Dimerization hydroalumination

It has been suggested that alkene or alkyne complexes are mechanistically important intermediates in hydroalumination and carboalumination reactions (124-126). Clear spectroscopic evidence for n interactions stems from investigations of alkenylaluminum compounds having a suitable intramolecular separation between the aluminum center and the double bond (127). IR and NMR data of these compounds show comparably lower alkene stretching frequencies and deshielded vinylic protons. Furthermore, these molecules are monomeric in solution this indicates that the tendency toward 7i-complex formation is stronger than that toward the dimeric bonding usual in aluminum alkyls. 

The direct interaction of Al, H2, and olefin (hydroalumination) is used to give either the dialkyl hydrides or the trialkyls. These are also accessible by reduction of organoaluminum halides and exist as dimers or trimers, depending on the steric requirements of the alkyl or aryl group. Examples are [Bu2A1H]3, [mes AlH2]2, and mes GaH2.24... 

For main group metals, which exhibit relatively small differences between M—H and M—C bond strengths and no strong metal-alkene interactions, the thermodynamics of hydrometallation should be even more favorable than for early transition metals. This does not appear to be the case, especially for hydroalumination (Volume 8, Chapter 3.11). The reason is almost certainly the additional stability of the metal hydride reagent conferred by aggregation organoaluminum hydrides exist as rather tight dimers. 

The extent and types of interfering reactions during hydroalumination of alkynes were examined in Section 3.11.2.2 (equations 18-24). To a lesser degree two of these reactions are also encountered in A1—H additions to alkenes and cycloalkenes. Analogous to the reductive dimerization of alkynes (equation 23) is the dimerization of a-alkenes by Bu 2AlH, which can be conducted catalytically in hydride to give high yields of dimer (equation 30). ... 

A variety of conjugated dienes can be synthesized stereospecifically via the hydroalumination of al-kynes. For example, the first-formed adduct from an alkyne can insert a second mole of alkyne and upon hydrolysis yield the ( , )-1,3-alkadiene (equation 23). Alternatively, the first-formed adduct can directly oxidatively dimerize to the ( , )-1,3-diene (equation 64). ... 

Hindered t-alkyl halides with lithium triethylborohydride in the presence of chromous chloride furnish highly crowded coupled products in good yield (equation 111) . Alkanes obtained by hydroalumination of alkenes with LAH in the presence of titanium tetrachloride can be dimerized in the presence of cupric acetate to furnish alkanes (equation 112) ... 

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). ... 

In these associations, the alkylbridge bonds are very weak compared with the other bridge bonds of hydrogen, halogen and hetero atoms [16,24]. For example, MeAlCb forms a dimer with the bridge bond of chlorine atom (the methyl group does not form the bridge bond [25]). The Al—H and Al—C bonds of the aluminum compounds show characteristic reactivities, and their reactivities are industrially utilized. The most representative reaction is hydroalumination. The reverse reaction is an olefin elimination. Ethylene is the most reactive of the olefins in the hydro-... 

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