A search for carbene-solvent interactions using time-resolved infrared spectroscopy

Article abstract of DOI:10.1021/ol034236u

(Matrix presented) The time-resolved infrared (TRIR) spectra of chlorophenylcarbene (CPC) and fluorophenylcarbene (FPC) were recorded in heptane at ambient temperature. The C-C and C-F vibrational frequencies involving the carbene carbon were obtained in heptane, benzene, and acetonitrile and in heptane containing 0. 1M tetrahydrofuran or benzene. It is concluded that carbene-solvent interactions of CPC and FPC are quite weak.

Full text of DOI:10.1021/ol034236u

ORGANIC
LETTERS
2
003
Vol. 5, No. 8
305-1307
A Search for Carbene−Solvent
Interactions Using Time-Resolved
Infrared Spectroscopy
1
Ying Sun, Eric M. Tippmann, and Matthew S. Platz*
Department of Chemistry, The Ohio State UniVersity, 1118 Newman-Wolfrom,
100 West 18th AVenue, Columbus, Ohio 43210
platz.1@osu.edu
Received February 10, 2003
ABSTRACT
The time-resolved infrared (TRIR) spectra of chlorophenylcarbene (CPC) and fluorophenylcarbene (FPC) were recorded in heptane at ambient
temperature. The C−C and C−F vibrational frequencies involving the carbene carbon were obtained in heptane, benzene, and acetonitrile and
in heptane containing 0. 1M tetrahydrofuran or benzene. It is concluded that carbene−solvent interactions of CPC and FPC are quite weak.
1
Singlet carbenes have a zwitterionic character, which has
led chemists to consider the possibility of their specific
solvation as shown in the abstract for chlorophenylcarbene
prominent vibrational bands at 1590, 1225, 1170, and 848
-1
cm . The vibrational frequencies of CPC are well-simulated
using density functional theory (DFT) calculations, which
-
1
(
CPC) and tetrahydrofuran and flurophenylcarbene (FPC) and
benzene. Tomioka and co-workers reported the effect of
dioxane on carbene selectivity. In recent years, Khan and
indicate that the 1225 cm band largely involves stretching
of the partial double bond between the carbene carbon and
2
6,7
-1
aromatic ring (Scheme 1). The band at 1170 cm is
Goodman concluded that singlet methylene forms a charge-
3
4
transfer complex with benzene. Ruck and Jones discovered
dramatic solvent effects on the distribution of rearrangement
products formed from tert-butylcarbene and Moss, Yan, and
Scheme 1. Carbene Electron Delocalization
5
Krogh-Jesperson discovered experimental and computational
evidence in favor of halocarbene-benzene complexes.
The matrix IR spectrum of CPC is known and has been
6
analyzed by Sheridan and co-workers. In argon, CPC has
(
1) (a) Hoffmann, R.; Zeiss, G. D.; Van Dine, G. W. J. Am. Chem. Soc.
1
968, 90, 1485. (b) Hoffmann, R. J. Am. Chem. Soc. 1968, 90, 1475.
(
(
(
(
2) Tomioka, H.; Ozaki, Y.; Izawa, Y. Tetrahedron 1985, 41, 4987.
3) Khan, M. I.; Goodman, J. L. J. Am. Chem. Soc. 1995, 117, 6635.
4) Ruck, R. T.; Jones, M., Jr. Tetrahedron Lett. 1998, 39, 2277.
5) (a) Moss, R. A.; Yan, S.; Krogh-Jesperson, K. J. Am. Chem. Soc.
assigned to bending of aromatic ring hydrogens on the basis
of the DFT calculations. Intuition suggests that electron pair
1
998, 120, 1088. (b) Krogh-Jesperson, K.; Yan, S.; Moss, R. A. J. Am.
Chem. Soc. 1999, 121, 6269.
6) Ganzer, G. A.; Sheridan, R. S.; Liu, M. J. Am. Chem. Soc. 1986,
08, 1517.
(
(7) Pliego, J. R., Jr.; DeAmedia, W. B.; Celebi, S.; Zhu, Z.; Platz, M. S.
J. Phys. Chem. 1999, 103, 7481.
1
1
0.1021/ol034236u CCC: $25.00 © 2003 American Chemical Society
Published on Web 03/26/2003

donation from a coordinating solvent, such as THF, aceto-
nitrile, or benzene into the empty p orbital of CPC will reduce
the partial double bond character to the carbene center. This
argument predicts that solvation will shift the vibrational
frequency to a lower value.
Table 1. Experimental and DFT Calculations for FCP and Its
Precursor (B3LYP/6-31G*; scaled)
compound
expt (cm-1)
calcd (cm-1
)
vibration
8
FC(N )Ph
FCP
FCP
1165
1220-1230
1110
1163
1199
1102
[C-F]
[Ph-C]
[C-F]
Thus, chlorophenyl diazirine was pulsed with 355 nm
2
9
10
radiation and the time-resolved infrared (TRIR) spectrum
of the carbene was recorded at ambient temperature as a
function of solvent. TRIR spectra were recorded in neat
heptane, acetonitrile, and benzene and in heptane containing
(
Table 1) predict that the C-F bands of the carbene and
0.1 M benzene or 0.1 M THF. Figure 1 shows that at a
-
1
precursor will be observed at 1102 and 1163 cm , respec-
tively. Benzene and acetonitrile slightly shift and broaden
the carbene bands (Figure 2). However, the C-F vibration
of the diazirine precursor also shifts slightly in these solvents,
suggesting that a bulk solvent effect rather than complexation
is responsible.
Thus, there is no compelling evidence by TRIR spectros-
copy of specific solute-solvent interactions of CPC and FPC
using either low or high concentrations of potential ligands.
Specific solvation of nucleophiles by hydrogen bonding is
evident in the reduction of rate of SN
2
reactions in aprotic
1
4
solvents upon addition of water. Previously, Celebi, Tsao,
1
5
and Platz demonstrated that chelating solvents have little
influence on the reactivity of CPC and chloro-p-nitrophen-
ylcarbene (CNP) with tetramethylethene (TME). Inspection
of Table 2 reveals that chelating solvents have little influence
Figure 1. Time-resolved IR spectra for chlorophenylcarbene. The
spectra were recorded over a window of 1000 ns, 50 ns after the
laser pulse. (Dashed line spectra are offset on the y-axis.)
Table 2. Absolute Rate Constants of Selected
Arylhalocarbenes with Tetramethylethene (kTME) as a Function
of Solvent
modest ligand concentration (0.l M) there is little effect on
the carbene bands. Dramatic changes in the TRIR spectra
were not observed in neat solvent either.
kTME (× 10 M s-1
8
-1
)
CPC15
CNP15
solvent
FPC
The effect of a fluorine substituent was also examined.
Fluorophenylcarbene was produced by flash photolysis of
fluorophenyl diazirine.11 FPC has the analogous C-C
acetonitrile
tetrahydrofuran
ethyl acetate
anisole
benzene
isooctane
1.0
0.8
11
7.4
0.71
0.89
1.3
0.99
2.8
2
-
1
vibration in heptane at 1220 cm , but additionally, a C-F
-
1
12,13
vibration at 1108 cm (Figure 2). DFT calculations
1.3
1.6
2.2
12
12
pentane
Freon-113
1.8
on the absolute reactivity of FPC with TME. Together with
the TRIR data, these observations argue against strong
(
(
(
8) Graham, W. H. J. Am. Chem. Soc. 1965, 87, 4396.
9) Tsao, M.-L.; Zhu, Z.; Platz, M. S. J. Phys. Chem. A 2001, 105, 8413.
10) (a) Yuzawa, T.; Kato, C.; George, M. W.; Hamaguchi, H. Appl.
Spectrosc. 1994, 48, 684. (b) Toscano, J. P. AdV. Photochem. 2001, 26, 41.
11) Moss, R. A.; Terpinski, J.; Cox, D. P.; Denny, D. Z.; Krogh-
Jesperson, K. J. Am. Chem. Soc. 1985, 107, 2743.
12) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang,
(
(
W.; Parr, R. G. Phys. ReV. B 1998, 37, 785. (c) Hariharan, P. C.; Pople, J.
A. Theor. Chim. Acta 1973, 28, 213.
(13) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratman, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.;
Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.;
Cammi, R.; Menucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski,
J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.;
Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Oritz, J.
Figure 2. Time-resolved IR spectra for fluorophenylcarbene. The
spectra were recorded over a window of 1000 ns, 50 ns after the
laser pulse. (Dashed line spectra are offset on the y-axis.)
1306
Org. Lett., Vol. 5, No. 8, 2003

carbene-solvent interactions of CPC and FPC. It is possible
that the aryl halo carbenes are so well-stabilized that these
carbenes have particularly weak interactions with solvent.
Thus, we are extending the TRIR method to more electron-
demanding carbenes.
V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng,
C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M.; Johnson, B.; Chen,
W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-Gordon, M.; Repogle,
E. S.; Pople, J. A. Gaussian 98; Gaussian, Inc.: Pittsburgh, 1998.
Acknowledgment. Support of this work by the National
Science Foundation is gratefully acknowledged. E.M.T.
gratefully acknowledges GAANN support.
(14) Heyding, R. D., Winkler, C. A. Can J. Chem 1951, 29, 790.
15) Celebi, S.; Tsao, M., Platz, M. S. J. Phys. Chem. A 2001, 105,
(
1
158.
OL034236U
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