Addition von Haloformen an polarisierte Doppelbindungen

Abstract

Trihalogen-methide, die durch Eliminierung des Protons aus Haloformen mittels Basen oder durch Thermolyse von Natriumtrichloracctat 1) sowie nach einigen weiteren, spezielleren Methoden 2 ) als Zwischenstufen bei der Erzeugung von Dihalogencarbenen entstehen, konnen mit geeignet substituierten Olefinen in Konkurrenzreaktion zur Cyclopropanbildung nucleophile Additionen eingehen. So fand Wngner3), daR beim Erhitzen von Natriumtrichloracetat in 1.2-Dimethoxy-athan in Gegenwart von Vinylacetat sowohl I . 1-Dichlor-2-acetoxy-cyclopropan (10 %) als auch I-Trichlormethyl-Hthylacetat (10 %) gebildet wird. Fur nucleophile Additionen an Enolacetate gibt es bisher keine anderen Beispiele in der Literatur. De Selnls4) stellte hei ahnlichen Versuchen mit substituierten Vinylacetaten ein Anwachsen der Ausbeuten an Trichlormethid-Addukt mit steigender Elektronenacceptorwirkung des Carboxylanteils fest, was einen Primarangriff des Anions wahrscheinlich macht, und klarte die Herkunft des zur Neutralisation des Addukts erforderlichen Protons.

Formation of Five‐Membered Cyclic Orthoesters from Tribromides with Participation of a Neighboring Carbonyl Group

“…[14][15] Whereas the LDA-mediated reaction provided dibromomethylenated nitroalkenes 3 at low temperature (-78°C), the Mg-mediated reaction furnished nitrotribromides 2 at 0°C to room temperature, which underwent elimination under reflux conditions to form dibromides 3.…”

Electrocyclic Ring-Opening Reactions of gem-Dibromocyclopropanes in the Synthesis of Natural Products and Related Compounds

“…82 The reaction is complicated, however, by the production of conjugated diene from 1,4-elimination. More selective is the application of the cyclopropyl-allyl rearrangement to methylidenecyclopropanes.…”

“…Sequential gem-dibromocyclopropane synthesis, electrocyclic ring opening, and capture of the ensuing allylic cation has been made into a 'one-pot' procedure for use with ethene oxide as illustrated in Scheme 28. 82 The reaction is complicated, however, by the production of conjugated diene from 1,4-elimination. More selective is the application of the cyclopropyl-allyl rearrangement to methylidenecyclopropanes.…”

Facile Synthesis of β-Tribromomethyl and Dibromomethylenated Nitroalkanes via Conjugate Addition of Bromoform to Nitroalkenes

“…Haloform is a ready source of trihalomethyl and dihalomethylene and the reaction of haloform with alkenes under anionic − and radical conditions is well-established. The base mediated reaction, in the absence and more often in the presence − of phase transfer catalysts (PTC), is the method of choice for the cyclopropanation of alkenes.…”

“…Haloform is a ready source of trihalomethyl and dihalomethylene and the reaction of haloform with alkenes under anionic − and radical conditions is well-established. The base mediated reaction, in the absence and more often in the presence − of phase transfer catalysts (PTC), is the method of choice for the cyclopropanation of alkenes. While the formation of cyclopropane derivatives from unactivated alkenes presumably takes place via dibromocarbene addition, such product formation from activated alkenes appears to proceed via an initial Michael addition of the trihalomethyl carbanion followed by intramolecular cyclization pathway. , In the case of activated alkenes, the Michael adducts and their β-elimination products, the alkylidene dibromides, are also isolated …”