Hydrides families

Recent mechanistic studies on transition metal-catalysed hydrogen transfer reactions have been reviewed. Experimental and theoretical studies showed that hydrogen transfer reactions proceed through different pathways. For transition metals, hydridic routes are the most common. Within the hydridic family there are two main groups the monohydride and dihydride routes. Experimentally, it was found that whereas rhodium and iridium catalysts favour the monohydride route, the mechanism for ruthenium catalysts proceeds by either pathway, depending on the ligands. A direct hydrogen transfer mechanism has been proposed for Meerwein-Ponndorf-Verley (MPV) reductions.352... 

At first, different hydride families will be briefly presented together with the currently adopted synthetic strategies, the most useful characterisation methods, and their reactivity. As the use of these complexes in stoichiometric processes is aprioristically forbidden by their very high cost, only their catalytic applications will be, in a second part, reviewed. Finally, in the third part of the review, some of the more important and characteristic features of the organolanthanide hydrides will be summarised. 

Storage as Hydrides. The discovery of metal compounds that reversibly absorb hydrogen is relatively recent. In the 1970s, the AB and AB family of alloys, which reversibly absorb hydrogen at room temperature and low pressure, were identified (205). Both A and B are metals. As of this writing many such compounds are known LaNi and TiFe are examples. 

One family of porphyrin complexes that will be treated in the review, even though they do not contain metal-carbon bonds, are metalloporphyrin hydride and dihydrogen complexes. As in classical organometallic chemistry, hydride complexes play key roles in some reactions involving porphyrins, and the discovery of dihydrogen complexes and their relationship to metal hydrides has been an important advance in the last decade. 

Lithium aluminum hydride is an exceptionally powerful reducing agent, whereas sodium borohydride is a mild one. We undertook to develop a family of reducing agents with capabilities between these extremes (27-30). 

One very fascinating domino reaction is the fivefold anionic/pericydic sequence developed by Heathcockand coworkers for the total synthesis of alkaloids of the Daphniphyllum family [351], of which one example was presented in the Introduction. Another example is the synthesis of secodaphniphylline (2-692) [352]. As depicted in Scheme 2.154, a twofold condensation of methylamine with the dialdehyde 2-686 led to the formation of the dihydropyridinium ion 2-687 which underwent an intramolecular hetero- Diels-Alder reaction to give the unsaturated iminium ion 2-688. This cydized, providing carbocation 2-689. Subsequent 1,5-hydride shift afforded the iminium ion 2-690 which, upon aqueous work-up, is hydrolyzed to give the final product 2-691 in a remarkable yield of about 75 %. In a similar way, dihydrosqualene dialdehyde was transformed into the corresponding polycyclic compound [353]. 

Plasma Synthesis The use of plasma methods has lead to a new range of materials having unique properties. An example is the family of amorphous elemental hydrides (eg cr-C H Of -Si H or-P H) which contain a variable proportion of H from almost zero to 50 atomic %. The carbon films, known variously as "hard carbon", "diamond-like carbon", " a-carbon" etc (9 ) - These layers are of considerable interest because of their optical and abrasion-resistant properties etc (Table I). The properties of these Gr-carbon films, can be tailored by modifying the plasma parameters. 

Main-group elements X such as monovalent F, divalent O, and trivalent N are expected to form families of transition-metal compounds MX (M—F fluorides, M=0 oxides, M=N nitrides) that are analogous to the corresponding p-block compounds. In this section we wish to compare the geometries and NBO descriptors of transition-metal halides, oxides, and nitrides briefly with the isovalent hydrocarbon species (that is, we compare fluorides with hydrides or alkyls, oxides with alkylidenes, and nitrides with alkylidynes). However, these substitutions also bring in other important electronic variations whose effects will now be considered. 

A number of hydrides which carry chiral groups have been prepared with the aim of using them in asymmetric synthesis.284 One family of these hydrides carries the (lR,2S,5R)-menthyl (Men ) group,431 as in, for example, Men Me2SnH,432 Men 2MeSnH,433 Men Ph2SnH, and Men 2PhSnH,431 and a second family is based on the chirality of the 1,1 -binaphthyl derivatives.172-174... 

A short description of these different hydride categories will be given here below. However, before going any further, it may be useful to quote from Greenwood and Earnshaw (1997) a few remarks which may be relevant also when considering the description and the classification of other families of compounds. Greenwood and Earnshaw underlined that one criterion usually adopted for grouping the binary... 

A summary of the various families of hydrides is reported in the following. 

Probably the most extensively studied enzymes are those from alcohol dehydrogenase family. One enzyme from this series which has been thoroughly examined both experimentally and theoretically is liver alcohol dehydrogenase (LADH). It catalyzes the reversible conversion of an alcohol to an aldehyde by transferring hydride from substrate to the cofactor (NAD+) ... 

One of palladiums unique characteristics is its abihty to absorb 900 times its own volume of hydrogen gas. When the surface of the pure metal is exposed to hydrogen gas (H ), the gas molecules break into atomic hydrogen. These hydrogen atoms then seep into the holes in the crystal structure of the metal. The result is a metallic hydride (PdH that changes palladium from an electrical conductor to a semiconductor. The compound palladium dichloride (PdCl ) also has the ability to absorb large quantities of carbon monoxide (CO). These characteristics are useful for many commercial applications. Palladium is the most reactive of all the platinum family of elements (Ru, Rh, Pd, Os, Is, and Pt.)... 

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