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ElectrophilicityЦNucleophilicity Scale Also in the Gas Phase.

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Angewandte
Chemie
Gas-Phase studies
DOI: 10.1002/ange.200503235
Electrophilicity–Nucleophilicity Scale Also in the
Gas Phase
Chagit Denekamp* and Yana Sandlers
Quantification of the known characteristics of organic
substances is an ongoing objective for organic chemists. One
of the important properties of organic cations is their
electrophilicity. This property, which defines the reactivity
of cationic intermediates towards nucleophiles is not easy to
access. In fact, it is primarily necessary to establish whether or
not electrophilicity can be defined without taking into
account the counterpart nucleophile, the solvent, temperature, or any other experimental factor. There have been
several attempts to quantify electrophilicity and nucleophilicity and recently it has been demonstrated that the rates of
reactions of carbocations with uncharged nucleophiles obey a
linear free-energy relationship (log k = s(E+N), where E and
N denote electrophilicity and nucleophilicity parameters,
respectively, and s is a nucleophile-specific slope factor).[1–10]
Subsequently it has been unambiguously shown that the
relative rate constants for electrophile–nucleophile coupling
reactions follow linear free-energy relationships and that an
electrophilicity parameter can be defined and measured
regardless of the nucleophile and with some solvent effect.
Good correlation is found between the experimental electrophilicity scale and a theoretical electrophilicity index, developed by P+rez and co-workers for a series of benzhydryl
cations.[11, 12]
[*] Dr. C. Denekamp, Y. Sandlers
Department of Chemistry
Technion—Israel Institute of Technology
Haifa 32000 (Israel)
Fax: (+ 972) 4829-3736
E-mail: chchagit@tx.technion.ac.il
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. 2006, 118, 2147 –2150
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2147
Zuschriften
Mass spectrometry is a powerful tool for studying the
kinetics and mechanisms of organic reactions in the gas
phase[13–16] and in particular of ion–molecule reactions.[17]
Data from such studies deepened the understanding of the
reaction chemistry of a wide range of reactive organic ions.
Ion chemistry in the condensed phase is often dominated by
ion pairing and solvation interactions which can mask the
intrinsic reactivity of the reaction partners. In the absence of
solvation and counter ions, gas-phase studies can reveal the
details of reaction mechanisms and clearly characterize the
intrinsic reactivity of ionic and neutral species. The purpose of
this work was to study the reactivity of electrophilic intermediates in a solvent-free environment and compare these
gas-phase results with the corresponding reactions in solution.
Ion–molecule reactions were carried out with the aid of an
FT-ICR mass spectrometer.
A schematic description of the experimental setup is
presented in Figure 1: benzhydryl cations are generated from
Figure 2. Products of the reaction of cations 3 (m/z 167) and 5
(m/z 181) with pyrrolidine after 1 (a) and 5 s (b).
Figure 1. Sequence of events for an ion–molecule reaction in the FTICR mass spectrometer. ESI: electrospray ionization; FT: Fourier transformation; ICR: ion cyclotron resonance.
corresponding alcohols in the electrospray ion source. These
ions are transferred and trapped in an ICR cell that is
previously evacuated of any ions and flooded with a constant
pressure of a neutral nucleophile. Any ionic impurities that
are present with the desired ions are selectively ejected out of
the cell. The ions react with the neutral gas during a variable
reaction time after which all ions are detected. The decrease
in relative intensity of the reacting cation with time is
recorded and, assuming a pseudo-first-order reaction with
respect to this cation and that the pressure of the neutral gas is
unchanged, rate constants are derived.
To reduce errors, relative reaction rates were measured.
The chosen reference cation is 4-methylbenzhydryl cation 5.
In the example shown in Figure 2, benzhydryl cations 3 and 5
reacted with pyrrolidine, forming products of m/z 238 and
252, respectively. The reactions were repeated at various
reaction delays t and thus reaction rates could be derived from
slopes of the curves representing the logarithm of the relative
intensity of the cation peak as a function of time (Figure 3).
Table 1 lists the reaction rates that were measured for
benzhydryl cations 1–9 in the presence of various amine
2148
www.angewandte.de
Figure 3. Pseudo-first-order disappearance of ions 3 and 5 in the
presence of pyrrolidine.
nucleophiles. The experimental results support the assumption of pseudo-first-order reactions and linear regressions
with R2 > 0.985 were found for all the experiments. When it
was not possible to use cation 5 as reference due to a large
difference in reactivity, experimental results were normalized
according to the rate constant of a mediator. Compound 1 was
used as a mediator for 2, and compound 8 as a mediator for 5.
To compare the gas-phase rate constants with those
obtained in solution and assess the relation between electrophilicity, nucleophilicity, and reaction rate (log k = s(E+N)),
cation 5 was chosen as reference a electrophile for which E =
0 and piperidine was chosen as reference nucleophile for
which N = 0. These are not the references chosen by Mayr and
co-workers, however, since the experimental limitations in
our work are different from theirs is was necessary to choose
different anchor points. All reaction rates were plotted
against the rates of the chosen reference resulting in a series
of curves from which electrophilicity and nucleophilicity
parameters could be derived (Figure 4).
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. 2006, 118, 2147 –2150
Angewandte
Chemie
Table 1: Relative reaction rates of ion–molecule reactions between cations 1–9 and neutral nucleophiles.
Nucleophile
1
2
3
4
Cation
5[a]
6
7
8
9
cyclohexylamine
propylamine
diisopropylamine
morpholine
pyridine
piperidine[a]
pyrrolidine
0.69
0.98
0.77
1.8
3.0
2.2
3.0
0.93
–
0.97
2.2
3.5
2.0
5.2
0.40
0.50
0.25
0.75
0.93
1.25
1.8
0.35
0.37
0.23
0.65
1.0
1.20
1.5
0.26
0.32
0.089
0.46
0.59
1.0
1.2
0.15
0.23
0.046
0.24
0.41
0.69
0.79
–
–
–
–
0.14
0.36
0.47
–
–
–
–
0.099
0.31
0.34
–
–
–
–
–
0.080
0.076
[a] Cation 5 is the reference electrophile and piperidine the reference nucleophile.
The correlation between the nucleophilicities and pKa values
of carbanions is quite poor[9] much like the correlation
between the Ngas values in Table 2 and gas-phase basicities.
However, the correlation between Mayr?s E values and the
Egas values in Table 2 is reasonable for most of the electrophiles (Figure 5, R2 = 0.978). An exception is 4,4’-dimethoxy-
Figure 4. Free-energy correlations for the reactions of 1–9 (k) and the
reference cation 5 (kref ) with different nucleophiles.
Table 2 summarizes the experimental results, where Egas
values are electrophilicity parameters in the gas phase and
Ngas are nucleophilicity values accordingly. Considering our
Table 2: Egas and Ngas values that are derived from the experimental data
presented herein.
Electrophile
1
2
3
4
5
6
7
8
9
Egas
0.79
0.69
0.22
0.18
0
0.37
1.0
1.2
2.5
Ngas
Nucleophile
cyclohexylamine
propylamine
diisopropylamine
morpholine
pyridine
piperidine
pyrrolidine
0.87
0.83
0.78
0.39
0.15
0
0.23
experimental error (that we estimate as 12 %), a good
agreement with Mayr?s equation is observed. It is noteworthy
that cross reactions that were performed between different
ions indicate an error of 13 %.
According to the work of Mayr and co-workers, an
electrophilicity parameter can be defined as long as a
reference reaction is chosen. These E values are valid
regardless of the reaction type and solvent. On the other
hand, nucleophilicity can be affected by the solvent. This
effect can also be quantified and subsequently the Mayr
equation is valid with a corrected nucleophilicity parameter.
Angew. Chem. 2006, 118, 2147 –2150
Figure 5. Correlation between Mayr’s electrophilicity parameter E and
our Egas values.
benzhydryl cation 9 that appears too unreactive in the gas
phase when compared with solution and other cations. As
mentioned above, for extreme points it was not possible to use
cation 5 as a reference owing to large differences in reaction
rates and experimental results were normalized according to
the rate constant of a mediator and this may be a source for
considerable error.
As only three of the amines used herein were also used by
Mayr and co-workers, it is currently impossible to examine
the correlation between N and Ngas parameters.
In conclusion, we found that the kinetics of electrophile–
nucleophile gas-phase reactions correlate well with values
known for solution although the actual electrophilicity
parameters are considerably smaller in the gas phase. The
differences in reactivity of the cations under study may be
surprising if one assumes that the association between them
and the neutral amines occurs without barrier and should be
controlled by the collision rate. There could be several
explanations to the apparent results and calculations are now
undertaken in order to gain better insight into this process.
We are aware that association reactions may be controlled by
the infrared emission rate, however, in such a case the
substituent effect should not necessarily correlate with the
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.de
2149
Zuschriften
substituent effect in solution. Preliminary calculations indicate that the formation of several ion–molecule complexes
may interfere with the formation of stable adducts and affect
the kinetic behavior of the system under study.
Experimental Section
[18] Y. Senda, N. Kikuchi, A. Inu, H. Itoh, Bull. Chem. Soc. Jpn. 2000,
73, 237.
[19] Vogel1s Textbook of Practical Organic Chemistry (Eds.: B. S.
Furniss, A. J. Hannaford, P. W. G. Smith, A. R. Tathchell), 5th
ed., Longman, 1989.
[20] P. Knochel, W. Dohle, N. Gommermann, F. F. Kneisel, F. Kopp,
T. Korn, I. Sapountzis, V. A. Vu, Angew. Chem. 2003, 115, 4438;
Angew. Chem. Int. Ed. 2003, 42, 4302.
Benzhydryl alcohol was purchased from Fluka (Sigma-Aldrich Co.,
Israel). Other alcohols were synthesized from the corresponding
commercial benzophenones (2, 4, 6, and 9) or aldehydes (5, 7, and 8)
according to known procedures[18–20] (see the Supporting Information
for further experimental details).
Ion–molecule reactions were carried out with a Bruker BioAPEX
III 47e FT-ICR mass spectrometer (Bruker Analytical Systems, Inc.,
Billerica, MA) equipped with a 4.7-T superconducting magnet, an
external source (Apollo ESI Source), and an infinity analyzer cell.
Samples were dissolved in CH3OH (0.01 mg mL 1) and introduced
into the ESI source at a flow rate of 0.3 mL min 1. Ions were detected
using the broadband detection mode covering a mass range from
m/z 50 to 1000. Typically, eight individual transients were accumulated to improve the signal-to-noise ratio. Precursor ions were
isolated using a series of “shot” frequency ejection pulses of 1000 ms
duration to eject all other ions and avoid activation of precursor ions.
Neutral amines were introduced into the ICR cell through a leak
valve maintaining a constant pressure of ca. 4.2 H 10 9 mbar (indicated). Spectra were recorded after various reaction times with
intervals depending on the reactivity.
Received: September 12, 2005
Revised: December 8, 2005
Published online: February 22, 2006
.
Keywords: electrophilicity · ion–molecule reactions · kinetics ·
linear free-energy relationships · mass spectrometry
[1] H. Mayr, M. Patz, Angew. Chem. 1994, 106, 938; Angew. Chem.
Int. Ed. Engl. 1994, 33, 990.
[2] M. Roth, H. Mayr, Angew. Chem. 1995, 107, 2428; Angew. Chem.
Int. Ed. Engl. 1995, 34, 2250.
[3] H. Mayr, T. Bug, M. F. Gotta, N. Hering, B. Irrgang, B. Janker, B.
Kempf, R. Loos, A. Ofial, G. Remennikov, H. Schimmel, J. Am.
Chem. Soc. 2001, 123, 9500.
[4] R. Lucius, R. Loos, H. Mayr, Angew. Chem. 2002, 114, 97;
Angew. Chem. Int. Ed. 2002, 41, 92.
[5] S. Minegishi, H. Mayr, J. Am. Chem. Soc. 2003, 125, 286.
[6] H. Mayr, Angew. Chem. 1990, 102, 1415; Angew. Chem. Int. Ed.
Engl. 1990, 29, 1371.
[7] H. Mayr, K.-H. MLller, D. Rau, Angew. Chem. 1993, 105, 1732;
Angew. Chem. Int. Ed. Engl. 1993, 32, 1630.
[8] M. Patz, H. Mayr, J. Maruta, A. Fukuzumi, Angew. Chem. 1995,
107, 1351; Angew. Chem. Int. Ed. Engl. 1995, 34, 1225.
[9] R. Lucius, H. Mayr, Angew. Chem. 2000, 112, 2086; Angew.
Chem. Int. Ed. 2000, 39, 1995.
[10] H. Mayr, O. Khun, M. F. Gotta, M. Patz, J. Phys. Org. Chem.
1998, 11, 642.
[11] L. R. Domingo, M. ArnM, R. Contreras, P. P+rez, J. Phys. Chem.
A 2002, 106, 952.
[12] P. J. P+rez, A. Toro-Labb+, A. Aizman, R. Contreras, J. Org.
Chem. 2002, 67, 4747.
[13] C. Denekamp, A. Mandelbaum, J. Mass Spectrom. 2001, 36, 422.
[14] C. Denekamp, A. Stanger, Chem. Commun. 2002, 236.
[15] C. Denekamp, E. Tenetov, Y. Horev, J. Am. Soc. Mass Spectrom.
2003, 14, 790.
[16] C. Denekamp, Y. Sandlers, J. Mass Spectrom. 2005, 40, 1055.
[17] S. Gronert, Chem. Rev. 2001, 101, 329.
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2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. 2006, 118, 2147 –2150
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