Carbonium ions, metal complexes

The polymerization of aromatic olefins belongs to Class II for both types of metal halide if the co-catalyst is HX, but in this case the metal halide will probably be complexed not with the double bond but with the aromatic part of the molecule. Such complexes have been shown to exist in many different systems [37]. Provided that the aromatic ring is sufficiently close to the double bond the HX can react simultaneously with the metal halide complexed on to the aromatic ring and with the adjacent double bond to form the MX"n+1 and carbonium ions. For co-catalysts other than HX, polymerization might proceed by either or both mechanisms. 

Stern showed rather conclusively that the palladium does not depart to leave a carbonium ion but that both hydride migration and collapse to an aldehyde proceed simultaneously. The removal of the /3 hydrogen in a complexes by the heavier Group VIII metals has been documented. Thus Chatt and Shaw (63) showed that a platinum hydride complex could undergo the reversible addition of ethylene ... 

The polymerization of olefins in the presence of halides such as aluminum chloride and boron fluoride but in the absence of hydrogen halide promoter may also be described in terms of the complex carbonium ion formed by addition of the metal halide (without hydrogen chloride or hydrogen fluoride) to the olefin (cf. p. 28). These carbonium ions are apparently more stable than those of the purely hydrocarbon type the reaction resulting in their formation is less readily reversed than is that of the addition of a proton to an olefin (Whitmore, 18). Polymerization in the presence of such a complex catalyst, may be indicated as follows (cf. Hunter and Yohe, 17) ... 

Substituted ethanes, e.g., benzpinacol Carbonium ions Organometallic compounds ti-Complexes with transition metals, e.g., metallocenes... 

If this mechanism is correct, the aconitase reaction is an excellent illustration of the influence of the stereochemistry of the metal, as well as its charge, upon the course of a biochemical reaction. The charge on the iron is, of course, responsible for the formation of the resonating carbonium ions A and B from C, D, or E. In C and D the flow of electrons toward iron severs the bond between carbon and the hydroxyl group, whereas in E the proton is released from coordinated water and attached to one of the two ethylenic carbon atoms. The stereochemistry of the iron atom can be credited with holding the organic molecule and the hydroxide in their proper spatial relationship in A and B. It has been recently demonstrated that the complexes of the aconitase substrates with nickel have the structures postulated by Speyer and Dickman and shown in Figure 3 (19). 

The relative rate data closely parallel the results obtained in the solvolysis studies. Such a result might be expected from reactions proceeding through similar transition states. The observed order of relative rates may result from better overlap as the size of the central metal atom and the polarizability of its electron shell increase. This would result in increased stabilization and therefore ease of formation of the carbonium ions, proceeding from lighter to heavier metal complexes. 

The isolation of stable vinylidene complexes and elucidation of many of their reactions have given substance to speculations concerning their intermediacy in many reactions. Indeed, the reactions of many alkynes with a series of platinum(II) complexes were explained several years ago by considering the formation of metal-stabilized carbonium ions as nonisolable intermediates (10). Summarized below are several reactions that may reasonably be assumed to proceed via vinylidene complexes. 

The addition of a proton to a metal carbonyl compound may occur in either of two modes the formation of metal-hydrogen bond, or protonation of a ligand attached to the central metal atom. If the ligand protonated is an organic radical, a carbonium ion is produced, which may be stabilized by suitable delocalization of charge over the complex, including the central metal atom. Consequently, such protonated species may be legitimately considered as examples of cationic metal carbonyl compounds. 

The concepts of electron and ligand transfer can be applied to the oxidation and reduction of organic substrates by metal complexes,61-64 since one-equivalent changes in the oxidation states of metals in inorganic redox reactions also have analogies in organic chemistry. Thus, the interconversion of the series of species carbonium ion (R+), free radical (R ), and carbanion (R-) results from one-equivalent changes, namely,... 

In organic reactions there is abundant evidence for transient carbonium ions (R3C+), carbanions (R3C ), and carbenes ( CR2). Some stable carbonium ions like Ph3C+ and carbanions like C(CN)3 can be isolated as well as radicals like Ph3C In most of these cases the charge on the electron must be delocalized over the entire system for stability. Transition metal complexes with carbene or carbyne ligands, L M=CR2 and L M=CR, are discussed in Chapters 16 and 21. 

CH Activation is sometimes used rather too loosely to cover a wide variety of situations in which CH bonds are broken. As Sames has most recently pointed out, the term was first adopted to make a distinction between organic reactions in which CH bonds are broken by classical mechanistic pathways, and the class of reactions involving transition metals that avoid these pathways and their consequences in terms of reaction selectivity. For example, radicals such as RO- and -OH readily abstract an H atom from alkanes, RH, to give the alkyl radical R. Also in this class, are some of the metal catalyzed oxidations, such as the Gif reaction and Fenton chemistry see Oxidation Catalysis by Transition Metal Complexes). Since this reaction tends to occur at the weakest CH bond, the most highly substituted R tends to be formed, for example, iPr-and not nPn from propane. Likewise, electrophilic reagents such as superacids see Superacid), readily abstract a H ion from an alkane. The selectivity is even more strongly in favor of the more substituted carbonium ion product such as iPr+ and not nPr+ from propane. The result is that in any subsequent fimctionalization, the branched product is obtained, for example, iPrX and not nPrX (Scheme 1). 

The problem of metal participation in the properties of a-metallo-cenyl-carbonium ions Determination of absolute configurations of axial- and planar-symmetrical compounds Ferrocene-type complex compounds and organic synthesis Chemistry of ferrocenes... 

The skeletal isomerization of tetrabydrodicyclopentadiene into adamantane is an example of a very complex rearrangement diat is commercially carried out over strong Lewis acids with a hydride transfer initiator. The reaction can be catalyzed by rare earth (La, Ce, Y, Nd, Yb) exchanged faujasites (Scheme 1) in a Hj/HCl atmosphere at 25(yX3. Selectivities to adamantane of up to 50% have been reported, when a metal fimction, such as Pt, capable of catalyzing hydrogenation is added [54]. Initially acid catalyzed endo- to exo- isomerization of tetrahydro-dicyclopentadiene takes place and then a series of 1,2 alkyl shifts involving secondary and tertiary carbonium ions leads eventually to adamantane[55]. The possible mechanistic pathways of adamantane formation from tetrahydro-dicyclopentadiene are discussed in detail in ref [56]. 

From a mechanistic point of view, two different ionic mechanisms have to be considered (due to the presence of oxygen the radical chain mechanism plays no role in the technical process) first, the uncatalyzed reaction of ethylene and chlorine and second, the metal halide catalyzed reaction. Both routes compete in this process. The uncatalyzed halogenation was studied extensively for the bromina-tion of olefins [14, 15] (Scheme 4). It is commonly accepted that the halogenation of olefins starts with formation of a 1 1 -complex of halogen and alkene followed by formation of a bromonium ion. Subsequent nucleophilic attack of a bromine anion leads to the dibromoalkane. However, when highly hindered olefins (such as tetraneopentylethylene) are used, formation of a 2 1 r-complex, as an intermediate between 1 1 ir-complex and a bromonium ion, is detectable by UV spectroscopy. In the catalyzed reaction the metal halide polarizes the chlorine bond, thus leading to formation of a chloronium or carbonium ion. Subsequent nucleophilic attack of a chloride anion gives the dichloroalkane [12] (Scheme 5). 

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