Dialkylaluminum hydride

Propose a mechanism that could account for the overall four-thirds-order kinetics and the appearance of the dialkylaluminum hydride concentration to the one-third power. 

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]. 

Aluminum trialkyls dissolve aluminum with hydrogen under pressure affording dialkylaluminum hydrides. The reaction products add to olefins in the next step (b) to give 50% more aluminum alkyl than the amount originally employed, a sequence which finally leads to any desired amount. 

There are three side reactions which occur during the growth step (8). At higher temperatures (>120°C), aluminum alkyls crack to form dialkylaluminum hydride and a-olefins (thermal displacement). 

There are two important side reactions, particularly above 120°C (/) aluminum alkyls decompose to form dialkylaluminum hydrides and alpha olefins (the dialkylaluminum hydrides rapidly react with ethylene to regenerate a trialkyl aluminum) ... 

This is a convenient laboratory synthesis of dialkylaluminum hydrides from trialkylalanes , especially those with branched-chain alkyl groups. Diisobutylaluminum hydride is prepared by heating triisobutylalane under Nj for 12 h at 160-180°C, followed... 

Dialkylaluminum hydrides, important reagents industrially, are prepared by treatment" of dialkylaluminum halides with LiH, e.g., diisobutylaluminum hydride (or deuteride) may be prepared by treatment of (i-C4H,)jAlCl with LiH in ether LiH or LiD in ether is cooled to 0°C and (i-C,H,)jAlCl is added dropwise under N so that an xs of LiH exists. The mixture is heated to reflux for 48 h or until a Cl test shows negative. After workup (i-C,H,)2AlH is obtained in 67% yield. 

The hydroalumination of alkynes was first observed by Wilke and Muller in 1955, when they demonstrated that dialkylaluminum hydrides added to disubstituted alkynes stereospecifically in a syn fashion (13b). Subsequently, Zakharkin and coworkers were able to add NaAlH4 to phenylacetylene by employing 5% of Bu 2A1H and diglyme as a solvent, but did not ascertain the stereochemistry of reaction. Some 10 years later, in 1966, Slaugh found that LAH in refluxing THF-diglyme mixtures, hydroalumi-nates such alkynes exclusively in an anti manner (14 equation 6). ... 

Table 1 Scope of Suitable Alkenic Substrates for Hydroalumination with Dialkylaluminum Hydrides"- ...

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 3 Selective Hydroalumination of Multiunsaturated Substrates with Dialkylaluminum Hydrides...

There is a far-reaching analogy between the addition of dialkylaluminum hydrides to olefins (hydroalumination) and Brown s hydroboration reaction, and in particular the third stage of hydroboration. Hydroboration is more facile than hydroalumination, and proceeds by cis addition to the double bond. Cis addition of R2AI—H to the double bond may reasonably be presumed but is not definitely established, since there is no test of cis addition for the terminal olefins which are most commonly subjected to hydroalumination. 

The equilibrium in Eq. (b) is shifted to the left at elevated T, and a significant amount of dialkylaluminum hydride and free olefin is present when the alkyl group is branched at the position (e.g., in i-Bu3Al) . ... 

The reaction of dialkylaluminum hydrides with mono- or di-substituted acetylenes provides the best route to alkenyldialkylaluminums . The hydroalumination of acetylene itself gives polymers Monohydroalumination of terminal acetylenes can be conducted either neat or in hydrocarbon solvents (< 50°C). The products are (E)-alkenylalanes as the result of a regio and stereoselective cis addition of Al—H to the C=C bond i- ... 

Trialkylaluminums eliminate olefin leading to a dialkylaluminum hydride as shown for the most important example, i-Bu3Al ... 

A further synthesis of certain trialkylaluminums and dialkylaluminum hydrides is afforded by the observation that trialkylaluminums, e.g., the tri-isobutylaluminum that is readily available by the above process, undergoes exchange reactions when warmed with other olefins 246,249... 

The procedure can also be varied so that the olefin is added slowly then at any one moment more isobutene is split off than olefin is added, and the solution always contains free dialkylaluminum hydride, whereby secondary reactions are avoided. Finally the temperature is lowered to 70-80° and the remaining Al-H bonds are broken by adding the remainder of the olefin. 

An excess of the olefin must be used when it does not react completely with dialkylaluminum hydrides but instead enters into an equilibrium ... 

By the use of deuterated dialkylaluminum hydride for the addition and/or heavy water for hydrolysis, specifically deuterated olefins may be prepared. Likewise, hydroboration of certain olefins has now been shown to occur in a cis-fashion (16). 

In the displacement reaction, reversible -hydride elimination of an alkene from an aluminum alkyl (equation 5) is followed by insertion of a different alkene into the Al-H bond of the resulting dialkylaluminum hydride (equation 6). Three repetitions of this process yield the new trialkylaluminum compound as in equation (7). Tri-Ao-butylaluminum obtained from the direct synthesis is a convenient starting aluminum alkyl for this process. 

In contrast to the dimeric trialkylaluminum compounds, many dialkylaluminum hydrides prefer trimeric structures with bridging hydrides as shown for Bu2AlH (3). Uhl and coworkers have prepared and structurally characterized the novel tetrameric sesquihydride [ Bu3Al2H3]2 (4), which also exhibits bridging hydrides with large Al-H-Al angles of 149(2) and 155(2)°. ... 

The polymerisation of dienes occurs when the lanthanide catalyst contains an organometallic component (usually, trialkylaluminum or dialkylaluminum hydride). Systems LnHal3 nL-AlR2lTal do not reveal catalytic activity. Moreover, dialkylaluminum... 

Organoaluminum reagents also undergo a variety of synthetically useful reactions with alkynes. The addition of dialkylaluminum hydrides to alkynes is a stereospecific cis addition. Vinylalanes that are capable of being converted to substituted alkenes by reaction with electrophilic reagents are formed. 

Reactions of dialkylaluminum hydrides with acetylenes give addition products ... 

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