Branchi et al.
hydride in anhydrous tetrahydrofuran; purified by column
chromatography (silica gel, eluent hexane/ethyl acetate 4:1)
reaction proceeds through path b. In this context, it is
important to point out that even though 1 and 3 should
be characterized by comparable base strengths, with the
latter substrate R-C-H deprotonation from 3•+ leads, via
methanol loss, to 1ct from which, however, formation of
products 1a-c is expected to be an unfavorable process
since it would require cleavage of an O-CH3 bond with
formation of a very unstable methyl cation. Accordingly,
in the TPPBF4-photosensitized reaction of 3 no products
deriving from an intermediate cation have been observed
but only 4-methoxybenzaldehyde and methyl 4-methoxy-
benzoate suggested to be formed after R-C-H deproto-
nation from 3•+ as described in Scheme 7.
1
and identified by H NMR and GC-MS.5
Bis(4-methoxybenzyl) ether (1a) was prepared carrying out
the TPPBF4-photosensitized reaction of 4-methoxybenzyl al-
cohol (1) on a preparative scale. After workup, the residue was
purified by column chromatography (basic alumina, eluent
petroleum ether/ethyl acetate 5:1) and identified by GC-MS.
GC-MS m/e (relative abundance): 258 [M+], 151, 137, 136,
122, 121 (100), 107, 77.
4-Methoxydiphenylmethanol (10) was prepared by reduction
of 4-methoxybenzophenone with NaBH4 in 2-propanol, purified
by column chromatography (basic alumina, eluent petroleum
ether/ethyl acetate 10:1), and identified by 1H NMR and GC-
MS.38
The observation that in the LFP of 11 no evidence for
the formation of the corresponding diarylmethyl cation
(up to 5 ms after the laser flash) but only of the
corresponding radical cation 11•+ was obtained is likely
to reflect the high stability of 11•+,13,27 which accordingly
is expected to undergo a very slow deprotonation reaction.
The relatively less stabilized radical cations 9•+ and 10•+
may instead undergo significantly faster deprotonation
reactions,38 and formation of the corresponding cations
9ct and 10ct (according to the mechanism described in
Scheme 9) has been observed in both cases (see Figure
S4 of the Supporting Information and Figure 4, respec-
tively).
In conclusion, on the basis of the results of steady-state
and laser flash photolysis experiments on the TPPBF4-
photosensitized oxidation of ring-methoxylated benzyl
alcohols in organic solution, clear evidence in favor of the
involvement of benzyl alcohol radical cations and benzyl
cations in these processes is provided, leading to a
general mechanism for these reactions. The product
distributions observed in the steady-state irradiation
experiments are interpreted in terms of the interplay
between radical cation and benzyl cation stability. Quite
importantly, in contrast with previous studies where
formation of the 4-methoxybenzyl cation was suggested
to occur from the neutral 4-methoxybenzyl alcohol through
excited TPP+ (TPP*) acting as a Lewis acid,3 the experi-
mental data presented above allow to rule out such
hypothesis and benzyl cation formation is now rational-
ized in terms of R-C-H deprotonation of the radical
cation (formed after electron transfer from the substrate
to TPP*) by the R-OH group of the parent substrate
which acts as the deprotonating base.
2,4,4′,5-Tetramethoxydiphenylmethanol (11) was prepared
by reaction of 4-methoxyphenylmagnesium bromide with 2,4,5-
trimethoxybenzaldehyde in anhydrous tetrahydrofuran, puri-
fied by column chromatography (basic alumina, eluent petro-
1
leum ether/ethyl acetate 3:1) and identified by H NMR and
GC-MS.
1H NMR (CDCl3): δ 3.77 (s, 3H), 3.79 (s, 6H), 3.88 (s, 3H),
5.98 (s, 1H), 6.52 (s, 1H), 6.84 (s, 1H), 6.83-6.87 (m, 2H), 7.26-
7.31 (m, 2H).
GC-MS m/e (relative abundance): 304 [M+] (100), 287, 271,
257, 195, 169, 135, 121, 109.
Product Studies. Air- or oxygen-saturated CH2Cl2 solu-
tions containing TPPBF4 (5 × 10-4 M) and the substrate (1,
1a, 2-8, 10; between 2.5 and 5 × 10-3 M) were irradiated for
times varying between 1 and 60 min employing a Photochemi-
cal Multirays Reactor equipped with 10 × 15 W 365 nm lamps.
The reactor was a cylindrical flask equipped with a water
cooling jacket thermostated at T ) 25 °C. After irradiation,
the reaction mixture was washed with a saturated NaCl
solution and then with a 10% NaHCO3 solution, and the
solvent was evaporated under vacuum. Addition of diethyl
ether to the residue allowed removal of the insoluble TPPBF4
through filtration. The quantitative analysis of the ethereal
filtrate was performed by GC in the presence of an internal
standard (bibenzyl). Unreacted substrate and reaction prod-
1
ucts were generally identified by H NMR, GC-MS, and GC
by comparison with authentic samples. Good to excellent mass
balances (g85%) were obtained in all experiments.
Blank experiments carried out in the absence of irradiation
or irradiating solutions without TPPBF4 showed no substrate
consumption.
Laser Flash Photolysis Studies. Laser flash photolysis
experiments were carried out with a laser kinetic spectrometer
using the third armonic (355 nm) of a Q-switched Nd:YAG
laser. The laser energy was adjusted to ∼10 mJ at 355 nm by
the use of the appropriate filter. A 3 mL Suprasil quartz cell
(10 mm × 10 mm) was used for all experiments. Air-saturated
CH2Cl2 solutions containing TPPBF4 (2.5 × 10-5 M, A355nm
≈
1) and the substrate (1, 5, 6, 8-11; between 5 × 10-3 M and
0.1 M) were used. All the experiments were carried out at T
) 25 ( 0.5 °C under magnetic stirring. Rate constants were
obtained by averaging three or four values. The stability of
the solution to the experimental conditions was checked
spectrophotometrically comparing the spectrum of the solution
before irradiation with that obtained after irradiation.
With substrates 6, 8, and 11, the decay kinetics were
monitored at 460, 470 and 550 nm. At 460 and 470 nm, the
best fit to the experimental data was obtained using consecu-
tive first-order second order kinetics, assigned, respectively
to the decay of triplet TPP+ and of the substrate radical cation
(6•+, 8•+, and 11•+). At 550 nm, the experimental data were
fitted according to second-order kinetics, assigned to the decay
of TPP•.
Experimental Section
Materials. Commercial samples of 4-methoxybenzyl alcohol
(1), 4-methoxytoluene (2), 2-methoxybenzyl alcohol (4), 2,4-
dimethoxybenzyl alcohol (5), 2,5-dimethoxybenzyl alcohol (6),
3,4-dimethoxybenzyl alcohol (7), 2,4,5-trimethoxybenzyl alco-
hol (8), 4,4′-dimethoxydiphenylmethanol (10), pyridine, 1,2-
epoxybutane, and biphenyl of the highest quality available
were used as received. CH2Cl2 was purified on a basic alumina
column prior to use. 2,4,6-Triphenylpyrylium tetrafluoroborate
was recrystallized twice from ethanol before use.39 4,4′-
Dimethoxydiphenylmethane (1b) was available from previous
work.38
4-Methoxybenzyl methyl ether (3) was prepared by reaction
of 4-methoxybenzyl alcohol with methyl iodide and sodium
In the global kinetic analysis40 (for LFP experiments of
substrates 6 and 8) the deconvolution of the time-resolved
spectra of transients was carried out employing the Pro-K 4.0
program, according to the following model: a first-order
(38) Bietti, M.; Lanzalunga, O. J. Org. Chem. 2002, 67, 2632-2638.
(39) Ramamurthy, P.; Morlet-Savary, F.; Fouassier, J. P. J. Chem.
Soc., Faraday Trans. 1993, 89, 465-469.
8884 J. Org. Chem., Vol. 69, No. 25, 2004