Olefin transition-metal complexes Subject

Electron-rich olefins are nucleophilic and therefore subject to thermal cleavage by various electrophilic transition metal complexes. As the formation of tetraaminoethylenes, i.e., enetetramines, is possible by different methods, various precursors to imidazolidin-2-ylidene complexes are readily available. " Dimerization of nonstable NHCs such as imidazolidin-2-ylidenes is one of the routes used to obtain these electron-rich olefins [Eq. (29)]. The existence of an equilibrium between free NHC monomers and the olefinic dimer was proven only recently for benzimidazolin-2-ylidenes. In addition to the previously mentioned methods it is possible to deprotonate imidazolidinium salts with Grignard reagents in order to prepare tetraaminoethylenes. " The isolation of stable imidazolidin-2-ylidenes was achieved by deprotonation of the imidazolidinium salt with potassium hydride in THF. ... 

The chemistry of //// //-metallocene compounds has been the subject of several reviews. Structural aspects affecting the catalytic activity and the application of these complexes as catalysts for the homo- and co-polymerization of olefins have been considered.323 The evolution of the //// //-bridge complexes in terms of the various synthetic approaches used to construct the bridged ligand framework, the variety of bridges introduced, and the effect of the bridge on the structure and reactivity of ////.y//-titanocene and other transition metal complexes as compared with their unbridged counterparts has been reviewed.1634... 

A more detailed consideration of the Woodward-Hofimann postu-ulates for olefinic systems in the presence of a transition metal indicates that the thermally forbidden dimerization of two ethylene molecules to cyclobutane becomes allowed if the orbitals of the olefins can interact symmetrically with the dxt and dyz orbitals of the transition metal catalyst (53). One would consequently also expect transition metal complexes to catalyze the conversion of quadricyclene (IV) back to norbornadiene. This has been reported to be the case (54). The reactions leading to the formation of VI, XXX, and XXXI are examples of processes in which thermally allowed sigmatropic reactions become subject to catalysis by transition metal complexes. The catalysts thus display the dual role of removing symmetry restrictions and of generally lowering activation energies. 

The sheer size and value of the polyethylene industry ensure that there is continued research, progress, and development in catalysis, for their potential commercial impact. Although this whole subject is not within the scope of this chapter, we mention a couple of aspects of the progress, which offer the potential to impact this industry. In 1995, DuPont introduced work, carried out with them at the University of North Carolina—via the largest patent applicafion ever in the USA. They disclosed what are described as post-metallocene catalysts. These are transition and late transition metal complexes with di-imine ligands, which form part of the DuPont Versipol technology. Such catalysts create highly branched to exceptionally linear ethylene homopolymers and linear alpha-olefins. Late transition metals offer not only the potential for the incorporation of polar comonomers, which until now has only been possible in LDPE reactors, but also their controlled sequence distribution, compared to the random composition of free radical LDPE copolymers. Such copolymers account for over 1 million tons per annum [20]. Versipol has so far only been cross-licensed and used commercially by DuPont Dow Elastomers (a former joint venture, now dissolved) in an EPDM plant. 

Interestingly, the first NHC complexes were reported with chromium (0) carbonyl by Ofele in 1968. Relatively few NHC early-transition metal complexes were then reported in the 1990s and this number steadily increased over the past decade. This subject is now mature moreover, the coordination chemistry of NHC has been investigated with alkali metals, alkaline earth metals, lanthanides or group 13-15 metals. Applications of these NHC complexes in catalysis now include, most notably, olefin polymerization or ring-opening polymerization of cyclic esters. Some of these complexes display high activity and selectivity and, in some instances, may compete with the best systems in the field. 

One of the most defining characteristics of the late metal a-diimine polymerization systems is the uniquely branched polyolefins that they afford. This arises from facile p-hydride elimination that late transition metal alkyl complexes undergo. The characteristics of the isomerization process have been the subject of much investigation, particularly with the more easily studied Pd(II) a-diimine system. The process is initiated by P-hydride elimination from the unsaturated alkyl agostic complex 1.17, followed by hydride reinsertion into olefin hydride intermediate 1.18 in a non-regioselective manner (Scheme 5). In doing so, the metal center may migrate... 

Metathesis, which is reversible and can be catalyzed by a variety of organometallic complexes, has been the subject of considerable investigation, and many reviews on this topic have been published.In 1970, Herisson and Chauvin proposed that these reactions are catalyzed by carbene (alkylidene) complexes that react with alkenes via the formation of metallacyclobutane intermediates, as shown in Figure 14-20. This mechanism, now known as the Chauvin mechanism, has received considerable support and is believed to be the pathway of the majority of transition metal-catalyzed olefin metathesis reactions. 

During the past few decades, a wide variety of molecules with transition metal-carhon mulhple bonds have been studied. The chemistry of doubly bonded species - carbenes - is particularly interesting because it leads to several synthetically important transformations, and for this reason, metal carbenes are the main subject of this chapter. Our discussion begins with a classification of metal-carbene complexes based on electronic structure, which provides a way to understand their reactivity patterns. Next, we summarize the mechanistic highlights of three metal-carbene-mediated reactions carbonyl olefinafion, olefin cyclopropanafion, and olefin metathesis. Throughout the second half of the chapter, we focus mainly on ruthenium-carbene olefin metathesis catalysts, in part because of widespread interest in the applications of these catalysts, and in part because of our expertise in this area. We conclude with some perspectives on the chemistry of metal carbenes and on future developments in catalysis. 

Various unsaturated compounds can be inserted into the metal alkyl, aryl, and alkenyl complexes to give new organometallic complexes having various functional groups. The insertions of carbon monoxide (CO) and isocyanide (CNR) into transition metal-carbon a-bond are particularly important processes, since a carbon unit can be increased in the process and the acyl type complexes formed by the insertion processes can be subjected to further transformations to synthesize useful organic compounds. For example, the CO inserhon constitutes a fundamental step in industrially important processes such as hydroformylation of olefins, acetic acid synthesis from methanol and CO, Fischer-Tropsch process, amidocarbonylation, olefin and CO copolymerizahon processes as well as in a variety of laboratory syntheses of carbonyl containing compounds. 

The oxidative functionalization of olefins mediated by transition metal oxides leads to a variety of products including epoxides, 1,2-diols, 1,2-aminoalcohols, and 1,2-diamines [1]. Also the formation of tetrahydrofurans (THF) from 1,5-dienes has been observed, and enantioselective versions of the different reactions have been developed. Although a lot of experimental data has been available, the reaction mechanisms have been a subject of controversial discussion. Especially, osmium (VIII) complexes play an important role there, as the proposal of a stepwise mechanism [2] for the dihydroxylation (DH) of olefins by osmium tetroxide (OSO4) had started an intense discussion about the mechanism [2—11],... 

The addition of HCN to olefins catalyzed by complexes of transition metals has been studied since about 1950. The first hydrocyanation by a homogeneous catalyst was reported by Arthur with cobalt carbonyl as catalyst. These reactions gave the branched nitrile as the predominant product. Nickel complexes of phosphites are more active catalysts for hydrocyanation, and these catalysts give the anti-Markovnikov product with terminal alkenes. The first nickel-catalyzed hydrocyanations were disclosed by Drinkard and by Brown and Rick. The development of this nickel-catalyzed chemistry into the commercially important addition to butadiene (Equation 16.3) was conducted at DuPont. Taylor and Swift referred to hydrocyanation of butadiene, and Drinkard exploited this chemistry for the synthesis of adiponitrile. The mechanism of ftiis process was pursued in depth by Tolman. As a result of this work, butadiene hydrocyanation was commercialized in 1971. The development of hydrocyanation is one of tfie early success stories in homogeneous catalysis. Significant improvements in catalysts have been made since that time, and many reviews have now been written on this subject. ... 

Cyclopentadiene [542-92-7] (CPD), CsHe, (1), and its more stable dimer, dicy-clopentadiene [77-73-6] (DCPD), C10H12, (2), are the major constituents of hydrocarbon resins, cyclic olefin polymers, and a host of specialty chemicals. They can be transformed into many chemical intermediates used in the production of pharmaceuticals, pesticides, perfumes, flame retardants, and antioxidants. Because of their wide industrial uses, their chemistry has been extensively investigated and documented. Numerous reviews (1-12) have been published on the subject. The production processes and industrial uses of CPD and DCPD are summarized in Reference 13. In addition to the classical organic reactions, CPD forms organic metallic complexes, ferrocene, with transition metals (14). Some of these complexes have been established as excellent olefin polymerization catalysts. Several reviews have been published on this rapid growing field (15-19) (see Single-Site Catalysts). 

---