Journal article

Publication year: 2001

Pages: 2739-2744

Journal: Chemistry - A European Journal

Volume number: 7

Issue number: 13

ISSN: 0947-6539

Publisher: Wiley

Abstract: 
The rearrangement of the cubane radical cation (1(.+)) was examined both experimentally (anodic as well as (photo)chemical oxidation of cubane 1 in acetonitrile) and computationally at coupled cluster, DFT, and MP2 [BCCD(T)/cc-pVDZ//B3LYP/6-31G* + ZPVE as well as BCCD(T)/cc-pVDZ//MP2/6-31G* + ZPVE] levels of theory. The interconversion of the twelve C-2v degenerate structures of 1(.+) is associated with a sizable activation energy of 1.6 kcal mol(-1). The barriers for the isomerization of 1(.+) to the cuneane radical cation (2(.+)) and for the C-C bond fragmentation to the secocubane- 4,7-diyl radical cation (10(.+)) are virtually identical (DeltaH(0)(double dagger) = 7.8 and 7.9 kcal mol(-1), respectively). The low-barrier rearrangement of 10(.+) to the more stable syn-tricyclooctadiene radical cation 3(.+) favors the fragmentation pathway that terminates with the cyclooctatetraene radical cation 6(.+). Experimental single-electron transfer (SET) oxidation of cubane in acetonitrile with photoexcited 1,2,4,5-tetracyanobenzene, in combination with back electron transfer to the transient radical cation, also shows that 1(.+) preferentially follows a multistep rearrangement to 6(.+) through 10(.+) and 3(.+) rather than through 2(.+). This was confirmed by the oxidation of syn-tricyclooctadiene (3), which, like 1, also forms 6 in the SET oxidation/rearrangement/electron-recapture process. In contrast, cuneane (2) is oxidized exclusively to semibullvalene (9) under analogous conditions. The rearrangement of 1(.+) to 6(.+) via 3(.+), which was recently observed spectroscopically upon ionization in a hydrocarbon glass matrix, is also favored in solution.