Table 1. Product Distribution upon Photolysis of Compounds
1a-e Encapsulated in Octa Acida
photoproducts
substrate
2
3
4
5
6 + 7
1a
1b
1c
1d
1e
49
41
45
15
0
38
13
23
23
34
20
36
32
51
80
a Yields are average of at least three different runs and are estimated by
GC analysis. Average conversion of each reaction was maintained between
25-35%. Photochemistry of 1a was reported earlier.5 The mass balance in
all reactions was in the range of 80-100%.
1
Figure 2. Alkyl region of H NMR (500 MHz, D2O) spectra of
2:1 complexes of octa acid with (i) 1b, (ii) 1c, (iii) 1d, and (iv) 1e.
as well as RP-1 during their entire lifetime. Absence of AA
and BB is consistent with the postulate that the capsule does
not come apart during the lifetime of RP-2. Although the
exceptionally high (100%) cage effect (AB/(AB + BB +
AA)) in the capsule is interesting, we focus our attention on
the para-rearrangement product.
unsubstituted benzyl ring leaves us unable to verify if its
position is dependent on the remote alkyl chain. The observed
additional 1 ppm chemical shift for the methyl signal (∆δ
) -4.4 ppm) with a pentyl chain as the substituent suggests
either a greater interaction between this ketone and the phenyl
groups of the cavitand or its insertion between the phenyl
rings of the cavitand and ring of the guest ketone. Interest-
ingly, in this case the NOESY spectra suggested that the
methyl group interacts only with Hg and not even with Hf
suggesting that the chain might be folded in this case. The
Formation of para-rearrangement product, especially whose
yield depends on the length of the alkyl chain, to our
knowledge is previously less known.6 We believe an analysis
of this process would provide an insight on understanding
reactions even at enzyme active sites whose “reaction
pockets” are similar in size to the reactant and product
molecules. Within the capsule, as illustrated in Scheme 2,
formation of the rearrangement product 2 from RP-1 is
dictated by the ability of the benzyl radical to rotate while
the nearby arylacyl radical remains stationary. Questions
remaining to be addressed include: (a) What drives the
reorientation of the benzyl radical within the capsule? (b)
What is the dependency of benzyl radical reorientation on
the p-alkyl substitution on the arylacyl radical? (c) What is
the correlation between the yield of 2 and the length of the
alkyl chain of the arylacyl radical that is not involved in the
rotation.
1H
NMR
data
suggest
that
1-(4-butylphen-
yl)-3-phenylpropan-2-one is the longest molecule that could
fit in a linear fashion with probable tail bending within the
capsule. Consistent with this hypothesis, with longer (19.05
Å) 1-(4-hexylphenyl)-3-phenylpropan-2-one even after soni-
cation for over 2 h at 50 °C no inclusion within OA capsule
occurred.
As expected, photolysis of 1b-e in hexane (30 min under
nitrogen saturated conditions) resulted exclusively in the three
diarylethanes AA (3), AB (4), and BB (5), in the statistical
ratio of 1:2:1 without any influence of the alkyl chain on
the products or their distribution.3 Following irradiation (30
min) under identical conditions of 1b-e inclusions in OA
capsule (1:2 complex) in borate buffer, products were
extracted with chloroform, analyzed by GC, GC-MS, and
1H NMR and identified by comparison with authentic
samples (Table 1 and for details, see the Supporting
Information). The noteworthy influences of OA capsule on
product distribution include: (a) The exclusive formation
of diarylethane AB as product with the absence of both AA
and BB. Such a high cage effect hitherto unobserved in
solution with any host to our knowledge, is not unusual in
crystals and in zeolites where diffusion of radicals is
restricted.7 (b) Rearrangement products 2b-e absent from
hexane or borate buffer reactions were formed in varying
yields within OA capsule and (c) most importantly, their
yield was dependent on the length of alkyl chain at the para
position (Table 1).
Rotation of the benzyl radical before decarbonylation of
the partner arylacyl radical is a key step in the formation of
the para-rearrangement product within the OA capsule
(Scheme 2). The benzyl radical, an independent species, can
on generation find the best position within the capsule
irrespective of the second species, arylacyl radical. We
believe that space filling (van der Waals) interaction and the
binding preference of the benzyl radical where the methyl
group occupies the narrow portion of the cavity are the
primary sources for the reorientation of the reactive benzyl
radical. The observed reorientation product can easily be
visualized as illustrated in Scheme 2 that involves R-cleavage
from the triplet state (i), inter system crossing before or after
reorientation (ii) or (iii), coupling and 1,5 hydrogen shift.
The control exerted by the partner radical’s alkyl group
(7) (a) Turro, N. J. Acc. Chem. Res. 2000, 33, 637-646. (b) Quinkert,
G.; Tabata, T.; Hickmann, E. A. J.; Dobart, W. Angew. Chem., Int. Ed.
Engl. 1971, 10, 199-200. (c) Veerman, M.; Resendiz, M. J. E.; Garcia-
Garibay, M. A. Org. Lett. 2006, 8, 2615-2617.
Formation of only AB (4) (and rearranged AB 6 and 7) is
suggestive of incarceration within the capsule of both RP-2
Org. Lett., Vol. 9, No. 18, 2007
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