Reactivity, patterns decomposition

Olefin metathesis is one of the most important reaction in organic synthesis [44], Complexes of Ru are extremely useful for this transformation, especially so-called Grubbs catalysts. The introduction of NHCs in Ru metathesis catalysts a decade ago ( second generation Grubbs catalysts) resulted in enhanced activity and lifetime, hence overall improved catalytic performance [45, 46]. However, compared to the archetypal phosphine-based Ru metathesis catalyst 24 (Fig. 13.3), Ru-NHC complexes such as 25 display specific reactivity patterns and as a consequence, are prone to additional decomposition pathways as well as non NHC-specific pathways [47]. 

Qualitatively the saipe reactivity pattern was observed for the decomposition of sym. azonitriles 20 (R1 = CN, R2, R3 = alkyl)29 and several symmetrically and un-symmetrically substituted azo compounds30. A selection of these results is found in Table 2. It is apparent from these data that the thermal stability of 20 decreases as the size of the groups R1—R3 increases. Riichardt et al. have observed that a linear relationship exists between the thermolysis rates of Table 2 and the SN 1-solvolysis rates of corresponding f-alkyl-p-nitrobenzoates 21 in 80% acetone-water28d). The... 

Vanoppen et al. [88] have reported the gas-phase oxidation of zeolite-ad-sorbed cyclohexane to form cyclohexanone. The reaction rate was observed to increase in the order NaY < BaY < SrY < CaY. This was attributed to a Frei-type thermal oxidation process. The possibility that a free-radical chain process initiated by the intrazeolite formation of a peroxy radical, however, could not be completely excluded. On the other hand, liquid-phase auto-oxidation of cyclohexane, although still exhibiting the same rate effect (i.e., NaY < BaY < SrY < CaY), has been attributed to a homolytic peroxide decomposition mechanism [89]. Evidence for the homolytic peroxide decomposition mechanism was provided in part by the observation that the addition of cyclohexyl hydroperoxide dramatically enhanced the intrazeolite oxidation. In addition, decomposition of cyclohexyl hydroperoxide followed the same reactivity pattern (i.e., NaY < BaY... 

The growth of filamentous carbon along with the gas phase product analysis has been used to determine the influence of sulfur on the iron catalyzed decomposition of carbon containing gas mixtures at 600°C. Pretreatment of the metal in H2S was found to initially suppress the reactions leading to carbon deposition from the decomposition of CO/H2 mixtures. After a short time the activity was restored to approximately the same level as that exhibited by an unadulterated iron powder, suggesting that most of the sulfur atoms were being removed from the surface. The small residual fraction of adatoms did, however, induce modifications in the structural characteristics of the filamentous carbon deposit and also altered the reactivity pattern of the iron towards decomposition of a CO/C2H4/H2 mixture. 

In contrast to the behavior of CO, the decomposition of ethylene is a facile process when performed on a nickel catalyst, but does not occur when the hydrocarbon is passed over iron. Based on these data we can rationalize the observed deactivation behavior observed in the present investigations according to the notion that at 725°C, the surface of the bimetallic particles become enriched in nickel, a condition that favors decomposition of adsorbed ethylene molecules, but is inert with regard to catalyzed disproportionation of CO. Subsequent lowering of the temperature to 600°C results in the restoration of the original surface composition and the concomitant attainment of the initial catalytic reactivity pattern. 

A further study by Ramirez et al. (1982) also utilized the effect of solvent and ionic state in controlling reactivity patterns. A number of reactions, including the decomposition and nucleophilic substitution processes of p-nitrophenyl phosphate in acetonitrile, were examined. In the absence of added nucleophiles cyclic trimetaphosphate is produced. Both t-butyl alcohol and phenol are phosphorylated under these conditions. A nonaqueous solvent of low polarity should be better than water in promoting the conversion of the highly ionic product to the less polar transition state that would produce metaphosphate. The low steric sensitivity of the reaction and the increased rate of the dianion relative to the monoanion are consistent with either any extended associative transition state or the dissociative transition state. Stereochemical studies of these reactions by Knowles... 

The mechanism for the decomposition of NH4NO3 is similar. Once again, the nucleophile ammonia is generated via proton transfer from NH4" to N03. It may seem strange that nitrate, an extremely weak base, should pick up a proton from NH4. Remember, however, that this is a heat-induced solid-state reaction reactivity patterns are expected to be different under such conditions, relative to lower temperature solution-phase chemistry. Once the N-N bond has formed, we need to do a few proton transfers and eliminate two water molecules to arrive at the final product N2O, as depicted below ... 

While a number of interesting reactivity patterns have been observed, these systems suffer from poor solubility and decomposition via abstraction of cyclopentadienyl ring hydrogen atoms. Permethylation of the ligand (79) circumvents a number of these problems, and for X = Si(CH3)2 a new series of coordinatively unsaturated, thermally stable, and highly reactive actinide hydro-carbyls can be prepared [71, 72] ... 

Interaction of a carbonyl group with an electrophilic metal carbene would be expected to lead to a carbonyl ylide. In fact, such compounds have been isolated in recent years 14) the strategy comprises intramolecular generation of a carbonyl ylide whose substituent pattern guarantees efficient stabilization of the dipolar electronic structure. The highly reactive 1,3-dipolar species are usually characterized by [3 + 2] cycloaddition to alkynes and activated alkenes. Furthermore, cycloaddition to ketones and aldehydes has been reported for l-methoxy-2-benzopyrylium-4-olate 286, which was generated by Cu(acac)2-catalyzed decomposition of o-methoxycarbonyl-m-diazoacetophenone 285 2681... 

The tripeptides in Fig. 6.17 underwent a few breakdown reactions (N-ter-minus elimination, Qm formation, peptide bond hydrolysis), some of which will be considered later in this section. Of relevance here was that, of the two amidated tripeptides, the amide at the C-terminus underwent deamidation predominantly (Fig. 6.17, Reaction a), which, perhaps, explains the somewhat lesser stability compared to the free carboxylic acid forms. While the hexapeptide (6.52, Fig. 6.17) followed a different pattern of decomposition [76b], deamidation was also a predominant hydrolytic reaction at all pH values. Thus, the procedure to extrapolate results from small model peptides to larger medicinal peptides appears to be an uncertain one, since small modifications in structure can cause large differences in reactivity. 

A well-known tool for the estimation of reactivity hazards of organic material is called CHETAH [5]. The method is based on pattern recognition techniques, based on experimental data, in order to infer the decomposition products that maximize the decomposition energy, and then performs thermochemical calculations based on the Benson group increments mentioned above. Thus, the calculations are valid for the gas phase, but this may be a drawback, since in fine chemistry most reactions are performed in the condensed phase. Corrections must be made, but in general they remain small and do not significantly affect the results. 

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