Why are vinyl cations sluggish electrophiles?

Article abstract of DOI:10.1021/jacs.6b10889

The kinetics of the reactions of the vinyl cations 2 [Ph₂C=C⁺-(4-MeO-C₆H₄)] and 3 [Me₂C=C⁺-(4-MeO-C₆H₄)] (generated by laser flash photolysis) with diverse nucleophile

Full text of DOI:10.1021/jacs.6b10889

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Article
Why are Vinyl Cations Sluggish Electrophiles?
Peter A Byrne, Shinjiro Kobayashi, Ernst-Ulrich Würthwein, Johannes Ammer, and Herbert Mayr
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Why are Vinyl Cations Sluggish Electrophiles?
Peter A. Byrne,a Shinjiro Kobayashi,a ErnstꢀUlrich Würthwein,b Johannes Ammer,a Herbert
Mayra*
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a) Department Chemie, LudwigꢀMaximiliansꢀUniversität München,
Butenandtstr. 5ꢀ13, 81377 München, Germany
.
b) OrganischꢀChemisches Institut, Westfälische WilhelmsꢀUniversität,
48149 Münster, Germany
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Abstract. The kinetics of the reactions of the vinyl cations 2 [Ph2C=C+ꢀ(4ꢀMeOꢀC6H4)] and 3
[Me2C=C+ꢀ(4ꢀMeOꢀC6H4)] (generated by laser flash photolysis) with diverse nucleophiles
(e.g. pyrroles, halide ions, alcoholic and aqueous solvents) have been determined
photometrically. It was found that the reactivity order of the nucleophiles toward these vinyl
cations is the same as that towards diarylcarbenium ions (benzhydrylium ions). However, the
reaction rates of vinyl cations are affected only half as much by variation of the nucleophiles
as those of the benzhydrylium ions. For that reason, the relative reactivities of vinyl cations
and benzhydrylium ions depend strongly on the nature of the nucleophiles. It is shown that
vinyl cations 2 and 3 react, respectively, 227 and 14 times more slowly with trifluoroethanol
than the parent benzhydrylium ion (Ph)2CH+, even though in solvolysis reactions (80 %
aqueous ethanol at 25 °C) the vinyl bromides leading to 2 and 3 ionize much more slowly
(halfꢀlives 1.15 yrs and 33 days) than (Ph)2CHꢀBr (halfꢀlife 23 s). The origin of this counterꢀ
intuitive phenomenon was investigated by highꢀlevel MO calculations. We report that vinyl
cations are not exceptionally high energy intermediates, and that high intrinsic barriers for the
sp2
sp rehybridizations account for the general phenomenon that vinyl cations are formed
slowly by solvolytic cleavage of vinyl derivatives, and are also consumed slowly by reactions
with nucleophiles.
Introduction
Over the course of the last century, the properties of carbocations (which frequently are
reactive intermediates) have been investigated by diverse techniques in order to understand
their role in organic chemical reactions.1,2,3,4,5,6,7,8,9,10,11 Although there exist several means
for quantifying the stabilities of carbocations in solution and in the gas phase,2ꢀ10,12,13 general
stability scales for carbocations (R+) do not exist, i.e., the absolute stability of a given
carbocation cannot be uniquely defined.2,14 Frequently, the equilibrium constants of their
reactions with a certain reference Lewis base X− (eq. 1a), i.e. their Lewis acidities with
respect to X−, are employed as a measure of their relative stabilities.2ꢀ9,12,15,16,17 Alternatively,
carbocations have been ranked according to their Brønsted acidities with respect to a certain
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Brønsted base (eq. 1b),2b,18 and most Brønsted acidity scales refer to the reaction medium
(solvent) as the reference base.
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Solvolysis reactions often proceed with rateꢀdetermining heterolytic cleavage of RꢀX (reverse
of reaction 1a). The initially formed intermediate carbocation R+ is then immediately trapped
by the solvent and does not recombine with the leaving group.1,19 As the transition state of
this cleavage is generally assumed to be carbocationꢀlike (Hammond’s postulate), the rates of
the solvolysis reactions of RꢀX have frequently been considered to be a measure for the
relative stabilities of the intermediate carbocations.1,6c
In several series of solvolysis reactions, linear relationships between the measured solvolysis
rates and the Lewis acidities of the intermediate carbocations (which are derived from
equilibrium measurements) have been observed, but deviations from such rateꢀequilibrium
relationships have also been reported.12,15,20,21,22 In our investigations of the reactivities of
benzhydryl derivatives, we have observed, for example, that benzhydrylium ion 1a is formed
22 times faster than 1b during solvolysis of the corresponding benzoates (Scheme 1)20 even
though 1a is the stronger Lewis acid according to equilibrium studies in solution and
computational studies for the gas phase (Scheme 1).12,15
Scheme 1. Rate constants (k) and Gibbs energies of reaction (ꢁG°) for heterolysis reactions giving
benzhydrylium ions 1a and 1b.
a
Experimental rate constant for reaction at 25 °C in 80:20 MeCN/H2O.20
Gibbs reaction energy ꢁG° at 20 °C calculated using lg K = LA + LB,12 in which the LA values of 1a and 1b in
b
CH2Cl2 (−5.39 and −5.72, respectively), and the LB value of benzoate in MeCN (17.45) were employed.
Negative of the calculated gas phase methyl anion affinity of the benzhydrylium ion at 20 °C (see reference
c
15).
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What is the origin of this discrepancy? Direct rate measurements in the solvents used for the
solvolysis studies showed that most flash photolytically generated benzhydryl cations react
with chloride and bromide ions under diffusion control.19 As there is obviously no barrier for
the ion combination, one can conclude that in the reverse reaction (first step of an SN1
reaction) the transition state also corresponds to the ion pair (Figure 1).
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Figure 1. First step of the SN1 reaction of a benzhydryl halide giving benzhydrylium ion and halide anion.
On the other hand, we have measured significant barriers for the combinations of
benzhydrylium ions with carboxylate anions (as depicted for the reactions from right to left in
Figure 2).21 From the principle of microscopic reversibility one can now derive that the first
step of an SN1 reaction of a benzhydryl carboxylate also must proceed via a transition state
that is higher in energy than the carbocation (see reactions from left to right in Figure 2).
According to Marcus (eq. 2),23aꢀg the Gibbs activation energy (ꢁG‡) of a chemical reaction
can be expressed by a combination of the Gibbs reaction energy (ꢁG°) and the intrinsic
barrier (ꢁG‡ ), the latter of which corresponds to ꢁG‡ for an identity reaction. For reaction
0
series where identity reactions cannot be established, e.g. carbocationꢀanion combinations,
the intrinsic barrier ꢁG‡ is commonly obtained by extrapolations to reactions with ꢁG° =
0
0.23h
2
ꢀG°
2
(
ꢀG°
)
‡
ꢀG‡ = ꢀG0 +
+
(2)
‡
16ꢀG0
Consider two heterolysis reactions, involving different species AꢀX and BꢀX, that have
identical Gibbs energies of reaction (ꢁG°) but different Gibbs energies of activation (ꢁG‡(A)
< ꢁG‡(B)), as shown in Figure 2. Since the thermodynamic contribution (ꢁG°) to the Gibbs
energy of activation in both cases is identical, the difference between ꢁG‡(A) and ꢁG‡(B)
must arise exclusively from the differences between the intrinsic barriers ꢁG‡ (A) and
0
ꢁG‡ (B) (not shown explicitly in Figure 2). Thus, in the case depicted in Figure 2, the
0
intrinsic barrier ꢁG‡ (B) must be greater than ꢁG‡ (A). The energy profiles in Figure 2 now
0
0
illustrate the counterintuitive conclusion that in cases where the difference of the heterolysis
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rates is mostly due to a difference in intrinsic barriers, the carbocation that is formed more
quickly, A+, also reacts more quickly than B+ with X−.
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Figure 2. Comparison of ionizations of the alkyl halides AꢀX and BꢀX to give carbocations A+ and B+ of equal
Lewis acidity with different rates.
Let us now return to the example illustrated in Scheme 1. As the Lewis acidity of carbocation
1a is slightly higher than that of 1b, the higher solvolysis rate of 1a-X compared to 1b-X
must be due to the lower intrinsic barrier for the ionization of 1a-X than of 1b-X. With this
conclusion, we can explain why carbocation 1a, which is formed faster in SN1 reactions than
1b, has also been found to generally react faster with nucleophiles than 1b.24
What is the consequence of these considerations for vinyl cation chemistry? The very low
SN1 reactivities of vinyl halides and vinyl tosylates25,26,27 have commonly been ascribed to
low thermodynamic stabilities of vinyl cations due to the spꢀhybridization of the carbenium
center.28,29 However, when one considers that the 1ꢀphenylvinyl cation has a similar hydride
affinity to the benzyl cation,30,31,32 only 2.9 kcal mol−1 greater than that of the tertꢀbutyl
cation,30 the question arises whether the low solvolysis rates of vinyl halides are really mostly
due to the low thermodynamic stabilities of dicoordinated carbenium ions. If instead high
intrinsic barriers for the sp2
sp rehybridization were responsible for the slow solvolyses of
vinyl derivatives, the discussion of Figure 2 implies that also the reverse reaction should be
slow, and vinyl cations should not be extraordinarily reactive intermediates but rather
sluggish electrophiles. Though rate constants for the reactions of vinyl cations generated by
laserꢀflash photolysis with a variety of nucleophiles have previously been reported,33,34
a
systematic comparison of the electrophilic reactivities of vinyl cations and tricoordinated
carbenium ions has not yet been performed. We now report on a systematic analysis of the
reactivities of vinyl cations and show that exceptionally high intrinsic barriers account for
their extraordinarily slow formation in SN1 reactions as well as for their slow reactions with
nucleophiles.
Results
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Vinyl cations 2 and 3 (Chart 1), which can be generated by laser flash photolysis from 4-Br
and 5-Br, were selected as representative vinyl cations to study the rates of the reactions with
nucleophiles 6ꢀ22 (Table 1) in MeCN (13 and 14 in 2,2,2ꢀtrifluoroethanol, TFE) and with the
solvent mixtures 23-41 listed in Table 2.
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Chart 1. Structures of vinyl cations 2 and 3 and vinyl derivatives 4-X and 5-X.
Table 1. Structures and values of the nucleophileꢀspecific parameters N and sN (in MeCN
unless otherwise indicated) for nucleophiles 6-22.
Nucleophile
#
N
sN
Ref
35
Nucleophile
tBuNH2
#
N
sN
Ref
37
6
3.76
0.91
15 12.35 0.72
16 13.77 0.70
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8
9
4.41
5.21
5.85
0.96
1.00
1.03
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24
36
iPrNH2
Et2NH
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37
17 15.10 0.73
18 15.65 0.74
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8.01
9.11
0.96
0.88
36
24
37
19 17.35 0.68
20 16.90 0.75
37
21
10.13 0.75
21
22
ꢀ
ꢀ
ꢀ
ꢀ
13
14
10.3 a 0.60
11.7 a 0.60
19
19
ꢀ
ꢀ
a Solvent = 2,2,2ꢀtrifluoroethanol (TFE).
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Table 2. Nucleophileꢀspecific reactivity
parameters N1 and sN for solvents and solvent
mixtures (v/v) 23-41.38
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Nucleophile
TFE a
N1
sN
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1.11 a 0.96 a
TFE/H2O (90:10)
TFE/H2O (60:40)
MeCN/H2O (90:10)
MeCN/H2O (80:20)
MeCN/H2O (67:33)
MeCN/H2O (50:50)
MeCN/EtOH (90:10)
MeCN/EtOH (80:20)
MeCN/EtOH (67:33)
MeCN/EtOH (50:50)
MeCN/EtOH (33:67)
MeCN/EtOH (20:80)
MeCN/EtOH (10:90)
EtOH/H2O (40:60)
EtOH/H2O (50:50)
EtOH/H2O (60:40)
EtOH/H2O (80:20)
EtOH/H2O (90:10)
2.93
3.42
4.56
5.02
5.02
5.05
5.19
5.77
6.06
6.37
6.74
6.94
7.10
5.81
5.96
6.28
6.68
7.03
0.88
0.90
0.94
0.89
0.90
0.89
0.96
0.92
0.90
0.90
0.89
0.90
0.90
0.90
0.89
0.87
0.85
0.86
a
TFE = 2,2,2ꢀtrifluoroethanol; N1 and sN are taken
from ref 35.
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Product Characterization
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Scheme 2. Reactions of vinyl cations 2 and 3, generated by ionization of 4-OMs and 5-Br, respectively, in TFE
in the presence of different nucleophiles.
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Vinyl cations 2 and 3 can be transiently generated in solvents of high ionizing power by
heterolysis of precursor vinyl halides or pseudohalides 4ꢀX and 5ꢀX.25,27,39,40a,41 We have
found that heterolyses of 4-OMs and 5-Br occur at convenient rates at 40 °C in 2,2,2ꢀ
trifluoroethanol (TFE), a solvent of low nucleophilicity (N1 = 1.11).35 When the vinyl cations
are generated in aqueous TFE, the ketones 42 and 43 are formed quantitatively, as shown in
Scheme 2a.
In order to verify that the vinyl cations generated in TFE solution can also be intercepted by
other nucleophiles, 4-OMs and 5-Br were dissolved in TFE containing ≥ 1 mol L−1 of 10 or
20 as representative nucleophiles. Following our previously published methodology for
carrying out FriedelꢀCraftsꢀtype chemistry in neutral aqueous or alcoholic solutions,42 high
nucleophile concentrations were employed to avoid trapping by trifluoroethanol or traces of
water. In this way, the reactions of solvolytically generated 2 and 3 with pyrrole 10 (N = 8.01
in MeCN) resulted in high conversion to 3ꢀvinylpyrroles 44 and 45, respectively (Scheme
2b), without formation of hydrolysis products.43 The reaction of 2 with [nBu4N]OAc (20; N =
16.9 in MeCN) gave vinyl acetate 46 (Scheme 2c) as the major product, with the formation of
a small amount (5%) of hydrolysis product 42.
ipsoꢀSubstitution (at Cꢀ4 of the anisyl group), which has been observed in reactions of
photochemically generated vinyl cations 2 with cyanide or alkoxide,44,45 but not with
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alcohols,45e,f did not occur under the conditions described in Scheme 2. We therefore
conclude, that all nucleophiles added to the cationic spꢀcenter of 2 and 3 in our kinetic
experiments, in line with results from earlier studies carried out under similar
conditions.33,34,39,40,45e,45f,46,47,48,49
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Vinyl cations 2 and 3 were generated in MeCN, 2,2,2ꢀtrifluoroethanol, or solvent mixtures
(Tables 1 and 2) by irradiation of the precursor vinyl bromides 4-Br and 5-Br with a 7ꢀns
laser pulse of λ = 266 nm. A single signal is observed in the UVꢀVis spectrum of each
transient cation.50 In the presence of a large excess of the nucleophiles 6 – 22 (pseudo firstꢀ
order conditions), the absorbance of the vinyl cation was generally observed to undergo
monoꢀexponential decay, as shown for the reaction of 2 with 9 (Scheme 3) in Figure 3a.51
obst
Leastꢀsquares fitting of the singleꢀexponential function At = A0e−k + C (A0 and At are the
absorbances at time 0 and time t, respectively, and C is a constant) to the absorbance decay
curve for the reaction of 2 or 3 with a nucleophile yielded kobs (s−1)for the particular
concentration of nucleophile.
Scheme 3. Generation of vinyl cation 2 by laser flash photolysis of 4-Br in MeCN, and subsequent reaction
with pyrrole 9.
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Figure 3. (a) Decay curve (λ = 350 nm) for the reaction of vinyl cation 2 with pyrrole 9; (b) The second order
rate constant k was obtained from the slope of the plot of kobs vs. [9]0.
Plots of the pseudoꢀfirstꢀorder rate constants kobs vs concentrations of the nucleophile were
linear (see Figure 3b) and can be expressed by equation 3:
kobs = k[Nu] + ksolv
(3)
where [Nu] is the molar concentration of the nucleophile, k (the slope of a plot of kobs vs.
[Nu]) is the secondꢀorder rate constant for the reaction of vinyl cation with nucleophile
(values in Table 3), and the intercept is the firstꢀorder rate constant for the reaction of the
vinyl cation with solvent (ksolv). Nine of the 14 correlations in acetonitrile showed intercepts
in the range (1.4 – 1.7) × 105 s−1, which we ascribe to the reaction of 2 with acetonitrile. The
only strong deviation from this value (6 × 106 s−1, for the reaction of 2 with the strong
nucleophile 20) is likely to be a consequence of the problematic extrapolation of the very
large rate constants to the concentration [20] = 0. Whereas the intercept for the correlation of
kobs vs. [Bu4N]Cl (13) in trifluoroethanol agrees exactly with the previously reported rate
constant for the reaction of 2 with trifluoroethanol (nucleophile 23), for unknown reasons the
corresponding plot of kobs vs. [Bu4N]Br (14) gives an intercept which is two times larger.
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Table 3. Secondꢀorder rate constants, k (20 °C), for the reactions of vinyl cation 2 with
nucleophiles 6-22 (solvent MeCN unless otherwise indicated).
k
k
Nucleophile
Nucleophile
tBuNH2
(L mol−1 s−1)
(L mol−1 s−1)
6
7
1.64 × 105
3.10 × 105
15
16
1.23 × 107
4.64 × 107
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iPrNH2
8
9
1.30 × 106
1.07 × 108
Et2NH
17
18
1.28 × 108
2.18 × 108
10
11
3.17 × 108
1.50 × 107
19
20
6.88 × 108
4.79 × 109
12
13
14
6.01 × 106
8.06 × 105 a
3.67 × 106 a
21
22
5.99 × 107
ca. 4.4 × 109 b
a
b
Solvent = 2,2,2ꢀtrifluoroethanol (TFE).
Approximate rate constant derived from an experiment with a single concentration of [Bu4N]I. See details in
Supporting Information, p S25.
Monoexponential decays of the absorbances of the vinyl cations 2 and 3 were also observed
when they were generated in trifluoroethanol (23) and in the solvent mixtures 24ꢀ41 (see
Figure 4 for an example), and the firstꢀorder rate constants kobs were obtained from fitting of
At = A0e−k + C to the decay curves. However, kobs did not increase linearly with [H2O]
obst
(Figure 5) and remained almost constant when the water content was raised beyond 20% v/v
H2O, in line with previous reports on the consumption of benzhydrylium38,52 and tritylium
ions52 in MeCN/H2O mixtures. Since a similar situation was also observed for other solvent
mixtures, Table 4 lists the firstꢀorder rate constants k = kobs for reactions of 2 with the solvent
nucleophiles 23ꢀ41.
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31
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47
48
49
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53
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55
56
57
58
59
60
ꢀ0.5
0
0.5
1
1.5
t / ꢁs
2
2.5
3
3.5
Figure 4. Decay curve (λ = 350 nm) from the reaction of vinyl cation 2 with 67:33 MeCN/EtOH (32).
6.0 × 105
50:50
67:33
4.0 × 105
80:20
90:10
MeCN/H2O
2.0 × 105
0
0
10
20
30
[H2O] (mol L−1) in mixed
MeCN/H2O solvents
Figure 5. Plot of kobs for the consumption of 2 in MeCN/H2O solvents vs. concentration of H2O.
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Table 4. Observed firstꢀorder rate constants, k
(20 °C), for the reactions of vinyl cation 2 with
solvent nucleophiles 23-41.
Nucleophile
k (s−1)
7
8
9
TFE
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
1.4 × 104 a
2.47 × 104
6.86 × 104
3.22 × 105
4.22 × 105
4.80 × 105
5.24 × 105
6.06 × 105
1.05 × 106
2.01 × 106
3.02 × 106
4.13 × 106
5.14 × 106
6.93 × 106
1.47 × 106
1.61 × 106
1.77 × 106
2.88 × 106
4.03 × 106
TFE/H2O (90:10)
TFE/H2O (60:40)
MeCN/H2O (90:10)
MeCN/H2O (80:20)
MeCN/H2O (67:33)
MeCN/H2O (50:50)
MeCN/EtOH (90:10)
MeCN/EtOH (80:20)
MeCN/EtOH (67:33)
MeCN/EtOH (50:50)
MeCN/EtOH (33:67)
MeCN/EtOH (20:80)
MeCN/EtOH (10:90)
EtOH/H2O (40:60)
EtOH/H2O (50:50)
EtOH/H2O (60:40)
EtOH/H2O (80:20)
10
11
12
13
14
15
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19
20
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23
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30
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40
41
42
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44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
EtOH/H2O (90:10)
a
Rate constant taken from ref 33.
Secondꢀorder rate constants for the reactions of 3 with pyrroles 9 and 10 and firstꢀorder rate
constants for the reactions of 3 with the solvents 23-27, 30, and 31 were derived in a similar
manner to that described above for vinyl cation 2. The rate constants determined in this way
are shown in Table 5.
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Table 5. Rate constants, k (20 °C), for the
reactions of vinyl cation 3 with πꢀnucleophiles 9
and 10 and with solvent nucleophiles 23-27, 30,
and 31.
7
8
Nucleophile
k
3.19 × 108
9
9
10
11
12
13
14
15
16
17
18
19
20
21
22
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31
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47
48
49
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52
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58
59
60
L mol−1 s−1 a
H
1.21 × 109
N
Me
Me
10
L mol−1 s−1 a
TFE
23
24
25
26
27
30
31
2.3 × 105 s−1 b
5.85 × 105 s−1
1.76 × 106 s−1
1.30 × 107 s−1
1.46 × 107 s−1
1.62 × 107 s−1
2.70 × 107 s−1
TFE/H2O (90:10)
TFE/H2O (60:40)
MeCN/H2O (90:10)
MeCN/H2O (80:20)
MeCN/EtOH (90:10)
MeCN/EtOH (80:20)
a Solvent = MeCN
b
Rate constant taken from ref 33
trifluoroethanol.
.
TFE = 2,2,2ꢀ
Correlations and Discussion
In numerous investigations we have shown that the secondꢀorder rate constants k for the
reactions of electrophiles with nucleophiles at 20 °C may be calculated using equation 4,
lg k (20 °C) = sN (E + N)
(4)
where E characterizes the electrophilicity of the electrophile (treated as being solventꢀ
independent), while N represents the nucleophilicity of the nucleophile, and sN is a
nucleophileꢀspecific susceptibility parameter.24,53 Whereas new N and sN parameters of
nucleophiles are derived from linear plots of lg k vs. the known E parameters of the reference
electrophiles, new E parameters of electrophiles have been derived from linear correlations
between (lg k)/sN and N of the reference nucleophiles.
Following this procedure, (lg k)/sN was plotted vs. N for all investigated nucleophiles with
known nucleophilicity parameters (Figure 6). As the published reactivity parameters N and
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sN for nucleophiles 6 – 20 refer to secondꢀorder rate constants in acetonitrile, while those for
solvents 23 – 41 (designated N1) refer to firstꢀorder rate constants, it is possible to plot the
logarithms of the secondꢀorder rate constants for the πꢀsystems 6 – 11, the amines 12 and 15
– 19, and the anions 13, 14, 20 as well as those of the firstꢀorder rate constants for the
solvents 23 – 41 side by side in Figure 6. The remarkably good correlation for these diverse
nucleophiles over a reactivity range of more than 16 orders of magnitude shows that the
nucleophilic reactivities toward vinyl cation 2 (spꢀelectrophile) follow the same pattern as
those toward benzhydrylium ions. However, equation (4) is not fulfilled because the slope of
this correlation is 0.53 and not 1.0, as required by equation (4), showing that 2 is substantially
less sensitive to changes in the reactivity of the nucleophile than benzhydrylium ions (sp2ꢀ
electrophiles).
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48
49
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60
16
14
14
13
10
19
12
9
18
27
10
8
26
20
6
16
17
15
6
28-41
12
25
24
11
4
(lg k)/sN = 0.53N + 3.76
23
R² = 0.9663
8
7
2
0
0
2
4
6
8
10
12
14
16
18
Nucleophilicity, N
Figure 6. Plot of (lg k/sN) vs. N for the reactions of vinyl cation 2 with nucleophiles 6-20 and 23-41.
A similar plot, shown in Figure 7, was constructed using the firstꢀ and secondꢀorder rate
constants k (listed in Table 5) for the reactions of vinyl cation 3 with a variety of
nucleophiles. Again, a strong linear correlation is observed over a wide range of reactivity,
and again a slope of much less than 1 is obtained. It is remarkable that even pyrrole 10 fits
this correlation though the rate constant of 1.21 × 109 L mol−1 s−1 is already close to the
diffusion control limit. Vinyl cation 3 thus shows very similar behavior to 2. Its higher
electrophilic reactivity may be due to reduced steric shielding of the cationic carbon center.
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30
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(10)
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31
27
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4
24
23
(lg k)/sN = 0.55N + 4.97
2
R2 = 0.9776
0
0
2
4
6
8
Nucleophilicity, N
Figure 7. Plot of (lg k/sN) vs. N for vinyl cation 3, from its reactions with various nucleophiles.
Analogous linear correlations of (lg k/sN) vs. N with slopes much smaller than 1 were
previously found for SN2 reactions of alkyl halides,54 indicating that variation of the
nucleophiles also had a smaller influence on the rate constants of the SN2 reactions than on
those for the reactions with benzhydrylium ions. Although the correlations shown in Figures
6 and 7 might also be mathematically expressed by adding an additional electrophileꢀspecific
susceptibility parameter sE to eq. (4),54 we refrain from deriving electrophilicity parameters E
from the extended correlations. The reason is that E determined in this way would represent
an approximate reactivity ranking toward very weak nucleophiles which react with rate
constants close to 1 (lg k = 0), i. e., reactions which have little relevance in practice, because
commonly used solvents react much faster.
For that reason, let us directly compare rate constants for the reactions of nucleophiles with
vinyl cations 2 and 3 and with benzhydrylium ions 47 – 49 (Chart 2).
Chart 2. Benzhydrylium ions 47 ꢀ 49.
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In Figure 8, the lg k values for the reactions of 2 (values of k taken from Tables 3 and 4) are
plotted against the corresponding lg k values for the reactions of benzhydrylium ion 47 (k(47)
from Table S1 on p. 43ꢀ44 of the Supporting Information).55 The plot shows a fair correlation
between the two data sets and (similar to Figure 6) that the rate constants for the reactions
with the vinyl cation 2 are less affected by variation of the nucleophiles than the
corresponding rate constants for 47 (slope = 0.48). If we neglect the two pyrroles (9 and 10),
which deviate significantly from the correlation, one can see that the vinyl cation 2 reacts
faster than the dimethoxybenzhydrylium ion 47 with weak nucleophiles (k < 5 × 106 s−1 or L
mol−1s−1), while stronger nucleophiles react faster with benzhydrylium ion 47. Overall,
however, the vinyl cation 2 and the dimethoxybenzhydrylium ion 47 have comparable
electrophilic reactivities.
9
10
11
12
13
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47
48
49
50
51
52
53
54
55
56
57
58
59
60
9
10
9
7
5
lg k(2) = 0.48(lg k47))
3
+ 3.43
lg k(47)
1
1
3
5
7
9
lg k(47)
Figure 8. Correlation of lg k for the reactions of vinyl cation 2 with various nucleophiles (6-16, 23ꢀ41) with lg k for the
analogous reactions of the 4,4’ꢀdimethoxybenzhydrylium ion 47(from Table S1 in SI). The red line is a plot of lg k(47)
against itself to highlight the crossing range where nucleophiles react with equal rates with 2 and 47.
An analogous comparison can be made between vinyl cation 3 and benzhydrylium ion 48. A
plot of lg k for the reactions of 3 with various nucleophiles vs. lg k for the corresponding
reactions of 48 (k(48) from Table S2 on p. S44 of the Supporting Information) shows a good
linear correlation (Figure 9),56 with a slope significantly less than 1 (slope = 0.56). Vinyl
cation 3 thus has an electrophilicity comparable to that of 48.
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59
60
6
lg k(3) = 0.56(lg k(48))
+ 2.82
lg k(48)
4
4
6
8
10
lg k(48)
Figure 9. Correlation of lg k for the reactions of vinyl cation 3 with various nucleophiles (9, 23-27, 30, 31) with lg k for the
analogous reactions of the 4,4’ꢀdimethylbenzhydrylium ion 48 (from Table S2 in SI). The red line is a plot of lg k(48)
against itself to highlight the crossing range where nucleophiles react with equal rates with 3 and 48.
.
The observation that vinyl cations 2 and 3 show reactivities toward nucleophiles that are
similar to the corresponding reactivities of the donorꢀstabilized benzhydrylium ions 47 and 48
appears surprising at first glance, as the latter species are formed much faster in SN1 reactions
than the corresponding vinyl derivatives. In fact, the ionizations of the benzhydryl bromides,
which yield the highly stabilized benzhydryl cations 47 and 48, are so fast in aqueous ethanol
that it is not possible at present to measure their solvolysis rates.
Therefore, we compare here the solvolyses of the vinyl bromides 4-Br and 5-Br with that of
Ph2CHBr (for which experimental data are available). Scheme 4 shows that the benzhydryl
bromide solvolyzes 105 to 106 times faster in 80% aqueous ethanol at 25 °C than the vinyl
bromides 4-Br and 5-Br. Despite its much faster rate of formation, the parent benzhydrylium
ion reacts one to two orders of magnitude faster with trifluoroethanol at 20 °C than 2 or 3
(compare the second reactions in each of Scheme 4a, 4b and 4c). Similar rate ratios have been
found for activationꢀcontrolled reactions of these electrophiles with numerous other
nucleophiles (i. e. reactions which do not proceed with diffusion controlled rates).
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c
Scheme 4
.
Solvolysis reactions of (a) 4-Br,a (b) 5-Br,b and (c) Ph2CHBr in 80:20 EtOH/H2O at 25 °C, and
reactions of cations 2, 3, and 49 with TFE at 20 °C.
3
4
5
6
7
8
9
10
11
12
13
14
15
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19
20
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22
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41
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45
46
47
48
49
50
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52
53
54
55
56
57
58
59
60
a
For the first step of reaction (a), a value of k = 1.92 × 10−4 s−1 was measured at 120 °C;39d reported activation
parameters allow extrapolation to value at 25 °C given in the Scheme. The rate constant for the second step is
taken from reference 33.
For the first step of reaction (b), a value of k = 8.78 × 10−4 s−1 was measured at 120 °C;40c reported activation
parameters allow extrapolation to value at 25 °C given in the Scheme. The rate constant for the second step is
taken from reference 33.
The rate constant for the first step of (c) is from reference 57. The rate constant for the second step is from
b
c
reference 58.
Laser flash experiments have shown that the reactions of the parent benzhydrylium ion 49
with Br− are diffusionꢀcontrolled in all alcoholic solvents investigated.19 As there is no barrier
for the ion combination, the principle of microscopic reversibility implies that the transition
state for the heterolytic cleavage of Ph2CHBr corresponds to the ionꢀpair, as illustrated in
Figure 10.
In contrast, the reaction of Br− with vinyl cation 2 is activation controlled, proceeding with a
rate constant of 3.7 × 106 L mol−1 s−1 in TFE (Table 3). For the same reaction in 80 %
aqueous ethanol one can calculate k = 7.4 × 106 L mol−1 s−1 from N = 14.5 , sN = 0.6 (for Br−
in 80 % aqueous ethanol)19 using the correlation equation given in Figure 6. As this reaction
is not diffusion controlled, transition state theory can be applied to calculate an activation
energy of ꢁG‡ = 8.1 kcal mol−1 for the ion combination in 80 % aqueous ethanol (Figure 10,
right hand side). The kinetic data implies that the heterolyses of Ph2CHBr and 4-Br have
almost identical ꢁG° values, as shown in Figure 10.
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Figure 10 clearly illustrates that the major reason for the different solvolysis rates of
Ph2CHBr and 4-Br is the difference in the intrinsic barriers (since the Gibbs energies of
reaction ꢁG° are very similar). Hence, the transition state of the first step of the SN1 reaction
of 4-Br cannot be carbocationꢀlike, i.e., it does not correspond to the ion pair of Br− with
vinyl cation 2. We demonstrate explicitly below using quantum chemical calculations that
9
10
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60
ꢁꢁG° = ꢁG°Ph CHBr − ꢁG°4-Br is indeed negligible (vide infra), as derived from kinetic data
2
for the construction of Figure 10.
Figure 10. Schematic Gibbs energy profiles (kcal molꢀ1) for the ionization of benzhydryl bromide and vinyl
bromide 4-Br in 80% aqueous ethanol at 25 °C. ꢁG‡ values were calculated using the Eyring equation (for the
solvolysis reactions, rate constants from Scheme 4a and 4c were used; see main text for details of the calculation
for addition of Br− to 2).
Common Ion Rate Depression
Although SN1 reactions are usually accelerated when the polarity of the solvent is increased
by salt additives,1c,d,h,59 in certain cases, the opposite effect is observed. Common ion rate
depression is a phenomenon whereby the rate of a solvolysis reaction kobs is slowed by
addition of a salt containing the anion of the leaving group (X−).1h,19,27,60 It is observed when
the recombination of the carbocation R+ with X− is faster than addition of the solvent to R+
(with first order rate constant ksolv); i.e., when k−1[X−] ≥ ksolv (see Scheme 5, and equation (5),
in which the full expression for kobs is given, and α = k−1/ksolv).
Scheme 5. Reversible heterolysis of RX to give R+, which may react with solvent (completing the solvolysis)
or revert to RX by reaction with X−.
k1ksolv
k1
(5)
kobs
=
=
ksolv + k−1[X− ] 1+ α[X− ]
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As highly reactive carbocations are immediately trapped by the solvent, which is present in
high concentration, common ion return is only observed when highly stabilized carbocations
are generated in solvents of low nucleophilicity. Since vinyl cations had been considered to
be highly reactive because of their slow formation in SN1 processes, the observation of
common ion rate depression in SN1 processes was highly surprising, and the numerous
attempts to rationalize this phenomenon have been summarized.60b With the knowledge that
vinyl cations 2 and 3 have relatively low electrophilic reactivities, similar to those of the
highly stabilized benzhydrylium ions 47 and 48 (see Figures 8 and 9), two systems for which
common ion depression has generally been observed,19 it is no longer surprising that this
effect was also found for the solvolyses of 4-X and 5-X (X = Cl, Br).60b
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59
60
In previous work we have demonstrated that the occurrence of common ion rate depression
can be derived from the directly measured rate constants of the reactions of the independently
generated carbocations with the anionic leaving groups (i. e. X−) and the solvent.19 From the
rate constants in Tables 3 and 4 for the reactions of 2 with Cl− in TFE (8.06 × 105 L mol−1
s−1), Br− in TFE (3.67 × 106 L mol−1 s−1), and TFE (1.40 × 104 sꢀ1), respectively, one can
calculate α values of 58 and 262, respectively, in good agreement with common ion rate
depressions observed for similar systems under comparable conditions.61
Computational Analysis
Directly measured rate constants as well as the observation of common ion rate depression
thus indicate that vinyl cations are sluggish electrophiles, despite their very slow formation in
SN1 reactions. By combining the experimentally determined rate constants for the solvolysis
reactions and ion recombinations (see Figure 10 and associated discussion above), we had
derived that ꢁG° for the heterolytic cleavage of the CꢀBr bond in Ph2CHBr is almost the
same as that for heterolysis of 4-Br. This conclusion is in line with quantum chemical
calculations, as we show below.
Recently, we have calculated62 gas phase methyl anion affinities of benzhydrylium ions as a
measure for their relative Lewis acidities (Scheme 6, table entries 1 – 3).12 Comparison with
entries 4 and 5 shows that the methyl anion affinities of vinyl cations 2 and 3 are closely
similar to that of the unsubstituted benzhydrylium ion (49), as derived from the rate constants
for forward and backward reactions in Figure 10.63 Kinetic data and quantum chemically
calculated Lewis acidities thus agree on the conclusion that the differences in intrinsic
barriers account for the fact that vinyl cations 2 and 3, despite their much higher Lewis
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acidities, are not more electrophilic than the highly stabilized benzhydrylium ions 47 and 48,
3
4
respectively (Figures 8 – 9 and associated discussion).
5
6
7
Scheme 6. Calculated Gibbs energies of reactionsa of methyl anion with benzhydrylium ions 47ꢀ49 and of vinyl
cations 2 and 3 (methyl anion affinities) in the gas phase.
8
9
10
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60
a
Computational method and basis set employed for all values of ꢁG°: B3ꢀLYP/6ꢀ311++G(3df,pd)//B3LYP/6ꢀ
31G(d,p).62,64
b Calculated value reported in reference 12.
c This work.62,63,64
The calculations shown in Table 6 confirm that in the gas phase, Ph2CH+ and 2 also have
almost equal affinities toward Br− (ꢁꢁG° = 0.9 kcal molꢀ1, entries 1, 2) as well as toward Br−
solvated by one molecule of water (ꢁꢁG° = 1.1 kcal molꢀ1, entries 3, 4), in agreement with
our observations based on the kinetic data (Figure 10, above). As expected, the ion
combinations are calculated to be much less exergonic in solution (entries 5 – 8), but the very
small calculated values of ꢁG° in aqueous solution indicate that the PCM model significantly
overestimates the ion solvation energies. 65
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Table 6. Calculated Gibbs energies of the reactionsa of Br−
(and Br− solvated by 1 water molecule) with the parent
benzhydrylium ion (49) and vinyl cation 2 in the gas phase
and in aqueous solution.b
7
8
9
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60
Gas
Phase /
Solvent (kcal mol−1)
Model
∆G°
X−
Entry
R+
1
2
3
Ph2CH+
2
Gas
Gas
Gas
−113.9
−114.8
−102.1
Ph2CH+
4
2
Gas
−103.2
5
6
7
Ph2CH+
2
Water
Water
Water
−9.5
−15.3
−5.8
Ph2CH+
8
2
Water
−11.6
a Computational method and basis set employed for all values of ꢁG°:
TPSS/def2TZVP+GD3.66,67
b
PCM; method TPSS/def2TZVP+GD3. See Table S7 (p S83 of
Supporting Information) for further quantities determined from these
calculations.
When trying to localize the transition states for the heterolytic cleavage reactions of 4-Br and
Ph2CHBr in aqueous solution (i.e., the reactions in Figure 10) by the DFT method
TPSS/def2TZVP+GD3 using the PCM continuum solvent model,68 we observed a continuous
increase of the potential energy (Etot) as the CꢀBr bond length was elongated, and the ion pair
was reached without passing through a maximum in Etot (Figure 11). The same result was
found when one molecule of water was explicitly considered (see Figure S4 in SI, p S84). In
both cases, the gradient was much larger for the vinyl bromide due to the higher force
constant of the Csp2ꢀBr bond.
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Figure 11. Intrinsic reaction coordinate calculations for the cleavage of the CꢀBr bond in 4-Br and Ph2CHBr
(Total energies Etot; TPSS/def2TZVP+GD3; PCM (solvent = water)).
The fact that the maximum of total energy in the 4-Br graph of Figure 11 is slightly lower
than ꢁEtot calculated for the reaction of 2 with Br− in water (−24.9 kcal mol−1, see Table S7 in
SI on p S83) may be due to one or both of the following: (i) the PCMꢀmodel (used for the
calculations on the heterolysis reactions as well as for those on the separated species), and (ii)
the fact that for longer CꢀBr distances the closedꢀshell restriction does not apply.
A computational assessment of the transition state for the vinyl bromide heterolysis (for
which the activation barrier was unequivocally deduced from kinetic data – see Figure 10 and
associated discussion) might be possible by explicit consideration of more solvent molecules.
However, general problems of this type of treatment have recently been pointed out by
Singleton,69 and in any case, this approach is beyond the scope of this investigation.
‡
As mentioned in the Introduction, the intrinsic barrier in the Marcus equation (∆G0 )
corresponds to the Gibbs activation energy of an identity reaction. Since there are no identity
reactions for the ion combinations in eq. (1), we analyzed the intrinsic barriers for the identity
hydride transfers illustrated in Scheme 7 as models for the transition states of sp3
sp2 sp rehybridizations (Table 7).70
sp2 and
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Scheme 7. Formation of (a) tricoordinated and (b) dicoordinated hydridoꢀbridged carbenium ions [RꢀꢀꢀHꢀꢀꢀR]+
from the isolated carbenium ion R+ and its parent compound, RH (without considering species showing
aromatic interactions (πꢀstacking)).
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Table 7 allows comparison of quantum chemically calculated and experimental gasꢀphase
hydride affinities for several triꢀ and diꢀcoordinated carbenium ions.71 While the absolute
hydride affinities calculated with different basis sets differ considerably (see Supporting
Information, p. S69ꢀ70), there is good agreement between relative hydride affinities obtained
with different computational methods and experimental data.
Table 7. Comparison of Calculated and Experimental Gas Phase Values ꢁHHA (298 K) and
ꢁGHA (298 K) for Additions of H− to Various Carbenium Ions (Hydride Affinity = −ꢁHHA),
and Calculated Intrinsic Barriers ∆Hbridge and ∆Gbridge for the Hydride Transfer Reactions:
Calculated Quantities b
Experimental
Entry
R+
a
b
b
b
b
∆HHA
∆HHA
∆GHA
∆Hbridge
∆Gbridge
1
2
3
4
5
6
−251.8
−264.9
−225.1
−239.7
−239.3
ꢀ
−259.3
−272.4
−234.5
−247.5
−246.8
−220.9
−251.8
−264.2
−228.0
−240.1
−240.6
−212.8
−18.5
−8.1
−14.9
−12.2
−3.8
−2.9
+0.3
+8.2
−1.9
+3.3
−12.9
−8.0
a
Experimental data (kcal mol−1) listed in ref 30.
b
Calculated
values
from
this
work.
Method
and
basis
set
employed:
TPSSTPSS/def2TZVP+GD3//TPSSTPSS/def2TZVP+GD3. Units are kcal mol−1.
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Table 7 shows, for example, that successive replacement of the methyl groups in the
isopropyl cation by phenyl (i.e., giving first phenethyl cation and then benzhydryl cation)
reduces the hydride affinities by 25 and then 15 kcal mol−1 in calculated ꢁHHA and ꢁGHA
(entries 1, 3, 6) and in experimentally determined ꢁHHA. The hydride affinities of the vinyl
cations (entries 2 and 4) are 13 kcal mol−1 larger than those of the corresponding saturated
analogues (entries 1 and 3).
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Hydride transfer to triꢀ or diꢀcoordinated carbenium ions from their parent alkanes or alkenes
occurs through hydridoꢀbridged species (Scheme 7). The enthalpies and Gibbs energies
(ꢁHbridge and ꢁGbridge, respectively) of formation of these entities from the isolated reactants,
i. e., the intrinsic barriers for the hydride transfers are shown in the right hand columns of
Table 7. Whereas the hydridoꢀbridged species from isopropyl cation/propane (entry 1) and
benzyl cation/toluene (entry 5) are minima on the potential energy surface (gas phase), all
other hydridoꢀbridged species correspond to transition states (Table 7, and Tables S4ꢀS6 in
Supporting Information p. S71ꢀS75). All minima and all transition states have a negative
ꢁHbridge with respect to the isolated reactants. As expected, the tendency to undergo hydridoꢀ
bridging decreases with decreasing hydride ion affinity, i.e., for tricoordinated carbenium
ions, ꢁGbridge increases in the order 1 < 5 ≈ 3 < 6 and in the order entry 2 < entry 4 for
dicoordinated carbenium ions.
In the context of our analysis, the comparisons of entries 2 and 1 and of entries 4 and 3 are of
particular importance. Although the vinyl cations in entries 2 and 4 have significantly higher
hydride affinities compared to their saturated analogs in entries 1 and 3, respectively, their
hydridoꢀbridging tendencies are much smaller. As the hydridoꢀbridged complexes represent
models for the transition states of sp3 sp2 and sp2
sp rehybridizations (Scheme 7), the 8
kcal/mol lower tendency towards hydridoꢀbridging of the phenylꢀsubstituted vinyl cation in
entry 4 compared to the saturated analog in entry 3 reflects the high intrinsic barriers which
are responsible for the low rates of vinyl halide heterolyses (low electrofugalities15 of vinyl
cations) and the low rates of nucleophilic additions to vinyl cations (low electrophilicities15 of
vinyl cations).
Conclusion
Discussions of carbocation reactivities are generally based on the assumption that the
transition states of SN1 reactions are carbocationꢀlike (Hammond postulate), and that the
slower a carbocation is formed in an SN1 process, the faster it will react with a nucleophile.
Although deviations from this rule have previously been reported, including literature reports
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on comparatively low absolute rate constants for reactions of vinyl cations with nucleophiles
and common ion rate depression in solvolyses of vinyl derivatives (typical for carbocations of
relatively low reactivity), the consequences of these observations for the interpretation of
vinyl cation chemistry have rarely been considered.
8
9
We have now shown that the transition states of vinyl halide solvolyses are often not
carbocationꢀlike. Consequently, the influence of intrinsic barriers on this step cannot be
neglected. The approximation to consider only the thermodynamic term (i. e., ꢁG° of the
ionization step) for analyzing the kinetic behavior of carbocations, which works well for the
reactions of most weakly stabilized tricoordinated carbenium ions, cannot be applied for
vinyl cations. Vinyl cations (i.e., dicoordinated carbenium ions) are weaker electrofuges as
well as weaker electrophiles than tricoordinated carbenium ions of similar Lewis acidity
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because of the high intrinsic barriers for sp2
sp rehybridizations.
Supporting Information
Experimental procedures and characterization of representative products; details of kinetic
experiments with vinyl cations 2 and 3; literature rate constants used for the construction of
Figures 8 and 9; copies of NMR spectra; details of quantum chemical calculations. The
Supporting Information is available free of charge on the ACS Publications website at DOI:_
Author Information
Corresponding Author: * herbert.mayr@cup.uniꢀmuenchen.de
Notes: The authors declare no competing financial interest.
Acknowledgements
We thank the Deutsche Forschungsgemeinschaft (SFB 749, Project B1) for financial support,
the Humboldt foundation for the provision to PB of a Humboldt Research Fellowship for
Postdoctoral Researchers, Giulio Volpin, Dr. David Stephenson, and Claudia Dubler for
assistance with NMR spectroscopy, and Dr. A. R. Ofial for helpful comments.
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References and Notes
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8
9
1
General Reviews: (a) Carbonium Ions, Vol. 1 – 5; Olah, G. A.; Schleyer, P. v. R., Eds.; Interscience: New
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12, 173 – 212; (c) Streitwieser Jr., A. Solvolytic Displacement Reactions; McGrawꢀHill: New York, 1962;
(d) Ingold, C. K. Structure and Mechanism in Organic Chemistry, 2nd ed.; Cornell University Press: Ithaca,
NY, 1969; (e) Vogel, P. Carbocation Chemistry; Elsevier: Amsterdam, 1985; (f) Advances in Carbocation
Chemistry, Vol. 1; Creary, X., Ed.; JAI Press: Greenwich, CT, 1989; (g) Advances in Carbocation
Chemistry, Vol. 2; Coxon, J. M., Ed.; JAI Press: Greenwich, CT, 1995; (h) Raber, D. J.; Harris, J. M.;
Schleyer, P. v. R. in Ions and Ion Pairs in Organic Reactions; Wiley and Sons: New York; 1974; Volume 2,
pp 247 – 374.
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therein; (b) Mayr, H. in Seminars in Organic Synthesis, 40th "A. Corbella" International Summer School,
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– 3051; (b) Deno, N. C.; Schriesheim, A. J. Am. Chem. Soc 1955, 77, 3051 – 3054; (c) Arnett, E. M.;
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4
Gas phase hydride affinities of carbenium ions (X− = H−): (a) Wolf, J. F.; Staley, R. H.; Koppel, I.;
Taagepera, M.; Mclver, R.T.; Beauchamp, J. L.; Taft, R. W. J. Am. Chem. Soc. 1977, 99, 5417 – 5429; (b)
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Beauchamp. J. I. J. Am. Chem. Soc.1984, 106, 3917 – 3927; (d) Karaman, R.; Huang, J.ꢀT. L.; Fry, J. L. J.
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5
6
Solution hydride affinities of carbenium ions (X− = H−): (a) Cheng, J.ꢀP.; Handoo, K. L.; Parker, V. D. J.
Am. Chem. Soc.1993, 115, 2655 – 2660; (b) Arnett, E. M.; Flowers, R. A.; Ludwig, R. T.; Meekhof, A. E.;
Walek, S. A. J. Phys. Org. Chem. 1997, 10, 499 – 513; (c) See also reference 3i; (d) See also reference 4e.
Calorimetric determination of enthalpies of ionization of alkyl halides in SbF5/SO2ClF superacid (formally
the reverse of the reaction in equation (1b); X− = halide): (a) Arnett, E. M.; Petro, S. C. J. Am. Chem. Soc.
1978, 100, 2563 – 2564; (b) Arnett, E. M.; Petro, S. C. J. Am. Chem. Soc. 1978, 100, 5402 – 5407, (c)
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1980, 102, 3329 – 3334; (f) Arnett, E. M.; Hofe1ich.T. C. J. Am. Chem. Soc. 1983, 105, 2889 – 2895; (g)
Arnett, E. M.; Hofelich, T. C. J. Am. Chem. Soc. 1982, 104, 3522 – 3524.
7
8
Equilibrium constants for ionization of benzhydryl chlorides (X− = halide): Schade, C.; Mayr, H.; Arnett, E.
M. J. Am. Chem. Soc. 1988, 110, 567 – 571.
Enthalpies and equilibrium constants for reactions of carbocations with carbanions (X− = carbanion): (a)
Troughton, E. B.; Molter, K. E.; Arnett, E. M. J. Am. Chem. Soc. 1984, 105, 6726 – 6735; (b) Arnett, E. M.;
Chawla, B.; Molter K.; Amarnath, K.; Healy, M. J. Am. Chem. Soc. 1985, 107, 5288 – 5289; (c) Arnett, E.
M.; Molter, K. E. Acc. Chem. Res. 1985, 18, 339 – 346.
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Gibbs energies of reaction for production of carbocations from neutral precursors in the gas phase,
determined using ion cyclotron resonance techniques: (a) Aue, D. H.; Bowers, M. T. in Gas Phase Ion
Chemistry; Academic Press: New York, 1979; Vol. 2, Chapter 9; (b) Walder, R.; Franklin, J. L. Int. J. Mass
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68, 3786 – 3796.
9
10 Destabilized carbocations: Tidwell, T. T. Angew. Chem. 1984, 96, 16 – 28; Angew. Chem., Int. Ed. Engl.
1984, 23, 20 – 32.
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44
45
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51
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11 Gas phase IRꢀspectroscopy: (a) Crestoni, M. E.; Fornarini, S. Angew. Chem. 2012, 124, 7487 – 7489;
Angew. Chem., Int. Ed. 2012, 51, 7373 – 7375; (b) Chiavorino, B.; Crestoni, M. E.; Dopfer, O.; Maitre, P.;
Fornarini, S. Angew. Chem. 2012, 124, 5031 – 5033; Angew. Chem., Int. Ed. 2012, 51, 4947 – 4949.
12 Mayr, H.; Ammer, J.; Baidya, M.; Maji, B.; Nigst, T. A.; Ofial, A. R.; Singer, T. J. Am. Chem. Soc. 2015,
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13 Abboud, J.ꢀL. M.; Alkorta, I.; Dávalos, J. Z.; Müller, P.; Quintanilla, E. Adv. Phys. Org. Chem. 2002, 37 57
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14 Hine, J. Structural Effects on Equilibria in Organic Chemistry; Robert E. Krieger Publishing Company;
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15 Mayr, H.; Ofial, A. R. Acc. Chem. Res. 2016, 49, 952 − 965.
16 Equilibrium constants have been reported for reversible CꢀC bond heterolysis in DMSO to give hydrocarbon
salts (X− = carbanion) in reference 3e.
17 Carbocation reduction potentials (Ered) have also been used as a measure of their stability: See references 3e,
3i, 3j, and 5b.
18 (a) Stewart, R. The Proton: Applications to Organic Chemistry; Academic Press: Orlando; 1985, p 125; (b)
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23 For details on Marcus theory, see: (a) Marcus, R. A. Annu. Rev. Phys. Chem. 1964, 15, 155 – 196; (b)
Marcus, R. A. J. Phys. Chem. 1968, 72, 891 – 899; (c) Marcus, R. A. J. Am. Chem. Soc. 1969, 91, 7224 –
7225; (d) Albery, W. J.; Kreevoy, M. M. Adv. Phys. Org. Chem. 1978, 16, 87 – 157; (e) Albery, W. J. Annu.
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24 Mayr, H.; Bug, T.; Gotta, M. F.; Hering, N.; Irrgang, B.; Janker, B.; Kempf, B.; Loos, R.; Ofial, A. R.;
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25 Vinyl Cations; Stang, P.; Rappoport, Z.; Hanack, M.; Subramanian, L. R., Eds.; Academic Press: New York,
1979.
26 (a) Dicoordinated Cations; Z. Rappoport, P. J. Stang, Eds.; Wiley: New York, 1997; (b) Grob, C A. in
reference 26a Chapter 1, pp 1 – 7; (c) Stang, P. J. in Progress in Physical Organic Chemistry; Streitwieser
Jr., A. J.; Taft, R. W., Eds.; Wiley and Sons: New York; 1973; Vol. 10, pp 205 – 325; (d) Rappoport, Z. in
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Modena, G.; Tonellato, U. Adv. Phys. Org. Chem. 1971, 9, 185 – 280.
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27 Kitamura, T.; Taniguchi, H. in reference 26a, Chapter 7, pp 321 – 376.
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28 Generation of βꢀsilylꢀsubstituted vinyl cations that are stable at low temperature in superꢀacidic media: (a)
Siehl, H.ꢀU.; Kaufmann, F.ꢀP.; Apeloig, Y.; Braude, V.; Danovich, D.; Berndt, A.; Stamatis, N. Angew.
Chem. 1991, 103, 1546 – 1549; Angew. Chem., Int. Ed. Engl. 1991, 30, 1479 – 1482; (b) Siehl, H.ꢀU.;
Müller, T.; Gauss, J.; Buzek, P.; Schleyer, P. v. R. J. Am. Chem. Soc. 1994, 116, 6384 – 6387; (c) F.ꢀP.
Kaufmann, H.ꢀU. Siehl, J. Am. Chem. Soc. 1992, 114, 4937 – 4939; (d) H.ꢀU. Siehl, F.ꢀP. Kaufmann, K.
Hori, J. Am. Chem. Soc. 1992, 114, 9343 – 9349.
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29 Generation of room temperatureꢀstable silylꢀsubstituted vinyl cations: (a) Klaer, A.; Syha, Y.; Nasiri, H. R.;
Müller, T. Chem. Eur. J. 2009, 15, 8414 – 8423; (b) Müller, T.; Meyer, R.; Lennartz, D.; Siehl, H.ꢀU.
Angew. Chem. 2000, 112, 3203 – 3206; Angew. Chem., Int. Ed. 2000, 39, 3074 – 3077; (c) Müller, T.;
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Angew. Chem. 2004, 116, 1569 – 1572; Angew. Chem., Int. Ed. 2004, 43, 1543 – 1546.
30 Aue, D. H. in Dicoordinated Carbocations. Rappoport, Z., Stang, P. J., Eds.; Wiley: New York, 1997; pp
105 – 156. In particular, see pp 126 – 129, Table 2.
31 Calculations on stabilities of vinyl cations: (a) Apeloig, Y.; Franke, W.; Rappoport, Z.; Schwarz, H.; Stahl,
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when the reaction was stopped.
29
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