Hula‐Twist. A Stereoselective and Regioselective Photoisomerization Reaction Mechanism

Abstract

Hula-twist is a rapid diabatic process of photoisomerization. It is not volume-demanding and its stereochemical consequence is simultaneous isomerization of a double and a single bond. Recent new examples include diene, styrene, stilbene, diphenylbutadiene and other derivatives in organic glasses at liquid nitrogen temperature. In systems where two different modes of HT are possible, a high regioselectivity was observed. Possible involvement of the volume-conserving mechanism of isomerization in primary processes of photosensitive biopigments is discussed.

Theoretical Study of the Photoisomerization Mechanism of All-Trans-Retinyl Acetate

“…The simultaneous rotation around the C3=C4 double bond and the C4–C5 bond, which takes the molecule from the S 1 -min ( tEtEt ) structure to the S 1 /S 0 -CI, is an instance of a hula-twist rotation − a concerted rotation around a pair of consecutive double and single bonds. This type of intramolecular rotation has also been implicated in the photoisomerization processes of other chromophores such as protonated Schiff bases, ,− ,− the green fluorescent protein chromophore, − and stilbene and its derivatives. − …”