Metallation of Ethene

Apparatus 11 three-necked, round-bottomed flask, fitted with a gas inlet tube, a mechanical stirrer and a combination of a gas outlet and a thermometer. 

For derivatization reactions (showing the high efficiency of the metallation) see Ref. [21]. 

25 Metallation of Norbornene and Norbornadiene with BuLi f-BuOK TMEDA 

7 describes the metallation of the olefins with a 1 1 complex of BuLi and t-BuOK (prepared in situ) in a THF-hexane mixture. The deprotonations proceed with high efficiency, provided that a large excess of the olefins are used to suppress the competitive attack of THF by the strongly basic reagent. The system BuLi t-BuOK TMEDA is soluble in hexane and has a reasonable stability below —20 °C. The metallations can be carried out with excellent results in a relatively short time (up to 2 h), using a moderate excess of the olefins. 

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Reactions between KCH2SiMe3 and cyclohexene or methylcyclohexene gave white solids formulated as 59 or 60. They were characterized by reaction with CHzO and oxidized with (COCl)2, but no structural data have been reported. Reactions with cyclooctene were similar.45 The role of tertiary amine in the metallation of ethene, n-hexene, and a-pinene has been studied.15... 

Although some studies dealing with the vinylic metallation of ethene homologues by alkyl sodium have been reported [63, 64], this direct metallation is not interesting from a synthetic point of view, because allylic deprotonation occurs to a considerable extent [65]. Interaction between Lbutylethene (ten-fold excess) and BuLi TMEDA in hexane at reflux temperature, followed by quenching with dimethyl disulfide, resulted in a low yield of the expected vinylic sulfide (only the E-isomer was isolated) [9] ... 

This year has seen some interesting developments in the area of alkenyl anions, including the first direct metallation of ethene, which was achieved using a new mixed-based metallation system (Scheme 15). Lithioethene produced in this way was trapped in good yield by benzaldehyde and diphenyldisulphide. ... 

A few n-butyllithium/potassium tert-butoxide metalations of vinylic C-H bonds are known. For example, the metalation of ethene itself, or of l,7,7-trimethylbicyclo[2.2.1]hept-2-ene. 

The first example of homogeneous transition metal catalysis in an ionic liquid was the platinum-catalyzed hydroformylation of ethene in tetraethylammonium trichlorostannate (mp. 78 °C), described by Parshall in 1972 (Scheme 5.2-1, a)) [1]. In 1987, Knifton reported the ruthenium- and cobalt-catalyzed hydroformylation of internal and terminal alkenes in molten [Bu4P]Br, a salt that falls under the now accepted definition for an ionic liquid (see Scheme 5.2-1, b)) [2]. The first applications of room-temperature ionic liquids in homogeneous transition metal catalysis were described in 1990 by Chauvin et al. and by Wilkes et ak. Wilkes et al. used weekly acidic chloroaluminate melts and studied ethylene polymerization in them with Ziegler-Natta catalysts (Scheme 5.2-1, c)) [3]. Chauvin s group dissolved nickel catalysts in weakly acidic chloroaluminate melts and investigated the resulting ionic catalyst solutions for the dimerization of propene (Scheme 5.2-1, d)) [4]. 

Formation of a <7-bond by donation from the 7r-orbital of ethene into a vacant metal dsp2 hybrid orbital... 

Olefin metathesis is the transition-metal-catalyzed inter- or intramolecular exchange of alkylidene units of alkenes. The metathesis of propene is the most simple example in the presence of a suitable catalyst, an equilibrium mixture of ethene, 2-butene, and unreacted propene is obtained (Eq. 1). This example illustrates one of the most important features of olefin metathesis its reversibility. The metathesis of propene was the first technical process exploiting the olefin metathesis reaction. It is known as the Phillips triolefin process and was run from 1966 till 1972 for the production of 2-butene (feedstock propene) and from 1985 for the production of propene (feedstock ethene and 2-butene, which is nowadays obtained by dimerization of ethene). Typical catalysts are oxides of tungsten, molybdenum or rhenium supported on silica or alumina [ 1 ]. 

In 1954, Ziegler and coworkers observed that the course of the reaction of ethene with trialkylalanes was drastically altered by the presence of traces of nickel salts [25]. Instead of low molecular weight polyethylene, the only product was 1-butene. Obviously, the transition metal strongly supports the displacement reaction of the alkyl group bonded to the aluminum by ethylene, a reaction which can be formally described as transfer of a hydridoalane. 

Ni(sacsac)P(nBu3)Cl (sacsac = pentane-2,4-dithionate) was activated by AlEt2Cl to form a catalytically active species for the oligomerization of ethene and propene. This study is noteworthy in that it uses in situ UV-VIS spectroscopy to monitor the course of the polymerization. In this reaction, the aluminum reagent serves both to activate the transition metal and to scavenge any moisture present. 

Extended Hiickel theory (EHT) was applied to study the decomposition of the five-membered metallacycle intermediate proposed by Mimoun for the epoxidation by Mo bisperoxo complexes [44, 45]. Another EHT study [40] proposed the coordination of ethene to the metal center of an MoO(02)2 complex as the first step, followed by a slipping motion of ethene toward... 

The direct attack of the front-oxygen peroxo center yields the lowest activation barrier for all species considered. Due to repulsion of ethene from the complexes we failed [61] to localize intermediates with the olefin precoordinated to the metal center, proposed as a necessary first step of the epoxidation reaction via the insertion mechanism. Recently, Deubel et al. were able to find a local minimum corresponding to ethene coordinated to the complex MoO(02)2 OPH3 however, the formation of such an intermediate from isolated reagents was calculated to be endothermic [63, 64], The activation barriers for ethene insertion into an M-0 bond leading to the five-membered metallacycle intermediate are at least 5 kcal/mol higher than those of a direct front-side attack [61]. Moreover, the metallacycle intermediate leads to an aldehyde instead of an epoxide [63]. Based on these calculated data, the insertion mechanism of ethene epoxidation by d° TM peroxides can be ruled out. 

Many other metal-catalysed polymerizations may be carried out in water including the copper-catalysed polymerization of methacrylates, the palladium-and nickel-catalysed polymerization of ethene and other alkenes and the rhodium-catalysed polymerization of butadiene [22],... 

Most catalysts are based on chromium that has been studied for this purpose since the mid-seventies, probably started by Union Carbide Corporation. Chromium is the metal of the Phillips ethene polymensation catalysts and presumably it was discovered accidentally that under certain conditions 1-hexene was obtained as a substantial by-product. Neither the precise catalytic cycle nor the intermediate complexes or precursors are known. It is generally accepted that an alkyl aluminium compound first reduces the chromium source and that coordination of two molecules of ethene is followed by cyclometallation, giving a chromocyclopentane. During the cyclometallation the valence of chromium goes up by two and thus a starting valence of either one or zero seems reasonable. This cyclic mechanism explains why such high selectivity is obtained [5],... 

The first steps involve coordination and cycloaddition to the metal. Insertion of a third molecule of ethene leads to a more instable intermediate, a seven-membered ring, that eliminates the product, 1-hexene. This last reaction can be a (3-hydrogen elimination giving chromium hydride and alkene, followed by a reductive elimination. Alternatively, one alkyl anion can abstract a (3-hydrogen from the other alkyl-chromium bond, giving 1-hexene in one step. We prefer the latter pathway as this offers no possibilities to initiate a classic chain growth mechanism, as was also proposed for titanium [8]. The byproduct observed is a mixture of decenes ( ) and not octenes. The latter would be expected if one more molecule of ethene would insert into the metallocycloheptane intermediate. Decene is formed via insertion of the product hexene into the metallo-cyclopentane intermediate followed by elimination. 

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