Fluorescent quenching of the 2-naphthoxide Anion by aliphatic and aromatic halides. Mechanism and consequences of electron transfer reactions

Article abstract of DOI:10.1021/jo026518y

The fluorescent excited state of the 2-naphthoxide ion (1) is quenched by aliphatic and aromatic halides according to an electron-transfer mechanism, with generation of the corresponding alkyl and aryl radicals by a concerted or consecutive C-X bond fragmentation reaction. Whereas bromo-and iodobenzene follow a concerted ET mechanism (C-X, BDE control), 1-bromonaphthalene exhibits a stepwise process (π LUMO control). The photoinduced reaction of anion 1 with 1-iodoadamantane (2) in DMSO affords substitution products on C3, C6, and C8, 1-adamantanol, 1-adamantyl 2-naphthyl ether, and adamantane (3.2, 13.2, 12.2, 2.8, 2.5, and 14.1% yields, respectively). A complex mixture is also observed in the photochemical reaction of neopentyl iodide (3) with anion 1, which renders substitution on C1, C3, C6, C8, and 2-naphthyl neopentyl ether (8.1, 1.3, 19.1, 31.1, and 2.8% yields, respectively). The absence of reaction in the dark and the inhibition of the photoinduced reaction by the presence of the radical traps di-tert-butylnitroxide (DTBN) and 1,4-cyclohexadiene are evidence of a radical chain mechanism for these substitutions. On the other hand, only coupling at C1 is achieved by the photostimulated reaction of anion 1 with iodobenzene (5), to afford 41.9% of 1-phenyl-2-naphthol and 5.4% of disubstitution product. The regiochemistry of these reactions can be ascribed to steric hindrance and activation parameters.

Full text of DOI:10.1021/jo026518y

F lu or escen t Qu en ch in g of th e 2-Na p h th oxid e An ion by Alip h a tic
a n d Ar om a tic Ha lid es. Mech a n ism a n d Con sequ en ces of Electr on
Tr a n sfer Rea ction s
J uan E. Argu¨ello and Alicia B. Pen˜e´n˜ory*
INFIQC, Departamento de Quı´mica Orga´nica, Facultad de Ciencias Qu´ımicas,
Universidad Nacional de Co´rdoba, Ciudad Universitaria, 5000 Co´rdoba, Argentina
penenory@dqo.fcq.unc.edu.ar
Received October 3, 2002
The fluorescent excited state of the 2-naphthoxide ion (1) is quenched by aliphatic and aromatic
halides according to an electron-transfer mechanism, with generation of the corresponding alkyl
and aryl radicals by a concerted or consecutive C-X bond fragmentation reaction. Whereas bromo-
and iodobenzene follow a concerted ET mechanism (C-X, BDE control), 1-bromonaphthalene
exhibits a stepwise process (π LUMO control). The photoinduced reaction of anion 1 with
1-iodoadamantane (2) in DMSO affords substitution products on C3, C6, and C8, 1-adamantanol,
1-adamantyl 2-naphthyl ether, and adamantane (3.2, 13.2, 12.2, 2.8, 2.5, and 14.1% yields,
respectively). A complex mixture is also observed in the photochemical reaction of neopentyl iodide
(3) with anion 1, which renders substitution on C1, C3, C6, C8, and 2-naphthyl neopentyl ether
(8.1, 1.3, 19.1, 31.1, and 2.8% yields, respectively). The absence of reaction in the dark and the
inhibition of the photoinduced reaction by the presence of the radical traps di-tert-butylnitroxide
(DTBN) and 1,4-cyclohexadiene are evidence of a radical chain mechanism for these substitutions.
On the other hand, only coupling at C1 is achieved by the photostimulated reaction of anion 1 with
iodobenzene (5), to afford 41.9% of 1-phenyl-2-naphthol and 5.4% of disubstitution product. The
regiochemistry of these reactions can be ascribed to steric hindrance and activation parameters.
In tr od u ction
by the SRN1 mechanism, and involve the participation of
radicals and radical anions as intermediates (eq 1).
Photoinduced electron transfer (PET) stimulates wide
interest in areas that range from biological to technolog-
ical processes.1 The rate of the process and the formation
of free fragments are strongly dependent on the nature
of the donor-acceptor pair. Aliphatic and aromatic free
radicals produced in electron-transfer processes can
initiate a chain reaction mechanism of substitution (SRN
1
mechanism) to afford substitution in aromatic (ArX) and
aliphatic halides (RX) where the classical mechanisms,
i.e., SN1, SN2, and SNAr, are not possible for steric
hindrance or energetic reasons.2
On the other hand, an efficient O-alkylation occurs in
the SRN1 reaction of 1-methyl-2-naphthoxide ions4 with
R,p-dinitrocumene and R-chloro-p-nitrocumene and phen-
oxide ions5 with R,p-dinitrocumene. The change in the
regiochemistry of the coupling reactions of these ambi-
dent anions depends on the radical nature. This was
ascribed to differences in the activation energy for the
two possible pathways. Thus, the C-C coupling route is
faster than the C-O route for aryl radicals, and the
inverse occurs for the radicals with EWG.6 To the present,
It is known that the 2-naphthoxide ion reacts with a
variety of aryl halides under photostimulation in liquid
ammonia to yield 1-aryl-2-naphthoxides as substitution
products.3 These reactions have been proposed to occur
* Address correspondence to this author. Fax: 54-351-4333030/
4334174. Phone: 54-351-4334170/73.
(1) (a) Fox, M. A.; Chanon, M., Eds. Photoinduced Electron Transfer;
Elsevier: The Hague, 1988. (b) The complete volumes of the following
recent journals: Chem. Rev. 1992, 92. J . Photochem. Photobiol A.
Chem. 1994, 82.
(2) (a) Rossi, R. A.; Pierini, A. B.; Palacios, S. M. Advances in Free
Radical Chemistry; Tanner, D. D., Ed.; J ai Press: London, UK, 1990;
pp 193-252. (b) Lund, H.; Daasbjerg, K.; Lund, T.; Pedersen, S. U.
Acc. Chem. Res. 1995, 28, 313-319. (c) Rossi, R. A.; Pierini, A. B.;
Pen˜e´n˜ory, A. B. In The Chemistry of the Functional Groups; Patai, S.,
Rappoport, Z., Eds.; Wiley: Chichester, UK, 1995; Suppl. D2, Chapter
24, pp 1395-1485. (d) Rossi, R. A.; Pierini, A. B.; Pen˜e´n˜ory, A. B. Chem.
Rev. 2003, 103, 71-167.
(3) Pierini, A. B.; Baumgartner, M. T.; Rossi, R. A. J . Org. Chem.
1991, 56, 580-586.
(4) Kornblum, N.; Cheng, L.; Davies, T. M.; Earl, G. W.; Holy, N.
L.; Kerber, R. C.; Kestner, M. M.; Manthey, J . W.; Musser, M. T.;
Pinnick, H. W.; Snow, D. H.; Stuchal, F. W.; Swiger, R. T. J . Org. Chem.
1987, 52, 196-204.
(5) Kornblum, N.; Davies, M. T.; Earl, G. W.; Greene, G. S.; Holy,
N. L.; Kerber, R. C.; Manthey, J . W.; Musser, M. T.; Snow, D. H. J .
Am. Chem. Soc. 1967, 89, 5714-5715.
(6) Pierini, A. B.; Borosky, G. L.; Baumgartner, M. T. Int. J .
Quantum Chem. 1992, 44, 759-772.
10.1021/jo026518y CCC: $25.00 © 2003 American Chemical Society
Published on Web 02/20/2003
2362
J . Org. Chem. 2003, 68, 2362-2368

Fluorescent Quenching of the 2-Naphthoxide Anion
TABLE 1. F lu or escen t Qu en ch in g of An ion 1 by Alk yl
a n d Ar yl Ha lid es
F IGURE 1. Fluorescent quenching of ion 1 by 2.
there are neither reports on the reactivity of these anions
with aliphatic halides without EWG nor quantitative
photochemical studies of these reactions. Considering
that the photophysics of the 2-naphthoxide anion (1) was
determined by Soumillion and co-workers,7 we undertook
a systematic study of the photoinduced reaction of ion 1
with 1-adamantyl and neopentyl halides. The product
distribution as well as the regiochemistry of the coupling
reaction between the radicals and anion 1 was deter-
mined.
Resu lts
a
Reduction peak potential at 0.1 V s-1, 0.1 M Bu4NBF4 in
F lu or escen t Qu en ch in g of th e 2-Na p h th oxid e Ion
(1) by RX a n d Ar X. A solution of ion 1 in DMSO has a
pale yellow color and absorbs maximally at 396 nm,
whereas most of the alkyl and aryl halides show absorp-
tion below 300 nm. When different RX or ArX are added
to a solution of 1, no changes are observed in the
absorption spectra.
b
DMSO. LUMO energy in eV obtained by using the AM1 method.
c Stern-Volmer constant. kq ) Ksv/τ0; τ0 ) lifetime of the excited
d
singlet of 1. e Quenching Φ for [Q] ) 0.04 M. f J aworski, J . E.;
Leszczynski, P.; Kalinowski, M. K. Pol. J . Chem. 1995, 69, 688-
g
692. Energy of the σ* C-X bond.
C-X, which is not the case for aryl halides. However,
for all the monohalide substrates studied with the
exception of 1-bromonaphthalene, a parabolic correlation
is found between the quenching rate constant and the
calculated σ* C-X, including the phenyl derivatives
(Figure 2). Whereas bromobenzene exhibits a quenching
rate constant of 1.66 × 108 M-1 s-1 (C-X BDE control),
1-bromonaphthalene shows a kq of 10.4 × 109 M-1 s-1 (π
LUMO control).
On the other hand, the fluorescent excited state of
anion 1 (φF ) 0.93 in DMSO)7 is quenched by RX and
ArX following Stern-Volmer linear plots, which allow us
to evaluate the quenching rate constants (kq) and the
quantum yield for the fluorescent quenching (φq). As an
example, the effect of adding increasing concentrations
of 1-iodoadamantane (2) on the fluorescent spectra of
anion 1 is shown in Figure 1. The quenching rate
constants have a good correlation with the reduction peak
potentials for the halides measured by cyclic voltametry
and with the theoretical values of their LUMO energy
calculated by using the AM1 method (Table 1). Thus,
iodide 2 inhibits the fluorescence of anion 1 with a rate
constant near diffusion (kdiff for DMSO ) 3.3 × 109 M-1
s-1).8 For the alkyl halides the LUMO orbital is the σ*
Under similar conditions chlorobenzene was unreac-
tive.
P h ot oin d u ced R ea ct ion s of Ion 1 w it h R I a n d
P h I. From stationary photolysis experiments we per-
formed a detailed analysis of the reaction products. Thus,
the photoinduced reaction of 1 with R-X ) 1-iodo-
adamantane (2), 1-iodo-2,2-dimethylpropane (3), and
1-iodo-2,2-dimethyl-3-phenylpropane (4) in DMSO ren-
dered a mixture of the respective alkanes R-H (from the
hydrogen abstraction of the radical intermediates), sub-
stitution products (which arise from the addition of the
radicals to the 3, 6, and 8 positions of the ion), as well as
(7) (a) Soumillion, J . Ph.; Vandereecken, P.; Van Der Auweraer, M.;
De Schryver, F. C.; Schanck, A. J . Am. Chem. Soc. 1989, 111, 2217-
2225. (b) Legros, B.; Vandereecken, P.; Soumillion, J . Ph. J . Phys.
Chem. 1991, 95, 4752-4761.
(8) Murov, S. L.; Carmichael, I.; Hung, G. L. Handbook of Photo-
chemistry, 2nd ed.; Marcel Dekker: New York, 1993.
J . Org. Chem, Vol. 68, No. 6, 2003 2363

Argu¨ello and Pen˜e´n˜ory
TABLE 3. Qu a n tu m Yield s of th e P ET Rea ction s of Ion
1 w ith 2a
Φc
added compd
entry
(M)
I- b
6
8
9
10
11
1
1.08 0.071 0.076 0.103 0.222 0.304
2d
3f
4
DTBN (0.04)
DTBN (0.08)
1,4-cyclohexa-
diene (0.12)
e
e
e
0.078 0.006 0.040 0.017
0.014 0.004 0.024 0.012
0.222 0.057 0.049 0.183 0.061
a
Performed under nitrogen atmosphere, using 0.04 M solutions
of substrate and 0.12 M solutions of the anion, irradiation at λ
max 350 nm. b Determined potentiometrically. c Determined by GC,
using the internal standard method, error 5%, with potassium
d
ferrioxalate as actinometer. In this reaction the Φ of the adduct
12 is 0.147. e Not quantified. f Φ of the adduct 12 is 0.094.
13.4% yield (Table 2, entry 3). It was previously reported9
that the photolysis of 2 in polar solvents affords solvolytic
products coming from a carbocation intermediate. Thus,
1-adamantanol is the main product observed in DMSO.9
The quantum yield determined (λ ) 350 nm) for the
reaction of ion 1 with iodide 2 was near 1, evaluated as
the quantum yield for the iodide ion formation or as the
addition of the quantum yields for the formations of
products 6-11 (Table 3, entry 1). This photochemical
reaction is inhibited by a radical trap as di-tert-butyl-
nitroxide (DTBN) with formation of adduct 12. In the
F IGURE 2. Correlation between the rate constant for the
quenching and the MO energy.
TABLE 2. Rea ction s of th e 2-Na p h th oxid e Ion (1) w ith
Iod id es in DMSOa
product distribution (%)c
substitution products on
substr reac
entry
RI
cond I- (%)b RH ROH
O
C1 C3 C6
C8
1
2
3d
4
5
6
7
8
2
hv
dark
hv
hv
dark
hv
85
<2
27
83
<2
89
14.1 2.8
2.5
3.2 13.2 12.2
13.4
3
e
e
2.8 8.1
1.3 19.1 31.1
4
5
8.9
e
e
e
e
e
e
presence of 1,4-cyclohexadiene as hydrogen atom donor,
the quantum yields for the reaction products 8-11
decrease whereas the formation of adamantane consider-
ably increases (Table 3, entries 2-4). For the quantum
yield determination the reactions are performed at very
low conversion, and not even traces of 1-adamantanol are
observed.
hv
dark
54
<2
41.9f
a
Substrates 0.04 M, 2-naphthoxide ion (1) 0.12 M, irradiation
for 1 h with a medium-pressure Hg lamp under nitrogen atmo-
sphere. Determined potentiometrically. c Determined by GC, us-
b
d
ing the internal standard method, error 5%. Without the pres-
ence of ion 1. e Not quantified. f Together with 5.4% yield of a
disubstitution product.
Rea ction of An ion 1 w ith 1-Iod o-2,2-d im eth ylp r o-
p a n e (3). When a solution of anion 1 with iodide 3 was
irradiated for 1 h, a mixture of the substitution products
on carbons 1, 3, 6, and 8 of the naphthoxide anion and
the ether derivative were formed with 83% yield of iodide
ion (eq 3) (Table 2, entry 4).
minor amounts of the corresponding alcohols (R-OH)
and 1-alkyl-2-naphthyl ether derivatives. The photo-
induced reaction of anion 1 with iodobenzene (5) was also
studied in DMSO.
Rea ction s of An ion 1 w ith 1-Iod oa d a m a n ta n e (2).
The photoinduced reaction of anion 1 with iodide 2 yields
after 1 h a mixture of products: adamantane (6), 1-ada-
mantanol (7), 1-adamantyl 2-naphthyl ether (8), and the
substitution products on carbons 3, 6, and 8 of the
naphthyl ring (9-11) (eq 2). This reaction does not occur
in the dark (Table 2, entries 1 and 2).
There is no reaction in the dark between 1 and 3. To
improve the mass balance by quantification of the
product from hydrogen atom abstraction of the radical
intermediate, the photoinduced reaction was performed
with 1-iodo-2,2-dimethyl-3-phenylpropane (4), a heavier
As a control experiment, a solution of iodide 2 was
irradiated for 1 h in DMSO to give 1-adamantanol in
(9) Perkins, P. R.; Pincock, R. E. Tetrahedron Lett. 1975, 11, 943-
946.
2364 J . Org. Chem., Vol. 68, No. 6, 2003

Fluorescent Quenching of the 2-Naphthoxide Anion
A dissociative mechanism12 is assumed for the ET from
the exited state of anion 1 to aliphatic halides without
electron-withdrawing groups such as 1-iodoadamantane
(eq 5 and eq 6), as evidenced by the observed good
correlation between the log kq and the values of σ* C-X
for most of the compounds studied (Figure 2).
SCHEME 1
derivative of 3. Thus, after 1 h of irradiation a solution
of anion 1 with 4 renders 89% yield of iodide ion with
8.9% yield of 2,2-dimethyl-3-phenylpropane determined
by GC, with a complex mixture of substitution products,
which were not isolated (Table 2, entries 5 and 6).
Rea ction s of An ion 1 w ith Iod oben zen e (5). The
photostimulated reaction of anion 1 with iodide 5 affords
mainly the substitution product at C1 (18) together with
a minor amount of a disubstitution product 19 (41.9%
and 5.4% yield respectively) (eq 4). Due to the complexity
of the signals in the proton and carbon NMR spectra, it
was not possible to assign the structure of product 19.
This reaction does not occur in the dark (Table 2, entries
7 and 8).
The fact that for the bromo- and iodobenzene kq
correlates with the σ* C-X values and not with the
LUMO energy (for which a higher kq should be expected)
allows us to propose a concerted mechanism for the ET
and the fragmentation of the C-X bond for both halides
(C-X BDE control). Whereas 1-chlorobenzene was un-
reactive, 1-bromonaphthalene showed a faster quenching
rate constant than bromobenzene, which correlates with
the aromatic LUMO. This result suggests a stepwise
behavior for the naphthalene bromide (π LUMO control).
A transition between a concerted and stepwise mech-
anism is possible by changing the driving force of the
experimental conditions.13,14 For example, by increasing
the scan rate of cyclic voltametry15 or by modifying the
temperature.16 For iodobenzene the mechanism passes
from concerted to stepwise as the driving force increases.
In contrast, bromobenzene and 1-iodonaphthalene follow
a stepwise mechanism over the whole range of scan
rate.17
The fluorescent quenching of the 2-naphthoxide ion by
the aromatic halides showed a clear change of mechanism
from concerted to stepwise going from bromobenzene to
1-bromonaphthalene, by changing the driving force of the
reaction, that is making the process more exergonic.
Our experimental results also indicate that the calcu-
lated energy order for the virtual molecular orbitals
obtained by AM1 is erroneous. This statement is in
agreement with the recently ab initio theoretical studies
with electronic correlation, showing that the LUMO for
iodobenzene has σ symmetry (σ* C-I) and that for
chlorobenzene is of π nature, whereas for bromobenzene
both orbitals σ (σ* C-Br) and π have similar energy.18
Qu a n tu m Yield for F lu or escen t Qu en ch in g. For
the photochemical initiation by an excited donor, the
occurrence of a concerted ET/bond-breaking reaction
Discu ssion
Mech a n ism for t h e P E T R ed u ct ion s of R X a n d
Ar X. The formation of a CTC between the naphthoxide
anion and the halides was not observed by UV-vis
spectroscopy, which allowed us to disregard this possibil-
ity as a mechanism for the ET reaction.
However, the rate constant for the fluorescent quench-
ing of anion 1 by the halides shows a good correlation
with their reduction potential or the LUMO energies.
Thus, as the reduction potential of RX is made more
negative or the LUMO energy is higher, a decrease in
the quenching rate constant is found. On the basis of the
observed correlation we propose an electron-transfer
mechanism for this fluorescent quenching of anion 1 by
the electron acceptors (RX and ArX).
Two different mechanisms are possible for the ET: a
stepwise process with consecutive ET and fragmentation
of the C-X bond in which the radical anion is an
intermediate, or a concerted one, namely dissociative ET.
This dichotomy is presented in Scheme 1.
Both mechanisms are well described in the literature
for homogeneous and heterogeneous (electrochemical)
reactions.10 For aromatic halides such as iodo-, bromo-,
and chlorobenzene it was considered that the ET affords
a π radical that after an intramolecular ET to the σ state
dissociates into a Ph• radical.11 On the other hand, for
alkyl halides such as 1-iodo- and 1-bromoadamantane
and iodo- and bromoneopentane, the ET and fragmenta-
tion is proposed to occur simultaneously in a concerted
mechanism.
(12) (a) Save´ant, J . M. J . Am. Chem. Soc. 1987, 109, 6788-6795.
(b) Save´ant, J . M. J . Am. Chem. Soc. 1992, 114, 10595-10602.
(13) Save´ant, J .-M. Adv. Phys. Org. Chem. 2000, 35, 117-192.
(14) (a) Antonello, S.; Maran, F. J . Am. Chem. Soc. 1997, 119,
12595-12600. (b) Daasbjerg, K.; J ensen, H.; Benassi, R.; Taddei, F.;
Antonello, S.; Gennaro, A.; Maran, F. J . Am. Chem. Soc. 1999, 121,
7150-7151.
(15) Andrieux, C. P.; Robert, M.; Saeva, F. D.; Save´ant, J . M. J . Am.
Chem. Soc. 1994, 116, 7864-7871.
(10) (a) Save´ant, J . M. Adv. Phys. Org. Chem. 1990, 26, 1-130. (b)
Save´ant, J . M. Adv. Electron-Transfer Chem. 1994, 4, 53-116. (c)
Save´ant, J . M. Acc. Chem. Res. 1993, 26, 455-461. (d) Lund, H.;
Daasbjerg, K.; Lund, T.; Occhialini, D.; Pedersen, S. U. Acta Chem.
Scand. 1997, 51, 135-144.
(11) Pierini, A. B.; Duca, J . S., J r.; Baumgartner, M. T. THEOCHEM
1994, 311, 343-352 and references therein.
(16) Andrieux, C. P.; Save´ant, J . M.; Tardy, C. J . Am. Chem. Soc.
1997, 119, 11546-11547.
(17) Pause, L.; Robert, M.; Save´ant, J .-M. J . Am. Chem. Soc. 1999,
121, 7158-7159.
(18) Vera, M. PhD Thesis, adviser Prof. Dra. Adriana B. Pierini,
Departamento de Qu´ımica Orga´nica, Facultad de Ciencias Qu´ımicas,
Universidad Nacional de Co´rdoba, 2001.
J . Org. Chem, Vol. 68, No. 6, 2003 2365

Argu¨ello and Pen˜e´n˜ory
SCHEME 2
intuitively appears as an ideal situation in which the
quantum yield quenching of fragmentation should be
unity. This view is based on back ET between the
fragments in the solvent cage having an activation
barrier too high to compete with their diffusion out of
the cage.13,19 On the other hand, quenching quantum
yields are expected to be lower for a stepwise photo-
induced ET due to the higher probability of back ET from
the intermediate radical anion formed.13,19 However, it
has been shown on theoretical grounds that photoinduced
dissociative ETs do not necessarily mean a quantum yield
equal to unity. Partition between back ET and fragmen-
tation when the upper first-order potential energy of the
system intersects the reactants and fragments zero order
surfaces is the main reason for observing a less than
unity quantum yield.13,19 Thus, the experimental quench-
ing quantum yields measured for the reduction of CCl4
by the excited singlets of perylene and 2-ethyl-9,10-
dimethoxyanthracene (EDA) are 0.70 and 0.77, respec-
tively.20a Similarly the experimental quenching quantum
yields measured for the ET from the excited singlets of
perylene to 4-cyano-trifluoromethyl-benzene and from
EDA to dimethylphenyl sulfonium, to 4-cyanobenzyl-
methylphenyl sulfonium, and to 4-cyanobenzyl chloride
were 0.25, 0.35, 0.77, and 0.55, respectively.20b On the
basis of electrochemical and photochemical experimental
results, the first two values are assigned to a stepwise
ET mechanism; the last one (0.55) is assigned to a
concerted mechanism and a value of 0.77, corresponding
to 4-cyanobenzylmethylphenyl sulfonium, a situation in
which both mechanism are considered competitive.20b
Our results are consistent with the statement that a
concerted photoinduced electron transfer is not neces-
sarily endowed with a unity quantum yield.13,19 In fact,
a quantum yield as low as 0.5 can be envisaged from
theoretical calculations.19a Thus, for the dissociative PET
reactions between anion 1 and iodides 2, 3, and 5 the
quantum yields measured are 0.75, 0.75, and 0.77,
respectively.
Nu cleop h ilic Su bstitu tion s Rea ction s of An ion 1
w ith Alk yl a n d Ar yl Ha lid es. There is no reaction
between anion 1 and iodide 2 in the dark whereas a
mixture of substitution on C3, C6, and C8 (9-11),
reduction (6), and the solvolytic 1-adamantanol and the
ether derivative (7 and 8) are produced in the photo-
stimulated process, which is strongly inhibited by DTBN.
Also, inhibition in the formation of the substitution
products is observed in the presence of a hydrogen atom
donor such as 1,4-cyclohexadiene, with an increase in the
yield of adamantane. These results can be rationalized
by a radical chain mechanism of substitution (SRN1) as
follows.
The adamantyl radical (20) formed by a dissociative
PET (eqs 5 and 6) reacts with 1 at the 3, 6, and 8
positions to yield the radical anion intermediates. No
addition to C1 is observed. These radical anions transfer
their odd electron to the substrate to continue the
propagation cycle to give the radical 20 and the products
21-23. Under the basic reaction conditions, the latter
yield the more stable enol tautomers (products 9-11)
(Scheme 2).
To measure the efficiency and chain length of the SRN1
reactions, it is necessary to know the quantum yield for
the initiation step as well as the quantum yield for the
formation of the substitution products (Φglobal). Thus, it
is possible to obtain the Φpropagation (that is, the chain
length) from the ratio of the overall quantum yield and
the quantum yield of the initiation step.21
In an ideal situation, the photochemical efficiency of
the initiation process will be the fluorescent quenching
quantum yield measured. To evaluate only the quantum
yield of the initiation process, it was necessary to carry
out the reactions in the presence of a radical trap such
as DTBN, which inhibits the propagation steps. Although
the concentration of DTBN was enhanced from 0.008 to
0.08 M, the complete inhibition of the chain reaction was
not observed. Considering the value obtained for a
concentration of 0.08 M as the photochemical efficiency
of the initiation process (Table 3, entry 3), it is possible
to estimate an approximate value of 7 for the chain length
or Φpropagation for the reaction of 2-naphthoxide ion 1 with
iodide 2.
The formation of 1-adamantanol (7) and 1-adamantyl
2-naphthyl ether (8) is evidence of the presence, although
in a lower proportion, of the 1-adamantyl cation (25) as
an intermediate in this reaction. This cation can be
generated by photolysis of 2 (Table 2, entry 3); neverthe-
less, a decrease in the quantum yield of 8 is observed by
the addition of DTBN and 1,4-cyclohexadiene (Table 3,
entries 3 and 4). These latter results can be ascribed to
the participation of 1-adamantyl radicals (20) in the
formation of the corresponding cation. Thus, 25 can be
produced by photolysis of 2 or by ET from radical 20 to
a good acceptor such as the naphthoxyl radical (24)
(Scheme 3).
The photoinduced reaction of anion 1 with neopentyl
iodide (3) yields substitution products on C1, C3, C6, C8,
and the oxygen. However, not even a trace of the alcohol
derivative is formed, which allows us to disregard the
participation of the neopentyl cation in this reaction. On
the other hand, it is well-known that the neopentyl cation
gives a very fast rearrangement to the 2-methyl-2-butyl
cation. These results indicate that the reaction proceeds
(19) (a) Robert, M.; Save´ant, J .-M. J . Am. Chem. Soc. 2000, 122,
514-517. (b) Costentin, C.; Robert, M.; Save´ant, J .-M. J . Phys. Chem.
A 2000, 104, 7492-7501.
(20) (a) Pause, L.; Robert, M.; Save´ant, J .-M. Chem. Phys. Chem.
2000, 1, 199-205. (b) Pause, L.; Robert, M.; Save´ant, J .-M. J . Am. Chem.
Soc. 2001, 123, 4886-4895.
(21) Argu¨ello, J . E.; Pen˜e´n˜ory, A. B.; Rossi, R. A. J . Org. Chem. 2000,
65, 7175-7182.
2366 J . Org. Chem., Vol. 68, No. 6, 2003

Fluorescent Quenching of the 2-Naphthoxide Anion
SCHEME 3
TABLE 4. AM1 (k ca l/m ol) Hea t of F or m a tion for th e
Ra d ica l An ion P r od u ct in th e Rea ction of An ion 1 w ith
P h en yl, Ad a m a n tyl, a n d Neop en tyl Ra d ica ls
position at anion 1
F IGURE 3. Radical anion products for the coupling at C1 of
the 2-naphthoxide ion with 1-adamantyl, phenyl, and neo-
pentyl radicals.
radical
phenyl
C1
C3
C6
C8
O
-1.729
0.297
5.665
2.206
14.730
1-adamantyl (20) -58.611 -57.412 -51.968 -54.575 -44.275
neopentyl -53.001 -51.123 -43.802 -49.824 -38.025
coupling at C1. These factors could be responsible for a
higher activation energy for coupling with these radicals,
which will not be the case with the phenyl radicals.
On these basis coupling at position 1 with phenyl
radicals will be under thermodynamic control while
kinetic control will determine the coupling with the more
sterically hindered radicals. Calculations with a higher
theoretical level and with inclusion of solvent effects to
clarify this regiochemistry are in progress.
by a radical chain mechanism with the neopentyl radical
as the intermediate. Coupling of this radical on the
oxygen or the C1, C3, C6, and C8 results in the observed
products 13-17.
Iodobenzene reacts with anion 1 under irradiation to
yield only monosubstitution on C1 (18) and disubstitution
as a minor product (19). This reaction proceeds by the
SRN1 mechanism as previously reported, using liquid
ammonia as solvent.3,22 Deprotonation of 18 by anion 1
affords the 1-phenyl-2-naphthoxide anion, which is able
to react further as a nucleophile with phenyl radicals to
yield the disubstituted product 19.
Regioch em istr y of th e Cou p lin g Rea ction . The
regiochemistry observed in the reaction of phenyl radicals
with anion 1 is in agreement with those previously
reported,22 although the concentrations used in DMSO
are higher than those employed in liquid ammonia. Thus,
in this reaction only coupling at C1 is observed for the
monosubstitution.
On the other hand, 1-adamantyl radicals (20) gener-
ated by PET from anion 1 to 2 showed a very low
reactivity yielding coupling at C3, C6, and C8. To
rationalize the observed regiochemistry, preliminary
theoretical results obtained from the AM1/UHF method23
show that for all the cases studied the substitution on
C1 yields the more stable radical anion (Table 4). This
is also valid for the 1-adamantyl radical, which does not
present reactivity at that position. Only the phenyl
radical yields exclusive coupling at C1.
Con clu sion s
The fluorescent excited state of the 2-naphthoxide ion
(1) is quenched by aliphatic and aromatic halides as
electron acceptors according to an electron-transfer mech-
anism, which is proposed on the basis of the following:
(a) decreasing the quenching rate constant as the reduc-
tion potential of the halides is made more negative,
following a typical behavior for an electron-transfer
reaction, and (b) from a detailed analysis of the photo-
chemical reaction products. The ET and the fragmenta-
tion of the C-X bond for 1-adamantyl, neopentyl, and
phenyl iodide and bromide are assumed to be concerted
due to the observed good correlation between the kq with
the σ* C-X energies, whereas for 1-bromonaphthalene
a stepwise behavior is suggested.
Aliphatic halides (such as 1-iodoadamantane and neo-
pentyl iodide) and iodobenzene react with 2-naphthoxide
ions by the SRN1 mechanism in DMSO. Mechanistic
evidence is provided by the stimulation of these reactions
by light and the inhibition of the photoinduced reactions
in the presence of 1,4-cyclohexadiene and DTBN. The
photostimulated reactions of the 2-naphthoxide ion with
different radicals afford a mixture of substitution prod-
ucts. The addition of the 1-adamantyl radical gives
substitution at the 3, 6, and 8 positions of the ion as well
as 1-adamantanol and minor amounts of 1-adamantyl
2-naphthyl ether. With a primary radical a similar
behavior is observed as well as formation of the expected
substitution at the one-position of the naphthyl moiety;
meanwhile the aromatic radical renders almost exclu-
sively the product coming from the substitution at the
one-position. These last results can be rationalized in
terms of steric effects.
The radicals anions calculated for coupling at C1 are
presented in Figure 3. As can be seen from the figures
the intermediate formed by coupling with radical 20 has
a considerably steric hindrance between the methylene
group of the radical and the oxygen and the hydrogen at
position 8 of anion 1. The primary neopentyl radical
presents low regioselectivity with a minor steric hin-
drance as is shown by the formation of product from
(22) (a) Pierini, A. B.; Baumgartner, M. T.; Rossi, R. A. Tetrahedron
Lett. 1988, 29, 3451-3454. (b) Petrillo, G.; Novi, M.; Dell’Erba, C.
Tetrahedron Lett. 1989, 30, 6911-6912. (c) Beugelmans, R.; Chbani,
M. New J . Chem. 1994, 18, 949-952.
(23) (a) Dewar, M. J . S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J . J .
P. J . Am. Chem. Soc. 1985, 107, 3902-3909. (b) Semiempirical
calculations were performed by using the AM1/UHF method as
implemented in the AMPAC package, from the Quamtum Chemistry
Program Exchange (QCPE), Program No. 506.
Exp er im en ta l Section
Meth od s. The fluorescent quenching study was performed
by using stationary fluorescent techniques at room tempera-
ture (25 ( 1 °C). The reduction peak potentials were measured
J . Org. Chem, Vol. 68, No. 6, 2003 2367

Argu¨ello and Pen˜e´n˜ory
by cyclic voltametry. The general methods and procedure for
the photoinduced reaction are the same as previously pub-
lished.21
8-(1-Ad a m a n tyl)-2-n a p h th ol (11) was isolated by several
radial chromatography techniques with petroleum ether/
diethyl ether (95:5) as eluent from the crude product reaction
mixture of anion 1 and 2: 1H NMR (CDCl3) δ 1.88 (6H, m),
2.19 (3H, m), 2.30 (6H, m), 5.38 (1H, s), 7.06 (1H, dd, J ) 8.8,
2.2 Hz), 7.27 (1H, t, J ) 7.7 Hz), 7.42 (1H, dd, J ) 7.5, 1.3
Hz), 7.63 (1H, d, J ) 8 Hz), 7.77 (1H, d, J ) 8.7 Hz), 8.01 (1H,
d, J ) 2.2 Hz); 13C NMR (CDCl3) δ 29.70, 37.19, 38.40, 42.34,
109.69, 116.21, 123.17, 123.98, 127.24, 130.53, 131.52, 132.44,
144.46, 151.79. MS (EI+) 279 (22), 278 (100), 221 (48). HRMS
(EI+) calcd 278.1671, expt 278.1670.
Cou plin g pr odu ct fr om 1-adam an tyl r adical an d DTBN
(12) was isolated by radial chromatography with petroleum
ether/diethyl ether (95:5) as eluent from the crude product
reaction mixture of anion 1 and 2 in the presence of DTBN:
1H NMR (CDCl3) δ 1.06 (18H, s), 1.615 (6H, m), 1.76 (6H, m),
2.11 (3H, m); 13C NMR (CDCl3) δ 27.19, 29.70, 30.56, 36.68,
41.13, 53.79. MS (EI+) 135 (100), 93 (30), 79 (26).
Neop en tyl 2-n a p h th yl eth er (13) was isolated by radial
chromatography with petroleum ether as eluent from the crude
product reaction mixture of anion 1 and 3: 1H NMR (CDCl3) δ
1.08 (9H, s), 3.71 (2H, s), 7.11-7.44 (4H, m), 7.69-7.76 (3H,
m); 13C NMR (CDCl3) δ 26.71, 31.91, 77.92, 106.54, 119.13,
123.38, 126.24, 126.64, 127.62, 128.88, 129.21, 134.67, 157.59.
MS (EI+) 215 (5), 214 (31), 145 (19), 144 (100), 116 (11), 115
(39).
1-Neop en tyl-2-n a p h th ol (14) was isolated by radial chro-
matography with petroleum ether/diethyl ether (98:2) as eluent
from the crude product reaction mixture of anion 1 and 3: 1H
NMR (CDCl3) δ 1.09 (9H, s), 3.06 (2H, s), 7.11 (1H, d, J ) 8.8
Hz), 7.31-7.39 (1H, m), 7.45-7.59 (1H, m), 7.68 (1H, d, J )
9.1 Hz), 7.79 (1H, dd, J ) 8, 1.1 Hz), 8.06 (1H, d, J ) 8.8 Hz);
13C NMR (CDCl3) δ 30.53, 34.39, 37.30, 117.72, 117.91, 122.77,
124.52, 125.70, 128.02, 128.40, 151.63. MS (EI+) 215 (5), 214
(31), 158 (80), 157 (100), 129 (55), 128 (45), 127 (19).
1-P h en yl-2-n a p h th ol (18) was isolated by radial chroma-
tography with petroleum ether/diethyl ether (95:5) as eluent
from the crude product reaction mixture of anion 1 and 4: 1H
NMR (CDCl3) δ 5.15 (1H, s), 7.18-7.82 (11H, m); 13C NMR
(CDCl3) δ 117.35, 120.96, 123.25, 124.57, 126.43, 127.97,
128.40, 128.91, 129.45, 129.56, 131.15, 133.25, 134.19, 150.12.
MS (EI+) 221 (15), 220 (100), 219 (46), 191 (20), 198 (12), 189
(30). HRMS (EI+) calcd 220.0888, expt 220.0886.
Mater ials. t-BuOK, 2,2-dimethyl-3-phenyl-1-propanol, bromo-
neopentane, iodoneopentane, bromobenzene, iodobenzene,
1-bromoadamantane, 1-iodoadamantane, DTBN, and 1,4-
cyclohexadiene were all high purity commercial samples which
were used without further purification. The commercially
available 2-naphthol was recrystallized from water. DMSO
was distilled under vacuum and stored over molecular sieves
(4 Å). The supporting electrolyte Bu4NBF4 was prepared
following standard procedures. The 2-naphthoxyde ion (1) was
generated in situ by acid-base deprotonation with use of
t-BuOK. 1-Iodo-2,2-dimethyl-3-phenylpropane24 was synthe-
sized by reaction of the corresponding tosylate with KI or LiCl
in DMF. The tosylate and benzene sulfonate were prepared
by standard procedures. The di- and trihaloneopentyl deriva-
tives such as 1-iodo-2-iodomethyl-2-methylpropane, 1-bromo-
2-bromomethyl-2-methylpropane, 1-chloro-2-chloromethyl-2-
methylpropane, and 1-bromo-2,2-dibromomethylpropane were
synthesized by reaction of the corresponding tosylates with
KI, NaBr, or LiCl in diethylene glycol following a procedure
similar to that reported in the literature.25 1,3-Diiodoadaman-
tane and 1,3-dibromoadamantane were obtained by oxidation
of the corresponding haloadamantane with CrO3 and further
reaction of the 3-hydroxy-1-haloadamantane with HI or HBr.26
1-Ad a m a n tyl 2-n a p h th yl eth er (8) was isolated by radial
chromatography with petroleum ether as eluent from the crude
product reaction mixture of anion 1 and 2: 1H NMR (CDCl3)
δ 1.62 (6H, m), 1.95 (6H, m), 2.18 (3H, m), 7.15-7.82 (7H, m);
13C NMR (CDCl3) δ 29.08, 37.11, 37.25, 40.75, 110.82, 123.84,
125.03, 125.81, 126.27, 127.83, 129.15, 132.74, 138.64, 153.62.
MS (EI+) 279 (2), 278 (9), 135 (100), 93 (19), 79 (21). HRMS
(EI+) calcd 278.1671, expt. 278.1672.
3-(1-Ad a m a n tyl)-2-n a p h th ol (9) was isolated by radial
chromatography with petroleum ether/diethyl ether (98:2) as
eluent from the crude product reaction mixture of anion 1 and
2: 1H NMR (CDCl3) δ 1.82 (6H, m), 2.13 (3H, m), 2.23 (6H, m),
5.06 (1H, s), 6.99 (1H, s), 7.24-7.40 (2H, m), 7.64 (1H, s), 7.58-
7.77 (2H, m); 13C NMR (CDCl3) δ 29.67, 36.17, 42.90, 78.27,
120.96, 124.52, 125.59, 125.92, 127.21, 127.53, 128.29, 130.55,
134.00, 151.90; MS (EI+) 279 (21), 278(100), 221(37). HRMS
(EI+) calcd 278.1671, expt 278.1672.
6-(1-Ad a m a n tyl)-2-n a p h th ol (10)27 was isolated by several
radial chromatography techniques with petroleum ether/
diethyl ether (95:5) as eluent from the crude product reaction
mixture of anion 1 and 2: 1H NMR (CDCl3) δ 1.80 (6H, m),
2.00 (6H, m), 2.13 (3H, m), 4.91 (1H, s), 7.02-7.11 (2H, m),
7.49-7.73 (4H, m); 13C NMR (CDCl3) δ 29.00, 36.14, 36.90,
43.18, 109.10, 117.48, 122.71, 124.81, 126.02, 128.99, 129.85,
132.79, 146.56, 152.92. MS (EI+) 279 (20), 278(100), 221(65).
HRMS (EI+) calcd 278.1671, expt 278.1668.
Ack n ow led gm en t. We especially acknowledge Prof.
Kim Daasbjerg from Aarhus University for providing
the facilities to make the electrochemical measure-
ments. This work was supported in part by the Consejo
Nacional de Investigaciones Cient´ıficas y Te´cnicas
(CONICET) and SECYT-Universidad Nacional de Co´r-
doba, Argentina. J .E.A. gratefully acknowledges the
receipt of a fellowship from CONICET.
(24) Yuan, K.; Scott, W. J . J . Org. Chem. 1990, 55, 6188-6194.
(25) Organic Syntheses; Baumgarten, Ed.; J ohn Wiley & Sons: New
York, 1963; Collect. Vol. IV, p 753.
(26) Lukach, A. E.; Santiago, A. N.; Rossi, R. A. J . Phys. Org. Chem.
1994, 7, 610-614.
(27) Rosowsky, A.; Chen, K. N.; Papathanasopoulos, N.; Modest, E.
J . J . Heterocycl. Chem. 1972, 9, 263-273.
Su p p or t in g In for m a t ion Ava ila b le: 1H NMR and 13C
NMR spectra of compounds 8-14, and 18. This material is
available free of charge via the Internet at http://pubs.acs.org.
J O026518Y
2368 J . Org. Chem., Vol. 68, No. 6, 2003