Pathway dependence

With unsymmetrical ketones two different bonds are available for photolytic cleavage the actual cleavage pathway depends on the relative stability of the possible radical species R and R. ... 

The terpenoid precursor isopentenyl diphosphate, formerly called isopentenyl pyrophosphate and abbreviated IPP, is biosynthesized by two different pathways depending on the organism and the structure of the final product. In animals and higher plants, sesquiterpenoids and triterpenoids arise primarily from the mevalonate pathway, whereas monoterpenoids, diterpenoids, and tetraterpenoids are biosynthesized by the 1-deoxyxylulose 5-phosphate (DXP) pathway. In bacteria,... 

These examples clearly show that the initiation pathways depend on the structures of the radical and the monomer. The high degree of specificity shown by a radical e.g. i-butoxy) in its reactions with one monomer (e.g. S) must not be taken as a sign that a similarly high degree of specificity will be shown in reactions with all monomers (e.g. MMA). 

The relative importance of the various pathways depends on the alkyl groups (R). The rate constants for scission of groups (R ) from /-aikoxy radicals (RR C-O) increase in the order isopropylalkyl radical is less important when R is methyl than when R is a higher alkyl group, if the pathway to alkylperoxy radicals is dominant, the resultant polymer is likely to have a proportion of peroxy end groups.200 211... 

Quinones may react with carbon-centered radicals by addition at oxygen or carbon, or by electron transfer (Scheme 5.]6).l74, fi2 195 201 202 The preferred reaction pathway depends both on the attacking radical and the particular quinone (halogenated quinones react preferentially by electron transfer). The radical formed may then scavenge another radical. There is also evidence that certain quinones e.g. chloranil, benzoquinone (38)] may copolymerize under some conditions. ... 

As the reactions proceed, the disappearance of phenol is delayed due to competition for reaction with formaldehyde between phenol and the faster reacting hydroxymethyl-substituted phenols. Competition also exists between formaldehyde substitution reactions and condensation reactions between rings. Condensation reactions between two ortho-hydroxymethyl substituents are the least favorable condensation pathway. Depending on the reaction conditions, substitutions occur... 

Some years ago, we tackled (ref. 7) the particular question of bromine bridging, related mainly to stereochemistry, postulating that bromonium ions and bromo-carbocations are formed in separate pathways as shown in Scheme 3. The relative rates of reaction by these pathways depend on the olefin structure. As demonstrated later... 

The range of chemicals capable of reducing Pd to the zero oxidation state is vey wide however it is remarkable that the desired selectivity towards the coupling pathway depends in a large measure on the choice of reducing agent. 

The reductive couphng of imines can follow different pathways, depending on the nature of the one-electron reducing agent (cathode, metal, low-valent metal salt), the presence of a protic or electrophihc reagent, and the experimental conditions (Scheme 2). Starting from the imine 7, the one-electron reduction is facihtated by the preliminary formation of the iminiiim ion 8 by protonation or reaction with an electrophile, e.g., trimethylsilyl (TMS) chloride. Alternatively, the radical anion 9 is first formed by direct reduction of the imine 7, followed by protonation or reaction with the electrophile, so giving the same intermediate a-amino radical 10. The 1,2-diamine 11 can be formed from the radical 10 by dimerization (and subsequent removal of the electrophile) or addition to the iminium ion 8, followed by one-electron reduction of the so formed aminyl radical. In certain cases/conditions the radical 9 can be further reduced to the carbanion 12, which then attacks the... 

The above-postulated overall mechanism considers two alternative pathways depending on the nature of the acetylene derivative. Region A outlines a proposal in which the formation of the a-complex intermediate is supported by the fact that the treatment of aliphatic terminal acetylenes with FeCl3 led to 2-chloro-l-alkenes or methyl ketones (Scheme 12). The catalytic cycle outlined in region B invoked the formation of the oxetene. Any attempt to control the final balance of the obtained... 

The metabolism of fluorobenzoates has been examined over many years. Early studies using Nocardia erythropoUs (Cain et al. 1968) and Pseudomonas fluorescens (Hughes 1965) showed that although the rates of whole-cell oxidation of fluorobenzoates were less than for benzoate, they were comparable to, and greater than for, the chlorinated analogs. As for their chlorinated analogs, both dioxygenation and hydrolytic pathways may be involved, and studies have revealed that the different pathways depended on the positions of the fluorine substituents. 

Hirschorn SK, Ml Dinglasan, M Eisner, SA Mancini, G Lacrampe-Couloume, EA Edwards, BS Lollar (2004) Pathway dependent isotopic fractionation during aerobic biodegradation of 1,2-dichloroethane. Environ Sci Technol 38 4775-4781. 

Signals from neighbouring cells or tissues will instruct a particular cell to proliferate, differentiate into another cell type or to commit suicide by programmed cell death. Figure 10.4 shows that NFKB can play a major role in countering programmed cell death and hence survival of the cell after stimulation of the TNFa pathway. Such a pathway depends upon the activation of NFKB which will activate a variety of anti-apoptopic genes. 

Table 22.1 Partitioning of aldolization and 1,4-reduction pathways depends critically on the use of cationic Rh-complexes and mildly basic additives.a)...

Thus, in summary, we infer that there is not a simple answer to the question which is the primary product, CO or CO2 . The complexity might suggest that the reaction pathway depends not only on the catalyst composition but also on the reaction conditions. 

As can be appreciated from the curve in Fig. 25, the amplitudes vary more for the sequential cross peaks than for the intraresidual pathway, depending on the secondary structure. This is because we have intentionally optimized the 15N-13C transfer step for the intraresidual correlations. Although the sequential connectivities also appear in the HN(CA)CO-TROSY... 

The reaction pathway depends on the steric hindrance and/or the electron density of the allene moiety. With alcohols bearing a less or unsubstituted allene moiety, the addition of CaC03 may help the cyclization [166,167]. 

Figure 4 gives an alternative scheme of possible biomass conversion pathways. Depending on the type of available biomasses and the objective products, each biorefinery will implement a different production and conversion scheme. 

The relative importance of the three pathways depends on internal and external factors. External factors such as pH will be discussed in the pages to follow, as will internal factors such as the nature of the adjacent residue. There is, however, one internal factor that can be deduced from Fig. 6.27, namely that Pathway b is priviledged relative to Pathway c since the formation of a five-membered ring is entropically more favorable than that of a six-membered ring [93] [96]. 

The presence of the OH group in alcohols makes alcohol combustion chemistry an interesting variation of the analogous paraffin hydrocarbon. Two fundamental pathways can exist in the initial attack on alcohols. In one, the OH group can be displaced while an alkyl radical also remains as a product. In the other, the alcohol is attacked at a different site and forms an intermediate oxygenated species, typically an aldehyde. The dominant pathway depends on the bond strengths in the particular alcohol molecule and on the overall stoichiometry that determines the relative abundance of the reactive radicals. 

---