TRIBENZACEPENTALENE DIANION AND 4,7-DISUBSTITUTED TRIBENZODIHYDROACEPENTALENE DERIVATIVES - FORMATION, REACTIONS, AND STRUCTURAL-PROPERTIES OF POTENTIAL TRIBENZACEPENTALENE PRECURSORS

Upon treatment of tribenzotriquinacene (4a) (R = H) and its centro-alkyl-substituted derivatives 4b-d (R CH3, C2H5, CH(2)Ph) with the strongly basic mixture of n-BuLi and KO(i)Pen, 2-fold deprotonation combined with (formal) elimination of RH from positions C(1)-C(10) occurs to generate tribenzacepentalene dianion 5-K-2 with varying ease. Dianion 5-K-2 can be trapped with various electrophiles to give 4,7-disubstituted tribenzodihydroacepentalenes 6c-f in good yields. Compounds 6a-f contain an extremely out-of-plane bent C(1)=C(10) double bond, as shown by X-ray structure analyses of 6b-d, and therefore possess an increased reactivity. 4,7-Dihydrotribenzacepentalene (6a) obtained upon protonation of 5 at -78 degrees C dimerizes at higher temperatures (greater than or equal to 0 degrees C) toward the head-to-head [2 + 2] dimer 22. At elevated temperatures (greater than or equal to 220 degrees C), 22 is cleaved to regenerate 6a which can be trapped with anthracene and tetracyclone to give the highly condensed Diels-Alder adducts 23 and 24. Likewise, 6a generated from 5 at low temperatures can be trapped with 1,3-diphenylisobenzofuran, yielding 25. X-ray structure analyses of dimer 22 and adduct 23 revealed strongly elongated 1,1,2,2-tetraarylethane C-C bonds, which are attributed to through-bond pi-sigma* couplings in these rigid frameworks. The 4,7-bis(trimethylstannyl)tribenzodihydroacepentalene (6f) undergoes clean transmetalation with methyllithium to give pure dilithium tribenzacepentalenediide (5-Li-2.) Crystal structure analysis reveals that the dianionic fragments 5-Li-2 in these solvent-separated ion pairs are considerably curved.