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

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.