Catalysts alanate

Common reducing agents are hydrogen in the presence of metallic or complex catalysts (e.g. Ni, Pd, Pt, Ru, Rh), hydrides (e.g. alanes, boranes, LIAIH, NaBHJ, reducing metals (e.g. Li, Na, Mg, Ca, Zn), and low-valent compounds of nitrogen (e.g. NjHj, NjHJ, phosphorus (e.g. triethyl phosphite, triphenyiphosphine), and sulfur (e.g. HO-CHj-SOjNa = SFS, sodium dithionite = Na S O. ... 

Al—Ti Catalyst for cis-l,4-PoIyisoprene. Of the many catalysts that polymerize isoprene, four have attained commercial importance. One is a coordination catalyst based on an aluminum alkyl and a vanadium salt which produces /n j -l,4-polyisoprene. A second is a lithium alkyl which produces 90% i7j -l,4-polyisoprene. Very high (99%) i7j -l,4-polyisoprene is produced with coordination catalysts consisting of a combination of titanium tetrachloride, TiCl, plus a trialkyl aluminum, R Al, or a combination of TiCl with an alane (aluminum hydride derivative) (86—88). 

Another group of isoprene polymerization catalysts is based on alanes and TiCl. In place of alkyl aluminum, derivatives of AlH (alanes) are used and react with TiCl to produce an active catalyst for the polymerization of isoprene. These systems are unique because no organometaHic compound is involved in producing the active species from TiCl. The substituted alanes are generally complexed with donor molecules of the Lewis base type, and they are Hquids or soHds that are soluble in aromatic solvents. The performance of catalysts prepared from AlHCl20(C2H )2 with TiCl has been reported (101). 

The aziridine-2-carboxaldehyde 56 can also serve as synthon for the synthesis of sphingosines, which are important biomembrane constituents [64]. One possible route involves the addition of an alanate to the aldehyde. In a later stage of this synthetic plan the aziridine can be opened, either via the intermediacy of an oxazoline or directly with dilute acid. Unfortunately, the reaction of aldehyde 56 with a vinylalanate has a poor diastereoselectivity of 3 2. Therefore, an alternative approach was considered, namely one involving the addition of a vinylzinc reagent to the aldehyde thereby employing our N-tritylaziridinediphenyl-methanol 51 as the chiral catalyst. Gratifyingly, only one diastereomer was obtained. Reductive removal of the trityl function, acetylation of the hydroxy... 

The decomposition is significantly accelerated and the temperature of the first decomposition reaction is lowered to 120°C (Fig. 19.7). The decomposition rate is relatively low compared with other titanium-based dopants. The highest activity of a titanium catalyst used in alanate decomposition was observed for ligand-stabilized colloidal titanium metal [42]. 

Bis(diamino)alanes (R2N)2A1H were used for the hydroalumination of terminal and internal alkenes [18, 19]. TiCb and CpjTiCb are suitable catalysts for these reactions, whereas CpjZrCb exhibits low catalytic activity. The hydroaluminations are carried out in benzene or THF soluhon at elevated temperatures (60°C). Internal linear cis- and trans-alkenes are converted into n-alkylalanes via an isomerization process. Cycloalkenes give only moderate yields tri- and tetrasubstituted double bonds are inert. Hydroaluminahon of conjugated dienes like butadiene and 1,3-hexa-diene proceeds with only poor selechvity. The structure of the hydroaluminahon product of 1,5-hexadiene depends on the solvent used. While in benzene cyclization is observed, the reaction carried out in THF yields linear products (Scheme 2-10). 

This interpretation was supported by further investigations by GiacomelH and coworkers [73]. Racemic 4-phenyl-l-hexene was kineticaUy resolved by isomerization of the double bond using a catalyst system consisting of Al Buj, (R)-N,N-di-methyl-l-phenylethylamine and Ni(mesal)2. Very poor enantioselectivities (ee < 0.3%) were observed for both the isomerization product and the unreacted alkene. The authors note that it is essential to first react the alane with the chiral amine. No... 

T. H. Tsai, J. W. Lane, and C. S. Lin Temperature-Programmed Reduction for Solid Materials Characterization, Alan Jones and Brian McNichol Catalytic Cracking Catalysts, Chemistry, and Kinetics,... 

The use of water-soluble reagents and catalysts allows reactions to be performed in aqueous buffered solutions. PEG-supported triarylphos-phine has been used in a Wittig reaction under mildly basic aqueous conditions (Eq. 8.115). The PEG-supported phosphine oxide byproduct can be easily recovered and reduced by alane to regenerate the starting reagent for reuse.312 The aqueous Wittig reaction has also been used in... 

Titanium tetrachloride is a very effective catalyst for the addition of LiAlH4 or alane to the olefinic double bond. The mechanism of this reaction involves intermediate transition metal hydrides, as in the case of reaction of NaBPU and Co11-salts. The hydrotitanation of the double bonds is probably followed by a rapid metal exchange reaction (Scheme 3)94. 

Alan Goldman developed much improved catalysts and also obtained detailed mechanistic information in the 1990s [35, 36]. An unexpected variant developed by Saito, by our own group and most extensively by Goldman, was acceptorless dehydrogenation. It turned out that the reaction could be driven by reflux because the hydrogen was continuously pumped out of the solvent by the reflux action [37-39]. 

G. Sandrock, K. Gross, G. Thomas, Effect of Ti-catalyst content on the reversible hydrogen storage properties of the sodium alanate , J. Alloys Compd. 339 (2002) 299-308. 

Many more examples exist for reduction of the carhonyl only. Over an osmium catalyst [763] or platinum catalyst activated by zinc acetate and ferrous chloride [782] cinnamaldehyde was hydrogenated to cinnamyl alcohol. The same product was obtained by gentle reduction with lithium aluminum hydride at —10° using the inverse technique [609], by reduction with alane (prepared in situ from lithium aluminum hydride and aluminum chloride)... 

The practical use of the desorption reaction requires a catalyst for the improvement of the kinetics. The first work on catalyzed alanates at MPI - Miilheim was derived from studies that used transition-metal catalysts for the preparation of MgH2- The NaAlH4 was doped with Ti by solution chemistry techniques whereby nonaqueous liquid solutions or suspensions of NaAlH4 and either TiCl3 or the alkoxide Ti(OBu )4 [titanium(IV) w-butoxide] catalyst precursors were decomposed to precipitate solid Ti-doped NaAlH4 [57, 58]. 

Reaction 5.45 is at least partly hypothetical. Evidence that the Cl does react with the Na component of the alanate to form NaCl was found by means of X-ray diffraction (XRD), but the final form of the Ti catalyst is not clear [68]. Ti is probably metallic in the form of an alloy or intermetallic compound (e.g. with Al) rather than elemental. Another possibility is that the transition metal dopant (e.g. Ti) actually does not act as a classic surface catalyst on NaAlH4, but rather enters the entire Na sublattice as a variable valence species to produce vacancies and lattice distortions, thus aiding the necessary short-range diffusion of Na and Al atoms [69]. Ti, derived from the decomposition of TiCU during ball-milling, seems to also promote the decomposition of LiAlH4 and the release of H2 [70]. In order to understand the role of the catalyst, Sandrock et al. performed detailed desorption kinetics studies (forward reactions, both steps, of the reaction) as a function of temperature and catalyst level [71] (Figure 5.39). 

The identity of the effective catalyst is not known yet. XRD and microscopic analyses have not led to its identification, probably because it is amorphous, too fine or is substantially located in the alanate lattice itself. Can we produce effective catalysts only by thein situ decomposition of a precursor ... 

Alan J. Chalk I should like to comment on the point raised by Dr. Dessy on coordinately unsaturated catalysts and on some points in the paper. 

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