Diastereomeric Discrimination in the Lifetimes of Norrish Type II Triplet 1,4‐Biradicals and Stereocontrolled Partitioning of Their Reactivity (Yang Cyclization versus Type II Fragmentation)

Abstract

The stereochemistry at C2 and C3 carbons controls the partitioning of triplet 1,4-biradicals of ketones 2 among various pathways. Differences in the major reaction pathways, for example, cyclization (syn) and fragmentation (anti), adopted by the diastereomeric 1,4-radicals of ketones 2 have permitted unprecedented diastereomeric discrimination in their lifetimes to be observed by nanosecond laser flash photolysis. From quantum yield measurements and transient lifetime data, the absolute rate constants for cyclization and fragmentation of a pair of diastereomeric triplet 1,4-biradicals have been determined for the first time.

Aromatic aldols and 1,5-diketones as optimized fragrance photocages

“…20 From this series, octanal, melonal and α-ionone are widely used as fragrances. Aldols have been prepared several times as precursors to functionalized Norrish II precursors 21,22 but to the best of our knowledge the photocleavage of aldols has yet not been investigated. Prior to the photochemical experiments, the thermal stability under the reaction conditions was checked and no thermal contribution to cleavage was detected, only increasing amounts of dehydration after several hours at 60 °C in benzene solution.…”

Diastereomerically Differentiated Excited State Behavior in Ruthenium(II) Hexafluoroacetylacetonate Complexes of Diphenyl Thioindigo Diimine

“…Select organic compounds containing phenone or benzophenone moieties have been reported to display diastereomeric transient lifetime discrimination in the literature. − In these cases, the difference was attributed to the ability of one diastereomer to adopt a conformation that led to either an additional quenching pathway or a lower energy transition state for the quenching mechanism. Our data suggests that the diastereomeric lifetime differentiation observed in the diruthenium complexes 1a , b may be caused by a similar phenomenon.…”

Template effects and supramolecular control of photoreactions in solution

“…A detailed mechanistic study of the partitioning of 1,4-biradicals due to conformational constraints (in diastereomers) was conducted by Moorthy et al and provides a compelling background on which to rationalize template effects by molecular capsules on these reactions. 257 Calixarenes also encapsulate aryl ketones and have been shown to promote Norrish II products in benzoin alkyl ethers through ground-state conformational effects. 258 Further studies on the use of dendrimers as molecular hosts for the Norrish I reaction of dibenzylketone show that correct choice of the dendron arms has a greater impact than the dendrimer generation.…”