Transition Metal-Mimicking Relay Catalysis by a Low-Valent Phosphorus Compound.

Low-valent main group species have been evolving as powerful alternatives to transition metals over the years due to their advantages such as low toxicity and high abundance. However, the inability of main group elements to mimic the redox-switching property of transition metals often limits their role as catalysts. Here, we demonstrate the use of a low-valent phosphorus(I) compound as an efficient metal-free catalyst for the synthesis of biologically relevant γ-butyrolactones through dual activation under ambient reaction conditions. The highly nucleophilic phosphorus(I) center plays a key role in leading to this transformation. Extensive experimental and theoretical studies suggest that the phosphorus center exhibits facile switching between its reduced state [P(I)] and its oxidized state [P(III)] during this transformation, mimicking the behavior of transition metals.

Highly Stable Self-Regenerating Organic Multi-Redox Systems derived from Bicyclic (Alkyl)(amino)carbenes (BICAACs).

An extended class of organic multi-redox systems was derived from bicyclic(alkyl)amino carbenes (BICAACs). The highly-conjugated system undergoes a total of 4 redox events spanning a 1.8 V redox range. These organic compounds exhibited four different stable redox states (dication, radical cation, neutral and radical anion), and all of them were characterized either by single crystal X-ray study and/or various spectroscopic studies. Three of the four redox states are stable to air and moisture. The availability of stable multiple redox states demonstrated promise towards their efficacy in the symmetric H-cell charge/discharge cycling. Among various redox states, the dication/neutral state works efficiently and continuously for 1500 cycles in 2e- charge/discharge process outside glovebox in commercially available DMF with minimum capacity loss (retaining nearly 90 % Coulombic efficiency). Surprisingly, the efficiency of the redox cycle was retained even if the system was exposed to air for 30 days when it slowly regenerated to the initial deep blue radical cation, and it exhibited another 100 charge/discharge cycles with a minimal capacity loss. Such a stable H-cell cycling ability is not well known among organic molecule-based systems.

Mesoionic N-Heterocyclic Imines as Super Nucleophiles in Catalytic Couplings of Amides with CO2.

An extended class of stable mesoionic N-heterocyclic imines (mNHIs), containing a highly polarized exocyclic imine moiety, were synthesized. The calculated proton affinities (PA) and experimentally determined Tolman electronic parameters (TEPs) reveal that these synthesized mNHIs have the highest basicity and donor ability among NHIs reported so far. The superior nucleophilicity of newly designed mNHIs was utilized in devising a strategy to incorporate CO2 as a bridging unit under reductive conditions to couple inert primary amides. This strategy was further extended to hetero-couplings between amide and amine using CO2 . These hitherto unknown catalytic transformations were introduced in the diversification of various biologically active drug molecules under metal-free conditions. The underlying mechanism was explored by performing a series of control experiments, characterizing key intermediates using spectroscopic and crystallographic techniques.

Benzimidazolylidene-Stabilized Borenium Ion for Catalytic Hydrogenation of N-Heterocycles.

Herein, we report the synthesis of a benzimidazolylidene-stabilized borane adduct and its borenium ion. This borenium ion was used as a metal-free catalyst for hydrogenating various substituted quinoline N-heterocycles under ambient conditions. Furthermore, this method was utilized to synthesize two drug molecules: galipinine and angustureine. A detailed DFT study was performed to understand this metal-free catalytic hydrogenation.

Mesoionic N-heterocyclic olefin catalysed reductive functionalization of CO2 for consecutive N-methylation of amines.

A mesoionic N-heterocyclic olefin (mNHO) was introduced as a metal-free catalyst for the reductive functionalization of CO2 leading to consecutive double N-methylation of primary amines in the presence of 9-borabicyclo[3.3.1]nonane (9-BBN). A wide range of secondary amines and primary amines were successfully methylated under mild conditions. The catalyst sustained over six successive cycles of N-methylation of secondary amines without compromising its activity, which encouraged us to check its efficacy towards double N-methylation of primary amines. Moreover, this method was utilized for the synthesis of two commercially available drug molecules. A detailed mechanistic cycle was proposed by performing a series of control reactions along with the successful characterisation of active catalytic intermediates either by single-crystal X-ray study or by NMR spectroscopic studies in association with DFT calculations.

NHC-catalyzed silylative dehydration of primary amides to nitriles at room temperature.

Herein we report an abnormal N-heterocyclic carbene catalyzed dehydration of primary amides in the presence of a silane. This process bypasses the energy demanding 1,2-siloxane elimination step usually required for metal/silane catalyzed reactions. A detailed mechanistic cycle of this process has been proposed based on experimental evidence along with computational study.