Reaktivität bei intramolekularen Radikalreaktionen – Vergleich zwischen Wasserstofftransfer und Alkenaddition

Abstract

Bei Ionen‐ wie bei Radikalreaktionen scheint generell die effektive Molarität EM (= kintra/kinter) für Additionen sehr viel größer als für den Wasserstofftransfer zu sein. Werte für Radikalreaktionen wurden an Acyl‐tert‐butylnitroxiden ermittelt: Für den Transfer eines benzylischen H‐Atoms in 1 ergibt sich aus dem Vergleich mit der entsprechenden intermolekularen Reaktion ein EM‐Wert von 44 M, für die Addition von Radikalen an Alkene erhält man analog EM‐Werte von ca. 105 M.

Metal‐Free, Aerobic Dioxygenation of Alkenes Using Hydroxamic Acids

“… Spontaneous aerobic radical cyclization of stilbene‐substituted tert ‐butyl hydroxamic acid 1 as reported by Perkins and co‐workers 11…”

“…During their pioneering work on the fundamental reactivity of amidoxyl radicals with alkenes, Perkins and co‐workers observed a single remarkable example of an amidoxyl radical cyclization followed by oxygenation (Scheme ) 11. While attempting to prepare the highly conjugated stilbene‐substituted tert ‐butyl amidoxyl radical 2 , the parent hydroxamic acid 1 underwent spontaneous oxidation and intramolecular cyclization followed by radical oxygenation to deliver hydroperoxide 3 as a mixture of diastereomers 12.…”

Metal‐Free, Aerobic Dioxygenation of Alkenes Using Hydroxamic Acids

“…For example, radical decarboxylation, [7] dehalogenation, [8] demercuration, [9] and carbocyclization [10] processes have all utilized molecular oxygen to selectively deliver radical oxidation products.During their pioneering work on the fundamental reactivity of amidoxyl radicals with alkenes, Perkins and coworkers observed a single remarkable example of an amidoxyl radical cyclization followed by oxygenation (Scheme 1). [11] While attempting to prepare the highly conjugated stilbene-substituted tert-butyl amidoxyl radical 2, the parent hydroxamic acid 1 underwent spontaneous oxidation and intramolecular cyclization followed by radical oxygenation to deliver hydroperoxide 3 as a mixture of diastereomers. [12] These nitroxyl radicals, which contain electronwithdrawing acyl groups, are destabilized relative to persistent dialkyl nitroxyl radicals (e.g.…”

“…Spontaneous aerobic radical cyclization of stilbene-substituted tert-butyl hydroxamic acid 1 as reported by Perkins and coworkers. [11]…”

“…Spontaneous aerobic radical cyclization of stilbene-substituted tert-butyl hydroxamic acid 1 as reported by Perkins and coworkers. [11] mild in situ reductive work up with PPh 3 ( Encouraged by these initial findings, we explored the generality of this metal-free aerobic alkene dioxygenation in a variety of synthetic contexts ( Table 2). The difunctionalization of substrate 6 by 5-exo ring closure proceeded well, delivering isoxazolidinone 7 in 88 % yield ( Table 2, entry 1).…”