Radical cycloaromatization reactions

Scheme 1 Schematic representation of cycloaromatization reactions. Double lines correspond to the out-of-plane re-systems of a bis-alkyne reagent. Only orbitals of the in-plane re-system in the reactant and of new

These results, of course, contrast the significant effect of benzannelation in radical-anionic cycloaromatization reactions discussed above12 where not just the formation of a new aromatic cycle but also restoration of aromaticity in the previously existing cycle occurs at the same time in the cyclorearomatization process. 

Kovalenko and Alabugin reported that Cj-C cyclization of benzannelated enediynes with tetra-fluoropyridinyl (TFP) substituents at the terminal alkyne carbons forms indenes rather than fulvenes (Scheme 30.22). The radical/anionic Cj-Cj cyclization of enediynes represents a new type of cycloaromatization reaction—the cyclorearomatization process driven by rearomatization in the vicinity of the TS. This process is triggered via photoinduced electron transfer (PET) [35-38]. Expaimental work unambiguously established PET as the triggering event for the Cj-C cascade and the intermediacy of the second PET step in the indene-forming cascade [39]. Unlike the stable benzene product... 

The radical-initiated reaction of substituted enediynes leads to the creation of fused rings such as fluoranthene and acephenanthrylene in a single step. For example, orr/70-substituted biphenyl (radical target) is prepared by coupling homoenediyne 3.647 with l-bromo-2-iodobenzene. Desilylation of the formed biphenyl derivative gives the enediyne 3.648. Next, its reaction with tin hydride/AIBN at 80°C generates free radicals which initiates cycloaromatization to form only one product, the fluoranthene derivative 3.649 (Scheme 3.60) [305]. 

With the presence of two methyl substituents at the allenic terminus of 20a, the a,3-didehydrotoluene biradical 21 having a tertiary benzylic radical center was generated after cycloaromatization (Eq. 20.1). As a result, the half-life of the reaction is only -70 min at 37 °C, which is significantly shorter than that of 8. 

Alternatively, a retro-ene reaction cleaves the O10-C11 bond and gives a highly unsaturated ketene. The ketene can undergo cycloaromatization to give a diradical intermediate. H- abstraction and radical-radical recombination then give the product. 

The major classes of nonchain free-radical reactions are photochemical reactions, reductions and oxidations with metals, and cycloaromatizations. 

Cumulene structures also undergo the Myers-Saito reaction. Cyclization of acyclic enyne[3]cumulenes, on the activation of Z-configured dienediyne 38 via acid solvolysis, has been described by Bruckner et al. It has been found that 38 dissolved in /-BuSH/dichloro-methane and treated with a catalytic amount of triflic acid forms the monocyclic cumulene 39. Storage of the mixture for 4 days at room temperature gave the corresponding styrene derivatives 40 and 41 these products form as a result of cycloaromatization via path A (benzoid radical). Independently, after... 

An aza-variant of the cycloaromatization of propargyl azaeneynes, such as 50, via azaenyne-allenes 51, has been reported by Kerwin et al. The aza-Myers-Saito cyclization provides a,5-didehydro-3-picoline diradical 52, which affords either polar or radical-based trapping products 53 and 54, depending on the reaction solvent. The facility of the aza-Myers-Saito cyclization relative to the parent Myers-Saito cyclization was predicted based on DFT calculations these results also indicate that the corresponding C2-C6 (aza-Schmittel) cyclization, although disfavored in the case of 51, is... 

Cycloaromatization may be initiated by adding radical species to arenediynes [303-314]. The cyclization of diarylenediynes 3.624 by the addition of Bu3SnH proceeds via formation of BusSn radicals which can add to a weakly polarized triple bond by the formation of free a-radical 3.625 or p-radical 3.626 (Scheme 3.54) [308]. Although the P-radical of 1,2-diethynylbenzene is less stable than the a-radical, in the case of enediynes 3.624 this difference is compensated by stabilization of the terminal aryl substituent. The reaction selectivity indicates that 5-exo-dig-cyclization has a lower activation barrier than 6-endo-dig- and 5-endo-dig-cjdiz3.tion (Scheme 3.54) [308]. 

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