“…The reaction has also been studied intramolecularly, in systems containing an aromatic ketone linked to a phenol via spacer groups of varying chain lengths. Das, Levin, and Hayashi 39 and their groups have reported that hemipinacol/phenoxyl biradicals are formed by exciting benzophenone−phenol systems linked by 5−12 atom chains, while our group has studied acetophenone−, indanone−, and benzophenone−phenol systems linked by a three-atom (oxyethyl) spacer through the para positions of the two aromatic rings ( 1 − 4 ). ,,, The triplet lifetimes of 1 − 4 are in every case controlled by the rate of intramolecular phenolic hydrogen abstraction, which produces the corresponding hemipinacol/phenoxyl biradical. This process is slightly faster in 1 (a lowest π,π* triplet) than in 4 (a lowest n,π* triplet), in accord with the relative rate constants for bimolecular hydrogen abstraction by 4-methoxyacetophenone and 4-methoxybenzophenone from p -cresol.…”
The photochemistry of a series of alkoxyacetophenone derivatives bearing remote phenolic groups
has been studied using laser flash photolysis techniques. The compounds are structured with a p- or m-phenolic
moiety attached via a m- or p-oxyethyl linkage to the carbonyl chromophore, and each have a lowest triplet
state of π,π* configuration. The corresponding methoxy-substituted compounds have also been examined.
The triplet lifetimes of the phenolic ketones vary with the positions of attachment (meta or para) of the oxyethyl
spacer to the carbonyl and phenolic moieties, indicating a very strong dependence of the rate of intramolecular
H-abstraction on geometric factors. For example, the para,para‘-linked phenolic ketone has a lifetime τT ≈ 12
ns in dry MeCN solution at room temperature due to rapid intramolecular H-abstraction, while the meta,meta‘-derivative exhibits a lifetime τT ≥ 11.5 μs at infinite dilution and no detectable intramolecular reactivity. The
presence of as little as 0.03 M water in the solvent leads to a significant increase in triplet decay rate in all
cases, in contrast to its retarding effect on the rate of bimolecular phenolic H-abstraction in model compounds.
Semiempirical (PM3) calculations have been carried out to determine the optimum conformation for abstraction
in each molecule as a function of substitution pattern. The variation in the rate constants for intramolecular
H-abstraction throughout the series is consistent with a mechanism involving coupled electron/proton transfer
within a hydrogen-bonded triplet exciplex, the overall rate of which depends critically on orbital overlap factors
between the aromatic rings in the ketone and the phenol.
“…The reaction has also been studied intramolecularly, in systems containing an aromatic ketone linked to a phenol via spacer groups of varying chain lengths. Das, Levin, and Hayashi 39 and their groups have reported that hemipinacol/phenoxyl biradicals are formed by exciting benzophenone−phenol systems linked by 5−12 atom chains, while our group has studied acetophenone−, indanone−, and benzophenone−phenol systems linked by a three-atom (oxyethyl) spacer through the para positions of the two aromatic rings ( 1 − 4 ). ,,, The triplet lifetimes of 1 − 4 are in every case controlled by the rate of intramolecular phenolic hydrogen abstraction, which produces the corresponding hemipinacol/phenoxyl biradical. This process is slightly faster in 1 (a lowest π,π* triplet) than in 4 (a lowest n,π* triplet), in accord with the relative rate constants for bimolecular hydrogen abstraction by 4-methoxyacetophenone and 4-methoxybenzophenone from p -cresol.…”
The photochemistry of a series of alkoxyacetophenone derivatives bearing remote phenolic groups
has been studied using laser flash photolysis techniques. The compounds are structured with a p- or m-phenolic
moiety attached via a m- or p-oxyethyl linkage to the carbonyl chromophore, and each have a lowest triplet
state of π,π* configuration. The corresponding methoxy-substituted compounds have also been examined.
The triplet lifetimes of the phenolic ketones vary with the positions of attachment (meta or para) of the oxyethyl
spacer to the carbonyl and phenolic moieties, indicating a very strong dependence of the rate of intramolecular
H-abstraction on geometric factors. For example, the para,para‘-linked phenolic ketone has a lifetime τT ≈ 12
ns in dry MeCN solution at room temperature due to rapid intramolecular H-abstraction, while the meta,meta‘-derivative exhibits a lifetime τT ≥ 11.5 μs at infinite dilution and no detectable intramolecular reactivity. The
presence of as little as 0.03 M water in the solvent leads to a significant increase in triplet decay rate in all
cases, in contrast to its retarding effect on the rate of bimolecular phenolic H-abstraction in model compounds.
Semiempirical (PM3) calculations have been carried out to determine the optimum conformation for abstraction
in each molecule as a function of substitution pattern. The variation in the rate constants for intramolecular
H-abstraction throughout the series is consistent with a mechanism involving coupled electron/proton transfer
within a hydrogen-bonded triplet exciplex, the overall rate of which depends critically on orbital overlap factors
between the aromatic rings in the ketone and the phenol.
“…The observation of a deuterium kinetic isotope effect ( k H / k D = 1.8) on the triplet decay rate of 3 in H(D) 2 O-saturated benzene solution is consistent with phenolic hydrogen abstraction being the rate determining process leading to deactivation of the triplet state, suggesting that in fluid solution, the conformational motions required for the molecule to achieve an appropriate geometry for intramolecular hydrogen abstraction (eq 2) occur over a somewhat faster time scale than the actual abstraction process. Indeed, dissolution or inclusion of 3 in organized media such as liquid crystals or cyclodextrins results in substantial increases in the triplet lifetime, presumably owing to the restrictive effects of these media on the conformational mobility of the molecule. …”
Nanosecond laser flash photolysis studies have been
carried out of the kinetics of inter- and intramolecular
phenolic hydrogen abstraction by alkoxyacetophenone, 5-alkoxyindanone,
and 4-alkoxybenzophenone triplets in
acetonitrile and benzene solution. Information on the geometric
requirements for abstraction by carbonyl n,π* and
π,π* triplets is derived from the results for a series of ketones
which contain a para-phenolic moiety attached
via
a para-oxyethyl linkage. For all of these compounds,
the deuterium kinetic isotope effect on the triplet lifetime
in
acetonitrile solution indicates that triplet decay is determined by the
rate of intramolecular abstraction of the remote
phenolic hydrogen, which yields the corresponding
phenoxyl−hemipinacol biradical. The biradicals have also
been
detected, and are about an order of magnitude longer-lived than the
triplet in each case. For three of the compounds,
the rates of the intramolecular process follow the same trend as that
observed in the rates of bimolecular quenching
of the parent methoxy-substituted ketones by p-cresol.
Deviation from this trend is observed for the
alkoxyindanone
derivative, where an in-plane approach of the phenolic hydrogen to the
carbonyl n-orbital is not possible. The
trends in the rate constants for bimolecular quenching of a series of
substituted benzophenones by p-cresol indicate
that for n,π* triplet abstractions, the quenching mechanism is
different for electron donor-substituted and electron
acceptor-substituted ketones. For π,π* triplets and
donor-substituted (n,π* triplet) benzophenones, abstraction
is
proposed to occur by a mechanism involving the intermediacy of a
hydrogen-bonded exciplex, which yields the
corresponding radicals by sequential electron- and proton-transfer.
The rate constant for quenching by this mechanism
thus depends mainly on the basicity of the ketone triplet state and the
oxidation potential of the phenol.
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