Hydroaluminations lithium aluminum hydride

Hydroalumination of 1-chloro-l-alkynes. Lithium aluminum hydride adds to 1-chloro-l-alkynes (1) regio- and stereoselectively to form the a-chlorovinyl alanates 2, which are moderately stable at 0°C. On methanolysis they are converted into (E)-l-chloro-l-alkenes (3). They can also be converted into (Z)-l-bromo-l-chloro-l-alkenes (5) and into (Z)-l-chloro-l-iodo-l-alkenes (6). 

As a class alkynes are much more reactive in hydroalumination than are alkenes. Hence, both terminal and internal alkynes react at feasible rates with both dialkylaluminum hydrides in alkanes and lithium aluminum hydrides (LiAlRnH4-n) in ethers. Selected examples of such additions are presented in Table 2. With alkyl or aryl substituents, it should be noted that R2AIH adds in a kinetically syn manner, (5 equation 2) and (7 equation 3), and LAH yields the anti adduct (14 equation 6). 

HYDROALUMINATION Dichlorobis-(cyclopentadienyl)titanium. Dichloro-bis(cyclopentadienyl)zirconium . Lithium aluminum hydride-Dichlorobis(cyclo-pentadienyl)zirconium. Borane-Di-methyl sulfide. 

Related hydroalumination reactions have been reported using Lithium Aluminum Hydride. 

Lithium trialkylaluminum hydrides, such as lithium diisobutyl(methyl)aluminum hydride (15), which can be readily prepared by the interaction of diisobutylaluminum hydride with methyllithium in DME, are useful for the anti hydroalumination of internal alkynes (equation 9). ... 

Anhydrous diethyl ether, freshly obtained from a commercial supplier, is preferred for the reactions involving lithium hydro-aluminaie. The ether must be peroxide-free. Carbon dioxide must be rigorously excluded from these reaction systems. Explosions have been observed during evaporation of solutions of aluminum hydride and related compounds when carbon dioxide is present as an impurity.If it is necessary to concentrate ether solutions of lithium hydroaluminate by distillation, the following precautions must be observed. A large fluid volume must be maintained in the distillation flask such solutions should never be concentrated so far that very little ether remains in the reaction flask. Distillations must be effected in a protective atmosphere. Cyclic ethers, and especially tetrahydropyran solutions of lithium hydroaluminate, present a far greater explosion risk than diethyl ether solutions. 

Hydroalumination is a less often used reaction but the resulting organoalanes are more reactive than either organosilanes or boranes and less reactive than the traditional organometallic reagents derived from lithium or magnesium. Various aluminum hydrides are commercially available or readily prepared by reduction of aliuninum halides [10]. 

With the purpose of employing a more soluble form of complexed lithium hydride in such a process, Ziegler and Gellert heated ethylene in ether under pressure at 120-140 °C with LAH, which had been prepared for the first time by Schlesinger and coworkers. To Ziegler s gratification LAH was smoothly converted into lithium tetraethylaluminate (10), which could be isolated in high yield as a crystalline adduct. As proof of structure, the same adduct was also synthesized by the admixture of the lithium and aluminum ethyl derivatives in a 1 1 ratio (Scheme 2). This study represents the first authenticated instance of hydroalumination. 

This study opens up the possibility that hydroaluminations by LAH or LiAlBu 2MeH actually occur by the dissociation of such complex hydrides into AIH3 and R2AIH at higher temperatures. Steps of conventional syn hydroalumination, aluminum exchange and vinylic lithium isomerization could then lead to the anti adduct (Scheme 10). 

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