COMMUNICATION
www.rsc.org/obc | Organic & Biomolecular Chemistry
Photo-Fries reaction in water made selective with a capsule†
a
b
b
a
Lakshmi S. Kaanumalle, Corinne L. D. Gibb, Bruce C. Gibb* and V. Ramamurthy*
Received 21st November 2006, Accepted 22nd November 2006
First published as an Advance Article on the web 29th November 2006
DOI: 10.1039/b617022f
The water soluble capsule formed by a deep cavity cavitand
with eight carboxylic acid groups controls product distribu-
tion during photo-Fries rearrangement of naphthyl esters in
water by restricting the mobility of primary singlet radical
pair.
sub-microsecond time-scale processes, we report here on photo-
3–5
Fries rearrangement reactions, excited state processes that occur
via singlet radical pairs. Specifically, we focus on naphthyl esters
2a–c, molecules that in solution form a bewildering array of
products in hydrocarbon solvents (Scheme 1).
As summarized in Table 1, upon excitation in hexane, b-
cleavage (with respect to the naphthyl ring) of 2a yields ortho
and para rearranged naphthol ketones 3a and 4a as well as a small
amount of naphthol (5) (Table 1). Esters 2b and 2c form virtually
every compound depicted in Scheme 1 in hexane. Reactions were
conducted in aqueous basic (pH ∼ 8.9) conditions to serve as
a control in the host 1. Under these conditions naphthyl esters
The multitude of products from photoreactions in general,
preclude their use in routine and commercial synthesis of organic
compounds. With the aim of devising strategies to control product
distributions in photochemical reactions, particularly those that
function in aqueous media, we have been exploring the use of water
soluble confined assemblies. In the current study we have used
the host deep-cavity cavitand 1, a molecule with eight carboxylic
acid groups located at the periphery that engender water-solubility
under basic conditions. Previously, 1 has been shown to form a
2a–c are thermally hydrolyzed to naphthol and the corresponding
acids within a few hours at room temperature (2c being least prone
to hydrolysis). Although irradiation in basic aqueous solution
gave less photoproducts, a side reaction namely thermal hydrolysis
complicated the photoprocess. Although in hexane hydrolysis was
not a problem, multiple product formation made the photo-Fries
reaction of 2a–c less useful. As described below cavitand 1 was
able to prevent thermal hydrolysis in basic aqueous solution and
encourage formation of a single photoproduct. Thus the results
described below are novel both with respect to hexane as well as
aqueous solution.
1
capsule in the presence of hydrophobic guest molecules (Fig. 1).
However, in the presence of two equivalents of 1, no hydrolysis of
these esters 2a–c occurred suggesting encapsulation and protection
from the basic aqueous solution. Confirmation of encapsulation
1
within 1
2
came from H NMR analysis of 2c and its 2c@1
2
complex
(
Fig. 2). Integration identified the stoichiometry as 2 : 1, while
peak shifts of the guest and signal splitting of the host confirmed
encapsulation. Thus, characteristic of encapsulation is the 0.5 ppm
difference between the methyl signals of the free and bound guest
(
Fig. 2a and 2c); a phenomenon induced by the magnetic shielding
by the aromatic walls of 1 . Furthermore, the signals of the free
host 1 (Fig. 2b) are split in the complex (Fig. 2c) because the
symmetric guest spins rapidly (on the 500 MHz timescale)
around the C axis of the cavity, but tumbles slowly around the
pseudo C rotation axis of the capsule. In other words at any
2
C
s
4
Fig. 1 a) Deep-cavity cavitand 1. b) Schematic of the templated dimer-
ization of 1.
2
given moment, one hemisphere of the cavitand binds the guest’s
naphthalene ring while the other binds the phenyl ring, hence
presenting magnetically unique areas (inset).
All photoreactions were conducted in the presence of four
equivalents of host 1 per guest molecule to ensure the absence
of any free ester in solution. Results of the photolyses presented in
Table 1 reveal that in contrast to the multiple products produced
in hexane, all three encapsulated guests gave the corresponding
ortho rearranged naphthol (3a, b or c) as the exclusive product.
To rule out the possibility of selective destruction of the para
More recently we have established that on the microsecond
time-scale 1
2
can restrict the translational—but not rotational—
2
freedom of the triplet radical pair derived from dibenzylketones.
To explore the efficacy of the capsule in restricting (templating)
a
Department of Chemistry, University of Miami, Coral Gables, Florida,
3155, USA. E-mail: murthy1@miami.edu; Fax: +1 305 284 4571; Tel: +1
05 284 1534
3
3
b
Department of Chemistry, University of New Orleans, New Orleans,
Louisiana, 70148, USA. E-mail: bgibb@uno.edu; Tel: +1 504 280 3152
products 4 during the irradiation, the progress of the photolysis
1
of 2a@1
2
was monitored with H NMR. In this experiment, only
†
Electronic supplementary information (ESI) available: Experimental
1
details and H NMR spectra . See DOI: 10.1039/b617022f
one significant guest was observed at each sampling time, and
2
36 | Org. Biomol. Chem., 2007, 5, 236–238
This journal is © The Royal Society of Chemistry 2007
Kaanumalle, Lakshmi S.
