Hydroaluminations diisobutylaluminum hydride

Hydrometallation reactions of carbon-carbon MULTIPLE BONDS Hydroalumination Diisobutylaluminum hydride, 189 Hydroboration General methods Dibromoborane-Dimethyl sulfide,... 

In addition to the regiochemical course, the stereochemistry of these additions can be highly selective. Illustrative of this characteristic is the behavior of 1-alkynylsilanes (5) toward diisobutylaluminum hydride. In the presence of a Lewis base, such as N-methylpyrrolidine, the hydroalumination gives >95% of the syn adduct (4) in the absence of a donor, the adduct has >95% of the anti configuration (6 equation 2). 

Finally, the high locoselectivity or chemoselectivity of hydroalumination toward substrates having both C==C and C C bonds is exemplified by the reaction of diisobutylaluminum hydride with 1-octen-7-yne (7). The only organoaluminum product detected (8) was that of a regioselective syn stereospecific addition to the bond (equation 3). 

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). ... 

Table 4 Relative Reactivity of Alkynes towards Hydroalumination with Diisobutylaluminum Hydride...

Hydroalumination. The treatment of alkynes with diisobutylaluminum hydride in hydrocarbon solvents results in a aT-addition of the Al-H bond to the triple bond to produce stereodefined alkenylalanes. The hydroalumination of alkynes is more limited in scope than the corresponding hydroboration reaction of alkynes (see Chapter 5) with regard to accommodation of functional groups and regioselectivity. Whereas hydroalumination of 1-alkynes is highly regioselective, placing the aluminum at the terminal position of the triple bond, unsymmetrically substituted alkynes produce mixtures of isomeric alkenylalanes. 

The organometallic derivatives required for the introduction of a 15-deoxy p-chain, not otherwise oxygenated, were obtained most conveniently by hydroalumination of a terminal acetylene with diisobutylaluminum hydride and conversion in situ to the lithio alanate 1 required for conjugate addition. In the absence of other oxy functions in the p-chain this process produces 15-deoxyprostaglandins such as 1 and IM conveniently and in excellent yield, as high as 80-90% for the 11,15-bisdeoxy series (Scheme 18) (25,54). 

Diisobutylaluminum hydride I metliijllithium Ethylene from acetylene derivatives via hydroalumination... 

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