A mild and efficient protocol for the assembly of tetra-substituted 5-trifluoromethylpyrazoles is presented, involving halogenation at the 4-position of readily prepared tri-substituted 5-trifluoromethylpyrazoles to give 4-halo-1-phenyl-5-trifluoromethyl pyrazoles, and subsequent palladium-catalyzed Negishi or Buchwald-Hartwig cross-couplings to install carbon or nitrogen-based 4-substituents. Key to the success of these challenging cross-couplings is the use of XPhos and JosiPhos CyPF-tBu ligands, respectively.
The arylative oxygenation of the
electron-rich olefins styrene,
α-methylstyrene, vinyl pyrrolidinone, and vinyl oxazolidinone
was accomplished using arenediazonium salts and catalytic amounts
of FeSO4 in an effective single electron transfer radical
process. A broad range of aryldiazonium salts was tolerated using
water, methanol, or their combination with acetonitrile to furnish
the corresponding carbohydroxylated and carbomethoxylated products
(42 examples), including functionalized dihydroisocoumarin and dihydrobenzofuran
systems in good to excellent yields (up to 88%). The protocols developed
for the Fe(II)-catalyzed carbohydroxylation were also compared to
Ru(II) and Ir(III) photoredox carbooxygenations of these electron-rich
olefins. The Fe(II)-catalyzed process proved to be highly competitive
compared to the photoredox and the uncatalyzed processes. The proposed
mechanism for the Fe(II)-catalyzed reactions involves the synergic
combination with an effective Fe+2/Fe+3 redox
system and a radical polar crossover mechanism featuring an unprecedented
capture of the reactive N-acyliminium in the case
of vinyl pyrrolidinone and vinyl oxazolidinone.
The nickel-catalyzed cyclization of in situ generated ortho-chlorobenzophenone hydrazone derivatives, to afford 3-(hetero)aryl-1H-indazoles, is documented for the first time. The product 1H-indazoles can be transformed subsequently in a one-pot procedure into 1,3-di(hetero)aryl-1H-indazoles via copper-catalyzed N-arylation with (hetero)aryl bromides.
The first application of 4-alkoxy-1,1,1-trifluoroalk-3-en-2-ones in a three-component condensation protocol for the synthesis of 3-acyl-4-aryl-2-(trifluoromethyl)-2-hydroxy-3,4,7,8-tetrahydro-2H-chromen-5(6H)-ones
An efficient one-pot transition metal-free procedure for the synthesis of new pyrazolo[1,5-a]quinoxalin-4(5H)-ones from easily prepared 1-(2-chlorophenyl-5-ethylcarboxylate)pyrazoles and various primary alkylamines is described. The key steps involved in the synthesis of the new 5,6-fused ring system are the formation of an amide intermediate followed by an intramolecular N-arylation reaction via nucleophilic aromatic substitution.
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