Aufbau reaction

Aluminium is used in a number of important chemical transformations. Perhaps the best-known example is the Aufbau reaction, discovered in 1950 by Ziegler [1] and still in use today ... 

Under the reaction conditions employed, the oligomerization is living the aluminium is removed from the chain in a separate process step [2]. Further research on the Aufbau reaction led to the discovery of transition metal catalyzed olefin polymerization (Ziegler-Natta catalysis) [3]. 

At lower temperatures (or in solution) and at high monomer concentration, a second chain termination process that could occur is direct j -hydrogen transfer to a second molecule of monomer. This kind of chain transfer step is now generally accepted for many transition-metal-catalyzed polymerizations, where direct /1-elimination would be too much uphill to explain the observed molecular weights, for olefin oligomerization at aluminium, a similar situation applies. Since insertion and j -hydrogen transfer have an identical concentration dependence, their ratio does not depend much on the reaction conditions (except temperature) and hence limits the molecular weight attainable in the Aufbau reaction. 

Gallium, on the other hand, shows a clearly increased barrier for olefin insertion, but smaller increases for j -elimination and j -hydrogen transfer. Thus, the Aufbau reaction at gallium should be much slower and have a much lower limit on the attainable molecular weight. 

Variations of this mechanism included the suggestion of a partially bonded alkene molecule,299 the participation of a titanium-aluminum ion pair,300 and a concerted alkene insertion.301 The development of the activator-alkyl mechanism was probably strongly influenced by the Aufbau reaction, studied originally by Ziegle.102 He observed that Group I—III alkyl compounds such as Et3Al catalyzed the oligomerization of ethylene to terminal alkenes. Additional evidence of such mechanism comes from the fact that alkylaluminum compounds exist in dimeric... 

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... 

Ziegler and coworkers at the Max Planck Institut fiir Kohlenforschung (Coal Research) in what was then Mulheim, West Germany were working to expand the scope and utility of the aufbau reaction. It was during this endeavor in 1953 that they accidentally discovered the "nickel effect." This term stemmed from the observation that nickel in combination with triethylaluminum catalyzes dimerization of ethylene to produce 1-butene. Accounts vary on the source of nickel in the formative experiments. It was ultimately attributed to trace nickel extracted from the surface of the stainless steel reactor in which early reactions were conducted. 

This approach was first accomplished by Ziegler in his early work when nickel was introduced with titanium, either purposely or through autoclave corrosion as part of the investigation of the Aufbau reaction. The nickel dimerized ethylene to butene, while the titanium incorporated it at low levels yielding poly ethylenes with low levels of ethyl branches. We have been told that this work appears in early notebooks from the Ziegler laboratory, but we have been unable to find a reference in the open literature. 

Although limited experimental evidence does lend support to this concept, major objections have been voiced by Ziegler, who is of the opinion that as dimeric aluminum alkyls are inefficient catalysts in the Aufbau reaction, the Ti-Al complex is not likely to be the effective catalytic agent. Other more recent work also favors the second and simpler alternative, the monometallic mechanism. 

Ziegler presentation in 1952 on the Aufbau reaction started collaboration with Natta. The 2-methyl-1-pentene was the starting material Ru industrial synthesis of isoprene, while the 2-ethyl-1-hexene opened up the way to terephthalic acid... 

Eventually, the product of the aufbau reaction added more ethylene molecules and a verdangung (displacement) reaction occurred which produced a mixture of linear alkanes. Ziegler applied for patents, published and discussed his results in frequent lectures. 

Subsequently, 1-butene was obtained exclusively in the aufbau reaction. Further investigations by Ziegler and coworkers E Holzkamp, H. Briel, and H. Martin showed that a trace of nickel from the stainless steel autoclave was responsible for the change in the reaction product. An investigation of salts of other related metals in 1953 showed that elimination of butene did not occur in the presence of titanium salts but that the aufbau reaction... 

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. 

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