14 Catalysis at the Atomic Level
Downloaded via COLUMBIA UNIV on September 3, 2019 at 00:20:57 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
Dimerization and Polymerization of Ethylene with Nickelocene and Dibenzenechromium M . TSUTSUI, J. ARYOSHI, T. KOYANO, and M . N . L E V Y a
b
c
New York University, University Heights, New York, Ν. Y. 10453 Recent developments in catalysis at the atomic level are described. The use of transition metal halides which form intermediate metal π-complexes can promote both the cata lytic and stoichiometric coupling of aryl Grignard reagents, the stereospecific coupling of 1,2 disubstituted vinyl groups and the double coupling of ethylene to dimers of butadiene. In the most recent work, nickelocene was decomposed at 200°C. under 600 lb./sq. in. pressure and ethylene thereby selectively dimerized to butene. Similarly, dibenzenechro mium was decomposed at 250°C. under 2800 lb./sq. in. pressure, and ethylene was polymerized to a relatively high density polyethylene.
>Tphe concept of catalysis at the atomic level involves a new approach -*· to stimulate reaction partly by creating an active metal atom in a favorable environment. The creation of the active zero-valent metal can be accomplished in several ways. It has been shown that homolytic cleavage of σ-bonded aryl metal compounds or homolytic cleavage of an alkyl-metal bond produce an active zero-valent metal. In addition, its creation from the decomposition of either a transitory, unstable, or even stable metal 7r-complex has been demonstrated. The metal atom thus formed is essential to the catalysis of a given system. In 1914 Bennett and Turner ( I ) failed in their attempt to prepare σ-bonded triphenylchromium by refluxing an etheral solution of phenylPresent address: Arakawa Forest Chemical Co., 21-1, Hiranomachi, Higashi-ku, Osaka, Japan. Present address: Toa Nenryo Co., Oi-mura, Iruma-gum, Saitama-ken, Japan. Present address: Chemical and Solvent Distillers Co., Inc., East Elmhurst, Ν. Y.
a
b c
266 Luberoff; Homogeneous Catalysis Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
14.
TSUTSUI ET AL.
Catalysis at the Atomic Level
Figure 1.
267
Stoichiometric aryl coupling reaction
magnesium bromide with chromic chloride. They obtained biphenyl in nearly quantitative yield (Figure 1 ) . Largely through the efforts of Zeiss and Tsutsui (6, 14, 15), this simple aryl coupling reaction was ex panded, and a mechanism involving intermediate metal ττ-complexes was proposed (Figure 2). B y varying the reaction conditions, either biphenyl and/or stable metal ττ-complexes can be isolated. In Figure 2 the divalent state of the metal is used for simplicity. The first step of this stoichiometric aryl coupling reaction involves the formation of a σ-bonded diphenylchromium compound, I. Homolytic cleavage, II, results in the formation of a hypo thetical diradical π-complex, III, or a half-sandwich π-complex, IX. Diva lent chromium is reduced to zero-valent chromium at this stage, a twoelectron reduction. In the final stage of the reaction biphenyl is produced under vigorous reaction conditions at room temperature or by refluxing. Under milder conditions (— 1 0 ° C . ) or in the presence of additional phenylmagnesium bromide, mixed arene π-complexes and biphenyl are isolated. A mechanism proposed for a catalytic coupling reaction is illustrated in Figure 3. This reaction, reported by Kharasch and Fields ( 4 ) involves the formation of biphenyl by adding an organic halide such as bromobenzene to a mixture of phenylmagnesium bromide and a trace of a transition metal halide. The reaction is striking since ( a ) organic halides do not react with arylmagnesium compounds or with metal halides, and
Luberoff; Homogeneous Catalysis Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
268
HOMOGENEOUS CATALYSIS
Figure 3.
Mechanism of catalytic aryl coupling reaction
(b) metallic halides couple stoichiometrically not catalytically with aryl Grignard reagents. Diphenylcobalt may cleave as described for the stoichiometric mechanism giving biphenyl and an active zero-valent cobalt atom. The cobalt atom can react with bromobenzene, producing
Luberoff; Homogeneous Catalysis Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
14.
TSUTSUI ET AL.
Catalysis at the Atomic Level
269
phenyl radicals and cobaltous bromide. The trace quantity of transition metal halide is maintained, and the catalytic role of the transitory radical cobalt τΓ-complex and cobalt metal is sustained. It is not unreasonable that the bromobenzene may attack the cobalt complex directly. Dibenzenecobalt is not expected to possess any degree of stability since cobalt does not easily accept 12 electrons from the two benzene rings. However, even a transitory existence can be postulated since it has been shown that the biphenyl phenyl groups originate from phenylmagnesium bromide and not from bromobenzene. The over-all concept of catalysis by zero-valent transition metals is also being developed by others (2,16). W e have thus far assumed that the initial step in all of these re actions involves the formation of a
