A Lewis base catalyst Trip-SMe (Trip = triptycenyl) for electrophilic aromatic halogenation using Nhalosuccinimides (NXS) is introduced. In the presence of an appropriate activator (as a noncoordinating-anion source), a series of unactivated aromatic compounds were halogenated at ambient temperature using NXS. This catalytic system was applicable to transformations that are currently unachievable except for the use of Br 2 or Cl 2 : e.g., multihalogenation of naphthalene, regioselective bromination of BINOL, etc. Controlled experiments revealed that the triptycenyl substituent exerts a crucial role for the catalytic activity, and kinetic experiments implied the occurrence of a sulfonium salt [Trip-S(Me)Br][SbF 6 ] as an active species. Compared to simple dialkyl sulfides, Trip-SMe exhibited a significant charge-separated ion pair character within the halonium complex whose structural information was obtained by the single-crystal X-ray analysis. A preliminary computational study disclosed that the π system of the triptycenyl functionality is a key motif to consolidate the enhancement of electrophilicity.
Transition-metal-catalyzed C–H activation and
subsequent
oxidative cyclization with alkynes has been a powerful tool for the
synthesis of polycyclic aromatic compounds. Despite the substantial
progress in this field, it is still a significant challenge to establish
synthetic methodologies for the construction of nonsubstituted vinylene-fused
aromatics. We herein report a Rh(III)-catalyzed C–H/N–H
annulation with vinylene carbonate as an acetylene surrogate. Vinylene
carbonate also acts as an internal oxidant to regenerate the Rh(III)
species in situ; thus, no external oxidant is required to trigger
the oxidative annulation. This protocol is applicable to the direct
synthesis of various N-heteroaromatics.
Snipping tool: Zn(OTf)2 is an efficient catalyst for selective cleavage of amides bearing a β‐hydroxyethyl group on the nitrogen atom. The mechanism involves an N,O‐acyl rearrangement and transesterification. This new catalytic system can be applied to sequence‐specific peptide bond scission at the amine side of a serine residue. Tf=trifluoromethanesulfonyl.
The palladium-catalyzed intramolecular oxidative C-H/C-H coupling of 2-aryloxypyridines as challenging substrates is investigated to construct an important class of N,O-mixed heteroacenes, i.e. benzofuropyridines. It is found that 2,6-diaryloxypyridines efficiently undergo double cyclization to provide bisbenzofuro[2,3-b:3',2'-e]pyridines of interest in materials chemistry.
Transition-metal-catalyzed activation of inert C−H bonds and subsequent C−C bond formation have emerged as powerful synthetic tools for the synthesis of elaborate cyclic molecules. In this report, we introduce an efficient synthetic method of 3,4-unsubstituted isocoumarins adopting an electron-deficient Cp E Rh complex as the catalyst. The use of vinylene carbonate as a vinylene transfer reagent enables the direct construction of isocoumarins from readily available benzoic acids, without any external oxidants as well as bases. The reaction mechanism is evaluated by computational analysis to find an unprecedented "rhodium shift" event within the catalytic cycle.
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