Ethene 57-58 metallacycle intermediates

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. 

Overall, the alkene metathesis reaction between 70 and ethene is energetically feasible. The largest free-energy difference between minima and transition states was found to be between the most stable metallacycle intermediate and the transition state for ethene de-coordination, similar to what was computed for the Mo,... 

The role of metallacycles, as intermediates in chromium-catalysed ethylene trim-erisation, is highlighted in this article. It is also shown that when 37 is treated with ethene in a protonolysis reaction, hexane and butane are liberated (1 3 ratio hexane butane). This observation gives strength to the argument that metallacycles are intermediates in chromium-catalysed ethylene trimerisation. Scheme 13 provides a simplified representation for chromium-based ethylene trimerisation. 

Since the mechanism of ethene polymerisation commonly suggested for transition-metal catalysts involves the formation of metallacycle, development of this type of catalyst was performed in order to elucidate the type of intermediate involved in the reaction. Binuclear Cr(ii) metallacycles showed little production of 1-hexene when reacted with ethene at room temperature, even after reacting for over 24 h. However, mononuclear Cr(m) metallacycles were considered possible intermediates as these complexes are able to trimerise ethene. Thus, Monillas et al. concluded this to be the likely intermediate involved in the reaction. Nonetheless, the use of the Cr(i) dinitrogen complex yields the desired product, 1-hexene, in a mechanism... 

Fischer R, Walther D, Gebhardt P, Gorls H (2000) Reactive intermediates of the catalytic carbomagnesiation reaction isolation and structures of [Cp2ZrEt]2(lt-ethene), [Cp2Zr(ethene) (L)] (L = THF, pyridine), and [(indenyl)2Zr(ethene)(THF)] and of metallacycles with norbomene. Organometallics 19 2532-2540... 

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