Generation and reactivity of the 4-aminophenyl cation by photolysis of 4-chloroaniline

Article abstract of DOI:10.1021/jo0104680

4-Chloroaniline and its N,N-dimethyl derivative are photostable in cyclohexane but undergo efficient photoheterolysis in polar media via the triplet state and give the corresponding triplet phenyl cations. CASSCF and UB3LYP calculations show that the 4-aminophenyl triplet cation has a planar geometry and is stabilized by > 10 kcal mol^-1 with respect to the slightly bent singlet. The triplet has a mixed carbene-diradical character at the divalent carbon. This species either adds to the starting substrate forming 5-chloro-2,4′-diaminodiphenyls (via an intermediate cyclohexadienyl cation) or is reduced to the aniline (via the aniline radical cation) in a ratio depending on the hydrogen-donating properties of the solvent. Transients attributable to the triplet aminophenyl cation as well as to the ensuing intermediates are detected. Chemical evidence for the generation of the phenyl cation is given by trapping via electrophilic substitution with benzene, mesitylene, and hexamethylbenzene (in the last case the main product is a 6-aryl-3-methylene-1,4-cyclohexadiene). Relative rates of electrophilic attack to benzene and to some alkenes and five-membered heterocycles are measured and span over a factor of 15 or 30 for the two cations. The triplet cation formed under these conditions is trapped by iodide more efficiently than by the best π nucleophiles, However, in contrast to the singlet cation, it does not form ethers with alcohols, by which it is rather reduced.

Full text of DOI:10.1021/jo0104680

J . Org. Chem. 2001, 66, 6353-6363
6353
Gen er a tion a n d Rea ctivity of th e 4-Am in op h en yl Ca tion by
P h otolysis of 4-Ch lor oa n ilin e
Benedetta Guizzardi, Mariella Mella, Maurizio Fagnoni, Mauro Freccero, and Angelo Albini*
Department of Organic Chemistry, University of Pavia, Via Taramelli 10, 27100 Pavia, Italy
albini@chifis.unipv.it
Received May 7, 2001
4-Chloroaniline and its N,N-dimethyl derivative are photostable in cyclohexane but undergo efficient
photoheterolysis in polar media via the triplet state and give the corresponding triplet phenyl
cations. CASSCF and UB3LYP calculations show that the 4-aminophenyl triplet cation has a planar
geometry and is stabilized by >10 kcal mol-1 with respect to the slightly bent singlet. The triplet
has a mixed carbene-diradical character at the divalent carbon. This species either adds to the
starting substrate forming 5-chloro-2,4′-diaminodiphenyls (via an intermediate cyclohexadienyl
cation) or is reduced to the aniline (via the aniline radical cation) in a ratio depending on the
hydrogen-donating properties of the solvent. Transients attributable to the triplet aminophenyl
cation as well as to the ensuing intermediates are detected. Chemical evidence for the generation
of the phenyl cation is given by trapping via electrophilic substitution with benzene, mesitylene,
and hexamethylbenzene (in the last case the main product is a 6-aryl-3-methylene-1,4-cyclohexa-
diene). Relative rates of electrophilic attack to benzene and to some alkenes and five-membered
heterocycles are measured and span over a factor of 15 or 30 for the two cations. The triplet cation
formed under these conditions is trapped by iodide more efficiently than by the best π nucleophiles.
However, in contrast to the singlet cation, it does not form ethers with alcohols, by which it is
rather reduced.
Aryl cations1 are potentially useful synthetic interme-
diates, but their chemical behavior is poorly known at
present. This is mainly because of their limited acces-
sibility. The dediazoniation of diazonium salt normally
occurs under reducing conditions,2 and thus useful reac-
tions such as the Gomberg-Bachmann synthesis of
biaryls and the Meerwein arylation of alkenes involve
aryl radicals rather than cations.2,3 The phenyl cation and
some of its derivatives have been generated by decay of
tritium-marked benzenes,4 by solvolysis of some perfluo-
roalkylsulfonic aryl esters,5 and by solvolytic cyclization
of (trifluoromethanesulfonyl)oxydienynes6 as well as by
carrying out under controlled conditions the thermal7 or
photochemical (in solution8 or in matrix9a-c) decomposi-
tion of diazonium salts. Quite recently, the phenyl cation
has been shown to be among the products from the
photolysis of bromo- and iodobenzene in matrix at 4 K.9d
From the evidence available, phenyl cations are highly
reactive and indiscriminate reagents, behaving both as
electrophiles and as radical hydrogen abstractors. How-
ever, the conditions required for their generation via the
noncatalyzed decomposition of diazonium salts limit the
choice of experimental parameters and the other methods
have a purely mechanistic interest. The scope of their
reactivity and the possibility of synthetic application have
thus not been examined. It has been recently proposed
that some electron-donating substituted haloaromatics
undergo heterolytic photoinduced fragmentation10,11 and
(5) (a) Himeshima, Y.; Kobayashi, H.; Sonoda, T. J . Am. Chem. Soc.
1985, 107, 5286. (b) Apeloig, Y.; Arad, D. J . Am. Chem. Soc. 1985,
107, 5285. (c) Subramanian, L. R.; Hanack, M.; Chang, L. W.; Imhoff,
M. A.; Schleyer, P. v. R.; Effenberger, F.; Kurtz, W.; Stang, P. J .;
Dueber, T. E. J . Org. Chem. 1976, 41, 4099. (d) Laali, K.; Szele, I.;
Yoshida, K. Helv. Chim. Acta 1983, 66, 1710.
(6) (a) Holweger, W.; Hanack, M. Chem. Ber. 1984, 117, 3004. (b)
Bleckmann, W.; Hanack, M. Chem. Ber. 1984, 117, 3021. (c) Hanack,
M.; Rieth, R. Chem. Ber. 1987, 120, 1659.
(7) (a) Romsted, L. S.; Zhang, J .; Zhang, L. J . Am. Chem. Soc. 1998,
120, 10046. (b) Chauduri, A.; Loughlin, J . A.; Romsted, L. S.; Yao, J .
J . Am. Chem. Soc. 1993, 115, 8351. (c) Swain, C. G.; Sheats, J . E.;
Harbison, K. G. J . Am. Chem. Soc. 1975, 97, 783. (d) Bergstrom, R.
G.; Landells, R. G. M.; Wahl, G. W.; Zollinger, H. J . Am. Chem. Soc.
1976, 98, 3301.
(8) (a) Zollinger, H. Diazochemistry I; VCH: New York, 1995. (b)
Gasper, S. M.; Devadoss, C.; Schuster, G. B. J . Am. Chem. Soc. 1995,
117, 5206. (c) Steenken, S.; Askokkuna, M.; Maruthamuthu, P.;
McClelland, R. A. J . Am. Chem. Soc. 1998, 120, 11925.
(9) (a) Ambroz, H. B.; Kemp, T. J . J . Chem. Soc., Perkin Trans. 2
1980, 768. (b) Ambroz, H. B.; Kemp, T. J . Prybytniak, G. K. J .
Photochem. Photobiol., A 1997, 108, 149. (c) Ambroz, H. B.; Kemp, T.
J . Chem. Soc. Rev. 1979, 8, 353. (d) Winkler, M.; Sander, W. Angew.
Chem., Int. Ed. Engl. 2000, 39, 2014. (e) Cox, A.; Kemp, T. J .; Payne,
D. R.; Symons, M. C. R.; de Moira, P. P. J . Am. Chem. Soc. 1978, 100,
4779. (f) Ambroz, H. B.; Kemp, T. J . J . Chem. Soc., Perkin Trans. 2
1969, 1420.
* Corresponding author. Fax: +39-0382-507323.
(1) (a) Stang, P. J . In Dicordinated Carbocations; Rappoport, Z.,
Stang, P. J ., Eds.; Wiley: New York, 1997; p 451. (b) Hanack, M.;
Subramanian, L. R. In Methoden Org. Chemie; Hanck, M., Ed.;
Thieme: Stuttgart, Germany, 1990; Vol. E19C, p 249.
(2) (a) Galli, C. Chem. Rev. 1988, 88, 765. (b) Zollinger, H. Angew.
Chem., Int. Ed. Engl. 1978, 17, 141. (c) Metger, J . O. In Methoden Org.
Chemie; Regitz, M., Giese, B., Eds.; Thieme: Stuttgart, Germany, 1989;
Vol. E19A, p 121. (d) The distinction of the mechanistic alternatives
between heterolytic decomposition of the diazonium cation to yield the
phenyl cation and catalyzed (e.g. Sandmayer) decomposition to yield
the phenyl radical has been first clearly formulated by Waters.2e (e)
Waters, W. A. J . Chem. Soc. 1942, 266.
(3) (a) Hey, D. H. In Advances in Free Radical Chemistry; Williams,
G. H., Ed.; Academic Press: New York, 1980; Vol. 2, p 47. (b)
Rondesvedt, C. S. Org. React. 1976, 24, 225. (c) Ghosez, A.; Giese, B.;
Zipse, H. In Methoden Org. Chemie; Regitz, M., Giese, B., Eds.;
Thieme: Stuttgart, Germany, 1989; Vol. E19A, p 1216.
(4) (a) Speranza, M. Chem. Rev. 1993, 93, 2933. (b) Hrusa´k, J .;
Schro¨der, D.; Iwata, S. J . Am. Chem. Soc. 1993, 115, 2015. (c) Filippi,
A.; Lilla, G.; Occhiucci, G.; Sparapani, C.; Ursini, O.; Speranza, M. J .
Org. Chem. 1995, 60, 1250. (d) Speranza, M. In ref 1a, p 157. (e)
Shchepina, N. E.; Nefedov, V. D.; Toropova, M. A.; Avrorin, V. V.;
Lewis, S. B.; Mattson, B. Tetrahedron Lett. 2000, 41, 5303.
10.1021/jo0104680 CCC: $20.00 © 2001 American Chemical Society
Published on Web 08/25/2001

6354 J . Org. Chem., Vol. 66, No. 19, 2001
Guizzardi et al.
Ta ble 1. P r od u cts fr om th e Ir r a d ia tion of
Ch lor oa n ilin es 1 a n d 4a
Sch em e 1
aniline
additive products (% yield)
1
1
1
1
1
4
4
4
none
2 (47), 3 (30)
7a b
2 (20), 3 (11), 8a (44)
2 (12), 3 (5), 8b (65)
2 (12), 3 (5), 9 (40), 10 (18)
2 (5), 15 (60)
5 (45), 6 (31)
5 (21), 6 (9), 11 (40)
5 (5), 16 (51)
7bb
7cc
NaI, 0.05 Md
none
Sch em e 2
7a b
NaI, 0.05 Md
a
By irradiation of a 0.05 M solution of the anilines in MeCN
for 1 h in the case of 1 and 2 h in the case of 4; conversion was ca.
85% except when otherwise noted. In the case of 4 a solution
containing 0.1 M triethylamine was irradiated to avoid crystal-
lization of the hydrochlorides; similar results were obtained by
b
stirring solid potassium carbonate in the reaction vessel. 1 M.
this may be a potentially more versatile source of aryl
cations, as it is the case for the better known generation
of vinyl cations by photoheterolysis of vinyl halides.12 The
reaction has been studied in detail for the case of
halophenols (mostly in water), where however deproto-
nation leads to a further different intermediate, a carbene
(see Scheme 1).11 We report in the following that 4-chlo-
roaniline and its N,N-dimethyl derivative undergo ef-
ficient photodechlorination and that useful arylation
reactions via the corresponding phenyl cation can be
obtained in this way.13
At 0.1 M the yield of adducts such as compounds 8 or 11 did not
exceed 10%. c In this case a 5 × 10-3 M solution of 1 in the presence
d
of 5 × 10-2 M 7c was irradiated. At ca. 50% conversion.
Sch em e 3
Resu lts
P h otod ecom p osition of Ch lor oa n ilin es. Irradia-
tion of N,N-dimethyl-4-chloroaniline (1, 0.05 M) in ac-
etonitrile caused decomposition of the substrate with
evolution of hydrogen chloride and formation of N,N-
dimethylaniline (2) and of a lesser amount of 5-chloro-
2,4′-bis(dimethylamino)biphenyl (3) (see Scheme 2, Table
1). The irradiation could be carried out to high (>85%)
conversion (see Table 1). The photodecomposition took
place in various solvents (alcohols, perfluoroalcohols,
ethyl acetate, chlorinated hydrocarbons)14 with essen-
tially the same course, although the ratio between
products 2 and 3 and the irradiation time required for
reaction changed significantly. In particular, the photo-
reaction of compound 1 was much slower in cyclohexane
(compare the quantum yield measurements below).
4-Chloroaniline (4) was likewise photoreactive. Aniline
(5) and 5-chloro-2,4′-diaminobiphenyl (6) were the prod-
ucts, similarly to the case of the N,N-dimethyl derivative
1 (Scheme 2, Table 1). Hydrogen chloride was evolved
during the reaction, and particularly with 4, this caused
partial precipitation of the amine hydrochloride. Some
of the irradiations, particularly in the presence of acid-
sensitive traps (see below), were thus carried out in the
presence of 0.1 M triethylamine or by stirring solid
potassium carbonate in the irradiation vessel. This
caused no major change in the product distribution.
Rea ction w ith Ar en es. When aniline 1 was irradi-
ated (310 nm) in acetonitrile containing 1 M benzene (7a ),
the amount of the above-mentioned products was strongly
reduced and the biphenylamine 8a was formed in 43%
yield (Scheme 3, Table 1). In a 0.1 M benzene solution
compound 8a was a minor product and the yields of
products 2 and 3 only slightly diminished with respect
to neat acetonitrile. With 1 M mesitylene (7b) the
corresponding biphenylamine 8b was again the main
product.
Experiments with hexamethylbenzene (7c) required
some modification, due to the limited solubility of this
compound in acetonitrile. Satisfactory trapping was
obtained by irradiating a 5 × 10-3 M solution of 1 in the
presence of 5 × 10-2 M 7c. Under these conditions two
new products were formed, and their spectroscopic
properties allowed us to assign the structure of the triene
9 to the major one and that of the diphenyl derivative
10 to the minor one.
(10) (a) Zhang, G.; Wan, P. J . Chem. Soc., Chem. Commun. 1994,
19. (b) Yang, N. C.; Huang, A.; Yang, D. H. J . Am. Chem. Soc. 1989,
111, 8060. (c) Othmen, K.; Boule, P. Bull. Environ. Contam. Toxicol.
1997, 59, 924. (d) Hori, K.; Sonoda, T.; Harada, M.; Yamazaki-Nishida,
S. Tetrahedron 2000, 56, 1429.
(11) (a) Durand, A. P.; Brown, R. G.; Worrall, D.; Wilkinson, F. J .
Chem. Soc., Perkin Trans. 2 1998, 365. (b) Grabner, Richard, C.;
Ko¨hler, G. J . Am. Chem. Soc. 1994, 116, 11470. (c) Bonnichon, F.;
Grabner, G.; Guyot, G.; Richard, C. J . Chem. Soc., Perkin Trans. 2
1999, 1203. (d) Arnold, B. R.; Scaiano, J . C.; Bucher, G. F.; Sander,
W. J . Org. Chem. 1992, 57, 6469.
(12) Lodder, G. In ref 1a, p 377.
(13) Preliminary communication: Fagnoni, M.; Mella, M.; Albini,
A. Org. Lett. 1999, 1, 1299.
(14) In solvents of low polarity the chloridrate of the aniline
precipitated out during irradiation unless the acidity set free was
buffered (see below the reactions with 4).
Nonmethylated aniline 4 reacted in a way similar to 1
and gave biphenylamine 11 in the presence of benzene.
The reaction with arenes could be carried also in other
solvents. Thus, biphenylamine 8a was obtained by ir-
radiation of 1 in ethyl acetate containing 1 M benzene
provided that the irradiation was twice as long as in
acetonitrile, as well as by irradiation in neat benzene,

Generation/Reactivity of the 4-Aminophenyl Cation
J . Org. Chem., Vol. 66, No. 19, 2001 6355
Ta ble 2. Qu a n tu m Yield of Rea ction a n d P r od u cts F or m ed by Ir r a d ia tion of Ch lor oa n ilin es 1 a n d 4 (Low -Con ver sion
Ru n s)
N,N-dimethyl-4-chloroaniline (1)
4-chloroaniline (4)
solvent
MeCN
MeCN, 1 M C6H6
MeCN, 0.02 M NaI
MeOH
Φr
Φ2
Φ3
Φother
Φr
Φ5
Φ6
Φother
0.87
0.78
0.92
0.95
0.50
0.90
0.70
0.50
0.05
0.03
0.5
0.33
0.07
0.44
0.45
0.70
0.50
0.25
0.60
0.29
0.13
0.03
0.02
0.23
0.04
0.05
0.45
0.15
0.48
0.08
0.1
0.17
0.03
0.19
0.08
0.83
0.2
0.36 (8a )
0.81 (15)
0.33 (11)
0.6 (16)
<0.01
0.08
0.03
0.06
0.06
0.17
0.02
CF3CH2OH
i-PrOH
0.23
0.85
0.37
0.29
<0.01
t-BuOH
0.3
AcOEt
0.14
C6H6
0.03 (8a )
0.02 (11)
C6H12
0.02
0.02
Sch em e 4
>20 in going from MeCN to cyclohexane) but essentially
independent from the presence of additives. As an
example, the reaction quantum yield for 1 in the presence
of benzene, 1-hexene, or pyrrole up to 1 M very little
changed (within 10%) with respect to neat acetonitrile,
despite the fact that under these conditions a major
change in the product distribution occurred, since the
main products became 8a , 12 (mixture of the two regioi-
somers), and 13 (X ) NH) respectively, in the place of 2
and 3 obtained in the absence of the additives. Also in
the presence of salts the effect was generally small. The
reaction quantum yield for chloroaniline 1 in acetonitrile
containing NaI little changed (5%), while it increased
significantly (by ca. 40%) in the case of compound 4.
Com p etition Exp er im en ts. The above results sug-
gested to compare the relative efficiency of the arylation
reactions with various substrates in direct competition
experiments. Since substitution of a iodo for a chloro atom
was efficient already at low iodide concentration (down
to 2 × 10-3 M), we found it expedient to compare the
formation of iodoanilines with that of arylated organic
substrates at 1 M concentration of the organic trap and
varying the concentration of iodide in the appropriate
range (over at least 1 order of magnitude). In this way
the bulk properties of the medium were varied as little
as possible, since the amount of the apolar trap remained
constant and the concentration of the added salt was
small. The competition experiments were carried out at
low (<20%) conversion and in the presence of a small
amount (5 × 10-3 M) of triethylamine. This low concen-
tration ensured that no acid-catalyzed process intervened
while barely enhancing the yield of anilines 2 and 5.
albeit in this case the reaction was so slow as to make
the reaction unpractical for preparative purposes.
Besides the benzene derivatives discussed above, sev-
eral other substrates were efficiently arylated during the
photodecompostion of the chloroanilines 1 and 4. In
particular alkenes gave either â-chloroalkylarenes (12)
or vinyl or allylarenes while five-membered heterocycles
gave arylheterocycles (13) (see Scheme 4a). The prepara-
tive scope and the mechanism of such reactions are
discussed separately.15
Rea ction in Alcoh ols a n d in th e P r esen ce of Sa lts.
As mentioned above, irradiation of anilines 1 or 4 in
alcohols led to the same products as in acetonitrile,
though with a change in the ratio. In methanol and in
2-propanol the proportion of dehalogenated anilines 2 and
5 increased (see Table 1 as well as the quantum yield
data in Table 2). In tert-butyl alcohol and in trifluoroet-
hanol the product distribution was essentially the same
as that in MeCN, with some increase of self-arylation in
one case (4 in tert-BuOH; see Table 2). However, with
all of the alcohols tested the corresponding alkoxyanilines
14 were formed at most in trace amounts (see Scheme
4b).
Addition of some salts had a major effect. Thus,
irradiation of either 1 or 4 in acetonitrile containing 0.05
M NaI gave 4-iodoanilines 15 and 16 as by far the main
products, at least at a moderate conversion (see Scheme
4b and Table 1).
Rea ction Qu a n tu m Yield . The decomposition quan-
tum yield of anilines 1 and 4 was measured in low (<20%)
conversion experiments. Representative results are re-
ported in Table 2. The quantum yield was dependent on
the structure (in MeCN Φ ) 0.87 for 1 and 0.44 for 4;
see Table 2) and on the solvent (a decrease by a factor
The substrates chosen were benzene, 1-hexene, and
2,3-dihydrofuran as a representative enol ether as well
as two heteroaromatic substrates, viz. pyrrole and
thiophene. A doubly reciprocal plot of the iodoaniline
formed vs the iodide concentration was found to be linear
in every case giving two families of straight lines with
common intercept for chloroanilines 1 and 4, respectively
(shown in Figure 1 for the case of compound 1). The
intercept/slope ratios from such plots are reported in
Table 3.
P h otop h ysica l Exp er im en ts. Chloroanilines 1 and
4 showed a well-detectable phosphorescence in a glassy
matrix. Both the form of the spectrum (λmax 425 nm) and
the efficiency of the emission were similar to those
observed for the corresponding nonhalogenated anilines
2 and 5 (λmax 405 nm) and underwent no major change
in passing from a polar (ether-pentane-alcohol) to an
apolar (3-methylpentane) glass.
(15) For the trapping with alkenes see the accompanying article.
With heterocycles see: Guizzardi, B.; Mella, M.; Fagnoni, M.; Albini,
A. Tetrahedron 2000, 56, 8691.
Nanosecond laser flash photolysis experiments were
carried out, again in comparison with nonhalogenated

6356 J . Org. Chem., Vol. 66, No. 19, 2001
Guizzardi et al.
F igu r e 1. Double reciprocal plot of N,N-dimethyl-4-iodoa-
niline (15) formed vs sodium iodide concentration by irradia-
tion of N,N-dimethyl-4-chloroaniline (1) in the presence of 1
M additives: (9) benzene; (]) pyrrole; (0) thiophene; ([)
1-hexene; (2) 2,3-dihydrofuran. The intercept vs slope ratios
obtained from this plot are reported in Table 3. Similar plots
are obtained in the case of 4-chloroaniline (4).
Ta ble 3. F or m a tion of Iod oa n ilin es vs Alk yl- or
Ar yla n ilin es in Com p etition Exp er im en ts in Aceton itr ile
dimethylaniline 1
intercept/ relative
slope, Ma to benzene slope, Mb to benzene
aniline 4
intercept/ relative
substrate
1-hexene
2,3-dihydofuran
benzene
pyrrole
thiophene
73.4
39.1
163.3
10.4
2.2
4.2
1
16.3
1.2
19.9
18.8
109.5
3.5
5.5
5.8
1
F igu r e 3. Oscilloscope traces at 390 (A), 430 (B), and 480 (C)
nm after flashing (266 nm) a 2 × 10-4 M solution of N,N-
dimethyl-4-chloroaniline (1) in acetonitrile
31.3
132.7
126
.87
acetonitrile the result was completely different. The end-
of-pulse absorption (50 ns) was a strong band peaking
at 430 nm which evolved into further intense and
persistent absorptions at both shorter and longer wave-
length (τ several tenths of microseconds) (see decay
profiles and spectra in Figures 3 and 4). Parallel results
were obtained with the nonmethylated chloroaniline 4.
A similar behavior was observed in all of the polar
solvents used, with some change in the wavelength
distribution. In particular in methanol and 2-propanol
the persistent transient corresponded to the long-
wavelength part of the spectrum in acetonitrile (Figure
5). In the presence of 0.3 or 0.6 M mesitylene, on the
contrary, the long-lived transient at 460 nm was much
reduced and a different transient appeared, with the
strongest absorption around 350 nm (Figure 6).
a
b
From the plot in Figure 1. From a plot analogous to that in
Figure 1 (not shown) using aniline 4 in the place of 1.
Com p u ta tion a l Resu lts. Since the 4-aminophenyl
cation was suspected to be an intermediate in the above
reactions, calculations were carried out to determine the
structure of such species. The singlet and triplet states
of the H2NC6H4+ cation were optimized by the standard
(U)B3LYP method, using 6-31G(d) and 6-311+G(d,p)
basis sets. The computed relative electronic energies for
F igu r e 2. Transient observed 200 ns after flashing (266 nm)
a 1 × 10-4 M solution of N,N-dimethyl-4-chloroaniline (1) in
cyclohexane (inset, decay at 460 nm, time unit 1 µs).
substrates. Aniline (2) in cyclohexane showed a conspicu-
ous transient absorption with maximum in the visible
at 450 nm and a lifetime of ca. 2 µs. This has been
previously observed and assigned to the T-T absorp-
tion.16 In acetonitrile the same transient was observed
with a similar intensity and lifetime. The lifetime of the
transient decreased upon increasing the concentration,
with a stronger effect in C6H12 than in MeCN.
The introduction of a chlorine atom did not change the
situation in cyclohexane, where chloroaniline 1 showed
a transient absorption in the same wavelength range as
nonchlorinated 2 (see Figure 2), again with some decrease
in the lifetime at higher concentration. However, in
H2NC6H4 singlet (1A′) and triplet (3B2) states and the
+
relative Gibbs free energies obtained from gas-phase
vibrational frequencies are listed in Table 4.
To check the viability of such a single-determinant
approach and to describe the nature of the singlet and
triplet H2NC6H4+, CASSCF calculations with an active
space comprising eight electrons in eight active orbitals
were then performed. The active space consisted of six π
orbitals, the nitrogen lone pair (pN), and the σ orbital
located at the divalent carbon atom (C1 in Figure 7). It
was found that in the case of 3B2 state the CASSCF wave
function was well described by the ground-state reference
(16) Previtali, C. M. J . Photochem. 1985, 233.

Generation/Reactivity of the 4-Aminophenyl Cation
Ta ble 4. Electr on ic En er gies a n d Gibbs F r ee En er gies (k ca l m ol-1) for Sin gle a n d Tr ip let Sta tes
∆E ∆G(298 K), 1 atm
J . Org. Chem., Vol. 66, No. 19, 2001 6357
species
state
pt group
CASSCFa
(U)B3LYPb
CASSCFa
(U)B3LYPb
+
NH2C6H4
1A′
Cs
C2v
16.12b
0.00
10.92b (9.84)c
0.00 (0.00)c
15.29b,d
0.00
10.09b,d (9.01)c,d
0.00 (0.00)
3B2
a
d
CASSCF(8,8)76-31G(d). b (U)B3LYP/6-31G(d). c (U)B3LYP/6-311+G(d,p). Adding the thermal correction to Gibbs free energy computed
from frequency calculation at the (U)B3LYP/6-31G(d) level of theory.
F igu r e 6. Transient observed 480 ns after flashing (308 nm)
a 2 × 10-4 M solution of N,N-dimethyl-4-chloroaniline (1) in
acetonitrile containing 0.3 M mesitylene (inset, decay at 350
nm, time unit 1 µs).
F igu r e 4. Transient observed 50 ns (9) and 1 µs ([) after
flashing (266 nm) a 2 × 10-4 M solution of N,N-dimethyl-4-
chloroaniline (1) in acetonitrile.
limitation of the CASSCF approach concerning the
quantitative energy assessment of different states, due
to the missing dynamic correlation energy term. Further
efforts toward the exact computation of the triplet-
singlet gap were not carried out, since the main aim of
this work was rather to allow an appreciation of the wave
functions involved and of the nature of the two states to
facilitate the rationalization of the observed chemistry.
At any rate it was checked that the extension of the basis
set from 6-31G(d) to 6-311+G(d,p) at the (U)B3LYP level
of theory slightly reduced the gap (from 10.92 to 9.84 kcal
mol-1; see Table 4, values in parentheses). Inclusion of
nonpotential energy terms and entropy did not modify
the energy gap, as demonstrated by an identical gap in
the Gibbs free energies (see ∆G in Table 4).
F igu r e 5. Transient observed 200 ns after flashing (266 nm)
a 2 × 10-4 M solution of N,N-dimethyl-4-chloroaniline (1) in
methanol (inset, decay at 460 nm, time unit 0.5 µs).
The geometry of the two states as obtained by both
methods is depicted in Figure 7, and the corresponding
charge and spin densities are reported in Table 6. To
facilitate the discussion on the chemical properties of the
cation, the key orbitals Φ4 and Φ5 and their contribution
to the two states are depicted in Figure 8
determinant, while the 1A′ state was poorly described by
any single configuration. Clearly, a balanced comparison
of the geometries and nature of the two states could only
be obtained through a multiconfigurational approach.
Both methods agreed in establishing that the triplet
state was more stable than the singlet state in the gas
phase by at least 10 kcal mol-1. (U)B3LYP/6-31G(d)
underestimated such a gap in comparison to the CASSCF/
Discu ssion
P h otoin d u ced Clea va ge. 4-Chloroanilines 1 and 4
undergo efficient photodecomposition in a variety of polar
6-31G(d) (16.12 kcal mol-1) method by 5.2 kcal mol-1
.
(22) (a) Bunce, N. J . In CRC Handbook of Organic Photochemistry
and Photobiology; Horspool, W. M., Song, P. S., Eds.; CRC Press: Boca
Raton, FL, 1994; p 1181. (b) Bunce, N. J .; Gallacher, J . C. J . Org. Chem.
1982, 47, 1955. (c) Sammes, P. G. In The Chemistry of the Carbon-
Halogen Bond; Patai, S., Ed.; Wiley: New York, 1973; Part II, Chapter
11. (d) Lodder, G.; Cornelisse, J . In The Chemistry of the Carbon-
Halogen Bond; Patai, S., Rappoport, Z., Eds.; Wiley: New York, 1995;
Supplement D, Chapter 29. (e) Grimshaw, J .; de Silva, A. P. Chem.
Soc. Rev. 1981, 10, 181. (f) D’Auria, M.; De Luca, B.; Mauriello, G.;
Racioppi, R. J . Chem. Soc., Perkin Trans 1 1998, 271. (g) Mangion,
D.; Arnold, D. R. Can. J . Chem. 1999, 77, 1655.
(23) Arnold, D. R.; Wong, P. C. J . Am. Chem. Soc. 1977, 99, 3363.
Ichimura, T.; Mori, Y. J . Chem. Phys. 1973, 58, 288.
(24) Nicolaides, A.; Smith, D. A.; J ensen, F.; Radom, L. J . Am. Chem.
Soc. 1997, 119, 8083-8088.
(25) (a) Aschi, M.; Harvey, J . N. J . Chem. Soc., Perkin Trans. 2 1999,
1059. (b) Mendez, F.; Garcia-Garibay, M. A. J . Org. Chem. 1999, 64,
7061.
Such a result agreed with previous investigations25
focused on this aspect where it had been recognized that
B3LYP underestimated the S-T gap by 2-4 kcal mol-1
On the other hand one has to keep in mind the possible
.
(17) Othmen, K.; Boule, P. J . Photochem. Photobiol., A 1999, 121,
161.
(18) Bunce, N. J .; Bergsma, J . P.; Bergsma, M. D.; De Graaf, W.;
Kumar, Y.; Ravanal, L. J . Org. Chem. 1980, 45, 3708.
(19) Othmen, K.; Boule, P.; Szczepanik, B.; Rotkiewicz, K.; Grabner,
G. J . Phys. Chem. A 2000, 104, 9525.
(20) Miller, G. C.; Crosby, D. G. Chemosphere 1983, 12, 1217.
Szczepanik, B.; Latowski, T. Pol. J . Chem. 1997, 71, 807.
(21) Kouteck, B.; Musil, L.; Velek, J .; Soucek, M. Collect. Czech.
Chem. Commun. 1985, 50, 1985.

6358 J . Org. Chem., Vol. 66, No. 19, 2001
Guizzardi et al.
Ta ble 5. F r a ction a l P op u la tion γi for th e Eigh t Active Or bita ls for Sin gle a n d Tr ip let Sta tes fr om th e CAS-SCF (8,8)/
6-31G* Level of Th eor y
state
Φ1(π)
Φ2(π)
Φ3(π)
Φ4(pN)
Φ5(σ at divalent C1)
Φ6(π*)
Φ7(π*)
Φ8(π*)
1A′
1.96
1.96
1.94
1.92
1.89
1.90
1.46
1.00
0.54
1.00
0.11
0.07
0.06
0.097
0.03
0.034
3B2
F igu r e 8. Schematic depiction of the nature of triplet (3B2)
and singlet (1A′) 4-aminophenyl cation. The coefficients (Ci) of
the triplet state lowest eigenvector and the coefficients of the
two main electronic configurations for the singlet state of
4-aminophenyl cation are reported. In the case of the singlet,
the most important configuration has two spin-coupled elec-
trons in a π orbital (Φ4) and an empty σ orbital (Φ5). The
second most important configuration involves double excitation
of the above electrons from the π orbital (Φ4) to the σ orbital
(Φ5).
F igu r e 7. Bond lengths (Å), angles ( ; in deg), and dihedral
angles ( ; in deg) for singlet (1A′) and triplet (3B2) 4-aminophe-
nyl cation as obtained by B3LYP/6-31G(d) (bold characters)
and CASSCF(8,8)/6-31G(d) (plain characters) calculations.
Ta ble 6. Mu llik en Ch a r ges (w ith Hyd r ogen s Su m m ed
in to Hea vy Atom s) a n d Sp in Den sity for Sin gle a n d
Tr ip let Sta tes
vated previous studies,10c,11,17,19,20 but also for exploring
new mild C-C bond-forming reactions.
The initiating photochemical step involves C-Cl bond
fragmentation. Various pieces of evidence support that
the cleavage proceeds from the triplet. Thus, the inter-
system crossing efficiency for both chlorinated and non-
chlorinated anilines such as 1, 2, 4, and 5 has been
estimated to be around 0.9 and to be little affected by
the solvent.21 This precludes the possibility of a role of
an excited singlet in an efficient reaction such as the
present one. Furthermore, the reaction quantum yield
increases by a factor >20 in going from cyclohexane to
acetonitrile. There is no corresponding decrease in the
fluorescence lifetime (actually reported to increase by a
factor of ca. 2 from isooctane to MeCN),21 while there is
a major change in the transients observed. Thus, in
cyclohexane a short-lived transient is observed with both
1 or 4 (τ in the microsecond range). The spectra are very
similar to those that the nonchlorinated anilines 2 and
5 exhibit in both polar and apolar solvents (as well as
that shown by 4-chlorophenol in cyclohexane)11b and are
confidently assigned to the T-T absorption in accordance
with Othmen et al.19
1A′
3B2
3B2: UB3LYP
atom CASSCF B3LYP CASSCF UB3LYP (spin density)
C1
C2
C3
C4
N7
0.133
0.073
0.100
0.346
0.176
0.162
0.150
0.048
0.425
0.016
0.121
0.083
0.092
0.368
0.160
0.138
0.086
0.097
0.379
0.115
1.340
-0.134
0.242
0.083
0.350
organic solvents. Although the photoreaction of 4 in water
(to give 4-aminophenol and 2,4′-diamino-5-chlorobiphe-
nyl) has been previously reported,17 this result is some-
what surprising particularly when the rather high quan-
tum yields, 0.84 and 0.44 for 1 and 4, respectively, in
MeCN and the strong solvent effect are considered. Such
an effect of the solvent polarity on the photoreaction
efficiency has not previously observed for halobenzenes;
e.g., fragmentation of chlorobenzene occurs efficiently in
cyclohexane.18 However, a known exception is that of
halophenols, which are photoreactive only in alcohols or
water,11 and after the submission of this work a report
by Othmen et al.19 appeared showing that chloroanilines
1 and 4 fragment in water and in polar organic solvents
but not in n-hexane.
On the other hand, in all of the polar solvents tested
such transients are substituted by a strong transient at
430 nm, which clearly cannot be attributed to the same
species and gives place to further long-lived and intensive
signals (τ in the tenths of microseconds range). Thus, the
triplet undergoes a fast decomposition in polar solvents,
and actually there is a reverse relationship between the
efficiency of the photoreaction and solvent polarity (see
The present study shows that aminoarylation reactions
occur efficiently upon irradiation of chloroanilines in the
presence of various substrates. Thus, the reaction has
some synthetic interest and recognizing the mechanism
of fragmentation is important not only for the photodeg-
radation of chloroanilines and phenols, which has moti-

Generation/Reactivity of the 4-Aminophenyl Cation
J . Org. Chem., Vol. 66, No. 19, 2001 6359
Table 2). This notion is strengthened by the demonstra-
tion by Othmen et al. that T-T energy transfer from 43*
is more that 1 order of magnitude slower in ethanol than
in n-hexane.19
As for the mode of fragmentation, the body of data
accumulated on the photoreactions of haloaromatics has
been rationalized in terms of a homolytic fragmentation
(eq 1).18,21,23
confirmed that in the case of the parent cation the
ground-state reference determinant contributed more
than 90% (the coefficient of the lowest eigenvector is 0.94
and 0.95 for the singlet and triplet state, respectively).27
Therefore, in that case a single-determinant reference
wave function such as B3LYP is fairly adequate to
provide a correct description of the molecular character-
istics.27
However, in the case of the 4-aminophenyl cation this
is reasonable only for the 3B2 state for which the CASSCF
wave function is described for more than 90% by the
reference determinant and no individual excited config-
uration contributes significantly. On the other hand, the
1A′ state of the amino-substituted cation is poorly de-
scribed by any single configuration. Therefore, it is not
surprising that the geometric parameters (in Figure 7)
and charges (in Table 6) determined by the two methods
are in excellent agreement only for the 3B2 state of
4-aminophenyl cation.
Ar-Cl + hν f Ar• + Cl•
Ar-Cl + hν f Ar+ + Cl-
(1)
(2)
However, the present reaction appears to involve a
heterolytic cleavage (eq 2). This is supported by the
consideration of the thermodynamics of the process and
of the solvent effect. Indeed, it is unlikely that triplet 1
or 4 (ET ) 72 kcal mol-1, as evaluated from the phos-
phorescence spectrum) undergoes cleavage of the C-Cl
bond (BDE ≈ 85-91 kcal mol-1) with an equal to or
greater efficiency than the more energetic triplet chlo-
robenzene (ET ≈ 82 kcal mol-1), while it has been
consistently observed that the photohomolysis quantum
yield decreases, as one may expect, with the excited-state
energy.22a When the cleavage is homolytic, it occurs also
in an apolar medium (e.g. for chlorobenzene Φr ) 0.42
when 0.02 M in cyclohexane; the yield is somewhat
concentration dependent)18 while 1 and 4 have Φr ca. 0.02
in cyclohexane and quantum yields are high only in a
polar medium.
The ππ* S1 of 1 and 4 has an internal charge-transfer
character, as indicated by the bathochromic shift in going
from isooctane to MeCN (1700 cm-1),21 and this reason-
ably holds also for the emitting triplet, again a ππ* state
as indicated by the phosphorescence shape and intensity.
In the case of the homolysis of chlorobenzene, the possible
intermediacy of a nonemitting, somewhat higher-lying
σσ* state has been considered.23 We have no support for
the role of such a state in the fragmentation of anilines.
It appears that a polar medium is determining for the
efficiency of the cleavage by increasing the CT character
of the excited state and stabilizing the ionic fragments.
In particular, this is indicated by the requirement of at
least a moderately polar environment (with ethyl acetate,
ꢀ ) 6, Φr is about a half of the maximum value, reached
by all solvent with ꢀ > 12) and by the fact that with the
strongly polar but significantly acidic trifluoroethanol as
the solvent Φr markedly decreases, indicating that less-
ening CT through amino group protonation disfavors the
efficiency of the C-Cl bond cleavage.
There is little precedent for such a strong solvent
polarity effect on a photochemical reaction, but we see
no other explanation for the unusual solvent dependence.
The behavior appears to be typical of chlorophenols
(where the reaction requires protic, not merely polar,
solvents)11 and of chloroanilines, as reported also by
Othmen et al.19
Str u ctu r e of th e Am in op h en yl Ca tion . The proper
choice of the method was an obviously important issue.
In the frame of density functional theory the B3LYP
method appeared to be a good choice, because, while
being computationally affordable, it has been used for the
parent phenyl cation24,25a and shown to fairly well account
for the singlet-triplet splitting25a,26 and to yield very
similar geometries as those obtained by the RCCSD(T)-
(full)/6-31G(d) and CASSCF(6,7)/6-31G(d) methods. We
Both methods recognize the triplet as the lowest energy
state by at least 10 kcal mol-1 in the gas phase, in accord
with a previous evaluation.25 Both the ring an the
nitrogen atom remain planar in the triplet, while the
singlet has the nitrogen slightly pyramidalized and C1
slightly bent out of plane and puckered (Figure 7).
Ch em ica l Rea ction s of th e Am in op h en yl Ca tion .
Consideration of the chemistry observed both in neat
solvent and in the presence of a trap offers alternate
evidence for the structure of the intermediate. The
reaction quantum yields in the presence of increasing
concentrations of the traps do not differ from those
observed in neat solvent. Furthermore, Φ remains the
same also in dilute solutions of the chloroaniline, where
quenching of the short-lived triplet is excluded. Thus the
present reaction does not involve electron-transfer quench-
ing of the excited aniline or formation of an exciplex, as
in other arylation processes,22c-e,g but rather trapping of
an intermediate arising directly from the triplet state.
The structure of the final products in the reactions with
benzene and methylbenzenes implies the intermediacy
of an electrophilic species and is best accounted for by
an aryl cation (17) reacting through the usual addition-
elimination mechanism via the cyclohexadienyl cation 18
(Scheme 5). Thus, electrophilic substitution takes place
both with benzene (7a ) and with mesitylene (7b), with
no competing hydrogen abstraction in the latter case (the
proportion of aniline is lower with 7b than with 7a ). This
again disproves a role of an aryl radical. Most notably
the reaction with hexamethylbenzene (7c) involves ring
attack. The methylenecyclohexadiene 9 and the pentam-
ethylbenzene 10 are the expected products from the
fragmentation of the cyclohexadienyl cation 18′. Indeed
a 1-alkyl-4-methylenecyclohexadiene and a 1-alkylpen-
tamethylbenzene are formed in a similar yield in the
alkylation of 7c by alkyl halides and AlCl3.28
(26) Hrusak, J .; Schroder, D.; Iwata, S. J . Chem. Phys. 1997, 106,
7541-7549.
(27) The correctness of (U)B3LYP method choice in studying un-
substituted phenyl cation is corroborated by CASSCF(6,7)/6-31G(d)
results performed by one of us (M.Fr.) in the frame of a comprehensive
study of substituted phenyl cations, including the solvent effect. These
will be submitted in due course.
(28) (a) Mindl, J .; Sebastiani, G. V. Collect. Czech. Chem. Commun.
1987, 52, 2936. (b) See also: Shubin, V. G.; Tabatskaya, A. A.;
Darendayaev, B. G.; Korchagina, D. V.; Koptyung, V. A. Zh. Org. Khim.
1970, 6, 2072. Hu¨nig, S.; Schilling, P. Chem. Ber. 1975, 108, 3355. (c)
For an analogy in the gas phase, see: Attina`, M.; Cacace, F.; de Petris,
G.; Fornarini, S.; Giacomello, P. J . Am. Chem. Soc. 1985, 107, 2297.

6360 J . Org. Chem., Vol. 66, No. 19, 2001
Guizzardi et al.
Sch em e 5
compressed range, the known order of reactivity with
electrophiles with these substrates and is roughly pro-
portional to the ionization potentials. Likewise, an enol
ether as 2,3-dihydrofuran is a more efficient trap than a
simple alkene.
With this mechanism, it is expected that, in the
absence of a purposely added trap, cation 17 attacks
activated substrates such as the starting anilines yielding
the biphenyldiamines 3 and 6.
Reduction to anilines 2 and 4 is competitive with the
above reactions. This depends on the hydrogen-donating
properties of the solvent. The ratio biphenyldiamine/
reduced aniline is ca. 1 or larger in acetonitrile, tert-butyl
alcohol, or trifluoroethanol, while it is reduced to 0.1 with
better hydrogen donors such as 2-propanol or methanol
(down to 0.01 with 1 in 2-propanol).
Tr a n sien t Attr ibu tion . The transients observed can
be rationalized within this framework. In polar solvents,
differently from cyclohexane, the triplet is too short-lived
for detection. The intensive end-of-pulse transient (λmax
430 nm; see Figure 3) is attributed to cation 17 resulting
from dechlorination. The same identification has been
given by Othmen et al., though these author refers to
this species as a (protonated) carbene (see above).19 The
absorption fits with that observed by photolysis of 4-
(dialkylamino)phenyldiazonium salts in glass at low
temperature9a-c and identified as the triplet phenyl cation
also on the basis of EPR measurements.9e,f In contrast, a
previous search for this cation by photolysis of 4-(dialky-
lamino)phenyldiazonium tetrafluoborate at room tem-
perature had revealed only a weak transient in the
picosecond time scale in this spectral region.8b This is not
in contrast with the present results, since direct pho-
tolysis of the diazonium salt generates the singlet cation,
while dechlorination of triplet chloroaniline gives the
triplet species (see further below).
Evolution of the primary intermediates leads to long-
lived (>10 µs in MeCN) transients in two spectral
regions, around 340-370 and around 460 nm. We suggest
that the first one corresponds to the cyclohexadienyl
cation 19 formed by addition of the 4-aminophenyl cation
17 to the starting aniline. The stabilization by the amino
group explains the persistence of this species.
On the other hand, hydrogen transfer from the solvent
generates radical cation 20. The longer-wavelength tran-
sient is quite similar in shape to the spectrum observed
by Mariano upon electron transfer from dimethylaniline
to 1,4-dicyanobenzene (λmax 430-470 nm in MeOH-
MeCN).29 Under the present conditions (organic solvent,
weak laser power) the radical cation is not formed by
photoionization of the anilines, although this may be the
case under different circumstances.30 This is shown by
the fact that no radical cation is formed from nonchlori-
nated aniline 2 under these conditions. Since no easy
reduction path is available, this species is quite long-lived
under the present case.
The observed aromatic substitution occurs with some
selectivity. This has been studied by means of competition
experiments. In neat MeCN, the corresponding anilines
and biphenyldiamines are formed (eq 3).
1 (or 4) f 2 (or 5) + 3 (or 6) rate kMeCN
1 (or 4) + I- f 15 (or 16) rate kI
(3)
(4)
1 (or 4) + trap f trapping product(s) rate kTr (5)
In the presence of iodide the iodoanilines 15 or 16 are
obtained (eq 4). In the presence of an aromatic substrate,
and analogously with heteoaromatics and alkenes (“traps”),
trapping products are formed (e.g. 8a from 1 and ben-
zene; see eq 5). In the presence of both iodide and trap,
the amount of iodoaniline 15 (or 16) formed in a given
amount of time is expressed by eq 6 if, as we suggest, all
of the products arise from the same intermediate.
kI[I-]
[15] ) K
(6)
kMeCN + kI[I-] + kTr[trap]
In the reciprocal form and by consideration of only
experiments at 1 M trap, where, as it appears from Table
2, trapping is efficient and kTr[trap] > kMeCN, eq 6 modifies
to eq 7.
kTr[trap]
[15]-1 = K-1 1 +
(7)
kI[I-]
(
)
Figure 1 shows the verification of eq 7 for a range of traps
and the appropriate range of iodide concentrations. This
allows one to evaluate the ratio kI/kTr as reported in Table
3.
Consistently with this identification, little if any
transient absorption is observed below 430 nm upon flash
photolysis in methanol (Figure 5) or 2-propanol, while
the long-wavelength band corresponding to the radical
cation (somewhat red-shifted and markedly shorter-lived)
As it appears from Table 3, the organic substrates
tested react with the aryl cations from 3.5 to 165 times
slower than iodide. Among organic traps, thiophene is
essentially as reactive as benzene while pyrrole is 31
times more reactive than benzene in the case of the
amino cation derived from 4 and 16 times with the
dimethylamino cation from 1. This reflects, though in a
(29) Zhang, X.; Yeh, S. R.; Hong, S.; Freccero, M.; Albini, A.; Falvey,
D. E.; Mariano, P. S. J . Am. Chem. Soc. 1994, 116, 4211.
(30) Latowski, T. Ann. Soc. Chim. Pol. 1966, 40, 231. Latowski, T.
Ann. Soc. Chim. Pol. 1968, 42, 99.

Generation/Reactivity of the 4-Aminophenyl Cation
J . Org. Chem., Vol. 66, No. 19, 2001 6361
is apparent: in alcohols, reduction to the aniline strongly
predominates over formation of the biphenyldiamines
(see further below). Conversely, in the presence of mesi-
tylene, where attack to the aromatic is the dominant
process, the two above transients are substituted (only
the 470 nm transient is detected as a weak signal) by a
different absorption with a maximum at 350 nm (τ 2 µs;
see Figure 6). This is similar in shape to the transient
observed by Steenken and McClelland upon photolysis
of phenyldiazonium tetrafluoborates in the presence of
the same trap and attributed to the cyclohexadienyl
cation 21 (X ) H).8c In our case the corresponding amino-
This led to the triplet state of the aryl cation which
abstracted hydrogen from the solvent. In the other cases,
the singlet excited state of the diazonium salt underwent
dediazoniation before ISC leading to the singlet cation
and the ether from it.
It has been well established in the present and in
previous studies10d,25,27,31 that the singlet is the lowest
state with the parent phenyl cation, but the energy
ordering depends on the substituent. As mentioned
above, the triplet is markedly lower in energy (by >10
kcal mol-1) with the 4-amino-substituted derivative.
Under such conditions ISC is expected to be slow, as
suggested by the computed energy difference between the
triplet cation and the minimum energy crossing point
with the singlet surface (9.1 kcal mol-1).25a It may well
be that, analogously to what happens with isoelectronic
phenylnitrene,32 for which a similar situation applies, the
chemical reaction (expected to involve essentially no
activation energy) proceeds from the first formed spin
state before ISC. Thus, the aminophenyl cation reacts
from the singlet state when generated in this state by
photolysis the diazonium salt, while heterolysis of chlo-
roanilines proceeds, as indicated above, from the triplet
state and leads to triplet phenyl cation.
substituted derivative (21, X ) NMe2 ≡ 18, R ) Me) is
formed and since the amino group is not conjugated, no
major difference is expected on the spectrum of the cation
or on its persistence (explaining the much shorter lifetime
with respect to stabilized 19). Indeed, according to
Steenken and McClelland8c the spectrum of the (4-
methoxyphenyl)cyclohexadienyl cation (21, X ) OMe, λmax
356 nm in hexafluoropropanol) was only slightly different
from that of the parent cation (21, X ) H, λmax 350 nm).
Sp in -Dep en d en t Ch em istr y of th e Ca tion . What
is perhaps unexpected in the chemistry reported above
is the selectivity. Electrophilic attack to (hetero)arenes
and alkenes and hydrogen abstraction are consistent with
the “intuitively” expected reactivity of aryl cations. Less
obvious is that, while being efficiently trapped by iodide
to give iodoanilines (ca. 90% at [I-] ) 0.02 M), cation 17
does not react with σ neutral nucleophiles. Strikingly,
only trace amounts of alkoxyanilines 14 have been
detected in the irradiation we carried out in various
alcohols. This should be compared with the previous
reports on the photodecomposition of diazonium salts in
alcohols. Steenken and McClelland obtained both a
biphenyl and an ether by photodecomposition of phenyl-
diazonium tetrafluoborates in hexafluoropropanol con-
taining 0.05-0.5 M benzene derivatives (eq 8),8c appar-
ently indicating competition between the attack at the π
and at the σ nucleophile (quite a weak one in this case).
Schuster obtained substituent-dependent results from
the photolysis of 4-substituted phenyldiazonium tet-
rafluoroborates in trifluoroethanol.8b With dialkylamino
derivatives the ethers accounted for ca. 90% of the
products (eq 9), whereas with the 4-acetyl derivatives
reduction was by far the main process (>90%, eq 10) and
the ether was formed in traces. As indicated in the
equations, this was rationalized as being due to fast ISC
induced by the keto group in the last case.
The above computational results offers a clue for
understanding the contrasting chemistry. The analysis
3
of the CI wave function (Ψ) for B2 state can be carried
out in terms of the coefficient of the lowest eigenvector,
or equivalently of populations of the active orbitals (γi,
in Table 5). In this case, a single configuration dominates
Ψ. In fact, the highest coefficient (Ci) is >0.93. This is
reflected in the values of the fractional populations γi,
which are close to 0, 1, and 2 respectively for the eight
active orbitals (Table 5). The σ and a π orbitals (Φ4, Φ5;
see Figure 8 with the corresponding Ci) are both singly
occupied. The triplet state can be viewed as a classical
triplet carbene, with the electrons in orthogonal spaces.
However, the electron in the σ space is highly localized
while the electron in the π space is delocalized on the
ring and on the nitrogen atom. This consideration is
based on CASSCF data and is also supported by UB3LYP
spin density data (see Table 6) showing that the N atom
has the second highest spin density (0.35). Therefore, the
3B2 state has a 65% diradical, 35% carbene character at
divalent C1 atom. As for charge distribution, CASSCF
and UB3LYP data are very similar (see Table 2) and the
highest positive charge is on the C4 atom (+0.37). Thus,
the 3B2 state involves no cation at C1 and it is not
surprising that it does not react with a neutral σ
nucleophile, while it easily adds to a π nucleophile
behaving as an electrophilic radical.
Contrary to 3B2 state, the singlet 4-aminophenyl cation
(1A′) cannot be properly described by a single-determi-
(31) Dill, J . D.; Schleyer, P. v. R.; Binkley, J . S.; Seeger, R.; Pople,
J . A.; Haselbach, E. J . Am. Chem. Soc. 1976, 98, 5428. Dill, J . D.;
Schleyer, P. v. R.; Pople, J . J . Am. Chem. Soc. 1977, 99, 1. Castenmiller,
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Perkin Trans. 2 1980, 492. Alcock, N. W.; Greenhough, T. J .; Hirst, D.
M.; Kemp, T. J .; Payne, D. R. J . Chem. Soc., Perkin Trans. 2 1980, 8.
Tasaka, M.; Ogata, M.; Ichikawa, H. J . Am. Chem. Soc. 1981, 103,
1885. Schleyer, P. v. R.; Kos, A. J .; Raghavachari, K. J . Chem. Soc.,
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Am. Chem. Soc. 1994, 116, 10129.
hν
-
C6H5N2+BF4
8 C6H5+ f
(CF3)2CHOH, Ar′H
C6H5Ar′ + C6H5OCH(CF3)2 (8)
hν
4-R2NC6H4N2+BF4
8 4-R2NC6H4+ (S) f
-
CF3CH2OH
4-R2NC6H4OCH2CF3 (9)
hν, ISC
-
4-MeCOC6H4N2+BF4 CF CH OH8
3
2
4-MeCOC6H4+ (T) f 4-MeCOC6H5 (10)
(32) Platz, M. S. Acc. Chem. Res. 1995, 28, 487.

6362 J . Org. Chem., Vol. 66, No. 19, 2001
Guizzardi et al.
Sch em e 6
nucleophiles, a quantitative comparison with the rate of
reaction with anions is premature, since the mechanism
may be not be exactly the same in that case.
It should also be noted that the chemical results
obtained with chloroaniline 4, both as far as the product
distribution and as far as the relative rates are con-
cerned, differ very little from what found with the
corresponding N,N-dimethyl derivative 1. Presumably,
the aminophenyl cation reacts with both hydrogen donors
and π nucleophiles faster than it deprotonates. Actually,
there is no need to invoke carbene 22 as an intermediate
nant approach such as B3LYP. Thus, this question will
be addressed on the basis of CASSCF data. There are
two large coefficients in the lowest CI engenvector,
respectively 0.81 and -0.47 (see Figure 8). Correspond-
ingly, the populations of the active orbitals Φ4 and Φ5
are 1.46 and 0.54 (see Table 5). The most important
configuration has the 8 electrons in four orbitals, three
π and the pN, while the σ orbital on C1 (Φ5) is empty.
However, the second configuration has a role, and results
from double excitation from Φ4 to Φ5, both electrons
being promoted from the pN to the σ orbital. The first
configuration corresponds the conventional representa-
tion of a phenyl cation with an empty σ orbital (π6σ0)
while the latter one emphasizes the important singlet
carbene character (planar sp2-hybridized carbon, filled
σ, and empty p orbital) of this species. Even if the charge
is delocalized also in this case, the singlet carbene
character supports that addition to σ nucleophiles is easy.
This is not relevant in the present case since, as shown
above, with chloroanilines ISC precedes heterolysis and
leads directly to the triplet phenyl cation.
Thus, photolysis of an aminophenyldiazonium salt in
alcohols yields an aminophenyl ether, since the singlet
excited diazonium salt decomposes to the singlet phenyl
cation, while only reduced aniline is obtained from the
heterolysis of chloroaniline, via the triplet phenyl cation.
As discussed above, this species has a radical-carbene
character, indicated in Scheme 6 through mesomeric
formula 17′, and hydrogen abstraction is fast in neat
alcohols (e.g. the intermediate radical cation 20, see
Scheme 6, is formed within 1 µs in MeOH; see Figure 3).
This leaves no room for ether formation, and the rate for
such process is at least 2 orders of magnitude slower than
that of hydrogen transfer from the activated alcoholic
C-H. In accordance with this mechanism, photoreduction
of the chloroanilines is less efficient both in R trisubsti-
tuted tert-butyl alcohol and in electron-poor trifluoroet-
hanol, but then it is the attack on anilines to give
products 3 and 6 which competes and not ether formation
(see Table 2).
since triplet cation 17 has a carbene-diradical character
which fully explains the observed chemistry. Since some
of the arylations discussed here and in the accompanying
paper15 may have some synthetic significance, this is a
relevant point since it leads to the expectation that both
N-alkylated and nonalkylated anilines may be used as a
substrates for photochemical reactions of this type.
Con clu sion
The present data show that the 4-amino derivatives
of the elusive phenyl cation are efficiently generated in
solution by photoheterolysis of 4-chloroaniline and its
dimethyl derivative in moderately polar or polar solvents.
Under these conditions the triplet cation is generated.
CASSCF and UB3LYP calculations show that this species
has a mixed carbene-diradical character and very little
positive charge at C1. In accordance with this representa-
tion, it reacts with π nucleophiles but not with neutral σ
nucleophiles. Importantly from the synthetic point of
view, irradiation of chloroanilines offers a smooth and
versatile method for the arylation of arenes. As discussed
separately, the arylation of alkenes can be carried out
similarly and offers a variety of synthetically useful
paths.15 Furthermore, preliminary studies show that
photoinduced arylations are not limited to haloanilines
but can be extended to other electron-donating substi-
tuted aromatics.33
Exp er im en ta l Section
Gen er a l Meth od s. N,N-4-chlorodimethylaniline (1, pre-
pared by methylation of the aniline) and the other reagents
(of commercial origin) were distilled or recrystallized before
use. For the irradiations, spectroscopic grade solvents were
used as received.
P r ep a r a tive Ir r a d ia tion s. In a typical experiment a
solution of 780 mg of aniline 1 or 625 mg of aniline 4 (0.05 M)
in 100 mL of acetonitrile (containing a trap when required)
was irradiated for 3 h in an immersion well apparatus fitted
While an uncharged σ nucleophile is ineffective, a
charged species such as bromide or iodide does trap the
cation also in the triplet state. Indeed, iodide is a better
trap than the best π nucleophile, pyrrole, reacting 10 or
3.5 times faster respectively with the two cations con-
sidered. In the case of aniline 4 (not with the more
reactive 1), there is also a moderate increase in the
quantum yield of reaction, contrary to what observed
with all the other additives. It is possible that in this case
a further path via electron transfer to the triplet con-
tributes to the decomposition of the chloroaniline add-
ing to the usual unimolecular fragmentation. There-
fore, while the ratios in Table 3 give the order of π
with
a high-pressure mercury arc (125 W, water-cooled
through a quartz jacket) after 15 min of flushing with argon
and maintaining a slow gas flux during the irradiation. In the
experiments with acid-sensitive substrates anhydrous potas-
sium carbonate (2 g) was added and the solution magnetically
stirred during the experiment. In an alternative procedure,
irradiations were carried out by using 20 mL portions of the
same solutions in a number of quartz tubes which were capped
after flushing with argon for 15 min and externally irradiated
(33) Unpublished from the present laboratory.

Generation/Reactivity of the 4-Aminophenyl Cation
J . Org. Chem., Vol. 66, No. 19, 2001 6363
by means of 6 × 15 W phosphor-coated lamps for 3 h in a
merry-go-round apparatus. In this case, the solution was made
0.1 M in triethylamine when a buffer was required. The
progress of the reaction was monitored by GC and GC/MS.
The traps used are listed in Table 1.
peaks were found in the NOESY spectrum: 7.02 with 2.01;
2.01 with 2.25. NMR: δC (CDCl3) 16.7 (CH3-4), 18.3 (CH3-2
and CH3-6), 30.8 (CH3-3 and CH3-5), 40.6 (NMe2), 112.2 (CH),
126.5, 130.1 (CH), 131.9, 132.3, 133.5, 140.1, 142.5. GC/MS:
tR 19.16 min; m/z 267 (100), 252 (20), 237 (8).
P r od u cts Isola tion a n d Id en tifica tion . The irradiated
solution was evaporated under reduced pressure and the
residue chromatographed on silica gel 60 HR by eluting with
cyclohexane-ethyl acetate mixtures. The products were ob-
tained as solids or oils from the fractions (by repeating the
chromatography in the case of unsatisfactory separation) and
characterized by elemental analysis, GC/MS, and NMR as
detailed in the following.
Low -Con ver sion Exp er im en ts. Low-conversion experi-
ments were carried out for quantum yields measurements
(Table 2) and competition experiments (Table 3). The solutions
(5 mL) were prepared and irradiated under the same condi-
tions as in the preparative experiments in a merry-go-round
apparatus. Alternatively, 2 mL portions were irradiated in
spectrophotometric couvettes by means of a 200 W focalized
high-pressure mercury arc (interference filter, λtr 313 nm). In
every case the conversion was limited to 20%. Product forma-
tion was assessed by GC and HPLC. The light flux was
measured by ferrioxalate actinometry.
4-Arylanilines 8a ,34 8b,35 and 1136 and iodoanilines 15 and
16 were previously reported and were recognized either by
comparison with an authentic sample or by comparison of the
spectral properties. 1H and 13C NMR spectra were recorded
with a 300 MHz spectrometer, and the chemical shifts are
reported relative to TMS. The structure of new compounds
were deduced from the results of 1H, 13C, DEPT-135, and 2D-
correlated experiments. The GC-MS analyses were performed
using a DB-5 column, 30 m × 0.25 mm with film thickness
0.25 µm. The carrier was helium at 0.6 mL/min. The total run
time was 30 min with the initial oven temperature 80 °C (4
min.), rising at a rate of 10 °C/min to 250 °C.
N,N-Dim eth yl-2-(4′-(d im eth yla m in o)p h en yl)-4-ch lor o-
a n ilin e (3) formed as a colorless oil which solidifies on
standing, mp 40-42 °C (lit.37 oil). Anal. Found: C, 77.82; H,
7.83; N, 10.05. Calcd for C16H19 N2Cl: C, 77.87; H, 7.76; N,
10.19. NMR: δH (CDCl3) 2.55 (s, 6H, NMe2), 3.05 (s, 6H,
-NMe2), 6.9 (d, J ) 8 Hz, 1H), 6.72 and 7.45 (AA′XX′, 4H,
aromatics), 7.14 (dd, J ) 2, 8 Hz, 2H), 7.18 (dd, J ) 2 Hz, 1H);
δC (CDCl3) 40.4 (NMe2), 43.1 (NMe2), 112.3 (CH), 118.6 (CH),
126.3, 126.5 (CH), 128,9, 129.0 (CH), 130.9 (CH), 135.8, 149.4,
149.8. GC/MS: tR 11.21 min; m/z 274 (100), 276 (33), 259 (25).
2-(4′-Am in op h en yl)-4-clor oa n ilin e (6) was obtained as
an oil.38 Anal. Found: C, 65.85; H, 5.10; N, 12.72. Calcd for
Em ission Mea su r em en ts. Emission spectra were mea-
sured by means of an Aminco Bowman spectrofluorometer,
fitted by a rotating phosphoroscope for phosphorescence
measurements (made at 77 K).
F la sh P h otolysis. The laser flash photolysis studies were
carried out by using the fourth (266 nm) harmonics of a
Q-switched Nd:YAG laser (model HY 200, J K Laser Ltd.
Lumonics). This delivered 3 mJ pulses with a duration of ca.
10 ns. The monitor system, arranged in cross-beam configu-
ration, consisted of a laser kinetic spectrophotometer (model
K 347, Applied Photophysics) fitted with a 200 W Xe arc lamp,
an F/3.4 monochormator, and a five-stage photomultiplier. The
signals were captured by a Hewlett-Packard 54510A digitizing
oscilloscope, and the data were processed on a 286-based
computer system using software developed by Prof. C. Long
(Dublin). The experiments with excitation at 308 nm (excimer
laser) were carried out at the Department of Chemistry of the
University of Perugia. We thank Prof. F. Elisei for assistance
in these measurements.
Ca lcu la tion s. The singlet (1A′) and triplet (3B2) states of
+
the H2NC6H4 cation were optimized by the standard (U)-
C
12H11 N2Cl: C, 65.91; H, 5.07; N, 12.87. NMR: δH (CDCl3)
B3LYP method, using 6-31G(d) and 6-311+G(d,p) basis sets,
as implemented in the Gaussian 94 program.39 Both states
were characterized by harmonic frequency calculations at
B3LYP/6-31G(d) level. To confirm the nature of the stationary
points and to produce theoretical parameters, vibrational
frequencies (in the harmonic approximation) were calculated
by B3LYP/6-31G(d) and used with no scaling for computing
the zero point energies and their contributions to Gibbs free
energies. CASSCF(8,8)/6-31(d) calculations were carried out,
and the active space consisted of six π orbitals, the nitrogen
lone pair (pN), and the σ orbital located at the divalent carbon
atom.
3.9 (4H, broad, exch, NH2), 6.68 (1H, dd, J ) 8, 1.5 Hz), 6.79
and 7.25 (4H, AA′XX′, aromatics), 7.08 (2H, m,); δC (CDCl3)
115.2 (CH), 116.3 (CH), 122.9, 127.4 (CH), 128.1, 129.4, 129.7
(CH), 129.8 (CH), 142.2, 145.7.
6-(4′-(Dim et h yla m in o)p h en yl)-3-m et h ylen e-1,2,4,5,6-
p en ta m eth ylcycloh exa d ien e (9) was obtained as a mixture
with N,N-dim eth yl-4-(2,3,4,5,6-pen tam eth ylph en yl)an ilin e
(10) in a 3:2 ratio. The assignment of proton and carbon signals
for each compound resulted from 2D-NOESY and 2D-HSQC
NMR experiments. NOE correlations allowed the attribution
of methyl signals and proved the position of the methylene
group in cyclohexadiene derivative. Then the carbon signals
were completely attributed on the basis of 1H-13C correlations.
9: NMR δH (CDCl3) 1.45 (s, 3H, CH3-6), 1.55 (s, 6H, CH3-1
and CH3-5), 1.9 (s, 6H, CH3-2 and CH3-4), 2.95 (s, 6H, -NMe2),
5.0 (bs, 2H, CH2-3), 6.7 and 7.08 (4H, AA′XX′, aromatics). The
following cross-peaks were found in the NOESY spectrum: 5.0
with 1.9; 7.08 with 1.43 and 1.55; 1.55 with 1.9. NMR: δC
(CDCl3) 14.5 (CH3-2 and CH3-4), 16.6 (CH3-1 and CH3-5), 21.8
(CH3-6), 40.5 (NMe2), 49.7 (C-6), 102.7 (CH2-3), 112.6 (CH),
124.2, 127.9 (CH), 132.1, 132.3, 138.4, 142.5. GC/MS: tR 18.42
min; m/z 281 (48), 266 (85), 251 (100), 237 (15), 146 (15). 10:
NMR δH (CDCl3) 2.01 (s, 6H, CH3-2 and CH3-6), 2.25 (s, 6H,
CH3-3 and CH3-5), 2.3 (s, 3H, CH3-4), 3.05 (s, 6H, -NMe2),
6.9 and 7.02 (4H, AA′XX′, aromatics). The following cross-
Ack n ow led gm en t. Partial support of this work by
the CNR and MURST, Rome, is gratefully acknowl-
edged.
Su p p or tin g In for m a tion Ava ila ble: Complete compu-
tational results in the form of tables of Z-matrixes with the
computed total energies. This material is available free of
charge via the Internet at http://pubs.acs.org.
J O0104680
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