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 Ph 3 P 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.