Metal complex catalyzed organic reactions

These characteristics of thiolate ligands means they are ideally suited for applications where two or more metal centers are to be brought together in a well-defined way. Such is the case in a number of metal complex catalyzed organic reactions where organometallic reagents are employed for C—C cou-... 

Although the utiHty of rare earth metal complexes as Lewis acid catalysts in organic synthesis has received much attention, only a limited number of investigations has been reported on isolable chiral rare earth metal complex-catalyzed asymmetric reactions [ 11,27,28,29,30]. 

Cyanoacrylate adhesives cure by anionic polymerization. This reaction is catalyzed by weak bases (such as water), so the adhesives are generally stabilized by the inclusion of a weak acid in the formulation. While adhesion of cyanoacrylates to bare metals and many polymers is excellent, bonding to polyolefins requires a surface modifying primer. Solutions of chlorinated polyolefin oligomers, fran-sition metal complexes, and organic bases such as tertiary amines can greatly enhance cyanoacrylate adhesion to these surfaces [72]. The solvent is a critical component of these primers, as solvent swelling of the surface facilitates inter-... 

Precious metals have faced a significant price increase and the fear of depletion. By contrast, iron is a highly abundant metal in the crust of the earth (4.7 wt%) of low toxicity and price. Thus, it can be defined as an environmentally friendly material. Therefore, iron complexes have been studied intensively as an alternative for precious-metal catalysts within recent years (for reviews of iron-catalyzed organic reactions, see [12-20]). The chemistry of iron complexes continues to expand rapidly because these catalysts play indispensable roles in today s academic study as well as chemical industry. 

Transition metal complex-catalyzed carbon-nitrogen bond formations have been developed as fundamentally important reactions. This chapter highlights the allylic amination and its asymmetric version as well as all other possible aminations such as crosscoupling reactions, oxidative addition-/3-elimination, and hydroamination, except for nitrene reactions. This chapter has been organized according to the different types of reactions and references to literature from 1993 to 2004 have been used. 

Organic reactions are extensively catalyzed by metal complexes. The catalytic cycle of a metal complex-catalyzed reaction is illustrated in Scheme 13. 

A second basic interaction pathway between transition metal complexes and organic substrates is SET (Path B). The overall processes can involve one individual or several sequential SET steps. For the latter, timing and direction of SET steps determine the reaction outcome significantly. The catalyzed reaction can proceed either as redox-neutral processes, in which oxidative and reductive SET steps are involved in the catalytic cycle, or as overall oxidative or reductive catalytic reactions, where two oxidative or reductive SET steps occur consecutively in the catalytic cycle. The third pathway (Path C) consists of a direct atom or group abstraction by the metal complex, which is possible for a weak R-X bond. 

The facility with which metal complexes bring about reactions 8.16 and 8.17 depends on several factors, one of the important ones being the half-cell potential (E°) of the M"+/M(n+1)+ couple. It should be remembered, however, that most E° values for metal ions have been measured in an aqueous environment. On complexation and in an organic liquid these values are expected to change substantially. The initial hydroperoxide required for metal-catalyzed decomposition, reactions 8.16 and 8.17, is normally present in trace quantities in most hydrocarbons. 

CH Activation is sometimes used rather too loosely to cover a wide variety of situations in which CH bonds are broken. As Sames has most recently pointed out, the term was first adopted to make a distinction between organic reactions in which CH bonds are broken by classical mechanistic pathways, and the class of reactions involving transition metals that avoid these pathways and their consequences in terms of reaction selectivity. For example, radicals such as RO- and -OH readily abstract an H atom from alkanes, RH, to give the alkyl radical R. Also in this class, are some of the metal catalyzed oxidations, such as the Gif reaction and Fenton chemistry see Oxidation Catalysis by Transition Metal Complexes). Since this reaction tends to occur at the weakest CH bond, the most highly substituted R tends to be formed, for example, iPr-and not nPn from propane. Likewise, electrophilic reagents such as superacids see Superacid), readily abstract a H ion from an alkane. The selectivity is even more strongly in favor of the more substituted carbonium ion product such as iPr+ and not nPr+ from propane. The result is that in any subsequent fimctionalization, the branched product is obtained, for example, iPrX and not nPrX (Scheme 1). 

Organic synthesis via transition metal complex-catalyzed electrochemical and photochemical reduction of CO2 has been developed [2,122b, 145-147]. Among transition metal complexes, ruthenium bipyridine complexes show high catalytic activity a typical reaction is shown in Eq. 11.79. [Ru(bpy)2(CO)2] and [Ru(bpy)2(CO)Cl] efficiently catalyze the electrochemical reduction of CO2 to CO and HC02. The nature of the products is dependent upon the pH of the solution. A catalytic cycle involving [Ru(bpy)2(CO)]°, ]Ru(bpy)2(C0)(C02 )] and [Ru(bpy)2(C0)C02H] was proposed (Eq. 11.79) [1461]. 

Lewis acids act as electron pair acceptors. The proton is an important special case, but many other compounds catalyze organic reactions by acting as electron pair acceptors. The most important Lewis acids in organic reactions are metal cations and covalent compounds of metals. Metal cations that function as Lewis acids include the alkali metal monocations Li+, Na+, K+, di- and trivalent ions such as Mg +, Ca, Zn +, Sc, and Bi + transition metal cations and complexes and lanthanide cations, such as Ce + and Yb. Neutral electrophilic covalent molecules can also act as Lewis acids. The most commonly employed of the covalent compounds include boron trifluoride, aluminum trichloride, titanium tetrachloride, and tin(IV)tetrachloride. Various other derivatives of boron, aluminum, titanium, and tin also are Lewis acid catalysts. 

Collectively, these studies illustrate the broad range of possible applications of SCFs in metal-mediated organic synthesis, and the challenge is now to make efficient use of these methodologies. Investigations towards the understanding of coordination chemistry in SCFs (Chapter 4.2) will stimulate the elucidation of metal-complex-catalyzed reactions. However, it should be evident from this chapter that Ae field of complex-catalyzed SFRs is far from being complete, and much remains to be done. The potential of the technique has been hinted at, but many new ways to exploit the special properties of SCFs in metal complex catalysis are still to be discovered. 

Because various important industrial organic processes utilize olefins, convenient methods to convert olefins into various products are vital. Transition metal catalysts with proper ligands have proved most useful in controlling the course of these reactions. Transition metal complexes catalyze skeletal isomerization, double bond isomerization, polymerization, and other processes. Insertion of a terminal olefin into a transition metal hydride bond by 1,2-inserfion or... 

Arylboronic and alkenylboronic acids undergo transition metal complex-catalyzed synthetic organic reactions such as cross-coupling with organic halides [58-60], 1,4-addition to a,/I-unsaturated ketones [61-63], and ring-opening addition to vinyl oxirane [64]. Scheme 5.8 depicts the mechanism proposed for the... 

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