Jeganmohan Masilamani

Olefins are widely available feedstocks and can be readily functionalized into complex organic molecules. In particular, the regioselective allylic C–H functionalization of olefins can provide highly valuable allylic functionalized organic molecules in a highly atom- and step-economical manner. This present review describes the recent advancement on allylic C(sp3)–H functionalization of olefins catalyzed by Rh(III) or Ir(III) complexes. This review covers the advancement in the allylic C–H amination, arylation and etherification of olefins catalyzed by Rh(III) or Ir(III) complexes.

A versatile and site-selective rhodium(III)-catalyzed aerobic oxidative alkenylation of arylacetamides including primary, secondary, and tertiary amides having a weak O-coordinating acetamide directing group with alkenes is described. In the reaction, air was utilized as a sole oxidant. The reaction was compatible with activated alkenes and maleimides.

An efficient Rh(III)-catalyzed aerobic oxidative C-H alkenylation of arylamides with unactivated alkenes is described. The olefination reaction was compatible with various substituted arylamides including primary, secondary, and tertiary as well as functionalized unactivated olefins. Meanwhile, ortho mono/bis-alkylated arylamides were synthesized in the reaction of arylamides with norbornene. In the alkenylation reaction, molecular oxygen along with organic acid was used to regenerate the active catalyst for the next catalytic cycle. A possible reaction mechanism involving C-H activation/insertion/β-hydride elimination followed by aerobic oxidation was proposed and supported by the deuterium labeling studies.

A Rh(III)-catalyzed weak enone carbonyl/ketone-assisted aerobic oxidative C-H olefination of aromatics with unactivated alkenes has been developed. This protocol involves cross-dehydrogenative Heck-type olefination reaction of various substituted biologically relevant chalcones and aromatic ketones such as acetophenones and chromones with various functionalized unactivated olefins in moderate to good yields. Further, ortho-alkylation of chalcones with norbornene is also demonstrated. A possible reaction mechanism involving weak chelation-assisted C-H activation/insertion/β-hydride elimination was proposed and supported by the deuterium labeling studies.

An efficient and new route for the synthesis of (E)-4-benzylideneisochroman-1-ones through tandem cascade annulation of benzoic acids with alkylidenecyclopropanes using Ru(II) as a catalyst is demonstrated. It is important to note that the reaction delivers selectively E-stereoselective 4-benzylideneisochroman-1-one derivatives in moderate to good yields, which has completely diverse selectivity as compared with previous methods. Further, the annulation was explored with less-reactive β C-H activation of vinylic acids with alkylidenecyclopropanes, leading to the highly useful α-pyrone derivatives in the presence of an Rh(III) catalyst.

A rhodium(III)-catalyzed ortho-alkenylation of substituted anilides with maleimides providing 3-arylated maleimides in good to excellent yields is described. In the reaction, a minor amount of Michael-type addition product, 3-arylated succinimides, was also observed. Later, 3-arylated succinimides was converted into 3-arylated maleimides in the presence of diisopropyl azodicarboxylate and K 2 CO 3 . Generally, a Michael-type 3-arylated succinimides was observed in the reaction of maleimides with substituted aromatics via C-H bond activation. In the present reaction, unusually, a Heck-type alkenylated product was observed in the major amount. A possible reaction mechanism was proposed and supported by the deuterium labelling studies and computational experiments. The mechanistic investigation and DFT calculations revealed that the time interval for the regeneration of Rh(I) to Rh(III) could be the factor for the observation of a minor ortho alkylated aromatics which was formed via a redox neutral Rh(III) version.

An efficient ruthenium(II)-catalyzed tandem C-C/C-N bond formation with aryl amides and 7-azabenzonorbornadienes has been developed to synthesize cis-fused dihydrobenzo[c]phenanthridinones. The amide group functions as a directing group as well as a leaving group and provides an easy access to the pharmaceutically useful benzo[c]phenanthridine alkaloids such as nitidine and fagaronine analogues. The present methodology is compatible with various functional groups with respect to azabicyclic alkenes and aromatic amides. The reaction mechanism involving directing-group-assisted C-H activation was proposed and supported by the deuterium labeling studies.

A highly regio- and chemoselective three-component assembling of N-pyrimidyl indoles with dienes and formaldehyde in the presence of a Co(iii) catalyst was demonstrated. The scope of the reaction was investigated with a variety of indole derivatives to synthesize substituted homoallylic alcohols. Both butadiene and isoprene units were compatible with the reaction. To understand the reaction mechanism, various investigations were carried out, and suggested the plausibility of a reaction mechanism involving C-H bond activation as a key step.

The reaction of N-aryl acrylamides with maleimides in the presence of a ruthenium/rhodium catalyst and Cu(OAc)2·H2O provided succinimide-substituted oxindole derivatives in good to excellent yields. In the reaction, 3,3′-disubstituted oxindoles are produced via a C-H alkylation and intramolecular oxidative cyclization sequence. The proposed mechanism was supported by competition experiments, deuterium labeling studies, and radical scavenger experiments.

1H-Isoindoles and 2H-isoindoles are synthesized via a ruthenium-catalyzed oxidant-free cyclization of benzimidates with alkenes at room temperature with the liberation of H2. Later, 1H-isoindoles were converted into nitrogen-containing heterocycles. The proposed reaction mechanism was strongly supported by experimental evidence and DFT calculations.