Studies of rearrangement reactions of protonated and ... · Derivatives by MALDI-FT-ICR Mass Spectrometry Hao-Yang Wang and Yin-Long Guo Shanghai Mass Spectrometry Center, Shanghai

Studies of Rearrangement Reactionsof Protonated and Lithium Cationized2-Pyrimidinyloxy-N-ArylbenzylamineDerivatives by MALDI-FT-ICR MassSpectrometry

Hao-Yang Wang and Yin-Long GuoShanghai Mass Spectrometry Center, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,Shanghai, People’s Republic of China

Long LuKey Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academyof Sciences, Shanghai, People’s Republic of China

The rearrangement reactions of protonated and lithium-cationized 2-pyrimidinyloxy-N-arylbenzylamine derivatives were studied by Fourier transform ion cyclotron resonance massspectrometry (FT-ICRMS) and infrared multiphoton dissociation mass spectrometry (IRMPD).Our results show that three kinds of rearrangement reactions occur in IRMPD processes. First,nearly all protonated 2-pyrimidinyloxy-N-arylbenzylamine derivatives undergo Pathway A toform the K ion series. It is proposed that this rearrangement (migration of a substituted benzylgroup) proceeds by way of a gas-phase intramolecular SN2 reaction. Second, a gas phaseintramolecular SNAr type rearrangement mechanism is proposed to explain the formation ofthe F ion series from protonated and lithium-cationized 5 (or 6). This skeletal rearrangementreaction competes with the SN2 reaction of the Pathway A, which produces the K ion series, inIRMPD of protonated 5 and 6. Third, the formation pathway of the W ion series is explainedby a gas phase Cope type rearrangement mechanism. (J Am Soc Mass Spectrom 2004, 15,1820–1832) © 2004 American Society for Mass Spectrometry

The development of environmentally-benign herbi-cides has become a major focus in herbicide re-search. 2-Pyrimidinyloxy-N-arylbenzylamine deriv-

atives with favorable properties, such as a low dose rate ofapplication and a high degree of (bio)degradation, weredeveloped by the Shanghai Institute of Organic Chemistrytogether with Zhejiang Chemical and Industrial Technol-ogy Ltd. This novel, efficient herbicide has been applied inrape fields successfully. For simplicity in this discussion,the eight compounds studied here are each assigned anumber (1 to 8), as shown in the Scheme 1.Previous EI mass spectrometry studies have demon-

strated that these compounds have some unique gas-phase chemistry [1]. No systematic study, however, hasbeen reported for MS/MS data of the protonated andlithium-cationized ions of these compounds. A thoroughunderstanding of the MS/MS and MSn data of theseherbicides is necessary for further development of thesecompounds [2]. Protonated ions often undergo gas-phase

rearrangement [3–13]. The gas-phase rearrangement reac-tions of peptides and modified peptides have been inves-tigated in many previous reports [3–9]. Gas-phase in-tramolecular nucleophilic displacement [3–9] and ion–neutral complex mechanisms [13] are used to interpret thegas-phase rearrangement processes. For example, in thefragmentation reaction of protonated cysteine, the thiol-neighboring group facilitates loss of NH3 [3]. More re-cently, the gas-phase rearrangement of farnesyl trans-ferase inhibitor has been reported; this process involves agas-phase intramolecular SN2 reaction [10].

Published online November 2, 2004Address reprint requests to Dr. Y.-L. Guo, Shanghai Mass SpectrometryCenter, Shanghai Institute of Organic Chemistry, Chinese Academy ofSciences, Shanghai 200032, P. R. China. E-mail: [email protected]

© 2004 American Society for Mass Spectrometry. Published by Elsevier Inc. Received April 26, 20041044-0305/04/$30.00 Revised August 20, 2004doi:10.1016/j.jasms.2004.08.014 Accepted August 24, 2004

IRMPD has been shown to be a viable alternative to CADfor probing molecular structures [4–19]. IRMPD does notappreciably alter the kinetic energy or stable orbits of thetrapped ions.Analysis of the IRMPD data reveals interesting and

remarkable gas-phase reactions of these compounds,which include gas-phase intramolecular SN2 reactions,a SNAr type skeletal rearrangement reaction, and a

gas-phase Cope type rearrangement reaction. Herein,the fragmentation patterns of the protonated 2-pyrim-idinyloxy-N-arylbenzylamine derivatives are general-ized into three main modes (Pathways A, B, and C) andthe structures of three kinds of ions (K, P, and F,respectively) are also proposed.Of note is the finding that the ESI-in-source CAD

spectrum of protonated 5 (or 6) contains only the ion at

Figure 1. The IRMPD spectra of protonated 1 to 8: (a) Protonated 2 at m/z 424; (b) protonated 2 at m/z458; (c) protonated 3 at m/z 449; (d) protonated 4 at m/z 471; (e) protonated 5 at m/z 417; (f) IRMPD ofprotonated 6 at m/z 451; (g) protonated 7 at m/z 416; (h) protonated 8 at m/z 457.

Scheme 1. Chemical structures of 2-Pyrimidinyloxy-N-arylbenzylamine derivatives (1 to 8, molec-ular weight in Da.).

1821J Am Soc Mass Spectrom 2004, 15, 1820–1832 REARRANGEMENTS OF HERBICIDE IN FTMS

Scheme 2. Proposed three main fragment pathways (A, B, and C) and the structures of the threetypes of product ions (K, P, and F) observed in IRMPD processes of protonated 1 to 8.

Table 1. Comparison of the results of accurate mass determinations by FTMS and the actual masses for the proposed structures ofmain product ions of protonated 1 to 8

Compounds Ions element composition Mass detected Actual mass Relative error (PPM)

1 C23H26N3O5 424.1874 424.1867 1.7C20H18N3O4 364.1289 364.1292 �0.8C13H13N2O3 245.0912 245.0921 �3.5

2 C23H25N3O5Cl 458.1495 458.1477 3.9C20H17N3O4Cl 398.0905 398.0902 0.7C13H12N2O3Cl 279.0524 279.0531 �2.5

3 C25H29N4O4 449.2172 449.2183 �2.5C20H18N3O4 364.1290 364.1292 �0.5C13H13N2O3 245.0925 245.0921 1.6

4 C24H28N4O4Cl 471.1808 471.1794 3.0C20H17N3O4Cl 398.0896 398.0902 �1.5C17H23N4O3 331.1755 331.1765 �2.9C13H12N2O3Cl 279.0522 279.0531 �3.2C13H12N3O3 258.0862 258.0873 �4.3

5 C18H18N4O3Br 417.0559 417.0557 0.5C11H12N4O2Br 311.0130 311.0138 �2.6C13H13N2O3 245.0911 245.0921 �4.1

6 C18H17N4O3BrCl 451.0161 451.0167 �1.4C11H12N4O2Br 311.0124 311.0138 �4.5C13H12N2O3Cl 279.0519 279.0531 �4.3

7 C19H19N3O3Br 416.0617 416.0604 3.1C13H13N2O3 245.0924 245.0921 1.2

8 C26H25N4O4 457.1885 457.1870 3.2C26H23N4O3 439.1779 439.1765 3.3C13H13N2O3 245.0922 245.0921 0.4

1822 WANG ET AL. J Am Soc Mass Spectrom 2004, 15, 1820–1832

m/z 245 (or 279, when R2 � Cl) and the ESI-in-sourceCAD spectrum of protonated 9 (or 10) contains only theion at m/z 311. However, the IRMPD spectrum ofprotonated 5 (or 6) contains not only the ion at m/z 245(or 279, when R2 � Cl) but also the ion at m/z 311.Careful analysis of results from two different dissocia-tion techniques shows that a SNAr type skeletal rear-rangement reaction of protonated 5 (or 6) may occurunder IRMPD in the FT-ICR cell. During IRMPD, it isproposed that the protonated 5 (or 6) ions rearrange tothe protonated 9 (or 10) ion, and then dissociate into theF ion at m/z 311. The rearrangement reaction describedin this paper is based on the results of FT-ICR MS.

Experimental

Materials

MALDI matrix, 2,5-dihydrobenzoic acid (DHB), was pur-chased from Sigma (Sigma-Aldrich Co., St. Louis, MO).Water was purified with a Millipore plus system fromMillipore (Millipore, ELPaso, TX). Compounds (1 to 10)were synthesized and verified by NMR, IR, and MS. Allsample solutions were prepared at approximately 1mg/ml with methanol. For lithium adduct experiments,the 0.5 mM LiCl solution was added to the samplesolution. The DHBmatrix solution was deposited onto theprobe tip using Eppendrof GEL loader pipette tips (Brink-ermann Instruments Inc., Westbury, NY) producedmicro-crystalline layers ofmatrix, followed by sample depositiononto the preformed matrix layer. Samples and DHB weredissolved in the D4- methanol in the H™D exchangeexperiment.

MALDI-IRMPD Analysis

The experiments were performed on a FT-ICR MS(Ionspec, Irvine, CA) equipped with a 4.7 T activelyshielded super-conducting electromagnet (Cryomag-netics, Oak Ridge, TN). The external Ionspec MALDIion source used an air-cooled ND:YAG laser (355 nm,New Wave, Fremont, CA) with a gradient filter foradjusting the UV-laser power. Ions, generated from aMALDI source, were transferred via a quadrupole ionguide to the capacitively coupled closed cylindrical cell.Ions of interest were isolated using the arbitrary

waveform generator, with parameters adjusted to eject

unwanted ions (including the isotopic peak) with min-imal loss of the intended precursor ion. The arbitraryamplitude was set to 60 Vb-p and the mass isolationwindow was set as 0.8 m/s to eliminate the unwantedions, so all masses listed here and throughout the paperare the masses of the monoisotopic peak.IRMPD experiments were performed with a model

SYNRAD series 48 CO2 laser (75 MAX). The intensity ofIRMPD laser irradiation was varied between 50� 100%as needed at a flux of 75 W/cm2. The IRMPD laserirradiation pulse time was set as 500 ms for protonatedcompounds and 1500 ms for lithium-cationized com-pounds to give sufficient energy and time for dissocia-tion. The SORI-CAD experiments were also performedand the results show no obvious differences betweenIRMPD and SORI-CAD. Therefore the SORI-CAD ex-periments are not discussed further.

ESI In-Source CAD Analysis

The ESI in-source CAD (Collision Induced Dissociationin ESI ion source) experiments were performed with aquadruple mass spectrometer (Agilent LCMSD-SL, Agi-lent Technologies, Palo Alto, CA). The spray tip poten-tial was set to 4000V. The nebulizing gas flow rate andcurtain gas pressure were 10 ml min�1 and 35 psi,respectively. The fragmentation voltage was set at 250 Vto give sufficient energy for dissociation.

Figure 2. The IRMPD spectra of protonated 8 and the ion at m/z439: (a) Protonated 3 at m/z 457; (b) the ion at m/z 439.

Scheme 3. Formation pathway of K ion at m/z 245 from IRMPD of protonated 7.


Results and Discussion

IRMPD of Protonated 2-Pyrimidinyloxy-N-Arylbenzylamine Derivatives

IRMPD is a general method for activating ions trapped ina FT-ICR cell to generate fragment ions and is frequentlyused for structural analysis and gas-phase chemistry re-search. The IRMPD spectra of protonated 2-pyrimidinyl-oxy-N-arylbenzylamine derivatives obtained using Ion-Spec FT-ICRMS are depicted in Figure 1. According to theIRMPDdata, threemajor dissociation pathways (A, B, andC) and the structures of three major types of product ions(K, P, and F) from IRMPD of the protonated compoundsare proposed and shown in Scheme 2. The structures ofthe product ions are fully supported by the accurate massmeasurements obtained using FT-ICR MS. Table 1 showsa comparison of the masses determined and the actualmasses of the proposed structures, and the correspondingelement composites of the main fragment ions. The rela-tive errors are all less than 5 ppm, so the results indicatethat the proposed structures have the only reasonableelemental compositions. The mechanisms of these path-ways will be discussed in detail below.

Migration of Benzyl Group Initiated byIntramolecular SN2 Reaction

Analyses revealed that the protonated 1 to 8 can allundergo Pathway A to produce the characteristic ionseries of these compounds, the K ion series at m/z 245 (or279, when R2 � Cl). Protonated 7 is selected to illustratePathway A, which is the only dissociation Pathway ofprotonated 7. The mechanism of this rearrangement isproposed to be a gas-phase intramolecular nucleophilicdisplacement reaction (SN2) and is shown in Scheme 3.

The tertiary amine of pyrimidine attacks the benzyliccarbon as the nucleophile and the protonated phenylamine act as a leaving group, which leads to the formationof the ion at m/z 245 by the cleavage of the C™N bondconnecting the benzylic carbon and phenyl amine. Thisrearrangement (migration of the benzylic group fromphenyl amine to pyrimidine amine) is similar to therearrangement of farnesyl transferase inhibitor [10]. Sucha SN2 reaction is facilitated by the fact that pyrimidine is agas-phase nucleophile and protonated phenyl amine is agas-phase leaving group. The mass of the K ion series cangive useful information about the R2 substituent group.

An Interesting Intramolecular DehydrationReaction of Protonated 8

The IRMPD spectrum of protonated 8 contains two prod-uct ions at m/z 245 and 439. The accurate mass determi-nation result shows that the ion at m/z 439 arises bydehydration from protonated 8. To investigate this pro-cess, the IRMPD spectrum of the ion at m/z 439 wasobtained and shown in Figure 2, which contains only oneproduct ion at m/z 245. Therefore, the structural unit,

Scheme 4. Formation pathways of the ion at m/z 245 and 439 from IRMPD of protonated 8.

Scheme 5. The chemical structure of 9 and 10 (molecular weightin Da.).


which can produce the K ion at m/z 245, must remainunchanged during the dehydration process. The twocompetitive rearrangement/ fragmentation mechanismsof protonated 8 are shown in Scheme 4. The interestingdehydration process contains two steps. First, H rear-ranges from arylbenzylamine to the carbonyl of amide. Inthe second step, the arylbenzylamine adds to the proton-ated carbonyl of amide as the nucleophile to form theunstable intermediate ion; the hydroxyl of the intermedi-ate ion is protonated, which gives rise to the iminium ionat m/z 439 by loss of H2O. The iminium ion at m/z 439 hasa benzoimidazole structure. In the IRMPD of the ion atm/z439, the pyrimidine amine attacks the benzylic carbon andthe benzoimidazole type iminium ion acts as a goodleaving group to form the K ion at m/z 245 by cleavage ofthe C™N bond connecting the benzylic carbon and thebenzoimidazole amine.

Rearrangement of Protonated 5 and 6 Promotedby Intramolecular SNAr Reaction

The IRMPD spectrum of protonated 5 (or 6) shows thatprotonated 5 (or 6) has two competitive rearrangementpathways: (1) Pathway A, which gives rise to the K ionseries at m/z 245 (at m/z 279, when R2 � Cl); (2) PathwayC, which gives rise to the F ion at m/z 311. Theformation of F ion at m/z 311 cannot be rationalizedwithout invoking skeletal rearrangement.The recent finding during LC-MS studies of the

degradation products of 5 and 6 provides a valuableclue about the formation pathway of the F ion at m/z311. Our recent studies showed that the 2-pyrimidi-nyloxy-N-arylbenzylamine derivatives could un-dergo rearrangement during chemical degradation.Compounds 9 and 10 are the rearrangement productsof 5 and 6, respectively. The chemical structures ofCompounds 9 and 10 are shown in Scheme 5. In-source CAD was used to obtain structural informa-

tion for the compounds in the LC-MS analysis. To oursurprise, the ion at m/z 245 (or 279, when R2 � Cl) isthe only product ion from ESI in-source CAD ofprotonated 5 (or 6). Furthermore, protonated 9 (or 10)gives the product ion at m/z 311 from ESI in-sourceCAD, which is consistent with the F ion at m/z 311from IRMPD of protonated 5 (or 6). The ESI in-source-CAD spectra of protonated 5 (and 6) andprotonated 9 (and 10) and IRMPD spectra of proton-ated 5 (and 6) are shown in Figures 3 and 4, respec-tively. These results, obtained from two dissociationtechniques, shed light on the mechanism of PathwayC. It is proposed that the formation pathway of the Fion at m/z 311 under IRMPD in the FT-ICR cell is thatthe protonated 5 (or 6) first rearranges to protonated9 (or 10), and then dissociates into the ion at m/z 311.The different timescales of the two dissociation tech-

niques can be used to explain the difference in theproduct ion spectra. During the in-source CAD process,the ions generated by ESI were immediately transferredvia collision skimmer to the quadrupole mass detector.Generally, the product ions of in-source CAD reflect thestructure of precursor ion and the original charge site ofthe ion. However, the comparatively long duration ofIRMPD allows the proton of the activated protonatedion to migrate to other basic sites, which can providealterative rearrangement/fragmentation pathways. Allthe information mentioned above shows that proton-ated 5 and 6 could undergo rearrangement duringIRMPD, just as the rearrangement reaction occurs in thecondensed phase.A two-step mechanism is proposed to explain Path-

way C. In the first step, protonated 5 (or 6) rearranges toprotonated 9 (or 10). It is proposed that this rearrange-ment is initiated by a gas-phase intramolecular aromaticaddition and elimination reaction (SNAr) [20]. ThisSNAr type rearrangement mechanism is shown inScheme 6. In this process, H migrates from the arylben-zylamine group to the neighboring pyridine amine, so

Figure 3. Comparison of the product ion spectra between in-source CAD and IRMPD: (a) The in-source CAD spectrum ofprotonated 5 at m/z 417; (b) the in-source CAD spectrum ofprotonated 9 at m/z 417; (c) the IRMPD spectrum of protonated 5at m/z 417.

Figure 4. Comparison of the product ion spectra between in-source CAD and IRMPD: (a) the in-source CAD spectrum ofprotonated 6 at m/z 451; (b) the in-source CAD spectrum ofprotonated 10 at m/z 451; (c) the IRMPD spectrum of protonated 6at m/z 451.


the neutral arylbenzylamine group can attack the 6-po-sition carbon of pyrimidine as the nucleophile; simul-taneously, cleavage of the C™O bond occurs with the Hmigration from the arylbenzylamine to the phenoloxygen to form the phenol hydroxyl group of proton-ated 9 (or 10).The second step is the formation of the F ion at m/z

311 from protonated 9 (or 10). Ion–neutral complexesare invoked to explain this pathway. This mechanismis shown in Scheme 7. Ion–neutral complexes havebeen increasingly proposed as important intermedi-ates in unimolecular fragmentation/rearrangement

in MS/MS [13]. The ion–neutral complex is formedafter an intramolecular SN2 reaction. In this SN2reaction, the oxygen of phenol hydroxyl group at-tacks the benzylic carbon as the nucleophile and theprotonated phenyl amine acts as a leaving group; thisinduces cleavage of the C™N bond connecting thebenzylic carbon and protonated phenyl amine toform the ion–neutral complexes. In this ion–neutralcomplex, the protonated phenol ether is bonded tophenyl amine by hydrogen bonding, and then Hrearrangement from the protonated phenol ether tophenyl amine occurs to form the F ion at m/z 311.

Scheme 6. The mechanism of the SNAr type rearrangement.

Scheme 7. Formation pathway of the F ion at m/z 311 from IRMPD of protonated 9 (or 10). The datain parentheses are those observed, when R2 is Cl substitute group.

Scheme 8. Formation pathways of the ions at m/z 331, 258, and 398 from IRMPD of protonated 4.


Compared with 5, 6 has a chlorine substituent at C-6of the benzene of benzylamine, so the chlorine sub-stituent can be regarded as a marker to support therearrangement process.However, the reason that 5 (or 6) can undergo this

reaction to such a great extent can be explained asfollows: 5 and 6 have a tertiary amine of pyridine asneighboring functional groups of arylbenzylamine withsimilar PA (proton affinity); therefore, the H migrationcan readily occur to release the neutral arylbenzylamineas a nucleophile to induce the SNAr type gas-phaserearrangement. When no other neighboring functionalgroup is available to accept H, Pathway A will be thedominant dissociation process, just as for compound 7.Protonated 4 can also undergo Pathway C to form Fions (at m/z 331). The proposed mechanism of thisprocess is shown in Scheme 8. The amide group ofCompound 4 plays a similar role as the tertiary amine ofthe pyridine group in 5 (or 6) to stabilize the proton.However, it is only a minor process compared with therearrangement reaction of protonated 5 and 6, because

of the long distance between the arylbenzylamine andamide group in Compound 4.

Gas-Phase Cope Type Rearrangement of P Ionsfrom Protonated 1 and 2

Pathway B involves loss of a neutral molecule. Theprotonated 1, 2, 3, and 4 ions can undergo the Pathway Bproduce the P ion at m/z 364 (or 398, when R2 � Cl). Ofnote is the finding that the IRMPD spectra of the P ion atm/z 364 (or 398, when R2 � Cl) from protonated 1 (or 2),show abundant peaks of the W ion series by an NH3elimination pathway. The assignment of W ions wassupported by comparison between the average mass shiftfrom P ions (17.0263 Da) and the accurate mass of NH3(17.0265 Da). This NH3 elimination pathway is an unusualprocess for the P ions, so the formation of the W ions wasmore fully explored. To reveal the whole process, theMS/MS spectra of the deuterium-labeled molecule (D2-1)at m/z 426 and the ion at m/z 366 were obtained by H™D

Figure 5. The IRMPD spectra of deuterated 1 and the ion at m/z 366: (a) deuterated 1 at m/z 426; (b)the ion at m/z 366.

Scheme 9. Formation pathways of P1 ion and P2 ion from IRMPD of [D2 � 1]�.


exchange experiments and shown in Figure 5. The twoproduct ions at m/z 365 and m/z 366 show that the P ionsmay have two different structures, the P1 ions and the P2

ions. The proposed mechanism and the structures of theP1 ions and the P2 ions are shown in Scheme 9. The P1 ionat m/z 366 is formed, when H to form propan-2-ol comesfrom the ortho-position H of the benzene. The P2 ion atm/z 365 is formed, when D is located at the oxygen of theester.The ND2H loss pathway from the ion at m/z 366

shows that the nitrogen of neutral NH3 loss is from thenitrogen of arylbenzylamine. But the source of the H inthe ND2H is the most important clue to illustrate theNH3 elimination process. There is no active H near thearylbenzylamine, so a complicated rearrangement pro-cess may be involved. A gas-phase Cope type rearrange-ment [21–25] is proposed to explain the formation of theW ion and the proposed rearrangement mechanism isshown in Scheme 10. The NH3 elimination processcontains two steps. The first step is an atypical gas-phase Cope rearrangement process. In this process, anew C™C bond connecting two aromatic rings isformed, and simultaneously, the C™N bond connectingbenzylic carbon and protonated phenyl amine is bro-ken, which occurs with the migration of double bonds.In the second step, the H rearrangement leads to theformation of protonated aniline, and then the NH3elimination occurs with the bonding of the benzyliccarbon to the neighboring benzene to form the W ion.The IRMPD spectrum of the P1 ion at m/z 364 from

protonated 1 and IRMPD spectrum of the P1 ion at m/z398 from protonated 2 is shown in Figure 6. The P1 ionsfrom two compounds were dissociated at the same theenergy, the relative abundance of the W ion (31%) of 1is higher than the relative abundance of theW ion (20%)of 2. This occurs since the steric hindrance caused bychlorine substitution at C-6 of the benzene blocks theCope type rearrangement process, which in turn showsthat the rearrangement process is reasonable. The Uions at m/z 209 (or 243, when R2 � Cl), which aregenerated by loss of pyrimidine from the W ions andthe Y ion at m/z 319 (or 353, when R2 � Cl), which aregenerated by loss of CO, can also be used to support theformation process of the W ions.

Figure 6. The IRMPD spectra of the P ion from protonated 1 andprotonated 2: (a) The ion at m/z 364 from protonated 1; (b) the ionat m/z 398 from protonated 1.

Scheme 10. Formation pathways of the W ion series, U ion series and Y ion series from IRMPD ofprotonated P ion series. The data in parentheses are those observed, when R2 is Cl substitute group.


IRMPD of Lithium-Cationized 2-Pyrimidinyloxy-N-Arylbenzylamine Derivatives

Alkali metal cationization has been explored in recentyears as an alternative to protonation to change thesite of charge localization and promote differentfragmentation processes [19, 26]. In the presence ofalkali metal ions, MALDI spectra of these compoundsyield abundant adduct ions; however, only the lithi-ated adduct ions of 1, 2, 5, and 6 yield informativefragment ions for structural determination. The spec-tra are shown in Figure 7. The analysis of IRMPD dataof the lithium cationized compounds shows the majorpathways mentioned above are still useful for inter-preting the product ions. The proposed structures ofthe product ions are supported by the accurate massdeterminations. Table 2 shows a comparison of themasses determined and the actual masses of theproposed structures, and the corresponding elemen-tal compositions of the main fragment ions. Therelative errors are all less than 5 ppm, so the resultsindicate the proposed structures of the product ionshave the only reasonable elemental compositions.

Multi-Stage IRMPD of Lithium-Cationized 1 and 2

The effect of lithium cationization on the dissociation oflithium cationized 1 (or 2) is striking. There are twonotable changes in the dissociation pattern of lithium-cationized 1 (or 2). First, the IRMPD spectrum oflithium-cationized 1 (or 2) shows more fragment ions,presumably since the lithium ion changes the site ofcharge localization and the coordination effect of lith-ium ion promotes different fragmentation processes[19]. The ion generated from the loss of propene at m/z388 (or 422, when R2 � Cl) and the ion formed byfurther loss of CO2 at m/z 344 (or 378, when R2 � Cl),were observed with high abundance instead of the ionfrom loss of propan-2-ol seen for protonated 1 (or 2).Second, the ions at m/z 238 arising from Pathway C canbe observed in the IRMPD spectrum of lithium-contain-ing ions at m/z 344 (or 378, when R2 � Cl). Interestingly,the formation of the K ion at m/z 245 (or 279, when R2 �Cl) can still be observed in the IRMPD spectrum oflithium-cationized 1 (or 2). However, this pathway isminor compared with that of protonated 1 (or 2). Thegenealogical relationships of dissociation of lithium-

Figure 7. The IRMPD spectra of lithium-cationized 1, 2, 5 and 6: (a) lithium-cationized 1 at m/z 430;(b) lithium-cationized 2 at m/z 464; (c) lithium-cationized 5 at m/z 423; (d) lithium-cationized 6 at m/z457.

Table 2. Comparison of the results of accurate mass determinations by FTMS and the actual masses for the proposed structures ofmain product ions of lithium-cationized compounds 1, 2, 5, and 6

Compounds Ions element composition Mass detected Actual mass Relative error (PPM)

1 C23H25N3O5Li 430.1956 430.1949 1.7C20H19N3O5Li 388.1483 388.1479 1.0C19H19N3O3Li 344.1581 344.1581 0.0C13H13N2O3 245.0913 245.0921 �3.1C12H13N3O2Li 238.1155 238.1162 �3.1

2 C23H24N3O5ClLi 464.1560 464.1559 0.2C20H18N3O5ClLi 422.1096 422.1090 1.5C19H18N3O3ClLi 378.1198 378.1191 1.8C13H12N2O3Cl 279.0536 279.0531 1.8C12H13N3O2Li 238.1166 238.1162 1.6

5 C18H17N4O3BrLi 423.0645 423.0639 1.5C11H11N4O2BrLi 317.0209 317.0220 �3.4

6 C18H16N4O3BrClLi 457.0261 457.0249 2.6C11H11N4O2BrLi 317.0232 317.0220 3.8


cationized 1 and lithium-cationized 2 were demon-strated clearly by multi-stage IRMPD, and the multi-stage IRMPD spectra of lithium-cationized 1 andlithium-cationized 2 are shown in Figures 8 and 9,respectively. The proposed fragmentation genealogicalrelationship patterns for the lithium-cationized speciesare shown in Scheme 11.

Rearrangement of Lithium-Cationized 5 and 6in IRMPD

The IRMPD spectrum of lithium-cationized 5 and 6contains only the ion at m/z 317. The proposed forma-

tion pathway of the ion at m/z 317 is that the lithium-cationized 5 (or 6) first rearranges to lithium-cationized9 (or 10) and then dissociates into the ion at m/z 317.This mechanism is similar to that of protonated 5 and 6.However, there are two different points: (1) the multi-site coordination effect of lithium ion may producelithium-containing chelate bond connecting pyridineamine and pyrimidine amine, which plays an importantrole in the rearrangement and can be used to explainwhy this pathway is the exclusive dissociation pathwayof lithium-cationized 5 (or 6). The proposed mechanismfor the SNAr type rearrangements from lithium-cation-ized 5 (or 6) is shown in Scheme 12; (2) the formation of

Figure 8. The IRMPD spectra of lithium-cationized 1, the ion atm/z 388 and the ion at m/z 344: (a) Lithium-cationized 1 at m/z 430;(b) the ion at m/z 388; (c) the ion at m/z 344.

Figure 9. The IRMPD spectra of lithium-cationized 2, the ion atm/z 422 and the ion at m/z 378: (a) Llithium-cationized 1 at m/z 464;(b) the ion at m/z 422; (c) the ion at m/z 378.

Scheme 11. Formation pathways of the product ion at m/z 388 (or 422), 344 (or 378), 245 (or 279), and238 from IRMPD of lithium-cationed 1 (or 2). The data in parentheses are those observed, when R2 isCl substitute group.


the ion at m/z 317 from lithium-cationized 9 (or 10) maynot involve an ion–neutral complex process. It is pro-posed that H rearranges from the form phenol hydroxylto phenyl amine first and then the oxygen attacks thebenzylic carbon, which induces the cleavage of the C™Nbond connecting the benzylic carbon and phenyl amineto form the ion at m/z 317.

Conclusions

This study demonstrates that the protonated and lithi-um-cationized compounds (1 to 8) undergo compli-cated rearrangements and fragmentations underIRMPD in a FT-ICR cell. It has been shown that thegas-phase intramolecular SN2 reactions play an impor-tant role in the formation of the K ion series; an adjacentfunctional group capable of accepting H provides analternative gas-phase SNAr type skeletal rearrangementpathway to form F ion series. Based on comparison ofIRMPD spectra and the in-source CAD spectra, thegas-phase rearrangement of protonated 5 and 6 waselucidated and highlighted. It is proposed that theformation of the F ion at m/z 311 from protonated 9 (or10) proceeds by way of ion–neutral complexes. More-over, a gas-phase Cope type rearrangement mechanismis proposed to explain the formation of the W ion seriesand is supported by the H™D exchange experiment. Fullunderstanding of the fragment pathway is importantfor the further metabolism and degradation research ofthese compounds. These rearrangement reactions mayhave significance for understanding gas-phase rear-rangements of protonated species in general.

AcknowledgmentsThe authors gratefully acknowledge financial support by theNational Natural Science Foundation of China (no. 20175034) andby the Chinese Academy of Sciences.

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Studies of rearrangement reactions of protonated and ... · Derivatives by MALDI-FT-ICR Mass Spectrometry Hao-Yang Wang and Yin-Long Guo Shanghai Mass Spectrometry Center, Shanghai