Photochemistry of racemic and resolved 2-iodooctane. Effect of solvent polarity and viscosity on the chemistry

Article abstract of DOI:10.1021/jo020472r

The photochemistry of racemic and resolved 2-iodooctane was examined in cyclopentane, methanol, and 2-methyl-2-propanol, media with differing polarities and viscosities. The photochemistry of racemic 2-iodooctane was also examined in the gas phase. The photochemistry of 2-deuterio- and 1,1,1-trideuterio-2-iodooctane in cyclopentane and methanol was also studied. The photoreactions in cyclopentane, 2-methyl-2-propanol, and the gas phase occurred exclusively through homolytic reactions, while in methanol, they occurred predominantly (> 53%) through heterolytic reactions. By comparing the disappearance of the optically active substrate with its loss of optical activity, F, the fraction of the initially formed radical pair (RP) or ion pair (IP) resulting in product was determined for the three solvents. Because F contains contributions of both escape of the partners in the RP or IP into the bulk of the solvent and reaction within the RP or IP to yield products other than the substrate, there was no correlation between F and solvent viscosity. The F values will be valuable in assessing the photochemistry of 2-iodooctane in the same media with circularly polarized light.

Full text of DOI:10.1021/jo020472r

P h otoch em istr y of Ra cem ic a n d Resolved 2-Iod oocta n e. Effect of
Solven t P ola r ity a n d Viscosity on th e Ch em istr y
Fang Gao,† David Boyles,† Rodney Sullivan,‡ Robert N. Compton,†,‡ and Richard M. Pagni*,†
Departments of Chemistry and Physics, University of Tennessee, Knoxville, Tennessee 37996-1600
rpagni@utk.edu
Received J uly 15, 2002
The photochemistry of racemic and resolved 2-iodooctane was examined in cyclopentane, methanol,
and 2-methyl-2-propanol, media with differing polarities and viscosities. The photochemistry of
racemic 2-iodooctane was also examined in the gas phase. The photochemistry of 2-deuterio- and
1,1,1-trideuterio-2-iodooctane in cyclopentane and methanol was also studied. The photoreactions
in cyclopentane, 2-methyl-2-propanol, and the gas phase occurred exclusively through homolytic
reactions, while in methanol, they occurred predominantly (>53%) through heterolytic reactions.
By comparing the disappearance of the optically active substrate with its loss of optical activity, F,
the fraction of the initially formed radical pair (RP) or ion pair (IP) resulting in product was
determined for the three solvents. Because F contains contributions of both escape of the partners
in the RP or IP into the bulk of the solvent and reaction within the RP or IP to yield products other
than the substrate, there was no correlation between F and solvent viscosity. The F values will be
valuable in assessing the photochemistry of 2-iodooctane in the same media with circularly polarized
light.
In tr od u ction
ordinarily destroys most of R/S before a large ee is
obtained. This is a consequence of the fact that the
differences in ꢀ [∆ꢀ; determined by circular dichroism
(CD)] are usually exceedingly small. Photolysis of racemic
camphor (1) with CPL, for example, afforded recovered
One of the triumphs of modern organic synthesis is the
ability to make optically active molecules with high
enantiomeric purity. The 2001 Nobel Prize in chemistry
was awarded to three chemists who contributed signifi-
cantly to this goal. Most of these asymmetric reactions
are thermally activated.
Photochemistry has also been used for asymmetric
synthesis, with mixed results. The most successful ap-
proach to date involves the photolysis of optically active
crystals made up of optically inactive molecules.1 This
method works well but is limited to those relatively rare
cases where enantiomorphic crystals are formed from
achiral molecules. Optically active sensitizers have also
proven useful in certain cases.2 Photoreactions initiated
with circularly polarized light (CPL), an easily prepared
chiral form of light, is another approach. There are
myriad ways in which CPL could be used in asymmetric
synthesis, but this initial discussion will only consider
two.
If one photolyzes a racemic mixture R/S with one
enantiomeric form of CPL [left-handed CPL (LCPL) or
right-handed CPL (RCPL)], one enantiomer of R/S will
degrade more rapidly than the other, because it will have
a slightly greater extinction coefficient (ꢀ) than the other.
In fact, the enantiomeric excess (ee) of recovered R/S
approaches 100% as the time of the reaction becomes
large. The problem with this approach is that one
camphor with an ee of 20% after 99% of the camphor had
been degraded.3 Two-photon photochemistry of racemic
tartaric acid (2) with CPL afforded a maximum ee for
recovered tartaric acid of 7.5%.4
If one instead has a system in which R and S of the
racemic mixture equilibrate photochemically but do not
otherwise degrade, the ee at the photostationary state
(pss) is gA/2, where gA ) (ꢀR - ꢀL)/(ꢀR + ꢀL), with ꢀR and ꢀL
being the extinction coefficients for the R and S enanti-
omers at a given wavelength. This system is desirable
in that R/S is not degraded, but it has the negative effect
of yielding very small ee values because gA is usually very
small (ꢀR - ꢀL is small and ꢀR + ꢀL is large). For example,
enone 3 underwent isomerization about the C-C double
† Department of Chemistry.
‡ Department of Physics.
(1) Feringa, B. L.; van Delden, R. A. Angew. Chem., Int. Ed. Engl.
1999, 38, 3418.
(2) Inoue, Y.; Wada, T.; Asaoka, S.; Sato, H.; Pete, J .-P. Chem.
Commun. 2000, 250.
bond photochemically and yielded an ee of 1.6% when
10.1021/jo020472r CCC: $22.00 © 2002 American Chemical Society
Published on Web 12/05/2002
J . Org. Chem. 2002, 67, 9361-9367
9361

Gao et al.
photoequilibrated with CPL.5 Photoequilibration of (E)-
and (Z)-cyclooctene with CPL (190 nm from a synchotron
source) afforded (E)-cyclooctene with a small enantio-
meric enrichment.6
TABLE 1. P r op er ties of th e Rea ction P h a ses
bond dissociation
energy/kcal mol-1 c,d
viscosity/cp
(25 °C)b
phase
ET(30)a
C-H
O-H
The first case, i.e., R/S f P, is desirable because large
ee values can be obtained regardless of the values of ꢀR
and ꢀL, but only at the expense of degrading R/S
significantly. The second case, i.e., R h S, is desirable
because R/S is not degraded, but ee values will rarely be
large. What may prove useful is a system that contains
desirable features of both cases.
gas
cyclopentane
methanol
2-methyl-2-propanol
low
31
55.4
43.3
low
0.416
0.551
4.438
94.5
94
104.4
105.1
100.5e
a
Reichardt, C. Solvents and Solvent Effects in Organic Chem-
b
istry, 2nd ed.; VCH: Weinheim, Germany, 1988. Riddick, J . A.;
Bunger, W. B.; Sakuno, T. K. Organic Solvents. Physical Properties
and Methods of Purification, 4th ed.; Wiley: New York, 1986.
c Benson, S. W. Thermochemical Kinetics, 2nd ed.; Wiley: New
Reactions going through radical pairs (RPs) and/or ion
pairs (IPs) have features of both cases (eq 1). The
d
York, 1976. CRC Handbook of Chemistry and Physics, 63rd ed.;
Weast, R. C., Ed.; CRC Press: Boca Raton, FL, 1982. e The value
for neopentane (Bartmess, J ., personal communication).
Resu lts a n d Discu ssion
Cyclopentane, methanol, and 2-methyl-2-propanol were
chosen as reaction media for this study. Cyclopentane
and methanol have similar viscosities but different
polarities, while 2-methyl-2-proponol is much more vis-
cous and exhibits an intermediate polarity (Table 1).
Because 2-iodooctane is volatile at room temperature, its
photochemistry was also examined in the gas phase.
Cyclopentane is a low-boiling, nonpolar liquid that should
enantiomers, R and S, photoequilibrate through the
transient radical or ion pair (IN ) intermediate) but
ultimately yield products (P) through IN. It can be shown
that the ee of recovered R/S depends on F, the fraction
of IN that yields P.7 When F ) 1, i.e., IN f P 100% of
the time, the system reverts to R/S f P. When F ) 0,
i.e., IN never yields P, the system reverts to R h S. IN
can yield P in two ways: (1) by the two components of
the radical or ion pair diffusing apart and then reacting
with solvent, for example, and (2) the two components of
the radical or ion pair reacting with one another in a way
that does not re-form R or S. Because F, in part, requires
the two components of the intermediate to diffuse apart,
F should depend on solvent viscosity.
favor the formation of the RP, CH3(CH2)5 CHCH3 I•. The
•
organic radical can abstract hydrogen, dimerize, and
disproportionate. The iodine atom is less prone to ab-
stract H, because the bond dissociation energy of HI is
so much less than the value of a typical C-H bond. The
iodine atom could abstract H from the octyl radical in
the RP, however; it could also dimerize. Because all 10
of its C-H bonds are equivalent, the number of products
arising from cyclopentane will be minimized. Methanol,
on the other hand, is a low-boiling, polar liquid that
should favor the formation of the IP, CH3(CH2)5C+HCH3 I-.
The 2-octyl cation can, of course, be trapped by the
solvent or isomerize to the 3-octyl cation, which in turn
can be trapped. Trapping yields an ether and HI which
could also be generated from iodide or other bases
abstracting a proton from the carbocation. 2-Methyl-2-
propanol has the highest viscosity of the three solvents,
which should favor a chemistry occurring within the
solvent cage. Unlike the other two solvents, its C-H and
O-H bond strengths are sufficiently large to ensure that
the 2-octyl radical will not likely abstract hydrogen from
this solvent.
Photolysis of racemic 2-iodooctane in the gas phase,
where a radical pair yielding F ) 1 is expected, afforded
six hydrocarbon products: 1-octene, octane, (E)- and (Z)-
2-octene, and meso- and d,l-7,8-dimethyltetradecane9
(Table 2, Scheme 1). Although the yields of HI and I2 were
not determined, their yields could be estimated from the
product distribution and hydrogen atom balance. About
80% of the time the iodine atom dimerizes to I2 and 20%
of the time it abstracts a hydrogen atom from C-1 or C-3
of the 2-octyl radical to form HI and the octenes. Octane
(and a comparable amount of octenes) can also arise by
disproportionation of the 2-octyl radical. The ratio of
2(octane)/tetradecanes of 0.19 is what is expected for the
disproportionation of a secondary radical.10
The molecule chosen for this investigation was 2-
iodooctane. It is easily prepared in racemic and optically
active form.8 Its chirooptical properties and thus gA are
easily measured by UV and CD spectroscopies. As shown
herein, it yields an RP or IP, depending on solvent
polarity, and a rich array of products. Its F values can
be measured in a straightforward manner. This study is
designed to provide a wealth of information on the
dynamics of RPs and IPs as a function of solvent polarity
and viscosity. Finally, the effect of gA and F on the
photolysis of racemic material with CPL can be probed.
This first paper will concentrate on the photochemistry
of the racemic and optically active substrate with unpo-
larized light in four phases varying in polarity and
viscosity and the measurement of F values. The results
of the photochemistry with CPL will be reported else-
where.
(3) Kagan, H. B.; Balavoine, G.; Moradour, A. J . Mol. Evol. 1974, 4,
41.
(4) (a) Shimizu, Y.; Kawanishi, S. Chem. Commun. 1996, 819. (b)
Shimizu, Y.; Kawanishi, S. Chem. Commun 1996, 1333. (c) Shimizu,
Y. J . Chem. Soc., Perkin Trans. 1 1997, 1275.
(5) Zhang, Y.; Schuster, G. B. J . Org. Chem. 1995, 60, 7192.
(6) Inoue, Y.; Tsuneisci, H.; Hakashi, T.; Yagi, K.; Awazu, K.; Onuki,
H. Chem. Commun. 1996, 2627.
(7) The derivation assumes that the disappearance of R and S is
first-order.
(8) San Filippo, J ., J r.; Romano, L. J . J . Org. Chem. 1975, 40, 1514.
(9) We do not know which diastereomer is which.
9362 J . Org. Chem., Vol. 67, No. 26, 2002

Photochemistry of Racemic and Resolved 2-Iodooctane
TABLE 2. P h otop r od u cts a n d Yield s (%) fr om th e
P h otoch em istr y of 2-Iod oocta n ea
SCHEME 2
reaction phase
cyclo-
2-methyl-
photoproducts
1-octene
gas pentane methanolb 2-propanolc
22
6
4
12
22
29
14
4
17
10
6
54
2
28
16
octane
(E)-2-octene
(Z)-2-octene
7,8-dimethyltetradecane
isomer 1d
4
21
43
4
14
5
1
2
isomer 2d
iodocyclopentane
unknown
7
2-octanol
trace
trace
3-methoxyoctane
2-methoxyoctane
2-octanone
iodine
hydrogen iodide
10
38
5
76e
24e
45
3
2e
50f
97f
98e
a
Reactions usually contained unreacted starting material.
Products normalized to 100%. b CH3I is also formed in the reaction.
d
c (CH3)3CI is also a product of the reaction. Meso or d,l isomer,
shown in order of elution. e Estimated from product distribution
and hydrogen atom balance. f Inferred from the amount of I2
formed in reaction.
be approximately isoenergetic. This interprepation is
confirmed by the presence of iodocyclopentane in the
product mixture. Of note is the lack of other cyclopentane-
containing products including bicyclopentyl in the prod-
uct mixture. Low-boiling products such as cyclopentene
would have escaped detection, however.
SCHEME 1
Much more HI is formed in cyclopentane than in the
gas phase. In the gas phase, no RP is formed (F ) 1),
and the 2-octyl radical and iodine atom escape each other
instantly. In cyclopentane, the two radicals are formed
in a cage. The radicals can react to regenerate the
reactant or to produce HI and octenes and escape the
cage. On the basis of the yields of I2 and the two
dimethyltetradecanes, escape products, one sees that
about one-half of the radical pairs escape into the bulk
solvent. Thus, F contains two contributions, reaction to
yield HI and octenes and escape.
The photoreaction of 2-iodooctane in deoxygenerated
methanol, which is faster than in cyclopentane, especially
at longer reaction times, also yields the six hydrocarbons
seen in the gas phase and cyclopentane (Table 2, Scheme
2). The reaction also yields 2-methoxyoctane (major
product), 3-methoxyoctane, 2-octanol, 2-octanone, and an
unknown compound. Trace amounts of unidentified
compounds are also found. On the basis of their gas
chromatographic retention times, some of these are likely
(E)- and (Z)-3- and 4-octene and 4-methoxyoctane. UV/
vis spectroscopy of the reaction solution shows the
presence of triiodide, but not I2. Thus, HI, which is soluble
in methanol, is formed in greater amounts than I2. Very
little meso- and d,l-7,8-dimethyltetradecane are formed
here, implying that little of the octane is formed by the
disproportionation of the 2-octyl radical. Most of the
octane undoubtedly arises by the 2-octyl radical abstrac-
tion of the C-H hydrogen atom from the solvent, a
reaction which is approximately isoenergetic. Unlike
octane, 2- and 3-methoxyoctane, 2-octanol, and 2-
octanone arise via heterolytic chemistry, as shown below.
The 2-octanol is undoubtedly formed by cleavage of
2-methoxyoctane by HI; the alcohol in turn is photooxi-
dized by I2 to form 2-octanone.11
The photoreaction of 2-iodooctane in deoxygenerated
cyclopentane, where a radical pair and F * 1 are
expected, occurs rapidly at first but becomes progres-
sively slower as the reaction progresses. This is undoubt-
edly due in some fashion to the buildup of I2, a major
photoproduct in the reaction. The reaction yields the
same six hydrocarbons seen in the gas-phase plus iodo-
cyclopentane (Table 1). The reaction also yields signifi-
cant amounts of I2. HI is also a product of the reaction,
although it could not be detected in situ (lack of triiodide
bands between 300 and 400 nm) because it is insoluble
in cyclopentane. However, it could be trapped, in part,
and detected in water (pH, AgI precipitate).
The free radical reaction in cyclopentane affords con-
siderably more octane than observed in the gas phase
reaction. If the disproportion-to-dimerization ratio of 0.2
holds here, only about 5% of the octane is formed from
the disproportionation of two 2-octyl radicals. The re-
mainder must arise by the 2-octyl radical abstraction of
a hydrogen atom from the solvent. This reaction should
(10) Lowry, T. H.; Richardson, K. S. Mechanisms and Theory in
Organic Chemistry, 3rd ed.; Harper and Row: New York, 1987; chapter
9.
(11) Itoh, A.; Kodamu, T.; Masaki, Y. Chem. Lett. 2001, 686.
J . Org. Chem, Vol. 67, No. 26, 2002 9363

Gao et al.
TABLE 3. % Deu ter iu m Con ten t of Selected P h otop r od u cts
photolysis of 2-iodo-2-deutereooctane
photolysis of 2-iodo-1,1,l-trideuteriooctane
cyclopentane methanol
d3 d2
cyclopentane
d1 d0
methanol
product
1-octene
d1
d0
d3
d2
d1
d0
100
53
0
100
66
70
66
100
100
0
4
30
34
0
100
0
0
0
100
100
30
51
20
34
70
73
52
32
54
50
30
27
17
16
26
16
octane
47
∼4
∼4
100
(E)-2-octene
(Z)-2-octene
2-methoxyoctane
3-methoxyoctene
∼96
∼96
0
0
SCHEME 3
SCHEME 5
other, as this would generate octane with four Ds.
Transfer of an H at C-3 is consistent with the results.
Interestingly, there is much less octane formed here (7%)
than is seen in the nondeuterated case (22%).
SCHEME 4
Photolysis of 2-iodooctane-d1 in cyclopentane is also
informative. As in the previous case, only a small amount
of octane is produced (5%), 47% of which contained no
deuterium. This is consistent with the photoreaction
yielding a small amount of the 2-octyl carbene (Scheme
5). This result has precedent in Kropp’s study of the
photochemistry of alkyl halides, where carbenes are often
observed.12 Once formed the carbene could abstract
hydrogen from the solvent to form octane or isomerize
to (E)- and (Z)-2-octene, all with no deuterium. The
2-butyl carbene, an analogue of the 2-octyl carbene, is
known to preferentially yield the two 2-butenes.13
Unlike in cyclopentane, the photochemistry of 2-iodo-
octane-d1 in methanol yields considerable amounts of
octane, approximately 34% of which contains no deu-
terum (Scheme 6). Thus, more carbene is formed in
methanol than in cyclopentane. Some of the unlabeled
(E)- and (Z)-2-octene must arise from the 2-octyl carbene.
The rest arises from 1,2-hydride shifts starting from the
2-octyl cation. Interestingly neither 3-methoxyoctane nor
2-methoxyoctane has any D, which is expected for their
formation from the 3-octyl and 2-octyl cations, respec-
tively. This yields the rather surprising result that none
of the 2-methoxyoctane arises from the insertion of the
2-octyl carbene into the solvent’s O-H bond.
The product distribution reveals that at least 53% of
the reaction occurs through the 2-octyl cation and at least
another 20% through the 2-octyl radical. How the re-
maining 27% of the product is formed is unclear, because
all of the octenes can be formed from the RP or IP.
Although the RP and IP may be formed independently
from the excited state of 2-iodooctane, it is reasonable to
assume that they form sequentially, as shown in Scheme
3. The low yields of I2 and the dimethyltetradecanes
suggest that very little of the radicals in the RP escape
into the bulk solvent. On the basis of the extensive
literature on solvolysis reactions and their associated ion
pairs, it is reasonable to assume that most of the ionic
chemistry takes place within the IP and little of the
relatively unstable 2-octyl cation and iodide escape into
the bulk solvent.
The photochemistry of the substrate in 2-methyl-2-
propanol, whose polarity is intermediate between that
of cyclopentane and methanol, occurs surprisingly com-
pletely through free radicals (Table 2). As little I2 and
no dimethyltetradecanes are formed here, it is reasonable
to assume that virtually all the chemistry takes place
within the RP.
The photochemistry of 2-iodooctane-d3 in methanol is
also informative. The products show evidence for the loss
of one, two, and even three deuteriums. This is a
consequence of the fact that HI is generated in the
reaction and it can re-add to the octene to re-form
2-iodooctane (3-iodooctane is not seen) (Scheme 7).14
Thus, 1-octene-d2 formed in the deprotonation of the
2-octyl cation forms, in addition to HI, 2-iodooctane-d2,
which can then react photochemically again. This hy-
The photochemistry of two deuterated analogues of
2-iodooctane, 2-deuterio-, and 1,1,1-trideuterio-, whose
syntheses are shown in Scheme 4, was investigated in
cyclopentane and methanol.
Photolysis of 2-iodooctane-d3 in cyclopentane gives the
expected results: 1-octene with two Ds and (E)- and (Z)-
2-octene with three Ds (Table 3). Octane is formed with
three Ds only.
(12) Kropp, P. J .; Sawyer, J . A.; Snyder, J . J . J . Org. Chem. 1984,
49, 1583.
(13) Platz, M. S. In Advances in Carbene Chemistry; Brinlar, V. H.,
Ed.; J A1 Press: Stanford, CT, 1998; Vol. 2, p 131.
Any octane formed in this reaction by disproportion-
ation does not transfer a D at C-1 in one radical to the
9364 J . Org. Chem., Vol. 67, No. 26, 2002

Photochemistry of Racemic and Resolved 2-Iodooctane
SCHEME 6
eq 1, it is easy to show that, under first-order conditions,
the disappearance of substrate, i.e. [R] + [S], and the loss
of optical activity, as measured by [R] - [S], yields,
respectively
[R] + [S] ) [R]oe-kFt
[R] - [S] ) [R]oe-kt
where [R] ) [R]o and [S] ) 0 at t ) 0, k is the
phenomenological rate constant, and F is the fraction of
IN going to product.18 Plots of ln([R] + [S])/[R]o and ln-
([R] - [S])/ [R]o versus time yield straight lines with
slopes of kF and k. If the reactions have the appearance
of being zero-order, plots of ([R] + [S])/[R]o and ([R] -
[S])/ [R]o versus time yield straight lines whose ratio of
slopes yields F.
Photolysis of optically active (R)-2-iodooctane8 in metha-
nol, cyclopentane, and 2-methyl-2- propanol was followed
by quantitative GC/MS and polarimetry as a function of
time.19 The normalized results are shown in the table in
the Supporting Information. The methanol data afforded
linear (zero-order) plots of ([R] + [S])/ [R]o and ([R] - [S])/
[R]o versus time for the first half of the reaction. From
the slopes, one obtains F(CH3OH) ) 0.60 ( 0.05. A like
analysis of the 2-methyl-2-propanol data yields F[(CH3)3-
COH] ) 0.74 ( 0.05. A similar analysis of the cyclopen-
tane data was difficult, because the log and nonlog plots
deviated quite early in the reaction. It is clear from the
data that F(cyclopentane) is close to 1. Using the data
points through 1 h of the reaction yields F(cyclopentane)
) 0.70 ( 0.12, although a value larger than 0.80 is likely.
The photolysis solutions have rotations close to or at
0° when the reactions are near completion. The unreacted
2-iodooctane is racemic at this time. The chiral 7,8-
dimethyltetradecane isomer must also be racemic. This
is not surprising, as the hydrocarbon arises by dimer-
ization of the achiral 2-octyl radical that has escaped the
RP cage. 2- And 3-methoxyoctane must also be racemic
or nearly so.
It is possible to dissect the behavior of the radical and
ion pairs in three solvents (Table 4). In the gas phase
there is, of course, no caged intermediates. Except in
methanol, where a sizable fraction of the reaction pro-
ceeds through ion pairs, the chemistry is dominated by
free radicals. F is large in all cases through varying
degrees of escape of the caged intermediates into the bulk
solvent and reaction within the cage to form products
other than the reactant. Escape of the radicals in the RP
in cyclopentane is large, while very small in 2-methyl-
SCHEME 7
pothesis is also born out by the fact that recovered
2-iodooctane consists of 63%-d3 and 37%-d2 after most of
the overall reaction is complete (>99%).
Solvolysis reactions yielding IPs have been character-
ized, in part, by various kinds of rate constant measure-
ments including kt, from the titration of acid formed in
the reaction; kR, from the loss of optical activity of the
solution; and krac, from the loss of optical activity in the
reactant. F, the fraction of the IP going to product, can
be deduced from these measurements. Similar data for
photoreactions proceeding through RPs and/or IPs will
also yield F values.
The kinetics of unimolecular photochemical reactions
has been worked out by Kagan,15 Inoue,16 and others.17
If the absorbance (A) of the substrate and products is
small (A < 0.01), the rate of disappearance of reactant is
close to first-order. At higher values of A, the dependence
of the rate on substrate concentration is quite complex,
although it may appear to be zero-order when A is large.16
If an optically active substrate R reacts photochemi-
cally with unpolarized light according to the scheme in
(15) Balavoine, G.; Moradpour, A.; Kagan, H. B. J . Am. Chem. Soc.
1974, 96, 5152.
(16) Nakamura, A.; Nishimo, H.; Inoue, Y. J . Chem. Soc., Perkin
Trans. 2 2001, 1701.
(17) Mauser, H.; Gauglitz, G. Photokinetics-Theoretical Fundamen-
tals and Applications. In Comprehensive Chemical Kinetics; Compton,
R. G., Hancock, G., Eds.; Elsevier: Amsterdam, 1998; Vol. 36, p 1.
(18) Equation 1 is the simplest scheme for describing the photo-
chemistry of resolved 2-iodooctane. A more involved scheme would
involve “fight” and “loose” ion/radical pairs, each of which could give
P. Intimate and solvent-separated ion pairs are well-known from
solvolysis chemistry. The mathematics for the more complex scheme
but gives solutions similar in form to those described in the text.
Because there is no evidence for the more complex behavior, the
simplest scheme was used.
(14) HI does not add to 1-octene in methanol at room temperature.
The temperature inside the photoreactor is approximately 31-32 °C.
The addition must be photoactivated. HI forms H• and I• under our
reaction conditions: Calvert, J . G.; Pitts, J . N., J r. Photochemistry;
Wiley: New York, 1966.
(19) The HI formed in the reactions does not racemize the (R)-2-
iodooctane.
J . Org. Chem, Vol. 67, No. 26, 2002 9365

Gao et al.
TABLE 4. F r a ction a l Beh a vior of 2-Octyl/Iod in e
Ra d ica l/Ion P a ir
to product, the data from the experiments herein do not
directly address their nature.
reaction in
Although a lot has been learned about RPs and IPs
from time-resolved experiments, labeling experiments,
and the like, the F values determined here are the first
reported for RPs and the first for IPs generated in
photochemical reactions. The F value for methanol is a
composite, containing contributions from both its RP and
IP. The experiments were not designed to resolve the F
values into its contributors. The F values for the three
solvents do not correlate with solvent viscosity. This is a
consequence of the fact that the caged intermediates
cannot only re-form the reactant and escape but also
react within the cage to form the octenes. Nonetheless,
it will now be possible to assess how the F values
influence the photochemistry of 2-iodooctane in the three
solvents with circularly polarized light.
fraction of
reaction that
is free radical
return to
reactant
cage to form
octenes
phase
F
(1 - F) escape and octane
gas
1
1
1a
0a
1
cyclopentane
methanol
>0.80 <0.20 ∼0.50
∼0.40
0.20 < fraction 0.60
<0.47
2-methyl-2- ∼1
0.40
0.26
0.03b
0.02
0.57
0.74
0.72
propanol
a
b
Assumed. From RP. Escape from IP unknown but assumed
to be 0.
2-propanol, reflecting the 10-fold difference in viscosity
of the two media. The F value in methanol, a solvent with
a viscosity similar to that of cyclopentane, is smaller than
in the other two solvents. This is a reflection of the fact
that the reaction in methanol is dominated by an IP,
where escape of the ions likely does not occur.
Exp er im en t Section
Con clu d in g Rem a r k s
Sa m p les. Racemic and optically active (R)-2-iodooctane
were prepared from the corresponding alcohol by a literature
procedure.8 2-Deuterio-2-octanol was prepared by reduction of
2-octanone with NaBD4; further treatment afforded 2-deuterio-
2-iodooctane. 1,1,1-Trideuterio-2-octanol was prepared by the
reaction of heptanal with CD3MgBr; further treatment af-
forded 1,1,1-trideuterio-2-iodooctane. Both substrates were
isotopically pure by mass spectrometry. (Z)-2-octene was
prepared by the direct irradiation of (E)-2-octene.
There have been many studies of photoreactions in
solution yielding RPs and/or IPs. The extensive work of
Kropp on alkyl halides,12,20,21 Givens on phosphate es-
ters,22 and Pincock on arylmethyl esters23 comes to mind,
with the work of Kropp having the closest relationship
to the work reported herein.24
2-Iodooctane has a weak absorption band at ca. 270
nm, which is associated with an n f σ* transition.25 For
the related compounds CH3I and CH3CH2I, this results
in homolytic cleavage of the C-I bond in the gas phase.26
Similar behavior is seen for 2-iodooctane in the gas phase,
where significant amounts of the dimeric meso- and d,l-
7,8-dimethyltetradecane are formed. Similar behavior is
also seen for the substrate in the nonpolar cyclopentane
and the much more polar 2-methyl-2-propanol. Dimers
are seen in cyclopentane but not in the more viscous
2-methyl-2-propanol. Interestingly, dimers are not formed
in the photochemistry of 1-iodooctane.12,24 A small amount
of carbene-derived products are formed in cyclopentane.
Kropp has shown in the case of 1-iodooctane, where some
1-octyl carbene is formed, that the carbene does not arise
in a concerted loss of HI, but instead may derive from
the RP or IP.12,24 In methanol, considerably greater than
half of the photoreaction occurs through an IP, with the
rest through an RP. A small amount of the 2-octyl dimers
are formed in methanol. 2-Iodooctane affords more car-
bocation in methanol than does 1-iodooctane; the second-
ary 2-octyl cation is, of course, more stable than the
1-octyl cation. It is known that many solvolysis reactions
proceed through two types of ion pairs: intimate and
solvent-separated. Although it is clear that they are
formed during the photolysis of 2-iodooctane in methanol,
after all the “ion pair” goes back to reactant and forward
P h otolyses. Photolyses were carried out in a reactor using
2537Å lamps. 2-Iodooctane has λmax at 266 nm in cyclopentane.
The samples in quartz tubes were degassed before photolysis.
The gas-phase photolysis was carried out on a degassed, neat
sample of the 2-iodooctane in the quartz tube. Because the
substrate has a vapor pressure at room temperature, the
quartz tube always contained equilibrated gaseous substrate.
The light passed through the nonliquid-containing portion of
the tube. Because there are no cage effects here, the gas-phase
reaction occurred almost as fast as the solution phase ones.
P r od u ct Id en tifica tion . The photoproducts were sepa-
rated by GC/MS. The products were identified using known
samples and literature mass spectra data and retention times.
Deuterium analysis of the products obtained from the irradia-
tion of the deuterated substrates was carried out by MS.
Con tr ols. Experiments showed that HI did not react with
1-octene in methanol in the absence of light. It also did not
racemize optically active 2-iodooctane under the reaction
conditions.
UV/Vis Exp er im en ts. I2 from photolyses in cyclopentane
was quantified using the extinction coefficient determined in
-
these laboratories. Likewise I3 in methanol was quantified
using the extinction coefficient determined in these laborato-
ries. Anhydrous HI was prepared by the method of Hoffman.27
Bubbling HI into I2 in cyclopentane did not yield I3-. Bubbling
HI into I2 in methanol did yield the characteristic bands of
-
I3
.
P h otolysis of (R)-2-Iod oocta n e. The disappearance of the
substrate was followed by GC/MS, while the change in optical
rotation was followed by polarimetry. Rotations of 0.001° could
be measured on carefully equilibrated samples. How the data
were analyzed is found in the Supporting Information.
Photolysis of a degassed solution of 2-octanol and I2 in
methanol under the conditions described above afforded 2-oc-
tanone. Similar behavior is known for benzylic and allylic
alcohols.28
(20) Kropp, P. J . Acc. Chem. Res. 1984, 17, 131.
(21) Kropp, P. J .; Adkins, R. L. J . Am. Chem. Soc. 1991, 113, 2709.
(22) Givens, R. S.; Kueper, L. W., III. Chem. Rev. 1993, 93, 55.
(23) Pincock, J . A. Acc. Chem. Res. 1997, 30, 43.
(24) Kropp, P. J .; Poindexter, G. S.; Pienta, N. J .; Hamilton, D. C.
J . Am. Chem. Soc. 1976, 98, 8135.
(25) J affe´, H. H.; Orchin, M. Theory and Applications of Ultraviolet
Spectroscopy; Wiley: New York, 1962.
(26) Calvert, J . G.; Pitts, J . N., J r. Photochemistry; Wiley: New York,
1966.
(27) Hoffman, C. J . Inorg. Synth. 1963, 7, 180.
(28) Itoh, A.; Kodama, T.; Masaki, Y. Chem. Lett. 2001, 686.
9366 J . Org. Chem., Vol. 67, No. 26, 2002

Photochemistry of Racemic and Resolved 2-Iodooctane
derivation of the relevant mathematical equations. This mate-
rial is available free of charge via the Internet at
http://pubs.acs.org.
Ack n ow led gm en t. We thank the National Science
Foundation for support of this work.
Su p p or tin g In for m a tion Ava ila ble: The data analysis
from the photolysis of the optically active substrate and the
J O020472R
J . Org. Chem, Vol. 67, No. 26, 2002 9367