Reversible photo-valence isomerization of troponoids

Chapter 10. Aromatic compounds

“…The He(r1) photoelectron spectrum of p-quinodimethane formed by the flash vacuum pyrolysis of [2,2]paracyclophane has been observed, and it indicates that the ground-state structure is that of a polyene. '22 Photodecarbonylation of 1,3diphenylinden-2-one-substituted maleimide adducts gave the o-quinodimethanes (37), which undergo a thermal 1,5-sigmatropic shift of the imido-group rather than alkyl migration.…”

“…Cyclophanes.-The reaction involving thermal extrusion of sulphur dioxide to form ethano-bridges has been extended to give a new method for synthesis of medium and large hydrocarbon rings,128 and this has been applied to the synthesis of [4](4,4")orthoterphenylophane. Some [2,2]cyclophanes are now much more readily prepared.12'" The experimental procedure for production of dithia[3,3]cyclophanes has been markedly improved, and subsequent Wittig rearrangement to substituted [2,2]cyclophanes is a route to be preferred to that involving Stevens rearrangement. For paracyclophanes, and metacyclophanes where 1,6-elimination is a possibility, the conversion is best accomplished by the 'benzene-Stevens' route (Scheme 2).129b by the photochemical sulphur-extrusion procedure and two by transannular carbenoid insertion reactions.…”

“…For paracyclophanes, and metacyclophanes where 1,6-elimination is a possibility, the conversion is best accomplished by the 'benzene-Stevens' route (Scheme 2).129b by the photochemical sulphur-extrusion procedure and two by transannular carbenoid insertion reactions. [2,2](2,7)Pyrenophane ( 47) and [2,2]( 1,3)pyrenophane (48) have been prepared by routes initially involving [2,2]metacyclophane derivatives and the formation of the pyrene structure by transannular ~e a c t i 0 n . l ~~ Attempts14' to synthesize the triple-layered compound (49) produced the compound with structure (50).…”

“…Energies for the axial-equatorial conformational change of the ethylene bridge protons have been established for the series. Studies of a series of new disubstituted[2,2]paracyclophanes in which each ring bears one substituent have been carried out 132. Spectral characteristics of such compounds were attributed to transannular electronic effects, but no transannular directive effects were observed for the nitration of 4-cyano-or 4-acetyl-[2,2]paracyclophane.Tetra-and octa-fluoro[4,2]paracyclophanes have been prepared,'33 and comparison with the corresponding octafluoro[2,2]paracyclophane may indicate ring distortions in the latter with the increased proximity of the aromatic rings.…”

“…Studies of the hydrolysis of cyclooctatetraenylcarbinyl chloride have produced2286 no evidence for the involvement of the homotropylium ion, but allylic stabilization, as in (96), is more likely. Evidence has been presented228c that #I-cyclo-octatetraenylethyl brosylates undergo solvolysis to give initially spiro [7,2]nonatrienyl cations [e.g. (97)] and, finally, tetrahydroazulenyl acetates.…”