An air-stable oxyallyl radical cation

Article abstract of DOI:10.1002/anie.201302841

Easy does it! Simply take two carbenes, add carbon monoxide, then HCl, and open the flask to air, and an oxyallyl radical cation is formed. Copyright

Full text of DOI:10.1002/anie.201302841

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DOI: 10.1002/anie.201302841
Stable Radicals
An Air-Stable Oxyallyl Radical Cation**
David Martin, Curtis E. Moore, Arnold L. Rheingold, and Guy Bertrand*
Regarded as laboratory curiosities at the time of their
discovery,[1] stable carbenes were later recognized as powerful
tools in organometallic chemistry since the corresponding
complexes led to numerous breakthroughs in homogeneous
catalysis.[2] In main-group chemistry, stable carbenes have
only recently found application. Carbenes can coordinate
silicon, phosphorus, and their heavier congeners in their zero
oxidation state,[3] and they can stabilize a variety of main-
group-element-centered radicals.[4,5] So far, the applications
of stable carbenes in organic chemistry have been limited to
merely their role as organocatalysts.[6] Here we report that
stable carbenes can also be used to prepare organic para-
magnetic species. We show that oxyallyl radical cations, which
were hitherto believed to be only transient intermediates,
only observable by mass spectroscopy[7] or in a glassy matrix
at 77 K,[8] can be isolated at room temperature and are even
stable towards air and moisture.
Oxyallyl derivatives I (Scheme 1) are non-Kekulꢀ mole-
cules, which have been postulated as intermediates in several
rearrangements and cycloaddition reactions,[9,10] including the
in vivo biosynthesis of prostaglandins.[11] Alkyl-substituted
oxyallyl derivatives have been calculated to be singlet
biradicals (Ia), whose ring closure to give the corresponding
cyclopropanones II occurs with a negligible barrier.[12,13]
These predictions are in line with a recent report describing
the first observation of a dialkyloxyallyl species, which has
a half-life at 298 K of 42 min in the solid state, but a few
picoseconds in solution.[14] Importantly, ab initio studies[15]
showed that substituents with a strong + M effect, such as
amino groups,[16] decrease the biradical character and there-
fore favor the zwitterionic mesomeric form Ib, which leads to
less reactive species. Indeed, Siemeling et al. reported the
isolation of compound III,[17] which they described as
a zwitterionic enolate since it features one short and one
À
long C CCO bond, and a dihedral angle of nearly 908 between
the two amidinium units.
Not surprisingly, the redox chemistry of oxyallyl deriva-
tives I is terra incognita, and due to the presence of iron
centers, the oxidation of III is unlikely to occur on the organic
fragment. Compound III was prepared by the addition of two
ferrocene-based diaminocarbenes to CO, but the general-
ization of this synthetic approach is not straightforward.[18]
Indeed, classical N-heterocyclic carbenes (NHCs) are not
electrophilic enough to react with CO.[19] On the other hand,
alkyl(amino)carbenes[20] and diamidocarbenes[21] add to
carbon monoxide, but afford monocarbene adducts, namely
ketenes.[22] It is quite likely that the addition of a second of
these carbenes, and thus the formation of the oxyallyl
derivative, is precluded by their steric bulk. Recently, we
described a novel type of cyclic diaminocarbene, which
features enhanced electrophilicity due to the strained bridge-
head position of one of the nitrogen atoms (anti-Bredt
NHCs).[23] We reasoned that the electronic properties of this
carbene coupled with virtually no steric hindrance around the
bent nitrogen, should make it possible to prepare the desired
oxyallyl derivative.
In a first experiment, carbon monoxide was bubbled at
À788C into a THF solution of the anti-Bredt NHC 2
(prepared by in situ deprotonation of amidinium triflate
1 with potassium hexamethyldisilylazide) (Scheme 2). The
reaction mixture immediately turned deep blue; its 13C NMR
spectrum recorded at À408C displayed signals for a new
compound 3, featuring a single type of carbene unit.
Scheme 1. Oxyallyl I, cyclopropanone II, and zwiterionic enolate III.[17]
[*] Dr. D. Martin, Prof. G. Bertrand
UCSD-CNRS Joint Research Laboratory (UMI 3555)
Department of Chemistry and Biochemistry
University of California San Diego
La Jolla, CA 92093-0343 (USA)
E-mail: guybertrand@ucsd.edu
Homepage: http://bertrandgroup.ucsd.edu
Dr. C. E. Moore
UCSD Crystallography Facility
Department of Chemistry and Biochemistry
University of California San Diego
La Jolla, CA 92093-0343 (USA)
[**] D.M. and G.B. acknowledge financial support from the NSF (CHE-
1112133 and CHE-1316956), and Maria Angelella and Michael
Tauber for assistance with ESR experiments. D.M. acknowledges the
Extreme Science and Engineering Discovery Environment (XSEDE),
which is supported by NSF (OCI-1053575).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201302841.
Scheme 2. Formation and reactivity of oxyallyl 3.
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Table 1: X-ray crystal structures[a] with selected bond lengths [ꢀ] and angles [8] of 5 (left), d,l-6 (middle), and meso-6 (right).
O1-C1
C1-C2
N1-C2
N2-C2
C1-C2’
N1’-C2’
N2’-C2’
N1-C2-N2
N1’-C2’-N2’
5
1.3676(19)
1.287(4)
1.2569(18)
1.419(2)
1.435(5)
1.422(2)
1.3534(19)
1.340(4)
1.3600(18)
1.384(2)
1.388(4)
1.4060(18)
1.424(2)
1.447(5)
1.5070(19)
1.351(2)
1.343(4)
1.3200(17)
1.380(2)
1.378(4)
1.3498(19)
117.70(13)
118.3(3)
118.81(12)
117.29(13)
117.6(3)
119.54(12)
d,l-6
meso-6
[a] Thermal ellipsoids drawn at 50% probability level. Isopropyl groups, hydrogen atoms, cocrystallized solvents and triflate anions are omitted for
clarity.
Importantly no signal above d = 200 ppm could be observed,
demonstrating that 3 was not an aminoketene.[20] When the
solution was warmed to À108C, it became red, and a new set
of 13C NMR signals appeared. After workup, the 3,3-diamino-
2-aryloxyacrylimidamide 4 was isolated in 70% yield as a pale
yellow solid, and fully characterized (including an X-ray
diffraction study).[24] This result suggests that the deep-blue
intermediate was the desired oxyallyl derivative 3, which
rearranged through the migration of the 2,6-diisopropyl-
phenyl (Dipp) group. DFT calculations (B3LYP/6-311g**
level of theory) supported this hypothesis by predicting
a chemical shift of d = 168 ppm for the two equivalent NCN
carbons of oxyallyl 3, close to the observed experimental
value of d = 160 ppm; in contrast, the signal of central
carbonyl carbon was expected around d = 137 ppm, among
the signals of the quaternary carbons of the Dipp substituents.
In order to ascertain further the formation of oxyallyl 3,
a freshly prepared THF solution of 3 was reacted with one
equivalent of HCl at À788C. After workup, the hydroxycya-
nine triflate 5 was isolated in 47% yield. X-ray diffraction
analysis (Table 1, left) revealed that the CO and carbene
fragments are nearly coplanar (dihedral angles: N1-C2-C1-
In line with the experimental results, DFT calculation
confirmed that 5 is an excellent hydrogen-radical donor, since
À
the O H bond dissociation energy was predicted to be only
66 kcalmolÀ1. This value is significantly lower than that for
alkyl alcohols (100–110 kcalmolÀ1) and phenols (80–90 kcal
molÀ1), and is even in the range of the value for TEMPO·H
(68–71 kcalmolÀ1).[27] The SOMO of d,l-6 is a bonding
combination of the p*(CO) molecular orbital and the
LUMO of the carbenes, with significant electronic density
on all conjugated atoms (Figure 1, top right). A simple
Mulliken analysis confirmed the delocalization of the spin
density across the carbonyl moieties (40% on oxygen and
12% on carbon) and the two carbene units (15% on N1 and
N1’, 11% on N2 and N2’, and 19% on C2 and C2’).
Interestingly, we have also isolated a few crystals of the
meso diastereomer of 6 (Table 1, right). In marked contrast
À
O1: 158 and N1’-C2’-C1-O1: 108); the C O bond length
(136.8 pm) was in the range observed for related com-
pounds.[25,26]
Interestingly, compound 5 appeared to be highly sensitive
toward oxygen, turning deep green–blue upon brief exposure
to air, even in the solid state. A dichloromethane solution of 5
was stirred overnight in a flask opened to air, yielding an
NMR-silent product. An X-ray diffraction study revealed the
formation of the oxyallyl radical triflate salt d,l-6 (Table 1,
middle). The absence of a proton on the central oxygen atom
À
was confirmed by a significantly shorter C O bond length
(d,l-6: 128.7 pm; 5: 136.8 pm), and the absence of residual
electronic density around the oxygen atom. Cyclic voltam-
metry showed that 6 is both a weak reducing agent (E =
À0.116 V vs. Fc+/Fc, see the Supporting Information) and
a weak oxidant (E = À1.276 V), illustrating further the
thermodynamic stability of this compound.
Figure 1. Left: X-band EPR spectra recorded in benzene solution at
room temperature for: a) d,l-6 (g=2.0051) and b) meso-6 (g=2.0048).
c) Simulated EPR spectrum of meso-6 with the following set of hyper-
fine coupling constants: a(1H)=28.3, 20.4, 3.0, 2.9 (2H), and
2.5 MHz, a(14N)=9.4, 7.9, 3.0, and 4.0 MHz. Right: Representation of
the SOMO of d,l-6 (top) and meso-6 (bottom) with isosurfaces at
0.05 a.u.
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6: Compound 5 was prepared in situ from amidinium 1 (693 mg,
1.6 mmol), as described above. The volatiles were removed in vacuo
and technical-grade dichloromethane (30 mL) was added. The
resulting solution was vigorously stirred overnight in a round-
bottom flask opened to air, then washed with water (3 ꢁ 10 mL),
and dried over anhydrous magnesium sulfate. The volatiles were
removed in vacuo, affording a deep green–blue NMR-silent solid.
380 mg, 63% yield. MS (m/z): [M·+] calcd. for C39H56N4O, 596.4454;
found, 596.4455. Slow evaporation of a solution of 6 in toluene
afforded crystals of d,l-6 as green–blue needles. 160 mg. M.p.: 204–
2068C. Slow diffusion of pentane into the mother liquor afforded
a mixture of similar crystals (80 mg), mixed with large red–brown
crystalline blocks; 15 mg of the latter was separated manually and
identified as meso-6 by X-ray diffractometry. M.p.: 211–2138C.
with d,l-6, the solid-state structure of meso-6 is significantly
twisted (dihedral angle: meso-6: N1-C2-C1-O1: 58 and N1’-
C2’-C1-O1:578; d,l-6: 128 and 168, respectively). Thus, one
carbene unit is only weakly conjugated with the carbonyl
À
moieties, as also shown by the C1 C2’ bond length of
150.7 pm, typical of a single CN bond. Furthermore, whereas
the hyperfine X-band EPR signal of d,l-6 is unresolved, the
spectrum of meso-6 features a complex pattern, which could
be simulated only by assuming two different carbene units
(Figure 1, left).[28] Calculations reveal that the SOMO of
meso-6 involves only one carbene unit and confirm that most
of the spin density is distributed between the carbonyl moiety
(O1: 34%, C1: 14%) and the conjugated carbene unit (46%,
i.e. C2: 21%, N1: 21%, and N2: 4%). Interestingly, the
difference in energy between meso-6 and d,l-6 is only
+ 4 kcalmolÀ1, thus confirming that in such a twisted arrange-
ment the energetic loss due to decreased spin delocalization
can be compensated by the reduction of unfavorable steric
interactions between the two carbene units.
Received: April 5, 2013
Published online: May 29, 2013
Keywords: carbenes · carbon monoxide · non-Kekulꢁ systems ·
.
oxidation · radicals
This work demonstrates that stable carbenes can be used
for the preparation and isolation of oxyallyl radical cations
under aerobic conditions. Interestingly, the meso isomer of 6,
which benefits from the stabilization of only one “carbene”
unit, is stable. This suggests that a variety of carbene-
stabilized organic radicals should be isolable at ambient
temperature.
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Experimental Section
Experiments were performed under an atmosphere of dry argon using
standard Schlenk techniques, unless otherwise stated. Solvents were
dried by standard methods and distilled under argon. 1H and
13C NMR spectra were recorded on Varian Inova 400, 500, and
Bruker 300 MHz spectrometers. Low-temperature NMR spectra
were recorded on a JEOL 500 MHz spectrometer. NMR multi-
plicities are abbreviated as follows: s = singlet, d = doublet, t = triplet,
sept = septet, m = multiplet, br= broad signal. Melting points (uncor-
rected) were measured with an Electrothermal MEL-TEMP appara-
tus. Electrochemical experiments were performed with an analyzer
from CH Instruments (Model 620E). EPR spectra were obtained
using an X-band Bruker E500 spectrometer. Field calibration was
accomplished by using a standard of solid 2,2-diphenyl-1-picrylhy-
drazyl (DPPH), g = 2.0036.
5: A solution of 1 (850 mg, 1.95 mmol) in THF (15 mL) was added
dropwise to a stirred solution of KHMDS (430 mg, 2.15 mmol) in
THF (3 mL) at À788C. After 5 min, carbon monoxide was bubbled in
the solution for 20 min. Then HCl (0.5 mL, 2m in diethyl ether) was
added dropwise. After the reaction mixture had warmed to room
temperature, the volatiles were removed in vacuo and the residue was
extracted four times with benzene (10 mL). The solution was
concentrated in vacuo. Colorless crystals were obtained by slow
diffusion of layered pentane. 5: 340 mg, 47% yield. M.p.: 190–2008C
(decomp.). 1H NMR (CDCl3, 400 MHz): d = 7.23 (t, J = 7.5 Hz, 2H),
7.10 (dd, J = 2 and 7.5 Hz, 2H), 7.00 (dd, J = 2 and 7.5 Hz, 2H), 3.55–
3.39 (m, 8H), 3.28 (m, 2H), 2.86 (sept, J = 8.4 Hz, 2H), 2.65 (sept, J =
8.4 Hz, 2H), 2.58 (m, 2H), 2.09 (m, 2H), 1.81 (broad d, J = 13 Hz, 4H)
1.7–1.5 (m, 2H), 1.37–1.28 (m, 2H), 1.23 (d, J = 8.4 Hz, 6H), 1.22 (d,
J = 8.4 Hz, 6H), 1.15 (d, J = 8.4 Hz, 6H), 0.58 ppm (d, J = 8.4 Hz,
6H); 13C NMR (CDCl3, 100 MHz): d = 23.6 (CH3), 24.3 (CH3), 24.5
(CH2), 24.9 (CH3), 25.3 (CH3), 28.6 (CH), 29.4 (CH2), 29.5 (CH), 30.4
(CH). 49.5 (CH2), 57.6 (CH2), 60.1 (CH2), 109.7 (C), 124.2 (CH), 125.2
(CH), 128.9 (CH), 139.1 (C), 142.6 (C), 145.2 (C), 164.6 ppm (C).
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