SummaryCarbon–oxygen single bonds are ubiquitous in natural products whereas efficient methods for their reductive defunctionalization are rare. In this work an environmentally benign protocol for the activation of carbon–oxygen single bonds of alcohols towards a reductive bond cleavage under visible light photocatalysis was developed. Alcohols were activated as 3,5-bis(trifluoromethyl)-substituted benzoates and irradiation with blue light in the presence of [Ir(ppy)2(dtb-bpy)](PF6) as visible light photocatalyst and Hünig’s base as sacrificial electron donor in an acetonitrile/water mixture generally gave good to excellent yields of the desired defunctionalized compounds. Functional group tolerance is high but the protocol developed is limited to benzylic, α-carbonyl, and α-cyanoalcohols; with other alcohols a slow partial C–F bond reduction in the 3,5-bis(trifluoromethyl)benzoate moiety occurs.
Photoredox catalyzed intermolecular couplings of a-bromochalcones to olefins have been developed. Employing 1 mol% of the iridium complex IrA C H T U N G T R E N N U N G (ppy) 3 as photocatalyst, vinyl radicals are generated from a-bromochalcones as the key intermediate, which efficiently engage in a formal [4+2] cyclization with various alkenes. The resulting 3,4-dihydronaphthalenes can be readily transformed to the corresponding naphthalenes and further cyclized to 5H-benzo[c]fluorenes. Alternatively, Heck-type coupling products are obtained with sterically more hindered alkenes or allylated products if the alkene possesses a suitable leaving group in the allylic position.
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...
The synthesis of chiral tetrahydrofurans and pyrrolidines starting from 1,2‐diols or β‐amino alcohols, respectively, by visible‐light‐mediated deoxygenation is described. Easily accessible monoallylated/propargylated substrates were activated either as inexpensive ethyl oxalates or as recyclable 3,5‐bis(trifluoromethyl)benzoates to generate alkyl radicals suitable for 5‐exo‐trig/5‐exo‐dig cyclizations under visible‐light irradiation.
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