Organoaluminum reagents alkenes

In all of the above processes, the organoaluminum compounds serve as cocatalysts that activate a transition metal for the desired organic transformations. There are several important processes that do not involve transition metals and in which the organoaluminum reagents acts as a catalyst or stoichiometric reagent. The two most important of these are the formation of fatty alcohols and terminal alkenes from ethylene. These capitalize on the Aufbau reaction for formation of alkyl chains that can reach to C200, but the commercially important alkyls are those from C14 to C20 Oxidation of the aluminum alkyl followed by acidic hydrolysis yields predominately C14 to C20 alcohols and alumina (equation 36). The alcohols are converted to... 

The efficient activation of oxime sulfonates by organoaluminum reagents enables the intramolecular cyclization of alkenic oxime mesylates, which involves the electrophilic addition of the intermediate ni-trilium ion to the double bond. This results in the direct formation of a wide variety of structurally diverse carbocyclic and heterocyclic systems. Four distinct cyclization modes, i.e. endo(B)-endo, endo(B)-exo, exo(B)-endo and exo(B)-exo are possible, as shown in Scheme 4P The values in parentheses refer to the yields obtained using SnCU. 

Mixtures of Ni and Co salts with organoaluminum reagents catalyze the polymerization of 1,3-butadiene, whereas these catalysts are not effective for alkene polymerization due to facile chain transfer by /1-hydrogen elimination of the growing polymer. The polymerization of 1,3-butadiene usually proceeds via the intermediate n-allyl complexes, which are more stable than the alkyl metal complexes and hardly cause any /1-hydrogen elimination [56]. The late transition metal compounds polymerize 1,3-butadiene even in... 

Chloral. Yamamoto has found that the organoaluminum reagent prepared from MeaAl and (/ )-(+)-3,3 -bis(triphenylsilyl)binaphthol catalyzes the ene reaction of chloral and pentafluorobenzaldehyde with 1,1-disubstituted alkenes at -78 °C, giving the expected ene adducts in 40-90% yield and 60-90% enantiomeric excess. Use of the sterically hindered chiral auxiliary is necessary. Low yields of racemic products are obtained with 3,3 -diphenylbinaphthol. 

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. 

The ate complexes that result may be converted to vinyl halides or a,p-unsaturated carboxylic acids by procedures analogous to those employed in the previous examples. Organoaluminum reagents possess the characteristic of versatility, in that an alkyne may be converted stereospecifically to a cis- or trans-substituted alkene by appropriate choice of reagents. In the case of a,)8-unsaturated acids, the flexibility is even more pronounced, in that it is simply the order of addition of reagents that determines the stereochemistry of the product. 

Organoaluminum reagents are important in Ziegler-Natta catalysts (Section 11.5), but are not widely used in organic synthesis. They can be violently pyrophoric and water-sensitive and can add readily to alkenes. The Aufbau reaction (Eq. 14.15) is a commercial synthesis of C12-C16 linear alcohols that are useful in detergents. 

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. 

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