Generation and Mesolysis of PhSeSiR<sub>3</sub>]<sup>•-</sup>: Mechanistic Studies by Laser Flash Photolysis and Application for Bimolecular Group Transfer Radical Reactions
作者:Ganesh Pandey、K. S. Sesha Poleswara Rao、D. K. Palit、J. P. Mittal
DOI:10.1021/jo972345e
日期:1998.6.1
The investigation presented in this paper explores the mechanistic aspects and synthetic potentials of PET promoted reductive activation of selenosilane la to its radical anion la(-.). PET activation of la 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 la is suggested by estimating negative Delta G(et) (-51 and -43.46 kcal mol(-1), respectively) values and nearly diffusion-controlled fluorescence quenching rate constants (k(q)TR) 0.36 x 10(10) M-1 s(-1) and 0.28 x 10(10) M-1 s(-1), respectively, from time-resolved fluorescence quenching study. The transient absorption spectra of DMN.+, DMA(.+), and la(.-) 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 la(.-), 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 la in acetonitrile in the presence of DMN or DMA leads to the formation of 5 and 6, confirming the fragmentation pattern of la(.-). The overall ET rate constants (K-r(DMN) = 0.99 x 10(10) M-1 s(-1) and k(r)(DMA) = 1.62 x 10(10) M-1 s(-1)) and limiting quantum yields (phi(lim)DMN) = 0.034 and phi(lim)(DMA) = 0.12) are estimated from the inverse plot (1/[la] vs 1/phi(dis)) Obtained by measuring the dependence of photodissociation quantum yields of la at its maximum concentration in the presence of DMN or DMA. Silicon-centered radical species generated from the mesolysis of la(.-) 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.