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.

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.

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.

A Co(III)-catalyzed redox-neutral [4 + 2] annulation of N-chlorobenzamides/acrylamides with substituted alkenes at ambient temperature is demonstrated. Using this protocol, pharmaceutically important 3,4-dihydroisoquinolinone derivatives were synthesized in good yields. Intriguingly, the synthetically useful functional group of allylic coupling partners such as sulfonyl, carbonate, acetate, phosphate, amide, nitrile, and silane were retained in the final cyclized product. The present annulation reaction was compatible with various substituted benzamides and allylic coupling partners. To support the proposed reaction mechanism, competition experiments, deuterium labeling studies, and kinetic isotope effect studies were performed.

A palladium-catalyzed [3 + 2] annulation of substituted aromatic amides with maleimides providing tricyclic heterocyclic molecules in good to moderate yields through weak carbonyl chelation is reported. The reaction proceeds via a dual C-H bond activation where the first C-H activation takes place selectively at the benzylic position followed by a second C-H bond activation at the meta position to afford a five-membered cyclic ring. An external ligand Ac-Gly-OH has been used to succeed in this protocol. A plausible reaction mechanism has been proposed for the [3 + 2] annulation reaction.

Co(III)-catalyzed regio- and chemoselective redox-neutral C-H annulation of arylamides/acrylamides with 1,3-dienes is described. The present annulation reaction was well suited with a less-reactive 1,3-butadiene. By employing this protocol, pharmaceutically important 3,4-dihydroisoquinolinones were synthesized in good yields. Furthermore, the prepared 3,4-dihydroisoquinolinones were converted into highly important oxirane derivatives in good yields. A plausible mechanistic cycle is proposed and supported by a competition experiment and kinetic isotopic effect (KIE) studies.

Rh(iii)-catalyzed allylic C-H amidation of substituted alkenes within situgenerated iminoiodinanes is demonstrated. The presented protocol is compatible with differently functionalized unactivated terminal alkenes and internal alkenes. In terminal alkenes, branch selectivity was observed exclusively. Based on the detailed mechanistic investigation, a possible reaction mechanism involving thein situgenerated π-allyl as well as metal-nitrene intermediates has been proposed.