Dead-end intermediate

Tliis rate expression is conslstenl widi die reaction sclieme shown in Eq. 10.6, fot-niLilaled on die basis of die Ktauss-Smidi paper. Tlius, die inilially formed cuptate diniet/enone complex widi lidiium/catbonyl and coppet/olefin cootdinalions [71, 72] Itansfbrms inlo die product via an intermediate ot inlermediales. A lidiium/ carbonyl complex also forms, bul diis is a dead-end intermediate. Tliougli detailed... 

One key feature of the model is that the [Fe2(0H)S03]3+ species is a dead-end intermediate with respect to the overall redox process. The actual redox reaction consists of only two steps ... 

This rate expression is consistent with the reaction scheme shown in Eq. 10.6, formulated on the basis of the Krauss-Smith paper. Thus, the initially formed cuprate dimer/enone complex with lithium/carbonyl and copper/olefin coordinations [71, 72] transforms into the product via an intermediate or intermediates. A lithium/ carbonyl complex also forms, but this is a dead-end intermediate. Though detailed... 

Formation of Dead-end Intermediate Oxidation States, Particularly... 

In this context, in the case of catalase the Fe=0 complex may be considered as a dead-end intermediate, though for peroxidases it most likely represents the catalytic site for one-electron oxidation. 

The fragment [Pt(PPh2-/-Bu)2] was found to insert into the C-C bond of biphenylene, and could slowly catalyze the formation of tetraphenylene [16]. Here, however, intramolecular cyclometallation of the phosphine phenyl ring leads to an off-cycle dead-end intermediate, resulting in slow catalysis (5). 

The isomerization barrier of 15.0-20.0 kcal mol-1 (AG ) can be considered to be large enough to allow isolation and characterization of bis(q3-<2 /),A- nms-dodecatrienediyl-Nin stereoisomers of 7b41 as reactive intermediates in the stoichiometric cyclotrimerization process. Furthermore, the trans orientation of the two allylic groups gives rise to an insurmountable barrier for reductive elimination for these cases, which prevents these species from readily leaving the thermodynamic sink via a facile reductive elimination. The isolated intermediates clearly constitute dead-end... 

Another piece of mechanistic evidence was reported by Snapper et al. [14], who describe a ruthenium catalyst caught in action . During studies on ring opening metathesis, these authors were able to isolate and characterize carbene 5 in which a tethered alkene group has replaced one of the phosphines originally present in Id. Control experiments have shown that compound 5 by itself is catalytically active, thus making sure that it is a true intermediate of a dissociative pathway rather than a dead-end product of a metathetic process. 

An interesting dinically useful prodrug is 5-fluorouracil, which is converted in vivo to 5-fluoro-2 -deoxyuridine 5 -monophosphate, a potent irreversible inactivator of thymidylate synthase It is sometimes charaderized as a dead end inactivator rather than a suicide substrate since no electrophile is unmasked during attempted catalytic turnover. Rathei since a fluorine atom replaces the proton found on the normal substrate enzyme-catalyzed deprotonation at the 5 -position of uracil cannot occur. The enzyme-inactivator covalent addud (analogous to the normal enzyme-substrate covalent intermediate) therefore cannot break down and has reached a dead end (R. R. Rando, Mechanism-Based Enzyme Inadivators , Pharm. Rev. 1984,36,111-142). 

One or more steps may form a dead end in the form of an intermediate formed through an elementary reaction and consumed exclusively by the reverse of this step. Although the dead-end will not contribute to the overall reaction rate, the step may affect the kinetics if the intermediate is strongly adsorbed on the surface. The poisonous effect of H2O in ammonia synthesis is an example. 

Beware of thinking that the occurrence of the lithiobetaines A and B must have stereochemical implications. Until fairly recently, lithium /ree betaines were incorrectly considered intermediates in the Wittig reaction. Today, it is known that lithima-containing betaines are formed in a dead-end side reaction. They must revert back to an oxaphosphetane—which occurs with retention of the configuration—before the actual Wittig reaction can continue. 

As with HZSM5-la, we attribute the initial deactivation to blocking of catalytically active sites by adsorbed xylene molecules preventing toluene methylation to occur at these sites. The longer residence time of the bulkier xylene isomers in the larger crystals of HZSM5-2 (see Table 1) seems to favour further alkylation of m- and o-xylene to trimethylbenzenes over isomerization to p-xylene Once trimethylbenzene is formed, dealkylation is rather difficult at 573 K and its rate of transport is too low to be able to diffuse out of the zeolite pores. It forms, thus, a dead end product that decreases the availability of active sites and reaction intermediates (leading to slow deactivation). 

The formation of the formate complex 14 as an alternate CO2 adduct has been noted above. No CO is directly produced from this complex, however, and also due to its low photocatal5rtic ability, 14 itself is seen as a dead end product rather than a reaction intermediate (60,62). Recently, Fujita et al. (95) have proposed a possibility that there is a switch between the two CO2 adducts as the photocatalytic reaction proceeds. That is, a mechanism in which dimer 16 is the main intermediate early in the photocatalytic reaction, but due to the increase in H+ concentration as the reaction proceeds, the carboxylate complex 17 becomes the intermediate (95). However, no report detecting either CO2 adduct in photocatalytic reaction solution has been published, indicating a need for further research. 

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