Tapan Maji

L‐Proline/CoCl2‐Catalyzed Highly Diastereo‐ and Enantioselective Direct Aldol Reactions

The CoCl(2)/L-proline (1:2) system was found to be an excellent catalyst for direct aldol reactions. Excellent yields (up to 93%) and a significant improvement in diastereoselectivity (anti/syn up to 45:1) as well as enantioselectivity (up to more than 99% ee) compared with using proline as the sole catalyst were observed. This catalyst system was successfully applied to both cyclic and acyclic ketones in combination with aromatic and aliphatic aldehydes. In situ chelation of CoCl(2) and proline (1:2) is proposed to promote the reaction through a six-membered Zimmermann-Traxler type transition state involving the positioning of proline-enamine and the aldehyde through chelation to Co(II).

Development of Asymmetric Deacylative Allylation

Herein we present the development of asymmetric deacylative allylation of ketone enolates. The reaction directly couples readily available ketone pronucleophiles with allylic alcohols using facile retro-Claisen cleavage to form reactive intermediates in situ. The simplicity and robustness of the reaction conditions is demonstrated by the preparation of >6 g of an allylated tetralone from commercially available materials. Furthermore, use of nonracemic PHOX ligands allows intermolecular formation of quaternary stereocenters directly from allylic alcohols.

Catalytic α-Monoallylation of Aryl Acetonitriles

α-Cyano aldehydes undergo selective transition-metal-catalyzed monoallylation to provide α-allylated nitriles. The transformation leads to linear substitution products with palladium catalysts or branched allylated nitriles using an iridium catalyst. Facile TBD-catalyzed retro-Claisen cleavage is leveraged to attain selective monoallylation.

Synthesis of Spirooxindoles via the tert-Amino Effect

A new method is developed for the synthesis of spirooxindoles from amines and isatins via C-H functionalization. The reaction leverages the tert-amino effect to form an enolate-iminium intermediate via [1,5]-hydride shift followed by cyclization. Interestingly the hydride migrates to the N atom of a C═N, which is atypical for hydride additions to imines.

Palladium-Catalyzed Double-Decarboxylative Addition to Pyrones: Synthesis of Conjugated Dienoic Esters.

An interceptive decarboxylative allylation protocol has been developed utilizing pyrone as a C4 synthon. This palladium-catalyzed transformation difunctionalizes the pyrone moiety by in situ generation and activation of both the electrophile and nucleophile via a double decarboxylation pathway. Ultimately, allyl carbonates react smoothly with 2-carboxypyrone under mild reaction conditions to generate synthetically useful acyclic dienoic esters, forming carbon dioxide as the sole byproduct.

Catalytic Addition of Fluorinated Benzoic Acids to Butadiene

The rhodium‐catalyzed addition of polyfluorobenzoic acids to butadiene led to a range of branched and linear allylic esters, with preference for the formation of the branched isomer. This method provided branched fluorinated esters in an atom‐economical catalytic addition reaction. The observation that the yield of the addition product depended on the acidity of the starting acid led to the hypothesis that the addition reaction was reversible and thus thermodynamically controlled. Deuterium‐labeling and crossover experiments supported this hypothesis. Ultimately, the dependence of yield on the pKa as reported in this paper may help to guide approaches to high‐yielding additions of acids to dienes.