Cyclisch gekreuzt‐konjugierte Bindungssysteme, 38¹⁾Vierzehn‐Elektronen‐Elektrocyclisierung des vinylogen Sesquifulvalens – Phenazulen

Abstract

Das vinyloge Sesquifulvalen 1 geht thermisch leicht (E, = 100 * 2 kJ . mol-', log A = 12.0) und perispezifisch die 14-Elektronen-Elektrocyclisierung ein. Unter dem Zwang der sterischen Verhaltnisse erfolgt diesesehr wahrscheinlich ausschlieRIichi m ,,symmetrieverbotenen", konrotatorischen Sinn zum trans-lOa,lOb-Dihydrophenazulen 2a. Dieses ist wegen der raschen Isomerisierung in die benzoiden 1,s-und 3,8-Dihydrocyclohept[e]indene 12/15 direkt nicht nachweisbar. Die frans-Stereochemie in 2a wird durch Rthtgenstrukturanalyse seines 1 : 1-Addukts mit Acetylendicarbonsaure-dimethylester (20) bewiesen. Die aus 12/15 hergestellten Benzylidencyclohept[e]indene 28a -c konnten prototrop nicht zu den Bemylphenazulenen 30ac isomerisiert werden. Phenazulen-Derivate (34) entstehen auch nicht bei der Umsetzung des aus 28a gewonnenen Kations 31 mit Nucleophilen. Die Phenazulenium-Sake 38/39 sind je nach Gegenion in Losung (BFP, ClOP) bzw. kristallisiert aus 12 (15) herstellbar (SbClp, l/2PtC1i0). Aus dem BFP-Salz wird eine bei -60°C und groBem DBN-UberschuR bestlndige, tiefblaue Verbindung freigesetzt, die oberhalb 0°C bzw. im Kontakt mit SiO, oder A120, ( -60°C) rasch polymerisiert. Es wird argumentiert, da8 es sich hierbei um Phenazulen (3) handelt. Cyclic Cross-Conjugated Bond Systems, 38 *) Fourteen Electron Electrocyclisation of the Vinylogous Sesquifulvalene -PhenazuleneThe vinylogous sesquifulvalene 1 thermally undergoes 14-electron electrocyclisation with ease (E, = 100 + 2 kJ . mol-', log A = 12.0) and perispecifically. Due to the steric constraints imposed by the system it takes placealmost certainly exclusivelyin the "symmetry-forbidden", conrotatory sense to give frans-lOa,lOb-dihydrophenazulene 2a. This is rapidly isomerized into the benzenoid 1,s-and 3,8-dihydrocyclohept[e]indenes 12/15 and is therefore not directly detectable. The trans-stereochemistry in 2a is verified by x-ray crystal analysis of its 1 : 1-adduct with dimethyl acetylenedicarboxylate (20). The benzylidenecyclohept[e]indenes 28a -c (obtainable from 12/15) could not be isomerised prototropically to the benzylphenazulenes 30a -c. Treatment of the cation 31 (generated from 28a) with nucleophiles also fails to produce phenazulene derivatives (34). The phenazulenium salts 38/39 are, depending on the counteranion, obtainable from 12 (15) in solution (BFP, C l o p ) or crystalline (SbClp, 112 PtClaa). From the B F P salt a deep-blue compound is generated which is stable below -60°C and in the presence of a large excess of DBN, but polymerises above 0°C or in the presence of SiO, or N,O, ( -60°C). It is concluded, that this blue species is phenazulene (3).Das vinyloge Sesquifulvalen 1 war in der Absicht synthetisiert worden'), iiber die a,oElektrocyclisierung zum Dihydrocyclohept[e]inden 2 letztlich zum noch unbekannten ,,Phenazulen" (Cyclohept[eJinden) 3 zu kommen. Diesem praparativen Ziel hat sich die auch von der Theorie aufgeworfene Frage nach der Peri-und Stereoselektivitat2) potentiell konkurrierender elektrocyclischer Prozesse iiberlagert. Die dem...

The road to bis‐periazulene (cyclohepta[def]fluorene): Realizing one of the long‐standing dreams in nonalternant hydrocarbons

“…The pristine cyclohepta[e]indene 7 was synthesized via the electrocyclization of a fulvadiene 15, followed by the hydride elimination and the subsequent deprotonation with base (Figure 4B). The pristine 7 was an unstable blue molecule, which survived for only a few hours at À60 C. [44,45] The same group successfully synthesized and characterized the stable derivatives of 7 by introducing several substituents. [46] The [12 + 2] 4C).…”

“…The pristine 7 was an unstable blue molecule, which survived for only a few hours at −60°C. [ 44,45 ] The same group successfully synthesized and characterized the stable derivatives of 7 by introducing several substituents. [ 46 ] The [12 + 2] addition of 8,9‐dicyanosesquifulvalene 16 with enol ethers 17 followed by the in situ aromatization afforded dihydroprecursors 18 as the mixture of several tautomers.…”

Non‐Alternant Non‐Benzenoid Aromatic Compounds: Past, Present, and Future

“…For the 5‐6‐7 (14 π‐electron) system, the linearly fused cyclohepta[ f ]indene ( 6 ) is unknown . For angularly fused cyclohepta[ e ]indene ( 7a , also called “phenazulene”), the parent hydrocarbon turned out to be unstable and survived for only several hours at −60°C; only its derivatives 7b and 7c bearing cyano groups were isolated . No attempts seem to have been made to prepare the 7‐6‐7 (16 π‐electron) ring conjugated systems 8 and 9 .…”

Violations. How Nature Circumvents the Woodward–Hoffmann Rules and Promotes the Forbidden Conrotatory 4n + 2 Electron Electrocyclization of Prinzbach’s Vinylogous Sesquifulvalene

“…The electrocyclization of vinylogous sesquifulvalene was investigated experimentally by Prinzbach and co-workers in 1978 (Figure a). , For this 14π electrocyclization, Prinzbach and co-workers observed that the symmetry-forbidden conrotatory pathway occurred “almost certainly exclusively” to form the trans product ( 2- trans ) . The cis product ( 2- cis ) that would be formed from a symmetry-allowed disrotatory pathway was not observed.…”