Gas-phase arylmethyl transfer and cyclodeamination of argentinated N-arylmethyl-pyridin-2-ylmethanimine

Article abstract of DOI:10.1007/s13361-013-0778-7

In collisional activation of argentinated N-arylmethyl-pyridin-2- ylmethanimine, a neutral molecule of AgNH₂ is eliminated, carrying one hydrogen from the methylene and the other one from the ortho position (relative to the ipso carbon) of the aryl ring. Taking argentinated N-benzyl-pyridin-2-ylmethanimine for example, the proposition that the AgNH ₂ loss results from intramolecular arylmethyl transfer combined with cyclodeamination is rationalized by deuterium labeling experiments, blocking experiments, and theoretical calculations. The structure of the final product ion from loss of AgNH₂ was confirmed further by multistage mass spectrometry. [Figure not available: see fulltext.]

Full text of DOI:10.1007/s13361-013-0778-7

B American Society for Mass Spectrometry, 2013
J. Am. Soc. Mass Spectrom. (2014) 25:169Y175
DOI: 10.1007/s13361-013-0778-7
RESEARCH ARTICLE
Gas-Phase Arylmethyl Transfer and Cyclodeamination
of Argentinated N-Arylmethyl-Pyridin-2-Ylmethanimine
Hezhi Sun, Lin Wang, Yuanjiang Pan
Department of Chemistry, Zhejiang University, 38 Zheda Road, Hangzhou, 310027, China
Abstract. In collisional activation of argentinated N-arylmethyl-pyridin-2-
ylmethanimine, a neutral molecule of AgNH2 is eliminated, carrying one hydrogen
from the methylene and the other one from the ortho position (relative to the ipso
carbon) of the aryl ring. Taking argentinated N-benzyl-pyridin-2-ylmethanimine for
example, the proposition that the AgNH2 loss results from intramolecular
arylmethyl transfer combined with cyclodeamination is rationalized by deuterium
labeling experiments, blocking experiments, and theoretical calculations. The
structure of the final product ion from loss of AgNH2 was confirmed further by
multistage mass spectrometry.
Key words: Collisional activation, Arylmethyl transfer, Cyclodeamination, AgNH2 loss
Received: 15 July 2013/Revised: 16 October 2013/Accepted: 23 October 2013/Published online: 20 December 2013
fragmentation pathways (compared with the protonated
Introduction
species) [14–16].
lectrospray ionization mass spectrometry (ESI-MS) is
Generally, in the vast majority of unimolecular
E
efficient in producing gas-phase metal-containing ions
dissociation reactions involving coordination of silver
ions with organic ligands, product ions retaining the
silver ion are commonly observed except when elimi-
nation of a neutral molecule such as AgR (R = CH3,
C6H5) [8], RCOOAg [10], or AgH [4–9, 17] takes
place. However, in the collisional activations of silver/
chalcone complexes, another example of AgH loss has been
discovered resulting from Oxo-Diels-Alder reaction rather than
1,2-elimination, while a peculiar AgOH loss is confirmed as
the consequence of Nazarov cyclization [18].
The gas-phase chemistry of compounds containing
benzyl cation has been extensively and systematically
investigated in our group [19–22], and reaction observed
in mass spectrometry includes hydride transfer, proton
transfer, electron transfer, nucleophilic aromatic substitu-
tion, and electrophilic aromatic substitution. Accompa-
nied by these gaseous reactions, particularly nucleophilic
and electrophilic aromatic substitutions, benzyl cation
transfer takes place during the fragmentations of a
variety of benzylated cations [20–26], in which the
benzyl cation is generated with ease, although tropylium
also can be formed in some special cases [27–29].
The fragmentations of silver-containing compounds
normally provide structural information complementary
to that obtained from protonated ones. In an attempt to
access the effect of silver cation on altering fragmenta-
and has become a powerful tool in studies on the interaction
between metal ions and organic molecules, such as the
biological function of enzymes to coordination chemistry
and catalysis.
Among those metal cations studied, including main
group metal (Li, Na, K, Mg, Al) and transition-metal
(Fe, Co, Ni, Cu), addition of silver cation for mass
spectrometric analysis is an especially popular choice
because of its affinity for numerous organic compounds
along with its characteristic isotopic abundances, which
makes the signals of silver-containing ions in mass
spectra much easier to recognize. The electrospray mass
spectrometry of Ag+-containing ions has been studied in
the analysis of aromatic hydrocarbons [1, 2], amines [3–
9], unstaturated lipids [10], and saccharides [11] over the
years. Aside from simply enabling ionization of nonpolar
compounds or improving the selectivity and sensitivity of
the mass spectrometric methods, silver complexation can
enhance isomer differentiation [12, 13] and even assist
peptide sequencing to a certain extent by altering
Electronic supplementary material The online version of this article
(doi:10.1007/s13361-013-0778-7) contains supplementary material, which
is available to authorized users.
Correspondence to: Yuanjiang Pan; e-mail: panyuanjiang@zju.edu.cn

170
H. Sun et al.: Arylmethyl Transfer and Cyclodeamination
tions, we have examined the collisional activations of
argentinated N-arylmethyl-pyridin-2-ylmethanimines. An
interesting AgNH2 loss is discovered and interpreted,
which is not observed with the protonated molecules
(Supplementary Figure S1). The reaction involves
arylmethyl transfer along with cyclodeamination.
Mass Spectrometry
The CID experiments were performed in the positive ion
mode using a Bruker Esquire 3000plus mass spectrometer
equipped with an ESI source and an ion trap analyzer.
Nitrogen was used as the drying gas at a flow rate of
5 L min–1 and the nebulizing gas at a pressure of 10 psi. The
drying gas temperature was set at 250 °C and the capillary
voltage was set at –4000 V. The CID mass spectra were
obtained with helium as the collision gas at appropriate
collision energy after isolation of the desired precursor ion.
All compounds were purified after synthesis and mixed
with 1 equiv of silver nitrate aqueous solution in methanol.
The mixtures were infused to the mass spectrometer at a
final concentration of approximate 1 μg mL–1, with a
syringe pump at a flow rate of 180 μL h–1. All product ion
spectra involving Ag+ are those of the 107Ag isotope and no
subsequent isotope identification was made.
Experimental
Chemicals
The N-arylmethyl-pyridin-2-ylmethanimines (series I), shown
in Table 1, were obtained in the same way as in a previous
paper [30]. N-arylmethyl-phenylmethylideneamines, N-
arylmethyl-furylmethylideneamines, and N-arylmethyl-
thienylmethylideneamines were synthesized in a similar
manner with corresponding aromatic aldehyde and
amines. The acridizinium was synthesized as described
elsewhere [31]. The structures of all compounds were
1
confirmed by H and 13C NMR and high resolution mass
Theoretical Calculations
spectrometry. N-benzyl-pyridin-2-ylmethanimine, 1H
NMR (500 MHz, CDCl3): δ (ppm) = 8.67 (d, 1H),
8.52 (s, 1H), 8.09 (d, 1H), 7.76 (d, 1H), 7.38–7.29 (m,
6H), 4.91 (s, 2H); 13C NMR (150 MHz, CDCl3): δ
(ppm) = 163.0, 154.7, 149.6, 138.9, 136.8, 128.8, 128.4,
127.4, 125.1, 121.6, 65.1. Acridizinium, 1H NMR
(500 MHz, DMSO-d6): δ (ppm) = 10.53 (s, 1H), 9.32
(d, 1H), 9.27 (s, 1H), 8.59 (d, 1H), 8.49 (d, 1H), 8.41
( d, 1H), 8.15 (t, 1H), 8.10 ( t, 1H), 8.01 ( t, 1H), 7.97
(t, 1H): 13C NMR (DMSO-d6): δ (ppm) = 140.8, 138.0,
135.8, 135.1, 134.7, 131.5, 131.5, 128.6, 127.6, 127.3,
126.3, 125.2, 122.8.
Theoretical calculations were carried out using the
Gaussian 03 package of programs [32]. All structures
were optimized without symmetry constraints by density
functional theory (DFT) using the RB3LYP hybrid
method: LANL2DZ basis set for silver while 6-311 +
G(d,p) basis set for the other atoms. The structures of the
two minima associated with each transition state were
established by performing intrinsic reaction coordinate (IRC)
calculations. The vibrational frequency analysis of all opti-
mized structures was calculated to ensure they corresponded to
either true minima (no imaginary frequencies) or transition
Table 1. Major Ions in the CID Mass Spectra of the [M + Ag]+ Ions of N-Arylmethyl-Pyridin-2-Ylmethanimines
Cmpd.
1
R
[M+Ag]+
303
Ion A
Ion B
Ion C
Other ions
-
180 (16)a 195 (100) 196 (10)
2
3
293
309
170 (6)
185 (91) 186 (100) 213 (14)
186 (22) 201 (100) 202 (70)
-
4
5
353
253
230 (8) 245 (100)
-
229 (30)
-
130 (12) 145 (100) 146 (14)
a
m/z (relative abundance, %)

H. Sun et al.: Arylmethyl Transfer and Cyclodeamination
171
Figure 1. CID mass spectra of [M + Ag]+ ion of (a) N-benzyl-
pyridin-2-ylmethanimine, (b) 1-phenyl-N-(pyridin-2-
ylmethylene)methanimine-1,1-d2, and (c) 1-( phenyl- d5)-N-
(pyridin-2-ylmethylene)methanamine
Figure 3. CID mass spectra of argentinated (a) N-benzyl-
phenylmethanimine, (b) N-benzyl-furan-2-ylmethanimine,
and (c) N-benzyl-thiophen-2-ylmethanimine
states (one imaginary frequency). Considering the existence of
the weakly bonded interactions involved in this paper, a
vibrational frequency scaling factor of 0.967 is applied to the
B3LYP/6-311G + (d,p) harmonic vibrational frequencies,
which is taken from the NIST database available on the
Internet [33]. Zero-point energies (ZPE) for all the key species
were calculated at the same level of theory. The effect of basis
set superposition error (BSSE) was analyzed as well by means
of the counterpoise correction method. The energies discussed
below are the Gibbs free energies.
Figure 2. CID mass spectra of [M + Ag]+ ions of N-benzyl-pyridin-2-ylmethanimines with different chlorine substitution on
benzyl ring: (a) 4′-chlorobenzyl, (b) 3′,5′-dichlorobenzyl, (c) 2′,6′-dichlorobenzyl, and (d) 2′,4′-dichlorobenzyl. The signals
corresponding to AgNH2 loss are marked with asterisks for clarity

172
H. Sun et al.: Arylmethyl Transfer and Cyclodeamination
Scheme 1. Proposed mechanism of AgNH2 loss from aromatic cyclodeamination reaction combined with benzyl transfer
under the effect of silver cation as Lewis acid
ascertains that one hydrogen employed in the process of
building a molecular AgNH2 comes from the methylene
Results and Discussion
The CID spectra of a series of argentinated N-arylmethyl-
pyridin-2-ylmethanimines are summarized in Table 1. A
significant elimination of AgNH2 (ion A) and a dominating but
previously reported elimination of AgH (ion B) were observed,
besides the loss of silver atom (ion C). In such a loss of AgNH2,
the mechanism is not straightforward since the argentinated
molecule should undergo a complicated skeletal rearrangement
to break the three bonds attached to the imino nitrogen.
For simplification, the argentinated N-benzyl-pyridin-2-
ylmethanimine is used as the model complex. All the
fragment ions observed in the spectrum of the model
complex (Figure 1a) have been confirmed by the accurate
mass determined on an ESI-FTICR mass spectrometer
(Supplementary Table S1).
while the other one is from the phenyl ring.
To further understand which hydrogen on the phenyl ring
is involved, the effect of chlorine substitution on the AgNH2
loss was studied (Figure 2). When para or meta sites of the
phenyl ring are occupied by chlorine, the signal correspond-
ing to the AgNH2 loss can be observed (m/z 214 in Figure 2a
and m/z 248 in Figure 2b). However, it disappears only
when both ortho sites are blocked (Figure 2c and d). Hence,
the arylmethyl moieties should provide at least one ortho
aromatic hydrogen for the elimination of AgNH2. Support
for this proposition was also obtained from the AgNH2
elimination in CID fragmentations of argentinated Com-
pounds 2, 3, and 4 (Table 1), where the phenyl ring is
replaced by furyl, thienyl, and naphthyl. For a
heteroaromatic ring, ortho site refers to the neighboring site
to the ipso carbon.
When either the methylene or phenyl ring of the model
complex are deuterated (as depicted in Figure 1b and c), one
deuterium atom is exclusively eliminated as AgNHD in each
case (signal at m/z 181 and 184, respectively), which
On the other hand, when the pyridyl moiety of the model
complex is replaced by phenyl, furyl, or thienyl (Figure 3),
Scheme 2. Proposed mechanism of AgNH2 loss from aromatic cyclodeamination reaction combined with benzyl transfer with
silver-mediated C-H activation involved

H. Sun et al.: Arylmethyl Transfer and Cyclodeamination
173
Figure 4. DFT potential energy surfaces of different pathways for elimination of AgNH2. Path (A) is aromatic cyclodeamination
coming after benzyl transfer under the effect of silver cation as Lewis acid. Path (B) is silver-mediated C-H activation followed
by benzyl transfer and aromatic cyclodeamination
the formation of benzyl cation (m/z 91) occurs instead of the
elimination of AgNH2, implying that the pyridyl nitrogen
plays a part in AgNH2 loss.
The ortho phenyl hydrogen is first activated by silver cation
and transfers to imino nitrogen, where methylene hydrogen
transfer follows as well after benzyl transfer. Lastly,
cyclodeamination results in the AgNH2 loss. Despite the
sequence of two proton transfers (from methylene and ortho
site of phenyl ring to the imino nitrogen), the benzyl transfer
together with cyclodeamination are the common steps to
eliminate the AgNH2 in those two mechanisms.
To quantitatively describe the energy requirements of two
pathways of AgNH2 loss, theoretical calculations were
performed and potential energy surfaces were obtained, as
shown in Figure 2. Because the product ions arising from
path A and path B possess the same structures at
21.8 kcal mol–1, the transition states for those two possible
In order to rationalize the present experimental results, a
plausible mechanism based on the known benzyl rearrange-
ment in the gas-phase fragmentation of numerous benzylated
cations is postulated in Scheme 1, where the pyridyl nitrogen
functions as the site for primal benzyl transfer. After the
benzyl transfer, the imino nitrogen carries out nucleophilic
attack on the methylene hydrogen via a six-membered ring,
under the role of silver cation as Lewis acid. Eventually, the
AgNH2 is eliminated through cyclodeamination.
Considering the possibility of silver-mediated C-H
activation [34], another mechanism is shown in Scheme 2.

174
H. Sun et al.: Arylmethyl Transfer and Cyclodeamination
virtually identical CID spectra of these two ions (Figure 5)
verifies the structure of ion A as acridizinium, which is a
compelling experimental evidence for the existence of
benzyl rearrangement and cyclodeamination during the
elimination of AgNH2.
Conclusions
Argentinated N-arylmethyl-pyridin-2-ylmethanimine investi-
gated by ESI-MS/MS equipped with an ion trap cell
eliminates a molecular AgNH2, two hydrogens of which
derive from the methylene and ortho position (relative to the
ipso carbon) of the aryl ring, respectively. The arylmethyl
transfer combined with cyclodeamination are theoretically
and experimentally verified as the reason for AgNH2 loss.
The most convincing evidence lies in the structural confir-
mation of the product ion, which was confirmed as
acridizinium for its highly similar fragmentations. Instead
of the direct dissociation to form arylmethyl cation in
fragmentation of protonated N-arylmethyl-pyridin-2-
ylmethanimine, these compounds show a notable gas-phase
cyclodeamination combined with arylmethyl transfer after
complex with silver cation. In addition to highlighting the
vital role that the attached cation plays in fragmentation
patterns, this research contributes to the development of
arylmethyl transfer and cyclodeamination as well.
Figure 5. CID spectra of the ion at m/z 180 derived from (a)
the fragmentation of the argentinated N-benzyl-pyridin-2-
ylmethanimine, and (b) acridizinium
pathways take control of the reactions regardless of the
product thermochemistry.
For the phenyl hydrogen transfer to imino nitrogen, the
transition-state energy barrier is 49.7 kcal mol–1 in path B by
silver-mediated C-H activation (TS2-0), lower (by
24.1 kcal mol–1, Figure 2) than that in path A by 1,3-proton
transfer (TS1-4). As for the 1,6-transfer of methylene
hydrogen to imino nitrogen, a significant barrier up to
74.9 kcal mol-1 exists in path B (TS2-2), higher (by
21.5 kcal mol–1, Figure 2) than that in path A (TS1-2).
With respect to cyclodeamination, cyclization in path B
proceeds via a considerably higher barrier (TS2-3) of
81.5 kcal mol–1, which is 10.7 kcal mol–1 above TS1-3 in
path A, whereas the energies of deamination in path A (TS1-
5) and path B (TS2-4) differ only by 5.6 kcal mol-1 from
each other. Briefly, the overall energy barrier for the
elimination of AgNH2 in path A (TS1-4, 73.8 kcal mol–1)
is lower by 7.7 kcal mol–1 than that in path B (TS2-3,
81.5 kcal mol–1), which suggests that the kinetically
preferred pathway is via path A or Scheme 1 (Figure 4).
By comprehensively comparing the energies of the
products and transition states bewteen AgNH2 loss and
AgH loss (Supplementary Figure S2), the model complex
prefers the elimination of AgH. This theoretical result is
consistent with the experimental distribution of the product
ions (Figure 1a), where the abundance of ion A (m/z 180) is
much lower than that of ion B (m/z 195).
Acknowledgment
The authors gratefully acknowledge financial support from
the National Science Foundation of China (21025207 and
21372199).
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