Hydroalumination vinylalanes

Alkynes are much more reactive toward hydroalumination than alkenes. Hence, they readily react with both dialkylaluminum hydrides and LiAlH4 under mild conditions in the absence of a catalyst [1]. However, it is not always possible to avoid side reactions and subsequent transformation of the vinylalanes formed in this transformation [81, 82]. In addition, ds-trans-isomerization of the metallated C=C bond can take place, thereby reducing the stereoselectivity of the overall reaction [83]. 

In general, transition metal-catalyzed hydroaluminations of alkynes occur in a syn fashion, i.e., both aluminum and hydride are added to the same face of the 7i-bond. Isomerization of the initially formed vinylalane is usually not observed under the mild reaction conditions used for these transformations. 

A final group of fp -carbanions which have been found to react well with at least monosubstituted epoxides are the vinylalanes formed either by direct hydroalumination or by zirconium-catalyzed car-boalumination of a terminal alkyne. Both species are insufficiently reactive to couple with epoxides and must first be converted into their corresponding ate complexes, usually by treatment with n-butylli-thium (Scheme 30). 

Hydroalumination of alkynes. The hydride reacts with alkynes in a hydrocarbon solvent by cis addition to yield a tran.t-vinylalane (1) from 1-alkynes and a cis-vinylalane from disubstituted alkynes. Treatment of the vinylalane with methyl-lithium in ether in a 1 1 ratio to give an ate complex (2) followed by carbonation... 

An alternative solution to the same problem uses mixed vinylalanes that contain silicon in the a-position. Since these derive from a diisobutylaluminum (DIBAL)-based hydroalumination of silylalkynes, the resulting species are diisobutyl(alkenyl)-alanes. These undergo transmetalation in the presence of an NHC-Cu complex, derived from CuCb and, importantly, a bidentate, dimeric silver NHC precursor (illustrated below). Silyl moieties can be TMS or TBS, although the latter, bulkier group in the starting alane requires room temperature for 1,4-addition to ensue. Both 5- and 6-membered enones can be used representative examples are shown below. The resulting vinylsilane in each product can be easily converted to alternative functionality, such as a ketone via oxidation with MCPBA), a halide e.g., an iodide, with NIS), and an alkene yia protodesilylation with TFA). ... 

A valuable method for the synthesis of isomerically pure a/5-unsaturated nitriles via the hydroalumination of alkynes has been reported by Zweifel, Snow and Whitney [107]. Addition of di-isobutylaluminium hydride to an alkyne gives the trans-vinylalane (33) in 90% yield. Compound (33) does not react with cyanogen at room temperature but gives the vinylalanate (34) with methyl lithium. The latter reacts with cyanogen to given the trans-a,i3-unsaturated nitrile. 

Unlike alkenes, disubstituted acetylenes are hydroaluminated under mild conditions by means of fBu2AlH and IBusAl without a catalyst However, often it is impossible to avoid undesired side reactions and the subsequent transformations of vinyl alanes. Hydrolysis of the vinylalanes gives rise to 1,2-disubstituted olefins of Z- or -configuration depending on the namre of the hydroaluminating reagent used [58], the structure of the initial acetylene [59], and the solvent [59]. 

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