Desulfonylation of Tosyl Amides through Catalytic Photoredox Cleavage of NS Bond Under Visible‐Light Irradiation

Abstract

All lit up: A novel and efficient desulfonylation method of tosyl amides has been developed by means of visible-light-promoted reductive cleavage of N-S bonds. This method has a broad substrate scope, good functional group tolerance, and excellent yields.

N‐Radical‐Initiated Cyclization through Insertion of Sulfur Dioxide under Photoinduced Catalyst‐Free Conditions

“…Atandem radicalprocess is involved through the iminyl radical-mediated cyclizationw ith the insertion of sulfur dioxide;t his processs hows high efficiency and good selectivity.Recently,t he synthetic methods of N-centered radical species under visible light photocatalysish ave been observed. [1][2][3] Usually,p hotoredox-catalyticr eductiveo ro xidative cleavage of the NÀX( X= O, S, N, Cl, Br,H )b ond would occur,t og ive rise to the corresponding N-radicals. For instance, Studer described the photoinduced amidation of arenes and heteroarenes by using N-aminopyridiniums alts as the N-radical precursors.…”

“…For instance, sulfonated indolin-2-ones [7k] and coumarins [7a] could be prepared efficiently with radical sulfonylativer eactions. Promptedb yt hese results and the advancement of N-radicalc hemistry, [1][2][3] we conceived that sulfonated N-heterocycles could be obtainedt hrough N-radical-mediated cyclization by insertion of sulfur dioxide. Ap roposeds ynthetic route is shown in Scheme 1.…”

Visible‐Light‐Induced CS Bond Activation: Facile Access to 1,4‐Diketones from β‐Ketosulfones

“…Very recently, we have demonstrated that the NÀS bond of tosyl amides can be activated under visible light irradiation in the presence of photosensitizers and a subsequent reductive cleavage reaction finished the desulfonylation procesess of tosyl amides with high chemoselectivity under very mild conditions. [11] We therefore reasoned that the CÀS bond in the structural analog b-ketosulfone could proceed a similar process under visible light photocatalytic conditions to generate a carbon-centered radical intermediate A (Scheme 1). The latter will subsequently undergo a radical-radical coupling reaction to give a 1,4-diketone, an important synthetic building block for the construction of various carbocyclic and heterocyclic structural motifs.…”

“…To further improve the efficiency of this process, various basic additives [16] were evaluated in order to improve the isomerization of the b-ketosolfones to their enolate form (Scheme 3, process 1 to B) and the yield increased to 75 % when two equivalents of K 2 CO 3 were added to the system ( Table 1, entries [8][9][10][11][12]. Notably, decreasing the concentration of the mixture slightly enhanced the yield of 2 a to 77 % ( Table 1, entry 13).…”

“…To better understand this selectivity, a theoretical calculation was performed and the bond length of CH 2 ÀS bond (1.835 ) was found longer than PhÀS bond (1.807 ), which indicated the former more likely to undergo reductive cleavage process. [11,13] On the other hand, another molecule of b-ketosulfone 1 would facilely isomerize to their enolate form B under basic conditions. Subsequently, radical addition of A to the aposition of B generated radical anion C, wherein the elimination of a phenylsulfonyl radical afforded enolate D. [20] Finally, protonation of D gave the desired CÀC formation product 2, whereas competitive hydrogen abstraction of A would provide acetophenone as the byproduct.…”