Phenalenyl radical stability

Why does a stable bis-allyl analog exist on the Cope reaction surface of 27 In the prototype Diels-Alder reaction of 1,5-hexadiene, the possible bis-allyl intermediate (i.e., two isolated allyl radicals) is about 26 kcal mol higher in energy than the Cope transition state. Only with significant radical stabilization might one expect a bis-aUyl intermediate to occur. One can consider 28 as composed of two bridged phenalenyl radicals (29), which affords a rather dramatic stabilization of each radical. 

As shown in Scheme 1, the synthesis of fert-butyl phenalenyl radical 10 started from 2,7-di-ferf-butylnaphthalene in ten steps. Bromination of 2,1-di-tert-butylnaphthalene gave 6 in high yield, which was converted into aldehyde by lithiation followed by reaction with DMF. Successive Reformatsky reaction afforded ester 7, which upon reduction, hydrolysis, and Friedel-Crafts acylation reaction gave the phenalanone 8. The key intermediate 9 was then obtained as pale yellow crystals by reduction of 8 and subsequent dehydration. Oxidation of 9 with p-chloranil in degassed toluene led to a blue neutral radical solution while similar treatment in hexane gave deep blue needles. This crystal showed high stability in the absence of air, while changing into phenalanone derivatives and other byproducts in 1 week in air. 

Beer L, Mandal SK, Reed RW, Oakley RT, Tham FS, Donnadieu B, Haddon RC (2007) The first electronically stabilized phenalenyl radical effect of substituents on solution chemistry and solid-state structure. Cryst Growth Des 7 802-809... 

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