Semiconductor photocatalysis type B: synthesis of unsaturated α-amino esters from imines and olefins photocatalyzed by silica-supported cadmium sulfide

Abstract

Novel unsaturated N-phenyl-alpha-amino esters were synthesized in isolated yields of 50 to 10% by visible light irradiation of methanolic suspensions of silica-supported cadmium sulfide in the presence of methyl (2Z)-phenyl(phenylimino)acetate and various cyclic olefins. A semiconductor photocatalysis mechanism is proposed for this linear addition reaction. The light-generated electron-hole pair in the oxidative step induces a dissociative electron transfer from the olefin to CdS affording a proton and an allylic radical, whereas in the reductive step an alpha-aminobenzyl radical is formed in a proton coupled reaction. Heterocoupling of these intermediate radicals leads to the corresponding addition products. As the only by-product the hydrogenated imine is obtained in comparable amounts through the subsequent photoreduction of the alpha-aminobenzyl radical. Supporting the catalyst on silica made the reaction three times faster as compared to neat CdS. When the surface OH groups of CdS were removed through alkylation, the reaction between ArCH=NAr (Ar = p-C1C6H4) and cyclopentene was completely inhibited, but occurred again upon replacing the aldimine by its hydrochloride salt. This indicates that the protonated aldimine is involved in the reductive reaction step. Protonation makes this reduction easier by 0.1 V as indicated by the shift of the reduction potential of methyl (2Z)-phenyl(phenylimino)acetate upon addition of glacial acetic acid to the acetonitrile solution.

Visible‐Light‐Driven Cleavage of C−O Linkage for Lignin Valorization to Functionalized Aromatics

“…Additionalc ontrol experiments were conducted to confirm the reactionp athway by tracing the reductive cleavage of C a =Oi ntermediate in the presenceo fC a ÀOH reactant. [37,38] Thus, furthers tudies were performed to understand the effect of ÀSH groups on the Zn 4 In 2 S 7 surface. If PP-one was added with methoxy-PP-ol as ar eactant, the conversion of PP-one was only 17 %( Scheme 1, entry 6).…”

“…If methoxy-PP-one was added with PP-ol as ar eactant, the conversion of methoxy-PP-one was only 24 %( Scheme 1, entry 5). Bromide derivatives can reactw ith ÀSH groups efficiently through an ucleophilic displacementr eaction, [38,39] and thus can be used as inhibitors of ÀSH groups on the Zn 4 In 2 S 7 surface. Both resultsf urther confirm that the major process for cleavage of the b-O-4 bond over Zn 4 In 2 S 7 is the one-step C a radical intermediate pathway;t he two-step C a =Oi ntermediate pathway is am inor process for cleavage of the b-O-4 bond ( Figure 4).…”

“…For the inhibition experiment of ÀSH groups on the Zn 4 In 2 S 7 surface, Zn 4 In 2 S 7 catalyst (100 mg) was treated with ac ertain amount of BPTMOS in cyclohexane (10 mL) at 80 8Cf or 6h; [38] then, the powder was collected by centrifugation, rinsed with cyclohexane, and dried in vacuum at 60 8Co vernight. The catalytic performance of the treated catalysts was evaluated for the transformation of PPol.…”

Visible‐Light‐Promoted Stereoselective Alkylation by Combining Heterogeneous Photocatalysis with Organocatalysis

“…Yellow CdS (5) has a band gap of 2.4 eV and was prepared as previously reported. [26] Table 1: Enantioselective alkylations using MacMillan's chiral secondary amine and inorganic semiconductors as photocatalysts.…”

Factors affecting the selectivity of the photocatalytic conversion of nitroaromatic compounds over TiO2 to valuable nitrogen-containing organic compounds

“…9 A variety of organic transformations mediated by irradiation of semiconductors has been reported including oxidation and/ or reduction, and some coupling reactions. 6,7,[10][11][12] Among the photocatalytic reductions, the reduction of nitroaromatic compounds has been studied most extensively. In this case, the presence of dioxygen (O 2 ) must be avoided since O 2 competes with the nitroaromatic compound for electrons.…”