Intramolecular hydroxycarbene C-H-insertion: The curious case of (o-methoxyphenyl)hydroxycarbene

Article abstract of DOI:10.3762/bjoc.6.121

The first C-H insertion of a hydroxycarbene species in the gas phase has been observed experimentally by means of high vacuum flash pyrolysis (HVFP) and subsequent matrix isolation: (o-Methoxyphenyl)glyoxylic acid gives non-isolable (o-methoxyphenyl)hydroxycarbene upon pyrolysis at 600 °C, which rapidly inserts into the methyl C-H bond. The insertion product, 2,3-dihydrobenzofuran- 3-ol, was trapped in an excess of Ar at 11 K and characterized by infrared spectroscopy. The insertion process kinetically outruns the alternative [1,2]H-tunneling reaction to o-anisaldehyde, a type of reaction observed for other hydroxycarbenes. Traces of the dehydration product, benzo[b]furan, were also detected. The potential energy hypersurface including the insertion and hydrogen migration processes was computed at the all-electron coupled-cluster level of theory encompassing single and double substitutions and perturbatively included triple excitations [AE-CCSD(T)] in conjunction with a correlation-consistent double-ζ basis set (cc-pVDZ) by utilizing density functional theory (DFT) optimized geometries (M06-2X/ccpVDZ) with zero-point vibrational energy (ZPVE) corrections. Exchange of the methoxy for a trifluoromethoxy group successfully prevents insertion and (o-trifluoromethoxy) benzaldehyde is produced instead; however, the carbene cannot be observed under these conditions. Thermal decomposition of (o-methoxyphenyl)glyoxylic acid in refluxing xylenes does not give the insertion product but yields o-anisaldehyde. This unanticipated outcome can be rationalized by protonation of the hydroxycarbene intermediate leading to the tautomeric formyl group. Thermochemical computations at M06-2X/cc-pVDZ in conjunction with a self-consistent solvent reaction field model support this suggested reaction pathway.

Full text of DOI:10.3762/bjoc.6.121

Intramolecular hydroxycarbene C–H-insertion: The
curious case of (o-methoxyphenyl)hydroxycarbene
Dennis Gerbig, David Ley, Hans Peter Reisenauer and Peter R. Schreiner*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2010, 6, 1061–1069.
Institut für Organische Chemie, Justus-Liebig University Giessen,
Heinrich-Buff-Ring 58, 35392 Giessen, Germany
doi:10.3762/bjoc.6.121
Received: 07 August 2010
Accepted: 21 October 2010
Published: 11 November 2010
Email:
Dennis Gerbig - Dennis.Gerbig@org.chemie.uni-giessen.de;
David Ley - David.Ley@org.chemie.uni-giessen.de;
Hans Peter Reisenauer -
Guest Editor: J. Murphy
Hans.P.Reisenauer@org.chemie.uni-giessen.de;
Peter R. Schreiner* - prs@org.chemie.uni-giessen.de
© 2010 Gerbig et al; licensee Beilstein-Institut.
License and terms: see end of document.
* Corresponding author
Keywords:
benzofuran; C–H-insertion; hydroxycarbene; singlet carbene;
tunneling
Abstract
The first C–H insertion of a hydroxycarbene species in the gas phase has been observed experimentally by means of high vacuum
flash pyrolysis (HVFP) and subsequent matrix isolation: (o-Methoxyphenyl)glyoxylic acid gives non-isolable
(o-methoxyphenyl)hydroxycarbene upon pyrolysis at 600 °C, which rapidly inserts into the methyl C–H bond. The insertion pro-
duct, 2,3-dihydrobenzofuran-3-ol, was trapped in an excess of Ar at 11 K and characterized by infrared spectroscopy. The insertion
process kinetically outruns the alternative [1,2]H-tunneling reaction to o-anisaldehyde, a type of reaction observed for other
hydroxycarbenes. Traces of the dehydration product, benzo[b]furan, were also detected. The potential energy hypersurface
including the insertion and hydrogen migration processes was computed at the all-electron coupled-cluster level of theory encom-
passing single and double substitutions and perturbatively included triple excitations [AE-CCSD(T)] in conjunction with a correla-
tion-consistent double-ζ basis set (cc-pVDZ) by utilizing density functional theory (DFT) optimized geometries (M06-2X/cc-
pVDZ) with zero-point vibrational energy (ZPVE) corrections. Exchange of the methoxy for a trifluoromethoxy group successfully
prevents insertion and (o-trifluoromethoxy)benzaldehyde is produced instead; however, the carbene cannot be observed under these
conditions. Thermal decomposition of (o-methoxyphenyl)glyoxylic acid in refluxing xylenes does not give the insertion product but
yields o-anisaldehyde. This unanticipated outcome can be rationalized by protonation of the hydroxycarbene intermediate leading to
the tautomeric formyl group. Thermochemical computations at M06-2X/cc-pVDZ in conjunction with a self-consistent solvent
reaction field model support this suggested reaction pathway.
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Beilstein J. Org. Chem. 2010, 6, 1061–1069.
Introduction
Hydroxycarbenes have been the subject of many theoretical and insertion both under matrix isolation and solution conditions
experimental studies since the early years of last century, (Scheme 3). The generation of such carbenes in solution in
however, these proved to be elusive for a long time [1]. Whilst a high-boiling solvents would also provide convenient prepara-
large variety of hydroxycarbene ligands have been prepared as tive access to dihydrobenzofuranols from readily accessible
fairly stable Fischer-type carbene–metal complexes with group α-keto acids as the starting materials. Our results were rational-
VI, VII, VIII, and X elements [2-12] it was only very recently ized by quantum chemical computations.
that free hydroxycarbenes were generated by high vacuum flash
pyrolysis (HVFP) followed by immediate matrix isolation and
thoroughly characterized by means of IR- and UV-spec-
troscopy [13-15]. Very surprisingly, several hydroxycarbenes
exhibit remarkable [1,2]H-tunneling under cryogenic condi-
tions in solid noble gas matrices, even at temperatures as low as
11 K: Hydroxymethylene (1) [13] and phenylhydroxycarbene
(3) [14] yield formaldehyde (2) and benzaldehyde (4), respec-
tively, as a result of facile [1,2]hydrogen tunneling from the
hydroxy group to the carbene center. Dihydroxycarbene (a) [15]
and methoxyhydroxycarbene (c) [15], however, do not undergo
[1,2]H-tunneling under the same conditions: Their respective
products, formic acid (b) and methyl formate (d) were not
detected in matrix isolation experiments (Scheme 1).
Scheme 2: A selection of heterocarbenes that undergo intramolecular
C–H insertions.
Results and Discussion
Matrix isolation studies on
(o-methoxyphenyl)hydroxycarbene
Scheme 1: Unimolecular reactivity of hydroxycarbenes under cryo-
genic conditions: [1,2]H-Tunneling of 1 and 3 (τ1/2: half-life).
In the course of our ongoing investigations regarding the nature
of the fascinating [1,2]H-tunneling mechanism in phenylhy-
Ring insertions characteristic for other singlet phenylcarbenes,
i. e., phenylmethylcarbene [16] and phenylchlorocarbene [17],
were not experimentally observed for 3, and we report herein
the first C–H-bond insertion reaction of a hydroxycarbene that
is akin to other heterocarbenes [18-20] (Scheme 2).
(o-Methoxyphenyl)hydroxycarbene (5) serves as the model
compound for studying the intramolecular carbene C–H-bond
droxycarbenes (with parent 3, Scheme 1), we sought to study
the behavior of derivatives of 3 in Ar matrices at temperatures
as low as 11 K. We attempted to generate novel o-methoxy-
substituted carbene 5 by extrusion of carbon dioxide from
(o-methoxyphenyl)glyoxylic acid (6) by HVFP and subsequent
condensation and isolation of the pyrolysis products in an
excess of Ar (Scheme 3). However, neither 5 nor its tunneling
Scheme 3: Attempted generation of 5 and d-5 as well as their corresponding insertion products.
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Beilstein J. Org. Chem. 2010, 6, 1061–1069.
product, o-anisaldehyde (7), could be detected: Comparison proposed mechanism for the formation of the insertion product
with an authentic spectrum of matrix-isolated 7 showed that this 8 is presented in Scheme 4. Matrix isolation spectra of both
compound was not present among the pyrolysis products. Even pyrolyses (6 and d-6, respectively) are presented in Figure 1 and
after more than 12 h in the dark at 11 K, the IR spectrum was Figure 2, along with an assignment of all signals. A collection
unchanged, indicating that there is no compound among the of all the matrix spectra is contained in Supporting Information
pyrolysis products that is susceptible to a tunneling decay File 1.
mechanism. The TD-DFT (B3LYP/cc-pVDZ) absorption
maximum of 5 lies at 591 nm [21,22]. To rule out the existence
of a persistent, non-tunneling carbene, irradiation experiments
were conducted: Subsequent irradiation with a high-pressure
Hg-lamp at λ = 577 nm, 546 nm, and 313 nm did not lead either
to the appearance of new or the disappearance of existing
signals, thereby verifying the complete absence of 5.
Analysis of the IR spectrum showed that 8 was the main pro-
duct instead. To confirm the C–H-insertion into the neigh-
boring methyl group, to yield 2,3-dihydrobenzofuran-3-ol (8), a
sample of 8 was prepared by the reduction of commercially
Figure 1: Unmodified matrix IR spectrum (Ar, 11 K) of the pyrolysis
(600 °C) of 5. Traces of 9 are indicated by magenta dots.
available 3-coumaranone and subjected to matrix isolation
studies. All intense signals in the original pyrolysis spectrum of
6 could be shown to originate from 8. As a side product, traces
of the dehydration product of 8, benzo[b]furan (9), were found
and identified by comparison with an authentic spectrum of
matrix-isolated 9 (Scheme 3 and Scheme 4).
The [1,2]H-tunneling reactions in hydroxycarbenes can be
suppressed by an exchange of hydrogen for deuterium. Hence,
carbene signals can readily be identified by prolonged irradi-
ation at or near the maximum absorption wavelength of the
carbene. Thus, the pyrolysis experiment was repeated with the
mono-deuterated acid d-6 (o-MeOC6H4COCOOD) to yield d-8
(OD). Again minor amounts of 9 (without any deuterium
Figure 2: Unmodified matrix IR spectrum (Ar, 11 K) of the pyrolysis
(600 °C) of d-5. Traces of 9 are indicated by magenta dots.
incorporation) were formed. In accordance with prior results,
neither d-5 nor d-7 could be detected after irradiation. A
Scheme 4: Proposed mechanism for the generation of 8 and 9. The [1,2]H-tunneling process apparently cannot compete with C–H-insertion (τ1/2:
half-life).
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Beilstein J. Org. Chem. 2010, 6, 1061–1069.
Scheme 5: Decay of the 2,3-dihydrobenzofuran-3-ol molecular radical cation (8+•).
After warming of the matrix to room temperature, a sample of fied. On repeating the experiment with deuterated acid (OD)
the pyrolysis products of 6 was collected from the matrix d-10, no deutero-carbene d-12 was likewise produced, as veri-
window. The molecular ions of 8 and 9 were found in the fied by subsequent irradiation (577 nm, 313 nm). The computed
corresponding electron-impact mass spectrum, most probably [1,2]H-tunneling half-lives of 5 and 12, based on a) an Eckart
generated through the decay mechanism proposed by Florêncio barrier approach, and b) the Wentzel–Kramers–Brillouin
et al. [23] (Scheme 5).
(WKB) approximation (see Computational methods for details),
are summarized in Table 1, together with the computed half-life
of 3 for comparison.
Matrix isolation studies on [(o-trifluoro-
methoxy)phenyl]hydroxycarbene
Exchanging the methoxy group for a trifluoromethoxy moiety
should prohibit the insertion reaction due to the much stronger
carbon–fluorine bond compared to the carbon–hydrogen bond.
HVFP of [(o-trifluoromethoxy)phenyl]glyoxylic acid (10) at
600 °C (500 °C, 800 °C) and subsequent matrix isolation gave
(o-trifluoromethoxy)benzaldehyde (11); as expected, no inser-
tion product (13) was detected (Scheme 6).
Table 1: Computed half-lives (unscaled) for the [1,2]H-tunneling reac-
tion in carbenes 3, 5, and 12 at 11 K. The measured half-life of 3 is 2.5
h at 11 K. The thermal barrier for the [1,2]H-shift was computed at
AE-CCSD(T)/cc-pVDZ // M06-2X/cc-pVDZ; (cf. Computational
methods).
∆H‡
for [1,2]H-shift Eckart WKB
[kcal mol–1]
[h]
[h]
PhCOH (3)
28.9
27.1
29.3
3.7
0.6
1.2
4.8
0.5
1.1
o-MeOC6H4COH (5)
o-F3COC6H4COH (12)
Based on the computed tunneling half-lives, 12 should be
observable in matrix-isolation experiments if it survives the for-
mation conditions in the gas phase. The same holds true for
d-12.
Pyrolysis of (o-methoxyphenyl)glyoxylic acid
(6) in solution
Surprisingly, the reactivity of 5 in solution is quite different
from that in our matrix isolation experiments as o-anisaldehyde
7 forms instead of 8 upon refluxing 6 in xylenes (bp
139–140 °C) for 12 h in almost quantitative yield (Scheme 7).
This unanticipated outcome is in stark contrast to the reactions
of other heterocarbenes that have been shown to insert into
comparable methyl C–H bonds in solution with ease [24]. When
Scheme 6: Attempted generation of 12 and the actual pyrolysis pro-
duct 11.
Contrary to our findings on hydroxycarbenes, [(o-trifluoro-
methoxy)phenyl]hydroxycarbene (12) was not detected.
Instead, only the substituted anisaldehyde 11 could be identi-
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Beilstein J. Org. Chem. 2010, 6, 1061–1069.
the reaction was carried out in an even higher boiling solvent, condensed phase reactivity of 5, M06-2X/cc-pVDZ computa-
i.e., nitrobenzene (bp 211 °C), a complicated mixture of decom- tions including a solvent model were performed. These indicate
position products resulted.
a thermodynamic preference of −12 kcal mol–1 for the insertion
product 8 over the aldehyde 7 as well as a 5 kcal mol–1 lower
activation energy for the formation of 8.
Based on this potential energy hypersurface, it seems surprising
that 7 is produced by pyrolysis of 6 in the condensed phase as 8
is both favored both thermodynamically and kinetically due to a
lower thermal barrier. As a result, another mechanism must be
implicated for the transformation of 6 to 7. A possibly alter-
native pathway, involving an acid-catalyzed formation of the
aldehyde, is presented in Scheme 9: Protonation of 5 at the
carbene center by 6 leads to a carbonyl-protonated aldehyde
[7+H]+, which is then deprotonated at the oxygen atom in the
follow-up reaction, regenerating 6 and producing 7. As the
proton affinity Epa of 5 was computed to be 262 kcal mol–1,
which is comparable to those of alkali metal hydroxides, this
seems a viable possibility.
Scheme 7: Unanticipated reaction of 6 upon heating in xylenes.
In order to probe for a chemical connection between 7 and 8,
insertion product 8 was also refluxed in xylenes for 12 h.
However, only pure 8 was recovered. As expected on thermo-
dynamic and kinetics grounds, the direct interconversion of 7
and 8 can therefore be excluded.
Computational considerations
AE-CCSD(T)/cc-pVDZ single point computations (with DFT
ZPVEs; Scheme 8, for computational details see Computa- Conclusion
tional Methods below) on M06-2X/cc-pVDZ geometries do not The formation of a 2,3-dihydrobenzofuran derivative from
indicate a thermodynamic preference for either 7 or 8. The carbene 5 in the gas phase is the first C–H insertion reaction
thermal barrier for the reaction of 5 via conformer 5i leading to observed for a hydroxycarbene derivative. Hence, the general
8 is lower (18 kcal mol–1, via TS3) than that leading to 7 reactivity pattern of 5 under HVFP conditions is similar to that
(27 kcal mol–1, via TS1). These results help rationalize the of other hetereocarbenes. However, synthetic access to substi-
observed reactivity under HVFP conditions. The thermal barrier tuted 2,3-dihydrobenzofuran derivatives through the in situ
for the dehydration of 7 is quite high (in excess of 60 kcal generation of various (o-methoxyphenyl)hydroxycarbenes from
mol–1, via TS4), which was confirmed by HVFP of 7 at 600 °C (o-methoxyphenyl)glyoxylic acids in high-boiling solvents is
and subsequent matrix isolation of the products: While 8 was not possible, because an o-anisaldehyde derivatives are formed.
more abundant at this temperature than in the pyrolysis of 6, 7 We suggest that this dichotomy in reactivity between the high-
was for the most part unchanged. In order to assess the temperature HVFP and the high-temperature solution experi-
Scheme 8: Potential energy hypersurface of (o-methoxyphenyl)hydroxycarbene (5) (not drawn to scale; ZPVE included); legend: 5c: carbene, cis
conformation; 5t: carbene, trans conformation; 5i: conformer that undergoes insertion.
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Beilstein J. Org. Chem. 2010, 6, 1061–1069.
Scheme 9: Acid-catalyzed generation of 7 by unreacted 6.
ments derives from protonation of the intermediate hydroxycar- 1H NMR (400 MHz, d6-DMSO): δ = 3.85 (s, 3H, –OMe), 7.13
bene by the starting material in solution; this inevitably leads to (t, 1H, 3J = 7.5 Hz, p-H to –OMe), 7.24 (d, 1H, 3J = 8.4 Hz,
the aldehyde upon deprotonation. Although preliminary o-H to –OMe), 7.66–7.78 (m, 2H), 14.01 (s, 1H, acid-H);
tunneling computations indicate that carbene 12 should be 13C NMR (100 MHz, d6-DMSO): δ = 56.2 (–OMe), 113.1 (o-C
observable, it has not yet been detected experimentally.
to –OMe), 121.2 (p-C to –OMe), 122.2 (ipso-C to –COCOOH),
126.1 (o-C to –COCOOH), 136.6 (p-C to –COCOOH), 160.0
(ipso-C to –OMe), 166.8 (–COCOOH), 188.1 (–COCOOH).
Experimental
Matrix-isolation studies. An APD Cryogenics HC-2 closed-
cycle cryostat system with an inner CsI window was used for IR (o-Methoxyphenyl)glyoxylic acid-O-d (d-6) was obtained by
measurements. Spectra were recorded with a Bruker IFS 55 repeated dissolution of (o-methoxyphenyl)glyoxylic acid in
FT-IR spectrometer (4500–300 cm–1 spectral range with a reso- excess deuterium oxide followed by evaporation of the solvent
lution of 0.7 cm–1). For the combination of high-vacuum flash in vacuo.
pyrolysis with matrix isolation, a small, custom-built, water-
cooled oven was used, which was directly connected to the 2,3-Dihydrobenzofuran-3-ol (8) [26]. To a solution of 1.96 g
vacuum shroud of the cryostat. The pyrolysis zone consisted of (5 mmol) 3-coumaranone in 40 mL of absolute methanol,
an empty quartz tube with an internal diameter of 8 mm and a sodium borohydride was added in small portions at –10 °C until
length of the heating zone of 50 mm, which was resistively the solution solidified. After the succession of hydrogen evolu-
heated by a coax wire. The temperature was controlled with a tion, 20 mL of 0.2 N hydrochloric acid was added to the reac-
Ni/CrNi thermocouple. (o-Methoxyphenyl)glyoxylic acid (6) tion mixture. The mixture was then extracted with chloroform.
was evaporated at room temperature from a small storage tube The combined organic layers were washed with brine and dried
into the pyrolysis tube. All pyrolysis products were immedi- over anhydrous sodium carbonate. Filtration and removal of the
ately co-condensed with a large excess of argon (typically 30 to solvent gave a brown oily liquid that was immediately purified
80 mbar from a 2000 mL storage bulb) on the surface of the by flash chromatography on a short column of silica gel with
11 K matrix window at a distance of approximately 50 mm. A tert-butyl methyl ether as the eluent. The purified product was
high-pressure mercury lamp (HBO 200, Osram) with a mono- obtained as the second fraction (Rf ≈ 0.6) in 30% yield.
chromator (Bausch & Lomb) was used for irradiation. Experi-
ments with deuterated acid were conducted accordingly.
1H NMR (400 MHz, CDCl3): δ = 1.91 (s, 1H, –OH), 4.35 (dd,
1H, 2J = 10.6 Hz, 3J = 2.5 Hz, –O–CH2–), 4.44 (dd, 1H, 2J =
(o-Methoxyphenyl)glyoxylic acid (6) [25]. To a solution of 10.6 Hz, 3J = 6.5 Hz, –O–CH2–), 5.26 (dd, 1H, 3J = 6.5 Hz, 2.5
3.00 g (20.0 mmol) o-methoxyacetophenone in absolute pyri- Hz,–CHOH–), 6.81 (d, 1H, 3J = 8.1, o-H to –O–CH2–), 6.87 (t,
dine, was added 3.33 g (30.0 mmol) of selenium dioxide. The 1H, 3J = 7.04, p-H to –O–CH2–), 7.17-7.23 (m, 1H, p-H to
mixture was stirred at 80 °C for 4 h. After filtration, concentra- –CHOH–), 7.34 (d, 1H, 3J = 7.5 Hz, o-H to –CHOH–);
tion of the solution gave a brown oil that was dissolved in 5% 13C NMR (100 MHz, CDCl3): δ = 106.5 (–O–CH2–), 111.4
sodium hydroxide solution and washed three times with small (–CHOH–), 121.4 (o-C to –O–CH2–, m-C to –CHOH–), 122.7
portions of diethyl ether. The aqueous layer was acidified with (p-C to –O–CH2),124.2 (o-C to –CHOH–, m-C to –O–CH2–),
dilute hydrochloric acid and extracted with ethyl acetate. The 127.4 (ipso-C –CHOH–), 144.9 (p-C to –CHOH–), 154.9 (ipso-
organic layer was then dried over sodium sulfate. Filtration and C –O–CH2–).
concentration gave a brown oil, which crystallized on standing.
(o-Methoxyphenyl)glyoxylic acid (3.30 g,18.4 mmol) was 2,3-Dihydrobenzofuran-3-ol-O-d. H–D-exchange with D2O
obtained as a brown solid. Sublimation in vacuo afforded the proved to be unsuccessful on the preparative scale. As a conse-
pure title compound as a white to yellowish powder in 60% quence, the deuterated compound was prepared in the same way
yield.
as its protium analogue: To a solution of 0.55 g (1.3 mmol) of
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Beilstein J. Org. Chem. 2010, 6, 1061–1069.
3-coumaranone in 20 mL of methanol-O-d, sodium borohy-
dride was added in small portions at −10 °C until the solution
solidified. After the succession of hydrogen evolution, 10 mL of
D2O were added to the reaction mixture. The mixture was then
extracted with CDCl3. The combined organic layers were
washed with brine and dried over anhydrous sodium carbonate.
Filtration and removal of the solvent gave a yellow liquid in
60% yield.
,
and
.
The half-life τ1/2 was obtained by employing the rate law of
first-order kinetics:
1H NMR (400 MHz, CDCl3): δ = 4.35 (dd, 1H, 2J = 10.6 Hz, 3J
= 2.5 Hz, –O–CH2–), 4.44 (dd, 1H, 2J = 10.6 Hz, 3J = 6.5 Hz,
–O–CH2–), 5.26 (dd, 1H, 3J = 6.5 Hz, 2.5 Hz, –CHOH–), 6.81
(d, 1H, 3J = 8.1, o-H to –O–CH2–), 6.87 (t, 1H, 3J = 7.04, p-H
.
to –O–CH2–), 7.17–7.23 (m, 1H, p-H to –CHOH–), 7.34 (d, 1H, Computations at B3PW91/cc-pVDZ were found to produce rea-
3J = 7.5 Hz, o-H to –CHOH–).
sonable half-lives for PhCOH [21,39,40], for which experi-
mental data are available.
Benzo[b]furan was purchased from Sigma–Aldrich and used
without further purification. For matrix isolation studies, the Wentzel–Kramers–Brillouin approximation
sample was cooled to −35 °C to reduce its vapor pressure.
The intrinsic reaction path, IRP [41], (or minimum energy path,
MEP) for each molecule was established at M06-2X/6-
311++G(d,p) using the Hessian-based predictor-corrector algo-
Computational methods
All computations were performed with the Gaussian09 [27] or rithm [42] as implemented in Gaussian09, with tight conver-
Cfour suite [28] of programs. All structures were computed gence criteria. An augmented triple-ζ basis is necessary to
employing the M06 density functional [29,30] with doubled achieve an IRP as accurate as possible (as far as the system size
(50%) HF-exchange (M06-2X), developed by Truhlar and permits). The single point energies along the reaction path were
co-workers, in conjunction with a Dunning-type correlation- vibrationally zero-point corrected by adding the energy contri-
consistent double-ζ (zeta) basis set [31] (cc-pVDZ). Single bution of the projected frequencies along the path, i.e., a zero-
point energies of the DFT structures were evaluated with the point correction excluding the frequency νξ , corresponding to
coupled cluster method, incorporating singles and doubles as the reaction coordinate. The thus obtained corrected potential
well as perturbative triples and taking into account both valence along the reaction coordinate ξ was then characterized by an
and core electrons [32-34]. Again, cc-pVDZ was used as the interpolating function V(ξ). The attempt frequency of barrier
basis set (AE-CCSD(T)/cc-pVDZ). For the elucidation of penetration, νξ, was identified by comparing the starting ma-
solvent effects, M06-2X/cc-pVDZ-computations with the terial’s frequencies and the projected frequencies [43] along the
Polarization Continuum Model [35] (PCM) were performed, IRC. The barrier penetration integral σ between the classical
containing radii based on the United Atom Topological Model turning points s1,2 , where V(ξ) = ε and
as implemented in Gaussian09. Tunneling half-lives τ1/2 of
carbenes 5 and 12 were estimated employing a) a simple Eckart
barrier methodology [36,37], and b) the one-dimensional
Wentzel–Kramers–Brillouin approximation [13,14,37,38].
, was then computed as
.
Eckart-barrier approach
With σ at hand, the transition probability P could be computed
For evaluation of the rate constant k, the activation barriers of as
the forward and back reaction, Vf and Vb, were computed,
along with the imaginary frequency νi of the transition state and
the frequency νξ, corresponding to the reaction coordinate. All
.
energies were vibrationally zero-point corrected, excluding νi The half-life τ1/2 was again obtained by employing the rate law
and νξ. The transmission probability P was computed as
of first-order kinetics:
, with the three parameters being
.
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Beilstein J. Org. Chem. 2010, 6, 1061–1069.
23.Florênico, H.; Heerma, W. Org. Mass Spectrom. 1978, 13, 368–370.
doi:10.1002/oms.1210130614
All mathematical operations were carried out with the Mathe-
matica software package [44].
24.Holdroyd, R. S.; Page, M. J.; Warren, M. R.; Whittlesey, M. K.
Tetrahedron Lett. 2010, 51, 557–559. doi:10.1016/j.tetlet.2009.11.090
25.Domagala, J. M.; Haskell, T. H. J. Org. Chem. 1981, 46, 134–140.
doi:10.1021/jo00314a029
Supporting Information
26.Ghosh, S.; Datta, I.; Chakraborty, R.; Das, T. K.; Sengupta, J.;
Sarkar, D. C. Tetrahedron 1989, 45, 1441–1446.
doi:10.1016/0040-4020(89)80142-5
Supporting Information File 1
Full matrix isolation spectra.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-6-121-S1.pdf]
27.Gaussian 09, Revision A.02; Gaussian, Inc.: Wallingford CT, 2009.
28.CFOUR; , http://www.cfour.de.
Coupled-Cluster techniques for Computational Chemistry;
quantum-chemical program package.
29.Zhao, Y.; Truhlar, D. G. Theor. Chem. Acc. 2008, 120, 215–241.
doi:10.1007/s00214-007-0310-x
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