Neue Wege zu Pentalen‐Vorstufen

Abstract

New Pathways to Precursors of PentalenePentalene dimers 2 and 3 are easily available in moderate yields by CuCI,-induced oxidative coupling of dilithium-pentalenediide (5) (Scheme f). On the other hand, NBS bromination of 1,5-dihydropentaIene (4) or of 1.2-dihydropentalene (8) gives unstable 1 h o m o -I ,2-dihydropentalene (9), while subsequent in-siru elimination with Et,N exclusively gives syn-cis-pentalene dimer 2 in moderdle yields (Scheme 3). NMR-Spectroscopic evidence for compounds 2,3, and 9 is presented, and mechanistic alternatives for the formation of pentalene dimers 2 and 3 are discussed. 1.Einleitung. -Pentalen (1)2)3) hat sowohl die synthetischen als auch die theoretischen organischen Chemiker seit mehr als vier Jahrzehnten fasziniert. Trotz zahlreicher Syntheseversuche konnte jedoch der Grundkorper 1 bisher spektroskopisch nicht charakterisiert werden, und alle bis heute isolierten substituierten Pentalene waren entweder sterisch oder elektronisch stabilisiert. Hexaphenylpentalen war das erste einfache Pentalen, welches 1962 durch Le Gofjrsynthetisiert wurde [4]; spater konnten die elektronisch stabilisierten Pentalene 1,3-Bis(dimethylamino)pentalen [5] und 1,4-Diamino-3,6-dime-

Antiaromatic Dicyclopenta[b,g]/[a,f]naphthalene Isomers Showing an Open-Shell Singlet Ground State with Tunable Diradical Character

“…Antiaromatic compounds have gained increasing interest in organic electronics due to their small HOMO–LUMO energy gap and redox amphotericity . A typical example is pentalene (Figure a), which is highly reactive . However, its sterically and/or electronically stabilized derivatives can be isolated .…”

Further Synthetic Attempts towards Calicene

“…15 ) We believe that the yield of step 12 3 13 can be considerably improved if twofold dehydration can be achieved at lower temperature (see Scheme 4; 4,4'-di(cyclopentadienyl)biphenyl has been produced in a yield of 85% by chromatography of the corresponding di(cyclopentenol) over acidic silica gel at À 158). 16 ) It is impossible to assign these multiplets to HÀC(2) and HÀC(3) from the splitting pattern. Due to the fact that, in several other examples with confirmed assignment [9] [1], HÀC(2) absorbs in the range of 6.0 ± 6.2 ppm, we tentatively assigned the multiplet at 6.06 ppm to HÀC(2).…”

“…), which shows signals of vinylic Hatoms at 6.06 and 5.76 ppm (both as dt with J 5.5 and J' 2.2 Hz, resp. 16 …”