Lewis acids hydroalumination

Since hydroalumination by neutral aluminum hydrides is an electrophilic attack on a C= or C=C linkage, the reaction can be accelerated by Lewis acids such as aluminum halides, and be retarded by Lewis bases like R3N, R2O or even unsaturated R3AI cf. equation 38). Such reagents also exert an effect on the syn or anti character of the A1—H adduct. Evidence suggests that Lewis acids or bases principally affect the rate of isomerization of the initial syn adduct into the generally more stable anti adduct Lewis bases retard such isomerizations, while Lewis acids promote them. The presence of ethers or tertiary amines stabilize the syn adducts of alkynyl-silanes and -germanes (47) and permit such adducts to be formed in >95% geometrical purity (Scheme 12). ... 

This reagent [BK5, Wl, W3, YGl] is both soluble and stable in toluene or hexane. It is also soluble in ethers (diethylether, THF, DME, glymes), but these solutions are stable only at low temperature. It is a particularly strong Lewis acid. At high temperature, DIBAH hydroaluminates carbon-carbon double and triple bonds [HHl]. The usual workup after reduction consists of addition of methanol then water to the solution, followed by separation of the aluminum salts that have precipitated. Alternatively, the mixture can be treated with dilute aqueous HCl followed by extraction, or else addition of tartaric acid in ethanol followed by addition of NaS04 and celite and then filtration [BL2]. 

This method was employed in the stereospecific construction of a silyl diene in a synthesis of chlorotri-cholide (Scheme 2.80). The boronic acid partner 2.233 was prepared by hydroboration and hydrolysis. The vinyl iodide was 2.235 prepared by hydroalumination-iodination. The silyl group was included in order to boost stereoselectivity in the later Diels-Alder reaction, but also served to facilitate the synthesis of the vinyl iodide coupling partner. The Suzuki coupling yielded the diene 2.236 with retention of the stereochemistry of both alkenes. Thallium hydroxide was employed as the Lewis base. Thallium-containing Lewis bases have been found to be advantageous in a number of cases, but the toxicity of thallium is a serious concern. 

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