Activation of the B?F Bond by Diphenylcarbene: A Reversible 1,2-Fluorine Migration between Boron and Carbon

Article abstract of DOI:10.1002/anie.201610179

Experiments in low-temperature matrices reveal that triplet diphenylcarbene inserts into the very strong B?F bond of BF₃in a two-step reaction. The first step is the formation of a strongly bound Lewis acid–base complex between the singlet stat

Full text of DOI:10.1002/anie.201610179

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201610179
German Edition: DOI: 10.1002/ange.201610179
Carbenes
À
Activation of the B F Bond by Diphenylcarbene: A Reversible
1,2-Fluorine Migration between Boron and Carbon
Paolo Costa, Joel Mieres Perez, Nesli ꢀzkan, and Wolfram Sander*
Dedicated to Prof. Rolf Gleiter on the occasion of his 80th birthday
Abstract: Experiments in low-temperature matrices reveal that
À
triplet diphenylcarbene inserts into the very strong B F bond
of BF3 in a two-step reaction. The first step is the formation of
a strongly bound Lewis acid–base complex between the singlet
state of diphenylcarbene and BF3. This step involves an
inversion of the spin state of the carbene from triplet to singlet.
The second step requires visible-light photochemical activation
to induce a 1,2-F migration from boron to the adjacent carbon
atom under formation of the formal insertion product of the
carbene center into BF3. The 1,2-F migration is reversible
under short-wavelength UV irradiation, thus leading back to
the Lewis acid–base adduct.
Scheme 1. Carbene–BF3 complexes.
W
ith a bond dissociation energy of more than 170 kcal
molÀ1,[1] the B F bond in BF3 belongs to the strongest known
single bonds between any elements, and the activation of this
bond by direct insertion is a challenge. Since BF3 is a strong
Lewis acid, it forms stable complexes with a large variety of
Lewis bases. The substitution of F atoms in BF3 proceeds by
an addition-elimination mechanism: the first step is addition
of XÀ to form a tetracoordinated borate BF3XÀ followed by
elimination of FÀ.
with dimesitylfluoroborane, thus resulting in a borylated
À
phosphorus ylide.[7] Braunschweig et al. observed that Lewis-
basic platinum(0) complexes undergo oxidative additions into
BF3.[8] According to recent DFT calculations by Kameo and
Sakaki, this reaction is endergonic and has a large activation
barrier if only one BF3 molecule is involved. A second
molecule of BF3 is necessary to reduce the activation barrier
and to make the reaction thermodynamically feasible.[9]
The diphenylcarbene 1 is a prototypical transient car-
bene[10] with a triplet ground state and has been subject to
a large number of mechanistic studies using time-resolved or
low-temperature spectroscopy.[11–14] Recently, we found that
the interaction of the triplet state of 1 (T-1) with either H2O[15]
or CH3OH[16] results in a switching of the spin state of the
carbene from triplet to singlet (S-1) upon formation of the
Imidazol-2-ylidenes (Arduengo carbenes) are strong
Lewis bases which have been shown by Kuhn et al.[2] and by
Arduengo et al.[3] to replace the ether in BF3 etherate to form
À
stable Lewis acid–base adducts with C B bond distances
between 1.63 and 1.67 ꢀ (Scheme 1). The bonding in these
adducts is similar to that in other Lewis acid–base complexes:
the BF3 unit is strongly negatively charged and the imidazo-
lium ring carries a corresponding positive charge. Carbene–
BF3 complexes were also synthesized by thermolysis of solid
imidazoliumtetrafluoroborates at 300–4008C under vacuum,
and produces the complexes under elimination of HF in high
yields (Scheme 1).[4,5] Li and Gabbai et al. observed that the
hydrolysis of BF3 is suppressed in the adducts with Arduengo
carbenes, which makes these adducts interesting as water-
stable probes for 18F-positron emission tomography (PET).[6]
There are only very few reports on insertion or addition
À
strongly hydrogen-bonded complexes S-1···H OH and
À
S-1···H OCH3, respectively (Scheme 2). A similar switching
of the spin states was found when 1 interacted with ICF3, and
in this case the halogen-bonded complex S-1···I-CF3 is
formed.[17]
The strong stabilization of S-1 by interaction with hydro-
gen- or halogen-bond donors results from the strong Lewis
À
reactions involving the B F bond. Bertrand et al. described
a formal 1,2-addition of a stable (phosphanyl)(silyl)carbene
[*] Dr. P. Costa, J. M. Perez, N. ꢀzkan, Prof. Dr. W. Sander
Lehrstuhl fꢁr Organische Chemie II, Ruhr-Universitꢂt Bochum
44780 Bochum (Germany)
E-mail: wolfram.sander@rub.de
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under http://dx.doi.org/10.
1002/anie.201610179.
Scheme 2. Hydrogen- and halogen-bonded complexes of 1.
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basicity of the singlet state S-1. In contrast, T-1 is much less
polar, and consequently only weakly interacts with Lewis
acids. BF3 is a much stronger Lewis acid than either H2O,
CH3OH, or CF3I mentioned above, and therefore is expected
to interact more strongly with S-1. This is confirmed by (gas
phase) DFT calculations at the B3LYP-D3/6-311 ++ G(d,p)
level of theory for the complexes of 1 (both S and T states)
with H2O, CH3OH, CF3I, and BF3 (Figure 1). The interaction
Figure 2. IR spectra showing the reaction of 1 with BF3. a) IR spectrum
of T-1 in argon doped with 1% of BF3 at 3 K generated by 530 nm
photolysis of 2. b) Difference IR spectrum of the same matrix showing
changes after annealing for 10 min at 30 K. Bands pointing downwards
assigned to T-1 and BF3 are disappearing, and bands pointing upwards
assigned to 4 and oligomers of BF3 are appearing. c) Difference IR
spectrum of the same matrix showing changes after irradiation with
l=405 nm at 3 K. Bands pointing downwards assigned to 4 are
disappearing, and bands pointing upwards assigned to 5 are appear-
ing. d) IR spectrum of an argon matrix containing 1% of BF3 annealed
from 3 K to 30 K for 10 minutes. e) Difference IR spectrum 5-4
calculated at the B3LYP-D3/cc-pVTZ level of theory.
882, 1073, 1080, 1350, and 1586 cmÀ1 are assigned to the Lewis
acid–base complex between 1 and BF3 (Figure 2, Table 1). If
the carbene center in 1 is 13C labelled, two IR bands of the
complex show large isotopic shifts: the band at 882 cmÀ1 is
red-shifted by 11 cmÀ1 and the band at 1350 cmÀ1 by 19 cmÀ1
(see Figure S2 in the Supporting Information). These vibra-
tions obviously involve the carbon atom of the carbene
Figure 1. S–T gaps (kcalmolÀ1) of 1 and its most stable complexes
with H2O (red), CH3OH (blue), CF3I (cyan), and BF3 (green) computed
at the B3LYP-D3/6-311+ +G(d,p) level of theory (all energies includ-
ing ZPE correction). DES and DET (dashed lines) refer to the binding
energies for singlet and triplet complexes, respectively.
energy of these four Lewis acids with T-1 lies in a narrow
range between 4 and 5 kcalmolÀ1, and most of this stabiliza-
tion energy results from dispersion effects. For S-1 the
differences in stabilization energies are much more pro-
nounced: hydrogen bonding results in a stabilization of 10–
11 kcalmolÀ1, halogen bonding in 15 kcalmolÀ1, while the BF3
complex is stabilized by 40.0 kcalmolÀ1. Consequently, for the
complex S-1···BF3 the singlet state is predicted to be
30.4 kcalmolÀ1 more stable than the triplet state.
To study the reaction between 1 and BF3, experiments
were performed in argon matrices doped with small amounts
of diphenyldiazomethane (2), as a precursor of 1, and 1% of
BF3. Visible-light irradiation of these matrices at 3 K results in
the formation of T-1 with characteristic IR absorptions[13] at
673 and 743 cmÀ1 (Figure 2). In addition, very strong bands of
matrix-isolated BF3 are observed at 677, 1441, and 1493 cmÀ1.
Subsequent annealing at 25–30 K allows the BF3 to diffuse in
the argon matrix, and results in a decrease of all bands
assigned to T-1 and BF3, and formation of new strong bands at
Table 1: IR spectroscopic data of 4 (S-1···BF3).
Mode Calculated[a] Argon[b]
n/cmÀ1 (Iabs
)
n/cmÀ1 (Irel) 13C shift (ppm)[c] Assignment[d]
20 (A) 619.7 (35)
26 (A) 713.4 (76)
28 (A) 814.9 (30)
31 (A) 893.7 (121)
33 (A) 955.4 (155)
35 (A) 989.7 (44)
44 (A) 1084.5 (246) 1072.8 (33)
47 (A) 1140.7 (232) 1121.0 (37)
49 (A) 1201.7 (9)
53 (A) 1321.0 (51)
600.2 (6)
693.6 (19)
791.7 (4)
882.3 (32)
941.7 (19)
976.9 (21)
À3.1 (À3.1)
À2.1 (À3.8)
À5.0 (À4.7)
À11.5 (À9.3)
À1.6 (À4.9)
À2.0 (À2.6)
À0.2 (À0.7)
À0.6 (À0.1)
À0.8 (À0.0)
À0.9 (À1.5)
BF3 str.
À
C H def.
À
C H def.
BF3 str.
À
C B str.
À
C H def.
À
B F str.
À
B F str.
À
1190.8 (7)
1305.5 (11)
C H def.
À À
C C C def.
À À
55 (A) 1370.5 (242) 1349.5 (100) À18.6 (À19.7)
C C C str.
=
58 (A) 1470.8 (35)
60 (A) 1521.1 (33)
1430.0 (28)
1483.0 (22)
0.2 (À0.3)
À1.9 (À1.1)
0.4 (0.0)
C C str.
=
C C str.
=
64 (A) 1631.0 (205) 1585.5 (45)
C C str.
[a] Calculated at the B3LYP-D3/cc-pVTZ level of theory. [b] Argon matrix
at 3 K. [c] 13C-2 isotopic shift (values within parentheses refer to the
calculated shifts). [d] Tentative assignment.
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center. The band positions, relative intensities, and 13C
isotopic shifts are in excellent agreement with the IR
spectroscopic data calculated for S-1···BF3 (Table 1). The IR
frequencies of the degenerate (E symmetrical) CF stretching
vibrations in the BF3 moiety depend strongly on the charge at
À
BF3. In matrix-isolated BF3 this vibration is found at
1026 cmÀ1, in BF3 at 1448 cmÀ1, and in BF3
at
+
1790 cmÀ1.[18,19] In S-1···BF3 these vibrations are found at
BF3 fragment, in accordance with a structure of a reversed
1073/1080 cmÀ1, close to the value of BF3 (Table 1), thus
ylide, Ph2C+BF3À (4).
À
indicating a large negative charge at the BF3 fragment (see
Figure S4).
The ylide 4 proved to be photolabile, and visible light (l =
405 nm) photolysis results in the disappearance of all IR and
UV-vis absorptions assigned to this species (Figure 2c and
Figure 3d). In the UV-vis region, no new absorptions are
formed, thus indicating that the photoproduct has no
prominent absorptions in the visible and near UV region of
the spectrum. The IR spectrum shows the formation of a new
compound with the strongest bands at 1394, 1318, and
768 cmÀ1 (Figure 2c, Table 2). By comparison with DFT
The interaction between T-1 and BF3 was also investigated
in the UV-vis spectral region. Under reaction conditions
similar to the IR experiments described above, annealing of
an argon matrix containing T-1 and BF3 at 25 K results in
a decrease in intensity of the band at l = 457 nm, assigned to
T-1, and formation of a new, very strong band with lmax
=
438 nm assigned to S-1···BF3 (Figure 3c). The absorption
Table 2: IR spectroscopic data of 5.
Mode Calculated[a] Argon[b]
n/cmÀ1 (Iabs
)
n/cmÀ1 (Irel) 13C shift (ppm)[c] Assignment[d]
20 (A) 599.0 (52)
26 (A) 719.2 (46)
27 (A) 774.4 (33)
32 (A) 928.6 (13)
34 (A) 965.7 (11)
51 (A) 1228.9 (4)
600.2 (6)
À2.2 (À1.3)
À1.7 ( 0.0)
À0.9 (À0.2)
BF3 str.
C-H def.
700.9 (23)
768.0 (24)
954.0 (10)
1015.8 (8)
1226.5 (12)
À
C H def.
À
C H def.
À À
À10.7 (À9.9)
C C C def.
À
C H def.
À
52 (A) 1312.5 (135) 1301.7 (46)
54 (A) 1336.6 (88) 1317.5 (32)
À4.0 (À4.6)
À4.2 (À4.0)
C B str.
=
C C str.
À
57 (A) 1411.7 (261) 1393.8 (100) À0.7 (À0.5)
B F str.
=
59 (A) 1486.7 (6)
60 (A) 1530.9 (17)
1495.0 (8)
1523.8 (7)
À1.5 (À0.6)
À2.5 (À0.2)
C
str.
=
C C str.
[a] Calculated at the B3LYP-D3/cc-pVTZ level of theory. [b] Argon matrix
at 3 K. [c] 13C-2 isotopic shift (values in parenthesis refer to the calculated
shifts). [d] Tentative assignment.
calculations at the B3LYP-D3/cc-pVTZ level of theory, the
new compound is assigned to difluoro(fluorodiphenylme-
À
thyl)borane (5), the formal insertion product of 1 into a B F
Figure 3. UV-vis spectra showing the formation of 4 and the photo-
chemical interconversion between 4 and 5 in an argon matrix at 8 K.
a) UV-vis spectrum of 2 in an argon matrix doped with 1% of BF3.
b) UV-vis spectrum of 1 generated by photolysis of 2 with l>530 nm
light. c) UV-vis spectrum of the same matrix after annealing from 8 K
to 25 K for 10 minutes. d) UV-vis spectrum of the same matrix after
irradiation with l=405 nm for 10 minutes. e) UV-vis spectrum of the
same matrix after irradiation with l=254 nm for 10 minutes. f) UV-vis
spectrum of the benzhydryl cation 3 generated by photolysis of 2 in
LDA water ice matrix.[15] g) Spectral transitions of 5 calculated at the
B3LYP-D3/cc-pVTZ level of theory. h) Spectral transitions of 4 calcu-
lated at the B3LYP-D3/cc-pVTZ level of theory.
bond of BF3. Thus, visible-light irradiation induces a 1,2-
fluorine migration from the negatively charged BF3 fragment
to the cationic center in 4. Even more remarkably, this
fluorine shift is reversible, and UV irradiation at l = 254 nm
results in the rearrangement of 5 back to 4 (Scheme 3 and
Figure 3, see Figure S3 in the Supporting Information).
Scheme 3. Reaction of T-1 with BF3.
maximum and the shape of this band is very similar to that of
the matrix-isolated benzhydryl cation 3 (lmax = 435 nm),[15]
and reveals that the Ph2C fragment in S-1.···BF3 is positively
charged. The UV-vis absorptions of S-1···BF3 can be nicely
reproduced by TD-DFT calculations (Figure 3h). IR and UV-
vis spectra of S-1···BF3, as well as the DFT calculations,
indicate a positively charged Ph2C and a negatively charged
To gain insight into the energetics of the fluorine
migration, the rearrangement was calculated at the B3LYP-
D3/cc-pVTZ level of theory (Figure 4). The migration of
a fluorine atom from boron to the adjacent carbon atom in 4
À
requires the alignment of a C F bond with the vacant p
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tional[21–23] with D3 dispersion corrections,[24] employing the cc-
pVTZ and 6-311 ++ g(d,p) basis sets as implemented in Gaussian
09 (Revision D.01).[25]
Acknowledgements
This work was supported by the Cluster of Excellence
RESOLV (EXC 1069) funded by the Deutsche Forschungs-
gemeinschaft (DFG).
Conflict of interest
The authors declare no conflict of interest.
Figure 4. Rearrangement of 4 (S-1.BF3) to 5 calculated at the B3LYP-
Keywords: carbenes · fluorine · IR spectroscopy ·
matrix isolation · rearrangements
À
À
À
D3/cc-pVTZ level of theory. The bonds length B F, C F, and C B are
given for comparison. For each structure two different views are
drawn. All attempts to locate a minimum structure between the
transition states TS-1 and TS-2 resulted in the formation of 4.
orbital at the cationic center. The transition-state TS-1 for the
fluorine shift is calculated to lie only 13.4 kcalmolÀ1 above 4.
The IRC from TS-1 towards the product leads to transition-
state TS-2, from which the final structure 5 is reached by
rotation of the planar BF2 group. As a consequence of the
bifurcation of the potential energy surface (and the presence
of a valley-ridge inflection point)[20] TS-2 is not reachedat-
tained during the reaction. The overall reaction from 4 to 5 is
predicted to be mildly endothermic by 5.5 kcalmolÀ1. The
fluorine migration results in a switch of the philicities of the
adjacent B and C atoms: while in 4 the C atom is highly
electrophilic, in 5 the B atom is the electrophilic center.
In conclusion, 1 inserts into the BF bond of BF3 under
formation of 5 by a two-step reaction sequence: first, T-1 and
BF3 react to give 4 in a highly exothermic (calculated to
À30.4 kcalmolÀ1, Figure 1), formally spin-forbidden, reaction.
This step is followed by a slightly endothermic (+ 5.5 kcal
molÀ1) 1,2-fluorine migration from B to C via a moderate
activation barrier (13.4 kcalmolÀ1). Since the migration of the
fluorine atom in both directions creates a highly electrophilic
center at the atom it leaves from, the rearrangement becomes
reversible. To overcome the activation barrier at the cryo-
genic temperatures used in our experiments, photochemical
excitation is necessary, while at elevated temperatures the
reaction is expected to proceed thermally. By tuning the
electronic properties of the carbene by substitution it should
be possible to design systems with exothermic fluorine
migrations of potential synthetic use.
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Experimental Section
Matrix isolation spectroscopy. Matrix isolation experiments were
performed using standard techniques as reported previously.[15,16]
Matrices were generated by co-deposition of diphenyldiazomethane
2 and 1% of BF3 with a large excess of argon (99.99%). The matrices
were irradiated with LEDs at 530 nm or 405 nm, or at 254 nm using
a low pressure mercury arc lamp.
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Manuscript received: October 18, 2016
Revised: October 29, 2016
Final Article published: && &&, &&&&
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Communications
Carbenes
In short: Triplet diphenylcarbene inserts
À
into the B F bond of BF3 molecule by
P. Costa, J. M. Perez, N. ꢁzkan,
W. Sander*
reversible fluorine migration despite
a bond dissociation energy of 170 kcal
molÀ1. The 1,2-F migration is reversible
under short-wavelength UV irradiation.
&&& — &&&
À
Activation of the B F Bond by
Diphenylcarbene: A Reversible 1,2-
Fluorine Migration between Boron and
Carbon
6
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