Transition state multicentered

One common feature of all M + hydrocarbon systems mentioned in Sec. 1.2.2 is that none of the products resulted from cleavage of a C-C bond. This is a result of several factors. First, C-H bonds are less directional than C-C bonds (Sec. 1.1), allowing for multicentered bonding at the transition state, which tends to lower the barrier for C-H insertion relative to C-C insertion.2,18,22 Second, since M-H bonds are usually stronger than M-C bonds, intermediates resulting from insertion into a C-H bond are usually thermodynamically favored.141 Third, there are typically more C-H bonds in hydrocarbons than C-C bonds, so C-H insertion is also statistically favored. Finally, C-H bonds are more accessible to an incoming metal atom and are therefore more susceptible to insertion. 

It could be suggested that silylation of AN occurs through multicenter transition states A or B(Chart 3.1). 

The conspicuously high speed of hydrogenolysis of allylic and vinylic halogens as compared with those removed further from the multiple bonds or halogens in alkanes implies that the multiple bond participates in a multicenter transition state [63]. Such a mechanism would account even for the surprisingly easy hydrogenolysis of allylic and vinylic fluorine [66, 528, 529, 530]. 

The only other types of system to be considered are those with multicenter bonds other than tt bonds, and non-classical even conjugated systems. Most examples of multicenter bonds involve three-center orbitals formed by mutual overlapping of AO s of three different atoms diborane and the transition state of an S 2 reaction, provide good examples. An even conjugated system can have more than one classical structure only if it is cyclic compounds of this kind are considered in the following section. 

Propagation proceeds by the electrophilic addition of carbenium ions to double bonds with the regeneration of carbocations. The transition state is relatively late, and it was estimated that approximately half of the charge is transferred into the developing carbocation (Chapter 2). This may explain the fact that dormant species (covalent esters and onium ions) do not react directly with alkenes. The charge on the a-C atoms in the dormant species is not sufficient for the formation of the transition state. A multicenter rearrangement process is additionally disfavored by entropy. In contrast, a two-step process in which carbocations are formed and then very rapidly add to alkenes is free of this difficulty. 

The localized multicenter orbital holds a certain intermediate ground, and is particularly useful when there are more valence orbitals then electrons in a molecule or transition state. First widely used in the boron hydrides and car-boranes, these three-center and multicenter orbitals provide a coherent and consistent description of much of the structure and chemistry of the upper left side of the periodic table, and of the interactions of metallic ions with other atoms or molecules. 

However, the intense fragment peak immediately below the molecular ion has an even mass and corresponds not to a simple cleavage product but to the multicenter elimination of water. Such an ortho effect is not uncommon in mass spectrometry and can be rationalized by a six-membered transition state. The subsequent loss of CO may occur by expulsion of the group from the ring or from the new ketene function (Equation 2.99). Meta and para isomers of this compound fragment in a manner parallel to that of benzoic acid loss of a hydroxy radical followed by loss of CO (Equation 2.100). 

Saville s rules serve to indicate optimal catalytic conditions. However, there exist many facile multicenter reactions which do not fall into either of the two categories. The hydrolysis rates for acetylimidazole (62) rise with increasing imidazole buffer concentration, and this observation suggests a (S)- - - transition state. 

This thermodynamically highly unfavorable reaction is mediated by the presence of U02 and the olefin providing a low energy path to the products. Similarly as in the case of the Fe(III) assisted reaction discussed above, the authors propose a multicenter transition state complex comprising... 

In a concerted step, the new cr bonds may be formed in a multicenter mechanism. In this case, no intermediates would form, only a transition state. Thus, there would be only one activation barrier in the energy profile. 

The description of 3c-4e bonds, the sort which Musher (35) has called hypervalent, in terms of linear combinations of orbitals of the three colinear atomic centers is pictured for the product sulfurane (2i ) at the right below. The 5c-6e bond i stu-lated for transition state 23 is illustrated in similar terms in the MO diagram to the left. Arduengo and Burgess (36) have discussed related multicenter bonds. 

It has been fashionable to postulate multicenter mechanisms for reactions of organometallic compounds. In the present case a four-center transition state (XII) would be an attractive one. On the other hand, by analogy with... 

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