Designing photosystems for harvesting photons into electrons by sequential electron-transfer processes: reversing the reactivity profiles of α,β-unsaturated ketones as carbon radical precursor by one electron reductive β -activation

Abstract

Two photosystems are developed to harvest visible-light photons into electrons via sequential electron transfer processes. Photosystem-A (PS-A) consisted of DCA as light harvesting electron acceptor and Ph₃P as sacrificial electron donor, whereas photosystem-B (PS-B) employed DCA as usual electron acceptor, DMN as a primary electron donor, and ascorbic acid as a secondary and sacrificial electron donor. α ,β -Unsaturated ketones are utilized as secondary electron acceptors. The design of these photosystems is based on the thermodynamic feasibility of electron transfer between each participating components. Electron transfer from DCA^·- to α ,β-unsaturated ketones leads to their β-activation as carbon centered radicals which cyclizes efficiently to tethered activated olefins. Cyclization with a nonactivated olefin is found to be moderate. The cyclization stereochemistries have been illustrated by studying the PET activation of 5 and 21. The exclusive trans-stereochemistry observed in 8 is explained by considering the thermodynamic equilibration of initially formed syn-intermediate 10 from 5. The isolation of trace amount of 9 in this reaction substantiates the syn-intermediacy as primary intermediate which is further confirmed by the isolation of 25 from 21. Formation of 25 suggests that wherever the syn-intermediate is thermodynamically more stable, it invariably undergoes further cyclization to geometrically well-placed enolate double bond. An interesting observation is made by isolating 9 as a major product from the PET activation of 5 using PS-B. Stabilization of 10 by ascorbic acid is suggested to be the plausible explanation for this unusual observation. Radicals produced by the reductive β -activation of α ,β -unsaturated ketones follow well established radical cyclization rules which is exemplified by studying the reactions of 39 and 40. Generality of these cyclizations is demonstrated from the PET reactions of 29-32. Synthesis of 49, an important structural framework of biologically active angularly fused triquinanes, from 48 is included in this study to demonstrate the varied applicability of this strategy.