Photoinduced electron transfer reactions of anthracene in CF3SO3H- CF3CO2H

Article abstract of DOI:10.1039/a903072g

Photolysis of anthracene in a mixture consisting of 2% CF₃SO₃H in CF₃CO₂H (w/w), where anthracene is partially protonated, gives a mixture of oxidized, neutral and reduced monomeric and dimeric products, initiation of this chemistry being photoinduced electron transfer from anthracene to protonated anthracene.

Full text of DOI:10.1039/a903072g

View Article Online / Journal Homepage / Table of Contents for this issue
486 J. CHEM. RESEARCH (S), 1999
J. Chem. Research (S),
1999, 486^487y
Photoinduced Electron Transfer Reactions of
Anthracene in CF3SO3H CF3CO2Hy
Richard M. Pagni,* Gleb Mamantov,z George Hondrogiannis
and Aditya Unni
Department of Chemistry, University of Tennessee, Knoxville, TN 37996-1600, USA
Photolysis of anthracene in a mixture consisting of 2% CF3SO3H in CF3CO2H (w/w), where anthracene is
partially protonated, gives a mixture of oxidized, neutral and reduced monomeric and dimeric products, initiation
of this chemistry being photoinduced electron transfer from anthracene to protonated anthracene.
Two types of BrÖnsted superacids are known: those consist-
ing of £uorine-containing BrÖnsted acids with or without
Lewis acids1 such as HF^SbF5 and the less well known
bromine- and chlorine-containing room temperature molten
the singlet excited state of 1b to 2.7 The photochemistry
of the less easily oxidized An in the same medium, on
the other hand, a¡ords the 4 ‡ 4 dimer 3 exclusively,8
a
result identical to that previously observed in numerous
other solvents.9 Photochemistry of An in acidic
EMIC^AlCl3 (55 mol% AlCl3), which contains 2, AlCl4
and Al2Cl7 , a powerful Lewis acid, takes an entirely dif-
ferent course a¡ording a series of oxidized, neutral and
reduced monomers and dimers: 3^9.8 This latter chemistry
is initiated by electron transfer from the singlet excited state
salts such as HCl (1 atm) in AlCl3^l-ethyl-3-methyl-
4
imidazolium chloride (EMIC; excess AlCl3),2
(1 atm)
in AlCl3^EMIC (excess AlCl3 bu¡ered with LiCl or NaCl),5
and HBr^AlCl3^Me3SBr.6 Because the two types of
superacids have high acidity and low basicity/nucleo-
philicity in common, it should be possible to observe the
same organic chemistry in both. One in fact can adjust
the acidity of a mixture of CF3SO3H and CF3CO2H so that
the same, complex photochemistry of anthracene (An) 1a
occurs in it as in the room temperature molten salt:
HC1 in AlCl3^EMIC.
‡
‡
of An, not to EMI but to protonated anthracene (AnH ),
which is formed by protonation of An by trace amounts
of HCl in the molten salt (Scheme 1).8 This unusual set
of products arises by a series of bimolecular coupling,
hydrogen-transfer, and electron-transfer reactions. Reaction
of AnÁ‡ with solvent components does not occur here
because there are no good bases in the acidic medium with
which AnÁ‡ can react. Instead AnÁ‡ reacts with itself, An
and species derived from them.
R
+
N
N
Me
Et
Acidic Molten Salt
2
1a R = H
1b R = Me
An + HCl
AnH+ + Cl –
H
H
hν
*
An
An 1
*
An 1 + AnH+
An•+ + AH•
An•+, AnH•
Products
H
H
H
4
3
Scheme 1
H
If one is to observe the same photochemistry of An in a
BrÖnsted acid as occurred in the acidic molten salt, one
requires a medium that is poorly nucleophilic and will sus-
tain a low concentration of AnH . The pKa, of AnH ,
which is 3.2 in HF,10 9:1 in MeCN,11 and 9:2 in HCl
(1 atm)^EMIC^AlCl3 (acidic),2;3 provides some guidance
in the choice of acid. A mixture of CF3CO2H, with
H0 ˆ 2:7, and CF3SO3H, with H0 ˆ 14:0, with an H0
between 2:7 and 14:012 in fact provides the appropriate
acidity and nucleophilicity.
H
‡
‡
5
6
H
H
H
3
hν
CF3CO2H
H
An
7
8
CF3SO3H
hν
AnH+
No reaction
Consider ¢rst the following published results. The
Scheme 2
photochemistry of 9-methylanthracene 1b in degassed basic
‡
EMICl^AlCl3 (55 mol% EMIC), which contains EMI (2),
Photolysis of a degassed solution of An in CF3CO2H
Cl and AlCl4 , is initiated by electron transfer from
‡
(very faint pink coloration); which contains no AnH
(1H NMR; UV^VIS), proceeds in the classic manner to
* To receive any correspondence.
a¡ord the 4 ‡ 4 dimer 3 exclusively (Scheme 2). As there
y This is a Short Paper as de¢ned in the Instructions for Authors,
Section 5.0 [see J. Chem. Research (S), 1999, Issue 1]; there is
therefore no corresponding material in J. Chem. Research (M).
z Deceased March 12th, 1995.
‡
is no AnH in the CF3CO2H, there is no electron acceptor
available to initiate the photochemistry observed in the
acidic molten salt. Photolysis of a solution of An in

View Article Online
J. CHEM. RESEARCH (S), 1999 487
Table 1 Product distribution from the photolysis of anthracene
Experimental
in CF3SO3H CF3CO2Ha and acidic EMIC^AlCl3
b
The CF3SO3H and CF3CO2H were of the highest purity and used
as received. The acid solutions, ca. 0.1 M in An, were deoxygenated
by purging with dry N2 and photolyzed in a Rayonet reactor using
3500 Ð lamps. Work-up of the reactions and separation, identi¢cation
and quantitation of products were identical to that found in the
Supplementary Material of ref. 8.
Relative product yield(%)
Solvent
4
5
6
7
8
9c
3
CF3SO3H CF3CO2Hd 36.2 23.3 5.4 11.7 1.6 14.8 7.0
EMIC^AlCl3 32.2 30.0 10.1 6.6 4.4 7.9 8.8
aPhotolyzed for 28 h; 52% of An consumed. The material balance
was 25.7%. bPhotolyzed for 24h; 60.4% of An was consumed. The
material balance was 22.7%. cAn unknown condensed product
similar in structure to 8. d9;90-Bianthracene and two other
unknown dimeric products were also formed in this reaction.
This research was supported by the Air Force O¤ce of
Scienti¢c Research. A.U. thanks the University of
Tennessee Science Alliance Center of Excellence for
support.
‡
CF3SO3H (deep green coloration), which contains AnH
and no An (1H NMR), on the other hand, yields no
photoproducts; An is recovered quantitatively on workup.
In this case the electron acceptor (AnH ) is present but
Received, 19th April 1999; Accepted, 13th May 1999
Paper E/9/03072G
‡
the donor (An) is absent.
hν
An, AnH+
3–9
CF3SO3H–CF3CO2H
References
Scheme 3
1
2
3
4
5
6
7
8
9
G. A. Olah, G. K. Surya Prakash and J. Sommer, Superacids,
Wiley, New York, 1985.
G. P. Smith, A. S. Dworkin, R. M. Pagni and S. P. Zingg, J.
Am. Chem. Soc., 1989, 111, 525.
G. P. Smith, A. S. Dworkin, R. M. Pagni and S. P. Zingg, J.
Am. Chem. Soc., 1989, 111, 5075.
J. L. E. Campbell and K. E. Johnson, J. Am. Chem. Soc., 1995,
117, 7791.
D. King, R. Mantz and R. Osteryoung, J. Am. Chem. Soc.,
1996, 118, 11933.
M. Ma and K. E. Johnson, J. Am. Chem. Soc., 1995, 117,
1508.
C. Lee, T. Winston, A. Unni, R. M. Pagni and G. Mamantov,
J. Am. Chem. Soc., 1996, 118, 4919.
G. Hondrogiannis, C. W. Lee, R. M. Pagni and G. Mamantov,
J. Am. Chem. Soc., 1993, 115, 9228.
‡
The pKa of AnH in CF3SO3H CF3CO2H clearly lies
between 2:7, where An is unprotonated, and 14:0, where
An is completely protonated. When the 1H NMR and
‡
UV^VIS detection limits of An and AnH are taken into
1
account {1% An in CF3SO3H ꢀ H NMR† and 0.1%
‡
‡
AnH
in CF3CO2H (UV^VIS ‰eꢀAnH † & 1  104
dm3 mol 1 cm Š†}, the pKa range is narrowed to between
5:7 and 12:0. Thus, photolysis (Scheme 3) of a degassed,
pale green solution of An in 2% CF3SO3H 98% CF3CO2H
(w/w), which has H0 ˆ 8:112 and contains both An and
1
‡
AnH , yields 3^9, the same products as found in acidic
EMIC^AlCl3, and a couple of other minor products (Table
1). The distribution of products in CF3SO3H CF3CO2H
is very similar to that found in the molten salt (Table
1). Because the molten salt reaction was conclusively shown
(a) D. O. Cowan and R. L. Drisko, Elements of Organic
Photochemistry, Plenum, New York, 1976; (b) H.-D. Becker,
Chem. Rev., 1993, 93, 145 and references cited therein.
10 (a) E. L. Makor, A. Hofstra and J. H. van der Waals, Trans.
Faraday Soc., 1958, 54, 66, 186; (b) D. M. Brouwer, E. L.
Makor and C. MacLean, Carbonium Ions, ed. G. A. Olah
and P. von R. Schleyer, Wiley-Interscience, New York, 1970,
vol. II, ch. 20.
11 T. Nagaoka, A. B. Berinstain, D. Griller and D. D. M.
Wayner, J. Org. Chem., 1990, 55, 3707.
12 S. Saito, S. Saito, T. Ohwada and K. Shudo, Chem. Pharm.
Bull., 1991, 39. 2718.
13 A. Streitwieser, Jr., Molecular Orbital Theory for Organic
Chemists, Wiley, New York, 1961, ch. 11.
to be initiated by electron transfer from An*1 to AnH , the
‡ 8
reaction in CF3SO3H CF3CO2H must begin in the same
way. The subsequent sequence of bimolecular coupling,
hydrogen- transfer and electron-transfer reactions must
be similar as well.
Because many aromatic hydrocarbons have basicities
similar to that of An,13 photoelectron transfer between
an aromatic hydrocarbon and its conjugate acid should
be a common reaction in strong acids.