Exploration of a Chiral Cobalt Catalyst for Visible‐Light‐Induced Enantioselective Radical Conjugate Addition

Abstract

Chiral catalysts tolerating photochemical reactions are in great demand for the vast development of visible-lightinduced asymmetric synthesis.N ow,c hiral octahedral complexes based on earth-abundant metal and chiral N 4 ligands are reported. One well-defined chiral Co II -complex is shown to be an efficient catalyst in the visible-light-induced conjugated addition of enones by alkyl and acyl radicals,p roviding synthetically valued chiral ketones and 1,4-dicarbonyls in 47-> 99 %yields with up to 97:3 e.r.Supportinginformation and the ORCID identification number(s) for the author(s) of this article can be found under: https://doi.

Photochemical generation of acyl and carbamoyl radicals using a nucleophilic organic catalyst: applications and mechanism thereof

“… 13 , 16c – e Catalytic methods for acyl radical formation mainly rely on hydrogen atom transfer (HAT) processes 17 or electron transfer mechanisms. 18 , 19 While the HAT process from aldehydes provides an elegant and atom-economical entry to acyl radicals, this approach is biased by regioselectivity issues when it is applied to complex molecules. 17 c Photoredox catalytic methods 20 require purposely designed substrates adorned with redox active auxiliaries.…”

Exploration of a Chiral Cobalt Catalyst for Visible‐Light‐Induced Enantioselective Radical Conjugate Addition

“…When the reaction was performed at 0 8 8C, the enantioselectivity can be improved, albeit with aprolonged time (7o:80%yield and 94.5:5.5 e.r.). Ap yranose-derived HE could be used to achieve an ew pyranose derivative 7q [9] in 51 %y ield with complete stereoselectivity.M oreover,o ther enones were tested under the standard conditions.B oth the alkyl and aryl substituent at the b-position of enones were compatible, achieving chiral ketones 7r-7z in 49-81 %y ields and up to 95.5:4.5 e.r. Thea bsolute configuration of these alkylated products was established by comparing the optical rotation data with the reported ones.…”

“…Thea bsolute configuration of chiral product 9p was established via X-ray crystallographic analysis. [9] Furthermore,t he directing group on product 9a can be facilely removed in aquantitative yield without obvious effects on the optical purity (9a' ': > 99 % yield and 93:7 e.r. ).…”

“…[8] The sequential acylation/reduction operation afforded am ethylated diamine 2a;f urthermore,t he condensation of odiaminobenzene derivative 3a and ethyl chloroacetate provided chloromethyl-substitutedb enzimidazole 4a.T he subsequent alkylation of 2a with 4a under basic conditions gave an ew chiral N 4 ligand L1.F inally,c oordination of L1 with CoCl 2 ·(H 2 O) 6 in CH 3 CN completed the synthesis of designed chiral octahedral complex DCo1,which was confirmed by Xray crystallographic analysis. [9] As part of our ongoing research interest in visible-lightinduced asymmetric photochemical reaction, [10] we planned to examine the catalytic performance of the well-defined Co complex, DCo1,a sachiral Lewis acid catalyst in the benchmark photo-Giese reaction (conjugated addition to electron-deficient alkenes by photogenerated radicals). [11,12] Initially,w el aunched this study by using enone 5a and 4benzyl-substituted Hantzsch ester (HE) 6a as model substrates and Mes-AcrClO 4 (PC)a sap hotocatalyst under irradiation of two 3Wblue LEDs.I nt he presence of chiral Co catalyst, chiral ketone 7a was produced in good yield with moderate enantioselectivity ( Table 1, entry 1: 80 %y ield, 70:30 e.r.…”

Recent developments in enantioselective photocatalysis

“…Recently, Xiao et al developed a similar catalyst 317, using a cobalt-based system where the chirality was conferred by the use of chiral ligands rather than metal-centred chirality, which is the modus operandi of Meggers' technology (Scheme 53) [124]. They applied 317 to the enantioselective RCA reaction of enones 318 using DHPs 319 as radical precursors.…”