Two Green Protocols for Halogenative Semipinacol Rearrangement.

Semipinacol rearrangement is a special type of Wagner-Meerwein rearrangement that involves carbocation 1,2-rearrangement to provide carbonyl compounds with an α-quaternary carbon center. It has been strategically used for natural product synthesis and construction of highly congested quaternary carbons. Herein, we report a safe and green protocol that uses oxone/halide and Fenton bromide to achieve halogenative semipinacol rearrangement. The key feature of this method is the green in situ generation of reactive halogenating species from oxidation of halide with oxone or H2O2, which produces a nontoxic byproduct (potassium sulfate or water). Easy operation (insensitive to air and moisture) at room temperature without using special equipment adds additional advantage over previous methods.

Environmentally Benign and User-Friendly In Situ Generation of Nitrile Imines from Hydrazones for 1,3-Dipolar Cycloaddition.

Nitrile imines are highly reactive and versatile dipoles and conventionally generated in situ from unstable hydrazonyl halides under basic conditions. Herein, we report the first green and user-friendly protocol for in situ generation of nitrile imines from Oxone-KBr oxidation of hydrazones and base-promoted dehydrobromination. The nitrile imines were demonstrated for 1,3-dipolar cycloaddition with various dipolarophiles, including alkene and alkyne groups. With its green nature, ease of operation, and air and moisture tolerance, we expect our method will find wide applications in organic synthesis.

Recent Advances in the Reactions of Cyclic Carbynes.

The acyclic organic alkynes and carbyne bonds exhibit linear shapes. Metallabenzynes and metallapentalynes are six- or five-membered metallacycles containing carbynes, whose carbine-carbon bond angles are less than 180°. Such distortion results in considerable ring strain, resulting in the unprecedented reactivity compared with acyclic carbynes. Meanwhile, the aromaticity of these metallacycles would stabilize the ring system. The fascinating combination of ring strain and aromaticity would lead to interesting reactivities. This mini review summarized recent findings on the reactivity of the metal-carbon triple bonds and the aromatic ring system. In the case of metallabenzynes, aromaticity would prevail over ring strain. The reactions are similar to those of organic aromatics, especially in electrophilic reactions. Meanwhile, fragmentation of metallacarbynes might be observed via migratory insertion if the aromaticity of metallacarbynes is strongly affected. In the case of metallapentalynes, the extremely small bond angle would result in high reactivity of the carbyne moiety, which would undergo typical reactions for organic alkynes, including interaction with coinage metal complexes, electrophilic reactions, nucleophilic reactions and cycloaddition reactions, whereas the strong aromaticity ensured the integrity of the bicyclic framework of metallapentalynes throughout all reported reaction conditions.