Self-Initiated Autoxidation of a Sterically Crowded Cycloheptatriene Derivative via Norcaradienyloxyl Radicals

Abstract

An extremely crowded cycloheptatriene derivative, 1,3,5-tri-tert-butyl-7-(9-phenyl-9-fluorenyl)-1,3,5cycloheptatriene (1), underwent autoxidation at 25 °C in cyclohexane to give 4-tert-butyl-2pivaloylphenol (3) (33%), 5-tert-butyl-2-pivaloylphenol (4) (11%), 1,3,5-tri-tert-butylbenzene (5) (5%), tert-butyl 9-phenyl-9-fluorenyl peroxide (6) (44%), bis(9-phenyl-9-fluorenyl) peroxide (7) (7%), 1,1′,3,3′,5,5′-hexa-tert-butyl-7,7′-bicycloheptatriene (8) (2%), and carbon monoxide (4%). ESR studies showed that 1 dissociates into 1,3,5-tri-tert-butyltropyl radical (11) and 9-phenylfluorenyl radical (12) at 25-85 °C. The enthalpy and entropy of dissociation were determined to be 23.3 kcal/mol and 22.0 cal/mol‚K, respectively. The formation of 3-5 can be explained by a mechanism involving attack of molecular oxygen to 11 and subsequent valence tautomerism of cycloheptatrienyloxyl radicals to norcaradienyloxyl radicals.

Allyl Radical Nature of a Phenylcyclopentadienyl Radical Annelated with Two Homoadamantene Frameworks

“…In accord with this result, GIAO calculations predicted low-field shift of the 13 C NMR signals of these carbons (δ 175.9, 189.8, and 193.6, respectively), which is in agreement with experimental observation of three deshielded signals at δ 184.3, 179.0, and 166.8. The proposed mechanism of the oxidation begins with the addition of dioxygen to the Cp ring to form a peroxy radical 6 • , the intermolecular reaction of which brings about the loss of an oxygen atom to lead to cyclopentadienyloxy radical 7 • . The subsequent rearrangement forms the pyrylium radical 8 • , followed by oxidation by Ag + to afford a stable 6π aromatic cation 8 + .…”

Solvent‐Mediated Generation of Cobalt‐Cluster‐Stabilised Propargyl Cations and Radicals: Allyl Migration versus Peroxide Formation

“…Although these are, to our knowledge, the first metal-complexed fluorenyl peroxides to have been synthesised, there are several known metal-free difluorenyl peroxides, including phenyl-, isopropyl-and benzyl-substituted systems. [33,34] It should be emphasised that organometallic peroxides are relatively uncommon: not only are preformed peroxide ligands insufficiently thermally stable to survive the conditions required for complexation by a metal, but also low-valent transition metals generally bring about cleavage of the peroxy linkage. Previously known examples include numerous mercuric peroxides [35] and transition-metal-containing porphyrins.…”

Circumambulatory rearrangements of cyclopolyenes containing element-centred migrants

“…Thermal dissociation of C7C s bonds in hydrocarbon molecules generally involves their homolytic cleavage. For instance, sterically overcrowded 1,3,5-tri-tert-butyl-7-(9-fluorenyl-9-phenyl)cyclohepta-1,3,5-triene (73a) dissociates upon heating in m-xylene or cyclohexane to form the 1,3,5-tri-tert-butyltropyl (73b) and 9-phenylfluorenyl (73c) radicals (DH = = 23 kcal mol 71 , DS = = 22.0 cal mol 71 deg 71 ) 200 detected by EPR spectroscopy. Evidently, the recombination step proceeds randomly.…”