Thermal rearrangement of spiro[naphthalene(naphthopyranofurazan)]oxides to spiro[naphthalene(phenalenofurazan)oxides. A probable furazan oxide triggered tandem isomerisation process
Thermal rearrangement of spiro[naphthalene(naphthopyranofurazan)]oxides to spiro[naphthalene(phenalenofurazan)oxides. A probable furazan oxide triggered tandem isomerisation process
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Supsana, P.
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Πανεπιστήμιο Ιωαννίνων. Σχολή Θετικών Επιστημών. Τμήμα Χημείας
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Supsana, P.
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Thermal rearrangement of (+/-)-spiro {naphthalene-1(2H)4'-(naphtho-[2', 1': 2,3]pyrano[4,5-c]furazan)]-2-one-11'-oxides in DMF or acetic anhydride at 140 degrees C gave an isomeric mixture of (+/-)-spiro {naphthalene-1 (2H), 1'-(5'-hydroxyphenalene[1,2-c]furazan)}-2-one-2'-oxides and 4'-oxides. The rearranged structure was confirmed from X-ray analysis and was consistent with the through space NOE data. The rearrangement is suggested to be an overall tandem isomerization process. Using variable temperature H-1 NMR spectroscopy the lower limit for the isomerisation barrier for a pair of tautomers was calculated to be 22 kcal mol(-1) at 423 K. The isomerisation equilibrium for a pair of isomers was studied by variable temperature 1H NMR. The lower limit for the isomerisation barrier was calculated to be 22 kcal mol(-1) at 423 K. This low value may be indicative of the difficulty encountered in separating the isomers by chromatography. Semiempirical AMI and molecular mechanics calculations suggest that the (+/-)-spiro {naphthalene-1(2H), 1'-(5'-hydroxyphenalene[1,2-c]-furazan) 1-2-one-2'-oxides are more stable than their 4'-oxide counterparts, in accordance to the X-ray structure. The lower population of the 4'-oxide isomers relative to that of the 2'-oxide isomers was explained in terms of an unfavourable intramolecular steric interaction found in the low energy structure of the former. (c) 2005 Elsevier Ltd. All rights reserved.
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