Reaction pathway studies

Kinetic models which consider demetallation as a complex reaction network of consecutive and parallel reactions taught by model compound studies have been recognized with real feedstocks. Tamm et al. (1981) suggest a sequential mechanism where the metal compounds are activated by H2S. Model compound reaction pathway studies in the absence of H2S, discussed in Section IV,A,1, and experiments in which H2S was present in excess (Pazos et al., 1983) indicate that sequential reactions are inherent to the chemistry of the metal compounds irrespective of the presence of H2S. However, it is possible that both mechanisms contribute to metal removal. 

Polyhaloalkyl-substituted chromones and 7-pyrones react with salicylaldehydes in the presence of piperidine to give a variety of fused 277-chromenes in good yields (Scheme 58) <2006JOC4538>. Although it is conceivable that this reaction could proceed through a Baylis-Hillman reaction pathway, studies of this reaction point to the mechanism being a tandem intramolecular oxa-Michael addition and subsequent Mannich condensation. 

The two reaction pathways studied involve a route from the carboxylic acid to the isocyanate via the acyl chloride and azide (Sodium Azide Pathway, Pathway A), Figure 4 and a route from the carboxylic acid to the isocyanate via the hydrazide (Hydrazide Pathway, Pathway B), Figure 5. The reactions were monitored by isolating the intermediate reaction products and characterizing them by melting points, elemental composition, or IR spectroscopy. 

Reaction Pathway Studies by the Determination of the Position of the Label... 

Scheme 2.1 Boronate ester formation reaction pathways studied by Pizer and Tihal.

This genera] scheme could be used to explain hydrogen exchange in the 5-position, providing a new alternative for the reaction (466). This leads us also to ask whether some reactions described as typically electrophilic cannot also be rationalized by a preliminary hydration of the C2=N bond. The nitration reaction of 2-dialkylaminothiazoles could occur, for example, on the enamine-like intermediate (229) (Scheme 141). This scheme would explain why alkyl groups on the exocyclic nitrogen may drastically change the reaction pathway (see Section rV.l.A). Kinetic studies and careful analysis of by-products would enable a check of this hypothesis. 

The mechanism and the stereochemistry of the reaction was studied using borodeuteride andfor deuterium oxide (480) and a reaction pathway was suggested (Scheme 93). 

The reactivity of isoxazole in the presence of light, heat or electron impact has been well studied and the various transformations analyzed in terms of reaction pathways and of the potential intermediates. These studies have also been extended to a large variety of substituted derivatives (79AHC(25)147). 

Although the rationalization of the reactivity and selectivity of this particular substrate is distinct from that for chiral ketals 92-95, it still agrees with the mechanistic conclusions gained throughout the study of Simmons-Smith cyclopropa-nations. StOl, the possibility of the existence of a bimetallic transition structure similar to v (see Fig. 3.5) has not been rigorously ruled out. No real changes in the stereochemical rationale of the reaction are required upon substitution of such a bimetallic transition structure. But as will be seen later, the effect of zinc iodide on catalytic cyclopropanations is a clue to the nature of highly selective reaction pathways. A similar but unexplained effect of zinc iodide on these cyclopro-panation may provide further information on the true reactive species. 

It can be concluded from the study of Grubbs and Brunck that indeed a metal-carbon cr-complex might be the key intermediate in the metathesis reaction. For the conversion of I into II several reaction pathways can be... 

Meinwald and coworkers71 studied the chemistry of naphtho[l, 8-bc]thiete and its S-oxides. The reaction of the sulphone 2 with LAH (equation 29) is of particular and direct relevance to this section since it is different from the reductions that have been discussed thus far, because the major reaction pathway is now cleavage of an S—C bond, rather than a deoxygenation of the sulphur atom. The major product (equation 29) was isolated in 65% yield two minor products accounted for a further 15% yield. One of the minor products is 1-methylthionaphthalene and this was most probably produced by an initial reduction of the strained 1,8-naphthosulphone, 2, to the thiete, which was then cleaved to the thiol and subsequently methylated. Meinwald also showed71 that the thiete was subject to cleavage by LAH as well as that both molecules were susceptible to attack and cleavage by other nucleophiles, notably methyllithium. These reactions are in fact very useful in attempts to assess a probable mechanism for the reduction of sulphones by LAH and this will be discussed at the end of this section. 

Reaction pathways involved in the curing of novolacs with HMTA have been extensively investigated by Solomon and co-workers.43-50 In a series of model studies where 2,6-xylenol and/or 2,4-xylenol was reacted with HMTA, these workers found that the types of linkages formed were affected by the initial... 

The high conversion test is operated to ensure that essentially complete conversion of the HBr is possible, and to study the fate of the feed contaminants. In this test, the conditions are selected to ensure complete conversion of the HBr. Several reaction pathways are then available to feed contaminants. They may undergo combustion, react with HBr, or react with the bromine formed. The extent of reaction via any of these pathways will depend on the nature of the contaminants and the temperature. Information concerning the fate of the contaminants can then be gained by analyzing the gas, bromine, and aqueous phases exiting the reactor. 

No attempt was made to measure CO2 in these experiments. By increasing the temperature to 320°C, catalyst deactivation was prevented, and no carbon residue could be detected on the spent catalyst. Thus, temperature can be expected to significantly shift the reaction pathways of organic contaminants. In this study, and in all other studies, excellent corrosion resistance was observed for the corrosion coupons. 

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