Investigation of Group Migration in the Fragmentation of Bistrimethylsilyl Ethers of Diols Separated by Rigid Groups

Abstract

Two fragmentations of (CH3)3SiO-CR2-X-CR2-OSi(CH3)3, where X is a rigid group such as a triple bond or an aromatic ring, are losses of a methyl or R group (where R is H or alkyl). The metastable-ion dissociations of [M-R]+ and [M-CH3]+ ions include the rearrangement of a trimethylsilyl (TMS) cation and a (CH3)2Si = O neutral species through an ion-neutral complex. On the basis of tandem mass spectrometry (MS/MS), exact mass measurement and isotopic labeling experiments, it has been established that the two trimethylsilyloxy groups in the TMS ethers interact across a wide range of distances via an ion-neutral complex. The migration of a TMS cation occurs when the group that is bound to carbon is expelled as a radical by an oxygen-directed cleavage to give a trimethylsilylated oxonium ion. If, on the other hand, a methyl radical is lost from the silicon atom, then (CH3)2Si = O migrates. The mobilities of the TMS cation and the neutral (CH3)2Si = O are governed by the capability of the rigid group to delocalize charge.

Derivatization in Mass Spectrometry—1. Silylation

“…These ions are formed even for compounds with greatly separated TMS groups as, for example, 3,17-di-TMS ethers of steroidal phosphate derivatives. 77 For several trimethylsilylated derivatives of rather rigid benzenedimethanols and acetylenic diols, it was shown 78 that these ions are formed from the [M -CH 3 ] + ions due to a migration of the neutral species (CH 3 ) 2 Si = O most likely through an ion-neutral complex. Further decomposition of the m/z 147 ions proceeds via the loss of CH 2 O (to m/z 117) or CH 4 (to m/z 131).…”

Gas-Phase Reaction of Acetaldehyde with Silicon-Containing Ions

“…8 On the one hand, the experimental study shows that both metastable ions fragment to give the m/z 131 ion (1) by the loss of an ethyl radical and by the loss of a methane molecule, respectively. On the other hand, calculations indicate that the more stable structure of m/z 131 thus formed is not the open structure 1a as previously presumed, 6,7 but the cyclic structure 1b (Scheme 1). In order to generate new silicon-containing ions by using common reagents such as hexamethyldisiloxane or hexamethyldisilane, the ion-molecule reactions of the m/z 147 and m/z 131 ions with neutral molecules, such as water, 7 ammonia and carbonyl compounds 9 are currently under study in this laboratory.…”

“…4 Trimethylsilylated ions undergo rearrangements involving the intermediacy of ion-neutral complexes, as shown by Longevialle 5 in the case of the trimethylsilyl derivatives of hydroxysteroids. For instance, Gross and coworkers 6 have shown that ionized bis(trimethylsilyl) ethers of diols yield the (CH 3 ) 3 SiOSi(CH 3 ) 2 + (m/z 147) ion. This ion is identical to that formed by loss of a methyl group from ionized hexamethyldisiloxane (CH 3 ) 3 SiOSi(CH 3 ) 3 (m/z 162).…”

Gas-phase ion chemistry of silyl cations obtained from hexamethyldisilazane

“…The idea that ion-dipole complexes can have an important mechanistic participation in this type of systems has been previously advanced to account for the trimethylsilyl group migration observed in mass spectral fragmentations of trimethylsilyl ethers 33 , and for the outcome of ion-molecule reactions between silyl cations and ketones 34 . This ease of migration by methylsilyl cations is supported by preliminary ab initio calculations on model silyl systems that show such rearrangements to occur without any formal activation energy 35 .…”