Development and Mechanistic Studies of the Iridium-Catalyzed C-H Alkenylation of Enamides with Vinyl Acetates: A Versatile Approach for Ketone Functionalization

Bioinspired Functionalization of Carbonyl Compounds Enabled by Metal Chelated Bifunctional Ligands

In Nature, enzymatic reactions proceed through exceptionally ordered transition states giving rise to extraordinary levels of stereoselection. In those reactions, the active site of the enzyme plays crucial roles - through one position, it holds the substrate in the proximity to the reaction epicentre that facilitates both the reactivity and stereoselectivity of the chemical process. Inspired by this natural phenomenon, synthetic chemists have designed bifunctional ligands that not only coordinate to a metal centre but also preassociate with an organic substrate, for example aldehyde and ketone, and exerts stereodirecting influence to accelerate the attack of the incoming reacting partner from a particular enantiotopic face. The chief goal of the current review is to give an overview of the recently developed approaches enabled by privileged bio-inspired bifunctional ligands that not only bind to the metal catalyst but also activates carbonyl substrates via organocatalysis, thereby easing in the new bond forming step. As carbonyl α-functionalizations are dominated by enamine and enolate chemistry, the current review primarily focusses on enamine- and enolate-metal catalysis by bifunctional ligands. Thus, developments based on traditional cooperative catalysis occurring through two directly coupled but independent catalytic cycles of an organocatalyst and a metal catalyst are not covered.

Synthesis of 3-aminoindenes and cis-1-aminoindanes by Zn(OTf)2-catalyzed cyclization of o-alkynylbenzaldehydes with tertiary alkyl primary amines

Using Zn(OTf)2 as catalyst, a highly regio- and chemo-selective cyclocarboamination of o-alkynylbenzaldehydes with tertiary alkyl primary amines was realized to access 3-aminoindenes with different substitution patterns from previously reported methods. The full reduction of the iminoindenone intermediates affords cis-1-amino-2-arylindanes with excellent diastereoselectivity. Mechanistically, the reaction involves the rearrangement of 1-amino-3-arylidene-isoindolines and isomerization of 1-aminoindenes to 3-aminoindenes.

Iridium-Catalyzed Enantioselective Alkene Hydroalkylation via a Heteroaryl-Directed Enolization-Decarboxylation Sequence

Upon exposure to a cationic Ir(I)-complex modified with the chiral diphosphine DuanPhos, hydroalkylations of styrenes and α-olefins with diverse heteroaryl tert-butyl acetates occur with complete branched selectivity and very high enantioselectivity. The initial adducts undergo acid promoted decarboxylation in situ to provide alkylated heteroarenes possessing defined β-stereocenters. The processes are postulated to proceed via a stereodefined chiral Ir-enolate, which arises upon heteroarene directed enolization of the heteroaryl acetate precursor. The method can be classified as an enantioselective decarboxylative C(sp3)-C(sp3) cross-coupling.

Iridium-Catalyzed Branch-Selective Hydroalkylation of Simple Alkenes with Malonic Amides and Malonic Esters

We report the iridium-catalyzed branch-selective hydroalkylation of simple alkenes such as aliphatic alkenes and aromatic alkenes with malonic amides and malonic esters under neutral reaction conditions. A variety of aliphatic alkenes and aromatic alkenes bearing bromine, chlorine, ester, 2-thienylcarboxylate, silyl, and phthalimide groups were all found to be suitable for this hydroalkylation. The combination of this method with Krapcho dealkoxycarbonylation realized a one-pot synthesis of β-substituted amide and ester from β-amide ester and malonic ester. The hydroalkylated products derived from malonic amides are suitable for further transformation. The finely tuned reaction conditions realized the selective transformation of hydroalkylated products to 1,3-diamines or monoamides with the same reagent. Deuterium labeling experiments and measurement of the kinetic isotope effect indicated that the catalytic cycle involves a reversible step and cleavage of the C-H bond is not a rate-determining step. Density functional theory calculations provided insight into the reaction mechanism, where the carboiridation step is followed by C-H reductive elimination.

Iridium-Catalyzed Branch-Selective and Enantioselective Hydroalkenylation of α-Olefins through C-H Cleavage of Enamides

Catalytic branch-selective hydrofunctionalization of feedstock α-olefins to form enantioenriched chiral compounds is a particularly attractive yet challenging transformation in asymmetric catalysis. Herein we report an iridium-catalyzed asymmetric hydroalkenylation of α-olefins through directed C-H cleavage of enamides. This atom-economical addition process is highly branch-selective and enantioselective, delivering trisubstituted alkenes with an allylic stereocenter. DFT calculations reveal the origin of regio- and enantioselectivity.

Exploring the regioselectivity of the cyanoalkylation of 3-aza-1,5-dienes: photoinduced synthesis of 3-cyanoalkyl-4-pyrrolin-2-ones

Regioselective cyanoalkylalkenylation of 3-aza-1,5-dienes with oxime esters induced by visible light.