4‐exo‐Cyclisierungen durch Templatkatalyse

Abstract

Kleine Ringe durch große Template: Die Zwei‐Punkt‐Anbindung des Radikals an das kationische Templat ist für den Erfolg einer sonst unmöglichen 4‐exo‐Cyclisierung entscheidend (siehe Schema; Bn=Benzyl). Es werden mit hoher Stereoselektivität Cyclobutane erhalten, die sich auf einfache Weise weiter funktionalisieren lassen.

Synthesis of Dihydropyrrolizine and Tetrahydroindolizine Scaffolds from Pyrroles by Titanocene(III) Catalysis

“…To rule out the unlikely generation of a tertiary cation, the radical was trapped with t ‐butyl acrylate to give 4 at room temperature and at 66 °C. In agreement with a study on the radical arylation with anilines,, and comparison with slow cyclizations,, the results suggest an upper limit for the rate constant of radical addition of 10 3 at 298 K. Therefore, radical addition to pyrroles is relatively slow. The high yields of 3 highlight the fact that the alkyl radicals generated under our reaction conditions are fairly persistent.…”

Radical 4‐exoCyclizations via Template Catalysis

“…Computational study of the reaction mechanism : In this computational study, the cyclohexylidene group in 7 and 8 was replaced by a gem ‐dimethyl group and the NHBn group by an NHMe group to reduce the complexity of the system. This is justified because the experimental results are essentially identical for titanocenes containing such changes 8…”

“…This is justified because the experimental results are essentially identical for titanocenes containing such changes. [8] Single-point binding of the radicals: The simplest starting point of the study of the 4-exo cyclization is radical 11 featuring a single-point binding of the radical to the titanocene. It should be noted that this single-point binding results in a reaction similar to that of free-radical reactions where the titanocene acts as a radical generating agent and bulky group as potential passive control element of diastereoselectivity.…”

“…Recently, we described the first examples of such 4‐ exo cyclizations without assistance by the gem ‐dialkyl effect 8. The use of our novel titanocene derived cationic catalyst 2 9 was essential as shown for substrate 1 in Scheme .…”

“…[7] Recently, we described the first examples of such 4-exo cyclizations without assistance by the gem-dialkyl effect. [8] The use of our novel titanocene derived cationic catalyst 2 [9] was essential as shown for substrate 1 in Scheme 1. The opening of a coordination site for a two-point radical binding was postulated to be essential.…”