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Since direct irradiation led to singlet chemistry almost
exclusively with salts 1c–e and for a large part with 1b, we
turned to sensitisation and used xanthone, because diazonium
salts are known as ketone triplet quenchers.12 In all cases,
formation of the acetanilides was greatly reduced or eliminated
and reduction to compounds 2 became the major pathway in
neat MeCN. In the presence of ATMS or benzene, arylated
products 6 and respectively 7 became in most cases the main
products. Clearly, a different chemistry was occurring and
involved reduction and addition to p nucleophiles, viz the
reactions expected from the triplet phenyl cation.
Thus, sensitisation leads to triplet diazonium salt, fragmenta-
tion occurs in this manifold and the thus formed triplet phenyl
cation (5 ) reacts before ISC to the singlet takes place, even
Scheme 3
3
when the latter spin state is the lowest one, as it is the case for
all the compounds examined except the amino substituted 1e.
The triplet abstracts hydrogen even from a poor donor such as
acetonitrile rather than forming an ylide as the singlet does, and
reacts with p nucleophiles via an addition–elimination mecha-
nism as represented in Scheme 2 (the adduct with ATMS is
indicated as a phenonium cation, probably the most stable
configuration).9
not surprising in view of the fast ISC generally occurring with
nitroaromatics.14 The case of cyano derivative 1b is somewhat
more complicated, in that some arylation products 6b, 7b are
found also upon direct irradiation, and the sensitised reaction
differs essentially in the non-formation of acetanilides. The
simplest explanation is that ISC from the excited diazonium
1
singlet 1b
the phenyl cation is formed in both spin states. However, the
triplet cation 5b seems to be a poor hydrogen abstracting
* competes to some degree with fragmentation and
The material balance is satisfactory and, except in the case of
3
1d (X = tert-butyl), where reduction remains the main path also
species, and in neat MeCN it intersystem crosses to the lower-
lying singlet, although the presence of p traps such as ATMS or
benzene reveals its presence (Scheme 3).
in the presence of benzene, reaction with p nucleophiles is
efficient and the synthetically interesting products 6 and 7 are
formed in good yield. These reactions are ionic analogues of the
Meerwein and Gomberg–Bachmann arylations and are clearly
distinguished from them, which, as it is well known, involve the
aryl radical. In fact the present arylations are not chain reactions
Summing up, a rivisitation of the direct and sensitized
photochemistry of diazonium salts suggests that controlled
conditions may be found for the generation of phenyl cations in
either spin state each of which have a distintive chemistry.
These as yet elusive species may develop from spectroscopic
curiosities to synthetically useful intermediates, in particular
with regard to the triplet states which lend themselves to C–C
bond forming reactions.
(
F = 0.5 for 1c–e, 0.25 for 1b and 50.1 for 1a). Photoinitiated
chain reactions are known for easily reducible diazonium salts
such as 1a in the presence of good hydrogen donors, e.g.
aqueous methanol as the solvent, where reduction occurs with F
≈
3.5–8.13 Furthermore, the present reactions involve attack to
nucleophilic, not electrophilic, olefins. We checked that
generating the 4-nitrophenyl radical from 1a under the classical
Notes and references
Meerwein conditions (CuCl
(
(
2
) leads to arylation of acrylonitrile
forming the 3-phenyl-2-chloropropionitrile) but not of ATMS
product 6a 10%) and conversely photodecomposition of 1a in
1
H. Zollinger, Diazo Chemistry, VCH, Weinheim, vol. 1, 1994.
2 The feasibility of thermal reaction between diazonium salts and
nucleophiles can be evaluated from Fig. 10 reported by Mayr et al.: see:
H. Mayr, M. Hartnagel and K. Grimm, Liebigs Ann., 1997, 55.
the presence of acrylonitrile does not lead to arylation, contrary
to the case of ATMS. At any rate, it may prove difficult to
exclude some participation of a phenyl radical in the case of the
easily reduced 1a, but this does not applies to the other salts.
Thus, photosensitisation is a general method for arriving at
the triplet phenyl cation, when, as occurs with most diazonium
salts, direct irradiation yields the singlet. In contrast, the nitro-
3
A. F. Hegarty, in The chemistry of Diazonium and Diazo Groups, ed. S.
Patai, Wiley, Chichester, 1978, vol. 2, p. 545.
(a) C. Galli, Chem. Rev., 1988, 88, 765; (b) A subcase of radical
arylation is that reported by Kochi through the irradiation of the EDA
complexes between some diazonium salts and aromatics, see: T. M.
Bockman, D. Kosynkin and J. K. Kochi, J. Org. Chem., 1997, 62,
4
5
811.
1
substituted 1a * apparently undergoes ISC to the diazonium
5 (a) P. S. J. Canning, K. McCrudden, H. Maskill and B. Sexton, J. Chem.
Soc., Perkin Trans. 2, 1999, 12, 2735; (b) aryl cations have been
generated in matrices, see: H. B. Ambroz, T. J. Kemp and G. K.
Prybytniak, J. Photochem. Photobiol., A, 1997, 108, 149 and references
therein.
triplet before decomposition and generates the phenyl cation in
the triplet state also upon direct irradiation. The same behavior
has been previously found for the 4-benzoyl derivative and is
8
6
7
8
9
P. J. Stang, in Dicordinate Carbocations, ed. Z. Rappoport and P. J.
Stang, Wiley, New York, 1997, p. 451.
M. Aschi and J. N. Harvey, J. Chem. Soc., Perkin Trans. 2, 1999,
1
059.
S. M. Gasper, C. Devadoss and G. B. Schuster, J. Am. Chem. Soc., 1995,
17, 5206.
1
M. Mella, P. Coppo, B. Guizzardi, M. Fagnoni, M. Freccero and A.
Albini, J. Org. Chem., 2001, 66, 6344.
1
0 S. Steenken, M. Askokkuna, P. Maruthamuthu and R. A. McClelland, J.
Am. Chem. Soc., 1998, 120, 11295.
1
1 Appropriate blank tests showed no appreciable thermal decomposi-
tion.
1
1
2 J. C. Scaiano and N. Kim-Thuan, Can. J. Chem., 1982, 60, 2286.
3 J. E. Packer, D. B. House and E. J. Rasburn, J. Chem. Soc. B, 1971,
1
574.
1
4 D. Doepp in Handbook of Organic Photochemistry and Photobiology,
Scheme 2
ed. W. Horspool and P.-S. Song, CRC Press, 1995, p. 1019.
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