Role of free space and conformational control on photoproduct selectivity of optically pure α-alkyldeoxybenzoins within a water-soluble organic capsule

Article abstract of DOI:10.1021/jo3023928

Optically pure α-alkyl deoxybenzoins resulting in products of Norrish Type I and Type II reactions upon excitation has been investigated within the octa acid (OA) capsule in water. The product distribution was different from that in an organic solvent and was also dependent on the length of the α-alkyl chain. Most importantly, a rearrangement product not formed in an organic solvent arising from the triplet radical pair generated by Norrish Type I reaction was formed, and its yield was dependent on the alkyl chain length. In an organic solvent, since the cage lifetime is shorter than the time required for intersystem crossing (ISC) of the triplet radical pair to the singlet radical pair the recombination with or without rearrangement of the primary radical pair (phenylacetyl and benzyl) does not occur. Recombination without rearrangement within the capsule as inferred from monitoring the racemization of the optically pure α-alkyl deoxybenzoins suggesting the capsule's stability for at least 10^-8 s (the time required for ISC) is consistent with our previous photophysical studies that showed partial opening and closing of the capsule in the time range of microseconds.

Full text of DOI:10.1021/jo3023928

Article
pubs.acs.org/joc
Role of Free Space and Conformational Control on Photoproduct
Selectivity of Optically Pure α‑Alkyldeoxybenzoins within a Water-
Soluble Organic Capsule†
Revathy Kulasekharan, Murthy V. S. N. Maddipatla, Anand Parthasarathy, and V. Ramamurthy*
Department of Chemistry, University of Miami, Coral Gables, Florida 33124, United States
S
* Supporting Information
ABSTRACT: Optically pure α-alkyl deoxybenzoins resulting in products of
Norrish Type I and Type II reactions upon excitation has been investigated
within the octa acid (OA) capsule in water. The product distribution was
different from that in an organic solvent and was also dependent on the length of
the α-alkyl chain. Most importantly, a rearrangement product not formed in an
organic solvent arising from the triplet radical pair generated by Norrish Type I
reaction was formed, and its yield was dependent on the alkyl chain length. In an
organic solvent, since the cage lifetime is shorter than the time required for
intersystem crossing (ISC) of the triplet radical pair to the singlet radical pair the
recombination with or without rearrangement of the primary radical pair
(phenylacetyl and benzyl) does not occur. Recombination without rearrange-
ment within the capsule as inferred from monitoring the racemization of the
optically pure α-alkyl deoxybenzoins suggesting the capsule’s stability for at least
10−8 s (the time required for ISC) is consistent with our previous photophysical studies that showed partial opening and closing
of the capsule in the time range of microseconds.
due to Norrish Type I reaction of carbonyl compounds yields a
radical pair. The short 0.1 ns cage lifetime in solution prohibits
the intersystem crossing (ISC) of the triplet radical pair within
a solvent cage.20 It normally does not recombine with or
without rearrangement within the solvent cage. Interestingly,
irradiation of dibenzyl ketone,21 short-chain α-alkyl dibenzyl
ketones, and p-alkyl dibenzylketones22 included within OA
although underwent Norrish Type I reaction they yielded
significant amount of a rearrangement product of the Type I
primary radical pair (phenylacetyl and α-alkyl benzyl pair) in
competition with decarbonylation of phenylacetyl radical (three
examples chosen from our previous studies are highlighted in
Scheme 1). This is consistent with the notion that unlike in
organic solvents, the cage lifetime within a capsule was longer
(>5 μs)23 and the singlet radical pair was generated via ISC of
the initially formed triplet radical pair before it could escape to
the aqueous solution. This permitted the singlet radical pair to
give recombination products (with rearrangement) that are not
obtained in organic solvents. Although formation of rearranged
product suggested that the primary radical pair following
Norrish Type I recombined following tumbling of the α-alkyl
benzyl radical it was not clear whether the phenylacetyl and
benzyl radical pair recombined without rearrangement to
generate the reactant ketone. We felt it is important to learn
whether the recombination process was possible without
rearrangement within the OA capsule.
INTRODUCTION
■
Excited-state process manipulation through supramolecular
effects has drawn considerable attention during the last three
decades.1−3 The controlling factors of the product distribution
have been understood in terms of cage, conformational,
preorientational, and local-concentration factors4 with the
effects attributed to weak intermolecular forces and confine-
ment provided by the supramolecular assemblies. While early
investigations of supramolecular photochemistry were confined
to crystals,5 with time the emphasis shifted to aqueous
medium.6,7 In this context, water-soluble hosts such as micelles,
cyclodextrins, cucurbiturils, calixarenes, and Pd-nanocage and
various organic cavitands have played a significant role as
reaction containers.3,8−15 Efficient exploitation of the hosts as
reaction containers requires an understanding of the transla-
tional and rotational mobility of molecules within these
confined spaces. In this paper, we explore one such motion
by examining the excited- state behavior of optically pure α-
alkyl deoxybenzoins included within a water-soluble organic
capsule.
From our investigations on the utility of a synthetic water-
soluble deep cavity cavitand octa acid (Scheme 1; OA)16 as a
host for manipulating the excited state processes of organic
guest molecules,17,18 we have demonstrated that the photo-
chemistry of α-alkyl dibenzyl ketones could be controlled
within this host.19 In organic media, these ketones undergo
conformation dependent Norrish Type I and Type II reactions.
The reaction of immediate concern to the question addressed
here is Norrish Type I reaction. In general, α-cleavage reaction
Received: November 2, 2012
Published: December 29, 2012
© 2012 American Chemical Society
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Scheme 1. Examples Illustrating the Difference in Product Distribution between Solution and OA Photochemistry
Scheme 2. Chemical Structure of OA and α-Alkyldeoxybenzoins
To address the above question we have examined the
photochemistry of optically pure α-alkyl deoxybenzoins 1a−d
(Scheme 2; ADB) that could be readily synthesized following a
recently reported procedure.24,25 Similar to α-alkyl dibenzyl
ketones, these deoxybenzoins are established to undergo both
Type I and Type II reactions (Scheme 3) from excited triplet
state.26,27 By HPLC monitoring of the racemization of
photoexcited optically pure 1a−d we have established the
cage recombination of the primary radical pair to yield the
starting ketone within the OA capsule regardless of the alkyl
chain. The results suggest the lifetime of the capsule confined
triplet radical pair to be long enough to intersystem cross to the
singlet radical pair and recombine to the racemic reactant. As
anticipated, product distributions monitored by GC and HPLC
revealed that the excited state behavior of the ADBs and their
corresponding radical intermediates were distinctly different
within OA capsule solubilized in aqueous solution from that in
benzene solution and are discussed below.
To appreciate the results presented here it is important to be
aware of the following characteristics of the supramolecular
assemblies of OA and guests: (a) OA forms 1:1 cavitandplex
with guest molecules that contain polar head groups such as
ammonium or carboxylic acid. With hydrophobic guest
molecules, depending on the size it forms either a 1:2 or 2:2
(guest to host) complex that is termed “capsule”.28 (b) Within
the capsule the guest molecule depending on its size and shape
have some mobility; in other words, they are not stationary.29
(c) The capsule itself partially opens and closes in the time
scale of 0.5 μs23 and fully opens in ∼2.7s.30 Therefore, in the
excited time scale of the carbonyl compounds investigated here
the capsule is expected to be stable. (d) Using photophysical
probes and EPR techniques the internal polarity of the capsule
is determined to be close to that of benzene.31,32 This is not
surprising as the internal cavity of OA is laced with substituted
benzenes. Determined polarity suggests that there are no water
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Scheme 3. Product Distribution upon Irradiation of α-Alkyldeoxybenzoins
Scheme 4. Synthetic Scheme for the Preparation of Racemic and Optically Pure α-Alkyl Deoxybenzoins
molecules within the capsule and the capsule is stable in the
excited-state time scale.
Capsular host−guest assemblies (2:1) of OA and 1a−d were
formed upon stirring an aliquot of DMSO stock solution of
water-insoluble 1a−d with OA in the ratio of 1:2 in borate
1
buffer solution (pH ∼9). In Figure 1 H NMR spectra of the
RESULTS AND DISCUSSION
■
alkyl part of ADB@OA2 are provided. Evidently, all alkyl
proton signals were upfield shifted with respect to that in
CDCl3, most dramatically in propyl-, hexyl-, and octyl-
substituted ones. This is an indication of inclusion of guests
within the OA host.28 1H NMR titration experimental data
(Figures S2−S5, Supporting Information) as well as the
measured diffusion constants by DOSY (diffusion ordered
spectroscopy) experiments (Table S1, Supporting Information)
suggested the 1a−d location within the capsule formed by two
molecules of OA. During the titration experiments as followed
by 1H NMR there was only one set of upfield-shifted signal for
the guest molecules, suggesting that there are no free molecules
in solution. Furthermore, the chemical shift for the guest
molecules was constant independent of the ratio of OA to the
guest, suggesting that there is no exchange between free and
complexed guest molecules in the NMR time scale.
Optically pure ADBs were synthesized by following the recently
reported procedure for propyl deoxybenzoin (1b) (Scheme
4).25 As expected, the synthesized ADBs ((R)-1) gave a single
peak upon analysis by HPLC (Chiralpak ADH, hexane/2-
propanol 98:2, 0.35 mL/min, T = 25 °C, λ = 254 nm) as
opposed to the racemic ADBs (1) with two peaks (Figure S1,
Supporting Information). As shown in Scheme 4, optically pure
(R,R)-diastereomer (8) was prepared by asymmetric hydro-
genation reaction via dynamic kinetic resolution with excellent
enantio- and diastereoselectivities by using the chiral catalyst
(RuCl2[(S)-(DM-SEGPHOS)][(S)-DAIPEN]). The absolute
stereochemical integrity of the α-carbon center was also
maintained when 8 (99% ee) was subjected to Dess−Martin
oxidation reaction.33 Product (R)-1 thus obtained was optically
pure.
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increasing chain length (methyl, propyl, hexyl and octyl) would
decrease the free volume of the capsular assembly and
consequently limit the freedom of the guest molecules therein
(i.e., methyl deoxybenzoin would be more mobile than octyl
deoxybenzoin) and be reflected in the product distribution
upon excitation of the ADBs@OA2
In Table 1, results of photolysis of (R)-ADBs in benzene and
(R)-ADBs@OA2 in borate buffer solution are presented.
Consistent with literature reports all four ADBs gave products
of Type I reaction in benzene (Scheme 3).20,24,26,27,34 As
expected, while methyl deoxybenzoin (1a) did not give Type II
products, ADBs 1b−d gave products of both Type I and Type
II reactions. The alkyl chain-length dependent photochemistry
of ADBs@OA2 was significantly different from that in benzene.
Notably unlike in solution, methyl and propyl deoxybenzoins
(1a and 1b) within OA underwent Type I reaction yielding
rearrangement product 6 (Scheme 3). On the other hand,
capsule included 1c and 1d upon excitation preferentially
yielded Type II products (2 and 3, Table 1). No rearrangement
products of the starting ketone were detected by GC analysis.
We believe these observations result from the influence of the
OA capsule on the conformation of ADBs’ alkyl chain in a
manner similar to that of the α-alkyl dibenzyl ketones.19
To fully understand the influence of OA on the products
distribution in the above systems, we irradiated optically pure
1a−d included within OA capsule for 10 min (conversion was
less than 20%) under identical conditions and analyzed the
Figure 1. Partial 1H NMR (500 MHz) spectra of various complexes of
doexybenzoins with OA: (i) 1a@OA2, (ii) 1b@OA2, (iii) 1c@OA2,
and (iv)1d@OA2. The protons of the aliphatic chain of guests are
numbered with methyl as 1.
Consistent with the observed chemical shifts of the methyl
groups in methyl deoxybenzoin and octyldeoxybenzoin (δ ∼0.2
and −3.6 ppm, respectively), the NOESY correlations shown in
Figure 2 suggested that the methyl group in the former was
closer to the middle region while being buried at the narrower
part of the capsule in the latter. Based on the chemical shifts of
the methyl and methylene signals and their NOESY
correlations with the host OA protons, we believe that the
alkyl chains in 1b, 1c, and 1d were linearly placed along the
long axis of the capsule (Figures S6−S11, Supporting
Information). Based on the above data, we visualized that
Figure 2. (Left) Comparison of NOESY correlations of (ia) 1a@OA2 and (iia) 1d@OA2. (Right) Pictorial representation of the NOESY
interactions in the above systems is shown in (ib) and (iib).
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the rotational motion of the radical pair resulting via the Type I
process and keeps them within the capsule.
Table 1. Product Distribution upon Photolysis of α-ADBs in
Benzene and as Complexes within the Capsule in Borate
Buffer
Upon examination of the results obtained upon photolysis of
optically pure ADBs presented in Table 1, one should note
three important observations: (a) Consistent with literature
reports there was very little racemization when the ketones
were irradiated in benzene.20 (b) When the ADBs included in
OA capsule were irradiated the optical purity of the ketone was
lost in all cases. (c) The extent of loss of optical purity was
dependent on the alkyl chain length. For example, under
identical conditions and irradiation timing α-methyldeoxyben-
zoin racemized more readily (24% S-isomer) than α-
octyldeoxybenzoin (12% S-isomer). The racemization of 1c
and 1d despite not giving any rearrangement product 6 via
recombination of the primary radical pair (benzoyl−alkyl
benzyl radical pair) clearly suggested that enhancement of the
Type II products within OA was not entirely due to
suppression of the Type I reaction. These results imply that
the influence of OA capsule on product distribution occurs by
controlling the conformation of the reactant ketone and not by
prohibiting the Type I process. The 100% rearranged product
yield from α-methyl deoxybenzoin and none from α-octyl
deoxybenzoin is suggestive of the importance of the free space
within the cavity in controlling the behavior of the radical pair.
The definition of free space within a reaction cavity formed by
an organized assembly is illustrated in a cartoon fashion in
Figure 3.35 The long α-octyl benzyl radical nearly filling the
composition of
type II
optical
b,
c
type I products
products
isomers
alkyldeoxybenzoin/
R
S
a
medium
4
5
6
2
3
isomer isomer
1a/benzene
26
74
95
75
96
80
96
84
96
88
5
25
4
1a@OA2/buffer
1b/benzene
100
60
12
12
6
17
24
49
54
34
36
83
24
46
17
6
1b@OA2/buffer
1c/benzene
20
4
32
17
21
54
1c@OA2/buffer
1d/benzene
16
4
1d@OA2/buffer
12
a
The samples were irradiated in Pyrex tubes (>280 nm) using a
medium-pressure Hg lamp for 10−30 min (to ensure at least 20−25%
conversion); all products were analyzed by GC using an HP-5 column.
To begin, all chiral alkyl deoxybenzoins were of 99% optical purity.
Analyzed by HPLC (Chiralpak ADH, hexane/2-propanol 98:2, 0.35
b
c
mL/min, T = 25 °C, λ = 254 nm).
optical purity of the isolated reactant ketones by HPLC
(Chiralpak ADH column). We hypothesized Type I cleavage as
illustrated in Scheme 5 would lead to racemization of the
optically pure ketone provided the OA capsule did not prohibit
Scheme 5. Photoracemization of α-Alkyldeoxybenzoins Occurring within the Capsular Assembly of OA
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molecules in understanding the molecular motions in confined
spaces. This approach along with NMR and molecular
modeling could be of great value in understanding time-
dependent host−guest structures.
EXPERIMENTAL SECTION
■
Materials and Methods. Octa acid was synthesized according to
the reported procedure.16 Racemic as well as chiral guest molecules
(1a−d) were prepared by adopting the procedure detailed below.25
General Synthetic Protocol for Chiral α-Alkyl Deoxybenzoin.
Synthesis of 1. To a suspension of potassium tert-butoxide (1.85 g,
16.5 mmol) in tetrahydrofuran (THF) was added a solution of
deoxybenzoin (2) (2.5 g, 12.7 mmol, 1 equiv) in THF dropwise over a
period of 45 min at 0 °C. Iodoalkane (1.1 equiv) solution in THF was
added gradually to keep the internal temperature <3 °C. The reaction
was then allowed to warm to room temperature. After 2 h of stirring at
room temperature, the reaction mixture was slowly quenched with 2 N
HCl and diluted with ethyl acetate. The aqueous layer was removed
and the organic layer washed with water and brine and purified by flash
chromatography (ethyl acetate/hexane).
Figure 3. Cartoon representation of the reaction cavity in an organized
assembly. (a) General representation showing the reaction cavity and
the guest inside. Available free space following the occupation of the
guest is projected with a white space indicated by an arrow. (b)
Capsule has more free space when α-methyl deoxybenzoin is the guest.
(c) Capsule has less free space when α-octyl deoxybenzoin is the guest.
Synthesis of 8.25 In a glovebox, chiral catalyst (RuCl2[(S)-(DM-
SEGPHOS)][(S)-DAIPEN]) (0.015 g, 0.012 mmol, 0.001 equiv),
potassium tert- butoxide (0.22 g, 0.2 mmol), and 2-propanol (3 mL)
were mixed together in a flask and stirred at room temperature for 2 h.
The resulting activated yellow catalyst solution was charged into a
hydrogenation glass reactor vessel, and a solution of α-alkyl
deoxybenzoin (1 equiv) in 2-propanol (13 mL) was added to it.
The vessel was capped and hydrogenated at 100 psi hydrogen for 40 h
at 25 °C. The reaction mixture was treated with 0.5 g of Norit-A
decolorizing carbon and stirred at room temperature for 40 min. The
mixture was filtered over a Celite pad and washed with 10 mL of 2-
propanol. The volume of 2-propanol was adjusted to about 40 mL. To
this 80 mL of H2O was added slowly over 45 min to crystallize the
product. The resultant slurry was filtered by gravity filtration (without
applying vacuum) and washed with 1:2 2-propanol/H2O (40 mL), and
the wet cake was air-dried and further dried in high vacuum to afford
anti-8 (R,R diastereomer).
entire cavity very likely could not tumble within the capsule as
freely as the small α-methyl benzyl radical and proceeded
through a Type I reaction to yield the starting ketone. For easy
visualization, the variation of free space with alkyl chain length
is illustrated in Figure 3.
The rearrangement of the primary Norrish Type I radical
pair could occur by tumbling of the α-alkyl benzyl radical or
sliding of the benzoyl radical within the capsule. Both motions
require free space within the capsule. The availability of the free
space within the capsule is dependent on the length of the alkyl
chain. Once the capsule is filled by the alkyl chain of the guest
as in the case of α-octyl deoxybenzoin neither of these motions
are likely. The absence of rearranged product 6 in the case α-
octyl deoxybenzoin is supportive of this model. Epimerization
of optically pure α-octyl deoxybenzoin in spite of the absence of
rearrangement product suggests that rotational motion of the
radical pair along the long axis of the capsule is not prohibited
although tumbling and sliding of the radical pair seem to be
curtailed. To gain better insight into the role of the length of
the alkyl chain on the racemization process quantum yield data
would be needed that we do not have at this stage.
Synthesis of (R)-(−)-1.33 (1R,2R)-1,2-Diphenylalkan-1-ol (1 equiv)
was dissolved in dichloromethane, Dess−Martin periodinane (1.4
equiv) was added, and the mixture stirred for 45 min at room
temperature. The reaction mixture was filtered over a Celite pad, and
the filtrate was washed with sodium bicarbonate solution. The
resultant organic layer was distilled to yield an off-white crystalline
solid.
HPLC Analysis. Chiralpak AD-H, hexane/ethanol 98/2, 0.35 mL/
min, T = 25 °C, λ = 254 nm. Retention times of racemic mixture (1a):
23.4 and 26.9 min, chiral (1a): 26.9 min. Retention times of racemic
mixture (1b): 19.8 and 22.5 min, chiral (1b): 22.5 min. Retention
times of racemic mixture (1c): 19.7 and 21.6 min, chiral (1c): 21.7
min. Retention times of racemic mixture (1d): 18.9 and 20.9 min,
chiral (1d): 21.0 min (Figure S1, Supporting Information).
1H NMR and MS Data of the Guests. All 1H NMR spectra were
recorded on a 500 MHz NMR spectrometer. ESI-MS spectra were
obtained using mass spectrometers equipped with time-of-flight
(TOF) and ion trap analyzers.
1a: (300 MHz, CDCl3) δ 8.04 (d, 2H), 7.22−7.50 (m, 8H), 4.57 (t,
1H), 1.69 (d, 3H); HRMS (ESI-LC-MS) solution of 1a in hexane/
acetone/acetonitrile (25:25:50) + 0.1% formic acid) calcd C15H14O
[M + Na]+ 233.0942, found 233.0917.
1b: (300 MHz, CDCl3) δ 7.95 (d, 2H), 7.15−7.46 (m, 8H), 4.53 (t,
1H), 2.15 (m, 1H), 1.80 (m, 1H), 1.28 (m, 2H), 0.90 (t, 3H); HRMS
(ESI-LC-MS) solution of 1b in hexane/acetone/acetonitrile
(25:25:50) + 0.1% formic acid) calcd C17H18O [M + Na]+
261.1255, found 261.1239.
SUMMARY
■
The above photochemical studies have established that the OA
capsule holds the primary radical pair generated by the Type I
process for longer than 10−8 s, a period long enough for the
triplet radical pair to undergo intersystem crossing. In this time
period the α-alkyl benzyl radical tumbles and rotates to yield
the racemic reactant and rearranged product. The extent of
product formation via Type I and Type II reactions within the
capsule is dictated by the length of the α-alkyl chain that
determines the available free space within the capsule and also
the conformation of the reactant ketone. All molecules
independent of the chain length underwent racemization
suggesting that the radical pair generated by the Norrish
Type I reaction has enough free space to tumble and rotate
within the confined space. Dependence of racemization on the
alkyl chain length highlights the role of the available free space
on rotational mobility with the reaction cavity. The current
study has unequivocally established the value of confining
molecules within a molecular container to achieve selectivity in
photoreactions. This study also brings out the value of classical
photochemical investigations with optically pure organic
1c: (300 MHz, CDCl3) δ 8.04 (d, 2H), 7.26−7.62 (m, 8H), 4.65 (t,
1H), 2.24 (m, 1H), 1.90 (m, 1H), 1.35 (m, 8H), 0.89 (t, 3H); HRMS
(ESI-LC-MS) solution of 1c in hexane/acetone/acetonitrile
(25:25:50) + 0.1% formic acid), calcd C20H24O [M + Na]+
303.1725, found 303.1714.
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1d: (300 MHz, CDCl3) δ 8.02 (d, 2H), 7.22−7.59 (m, 8H), 4.62 (t,
1H), 2.26 (m, 1H), 1.87 (m, 1H), 1.35 (m, 12H), 0.88 (t, 3H); HRMS
(ESI-LC-MS) solution of 1d in hexane/acetone/acetonitrile
(25:25:50) + 0.1% formic acid) calcd C22H28O [M + Na]+
331.2030, found 331.2021.
thank Professor C. Hoff for help with the synthesis of optically
pure ADBs.
DEDICATION
■
†This paper is dedicated to the memory of Professor N. J.
Procedure for Preparation of Octa Acid−guest Complex,
Photolysis and Analysis of Photoproducts. Stock solution of the
guest (1a−d) was prepared in DMSO-d6. Octa acid solution (5 mM)
was prepared in sodium tetraborate buffered D2O. Aliquots of the
guest solution were added to octa acid solution maintaining a host−
guest ratio of 2:1, and the solution was sonicated for 30 min. NMR
analysis of the solution showed formation of a complex. The solution
was then bubbled with nitrogen for 30 min and irradiated using a
medium-pressure Hg lamp for 10−30 min (to ensure at least 20−25%
conversion). The photoproducts were extracted from the aqueous
solution with chloroform and the organic layer was analyzed by GC.
Retention times of each guest and its products are as given below.
Substrate 1a. Column: HP-1, temperature program: initial temp,
70 °C; initial time, 1 min; rate, ramp 10 °C/min; ramp ends, 220 °C;
ramp time, 1 min; rate, 3 °C/min final temp, 270 °C; final time, 10
min. Retention times: 1a, 14.00 min; 4, 14.93 min; 5a, 12.56, 12.913
min; 6a, 15.52 min. Irradiation time: 15 min.
Turro, an outstanding teacher and selfless mentor.
REFERENCES
■
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Substrate 1b. Column: HP-1, temperature program: initial temp,
70 °C; initial time, 1 min; rate, ramp 10 °C/min; ramp ends, 220 °C;
ramp time, 1 min; rate, 3 °C/min final temp, 270 °C; final time, 10
min. Retention times: 1b, 17.99 min; 2b, 15.78 min; 3, 14.13 min; 4,
14.93 min; 5b, 16.19, 16.39 min; 6b, 18.01 min. Irradiation time: 15
min.
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Substrate 1c. Column: HP-1, temperature program: initial temp, 70
°C; initial time, 1 min; rate, ramp 10 °C/min; ramp ends, 220 °C;
ramp time, 1 min; rate, 3 °C/min final temp, 270 °C; final time, 10
min. Retention times: 1c, 19.42 min; 2c, 19.49, 19.69 min; 3, 14.13
min; 4, 14.93 min; 5c, 22.27, 23.14 min. Irradiation time: 15 min.
Substrate 1d. Column: HP-1, temperature program: initial temp,
70 °C; initial time, 1 min; rate, ramp 10 °C/min; ramp ends, 220 °C;
ramp time, 1 min; rate, 3 °C/min final temp, 270 °C; final time, 10
min. Retention times: 1d, 22.23 min; 2d, 22.29, 22.52 min; 3, 14.13
min; 4, 14.93 min; 5d, 26.14, 26.45 min. Irradiation time: 15 min.
GC−MS (EI) (m/z, relative intensities) of photoproducts: GC−MS
analysis. GC−MS analyses were performed on a GC instrument fitted
with a HP-5 column and EI detector. 1a: 210 (M+, 6), 196 (11), 105
(100), 91 (15). 1b: 238 (M+, 2), 196 (10), 165 (2), 105 (100), 91
(32). 1c: 280 (M+, 1), 196 (55), 176 (45), 134 (100), 120 (30), 105
(16), 91 (14). 1d: 308 (M+, 1), 196 (8), 134 (16), 105 (100), 91 (39).
3: 196 (M+, 3), 105 (100), 91 (8). 2b: 238 (M+, 0), 221 (8), 196 (67),
148 (100), 133 (45), 105 (85), 91 (28). 2c: 280 (M+, 1), 262 (3), 196
(61), 148 (100), 133 (45), 105 (71), 91 (31). 2d: 308 (M+, 0), 291
(11), 205 (4), 196 (51), 148 (100), 133 (49), 105 (63), 91 (41). 6a:
210 (M+, 10), 181 (100), 105 (45). 6b: 238 (M+, 14), 196 (20), 181
(100), 105 (35).
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ASSOCIATED CONTENT
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S
* Supporting Information
1H NMR titration spectra, 2-D COSY and NOESY spectra of
host−guest complexes. This information is available free of
charge via the Internet at http://pubs.acs.org.
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Ramamurthy, V. Langmuir 2009, 25, 10575.
AUTHOR INFORMATION
Corresponding Author
*Tel: 305 2841534. E-mail: murthy1@miami.edu.
Notes
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(29) Kulasekharan, R.; Jayaraj, N.; Porel, M.; Choudhury, R.;
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N.; Parthasarathy, A.; Ramamurthy, V. Langmuir 2009, 25, 3473.
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
V.R. is grateful to the National Science Foundation for
generous financial support (CHE-0848017) and for the funds
toward the purchase of a LC-ESI-MS (CHE-0946858). We
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The Journal of Organic Chemistry
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