Generation and Characterization of New Fluoro-Substituted Carbenes

Article abstract of DOI:10.1021/jp0363848

Singlet fluorocarbenes substituted with diphenylphosphanyl, phenylsulfanyl, and trimethylsilyl groups were generated upon photolysis of the corresponding tricyclo[4.3.1.0]decadiene precursors 1a-c. Diphenylphosphanyl fluoro carbene 2a was directly observed by transient UV-Vis spectroscopy. Its lifetime in several solvents ranged from 1 to 10 μs. A study of its reactivity toward methyl acrylate and tetramethylethylene along with DFT calculations did not indicate specific solvation of the carbene center. Fluorothiophenyl carbene 2b and fluorotrimethylsilyl carbene 2c were studied by their reaction with pyridine to form ylides and have lifetimes of 1 μs (cyclohexane) and 33 ns (carbon tetrachloride), respectively, at room temperature. Transient UV-Vis spectroscopy supported by DFT calculations showed that, in the case of fluorothiophenyl carbene 2b, carbene formation is in competition with the generation of the diradical intermediate formed by the cleavage of only a single C-C bond of the cyclopropane ring of the precursor.

Full text of DOI:10.1021/jp0363848

J. Phys. Chem. A 2004, 108, 1033-1041
1033
Generation and Characterization of New Fluoro-Substituted Carbenes
Christophe Buron, Eric M. Tippmann, and Matthew S. Platz*
Department of Chemistry, The Ohio State UniVersity, 1118 Newman-Wolfrom, 100 West 18th AVenue,
Columbus, Ohio 43210
ReceiVed: August 11, 2003; In Final Form: December 1, 2003
Singlet fluorocarbenes substituted with diphenylphosphanyl, phenylsulfanyl, and trimethylsilyl groups were
generated upon photolysis of the corresponding tricyclo[4.3.1.0]decadiene precursors 1a-c. Diphenylphos-
phanyl fluoro carbene 2a was directly observed by transient UV-Vis spectroscopy. Its lifetime in several
solvents ranged from 1 to 10 µs. A study of its reactivity toward methyl acrylate and tetramethylethylene
along with DFT calculations did not indicate specific solvation of the carbene center. Fluorothiophenyl carbene
2b and fluorotrimethylsilyl carbene 2c were studied by their reaction with pyridine to form ylides and have
lifetimes of 1 µs (cyclohexane) and 33 ns (carbon tetrachloride), respectively, at room temperature. Transient
UV-Vis spectroscopy supported by DFT calculations showed that, in the case of fluorothiophenyl carbene
2b, carbene formation is in competition with the generation of the diradical intermediate formed by the cleavage
of only a single C-C bond of the cyclopropane ring of the precursor.
I. Introduction
SCHEME 1
Vogel-Jones type precursors (e.g., 1) have proven to be useful
photochemically activated sources of halocarbenes 2 (Scheme
1
1
). This type of precursor can often be more convenient than
the more common diazirine and diazo precursors for certain
2
carbenes. Herein, we report the preparation of three new
precursors, 1a-c, studies of their photochemistry, and kinetic
spectroscopy of related carbenes 2a-c. This permits the first
generation and characterization of fluorocarbenes substituted
with phosphorus, sulfur, and silicon.
II. Experimental Section
Synthesis. Dihydroindan 4 was prepared by Birch Reduction
3
of indan 3 as described in the literature. All chemical reagents
were used as purchased from Sigma Aldrich. All reaction
solvents were distilled under a dry argon atmosphere. THF was
distilled over sodium and benzophenone, diisopropylamine over
calcium hydride, and triethylamine over potassium hydroxide.
NMR spectra were recorded using a Brucker DPX-250 or DPX-
4
00 MHz spectrometers as noted. Chemical shifts are reported
positively downfield and are expressed in ppm relative to
1
13
19
triethylsilane ( H and C), CF3COOH ( F), or 10% H3PO4
3
1
(
P).
1
1
,6
0-Chloro-10′-fluorotricyclo[4.3.1.0 ]deca-3-ene (5). A
4
procedure of Dolbier and Burkholder was followed closely.
To a 1L RBF fitted with a mechanical stirrer and condenser
was added 600 mL of dry THF. The flask was cooled to 0 °C,
and 50 mL of TiCl4 was added dropwise, under an argon
atmosphere, over 1 h. HCl gas was evolved and the flask was
open to the atmosphere to prevent pressure buildup. The reaction
mixture turned bright yellow. LiAlH4 (17.26 g) was added as a
powder to the reaction in 2.5 g portions over 45 min. Gas was
evolved and the flask was vented to the atmosphere. The reaction
mixture turned dark brown. The reaction was allowed to warm
to RT and was stirred 30 min. The reaction was cooled to 0 °C,
and dihydroindan 4 (21.5 mL) was added slowly. Using a
(
a) (i) Na(s)/NH3(l) -50 °C (ii) MeOH; (b) (i)TiCl
CFCl 1.1 eq. 0 °C THF; (c) MCPBA, CH Cl , RT; (d) (i) diiso-
proplyamine/n-BuLi 3 eq. (ii) H O 0 °C THF; (e) (i) 3.5 eq. NEt (ii)
4 4
/LiAlH , 2 eq. (ii)
3
2
2
2
3
+
3.5 eq. MsCl 0 °C THF; (f) (i) n-BuLi, -78 °C (ii) electrophile (X ).
syringe pump, a solution of 19 mL of CFCl3 in 60 mL of THF
was added to the reaction over 2 h. The reaction was stirred an
additional 1 h then poured into 400 mL of cold aqueous 10%
HCl. The acidic aqueous solution was extracted with ether (2
*
Corresponding author. E-mail: Platz.1@osu.edu.
1
0.1021/jp0363848 CCC: $27.50 © 2004 American Chemical Society
Published on Web 01/15/2004

1
034 J. Phys. Chem. A, Vol. 108, No. 6, 2004
Buron et al.
×
500 mL) and CH2Cl2 (1 × 500 mL). The combined organic
0 °C followed by dropwise addition of 10 mL of methane
sulfonyl chloride. The reaction was stirred overnight at 0 °C to
RT and a white solid precipitated. The reaction was poured into
150 mL of ice water and separated. The organic layers were
washed with 2 M HCl (1 × 75 mL) and then the aqueous phase
was extracted with CH2Cl2 (2 × 75 mL). The organic phase
was dried over MgSO4, filtered, and concentrated by rotary
evaporation to yield 10.5 g of a reddish-orange mixture.
Chromatography (silica gel, hexanes elution) afforded 8.5 g of
a mixture of isomers (75%) as a clear oil. A more stringent
separation was able to completely separate the isomers but
typically the subsequent reactions were performed with the
phase was washed with 20% NaHCO3 (1 × 150 mL). The
solution was dried over MgSO4, filtered, and then the solvent
was removed by rotary evaporation to yield a red-brown oil.
The product was passed over silica gel with pentane elution to
yield 29.5 g (90%) of clear oil of the two isomers and a small
amount of starting material. 19F NMR (235 MHz, CDCl3): δ
1
-
139.1 (s, 1F); -146.5 (s, 1F). H NMR (400 MHz, CDCl3):
13
δ 1.6-2.7 (m, 20H); 5.81 (s, 2H); 5.86 (s, 2H). C NMR (101
MHz, CDCl3): δ 24.3; 24.4; 24.6; 24.9; 25.6; 25.7; 33.3; 34.1;
3
1
4.2; 36.0; 36.1; 36.2; 123.2; 123.3; 124.7; 126.9. MS (EI) m/z
86 (5%); 151 [-Cl] (100%).
1
9
1,6
isomeric mixture. F NMR (235 MHz, CDCl ): δ -130.2 (s,
1
0-Chloro-10′-fluorotricyclo[4.3.1.0 ]deca-3-oxirane (6).
3
1
1
(
6
F), -164.9 (s, 1F). H NMR (400 MHz, CDCl3): δ 1.6-1.8
To a round-bottom flask containing 225 mL of CH2Cl2 cooled
in an ice bath was added 29.5 g of alkene (5) followed by 35.5
g of solid m-chloroperoxybenzoic acid (MCPBA) in four
portions, with magnetic stirring. After the last portion of
MCPBA was added, the reaction was brought to room temper-
ature and stirred 2 h. The reaction was cooled to -20 °C, and
the benzoic acid was removed by filtration. Unreacted MCPBA
was removed by washing with 10% aqueous sodium bisulfite.
The filtrate was then washed with 10% sodium carbonate, dried
with MgSO4, filtered, and rotavapped to yield a clear oil. The
crude product was passed over silica gel with hexane elution
to remove all nonpolar impurities followed by 1:20 ether/
hexanes to elute the product. The appropriate fractions were
m, 2H); 1.9-2.1 (m, 2H); 2.3-2.5 (m, 2H); 5.86 (m, 2H);
1
3
.05-6.13 (m, 2H). C NMR (101 MHz, CDCl3): δ 123.5 (d,
J ) 1.1 Hz); 119.5 (d, J ) 9.4 Hz); 102.6 (d, J ) 412 Hz);
5
0.7 (d, J ) 11.1 Hz); 34.8 (s); 26.2 (d, J ) 2.2 Hz). MS (EI)
m/z 183 (2.5%), 149 [-Cl] (100%).
Synthesis of Compounds 1a-c. General Procedure. A
mixture of compounds 8 and 9 under an inert atmosphere of
argon was dissolved in dry THF and cooled to -78 °C. The
1
9
relative ratio of 8:9 was determined by either F NMR or GC/
MS. Then 1.1 equivalents (relative to 8) of n-butyllithium in
hexanes were added in one portion. The mixture was stirred at
low temperature for 2 h. Then 1 equivalent of electrophile was
introduced (e.g., chlorodiphenylphosphine, benzenesulfanyl
chloride, or chlorotrimethylsilane). The cold bath was removed
and the solution was allowed to warm to room temperature.
THF was evaporated and the mixture was extracted with diethyl
ether. After evaporation, silica gel chromatography (hexanes/
Et2O: 98/2; hexanes/Et2O: 98/2, hexanes/Et2O: 90/10, respec-
tively) yielded the corresponding compounds 1a-c.
1
9
collected and concentrated for a 94% yield of two isomers.
F
NMR (235 MHz, CDCl3): δ -133.7 (s, 1F); -148.1 (s, 1F).
1
H NMR (400 MHz, CDCl3): δ 1.5-1.7 (m, 8H); 1.8-2.3 (m,
1
2H); 3.06 (d, 4H, J ) 6.0 Hz). 13C NMR (101 MHz, CDCl3):
δ 20.9; 21.0; 23.0; 23.4; 29.2; 24.4; 31.9; 32.2; 32.3; 36.4; 36.5;
0.2; 50.3; 50.4. MS (EI) m/z 202 (2%), 167 [-Cl] (56%), 146
-F] (100%).
0-Chloro-10′-fluorotricyclo[4.3.1.0 ]deca-4-ene-2-ol (7).
5
[
1,6
1
endo-10-Fluoro-exo-10′-diphenylphosphanyltricyclo-
1
,6
1
To a flame-dried RBF was introduced 50 mL of dry THF. This
was followed by the addition of 26 mL of diisopropylamine.
The flask was cooled to -5 °C, then 116 mL of n-BuLi (1.6 M
in hexanes) was added dropwise over 30 min and stirred 25
min. The starting epoxide (6, 10 g, 0.187 mol) was dissolved
in 25 mL of dry THF and added to the reaction over 1 h via
syringe pump. The reaction became increasingly reddish during
the addition and was stirred for 2 h. The reaction was quenched
with the dropwise addition of 25 mL of water, and then poured
into 700 mL of ice water. The reaction was neutralized by
adding 10% aqueous HCl. The aqueous phase was extracted
with CH2Cl2 (2 × 300 mL). The combined organic phase was
washed with water (2 × 250 mL), dried (MgSO4), filtered, and
concentrated to yield 18 g of a crude reddish oil. Chromatog-
raphy (silica gel, CH2Cl2 elution) yielded 10.6 g of a clear oil
[4.3.1.0 ]deca-1,3-diene (1a). Yield: 70%. H NMR (400
MHz, CDCl3): δ 1.68 (m, 1H), 1.99 (m, 3H); 2.45 (m, 2H);
5.85 (dd, 2H, J ) 11.5 Hz, J ) 3.9 Hz), 6.05 (dd, 2H, J ) 11.7
1
3
Hz, J ) 3.9 Hz), 7.21 (m, 6H), 7.39 (m, 4H); C NMR (101
MHz, CDCl3): δ 133.7 (dd, J ) 8.3 Hz, J ) 5.2 Hz); 132.4 (d,
J ) 19.3 Hz); 127.7 (s); 127.4 (d, J ) 6.7 Hz); 122.3 (s); 122.3
(d, J ) 5.2 Hz); 73.6 (dd, J ) 278.0 Hz, J ) 44.4 Hz); 48.4
(dd, J ) 10.3 Hz, J ) 7.3 Hz); 34.5 (d, J ) 13.9 Hz); 23.6 (d,
3
1
J ) 16.1 Hz). P NMR (101 MHz, CDCl3): δ -10.8 (d, J )
1
9
5.2 Hz). F NMR (235 MHz, CDCl3): δ -180.5 (d, J ) 7.0
Hz). Exact mass for C22H20FP (amu): Calcd: 334.128116;
Found: 334.1291.
1,6
endo-10-Fluoro-exo-10′-thiophenyltricyclo[4.3.1.0 ]deca-
1,3-diene (1b). Yield: 58%. H NMR (400 MHz, CDCl ): δ
1
3
7.35(dm, 2H, J ) 7.3 Hz); 7.22 (tm, 2H, J ) 7.6 Hz); 7.15
(dm, 1H, J ) 7.2 Hz); 6.09 (dd, 2H, J ) 7.2 Hz, J ) 2.6 Hz);
5.86 (dd, 2H, J ) 7.1 Hz, J ) 2.5 Hz); 2.09 (m, 2H); 1.85 (m,
(82% yield) of isomers. Further purification can be achieved
by chromatography a second time on silica with a 10:90 mixture
of ether/hexanes followed by elution with 30:70 ether/hexanes.
1
3
2H); 1.69 (m, 1H); 1.58 (m, 1H). C NMR (63 MHz, CDCl3):
δ 135.5 (d, J ) 3.1 Hz); 129.5 (s); 128.0 (d, J ) 1.7 Hz); 126.8
(s); 124.1 (d, J ) 1.8 Hz); 121.6 (d, J ) 3.7 Hz); 84.3 (d, J )
268.3 Hz); 50.0 (d, J ) 12.6 Hz); 35.2 (s); 25.1 (d, J ) 2.5
1
9
F NMR (235 MHz, CDCl3): δ -132.3 (s, 1F); -139.2 (s,
1
1
1
F). H NMR (400 MHz, CDCl3): δ 1.69-1.71 (m, 2H); 1.84-
.95 (m, 4H); 2.0-2.2 (m, 8H); 2.48 (q, J ) 7.8 Hz); 2.56 (q,
1
9
J ) 7.8 Hz); 4.18 (m, 1H); 4.29 (m, 1H); 5.79 (dd, J ) 2.3 Hz,
J ) 3.8 Hz); 5.85 (m); 6.0 (d, J ) 10.0 Hz). 13C NMR (101
MHz, CDCl3): δ 26.0; 26.1; 26.3; 26.4; 30.9; 31.9; 32.1; 32.6;
Hz). F NMR (235 MHz, CDCl ): δ -144.4. Exact mass for
3
C16H15FS (amu): Calcd: 258.087299; Found: 258.0865.
endo-10-Fluoro-exo-10′-trimethylsilyltricyclo[4.3.1.01,6]-
3
1
5.4; 37.7; 36.8; 36.9; 38.3; 38.4; 64.1; 65.0; 123.9; 124.0;
27.3; 131.8; 132.5.
1
deca-1,3-diene (1c). Yield: 31%. H (400 MHz, CDCl3): δ
6
.04 (dd, 2H, J ) 7.1 Hz, J ) 2.5 Hz); 5.82 (dd, 2H, J ) 7.0
1,6
1
0-Chloro-10′-exo-fluorotricyclo[4.3.1.0 ]deca-2,4-diene
Hz, J ) 2.6 Hz,); 2.05 (m, 2H); 1.89 (m, 2H); 1.56 (m, 1H);
1.35 (m, 1H); 0.22 (d, 9H, J ) 1.0 Hz). C NMR (63 MHz,
1,6
13
(8) and 10-exo-Chloro-10′-fluorotricyclo[4.3.1.0 ]deca-2,4-
diene (9). The alcohol 7 (20 g), was dissolved in 75 mL of
CH2Cl2. A quantity of 17.5 mL of triethylamine was added at
CDCl3): δ 122.5 (d, J ) 2.2 Hz); 121.6 (d, J ) 6.7 Hz); 69.3
(d, J ) 237.8 Hz); 49.9 (d, J ) 9.7 Hz); 35.4 (s); 27.6 (s);

Generation and Characterization of Fluorocarbenes
0.2 (d, J ) 3.8 Hz). 19F NMR (235 MHz, CDCl3): δ -185.4.
J. Phys. Chem. A, Vol. 108, No. 6, 2004 1035
-
Exact mass for C13H19FSi (amu): Calcd: 222.123456; Found:
2
22.1233.
Laser Flash Photolysis (LFP) Studies. For LFP studies, a
stock solution of 1a-c in the desired solvent was prepared to
an optical density of 1.0-1.5 at 308 nm and placed in each
cuvette (2 mL per cuvette) with various amounts of trapping
agents. Suprasil quartz fluorescence free static cells and a special
sample holder were used to avoid scattering of the laser beam.
The LFP apparatus utilized a Lambda Physik LPX 100/200
excimer laser (308 nm, 150 mJ, 17 ns, XeCl excimer). Samples
used in LFP experiments were deoxygenated by passing a flow
of dry argon through the sample for 2 min. The analysis of the
data was performed with the program IgorPro developed by
Wavemetrics. Transient absorption spectra were obtained on an
EG&G PARC 1460 optical multichannel analyzer fitted with
an EG&G PARC 1304 pulse amplifier, an EG&G PARC 1024
UV detector, and a Jarrell-Ash 1234 grating. The monochro-
mator used was an Oriel 77200. Signals were obtained with a
photomultiplier tube detector and were digitalized by a Tektronix
Figure 1. Transient UV spectrum produced by LFP (308 nm) of 1a
in several solvents. The spectrum was recorded immediately after the
3 6 6
laser pulse over a window of 20 ns. (1) CH CN, (2) EtOAc, (3) C H ,
(
4) CH Cl , (5) C 12, (6) THF.
2
2
5
H
5
818A A/D transient digitizer. The spectrometer has been
5
described elsewhere in detail.
Protocols for Photolysis in Glassy Matrixes. A solution of
1
a-c in the desired solvent was prepared to an optical density
of 1.0-1.5 at 300 nm and placed in each cuvette (3 mL per
cuvette). Samples were deoxygenated by passing a flow of dry
argon through the sample for 2 min. The cuvette was placed in
a transparent quartz dewar and the temperature was cooled by
the addition of liquid nitrogen. Photolysis was performed in a
rayonet photochemical reactor using 300 nm UV lamps. The
UV spectrum was recorded using a Perkin-Elmer lambda 6 UV/
VIS spectrophotometer.
Density Functional Calculations. Geometries were fully
optimized at the B3LYP level using the 6-31G(d) basis set, and
harmonic frequencies were calculated at the same level. All the
calculations were carried out with the Guassian98 software
6
package. Energies and zero-point vibrational energy (ZPVE)
corrections were scaled by 0.9806 as noted.
III. Results
Figure 2. Differential UV spectrum produced by photolysis (300 nm)
of 1a at 77 K in 3-methylpentane. (1) 1 min, (2) 2 min, (3) 4 min, (4)
III.1. Fluorodiphenylphosphoryl Carbene (2a). We syn-
thesized 10-chloro-10-fluorotricycloundecadiene as an endo/exo
mixture at C-10 as shown in Scheme 1. n-Butyllithium
undergoes metal-halogen exchange exclusively with the exo-
chloride which was quenched with either diphenylphosphoryl
chloride, phenylchlorosulfide, or trimethylsilylchloride to form
7
min, (5) 10 min, (6) 15 min, (7) 20 min, (8) after annealing to RT.
1
a-1c, respectively.
Transient and Glassy Matrix Spectroscopy. Laser flash
photolysis (LFP) studies were performed using 308 nm pulses
from a XeCl excimer laser. The transient spectra produced upon
LFP of 1a are given in Figure 1. All the transient spectra exhibit
an average maximum absorbance around 330 nm. In the case
of acetonitrile, a broad absorption band at higher wavelength
Figure 3. Results of DFT calculations (b3lyp/6-31 g(d)) on carbene
2
a. Selected calculated bond lengths and angles: d(FC) ) 1.34 Å; d(CP)
) 1.77 Å; FCP ) 111.4 °. TD-DFT: 514.84 nm (f ) 0.0040); 323.16
nm (f ) 0.0486); 300.66 nm (f ) 0.0341); 280.70 nm (f ) 0.0032);
(
410 nm) is also observed. When excess oxygen is bubbled
2
0
77.40 nm (f ) 0.0164); 271.06 nm (f ) 0.0223); 267.84 nm (f )
.0096); 265.06 nm (f ) 0.0153).
through the sample solution, the transient absorption at 330 nm
is not modified, indicating that the carrier of this absorption is
neither a radical, nor a triplet carbene nor a triplet excited state
nor a triplet biradical.
is generated during the photolysis. The 325 nm peak is
associated with a reactive species
Similar results were obtained upon photolysis (300 nm) of
1
a in glassy 3-methylpentane at 77 K. The data (Figure 2) show
Density Functional Theoretical (DFT) Calculations. Density
functional theoretical calculations on carbene 2a were performed
using the b3lyp/6-31 g(d) level of theory (Figure 3). The
predicted geometry of singlet 2a reveals a bent carbene (the
angle at the carbene center is 111.4°). Furthermore, the geometry
the growth of two absorption bands at 295 and 325 nm. After
warming the sample to room temperature and re-cooling, the
final spectrum (labeled RT) exhibits only a peak at 295 nm.
Thus, the short wavelength band is due to a stable product that
6

1036 J. Phys. Chem. A, Vol. 108, No. 6, 2004
Buron et al.
Figure 4. Transient UV spectrum produced by LFP (308 nm) of 1a
in the presence of pyridine in several solvents. The spectrum was
recorded immediately after the laser pulse over a window of 20 ns. (1)
3 6 6 2 2
CH CN, (2) EtOAc, (3) C H , (4) CH Cl , (5) pentane, (6) THF.
Figure 5. Plots of the observed rate constant of transient decay (330
nm) and absorption (414 nm) versus the concentration of pyridine
produced by LFP (308 nm) of 1a in cyclohexane.
around the phosphorus atom is almost planar (the dihedral angle
around the phosphorus atom of 34° and sum of the angles around
3
3
TABLE 1: Rate Constants and Lifetime of Carbene 3A in
Several Solvents
it is 338°) which argues against σ λ character of phosphorus.
The PC bond length is calculated to be 1.77 Å. This value is
intermediate between that of typical carbon-phosphorus single
solvent
lifetime (µs)
7
3 4
and double bond lengths and is consistent with a σ λ character
of the phosphorus atom. These facts indicate that the lone pair
on phosphorus is delocalized on the vacant orbital on the carbene
center (a representation of the LUMO orbital is available in the
Supporting Information and shows the corresponding inter-
cyclohexane
cyclohexane-d12
pentane
acetonitrile
dichloromethane
benzene
1.0
2.0
1.2
8.5
9.8
7.2
8.5
2.5
8
action). Time-dependent density functional theory with the
tetrahydrofuran
valeronitrile
same basis set predicts two strong vertical UV absorptions for
carbene 2a at 323 and 301 nm. Thus, the calculations suggest
that the transient absorption at 325 nm observed by LFP and
the carrier of the persistent species absorbing at the same
wavelength in 3-methylpentane should be attributed to carbene
The magnitude of the isotope effect is consistent with that of
1
0
other singlet carbenes. When nonpolar solvents such as
cyclohexane or pentane are employed, the lifetimes of the
carbene are 1.0 and 1.2 µs, respectively.
2a.
One can intuitively predict that carbene 2a will be strongly
stabilized due to the interaction of the lone pair on the
phosphorus atom with the vacant orbital of the carbene carbon
atom, resulting in the mesomeric structure shown below. TD-
DFT calculations are consistent with this hypothesis. As
mentioned previously, the calculated C-P bond length is shorter
than that expected for a normal C-P single bond. Furthermore,
Pyridine Ylides. Compound 1a was studied by LFP in several
solvents in the presence of pyridine (Figure 4). The formation
of a pyridine ylide is observed in every case with a characteristic
maximum absorbance at 414 nm.
The decay of transient absorbance at 330 nm was monitored
along with the growth of absorbance at 414 nm in the presence
of pyridine. Figure 5 demonstrates that there is an excellence
correspondence of the dynamics at the two wavelengths. There
is no doubt that the carrier of transient absorption of 325 nm
reacts with pyridine to form a new compound that absorbs at
1
1
a Natural Population Analysis was performed. This analysis
indicates that the phosphorus atom has a net positive charge of
3
4
+
0.90 consistent with its σ λ character. There is corresponding
negative charge on the carbene center (-0.03), fluorine atom
414 nm. At 414 nm, we can reasonably assume that the
(
-0.33) and the two ipso carbon atoms (-0.38 and -0.36),
formation of the carbene-pyridine ylide 10 is observed. We
justifying the mesomeric structure below.
therefore conclude that the signal observed at 330 nm is singlet
8
-1 -1
carbene 2a. We find that kPYR ) 5 × 10 M s , a value
9
similar to that of other ground-state singlet carbenes.
We hypothesized that this delocalization and the resulting
charge separation might lead to strong interactions of the carbene
with solvent. Indeed, when more polar solvents are used (such
as acetonitrile, dichloromethane, and THF), the carbene lifetime
is lengthened by a factor of ≈10. One can speculate that the
carbene center is stabilized either by the polarity of the solvent,
or by some stronger solvent-carbene interaction.
SolVent Effects on Carbene Lifetimes. The lifetimes of carbene
2
1
2
a in several solvents, monitored at 330 nm, are listed in Table
. The carbene lifetime of 1 µs in cyclohexane is extended to
µs in cyclohexane-d12. This indicates that the lifetime of 2a
is limited in part by insertion into C-H bonds of the solvent.

Generation and Characterization of Fluorocarbenes
J. Phys. Chem. A, Vol. 108, No. 6, 2004 1037
When benzene is employed, the observed lifetime of the
carbene is 7.2 µs. Small amounts of THF and benzene in
cyclohexane do not increase the lifetime, suggesting that there
is no stabilization of the carbene by specific solvation as shown
above.
Calculated Carbene-Acetonitrile Complexes. To better un-
derstand the solvent effects on the lifetime of carbene 2a, we
performed DFT calculations on the hypothetical complex
between carbene 2a and acetonitrile at the b3lyp/6-31 g(d) level
of theory. In these calculations, the bond length between the
carbenic carbon and the acetonitrile nitrogen atom was fixed
between 1.4 and 4.4 Å and the structures were optimized. These
structures do not correspond to stationary points, but were used
in the construction of a Morse potential. The results are
presented in Figures 6 and 7. The two widely separated
molecules (2a and acetonitrile) were chosen as the reference
energies. At 5.03 Å, we predict the formation of a weak
hydrogen-bonded complex. No evidence was obtained for the
formation of a π complex between the lone pair of electrons
on nitrogen and the carbene center.
Figure 7. Calculated geometries (b3lyp/6-31 g(d) during the approach
of an acetonitrile molecule to carbene 2a with nitrile-ylide forma-
tion: ylide (d ) 1.273 Å), transition state (d ) 2.011 Å), and weak
complex (d ) 5.030 Å).
DFT calculations predict there will be a substantial barrier
of 10.43 kcal/mol to forming ylide 11. We propose that this
process requires the rotation of the acetonitrile molecule
resulting in the breaking of the calculated C-H bonding
interaction and the loss of stabilization due to the interaction
of the phosphorus lone pair and the carbene vacant orbital. Thus,
ylide formation with carbene 2a is predicted to be a slow process
relative to other carbene reactions.
As mentioned previously, LFP of diene 1a in acetonitrile
produces a long wavelength absorption band at 410 nm.
Unfortunately, its intensity is low and we were not able to
resolve the kinetics of its formation at this specific wavelength.
TD-DFT calculations predict that ylide 11 will have vertical
absorption bands below 360 nm with a maximum absorbance
at 306 and 289 nm. On the basis of this computational evidence,
we suggest that the carrier of the 410 nm band is not ylide 11
and, at this time, this transient species is not identified.
Calculated Singlet-Triplet Energy Separation of 2a. The
relative energies of the singlet and triplet states of fluorodiphen-
Figure 6. Calculated energy changes (b3lyp/6-31 g(d)) during the
approach of one molecule of acetonitrile to carbene 2a.

1038 J. Phys. Chem. A, Vol. 108, No. 6, 2004
Buron et al.
Figure 8. A plot of the observed rate constant of transient decay (330
nm) versus the concentration of methanol produced by LFP (308 nm)
of 1a in cyclohexane.
TABLE 2: Calculated Relative Energies (kcal/mol, scaled,
b3lyp/6-31 g(d)) of the Singlet and Triplet States of
Carbene 2a in Several Solvents and in the Gas Phase (The
singlet is predicted to be the ground state in the gas phase
and in all solvents examined.)
E (singlet)
E (triplet)
∆E(S/T)
gas phase
c-hexane
benzene
THF
dichloromethane
acetonitrile
0
9.905
8.785
8.647
7.359
7.260
6.807
9.905
10.283
10.263
10.771
10.973
11.182
-1.498
-1.616
-3.413
-3.713
-4.376
Figure 9. Plots of the observed rate constant of transient decay (330
nm) versus the concentration of methyl acrylate produced by LFP (308
nm) of 1a in several solvents.
ylphosphorylcarbene were computed using DFT methods with
the b3lyp/6-31 g(d) basis set.
As expected for a halocarbene, the singlet is predicted to be
the ground state in the gas phase (the singlet/triplet gap ∆ES/T
)
9.9 kcal/mol). This level of theory is known to overestimate
1
2
the stability of triplet carbenes relative to singlet carbenes. In
the case of methylene (CH2) this level of theory predicts a
singlet-triplet energy separation of 13 kcal/mol even though
the experimental value is 9.05 kcal/mol.
The singlet carbene has a calculated dipole moment of 3.6770
D, the triplet carbene dipole moment is predicted to be 2.4388
D. Thus, these calculations were repeated in a solvent force-
1
2
field using the Polarized Continuum Model of Tomasi et al.
The results are given in Table 2. Polar solvents are found to
differentially stabilize the singlet carbene although the effect is
rather small. ∆ES/T increases from 10.3 kcal/mol in cyclohexane
to 11.2 kcal/mol in acetonitrile showing that polar solvents better
stabilize the singlet than the triplet state. Hadad and co-workers
have reported related findings with arylcarbenes.14
Bimolecular Reactions of 2a. Specific solvation is often
manifested as a retardation of the rate of bimolecular chemical
reactions. As shown in Figure 8, methanol accelerates the rate
of decay of fluorodiphenylphosphorylcarbene in cyclohexane.
Figure 10. Plots of the observed rate constant of transient growth (414
nm) versus the concentration of tetramethylethylene produced by LFP
15
(308 nm) of 1a in several solvents containing 1.978 mM of pyridine.
TABLE 3: Absolute Rate Constants of Reaction of Carbene
2A with Alkenes Measured in Several Solvents
A plot of kOBS versus [methanol] is linear with kCH OH ) 2 ×
3
7
-1 -1
1
0 M
s
in cyclohexane. The absolute rate constants of
1
reaction of 2a with methyl acrylate (Figure 9) and tetrameth-
ylethylene (Figure 10) were determined in cyclohexane, aceto-
nitrile, and benzene. The results are collected in Table 3. It is
clear that the cycloaddition reaction rate constants of 2a show
little variation with solvent. Thus, there is no kinetic evidence
of specific solvation of fluorodiphenylphosphorylcarbene 2a.
III.2. Fluorothiophenylcarbene (2b). DFT calculations
solvent
k (methyl acrylate) (M- s-1
)
5
cyclohexane
benzene
3.0 × 10
5
3.9 × 10
5
3
CH CN
1.8 × 10
1
solvent
k (TME) (M- s-1
)
6
cyclohexane
benzene
2.0 × 10
6
2.0 × 10
(b31yp/6-318/d) were performed on carbene 2b in its singlet
5
CH
3
CN
9.0 × 10
state. The geometry of fluorothiophenylcarbene and key pa-
rameters are given in Figure 11 along with the UV-Vis
spectrum of 2b predicted by TD-DFT calculations. According
to the TD-DFT calculations, the sulfur-substituted carbene 2b
will not absorb significantly above 300 nm and will be more
difficult to observe directly by transient UV-Vis spectroscopy
than the phosphorus analogue 2a.
LFP of 1b (308 nm) produced transient spectra as a function
of solvent (Figure 12). A maximum transient absorption at 360
nm was observed in every solvent. The carrier of this transient
absorption was shown to react rapidly with oxygen (τ(N2)/ τ-
(O2) ) 19 in pentane), in contrast to phosphoryl carbene 2a,
whose lifetime is not appreciably influenced by oxygen.

Generation and Characterization of Fluorocarbenes
J. Phys. Chem. A, Vol. 108, No. 6, 2004 1039
Figure 11. Results of DFT calculations on carbene 2b (b3lyp/6-31
g(d)). Selected calculated bond lengths and angles: d(FC) ) 1.336 Å;
d(CS) ) 1.75 Å; FCS ) 105.3 °. TD-DFT: 394.16 nm (f ) 0.0045);
2
0
88.77 nm (f ) 0.0508); 272.09 nm (f ) 0.0072); 240.51 nm (f )
.0351); 235.31 nm (f ) 0.0025); 226.90 nm (f ) 0.0124); 215.34 nm
(f ) 0.0245); 212.11 nm (f ) 0.0960).
Figure 14. Transient UV spectrum produced by LFP (308 nm) of 1b
in the presence of pyridine. The spectrum was recorded immediately
3
after the laser pulse over a window of 20 ns. (1) CH CN, (2) EtOAc,
(
3) C , (4) CH Cl , (5) C 12, (6) THF.
6
H
6
2
2
5
H
Figure 15. A plot of the observed rate constant of transient decay
406 nm) versus the concentration of pyridine produced by LFP (308
nm) of 1b in cyclohexane.
Figure 12. Transient UV spectrum produced by LFP (308 nm) of 1b.
The spectrum was recorded immediately after the laser pulse over a
(
window of 20 ns. (1) CH
12, (6) THF.
3 6 6 2 2
CN, (2) EtOAc, (3) C H , (4) CH Cl , (5)
5
C H
Figure 16. A plot of the observed rate constant of transient decay
Figure 13. A plot of the observed rate constant of transient decay
360 nm) versus the concentration of methyl methacrylate produced
by LFP (308 nm) of 1b in cyclohexane.
(406 nm) versus the concentration of methyl methacrylate produced
(
by LFP (308 nm) of 1b in cyclohexane containing 0.247 M of pyridine.
The transient obtained by LFP of 1b reacts with methyl
methacrylate. A plot (Figure 13) of 1/τ versus [alkene] is linear
intercept of the plot (1/τ) indicates that the carbene lifetime in
cyclohexane is 1 µs. A plot of kOBS versus [methyl methacrylate]
at a constant pyridine concentration of 0.247 M is given in
Figure 16. The slope of this plot is the absolute rate constant of
reaction of carbene 2b with methyl methacrylate, and is 2.0 ×
5
-1 -1
with a slope of 2.8 × 10 M
s
in cyclohexane at ambient
temperature.
LFP of 1b in several pyridine-containing solvents produces
the transient spectra shown in Figure 14. The carrier of the broad
intense pyridine-dependent transient at 406 nm is assigned to
ylide 12 by analogy to the behavior of 2a.
6
-1 -1
10 M s .
The data obtained by the pyridine ylide probe method is in
poor agreement with the data obtained by direct observation of
the effect of methyl methacrylate on the rate of decay of the
transient absorbing at 360 nm, produced by LFP of 1b in the
absence of pyridine. Furthermore, we found that the yield of
pyridine ylide is unaffected by the presence of oxygen in the
solution (as expected for the reaction of a singlet carbene). We
conclude, therefore, that the oxygen-sensitive carrier of the
transient absorption at 360 nm is not carbene 2b.
We speculate that the carrier of transient absorption at 360
nm is triplet biradical 13, derived from one-bond cleavage of
precursor 1b, in competition with two-bond cleavage to form
A plot of kOBS to the formation of ylide 12 versus [pyridine]
is linear (Figure 15) with slope kpyr ) 2.0 × 10 M s . The
7
-1 -1

1040 J. Phys. Chem. A, Vol. 108, No. 6, 2004
Buron et al.
Figure 19. Results of DFT calculations (b3lyp/6-31 g(d)) on carbene
2
c. Selected calculated bond lengths and angles: d(FC) ) 1.335 Å;
d(CSi) ) 1.950 Å; FCS ) 109.1 °. TD-DFT: 1120.79 nm (f ) 0.0002);
2
0
47.39 nm (f ) 0.0144); 234.16 nm (f ) 0.0345); 206.61 nm (f )
.0714); 194.74 nm (f ) 0.0122); 184.73 nm (f ) 0.0200); 176.80 nm
(f ) 0.0223); 175.23 nm (f ) 0.0754).
Figure 17. Transient UV spectrum produced by LFP (308 nm) of 1c.
The spectrum was recorded immediately after the laser pulse over a
6 6 4 2 2 3
window of 20 ns. (1) AcOEt, (2) C H , (3) CCl , (4) CH Cl , (5) CH -
CN, (6) C 12, (7) THF.
5
H
Figure 20. A plot of the observed rate constant of transient decay
380 nm) versus concentration of pyridine produced by LFP (308 nm)
(
of 1c in carbon tetrachloride.
sensitivity of the transient toward oxygen and its reactivity
toward methyl methacrylate. It is possible that the photolysis
of diene 1a produces both phosphorus carbene 2a and a biradical
analogue 13. However, since carbene 1a absorbs strongly
between 300 and 350 nm, it is possible that the transient
spectrum of such a biradical is masked.
III.3. Fluorotrimethylsilylcarbene (2c). LFP (308 nm) of
1
c produces the transient spectra of Figure 17. The transient
lifetime monitored at 340 nm is 1 µs. The lifetime of the
transient is not sensitive to oxygen (k(N2)/k(O2) ) 0.99 in
pentane).
LFP (308 nm) of 1c in several pyridine-containing solvents
produces the transient spectra presented in Figure 18. The
transient absorption is attributed to ylide 14.
Figure 18. Transient UV spectrum produced by LFP (308 nm) of 1c
in the presence of pyridine. The spectrum was recorded immediately
after the laser pulse over a window of 20 ns. (1) AcOEt, (2) C
6 6
H , (3)
CCl , (4) CH Cl , (5) CH CN, (6) C 12, (7) THF.
4
2
2
3
5
H
TABLE 4: Relative Energies and TD-DFT Calculations on
Several Possible Intermediates at B3Lyp/6-31 g(d)
13
E + ZPE (scaled)) 0 kcal/mol
TD-DFT: 449 (f ) 0.0007); 416 (f ) 0.0019);
3
3
60 (f ) 0.0014);355 (f ) 0.0042); 343 (f ) 0.036);
36 (f ) 0.0073); 331 (f ) 0.0071); 319 (f ) 0.057)
The yield of ylide 14 is slightly sensitive to the presence of
oxygen (Absmax(N2)/Absmax(O2) ) 1.25 in pentane). DFT
calculations of carbene 2c (Figure 19) and TD-DFT calculations
predict that 2c has no significant absorption above 250 nm. This
is evidence that the transient detected at 340 nm in the absence
of pyridine is not carbene 2c.
indan + 2b
E + ZPE (scaled)) -17.8 kcal/mol
TD-DFT: Indan: 233 (f ) 0.0178); 206 (f ) 0.0082);
1
79 (f ) 0.3698); 178 (f ) 0.6898)
TD-DFT: 3b: 394 (f ) 0.0045); 289 (f ) 0.0508);
2
72 (f ) 0.0072); 240 (f ) 0.0351)
A plot of the observed rate constant of formation of ylide 14
versus [pyridine] is given in Figure 20. The slope of this plot is
carbene 2b and indan. Such intermediates, although never
observed, were postulated to explain the photochemical isomer-
ization of certain cyclopropanes.16
The energies of indan plus 2b and triplet 13 are given in
Table 4 along with the calculated (TD-DFT) UV-Vis spectrum
of triplet biradical 13. The TD-DFT calculations predict that,
unlike carbene 2b, triplet biradical 13 will absorb strongly
between 300 and 350 nm, the active region observed upon LFP
of 1b in the absence of pyridine. This assignment explains the
9
-1 -1
kPYR ) 2 × 10 M s . The intercept of this plot indicates
that the lifetime of carbene 2c is 33 ns in carbon tetrachloride
in the absence of pyridine.
TD-DFT calculations (b3lyp/6-31 g(d)) predict that diradical
intermediate 15 has vertical absorption bands at 271, 288, 320,
and 339 nm. The latter value is consistent with the observed
value of 340 nm. Indeed, photolysis (CF2ClCFCl2, 300 nm) of
1c leads to the formation of a stable isomeric photoproduct

Generation and Characterization of Fluorocarbenes
J. Phys. Chem. A, Vol. 108, No. 6, 2004 1041
References and Notes
detected by GC-MS in low yields. It is tempting to assign the
transient signal absorbing at 340 nm to biradical 15, which forms
the product of epimerization of 1c. However, the oxygen
independence of the transient lifetime does not appear to be
consistent with the presence of a biradical intermediate.
Therefore, the identity of the transient observed upon LFP of
(
1) Hartwig, J. F.; Jones, M., Jr.; Moss, R. A.; Lawrynowicz, W.
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(
(
(
1
c cannot be assigned with confidence at this time
(
W.; Parr, R. G. Phys. ReV. B 1998, 37, 785. (c) Frisch, M. J.; Trucks, G.
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IV. Conclusion
Laser flash photolysis (LFP, 308 nm) of endo-10-fluoro-exo-
1
0-substituted tricyclo[4.3.1.0]-decadienes produces indan and
singlet fluorocarbenes substituted with diphenylphosphoryl,
phenylsulfanyl, and trimethylsilyl groups. All three carbenes
react with pyridine to form UV-Vis active ylides. The ylides
are formed exponentially with observed rate constants of kOBS.
Plots of kOBS versus [pyridine] are linear with slopes equal to
the absolute rate constant of reaction of carbene with pyridine
8
-1 -1
of kPYR ) 5 × 10 M
s
(fluorodiphenylphosphorylcarbene),
(fluorophenylsulfanylcarbene) and
(fluorotrimethylsilylcarbene). Fluo-
7
-1 -1
(7) Corbridge, D. E. C. Phosphorus: An outline of its chemistry,
biochemistry and technology (5th ed.); Elsevier: Amsterdam, 1995; p 54.
kPYR) 2 × 10 M
s
9
-1 -1
kPYR) 2 × 10 M
s
(
8) Casida, M. E.; Jamorski, C.; Casida, K. C.; Salahub, D. R. J. Chem.
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through absorption at 325 nm. Its lifetime and reactivity are
slightly modulated by solvent.
(
Reactions in Solution; Landolt-Bornstein, Group II, Volume 18, Subvolume
E2; Springer: Berlin, Germany, 1998; p 141. (b) Ge, C.-S.; Jang, E. G.;
Jefferson, E. A.; Liu, W.; Moss, R. A.; Wlostowska, J.; Xue, S. Chem.
Commun. 1994, 1479.
Acknowledgment. Support of this work by the National
Science Foundation, a GAANN Fellowship of the Department
of Education (E.M.T.) and the Ohio Supercomputer Center is
gratefully acknowledged.
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99.
(
11) Reed, A. E.; Weinhold, F. A.; Curtiss, C. A. Chem. ReV. 1988, 88,
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(
13) Tomasi, J.; Persico, M. Chem. ReV. 1994, 94, 2027.
Supporting Information Available: Optimized geometries,
energies, zero-point and thermal corrections and IR frequencies
for compounds 2a-c, 13, 15, and compounds presented in
Figure 7. This material is available free of charge via the Internet
at http://pubs.acs.org.
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(
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