Abstract:Bei der Belichtung von Pivalophenon (1a) in Benzol erfolgt α‐Spaltung unter bevorzugter Bildung von Benzaldehyd, Isobutan und Isobutylen. 4‐tert.‐Butyl‐ und 2.4.6‐Trimethyl‐pivalophenon (1b und 1c) reagieren analog, während 2.4‐Dimethyl‐pivalophenon (1d) überwiegend zum Benzocyclobutenol 5 isomerisiert. In Propanol‐(2) unterliegen 1a und 1b der Photoreduktion in Konkurrenz zur α‐Spaltung. Statt der erwarteten Pinakole entstehen die zugehörigen Benzylalkohole 7a und 7b. Pivalonaphthon‐(I) ist unter den Reaktion… Show more
“…31 The small variation in cyclization quantum yield (compared with that for elimination quantum yield) with a substitution suggests that eclipsing interactions destabilize the conformation for biradical cyclization (18) less than that for elimination (15). A nonplanar cyclization transition state can readily accommodate a single (equatorial) /3-methyl substituent (19). However a second ß substituent will introduce a 1,3-diaxial interaction.…”
The effects of methyl substituents upon the photochemical behavior of butyrophenone have been investigated. ierr-Alkyl phenyl ketones undergo competitive a-(type I) cleavage and type II photoelimination and cyclization. The rate constant for -cleavage (~1 X 107 sec-1) is considerably slower than for a-cleavage of triplet ferr-butyl alkyl ketones owing to a difference in the energetics of the reactions. The reactivity of the aromatic carbonyl triplet state toward 7-hydrogen abstraction is determined primarily by 7-substitution and is less affected by a and ß substituents, a substituents increase the percentage of cyclization products whereas ß substituents have the opposite effect. The effects of substituents on the behavior of the 1,4-biradical intermediates are explained in terms of the transition states for biradical cyclization and elimination.
“…31 The small variation in cyclization quantum yield (compared with that for elimination quantum yield) with a substitution suggests that eclipsing interactions destabilize the conformation for biradical cyclization (18) less than that for elimination (15). A nonplanar cyclization transition state can readily accommodate a single (equatorial) /3-methyl substituent (19). However a second ß substituent will introduce a 1,3-diaxial interaction.…”
The effects of methyl substituents upon the photochemical behavior of butyrophenone have been investigated. ierr-Alkyl phenyl ketones undergo competitive a-(type I) cleavage and type II photoelimination and cyclization. The rate constant for -cleavage (~1 X 107 sec-1) is considerably slower than for a-cleavage of triplet ferr-butyl alkyl ketones owing to a difference in the energetics of the reactions. The reactivity of the aromatic carbonyl triplet state toward 7-hydrogen abstraction is determined primarily by 7-substitution and is less affected by a and ß substituents, a substituents increase the percentage of cyclization products whereas ß substituents have the opposite effect. The effects of substituents on the behavior of the 1,4-biradical intermediates are explained in terms of the transition states for biradical cyclization and elimination.
“…Having failed to synthesize azo or perester precursors to 2 , we turned to the Norrish Type I photochemical cleavage of α-azoxy ketone 4 , a previously unknown structural type. Photolysis of some phenones and benzyl ketones yields alkyl radicals in solution; − hence, it is not unreasonable that 4a − c might also exhibit α-cleavage. …”
[reaction: see text] Although some aspects of azoxy group radical chemistry have been investigated, unhindered alpha-azoxy radicals remain poorly understood. Here we report the generation of alpha-azoxy radicals under mild conditions by irradiation of alpha-azoxy ketones 4a,b. These compounds undergo alpha-cleavage to yield radicals 5a,b, whose oxygen atom then recombines with benzoyl radicals to produce presumed intermediate 15. Formal Claisen rearrangement gives alpha-benzoyloxyazo compounds 8a,b, which are themselves photolabile, leading to both radical and ionic decomposition. The ESR spectrum of 5a was simulated to extract the isotropic hyperfine splitting constants, which showed its resonance stabilization energy to be exceptionally large. Azoxy compounds have been found for the first time to be good quenchers of triplet excited acetophenone, the main sensitized photoreaction of 7Z in benzene being deoxygenation. While this reaction has been reported previously, it was always in hydrogen atom donating solvents, where chemical sensitization occurred. The principal direct irradiation product of 4bZ and model azoxyalkane 7Z is the E isomer, whose thermal reversion to Z is much faster than that of previously studied analogues.
“…We were stimulated by the simplicity of this approach to prepare benzocyclobutenols from relatively inexpensive feedstock, i.e . aryl halides and THF, and expect subsequent oxidation 19 would provide a rapid way to access benzocyclobutenones. Hence, our study began with optimizing the reaction conditions for the synthesis of benzocyclobutenol, then investigating the influence of functional groups including alkenes and alkynes, and finally employing this approach to prepare functionalized benzocyclobutenones via subsequent alcohol oxidation.…”
A concise approach to access functionalized benzocyclobutenones from 3-halophenol derivatives is described. This modified synthesis employs a [2+2] cycloaddition between benzynes generated from dehydrohalogenation of aryl halides using LiTMP and acetaldehyde enolate generated from n-BuLi and THF, followed by oxidation of the benzocyclobutenol intermediates to provide benzocyclobutenones. The [2+2] reaction can be run on a 10-gram scale with an increased yield. A number of functional groups including alkenes and alkynes are tolerated. Coupling of benzynes with ketene silyl acetals to give 8-substituted benzocyclobutenones is also demonstrated.
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