Date of Award

1-1-1987

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

Abstract

Introduction. The reductive cleavage of diaryl and arylalkyl ethers has been postulated in the literature to occur by either a radical-anion or dianion mechanism. Clarification of this mechanism problem is important if reductive cleavage conditions are to be used for degradative structural determination of natural products such as coals or lignins. The purpose of this study was to determine the mechanisms of the chemical reductive cleavages (with alkali metals and electron transfer agents) of some diaryl and arylalkyl ethers by trapping the highly reactive intermediates. The model ethers in this study were prepared with attached intramolecular mechanistic probes; the diaryl ethers had the 2-(3-butenyl) probe, and the arylalkyl ethers had the 6-hepten-2-yl alkyl probe attached to them.The phenyl- and naphthyl- anions were observed to cyclize slowly (k$\sb{\rm c}$ of ca. 10$\sp{-3}$ and 10$\sp{-4}$ s$\sp{-1}$, respectively) at room temperature with the 2-(3-butenyl) probe to yield 1-methylindan and 1-methylbenz (e) indan, respectively. These anions did not cyclize at low temperature ($-78\sp\circ$C), where the corresponding radicals were found to readily cyclize.Cleavage of Diaryl Ethers. The conditions required for a valid test of aryl radical intermediacy were utilized during the cleavage of diphenyl ether and 1-phenoxynaphthalene. Diphenylether was determined to cleave by a unimolecular radical-anion pathway, which yielded phenoxide ion and the phenyl radical.In contrast, the chemical reductive cleavage of 1-phenoxynaphthalene did not yield a naphthyl radical after loss of phenoxide ion. The behavior of the naphthyl moiety resembled that of a naphthyl sigma-anion after reductive cleavage. A dianion mechanism of cleavage was proposed.Cleavage of Arylalkyl Ethers. The phenoxy ether with the 6-hepten-2-yl alkyl probe cleaved by unimolecular decomposition of the radical anion, which was demonstrated by diagnostic radical-like behavior of the alkyl fragment. The 6-hepten-2-yl radical probe was found to cyclize exclusively to cis-1,2-dimethylcyclopentane (in 63% yield) at low temperature ($-78\sp\circ$C). In contrast, the 1-(naphthoxy)alkyl ether cleaved primarily by a different pathway. The 6-hepten-2-yl fragment underwent large amounts of 1,4-prototropic shifts to yield mostly 2-heptenes (67% of the C$\sb7$ hydrocarbons). Therefore, a dianion cleavage mechanism was proposed for the naphthylalkyl ethers.

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