Mechanistic Insights into Transition-Metal-Catalyzed C1 Polymerization of Diazo Compounds

The monomer 5‐[(5‐ethynyl‐1‐naphthyl)ethynyl]‐N,N‐dimethylnaphthalen‐1‐amine was satisfactory obtained through the heterocoupling reaction of 5‐ethynyl‐N,N‐dimethylnaphthalen‐1‐amine and 4‐(5‐iodo‐1‐naphthyl)‐2‐methyl‐3‐butyn‐2‐ol catalyzed by a palladium–copper system, followed by acetone elimination. Poly{5‐[(5‐ethynyl‐1‐naphthyl)ethynyl]‐N,N‐dimethylnaphthalen‐1‐amine} was obtained through the reaction of the acetylene monomer with homogeneous rhodium and palladium catalyst complexes. The structure of the polymers always showed a trans–cisoidal chain configuration on the basis of IR and NMR spectra. Moreover, only for the rhodium catalyst complex in methanol was a dimeric product isolated in a very low yield, having a conjugated terminal ene–yne structure, which permitted the consideration of a metallated chain‐transfer intermediate in the polymer propagation. The mass determination of the polymers, by osmometry and gel permeation chromatography techniques, showed low average molecular weights. The kinetics of the catalyzed polymerization were analyzed. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 2038–2047, 2007

The physical properties of synthetic macromolecules are strongly coupled to their molecular weight (MW), topology, and polydispersity index (PDI). Factors that contribute to their utility include the control of functionality at the macromolecule termini and copolymer composition. Conventional polymerization reactions that produce carbon backbone polymers (ionic, free radical, and coordination) provide little opportunity for controlling these variables. Living polymerizations, sometimes referred to as controlled polymerizations, have provided the means for achieving these goals. Not surprisingly, these reactions have had a profound impact on polymer and materials science. Three basic reaction types are used for the synthesis of most carbon backbone polymers. The first examples of "living" polymerizations were developed for ionic polymerizations (cationic and anionic). These reactions, which can be technically challenging to perform, can yield excellent control of molecular weight with very low polydispersity. The second reaction type, free radical polymerization, is one of the most widely used polymerizations for the commercial production of high molecular weight carbon backbone polymers. Nitroxide mediated polymerization (NMP), reversible addition-fragmentation chain transfer polymerization (RAFT), and atom transfer radical polymerization (ATRP) have emerged as three of the more successful approaches for controlling these reactions. The third type, transition metal mediated coordination polymerization, is the most important method for large-scale commercial polyolefin production. Simple nonfunctional hydrocarbon polymers such as polyethylene (PE), polypropylene, poly-α-olefins, and their copolymers are synthesized by high pressure-high temperature free radical polymerization, Ziegler-Natta or metallocene catalysts. Although these catalysts of exceptional efficiency that produce polymers on a huge scale are in common use, control that approaches a "living polymerization" is rare. Although the controlled synthesis of linear "polyethylene" described in this Account is not competitive with existing commercial processes for bulk polymer production, they can provide quantities of specialized materials for the study of structure-property relationships. This information can guide the production of polymers for new commercial applications. We initiated a search for novel polymerization reactions that would produce simple hydrocarbon polymers with the potential for molecular weight and topological control. Our research focused on polymerization reactions that employ nonolefin monomers, more specifically the polymerization of ylides and diazoalkanes. In this reaction, the carbon backbone is built one carbon at a time (C1 polymerization). These studies draw upon earlier investigations of the Lewis acid catalyzed polymerization of diazoalkanes and build upon our discovery of the trialkylborane initiated living polymerization of dimethylsulfoxonium methylide 1.

Hydroformylation of vinyl acetate (VAM) has been studied as a key step in the alternative route for the synthesis of 1,2-propanediol (1,2-PDO) and 1,3-propanedol (1,3-PDO) using homogeneous rhodium (Rh) and cobalt (Co) complex catalysts. The feasibility of the VAM hydroformylation route has been demonstrated, and a detailed study has been reported on the key hydroformylation step using homogeneous Rh and Co catalysts. The roles of the catalyst precursors, ligands, and solvents in the activity and regioselectivity of the aldehyde products, i.e., 2-acetoxy propanal (2-ACPAL) and 3-acetoxy propanal (3-ACPAL), and the effect of reaction conditions have been investigated. With Rh−phosphine catalysts, 2-ACPAL is obtained with a selectivity of >90%, while with cobalt carbonyl catalyst, 2-ACPAL and 3-ACPAL are formed with comparable selectivities (∼50% each) thus substantially improving the selectivity of the linear aldehyde, a precursor for 1,3-PDO. In halogenated solvents with cobalt carbonyl catalyst, the selectivity to 3-ACPAL was found to increase still further (58%). A possible mechanism to explain the variation in regioselectivity for the Rh and Co catalysts has been discussed. In the presence of pyridine as a ligand in the Co-catalyzed hydroformylation of VAM, the rate of reaction was found to be enhanced 4-fold. The hydrogenation of acetoxypropanal isomers using Raney-Ni catalyst followed by hydrolysis using Amberlite IR-120 resin catalyst gave quantitative conversion to the mixture of 1,2- and 1,3-PDOs (>90% yield).

This review describes recent progresses in the selective reduction of aromatic nitro-groups to amines catalyzed by homogeneous ruthenium or rhodium catalysts under CO/H2O conditions. These catalytic reactions are useful from both synthetic and industrial viewpoints, because the after-treatment of by-products can be simplified in comparison to the conventional methods, since the reaction proceeds with extremely high selectivity. Efficient and convenient palladium–phosphine catalysts for hydrocarbonylation of chlorobenzenes affording benzoic acid derivatives in high yields are also described.

1. A study was made of the activity and selectivity of action of some rhodium catalysts, representing heterogeneous and homogeneous complexes with N-phenylanthranilic acid and L-tyrosine. 2. The direction of hydrogen addition to a system of conjugated double bonds, the ratio in the rates of the reaction for the isomerization and hydrogenation of the C=C bond of the studied olefins, and also the selectivity of the process, are all independent of the phase condition of the catalyst.

The reasons for industrial interest in metal clusters and cluster catalysis are described. These may be summarized as attempting to bridge the gap between homogeneous and heterogeneous catalysis, specifically by combining the high selectivities typical of the former with the high activities associated with the latter. Progress towards the realization of this objective is illustrated by using examples of our work in both areas. Thus, some results from an investigation of homogeneous ruthenium and rhodium catalysts (as both separate components and mixtures) for the synthesis of oxygenated products from CO-H2are summarized and correlated with high-pressure infrared spectroscopic measurements. The cluster anion [Rh5(CO)15] - is shown from spectroscopic evidence to be very closely related to the catalytically active species in the rhodium-catalysed reactions. Also, the distinction between the behaviour of supported catalysts derived from Group VIII metal cluster compounds and those obtained by more conventional methods of heterogeneous catalyst preparation is discussed. For example, in the case of ruthenium, cluster-derived catalysts are shown to display greatly enhanced activity for the complete hydrogenolysis of straight-chain aliphatic hydrocarbons to methane and provide a temperature advantage of 150°C relative to conventionally prepared ruthenium catalysts, where only moderate hydrocarbon conversions are noted. The increased activity superficially correlates with the smaller metal crystallite sizes (15-20 A; 1.5-2.0 nm) reproducibly obtainable with metal cluster compounds as catalyst precursors.

The use of copper and rhodium catalysts separately and in combination directs reactions between vinyldiazoacetates 3 and cinnamaldehydes 2 to from formal [4+3]-cycloaddition (epoxidation followed by Cope rearrangement), intramolecular cyclopropanation, and Mukaiyama-aldol reactions selectively and in high yield.

Good levels of enantioselectivity have been achieved in intramolecular C–H insertion reactions of α-diazocarbonyl compounds leading to six-membered oxygen heterocycles (chromanones) through the use of chiral rhodium(II) carboxylates as catalysts. Competition between C–H insertion and sigmatropic rearrangement, the latter leading to five-membered oxygen heterocycles (furanones), was observed with precursors containing a proximal O-allyl side chain. Whereas rhodium carboxylates produced C-H insertion products predominantly, a copper catalyst produced sigmatropic rearrangement products exclusively. A precursor with an S-allyl side chain exhibited cyclisation via sigmatropic rearrangement with both copper and rhodium catalysts.

A family of polymer-attached phenanthrolines was prepared from solvothermal copolymerization of divinylbenzene with N-(1,10-phenanthroline-5-yl)acrylamide in different ratios. The polymer-supported copper catalysts were obtained through typical impregnation with copper(II) salts. The polymers and supported copper catalysts have been characterized by N2 adsortion, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and thermogravimetric analysis (TG); they exhibit a high surface area, hierarchical porosity, large pore volume, and high thermal and chemical stabilities. The copper catalyst has proved to be highly active for Glaser homocoupling of alkynes and Huisgen 1,3-diolar cycloaddition of alkynes with benzyl azide under mild conditions at low catalyst loading. The heterogeneous copper catalyst is more active than commonly used homogeneous and nonporous polystyrene-supported copper catalysts. In particular, the catalyst is easily recovered and can be recycled at least ten times without any obvious loss in catalytic activity. Metal leaching was prevented due to the strong binding ability of phenanthroline and products were not contaminated with copper, as determined by ICP analysis.

Homogeneous hydrogenation and hydrosilylation of the 2‐methylquinoxaline (I) is achieved in the presence of the rhodium catalyst (II), producing the methyltetrahydroquinoxaline (III) and the bissilyl derivative (V) which is desilylated, directly forming (III).

Asymmetric hydrogenations of dehydroaminoacids with homogeneous rhodium catalysts are reviewed. High enantiomeric excess up to 100% is achieved with rhodium catalysts including chiral ligands such as phosphines, phosphinittes, and aminophosphines.

Isomerization of vinylcyclopropanes by a homogeneous rhodium catalyst

The Huisgen cycloaddition, often referred to as 1,3-Dipolar cycloaddition, is a well-established method for synthesizing 1,4-disubstituted triazoles. Originally conducted under thermal conditions [3+2] cycloaddition reactions were limited by temperature, prolonged reaction time, and regioselectivity. The introduction of copper catalyzed azide-alkyne cycloaddition (CuAAC) revitalized interest, giving rise to the concept of "click chemistry". The CuAAC has emerged as a prominent method for producing 1,2,3-triazole with excellent yields and exceptional regioselectivity even in unfavorable conditions. Copper catalysts conventionally facilitate azide-alkyne cycloadditions, but challenges include instability and recycling issues. In recent years, there has been a growing demand for heterogeneous and porous catalysts in various chemical reactions. Chemists have been more interested in heterogenous catalysts as a result of the difficulties in separating homogenous catalysts from reaction products. These catalysts are favored for their abundant active sites, extensive surface area, easy separation from reaction mixtures, and the ability to be reused. Heterogeneous catalysts have garnered significant attention due to their broad industrial utility, characterized by cost-effectiveness, stability, resistance to thermal degradation, and ease of removal compared to their homogeneous counterparts. The present review covers recent advancements from year 2018 to 2023 in the field of click reactions for obtaining 1,2,3-triazoles through Cu catalyzed 1,3-dipolar azide-alkyne cycloaddition and the properties of the catalyst, reaction conditions such as solvent, temperature, reaction time, and the impact of different heterogeneous copper catalysts on product yield.

In this study, a new, efficient and inexpensive strategy has been developed based on the heterogeneous copper catalyst for the synthesis of different biaryl compounds via the Suzuki–Miyaura cross‐coupling reaction. In this respect, the core@shell modified Fe 3 O 4 @SiO 2 magnetic nanoparticles (MNPs) has been prepared by immobilization of aminoguanidine and then the copper (II) ion into the surface of magnetic nanoparticles. After full characterization of the prepared catalyst by various technique such as TEM, SEM, EDX, TGA, XRD, VSM, ICP and FT‐IR, its catalytic activity has been investigated in the synthesis of different biarly compounds through the Suzuki‐Miyaura cross‐coupling reaction. Interestingly, similar to Pd‐based catalysts, this new heterogeneous Cu‐based catalyst can catalyze the Suzuki–Miyaura reaction in solvent free condition with good to excellent yields. Besides, this heterogeneous nano copper catalyst was able to separate from the reaction mixture by easy separation procedures and can reuse more than six runs.

The polymerization of nitrogen-substituted vinyl monomers by halogens and their compounds has been the subject of active research over the last 12 years. Several interesting features including, in particular, the development of the concepts of charge transfer polymerization [1, 2] have resulted from such studies. This article is intended to highlight the significant developments to date in the field of polymerization of nitrogen vinyl monomers by halogens and halogen-containing electron transfer agents with relevant reference to earlier work in these fields [1, 3]. In addition, the controversial developments in polymerization of these monomers by hydrogen halides and other organic halides will also be discussed. However, the general features of polymerization of these monomers by metal halides, recently reviewed by Biswas and Chakravorty [4], will not be included in the discussion.