A facile formal [2+1] cycloaddition of styrenes with alpha-bromocarbonyls catalyzed by copper: efficient synthesis of donor–acceptor cyclopropanes

Abstract

Reactions of 2-bromoesters and styrenes underwent a [2+1] cycloaddition reaction, i.e., cyclopropanation, to produce donor-acceptor (D-A) cyclopropanes through a radical addition-ring closure process. In this reaction, the combination of copper(ii) complex and amines is found to be a good catalyst system to obtain cyclic compounds in high yields. This reaction provides an efficient protocol to synthesize D-A cyclopropanes, which are very important building blocks in organic synthesis.

Desymmetrization of 1-Symmetrical Donor–Acceptor (D–A) Cyclopropanes via Reactions with 1,3-Cyclodiones

“…Donor–acceptor (D–A) cyclopropanes 2 used in this study are all were prepared according to the literature procedures from the corresponding aromatic aldehydes through a sequence of Knoevenagel condensation followed by a Corey-Chaykovsky cyclopropanation . The compounds 2a , 2c–2e , 2f , 2g , 2h , 2i are known compounds, and the characterization data all corresponded to the reported values. The compound 2j is a known compound but not fully characterized in the literature.…”

Developments in the Chemistry of α‐Carbonyl Alkyl Bromides

“…After that,oxidation of the new alkyl radical intermediate affords the cationic inter-mediate, which undergoes an electrophilica lkylation to give the product. Notably,t he proposed mechanism was supported by 18 O-labeling experiments, reaction profiles of competitive reactions, and kinetic isotope effect (KIE) experiments.…”

“…Nishikata and co‐workers developed reactions between 2‐bromoesters and styrenes, which underwent a [2+1] cycloaddition reaction, that is, cyclopropanation, to produce donor–acceptor (D–A) cyclopropanes through a radical addition/ring‐closure process (Scheme ) . In this reaction, the combination of the copper(II) complex and amines is found to be a good catalyst system to obtain cyclic compounds in high yields.…”

“…Recently, α‐carbonyl alkyl halides, especially the readily available α‐carbonyl alkyl bromides, have emerged as efficient substrates providing alkyl radicals to initiate coupling reactions or functionalization reactions with unsaturated hydrocarbons (for example, alkenes, alkynes, 1, n ‐enynes, and divinyls). Furthermore, monofunctionalization and difunctionalization of unsaturated hydrocarbons with α‐carbonyl alkyl bromides have been achieved in recent years …”

Indole‐derived Donor‐acceptor Cyclopropanes

“…Very recently, Nishikata et al . reported cyclopropanation of arylethenes via Cu(II)‐catalyzed reaction with α‐bromocarbonyl compounds . Using this approach, they synthesized IDDA cyclopropane 39 (Scheme ) with a yield that is identical to that obtained in the Rh 2 (OAc) 4 ‐catalyzed reaction between the same alkene 38 and diazomalonate …”