Ring-closing C-O/C-O metathesis of ethers with primary aliphatic alcohols

Double strain-release enables formal C-O/C-F and C-N/C-F ring-opening metathesis

Metathesis reactions have been established as a powerful tool in organic synthesis. While great advances were achieved in double-bond metathesis, like olefin metathesis and carbonyl metathesis, single-bond metathesis has received less attention in the past decade. Herein, we describe the first C(sp3)-O/C(sp3)-F bond formal cross metathesis reaction between gem-difluorinated cyclopropanes (gem-DFCPs) and epoxides under rhodium catalysis. The reaction involves the formation of a highly electrophilic fluoroallyl rhodium intermediate, which is capable of reacting with the oxygen atom in epoxides as weak nucleophiles followed by C-F bond reconstruction. The use of two strained ring substrates is the key to the success of the formal cross metathesis, in which the double strain release accounts for the driving force of the transformation. Additionally, azetidine also proves to be a suitable substrate for this transformation. The reaction offers a novel approach for the metathesis of C(sp3)-O and C(sp3)-N bonds, presenting new opportunities for single-bond metathesis.

Late-Stage C-H Nitration of Unactivated Arenes by Fe(NO3)3·9H2O in Hexafluoroisopropanol

Operationally simple and generally applicable arene nitration with cheap and easily accessible chemicals has been a long-sought transformation in the synthetic organic community. In this work, we realized this goal with nontoxic and inexpensive Fe(NO3)3·9H2O as the nitro source and easily recyclable solvent hexafluoroisopropanol as the promotor via a network of hydrogen-bonding interactions. As a result of the relative mildness and high reliability of this protocol, late-stage nitration of various highly functionalized natural products and commercially available drugs was realized.

Enantioselective Ring-Closing Aminomethylamination of Allylic Aminodienes with Aminals Triggered by C-N Bond Metathesis

A conceptually novel strategy utilizing a cyclopalladated complex as an electrophile to activate the C-N bond for the C-N bond metathesis between allylamines and aminals is developed, which enables an efficient ring-closing aminomethylamination of allylic aminodienes and aminals. The reaction proceeds under mild reaction conditions and displays a remarkable scope. Utilizing a modified Trost-type diphosphine as the ligand, this method enables the efficient synthesis of 5-10-membered aminoallylated chiral N-heterocycles in good yields with high enantiomeric excess values.

An Intermolecular Hydroarylation of Unactivated Arylcyclopropane via Re2O7/HFIP-Mediated Ring Opening

In this paper, we describe a Re2O7-mediated ring-opening arylation of unactivated arylcyclopropane because of its functionalization with various arenes via Friedel-Crafts-type reactivity. This protocol allows facile access to functionalized 1,1-diaryl alkanes and is characterized by a broad substrate scope, mild reaction conditions, high efficiency, and high atom economy. Both density functional theory calculations and deuterium labeling experiments were carried out to justify the indispensable role of HFIP in this transformation and pointed to Re2O7-mediated ring opening being the rate-determining step.

Route to Functionalized Tetrahydrobenzo[d]azepines via Re2O7-Mediated Intramolecular Friedel-Crafts Reaction

We describe a Re2O7-mediated intramolecular dehydrative Friedel-Crafts reaction for the efficient synthesis of various benzo-fused heterocycles such as benzazepines and benzazocines. This process is characterized by a broad substrate scope, mild reaction conditions, high efficiency, and high atom economy. The potential application of this methodology was exemplified by the facile preparation of a NMDA antagonist as well as a key intermediate en route to SKF 38393.

Transition-metal-free and additive-free intermolecular hydroarylation of alkenes with indoles in hexafluoroisopropanol

Hydroarylation of alkenes is one of the most straightforward and atom-economical strategy for the construction of multi-aryl-substituted alkanes, but systematic studies have been limited to transition metal catalysis. Here we report a hexafluoroisopropanol (HFIP)-promoted hydroarylation of alkenes with indoles without the presence of transition metal catalysts or any additive. HFIP was the only reagent used in this work, and could be easily removed via evaporation, and recovered via distillation in industry settings. This reaction was shown to provide an efficient, clean and operationally simple procedure with a remarkable substrate scope and versatile transformations, delivering a variety of multi-aryl alkanes incorporating the indole motif. In preliminary studies, several of these products showed biologically activity against cells from an array of human cancer cell lines. A mechanistic study was also carried out and suggested that the quinone methide might be the key intermediate. And in contrast to the conclusions of a previous report, the current work suggested that protonation by HFIP might not be the rate-determining step.

Diastereoselective Reductive Etherification Via High-Throughput Experimentation: Access to Pharmaceutically Relevant Alkyl Ethers

This manuscript describes the development of the first diastereoselective intermolecular synthesis of alkyl ethers via reductive etherification of diverse ketones or aldehydes with alcohols. Key to this development was the use of low-temperature high-throughput experimentation (HTE) technologies that enabled rapid reaction optimizations and parallel synthesis. A broad scope of pharmaceutically relevant substrates was surveyed, which formed alkyl ethers effectively. In addition, we demonstrated that the diastereoselectivity of this transformation can be readily modulated by prudent selection of the reductant.