Spectroscopic identification of dihydroxycarbene

Article abstract of DOI:10.1002/anie.200802105

Out and about: CO₂ extrusion from simple α-keto carboxylic acids can be used to prepare hitherto unknown hydroxycarbenes. High-vacuum flash pyrolysis of oxalic acid or its monomethyl ester enabled the preparation and characterization of dihydro

Full text of DOI:10.1002/anie.200802105

Angewandte
Chemie
DOI: 10.1002/anie.200802105
Reactive Intermediates
Spectroscopic Identification of Dihydroxycarbene**
Peter R. Schreiner* and Hans Peter Reisenauer
Dihydroxycarbene (HO-C-OH, 1, Scheme 1) is a prototypical
compound that plays a fundamental role in the class of
synthetically valuable heterosubstituted carbenes that act as
reagents and metal ligands.In tribute to the discoverer of such
this is that carbenes were just being established as viable
chemical entities at that time.Lapidus et al.proposed the
direct decomposition of 2 to CO2 and H2 by an intramolecular
hydrogen transfer that implies the formation of a divalent
species, although it was neither drawn nor explicitly
stated.[7] The proposed intermediacy of 1 in the
thermal decomposition of 2 in the gas phase came in
1985 when Yamamoto and Back noted a marked
difference in the rate of formation of CO2 in the
photochemical and thermal reactions of 2.[8]
A
subsequent MP2/4-31G//HF/3-21G computational
study emphasized the key role 1 plays in the thermal
reactions of 2,[9] and confirms the singlet ground
state of 1 predicted earlier by Feller et al.[10] These
authors noted that besides the Woodward—Hoff-
mann-allowed C2v-symmetric decomposition of 1
into CO2 and H2, the radical path through HC +
COCOH and recombination may also be important,
and that the hydrogen shift of 1 to give 3 is formally
a forbidden process.
Scheme 1. Generation of dihydroxycarbene (1; tt refers to the s-trans,s-trans
rotamer, tc denotes the s-trans,s-cis rotamer)through CO 2 extrusion from oxalic
acid (2). Irradiation near the absorption maximum of 1 quickly gives water and
CO.
metal–carbene complexes, these types of structures are now
Indirect evidence for the successful experimental prepa-
referred to as Fischer carbenes.[1] However, attempts to
prepare free hydroxycarbenes through the thermal decom-
position of metal carbenes have failed.[2] Like its smaller
cousin hydroxycarbene, 1 also plays an important role in
deciphering the organic chemistry of small organic molecules
in prebiotic Earth and extraterrestrial environments, since 1 is
implied in the thermal decomposition of oxalic acid (2), which
gives rise to formic acid (3), CO, CO2, H2, and H2O.[3–5] Here
we show that 1 can be generated thermally through high-
vacuum flash pyrolysis (HVFP), subsequently be trapped in
an argon matrix at 10 K, and be fully characterized spectro-
scopically.
The thermal decomposition of 2 is a long-known reaction
that has been used to generate CO2.Wobbe and Noyes
apparently presented the first kinetic study of this reaction as
early as 1926,[3] and numerous publications followed by
Clark[4] in the 1950s as well as by Lapidus et al.in the
1960s.[6] None of these, however, mention the possible
involvement of free carbene 1 along the decomposition
pathway to give predominantly CO2 and H2; the reason for
ration of 1 through neutralization of the radical cation of 1
(1C+, generated from 2C+) comes from gas-phase studies by
Burgers et al., who employed neutralization-reionization
mass spectrometry.[11] Wiedmann et al.reported that 1 must
survive intact for at least 0.3 ms, and that 1 should indeed
possess a singlet ground state.[12] Theoretical studies empha-
size that singlet 1 indeed lies in a deep potential well and
should therefore be observable.[13,14]
Since we were successful recently in the preparation of
hydroxymethylene by the thermal decomposition of glyoxylic
acid,[15] we attempted the generation of 1 by thermal extrusion
of CO2 from 2 (Scheme 1) by using HVFP followed by
immediate matrix isolation at 10 K in solid argon.The starting
material 2 is sufficiently volatile to enable gas-phase decom-
position in a heated quartz tube.After several attempts and
optimization of the reaction temperature we were successful
in the generation and trapping of 1 (see the Experimental
Section for details).
We characterized the two populated rotamers 1tt and 1tc
of 1 (Scheme 1) by their measured difference IR spectra
(Figure 1) and by comparison with the spectrum obtained by
high-level coupled cluster theory computations including all
electrons (AE) at the AE-CCSD(T)/cc-pVTZ level (see the
Experimental Section for details).The remarkable agreement
between the measured and computed IR absorptions pro-
vides convincing evidence for the successful preparation of 1.
The generation of two of the three possible rotamers of 1 (the
last one is C2v-symmetric, namely, s-cis,s-cis, 1cc) derives from
the mechanism of preparation, as indicated by the arrows in
Scheme 1.Although the depicted C2h form of 2 is most stable,
less-symmetric forms are close in energy (within several
[*] Prof. Dr. P. R. Schreiner, Dr. H. P. Reisenauer
Institut für Organische Chemie
Justus-Liebig-Universität
Heinrich-Buff-Ring 58, 35392 Gießen (Germany)
Fax: (+49)641-99-34309
E-mail: prs@org.chemie.uni-giessen.de
Homepage: http://www.chemie.uni-giessen.de/schreiner
[**] This work was supported by the Fonds der Chemischen Industrie.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200802105.
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Figure 3. UV spectrum of 1 (argon matrix, 10 K). The spectrum was
obtained by subtracting the spectrum of the irradiated (32 min,
254 nm)from that of the un-irradiated matrix-isolated pyrolysis
products of 2.
Figure 1. Dihydroxycarbene rotamers 1tt and 1tc (see Scheme 1)
identified in an argon matrix at 10 K. Lower trace: Difference IR
spectrum obtained by subtracting the IR spectrum of the irradiated
(6 min, 254 nm)from the un-irradiated matrix-isolated pyrolysis pro-
ducts of oxalic acid (2). Upper trace: Computed IR spectrum of a 1:1
mixture of 1tt and 1ct at the CCSD(T)/cc-pVTZ level. For the full IR
spectrum see the Supporting Information.
bene (lmax = 257 nm), and emphasizes the close relationship
between the parent and substituted alkyoxycarbenes.[16]
In marked contrast to hydroxymethylene,[15] which has a
half-life of only two hours under comparable conditions (with
its isotopologue surviving for at least several days), both 1 and
[D2]-1 are persistent for days in the matrix: while hydrogen
tunneling is pronounced for hydroxymethylene, there is no
indication for such a process for 1.This finding may be due to
strong p donation of the lone pairs of electrons on the oxygen
atoms into the empty p orbital on the carbon atom, which
greatly reduces its electron deficit.This proposal is substan-
kcalmolÀ1, see below),[14] and can readily be interconverted
before CO2 extrusion in the hot quartz tube.As found for
hydroxymethylene, the OH stretching vibrations are highly
anharmonic and the quantitative deviation from the com-
puted harmonic absorptions is larger than usual (we have not
applied scaling).[15]
To provide further evidence for the successful preparation
of 1, we also studied the deuterated species by thermal
decomposition of deuterated oxalic acid [D2]-2.The agree-
ment between the computed and measured IR spectra
(Figure 2) is again excellent.
À
tiated by the rather short computed C O bond lengths of
1.325 Š in 1tt.The three-center-two-electron delocalized
nature of the respective molecular orbitals of 1tt underline
this conclusion (Figure 4).
We also obtained the UV spectrum of 1 (Figure 3) which
showed a maximum absorption at l = 256 nm.This absorption
maximum essentially coincides with that of dimethoxycar-
Figure 4. Molecular orbitals of 1tt involved in the p delocalization and
À
shortening of the C O bond. HOMO=highest occupied molecular
orbital.
Not only the heteroallyl-type HOMO-2 but also the
HOMO—formally comprised of the sp2 lone pair of electrons
on the carbon atom and the p in-plane lone pairs of electrons
on the oxygen atoms—contribute to the internal stabilization
À
of 1tt, and this explains the significant shortening of the C O
bond as compared to, for example, that in methanol
(1.427 Š).[17] This stabilization results in a rather large rota-
tional barrier for the interconversion of the rotamers of 1
(Figure 5).
To determine the energetics for 1 and its rearrangement
products we computed the respective potential energy hyper-
surface (DH0) at the CCSD(T)/cc-pVTZ + ZPVE level
Figure 2. Deuterated dihydroxycarbene rotamers [D2]-1tt and [D2]-1tc
(see Scheme 1)identified in an argon matrix at 10 K. Lower trace:
Difference IR spectrum obtained by subtracting the IR spectrum of the
irradiated (6 min, 254 nm)from the un-irradiated matrix-isolated
pyrolysis products of deuterated oxalic acid ([D2]-2). Upper trace:
Computed IR spectrum of a 2:1 mixture of [D2]-1tt and [D2]-1ct at the
CCSD(T)/cc-pVTZ level.
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À94.05 kcalmolÀ1) and H2 or from
formic acid (3, DHf0 = À90.49); these
approaches give a DHf0(1tt) value of
À47.26 kcalmolÀ1 and À47.67 kcal
molÀ1, respectively.To underline the
generality
of
the
experimental
approach to the preparation of carbenes
by CO2 extrusion from substituted car-
boxylic acids, we also generated the
hitherto unknown hydroxymethoxycar-
bene (4) from oxalic acid monomethyl
ester (5; Scheme 2); the conditions
were virtually the same as before.
The difference IR spectrum of the
pyrolysate before and after irradiation
(Figure 6) and comparison with
CCSD(T)/cc-pVTZ
computational
data show that the s-trans,s-trans form
(4tt) dominates over the s-trans,s-cis
(4tc) and s-cis,s-trans (4ct) rotamers.
There is no indication of the presence of
an s-cis,s-cis form, as expected from its
lower stability and the mechanism of its
generation (see above).Remarkably,
the thermal reaction (during HVFP)
leads to methyl formate (6), while
irradiation at 254 nm produces acetic
acid (7).
Figure 5. Potential energy hypersurface (DH0)of 1 and related species at the CCSD(T)/cc-
pVTZ+ZPVE level in kcalmolÀ1. The main reaction pathways are highlighted.
Extrusion of CO2 from a-keto car-
boxylic acids provides direct access to
hydroxycarbenes.This has been shown
[8,9,13,14]
(Figure 5).As predicted,
all rotamers of 1 lie in
for hydroxycarbene,[15] dihydroxycarbene, and hydroxyme-
thoxycarbene, and is likely to be of broad general utility.
Given that the energies required are around 30 kcalmolÀ1
(Figure 5), this approach may in the future also be amenable
to preparative chemistry involving some metal-free Fischer
carbenes.
sizeable potential energy wells that cannot be overcome
once they are trapped in the matrix at low temperatures.
Somewhat surprising are the virtually isoenergetic escape
routes for the rotamers of 1, which lead to the observed
rearrangement and decomposition products upon irradiation
of the matrix around the UV absorption maximum.
In terms of the conservation of orbital symmetry, we see
that there is no dominant energetic preference for the
symmetry-allowed H2 extrusion and the forbidden hydrogen
shifts.In marked contrast to earlier (necessarily lower level)
theoretical studies,[10,18] the dissociation of 1 to HC + COCOH
Experimental Section
Matrix isolation experiments: Anhydrous oxalic acid (2) was pre-
pared by heating the dihydrate under vacuum to 908C.Monomethyl
oxalate (5) was prepared by reaction of equimolar amounts of
anhydrous oxalic acid with anhydrous methanol, and was purified by
fractional trap to trap distillation under vacuum.The cryostat used for
the matrix isolation studies was an APD Cryogenics HC-2 closed-
cycle refrigerator system fitted with CsI windows for IR spectroscopic
measurements and BaF2 windows for UV/Vis measurements.The IR
spectra were recorded with a Bruker IFS 55
(+ 44.4 kcalmolÀ1) is highly unfavorable because of the high
[19]
À
dissociation energy of the O H bond.
The computational data also allow the derivation of the
enthalpy of formation (DHf0) of 1tt from either CO2 (DHf0 =
FTIR spectrometer (4500–300 cmÀ1, resolution
0.7 cmÀ1), and UV/Vis spectra with an Agilent
HP 8453 Diode-Array spectrometer and a
JASCO V-670 spectrophotometer.For the
combination of HVFP with matrix isolation
we employed
a small water-cooled oven
directly connected to the vacuum shroud of
the cryostat.The pyrolysis zone consisted of an
empty quartz tube (inner diameter 8 mm,
length of heating zone 50 mm) resistively
heated by a coax wire.The temperature was
controlled by a Ni/CrNi thermocouple.The
Scheme 2. Generation of hydroxymethoxycarbene (4, s-trans,s-trans (4tt), s-trans,s-cis (4tc), and
s-cis,s-trans (4ct)) through CO2 extrusion from oxalic acid monomethyl ester (5). The
rearrangement product methyl formate (6)also forms in the reaction.
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Communications
Keywords: carbenes · density functional calculations ·
.
IR spectroscopy · matrix isolation · thermal decomposition
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Received: May 5, 2008
Published online: July 29, 2008
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Angew. Chem. Int. Ed. 2008, 47, 7071 –7074