7
918
J . Org. Chem. 1996, 61, 7918-7921
F lu or escen ce fr om Sa m a r iu m (II) Iod id e a n d Its Electr on Tr a n sfer
Qu en ch in g: Dyn a m ics of th e Rea ction of Ben zyl Ra d ica ls w ith
Sm (II)
W. G. Skene, J . C. Scaiano,* and F. L. Cozens
Department of Chemistry, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada
Received March 8, 1996X
2
The luminescence from SmI in THF can be readily quenched by a variety of electron acceptors.
In the case of organohalides, the reaction is quite fast; for example, for dichloromethane the rate
8
-1 -1
constant is 2.7 × 10 M
s . Electron transfer leads to halide loss and formation of the carbon-
centered radical. In the case of benzyl chloride, the benzyl radicals produced can be readily detected
using laser flash photolysis techniques. This electron-transfer reaction has been used as a source
of benzyl radicals in order to determine the rate constant for their reaction with SmI
2
; the value
in THF at room temperature. The effect of HMPA on the
2
has also been examined.
7
-1 -1
obtained is (5.3 ( 1.4) × 10 M
s
spectroscopic properties of SmI
In tr od u ction
where we could photodecompose dibenzyl ketone without
predominantly irradiating SmI As our experiments
progressed and we attempted to understand better the
photochemistry of Sm(II), we realized that the electron-
transfer reactions of SmI provided an alternate clean
2
way of generating benzyl radicals from the corresponding
benzyl halides.
2
.
Samarium iodide (SmI
2
) is widely employed as a
1
-11
reducing agent in organic chemistry.
are believed to be mediated by free radicals and are
occasionally initiated photochemically, although the pho-
The reactions
tochemical properties of SmI
2
have not been examined
2-14
in any detail. SmI
2
in THF is known to be fluorescent,1
In the work reported here we have performed a time-
resolved study of the excited states of Sm(II). For these
measurements, we have utilized nanosecond laser pho-
tolysis techniques combined with time-resolved emission
spectroscopy and transient absorption detection. Excited
Sm(II) is an excellent electron donor,12 and we have
studied its reactivity toward a variety of substrates,
predominantly electron acceptors. The reactions with
benzyl halides provide access to a viable source of benzyl
radicals, thus allowing the determination of the absolute
rate constant for their reaction with Sm(II), and, in the
process, to an understanding of some aspects of the
photochemistry and photophysics of SmI2.
but to the best of our knowledge, its lifetime and
quenching characteristics have not been reported. The
emission is believed to involve the transition from the
5
6
13-16
4
f 5d to the 4f state.
One key step in any mechanism involving the reduction
of organic substrates by SmI is the reaction of free
2
radicals with Sm(II). In the case of primary alkyl
radicals, Hasegawa and Curran have estimated a rate
6
-1 -1
constant of 7 × 10 M
s
in the presence of an excess
1
7
of HMPA (hexamethylphosphoramide).
We initiated
our work in this area in the hope of determining an
absolute rate constant for the reaction of carbon-centered
•
free radicals, in particular C
6
H
5
CH
2
(which is readily
detectable) with Sm(II). Our original goal was to deter-
mine this value by generating benzyl radicals from
photodecomposition of dibenzyl ketone in the presence
Exp er im en ta l Section
of various concentrations of SmI
impossible with our equipment to find a wavelength
2
. Unfortunately, it was
Samarium(II) iodide was purchased from Aldrich as a 0.1
M solution in tetrahydrofuran (THF) and used as received. The
standard solvent for this work was THF (from Aldrich), which
was dried by distillation over sodium-potassium alloy under
X
Abstract published in Advance ACS Abstracts, October 1, 1996.
2
nitrogen and used immediately. While SmI can be usually
(
(
(
(
1) Curran, D. P.; Totleben, M. J . J . Am. Chem. Soc. 1992, 114, 6050.
2) Molander, G. A.; McKie, J . A. J . Org. Chem. 1992, 57, 3132.
3) Sturino, C. F.; Fallis, A. G. J . Am. Chem. Soc. 1994, 116, 7447.
4) Curran, D. P.; J asperse, C. P.; Totleben, M. J . J . Org. Chem.
handled with simple “sure-seal” techniques at the concentra-
tions used for synthetic work, spectroscopic work with milli-
molar concentrations of Sm(II) requires drybox handling under
oxygen-free and water-free conditions. Hexamethylphosphor-
amide (HMPA) and dibenzyl ketone (from Aldrich) were used
as received. The electron acceptors used as quenchers for
Sm(II) fluorescence were obtained from Aldrich and used as
1
991, 56, 7169.
5) Curran, D. P.; Fevig, T. L.; J asperse, C. P.; Totleben, M. J .
Synlett. 1992, 943.
(
(
(
6) Hasegawa, E.; Curran, D. P. J . Org. Chem. 1993, 58, 5008.
7) Inanaga, J .; Ishikawa, M.; Yamaguchi, M. Chem. Lett. 1987,
1
485.
8) Kunishima, M.; Hioki, K.; Kono, K.; Sakuma, T.; Tani, S. Chem.
Pharm. Bull. 1994, 42, 2190.
received except for drying them with MgSO
activated alumina.
4 2 2 5
, CaCl , P O , or
(
(
(
(
9) Molander, G. A.; Harris, C. R. J . Am. Chem. Soc. 1995, 117, 3705.
10) Lange, G. L.; Gottardo, C. Tetrahedron Lett. 1994, 35, 6607.
11) Kondo, T.; Akazome, M.; Watanabe, Y. J . Chem. Soc., Chem.
In our work, we have employed steady-state and time-
resolved techniques to study the luminescence from SmI in
2
THF. Steady-state experiments were performed with a Per-
kin-Elmer LS-50 spectrofluorimeter. Time-resolved lumines-
cence work used attenuated 355 or 532 nm pulses from the
harmonics of a Surelite Nd/YAG nanosecond laser for excita-
tion (e20 mJ per 6 ns pulse). The emission was monitored
with a system consisting of a spectrograph and a Hamamatsu
C-4334 Streakscope, capable of simultaneous spectral and
time-resolved data acquisition with nano- or picosecond resolu-
tion. All experiments were carried out at room temperature
Commun. 1991, 757.
(12) Kamenskaya, A. N. Russ. J . Inorg. Chem. 1984, 29, 251.
(13) Okaue, Y.; Isobe, T. Inorg. Chim. Acta 1988, 144, 143.
(14) Wang, S. H.; King, G.; Lin, S. H.; Brown, T. J . Solid State Chem.
1
987, 69, 224.
15) Kamenskaya, A. N.; Bukietynska, K.; Mikheev, N. B.; Spitsyn,
V. I.; J ezowska-Trzebiatowska, B. Russ. J . Inorg. Chem. 1979, 24, 1339.
16) Kamenskaya, A. N.; Mikheev, N. B.; Kholmogorova, N. P. Russ.
J . Inorg. Chem. 1983, 28, 1420.
17) Hasegawa, E.; Curran, D. P. Tetrahedron Lett. 1993, 34, 1717.
(
(
(
S0022-3263(96)00473-2 CCC: $12.00 © 1996 American Chemical Society