Photochemistry of aliphatic thioketones in the gas phase

Article abstract of DOI:10.1021/jp973360q

The solution and gas-phase photophysical and photochemical properties of a series of bicyclic and alicyclic thioketones (apothiocamphor (1), thiocamphor (2), thiofenchone (3), endo-5,6-trimethylene-2-norborneanthione (4), 3,3-diethylbicyclo[3.2.1]octane-2-thione (5), 2,2-diethyl-5,5-dimethylcyclopentanethione (6), 2-ethyl-2,6,6-trimethylcyclohexanethione (7), and 2,4,4-trimethyl-3-hexanethione (8)) are reported. Photolysis in solution typically gives rise to products arising from insertion into β, γ, and, in one case (4), δ carbons to form cyclic thiols. This chemistry is analogous to that observed in earlier studies. Novel photochemistry is found in the gas phase where Norrish type II products are also isolated from several substrates (1, 2, 5, 6, and 7). The effect of the quencher gas, butane, on both the spectral and photochemical properties of 2 in the gas phase provide evidence to support the proposal that the Norrish type II chemistry arises from initially populated vibrationally excited levels of S₂.

Full text of DOI:10.1021/jp973360q

J. Phys. Chem. A 1998, 102, 5421-5432
5421
Photochemistry of Aliphatic Thioketones in the Gas Phase1,2
Harry Morrison,* Yuelie Lu, and Dean Carlson
Department of Chemistry, Purdue UniVersity, West Lafayette, Indiana 47907-1393
ReceiVed: October 15, 1997; In Final Form: December 31, 1997
The solution and gas-phase photophysical and photochemical properties of a series of bicyclic and alicyclic
thioketones (apothiocamphor (1), thiocamphor (2), thiofenchone (3), endo-5,6-trimethylene-2-norborneanthione
(4), 3,3-diethylbicyclo[3.2.1]octane-2-thione (5), 2,2-diethyl-5,5-dimethylcyclopentanethione (6), 2-ethyl-2,6,6-
trimethylcyclohexanethione (7), and 2,4,4-trimethyl-3-hexanethione (8)) are reported. Photolysis in solution
typically gives rise to products arising from insertion into â, γ, and, in one case (4), δ carbons to form cyclic
thiols. This chemistry is analogous to that observed in earlier studies. Novel photochemistry is found in the
gas phase where Norrish type II products are also isolated from several substrates (1, 2, 5, 6, and 7). The
effect of the quencher gas, butane, on both the spectral and photochemical properties of 2 in the gas phase
provide evidence to support the proposal that the Norrish type II chemistry arises from initially populated
vibrationally excited levels of S2.
TABLE 1: Spectroscopic Data for the Aliphatic Thioketone
and Ketone Groups
Introduction
The photochemistry of thiocarbonyl compounds has received
increasing attention over the last two decades.3 This is largely
due to the propensity of the electronically excited states of
thiocarbonyl compounds to undergo photochemical reactions
unlike those observed from structurally analogous carbonyl
compounds. “Classic” examples include the reaction of an
excited thioketone with oxygen4 and the photodimerization of
thiophosgene.5 More recent systematic investigations on aro-
matic, aryl alkyl, and alicyclic thiocarbonyl compounds have
demonstrated a wide variety of reactions in solution, including
cycloaddition, cyclization, dimerization, oxidation, and reduc-
tion.3
Perhaps the most dramatic difference between the thiocar-
bonyl and carbonyl functionalities is the wavelength-dependent
photochemistry that characterizes, for example, thioketones. A
study by Oster and co-workers6 of both the photochemical
oxidation and reduction of thiobenzophenone contained the first
report of wavelength dependence in the solution photochemistry
of a thiocarbonyl compound. As shown in Table 1, aliphatic
thioketones exhibit two distinct absorption bands that occur at
ca. 240 and 490-555 nm. The former is intense and is
attributed to an allowed π f π* transition (S0 f S2).7 The
latter is responsible for the red color of thioketones and is
attributed to the dipole-forbidden n f π* transitions (S0 f S1
and S0 f T1).7 These differences between the ketone and
thioketone functional groups in both the energies and the
spacings of their excited states leads to substantial differences
between them in their photochemical and photophysical proper-
ties. For small thioketones the relatively large electronic energy
gaps (40-50 kcal/mol) between the S1 and the S2 states lead to
slow S2 radiationless decay and relatively long-lived S2 states.
The S2 states of larger aliphatic thioketones are weakly emissive
and have lifetimes that are too short to be measured. Steer and
co-workers have estimated that the upper state lifetimes are less
than 1 ps, based on the small quantum yields of S2 f S0
fluorescence in perfluoroalkanes and the radiative lifetimes
derived from the fluorescence using the Strickler-Berg equa-
tion.8
λ
max (nm) ꢀmax (L/mol‚cm)
λmax (nm) ꢀmax (L/mol‚cm)
190
∼1000
270-300
16-25
C
C
O
S
240
∼10000
490-555
2-10
Our own attention was drawn to this area through our interest
in the photochemistry from upper excited states that can
oftentimes be observed uniquely in the gas phase.9 In fact,
surprisingly little attention has been given to the gas-phase
photochemistry of thiocarbonyl compounds with the exception
of some small molecules such as thiophosgene.10 We therefore
undertook exploration of the gas-phase photochemistry and
photophysics of aliphatic cyclic thioketones under a variety of
conditions to determine in what ways this photochemistry
differed from (and perhaps helped rationalize) that observed in
solution. The thioketones discussed herein are apothiocamphor
(1), thiocamphor (2), thiofenchone (3), endo-5,6-trimethylene-
2-norborneanthione (4), 3,3-diethylbicyclo[3.2.1]octane-2-thione
(5), 2,2-diethyl-5,5-dimethylcyclopentanethione (6), 2-ethyl-
2,6,6-trimethylcyclohexanethione (7), and 2,4,4-trimethyl-3-
hexanethione (8).
S1089-5639(97)03360-4 CCC: $15.00 © 1998 American Chemical Society
Published on Web 02/14/1998

5422 J. Phys. Chem. A, Vol. 102, No. 28, 1998
Morrison et al.
TABLE 2: UV-Vis Absorption Maxima for Aliphatic
Thioketones
TABLE 3: Summary of Emission Maxima for Aliphatic
Thioketones
wavelength maxima,
wavelength maxima (nm)
compd
conditions
nm (log ꢀ)
compd
conditions
S2
∆λa
T2
T1
1
cyclohexane
215 (3.72)
241 (3.9)
1
2
cyclohexane
PFMC
vapor
cyclohexane
cyclohexane
cyclohexane
cyclohexane
cyclohexane
cyclohexane
323
342
298
343
324
319
325
324
322
82
87
39
103
82
76
85
84
87
580
569
625
608
630
603
622
647
631
680
647
499 (1.09)
562 (0.26)
213 (3.76)
240 (4.08)
490 (7.08)
554 (0.02)
219 (A ) 0.014)
235 (A ) 0.009)
216 (3.61)
240 (4.00)
487 (1.12)
548 (0.19)
213 (3.71)
242 (4.03)
500 (1.08)
548 (0.19)
243 (3.92)
517 (1.07)
210 (3.66)
240 (3.96)
503 (1.08)
569 (0.06)
240 (3.91)
534 (1.04)
213 (3.94)
235 (4.14)
517 (1.33)
204 (A ) 0.179)
218 (A ) 0.214)
230 (A ) 0.173)
3
4
5
6
7
8
570
576
603
590
631
607
2
pentane
vapor
a Stokes’ shift.
3
4
cyclohexane
cyclohexane
5
6
cyclohexane
cyclohexane
7
8
cyclohexane
cyclohexane
vapor
Figure 2. Total emission spectra of thiocamphor in the gas phase (solid
line) and in perfluoromethylcyclohexane (dotted line): “a” ) Fluo-
rescence; “b” ) phosphorescence.
Figure 1. UV-vis absorption spectra of thiocamphor in pentane (solid
line) and in the gas phase (dotted line).
Figure 3. Excitation and absorption spectra of thiocamphor vapor.
Results
latter includes emission from two triplets, with thermally
activated phosphorescence appearing as a shoulder on the high-
energy portion of the “normal” phosphorescence band.8b,11 The
data are given in Table 3, and the full spectrum for 2 is shown
in Figure 2.
The fluorescence excitation spectrum for 2 in the gas phase
is presented in Figure 3. The enhancement of emission on the
red edge of the absorption curve is obvious. We have also
studied the effect of an inert quencher gas on the gas-phase
emission, and these results are shown in in Figure 4. Excitation
spectra with and without butane are shown in Figure 5.
Photophysical Studies. UV-Vis Absorption Spectra. The
two transitions noted in the Introduction are observed for each
of the substrates; the band maxima in hydrocarbon media are
listed in Table 2, and the spectrum for 2 is displayed in Figure
1. Only the UV absorption bands are observed in the gas-phase
spectra of, for example, compounds 2 and 8.
Total Emission Spectra. Excitation of the thioketones into
S2 (260 nm), at room temperature, gives rise to both S2
fluorescence (300-450 nm in solution, 265-320 nm in the gas
phase) and phosphorescence (500-800 nm in both media). The

Photochemistry of Gas-Phase Aliphatic Thioketones
J. Phys. Chem. A, Vol. 102, No. 28, 1998 5423
pentamethyltricyclo[2.2.1.02,6]heptane-2-thiol) with yet lower
quantum efficiencies of Φdis ) 0.044 and Φpdt ) 0.040.
Photolysis of 4 in Pentane. Photolysis of a deoxygenated
3.2 mM solution of 4 as described above gave complete
conversion to a mixture of one major product (4A) containing
a small amount (2.5%) of the ketone corresponding to 4. The
major product is assigned as tetracyclo[3.2.2.1.02,9]nonane-2-
thiol on the basis of its spectral data (cf. eq 3).
Figure 4. Total emission spectra of thiocamphor vapor with (dotted
line) and without (solid line) butane.
Photolysis of 5 in Pentane. Photolysis of a deoxygenated
13 mM solution of 5 as described above gave an 81% conversion
to three major products in a ratio of 3.3:1.2:1.0. The major
and minor products (5A and 5C) were assigned, on the basis
of their spectral data, as 3,3-diethyltricyclo[3.2.1.02,7]octane-2-
thiol and 2,3-dimethylene-3-ethylbicyclo[3.2.1]octane-2-thiol,
respectively. Only GC-MS data are available for compound
5B because it could not be obtained in sufficient purity for NMR
spectral analysis. It is an isomer of 5 and considered likely to
be 3-ethyl-9-methyltricyclo[3.2.1.12,3]nonane-2-thiol (cf. eq 4).
Figure 5. Excitation spectra of thiocamphor vapor with (dotted line)
and without (solid line) butane.
Photochemical Studies. Solution Photolyses. Photolysis of
Apothiocamphor (1). Photolysis of a deoxygenated 6.98 mM
solution of 1 at room temperature in pentane in a quartz tube
using 254-nm light led to a 99% conversion to one major
product, 7,7-dimethyltricyclo[2.2.1.02,6]heptane-2-thiol (1A),
analogous to the tricyclenes previously reported12 (eq 1).
Photolysis of 6 in Pentane. Photolysis of a deoxygenated
13 mM solution of 6 as described above gave a 93% conversion
to two products in a ratio of 1.8:1.0. These were assigned on
the basis of their spectral data as 2-ethyl-5,5,6-trimethylbicyclo-
[3.1.0]hexane-1-thiol (6A) and 2-ethyl-5,5-dimethylbicyclo-
[3.2.0]heptane-1-thiol (6B), respectively (cf. eq 5).
Photolysis of Thiocamphor (2) and its Analogues. Photolysis
of a 7.29 mM solution of 2 as described above for 1 gave a
96% conversion to one major product, 1,7,7-trimethyltricyclo-
[2.2.1.02,6]heptane-2-thiol (2A), as previously reported12 (eq 2).
The quantum efficiencies of loss of starting material and
formation of 2A were determined to be 0.089 and 0.081,
respectively. The 3-endo-methylthiocamphor (2a) was photo-
lyzed in a like manner and gave the corresponding cyclopro-
panethiol (3-endo-methyl-1,7,7-trimethyltricyclo[2.2.1.02,6]heptane-
2-thiol). In this case, the quantum efficiencies were slightly
lower, Φdis ) 0.056 and Φpdt ) 0.051. 3,3-Dimethylthiocam-
phor (2b) similarly provided the cyclopropanethiol (1,3,3,7,7-
Photolysis of 7 in Pentane. Photolysis of a deoxygenated
13 mM solution of 7 as described above gave complete
conversion to a mixture of three products in a ratio of 5.6:2.0:
1.0. Complete spectral and analytical data are available for 7a
and 7B, and these products are assigned as 2,6,6,7-
tetramethylbicyclo[4.1.0]heptane-1-thiol (7A) and 2,6,6-
trimethylbicyclo[4.2.0]octane-1-thiol (7B). Only GC-MS data

5424 J. Phys. Chem. A, Vol. 102, No. 28, 1998
Morrison et al.
are available for 7c, but the presence of a M - CH2CH3
fragment ion supports the assignment as 2-ethyl-2,6-
dimethylbicyclo[4.1.0]heptane-1-thiol (7C) (cf. eq 6).
No products were detected by capillary GC when ∼650 mTorr
of 2 was photolyzed with a medium-pressure mercury lamp (λ
> 340 nm) for 18.23 h.
When the 3-endo analogue (2a) was photolyzed as described
for 1, the corresponding tricyclic thiol and ring-opened cyclo-
hexene were formed in a ratio of 2:1, respectively. The same
ratio of products was obtained when the 3-methyl group was
fully deuterated. 3,3-Dimethylthiocamphor (2b) gave primarily
the dimethyl tricyclic thiol with only 3% of the dimethylated
cyclohexene.
Photolysis of 3. Compound 3 was photolyzed as described
for 1 to produce a single product with conversions of ca. 15%.
The product was isolated from a preparative photolysis and
1
assigned by H NMR spectroscopy as 1,3,3-trimethyltricyclo-
[2.2.1.02,6]heptane-2-thiol, (the solution-phase product previously
reported).12
Photolysis of 8 in Pentane. Photolysis of a deoxygenated
13 mM solution of 7 as described above gave a 95% conversion
to two major products in a ratio of 1.2:1.0. Compound 8A was
assigned on the basis of its spectral data as 2,2,3-dimethyl-1-
isopropyl-1-cyclopropylthiol. Compound 8B showed a molec-
ular weight corresponding to the reduction product (cf. eq 7).
Photolysis of 4. Compound 4 was photolyzed as described
for 1 to give a mixture of products with an overall conversion
of 66%. One product, constituting 70% of the mixture, was
identified as 4A by comparison of its GLC retention time to
the solution-phase product. In addition, two products (4B and
4B′) were formed as 16% of the mixture; GC-mass spectral
analysis confirmed that these were isomers of 4 and, on the
basis of the MS fragmentation pattern, they are postulated to
be 3-cyclopenten-3′-yl-1-cyclopentene-1-thiol and 3-cyclopen-
tenyl-1-cyclopentene-1-thiol. An additional minor component
(10.3%) was identified by GC-MS as a reduction product,
presumed to be 4C. The results are summarized in eq 10.
Gas-Phase Photolyses. Photolysis of 1. Typically 10-20-
mg samples of 1 were photolyzed under “flowing conditions”
(see Experimental Section) in a modified Rayonet reactor at
254 nm at room temperature. The photolysis produced two
products in a ratio of 1.0:1.1 with conversions of ca. 49%. The
products were isolated from a preparative photolysis, analyzed
1
by H NMR spectroscopy and by comparison of GC retention
time with the solution-phase product, and identified as 1A and
5-isopropenyl-1-mercapto-1-cyclohexene (1B) (cf. eq 8).
Photolysis of 5. Compound 5 was photolyzed as described
for 1 and gave a 73% conversion to five products (5A-E) in a
ratio of 6.2:3.4:4.4:13.8:1.0. Products 5A, 5B, and 5C were
identified by comparison of GLC retention times with the
solution-phase products (vide supra). Product 5D is only formed
in the gas phase and was assigned as 3-ethylbicyclo[3.2.1]oct-
2-ene-2-thiol by 1H NMR spectroscopy. Compound 5E is also
unique to the gas phase and an isomer of 5, but we are unable
to determine its structure at this time. The results are sum-
marized in eq 11.
Photolysis of Thiocamphor (2) and Its Analogues. Compound
2 was photolyzed as described for 1 and produced two products
in a ratio of 1.0:1.0. Conversions were typically ca. 50%. The
products were isolated from a preparative photolysis, analyzed
by H NMR spectroscopy, and identified as 2A and 5-isopro-
penyl-2-methyl-1-mercapto-1-cyclohexene (2B) (cf. eq 9).
1
A “static” photolysis of 2 at ∼800 mTorr (see Experimental
Section) with 254-nm light produced no new products but
changed the ratio of 2A:2B from 1:1 to 8:1. Photolysis of 2 at
∼800 mTorr in the presence of 50 Torr of n-butane, also with
254-nm light, further changed the ratio of 2A:2B from to 10:1.

Photochemistry of Gas-Phase Aliphatic Thioketones
J. Phys. Chem. A, Vol. 102, No. 28, 1998 5425
Photolysis of 6. Compound 6 was photolyzed as described
for 1 and gave a 37% conversion to four products (6A-D) in
a ratio of 2.7:3.1:1.0:1.1 Products 6A and 6B were identified
by 1H NMR spectroscopy and by a comparison of GLC retention
times with the solution-phase products. Compound 6C was
assigned as 2-ethyl-5,5,-dimethyl-1-cyclopentene-1-thiol on the
basis of its NMR and mass-spectral data. Compound 6D could
not be isolated in sufficient purity for structure determination.
GC-mass spectral analysis indicated it to be an isomer of 6,
and we believe it likely to be the product of insertion into one
of the geminal methyl groups. The results are summarized in
eq 12.
550-600 nm combines an S0 f Tn transition superimposed on
the red edge of a very broad S0 f S1 transition. Both of these
have been ascribed to n f π* transitions.8b,11c For the substrates
5, 7, and 8 the S0 f S1 transition maximum is red-shifted with
respect to the norbornane system and a distinguishable singlet
to triplet transition is not observed. Likewise, singlet-to-triplet
transitions are, in general, not observed in the gas phase because
of their extremely weak intensities. The gas-phase spectra for
compounds 2 and 8 reveal additional transitions at ca. 218-
219 nm, which, by analogy with thioacetone and other aliphatic
thioketones, are assignable to the n f 4s Rydberg transition.3a
These obvious features are missing from the spectra taken in
solution, but the solution spectra do exhibit broad features on
the short-wavelength side of the π f π* transition that are likely
to be of Rydberg character. Given the 254-nm wavelength used
in the photochemical studies, these higher singlet states are
unlikely to play a role in the photochemistry described in this
work.
The total emission spectra were obtained via excitation into
the S2 manifold at 260 nm, and the results are summarized in
Table 3. As has been observed by others for such alicyclics,8b,11c
emission from S2 in these compounds is associated with large
Stokes’ shifts because of the significant change in bond order
associated with excitation into this state. Fluorescence from
S1 is too weak to be measured, presumably the consequence of
its rapid intersystem crossing to the triplet manifold.3g Strong
phosphorescence is observed from all of the thioketones. A
shoulder on the blue edge of the room temperature spectrum
can be assigned to thermally activated emission by analogy with
this assignment from temperature-dependent emission
measurements.11c This emission has been attributed to thermally
activated emission from T2, which has been placed ca. 500 cm-1
above T1 in nonpolar solvents.11c The solution-phase absorption
and emission spectra of adamantanethione, thiocamphor (2), and
thiofenchone (3) have been analyzed in detail by Falk and
Steer.8b,11c In summary, they observe that the S2 state is weakly
emissive (φf values are ca. 3-17 × 10-5) and extremely short-
lived. Phosphorescence quantum efficiencies are ca. 0.02-0.06
when extrapolated to infinite dilution, with T1 and T2 closely
spaced and both lying below S1. The location of the π, π* triplet
Photolysis of 7. Compound 7 was photolyzed as described
for 1 and gave a 26% conversion to four products (7A-D) in
a ratio of 0.7:0.2:0.1:1.0. Compounds 7A-7C were shown to
1
be identical to those formed in solution by H NMR spectros-
copy or by comparison of capillary GC retention times. The
gas-phase product (7D) was identified as 2,6,6-trimethyl-1-
1
cyclohexene-1-thiol based on H NMR and GLC-mass spec-
trometry. The results are summarized in eq 13.
at this level is indicative of a very large (ca. 19 000 cm-1
)
singlet/triplet splitting for alicyclic thioketones.8b,11c On the
basis of phosphorescence lifetime studies,11c these workers
conclude that the rate of T1/T2 interconversion is very rapid,
with the two states in thermal equilibrium at room temperature.
The gas-phase excitation and emission spectra of thiocamphor
(2) upon S2 excitation (260 nm) at room temperature are shown
in Figure 2 and serve as a prototype for the aliphatic thioketones.
As in solution, one observes S2 fluorescence as well as
phosphorescence consisting of “normal” and thermally activated
emission. It is particularly noteworthy that the S2 fluorescence
dominates the total emission spectrum in the gas phase, by
contrast with the phosphorescence-dominated emission in solu-
tion. As noted in the Introduction, weak emission from the S2
state in solution is a consequence of its very short (subpico-
second) lifetime. Clearly, one or more of the radiationless decay
paths which contribute to such a short lifetime are significantly
inhibited in the gas phase. Note that both states are potentially
susceptible to quenching by the hydrocarbon solvent.13
Photolysis of 8. Compound 8 was photolyzed as described
for 1 and gave one major product. This was identified as 8A,
a product obtained from the solution-phase photolysis (vide
supra). The results are summarized in eq 14.
Discussion
Photophysics. The wavelengths and intensities of the singlet
and triplet transitions in the absorption spectra of the substrates
studied in this work are summarized in Table 2. We have noted
in the Introduction that it is typical for such thioketones to have
an absorption band with a λmax at ca. 240 nm due to the S0 f
S2 (π f π*) transition. The weak long-wavelength feature at
A related set of observations involves the fluorescence
excitation spectrum of 2 and the effect of the quenching gas,
butane, on this spectrum. The maximum of the fluorescence
excitation spectrum of 2 in the gas phase is red-shifted about
20 nm relative to the S0 f S2 absorption band (cf. Figure 3).

5426 J. Phys. Chem. A, Vol. 102, No. 28, 1998
Morrison et al.
Taken at face value, this would indicate that, in the absence of
solvent, the upper vibrational levels of the S2 state are not readily
deactivated to the lower fluorescent levels. Obviously, such a
mismatch between absorption and excitation spectra raises
concerns about the possible role of impurities in the latter.
However, S2 fluorescence is significantly enhanced, but phos-
phorescence relatively less so, by the addition of butane (Figure
4) and there is an associated butane-induced blue-shift in the
gas-phase excitation spectrum (Figure 5). It is reasonable to
assume that the butane is enhancing fluorescence by collisional
deactivation of initially populated upper vibrational levels.
Analogous observations have been made with some small
molecular weight thiocarbonyls such as thiophosgene.14 In
summary, though emission from S2 is more readily observed
in the gas phase than in solution, there are efficient radiationless
decay pathways available to the upper vibrational manifold of
this state that deactivate S2 at higher excitation energies.
studies indicate that excitation into S2 ultimately leads to the
triplet manifold with an efficiency comparable to that obtained
from S1, Falk and Steer likewise rule out direct intersystem
crossing from S2 f Tn since both T1 and T2 lie below S1 and
therefore should be accessed with equally slow radiationless
decay rates. Since sensitization and quenching studies appear
to rule out a triplet precursor to the homothienols, the necessary
conclusion is that S2 rapidly transforms into a species that these
workers designate as “X”. They propose that it is this species
that goes on to product, decays to S1 or Tn, or rapidly decays
back to starting material. This intermediate is thought to be
equivalent to a ca. 100-200-ps singlet intermediate that de
Mayo had deduced from quenching and sensitization experi-
ments as preceding the intermolecular photochemistry of
adamantanethione.18 Falk and Steer8b analyzed the intramo-
lecular insertion to homothioenol by adapting a natural correla-
tion analysis used by Bigot19 to study the intermolecular
hydrogen atom abstraction by thioformaldehyde from methane.
Their analysis indicates that there is a minimal barrier between
S2 and X and that X ultimately both correlates with a diradical
product and intersects with the T2 surface.
Photochemistry. Solution. The photolysis of the aliphatic
thioketones 1-8 in pentane leads primarily to net â-insertion
reactions to form cyclopropanethiols (cf. eqs 1, 2, and 4-7).
In one case (eq 7) we observe reduction of the thioketone to
the mercaptan, a reaction that de Mayo has attributed to
secondary photochemistry on the basis of studies at varied
conversions.12b The insertion chemistry has been extensively
described by de Mayo.12 In general, the quantum efficiencies
of ca. 4-9% are comparable to those determined for insertion
chemistry in a number of aryl alkyl thioketones.15b,16,17
Gas Phase. By contrast with the relatively extensively
explored photochemistry of these thioketones in solution, to our
knowledge the gas-phase experiments outlined above are the
first studies of molecules of this complexity in this medium. In
that vein, the results shown in eq 8 are quite striking; i.e.,
excitation of apothiocamphor (1) into S2 is found to lead not
only to the insertion product (1A) but also to a new ring-opened
product (1B). The two products are formed in equivalent
amounts and with quite reasonable conversions. A similar ring-
opening reaction is observed upon gas-phase photolysis of
thiocamphor (2), the solution-phase homothioenol (2A) and the
“limonene-like” thioenol (2B) again being formed in equal
amounts (eq 9). Thioketones giving analogous hydrogen
abstraction chemistry were 5 (cf. 5D as the major product in eq
11), 6 (cf. 6C in eq 12) and 7 (cf. 7D as the major product in
eq 13). Most striking is the fact that each of these products
may be thought of as resulting from the equivalent of a Norrish
type II fragmentation, hitherto unobserved in the alicyclic
thioketones and rare among other thiocarbonyl substrates.3,16b
The propensity for â insertion is quite strong, and previous
workers have reported that this chemistry occurs exclusively
even in norbornanethiones containing ethyl, propyl and butyl
groups at the 3-position.12b This contrasts with aryl alkyl
thioketones for which δ insertion is actually preferred.15 In fact,
the somewhat more flexible 3:2:1 substrate 5 does give some
γ-insertion product to form a cyclobutanethiol (cf. eq 4), and
this chemistry is also seen for the cyclopentanthione 6 and the
cyclohexanthione 7 (eqs 5 and 6, respectively). The latter
observations are analogous to those in an earlier report wherein
cyclobutanethiols were isolated from R-butyl cyclopentane- and
cyclohexanethiones.12b As one might expect, for 6 and 7 we
observe â insertion to be dominant in competition with γ
abstraction of the primary hydrogen of the methyl group,
whereas de Mayo found γ insertion into the secondary meth-
ylene to be dominant. The chemistry reverts to exclusive â
insertion in the acyclic substrate 8 (eq 7).
We have already alluded to the interesting effects of butane
as a quencher gas on the fluorescence-emission properties of
the S2 state. Butane was also added to the thiocamphor (2)
“static” photolyses, and the results are consistent with the
spectral observations as well as with the fact that, as in solution,
there is no observable photochemistry upon excitation of 2 into
S1. Thus, the addition of 50 Torr of butane selectively quenched
the formation of the gas-phase product 2B, changing the ratio
of 2A:2B from 1:1 to 10:1. A similar effect was observed when
the pressure of 2 was increased in the absence of butane. Thus,
though we cannot be sure that we ever achieved a sufficiently
“collision-free” environment to totally eliminate deactivation,
the product ratio observed under “flowing conditions” maxi-
mized the amount of “gas-phase” product under the several
photolysis conditions we employed. Clearly, the precursor of
the type II product is sufficiently long-liVed to be Vibrationally
relaxed by collision with the thioketone ground-state or foreign
gases. A calculation based on the kinetic theory of gases
indicates that the time between collisions of a molecule of 1
Torr of thiocamphor vapor under the static photolysis conditions
is ca. 10.3 ns. This is reduced to ca. 2.3 ns by the addition of
50 Torr of butane.
Of particular interest is the observation that photolysis of 4
only gives the δ-insertion product, 4A, with no trace of a
γ-insertion product, despite the availability of the appropriate
hydrogens (eq 3). The NMR spectra for this compound were
relatively difficult to assign due to several instances of chemical
shift near-coincidence in both the proton and carbon spectra.
Although at first puzzling for such a small molecule, an
explanation for the complication became apparent once the
structure took shape. If one rotates a model of the compound
120° about an axis involving what was originally C3 and C6,
another norbornyl structure is obtained such that the overall
structure is, excepting the position of the mercaptan, the mirror
image of the original (see details in the Experimental Section).
The mechanistic details of the solution-phase photochemistry
have been well-summarized by Falk and Steer.8b Key is the
fact that this chemistry is not generated by excitation into S1,
an observation that we have confirmed in the current study.
Furthermore, though the S2 lifetime is too short to be measured,
it is unlikely to be decaying with rapidity to S1 because of the
large energy gap between these states. Though phosphorescence
In summary, the collisional enhancement of S2 fluorescence
is accompanied by the quenching of gas-phase photoproduct.

Photochemistry of Gas-Phase Aliphatic Thioketones
J. Phys. Chem. A, Vol. 102, No. 28, 1998 5427
SCHEME 1: Proposed Mechanism of Photolysis of
Thiocamphor Vapor
Figure 6. UV absorption spectra of 3,3-diethylbicyclo[3.2.1]octane-
2-thione (5, “B”) and 3-ethylbicyclo[3.2.1]oct-2-ene-2-thiol (5D, “A”)
in cyclohexane.
hexane, and pentane were from Burdick & Jackson; they were
stored under argon and used without further purification.
Perfluoromethylcyclohexane (PCR) was distilled under nitrogen.
Tetrahydrofuran and diethyl ether used in synthetic reactions
were distilled under nitrogen from sodium benzophenone ketal.
Benzene was distilled under nitrogen from sodium. Nitrogen,
hydrogen, and argon were obtained from Airco. Hydrogen
chloride (99.0% pure), and hydrogen sulfide (99.0% pure) were
from Matheson and used as received. The synthesis of hitherto
unknown substrates are detailed below; synthetic details and
spectral data for all others are available in the doctoral
dissertation of Y.L.2b
Gas Chromatography. Analytical separations were per-
formed on a capillary gas chromatograph (Varian model 3700)
equipped with a flame-ionization detector and a Hewlett-Packard
model 3390A integrator. The chromatograph was operated with
helium, hydrogen, and air flow rates of 0.8-1.0, 30, and 240
mL/min, respectively. The following 0.25-µm capillary columns
from J & W Scientific were used in this work: A, 15 m × 0.25
mm i.d., DB-1; B, 12.5 m × 0.25 mm i.d., DB-1; C, 10 m ×
0.25 mm i.d., DB-1. Preparative separations were performed
on a Varian model 3300 gas chromatograph equipped with a
thermal conductivity detector and Hewlett-Packard model 3390A
or 3393A integrators. Helium was used as the carrier gas at
flow rates of 40-60 mL/min. The following columns were used
in this work: E, 5 ft × 0.25 in., 15% SE-30 on 40/60 AW-
DMCS Chromasorb W; F, 10 ft × 0.25 in., 10% SE-30 on 40/
60 AW-DMCS Chromasorb W; G, 10 ft × 0.25 in., 5% OV-17
on 40/60 AW-DMCS Chromasorb W.
Adsorption Chromatography. Analytical thin-layer chro-
matography was performed on silica gel 60A, 0.25 mm, coated
on glass support from Whatman. For sulfur-containing com-
pounds a special spray reagent consisting of 0.5% aqueous PdCl2
with a few drops of concentrated HCl was used for visualiza-
tion.24 Flash chromatography utilized silica gel 60 (E. Merck
9285, 230-400 mesh) according to the published method.25
Column chromatography was performed on 60-200 mesh silica
gel (EM Science).
Spectroscopy. Infrared spectra were recorded on either a
Perkin-Elmer model 1420 Ratio Recording spectrophotometer
or a Perkin-Elmer 1800 Fourier Transform infrared spectro-
photometer. Routine 1H NMR spectra were acquired on a
General Electric QE 300 MHz spectrometer. Proton spectra
The simplest interpretation of these facts would be that the gas-
phase photofragmentation originates from S2vib. In fact, it may
well be (reversible?) chemistry along the fragmentation pathway
vib
that leads S2 back to the ground state bypassing S2. Con-
versely, since â insertion is specific to S2 excitation, is observed
in solution, and is enhanced relative to type II fragmentation
by collision in the gas phase, this chemistry must clearly arise
from vibrationally relaxed S2.20,21 This proposal is outlined in
Scheme 1.
An interesting consequence of our discovery of the gas-phase
type II photochemistry is that we have been able to prepare
thioenols uncontaminated by the corresponding thioketones and
to characterize them by UV and NMR spectroscopies. Aliphatic
thioenols are tautomerically stable isomers of thioketones, but,
in contrast with the oxygen enol series, thioenols have only
previously been prepared as mixtures with their isomeric
thioketones.22 For example, the UV absorption spectra of 5
and 5D are displayed in Figure 6. The blue-shifted absorption
band with a maximum at 220 nm in 5D is characteristic of the
thioenol double bond.23 Thus, the gas-phase Norrish type II
photofragmentation reaction constitutes a potentially syntheti-
cally useful method to prepare alicyclic thioenols for further
study. Such studies are in progress.
Experimental Section
The detailed experimental procedures for this work may be
found in the doctoral dissertation of Y.L.2b The salient features
are summarized below.
Materials. The following chemicals from Aldrich were used
as received and were stored at room temperature unless
otherwise noted: (1R)-(+)-camphor; (1R)-(-)-fenchone; (1R)-
(+)-nopione; endo-dicyclopentadiene; platinum(IV) oxide;
iodomethane, stored at -20 °C; iodoethane; 2.0 M lithium
diisopropylamide in heptane/tetrahydrofuran/ethylbenzene, stored
at -20 °C; 2,2-dimethylcyclopentanone; 2,2,6-trimethylcyclo-
hexanone; 2,4-dimethyl-3-pentanone; bicyclo[3.2.1]octane-2-
one; 1.0 M boron trichloride in p-xylene, stored at -20 °C;
hexamethyldisilathiane; phosphorus pentasulfide; pyridinium
chlorochromate. Boron sulfide (Alfa) was stored at room
temperature and used as received. Spectrograde cyclohexane,

5428 J. Phys. Chem. A, Vol. 102, No. 28, 1998
Morrison et al.
were acquired in either deuterated chloroform or deuterated
benzene (both from Aldrich) relative to tetramethylsilane at 0.0
ppm. Benzene as solvent causes large changes in the observed
chemical shifts of the solute when compared with spectra run
in chloroform. Routine carbon-13 and APT26 (“Attached Proton
Test”) spectra were acquired on the QE-300 operating at 75.6
MHz. Carbon chemical shifts were recorded relative to either
CDCl3 at 77.0 ppm or C6D6 at 128.0 ppm. Decoupling studies
were performed on the QE-300 spectrometer using a decou-
pling field of ca. 30 Hz for proton spectra and 3.54 Hz for carbon
spectra. High-resolution proton spectra and carbon-13 spectra
were obtained on Varian VXR 500 MHz or VXR 600 MHz
instruments. Routine mass spectra were recorded on a Finnigan
4000 mass spectrometer operating with a source temperature
of 250 °C and interfaced to a gas chromatograph containing a
30 m × 0.25 mm i.d., 0.25 µm DB-1 capillary column (J & W
Scientific). Electron impact (EI) mass spectra were recorded
at 70 eV. Chemical ionization (CI) spectra were recorded at
70 eV with isobutane gas at a pressure of 0.30 Torr. High-
resolution mass spectra were obtained on a Kratos model MS-
50 instrument operated at 70 eV for EI spectra. Ultraviolet
spectra were recorded on a Perkin-Elmer model Lamda 3B
spectrophotometer that was interfaced to a microcomputer
controlled by Perkin-Elmer Computerized Spectroscopy Soft-
ware (PECSS). Solution absorbance measurements were ob-
tained in matched square quartz cells (Wilmad) with 1-cm path
lengths. Gas-phase absorbance measurements were obtained
in a 1-cm square quartz fluorescence cell (NSG Precision Cells)
with an attached Teflon vacuum stopcock (0-4-mm bore from
Kontes).
with boron sulfide powder (method A)29 or reaction with
hexamethyldisilathiane catalyzed by boron chloride (method
B).30
endo-3-Methylthiocamphor (2a). (1R)-(+)-Camphor (99%)
was converted to (1R)-(+)-3-methylcamphor using LDA pro-
moted methylation. The 3-methylcamphor, consisting of a
mixture of exo and endo-conformers, was isomerized to the
endo-conformer31 and converted to 2a using method B for
thionation. Analysis of the products by capillary GC on column
A at 80 °C showed the presence of 2a (92%) at a retention
time of 12.29 min and exo-3-methylthiocamphor (6%) at 12.86
min. Compound 2a was purified by preparative GC on column
E at 170 °C. NMR: 1H (C6D6, 300 MHz) δ 2.65-2.74 (m,
1H, H3exo); 2.07-2.10 (t, 1H, bridgehead H, J(3Hexo-4H) ) 4.0
Hz); 1.55-1.81 (m, 4H); 1.24-1.27 (d, 3H, 3-endo-methyl,
J(3exoH -3(CH )en ) 7.0 Hz); 1.08 (s, syn-methyl and bridgehead
3
methyl, 6H); 0.83 (s, anti-methyl, 3H). 13C (CDCl3, 75.6 MHz)
δ 13.98, 16.46, 19.38, 20.03, 20.42, 34.94, 48.50, 50.34, 55.07,
70.43, 269.66 (CdS). MS (m/e), EI: 182 (M+), 139 (base
peak). HRMS (m/e): calcd for C11H18S 182.1129, found
182.1126.
endo-5,6-Trimethylene-2-norborneanethione (4). endo-5,6-
Trimethylene-exo-2-norbornanol32 was prepared by oxymer-
curation-demercuration of endo-dicyclopentadiene followed by
catalytic hydrogenation. It was then oxidized by adding,
dropwise with stirring, 100 mL of aqueous chromic acid
(Na2Cr2O7‚2H2O, 9.4 g, 31.56 mmol and H2SO4, 10.3 g, 104.9
mmol) from an addition funnel at room temperature to a 250-
ml round-bottom flask (equipped with a magnetic stir bar)
containing a solution of ca. 12 g of the alcohol in 50 mL of
acetone. The mixture was stirred overnight at room temperature.
The color of the solution changed from red-orange to green-
brown. The aqueous layer was extracted four times with ether.
The combined organic layers were washed four times with 50
mL of NaHCO3 and twice with 50 mL of brine solution and
then dried over Na2SO4. The organic solvent was removed
under vacuum. The ketone product was purified by preparative
GC on column E at 165 °C. GS-MS (m/e), EI: 150 (M+), 79
(base peak).
Fluorescence spectra were obtained on an SLM-AMINCO
(model SPF-500C) spectrofluorimeter. The spectra are corrected
for both photomultiplier response and the variation of lamp
intensity with excitation wavelength. Solution fluorescence
spectra were obtained in 1-cm square quartz fluorescence cells
(Wilmad). Spectroquality hydrocarbon solvents (Burdick &
Jackson) were generally used, and all solutions were deoxy-
genated prior to the fluorescence measurements. Gas-phase
fluorescence spectra of deoxygenated samples were obtained
in the 1-cm square quartz cell.
endo-5,6-Trimethylene-2-norbornanone was directly con-
verted to 4 using method B for thionation. The final product
was isolated by flash chromatography and further purified by
preparative GC on column F at 190 °C. NMR: 1H (C6D6, 300
MHz) δ 3.03-3.04 (d, 1H, bridgehead proton, J ) 3.9 Hz);
2.34-2.42 (d, q, 1H, syn-proton, Jd ) 20.3 Hz, Jq ) 4.2 Hz);
2.13-2.23 (m, 2H, two methine protons, Jd ) 3.9 Hz); 2.03
(bs, 1H, bridgehead proton); 1.78-1.86 (d, q, 1H, anti-proton,
Jd ) 20.3 Hz, Jq ) 4.6 Hz), 1.52-1.59 (m, 1H), 1.10-1.37
(m, 7H). 13C (C6D6, 75.6 MHz) δ 27.10, 27.32, 28.09, 41.45,
43.55, 48.47, 52.16, 70.23, 265.66 (CdS) (one carbon un-
accounted for). The HETCOR spectrum showed that the two
methine carbons are coincident at 48.47 ppm. GC-MS (m/e),
EI: M+ ) 166, base peak ) 98. HRMS (m/e): calcd for
C10H14S 166.0816, found 166.0823.
Photolyses. Gas-phase photolyses were performed as previ-
ously described9,27 either in a “flowing” mode or in a static mode
using a modified model RPR-100 Southern New England
Ultraviolet Co. Rayonet reactor with a 0.5-L cylindrical quartz
vessel equipped with a needle valve and a vacuum attachment
to hold the sample. For solution studies the reactor was
equipped with 16 low-pressure mercury lamps, a cooling fan,
and a merry-go-round turntable that positioned the photolysis
tubes approximately 2 cm from the lamps. Samples for gas-
phase photochemical and photophysical experiments were
deoxygenated by freeze-pump-thaw degassing. Solution
samples in photolysis tubes were deoxygenated by bubbling with
argon for 15-20 min. Quantum efficiencies were determined
using E-1-phenyl-2-butene actinometry.28
Syntheses. Ketones were alkylated by one of two general
methods. Method I involved the reaction of the ketone
(dissolved in THF) with 1.1 equiv of LDA (in heptane/THF/
ethylbenzene) at -78 °C. After stirring for 30 min, 1.1 equiv
of haloalkane was added, and the mixture stirred for 30 min at
-78 °C and 1 h at room temperature. Method II involved the
preparation of a tert-butyl alcohol solution of potassium tert-
butoxide to which was added the ketone followed by a large
excess of iodomethane. The solution was heated at reflux for
2.5 h. The thionation reactions involved reaction of the ketone
3,3-Diethylbicyclo[3.2.1]octane-2-thione (5). Bicyclo[3.2.1]-
octane-2-one (85%) was ethylated to give 3,3-diethylbicyclo-
[3.2.1]octane-2-one, which was purified by flash chromatogra-
phy and preparative GC on column F at 190 °C. Analysis of
the product by capillary GC on column B at 80 °C showed pure
material with a retention time of 12.12 min. NMR: 1H (CDCl3,
300 MHz) δ 2.59-2.60 (bs, 1H, bridgehead proton); 1.97-
1.98 (bs, 1H, bridgehead proton); 1.59-1.71 (m, 1H); 1.07-
1.46 (m, 11H); 0.60-0.70 (q, 6H, two methyl). 13C (CDCl3,
75.6 MHz) δ 8.73, 8.82, 27.16, 29.28, 31.05, 33.62, 34.59,

Photochemistry of Gas-Phase Aliphatic Thioketones
J. Phys. Chem. A, Vol. 102, No. 28, 1998 5429
34.75, 39.27, 49.36, 51.14, 214.26 (CdO). GC-MS (m/e),
EI: 180 (M+), 67 (base peak). HRMS (m/e): calcd for C12H20O
181.1592, found 181.1589.
Conversion to the thioketone by method B gave a product
that was purified by flash chromatography and by preparative
GC on column F at 220 °C. NMR: 1H (C6D6, 200 MHz) δ
3.73-3.76 (bs, 1H, bridgehead proton); 2.00-2.05 (bs, 1H,
bridgehead proton); 1.81-1.92 (m, 1H); 1.30-1.58 (m, 10H);
1.07-1.15 (m, 1H); 0.64-0.72 (q, 6H, two methyl). 13C (C6D6,
75.6 MHz) δ 8.67, 8.76, 29.34, 30.51, 35.20, 35.33, 35.82,
37.97, 40.00, 56.27, 63.91, 282.94 (CdS). GC-MS (m/e), EI:
196 (M+), 167 (base peak). HRMS (m/e): calcd for C12H20S
196.1286, found 196.1287.
2,2-Diethyl-5,5-dimethylcyclopentanethione (6). 2,2-Di-
methylcyclopentanone (98%) was ethylated to give 2,2-diethyl-
5,5-dimethylcyclopentanone admixed with unreacted starting
material and monoethylated product. The mixture was con-
verted to 6 using method B for thionation. The crude product
was isolated by flash chromatography and further purified by
preparative GC on column F at 180 °C. NMR: 1H (C6D6, 300
MHz) δ 1.56-1.53 (t, 4H, J ) 5.3 Hz, four methylene protons);
1.41-1.48 (q, 4H, J ) 7.3 Hz, four methylene protons); 1.03
(s, 6H, two methyl); 0.66-0.62 (t, 6H, J ) 7.4 Hz, two methyls).
13C (C6D6, 75.6 MHz) δ 8.65, 29.52, 30.20, 32.49, 37.17, 57.35,
65.15, 282.04 (CdS). GC-MS (m/e), EI: 184 (M+), 121 (base
peak). HRMS (m/e): calcd for C11H20S 184.1286, found
184.1277.
2-Ethyl-2,6,6-trimethylcyclohexanethione (7). 2,6,6-trimethyl-
cyclohexanone (98%) was ethylated to give a mixture of 2-ethyl-
2,6,6-trimethylcyclohexanone and unreacted starting material.
The mixture was thionated using method A. The crude thione
was isolated by flash chromatography and further purified by
preparative GC on column F at 180 °C. NMR: 1H (C6D6, 300
MHz) δ 1.27-1.74 (q and m, 8H, J ) 7.4 Hz, four methylene
protons); 1.23 (s, 3H, methyl); 1.14-1.15 (s,s, 6H, two methyls);
0.64-0.69 (t, 3H, J ) 7.4 Hz, methyl). 13C (C6D6, 75.6 MHz)
δ 8.77, 17.75, 31.69, 31.89, 34.31, 35.59, 36.11, 39.10, 52.16,
55.61, 277.38 (CdS). GC-MS (m/e), EI: 184 (M+; base peak);
CI: 185 (M + H), 151 (base peak). HRMS (m/e): calcd for
C11H20S 184.1286, found 184.1285.
Figure 7. Numbering scheme for photoproduct 4a.
coupling with H3endo); 1.44-1.41 (d, 1H, H5exo, Jd ) 10.3 Hz,
vicinal coupling with H5endo); 1.22-1.18 (d, 1H, H3endo, Jd )
10.4 Hz); 1.11 (s, 1H, bridgehead H4), 1.09 (s, 1H, -SH), 1.06-
1.09 (d, 1H, H5endo, Jd ) 10.3 Hz); 0.93-0.91 (m, 1H,
cyclopropyl H6); 0.78 (s, 3H, syn-CH3); 0.66 (s, 3H, anti-CH3).
GC-MS (m/e), EI: 154 (M+), 139 (M - CH3), 121 (M-SH),
111 (M - C3H7, base peak). HRMS (m/e): calcd for C9H14S
154.0816, found 154.0776.
Photoproduct of 2a. The sole photoproduct was purified by
preparative GC either on column E or G at 140 °C and found
to be pure by capillary GC analysis on column A at 85 °C (rt
6.23 min). NMR: 1H (CDCl3, 300 MHz) δ 2.06-1.99 (q and
d, 1H, H3exo, Jq ) 6.8 Hz (coupling with 3-endo-CH3), Jd )
1.59 Hz (long-range coupling with H5exo); 1.55 (s, 1H, -SH);
1.54-1.53 (d, 1H, H5exo, Jd ) 6.1 Hz, vicinal coupling with
H5endo); 1.47 (bs, 1H, bridgehead H4); 1.30 (b, 1H, H5endo)); 1.13
(s, 3H, syn-CH3); 0.92-0.90 (d, 3H, 3-endo-CH3, Jd ) 6.8 Hz);
0.88-0.86 (b, 7H, cyclopropyl H, anti-CH3, and bridgehead
CH3). 13C (CDCl3, 75.6 MHz) δ 65.85, 59.10, 47.37, 44.44,
42.24, 29.41, 27.75, 21.73, 20.22, 13.25, 8.90. GC-MS (m/
e), EI: 182 (M+), 167 (M - CH3), 139 (base peak; M - C3H7).
HRMS (m/e): calcd for C11H18S 182.1129, found 182.1120.
Photoproduct of 2b. The sole photoproduct was purified by
preparative GC on column F at 190 °C and found to be pure by
capillary GC analysis on column A at 80 °C (rt 10.95 min).
NMR: 1H (CDCl3, 300 MHz) δ 1.61-1.58 (d, 1H, H5exo, Jd )
8.7 Hz, vicinal coupling with H5endo); 1.52 (s, 1H, -SH); 1.38
(s, 1H, bridgehead H4); 1.28 (b, 1H, H5endo); 1.15 (s, 3H, CH3);
1.13 (s, 3H, CH3); 1.11 (s, 3H, CH3); 0.98 (s, 3H, CH3); 0.92
(s, 3H, CH3); 0.83 (s, 1H, cyclopropyl H). GC-MS (m/e), EI:
196 (M+, base peak), 181 (M - CH3), 153 (M - C3H7). HRMS
(m/e): calcd for C12H20S 196.1286, found 196.1282.
Photoproduct of 4. The sole photoproduct was purified by
preparative GC on column F at 200 °C and found to be pure by
capillary GC analysis on column C at 80 °C (rt 3.92 min). GC-
MS (m/e), EI: 166 (M+), 133 (M - SH), 98 (base peak, M -
C5H8). HRMS (m/e): calcd for C10H14S 166.0816, found
166.0823. The following discussion employs the numbering
shown in Figure 7.
2,4,4-Trimethylhexane-3-thione (8). 2,4-Dimethyl-3-pen-
tanone (98%) was methylated and then converted to 8 using
method A for thionation. The final product was isolated by
flash chromatography and further purified by preparative GC
1
on column F at 145 °C. NMR: H (CDCl3, 300 MHz) δ 3.77-
3.86 (m, 1H, J ) 6.49 Hz, isopropyl proton); 1.70-1.78 (q,
2H, J ) 7.5 Hz, methylene proton); 1.27 (s, 6H, two methyls);
1.16-1.18 (d, 6H, J ) 6.4 Hz, two isopropyl methyls); 0.76-
0.81 (t, 3H, J ) 7.5 Hz, methyl). GC-MS (m/e), EI: 158 (M+),
87 (base peak). HRMS (m/e): calcd for C9H18S 158.1129,
found 158.1121.
Isolation and Identification of Photoproducts. After
preparative photolyses of compounds 1-8 in pentane, the major
photoproducts were first isolated by flash chromatography with
230-400 mesh silica gel using cyclohexane or pentane as eluent
and then purified by preparative GC. The purified products
were identified by capillary GC retention time, mass spectrom-
etry (low and high resolution), and NMR spectroscopy (proton
and carbon).
Photoproduct of 1. The photoproduct, 1A, was purified by
preparative GC on column F at 170 °C and found to be pure by
capillary GC analysis on column B at 70 °C (retention time (rt)
2.43 min). NMR: 1H (C6D6, 300 MHz) δ 1.75 (s, 1H,
bridgehead H1); 1.74-1.70 (d, 1H, H3exo, Jd ) 10.7 Hz, vicinal
The assignment of structure to this compound required
extensive high-field NMR spectroscopy involving a number of
2D experiments. APT experiments were performed on a Varian
VXR-500 MHz spectrometer, while other analyses utilized a
Varian Unity-plus 600 MHz spectrometer. The spectra were

5430 J. Phys. Chem. A, Vol. 102, No. 28, 1998
Morrison et al.
obtained at ambient temperature (19-21°) and referenced to
the residual solvent lines as 7.15 δ 1H and 128.0 δ 13C. Proton
connectivities were assigned using a PFG COSY experiment
and a phase-cycled proton-proton double-quantum experiment
optimized for 7 Hz. In addition to the geminal correlations,
several vicinal and long-range correlations were observed. The
double-quantum experiment was useful in clarifying some areas
of overlap in the COSY spectrum. Correlations such as those
between H2-H9S (1.668, 0.893 δ), 3S-5R (1.442, 1.580 δ),
and H4-H6 (2.100, 2.001 δ) were later found to be consistent
with W-plan coupling. The apparent coupling constant and
chemical shifts were extracted via a phase-sensitive homonuclear
J-resolved experiment using an adiabatic purging pulse and a
C4-symmetrization procedure.33
5B: GC-MS (m/e), EI: 196 (M+), 167 (base peak, M -
CH2CH3), 163 (M - SH). CI: 197 (M + H), 163 (base peak,
M + H - H2S).
Photoproducts of 6. Products 6A and 6B were separated by
preparative GC on column F at 180 °C.
6A: NMR: 1H (CDCl3, 300 MHz) δ 1.76-1.27 (m, 7H
including cyclopropyl H); 1.03 (s, 3H, CH3); 1.05-1.07 (s and
d, 4H, -SH and CH3, Jd ) 6.2 Hz (coupling with cyclopropyl
proton)); 0.92-0.97 (s and t, 6H, CH3 and CH3 of ethyl group,
Jt ) 7.5 Hz (coupling with methylene protons of ethyl group)).
GC-MS (m/e), EI: 184 (M+), 155 (M - CH2CH3), 151 (M -
SH), 121 (base peak). HRMS (m/e): calcd for C11H20S
184.1286, found 184.1286.
6B: NMR: 1H (CDCl3, 300 MHz) δ 2.17-2.28 (m, 1H);
1.80-1.99 (m, 3H); 1.41-1.64 (m, 6H, containing CH2 of ethyl
group, J ) 7.8 Hz (coupling with methyl protons of ethyl
group)); 1.22-1.28 (m and s, 2H); 0.96 (s, 3H, methyl); 0.80-
0.84 (s and t, 6H, two CH3, Jt ) 7.7 Hz (coupling with
methylene protons of ethyl group)). GC-MS (m/e), EI: 184
(M+), 156 (M - CH2dCH2), 141 (M - CH2dCH2 - CH3),
102 (base peak). HRMS (m/e): calcd for C11H20S 184.1286,
found 184.1284.
One bond H-C connectivity was established by an HMQC
experiment at 600 MHz. The use of 1024 t1 increments resolved
all but the C2 and C5 (33.970, 33.965 δ) carbons, which were
distinguished by APT experiments at 125 MHz. A low S/N
150-MHz INADEQUATE spectrum (ca. 80 mg of crude
material doped to ca. 0.1 M with Cr(acac)3) was enhanced by
processing with a digital filter optimized for 36 Hz34 and used
to establish the carbon skeleton. All C-C connectivities except
the C2-C3 (33.965, 33.111 δ) bond were unambiguously
obtained from these data; the C2-C3 bond was postulated as
buried in overlap and later found to be consistent with the COSY
and HMBC data. Comparison of the HMQC data with the high-
resolution 1-D proton spectrum indicated that a sharp singlet at
1.574 δ was not directly attached to carbon. This signal was
assigned to the mercaptan proton. A PFG HMBC experiment
optimized for 7-Hz long-range JCH provided many correlations
and, in combination with the COSY and INADEQUATE data,
allowed the stereospecific assignments of all proton signals. The
HMBC data showed several instances (e.g., C1-H3R; C2-H9S;
C3-H5R) where one proton of a geminal pair was much more
efficiently coupled to a carbon three bonds away than the other.
In all cases, the observations were consistent with the dihedral
angles present in the structure proposed. NMR: 1H (600 MHz,
C6D6 ) δ 7.15) 2.110 (bs, H4); 2.001 (dm; H6); 1.987 (m; H1);
1.981(m; H7); 1.876 (bddd; H9pro-R); 1.668 (bddd; H2); 1.580
(bddd; H5pro-R); 1.574 (s; H-S); 1.442 (bddd; H3pro-S); 1.159
(bd; H10pro-S); 1.109 (bddd; H10pro-R); 0.893 (d; H9pro-S); 0.877
(bd; H5pro-S); 0.627 (dd; H3pro-R). 13C (C6D6 ) δ 128.0) 59.358
(C7, CH2); 50.863 (C4, CH); 48.728 (C6, CH); 48.519 (C8,
C-SH); 43.580 (C9, CH2); 40.448 (C1, CH); 34.856 (C10, CH2);
33.970 (C5, CH2); 33.965 (C2, CH); 33.111 (C3, CH2).
Photoproducts of 7. Products 7A and 7B were purified by
preparative GC on column F at 180 °C.
7A: NMR: 1H (C6D6, 300 MHz) δ 1.45-1.54 (m, 3H);
1.15-1.35 (m, 3H); 1.12 (s, 3H, CH3); 1.05 (s, 4H, CH3 and
-SH); 0.95 (d, 3H, CH3, Jd ) 6.2 Hz (coupling with cyclopropyl
proton)); 0.94 (s, 3H, CH3); 0.5-0.64 (q, 1H, cyclopropyl H,
Jq ) 7.5 Hz (coupling with methyl protons)). GC-MS (m/e),
EI: 184 (M+), 169 (M - CH3), 151 (M - SH), 95 (base peak).
HRMS (m/e): calcd for C11H20S 184.1286, found 184.1286.
7B: NMR: 1H (CDCl3, 300 MHz) δ 1.10-1.52 (m, 11H);
1.06 (s, 3H, CH3); 0.87 (s, 3H, CH3); 0.81 (s, 3H, CH3). GC-
MS (m/e), EI: 184 (M+), 169 (M - CH3), 156 (M -
CH2dCH2), 151 (M - SH), 141 (M - C3H7), 102 (base peak).
HRMS (m/e): calcd for C11H20S 184.1286, found 184.1288.
7C: GC-MS (m/e), EI: 184 (M+), 155 (M - CH2CH3), 151
(M - SH), 121 (base peak). Particularly noteworthy is the M
- CH2CH3 ion which is not seen in products 7A and 7B.
Photoproducts of 8. Product 8A was isolated by preparative
GC on column F at 140 °C. Product 8B could only be
characterized by GC-mass spectrometry.
8A: NMR: 1H (CDCl3, 300 MHz) δ 1.45-1.52 (m, 1H,
isopropyl H, J ) 6.6 Hz (coupling with two isopropyl methyl
protons)); 1.16 (s, 3H, CH3); 1.12 (s, 3H, CH3); 1.05-1.07 (d,
3H, CH3, Jd ) 6.4 Hz (coupling with cyclopropyl proton)); 1.00
(s, 1H, -SH); 0.99-0.95 (d, d′, 6H, two isopropyl methyls, Jd
) 6.5 Hz, Jd′ ) 6.6 Hz (coupling with isopropyl protons));
0.49-0.43 (q, 1H, cyclopropyl H, Jq ) 6.4 Hz (coupling with
methyl protons)). GC-MS (m/e), EI: 158 (M+), 143 (M -
CH3), 125 (M - SH), 115 (M - C3H7), 87 (base peak). CI:
159 (M+H), 125 (base peak, M + H - H2S). HRMS (m/e):
calcd for C9H18S 158.1129, found 158.1136.
Photoproducts of 5. Products 5A and 5C were purified by
preparative GC on column F at 200 °C.
5A: NMR: 1H (C6D6, 300 MHz) δ 1.69 (s, 1H, bridge head
H1); 1.54-1.56 (m, 1H, bridge head H5); 1.36-1.46 (m, 9H,
containing two CH2s of ethyl groups, Jq ) 7.4 Hz (coupling
with methyl protons of ethyl groups); 1.00-1.02 (m, 2H, H4e
and H4a); 0.81 (s, 1H, cyclopropyl H); 0.75-0.80 (t, 6H, two
CH3, Jt ) 7.5 Hz (coupling with methylene protons of ethyl
group). GC-MS (m/e), EI: 196 (M+), 167 (base peak, M -
CH2CH3), CI: 197 (M+H), 163 (base peak, M + H - H2S).
HRMS (m/e): calcd for C12H20S 196.1286, found 196.1288.
8B: GC-MS (m/e), EI: 160 (M+), 71 (base peak).
Gas-Phase Photolyses. For preparative work, approximately
100-mg samples of each thioketone were photolyzed at 254 nm
with the sample vessel maintained at ambient temperature. The
sample was allowed to evaporate under vacuum into a quartz
photolysis vessel and then to a cold trap. The major photo-
products were purified first by flash chromatography and then
by preparative GC. For studies with added butane the static
gas-phase cell was filled with ca. 800 mTorr of 2 and then with
1
5C: NMR: H (C6D6, 300 MHz) δ 2.51-2.21 (m, 1H, bridge
head H1); 2.38 (b, 1H, bridgehead H5); 2.20-1.40 (m, 15H);
0.75-0.80 (t, 3H, CH3, Jt ) 7.5 Hz (coupling with methylene
protons of ethyl group)). GC-MS (m/e), EI: 196 (M+), 168
(M - CH2dCH2), 139 (base peak, M - CH2dCH2 - CH2CH3).
CI: 163 (base peak M + H - H2S). HRMS (m/e): calcd for
C12H20S 196.1286, found 196.1291.

Photochemistry of Gas-Phase Aliphatic Thioketones
J. Phys. Chem. A, Vol. 102, No. 28, 1998 5431
ca. 50 Torr of n-butane. The gases were allowed to mix for 1
h, and the gaseous mixture was then photolyzed for 5 min at
254 nm.
4B and 4B′: (m/e), EI: 166 (M+), 99 (M - C5H7; base peak),
67 (M - C5H7S); CI: 167 (M + H; base peak), 133 (M + H
- H2S), 99 (M + H - C5H7).
4C: (m/e), EI: 168 (M+), 135 (M - SH; base peak).
Photoproducts of 5. Analysis of the product mixture by
capillary GC on column B at 100 °C revealed that one major
and four minor products are formed. Products 5A, 5B, and 5C
were shown to be identical to products formed in solution. The
major product (5D) is unique to the gas-phase photolysis. It
was purified by flash chromatography followed by preparative
GC on column F at 200 °C.
Photoproducts of 1. Analysis of the product mixture by
capillary GC on column B at 70 °C indicated that one product
had the same retention time as 1A. This product was isolated
1
and gave H NMR and GC-mass spectral data identical with
that obtained for 1A isolated from the solution-phase study.
1B: 1H NMR analysis indicated the presence of two isomers
in a ratio of 1:1 resulting from a thermally induced double-
bond migration during the GC purification. Characteristic
multiplets at δ 5.76 (s) and 5.66-5.65 (m) are indicative of the
terminal vinyl protons and another characteristic multiplet at δ
4.77-4.69 (m) is indicative of the endocyclic olefinic protons.
Two singlets at δ 1.74 and 1.71 are characteristic of two
isopropenyl methyl groups. Analysis by GC-mass spectrom-
etry prior to GC purification gave the following results: (m/e),
EI: 154 (M+), 139 (M - CH3), 121 (M - SH), 111 (M -
C3H7), 79 (base peak). CI: 155 (M + H, base peak), 121 (M
+ H - H2S). HRMS (m/e): calcd for C9H14S 154.0816, found
154.0776.
5D: NMR: 1H (C6D6, 300 MHz) δ 1.88-2.15 (m, 7H,
including CH2 of ethyl group, J ) 7.63 Hz (coupling with
methyl protons)); 1.41-1.66 (m, 4H); 1.16-1.27 (m, 2H);
0.81-0.86 (t, 3H, CH3, Jt ) 7.5 Hz, coupling with methylene
protons of ethyl group). GC-MS: (m/e), EI: 168 (M+, base
peak), 153 (M - CH3), 139 (M - CH2CH3), 135; CI: 169 (M
+ H; base), 135 (M + H - H2S). HRMS (m/e): calcd for
C10H16S 168.0973, found 168.0963.
Photoproducts of 6. The photoproduct mixture was analyzed
by capillary GC on column C at 60 °C. There are four products
of which 6C and 6D are unique to the gas-phase photolysis.
Products 6A and 6b were shown to be identical to these products
formed in solution. Product 6C was further purified by flash
chromatography and preparative GC on column F at 180 °C.
6C: NMR: 1H (CDCl3, 300 MHz) δ 2.24-2.29 (s and t, 3H,
-SH and CH2 in the ring, Jt ) 6.3 Hz (coupling with another
CH2 in the ring)); 2.12-2.20 (q, 2H, CH2 of ethyl group, Jq )
7.6 Hz, coupling with methyl protons of ethyl group); 1.69-
1.74 (t, 2H, CH2 in the ring, Jt ) 6.6 Hz, coupling with another
CH2 in the ring); 1.05 (s, 6H, two CH3s); 0.95-0.98 (t, 3H,
CH3, Jt ) 7.60 Hz, coupling with CH2 of ethyl group). 13C
(CDCl3, 75.6 MHz) δ 140.79, 129.71, 48.09, 38.18, 31.94,
26.38, 23.03, 14.02, 11.89. GC-MS (m/e), EI: 156 (M+), 141
(M - CH3; base peak); HRMS (m/e): calcd for C9H16S
156.1129, found 156.1129.
Photoproducts of 2. Analysis of product mixture by capillary
GC on column A at 80 °C indicated that one product had the
same retention time as 2A. This product was isolated and gave
1H NMR and GC-mass spectral data identical with that
obtained for 2A isolated from the solution-phase study.
2B: NMR: 1H (C6D6, 600 MHz) δ 4.75 (m, 1H); 4.71 (m,
1H); 2.11 (bs, 1H, SH); 2.11 (m, 1H); 2.01 (m, 1H); 2.00 (m,
1H); 1.81 (m, 1H);1.76 (m, 1H); 1.68 (bs, 3H, methyl); 1.55
(bs, 3H, methyl); 1.53 (m, 1H); 1.24 (m, 1H). 13C (C6D6, 150
MHz) δ 148.80, 129.59, 119.93, 109.40, 42.70, 40.75, 32.57,
28.01, 20.63, 20.70. GC-MS (m/e), EI: 168 (M+), 153 (M -
CH3), 125 (base peak, M - C3H7). HRMS (m/e): calcd for
C10H16S 168.0973, found 168.0963.
High-resolution NMR spectroscopy was carried out at 500
Analysis of 6D by low-resolution GC-mass spectrometry
gave the same fragmentation pattern as 6A.
1
and 600 MHz for H spectra and 125 and 150 MHz for 13C
spectra. Experiments included APT, TOCSY, NOESY, HMQC,
and HMBC analyses in C6D6. The numbering used in the
following discussion is shown in Figure 8. The 13C spectrum
is consistent with that for limonene,35 allowing for the effect of
the mercaptan group on the chemical shifts. The existence of
the -SH moiety is implicated by HMQC evidence for only one
C-H linkage in the 2.1 ppm region and confirmed by exchange
with D2O. Evidence for the isopropenyl group at C-5 is
provided by (1) TOCSY correlations between the vinyl hydro-
gens at 4.72 ppm and the C-9 methyl protons and (2) HMBC
long-range carbon-proton correlations between carbons 5, 7,
and 8 and the C-9 methyl protons. The -CH2CH2CHCH2-
relationship follows from the TOCSY and HMBC data. Finally,
appendage of the C-10 methyl group on C-2 is evidenced by a
positive NOE involving the methyl hydrogens and the hydrogens
at C-3 and is confirmed by an HMBC correlation between the
C-10 methyl protons and the C-3 protons.
Photoproducts of 7. Analysis of the photoproduct mixture
capillary GC on column C at 70 °C indicated the formation of
four products. Three (7A-C) were identical with products
formed in solution. Product 7D is unique to the gas-phase
photolysis. It was purified by flash chromatography followed
by preparative GC on column F at 180 °C.
7D: NMR 1H (CDCl3, 300 MHz) δ 2.01-2.05 (t, 2H); 1.78
(s, 3H, vinyl CH3); 1.55-1.58 (b, 5H); 1.12 (s, 6H, two CH3).
GC-MS (m/e), EI: 156 (M+), 141 (M - CH3; base peak), 123
(M-SH). CI: 157 (M+H), 123 (M + H-H2S).
Photoproduct of 8. The photoproduct mixture was analyzed
by capillary GC on column B at 45 °C. In addition to the ketone
precursor of 8, there were several minor products and one major
product identified as 8A which is formed in solution.
Acknowledgment. We are grateful to the National Science
Foundation (Grant CHE 9311828) for support of this research.
Photoproducts of 4. Analysis of the product mixture by
capillary GC on column C at 80 °C indicated one major product
with the same retention time as 4A. This product was isolated
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and gave H NMR and GC-mass spectral data identical with
that obtained for 4A isolated from the solution-phase study.
Isolation of three minor products was unsuccessful due to their
very small quantity. Two of the products (4B and 4B′) gave
low-resolution GC-mass spectra with similar fragmentation
patterns; the third appears to correspond to the reduction of 4.

5432 J. Phys. Chem. A, Vol. 102, No. 28, 1998
Morrison et al.
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