Palladium(II)/Brønsted Acid‐Catalyzed Enantioselective Oxidative Carbocyclization–Borylation of Enallenes

Abstract

An enantioselective oxidative carbocyclization–borylation of enallenes that is catalyzed by palladium(II) and a Brønsted acid was developed. Biphenol-type chiral phosphoric acids were superior co-catalysts for inducing the enantioselective cyclization. A number of chiral borylated carbocycles were synthesized in high enantiomeric excess.

Mechanistic Insight into Enantioselective Palladium‐Catalyzed Oxidative Carbocyclization–Borylation of Enallenes

“…The carbocyclization product 2 was prepared according to the previously reported procedure, [8] andi tw as obtained as av iscous oil in 93 %y ield, in 85 % ee. Preliminary attempts to crystallize product 2 from variouss olvents wereu nsuccessful, but after leaving it in ap entane/Et 2 O( 5:1) solution in af reezer (À20 8C) for 3days, crystals were obtained.…”

“…[8] In this reaction [Eq. (1))] ac hiral phosphoric acid was employeda sachiral catalytica dditive, which generatesa chiralp hosphate as counter-anion to palladium(II). This is not a simple task since,u nder the oxidative reaction conditions employed, phosphine ligands cannot be used and,f urthermore, most chiral ligands will suppresst he reactivity of the Pd II and shut down the reaction.…”

“…[8] To obtain more mechanistic insighti nto this enantioselective carbocyclization [Eq. (1)] we have now determined the absolute configuration of the product and carried out DFT calculations that allow us to rationalize the formation of the enantiomer observed. [8] To obtain more mechanistic insighti nto this enantioselective carbocyclization [Eq. (1)] we have now determined the absolute configuration of the product and carried out DFT calculations that allow us to rationalize the formation of the enantiomer observed.…”

Enantioselective Palladium‐Catalyzed Carbonylative Carbocyclization of Enallenes via Cross‐Dehydrogenative Coupling with Terminal Alkynes: Efficient Construction of α‐Chirality of Ketones

“…On the other hand, cascade carbon–carbon (C−C) bond formation involved in carbocyclizations is highly attractive, due to its atom economy and step efficiency. Previous work within our research group has shown that enantioselective oxidative carbocyclization of unsaturated structures can be achieved by exchanging the anion of a Pd II salt into chiral one . On the basis of this concept, the strategy using a cascade insertion of carbon monoxide (CO) and an olefin would give cyclic carbonyl compounds with high efficiency (Scheme a, right).…”

Highly Efficient Cascade Reaction for Selective Formation of Spirocyclobutenes from Dienallenes via Palladium-Catalyzed Oxidative Double Carbocyclization–Carbonylation–Alkynylation

“…8−10 An extension of these carbocyclizations to formation of spirocarbocycles would be highly interesting because spirocarbocycles bearing a fully carbon-substituted quaternary carbon center are challenging synthetic targets. 11 One class of compounds that we considered were spirocarbocycles bearing a cyclobutene ring.…”