Transition metals catalyzed C–C and C–O bonds formation: facile synthesis of flavans and benzoxepines

α-Alkylation and Asymmetric Transfer Hydrogenation of Tetralone via Hydrogen Borrowing and Dynamic Kinetic Resolution Strategy Using a Single Iridium(III) Complex

Here we present a novel strategy for the synthesis of enantiomerically enriched tetrahydronaphthalen-1-ols. The reaction proceeds via an alkylation (via hydrogen borrowing) and ammonium formate-mediated asymmetric transfer hydrogenation (via dynamic kinetic resolution), giving alkylated tetralols in high yields and good enantio- and diastereoselectivity across a diverse range of both alcohol and tetralone substrates. Additionally, these products were successfully derivatized to several complex molecules, demonstrating the utility of the tetrahydronaphthalen-1-ol.

One pot tandem dehydrogenative cross-coupling of primary and secondary alcohols by ruthenium amido-functionalized 1,2,4-triazole derived N-heterocyclic carbene complexes

One-pot tandem dehydrogenative cross-coupling of primary and secondary alcohols was catalyzed by three ruthenium complexes [1-(R)-4-N-(furan-2-ylmethyl)acetamido-1,2,4-triazol-5-ylidene]Ru(p-cymene)Cl [R = Et (1b), i-Pr (2b), Bn (3b)], of amido-functionalized 1,2,4-triazole derived N-heterocyclic carbene (NHC) ligands. Density Functional Theory (DFT) calculations were employed for the ruthenium (1b) precatalyst to understand this reaction mechanism completely, and the mechanisms adapted are divided categorically into three steps (i) nucleophilic substitution of chloride ions by alcohols, (ii) dehydrogenation of primary and secondary alcohols, and (iii) olefin and ketone hydrogenation. Our mechanistic study reveals that the formation of a deprotonated Ru-alcoholate (A) or (E) intermediate is favorable compared to the protonated form (A') or (E') from (1b) by associative nucleophilic substitution. Though an ionic pathway that proceeds through (A') or (E'), has less barriers in the dehydrogenation and olefin/ketone hydrogenation steps than that of the neutral pathway, proceeding through (A) or (E), a steep energy barrier was observed in the first nucleophilic substitution step, prohibiting the reaction to proceed via the intermediate (A') or (E'). Thus, our thorough mechanistic study reveals that the reaction proceeds via deprotonated Ru-alcoholate (A) or (E) species. Furthermore, the 1,4 addition of an α,β-unsaturated carbonyl compound is kinetically and thermodynamically favorable over the 1,2 addition, and the experiments support these observations. As a testimony towards practical application in synthesizing bio-active flavonoid based natural products, five different flavan derivatives (16-20), were synthesized by the dehydrogenative coupling reaction using the neutral ruthenium (1-3)b complexes.

Deaminative carbonylative coupling of alkylamines with styrenes under transition-metal-free conditions

Transition-metal-free deaminative carbonylation through C–N bonds activation via Katritzky salts has been successful developed. Various α,β-unsaturated ketones were obtained in moderate to good yields with alkylamines and styrenes as the substrates.

Enantioselective Synthesis of β-Quaternary Carbon-Containing Chromanes and 3,4-Dihydropyrans via Cu-Catalyzed Intramolecular C-O Bond Formation

A copper-catalyzed efficient enantioselective construction of chiral quaternary carbon-containing chromanes and 3,4-dihydropyrans is reported. The desymmetric C-O coupling is enabled by a chiral dimethylcyclohexane-1,2-diamine ligand and provides the desired products in good yields with high enantioselectivities. This method presents a broad substrate scope and is applicable to diversely substituted aryl bromides and alkenyl bromides. The application is demonstrated by a gram-scale synthesis and derivatization of the products toward valuable building blocks.

Access to Electron-Deficient 2,2-Disubstituted Chromanes: A Highly Regioselective One-Pot Synthesis via an Inverse-Electron-Demand [4 + 2] Cycloaddition of ortho-Quinone Methides

We report the one-pot synthesis of 2,2-disubstituted chromanes with electron-withdrawing substituents. This reaction provides a simple yet efficient route to a wide range of electron-deficient chromanes in high yield and excellent regioselectivity. The reaction of salicylaldehyde with 1,1-disubstituted ethylenes smoothly furnishes these electron-deficient chromanes, which can be further transformed into functionalized chromanes or chromene. For example, BW683C was effectively synthesized from 5-chlorosalicylaldehyde with 4-chlorostyrene in two steps in excellent yield. The present reaction thus provides versatile access to functionalized electron-deficient chromanes and chromenes and therefore constitutes a promising tool for the synthesis of biologically and photochemically active molecules.

Sequential one-pot approach for the synthesis of functionalized phthalans via Heck-reduction–cyclization (HRC) reactions

An efficient and practical method is described for the direct synthesis of 1,3-dihydroisobenzofurans, an important structural motif present in biologically active natural or synthetic compounds.

Lewis acid promoted C–C and copper-catalyzed C–O bond formation: synthesis of neoflavans

An intramolecular [Cu]-catalyzed C–O bond formation for the synthesis of neoflavans is presented.