“On-Water,” Microwave-Assisted, Pd-Catalyzed Synthesis of Indoles from Imines and o-Difunctionalized Arenes

Palladium-Catalyzed Allylation of Endocyclic 1-Azaallyl Anions

Endocyclic 1-azaallyl anions engage allyl acetates in a palladium-catalyzed allylation followed by reduction to give unprotected 2-(hetero)aryl-3-allylpiperidines and 2-allyl-3-arylmorpholines, products not easily accessible by other means. The allyl group is then readily transformed into a variety of functional groups. Preliminary studies on the asymmetric variant of the reaction using an enantiomerically pure BI-DIME-type ligand provide the product with moderate enantioselectivity. Computational studies suggest that energy barriers of inner-sphere reductive elimination and outer-sphere nucleophilic substitution are almost the same, which makes both of them possible reaction pathways. In addition, the inner-sphere mechanism displays an enantiodiscriminating C-C bond forming step, while the outer-sphere mechanism is much less selective, which combined to give the asymmetric variant of the reaction moderate enantioselectivity.

Palladium-Catalyzed Arylation of Endocyclic 1-Azaallyl Anions: Concise Synthesis of Unprotected Enantioenriched cis-2,3-Diarylpiperidines

Unprotected cis-2,3-diarylpiperidines are synthesized through an unprecedented palladium-catalyzed cross-coupling reaction between aryl halides and elusive endocyclic 1-azaallyl anions. These intermediates are generated in situ by the deprotonation of 2-aryl-1-piperideines, precursors that are readily prepared in two operations from simple piperidines. An asymmetric version of this reaction with (2R, 3R)-iPr-BI-DIME as the ligand provides products in moderate to good yields and enantioselectivities. This study significantly expands the synthetic utility of endocyclic 1-azaallyl anions.

Nickel-Catalyzed Mizoroki-Heck/Amination Cascade Reactions of o-Dihaloarenes with Allylamines: Synthesis of Indoles

An efficient Mizoroki-Heck/amination cascade reaction of o-dihaloarenes with allylamines has been developed using nickel and IPr carbene ligand as catalyst. This protocol enables the synthesis of a broad range of substituted indoles by a cascade process, from readily available starting materials. Mechanistic studies suggest that the Mizoroki-Heck reaction occurred first under IPr-nickel catalysis.

Dehydrogenative [2 + 2 + 1] Heteroannulation Using a Methyl Group as a One-Carbon Unit: Access to Pyrazolo[3,4-c]quinolines

A practical and straightforward access to pyrazolo[3,4-c]quinolines by molecular sieve mediated dehydrogenative [2 + 2 + 1] heteroannulation of N-(o-alkenylaryl)imines with aryldiazonium salts is described using a sp(3)-hybrid carbon atom as a one-carbon unit. The reaction enables the formation of three new chemical bonds, a C-C bond and two C-N bonds, in a single reaction and features simple operation and excellent functional group tolerance.

A practical synthesis of indoles via a Pd-catalyzed C-N ring formation

A method for the synthesis of N-functionalized C2-/C3-substituted indoles via Pd-catalyzed C-N bond coupling of halo-aryl enamines is described. The general strategy utilizes a variety of amines and β-keto esters which are elaborated into halo-aryl enamines as latent precursors to indoles. The preferred conditions comprising the RuPhos precatalyst and RuPhos in the presence of NaOMe in 1,4-dioxane tolerate a variety of substituents and are scalable for the construction of indoles in multigram quantities.