Solution and gas-phase investigations of trimethylsilylpropyl- Substituted pyridinium ions. Manifestation of the silicon δ eff

Article abstract of DOI:10.1021/om200510a

Computational studies on the N-methyl-2-trimethyl- M-propylpyridinium ions 15a (M = Si), 15b (M = Ge), 15c (M = Sn), and 15d (M = Pb) and N-methyl-4-trimethyl-Mpropylpyridinium ions 16a (M = Si), 16b (M = Ge), 16c (M = Sn), and 16d (M = Pb) provide evidence for a significant through-bond (double hyperconjugative) interaction between the M-CH₂ bond and the low-lying π* orbital of the pyridinium ion. The strength of this interaction increases in the order Si < Ge < Sn < Pb, in line with the σ-donor abilities of the C-M bond. The through-bond interaction for M = Si has been studied in solution using ¹³C and ²⁹Si NMR studies; however, the effect is small. The collision-induced dissociation fragmentation reactions of 15a and 16a are strongly influenced by the through-bond interaction with the major fragmentation pathway proceeding via extrusion of ethylene to yield the trimethylsilylmethyl-substituted pyridinium ions 1a and 2a.

Full text of DOI:10.1021/om200510a

ARTICLE
pubs.acs.org/Organometallics
Solution and Gas-Phase Investigations of Trimethylsilylpropyl-
Substituted Pyridinium Ions. Manifestation of the Silicon δ Effect
Asimo Karnezis, Richard A. J. O’Hair, and Jonathan M. White*
School of Chemistry, Bio21 Institute of Molecular Science and Biotechnology, University of Melbourne, Victoria 3010, Australia
S Supporting Information
b
ABSTRACT: Computational studies on the N-methyl-2-tri-
methyl-M-propylpyridinium ions 15a (M = Si), 15b (M = Ge),
15c (M = Sn), and 15d (M = Pb) and N-methyl-4-trimethyl-M-
propylpyridinium ions 16a (M = Si), 16b (M = Ge), 16c (M =
Sn), and 16d (M = Pb) provide evidence for a significant
through-bond (double hyperconjugative) interaction between
the MꢀCH2 bond and the low-lying π* orbital of the pyridi-
nium ion. The strength of this interaction increases in the order
Si < Ge < Sn < Pb, in line with the σ-donor abilities of the CꢀM
bond. The through-bond interaction for M = Si has been
studied in solution using 13C and 29Si NMR studies; however, the effect is small. The collision-induced dissociation fragmentation
reactions of 15a and 16a are strongly influenced by the through-bond interaction, with the major fragmentation pathway proceeding
via extrusion of ethylene to yield the trimethylsilylmethyl-substituted pyridinium ions 1a and 2a.
’ INTRODUCTION
The silicon β effect describes the well-known stabilizing effect
that silicon substituents have on positive charge at the β
position.1 The origin of this stabilization is due to strong hyper-
conjugation between the high-lying polarizable CꢀSi bond and
the carbocation p orbital and has been demonstrated in a number
of model systems. These range from trialkylsilylmethylbenzene
derivatives with low electron demand2 through to cations with
substantially higher electron demand, including the silyl-substi-
tuted tropylium ion3 and a range of vinyl cations.4 The trialk-
ylsilylmethyl-substituted pyridinium cations 1 and 2 provide a
convenient system for investigating CꢀSi hyperconjugation in
cationic systems with relatively low electron demand, and these
studies have been extended to related organogermanium
systems.5 These crystalline cations are relatively easy to handle
and are readily prepared by methylation of the corresponding
neutral substituted-pyridine precursors 3 and 4.
The δ effect of silicon, which is substantially smaller in
magnitude than the β effect, describes the stabilizing effect of
silicon substituents at the δ position and arises from a through-
In the solution phase CꢀSi π hyperconjugation reveals itself
in the 29Si NMR spectrum, with a downfield 29Si chemical shift
bond (σCSiꢀσCCꢀp) interaction between the CꢀSi bond and a
13
and a decrease in the 29Siꢀ C one-bond coupling constant
carbenium p orbital, which can be represented by the resonance
upon methylation of the precursor pyridine derivatives 3 and 4.5
forms shown in Figure 1. This type of interaction is also referred
to as double hyperconjugation.6
Both effects are larger in magnitude for the 2-substituted deriva-
tives 1, in qualitative agreement with calculations. In the solid
The δ effects of silicon, germanium, and tin are demonstrated
by the unimolecular solvolyses of 4-MMe3-substituted (M = Si,
Ge, Sn) bicyclo[2.2.2]oct-1-yl mesylates 5ꢀ7, which occur at
state X-ray analysis reveals that CꢀSi hyperconjugation in 1 and
2 manifests itself in lengthening of the CH2ꢀSi bond distance
and shortening of the CH2ꢀC(Ar) bond distance, relative to
those in 3 and 4, reflecting contributions of the double-bondꢀ
no-bond resonance forms 10 and 20 to the ground-state structures
of these ions.
Received: June 15, 2011
Published: October 04, 2011
r
2011 American Chemical Society
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rates that are 49, 71, and 2841 times faster than that of the parent
system,7 8.
Figure 1. Double hyperconjugation in δ-silyl carbenium ions.
conformation adopted by these ions allows for double hyper-
conjugation between the CꢀM bond and the low-lying π* orbital
of the pyridinium ions. Thus, the C2ꢀC3 bond is essentially
orthogonal to the plane of the aromatic ring and can therefore
overlap effectively with the π* orbital of the aromatic ring, while
the antiperiplanar arrangement about the C2ꢀC1 bond allows
for the C1ꢀM σ bonding orbital to overlap with the C2ꢀC3 σ*
orbital. The alternative conformation in which the C2ꢀC3 bond
lies in the plane of the pyridinium cation ring was found to be
4ꢀ5 kJ molꢀ1 higher in energy in all cases.
The dependence of the δ effect on stereochemistry is provided
by the solvolysis of the E and Z isomers of 5-(trimethylsilyl)- and
5-(trimethylstannyl)-2-adamantyl brosylate. Rate accelerations
were observed for the E isomers 9 and 10 (with Me3Sn > Me3Si)
(for which the stereoelectronic requirements of the through-
bond interaction are fulfilled), in comparison with the Z isomers
11 and 12 (for which concerted through-bond participation in
the developing carbonium ion is not possible).8 A similar result
was obtained from solvolysis of cis- and trans-4-trimethylstan-
nylcyclohexyl tosylates 13 and 14.9
The structural effects of double hyperconjugation which are
manifested upon conversion of the neutral pyridine precursors
17 and 18 into their corresponding pyridinium ions 15 and 16
are apparent upon inspection of the calculated bond distances,
which are presented in Table 1 for the 2-substituted derivatives
15 and 17 and in Table 2 for the 4-substituted derivatives 16 and
18. It is most revealing to consider the changes that occur to these
selected bond distances upon conversion of the neutral pyridine
precursors to the pyridinium ions; these are presented in Table 3.
Thus, upon conversion of the neutral pyridines 17 and 18 (M =
Si, Ge, Sn, Pb) into the pyridinium ions 15 and 16 an increase in
the MꢀC1 distance, a decrease in the C1ꢀC2 distance, an in-
crease in the C2ꢀC3 distance, and a decrease in the C3ꢀAr
bond distance are observed. Although the individual bond length
changes are small, the qualitative trends are consistent with
increased contribution of the resonance forms 150, 1500 and
160, 1600 to the ground-state structures of these ions (Figure 3).
The magnitude of these structural effects increases from M = Si
down to M = Pb, consistent with the increasing σ-donor abilities
of the CꢀM bond with Si < Ge < Sn < Pb.2c
Thus, interest lay in determining the properties of 2-trimethyl-
metalpropyl- and 4-trimethylmetalpropyl-substituted pyridi-
nium ions, 15aꢀd and 16aꢀd, in particular to determine the
magnitude of the stabilization provided to these ions by the
silicon substituent and to find whether the structural effects
arising from double hyperconjugation between the CꢀSi bond
and the electron-deficient pyridinium ion π system would be
observable in the solution-phase and solid-state structures.
NBO Analysis. NBO analysis of the interacting orbitals12 (σ(Mꢀ
C1), σ(C2ꢀC3), π*) provides some insight into the nature and
magnitude of the donorꢀacceptor interactions which characterize the
through-bond interaction between the CꢀM bond and the pyridi-
nium π* orbital for M = Si, Ge, Sn, Pb. The interaction energies
between the σMꢀC1 donor orbital and the σ*C2ꢀC3 acceptor orbital in
addition to the interaction energy between the σC2ꢀC3 donor orbital
and the π* acceptor orbital of the pyridinium cation are presented in
Tables 4 and 5 for the 2- and 4-substituted pyridinium ions 15 and 16,
respectively. These orbital interactions are slightly stronger in the
2-substituted pyridinium cations 15 than in the 4-substituted deriva-
tives 16, in support of our conclusions from previous studies on β-
silyl-substituted pyridinium cations 1a and 2a that the pyridinium
cation exerts stronger electron demand at the 2-position than at the 4-
position. The donorꢀacceptor interactions in 15 and 16 (M=Si,Ge)
are significantly weaker in magnitude to those calculated for the 4-silyl-
and 4-germyl-substituted cyclohexyl cation 19, consistent with
the much higher electron demand of the secondary carbenium
ion p orbital in 19.13
’ COMPUTATIONAL STUDIES
To gain some initial insight into the structures and properties of these
ions, we carried out ab initio calculations on the model systems 15 (M = Si)
and 16 (M = Si) and the heavier group IV congeners where M = Ge, Sn, Pb
for comparison. In addition, the neutral pyridine precursors 17 and 18 (M
= Si, Ge, Sn, Pb), for which the through-bond interaction should be less
pronounced, were also calculated. Calculations were performed at the
B3LYP/GEN (where C, H, and N were calculated at 6-311g(dp)10 and Si,
Ge, Sn, and Pb were calculated at SDD) level of theory.11
’ RESULTS AND DISCUSSION
The strength of both the σM‑C1ꢀσ*C1ꢀC2 and σC2ꢀC3ꢀπ*
orbital interactions increases from M = Si to Ge to Sn but then
appears to level off, with interactions for M = Pb decreasing slightly.
The optimized geometries of 15 and 16 and the precursors 17
and 18 (for M = Si) are shown in Figure 2.19 The preferred
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Figure 2. Calculated structures of 15 (M = Si) and 16 (M = Si) at the B3LYP/GEN level, where the basis set used was 6-311g(d,p) for C, H, and N and
SDD ECP for Si, Ge, Sn, and Pb.
Table 1. Selected Bond Distances for the Neutral Species 17 and Methylated 2-Trimethylmetal-Substituted Pyridinium Ions 15,
Where the Basis Set Used Was 6-311g(d,p) for C, H, and N and SDD ECP for Si, Ge, Sn, and Pb
17
C3ꢀAr (Å)
15
C3ꢀAr (Å)
M
MꢀC1 (Å)
C1ꢀC2 (Å)
C2ꢀC3 (Å)
M
MꢀC1 (Å)
C1ꢀC2 (Å)
C2ꢀC3 (Å)
Si
1.925
2.005
2.196
2.281
1.510
1.510
1.510
1.510
1.536
1.533
1.534
1.530
1.542
1.543
1.544
1.545
Si
1.942
2.026
2.222
2.310
1.500
1.499
1.498
1.500
1.532
1.529
1.528
1.525
1.557
1.559
1.562
1.565
Ge
Sn
Pb
Ge
Sn
Pb
Table 2. Selected Bond Distances for the Neutral Species 18 and Methylated 4-Trimethylmetal-Substituted Pyridinium Ions 16,
Where the Basis Set Used Was 6-311g(d,p) for C, H, and N and SDD ECP for Si, Ge, Sn, and Pb
18
C3ꢀAr (Å)
16
C3ꢀAr (Å)
M
MꢀC1 (Å)
C1ꢀC2 (Å)
C2ꢀC3 (Å)
M
MꢀC1 (Å)
C1ꢀC2 (Å)
C2ꢀC3 (Å)
Si
1.927
2.006
2.199
2.284
1.509
1.509
1.509
1.509
1.535
1.533
1.534
1.530
1.544
1.545
1.546
1.548
Si
1.940
2.023
2.219
2.307
1.497
1.496
1.495
1.494
1.532
1.529
1.528
1.524
1.557
1.560
1.566
1.566
Ge
Sn
Pb
Ge
Sn
Pb
The overall stabilization afforded to the 2- and 4-substituted
pyridinium cations 15 and 16 by the presence of the group 4
metal substituent was assessed by calculating the enthalpy change
in the isodesmic equations (eqs 1 and 2 in Table 6). The
stabilization is small and may be close to the significance level
for the level of theory used in these calculations; however, the
errors associated with this level of theory would substantially
cancel in these equations. In all cases the degree of stabilization
provided by the metal substituent is greater at the 2-position than
at the 4-position, consistent with greater electron demand at the
2-position. The degree of stabilization increases down the group
Pb > Sn > Ge > Si, consistent with the increasing ability of the
CꢀM bond to donate electron density by hyperconjugation.
Synthesis. Preparation of 2- and 4-trimethylsilylpropyl pyr-
idines 15 and 16 for the experimental studies was achieved
according to the method outline in Scheme 1. Thus, bromopyr-
idines 20 and 21 were treated with propargyltrimethylsilane
under Sonogashira conditions,14 followed by hydrogenation of
the intermediate pyridine derivatives 22 and 23 to give 2- and
4-trimethylsilylpropyl pyridines 17 and 18 in 41% and 13%
yields, respectively. Conversion of the neutral pyridines 17 and
18 into the corresponding pyridinium ions 15 and 16 was
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achieved by treatment with methyl iodide, methyl tosylate, or
methyl triflate.
on the structures of these ions in the solid state, several attempts
were made to obtain crystals of the N-methylpyridinium ions 15
and 16. However, with a variety of counterions and solvents these
all resulted in oily liquids. Low-quality crystals could however be
obtained for the protonated salt of the pyridine precursor 17a as
its picrate salt. The structure of 17aH+ within the picrate salt is
presented in Figure 4.
Although the quality of the structural determination for 17aH
precludes any useful comments being made regarding the bond
distances in the structure, it does however establish that the
molecule exists in the solid state in a conformation which allows
for the through-bond interaction to occur and is in qualitative
agreement with the lowest energy computed structure for the
corresponding pyridinium ion 15 (Figure 2).
Gas-Phase Studies. To determine the impact that through-
conjugation between the trimethylsilyl substituent and the
pyridinium cation might have on the fragmentation behavior,
15 and 16 were subjected to electrospray ionization mass spectro-
metry followed by low energy collision-induced dissociation (CID).
The substituted precursors 17 and 18 were methylated with
either CH3I or CD3I immediately prior to ESI/MS. The N-CH3
pyridinium ions 15 and 16 both gave rise to the precursor ions at
m/z 208; subsequent isolation and CID of these pyridinium ions
gave rise to product ions at m/z 180, 108, and 73 (Figures 5A and
6A). Similarly the N-CD3 pyridinium ions 15d and 16d gave
precursor ions at m/z 211, which upon CID gave rise to product
ions m/z 183, 111, and 73 (Figures 5B and 6B).
Conversion of the neutral pyridines 17 and 18 into pyridinium
ions 15 and 16 results in a small and, in the case of 16, barely
13
significant decrease in the 29Siꢀ C one-bond coupling constant
for the SiꢀCH2 bond (Table 7). In contrast, the one-bond 29Siꢀ
13C coupling constants for the SiꢀCH3 bonds, which are not
involved in this interaction, remain essentially unchanged, while
the 29Si chemical shifts undergo only a slight, but measurable,
downfield shift. These spectroscopic effects, which are consistent
with small contributions of the resonance forms 150, 1500 and 160,
1600 (Figure 3) to the ground-state structures, are substantially
smaller than those observed for the silylmethylpyrininium ions
1aꢀc and 2aꢀc, where conversion from the neutral pyridine
13
precursors results in a decrease in the 29Siꢀ C one-bond
coupling constant by ca. 7ꢀ9 Hz, while the 29Si chemical shifts
move downfield by 7ꢀ12 ppm. This is consistent with the β
effect of silicon being substantially stronger than the δ effect.9
With the view to investigating whether the through-bond
interaction between the trimethylsilyl substituent and the
electron-deficient pyridinium ion would lead to observable effects
Table 3. Changes to Selected Bond Distances That Occur
upon Conversion of the Neutral Precursors 17 and 18 into the
Pyridinium Ions 15 and 16
Δ(15ꢀ17)
M
MꢀC1
C3ꢀC(Ar)
C1ꢀC2
C2ꢀC3
Table 4. NBO Interaction Energies for the 2-Me3M(CH2)3-
Substituted Pyridinium Ions 15a
Si
+0.017
+0.022
+0.025
+0.028
ꢀ0.010
ꢀ0.010
ꢀ0.010
ꢀ0.030
ꢀ0.004
ꢀ0.004
ꢀ0.005
ꢀ0.005
+0.015
+0.017
+0.019
+0.020
Ge
Sn
Pb
Δ(16ꢀ18)
M
σMꢀC1ꢀσ*C1ꢀC2 (kJ molꢀ1
)
σC2ꢀC3ꢀπ* (kJ molꢀ1
)
MꢀC1
C3ꢀC(Ar)
C1ꢀC2
C2ꢀC3
Si
16.4 (13.8)
19.4 (16.3)
23.7 (19.7)
22.3 (20.5)
27.7 (16.9)
28.1 (16.9)
28.5 (16.9)
28.2 (16.9)
Si
+0.013
+0.016
+0.021
+0.023
ꢀ0.013
ꢀ0.013
ꢀ0.014
ꢀ0.015
ꢀ0.004
ꢀ0.004
ꢀ0.005
ꢀ0.006
+0.013
+0.015
+0.020
+0.018
Ge
Sn
Pb
Ge
Sn
Pb
a Energies for the neutral pyridine precursor 17 are given in parentheses.
Figure 3. Resonance forms representing the through-bond interactions in the pyridinium ions 15 and 16.
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When the product ions at m/z 180 (Figures 5A and 6A) (or
m/z 183 (Figures 5B and 6B)) derived from CID of 15 and 16
(or 15d and 16d) were isolated and then subjected to further
CID, the spectra shown in parts A and B of Figure 7 were
recorded. These spectra are essentially identical with those
previously reported for the CID of 2- and 4-trimethylsilylmethyl
pyridinium ions 1a and 2a (and their corresponding N-CD3
derivatives).15 Thus, we propose that the ions m/z 180 derived
from 15 and 16 correspond to ions 1a and 2a by formal loss of
ethylene from the parent ion. The fragmentations that result
upon CID of these ions (m/z 180) are summarized in Schemes 2
and 3 respectively.
Thus, CID of m/z 180 derived from 15 gave rise to ions at m/z
108 and 106, which have previously been assigned to the
structures 24 and 25, in addition to ions m/z 73 and 91, which
correspond to the Me3Si+ ion and its H2O adduct, respectively
(Scheme 2). These ions arise from competing proton transfer
(path a), hydride transfer (path b) between the enamine 26 and
Me3Si+ and dissociation (path c) from the ionꢀmolecule complex
Table 5. NBO Interaction Energies for the 4-Me3M(CH2)3-
Substituted Pyridinium Ions 16a
13
13
Table 7. 29Siꢀ CH2 and 29Siꢀ CH3 Coupling Constants
((0.5 Hz) and 29Si NMR Data ((0.01 ppm) for Pyridinium
Ions 15 and 16 and Their Neutral Precursors 17 and 18
M
σMꢀC1ꢀσ*C1ꢀC2 (kJ molꢀ1
)
σC2ꢀC3ꢀπ* (kJ molꢀ1
)
compd
1J(29Siꢀ13CH2)
1J(29Siꢀ13CH3)
δ(29Si)
Si
Ge
Sn
Pb
16.0 (13.8)
18.9 (16.3)
23.1 (19.9)
21.9 (20.8)
24.9 (18.3)
25.2 (18.2)
25.6 (18.4)
25.4 (18.4)
17 (M = Si)
18 (M = Si)
15 (M = Si)
16 (M = Si)
51.6
51.2
49.7
50.3
50.2
50.6
50.9
50.8
1.25
1.36
1.55
1.42
a Energies for the neutral pyridine precursor 18 are given in parentheses.
Table 6. Isodesmic Equations for Me3M(CH2)3-Substituted (M = Si, Ge, Sn, Pb) Pyridinium Cations
ΔE (kJ molꢀ1
)
M
eq 1
eq 2
Si
ꢀ5.8
ꢀ6.5
ꢀ7.8
ꢀ8.8
ꢀ5.4
ꢀ6.4
ꢀ7.7
ꢀ8.7
Ge
Sn
Pb
Scheme 1. Synthesis of the Pyridine Precursors 17 and 18a
a Reagents and conditions: (a) HCCCH2SiMe3, CuI, PdCl2(Ph3P), Et3N; (b) H2, Pd 10% on carbon, MeOH; (c) MeI.
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Figure 4. Thermal ellipsoid plot for 17aH+ picrate. Ellipsoids are
shown at the 20% probability level. The picrate counterion has been
omitted for clarity.
Figure 6. LTQ low-energy CID spectra of (A) N-CD3-2-trimethylsi-
lylpropylpyridine 15d and (B) N-CD3-4-trimethylsilylmethylpyridi-
nium 16d. The red star denots the mass-selected precursor ion.
Figure 5. LTQ low-energy CID spectra of (A) N-CH3-2-trimethylsi-
lylpropylpyridine 15 and (B) N-CH3-4-trimethylsilylmethylpyridinium
16. The red star denotes the mass-selected precursor ion.
(IMC). CID of ion m/z 180 derived from 16, assigned as 2a,
gives rise to similar types of fragment ions at m/z 108 and 106
(paths a and b), identified as 27 and 28, in addition to Me3Si+ ion
and its H2O addition peak at m/z 73 and 91, respectively (path
c), which arise from the ionꢀmolecule complex (Scheme 3); in
addition, the radical cation 29 arises directly by loss of CH3
radical from 2a (path d).
Isolation and CID of the ions m/z 208 derived from 15a and
16a give ions at m/z 180 which correspond to the trimethylsi-
lylmethyl-substituted pyridinium ions 1a and 2a. In addition,
ions at m/z 108, 73, and 91 are isolated, which must arise directly
as a result of excitation of the parent ions m/z 208. To accom-
modate these observations, we propose that CID of the parent
ion m/z 208 for 15a results in the formation of ionꢀmolecule
complex 1 (IMC1; Scheme 4, path a, with an analogous ionꢀ
molecule complex being formed from 16b), consisting of the
enamine 26 and the stabilized β-silylethyl cation (as either an
open or bridged cation); notably this ionꢀmolecule complex
Figure 7. LTQ low-energy MS3 CID spectra of (A) N-CH3-2-tri-
methylsilylpropylpyridine and (B) N-CH3-4-trimethylsilylpropylpyridi-
nium. The red star denotes the mass-selected precursor ion. The water
adduct [73 + H2O] is denoted by f.
resembles the resonance form 15a (Figure 3), which charac-
terizes the through-bond interaction between the SiꢀCH2 bond
and the pyridine π* orbital. Loss of ethylene from the β-silylethyl
cation within IMC1 could in principle give IMC2 (path b),
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Scheme 2. Suggested Pathway for the CID Reaction of N-Methyl-2-trimethysilylpropylpyridinium Ion 15
Scheme 3. Suggested Pathway for the CID Reaction of N-Methyl-4-trimethysilylpropylpyridinium Ion 16
Scheme 4. Potential Fragmentation Pathways of N-Methyl-2-trimethylsilylpropylpyridinium Ion 15a
consisting of Me3Si+ ion, ethylene, and the enamine 26; however,
if IMC2 is formed, then it is unlikely that it has a sufficient life-
time for further reaction within this complex before dissociation
occurs to give the Me3Si+ ion and its H2O adduct at m/z 73 and
91, respectively, otherwise the ion 25 at m/z 106 should have
been observed (by analogy with Scheme 2). Thus, we also believe
that neither the ion 1a at m/z 180 nor the ion 24 at m/z 108 arise
from IMC2. There are plausible pathways for the formation of
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Scheme 5. Proposed π-Ligand Exchange between Enamine
26 and Ethylene
2-Trimethylsilyl(propargyl)pyridine (22). To a solution of 2-bromo-
pyridine (0.32 g, 2.02 mmol) in dry triethylamine (10 mL) and
trimethyl(propargyl)silane (0.25 g, 2.23 mmol) at 0 °C under N2 were
added copper iodide (0.10 mmol, 0.02 g) and dichloro[bis(triphenyl-
phosphine)]Pd(II) (0.03 g, 0.04 mmol), and the resulting solution was
stirred for 1 h. The reaction mixture was then warmed to room temper-
ature and stirred for 3 days. The mixture was diluted with H2O (20 mL)
and extracted with ether (2 ꢁ 50 mL). The organic layer was washed
with H2O (3 ꢁ 30 mL) and then with saturated ammonium chloride
solution. The solution was dried (MgSO4), and the solvent was removed
under reduced pressure and the product purified by dry-flash column
chromatography (n-hexane/ether) to yield 2-trimethyl(propargyl)pyridine
(22; 0.10 g, 0.5 mmol, 25% yield). 1H NMR (500 MHz, CDCl3): δ 8.52
(1H, d, J = 4.9 Hz), 7.59 (1H, ddd, J = 7.8, 7.7, 2.0), 7.14 (1H, m), 7.3
(1H, d, J = 7.8 Hz), 1.75 (2H, s), 0.18 (9H, s, Si(CH3)3). 13C NMR (125
MHz, CDCl3): δ 149.8, 143.9, 136.0, 126.6, 122.3, 86.7, 79.6, 4.3,
1.3. ESI/MS: [M + CH3]+ 204.120 30 (calculated for C12H18SiN+
204.120 85).
Scheme 6. Grob Fragmentation of Pyridinium Ion 15
4-Trimethylsilyl(propargyl)pyridine (23). To a solution of 4-bromo-
pyridineꢀHCl (0.64 g, 4.04 mmol) in water (20 mL) was added NaOH
until the solution was basified. The resulting 4-bromopyridne was then
extracted into ether (2 ꢁ 20 mL) and dried (MgSO4) and the solvent
removed under reduced pressure. The 4-bromopyridine was then added
to dry triethylamine (10 mL), and trimethyl(propargyl)silane (0.5 g,
4.43 mmol, 0.66 mL) under N2, copper iodide (0.20 mmol, 0.04 g), and
dichloro[bis(triphenylphosphine)]palladium(II) (0.06 g, 0.08 mmol)
were added at 0 °C, and the resulting mixture was stirred for 1 h. The
reaction mixture was then warmed to room temperature and stirred for
3 days. The solution was quenched with H2O (20 mL) and extracted
with ether (50 mL). The organic layer was washed with H2O (3 ꢁ 30 mL)
and then with saturated ammonium chloride solution. The solution was
dried (MgSO4), and the solvent was removed under reduced pressure
and the compound purified by dry-flash column chromatography (n-
hexane/ether) to yield 4-trimethyl(propargyl)pyridine (23; 0.15 g, 0.8
mmol, 20% yield). 1H NMR (500 MHz, CDCl3): δ 8.49 (2H, d, J = 5.2
Hz), 7.47 (2H, m), 1.73 (2H, s), 0.16 (9H, s, Si(CH3)3). 13C NMR (125
MHz, CDCl3): δ 150.7, 149.4, 127.2, 74.4, 64.7, 7.7, ꢀ2.03. ESI/MS:
[M + H]+ 190.104 44 (calculated for C11H16SiN+ 190.104 65).
2-Trimethylsilylpropylpyridine (17). To a solution of 2-trimethylsilyl-
(propargyl)pyridine (22; 0.10 g, 0.5 mmol) in methanol (50 mL) was
added a catalytic amount of Pd (10 wt % on carbon) and the system
stirred under an atmosphere of H2 for 48 h. The solution was filtered
through Celite, the solvent was removed under reduced pressure, and
the compound was purified via an acidꢀbase extraction to yield
these two ions by reaction within IMC1. Thus, ion 24 (m/z 108)
can arise by proton transfer from the trimethylsilylethyl cation to
the basic enamine 26 with the formation of vinyltrimethylsilane
(path c), while the ion at m/z 180 (1a) can arise by formal π-
ligand exchange of Me3Si+ between ethylene and the enamine 26
(path d), as shown in Scheme 5.
We also cannot discount direct conversion of the parent ion 15
at m/z 208 into the IMC2 (Scheme 4, path e) by Grob frag-
mentation (Scheme 6) with subsequent dissociation to give ion
m/z 73.
’ CONCLUSION
Computational studies on the pyridinium ions 15 and 16 (M =
Si, Ge, Sn, Pb) provide evidence for a weak through-bond (double
hyperconjugative) interaction between the MꢀCH2 bond and
the low-lying π* orbital of the pyridinium ion. The strength of
this interaction increases in the order Si < Ge < Sn < Pb, in line
with the σ-donor abilities of the CꢀM bond. Some evidence
for the through-bond interaction for M = Si in solution is
provided by 13C and 29Si NMR studies; although the changes
in the 29Si chemical shift are measurable, the changes in the
29
13Cꢀ Si coupling constants are barely so. Fragmentation
pathways followed by the ions 15 and 16 in the gas phase
under CID, however, are strongly influenced by the through-
bond interaction, such that the bonds which are weakened by
this interaction are the most readily fragmented. This is
consistent with the expectation that hyperconjugative effects
on transition states are substantially larger than the effects on
the ground state.
1
2-trimethylsilylpropylpyridine (17; 0.04 g, 0.2 mmol, 41% yield). H
NMR (500 MHz, CDCl3): δ 8.50 (1H, d, J = 4.5 Hz), 7.57 (1H, ddd, J =
7.5, 7.5, 2.0 Hz), 7.12 (1H, d, J = 7.5 Hz), 7.08 (1H, m), 2.78 (2H, t, J =
8.0, 7.5 Hz), 1.72 (2H, m), 0.55 (2H, m) ꢀ0.04 (s, 9H). 13C NMR (125
MHz, CDCl3): δ 161.3, 149.1, 136.2, 127.8, 120.8, 42.2, 24.6, 16.7
(1J(CꢀSi(CH3)3) = 55.3 Hz), ꢀ1.75 (1J(Si(CH3)3) = 50.2 Hz). ESI/
MS: [M + CH3]+ 208.151 55 (calculated for C12H22NSi+ 208.151 60).
29Si NMR (80 MHz, CDCl3): δ 1.25.
’ EXPERIMENTAL SECTION
4-Trimethylsilylpropylpyridine (18). To a solution of 4-trimethylsilyl-
(propargyl)pyridine (23; 0.15 g, 0.8 mmol) in methanol (50 mL) was
added a catalytic amount of Pd (10 wt % on carbon) and the system stirred
under H2 for 48 h. The solution wasfiltered through Celite, the solvent was
removed under reduced pressure, and the compound was purified via an
acidꢀbase extraction to yield 4-trimethylsilylpropylpyridine (18; 0.02 g,
0.11 mol, 13% yield). 1H NMR (500 MHz, CDCl3): δ 8.48 (2H, broad
doublet), 7.08 (2H, broad doublet), 2.6 (2H, m), 1.62 (2H, m), 0.51 (2H,
m), ꢀ0.033 (9H, s). 13C NMR (125 MHz, CDCl3): δ 151.3, 149.3, 123.9,
39.0, 24.6, 16.3 (1J(CꢀSi(CH3)3) = 51.2 Hz), ꢀ1.67 (1J(Si(CH3)3) =
50.6 Hz). ESI/MS: [M + H]+ 194.133 64 (calculated for C11H22NSi+
194.133 64). 29Si NMR (80 MHz, CDCl3): δ 1.36.
Synthesis. All reactions were conducted in oven-dried glassware
under a nitrogen atmosphere. Solvents were anhydrous ,and all fine
chemicals were used as received from Sigma-Aldrich Chemical Co.
Petroleum spirit refers to the fraction that boils at 40ꢀ60 °C. Analytical
thin layer chromatography (TLC) was carried out using aluminum-
backed 2 mm thick Merck silica gel 60 GF256. Compounds were
1
visualized under UV 365 nm light or by iodine. H and 13C NMR
spectra were recorded as solutions in solvents indicated on an Inova 400
or Inova 500 spectrometer. 1H and 13C spectra were reported as
chemical shift (ppm), followed by (in parentheses, where applicable)
integration, multiplicity, coupling constant, and peak assignment.
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Organometallics
ARTICLE
N-Methyl-2-trimethylsilylpropylpyridinium Tosylate (15 Ots). A
’ ASSOCIATED CONTENT
3
solution of the pyridine derivative 17 (50 mg) in deuteriochloroform
(0.5 mL) was treated with neat methyl tosylate (1.05 equiv) and the
solution left overnight. 1H NMR (500 MHz, CDCl3): δ 8.76 (1H, m),
8.34 (1H, t, J = 7.9, 7.8 Hz), 7.78 (2H, m), 3.05 (1H, m), 1.76 (1H, m),
0.63 (2H, m), ꢀ0.01, (9H, s). 13C NMR (125 MHz, CDCl3): δ 158.6,
146.6, 145.2, 128.1, 22.2, 16.4 (1J(CꢀSi(CH3)3) = 49.7 Hz), ꢀ2.1
(1J(Si(CH3)3) = 50.9 Hz), 45.6. 29Si NMR (80 MHz, CDCl3): δ 1.55.
S
Supporting Information. Full listings of geometries and
b
energies (Gaussian Archive entries) for ground-state structures and
a CIF file giving crystallographic data for 17aH+. This material is
available free of charge via the Internet at http://pubs.acs.org. The
crystal data have also been deposited at the Cambridge Crystal-
lographic Data Centre and assigned CCDC code 845803.
N-Methyl-4-trimethylsilylpropylpyridinium Tosylate (16 Ots). A
3
solution of the pyridine derivative 18 (50 mg) in deuteriochloroform
(0.5 mL) was treated with neat methyl tosylate (1.05 equiv) and the
solution left overnight. 1H NMR (500 MHz, CDCl3): δ 8.96 (2H, d, J =
6.5 Hz), 7.71 (2H, m), 7.56 (2H, d, J = 6.5 Hz), 7.30 (2H, m), 4.34 (3H,
s, NꢀCH3), 3.67 (2H, s), 2.74 (2H, m), 2.39 (3H, s), 2.26 (3H, s), 1.58
(2H, m), 0.45 (2H, m), ꢀ0.08 (s, 9H). 13C NMR (125 MHz, CDCl3): δ
162.0, 144.8, 131.8, 129.7, 128.4, 127.7, 127.4, 56.2, 48.0 (NꢀCH3).
39.4, 24.8, 21.6, 15.7 (1J(CꢀSi(CH3)3) = 50.3 Hz), ꢀ2.5 (1J(Si(CH3)3) =
50.8 Hz). 29Si NMR (80 MHz, CDCl3): δ 1.42.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: whitejm@unimelb.edu.au.
’ ACKNOWLEDGMENT
We thank the Australian Research Council for financial
support (DP0770565) and an award of an APA to A.K. We also
thank the Victorian Partnership for Advanced Computing and
the Victorian Institute for Chemical Sciences High Performance
Computing Facility for computational time.
NMR Measurements To Determine the Influence of the
13
Silicon γ Effect on 29Si Chemical Shifts and 29Siꢀ C Cou-
pling Constants. Both 13C and 29Si NMR were run for 16 h for
compounds 15a, 16a, 17a, and 18. The resolution of the spectra can be
calculated by dividing the width (in Hz) by the square of the FT size. The
square of the FT size is required because the FT is composed of real and
imaginary points and the spectrum consists of real points only. Thus, for
the 13C NMR the digital resolution is 30 188.7/(131 072) ꢁ 2 = 0.46 Hz
and for 29Si the digital resolution is 59 612.5/(131 072) ꢁ 2 = 0.91 Hz.
Crystallography. Intensity data were collected with an Oxford
Diffraction SuperNova CCD diffractometer using Cu Kα radiation
(graphite crystal monochromator, λ = 1.541 84 Å); the temperature
during data collection was maintained at 130.0(2) K using an Oxford
Cryosystems cooling device.
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