Olefin theoretical treatment

Experimental results [1361] and theoretical treatment [28] indicate that the cyclo-propanation of alkenes by electrophilic carbene complexes is a concerted process. Z-Olefins normally lead to the formation of the corresponding c7. -cyclopropanes, and -olefins yield fran -cyclopropanes. The relative configuration of the carbene-bound substituent and the substituents of the alkene in the final cyclopropane seems to be mainly determined by the steric bulk of these groups. In cyclopropanations of terminal alkenes with ethyl diazoacetate low diastereoselectivities are often observed [1024,1351]. These can be improved by increasing the steric demand of the substituents at the carbene or at the alkene [1033,1362]. High diastereoselectivities can, e.g., often be achieved with terf-butyl, neopentyl or 2,6-di(rerr-butyl)phenyl diazoacetate [1362] as carbene complex precursors (Figure 4.19). 

Up to now, the solubility decrease of cobalt complexes with these modiflers has not been explained satisfyingly. It is assumed that the changes in the solvatization characteristics observed are caused by different interactions of the solute with the mixture of organic components and CO2 the modifier-solute (olefin/aldehyde-complex) interaction probably is stronger than the solute-scC02 interaction. Future theoretical treatment may also improve the... 

In the last decades, introduction of new synthetic methods66,146,147 has increased the availability of crowded organic molecules containing C=C bonds, and methods for preparing and studying labile molecules have been developed and refined. The deformations resulting from the strain in C=C bonds in different environments and the limits for existence of strained olefins have been explored in depth, and the theoretical treatment of strained systems by EFF and quantum chemical methods has developed greatly. 

This is the case, for example, in the copolymerization of carbon monoxide and ethylene where the CO will not add to itself but does copolymerize with the olefin monomer. General theoretical treatments have been developed for such cases, taking into account temperature and penultimate effects. Again, the superiority of these more complicated theories over the simpler copolymer model is not proved for all systems to which they have been applied. 

In the olefin complex the carbon-carbon double bond is believed to be perpendicular to one of the valency directions of the transition metal. The theoretical treatment of Tr-bonding by Chatt and Duncanson (77) furnishes the basic idea underlying the molecular orbital treatment for a variety of complexes between transition elements and unsaturated hydrocarbons. 

There are further subtle infiuences of structure on the strength of allylic C—H bonds. Oxidation reactions in which there is initial removal of allylic hydrogen proceed with probabilities governed in part by the relative strength of this bond. Detailed experimental data are beginning to appear, as noted later, on the more subtle effects of olefin structure, including substituent effects, steric effects, and charge effects. Theoretical treatments of such effects are not far advanced indeed, these will... 

Coke formation [911-915] is a very important phenomenon in most hydrocarbon conversions on zeolites. Also, reactions of olefins on zeolites (in particular isomerization and oligomerization) were frequently subject of in-situ IR experiments [916-919] and theoretical treatment [740, 920], in many cases because of the close relation to coke formation, for instance, during isomerization and alkylation reactions [835,916,917]. Further examples are presented in... 

Interestingly theoretical treatment has established that the energetics of the titanacyclobutane is more stable than the olefin-carbene complex resulting of its rearrangement [108] and a titanacyclobutane is in fact postulated as a titanocene methylene precursor in reaction (43). 

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