Reactions of nitrophenide and halonitrophenide ions with acrylonitrile and alkyl acrylates in the gas phase: The case of [M-2]- ion formation

Article abstract of DOI:10.1016/j.ijms.2011.12.015

The structure of [M-2]^- ions formed in reactions between α,β-unsaturated compounds (acrylonitrile and alkyl acrylates) and nitro- and p-halonitro-phenide ions have been studied both experimentally and theoretically. Although it is not possible to prove the origin of the [M-2] ^- ions directly clear evidence for intermediate β-adduct formation during reaction was found. More specifically, the mechanism involves transformation of the β-adduct comprising an O-atom transfer via a six-membered intermediate, followed by the loss of an HCOR molecule (R = CN, CO₂Me, CO₂Et). This sequence leads eventually to an [M-2]^- anion with an o-nitrosobenzylic anion structure. Further transformation of the o-nitrosobenzylic anion to the more stable o-aminobenzaldehyde anion was also considered but was ruled out.

Full text of DOI:10.1016/j.ijms.2011.12.015

International Journal of Mass Spectrometry 316–318 (2012) 76–83
Contents lists available at SciVerse ScienceDirect
International Journal of Mass Spectrometry
journal homepage: www.elsevier.com/locate/ijms
Reactions of nitrophenide and halonitrophenide ions with acrylonitrile and alkyl
acrylates in the gas phase: The case of [M−2]− ion formationଝ
Magdalena Zimnickaa,∗, Osamu Sekiguchib, Einar Uggerudb, Witold Danikiewicza
a Institute of Organic Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warszawa, Poland
b Centre for Theoretical and Computational Chemistry and Mass Spectrometry Laboratory, Department of Chemistry, University of Oslo, P.O. Box 1033, Blindern, N - 0315 Oslo, Norway
a r t i c l e i n f o
a b s t r a c t
The structure of [M−2]− ions formed in reactions between ␣,␤-unsaturated compounds (acrylonitrile
and alkyl acrylates) and nitro- and p-halonitro-phenide ions have been studied both experimentally and
theoretically. Although it is not possible to prove the origin of the [M−2]− ions directly clear evidence for
intermediate ␤-adduct formation during reaction was found. More specifically, the mechanism involves
transformation of the ␤-adduct comprising an O-atom transfer via a six-membered intermediate, fol-
lowed by the loss of an HCOR molecule (R = CN, CO2Me, CO2Et). This sequence leads eventually to an
[M−2]− anion with an o-nitrosobenzylic anion structure. Further transformation of the o-nitrosobenzylic
anion to the more stable o-aminobenzaldehyde anion was also considered but was ruled out.
© 2011 Elsevier B.V. All rights reserved.
Article history:
Received 28 October 2011
Received in revised form
16 December 2011
Accepted 21 December 2011
Available online 27 December 2011
This paper is dedicated to Professor Alex
Harrison on the occasion of his 80th
birthday.
Keywords:
Gas-phase reaction
(Halo)nitrophenide ion
␣,␤-Unsaturated compound
␤-Adduct
1. Introduction
nitromethyl anion and phenyl anion) [7] give rise to adduct for-
mation and subsequent rearrangement into conversion products.
Taking into account the importance of acrylonitrile and acry-
lates in the manufacture of acrylic emulsion polymers used in
paints, surface coatings for textiles, paper and leather, adhesives
and sealants, acrylic fibers, chemical intermediates, fragrance and
flavoring agent, in the synthesis of vitamin B1 [1] it is mandatory
to understand the factors that determine the outcome of reactions
involving these compounds.
Although such activated olefins are extensively used in organic
synthesis as reactants in condense-phase nucleophilic conju-
gate addition (Michael reaction) [2–4] their intrinsic gas-phase
reactivity, in particular with nucleophiles, is still incompletely
known limited to published studies by Bartmess [5], Bowie [6]
and Bernasconi [7]. One computational [8] study results showed
that the stability of the initially formed ␤-adduct is decisive in
determining reactivity. Only electron-deficient olefins reacting
with nucleophiles having stabilizing groups (cyanomethyl anion,
Adapting this insight we performed a number of reactions of
(halo)nitrophenide ions with ␣,␤-unsaturated compounds like
acrylonitrile, methyl acrylate and ethyl acrylate as gaseous neutral
reagents. The results of these reactions including product struc-
ture determination and elucidation of the corresponding reaction
pathways were reported in our recent paper [9]. The most sig-
nificant reaction pathways are presented in Scheme 1. It was
also reported in this paper, that the gas-phase ion-molecule reac-
tions of ␣,␤-unsaturated compounds with nitrophenide ions and
p-halonitrophenide ions lead to products having m/z values nom-
inally two mass units lower than the (halo)nitrophenide ion (M).
The mechanistic and structural details associated with ([M−2]−)
ion formation is the topic of the present work, and we report the
results of mass spectrometric experiments interpreted by means of
computational quantum chemistry.
2. Experimental
ଝ
Parts of this work were presented during the 2nd EuCheMS Chemistry Congress,
2.1. Mass spectrometry and quantum chemical calculations
Torino, Italy and during the 1st Conference of Polish Mass Spectrometry Society,
Puławy, Poland.
∗
The mass spectrometric methodology applied here was the
same as in our previous paper [9], including tandem mass
Corresponding author.
E-mail address: magdalena.zimnicka@icho.edu.pl (M. Zimnicka).
1387-3806/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ijms.2011.12.015

M. Zimnicka et al. / International Journal of Mass Spectrometry 316–318 (2012) 76–83
77
Scheme 1. General scheme of plausible pathways for the most significant reactions of ␣,␤-unsaturated compounds with isomeric nitrophenide ions and p-halonitrophenide
ions. Pathway (a) presents the addition of the respective nitrophenide ion to the carbonyl group of acrylate followed by the elimination of an alcohol molecule. Pathway (b)
comprises the ␤-adduct formation followed by the intramolecular substitution of the halogen atom.
spectrometry and accurate high resolution mass measurements
and is described briefly.
Reaction potential energy profiles and molecular properties
were computed applying density functional theory methods, at the
PBE1PBE/6-311+G(d,p)//PBE1PBE/6-31+G(d) level, as described in
the previous paper [9].
The gas-phase ion-molecular reactions of (halo)nitrophenide
ions with ␣,␤-unsaturated compounds as well as the fragmen-
tations of the reaction products were studied employing an API
365 triple quadrupole mass spectrometer (Applied Biosystems)
equipped with a TurboIonSprayTM electrospray ion source oper-
ated in the standard ESI mode. The examined phenide ions were
generated in the ion source from appropriately substituted ben-
zoate anions and under such ion source parameters to obtain the
highest possible abundance of the (halo)nitrophenide ions. After
mass selection with the use of the first quadrupole mass filter, the
anions were subject to the reactions with ␣,␤-unsaturated com-
pounds admixed to the nitrogen introduced into the collision cell.
The collision gas inlet has been modified to accommodate the intro-
duction of vapors independently from the curtain gas of the ESI
source. It is difficult to estimate the concentration of the substrate
present in the collision cell with sufficient accuracy to obtain the
corresponding coefficients, so our results are reported in a quali-
tative fashion only. However for each gas-phase reaction, we tried
to keep the same increase of pressure in mass spectrometer. The
nominal cell voltage was set to −5 eV relative to the immediate
surroundings to suppress collisionally induced dissociation (CID).
In order to obtain product CID spectra the ␣,␤-unsaturated com-
pound was injected directly into the medium pressure section of
the ESI ion source.
2.2. Reagents and reference compounds
Reagents and solvents (HPLC grade) were obtained from
commercial suppliers or were synthesized according to known
procedures. One reference compound which is not commercially
available was synthesized as described below.
(o-Nitrobenzyl)-cyanoacetic acid – This compound was obtained
in a two-step synthesis. In the first step, ethyl ester of the
(o-nitrobenzyl)-cyanoacetic acid was prepared. To the acetone
solution containing the 1.7 g (10 mmol) 2-nitrobenzyl chloride,
2.3 g (20 mmol) ethyl cyanoacetate and 0.2 g (0.5 mmol) tetrabuty-
lammonium bromide the potassium carbonate was added. The
solution was stirred at ambient temperature for 4 h and ethyl
acetate was then added. Thereafter the resulting solution was
washed with water and brine. The solvent was removed under
reduced pressure and the residue was purified by chromatography
applying silica gel using hexane–ethyl acetate (5:1) as eluent. The
product obtained in this first step, the ethyl ester of (o-nitrobenzyl)-
cyanoacetic acid, was then subject to the alkaline hydrolysis. The
resulting methanol solution of the ethyl ester of (o-nitrobenzyl)-
cyanoacetic acid (0.4 g (2 mmol)) was treated with an aqueous
solution of NaOH (0.2 g NaOH in 1 ml of water). After the hydrol-
ysis was completed the resulting solution was acidified with 10%
hydrochloric acid. The precipitate was collected and washed with
water to give 210 mg of the title compound.
Accurate high resolution mass measurements were performed
using a Bruker Apex 47, FT-ICR mass spectrometer (Billerica, MA)
equipped with an electrospray ion source. Ions produced in the ESI
source were transferred to the FT-ICR cell, and all ions except the ion
of interest were ejected using sweep pulses followed by clean up
shot pulses. The substrate was charged into the FT-ICR cell through
a leak valve at a constant pressure in the range 5 × 10−8 to 1 × 10−7
mbar, and mass spectra were recorded after a variable reaction time
(from 5 to 10 s reaction delay).
1H NMR (500 MHz, DMSO-d6): ı [ppm] 3.38 (dd, 1H, J = 14.0,
J = 9.4 Hz); 3.61 (dd, 1H, J = 14.0, J = 5.8 Hz); 4.43 (dd, 1H, J = 9.4,
J = 5.8 Hz); 7.59–7.64 (m, 2H); 7.76 (td, 1Hc, J = 7.5, Ja–c = 1.3 Hz); 8.07
(dd, 1Ha, Ja–b = 8.2, Ja–c = 1.3 Hz).

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M. Zimnicka et al. / International Journal of Mass Spectrometry 316–318 (2012) 76–83
Fig. 1. Q3 mass spectra recorded for the products of the reactions in a collision
cell (Q2) of (a) p-(trifluoromethyl)-phenide ion with methyl acrylate; (b) 2,5-
dinitrophenide ion with acrylonitrile (A− – adduct ion (the formation of different
types of adducts in the reactions of (halo)nitrophenide ions with ␣,␤-unsaturated
compounds is discussed in our previous paper [9])).
13C NMR (125 MHz, DMSO-d6): ı [ppm] 117.2; 125.0; 128.9;
131.3; 132.8; 133.7; 148.8; 166.9.
3. Results and discussion
3.1. Structural requirements for [M−2]− ion formation
As a first step in a systematic effort to identify the factors that
govern [M−2]− ion formation we conducted a series of reaction
studies including an array of substituted olefins and substituted
phenide ions. From these measurements it appears clear that the
presence of a nitro group in the phenide ion is prerequisite. The
situation is illustrated by the mass spectra reproduced in Fig. 1,
resulting from the reaction of p-(trifluoromethyl)phenide ion with
methyl acrylate (Fig. 1a) and the reaction of 2,5-dinitrophenide
ion with acrylonitrile (Fig. 1b). In the first case no [M−2]− ion
is observed. Instead, the product spectrum is dominated by the
adduct (m/z 231), and its methanol elimination product (m/z 199)
as discussed in our previous article [9]. In the second case formation
of [M−2]− (m/z 165) is clearly evident. These two examples briefly
summarizes the general observation that [M−2]− is highly specific
to nitrophenide ions. In addition, the observation of [M−4]− (m/z
Fig. 2. Q3 mass spectra recorded for the products of the reactions in a collision cell
(Q2) of acrylonitrile with Q1-selected (a) 5-fluoro-2-nitrophenide ion, (b) 2-fluoro-
5-nitrophenide ion and (c) reaction of 3,5-dichloro-4-nitrophenide ion with ethyl
acrylate. Collision energy was set to −5 eV in all cases.
163) in the second case shows that the presence of the additional
nitro group gives rise to two successive reactions of the same kind.
Comparison of the intensities of the peaks corresponding to
the [M−2]− ions with other products formed during the reactions
of isomeric p-halonitrophenide ions with acrylonitrile and ethyl
acrylate (Fig. 2) gives another piece of important information. The

M. Zimnicka et al. / International Journal of Mass Spectrometry 316–318 (2012) 76–83
79
Scheme 2. Scheme of the formation of ␤-adduct of 2-nitrophenide ion to acryloni-
trile.
spectra show that p-halonitrophenide ion with the negative charge
situated in the ortho position to the nitro group gives abundant
[M−2]− ions, in stark contrast to p-halonitrophenide ions in which
the negative charge occupies the meta position with respect to the
nitro group. This result suggests that in the latter case a proton
transfer, necessary for rearrangement into the anion having the
negative charge in the ortho position, has to proceed prior to for-
mation of the [M−2]− ion. To clarify this hypothesis we performed
an additional experiment with 3,5-dichloro-4-nitrophenide ion
(Fig. 2c). According to the hypothesis this anionic reagent in which
both ortho positions of the nitro group are occupied by chlorine
atoms should not give [M−2]− ions in the reactions with exam-
ined ␣,␤-unsaturated compounds. The spectrum recorded during
the reaction of 3,5-dichloro-4-nitrophenide ion with ethyl acrylate
(Fig. 2c) confirms this suggestion. It shows the peaks corresponding
to the adduct (m/z 290) and the product of elimination of ethanol
molecule from the adduct (m/z 244) while there is no peak for
[M−2]−.
Fig. 3. The CID (CE = −5 eV) negative ion mass spectrum recorded for the (2-
nitrobenzyl)-cyanoacetic acid anion (m/z 219).
group give the most abundant peaks corresponding to the [M−2]−
ions.
To investigate this presumption we synthesized (2-
nitrobenzyl)-cyanoacetic acid, which in the negative ion ESI
mode after decarboxylation, was expected to give the ␤-adduct
with the structure shown in Scheme 2. We hypothesized that
transformation of this adduct could in turn lead to the [M−2]−
ion. As anticipated, in the CID mass spectrum recorded for
(2-nitrobenzyl)-cyanoacetic acid anion (m/z 219) presented in
Fig. 3, the peak at m/z = 175 corresponding to decarboxylation
is observed. Unfortunately, this fragment does not constitute
the major fragmentation pathway in the represented CID mass
spectrum. All attempts to generate ion with m/z 175 in reasonable
abundance to enable recording its CID spectrum were ineffective.
On the other hand, one likely product of the m/z = 175 ion would
be m/z 120, as actually seen in the CID spectrum of m/z 219, suggest-
ing the intermediacy of m/z 175. As a matter of fact, the CID spectra
of m/z 120 – resulting from the decomposition of (2-nitrobenzyl)-
cyanoacetic acid and from the ion-molecule gas-phase reaction of
2-nitrophenide ion with acrylonitrile – give very similar spectra, as
evident from Fig. 4.
We further studied ␣,␤-unsaturated compounds without elec-
tron withdrawing groups, more specifically allyl chloride and allyl
iodide. Neither the adducts nor any decomposition product was
observed, indicating that the presence of a strongly electron with-
drawing group is required to make the formation of [M−2]−
possible.
3.2. Discussion on the ˇ-adduct formation
On the basis of our findings we speculate that [M−2]− formation
involves formation of a ␤-adduct in the first step of the reaction
(Scheme 2). Formation of the ␤-adduct occurs in the ortho posi-
tion in accordance with the experimental observations, while in
fact, according to the earlier remark, the p-halonitrophenide ions
with the negative charge situated in ortho position to the nitro
Fig. 4. CID spectra (recorded at CE = −20 eV) of ionic product (m/z 120) resulted from: (a) the reaction of 2-nitrophenide ion with acrylonitrile; (b) in-source fragmentation
of (2-nitrobenzyl)-cyanoacetic acid anion.

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M. Zimnicka et al. / International Journal of Mass Spectrometry 316–318 (2012) 76–83
Scheme 3. Postulated mechanism of the [M−2]− ion formation in the gas-phase reaction of 2-nitrophenide ion with acrylonitrile.
3.3. Structural properties of [M−2]− ion formation
found to be essential in picturing out the mechanistic details, clearly
showing the direct involvement of the nitro group during the order
of events leading to the product formation.
The next set of experiments were aimed at determining the
structural properties of the [M−2]− ions. The proton affinity of the
[M−2]− ion resulting from the reaction of 5-chloro-2-nitrophenide
ions with ethyl acrylate was bracketed by observing in-cell reac-
tions with the following acids (PA of corresponding anion): acetone
(369 kcal/mol), chloroacetonitrile (358 kcal/mol) and acetic acid
(347 kcal/mol). Only the reaction with acetic acid gave reaction,
evident from acetate ion formation, indicating a value of the pro-
ton affinity of the [M−2]− anion in the range 347–358 kcal/mol. The
low reaction efficiency observed even with acetic acid indicates a
value in the low end of the indicated PA range.
3.4. Mechanism of [M−2]− ion formation
The results of the experiments described above now allow us
to propose a putative mechanism leading to [M−2]−, shown in
Scheme 3. It consists of three steps, (i) formation of the ␤-adduct,
(ii) cyclization of the ␤-adduct comprising an intramolecular nucle-
ophilic attack of the oxygen atom of the nitro group, (iii) eventually
followed by the loss of formyl cyanide molecule leading to the
2-nitrosobenzylic anion. According to the previous remark the
(halo)nitrophenide ions with the negative charge situated in meta
or para position to the nitro group have first to isomerize to the
anion with negative charge located in ortho position with respect
to the nitro group to make the reaction possible. Reactions involv-
ing nucleophilic addition to the oxygen atom of the nitro group
are well known, as in vinyl Grignard reactions with nitroarenes
[10,11]. The mechanism presented in Scheme 3 resembles the
mechanism of rearrangement of 2-nitrosobenzyl compounds
To verify the 2-nitrosobenzylic anion structure proposed for
[M−2]− ion we computed the proton affinity of 5-chloro-2-
nitrosobenzylic anion, resulting in a value 347.1 kcal/mol, only
slightly higher than the PA of the acetate anion, in accordance with
the bracketing experiments.
Accurate mass measurements of [M−2]− product gave the evi-
dence that its formation is consistent with an isotopic mass shift
corresponding to the replacement of O with CH2. This finding was
Fig. 5. Overall profile of the E (0 K) surface (in kcal/mol) for [M−2]− formation calculated using PBE1PBE/6-311+G(d,p)//PBE1PBE/6-31+G(d).

M. Zimnicka et al. / International Journal of Mass Spectrometry 316–318 (2012) 76–83
81
˚
Fig. 6. PBE1PBE/6-31+G(d) optimized geometries of anions (1), (2) and TS connecting them. The C O bond length in the transition state is computed to be 1.87 A. It has to
be noted that anions (1) and (2) are conformationaly flexible and a number of other structures with similar energy can be found.
proposed by Il’ichev and Wirz [12]. In their experimental [13–15]
and theoretical [12,16] studies on the photoinduced isomeriza-
tion reaction of 2-nitrosobenzyl compounds they also took under
discussion the anionic version of the reaction, i.e., the isomer-
ization of 2-nitrotoluene anion to 2-nitrosobenzyl alcohol anion
via formation of bicyclic intermediate. The transformation of 2-
nitrotoluene anion into 2-nitrosobenzyl anion considered formally
as cyclization of deprotonated quinoid intermediate (also known
as nitronic acids or aci-form) was found to occur with activation
barrier of 31.3 kcal/mol (B3LYP) and 36.1 kcal/mol (BHLYP) [12].
The activation barrier for the cyclization of deprotonated species
was predicted to encounter a much higher activation barrier than
the analogous isomerization reaction of the quinoid intermediate.
Although the relatively high activation barrier of the isomerization
of 2-nitrotoluene anion was found, we suppose that in our case the
transformation of ␤-adduct leading to the product of O-atom trans-
fer can proceed with substantial lower activation energy due to the
presence of the stabilizing group like CN, CO2Me and CO2Et.
The formation of six-membered ring instead of five-membered
ring should also influent on the stabilization of the transition
state.
the spectra presented below, are characteristic for the reactions of
(halo)nitrophenide ions with activated olefins and were described
in detail in the previous paper.
The last important problem which has to be addressed is
the possibility of the rearrangement of the o-nitrosobenzylic
More significant to the discussion is the computed potential
energy diagram reproduced in Fig. 5. This diagram shows that the
suggested mechanism is energetically feasible, and is typical for
fast addition/elimination reactions of ions in the gas-phase. Accord-
ing to the results of our calculations, both the ␤-adduct (1) and
the intermediate O-atom transfer product (2), as well as the con-
necting transition structure TS(1-2) all lie significantly below the
energy of reactants. The six-membered transition structure TS 1-2
lies 10.5 kcal/mol above the ␤-adduct (1). The optimized geome-
tries of anions (1), (2) and transition structure connecting them are
presented in Fig. 6.
The mechanism of [M−2]− ion formation was further supported
in experiments with methyl methacrylate and crotonitrile serving
as the ␣,␤-unsaturated reagents. According to the expected reac-
tion pathways depicted in the Scheme 4 which are analogous to
the proposed mechanism of Scheme 3, the reaction with methyl
methacrylate (pathway a) should lead to the same anionic product
as in the case of the reactions with methyl and ethyl acrylates, while
in the case of the reaction with crotonitrile (pathway b) a peak at
m/z 168 should be expected.
As anticipated, in the spectra recorded during the reactions of 5-
fluoro-2-nitrophenide ion with methyl methacrylate (Fig. 7a) and
5-chloro-2-nitrophenide ion with crotonitrile (Fig. 7b) the peaks
observed at m/z 138 and m/z 168 confirm the suggested mech-
anistic pathway. Moreover, the peaks corresponding to the ionic
products [A−MeOH]−, [A−HX]− and A− which are also observed in
Fig. 7. Q3 mass spectra recorded for the products of the reactions of (a) 5-fluoro-
2-nitrophenide ion with methyl methacrylate and (b) 5-chloro-2-nitrophenide ion
with crotonitrile.

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M. Zimnicka et al. / International Journal of Mass Spectrometry 316–318 (2012) 76–83
Scheme 4. The expected products of the reactions of (a) 5-fluoro-2-nitrophenide ion with methyl methacrylate and (b) 5-chloro-2-nitrophenide ion with crotonitrile.
Fig. 8. Calculated energy profile (in kcal/mol) for the proposed further transformation of o-nitrosobenzylic anion into o-aminobenzaldehyde anion.
anion to the probably more stable o-aminobenzaldehyde anion
(Fig. 8). Our quantum chemical calculations showed that o-
aminobenzaldehyde anion indeed is 51.4 kcal/mol more stable than
the o-nitrosobenzylic anion but the activation energy of the first
reaction step is 23.7 kcal/mol and of the second step is 56.8 kcal/mol
(52.2 kcal/mol respective to the energy of the substrate). This sec-
ond value shows that this rearrangement is unlikely.
reactions of (halo)nitrophenide ions with acrylonitrile, methyl
acrylate and ethyl acrylate. The [M−2]− ion was found to have
the structure of o-nitrosobenzylic anion. The mechanism involves
initial formation of a ␤-adduct of the nitrophenide ion with the
activated olefin. The formation of ␤-adduct at the ortho position
relative to the nitro group of the phenyl ring is prerequisite for
further reaction along this pathway. The ␤-adducts are kinetically
unstable and rearrange to derivatives of o-nitrosobenzylic anion
accompanied by the loss of HCOR (R = CN, CO2Me, CO2Et).
4. Conclusions
The results presented in this part of work clearly show that the
formation of [M−2]− ions is characteristic of nitrophenide ions.
On this basis this reaction may be useful for analytical purposes,
The study presented here provides a consistent picture of
the mechanism leading to [M−2]− formation during gas-phase

M. Zimnicka et al. / International Journal of Mass Spectrometry 316–318 (2012) 76–83
83
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