Activation of hydrides

III. Catalytic Activity of Hydride Phases of Palladium and Its Alloys with... 

Molar mass reduction with DIBAH was significantly more effective than with TIBA. The higher activity of DIBAH was assigned to the hydride group. The differences in the relative activities of hydride and isobutyl moieties towards substitution by living polybutadienyl chains were qualitatively described by different equilibrium positions (Scheme 33) [179,180,205]. 

Palczewska [3] has discussed the role and activities of hydride phases in metal-catalyzed reactions. Attention is drawn to the influences of crystallite sizes and of impurities bonded to active sites on the stability of the hydride phases. 

Copolymers with silanes have attracted much interest. These include a range of methacrylic acid copolymers with disilanes, secondary silanes, phenylsi-lane and fumarate terminated poly(dimethylsiloxanes). A platinum(II) bis(a-cetylacetonato) catalyst has been found to be highly effective for enhancing the activities of hydride and vinyl polymer end groups in step polymerisation reactions. ... 

Spectroscopic Signatures of Entropic Activation of Hydride Transfer... 

Specific activities of hydrides of IMC, such as LaNis or LaCos, are ten times higher, than activities of supported catalysts, such as 5% Ni-Ce02... 

Some Recent Developments in Hydrogenation Activation of Hydrides by Transition Metal Derivatives... 

Activation of hydride reagents such as NaH, LiAlH4, NaBH4, etc. by TM complexes has attracted much attention in recent years and considerably extended the scope of their use. Not only are such reagents attractive because of their ease of handling and low costs, but also because of their versatility and the many variations possible. By careful choice of catalyst system, a remarkable degree of selectivity can be attained, often under mild conditions. 

Although a few simple hydrides were known before the twentieth century, the field of hydride chemistry did not become active until around the time of World War II. Commerce in hydrides began in 1937 when Metal Hydrides Inc. used calcium hydride [7789-78-8J, CaH2, to produce transition-metal powders. After World War II, lithium aluminum hydride [16853-85-3] LiAlH, and sodium borohydride [16940-66-2] NaBH, gained rapid acceptance in organic synthesis. Commercial appHcations of hydrides have continued to grow, such that hydrides have become important industrial chemicals manufactured and used on a large scale. 

Squalene monooxygenase, an enzyme bound to the endoplasmic reticulum, converts squalene to squalene-2,3-epoxide (Figure 25.35). This reaction employs FAD and NADPH as coenzymes and requires Og as well as a cytosolic protein called soluble protein activator. A second ER membrane enzyme, 2,3-oxidosqualene lanosterol cyclase, catalyzes the second reaction, which involves a succession of 1,2 shifts of hydride ions and methyl groups. 

Casuscelli and colleagues (95T8605) reported the conversion of the 3-carboxy-butyl-substituted isoxazolidines 60 (R = alkyl, CH2OH R = H, Me) into 3-me-thylamino-2(5// )-furanones 61 by the activity of sodium hydride in tetrahydrofuran... 

Its structure shows the trans-influence of hydride and pronounced distortion from square planar geometry (H—Rh-P 70.7°) owing to steric crowding. (RhH(PPh3)3 is rather less distorted (H-Rh-P 75.8°) [118a], This 16-electron complex shows no tendency to add an extra molecule of phosphine, unlike the less hindered RhH(PEt3)3. It is, however, an active... 

IV. The Effect of Transformation into Hydride on the Catalytic Activity of... 

V. Catalytic Activity of Other Metal Hydrides in Test Reaction of Hydrogen. 283... 

The screened proton model of nickel or palladium hydrides and Switendick s concept of the electronic structure do not constitute a single approach sufficient to explain the observed facts. In this review, however, such a model will be used as the basis for further discussions. It allows for the explanation and general interpretation of the observed change of catalytic activity of the metals, when transformed into their respective hydrides. 

However this was not always the case. It is possible to demonstrate, on the basis of selected examples from the literature representing the experimental evidence and the authors original interpretation, that the catalytic activity of palladium or its alloys changes sometimes dramatically, when there is a possibility of their being converted into the corresponding hydrides. 

The catalytic activity of the pure /3-palladium hydride has been studied under the appropriate temperature and pressure conditions. The palladium sample was converted into the hydride in a manner which bypassed the area of coexistence of the phases. This was achieved by suitably saturating the metal with hydrogen at 35 atm above the critical temperature and then subsequently cooling the sample to the required temperature and reducing the hydrogen pressure. This method of sample prepare tion allowed one to avoid cracking the palladium crystallites, which would... 

Fig. 7. Changes of the coefficient of recombination, y, of H atoms on the surface of Pd-Au alloy foil catalysts at room temperature. O, Initial values of log y, final values representing catalytic activity of Pd and its alloys containing absorbed hydrogen. Broken line denotes the alloy Pd40Au60 which represents the upper limit of gold content in Pd-Au alloys closing the region of Pd-Au hydride formation. After Dickens et al. (86).

---