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Ring contraction aromatics

Only a few reactions of benzodithiadiazines have been investigated. In common with dithiatriazines 12.8, the anti-aromatic system 12.12 (R = H) undergoes a reversible 5,5 -cycloaddition with norbornadiene. The reaction of 12.12 (R = F) with triphenylphosphine results in a ring contraction to give the imino 2 -phosphane 12.13. ... [Pg.246]

On heating, the dichlorooxazolo[3,4- ]azepine 26, for which dimerization is prevented by the chloro groups, undergoes ring contraction and aromatization, involving a [l,2]-chlorine shift, to 5,8-dichloro-l,4-dihydro-2//-3,l-benzoxazin-2-one (27).11,153... [Pg.183]

By analogy with die acid-induced ring contraction of cycloheptatrienyl ketones 233, high-yield rearrangement to 2-tetralones is possible 233 >. As Scheme 32 shows, substituents on the aromatic nucleus determine the regioselectivity of cyclopropa-... [Pg.178]

Tri- tert-butyl-1,3,5 -triphosphabenzene (128, Scheme 53) undergoes a ring contraction over a potassium mirror,159 with the elimination of potassium phosphine and formation of the aromatic 2,4,6-tri-tot-butyl-1,3-diphosphacyclopentadienide anion 129 with three bulky substituents (Scheme 53). The driving force of this reaction could be the higher aromaticity of the five-membered ring. [Pg.22]

Skeletal ring contraction steps of primary C7 and Cg rings are more probable than bicyclic intermediates (132b). Aromatization of methylcyclo-pentane indicated no carbonium mechanism with a nonacidic catalyst. Instead, Pines and Chen (132b) proposed a mechanism similar to that defined later as bond shift. This is a methyl shift. Two additional isomerization pathways characteristic of chromia have also been demonstrated vinyl shift (94) and isomerization via C3 and C4 cyclic intermediates (90a). These were discussed in Section III. 1,1-Dimethylcyclohexane and 4,4-dimethyl-cyclohexene gave mainly toluene over various chromia catalysts. Thus, both skeletal isomerization and demethylation activities of chromia have been verified. The presence of an acidic almnina support enhances isomerization dual function effects are thus also possible. [Pg.317]

Other approaches to alkylidenecycloproparenes have been attempted without success. Aromatization of appropriate alkylidenecyclopropanes or their precursors could not be realized, and flash vacuum pyrolysis of methylene phthalide and 3-methylene-2-coumaranone afforded rearrangement products rather than alkylidenecycloproparenes via extrusion of 002. The photochemical or thermal decomposition of the sodium salt of benzocyclobutenone p-toluenesulfonyl hydrazone led to products derived from dimerization of the intermediate benzocy-clobutenylidene, or from its reaction with the solvent, but no ring-contracted products were observed. When the adduct of methylene-l,6-methano[10]annulene to dicyanoacetylene (249) was subjected to Alder-Rickert cleavage, phenylacety-lene (250) was formed, which derives reasonably from the parent 234. ... [Pg.67]

Table 4 shows that benzene is formed at almost identical rates from cyclohexene, hexadiene, methylcyclohexene and cyclohexadiene react under low partial pressure over Ga-HZSM-5. which suggests that over these catalysts benzene is formed from the same intermediate. In contrast over H-ZSM-5, under identical experimental conditions, the rate of benzene formation from the hydrocarbons cited was one to two orders of magnitude lower. These results prove again that gallium plays a decisive role in aromatization. Over H-ZSM-5 the major hydrocarbon formed is methylcyclopentene from cyclohexene (ring contraction)... [Pg.277]

Partially unsaturated azepines, like the fully conjugated systems, tend to be unstable in acid solution and undergo either rearrangement or ring contraction, generally to an aromatic system. Protonation takes place at nitrogen or, as in those hydroazepines in which delocalization of the nitrogen lone pair is possible, at the /3 -carbon of the enamine system. [Pg.509]

Huisgen found that 2-amino-3H-azepines are unstable under normal acylating conditions. Schotten-Baumann benzoylation, for example, brings about aromatization and, ultimately, formation of 2-(benzamido)diphenylamine, whereas p-nitrobenzoyl chloride in cold pyridine yields 2-(p-nitrophenyl)-l-phenylbenzimidazole. Benzenesulfonylation, however, proceeds normally at the exocyclic nitrogen. The mechanistic rationale for these ring contractions is outlined in Scheme 8 (b-69MI516oo). [Pg.511]

Other fluoroaromatic compounds such as fluorobenzene, trifluorotoluene and their derivatives may be elaborated to more complex aromatic compounds by directed metallation reactions (in itself, the subject of another large review [322-324]). Wakselman and co-workers described the conversion of 3-tri-fluoromethylphenol into 2-(trifluoromethyl)-l,3-cyclopentadienone, an intermediate they used to synthesise angularly trifluoromethylated steroid analogues [325]. The reaction, which involved an interesting ring contraction reaction, occurred with rather low efficiency (Eq. 122) [326]. [Pg.175]

Whereas, photodecomposition of diazoketones in the presence of aromatic nuclei induces an enlargement of the ring system, irradiation of cyelic-diazoketones results in ring contraction with loss of nitrogen,... [Pg.114]


See other pages where Ring contraction aromatics is mentioned: [Pg.414]    [Pg.308]    [Pg.220]    [Pg.530]    [Pg.775]    [Pg.911]    [Pg.73]    [Pg.110]    [Pg.573]    [Pg.143]    [Pg.143]    [Pg.5]    [Pg.256]    [Pg.256]    [Pg.216]    [Pg.82]    [Pg.33]    [Pg.45]    [Pg.47]    [Pg.48]    [Pg.27]    [Pg.208]    [Pg.247]    [Pg.306]    [Pg.307]    [Pg.310]    [Pg.527]    [Pg.498]    [Pg.499]    [Pg.434]    [Pg.1122]    [Pg.542]    [Pg.745]    [Pg.503]    [Pg.509]    [Pg.511]    [Pg.523]    [Pg.641]    [Pg.542]   
See also in sourсe #XX -- [ Pg.19 , Pg.66 ]




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