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.
The Hofmann elimination of ammonium ions having a single positive charge is demonstrated to exhibit stereospecificity with regard to expulsion of neutral alkene. For the 3-hexyl series of threo and erythro 4-monodeuterated ions (3-hexylammonium ion; N,N,N-trimethyl- d-3-hexylammonium ion; N-ethyl-3-hexylammonium ion; N-methyl, N-ethyl-3-hexylammonium ion; N,N-dimethyl, N-ethyl-3-hexylammonium ion), the upper limit of the E:Z ratio of the expelled alkene (r) approaches 2 (the stereospecificity) with a deuterium isotope effect close to 2.0, although the effects of isotopic substitution diminish the E:Z ratio somewhat. Two fragmentations compete with that reaction: hydride shift (which gives the same products but with hydrogen scrambling) and loss of a neutral amine to give an alkyl cation. These competing reactions render our calculation approximate, but the results suggest a value not too far from the upper limit.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.