Ethylene chemistry, recent developments

T thylene chemistry encompasses such a broad scope that this discussion must be limited to new developments which have industrial or commercial significance. Therefore, only recent developments related to the production of heavy organic chemicals (petrochemicals) from ethylene are considered. 

These are a few of the recent developments in ethylene chemistry which have, or will have, an impact on the production of tonnage organic chemicals. As larger ethylene capacity is installed, greater impetus is given to further replacement of acetylene as a raw material, and the search for new or improved uses for ethylene becomes intensified. 

Recent developments in the chemistry of mesoionic compounds85 include cycloaddition-elimination reactions, which afford novel synthetic routes to a variety of heterocyclic systems. These reactions may be seen as involving 1,3-dipolar cycloadditions, following Huisgen,86 or alternatively as 1,4-cycloadditions to heterodiene systems,87 depending on the choice of canonical structure to represent the mesoionic compound. Benzyne has been employed in such reactions less frequently than more stable acetylenic or ethylenic dipolarophiles. 

Functionalized polyethylene would be of great industrial importance, and if synthetic methods to control the microstructure of functionalized polymers using transition-metal-based catalysis are developed, it would significantly broaden the utility and range of properties of this class of polymers. Recent progress in the field of late transition metal chemistry, such as Brookliart s use of nickel-based diimine catalysts, has enabled the copolymerization of ethylene with functional a-olefins.29 However, these systems incorporate functionalized olefins randomly and with limited quantity (mol percent) into the polymer backbone. 

Enyne metathesis is unique and interesting in synthetic organic chemistry. Since it is difficult to control intermolecular enyne metathesis, this reaction is used as intramolecular enyne metathesis. There are two types of enyne metathesis one is caused by [2+2] cycloaddition of a multiple bond and transition metal carbene complex, and the other is an oxidative cyclization reaction caused by low-valent transition metals. In these cases, the alkyli-dene part migrates from alkene to alkyne carbon. Thus, this reaction is called an alkylidene migration reaction or a skeletal reorganization reaction. Many cyclized products having a diene moiety were obtained using intramolecular enyne metathesis. Very recently, intermolecular enyne metathesis has been developed between alkyne and ethylene as novel diene synthesis. 

A new development in silsesquioxane chemistry is the combination of sil-sesquioxanes with cyclopentadienyl-type ligands. Recently, several synthetic routes leading to silsesquioxane-tethered fluorene ligands have been developed.86,87 The scenario is illustrated in Scheme 47. A straightforward access to the new ligand 140 involves the 1 1 reaction of 2 with 9-triethoxysilylmethylfluorene. Alternatively, the chloromethyl-substituted c/ovo-silsesquioxane derivative 141 can be prepared first and treated subsequently with lithium fluorenide to afford 140. Compound 141 has been used as starting material for the preparation of the trimethylsilyl and tri-methylstannyl derivatives 142 and 143, respectively, as well as the novel zirconocene complex 144. When activated with MAO (methylalumoxane), 144 yields an active ethylene polymerization system. 

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