Unexpected Hofmann elimination in the benzophenone-(phenylthio)acetic tetrabutylammonium salt photoredox system

Article abstract of DOI:10.1021/ja036196v

The triplet state of benzophenone was quenched by the tetrabutylammonium salt of (phenylthio)acetic acid in acetonitrile solutions. The quenching event, following laser flash photolysis, resulted in the formation of a transient ion pair consisting of the benzophenone radical anion and the tetrabutylammonium cation. Subsequently this ion pair decays with the quaternary ammonium cation undergoing a Hofmann elimination to form butane-1 and tributylamine, which were detected in steady-state irradiation. This appears to be the first report of an ion pair consisting of a benzophenone radical anion and an organic cation (nonradical), in addition to the first report of a photoinduced Hofmann elimination in quaternary ammonium ions. Copyright

Full text of DOI:10.1021/ja036196v

Published on Web 08/20/2003
Unexpected Hofmann Elimination in the Benzophenone-(Phenylthio)acetic
Tetrabutylammonium Salt Photoredox System
Andrzej Wrzyszczyn´ski,† Marek Pietrzak,† Jarogniew Bartoszewicz,‡ Halina Kozubek,‡
Gordon L. Hug,*,§ Bronislaw Marciniak,*,‡ and Jerzy Pa¸czkowski*,†
Faculty of Chemical Technology and Engineering, UniVersity of Technology and Agriculture,
Bydgoszcz, Poland, Faculty of Chemistry, A. Mickiewicz UniVersity, Poznan´, Poland, and
Radiation Laboratory, UniVersity of Notre Dame, Indiana 46556
Received May 16, 2003; E-mail: hug.1@nd.edu; marcinia@amu.edu.pl; paczek@atr.bydgoszcz.pl
Photoinduced electron transfer has features that make it attractive
in initiating free-radical polymerizations.1 It is generally a fast
process, and it leads directly to radical ions that often fragment to
form a variety of secondary radicals that are, themselves, useful
polymerization initiators. Electron donors, such as thioethers, are
of interest as potential co-initiators because of the tendency of their
sulfur-centered radical cations to fragment,2 giving the radicals that
can start free radical polymerization.3
Radical cations from sulfur-containing aromatic carboxylic acids
(SCCA) are good candidates as co-initiators of photopolymeriza-
tions. They are susceptible to decarboxylation and deprotonations,
leading to carbon-centered radicals that are efficient initiators of
free-radical polymerization. For such a co-initiator to be soluble in
nonpolar and medium polar monomers, it is prudent to introduce
the SCCA as its organic salt. The co-initiator system under study
in this work consists of the SCCA’s tetraalkylammonium salt plus
a photosensitizer that should, itself, be a good electron acceptor,
as well as being an efficient light absorber. Such a system is
reminiscent of photoinitiators studied by Neckers et al.4 (see I).
Figure 1. Transient absorption spectra of intermediates following the
quenching of benzophenone triplet by Ph-S-CH2-COO-N+(C4H9)4 (0.01
M). Inset: kinetic trace at 710 nm.
than 200 ns following the laser pulse, the 710-nm kinetic traces
(see inset to Figure 1) clearly indicate the formation of a transient
whose growth rate is identical (within experimental error) to the
decay rate of the triplet state. This observation suggests that the
intermediate absorbing at ab. 710 nm is a primary product from
the quenching of the BP triplet by Ph-S-CH2-COO-N+(C4H9)4.
There are some observations and facts that are pertinent to an
assignment of the 710-nm transient. (i) No 710-nm transients were
seen in laser flash experiments performed on either the reaction
There are two significant structural differences between the main
mixture of BP (0.002 M) - Ph-S-CH2-COOH (0.02 M) -
system under study in the current work (II) and (I). First the tertiary
N(C4H9)4Cl (0.02 M) or in the system containing BP (0.002 M)
amine group is not covalently attached to a chromophore in our
system, and second we use the (phenylthio)acetate anion as the
electron donor, instead of a n-butyltriphenylborate anion.
and Ph-S-CH2-COOH (0.02 M). However, both of these
mixtures yielded the benzophenone ketyl radical (BPH• transient
with an absorption maximum at 550 nm). (ii) The 710-nm
The photoreduction of benzophenone (BP) by Ph-S-CH2-
intermediate cannot be assigned to solvated electrons because any
COOH and Ph-S-CH2-COO-N+(C4H9)4 was studied by transient
electrons would be scavenged by acetonitrile.6 (iii) The benzophe-
nanosecond laser flash spectroscopy5 and steady-state photolysis.
none radical anion (BP•-) in acetonitrile shows a broad band with
Laser flash excitation (337 nm) of BP (2 × 10-3 M) in the presence
a maximum around 650-720 nm depending on experimental
of Ph-S-CH2-COOH (0.02 M) resulted in the appearance of an
conditions.7 (iv) The formation of a 710-nm transient occurs only
absorption corresponding to the presence of the benzophenone ketyl
radical (BPH•, λmax ) 550 nm). For Ph-S-CH2-COO- N+(C4H9)4
a new absorption band appears in the red region of the transient
absorption spectra (see Figure 1).
At high concentrations of Ph-S-CH2-COO-N+(C4H9)4 (0.01
M), such that the triplet state of BP is quenched within a time shorter
when the (phenylthio)acetate anion and the N+(C4H9)4 cation are
used in experiments as the Ph-S-CH2-COO-N+(C4H9)4 salt.
Thus, the transient absorption with a maximum at ab. 710 nm can
be assigned as the BP•- present in the [BP•-‚‚‚N+(C4H9)4] ion
pair.7b,8
From the photochemical point of view, the most striking feature
of the time-resolved transient spectra (Figure 1) is the apparent
growth of the long-lived BPH• at the expense of the [BP•-‚‚‚N+-
(C4H9)4] intermediate. This finding suggests that BPH• is formed
† Faculty of Chemical Technology and Engineering, University of Technology
and Agriculture.
‡ Faculty of Chemistry, A. Mickiewicz University.
§ Radiation Laboratory, University of Notre Dame.
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J. AM. CHEM. SOC. 2003, 125, 11182-11183
10.1021/ja036196v CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
of BP•- and N+(C4H9)4 may result in an effective proton abstraction
from the â carbon to the nitrogen, yielding butene-1 and tributyl-
amine as products. This reaction is quite analogous to the Hofmann
elimination observed in quaternary ammonium ions.11
Scheme 1
The earlier mentioned photoinitiating systems, studied by Neckers
et al.4 (I) behave quite differently. In those systems, electron transfer
from borate anions to the BP triplet state gives the (benzophe-
nonylmethyl)-tri-n-butylammonium radical anion (not seen on the
nanosecond time scale). A further sequence of reactions yields the
4-benzoylbenzyl radical. In our supporting experiments with the
n-butyltriphenylborate tetrabutylammonium salt as the electron
donor, the laser flash photolysis studies clearly show again the
formation of a long-lived benzophenone radical anion. This
observation indicates that the observed phenomena are general and
that the mechanism of the reaction likely applies to other electron
donors that can exist as ion pairs in solution.
In summary, we have demonstrated a specific sequence of
photoinitiated reactions involving the Hofmann elimination, yielding
typical products for this type of elimination. We have also shown
that this type of reaction requires the presence of substrates in a
specific form, namely the electron donor is present as the (phe-
nylthio)acetic tetrabutylammonium salt instead of as the carboxylic
acid.
directly from BP•- within the [BP•-‚‚‚N+(C4H9)4] ion pair. As-
suming that the decay of the [BP•-‚‚‚N+(C4H9)4] ion pair intermedi-
ate leads exclusively to the BPH• radical, one can estimate the molar
absorption coefficient of the ion pair at 710 nm as ꢀ710 ) 18000
M-1 cm-1 taking the value of ꢀ550 ) 3400 M-1 cm-1 as the molar
absorption coefficient of the ketyl radical in acetonitrile.5 Further-
more, by applying experimental conditions described in ref 5, one
can establish the quantum yield of [BP•-‚‚‚N+(C4H9)4] formation,
as equal to 0.35 ( 0.05. This value, based on our mechanistic
assumption, also corresponds to the benzophenone ketyl radical
quantum yield.
Acknowledgment. This work was supported by the State
Committee for Scientific Research (Grant 4T09A 051 22 and BW-
20/01), the A. Mickiewicz University, and the Office of Basic
Energy Sciences of the U.S. Department of Energy. This is
Document No. NDRL-4450 from the ND Radiation Laboratory.
References
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Steady-state irradiation of the BP/Ph-S-CH2-COO-N+(C4H9)4
photoredox system in MeCN gives CO2 (ΦCO ) 0.45 ( 0.09),
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2
benzopinacole, thioanisole, 1,2-bis(phenylthio)ethane, and two
unexpected products, i.e., butene-1 (Φ ≈ 0.26) and tributylamine
(Φ ≈ 0.20). In addition, during the steady-state irradiation, the
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0.42 ( 0.05.9 The equality of ΦCO and ΦBP is evidence that the
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2
potential formation of BPH• via reaction b can be neglected (Scheme
1) because reaction b does not lead to the formation of CO2. From
comparisons of the quantum yield of [BP•-‚‚‚N+(C4H9)4] formation,
the quantum yield of decarboxylation, and the quantum yield of
BP consumption, one can conclude that the benzophenone ketyl
radical can only be formed from [BP•-‚‚‚N+(C4H9)4].
(5) Experimental conditions of nanosecond flash photolysis: 337 nm nitrogen
laser excitation, [BP] ) 2 mM, [Ph-S-CH2-COO-N+(C4H9)4] ) 10
mM, argon saturated solutions; quantum yields of [BP•-‚‚‚N+(C4H9)4] and
of BPH• formation were determined using external actinometry: ben-
zophenone triplet state in acetonitrile ꢀ520 ) 6500 M-1 cm-1 (Baral-Tosh,
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taking molar absorption coefficients of [BP•-‚‚‚N+(C4H9)4] as ꢀ710
)
18 000 M-1 cm-1 and of BPH• as ꢀ550 ) 3400 M-1 cm-1 of the ketyl
radical in acetonitrile (Sudhindra, N.; Bhattacharyya, N.; Das, P. K. J.
Chem. Soc., Faraday Trans. 2 1984, 80, 1107).
On the basis of the known photochemistry of sulfur-containing
aromatic carboxylic acids2,3 and the experimental data, we propose
the mechanism in Scheme 1 that is consistent with the observations
described above.
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(9) The steady-state measurement conditions: λir ) 365 nm; light intensity
I0 ) 1.76 × 10-4 einstein dm-3 min-1 measured using benzophenone-
benzhydrol actionometry (see: Murov, S. L.; Carmichael, I.; Hug, G. L.
Handbook of Photochemistry; Marcel Dekker: New York, 1993),
concentrations of BP and Ph-S-CH2-COO-N+(C4H9)4 were equal to 4.0
and 4.8 mM, respectively. Stable products were analyzed by GC and HPLC
methods.
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In this scheme, electron transfer from the sulfur of the Ph-S-
CH2-COO-N+(C4H9)4 gives the benzophenone radical anion and
the sulfur-centered radical cation Ph-S•+-CH2-COO-N+(C4H9)4.
Experimental data suggests that the electron transfer is followed
by decarboxylation that occurs on the nanosecond time scale.10
•
Decarboxylation yields an R-alkylthio-type radical (R-S-CH2 )
that escapes from the cage leaving a BP•- forming the ion pair
with the tetrabutylammonium cation. The [BP•-‚‚‚N+(C4H9)4] ion
pair remaining in the solvent cage is organized and stabilized by
electrostatic interaction of the BP•- and the tetrabutylammonium
cation. This interaction is probably responsible for the specific
properties of the benzophenone radical anions in the ion pair. In
addition, since BP•- can be considered as a base, the close proximity
JA036196V
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