
diverse research fields, including electronic devices, it is of
interest to explore whether other families of electron-rich olefins
can be prepared with similar or larger electronic coupling
between their first and second oxidation events. If realized, one
can expect the isolation of redox-amphoteric radical cations and
blocks, with the anticipation that the NHC will attack the
electrophilic carbon center of the cyclic iminium salt, leading to
1
0,17
the conjugate acids of NHC−CAAC derived alkenes.
A
similar strategy was used for the dimerization of diaminocarbenes
18
Me
iPr
6
to obtain tetraazaalkenes. Accordingly, 1 and 1 were
their subsequent use in various applications. Cyclic alkyl amino
iPr19
7
reacted separately with 2
in THF at rt, leading to the
carbene (CAAC) ligands appear to be extremely suitable for
8
stabilizing C-center based radicals and other entities possessing
9
reactive functionalities. Bertrand and co-workers have recently
Received: September 2, 2017
reported the isolation of a NHC−CAAC heterodimer, III
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b02721
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
MeiPr
formation of the conjugated acids of triazaolefins, 3
(93%)
Encouraged by the electrochemical studies and with a desire to
iPriPr
19
MeiPr
iPriPr
isolate the oxidized products, we reacted 4
and 4 1 with
and 3
(91%) as colorless solids (Scheme 2).
9
AgOTf in a 1:1 and 1:2 ratio, respectively (Scheme 4). In the
Scheme 2. Syntheses of 3
Scheme 4. Syntheses of 5 and 6
1
MeiPr
The H NMR spectra of compounds 3
and 3
(δ = 4.48 ppm)
iPriPr
(δ = 4.70 ppm) reveal a characteristic singlet for the
“
C−H” unit. These chemical shifts are considerably upfield
case of the 1:1 reaction, we observed an immediate visible color
change from colorless to deep orange red, and we isolated the
shifted when compared with that of the starting cyclic iminium
salt 2 (δ = 9.23 ppm).
iPr
MeiPr
iPriPr
corresponding radical cations 5
X-band EPR measurement of 5
(86%) and 5
(70%).
MeiPr
19
and 3iPriPr were
MeiPr
iPriPr
The solid-state molecular structures of 3
and 5
in THF solution
confirmed by single-crystal X-ray diffraction studies (Figure 1),
at rt shows that the unpaired electron in these compounds is
coupled with all three nitrogen atoms, as indicated by the
19
presence of hyperfine coupling in the EPR spectra (Figure 2).
Figure 2. Experimental and simulated EPR spectra of 5MeiPr and 5iPriPr in
THF.
Figure 1. Molecular structure of 3MeiPr in the solid state (thermal
ellipsoids at 50%, triflate anion, and H atoms except C4−H omitted for
clarity). Selected bond lengths (Å) and angles (°): C3−C4 1.517(3);
N2−C3−N1 107.2(2), N3−C4−C7 105.8(2).
Simulation of the EPR spectra yielded 16.8, 13.2, and 12.0
MHz and 19.8, 12.7, and 10.4 MHz as the hyperfine coupling
MeiPr
iPriPr
constants involving the three nitrogen atoms in 5
and 5
,
21
which are consistent with their solution behavior. The bond
distances between the connecting carbon centers of the two
respectively. For both compounds, an isotropic g tensor of
2.006 was estimated. One large and two smaller isotropic
hyperfine constants are noted in both cases, which clearly
suggests that the spin density is delocalized from the carbon atom
to all three nitrogen atoms. We have also estimated the g and
hyperfine tensors of these compounds using DFT methods
MeiPr
heterocyles are 1.517(3) and 1.510(3) Å, respectively, for 3
and 3iPriPr.
Subsequent deprotonation of 3MeiPr and 3iPriPr with potassium
bis(trimethylsilyl)amide (KN(TMS) ) and lithium-2,2,6,6-tetra-
2
MeiPr
iPriPr
22
methylpiperidide (LiTMP) afforded 4
(92%) and 4
employing the ORCA suite of programs, affording values 14.3,
MeiPr
iPriPr
1
4.2, and 10.7 MHz (5
) and 14.1, 14.1, and 7.7 MHz (5
)
Scheme 3. Syntheses of 4
for N , N , and N , respectively. These estimated values are
1
2
3
consistent with the simulations, and the differences in the
hyperfine coupling are found to arise from the spin-dipolar term.
MeiPr
iPriPr
Radical cations of 5
and 5
showed absorption in the
MeiPr
iPriPr
UV/vis region, whereas the parent alkenes 4
absorption only in the UV region. In the UV/vis spectra, 5
and 4
show
MeiPr
shows two absorption bands at λ (ε) = 379 (1399.0) and 452
max
(
349.4) nm (L mol− cm ), whereas 5
1
−1
iPriPr
exhibits three
1
9
(
46%), respectively (Scheme 3). Solution-state 1H and
C{ H} NMR spectra of 4
absorption bands at λmax(ε) = 280 (1556.8), 371 (1913.2), and
1
3
1
MeiPr
and 4iPriPr are consistent with
−1
−1
465 (494.7) nm (L mol cm ). These values are blue-shifted
when compared with III (λmax(ε) = 530 nm). TD-DFT
calculations show that the longest wavelength absorptions of
their expected chemical structures; both are stable in solution
under inert atmosphere, and we did not observe any kind of
change even up to 80 °C.
MeiPr
iPriPr
5
at 452 nm and of 5
at 465 nm correspond to the
The cyclic voltammogram of 4MeiPr showed two reversible
one-electron redox processes with an electronic coupling of
transition of an α-spin of the SOMO (singly occupied molecular
orbital) to an α-spin of the LUMO. Adjacent transitions at 379
and 371 nm in 5
MeiPr
19
MeiPr
iPriPr
Δ[E (4
)] = 0.50 V at −20 °C, which is larger than that of
and 5
, respectively, correspond to a β-
1/2
20
10
19
iPriPr
II (Δ[E ] = 0.37 V) and III (Δ[E ] = 0.34 V).
spin of HOMO to β-spin of LUMO transition. In case of 5
,
1/2
1/2
B
DOI: 10.1021/acs.orglett.7b02721
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
the signal at 280 nm is due to the transition of an α-spin of the
1
9
SOMO to an α-spin of the LUMO+1.
Solid-state structure analysis of 5
MeiPr19
and 5iPriPr shows that
the central C−C bond lengths are 1.484(3) and 1.444(8) Å,
respectively (Figure 3). These are longer than standard C−C
Figure 4. Molecular structure of 6MeiPr in the solid state (thermal
ellipsoids at 30%, triflate anion, and H atoms omitted for clarity).
Selected bond lengths (Å) and angles (°): C1−C4 1.482(2); N1−C1−
N2 108.15(13), N3−C4−C5 113.44(14).
Scheme 5. Comproportionation of 4 and 6 to 5
Figure 3. Molecular structure of 5iPriPr in the solid state (thermal
ellipsoids at 30%, triflate anion, and H atoms omitted for clarity).
Selected bond lengths (Å) and angles (°): C4−C5 1.444(8); N1−C4−
C3 111.4(5), N2−C5−N3 106.6(5).
Scheme 6. Reduction of 7 by 4MeiPr
double bond lengths, suggesting that the oxidized products are
formed by the removal of an electron from the bonding orbital.
MeiPr
The torsion angles between the two heterocycles of 5
5
and
iPriPr
are 73.77 and 57.73°, respectively. The differences in these
angles presumably arise due to the steric differences between the
Me and Pr groups on the N-centers of the NHC moiety. The
In summary, we have disclosed N-peralkyl-substituted NHC−
CAAC derived electron-rich triazaolefins, which exhibit three
i
sum of the bond angles around the N-centers of the CAAC
stable oxidation states with ΔE1 = 0.50 V, larger than that of
/2
MeiPr
iPriPr
moiety of 5
and 5
is 344.86 and 356.30°, respectively,
TTF and therefore showing potential for future applications. The
reversible nature of the electron transfer process was
demonstrated both electrochemically as well as chemically
(comproportionation reaction). Currently, we are extending the
scope of these studies to prepare a larger variety of such
compounds and also to explore their properties further.
indicating some degree of pyramidalized geometry, which is quite
unlike what is found in NHCs.
MeiPr
iPriPr
In the 1:2 reaction of 4
and 4
with AgOTf, we
MeiPr
and 6iPriP in good yields.
obtained corresponding dications 6
Analysis of solid-state molecular structures of 6
MeiPr
iPriPr19
and 6
reveal that upon oxidation of the radical cation to the dication the
MeiPr
central C−C bond length is almost unchanged (5
(1.484(3)
(1.444(8) Å) to 6
ASSOCIATED CONTENT
Supporting Information
■
MeiPr
iPriPr
iPriPr
Å) to 6
(1.482(2) Å) and 5
*
S
(
1.487(5) Å) (Figure 4). The torsion angles between the two
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.7b02721.
MeiPr
iPriPr
heterocycles of the dications 6
and 6
are 89.59 and
8
8.48°, which are very close to each other.
Experimental and computational details (PDF)
Crystallographic details and data (CIF)
After the isolation of three oxidation states of NHC−CAAC
derived alkenes, we carried out stoichiometric reactions between
neutral and dicationic NHC−CAAC heterodimers (Scheme
1
9
AUTHOR INFORMATION
Corresponding Authors
5
). The formation of the radical cations in this comproportio-
■
*
*
*
*
nation reaction is another strong indication, in addition to cyclic
voltammetry results, of the reversible nature of the electron
transfer.
E-mail: rajaraman@chem.iitb.ac.in.
E-mail: carola.schulzke@uni-greifswald.de.
E-mail: vc@tifrh.res.in.
E-mail: ajana@tifrh.res.in.
ORCID
Finally, to demonstrate the reductant properties of the newly
synthesized electron-rich olefins, we considered the reduction of
MeiPr
4-methoxybenzenediazonium tetrafluoroborate, 7 with 4
. At
rt, involving a 1:1 reaction, the formation of anisole 8 under
19
Gopalan Rajaraman: 0000-0001-6133-3026
Vadapalli Chandrasekhar: 0000-0003-1968-2980
Anukul Jana: 0000-0002-1657-1321
elimination of N gas was indeed detected (Scheme 6). We
2
have not attempted to optimize the reaction conditions, though
MeiPr
the electron donor capability of 4
has been established.
C
DOI: 10.1021/acs.orglett.7b02721
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Notes
(12) For selected references: (a) Jana, A.; Huch, V.; Rzepa, H. S.;
Scheschkewitz, D. Organometallics 2015, 34, 2130−2133. (b) Sidir-
opoulos, A.; Jones, C.; Stasch, A.; Klein, S.; Frenking, G. Angew. Chem.,
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Merkel, S.; Henn, J.; Stalke, D. Angew. Chem., Int. Ed. 2009, 48, 5683−
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
5
686. (d) Filippou, A. C.; Chernov, O.; Schnakenburg, G. Angew. Chem.,
This work is supported by the TIFR Centre for Interdisciplinary
Science Hyderabad, Hyderabad, India, and SERB-DST (EMR/
Int. Ed. 2009, 48, 5687−5690. (e) Filippou, A. C.; Lebedev, Y. N.;
Chernov, O.; Straßmann, M.; Schnakenburg, G. Angew. Chem., Int. Ed.
2
014/001237), India. V.C. is thankful to the Department of
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013, 52, 6974−6978.
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F. J.; Ku
Science and Technology, New Delhi, India, for a National J.C.
(
Bose fellowship. G.R. would like to thank SERB-DST EMR/
̈
hn, F. E. Chem. Rev. 2017, 117, 1970−2058. (b) Ghadwal, R. S.;
2
014/000247 for funding. We are indebted to Prof. Ranjan Das,
Azhakar, R.; Roesky, H. W. Acc. Chem. Res. 2013, 46, 444−456.
TIFR Mumbai, India, for the measurement of EPR. We are
thankful to Prof. David Scheschkewitz, Saarland University,
(c) Bourissou, D.; Guerret, O.; Gabbaï, F. P.; Bertrand, G. Chem. Rev.
2
(
000, 100, 39−91.
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iPr
IIT Kharagpur, India, for the single-crystal X-ray data of 2 and
MeiPr
3
, and Dr. Rahul Bannerjee, NCL Pune, India, for the single-
crystal X-ray data of 6MeiPr.
(
d) Rupar, P. A.; Staroverov, V. N.; Ragogna, P. J.; Baines, K. M. J. Am.
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DOI: 10.1021/acs.orglett.7b02721
Org. Lett. XXXX, XXX, XXX−XXX