Effect of alkali metal cationization on the collision-induced decomposition of alkyl per-O-acetyl-2-deoxy-2-halo-α-O-mannopyranosides

The effect of alkali metal cationization on the collision-induced decomposition of alkyl per-O-acetyl-2-deoxy-2-bromo-and-iodo-α-O-mannopyranosides was studied. The bromo sugars gave fairly abundant MH⁺, whereas for the iodo sugars the MH⁺ ions were insignificant. However, both the bromo and the iodo derivatives gave abundant M + alkali metal ion complexes. In contrast to the behaviour of the MH⁺ ion, the [M + Li]⁺, [M + Na]⁺ and [M + K]⁺ ions of these compounds do not decompose by loss of the C(1) substituent. Elimination of AcOH is the preferred fragmentation pathway of [M + Cat]⁺. Elimination of HX occurs only after loss of AcOH and CH₂CO from MH⁺, whereas [M + Cat]⁺ directly loses HX. The elimination of HX is more pronounced from [M + Na]⁺ and [M + K]⁺ than from [M + Li]⁺. Loss of AcOLi is an additional fragmentation route observed in the case of the decomposition of [M + Li]⁺ ion.     

Collision-induced decompositions of metal-cationized methyl-6-deoxy-6-bromo-α-D-glucopyranoside derivatives

Collision-induced decompositions (CIDs) of the [M + H]⁺, [M + Li]⁺, [M + Na]⁺, [M + K]⁺ and [M + Ag]⁺ ions of some methyl-6-deoxy-6-bromo-α-D-glucopyranoside derivatives are discussed. Elimination of MeOH resulting in the glycosidyl cation is the predominant reaction of the [M + H]⁺ ion. This process is completely suppressed during CID of the metal-cationized species, which, surprisingly, show elimination of the added metal in the form of RCOO-metal and metal bromide in the case of the ester derivatives. These reactions appear to be assisted by neighbouring group participation. Because of the proximity of the C(3)-oxygen with C(6), the benzyl ether derivative is characterized by the loss of PhCH₂Br from the [M + metal]⁺ ion.     

Collision‐induced loss of AgH from Ag+ adducts of alkylamines,aminocarboxylic acids and alkyl benzyl ethers leads exclusively to thermodynamically favored product ions

The loss of AgH from [M + Ag]⁺ precursor ions of tertiary amines, aminocarboxylic acids and aryl alkyl ethers is examined by deuterium labeling combined with collision activation (CA) dissociation experiments. It was possible to demonstrate that the AgH loss process is highly selective toward the hydride abstraction. For tertiary amines and aminocarboxylic acids, hydrogen originates from the α‐methylene group carrying the nitrogen function (formation of an immonium ion). In all cases examined, the most stable, i.e. the thermodynamically favored product ion is formed. In the AgH loss process, a large isotope effect operates discriminating against the loss of D. The [M + Ag]⁺ ion of benzyl methyl ether loses a hydride ion exclusively from the benzylic methylene group supporting the experimental finding that the AgH loss reaction selectively cleaves the weakest C? H bond available. Copyright © 2008 John Wiley & Sons, Ltd.     

Siloxyaluminate and Siloxygallate Complexes as Models for Framework and Partially Hydrolyzed Framework Sites in Zeolites and Zeotypes

Anionic molecular models for nonhydrolyzed and partially hydrolyzed aluminum and gallium framework sites on silica, M[OSi(OtBu)₃]₄⁻ and HOM[OSi(OtBu)₃]₃⁻ (where M=Al or Ga), were synthesized from anionic chlorides Li{M[OSi(OtBu)₃]₃Cl} in salt metathesis reactions. Sequestration of lithium cations with [12]crown-4 afforded charge-separated ion pairs composed of monomeric anions M[OSi(OtBu)₃]₄⁻ with outer-sphere [([12]crown-4)₂Li]⁺ cations, and hydroxides {HOM[OSi(OtBu)₃]₃} with pendant [([12]crown-4)Li]⁺ cations. These molecular models were characterized by single-crystal X-ray diffraction, vibrational spectroscopy, mass spectrometry and NMR spectroscopy. Upon treatment of monomeric [([12]crown-4)Li]{HOM[OSi(OtBu)₃]₃} complexes with benzyl alcohol, benzyloxide complexes were formed, modeling a possible pathway for the formation of active sites for Meerwin–Ponndorf–Verley (MPV) transfer hydrogenations with Al/Ga-doped silica catalysts.     

HPLC‐DAD and HPLC‐ESI‐MS separation,determination and identification of the spin‐labeled diastereoisomers of podophyllotoxin

Spin‐labeled nitroxide derivatives of podophyllotoxin had better antitumor activity and less toxicity than that of the parent compounds. However, the 2‐H configurations of these spin‐labeled derivatives cannot be determined by nuclear magnetic resonance (NMR) methods. In the present paper, a high‐performance liquid chromatography‐diode array detection (HPLC‐DAD) and a high‐performance liquid chromatography‐electrospray ionization tandem mass spectrometry (HPLC‐ESI/MS/MS) method were developed and validated for the separation, identification of four pairs of diastereoisomers of spin‐labeled derivatives of podophyllotoxin at C‐2 position. In the HPLC‐ESI/MS spectra, each pair of diastereoisomers of the spin‐labeled derivatives in the mixture was directly confirmed and identified by [M+H]⁺ ions and ion ratios of relative abundance of [M‐ROH+H]⁺ (ion 397) to [M+H]⁺. When the [M‐ROH+H]⁺ ions (at m/z 397) were selected as the precursor ions to perform the MS/MS product ion scan. The product ions at m/z 313, 282, and 229 were the common diagnostic ions. The ion ratios of relative abundance of the [M‐ROH+H]⁺ (ion 397) to [M+H]⁺, [A+H]⁺ (ion 313) to [M‐ROH+H]⁺, [A+H‐OCH₃]⁺ (ion 282) to [M‐ROH+H]⁺ and [M‐ROH‐ArH+H]⁺ (ion 229) to [M‐ROH+H]⁺ of each pair of diastereoisomers of the derivatives specifically exhibited a stereochemical effect. Thus, by using identical chromatographic conditions, the combination of DAD and MS/MS data permitted the separation and identification of the four pairs of diastereoisomers of spin‐labeled derivatives of podophyllotoxin at C‐2 in the mixture.     

Synthesis of per-O-(carboxymethyl)calix[4]pyrogallols and their complexation with some alkaline metal and lanthanide ions

Complexones of a new class, viz., carboxy-functionalized calix[4]pyrrogallols, were synthesized. The per-O-(carboxymethyl)calix[4]pyrogallols obtained were established to exist in the (rel, cis, trans, trans)-configuration by 2D NMR spectroscopic data. According to the pH-potentiometric data, the interaction of these compounds with alkaline metal ions (Li⁺, Na⁺, K⁺, Cs⁺) and lanthanide ions (La³⁺, Gd³⁺, Lu³⁺) in a water—DMSO system produces 1 : 1 complexes. The specific features of complexation of per-O-(carboxymethyl)calix[4]pyrogallols, as compared to their acyclic analogs, with alkaline metal and lanthanide ions are due to the cooperative effect of donor groups preorganized on the calixarene matrix.     

Selective reagents in chemical ionization mass spectrometry: Tetramethylsilane with ethers

Chemical ionization mass spectra of several ethers obtained with He/(CH₃)₄Si mixtures as the reagent gases contain abundant [M + 73]⁺ adduct ions which identify the relative molecular mass. For the di-n-alkyl ethers, these [M + 73]⁺ ions are formed by sample ion/sample molecule reactions of the fragment ions, [M + 73 ? C_nH_2n]⁺ and [M + 73 ? 2CnH_2n]⁺. Small amounts of [M + H]⁺ ions are also formed, predominantly by proton transfer reactions of the [M + 73 ? 2CnH_2n]⁺ or [(CH₃)₃SiOH₂]⁺ ions with the ethers. The di-s-alkyl ethers give no [M + 73] ⁺ ions, but do give [M + H]⁺ ions, which allow the determination of the relative molecular mass. These [M + H]⁺ ions result primarily from proton transfer reactions from the dominant fragment ion, [(CH₃)₃SiOH₂]⁺ with the ether. Methyl phenyl ether gives only [M + 73]⁺ adduct ions, by a bimolecular addition of the trimethylsilyl ion to the ether, not by the two-step process found for the di-n-alkyl ethers. Ethyl phenyl ether gives [M + 73]⁺ by both the two-step process and the bimolecular addition. Although the mass spectra of the alkyl etherr are temperature-dependent, the sensitivities of the di-alkyl ethers and ethyl phenyl ether are independent of temperature. However, the sensitivity for methyl phenyl ether decreases significantly with increasing temperature.     

Fast atom bombardment mass spectrometric study of some unsaturated aryl C-glycosides

Fast atom bombardment (FAB), FAB mass-analysed ion kinetic energy (FAB MIKE) and collision-activated dissociation (FAB CAD-MIKE) mass spectra were obtained for two series of unsaturated anomeric aryl C-glycosides. These tandem mass spectrometric techniques allowed the differentiation of the anomers by analysing either the [M + H]⁺ ion or the [M + met]⁺ ion (met=Li, Na).     

Characterization of N‐Boc/Fmoc/Z‐N′‐formyl‐gem‐diaminoalkyl derivatives using electrospray ionization multi‐stage mass spectrometry

N‐Boc/Fmoc/Z‐N′‐formyl‐gem‐diaminoalkyl derivatives, intermediates particularly useful in the synthesis of partially modified retro‐inverso peptides, have been characterized by both positive and negative ion electrospray ionization (ESI) ion‐trap multi‐stage mass spectrometry (MSⁿ). The MS² collision induced dissociation (CID) spectra of the sodium adduct of the formamides derived from the corresponding N‐Fmoc/Z‐amino acids, dipeptide and tripeptide acids show the [M + Na‐NH₂CHO]⁺ ion, arising from the loss of formamide, as the base peak. Differently, the MS² CID spectra of [M + Na]⁺ ion of all the N‐Boc derivatives yield the abundant [M + Na‐C₄H₈]⁺ and [M + Na‐Boc + H]⁺ ions because of the loss of isobutylene and CO₂ from the Boc protecting function. Useful information on the type of amino acids and their sequence in the N‐protected dipeptidyl and tripeptidyl‐N′‐formamides is provided by MS² and subsequent MSⁿ experiments on the respective precursor ions. The negative ion ESI mass spectra of these oligomers show, in addition to [M‐H]^?, [M + HCOO]^? and [M + Cl]^? ions, the presence of in‐source CID fragment ions deriving from the involvement of the N‐protecting group. Furthermore, MSⁿ spectra of [M + Cl]^? ion of N‐protected dipeptide and tripeptide derivatives show characteristic fragmentations that are useful for determining the nature of the C‐terminal gem‐diamino residue. The present paper represents an initial attempt to study the ESI‐MS behavior of these important intermediates and lays the groundwork for structural‐based studies on more complex partially modified retro‐inverso peptides. Copyright © 2013 John Wiley & Sons, Ltd.     

Dehydrogenation and dehalogenation of amines in MALDI‐TOF MS investigated by isotopic labeling

Secondary and tertiary amines have been reported to form [M–H]⁺ that correspond to dehydrogenation in matrix‐assisted laser desorption ionization time of flight mass spectrometry (MALDI‐TOF MS). In this investigation, we studied the dehydrogenation of amines in MALDI‐TOF MS by isotopic labeling. Aliphatic amines were labeled with deuterium on the methylene of an N‐benzyl group, which resulted in the formation of [M–D]⁺ and [M–H]⁺ ions by dedeuteration and dehydrogenation, respectively. This method revealed the proton that was removed. The spectra of most tertiary amines with an N‐benzyl group showed high‐intensity [M–D]⁺ and [M–H]⁺ ion peaks, whereas those of secondary amines showed low‐intensity ion peaks. Ratios between the peak intensities of [M–D]⁺ and [M–H]⁺ greater than 1 suggested chemoselective dehydrogenation at the N‐benzyl groups. The presence of an electron donor group on the N‐benzyl groups enhanced the selectivity. The dehalogenation of amines with an N‐(4‐halobenzyl) group was also observed alongside dehydrogenation. The amino ions from dehalogenation can undergo second dehydrogenation. These results provide the first direct evidence about the position at which dehydrogenation of an amine occurs and the first example of dehalogenation of haloaromatic compounds in MALDI‐TOF MS. These results should be helpful in the structural identification and elucidation of synthetic and natural molecules. Copyright © 2013 John Wiley & Sons, Ltd.     

Electrospray ionization multiple-stage linear ion-trap mass spectrometry for structural elucidation of triacylglycerols: Assignment of fatty acyl groups on the glycerol backbone and location of double bonds

Linear ion-trap multiple-stage mass spectrometric approach (MS ⁿ ) towards nearly complete structural elucidation of triacylglycerol (TAG) including (1) assignment the fatty acid substituents on the glycerol backbone and (2) location of the double bond(s) on the unsaturated fatty acyl groups is reported. The characterization is established by the findings that MS² on the [M+Li]⁺ ions of TAG yields more abundant ions reflecting losses of the outer fatty acid substituents either as free acids (i.e., [M+Li-R₁CO₂H]⁺ and [M+Li-R₃CO₂H]⁺ ions) or as lithium salts (i.e., [M+Li-R₁CO₂Li]⁺ and [M+Li-R₃CO₂Li]⁺ ions) than the ions reflecting the similar losses of the inner fatty acid substituent (i.e., [M+Li-R₂CO₂Li]⁺ and [M+Li-R₂CO₂Li]⁺ ions). Further dissociation (MS³) of [M+Li-R _n CO₂H]⁺ (n=1, 2, or 3) gives rise to the ion series locating the double bonds along the fatty acid chain. These ions arise from charge-remote fragmentations involving β-cleavage with γ-H shift, analogous to those seen for the unsaturated long-chain fatty acids characterized as initiated ions. Significant differences in abundances in the ion pairs reflecting the additional losses of the fatty acid moieties, respectively, were also seen in the MS³ spectra of the [M+Li-R _n CO₂H]⁺ and [M+Li-R _n CO₂Li]⁺ ions, leading to confirmation of the fatty acid substituents on the glycerol backbone. MS ⁿ on the [M+Na]⁺ and [M+NH₄]⁺ adduct ions also affords location of fatty acid substituents on the glycerol backbone, but not the position of the double bond(s) along the fatty acid chain. Unique ions from internal losses of the glycerol residues were seen in the MS³ spectra of [M+Alk-R _n CO₂H]⁺ (n=1, 2, 3) and of [M+Alk-R _n CO₂Alk]⁺ (Alk=Li, Na, NH₄; n=1, 3). They are signature ions for glycerides and the pathways leading to their formation may involve rearrangements.     

Studies on phosphatidylserine by tandem quadrupole and multiple stage quadrupole ion-trap mass spectrometry with electrospray ionization: Structural characterization and the fragmentation processes

Low-energy CAD product-ion spectra of various molecular species of phosphatidylserine (PS) in the forms of [M−H]⁻ and [M−2H+Alk]⁻ in the negative-ion mode, as well as in the forms of [M+H]⁺, [M+Alk]⁺, [M−H+2Alk]⁺, and [M−2H+3Alk]⁺ (where Alk=Li, Na) in the positive-ion mode contain rich fragment ions that are applicable for structural determination. Following CAD, the [M−H]⁻ ion of PS undergoes dissociation to eliminate the serine moiety (loss of C₃H₅NO₂) to give a [M−H−87]⁻ ion, which equals to the [M−H]⁻ ion of a phoshatidic acid (PA) and give rise to a MS³-spectrum that is identical to the MS²-spectrum of PA. The major fragmentation process for the [M−2H+Alk]⁻ ion of PS arises from primary loss of 87 to give rise to a [M−2H+Alk−87]⁻ ion, followed by loss of fatty acid substituents as acids (R_xCO₂H, x=1,2) or as alkali salts (e. g., R_xCO₂Li, x=1,2). These fragmentations result in a greater abundance of [M−2H+Alk−87−R₂CO₂H]⁻ than [M−2H+Alk−87−R₁CO₂H]⁻ and a greater abundance of [M−2H+Alk−87−R₂CO₂Li]⁻ than [M−2H+Alk−87−R₁CO₂Li]⁻; while further dissociation of the [M−2H+Alk−87−R_{2(or 1)}CO₂Li]⁻ ions gives a preferential formation of the carboxylate anion at sn-1 (R₁CO₂⁻) over that at sn-2 (R₂CO₂⁻). Other major fragmentation process arises from differential loss of the fatty acid substituents as ketenes (loss of R_x′CH=CO, x=1,2). This results in a more prominent [M−2H+Alk−R₂′CH=CO]⁻ ion than [M−2H+Alk−R₁′CH=CO]⁻ ion. Ions informative for structural characterization of PS are of low abundance in the MS²-spectra of both the [M+H]⁺ and the [M+Alk]⁺ ions, but are abundant in the MS³-spectra. The MS²-spectrum of the [M+Alk]⁺ ion contains a unique ion corresponding to internal loss of a phosphate group probably via the fragmentation processes involving rearrangement steps. The [M−H+2Alk]⁺ ion of PS yields a major [M−H+2Alk−87]⁺ ion, which is equivalent to an alkali adduct ion of a monoalkali salt of PA and gives rise to a greater abundance of [M−H+2Alk−87−R₁CO₂H]⁺ than [M−H+2Alk−87−R₂CO₂H]⁺. Similarly, the [M−2H+3Alk]⁺ ion of PS also yields a prominent [M−2H+3Alk−87]⁺ ion, which undergoes consecutive dissociation processes that involve differential losses of the two fatty acyl substituents. Because all of the above tandem mass spectra contain several sets of ion pairs involving differential losses of the fatty acid substituents as ketenes or as free fatty acids, the identities of the fatty acyl substituents and their positions on the glycerol backbone can be easily assigned by the drastic differences in the abundances of the ions in each pair.     

Charge localization and aromatic stabilization in large ring ions: Mass spectra of ms-porphyrins

The major mass spectrometric fragments of ms-tetraphenylporphin and ms-tetra(p-chloro)phenylporphin are [M ? H]⁺˙ and [M ? Cl]⁺˙, respectively. Metal derivatives of these compounds give a modified characteristic fragmentation pattern with peak groups ending in the ions [M ? 4H]⁺˙, [M ? ? ? 5H]⁺˙ and [M ? 2? ? 2H]⁺˙ for the metallo ms-tetraphenylporphins, and [M ? ?Cl ? 2Cl ? 3H]⁺˙ and [M ? 2?Cl ? Cl ? H]⁺˙ for Mgms-tetra(p-chloro)phenylporphin. Deuterated metal derivatives indicate random hydrogen loss from both phenyl and pyrrole carbons. However, metal substituents do not significantly modify the fragmentation pattern in the case of ms-tetra(p-methoxy)phenylporphin. These patterns can be explained in terms of aromatic stabilization of the fragmentation products, coupled with charge localization on the π system in the free base, on the metal atom in the metallo derivatives and on the methoxy function in the p-methoxyphenyl derivative.     

Collision-induced decompositions of [M – H]− and [M + Li]+ ions from fast atom bombardment of dinucleoside phenylphosphonates

The collision-induced decompositions of the [M – H]^? and [M + Li]⁺ ions of a few dinucleoside phenylphosphonates were studied using fast atom bombardment and linked scanning at constant B/E. Deprotonation takes place on the base or sugar moieties. The [M – H]^? ion decomposes mainly by cleavage on either side of the phosphonate linkage, leading to the formation of mononucleotide fragment ions and also by cleavage of the basesugar bond. Rupture of the 3′-phosphonate bond is preferred. Unlike the normal charged nucleotides, these neutral nucleotides do not eliminate a neutral base from the [M – H]^? ion. However, the mononucleotide fragment ions which can have the charge on the phosphorus oxygen eliminate neutral bases by charge-remote fragmentation. The 4,4′-dimethoxytrityl (DMT)-protected nucleotides show the additional fragmentation of loss of DMT. Li⁺ attachment can occur at several sites in the molecule. As observed for the [M – H]^? ion, the major cleavage occurs on either side of the phosphonate bond in the fully deprotected nucleotides, cleavage of the ester bond on C(3′) being preferred. Cleavage of the 5′-phosphonate bond is not observed in the DMT-protected nucleotides. Many of the fragmentations observed can be explained as arising from charge-remote reactions.     

Collisional activation of ions formed by [Li]+ ion attachment

Since no unimolecular fragmentation is observed with [M+Li]⁺ ions under normal operating conditions the collisional activation method was used to study the fragmentation behaviour of these ions. It was found that the liberation of the [Li]⁺ ion is a dominant process only with smaller molecules. In addition, direct bond cleavages and new types of rearrangement reactions lead to fragment ions in which the lithium is normally retained. The decomposition behaviour of [M+Li]⁺ ions represents an intermediate case between that of [M]+ ions and excited neutral molecules and is quite different from that of [M+H]⁺ ions.     

Identification of aminophospholipid stereomers by positive-ion fast atom bombardment combined with collisional activation mass-analysed ion kinetic energy analysis and high-performance liquid chromatography

Two methods for the identification of aminophospholipid stereomers are described. After a chemical derivatization, 9-fluorenylmethoxycarbonyl derivatives of diacyl-sn-2- and diacyl-sn-3-phosphoserine and diacyl-sn-2- and diacyl-sn-3-phosphoethanolamine can be characterized by positive-ion fast atom bombardment combined with collisional activation mass-analysed ion kinetic energy analysis based on the differentiation of relative abundances [M + H + diethanolamine ? 89]⁺ and [M + H]⁺ fragments derived from [M + H + diethanolamine]⁺ ions, the protonated solvated molecules, and normal phase high-performance liquid chromatography on the basis of different elution times of the derivatives of the aminophospholipid stereomers on an aminopropyl-bonded column.     

Silver (Ι)‐assisted enantiomeric analysis of ginsenosides using electrospray ionization tandem mass spectrometry

For identification of ginsenoside enantiomers, electrospray ionization mass spectrometry (ESI‐MS) was used to generate silver complexes of the type [ginsenoside + Ag]⁺. Collision induced dissociation of the silver‐ginsenoside complexes produced fragment ions by dehydration, allowing differentiation of ginsenoside enantiomers by the intensity of [M + Ag ? H₂O]⁺ ion. In the meanwhile, an approach based on the distinct profiles of enantiomer‐selective fragment ion intensity varied with collision energy was introduced to refine the identification and quantitation of ginsenoside enantiomers. Five pairs of enantiomeric ginsenosides were distinguished and quantified on the basis of the distribution of fragment ion [M + Ag ? H₂O]⁺. This method was also extended to the identification of other type of ginsenoside isomers such as ginsenoside Rb2 and Rb3. For demonstrating the practicability of this novel approach, it was utilized to analyze the molar ratio of 20‐(S) and 20‐(R) type enantiomeric ginsenosides in enantiomer mixture in red ginseng extract. The generation of characteristic fragment ion [M + Ag ? H₂O]⁺ likely results from the reduction of potential energy barrier of dehydration because of the catalysis of silver ion. The mechanism of enantiomer identification of ginsenosides was discussed from the aspects of computational modeling and internal energy. Copyright © 2012 John Wiley & Sons, Ltd.     

Collisional activation spectra of [M + Li]+, [M + Na]+ and [M + K]+ ions formed by field desorption of some monosaccharides

The collisional activation spectra of monosaccharide ions formed by [Li]⁺, [Na]⁺ and [K]⁺ ion attachment under field desorption conditions are reported. It is shown that the elimination of the alkali ions is determined by the alkali ion affinities of the molecules (M) and competes with a fragmentation of M which is almost independent of the alkali ion attached. Correspondingly the alkali ion is predominantly retained in the fragment ions. The usefulness of this method for the differentiation of underivatized isomers is demonstrated.     

Influence of alkali and alkaline earth ions on the O -alkylation of the lower rim phenolic-OH groups of p-tert -butyl-calix[4]arene to result in amide-pendants: Template action of K+ and the structure of K+ bound tetra-amide derivative crystallized with a p-tert -butyl-calix[4]arene anion

Role of alkali and alkaline earth ions on the formation of calix[4]arene-amide derivatives through O-alkylation of the lower rim phenolic-OH groups in general and template action of K⁺ in particular have been explored. Na⁺ and K⁺ ions among alkali, and Ca²⁺ and Sr²⁺ ions among alkaline earth have shown tetra-amide derivatives bound to metal ion species. Among all these, potassium salts act as template and yields a K⁺ bound tetra-amide derivative where the charge is counter balanced by a calix[4] arene-monoanion and the product is crystallographically characterized. Change in the amide precursor used in these O-alkylation reactions has no effect on the type of the amide derivative formed. Also demonstrated is a direct one-step reaction for the preparation of 1,3-di-amide derivative in high yield and low reaction period using CsHCO₃.     

The [M − 1 ]+ quasi-molecular ion in chemical ionization mass spectrometry. Fragmentation of bis(benzyloxy)silanes by intramolecular reactions

The unusual chemical ionization mass spectrometric behavior of bis(benzyloxy)silanes showing heavy fragmentation and an [M ? 1]⁺ fragment, instead of the expected [M + 1]⁺ ion with isobutane as the reactant gas, was investigated by means of (chloromethyl)bis(benzyloxy)methylsilane and model compounds. It could be shown that probably an intramolecular hydride transfer to an adjacent proton via a six-membered transition state gives rise to the uncommon [M ? 1]⁺ quasi-molecular ion. Similar intramolecular reactions transferring a hydride, a chloro-methyl, a phenyl or a benzyl group via cyclic transition states to a neighboring electrophile are held responsible for five of the six additional major fragments. The results demonstrate that fragmentation reactions become important in chemical ionization mass spectrometry if a molecule or an initially formed cluster ion possesses reactive groups which are in close proximity to each other.