Intramolecular hydrogen transfer in mass spectra. II. McLafferty rearrangement and related reactions

This paper, describing electron ionization mass spectral fragmentation of some hydroxycarboxylic and oxocarboxylic acid trimethylsilyl derivatives, focuses on the formation of fragment ions resulting from the interactions between the two functionalities of these compounds. These interactions result in the formation of fragment ions at [CH 2 ilylation is by far the major derivatization procedure for GC/MS analyses [1][2][3]. Silyl derivatives are formed when active proton displacement (in ϪOH, ϪSH, or ϪNH groups) by an alkylsilyl group occurs. The most common silylation procedure is trimethylsilylation. Trimethylsilyl derivatives combine thermal stability and high volatility. In general, electron ionization (EI) mass spectra of trimethylsilyl ethers or esters exhibit a significant [M Ϫ 15] ϩ ion formed by loss of a methyl group bonded to silicon, which is very useful in determining the molecular mass. It is very important to note that EI mass spectra of trimethylsilyl derivatives may be employed not only for molecular weight determinations, but also for structural deductions [4].Trimethylsilyl group migrations during EI fragmentation process are common [5,6] and take place by means of ion-neutral complexes [7]. These rearrangements are generally considered useful for deducing structures of unknown spectra [8]; however, in some cases, misleading conclusions may be drawn. Though trimethylsilyl group migrations usually occur through five-to eight-member-ring transition states [9], intense long-range migration of the trimethylsilyl group was previously observed in the case of trimethylsilyl derivatives of some steroid compounds [8, 10 -13]. The capability of the trimethylsilyl group to migrate over a wide range of distances was also demonstrated in the case of semirigid or rigid molecules [5]. Recently, a significant transfer of trimethylsilyl group towards the carbonyl group was also observed during EI fragmentation of the trimethylsilyl derivatives of long-chain -hydroxycarboxylic and -dicarboxylic acids [14], showing that such migrations may also occur in the case of nonrigid structures.Hydrogen transfers between distant functionalities during EI fragmentation processes are also well known [6,15,16], for this reaction hydrogen activation is needed and is caused by groups such as keto, amino, ether, trimethylsilyloxy, as well as by chain branching and unsaturation.In the present work, which is a part of a more general study of the synergistic effects during EI mass fragmentation of polyfunctional compounds [14,[17][18][19], we describe EI mass spectral fragmentation of several hydroxycarboxylic and oxocarboxylic acid trimethylsilyl derivatives, which involves trimethylsilyl and hydrogen transfers.The characterization of such compounds appears to be of great environmental importance [20,21]. Indeed, it was previously demonstrated that autoxidative processes could induce a bias during alkenone-based paAddress reprint requests to Dr.

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confidence: 88%

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confidence: 99%

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Hydrogen and trimethylsilyl transfers during EI mass spectral fragmentation of hydroxycarboxylic and oxocarboxylic acid trimethylsilyl derivatives

Rontani

Aubert²

2008

“…However, clear-cut difference in the low mass region of the spectra is observed in all the compounds, except in 6 and 8. The N,N-diethyl isomers (10 -13) show specific fragment ions at m/z 124 and 92, whereas the fragment ions at m/z 120 and 94 are diagnostic to N-isopropyl-N-methyl isomers (18)(19)(20)(21). The ions at m/z 124 and 92 prove to be characteristic of N,N-diethyl group based on metastable ion data as summarized in Scheme 7.…”

Section: Nn-diethyl and N-isopropyl N-methyl Groups Containing Isomersmentioning

confidence: 99%

Mass spectral study on O,O-dialkyl N,N-dialkyl phosphoramidates under electron impact conditions

Reddy

Prabhakar

Saradhi

et al. 2004

A series of O,O-dialkyl N,N-dialkyl phosphoramidates (1-25) were analyzed under GC-EIMS conditions. Clear-cut differences are found in the fragmentation of O,O-dialkyl N,N-dimethyl phosphoramidates (Series 1) and O,O-dimethyl N,N-dialkyl phosphoramidates (Series 2). The phosphoramidates comprising of mixed/crossed alkyl groups on nitrogen and oxygen (Series 3) showed mixed fragmentation pattern corresponding to both Series 1 and 2 depending on the nature of alkyl groups. All the possible isomers among the studied compounds showed distinguishable EI mass spectra. Although the major ions in the EI mass spectra for the isomers containing O-n-propyl or O-isopropyl and N,N-diethyl or N-isopropyl N-methyl are similar, the isomers could be distinguished by characteristic ions of low abundance at low mass region. The differences are prominent in the metastable ion spectra of characteristic ions. (J Am Soc Mass Spectrom 2004, 15, 547-557)

“…If 3 is a stable species, given that the transition-state for 1 ¡ 3 is higher in energy than 3 ¡ 2, 3 formed by 1 ¡ 3 would contain enough energy to continue on to 2. The reaction 1 ¡ 3 ¡ 2 would be analogous to the sequence RCH 2 CH 2 C(ϭO [4][5][6][7][8] , a seminal proposal that was later supported by a more detailed analysis of the dissociation of deuterium labeled forms of 1 by Veith and Gross [10]. We conclude that this reaction occurs in three temporally distinct stages, CN bond stretching to form an ion-neutral complex, isomerization from n-to i-C 3 H 7 ϩ in an ionneutral complex and H-transfer from i-C 3 [28]; however, we could not locate such a species by minimum-seeking efforts.…”

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confidence: 99%

Characterization by theory of H-transfers and onium reactions of CH₃CH₂CH₂N⁺H=CH₂

McAdoo

2007

H-transfers by 4-, 5-, and 6-membered ring transition states to the -bonded methylene of CH 3 CH 2 CH 2 NH ϩ ϭCH 2 (1) are characterized by theory and compared with the corresponding transfers in cation radicals. Four-membered ring H-transfers converting 1 to CH 3 CH 2 CHϭN ϩ HCH 3 (2) and CH 3 N ϩ HϭCH 2 to CH 2 ϭNH ϩ CH 3 are high-energy processes involving rotation of the source and destination RHCϭ groups (R ϭ H or C 2 H 5 ) to near bisection by skeletal planes; migrating hydrogens move near these planes. The H-transfer 1 ¡ CH 3 C ϩ HCH 2 NHCH 3 (3) has a higher energy transition-state than 1 ¡ 2, in marked contrast to the corresponding relative energies of 4-and 5-membered ring H-transfers in cationradicals. Six-membered ring H-transfer-dissociation (1 ¡ CH 2 ϭCH 2 ϩ CH 2 ϭN ϩ HCH 3 ) is a closed shell analog of the McLafferty rearrangement. It has a lower energy transition-state than either 1 ¡ 2 or 1 ¡ 3, but is still a much higher energy process than 6-membered ring H-transfers in aliphatic cation radicals. In contrast to the stepwise McLafferty rearrangement in cation radicals, H-transfer and CC bond breaking are highly synchronous in 1 ¡ CH 3 N ϩ HϭCH 2 ϩ CH 2 ϭCH 2 . H-transfers in propene elimination from 1 are ion-neutral complex-mediated:Intrinsic reaction coordinate tracing demonstrated that a slight preference for H-transfer from the methyl containing the carbon from which CH 2 ϭNH is cleaved is due to CH 2 ϭNH passing nearer this methyl than the other on its way to abstracting H, i.e., some memory of the initial orientation of the partners accompanies this reaction. (J Am Soc Mass Spectrom 2007, 18, 270 -278)