Consequences of controlling free space

PAPER www.rsc.org/pps | Photochemical & Photobiological Sciences
Consequences of controlling free space within a reaction cavity with a remote
alkyl group: photochemistry of para-alkyl dibenzyl ketones within an organic
capsule in water†‡
Arun Kumar Sundaresan and V. Ramamurthy*
Received 12th August 2008, Accepted 11th September 2008
First published as an Advance Article on the web 20th October 2008
DOI: 10.1039/b814001d
With the aim of controlling free space available for the reactants, photochemical reactivity of several
4-alkyl dibenzyl ketones included within a water-soluble self-assembled organic capsule (octa acid) has
been investigated. One and two-dimensional NMR spectroscopic techniques were employed to
characterize the structure of the guest@host complexes. Free space controlled by the remote alkyl
group influences the distribution of photoproducts. Correlation between the structures of the
guest@host complexes and products selectivity suggested that the free space that could be controlled by
a remote tether has a determining role during a photoreaction within the restricted space of the capsule.
The fact that the same alkyl tether has no effect on the photobehavior of 4-alkyl dibenzyl ketones in
hexane solution suggests that the role of substituents in free (solution) and restricted space
(supramolecular assemblies) is different. The current observation suggests that rules of physical organic
chemistry and photochemistry developed based on solution chemistry cannot be simply extended to
supramolecular assemblies.
Introduction
This presentation pertains to our continued exploration of
supramolecular assemblies to control excited state chemistry. One
of our long-range scientific goals is to develop, on the basis of
well-established rules of molecular organic photochemistry and
supramolecular chemistry, a model to predict the photobehavior
of organic molecules in restricted spaces, in general. In this
presentation we focus on a special class of supramolecular
assemblies namely, guest@host complexes in aqueous solution.
Some time ago, one of us jointly with Weiss and Hammond1,2 and
independently, Turro and Garcia-Garibay3 have presented general
models to understand and predict photoreactions in restricted
spaces. The host reaction cavity concept developed by these
workers emphasized the size and shape changes that occur as
the reactant guest is transformed into the product and how this
phenomenon is commensurate or not with the available space
of the reaction cavity. One of the features of the reaction cavity
concept is the ‘free space’.4 The free space within a reaction
cavity relative to the size and shape of the guest is an important
parameter:thecomplementaryguestandhostshape,size,location,
directionality and dynamics control in large part the extent to
which the host can influence a photoreaction that involves shape
changes from reactant to product(s). When the atoms/molecules
constituting the walls of the reaction cavity are stationary and
relatively rigid (i.e., possess time-independent positions on the
time scale of the guest reaction; e.g., solids or crystals), the free
Department of Chemistry, University of Miami, Coral Gables, FL, 33124,
USA
† This paper was published as part of the themed issue in honour of
Nicholas Turro.
‡ Electronic supplementary information (ESI) available: Experimental
details and NMR spectra of the complexes. See DOI: 10.1039/b814001d
space necessary to allow the conversion of a guest molecule to
its photoproducts must be built into the reaction cavity (e.g.,
zeolites).5 On the other hand, in systems where the walls of the
host are relatively flexible (e.g., micelles), the free volume may
adjust during the course of a reaction. For such media, the free
space of a reaction cavity is modified by structural fluctuations of
the medium and cannot be readily represented by static molecular
models. Unlike micelles, unimolecular organic hosts have fixed
internal shape, size and geometry. Therefore one would expect
that photoreactions of guest@host assemblies although occur in
isotropic aqueous solution it would be controlled by shape and
free space of the host reaction cavity. In this study we illustrate
that one could predictably control the free space within an organic
host and thus alter the course of a photoreaction. For this purpose
we use dibenzyl ketones6,7 as guests and an organic host known as
octa acid as host.8
During the last few years, we have examined, in addition to
crystals and zeolites, the use of water-soluble hosts cucurbiturils
(CB),9–12 natural and functionalized cyclodextrins (CD),13 calixarenes (CA),14,15 Fujita’s Pd host (FPdH),16,17 dendrimers18,19 and
micelles20 as reaction media. Experimental problems associated
with CB, CD, and CA make them less appealing as photoreaction
vessels. For example, guest@cyclodextrin complexes have limited
solubility in water; CBs have limited ability to solubilize neutral
organic molecules in water; CAs functionalized with sulfonic acid
groups are too flexible to form strong and rigid guest@host
complexes in water. In this context, a water-soluble cavitand,
called octa acid (OA) whose synthesis was recently reported by
Gibb’s group, has attracted our attention as a reaction medium.8
As shown in Fig. 1, CD, CB, CA and have similar structural
features varying slightly in the cavity dimensions, portal openings
and its functionality. While the guest is only partially surrounded
by the first three hosts (as 1 : 1 or 2 :1 guest@host complexes), two
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Fig. 1 Representation of structures of some water-soluble molecules used as hosts for photochemical reactions.
molecules of OA self-assemble and encapsulate the guest (as either
a 2:2 or 1 : 2 guest@host capsule)21–23 (Fig. 2). Thus the OA capsule
provides a relatively ‘rigid’, solid (crystal- and zeolite-like) reaction
cavity. Studies with this host as a reaction medium provided high
levels of selectivities for a wide range of photochemical reactions
that are unparalleled by any solution-based host. We show in this
report that one could predictably alter the free space within the
capsule and thus control the photoreactions of a guest molecule.
The use of dibenzyl ketones as probes for studying the effect of
organized media and restricted environments follows from their
ability to report on events that cover relatively large spatial and
dynamic ranges.3 Properties of numerous organized media like
micelles,24,25 cyclodextrins,26–28 zeolites29–31 and polymer films32,33
have been investigated using dibenzyl ketones. We have reported
the photochemistry of dibenzyl ketone in OA34 and have shown
the influence of the shape of OA in the product distribution.
In this report, we present our results on the characterization
of the complexes formed between 1-(4-alkyphenyl)-3-phenyl-2-
propanones (4-alkyl dibenzyl ketones, 1c–f; Fig. 2) and OA
using 1H NMR spectroscopy techniques and the photochemical
reactivities of the included ketones. The striking effect of a remote
alkyl tether on the observed product selectivity within a confined
space is explored using the above 4-alkyl dibenzyl ketones. Two
parallel reactions of dibenzyl ketones, viz. decarbonylation and
rearrangement product formation occurred with short paraalkyl-substituents. Elongation of the substituent by one or two
methylene groups effectively inhibited formation of rearrangement
product, although the substituent is well separated from the
Fig. 2 Top: structure of octa acid and the pictorial representation of octa acid molecule. Bottom: schematic formation of 2 : 1 complex formed between
OA and guests 1c–f.
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reaction centre. Lack of formation of the rearrangement product
has been attributed to the lack of free space required for the
intermediateradicaltorotateinsidethecavityofthehostmolecule.
Experimental
Procedure for NMR experiments
Octa acid and guests 1c–1f were synthesized using reported
procedures.8,35 Stoichiometry of the complexes formed between
octa acid and ketones 1c–1f were monitored using 1H NMR
analysis. Aliquots of DMSO-d6 solution of the guest were added
to 0.6 mL of 10-3 M octa acid and 10-2 M sodium tetraborate
in D2O. 1H NMR analysis of the complex was performed after
each addition. Fig. 3 shows the 1H NMR spectra of free OA and
1d–OA2 complex after addition of 1d. Complete complexation
occurred after addition of 0.5 equiv of 1d. Further addition of
the guest showed signals corresponding to free guest in the NMR
spectrum.Basedonthisobservation,thestoichiometryofthehost
and guest in the complex was determined as 2:1. To confirm the
stability of the complex, 1H NMR spectra of the same sample of
1d@OA2 complexwererecordedafter48handtwoweeksandwere
identical to the spectrum recorded immediately after addition,
shown in Fig. 3(v), confirming that the complex was stable. Based
onthis,thehost:guestratiowasestablishedas2:1.Formationofa
2:1 complex between octa acid and 1c–f is pictorially represented
in Fig. 2. 1H NMR spectra of all the complexes are provided
in the electronic supplementary information, ESI, (Fig. SI 2,
SI 5, SI 8 and SI 11).‡ COSY (Correlation Spectroscopy) and
Nuclear Overhauser Effect Spectroscopy (NOESY) experiments
were performed using 5 ¥ 10-3 M solution of the complexes (2.5 ¥
10-3 M guest). The spectra are included in the ESI (Fig. SI 3, SI 6,
SI 9 and SI 12)‡ and the data are interpreted in the discussion
section below.
Procedure for photochemical experiments
Solutionsoftheguestinhexane(10-3 M),takeninaPyrextesttube,
werebubbledwithnitrogenfor30min,sealedwitharubberseptum
and irradiated using a medium pressure Hg lamp for 30 min. GC
(HP-5890 series with SE-30 capillary column) and GC-MS (HP-
6890 series fitted with 5975B MSD, HP-5 column) analyses were
performed to identify the photoproducts formed.
Octa acid complex of guests 1c–1f were prepared by addition
of DMSO-d6 solution of the guest to 2 ¥ 10-3 M solution of octa
acid dissolved in buffered D2O/H2O. The mixture was agitated
for 30 min to ensure complete binding, bubbled with nitrogen
for 30 min, sealed with septum and irradiated for 30 min using
a medium pressure Hg lamp. The conversion attained over this
irradiation time period was 25% to 35%. 1H NMR spectra of
the irradiated complexes in D2O were recorded to monitor the
reaction.Theaqueoussolutionwasextractedwithchloroformand
the organic layer was analyzed by GC and GC-MS. Identity of the
products 3 and 7 were established by comparison with authentic
samples synthesized independently.
Results
1H NMR Analysis
Complexesofketones(1c–f,Scheme1)andOAwerecharacterized
by 1H NMR, NOESY NMR and Pulsed-field Gradient Spin
Fig. 3 1H NMR spectra (500 MHz, D2O, 10-2 M sodium tetraborate) of (i) 10-3 M OA, and the complex of OA and 1d in (ii) 2:0.25, (iii) 2:0.5,
(iv) 2:0.75 and (v) 2:1 ratio. The signals of 1d bound to OA are numbered as shown in the structure of 1d (Fig. 2(ii)). OA signals are labelled a–g and
aromatic signals of the guest are denoted using *. Residual water signal is marked with ∑.
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Echo (PGSE) NMR analysis. The complexes formed between octa
acid and ketones 1c–f showed upfield shift of the included guest
signals.36 NMR spectral titration data for 1d@OA2 are provided in
Fig. 3. The aliphatic signals of 1d are shielded by the aromatic walls
of the host and are shifted to 0 ppm to -2.5 ppm (Fig. 3(v)). Since
the two halves of the capsular assembly formed are dissimilar,
symmetrical 1H NMR spectral pattern of OA (Fig. 3(i)) is replaced
by multiple signals (Fig. 3(v)). NOESY correlations between OA
and the guest (Fig. 5), and PGSE NMR helped to establish the
structure of the complex. 1H NMR, NOESY NMR and COSY
NMR spectra of 1c@OA2, 1d@OA2, 1e@OA2 and 1f@OA2 are
provided in the ESI (Fig. SI 1 to SI 13).‡
Photochemical results
The primary photochemical process of dibenzyl ketone is the Norrish type I (a-cleavage) reaction (Scheme 1).6,7 Upon irradiation
of the ketone, the primary radical pair (RP-1) consisting of benzyl
and phenylacyl radicals is formed.24 In principle a-cleavage on
either side of the carbonyl chromophore could occur leading to the
two radical pairs shown in Scheme 1. When the reaction is carried
out in a solvent like hexane, the primary pathway for both radical
pairs is the loss of CO from the phenylacyl radical to yield RP-2.
Radical recombination occurs randomly to yield photoproducts 2
(AA), 3 (AB) and 4 (BB) in 1:2 : 1 ratio (Table 1). In homogeneous
Fig. 4 Selected region of the 1H NMR spectra (500 MHz, D2O, 5 mM
OA) of (i) 1c@OA2, (ii) 1d@OA2, (iii) 1e@OA2 and (iv) 1f@OA2.
solvents, neither the rearrangement products (7, 8 or 9 in Scheme 1)
nor the isomers of 3 viz., 5 and 6 are formed.
When ketones 1c–f were irradiated as octa acid complexes
and the photoproducts were analyzed by GC and GC-MS, the
only products formed from the decarbonylation reaction were
Scheme 1 Steps involved in the photochemical reaction of dibenzyl ketones 1a–f and structures of the photoproducts.
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Fig. 5 Pictorial representation of the NOE interactions observed between octa acid and (i) 1c, (ii) 1d, (iii) 1e, and (iv) 1f.
3 and its isomers 5 and 6. Cage escape products AA and BB
(2 and 4 respectively) were not observed. Unlike the solution
reaction, the para-rearrangement product 7 was obtained from
irradiation of the octa acid complexes (Table 1). Importantly, a
correlation between the alkyl chain length and the yield of 7 was
evident. While 1c and 1d yielded 7 in 41% and 45% respectively,
only 15% of 7 was formed from 1e. Finally, only decarbonylated
photoproducts were isolated from irradiation of 1f@OA2 complex.
Yield of decarbonylated products increased correspondingly from
85% (1d@OA2) to 100% (1f@OA2) (Table 1).
Table 1 Relative yields of photoproducts obtained from irradiation of
ketones 1c–f in hexane and as OA complex
Photoproducts(%)a,b
Substratea Medium 2 3 4 5 + 6 7
1ac Buffer–OA 38 — — 13 49
1bc Buffer–OA — 41 — 15 44
1c Hexane 19 50 31 — —
Buffer–OA — 36 — 23 41
1d Hexane 17 50 33 — —
Buffer–OA — 32 — 23 45
1e Hexane 16 53 31 — —
Buffer–OA — 51 — 34 15
1f Hexane 13 52 35 — —
Buffer–OA — 80 — 20 0
a Structure of reactant ketones and photoproducts are given in Scheme 1.
b The yields reported are based on GC analysis of at least three reactions.
Average conversion of 25–30% was maintained. c Yields of 1a and 1b are
reproduced from ref. 34.
Discussion
Analysis of 1H NMR spectra
Analysis of 1D NMR data is divided into two sections, one related
to the host signals and the other concerning the guest signals. 1H
NMR spectrum of the free octa acid is shown in Fig. 3. The six
aromatic signals shown in the NMR spectrum correspond to each
aromatic hydrogen (Ha–Hf) as assigned in Fig. 2.8 After addition
of the guest, two octa acid molecules that form the complex,
encapsulate different parts of the guest. As a result, the two octa
acid molecules that form the capsule are not symmetrically related
and therefore multiple signals of the host’s hydrogen atoms are
recorded in the NMR spectrum of the complex. In addition,
chemical shift of a few internal hydrogens are altered due to the
presence of the guest. For example, the single signal corresponding
to H
e in Fig. 3(i) is split into two signals after binding to any one
of the guests 1c–f (Fig. 3 and Fig. SI 2, SI 5, SI 8 and SI 11
in ESI‡). Aromatic walls of the host have a shielding effect on
the guest resonances.36,37 While the aliphatic signals of the guest
are shifted to <0 ppm, the aromatic signals of the guest remain
between 5 and 6 ppm. Fig. 4 shows the high field region of the 1H
NMR of all four compounds bound to octa acid. The two signals
labeled ‘3’ and ‘4’ in Fig. 4(i) correspond to CH2 and CH3 groups
of 1c. Loss of coupling of the guest signals could be attributed to
the presence of different conformers of the guest, which exchange
slowly when bound to the host. The shielding effect of the host
on the guest is governed to a great extent by the position of the
latter. Consequently, slow exchange of guest conformers is also
accompanied by minor changes in their chemical shifts, relative
to their position inside the cavity. It is likely that broad signals in
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Table 2 Change in chemical shift of guest’s aliphatic signals upon binding
to OA
Change in chemical shift (ppm)b
Dd = d CDCl3 – d octa acid complex
Guesta H1, H2 H3 H4 H5 H6 H7
1c 2.3 2.5 3.0 — — —
1d 2.4 2.5 2.7 3.5 — —
1e 2.5 2.4 2.6 2.7 3.4 —
1f 2.5 2.3 2.4 2.3 2.1 4.3
a Refer Fig. 4 for structure and numbering of guests 1c–f. b For 1H NMR
spectra of OA bound and free guests in CDCl3, see ESI (Fig. SI 1, SI 4, SI
7 and SI 10 for spectra in CDCl3; Fig. SI 2, SI 5, SI 8 and SI 11 for OA
complexes).
Fig. 4 are caused by the guest dynamics. Of vital importance is the
chemical shift of these signals since the chemical shift differences
between guests in the bulk solution and those in the capsule are
related to their positions in the cavity. In the 1H NMR spectrum
of 1c recorded in CDCl3 shown in ESI (Fig. SI 1),‡ the quartet
corresponding to CH2 signal is at 2.6 ppm and the CH3 triplet is at
1.2 ppm. Upon binding, the signals shift to 0.1 ppm and -1.8 ppm
respectively, a relative upfield shift of 2.5 ppm and 3 ppm. Thus, for
1c@OA2, while both the CH2 and the CH3 signals move upfield,
the relative shift is greater for CH3 than CH2. Table 2 lists the
change in chemical shift of the aliphatic signals of the guests 1c–f
after binding, which were assigned based on COSY correlations.
The information provided in Fig. 4 and in Table 2 gives an
insight into the structure of the guest inside the cavity of the host.
High field shifted signals of the aliphatic chains confirms that the
aliphatic portion of the guest is well shielded by the aromatic parts
of the host. Further, in each complex, the maximum shift (Dd) is
noted for the terminal methyl group. For example, the methylene
signal of 1c (H3 in Table 2) is shifted by 2.5 ppm and the methyl
signal is shifted by 3 ppm. Similarly, the two methylene signals in
alkyl chain of 1d are shifted by 2.5 ppm and 2.7 ppm, respectively,
and the methyl signal by 3.5 ppm. Since the methyl signal is shifted
most (more than the corresponding methylene signals), it can be
concluded that the methyl group occupies the deeper end of the
cavity (formed by four phenyl rings) in all complexes. Further,
comparison of relative shifts of the methyl groups provides further
details on the arrangement of the guest. The relative shifts (Dd) for
the methyl groups of 1c@OA2, 1d@OA2, 1e@OA2 and 1f@OA2 are
3 ppm, 3.5 ppm, 3.4 ppm and 4.3 ppm respectively. In 1f@OA2, the
methyl signal is shifted additionally by 0.9 ppm, when compared to
1e@OA2. This additional shielding of the methyl signal in 1f@OA2
suggests that the position of the methyl group in this complex
is different from the other complexes. Also, the three methylene
signals are closer to each other (signals 4, 5 and 6 in Fig. 4(iv)),
indicating that they are in equivalent magnetic environment in the
cavity, most likely in a coiled conformation as shown in Fig. 6(iv).
Comparison of the benzylic methylene signals (signals 1, 2 and
3 in Fig. 4 and Table 2) provides information about the position
of the aromatic rings of the guest, since the methylene groups are
attached to the phenyl ring. Across all four guests, chemical shifts
of signals 1, 2 and 3 and the relative shift of their signals (Dd) are
almost constant. Chemical shifts of H1/H2 signals of all four guests
bound to octa acid are between 1.5 ppm to 1.2 ppm. Based on this
Fig. 6 Structures of the complexes (i) 1c@OA2, (ii) 1d@OA2,
(iii) 1e@OA2, and (iv) 1f@OA2 proposed based on NMR analysis.
observation, it can be concluded that the two methylene groups of
each guest (methylene groups marked 1 and 2 in Fig. 4) occupy
similar positions inside the cavity. Similarly, chemical shift of H3
signal of all four octa acid bound guests are between 0.3 ppm
and 0.2 ppm, confirming that its position inside the cavity is
comparable for all guests. Although the length of the aliphatic
chain increases considerably from 1c to 1f, coiled conformation of
the aliphatic chain allows the benzylic methylene groups to be in
a similar position. Since the methylene groups 1, 2 and 3 (Fig. 4)
are linked to the phenyl rings of the guest, it can be concluded
that position of the aromatic groups of the guest is not altered
substantially in spite of the change in alkyl chain lengths.
NOESY NMR experiments were performed to obtain additional information on the nature of complexes and the spectra are
included in the ESI (Fig. SI 3, SI 6, SI 9 and SI 12).‡ Fig. 5(i)–
(iv) provide pictorial representation of the intermolecular NOESY
correlations observed in each complex. In the 1c@OA2 complex
(Fig. 5(i)), both the CH3 and CH2 groups show strong correlations
with H
g, He and Hf of the host. As the individual aromatic signals
of the guest were not identified in the spectra, correlations between
the host and aromatic hydrogens of the guest were not explored.
NOESY spectra of complexes of 1d–f with octa acid also showed
NOE correlations between H
g of the host and CH3 and CH2
groups of the guest (Fig. 5 and Fig. SI 3, SI 6, SI 9 and SI 12
in ESI‡). The important difference in the NOESY spectra is that
while through space interaction between the aromatic hydrogens of
the host and the aliphatic methylene and methyl units of the guest
were prominent in 1c@OA2, 1d@OA2 and 1e@OA2, CH3 of 1f and
aromatic hydrogens of the host did not show such correlation. As
mentioned earlier, there is an additional 1ppm upfield shift for CH3
signal of this complex (Fig. 4(iv)). Lack of NOESY correlation
between the CH3 group of 1f and the aromatic signals of the guest
corroborates the conclusion based on 1H NMR result that methyl
group of 1f is placed deeper inside the cavity than the methyl
groups of other guests.
Pulsed-Field Gradient Spin Echo (PGSE) NMR data provided
information for formation of stable complexes between OA and
1c–f. While the diffusion coefficient of free octa acid was calculated
to be 1.82 ¥ 10-10 m2 s-1, diffusion coeffecient of each complex was
significantly lower. Diffusion coefficients for the complexes were
1.61 ¥ 10-10 m2 s-1 (1c@OA2), 1.60 ¥ 10-10 m2 s-1 (1d@OA2), 1.57 ¥
10-10 m2 s-1 (1e@OA2), and 1.62 ¥ 10-10 m2 s-1 (1f@OA2). The
diffusion coefficients were not only less than that of free host, but
were comparable to complexes formed with octa acid and other
guests in 2 : 1 host : guest ratio.38,39
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In summary, the 1D and 2D NMR analysis of the complexes
of octa acid and guests 1c–f showed that all the four guests form
stable complexes in 2:1 host:guest ratio with octa acid. Further,
the arrangement of the guest in the complex is such that aliphatic
chain of the guest occupies one end of the cavity. The CH3 group
is placed deeper than the methylene groups of the alkyl chain as
shown by COSY NMR. The unsubstituted phenyl ring occupies
the opposite end of the cavity. The phenyl bearing the substituent
is wedged in the central portion of the cavity. With 1f as guest,
length of the molecule forces the alkyl chain to coil in the cavity.
The possible orientation of each guest in the host’s cavity is shown
in Fig. 6.
Photochemical results
Photolysis of compounds 1c–f as octa acid complexes as well
as in hexanes was performed in nitrogen-saturated solutions.
To confirm that the starting ketones and the photoproducts
were bound to octa acid during the reaction, 1H NMR spectra
of 1d@OA2 and 1f@OA2 were recorded prior to, and after
irradiation.InthehighfieldregionoftheNMRspectra(Fig.7and
8),followingirradiationmorethanonesetofsignalswasobserved.
WhiletheinitialNMRspectrumexisted(shownwithdottedlines),
the other signals are attributed to the photoproducts. To confirm
whether the new signals were indeed from the photoproducts,
compounds 3d, 3f, and 7d were synthesized and 1H NMR spectra
Fig. 7 Top: 1HNMRspectra(300MHz,D2O,10-3 MOA,10-2 Msodium
tetraborate) of (i) 1d@OA2, (ii) 1d@OA2 after irradiation, (iii) 3d@OA2
and (iv) 7d@OA2. Bottom: structures of 1d, 3d and 7d and the numbering
used in the NMR spectra.
Fig. 8 Top: 1HNMRspectra(300MHz,D2O,10-3 MOA,10-2 Msodium
tetraborate) of (i) 1f@OA2, (ii) and (iii) 1f@OA2 after irradiation, and
(iv) 3f@OA2. Bottom: structures of 1f, and 3f and the numbering used in
the NMR spectra.
of the octa acid complexes of these photoproducts were recorded
(Fig.7and8).ShowninFig.7arethe 1HNMRspectraof 1d@OA2
complex before (Fig. 7(i)) and after irradiation (Fig. 7(ii)). The
signals marked 3, 4 and 5 correspond to the methylene and
methyl signals of 1d (numbering is shown in Fig. 7, bottom). The
NMR spectrum of 1d@OA2 recorded after irradiation is shown
in Fig. 7(ii). Additional signals in Fig. 7(ii) marked 3¢, 4¢ and 5¢,
correlated well with the 1H NMR spectrum of the decarbonylated
product 3d shown in Fig. 7(iii). Likewise, signals marked 4† and 5†
in Fig. 7(ii) were assigned to the rearrangement product 7d shown
in Fig. 7(iv). Signal marked with * in Fig. 7(ii) is expected to be
from isomer of 3d (photoproduct 5 or 6 in Scheme 1).
Similarly, in the 1H NMR spectrum of 1f@OA2 recorded after
irradiation(Fig.8(ii)and8(iii)),signalsofboththestartingketone
anditsphotoproductswereobserved.Signalsmarked3,4,5,6and
7wereassignedtothestartingketonebasedonFig.8(i).Signals4¢,
5¢, 6¢ and 7¢ were assigned to the decarbonylated product 3f. The
1H NMR spectrum of the 3f@OA2 complex is shown in Fig. 8(iv).
Signals from isomer of 3f are marked with * in Fig. 8(ii) and 8(iii).
The photoproducts were extracted from the aqueous solution
with chloroform and were characterized by GC and GC-MS
analyses. Photoproducts isolated from hexane irradiations and
independently synthesized were used to identify the products
formed upon irradiation of the complexes. Relative yields of the
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products obtained from ketones 1c–f are summarized in Table 1.
When irradiated as hexane solutions, guests 1c–f yielded only
products 2, 3 and 4 in 1 :2 : 1 ratio (Table 1). When octa acid
complexes were irradiated, decarbonylated products 3, 5, and 6,
and rearrangement product 7 were formed. Cage escape products
viz., 2 (AA) and 4 (BB) were absent from all complexes. Based
on this result and the NMR spectra recorded after irradiation, it
can be confirmed that the radical pairs remain caged during the
course of the reaction. That is, the two OA molecules that form
the capsule do not fall apart even when the guest ketone exists as
two individual radicals.
While all the guests studied showed formation of in-cage
products only, the ratio of decarbonylated products versus rearranged products differed for each substrate. The yields of 7
from complexes 1b@OA2, 1c@OA2 and 1d@OA2 were 49%, 41%
and 45% respectively (Table 1). This result was comparable to
the 44% yield of 7 from 1a@OA2. But, yield of 7 from 1e@OA2
decreased to 15%. Irradiation of the 1f@OA2 complex yielded
only the decarbonylated 3 (80%) and its isomers 5 and 6 (20%).
Two factors play a role in the observed lower yield of 7 from
guests bearing longer aliphatic substituent (1e and 1f). Scheme 2
shows the intermediate steps involved in the formation of 7 over
the course of the reaction. The first step is the a-cleavage process
leading to formation of RP-1 (Scheme 2(i)). The second step is the
~90rotation of the benzyl radical with respect to the phenylacyl
radical (Scheme 2(ii)). Recombination of the radicals followed by
1,5-hydrogen shift yields 7 (Scheme 2 (iii) and (iv)).
Based on NMR data we concluded that the initial orientation
of each guest in the complex is such that the aliphatic part is coiled
into one of the narrower ends of the cavity, with the CH3 group
at the bottom (Fig. 6). Position of the two phenyl rings in the
cavity is different due to the presence of para-alkyl substituent
on one phenyl ring. To distinguish the two rings, the aliphatic
substituent and the ring bearing it is referred as ‘B’, while the
unsubstituted phenyl ring is called ‘A’ and the dibenzyl ketone
is represented as A-CO-B. Initially, group A is closer to one end
of the cavity (top half of the complexes shown in Fig. 6), while
group B is pushed towards the central portion by the aliphatic
substituent (bottom half of the complexes shown in Fig. 6). The
difference in orientation between the guests 1c–f is that when the
aliphatic chain length is increased, the phenyl ring of group B
is progressively closer to the central portion of the cavity. The 1H
NMR chemical shifts of the benzylic methylene group (‘3’ in Fig. 4
and Table 2) showed a downfield shift of 0.1 ppm and 0.2 ppm for
1e@OA2 and 1f@OA2 respectively. The downfield shift is a result
of the marginal shift of the methylene group towards the center of
the capsular assembly. This also points to a marginal shift of the
phenyl ring bearing that group towards to central portion of the
cavity.
Upon photolysis, once RP-1 is formed, the two intermediate
radicals can be accommodated into two halves of the cavity
independently. At this stage, octa acid templates the radicals into a
specific orientation and alters the product distribution. The radical
‘A’ rotates to a stable orientation inside the cavity, in which CH3
group occupies the tapering end of the cavity. The principal driving
force for rotation of benzyl radical ‘A’ is that the CH3 group better
complements the narrow end of the cavity. Other factors such as
CH–p interaction between the methylene group of the guest and
the aromatic rings of OA and van der Walls interaction between
the guest radical and the host may also stabilize this orientation.
For 7 to form, the rate of rotation of ‘A’ must be greater than rate
of decarbonylation of ‘B’ (~106 s-1).40,41 Comparison of yields of
7 from 1c and 1d (41 and 45% respectively) and decarbonylated
products 3, 5 and 6 (59 and 55% respectively) shows that the
rate of rotation of ‘A’ is comparable to rate of decarbonylation
(Table 1). It can be concluded that yield of 7 is a reflection of
the relative rate of rotation of ‘A’. During the rotation of benzyl
radical ‘A’, the phenylacyl radical ‘B’ remains stationary since the
para-substituent anchors the phenyl ring in a fixed geometry.
The yield of 7 obtained from irradiation of 1e@OA2 was 15%,
with the rest being decarbonylated products (3, 5 and 6) (Table 1).
The observed yield of 7 from 1e@OA2 was lower than the 41% and
45% yield from 1c@OA2 and 1d@OA2. This result is significant,
since the reaction mechanism is not likely to be altered by the
change in aliphatic chainlength at the para position. The rate of
decarbonylation is also not expected to be different for guests
1c–f. Hence, the additional CH2 unit in 1e alters the reaction
pathway indirectly. Since, formation of 7 is a direct reflection of
the ability of the benzyl radical to rotate, it can be concluded that
with 1e as guest, rotation of the benzyl radical ‘A’ is slower than
with guests 1a–d. It is likely that the 4-butylphenylacyl radical
(B–CO∑) of 1e extends beyond one-half of the capsule, into the
opposite end of the cavity to a greater extent than the other shorter
radicals. Consequently, the corresponding benzyl radical ‘A’ does
not posses the same amount of free space it had in the cases of
1a–d. Most likely the reduction in free space plays a crucial role in
the decreased yield of the rearrangment product 7e.
Available free space inside the cavity is further diminished
in case of 1f@OA2 and no rearrangement product is observed.
Decarbonylated AB and its isomers are the only products formed.
It is very likely that the free space determined by the alkyl group
present at the para position of the reactant ketone that allows the
Scheme 2 Conversion of ketone 1 (a–f) to the photoproduct 7 is expected to occur through the following steps shown in this scheme: (i) formation of
RP-1 followed by the reorientation of the benzyl radical to form the structure (ii). Radical recombination followed by 1,5-hydrogen shift results in 7.
1562 | Photochem. Photobiol. Sci., 2008, 7, 1555–1564 This journal is © The Royal Society of Chemistry and Owner Societies 2008
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benzyl radical to rotate is severely reduced with pentyl substituted
ketone 1f to allow formation of any rearrangement product. Fig. 9
shows a schematic representation of the intermediate radical pairs
of complexes 1c@OA2, 1d@OA2, 1e@OA2 and 1f@OA2. As shown
in Fig. 9, the substituted benzyl radical (B) extends beyond the
midpoint of the cavity with increase in the alkyl chain length.
Consequently, the space available for the unsubstituted benzyl
radical (A) is gradually reduced. The outcome is the observed
greater yield of decarbonylated products compared to the rearrangement products from substrates bearing longer substituents.
Thus, length of the para-alkyl tether, which is separated from
the reaction center, controls the changes at a distant location.
Whether the guest is 1a or 1d, comparable yield of 7 illustrates the
rotational freedom experienced by the benzyl radical. However,
one additional methylene group in 1e controls the reaction by
sterically inhibiting the rotation of the other radical pair.
The role of cavity volume and its effect on molecular recognition
has been reported in closely related self-assembled complexes.42
The ideal binding ratio is expected when the guest volume is
about 55% of the host volume. Considering that the volume of the
starting dibenzyl ketone and the volumes of the two rearrangement
products are identical, preference for formation of one of the two
rearrangement products is not driven by their volume, but by their
shape.
Fig. 9 Representation of the caged radical pairs and the corresponding yields from each complex.
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The aforementioned discussion is valid when the a-cleavage
reaction yields A and CO-B radicals. But, the a-cleavage reaction
yields both the A-CO/B radical pair and A/CO-B radical pair.
When the reaction is carried out in octa acid, and when A-CO/B
radical pair is formed, alkyl chain of B radical is initially in its
preferred orientation, i.e., in the narrow end of the cavity. Rotation
of this (B) radical to yield ortho rearrangement product is not
expected, and ortho rearrangement product is not observed from
the reaction. It is likely that the A-CO radical decarbonylates to
form the A radical and yield products 3, 5 and/or 6. We believe
that from the data on hand further speculation on this process is
fruitless.
1H NMR analysis of the complexes before and after irradiation
supports this conclusion. 1H NMR spectra of 1d@OA2 and
1f@OA2 are shown in Figs. 7 and 8 respectively. Initially, the
CH3 group of bound 1f is at -3.4 ppm (Fig. 8(i)) whereas the
CH3 signal of 1d is at -2.4 ppm (Fig. 7(i)). The additional 1 ppm
upfield shift was attributed to the orientation of the aliphatic chain
where the CH3 group of 1f is placed deeper inside the cavity. After
irradiation, signals of the photoproducts formed from 1f show a
downfield shift when compared to the signals of 1f. So, the snug fit
of 1f@OA2 is slackened with loss of CO and the products are not
placed as deep in the cavity as the starting ketone. In comparison,
1H NMR signals of the photoproducts of 1d@OA2 show an upfield
shift (Fig. 6). For 1d@OA2 complex, rotation of the benzyl radicals
leads to better binding with the host than the starting ketone itself
and is reflected on the upfield shift of the product signals.
Overview
Results presented in this study highlight that photoreactions
within a confined space are subject to the availability of free space
for the reacting guest molecule. We have shown that one could
control the free space and thereby the photoproducts by a remote
alkyl tether. The remote tether which has no effect on the excited
state chemistry of reactants in isotropic solution has a determining
role in a confined space. We have used the photochemistry of paraalkyl dibenzyl ketones to bring out the importance of free space
within a confined space.
Acknowledgements
We thank the National Science Foundation, USA for financial
support (CHE-0213042 and CHE-0531802).
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1564 | Photochem. Photobiol. Sci., 2008, 7, 1555–1564 This journal is © The Royal Society of Chemistry and Owner Societies 2008
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