Quantum Yields of the Initiation Step and Chain Propagation Turnovers in SRN1 Reactions: Photostimulated Reaction of 1-Iodo-2-methyl-2-phenyl Propane with Carbanions in DMSO

Article abstract of DOI:10.1021/jo001141m

Neophyl radicals were generated by photoinduced electron transfer (PET) from a suitable donor to the neophyl iodide (1, 1-iodo-2-methyl-2-phenylpropane). The PET reaction of 1 with the enolate anion of cyclohexenone (2) afforded mainly the reduction products tert-butylbenzene (5) and the rearranged isobutylbenzene (6), arising from hydrogen abstraction of the neophyl radical (15) and the rearranged radical 16 intermediates, respectively. The photostimulated reaction of 1 with 2 in the presence of di-tert-butylnitroxide, as a radical trap, afforded adduct 10 in 57% yield. The photoinduced reaction of the enolate anion of acetophenone (3) with 1 gave the substitution products 11 (50%) and 12 (16%), which arise from the coupling of 3 with radicals 15 and 16, respectively. The rate constant obtained for the addition of anion 3 to radical 15 was 1.2 × 10⁵ M^-1 s^-1, by the use of the rearrangement of this radical as a clock reaction. The anion of nitromethane (4) was almost unreactive at the initiation step, but in the presence of 2 under irradiation, it gave high yields (67%) of the substitution product 13 and only 2% of the rearranged product 14. When the ratio of 4 to 1 was diminished, it was possible to observe both substitution products 13 and 14 in 16% and 6.4% yields, respectively. These last results allowed us to estimate the coupling rate constant of neophyl radicals 15 with anion 4 to be at least of the order of 10⁶ M^-1 s^-1. Although the overall quantum yield determined (λ = 350 nm) for the studied reactions is below 1, the chain lengths (Φ_propagation) for the reaction of 1 with anions 3 and 4 are 127 and 2, respectively.

Full text of DOI:10.1021/jo001141m

J . Org. Chem. 2000, 65, 7175-7182
7175
Qu a n tu m Yield s of th e In itia tion Step a n d Ch a in P r op a ga tion
Tu r n over s in SRN1 Rea ction s: P h otostim u la ted Rea ction of
1
-Iod o-2-m eth yl-2-p h en yl P r op a n e w ith Ca r ba n ion s in DMSO
J uan E. Arg u¨ ello, Alicia B. Pe n˜ e´ n˜ ory,* and Roberto A. Rossi*
INFIQC, Departamento de Qu ı´ mica Org a´ nica, Facultad de Ciencias Qu ´ı micas, Universidad Nacional de
C o´ rdoba, Ciudad Universitaria, 5000 C o´ rdoba, Argentina
rossi@dqo.fcq.unc.edu.ar
Received J uly 27, 2000
Neophyl radicals were generated by photoinduced electron transfer (PET) from a suitable donor to
the neophyl iodide (1, 1-iodo-2-methyl-2-phenylpropane). The PET reaction of 1 with the enolate
anion of cyclohexenone (2) afforded mainly the reduction products tert-butylbenzene (5) and the
rearranged isobutylbenzene (6), arising from hydrogen abstraction of the neophyl radical (15) and
the rearranged radical 16 intermediates, respectively. The photostimulated reaction of 1 with 2 in
the presence of di-tert-butylnitroxide, as a radical trap, afforded adduct 10 in 57% yield. The
photoinduced reaction of the enolate anion of acetophenone (3) with 1 gave the substitution products
1
1 (50%) and 12 (16%), which arise from the coupling of 3 with radicals 15 and 16, respectively.
5
-1 -1
The rate constant obtained for the addition of anion 3 to radical 15 was 1.2 × 10 M
s , by the
use of the rearrangement of this radical as a clock reaction. The anion of nitromethane (4) was
almost unreactive at the initiation step, but in the presence of 2 under irradiation, it gave high
yields (67%) of the substitution product 13 and only 2% of the rearranged product 14. When the
ratio of 4 to 1 was diminished, it was possible to observe both substitution products 13 and 14 in
1
6% and 6.4% yields, respectively. These last results allowed us to estimate the coupling rate
6
-1 -1
constant of neophyl radicals 15 with anion 4 to be at least of the order of 10 M
s . Although the
overall quantum yield determined (λ ) 350 nm) for the studied reactions is below 1, the chain
lengths (Φpropagation) for the reaction of 1 with anions 3 and 4 are 127 and 2, respectively.
The radical nucleophilic substitution reaction or SRN
1
is a well-documented process,1 which is proposed to
include an initiation, a propagation, and a termination
step in a chain process. The propagation cycle is depicted
in eqs 1-3.
,2
In the thermal SRN1 reaction of 4-nitrocumyl chloride
and the 2-nitropropane ion, a dissociative ET as in eq 4
has also been suggested.3
Because the SRN1 is a chain mechanism, the overall
reactivity depends on the efficiency of the initiation,
propagation, and termination steps. For the process to
work efficiently, the initiation step may be slow; however,
the chain propagation must be fast and efficient to allow
for long chains to build up.
When RX is an alkyl halide without electron-with-
drawing groups,1c,d the electron transfer (ET) from (RNu)
•-
Few systems are known to react through a thermal (or
spontaneous) SRN1 reaction.1 Most systems need to be
initiated by other means. The most widely used method
to initiate the chain process is photostimulation in
,3,4
2
to RX is dissociative, giving the substitution product and
•
the alkyl radical R that propagates the chain cycle (eq
4
).
1
aromatic as well as in aliphatic SRN1 reactions. Other
(1) For reviews, see: (a) Rossi, R. A.; de Rossi, R. H. Aromatic
methods that have been used in aromatic SRN1 reactions
Substitution by the SRN1 Mechanism; ACS Monograph 178; American
Chemical Society: Washington, DC, 1983. (b) Norris, R. K. In
Comprehensive Organic Synthesis; Trost, B. M., Ed.; Pergamon Press:
New York, 1991; Vol. 4, Chapter 2, p 451. (c) Rossi, R. A.; Pierini, A.
B.; Palacios, S. M. Adv. Free Rad. Chem. (Greenwich, Conn.) 1990, 1,
5
are solvated electrons or sodium amalgam in liquid
ammonia,6 reactions induced electrochemically, and
2
1
93. (d) Rossi, R. A.; Pierini, A. B.; Pe n˜ e´ n˜ ory, A. B. Recent Advances
(3) Costentin, C.; Hapiot, P.; M e´ debielle, M.; Sav e´ ant, J .-M. J . Am.
Chem. Soc. 1999, 121, 4451.
in the Reaction of Organic Halides. In The Chemistry of
S
RN
1
Functional Group; Patai, S., Rappoport, Z., Eds.; Wiley: Chichester,
U.K., 1995; Suppl. D2, Chapter 24, p 1395. (e) Rossi, R. A.; Pierini, A.
B.; Santiago, A. N. Aromatic Substitution by the SRN1 Reaction. In
Organic Reactions; Paquette, L. A., Bittman, R., Eds.; J ohn Wiley and
Sons: New York, 1999; Vol. 54, Chapter 1, pp 1-271.
(4) (a) Scamehorn, R. G.; Bunnett, J . F. J . Org. Chem. 1977, 42,
1449. (b) Carver, D. R.; Komin, A. P.; Hubbard, J . S.; Wolfe, J . F. J .
Org. Chem. 1981, 46, 294. (c) Carver, D. R.; Hubbard, J . S.; Wolfe, J .
F. J . Org. Chem. 1982, 47, 1036. (d) Scamehorn, R. G.; Hardacre, J .
M.; Lukanich, J . M.; Sharpe, L. R. J . Org. Chem. 1984, 49, 4881. (e)
Kornblum, N. Angew. Chem., Int. Ed. Engl. 1975, 14, 734.
(5) Palacios, S. M.; Asis, S. E.; Rossi, R. A. Bull. Soc. Chim. Fr. 1993,
130, 111 and references therein.
(
2) For reviews on reactions induced electrochemically, see: (a)
Sav e´ ant, J .-M. Tetrahedron 1994, 50, 10117. (b) Sav e´ ant, J .-M. Acc.
Chem. Res. 1993, 26, 455. (c) Sav e´ ant, J .-M. New J . Chem. 1992, 16,
1
31. (d) Sav e´ ant, J .-M. Adv. Phys. Org. Chem. 1990, 26, 1. (e) Sav e´ ant,
(6) (a) Austin, E.; Ferrayoli, C. G.; Alonso, R. A.; Rossi, R. A.
Tetrahedron 1993, 49, 4495. (b) Austin, E.; Alonso, R. A.; Rossi, R. A.
J . Org. Chem. 1991, 56, 4486.
J .-M. Adv. Electron Transfer Chem. 1994, 4, 53. (f) Andrieux, C. P.;
Hapiot, P.; Sav e´ ant, J .-M. Chem. Rev. 1990, 90, 723.
1
0.1021/jo001141m CCC: $19.00 © 2000 American Chemical Society
Published on Web 10/13/2000

7
176 J . Org. Chem., Vol. 65, No. 21, 2000
Arg u¨ ello et al.
7
inorganic salts such as SmI
2
. Aromatic and aliphatic
or with 2-naphthoxide ions by KI has been explained on
the basis of an ET through the exciplex formed between
halides also can be initiated by Fe2 salts. Data are now
available which characterize both the propagation cycle
and the termination steps of this reaction.1
+
8
-
14
-
the aryl halide and the I ions. Catalysis by I ions was
observed in the photoinduced reaction of bromoarenes
with diethyl phosphite ions in the acetonitrile-tetrahy-
drofuran mixture.15
,2
A photostimulated reaction between a nucleophile and
a substrate may be rather unreactive at initiation but
quite reactive at propagation. The addition of minute
amounts of another nucleophile that is more reactive at
initiation increases the generation of the reactive inter-
mediates and allows the less reactive initiation nucleo-
phile to start its own propagation. The process, thus, can
afford the substitution of an unreactive nucleophile
through an entrainment reaction, which is also a support
for the chain mechanism.1
-
ET from an excited Nu is another possibility for
initiation. This is highly probable because nucleophiles
are the best candidates for absorption in the usual
reaction conditions (λ > 350 nm). An example of the
involvement of the anion in its excited state is the
reaction of organic sulfides under laboratory light with
-
Ph
2
P
ions, which absorb strongly in the visible region
12
(λ ) 475 nm). Other evidence for this type of initiation
is the fluorescence quenching of the diphenylindenyl
anion by PhBr.16
Even though photostimulation is a widely used initia-
tion method (eq 5) there are not many studies on the
mechanism involved.
Initiation of the reaction of the 1-chloro-2-naphthoxide
anion with sodium sulfite has been proposed to occur by
ET between the excited triplet state of the substrate and
1
7
its ground state, with a quantum yield of about 50. For
the 1-bromo derivative, photohomolytic C-Br dissocia-
1
7
These photoinduced electron transfers (PET) can con-
ceivably be accomplished in one of the followings ways:
tion is proposed, with a quantum yield of about 20. The
substitution of 1-chloro-2-naphthoxide ions with sodium
sulfite can be dye photoinitiated18 or initiated by visible
light with a Ru complex as sensitizer and a Co complex
as the intermediate electron carrier.19
(
1) homolytic cleavage of the C-X bond; (2) ET from the
-
Nu to the excited ArX, generating a radical anion that
enters the cycle; (3) ET within an excited charge-transfer
complex (CTC); and (4) photoejection of an electron from
In the photostimulated reaction of 1-iodoadamantane
-
-
the excited Nu . Depending on the nature of the RX, the
with PhS ions in acetonitrile, the mechanism of photo-
-
-
Nu , and the experimental conditions, any of these
initiation involves excitation of PhS ions with concomi-
mechanisms could probably be the initiation step.
The proposal that initiation occurs by photoexcitation
of a CTC formed between the Nu- and the substrate is
reinforced by studies of the wavelength dependence of
the quantum yield. This type of initiation has been
proposed for the reaction of acetone enolate anions with
PhI and PhBr in DMSO,9 whereas when potassium
diethyl phosphite is used as a nucleophile, homolysis of
PhI and PET within the CTC is competitive.9 Further-
more, the CTC initiation has been postulated in the
reaction of nitrile carbanions with haloarenes such as
tant photoejection of electrons to the solvent, with a
quantum yield for photoejection of 0.43-0.75 at λ ) 308
20
nm. The reaction does not follow a chain mechanism.
Varying the experimental conditions, we can change
the reactivity of the pair substrate-nucleophile from an
unreactive system to a reactive one. For instance, PhI
did not react under irradiation (Pyrex-filtered flask) with
2
1
acetophenone enolate anion in liquid ammonia, but it
,10
22
does react in DMSO. However, a reaction was observed
in liquid ammonia when irradiation was conducted in an
immersion well.23
2
-bromonaphthalene or p-bromobiphenyl in liquid am-
The photoinduced initiation reaction may have a low
quantum yield, because of a fast backward ET which
annihilates the ion pair before cage separation occurs.
This will result in a poorly efficient source of radicals.
The measured quantum yield depends on the efficiency
of both initiation and turnover in the propagation steps.
The magnitude of the chain length of any SRN1 process
can easily be derived from a comparison of the quantum
yield of the initiation step and an overall reaction
quantum yield. It would therefore be possible to obtain
quantum yields for the initiation step, provided that the
propagation process is eliminated or reduced significantly
in comparison with the initiation step.
monia. The quantum yields of these reactions (λ > 313
nm) range from 7 to 31.11 Formation of a CTC has also
been proposed in the reaction of ethyl phenyl sulfide with
diethyl phosphite ions.12
The quantum yields for the SRN1 substitution reactions
of p-nitrocumyl chloride with quinuclidine (3.5) and azide
ions (6000) have been determined.13 Furthermore, by
studying the wavelength dependence on the quantum
yields, evidence has been obtained that the photochemical
initiation proceeds by means of a CTC.
Acceleration of the substitution reaction of function-
alized aryl bromides with potassium diethyl phosphite
(
(
7) Nazareno, M. A.; Rossi, R. A. Tetrahedron Lett. 1994, 35, 5185.
8) (a) Galli, C.; Bunnett, J . F. J . Org. Chem. 1984, 49, 3041. (b)
(14) Beugelmans, R.; Chbani, M. New J . Chem. 1994, 18, 949.
(15) Boumekouez, A.; About-J audet, E.; Collignon, N.; Savignac, P.
J . Organomet. Chem. 1992, 440, 297.
(16) Tolbert, L. M.; Siddiqui, S. J . Org. Chem. 1984, 49, 1744.
(17) Ivanov, V. L.; Eggert, L.; Kuzmin, M. G. High Energy Chem.
1987, 21, 284; Chem. Abstr. 1988, 109, 169594b.
(18) Ivanov, V. L.; Aurich, J .; Eggert, L.; Kuzmin, M. G. J . Photo-
chem. Photobiol., A 1989, 50, 275.
Galli, C.; Gentili, P. J . Chem. Soc., Perkin Trans. 2 1993, 1135. (c)
van Leeuwen, M.; McKillop, A. J . Chem. Soc., Perkin Trans. 1 1993,
433. (d) Nazareno, M. A.; Rossi, R. A. J . Org. Chem. 1996, 61, 1645.
e) Murgu ´ı a, M. C.; Rossi, R. A. Tetrahedron Lett. 1997, 38, 1355. (f)
Baumgartner, M. T.; Gallego, M. H.; Pierini, A. B. J . Org. Chem. 1998,
2
(
6
3, 6394.
(
9) Fox, M. A.; Younathan, J .; Fryxell, G. E. J . Org. Chem. 1983,
(19) Savvina, V. S.; Ivanov, V. L. High Energy Chem. 1990, 24, 205.
(20) Ahbala, M.; Hapiot, P.; Houmam, A.; J ouini, M.; Pinson, J .;
Sav e´ ant, J .-M. J . Am. Chem. Soc. 1995, 117, 11488.
(21) Bunnett, J . F.; Sundberg, J . E. J . Org. Chem. 1976, 41, 1702.
(22) Borosky, G. L.; Pierini, A. B.; Rossi, R. A. J . Org. Chem. 1992,
57, 247.
4
8, 3109.
(
(
10) Hoz, S.; Bunnett, J . F. J . Am. Chem. Soc. 1977, 99, 4690.
11) Wu, B.; Zeng, F.; Ge, M.; Cheng, X.; Wu, G. Sci. China, Ser. B
1
991, 34, 777; Chem. Abstr. 1992, 116, 58463h.
(
12) Cheng, C.; Stock, L. M. J . Org. Chem. 1991, 56, 2436.
(
13) Wade, P. A.; Morrison, H. A.; Kornblum, N. J . Org. Chem. 1987,
(23) Semmelhack, M. F.; Bargar, T. M. J . Am. Chem. Soc. 1980, 102,
7765.
5
2, 3102.

Quantum Yields in SRN1 Reactions
Ta ble 1. Rea ction s of Neop h yl Iod id e (1) w ith Ca r ba n ion s in DMSOa
product distributions (%)b
other products
J . Org. Chem., Vol. 65, No. 21, 2000 7177
conditions,
added compound (M)
entry
carbanion
I- (%)c
reduction (5 + 6)
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
2
2
2
2
2
2
3
3
3
3
3
4
4
4
4
dark
hv
hv
hv
9.3
84
80
77
74
5 (1.8), 6 (0.3)
5 + 6 (49)
5 (21), 6 (10)
5 (36), 6 (7.3)
5 (27), 6 (6.7)g
5 (10)
d
7 (6.0), 8 (9.0), 9 (10.4), phenol (1.2)
7 (2.9), 8 (4.6), 9 (7.7), phenol (2.5)
9 (9.1), phenol (11.5)
10 (57)
11 (5.7)
e
f
hv
hv, DTBN (0.004)
dark
dark, DTBN (0.008)
hv
74
7.2
6.7
73
74
13
<1
4.5
79
44
69
11 (6.5)
11 (45) , 12 (16)
h
h
5 + 6 (3.6)
i
i
1
1
1
1
1
1
1
hv
11 (55), 12 (16)
11 (5.5), 10 (2.5)
DTBN (0.008)
dark
hv
hv, 2 (0.04)
hv, 2 (0.12)
hv, 2 (0.12)
13 (3.1)
13 (67), 14 (2.0)
13 (35), 14 (1.3)
13 (16), 14 (6.4), 7 (0.8), 8 (2.2)
j
5 + 6 (3.9)
5 + 6 (2.7)
5 (26), 6 (2.9)
k
4
a
, 0.04 M; carbanions, 0.12 M. irradiation for 1 h under nitrogen atmosphere, unless otherwise indicated. b Determined by GC using
1
c
the internal standard method, error 5%; 5 and 6 were quantified together in a ca. 3:1 ratio, unless otherwise indicated. Determined
potentiometrically. 2, 0.04 M; reaction time 2 h. The reaction was conducted under a nitrogen atmosphere in a closed system, and the
d
e
f
g
internal standard was added before irradiation. The reaction was performed in 4.0 mL of DMSO-d6. In the reduction products 5 and
i
j
k
6
, 12.7% of incorporation of deuterium was detected by GC-MS. h Isolated product. Not quantified. Reaction time 2 h. 4, 0.012 M.
It has been reported that 1-iodo-2,2-dimethylpropane
(
neopentyl iodide) reacts under irradiation with carban-
ions in DMSO to give the substitution product by the
RN1 mechanism24 and that 1-chloro-2-methyl-2-phenyl-
propane (neophyl chloride) is more reactive than neo-
S
-
pentyl iodide toward Ph
2
P
ions in photostimulated
2
5
competition experiments run in liquid ammonia. In this
context, we decided to study the photoinduced reaction
of neophyl iodide with different carbanions in DMSO. In
this system, we could measure the quantum yield of the
photoinitiation, the rate of coupling of the neophyl radical
with acetophenone enolate anion, and the turnover of the
propagation cycle of these SRN1 reactions.
Resu lts
P h otoph ysical P r oper ties of Car ban ion s an d Neo-
p h yl Iod id e (1). CTC F or m a tion . The enolate anion
of cyclohexenone (2) absorbs maximally at 339 nm,
whereas the neophyl iodide (1) shows absorption below
3
00 nm (Figure 1). When increasing amounts of 1 are
F igu r e 1. UV-vis spectra of carbanion 2 and iodide 1 and
CTC formation (measured at λ ) 338.6 nm).
added to a solution of 2, an enhancement of the absorp-
tion maximum of the anion is observed. However, a new
absorption band is not apparent; therefore, this increase
can be ascribed to an interaction between 1 and the
Rea ction s of 1 w ith En ola te An ion 2. There is
almost no reaction of 1 with enolate anion 2 after 1 h in
the dark in DMSO. When a solution of 1 and 2 is
irradiated for 1 h at 365 nm, a mixture of tert-butylben-
zene (5) and isobutylbenzene (6) is produced in 49% yield
2
6
anion, i.e., the formation of a CTC between both species.
2
7
The equilibrium constant for the formation of this
complex is 15 M-1
.
For the enolate anion of acetophenone (3), which shows
a maximum of the absorption band at 337 nm, a similar
phenomenon was observed when increasing amounts of
(Table 1, entries 1 and 2). Product 6 is formed through
the rearrangement of the neophyl radical. No products
from the addition of radicals to 2 were found in the
reaction mixture. Both reactions were performed using
a 2:1 ratio of 1:3. When equimolar amounts of 1 and 2
were employed, the reaction was slower, and 2 h of
irradiation were necessary to reach the same conversion.
As control experiments, the photoinduced reaction was
conducted under nitrogen atmosphere in a closed system
and the internal standard for GC-quantification was
added before irradiation. It made no difference whether
the experiment was conducted in a closed system under
nitrogen or with nitrogen circulation. Small amounts of
olefins (2-methyl-3-phenylpropene, 7, and 2-methyl-1-
1
are added (not shown). This can also be ascribed to the
formation of a CTC with an equilibrium constant of 51
M
-1
.
For the nitromethane anion (4) which absorbs maxi-
mally at 290 nm similar to the absorption of 1, no
interaction between 1 and 4 could be determined.
(
(
24) Pe n˜ e´ n˜ ory, A. B.; Rossi, R. A. Gazz. Chim. Ital. 1995, 125, 605.
25) Duca, J . S.; Gallego, M. H.; Pierini, A. B.; Rossi, R. A. J . Org.
Chem. 1999, 64, 2626.
26) Foster, R. Organic Charge-Transfer Complexes; Academic
Press: New York, 1969.
(
-
(
27) It was obtained from the following equation: [Nu ]/(Aobs -
Afobs) ) 1/∆ꢀ + 1/∆ꢀ Kctc [1]
-1
.

7
178 J . Org. Chem., Vol. 65, No. 21, 2000
Arg u¨ ello et al.
propene, 8) and neophyl dimers (9) were also formed (eq
Ta ble 2. Qu a n tu m Yield s a n d Ch a in Len gth s of th e P ET
Rea ction s of 1 w ith Ca r ba n ion sa
6
4
). Traces of phenol are observed. (Table 1, entries 3 and
).
b
chain lengthc
entry
1
carbanion
φproducts
2
3
0.17 ( 0.03 (5 + 6)
0.124 ( 0.011 (11)
d
2
127
0
.0027 ( 0.0005 (12)
3e
3
0.0010 ( 0.0002 (10)
0.33 ( 0.05 (13)
d
4
4 (2)
2
a
Performed under nitrogen atmosphere, using 0.04 M solutions
b
of substrate and 0.12 M solutions of the nucleophiles. Determined
at λ max ) 350 nm. Determined by eq 25. d Duplicate experiments.
c
e
In the presence of 20 mol % of DTBN.
When the photochemical reaction was performed in
DMSO-d , 12.7% of deuterium incorporation was detected
6
in the reduction products 5 and 6 by GC-MS. In this
reaction, the amount of phenol increases up to 11.5%, and
incorporation of deuterium was also observed. In the
presence of a very efficient radical trap like di-tert-
butylnitroxide (DTBN), the main product is the corre-
sponding adduct 10 in 57% yield and 5 as minor product
in 10% yield (Table 1, entries 5 and 6).
of 4 affords the expected 13 and the rearranged 14
substitution products in 16 and 6.4% yields, respectively
(Table 1, entries 15 and 16).
Qu a n tu m Yield Mea su r em en ts a n d Deter m in a -
tion of th e Ra te Con sta n t for Nu cleop h ilic Ad d i-
tion . The quantum yields determined (λ ) 350 nm) for
the reactions of carbanions 2 and 3 with iodide 1 were
-
3
below 1. A quantum yield of ca. 10 was obtained for
the formation of adduct 10 in the reaction with 3 in the
presence of DTBN.28 Product 13 was formed with a
quantum yield of 0.33 when 1 is made to react with 4 in
the presence of 2 as initiator (Table 2).
The rearrangement of the neophyl radical (15) to the
tertiary radical 16 is a well-known process, and there is
an abundance of evidence that supports phenyl migration
by a cyclic transition state.29 The Arrhenius function for
this reaction has recently been determined by ESR
methods30 (eq 9). Therefore, the rate constant for the
coupling reaction of radical 15 with a nucleophile may
be determined by using its rearrangement as a “radical
clock”.31
Rea ction of 1 w ith th e En ola te Ion of Acetop h e-
n on e (3). The dark reaction of 1 with enolate anion 3 in
DMSO yielded, after 1 h, only 5.7% of the substitution
product 11. This sluggish reaction was not inhibited in
the presence of 20 mol % of DTBN. When the reaction
was irradiated for 1 h, it afforded the expected substitu-
tion product 11 in 55% yield, the rearranged substitution
product 12 (16%), and small amounts of 5 and 6 (eq 7;
Table 1, entries 7-10).
This reaction showed a strong inhibition after 1 h upon
addition of 20 mol % of DTBN, affording only products
derived from the neophyl radical, the substitution product
The quantum yields for substitution products 11 and
2 were determined (λ ) 350 nm) and were found to be
dependent on the concentration of the nucleophile. A plot
1
1
1 (5.5%), and the adduct 10 (2.5%).
Rea ction of 1 w ith Nitr om eth a n e An ion (4). There
of the 11:12 quantum yield ratio against the concentra-
is no dark reaction of 1 with 4 (1 h). The photoinduced
reaction of 1 with 4 yields only 3.1% of the substitution
product 13 after 1 h. When the reaction was performed
in the presence of 2, which is a good electron donor (1:
(28) Di-tert-butylnitroxide (DTBN) absorbs maximally at 440 nm and
shows a minimum at 340 nm with a ꢀ350 ≈ 2, whereas the anions 2
and 3 have molar extinction coefficients at the absorption maximums
of 4000 and 7700, respectively.
4
:2 ratio of 1:3:1), the substitution product 13 is obtained
(29) March, J . Advances in Organic Chemistry, 4th ed.; J ohn Wiley
Sons: New York, 1992.
&
in 67% yield and only 2% of the rearranged substitution
product 14 is observed (eq 8; Table 1, entries 12-14).
When the amount of anion 2 was increased to a 1:3:3
ratio (1:4:2), the conversion of the reaction markedly
diminishes after 2 h of irradiation. The photoinduced
reaction of 1 with a 10-fold decrease in the concentration
(
30) Weber, M.; Fisher, H. J . Am. Chem. Soc. 1999, 121, 7381.
(31) General references for free radical clocks: (a) Beckwith, A. L.
J .; Ingold, K. U. In Rearrangements in Ground and Excited States; de
Mayo, P., Ed.; Academic: New York, 1980; Vol. 1, Essay 4. (b) Surzur,
J . M. In Reactive Intermediates; Abramovitch, R. A., Ed.; Plenum: New
York, 1981; Vol. 2, Chapter 2. (c) Newcomb, M. Tetrahedron 1993, 49,
1151.

Quantum Yields in SRN1 Reactions
J . Org. Chem., Vol. 65, No. 21, 2000 7179
process. Under the basic reaction conditions, 17 affords
the phenoxide anion 18 (eq 16).
F igu r e 2. Rate constant for the coupling of carbanions 3 with
radical 15. The reactions were performed under a nitrogen
atmosphere, using 0.04 M solutions of substrate, and varying
concentrations of anion 3. The quantum yields were deter-
mined at λ max ) 350 nm, and the rate constant was obtained
by the equation in ref 32.
tion of anion 3 is linear. From the slope of this plot and
taking into account the rearrangement rate of the neo-
-
1
30
phyl radical (k ) 402 s ), the rate constant of the
r
coupling of the neophyl radical with 3 was calculated to
5
-1 -1
32
be k
c
) 1.2 × 10 M
s
(Figure 2).
Discu ssion
The lack of inhibition by a radical trap, the presence
of only traces of phenol33 (below 2.5%) in the reaction
mixture, and the deuterium incorporation when labeled
DMSO is used allow us to rule out this alternative
pathway, suggesting that the mechanism for the reduc-
tion is hydrogen atom abstraction from the solvent in a
nonchain reaction. When the reaction is conducted in
A detailed mechanistic study of the photochemical
reactions of iodide 1 with carbanions 2-4 was conducted
in DMSO. These three carbanions have distinct reactivity
on the different reaction pathways of the SRN1 mecha-
nism.
P ET Red u ction of Iod id e 1 by 2. The enolate anion
2
reacts only under photostimulation with 1 to afford
6
DMSO-d the rate for atom abstraction from the solvent
mainly the reduction products 5 and 6 together with
small amounts of olefins and dimers. The intermediacy
of the neophyl radicals in this reaction was confirmed
by the observation of product 6 which arises from
rearrangement, the efficiency of the DTBN in affording
the adduct 10, and the deuterium incorporation in the
reduction products when using labeled DMSO.
A possible mechanism to account for this reductive
process involves an initial PET from anion 2 to the
substrate 1 to yield radical 15 (eq 10). Three competitive
reactions’ pathways can take place for the neophyl radical
slows down, and the reduction process by anion 2
becomes more important with the concomitant increase
in the amount of phenol. Small amounts of olefins are
formed because of a disproportionation process. Thus, the
quantum yield measurement for the formation of the
reduction products renders the efficiency of this process
(Φinitiation, see below).
Nu cleop h ilic Su bstitu tion Rea ction of Iod id e 1.
The slow dark reaction of 1 with enolate anion 3 does
not show inhibition in the presence of a radical trap,
which indicates that it may proceed by a polar mecha-
1
5 thus formed: (1) reduction to 5 by hydrogen abstrac-
34
nism.
tion from the solvent (eq 11); (2) rearrangement to a
tertiary radical 16 which affords 6 after hydrogen ab-
straction (eq 12); or (3) in the presence of DTBN,
formation of the corresponding adduct 10 with a rate
The photoinduced reaction of 1 with 3 gives the
expected and the rearranged substitution products 11
and 12 in good yields and minor amounts of the reduction
products 5 and 6. This reaction, strongly inhibited by
DTBN, occurs by a radical chain SRN1 mechanism and
can be rationalized in the terms of PET from nucleophile
3 to 1 (initiation step eq 17). Radical 15, thus formed,
9
-1 -1
31c
constant of approximately 10 M
s
(eq 13).
An alternative mechanism for the reduction of 15 could
be hydrogen atom abstraction from enolate anion 2,
which yields the cyclohexadienone radical anion (eq 14).
•-
reacts with 3 to give the radical anion intermediate 11 ,
which by an ET to 1 affords the substitution product 11
•
-
This radical anion intermediate (17 ), could transfer its
odd electron to another molecule of 1 (eq 15) to render
1
5 and 17 and build a chain mechanism for this reductive
(33) In a control experiment, phenol in a concentration of 0.04 M
was recovered in ca. 85%.
(
34) Slow substitution of 1 by t-BuO- ion by a polar mechanism was
(
32) The equation used in the evaluation of the coupling rate
previously observed: Arg u¨ ello, J . E.; Pe n˜ e´ n˜ ory, A. B.; Rossi, R. A. J .
Org. Chem. 1999, 64, 6115. This reaction was not observed at very
short times, i.e., the employed time for the quantum yield measure-
ments.
constant of ion 3 with radical 15, assuming that the concentration of
-
Nu is constant during the reaction, is the following: Φ11/Φ12 )
-
a[Nu ] + b, where a is equal to k
c
/k
r
and b ) constant.

7
180 J . Org. Chem., Vol. 65, No. 21, 2000
Arg u¨ ello et al.
and radical 15, which may continue with the propagation
cycle (eq 18) or be reduced to 5 (eq 11). Radical 15
rearranges to give radical 16, which is also able to react
with 3 to render the radical anion intermediate 12 that
yields through an ET to 1 the rearranged product 12 and
radical 15 (eqs 19 and 20). Finally, radical 16 affords the
reduction product 6 by hydrogen atom abstraction from
the solvent (eq 12).
the HOMO energy, and it increases with an increase in
the pK
a a
of the carbanion36 (the pK
in DMSO are as
follows: cyclohexenone, undetermined; acetophenone,
•-
37
24.7; and nitromethane, 17.2). Thus, the enolate anions
2 and 3 are more reactive than anion 4 in the initiation
2
2,38
step. Furthermore, it has been previously proposed
that, in the addition step, the reactivity of the carbanions
would depend in part on their HOMO (pK ) as well as
a
on the SOMO of the radical anion intermediate. Thus,
anion 4 is more reactive in the coupling reaction with
the radicals than the enolate anions 2 and 3. For these
carbanions, the reactivity increases with a decrease in
a
the pK of the conjugated acids (HOMO energy).
The coupling reaction between the nucleophile and the
radical is the most important step in determining whether
an SRN1 process takes place. If this reaction cannot
compete efficiently with the termination steps, the chain
will be short or even nonexistent. For aromatic radicals,
absolute rate constants for the addition of several nu-
cleophiles have been determined by cyclic voltammetry,
and a large number of these values are near the diffusion
2
limit. Nevertheless, little is known about the rates of
these reactions in aliphatic systems.39
The observation of products, which arises from addition
of ion 3 to radical 15 and the rearranged radical 16, allow
5
-1 -1
us to obtain a value of 1.2 × 10 M
s
for the rate
The nitromethane anion 4 does not react with 1 in the
dark but upon irradiation affords the substitution product
constant for the coupling reaction of radical 15 with this
nucleophile.
1
3 in very low yield (see above).
The nitronate anions are poor electron donors, and they
The fact that only traces of 14 were detected suggests
that the reaction of radical 15 with 4 is faster than its
rearrangement to give radical 16 at least by 3 orders of
magnitude, so this is not an appropriate clock for
determining the addition rate constant. When the con-
centration of 4 was decreased by 10-fold (to slow the
coupling reaction with radical 15), both products 13 and
14 were formed, so we can estimate the rate constant of
are unable to transfer one electron to nonactivated alkyl
halides to initiate an SRN1 process.35 Nevertheless, the
photoinduced reaction of 1 with 4 in the presence of the
enolate anion 2 (entrainment reagent) renders the sub-
stitution product 13 in high yield and the rearranged
product 14 only in traces.
Carbanion 2 is able to transfer one electron to 1 under
irradiation to yield radical 15 (eq 10), but this anion does
not add to the radical (see the above results). Further-
more, 4 is more reactive in the coupling reaction, and
the only product formed is the substitution product from
the addition of 4 to radicals 15 (eq 21) and 16 (eqs 22
and 23).
6
-1
the coupling reaction between 15 and 4 to be ca. 10 M
-1
s
. This is an approximate value, which is about 2 orders
of magnitude below the previously reported rate constant
for the addition of 4 to methyl radicals determined by
pulse radiolysis,3 and to achieve a more realistic one it
should be necessary to use a faster radical clock. Finally,
the results obtained herein for the addition of these
carbanions to 15 are in complete agreement with their
previously reported relative reactivity toward adaman-
9c
3
8
22
tyl and phenyl radicals.
Efficien cy a n d Ch a in Len gth in SRN1 Rea ction s.
A photoinduced SRN1 mechanism involves, as an initia-
tion step, a PET reaction from the nucleophile to the
substrate in the majority of the cases, followed by a
propagation cycle and termination steps. For enolate
anion 3 which is reactive at both the initiation and the
propagation pathways, the quantum yield for the forma-
(36) Bordwell, F. G.; Clemens, A. H.; Smith, D. E.; Begemann, J .
J . Org. Chem. 1985, 50, 1151.
(
37) Bordwell, F. G. Acc. Chem. Res. 1988, 21, 456.
(38) Rossi, R. A.; Pierini, A. B.; Borosky, G. L. J . Chem. Soc., Perkin
Trans. 2 1994, 2577.
-
(39) (a) The coupling rate constant of the PhS ion with the 2,2-
8
-1
s-1 in acetonitrile at
dimethyl-5-hexenyl radical is k ) 1.2 × 10
M
The three carbanions studied showed different reactiv-
2
1
5 °C. See: Beckwith, A. L. J .; Palacios, S. M. J . Phys. Org. Chem.
991, 4, 404. (b) The coupling rate constant of the 2-nitropropane anion
ity in the initiation and propagation steps of the SRN
1
6
-1 -1
process. The PET from the carbanion to 1 depends on
with the 5-hexenyl radical is k ≈ 1 × 10
M
s
in DMSO at 40 °C.
See: Russell, G. A.; Guo, D. Tetrahedron Lett. 1984, 25, 5239. (c) The
the oxidation potential of the anion, which is related to
coupling rate constants of nitronate ions with methyl radicals in DMSO
8
at room temperature are the following: nitromethane (k ) 1.35 × 10
-
1
-1
7
-1 -1
(35) Nevertheless, with a better electron acceptor (p-nitrobenzyl-
M
s
), nitroethane (k ) 1.6 × 10
M
s ), 1-nitropropane (k )
6 -1 -1
7
-1 -1
halides, R-nitrohalo alkanes, and related compounds), the PET is very
efficient. See ref 1d.
1.35 × 10 M
s
), and 2-nitropropane (k ) 2.35 × 10
M
s ). See:
Veltwisch, D.; Asmus, K.-D. J . Chem. Soc., Perkin Trans. 2 1982, 1143.

Quantum Yields in SRN1 Reactions
J . Org. Chem., Vol. 65, No. 21, 2000 7181
tion of the substitution product (Φglobal) encompasses both
processes (eq 24). From this expression, it is possible to
obtain the Φpropagation (that is, the chain length) by the
ratio of the overall quantum yield and the quantum yield
of the initiation step (eq 25).
plexes. Because the nucleophiles and the complexes show
the same absorption band, it is not possible to rule out
ET from the excited nucleophile by measurements of the
quantum yields of the reaction varying the wavelength.
The reaction of 1 with enolate anion 3 gives the
expected and the rearranged substitution products,
whereas the reaction with anion 4 yields mainly the
unrearranged substitution product. The use of the rear-
rangement rate constant of the neophyl radical as a clock
reaction has allowed us to obtain the rate constant for
the addition of nucleophile 3 to a primary alkyl radical.
For the first time, the Φpropagation (that is, the chain
length) for a SRN1 process was measured, demonstrating
that, although the overall quantum yields for these
The quantum yield measured for the formation of both
substitution products 11 and 12 is 0.127 (Table 2). 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. This reaction has a quantum yield of
reactions are below 1, they occur by a chain SRN
mechanism.
1
-3
1
0
, which represents the photochemical efficiency of the
Exp er im en ta l Section
initiation process.
Gen er a l Meth od s. Irradiation was conducted with a 125-W
medium pressure Hg lamp, emitting maximally at 365 nm
Applied Photophysics Limited). The quantum yield for the
reactions of 1 with the carbanions was determined using
Thus, the chain length or Φpropagation for the reaction of
iodide 1 with enolate anion 3 obtained from eq 25 is
ca.127. As far as we know, this reports represents the
first measurement for the efficiency of the propagation
step of a SRN1 process. This result is important not only
because it means a very effective chain with more than
(
ferrioxalate actinometry at 350 nm with a 125-W Philips HPW
4
0
lamp, according to the procedure described in the literature.
The conversion of the actinometer was monitored by measur-
ing the absorbance of the complex Fe-(o-phenanthroline) at
1
00 cycles but also because a reaction having an overall
-
7
5
10 nm. A radiant power of ca. 10 einstein/min was mea-
quantum yield below 1 does not necessarily mean a
nonchain process. Thus, many reactions that are thought
to proceed by a nonchain reaction, e.g., geminate coupling
of a pair of radicals, with the only evidence that the
overall quantum yields are below one, should now be
revised in the light of our results.
sured (a mean value among 5, 10, 15, and 20 min of irradia-
tion). The sample and the actinometer were placed equidistant
1
13
to the lamp in a merry-go-round equipment. H and C NMR
spectra were recorded at 200.00 and 50.288 MHz, respectively,
on a Brucker AC-200 spectrometer, and all spectra are
4 3
reported in δ (ppm) relative to Me Si, with CDCl as the
solvent. HR-MS analyses were recorded at the Facultat de
Farmacia Universitat de Bercelona, Barcelona, Spain. Gas
chromatographic analyses were performed on a Hewlett-
Packard 5890 Series II with a flame-ionization detector and
the data system Hewlett-Packard 3396 Series II integrator,
on a HP-17 10 m capillary column packed with 50% phenyl
and 50% methylpolysiloxane gum of a 0.53 mm × 2.00 µm film
thickness. GC-MS analyses were carried out on a Shimadzu
GC-MS QP 5050 spectrometer, employing a 30 m × 0.12 mm
DB-5 MS column. The column chromatography was performed
on silica gel (70-230 mesh ASTM). UV-vis spectra were
recorded on a UV-vis spectrophotometer Shimadzu UV-260.
Potentiometric titration on halide ions was performed in a pH
The anion 4 does not transfer one electron to iodide 1
in the dark or upon irradiation, but it is reactive in the
addition reaction to alkyl radicals generated by PET from
2
(unreactive anion in the coupling reaction). We have
measured a quantum yield of 0.33 for the reaction of 1
with nitronate 4 in the presence of 2 as photoinitiator.
This allowed us to evaluate the chain length of this
reaction, assuming that the Φinitiation is the same as that
of the reaction of anion 2. Thus, the reaction of 1 with 4
has a chain length of ca. 2.
The chain length for these reactions depends on the
efficiency of the propagation steps in relation to the
possible termination pathways. For carbanion 3, the most
probable termination step is the reduction of the radical
by hydrogen abstraction. In this latter reaction, the
coupling pathway is faster than the reduction process,
so that the chain length is ca. 127. In the case of anion
+
meter (Orion, model 420 A), using an Ag/Ag electrode and a
AgNO solution as a standard. Melting points were obtained
3
on a B u¨ chi 510 apparatus and were not corrected.
Ma ter ia ls. Iodide 141 was synthesized by a procedure
similar to thatdescribed in the literature regardless of the
starting material, neophyl alcohol, which was prepared by air
oxidation of the corresponding Grignard reactive from neophyl
chloride. The product presents the same spectroscopic data as
that given in the literature. t-BuOK (Aldrich) and tert-
butylbenzene (Aldrich) were commercially available and used
as received. DMSO (Carlo Erba) was distilled under vacuum
and stored over molecular sieves (4 Å). Acetophenone (Aldrich),
4
, although the coupling reaction is faster, the ET from
•-
the stable radical anion of the substitution product (13 )
to the substrate is slower, and a competitive ET to other
acceptor molecules can take place (for example, ET to
radicals 15 or 16 or the radical of the nucleophile formed
in the initiation step, which are considered termination
steps). This accounts for the short chain length, i.e., 2.
2
-cyclohexen-1-one (Aldrich), and nitromethane (Aldrich) were
distilled and dried over molecular sieves (4 Å). The carbanions
were generated in situ by acid-base deprotonation using
t-BuOK.
Con clu sion s
Gen er a l P r oced u r es for th e P h otoch em ica l Rea ction
of 1 w ith Ca r ba n ion s. The reactions were carried out in a
The main results presented herein can be summarized
as follows:
2
0-mL three-necked Schlenk tube, equipped with a nitrogen
gas inlet, a condenser with a cooling jacket, and a magnetic
The formation of a CTC between the enolate anions 2
and 3 and iodide 1 was confirmed by UV-vis spectros-
copy. Taking into account the magnitude of the equilib-
rium constant for the formation of a CTC, we found that
it is probable that the PET occurs within these com-
(40) (a) Hatchard, C. G.; Parker, C. A. J . Chem. Soc. 1956, 518-
5
36. (b) Murov, S. L.; Carmichael, I.; Hung, G. L. Handbook of
Photochemistry, 2nd ed.; Marcel Decker Inc.: New York, 1993.
41) Collman, J . P.; Brauman, J . I.; Madonik, A. M. Organometallics
1986, 5, 310.
(

7
182 J . Org. Chem., Vol. 65, No. 21, 2000
Arg u¨ ello et al.
+
+
stirrer. The tube was dried under vacuum and filled with
nitrogen and then loaded with 10 mL of dried DMSO. To the
degassed solvent under nitrogen was added 1.2 mmol of
t-BuOK, 1.2 mmol of the nucleophile precursor, and 0.4 mmol
of 1. After 1 h of irradiation, the reaction was quenched with
addition of ammonium nitrate in excess and 10 mL of water,
and then the mixture was extracted with diethyl ether. The
iodide ions in the aqueous solution were determined poten-
tiometrically. The ether extract was washed twice with water
and dried, and the products were quantified by GLC with the
internal standard method.
(EI ) 193 (5.40, M ), 131 (11.42), 119 (100.00), 92 (5.42), 91
+
(71.72), 77 (12.46); HRMS (EI ) calcd 193.1103, obsd 193.1099.
3
-P h en yl-2,2-d im eth yl-1-n itr op r op a n e (14). Compound
4 was isolated by silica gel chromatography with petroleum
ether/diethyl ether (90:10) as eluent from the crude product
1
1
reaction: H NMR (CDCl
2H, s), 7.17-7.35 (5H, m); C NMR (CDCl ) δ 25.25, 36.22,
5.73, 84.79, 126.72, 128.18, 130.63, 136.67; MS (EI ) 132
9.60, M ), 117 (6.41), 107 (5.77), 105 (11.12), 91 (100.00), 65
14.21); HRMS (EI ) calcd 193.1103, obsd 193.1103.
3
) δ 1.07 (6H, s), 2.71 (2H, s), 4.21
13
(
4
(
3
+
+
+
(
Cou p lin g P r od u ct fr om Ra d ica l 15 a n d DTBN (10). The
product was isolated by silica gel chromatography with
petroleum ether/diethyl ether (90:10) as eluent from the crude
P h en yl(3-p h en yl-3-m eth yl-1-bu tyl)k eton e (11). Com-
pound 11 was isolated in 44.9% yield by silica gel chromatog-
raphy with petroleum ether/diethyl ether (90:10) as eluent
product reaction mixture of 1 and 2 in the presence of DTBN:
from the crude product reaction of 1 and 3: mp 65-66 °C
1
1
H NMR (CDCl
3
) δ 1.14 (18H, s), 1.39 (6H, s), 3.74 (2H, s),
(
(
(
1
(
Table 1, entry 9); H NMR (CDCl
3
) δ 1.38 (6H, s), 2.04-2.12
7.11-7.41 (5H, m); 13C NMR (CDCl
) δ 25.84, 29.97, 39.19,
1
3
3
2H, m), 2.65-2.74 (2H, m), 7.1-7.9 (10H, m); C NMR
CDCl ) δ 29.00, 34.50, 37.44, 38.38, 125.78, 125.84, 128.02,
28.29, 128.45, 132.82, 136.94, 148.31, 200.57; MS (EI ) 252
2.51, M ), 133 (10.37), 132 (23.98), 120 (34.10), 119 (100.00),
05 (39.56), 92 (14.84), 91 (39.30), 77 (36.63); HRMS (EI ) calcd
52.1514, obsd 252.1512.
+
6
2.71, 84.76, 125.73, 126.24, 127.72, 147.56; MS (EI ) 145
3
+
(33.85), 91 (100.00), 89 (99.70), 74 (89.48), 57 (99.59). The
compound is so unstable at the MS conditions that the
molecular ion cannot be observed.
+
+
1
2
P h en yl(3-p h en yl-2,2-d im et h yl-1-p r op yl)k et on e (12).
Compound 12 was isolated in 15.5% yield by silica gel
chromatography with petroleum ether/diethyl ether (90:10) as
Ack n ow led gm en t. This work was supported in part
by the Consejo de Investigaciones de la Provincia de
C o´ rdoba (CONICOR), the Consejo Nacional de Investi-
gaciones Cient ´ı ficas y T e´ cnicas (CONICET), SECYT,
Universidad Nacional de C o´ rdoba, and FONCYT, Ar-
gentina. J .E.A. gratefully acknowledges receipt of a
fellowship from CONICET.
1
eluent from the crude product reaction: H NMR (CDCl
3
) δ
0
.98 (6H, s), 2.71 (2H, s), 2.75 (2H, s), 7.06-7.9 (10H, m); 13
C
NMR (CDCl ) δ 27.70, 34.87, 47.51, 48.00, 126.00, 127.80,
3
1
(
(
28.02, 128.48, 130.66, 132.68, 138.61, 138.83, 200.33; MS
+
EI ) 133 (4.48), 132 (37.26), 117 (13.69), 106 (7.84), 105
100.00), 91 (37.27), 77 (54.04); HRMS (EI ) calcd 252.1514,
+
obsd 252.1508.
-P h en yl-3-m eth yl-1-n itr obu ta n e (13). Compound 13
was isolated by silica gel chromatography with petroleum
Su p p or t in g In for m a t ion Ava ila b le: 1H and 13C NMR
3
and mass spectra of compounds 10-14. UV-vis spectra of
CTC. Figure S1 UV-vis spectra of carbanion 3 and iodide 1
and CTC formation. This material is available free of charge
via the Internet at http://pubs.acs.org.
ether/diethyl ether (90:10) as eluent from the crude product
1
reaction: H NMR (CDCl
4
2
3
) δ 1.31 (6H, s), 2.28-2.36 (2H, m),
1
3
.00-4.09 (2H, m), 7.11-7.28 (5H, m); C NMR (CDCl
8.70, 36.70, 40.99, 72.79, 125.38, 126.35, 128.58, 146.37; MS
3
) δ
J O001141M