Olefin metathesis definition

Stereospecificity as applied to olefin metathesis may be considered in two ways (a) How does the cis/trans isomer ratio of a product olefin compare with its equilibrium ratio, or (b) how does this cis/trans value differ from 1.0, which is the statistically expected value in terms of probabilities. In the present discussion, the latter definition applies. 

An approach to inducing an ot-helical conformation in peptides that is more definitive than relying on salt-bridging interactions involves chemical ligation of side-chain residues using either ring-closing olefin metathesis or Cu-catalyzed azide cycloaddition to an alkyne (click chemistry) [147-153]. This approach,... 

This definition is taken from T.M. Trnka and R.H. Grubbs (2001) Accounts of Chemical Research, vol. 34, p. 18 - The development of L2X2Ru=CHR olefin metathesis catalysts An organometallic success story . 

In this chapter, theoretical studies on various transition metal catalyzed boration reactions have been summarized. The hydroboration of olefins catalyzed by the Wilkinson catalyst was studied most. The oxidative addition of borane to the Rh metal center is commonly believed to be the first step followed by the coordination of olefin. The extensive calculations on the experimentally proposed associative and dissociative reaction pathways do not yield a definitive conclusion on which pathway is preferred. Clearly, the reaction mechanism is a complicated one. It is believed that the properties of the substrate and the nature of ligands in the catalyst together with temperature and solvent affect the reaction pathways significantly. Early transition metal catalyzed hydroboration is believed to involve a G-bond metathesis process because of the difficulty in having an oxidative addition reaction due to less available metal d electrons. 

There is some ambiguity as to whether the alkenyl- and alkynylsilane-forming reactions are true dehydrocoupling reactions. According to the definition as embodied in Eq. (1), this would require the direct activation of a C-H bond, such as the = C—H bond of an olefin. The general consensus is that the alkenyl- and alkynylsilane-forming reactions are more likely to take place via an insertion of the olefin into an M-Si bond, followed by /3-hydride elimination [Eqs. (18) and (19)] in the case of electron-rich metal complexes,1616 or by bond metathesis of M-Si and H -C= in the case... 

Noncatalytic hydrogenation of olefins under the conditions of hydropyrolysis is a definite possibility. Such a reaction could proceed by a chain mechanism involving addition of a hydrogen atom to a double bond, followed by metathesis between the radical produced and molecular hydrogen ... 

In contrast to many other methods commonly employed for stereoselective alkene synthesis such as elimination, alkenylation, alkene metathesis, alkyne addition, the JuUa olefination, and the Peterson olefination [1-3], the olefination reactions of phosphorus-stabilized carbon nucleophiles remain very powerful for modem stereoselective alkene synthesis owing to their convenience, complete positional selectivity, and generally high levels of geometrical control. However, further modifications of these olefination reactions are definitely needed to broaden substrate scope, enhance stereoselectivity, and improve environmental impacts. 

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