1
1
0376 J. Am. Chem. Soc., Vol. 120, No. 40, 1998
Richard et al.
1-propenyl ether and water with (4-ClAr)2CH+ is consistent
31
). By contrast, electron donation from the lone annular sulfur
at thiophene does not result in a π-nucleophile that is sufficiently
with kNu/kHOH < 6 in an aqueous solvent (Table 1).
+
nucleophilic to compete with solvent for addition to 1 (kNu/ks
Structure-Reactivity Correlations. Mayr and co-workers
-
1
have shown that most or all of the rate constants, k , for the
<
0.2 M , Table 1). Similarly, the following π-nucleophiles,
Nu
activation-limited reactions of nucleophilic reagents with reso-
each of which are activated by only a single oxygen, are
insufficiently nucleophilic to compete with a solvent of 50:50
nance-stabilized carbocations show a good fit to a three-
2
+
parameter equation (eq 5). In this equation, N is the Mayr
(
v:v) acetonitrile/water for addition to 1 (Table 1): furan (kNu/
nucleophilicity parameter, E is the electrophilicity parameter,
which has an arbitrary value of 0 for the bis(4-methoxyphenyl)-
methyl carbocation, and s is the sensitivity of the nucleophilic
addition reaction to changes in the sum of E and N, which has
an arbitrary value of 1 for the “standard” alkene 2-methyl-1-
-1
ks < 0.2 M ); 1-cyclohexenyl trimethylsilyl ether (a representa-
tive silyl enol ether, kNu/ks < 2 M ); and ethyl 1-propenyl ether
a representative vinyl ether, kNu/ks < 0.2 M ).
-1
-
1
(
-1
The partitioning ratio kNu/ks ) 23 M for reaction of pyrrole
+
2
and solvent with 1 in 50:50 (v:v) acetonitrile/water can be
combined with [H2O] ) 27.8 M in this solvent to give the
dimensionless ratio of second-order rate constants kNu/kHOH )
pentene. Equation 6 describes the constant selectivity relation-
ship for electrophile-nucleophile combination reactions estab-
3
2
lished by Ritchie, where ks is the rate constant for reaction of
the electrophile with water (or an aqueous solvent) and N+ is
the Ritchie nucleophilicity parameter. The relatively unreactive
6
40 (Table 1). Thus, despite the lower Brønsted basicity of
27
pyrrole (pKa ) -3.8) than of water (pKa ) -1.7), C-2 of
pyrrole is 640-fold more reactive than water toward 1 . This
+
+
carbocation 1 is expected to adhere, at least approximately, to
the Ritchie N+ scale, provided its reactions with nucleophiles
are activation limited, with rate constants below the diffusion-
controlled limit. This criterion is met for the alkenes examined
is consistent with the known preference of “soft” acids and bases
+
(
e.g., 1 and the π-system of pyrrole) and “hard” acids and
bases (e.g., the proton and water) to react with one another,
and it can be rationalized within the framework developed to
-
1
in this work, because the selectivities kNu/ks (M ) for their
+
reaction with 1 in 50:50 (v:v) acetonitrile/water are at least
2
8
explain this general trend of organic and inorganic reactivity.
3
-1
1
0 -fold smaller than kaz/ks ) 100 000 M , and the rate constant
+
for the reaction of 1 with azide ion in 50:50 (v:v) methanol/
The partitioning of the benzyl carbocation, generated from
the decomposition of N-benzyl-N-nitrosobenzamide or the
protonation of phenyldiazomethane by benzoic acid, between
nucleophilic addition of solvent and trapping by benzoate ion
within an ion pair results in a higher yield of the solvent adduct
7
-1 -1 23
water is kaz ) 1.0 × 10 M s .
log kNu ) s(E + N)
log (k /k ) ) N
(5)
(6)
Nu
s
+
2
9
in a solvent of pyrrole than in a solvent of methanol. This
observation is in qualitative agreement with the results reported
here that pyrrole is more reactive than the hydroxylic solvent
N + log k - sE
+
s
N )
(7)
(8)
s
+
water toward 1 . However, despite the early report of the facile
formation of pyrrole adducts from the reaction of triarylcarbinols
with pyrrole in acetic acid,30 there have been surprisingly few
reports of the preparation of benzylated pyrroles via nucleophilic
addition of pyrrole to carbocations generated in nucleophilic
media.
Ncalc ) N + 5.8
+
Equations 5 and 6 can be combined to give eq 7, which relates
the Mayr and the Ritchie scales of nucleophilicity. The
reactivity of the bis(4-methoxyphenyl)methyl carbocation, for
which E ) 0, toward a solvent of 1:2 (v:v) acetonitrile/water is
A comparison of the upper limits on the rate constant ratios
5
-1 25
ks ) 1.3 × 10 s , which is somewhat larger than ks ) 180
kNu/kHOH for the reaction of representative vinyl and silyl enol
-1
+
s
for the reaction of 1 with a solvent of 50:50 (v:v)
+
ethers and water with 1 in 50:50 (v:v) acetonitrile/water (Table
23
acetonitrile/water. These values of ks can be substituted into
1
) with the corresponding rate constant ratios calculated from
+
the expressions given by eq 5 for the reactions of 1 and the
the second-order rate constants determined for reaction of these
alkenes and water with ring-substituted diarylmethyl carbo-
cations generated by laser flash photolysis in acetonitrile shows
that the change from an aqueous solvent to acetonitrile results
in either little change or in an increase in kNu/kHOH. For example,
bis(4-methoxyphenyl)methyl carbocation to give E ) [log (180/
5
(1.3 × 10 ))]/0.80 ) -3.57 as the electrophilicity parameter
11
+
for 1 , where s ) 0.80 is the sensitivity parameter for reaction
2
of the nucleophile water. The values of log ks ) 2.26 and E
+
)
-3.57 for 1 , together with s ) 1.0 determined for the
2
8
-1 -1
reactions of many π-nucleophiles, can then be substituted into
eq 7 to give eq 8. This equation provides values of Ncalc, the
the values of kNu ) 1.3 × 10 M
s
for the reaction of
3
3
11
6
-1
1
-cyclohexenyl trimethylsilyl ether and kHOH ) 4.3 × 10 M
-
1 25
estimated Mayr nucleophilicity parameter for the reaction of
s
for the reaction of water with the diarylmethyl carbocation
+
+
π-nucleophiles with the carbocation 1 in the aqueous solvent
(
4-MeAr)2CH give kNu/kHOH ) 30, which is consistent with
5
0:50 (v:v) acetonitrile/water (Table 1).
kNu/kHOH < 60 in 50:50 (v:v) acetonitrile/water determined in
this work (Table 1). Similarly, kNu/kHOH ≈ 6 estimated for
reaction of a 1:1 mixture of the cis and trans isomers of ethyl
+
The selectivity of 1 for reaction with pyrrole (kNu/ks ) 23
-
1
M , Table 1) gives an estimated value of N+ ) 1.4 for this
π-nucleophile (eq 6) that can be substituted into eq 8 to give
Ncalc ) 7.2 as the estimated Mayr nucleophilicity parameter for
pyrrole in an aqueous solvent (Table 1). This is in good
(27) Chiang, Y.; Whipple, E. B. J. Am. Chem. Soc. 1963, 85, 2763. The
thermodynamically favored position of protonation of pyrrole is C-2: Joule,
J. A.; Smith, G. F. Heterocyclic Chemistry; Van Nostrand: London, 1972;
pp 194-195.
(31) Calculated using kNu ) 1 × 10 M s-1 for reaction of a mixture
9
-1
of the cis and trans isomers of ethyl 1-propenyl ether, which is an average
8
-1 -1
9
-1 -1
(28) Hard and Soft Acids and Bases; Pearson, R. G., Ed.; Dowden,
of the values of kNu ) 7.7 × 10 M
s
and kNu ) 1.3 × 10 M
s
for
8
Hutchinson and Ross: Stroudsberg, PA, 1973. Pearson, R. G. Chemical
Hardness; John Wiley & Sons: New York, 1997.
reaction of the cis and trans isomers, respectively, and kHOH ) 1.7 × 10
-1 -1 11
M
s .
(
29) Darbeau, R. W.; White, E. H. J. Org. Chem. 1997, 62, 8091-8094.
(32) Ritchie, C. D. Acc. Chem. Res. 1972, 5, 348-354. Ritchie, C. D.
Can. J. Chem. 1986, 64, 2239-2250.
(30) Khotinsky, E.; Patzewitch, R. Ber. 1909, 42, 3104-3106.