The role of the leaving group in the dissociation of radical anions of 9-(aryloxymethyl)anthracenes [4]

Full text of DOI:10.1021/ja0028929

3824
J. Am. Chem. Soc. 2001, 123, 3824-3825
Table 1. Rate Constants for Dissociation of the Radical Anions
of 1a-f
The Role of the Leaving Group in the Dissociation of
Radical Anions of 9-(Aryloxymethyl)anthracenes
Y
k/s-1
4.1 × 105
5.8 × 105
6.3 × 105
1.1 × 106
1.4 × 106
3.8 × 106
Norio Kimura
p-MeO
p-Me
p-Et
H
p-F
m-F
The Institute of Scientific and Industrial Research
Osaka UniVersity, 8-1 Mihogaoka, Ibaraki
Osaka 567-0043, Japan
ReceiVed August 4, 2000
ReVised Manuscript ReceiVed January 18, 2001
The formation and subsequent dissociation of radical anions
(RX-•) are important processes in connection with the SRN
1
mechanism.1 The dissociation reaction is accompanied by the
redistribution of electron density to the forming fragments.
Considering the electron apportionment, two distinct modes such
as a homolysis (to give R- + X•) and a heterolysis (to give R• +
X-) are possible,2 but a homolysis route is a rare event in organic
radical anions containing a carbon-heteroatom bond (e.g.,
carbon-halogen and carbon-phenoxy). Maslak and Guthrie2b
have shown that the substituent on the phenoxy ring influences
the lifetime of the radical anion of p-nitrobenzyl phenyl ether.
No attempt has been made to determine the absolute rate for
dissociation of arylmethyl phenyl ether radical anions.
In this communication the kinetics for dissociation of the radical
anions of substituted 9-(phenoxymethyl)anthracenes (1a-f)3 have
been determined by using the pulse radiolysis technique. The
kinetic data show a significant influence of substituent on the
dissociation of the radical anion.
Figure 1. Hammett plot of log(kY/kH) versus σ for the dissociation
reaction of the radical anions of 1a-f in 1-methyl-2-pyrrolidone.
Scheme 1
Scheme 2
1-Methyl-2-pyrrolidinone (NMP) solutions of 1a-f held in
Suprasil quartz cells were irradiated with an 8-ns pulse of 28 MeV
electrons at room temperature and the transient absorption spectra
were recorded at various times after the pulse.4 In all cases the
spectrum observed immediately after the pulse had an absorption
maximum at around 710 nm and was assigned to the correspond-
ing anthracene radical anions. All these spectra changed with time
to result in a 430-nm peak, which was identical with those of
1a-f. This absorption was assigned to the (9-anthryl)methyl
radical by comparison with the spectrum observed by the pulse
radiolysis of 9-(chloromethyl)anthracene.5 On the basis of these
observations, the sequence of events in the reductive cleavage
reaction of 9-(phenoxymethyl)anthracene (AnCH2OPh) can be
depicted as shown in Scheme 1. That is, the reaction involves
two successive distinct steps, addition of an electron to give the
corresponding anthracene radical anion and then dissociation of
the C-O bond to form the (9-anthryl)methyl radical (AnCH2 )
•
and phenoxide ion (PhO-). The rate of the latter process was
determined by monitoring the rate of decay of the 710-nm
absorption and the rate of the concomitant buildup of the 430-
nm absorption. The rates were found to be first-order and
independent of the solute concentration (1-10 mM). The kinetic
data gathered in Table 1 show the variation of the rate constant
(kY/s-1) with the substituent. A Hammett plot of log(kY/kH) with
the σ values of the substituents results in an excellent correlation
(Figure 1)6 and gives the reaction constant (F) of 1.62. The value
of F indicates that the substituent electronic effect is important
in the dissociation reaction.
(1) (a) Kornblum, N. Angew. Chem., Int. Ed. Engl. 1975, 14, 734. (b)
Bunnett, J. F. Acc. Chem. Res. 1978, 11, 413. (c) Bowmann, W. R. Chem.
Soc. ReV. 1988, 17, 283. (d) Saveant, J.-M. Tetrahedron 1994, 50, 10117.
(2) (a) Maslak, P.; Guthrie, D. R. J. Am. Chem. Soc. 1986, 108, 2628. (b)
Maslak, P.; Guthrie, D. R. J. Am. Chem. Soc. 1986, 108, 2637. (c) Guthrie,
R. D.; Shi, B. J. Am. Chem. Soc. 1990, 112, 3135. (d) Maslak, P.; Vallombroso,
T. M.; Chapman, W. H., Jr.; Narvaez, J. N. Angew. Chem., Int. Ed. Engl.
1994, 33, 73.
We previously reported that, as shown in Scheme 2, the radical
anion of 1-(p-biphenylyl)-4-chlorobutane (2, λmax ) 410 nm),
generated by the reaction with the solvated electron, reacts
unimolecularly to give a mixture of the dissociative ET and SN2
(3) (a) The ethers 1a-f were synthesized according to the literature
procedure.3b (b) Tamura, Y,; Yamamoto, G.; Oki, M. Bull. Chem. Soc. Jpn.
1987, 60, 1781.
(4) The apparatus for the pulse radiolysis experiments was described in a
previous paper.6b
(5) (a) The reductive cleavage of 9-(chloromethyl)anthracene occurs via a
concerted electron transfer-bond breaking mechanism.9b (b) Andrieux, C. P.;
Le Gorande, A.; Saveant, J.-M. J. Am. Chem. Soc. 1992, 114, 6892.
(6) (a) Such a correlation is observed for radical anions of arylhalide6b and
1-benzoyl-ω-haloalkanes.6c (b) Saveant, J.-M. Tetrahedron 1994, 50, 10117.
(c) Kimura, N.; Takamuku, S. J. Am. Chem. Soc. 1994, 116, 4087.
10.1021/ja0028929 CCC: $20.00 © 2001 American Chemical Society
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Communications to the Editor
J. Am. Chem. Soc., Vol. 123, No. 16, 2001 3825
type products such as 1-(p-biphenylyl)-4-butyl radical (3,
λmax , 300 nm) and 8-phenylspiro[4.5]-6,8-decadienyl radical
(4, λmax ) 330 nm).7 It was found that the rate constants (k410
and k330, s-1) determined from the decay of the 410-nm absorption
and the concomitant buildup of the 330-nm absorption have the
same value, irrespective of the product ratio, and increase linearly
with an increase in the polarity of the solvent. The results led to
the conclusion that the formation processes of 3 and 4 involve a
common rate-determining step (i.e., concerted electron-transfer
and bond-dissociation process), and hence the radical anion, at
first of the π-type would acquire σ-character (i.e., carbanion-like)
at the transition state.8 If the electron transfer takes place from a
π* radical anion, 4 is not formed because a terminal carbon radical
(i.e., 3) does not react with the aromatic moiety.
Similarly, the dissociation rate of the radical anion was found
to be dependent on the nature of the solvent. Empirical parameters
of solvent polarity are as useful as the empirically derived
Hammett constants used in the prediction of substituent effects
in chemical reactions. Two empirical parameters introduced by
Gutmann9 are the donor number (DN) and the acceptor number
(AN), which reflect the nucleophilic and electrophilic properties
of the solvent. Since the reactant and product in the present
reaction are anions, the parameter of interest is AN. Figure 2
illustrates the variation of the dissociation rate constant (log kH)
with AN. It is apparent that the dissociation rate increases with
increasing AN, namely, solvent polarity. This trend is similar to
that found for the intramolecular reaction (Scheme 2). A similar
dependence of solvent influence on the reaction rates suggests
Figure 2. Variation of the dissociation rate constant (log kH, s-1) of the
radical anion of 1a with the acceptor number (AN) of the solvents:
hexamethylphosphoric triamide (HMPA), NMP, dimethylacetamide
(DMA), dimethylformamide (DMF), and dimethyl sulfoxide (DMSO).
the presence of a common mechanism as the rate-determining
step. The positive slope for the reaction rate indicates that, on
the basis of the Hughes-Ingold rules for solvent effects,10 the
negative charge is more localized in the transition state subjected
to intense solvation than in the initial state of the radical anion.
The results of solvent and substituent effects suggest the
participation of a polarized configuration in the reaction.11 The
dissociation mechanism of the radical anion of 9-(phenoxy-
methyl)anthracene is considered to be the same as that described
for the intramolecular reaction in Scheme 2.
JA0028929
(7) (a) Kigawa, H.; Takamuku, S.; Toki, S.; Kimura, N.; Takeda, S.;
Tsumori, K.; Sakurai, H. J. Am. Chem. Soc. 1981, 103, 5176. (b) Kimura, N.;
Takamuku, S. Bull. Chem. Soc. Jpn. 1992, 65, 1662.
(10) Ingold, C. K. Structure and Mechanism in Organic Chemistry; Cornell
University Press: New York, 1969.
(11) (a) In analogy with the solvent effect, the heterolytic dissociation is
very much dependent on the electrophilic assistance by the counterion.11b (b)
Casado, F.; Pisano, L.; Farriol, M.; Gallardo, I.; Marquet, J.; Melloni, G. J.
Org. Chem. 2000, 65, 322 and references therein.
(8) (a) Similarly, a quantum mechanical study led to the conclusion of a
substantial σ character of the radical anion in the dissociation reaction.8b (b)
Moreno, M.; Gallard, I.; Bertian, J. J. Chem. Soc., Perkin Trans. 2 1989,
2017.
(9) Gutmann, V. Electrochim. Acta 1976, 21, 66.