Metal hydride initiators

In anionic polymerization the reaction is initiated by a strong base, eg, a metal hydride, alkah metal alkoxide, organometaHic compounds, or hydroxides, to form a lactamate ... 

Anhydrous HX are versatile and vigorous reagents for the halogenation of metals, non-metals, hydrides, oxides and many other classes of compound, though reactions that are thermodynamically permissible do not always occur in the absence of catalysts, thermal initiation or photolytic encouragement, because... 

Nickel-Hydrogen, Nickel-Iron, and Nickel-Metal Hydride. First developed for communication satellites in the early 1970s, nickel-hydrogen batteries are durable, require low maintenance, and have a long life expectancy. The major disadvantage is the high initial cost. For these batteries to be a viable option for electric vehicles, mass production techniques will have to be developed to reduce the cost. 

In normal battery operation several electrochemical reactions occur on the nickel hydroxide electrode. These are the redox reactions of the active material, oxygen evolution, and in the case of nickel-hydrogen and nickel-metal hydride batteries, hydrogen oxidation. In addition there are parasitic reactions such as the corrosion of nickel current collector materials and the oxidation of organic materials from separators. The initial reaction in the corrosion process is the conversion of Ni to Ni(OH)2. 

The mechanism of the poisoning effect of nickel or palladium (and other metal) hydrides may be explained, generally, in terms of the electronic theory of catalysis on transition metals. Hydrogen when forming a hydride phase fills the empty energy levels in the nickel or palladium (or alloys) d band with its Is electron. In consequence the initially d transition metal transforms into an s-p metal and loses its great ability to chemisorb and properly activate catalytically the reactants involved. 

After the initial two reports of Rh- and Co-catalyzed reductive aldol couplings, further studies did not appear in the literature until the late 1990s. Beyond 1998, several stereoselective and enantioselective reductive aldol reactions were developed, which are catalyzed by a remarkably diverse range of metal complexes, including those based upon Pd, Cu, Ir, and In. In this chapter, transition metal-catalyzed aldol, Michael, and Mannich reactions that proceed via transition metal hydride-promoted conjugate reduction are reviewed. 

It is also worth emphasizing that the initiation and termination steps are not included in the central chain process. For instance, in metal hydride-promoted domino reactions the initial halogen abstraction (or SePh displacement, etc.) and the final hydrogen abstraction from R MH are not classified as part of the domino sequence. More precisely, only the propagation steps within the mechanism of this process will be considered as a strict integral part of the domino reaction. 

High-power lithium-ion batteries are promising alternatives to the nickel metal hydride batteries which are currently used for energy storage in hybrid electric vehicles (HEVs). Currently, Li(Ni,Co)02-based materials are the most widely studied cathode materials for the high-power lithium-ion batteries [1-4]. Although Li(Ni,Co)02-based materials meet the initial power requirement for the HEY application, however, it has been reported that they... 

The reactions covered in Scheme 2 are initiated by protonation but a hydride could form on the metal as intermediate. In some instances, cationic metal hydrides have been shown to be actually involved. See, for example, the addition of [HNi (POEt)3 4+] to butadiene (54) or of [HNi(Ph3P)3(7r-C3H5)] to olefins (10c, Vol. II, p. 25). Thus the reaction of olefins or dienes with acids in the presence of zero-valent nickel may be considered proton-promoted as well as hydride-promoted. 

Hydrogenation of 1,3-dienes to terminal olefins is catalyzed by HRh(PPh3)4 and [Rh(CO)2(PPh3)2]2 in the presence of excess phosphine diene insertion into a metal- hydride bond to give a-alkenyl rather than 7r-allyl intermediates was postulated for the initial step (141). Mechanistic studies of the HRh(PPh3)4 catalyst (142) and a more reactive phosphole analog (143) HRh(DBP)4 [5-phenyl-5//-dibenzophosphole (DBP), 7] for... 

Osborn and Green s elegant results are instructive, but their relevance to metathesis must be qualified. Until actual catalytic activity with the respective complexes is demonstrated, it remains uncertain whether this chemistry indeed relates to olefin metathesis. With this qualification in mind, their work in concert is pioneering as it provides the initial experimental backing for a basic reaction wherein an olefin and a metal exclusively may produce the initiating carbene-metal complex by a simple sequence of 7r-complexation followed by a hydride shift, thus forming a 77-allyl-metal hydride entity which then rearranges into a metallocyclobutane via a nucleophilic attack of the hydride on the central atom of the 7r-allyl species ... 

These observations illustrate that there are two transformations open to metallocarboxylic acid intermediates reversible loss of OH" accompanied by oxygen exchange, and metal-hydride formation with expulsion of C02. Our entry into this area of chemistry was in 1975 when extensive studies of oxygen lability in metal carbonyl cations were initiated (10). These... 

To summarize briefly, our approach involves initial attack by a relatively nucleophilic metal hydride on coordinated CO. Such reactivity has been demonstrated repeatedly for main-group metal hydrides perhaps the most elegantly worked-out system involves CpRe(C0)2(N0)+ (Cp = Tl-C H ) which, under varying conditions, can be converted to an entire range of products containing CO at different stages of reduction, including formyl, carbene, hydroxymethyl and methyl species (Scheme l). Reactions lead-... 

The addition of an R-M moiety to the triple bond gives the corresponding vinylmetal intermediate 241, which is activated enough to react with the alkene moiety. Depending upon the nature of the R1 group, several options are open. In the case of an initial hydridometallation by a metal hydride, which is most often formed in situ through the oxidative addition to acetic acid (R-R1 = H-OAc), the resulting cyclization product 243 will liberate its metal component by... 

Many of these catalysts are derived from metal complexes which, initially, do not contain metal hydride bonds, but can give rise to intermediate MH2 (al-kene) species. These species, after migratory insertion of the hydride to the coordinated alkene and subsequent hydrogenolysis of the metal alkyl species, yield the saturated alkane. At first glance there are two possibilities to reach MH2 (alkene) intermediates which are related to the order of entry of the two reaction partners in the coordination sphere of the metal (Scheme 1.2). 

Metal-free initiators, 14 258-259 Metal fullerenes, 2 718-719 Metal-halogen exchange, in pyridine chemistry, 22 107-108 Metal hydrazides, 23 567 Metal hydrides, 23 611-613 amines by reduction, 2 493 hydrogen storage and, 23 851 nitriding, 27 206-207 storage of, 23 786... 

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