Concerted mechanisms of the reactions of ethyl S-aryl thiocarbonates with substituted phenoxide ions

Article abstract of DOI:10.1021/jo981956j

The reactions of 4-nitrophenyl, 2,4-dinitrophenyl, and 2,4,6- trinitrophenyl O-ethyl thiolcarbonates with substituted phenoxide ions are subjected to a kinetic study in water, 25.0 °C, ionic strength 0.2 M (KCl). By following the reactions spectrophotometrically, pseudo-first-order rate coefficients (k(obsd)) are found under excess of the nucleophile. Plots of k(obsd) vs phenoxide anion concentration at constant pH are linear, with the slope (k(N)) independent of pH. The Bronsted-type plots (log k(N) vs pK(a) of the phenols) are linear with slopes β = 0.92, 0.77, and 0.61 for the reactions of the 4-nitrophenyl, 2,4-dinitrophenyl, and 2,4,6-trinitrophenyl derivatives, respectively. For these reactions, a concerted mechanism is proposed since the slope values are similar to those found in the concerted phenolysis of aryl acetates; the slope magnitudes are not consistent with a stepwise mechanism where the formation of a tetrahedral intermediate is rate limiting. Our results are in line with the finding that the reactions of the 2,4-dinitrophenyl and 2,4,6-trinitrophenyl derivatives with secondary alicyclic amines in water are concerted. In contrast, the reactions of the same substrates with pyridines are stepwise, which means that substitution of a pyridine moiety in a tetrahedral intermediate with a benzenethio group by a phenoxy group destabilizes the intermediate.

Full text of DOI:10.1021/jo981956j

2310
J . Org. Chem. 1999, 64, 2310-2313
Con cer ted Mech a n ism s of th e Rea ction s of Eth yl S-Ar yl
Th ioca r bon a tes w ith Su bstitu ted P h en oxid e Ion s
Enrique A. Castro,* Paulina Pavez, and J ose´ G. Santos*
Facultad de Quı´mica, Pontificia Universidad Cato´lica de Chile, Casilla 306, Santiago 22, Chile
Received September 28, 1998
The reactions of 4-nitrophenyl, 2,4-dinitrophenyl, and 2,4,6-trinitrophenyl O-ethyl thiolcarbonates
with substituted phenoxide ions are subjected to a kinetic study 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 excess of the nucleophile. Plots of kobsd vs phenoxide anion concentration at
constant pH are linear, with the slope (kN) independent of pH. The Bro¨nsted-type plots (log kN vs
pKa of the phenols) are linear with slopes â ) 0.92, 0.77, and 0.61 for the reactions of the
4-nitrophenyl, 2,4-dinitrophenyl, and 2,4,6-trinitrophenyl derivatives, respectively. For these
reactions, a concerted mechanism is proposed since the slope values are similar to those found in
the concerted phenolysis of aryl acetates; the slope magnitudes are not consistent with a stepwise
mechanism where the formation of a tetrahedral intermediate is rate limiting. Our results are in
line with the finding that the reactions of the 2,4-dinitrophenyl and 2,4,6-trinitrophenyl derivatives
with secondary alicyclic amines in water are concerted. In contrast, the reactions of the same
substrates with pyridines are stepwise, which means that substitution of a pyridine moiety in a
tetrahedral intermediate with a benzenethio group by a phenoxy group destabilizes the intermediate.
In tr od u ction
DNPTC, and TNPTC, respectively) with the aim to shed
more light on the reaction mechanisms of thiol com-
pounds; we also compare these mechanisms with those
for the aminolysis and pyridinolysis of the same sub-
strates8 and with those found in the phenolysis of other
carbonyl and thiocarbonyl compounds.10-13
Although much attention has been drawn to the
kinetics and mechanism of the aminolysis and pyridi-
nolysis of carbonyl1-4 and thiocarbonyl5-9 compounds, the
reactions of phenoxide anions with the latter substrates
have been less studied.10
In the present work, we describe a kinetic and mecha-
nistic study of the reactions of phenoxide anions with
ethyl S-(4-nitrophenyl), ethyl S-(2,4-dinitrophenyl), and
ethyl S-(2,4,6-trinitrophenyl) thiocarbonates (NPTC,
Exp er im en ta l Section
Ma ter ia ls. The phenols (Aldrich) were purified either by
distillation or recrystallization. NPTC, DNPTC, and TNPTC
were obtained as previously.8a-c Ethyl phenyl carbonate (EtO-
COOPh) was prepared by the same procedure used for methyl
phenyl carbonate.3d
Deter m in a tion of p Ka . The pKa values of the phenols were
determined spectrophotometrically in water, at 25.0 ( 0.1 °C,
ionic strength 0.2 M (KCl).
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: aqueous solution, at 25.0 (
0.1 °C, ionic strength 0.2 M (KCl), and in some cases, borate
buffer 0.005 M. The reactions were followed at 400 nm
(appearance of the substituted benzenethiolate anions).
(1) Satterthwait, A. C.; J encks, W. P. J . Am. Chem. Soc. 1974, 96,
7018.
(2) Gresser, M. J .; J encks, W. P. J . Am. Chem. Soc. 1977, 99, 6963,
6970.
(3) (a) Fife, T. H.; Hutchins, J . E. C. J . Am. Chem. Soc. 1981, 103,
4194. (b) Brunelle, D. J . Tetrahedron Lett. 1982, 23, 1739. (c) Castro,
E. A.; Gil, F. J . J . Am. Chem. Soc. 1977, 99, 7611. (d) Castro, E. A.;
Freudenberg, M. J . Org. Chem. 1980, 45, 906. (e) Castro, E. A.; Iban˜ez,
F.; Lagos, S.; Schick, M.; Santos, J . G. J . Org. Chem. 1992, 57, 2691.
(f) Hibbert, F.; Malana, M. A. J . Chem. Soc., Perkin Trans. 2 1990,
711.
(4) Kovach, I. M.; Belz, M.; Larson, M.; Rousy, S.; Schowen, R. L. J .
Am. Chem. Soc. 1985, 107, 7360. Neuvonen, H. J . Chem. Soc., Perkin
Trans. 2 1987, 159. Yoh, S.-D.; Kang, J .-K.; Kim, S.-H. Tetrahedron
1988, 44, 2167. Knowlton, R. C.; Byers, L. D. J . Org. Chem. 1988, 53,
3862. Koh, H. J .; Lee, H. C.; Lee, H. W.; Lee, I. Bull. Korean Chem.
Soc. 1995, 16, 839.
(5) Campbell, P.; Lapinskas, B. A. J . Am. Chem. Soc. 1977, 99, 5378.
Um, I.-H.; Kwon, H.-J .; Kwon, D.-S.; Park, J .-Y. J . Chem. Res., Synop.
1995, 301. Um, I. H.; Choi, K. E.; Kwon, D. S. Bull. Korean Chem.
Soc. 1990, 11, 362. Lee, I.; Shim, C. S.; Lee, H. W. J . Chem. Res., Synop.
1992, 90. Oh, H. K.; Shin, C. H.; Lee, I. Bull. Korean Chem. Soc. 1995,
16, 657. Oh, H. K.; Shin, C. H.; Lee, I. J . Chem. Soc., Perkin Trans. 2
1995, 1169.
(6) (a) Castro, E. A.; Ureta, C. J . Chem. Soc., Perkin Trans. 2 1991,
63. (b) Castro, E. A.; Iba´n˜ez, F.; Santos, J . G.; Ureta, C. J . Org. Chem.
1992, 57, 7024.
(9) (a) Castro, E. A.; Cubillos, M.; Santos, J . G. J . Org. Chem. 1996,
61, 3501. (b) Castro, E. A.; Cubillos, M.; Santos, J . G.; Te´llez, J . J .
Org. Chem. 1997, 62, 2512. (c) Castro, E. A.; Santos, J . G.; Te´llez, J .;
Uman˜a, M. I. J . Org. Chem. 1997, 62, 6568.
(10) (a) Hupe, D. J .; J encks, W. P. J . Am. Chem. Soc. 1977, 99, 451.
(b) Pohl, E. R.; Wu, D.; Hupe, D. J . J . Am. Chem. Soc. 1980, 102, 2759.
(c) Pohl, E. R.; Hupe, D. J . J . Am. Chem. Soc. 1980, 102, 2763. (d)
Um, I.-H.; Chun, S.-E.; Kwon, D.-S. Bull. Korean Chem. Soc. 1991,
12, 510.
(11) (a) Ba-Saif, S.; Luthra, A. K.; Williams, A. J . Am. Chem. Soc.
1987, 109, 6362. (b) Ba-Saif, S.; Luthra, A. K.; Williams, A. J . Am.
Chem. Soc. 1989,111, 2647. (c) Ba-Saif, S. A.; Colthurst, M.; Waring,
M. A.; Williams, A. J . Chem. Soc., Perkin Trans. 2 1991, 1901. (d)
Colthurst, M. J .; Nanni, M.; Williams, A. J . Chem. Soc., Perkin Trans.
2 1996, 2285. (e) Colthurst, M. J .; Williams, A. J . Chem. Soc., Perkin
Trans. 2 1997, 1493.
(7) (a) Castro, E. A.; Cubillos, M.; Iba´n˜ez, F.; Moraga, I.; Santos, J .
G. J . Org. Chem. 1993, 58, 5400. (b) Castro, E. A.; Araneda, C. A.;
Santos, J . G. J . Org. Chem. 1997, 62, 126.
(8) (a) Castro, E. A.; Iba´n˜ez, F.; Salas, M.; Santos, J . G. J . Org. Chem.
1991, 56, 4819. (b) Castro, E. A.; Salas, M.; Santos, J . G. J . Org. Chem.
1994, 59, 30. (c) Castro, E. A.; Cubillos, M.; Santos, J . G. J . Org. Chem.
1994, 59, 3572. (d) Castro, E. A.; Pizarro, M. I.; Santos, J . G. J . Org.
Chem. 1996, 61, 5982.
(12) (a) Kwon, D. S.; Lee, G. J .; Um I. H. Bull. Korean Chem. Soc.
1990, 11, 262. (b) Buncel, E.; Um, I. H.; Hoz, S. J . Am. Chem. Soc.
1989, 111, 971.
(13) Stefanidis, D.; Cho, S.; Dhe-Paganon, S.; J encks, W. P. J . Am.
Chem. Soc. 1993, 115, 1650.
10.1021/jo981956j CCC: $18.00 © 1999 American Chemical Society
Published on Web 03/06/1999

Reactions of Ethyl S-arylthiocarbonates
J . Org. Chem., Vol. 64, No. 7, 1999 2311
Ta ble 1. 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 Eth yl
S-(4-Nitr op h en yl) Th ioca r bon a tea
Ta ble 3. 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 Eth yl
S-(2,4,6-Tr in ir op h en yl) Th ioca r bon a tea
phenoxide
substituent
no. of
runs
phenoxide
substituent
no. of
runs
pH
102[ArOH]totb/M
104kobsd/s-1
pH
102[ArOH]totb/M
103kobsd/s-1
4-methoxy
none
9.44c
0.5-4.0
0.5-7.0
1.0-9.0
0.5-7.0
0.5-7.0
1.0-7.0
1.0-5.0
1.0-9.0
0.5-5.0
1.0-9.0
1.0-9.0
1.0-9.0
4.7-17
5.4-42
16-137
2.3-12
2.5-19
6.0-34
2.1-7.3
3.8-14
2.5-15
0.04-0.3
0.08-0.5
0.1-0.6
4
6
6
6
6
5
4
6
5
6
6
6
4-methoxy
none
9.44c
0.1-4.0
0.1-2.5
0.1-1.0
0.5-4.0
0.5-7.0
0.5-4.0
1.0-7.0
0.1-4.0
0.5-7.0
5.0-9.0
1.0-9.0
1.0-9.0
1.0-9.0
1.0-9.0
1.0-9.0
8.1-107
8.4-73
19-94
5.1-39
6.6-92
12-82
7.7-37
4.0-32
8.7-84
1.7-2.9
0.8-4.2
1.1-5.7
0.21-1.0
0.23-1.1
0.26-1.0
5
4
3
4
6
4
5
5
6
3
5
6
6
6
6
9.74c
9.74c
10.4
9.44
9.74
10.4
9.44
9.74
10.4
10.4
4-chloro
4-cyano
8.96
9.26
9.56
7.50
7.80
8.10
4-chloro
4-cyano
8.96
9.26
9.56
7.50
7.80
8.10
8.00c
8.50c
9.00c
pentafluoro
a
b
In water, at 25.0 °C. Ionic strength 0.2 M (KCl). Total
concentration of substituted phenol (acid plus conjugate base).
c Buffer borate 0.005 M.
a
b
In water, at 25.0 °C. Ionic strength 0.2 M (KCl). Total
concentration of substituted phenol (acid plus conjugate base).
c Buffer borate 0.005 M.
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 Eth yl
S-(2,4-Din itr op h en yl) Th ioca r bon a tea
coefficient, ko and kN are the rate coefficients for hydroly-
phenoxide
substituent
no. of
runs
pH
102[ArOH]totb/M
103kobsd/s-1
kobsd ) ko + kN[ArO-]
(1)
4-methoxy
none
9.44c
0.5-5.0
0.3-4.0
0.3-1.0
0.5-7.0
0.5-7.0
0.5-7.0
1.0-7.0
0.1-5.0
0.5-7.0
1.0-9.0
1.0-9.0
1.0-9.0
2.5-9.0
1.0-9.0
1.0-9.0
7.5-64
8.3-94
5
5
3
6
6
6
5
6
6
6
6
6
5
6
6
9.74c
10.4
9.44
9.74
20-53
sis and phenolysis, respectively, and ArO- represents a
phenoxide ion. The values of ko and kN were obtained as
the intercept and slope, respectively, of linear plots of
kobsd vs [ArO-] at constant pH. For all the reactions the
same straight line for three different pH values was
found, except those of DNPTC with pentafluorophenoxide
where three parallel lines were obtained for the different
pH values. These results show that both the kN and ko
values are independent of pH, except the ko values for
the latter reactions.
3.4-26
3.0-37
10.4
11-94
4-chloro
4-cyano
8.96
9.26
9.56
7.50
7.80
8.10
8.00c
8.50c
9.00c
4.1-19
1.4-29
4.7-51
0.06-0.13
0.10-0.18
0.12-0.22
0.4-1.2
0.2-2.1
0.3-2.6
pentafluoro
In the 7.5-8.1 pH range, the ko values obtained are 4
× 10-6, 1 × 10-4, and 6 × 10-4 s-1 for the hydrolysis
reactions of NPTC, DNPTC, and TNPTC, respectively,
showing the dependence of ko on the basicity of the
leaving group. On the other hand, in the reaction of
DNPTC with pentafluorophenoxide, the ko values are 5
× 10-5, 9 × 10-5, and 1.1 × 10-4 s-1 at pH 8.0, 8.5, and
9.0, respectively.
The kN values are shown in Table 4 together with the
pKa of the phenols. Figure 1 shows the Bro¨nsted-type
plots obtained with the data in Table 4. The Bro¨nsted
slopes of the plots in Figure 1 are â ) 0.92 ( 0.09, 0.77
( 0.06, and 0.61 ( 0.09 for the reactions of NPTC,
DNPTC, and TNPTC, respectively. The values of the
slopes for the reactions of the two latter substrates are
in agreement with those found in the concerted reactions
of phenoxide ions with phenyl esters and similar com-
pounds: 1-acetoxy-8-hydroxynaphthalene (â ) 0.48),3f
4-chloro-2-nitrophenyl acetate (â ) 0.64 ( 0.05),11b acetic
anhydride (â ) 0.58 ( 0.05),11c 2,4-dinitrophenyl acetate
(â ) 0.57 ( 0.03),11c 3-nitrophenyl, 4-nitrophenyl, and 3,4-
dinitrophenyl formates (â ) 0.64, 0.51, and 0.43, respec-
tively),13 and the corresponding acetates (â ) 0.66, 0.59,
and 0.53, respectively).13 All these reactions have been
found to be governed by concerted mechanisms.
a
b
In water, at 25.0 °C. Ionic strength 0.2 M (KCl). Total
concentration of substituted phenol (acid plus conjugate base).
c Buffer borate 0.005 M.
All reactions were studied under excess of the phenol over
the substrate (10-fold at least). The initial substrate concen-
tration was 2 × 10-5 M.
Pseudo-first-order rate coefficients (kobsd) were found through-
out, mostly by means of the “infinity” method; for the slowest
reactions (NPTC with 4-cyanophenoxide) the initial rate
method was used.11a The experimental conditions of the
reactions and the kobsd values are shown in Tables 1-3.
P r od u ct Stu d ies. For the reactions of the substrates with
the different phenoxide ions, one of the products was identified
as the corresponding benzenethiolate; this was achieved by
comparison of the UV-vis spectra after completion of the
reactions with those of authentic samples under the same
conditions. The other product of the reactions of phenoxide ion
with the three substrates was identified as ethyl phenyl car-
bonate, as indicated by HPLC comparison of an authentic sam-
ple with those after completion of the reactions. HPLC condi-
tions: column Eurospher C-18 (10 cm, 7 µm), eluant acetoni-
trile/water ) 70/30, isocratic mode 0.7 mL/min. The yield of
ethyl phenyl carbonate obtained in these reactions was in
accordance with a nucleophilic attack of phenoxide ion on the
substrates. A general base-catalyzed pathway cannot be rigor-
ously excluded, but it would amount to no more than 5% (with-
in the error of the analysis) of the total phenoxide reaction.
If the phenolyses of DNPTC and TNPTC were step-
wise, the rate-determining step would be the formation
of the intermediate in view of the greater nucleofugality
from the putative tetrahedral intermediate of 2,4-dinitro-
and 2,4,6-trinitrobenzenethiolate anions (pKa 3.4 and 1.4,
Resu lts a n d Discu ssion
The kinetic law obtained in the present reactions is
given by eq 1, where kobsd is the pseudo-first-order rate

2312 J . Org. Chem., Vol. 64, No. 7, 1999
Castro et al.
Ta ble 4. Va lu es of p Ka of P h en ols a n d kN for th e Rea ction s of P h en oxid es w ith Eth yl S-Ar yl Th ioca r bon a tesa
102kN/s-1 M-1
nucleophile anion
pKa of phenol
NPTC
DNPTC
TNPTC
4-methoxyphenoxide
phenoxide
4-chlorophenoxide
4-cyanophenoxide
pentafluorophenoxide
10.3
9.9
9.4
7.8
5.3
27.2 ( 0.6
5.7 ( 0.1
1060 ( 30
1820 ( 90
260 ( 8
135 ( 5
121 ( 5
3.8 ( 0.2
197 ( 5
0.11 ( 0.04
4.8 ( 0.2
0.094 ( 0.004
9.4 ( 0.5
1.1 ( 0.4
a
Both pKa and kN values were determined in water, at 25.0 °C, ionic strength 0.2 M (KCl).
destabilization of the intermediate, as explained below.
Thereactionsofsubstitutedphenylacetates,11 benzoates,11b,d
formates,13 and chlorothionoformates,14 as well as those
of acetic anhydride11c with phenoxide ions are concerted,
in contrast to those with amines that are known to
proceed through a tetrahedral intermediate in a two-step
reaction.1,15 Therefore, in all these reactions substitution
of an amine in the tetrahedral intermediate by a substi-
tuted phenoxy group results in destabilization of the
intermediate.
F igu r e 1. Bro¨nsted-type plots obtained in the reactions of
4-nitrophenyl, 2,4-dinitrophenyl, and 2,4,6-trinitrophenyl O-
ethyl thiolcarbonates (NPTC, DNPTC, and TNPTC, respec-
tively) with a series of phenoxide anions in aqueous solution,
25.0 °C, ionic strength 0.2 M (KCl).
Concerning the phenolysis of NPTC, the Bro¨nsted slope
(â ) 0.92) of Figure 1 is in accord with those found in
the concerted phenolysis of 4-nitrophenyl, 4-formylphe-
nyl, and 3-nitrophenyl acetates, â ) 0.75, 0.79, and 1.04,
respectively.11b Had the phenolysis of NPTC been step-
wise, the formation of the tetrahedral intermediate would
have been rate limiting since 4-nitrobenzenethiolate is
a much better nucleofuge from the intermediate than the
four phenoxide nucleophiles studied (pKa ca. 8-10 for the
latter phenols compared to 4.6 for 4-nitrobenzenethiol6a).
Although the 4-nitrobenzenethio group should stabilize
the intermediate relative to the 2,4-dinitro- and 2,4,6-
trinitrobenzenethio groups, we are more inclined toward
the concerted mechanism for the phenolysis of NPTC
since its â value is far too great for a stepwise mechanism
where the formation of the intermediate is rate deter-
mining. Moreover, for the reactions studied in this work,
a plot of âN vs pKa of the leaving group is linear (not
shown) with slope 0.09 and correlation coefficient 0.994
(albeit three points), which suggests that these three
reactions are governed by a common mechanism.
The mechanism of the aminolysis of NPTC is stepwise;8c
therefore, the tetrahedral intermediate 4 (NP ) 4-nitro-
phenyl) exists; a change of the amine by a phenoxy group
leads to compound 5, which according to what has been
discussed previously should be more unstable than 4.
respectively)6a relative to the series of phenoxide nucleo-
philes. The Bro¨nsted slopes usually found for the step-
wise thiolysis of aryl acetates10a and aminolysis of aryl
acetates,1 diaryl carbonates,2 and their thioanalogues6,7
is âN ) 0.1-0.3, when the formation of the tetrahedral
intermediate is rate determining. These â values are not
in agreement with those found in this work for the
phenolysis of DNPTC and TNPTC (Figure 1). Therefore,
it is likely that these reactions are concerted, through
transition state 1 (Ar1 ) 2,4-dinitrophenyl or 2,4,6-
trinitrophenyl).
In the reactions of DNPTC and TNPTC with secondary
alicyclic amines,8a,b under the same conditions as this
study, the magnitudes of â together with the fact that
the predicted hypothetical breaks for stepwise reactions
were not observed led to the conclusion that these reac-
tion mechanisms are concerted. This means that the
tetrahedral intermediate 2 (Ar ) 2,4-dinitrophenyl or
2,4,6-trinitrophenyl) is either too unstable to exist or it
is very unstable and the reaction bypasses it, choosing a
path of lower free energy barrier (the concerted proc-
ess). Therefore, it is very likely that the putative inter-
mediate formed in the phenolysis of the same substrates
(this work, 3) is more unstable than 2 because substitu-
tion of amine by phenoxy should result in a further
(14) Castro, E. A.; Cubillos, M.; Santos, J . G. J . Org. Chem. 1998,
63, 6820.
(15) Castro, E. A.; Valdivia, J . L. J . Org. Chem. 1986, 51, 1668. Yang,
C. C.; J encks, W. P. J . Am. Chem. Soc. 1988, 110, 2972. Castro, C.;
Castro, E. A. J . Org. Chem. 1981, 46, 2939. Castro, E. A.; Cubillos,
M.; Santos, J . G., J . Org. Chem. 1997, 62, 4395.

Reactions of Ethyl S-arylthiocarbonates
J . Org. Chem., Vol. 64, No. 7, 1999 2313
It is known that in the reactions of phenoxide ions
with 4-nitrophenyl acetate the mechanism is concerted;
therefore, the tetrahedral intermediate 6 either does not
exist or it is very unstable.11b It is also known that the
change of methyl by an ethoxy group destabilizes the tet-
rahedral intermediate;8a,b therefore, the intermediate 7
should be more unstable than 6. On the other hand, since
a stepwise mechanism was found for the thiolysis of aryl
acetates,10a it means that tetrahedral compound 8 has a
lifetime long enough to qualify as an intermediate.
A comparison of intermediates 6 and 8 suggests that
a change of phenoxy by a benzenethio group stabilizes
the intermediate. Therefore the tetrahedral intermedi-
ate 5 should be more stable than 7, in line with the
stabilization of tetrahedral intermediates caused by sub-
stitution of O groups attached to the central carbon by S
groups.16
Ack n ow led gm en t. We thank the Fondo Nacional
de Investigaciones Cient´ıficas y Tecnolo´gicas (FOND-
ECYT) of Chile for financial support to this work.
J O981956J
(16) Capon, B.; Ghosh, A. K.; Grieve, D. M. A. Acc. Chem. Res. 1981,
14, 306. Capon, B.; Dosunumu, M. I.; de Matus-Sanchez, M. N. Adv.
Phys. Org. Chem. 1985, 21, 37 and references therein.