Four-center process

The formation and decomposition of benzenediazoazide and phenylpentazole can be described by a mechanism alternative to the one discussed in Section III, A, 1 [Eq. (3)]. In contrast to the tacitly assumed independent formation and decomposition of phenylpentazole, e.g. one-step four-centered processes as described by 21 and 22,... 

It has become increasingly clear that carbometallation reactions are mechanistically diverse. Although most of the synthetically interesting carbometallation reactions of organotransition metals appear to involve concerted four-centered processes in which the presence or ready availability of a low-lying metal-empty orbital is critically important (Scheme 3), many other processes including radical and polar processes are also known. 

Aside from two-center (Patterns 1 and 2) and three-center (Patterns 3, 4, 11, and 12) processes, most of the processes shown in Scheme 1.3 are four-center processes involving either addition (Patterns 5—10) or 0-bond metathesis (Pattern 13). In this context, it should be noted that addition is simply a four-center metathesis in which one molecule happens to be multiply-bonded. In addition to these metathetical processes, there is yet another fundamentally important four-center metathetical process termed migratory insertion and deinsertion (Patterns 14 and 15). It should be clear from Patterns 14 and 15 shown in Scheme 1.3 that distinction between insertion and deinsertion is only a relative and semantic issue. In the current discussion, a process involving cleavage of the C—Zr bond is termed migratory insertion, while the reverse process is termed migratory deinsertion. 

The reaction mechanism will be assumed to be a four-center process ... 

A kinetic study of the reaction was also performed in which NMR-obtained rate data were correlated with mercurial structure changes (12). This study revealed a quite distinct reactivity order which, coupled with a 1 1 reactant stoichiometry, indicates a 1,3-dipolar electrophilic attack by ozone via a SE2 or four-center process. Although the exact mechanism was not conclusively proved, it is certain that neither the SE1 or SEi processes were operative during these reactions. 

A slightly different mechanism has been proposed by Cairncross and Sheppard (37) for the reaction of fluoroarylcopper compounds with substituted alkyl halides. Pentafluorophenylcopper can form a complex with bicyclooctyl bromide by coordination with the halogen atom. Such a complex may go directly to coupled product in a four-center process, or, depending on the nature of the group attached and the nature of the alkyl moiety, may form an ion pair which collapses to the coupled... 

One can conclude that with some alkynes, if strong t acceptors are present in the catalyst, as CO or PX3, a catalytic multicenter process can take place in which as many as four alkyne molecules are bound to nickel (0). However, when a coordination site is blocked by a ligand difficult to replace, the four-center process changes to a three-center. 

Many of these reactions occur by the free radical chain illustrated earlier (Scheme 1). They can also occur by a concerted four-center process, but the evidence for such a mechanism is large qualitative. The relative weakness of the M-H bonds and their extremely rapid H atom transfer rates make the free radical mechanisms the more likely route. 

This is unusual since in the reduction of monocyclic and bicyclic ketones, bulkier hydroboranes produce the less stable of the two possible alcohols in accordance with steric approach control. This suggests that the transition state for the catalytic hydrosiiyiation cannot be accommodated by a simple four-centered process. The mechanism must take into account the intermediacy of an a-siloxyalkylrhodium complex, which forms according to the soft-hard concept and must be characteristic of the ketone hydrosiiyiation. 

Figure 2 illustrates several reaction coordinate diagrams that allow exothermic oxaphosphetane formation. Option a (four-center process) and the kinetically equivalent b (transient betaine precursor of the oxaphosphetane two-step mechanism) are consistent with the observation that oxaphosphetanes are formed rapidly and decompose slowly when R = alkyl. Since the barrier AGjJgc decomposition to the alkene is smaller than AGJ y, there will be little reversal or loss of stereochemistry in option a. Reversal should become less likely if the a-substituent R is unsaturated (CH=CH2 or aryl), a situation that would decrease AG g by weakening the P—Cj bond (reaction profile c). If substituents are present that retard the rate of decomposition relative to reversal (as in options d or e), then oxaphosphetane reversal and equilibration of stereochemistry become possible, as discussed in a later section. However, this behavior has not been demonstrated for members of the Ph3P=CHR ylide family in the absence of lithium salts. 

Several mechanistic variations might be possible under either of the main options (1 or 2). For example, the four-center process might involve a direct conversion from the P=C and the C=0 reactants into the oxaphosphetane (asynchronous cycloaddition) (18,59,66,219,220). In this case, there would be no other intermediates and no energy minima between the reactants and... 

Ionic mechanisms based on betaine intermediates or TS are difficult to reconcile with the absence of solvent effects on lithium-free nonstabilized ylide reactions (Table 12) or reactivity-selectivity considerations (15). Also, there is no apparent reason why the reactants should prefer to form a high-energy intermediate such as 93 when the direct conversion to a more stable oxaphosphetane 97 is possible. Orbital symmetry should not interfere with the four-center process since phosphorus can provide 3d orbitals of appropriate symmetry for a 2s - - 2s cycloaddition. Nevertheless, the betaine mechanism has persisted in the literature because there was no direct evidence against the formation of 93 as a transient intermediate until recently (229). 

A number of allylic compounds, including allyl vinyl ethers, allyJic azides, allylic thiocyanates, and allylic sulfinates, undergo unimolecular thermal isomerization reactions. Most of these reactions are concerted, four-center processes and in most of them the allylic substituent, as well as the allylic carbon skeleton, undergoes alteration. 

C.i.a. Four-Centered Processes. The carbopalladation of a C,C multiple bond with a carbon-palladium single bonds is the key step in the catalytic cycle of the standard Heck reaction, the intermolecular version of which has been used extensively since its discovery for the functionalization and derivatization of aryl and alkenyl halides, as well as alkenyl triflates or the more reactive nonafiates, which are readily available from the corresponding ketones (Scheme 2) (Sect. IV.2.1.2). 

Since base-catalyzed cleavage of the hydroboration products of cis- and trans-a, a -dimethylstilbene gives meso and dl hydrocarbon, respectively, this has been taken as evidence for a four-center process. 

Figure 6. Energetics of the reaction pathways for the conversion of nitramine, NH2NO2, to N2O and H2O. As a unimolecular four-centered process with no water molecule present (n = 0), the activation energies are high. On the other hand, as a six-centered bimolecular process involving another water molecule (n = 1), the activation energies are greatly lowered. Energies are in kcal-mol l.

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