Radical cascade cyclization for the green and simple synthesis of silylated indolo[2,1-a]isoquinoline derivatives via visible light-mediated Si–H bonds activation

Photocatalytic and photoinduced silyl radicals cascade cyclization procedures for the green and simple preparation of fused tetracyclic skeleton silylated indolo[2,1-a]isoquinoline-6(5H)-ones from 2-aryl-N-acryloyl indoles with hydrosilanes are developed.The photocatalytic reaction is carried out with 9,10-dicyanoanthracene(DCA)as an organophotocatalyst and 3-acetoxyquinuclidine as hydrogen atom transfer(HAT)catalyst at room temperature under metal-and oxidant-free conditions.The keys to the success of photoredox-catalytic conversion include(1)the reductive quenching of DCA^(*)[E_(1/2)(*P/P^(-))=+1.97 V vs.SCE in MeCN]by 3-acetoxyquinuclidine(E_(p)=+1.22 V vs.SCE in MeCN),and(2)the thermodynamic feasibility of hydrogen atom abstraction from hydridic Si-H bond by electrophilic N^(+·).Particularly,the simple photoinduced cascade cyclization using(TMS)3SiH with 2-aryl-N-acryloyl indoles was exploited via an electron-donor-acceptor(EDA)complex under visible light irradiation.

Chloride-Promoted Photoelectrochemical C-H Silylation of Heteroarenes

A photoelectrochemical approach for the C-H silylation of heteroarenes through dehydrogenation cross-coupling with H2 evolution has been developed.The photoelectrochemical C-H silylation depends on hydrogen atom transfer(HAT)from silanes to Cl-radical generated through the light-induced homolytic cleavage of Cl2,in which Cl2 was produced by electrochemical oxidation of chloride.A large number of silylated heterocyclic molecules are rapidly constructed in satisfactory yields without relying on oxidants and metal reagents.

Recent advances in synthesis of organosilicons via radical strategies

Organosilicon compounds play an important role in the fields of materials science,pharmacy,and organic synthesis.The development of effective approaches for the preparation of these compounds have also become a research focus in organic synthesis.In recent years,free radical synthesis of organosilicons has been vigorously developed,which generally has the advantages of milder synthesis conditions,higher yields and selectivity,and free of precious metal catalysts compared with traditional strategies.This article reviews research progresses in the synthesis of organosilico n compounds by free radical pathways since 2016.In most cases,the radical silylation is achieved based on the reaction of silyl radicals,which are triggered by four routes including peroxide,transition-metal-induced peroxide decomposition,alkali,photocatalysis.The alkyl radicals can also initiate the radical silylation for the generation of C(sp^(3))—Si bonds.

α-Substituent Effects on Si--H, P- H and S--H Bond Dissociation Energies

CBS-Q and G3 methods were used to generate a large number of reliable Si--H, P---H and S--H bond dissociation energies （BDEs） for the first time. It was found that the Si--H BDE displayed dramatically different substituent effects compared with the C--H BDE. On the other hand, the P---H and S--H BDE exhibited patterns of substituent effects similar to those of the N--H and O--H BDE. Further analysis indicated that increasing the positive charge on Si of XSiH3 would strengthen the Si--H bond whereas increasing the positive charge on P and S of XPH2 and XSH would weaken the P---H and S--H bonds. Meanwhile, increasing the positive charge on Si of XSiH2^＋ stabilized the silyl radical whereas increasing the positive charge on P and S in XPH＂ and XS＊ destabilized P- and S-centered radicals. These behaviors could be reasonalized by the fact that Si is less electronegative than H while P and S are not. Finally, it was demonstrated that the spin-delocalization effect was valid for the Si-, P- and S-centered radicals.