Photochemistry of trans-2,3-Diphenyloxirane
J . Org. Chem., Vol. 62, No. 8, 1997 2411
was further identified by X-ray crystallography.19 The NMR
assignments are as follows: Adduct 1 (rac-(2R,3R,4R,5R)-
Ta ble 1. P seu d o-F ir st Or d er Ra te Con sta n ts Mea su r ed
for th e Rea ction betw een Eith er Ma leic An h yd r id e or
F u m a r on itr ile a n d th e Tr a n s-Ylid e in Eith er Aceton itr ile
or Cycloh exa n e
,4-dicyano-2,5-diphenyltetrahydrofuran): 1H (CDCl
3
7
2
3
) δ 7.6-
.4 (m, 10H, aromatic), 5.6-5.4 (m, 2H, benzylic), 3.6-3.4 (m,
H, methylenic). Adduct 2 (rac-(2S,3R,4R,5R)-3,4-dicyano-2,5-
acetonitrile
cyclohexane
1
diphenyltetrahydrofuran): H (CDCl
3
) δ 7.6-7.4 (m, 10H,
maleic
anhydride
(3.32 ( 0.04) × 10 M-1 s-1 (5.36 ( 0.07) × 10 M s-1
9
9
-1
-1
aromatic), 5.34 (d, J ) 6.7 Hz, 1H, benzylic), 5.15 (d, J ) 7.8
Hz, 1H, benzylic), 3.86 (dd, J ) 6.7 Hz, 5.2 Hz, 1H, methylenic),
fumaronitrile (1.57 ( 0.02) × 10 M s-1 (3.69 ( 0.04) × 10 M s-1
9
-1
9
3
(
.40 (dd, J ) 7.8 Hz, 5.1 Hz, 1H, methylenic). Adduct 3 (rac-
1
2R,3S,4S,5R)-3,4-dicyano-2,5-diphenyltetrahydrofuran):
H
Ta ble 2. Qu a n tu m Yield s for th e F or m a tion of th e
P r od u cts Listed F ollow in g P h otolysis of TDP O in
Cycloh exa n e in th e P r esen ce of Qu en ch er (Q)
(CDCl
3
) δ 7.6-7.4 (m, 10H, aromatic), 5.81 (d, J ) 4.8 Hz, 2H,
benzylic), 3.93 (d, J ) 4.8 Hz, 2H, methylenic). We have been
unable to model the splitting between the benzylic and
methylenic protons in Adduct 1. The NMR spectra of all three
adducts are included as supporting information.
[Q] (M)
Φ
CDPO
Φ
benzaldehyde
Φ
deoxybenzoin
φ
adduct 1
0
0
0
0
0
0.10 ( 0.009 0.47 ( 0.04 0.077 ( 0.002
-
-
-
-
-
.004a
.002a
0
0.44 ( 0.07 0.072 ( 0.006
0.47 ( 0.07 0.077 ( 0.006
0.46 ( 0.07 0.076 ( 0.006
0.45 ( 0.07 0.074 ( 0.006
2
.2. Qu a n tu m Yield s. All samples were agitated rapidly
c
-
-
-
by magnetic stirrer in sealed 1 cm × 1 cm quartz cuvettes
during photolysis. Oxirane solutions were bubbled with argon
for 10 min prior to photolysis. Ferrioxalate solutions, used
for actinometry, were prepared as per the literature.2
Oxirane solutions were 0.01 M TDPO in cyclohexane or
tetradecane, 0.005 M tetradecane or hexadecane as internal
GC standard, 0 to 0.01 M maleic anhydride or fumaronitrile
as quencher. At a 1 cm pathlength, 0.01 M TDPO solution is
optically opaque at 266 nm. The 0.01 M maleic anhydride
solutions in cyclohexane absorbs 266 nm light with an optical
density of 0.04; fumaronitrile in cyclohexane is transparent
at 266 nm.
a
c
c
.0013
.001a
b
0.010
0
0.47 ( 0.07 0.077 ( 0.006 0.099 ( 0.014
0,21
mator (Instruments S.A. Inc, Model H10) and detected as a
function of time by a photomultiplier (Electron Tubes, Inc.,
model 9816, four dynodes wired). The output current of the
photomultiplier is dropped over 50 Ω to ground, and the
resulting voltage is measured with a digital oscilloscope
(Tektronix, Model TDS350) and stored and processed on a
PowerMac 7100.
For these studies, all kinetic traces are modeled as first-
We split the fourth harmonic of a Nd:YAG laser (Quanta-
Ray DCR-I, 266 nm, 6 ns fwhm, 10 Hz) with a series of quartz
flats such that a pair of samples receive roughly equivalent
photon intensities (split ratio ca. 1.2:1). Laser power varied
from 100 to 500 µJ /pulse from photolysis to photolysis and
photolyses were carried to <3% conversion. In the regions
sampled, we measured linear effects of both photon intensity
and conversion on product quantum yields.
order decays, and fits are minimized with the Levenberg-
Marquardt algorithm.24
Samples were prepared so as to exhibit optical densities of
0.3 to 0.6 at 266 nm and were bubbled for 10 min prior to the
experiment with solvent-saturated argon. In agreement with
1
the literature, atmospheric oxygen has no effect on the
lifetimes or quenching rates we measure. We probe the trans-
ylide produced upon photolysis of TDPO at 470 nm. Typically,
the average of five laser pulses yielded acceptable signal to
noise. Samples were not stirred during the experiment.
Concentrations of maleic anhydride or fumaronitrile as
quencher were varied from 0 to 0.01 M in cyclohexane and
acetonitrile. We report the pseudo-first-order rate of quench-
We measured the photoconversion of Fe3+ f Fe by the
2+
2
+
absorption of the 1,10-phenanthroline complex of Fe (HP8452A
diode array spectrophotometer). Each optical density reported
is the average of four measurements. Our uncertainty in the
split ratio is given as one standard deviation from the average
of nine photolyses of ferrioxalate solution and is (6%.
ing (k
as (3.32 ( 0.04) × 10 M
-1
co-workers.1 Errors reported are one standard deviation in
Q
) of the trans-ylide by maleic anhydride in acetonitrile
9
-1 -1
s
, in acceptable agreement with
Yields of photolysis products from oxirane solutions were
measured by gas chromatography (HP5890, J &W Scientific
DB5 or DB17 column, flame ionization detection (FID)). The
peaks due to benzaldehyde, tetradecane, hexadecane, CDPO,
TDPO, deoxybenzoin, and adducts 1, 2, and 3 were identified
by injection of known samples and by GC-MS. FID response
factors were measured relative to tetradecane. Phenylcyclo-
hexylmethane, the product of phenylmethyl carbene insertion
into cyclohexane was identified by GC-MS and by the observa-
tion that photolysis of TDPO in hexanes or heptane yielded
different peaks, identifiable by their masses as carbene inser-
tion into the respective solvents. We synthesized phenylcy-
clohexylmethane by condensation of cyclohexylmagnesium
bromide (Aldrich) with benzyl chloride (Aldrich) in 20% yield.
We were unable to purify the product to greater than 84%
purity, however, and we therefore do not report quantum
yields for this product.
9
-1
the 3.0 × 10
M
s
previously reported by Das and
the linear fit to a Stern-Volmer lifetime analysis.
3. Resu lts
3.1. Qu en ch in g Ra te Con sta n ts. Table 1 lists the
pseudo first order rates (k ) by which maleic anhydride
Q
and fumaronitrile quench the transient absorption as-
signed to the trans-ylide in acetonitrile and cyclohexane.
The value we report for maleic anhydride in acetonitrile
9
-1 -1
agrees with the 3.0 × 10 M
s
reported by Das and
co-workers.1
3.2. Qu a n tu m Yield s. Table 2 lists the quantum
yields both in the absence and presence of quencher for
photolysis of TDPO in cyclohexane. Because two prod-
ucts of the photolysis, benzaldehyde, and deoxybenzoin
Quantum yields reported are the average of three photoly-
ses, two GC injections per photolysis. Reported errors are one
standard deviation from the average of multiple photolyses.
(Scheme 2) have slightly larger extinction coefficients at
2
.3. La ser F la sh P h otolysis. Our laser flash photolysis
2
7
66 nm than does TDPO (ꢀbenz ≈ ꢀdeox ≈ 900; ꢀTDPO
)
2
2
experiment is similar to many reported in the literature and
will therefore only be described briefly. Samples are excited
by the fourth harmonic of a Nd:YAG laser (typically 0.5 to 5
mJ /pulse) and probed by a pulsed high pressure mercury
14,20
40),
we were unable to take the photoreaction to
large enough conversion to confidently measure the
quantum yield of disappearance of TDPO. Product
quantum yields do not add to 1.
2
2,23
lamp.
Probe light wavelength is selected by a monochro-
Upon addition of quencher, under conditions where
CDPO is formed in concentrations too small to quantify,
(19) The author has deposited atomic coordinates for this structure
with the Cambridge Crystallographic Data Centre. The coordinates
can be obtained, on request, from the Director of Cambridge Crystal-
lographic Data Centre, 12 Union Road, Cambridge, CB2 1EZ, UK.
(22) CRC Handbook of Organic Photochemistry; Scaiano, J . C., Ed.;
CRC Press: Boca Raton, 1989.
(
20) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photo-
chemistry; 2nd ed.; Marcel Dekker: New York, 1993.
21) Schwarzenback, G.; Flashka, H. In Complexometric Titrations;
Methuen and Co.: London, 1969; p 235.
(23) Schanze, K. S. News. Inter-Am. Photochem. Soc. 1993, 16, 40-
42.
(
(24) Press, W. H. Numerical Recipes: The Art of Scientific Comput-
ing; Cambridge University Press: New York, 1986.