Corannulene, reduction dianion

An early electrochemical study of corannulene revealed the presence of two well-defined polarographic waves with half-wave potentials of-1.88 and -2.36 V (r-butylammonium perchlorate in acetonitrile). The first wave represented a reversible, one-electron reduction leading to radical anion formation (emerald green solution) further characterized by UV-VIS and ESR. The second wave was reported to be associated with the formation of a bright red species which is not paramagnetic, but it is not believed to be the dianion, but rather some decay product of it. Treatment of THF solutions of 8 with sodium and potassium metals also led to the formation of the same species. ... 

The reduction of several annelated corannulene derivatives has been performed using lithium and potassium metals.7 It has been found that annelation affects the annulenic character of corannulene by changing its charge distribution the dianions of derivatives that are annelated with six-membered rings have less annulenic character and are less paratropic than corannulene dianion. 

It also appears that the addition of alkali metals may have an effect on the bowl-to-bowl inversion of 1. An interesting study of the alkali metal reduction of S9, in which two corannulene moieties are tethered by an eight carbon atom chain, allowed for the estimation of inversion barriers in the corannulene dianion [49]. The barrier for the doubly charged corannulene moieties (8.8 and 9.2 kcal mol at 186 and 196 K for K and Cs salts, respectively) is significantly lower than for neutral corannulene (ca. 11 kcal mol ), and closely agrees with our theoretical prediction (7.9-9.2 kcal mol [48]) published earlier. 

The chiral penta-substituted corannulene, 1,3,5,7,9-penta-terfbutylcorannulene (37), which shows anomalous dynamic behavior [116], was reduced with Li and four reduction stages were observed [117]. The behavior of the anti-aromatic dianion and the aromatic tetraanion resemble that of the parent corannulene. In the final stage of the reduction, three distinct tetraanionic species could be detected. Two of the species are sandwich-type dimers, as in the case of corannulene, and are diastereomers, due to the chirality of 37 (RR/SS and meso - RS/SR). The third species was assigned as a tetraanionic monomer, which slowly disappears. 

The total change in the carbon chemical shifts upon reduction (SA<5, Eq. (1)) shows smaller values for both 40 and 41 compared to corannulene, although negative charge was added (2A<5 = 336, 50, —178, for 2 /2Li+, 40 /2Li+, 41 /2Li, respectively). This comparison shows that the annulenic character of dianions of 40 and 41 is less pronounced than that in 2. It also suggests that the annulenic character in 40, which contains two benzannelated rings, is more significant than in 41 , which contains an additional 5MR. 

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