Oxidative addition alane

Selective partial hydroalumination of dienes and trienes or their mixtures is possible because of the differences in the rate of A1H addition to various sorts of C=C double bonds. Further transformations of the alkenyl alanes open the way to some interesting possibilities in preparative work. For example, by partial hydroalumination of 4-vinylcyclohexene or limo-nene, wherein the A1—H bond reacts with the terminal double bond, followed by air oxidation, it is possible to obtain cyclohexenylethanol or 18-terpineol in good yield 312). A further example from terpene chemistry is the preparation of citronellol 225). 

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). ... 

An improved procedure for the synthesis of alkyl iodides from organoboranes (and thus ultimately from alkenes), involving the use of iodine and sodium meth-oxide, has been shown to entail inversion at the C-B bond, and is employed in a new route to optically active iodides from alkenes e.g. Scheme 30). A similar sequence for conversion of alkenes to primary halides via hydrozirconation (Scheme 31) has been reviewed. Zr also catalyses addition of lithium aluminium hydride to alkenes under mild conditions, and the alanes so formed can likewise be converted into alkyl halides (Scheme 32). 

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