Hydroalumination reactivity

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]. 

Among the hydrometallations reported to date, hydroboration and hydrosilylation have been the most widely investigated while hydroalumination has received less attention. The resulting organoalanes from hydroalumination are often more reactive than the organoboranes or organosilanes. While the first metal-catalyzed hydroalumination... 

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... 

The hydroalumination of cycloalkenes exhibits two interesting minima in relative rates toward diethyl-aluminum hydride. The half-life (in min) of a 3 1 ratio of Et2AlH and cycloalkene at 78 °C varied thus with ring size C4, 40 C5, 200 Ce, 1880 C7, 110 Cg, 86 C9, 280 Cio, 2500 Cn, 1350. The reactivity of cycloundecene is approximately that of an open-chain internal alkene. The low reactivity of cyclohexene can be ascribed to the eclipsing repulsions generated in the transition state by the syn attack of Et2AlH on the C=C bond (equation 17). ... 

As a class alkynes are much more reactive in hydroalumination than are alkenes. Hence, both terminal and internal alkynes react at feasible rates with both dialkylaluminum hydrides in alkanes and lithium aluminum hydrides (LiAlRnH4-n) in ethers. Selected examples of such additions are presented in Table 2. With alkyl or aryl substituents, it should be noted that R2AIH adds in a kinetically syn manner, (5 equation 2) and (7 equation 3), and LAH yields the anti adduct (14 equation 6). 

Table 4 Relative Reactivity of Alkynes towards Hydroalumination with Diisobutylaluminum Hydride...

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 is a less often used reaction but the resulting organoalanes are more reactive than either organosilanes or boranes and less reactive than the traditional organometallic reagents derived from lithium or magnesium. Various aluminum hydrides are commercially available or readily prepared by reduction of aliuninum halides [10]. 

The reactivity of the alkenes toward LiAlH -TiCl decreases in the series CH2=CHR > CH2=CR2 > RCH=CHR. Thus dienes can be hydroaluminated selectively at the terminal double bond , e.g. ... 

Deuterolysis gives the trans-deuterio-olefin, and small amounts of 1-deuterio-hex-l-yne (metallation) and 1,1-dideuterio-n-hexane (bis-hydroalumination). The predictable stereochemistry, the available starting materials and the known reactivity pattern of Al—C= bonds encourages synthetic application of this reaction - " . The R group at the C=C bond influences reaction (m) secondary or tertiary alkyl groups increase the rate more acidic alk-l-ynes (e.g. R = Ph, olefin) increases the amount of metallation. ... 

Hydroalumination followed by protonolysis is less convenient for stereoselective cis hydrogenation of the double bond. Simple, vicinally disubstituted double bonds sluggishly undergo hydroalumination and the carbon-aluminum bond undergoes inversion in hydrocarbon solvents at moderate temperatures. Strained olefins are more reactive and can react stereoselec-tively under carefully controlled conditions126. 

Alumination-addition. While DIBAL usually reacts with alkynes to give hydroalumination products, the addition of a small quantity of triethylamine leads to the alumination of the terminal alkyne. The resulting alkynylorganoaluminum reagent has then been used for diastereoselective aIk3mylation of oxazolidines (eq 41). Similar reactivity was observed with AlMes. 

Despite the low intrinsic reactivity of alkylboranes, Pd-catalyzed all lation with alkylboron compounds holds considerable promise as a unique and chemoselective alkylation method. It is unique in part because hydioboration of alkenes is by far the most general and dependable method for stoichiometrically converting alkenes into organometals and in part because alkylboron compounds thus generated can satisfactorily participate in Pd-catalyzed alkylation under basic conditions. In this connection, it should be noted that the current scopes of hydroalumination and hydiozirconation of alkenes are much more limited than... 

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-... 

Thermal hydroalumination of alkynols using LiAlHa in diglyme at 150°C results in the formation of (E)-alkenols [65]. When this reaction is catalyzed by Cp2TiCl2 (10 mol%), (Z)-alkenols are formed as major products (Z/ 10 1) [66] (Scheme 9). It was suggested [66] that under conditions of the above reaction, reactive complexes [Ti]-H are generated and hydrotitanate the triple bond. The subsequent transmetallation of the Ti alkenyl complexes results in (syn)-alkenyl alanes and regeneration of the hydride complexes [Ti]-H. 

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