Alkali metal hydride, activity

The isomerization of 5-vinyl-2-norbornene to 5-ethylidene-2-norbornene has been performed using a catalytic system consisting of an alkali metal hydride and an amine. The activity of the alkali metal hydride increased with increasing size of the alkali metal KH > NaH > LiH. Among the various amines tested, only aliphatic 1,2-diamines exhibited the activity for the isomerization. Electron paramagnetic resonance (EPR) and UV-visible spectroscopic experiments on the active species suggest that the isomerization of 5-vinyl-2-norbornene proceeds through a radical mechanism.167... 

The system is not a homogeneous one, as the organometallic compounds and alkali metal hydrides are insoluble in benzene, the solvent used however, the rate of reaction was found not to be affected by the addition of platinum or palladium catalysts. The reaction is formally similar to the base-catalyzed activations of hydrogen studied by Wilmarth this becomes clear if Eq. (45) is written as... 

The best results are obtained with the use of alkali metal hydrides (NaH, KH) in THF, DME, or DMF. The reaction works well in THF or DME with activated halides such as ethyl bromoac-etate, ten-butyl bromoacetate, - ethyl 2-bromobutyratc, ethyl T-broniobulyrale.- (iodom-ethyl)trimethylstannane, " (iodomethyl)trimethylsilane, benzoyl bromide,- benzyl bro-mide, - farnesyl bromide,- " alkyl 4-bromocrotonates, l-(bromomethyl)naphtalene,- andN-bromomethylphthalimide but gives poor results with primary alkyl halides.- Primary and secondary alkyl halides, bromides and iodides (Scheme 8.16), react satisfactorily in dMF or DMSO, although bulky electrophiles give poor results. In DMSO the expected product is frequently contaminated by the dialkylation product. ... 

Class D fires involve strong reducing agents such as active metals (magnesium, titanium, zirconium, and alkali metals), metal hydrides, and organome-tallics. Special dry-chemical fire extinguishers are available for these fires (e.g., Ansul Co.). Sand is also useful for small fires of this type. Water should be avoided because it promotes the fire by liberation of hydrogen or hydrocarbons. 

Various modes of termination of anionic polymerization can be visualized. The growing chain end could split out a hydride ion to leave a residual double bond. This is, however, a high activation energy process and has not as yet been reported in the cases where alkali metal cations are present. It is important in systems involving Al—C bonds, however (73). A second possibility is termination through isomerization of the carbanion to an inactive anion. Proton transfer from solvent, polymer, or monomer would also cause termination of the growing chain. Lastly, the carbanion could undergo an irreversible reaction with solvent or monomer. The latter three types have been shown or postulated as termination or transfer reactions. 

The simplest and most used industrial synthesis of NaH and KH consists in reacting 2-3 atm of Hj (at ca. 300°C) with a dispersion of 50% Na or 25% K (by wt) in an inert liquid (mineral oil or kerosene), also at ca. 300 C" . The conversion efficiency can reach 98%. The hydride can be conserved and manipulated in the form of a dispersion in the inert liquid. To obtain it dry, it is filtered and washed with a volatile solvent, such as petroleum ether. The synthesis can be accelerated by adding activating agents to the oil, such as anthracene or ortho-xylene, which act through the intermediate formation of tt complexes with the alkali metal. 

Reductions of cyclic ketones by dissolving metals are frequently highly stereoselective and these reductions have been used to obtain secondary alcohols which are difficult or impossible to prepare by metal hydride reduction. In terms of yield, the best results are usually obtained either by reductions with alkali metals (commonly Li) in liquid NH3 in the presence of proton donors or with active metals in an alcohol. Although a number of explanations have been advanced for the stereoselectivity of these reductions, they are all rationalizations with dubious predictive value." There are, however, a number of empirical generalizations which are based on a considerable body of experimental data, specifically ... 

Triple bonds can also be selectively reduced to double bonds with diisobutyla-luminum hydride (Dibal-H), ° with activated zinc (see 12-38), with hydrogen and Bi2B-borohydride exchange resin, or (internal triple bonds only) with alkali metals (Na, Li) in hquid ammonia or a low-molecular-weight amine. Terminal alkynes are not reduced by the Na NH3 procedure because they are converted to acetylide ions under these conditions. However, terminal triple bonds can be reduced to double bonds by the addition to the Na—NH3 solution of (NH4)2S04, which liberates the free ethynyl group. The reaction of a terminal alkyne with... 

Much faster reaction can be achieved with strong bases (/07). The chain is started by an N-acylimide which may be N-caproyl-caprolactam produced in a slow reaction from the monomer during an induction period or an N-acylamid produced by action of a cocatalyst like for example carboxylic acid chlorides or anhydrides on caprolactam. The cocatalyst action speeds up the reaction such, that fast polymerization below the melting point of the polymer becomes possible. The strong base, such as alkali metal, metal hydride, metal amid, or oiganometallic compound, activates the monomer by lactam anion formation ... 

The finely divided alkali metal or its hydride will react with activated aluminum at an elevated temperature in an autoclave under hydrogen pressure. The solvent plays a decisive role. While dialkyl ethers or polyethers are unsuitable, the synthesis goes particularly well in absolute tetrahydrofuran (6, 15, 44, 214). When using aliphatic or aromatic hydrocarbons it is necessary to add 5-10% aluminum triethyl to the reaction mixture (5, 275). 

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