6106 J . Org. Chem., Vol. 63, No. 18, 1998
Dauben and Hecht
1.52 (s, 3H), 2.4 (d, J ) 17 Hz, 1H), 2.58 (d, J ) 17 Hz, 1H),
2.8 (q, J ) 17 Hz, 1H), 3.66 (s, 3H), 4.03 (s, 3H); 13C NMR
(CDCl3) δ 189.9, 159.8, 136.4, 122.3, 61.0, 59.4, 46.9, 39.2, 32.5,
25.3; IR (film) 2956, 2200, 1675, 1623, 1150 cm-1; UV (MeOH)
λmax (ꢀ) 270 nm (1700 L/mol cm); HRMS calcd for C10H13NO3
195.089 54, found 195.089 01.
F lu or escen ce a n d P h osp h or escen ce Mea su r em en t s.
The chromatographed compounds were recrystallized twice
from hexane/ethyl acetate, and no impurity (<0.1%) was
detected by GC. At room temperature, the fluorescence
experiments were carried out in a 1 cm quartz cuvette,
whereas low-temperature experiments (77 K) were performed
with ethanol glass matrices in quartz tubes. In the phospho-
rescence measurements, a perpendicular experimental ar-
rangement with an internal chopper was used in order to
eliminate fluorescence. The concentrations were adjusted to
approximately 10-3 mol/L. The phosphorescence spectra were
corrected with an obtained sensitivity-wavelength calibration
function of the detector, and the background of a sample
containing neat solvent was subtracted.
[1.2]migration yields phenolic products, can occur in this
particular system.
Con clu sion
Wavelength-dependent photochemistry of 4-methoxy-
bicyclo[3.1.0]hexenones has been demonstrated, and a
reasonable mechanistic picture is suggested. The crucial
factor for achieving wavelength dependency is the 4-meth-
oxy substitution pattern, which dramatically lowers the
πfπ* triplet state.29 As a result, two competing pro-
cesses, i.e., photoisomerization and phenol formation,
become apparent. The product distribution, which is
dictated by the population of the two different triplet
excited states, depends mainly on efficiencies of ISC and
IC starting from the excited singlet states. Herein, the
wavelength dependency has its origin.
Dir ect a n d Sen sitized Ir r a d ia tion s. Preparative ir-
radiation experiments were carried out using a 450 W Hg lamp
employing uranium (λ50%T ) 340 nm, λ10%T ) 320 nm) as well
as Corex (λ50%T ) 290 nm, λ10%T ) 270 nm) filters. For
analytical runs, constant light conditions were established and
the runs were monitored by GC or HPLC. In sensitized
experiments, the concentrations were adjusted to guarantee
>99% absorption of the light by the sensitzer. All irradiations
were performed under deaerated conditions (thoroughly de-
gassing with dry nitrogen).
Exp er im en ta l Section
Gen er a l Meth od s. 1H NMR and 13C NMR were recorded
at 300 and 400 MHz in CDCl3 solutions calibrated with TMS.
Gas chromatography was performed on a gas chromatograph
equipped with a flame-ionization detector and a 25 m × 0.25
µm DB1-30N fused silica capillary column. HPLC was done
using a 250 × 4.6 mm packed silica column with elution
mixtures of hexanes and ethyl acetate. UV detection was
employed. Response factors were developed against internal
standards for GC and HPLC for each compound quantified.
The estimated error of the response factors is (5%. Flash
chromatography was performed on 230-400 mesh silica gel
applying medium pressure.30 All solvents used in spectros-
copy, in irradiation and trapping experiments, and in quantum
yield measurements were of spectroscopic grade and used as
received. Benzene was distilled over LiAlH4 prior to use.
2,5-Cycloh exadien on es (1) an d Bicyclo[3.1.0]h exen on es
(2 a n d 3). 2,5-Cyclohexadienones (1a -c) were prepared via
Birch reduction followed by allylic oxidation using a mixture
of tert-butyl hydroperoxide and PDC31 according to a procedure
by Schultz.2a For 1c, the furan tether was prepared in a five-
step sequence starting from furfural.32 Irradiation yielded the
corresponding bicyclo[3.1.0]hexenones (2a -c, 3a -c) already
described previously.2a Characterization of phenol 4a and
trapping product 9c were in good agreement with the
literature.2a,13 All compounds were chromatographed and
recrystallized from hexane/ethyl acetate. 7a could not be
isolated, and only GC-MS analysis is available.
2-Cya n o-5-m eth oxy-3-m eth ylp h en ol (4b). A solution of
2b (0.3 g, 1.8 mmol) in 10 mL of acetonitrile was purged with
nitrogen for 10 min and irradiated with a 450 W Hanovia high-
pressure mercury lamp equipped with a Corex filter until
complete conversion (GC > 97%). Removal of the solvent in
vacuo and column chromatography (silica gel, ether/hexane
4:1) gave 4b as an oil (86 mg, 29% yield): 1H NMR (CDCl3) δ
2.45 (s, 3H), 3.8 (s, 3H), 6.32 (d, J ) 2 Hz, 1H), 6.39 (d, J ) 2
Hz, 1H); 13C NMR (CDCl3) δ 165.1, 159.6, 144.0, 116.0, 108.1,
99.1, 92.8, 54.1, 21.3; HRMS calcd for C9H9NO2 163.06333,
found 163.06337.
Dir ect a n d Tr ip let Qu a n tu m Yield s. Direct quantum
yields were measured by a PTI electronic actinometer33
calibrated with standard potassium ferrioxalate actinometry34
at each wavelength. A 1000 W Xe-Hg lamp equipped with a
monochromator resulting in a bandwidth of 2 nm was used.
Sample and actinometer cells were sequentially irradiated, and
from the actinometer cell the photon flux was determined. The
light flux remained constant. Quantification was done by GC.
Biphenyl was used as internal standard. The samples (1 mM
in benzene or acetonitrile) were irradiated to 2-5% conversion.
Each irradiation mixture was analyzed with five separate
injections, and the runs were repeated at least three times.
Triplet quantum yield measurements were performed on
solutions with sensitizer-compound absorbance ratios of ∼100.
Maximum TTET efficiency16,35 was guaranteed by obtaining
a quantum yield-sensitizer concentration-function (satura-
tion curve) for each sensitizer and each compound. Two
different experimental approaches were applied: in method
a, the triplet quantum yields are determined analogous to the
direct quantum yields, whereas in method b, an experimental
arrangement consisting of a Rayonet reactor equipped with
RPR-3500 Å lamps and a combination of a uranium filter and
a solution filter36 (λ50%T ) 360 nm, λ10%T ) 375 nm) in order to
isolate the 350 nm Hg line was used. In method b, ferrioxalate
actinometry was performed sequentially to the irradiation. All
quantum yield determinations were carried out after rigorous
purging with dry nitrogen. The estimated error is (10%.
Tr a p p in g Exp er im en ts. Intermolecular and intramolecu-
lar trapping experiments were performed using a 450 W Hg
lamp equipped with either a Corex or a uranium filter.
Concentrations were ∼10 mM, and in sensitized runs it was
verified that >99% of the light was absorbed by the sensitizer.
In the intermolecular experiments, neat methanol was used
as solvent. Quantification was done by GC or HPLC, and
degassed conditions were applied in all experiments.
2,3-Dim eth yl-5-cya n o-5′-m eth yl-2-cycloh exen on e (7b).
A solution of 2b (0.25 g, 1.5 mmol) in 10 mL of methanol was
degassed with nitrogen for 10 min and irradiated with a 450
W Hanovia Hg lamp equipped with a Corex filter until
complete conversion (GC > 98%). Removal of the solvent in
vacuo and column chromatography (silica gel, ether/hexane
4:1) gave 7b as an oil (54 mg, 22% yield): 1H NMR (CDCl3) δ
Com p u ta tion s. Semiempirical calculations were done
(33) Amrein, W.; Gloor, J .; Schaffner, K. Chimia 1974, 28, 185.
(34) (a) Calvert, J . G.; Pitts, J . N. Photochemistry; Wiley: New York,
1966; p 783. (b) Kuhn, H. J .; Braslavsky, S. E.; Schmidt, R. Pure Appl.
Chem. 1989, 61, 187.
(35) (a) Lamola, A. A.; Hammond, G. S. J . Chem. Phys. 1965, 43,
2129. (b) Saltiel, J .; Marchand, G. J . Am. Chem. Soc. 1991, 113, 2702.
(36) Kopecky, J . Organic PhotochemistrysA visual Approach;
VCH: New York, 1992; p 221.
(29) Zimmerman, H. E.; Lynch, D. C. J . Am. Chem. Soc. 1985, 107,
7745.
(30) Still, W. C.; Kahn, M.; Mitra, A. J . Org. Chem. 1978, 43, 2923.
(31) Schultz, A. G.; Taveras, A. G.; Harrington, R. E. Tetrahedron
Lett. 1988, 29, 3907.
(32) Harmata, M.; Gamlath, C. B.; Barnes, C. L. Tetrahedron Lett.
1990, 31, 5981.