Photogeneration of benzhydryl cations by near-UV laser flash photolysis of pyridinium salts

Article abstract of DOI:10.1021/jp3049247

Laser flash irradiation of substituted N-benzhydryl pyridinium salts yields benzhydryl cations (diarylcarbenium ions) and/or benzhydryl radicals (diarylmethyl radicals). The use of 3,4,5-triamino-substituted pyridines as photoleaving groups allowed us to employ the third harmonic of a Nd/YAG laser (355 nm) for the photogeneration of benzhydryl cations. In this way, benzhydryl cations can also be photogenerated in the presence of aromatic compounds and in solvents which are opaque at the wavelength of the quadrupled Nd/YAG laser (266 nm). To demonstrate the scope and limitations of this method, the rate constants for the bimolecular reactions of benzhydryl cations with several substituted pyridines were determined in acetonitrile and with water in acetone. The obtained data agree with results obtained by stopped-flow UV-vis spectroscopic measurements. The rate constants for the reaction of the 4,4'-bis[methyl(2,2,2- trifluoroethyl)amino]benzhydrylium ion with 4- (dimethylamino)pyridine were also determined in dimethyl sulfoxide, N,N-dimethylformamide, and acetone. From the secondorder rate constants, we derived the nucleophilicity parameters N and sN for the substituted pyridines, as defined by the linear free energy relationship, log k2 = sN(N + E).

Full text of DOI:10.1021/jp3049247

Article
pubs.acs.org/JPCA
Photogeneration of Benzhydryl Cations by Near-UV Laser Flash
Photolysis of Pyridinium Salts
Tobias A. Nigst, Johannes Ammer, and Herbert Mayr*
Department Chemie, Ludwig-Maximilians-Universitat Munchen, Butenandtstr. 5-13 (Haus F), 81377 Munchen, Germany
̈
̈
̈
S
* Supporting Information
ABSTRACT: Laser flash irradiation of substituted N-benzhydryl pyridinium salts
yields benzhydryl cations (diarylcarbenium ions) and/or benzhydryl radicals
(diarylmethyl radicals). The use of 3,4,5-triamino-substituted pyridines as photo-
leaving groups allowed us to employ the third harmonic of a Nd/YAG laser (355 nm)
for the photogeneration of benzhydryl cations. In this way, benzhydryl cations can
also be photogenerated in the presence of aromatic compounds and in solvents which
are opaque at the wavelength of the quadrupled Nd/YAG laser (266 nm). To demonstrate the scope and limitations of this
method, the rate constants for the bimolecular reactions of benzhydryl cations with several substituted pyridines were determined
in acetonitrile and with water in acetone. The obtained data agree with results obtained by stopped-flow UV−vis spectroscopic
measurements. The rate constants for the reaction of the 4,4′-bis[methyl(2,2,2-trifluoroethyl)amino]benzhydrylium ion with 4-
(dimethylamino)pyridine were also determined in dimethyl sulfoxide, N,N-dimethylformamide, and acetone. From the second-
order rate constants, we derived the nucleophilicity parameters N and sN for the substituted pyridines, as defined by the linear
free energy relationship, log k2 = sN(N + E).
Scheme 1. Photoheterolysis of Substrates R-PLG
(Photoleaving Group PLG− = Cl−, PR′3, etc.) and
Subsequent Trapping of the Generated Carbocations (R+)
by Nucleophiles (Nu)
INTRODUCTION
■
Pyridinium salts have a rich photochemistry1 which includes
their use as photoinitiators in cationic polymerizations.2−4
However, the heterolytic cleavage of the exocyclic C−N bond
of N-alkylated pyridinium ions was only found in a few cases.4
The observation that photolyses of related onium salts, such as
phosphonium5,6 or ammonium salts,7,8 gave excellent yields of
carbocations thus prompted us to investigate substituted
pyridinium salts as precursors for the carbocations. The use
of highly substituted aminopyridines as photoleaving groups
promised a good wavelength tunability, which would be
interesting for applications as photoinitiators9,10 and for kinetic
investigations of carbocation reactivities.
266 nm laser pulse; subsequently, the UV−vis absorption
decays of the carbocations were monitored in the presence of
the nucleophilic reaction partners.8,13
This procedure is problematic when the reaction partners
also have considerable absorbances at the excitation wavelength
of 266 nm. In this case, the laser pulse may also generate
reactive intermediates from the nucleophilic reaction partner,
and we can no longer be sure which process is causing the
decay of the carbocation. In the usual experimental setup11
featuring a 90° angle between pump and probe light, the
opacity of the sample solutions for the 266 nm pump pulse also
prevents sufficient excitation of the precursor molecules to
generate the carbocations. The same problems not only occur
when the nucleophilic reaction partner absorbs at 266 nm but
also occur when experiments are carried out in solvents with a
high UV cutoff, such as N,N-dimethylformamide (DMF),
dimethyl sulfoxide (DMSO), or acetone.
Many bimolecular reactions of carbocations proceed on a
timescale from nanoseconds to microseconds, which requires
short pump pulses for time-resolved measurements. The most
common and most affordable source of light pulses with pulse
widths of a few nanoseconds is the Nd/YAG laser.11 Its
fundamental emission in the infrared region (1064 nm) can be
converted to the second, third, or fourth harmonic to generate
laser pulses in the visible (532 nm) or UV range (355 or 266
nm). In the following discussion, we used the Nd/YAG laser as
an example to address some problems related to the choice of
excitation wavelength in laser flash photolysis experiments, but
similar problems will also be encountered with other types of
lasers (including tunable lasers). In previous investigations, we
have generated benzhydryl cations 1 and other carbocations by
heterolytic photocleavage of precursor molecules such as
arylmethyl halides12 or arylmethyl triarylphosphonium salts6
which have significant UV absorbances at 266 nm, but not at
355 nm (Scheme 1). Typically, the carbocations were obtained
by irradiating precursor solutions with A266 nm ≈ 0.1−1.0 with a
In such cases, the third harmonic of the Nd/YAG laser (355
nm) would be suitable as a pump, but onium salts do not
usually absorb at this wavelength. The use of photosensitizers, a
common strategy for photoinitiator systems,9,10 cannot be
Received: May 21, 2012
Revised: July 30, 2012
Published: July 31, 2012
© 2012 American Chemical Society
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employed for kinetic studies because the diffusion-limited
bimolecular excitation transfer is too slow. It was, therefore,
desirable to develop a carbocation precursor that can be
irradiated at 355 nm. For this purpose, we took advantage of
the fact that amino substitution of pyridinium salts causes red
shifts of the UV absorptions.1a In this work, we report the
generation of benzhydryl cations 1f−n by 355 nm irradiation of
benzhydryl pyridinium salts 2f−n-R derived from 3,4,5-
triamino-substituted pyridines 3-R, the so-called super-
DMAPs14,15 (Scheme 2).
azaprefulvenes or Dewar-pyridines upon ∼254 nm irradiation.16
Therefore, their reactivities cannot be characterized readily with
a method that uses 266 nm pump pulses. Rate constants of the
reactions of photolytically generated carbocations with
pyridines could previously only be determined when the
employed precursors incorporated a highly conjugated π system
which absorbed at λ > 300 nm.17 The reactivity data acquired in
this study will subsequently be employed to determine the
nucleophilicity parameters, N and sN, for the pyridines 4−8
according to the linear free energy relationship eq 1, which
allows us to predict second-order rate constants, k2, for polar
organic reactions with one electrophile-specific parameter, E,
and two solvent-dependent nucleophile-specific parameters, N
and sN.18
Scheme 2. Generation of Benzhydrylium Ions 1f−n by 355
nm Laser Flash Photolysis of Pyridinium Ions 2f−n-R with
a
Different Substituents (R) on the Pyridine Moiety
log k2 = sN(N + E)
(1)
RESULTS AND DISCUSSION
■
Photogeneration of Benzhydrylium Ions. Since the
photoelectrofuges (i.e., carbocations-to-be) do not absorb at
355 nm, the excitation must occur at the photonucleofuge (i.e.,
PLG moiety in Scheme 1). Our recent investigation of 3,4,5-
triamino-substituted pyridines 3-R (Chart 2) showed that these
compounds are strong Lewis bases and that the pyridinium ions
2-R (i.e., their adducts with benzhydrylium ions 1) show UV
absorptions around 355 nm.15
a
For substitution patterns f−n of the benzhydryl moiety, see Table 1.
−
Counteranion X− = Cl− or BF4 .
For testing our approach, we measured the rate constants of
the reactions of benzhydrylium ions 1 (Table 1) with pyridines
Table 1. List of the Reference Electrophiles 1 Used in This
Study
Chart 2. Pyridines Employed as Photoleaving Groups in this
Study
a
a
dmaBz = 4-(dimethylamino)benzoyl.
The pyridines 3-R were obtained in four to six steps from 4-
pyridone as described by David et al.15 In order to use the
costly materials efficiently, we did not isolate the benzhydryl
−
pyridinium tetrafluoroborates 2f−j-R (X− = BF4 ) but
generated them in solution by combining the pyridines 3-R
with solutions of the isolated18a benzhydrylium tetrafluorobo-
−
rates 1f−j BF4 (i.e., the reverse of the reaction depicted in
a
Electrophilicity parameters, E, were taken from ref 18a, unless noted
Scheme 2). The formation of the pyridinium salts 2f−j-R was
indicated by the immediate disappearance of the color of 1f−j
and the appearance of UV−vis absorption bands at 340−380
nm. The benzhydryl pyridinium chlorides 2j−o-R (X− = Cl−)
were obtained from reactions of 3-R with the corresponding
benzhydryl chlorides, Ar2CH-Cl. The syntheses of 2n-R and
2o-R by this method required longer reaction times of 2 h or
overnight, as revealed by the full development of the
pyridinium bands in the UV−vis spectrum.
b
otherwise. From ref 6b.
4−8 (Chart 1). Pyridines have strong UV absorptions below
300 nm and are known to undergo photoisomerizations via
Chart 1. Pyridines Employed as Nucleophiles in This Study
Figure 1 shows the UV−vis absorption spectra of several 1-
benzhydryl pyridinium salts 2l-R derived from the pyridines 3-
R. The parent compound 2l-H (not shown) and the N-alkyl-
substituted derivatives, 2l-Bn, 2l-Me (Figure 1), and 2l-Et (not
shown), have absorption maxima near 355 nm. The absorption
maxima of the N-acyl-substituted compounds 2l-Ac and 2l-Bz
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Figure 1. UV−vis absorption spectra of pyridinium salts 2l-R (X−
Cl−) with different substituents.
=
(not shown) are too far in the UV but can be red shifted by the
introduction of the NMe2 substituent on the benzoyl group (→
2l-dmaBz, Figure 1). In agreement with previous reports,1a the
absorption maximum of the pyridinium salts does not depend
on the substituent at the pyridine nitrogen atom (see Figure S1
of the Supporting Information).
When we irradiated ∼1.2 × 10−4 M solutions of the parent
pyridinium salt 2i−l-H or the N-alkyl derivatives 2i−l-Bn, 2i−l-
Me, or 2i−l-Et in CH3CN with a 7 ns laser pulse from the
frequency-tripled Nd/YAG laser (λexc = 355 nm, ∼50 mJ/
pulse), we detected the previously described12 UV−vis
absorption bands of the moderately stabilized benzhydrylium
ions 1i−l at λmax = 450−600 nm in the transient spectra and
observed the disappearance (bleaching) of the pyridinium salts
2i−l-R at ∼350−370 nm. Figure 2 shows the transient spectra
obtained by irradiation of the N-benzyl-substituted pyridinium
salts 2f−n-Bn with different substituents on the benzhydryl
moiety. The different absorbances of the benzhydrylium ions
1f−n in Figure 2(panels a−d) are mostly due to the different
absorption coefficients and correspond to concentrations of the
benzhydrylium ions of 0.6−2.3 × 10−6 M. The spectrum
obtained by irradiation of 2n-Bn (X− = Cl−) additionally shows
a small band at ∼338 nm, which we assign to the phenyl(p-
tolyl)methyl radical by comparison with its previously
published spectrum.12 More electrophilic carbocations such as
the parent benzhydrylium ion 1o could not be obtained in
concentrations which were sufficient for kinetic investigations;
in these cases we mainly observed the benzhydryl radicals.
The yields and lifetimes of the carbocations 1i−l obtained by
photolysis of the parent pyridinium salts 2i−l-H and the N-
alkylated derivatives 2i−l-Bn, 2i−l-Me, and 2i−l-Et were
almost independent of the nature of the photonucleofuge 3-
R. Irradiation of the N-acyl derivative 2l-dmaBz, however, did
not yield any carbocations 1l. For our kinetic investigations, we
selected the N-benzyl derivatives 2-Bn because the pyridine 3-
Bn has the highest Lewis basicity in the series15 and, therefore,
forms the most stable pyridinium ions (see below).
In the photolyses of benzhydryl triphenylphosphonium salts,
the yields and lifetimes of the benzhydryl cations greatly
depend on the counteranion of the precursor salt.6a The
generation of benzhydryl radicals by electron transfer in
electronically excited phosphonium chloride and bromide ion
pairs was described, but this process is unimportant at low
concentrations of the precursor salts in CH3CN, as the
phosphonium halides are mostly unpaired under these
conditions.6a However, the lifetimes of the photogenerated
benzhydryl cations are reduced significantly by the diffusion-
controlled reactions with the halide counteranions.6a Therefore,
we also investigated the influence of the counteranion in the
photolysis of 2j-Bn on the lifetime of the photogenerated
carbocation 1j. When we irradiated a 1.2 × 10−4 M solution of
Figure 2. Transient UV−vis spectra obtained by irradiation of 1.2 ×
−
−
10−4 M solutions of (a) 2f-Bn (X− = BF4 ), (b) 2i-Bn (X− = BF4 ),
(c) 2k-Bn (X− = Cl−), or (d) 2n-Bn (X− = Cl−) in acetonitrile with a 7
ns laser pulse (λexc = 355 nm).
the pyridinium halide 2j-Bn (X− = Cl−), we observed a
monoexponential decay of the benzhydryl cation 1j with a rate
constant of kobs = 1.9 × 106 s−1, which corresponds to a second-
order rate constant of 1.6 × 1010 M−1 s−1 for the reaction of 1j
with 2j-Bn Cl−. As this value is slightly larger than the
previously reported value of 9.39 × 109 M−1 s−1 for the reaction
of 1j with Cl− in acetonitrile,19 one might assign this decay to
the combination of 1j with chloride ions. On the other hand,
when we employed the pyridinium tetrafluoroborate 2j-Bn (X−
−
= BF4 ) as a precursor, the decay of the benzhydryl cation 1j
was not monoexponential, and we found an initial fast decay
with a rate constant comparable to that obtained with 2j-Bn
Cl−, which was followed by a slower decay on the microsecond
timescale. We explain this behavior by a fast reversible reaction
of 1j with the tertiary amine functions of the precursor salt 2j-
Bn, which is followed by a slower unidentified subsequent
reaction. This behavior is analogous to that of other tertiary
amines, where we also observed fast reversible reactions
followed by slower decays due to unknown subsequent
reactions.8 Due to the high nucleophilic reactivity of the
tertiary amine centers of the precursors 2-Bn, the choice of the
counteranion X− is thus not so important, and we did not
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convert the chlorides 2j−n-Bn (X− = Cl−) to the tetrafluor-
oborates. Because of the proximity of the diffusion limit, the
reactions of 1j−n with chloride19 or tertiary amines8 proceed
with almost the same rate constants, and we observed similar
decay rates for all investigated benzhydrylium ions obtained
Table 2. Second-Order Rate Constants, k2, for the Reactions
of the Reference Electrophiles 1 with the Pyridines 4−8 at
20 °C and Resulting N and sN Parameters
from different precursors 2j−n-Bn (X− = Cl− or BF4 ).
−
Kinetic Investigations. With a method at hand to generate
the benzydrylium ions 1 using 355 nm laser pulses, we
determined rate constants of reactions of 1 with nucleophiles.
After irradiation of the pyridinium salts 2f−l-Bn (∼1.2 × 10−4
M) in the presence of a large excess of added nucleophiles (7 ×
10−4 to 1 × 10−1 M), we observed monoexponential decays of
the absorbances, A, of the photogenerated benzhydrylium ions
1f−l, as illustrated in Figure 3 for the reaction of 1h with
Figure 3. Plot of the absorbance decay of 1h at λ = 612 nm observed
−
after irradiation of 2h-Bn (1.2 × 10−4 M, X− = BF4 ) with a 7 ns laser
pulse (λexc = 355 nm, ∼50 mJ/pulse) in the presence of 4-
(dimethylamino)pyridine ([4] = 2.12 × 10−3 M). Inset: plot of kobs
vs [4]; kobs = 1.37 × 106 [4] + 857 (R2 = 0.9996).
DMAP (4). The decays of the absorbances were fitted with the
obst
exponential function A(t) = A0e−k + C to obtain the first-
order rate constants kobs (s−1). Plots of kobs versus the
nucleophile concentrations were linear in all cases, and the
slopes of these plots provided the second-order rate constants
k2 (M−1 s−1) for the reactions of 1f−l with the nucleophiles.
In order to study some systems of known reactivity, we
measured the second-order rate constants, k2, for the reactions
of some benzhydrylium ions 1 with 4-(dimethylamino)pyridine
(DMAP, 4) in acetonitrile (Table 2). The second-order rate
constant for the reaction of 4 with 1f was previously
determined with the stopped-flow method,20 and we wanted
to reproduce this measurement with our new method.
However, due to the similar electrofugalities of 1f (Ef = 4.84)
and 1c (Ef = 4.83),21 the formation of 1f by thermal
dissociation of 2f-Bn in acetonitrile solutions is expected to
occur with a rate constant similar to that reported for the
dissociation of 2c-Bn (9.6 × 10−2 s−1).15 In the presence of high
concentrations of DMAP (4), the benzhydrylium ions 1f will
then be trapped by 4 to give a pyridinium salt with no
absorption at 355 nm. As a consequence, we had to work
quickly and keep the nucleophile concentration below ca. 5 ×
10−3 M in order to maintain a sufficient concentration of 2f-Bn,
which is needed to generate the carbocation 1f with the 355 nm
laser pulse. The rate constant for the reaction of 4 with 1f (2.97
× 105 M−1 s−1) determined in this way is in fair agreement with
the previously reported value from stopped-flow experiments
(2.31 × 105 M−1 s−1).20
a
b
Laser flash photolysis of 2-Bn, unless noted otherwise. Stopped-flow
c
d
UV−vis measurement. From ref 20. The average of both rate
constants from laser flash photolysis and stopped-flow measurements
was used for the correlation analysis. With <1% acetonitrile as
cosolvent.
e
Figure 4. Plot of log k2 for reactions of benzhydrylium ions 1 with
substituted pyridines 4−8 vs the electrophilicity parameters E of the
benzhydrylium ions. The plot for 6 is not shown because it overlaps
with 8; it is shown in the Supporting Information.
The rate constants for the reactions of 4 with 1h and 1i are
too fast for the stopped-flow method and could not be
measured with the established laser flash photolytic method
using 266 nm laser pulses because 4 absorbs at the excitation
wavelength. These rate constants could now be determined by
laser flash photolysis of 2-Bn at 355 nm (Table 2), and Figure 4
shows that these rate constants extend the correlation line for
log k2 (1a−d), which was determined by stopped-flow
experiments. If all available data are used to calculate the
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nucleophilicity parameters of 4, we obtain N = 15.51 and sN =
0.62 (Table 2), which deviate only slightly from the previously
published values.20
such as 1o cannot be generated efficiently, and the precursors
2-Bn of carbocations, which are less Lewis-acidic than 1f, are
not stable in the presence of nucleophiles that trap the small
concentrations of benzhydrylium ions, which exist in solutions
of 2a−f-Bn in the dark. Nevertheless, we could generate
benzhydrylium ions 1f−n covering more than 10 orders of
reactivity and investigate the rates of their reactions with
nucleophiles. We employed this method to characterize the
nucleophilic reactivities of the electron-rich pyridines 5−8,
which will be used for further studies in our group. Moreover,
we have demonstrated that the third harmonic of the Nd/YAG
laser (355 nm) can be employed to generate benzhydrylium
ions 1 in solvents with a high UV cutoff such as acetone (cutoff
= 330 nm)25 which is opaque to the quadrupled Nd/YAG laser
(266 nm) and the XeCl excimer laser (308 nm). Laser flash
photolysis of substituted pyridinium salts at 355 nm thus
supplements the established kinetic methods and is particularly
useful for characterizing nucleophiles which do not react with
stabilized carbocations for thermodynamic reasons and which
cannot be studied with 266 nm laser flash photolytically
generated carbocations due to the absorbance of the sample
solutions.
Encouraged by the results with 4, we determined second-
order rate constants k2 > 105 M−1 s−1 for the reactions of 1f−k
with the substituted pyridines 5−8 (Table 2) in acetonitrile
with the 355 nm laser flash photolytic method.22 These data are
supplemented with rate constants for slower reactions (k2 < 106
M−1 s−1), which were determined with the stopped-flow
method under first-order conditions by using the nucleophiles
in a large excess (>10 equiv over the benzhydrylium
tetrafluoroborates 1a−i) as described previously18a (Table 2).
Again we observed good linear correlations of log k2 versus E
(Figure 4) for the data from both methods. As previously
observed, 1g8,23a,b (not shown) and 1j23c always react
somewhat faster in acetonitrile solution than expected from
their E parameters which have been determined in CH2Cl2
solutions.
In the case of 7, only the reactions with 1h and 1i can be
followed with the stopped-flow method, as the better stabilized
carbocations show no conversion in reactions with 7 due to its
low Lewis basicity. We have now been able to characterize the
nucleophilicity of 7 over a wider reactivity range by including
the data determined by the laser flash photolysis method.
From the correlations in Figure 4, we derived the
nucleophilicity parameters of 4−8 listed in Table 2. As
expected, the nucleophilicities of the 4-amino-substituted
pyridines 4−7 decrease with the electron-withdrawing character
of the substitutents (Table 2 and Figure 4). The N-acetyl-4-
aminopyridine (8) is of similar reactivity as the 3-bromo-
substituted DMAP 6.
ASSOCIATED CONTENT
* Supporting Information
■
S
Additional UV−vis spectra and details of the kinetic studies.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: herbert.mayr@cup.uni-muenchen.de. Tel: (+49) 89-
2180-77719. Fax: (+49) 89-2180-77717.
■
To demonstrate the applicability of the method in solvents
with high UV cutoff, we also measured the second-order rate
constants for the reactions of 1i with 4 in DMSO, DMF, and
acetone (Table 2). The order of reactivity (acetone >
acetonitrile > DMF > DMSO) agrees with that previously
reported for the reactions of 4 with 1f.20 Furthermore, we
determined first-order rate constants for the reactions of 1l with
80% and 90% aqueous acetone (Table 3) and found good
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Deutsche Forschungsgemeinschaft (SFB 749)
and the Fonds der Chemischen Industrie for financial support
and Raman Tandon for samples of 3-Me and 3-Et.
Table 3. Comparison of Experimental and Calculated First-
Order Rate Constants (s−1) for Reactions of 1l with
Acetone/Water Mixtures
ABBREVIATIONS
■
a
b
c
k1 (s−1
)
kcalc (s−1
)
k2/kcalc
Bn, benzyl; Bz, benzoyl; dmaBz, 4-(dimethylamino)benzoyl;
PLG, photoleaving group; DMAP, 4-(dimethylamino)pyridine;
DMF, N,N-dimethylformamide; DMSO, dimethyl sulfoxide;
YAG, yttrium aluminum garnet
solvent
90A10W
80A20W
6.23 × 106
6.61 × 106
4.35 × 106
7.17 × 106
1.43
0.92
a
b
Mixtures given in v/v (A = acetone, W = water). Laser flash
c
photolysis of 2l-Bn. Calculated from eq 1 using the previously
published N1 and sN parameters of acetone/water mixtures (90A10W:
N1 = 5.70, sN = 0.85; 80A20W: N1 = 5.77, sN = 0.87).24
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■
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CONCLUSION
■
We have demonstrated that the 355 nm laser flash photolysis of
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1
dimethoxybenzhydryl chloride in CD3CN was demonstrated by H
NMR experiments (see Supporting Information for details).
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(13) Further recent examples: (a) Ammer, J.; Mayr, H.
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Eur. J. Org. Chem. 2011, 6470−6475. (g) Troshin, K.; Schindele, C.;
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David, O. R. P. Chem.Eur. J. 2011, 17, 12852−12871.
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(18) (a) Mayr, H.; Bug, T.; Gotta, M. F.; Hering, N.; Irrgang, B.;
Janker, B.; Kempf, B.; Loos, R.; Ofial, A. R.; Remennikov, G.;
Schimmel, H. J. Am. Chem. Soc. 2001, 123, 9500−9512. (b) Mayr, H.;
Kempf, B.; Ofial, A. R. Acc. Chem. Res. 2003, 36, 66−77. (c) For a
comprehensive database of nucleophilicity and electrophilicity
parameters, see: (http://www.cup.lmu.de/oc/mayr/DBintro.html).
(19) Minegishi, S.; Loos, R.; Kobayashi, S.; Mayr, H. J. Am. Chem.
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(20) Brotzel, F.; Kempf, B.; Singer, T.; Zipse, H.; Mayr, H. Chem.
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(21) Streidl, N.; Denegri, B.; Kronja, O.; Mayr, H. Acc. Chem. Res.
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