Photochemical Generation of BQM Intermediates
A R T I C L E S
freshly distilled over CaH2 for use in fluorescence measurements.
Anhydrous tetrahydrofuran (THF) and diethyl ether were obtained by
distillation over sodium, with benzophenone as an indicator. 2,2,2-
Trifluoroethanol (Aldrich) was distilled prior to use to remove impurities
that appeared in the UV-vis spectrum. UV-vis spectra were recorded
on a Varian Cary 1, 5, or 50 spectrophotometer.
CN-H2O; 8 lamps, 254 nm, 1.5 min) ranged from 33% conversion
(1:1) to 0% conversion (9:1). pH studies (at pH 7 (0.01 M phosphate
buffer), 9.3, 10.2, 11.2, and 12.5) were conducted in 1:2 H2O/CH3CN
with 15.8 mg of 12 (254 nm, 1.5 min, 8 lamps).
Photolysis of 9 in 1:1 H2O-CH3OH. Solutions of 20 mg of 9 in
100 mL of 1:1 H2O-CH3OH were irradiated for times up to 30 min
(254 nm, 16 lamps) to yield the methyl ether 20 as the only product
(26% yield after 30 min). Similar irradiations were carried out in 1:1
CH3OH-CH3CN for up to 2 h (254 nm, 16 lamps). Preparative TLC
(3:1 hexanes/ethyl acetate) was used to isolate a pure sample of the
product ether: 1H NMR (300 MHz, CDCl3) δ 1.87 (s, 3H), 3.17 (s,
3H), 4.8 (br s, 1H), 6.86 (d, 2H, J ) 8.8 Hz), 7.24-7.34 (m, 3H),
7.35-7.5 (m, 8H); MS (CI) m/z 273 (M+ - OCH3). pH studies (at pH
1.2, 3.4, 5.3, 7 (0.01 M phosphate buffer), 9.3, 10.2, 11.2, and 12.5)
were conducted on 9 in 1:1 H2O/CH3OH (20 mg, 254 nm, 5 min, 16
lamps).
Materials. The required biphenyl alkenes and alcohols (7-14) were
all readily synthesized via standard synthetic procedures from readily
available compounds, as described in the Supporting Information.
Product Studies. All preparative photolyses were carried out in a
Rayonet RPR 100 photochemical reactor equipped with 254 or 300
nm lamps. The solutions were contained in quartz tubes (100-200 mL),
which were cooled to e 15 °C with tap water by means of an internal
coldfinger. All solutions were purged with argon 5 min prior to
irradiation and for the entire time of the irradiation, to promote stirring
and maintain the atmosphere. Photolysis times varied from 30 s to 2 h.
Workup involved extraction of the photolyzed solution with CH2Cl2,
followed by drying of the combined organic layer and removal of the
solvent under reduced pressure. All pH studies list the pH of the aqueous
phase prior to mixing with CH3CN or CH3OH. The samples being
studied were dissolved in the organic solvent before being mixed with
the aqueous component. In these studies, the solution was brought to
∼pH 7 following irradiation by the addition of NH4Cl or 0.1 M pH 7
buffer solution, prior to extraction. The irradiation products were
separated (if necessary) by preparative thin-layer chromatography (TLC)
and then analyzed by MS and 1H NMR. In all cases, control experiments
at e15 °C and in the absence of light showed no conversion to products,
suggesting that thermal reaction does not play a role in the observed
chemistry.
Photolysis of 13 in 1:1 H2O-CH3OH. Irradiation of a solution of
13 (21 mg, 254 nm, 16 lamps, 100 mL of 1:1 H2O-CH3OH) gave
<6% conversion to the respective methanolysis product after 30 min.
Photolysis of 10 in 1:1 H2O-CH3OH. Irradiation of 20 mg of 10
in 1:1 H2O-H3OH for times up to 20 min (52% conversion) gave the
methyl ether 21 as the only product. A pure sample of 21 was isolated
by preparative TLC (3:1 hexanes/ethyl acetate): 1H NMR (300 MHz,
CDCl3) δ 1.89 (s, 3H), 3.18 (s, 3H), 4.8 (br s, 1H), 6.86 (d, 2H, J )
8.1 Hz), 7.18-7.51 (m, 10H), 7.55 (s, 1H); MS (EI) m/z 272 (MA+
-
HOCH3). pH studies (at pH 1.2, 3.4, 5.3, 7 (0.01 M phosphate buffer),
9.3, 10.2, 11.2, and 12.5) were conducted on 9 in 1:1 H2O-H3OH (20
mg, 254 nm, 16 lamps, 5 min).
Photolysis of 14 in 1:1 H2O-CH3OH. A solution of 21 mg of 14
in 100 mL of 1:1 H2O-CH3OH gave <2% conversion to the respective
methanolysis product (254 nm, 8 lamps, 10 min).
Photolysis of 7 in 1:2 H2O-CH3CN. A solution of 15 mg of 7 in
90 mL of 1:2 H2O-CH3CN was irradiated at 300 nm (8 lamps) for
1
times up to 20 min. H NMR indicated that 9 was the only product
Photolysis of 9 in 1:1 H2O-CH3CN in the Presence of Ethanol-
amine. A solution of 20 mg of 9 in 80 mL of 1:1 H2O-CH3CN in the
presence of 0.2 M ethanolamine was photolyzed (254 nm, 8 min). After
workup, the CH2Cl2 extract was washed with water to remove residual
ethanolamine. 1H NMR of the product mixture after 8 min of photolysis
gave ∼40% yield of 28, as identified by its characteristic methylenes.
Quantitative conversion to 28 was achieved by photolysis for 20-30
min, which gave upon workup essentially pure 28: 1H NMR (300 MHz,
acetone-d6): δ 2.56 (t, J ) 7 Hz, 2H, -CH2N-), 3.62 (t, J ) 7 Hz, 2H,
-OCH2-), 2.85 (br s, -OH, -NH-, exchangeable with D2O), 4.88 (s, 1H,
ArCH-), 6.86 (dt, J ) 7.8 Hz, 2H, ArH), 7.16 (t, J ) 6.8 Hz, 1H,
ArH), 7.26 (t, 2H, J ) 7.8 Hz, ArH), 7.43-7.53 (m, 8H, ArH), 8.52
(br s, 1H, ArOH, exchangeable with D2O); HRMS calcd for C22H23O2N
333.1729, found 333.1730.
UV-Vis Studies. The molar absortivities (ꢀ) for 7 and 12 were
determined at 300 and 237 nm, respectively. In each case, a known
mass of sample was dissolved in a 500 mL volumetric flask containing
1:1 H2O/CH3CN. Three milliliters of this stock solution was then
pipetted into a quartz cuvette and the UV-vis spectrum was recorded
on the Cary 5. This was repeated three times for each sample. ꢀ was
calculated from the Beer-Lambert relationship (A ) ꢀcl).
formed even at high conversions (>87%) (N2 or O2), on the basis of
comparison to the spectrum of the authentic material. Relative Φp tests
for irradiation in 1:1, 1:2, 1:4, and 1:9 H2O/CH3CN showed essentially
no difference between the solvent systems ((4%). pH studies [at pH
1, 3, 5, 7 (0.01 M phosphate buffer), 9, 10, 11, and 12] were conducted
in 1:2 H2O/CH3CN (90 mL total volume, 300 nm, 8 lamps, 10 min)
with 15 mg of 7.
Photolysis of 11 in 1:2 H2O-CH3CN. A 15.8 mg sample of 11
irradiated for 10 min at 300 nm (8 lamps) in 1:2 H2O-CH3CN (N2 or
O2) gave ∼12% conversion to the alcohol 13 with no other products
being observed. The identity of the product was confirmed by
comparison to the 1H NMR of the authentic material. Relative Φp
measurements were conducted in 1:2, 1:4, and 1:9 H2O/CH3CN (100
mL total volume). pH studies [at pH 7 (0.01 M phosphate buffer), 9.7,
10.5, 11.3, and 12.5) were conducted in 1:2 H2O/CH3CN (300 nm, 8
lamps, 10 min) with 15.8 mg of 11.
Photolysis of 15 in 1:2 H2O-CH3CN. Fifteen milligrams of 24
was dissolved in 90 mL of 1:2 H2O/CH3CN and irradiated at 300 nm
for 5 min. Analysis by 1H NMR indicated no conversion to the
photohydrated product.
Photolysis of 8 in 1:1 H2O-CH3CN. A solution of 15 mg of 8 in
90 mL of 1:1 H2O-CH3CN was irradiated (254 nm, 8 lamps) for times
up to 5 min. Even at high conversion (>80%), 10 was the only product,
Quantum Yields. Relative quantum yields were determined by
comparison of the yields of photosolvolyzed product between a
compound with a known quantum yield to those of the compound under
study. A known amount of compound was dissolved in a given solvent
system (abs at λex > 3) in a quartz tube and irradiated long enough to
give a low conversion. Immediately following this, an equimolar amount
of the compound with the known quantum yield was irradiated under
the same conditions. Integration from 1H NMR was used to determine
the relative conversions of the photosolvated products. Each system
was maintained at ∼15 °C by means of an internal coldfinger and was
purged with argon for 5 min prior to irradiation. All values are the
result of at least three independent irradiations.
1
as shown by comparison to the H NMR of authentic material. The
relative Φp was calculated by comparison to the yield of 14 from 12
as described below. The relative Φp was found to vary little (<5%)
between irradiation in 1:1, 1:2, 1:4, and 1:9 H2O/CH3CN (15 mg, 254
nm, 8 lamps, 1.5 min, 100 mL total volume). pH studies (at pH 7 (0.01
M phosphate buffer), 9.3, 10.2, 11.2, and 12.5) were conducted in both
1:1 and 1:2 H2O/CH3CN (254 nm, 1.5 min, 8 lamps) with 15 mg of 8.
Photolysis of 12 in 1:1 H2O-CH3CN. Irradiation of 15.8 mg of
12 in 100 mL of 1:1 H2O-CH3CN (254 nm, 1.5 min, 8 lamps) gave
14 as the only observable product in 33% yield. Relative yield
experiments in various solvent systems (1:1, 2:1, 4:1, and 9:1 CH3-
The product quantum yield for methyl ether formation from m- and
p-hydroxybenzhydrol have been previously determined11 (Φp) 0.40
9
J. AM. CHEM. SOC. VOL. 125, NO. 42, 2003 12969