Bietti et al.
1
1
SCHEME 8
identified by H NMR and GC-MS. H NMR (CDCl
3
): δ 7.36-
7
3
.33 (m, 2H), 6.90-6.87 (m, 2H), 3.96 (d, 1H, J ) 8 Hz), 3.80 (s,
H), 1.26-1.14 (m, 1H), 0.64-0.47 (m, 3H), 0.46-0.31 (m, 1H).
+
GC-MS (m/z, relative abundance): 178 (M ), 150 (100), 135, 121,
109, 91, 77, 51.
R,R-Dicyclopropyl(4-methoxyphenyl)methanol (4) was prepared
by the reaction of cyclopropyl(4-methoxyphenyl) ketone with
cyclopropylmagnesium chloride in anhydrous tetrahydrofuran,
purified by column chromatography (silica gel; eluent, hexane/ethyl
1
1
acetate, 5:1), and identified by H NMR and GC-MS. H NMR
(
44
3
CDCl ): δ 7.51-7.48 (m, 2H), 6.88-6.85 (m, 2H), 3.81 (s, 3H),
mentioned above, a rate constant around 108
s
-1
can be
1.43 (s, 1H), 1.23-1.13 (m, 2H), 0.57-0.48 (m, 4H), 0.41-0.34
•
9
+
reasonably expected for the 1,2-H-atom shift in 3r , and, thus,
this intermediate escapes detection under the experimental
conditions employed in the present study.
(m, 4H). GC-MS (m/z, relative abundance): 218 (M ), 190, 177,
173, 159, 135 (100), 121, 115, 91,77,69,55.
The purity of the synthesized compounds 3 and 4 was in both
cases >99%.
In the reaction of 4, the formation of cyclopropyl(4-
methoxyphenyl) ketone can be explained as described previously
for the 4-methoxycumyl alcohol radical cation (see Scheme 2)
Product Studies. In a typical experiment, 10 mL of an argon-
saturated aqueous solution (pH ) 3.5 or 9.5) containing 10% MeCN
(to improve the solubility of substrates 1 and 2), the substrate (1-
3
-
•+
in terms of the OH-induced R-OH deprotonation of 4 to give
•
2 2 8
4, 1 mM), and K S O (0.1 M) were irradiated for 30 s, employing
an alkoxyl radical (4r ) which then undergoes C-cyclopropyl
a photochemical reactor equipped with 4 × 15 W lamps with the
emission at 254 nm. The reactor was a cylindrical flask equipped
with a water cooling jacket thermostated at T ) 25 °C. To minimize
undesired photoreactions of the first-formed ring-conjugated car-
bonyl products, the irradiation times were chosen in such a way as
to obtain a limited substrate conversion (≈30%). In acidic solution,
7
â-scission, leading to the ketone. Because a rate constant >10
-1
•
s
can be reasonably expected for the â-scission of 4r in
42
aqueous solution, in this case also, it was not possible to detect
this intermediate under the experimental conditions employed.
In conclusion, by means of time-resolved studies carried out
in alkaline aqueous solution, direct spectroscopic evidence for
the formation of 1,1-diarylalkoxyl radicals 1r and 2r , following
OH-induced R-OH deprotonation of the radical cations gener-
4
the pH was adjusted to 3.5 with HClO . In basic solution, 10 mM
•
•
Na B O × 10 H O was added, and the pH was adjusted to 9.5
2
4
7
2
-
with NaOH. After irradiation, the reaction mixture was extracted
with Et
2
O (3 × 10 mL), and the combined organic layers were
ated from cyclopropyl(4-methoxyphenyl)phenylmethanol (1) and
cyclopropyl[bis(4-methoxyphenyl)]methanol (2), has been ob-
tained, in agreement with the results obtained previously for
the 4-methoxycumyl alcohol radical cation. This mechanistic
picture is also well-supported by the results of product studies
carried out after the one-electron oxidations of 1 and 2 at pH
dried over anhydrous sodium sulfate. Reaction products were
identified by GC (comparison with authentic samples) and GC-
MS and quantitatively determined by GC using bibenzyl as an
internal standard. Good mass balances (>90%) were obtained in
all experiments. At least three independent product studies were
carried out for every substrate under identical experimental condi-
tions. Blank experiments performed in the absence of irradiation,
)
9.5. In particular, as opposite kinetic solvent effects have
been observed for the O-neophyl shift and the C-C â-scission
in arylcarbinyloxyl radicals, the product distributions observed
in the reactions of 1 and 2 under these conditions are in line
with a water-induced competition between the O-neophyl shift
and the C-cyclopropyl â-scission in the intermediate 1,1-
2 2 8
or by irradiating in the absence of K S O , resulted in an almost
complete (g98%) substrate recovery accompanied by the formation
of negligible amounts (<1%) of reaction products.
Time-Resolved Studies. The PR experiments were performed
using a 10 MeV electron linear accelerator that supplied 300-ns to
1-µs pulses, with doses such that 1-10 µM radicals were produced.
•
•
diarylalkoxyl radicals 1r and 2r .
Experiments were performed at room temperature using argon-
saturated aqueous solutions containing the substrate (0.1-1.0 mM),
potassium peroxydisulfate (10 mM), and 2-methyl-2-propanol (0.1-
Experimental Section
4
0.5 M). The pH of the solutions was adjusted with HClO or NaOH.
Reagents. Potassium peroxydisulfate, sodium hydroxide, di-
sodium tetraborate decahydrate, perchloric acid, acetonitrile, and
A flow system was employed in all the experiments. The rate
constants (kdec) were obtained by averaging 4 to 8 values, each
consisting of the average of 3 to 10 shots and were reproducible to
within 10%.
2-methyl-2-propanol were of the highest commercial quality
available. Milli-Q-filtered (Millipore) water was used for all
solutions.
The second-order rate constants for the reaction of the radical
A commercial sample of cyclopropyl(4-methoxyphenyl)phenyl-
methanol (1) was used as received. Cyclopropyl[bis(4-methoxy-
phenyl)]methanol (2) was available from a previous study.10
R-Cyclopropyl(4-methoxyphenyl)methanol (3) was prepared by the
reaction of 4-methoxybenzaldehyde with cyclopropylmagnesium
chloride in anhydrous tetrahydrofuran,43 purified by column chro-
matography (silica gel; eluent, hexane/ethyl acetate, 5:1), and
-
cations with OH (k-OH) were obtained from the slopes of the plots
of the observed rates (kobs) versus the concentration of NaOH. For
these experiments, argon-saturated solutions containing 0.4-1.0
mM substrate, 10 mM potassium peroxydisulfate, 0.1-0.5 M
2
-methyl-2-propanol, and 1 mM Na
2
B
4
O
7
× 10 H
2
O were
employed.
LFP experiments were carried out with a laser kinetic spectrom-
eter that supplied 8-ns pulses, using the fourth harmonic (266 nm)
of a Q-switched Nd:YAG laser. The laser energy was adjusted to
e10 mJ/pulse by the use of the appropriate filter. A 3-mL Suprasil
quartz cell (10 mm × 10 mm) was used for all experiments. Argon-
or oxygen-saturated aqueous solutions (pH ) 3.5) containing the
(
42) By comparing the rate constants for C-methyl â-scission for the
cumyloxyl radical measured in acetonitrile, trifluoroethanol, and water (k
5
6
7
-1
)
7.1 × 10 , 6.1 × 10 , and 1.0 × 10 s , respectively (see refs 30 and
9)) with those for C-cyclopropyl â-scission in the R,R-dicyclopropyl-
phenylmethoxyl one measured in acetonitrile and trifluoroethanol (k ) 1.3
3
6
7 -1
×
10 and 1.6 × 10 s , respectively (see ref 30)), a rate constant greater
2 2 8
substrate, 1-4 (0.1-0.5 mM), K S O (0.1 M), and 2-methyl-2-
7
-1
than 1.6 × 10 s can be reasonably expected for â-scission of the R,R-
•
dicyclopropyl(4-methoxyphenyl)methoxyl radical 4r in water.
(
43) McCormick, J. P.; Fitterman, A. S.; Barton, D. L. J. Org. Chem.
(44) Olah, G. A.; Surya Prakash, G. K.; Liang, G. J. Org. Chem. 1977,
42, 2666-2671.
1
981, 46, 4708-4712.
3174 J. Org. Chem., Vol. 71, No. 8, 2006