Expulsion of Leaving Groups from Zwitterionic Intermediates Generated via Photochemical Electrocyclic Ring Closure Reactions and an Investigation on TPE Oligomers

Photochemical cleavage reactions have been found widespread use in biological applications that require intracellular photochemical release of biologically active substrates such as peptides, proteins, neurotransmitters, or nucleotide phosphates. The research is to devise photoremovable protecting groups (cage compounds) that can be used to deprotect those biological substrates by irradiation. A number of photocleavage reactions have been developed in recent years which release chlorides, hydroxides, carboxylates, thiolates, and phenolates leaving groups for use in such applications. Nevertheless, photochemical elimination reactions that expel such leaving group anions remain quite uncommon. The cleavage of cage compounds via photolysis requires heterolysis of a bond to the substrate or leaving group. Heterolysis of a C-Cl, C-O or C-S σ-bonds in a zwitterionic intermediate is proposed. The study also involves benzothiophene carboxanilides. The photolysis wavelength can be 365 nm by introducing a benzoyl group into the para position of the carboxanilide. The electrocyclization appears to be a triplet excited state reaction, according to the quenching studies. Heavy atom effect has been investigated for this system by introducing Br-atom at C-6 position of benzothiophene ring and also leading to a very efficient reaction. Chloride, hydroxyl, carboxylate, thiolate and phenolate leaving groups can be released photolytically. The proposed mechanism involves excited state electrocyclization to produce a ground state zwitterionic intermediate which eliminates the leaving group. We found quantum yields for leaving group expulsion decrease with the increased basicity of leaving groups, consistent with elimination of the leaving group directly from the zwitterionic intermediate. The zwitterion also likely undergoes competing ring dissertationing to regenerate the photoreactant. Moreover, the inclusion of a carboxylate group at the C-6 position of the benzothiophene ring improves solubility in aqueous media. In addition, the ionic behavior of di-phenyl substituted poly(phenylene vinylene) as well as well-defined tetraphenyl ethylene (TPE) oligomers have been investigated as in another part of my research