Dienophiles with a difference: 2‐Aryl 1,1‐bis(alkoxycarbonyl) cyclopropanes undergo the title reaction under the catalysis of ytterbium triflate (Yb(OTf)3) in excellent yield (see scheme). Under mild reaction conditions, the major product is the less stable exo isomer. At higher temperatures, the endo isomer is obtained exclusively, as the exo cycloadduct decomposes through cycloreversion.
Halogenated trifluoroacetylacetylenes have been synthesized for the first time. It was shown that they undergo formally forbidden by the orbital symmetry rules [2+2] cycloaddition reac tions with nonactivated alkenes in the absence of light and catalyst. A possible mechanism for this transformation is suggested.
Manche mögen's heiß – andere nicht: 2‐Aryl‐1,1‐bis(alkoxycarbonyl)cyclopropane gehen unter Katalyse von Yb(OTf)3 in hervorragender Ausbeute die Titelreaktion ein (siehe Schema). Das Hauptprodukt unter milden Reaktionsbedingungen ist das weniger stabile exo‐Isomer. Bei höheren Temperaturen wird hingegen ausschließlich das endo‐Isomer erhalten, weil das exo‐Cycloaddukt durch Cycloreversion zersetzt wird.
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