E/Z-photoisomerizations within molecular crystals are varied. Existing cases are summarized. They require crystal lattices that allow for long-range molecular movements in the nontopotactic solid-state mechanism. Reactivity and directionality can be foreseen on the basis of the crystal packing. The reacting crystal changes continuously by phase rebuilding, phase transformation and disintegration. Two possibilities for the chemical mechanism exist: (1) highly space-demanding (cooperative) double-bond rotations; and (2) space-conserving hula-twist (HT) motions while the substituents move within their planes and only one C—H unit undergoes out-of-plane translocation. If internal rotation cannot be reasonably modeled within the crystal lattice, HT remains the only choice, as in the case of trans-1,2-dibenzoylethene. Direct experimental proof is still lacking because the differences in the conformational outcome could not be assessed in the studied examples. Density functional theory calculations of cis-1,2-dibenzoylethene revealed very low differences in energy content of the helical (s-cis, s-cis)- and the almost orthogonal (s-cis, s-trans)-cis-conformers. The almost orthogonal (s-cis, s-trans)-cis-conformer that is found in the pure crystal is very similar to the calculated counterpart. It is suggested that more favorable initial conformers might be obtained by proper vinylic substitution. The stereochemical outcome of highly space-demanding thermal vinylic-bond rotations followed by cyclizations of conjugated bisallenes to give bismethylene cyclobutenes excludes the alternative HT mechanism (double-bond isomerization) in the present cases. But space-conserving HT might be a mechanistic alternative in less-substituted cases under photoexcitation. The stereochemical consequences are discussed.
You have requested a machine translation of selected content from our databases. This functionality is provided solely for your convenience and is in no way intended to replace human translation. Neither BioOne nor the owners and publishers of the content make, and they explicitly disclaim, any express or implied representations or warranties of any kind, including, without limitation, representations and warranties as to the functionality of the translation feature or the accuracy or completeness of the translations.
Translations are not retained in our system. Your use of this feature and the translations is subject to all use restrictions contained in the Terms and Conditions of Use of the BioOne website.
Vol. 76 • No. 6