Stereochemical Preferences in 4-Center Syn-Eliminations from Gaseous Ions

Abstract

Concerted unimolecular eliminations from ionized sec-alkyl aryl ethers (ROAr (+*)) display a preference for producing double bonds with trans geometry. This preference can be assessed quantitatively, provided that a regioselective variant can be found. Expulsion of neutral alkenes via syn-elimination to give ionized ArOH does not exhibit a pronounced preference with regard to the direction of elimination. By contrast, ionized 2-hexyl p-trifluoromethylphenyl ether eliminates neutral ArOH regioselectively, giving ionized 2-hexenes rather than ionized 1-hexene. Vicinally monodeuterated 2-hexyl and 3-hexyl ethers were prepared as pure diastereomers. Metastable ion decompositions of their gaseous radical cations are compared over two different time windows. The regioselectivity for the 2-hexyl ether allows the geometric preference for the double bonds to be estimated based on the difference between the erythro and threo monodeuterated diastereomers ( trans/ cis = 2.0 for producing ionized 2-hexene from parent ions with the lowest internal energies). The 3-hexyl ethers and ionized 2- and 3-phenoxyoctanes also undergo stereoselective elimination but give experimental values that reflect their lack of regioselectivity. Examination of erythro/ threo combinations shows that GC/MS/MS has the ability to quantitate stereochemistry in mixtures containing both positional and stereoisomers.

Stereospecific collision-induced dissociation and vibrational spectroscopy of protonated cyclo (Tyr-Pro)

“…Protonated peptides have also been extensively studied by mass spectrometry coupled to laser fragmentation spectroscopy, either in the IR or in the UV range. [9][10][11][12][13][14][15][16][17] In addition to optical spectroscopy measurements, important structural information on the amino-acid sequence and the secondary structure is gained from the analysis of the collision-induced dissociation (CID) or electron-capture dissociation (ECD) fragments, [18][19][20][21][22][23][24][25][26][27][28] in particular for assessing the influence of stereochemical aspects on the structures. [29] Among the factors structuring the supramolecular shape of peptides in a well-defined and orderly manner, the absolute configuration of the sub-units plays an important role.…”

Stereospecific control of peptide gas‐phase ion chemistry with cis and trans cyclo ornithine residues

“…For these ions, stereoselectivity has been achieved by utilizing non‐covalent complexes with transition metal ions where a chiral auxiliary ligand provided the diastereoisomeric distinction . The lack or low level of stereospecificity is directly related to the peptide ion dissociation mechanisms. Peptide ion dissociations are accompanied by proton transfers from the original position on a basic residue (Lys, Arg, or His) to amide or side‐chain groups of the backbone amino acid residues.…”

Stereospecificity of the Hofmann elimination in the gas phase

“…Isotope effects for unimolecular ion fragmentations exhibit almost no sensitivity to internal energy. 24 Prior work shows that protonated neopentylamines (6-8) decompose to yield the two products of path ii in Scheme 3 via ion-neutral complexes. 16 Since deuterating the 3-hexyl group ought not to affect those combined rates, the yield of C 5 H 11 þ plus protonated 3-hexylamine provides a standard for gauging the rate of path i (the sum of the conjugate acids of neopentylamine from H þ or D þ transfer, immonium ions via hydride abstraction, and hexyl ions).…”

“…Isotope effects for unimolecular ion fragmentations exhibit almost no sensitivity to internal energy. 24…”