Electron-transfer nucleophilic substitution reactions on neopentyl- and phenyl-substituted alkyl chlorides. Effect of the bridge length on the intramolecular electron-transfer catalysis

Article abstract of DOI:10.1021/jo980657r

The nucleophilic substitution reaction of the chlorides RMe₂CCH₂Cl (R = Me, 4; Ph, 5a; PhCH₂, 5b) and their relative reactivities toward diphenyl phosphide ions were studied under irradiation in liquid ammonia. The relative reactivities determined were k_5a/k₄ ? 9 and k_5b/k₄ ? 0.85. These reactions are proposed to occur through the S_RN1 mechanism. The higher reactivity of 5a is explained on the basis of its higher electron affinity due to the phenyl substitution and the efficient intramolecular electron transfer from this group to the C-Cl σ* bond (intra-ET catalysis). Although 5b also has a phenyl ring, its lower reactivity is ascribed to a decrease in the rate of the intra-ET by elongation of the bridge in one methylene unit. The relative reactivity of 5a versus 5b (k_5a/k_5b, ? 6.4) is proposed to indicate the ratio of the intra-ET rates of the radical anions of both compounds. AMI calculations performed on the system are in agreement with the experimental results.

Full text of DOI:10.1021/jo980657r

2626
J . Org. Chem. 1999, 64, 2626-2629
Electr on -Tr a n sfer Nu cleop h ilic Su bstitu tion Rea ction s on
Neop en tyl- a n d P h en yl-Su bstitu ted Alk yl Ch lor id es. Effect of th e
Br id ge Len gth on th e In tr a m olecu la r Electr on -Tr a n sfer Ca ta lysis
J ose´ S. Duca, J r., Mariana H. Gallego, Adriana B. Pierini,* and Roberto A. Rossi*
INFIQC, Departamento de Quı´mica Orga´nica, Facultad de Ciencias Qu´ımicas, Universidad Nacional de
Co´rdoba, Ciudad Universitaria, 5000 Co´rdoba, Argentina
Received April 8, 1998
The nucleophilic substitution reaction of the chlorides RMe2CCH2Cl (R ) Me, 4; Ph, 5a ; PhCH2,
5b) and their relative reactivities toward diphenyl phosphide ions were studied under irradiation
in liquid ammonia. The relative reactivities determined were k5a /k4 = 9 and k5b/k4 = 0.85. These
reactions are proposed to occur through the SRN1 mechanism. The higher reactivity of 5a is explained
on the basis of its higher electron affinity due to the phenyl substitution and the efficient
intramolecular electron transfer from this group to the C-Cl σ* bond (intra-ET catalysis). Although
5b also has a phenyl ring, its lower reactivity is ascribed to a decrease in the rate of the intra-ET
by elongation of the bridge in one methylene unit. The relative reactivity of 5a versus 5b (k5a /k5b
= 6.4) is proposed to indicate the ratio of the intra-ET rates of the radical anions of both compounds.
AM1 calculations performed on the system are in agreement with the experimental results.
It is known that neopentyl halides have a low reactivity
1,3). Formation of the uncyclized 2 and cyclized 3
substitution products in the reactions of the radical
probes 1a -c with PhS-, Ph2P-, or Me3Sn- ions has been
used as evidence of the presence of radicals as intermedi-
ates in these reactions (eq 4).7
toward nucleophilic substitutions by polar mechanisms.1
They belong to a family of aliphatic compounds (neopen-
tyl, cycloalkyl, and bicycloalkyl halides) that can be
substituted through the SRN1 mechanism.2 Substitution
of neopentyl bromide can be achieved with the nucleo-
philes PhS-, Se2-, Ph2P-, and Ph2As- under photostimu-
lation in liquid ammonia.3 Carbanions such as the enolate
ion of acetophenone react with neopentyl iodide in DMSO
under irradiation4 or in the presence of FeBr2.5 The
propagation cycle of this mechanism is presented in the
following equations:
In those cases in which a better acceptor group is
present in the molecule in addition to the C-halogen
bond, the reactivity can be increased as a result of an
intramolecular ET (intra-ET) catalysis. For example, it
has been shown that 2-oxo-3,3-dimethyl-bicyclo[2.2.2]oct-
1-yl chloride8 and 2- and 3-oxo-bicyclo[2.2.1]hepta-1-yl
chlorides9 react with Ph2P- ions under irradiation by the
SRN1 mechanism, whereas the unsubstituted chloro
compounds do not react under the same experimental
conditions.
When a π acceptor such as an aromatic ring is present
in the substrate, the intermolecular ET (inter-ET) could
form a radical anion intermediate with the extra electron
localized in the π* system (eq 5). The intra-ET from this
The radical anion of the substrate (RX-•) is usually
formed in the initiation step by a photostimulated
electron transfer (ET) from the nucleophile.2 This reaction
is proposed as dissociative for unsubstituted and un-
strained aliphatic halides,2b,6 leading directly to the
aliphatic radical and the anion of the leaving group (eq
(1) March, J . Advanced Organic Chemistry, 4th ed.; J ohn Wiley &
Sons: New York, 1992. Carey, F. A.; Sundberg, R. J . Advanced Organic
Chemistry, 3rd ed.; Plenum Press: New York, 1990, Part A.
(2) For reviews on aliphatic SRN1, see: (a) Rossi, R. A.; Pierini, A.
B.; Palacios, S. M. Advances in Free Radical Chemistry; Tanner, D.
D., Ed.; J AI Press: Greenwich, CT, 1990; p 193; J . Chem. Educ. 1989,
66, 720. (b) Savea´nt, J . M. Adv. Phys. Org. Chem. 1990, 26, 1. (c) Rossi,
R. A.; Pierini, A. B.; Pen˜e´n˜ory, A. B. In The Chemistry of Functional
Group; Patai, S., Rappoport, Z., Eds.; Wiley: Chichester, 1995; Supl.
D2, Chapter 24.
(6) (a) Save´ant, J . M. J . Am. Chem. Soc. 1992, 114, 10595. (b)
Savea´nt, J . M. Acc. Chem. Res. 1993, 26, 455. (c) Compton, R. N.;
Reinhart, P. W.; Cooper, C. C. J . Chem. Phys. 1978, 68, 4360. (d)
Raymonda, J . M.; Edwards, L. O.; Russel, B. B. J . Am. Chem. Soc.
1974, 96, 1708.
(7) (a) Beckwith, A. L. J .; Palacios, S. M. J . Phys. Org. Chem. 1991,
4, 404. (b) Ashby, E. C.; Gurumurthy, R.; Ridlehuber, R. W. J . Org.
Chem. 1993, 58, 5832. (c) Ashby, E. C.; Deshpande, A. K. J . Org. Chem.
1994, 59, 7358.
(3) (a) Bornancini, E. R. N.; Palacios, S. M.; Pen˜e´n˜ory, A. B.; Rossi,
R. A. J . Phys. Org. Chem. 1989, 2, 255. (b) Pierini, A. B.; Pen˜e´n˜ory, A.
B.; Rossi, R. A. J . Org. Chem. 1985, 50, 2739.
(8) Santiago, A. N.; Takeuchi, K.; Ohga, Y.; Nishida, M.; Rossi, R.
A. J . Org. Chem. 1991, 56, 1581.
(4) Pen˜en˜ory, A. B.; Rossi, R. A. Gazz. Chim. Ital. 1995, 125, 605.
(5) Nazareno, M. A.; Rossi, R. A. J . Org. Chem. 1996, 61, 1645.
(9) Lukach, A. E.; Morris, D. G.; Santiago, A. N.; Rossi, R. A. J . Org.
Chem. 1995, 60, 1000.
10.1021/jo980657r CCC: $18.00 © 1999 American Chemical Society
Published on Web 04/16/1999

Intra- and Intermolecular ET Catalysis
J . Org. Chem., Vol. 64, No. 8, 1999 2627
Ta ble 1. Rea ction of 4, 5a , a n d 5b w ith
Dip h en ylp h osp id e An ion a
expt substrate, M × 103 Cl- (%) substitution productsb (%)
1c
2
4, 4.0
80
74
<5
<5
53
<5
<5
74
5a , 3.33
5a , 3.33
5a , 3.33
5b, 4.00
5b, 4.00
5b, 4.00
70 (61)d
3e
4g
5
f
f
49 (32)d
presence of p-DNB (Table 1, expts 3, 4, 6, 7). These
compounds are proposed to react by the SRN1 mechanism.
Rela tive Rea ctivities. The relative reactivities of
pairs of compounds toward the same nucleophile were
determined in reactions in which both substrates were
present in excess with respect to the nucleophile. Even
though 4 and 5a had similar reactivity with Ph2P- ions
in separated experiments, low substitution yield was
obtained with 4 when both substrates reacted in an
approximately 1:1 ratio. An average relative reactivity
k5a /k4 = 9 was obtained from reactions with higher 4 to
5a ratios (Table 2, expts 1-6) following the equation
previously reported.12 A relative reactivity k5b/k4 ) 0.85
was determined in competition reactions of 5b and 4
(Table 2, expts 7-10). Competition experiments between
5a and 5b led to the determination of a relative reactivity
k5a /k5b = 6.4 (Table 2, expts 11-13).
6e
7g
f
f
a
Reactions performed in liquid ammonia (300 mL). Ph2P- ions
with the same concentration of the substrates. Irradiation time
240 min. The substrates 5a and 5b were dissolved in 6 mL of
DMSO when added. b Quantified by GLC. c Ref 3a. Isolated yield.
d
e Dark reaction. f No substitution product was detected. g Reaction
performed in the presence of p-DNB.
system to the σ* C-halogen bond will be responsible for
the fragmentation of the intermediate into radicals (eq
6).10
To get new insights on intra-ET reactions and the effect
of the spacer length,11 we decided to study the relative
reactivity of 1-chloro-2,2-dimethyl-propane (neopentyl
chloride (4)), 1-chloro-2-methyl-2-phenyl propane (neo-
phyl chloride (5a )), and 1-chloro-2,2-dimethyl-3-phenyl
propane (5b) toward Ph2P- ions under irradiation.
Discu ssion
Semiempirical AM1 calculations13 were performed to
obtain information about the LUMO MOs of the com-
pounds under study, which can be correlated with their
reduction potentials,14 and the intermediates formed in
the inter-ET reaction.15 The calculated LUMO of 4 (1.52
eV) belongs to the σ* C-Cl bond and is of higher energy
than the π LUMOs of compounds 5a and 5b (0.31 and
0.37 eV, respectively). Furthermore, the anionic surface
of 4 showed an exothermic dissociative behavior with a
stabilization zone corresponding to the electrostatic loose
complex of the dissociated species (C-Cl distance ≈ 3.8
Å).16
The first member of the series is a simple neopentyl
chloride. In 5a , a methyl group of compound 4 is replaced
by a phenyl group, whereas in 5b, the methyl group is
replaced by a benzyl group, so the length of the spacer is
increased by one methylene unit. The family thus offers
the possibility to study the effect of the π substitution
on a simple alkyl halide and the effect of the spacer
length on the intra-ET reaction when it is mediated by a
flexible chain.
On the basis of the calculated LUMOs, 5a and 5b will
be reduced more easily than 4, and their π radical anions
could be formed in this step. These intermediates, which
(12) The equation used in the relative reactivity determination of
pairs of substrates toward a nucleophile is:
k1/k2 ) ln([RX1]0/[RX1]t)/ln([RX2]0/[RX2]t)
Resu lts
where [RX1]0 and [RX2]0 are initial concentrations and [RX1]t and [RX2]t
are concentration at time t of both substrates. This equation is based
on a first-order reaction of both substrates with the nucleophile. See:
Bunnett, J . F. In Investigation of Rates and Mechanisms of Reaction,
3rd ed.; Lewis, E. S., Ed.; Wiley-Interscience: New York, 1974; part
1, p 159.
(13) All the calculations were carried out with complete geometry
optimization, and the semiempirical AM1 method as implemented in
AMPAC. See: Liotard, D. A.; Healy, E. F.; Ruiz, J . M.; Dewar, M. J .
S. AMPAC version 2.1, Quantum Chemistry Program Exchange,
program 506. QCPE Bull. 1989, 9, 123.
Compounds 4,3a 5a , and 5b reacted with Ph2P- ions
under irradiation to give, after oxidation, the substitution
compounds 6, 7a , and 7b, respectively (eq 7) (Table 1,
expts 1, 2, 5).
The reactions of 5a and 5b did not occur in the dark,
and when photostimulated, they were inhibited by the
(10) Experimental and theoretical data have been used to support
the existence of π and σ radical anions. See: (a) Clarke, D. D.; Coulson,
C. A. J . Chem. Soc. A 1969, 169. (b) Dressler, R.; Allan, M.; Haselbach,
E. Chimia 1985, 39, 385. (c) Symons, M. C. R. Acta Chem. Scand. 1997,
51, 127. (d) Pierini, A. B.; Duca, J . S., J r. J . Chem. Soc., Perkin Trans.
2 1995, 1821. (e) Casado, J .; Gallardo, I.; Moreno, M. J . Electroanal.
Chem. 1987, 219, 197. (f) Andrieux, C. P.; Save´ant, J . M.; Zann, D.
Nouv. J . Chim. 1984, 8, 107.
(11) (a) Newton, M. D. Chem. Rev. 1991, 91, 767. (b) J ordan, K. D.;
Paddon-Row, M. N. Chem. Rev. 1992, 92, 395. (c) Paddon-Row, M. N.
Acc. Chem. Res. 1994, 27, 18. (d) Kuki, A. Structure and Bonding;
Springer-Verlag: Berlin, 1991; p 50. (e) Marcus, R. A.; Sutin, N.
Biochim. Biophys. Acta 1985, 811, 265. (f) Isied, S. S.; Ogawa, M. Y.;
Wishart, J . F. Chem. Rev. 1992, 92, 381. (g) Beratan, D. N.; Betts, J .
N.; Onuchic, J . N. Science 1991, 252, 12.
(14) Pierini, A. B.; Santiago, A. N.; Rossi, R. A. Tetrahedron 1991,
47, 941.
(15) (a) The AM1/UHF stationary points were characterized by
calculating their Hessian matrix, no negative eigenvalue for a minimun
energy structure, and one negative eigenvalue for a transition state.
(b) The CI calculations were performed with the AM1/RHF Hamilto-
nian within the subspace of all possible excitations of five electrons
into five molecular orbitals, from SOMO-2 up to SOMO+2 (CI ) 5).
(16) (a) Bertran, J .; Gallardo, I.; Moreno, M. Savea´nt, J . M. J . Am.
Chem. Soc. 1992, 114, 9576. (b) Tikhomirov, V. A.; German, E. D.;
Kuznetsov, A. M. Chem. Phys. 1995, 191, 25. (c) Benassi, R.; Bertarini,
C.; Taddei, F. Chem. Phys. Lett. 1996, 257, 633. (d) Clark, T. Faraday
Discuss. Chem. Soc. 1984, 78, 203. (e) Hotokka, M.; Roos, B. O.;
Eberson, L. J . Chem. Soc., Perkin Trans. 2 1986, 1979. (f) Canadell,
E.; Karafiloglou, P.; Salem. L. J . Am. Chem. Soc. 1980, 102, 855.

2628 J . Org. Chem., Vol. 64, No. 8, 1999
Duca et al.
Ta ble 2. Com p etition Rea ction s of th e P a ir s of
Su bstr a tes (5a :4), (5b:4), a n d (5a :5b) tow a r d Dip h en yl
P h osp h id e An ion a
Sch em e 1
substitution
substrates,
products
relative
reactivity
expt
M × 103
(%)b,c
average
1
2
3
5a , 1.96; 4, 16.4 7a (27); 6 (32)
5a , 1.80; 4, 20.0 7a (26); 6 (34)
5a , 1.84; 4, 24.0 7a (32); 6 (52)
k
k
k
5a /k4 ) 8.1
5a /k4 ) 9.4
5a /k4 ) 9.2
4d 5a , 1.76; 4, 24.3 7a (6); 6 (9)
k5a /k4 ) 8.8
5
6
5a , 1.78; 4, 28.0 7a (29); 6 (62)
5a , 1.84; 4, 32.2 7a (31); 6 (65)
k
k
5a /k4 ) 8.2
5a /k4 ) 9.5 k5a /k4
8.9 ( 0.6
)
7
8
9
5b, 1.79; 4, 2.48 7b (16); 6 (26)
5b, 1.79; 4, 3.19 7b (27); 6 (53)
5b, 2.55; 4, 1.88 7b (45); 6 (40)
k
k
k
k
5b/k4 ) 0.87
5b/k4 ) 0.89
5b/k4 ) 0.81
10 5b, 3.24; 4, 1.82 7b (47); 6 (31)
5b/k4 ) 0.83 k5b/k4
)
0.85 ( 0.04
11e,f 5a , 4.41; 5b, 3.65 7a (36); 7b (5.4) k5a /k5b ) 6.2
12f 5a , 1.89; 5b, 1.56 7a (56); 7b (8.5) k5a /k5b ) 6.8
13f,g 5a , 0.50; 5b 0.39 7a (57); 7b (8.1) k5a /k5b ) 6.1 k5a /k5b
)
6.4 ( 0.4
a
Performed in liquid ammonia-DMSO 3%. Irradiation time
(180 min or less if the solution turns colorless). Ph2P- (1.6 × 10-3
b
M). Calculated with respect to the mmoles of nucleophile.
c Quantified by GLC. Shorter reaction time; quenched in the
d
presence of Ph2P-. e Ph2P- (3.1 × 10-3 M). f Duplicated run.
Ph2P- (0.37 × 10-3 M).
g
On the basis of these calculations and the SRN1
propagation cycle, the mechanistic steps in Scheme 1 can
be proposed for competition reactions of compounds 5a
or 5b and 4 toward a given nucleophile.
The dissociative ET to 4 affords neopentyl radical 8
(eq 8a). On the other hand, the ET to compounds 5 can
form the radical anion 5-• (π type). Two reactions can be
proposed for 5-•, intra-ET to give the phenyl-substituted
radicals 9 (eq 8b) and inter-ET to 4 (8c).
Radicals 8 and 9 react with Ph2P- to form radical
anions 10-• and 11-•, respectively (eqs 9a and 9b). Both
reactions are estimated to occur at similar rates, because
they involve the coupling of similar radicals with the
same nucleophile to form radical anions of similar
stability (the extra electron would be located at the PPh2
group). These radical anions can transfer the extra
electron to the substrates to form the substitution
products 10 and 11 and the intermediates that will
continue the propagation cycle.
On the basis of the mechanism proposed and the higher
electron affinity of 5a and 5b compared to that of 4
toward the inter-ET steps of the propagation cycle (eqs
10), the difference in relative reactivity between these
compounds and 4 would depend mainly on the differences
in the intra-ET rates of their radical anions. The higher
reactivity of 5a (k5a /k4 = 9) with respect to 5b (k5b/k4 =
0.85) is thus indicating that the intra-ET reaction is
faster for 5a -•. The elongated bridge that mediates the
reaction in 5b-• increases the lifetime of this intermedi-
ate, favoring its inter-ET reaction to 4 (eq 8c), which thus
has a similar reactivity to 5b.
In the competition reactions between 5a and 5b, the
relative reactivity also will depend on the relative
fragmentation rate of their radical anions. As both
compounds have similar reduction potentials, the rate
of formation of the intermediates will be similar. The fact
that 5a is more reactive than 5b indicates that in this
F igu r e 1. AM1/UHF potential surface for the intra-ET
reaction of 5a-• (taken as representative): π radical anion
(dashed line); σ anionic surface (solid line).
have the negative charge and unpaired spin density on
the phenyl ring,10d were characterized as the most stable
form of both radical anions (5a -• and 5b-• henceforth) at
the AM1/UHF and RHF/CI level.15 The intra-ET of these
intermediates from the phenyl ring to the C-Cl bond to
give radicals occurs through the avoided crossing of their
π potential surface with the surface of the dissociated
anionic species, as shown in Figure 1 for 5a -•, taken as
representative.

Intra- and Intermolecular ET Catalysis
J . Org. Chem., Vol. 64, No. 8, 1999 2629
P h otostim u la ted Rea ction of 5b w ith P h 2P - in Liqu id
Am m on ia . The following procedure is representative of all
the reactions. Into a three-necked round-bottomed flask
equipped with a coldfinger condenser charged with dry ice-
ethanol, a nitrogen inlet, and a magnetic stirrer was condensed
300 mL of ammonia previously dried with Na metal under
nitrogen. Ph3P (1.6 mmol) and Na metal (3.2 mmol) were added
to form Ph2P- ions, and t-BuOH (3.2 mmol) was added to
neutralize the amide ions formed. To this solution was added
5b (1.6 mmol), dissolved in 6 mL of DMSO (stored under 4 Å
molecular sieves), and then the solution was irradiated for 240
min. The reaction was quenched by adding ammonium nitrate
in excess, and the ammonia was allowed to evaporate. The
residue was dissolved with water and extracted twice with Cl2-
CH2 (20 mL each). The products were oxidized with H2O2 and
then quantified by GLC with the internal standard method.
The same procedure was followed when the reaction was
performed in the presence of p-DNB, except that 20 mol %
p-DNB was added to the solution of nucleophile prior to the
substrate addition.
system an inter-ET between them and their radical
anions can be proposed to occur as a reversible reaction
with similar rates in both senses (exchange reaction) (eq
11).17
In addition to this exchange reaction, 5a -• and 5b-•
will render, by intra-ET, the radicals 9a and 9b, respec-
tively. Both radicals can react with Ph2P- ions at similar
rates to afford ultimately the substitution compounds.
At equilibrium, the determined k5a /k5b = 6.4 (Table 2)18
is indicative of the ratio of the fragmentation rates of the
radical anions of both compounds.19 In agreement with
this experimental reactivity, the semiempirical activation
energy evaluated for the intra-ET of 5a -• and 5b-• is
higher by ca. 2 kcal/mol for the elongated radical anion.
P h otostim u la ted Com p etition Rea ction of 5a a n d 5b
w it h P h 2P -. The procedure was similar to that previously
described, except that both substrates were added, dissolved
in DMSO, to the solution of nucleophile in liquid ammonia.
(2,2-Dim eth ylpr opyl)diph en ylph osph in e Oxide (6). Iso-
lated and characterized as described3b by comparison with an
authentic sample.
Exp er im en ta l Section
(2-Meth yl-2-p h en ylp r op yl)d ip h en ylp h osp h in e Oxid e
(7a ). White solid isolated by column chomatography on silica
gel (petroleum ether-diethyl ether as eluent) and recrystal-
lized from hexane-acetone: mp 134-135 °C; 1H NMR (acetone-
d6) δ 1.53 (6H, s), 2.83 (2H, d), 6.93-7.87 (15H, m); 13C NMR
δ 29.84, 29.92, 38.24, 38.33, 42.70, 44.06, 125.98, 126.07,
128.20, 128.52, 128.76, 130.68, 130.86, 131.21, 135.68, 137.60,
149.44, 149.61; MS (EI+) 334, 278, 277, 257, 215, 202, 201,
183, 152, 133, 119; HRMS calcd for C22H23OP 334.1480, found
334.1484.
(2,2-Dim eth yl-3-p h en ylp r op yl)d ip h en ylp h osp h in e Ox-
id e (7b). White solid isolated by chromatography on silica gel
(petroleum ether-diethyl ether as eluent) and recrystallized
from hexane-acetone: mp 139-141 °C; 1H NMR (acetone-d6)
δ 1.03 (6H, s), 2.42 (2H, d), 2.87 (2H, s), 7.10-7.97 (15H, m);
13C NMR δ 28.68, 28.83, 35.99, 36.08, 39.24, 40.63, 49.45,
49.60, 126.42, 128.08, 128.79, 128.99, 130.74, 130.92, 131.39,
131.45, 131.51, 136.30, 138.22, 139.29; MS (EI+) 348, 257, 215,
201, 183, 154, 125, 91, 77; HRMS calcd for C23H25OP 348.1634,
found 348.1634.
Gen er a l Meth od s. Irradiation was conducted in a reactor
equipped with two 400-W lamps emitting maximally at 350
nm (Philips Model HPT, air- and water-refrigerated). HRMS
were recorded at the Institute of Advanced Materials Study,
Kyushu University, J apan.
Ma ter ia ls. All materials, including neopentyl chloride,
neophyl chloride, and 2,2-dimethyl-3-phenyl-1-propanol (Ald-
rich), were commercially available and were used as received.
1-Chloro-2,2-dimethyl-3-phenylpropane (5b) was prepared
from 2,2-dimethyl-3-phenyl-propyl tosylate20 following the
procedure described for the neopentyl analogue.21 The product
was isolated as a colorless liquid by chromatography on silica
gel with petroleum ether as eluent. 1H NMR (acetone-d6) δ
0.98 (6H, s), 2.67 (2H, s), 3.39 (2H, s), 7.16-7.36 (5H, m); 13
C
NMR δ 25.31, 36.38, 44.79, 54.74, 126.22, 127.94, 130.39,
137.99; MS (EI+) 182, 147, 133, 105, 91, 77; HRMS calcd for
C
11H15Cl 182.0862, found 182.0863.
(17) Although there is no experimental data on the electron self-
exchange reaction between the benzene radical anion and benzene, it
has been calculated to be 1.4 × 109 at 298 K. See: Formosinho, S. J .;
Arnaut, L. G. Bull. Chem. Soc. J pn. 1997, 70, 977. For exchange
reaction of other aromatic radical anions, see: Ward, R. L.; Weissman,
S. I. J . Am. Chem. Soc. 1954, 76, 3612. Bergman, I. Trans. Faraday
Soc. 1954, 50, 829.
(18) This value is expected to decrease to k5a /k5b = 1 under high
dilution, conditions where the equilibrium is not attained, according
to simulations performed with the program Chemical Kinetics Simula-
tor 1.01 (International Business Machines Corporation, 1996). We were
unable to reach this limit experimentally.
Ack n ow led gm en t. J ose´ S. Duca, J r. and Mariana
H. Gallego are grateful recipients of fellowships from
the National Research Council of Argentina (CONICET).
Thanks are expressed to CONICET, Antorchas Founda-
tions, and the Council for Scientific Research of Co´rdoba
(CONICOR) for financial support. We thank Prof. S.
Mataka of the Graduate School of Engineering Sciences,
Kyushu University, J apan, for his help in recording the
HRMS.
(19) On the basis of the proposed mechanism and under equilibrium
conditions, the relative rate of formation of the substitution products
11a and 11b simplified to the following integrated equation:
Su p p or tin g In for m a tion Ava ila ble: AM1/UHF heat of
formation for the stationary points calculated, equilibrium
geometries of 5a -•, 5b-•, potential surfaces for the intra-ET of
kfrag5a -•/kfrag5b-• ) Kexchange (ln([5a ]0/[5a ]t)/ln([5b]0/[5b]t)
1
5b-• and the dissociative inter-ET to 4, and H and 13C NMR
(20) Bordwell, F. G.; Pitt, B. M.; Knell, N. J . Am. Chem. Soc. 1951,
73, 5004. Beringer, F. M.; Schultz, H. S. J . Am. Chem. Soc. 1955, 77,
5533.
(21) Stephenson, B.; Solladie´, G.; Mosher, H. S. J . Am. Chem. Soc.
1972, 94, 4184.
for 5b, 7a , and 7b. This material is available free of charge
via the internet at http://pubs.acs.org.
J O980657R