Ultrafast UV-visible and infrared spectroscopic observation of a singlet vinylcarbene and the intramolecular cyclopropenation reaction

Article abstract of DOI:10.1021/ja905992p

(Chemical Equation Presented) Ultrafast UV-vis/IR spectroscopies were used to study the photochemistry of a vinyl diazo ester PhCH=CHCN₂CO ₂CH₃ (1) in solution. The results indicate that singlet styrylcarbomethoxy carbene

Full text of DOI:10.1021/ja905992p

Published on Web 09/02/2009
Ultrafast UV-Visible and Infrared Spectroscopic Observation of a Singlet
Vinylcarbene and the Intramolecular Cyclopropenation Reaction
Yunlong Zhang,† Jacek Kubicki,‡ and Matthew S. Platz*,†
Department of Chemistry, The Ohio State UniVersity, 100 West 18th AVenue, Columbus, Ohio, 43210, and Quantum
Electronics Laboratory, Faculty of Physics, Adam Mickiewicz UniVersity, 85 Umultowska, Poznan 61-614, Poland
Received July 18, 2009; E-mail: platz.1@osu.edu
Singlet carbenes are reactive intermediates that undergo unique
inter- and intramolecular reactions.1 Intermolecular reactions of
arylcarbenes have been studied extensively.1 Vinylcarbenes undergo
intramolecular cyclization to form cyclopropenes.2 Triplet vinyl-
carbene has two geometric isomers which have been detected by
matrix isolation spectroscopy.3 Calculations indicate that singlet
vinylcarbene resides 12-13 kcal/mol above the triplet ground
state.4,5 Vinylchlorocarbene has a singlet ground state as demon-
strated by Sheridan and Zuev.6 The Moss and Sheridan groups7
directly detected two singlet vinylchlorocarbenes in solution and
reported that they have ns lifetimes and cyclize over barriers of
5.7-6.6 kcal/mol. Singlet vinylcarbenes have not been directly
observed when the triplet is the ground state because rapid
intersystem crossing (ISC) and intramolecular cyclopropenation
prevent their detection using either matrix isolation or nanosecond
laser flash photolysis. Thus, the detection of a singlet vinylcarbene
remains a challenging application of ultrafast time-resolved
techniques.
results. The absorption band of 12 decays exponentially with a time
constant of 25.7 ( 1.6 ps (Figure S1a). This result is consistent
with calculations that predict a singlet carbene lifetime of 30 ps in
the gas phase (SI, Figure S2), further supporting the assignment of
1
the transient to 2.
Methyl styryldiazoacetate8 (PhCHdCHCN2CO2CH3, 1, Scheme
1) was chosen for this inaugural fs/ps time-resolved study. The
phenyl group provides a UV-vis chromophore and the ester moiety
does the same for IR techniques. The anticipated carbene
PhCHdCHCCO2CH3 (2) is predicted to have triplet ground state
by B3LYP/6-31G(d) calculations9 and it was previously shown by
Schmitz that 12 can be trapped with neat methanol at a rate that is
competitive with cyclization.10
Figure 1. Transient UV-vis spectra produced upon 310 nm photolysis of
PhCHdCHCN2CO2CH3 in acetonitrile.
Scheme 1. Photochemical Reactions of PhCHdCHCN2CO2CH3
As the singlet carbene (12) decays, two very weak bands at 440
and 350 nm are revealed that have constant intensity for 3 ns.14
The long lifetimes are consistent with triplet vinylcarbene 32;
however, TD B3LYP calculation predicts triplet carbene absorption
at 392 nm (f ) 0.0028) and 342 nm (f ) 0.5761, Table S1), in
only fair agreement with the experimental data.
In methanol, the relaxed singlet carbene (12) decays with a
lifetime of 38.4 ( 2.4 ps (Figure S1b, Table S2). The observation
that the relaxed singlet carbene (12) decays faster in acetonitrile
than in methanol runs counter to most studies of singlet carbenes.12-15
We speculate that the singlet carbene lifetime in acetonitrile is
unusually short because of cyclopropene (3) formation, rather than
control by intersystem crossing and intermolecular reaction, as is
usually the case for the more thoroughly studied aryl carbenes in
this solvent.11,15,16 The rate of cyclization in methanol is slower
than in acetonitrile, probably because of more intimate solvation
of the carbene.17 Indeed, in a noncoordinating solvent (cyclohexane)
the lifetime of the singlet carbene is only 6.5 ( 1.1 ps (Figure
S1c, Table S2). Similar solvent effects on the rate of Wolff
rearrangement have been reported.18
Ultrafast UV-vis (λex ) 310 nm) of (1) in acetonitrile produced
the spectra shown in Figure 1. A broadly absorbing transient with
absorption maximum near 470 nm is formed within the laser pulse
(300 fs). As it decays, a new transient is observed with λmax ) 385
nm. The former band is attributed to an excited state of the diazo
precursor (1*) based on previous studies11 and the latter to singlet
vinylcarbene (12). The latter assignment is based in part on a TD
1
B3LYP calculation which predicts that 2 absorbs at 378 nm (f )
0.1220, Table S1), in excellent agreement with the experimental
† The Ohio State University.
‡ Adam Mickiewicz University.
9
13602 J. AM. CHEM. SOC. 2009, 131, 13602–13603
10.1021/ja905992p CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
The decay of singlet carbene (12) is not accompanied by the
growth of transient UV-vis absorption of cyclopropene 3, prompt-
ing the use of IR detection.
greatly affected by the overlapping hot ground state 1#. The long
lifetime of the triplet, relative to the singlet carbene, indicates that
the two spin isomers are not in rapid equilibrium, as predicted by
the large calculated energy separation (SI, ∆EST ) 7.9 kcal/mol).
In the diazo stretching region (Figure S6) the negative band
observed at 2085 cm-1 is due to the bleaching of the ground-state
precursor. As usual, the hot ground state is observed on its red
edge as a broad positive band, which then undergoes blue-shifting
and narrowing within 40 ps, a result typical of vibrational cooling.19
About 31% of the diazo bleaching band was recovered within 3
ns. The growth time constant of 18.4 ps (Figure S7), as probed at
2085 cm-1, is assigned to the vibrational cooling of diazo precursor.
The exact correspondence of the FTIR spectrum with the transient
spectrum at 2.5 ns, post flash, convinces us that a ketene is not
detected.
In summary, we have observed a singlet vinylcarbene (12)
produced directly from the excited state of a diazo precursor (1).
The singlet carbene undergoes intramolecular cyclopropenation
reaction to produce the cyclopropene product (3) in acetonitrile,
chloroform and cyclohexane, and undergoes intersystem crossing
to ground triplet carbene (32) to a small extent. The triplet carbene
is not in rapid equilibrium with the singlet due to a relatively large
singlet triplet separation.
Figure 2. Transient IR spectra produced upon 270 nm photolysis of
PhCHdCHCN2CO2CH3 in chloroform. The dotted curve is the FTIR
spectrum of PhCHdCHCN2CO2CH3 in chloroform. Bars represent the
calculated frequencies for singlet carbene (12), triplet carbene (32), and
cyclopropene (3).
Ultrafast photolysis (270 nm) of 1 in chloroform produces the
transient IR spectra shown in Figure 2. The negative band observed
in the 1750-1640 cm-1 region is due to the bleaching of the
carbonyl (CdO) stretch in the diazo precursor (1). However, the
carbonyl bleaching band reshapes over 100 ps, clearly indicating
that a new species absorbing around 1705 cm-1 is formed rapidly.
A positive band at 1770 cm-1 was also observed. This band narrows
and shifts to the blue within 80 ps of the laser pulse and retains the
same intensity over 3 ns. Identical kinetic behavior was observed
monitoring at 1770 and 1705 cm-1 which suggests that both bands
are associated with the same intermediate. After subtracting the
FTIR spectrum of the diazo precursor, two positive bands at 1770
and 1705 cm-1 are clearly observed and persist for at least 3 ns
(Figure S4). Both bands are assigned to the cyclopropene (3) formed
by the intramolecular cyclopropenation reaction (Scheme 1).
B3LYP/6-31G(d) calculations predict cyclopropene (3) has coupled
stretching modes of CdO and CdC at 1776 and 1723 cm-1 (Table
S3). The direct observation of both vibrational bands and the
excellent agreement with calculations indicate that the carrier of
the observed transient absorptions is indeed the cyclopropene
product (3).
Acknowledgment. The work was performed at The OSU Center
for Chemical and Biophysical Dynamics. Support of this work by
the National Science Foundation is gratefully acknowledged.
Supporting Information Available: A brief description of the
spectrometers, complete ref 9, Figures S1-S6, and Tables S1-S6. This
material is available free of charge via the Internet at http://pubs.acs.org.
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