(Sensitized) photolysis of diazonium salts as a mild general method for the generation of aryl cations. Chemoselectivity of the singlet and triplet 4-substituted phenyl cations

Article abstract of DOI:10.1021/jo048413w

(Chemical Equation Presented) The photolysis of a series of 4-X-benzenediazonium tetrafluoroborates is studied in MeCN. Loss of nitrogen occurs from the singlet excited state with X = H, t-Bu, and NMe₂ and leads to the singlet aryl cation. This adds to the solvent yielding the corresponding acetanilides. With other substituents, ISC competes with (X = Br, CN) or overcomes (X = COMe, NO₂) fragmentation and the aryl cation is formed in part or completely in the triplet state. In neat MeCN, this either abstracts hydrogen from the solvent (in most cases inefficiently) or undergoes intersystem crossing to the more stable singlet that reacts as above. In the presence of π nucleophiles (allyltrimethylsilane or benzene), the triplet aryl cation is efficiently trapped giving substituted allylbenzenes and biphenyls, respectively. By triplet sensitization by xanthone, the triplet cation and the products from it are obtained from the whole series considered. The direct or sensitized photodecomposition of diazonium fluoroborates, substituted with both electron-donating and -withdrawing substituents, in the presence of alkenes and arenes offers an access to an alternative arylation procedure.

Full text of DOI:10.1021/jo048413w

(Sensitized) Photolysis of Diazonium Salts as a Mild General
Method for the Generation of Aryl Cations. Chemoselectivity of
the Singlet and Triplet 4-Substituted Phenyl Cations
Silvia Milanesi, Maurizio Fagnoni,* and Angelo Albini*
Dipartimento di Chimica Organica, Universita`, V. Taramelli 10, 27100 Pavia, Italy
fagnoni@unipv.it
Received September 9, 2004
The photolysis of a series of 4-X-benzenediazonium tetrafluoroborates is studied in MeCN. Loss of
nitrogen occurs from the singlet excited state with X ) H, t-Bu, and NMe2 and leads to the singlet
aryl cation. This adds to the solvent yielding the corresponding acetanilides. With other substituents,
ISC competes with (X ) Br, CN) or overcomes (X ) COMe, NO2) fragmentation and the aryl cation
is formed in part or completely in the triplet state. In neat MeCN, this either abstracts hydrogen
from the solvent (in most cases inefficiently) or undergoes intersystem crossing to the more stable
singlet that reacts as above. In the presence of π nucleophiles (allyltrimethylsilane or benzene),
the triplet aryl cation is efficiently trapped giving substituted allylbenzenes and biphenyls,
respectively. By triplet sensitization by xanthone, the triplet cation and the products from it are
obtained from the whole series considered. The direct or sensitized photodecomposition of diazonium
fluoroborates, substituted with both electron-donating and -withdrawing substituents, in the
presence of alkenes and arenes offers an access to an alternative arylation procedure.
Introduction
The alternative (photo)fragmentation to give an aryl
cation (path e) has been less frequently exploited.9,10 Such
reactions include the Balz-Schiemann fluorination,11 the
synthesis of phenols in an aqueous medium,12,13 and the
synthesis of aryl ethers in alcohols11a,14,15 but not the more
interesting formation of aryl-C bonds.
Many synthetic applications of diazonium salts1,2 in-
volve dediazoniation reactions, which allow C-C bond
formation. Typically, these involve radicals generated
through single-electron-transfer reduction of the salt,3 as
in the Meerwein arylation of olefins4 (Scheme 1, path a),
the Gomberg-Bachmann arylations of aromatics5 (path
b), and Pschorr syntheses.6 Alternatively, aryl-carbon
bonds have been formed by the palladium-mediated
reactions with aryl boronic acids7 (path c) or with olefins
in the Heck synthesis8 (path d).
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10.1021/jo048413w CCC: $30.25 © 2005 American Chemical Society
Published on Web 12/29/2004
J. Org. Chem. 2005, 70, 603-610
603

Milanesi et al.
SCHEME 2
The mechanism of both thermal and photochemical
diazonium salt decomposition has been extensively in-
vestigated. The thermal decomposition involves either the
radical or the cation depending on the medium and the
salt used.16 Water has been considered the best solvent
for generating the cation,13 though the presence of
micelles can modify the mechanism.17 Changing the
medium from a water-alcohol mixture14 to neat alcohol
significantly increases the likelihood that a radical will
be formed.15 Among polar nonprotic solvents, acetoni-
trile18,19 favors the cations, whereas in DMF20 or DM-
SO18,21 radicals are formed. The mild photochemical
decomposition affords a more convenient entry to aryl
cations.22
aryl esters24 and of (trifluoromethanesulfonyl)oxydi-
enynes.25 To our knowledge, there is no direct evidence
for this elusive species in solution.26 Recently, Steenken
and McClelland reported indirect evidence of a phenyl
cation by detecting its adduct to 1,3,5-triisopropylbenzene
upon laser flash photolysis of 4-methoxyphenyl diazo-
nium tetrafluoroborate.27
SCHEME 1
It is important to take into account that aryl cations
exist either in the singlet state or in the triplet state (see
Scheme 2). Calculations show that the former species is
a localized cation with a vacant σ orbital at the dicoor-
dinated carbon atom,28a while the triplet state has both
diradical and carbene character with single occupancy
of that orbital and the charge delocalized on the aromatic
ring and on the substituents in the o- or p-positions.28,29
The difference in the electronic distribution is expected
to be reflected in the chemoselectivity, but until recently
this had little experimental support, due to the difficult
access to aryl cations. Our interest in this field stemmed
from the finding that p-chloro (or fluoro)anilines undergo
photoheterolysis and generate the 4-aminophenyl cation
in solution in the triplet state (Scheme 2, right-hand
side).30 This intermediate reacted selectively with π
nucleophiles31 in contrast with the unselective reaction
with the solvent expected for the singlet cation. Syntheti-
cally appealing arylation reactions were developed in this
way.
Apart from diazonium salts, aryl cations have been
generated only under particular conditions, viz., the
irradiation of bromo - or iodobenzene in a solid argon
matrix at 4 K23 and by solvolysis of perfluoroalkylsulfonic
However, photoinduced loss of halide is thermodynami-
cally inaccessible for aromatics not bearing a strong
(24) (a) Himeshima, Y.; Kobayashi, H.; Sonoda, T. J. Am. Chem.
Soc. 1985, 107, 5286-5288. (b) Apeloig, Y.; Arad, D. J. Am. Chem.
Soc. 1985, 107, 5285-5286. (c) Subramanian, L. R.; Hanack, M.;
Chang, L. W. K.; Imhoff, M. A.; Schleyer, P. v. R.; Effenberger, F.;
Kurtz, W.; Stang, P. J.; Dueber, T. E. J. Org. Chem. 1976, 41, 4099-
4103. (d) Laali, K.; Szele, I.; Yoshida, K. Helv. Chim. Acta 1983, 66,
1710-1720.
(14) Pazo-Llorente, R.; Bravo-Diaz, C.; Gonzalez-Romero, E. Eur.
J. Org. Chem. 2003, 3421-3428.
(15) (a) Bunnett, J. F.; Yijima, C. J. Org. Chem. 1977, 42, 639-
643. (b) Bunnett, J. F.; Yijima, C. J. Org. Chem. 1977, 42, 643-649.
(c) DeTar, D. F.; Turetzky M. N. J. Am. Chem. Soc. 1955, 77, 1745-
1750. (d) DeTar, D. F.; Turetzky M. N. J. Am. Chem. Soc. 1956, 78,
3925-3928. (e) Bergstrom, R. G.; Landells, R. G. M.; Wahl, G. H., Jr.;
Zollinger, H. J. Am. Chem. Soc. 1976, 98, 3301-3305.
(16) Zollinger, H. Angew. Chem., Int. Ed. Engl. 1978, 17, 141-150.
(17) (a) Cuccovia, I. M.; da Silva, M. A.; Ferraz, H. M. C.; Pliego, J.
R. Jr.; Riveros, J. M.; Chaimovich, H. J. Chem. Soc., Perkin Trans. 2
2000, 1896-1907. (b) Chauduri, A.; Loughlin, J. A.; Romsted, L. S.;
Yao, J. J. Am. Chem. Soc. 1993, 115, 8351-8361. (c) Bravo-Diaz, C.;
Romero-Nieto, M. E.; Gonzalez-Romero, E. Langmuir 2000, 16, 42-
48.
(25) (a) Holweger, W.; Hanack, M. Chem. Ber. 1984, 117, 3004-
3020. (b) Bleckmann, W.; Hanack, M. Chem. Ber. 1984, 117, 3021-
3033. (c) Hanack, M.; Rieth, R. Chem. Ber. 1987, 120, 1659-1666.
(26) (a) Scaiano, J. C.; Kim-Thuan, N. J. Photochem. 1983, 23, 269-
276. (b) Smith, D. M.; Maksic, Z. B.; Maskill, H. J. Chem. Soc., Perkin
Trans. 2 2002, 906-913.
(27) Steenken, S.; Ashokkumar, M.; Maruthamuthu, P.; McClelland,
R. A. J. Am. Chem. Soc. 1998, 120, 11925-11931.
(28) (a) Aschi, M.; Harvey, J. N. J. Chem. Soc., Perkin Trans. 2 1999,
1059-1061. (b) Nicolaides, A.; Smith, D. M.; Jensen, F.; Radom, L. J.
Am. Chem. Soc. 1997, 119, 8083-8088. (c) Hori, K.; Sonoda, T.; Harada,
M.; Yamazaki-Nishida, S. Tetrahedron 2000, 56, 1429-1436. (d) Laali,
K. K.; Rasul, G.; Surya Prakash, G. K.; Olah, G. A. J. Org. Chem. 2002,
67, 2913-2918.
(18) (a) Ishida, K.; Kobori, N.; Kobayashi, M.; Minato, H. Bull. Chem.
Soc. Jpn. 1970, 43, 285-286. (b) Kamigata, N.; Kobayashi, M.; Minato,
H. Bull. Chem. Soc. Jpn. 1972, 45, 2047-2050. (c) Kamigata, N.;
Hisada, R.; Minato, H.; Kobayashi, M. Bull. Chem. Soc. Jpn. 1973,
46, 1016-1018.
(19) Kobayashi, M.; Minato, H.; Yamada, E.; Kobori, N. Bull. Chem.
Soc. Jpn. 1970, 43, 215-219.
(20) Markgraf, J. H.; Chang, R.; Cort, J. R.; Durant, J. L., Jr.;
Finkelstein, M.; Gross, A. W.; Lavyne, M. H.; Moore, W. M.; Petersen,
R. C.; Ross, S. D. Tetrahedron 1997, 53, 10009-10018.
(21) Kobori, N.; Kobayashi, M.; Minato, H. Bull. Chem. Soc. Jpn.
1970, 43, 223-225.
(22) Zollinger, H. In The Chemistry of Triple-bonded Functional
Groups; Patai, S., Rappoport, Z., Eds.; Wiley & Sons: Chichester, 1983;
Part 1, pp 603-669.
(23) Winkler, M.; Sander, W. Angew. Chem., Int. Ed. 2000, 39,
2014-2016.
(29) Ambroz, H. B.; Kemp, T. J. Chem. Soc. Rev. 1979, 8, 353-365.
(30) (a) Guizzardi, B.; Mella, M.; Fagnoni, M.; Freccero, M.; Albini,
A. J. Org. Chem. 2001, 66, 6353-6363. (b) Mella, M.; Coppo, P.;
Guizzardi, B.; Fagnoni, M.; Freccero, M.; Albini, A. J. Org. Chem. 2001,
66, 6344-6352.
(31) (a) Guizzardi, B.; Mella, M.; Fagnoni, M.; Albini, A. Tetrahedron
2000, 56, 9383-9389. (b) Guizzardi, B.; Mella, M.; Fagnoni, M.; Albini,
A. J. Org. Chem. 2003, 68, 1067-1074. (c) Mella, M.; Esposti, S.;
Fagnoni, M.; Albini. A. Chem. Commun. 2003, 738-739. (d) Guizzardi,
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4886-4893.
604 J. Org. Chem., Vol. 70, No. 2, 2005

(Sensitized) Photolysis of Diazonium
SCHEME 3
electron-donating substituent such as the amino group
or a hydroxy(methoxy) group that stabilizes the positive
charge formed. A far better leaving group should make
diazonium salts suitable candidates for a generally
applicable generation of phenyl cations and, we hoped,
for C-C bond-forming reactions.
Previously, Schuster et al. showed that substituents
affect the result and irradiation of 4-diethylamino or
4-morpholino benzenediazonium salts gave products
resulting from the singlet phenyl cation, while the
4-benzoyl derivative underwent radical chain decomposi-
tion in 2,2,2-trifluoroethanol (TFE) initiated by triplet
phenyl cations.10
On the basis of the above, we surmised that a large-
scope ionic arylation based upon the photodecomposition
of diazonium salts in the presence of C-nucleophiles could
be developed provided that (a) thermal processes, whether
dediazoniation or thermal addition to the nucleophile, are
negligible with respect to photofragmentation in the time
scale used; (b) no reagent that may cause reduction of
the salt and the formation of the aryl radical is present;
and (c) the aryl cation is selectively generated in the
triplet state.
Therefore, we embarked on a systematic examination
of the photodecomposition of 4-substituted benzenedia-
zonium salts to allow the formation of either singlet or
triplet phenyl cations to be determined, and possibly
directed, and the scope of the arylation by this pathway
to be ascertained. In the following we report the photo-
decomposition of a series of diazonium tetrafluoroborates
1a-g in neat MeCN and in the presence of representative
π nucleophiles.32
TABLE 1. Products from the Irradiation of Diazonium
Fluoroborates 1 in MeCN
product yield %a
direct irradiation
sensitized irradiation
salt
X
2
3
4
2
3
4
1a
1b
1c
1d
1e
1f
NO2
CH3CO
CN
24
29
5
37
69
41
76
86
68
57
49
67
b
68
29
7
Br
3
H
t-Bu
NMe2
b
b
4
b
5
1g
2
a GC yields; no figure is reported when below 0.5%. b Not
determined.
SCHEME 4
Results
Seven 4-substituted benzenediazonium tetrafluorobo-
rates were selected for a study ranging from those
bearing electron-withdrawing groups (X ) NO2, 1a; X )
COCH3, 1b; X ) CN, 1c; X ) Br, 1d) to the parent
benzenediazonium salt (X ) H, 1e) to those bearing
electron-donating substituents (X ) t-Bu, 1f; X ) NMe2,
1g). Acetonitrile (0.1% water content) was chosen as the
reaction medium as a polar solvent, which effectively
dissolved all of the salts and reagents used (see Discus-
sion for further details). To assess the contribution of
thermal decomposition in the experiments, blank tests
were routinely carried out under the irradiation condi-
tions by covering the test tubes with aluminum foil. Only
in the case of 1a was there a slight consumption (<10%)
of the salt during the time required for complete photo-
decomposition. Photolyses were carried out in a multil-
amp apparatus by means of lamps with the maximum
of emission at 310 or 360 nm. Irradiation at 310 nm was
convenient for the direct photodegradation of the salts
(c ) 0.05 M), though 360 nm could be used equally well
for 1a, 1b, and 1g. In the sensitized experiments with
xanthone (or benzophenone), 360 nm lamps were used
(310 nm in the case of 1g). In these experiments, the
diazonium salt concentration was in some cases de-
creased (down to 0.01 M) in order to ensure a >95%
absorption by the sensitizer (see below and Experimental
Section).
Irradiation of 1a-g in MeCN at 20 °C caused effective
photodediazoniation (total conversion required 3 h for 1a
and 1 h for the other salts). Photolysis of nitroderivative
1a yielded nitrobenzene 2a in a modest yield as the only
product (Scheme 3 and Table 1).
Hydrodediazoniation giving 2b was observed also in
the case of 1b (29% acetophenone), but acetanilide 4b
was formed in a larger amount (37%) (see Scheme 3). In
view of the difference with 1a, photosensitization by
aromatic ketones was tested. With xanthone (absorbing
> 95% of the light), reduction was the exclusive path
(57% of 2b formed). Cyanophenyl diazonium salt 1c gave
acetanilide 4c as by far the main product (69%) ac-
companied by small amounts of benzonitrile 2c and
4-fluorobenzonitrile 3c (5 and 7%, respectively). Again,
(32) Preliminary communication of part of these data: Milanesi, S.;
Fagnoni, M.; Albini A. Chem. Commun. 2003, 216-217.
J. Org. Chem, Vol. 70, No. 2, 2005 605

Milanesi et al.
TABLE 2. Products from the Irradiation of Diazonium
SCHEME 5
Fluoroborates 1 in the Presence of π Nucleophiles
product yield %a
direct
irradiation
sensitized
irradiation
6
6
salt
X
nucleophileb (or 7)
2
3
4
(or 7)
2
3
4
1a NO2
1a NO2
1b CH3CO
1b CH3CO
1c CN
5
51
74, 87c
50
40
47
44
25
15
5
5
C6H6
5
6
14
18
5
C6H6
5
C6H6
5
C6H6
5
C6H6
5
xanthone sensitization made reduction to benzonitrile 2c
the exclusive path.
1
7
48
6
1c CN
4
9
35 84 10
The same trend as with 1c was followed in the
photodegradation of the remaining salts 1d-g. These
gave acetanilides 4d-g, with trace amounts of 2 and 3
by direct irradiation, while substitution of the diazo group
by a hydrogen predominated upon xanthone or, with
somewhat lower efficiency, benzophenone sensitization.
The reaction was then explored in the presence of two
π nucleophiles, viz., allyltrimethylsilane (5) (1 M) and
benzene (3.75 M). Since diazonium salts, particularly the
most electrophilic derivatives, are known to form com-
plexes with aromatic donors,33 the UV spectra in the
presence of the above additives were tested. No signifi-
cant variation of the UV spectra was observed in the case
of salts 1b and 1e-g, while a modest variation took place
with 1d. A more conspicuous variation occurred in the
case of 1c, as well as with 1a, but then only with 5 (see
Supporting Information).
With these additives, direct irradiation of 1a gave
p-allylnitrobenzene (6a) and p-nitrobiphenyl (7a), re-
spectively, both in >50% yield, while the yield of 2a was
reduced to 5% (in fact, 87% 7a was achieved with 7.5 M
benzene, despite the fact that 1a was not completely
dissolved in the initial reaction mixture) (Scheme 4 and
Table 2). Likewise with 1b, the arylation products 6b and
7b were formed at the expense of acetanilide 4b (sup-
pressed) and acetophenone 2b (decreased).
1d Br
1d Br
2
<2 41 58 29
7
62 54 26
81 47
60 32
1e
1e
H
H
d
d
d
d
d
d
d
6
5
9
9
d
1f t-Bu
1f t-Bu
1g NMe2
1g NMe2
2
4
89 48 34
70 17 61
87 55 13
C6H6
5
C6H6
12
2
12
3
25
2
63 41
2
a Isolated yields for compounds 6 and 7; GC yields in the other
cases; no figure is reported when below 0.5%. b Reaction in the
presence of either 1 M 5 or 3.75 M benzene. c Reaction in the
presence of 7.5 M benzene. d Not determined.
TABLE 3. Quantum Yield of Photodecomposition of
Diazonium Fluoroborates 1a-g in MeCN34
diazonium salt
quantum yield
1a, X ) NO2
1b, X ) COCH3
1c, X ) CN
1d, X ) Br
1e, X ) H
1f, X ) t-But
1g, X ) NMe2
<0.1
0.21
0.35
0.34
0.50
0.38
0.52
main product by sensitization. Acetanilide 4g remained
the main product from 1g by direct irradiation in the
presence of the above nucleophiles (87 and 63% yields,
respectively), along with 2% 6g and 25% 7g. However,
the yields of the last compounds increased to 55 and 41%,
respectively, upon sensitization.
Quantum Yield Measurement. Quantum yields of
decomposition of compounds 1a-g were determined by
photolysis of the salts in MeCN at 313 nm. The extent of
decomposition (10-20%) was determined by UV absor-
bance of the azo derivative formed after coupling with
â-naphthol (see Experimental Section). The results are
reported in Table 3.
With the cyanoderivative 1c, arylation to 6c and 7c
occurred in ca. 50% yield, with some reduction in the
amount of 4c, 2c, and 3c with respect to the experiments
in neat solvent. Triplet sensitization was effective and
increased the formation of aryl-carbon bond, particularly
for the case of 4-cyanobiphenyl (7c) in the presence of
benzene (84% yield compared to 44% by direct irradia-
tion, see Table 2). Similar to the experiment in neat
solvent, no acetanilide 4c was formed by photosensiti-
zation.
Arylated derivatives 6d and 7d were obtained in
modest yields (<25%) by direct irradiation of 1d, and the
presence of the nucleophiles did not decrease the forma-
tion of the acetanilide 4d in this case. Upon sensitization,
4d was not formed, and the yield of the 4-allyl- and
phenyl bromobenzenes (accompanied by ca. 30% 2d)
increased. Likewise, 1e gave 6e and 7e only by xanthone
sensitization (see Table 2).
Quantum yields were moderately affected by changing
the substituent X and in every case did not exceed unity.
Strong electron-withdrawing substituents such as the
nitro group and the acetyl group increased the photosta-
bility of the salts to some extent, especially in the case
of 1a (Φ < 0.1). The quantum yield value was slightly
higher in the case of salts 1a-c when measured at a
more extensive decomposition.
4-tert-Butyldiphenyl (7f) was formed in a low yield,
both upon direct and sensitized irradiation (with 61% 2f
in the latter case), while 4-tert-butylbenzene (6f) was the
Discussion
Choice of Solvent. The aim of this work is to explore
the conditions for the addition of phenyl cations to π
nucleophiles. The choice of solvent depends on the ability
to dissolve both the precursor diazonium salts and
benzene and allyltrimethylsilane, as well as to favor the
(33) (a) Bockman, T. M.; Kosynkin, D.; Kochi, J. K. J. Org. Chem.
1997, 62, 5811-5820. (b) Kosynkin, D.; Bockman, T. M.; Kochi, J. K.
J. Chem. Soc., Perkin Trans. 2 1997, 2003-2012. (c) Kosynkin, D.;
Bockman, T. M.; Kochi, J. K. J. Am. Chem. Soc. 1997, 119, 4846-
4855.
606 J. Org. Chem., Vol. 70, No. 2, 2005

(Sensitized) Photolysis of Diazonium
heterolytic rather than the homolytic decomposition of
the salt. Water and alcohols (e.g., MeOH or a fluorinated
alcohol such as TFE) are able to stabilize the charged
intermediates formed22 but have some drawbacks. As an
example, water does not dissolve most organic reagents
and may compete as a nucleophile for the cation. The
high hydrogen donor ability of methanol can lead to a
radical chain upon photolysis of diazonium salts, par-
ticularly of the most electrophilic ones. As an example,
4-nitrobenzenediazonium chloride was suggested to give
the aryl cation in water but radicalic species in EtOH or
MeOH.35 Furthermore, even the less electrophilic 4-me-
thylbenzenediazonium salt underwent free-radical reduc-
tion upon pulse radiolysis in MeOH.36 In TFE, the
formation of aryl radicals is less likely than in unfluori-
nated alcohols but cannot be excluded.10 The excellent
ion stabilization by TFE37 is in part counterbalanced by
its high price and the tendency to form aromatic fluorides
by fluoride transfer to the aryl cation intermediate.9b
Moreover, both alcohols and water may compete as
nucleophiles and give phenol13,38 and aryl ethers.15 Ac-
etonitrile was chosen as a reasonably polar solvent and
a poor hydrogen donor. Benzene and allyltrimethylsilane
were chosen as nucleophiles because they have been
previously shown to give clean reactions and high yields
in the trapping of (triplet) 4-aminophenyl cation.30
Mechanism and Scope of the Reaction. The reac-
tions were carried out at a relatively low temperature
(ca. 20 °C), and both the solvent and the diazonium salts
were freshly purified in order to avoid impurity-induced
photodecomposition side reactions. Under these condi-
tions, arylallylazo and diarylazo compounds, which may
be expected from the thermal reaction between 1a-g and
5 and benzene, respectively, were not formed during the
time required for decomposition. Mayr39a and others40
introduced parameters for the electrophilicity of diazo-
nium salts and compared such values with the nucleo-
philicity of olefins and aromatics. This allows the pre-
diction of which diazonium salt is able to attack a specific
nucleophile under thermal conditions.39a Accordingly,
benzene is not expected to react with any of the salts used
in this work, while azo-coupling involving 539 is expected
to occur with 1a,c41 but only marginally with 1d. The
analysis of the photoproducts evidences that in all of the
cases, including the best electrophile of the series, 1a,
photodediazoniation occurs faster than thermal azo-
coupling. Thus, arylation via photodecomposition of dia-
zonium salts can be extended, at least in principle, to
several nucleophilic olefins and (hetero)aromatics, pro-
vided that the competing thermal coupling is sufficiently
slow at room temperature (or below).
All of the photoproducts obtained can be rationalized
as arising from aryl cations, and the dependence of the
chemistry occurring on the structure of the 4-substituted
phenyl cation is the key point of this work. Calculations
show that π-donating substituents (F, OH, NH2) stabilize
the triplet state much more than the singlet state.28a
Actually, only the 4-aminophenyl cation has the triplet
state as the lowest state, while in the 4-hydroxy deriva-
tive the two spin states are close in energy. In all the
other cases, the singlet is the lowest state.28 Due to the
singlet multiplicity of the diazonium salt ground state
and the energy order of the aryl cations, in thermal
decomposition only the singlet aryl cation chemistry is
expected and indeed observed.13-21 This is not necessarily
the case for the photodecomposition.
Direct irradiation of 1b-g in neat acetonitrile leads
mainly to the corresponding acetanilides. These are
formed by reaction of the photogenerated aryl cation with
the solvent to yield a nitrilium ion, which adds moisture
present in the solvent, in a way analogous to the Ritter
reaction42 (see Scheme 5).
This has not been previously reported in photolyses but
has been in the thermal decomposition (at 40 °C) of
benzenediazonium tetrafluoroborate in acetonitrile.19,43
The reaction is safely attributed to the attack by the
singlet aryl cation to a poor nucleophile such as aceto-
nitrile. The overall yield of formation of product 4 is high
from 1c-g. In some cases, a small amount of fluorinated
compounds 3 is detected, reasonably via reaction with
the fluoroborate counterion, as established in related
cases.9b The presence of nitro or acetyl substituents
changes significantly the photoreactivity, and the aceta-
nilide is accompanied (in the case of 1b) or substituted
(in the case of 1a) by the hydrodediazoniated derivatives
2a,b. Photoreduction to 2c-g is a minor path for the
other salts but becomes the main reaction upon sensiti-
zation by aromatic ketones. The interaction between
triplet excited sensitizers and the diazonium salts has
been studied by two independent groups. The first
suggested an electron-transfer process causing reduction
of the diazonium salt and formation of an aryl radical.44
Thus, sensitization by aromatic ketones in acetonitrile
led to the corresponding aromatic hydrocarbons.45 How-
ever, Scaiano and co-workers demonstrated that energy
transfer occurs with high-energy triplet sensitizers such
as benzophenone or xanthone, while electron transfer
occurs with low-energy sensitizers such as anthracene
or pyrene.46
Under our conditions, photoreduction is the only
process in the presence of xanthone. The result is
completely different from direct irradiation and is at-
tributed to the triplet aryl cation. This abstracts a
hydrogen from the medium, forming a radical cation that
in turn is reduced to the hydrodediazoniated product 2
(34) Quantum yield values have been redetermined with respect to
the data in ref 32.
(35) Lee, W. E.; Calvert, J. G.; Malmberg, E. W. J. Am. Chem. Soc.
1961, 83, 1928-1934.
(36) Packer, J. E.; House, D. B.; Rasburn, E. J. J. Chem. Soc. B 1971,
1574-1578.
(42) Ho, T.-L.; Chein, R.-J. J. Org. Chem. 2004, 69, 591-592.
(43) Analoguous benzanilides were obtained from the irradiation of
imidate esters through the intermediacy of an N-arylbenzonitrilium
ion. See: Ruane, P. H.; Ahmed, A. R.; McClelland, R. A. J. Chem. Soc.,
Perkin Trans. 2 2002, 312-317.
(37) For more details on the use and the characteristics of fluori-
nated alcohols as solvents in organic synthesis, see: Be´gue´, J.-P.;
Bonnet-Delpon, D.; Crousse, B. Synlett 2004, 18-29.
(38) Lewis, E. S.; Holliday, R. E.; Hartung, L. D. J. Am. Chem. Soc.
1969, 91, 430-433.
(44) Baumann, H.; Becker, H. G. O.; Kronfeld, K. P.; Pfeifer, D.;
Timpe, H.-J. J. Photochem. 1985, 28, 393-403.
(39) (a) Mayr, H.; Hartnagel, M.; Grimm, K. Liebigs Ann. 1997, 58,
55-69. (b) Mayr, H.; Grimm, K. J. Org. Chem. 1992, 57, 1057-1059.
(40) Perez, P. J. Org. Chem. 2003, 68, 5886-5889.
(41) No data on 1b are reported in ref 39a, but we expect that the
behavior of this salt is similar to that of 1a and 1c.
(45) Timpe, H.-J.; Kronfeld, K.-P.; Schiller, M. J. Photochem. Pho-
tobiol. A 1991, 62, 245-259.
(46) Scaiano, J. C.; Kim-Thuan, N. Can. J. Chem. 1982, 60, 2286-
2291.
J. Org. Chem, Vol. 70, No. 2, 2005 607

Milanesi et al.
SCHEME 6
aryl cation intermediate from the solvent (SolvH )
MeOH15a,b or TFE10) has been envisaged (Scheme 7).
Initially, hydrogen abstraction leads to the formation
of Solv• and the radical cation of the hydrodediazoniated
product (eq a). In solution, easily oxidized radicals such
•
as CH2OH are known to reduce diazonium salts to the
corresponding radicals45,48 (eq b). A further hydrogen
abstraction gives rise to the reduced aromatic (ArH) and
radical Solv• is regenerated and continues the chain (eq
c). The termination of the chain involves a radical-
radical cation combination (eq d).
SCHEME 7
(Scheme 6). This path has been demonstrated for triplet
4-aminophenyl cation generated in MeCN by photolysis
of 4-chloroanilines.30
In the present case (SolvH ) MeCN), the decomposi-
tion quantum yields of 1a-g (Φ < 1 in all cases) support
that no radical chain process is involved, as indicated also
by the fact that addition of the nucleophiles has a
minimal effect on the reaction times (except for 1a). The
difference is that with a poor electron donor such as
•CH2CN, step b (Scheme 7) is much less efficient than
Alternatively, aryl cations in the triplet state are
trapped by 5 (forming products 6) or benzene (yielding
products 7) (Scheme 6). In the first case, a phenonium
ion can be envisaged as the intermediate31b,47 and gives
6 via elimination of the good positive leaving group
trimethylsilyl cation. Analogously, addition onto benzene
gives a cyclohexadienyl cation that deprotonates to 7.
Fast elimination of a proton or a trimethylsilyl group
from the adduct cation avoids competing paths (e.g.,
polymerization for olefins).
•
with CR2OH, at least with moderate electron acceptors
such as the present diazonium salts. Photodecomposition
in acetonitrile is generally less efficient than reported in
hexafluoro 2-propanol (Φ ca. 0.9 for all of the salts
investigated)27 but follows the same trend, with strong
electron-withdrawing substituents decreasing the pho-
toreactivity.12
On the basis of the above, a comprehensive mechanism
for the photodegradation of diazonium salts is outlined
in Scheme 8. Irradiation of compounds 1a-g leads to a
Efficient trapping requires a high concentration of the
π nucleophiles (1 M for 5 and 3.75 M for benzene) due to
the short lifetime of the intermediate.
Under our conditions, some complexation of most of
the diazonium salts 1 and π nucleophiles occurs, as
indicated by small shifts of the absorption spectra, more
apparent in the case of 1a,c (see the spectra at two
different concentrations in Supporting Information).
Since the effect is varied and at any rate moderate, while
the chemistry observed is uniform over the series, we
conclude that excitation of EDA complexes has no major
role. Thus, in the present case, the EDA route to aryl
radicals observed by Kochi33 with salts that are more
easily reduced than the present ones is not important.
There is indication in the literature that the photode-
composition of diazonium salts can occur with a quantum
yield significantly greater than unity. This has been
observed when the salt is strongly electrophilic and the
medium is a good hydrogen donor (e.g., an alcohol). As
an example, Kochi reported quantum yields ranging from
1.5 to 11 in the irradiation of 3,5-dinitro-, 2,4,6-trichloro-,
and 3,5-difluorobenzenediazonium salts in acetonitrile
via an excited complex in the presence of aromatic
hydrocarbons.33c Furthermore, a quantum yield up to
eight has been reported in the photodecomposition of a
4-methylbenzenediazonium derivative in aqueous solu-
tion when MeOH is added to the solution.36 A radical
chain reaction initiated by hydrogen abstraction by the
SCHEME 8
population of the singlet excited state (11). The nature
of the substituents present on the aromatic ring directs
the following steps of the reaction. In the case of 1e-g,
direct dediazoniation affords singlet aryl cations 18
(Scheme 8) and acetanilides 4 by addition to acetonitrile,
a n nucleophile. With the other salts, intersystem cross-
ing (ISC) from 1 to the corresponding triplet (31) takes
1
place in part (for salts 1c,d) or completely (for 1a and
1b) before fragmentation.
(47) B3LYP calculations have shown that the analogous 4-ami-
nophenyl cation adds to ethylene to form a triplet phenonium; the
corresponding singlet is much more stabilized, and intersystem cross-
ing back to the singlet surface reasonably occurs at this configuration.
See ref 30a.
(48) Scaiano, J., C.; Kim-Thuan, N. J. Photochem. 1983, 21, 167-
170.
608 J. Org. Chem., Vol. 70, No. 2, 2005

(Sensitized) Photolysis of Diazonium
Cleavage of the triplet yields the triplet aryl cation 38.
In neat solvent, hydrogen transfer from acetonitrile to
38 yields reduced derivatives 2 (Scheme 6). Since MeCN
is a poor hydrogen donor, the process is slow and
equilibration of the aryl cation to the more stable singlet
state is competitive with 38b and 38c. As a result, a
certain amount of compounds 4b and 4c is formed in
these cases. When π nucleophiles are added, there is no
change in the product distribution for 1e-g, since singlet
aryl cations are formed and then trapped by the solvent,
with only small amounts of compounds 6 and 7. Other
substituents favor ISC to 31 and enhance the triplet
cation chemistry. This is evidenced by addition onto π
nucleophiles, a much faster process than hydrogen
abstraction from acetonitrile, especially when a large
amount of the nucleophile is present. The bromine atom
in 1d and the cyano group in 1c exert such an effect. With
a nitro group (1a) or an acetyl group (1b), ISC is faster
scope. The general application of the SN1 reactions via
photogenerated aryl cations is currently under investiga-
tion in our laboratory.
Experimental Section
NMR spectra were recorded on a 300 MHz spectrometer.
Structure attributions were made on the basis of analytical
1
data and of H and 13C NMR (chemical shifts are reported in
parts per million downfield from TMS). The photochemical
reactions were performed at 20 °C in quartz tubes by using a
nitrogen-purged solution and a multilamp reactor fitted with
six 15 W phosphor-coated lamps (maximum emission of 310
or 360 nm) for the irradiation.
Diazonium salts were prepared by a known procedure10 and
purified by dissolving in acetonitrile or acetone and precipita-
tion by adding Et2O. Authentic samples of the acetanilides
4a-g were prepared from the corresponding anilines, crystal-
lized from ethanol, and used as standards. Acetonitrile was
used as received, and the water content was determined by
Karl-Fischer titration.52
The reaction course was followed by monitoring the disap-
pearance of the salt up to the point when no color change
occurred upon addition of some drops of an aqueous basic
solution of â-naphthol. Diluted aqueous NaHCO3 was added
to the irradiated solutions, and the product distribution was
monitored by GC analysis after extraction with diethyl ether.
Preparative experiments involved concentration in vacuo of
the photolyzed solution followed by chromatographic separa-
tion on silica gel.
Quantum Yield. Quantum yield measurements were made
on an optical bench fitted with a focalized high-pressure
mercury arc and an interference filter (transmission maximum
of 313 nm). The consumption of the salt was determined by
monitoring the absorption of the azo derivative formed upon
treatment with a few drops of an aqueous basic solution of
â-naphthol.
Thermal Stability of Salts 1. All irradiations reported
below were repeated under the same conditions with the tube
covered by aluminum foil. Diazonium salts 1 remained practi-
cally undecomposed in most cases (the maximal consumption
observed was in the case of 1a, <10%).
Irradiation of Diazonium Salts 1. General Procedure.
A 0.05 M solution of 1 in MeCN (prepared under shaded light)
was irradiated at 310 nm at 20 °C up to complete consumption
of the salt; the same experiments were carried out in the
presence of either allyltrimethylsilane (1 M) or benzene (3.75
or 7.5 M). In sensitized irradiation experiments, diazonium
salts 1 ((1-5) × 10-2 M) and xanthone ((4-6) × 10-2 M) were
dissolved in MeCN and irradiated at 360 nm (310 for the case
of 1g, see below). Benzophenone (5 × 10-2 M) was also used
in the place of xanthone in the photodegradation reactions of
salts 1e,f giving a lower yield of products 6 and 7. In the
synthesis of compounds 6g and 7g, the radiation was filtered
by means of a 1.5 × 10-2 M solution of o-nitroaniline in order
to prevent direct absorption by the diazonium salt.
4-(2-Propenyl)-nitrobenzene (6a). From 355 mg (1.5
mmol, 5 × 10-2 M) of 1a and 4.7 mL (30 mmol, 1 M) of 5 in 30
mL of MeCN irradiated at 310 nm. After column chromatog-
raphy (C6H12/EtAc 98/2), 124 mg of the title compound 6a (oil)
was isolated (51% yield). Spectroscopic data were in accordance
with the literature.50
1
than cleavage of 1 and the triplet aryl cation is exclu-
sively formed as revealed by the efficient reaction with
benzene and 5; however, in neat solvent, ISC from 38 to
18 occurs to some extent with 1b and gives some 4b along
with the hydrogen abstraction product 2b.
3
However, access to 8 can be made general by sensi-
tization by aromatic ketones, and under these conditions
reduction to 2 in neat solvent and arylation of benzene
and of 5 in the presence of such additives are the
predominant processes with all of the diazonium salts
considered.
Conclusion
The photodecomposition of a series of benzenediazo-
nium salts with substituents of various electronic nature
at position 4 is a convenient method for the generation
of aryl cations. This widens the scope of phenyl cation
photogeneration previously limited to the highly stabi-
lized 4-aminophenyl or 4-hydroxy(methoxy)phenyl cation
obtained by photolysis of chloroanilines30,31 and chlo-
rophenols(anisoles),49 respectively. The nature of the
substituents and the choice of direct or sensitized ir-
radiation direct the formation of phenyl cations in either
spin state, each of which shows a distinctive chemistry.
The triplet ion in every case can be obtained by photo-
sensitization.
The reaction of triplet aryl cations with π nucleophiles
offers a new aryl SN1 substitution with formation of an
aryl-carbon bond under mild, metal-free conditions. This
method can be applied to the synthesis of biaryls (a very
important topic at the moment, due to the wide variety
of applications of the products formed)6 and can be
considered both an ionic analogue of the Gomberg reac-
tion and a possible alternative to the palladium-mediated
Suzuki coupling. Furthermore, the reactions between aryl
cations and 5 offer a general method for the allylation of
aromatics, up to now obtained only through aryl radical50
and metal-mediated reactions,51 which have a different
4-Nitrobiphenyl (7a). From 355 mg (1.5 mmol, 5 × 10-2
M) of 1a and 10 mL (112.6 mmol, 3.75 M) of benzene in 20
mL of MeCN irradiated at 310 nm. After column chromatog-
raphy (C6H12/EtAc 98/2), 218 mg of the title compound 7a
(49) Protti, S.; Fagnoni, M.; Mella, M.; Albini, A. J. Org. Chem. 2004,
69, 3465-3473.
(50) Ek, F.; Axelsson, O.; Wistrand, L.-G.; Frejd, T. J. Org. Chem.
2002, 67, 6376-6381.
(51) (a) Littke, A. F.; Schwarz, L.; Fu, G. C. J. Am. Chem. Soc. 2002,
124, 6343-6348. (b) Gomes, P.; Gosmini, C.; Perichon, J. Org. Lett.
2003, 5, 1043-1045. (c) Ikegami, R.; Koresawa, A.; Shibatha, T.;
Takagi, K. J. Org. Chem. 2003, 68, 2195-2199.
(52) Use of acetonitrile with a higher water content (up to some
percent) did not affect the overall arylation yields in sensitized
experiments.
J. Org. Chem, Vol. 70, No. 2, 2005 609

Milanesi et al.
(colorless solid) was isolated (74% yield). Mp 112-114 °C (lit.53
112-113 °C). Spectroscopic data were in accordance with the
literature.54
4-(2-Propenyl)-acetophenone (6b). From 351 mg (1.5
mmol, 5 × 10-2 M) of 1b and 4.7 mL (30 mmol, 1 M) of 5 in 30
mL of MeCN irradiated at 310 nm. After column chromatog-
raphy (C6H12/EtAc 95/5), 120 mg of the title compound 6b (oil)
was isolated (50% yield). Spectroscopic data were in accordance
with the literature.55
4-Phenylacetophenone (7b). From 351 mg (1.5 mmol, 5
× 10-2 M) of 1b and 10 mL (112.6 mmol, 3.75 M) of benzene
in 20 mL of MeCN irradiated at 310 nm. After column
chromatography (C6H12/EtAc 98/2), 118 mg of the title com-
pound 7b (colorless solid) was isolated (40% yield). Mp 116-
118 °C (lit.56 119-120 °C). Spectroscopic data were in accor-
dance with the literature.57
4-(2-Propenyl)-benzonitrile (6c). From 325 mg (1.5 mmol,
5 × 10-2 M) of 1c, 4.7 mL (30 mmol, 1 M) of 5, and 235 mg
(1.2 mmol, 4 × 10-2 M) of xanthone in 30 mL of MeCN
irradiated at 366 nm. After column chromatography (C6H12/
EtAc 99/1), 102 mg of the title compound 6c (oil) was isolated
(48% yield). Spectroscopic data were in accordance with the
literature.58
4-Phenylbenzonitrile (7c). From 325 mg (1.5 mmol, 5 ×
10-2 M) of 1c, and 10 mL (112.6 mmol, 3.75 M) of benzene,
and 235 mg (1.2 mmol, 4 × 10-2 M) of xanthone in 20 mL of
MeCN irradiated at 366 nm. After column chromatography
(C6H12/EtAc 97.5/2.5), 225 mg of the title compound 7c
(colorless solid) was isolated (84% yield). Mp 85-87 °C (lit.59
86-87 °C). Spectroscopic data were in accordance with the
literature.54
4-(2-Propenyl)-bromobenzene (6d). From 162 mg (0.6
mmol, 2 × 10-2 M) of 1d, 4.7 mL (30 mmol, 1 M) of 5, and 235
mg (1.2 mmol, 4 × 10-2 M) of xanthone in 30 mL of MeCN
irradiated at 366 nm. After column chromatography (C6H12/
EtAc 95/5), 68 mg of the title compound 6d (oil) was isolated
(57.5% yield). Spectroscopic data were in accordance with the
literature.50
at 366 nm. After column chromatography (C6H12/EtAc 97/3),
83 mg of the title compound 6e (oil) was isolated (47% yield).
Spectroscopic data were in accordance with a commercial
sample (Fluka).
Biphenyl (7e). From 290 mg (1.5 mmol, 5 × 10-2 M) of 1e,
10 mL (112.6 mmol, 3.75 M) of benzene, and 235 mg (1.2 mmol,
4 × 10-2 M) of xanthone in 20 mL of MeCN irradiated at 366
nm. After column chromatography (neat C6H12), 74 mg of the
title compound 7e (colorless solid) was isolated (32% yield).
Mp 66-68 °C (Fluka catalog 68-70 °C). Spectroscopic data
were in accordance with a commercial sample (Fluka).
4-(2-Propenyl)-tert-butylbenzene (6f). From 372 mg (1.5
mmol, 5 × 10-2 M) of 1f, 4.7 mL (30 mmol, 1 M) of 5, and 235
mg (1.2 mmol, 4 × 10-2 M) of xanthone in 30 mL of MeCN
irradiated at 366 nm. After column chromatography (neat
C6H12), 125 mg of the title compound 6f (oil) was isolated (48%
yield). Spectroscopic data were in accordance with the litera-
ture.61
4-tert-Butylbiphenyl (7f). From 372 mg (1.5 mmol, 5 ×
10-2 M) of 1f, 10 mL (112.6 mmol, 3.75 M) of benzene, and
235 mg (1.2 mmol, 4 × 10-2 M) of xanthone in 20 mL of MeCN
irradiated at 366 nm. After column chromatography (neat
C6H12), 53 mg of the title compound 7f (colorless solid) was
isolated (17% yield). mp 49-51°C (lit.62 51-52°C). Spectro-
scopic data were in accordance with the literature.63
4-(2-Propenyl)-N,N-dimethylaniline (6g).64 From 140 mg
(0.6 mmol, 10-2 M) of 1g, 9.4 mL (60 mmol, 1 M) of 5, and 470
mg (2.4 mmol, 4 × 10-2 M) of xanthone in 60 mL of MeCN
irradiated at 310 nm. After column chromatography (C6H12/
EtAc 98/2), 53 mg of the title compound 6g (oil) was isolated
(55% yield). Data for 6g: 1H NMR (CDCl3) δ 6.7-7.05 (AA′BB′,
3
4 H), 5.9 (m, 1 H), 4.95-5.1 (m, 2 H), 3.3 (d, J (H,H) ) 6.5
Hz, 2 H), 2.95 (s, 6 H); IR (neat) ν (cm-1) 2924, 1638, 1345,
836.
4-N,N-Dimethylaminobiphenyl (7g). From 140 mg (0.6
mmol, 10-2 M) of 1g, 20 mL (112.6 mmol, 3.75 M) of benzene,
and 470 mg (2.4 mmol, 4 × 10-2 M) of xanthone in 40 mL of
MeCN irradiated at 310 nm. After column chromatography
(C6H12/EtAc 98/2), 48 mg of the title compound 7g (colorless
solid) were isolated (41% yield). Mp 118-120 °C (lit.65 121-
122 °C). Data for 7g: 1H NMR (CDCl3) δ 7.25-7.55 (m, 5 H),
6.82-7.5 (AA′BB′, 4 H), 3.02 (s, 6 H); IR (neat) ν (cm-1) 3051,
1353, 820.
4-Bromobiphenyl (7d). From 162 mg (0.6 mmol, 2 × 10-2
M) of 1d, 10 mL (112.6 mmol, 3.75 M) of benzene, and 235 mg
(1.2 mmol, 4 × 10-2 M) of xanthone in 20 mL of MeCN
irradiated at 366 nm. After column chromatography (neat
C6H12), 76 mg of the title compound 7d (colorless solid) was
isolated (54% yield). Mp 89-91 °C (lit.53 89-90 °C). Spectro-
scopic data were in accordance with the literature.60
3-Phenyl-1-propene (6e). From 290 mg (1.5 mmol, 5 ×
10-2 M) of 1e, 4.7 mL (30 mmol, 1 M) of 5, and 235 mg (1.2
mmol, 4 × 10-2 M) of xanthone in 30 mL of MeCN irradiated
Acknowledgment. Partial support of this work by
Murst, Rome, is gratefully acknowledged.
Supporting Information Available: UV spectra of salts
1a-g in the presence of both benzene (1.5 and 3.75 M) and 5
(0.5 and 1 M). This material is available free of charge via the
Internet at http://pubs.acs.org.
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