Steric Effects in Hula-Twist Photoisomerization.1,4-Dimethyl- and 2,3-Dimethyl-1,4-diphenylbutadienes

摘要

Hula-twist (HT) is a general mechanism of photoisomerization.It is a volume-conserving process of which a single C-H unit moves across the plane of an unsaturated molecule.In contrast,the traditional one-bond-flip (OBF) mechanism turns over one-half of the molecule:The question whether the process is limited to a small C-H unit has not been answered,although the absence of conformational equilibration of 2,3-dimethyl-l,3-butadiene in argon matrix (in contrast to the parent diene) has been interpreted as steric inhibition due to increased size at the HT reaction center.