The Journal of Organic Chemistry
Article
ice−water bath, acetophenone (2.5 mL, 20 mmol) was added. The
cooling bath was removed, and reaction mixture was refluxed for 12 h.
After cooling, the contents of the flask were adsorbed onto silica gel
and directly purified by column chromatography using hexanes as
1
eluent. Colorless solid (436 mg, 15% yield); mp: 135−136 °C; H
NMR (500 MHz, benzene-d6) δ 7.71 (d, J = 7.9 Hz, 1H), 7.67 (d, J =
7.9 Hz, 1H), 7.41−7.36 (m, 2H), 7.30 (dd, J = 7.5, 1.5 Hz, 1H), 7.23
(dd, J = 7.4, 1.6 Hz, 1H), 7.15−7.07 (m, 4H), 7.07−7.01 (m, 1H),
7.01−6.94 (m, 2H), 3.07 (br d, J = 7.8 Hz, 1H), 2.79 (br d, J = 7.9 Hz,
1H), 1.90 (br t, J = 2.0 Hz, 3H); 13C NMR (125 MHz, benzene-d6) δ
139.3, 133.1, 132.6, 129.7, 129.5, 129.1, 128.8, 128.0, 127.9, 127.8,
127.7, 127.5, 126.8, 126.3, 126.0, 124.2, 124.0, 123.5, 25.0, 20.6, 18.2;
FTIR: ν 3063, 2977, 1598, 1484, 1439 cm−1; LRMS (EI): m/z 294
(M+), 279, 178. HRMS: m/z (M+1)+ calcd for C23H19: 295.1487;
found: 295.1483
1-(1-Phenylethylidene)-1a,9b-dihydro-1H-cyclopropa[l]-
phenanthrene-13C (18-13C). The 13C-labeled precursor was prepared
using Ph13COCH3, instead of PhCOCH3, in the procedure described
above and using the same purification method. Colorless solid (131
mg, 13% yield); mp: 127−129 °C; 1H NMR (500 MHz, benzene-d6)
δ 7.73 (dd, J = 8.1, 1.2 Hz, 1H), 7.71−7.67 (m, 1H), 7.39 (m, 2H),
7.31 (dd, J = 7.5, 1.5 Hz, 1H), 7.24 (dd, J = 7.4, 1.5 Hz, 1H), 7.14−
7.08 (m, 4H), 7.08−7.02 (m, 1H), 6.98 (qd, J = 7.6, 1.5 Hz, 2H), 3.09
(br d, J = 8.2 Hz, 1H), 2.81 (br d, J = 7.8 Hz, 1H), 1.90 (dt, J = 6.4, 1.5
Hz, 3H); 13C NMR (125 MHz, benzene-d6) δ 123.98; FTIR: ν 3063,
2977, 1598, 1484, 1439 cm−1; LRMS (EI): m/z 295 (M+), 280, 178.
HRMS: m/z (M + 1)+ calcd for C2213CH19: 296.1520; found:
296.1519.
Figure 2. Two pathways for the rearrangement of 5s-pl into alkyne 6,
computed at CCSD(T)/cc-pVTZ//B3LYP/6-311+G**, showing
relative energies (in kcal/mol), T1 diagnostic values (CCSD),22 and
imaginary frequencies for the transition states.
at various levels of theory reveal that the singlet carbene has
two, nearly isoergetic conformers, both of which are
substantially lower in energy than the triplet. In accord with
observed experimental results, calculations also show that the
lower energy singlet carbene has a significantly smaller barrier
for a phenyl shift compared to that necessary for a methyl shift.
It is conceivable that phenanthrene-based methylene-
cyclopropane precursors, such as those described in this
work, could provide attractive photochemical routes to other
unsaturated carbenes. Research along these lines is currently
underway in our laboratory.
7-(1-Phenylethylidene)bicyclo[4.1.0]heptane (19). 7-Bromo-
bicyclo[4.1.0]heptane23 (1.07 g, 6.1 mmol) was dissolved in anhydrous
THF (80 mL) in an oven-dried round-bottom flask at −70 °C. A
solution of tert-butyllithium (8 mL, 1.7 M in pentanes, 14 mmol) was
added into the flask dropwise, while keeping the temperature below
−60 °C. The reaction was stirred at −70 °C for 30 min, and
bis(cyclopentadienyl)titanium(IV) dichloride (0.72 g, 2.8 mmol) was
added to the reaction mixture. After continued stirring for 5 min at this
temperature, the temperature was raised to 0 °C. Acetophenone (1.5
mL, 12 mmol) was added after 90 min of stirring at between 0 to 5 °C.
Reaction was refluxed at 70 °C for 22 h. The resulted mixture was
purified by silica gel column chromatography using hexanes as the
EXPERIMENTAL METHODS
■
1
General Experimental Procedures. Solvents and all other
reagents used were obtained from commercial sources. Tetrahydrofur-
an (THF) was dried by passage through two columns (2 ft ×4 in) of
activated alumina. Unless otherwise noted, all reactions were carried
out under an argon atmosphere in oven-dried glassware. The synthesis
of exo-1-bromo-1a,9b-dihydro-1H-cyclopropa[l]phenanthrene (17)11b
and 7-bromobicyclo[4.1.0]heptane23 was carried out following
literature procedures. Flash chromatography was performed on an
automated system with prepacked silica gel columns (70−230 mesh).
eluent. Pale yellow liquid (99 mg, 8.2% yield); H NMR (500 MHz,
Chloroform-d) δ 7.66 (dd, J = 8.3, 1.5 Hz, 2H), 7.52−7.36 (m, 2H),
7.27 (d, J = 7.3 Hz, 1H), 2.29 (s, 3H), 2.01 (ddt, J = 8.8, 6.8, 2.0 Hz,
1H), 1.96−1.81 (m, 4H), 1.70 (ddt, J = 8.9, 7.1, 1.6 Hz, 1H), 1.39−
1.26 (m, 3H), 1.26−1.16 (m, 1H); 13C NMR (125 MHz, Chloroform-
d) δ 140.4, 131.5, 128.2, 126.2, 125.4, 122.5, 22.4, 21.9, 21.5, 21.4,
18.8, 15.6, 11.0.; LRMS (EI): m/z 198 (M+), 183. HRMS: m/z (M
+1)+ calcd for C15H19: 199.1487; found: 199.1474.
Procedures for Photolysis. The precursor 18 (100 mg) was
dissolved in benzene-d6 (2 mL) in a glass vial. Photolysis was carried
out for 12 h in a Rayonet photochemical reactor equipped with 16 12-
in. 8 W lamps with output centered at ∼350 nm (range ∼315 to 400
nm). The progress of the photolysis was monitored periodically by
GC/MS. Following photolysis, an internal standard (4-bromoanisole)
was added and the yield of 1-phenylpropyne (6) was determined to be
73%. About 13% of 18 remained unreacted, and 11% had been
converted into nonphotolabile isomers of the precursor. The 13C-
labeled precursor (25 mg) was also photolyzed in benzene-d6 in an
analogous manner but in an NMR tube, and monitored periodically by
NMR spectroscopy. The identity of the alkyne in the photolysate was
confirmed as [2-13C]-1-phenylpropyne by comparison to the
previously reported spectra of this compound.24 For the trapping
experiment, 18 (76 mg) was dissolved in cyclohexene (5 mL) in a
glass vial and photolysis was carried out for 6 h. The identity of the
trapped product, 19, was established by comparing the GC/MS of the
photolysate with that of an independently synthesized authentic
sample. The yield of the adduct was determined to be ∼1.5% using an
internal standard. In all photolysis experiments, phenanthrene was
identified as a byproduct by GC/MS and/or NMR.
NMR spectra were recorded at 500 MHz for 1H and 125 MHz for 13
C
in the indicated solvent. The shifts are reported in δ ppm and
1
referenced to either tetramethylsilane (TMS) (for H NMR spectra)
or the signal from the solvent. FTIR spectra were acquired with an
attenuated total reflectance (ATR) accessory. GC/MS data were
obtained with a capillary gas chromatograph interfaced with a
quadrupole, triple-axis mass selective detector operating in the
electron impact (EI) mode. High resolution mass spectra were
obtained on a direct analysis in-real-time−time-of-flight (DART-TOF)
mass spectrometer. Melting points were recorded on a digital hot plate
and are uncorrected.
1-(1-Phenylethylidene)-1a,9b-dihydro-1H-cyclopropa[l]-
phenanthrene (18). A solution of exo-1-bromo-1a,9b-dihydro-1H-
cyclopropa[l]phenanthrene11b (2.70 g, 10 mmol) in anhydrous THF
(80 mL) was cooled to −70 °C with stirring. Then, tert-butyllithium
(13 mL, 1.7 M in pentanes, 22 mmol) was added dropwise, while
keeping the temperature below −60 °C. After stirring at −70 °C for 30
min, bis(cyclopentadienyl)titanium(IV) dichloride (1.1 g, 4.4 mmol)
was added to the reaction mixture. Stirring was continued at −70 °C
for another 15 min after which the reaction mixture was slowly allowed
to warm up to 0 °C. After 90 min of stirring between 0 and 5 °C in an
D
J. Org. Chem. XXXX, XXX, XXX−XXX