Chain amplification in photoreactions of N-alkoxypyridinium salts with alcohols: Mechanism and kinetics

Article abstract of DOI:10.1021/jo050726j

Photosensitized electron transfer from a variety of singlet- and triplet-excited donors to N-methoxypyridinium salts leads to N-O bond cleavage. Hydrogen atom abstraction by the resulting methoxy radical from an added alcohol generates an α-hydroxy radical that reduces another pyridinium molecule, thus leading to chain propagation. For example, thioxanthone-sensitized reactions of 4-cyano-N-methoxypyridinium, P1, with several aliphatic and benzyl alcohols gave quantum yields for products formation (an aldehyde or a ketone and protonated 4-cyanopyridinium) of ～15-20, at reactant concentrations of ～0.02-0.04 M. The reaction can also be sensitized with triplet benzopheone, which in this case acts as an electron donor. Energetic limitations on chain propagation are imposed by the relationship between the oxidation potential of the α-hydroxy radical and the reduction potential of the pyridinium salt. The chain reactions proceed despite ～0.25 eV endothermicity for the electron-transfer step. Chain reactions with the harder-to-reduce 4-phenyl-N-methoxypyridinium, however, are limited in scope because of increased endothermicity for electron transfer. The thioxanthone-sensitized reaction of P1 with benzhydrol was studied in detail by a combination of steady state and transient kinetics. The bimolecular rate constants for the chain propagation reactions:hydrogen atom abstraction by the methoxy radical and electron transfer from the diphenylketyl radical to P1 are ～6 × 10⁶ and 1.1 × 10⁶ M^-1 s^-1, respectively. The kinetic data indicate that deuterium atom abstraction by the methoxy radical from the solvent, acetonitrile-d₃, is a dominant chain-terminating process. Because of a large deuterium isotope effect, ～7, the quantum amplification is strongly suppressed when the reaction is carried out in acetonitrile.

Full text of DOI:10.1021/jo050726j

Chain Amplification in Photoreactions of N-Alkoxypyridinium
Salts with Alcohols: Mechanism and Kinetics
Deepak Shukla,* Wendy G. Ahearn, and Samir Farid*
Research Laboratories, Eastman Kodak Company, Rochester, New York 14650-2109
deepak.shukla@kodak.com
Received April 12, 2005
Photosensitized electron transfer from a variety of singlet- and triplet-excited donors to N-
methoxypyridinium salts leads to N-O bond cleavage. Hydrogen atom abstraction by the resulting
methoxy radical from an added alcohol generates an R-hydroxy radical that reduces another
pyridinium molecule, thus leading to chain propagation. For example, thioxanthone-sensitized
reactions of 4-cyano-N-methoxypyridinium, P1, with several aliphatic and benzyl alcohols gave
quantum yields for products formation (an aldehyde or a ketone and protonated 4-cyanopyridinium)
of ∼15-20, at reactant concentrations of ∼0.02-0.04 M. The reaction can also be sensitized with
triplet benzopheone, which in this case acts as an electron donor. Energetic limitations on chain
propagation are imposed by the relationship between the oxidation potential of the R-hydroxy radical
and the reduction potential of the pyridinium salt. The chain reactions proceed despite ∼0.25 eV
endothermicity for the electron-transfer step. Chain reactions with the harder-to-reduce 4-phenyl-
N-methoxypyridinium, however, are limited in scope because of increased endothermicity for electron
transfer. The thioxanthone-sensitized reaction of P1 with benzhydrol was studied in detail by a
combination of steady state and transient kinetics. The bimolecular rate constants for the chain
propagation reactions:hydrogen atom abstraction by the methoxy radical and electron transfer from
6
6
-1 -1
the diphenylketyl radical to P1 are ∼6 × 10 and 1.1 × 10 M s , respectively. The kinetic data
indicate that deuterium atom abstraction by the methoxy radical from the solvent, acetonitrile-d
3
,
is a dominant chain-terminating process. Because of a large deuterium isotope effect, ∼7, the
quantum amplification is strongly suppressed when the reaction is carried out in acetonitrile.
Introduction
tion of several reactant molecules to products. Well-
known among these are the photoinduced electron-
transfer reactions of electron donors, whose radical
cations undergo valence isomerization or 2 + 2 cycload-
dition/cycloreversion, where quantum yields of several
Amplification of photochemical processes plays an
important role in applications where high sensitivity is
required. Generation of a species that can be used as a
catalyst in a subsequent reaction is commonly used to
amplify the effect of the primary photochemical reaction.
Examples of these catalytic reactions are photogenerated
acids, used in lithographic applications, and photoge-
nerated clusters of a few silver atoms, acting as a latent
image in silver halide photography. Chain propagation
4
tens can be achieved in polar solvents. In these reactions,
the chain is propagated via electron transfer from a
reactant molecule to the radical cation of the product.
Whitten et al. reported an interesting chain-amplified,
photoinduced electron-transfer reaction in which oxida-
tive and reductive cleavage of two different pinacols
produce a reducing and an oxidizing radical, respectively,
capable of propagating the chain.5
1
2
in photoinitiated radical or cationic polymerization is
conceptually a different process, where the propagation
simply increases the molecular weight of the forming
3
polymer.
(
2) The Theory of the Photographic Process, 4th ed.; James, T. H.,
Only few concepts of chain-amplified photoreactions
are reported where one photon leads to the transforma-
Ed.; MacMillan: New York, 1977.
(3) See, for example: Reiser, A. In Photoreactive Polymers, The Sci-
ence and Technology of Resists; Wiley: New York, 1989; pp 102-177.
(4) (a) Ledwith, A. Acc. Chem. Res. 1972, 5, 133. (b) Evans, T. R.;
Wake, R. W.; Sifain, M. M. Tetrahedron Lett. 1973, 701. (c) Mattes, S.
L.; Farid, S. In Organic Photochemistry; Padwa, A., Ed.; Marcel
Dekker: New York, 1983; Vol. 6, p 233.
(
1) (a) Tessier, T. G.; Frechet, J. M. J.; Willson, C. G.; Ito, H. ACS
Symposium Series; American Chemical Society: Washington, DC,
985; pp 266, 269. (b) Stewart, M. D.; Patterson, K.; Somervell, M. H.;
Willson, C. G. J. Phys. Org. Chem. 2000, 13, 767.
1
1
0.1021/jo050726j CCC: $30.25 © 2005 American Chemical Society
Published on Web 07/16/2005
J. Org. Chem. 2005, 70, 6809-6819
6809

Shukla et al.
A different kind of chain reaction, also involving two
reactants, is that of onium salts. Upon one-electron
reduction, eq 1, an onium salt (Ar-X ) undergoes frag-
SCHEME 1
+
mentation to yield an aryl radical, eq 2, which in turn
abstracts a hydrogen atom from an alcohol to give an
R-hydroxy radical, eq 3. Chain propagation occurs via
electron transfer from the R-hydroxy radical to another
onium salt molecule, eq 4.
+
e-
+
•
Ar-X
8 Ar-X
(1)
(2)
(3)
•
•
Ar-X f Ar + X
•
•
Ar + RCH OH f Ar-H + R CHOH
2
•
+
+
•
R CHOH + Ar-X f RCHO + H + Ar-X (4)
This reaction sequence was first reported for diazonium
6
salts and was later extended to diaryl iodonium and to
7
triaryl sulfonium salts. In addition to alcohols such as
8
9
methanol, ethers (e.g., THF), triphenylphosphine, for-
1
0
11
mate, and hypophosphite were also found to undergo
similar chain reactions with onium salts.
In a similar reaction, bromoacetate esters (in place of
the onium salts) react with alcohols such as 2-propa-
nol to yield the acetate ester, hydrogen bromide, and
the oxidation product of the alcohol (acetone, in the
case of 2-propanol). The chain is propagated through
electron transfer from the hydroxy radical to a bromo-
acetate molecule, which fragments to a bromide ion and
a radical that abstracts a hydrogen atom from the
energetics, as well as the limits of chain amplification
based on a detailed kinetic studies to determine the
controlling rate constants for propagation and termina-
tion reactions.
The reaction that we sought to investigate, Scheme 1,
is based on one-electron reduction of N-methoxypyri-
dinium salts, eq 5, which leads to N-O bond cleavage,
eq 6. This fragmentation reaction yields a methoxy
radical that abstracts an R hydrogen atom from an
alcohol, eq 7, and if properly designed, the resulting
R-hydroxy radical could transfer an electron to another
molecule of N-methoxypyridinium salt, eq 8, and propa-
gate the chain.
1
2
alcohol.
We now show that a chain-amplified reaction, analo-
gous to that given by eqs 1-4, can be achieved in
photoreactions of N-alkoxypyridinium salts, which have
the advantage that their reduction potentials are readily
modulated and that easy-to-reduce derivatives can still
undergo fast fragmentation.1
3-15
Thus, the net reaction would be dehydrogenation of the
alcohol, with the two hydrogen atoms being formally
inserted into the N-O bond of the methoxypyridinium
salt to give protonated pyridinium and methanol, eq 9.
We also discuss several
sensitization/initiation schemes in terms of the reaction
(
5) Chen, L.; Lucia, L.; Whitten, D. G. J. Am. Chem. Soc. 1998, 120,
4
39.
(
6) (a) DeTar, F. D.; Turetzky, M. N. J. Am. Chem. Soc. 1955, 77,
1
(
745. (b) DeTar, F. D.; Kosuge, T. J. Am. Chem. Soc. 1958, 80, 6072.
c) Packer, J. E.; House, D. B.; Rasburn, E. J. J. Chem. Soc. B 1971,
1
2
5
1
574. (d) Packer, J. E.; Richardson, R. K. J. Chem. Soc., Perkin Trans.
1975, 751. (e) Smets, G.; Aerts, A.; Van Erum, J. J. Polym. 1980, 12,
39. (f) Kosynkin, D.; Bockman, T. M.; Kochi, J. K. J. Am. Chem. Soc.
997, 119, 4846. (g) Kosynkin, D.; Bockman, T. M.; Kochi, J. K. J.
Although usually exothermic, hydrogen atom abstrac-
tions by oxygen-centered radicals have rate constants
Chem. Soc., Perkin Trans. 2 1997, 2003.
(7) (a) Saeva, F. D. In Topics in Current Chemistry; Springer-
Verlag: Berlin, 1990; Vol. 156, p 59. (b) Knapczyk, J. M.; McEwen, W.
5
7
-1 -1
only in the range of 10 -10 M s , well below the
E. J. Org. Chem. 1970, 35, 2539. (c) Ledwith, A. Polymer 1978, 19,
diffusion-controlled limit.16 These reactions, however, can
1
217. (d) Ledwith, A. Makromol. Chem. Suppl. 1979, 3, 348. (e)
•
Ledwith, A. Pure Appl. Chem. 1979, 51, 159. (f) Baumann, H.; Timpe,
H.-J. Z. Chem. 1984, 24, 18. (g) Pappas, P. S.; Gatechair, L. R.; Jilek,
J. H. J. Polym. Sci., Polym. Chem. Ed. 1984, 22, 77. (h) Crivello, J. V.;
Jo, K. D. J. Polym. Sci., Part A: Polym. Chem. 1993, 31, 2143. (i)
Kampmeier, J. A.; Nalli, T. W. J. Org. Chem. 1994, 59, 1381.
be fast enough to become the dominant path for OMe.
The reducing power of the R-hydroxy radical should
be high enough for electron transfer to the pyridinium
salt to proceed at a rate well above those of potentially
competing processes. As shown below, this condition
could be met, even if the electron transfer is an endo-
thermic reaction.
(8) (a) Abdoul-Rasoul, A. M. F.; Ledwith, A.; Yagci, Y. Polym. Bull.
1
978, 1, 1. (b) Abdoul-Rasoul, A. M. F.; Ledwith, A.; Yagci, Y. Polymer
978, 19, 1219. (c) See also refs 7d, 7e, and 7f.
1
(
9) Kampmeier, J. A.; Nalli, T. W. J. Org. Chem. 1993, 58, 943.
10) Packer, J. E.; Richardson, R. K.; Soole, P. J.; Webster, D. R. J.
(
Chem. Soc., Perkin Trans. 2 1974, 1472.
(
3
11) (a) The chain reaction of diazonium salts with hypophosphite
PO
) was the first to be reported.10 (b) Kornblum, N.; Cooper, G.
D.; Taylor, J. E. J. Am. Chem. Soc. 1950, 72, 3013.
12) Fontana, F.; Kolt, R. J.; Huang, Y.; Wayner, D. D. M. J. Org.
Chem. 1994, 59, 4671.
13) Lee, K. Y.; Kochi, J. K. J. Chem. Soc., Perkin Trans. 2 1992, 7,
011.
(14) (a) Lorance, E. D.; Kramer, W. H.; Gould, I. R. J. Am. Chem.
Soc. 2004, 126, 14071. (b) Lorance, E. D.; Hendrickson, K.; Gould, I.
R. J. Org. Chem. 2005, 70, 2014.
(
H
2
(
(15) Gould, I. R.; Shukla, D.; Giesen D.; Farid, S. Helv. Chim. Acta
2001, 84, 2796.
(
(16) (a) Paul, H.; Small, R. D., Jr.; Scaiano, J. C. J. Am. Chem. Soc.
1978, 100, 4520. (b) Scaiano, J. C. J. Photochem. 1973, 2, 81.
1
6810 J. Org. Chem., Vol. 70, No. 17, 2005

Photoreactions of N-Alkoxypyridinium Salts with Alcohols
Results and Discussion
TABLE 1. Quantum Yields (ΦP) and Chain Lengths of
Photoreactions of Benzhydrol with
4-Cyano-N-methoxypyridinium (P1) at Equimolar
Concentrations of 0.04 M, Initiated by Different
Sensitizers in Acetonitrile
The reduction potential of the pyridinium salt sets the
limits on the sensitizers and alcohols that can be used.
In both cases the easier it is to reduce the pyridinium
salt, i.e., the less negative its reduction potential, the
broader the range of applicable sensitizers and alcohols.
Reduction potentials of N-methoxypyridinium salts are
not readily accessible through conventional electrochemi-
cal techniques because of the relatively fast reductive
cleavage of these compounds.14 However, on the basis of
differences in absorption maxima between CT complexes
of N-alkoxy- and N-alkyl-pyridinium salts, the N-alkoxy
derivatives were estimated to have reduction potentials
that are ∼0.14 V less negative than those of the N-alkyl
1
3
analogues. Similar differences in reduction potentials
between these sets of compounds are also supported by
1
5
additional data. Thus, the reduction potentials of N-
alkoxy pyridiniums can be estimated from those of the
corresponding N-alkyl analogues, many of which show
reversible potentials by conventional cyclic voltametry.13
We compared reactions of two pyridinium salts, P1 and
P2, that have a large difference in reduction potentials:
that of the 4-cyano derivative (P1) is estimated to be ca.
a
The measured quantum yield for formation of DCA•+ and
pyridyl radical is 0.027. A second chain is initiated through
oxidation of benzydrol by DCA
two chains are generated from the reaction of BP*, which is
formed with a quantum yield of 1.
•+
b
. Similar to the DCA reaction,
3
-
0.5 V vs SCE, and of the 4-phenyl analogue (P2) to be
15
ca. -1.0 V vs SCE.
nonpolar media. Thus, for the photoinduced electron
transfer to be exothermic (i.e., for the energy stored in
the excited state to exceed the energy stored in the
electron-transfer products),19 the relation shown in eq 10
has to be satisfied.
excit
ox
red
(E
) > (E ) - (E )A + ∆
(10)
D
D
The excitation energy of the donor, (Eexcit
)
D
, could be
Oxidation potentials of R-hydroxy radicals, which
define the structure of the alcohols to be used, are also
difficult to determine reliably because of sensitivity to
hydrogen bonding and to medium basicity. For example,
moderate to high concentrations of the parent alcohol
could lower the oxidation potential owing to coupled
electron/proton transfer.17 The oxidation potential of the
diphenyl ketyl radical, -0.25 V vs SCE, seems to be the
most carefully established.18 Interestingly, this R-hydroxy
radical has the same oxidation potential as that of the
corresponding methoxy radical. Because of the lack of
potential complication from proton-coupled electron trans-
fer in the case of the R-alkoxy radicals, their oxidation
potentials could provide an estimate for the oxidation
potentials of the corresponding R-hydroxy radicals.
that of the singlet or the triplet state, depending on which
of these states react with the pyridinium compound.
There are potential sources of inefficiencies in the initia-
tion step. In the case of reactions via the singlet-excited
state, energy-wasting return electron transfer can com-
pete with the fragmentation of the N-methoxypyridyl
radical, especially if the fragmentation of the pyridyl
radical is not very fast. In the case of reactions via the
triplet state, return electron transfer is spin forbidden
and is unlikely to compete with the fragmentation.
Inefficient intersystem crossing, however, will lower the
initiation efficiency.
We first carried out experiments with benzhydrol
because the R-hydroxy radical derived from this alcohol
is a well-characterized intermediate with an absorption
band at 540 nm that can be used to follow the reaction
kinetics by flash photolysis.21 In these experiments an
equal concentration of benzhydrol and P1 of ∼0.04 M was
used, and the conversion was kept under 20%. Irradia-
tions using a number of sensitizers, Table 1, were carried
1. Sensitization/Initiation. Based on the general
derivation for energetic requirements for photoinduced
electron transfer,19 the driving force for electron transfer
from an excited sensitizer to the methoxypyridinium salt
depends on (a) the excitation energy of the sensitizing
excit
ox
donor, (E
reduction potential of the pyridinium salt, (E
d) an energy increment ∆,20 which varies from near zero
in polar solvents, such as acetonitrile, to ca. 0.3 eV in
D D
) , (b) its oxidation potential, (E ) , (c) the
red
3
out in CD CN, and the degree of conversion to products
was determined by NMR. The three reaction products,
benzophenone, protonated cyanopyridinium and metha-
A
) , and
(
(
17) Lilie, J.; Beck, G.; Henglein, A. Ber. Bunsen-Ges. Phys. Chem.
971, 75, 458.
18) Lund, T.; Wayner, D. D. M.; Jonsson, M.; Larsen, A. G.;
Daasbjerg, K. J. Am. Chem. Soc. 2001, 123, 12590.
19) Weller, A. Z. Phys. Chem. (Munich) 1982, 130, 129.
(20) Arnold, B. R.; Farid, S.; Goodman, J. L.; Gould, I. R. J. Am.
Chem. Soc. 1996, 118, 5482.
(21) (a) Hurley, J. K.; Sinai, N.; Linschitz, H. Photochem. Photobiol.
1983, 38, 9. (b) Murov, S. L.; Carmichael, I.; Hug, G. L. In Handbook
of Photochemistry; Marcel Dekker: New York, 1993.
1
(
(
J. Org. Chem, Vol. 70, No. 17, 2005 6811

Shukla et al.
nol, were formed, as expected, in similar amounts; see
Experimental Section.
In Table 1, Φ is the measured product quantum yield
P
of the reaction determined by NMR, initiation efficiency
is the efficiency of the initial formation of the pyridyl
radical from the reaction of P1 with excited electron
donors, and the chain amplification is obtained from the
quantum yield divided by the initiation efficiency
Using 9,10-diphenylanthracene, DPA, as an electron-
transfer sensitizer via the singlet excited state,22
a
product quantum yield of 1.3 was obtained, experiment.
, Table 1. As shown by flash photolysis, the initiation
1
efficiency of this reaction was only 0.21 (see Experimental
Section for Quantum Yield for Initial Radical Formation).
Thus, after correction for inefficient initiation, the chain
length, in this case, is estimated to be ∼6.
3
FIGURE 1. Absorption spectra of triplet thioxanthone ( TX*)
•
+
and of thioxanthone radical cation (TX ), generated via
Although 9,10-dicyanoanthacene, DCA, is a widely
3
3
•+
electron transfer from TX* to P1. TX* and TX spectra
obtained 0.5 and 1 µs after laser pulse, respectively.
excit
red
used electron acceptor (E
vs SCE), its fluorescence is quenched by P1 at a rate
) 2.97 eV, E ) -0.91 V
2
3
9
-1 -1
constant, 9 × 10 M s , approaching the diffusion-
controlled limit. This suggests that, in this case, DCA is
acting as an electron donor, and using it as a sensitizer
for the benzhydrol/P1 reaction, a quantum yield of 1.3
was obtained, experiment 2. After correcting for an
initiation efficiency of 0.054,24 a chain amplification of
to the same species, see Experimental Section. A minor
source of inefficiency, in this case, is the quantum yield
of intersystem crossing, which is 0.88 for TX in acetoni-
26
trile. Thus, the measured quantum yield of 18 would
correspond to a chain length of 20.
In experiment 5, our original intent was to initiate the
benzhydrol/P1 chain reaction through the well-known
hydrogen-atom abstraction by triplet benzophenone,
2
4 is estimated.
In experiment 3, DCA was used in combination with
biphenyl as a cosensitizer.23 At high concentration of
biphenyl, most of the singlet-excited DCA is intercepted
by this electron donor, with only a small fraction of -
3
27
BP*, from benzhydrol. This reaction would yield two
diphenyl ketyl radicals and, thus, an initiation efficiency
of 2. Although the highest quantum yield was obtained
using BP as a sensitizer, the actual mechanism, however,
turned out to be a double electron-transfer reaction with
initiation efficiency of 2.
1
DCA* being intercepted by P1. Under the reaction
conditions, DCA• and biphenyl are formed with a
quantum yield of 0.65.23 Thus, in this case the reducing
agent is not an excited sensitizer, as in the previous two
-
•+
Whereas the bimolecular rate constant for hydrogen
•
-
experiments, but it is a radical anion, DCA , an interac-
tion that does not have a deactivation path. The mea-
sured quantum yield of this reaction was 11, which upon
correction for the initiation efficiency of 0.65 yields a
chain amplification of 17.
In experiment 4, a triplet sensitizer, thioxanthone (TX),
was used to eliminate inefficiencies resulting from return
electron transfer in the geminate pair, as encountered
in experiments 1 and 2. Triplet TX is quenched by P1 at
3
6
-1
abstraction from benzhydrol by BP* is 5.1 × 10 M
-1
s
(see Experimental Section), flash photolysis experi-
ments showed that BP* is quenched by P1 at a rate
3
8
-1 -1
28
constant of 1.7 × 10 M s , Figure 2.
Thus, at equimolar concentrations of benzhydrol and
3
P1, only 3% of BP* molecules are intercepted by ben-
zhydrol and 97% by P1. The latter process yields a
species with a transient absorption at ∼390 nm, Figure
•+
3, which we assign to BP , a product of electron transfer
9
-1
-1
3
29
a rate constant (8.8 × 10
diffusion-controlled limit in acetonitrile. Shown in Figure
is the transient spectrum from this reaction, with an
absorption maximum at 435 nm, similar to that reported
M
s ) approaching the
from BP* to P1. Consistent with this assignment is
its quenching by triphenylamine, TPA, at a rate constant
that matches the concurrent formation of TPA ; see
•
+
1
•
+ 25
for the thioxanthone radical cation, TX . An additional,
much stronger absorption band with ill-defined maxi-
mum at ca. 760 nm, which has not been reported before,
is also observed. For evidence that both bands are due
(26) (a) Intersystem-crossing quantum yield for TX of 0.85 has been
2
6b
measured in benzene. In polar solvents, however, triplet quantum
2
6c
yields ranging from 0.66 to 1 have been reported. In view of the
discrepancies in reported values, we measured Φisc for TX in acetoni-
trile and obtained a value of 0.88; see Experimental Section. (b)
Amirzadeh, G.; Schnabel, W. Makromol. Chem. 1981, 182, 2821. (c)
Allonas, X.; Ley, C.; Bibaut, C.; Jacques, P.; Fouassier, J. P. Chem.
Phys. Lett. 2000, 322, 483 and references therein.
22) (a) DPA: Eexcit ≈ 3 eV, Eox ) 1.22 V vs SCE. (b) Berlman,
22b
22c
(
I. B. In Handbook of Fluorescence Spectra of Aromatic Molecules;
Academic Press: New York, 1971. (c) Phelps, J.; Santhanam, K. V. S.;
Bard, A. J. Am. Chem. Soc. 1967, 89, 1752.
(27) Wagner, P. J. Mol. Photochem. 1981, 1, 71.
6
e
7h
(28) Smets and Pappas have proposed that reactions of onium
salts with alcohols were initiated by hydrogen atom abstraction by
triplet ketones. In light of our kinetic data, however, initiation of these
reactions through electron transfer from the triplet ketone to the onium
salt should not be ruled out.
(23) (a) Gould, I. R.; Ege, D.; Moser, J. E.; Farid, S. J. Am. Chem.
Soc. 1990, 112, 4291. (b) DCA fluorescence in CH
3
CN is quenched by
red
9
-1 -1
P2 (E ) -1.0 V vs SCE) at a rate constant of ∼6 × 10
M
s .
Thus, the oxidation potential of DCA is estimated to be e 1.9 V vs
(29) (a) Similar electron transfer occurs in reaction of 3BP* with
methyl viologen leading to the formation of methyl viologen radical
SCE.
29b
(24) The measured quantum yield of free ions is 0.027. The radical
cation, suggesting formation of benzophenone radical cation. In our
•+
cation of DCA can oxidize benzhydrol and initiate another chain; thus,
the effective initiation efficiency is double the measured ion yield.
experiment absorption by BP (∼390 nm) could be detected because
of the lack of contribution from other species in this spectral region.
In the methyl viologen reaction, this region is masked by absorption
by the reduced acceptor (viologen radical cation). (b) Das, P. K.
Tettrahedron Lett. 1981, 22, 1307.
(25) (a) Fouassier, J. P.; Burr, D.; Crivello, J. V. J. Photochem.
Photobio., A 1989, 49, 317. (b) Manivanan, F.; Fouassier, J. P.; Crivello,
J. V. J. Polym. Sci., Part A: Polym. Chem. 1992, 30, 1999.
6812 J. Org. Chem., Vol. 70, No. 17, 2005

Photoreactions of N-Alkoxypyridinium Salts with Alcohols
TABLE 2. Quantum Yieldsa of Photoreactions of Benzyl
Alcohols with N-Methoxypyridinium Salts P1 and P2 at
Equimolar Concentrations of 0.02 M, Initiated with
Thioxanthone (TX) in CD3CN
FIGURE 2. Observed decay rate constant of benzophenone
3
triplet ( BP*) as a function of added pyridinium salt P1.
a
b
Error margin ≈ 10%. The value in parentheses is the
quantum yield measured at 0.04 M concentration of both
reactants.
ergetic within 0.1 eV. Given that BP triplet energy is
3
.0 eV and the reduction potential of P1 is ca -0.5 V,
the oxidation potential of BP is likely to be ∼2.5 ( 0.1 V
vs SCE.30 Because of such high oxidation potential, the
BP radical cation should be a powerful enough oxidant
to oxidize benzhydrol. This was confirmed in a flash
photolysis experiment where a rate constant for intercep-
•
+
9
-1 -1
tion of BP by benzhydrol of ∼1 × 10 M
s
was
measured. One-electron oxidation of benzhydrol will yield
a radical cation, which upon deprotonation will yield a
ketyl radical, and this in turn will reduce P1 and start
another chain. Therefore, the initiation efficiency is most
likely ∼2, and thus the chain length is ∼18.
3
FIGURE 3. Absorption spectra of benzophenone triplet ( BP*)
•
+
and of benzophenone radical cation (BP ), generated via
At a concentration of benzhydrol and P1 of ∼0.04 M,
the chain amplification in experiments 2-5 seem to fall
within a narrow range of 17-24, regardless of the
sensitizer/initiator system. In the case of experiment 1,
the amplification is considerably less, which is probably
due to higher rates for termination reactions.
3
3
•+
electron transfer from BP* to P1. BP* and BP spectra
obtained 1.5 and 2 µs after laser pulse, respectively.
2
. Scope and Limitations. To explore the scope of
this chain process, we also investigated reactions with
other alcohols and the effect of substituents on the chain
propagation. An electron-donating substituent, such as
an alkoxy group, on the phenyl ring of a benzyl alcohol
lowers the oxidation potential of the corresponding R-hy-
droxy radical because of the increased stabilization of the
resulting cation.31 An R-methyl substituent also increases
the reducing power of the radical because, in general,
tertiary radicals have lower (more negative) oxidation
3
2
potentials than the corresponding secondary radicals.
An R-methyl substituent, and to a lesser degree a
FIGURE 4. Absorption spectra of the radical cations of
4-alkoxy group, will also lower the bond dissociation
•+
•+
benzophenone (BP ), 4,4′-dimethoxybenzophenone (DMPB ),
•
+
•+
•+
•+
and Michlers ketone (MK ). BP , DMPB , and MK spectra
(
30) (a) Independent experiments led, through bracketing, to a
obtained 1.2, 1.5, and 2 µs after laser pulse, respectively.
similar conclusion. The fluorescence of 2,6,9,10-tetracyanoanthracene
excit
red
23
(
E
) 2.87 eV, E ) -0.44 V vs SCE) is not quenched by BP, i.e.,
21b
ox
E
T
BP > 2.43 V. In addition, BP triplet (E ) 3.00 eV) is quenched
red 15
Supporting Information. For comparison with spectra of
radical cations of other well-characterized benzophenone
derivatives, see Figure 4. We believe this is the first
identification of benzophenone radical cation by transient
absorption.
by 4-cyano-N-methylpyridinium (E ) -0.64 V, reversible) at a rate
7
-1
-1, corresponding to endothermicity of e0.17
constant of 1.7 × 10
M
s
ox
eV, i.e., E BP e 2.53 V, hence our estimate of ∼2.5 V. A literature
value of 2.33 V is reported for the oxidation potential of BP.3 (b)
Raumer, M. v.; Suppan, P.; Jacques, P. J. Photochem. Photobiol., A
1997, 105, 21.
0b
3
(31) Workentin, M. S.; Wayner, D. D. M. Res. Chem. Intermed. 1993,
9, 777.
(
The fact that electron transfer from BP* to P1 occurs
1
with a rate constant that is ca. 1/100th of the diffu-
sion-controlled limit indicates that this reaction is isoen-
32) Wayner, D. D. M.; McPhee, D. J.; Griller, D. J. Am. Chem. Soc.
1988, 110, 132.
J. Org. Chem, Vol. 70, No. 17, 2005 6813

Shukla et al.
1 2 3 6
SCHEME 2. Chain Propagation (k and k ) and Chain Termination (k -k ) Steps in the Oxidation of
Alcohols by N-Methoxypyridinium Salts, Initiated by Photoexcitation of an Electron Donor with an
Efficiency r
energy of the R-C-H bond of the benzyl alcohol.33 Thus,
the energetics of the two propagation steps in our scheme,
the hydrogen atom abstraction from the alcohol by the
methoxy radical and the electron transfer from the
radical to the methoxypyridinium salt, will be more
favored by these substitution patterns.
a 0.025 M concentration for both reactants) a quantum
yield of 18 for products formation: acetone, protonated
4-cyanopyridinium, and methanol. Similarly, 2,4-pen-
tanediol and P1, both at 0.04 M concentration, sensitized
with BP, yielded 4-hydroxy-2-pentanone with a quantum
yield of 15, which upon extended exposure underwent
further oxidation to acetylacetone.
The oxidation potential of R-methoxybenzyl radical
(
-0.33 V vs SCE)31 is slightly more negative than that
3
. Kinetics and Chain Termination Processes.
of the diphenylketyl radical (-0.25 V vs SCE) which is
the chain-propagating species in the first set of experi-
ments, Table 1. If R-hydroxybenzyl radical has a similar
oxidation potential as that of the R-methoxybenzyl radi-
cal, see above, then the chain reaction between benzyl
alcohol and P1 should be feasible. Indeed, with thioxan-
thone as a sensitizer, a quantum yield of 10 was obtained
at 0.02 M concentrations of both benzyl alcohol and P1,
Table 2. Further increase in the quantum yield up to 20
was observed in reactions with benzyl alcohol derivatives
that produce radicals with more reducing power, Table
Having addressed the scope of the chain reactions of
N-alkoxypyridinium salts and different initiation schemes,
we now consider the kinetics of propagation and termina-
tion reactions, which are the parameters that control the
chain length, Scheme 2.
Chain termination can result from side reactions of the
intermediates (the methoxy radical and the R-hydroxy
3 4
radical), k and k , respectively. Other chain termination
processes could be side reactions of these intermediates
with the reactants or with impurities in the reactants,
k
5
and k
with the reactants, or with impurities therein, are
kinetically equivalent. In the latter case, k and k would
correspond simply to the rate constants times the molar
fraction of the impurities. The effect of k and k on the
quantum yield should decrease with increasing concen-
trations of the reactants, whereas that of k and k should
not be a function of the concentrations of the reactants.
The quantum yield for product formation, Φ , the
6
. Chain terminations resulting from reactions
2
.
With benzyl alcohol, the 4-alkoxy- or the R-methyl
5
6
analogues, no chain amplification was observed with the
phenylpyridinium salt P2, which is harder to reduce than
P1 by ∼0.5 V. This indicates that electron transfer from
the R-hydroxy radicals derived from these alcohols to P2
is too endothermic to proceed at a rate that competes with
the termination reactions. Chain propagation with P2
was observed only in the reaction with the most reducing
radical derived from this set of compounds, 4-alkoxy-R-
methylbenzyl alcohol. Thus, with proper substitution of
the alcohol, chain amplification can be achieved, even
with a pyridinium salt that has a reduction potential of
ca -1 V vs SCE.
3
4
5
6
P
conversion of methoxypyridinium to the protonated py-
ridinium with the concomitant dehydrogenation of
R CHOH (a primary or a secondary alcohol) to R CO (an
2 2
aldehyde or a ketone), according to Scheme 2, is given
by eq 11, which yields eq 12.
It is noteworthy that, in addition to the usual products,
the reactions of the 4-alkoxybenzyl alcohols with P1 also
lead to the formation of the corresponding dibenzyl
ethers. These benzyl alcohols are readily transformed to
the ethers in a fast (dark) acid-catalyzed reaction at room
temperature. This unintended reaction demonstrates the
potential for “double amplification”: a chain reaction
leading to the formation of a reagent that can be used to
catalyze another reaction. No ether formation was ob-
served in the reactions with P2, presumably because
protonated 4-phenylpyridinium is a weaker acid than
protonated 4-cyanopyridinium.
2
2
3 3
Φ ) Rpq (1 + p q + p q + ...)
(11)
(12)
P
R
1
)
- 1
ΦP pq
where
and
k [R CHOH]
1
2
p )
q )
(
k + k )[R CHOH] + k
1 5 2
3
4
To further explore the scope of these chain-amplified
reactions, we investigated the photoreaction of 2-propanol
with P1, sensitized with benzophenone, and obtained (at
+
k [MeO-Py ]
2
(
33) Shukla, D.; Young, R. H.; Farid, S. J. Phy. Chem. A 2004, 108,
+
(
k + k )[MeO-Py ] + k
1
0386.
2
6
6814 J. Org. Chem., Vol. 70, No. 17, 2005

Photoreactions of N-Alkoxypyridinium Salts with Alcohols
FIGURE 6. Plot of R/Φ
initiation efficiency of the reaction (equal to the intersystem
crossing yield of the sensitizer, TX, 0.88) and Φ is the
quantum yield for product formation, vs the reciprocal of
equimolar concentration of both reactants, the pyridinium salt
P1 and benzhydrol.
P
at [TX] ) 6 mM, where R is the
FIGURE 5. Plot of R/Φ
initiation efficiency of the reaction (equal to the intersystem
crossing yield of the sensitizer, TX, 0.88) and Φ is the
quantum yield for product formation, vs the reciprocal of the
concentration of the reactants: (0) the concentration of the
pyridinium salt, P1, was varied, keeping the concentration of
benzhydrol constant; (O) the concentration of benzhydrol was
varied keeping the concentration of P1 constant. The lines are
least-squares fits for the data points.
P
at [TX] ) 6 mM, where R is the
P
P
Equation 12 leads to the following very good ap-
proximation, eq 13, by eliminating terms with negligible
contributions:
R
ΦP
k5
k6
k3
1
≈
+
+
+
(
) ( )
k1 k2
k
[R CHOH]
k3 k
4
1
2
k
4
1
1
1
+
( k
)
+
(
)
+
k1 k [R CHOH]
2
[MeO-Py ]
2
2
[MeO-Py ]
13)
(
The kinetics of thioxanthone (TX)-sensitized reactions
of the pyridinium salt P1 with benzhydrol (i.e., R ) Ph)
were evaluated in terms of this model. The data indicated
that, within the concentration range we used (<0.1 M),
the magnitude of the last term in eq 13 is negligible
compared to the other terms. The contribution of this
quadratic term to the total value is no more than 2%,
well within experimental error. Equation 13, therefore,
can be further simplified to eq 14:
FIGURE 7. Plot of calculated vs measured quantum yield for
product formation according to eq 14, using the parameters
given in text.
The best correlation between measured and calculated
quantum yields, Figure 7, was obtained using the fol-
lowing values:
k5 k6
k4
-3
+
k2) ) 9 × 10
)
k5
k6
k2
k3
1
k4
1
(
(
)
k1
k2
R
ΦP
≈ (k1
+
+
1
+
2
[MeO-Py ]
14)
) ( )
( )
+
k
[R CHOH]
k
k4
2
-3
-4
1
.3 × 10
) 2.1 × 10
(
)
(
k2
The three parameters of eq 14 given in parentheses
can be determined by varying [R CHOH] while keeping
MeO-Py ] constant and vice versa, Figure 5, as well as
These parameters represent ratios of rate constants
2
of chain-termination reactions to those of chain propaga-
tion. Thus, the smaller these parameters become, the
higher the chain amplification. It is useful to identify the
sources of these chain-termination processes.
+
[
by varying an equimolar concentration of both reactants,
Figure 6.
•
As shown in Figure 5, the quantum yield is much less
dependent on the concentration of the pyridinium salt,
P1, than it is on the concentration of the reacting alco-
hol, benzhydrol. According to eq 14, estimates for the
fixed parameters can be obtained from the slopes and
intercepts of the linear plots. More accurate values for
these parameters were obtained, however, from a global
analysis of the combined data from the three experi-
ments.
A. Reactions of the Methoxy Radical (MeO ).
Hydrogen atom abstraction from benzhydrol by the
1
methoxy radical, k , Scheme 2, is one of the two key steps
in the chain reaction under investigation. Determined
from an independent experiment (see Experimental
6
-1 -1
Section), k
1
is ∼6 × 10 M s . This rate constant is
6 -1 -1
similar to the value of 6.7 × 10 M s , which is reported
for hydrogen atom abstraction by tert-butoxyl radical
from benzhydrol in benzene.16
J. Org. Chem, Vol. 70, No. 17, 2005 6815

Shukla et al.
According to eq 18, the slope of the linear plot of Figure
8
corresponds to ksens/k
1
[Ph
2
CHOH]. From the measured
-
1
6
slope (8.3 M ), the above-mentioned value of k
1
(6 × 10
-
1
-1
M
s ) and the concentration [Ph CHOH] used in this
2
experiment (0.019 M), the rate constant for the reaction
•
of MeO with thioxanthone, ksens (eq 16), is calculated to
5
-1 -1
be 9.5 × 10 M s .
The intercept of Figure 8, which is 0.039, corresponds
to
d
k5 k6
k4
k
[CD CN]
1
solv
3
+
+
+
+
{
(
) ( )
(
)
}
k1 k2
k
k [R CHOH]
1 2
2
[MeO-Py ]
From this intercept, the rate constant of the chain-
terminating reaction of the methoxy radical with deute-
rioacetonitrile, k , can be determined. The sum of the
FIGURE 8. Plot of R/Φ
of the reaction () (Φisc
product formation for reaction of P1 with benzhydrol vs the
concentration of the sensitizer, thioxanthone (TX). Both P1
and benzhydrol were at an average concentration (before and
after irradiation) of 0.019 M.
P
, where R is the initiation efficiency
TX) and Φp is the quantum yield of
d
solv
)
first two terms of the intercept is available from the data
mentioned above, (0.009) + (0.00021)/0.019 ) 0.02, and
from this, the third term is obtained, 0.039-0.02 ) 0.019.
6
-1 -1
From the values of k
1
(6 × 10 M s ) and the benz-
hydrol concentration (0.019 M), the pseudo first-order
•
d
In addition to potential reactions of MeO with impuri-
•
3
rate constant for the reaction of MeO with CD CN, k
solv
ties in the reactants, Figure 5 clearly shows that termi-
nation reactions involving this radical compete with the
propagation step. These are collectively expressed in
terms of a pseudo first-order rate constant, k
source for chain termination is deuterium abstraction
3
-1
[
k
CD
3
CN], is 2.2 × 10 s . Neat CD
3
CN is 19.2 M; thus,
d
2
-1 -1
(eq 15) is estimated to be 1.1 × 10 M s .
solv
In accordance with the conclusion that some of the
3
. A likely
chain termination is due to abstraction of a deuterium
atom from the solvent by the methoxy radical, a signifi-
cant drop in quantum yield was observed when the
from the solvent, CD
3
CN, by the methoxy radical to form
•
methanol and CD
2
CN, which is a nonreducing radical.
3 3
reaction was carried out in CH CN instead of CD CN.
This hypothesis is supported by the data given below,
which also indicates that there is an additional chain
termination from the reaction of MeO with the sensitizer,
thioxanthone (TX). Thus, k represents the sum of two
3
pseudo first-order processes, eqs 15-17:
As determined from the data in Figure 4, at an average
equimolar concentration of benzhydrol and P1 of 0.019
M, and at a TX concentration of 0.0023 M, the quantum
•
yield for product formation (Φ
the same conditions, the corresponding quantum yield
in CH CN is only 4.7. Three of the four terms on the
P 3
) in CD CN is 15.1. Under
3
kd
solv
right-hand side of eq 10 are the same in both experi-
ments, and their magnitudes are known from the data
•
•
MeO + CD CN
8 MeOD + CD CN (15)
3
2
3
given above. For the reaction in CD CN, the third term
ksens
•
MeO +
8 addition product
(16)
(17)
d
k
[CD CN]
3
solv
(
)
d
solv
k [R CHOH]
1 2
k3 ) k [CD CN] + ksens[TX]
3
as mentioned above, equals 0.019. Similarly the corre-
sponding term for the reaction in CH CN
We found that the quantum yield for product formation
decreases with the increasing concentration of TX. A plot
of R/ΦP vs [TX] at an average, equimolar concentration
of the reactants of 0.019 M, showed as expected, a linear
dependence with an intercept of 0.039 and a slope of 8.3
M , Figure 8. It is unlikely that this decrease in
quantum yield is due to interception of R
which would yield benzophenone and TX-H , because the
latter radical would also be effective in propagating the
chain by reducing P1 with no net change in the outcome.
The dependence of the quantum yield on [TX] is more
likely due to the reaction of MeO with TX and is given
by eq 18, which is derived from eqs 14 and 17:
3
h
solv
k
[CH CN]
3
(
)
k [R CHOH]
1
2
-
1
is calculated to be 0.147. This corresponds to a significant
deuterium isotope effect (k /k ) of 7.7 and a bimo-
lecular rate constant for hydrogen abstraction by MeO
•
2
C OH by TX,
•
h
d
solv solv
•
h
2
-1 -1
from CH
3
CN, k
(eq 19) of 8.5 × 10 M s . Reported
solv
deuterium isotope effect for hydrogen atom abstraction
•
by tert-butoxy radical from different substrates vary over
a wide range,3 up to ∼6 for the reaction with toluene,
4a
34
which is several orders of magnitude faster than that
with acetonitrile.
R
ΦP
k5
k6
k2
k4
1
≈
+
+
+
kh
{
(k1
) ( )
+
•
solv
•
k
2
[MeO-Py ]
MeO + CH CN
8 MeOH + CH CN (19)
3
2
d
solv
k
[CD CN]
ksens
3
+
[TX] (18)
B. Reactions of the r-Hydroxy Radical
(Ph C -OH). The second chain-propagating reaction is
2
(
)
}
k [R CHOH]
k [R CHOH]
•
1
2
1
2
6
816 J. Org. Chem., Vol. 70, No. 17, 2005

Photoreactions of N-Alkoxypyridinium Salts with Alcohols
SCHEME 3
SCHEME 4
•
the electron transfer from the ketyl radical, Ph
to the pyridinium salt (P1), k
whenever electron-transfer reactions are exothermic by
0.2 eV or more, they proceed at the diffusion-controlled
limit. The reduction potential of P1 is ca. -0.5 V vs
2
C -OH,
, Scheme 2. In general,
2
∼
limit, not unreasonable for an electron-transfer reaction
estimated to be endothermic by 0.25 eV. From the above-
15
SCE, see above, and the oxidation potential of the ketyl
-4
mentioned ratio of k
4
/k
2
of 2.1 × 10 M, the competing,
•
18
radical, Ph
2
C -OH, is -0.25 V vs SCE. Thus, electron
pseudo first-order rate constant, k
4
, is estimated to be
•
transfer from Ph
2
C -OH to P1 is estimated to be endo-
2
-1
2
.3 × 10 s . This chain-terminating process might
thermic by ∼0.25 eV.
represent a sum of more than one reaction. A possible
Based on the reaction scheme used in this work, the
•
reaction could be that of Ph
2
C -OH with residual oxygen
2
rate constant k is not readily determined by transient
kinetics because chain propagation will continue to
reform the ketyl radical during the course of the reaction.
We were able, however, to determine k by using another
2
approach to generate the ketyl radical in the absence of
in the argon-purged solutions used in these experiments.
Quenching of the ketyl radical by oxygen is likely to be
1
0
-1 -1
diffusion controlled (∼10
M
s ), and thus, an oxygen
-
8
concentration in the range of ∼10 M could account for
such chain-terminating process. It is also possible that
dimerization (radical/radical coupling) could contribute
to chain termination via the ketyl radical.
benzhydrol. On the basis of a reaction scheme that we
3
reported recently, triplet benzophenone ( BP*) transfers
energy to 1,2,4,5-tetracyanobenzene (TCB), which in an
-
electron-transfer reaction (e tr.) with benzopinacol,
followed by fragmentation of the pinacol radical cation
Summary
yields the ketyl radical, Scheme 3.35
A plot of the ketyl radical decay rate vs the concentra-
tion of added P1, Figure 9, gives a slope corresponding
Photosensitized electron-transfer reactions of N-meth-
oxypyridinium salts with alcohols of diverse structures
proceed via a chain process. Chain lengths of ∼10-20
can be achieved, even at modest reactant concentrations
of 0.02-0.04 M, and despite endothermicity of the critical
electron-transfer step from the intermediate R-hydroxy
radical to the pyridinium salt. The reactions can be
initiated by a number of singlet or triplet sensitizers, with
varying degrees of initiation efficiencies that can be as
high as 2.
Summarized in Scheme 4 are rate constants for the
propagation and termination steps in the thioxanthone-
sensitized reaction of the methoxypyridinium salt, P1,
with benzhydrol determined from a combination of
steady-state and transient kinetics.
The data reveal the different sources that affect the
propagation efficiency and clearly indicate that chain-
terminating reactions of the methoxy radical play a more
important role in limiting the extent of chain amplifica-
tion than those of the ketyl radical. To illustrate this
point, the measured quantum yield at 0.04 M concentra-
FIGURE 9. Observed rate constant for decay of the ketyl
radical (generated according to Scheme 3, monitored at 540
nm) vs concentration of added pyridinium salt, P1, in aceto-
nitrile. The slope corresponds to the bimolecular rate constant
for electron transfer from the ketyl radical to P1.
tion of both benzhydrol and P1 in CD
3
CN is ∼19. If the
6
-1
to k
2
of 1.1 × 10 M s-1. This rate constant is ca. 4
chain-terminating reactions of the methoxy radical (with
the solvent and the sensitizer) were eliminated, the
quantum yield would increase to ∼62. However, if the
chain-terminating reactions of the ketyl radical (probably
with residual oxygen and dimerization) were eliminated,
the quantum yield would be only ∼21. Because of a large
deuterium isotope effect for hydrogen-atom abstraction
by the methoxy radical from acetonitrile, the quantum
orders of magnitude lower than the diffusion-controlled
(
34) (a) Manchester, J. I.; Dinnocenzo, J. P.; Higgins, L.-A.; Jones,
J. P. J. Am. Chem. Soc. 1997, 119, 5069. (b) Kim, S. S.; Kim, S. Y.;
Ryou, S. S.; Lee, C. S.; Yoo, K. H. J. Org. Chem. 1993, 58, 192.
(
35) In a less polar solvent (1,2-dichloroethane), the geminate pair
TCB• /Ph
-
COH , Scheme 3, leads exclusively, via electron transfer,
+
2
•
to TCB and Ph
2
COH . In acetonitrile, however, out-of-cage separation
competes with the electron transfer. From the relative intensities of
•
•-
2
Ph COH and TCB (see Supporting Information) and their extinction
-1 -1, 33
yields in CD
CH CN.
3
CN are substantially higher than those in
coefficients 3,800 and 15,900 M cm respectively, the partitioning
between separation and electron transfer is estimated to be 30:70.
3
J. Org. Chem, Vol. 70, No. 17, 2005 6817

Shukla et al.
Experimental Section
λ
max at 630 nm (Figure 1) and τ ≈ 4 µs. Based on the rapid
quenching by oxygen and similarity to the previously reported
Materials. The pyridinium salts P1 and P2 were synthe-
38
spectrum, the transient species was assigned to triplet
1
3
sized according to literature procedure. The crude salts were
recrystallized from methanol. The sensitizers and alcohols
were commercially available. 4-Ethoxy-1-phenylethyl alcohol
was prepared by reduction of 4-ethoxyacetophenone with
3
3
thioxanthone ( TX*). The decay of TX* at 630 nm was
enhanced in the presence of P1 and from the slope of a linear
plot of the observed decay rate constant (kobs vs [P1]), a
9
-1 -1
quenching rate constant of 8.8 × 10 M
s
was obtained.
36
sodium borohydride in ethanol. The products were identified
by comparison with known authentic samples.
Steady-State Photolysis: Reaction Quantum Yields
Measurements. The irradiations were carried out with a PEK
3
At [P1] ) 0.05 M, the TX* spectrum was completely
quenched and a new species with a weak transient absorption
band, λmax of 435 nm, and a much stronger, broad long-
wavelength absorption with a λmax ≈ 760 nm was formed
(Figure 1). Identical decay kinetics for both bands showed that
they belong to a single species. Based on its rapid quenching
1
25-W super-high-pressure Hg lamp. For 365 nm, a combina-
tion of a Kodak Wratten ultraviolet filter, no. 18A, a Corning
-52 cutoff filter, and a 365 nm interference filter were used.
0
by electron donors, the transient was assigned to the thiox-
For 405 nm, a Corning 5-58 band-pass filter, a Corning 0-73
cutoff filter, and a 405 nm interference filter were used.
In a typical experiment, a 3-mL solution of an N-methoxy-
anthone radical cation (TX•+). In the presence of 1,4-dimeth-
oxybenzene (DMB) (10-3 M; Eox ) 1.30 V vs SCE), the decay
at 435 and 780 nm occurred with the same rate constant (9.7
9
-1 -1
pyridinium salt, an alcohol, and a sensitizer in CD
3
CN was
( 0.2 × 10 M
s
) as the growth of DMB radical cation at
3
9
placed in a 1 × 1 cm cell. The sensitizer concentration was 6
mM, unless otherwise stated. The solution was purged with
argon for ∼10 min and irradiated for 2-10 min to reach a
conversion of ∼8-18%. During photolysis, argon was continu-
ously bubbled above the exposed area to purge and stir the
solution. The percent conversion was determined from average
470 nm.
Triplet Benzophenone (BP) as Electron Donor. Laser
excitation of benzophenone (BP) at 360 nm in argon-purged
3
acetonitrile led to the formation of triplet benzophenone ( -
3
3,40
BP*) with characteristic absorption
having λmax at 525 nm
3
and τ of ∼9 µs. BP* was quenched by P1, and from a plot of
1
ratios of integrated H NMR signals of products and reactants.
the observed decay rate constant, k , vs [P1], a second-order
obs
1
8
-1 -1
For example, before photolysis the H NMR spectrum of an
quenching rate constant of 1.7 × 10 M
s
was obtained
equimolar solution of P1 and benzhydrol, with catalytic
(Figure 2).
amount of thioxanthone, TX, in acetonitrile-d
3
shows charac-
Quenching of BP* by P1 (ca. 0.04 M) was accompanied by
3
teristic signals due to P1 (δ 9.17 (m, 2H), 8.47 (m, 2H), 4.47
s, 3H)) and to benzhydrol (δ 7.45-7.20 (m, 10H), 5.81 (d, 1H),
the formation of a weakly absorbing transient at 390 nm, τ ≈
(
10 µs, assigned to BP•+ on the basis of fast quenching by
3
.90 (d, 1H)). After irradiation at 405 nm the 1H NMR
triphenylamine (TPA). The decay of BP at 440 nm in the
•+
spectrum of the photolyzate clearly showed appearance of new
diagnostic signals due to formation of N-H-4-cyanopyridinium
presence of added TPA and the concomitant rise of TPA radical
cation (TPA•+) at 680 nm occurred at the same rate; see
(
(
δ 8.92 (m, 2H) and 8.33 (m, 2H)), benzophenone (δ 7.83-7.50
m, 10H)) and CH
Supporting Information. The absorbance of BP•+ at 440 nm
3
OH (δ 3.30). The identity of these products
although low is enough to allow for measurement of its decay.
It was necessary to monitor decay at 440 nm since triphenyl-
amine radical cation has strong absorption below 400 nm. In
1
was established by comparison with H NMR spectra of
authentic samples. Integration of N-H-4-cyanopyridinium,
benzophenone, and methanol signals relative to P1 signals
clearly shows that the three products, as required by the
stoichiometery of the reaction, are formed in equal amounts.
Additional details of the NMR data are given in Supporting
Information.
3
a similar experiment, P2 (0.05 M) failed to quench BP*.
The triplet state of 4,4′-dimethoxybenzophenone was
9
-1 -1
quenched by P1 with a rate constant of 8.2 × 10 M
s , and
Michlers ketone triplet was quenched by P1 and by P2 with
10
10
-1
-1
a rate constant of 1.6 × 10 and of 1.2 × 10
M
s ,
The photon flux was determined by using the photocycload-
respectively. The spectra of the resulting radical cations are
shown in Figure 4.
dition reaction of phenanthrenequinone to trans-stilbene in
3
7
benzene as an actinometer. The light intensity was 7 to 9 ×
Hydrogen Abstraction by Triplet Benzophenone
-8
-1
3
3
10
Einstein min . The quantum yield for product formation
( BP*). The decay of BP* (λ
max
) 530 nm) was measured in
was determined from the percent conversion and the light
intensity, corrected for incomplete absorption by the sensitizer.
Laser Flash Photolysis. The samples in a 1 cm × 1 cm
quartz cell were excited at a right angle to the monitoring
beam. The excitation source was a Q-switched Nd:YAG laser
system. The laser can be tuned between 330 and 500 nm
acetonitrile at different concentrations of benzhydrol (λexcit )
360 nm). The reaction was monitored at 610 nm to avoid
absorption by the reaction product, diphenylketyl radical. A
6
-1 -1
rate constant of 5.1 × 10
M
s
was obtained, which is
6
-1
slightly lower than that reported in benzene, 8.9 × 10 M
-
1 27
s
.
(
pulses of 5 ns, 0.5-0.8 mJ) using the output of the 266 nm
Quantum Yield for Initial Radical Formation. The
pumped OPO. The excitation pulses were attenuated, when
necessary, using neutral density filters. A pulsed Oriel 150-W
xenon lamp was used as the monitoring beam. The analyzing
beam was collected and focused on the entrance slit (2 nm) of
a monochromator. A photomultiplier tube (PMT) was attached
to the exit slit of the monochromator. A computer-controlled
high-voltage power supply was used with the PMT. The signals
from the PMT were digitized using an oscilloscope and
transferred to a PC. A pulse generator provided TTL trigger
pulses to control the timing for the laser, lamp, and oscil-
loscope.
efficiency of the initial formation of the pyridyl radical from
the reaction of P1 with excited electron donors was measured
1
using the reaction of DCA* with durene (0.05 M) in aerated
23
acetonitrile as actinometer. In each case, 4,4′-dimethoxystil-
-
4
bene (DMS) was used as a monitor (5 × 10 M) to intercept
the primary radical cation. The concentration of the monitor
-
3
was increased up to 3.2 × 10 M when DCA was used as
•+
sensitizer in order to ensure complete interception of DCA .
The quantum yield of the free radical ions from the actinom-
2
3
eter is 0.239. Solutions having matched optical densities at
the excitation wavelength were irradiated at 400 nm and
monitored at 525 nm, where only DMS absorbs, with DCA
being quenched by molecular oxygen present in the aerated
solution. The quantum yields for the formation of the donor
•
+
•-
Triplet Thioxanthone (TX) as Electron Donor. Pulsed
laser excitation of TX at 400 nm in an argon-purged acetoni-
trile solution led to the formation of a transient species with
(
36) Furniss, B. S.; Hannaford, A. J.; Smith, P. W. G.; Tatchell, A.
(38) Yates, S. F.; Schuster, G. B. J. Org. Chem. 1984, 49, 3349.
(39) Shukla, D.; Schepp, N. P.; Mathivanan, N.; Johnston, L. J. Can.
J. Chem. 1997, 75, 5, 1820-1829.
(40) Peters, K. S.; Freilich, S. C.; Schaeffer, C. G. J. Am. Chem. Soc.
1980, 102, 5701.
R. In Vogel’s Text Book of Practical Organic Chemistry, 5th ed.; John
Wiley & Sons: New York, 1989.
(
37) Brown-Wensley, K. A.; Mattes, S. L.; Farid, S. J. Am. Chem.
Soc. 1978, 100, 4162.
6818 J. Org. Chem., Vol. 70, No. 17, 2005

Photoreactions of N-Alkoxypyridinium Salts with Alcohols
radical cation (and thus for that of the pyridyl radical) were
Kinetics of Electron Transfer from Diphenylketyl
Radical by P1. The ketyl radical was generated in the
absence of benzhydrol, as outlined in Scheme 3. Laser excita-
tion (355 nm) of benzophenone (6 mM) in argon-purged
acetonitrile in the presence of 1,2,4,5-tetracyanobenzene (TCB),
0.05 M, and benzpinacol, 0.05 M, gave two transient species:
0.21 for DPA and 0.027 for DCA.
Triplet Yield of Thioxanthone (TX). The quantum yield
for intersystem crossing of TX in acetonitrile was measured
using, as an actinometer, the energy transfer from triplet
2
1b
benzophenone (E
T
T
) 69.1 kcal/mol) to 1-cyanonaphthalene
21b
•-
•
(
E
) 57.4 kcal/mol).
The intersystem-crossing quantum
2
TCB and Ph C -OH with absorption bands at 465 nm and at
2
1b
yield for BP in acetonitrile was taken to be 1.0. Acetonitrile
solutions of BP and TX were matched for optical density (0.67)
at the excitation wavelength, 363 nm, and an equal amount
540 nm, respectively. The decay of the ketyl radical was
monitored as a function of added P1. The slope of a plot of the
observed decay rate constant vs [P1], Figure 9, corresponds
-
3
•
of 1-cyanonaphthalene (3.4 × 10 M) was added to each
sample. The solutions were thoroughly purged with argon and
the formation of triplet 1-cyanonaphthalene was monitored at
to the bimolecular electron-transfer rate constant from Ph
2
C -
6
-1 -1
•-
OH to P1 (1.13 × 10 M
s ). TCB was completely quenched
at the lowest concentration of P1 (0.02 M) because of a much
4
(
30 nm. An intersystem crossing quantum yield, Φisc, of 0.88
faster (probably diffusion controlled), exothermic electron
•
-
0.04 was obtained for TX in acetonitrile.
transfer from TCB to P1.
Kinetics of Hydrogen Abstraction from Benzhydrol
•
by MeO . The methoxy radical was generated by laser excita-
tion (355 nm) of N-methoxyphenanthridinium hexaflurophos-
phate in argon-purged acetonitrile. Cleavage of the N-O bond
yields a methoxy radical and phenanthridine radical cation.
Acknowledgment. We thank J. Dinnocenzo (Uni-
versity of Rochester) and H. Hall (University of Arizona)
for very valuable discussions.
1
,2-Dimethoxybenzene (0.01 M) was added to intercept the
Supporting Information Available: NMR data of the
radical cation and remove its absorption between 500 and 700
nm. The rate of hydrogen atom abstraction was determined
from the growth of the diphenylketyl radical at 530 nm in the
presence of varying concentrations of benzhydrol. A plot of the
observed growth rate at 530 nm, kobs, vs [benzhydrol] was
linear, and from the slope, a rate constant for hydrogen atom
•+
•+
pyridinium salts, figures for the decay of BP and TX with
the concurrent growth of the radical cation of added donors,
•
•-
2
and spectra of the Ph COH and TCB , generated according
to Scheme 3. This material is available free of charge via the
Internet at http://pubs.acs.org.
6
-1 -1
abstraction of 5.8 × 10 M
s
was obtained.
JO050726J
J. Org. Chem, Vol. 70, No. 17, 2005 6819