Copper sees the light of day: [Cu(dap)(2)Cl] proved to be an excellent photoredox catalyst for atom-transfer radical addition reactions, as well as for allylation reactions (see scheme), providing an attractive alternative to commonly used iridium- and ruthenium-based catalysts.
Cu(dap) 2 Cl has been utilized as a visible-light photoredox catalyst for the atom-transfer radical addition (ATRA) of benzyl halides to styrenes and silyl enol ethers. The resulting ATRA products can be readily converted into tetrahydroquinolines.The atom-transfer radical addition (ATRA) or Kharasch addition 1 of organic halides to olefins is a versatile tool for organic synthesis since it results in the formation of C-C and C-X bonds simultaneously. Commonly used initiators for ATRA reactions include peroxides, 1 triethylboron, 2 or organotin 3 reagents, which are not optimal with respect to operational safety, sensitivity, and environmental impact. Alternatively, transition-metal complexes of copper, 4 ruthenium, 5 iron, 6 or nickel 7 can be employed, nevertheless, the high catalyst loading necessary to achieve good yields leaves room for improvement. One solution to this problem has been proposed by adding reducing agents to regenerate the catalysts; 8 however, the reaction conditions necessary are generally quite harsh.Visible-light-driven photocatalysis has been recognized as a versatile tool for a growing number of organic transformations. 9 Stephenson et al. reported intermolecular ATRA reactions between perhaloalkanes or α-halocarbonyl compounds and olefins using oxidative or reductive quenching of photoredoxcatalysts based on ruthenium and iridium complexes. 10,11 Likewise, our group recently succeeded in the use of Cu(dap) 2 Cl [dap = 2,9-bis(p-anisyl)-1,10-phenanthroline; Sauvage's catalyst] 12 as visible-light photocatalyst for the same process utilizing the oxidative quenching pathway. 13 Cu(dap) 2 Cl is a stronger reductant (*Cu + /Cu 2+ = -1.43 V) than commonly used photocatalysts Ru(bpy) 3 Cl 2 or [Ir(dF(CF 3 )ppy) 2 (dtbbpy)](PF 6 ) no matter if the latter are utilized in the oxidative (*Ru 2+ /Ru 3+ = -0.86 V; *Ir 3+ /Ir 4+ = -0.89 V) or reductive quenching pathway (Ru + /Ru 2+ = -1.33 V; Ir 2+ /Ir 3+ = -1.21 V). Thus, *Cu(dap) 2 Cl can reduce substrates more effectively in the oxidative than Ru(bpy) 3 Cl 2 or [Ir(dF(CF 3 )ppy) 2 (dtbbpy)](PF 6 ) in the reductive quenching pathway with the additional advantage that no sacrificial electron donor has to be employed, making it an attractive alternative to ruthenium or iridium catalysts beside the economic advantage for visible-light-mediated ATRA reactions. 13 In this contribution, we demonstrate that also benzyl halides can be efficiently coupled with alkenes in ATRA reactions under visible-light catalysis (Scheme 1), offering a facile method for the synthesis of tetrahydroquinolines.
Scheme 1 Intermolecular atom-transfer radical addition of benzyl halides to styrenesThe activation of benzyl halides under visible-light photoredox conditions has only been shown in a few cases. Sauvage et al. demonstrated the dimerization of p-nitrobenzyl bromide under irradiation (λ ≥350 nm) in the presence of triethylamine as an electron donor and Cu(dap) 2 Cl, 12 while MacMillan developed the α-benzylation of aldehydes using fac-Ir(ppy) 3 (ppy = 2-phenylpyridin...
Cu(dap) 2 Cl] as an Efficient Visible-Light-Driven Photoredox Catalyst in Carbon--Carbon Bond-Forming Reactions. -Title catalyst is successfully applied in atom-transfer radical additions of alkyl halides towards olefins and allylation reactions of various nucleophiles, e.g. α-haloketones and amines. -(PIRTSCH, M.; PARIA, S.; MATSUNO, T.; ISOBE, H.; REISER*, O.; Chem. Eur. J. 18 (2012) 24, 7336-7340, http://dx.
Visible-Light-Induced Intermolecular Atom-Transfer Radical Addition of Benzyl Halides to Olefins: Facile Synthesis of Tetrahydroquinolines. -Radical addition of nitro-substituted benzyl halides to silyl enol ethers or styrenes provides ketones or halides which can be easily cyclized to produce tetrahydroquinoline derivatives. -(PARIA, S.; PIRTSCH, M.; KAIS, V.; REISER*, O.; Synthesis 45 (2013) 19, 2689-2698, http://dx.doi.org/10.1055/s-0033-1338910 ; Inst. Org. Chem., Univ. Regensburg, D-93053 Regensburg, Germany; Eng.) -Mais 08-047
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