The Advantages of Cyclic Over Acyclic Carbenes To Access Isolable Capto-Dative C-Centered Radicals

Article abstract of DOI:10.1002/chem.201700144

A cyclic and an acyclic di(amino)carbene as well as a cyclic and an acyclic (alkyl)(amino)carbene cleanly react with benzoyl chloride to give the corresponding adducts 1⁺_cyc, 1⁺_acy, 2⁺_cyc, and 2⁺_acy, respectively. The reduction of 1⁺_cyc and 2⁺_cyc derived from cyclic carbenes affords the corresponding radicals 1_cyc and 2_cyc that are stable at room temperature. In contrast, radicals 1_acy and 2_acy, derived from acyclic carbenes, cannot be isolated. It is shown that 1_acy is as thermodynamically stabilized as its cyclic counterpart 1_cyc, but its instability is the result of β-hydrogens of the nitrogen substituent, along with the enhanced flexibility around C?N bonds, which allow for a H^.-migration-elimination process. Radical 2_acy is thermodynamically unstable, and undergoes disproportionation into the corresponding iminium 2⁺_acy and enolate 2^?_acy. This is due to the excessive steric hindrance, which prevents electron-delocalization on the NCCO fragment, and thus, the capto-dative stabilization. This work suggests general guidelines for the design of highly persistent (amino)(carboxy)radicals, especially by emphasizing the key advantage of cyclic patterns.

Full text of DOI:10.1002/chem.201700144

A Journal of
Accepted Article
Title: On the Advantage of Cyclic over Acyclic Carbenes to Access
Isolable Capto-Dative C-Centered Radicals
Authors: Janell Mahoney, Rodolphe Jazzar, Guy Royal, David Martin,
and Guy Bertrand
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On the Advantage of Cyclic over Acyclic Carbenes to Access
Isolable Capto-Dative C-Centered Radicals
Janell K. Mahoney,a Rodolphe Jazzar,a Guy Royal,b David Martin*,b and Guy Bertrand*,a
Dedication ((optional))
Abstract: A cyclic and an acyclic di(amino)carbene as well as a
cyclic and an acyclic (alkyl)(amino)carbene cleanly react with
O
benzoyl chloride to give the corresponding adducts 1+cyc, 1+acy, 2+
O
cyc
H
O
Ph
and 2+acy, respectively. The reduction of 1+ and 2+ derived from
NMe2
Ph
2
Me2N
cyc
cyc
Ph
Me2N
cyclic carbenes affords the corresponding radicals 1cyc and 2cyc that
are stable at room temperature. In contrast, radicals 1acy and 2acy
derived from acyclic carbenes, cannot be isolated. It is shown that
1acy is as thermodynamically stabilized as its cyclic counterpart 1cyc
o
140 C
H
H
I
,
II
,
Figure 1. Dimerization of (dimethylamino)(carboxy) radical I.
but its instability is the result β-hydrogens of the nitrogen substituent,
along with the enhanced flexibility around C–N bonds, which allow
for
thermodynamically unstable, and disproportionates into the
corresponding iminium 2+ and enolate 2-acy. This is due to the
a
HŸ-migration-elimination process. Radical 2acy is
Stable N-substituted carbenes are ideal precursors for capto-
dative radicals such as IIIa-e (Figure 2). Fukuzumi et al. reacted
thiazolylidene[9] with benzaldehyde in the presence of a base
and performed a one-electron electrochemical oxidation of the
resulting enolate to afford capto-dative radical IIIa.[ 10 ] EPR
monitoring showed that the compound decayed within hours at
room temperature. More recently, we demonstrated that cyclic
(alkyl)(amino)carbenes (CAACs)[11,12] allow for a simple two-step
synthesis of the first structurally characterized monomeric C-
centered (amino)(carboxy)radicals IIIb, as well as related bi- and
tri-radicals.[13] These molecules were found to be perfectly stable
in solution at room temperature. In addition, we found that
carbonyl substituents with strong electronegative inductive
effects afford remarkably air-persistent radicals.[14] Hudnall et al.
also reported that radicals IIIc, IIId and IIIe, which are based on
acy
excessive steric hindrance, which prevents electron-delocalization
on the NCCO fragment, and thus the capto-dative stabilization. This
work suggests general guidelines for the design of highly persistent
(amino)(carboxy)radicals, especially by emphasizing the key
advantage of cyclic patterns.
Introduction
In recent years, stable carbenes[1,2] have emerged as powerful
tools[3] for the design of stable paramagnetic metal complexes[4]
and main-group derivatives.[5] They have also allowed for an
insightful reinvestigation into the stabilization of C-centered
radicals,[6] using the synergistic combination of electron donating
and withdrawing substituents, the so-called capto-dative effect.[7]
However, while this substitution pattern weakens the C–C bond
of the corresponding dimer, the monomer typically remains
thermodynamically disfavoured.[8] For instance, (dimethylamino)-
(carboxy)radical I exists in the solid state as the diamagnetic
dimer II (Figure 1).[8a] Interestingly, sufficient amounts of the
radical are formed in solution at 140 °C, which allowed for both
the detection of I by EPR and the determination of the
dimerization enthalpy (-25 kcal.mol-1 versus about -80 kcal.mol-1
for typical C–C bonds).
[16 ]
stable imidazol-2-ylidene,[15 ] cyclic(amido)(amino) carbene
and cyclic(diamido)carbene,[17] respectively, are also isolable at
room temperature.[18] Strikingly, only cyclic carbenes have been
used so far, and it has already been shown that their acyclic
versions differ significantly in terms of electronic properties, and
consequently chemical behaviour.[19] Herein, we explore stable
acyclic di(amino)carbene[20] and (alkyl)(amino) carbene[21] as
building blocks for (amino)(carboxy)radicals 1acy and 2acy
,
respectively, and we discuss their stability compared to their
cyclic counterparts 1cyc and 2cyc
.
O
O
O
O
O
O
N
O
N
Ph
Ph
N
O
N
N
Ph
Ph
Ph
IIIa
N
N
N
S
IIIb
IIIc
IIId
IIIe
[a]
[b]
J. K. Mahoney, Dr. R. Jazzar, Prof. G. Bertrand
UCSD-CNRS Joint Research Laboratory (UMI 3555), Department of
Chemistry and Biochemistry, University of California, San Diego, La
Jolla, California 92093-0358, United States.
E-mail: guybertran@ucsd.edu
O
Dipp
iPr
O
O
iPr
O
Dipp
N
N
N
N
Ph
iPr
Ph
Ph
iPr
Ph
N
N
iPr
iPr
1acy
Dipp
1cyc
Prof. G. Royal, Dr. D. Martin
2cyc
2acy
Université Grenoble-Alpes, CNRS, Département de Chimie
Moléculaire (UMR 5250), CS 40700, 38058 Grenoble Cedex 9,
France.
Figure 2. Previously reported stable radicals (IIIa-e) derived from stable
carbenes and cyclic and acyclic radicals 1 and 2 considered in this paper.
E-mail: david.martin@univ-grenoble-alpes.fr
Supporting information for this article is given via a link at the end of
the document.
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Results and Discussion
In both cases, the corresponding potentials were separated by
0.5–0.7 V, indicating that disproportionation of the redox-
amphoteric radicals could be excluded. Therefore, we undertook
the synthesis of radicals 1cyc and 1acy by electrochemical
Using the strategy previously employed,[13] the saturated cyclic
diaminocarbene (imidazolidin-2-ylidene)[
and its acyclic
22
]
reduction of 1+
and 1+acy, respectively, while monitoring the
counterpart, the bis(diisopropylamino)carbene,[20] were reacted
cyc
with benzoyl chloride to afford iminiums 1+ and 1+acy in 66 and
reaction by UV-Vis. In both cases, the stoichiometry (one
coulomb per mole of reagent) was consistent with a one-electron
process. Cyclic voltammograms, as well as rotating disk
electrode (RDE) voltammetry were similar for the product and
the starting material, demonstrating that neither 1cyc nor 1acy
underwent further chemical transformations after the electron
transfer. The isotropic EPR spectra of 1cyc and 1acy (Figure 4)
and UV-Vis spectroscopy gave definitive evidence for the
identity of the radicals. Indeed DFT calculations reproduced
experimental a(15N) isotropic hyperfine coupling constants[ 23 ]
(1cyc: exp., 6.6 and 5.4 MHz; calc. 5 and 4 MHz; 1acy: exp., 11.6
and 6.0 MHz; calc. 13 and 7 MHz), as well as the large UV-Vis
absorptions (1cyc: exp., 358 and 539 nm; calc. 350 and 515 nm;
1acy: exp., 504 nm; calc. 496 nm). Note that the chemical
cyc
78 % yield, respectively (Scheme 1).
O
Dipp
O
Dipp
O
Dipp
N
1e–
Cl
Dipp
Ph
N
N
Ph
Ph
+
N
N
Cl–
N
Dipp
O
Dipp
O
1+
1cyc
cyc
O
i-Pr2N
i-Pr2N
Cl
Ph
Ph
Ph
1e–
i-Pr2N
i-Pr2N
+
i-Pr2N Cl–
i-Pr2N
1+
1acy
acy
Scheme 1. Synthesis of radicals 1cyc and 1acy
.
reduction of 1+
and 1+
with a stoichiometric amount of
acy
cyc
cobaltocene similarly afforded radicals 1cyc and 1acy in 67 and
63% yield, respectively.
The cyclic voltammograms of 1+
reversible reduction waves, which were assigned to the
and 1+
feature two
cyc
acy
formation of the radical and enolate species, respectively (Figure
3).
Figure 4. Isotropic X-band EPR spectra of 1cyc and 1acy (top) and
corresponding simulated band shapes (bottom) with the following hyperfine
coupling constants: 1cyc, a(15N) = 6.6 and 5.4 MHz, a(1H) = 10.66 (2), 8.30 (2),
1.49, 2.08 and 3.21 MHz; 1acy, a(15N) = 11.6 and 6.0 MHz, a(1H) = 8.9, 6.1, 7.9,
3.9 and 3.2 MHz.
Under an inert atmosphere, and similarly to most
(amino)(carboxy)radicals that are derived from stable cyclic
carbenes, 1cyc was found to be stable at room temperature for
weeks in solution. In contrast, under the same conditions, radical
1acy decomposes within a few days. Attempts to obtain crystals
of 1acy from a cooled concentrated hexane solution led to the
isolation of the C–C dimer 4 of the (amino)(carboxy)radical 3
(Scheme 2).24 Clearly, 1acy undergoes a 1,3-H•-migration and a
Figure 3. Cyclovoltammetric and UV-Vis studies of 1+
(top) and 1+
cyc
acy
(bottom). a) Cyclic voltammogram in CH3CN + (n-Bu)4NPF6 0.1 mol⋅L–1 (carbon
electrode, Φ = 3 mm; E vs Fc/Fc+) before (black) and after (red) electrolysis. b)
UV-Vis monitoring (1 scan every 50 seconds) of the electrochemical reduction
on a reticulated vitreous electrode. c) Rotating disk electrode voltammetry
(RDE; 10 mV/s; 600 rpm) before electrolysis. d) RDE (10 mV/s; 600 rpm) after
one-electron electrolysis.
subsequent
fragmentation,
yielding
3
and
N-
isopropylacetonimine. Interestingly, the EPR spectrum of a THF
solution of 4 gives a strong isotropic X-band absorption (Figure 5,
left), which is fully consistent with radical 3 as it features large
isotropic hyperfine coupling constants with one nitrogen (20.6
MHz) and one hydrogen (22.8 MHz). Note that dimer II (Figure
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1) only releases detectable amounts of radical I at high
temperatures,[8a] which is due to the difference of steric
hindrance. DFT calculations confirmed this assumption with a
computed dissociation enthalpy of only +2 kcal·mol-1 for 4
turned progressively reversible (E1/2 = -1.22 V; ΔEp = 300 mV at
scan rate = 6.4 V⋅s-1, see Figure 6b) as the cyclovoltammetry
time scale surpassed the rate of the chemical transformation.
The complete electrochemical reduction of 2+
required two
acy
o
(experimental value reported for II in diphenyl ether at 140 C:
coulombs per mole. This ruled out the possibility for the isolation
+25 kcal·mol-1).
of the desired radical 2acy and, instead indicated the formation of
Radicals 1cyc and 1acy are both air-sensitive, just like previously
reported IIIa-e. The resulting decay of the latter has been
attributed to a reaction with dioxygen, although products of the
reaction have never been clearly identified. Interestingly, while
briefly exposing 1acy to air, we detected a transient paramagnetic
compound. Its EPR spectrum featured a 1:1:1 triplet of 1:2:1
triplets indicating coupling with one nitrogen (aN = 42.7 MHz)
and two hydrogens (aH = 12.2 MHz), respectively (Figure 5,
right). These values perfectly match those of the known
di(isopropyl)nitroxide radical.[25] The formation of this compound
is the first evidence of oxidative cleavage of N-C bonds resulting
from the air-oxidation of such capto-dative radicals (Scheme 2).
enolate 2–
through a two-electron transfer. Evaporation of
acy
volatiles and extraction of the residue with diethylether led to the
[26 ]
isolation of enol 2acy
H
as the final compound, and thus
identified protonation of 2–acy as the chemical transformation that
was previously evidenced by cyclovoltammetry experiments.
Scheme 3. A two-electron reduction occurs with 2+ giving the corresponding
acy
enolate 2-acy
.
O
O
days
2
i-Pr2N
i-Pr2N
i-Pr2N
Ph
1acy
Ph
Ni-Pr
H
O
–
3
O2
H
i-Pr2N
Ph
Ph
4
i-Pr2NO
Ni-Pr2
H
O
Scheme 2. In contrast to cyclic radical 1cyc
, which is stable at room
temperature for weeks, radical 1acy undergoes fragmentation into 3, which is in
equilibrium in solution with its dimer 4; Air oxidation of 1acy leads to the
di(isopropyl)nitroxide radical.
Figure 6. a) Cyclic voltammogram of 2+
(1 mmol⋅L–1) in CH3CN + (n-
acy
Bu)4NPF6 0.1 mol⋅L–1 (carbon electrode, Φ = 3 mm; E vs Fc/Fc+) with scan
rate v = 0.2 V.s–1; the dashed line corresponds to scan on a -1.6 – 1 V range.
b) Cyclic voltammogram of 2+acy in same conditions with scan rate v = 6.4 V.s–
1
.
Among other metrics,[27] a recognized measurement of the π-
accepting capabilities of stable carbenes is the 31P NMR
chemical shift of the corresponding (phenyl)phosphinidene
adducts (>C=PPh).[19c,d] Accordingly, the π-accepting properties
of cyclic carbenes increase from entry 1 to 6 of Table 1.
Concomitantly, the redox potential of the corresponding cyclic
cations/radicals follows the same trend, in line with recent
results from Hudnall et al.[18] Surprisingly, cations/radicals
derived from acyclic carbenes (entry 7 and 8) stand out
abnormally from the series. Indeed, acyclic di(amino)carbenes
and (alkyl)(amino)carbenes are among the most electron π-
accepting carbenes, and therefore one would have expected the
Figure 5. Isotropic X-band EPR spectra in acetonitrile (experimental and
simulated) of radicals 3 and iPr2NO.
Next, we reacted the acyclic (diisopropylamino)(tert-
butyl)carbene[21] with benzoyl chloride and isolated iminium 2+
acy
(Scheme 3). The cyclovoltammogram of 2+
at a scan rate of
acy
0.2 V.s-1 features an irreversible reduction wave at Ep = -1.30 V
versus Fc/Fc+, with the corresponding re-oxidation wave at Ep = -
0.28 V (Figure 6a). These data are indicative of an electron
reduction of corresponding iminiums 1+
(E1/2 = -1.24 V) and
acy
2+ (E1/2 = -1.22 V) to occur at more weakly negative potentials
acy
than those of the very weakly π-acceptor NHCs 1+
(E1/2 = -
cyc
transfer, followed by
a
chemical transformation. When
at Ep = -1.30 V
1.32 V) and 2+cyc (E1/2 = -1.21 V).
increasing the scan rate, the reduction of 2+
acy
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Table 1. Experimental measures of electronic properties of free carbenes
and of the corresponding radicals.
Table 2. Calculated radical stabilization energy RSE for 1cyc, 1acy, 2cyc, 2acy
and 3 (relative to H3C•) and corresponding deviation from additivity of RSE
(DARSE).
Entry
Carbene
Dipp
δ
31P NMR[a]
Cation
IIIc+
E1/2 (mV)[b]
RSE
DARSE
Entry
radical
(kcal.mol-1)
(kcal.mol-1)
N
1
-18.9
-1.32
1
2
3
4
5
1cyc
1acy
2cyc
2acy
3
41.9
41.6
36.5
21.4
37.7
9.7
11.9
6.3
N
Dipp
Dipp
Mes
Dipp
N
2
-10.2
1+
-1.21
cyc
N
-2.6
6.8
Mes
N
O
3
4
37.7
57
IIId+
IIIa+
-0.98
-0.95
N
Bn
As expected strong DARSE values were found for 1cyc, 1acy and
2cyc (Table 2, entries 1-3), but surprisingly the capto-dative
combination in 2acy has a clear detrimental effect (entry 4); in
addition the RSE for 2acy is much smaller than for the other three
radicals. We hypothesized that the mediocre thermodynamic
stabilization of radical 2acy was due to the excessive steric
hindrance around the radical center. Indeed, as shown in Figure
7, in contrast to 2cyc which features a nearly-planar conjugated
π-system (NC1C2O torsion angle: 6.7 o), there is a strong torsion
around the C1–C2 bond of 2acy (37.8 o) that prevents the
electron-delocalization on the NCCO fragment. The calculated
RSE (37.7 kcal.mol-1) and DARSE (6.8 kcal.mol-1) for radical 3
(Scheme 2), a much less bulky analogue of 2acy, confirmed our
assessment.
N
S
Dipp
N
5
6
68.9
83
2+
-0.93
-0.48
cyc
Mes
N
O
IIIe+
N
O
Mes
iPr
N
7
8
69.5
1+
-1.24
iPr
acy
N
iPr
iPr
iPr
N
126.3
2+
(-1.22)[c]
iPr
acy
[a] 31P NMR chemical shift (ppm) for carbene•PPh adducts as a measure of
the π-accepting properties of carbenes according to reference 19c. [b] Redox
potential for cation/radical. [c] Reduction potential for 2+acy /2-acy
.
We performed DFT calculations in order to have more insight
into the singular reduction behavior of acyclic cations 1+
and
acy
2+acy, especially with respect to their cyclic counterparts 1+
cyc
and 2+cyc. There is no unambiguous way to define radical
stabilization energy (RSE). However, numerous studies have
shown that the enthalpy of the hydrogen atom exchange of a C-
centered radical with CH4 is a conceptually simple and insightful
method for the study of the capto-dative effect.[28] For instance,
isodesmic reaction (1) provides a relevant estimate for the effect
on RSE of a set of (X,Y) substituents. It is well known that RSEs
hardly benefit from the combination of two donating groups. In
contrast, capto-dative combination of substituents is expected to
have a synergetic effect, resulting in a strong deviation (DARSE)
from the additivity of the RSE of each substituent, as defined by
equation (2):
Figure 7. View along the C1–C2 axis of the optimized geometry of 2cyc, 2acy
and 3 at the B3LYP/TZVP level of theory.
Conclusions
We have shown that the substitution pattern of stable acyclic
(amino)carbenes is detrimental for the stability of the
corresponding radicals 1acy and 2acy, but for different reasons.
Radical
2acy
is
thermodynamically
unstable,
and
and
disproportionates into the corresponding iminium 2+
acy
enolate 2-acy. This is due to the excessive steric hindrance,
which prevents the electron-delocalization on the NCCO
fragment, and thus capto-dative stabilization. In contrast, 1acy is
as thermodynamically stabilized as its cyclic counterpart 1cyc, but
suffers from the presence of β-hydrogens on the N-substituent
along with the enhanced flexibility around C–N bonds, which
allow for a HŸ-migration-elimination process. This work suggests
•CHXY + CH4
→
CH2XY + •CH3 ΔH=RSE(X,Y) (1)
DARSE(X,Y) = RSE(X,Y) - RSE(X,H) - RSE(H,Y) (2)
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Synthesis of iminium 2+
:
acy
A
solution of the acyclic
general guidelines for the design of highly persistent
(amino)(carboxy)radicals, especially by emphasizing the key
advantage of cyclic patterns. Their rigidity prevents
rearrangements, and offers an optimal +M donation of the amino
group allowing for a fully conjugated NCCO fragment, with a
planar local environment. The recent assumption[18] that the π-
accepting properties of carbenes parallel the reduction potential
of the corresponding (amino)(carboxy)radicals is in agreement
with our findings in the cyclic series. However, the acyclic
carbenes considered in this study seem to be out of range, but
other acyclic carbenes should be considered before drawing a
conclusion.
(amino)(alkyl)carbene in THF was prepared in situ from the
corresponding amidinium triflate (0.50 g, 1.64 mmol) and lithium 2,2,6,6-
tetramethylpiperidine (0.24 g, 1.64 mmol). The solution was cooled to -
78 °C, and benzoyl chloride (0.20 mL, 1.99 mmol) was added dropwise.
The solution was stirred at −78 °C for 30 minutes, warmed to room
temperature and stirred an additional 30 minutes. The supernatant was
removed via filtration, and the solid was then washed with diethyl ether (2
x 10 mL) and dried in vacuo. Extraction of the product in chloroform and
removal of the solvent in vacuo yielded a white solid. Yield: 255 mg
(38 %). mp: 179–180 °C. MS (m/z): [M+] calc. for C18H28NO+, 274.2165;
found, 274.2169. UV-Vis: Absorbance [nm] (ε) = 263 (9,058), 268 (9,427).
1H NMR (CDCl3, 500 MHz): δ = 8.10 (bs, 1H), 7.80 (bs, 3H), 7.71 (bs,
1H), 5.32 (sept, J = 6.6 Hz, 1H), 4.62 (sept., J = 7.0 Hz, 1H), 1.88 (d, J =
6.6 Hz, 3H), 1.71 (d, J = 6.6 Hz, 3H), 1.54 (s, 9H), 1.51 (d, J = 7.0 Hz,
3H), 1.30 (d, J = 7.0 Hz, 3H) ppm. 13C NMR (CDCl3, 125 MHz): δ = 197.8
(C), 189.0 (C), 137.3 (CH), 131.9 (C), 130.7 (CH), 127.5 (CH), 62.4 (CH),
40.5 (C), 29.3 (CH3), 23.7 (CH3) ppm. 19F NMR (CDCl3, 376 MHz): δ = -
78.18 (s) ppm.
Experimental Section
General Considerations: All manipulations were performed under an
inert atmosphere of dry argon, using standard Schlenk and drybox
techniques. Dry and oxygen-free solvents were employed. Benzoyl
Radical 1cyc: Iminium salt 1+
(0.400 g, 0.75 mmol) and cobaltocene
cyc
(0.14 g, 0.75 mmol) were combined under argon. Dichloromethane was
added to the flask, and the solution stirred for 30 minutes. Removal of the
solvent in vacuo and extraction in diethyl ether yielded 1cyc as a dark
purple solid. Yield: 250 mg (67 %). mp: 104–106 °C. The electrochemical
chloride was purified by distillation before use. Stable carbenes were
[13]
prepared as previously reported,[22,20,21] as well as radical 2cyc
.
1H, 13C,
and 19F NMR spectra were recorded on Bruker Advance 300, Varian VX
500 and Jeol ECA 500 spectrometers. Chemical shifts are given relative
to SiMe4 and referenced to the residual solvent signal (1H, 13C) or relative
to an external standard (19F: CFCl3). Melting points were measured with
Büchi Melting Point B–545 apparatus. Electrochemical experiments were
carried out with a Bio-logic SP-300 potentiostat. An Ag/0.01M AgNO3
reference electrode in CH3CN + 0.1M [Bu4N]PF6 was used. Cyclic
voltammetry experiments were performed in CH3CN + 0.1M [Bu4N]PF6 a
vitreous carbon disk (3 mm in diameter) as working electrode and a
platinum wire as auxiliary electrode. Ferrocene was used as standard,
and all reduction potentials are reported with respect to the E1/2 of the
Fc/Fc+ redox couple. Electrochemical reductions were performed using
reticulated vitreous carbon electrode, and were monitored with a Zeiss
MCS 501 UV-NIR Plus spectrometer. EPR spectra were obtained using
an X-band Bruker EMX Plus spectrometer.
reduction of iminium salt 1+
(6.9 mg, 0.013 mmol), in 10.0 mL of
cyc
acetonitrile containing 0.1M [Bu4N]PF6, was performed on a reticulated
vitreous carbon electrode at a potential of -1.50 V under an argon
atmosphere. Complete electrolysis (one electron exchanged per
molecule) yielded a purple solution. Absorbance [nm] (ε) =242 (9,120),
359 (5,643), 541 (2,991).
Radical 1acy: Iminium salt 1+
(0.200 g, 0.57 mmol) and cobaltocene
acy
(0.097 g, 0.52 mmol) were combined under argon. Dichloromethane was
added to the flask, and the solution stirred for 20 minutes. Removal of the
solvent in vacuo and extraction in hexanes yielded 1acy as a dark
reddish-purple solid. Yield: 62 mg (63 %). mp: 102–104 °C. The
+
electrochemical reduction of iminium salt 1acy (3.7 mg, 0.012 mmol), in
10.0 mL of acetonitrile containing 0.1M [Bu4N]PF6, was performed on a
reticulated vitreous carbon electrode at a potential of -1.40 V under an
argon atmosphere. Complete electrolysis (one electron exchanged per
molecule) yielded a reddish-purple solution. UV-Vis: Absorbance [nm] (ε)
Synthesis of iminium 1+cyc: Benzoyl chloride (0.37 mL, 3.17 mmol) was
added slowly to a toluene solution of the imidazolidin-2-ylidene (1.027 g,
2.88 mmol). The solution was stirred for 20 minutes and then filtered. The
solid was then washed with hexanes (3 x 20mL) to yield a white powder.
Recrystallization by diffusion of diethyl ether into a dichloromethane
=
236 (6,026), 505 (3,515). Crystallization of 1acy by cooling a
concentrated hexane solution yielded crystals of 4 as yellow blocks. mp:
109-111°C
+
solution of the product yielded 1cyc as colorless crystals. Yield: 942 mg
(66 %). mp: 261–263 °C. MS (m/z): [M+] calc. for C34H43N2O+, 495.3375;
found, 495.3372. UV-Vis: Absorbance [nm] (ε) = 269 (12,285). 1H NMR
(CDCl3, 300 MHz): δ = 7.96 (d, J = 6 Hz, 2H), 7.62 (t, J = 6Hz, 1H), 7.45
(t, J = 6 Hz, 2H), 7.32 (t, J = 6 Hz, 2H), 7.12 (d, J = 6 Hz, 4H), 5.07 (bs,
4H), 3.45 (sept, J = 3Hz, 4H), 1.33 (d, J = 3 Hz, 12H), 1.12 (d, J = 3 Hz,
12H) ppm. 13C NMR (CDCl3, 125 MHz): δ = 180.6 (C). 162.1 (C), 147.1
(C), 137.1 (CH), 131.7 (CH), 131.3 (C), 130.0 (CH), 129.9 (CH), 128.7
(C). 125.4 (CH), 55.7 (CH2), 29.4 (CH), 26.9 (CH3), 23.1 (CH3) ppm.
Electrochemical Synthesis of 2acyH: Iminium salt 2+ (4.5 mg, 0.011
acy
mmol), in 10.0 mL of acetonitrile containing 0.1M [Bu4N]PF6, was
reduced on a reticulated vitreous carbon electrode at -1.60V under an
argon atmosphere. Complete electrolysis (two electrons exchanged per
molecule) yielded a yellow solution. Removal of the solvent in vacuo and
extraction in diethyl ether yielded a yellow oil. UV-Vis: Absorbance [nm]
(ε) = 248 (13,561), 394 (618). 1H NMR (C6D6, 400 MHz): δ = 8.06 (s, 1H),
7.31-7.29 (m, 2H), 7.08-7.00 (m, 3H), 3.29 (sept, J = 6.4 Hz, 2H), 1.05 (d,
J = 6.4 Hz, 12H), 0.95 (s, 9H) ppm. 13C NMR (C6D6, 100 MHz): δ = 153.5
(C), 138.8 (C), 130.3 (CH), 129.7 (CH), 129.0 (CH), 124.3 (C), 49.3 (CH),
36.5 (C), 32.7 (CH3), 22.9 (CH3), 22.6 (CH3) ppm.
Synthesis of iminium 1+acy: Benzoyl chloride (0.122mL, 0.965 mmol)
was added to a hexane solution of bis(diisopropylamino)carbene (194 mg,
0.804 mmol) and the solution stirred for 20 minutes. The solvent was
filtered off and the solid washed with diethyl ether (2x10mL).
Recrystallization from diffusion of diethyl ether into a dichloromethane
solution of the product yielded a yellow crystalline material. Yield: 284 mg
(78%). mp: 209–210 °C. MS (m/z): [M+] calc. for C20H33N2O+, 317.2587;
found, 317.2590. UV-Vis: Absorbance [nm] (ε) = 272 (12,676), 367 (838).
1H NMR (CDCl3, 400 MHz): δ = 8.11 (bd, 2H, J = 7.2 Hz), 7.70-7.61 (m,
3H), 4.19 (sept, 4H, J = 6.8 Hz), 1.41 (d, 12H, J = 6.8 Hz), 1.37 (d, 12H, J
= 6.8 Hz) ppm. 13C NMR (CDCl3, 100 MHz): δ = 189.5 (C), 168.0 (C),
136.8 (CH), 133.8 (CH), 130.3 (CH), 56.9 (CH), 22.9 (CH3), 22.6 (CH3)
ppm.
Computational methods
The DFT calculations were carried at the B3LYP/TZVP level of theory,
unless otherwise stated, using the program package Gaussian09.[29 ]
Optimized structures were identified as energy minima by calculating the
vibrational frequencies. The solvent (acetonitrile) was taken into account
only for TD-DFT calculations, using the Polarizable Continuum Model
(PCM) method.
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Acknowledgements
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This work was supported by the NSF (CHE-1359809), the
French National Agency for Research (grant ANR-14-CE06-
0013-01) and the Partner University Fund, a program of the
French-American cultural Exchange foundation (FACE). The
authors wish to acknowledge “Centre de Calcul Intensif en
Chimie de Grenoble” (CECIC) for providing the computational
resources and the ICMG Chemistry Nanobio Platform of
Grenoble on which work has been performed. J. K. M. benefited
from a Chateaubriand Fellowship of the Office for Science &
Technology of the Embassy of France in the United States.
Thanks are also due to M. Gembicky, C. E. Moore and A. L.
Rheingold for their assistance with the X-ray diffraction study.
Keywords: Carbene • Radical • Capto-dative effect • EPR
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Stable or unstable capto-dative
radicals: The relative stability of
carbene derived cyclic and acyclic
capto-dative radicals is discussed in
J. K. Mahoney, R. Jazzar, G. Royal, D.
Martin, G. Bertrand
isolated ?
√
X
Page No. – Page No.
X
o
N
X
N
the
measurements, EPR and DFT
calculations. Decomposition
light
of
electrochemical
On the Advantage of Cyclic over
Acyclic Carbenes to Access Isolable
Capto-Dative C-Centered Radicals
Ph
Ph
o
pathways are observed with acylic
capto-dative radicals.
X
N
X
N
X =
C
N
,
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