Tunneling rearrangement of 1-azulenylcarbene

Article abstract of DOI:10.1021/ja3050376

1-Azulenylcarbene was synthesized by photolysis of 1-azulenyldiazomethane in argon or neon matrices at 3-10 K. The highly polar singlet carbene is only metastable and undergoes a tunneling rearrangement to 8-methylene-bicyclo[5.3.0] deca-1,3,5,6,9-pentaene. After substitution of the 4 and 8 positions with deuterium, the rearrangement is completely inhibited. This indicates a very large kinetic isotope effect, as expected for a tunneling reaction.

Full text of DOI:10.1021/ja3050376

Communication
pubs.acs.org/JACS
Tunneling Rearrangement of 1‑Azulenylcarbene
Stefan Henkel, Y-am Huynh, Patrik Neuhaus, Michael Winkler,* and Wolfram Sander*
Lehrstuhl fur Organische Chemie II, Ruhr-Universitat Bochum, D-44801 Bochum, Germany
̈
̈
S
* Supporting Information
with a negatively polarized five-membered ring and a positively
polarized seven-membered ring. In particular, C1 and C3 are
more electron-donating than the other positions, and a carbene
unit attached to these centers should result in stabilization of
the singlet state. Here, we describe the IR spectroscopic
characterization of carbene 4 and its rearranged products using
matrix isolation spectroscopy.
1-Azulenyldiazomethane was matrix-isolated in argon as a
mixture of the conformers 5a (major conformer) and 5b
(minor conformer, see Supporting Information (SI) for
details). Visible light irradiation (λ > 530 nm) of 5 produces
1-azulenylmethylene 4 (Scheme 2). A comparison of the matrix
ABSTRACT: 1-Azulenylcarbene was synthesized by
photolysis of 1-azulenyldiazomethane in argon or neon
matrices at 3−10 K. The highly polar singlet carbene is
only metastable and undergoes a tunneling rearrangement
to 8-methylene-bicyclo[5.3.0]deca-1,3,5,6,9-pentaene.
After substitution of the 4 and 8 positions with deuterium,
the rearrangement is completely inhibited. This indicates a
very large kinetic isotope effect, as expected for a tunneling
reaction.
uantum chemical tunneling is an important process that
Q can influence reaction rates of proton and hydrogen
Scheme 2. Synthesis and Rearrangements of 1-
Azulenylcarbene 4
atom transfer,1−5 but also of larger (“heavy”) atoms such as
carbon.6−9 In recent years, a number of singlet carbenes have
been described that are metastable and, even at cryogenic
temperatures, rearrange via tunneling processes.6,9,10 Since
tunneling rates are almost independent of temperature,
tunneling becomes rate determining at low temperatures,
when classical thermal barriers are prohibitively high.
Arylcarbenes such as phenylcarbene11,12 or the naphthyl-
carbenes 113,14 are important reactive intermediates with triplet
ground states that have been subject to a large number of
experimental and theoretical studies. The chemistry of these
carbenes is dominated by rearrangements (Scheme 1) to
Scheme 1. Rearrangements of Naphthylcarbenes 1
IR spectrum with spectra calculated (B3LYP/cc-pVTZ, vide
infra) for the syn- and anti-conformers 4a and 4b, respectively,
in both their lowest lying singlet and triplet states, reveals that 4
is formed in its closed-shell (1A′) singlet state. The spectra
3
calculated for the A″ states of both conformers 4a and 4b are
clearly distinct and allow one to rule out a triplet ground state
for 4 on the basis of its IR spectrum. Attempts to record an
EPR spectrum of matrix-isolated 4 failed, which also indicates a
singlet ground state of 4.
It is more subtle to distinguish between the singlet
strained allenes such as 2, cyclopropenes 3, or other carbenes.
The rich chemistry of arylcarbenes has been explored by matrix
isolation spectroscopy,11,15−17 time-resolved spectroscopy,18,19
and theoretical methods.20,21
In contrast to the naphthylcarbenes 1, their isomeric
azulenylcarbenes are basically unknown. Only one attempt to
trap 1-azulenylcarbene 4 with styrenes is reported in the
literature, and the results of those experiments are not
conclusive.22 Azulenylcarbenes are interesting systems, since
the spin state and the philicity of these carbenes should depend
on the position of the methylene group at the azulenyl moiety.
Azulene shows a large dipole moment (μ = 1.08 0.02 D)23
1
1
conformers A′-4a and A′-4b. Whereas the overall computed
IR patterns look quite similar for both conformers, there are
pronounced differences in the intensities of various bands,
which allows one to definitely identify 1A′-4b as the
predominant species present in the matrix.
To further elaborate on the ground state of 4, extensive ab
initio computations were carried out (see SI). In qualitative
agreement with the DFT results, computations at the
Received: June 2, 2012
Published: July 16, 2012
© 2012 American Chemical Society
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Communication
Figure 1. Rearrangements of carbene 4b on the singlet and triplet PES computed at the RCCSD(T)/cc-pVTZ//(U)B3LYP/cc-pVTZ (red) and
UB3LYP/cc-pVTZ levels (blue). Spin expectation values (UB3LYP), T1 diagnostics (CCSD), and imaginary frequencies for the transition states
(UB3LYP) are given for comparison.
CAS(6,6)-CISD+Q/cc-pVTZ//CASSCF (12,12)/cc-pVTZ
bending contributions artificially gain intensity. In particular, for
the CCH bending mode of 1A′-4b, relaxing the orbital
constraints in UB3LYP as compared to RB3LYP results in a
red-shift of 83 cm−1 and in an increase of the IR intensity from
42 to 502 km/mol. Thus, a meaningful comparison of
1
level indicate a A′ closed-shell singlet ground state for 4b
3
that is 1.8 kcal/mol more stable than A″-4b. In contrast, the
syn isomer 4a has a 3A″ ground state that is 1.6 kcal/mol more
1
stable than A′-4a.
1
The singlet and triplet states of 4a are less stable than the
corresponding states of 4b by 3.7 and 0.3 kcal/mol,
respectively; hence, the ground-state multiplicity depends on
the conformation. The destabilization of 4a as compared to 4b
is attributed to the repulsion between the hydrogen atom at the
carbene center and H8 of the azulene ring system in the syn
conformer. Because the distance between the hydrogens is
smaller in the singlet state (smaller CCH bond angle), the
destabilization is more pronounced in 1A′-4a than in 3A″-4a. In
contrast, 1A′-4b benefits from a stabilizing electrostatic
interaction between the negatively polarized carbene carbon
atom (vide infra) and H8, although the C···H8 distance is
clearly too large to be classified as a hydrogen bond.24 The
open-shell singlet states 1A″-4a and 1A″-4b are adiabatically less
computed and experimental IR data of A′-4 has to be based
on restricted B3LYP data, despite the small external
1
instabilities. The extraordinary high polarity of A′-4, close to
those of 1-nitroazulene (μ = 6.06
0.03 D)23 and related
compounds, highlights the unusual properties of the title
carbene.
There are two plausible pathways for the syn/anti isomer-
ization of 4: via rotation around the C−CH bond (C1) or via
in-plane flip of the carbene hydrogen (Cs). On the singlet
potential energy surface (PES), both transition states (TSs) are
similar in structure and energy, and the barriers computed at
the CCSD(T)/cc-pVTZ//UB3LYP/cc-pVTZ level amount to
35.2 and 32.2 kcal/mol (relative to 4b), respectively (Figure 1).
The latter is in excellent agreement with the CAS(6,6)-CISD
+Q/cc-pVTZ//CASSCF(12,12)/cc-pVTZ value of 33.6 kcal/
mol. Although the barrier heights obtained at the UB3LYP level
are significantly lower (16.6 and 16.9 kcal/mol, respectively;
note the strong spin contamination), they are still prohibitively
high for a thermal isomerization in low-temperature matrices.
On the triplet PES only one TS can be localized, and the barrier
is significantly lower (4.3 kcal/mol). We therefore assume that
4b is formed during photolysis of the preferred diazomethane
5a either in a secondary photochemical process (that might
involve transition to the triplet PES) or via a hot ground-state
reaction.
1
stable than A′-4b by 10.1 and 9.9 kcal/mol, respectively, with
3
similar CCH bond angles as in the A″ states.
At the equilibrium geometries, the closed-shell singlet states
of both isomers are rather well-behaved with little multi-
reference character. Accordingly, spin contaminations of the
1
UB3LYP Kohn−Sham solutions for A′-4a (⟨S2⟩ = 0.294) and
1A′-4b (⟨S2⟩ = 0.175) are moderate, and structures, energies,
and dipole moments computed at the RB3LYP level are very
close to those obtained with unrestricted DFT. The same is not
true for the vibrational spectra: because triplet contamination of
the UDFT solution rapidly increases with increasing CCH
bond angle at the carbene center, and because the 1A′ states of
Carbene 4b is metastable, and even at temperatures as low as
3 K it slowly rearranges to a new compound, which was
identified as 8-methylene-bicyclo[5.3.0]deca-1,3,5,6,9-pentaene
(6, Figure 2). The half-life time of 4b between 3 and 30 K is
3
4 are significantly more polar than the A″ states (RB3LYP,
μ(1A′-4b) = 5.77 D; UB3LYP, μ(1A′-4b) = 5.35 D; μ(3A″-4b)
= 1.06 D), those IR absorptions involving significant CCH
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ordering is supported by MRCI computations which show,
however, that the ground state of 6a is a two-determinantal 1A″
open-shell singlet state that is about 1 kcal/mol more stable
1
3
than A′-6a and 2−3 kcal/mol more stable than A″-6a. Thus,
all three states of 6a are very close in energy.
Table 1. Adiabatic (CASSCF(12,12)/cc-pVTZ geometries)
Electronic Energies (in kcal/mol) of the Lowest States of
a
1
Several Isomers of 4 Relative to A′-4b
b
c
c
c
c
state
RS2c
RS2
RS3
CISD+Q
AQCC
4b
4a
6a
7
1A′
3A″
1A″
3A″
1A′
1A″
1A″
1A′
3A″
3A″
1A″
1A′
1A′
0.0
−2.9
5.9
0.0
−2.0
5.4
0.0
2.6
0.0
1.8
0.0
0.4
10.7
2.9
9.9
8.1
−2.6
3.3
−1.8
4.9
2.1
0.5
4.3
3.7
3.3
Figure 2. Difference IR spectrum showing the tunneling rearrange-
ment of 1-azulenylcarbene in argon at 3 K. Bands pointing upward
increase in intensity; bands pointing downward decrease concom-
itantly.
5.8
5.5
10.8
−1.2
−0.3
2.6
10.1
−0.9
0.1
8.1
−6.6
−0.4
−4.8
−22.7
−21.2
−2.9
28.3
−2.9
3.9
−2.1
−1.1
0.5
−4.0
−24.3
−23.4
0.0
1.8
−17.9
−16.3
−4.7
32.2
−16.7
−15.7
−4.8
33.6
−18.3
−17.4
−5.2
33.0
approximately 12 h, independent of the temperature in this
range and of the matrix material (Ne, Ar, or Xe). A constant
rate over a large temperature range (here a factor of 10 in T)
points toward a tunneling process. Since a hydrogen atom is
migrating from position 8 to the carbene center, a large kinetic
isotope effect (KIE) can be expected for this process. This is
confirmed by experiment: after deuteration of the 4 and 8
positions, carbene d2-4b is completely stable under the
conditions of matrix isolation, and no rearrangement to 6 is
observed even after extended periods of time. Deuteration of
the carbene hydrogen atom (d1-4b), on the other hand, does
not lead to any noticeable KIE (Scheme 3).
d
TS
29.6
a
See SI for details. The most reliable energies are given in the last two
columns. CAS(12,12). CAS(6,6). Transition state 4a → 4b.
b
c
d
All attempts to locate an additional stationary point on the
singlet PES at the UB3LYP level failed and inevitably
1
converged to the (externally stable) A′ structure. Because
the barrier height for the 4b → 6a rearrangement is central to
the present work, we investigated the energies of various states
1
3
at the geometries of the A′- and A″-TSs using different
Scheme 3. Rearrangements of the Deuterated Isotopomers
of 1-Azulenylcarbene 4
multireference methods and different active spaces. At all levels
1
and geometries, the A′ state remains significantly below the
1A″ state, so that the A′-TS(4b/6a) computed at the B3LYP
1
level can safely be considered the lowest electronic singlet state
1
at the TS geometries, even though the A″ state is not directly
amenable to a description by DFT. Overall, the large calculated
barrier of >22 kcal/mol for the 4b → 6 rearrangement,
combined with the observation of a thermal reaction at 3 K, is
another clear evidence for a tunneling process.
Because the tunneling rearrangement of 4b is essentially
blocked in carbene d2-4b, this isotopomer can be generated in
much higher yields (free of d2-6) than the non-deuterated 4b.
This allows one to study its bimolecular reactions and its
photochemistry. The very slow bimolecular reaction with O2 in
oxygen-doped argon matrices is in accordance with reactions of
typical singlet carbenes25,26 and will be reported elsewhere.
UV irradiation (308 nm) of d2-4b results in the formation of
several photoproducts. The main product could be identified as
triplet carbene d2-7 by IR and EPR spectroscopy (see SI for
details). It is worth noting that the EPR spectrum of triplet d2-7
is clearly observable after UV irradiation of its EPR silent
precursor, the singlet carbene d2-4b. According to MRCI
The rearrangement of 1A′-4b to the chiral allene 6 is a rather
complex process. At the (U)B3LYP level, the barrier amounts
to 22.8 kcal/mol (CCSD(T)//(U)B3LYP, 21.9 kcal/mol) and
1
leads to the planar carbene 6a in its A′ state. This carbene is
the TS for the racemization of the chiral allene 6 with an
activation barrier of 9−12 kcal/mol (Figure 1). The formation
of chiral 6 from the planar carbene 4b requires that the
symmetry along the reaction coordinate is broken, resulting in a
branching point (VRI) on the potential energy surface.
3
computations, the A″ ground state of 7 is approximately 1
On the triplet PES, the barrier for the 4b−6a rearrangement
kcal/mol more stable than 1A″-7. The rearrangement 4b → 7 is
classified as a 1,2 carbon migration that is exothermic by
roughly 16 kcal/mol. The same photochemistry is also
observed for the non-deuterated carbene 4, although here the
yield is lower due to the competing tunneling rearrangement to
6.
is much higher (UB3LYP, 36.1 kcal/mol; CCSD(T)//
3
UB3LYP, 39.0 kcal/mol), whereas A″-6a is by 1.7 kcal/mol
more stable than 1A′-6a, according to DFT. At the CCSD(T)//
UB3LYP level this order is reversed, and the singlet is found to
be 3.9 kcal/mol more stable than the triplet. This latter
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(19) Wang, J.; Burdzinski, G.; Kubicki, J.; Platz, M. S.; Moss, R. A.;
Fu, X.; Piotrowiak, P.; Myahkostupov, M. J. Am. Chem. Soc. 2006, 128,
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(24) This point was brought to our attention by Prof. Wes Borden.
Note that the C8−H bond is of almost identical length in 1A′-4b and
in the less polar 3A″-4b. Still, a stabilizing electrostatic C···H
interaction is likely to contribute to the stabilization of 1A′-4b, as
can be conjectured from atomic NPA charges, indicating a polarization
of the C8−H bond. See SI for details.
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In summary, 1-azulenylcarbene 4 is a metastable carbene
with a singlet ground state that even at lowest temperatures
rearranges to cycloheptatetraene 6 via quantum chemical
tunneling. This assumption is confirmed by the following
observations: (i) the rates are independent of temperature
within a range of a factor of 10 in T, (ii) a very large kinetic
isotope effect is found, and (iii) a large activation barrier is
calculated that, without tunneling, should result in slow rates
even at room temperature. This tunneling rearrangement is of
particular interest since it is one of the rare cases where during
the rearrangement the molecular symmetry is broken, and a
pair of enantiomers is formed from the achiral precursor.
ASSOCIATED CONTENT
* Supporting Information
■
S
Materials and methods, spectroscopic data, and computational
details. This material is available free of charge via the Internet
at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
wolfram.sander@rub.de; michael.winkler@rub.de
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Prof. Wes Borden for his suggestions on the
tunneling mechanism. This work was financially supported by
the Deutsche Forschungsgemeinschaft (FOR 1175) and the
Fonds der Chemischen Industrie.
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