Enhanced reactivity in OH/NH/π polyfunctional systems through coupled proton/electron transfer in the excited state: The photocyclisation of 2-allyl-3-aminophenol

Article abstract of DOI:10.1039/b416625f

The photocyclisation rate of 2-allyl-3-(or 5-)aminophenols (1, 4 and 8) is dramatically enhanced, when compared with reference compounds, as a consequence of a coupled proton/electron transfer process. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b416625f

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Enhanced reactivity in OH/NH/p polyfunctional systems through
coupled proton/electron transfer in the excited state: the
photocyclisation of 2-allyl-3-aminophenol
Edgar A. Leo, Rosa Tormos and Miguel A. Miranda*
Received (in Cambridge, UK) 1st November 2004, Accepted 15th December 2004
First published as an Advance Article on the web 18th January 2005
DOI: 10.1039/b416625f
indolines are the respective final photoproducts (Scheme 1). Thus,
irreversible processes provide a photochemical (rather than
photophysical) fingerprint for the involved mechanism.
The photocyclisation rate of 2-allyl-3-(or 5-)aminophenols (1, 4
and 8) is dramatically enhanced, when compared with reference
compounds, as a consequence of a coupled proton/electron
transfer process.
Fig. 1 shows the fluorescence spectra of 1 and its reference
compounds phenol, aniline, 2-allylphenol and 2-allylaniline. They
were measured in acetonitrile, using isoabsorptive solutions
(A 5 0.1) at the excitation wavelength (280 nm). The emission
maximum of 1 was close to that of aniline, and its intensity was
extremely weak. It is known that ortho-allyl substitution of phenol
and aniline decreases the emission intensity, due to the occurrence
of photocyclisation.10,11 Thus, in the case of 1 a high photo-
reactivity was anticipated.
Excited state proton transfer (PT) and electron transfer (ET) are
two of the most important photochemical processes. Competition
between PT and ET (as well as coupled PT/ET) has recently
attracted considerable attention, both from the fundamental and
practical points of view. In this context, polyfunctional systems
that contain a proton donor, an electron donor and an accepting
moiety have been studied. For instance, 4-dialkylaminosalicylalde-
hyde and 49-dialkylamino-3-hydroxyflavone exhibit dual fluores-
cence associated with intramolecular proton transfer and charge
transfer in the excited singlet state.1–4
By contrast with the available photophysical and theoretical
studies on polyfunctional systems, the chemical reactions arising
from coupled PT/ET processes remain practically unexplored. In
the present work, a simple model (compound 1, Chart 1) has been
designed to address this issue by combining the known photo-
chemistry of 2-allylphenols (AP)5–10 and 2-allylanilines (AA)11,12 in
the same molecule. The former react via intramolecular PT, while
the latter undergo intramolecular ET; dihydrobenzofurans and
Scheme 1 Photocyclisation of 2-allylphenol and 2-allylaniline via PT or
ET, respectively.
Chart 1
Fig. 1 Emission spectra of 1 and some of its reference compounds.
Chem. Commun., 2005, 1203–1205 | 1203
*mmiranda@qim.upv.es
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Table 1 Photoproducts distribution found for compounds 1, 4, 6, 8, 10, 2-allylaniline and 2-allylphenol after 30 minutes irradiation
Entry
Compd
Conversion (%)
2
3
5
7
9
11
Indolinea
Dihydrobenzofurana
1
2
3
4
5
6
7
a
2-allylaniline
2-allylphenol
1
4
6
6
7
—
—
85
—
—
—
—
—
—
15
—
—
—
—
—
—
—
100
—
—
—
—
—
—
—
100
—
—
—
—
—
—
—
100
—
—
—
—
—
—
—
100
100
—
—
—
—
—
—
—
100
—
—
—
—
—
56
37
11
41
4
8
10
The photoproducts were 2-methylindoline for 2-allylaniline and 2,3-dihydro-2-methylbenzofuran for 2-allylphenol.
The photochemistry of
1
was compared with that of
the olefin moiety, to give a biradical ion. Protonation of the
anionic site by the phenolic group and subsequent back electron
transfer (BET) from the phenoxide to the aniline radical cation
would lead to the final product after the collapse of the 1,5-
biradical intermediate. The first and last steps of this mechanism
are identical to those that have been widely accepted for AA
derivatives, as shown in Scheme 1.{ The key step would be the
intramolecular PT at the biradical anion stage, where both phenol
and aniline radical cation may act as donors. Unambiguous
evidence for the predominance of the former was supported by the
behaviour of the dimethylamino derivative 4, that also underwent
a very fast photoreaction, to give 5 (see Table 1, entry 4, and
Fig. 2). By contrast, methylation of the phenol group as in 6
resulted in a much slower photocyclisation, to afford indoline 7
(Table 1, entry 5 and Fig. 2). The proposed BET from the
phenoxide to the aniline radical cation is thermodynamically
favourable in view of the redox potentials found in the
literature.13
monofunctional reference compounds under the same conditions.{
The results are shown in Table 1. Cyclisation to dihydrobenzo-
furan 2 (phenol-like behaviour) was the overwhelmingly major
process, although minor amounts of indoline 3 (aniline-derived
product) were also formed. This cannot be accounted for on the
basis of the relative reactivities of AP and AA (see Table 1, entries
1 and 2, and Fig. 2). Moreover, in the case of 1 the conversion was
much higher (Table 1, entry 3) and the reaction kinetics much
faster (Fig. 2) than in either of the reference compounds. Clearly,
combination of the hydroxy and amino substituents on the same
ring produces a dramatic synergetic effect.
A coupled PT/ET mechanism that reasonably explains the
enhanced photocyclisation to 2 is outlined in Scheme 2. It would
be initiated by excited state intramolecular ET from the aniline to
According to this mechanism, electron transfer should also
occur with an amino group located at the para-position. This was
confirmed by photolysis of compound 8, that proceeded very fast
to give 9 (Table 1, entry 6, and Fig. 2). By contrast, moving the
hydroxy substituent away from the allyl group as in 10 resulted in
a sharp decrease of the photoreactivity to give 11, that became
comparable to that of AA (Table 1, entry 7, and Fig. 2). Although
all irradiations reported in Table 1 were carried out under the
same conditions, the photocyclisation quantum yields were
determined for two selected compounds (1 and 2-allylphenol), in
order to obtain more quantitative data for comparison. The
values (obtained using the E/Z-isomerization of b-methylstyrene
as chemical actinometer)14 were 0.25 (for 1) and 0.02 (for
2-allylphenol). This is in good agreement with the preparative
data shown in entries 3 and 2 of Table 1 and confirms that the
reaction efficiency increases by one order of magnitude as a
consequence of coupled proton/electron transfer.
Fig. 2 Photoreaction kinetics of 1 (6), 4 (+), 6 (2), 8 (#), 10 (e),
2-allylphenol ($) and 2-allylaniline (X).
In summary, a dramatic enhancement of the photocyclisation
rate (when compared with the reference compounds AP and AA)
is observed in the case of 1, 4 and 8 as a consequence of a coupled
PT/ET process. This increases the potential interest of polyfunc-
tional systems containing proton donor, electron donor and
accepting moieties and allows one to obtain photochemical (in
addition to photophysical) information, through irreversible
reactions that constitute a fingerprint for the involved mechanism.
We are grateful for financial support from the Spanish MCYT
(Grant No. BQU2001-2725), the Generalitat Valenciana (Grupos
03/82), and the Universidad Polite´cnica de Valencia (fellowship to
E. A. L.).
Scheme 2 Coupled PT/ET mechanism explaining the photocyclisation
of 1.
1204 | Chem. Commun., 2005, 1203–1205
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Edgar A. Leo, Rosa Tormos and Miguel A. Miranda*
Departamento de Qu´ımica-ITQ UPV-CSIC, Universidad Polite´cnica de
Valencia, Camino de Vera, s/n, 46022, Valencia, Spain.
E-mail: mmiranda@qim.upv.es; Fax: +34 963879349;
Tel: +34 963877807
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{ Irradiations were performed through quartz in a multilamp photoreactor,
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at 254 nm, employing acetonitrile as solvent. The reaction course was
1
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1
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