Concerted mechanisms of the reactions of phenyl and 4-nitrophenyl chlorothionoformates with substituted phenoxide ions

Article abstract of DOI:10.1021/jo980284u

The title reactions are subjected to a kinetic study in 3% (v/v) dioxane in water, 25.0 °C, ionic strength 0.2 M (KC1). By following the reactions spectrophotometrically, pseudo-first-order rate coefficients (￡0bsd) are found under an excess of the nucleophile. Plots of ￡0bsd vs phenoxide anion concentration at constant pH are linear, with the slope (￡N) independent of pH. The Bro?nstedtype plots (log AN vs pKa of the phenols) are linear with slopes β= 0.55 and 0.47 for the reactions of the phenyl and the 4-nitrophenyl derivatives, respectively. These Bro?nsted slopes are in agreement with the ones found in the concerted reactions of the same nucleophiles with reactive phenyl esters and acetic anhydride in water. In contrast to the concerted mechanism of the title reactions that of the same substrates with secondary alicyclic amines is stepwise, which means that substitution of an amino moiety in a tetrahedral intermediate with a phenoxy group by another phenoxy group destabilizes the intermediate to the point that it no longer exists.

Full text of DOI:10.1021/jo980284u

6
820
J . Org. Chem. 1998, 63, 6820-6823
Con cer ted Mech a n ism s of th e Rea ction s of P h en yl a n d
-Nitr op h en yl Ch lor oth ion ofor m a tes w ith Su bstitu ted P h en oxid e
Ion s
4
Enrique A. Castro,* Mar ´ı a Cubillos, and J os e´ G. Santos*
Facultad de Qu ı´ mica, Pontificia Universidad Cat o´ lica de Chile, Casilla 306, Santiago 22, Chile
Received February 17, 1998
The title reactions are subjected to a kinetic study in 3% (v/v) dioxane in water, 25.0 °C, ionic
strength 0.2 M (KCl). By following the reactions spectrophotometrically, pseudo-first-order rate
coefficients (kobsd) are found under an excess of the nucleophile. Plots of kobsd vs phenoxide anion
concentration at constant pH are linear, with the slope (k
N
) independent of pH. The Br o¨ nsted-
type plots (log k vs pK of the phenols) are linear with slopes â ) 0.55 and 0.47 for the reactions
N
a
of the phenyl and the 4-nitrophenyl derivatives, respectively. These Br o¨ nsted slopes are in
agreement with the ones found in the concerted reactions of the same nucleophiles with reactive
phenyl esters and acetic anhydride in water. In contrast to the concerted mechanism of the title
reactions that of the same substrates with secondary alicyclic amines is stepwise, which means
that substitution of an amino moiety in a tetrahedral intermediate with a phenoxy group by another
phenoxy group destabilizes the intermediate to the point that it no longer exists.
In tr od u ction
Although much attention has been focused on the
synthesis was as follows: To a solution of 4-nitrophenol (2.02
g, 14.5 mmol) in THF (10 mL) in a Schlenk round-bottomed
flask was added a solution (9.1 mL, 14.5 mmol) of 1.6 M
butyllithium (Aldrich) slowly under nitrogen atmosphere. The
product, lithium 4-nitrophenolate, was rapidly transferred to
a compensation funnel, under nitrogen. In another Schlenk
round-bottomed flask, phenyl chlorothionoformate (Aldrich, 2.3
g) was dissolved in anhydrous THF (10 mL) under nitrogen
and the flask placed in an ethanol-liquid nitrogen bath. The
compensation funnel was attached to the flask and the lithium
1
2
kinetics and mechanism of the hydrolysis, alcoholysis,
3
and aminolysis of chloroformates, little is known on the
mechanisms of the reactions of chlorothionoformates.4
In the present work, we undergo a kinetic and mecha-
nistic study of the reactions of phenoxide ions with the
title substrates (PClTF and NPClTF) with the aim of
comparing them with the alcoholysis of substituted
4
-nitrophenolate solution added dropwise with stirring during
2
phenyl chloroformates and with the aminolysis of the
2 h. The mixture was left overnight with stirring under
nitrogen at ambient temperature. Chloroform (50 mL) was
added to this mixture and the solution washed with water.
title substrates.4
4
The organic layer was dried with MgSO and filtered under
C H OCSCl
4-NO C H OCSCl
2 6 4
6
5
vacuum and the solvent evaporated off. The crystallized
PClTF
NPClTF
6
PNPTOC melted at 187-189 °C (lit. mp 181-182 °C) and was
1
identified as follows: H NMR (200 MHz, CDCl
3
) δ 7.3-7.5
Exp er im en ta l Section
(m, 5H), 7.23 (sd, 2H, J ) 9.2 Hz), 8.36 (sd, 2H, J ) 9.2 Hz);
C NMR (50 MHz, CDCl ) δ 121.60 (C-2′/6′), 123.27 (C-2/6),
1
3
3
Ma ter ia ls. Dioxane (Merck, a.r.) was refluxed over ferrous
sulfate and then distilled.The phenols (Aldrich) were purified
either by distillation or recrystallization. Phenyl chlorothiono-
formate (Sigma) was used as purchased. 4-Nitrophenyl chlo-
1
1
1
25.48 (C-3/5), 127.2 (C-4′), 129.84 (C-3′/5′), 146.14 (C-4),
53.40 (C-1′), 157.52 (C-1), 193.39 (CdS); IR (KBr) 1487 and
2
520 (CNO ), 1225 (CdS), 865 (CH arom mono), 777 and 759
-1
(CH arom disust) cm
.
4
5
rothionoformate and bis(phenyl) thionocarbonate (BPTOC)
were synthesized as described.
Deter m in a tion of p K
a
a
. The pK of the phenols were
determined either by spectrophotometric or potentiometric
means, in 3% (v/v) dioxane in water, at 25.0 ( 0.1 °C, ionic
strength 0.2 M (KCl). The former method was used for
Phenyl 4-nitrophenyl thionocarbonate (PNPTOC) could not
be prepared in satisfactory yield by the method reported. This
6
4
-methoxy-, H-, 4-chloro-, and 3-chlorophenols at the wave-
(
1) For a review, see: Kevill, D. N. In The Chemistry of Acyl Halides;
lengths 235, 235, 243, and 240 nm, respectively, using either
carbonate or borate buffers, by the method reported.
Patai, S., Ed.; Interscience: New York, 1972; p 381. Queen, A. Can. J .
Chem. 1967, 45, 1619. Moodie, R. B.; Towill, R. J . Chem. Soc., Perkin
Trans. 2 1972, 184.
7
a
The potentiometric pK measurements were used for 4-cy-
anophenol and pentafluorophenol under the same conditions
(
2) (a) Hudson, R. F.; Loveday, G. J . Chem. Soc. 1962, 1055. (b) Yew,
K. H., Koh, H. J .; Lee, H. W.; Lee, I. J . Chem. Soc., Perkin Trans. 2
995, 2263. (c) Koo, I. S.; Yang, K.; Kang, K.; Oh, H. K.; Lee, I. Bull.
as above by the method already described.5
1
Kin etic Mea su r em en ts. These were carried out by means
of a Hewlett-Packard 8453 diode array spectrophotometer
under the following conditions: 3% (v/v) dioxane in water, at
Korean Chem. Soc. 1996, 17, 520. (d) Kevill, D. N.; D’Souza, J . J . Chem.
Soc., Perkin Trans. 2 1997, 1721. (e) Koo, I. S.; Yang, K.; Koo, J . C.;
Park, J . K.; Lee, I. Bull. Korean Chem. Soc. 1997, 18, 1017.
2
5.0 ( 0.1 °C, ionic strength 0.2 M (KCl), and phosphate buffer
(3) Hall, H. K., J r. J . Am. Chem. Soc. 1957, 79, 5439. Bond, P. M.;
Castro, E. A.; Moodie, R. B. J . Chem. Soc., Perkin Trans. 2 1976, 68.
King, J . F.; Guo, Z. R.; Klassen, D. F. J . Org. Chem. 1994, 59, 1095.
0.005 M. The pH values of some reaction solutions were
checked after completion of the reactions; no significant
variations were detected ((0.01 pH units). The reactions of
(4) Castro, E. A.; Cubillos, M.; Santos, J . G. J . Org. Chem. 1997,
6
1
1
2, 4395.
(5) Castro, E. A.; Santos, J . G.; T e´ llez, J .; Uma n˜ a, M.I. J . Org. Chem.
997, 62, 6568.
(7) Albert, A.; Serjeant, E. P. The Determination of Ionization
Constants; Chapman and Hall: London, 1971. Castro, E. A.; Steinfort,
G. B. J . Chem. Soc., Perkin Trans. 2 1983, 453.
(6) Al-Kazimi, H. R.; Tarbell, D. S.; Plant, D. J . Am. Chem. Soc.
955, 77, 2479.
S0022-3263(98)00284-9 CCC: $15.00 © 1998 American Chemical Society
Published on Web 09/12/1998

Reactions of Phenyl and 4-Nitrophenyl Chlorothionoformates
J . Org. Chem., Vol. 63, No. 20, 1998 6821
Ta ble 1. Exp er im en ta l Con d ition s a n d kobsd Va lu es for
Ta ble 2. Exp er im en ta l Con d ition s a n d kobsd Va lu es for
th e Rea ction s of P h en oxid e An ion s w ith 4-Nitr op h en yl
Ch lor oth ion ofor m a tea
th e Rea ction s of P h en oxid e An ion s w ith P h en yl
a
Ch lor oth ion ofor m a te
phenoxide
substituent
no. of
runs
phenoxide
substituent
no. of
runs
3
b
4
-1
3
b
4
-1
pH
10 [ArOH]tot /M
10 kobsd/s
pH
10 [ArOH]tot /M
10 kobsd/s
4
-methoxy
6.50
1.0-10
1.0-10
0.6-10
1.0-20
5.0-15
1.0-15
0.2-6.0
0.5-4.0
0.2-4.0
0.5-5.0
0.3-1.0
0.3-0.9
0.5-1.0
0.5-1.0
0.5-1.0
0.5-5.0
0.5-10
0.7-5.0
5.1-12
6.1-21
6.9-34
4.8-7.0
6.1-9.4
5.6-14
5.2-13
7.3-17
6.3-28
5.2-8.3
5.2-7.1
5.6-9.1
4.2-6.1
5.3-9.4
6.9-16
12-44
6
6
7
6
5
6
4
4
4
5
5
4
4
4
4
5
7
6
4-methoxy
none
6.50
6.80
7.10
6.50
6.80
7.10
6.50
6.80
7.10
6.50
6.80
7.10
6.50
6.80
7.10
6.50
6.80
7.10
1.0-10
1.0-10
0.6-8.0
1.0-20
0.4-15
1.0-15
0.2-4.0
0.2-4.0
0.2-4.0
0.3-5.0
0.3-1.0
0.3-1.0
0.5-1.0
0.5-1.0
0.5-1.0
0.5-4.0
0.5-1.7
0.5-2.0
7.2-16
10-26
11-33
7.5-25
9.4-31
13-49
8.0-15
8.5-22
9.8-34
8.2-14
8.9-12
10-16
9.2-12
11-17
17-24
30-126
38-75
34-74
7
6
6
6
7
6
3
6
6
5
5
5
4
4
4
5
6
5
6
7
.80
.10
none
6.50
6
7
.80
.10
4
3
4
-chloro
6.50
4-chloro
3-chloro
4-cyano
pentafluoro
6
7
.80
.10
-chloro
-cyano
6.50
6
7
.80
.10
6.50
6
7
.80
.10
pentafluoro
6.50
6
7
.80
.10
14-85
15-51
a
a
In 3% (v/v) dioxane in water at 25.0 °C, ionic strength 0.2 M
KCl), in the presence of phosphate buffer 0.005 M. b Total
In 3% (v/v) dioxane in water at 25.0 °C, ionic strength 0.2 M
b
(
(KCl), in the presence of phosphate buffer 0.005 M. Total
concentration of substituted phenol (acid plus conjugate base).
concentration of substituted phenol (acid plus conjugate base).
Ta ble 3. Va lu es of p Ka of P h en ols a n d kN for th e
phenyl and 4-nitrophenyl chlorothionoformates were followed
at 250 nm (disappearance of the substrate) and 350 nm
Rea ction s of P h en oxid es w ith P h en yl (P ClTF ) a n d
a
4
-Nitr op h en yl (NP ClTF ) Ch lor oth ion ofor m a tes
(appearance of the thionocarbonate product), respectively.
kN/s- M-1
1
All reactions were studied under excess of the phenol over
pKa of
the substrate (10-fold at least). The initial substrate concen-
nucleophile anion
phenol
PClTF
NPClTF
-
5
tration was 2 × 10 M.
4
-methoxyphenoxide
phenoxide
-chlorophenoxide
-chlorophenoxide
-cyanophenoxide
10.3
10.2
9.6
9.2
7.8
491 ( 13
150 ( 14
184 ( 7
543 ( 16
380 ( 8
Pseudo-first-order rate coefficients (kobsd) were found for all
6
the reactions, by means of the method reported. The experi-
4
3
4
204 ( 6
mental conditions of the reactions and the kobsd values are
shown in Tables 1 and 2.
P r od u ct Stu d ies. The products of the reactions of phe-
noxide ion with phenyl and 4-nitrophenyl chlorothionoformates
were identified as bis(phenyl) thionocarbonate (PhOCSOPh)
71 ( 3
102 ( 4
11.0 ( 0.4
3.0 ( 0.2
7.0 ( 0.5
0.80 ( 0.03
pentafluorophenoxide
5.4
a
Both pKa and kN values were determined in 3% (v/v) dioxane
in water at 25.0 °C, ionic strength 0.2 M (KCl).
and phenyl 4-nitrophenyl thionocarbonate (PhOCSOC
6
H
4
NO
2
),
respectively. This was achieved by comparison of the UV-
vis spectra after completion of the reactions with authentic
Nevertheless, there is one report on the reactions of these
nucleophiles with ethyl chloroformate in 85% aqueous
acetone, where a â value of 0.78 is reported, although no
7
samples of these products, under the same conditions.
2
a
Resu lts a n d Discu ssion
firm conclusions regarding the mechanism are drawn.
On the other hand, there are some reports on the
alcoholysis of phenyl chloroformates. The methanolysis
of these compounds in acetonitrile has been claimed to
be concerted,2b although the same reactions in water-
methanol mixtures are found to be governed by a carbo-
The kinetic law obtained in the present reactions is
given by eq 1, where kobsd is the pseudo-first-order rate
coefficient, k
O
and k
N
are the rate coefficients for hy-
-
kobsd ) k + k [ArO ]
(1)
2c
O
N
nyl addition-elimination pathway. The solvolysis of
phenyl chloroformate in aqueous ethanol solutions follows
drolysis and phenolysis, respectively, and ArO- repre-
sents a phenoxide ion. The values of k and k were
obtained as the intercept and slope, respectively, of plots
an addition-elimination route, with addition being rate
2
d
O
N
determining or possibly an enforced concerted variant.
In aqueous binary mixtures of methanol and ethanol, the
solvolysis of p-methoxyphenyl chloroformate was found
to be consistent with a general base-catalyzed carbonyl
-
of kobsd vs [ArO ] at constant pH. Both k
were independent of pH. The k values obtained for the
reactions of NPClTF and PClTF are (7.8 ( 0.2) × 10
, respectively; the same
o N
and k values
o
-
4
2e
addition-elimination mechanism.
-
1
-4 -1
s
and (4.2 ( 0.4) × 10
s
Nevertheless, we can compare the â values obtained
in this work with those found in the reactions of phe-
noxide ions with phenyl esters and similar compounds:
Our â values are in agreement with those for the
reactions with 1-acetoxy-8-hydroxynaphthalene (â )
values were obtained (within experimental error) in the
absence of phenol under the same conditions of buffer
and pH. The k
N
values are shown in Table 3 together
with the pK of the phenols. Figure 1 shows the Br o¨ n-
a
9
10
sted-type plots obtained with the data in Table 3.
The Br o¨ nsted slopes of the plots in Figure 1 are â )
.55 ( 0.05 and 0.47 ( 0.05 for the reactions of phenyl
0.48), 4-chloro 2-nitrophenyl acetate (â ) 0.64 ( 0.05),
acetic anhydride (â ) 0.58 ( 0.05),11 2,4-dinitrophenyl
0
and 4-nitrophenyl chlorothionoformates, respectively.
To our knowledge, there are no reports of â values for
the reactions of phenoxide ions with phenyl chlorofor-
mates in water, which could be compared to our values.
(8) Castro, E. A.; Ureta, C. J . Org. Chem. 1989, 54, 2153.
(
9) Hibbert, F.; Malana, M. A. J . Chem. Soc., Perkin Trans 2 1990,
11.
10) Ba-Saif, S.; Luthra, A. K.; Williams, A. J . Am. Chem. Soc. 1989,
111, 2647.
7
(

6
822 J . Org. Chem., Vol. 63, No. 20, 1998
Castro et al.
Guthrie has confirmed the conclusions of Williams that
the phenolysis of phenyl acetates are concerted, by the
application of an extension of Marcus theory to exchange
reactions of carboxylic esters.16 Hengge has reached the
same conclusion by measurement of the 15N isotope effect
on the leaving 4-nitrophenoxide ion in the reaction of
4
-nitrophenyl acetate with phenoxide anion.17 The reac-
tions of phenoxide anions with formate esters have been
1
8
confirmed to be concerted on theoretical grounds.
According to the above results, it seems reasonable that
the reactions subjected to the present study are con-
certed. This conclusion is based on (i) the similar
Br o¨ nsted slopes found in this work (ânuc ) 0.47-0.55) and
those in the concerted reactions of the same nucleophiles
with reactive esters and acetic anhydride (ânuc ) 0.43-
.68)9
-12
and (ii) the fact that the tetrahedral intermedi-
0
ate 1 is not formed in the concerted reactions of phenox-
ide ion with 3- or 4-nitrophenyl acetate, i.e., it is too
unstable to exist.1
0-16
Therefore, it is very likely that the
F igu r e 1. Br o¨ nsted-type plots obtained in the reactions of
phenyl and 4-nitrophenyl chlorothionoformates (PClTF and
NPClTF, respectively) with phenoxide anions in 3% (v/v)
dioxane in water, 25.0 °C, ionic strength 0.2 M (KCl).
putative intermediate formed in the phenolysis of phenyl
chlorothionoformate (this work, 2) be much more un-
stable than 1 for the following reasons:
The change of 3- or 4-nitrophenoxy by Cl should render
intermediate 3 more unstable kinetically than 1 in view
of the superior nucleofugality of Cl compared to the other
groups. Substitution of methyl by phenoxy should result
acetate (â ) 0.57 ( 0.03),11 3-nitrophenyl, 4-nitrophenyl,
and 3,4-dinitrophenyl formates (â ) 0.64, 0.51, and 0.43,
respectively),12 and the corresponding acetates (â ) 0.66,
1
2
0
.59, and 0.53, respectively). All these reactions have
9
-12
been found to be governed by concerted mechanisms.
Nevertheless, concerted phenolysis mechanisms with
larger â values are also known, such as those of 4-nitro-
phenyl, 4-formylphenyl, and 3-nitrophenyl acetates, â )
1
0
0
.75, 0.79, and 1.04, respectively, although these â
values are based on pK values at a different ionic
a
strength (0.1 M) than that of the kinetic measurements
(
1.0 M).12
in a further destabilization of intermediate 4 since it is
known that the similar change of methyl by methoxy or
ethoxy renders the tetrahedral intermediates more un-
In general, the above concerted mechanisms seem to
obey the reactivity-selectivity principle; i.e., the â values
are larger for the less reactive substrates,9
-12
although
stable.19 On the other hand, the change of O to S in a
-
-
there are some exceptions: in the phenolysis of 4-nitro-
phenyl N-methylisonicotinate, a value of â ) 0.90 has
been found compared to â ) 0.74 for the reactions of the
tetrahedral intermediate should stabilize the inter-
mediate,1
9c,20
although this stabilization should not be
enough to compensate for the greater destabilization
caused by the other two groups. Moreover, Williams has
argued that the presence of two phenoxy (or substituted
phenoxy) groups attached to the central carbon of a
tetrahedral intermediate is sufficient to destabilize this
same nucleophiles with 4-nitrophenyl acetate, despite the
higher reactivity of the former substrate.13
On the other hand, in solvents less polar than water,
larger â values are found for the concerted reactions of
phenoxides ions: â ) 1.32 and 1.6 for acetic anhydride
2
1
in acetonitrile and chlorobenzene, respectively.14
species in such way as to prevent its existence.
Obviously, substitution of one of the PhO groups in 2
by 4-nitrophenoxy should destabilize the “intermediate”
further, so that the reactions of phenoxide ions with
4-nitrophenyl chlorothionoformate should also be con-
certed.
Although the conclusions that some of the above
1
5
reactions are concerted have been challenged, the
1
5
arguments given by these authors have been rejected
by the original workers.1
1,12
(
11) Ba-Saif, S. A.; Colthurst, M.; Waring, M. A.; Williams, A. J .
Chem. Soc., Perkin Trans. 2 1991, 1901.
12) Stefanidis, D.; Cho, S.; Dhe-Paganon, S.; J encks, W. P. J . Am.
Chem. Soc. 1993, 115, 1650.
13) Colthurst, M. J .; Nanni, M.; Williams, A. J . Chem. Soc., Perkin
Trans. 2 1996, 2285.
14) Maude, A. B.; Williams, A. J . Chem. Soc., Perkin Trans.2 1997,
79.
(17) Hengge, A. J . Am. Chem. Soc. 1992, 114, 6575.
(18) Park, Y. S.; Kim, C. K.; Lee, B. S.; Lee, I.; Lim, W. M.; Kim, W.
K. J . Phys. Org. Chem. 1995, 8, 325.
(19) (a) Fersht A. R.; J encks, W. P. J . Am. Chem. Soc. 1970, 92,
5442. (b) Chrystiuk, E.; Williams, A. J . Am. Chem. Soc. 1987, 109,
3040. (c) Castro, E. A.; Ib a´ n˜ ez, F.; Salas, M.; Santos, J . G. J . Org. Chem.
1991, 56, 4819.
(
(
(
1
(
15) Kwon, D. S.; Lee, G. J .; Um I. H. Bull. Korean Chem. Soc. 1990,
(20) Castro, E. A.; Salas, M.; Santos, J . G. J . Org. Chem. 1994, 59,
30.
1
1, 262. Buncel, E.; Um, I. H.; Hoz, S. J . Am. Chem. Soc. 1989,111,
9
71.
(21) Ba-Saif, S.; Luthra, A. K.; Williams, A. J . Am. Chem. Soc. 1987,
109, 6362.
(16) Guthrie, J . P. J . Am. Chem. Soc. 1991, 113, 3941.

Reactions of Phenyl and 4-Nitrophenyl Chlorothionoformates
J . Org. Chem., Vol. 63, No. 20, 1998 6823
It has been claimed that thioanion attack on phenyl
acetates and phenyl thiolacetates can involve the par-
ticipation of a tetrahedral intermediate.21 When the
formation of this is rate determining a ânuc ) 0.3 has been
obtained.22 In the reactions described in the present
work, if a tetrahedral intermediate were formed its
formation would be rate limiting because chloride ion is
a very powerful nucleofuge, and therefore, its expulsion
from the putative intermediate would be faster than
expulsion of phenoxide anions. The fact that in the
present reactions ânuc is 0.47-0.55 is not in agreement
with a stepwise process with formation of a putative
tetrahedral intermediate as the rate-determining step.
In contrast to the concerted reactions exhibited by the
title substrates with phenoxide ions, the reactions of the
same substrates with secondary alicyclic amines are
known to proceed by stepwise mechanisms.4 In other
words, substitution of one of the PhO groups in the
putative intermediate 2 by a secondary amino group
should stabilize the “intermediate” and make possible its
existence as a discrete species. This is in accord with
the arguments by Williams concerning the stability of
different tetrahedral intermediates.21
The fact that the reactions of phenyl chlorothionofor-
mates with aryl oxide anions are concerted while those
with amines are stepwise is in agreement with the
behavior exhibited by the reactions of phenyl esters and
1
0-13,21
acetic anhydride. The reactions of phenyl acetates,
benzoates
1
1,23
12
and formates, as well as those of acetic
anhydride11 with phenoxide ions, are concerted, in con-
trast to those with amines that are known to proceed
through a tetrahedral intermediate in a two-step reac-
tion.24 This confirms that substitution of an amino
moiety in a tetrahedral intermediate that possesses a
phenoxy group by another phenoxy group brings about
such destabilization that the intermediate no longer
exists; i.e., its lifetime becomes shorter than a vibration
(
(
22) Hupe, D. J .; J encks, W. P. J . Am. Chem. Soc. 1977, 99, 451.
23) Colthurst, M. J .; Williams, A. J . Chem. Soc., Perkin Trans. 2
1
997, 1493.
24) Satterthwait, A. C.; J encks, W. P. J . Am. Chem. Soc. 1974, 96,
018. Castro, E. A.; Valdivia, J . L. J . Org. Chem. 1986, 51, 1668. Yang,
(
25
period.
7
C. C.; J encks, W. P. J . Am. Chem. Soc. 1988, 110, 2972. Castro, C.;
Castro, E. A. J . Org. Chem. 1981, 46, 2939.
Ack n ow led gm en t. We thank “Fondo Nacional de
Investigaciones Cient ´ı ficas y Tecnol o´ gicas” (FOND-
ECYT) of Chile for financial support to this work.
(
25) J encks, W. P. Acc. Chem. Res. 1980, 13, 161. J encks, W. P.
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