Regioselective halogenation of arenes and heterocycles with N-halosuccinimides in fluorinated alcohols is disclosed. Under mild condition reactions, a wide diversity of halogenated arenes are obtained in good yields with high regioselectivity. Additionally, the versatility of the method is demonstrated by the development of one-pot sequential halogenation and halogenation-Suzuki cross-coupling reactions.

Regioselective Halogenation of Arenes and Heterocycles in Hexafluoroisopropanol

1,1,1,3,3,3-Hexafluoroisopropanol (HFIP) is a polar, strongly hydrogen bond-donating solvent that has found numerous uses in organic synthesis due to its ability to stabilize ionic species, transfer protons, and engage in a range of other intermolecular interactions. The use of this solvent has exponentially increased in the past decade and has become a solvent of choice in some areas, such as C-H functionalization chemistry. In this review, following a brief history of HFIP in organic synthesis and an overview of its physical properties, literature examples of organic reactions using HFIP as a solvent or an additive are presented, emphasizing the effect of solvent of each reaction.

HFIP in Organic Synthesis.

An ortho-selective ammonium chloride salt-catalyzed direct C–H monohalogenation of phenols and 1,1′-bi-2-naphthol (BINOL) with 1,3-dichloro-5,5-dimethylhydantoin (DCDMH) as the chlorinating agent has been developed. The catalyst loading was low (down to 0.01 mol %) and the reaction conditions were very mild. A wide range of substrates including BINOLs were compatible with this catalytic protocol. Chlorinated BINOLs are useful synthons for the synthesis of a wide range of unsymmetrical 3-aryl BINOLs that are not easily accessible. In addition, the same catalytic system can facilitate the ortho-selective selenylation of phenols.

Ammonium Salt-Catalyzed Highly Practical Ortho-Selective Monohalogenation and Phenylselenation of Phenols: Scope and Applications

A Lewis base catalyst Trip-SMe (Trip = triptycenyl) for electrophilic aromatic halogenation using N-halosuccinimides (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 Br2 or Cl2: 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][SbF6] 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.

Triptycenyl Sulfide: A Practical and Active Catalyst for Electrophilic Aromatic Halogenation Using N -Halosuccinimides.

The Catalyst-Controlled Regiodivergent Chlorination of Phenols.

Novel regioselective aromatic chlorination via catalytic thiourea activation of N-chlorosuccinimide

A metal and acid-free electrophilic aromatic chlorination methodology involving catalytic thiourea activation of N-chlorosuccinimide (NCS) is reported herein. Moderate yields and regioselectivities of chlorinated aromatics were obtained using a combination of 1.05–1.15 equiv of NCS and 5 mol % of thiourea at room temperature in acetonitrile. A halogen bond between the sulfur atom of thiourea and the sigma-hole of the NCS chlorine atom is thought to enhance the electrophilic activity of the chlorine of NCS.

Pacharasak Limthavornlit

Papers

Novel regioselective aromatic chlorination via catalytic thiourea activation of N-chlorosuccinimide

Novel Regioselective Aromatic Chlorination via Catalytic Thiourea Activation of N-Chlorosuccinimide.