Properties of the [4]annulene system. Induced paramagnetic ring current

Abstract

Properties of the [4]Annulene System. Induced Paramagnetic Ring Current Sir:Recent remarkable achievements in annulene1 chemistry are, undoubtedly, the spectral characterization of [4]annulene (cyclobutadiene) (1) detained in a matrix at low temperatures2-6 and the isolation of its derivail) This term is defined by F. Sondheimer and R. Wolovsky, J. Amer.

Heterocycles with 7 and 8π electrons. Models for flavin and pteridine intermediates and candidates for novel inorganic rings

“…It should be noted that the 1,4-dihydropyrazine system with 4nr electrons can be stabilized in authentic form by organometallic substituents whereas sterically stabilized derivatives such as 6 have lost their 8r electron character because of conformational distortion. This situation is opposite to what is known for the much more rigid prototypical antiaromatic system, cyclobutadiene: stabilization by sterically shielding substituents [204] affects the character of this r system only slightly whereas r complexes of transition metals contain an electronically altered cyclobutadiene system [205]. The small ring size of cyclobutadienes permits only slight deviation from planarity [204], thus, the six-membered 1,4-dihydropyrazines stand between this fairly rigid four-membered 4r electron system and the extraordinarily flexible [92] eight-membered cyclooctatetraene ring (Figure 8).…”

A Diels–Alder super diene breaking benzene into C2H2 and C4H4 units

“…In fact, cyclobutadienes with small substituents are sufficiently reactive to dimerize via a Diels–Alder reaction and can also be trapped with a large variety of dienophiles 3 4 5 . Even isolable cyclobutadienes, with bulky substituents, undergo Diels–Alder cyclization with a variety of reagents 6 7 8 9 . However, aside from the dimerization process, cyclobutadienes almost exclusively react as electron-rich dienes.…”

Is cyclobutadiene really highly destabilized by antiaromaticity?

“…But is CBD really highly destabilized by antiaromaticity? Complexed 2 and substituted [3][4][5][6][7] CBD derivatives are known. However, Barbiou et al's recent claim of ''trapping'' 1,3-dimethylcyclobutadiene has raised much skepticism and debate.…”