Crystalline Monomeric Allenyl/Propargyl Radical

Article abstract of DOI:10.1021/jacs.7b09622

Reduction of alkynyl iminium salts derived from cyclic (alkyl)(amino)carbenes (CAACs) affords propargyl/allenyl radicals. Depending on the nature of the CAAC and alkyne substituents, these radicals can irreversibly dimerize, exist as monomers in solution

Full text of DOI:10.1021/jacs.7b09622

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Communication
A Crystalline Monomeric Allenyl/Propargyl Radical
Max M. Hansmann, Mohand Melaimi, and Guy Bertrand
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b09622 • Publication Date (Web): 18 Oct 2017
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A Crystalline Monomeric Allenyl/Propargyl Radical
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Max M. Hansmann, Mohand Melaimi and Guy Bertrand*
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UCSD-CNRS Joint Research Laboratory (UMI 3555), Department of Chemistry and Biochemistry, University of California,
San Diego, La Jolla, California 92093-0358, United States
Supporting Information Placeholder
6
of DDQ, then followed by treatment with NOSbF , afforded
iminium 3a in good yield.
ABSTRACT: Reduction of alkynyl iminium salts derived from
cyclic (alkyl)(amino)carbenes (CAACs) affords propargyl/allenyl
radicals. Depending on the nature of the CAAC and alkyne sub-
stituents, these radicals can irreversibly dimerize, exist as mono-
mers in solution but dimerize in the solid state, or can even remain
monomeric as solids. The first characterization of an allenyl radi-
cal by single crystal X-ray crystallography is reported.
Organic open shell molecules play a fundamental role in sever-
al areas of chemistry and biology, but their inherent high reactivi-
ty and thermal instability make them challenging targets for isola-
1
tion. α-Amino radicals have been postulated as key intermediates
in various chemical transformations, such as photo-redox catalytic
2
,3
processes. Although these radicals benefit from electron delo-
calization, they are usually not isolable and, even with the help of
4
the capto-dative effect, they still dimerize in the solid state as
illustrated by the well-known 2-oxomorpholin-3-yl radical I
5
(
(
Figure 1). Recently, our group and others have shown that cyclic
Figure 1. 2-oxomorpholin-3-yl radical I which dimerizes in the
solid state, CAAC-stabilized carboxy radicals II, parent al-
lenyl/propargyl radical III, and (amino)allenyl radicals IV.
6
,7
alkyl)(amino)carbene (CAAC) scaffolds are excellent for stabi-
lizing a variety of main-group and transition metal paramagnetic
species, and of particular interest, they allow for the isolation of
carboxy radicals II. The next question was whether the capto-
dative effect was a requirement for the isolation of CAAC-derived
carbon-centered radicals. To address this question, we chose
allenyl/propargyl radicals, depending on which canonical form is
8,9
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drawn, as targets. The parent compound III has been widely
studied because its dimerization is believed to be responsible for
13
the formation of benzene in combustion processes. Even more
sterically protected propargyl radicals such as IV are known to
spontaneously give rise to the propargyl/propargyl and propar-
gyl/allenyl dimers Va and Vb, respectively. However, this class
Scheme 1. Synthesis of (alkynyl)iminium salt 3a.
14
of radicals has been detected by EPR, but only in argon matrices
The cyclic voltammogram of 3a showed a reversible one elec-
tron reduction at E1/2 = -1.16 V vs. Fc /Fc (Figure 2). Upon chem-
15
below 100 K. Yet again, even the introduction of an amino
group does not allow for the isolation of radicals of type VI.
Indeed, their EPR spectra could only be recorded by irradiation of
the respective propargylic amines and di-butylperoxide in the
+
ical reduction with one equivalent of cobaltocene, or half an
equivalent of tetrakis(dimethylamino)ethylene, a red colored
solution, sensitive to air, was obtained. The EPR spectrum con-
firmed the generation of a paramagnetic species (4a) (see SI),
which proved to be short-lived at room temperature (Scheme 2).
This species dimerizes over a few hours via the allenyl carbon
giving 5a. Note that this allenyl/allenyl dimer is different from the
16
spectrometer cavity.
Herein we show that providing the right set of substituents, the
pyrrolidine scaffold makes allenyl radicals stable at room temper-
ature, and even allow for their isolation in the solid state.
14
dimerization product of IV, due to the protection provided by
the CAAC scaffold. Interestingly, radical 4a can be trapped by a
second equivalent of cobaltocene to give 6a. For both 5a and 6a,
the structural connectivity could clearly be established by single
crystal X-ray diffraction studies (Figure 3).
To access the desired allenyl radicals, we chose the reduction
method already used for carboxy radicals II. The desired al-
10
kynyl-iminium precursor 3a was prepared in two steps (Scheme
1
7
18
1
). As recently reported by Turner and our group, CAACs
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readily activate sp-hybridized C-H bonds at room temperature,
which allows for the installation of an alkyne fragment. Subse-
quently, hydride abstraction from 2a with a stoichiometric amount
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THF gave back a red solution, which gives an EPR signal identi-
cal to that observed for 4b.
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Figure 2. Cyclic voltammogram of (alkynyl)iminium salt 3a.
(nBu NPF 0.1M in THF, 100 mVs , vs. Fc /Fc).
4 6
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Figure 4. Synthesis of (amino)allenyl radical 4b, its experimental
and simulated EPR spectra, its calculated SOMO, and the solid-
state structure of its dimer 7b.
Scheme 2. Synthesis of short-lived (amino)allenyl radical
a, its dimer 5a and trapping product 6a.
4
We investigated the different dimerization pathways by DFT
calculations using the model compound 4 (Scheme 3). The al-
lenyl-allenyl dimerization process giving 5 is energetically highly
favorable (ΔG = -40.6 kcal/mol). The dimerization that involves
the allenyl and the para-benzene carbons to afford 7 is almost
thermo-neutral as a result of the loss of aromaticity (ΔG = -1.5
kcal/mol). This value readily rationalized the experimentally
observed monomer/dimer 4b/7b equilibrium. Note that the para-
para benzene dimerization that leads to 8 is highly unfavorable (+
34.3 kcal/mol) as two benzene rings are de-aromatized.
Figure 3. Solid-state structures of dimer 5a (left) and cobaltocene
adduct 6a (right). Selected bond lengths [Å] and angles [°]: 5a N-
C1 1.412(4); C1-C2 1.311(4); C2-C3 1.319(4); C3-C3’ 1.544(4),
C1-C2-C3 170.7(3); 6a N-C1 1.3947(17); C1-C2 1.3194(19); C2-
C3 1.3148(19); C1-C2-C3 166.84(14).
In the hope of preventing this dimerization process, we protect-
ed the allenyl position using a CAAC precursor that features the
bulky menthyl group. Upon reduction of 3b with cobaltocene, a
red solution was obtained which appeared to be EPR active (Fig-
20
ure 4). Simulation [a
N H H H
= 4.3 G; a = 2.9 G; a = 0.5 G; a = 2.8
G; a = 2.3 G] and DFT calculations of the EPR parameters are in
H
agreement with the desired radical species 4b (see SI). Note that
in addition to the expected hyperfine couplings with N and the
ortho, meta and para Hs, there is a coupling with one H atom of
the menthyl group; the latter is predicted by DFT. The spin densi-
ty is delocalized on the nitrogen, the propargyl and the allenyl
carbons as well as on the para and ortho positions of the benzene
ring (see SOMO). In contrast to 4a, radical 4b is stable in solution
for days. However, attempts to obtain single crystals led to the
dimeric species 7b. Because of the steric protection by the
menthyl group, the formation of the allenyl-allenyl dimer, analo-
gous to 5a, does not occur. Instead, the dimerization involves an
allenyl carbon of one molecule and the para-carbon of the phenyl
group of a second molecule, a process reminiscent to the dimeri-
Scheme 3. Dimerization pathways for model (ami-
no)allenyl radical 4 with free Gibbs energies calculated at
the B3LYP-D3/cc-pVDZ level.
To inhibit the dimerization processes observed with 4b, we first
replaced the phenyl substituent with a tert-butyl group. Reduction
of the corresponding iminium salt afforded 4c (Figure 5). This
allenyl radical is perfectly stable in solution for days, but all at-
tempts to obtain single crystals suitable for an X-ray diffraction
study failed.
21
zation of Gomberg radical. Dissolving yellow crystals of 7b in
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The isolation of the first CAAC-derived α-amino radicals, not
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stabilized by capto-dative effects, demonstrates the effectiveness
of the pyrrolidine scaffold, bearing bulky substituents, to stabilize
paramagnetic species. Importantly, this class of compounds is
readily
accessible
in
three-steps
from
cyclic
(al-
kyl)(amino)carbenes. We are currently using these stable versions
of α-aminoalkyl radicals to gain further insight into the mecha-
nism of photoredox catalytic reactions.
ASSOCIATED CONTENT
Supporting Information
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The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/xxx. CCDC 1573244
(
1
3a), 1573245 (4d), 1573246 (5a), 1573247 (7b), 1573248 (3d),
573249 (6a) contain the crystallographic data.
AUTHOR INFORMATION
Corresponding Author
Figure 5. Experimental and simulated EPR spectra (a
= 3.76 G) of 4c, with the SOMO and Mulliken spin densities
calculated at the UB3LYP/def2-TZVP level.
N
= 4.42 G,
*guybertrand@ucsd.edu
a
H
Notes
Tert-butyl and menthyl groups are notorious for making crystalli-
zation difficult, and thus we moved to a trityl substituent at the
alkyne moiety and ethyl groups on the CAAC. To our delight,
reduction of the corresponding iminium salt led to radical 4d,
which was fully characterized in solution, but also in the solid
The authors declare no competing financial interests.
ACKNOWLEDGMENTS
Thanks are due to the NSF (CHE-1661518) for financial support,
to the Alexander von Humboldt foundation for a Feodor-Lynen
scholarship (MMH), and to the Keck Foundation for funding the
KeckII computer center. We are thankful to Pr. M. Tauber and Dr.
D. Borchardt for their assistance with EPR experiments, and Drs.
M. Gembicky and C. E. Moore for their help with X-ray crystal-
lography.
state thanks to single crystals obtained from a pentane solution at -
o
4
0 C (Figure 6). Not surprisingly, when compared to the iminium
precursor 3d, there is a considerable lengthening of the N-C1
bond distance [3d: 1.281(9), 4d: 1.3850(14) Å]. More interesting-
ly, the C1-C2 [3d: 1.430(10), 4d: 1.3821(17) Å] and C2-C3 [3d:
1
.178(9), 4d: 1.2191(17) Å] bond are shorter and longer, respec-
tively. This highlights the importance of the allenyl resonance
form, which is confirmed by the calculated SOMO and Mulliken
spin densities.
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