Mehrstufige Redoxsysteme. XLIX. Mitteilung. Mit π‐Systemen substituierte 1,3‐Dimethylidencyclobutane: Synthese und Eigenschaften

Abstract

Multistep Redox Systems. 1,3-Dimethylidenecyclobutanes Substituted by n-Systems: Syntheses and PropertiesAs potential redox systems 1,3-dimethylidenecyclobutanes are synthesized in which the 'em'-methylidene group participates in a cyclopentadienyl (type C4) or cycloheptatrienyl (type A5) system. Benzo-annellated systems are also described. The formation of the C=C bond by the reaction of thioketones (3 and 31) with diazo compounds is extended to 1, 6, 7, 21, and 24, which so far had not been employed. In some cases, intermediate thiiranes can be isolated (e.g. 4) which are smoothly transformed into the target compounds of types C4 and A5 by extrusion of sulfur. The new C=C bonds may be introduced stepwise, thereby allowing the synthesis of unsymmetrically substituted 1,3-dimethylidenecyclobutanes (eg. 20 and 35). Reduction of 10 by Na does not produce bicyclobutanes but compounds 43 and 44 with one dihydrofulvene unit. Es lag daher nahe, die in Heterocyclen eingebauten (Al, A2) oder chinoid-artigen (C3) Methyliden-Gruppen durch in Carbocyclen eingebaute zu ersetzen, die beim moglichen Ubergang in das Bicyclobutan-System genugend Aromatisierungsenergie gewin- Einleitung

Attempted Synthesis of a Stable, Quintet, Tetraphenoxyl Tetraradical: Facile Rearrangement of a Substituted Bicyclobutane

“…A single crystal of the product, recrystallized from methanol, was submitted to X-ray analysis, definitively assigning the structure to the ring-opened bisgalvinol 7 (Figure ). Though Hünig's substituted bicyclobutanes are reported to ring-open in hot, polar solvents, − the methanol recrystallization is not responsible for the rearrangement here since unheated product displays the same NMR spectra.

9 X-ray structure of bisgalvinol 7 .

…”

“…It also seemed possible that tetraradical 2 could close to form substituted bicyclobutane biradical 4 , by analogy with observations of Hünig et al in similar closed-shell systems. − If so, it might be possible to observe “magnetooptical switching” between the ring-opened tetraradical 2 and the ring-closed biradical 4 , a process this group has demonstrated previously − with the triplet biradical 5 .…”

“…These products signal a rearrangement that must involve an unstable bicyclobutane intermediate such as 4 . While the bicyclobutane-to-butadiene rearrangement is known, it occurs for unsubstituted bicyclobutane at ∼200 °C and for Hünig's substituted bicyclobutanes at ∼40 °C. − In the present system, however, it occurs readily at room temperature and may occur at temperatures as low as −78 °Ca remarkable rate enhancement.…”

“…Anticipating possible rearrangment of the bicyclobutane at elevated temperatures, − , the base-assisted oxidation was conducted on a preparatory scale in a dry ice bath and at reduced temperature in the EPR cavity. Both yielded results essentially identical to the room-temperature studies.…”

Dynamic Redox Systems as Electrochromic Materials: Bistability and Advanced Response

“…Large peak separation in the cyclic voltammogram as well as the onewave two-electron process are diagnostic features for these special redox pairs. Fulvene-type hydrocarbons 30 as well as bis(quinonemethide)-type acceptors 31 containing the dimethylenecyclobutane unit also exhibit a similar 'intramolecular redox switching of single bonds'. 32…”