Carbonium ions, addition reactions selectivity

Organometallic complexes often activate the C-H either by oxidative addition (Fig. 6.3, path a) or a-bond metathesis, or a-CAM (path b). These reactions favor attack at a terminal C-H bond, leading to subsequent terminal functionalization (e.g., PrH n-PrX), or at an arene C-H bond (e.g., ArH ArX). This selectivity usefully contrasts with standard organic reactions via radicals or carbonium ions that are selective for the most highly substituted or benzylic CH bonds (e.g., PrH i-PrX ArMe ArCH2X). Species such as i-Pr- or i-Pr+ are more stable and more rapidly formed than n-Pr- or n-Pr+. Numerous organic synthetic applications of C-H activation continue to be found (Chapter 14). 

Since N-nitrosoimmonium ions seem to be involved in the hydrolysis of a-acetates, it should be possible to isolate such species as stable salts. For this purpose, we selected a system such as XVII in which the phenyl group should provide further stabilization of such a carbonium ion. After the reaction of nitrosyl chloride with the corresponding imines, addition of antimony pentachloride resulted in the precipitation of pale yellow solids these could be isolated and stored under nitrogen for several days at room temperature. ... 

A second important selectivity issue arises when there are several diffoent types of C—H bond in the molecule, typically, primary, secondary and tertiary C—bonds. Since tertiary radicals and carbonium ions are more stable than their secondary or primary analogs, many functionalization processes have an intrinsic selectivity pattern tertiary > secondary > primary. Steric effects fiivor attack at primaiy positions, which is seen for very bulky reagents or in reactions in which the C—H bond to be br en is brought side-on to the functionalizing groiqi, and therefore makes the transition state very sensitive to steric effects. The best example is oxidative addition to a transition metal conqilex. 

Equation (26) is the ideal copolymer composition equation suggested [203] early in the development of copolymerization theory but which had to be abandoned in favour of eqn. (23) as a general description of radical copolymerization. Only in this particular case are the rates of incorporation of each monomer proportional to their homopolymerization rates. It was shown that the reactivity of a series of monomers in stannic chloride initiated copolymerization followed the same order as their homopolymerization rates [202] and so eqn. (26) could be at least qualitatively correct for carbonium-ion polymerizations and possibly for reactions carried by carbanions. This, in fact, does not seem to be correct for anionic polymerizations since the reactivities of the ion-paired species at least, differ greatly. The methylmethacrylate ion-pair will, for instance, not add to styrene monomer, whereas the polystyryl ion-pair adds rapidly to methylmethacrylate [204]. This is a general phenomenon no reaction will occur if the ion-pair is on a monomer unit which has an appreciably higher electron affinity than that of the reacting monomer. The additions are thus extremely selective, more so than in radical copolymerization. There is no evidence that eqn. (26) holds and the approximate agreement with eqn. (25) results from other causes indicated below. 

A bimolecular reaction of C-ethanemethonium or C-proponium ions with propane molecule may result in formation of ethane and buthonium ion. The latter may further evolve to n-butane or i-butane via deprotonation and recovering the Brpnsted acid site. This reaction step may occur either via consecutive mechanism [15, 16] involving formation of CH3 carbenium ion bound to the zeolite framework, followed by its addition to propane molecule, or via concerted mechanism involving CHs transfer from carbonium ion to the propane molecule. This mechanistic pathway explains butanes primary formation. Ethane was also found to be a primary product, however selectivity to ethane formation was much lower comparetively to butanes. It may be due to the fact that ethane... 

We may thus conclude that in addition to carbonium ion route, other alternate reaction routes are plausible for the activation of C2H4 molecules over zeolite based catalysts leading to the formation of higher hydrocarbons. The effect of pretreatment temperature on the selectivity of reaction products (Fig.3) indicates an inq)ortant role played by the zeolite pore structure and the nature of the charge balancing cations. We are now investigating these aspects in detail. 

The general terms regioselective and regiospecific have been introduced to describe addition reactions that proceed selectively or exclusively in one direction with unsymmetrical alkenes. Markownikoff s rule then describes a general case of regioselectivity that operates because of the stabilizing effect of alkyl and aryl groups on carbonium ion centers. 

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