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Stereochemical preferences in 4-center syn-eliminations from gaseous ions.

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.