Chul-Ho Jun

Metal-organic cooperative catalysis in C-H and C-C bond activation and its concurrent recovery.

The development of an efficient catalytic activation (cleavage) system for C-H and C-C bonds is an important challenge in organic synthesis, because these bonds comprise a variety of organic molecules such as natural products, petroleum oils, and polymers on the earth. Among many elegant approaches utilizing transition metals to activate C-H and C-C bonds facilely, chelation-assisted protocols based on the coordinating ability of an organic moiety have attracted great attention, though they have often suffered from the need for an intact coordinating group in a substrate. In this Account, we describe our entire efforts to activate C-H or C-C bonds adjacent to carbonyl groups by employing a new concept of metal-organic cooperative catalysis (MOCC), which enables the temporal installation of a 2-aminopyridyl group into common aldehydes or ketones in a catalytic way. Consequently, a series of new catalytic reactions such as alcohol hydroacylation, oxo-ester synthesis, C-C triple bond cleavage, hydrative dimerization of alkynes, and skeletal rearrangements of cyclic ketones was realized through MOCC. In particular, in the quest for an optimized MOCC system composed of a Wilkinsons catalyst (Ph 3P) 3RhCl and an organic catalyst (2-amino-3-picoline), surprising efficiency enhancements could be achieved when benzoic acid and aniline were introduced as promoters for the aldimine formation process. Furthermore, a notable accomplishment of C-C bond activation has been made using 2-amino-3-picoline as a temporary chelating auxiliary in the reactions of unstrained ketones with various terminal olefins and Wilkinsons catalyst. In the case of seven-membered cyclic ketones, an interesting ring contraction to five- or six-membered ones takes place through skeletal rearrangements initiated by the C-C bond activation of MOCC. On the other hand, the fundamental advances of these catalytic systems into recyclable processes could be achieved by immobilizing both metal and organic components using a hydrogen-bonded self-assembled system as a catalyst support. This catalyst-recovery system provides a homogeneous phase at high temperature during the reaction and a heterogeneous phase at room temperature after the reaction. The product could be separated conveniently from the self-assembly support system by decanting the upper layer. The immobilized catalysts of both 2-aminopyridine and rhodium metal species sustained high catalytic activity for up to the eight catalytic reactions. In conclusion, the successful incorporation of an organocatalytic cycle into a transition metal catalyzed reaction led us to find MOCC for C-H and C-C bond activation. In addition, the hydrogen-bonded self-assembled support has been developed for an efficient and effective recovery system of homogeneous catalysts and could be successful in immobilizing both metal and organic catalysts.

Transition metal-catalyzed carbon–carbon bond activation

This tutorial review deals with recent developments in the activation of C-C bonds in organic molecules that have been catalyzed by transition metal complexes. Many chemists have devised a variety of strategies for C-C bond activation and significant progress has been made in this field over the past few decades. However, there remain only a few examples of the catalytic activation of C-C bonds, in spite of the potential use in organic synthesis, and most of the previously published reviews have dwelt mainly on the stoichiometric reactions. Consequently, this review will focus mainly on the catalytic reaction of C-C bond cleavage by homogeneous transition metal catalysts. The contents include cleavage of C-C bonds in strained and unstrained molecules, and cleavage of multiple C-C bonds such as C[triple bond]C triple bonds in alkynes. Multiple bond metathesis and heterogeneous systems are beyond the scope of this review, though they are also fascinating areas of C-C bond activation. In this review, the strategies and tactics for C-C bond activation will be explained.

Energy-Efficient Dehumidification over Hierachically Porous Metal–Organic Frameworks as Advanced Water Adsorbents

Water sorption technologies are widely used commercially in many contexts, including industrial or indoor desiccant applications such as desiccant dehumidifiers, gas dryers, adsorptive air conditioning systems, fresh water production, adsorption heat transformation, etc.[1] In recent years, the potential for energy savings through improved efficiency has received increased attention, particularly as low-grade thermal energy or solar energy could be utilized. Currently, silica gel and zeolites are widely utilized commercially, often formed into corrugated honeycomb rotors.[1] As these sorbents typically must be heated above 150 °C during the desorption step, these sorbents are far from ideal in terms of energy consumption. There are additional issues with the level of dehumidification that these materials are able to achieve.[1] Improved energy efficiency requires advanced water adsorbents that can be regenerated together with the removal of a large amount of water vapor from humid conditions.[1] If such materials could operate at or below 80 °C, they could utilize readily available waste heat, leading to further energy savings. Among the existing classes of porous solids, crystalline metal–organic frameworks (MOFs)[2] are currently of great

Chelation‐Assisted RhI‐Catalyzed ortho‐Alkylation of Aromatic Ketimines or Ketones with Olefins

Described herein is the Rh(I)-catalyzed ortho-alkylation of aromatic ketimines or ketones with olefins. This method showed high reactivity and selectivity to monoalkylation for a variety of olefins including 1-alkenes with an allylic proton, alpha,omega-dienes, and internal olefins. For a mechanistic study, H/D exchange experiments were carried out, which demonstrated that the ortho C-H bond could be easily cleaved even at the low temperature of 45 degrees C. The key step of this reaction is the formation of a stable five-membered metallacycle by a chelation-assisted ortho C-H bond activation. Furthermore, the direct ortho-alkylation of aromatic ketones with the Rh(I) complex was successfully achieved by adding 50 mol % of benzylamine as a chelation-assistant tool.

Chelation-assisted carbon-hydrogen and carbon-carbon bond activation by transition metal catalysts.

Herein we describe the chelation-assisted C-H and C-C bond activation of carbonyl compounds by Rh1 catalysts. Hydroacylation of olefins was accomplished by utilizing 2-amino-3-picoline as a chelation auxiliary. The same strategy was employed for the C-C bond activation of unstrained ketones. Allylamine 24 was devised as a synthon of formaldehyde. Hydroiminoacylation of alkynes with allylamine 24 was applied to the alkyne cleavage by the aid of cyclohexylamine.

Metal–Organic Cooperative Catalysis in C–H and C–C Bond Activation

Transition-metal-catalyzed activation of C-H and C-C bonds is a challenging area in synthetic organic chemistry. Among various methods to accomplish these processes, the approach using metal-organic cooperative catalytic systems is one of the most promising. In this protocol, organic molecules as well as transition metals act as catalysts to bring about reactions, which proceed with high efficiencies and selectivities. Various metal-organic cooperative catalytic systems developed for C-H and C-C bond activation reactions are discussed in this review. Also discussed are how each metal-organic cooperative catalyst affects the reaction mechanism and what kinds of substrates can be applied in each of the catalytic processes.

Design of Hydrophilic Metal Organic Framework Water Adsorbents for Heat Reallocation

A new hydrothermally stable Al polycarboxylate metal-organic framework (MOF) based on a heteroatom bio-derived aromatic spacer is designed through a template-free green synthesis process. It appears that in some test conditions this MOF outperforms the heat reallocation performances of commercial SAPO-34.

Directed C – C Bond Activation by Transition Metal Complexes

Directed metallation of organic molecules is an important tool for the C–C bond activation since this strategy solves the accessibility problem occurring between a metal and a C–C bond that is to be cleaved. Stability of the five-membered metallacyclic complexes derived from the coordinating substrate and transition metal complexes are a driving force for undergoing the C–C bond activation. In this reaction, nitrogen, oxygen, and phosphorus are the most commonly used heteroatoms for directing functionality. Depending on the heteroatom in the substrate, different types of C–C bond activations can be observed.

Post-grafting of silica surfaces with pre-functionalized organosilanes: new synthetic equivalents of conventional trialkoxysilanes

This article describes recent advances that have been made in the development of methods for post-grafting of silica surfaces using functionalized organosilanes. While procedures employing conventional trialkoxysilane precursors have been utilized to immobilize organic and biological molecules onto inorganic supports, such as silica and glass, they have intrinsic limitations including sensitivity to hydrolysis and slow reaction rates. In this context, new synthetic equivalents to conventional trialkoxysilanes and new grafting methods have been devised to overcome these drawbacks and improve post-grafting processes. A key feature of the new strategies is the stability of the immobilizing groups that enables the silane precursors to be functionalized and purified without decomposition before immobilization. Recent developments made in the design of immobilization methods, which employ non-catalytic and catalytic approaches, are described.

Application of C-H and C-C bond activation in organic synthesis*

Herein we describe the chelation-assisted C-H and C-C bond activation by Rh(I) catalysts and its application directed toward the formation of C-C bonds in organic synthesis.