GENERATION AND MESOLYSIS OF PHSESIR3](CENTER-DOT-) - MECHANISTIC STUDIES BY LASER FLASH-PHOTOLYSIS AND APPLICATION FOR BIMOLECULAR GROUP-TRANSFER RADICAL REACTIONS

The investigation presented in this paper explores the mechanistic asp ects and synthetic potentials of PET promoted reductive activation of selenosilane la to its radical anion la(-.). PET activation of la is a chieved through a photosystem comprising a light-absorbing electron-ri ch aromatic (ERA), such as DMN or DMA, as an electron donor and ascorb ic 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 ne arly diffusion-controlled fluorescence quenching rate constants (k(q)T R) 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 absorpt ion spectra of DMN.+, DMA(.+), and la(.-) are obtained initially by pu lse radiolysis in order to correlate the time-resolved absorption spec tral data. Laser flash photolysis studies in the nanosecond time domai n have confirmed the generation of la(.-), DMN.+, and DMA(.+), support ing the participation of the triplet state of DMN or DMA in the ET rea ction. Mesolytic cleavage of 1a(.-) produced a silyl radical and a phe nyl selenide anion. The preparative PET activation of la in acetonitri le in the presence of DMN or DMA leads to the formation of 5 and 6, co nfirming the fragmentation pattern of la(.-). The overall ET rate cons tants (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/ph i(dis)) Obtained by measuring the dependence of photodissociation quan tum yields of la at its maximum concentration in the presence of DMN o r DMA. Silicon-centered radical species generated from the mesolysis o f la(.-) are utilized for initiating a radical reaction by the abstrac tion of halogen atom from -C-X (X = Cl, Br) bonds, while PhSe- termina tes the radical sequences via PhSeSePh. This concept is successfully a pplied for the bimolecular group transfer (BMGT) radical reactions and intermolecular radical chain addition reactions.