Didehydrotoluene biradical

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. 

In the absence of 1,4-CHD, the biradical 55e undergoes an intramolecular 1,5-hydrogen shift to form 57, making it possible for an intramolecular radical-radical coupling to occur to produce 58 (Scheme 20.13). The fact that 58 was produced from 54e lends support to the formation of the a,3-didehydrotoluene biradical 55e as a transient reaction intermediate. It is also worth noting that the benzylic radical center in 55 is a stabilized triarylmethyl radical. 

Scheme 20.13 Intramolecular 1,5-hydrogen shift of an a,3-didehydrotoluene biradical.

Scheme 20.14 a,3-Didehydrotoluene biradicals having a triarylbenzylic radical center. 

The Myers cyclization of enyne-allenes under mild thermal conditions provides an easy access to the a,3-didehydrotoluene biradicals 10-18). Thermolysis of 22b in refluxing benzene generated the biradical 23 (Scheme 5). The phenyl radical center in 23 was then captured by the double bond intramolecularly, giving rise to the biradical 24. A subsequent 1,5-hydrogen shift produced the o-quinodimethane 25, which in turn underwent a [1,5] sigmatropic hydrogen shift to afford the indan 26. 

The ability of (Z)-l,2,4-heptatrien-6-ynes (enyne-allenes) and the benzannulated derivatives to undergo cyclization reactions under mild thermal conditions to produce biradicals has been the main focus of their chemical reactivities [1-5]. With the development of many synthetic methods for these highly conjugated allenes, a variety of biradicals are readily accessible for subsequent chemical transformations. Cyclization of the enyne-allene 1 could occur either via the C2-C7 pathway (Myers-Saito cyclization) leading to the a,3-didehydrotoluene/naphthalene biradical 2 [6-10] or via the C2-C6 pathway (Schmittel cyclization) producing the fulvene/benzofulvene biradical 3 [11] (Scheme 20.1). 

The prototype of this reaction is the Myers-Saito reaction, the rearrangement of eneyneallene (Z)-hepta-l,2,4-triene-6-yne (70) to a,3-didehydrotoluene (71). This C2—C7 cyclization yields a benzylic 7i-conjugated a,7t-biradical and is therefore... 

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