Zweistufig reversible Redoxsysteme, XXXI Pyridylsubstituierte Cyclobutane: Reversible Umwandlung von 1,3‐Bismethylencyclobutanen in Bicyclo[1.1.0]butane

Abstract

Die Synthesen der 1,3-Bismethylencyclobutane 1 a -l i sowie der 1,2-Bis(4-pyridyliden)ethane 6 (Modelle fur 1) werden beschrieben. electrode processes and are connected through two (valence tautomeric?) radical cations as part of a n ECE mechanism. For the 1 2 2 equilibrium the Ksem is estimated to be lo-", whereas for 6 * 8 the KSem is 1.1 . lo'. Redox potentials and UV spectra demonstrate that the 1,3-interaction in 1 involves ground state (HOMO) stabilization, whereas in 2 the LUMO energy is lowered by the pyridinium substituents. The energy diagram (Scheme 2) compares the magnitude of these effects with appropriate models which d o not possess such extra stabilizations. A qualitative frontier orbital model is presented (Fig. 5) to rationalize the unexpected finding that enlargement of the dihedral angle in the bicyclobutane moiety causes hypsochromic or bathochromic shifts depending o n the 1,3-substituents. The new bicyclobutanes 2a and 2c isomerize stereoselectively into the butadienes 15 and 16 already at 40°C. On photolysis 2a is converted into the cyclobutene 19.

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

“…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 .…”

Violene/Cyanine Hybrids as Electrochromic Systems: A New Variation of the General Structure

“…Starting from the tetracation 4 OX ϩ4 , the first reduction step produces 4 SEM ϩ3 , a violene radical ranging either from ring A to ring B, or from ring A to ring D. Its absorption maximum at 550 nm may be compared with violene 5 SEM ϩ (λ max ϭ 525 nm [12] ). On further reduction, the new species formed can be described as 4 RED/OX ϩ2 with a cyanine system (λ max ϭ 570 nm) extending from ring B to ring D, and likewise because of the thiophene ring, from ring A to ring D. The closest analogue could be found for 4 RED/OX ϩ2 in the cyclobutane derivative 6 (λ max ϭ 625 nm [13] ). Similar to 4 RED/OX ϩ2 , its absorption maximum is relatively broad, slowly decaying to longer wavelengths.…”

“…420 nm), since the isolated rings are expected to absorb at approximately 340 nm (cf. model 7 [13] ), and coupling between two of them should produce a λ max value of approximately 350 nm (cf. model 5 RED [12] ).…”

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

“…',C-NMR (CDCI,): cis-25: 21,71,34,Ol (2 9. 4 CH,);33,63 ( t ,C(lO,ll), C(10,ll"")); 52,64(s, C(2",4'1); 68,12(s, C(1", 3'7); 78,86(s,C(5,5"")); 125,63, 127,87, 129,11, 130,85(4 d, C(1,9), C(1"",9""), C (2,8), C (2,8""), C(3,7), C(3"",7""), C (4,6), C (4'"''')); 138,85,139,XO (2 s, C(4a,5a), C ( 4 a , 5 a ) , C(9a,l la), C(9""a,ll""a)); trans-25: 28,51 (q, 4 CH,); 33,79 (t, C(lO,ll), C(10"",11"")); 51,75 (s, C2",4")); 64,72 (s, C(1", 3'>); 78,69 (s. C(5S"")); 125,63, 127,87, 129,l 1, 131,66 (4 d, C(l,Y), C(1"",9), C (2,8), C(2"",8""), C(3,7), C(3"",7""), C (4,6), C(4,6"")); 137,95, 139,92 (2 s, C(4a,5a), C(4a,5""a), C(9a,l la), C(9""a,ll""a)).Anal. ber.furC, BH,,S,(556,8):C81,97,H6,52,S 11,52;gef.…”

“…So gelang es im Falle der Pyridin-Derivate A1 und B1 beide Stufen des Redox-Systems zu isolieren und reversibel ineinander umzuwandeln [5] [6], wahrend ausgehend vom Pyran-Derivat A2 die reversible Bildung von B2 nur voltammetrisch nachgewiesen werden konnte [7]. Durch Einsatz der Chinon-methide C3 wurde bewiesen, dass sich oxidierte und reduzierte Stufen vertauschen lassen: C3 geht durch Reduktion reversibel in B3 iiber (cyclische Voltammetrie [8]), das sich als Bis(trimethylsily1)ether abfangen l a s t [ 11.…”