Hydride base initiated

CHART 3 Dialkylaluminum halide/hydride based initiator systems. 

Hydride-based precatalysts (R = H) showed the highest initiation rates in a-ole-fin polymerization (Table 1, see p. 980) and this efficiency was later often assigned to the in situ formation of terminal Ln-H moieties. For 4(Sc) it was shown that the order of decreasing rate of ethylene insertion is Sc-H > Sc-CH2(CH2) CH3 (n > 2) > SC-CH2CH2CH3 > Sc-CHj > SC-CH2CH3 >... 

Aldehydes and ketones can be reduced using sodium borohydride or lithium aluminum hydride. The reaction is base-initiated with hydride ion as the nucleophile. One mole of sodium borohydride or lithium aluminum hydride reduces four moles of aldehyde or ketone the reaction mixture is then acidified to produce the neutral alcohol. 

As recounted, these studies demonstrate that two of the three expected intermediates in asymmetric hydrogenation may be directly observed, but the expected dihydride is too fleeting. There are two further experiments which are pertinent to this issue. A related diphosphine-iridium alkene complex reacts with dihydrogen at low temperatures and a series of alkene dihydrides are observed prior to the formation of the expected alkyl hydride. Based on the H-NMR chemical shifts of the respective Ir-H species, the initial addition (or to be more correct the initially observed species) possesses H trans to alkene and H trans to phosphine only at higher temperatures does this rearrange to the expected H trans to amide and H trans to phosphine structure (Fig. 9a) [36]. A more directly relevant experiment involves para-enriched hydrogen, and in the illustrated case a transient dihydride is observed. A problem is that the spectral characteristics are not entirely in accord with expectations for the proposed structure (the supposed trans-P-Rh-H coupHng is 4 Hz rather than ca. 120 Hz), but the presence of some transient Rh dihydride is definitive based on the evi-... 

A final set of catalysts that has largely historical importance consists of monohydride complexes that react by radical pathways. Insertion of the olefin into the hydride occurs through radical intermediates. Many of these catalysts have been shown to reduce dienes, acrylic acid derivatives, and fumarates. As discussed in Chapter 9, migratory insertion requires an open coordination site cis to the hydride for initial binding of the olefin. Thus, catalysts based on metal carbonyl compounds that dissociate CO slowly, or porphyrin ligands that occupy the sites cis to a hydride, react dirough radical pathways or direct transfer of a true "hydride." The basic mechanism for such direct formal insertions by radical processes is shown in Scheme 15.14. 

An extensive testing program has been Initiated to detenplne the burst strength, stress-rupture strength, and susceptibility to crack propagation of the process tube under conditions of elevated tenq rature and variable hydride concentration. Initially, these tests are made using samples of actual process tube under biaxial stress. These tests will provide the bases for comparison with similar tests made on samples of Irradiated process... 

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 ... 

Hydrosilation silicones or addition cure systems utilize a hydride functional crosslinker with a vinyl functional base polymer and a noble metal catalyst. While the cure can be initiated with UV [48,49], thermal cure versions dominate the commercial market [23,50]. In thermal cure systems, inhibitors are necessary for processing and anchorage additives are common. 

There is a lively controversy concerning the interpretation of these and other properties, and cogent arguments have been advanced both for the presence of hydride ions H" and for the presence of protons H+ in the d-block and f-block hydride phases.These difficulties emphasize again the problems attending any classification based on presumed bond type, and a phenomenological approach which describes the observed properties is a sounder initial basis for discussion. Thus the predominantly ionic nature of a phase cannot safely be inferred either from crystal structure or from calculated lattice energies since many metallic alloys adopt the NaCl-type or CsCl-type structures (e.g. LaBi, )S-brass) and enthalpy calculations are notoriously insensitive to bond type. 

The addition of a-(acylamino) esters to 3-aryl-2-propenoates, with sodium ethoxide in ethanol or sodium hydride in benzene as base, is a frequently ultilized procedure9-" A The initial Michael adducts cyclize to 3-aryl-5-oxo-2-pyrrolidinecarboxylic acids with modest to high trans diastereoselectivities 10°. 

There have been few satisfactory demonstrations that decompositions of hydrides, carbides and nitrides proceed by interface reactions, i.e. either nucleation and growth or contracting volume mechanisms. Kinetic studies have not usually been supplemented by microscopic observations and this approach is not easily applied to carbides, where the product is not volatile. The existence of a sigmoid a—time relation is not, by itself, a proof of the occurrence of a nucleation and growth process since an initial slow, or very slow, process may represent the generation of an active surface, e.g. poison removal, or the production of an equilibrium concentration of adsorbed intermediate. The reactions included below are, therefore, tentative classifications based on kinetic indications of interface-type processes, though in most instances this mechanistic interpretation would benefit from more direct experimental support. 

Change of base (pyridine, aniline, water) had no obvious effect on the steric course of the reaction (59). Rather surprisingly, it appears that the reactions of the cis and trans isomers of 1-bromopropene with the hydride [Co(CN)5H] give different products this eliminates an initial addition of Co—H to the double bond, since this would lead to the same product for the two isomers 105). Vinyl chloride apparently reacts only with [Co salen] , and not with the hydride, to form the CH2=CHCo complex 43) the same may be the case with the BAE complexes 40). 

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. 

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