DESIGNING PHOTOSYSTEMS FOR HARVESTING PHOTONS INTO ELECTRONS BY SEQUENTIAL ELECTRON-TRANSFER PROCESSES - REVERSING THE REACTIVITY PROFILES OF ALPHA,BETA-UNSATURATED KETONES AS CARBON RADICAL PRECURSOR BY ONE-ELECTRON REDUCTIVE BETA-ACTIVATION

Two photosystems are developed to harvest visible-light photons into e lectrons 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 asco rbic acid as a secondary and sacrificial election donor. alpha,beta-Un saturated ketones are utilized as secondary electron acceptors. The de sign of these photosystems is based on the thermodynamic feasibility o f electron transfer between each participating components. Electron tr ansfer from DCA(.-) to alpha,beta-unsaturated ketones leads to their b eta-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 i llustrated by studying the PET activation of 5 and 21. The exclusive t rans-stereochemistry observed in 8 is explained by considering the the rmodynamic, 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 confirme d by the isolation of 25 from 21. Formation of 25 suggests that wherev er the syn-intermediate is thermodynamically more stable, it invariabl y undergoes further cyclization to geometrically well-placed enolate d ouble bond. An interesting observation is made by isolating 9 as a maj or product from the PET activation of 5 using PS-B. Stabilization of 1 0 by ascorbic acid is suggested to be the plausible explanation for th is unusual observation. Radicals produced by the reductive beta-activa tion of alpha,beta-unsaturated ketones follow well established radical cyclization rules which is exemplified by studying the reactions of 3 9 and 40. Generality of these cyclizations is demonstrated from the PE T reactions of 29-32. Synthesis of 49, an important structural framewo rk of biologically active angularly fused triquinanes, from 48 is incl uded in this study to demonstrate the varied applicability of this str ategy.