Photocatalytic functionalizations of alkynes

Abstract

The visible light mediated functionalizations have significantly expanded the scope of alkynes by unraveling new mechanistic pathways and enabling their transformation to diverse structural entities. The photoredox reactions on alkynes...

Photoredox‐ and Nickel‐Catalyzed Hydroalkylation of Alkynes with 4‐Alkyl‐1,4‐dihydropyridines: Ligand‐Controlled Regioselectivity

“…A wide range of alkyl groups can be introduced to the alkyne to afford alkylated products that have not been achieved in previously reported catalytic hydroalkylation reactions. [6,7,[9][10][11][13][14][15][16][17]19] In addition to terminal alkynes, internal alkynes can also be converted into the desired product. We believe that the work described herein will expand the utility of 4-alkyl-1,4-dihydropyridine skeletons as mild formal alkylation reagents in combination with photoredox and cross-coupling catalysts for further organic syntheses under mild reaction conditions.…”

“…We have succeeded in the cooperative nickel‐ and photoredox‐catalyzed hydroalkylation of alkynes with 4‐alkyl‐1,4‐dihydropyridines at room temperature under irradiation with visible light, where the Markovnikov or anti ‐Markovnikov regioselectivity of the products is controlled by the coordination ligands bound to nickel species. A wide range of alkyl groups can be introduced to the alkyne to afford alkylated products that have not been achieved in previously reported catalytic hydroalkylation reactions [6,7,9–11,13–17,19] . In addition to terminal alkynes, internal alkynes can also be converted into the desired product.…”

“…High-resolution FAB mass spectra were measured on a JEOL JMS-700 mass spectrometer. 1 H NMR (400 MHz), 13 C{ 1 H} NMR (100 MHz), and 19 F NMR (376 MHz, referenced to CF 3 C 6 H 5 in CDCl 3 at δ -64.0) spectra were measured in CDCl 3 or CD 2 Cl 2 on a JEOL ECS-400 spectrometer with δ values in 1 H and 13 C{ 1 H} NMR calibrated by using residual peaks of CHCl 3 ( 1 H: 7.26; 13 C: 77.0) or CDHCl 2 ( 1 H, 5.32), respectively.…”

“…Among the vast range of methods to prepare functionalized alkenes, [2,3] hydroalkylation of alkynes [4–7] has been gaining attention as a convenient synthetic method to prepare functionalized alkenes with Markovnikov [8–11] or anti ‐Markovnikov [12–17] selectivity. In addition, functionalization of alkynes has been recently expanded by the introduction of visible light‐triggered generation of carbon‐centered radicals through a single electron transfer (SET) processes mediated by photoredox catalysts; [18] this has also been applied to the hydroalkylation of alkynes to form functionalized alkenes [6,7,9–11,13–17,19] . Indeed, both Markovnikov‐ [9–11] and anti ‐Markovnikov‐type [15,16] hydroalkylation of terminal alkynes mediated by dual photoredox and nickel catalysts has been reported, [20,21] in which alkyl carboxylic acids, [9,16] benzyl chorides, [10] cycloalkanols, [11] or dialkyl acyl peroxides [15] are used as alkylation reagents.…”

“…In addition, functionalization of alkynes has been recently expanded by the introduction of visible light-triggered generation of carbon-centered radicals through a single electron transfer (SET) processes mediated by photoredox catalysts; [18] this has also been applied to the hydroalkylation of alkynes to form functionalized alkenes. [6,7,[9][10][11][13][14][15][16][17]19] Indeed, both Markovnikov- [9][10][11] and anti-Markovnikov-type [15,16] hydroalkylation of terminal alkynes mediated by dual photoredox and nickel catalysts has been reported, [20,21] in which alkyl carboxylic acids, [9,16] benzyl chorides, [10] cycloalkanols, [11] or dialkyl acyl peroxides [15] are used as alkylation reagents. For example, MacMillan's and Rueping's groups have demonstrated visible light-mediated Markovnikov-or anti-Markovnikov-type decarboxylative hydroalkylation of terminal alkynes with alkyl carboxylic acids in the presence of an iridium-based photoredox catalyst by using [NiCl 2 (dtbbpy)] (dtbbpy = 4,4'-di-tert-butyl-2,2'bipyridine) or [NiCl 2 (dme)] as a co-catalyst, respectively; the addition of a base such as HNC(NMe 2 ) 2 or Cs 2 CO 3 was also required (Scheme 1a).…”