Reaction pathways, oxidative

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

Oxidative Reactions. The majority of pesticides, or pesticide products, are susceptible to some form of attack by oxidative enzymes. For more persistent pesticides, oxidation is frequently the primary mode of metaboHsm, although there are important exceptions, eg, DDT. For less persistent pesticides, oxidation may play a relatively minor role, or be the first reaction ia a metaboHc pathway. Oxidation generally results ia degradation of the parent molecule. However, attack by certain oxidative enzymes (phenol oxidases) can result ia the condensation or polymerization of the parent molecules this phenomenon is referred to as oxidative coupling (16). Examples of some important oxidative reactions are ether cleavage, alkyl-hydroxylation, aryl-hydroxylation, AJ-dealkylation, and sulfoxidation. 

Fig. 9. Reaction pathways for oxidized diearbocyanine dyes. Courtesy of the American Chemical Society.

Fig. 12-5. Ozone-propene reaction pathways showing oxidation products.

Radical and ionic nitrations are often competitive pathways in strong nitrating acid rmxtures. The predominant reaction pathway is determined by the composition of the nitrating medium. Oxides of nitrogen in the nitrating medium add to... 

Recently, solicon-tethered thastereoselecdve ISOC reactions have been reported, in which effective control of remote acyclic asymmetry can be achieved fEq 8 91) Whereas ISOC occur stereoselecdvely, INOC proceeds v/ith significandy lower levels of diastereoselecdon The reaction pathways presented in Scheme 8 28 suggest a plausible hypothesis for the observed difference of stereocontrol The enhanced selecdvity in reacdons of silyl nitronates may be due to l,3- illylic strain The near-linear geometry of nitnle oxides precludes such differendadng elements fScheme 8 28 ... 

In chlorinations either a substitution or an addition process can occur with the ultimate reaction pathway(s) determined by a combination of factors, which include the reaction conditions, the positions and natures of any substituents present, and the catalyst used. Uncatalyzed chlorination of benzothiadiazole is an exothermic reaction that gives rise to a mixture of isomeric tetrachloro addition products. These are converted in basic medium into 4,7-dichloro-2,1,3-benzothiadiazole (70RCR923). When an iron(III) catalyst is present 4- and 7-chloro substitution becomes the dominant process. Chlorination of a number of 4-substituted 2,1,3-benzothiadiazoles (43) using an oxidative process gave a combination of chlorinated and oxidized products. The 4-hydroxy, 4-amino-, 4-methyl-amino, and 4-acetoxy derivatives of 43 all formed the chloroquinones (44) (40-61% yields). With the 4-aIkoxy substrates both 44 and some 5,7-dichlorinated product were obtained (88CHE96). 

Whereas exo-norbornene oxide rearranges to nortricyclanol on treatment with strong base through transannular C-H insertion (Scheme 5.11), endo-norbornene oxide 64 gives norcamphor 65 as the major product (Scheme 5.14) [15, 22]. This product arises from 1,2-hydrogen migration very little transannular rearrangement is observed. These two reaction pathways are often found to be in competition with one another, and subtle differences in substrate structure, and even in the base employed, can have a profound influence on product distribution. 

Metalated epoxides can react with organometallics to give olefins after elimination of dimetal oxide, a process often referred to as reductive alkylation (Path B, Scheme 5.2). Crandall and Lin first described this reaction in their seminal paper in 1967 treatment of tert-butyloxirane 106 with 3 equiv. of tert-butyllithium, for example, gave trans-di-tert-butylethylene 110 in 64% yield (Scheme 5.23), Stating that this reaction should have some synthetic potential , [36] they proposed a reaction pathway in which tert-butyllithium reacted with a-lithiooxycarbene 108 to generate dianion 109 and thence olefin 110 upon elimination of dilithium oxide. The epoxide has, in effect, acted as a vinyl cation equivalent. 

The ultimate purpose of mechanistic considerations is the understanding of the detailed reaction pathway. In this connection it is important to know the structure of the active catalyst and, closely connected with this, the function of the cocatalyst. Two possibilities for the action of the cocatalyst will be taken into consideration, namely, the change in the oxidation state of the transition metal and the creation of vacant sites. In the following, a few catalyst systems will be considered in more detail. 

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. 

The [3S+1C] cycloaddition reaction with Fischer carbene complexes is a very unusual reaction pathway. In fact, only one example has been reported. This process involves the insertion of alkyl-derived chromium carbene complexes into the carbon-carbon a-bond of diphenylcyclopropenone to generate cyclobutenone derivatives [41] (Scheme 13). The mechanism of this transformation involves a CO dissociation followed by oxidative addition into the cyclopropenone carbon-carbon a-bond, affording a metalacyclopentenone derivative which undergoes reductive elimination to produce the final cyclobutenone derivatives. 

Monoalkylthallium(III) compounds can be prepared easily and rapidly by treatment of olefins with thallium(III) salts, i.e., oxythallation (66). In marked contrast to the analogous oxymercuration reaction (66), however, where treatment of olefins with mercury(II) salts results in formation of stable organomercurials, the monoalkylthallium(III) derivatives obtained from oxythallation are in the vast majority of cases spontaneously unstable, and cannot be isolated under the reaction conditions employed. Oxythallation adducts have been isolated on a number of occasions (61, 71,104,128), but the predominant reaction pathway which has been observed in oxythallation reactions is initial formation of an alkylthallium(III) derivative and subsequent rapid decomposition of this intermediate to give products derived by oxidation of the organic substrate and simultaneous reduction of the thallium from thallium(III) to thallium(I). The ease and rapidity with which these reactions occur have stimulated interest not only in the preparation and properties of monoalkylthallium(III) derivatives, but in the mechanism and stereochemistry of oxythallation, and in the development of specific synthetic organic transformations based on oxidation of unsaturated systems by thallium(III) salts. 

Second, apart from a different mode of activation, different modes of reactivity are observed. Hence, for FeCls reactions like oxidative C-C but also nonoxidative C-C couplings are as well observed as bond formation via Lewis-acid activation (Scheme 2) [11-16]. Depending on the reaction type, one or more mechanistic pathways are accessible at the same time, which makes it difficult to shed light into mechanistic details. 

Competitive Reaction Pathways in Propane Ammoxidation over V-Sb-Oxide Catalysts an IR and Flow Reactor Study... 

Investigation of direct conversion of methane to transportation fiiels has been an ongoing effort at PETC for over 10 years. One of our current areas of research is the conversion of methane to methanol, under mild conditions, using li t, water, and a semiconductor photocatalyst. Research in our laboratory is directed toward ad ting the chemistry developed for photolysis of water to that of methane conversion. The reaction sequence of interest uses visible light, a doped tungsten oxide photocatalyst and an electron transfer molecule to produce a hydroxyl i cal. Hydroxyl t cal can then react with a methane molecule to produce a methyl radical. In the preferred reaction pathway, the methyl radical then reacts with an additional wata- molecule to produce methanol and hydrogen. 

Concerning the reaction pathway, two routes have been proposed the sequence of total oxidation of methane, followed by reforming of the unconverted methane with CO2 and H2O (designated as indirect scheme), and the direct partial oxidation of methane to synthesis gas without the experience of CO2 and H2O as reaction intermediates. The results obtained by Schmidt and his co-workers [4, 5] indicate that the direct reaction scheme may be followed in a monolith reactor when an extremely short contact time is employed at temperatures in the neighborhood of 1000°C. However, the majority of previous studies over numerous types of catalysts show that the partial oxidation of methane follows the indirect reaction scheme, which is supported by the observation that a sharp temperature spike occurs near the entrance of the catalyst bed, and that essentially zero CO and H2 selectivity is obtained at low methane conversions (<25%) where oxygen is not fully consumed [2, 3]. A major problem encountered... 

The set of all intermediate steps is called the reaction pathway. A given reaction (involving the same reactants and products) may occur by a single pathway or by several parallel pathways. In the case of invertible reactions, the pathway followed in the reverse direction (e.g., the cathodic) may or may not coincide with that of the forward direction (in this example, the anodic). For instance, the relatively simple anodic oxidation of divalent manganese ions which in acidic solutions yields tetrava-lent manganese ions Mn +— Mn -l-2e , can follow these two pathways ... 

Nakabayashi, S., Yagi, 1., Sugiyama, N., Tamura, K. and Uosaki, K. (1997) Reaction pathway of four-electron oxidation of formaldehyde on platinum electrode as observed by in situ optical spectroscopy. Surf. Sci., 386, 82-88. 

The formation of halogenation products from Grignard reagents and sulfonic acid anhydrides is the result of an oxidative reaction pathway . This side-reaction can be reduced by using sulfonic acid esters, however, in these cases alkylations as well as twofold sulfonylations (cf. corresponding results with sulfonyl fluorides ) are competing (equations 64 and 65). 

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