Competition from other reaction pathways

Although many other reaction pathways are possible only a few by-products such as 21, resulting from a competitive ring closure step, are present to a small extent. 

Alternative paths for decomposition of the metal carboxylate can lead to ketones, acid anhydrides, esters, acid fluorides (1,11,22,68,77,78), and various coupling products (21,77,78), and aspects of these reactions have been reviewed (1,11). Competition from these routes is often substantial when thermal decomposition is carried out in the absence of a solvent (Section III,D), and their formation is attributable to homolytic pathways (11,21,77,78). Other alternative paths are reductive elimination rather than metal-carbon bond formation [Eq. (36)] (Section III,B) and formation of metal-oxygen rather than metal-carbon bonded compounds [e.g., Eqs. (107) (119) and (108) (120). Reactions (36) and (108) are reversible, and C02 activation (116) is involved in the reverse reactions (48,120). 

In addition to the favorable reaction cycle, the P-hydrogen elimination from Z7 leading to the formation of vinylborane side-products is also found to be competitive (Figure 7). In other words, side products are difficult to avoid in the associative reaction pathway. 

The development status of these molecules is not known. It will be interesting to note whether any differences emerge from the CAAX competitive versus FPP competitive molecules as more data become available for these compounds. Since FPP itself contributes to the CAAX peptide binding pocket, the interaction of FPP competitive FTIs with CAAX peptide competitive FTIs will be of interest. The selectivity of FPP competitive FTIs for the FTase pathway versus other biochemical pathways utilizing FPP, such as ubiquinone synthesis and the heme farnesyltransferase, has also not been reported. These other FPP reactions have important roles in mitochondrial function, which presents some risk for adverse events or possibly opportimities for modulating early apoptotic events. 

Thomas et al. (43) demonstrated that NO is depleted by cells other than red blood cells in an 02-dependent process of unknown mechanism. This reaction has implications for the diffusion distance of NO away from the point of origin. These pathways are important for confining the effects of NO to specific cellular regions by controlling migration to neighboring cells or tissues. Hence, processes that are mediated by NO must be competitive compared to the rates of other consumptive pathways in order to exhibit biological importance. 

These carbocations can undergo three reactions (i) single-electron oxidation, (ii) hydride abstraction and (iii) electrophilic addition. Thus, these compounds behave as mild single-electron oxidants towards reductants which do not react by other pathways. Actually, what is most interesting in these carbocations is the competition between electron-transfer and the other reactions. For instance, a hydride transfer can occur either directly or via the electron-transfer pathway using a trityl salt. From a synthetic standpoint, a hydride transfer which cannot be achieved directly for steric reasons may be attempted by means of the electron-transfer pathway. 

It will be noted that, in reactions 23-28, compound MH is consumed by two competitive processes, namely oxidation and protonation. Thus, these stoichiometries will be observed when the following two conditions are satisfied (0 the unoxidized complex MH is the strongest available base in the reaction medium and (ii) the rate of proton transfer is fast relative to the rate of oxidation of MH. When, on the other hand, the proton transfer is much slower than the electron transfer, then the oxidizing agent can finish its job and no more MH will be available to capture the proton from [MH]+. Under these conditions, [MH]+ can only deliver its proton (slowly) to an external base, if a sufficiently strong one is present, otherwise the product becomes kinetically stabilized toward deprotonation and can sometimes be isolated. However, other decomposition pathways may remain available, see following sections. 

This confirms the previously proposed mechanism of the monomethylamine imidiza-tion reaction except for the observed 1 1 dimethylamine/MMA stoichiometery. This stoi-chiometery indicates that the predominate reaction pathway of PMMA with dimethylamine probably involves two alkylations per anhydride (as in 2a -> 3a, Figure 2). The formation of amide at extreme reaction conditions could be due to a slower competitive amine acyl addition pathway (2c or addition to acid). Other possibilities include equilibrium reactions where high amine pressures could shift the equilibrium from, for instance, anhydride (3a and 3b) to amic-acid pair (4a and 4b). If the latter concentration is high enough, or leads to some statistically trapped groups, then some amides might survive the devolatilization conditions. 

In 1987, when rationalizing the formation of 24 from 21, one mechanistic pathway suggested by Overman and coworkers involved the formation of intermediate 22 which might undergo a concerted 2-oxonia[3,3]sigmatropic rearrangement followed by an intramolecular aldol reaction/ While other evidence in these studies favors an alternate mechanism, the oxonia-Cope mechanism suggested here has in fact been identified as a competitive, and sometimes favored pathway " in a number of transformations. 

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