Aluminum hydroalumination

Additioaal uses for higher olefias iaclude the productioa of epoxides for subsequeat coaversioa iato surface-active ageats, alkylatioa of benzene to produce drag-flow reducers, alkylation of phenol to produce antioxidants, oligomeriza tion to produce synthetic waxes (qv), and the production of linear mercaptans for use in agricultural chemicals and polymer stabilizers. Aluminum alkyls can be produced from a-olefias either by direct hydroalumination or by transalkylation. In addition, a number of heavy olefin streams and olefin or paraffin streams have been sulfated or sulfonated and used in the leather (qv) iadustry. 

In contrast to the related organoboranes, which are mostly used in the addition to non-polar carbon-carbon multiple bonds, aluminum hydrides have found their widest use in organic synthesis in the addition reaction to polar carbon-carbon and carbon-heteroatom multiple bonds including carbonyl, nitrile and imino groups as well as their a,(J-unsaturated analogs. Although these reduction reactions are also sometimes referred as hydroalumination reactions in the Hterature, they are outside the scope of this review. 

Nickel catalysts promote the hydroalumination of alkenes using trialkylalanes R3AI and dialkylalanes such as BU2AIH as the aluminum hydride sources [9, 29, 30, 33]. However, exhaustive studies of the range of substrates capable of hydroalumination with these reagents has not been carried out. Linear terminal alkenes like 1-octene react quantitatively with BU3AI at 0°C within 1-2 h in the presence of catalytic amounts of Ni(COD)2 [30]. Internal double bonds are inert under these conditions, whereas with 1,5-hexadiene cycHzation occurs. 

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. 

Computational studies of both hydroalumination and carboalumination have indicated a four-center TS for the addition.220 The aluminum reagents, however, have more nucleophilic character than do boranes. Whereas the TS for hydroboration is primarily electrophilic and resembles that for attack of CH3+ on a double bond, the... 

Mechanistically, the pre-catalyst Cp2TiCl2 104 is reduced to a [Ti]-H 105 species which is subsequently able to hydrotitanate the triple bond 106. A transmetallation from titanium to aluminum regenerated the [Ti]-H species to generate the (yy )-hydroaluminated compound 107 (Scheme 13). 

The results suggest that once hydroalumination has occurred at one end of the diene, the course of the reaction at the other double bond is significantly affected by the alkylaluminum group, possibly due to chelation in which a palladium-chloride-aluminum bond is thought to be important. 

Conjugate reductions. This combination (usually 1 3 ratio) effects conjugate reduction of a,p-acetylenic ketones or esters to a, 3-enones or unsaturated esters at -50° with moderate (E)-selectivity. The HMPT is believed to function as a ligand to aluminum and thus to promote hydroalumination to give a vinylaluminum intermediate, which can be trapped by an allylic bromide (equation I).1 The re-... 

The carbaalanes [8, 9] possess clusters formed by aluminum and carbon atoms. They represent a new class of compounds which, in some respects, may be compared to the important class of carbaboranes. Usually, they were obtained by the reaction of aluminum alkynides with aluminum hydrides (hydroalumination) and the release of trialkylaluminum derivatives (condensation). The first carbaalane, (AlMe)g(CCH2Ph)5H 3 [10], was synthesized by the treatment of dimethylalumi-num phenylethynide with neat dimethylaluminum hydride. The idealized stoichiometric ratio of the components is given in Eq. (2), which also shows a schematic drawing of the molecular structure. Compound 3 was isolated in the form of colorless crystals in 60% yield. While 3 is only slightly air-sensitive, the less sterically shielded propynide derivative 4, also shown in Eq. (2), is highly pyrophoric [11],... 

One remarkable derivative of these cube-like clusters was isolated with compound 9, in which one vertex of the Alx cube remained unoccupied [18]. It was obtained by a reaction similar to Eq. (2), however, a slightly better steric shielding was achieved by ethyl groups instead of methyl substituents attached to aluminum [Eq. (5)]. Orange-red crystals of 9 were isolated in 36% yield. It may be described as the product of an incomplete hydroalumination reaction and contains seven aluminum atoms, a hydrogen atom bridging only three metal atoms and two alkenyl groups with C=C double bonds that are located at the open face of the cluster. As... 

TN. Dymova, N.N. Mal tseva, V.N. Konoplev, A.I. Golovanova, D.P. Alexandrov, A.S. Sizareva, Solid-phase solvate-free formation of magnesium hydroaluminates Mg(AlH )2 and MgAlHj upon mechanochemical activation or heating of magnesium hydride and aluminum chloride mixtures , Russ. J. Coord. Chem. 29 (2003) 385-389. 

Hydroalumination. Titanocene dichloride is an effective catalyst for hydro-uluminution of alkenes and alkynes with his(dialkylamino)alancs5 and various complex aluminum hydrides. The adducts can be quenched with water or iodine. The reaction is satisfactory for terminal alkenes and internal alkynes, but is not clcun for internal alkenes and terminal alkynes. 

Aluminum hydrides add across carbon-carbon multiple bonds to form organo-alanes381-385 (hydroalanation or hydroalumination), which are usually further reacted with suitable electrophiles. The addition is thermal or may be catalyzed by metals, mainly titanium and zirconium compounds.386,387... 

Many of the characteristic features of hydroalanation of alkenes (reactivities, selectivities) are very similar to those of hydrosilylation. Terminal alkenes react readily in hydroalumination, whereas internal alkenes are much less reactive. Aluminum usually adds selectively to the terminal carbon. Hydroalumination of styrene, however, leads to a mixture of regioisomers.392 When hydroalumination of alkenes is followed by protolysis, saturated hydrocarbons are formed that is, net hydrogenation of the carbon-carbon double bond may be achieved. The difference in reactivity of different double bonds allows selective hydroalumination of the less hindered bond in dienes 393... 

Alkynes are more reactive in hydroalumination than alkenes. Hence, unlike internal alkenes, internal acetylenes readily undergo hydroalumination. Side reactions, however, may occur. Proton-aluminum exchange in terminal alkynes, for instance, leads to substituted products, and geminal dialuminum derivatives are formed as a result of double hydroalumination. In addition, nonsymmetric alkynes usually give mixtures of regioisomers. Appropriate reaction conditions, however, allow selective hydroalumination. Thus, the addition of isoBu2AlH to alkynes is a... 

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 addition of aluminum hydride and alkyls to non-activated C-C multiple bonds (C=C or C=C) is referred to as hydroaluminaton and carboalumination, respectively. The reactions involve metathesis of each C-C multiple bond with Al-H or Al-C bonds, giving an Al-C =C-H (or Al-C-C-H) or A/-C=C-C (or A/-C-C-6) structure, respectively. These methods are of broad utility in the construction of Al-C linkages, including Al-alkenyl and Al-alkyl bonds, which can be further converted into C-C bonds (Equations (42)-(45)).192-196 Therefore, a great contribution has been made in synthetic applications, in addition to the mechanistic studies, of hydroalumination and carboalumina-tion since 1993.197-199... 

The activation of mixtures of calcium and aluminum hydroxides taken at different ratios was studied in [13], It was stated that 3Ca0-Al203 6H20 was the main product of mechanochemical synthesis within a wide range of component ratios (from 2 1 to 1 2). When a planetary mill AGO-2 is used, the synthesis completes within 10 min (Fig. 6.2). An increase of activation time brings to decrease of the intensity of reflections, which is an evidence of product amorphization or the beginning of hydroaluminate decomposition. 

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