10738
J. Am. Chem. Soc. 2001, 123, 10738-10739
Communications to the Editor
Figure 1 shows the Bronsted plot for the bromine transfer
reactions with log k plotted against the ∆pKa of the acids derived
from the nucleophile and the substrate (when H substitutes for
Br). The figure, surprisingly, reveals two unique features which
are typical of proton-transfer reactions: the Pearson5 and Kresge6
nitro anomalies.
Halophilic Reactions: Anomalies in Bromine
Transfer Reactions
Judith Grinblat, Moshe Ben-Zion, and Shmaryahu Hoz*
Department of Chemistry, Bar-Ilan UniVersity
In 1953 Pearson and Dillon reported5 a linear correlation
between the kinetics and the thermodynamics of deprotonation
reactions of carbon compounds. The reactivity of nitroethane and
nitromethane deviated negatively from this linear correlation by
ca. 5 orders of magnitude. It is important to point out that this
behavior is unique to the case of a single nitro activating group.
The presence of another activating group largely attenuates this
anomaly. Thus, dinitromethane, for example, fits the linear
correlation well. Anomalous behavior similar to the above is found
in the bromine transfer reactions studied here. The largest rate
constants are associated with bromine transfer between cyano-
activated carbanions. The smallest relate to the removal of
bromine from the nitromethane and nitroethane moieties. Inter-
mediate within the same ∆pKa range is the bromine transfer
reaction involving phenylnitromethane in which the phenyl group
slightly attenuates the nitro anomaly effect.
Ramat-Gan, Israel 52900
ReceiVed April 23, 2001
While synthetic applications of halophilic reactions (nucleo-
philic attacks on halogens) are sporadically documented in the
literature,1 to the best of our knowledge, comprehensive mecha-
nistic studies are nearly nonexistent.2 This paucity of mechanistic
studies is glaring in light of the fact that nucleophilic displace-
ments are among the most thoroughly studied reactions in physical
organic chemistry. Herein we report a quantitative study of
nucleophilic attack on bromine, namely, its transfer between
carbanions (eq 1). The rate constants found, summarized in Table
R- + Br - R′ f R - Br + R′-
(1)
The second anomaly, reported by Kresge,6 relates to the series
nitromethane, ethane, and isopropane. Contrary to expectations,
in this series compounds with higher acidity undergo slower
deprotonation. Namely, the Bronsted plot displays a negative
slope.
1, were determined spectroscopically by following, in most cases,
the decrease in the absorption of the nucleophile. In the reactions
of HC(CN)2- with BrPhC(CN)2 we monitored the increase in the
absorption of the ionic product.
The progress of the reactions studied resulted in the simulta-
neous presence of PhC(CN)2- and BrPhC(CN)2, one as a reactant
and the other as a product. This led to a reaction between them
resulting in a dimer formation (eq 2).3However, since the
The Bronsted plot in Figure 1 clearly shows that unlike any
normal Bronsted plot which by definition displays a positive slope,
that for MeNO2 and EtNO2 is negatiVe. Thus, the resemblance
between proton and bromine transfer reactions is again borne out.
These surprising results are intelligible if one views bromine
transfer and deprotonation reactions as simple SN2 reactions on
Br and H, respectively. The anomalous behavior should then be
traced to the leaving group effect. Clearly, since deprotonation
reactions are not considered to involve an electron transfer
component, the observed similarity argues against the possibility
of the involvement of an electron transfer step in these bromine
transfer reactions. Alternatively, it will be very surprising to learn
that the nitro anomaly is manifested also under radical/radical-
anionic conditions.
dimerization is much faster than the other reactions (k ) (3 (
0.1) × 105 M-1 s-1, independently determined) it did not interfere
with the kinetic studies.
The reactions were conducted in buffered aqueous solutions
(sodium acetate, phosphate, and carbonate according to the
required pH values) under pseudo-first-order conditions with a
stopped flow spectrophotometer for the fast reactions.
It is interesting to further compare the reactivities of the same
carbanions in bromine-transfer and proton-transfer reactions.
Using the reversible reactions in Table 1 in combination with
the Marcus equation and assuming that the intrinsic barrier for
-
Clearly, the reaction between PhC(CN)2 and BrPhC(CN)2,
leading to dimer formation, involves electron transfer. It is not
clear, however, whether the reactions in Table 1 proceed by the
direct SN2 mechanism or also involve an electron-transfer step.
The fact that in these cases no dimerization was observed suggests
that free radicals are not involved. Yet, we were unable to rule
out the possibility of a cage process of an electron transfer yielding
the radical anion of the substrate (R-Br) that undergoes a
mesolytic cleavage4 to R- and Br•. Combination of the latter with
the radical derived from the nucleophile would provide the
product.
-
bromine transfer reaction between PhC(CN)2 and BrPhC(CN)2
is nearly identical to the intrinsic barrier for the bromine transfer
-
reaction between HC(CN)2 and BrCH(CN)2 (∆G0 for the first
entry reaction in Table 1 is only ≈3 kcal/mol), we calculated the
intrinsic barrier for the identity bromine transfer reactions between
substituted bromomalononitriles (RCBr(CN)2) and their corre-
sponding anions RC(CN)2- (Table 2). Since the intrinsic barrier
for a non-identity reaction (second row in Table 1) is the average
of the intrinsic barriers of the two relevant reactions, and since
one of the two (that for the malononitrile derivatives) is already
known (10.9 kcal/mol, Table 2), we can calculate the intrinsic
barrier for the Br exchange between PhCHBrNO2 and the anion
(1) Zefirov, N. S.; Makhon’kov, D. I. Chem. ReV. 1982, 82, 615 and
references cited under “Synthetic applications of X-philic reactions”.
(2) Following are some mechanistic studies of halophilic reactions: Beak,
P.; Allen, D. J. J. Am. Chem. Soc. 1992, 114, 3420. Differding, E.; Ruegg, G.
M. Tetrahedron Lett. 1991, 32, 3815. DesMarteau, D. D.; Xu, Z.-Q.; Witz,
M. J. Org. Chem. 1992, 57, 629. Li, X. Y.; Tu, M. H.; Jiang, X. K. Chin.
Chem. Lett. 1993, 4, 411. Umemoto, T.; Fukami, S.; Tomizawa, G.; Harasawa,
K.; Kawada, K.; Tomita, K. J. Am. Chem. Soc. 1990, 112, 8563.
(3) Hartzler, H. D. J. Org. Chem. 1966, 31, 2654.
-
PhCHNO2 (Table 2). The intrinsic barrier for proton-transfer
(5) Pearson, R. G.; Dillon, R. L. J. Am. Chem. Soc. 1953, 75, 2439.
(6) Kresge, A. J. Can. J. Chem. 1974, 52, 1897. For the latest work on
this issue see: Yamataka, H.; Mustanir; Mishima, M. J. Am. Chem. Soc.
1999, 121, 10223.
(4) Maslak, P.; Narvaez, J. N. Angew. Chem., Int. Ed. Engl. 1990, 29, 283.
10.1021/ja011014n CCC: $20.00 © 2001 American Chemical Society
Published on Web 10/03/2001