Self‐Terminating Radical Cyclizations: How Are Thiyl Radicals Performing?

Abstract

The performance of thiyl radicals RS · in "self-terminating radical cyclisations" was explored. Using the medium-sized cyclodecyne (1) as model system, the reaction of PhS · generated by photolysis of (PhS) 2 was used to study the intermolecular S-radical addition and subsequent intramolecular radical translocations. This reaction resulted in the formation of three stereoisomeric sulfides 17a in very good yield, which all possess the bicyclo[4.4.0]decane framework with either cis and trans ring fusion. The isomeric bicyclo[5.3.0]decane framework was not formed. Product identification was performed using a combination of techniques, e.g. synthesis of authentic samples, X-ray analysis and computational studies of the potential energy surface, which also revealed valuable insight into the mechanism of this radical cyclisation cascade. The (PhS) 2 /PhS · system provides an efficient source for in situ

Thiyl Radicals in Organic Synthesis

“…The radical cascade with benzenethiyl radicals led to a mixture of [4.4.0]- and [5.3.0]- bicyclic sulfides 258 and 259 in good overall yields (Scheme ) . Similar results were obtained for the reaction carried out with the corresponding disulfides RS–SR (R = alkyl, allyl, benzyl, aryl) . These results contrast with those of similar cascades reported by the same group using oxygen-centered radicals, which ended with the formation of a CO through a β-fragmentation (“self-terminating radical cyclization”) or, alternatively, oxidation or group transfer as suggested by Curran .…”

Alkenes as Alkyne Equivalents in Radical Cascades Terminated by Fragmentations: Overcoming Stereoelectronic Restrictions on Ring Expansions for the Preparation of Expanded Polyaromatics

“…The 2-center, 3-electron (2c-3e) delocalization between the alkyl radical center and lone pair of the adjacent heteroatom serves to stabilize the radical leaving group effectively allowing the cascade to “self-terminate”. This term was introduced by Wille et al to describe transformations where stable fragmenting radicals “exit” the reaction without exhibiting subsequent reactivity . Such 2c-3e interactions correspond to the formal bond order of 1/2 and can be referred to as “half-bonds”.…”

The Missing C1–C5 Cycloaromatization Reaction: Triplet State Antiaromaticity Relief and Self-Terminating Photorelease of Formaldehyde for Synthesis of Fulvenes from Enynes

“…In order to overcome the possible problems associated with trapping the diradical product of the elusive C 1 –C 5 cyclization of enynes, we have envisaged an innovative solution based on self-terminating radical cyclizations. In this recently reported class of organic transformations, the cyclization is terminated by a stereoelectronically promoted and thermodynamically favorable C–C bond fragmentation driven by aromatization and the formation of two-center/three-electron bonds .…”