1,4-addition four-centered mechanisms

There are some addition reactions where the initial attack is not at one carbon of the double bond, but both carbons are attacked simultaneously. Some of these are four-center mechanisms, which follow this pattern ... 

The relative reactivity profile of the simple alkenes toward Wacker oxidation is quite shallow and in the order ethene > propene > 1-butene > Zi-2-butene > Z-2-butene.102 This order indicates that steric factors outweigh electronic effects and is consistent with substantial nucleophilic character in the rate-determining step. (Compare with oxymercuration see Part A, Section 5.8.) The addition step is believed to occur by an internal ligand transfer through a four-center mechanism, leading to syn addition. 

The copper d-orbital being very low-lying (hence no redox chemistry available) [92], MeCu can undergo addition only through a four-centered mechanism (Eq. 10.8). 

In a study of the addition of nitrosyl chloride or nitrosyl bromide to norbor-nene and norbomadiene, it was observed that (a) there was no structural rearrangement during the reaction, (b) a cis addition had taken place, (c) nucleophilic solvents such as ethanol or acetic acid were not incorporated in the products. These facts seem to speak against an ionic addition mechanism, while a free radical initiated by NO radicals was considered unlikely since nitric oxide is inactive toward norbomadiene. Therefore a four-center mechanism has been suggested [70], However, when a relatively simple, unstrained olefin such as A9-octalin was subjected to the reaction, only blue, crystalline, monomeric 9-nitroso-10-chlorodecalin was produced. This product had a trans configuration. Thus it is evident that the structure of the olefin has a significant bearing on the steric course of the addition [71]. 

Most workers have presumed that the addition reaction proceeds via TT-donation from the olefin into the vacant/) orbitals of the diboron compound (55). Stereochemical evidence (discussed more fully below. Section III,B) is in general agreement with such a four-center mechanism, and there is as yet no convincing evidence for an alternative. The involvement of the boron p orbitals is clearly indicated by the lack of reactivity of B2Cl4-base complexes as well as by the failure to obtain addition with compounds such as the aminodiborons in which the coordinative saturation at the boron atoms can be removed or reduced by pir-pn bonding. 

The term insertion describes, in our understanding, the end result but not the actual process insertion does not have to proceed in one chemical step involving a four-center mechanism, but may occur stepwise by chain breaking, monomer addition, and recombination of the cation with a terminal hydroxyl group ... 

An objection that may be raised to a concerted four-center mechanism is the possibility of violation of orbital symmetry rules (68, pp. 65-78). Formally, the cyclization and cleavage may be viewed as forbidden + J2,] and [Jl, + J2,] cycloadditions (see transition state structure 41). The polarity of the bonds involved may alleviate the situation somewhat (21, 41). However, if the magnesium atom utilizes an additional orbital as in 42 (formally occupied partially by binding a solvent... 

Aside from the concentration dependence, most of the mechanistic discussion could be applied equally well to either the concerted four-center mechanism or the bimolecular carbanionic mechanism, since the latter appears to be described as essentially a concerted addition. It may be that a more complex kinetic scheme, or one with a tetrameric or more highly associated transition state, could fit the concentration dependence. 

A less-common activation parameter, the volume of activation (AF ), has been determined for a few Diels-Alder reactions carried out under pressure in liquid phase. The processes are cyclopentadiene dimerisation , isoprene dimerisation , addition of 2,3-dimethylbutadiene to butyl acrylate , addition of cyclopentadiene to dimethyl acetylene dicarboxylate , and addition of maleic anhydride to 1,3-cyclohexadiene, /rans-l-methoxybutadiene and isoprene . Activation volumes are negative, i.e. the reacting systems contract on passing from the initial to the transition state. In some cases the transition state appears to be even smaller than the adduct, independently of the solvent . Some of these experimental results gave rise to controversial interpretations however, the most recent ones favour a concerted four-center mechanism for the reaction. 

The elucidation of the role of double-bond isomerization activity in metathesis process is an example of the helpfulness of the four-center mechanism. As the scheme predicted, in certain applications the elimination, of double-bond isomerization activity (acidic isomerization sites were destroyed by various mild caustic treatments) prevented secondary metathesis reaction resulting in very high selectivity to specific products ( 5). In contrast, in other applications (e.g., linear olefin and neohexene processes) to obtain a high level of productive metathesis, the mechanistic scheme indicated a need for enhanced isomerization activity this was accomplished by addition of a very selective double-bond isomerization catalyst to the scheme ( ),... 

Hydrocarbons can also be activated through reactions that do not involve an oxidative addition to an unsaturated metal complex but rather proceed by a four-centered mechanism. This is usually called heterolytic activation of C-H bonds because these bonds are polarized in the transition state, by action of an electrophilic metal and a... 

The observation of consistent syn addition in the hydroboration step suggests that the mechanism for addition of BH3 is different from all of the addition mechanisms discussed so far except one. As discussed on page 580, a one-step, four-center mechanism was considered in the s)m addition of F2 to an alkene, and a similar mechanism can explain the stereochemistry of BH3 addition. Moreover, the regiochemistry of addition to imsymmetrical alkenes can be rationalized in terms of the steric and electronic effects present in such a transition structure. 

A simplified mechanism for the hydroformylation reaction using the rhodium complex starts by the addition of the olefin to the catalyst (A) to form complex (B). The latter rearranges, probably through a four-centered intermediate, to the alkyl complex (C). A carbon monoxide insertion gives the square-planar complex (D). Successive H2 and CO addition produces the original catalyst and the product ... 

A theoretical study of the reaction mechanism for addition of organozincate complexes to aldehydes was recently performed using density functional theory.298 It has been suggested that the addition takes place through formation of a four-centered transition state and, therefore, it can be considered a typical nucleophilic reaction. 

Four-centered addition of RCu to an enone was widely discussed in the 1960s (Scheme 10.3a) [51-53], while discussions on six-centered transition states have continued until recent times (Scheme 10.3b) [54]. These mechanisms do not, however, explain the formation of E/Z mixtures of enolate stereoisomers [20, 55] and must now be considered obsolete. 

The mechanism of this reaction is not well understood. It is a kind of three-or four-center addition. Some variations of this mechanism are ... 

Schlenk intermediates and the addition path have been made so far. However, our results seem to provide simple and realistic pathways. The Grignard addition occurs in a dimeric dichlorine-bridged form. A vicinal-magnesium bonding alkyl and C=0 interaction causes C—C bond formation via a four-center interaction as shown in Scheme 20 (polar mechanism). When the interaction is improbable owing to the steric effect, the Mg—0=C bond formation precedes the C—C bond formation and the Mg—C bond is ruptured (SET). 

In general, the ease of addition of H—X to simple alkenes follows their relative acidity, HI > HBr > HC1, but HF addition is often surprisingly easy. A diversity of mechanisms appear to be involved in these processes, ranging from relatively pure carbocation processes to those more reminiscent of four-center addition. Markovnikov addition is commonly observed when precautions are taken to exclude peroxide or other possible free radical initiators. Though strong acids are involved and rearrangements are not uncommon, the reaction conditions are otherwise quite mild and yields can be high. 

In addition, studies indicate that Zn(CH3)2 undergoes rapid exchange with Cd(CH3)2, and various postulates concerning the mechanism of this reaction have been made (57, 87). Recent studies of this system in methyl-cyclohexane solvent have clearly shown that the reaction is first order in each of the components and proceeds with an activation energy of 17 kcal/mole (57). This study indicates that the exchange process proceeds through a four-centered transition state... 

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