Flow‐Vacuum Pyrolysis of Polycyclic Compounds, 6. Pyrolysis of Dibenzo [CH]₈ Hydrocarbons

Abstract

The flow-vacuum pyrolysis of dibenzo [CHI8 hydrocarbons 2,3,4, and 5 are studied at 1 Torr and in the temperature range between 400 and 650°C. The following new transformations have been observed: 2 3 4,2-+5,3-+ 5,3 3 4,3+ 2,5-+ 11, the last three presenting analogies in the [CHI, and/or benzo [CHI, series. A reaction mechanism is suggested.1 2 3 ing of 7 to 8, described by Vogel in 1963[71. In this paper the results of the yet undescribed flow-vacuum pyrolysis of the remaining hydrocarbons 2, 3, 4, and 5 are discussed. 4 5 6 8 / 7 9 Chern. Ber. 1993, 126, 2513 -251 7 0 VCH Verlagsgesellschaft mbH, D-69451 Weinheim, 1993 0009

Stereochemistry – key role in the rearrangements of dibenzo-C8 and -C9 derivatives

“…26 Interesting information regarding the dibenzoC8 compounds is also obtained from the pyrolysis of hydrocarbons 26-28. 31 Inter-conversion of these hydrocarbons was experimentally proved, suggesting as intermediates radicals similar to the ions involved in the previously described solvolyses. During the pyrolysis, part of dibenzocyclooctatetraene (26) and dibenzosemibullvalene (27) rearranged to more stable compounds like dibenzobarrelene (28) (see Figure 5).…”

Synthesis and Reactivity of an X-Shaped Molecule: Reversible Formation and Cleavage of a Four-Membered Ring in Response to External Stimuli

“…Notably, in the 13 C NMR spectra, the signals corresponding to the bridgehead sp 3 carbons of 2 disappear, while new signals indicative of sp 2 carbons appear in the product. The anti - TOD derivatives are known to undergo eight-membered ring (8MR) formation through ring-opening reaction upon heating treatment. − Therefore, we consider that the product is an open-ring isomer 3 with an 8MR. The ring-opening reaction also proceeds in the solution phase (Figure S20).…”

“…DFT calculations confirm that 3 is more stable than 2 by 6.9 kcal/mol. In the case of TOD s, stepwise bond cleavage through a reaction intermediate has been suggested. − Thus, we consider that the central 4MR of 2 also opens in a stepwise manner upon heat and mechanical treatments.…”

“…Among various molecules, special focus has been placed on molecules featuring four-membered rings (4MRs) arranged in a ladder configuration because of their unique reactivities toward external stimuli, attributed to the inherent ring strain and weak carbon–carbon bonds of 4MRs. For instance, ladder-type molecules such as syn - and anti -tricyclo­[4.2.0.0 2,5 ]­octa-3,7-dienes ( TOD s), comprising three 4MRs, show the formation and cleavage of carbon–carbon bonds under thermal treatment and/or light irradiation, yielding a variety of molecular products (Figure a). − Despite these observations, the underlying reaction mechanisms remain subject to debate. − Xia and co-workers recently reported that ladder-type molecules or polymers containing 4MRs show a unique mechanochemical response associated with carbon–carbon bond cleavage. − …”

Regioselectivity of the Di-π-Methane Rearrangements of 1,4-Dialkoxy-9,10-bis(methoxycarbonyl)dibenzobarrelenes in Solution and in the Solid State

“…The thermal rearrangement of the parent dibenzosemibullvalene to dibenzocyclooctatetraene 4c has been proposed to proceed via biradicals BR2' and BR1' which are formed in steps of a retro-di-p-methane rearrangement. 7 Biradical BR1' is supposed to cyclize to 5c, which subsequently undergoes [2+2+2] cycloreversion reaction to afford dibenzocyclooctatetraene 4c (Scheme 3). 8 According to this mechanism, the C(8b)-C(8e) bond of dibenzosemibullvalene 2a must be cleaved on photolysis to give biradical BR2a if 4a is to be the final product.…”