The Mechanistic Puzzle of Transition‐Metal‐Catalyzed Skeletal Rearrangements of Enynes

Abstract

Three pathways actually compete in metal-catalyzed cyclizations of enynes in which the metal selectively activates the alkyne: an endocyclic process and two exo-cyclizations, one proceeding by anti attack of the alkene and a second one resulting in a syn addition. Although cyclobutenes may be formed in transition-metal-catalyzed cyclization of some enynes, particularly, 1,7-enynes, these compounds are not necessarily the intermediates in the skeletal rearrangement. Cyclobutenes are formed by ring expansion of syn-cyclopropyl metal-carbenes formed in the syn pathway.

Cycloisomerization of 1,n‐Enynes: Challenging Metal‐Catalyzed Rearrangements and Mechanistic Insights

“…Thorough investigations were conducted to understand the mechanistic rationale of these transformations. The isolation of a set of minor by-products, as well as DFT calculations, led the research groups of Fürstner [121,123] and Echavarren, [119] respectively, to present a comprehensive mechanistic picture that accounts for the observed regioselectivity (Scheme 27). Initial coordination of the platinum center occurs at the C À C triple bond to give an h 2 complex 299.…”

A Highly Efficient and Selective AuI‐Catalyzed Tandem Synthesis of Diversely Substituted Pyrrolo[1,2‐a]quinolines in Aqueous Media

“…[1] Among the numerous methods for this endeavor goldcatalyzed transformations are receiving much attention, [2,3] but examples of such reactions conducted in aqueous media are sparse. [3k,l,u] Water often exhibits an unprecedented effect on the rate and selectivity of organic reactions through hydrophobic interactions and the enrichment of organic substrates in the local hydrophobic environment.…”

Cycloisomerisierungen von 1,n‐Eninen: faszinierende metallkatalysierte Umlagerungen und mechanistische Einblicke

“…Echavarren und Mitarbeiter [115] haben die Cycloisomerisierung von Eninen wie 15 zu einem Gemisch der 1,4-Diene 16 a und 16 [121,123] bzw. Echavarren [119] als Grundlage für ein umfassendes mechanistisches Bild, das die beobachtete Regioselektivität erklärt (Schema 27). Zu Beginn koordiniert das Platinzentrum an die C-C-Dreifachbindung unter Bildung des h 2 -Komplexes 299.…”

“…[152] Das zu 300 (siehe Schema 27) äquivalente Cyclopropylcarben kann durch einfache oder doppelte Spaltung entstehen und zu den entsprechenden Dienen führen. [119] Die Isolierung und die Stabilität des Cyclobutens 380 sind ebenfalls mit dem vorgeschlagenen Mechanismus vereinbar [Gl. (124) …”