The Radical Anions of Some Benzo‐Derivatives of Dehydrocyclooctatetraenes

Abstract

In memory of Professor Franz Sondheimer (6.V. 83) SummaryThe radical anions of 5,6-didehydrobenzocyclooctene (3), 5,6,9,10-tetradehydrobenzocyclooctene (4) and 1,4,7,10-tetramethy1-5,6-didehydrodibenzo [u, el-cyclooctene (6) were prepared in situ from dibromo-precursors and have been characterized by ESR and ENDOR spectroscopy.Introduction. -Dehydrocyclooctatetraenes and their benzo-derivatives have received much attention in the last few years [I]. Insertion of a triple bond into the unsaturated eight-membered ring increases its tendency to flatten; concurrently, the propensity of the hydrocarbon to undergo addition, dimerization and rearrangement reactions is enhanced. The reactivity is, in general, lowered by introducing a second triple bond and by fusing benzene rings to the dehydrocyclooctatetraene 71-system 111.The transient existence of cyclooctatrienyne (l), the simplest member of the series, has been proved by trapping 1 with various reagents [2]. Attempts to synthesize cycloocta-1 ,5-diene-3,7-diyne (2) [3] and 5,6-didehydrobenzocyclooctene (3) [4] have failed so far, while 5,6,9,10-tetradehydrobenzocyclooctene (4) was obtained as a rapidly decomposing oil [ 5 ] . 5,6-Didehydrodibenzo [a, e]cyclooctene (5) and 5,6,11,12-tetradehydrodibenzo[a, e]cyclooctene (7) were both isolated in crystalline form 161, but only the diyne 7 is relatively stable at room temperature. In addition to 5, its 1,4,7,lO-tetramethyl derivative (6) has quite recently been prepared [7]. It is noteworthy that the first synthesis claimed for 6 [8] yielded, in fact, the isomeric 1,4,6,9-tetramethylbenzo [blbiphenylene 191.The radical anions 1 8 [lo], 5 9 [ l l ] and 7 9 [ l l ] have been characterized by ESR spectroscopy; the hyperfine data are consistent with a planar eight-membered

The Capture of sym-[8]Annuldiyne: The Cyclooctadienyne-η2-ynyl Potassium Zwitterionic Radical

“…Unfortunately, 3 and its anion radical are unknown. However, the anion radical of 3,4-benzo-[8]annulyne at ambient temperature was reported 1 For the anion radials of [8]annulyne and benzo-[8]annulene, the B3LYP/6-31G*-predicted carbon p z spin densities (shown in parentheses) are in extraordinary agreement with the experimental spin densities obtained from the a H values and the McConnell relationship ( a H = Q ρ with Q = 25.6 G).…”

Strained Dehydrobenzoannulenes

“…The radical anions of 11a and 12 were generated in solution through the corresponding neutral species prepared in situ and were characterized by ESR and ENDOR spectroscopy. [22] For doubly benzo-annelated systems, there are two possible modes of fusion; one is an a,e-fusion such as didehydroannulene 13a and tetradehydroannulene 14a (of D 2h point group) and the other is a C 2v symmetric a,c-fusion such as 15. While initial attempts to prepare 15 gave no reasonable evidence for its existence, [23a] its formation was later supported by the interception of its [4+2] adduct with a diene in the synthesis of cyclic phenylenes.…”

“…Note that both one-electron and two-electron reductions of 13a by alkali metal or by cyclic voltammetry could be carried out to form its radical anion and dianion (a 10π-electron system), respectively, while only one-electron reduction of 14a could be performed because irreversible polymerization took place upon further reduction. [24b,36] The radical anions of 13a, [37] 13b, [22] and 14a [37] thus formed were characterized by ESR spectroscopy. Cp*Co-cyclobutadienefused derivative 19 was isolated by Vollhardt during an attempt to synthesize [4]phenylene and its X-ray analysis revealed that the eight-membered ring is planar and the triple bond slightly less deformed [23.0°(average)] than that of 13a.…”