Enantioselective Synthesis of Chiral Cyclopent-2-enones by Nickel-Catalyzed Desymmetrization of Malonate Esters

Palladium(0)-Catalyzed Anti-Selective Addition-Cyclizations of Alkynyl Electrophiles

Palladium(0)/monophosphine complexes catalyze anti-selective alkylative, arylative, and alkynylative cyclizations of alkynyl electrophiles with organometallic reagents. The remarkable anti-selectivity results from novel oxidative addition, that is, the nucleophilic attack of electron-rich palladium(0) on the electrophile across the alkyne followed by transmetalation and reductive elimination ("anti-Wacker"-type cyclization). With regard to 5-alkynals, triphenylphosphine (PPh₃ )-ligated palladium(0) catalyzes the cyclization of terminal alkynes and conjugated alkenyl- or alkynyl-substituted ones to afford 2-cyclohexen-1-ol and 2-alkylidene-cyclopentanol derivatives, respectively. For 6-alkyl- or 6-aryl-5-alkynals, the cyclization does not proceed with the palladium/PPh₃ catalyst; however, it does proceed with palladium/tricyclohexylphosphine (PCy₃ ), to yield the former products predominantly. Remarkably, the latter catalyst completely switches the regioselectivity in the cyclization of the conjugated diyne-aldehydes. Notably, palladium/PPh₃ -catalyzed cyclizations also proceed with other organometallics or even without them.

Nickel-Catalyzed Arylative Cyclizations of Alkyne- and Allene-Tethered Electrophiles using Arylboron Reagents

The use of arylboron reagents in metal‐catalyzed domino addition–cyclization reactions is a well‐established strategy for the preparation of diverse, highly functionalized carbo‐ and heterocyclic products. Although rhodium‐ and palladium‐based catalysts have been commonly used for these reactions, more recent work has demonstrated nickel catalysis is also highly effective, in many cases offering unique reactivity and access to products that might otherwise not be readily available. This review gives an overview of nickel‐catalyzed arylative cyclizations of alkyne‐ and allene‐tethered electrophiles using arylboron reagents. The scope of the reactions is discussed in detail, and general mechanistic concepts underpinning the processes are described.

Precatalyst-Enabled Selectivity: Enantioselective NiH-Catalyzed anti-Hydrometalative Cyclization of Alkynones to Endo- and Heterocyclic Allylic Alcohols

A highly enantioselective NiH-catalyzed hydrocyclization of alkynones with unparalleled anti- and endocyclic selectivities is described. The choice of the precatalysts has significant influence in tuning the regio- and enantioselectivity. Using Ni(OTs)₂ /Phox as a precatalyst and (EtO)₂ MeSiH as a hydride source, an array of enantioenriched O-, N-, and S-containing heterocyclic tertiary allylic alcohols are obtained in 24-81 % yields with 80:20-99:1 er. Mechanistic investigations and synthetic application are also carried out. This study represents an efficient access to a set of allylic alcohols that are unable to access by the state-of-the-art coupling reactions.

Enantioselective Nickel-Catalyzed anti-Arylmetallative Cyclizations onto Acyclic Ketones

Domino reactions involving nickel-catalyzed additions of (hetero)arylboronic acids to alkynes, followed by cyclization of the alkenylnickel intermediates onto tethered acyclic ketones to give chiral tertiary-alcohol-containing products in high enantioselectivities, are described. The reversible E/Z isomerization of the alkenylnickel intermediates enables overall anti-arylmetallative cyclization to occur. The ring system of the products are substructures of certain diarylindolizidine alkaloids.