Recent developments in palladium-catalysed non-directed coupling of (hetero)arene C–H bonds with C–Z (Z = B, Si, Sn, S, N, C, H) bonds in bi(hetero)aryl synthesis

This review article describes the palladium-catalysed non-directed coupling of (hetero)arene C–H bonds with C_(Ar)–Z (Z = B, Si, Sn, S, N, C, H) bonds for facile bi(hetero)aryl synthesis in the past 10 years.

Application of primary halogenated hydrocarbons for the synthesis of 3-aryl and 3-alkyl indolizines

A transition-metal-free synthetic route to 3-aryl and 3-alkyl indolizines from electron-deficient alkenes, pyridines and primary halogenated hydrocarbons is reported for the first time using a tandem reaction.

Palladium-catalyzed C-3 desulfitative arylation of indolizines with sodium arylsulfinates and arylsulfonyl hydrazides

Derivatized indolizines were efficiently prepared by direct C-3 arylation of indolizines using sodium arylsulfinates and arylsulfonyl hydrazides in good yields.

Aza-Wacker-type reaction between electron-deficient olefins and N-alkylsulfonamides

A palladium-catalyzed oxidative amination protocol of electron-deficient olefins by aza-Wacker-type reaction with N-alkylsulfonamides was developed. The presence of the stoichiometric amount of methanesulfonic acid was crucial for the success of this transformation. The reactions were conducted in green solvent under mild conditions and scalable with excellent E-type stereoselectivity. In addition, a Pd(II)/Pd(0) catalytic cycle with the existence of a very strong oxidant (Selectfluor) was proposed.

Synthesis of pyrrolo[2,1,5-cd]indolizines through dehydrogenative Heck annelation of indolizines with diaryl acetylenes using dioxygen as an oxidant

A dehydrogenative Heck annelation reaction of indolizine with diaryl acetylene via dual C-H bond cleavage was developed. Oxygen gas was employed as a clean oxidant in this catalysis under base-free conditions. Diarylpyrrolo[2,1,5-cd]indolizines were synthesized with high atom economy. In addition, kinetic isotope experiments provided evidence for C-H bond metalation of the 5-position of the indolizine as the rate-limiting step.

Palladium-catalyzed direct and regioselective C–H acyloxylation of indolizines

A direct and highly regioselective C1-acyloxylation of indolizines was developed via palladium-catalyzed C–H functionalization. In this reaction, the regioselectivity was achieved in the absence of a directing group.

One-pot synthesis of indolizine via 1,3-dipolar cycloaddition using a sub-equivalent amount of K2Cr2O7 as an efficient oxidant under base free conditions

X = Cl or Br; EWG₁, EWG₂ = electron withdrow group, one-pot reaction, 29 examples, yields from moderate to high, gram scale-up potential confirmed.

Copper(II)-catalyzed indolizines formation followed by dehydrogenative functionalization cascade to synthesize 1-bromoindolizines

A one-pot, three-component cascade reaction between pyridine, α-acylmethylbromide, and maleic anhydride leading to direct access of 1-bromoindolizines in high yields has been developed. This protocol is accomplished via a reaction sequence of 1,3-dipolar cycloaddition of the pyridinium ylide with maleic anhydride, oxidative decarboxylation of the primary cycloadduct, and dehydrogenative bromination of the resulting 1-unsubstituted indolizine. Copper chloride was used as a catalyst and oxygen as the terminal oxidant. This reaction represents the first example of transition-metal-catalyzed direct dehydrogenative bromination of indolizine at the C-1 position. Moreover, the obtained 1-bromoindolizines can be transformed to other 1-substituted indolizines such as 1-arylindolizines via a simple reaction process.