Phenylhydroxycarbene

Article abstract of DOI:10.1021/ja9107885

Phenylhydroxycarbene (Ph-C-OH, 1), the parent of all arylhydroxycarbenes, was generated by high-vacuum flash pyrolysis of phenylglyoxylic acid at 600 °C and spectroscopically characterized (IR, UV-vis) via immediate matrix isolation in solid Ar at 11 K. The identity of 1 was unequivocally confirmed by the precise agreement between the observed IR bands and (unscaled) anharmonic vibrational frequencies computed from a CCSD(T)/cc-pVDZ quartic force field. The UV-vis spectrum of 1 displays a broad band with maximum absorption at 500 25 nm (2.5 ± 0.1 eV) that extends to ～640 nm (1.9 eV), in full accord with combined CCSD(T)/cc-pVQZ and EOM-CCSD/cc-pVTZ computations that yield a gas-phase vertical (adiabatic) excitation energy of 2.7 (1.9) eV. Unlike singlet phenylchlorocarbene, 1 does not undergo photochemical ring expansion. Instead, 1 exhibits quantum-mechanical hydroGenetunneling to benzaldehyde underneath a formidable barrier of 28.8 kcal mol^-1, even at cryogenic temperatures. The remarkable hydroGenetunneling mechanism is supported by the temperature insensitivity of the observed half-life (2.5 h) and substantiated by a comparable theoretical half-life (3.3 h) determined from high-level barrier penetration integrals computed along the intrinsic reaction path. As expected, deuteration turns off the tunneling mechanism, so d-1 is stable under otherwise identical conditions.

Full text of DOI:10.1021/ja9107885

Published on Web 05/12/2010
Phenylhydroxycarbene
Dennis Gerbig,§ Hans Peter Reisenauer,§ Chia-Hua Wu,‡ David Ley,§ Wesley D. Allen,*,‡ and
Peter R. Schreiner*,§
Justus-Liebig UniVersity, Heinrich-Buff-Ring 58, 35392 Giessen, Germany, and Center for Computational Chemistry
and Department of Chemistry, UniVersity of Georgia, Athens, Georgia 30602
Received December 22, 2009; E-mail: wdallen@uga.edu; prs@org.chemie.uni-giessen.de
Phenylhydroxycarbene (1) is a hitherto unobserved π-donor-
substituted singlet carbene of the recently discovered hydroxycar-
bene family.1 The intermediacy of 1 in thermal rearrangements of
hydroxyphenylcarbenes has been proposed.2 Structure 1 bears
resemblance to phenylcarbene (2a), phenylchlorocarbene (2b), and
phenylmethylcarbene (2c). Carbene 2b was first generated 50 years
ago by base-induced hydrogen chloride elimination from benzal
chloride;3 2a was produced shortly thereafter by thermal extrusion
of nitrogen from phenyldiazomethane,4 and 2c was obtained
similarly to 2a.5 Various thermolyses, photolyses, and mass
spectrometric studies followed,6 uncovering the intriguing singlet-
surface rearrangements of 2a and 2b to cyclohepta-1,2,4,6-tetraene
(3a) and chlorocyclohepta-1,2,4,6-tetraene (3b), respectively (Scheme
1). Singlet carbenes 12c and 12d, on the other hand, produce styrene
derivatives7 as a result of small barriers (∼5 kcal mol-1)8 for the
thermal [1,2] H-shifts, whose rates are likely to include tunneling
contributions.7c As demonstrated here, 1 does not undergo ring
expansion to hydroxycyclohepta-1,2,4,6-tetraene (3c) or its tropone
tautomer 3d but instead exhibits a strikingly different reactivity.
hydroxycarbenes.1,9 Thus, we prepared the s-trans carbene 1t and
its monodeuterated analogue Ph-C-OD (d-1t) by decarboxylation
of phenylglyoxylic acid (7) and d1-phenylglyoxylic acid (d-7)
through high-vacuum flash pyrolysis (HVFP) at 600 °C and instant
trapping in an argon matrix at 11 K (Scheme 3). In order to identify
all of the remaining signals of the starting material, 7 was evaporated
and matrix-isolated without pyrolysis for comparison. Although the
IR signals of carbenes 1t and d-1t were rather weak, all of the
characteristic absorptions of the hydroxycarbene moiety could be
identified unambiguously by precise matching with anharmonic
frequencies computed at a high level of theory (Table 1).
Scheme 3. Generation and Tunneling of Phenylhydroxycarbene
Table 1. Observed Vibrational Band Origins (cm-1) of 1t and d-1t
and Precise Matching with Computed Values; Theoretical Absolute
IR Intensities (Double-Harmonic, km mol-1) and Experimental
Relative Intensities Are Given in Parentheses
Scheme 1. Reactivities of Some Arylcarbene Derivatives
b
normal mode TED (%)a
symmetry
νtheor
νexpt
Ph-C-OH (1t)
OH stretch (100)
HOC bend (50)
a′
a′
3575 (166)
1332 (27)
1310 (26)
1289 (76)
1236 (218)
800 (99)
793 (24)
606 (12)
606 (9)
3570 (s)
1334 (m)
CC stretch/ring breathing (60)
HCC in-plane bends (56)
CO stretch (53)
a′
a′
a′
a′′
a′
a′
a′
} 1297 (m)
1219 (s)
HOCC torsion (82)
} 800 (m)
Building on our recent breakthroughs in preparing the first
members of the hydroxycarbene family, hydroxymethylene (4),1
dihydroxycarbene (5),9 and methoxyhydroxycarbene (6)9 (Scheme
2), we sought to prepare 1, the parent of all arylhydroxycarbenes.
Remarkably, 4 shows rapid (within 2 h at 10 K) H-tunneling through
a very large barrier (∼30 kcal mol-1)1 to form formaldehyde, while
neither 5 nor 6 disappears through such a process, despite
comparable [1,2] H-shift barriers. Hence, we were particularly
interested in the possible H-tunneling of 1 to yield benzaldehyde
(8), a phenomenon that would also affect its higher-temperature
behavior.
carbene CC stretch (27)
ring def (66) s CCO bend (26)
ring def (59) + CCO bend (17)
} 613 (w)
Ph-C-OD (d-1t)
OD stretch (100)
HCC in-plane bends (79)
CO stretch (67)
HCC in-plane bends (77)
DOC bend (62)
carbene CC stretch (25)
a′
a′
a′
a′
a′
a′
2645 (97)
1297 (20)
1258 (313)
1159 (11)
994 (33)
760 (26)
750 (74)
610 (19)
588 (27)
2641 (s)
1297 (w)
1256 (s)
1171 (m)
990 (m)
770 (m)
765 (m)
HCC out-of-plane bends (87)
ring def, out-of-plane (65)
CCO bend (41)
a′′
a′′
a′
} 595 (w)
Scheme 2. Presently Characterized Members of the
a Leading components of the total energy distribution (TED) of
the normal-mode vibrations.10 b (Unscaled) anharmonic fundamental
frequencies computed by applying second-order vibrational perturbation
theory (VPT2)11 (resonance cutoff ) 40 cm-1) to a quartic force field in
natural internal coordinates.12 The force field was evaluated with the
cc-pVDZ basis set13 as follows: CCSD(T)14 for (Fii, Fij, Fiii, Fiiii), MP2
for (Fiij, Fijk, Fiiij, Fiijj, Fiijk), and Fijkl set to 0. A complete tabulation of
theoretical frequencies and isotopic shifts is given in the SI.
Hydroxycarbene Family
The thermal decarboxylation of R-ketocarboxylic acids has
proved to be a versatile and reliable method for the generation of
The identification of 1t was complicated by the presence of
precursor conformer 7t (s-trans, nonplanar, non-hydrogen-bridged)
§ Justus-Liebig University.
‡ University of Georgia.
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C O M M U N I C A T I O N S
and its conversion to the lower-energy form 7c (s-cis, planar,
hydrogen-bridged) despite the cryogenic conditions, causing sig-
nificant temporal changes in the IR intensities. This process was
elucidated by enriching isomer 7t via 313 nm irradiation. The
measured first-order-decay half-life of 7t was τ ) 2.72 h ( 18
min at 11 K and τ ) 1.65 h ( 4 min at 20 K. It is likely that the
interconversion of 7t to 7c involves tunneling, as observed for the
conformers of formic acid15 and acetic acid.16
Irradiation at 546 nm of the matrix containing 1t led to the
disappearance of the carbene IR signals and an increase in the
intensities of the aldehyde in the IR difference spectrum (shown in
Figure 1 for d-1t and d-8). The complete absence of 3d was verified
with a matrix-isolated reference spectrum of 3d under the same
conditions [see the Supporting Information (SI)]. Tropone 3d also
proved to be resistant to HVFP conditions, and its intermediacy in
the reactions of 1 can be ruled out.
Figure 2. Experimental UV-vis difference spectra of 1t (black) and d-1t
(red) based on absorption changes after 19 h in the dark and 34 h of
irradiation at 546 nm, respectively. The black curve, but not the red one,
showed time dependence due to H-tunneling (see the SI). Inset: M06-2X/
cc-pVDZ computed MOs of the corresponding electronic transition.
The key stationary points on the potential energy hypersurface
(PES) of 1 were optimized at the M06-2X/cc-pVDZ level;19 very
similar structures were obtained from MP2/cc-pVDZ computations
(see the SI).20 M06-2X is an economical method that performs
well for thermochemistry and reaction barriers21 within systems
composed of main-group elements. Certainty in the final
energetics was ensured by rigorous CCSD(T)/cc-pVQZ single-
point computations with auxiliary core correlation corrections,
as reported in Scheme 4.
Scheme 4. Schematic PES Surrounding 1ta
Figure 1. Experimental IR difference spectrum of the products of d-7
pyrolysis at 600 °C instantly trapped in Ar at 11 K (center panel); absorption
differences were taken after irradiation at 546 nm for 34 h. The assignments
were established by anharmonic CCSD(T)/cc-pVDZ frequency computations
for d-1t (bottom panel) and the experimental reference spectrum of d-8
(top panel). Asterisks label remaining d-7 bands, CO2 (663 cm-1), or
measurement artifacts. The main CO2 signal near 2343 cm-1 was cut for
clarity. Detailed spectra of the parent 1t are given in the SI.
Despite low extinction coefficients, UV-vis difference spectra
of 1t and d-1t were successfully recorded (Figure 2). These spectra
display a broad band with maximum absorption at 500 ( 25 nm
(2.5 ( 0.1 eV) that extends to ∼640 nm (1.9 eV). A similar
spectrum was observed previously for 4.1 The associated S0(1A′)
f S1(1A′′) electronic transition, which induces decay of 1t and
conversion to 8, arises from an excitation from the HOMO localized
on the carbene moiety (Figure 2 inset). Our spectral assignments
were confirmed unequivocally by a series of careful electronic
structure computations that arrived at a gas-phase vertical (adiabatic)
excitation energy of 2.7 (1.9) eV for 1t, with an uncertainty of less
than 0.1 eV. To maximize accuracy, these computations were
dissected into a S0(1A′) f T1(3A′′) step treated by ROCCSD(T)
theory17 and a T1(3A′′) f S1(1A′′) step handled with the EOM-
CCSD method,18 applying high-quality cc-pVQZ and cc-pVTZ
basis sets,13 respectively. Moreover, an isogyric procedure (see the
SI) was employed using H-C-F as a reference with a precisely
known adiabatic excitation energy.
a Computed using CCSD(T)/cc-pVQZ single-point energies with (with-
out) CCSD(T)/cc-pCVTZ22 core correlation corrections and employing M06-
2X/cc-pVDZ optimized geometries and zero-point vibrational energies
(ZPVEs). The lowest triplet structure of 1 is 22.3 kcal mol-1 above 1t at
the same level. Selected M06-2X/cc-pVDZ bond lengths and angles are
given in Å and deg, respectively; full geometric structures are provided in
the SI. In contrast to 1t, 1c is nonplanar.
Structure 1t lies 50.8 kcal mol-1 above 8, but the corresponding
[1,2] H-shift barrier is large (28.8 kcal mol-1, TS1). The iso-
merization of 1t to 1c is blocked by a 22.7 kcal mol-1 barrier (TS2),
while a very high-lying transition state (TS3) prevents access to
the benzene + CO (9) products. The features of the PES are very
favorable for the isolation of 1t, as 1c, 8, and 9 are completely
inaccessible by thermal activation under matrix conditions. The
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facile [1,2] H-shift in 2c is not operative by thermal activation of
1t. In addition, we found no evidence that 1t undergoes the ring
expansion reactions that occur for 2a (photochemically or ther-
mally)6 and 2b (photochemically).6
a large barrier (nearly 30 kcal mol-1) with a half-life of a few hours
even at cryogenic temperatures. Our findings, together with the
published evidence for heavy-atom tunneling in carbenes,25 expands
our view of the importance and generality of tunneling mechanisms
in chemical reactions.
A reactivity previously undiscovered in the class of phenylcar-
benes is exhibited by 1t, even at the low temperatures of our
experiments. Close monitoring of the major IR bands at 3570, 1334,
1297, 1219, and 800 cm-1 revealed that 1t decays under matrix
isolation with half-lives (τ) of 2.46 h ( 8 min at 11 K and 2.55 h
( 8 min at 20 K; Figure 3 displays representative measurements.
In stark contrast, when d-1t signals were monitored, no changes
were detected, even after 96 h in the dark at 20 K. The dramatic
isotopic dependence and temperature insensitivity of the disap-
pearance of 1t, as well as the observation of concomitant increases
in signals due to 8, indicates that a quantum-mechanical tunneling
mechanism is at work to engender a remarkable [1,2] H-shift.
Acknowledgment. This work was supported by ERA-Chemistry
and the U.S. Department of Energy, Office of Basic Energy Sciences
(Grant DE-FG02-97ER14718).
Supporting Information Available: Spectra, geometries, PESs,
kinetic plots, and experimental and theoretical details. This material is
available free of charge via the Internet at http://pubs.acs.org.
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In conclusion, we have synthesized and characterized a new
member of the hydroxycarbene family, phenylhydroxycarbene (1t),
which has a rather unique reactivity. Carbene 1t does not show
signs of ring insertion reactions typical for other phenylcarbenes
such as 2a or 2b. Upon irradiation, it does not undergo ring
expansion but yields benzaldehyde (8) instead. However, neither
photoexcitation nor thermal activation is necessary for the isomer-
ization of 1t because it undergoes rapid H-tunneling to 8 through
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