Photooxidation of methylnaphthalenes

Article abstract of DOI:10.1021/jo040228v

(Chemical Equation Presented). Studies on the photooxidation of methyl-substituted aromatic hydrocarbons have revealed that whereas electron density is a determinant of endoperoxide formation, steric factors are most important in influencing the stability of the endoperoxide. Additional information on the energetics of the reactions and on the magnitude of the steric interactions was obtained using calculations at the B3LYP/6-311+G* level of theory.

Full text of DOI:10.1021/jo040228v

Photooxidation of Methylnaphthalenes
Harry H. Wasserman,* Kenneth B. Wiberg,* David L. Larsen, and Jonathan Parr
Department of Chemistry, Yale University, New Haven, Connecticut 06520-8107
harry.wasserman@yale.edu
Received July 7, 2004
Studies on the photooxidation of methyl-substituted aromatic hydrocarbons have revealed that
whereas electron density is a determinant of endoperoxide formation, steric factors are most
important in influencing the stability of the endoperoxide. Additional information on the energetics
of the reactions and on the magnitude of the steric interactions was obtained using calculations at
the B3LYP/6-311+G* level of theory.
Introduction
alkyl substitution also play an important role in the
stability of the resulting endoperoxides. Calculations at
the B3LYP/6-311+G* level provide additional informa-
tion on the energetic effects of methyl substitution and
on the magnitudes of the steric interactions.
Whereas endoperoxides derived from the photooxida-
tion of anthracenes, naphthacenes, pentacenes and other
polycyclic aromatic hydrocarbons have been studied for
many years,1 less attention has been paid to the forma-
tion of stable endoperoxides formed by the photooxidation
of naphthalenes.2 In this paper we will discuss further
developments in our studies on the photooxidation of
methylnaphthalenes and related compounds.
We have reported preliminary work on the dye-
sensitized reaction of alkylnaphthalenes with singlet
oxygen showing that the stability of the resulting endo-
peroxides is sensitive to the pattern of alkyl subsitution
on the naphthalene nucleus.3 The present paper provides
a fuller account of these studies showing that in addition
to consideration of the electronic density at specific
positions, steric factors connected with the pattern of
Results and Discussion
As reported earlier, the endoperoxides 1-5 may be
readily prepared by the photooxidation of the substituted
naphthalene in methylene chloride using methylene blue
as a sensitizer.3 In these reactions, it is advisable, par-
ticularly in the case of the less stable endoperoxides, to
maintain the solution below room temperature so as to
decrease the rate of decomposition of the endoperoxide.4
The naphthalene endoperoxides were characterized by
their spectroscopic properties. In the IR, the aromatic
band at 1580-1600 cm-1 disappears on photooxidation
and a strong band appears at 1200-1250 cm-1, and in
the UV spectrum, endoperoxide formation is indicated
by the disappearance of the naphthalene band usually
seen at 290 nm.
The NMR spectra are most useful for identification of
the oxidized products. Examination of the chemical shifts
of the methyl protons of the methylnaphthalenes allows
the assignment of the peaks for each methyl group on
the basis of a simple calculation.5 In most cases, it is also
possible to assign chemical shifts to the aromatic protons.
By observing the changes in the NMR spectra following
(1) (a) Aubry, J.-M.; Pierlot, C.; Rigaudy, J.; Schmidt, R. Acc. Chem.
Res. 2003, 36, 668-675. (b) Gollnick, K. Adv. Photochem. 1968, 6,
1-122. (c) Gollnick, K.; Schenck, G. O. Oxygen as a Dienophile. In
1,4-Cycloadition Reactions; Hamer, J., Ed.; Academic Press: New York,
1967; pp 255-344. (d) Baldwin, J. E.; Basson, H. H.; Krauss, H., Jr.
Chem. Commun. 1968, 984-985. (e) Rigaudy, J.; Dupont, R.; Cuong,
N. K. C. R. Acad. Sci. 1969, 269c, 416-419. (f) Wilson, R. Photochem.
Photobiol. 1969, 10, 441-449. (g) Rigaudy, J.; Guillaume, J.; Maurette,
D. Bull. Soc. Chim. Fr. 1971, 144-152. (h) Saito, I.; Matsuura, T. In
Singlet Oxygen; Wasserman, H. H., Murray, R. W., Eds.; Academic
Press: New York, 1979; Vol. 40, pp 511-569.
(2) (a) Rigaudy, J.; Deletang, C.; Basselier, J.-J. C. R. Acad. Sci.
1966, 263c, 1435-1438. (b) Rigaudy, J. Pure Appl. Chem. 1968, 16,
169-186. (c) Rigaudy, J.; Deletang, C.; Sparfel, D.; Cuong, N. K. C. R.
Acad. Sci. 1968, 267c, 1714-1717. (d) Rigaudy, J.; Deletang, C.;
Basselier, J.-J. C. R. Acad. Sci. 1969, 268c, 344-347.
(3) (a) Wasserman, H. H.; Larsen, D. L. Chem. Commun. 1972, 253-
254. (b) Larsen, D. L. Ph.D. Thesis, Yale University, New Haven, CT,
1973.
(4) Many of the naphthalene endoperoxides prepared in this work
(Table 1) show little evidence of decomposition after several months
storage at 0 °C.
(5) Yew, F. F.; Kurland, R. J.; Mair, B. J. Anal. Chem. 1964, 36,
843-845.
10.1021/jo040228v CCC: $30.25 © 2005 American Chemical Society
Published on Web 12/09/2004
J. Org. Chem. 2005, 70, 105-109
105

Wasserman et al.
TABLE 1. Stability of Naphthalene Endoperoxides
peroxide
t1/2 at 25 °C (h)a
∆Gq (kcal/mol)
1,4-dimethylnaphthalene-1,4-endoperoxide 1
1,8-dimethylnaphthalene-1,4-endoperoxide 2
1,2,4-trimethylnaphthalene-1,4-endoperoxide 3
1,4,5-trimethylnaphthalene-1,4-endoperoxide 4
1,2,3,4-tetramethylnaphthalene-1,4-endoperoxide 5
5
23.6
24.6
25.0
25.9
>26
30
70
290
indefinite
a Determined by 1H NMR using a CDCl3 solution initially 0.1 M in endoperoxide.
SCHEME 1. Preparation of Methyl Naphthyl
logue. An explanation for the observed result may be
found in the large steric strain associated with the
nonbonded peri interaction between the 1- and 8-methyl
groups.9,10 The bond rehybridization at C1 and C4 (sp2
to sp3) resulting from the formation of the endoperoxide
should significantly reduce the peri methyl-methyl
interaction in the endoperoxide compared to the parent
naphthalene.11 Examination of the stabilities of the
following pairs of naphthalenes serves to illustrate the
importance of the peri interaction in promoting stability.
Endoperoxides 1-5
photooxidation it was possible to assign structures for
the endoperoxides outlined in Scheme 1.
In this way, for example, the formation of a 1,4-
endoperoxide from 1,8-dimethylnaphthalene is indicated
by the change in chemical shift of the 1-methyl group
from δ ) 2.85 to δ ) 1.98, by the change in chemical shift
of the proton in the 4-position from the aromatic region
to δ ) 5.42, and by the change in chemical shift of the
protons in the 2- and 3-positions corresponding to the
change from aromatic to olefinic character in the bond.
The alkyl naphthalene endoperoxides react with hydri-
odic acid to liberate iodine and are inert to reduction by
sodium borohydride.6 Catalytic hydrogenation of the
endoperoxide with palladium reduces the isolated double
bond and forms a diol from the peroxide bridge.7
In related earlier work, the rates of reaction of substi-
tuted naphthalenes with singlet oxygen have been ex-
amined, and the results are in accord with the eletrophilic
nature of this reactive species.8 Thus, the rate of endo-
peroxide formation in the reaction of alkyl-substituted
naphthalenes with singlet oxygen has been correlated
with the electronic nature of the substrate.9
All of the endoperoxides formed from the alkyl naph-
thalenes in this group share the property of thermal
instability, regenerating singlet oxygen and the parent
naphthalene. The degree of instability varies widely, from
octamethylnaphthalene-1,4-endoperoxide,6 which re-
quires heating for extended periods in refluxing di-n-
butyl ether to bring about decomposition, to the very
unstable endoperoxides, which can only be prepared and
studied at very low temperatures.
This ortho effect can become very important in a case
such as 1,2,3,4-tetramethylnaphthalene where the cu-
mulative effect of four ortho methyl groups produces even
greater strain. The unusual stability of 1,2,3,4-tetra-
methylnaphthalene-1,4-endoperoxide 5 is due largely to
the relief of these interactions upon formation of the
endoperoxide.
The experimental results clearly indicate that steric
effects play an important role in determining the stability
of the endoperoxides. There are several energy terms of
interest with regard to the photooxidation of the substi-
tuted naphthalenes and their decomposition. Since ex-
perimental data are not available, we have made use of
B3LYP/6-311+G* calculations. They include geometry
optimizations and calculations of vibrational frequencies
in order to correct for differences in zero-point energies
and to convert the calculated energy differences to ∆H
and ∆G at 25 °C.12
Measurement of the rates of decompostion of the
endoperoxides listed in Table 1 at room temperature
reveals large differences. At this stage we need to address
the increased stability of 1,8-dimethylnaphthalene-1,4-
endoperoxide with respect to the 1,4-disubstituted ana-
An important question is concerned with the effect of
methyl substitution on the energy of naphthalene. It is
known that 1,8-dimethylnaphthalene has a higher energy
(6) It is noteworthy that octamethylnaphthalene-1,4-endoperoxide
is inert to both sodium borohydride and lithium aluminium hydride.
See: Hart, H.; Oku, A. Chem. Commun. 1972, 254-255.
(7) Hydrogenation of 1,4-dimethylnaphthyl-1,4-endoperoxide in the
presence of a platinum catalyst gives 1,4-dihydroxy-1,4-dimethyl-
1,2,3,4-tetrahydronaphthalene exclusively. See: Rigaudy, J.; Maurette,
D.; Cuong, N. K. C. R. Acad. Sci. 1971, 273c, 1553-1556.
(8) Singlet Oxygen; Wasserman, H. H., Murray, R. W., Eds.;
Academic Press: New York, 1979; Vol. 40.
(9) (a) van den Huevel, C. J. M.; Steinberg, H.; de Boer, Th. J. Recl.
Trav. Chim. Pays-Bas. 1980, 99, 109-113. (b) van den Huevel, C. J.
M.; Verhoeven, J. W.; de Boer, Th. J. Recl. Trav. Chim. Pays-Bas. 1980,
99, 280-284.
(10) The influence of peri interactions on the chemistry of naphtha-
lenes has been reviewed. See: Balasubramaniyan, V. Chem. Rev. 1966,
66, 567-641.
(11) The C1-C8 distance in naphthalene is 2.44 Å, the C1-C8A
bond length is 1.42 Å and the C1-C8A-C8 angle is slightly larger
than 120°. See: Cruickshank, D. W. J. Acta Crystallogr. 1957, 10, 504-
508.
(12) The total energies, zero-point energies, and changes in enthalpy
and free energy are available as Supporting Information.
106 J. Org. Chem., Vol. 70, No. 1, 2005

Photooxidation of Methylnaphthalenes
FIGURE 2. Calculated structures of naphthalene endoper-
oxides (upper) and of the transition state for decomposition to
naphthalene and singlet oxygen (lower).
FIGURE 1. Calculated energies of methyl transfer reactions
(kcal/mol).
than 1,4-dimethylnaphthalene as a result of methyl-
methyl repulsion in the former compound.13 The relative
energies of the substituted naphthalenes are conveniently
estimated using the isodesmic reaction shown in Figure
1. If there were no extra interactions, the conversion of
two methylnaphthalenes to naphthalene plus 1,4-di-
methylnaphthalene should lead to no significant energy
change. It can be seen that this expectation is realized.
On the other hand, a similar reaction leading to the
formation of 1,8-dimethylnaphthalene is endothermic by
6 kcal/mol. This agrees with a previous estimate of the
difference in energy.14 Similarly, 1,2,4-trimethyl substitu-
tion leads to little increase in steric interactions, whereas
1,4,5-substitution leads to a large change in energy. The
energy of 1,2,3,4-tetramethylnaphthalene is also quite
large.
FIGURE 3. Calculated energies of singlet oxygen transfer
reactions (kcal/mol).
a result the steric interaction of the methyl groups may
be changed. How does methyl substitution affect the
relative energies of the naphthalene peroxides? It is
expected that significant differences would be found in
accord with the observation that methyl-substituted
naphthalenes are more reactive than the unsubstituted
parent. This question can be explored using the oxygen
transfer reaction from naphthalene peroxide to a substi-
tuted naphthalene as shown in Figure 3. A methyl group
in the 1-position stabilizes the endoperoxide by 2 kcal/
mol, and the effect of two methyl groups at the 1,4- or
1,8-positions is about twice as large. It is interesting that
the two patterns give similar calculated energy changes
with the 1,8-dimethyl compound giving a slightly larger
effect than the 1,4-dimethyl derivative.14 Three methyl
groups arranged 1,2,4 or 1,4,5 give similar calculated
energy changes, which are about three times as large as
that for a single methyl group. Finally, 1,2,3,4-tetram-
ethyl substitution gives the largest energy change, prob-
ably due to relief of steric interaction between adjacent
methyl groups when the naphthalene ring is bent as a
result of endoperoxide formation.
When a naphthalene is converted to the peroxide, there
is a considerable change in geometry (Figure 2), and as
(13) In comparing 1,8- and 1,5-dimethylnaphthalene, the latter
cannot be taken as strain-free as there are methyl-hydrogen peri
interactions that, although probably small, cannot be neglected. The
heats of formation of 1- and 2-methylnaphthalenes as reported by
Rossini differ somewhat (Speros, D. M.; Rossini, F. D. J. Phys. Chem.
1960, 64, 1723-1727). On the basis of error limits for this determi-
nation, the strain energy for a 1-methyl to 8-hydrogen interaction lies
between zero and 900 cal/mol. Thus, two such interactions as in the
case of 1,5-dimethylnaphthalene could result in a total strain energy
of 1.8 kcal/mol, which when combined with the probable differences
between the 1,5- and 1,8-isomers implies that the strain associated
with a peri methyl-methyl interaction may be as great as 7.3 kcal/
mol. The results of Packer indicate that the strain in 1-methylnaph-
thalene, 1,2-dimethylnaphthalene, and 1,8-dimethylnaphthalene may
be 1.6, 3.4, and 7.6 kcal/mol, respectively (Packer, J.; Vaughan, J.;
Wong, E. J. Am. Chem. Soc. 1958, 80, 905-907). Although the
arguments of Packer et al. are not conclusive, these values may be
quite close to being correct.
(14) It should be noted that the ∆H values are somewhat more
reliable than ∆G values because the latter is influenced by changes in
methyl rotational barriers that were not explicitly calculated in this
study.
J. Org. Chem, Vol. 70, No. 1, 2005 107

Wasserman et al.
FIGURE 5. Eyring plot for the effect of temperature on the
rate of endoperoxide decomposition.
stabilized, and as expected, were found to undergo
moderately rapid decomposition. Trimethyl-substitution
leads to further stabilization and reduced rates of de-
composition, and tetramethyl-substitution with accom-
panying strong stabilization leads to a relatively stable
endoperoxide.
The calculations, of course, give only the overall energy
changes and do not necessarily correspond to the changes
found with the transition states for the decomposition of
the endoperoxides. As noted above, the rates of decom-
position of the peroxides have been studied, and the half-
lives and approximate free energies of activation are
given in Table 1. The rate of decomposition of 1,4,5-
trimethylnaphthalene endoperoxide 4 was studied as a
function of temperature. This leads to ∆Hq ) 29.2 kcal/
mol and ∆Sq ) 10.4 eu (Figure 5). The positive entropy
change indicates that the bonds are considerably weak-
ened on going to the transition state.
The transition state for the thermal decomposition of
naphthalene peroxide was located at the B3LYP/6-
311+G* level (Figure 2). The calculated activation pa-
rameters are ∆Hq ) 19.7 kcal/mol and ∆Sq ) 3 eu.
Because singlet oxygen is formed in the reaction and this
level of theory does not reproduce its energy very well, a
better agreement between experiment and calculations
is not expected.
FIGURE 4. Calculated energy changes for the decomposition
of naphthalene endoperoxides to naphthalene and singlet
oxygen (kcal/mol).
The energy changes resulting from the decomposition
of the endoperoxides yielding singlet oxygen and the
substituted naphthalene also were estimated (Figure 4).
Here, working with singlet oxygen presents a difficulty
in that high levels of electron correlation are needed in
order to calculate its energy accurately,15 and it is
impractical to use such a high level of theory for all of
the compounds in this report. Therefore an appropriate
energy for oxygen was estimated from the difference
between the B3LYP/6-311+G* energy and the results of
higher level calculations, giving an approximate value
of -150.32792 H. The calculated free energy changes are
the quantities of interest, and they differ from the largest
energy change, probably as a result of relief of steric
interaction between adjacent enthalpy changes because
of the overall positive entropy change resulting from the
decomposition of one molecule to form two molecules.
The results agree well with the experimental observa-
tions; naphthalene and 1-methylnaphthalene endoper-
oxides are predicted to be unstable with respect to the
corresponding naphthalene and singlet oxygen and can-
not be formed readily via photooxygenation. The dimeth-
yl-substituted endoperoxides are predicted to be weakly
Experimental Section
1,4-Dimethylnaphthalene-1,4-endperoxide (1). A sample
of 1,4-dimethylnaphthalene (1.0 g, 6.5 mmol) in 50 mL of
methylene chloride was photooxidized at 5 °C until the reaction
was ca. 50% complete as estimated by oxygen uptake (ap-
proximately 15 min). Removal of solvent allowed isolation of
the crude material, usually ca. 60-70% endoperoxide. Recrys-
tallization from pentane at -70 °C yielded 1,4-dimethylnaph-
thalene-1,4-endoperoxide as white crystals: yield 0.688 g, 58%;
1H NMR (CDCl3) δ 7.45-7.35 (m, 4 H, arom); 6.72 (s, 2H) 1.88
(s, 6 H, CH3). Anal. Calcd for C12H12O2: C, 76.57; H, 6.43.
Found: C, 76.38; H. 6.40.
1,8-Dimethylnaphthalene-1,4-endoperoxide (2). The
photooxidation of 1,8-dimethylnaphthalene under conditions
identical to those described for 1,4-dimethylnaphthalene led
to the endoperoxide, which could be separated from starting
(15) Shangguan, C.; McAllister, M. A. J. Mol. Struct. 1998, 422,
123-132.
108 J. Org. Chem., Vol. 70, No. 1, 2005

Photooxidation of Methylnaphthalenes
material by preparative thin-layer chromatography (C6H6,
silica gel). Photooxidation at -78 °C of 203 mg (1.3 mmol) of
1,8-dimethylnaphthalene in CH2Cl2 (20 mL) resulted in the
isolation of pure 1,8-dimethylnaphthalene-1,4-endoperoxide
after recrystallization at -78 °C from pentane: yield 0.202 g,
oxide was isolated as a white crystalline solid: yield 0.541 g,
83%; 1H NMR (CDCl3) δ 7.20 (s, 4H, arom.); 1.76 (s, 6H, CH3);
1.74 (s, 6H, CH3). Anal. Calcd for C14H16O2: C, 77.75; H, 7.46.
Found: C, 77.53; H, 7.47.
Calculations. The ab initio calculations were carried out
1
using Gaussian 99.16
83%; H NMR (CDCl3) δ 7.20-7.01 (m, 3H, arom); 6.90 (dd,
1H, J ) 5.5, 8.0, alkenyl); 6.59 (dd, 1H, J ) 1.8, 8.0, alkenyl);
5.42 (dd, 1H, J ) 1.8, 5.5, alkenyl); 2.48 (s, 3H, CH3); 1.98 (s,
3H, CH3). Anal. Calcd for C12H12O2: C, 76.57; H, 6.43. Found:
C, 76.47; H, 6.55.
1,2,4-Trimethylnaphthalene-1,4-endoperoxide (3). A
sample of 1,2,4-trimethylnaphthalene (1.13 g, 6.65 mmol) in
CH2Cl2 (60 mL) was photooxidized at 20 °C. After 15 min. the
theoretical volume of oxygen was absorbed. On workup, a pale
orange solid was isolated: yield 1.24 g, 93%; 1H NMR (CDCl3)
δ 7.20 (bs, 4H, arom.); 6.25 (s, 1H, alkenyl); 1.86 (s, 3H,
CH3)1.80 (s, 6H, CH3). Anal. Calcd for C13H14O2: C, 77.20; H,
6.98. Found: C, 77.21; H, 6.92.
1,4,5-Trimethylnaphthalene-1,4-endoperoxide (4). A
sample of 1,4,5-trimethylnaphthalene (1.2 g, 7.1 mmol) in CH2-
Cl2 (10 mL) was photooxidized at 15 °C. Oxygen uptake was
rapid, and the theoretical volume was absorbed in a short time.
After standard workup, the crude product was dissolved in
ether and passed through a short silica gel column. After
removal of solvent by evaporation, 1,4,5-trimethylnaphthalene-
1,4-endoperoxide was isolated as a fluffy white crystalline
solid: yield 0.678 g, 54%; 1H NMR (CDCl3) δ 7.00 (m, 3 H,
arom.); 6.53 (s, 2 H, arom.); 6.53 (s, 2H); 2.48 (s, 3H, CH3);
1.95 (s, 3H, CH3); 1.80 (s, 3H, CH3). Anal. Calcd for C13H14O2:
C, 77.20; H, 6.98. Found: C, 77.45; H, 6.81.
1,2,3,4-Tetramethylnaphthalene-1,4-endoperoxide (5).
A sample of 1,2,3,4-tetramethylnaphthalene (564 mg, 3 mmol)
in CH2Cl2 (70 mL) was photooxidized at 15 °C. Oxygen was
absorbed rapidly until 3 mmol (70 mL) was consumed. After
normal workup 1,2,3,4-tetramethylnaphthalene-1,4-endoper-
Acknowledgment. This study was supported by
grants from the National Institutes of Health and the
National Science Foundation. We thank Professor Nicho-
las Turro, Columbia University, for helpful discussions.
Supporting Information Available: General and pho-
tooxidation experimental details, experimental tables of total
energies and other energy quantities for the substituted
naphthalenes and their endoperoxides, and tables of calculated
energies and geometries for the compounds in this report. This
material is available free of charge via the Internet at
http://pubs.acs.org.
JO040228V
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