study of methylation of nitrogen-containing compounds in the gas phase

JOURNAL OF MASS SPECTROMETRYJ. Mass Spectrom. 2007; 42: 218–224Published online 18 December 2006 in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/jms.1154

Study of methylation of nitrogen-containingcompounds in the gas phase

Xiang Zhang, Haoyang Wang, Yuanxi Liao, Houwei Ji and Yinlong Guo∗

Shanghai Mass Spectrometry Center, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, China

Received 24 April 2006; Accepted 31 October 2006

The methyl migration between the protonated N,N-dimethylisopropylamine and other neutral aliphaticamines in the gas phase has been investigated by a hybrid external chemical ionization source ion trapmass spectrometer. Similar reactions have been found in the aqueous solution by Callahan and Wolfenden(J. Am. Chem. Soc. 2003; 125: 310). At the same time, density functional theory (DFT) calculations showthat in contrast with the neutral N,N-dimethylisopropylamine, protonated N,N-dimethylisopropylamineis a better methyl donor. These results indicate that protonation promotes the methyl migration betweenaliphatic amines both in the gas phase and in the aqueous solution. In addition, methyl transfer also existsbetween the protonated N,N-dimethylisopropylamine and some other nitrogen-containing compounds.Copyright 2006 John Wiley & Sons, Ltd.

KEYWORDS: methylation; nitrogen-containing compounds; ion trap mass spectrometry

INTRODUCTION

In the general area of ion–molecule reactions, methyl trans-fer is attracting increasing interest because of many rele-vant implications in chemistry1,2 and biochemistry.3 – 5 Thedimethylchlorinium ion,6 �CH3�2ClC, and methoxymethylcation,7 CH3OCH2

C, which are capable of transferring aCH3

C group via a simple SN2 transition state, have been usedto evaluate the gas-phase basicities and relative methylationnucleophilicities of some simple compounds, such as alco-hols, amines and thiols.8 The selectivity of the reactions of�CH3�2ClC with pyridine derivatives has been studied, andit has been found that the chloronium ion selectively methy-lates at the site with the greatest methyl cation affinity.9

Moreover, CH3OCH2C can react with glycine10 or cysteine,11

and limited regioselectivity was found in the competitionbetween addition at the oxygen and nitrogen atoms. Brod-belt and coworkers12 investigated the methylation of sp2- andsp3-nitrogens, and discovered the reversal of regiochemistryin the gas phase and in solution.

Methyl transfer reactions are important not only inorganic syntheses2 but also in a wide variety of biochem-ical processes,5 including the conversion of homocysteineto methionine, the methylation of glutamate residues inchemoreceptors, and in gene regulation.13 These processesare always carried out in solution. There are two basic typesof methyl transfer reactions involving neutral acceptors. Inthe first, the methyl transfer reagent is neutral, as in theMenshutkin reaction.14 Here, the transition state is dipolar

ŁCorrespondence to: Yinlong Guo, Shanghai Mass SpectrometryCenter, Shanghai Institute of Organic Chemistry, Chinese Academyof Sciences, Shanghai 200032, China. E-mail: [email protected]

and will be strongly stabilized by polar solvents.15

R3N : CMeX ��! [R3N-Me-X] ��! R3NMeC C X� �1�

In the second type, the methyl transfer reagent bears apositive charge. Here, the transition state for the reaction isless polar than the reactant because the charge is spread outover a large volume. Now, polar solvents should decreasethe rate of reaction.16,17

R3N : CMe3SC ��! [R3N-Me-SMe2]C

��! R3NMeC C Me2S �2�

Recently, Callahan and Wolfenden18 have reported thatmethyl groups migrated between aliphatic amines whenthey were incubated with their conjugate acids at elevatedtemperature in water, as shown below.

R3 R3 R4R5

R2

R1

R4

R5R2

R1

N NH H

H3C CH3N+ N++ + (3) �3�

In addition to the apparent novelty of these reactions, wethink it of interest to examine whether similar reactions occurin the gas phase. It is useful to understand the mechanism ofthe methyl transfer between aliphatic amines externally.

The mass spectrometer has been used for a long timeto study gas-phase reactions such as gas-phase polarcycloadditions,19 SN2 reactions,20 and other interestingreactions.21 – 26 Here, all experiments were performed in ahybrid, external-source ion trap mass spectrometer.

EXPERIMENTAL

MaterialsN,N-dimethylisopropylamine, N,N-dimethylbutylamine, tri-ethylamine, N,N,N0,N0-tetramethylethylenediamine,

Copyright 2006 John Wiley & Sons, Ltd.

Gas-phase methylation of nitrogen 219

n-propylamine, 2-(diphenylmethyl)pyrrolidine, dipheny-lamine, N,N-dimethylphenylamine, pyridine, imidazole,pyrrole, acetonitrile, guanine (G), adenine (A), uracil (U),and thymine (T) were purchased from Aldrich.

Mass spectrometerAll mass spectra were obtained using a hybrid, external-source ion trap mass spectrometer (Saturn 4000; Varian,Walnut Creek, CA, USA). The hybrid chemical ionization (CI)configuration is one of the three operational configurationsof the 4000 GC/MS system, shown in Fig. 1. The CI reagentions are generated in the external source and only thosereagent ions that are selected are stored in the ion trap.These trapped reagent ions are allowed to react with samplemolecules as they enter the ion trap, forming CI product ionsby ion–molecule reactions.

In the present work, N,N-dimethylbutylamine and N,N-dimethylisopropylamine were selected as CI reagent gasesbecause of their appropriate volatility. Other nitrogen-containing compounds were introduced into the ion trap byGC via the 1079 injection port. A VF-5 fused silica capillarycolumn (30 m ð 0.25 mm. i.d. DF D 0.25 µm) (Varian) wasused in the gas chromatograph. The temperature of the ionsource was 280 °C. The oven temperature program was: 3 minat 100 °C; 20 °C/min to 250 °C. The transfer line temperaturewas 200 °C; the carrier gas: He; and the flow mode: constantflow at 1 ml/min. The ion trap operated in the normalscan mode (scan range from m/z 50 to m/z 400, scan rate:1 scan/s).

Computational methodsThe nitrogen-containing compounds reported here wereoptimized with the hybrid density functional theory (DFT)using Becke’s three-parameter hybrid exchange-correlationfunctional27 containing the non-local gradient correctionof Lee, Yang, and Parr (B3LYP) within the Gaussian03program.28 All optimized structures were confirmed asminima by calculation of numerical vibrational frequencies.The basis set used for the remaining atomic species was6–31G with the important addition of the polarizationfunctions (d) for all atoms, including the hydrogens.

Figure 1. Schematic diagram of the hybrid ionizationconfiguration.

RESULTS AND DISCUSSION

The methyl migration between the aliphaticamines in the gas phaseWhen N,N-dimethylisopropylamine was selected as CIreagent gas, protonated N,N-dimethylisopropylamine atm/z 88 could be obtained through electron ionizationof the reagent gas in the external source and wasthen selected to be stored in the ion trap. Next, itcould react with other neutral aliphatic amines, suchas triethylamine, N,N,N0,N0 –tetramethylethylenediamine,n-propylamine, and 2-(diphenylmethyl)pyrrolidine in theion trap, where methyl transfer reactions might takeplace between the protonated N,N-dimethylisopropylamine(methyl donors) and neutral aliphatic amines (methyl accep-tors), as shown in Fig. 2. The corresponding mass spectra areshown in Fig. 3. In the mass spectrum of the ion–moleculereaction between protonated N,N-dimethylisopropylamineand triethylamine (Fig. 3(a)), the ion at m/z 88 due tothe protonated N,N-dimethylisopropylamine and the ionat m/z 116 due to the N-methylated triethylamine aredetected. Moreover, we also observe the protonated tri-ethylamine at m/z 102, which is generated by the protontransfer between protonated N,N-dimethylisopropylamineand triethylamine. Analogously, Fig. 3(b–d) shows themass spectra of the ion–molecule reactions between theprotonated N,N-dimethylisopropylamine and N,N,N0,N0-tetramethylethylenediamine, n-propylamine, and 2-(diphen-ylmethyl)pyrrolidine, respectively. The ion at m/z 131(Fig. 3(b)) due to the N-methylated N,N,N0,N0-tetramethyle-thylenediamine, the ion at m/z 74 (Fig. 3(c)) due to the N-methylated n-propylamine, and the ion at m/z 252 (Fig. 3(d))due to the N-methylated 2-(diphenylmethyl)pyrrolidine areobserved. In addition, we also detect the correspond-ing protonated aliphatic amines at m/z 117 (Fig. 3(b))and m/z 238 (Fig. 3(d)); however, the protonated n-propylamine cannot be observed (Fig. 3(c)). On the otherhand, when N,N-dimethylbutylamine was selected as CIreagent gas, methyl transfer could take place between theprotonated N,N-dimethylbutylamine and neutral aliphaticamines such as N,N,N0,N0-tetramethylethylenediamine andn-propylamine. In Fig. 3(e), the ion at m/z 131 due to theN-methylated N,N,N0,N0-tetramethylethylenediamine andthe ion at m/z 117 due to the protonated N,N,N0,N0-tetramethylethylenediamine are observed. In Fig. 3(f), onlythe ion at m/z 74 due to the N-methylated n-propylamine isdetected.

Since methyl migration can take place between the pro-tonated aliphatic amines and neutral aliphatic amines inthe gas phase, and a similar transfer occurs in the aque-ous solution, which has been studied by Callahan andWolfenden,18 it may be deduced that when the aliphaticamine bears a positive charge, for example, protonation,methyl migration takes place easily. Methyl migrationsbetween neutral amines are not expected to take place.DFT calculations at the B3LYP/6-31GŁ level on the methyltransfer have been performed in Gaussian 03. Figure 4shows the potential energy surface for the methyl transferbetween the protonated N,N-dimethylisopropylamine andtriethylamine in the gas phase, together with the B3LYP/

Copyright 2006 John Wiley & Sons, Ltd. J. Mass Spectrom. 2007; 42: 218–224DOI: 10.1002/jms

220 X. Zhang et al.

n-C4H9

n-C4H9 n-C4H9

n-C4H9

H

+

NH

NC2H5

C2H5

C2H5 C2H5

C2H5

C2H5

NH

H

H

NH

N

H

Hn-C3H7 n-C3H7

n-C3H7 n-C3H7

+

H

H+N

H

H

iso-C3H7

iso-C3H7

iso-C3H7

iso-C3H7

iso-C3H7

iso-C3H7

iso-C3H7

iso-C3H7

N+

N+

N+

N+

N+

N+

N+

N+

N+

N+

N+

N+

H NH

NH

NN N

NN N

+

H + +

H + +

H +NH

Ph

Ph

NH

+

Ph

PhH

+

H +

Figure 2. Ion–molecule reactions between aliphatic amines.

Figure 3. The mass spectra of the ion–molecule reactions between aliphatic amines (a) protonated N,N-dimethylisopropylamine andtriethylamine; (b) protonated N,N-dimethylisopropylamine and N,N,N0,N0-tetramethylethylenediamine; (c) protonated N,N-dimethyl-isopropylamine and n-propylamine; (d) protonated N,N-dimethylisopropylamine and 2-(diphenylmethyl)pyrrolidine; (e) protonatedN,N-dimethylbutylamine and N,N,N0,N0-tetramethylethylenediamine; (f) protonated N,N-dimethylbutylamine and n-propylamine.



Figure 4. Potential energy surface for the methyl transfer between the protonated N,N-dimethylisopropylamine and triethylamine inthe gas phase.

Figure 5. The mass spectra of the ion–molecule reactions between the protonated aliphatic amines and neutral aromatic amines(a) protonated N,N-dimethylisopropylamine and diphenylamine; (b) protonated N,N-dimethylisopropylamine andN,N-dimethylphenylamine.

6-31GŁ computed structures. It is well known that themethyl transfer proceeds via an SN2 transition state. Thefirst step corresponds to the formation the complex 1,and then the product complex 2 is formed via the rate-determining transition state TS1. Complex 2 is dissoci-ated subsequently to the separated products. The wholeprocess is exothermic by 0.7 kcal/mol and the activationenergy is 30.4 kcal/mol. Moreover, for the SN2 reaction,the entropy of activation better characterizes the activationprocess. The activation entropy of the methyl migrationbetween the protonated N,N-dimethylisopropylamine andneutral triethylamine is �45.4 cal/(K mol). The negativesign indicates that the methyl transfer is easy to takeplace.

On the other hand, calculations show that the pro-cess of the methyl transfer between the neutral N,N-dimethylisopropylamine and triethylamine in the gas phasehas a barrier of 90.8 kcal/mol, and that this process isendothermic by 171.0 kcal/mol. It is obvious that the methyltransfer between two neutral amines is unlikely. Accordingto the methyl transfer in the gas phase and the theoretical cal-culations, it might be concluded that protonation promotes

methyl migration between aliphatic amines both in the gasphase and in the aqueous solution.

As the methyl migration takes place between aliphaticamines, it is found that protonated N,N-dimethylisopropyla-mine can provide the methyl group for other compounds,such as �CH3�2ClC, CH3OCH2

C, and �CH3�3OC, etc. Pre-vious studies have indicated that �CH3�2ClC, �CH3�3SC,17

and �CH3�3OC 12can react with some nitrogen-containingcompounds. So, next we tried some ion–molecule reactionsbetween the protonated N,N-dimethylisopropylamine andother nitrogen-containing compounds.

+

iso-C3H7iso-C3H7

iso-C3H7iso-C3H7

N+ N+

N+N+

H NH

+

H + N

PhN

H

+

HN

Ph

Ph

Ph

Ph

H

Ph

Figure 6. Ion–molecule reactions between protonatedaliphatic amines and aromatic amines.


222 X. Zhang et al.

Figure 7. The mass spectra of ion–molecule reactions between (a) protonated N,N-dimethylisopropylamine and pyridine;(b) protonated N,N-dimethylisopropylamine and imidazole.

+

iso-C3H7iso-C3H7

iso-C3H7iso-C3H7

N+

N+

N+N+

H N

N

H

NH

+

H + +

N

NN

Figure 8. Ion–molecule reactions between protonatedaliphatic amines and pyridine and imidazole.

The methylation of aromatic aminesFirst we wondered whether the methyl transfer would takeplace between protonated aliphatic amines and neutral aro-matic amines. Protonated N,N-dimethylisopropylamine wasselected as the methyl donor, and diphenylamine and N,N-dimethylphenylamine were selected as the methyl acceptors.The mass spectra (Fig. 5) tell us that methyl can migratebetween the protonated aliphatic amines and neutral aro-matic amines, and the corresponding ion–molecule reactionsare shown in Fig. 6. In Fig. 6(a), the ion at m/z 184 due toN-methylated diphenylamine is observed; however, the pro-tonated diphenylamine is not observed. In Fig. 6(b), the ion atm/z 136 due to the N-methylated N,N-dimethylphenylamineand the ion at m/z 121 due to the radical cation of N,N-dimethylphenylamine are observed. That is to say, besidesmethyl transfer, electron transfer also occurs between theprotonated N,N-dimethylisopropylamine and neutral N,N-dimethylphenylamine.

The methylation of other nitrogen-containingcompounds, such as pyridine, imidazole,pyrimidine, purine, pyrrole, and acetonitrileBesides aliphatic amines and aromatic amines, we havealso tried some other nitrogen-containing compounds.Pyridine, imidazole, and pyrrole were selected as themethyl acceptors, and the corresponding mass spectra ofthe ion–molecule reactions are depicted in Fig. 7. The ion atm/z 94 (Fig. 7(a)) due to the N-methylated pyridine and theion at m/z 84 (Fig. 7(b)) due to the N-methylated imidazoleare detected. It is obvious that methyl can migrate betweenthe protonated aliphatic amine and pyridine (or imidazole)(Fig. 8). However, we cannot observe the N-methylatedpyrrole ion, that is to say, methyl cannot migrate betweenthe protonated aliphatic amine and pyrrole.

Next, we wanted to try G, A, U, and T. Figure 9 shows thecorresponding mass spectra. The ion at m/z 166 (Fig. 9(a))due to the N-methylated guanine ([G C CH3]C), the ionat m/z 150 (Fig. 9(b)) due to the N-methylated adenine([A C CH3]C), the ion at m/z 127 (Fig. 9(c)) due to the N-methylated uracil ([U C CH3]C), and the ion at m/z 141(Fig. 9(d)) due to the N-methylated thymine ([T C CH3]C)were observed. It is apparent that methyl can also migratebetween the protonated aliphatic amine and pyrimidine (orpurine) (Fig. 10). Finally, acetonitrile was selected as themethyl accepter; however, as in pyrrole, we could not obtainthe N-methylated ion, which meant that methyl could notmigrate between a protonated amine and acetonitrile.

In fact, between the protonated N,N-dimethylisopropyla-mine and other neutral nitrogen-containing compounds,there are four possible competing channels: (1) protontransfer, (2) methyl transfer, (3) isopropyl transfer, and(4) electron transfer. First, isopropyl transfer is not found inour experiments. For most nitrogen-containing compounds,methyl transfer can take place, except pyrrole and acetoni-trile, maybe because of their high methyl affinity, whichis related to the electron cloud delocalization on the �-system. Between aliphatic amines, besides methyl transfer,proton transfer can also take place. In addition, electrontransfer can also take place between the protonated N,N-dimethylisopropylamine and N,N-dimethylphenylamine.

CONCLUSIONS

The methyl migration between the protonated N,N-dimethylisopropylamine and other neutral aliphatic aminesin the gas phase has been studied. A similar trans-fer has been found in the aqueous solution. For N,N-dimethylisopropylamine, it is found that when it bears apositive charge, e.g. protonation, methyl migration will beeasy. DFT calculations also support this conclusion. There-fore, protonation promotes the methyl migration betweenaliphatic amines whether in the gas phase or in the aqueoussolution. In addition, methyl transfer also exists between theprotonated N,N-dimethylisopropylamine and some othernitrogen-containing compounds.

AcknowledgementsThe authors gratefully acknowledge financial support from theNational Natural Science Foundation of China (No. 20475059).



Figure 9. The mass spectra of ion–molecule reactions between (a) protonated N,N-dimethylisopropylamine and guanine;(b) protonated N,N-dimethylisopropylamine and adenine; (c) protonated N,N-dimethylisopropylamine and uracil; (d) protonatedN,N-dimethylisopropylamine and thymine.

+

iso-C3H7 iso-C3H7N+H N

H+ [A(G,U,T)+CH3]+A(G,U,T)

Figure 10. Ion–molecule reactions between the protonatedN,N-dimethylisopropylamine and pyrimidine (U, T) and purine(G, A).

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study of methylation of nitrogen-containing compounds in the gas phase

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