Triplet-Triplet Energy Transfer in Organic Synthesis: Efforts toward a Chiral Lewis Acid Catalyzed De Mayo Reaction and Investigations of a Photoactive Ruthenium(polypyridyl)-NHC Complex

Abstract

The fundamentals of energy transfer catalysis and its application in [2+2] photocycloadditions are thoroughly discussed. The De Mayo reaction is a UV light-promoted [2+2] photocycloaddition between enolic β-diketones and olefins that has been used to form α-substituted 1,5-diketones. Its introduction has enabled the assembly of complex macrocyclic scaffolds and been used as a key step in the synthesis of natural products. To address limitations imposed by direct UV light excitation the development of a visible-light variant(s) De Mayo reaction is essential. Herein, a report detailing a co-catalyzed system deploying the low-cost organic dye 4CzIPN in combination with a Lewis acid enables [2+2] photocycloaddition between β-diketones and styrenes to occur. Attempts toward an enantioselective De Mayo reaction via a chiral Lewis acid is also reported. In an effort to explore potential intramolecular energy transfer strategies the asymmetric imidazolium salt Ru(dmbpy)2(bpip)3 (bpip = 1-benzyl-3-isopropyl-1H-imidazo[4,5-f][1,10]phenanthrolinium), was prepared by a 4-step synthetic route. The isotopomer labelled with 13C at the C2 position of the bpip ligand was prepared from 1,10-phenanthroline-5,6-diamine and triethyl orthoformate-(formyl-13C). Deprotonation of the imidazolium salt at low temperatures in acetonitrile using KOtBu and KHMDS were monitored by NMR to investigate the formation of the free N-heterocyclic carbene.