550 J . Org. Chem., Vol. 62, No. 3, 1997
Buckley and Oppenheimer
in entropies for the NaN3 reaction between the sulfonium
and pyridinium substrates (∆∆Sq ) -15 gibbs/mol). We
interpret this difference to be the result of solvation of
the leaving group at the transition state.18 Introduction
of SMe2, which is hydrophobic, into solvent at the tran-
sition state would be expected to have little effect on the
organization of solvent and thus on the entropy, while
introduction of a very water soluble pyridine or sulfonic
acid would be expected to cause reorganization of solvent
at the transition state with a large loss in entropy.
Ha m m ett P lots. There is a severe break in the
Hammett plot for hydrolysis of 4-substituted benzyldi-
methylsulfoniums that is the result of a change from an
SN1 mechanism for 1 to direct solvent displacement for
substrates with less electron-donating substituents (see
value and the values calculated with eq 2 and eq 4 that
relate rate constants for the various processes and the
independently measured value of kN/kW for partitioning
of the 4-methoxybenzyl carbenium ion between nucleo-
phile and solvent under the reaction conditions. The
values of kN/kW for trapping of the carbenium ion by
-
pyridine-d5 or N3 are both larger for salt present, a
consequence of the lower SN1 rate constant at the higher
ionic strength.44 The fact that the values of [RNu]/[ROH]
are larger for pyridine-d5 in the presence of exogenous
salt than for pyridine-d5 alone, but are larger for nucleo-
phile alone for NaN3 and Na2SO4, is compatible only with
the intervention of a second faster bimolecular process
in which Nu-‚RSMe2 is an intermediate. In the pres-
+
ence of a large excess of exogenous salt, ion pairs such
as Cl-‚RSMe2+ would be in equilibrium with and largely
Figure 7A of ref 18).39 The Hammett plot for the N3
-
favored over the productive Nu-‚RSMe2 ion pair. The
+
reaction under constant ionic strength against σ+, based
on our data for 1 and data for 5-7 reported by Swain,
Rees, and Taylor,16c is also curved (Figure S5), a shape
often found for substitution reactions of both neutral40
and charged41 benzyl substrates. In this context the
shape of the plot is unremarkable, given the clean SN2
reaction for 1 under constant ionic strength. The argu-
ments made by Richard and Yeary42 to account for these
curves, cast in terms of the Pross-Shaik model,43 seem
entirely appropriate for this system as well. Certainly
they are adequate to explain substituent effects for
benzylpyridinium ion substrates.3 On the other hand,
the computational results for the benzylsulfonium ions
(see above) suggest that the frontier orbital effects that
control the rates are more subtle than can be accounted
for by valence bond theories. Clarification of this issue
awaits further study.
Su m m a r y a n d Con clu sion s. We have shown that
the reactions of 1 occur only for nucleophiles with
intermediate HSAB rank, apparently because of frontier
orbital interactions between the substituent and the
reaction center mediated in part by orbital symmetries.
For pyridine-d5, the substitution reactions with exog-
enous salt and with nucleophile alone are mixed SN1/SN2.
For NaN3 and Na2SO3 over most of the range of nucleo-
phile concentration in the presence of an equivalent
exogenous salt, the reactions are also mixed SN1/SN2. The
reactions with NaN3 and Na2SO3 alone, however, occur
by three pathways: SN1, Hughes-Ingold SN2, and pre-
association-concerted at low concentrations of nucleo-
phile, and primarily by the preassociation-concerted
mechanism at high concentrations of nucleophile. These
assignments of mechanism rest on the excellent cor-
respondence between the experimental [RNu]/[ROH]
increased rate of the preassociation-concerted mechanism
may be merely the result of Menger’s spatio-temporal
hypothesis45 in an intermolecular reaction.
With the results reported here and our work on the
(4-methoxybenzyl)-3′-cyanopyridinium ion substrate3 we
can evaluate J encks’s12 arguments regarding the impor-
tance of the stability of an intermediate and its ability
to become solvent equilibrated as determining factors for
the particular mechanism a substrate “chooses.” The
boldest statement of the ramifications of this view was
made by Knier and J encks:46 “All solvolysis and sub-
stitution reactions at saturated carbon that proceed
through SN2 displacement mechanisms do so simply
because the intermediate in the alternative SN1 mecha-
nism is too unstable to exist [italics added].” For the
NaN3 substitution reactions of 1, 4, and the (4-methoxy-
benzyl)-3′-cyanopyridinium ion, the putative carbenium
ion, nucleophile, and solvent are the same, and there are
no added cosolvents and therefore no chance of dif-
ferential solvation, differential hydrogen bonding of the
activated complex, or other manifestations of “solvent
sorting”. While the ground states will be solvated dif-
ferently, the structure of the solvation shell about the
putative carbenium ion will be the same. At constant
ionic strength, 4 reacts through an SN1 mechanism and
produces the carbenium ion as a stable, solvent-equili-
brated species that can select between water and N3-; 1
-
undergoes N3 substitution through a mixed SN1/SN2
mechanism; and (4-methoxybenzyl)-3′-cyanopyridinium
-
ion undergoes both hydrolysis and N3 substitution by
direct displacement with a very “loose” activated complex
for the N3 reaction (âLG ) -1.47).3 All of the activated
-
complexes have significant “carbenium ion character”,
(39) In contrast, the gas-phase relative rates for collisionally
activated dissociation of a series of 4-substituted benzyldimethylsul-
foniums19 give linear Hammett plots against either σ+ or δ∆G°, a gas-
phase scale for the equilibrium protonation of a series of 4-X-2-aryl
propenes to the corresponding tert-cumyl carbenium ions (Taft, R. W.;
Topsom, R. D. Prog. Phys. Org. Chem. 1987, 16, 2-83). While the
mechanism of the gas-phase dissociation could not be ascertained with
certainty, it appears to be a direct dissociation without the intervention
of an ion-neutral complex, unlike the corresponding benzylpyridinium
substrates that react through mixed ion-neutral complex/direct dis-
sociation mechanisms.19
(44) The fact that kN/kW is inverse for pyridine-d5 (0.42 M-1 and 0.80
M-1 for no salt and salt present, respectively) but not for NaN3 (17.3
M-1 and 20.5 M-1 for no salt and salt present, respectively) is a
-
consequence of the rates at which amines and N3 react with carbe-
nium ions. McClelland and his colleagues (McClelland, R. A.; Kana-
gasabapathy, V. M.; Banait, N. S.; Steenken, S. J . Am. Chem. Soc.
1992, 114, 1816-1823) found that rates for the reaction of amines with
stable carbenium ions decrease with increasing concentrations of water
(in acetonitrile/water mixtures) because of the need for equilibrium
desolvation of RNH2---OH2 before trapping can occur. Because the rate
constants for amines and water would be expected to be similar, mere
stoichiometry favors water capture. These authors had previously
shown (McClelland, R. A.; Kanagasabapathy, V. M.; Banait, N. S.;
Steenken, S. J . Am. Chem. Soc. 1992, 113, 1009-1014) that carbenium
(40) Hudson, R. F.; Klopman, G. J . Chem. Soc. 1962, 1062-1067.
(41) Lyoxide reactions of substituted benzyl dimethyl and phenyl-
methyl sulfoniums show V-shaped Hammett plots. Schowen, K. B. J .
Doctoral Dissertation (MIT, 1964).
-
ions such as the 4-methoxybenzyl ion react with N3 at the diffusion
(42) Richard, J . P.; Yeary, P. E. A Simple Explanation for Curved
Hammett Plots for Nucleophilic Substitution Reactions at Ring-
Substituted Benzyl Derivatives. Oral presentation at the 206th ACS
National Meeting, Chicago, IL, 22-27 August, 1993.
(43) Pross, A. Adv. Phys. Org. Chem. 1985, 21, 99-196. Pross, A.;
Shaik, S. S. Acc. Chem. Res. 1983, 16, 363-370.
rate while water reacts more slowly, which greatly favors formation
of RN3.
(45) Menger, F. M. Tetrahedron 1983, 39, 1013-1040; Menger, F.
M. Acc. Chem. Res. 1985, 18, 128-134.
(46) Knier, B. L.; J encks, W. P. J . Am. Chem. Soc. 1980, 102, 6789-
6796.