CID fragmentation of deprotonated N-Acyl aromatic sulfonamides. Smiles-type and nitrogen-oxygen rearrangements

Smiles-Type and Nitrogen −Oxygen Rearrangements

Research Article

CID Fragmentation of Deprotonated N‑Acyl Aromatic Sulfonamides.

Yuxue Liang,* Yamil Sim ó n-Manso, Pedatsur Neta, Xiaoyu Yang, and Stephen E. Stein

Cite This: J. Am. Soc. Mass Spectrom. 2021, 32, 806 −814

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sı Supporting Information

ABSTRACT: The NIST tandem mass spectral library (2020

version) includes over 800 aromatic sulfonamides. In negative

mode, upon collisional activation most benzenesulfonamides lose a

−

neutral SO molecule leading to an anilide anion (C H NH , m/z

2). However, for deprotonated N-benzoyl aromatic sulfonamides,

−

the phenoxide ion (C H O , m/z 93.0343) is the principal

product ion. A variety of N-acylbenzenesulfonamide derivatives

were also found to overwhelmingly produce the phenoxide ion as

the most intense product ion. A mechanism is proposed in which,

at low energy, a carbonyl oxygen atom (CO) is transferred to a

benzene ring, known as a Smiles-type rearrangement (the amide

oxygen atom attacks the arylsulfonyl group at the ipso position), in

parallel and determining the reaction at high energy a nitrogen−oxygen rearrangement mechanism leads to the formation of the

phenoxide ion. Tandem mass spectra of deprotonated N-benzoyl- O-benzenesulfonamide and N-thiobenzoyl-p-toluenesulfonamide

conﬁrmed the rearrangement since base peaks at m/z 95.0384 and 123.0270 which correspond to an O phenoxide ion

−

(

[C H O] ) and a 4-methylbenzenethiolate anion ([CH C H S] ) were observed, respectively. The parallel mechanism is

supported by the strong correlation between the observed product ion intensities and the corresponding activation energies obtained

by Density Functional Theory calculations. This is an example of a relatively simple ion with a complex path to fragmentation, being

a cautionary tale for indiscriminate use of in silico spectra in place of actual measurement.

INTRODUCTION

and negative modes, and these reference spectra have been

included into the latest version of the NIST tandem MS

library. Of these aromatic sulfonamides, N-acylsulfonamides

represent an important structural motif in drug synthesis as

several drugs including inhibitors of tRNA synthetases function

as antibacterial agents and therapeutic agents for Alzheimer’s

■

Aromatic sulfonamides and their derivatives play an important

role as organic intermediates for drug synthesis in the

pharmaceutical industry and have been widely used as

antibacterial compounds for the treatment of diseases in

livestock production since their antibacterial activity was ﬁrst

7,8

discovered in 1935. Consequently, the increased use of

disease.

sulfonamides causes residue accumulation in dairy, eggs, and

meat, which leads to the antibiotic resistance of many

In general, deprotonated aromatic sulfonamides are prone to

lose a neutral sulfur dioxide molecule to form an anilide anion

9,10

bacteria. Liquid chromatography in conjunction with electro-

in gas phase.

But a deprotonated N-benzoyl aromatic

spray tandem mass spectrometry (LC−MS/MS) provides an

sulfonamide was found to generate a phenoxide ion instead

which corresponds to a peak at m/z 93 as recorded in the

tandem mass spectrum. This required an oxygen transfer to the

benzene ring in an unusual rearrangement in collision-induced

dissociation (CID). Few papers reported similar rearrange-

ments, all of which occurred within fragmentation of

excellent analytical tool for characterization and quantiﬁcation

−5

of these sulfonamide compounds.

In order to enhance

unknown compounds analysis by mass spectrometry, spectral

libraries were employed for fast, reliable identiﬁcation of

compounds which have been measured by instruments with

diﬀerent fragmentation methods. For instance, the NIST

tandem MS library provides reference mass spectral data for

the identiﬁcation of various compounds through the

fragmentation of their ions generated by electrospray

ionization, and the current version (2020) includes 1.3 M

spectra of 186 K precursor ions from 31 K chemical

protonated sulfonamides. Jiang and co-workers reported a

Received: December 22, 2020

Revised: February 4, 2021

Accepted: February 4, 2021

Published: February 15, 2021

compounds. We have measured the tandem mass spectra of

many aromatic sulfonamides with higher energy collision

dissociation (HCD) and ion trap fragmentations in positive

2021 American Society for Mass

Spectrometry. Published by American

J. Am. Soc. Mass Spectrom. 2021, 32, 806−814

806

Journal of the American Society for Mass Spectrometry

Research Article

Scheme 1. Meyerson Rearrangement

novel oxygen transfer pathway in the dissociation of

protonated N-phenyl p-toluenesulfonamides, whereby an

oxygen atom of sulfonyl group is transferred to the ortho,

meta, or para position of the aniline ring in the CID process.

Zu reported an unprecedented reaction via N−O exchange

in the CID experiments of protonated N-(3-aminophenyl)-

benzamide. The carbonyl oxygen was transferred to the aniline

ring by a water molecule migration rearrangement. The

authors stated that the ﬁnding was the ﬁrst report of the N−

O exchange pathway of protonated N-(3-aminophenyl)-

benzamides observed through MS/MS experiments.

EXPERIMENTAL SECTION

■

Materials. Acetonitrile and water (HPLC grade) were

purchased from Honeywell-Burdick & Jackson (Muskegon,

MI, USA). Formic acid solution (50% in water) was purchased

from Fluka (Charlotte, NC). Thionyl chloride and benzene-

sulfonamide were purchased from Sigma-Aldrich (St. Louis,

MO). O -labeled benzoic acid was synthesized according to

the literature.

Preparation of Benzoyl ¹⁸O-Chloride. To a 5 mL glass

vial was added O -labeled benzoic acid (20 mg, 0.16 mmol).

Thionyl chloride (100 μL) was added to the vial and heated to

0 °C for 1 h. The reaction mixture was cooled to room

temperature and dried under nitrogen gas (Scheme 3).

In electron ionization, there are some reports about the

sulfonyl oxygen transfer to benzene ring. EI mass spectra of

diaryl sulfones and alkyl aryl sulfones have been interpreted

partly in terms of a rearrangement in which an aryl group

migrates from sulfur to oxygen. This rearrangement is also

General Procedure for Preparation of N-Acyl Ar-

omatic Sulfonamides. Under a nitrogen atmosphere, a

solution of benzenesulfonamide (20 mg, 0.13 mmol) and acyl

chloride (0.19 mmol) in acetonitrile (50 μL) was stirred at 60

°C and treated with 98% sulfuric acid (0.5 μL). The solution

was maintained at 60 °C for 1 h. The solvent was removed

under nitrogen and the residue was dried in vacuum.

3−15

known as Meyerson’s rearrangement (Scheme 1).

However, migrations of carbonyl oxygens are rare. Recently,

Irikura and Todua studied the fragmentation process of N-

acylarylsulfonamide radical cations under electron ionization

conditions and concluded that it follows a Smiles-type

rearrangement. The authors showed the amide oxygen atom

of N-acylarylsulfonamides attacks the arylsulfonyl group at the

Mass Spectrometry. N-Acyl aromatic sulfonamides were

dissolved in acetonitrile/water/formic acid (50:50:0.1, v/v/v)

at a concentration of about 0.1 mg/mL and directly infused

into an Orbitrap Fusion Lumos mass spectrometer (Thermo

Fisher Scientiﬁc, Waltham, MA) via a nano electrospray

source. Ion trap (IT-CID and FT-CID) and HCD (higher

energy collision dissociation) spectra were acquired in the

ipso position, displacing a molecule of SO . In synthetic

organic chemistry, the Smiles rearrangement reaction has been

known for several decades, and a simpliﬁed version of the base-

catalyzed Smiles rearrangement is presented in Scheme 2. In

negative mode. The gases N (99.999%) and He (99.999%)

Scheme 2. Base-Catalyzed Smiles Rearrangement

were utilized as collision gases for the HCD and IT spectra,

respectively. Ion-trap spectra were acquired at a normalized

collision energy (NCE) of 35%. The HCD spectra were

collected by using various normalized collision energies. The

resolution for MS was set at 30000. Spectra were acquired in

“

proﬁle” mode for both FT-CID and HCD.

Computational Methods. Several plausible reaction

mechanisms for the fragmentation of deprotonated N-

the LC−MS/MS ﬁeld, Bowie reported the gas-phase Smiles

benzoylbenzenesulfonamide were examined using the semi-

22−24

−

empirical PM3

potential energy surface. Then Density

rearrangement of ArX(CH ) O (X = O or S) ions with a joint

Functional Theory (DFT) calculations were performed on the

most satisfactory mechanisms using the hybrid density

theoretical and experimental approach where the extent of the

18,19

Smiles process decreases as n increases.

5,26

functional method B3LYP

in conjunction with the Pople’s

While the above mass spectrometry studies dealt with

protonated and radical cation of sulfonamides, we investigated

the fragmentation of deprotonated N-acyl aromatic sulfona-

mides and their derivatives in electrospray ionization and

propose a concomitant Smiles and N−O rearrangement

mechanisms which involve a carbonyl oxygen migrating to a

benzene ring.

basis set [6-311++g(d,p)] as implemented in Gaussian 09.

Frequency analysis at the same level of theory was used to

identify the optimized structures as minima or transition

structures on the potential energy surface. Intrinsic reaction

coordinate (IRC) calculations were performed to conﬁrm

that speciﬁc transition states were connected to the designated

local minima. These calculations have been used in previous

Scheme 3. Synthesis of N-Benzoyl- O-benzenesulfonamide

Journal of the American Society for Mass Spectrometry

J. Am. Soc. Mass Spectrom. 2021, 32, 806−814

the main peaks in the spectrum of deprotonated 1b on HCD

Research Article

Figure 1. Molecular structures of N-phenyl benzenesulfonamide 1a, N-benzoylbenzenesulfonamide 1b, N-thiobenzoylbenzenesulfonamide 1c, and

N-benzoyl- O-benzenesulfonamide compound 1d.

studies of ion fragmentation of small molecules under CID

conditions.

subsequent sulfur dioxide elimination to aﬀord the anilide

anion of m/z 92, which is the second highest peak.

0−32

When a carbonyl group is introduced into the benzene-

sulfonamide molecule, i.e., N-benzoylbenzenesulfonamide 1b,

the CID spectrum shows the phenoxide ion of m/z 93.0343 as

the most abundant ion and there is no anilide peak at m/z 92

RESULTS AND DISCUSSION

■

Comparison of Mass Spectra of N-Phenylbenzene-

sulfonamide 1a with N-Benzoylbenzenesulfonamide

b. Aromatic sulfonamides derived from primary amines

(

Figure 2 b). In addition to the phenoxide ion peak and the

deprotonated 1b precursor peak at m/z 260, ﬁve distinctive

tend to lose a proton to generate gaseous anions under

appropriate ESI conditions. However, compared with studies

of protonated forms, few investigations have been carried out

on the fragmentation of deprotonated sulfonamides. Generally,

such fragmentations are characterized as loss of a neutral sulfur

dioxide molecule by an intramolecular rearrangement process

product ion peaks are observed in the ESI tandem mass

−

spectrum: (1) the ion of m/z 241 ([C H NO S] , fragment

ion after loss of a H O molecule and a hydrogen radical from

−

precursor); (2) the ion of m/z 196 ([C13

10NO] , fragment

ion after loss of a SO molecule from precursor); (3) the ion of

−

m/z 157 ([C H O S] , a benzenesulfonate ion); (4) the ion of

−

m/z 141 ([C H O S] , a benzenesulﬁnate ion); and (5) the

and subsequent fragmentation to form an anilide anion at m/z

−

ion of m/z 120 ([C H CONH] , a benzamide anion formed

2. For example, in 2013, Attygalle reported that deproto-

6 5

via cleavage of sulfonamide bond).

Figure 3 shows the dependence of the relative intensities of

nated N-phenylbenzenesulfonamide 1a (Figure 1), at m/z 232,

undergoes the loss of a neutral SO to form the peak at m/z

68 which further fragments to generate the ion peak at m/z

66 by expelling a H molecule (Figure 2 a). Alternatively, the

m /z 232 precursor may also lose a benzyne and undergo

Figure 3. Relative product ion abundances as a function of

normalized collision energies in the HCD tandem mass spectrum of

deprotonated N-benzoylbenzenesulfonamide 1b in the orbitrap

instrument. Abundances are normalized to total ion current. Each

curve is identiﬁed by the m/z value of the corresponding product ion

(

m/z 260 is the precursor ion).

collision energy in the Orbitrap mass spectrometer. The ﬁrst

peak to appear at low collision energy is the phenoxide ion at

m/z 93. At the next higher energy, its relative intensity

increases to 33%, while the other peaks at m/z 141 and m/z

57 begin to appear concurrently. The intensity of the m/z 93

peak reaches its maximum level at 59% under a collision energy

of 44% and maintains the highest level across most of the

energy range. This phenomenon that the phenoxide ion

dominates in the spectrum of 1b indicates that the oxygen

transfer rearrangement to benzene ring occurs rapidly and is

Figure 2. (a) Tandem mass spectrum of deprotonated N-phenyl-

benzenesulfonamide 1a (precursor, m/z 232, top). (b) Tandem mass

spectrum of deprotonated N-benzoylbenzenesulfonamide 1b (pre-

cursor, m/z 260, bottom).

J. Am. Soc. Mass Spectrom. 2021, 32, 806−814

Journal of the American Society for Mass Spectrometry

Research Article

Scheme 4. Proposed Mechanism to Rationalize the formation of the Phenoxide Ion at m/z 93 and Other Product Ions from

Deprotonated N-Benzoylbenzenesulfonamide

energetically favorable. The ions at m/z 141 and 157 reach

their maximum abundances at about the same collision energy,

but the value for the former ion (relative intensity 18%) is

nearly seven times greater than that of the latter ion (relative

intensity 2%); i.e., the formation of benzenesulﬁnate ion is

more prominent than that of benzenesulfonate ion. The peak

at m/z 64 appears at much higher energy and is ascribed to

sulfur dioxide radical anion as conﬁrmed by high accuracy

HCD spectra. It is noteworthy that the m/z 157 ion is an

unexpected product ion since an additional oxygen atom is

attached to the benzenesulfonyl group to form the

benzenesulfonate ion which can then fragment to generate

an intramolecular nucleophilic reaction to generate the four-

membered ring intermediate anion 2c, which undergoes loss of

a neutral benzonitrile (C H CN) molecule under collision

dissociation conditions to form the benzenesulfonate ion at m/

z 157, whose MS spectrum shows rapid loss of a neutral SO

molecule to produce the phenoxide ion at m/z 93. The

intermediate anion 2c may also generate the benzenesulﬁnate

ion at m/z 141, which further fragments to form the phenoxide

ion by loss of a neutral SO via a Meyerson-type rearrange-

ment.

Tandem Mass Spectra of N-Thiobenzoylbenzenesul-

fonamide 1c. To support this hypothesis, we searched for

similar compounds in the newly released NIST tandem library

and found a variety of sulfonamide derivatives. One of the

search results is a compound 1c, where the benzoyl group of

1b is replaced with a thiobenzoyl group (Figure 1), which

allows us to investigate whether the sulfur atom could be

transferred to the ipso position or the sulfonyl group and

subsequently fragments to form the thiophenol anion in the

collision cell. The deprotonated 1c ion was subjected to

fragmention in ion trap at collision energy of 35%, followed by

analysis in Orbitrap analyzer, and the resulting spectrum is

shown in Figure 4 a. The most abundant ion peak is at m/z

123.0270 which corresponds to 4-methylbenzenethiolate anion

the ion of m/z 93 by a neutral loss of SO molecule, as

conﬁrmed by MS experiments. This pathway for formation

of the phenoxide ion represents another unique oxygen

transfer rearrangement as it cannot account for the observed

rapid formation of phenoxide at low collision energies as

shown in Figure 3. Since N-benzoylbenzenesulfonamide 1b

contains an additional carbonyl group as compared to N-

phenylbenzenesulfonamide 1a, we assume that the oxygen

atom of the carbonyl group is transferred to the benzene-

sulfonyl moiety to form the benzenesulfonate ion at m/z 157.

Reaction Mechanism. Based on these ﬁndings, we

propose the Smiles-type and N−O rearrangement mechanisms

to explain how the oxygen atom was transferred to form the

unexpected fragment ions. As shown in Scheme 4, when N-

benzoylbenzenesulfonamide 1b is subjected to ionization

within the ESI source in the negative mode it loses a proton

from the amide NH to produce the precursor ion 2a at m/z

−

([CH C H S] ), where the sulfur atom in CS is migrated to

the toluene ring. Similar to the spectrum of compound 1b, the

toluenesulﬁnate ion ([C H SO ] ) at m/z 155.0168 is

−

generated with a relative abundance of 18% comparable to

that of benzenesulﬁnate ion at m/z 141 mentioned before. We

−

60. Since the negative charge may be shifted from the

also noticed the benzenesulfonate-like anion ([C H S O ] ) at

nitrogen atom to the amide oxygen atom, which attacks the

arylsulfonyl group at the ipso position via a ﬁve-membered ring

intermediate 2b, it is a Smiles-type rearrangement with a low

m/z 186.9888, where a sulfur atom is attached to the sulfonyl

group to form S−S bond as shown in Figure 4 a. MS

experiments were carried out to further fragment the

−

energy pathway. Subsequent rapid neutral losses of SO and

C H S O anion in ion trap and subsequently analyzed in

benzonitrile result in the most abundant phenoxide ion at m/z

Orbitrap, the acquired spectrum exhibits the most intense peak

3. N−O rearrangement takes place with a high energy

at m/z 123.0275, indicating this ion can originate from the

−

pathway as the amide oxygen anion attacks the sulfur atom via

C H S O ion by loss of a SO molecule (Figure 4 b).

Journal of the American Society for Mass Spectrometry

J. Am. Soc. Mass Spectrom. 2021, 32, 806−814

Research Article

Figure 5. HCD tandem mass spectrum of deprotonated N-

benzoyl- O-benzenesulfonamide 1d (precursor, m/z 262.0434).

oxygen transfer mechanism to other compounds, we examined

a number of N-acyl aromatic sulfonamides. Some typical

compounds and their product ion abundances normalized to

total ion current are listed in Table 1.

Table 1. Various Compounds Which Exhibit the Smiles-

Type and N−O Exchange Rearrangements under Ion-Trap

Collision at Collision Energy 35% and Their Product Ion

Abundances

Figure 4. (a) Tandem mass spectrum of deprotonated N-thiobenzoyl-

p-toluenesulfonamide 1c (precursor, m/z 290.0304, top). (b) MS

spectrum of its product ion at m/z 186.9888 (bottom).

Methylphenoxide ion peak at m/z 107.0503 along with a

radical anion S O peak at m/z 95.9346 were also found with

relative lower intensities of 21% and 20%, respectively.

Tandem Mass Spectra of N-Benzoyl- O-benzenesul-

fonamide Compound 1d. In order to further support the

hypothesis that the oxygen atom of the observed phenoxide

ion is derived from the carbonyl group and not from the

sulfonyl group, we synthesized N-benzoyl- O-benzenesulfona-

mide compound 1d via reaction of benzoyl- O-chloride with

benzenesulfonamide. The HCD tandem mass spectrum of the

N-benzoyl- O-benzenesulfonamide shows a most intense peak

at m/z 95.0384 which corresponds to an ¹⁸O-phenoxide ion

accompanied by the phenoxide ion at m/z 93.0304 with a

relative abundance of 18% because of the above-mentioned

Meyerson-type rearrangement (Figure 5). The ions of m/z

−

(

22.0492 ([C H N O] ) and m/z 159.0000

6 5 2

7 6

−

[C H O S O] ) were also observed with relative lower

intensities. The phenomenon that these m/z values shift two

mass units upward is consistent with our suggestion that

carbonyl oxygen atom is transferred to benzene ring and sulfur

atom via the Smiles-type and N−O rearrangement. It should

be noted that other product ion peaks at m/z 141.0010 and m/

z 156.0118 without the ¹⁸O atom were also present in the

spectrum of 1d, which correspond to the respective

N-Acylbenzenesulfonamide Derivatives. For N-benzoyl-

benzenesulfonamide 1b, phenoxide ion abundance was

calculated as 38%. N-(4-Chlorobenzoyl)benzenesulfonamide

3, where a hydrogen atom on the benzoyl moiety is substituted

by a chlorine atom, shows a dramatic increase in the extent of

the rearrangement with the abundance value at 64%. When the

benzoyl is replaced with an aliphatic acyl group like acetyl, the

resulting N-acetylbenzenesulfonamide 4 undergoes fragmenta-

−

benzenesulﬁnate ion ([C H O S] ) and benzenesulfonamide

−

anion ([C H O SNH] ).

Tandem Mass Spectra of Other N-Acyl Aromatic

Sulfonamides. To extend the application of the amide

J. Am. Soc. Mass Spectrom. 2021, 32, 806−814

Journal of the American Society for Mass Spectrometry

the rearranged ion 2c ). The forty-point IRC plot in Figure 6

Research Article

tion to generate the phenoxide ion with moderate abundance

produced by two parallel rearrangements and each rearrange-

ment reaction contributes mainly to the intensity of one

product ion. In the following this is proved in two steps, (1) by

testing the feasibility of the rearrangement reactions using IRC

calculations and then (2) by correlating the experimental

intensities and the activation energies in a quantitative manner.

PM3 semiempirical relaxed scans were performed to

explore the potential energy surface using deprotonated N-

benzoylbenzenesulfonamide 2a as a model ion followed by

DFT calculations at the critical points. An IRC calculation was

performed to conﬁrm the located TS saddle point lies on the

minimum energy path between the assumed minima (2a and

(

57%). Similar mechanisms with larger aliphatic acyl groups,

such as propionyl 5 and pentanoyl 6, aﬀord higher product ion

abundances as 75% and 81%, respectively.

N-Acetyl-Substituted Benzenesulfonamide Derivatives.

Substitution eﬀects on the benzene ring of benzenesulfonyl

also were investigated, and N-acetyl-4-methylbenzenesulfona-

mide 7 and N-acetyl-4-bromobenzenesulfonamide 8 provide

their corresponding product ions with abundances at 35% and

6%, respectively. The results may be rationalized by the ability

of the Br to stabilize the negative charge on oxygen atom of the

product ion. However, the carboxyl group on the benzene ring

for 2-(N-acetylsulfamoyl)benzoic acid 9 provides an abun-

dance of the phenoxide ion at only 23%. The main reason is

that proton loss from the carboxyl group of compound 9

within the ESI source is very facile in negative mode, resulting

in the subsequent loss of a neutral carbon dioxide as the

predominant pathway.

Other Benzenesulfonamide Derivatives. The existence of

an amide bond (CONH ) in the compound structure is critical

for giving rise to the rearrangement reaction; in the case of 2-

oxo-S-phenyl-2-(piperidin-1-yl)ethane-1-sulfonamido 10

where a CH is inserted into the amide bond, only 2%

abundance of rearrangement ion was observed. Annular

sulfonamide like 1,1-dioxo-1,2-benzothiazol-3-one 11 does

not show the rearrangement behavior due to stability of the

ﬁve-membered ring. Even when the normalized collision

energy is raised to 100 no pronounced product ion peaks are

observed. Sulfonylurea compound 12 also does not give the

rearrangement product ion, instead forming the benzenesulfo-

namide and anilide anion as the dominated peaks such as N-

phenyl benzenesulfonamide 1a does.

Theoretical Calculations. As pointed out by Herman and

Figure 6. Intrinsic reaction coordinate (IRC) diagram of the N−O

Harrison, at low internal energies, the relative rates of

competing fragmentation reactions are determined largely by

their relative activation energies. Hence, intrinsic reaction

coordinate and activation energy calculations were used to

assess the feasibility of parallel rearrangement reactions

involved in the fragmentation of sulfonamides. Reactions

with multiple product channels have been studied before using

the direct-measurement kinetics method and theoretical

rearrangement and Smiles rearrangement showing the chemical

structures involved in the process, the activation energy (E ), and

the thermal result of the reaction. E and ΔH were estimated from the

electronic energies; other contributions are too small to be of any

signiﬁcance. The dashed lines do not represent calculated points and

only serve to guide the eye.

shows the N−O rearrangement is possible, and it produces a

physically reasonable activation energy of 247.5 kJ/mol. Also

note that the rearrangement is kinetically controlled because

both isomers are almost equally thermodynamically stable. The

reaction is barely exothermic with the rearranged ion 2c being

11 kJ/mol more stable than the original anion 2a. These same

calculations were performed for the diﬀerent rearrangement

reactions of the parent ion.

Figure 6 also shows the IRC path for the Smiles-type

rearrangement. The activation energy for this process is 165.5

kJ/mol, signiﬁcantly lower than for the N−O rearrangement,

so this mechanism dominates the low energy end of the

spectrum (initial stages), although the intermediate product 2b

(fragment ion + neutral) is thermodynamically less stable by

52.0 kJ/mol. Therefore, the spectrum of 2a is the result of at

least two major rearrangements of the parent ion, a Smiles-type

rearrangement and a N−O rearrangement. (The 3D structures

of the transition state and minima are shown in Figure 6, and

their electronic energies are given in Table S1 of the

Supporting Information). In addition, Meyerson’s rearrange-

ment directly originated from 2a provides a signiﬁcant

contribution to the intensity of the ion at m/z 93 in a more

convoluted way (see the discussion below). The spectrum also

calculations.

In this work, there are several experimental observations that

are not compatible with a single fragmentation pathway. For

−

example, Figure 3 shows that the ion at m/z 93 (C H O )

appears ﬁrst and it seems to be produced directly from the

parent ion in the initial stages. This observation goes along

with a Smiles-type rearrangement; however, the Smiles

rearrangement cannot explain the intense peaks at m/z 157

and 141 or the intense peak at m/z 93 in the spectrum of the

labeled compound (Figure 5). On the other hand, a N−O

rearrangement mechanism is appropriate to explain the

product ions at m/z 157 and 141 and it is expected to

contribute marginally to the intensity of the product ion at m/z

3. Also, there is no evidence that the peak at m/z 141 is

produced by the loss of an oxygen atom from benzenesulfonate

m/z 157). The production of these ions runs parallel at a rate

that is approximately an order of magnitude higher for the

product ion at m/z 141. It seems that both ions can be

produced from the same transition state of the N−O

rearrangement depending on the cleavage site to form

sulfonate ion or sulﬁnate ion. Based on this experimental

evidence, it is likely that the observed fragment ions are

(

J. Am. Soc. Mass Spectrom. 2021, 32, 806−814

Journal of the American Society for Mass Spectrometry

https://pubs.acs.org/doi/10.1021/jasms.0c00470.

Research Article

shows some small peaks from the sulfonamide-bond

fragmentation and other structural rearrangements of the

parent ion.

By combining the information in Figure 2b and Figure 5, the

contribution of diﬀerent rearrangements to the intensity of

each peak in the spectrum was estimated. For example, at low

energy, the peak at m/z 93 can be generated from a parent ion

that experiences a Smile-type rearrangement and with a minor

contribution of a Meyerson rearrangement. The Meyerson

rearrangement does not involve the carbonyl oxygen, so its

contribution to the intensity of the peak at m/z 93 is

approximately 18% according to the labeled spectrum (Figure

). It is worth noting that the Meyerson rearrangement

proceeds with negligible activation energy; however, the

activation energy of the rearranged ion to produce the

fragment ion at m/z 93 is 269.2 kJ/mol, signiﬁcantly higher

than the activation energy for the formation of products from

the Smiles-type or N−O rearrangements. At high energy, a

major contribution of Meyerson rearrangement to the

spectrum of 2a is via the fragmentation of the sulfonate ion

produced by the N−O rearrangement as discussed in ref 27.

The peaks at m/z 157 and 141 are almost exclusively produced

from the N−O-rearranged ion depending on the cleavage site,

either on the left or right of the carbonyl oxygen. Figure 2b

shows the intensity of peaks at m/z 157 and 141 are

approximately 7% and 15% of the base peak, respectively. The

peak at m/z 120 is originated from the direct cleavage of the

sulfonamide bond with a contribution of 6%. The activation

energy of the sulfonamide bond breaking is relatively high, 334

kJ/mol, and it explains its marginal contribution to the

spectrum. It is worth mentioning that there is a small peak at

m/z 241 that seems to be originated from the losses of atomic

hydrogen and water producing a radical anion. Theoretical

calculations of open-shell species require higher levels of

theory and are beyond the scope of this paper.

^Ii

Figure 7. Plot of log₍I₀

versus (E ) according to eq 2 of the

)

Supporting Information. The plot uses base 10 logarithm instead of

base e logarithm. The contributions to peak intensity values are listed

and explained in Table S2. The activation energies were also derived

from the electronic energies listed in Table S2.

intense peak. At higher energies, a novel N−O rearrangement

produces benzenesulfonate and benzenesulﬁnate ions. Specif-

ically, the Smiles-type rearrangement involves attack of the

amide oxygen anion on the arylsulfonyl group at the ipso

position followed by losses of SO and PhCN to form the

phenoxide ion. The N−O rearrangement occurs through an

intramolecular nucleophilic reaction of the amide oxygen with

the sulfur atom to generate the benzenesulfonate ion by loss of

PhCN, which can further lose SO to form the phenoxide ion.

The N−O rearrangement also may yield the benzenesulﬁnate

ion which can lose SO to form the phenoxide ion. The

proposed mechanisms are supported by substituting the

carbonyl of N-benzoyl benzenesulfonamide with a thiocarbon-

yl group experiment and by O isotopic labeling N-

benzoyl- O-benzenesulfonamide experiment which lead up

I_i

A plot of ln

versus (E ) (see Supporting Information for

(_I

)

to form the corresponding benzenethiolate anion and

a rationale for this graph) was drawn using the estimated

experimental values of peak intensities and the calculated

activation energies. (The activation energies were calculated

using the electronic energies listed in Table S1 in the

Supporting Information.) The linear relationship shown in

Figure 7 represents a validation of the assumption that the

most relevant peaks in the spectrum of 2a originate from

diﬀerent rearrangements of the parent ion (see also Table S2 in

the Supporting Information). Calculations also suggest that the

rearrangement reactions considered here have activation

energies that are considerably smaller than the direct breaking

of the sulfonamide bond.

phenoxide ion. The strong correlation between product ion

intensities and the corresponding calculated activation energies

suggests that the fragmentation of N-benzoylbenzenesulfona-

mide experiences the two major parallel rearrangement

reactions, the Smiles-type and N−O rearrangement, and also

minor contributions from the Meyerson rearrangement

reaction and the direct breaking of the sulfonamide bond.

This parallel mechanism is so compelling that should be

regarded as an example of the limitations of in-silico modeling

and the ever-increasing need for building experimental spectral

libraries. We also found certain compounds which demonstrate

the Smiles-type and N−O rearrangements in their spectra, and

this rule may be incorporated into NIST MS interpreter, CFM-

In an attempt to test the performance of in silico software

regarding complex mechanisms like the fragmentation of 2a

discussed in this work, we used the Competitive Fragmenta-

tion Modeling-ID software (CFM-ID) developed by the

ID and and other fragmentation modeling software to

enhance spectrum evaluation and thereby improve the quality

of the library.

University of Alberta for predicting the spectra of N-

benzoylbenzenesulfonamide. CFM-ID was not able to predict

the major peaks associated with either the N/O or the Smiles

rearrangement reactions (see Figure S2).

ASSOCIATED CONTENT

sı Supporting Information

The Supporting Information is available free of charge at

■

CONCLUSIONS

Unlike deprotonated N-phenylbenzenesulfonamide which

■

Analytical derivation of the correlation between peak

intensity and activation energy as shown in Figure 7;

details about the ab initio calculations; PM3 potential

energy surface (PES) of the N−O rearrangement

reaction (Figure S1); structural and energetic data

generally loses SO to form the anilide anion, deprotonated

N-benzoylbenzenesulfonamide mainly undergoes a Smiles-type

ipso rearrangement to form the phenoxide ion as the most

J. Am. Soc. Mass Spectrom. 2021, 32, 806−814

Journal of the American Society for Mass Spectrometry

Oldham, M. D.; Yue, W. Analogues of SB-203207 as inhibitors of

Research Article

related to the optimized geometries obtained using ab

initio methods (Table S1); calculated activation energies

for the rearrangement reactions (Table S2); head-to-tail

comparison between the predicted CFM-ID spectrum

and the NIST library spectrum (Figure S2) (PDF)

(7) Banwell, M. G.; Crasto, C. F.; Easton, C. J.; Forrest, A. K.;

Karoli, T.; March, D. R.; Mensah, L.; Nairn, M. R.; O ’Hanlon, P. J.;

tRNA synthetases. Bioorg. Med. Chem. Lett. 2000 , 10 , 2263 −2266.

8) Hasegawa, T.; Yamamoto, H. A practical synthesis of optically

active (R)-2-propyloctanoic acid: therapeutic agent for alzheimer ’s

disease. Bull. Chem. Soc. Jpn. 2000, 73, 423−428.

AUTHOR INFORMATION

Corresponding Author

Yuxue Liang − Mass Spectrometry Data Center, Biomolecular

Measurement Division, National Institute of Standards and

Technology, Gaithersburg, Maryland 20899, United States;

orcid.org/0000-0002-6430-915X ; Phone: 301-975-

(9) Hu, N.; Liu, P.; Jiang, K.; Zhou, Y.; Pan, Y. Mechanism study of

^S ^O 2 elimination from sulfonamides by negative electrospray

ionization mass spectrometry. Rapid Commun. Mass Spectrom. 2008,

■

10) Bowie, J. H. The fragmentations of even-electron organic

2, 2715−2722.

negative ions. Mass Spectrom. Rev. 1990, 9, 349−379.

11) Wang, S.; Guo, C.; Zhang, N.; Wu, Y.; Zhang, H.; Jiang, K.

Tosyl oxygen transfer and ion-neutral complex mediated electron

417; Email: yuxue.liang@nist.gov

transfer in the gas-phase fragmentation of the protonated N-phenyl p-

toluenesulfonamides. Int. J. Mass Spectrom. 2015, 376, 6−12.

Authors

Yamil Simón-Manso − Mass Spectrometry Data Center,

(12) Zu, C.; Mukhopadhyay, S.; Hanley, P. S.; Xia, S.; Bell, B. M.;

Biomolecular Measurement Division, National Institute of

Standards and Technology, Gaithersburg, Maryland 20899,

United States

Grigg, D.; Gilbert, J. R.; O ’Brien, J. P. Fragmentation of protonated

N-(3-aminophenyl)benzamide and its derivatives in gas phase. J. Am.

Soc. Mass Spectrom. 2016, 27, 917−926.

Pedatsur Neta − Mass Spectrometry Data Center,

Biomolecular Measurement Division, National Institute of

Standards and Technology, Gaithersburg, Maryland 20899,

United States

Xiaoyu Yang − Mass Spectrometry Data Center, Biomolecular

Measurement Division, National Institute of Standards and

Technology, Gaithersburg, Maryland 20899, United States

Stephen E. Stein − Mass Spectrometry Data Center,

Biomolecular Measurement Division, National Institute of

Standards and Technology, Gaithersburg, Maryland 20899,

United States

(13) Bowie, J. H.; Williams, D. H.; Lawesson, S. O.; Madsen, J. O.;

Nolde, C.; Schroll, G. Studies in mass spectrometry-XV ’ mass spectra

of sulfoxides and sulphones. Tetrahedron 1966 , 22 , 3515 −3525.

(14) Baarschers, W. H.; Krupa, B. W. Mass spectra of some sulfinate

esters and sulfones. Can. J. Chem. 1973, 51, 156 −161.

15) Meyerson, S.; Drews, H.; Fields, E. K. Mass spectra of diaryl

sulfones. Anal. Chem. 1964, 36, 1294 −1300.

16) Irikura, K. K.; Todua, N. G. Facile smiles-type rearrangement in

(17) Henderson, A. R. P.; Kosowan, J. R.; Wood, T. E. The truce-

radical cations of N-acylarylsulfonamides and analogs. Rapid Commun.

Mass Spectrom. 2014, 28, 829−834.

smiles rearrangement and related reactions: a review. Can. J. Chem.

017, 95, 483−504.

18) Eichinger, P. C. H.; Bowie, J. H.; Hayes, R. N. The gas-phase

smiles rearrangement: A heavy atom labeling study. J. Am. Chem. Soc.

989, 111, 4224−4227.

19) Wang, T.; Nibbering, N. M. M.; Bowie, J. H. The gas phase

Smiles rearrangement of anions PhO(CH ) O- (n = 2 −4). A joint

Complete contact information is available at:

https://pubs.acs.org/10.1021/jasms.0c00470

20) Xia, H.; Zhang, Y.; Attygalle, A. B. Experimental and theoretical

Notes

The authors declare no competing ﬁnancial interest.

theoretical and experimental approach. Org. Biomol. Chem. 2010, 8,

080−4084.

ACKNOWLEDGMENTS

studies on gas-phase fragmentation reactions of protonated methyl

■

We thank Dr. Karl K. Irikura of the National Institute of

Standards and Technology for his advice and comments.

Certain commercial equipment, instruments, or materials are

identiﬁed in this document. Such identiﬁcation does not imply

recommendation or endorsement by the National Institute of

Standards and Technology, nor does it imply that the products

identiﬁed are necessarily the best available for the purpose.

benzoate: concomitant neutral eliminations of benzene, carbon

dioxide, and methanol. J. Am. Soc. Mass Spectrom. 2018, 29, 1601−

21) Martin, M. T.; Roschangar, F.; Eaddy, J. F. Practical acid-

610.

catalyzed acylation of sulfonamides with carboxylic acid anhydrides.

Tetrahedron Lett. 2003 , 44 , 5461 −5463.

(22) Stewart, J. J. P. Optimization of parameters for semiempirical

REFERENCES

methods I. Method. J. Comput. Chem. 1989 , 10 , 209 −220.

■

(23) Stewart, J. J. P. Optimization of parameters for semiempirical

(

1) Stowe, C. M. The sulfonamides. In Veterinary Pharmacology and

Therapeutic; Jones, L. M., Ed.; Iowa State University Press: Ames, IA,

965; pp 438−502.

2) Neu, H. C. The crisis in antibiotic resistance. Science 1992, 257,

064−1073.

3) Hibbs, J. A.; Jariwala, F. B.; Weisbecker, C. S.; Attygalle, A. B.

methods II. Method. J. Comput. Chem. 1989 , 10 , 221 −264.

(24) Stewart, J. J. P. Optimization of parameters for semiempirical

methods. III Extension of PM3 to Be, Mg, Zn, Ga, Ge, As, Se, Cd, In,

Sn, Sb, Te, Hg, Tl, Pb, and Bi. J. Comput. Chem. 1991 , 12 , 320 −341.

(25) Becke, A. D. Density-functional thermochemistry. III. The role

of exact exchange. J. Chem. Phys. 1993 , 98 , 5648.

(

Gas-phase fragmentation of anions derived from N-phenyl benzene-

sulfonamides. J. Am. Soc. Mass Spectrom. 2013, 24, 1280−1287.

(26) Lee, C.; Yang, W.; Parr, R. G. Development of the colle-salvetti

correlation-energy formula into a functional of the electron density.

Phys. Rev. B: Condens. Matter Mater. Phys. 1988 , 37 , 785.

(27) Dunning, T. H., Jr. Gaussian basis sets for use in correlated

molecular calculations. I. The atoms boron through neon and

hydrogen. J. Chem. Phys. 1989, 90, 1007−1023.

(

4) Klagkou, K.; Pullen, F.; Harrison, M.; Organ, A.; Firth, A.;

Langley, G. J. Fragmentation pathways of sulphonamides under

electrospray tandem mass spectrometricconditions. Rapid Commun.

Mass Spectrom. 2003, 17, 2373−2379.

(5) Bialecki, J. B.; Weisbecker, C. S.; Attygalle, A. B. Low-energy

collision-induced dis-sociation mass spectra of protonated p-

toluenesulfonamides derived fromaliphatic amines. J. Am. Soc. Mass

Spectrom. 2014, 25, 1068−1078.

(28) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;

Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Petersson,

G. A.; Nakatsuji, H.; Li, X.; Caricato, M.; Marenich, A.; Bloino, J.;

Janesko, B. G.; Gomperts, R.; Mennucci, B.; Hratchian, H. P.; Ortiz, J.

V.; Izmaylov, A. F.; Sonnenberg, J. L.; Williams-Young, D.; Ding, F.;

6) NIST MS/MS library 2020, http://chemdata.nist.gov/mass-spc/

msms-search/ (accessed 2021-02-03).

J. Am. Soc. Mass Spectrom. 2021, 32, 806−814

Journal of the American Society for Mass Spectrometry

Research Article

Lipparini, F.; Egidi, F.; Goings, J.; Peng, B.; Petrone, A.; Henderson,

T.; Ranasinghe, D.; Zakrzewski, V. G.; Gao, J.; Rega, N.; Zheng, G.;

Liang, W.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;

Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.;

Throssell, K.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.;

Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V.

N.; Keith, T.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell,

A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Millam, J. M.;

Klene, M.; Adamo, C.; Cammi, R.; Ochterski, J. W.; Martin, R. L.;

Morokuma, K.; Farkas, O.; Foresman, J. B.; Fox, D. J. Gaussian 09,

Revision A.02; Gaussian, Inc.: Wallingford, CT, 2016.

29) Gonzalez, C.; Schlegel, H. B. An improved algorithm for

reaction path following. J. Chem. Phys. 1989, 90, 2154.

30) Neta, P.; Godugu, B.; Liang, Y.; Simón-Manso, Y.; Yang, X.;

(

Stein, S. E. Electrospray tandem quadrupole fragmentation of

quinolone drugs and related ions. On the reversibility of water loss

from protonated molecules. Rapid Commun. Mass Spectrom. 2010, 24,

31) Simón-Manso, Y.; Neta, P.; Yang, X.; Stein, S. E. Loss of 45 Da

271−3278.

from a2 ions and preferential loss of 48 Da from a2 ions containing

methionine in peptide ion tandem mass spectra. J. Am. Soc. Mass

Spectrom. 2011, 22, 280−289.

(32) Neta, P.; Simón-Manso, Y.; Liang, Y.; Stein, S. E. Loss of H2

and CO from protonated aldehydes in electrospray ionization mass

spectrometry. Rapid Commun. Mass Spectrom. 2014, 28, 1871−1882.

(33) Binkley, R. W.; Flechtner, T. W.; Tevesz, M. J. S.; Winnik, W.;

Zhong, B. Rearrangement of aromatic sulfonate anions in the gas

phase. Org. Mass Spectrom. 1993, 28, 769 −772.

(34) Herman, J. A.; Harrison, A. G. Energetics and structural effects

in the fragmentation of protonated esters in the gas phase. Can. J.

Chem. 1981, 59, 2133−2145.

(35) Manard, M. J.; Kemper, P. R.; Carpenter, C. J.; Bowers, M. T.

Dissociation reactions of diatomic silver cations with small alkenes:

experiment and theory. Int. J. Mass Spectrom. 2005, 241, 99−108.

(36) Foresman, J. B.; Frisch, A. E. Exploring chemistry with electronic

structure methods, 3rd ed.; Gaussian, Inc.: Wallingford, CT, 2015; pp

38−44, 255−58. ISBN: 978-1-935522-03-4.

37) Djoumbou-Feunang, Y.; Pon, A.; Karu, N.; Zheng, J.; Li, C.;

(

Arndt, D.; Gautam, M.; Allen, F.; Wishart, D. CFM-ID 3.0:

Significantly improved ESI-MS/MS prediction and compound

identification. Metabolites 2019, 9 , 72.

(38) NIST MS interpreter, https://chemdata.nist.gov/dokuwiki/

doku.php?id=chemdata:interpreter (accessed 2021-02-03).