PhSeSiR3-Catalyzed Group Transfer Radical Reactions
摘要:
A novel design for initiating radical-based chemistry in a catalytic fashion is described. The design of the concept is based on the phenylselenyl group transfer reaction from alkyl phenyl selenides by utilizing PhSeSiR3 (1) as a catalytic reagent. The reaction is initiated by the homolytic cleavage of -C-Se- bond of an alkyl phenyl selenide by the in situ generated alkylsilyl radical (R3Si.), obtained by the mesolysis of PhSeSiR3](.-) (1(.-)). The oxidative dimerization of counteranion PhSe- to PhSeSePh functions as radical terminator. The generation of 1(.-) is achieved by the photoinduced electron transfer (PET) promoted reductive activation of 1 through a photosystem comprising of a visible-light (410 nm)-absorbing electron rich DMA as an electron donor and ascorbic acid as a co-oxidant (Figure 1). The optimum mole ratio between the catalyst 1 and alkyl phenyl selenides for successful reaction is established to be 1:10. The generality of the concept is demonstrated by carrying out variety of radical reactions such as cyclization (10, 15-18), intermolecular addition (25), and tandem annulations (32).
<i>p</i>-Silylation of Arenes via Organic Photoredox Catalysis: Use of <i>p</i>-Silylated Arenes for Exclusive <i>o</i>-Silylation, <i>o</i>-Acylation, and <i>o</i>-Alkylation Reactions
Photocatalytic regiospecific p-silylation of arenes has been achieved by the coupling of in situ generated silyl radical with areneradicalcation. The strategy involves reductive activation of PhSe–SiR3 and single electron transfer from the electron rich arene to 9,10-dimethoxyanthracene radicalcation (DMA•+). p-Silyl arenes, thus formed, are further utilized for exclusive o-silylation reaction and
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.