C–H activation has surfaced as an increasingly powerful tool for molecular sciences, with notable applications to material sciences, crop protection, drug discovery, and pharmaceutical industries, among others. Despite major advances, the vast majority of these C–H functionalizations required precious 4d or 5d transition metal catalysts. Given the cost-effective and sustainable nature of earth-abundant first row transition metals, the development of less toxic, inexpensive 3d metal catalysts for C–H activation has gained considerable recent momentum as a significantly more environmentally-benign and economically-attractive alternative. Herein, we provide a comprehensive overview on first row transition metal catalysts for C–H activation until summer 2018.

3d Transition Metals for C-H Activation.

Lewis Acids: From Conventional Homogeneous to Green Homogeneous and Heterogeneous Catalysis

Heteropolyanions are polymeric oxoanions which are formed by the condensation of more than two different oxoanions [Eq. (1)l. Polyanions consisting of one kind of oxoanion are called isopolyanions [Eq. (2)]. Acidic elements such as Mo, W, V, Nb, and Ta are present as oxoanions in aqueous solutions and polymerize to form polyanions at low pH. Free acids (or acid forms) of these species are called heteropoly and isopoly acids, respectively. Here, the term “heteropoly compounds” is used for heteropoly acids and their salts.

Heterogeneous catalysis by heteropoly compounds of molybdenum and tungsten

Publisher Summary This chapter discusses the scope of Ziegler catalysis; stereoselectivity, kinetics, and mechanism of Ziegler catalysis; homogeneous Ziegler–Natta catalysts; and side reactions in homogeneous catalysts. Ziegler catalysis involves rapid polymerization of ethylene and α-ole-fins with the aid of catalysts based on transition-element compounds, normally formed by the reaction of a transition-element halide or alkoxide or alkyl or aryl derivative with a main-group element alkyl or alkyl halide. There are thousands of patents involving every combination of pure or mixed main-group alkyls with transition-element compounds, each claiming advantages. The result of the early work led to the development of “second-generation” Ziegler catalysts. Polymers produced with unmodified Ziegler catalysts showed extremely high molecular weight and broad distribution, and in some cases, there was evidence for “living polymer.” All homogeneous catalyst systems for ethylene polymerization become heterogeneous when polyethylene is formed. On using vanadium-based homogeneous catalysts, polymers consisting of syndiotactic stereo blocks and stereo-irregular blocks are obtained. Very high stereoselectivity is observed for racemic 4-methyl- 1-hexene and racemic 3,7-dimethyl-1-octene, where the asymmetric carbon atom is in the α-position relative to the double bond. Stereoselectivity is caused by the chirality of the catalytically active center, and not by chiral atoms in the growing chain. It must be concluded from the results that reactions take place, which change the number of active sites present, due to the different behavior of the polymers in solution. Study of these new catalysts is intensive. After a short induction period, the activity of polymerization increases as a function of the monomer concentration.

Ziegler-Natta Catalysis

Living polymerization techniques can be used to achieve a high degree of control over polymer chain architecture. Examples of the type of polymers that can be synthesized include block copolymers, comb-shaped polymers, multiarmed polymers, ladder polymers, and cyclic polymers. This control of structure, in turn, results in polymers with widely diverse physical properties, even though they are made from readily available low-cost monomers.

https://science.sciencemag.org/content/sci/251/4996/887.full.pdf

Living Polymerization Methods

"Living" Coordination Polymerization of Propene Initiated by the Soluble V(acac)3-Al(C2H5)2Cl System

The main kinetic behavior of the slurry polymerization of propene with a MgCl2-supported TiCl4/C6H5COOC2H5 catalyst, activated by Al(C2H5)3, was studied, Examination of the dependence of the polymerization rate on temperature and concentrations of Al(C2H5)3 and of propene resulted in a Langmuir-Hinshelwood rate law with the number of polymerization centers dependent on time. The Polymerization rate as function of the polymerization temperature shows a maximum, which is compatible with the rate law. The analysis of the phenomenon of an optimum temperature gave 15 KJ. mol−1 and 36 KJ. Mol −1 for the activation energy of the rate determining step and the adsorption energy of Al(C2H5)3, respectively. Examination of the rapid decay of the polymerization rate showed that the main part of the decay is represented by a second order decay independent of the amount of polymer produced, which can be understood by a second order decay of surface sites by Al(C2H5)3. The number of active centers of the catalyst in gas phase polymerization was estimated applying the inhibition method with carbon monoxide. The results show a constant value for the propagation rate constant, Kp, during the second order rate decay. The observed polymerization kinetics strongly suggest the existence of two kinds of polymerization centers (isotactic and atactic).

Propene polymerization with a magnesium chloride‐supported ziegler catalyst, 1. Principal kinetics

A kinetic argument for a quasi-living polymerization of propene with a MgCl2-supported catalyst

“Living” polymerization is of great utility both in understanding the mechanism of chain propagation reactions and in synthesizing well-defined block copolymers. In this review we deal with the development of soluble Ziegler-Natta catalysts for the living coordination polymerization of ethylene and α-olefins and discuss the possibility of synthesis of new block copolymers.

Synthesis of “living” polyolefins with soluble Ziegler-Natta catalysts and application to block copolymerization

The kinetic features of the molecular weight distributions (MWDs) of polypropylene produced with TiCl4/MgCl4/C6H5COOC2H5/Al(C2H5)3 as catalyst are investigated as functions of the polymerization time and the concentrations of Al(C2H5)3, C6H5COOC2H5 and hydrogen. The MWDs are dependent upon the concentrations of Al(C2H5)3 and C6H5COOC2H5, and are correlated with a change of the isotactic index of the produced polymers. In contrast, both MWD and the number-average molecular weight Mn remain unchanged during the polymerization from 5 s to 3 h. When hydrogen is added as a chain transfer reagent, Mn decreases as anticipated but the polydispersity index (Mw/Mn) is independent of the hydrogen concentration. The MWDs obtained with this supported catalyst are rather narrow (3≤ Mw /Mn ≤8)and are represented by a log-normal form or Wesslau equation, which is different from the MWDs (6≤ Mw /Mn ≤20) obtained with the traditional TiCl3 catalysts, which are represented by the Tung equation. Broadening of the MWD obtained with heterogeneous catalysts is also discussed. The constancy of Mw/Mn, independent of the hydrogen concentration leads to the important conclusion that non-uniform surface sites on heterogeneous catalysts are responsible for the broadening of the MWD. A surface heterogeneity of the propagation rate constant, kp, is demonstrated on the basis of the kinetic data, concerning the effect of CO-inhibition on the polymerization rate. A surface distribution function of kp, capable of accounting for the broad MWDs, is presented and discussed in terms of a distribution of the activation energy for the propagation step.

Tominaga Keii

Papers

"Living" Coordination Polymerization of Propene Initiated by the Soluble V(acac)3-Al(C2H5)2Cl System

Propene polymerization with a magnesium chloride‐supported ziegler catalyst, 1. Principal kinetics

A kinetic argument for a quasi-living polymerization of propene with a MgCl2-supported catalyst

Synthesis of “living” polyolefins with soluble Ziegler-Natta catalysts and application to block copolymerization

Propene polymerization with a magnesium chloride‐supported Ziegler catalyst, 2. Molecular weight distribution