Generation and mesolysis of PhSeSiR₃]^•-: mechanistic studies by laser flash photolysis and application for bimolecular group transfer radical reactions

Abstract

The investigation presented in this paper explores the mechanistic aspects and synthetic potentials of PET promoted reductive activation of selenosilane 1a to its radical anion 1a^•-. PET activation of 1a is achieved through a photosystem comprising a light-absorbing electron-rich aromatic (ERA), such as DMN or DMA, as an electron donor and ascorbic acid as a co-oxidant. The evidence for the ET from excited singlet states of DMN as well as DMA to 1a is suggested by estimating negative ΔG_et (-51 and -43.46 kcal mol^-1, respectively) values and nearly diffusion-controlled fluorescence quenching rate constants (k_qTR) 0.36 × 10¹⁰ M^-1 s^-1 and 0.28 × 10¹⁰ M^-1 s^-1, respectively, from time-resolved fluorescence quenching study. The transient absorption spectra of DMN^•+, DMA^•+, and 1a^•- are obtained initially by pulse radiolysis in order to correlate the time-resolved absorption spectral data. Laser flash photolysis studies in the nanosecond time domain have confirmed the generation of 1a^•-, DMN^•+, and DMA^•+, supporting the participation of the triplet state of DMN or DMA in the ET reaction. Mesolytic cleavage of 1a^•- produced a silyl radical and a phenyl selenide anion. The preparative PET activation of 1a in acetonitrile in the presence of DMN or DMA leads to the formation of 5 and 6, confirming the fragmentation pattern of 1a^•-. The overall ET rate constants (k_r(DMN) = 0.99 × 10¹⁰ M^-1 s^-1 and k_r(DMA) = 1.62 × 10¹⁰ M^-1 s^-1) and limiting quantum yields (φ _lim(DMN) = 0.034 and φ _lim(DMA) = 0.12) are estimated from the inverse plot (1/[1a] vs 1/φ _dis) obtained by measuring the dependence of photodissociation quantum yields of 1a at its maximum concentration in the presence of DMN or DMA. Silicon-centered radical species generated from the mesolysis of 1a^•- are utilized for initiating a radical reaction by the abstraction of halogen atom from -C-X (X = Cl, Br) bonds, while PhSe- terminates the radical sequences via PhSeSePh. This concept is successfully applied for the bimolecular group transfer (BMGT) radical reactions and intermolecular radical chain addition reactions.