Enantioselective Synthesis of Cyclobutane Derivatives via Cascade Asymmetric Allylic Etherification/[2 + 2] Photocycloaddition.

Chiral cyclobutane presents as a popular motif in natural products and biologically active molecules, and its derivatives have been extensively used as key synthons in organic synthesis. Herein, we report an efficient synthetic method toward enantioenriched cyclobutane derivatives. The reaction proceeds in a cascade fashion involving Ir-catalyzed asymmetric allylic etherification and visible-light induced [2 + 2] cycloaddition. Readily available branched allyl acetates and cinnamyl alcohols are directly used as the substrates under mild reaction conditions, providing a broad range of chiral cyclobutanes in good yields with excellent diastereo- and enantioselectivities (up to 12:1 dr, >99% ee). It is worth noting that all substrates and catalysts were simultaneously added without any separated step in this approach. The gram-scale reaction and diverse transformations of product further enhance the potential utility of this method.

Iridium-Catalyzed Asymmetric Allylic Benzylation with Photogenerated Hydroxy-o-Quinodimethanes.

An iridium-catalyzed asymmetric allylic benzylation of aryl vinyl carbinols under light irradiation is described. 2-Methylbenzophenone derivatives are employed and activated to hydroxy-o-quinodimethanes by an ultraviolet (UV) light. This approach enables asymmetric allylic benzylation with high enantioselectivity (up to 99 % ee) from readily available 2-methylbenzophenones without the utilization of strong bases, and pre-activation or pre-functionalization of the substrates. Moreover, deuterium experiments reveal the generation of nucleophilic benzyl species from 2-methylbenzophenone under UV irradiation.

C(alkenyl)-H Activation via Six-Membered Palladacycles: Catalytic 1,3-Diene Synthesis.

A catalytic method to prepare highly substituted 1,3-dienes from two different alkenes is described using a directed, palladium(II)-mediated C(alkenyl)-H activation strategy. The transformation exhibits broad scope across three synthetically useful substrate classes masked with suitable bidentate auxiliaries (4-pentenoic acids, allylic alcohols, and bishomoallylic amines) and tolerates internal nonconjugated alkenes, which have traditionally been a challenging class of substrates in this type of chemistry. Catalytic turnover is enabled by either MnO2 as the stoichiometric oxidant or co-catalytic Co(OAc)2 and O2 (1 atm). Experimental and computational studies were performed to elucidate the preference for C(alkenyl)-H activation over other potential pathways. As part of this effort, a structurally unique alkenylpalladium(II) dimer was isolated and characterized.

Rhodium-Catalyzed Atroposelective C-H/C-H Cross-Coupling Reaction between 1-Aryl Isoquinoline Derivatives and Indolizines.

A rhodium-catalyzed asymmetric oxidative C-H/C-H cross-coupling reaction between 1-aryl isoquinolines and indolizines is disclosed. With a matched pair of SCpRh complex and chiral carboxylic acid, enantioselective two-fold C-H/C-H cross-coupling reactions between 1-aryl isoquinolines and indolizines provide a variety of axially chiral bi(hetero)aryls in excellent yields and enantioselectivity (up to 96% yield and 98% ee). Mechanistic studies suggest that both C-H cleavages are likely reversible.

Time-dependent enantiodivergent synthesis via sequential kinetic resolution.

The preparation of both enantiomers of chiral molecules is among the most fundamental tasks in organic synthesis, medicinal chemistry and materials science. Achieving this goal typically requires reversing the absolute configuration of the chiral component employed in the reaction system that is being used. The task becomes challenging when the natural source of the chiral component is not available in both configurations. Herein, we report a time-dependent enantiodivergent synthesis, in which an Ir-catalysed allylic substitution reaction uses one catalyst sequentially to promote two kinetic resolution reactions, enabling the synthesis of both enantiomers of the product using the same enantiomer of a chiral catalyst. The appropriate permutation of individual reaction rates is essential for the isolation of the chiral products in opposite configurations with high enantiopurity when quenched at different reaction times. This work provides an alternative solution for the preparation of both enantiomers of chiral molecules.