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Title: The Hydrolysis of 4-acyloxy-4-substituted-2,5-cyclohexadienones: Limitations of Aryloxenium Ion Chemistry
Contributor(s): Novak, M (author); Glover, S  (author)
Publication Date: 2005
DOI: 10.1021/ja050899q
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Abstract: The title compounds serve as potential precursors to aryloxenium ions, often proposed, but primarily uncharacterized intermediates in phenol oxidations. The uncatalyzed and acid-catalyzed decomposition of 4-acetoxy-4-phenyl-2,5-cyclohexadienone, 2a, generates the quinol, 3a. 18O-Labeling studies performed in 18O-H2O, and monitored by LC/MS and 13C NMR spectroscopy that can detect 18O-induced chemical shifts on 13C resonances, show that 3a was generated in both the uncatalyzed and acid-catalyzed reactions by Calkyl-O bond cleavage consistent with formation of an aryloxenium ion. Trapping with N3- and Br- confirms that both uncatalyzed and acid-catalyzed decompositions occur by rate-limiting ionization to form the 4-biphenylyloxenium ion, 1a. This ion has a shorter lifetime in H2O than the corresponding nitrenium ion, 7a (12 ns for 1a, 300 ns for 7a at 30 C). Similar analyses of the product, 3b, of acid- and base-catalyzed decomposition of 4-acetoxy-4-methyl-2,5-cyclohexadienone, 2b, in 18O-H2O show that these reactions are ester hydrolyses that proceed by Cacyl-O bond cleavage processes not involving the p-tolyloxenium ion, 1b. Uncatalyzed decomposition of the more reactive 4-dichloroacetoxy-4-methyl-2,5-cyclohexadienone, 2b', is also an ester hydrolysis, but 2b' undergoes a kinetically second-order reaction with N3- that generates an oxenium ion-like substitution product by an apparent SN2' mechanism. Estimates based on the lifetimes of 1a, 7a, and the p-tolylnitrenium ion, 7b, and the calculated relative stabilities of these ions toward hydration indicate that the aqueous solution lifetime of 1b is ca. 3-5 ps. Simple 4-alkyl substituted aryloxenium ions are apparently not stable enough in aqueous solution to be competitively trapped by nonsolvent nucleophiles.
Publication Type: Journal Article
Source of Publication: Journal of the American Chemical Society, 127(22), p. 8090-8097
Publisher: American Chemical Society
Place of Publication: Washington, DC
ISSN: 0002-7863
Field of Research (FOR): 030505 Physical Organic Chemistry
Peer Reviewed: Yes
HERDC Category Description: C1 Refereed Article in a Scholarly Journal
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