Laser flash photolysis study of chlorofluorocarbene

Article abstract of DOI:10.1021/jp0345598

10-exo-Fluoro-10′-endo-chlorotricylco[4.3.1.0^1,6]decadi ene-2,4 (1) was synthesized. Continuous photolysis of 1 (300 nm) generates chlorofluorocarbene 2, which can be intercepted with tetramethylethylene (TME) to form the expected cyclopropane 3. Laser flash photolysis (LFP) of 1 (308 nm) in cyclohexane at ambient temperature does not result in the direct detection of 2 by time-resolved UV-vis spectroscopy. However, LFP of 1 in the presence of pyridine produces the intense transient absorption of ylide 4 at 450 nm. The absolute rate constant of reaction of 2 with pyridine is 8.0 × 10⁹ M^-1 s^-1 in cyclohexane. LFP (266 nm) of 1 in heptane at ambient temperature produces a transient species with a C-F vibration at 1142 cm^-1 and a lifetime of 2.5 μs at ambient temperature. The carrier of the transient absorption is assigned to carbene 2 on the basis of density functional theory (DFT) calculations and on the basis of its kinetic behavior. The absolute rate constant of reaction of carbene 2 with TME was determined by monitoring the transient absorption of ylide 4 (Freon-113) or by time-resolved IR spectroscopy (heptane) to give values of 1.1 × 10⁸ M^-1 s^-1 and 2.9 × 10⁸ M^-1 s^-1, respectively. This is the first measurement of a bimolecular ground-state singlet carbene reaction by both time-resolved UV-vis and IR spectroscopy. The value determined by the time-resolved IR measurement is more accurate than that from the ylide probe technique.

Full text of DOI:10.1021/jp0345598

J. Phys. Chem. A 2003, 107, 8547-8551
8547
Laser Flash Photolysis Study of Chlorofluorocarbene
Eric M. Tippmann and Matthew S. Platz*
Department of Chemistry, 100 West 18th AVenue, The Ohio State UniVersity, Columbus, Ohio 43210
ReceiVed: March 4, 2003; In Final Form: July 15, 2003
10-exo-Fluoro-10′-endo-chlorotricylco[4.3.1.01,6]decadiene-2,4 (1) was synthesized. Continuous photolysis of
1 (300 nm) generates chlorofluorocarbene 2, which can be intercepted with tetramethylethylene (TME) to
form the expected cyclopropane 3. Laser flash photolysis (LFP) of 1 (308 nm) in cyclohexane at ambient
temperature does not result in the direct detection of 2 by time-resolved UV-vis spectroscopy. However,
LFP of 1 in the presence of pyridine produces the intense transient absorption of ylide 4 at 450 nm. The
absolute rate constant of reaction of 2 with pyridine is 8.0 × 109 M-1 s-1 in cyclohexane. LFP (266 nm) of
1 in heptane at ambient temperature produces a transient species with a C-F vibration at 1142 cm-1 and a
lifetime of 2.5 µs at ambient temperature. The carrier of the transient absorption is assigned to carbene 2 on
the basis of density functional theory (DFT) calculations and on the basis of its kinetic behavior. The absolute
rate constant of reaction of carbene 2 with TME was determined by monitoring the transient absorption of
ylide 4 (Freon-113) or by time-resolved IR spectroscopy (heptane) to give values of 1.1 × 108 M-1 s-1 and
2.9 × 108 M-1 s-1, respectively. This is the first measurement of a bimolecular ground-state singlet carbene
reaction by both time-resolved UV-vis and IR spectroscopy. The value determined by the time-resolved IR
measurement is more accurate than that from the ylide probe technique.
I. Introduction
cycle cryostat (Air Products). Argon gas streaming over the
precursor was condensed on the surface of a CsI window held
at ca. 18 K. The IR spectra were recorded on a Perkin-Elmer
FT-IR 2000 interferometer at 0.2 cm-1. A Ray-O-Net reactor
was used to photolyze the sample.
Laser flash photolysis (LFP) studies of triatomic chlorocar-
benes have been reported. The absolute reactivities of chloro1
and dichlorocarbene2 were measured using the pyridine ylide
probe method. In this study, we report analogous studies of a
mixed dihalocarbene, CFCl, and demonstrate the utility of time-
resolved infrared (TRIR) spectroscopy for the direct observation
of fluorinated carbenes.3
Crystallography. See Table S1 of the Supporting Informa-
tion.
Time-Resolved Infrared (TRIR) Studies. TRIR experiments
were conducted with a JASCO TRIR-1000 dispersive-type IR
spectrometer with 16-cm-1 resolution following the method
described in the literature.5-7 Briefly, a reservoir of the
deoxygenated sample solution (10 mL of 5 mM 1) was
continuously circulated between two barium fluoride salt plates
with a 0.5-mm path length. The sample was excited by 266-
nm laser pulses of a Nd:YAG laser (40 Hz repetition rate, 0.5-
0.7 mJ/pulse power), which is crossed with the broadband output
of a MoSi2 IR source (JASCO). The intensity change of the IR
light induced by photoexcitation is monitored as a function of
time by an MCT photovoltaic IR detector (Kolmar Technologies,
KMPV11-1-J1) with a 50-ns rise time, amplified with a low-
noise NF Electronic Instruments 5307 differential amplifier and
digitized with a Tektronix TDS784D oscilloscope. The TRIR
spectrum is analyzed by the IGOR PRO program (Wavemetrics
Inc.) in the form of a difference spectrum:
II. Experimental Section
1
General Methods. H NMR spectra were obtained on a
Bruker DRX-250 (250 MHz) or DRX-400 (400 MHz) spec-
trometer. The GC-MS spectrometer was an HP-6890 series GC
system with an HP-1 methyl siloxane capillary column (40.0
m × 100 µm × 0.20 µm). The gas chromatograph was linked
to an HP 5973 mass-selective detector.
Tetrahydrofuran was purified by distillation from sodium
benzophenone under argon. Benzene, heptane (HPLC grade),
and cyclohexane were distilled from calcium hydride under
argon. Freon-113 (Aldrich Chemical Co.) was distilled under
argon from P2O5 using a 40-cm distillation column.
Laser Flash Photolysis Studies.4 For LFP studies of precur-
sor 1, a stock solution was prepared in Freon-113. Samples (1
mL) were prepared with an optical density of 0.1 at 308 nm.
The LFP apparatus utilized a Lambda Physik LPX-100 excimer
laser (308 nm, 120 mL, 10 ns). The analysis of data was
performed using the Igor Pro software package from Wavem-
etrics. Transient absorption spectra were obtained on an EG&G
PARC 1460 optical mutichannel analyzer fitted with an EG&G
PARC 1304 pulse amplifier, an EG&G PARC 1024 UV
detector, and a Jarrell-Ash 1234 grating.
∆At ) -log(1 + ∆It/I)
where ∆It is the intensity change induced by photoreaction at
time t and I is the IR intensity for the sample without
photoexcitation. Thus, the depletion of reactant and the forma-
tion of transient intermediates or products lead to negative and
positive signals, respectively.
Density Functional Calculations. Geometries of singlet 2,
precursor 1, triene 5, and biradical 6 were fully optimized at
the B3LYP level of theory with the 6-31G* basis set; harmonic
Matrix Isolation Spectroscopy. The precursor was placed
in a glass U tube that was connected directly to a helium closed-
* Corresponding author. E-mail: platz.1@osu.edu.
10.1021/jp0345598 CCC: $25.00 © 2003 American Chemical Society
Published on Web 09/23/2003

8548 J. Phys. Chem. A, Vol. 107, No. 41, 2003
Tippmann and Platz
Figure 1. Transient spectra produced by LFP of 1 in Freon-113 at ambient temperature in the presence of (top, 1 M) and absence of (bottom)
pyridine. The spectra were recorded 20 ns after the laser pulse over a window of 20 ns.
frequencies and UV-vis spectra (for 1 and 5) were calculated
at the same level of theory. All calculations were carried out
with the Gaussian 98 program package.8 Vibrational frequencies
were scaled by a factor of 0.9613.
III. Results
Jones’ dichloride9 was treated with butyllithium and N-
fluorobenzenesulfonamide to afford 1 as a highly crystalline
compound in good yield, and its structure was proven by X-ray
crystallography. (See Supporting Information Figure S1.)
Continuous photolysis (300 nm) of 1 in neat tetramethyleth-
ylene (TME) provides evidence for the formation of carbene 2
in solution because indan and cyclopropane 3 are formed in 97
and 77% yield, respectively.
Synthesis.10-exo-Fluoro-10-chlorotricyclo[4.3.1.01,6]decadiene-
2,4 (1). To a stirred solution of 10,10′-dichlorotricylco[4.3.1.01,6]-
decadiene-2,49 (1.69 g, 8.4 mmol) in 70 mL of THF maintained
at -78° C 1.1 eq of n-BuLi was added dropwise. After 1 h, a
solution of N-fluorobenzenesulfonamide (2.92 g, 9.2 mmol) in
20 mL of THF was added dropwise to the reaction with stirring.
The reaction was allowed to warm gradually to ambient
temperature and was poured into water (100 mL) and extracted
with diethyl ether (3 × 75 mL). The combined organics were
dried over MgSO4, filtered, and concentrated by rotary evapora-
tion to yield a crude red oil. The oil was passed over silica gel
(hexane elution) to yield a white crystalline compound in 85%
yield. 1H NMR (250 MHz, CDCl3, δ): 1.6-1.9 (m, 2H), 1.98-
2.1 (m, 2H), 2.42-2.55 (m, 2H), 5.87 (dd, 2H, J ) 7.6 Hz),
6.08 (dd, 2H, 7.3 Hz). 13C (126 MHz, CDCl3, δ): 25.7 (d, JCF
) 8.6 Hz), 33.9 (d, JCF ) 4.4 Hz), 49.9 (d, JCF ) 14.5 Hz),
96.6 (d, JCF )290.6 Hz), 121.9, 123.1 (d, JCF ) 2.7 Hz). 19F
NMR (235 MHz, CDCl3, δ): -164.97 (s).
The structure of 3 was confirmed by an independent synthesis
of this known compound.10
Adduct 3. Photochemical Preparation. Compound 1 (73 mg)
as a stirred solution in 2,3-dimethyl-2-butene (5 mL) was
irradiated with 300-nm light (Ray-O-Net reactor) for 13 h. Indan
and adduct peaks were identified by comparison with authentic
materials. Yields were determined from GC/MS analysis using
adamantane as an internal standard. The mixture was concen-
trated by rotary evaporation at 20 °C, filtered through silica
Laser Flash Photolysis. Laser flash photolysis (LFP, 308
nm) of 1 in cyclohexane at ambient temperature produces the
transient spectra of Figure 1 in the presence (top) and absence
(bottom) of pyridine. Indan does not absorb significantly above
300 nm and was not detected by this technique. The broad,
intense band absorbing at 450 nm detected in the presence of
pyridine is attributed to ylide 4, formed by trapping carbene 2.
1
with hexanes as the eluant, and analyzed by H NMR spec-
troscopy. 1H NMR (250 MHz, CDCl3, δ): 1.13 (d, J ) 2.1 Hz,
6H), 1.14 (d, J ) 1.6 Hz, 6H). 13C NMR (101 MHz, CDCl3,
δ): 15.9 (d, JCF ) 9.1 Hz), 19.2, 27.7 (d, JCF ) 9.8 Hz), 105
(d, JCF ) 290 Hz). 19F NMR (235 MHz, CDCl3, δ): -147.15.
MS (EI) m/z (relative intensity): 150 (5%), 135 (100%).
The absorption maximum of 4 is similar to that observed with
CCl2 (390 nm).2 Further support of this assignment is provided
by the fact that the pseudo-first-order rate constant of formation
(kobs) of the species absorbing at 450 nm is linearly dependent
on the concentration of pyridine (Figure 2). The absolute rate
constant of reaction of carbene 2 with pyridine is the slope of
this plot (kpyr ) 8.0 × 109 M-1 s-1); the carbene lifetime in the
Adduct 3. Chemical Preparation. Adduct 3 was prepared
according to a literature procedure.10 1H NMR (250 MHz,
CDCl3, δ): 1.13 (d, J ) 2.1 Hz, 6H), 1.14 (d, J ) 1.6 Hz, 6H).
13C NMR (101 MHz, CDCl3, δ): 15.88 (d, JCF ) 9.1 Hz), 19.2,
27.7 (d, JCF ) 9.8 Hz), 105 (d, JCF ) 290 Hz). 19F NMR (235
MHz, CDCl3, δ): -146.41.

Flash Photolysis Study of Chlorofluorocarbene
J. Phys. Chem. A, Vol. 107, No. 41, 2003 8549
Figure 2. Plot of kobs of formation of ylide 4 versus pyridine concentration in cyclohexane at ambient temperature.
Figure 3. Plot of kobs of formation of ylide 4 at constant pyridine concentration (0.002 M) as a function of [TME] in Freon-113 at ambient
temperature.
kZnBr2 ) 2.1 × 108 M-1 s-1, acetonitrile, see Supporting
Information Figures S2 and S3). The products expected in the
metal salt reactions are carbenoids.11,12 The carbenoid reaction
products were not observed by TRIR spectroscopy. The carbene
lifetime and kTME values obtained by TRIR spectroscopy are
similar to those obtained by LFP with UV-vis detection; thus
the carrier of transient absorption at 1142 cm-1 is attributed to
carbene 2.
At later times (>10 µs after the laser flash), a peak at 1160
cm-1 also appears and is assigned to the C-F stretch of the
product of carbene reaction with solvent. In all solvents studied
(benzene, acetonitrile, cyclohexane, and heptane), photolysis of
1 generates a peak at 1210 cm-1. The growth of this peak is
Figure 4. TRIR sprectrum produced by LFP (266 nm) of 1 in heptane
at ambient temperature (1 µs/div).
faster than the time resolution of our instrument (<200 ns), and
its decay exceeds the limit of our instrument (∼1 ms). The peak
is unchanged in either its intensity or its kinetics in the presence
absence of pyridine is 1.1 µs. These results are similar to those
of LiBr, TME, or oxygen. The carrier of this band will later be
attributed to triene 5.
reported for dichlorocarbene.2
The rate of formation of ylide 4 is accelerated by the addition
of tetramethylethylene (TME) at constant pyridine concentration
(0.002 M). An analysis of the data reveals that the absolute
rate constant of reaction of carbene 2 with TME is 1.1 × 108
M-1 s-1 in Freon-113 (Figure 3).
Matrix Isolation. Chlorofluorocarbene has been previously
isolated in a low-temperature matrix.13 The reported carbene
C-F vibration was reported by Smith et al. at 1146 cm-1. We
were able to deposit independent precursor 1 in an argon matrix
at 18 K. Photolysis (254 nm) bleaches the IR bands of precursor
1. We simultaneously observe the production of three photo-
product peaks assignable to a C-F vibration at 1210, 1160,
and 1115 cm-1 (Figure 6).
Time-Resolved Infrared Spectroscopy. LFP (266 nm) of 1
in heptane produces the transient IR difference spectrum of
Figure 4. The negative peak observed at 1180 cm-1 is due to
the consumption of precursor 1. The carrier of the positive TRIR
peak observed at 1142 cm-1 has a lifetime of 2.5 µs. The decay
of transient absorption at 1142 cm-1 is linearly dependent on
the concentrations of TME (kTME ) 3 × 108 M-1 s-1, heptane,
IV. Discussion
Computational Chemistry. Density functional theory (DFT)
calculations with the 6-31G* basis set were performed to
Figure 5) and either LiBr or ZnBr2 (kLiBr ) 5.3 × 108 M-1 s-1
;

8550 J. Phys. Chem. A, Vol. 107, No. 41, 2003
Tippmann and Platz
Figure 5. Plot of kobs of the decay of the transient absorbing at 1142 cm-1 as a function of [TME] in heptane at ambient temperature.
TABLE 2: Calculated (TD-DFT) Absorption Spectra of
Diene 1 and Triene 5
absorption
compound
maxima (nm)
f
1
4
5
277
389
285
0.0501
0.2238
0.007
The UV-vis spectra of diene 1 (∈270 ) 3663 M-1 cm-1
)
and triene 8 (∈268 ) 4242 M-1 cm-1; Supporting Information,
Figure S4) were recorded. It is clear that the triene absorbs
at longer wavelength than a related diene. This is confirmed
by TD-DFT calculations of 1 and 5 (Table 2). Thus, it seems
likely that the long-lived species formed by flash photolysis
of 1 and detected at 360 nm and 1210 cm-1 is triene 5. The
1210-cm-1 band observed in argon at 18 K is also assigned
to 5. Photolysis of 1 must lead to some isomerization to pro-
duce triene 5 in addition to some fragmentation to form carbene
2. This partitioning can occur via the intermediacy of biradical
6.
Figure 6. Argon matrix photolysis (254 nm) of 1 at 18 K. Arrows
mark the photoproduct peaks (cm-1).
TABLE 1: Calculated and Experimental νC-F Frequencies
experiment
(cm-1
theory
c
compound
)
(cm-1
)
1
2
5
6
1180a,b
1073
1169
1058
1019
1142a,1160b
1210a,b
1115b
A vibrational band was observed at 1210 cm-1 upon
photolysis of diene precursor 1 in both the argon matrix and
liquid-phase TRIR experiments. Thus, the carrier of this
vibrational band cannot be due to the reaction of carbene or
another species with solvent. The lack of reactivity of the carrier
of this spectral band with oxygen, a radical quencher, excludes
biradical 6 as the carrier of the 1210-cm-1 transient absorption.
These observations are consistent with our previous attribution
of the carrier of this band to triene 5, despite the fact that this
assignment is in poor agreement with DFT calculations.
A very similar reactivity pattern is seen in the case of 10-
exo-carbomethoxy-10-fluorotricyclo[4.3.1.01,6]decadiene 2,4,5.
In this system, a new long-lived IR band is produced upon
photolysis of a tricyclic precursor that is blue shifted from the
region of precursor bleaching.17
a TRIR. b Argon matrix. c B3LYP calculations with the 6-31G* basis
set after scaling by a factor of 0.9613.
validate the IR spectroscopic assignments.8 Minimum-energy
geometries (Supporting Information) of diene 1, carbene 2, triene
5, and triplet biradical 6 were calculated and are given in the
Supporting Information section. The C-F vibrational frequen-
cies of the compounds of interest, after scaling by a factor of
0.9613, are given in Table 1. The calculations are in fair to
poor agreement with our assignments of the experimental data.
Other groups have noted this level of modest agreement of
theory and experiment before with related fluoro compounds.14
At this time, we cannot confidently assign the carrier of the
1115-cm-1 band in argon. It is certainly not indan, whose
vibrational spectrum is known.18 (See also Supporting Informa-
tion.) It is conceivable that the carrier of the 1115-cm-1 band
in argon is triplet biradical 6, which is predicted to have its
C-F stretch at 1019 cm-1 after scaling by a factor of 0.9613.
The TRIR spectrum shown in Figure 4 contains two peaks
that persist 10 µs after laser flash photolysis of the precursor.
The growth of the peak at 1160 cm-1 is probably due to a C-H
insertion reaction of carbene with solvent to form a stable
product. This band is masked by its proximity to the bleaching
of the precursor (1180 cm-1) and the growth and decay of the
carbene (1142 cm-1).
Diene 7 spontaneously isomerizes to triene 8.15 The presence
of two strongly electronegative substituents on a cyclopropyl
ring is known to cause considerable increases in ring strain.15-17

Flash Photolysis Study of Chlorofluorocarbene
J. Phys. Chem. A, Vol. 107, No. 41, 2003 8551
References and Notes
Chlorofluorocarbene is found to have bimolecular reactivity
similar to that of chloro and dichlorocarbene.2 These three
carbenes react with pyridine with near diffusion-controlled rates.
Chlorofluorocarbene reacts with tetramethylethylene with rate
constants on the order of 108 M-1 s-1, which is about 10 times
slower than the corresponding reaction of dichlorocarbene with
this alkene.2 CFCl has a lifetime of microseconds in heptane.
This lifetime value is comparable to that of CCl2 but is much
longer than that of chlorocarbene.1
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Mixed dihalocarbene CFCl is produced upon photolysis of
1. In continuous photolysis experiments, the carbene is trapped
with tetramethylethylene (TME) to form the anticipated cyclo-
propane. In laser flash photolysis experiments, CFCl can be
trapped with pyridine to form an ylide (kpyr ) 8 × 109 M-1
s-1, τ ) 1.1 µs). The absolute rate constant of reaction of CFCl
with TME was found to be (kTME ) 1.1 × 108 M-1 s-1) using
the pyridine ylide probe technique in Freon-113. LFP of 1 with
IR detection produces CFCl with νC-F ) 1142 cm-1 and τ )
1-3 µs. The value of kTME determined by time-resolved infrared
spectroscopy is 2.9 × 108 M-1 s-1 in heptane and is in fair
agreement with the value determined by LFP in Freon-113. The
spectroscopic assignments are in fair to poor agreement with
density functional theory calculations.
(13) Smith, C. E.; Milligan, D. E.; Jacox, M. E. J. Chem. Phys. 1971,
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Acknowledgment. Support of this work by the National
Science Foundation is gratefully acknowledged. One of us
(E.M.T.) gratefully acknowledges a Graduate Assistantship in
the Area of National Needs Fellowship.
Supporting Information Available: The X-ray crystal
structure of 1 and an ORTEP drawing of 1 are provided. The
experimental and calculated IR spectra of indan are provided.
A plot of kobs versus [LiBr] and [ZnBr2] is provided along with
the UV-vis spectra of 1 and 8. This material is available free
of charge via the Internet at http://pubs.acs.org.