Tribenzacepentalene Dianion and 4,7-Disubstituted Tribenzodihydroacepentalene Derivatives: Formation, Reactions, and Structural Properties of Potential Tribenzacepentalene Precursors

Abstract

Upon treatment of tribenzotriquinacene (4a) (R = H) and its centro-alkyl-substituted derivatives 4b-d (R = CH3, C2H5, CH2Ph) with the strongly basic mixture of n-BuLi and KOTen, 2-fold deprotonation combined with (formal) elimination of RH from positions C( l)-C(lO) occurs to generate tribenzacepentalene dianion 5-K2 with varying ease. Dianion 5-K2 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( l)=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 "C dimerizes at higher temperatures (20 "C) toward the head-to-head [2 + 21 dimer 22. At elevated temperatures (1 220 "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 n-o* couplings in these rigid frameworks. The 4,7-bis(trimethylstannyl)-tribenzodihydroacepentalene (60 undergoes clean transmetalation with methyllithium to give pure dilithium tribenzacepentalenediide (5-Li2.) Crystal structure analysis reveals that the dianionic fragments 5-Li2 in these solventseparated ion pairs are considerably curved.

Syntheses, Structures, and Reactions of Highly Strained Dihydro- and Tetrahydroacepentalene Derivatives

“…On account of its low melting point, a suitable single crystal could only be obtained by the previously described capillary zone melting procedure. [34] The structural data of 7 d correlate well with those reported for the tribenzodihydroacepentalene derivatives; [25,29] however, 7 d shows the largest out-of-plane deformation (y 36.88) of any of the bent double bonds in this series. The pyramidalization of double bonds has best been characterized by Haddon [35] with the definition of the two angles y and F (see Figure 5).…”

“…[24,25] Stepwise deprotonation of all three allylic positions in 8 gives intermediates 11, 12, and 13. Subsequent elimination of the central hydrogen as a hydride ion generates the diide 10 a (Scheme 3).…”

“…In contrast to 14, the known tribenzodihydroacepentalene dimer is a [22] cycloadduct across the strained bridgehead ± bridgehead double bond. [25,29] The smooth and rapid formation of the 4,7-dihydroacepentalene dimer 14 also implies that the highly strained central double bond in 7 a is not sufficiently shielded by the two hydrogens on the other bridgehead positions in the monomer 7 a. The highly reactive monomer 7 a could not be observed by NMR spectroscopy, even at À 80 8C.…”

“…A similar bond lengthening was observed previously for the head-to-tail [22] cycloadduct dimer from 4,7-dihydrotribenzoacepentalene, which was attributed to a p ± s*-orbital interaction. [25] Figure 3. Structure of the 4,7-dihydroacepentalene ± anthracene adduct (16) in the crystal.…”

“…[25,29] In contrast to the latter which possess C s symmetry, the C(1) ± C(2) and C(1) ± C(9) bond lengths in 7 d are different, 144.4(6) and 150.4(7) pm, respectively. Also, the C(4) ± C(5) (154.3(6) pm) and C(6) ± C(7) (149.1(7) pm) are unequal in length.…”