Transition metal hydrides carbonyl type

To the best of our knowledge, this is the first report of a dealkylation of tliis type mediated by a transition metal cluster carbonyl. The formation of this hydride could explain in part the large amount of amine 57 formed during the catalytic reactions. This compound has in fact been considered a key intermediate in the catalytic hydrogenation of nitrobenzene to aniline [97]. However, as has been discussed in Chapters 4 and 6, the participation of hydride derivatives in this type of reduction of the nitro to amino group is doubtful. 

Reductive aldol reaction of a,(5-unsaturated esters and enones with aldehyde mediated by a transition metal hydride complex and a hydride source, such as hydrosilane, is a versatile process to produce p-hydroxy carbonyl compounds (Scheme 15a) [21]. This reaction is thought to be an alternative transformation of Lewis acid-catalyzed Mukaiyama-type aldol reaction with silyl enol ethers or silyl ketene acetals (Scheme 15b). 

Next to the cyclopropane formation, elimination represents the simplest type of a carbon-carbon bond formation in the homoenolates. Transition metal homoenolates readily eliminate a metal hydride unit to give a,p-unsaturated carbonyl compounds. Treatment of a mercurio ketone with palladium (II) chloride results in the formation of the enone presumably via a 3-palladio ketone (Eq. (24), Table 3) [8], The reaction can be carried out with catalytic amounts of palladium (II) by using CuCl2 as an oxidant. Isomerization of the initial exomethylene derivative to the more stable endo-olefin can efficiently be retarded by addition of triethylamine to the reaction mixture. 

Since these important findings near the end of the last century, metal carbonyls of a majority of the transition metals have been isolated and characterized. Many of these compounds are pure metal carbonyls, containing only the metal and CO. Many other types are also known, including metal carbonyl-halides, -hydrides, -anions, -7r-cyclopentadienyls, etc. A number of earlier reviews dealing primarily with metal carbonyls has been published 169, 197). Fortunately, there are also two recent reviews on the subject by Abel ) and Hileman ISO). A useful, recent... 

The terminal hydride (1) is the most common type. In these species, the M-H distance is close to the sum of the Covalent Radii. Neutron diffraction gives this distance reliably, but X-ray methods, because they detect electron density and not the nuclear positions, underestimate the M-H distance by about 0.1 A. It is not uncommon for the H to escape detection entirely by X-ray methods as a result of the proximity of the metal, always a strong diffractor of X rays. Typical M-H distances are 1.45 to 1.6 A for first-row elements, 1.6 to 1.7 A for the second row, and 1.65 to 1.75 A for the third row actinides have M-H distances near 2 A. Early transition metals have M-H distances about 0.1A longer than do late metals. Electron diffraction methods have been used for volatile carbonyl hydrides such as HMn(CO)s, HCo(CO)4, and H2Fe(CO)4 for the Mn compound, the e-diffraction and n-diffraction Mn-H bond lengths are in good agreement. 

The transition metals can be reduced in basic aqueous solutions via other mechanism. For example, the metal carbonyls could be attacked by the hydroxide ions and the metal reduced to metal hydride species by the elimination of carbon dioxide to yield a hydride, which could then be deprotonated with the excess of hydroxide ions (Figure 17). The reduction of the metal complexes by CO in aqueous phase is indeed a very important step in the Reppe-type catalysis and water-gas shift reactions. 

Hydroformylation catalysts typically consist of a transition-metal atom (M) to form metal carbonyl hydride species. In some cases, these complexes are modified by additional ligands (L). The general structure is H cM3,(CO)zL . This type of active hydroformylation species may be generated by precursors of different composition. Since the catalytic activity and selectivity are closely related to the metal atom and ligands, the improvement of catalyst performance can mainly be achieved by the variation of the center atom or the modification of the ligands [44,45]. 

Nucleophiles used in the seminal papers by Tsuji and co-workers were mostly stabilized carbon nucleophiles, and the method found an early synthetic application in a preparation of steroids." It soon became evident that many other types of nucleophiles could be used. In particular, hydride ion equivalents led to l-olefinsf ° " (see Sect. V.2.3.1), Silyl and stannyl enolates of simple ketones and aldehydes and esters can be aUylated, as well as allyl enol carbonates (see Sect. V.2.1.4), This is an indirect a-aUylation of ketones, aldehydes, and esters. Enol derivatives can take another reaction course under Pd(0) catalysis (Scheme 2). Thus, oxidation to a,/3-unsaturated carbonyl compounds ensues if reactions are performed in acetonitrile under precise sources of catalyst precursor. "" "" A full discussion on the dichotomy of allylation-oxidation has been published, as well as a comparison of the usefulness of several transition metals as catalysts in allylation of nucleophiles. ... 

This report deals with the chemistry of the metal carbonyls, metal carbonyl hydrides and metal carbonyl halides. Infrequently, complexes which are ostensibly outside the scope of this chapter are included for their importance and relevance to metal carbonyl chemistry. Ref.36 is a good example of this type of work. The general format remains similar to the 1991 report. Apart from the first and second sections, concerned with General Studies and Reviews and Theoretical and Spectroscopic Studies, the chapter is divided into sections for the transition metals of each group. A handful of references that appeared late in 1990 and were omitted from last year s report are included here. 

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