Substituent effects on the rate constants for the photo-claisen rearrangement of allyl aryl ethers

Article abstract of DOI:10.1021/ja011981y

The photochemistry of 11 substituted allyl 4-X- and 3-X-aryl ethers 3 (ArOCH₂-CH=CH₂) has been examined in both methanol and cyclohexane as solvents. The ethers react by the photo-Claisen rearrangement to give allyl substituted phenols as the major primary photoproducts, as expected from the well-established radical pair mechanism. The excited singlet state properties (absorption spectra, fluorescence spectra, fluorescence quantum yields, and singlet lifetimes) were compared with a parallel set of unreactive 4-X- and 3-X-anisoles 4. The excited-state properties of three substituted 4-X-aryl 4-(1-butenyl) ethers 14 (ArOCH₂CH₂-CH=CH₂) were also examined. The model compounds 4 and the reactive allyl ethers 3 have essentially identical rate constants for the excited-state processes with the exception of k_hom^s, the rate constant for homolytic cleavage from S₁ of the allyl ethers to give the radical pair. The difference between the fluorescence quantum yields and/or singlet lifetimes for 3 and 4 were used to obtain values of k_hom^s for all of the allyl ethers. These values exhibit a large substituent effect, spanning almost 2 orders of magnitude with electron-donating groups (CH₃O, CH₃) accelerating the reaction and electron-withdrawing ones (CN, CF₃) slowing it down. The parallel range of rate constants observed in both methanol and cyclohexane indicates that ion pairs are not important intermediates in these rearrangements. Quantum yields of reaction (Φ_r) for several of the more reactive ethers demonstrate that neither these values nor rate constants of reaction (k_hom^r) derived from them are reliable measures of the actual excited-state process. In fact, the k_hom^r values are significantly lower than the k_hom^s ones, indicating that the radical pairs undergo recombination to generate starting material. Finally, the k_hom^s rate constants were found to parallel a trend for the change in bond dissociation energy (ΔBDE) for the O-C (allyl) bond of the allyl ethers, indicating that other possible substituent effects are of minor importance.

Full text of DOI:10.1021/ja011981y

Published on Web 07/26/2002
Substituent Effects on the Rate Constants for the
Photo-Claisen Rearrangement of Allyl Aryl Ethers
Alexandra L. Pincock, James A. Pincock,* and Roumiana Stefanova
Contribution from the Department of Chemistry, Dalhousie UniVersity,
Halifax, NoVa Scotia, Canada, B3H 4J3
Received August 15, 2001. Revised Manuscript Received May 13, 2002
Abstract: The photochemistry of 11 substituted allyl 4-X- and 3-X-aryl ethers 3 (ArOCH2-CHdCH2) has
been examined in both methanol and cyclohexane as solvents. The ethers react by the photo-Claisen
rearrangement to give allyl substituted phenols as the major primary photoproducts, as expected from the
well-established radical pair mechanism. The excited singlet state properties (absorption spectra,
fluorescence spectra, fluorescence quantum yields, and singlet lifetimes) were compared with a parallel
set of unreactive 4-X- and 3-X-anisoles 4. The excited-state properties of three substituted 4-X-aryl 4-(1-
butenyl) ethers 14 (ArOCH2CH2-CHdCH2) were also examined. The model compounds 4 and the reactive
allyl ethers 3 have essentially identical rate constants for the excited-state processes with the exception of
kshom, the rate constant for homolytic cleavage from S1 of the allyl ethers to give the radical pair. The
difference between the fluorescence quantum yields and/or singlet lifetimes for 3 and 4 were used to obtain
values of kshom for all of the allyl ethers. These values exhibit a large substituent effect, spanning almost 2
orders of magnitude with electron-donating groups (CH3O, CH3) accelerating the reaction and electron-
withdrawing ones (CN, CF3) slowing it down. The parallel range of rate constants observed in both methanol
and cyclohexane indicates that ion pairs are not important intermediates in these rearrangements. Quantum
yields of reaction (Φr) for several of the more reactive ethers demonstrate that neither these values nor
rate constants of reaction (krhom) derived from them are reliable measures of the actual excited-state
process. In fact, the krhom values are significantly lower than the khsom ones, indicating that the radical pairs
undergo recombination to generate starting material. Finally, the kshom rate constants were found to parallel
a trend for the change in bond dissociation energy (∆BDE) for the O-C (allyl) bond of the allyl ethers,
indicating that other possible substituent effects are of minor importance.
phenylacetylenes),4 and σ•hν (on the basis of photochemical
quantum yields of homolytic σ bond cleavage for benzyl
Introduction
The effect that substituents (X) on aromatic rings have on
the rates of organic reactions of the type XC6H4Y-Z, i.e.,
Hammett Fσx correlations, continues to be a fundamental tool
for understanding reaction mechanisms in the ground electronic
state. Numerous σx scales have been developed as probes for
the changes in electron density that occur as the reactant
proceeds along the reaction coordinate to the rate-determining
transition state. The same statements cannot be made about σ
scales for photochemical reactions that proceed through elec-
tronically excited states. Therefore, there is a continuing interest
in measuring the effect that substituents have on the rates of
photochemical reactions. The four most extensive excited-state
σ scales are σex (on the basis of the excited singlet state pKa
values for substituted phenols with Fex defined as -3.1),1,2 σ*
(also on the basis of excited singlet state pKa values for phenols
but rescaled to give F* ) 1 for the ionization of benzoic acids
in the excited state),3 σhν (on the basis of the rate constants for
protonation of the excited state of substituted styrenes and
sulfides).5
We have been particularly interested in σ bond cleavage
reactions6,7 and have recently reported8 on the photochemical
reactivity of the substituted aryl tert-butyl ethers 1a-k in
methanol. The rate constants of product formation from the
excited singlet state, obtained from quantum yields of product
formation and singlet-state lifetimes, were correlated reasonably
well with σhν; F ) -0.77 (r ) 0.975). Unfortunately, strong
electron-withdrawing groups (CN, CF3) could not be included
in the correlation because σhν values are not known for these
substituents; the substituted styrenes are too unreactive in the
photoprotonation reaction used to establish the scale. Moreover,
(3) (a) Shim, S. C.; Park, J. W.; Ham, H. S. Bull. Korean Chem. Soc. 1982, 3,
13-18. (b) Shim, S. C.; Park, J. W.; Ham, H.-S.; Chung, J.-S. Bull. Korean
Chem. Soc. 1983, 4, 45-47.
(4) McEwen, J.; Yates, K. J. Phys. Org. Chem. 1991, 4, 193-206.
(5) Fleming, S. A.; Jensen, A. W. J. Org. Chem. 1996, 61, 7040-7044.
(6) Pincock, J. A. Acc. Chem. Res. 1997, 30, 43-49.
(7) Fleming, S. A.; Pincock, J. A. In Organic Molecular Photochemistry;
Ramamurthy, V., Schanze, K. S., Eds.; Marcel Dekker: New York, 1999;
Vol. 3, pp 211-281.
* Address correspondence to this author. Telephone: (902) 494-3324.
Fax: (902) 494-1310. E-mail: James.Pincock@Dal.ca.
(1) Wehry, E. L.; Rogers, L. B. J. Am. Chem. Soc. 1965, 87, 4234-4238.
(2) Baldry, P. J. J. Chem. Soc. 1979, 951-953.
(8) DeCosta, D. P.; Bennett, A.; Pincock, A. L.; Pincock, J. A.; Stefanova, R.
J. Org. Chem. 2000, 65, 4162-4168.
9
9768
J. AM. CHEM. SOC. 2002, 124, 9768-9778
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Photo-Claisen Rearrangement of Allyl Aryl Ethers
A R T I C L E S
we obtained evidence for the 4-cyano analogue 2 that some of
the products were ion pair rather than radical pair derived.
quantum yields of fluorescence for the substrate (Φf), eq 1,11
and the model (ΦMf ), eq 2, give eq 3; in the dynamic case, the
measured singlet lifetime of the substrate (τs) and the model
excited (τsM) give eq 4. In both cases the assumption is made
that all rate constants (kf, kic, kisc) for the singlet state except
kshom are the same for the substrate and the model. This method
has been used very successfully and extensively in studies of
intramolecular electron transfer in bichromophoric molecules
where the model chromophore is identical except for the absence
of the remote second chromophore.12
kf
kf
Φf )
)
) kfτs
(1)
kf + kic + kisc + kshom
kdt
kf
ΦMf )
) kfτsM
(2)
A generalized mechanism for the photocleavage of an aryl
ether (ArOR) is given in Scheme 1. Obtaining the rate constant
of interest, kshom, for the reaction of the excited singlet state is
not simple because of three complications.9 First, intersystem
crossing (kisc) from S1 gives T1 which may also be reactive
(kThom) and therefore radical-derived products will be obtained
from two pathways. Second, the excited singlet state may react
by heterolytic cleavage (kshet) to give ion pairs so that ion-
derived products are obtained. Also, because ion-derived
products can be obtained from initially formed singlet radical
pairs that undergo redox electron transfer (ket), the measured
yields of ion- and radical-derived products will not be a reliable
measure of the values for either kshet or khsom. Finally, both
radical pairs (krcom) and ion pairs (kicom) may undergo in-cage
recombination to form the starting substrate. Internal return has
been observed in every case where it has been tested for in
photochemical benzylic (ArCH2-X) cleavage reactions;7 ex-
amples are for X ) O-CO-R, +NH3, and O-PO-(OR)2. As
is well-known, if any of these complications occur, measured
quantum yields of product formation will not reflect the initial
reactivity of the excited state.
kf + kic + kisc
ΦfM
kshom
)
- 1 /τsM
(3)
(4)
(
)
Φf
1
1
kshom
)
-
s
τ
τMs
(
)
We now report on the application of this idea for determining
the rate constants for cleavage from the excited singlet state of
a set of substituted allyl aryl ethers 3a-k in both methanol and
cyclohexane. In both solvents, these substrates undergo an
efficient photo-Claisen reaction by the well-established radical
pair mechanism.13
This kind of comparison has been used previously in studies
of the photo-Claisen reaction, first in an early study by Carroll
and Hammond of the ether 5.14 A more recent study by Pohlers
et al.15,16 of the fluorescence of the allyl naphthyl ethers 6 and
7, compared to their unreactive methyl naphthyl ether analogues,
was used to verify photo CIDNP results that indicated that both
singlet and triplet states of 6 and 7 were reactive at 298 K but
by 373 K only the singlet state reacted. The effects that
substituents on the aromatic ring have on the efficiency or rate
constant of reaction were not reported in either of the above
examples.
Scheme 1. Mechanistic Scheme for the Photochemical σ Bond
Cleavage of a Generalized Aryl Ether
Results and Discussion
Photochemical Product Studies. The allyl aryl ethers 3c-k
were synthesized from the corresponding phenols by reaction
A solution to overcome these complications is possible if a
suitable unreactive model for the substrates of interest can be
found. The idea is that the model mimics the excited-state
properties of S1 in all ways except that it is unreactive. In this
case, either static or dynamic fluorescence spectroscopy can be
used to obtain kshom values directly.10 In the static case, the
(11) Equation 2 assumes that S1 reacts only by homolytic cleavage, kshom. If S1
also reacts by heterolytic cleavage, then kshet would also be in the sum of
rate constants for decay of S1 in the denominator. For the photochemical
reaction of the ethers described in this report, the results presented below
indicate that kshom . khset
.
(12) Wasieleaski, M. R. In Photoinduced Electron Transfer, Part A; Fox, M.
A., Chanon, M., Eds.; Elsevier: New York, 1988; pp 161-206.
(13) Gu, W.; Warrier, M.; Schoon, B.; Ramamurthy, V.; Weiss, R. G. Langmuir
2000, 16, 6977-6981 and references therein.
(9) These problems have been discussed in detail for excited-state σ bond
cleavage reactions in ref 7.
(10) As will be discussed below, kshom values will not be the same as krhom
values obtained from quantum yields of reaction.
(14) Carroll, F. A.; Hammond, G. S. Isr. J. Chem. 1972, 10, 613-626.
9
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A R T I C L E S
Pincock et al.
Table 1. Product Yields for the Photolysis of the Allyl Aryl Ethers
3a-j in Methanol and Cyclohexane
with allyl bromide using potassium carbonate in acetone; 3a:
X ) H and 3b: X ) 4-CH3O are available from Aldrich. As
expected from previous reports and shown in eq 5, the major
products in both methanol and cyclohexane are the correspond-
ing phenols 8 and the rearranged allyl substituted phenols
2-allyl-3-X, 4-allyl-3-X, and 2-allyl-5-X (9, 10, and 11) for the
3-substituted substrates and only 2-allyl-4-X (12) for the
4-substituted cases. Many of these products have been charac-
terized previously from studies of substituent effects on the rates
of thermal Claisen rearrangements: 12 for X ) 4-CH3O, 4-CH3,
and 4-CN;17,18 9 and 11 for X ) 3-CH3O, 3-CH3, and 3-CF3.19,20
The GC/FID yields, taken at low conversions (10-20%), are
reported in Table 1. At these conversions the mass balance was
always above 90%; the yields reported are normalized to 100%.
The photochemistry of 3a: X ) H and 3d: X ) 4-CH3 in both
methanol and cyclohexane for the former, but only in cyclo-
hexane for the latter, has been reported previously;21 as indicated
in Table 1, agreement with the literature yield values is good.
For 3c: X ) 3-CH3O, literature values are available in ethanol.22
At higher conversions, the product allyl ethers all react by
secondary photochemistry to give cyclopropyl derivatives, as
in eq 6. These were identified as isomers by GCMS and as
a M is methanol, C is cyclohexane. b Literature values, ref 21. c Literature
values, ref 22. d Actually, 3-allyl-4-methylphenol as shown in eq 7. e Isomer
assignment not certain, see Experimental Section.
chemistry24 of the cyclohexadienone 13, eq 7. This product was
also observed previously on photolysis in methanol.21 In this
case, the initially formed photo-Claisen product cannot enolize
to the phenol. Similar 2,5-cyclohexadienone derivatives have
always been proposed as the primary photoproducts in Claisen
reactions and have been isolated from irradiations of substituted
allyl aryl,21 benzyl aryl,25 and aryl naphthylmethyl26 ethers.
Products analogous to 12 are probably formed from others of
the para-substituted compounds 3, but if so, we were unable to
isolate them.
cyclopropyl derivatives by their high field signals in 1H NMR.
These cyclopropyl derivatives, expected from the aryl version
of the photochemical di-π-methane rearrangement,23 were not
characterized completely but could easily be seen increasing in
yield at the expense of the corresponding allyl compounds by
GC/FID.
Finally, the 3-cyano compound 3k proved to be so unreactive
photochemically that, by the time reasonable conversion of the
starting material had occurred, the product mixtures were
complicated by high yields of secondary photoproducts. These
mixtures could not be separated, and therefore the photoproducts
For 3d: X ) 4-CH3 one of the products was found to be
3-allyl-4-methylphenol (12), expected from secondary photo-
(15) Pohlers, G.; Grimme, S.; Dreeskamp, H. J. Photochem. Photobiol., A:
Chem. 1994, 79, 153-162.
(16) Pohlers, G.; Dreeskamp, H.; Grimme, S. J. Photochem. Photobiol., A:
Chem. 1996, 95, 41-49.
1
were not characterized. H NMR spectra of the crude reaction
(17) White, W. N.; Gwynn, D.; Schlitt, R.; Girard, C.; Fife, W. J. Am. Chem.
Soc. 1958, 80, 3271-3277.
mixtures indicated mainly cyclopropyl derivatives.
(18) Goering, H. L.; Jacobson, R. R. J. Am. Chem. Soc. 1958, 80, 3277-3285.
(19) White, W. N.; Slater, C. D. J. Org. Chem. 1961, 26, 3631-3638.
(20) White, W. N.; Slater, C. D. J. Org. Chem. 1962, 27, 2908-2914.
(21) Waespe, H.-R.; Heimgartner, H.; Schmid, H.; Hansen, H.-J.; Paul, H.;
Fischer, H. HelV. Chim. Acta 1978, 61, 401-429.
(22) Syamala, M. S.; Ramamurthy, V. Tetrahedron 1988, 44, 7223-7233.
(23) Zimmerman, H. E. In Organic Photochemistry; Padwa, A., Ed.; Marcel
Dekker: New York, 1991; Vol. 11, pp 1-36.
(24) Schultz, A. G. In Handbook of Organic Photochemistry and Photobiology;
Horspool, W. M., Song, P.-S., Eds.; CRC Press: Boca Raton, FL, 1995;
pp 685-700.
(25) Benn, R.; Schuchmann, H.-P.; von Sonntag, C. Z. Naturforsch., B: Chem.
Sci. 1979, 34b, 1002-1009.
(26) Yoshimi, Y.; Sugimoto, A.; Maeda, H.; Mizuno, K. Tetrahedron Lett. 1998,
39, 4683-4686.
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Photo-Claisen Rearrangement of Allyl Aryl Ethers
A R T I C L E S
Table 2. Absorbance and Fluorescence Data for the Allyl Aryl Ethers 3a-k and the Anisoles 4a-k in Methanol and Cyclohexane
compd
X
solventa
λ
max,b nm
ꢀ,b M-1 cm-1
λ
0,0,c nm
ES ,d kcal/mol
Φfe
τs,f ns
1
4a
3a
4a
3a
4b
3b
4b
3b
4c
3c
4c
3c
4d
3d
4d
3d
4e
3e
4e
3e
4f
H
H
H
H
M
M
C
271
271
271
272
290
290
289
288
274
274
274
274
279
278
279
279
273
273
273
273
280
279
281
281
269
269
269
269
270
269
270
270
277
277
267
276
271 (sh)
272 (sh)
274
274 (sh)
290
290
288
288
1680
1600
1930
1860
2820
2730
3200
2990
2200
2250
2220
2240
1880
1660
2170
1900
1660
1540
1810
1630
2670
2460
2900
2690
1590
1450
1650
1590
1060
860
1100
1060
2310
2250
2440
2300
2030
2250
1310
1480
2960
2870
2980
2840
281
282
281
281
305
305
304
304
285
286
284
284
289
290
290
288
283
285
282
283
291
291
290
290
279
280
278
278
279
280
280
280
288
289
289
288
286
286
286
287
305
305
300
301
102
101
102
102
94
94
94
94
100
100
101
101
99
99
99
99
101
100
101
101
98
98
99
99
103
102
103
103
103
102
102
102
99
0.24g
0.012
0.29h
0.0056
0.11
7.58 ( 0.06
7.8 ( 0.1
C
4-OCH3
4-OCH3
4-OCH3
4-OCH3
3-OCH3
3-OCH3
3-OCH3
3-OCH3
4-CH3
4-CH3
4-CH3
4-CH3
3-CH3
3-CH3
3-CH3
3-CH3
4-F
4-F
4-F
4-F
3-F
3-F
3-F
3-F
4-CF3
4-CF3
4-CF3
4-CF3
3-CF3
3-CF3
3-CF3
3-CF3
4-CN
4-CN
4-CN
4-CN
3-CN
3-CN
3-CN
3-CN
M
M
C
2.68 ( 0.02
3.02 ( 0.06
0.0069
0.12
C
0.0054
0.076
0.0095
0.093
0.0095
0.22
0.0065
0.33
0.0053
0.27
0.0088
0.37
0.0074
0.15
0.020
0.18
0.014
0.0077
0.0055
0.022
0.0081
0.093
0.028
0.13
0.021
0.11
0.067
0.16
0.064
0.075
0.048
0.11
0.044
0.14
M
M
C
2.42 ( 0.02
0.63 ( 0.04
2.48 ( 0.01
C
M
M
C
6.37 ( 0.07
7.48 ( 0.07
8.0 ( 0.1
C
M
M
C
8.79 ( 0.05
3.72 ( 0.04
3.86 ( 0.03
C
M
M
C
3f
4f
3f
C
4g
3g
4g
3g
4h
3h
4h
3h
4i
3i
4i
3i
4j
3j
4j
3j
4k
3k
4k
3k
M
M
C
0.8 ( 0.01
C
M
M
C
4.47 ( 0.05
1.47 ( 0.02
5.05 ( 0.06
0.93 ( 0.01
3.16 ( 0.02
2.03 ( 0.01
3.41 ( 0.03
1.63 ( 0.01
3.37 ( 0.04
2.38 ( 0.05
5.88 ( 0.09
2.45 ( 0.02
3.60 ( 0.04
3.10 ( 0.04
4.83 ( 0.03
3.90 ( 0.04
C
M
M
C
99
99
99
100
100
100
100
94
C
M
M
C
C
M
M
C
94
95
95
0.10
0.20
0.16
C
a M is methanol, C is cyclohexane. b Obtained from absorption spectra. c Obtained from the overlap of the absorption and fluorescence spectra. d Calculated
from ES ) 2.86 × 104/λ0,0
.
e Quantum yield of fluorescence relative to anisole in methanol (0.24) and cyclohexane (0.29), estimated error ( 10%. f Singlet
lifetime1by nanosecond single photon counting. The errors given are the standard deviation of the fit to the experimental counts. No value indicates that the
lifetime was less than 0.5 ns. g Reference 27. h Reference 28.
The conclusion from these observations is that these substi-
tuted aryl allyl ethers behave as expected according to the well-
established radical pair mechanism for the photo-Claisen
rearrangement.13
The absorbance spectra support the proposal that the anisoles
are good models for the allyl ethers. First, the absorbance
maximum (λmax) and molar absorptivity (ꢀ) are essentially the
same for both compounds in either solvent for all substitutents.
The ꢀ values for each compound are slightly, but consistently,
higher in cyclohexane than in methanol. The exception is for
the 4-cyano compounds, but their spectra are complicated
because of the strong conjugative interaction between the alkoxy
and cyano substitutents which shifts the S2 band to longer
wavelengths so that it overlaps with the S1 band. To strengthen
the case for similarity of the absorption spectra for the allyl
and methyl ethers, normalized absorption spectra comparing the
S0 to S1 band for the allyl aryl ethers 3 (dashed lines) and the
anisoles 4 (solid lines) for each substituent are included in the
Supporting Information, Figures S1 (methanol) and S2 (cyclo-
hexane). In all cases, the spectra are essentially superimposable
considering the 1 nm resolution of the spectrometer. The
4-trifluoromethyl compounds show the largest difference at the
short-wavelength end of the S0-S1 transition.
Photophysical Properties for the Allyl Aryl Ethers 3 and
the Corresponding Anisoles 4. The absorbance spectral data
(λmax, ꢀ for the long-wavelength S1 band) and fluorescence
emission data (λ0,0, ES (calculated from λ0,0), Φf, and τs) are
given in Table 2 for the two complete sets of allyl aryl (3a-k)
and aryl methyl (4a-k) ethers in both methanol (M) and
cyclohexane (C). The quantum yields of fluorescence (Φf) were
determined relative to literature values for anisole 4a: X ) H
in methanol (0.24)27 and cyclohexane (0.29).28 The singlet
lifetimes are too short to measure with our nanosecond system
for many of the allyl ethers 3 in both solvents. This fact will be
discussed in more detail below.
(27) Kohler, G.; Kittel, G.; Getoff, N. J. Photochem. 1982, 18, 19-27.
(28) Berlman, I. B. In Handbook of Fluorescence Spectra of Aromatic Molecules;
Academic Press: New York, 1971; p 139.
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A R T I C L E S
Pincock et al.
The fluorescence emission spectra also support the reliability
of the anisole model. This comparison of the fluorescence
spectra is critical in evaluating the excited-state properties of 3
and 4. The anisole model requires that any interaction between
the aromatic chromophore and the allyl group (relative to
methyl) is small so as not to interfere with the postulate that
leads to eqs 3 and 4 and reliable estimates of kshom. First, the
0,0 bands (from the overlap of absorbance and fluorescence
spectra) and derived excited singlet state energies (ES) are
essentially the same for all substituents in either solvent. Second,
complete normalized spectra comparing the two ethers 3 and 4
for each substituent are given in the Supporting Information,
Figures S3 (methanol) and S4(cyclohexane). The deviations
observed in the comparison of the spectra are small and are
only significant for the allyl ethers with very low quantum yields
of fluorescence, Φf (Table 2). The major reason for these
deviations is that the spectra of these very weakly emitting allyl
derivatives are perturbed by the superposition of the relatively
intense Raman scattering band.
good agreement with our experimental value in cyclohexane,
kd ) 9.7 × 107 s-1. The very short singlet lifetime for the anisole
4g: X ) 3-F is a result of a very high value for kd ) 120 ×
107 s-1, a factor of 3-17 higher than any of the other substrates.
The reason for this is unknown.
For the less reactive allyl ethers (4h: X ) 4-CF3, 4i: X )
3-CF3, 4j: X ) 4-CN, and 4k: X )3-CN), kf values can also
be obtained experimentally; see Table 3. In all cases, the
agreement between the value for the allyl ethers and the model
compounds, the methyl ethers 3, is very good. The largest
deviation is for the 4i: X ) 3-CF3/3i in cyclohexane and 4k:
X ) 3-CN/3k in methanol equal to 3.9 ns/3.2 ns or 1.22.
The conclusion from this section is that the unreactive anisoles
4 serve as excellent models for the reactive allyl ethers 3.
Presumably, the excited-state potential energy minimum is
essentially the same for both except that the activation barrier
for oxygen-carbon bond cleavage is lower and consequently
the reaction occurs faster for the weaker oxygen to allyl,
compared to oxygen to methyl, bond.
Calculation of Fluorescent Rate Constants, kf, for the
Ethers 3a-k and 4a-k in Methanol and Cyclohexane. The
theoretical relationship between the radiative lifetime (τf ) 1/kf)
and the absorbance spectrum is given in eq 8,30,31
A third fluorescence method in support of the anisole model
is through excitation spectra. These are shown as a function of
emission wavelength in the Supporting Information in Figures
S5a/5b (3a and 4a in methanol/cyclohexane, X ) H), S6a/S6b
(3f and 4f in methanol/cyclohexane, X ) 4-F), S7a/S7b (3j and
4j in methanol/cyclohexane, X ) 4-CN), S8a/S8b (3a/4a in
cyclohexane, X ) H), S9a/S9b (3f/4f in cyclohexane, X ) 4-F),
and S10a/S10b (3j/4j in cyclohexane, X ) 4-CN). These
compounds were chosen because they span the complete range
of reactivity for the allyl ethers. All of these excitation spectra
show essentially identical intensity versus wavelength depen-
dence, independent of the wavelength of observation. This
equivalence of the intensity of the vibrational modes helps to
establish that the S1 minima for the allyl and methyl ethers are
very similar.
In contrast to the identical wavelength dependence of the
fluorescence spectra, the fluorescent quantum yields are con-
sistently lower for the allyl aryl ethers 3 than the methyl ethers
4 in both solvents. As indicated by eqs 1 and 2, this is a
consequence of the reaction itself, kshom, increasing the total
rate of decay of S1. In agreement with this conclusion, in all
cases where the singlet state lifetimes could be obtained, the
allyl ethers have shorter lifetimes (τs) than the methyl ethers
(τsM).
For all but one (4g: X ) 3-F) of the substituted methyl ethers,
and for the less reactive allyl ethers (4h: X ) 4-CF3, 4i: X )
3-CF3, 4j: X ) 4-CN, and 4k: X) 3-CN) excited singlet state
lifetimes and fluorescence quantum yields can be used to obtain
other rate constants for that state, namely kf and kd ) 1/τs - kf.
The exception 4g: X ) 3-F has an unusually short lifetime,
almost at the limit of the time resolution (0.5 ns) of our flash
lamp lifetime equipment in cyclohexane (0.8 ns) and too short
to measure in methanol (∼0.3 ns). Table 3 gives these kf and
kd values for the other methyl ethers. In all cases, kd > kf. Likely
kd, the radiationless decay of the excited singlet state, is
dominated by intersystem crossing to the triplet state. For
kf ) 2.88 × 10-9n2 νjf-3 -1 ꢀ ln νj
(8)
∫
where n is the refractive index of the solvent,32,33 νf is the
expectation value for the frequency of the fluorescence spectrum,
and the final term is the integrated absorbance spectrum. The
constant gives kf in units of s-1 if the frequency values are
expressed in wavenumber (cm-1) and molar absorptivity ꢀ in
the usual units (M-1 cm-1). This equation is derived on the
basis of the oscillator strength model and the assumption that
the excited singlet state and the ground state have similar
geometries. It has been used successfully in the past for simple
aromatic compounds such as benzene, toluene, and σ-xylene.34
Calculated values of kf for all compounds, except the 4-cyano
and 4-trifluoromethyl ones are given in Table 3. Values of kf
for the 4-CF3 and 4-CN compounds were not calculated because
the absorption spectrum for the S0 to S1 transition could not be
integrated due to an overlap with the more intense S0 to S2
transition. Also given in parentheses in the same column are
ratios of kf(experimental)/kf(calculated) in cases where experi-
mental values could be obtained. The average of these ratios is
0.96 ( 0.13, an indication of a random distribution about a value
of 1. These calculations give us considerable confidence that
good estimates of kf can be made even in cases where
fluorescence lifetimes are too short to be measured. These
calculated values of kf can then be used to estimate kdt ) 1/τs
values using the measured quantum yields of fluorescence
because kdt ) kf/Φf. These values of kdt, in parentheses, along
with experimental ones are in Table 3.
Calculation of kshom Values for the Allyl Aryl Ethers, 3a-
k, in Methanol and Cyclohexane. Using eq 3 or 4 and the
(30) Strickler, S. J.; Berg, R. A. J. Chem. Phys. 1962, 37, 814-822.
(31) Birks, J. B.; Dyson, D. J. Proc. R. Soc. A 1963, 275, 135-148.
(32) The value of n ) 1.34 for methanol was estimated for 330 nm by
extrapolation from seven refractive index values from 656 to 404 nm; Wood,
S. E.; Langer, S.; Battino, R. J. Chem. Phys. 1960, 32, 1389-1393.
(33) The value of n ) 1.45 for cyclohexane was taken from ref 28, Appendix
7.2, p 422.
instance, the quantum yield of intersystem crossing (Φisc
)
0.64)29 for anisole 4a: X ) H along with the singlet lifetime
(τs ) 7.6 ns, Table 2) gives kisc ) Φisc/τs ) 8.4 × 107 s-1, in
(29) Murov, S. L.; Carmichael, T.; Haig, G. L. Handbook of Photochemistry,
(34) Cundall, R. B.; Pereira, L. C. J. Chem. Soc., Faraday Trans. 1972, 68,
1152-1163.
2nd ed.; Marcel Dekker: New York, 1993; p 13.
9
9772 J. AM. CHEM. SOC. VOL. 124, NO. 33, 2002

Photo-Claisen Rearrangement of Allyl Aryl Ethers
A R T I C L E S
Table 3. Excited-State Rate Constants for the Alkyl Aryl Ethers 3a-k and the Anisoles 4a-k in Methanol and Cyclohexane
kshom (s-1)/107 from eq 5
kshom (s-1)/107 from eq 4e
c
compd
X
solventa
kdt (s-1)/107 ) 1/τs b
k (s-1)/107 ) Φf/τs
k (s-1)/107 from eq 9
kd (s-1) /107 ) kdt − kd
f
f
f
4a
3a
4a
3a
4b
3b
4b
3b
4c
3c
4c
3c
4d
3d
4d
3d
4e
3e
4e
3e
4f
H
H
H
H
M
M
C
13.2
(270)
12.8
(660)
37.3
(580)
33.1
(720)
41.3
(330)
40.3
(400)
15.7
(530)
13.4
(830)
12.5
(380)
11.3
(570)
26.8
(200)
25.9
(330)
(300)
(418)
125
(345)
22.3
68
19.8
108
31.6
49.2
29.3
61.3
29.6
42.0
17.0
40.8
27.8
32.2
20.3
25.6
3.2
3.7
4.0
3.9
3.1
3.8
3.5
4.4
3.4
4.2
4.0
4.6
2.4 (1.33)
2.5
3.6 (1.03)
2.9
4.3 (0.93)
4.1
5.5 (0.71)
5.1
3.1 (1.00)
3.1
3.5 (1.09)
3.7
3.1 (1.12)
2.9
3.9 (1.13)
3.2
2.8 (1.21)
2.3
3.4 (1.24)
2.8
4.1 (0.98)
3.8
4.9 (0.94)
4.7
2.3
2.3
2.9 (1.00)
2.8
10
(17)
9.7
(7)
33
(46)
29
(25)
38
(37)
36
(46)
12
(17)
8.9
(7.7)
9.1
(7.6)
7.1
(0)
23
(26)
21
(25)
(300)
(300)
120
(132)
20
21
17
18
28
28
26
25
27
27
15
15
24
24
16
17
250
650
530
690
290
350
510
820
370
570
170
300
(120)
210
51
C
4-CH3O
4-CH3O
4-CH3O
4-CH3O
3-CH3O
3-CH3O
3-CH3O
3-CH3O
4-CH3
4-CH3
4-CH3
4-CH3
3-CH3
3-CH3
3-CH3
3-CH3
4-F
4-F
4-F
4-F
3-F
3-F
3-F
3-F
4-CF3
4-CF3
4-CF3
4-CF3
3-CF3
3-CF3
3-CF3
3-CF3
4-CN
4-CN
4-CN
4-CN
3-CN
3-CN
3-CN
3-CN
M
M
C
C
M
M
C
C
M
M
C
C
M
M
C
C
M
M
C
3f
4f
3f
C
4g
3g
4g
3g
4h
3h
4h
3h
4i
3i
4i
3i
4j
3j
4j
3j
4k
3k
4k
3k
M
M
C
2.9
C
M
M
C
2.1
1.9
2.6
2.3
3.5
3.3
3.2
3.9
2.2
2.0
1.9
1.8
3.9
3.2
4.1
4.1
46
88
18
32
12
24
6
C
100
20
M
M
C
3.7 (0.95)
4.0 (0.83)
4.4 (0.73)
4.0 (0.98)
C
44
M
M
C
16
C
26
M
M
C
4.5 (0.87)
4.6 (0.70)
5.1 (0.80)
4.6 (0.89)
11
C
6
11
a M is methanol, C is cyclohexane. b Values in parentheses are obtained from kf(eq 8)/Φf. c Values in parentheses are kf(experimental)/k(calculated).
d Values in parentheses are calculated from kdt - kf - khom e Estimated error ( 20%.
.
appropriate fluorescence quantum yields or lifetimes, respec-
tively, leads to an estimation of kshom, the rate constant for
homolytic cleavage for S1 of the allyl ethers. These values are
also given in Table 3. As discussed in the Introduction, triplet-
state reactivity or internal return by collapse of radical pairs
will not complicate these values because these values, deter-
mined by fluorescence data, are a measure of only excited singlet
state reactivity.
of fluorescence are also higher and therefore more reliable. For
the allyl ethers of higher reactivity and consequently singlet
lifetimes too short to measure, only the relative fluorescence
quantum yields and eq 3 can be used to obtain khsom
.
Finally, the self-consistency of this method of obtaining
kshom values by using the unreactive model can be assessed by
examining kd values for the excited singlet state. These are now
obtained from eq 9
In cases where fluorescence lifetimes are available experi-
mentally, eq 4 is probably more reliable. Quantum yields of
fluorescence, required for eq 3, are inherently more difficult to
measure accurately because they are obtained by comparison
of the relatively weak fluorescence spectra of the allyl aryl ethers
with a standard (anisole) using solutions of matched absorbance.
Very small amounts of the more strongly fluorescent corre-
sponding phenols, which are the synthetic precursors for the
allyl ethers, will significantly enhance these weak emissions.
However, as seen for the substituted ethers of lower reactivity,
where both eqs 3 and 4 can be applied, the two methods are in
good agreement. Of course, in these cases the quantum yields
kd ) kic + kisc ) kdt - kf - kshom
(9)
using measured values for the less reactive allyl ethers and
calculated values, in parentheses, for the more reactive substrates
where kf (calculated from eq 8) and Φf lead to kdt ) 1/τs. These
can then be compared to experimental kd values obtained for
the unreactive methyl ethers. The agreement is again very good,
validating the basic assumption of the method. The excited
singlet state properties of the reactiVe allyl aryl ethers and the
unreactiVe aryl methyl ether model compounds are essentially
9
J. AM. CHEM. SOC. VOL. 124, NO. 33, 2002 9773

A R T I C L E S
Pincock et al.
would be another process for internal conversion. The homolo-
gous aryl 4-butenyl ethers 14 were proposed as a better choice
for a model as they cannot undergo the photo-Claisen reaction
but do have a second vinyl functional group.
The problem with compounds 14 as models is that now
interaction between the two chromophores is essentially guar-
anteed according to the n ) 3 rule for the number of saturated
atoms that separate the two chromophores. This fact was first
demonstrated for 1,ω-diphenylalkanes where the localized
emission (If ) 4) was strongly quenched in cyclohexane and
excimer emission was observed for 1,3-diphenylpropane but not
for 1,2-diphenylethane (If ) 105) or 1,4-diphenylbutane (If )
100).36 This rule has been found to be not rigorously followed.
For instance, for 1,ω-dipyrenylalkanes excimer emission was
observable for a whole range (n ) 2-16) of insulating (CH2)n
linkers.37 However, even in these cases, the ratio of excimer to
monomer emission was high (>15) for n ) 3 and low (<0.5)
for n ) 2. Moreover, the emission was blue shifted for the n )
2 case relative to both the n ) 3 and the intermolecular pyrene
cases, suggesting that the excimer was of higher energy because
the shortness of the chain prevented a geometry that allowed
maximum interaction between the two chromophores.
Morrison and co-workers38 were the first to report a similar
effect for ω-arylalkenes. Thus, in cyclopentane, the fluorescence
(relative intensity If ) 0.05) of 15 (n ) 3) was strongly quenched
at room temperature relative to phenylhexane (If ) 0.90)
whereas that for 16 (n ) 2) was essentially the same (If ) 0.88).
Moreover, this rapid formation of an exciplex for 15 was
followed by intramolecular cycloaddition, a process (meta
photocycloaddition) which has since been observed for many
tethered alkenes and aromatics for n ) 3.39 To our knowledge,
there are no examples for n ) 2. We conclude that interaction
between the aryl and the vinyl group in the compounds 14, as
a consequence of exciplex formation, should be much greater
than for the allyl ethers 3.
Figure 1. Plot of log kshom(methanol) versus log kshom(cyclohexane) for the
photo-Claisen reaction of the allyl aryl ethers 3a-k.
the same in all respects except for the inclusion of an additional
process, kshom, for the reactiVe compounds.
Compound 3g: X ) 3-F is particularly interesting in this
regard. Although the model compound 4g is too short-lived in
methanol to measure its singlet lifetime, its lifetime can be
reliably estimated using Φf and kf (calculated from eq 8). Then
eq 3 can be used to calculate kshom ) 120 × 107 s-1 (in
parentheses in Table 3 because it is not obtained by direct
experiment). As will be shown below, this value is in good
agreement with correlations of rate constants for the other
substrates.
Several general observations concerning these rate constants
can be made. First, due to the experimental errors in determining
Φf and kf, these values are probably accurate only to about
(20%, particularly for the reactive ethers. Therefore, all rate
constants are reported to only two significant figures. Second,
the effect of the substituents is unusually large for a photo-
chemical reaction, spanning almost 2 orders of magnitude. Third,
the trend in the rate constants as a function of substitutents is
the same in methanol and cyclohexane. This observation is
reinforced by Figure 1, a linear log/log plot of the rates in
methanol versus cyclohexane. This is an important observation
because it demonstrates that ion pair intermediates are not
formed directly from S1 in methanol. The slope of 0.96 indicates
that the rate constants are greater in cyclohexane than in
methanol as is seen numerically in Table 3 for 3a-k. The
similarity in yields in methanol and cyclohexane (Table 1) also
indicates that ion pairs are not formed at a later stage after the
excited-state chemistry is finished, i.e., ket (Scheme 1).
Photophysical Properties for the Aryl 4-Butenyl Ethers,
14. At a conference presentation of these results,35 the valid
question was raised as to whether the allyl ethers are reliably
modeled by the anisoles. The point was made that a decrease
in fluorescence intensity or lifetime for the allyl ethers could
reflect an excited singlet state interaction (exciplex formation)
that does not lead to cleavage of the carbon-oxygen band but
is invisible because the excimer is nonemissive. Therefore, this
We have synthesized and measured fluorescence quantum
yields and singlet lifetimes in both methanol and cyclohexane
for three butenyl ethers 14a: X ) H, 14b: X ) 4-OCH3, and
14j: X ) 4-CN. These were chosen to span the range of
fluorescence quenching observed for the allyl ethers 3. For
instance, fluorescence is strongly quenched relative to the
corresponding anisole 4 for 3a: X ) H, Φ4F/ΦF3 ) 20 and 3b:
X ) 4-OCH3, Φ4/Φ3 ) 16, and quenched only slightly for 3j:
F
F
X ) 4-CN, Φ4F/Φ3F ) 1.6 (values are in methanol as solvent).
The three butenyl ethers also span the range of electron-donating
(X ) 4-OCH3) to electron-withdrawing (X ) 4-CN) groups to
test for any electron transfer component to exciplex formation.
The results are given in Table 4.
(36) Hirayama, F. J. Chem. Phys. 1965, 42, 3163-3171.
(37) Zachariasse, K.; Kuhnle, W. Z. Phys. Chem. Neue Folge 1976, 101, 267-
276.
(38) Ferree, W.; Grutzner, J. B.; Morrison, H. J. Am. Chem. Soc. 1971, 93,
5502-5512.
(39) Cornelisse, J. Chem. ReV. 1993, 93, 615-670.
(35) Presented at the IAPS Conference, Arizona State University, Tempe, AZ,
January 2002.
9
9774 J. AM. CHEM. SOC. VOL. 124, NO. 33, 2002

Photo-Claisen Rearrangement of Allyl Aryl Ethers
A R T I C L E S
Table 4. Absorbance and Fluorescence Data for the Aryl 4-(1-Butenyl) Ethers 14
compd
X
solventa
λ
0,0,b nm
ES ,c kcal/mol
Φfd
τs,e ns
1
4a
14a
4a
14a
4b
14b
4b
14b
4j
H
H
H
H
M
M
C
281
281
281
280
305
304
304
304
286
286
286
286
102
102
102
103
94
94
94
94
100
100
100
100
0.24f
0.043
0.29g
0.050
0.11
0.10
0.12
0.13
7.58 ( 0.06
1.64 ( 0.01
7.8 ( 0.1
C
1.73 ( 0.02
2.68 ( 0.02
2.58 ( 0.02
3.02 ( 0.06
2.96 ( 0.02
3.37 ( 0.04
1.38 ( 0.02
5.88 ( 0.09
3.40 ( 0.03
4-OCH3
4-OCH3
4-OCH3
4-OCH3
4-CN
4-CN
4-CN
4-CN
M
M
C
C
M
M
C
0.075
0.035
0.11
14j
4j
14j
C
0.061
a M is methanol, C is cyclohexane. b Obtained from the overlap of the absorption and flurorescence spectra. c Calculated from ES ) 2.86 × 104/λ0,0
single photon counting. The errors given are standard deviations of the fit to the experimental counts. f Reference 27. g Reference 28.
.
d Quantum yield of fluorescence relative to anisole in methanol (0.24) and cyclohexane (0.29), estimated error ( 10%. e Singlet life1time by nanosecond
Table 5. Quantum Yields and Derived Rate Constants for the
Photo-Claisen Reaction of Allyl Aryl Ethers 3
Reports on intramolecular photocycloadditions have appeared
for two of these compounds, 14a: X ) H and 14c: X ) 4-CN.
r
r
compd
X
solventa
Φrb
khom (s-1)/10-7 c
kshom/khom
In the former case,40 the process in cyclohexane was described
as “very inefficient; excessive polymer formation”;39 much better
yields have been obtained in dioxane as solvent, giving a mixture
of products derived from ortho and meta cycloadducts.41 In the
4-CN case, high yields of intramolecular ortho cycloadducts
were obtained on irradiation in all sovents used.42 For this
substrate, the cycloaddition products have been shown to arise
from the triplet state, first by Gilbert and co-workers42 and later
by Wagner and Smart.43
3a
3a
3b
3b
3c
3c
3d
3d
H
H
M
C
M
C
M
C
M
C
0.20 ( 0.02
0.14 ( 0.03
0.29 ( 0.01
0.21 ( 0.01
0.17 ( 0.02
0.08 ( 0.01
0.18 ( 0.02
0.13 ( 0.01
54
92
170
150
56
28
95
110
5
7
3
5
5
12
5
8
4-CH3O
4-CH3O
3-CH3O
3-CH3O
4-CH3
4-CH3
a M is methanol, C is cyclohexane. b Quantum yield at 25 °C, average
of two determinations for disappearance of allyl aryl ether. c Obtained from
Φrkdt.
No reports on intramolecular photoadducts for the butenyl
ether 14b: X ) 4-OCH3 are available (SciFinder). In agreement
with this fact, the quantum yields of fluorescence and singlet
lifetimes in both cyclohexane and methanol are essentially
identical to those of the anisole 4b: X ) 4-OCH3 (Table 4). In
contrast, for the allyl ether 3b: X ) 4-OCH3, the fluorescence
quantum yield is highly reduced, relative to the corresponding
anisole or butenyl ether. This is a consequence of its high
reactivity from the excited singlet state via homolytic bond
cleavage leading to photo-Claisen products. This observation
is particularly important because it is the very reactive com-
pounds like 3b that are critical in establishing the correlation
shown in Figure 3 (see below).
For the two other butenyl ethers 14a: X ) H and 14j: X )
4-CN, modest quenching of the fluorescence relative to the
anisole is observed, by a factor of about 4-5 for the former
and only about 2 for the latter. Because fluorescence quantum
yield and singlet lifetimes are decreased in a parallel fashion,
the kf values remain similar, as expected. Again, exciplex
formation should be much more favorable for the -OCH2CH2-
linker (n ) 3) in the butenyl ethers 14 than for the -OCH2-
(n ) 2) linker in the allyl ethers 3. Therefore, the small changes
for the butenyl ether indicate that the changes observed for the
allyl ethers, which are larger for X ) H and slightly smaller
for X ) 4-CN, are a consequence of singlet-state deactivation
by bond cleavage, not exciplex formation.
for singlet-state reactivity of the substituted allyl aryl ethers 3,
a brief discussion of measured quantum yields of reaction is
required. These are given in Table 5 for a few of the more
reactive ethers. The often invoked caution of using quantum
yields as a measure of excited-state reactivity certainly applies
here. For instance, 3d: X ) CH3 has a relatively low quantum
yield of reaction (Φr ) 0.14) in cyclohexane, but the highest
rate constant of reaction (kshom ) 690 × 107 s-1). Moreover,
the quantum yields of reaction are lower in cyclohexane but
the rate constants of reaction are higher for all cases studied.
These Φr values can be used to calculate an operational
value10 of krhom ) Φrkdt using the kdt values, also given in Table
3. In all cases, the krhom values (Table 5) calculated in this way
are significantly smaller than those obtained from the fluores-
cence method described above. The ratio of kshom/khr om averages
4.5 ( 0.7 in methanol and 8.0 ( 2.0 in cyclohexane. Because
of the observation that the rate constants parallel each other in
methanol and cyclohexane (Figure 1), ion pairs are not involved.
Therefore, the left-hand protion of Scheme 1 can be ignored
and only complications arising from kisc, potentially followed
by triplet reactivity (kThom), and singlet radical pair recombina-
tion (krcom) need consideration. The fact that the rate constants
(krhom) obtained from quantum yield results are lower than
those (kshom) obtained from the S1 fluorescence measurements
indicates, not surprisingly, that krcom is an important process.
Thus, a significant fraction of the singlet radical pairs generated
undergo recombination to starting material. This fact has been
previously demonstrated for the photo-Claisen reaction both by
CIDNP studies25,44 and by the observation of 1,3-migration in
Quantum Yields (Φr) of Reaction for the Allyl Aryl Ethers
3a-k. Before discussing the trends in rate constants obtained
(40) Gilbert, A.; Taylor, G. N. J. Chem. Soc., Perkin Trans. 1 1980, 1761-
1768.
(41) De Keukeleire, D.; He, S.-L.; Blakemore, D.; Gilbert, A. J. Photochem.
Photobiol., A: Chem. 1994, 80, 233-240.
(42) Al-Garadawi, S. Y.; Cosstick, K. B.; Gilbert, A. J. Chem. Soc., Perkin
Trans. 1 1992, 1145-1148.
(44) Adam, W.; Fischer, H.; Hansen, H.-J.; Heimgartner, H.; Schmid, H.;
(43) Wagner, P. J.; Smart, R. P. Tetrahedron Lett. 1995, 36, 5135-5138.
Waespe, H.-R. Angew. Chem. 1973, 85, 662-663.
9
J. AM. CHEM. SOC. VOL. 124, NO. 33, 2002 9775

A R T I C L E S
Pincock et al.
unsymmetrical (deuterium- or methyl-substituted) allyl deriva-
tives.21 Such migrations do not occur in the concerted thermal
Claisen rearrangement.45
The observation of lower rate constants of reaction by the
quantum yield measurements as opposed to the fluorescence
method also suggests that the triplet states of these ethers are
not reactive. If kisc followed by kThom were an important
process, then quantum yields of reaction would be expected to
lead to the determination of rate constants greater than those
obtained by the fluorescence method which monitors only the
reactivity of S1. In effect, products would be formed more
efficiently than expected on the basis of only excited singlet
state reactivity. In support of this conclusion, photo CIDNP
reults25 indicate that the excited singlet state is the precursor to
homolytic bond cleavage for 3a: X ) H. We have also
determined, by diene quenching studies, that triplet states are
not reactive for the aryl tert-butyl ethers 1.8
Substitutent Effects on kshom for the Photo-Claisen Reac-
tion of the Allyl Aryl Ethers 3a-k. The substitutent effects
on kshom are substantial, the ethers with electron-donating
groups (methoxy, methyl) reacting considerably faster than those
with electron-withdrawing groups (trifluoromethyl, cyano). In
all cases and in both solvents, the para and meta isomers react
at comparable rates although the para isomers are more reactive
for all substitutents in both solvents. Obviously, the photo-
chemical meta effect7,46 which predicts enhanced reactivity of
meta isomers relative to para is not operative here. For instance,
in one of the pioneering studies of the meta effect,47 3-
cyanophenyl trityl ether was found to react with a higher
quantum yield (0.68) than its 4-cyano analogue (0.30). However,
these reactions are probably occurring by excited-state hetero-
lytic cleavage to form ion pairs.
Figure 2. Plot of log kshom(cyclohexane) versus σex for the photo-Claisen
reaction of the allyl aryl ethers 3a-k.
(r ) 0.994), indicating again that electron-donating substituents
accelerate the reaction. The total span (a factor of about 20: k
) 1.03 × 10-5 and 21.3 × 10-5 s-1 for 4-NO2 and 4-CH3O,
respectively, in carbitol as solvent at 181 °C) of the rate
constants is considerably less than that for the photochemical
reactions. Because the transition state for this thermal reaction
would be expected to have a polarity with Oδ-‚‚‚Cδ+, this
correlation was also rather surprising. Rather convoluted argu-
ments were developed to explain this observation.
Since the first report in 1988,51 the bond dissociation energy
(BDE, kJ/mol) in phenols has been shown to be strongly
dependent on substitutents. This topic has been investigated
extensively and recently reviewed.52 The values are very well
correlated with σ+; F ) +28.1 (r ) 0.990) for ∆BDE (kJ/mol),
the change in bond dissociation energy relative to phenol, for
both para- and meta-substituted cases.52 This means that
electron-donating groups weaken the bond (∆BDE ) -22 kJ/
mol for 4-methoxy) and electron-withdrawing ones strengthen
the bond (∆BDE ) +18 kJ/mol for 4-cyano). Although there
has been considerable discussion about the reasons behind this
effect, i.e., changes in stability of the substituted phenols or
changes in the stability of the substituted phenoxy radicals, the
current arguments are strongly in favor of the latter factor.
Moreover, this effect has been clearly shown to hold true for
substituted alkyl aryl ethers such as anisoles and benzyl 4-X-
aryl ethers.53 For instance, X-C6H4O-CH2Ph BDEs have been
The data give a reasonable correlation with σex,2 the σ values
obtained from the excited-state pKa’s of substituted phenols.48,49
Figure 2 shows the plot for cyclohexane as solvent; F ) -0.81
(r ) 0.93). As expected on the basis of the parallel reactivity
in methanol and cyclohexane (Figure 1), the σex plot (not shown)
for the rate constants in methanol is similar; F ) -0.87 (r )
0.92). Surprisingly, this substitutent effect is in the opposite
50
direction of that for the defining phenol pKa values, even
though both might be expected to have similar polarities;
O-‚‚‚H+ for the phenols and Oδ-‚‚‚Cδ+ for the allyl ethers.
A possible reason for this order of reactivity for the allyl
ethers became apparent while examining reported substitutent
effects on the rate constants for the thermal Claisen reaction of
a similar set of allyl aryl ethers. For the 4-substituted cases,15,18
the rates were found to correlate very well with σ+; F ) -0.61
estimated by DFT calculations (46.7, 52.8, and 55.8 kcal mol-1
)
(45) Schmid, H. HelV. Chim. Acta 1957, 20, 13-26.
(46) Zimmerman, H. E. J. Phys. Chem. 1998, 102, 5616-5621 and references
therein.
and pyrolysis kinetics experiments (47.6, 53.1, and 56.3 kcal
mol-1) for X ) 4-CH3O, H, and 4-CF3, respectively. The
previous observation of the correlation of the rate constants of
the thermal Claisen rearrangement for allyl 4-X-aryl ethers with
σ+ now becomes obvious. The electron-donating substitutents
weaken the bond and increase the rate, as expected for a thermal
reaction.54
(47) Zimmerman, H. E.; Somasekhara, S. J. Am. Chem. Soc. 1963, 85, 922-
927.
(48) There is no σex value available for the 3-CN substituent.
(49) Not surprisingly, the correlation with the parallel3 but differently scaled
σ* value is similar; F ) -0.34 (r ) 0.92). Therefore the F value is lower
but still negative.
(50) There is some confusion about the sign of F values in σex correlations.
Baldry2 set F ) -3.10 but plotted pKa* ) -log Ka* rather than log Ka*.
The defining reaction for ground-state Hammett correlations, the ionization
of substituted benzoic acids, has F ) +1 using log Ka, not pKa. This double
reversal of sign means that the σ* values have the normal sign, positive
values for electron-withdrawing groups and negative for electron-donating
ones. Therefore, Baldry’s negative F should be positive whereas ours is
negative. Shim and co-workers3 set F* ) +1 for the excited-state ionization
of benzoic acids, using log Ka* and found F* ) 1.28, i.e., positive for the
excited-state ionization of phenols as expected.
(51) Mulder, P.; Saastad, O. W.; Griller, D. J. Am. Chem. Soc. 1988, 110, 4090-
4092.
(52) Borges dos Santos, R. M.; Simoes, J. A. M. J. Phys. Chem. Ref. Data
1998, 27, 707-739.
(53) Pratt, D. A.; de Heer, M. I.; Mulder, P. Ingold, K. U. J. Am. Chem. Soc.
2001, 123, 5518-5526.
9
9776 J. AM. CHEM. SOC. VOL. 124, NO. 33, 2002

Photo-Claisen Rearrangement of Allyl Aryl Ethers
A R T I C L E S
Conclusion
The rate constants for the homolytic cleavage from the excited
singlet state of the substituted allyl aryl ethers 3a-k in both
methanol and cyclohexane have been determined by a fluores-
cence (static and dynamic) comparison method with the
unreactive anisoles 4a-k. These values exhibit an unusually
large substituent effect for an excited-state reaction that follows
the same trend in both solvents. This effect is rationalized by
the similar large change in bond dissociation energy for the
carbon-oxygen bond that breaks in the rate-determining step
of this photo-Claisen rearrangement. This conclusion is surpris-
ing because the excitation energy available to the excited-state
S1 of these allyl ethers is some 45-50 kcal mol-1 (Table 2)
above the bond dissociation energy of ∼50 kcal mol-1, estimated
with the usual assumption that an allyl and benzyl ether will
have similar BDE values.56 To our knowledge, such an effect
is unprecedented in excited-state homolytic cleavage reactions.
Experimental Section
Figure 3. Plot of log kshom(methanol) versus ∆BDE (kJ/mol) for the allyl
aryl ethers 3a-k.
Preparation of the Allyl Aryl Ethers 3a-k. The ethers 3a: X )
H and 3b: X ) 4-CH3O were available from the Aldrich Chemical
Co. The others were all synthesized by a standard literature procedure
from the substituted phenol (Aldrich) using allyl bromide and potassium
carbonate in acetone. They were purified by flash chromatography on
silica gel followed by bulb-to-bulb distillation. They have all been
previously reported, some of them many times, but spectral data are
Because the rate constants for the thermal Claisen reaction
show the same trend in substitutent dependence as our photo-
chemical ones, we were forced to consider the effect of ∆BDEs
on the photochemical values. In fact, a plot of log kshom versus
∆BDE (kJ/mol) for phenols,52 Figure 3, shows a fair correla-
tion: F ) -0.04 (r ) 0.87), which should perhaps best be
described as a strong trend. We therefore propose that changes
in BDEs may be having a major effect on the photochemical
rate constants. This effect may dominate the other more usual
substituent effects (electron density changes on excitation,
resonance, inductive, etc.) for the σ bond cleavage reactions of
phenolic ethers because it is particularly large for XC6H4Y-Z
cases when Y ) O and Z ) H or C. For other cases where Y
) C and X ) H, C, or halogen,55 the substituent has only a
minor effect on the BDE. In those cases, the other effects of
the substituent may dominate.
In our previous study8 of the substituted aryl tert-butyl ethers
1a-j in methanol, we did not notice the dominance of the effect
of the BDEs. However, in that case we used the quantum yield
method for obtaining the excited-state rate constants. We now
know this is not a reliable method of obtaining kshom because of
the radical pair recombination problem. Using the fluorescence
lifetime method and eq 4, the kshom values for these tert-butyl
ethers span only a very narrow range of a factor of 6. Moreover,
we obtained evidence that some of the products were ion pair
derived in methanol, at least for the adamantyl analogue 2. We
did not study these substrates in cyclohexane. We are in the
process of making a set of substituted 1-adamantyl aryl ethers
in order to complete a similar study as is reported here in order
to probe the very different reactivities of the allyl versus tert-
alkyl ethers.
1
not in general available; H and 13C NMR, GCMS, and HRMS data
are given in the Supporting Information. The literature references given
are taken from publications where several of the ethers were prepared:
3c: X ) 3-CH3O,19 3d: X ) 4-CH3,15,57 3e: X ) 3-CH3,19 3f: X )
4-F,57 3g: X ) 3-F,20 3h: X ) 4-CF3,57 3i: X ) 3-CF3,58 3j: X )
4-CN,15 3k: X ) 3-CN.19
The aryl 4-butenyl ethers 14 were synthesized by the same method
using 4-bromo-1-butene and also purified by flash chromatography and
bulb-to-bulb distillation. NMR spectra obtained were identical to those
reported previously: 14a: X ) H,41 14b: X ) 4-OCH3,59 and 14j:
X ) 4-CN.42
Irradiation of Ethers. A solution of 0.5-0.7 g of the ether in 280
mL of methanol or cyclohexane was purged with nitrogen and then
irradiated with a Hanovia 450 W mediumpressure Hg lamp. The
progress of the reaction was monitored by GC/FID. In most cases, flash
chromatography on silica gel did not result in separation of the isomeric
allyl phenols and the cyclopropyl phenols (secondary photoproducts)
into analytically pure compoumds. However, the primary photoproducts,
the allyl phenols 9, 10, 11, and 12, were easily identified by GC/MS
and quantified by GC/FID. In most cases, their structures could be
1
determined by H NMR analysis of the partially separated mixtures.
The assignment of the positional isomers of the products 8-11 was
done by analyses of the coupling pattern of the aromatic hydrogens in
1H NMR spectra. However, for the substrates 3e: X ) 3-CH3 and 3g:
X ) 3-F the three allyl ethers 9, 10, and 11 could not be completely
separated, and therefore their aromatic hydrogens could not be analyzed
because of overlapping signals. In Table 1, the individual yields are
given but the assignments are not known. Because the product yields
are close to equal, the lack of individual assignments is not a concern.
Absorbance Measurements. Absorbance spectra were recorded at
1 nm resolution using a Cary 100 UV-vis spectrometer thermostated
(54) The explanation previously given for the substitutent effect on the thermal
Claisen reaction for meta or allyl 3-X-aryl ethers was even more convoluted.
This is not surprising because the substitutent will have an effect on the
BDE of the O-C bond that is breaking but also will likely have a strong
effect on the C-C bond that is forming at the ortho position (ortho or para
to X) in this concerted process.
(56) McMillen, D. F.; Golden, D. M. Annu. ReV. Phys. Chem. 1982, 33, 493-
532.
(57) Mullen, G. G.; Swift, P. A.; Marinyak, S. D.; Allen, S. D.; Mitchell, J. T.;
Kinsolving, C. R.; St. Georgiev, V. HelV. Chim. Acta 1988, 71, 718-731.
(58) Borgulya, J.; Mateja, R.; Fahrni, P.; Hansen, H.-J.; Schmid, H.; Barner, R.
HelV. Chim. Acta 1973, 56, 14-75.
(55) Pratt, D. A.; Wright, J. S.; Ingold, K. U. J. Am. Chem. Soc. 1999, 121,
4877-4882.
(59) Corey, E. J.; Guzman-Perez, A.; Noe, M. C. J. Am. Chem. Soc. 1995, 117,
10805-10816.
9
J. AM. CHEM. SOC. VOL. 124, NO. 33, 2002 9777

A R T I C L E S
Pincock et al.
at 25 °C. The integrated spectra needed for eq 8 were obtained by
summing the 1 nm incremental areas (the absorbance values converted
to ꢀ by dividing by the molar concentration) over the complete
absorption band. These areas are somewhat uncertain at the short-
wavelength end because of overlap with the long-wavelength tail of
the S2 band so that the absorbance does not reach zero. The wavelength
for the minimum in absorbance was used as the cutoff point for the S1
band. For 3 and 4h: X ) CF3 and 3 and 4k: X ) 4-CN, this overlap
was too extensive to allow integration of the S1 band.
Fluorescence Measurements. All samples were degassed by three
freeze-pump-thaw cycles and then thermostated at 25 °C. The
substituted anisoles used for comparison were all commercial samples
(Aldrich) and were distilled before use. Fluorescence intensity measure-
ments were done using a PTI-210M fluorescence spectrometer with
dual Model 101 monochromators, a 75 W xenon lamp, and a Model
814 photomultiplier detector. Corrected spectra were obtained. Fluo-
rescence quantum yields were determined using solutions of matched
absorbance by comparison with the known fluorescence quantum yield
of 0.2427 for anisole in methanol and 0.2928 in cyclohexane. The
expectation values necessary for eq 8 were obtained by summing the
1 nm incremental areas over the complete emission band.28 Singlet
lifetimes were measured by monitoring fluorescence decay using a PRA
time correlated single photon counting apparatus with a hydrogen flash
lamp of pulse width about 1.8 ns.
bench equipped with a 450 W high pressure Osram Hg/Xe lamp focused
onto open slits of a SPEX 1670 monochromator giving a band pass of
20 nm. The beam was split (approximately 12:1) into cylindrical sample
(5 cm long, 24 mL) and reference (9 mL) cells. The concentration of
the ether was adjusted to give absorbance values >2, and the cell was
thermostated at 25 °C. Ferrioxolate actinometry60 was used both to
measure the splitting ratio and to determine light intensity into the
reference cell during ether photolyses. Both the ferrioxalate actinometer
and the allyl ether solutions were purged with nitrogen and sealed before
photolysis. The disappearance of the ethers was determined by GC/
FID by duplicate injections on triplicate samples using naphthalene as
an internal standard. Percentage conversions were approximately 10%.
The values given are the average of two determinations.
Acknowledgment. We thank NSERC of Canada for financial
support and Sepracor Canada Ltd., Windsor, Nova Scotia, for
the donation of chemicals.
Supporting Information Available: 1H and 13C NMR,
GCMS, and HRMS spectral data for compounds 3c-3k and
absorbance, fluorescence, and fluorescence excitation spectra
for selected examples of 3 and 4. (PDF) This material is
available free of charge via the Internet at http://pubs.acs.org.
JA011981Y
Quantum Yield Measurements. The quantum yields of reaction
for the ethers were determined by irradiation at 270 nm using an optical
(60) Hatchard, C. G.; Parker, C. A. Proc. R. Soc. A 1956, 235, 518-536.
9
9778 J. AM. CHEM. SOC. VOL. 124, NO. 33, 2002