Reductive Bergman-type cyclizations of cross-conjugated enediynes to fulvene and fulvalene anions: the role of the substituent.

Various cross-conjugated enediynes undergo "Bergman-type" cycloaromatizations upon reduction with potassium metal, generating anions of fulvenes and fulvalene derivatives. This new anionic cyclization is considerably more facile than the classic Bergman cyclization with linear enediynes, creating highly reactive diradicals at -78 degrees C. Not all cross-conjugated enediynes yield cyclized dianions upon reduction; some give uncyclized, Y-shaped, cross-conjugated dianions, while others apparently yield radical-anions that either dimerize or persist as monomers.

Stereodynamics and characterization of the hexa(4-n-dodecylbiphenylyl)benzene hexaanion that includes a twisted benzene core.

Hexa(4-n-dodecylbiphenylyl)benzene (HDBB) was reduced by a series of alkali metals in THF under high vacuum. Three reduction states were identified by NMR spectroscopy, namely the dianion, tetraanion and hexaanion. The NMR spectra of HDBB(6-) revealed a remarkable distortion of symmetry, which is interpreted by adoption of a twisted conformation of the central benzene ring and a slow rotation of the inner phenylene rings of the biphenyl units.

Anionic cyclization of a cross-conjugated enediyne.

Cross-conjugated enediynes cannot follow the Bergman cycloaromatization as it involves a methylenediyne moiety with only five pi e(-), insufficient for aromatization. Under reductive conditions the cyclization is made feasible by generating a product with a Hückel number of pi electrons. We illustrate this principle and demonstrate for the first time an anionic cyclization of a cross-conjugated enediyne that results in formation of a five-membered ring.

Flexibility vs Rigidity of Singly and Doubly Tethered Biphenyls: Structure, Dynamic Stereochemistry, and Resolution of Tribenzo[a,c,f]cyclooctane, Tetrabenzo[a,de,h,kl]bicyclo[6.6.0]tetradecane, and Their Alkyl Derivatives.

The barrier for enantiomerization of tribenzo[a,c,f]cyclooctane (1a) as acquired from dynamic (1)H NMR experiments was found to be DeltaG() = 19.5-20.1 kcal mol(-)(1), and DeltaG() = 17.