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Benzene Loss from Trityl Cations— a Mechanistic Study

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Benzene Loss from Trityl Cations— a Mechanistic Study View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Elsevier - Publisher Connector Benzene Loss from Trityl Cations— A Mechanistic Study Chagit Denekamp and Moran Yaniv Department of Chemistry and Institute of Catalysis Science and Technology, Technion–Israel Institute of Technology, Haifa, Israel Triarylmethyl cations eliminate substituted benzene in the gas phase, upon activation. The mechanism of this process has been studied using deuterium labeling, substituent effects, and density functional theory calculations. It is shown that this apparently simple dissociation is in fact a complicated stepwise process that involves several consecutive hydride shifts. The combination of experimental evidence and computational results leads to a clear description of transition states and reaction intermediates. (J Am Soc Mass Spectrom 2006, 17, 730–736) © 2006 American Society for Mass Spectrometry riphenylmethane dyes form a very important the most abundant being the formation of a trityl cation. class of commercial dyes with applications in Another characteristic of these EI spectra is the presence Tmany fields [1]. Another function of the of antriph- ion at m/z165 that corresponds to the loss of 78 Da enylmethyl (trityl) groups is their usefulness as labile from the trityl cation of m/z 243. The authors suggest protecting groups in organic synthesis [2]. The thatapplica- the resulting ion of m/z 165 is a 9-fluorenyl cation. tions of trityl groups are based on their unique ability to It is also known that benzhydryl cations can afford an form stabilized cations. This is reflected in the relative analogous ion by the loss of H2 [10]. The loss of electrophilicity parameter for trityl cation [3] as molecularwell as hydrogen is supported by the presence of a ϩ hydride affinity [4] andR values k [5]. Trityl cations aremetastable ion. generated from alcohols in acidic conditions, but they Triphenylmethane has also been studied with nega- can also be generated from trityl halides in the presence tive chemical ionization, showing an abundant [M Ϫ Ϫ of alkali metal, alkaline earth metal ions [6], H]or ion.Lewis Upon high-energy collisional activation, two acids. A useful tool for the study of ionic species like major dissociation processes occur, involving the losses trityl cations is gas-phase ion chemistry using mass of C2H4 and C6H6 neutral fragments. The authors spectrometry. Mass spectrometry has been used to suggest that the loss of C6H6, a benzene ring, results in understand dissociation kinetics and mechanisms of the formation of a 9-fluorenyl anion. Isotopic labeling gas-phase molecular ions. Fragmentation pathways of experiments indicate extensive hydrogen-deuterium molecular ions are determined using tandem mass scrambling within this process [11]. spectrometry (MS/MS) isotopic labeling and molecular Modern desorption techniques enable the generation orbital calculations that provide valuable information. of stable trityl cations under relatively mild conditions. Owing to its general importance to synthetic organic In the present work, we studied the characteristic loss of chemistry, a large body of knowledge on the chemistry benzene or substituted benzene molecules from trityl of cationic aromatic compounds, like alkylbenzenium cations. A detailed mechanism for the formation of ions and benzyl cations has been accumulated in the 9-fluorenyl cation is proposed and supported by exper- ϩ past four decades. For example, the production of C7H7 imental work and theoretical calculations. from toluene molecular ion is one of the most exten- sively studied reactions in the field of gas-phase ion Results and Discussion chemistry [7]. Other studies concentrate on protonation, ion-neutral complex formation, and H/D exchange Protonated trityl alcohols eliminate water readily af- processes in aromatic systems. See for example fording[8] and trityl cations, under chemical ionization, atmo- references cited therein. spheric pressure chemical ionization, or electrospray Electron ionization spectra of several trityl com- ionization (ESI). Cation1 (Figure 1) andpara -substi- pounds were recorded by Shupe and Berlin tuted[9]. analogsThe were generated in the ESI source from the authors show several typical fragmentation pathways, corresponding alcohols in a 0.01 M TFA solution of methanol. The relative stability of the resulting trityl cations is evident by their inefficient reaction with Published online March 13, 2006 nucleophiles, at least under low pressures, in an FT-ICR Address reprint requests to Dr. C. Denekamp, Department of Chemistry Technion-Israel Institute of Technology, Haifa 32000, Israel. E-mail: cell. For comparison, reactions of analogous benzhydryl [email protected] and 9-fluorenyl cations with the same nucleophiles © 2006 American Society for Mass Spectrometry. Published by Elsevier Inc. Received November 30, 2005 1044-0305/06/$32.00 Revised February 2, 2006 doi:10.1016/j.jasms.2006.02.002 Accepted February 2, 2006 J Am Soc Mass Spectrom 2006, 17, 730–736 BENZENE LOSS FROM TRITYL CATIONS 731 + NHR2 R2NH + OH2 + -H O -C6H6 2 1 Scheme 1. Proposed formation of 9-fluorenyl cation from both cation 1 and 9-fluerenol. while the mass of unlabeled 9-fluorenyl cation is 165 Figure 1. CID spectrum of trityl cation 1. Da. In the presence of pyrrolidine (4.5 exp-9 mbar) the relative abundances of all parent ions decrease with time, affording four product ions at m/z 245, 241, 240, were carried out under similar conditions [12]. Never- and 236 (Figure 2). Figure 3 shows the decrease in the theless, despite this apparent stability towards associa- relative intensity of two precursor ions as a function of tion, it is found that 1 dissociates under relatively time, the 9-fluorenyl cation of m/z 165 and one of the low-energy, affording a cationic product of m/z 165 that labeled products of m/z 170. The two pseudo first-order corresponds to the loss of benzene. This process occurs rate constants are proportional to the measured slopes under collision induced dissociation (CID) but can also that are Ϫ0.49 (R2 ϭ 0.9990) and Ϫ0.48 (R2 ϭ 0.9976), be initiated in the source by increasing the voltage at the respectively, for the reaction with pyrrolidine. Reac- electrospray capillary (Figure 1). It is noteworthy that tions of the two cations of m/z 165 and 170 that were analogous diphenylmethyl cations also generate prod- carried out with benzylamine afford slopes of Ϫ0.31 (R2 uct ions of m/z 165, by the loss of two hydrogen atoms. ϭ 0.9948) and Ϫ0.32 (R2 ϭ 0.9854) for the two cations. As mentioned above, the loss of benzene from trityl These values are within our experimental error and cation was proposed to involve the formation of 9-flu- support the notion that these are identical cationic orenyl cation (Scheme1) [9, 10]. Before delving any structures. Consequently, we conclude that the product further into the dissociation mechanism of 1 it was ion of m/z 165 is indeed 9-fluorenyl cation (Scheme 1). essential to support this proposal by comparison of the The favored loss of benzene from 1 must involve CID behavior of this product ion with that of 9-fluore- either considerable rearrangement of the dissociating nyl cation produced directly from 9-fluorenol (Scheme cation before benzene loss or a stepwise loss of two 1). However, the two ions do not dissociate under the radicals. Three initial processes that were considered for experimental conditions chosen for this study (both in the reaction under study are illustrated in Scheme 2. an FTICR and in a triple stage quadruple mass spec- Routes (b) and (c) are rearrangement processes of 1 and trometer). Therefore, instead of CID spectra compari- both involve cyclization. Route (a) is the dissociation of son, ion-molecule association reactions of these two aCOC bond to form a complex between phenyl radical cations with amines were investigated. To follow the and a diphenylmethylene radical cation. DFT calcula- reaction of these two ions under identical conditions, tions at the B3LYP/6-31G(d) level were employed to deuterium labeled 1-d was prepared. 10 assess the viability of each process. D D D + D D D D D D D 1-d10 This approach has been previously used by Nibbering and coworkers to distinguish between benzyl and tro- pylium cations [13]. The elimination of benzene, upon activation of cation Figure 2. Ion-molecule reactions of the deuterium labeled cat- 1-d involves the losses of C H D, C D HorCD , 10 6 5 6 5 6 6 ions generated during CID of 1-d10 (m/z 169, 170, and 174) and affording ions of m/z 174, 170, and 169, respectively, 9-flurenyl cation (m/z165), with pyrrolidine. 732 DENEKAMP AND YANIV J Am Soc Mass Spectrom 2006, 17, 730–736 -0.5 -1.0 -1.5 ions) -2.0 -2.5 ln([165/170]/ -3.0 2.53.03.54.04.55.0 t (s) Figure 3. Pseudo first-order kinetics in the reaction of deuterium labeled cation generated during CID of 1-d10 (m/z170) with pyrrolidine (filled circles), benzylamine (filled triangles), reaction Figure 4. Partial CID spectrum of 1-d10, showing H/D exchange of 9-fluorenyl cation with pyrrolidine (filled squares), and ben- during the elimination of methyl. zylamine (filled inverted triangles). Route (a) was examined here by calculation of the tropylium cation. It is thus shown that the lowest energy path for rearrangement of benzyl cation to energy of cation 1 while increasing the bond length of Ϫ1 the dissociating COC bond. This bond length was tropylium requires 65.1 kcal mol and proceeds increased up to 2.45 Å while all other geometrical through a single stable intermediate [14]. Such a mech- parameters were optimized. This simple bond cleavage anism should exhibit hydrogen scrambling in the rear- is found to be highly endothermic with a barrier that ranging cation.
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