Production distribution, intermediate

Amines or ammonia replace activated halogens on the ting, but competing pyridyne [7129-66-0] (46) formation is observed for attack at 3- and 4-halo substituents, eg, in 3-bromopyridine [626-55-1] (39). The most acidic hydrogen in 3-halopyridines (except 3-fluoropyridine) has been shown to be the one in the 4-position. Hence, the 3,4-pyridyne is usually postulated to be an intermediate instead of a 2,3-pyridyne. Product distribution (40% (33) and 20% (34)) tends to support the 3,4-pyridyne also. 

Nonicosahedral carboranes can be prepared from the icosahedral species by similar degradation procedures or by reactions between boranes such as B H q and B H with acetylenes. The degradative reactions for intermediate C2B H 2 species (n = 6-9) have been described in detail (119). The small closo-Qr Yi 2 species (n = 3-5 are obtained by the direct thermal reaction (500—600°C) of B H using acetylene in a continuous-flow system. The combined yields approach 70% and the product distribution is around 5 5 1 of 2,4-C2B3H2 [20693-69-0] to l,6-C2B Hg [20693-67-8] to 1,5-C2B3H3 [20693-66-7] (120). A similar reaction (eq. 60) employing base catalysts, such as 2 6-dimethylpyridine at ambient temperature gives nido-2 >-(Z, ... 

SASOLII a.ndIII. Two additional plants weie built and aie in operation in South Africa near Secunda. The combined annual coal consumption for SASOL II, commissioned in 1980, and SASOL III, in 1983, is 25 x 10 t, and these plants together produce approximately 1.3 x lO" m (80,000 barrels) per day of transportation fuels. A block flow diagram for these processes is shown in Figure 15. The product distribution for SASOL II and III is much narrower in comparison to SASOL I. The later plants use only fluid-bed reactor technology, and extensive use of secondary catalytic processing of intermediates (alkylation, polymerisation, etc) is practiced to maximise the production of transportation fuels. 

The first possibility envisages essentially the same mechanism as for the second-order process, but with Bt2 replacing solvent in the rate-determining conversion to an ion pair. The second mechanism pictures Bt2 attack on a reversibly formed ion-pair intermediate. The third mechanism postulates collide of a ternary complex tiiat is structurally similar to the initial charge-transfer complex but has 2 1 bromine alkene stoichiometry. There are very striking similarities between the second-order and third-order processes in terms of magnitude of p values and product distribution. In feet, there is a quantitative correlation between the rates of the two processes over a broad series of alkenes, which can be expressed as... 

The product distribution can be shifted to favor the 1 -product by use of such milder brominating agents as the pyridine-bromine complex or the tribromide ion, Br3. It is believed that molecular bromine reacts through a cationic intermediate, whereas the less reactive brominating agents involve a process more like the AdgS and-addition mechanism. 

A recent paper by Singh et al. summarized the mechanism of the pyrazole formation via the Knorr reaction between diketones and monosubstituted hydrazines. The diketone is in equilibrium with its enolate forms 28a and 28b and NMR studies have shown the carbonyl group to react faster than its enolate forms.Computational studies were done to show that the product distribution ratio depended on the rates of dehydration of the 3,5-dihydroxy pyrazolidine intermediates of the two isomeric pathways for an unsymmetrical diketone 28. The affect of the hydrazine substituent R on the dehydration of the dihydroxy intermediates 19 and 22 was studied using semi-empirical calculations. ... 

There is some debate in the literature as to the actual mechanism of the Beirut reaction. It is not clear which of the electrophilic nitrogens of BFO is the site of nucleophilic attack or if the reactive species is the dinitroso compound 10. In the case of the unsubstituted benzofurazan oxide (R = H), the product is the same regardless of which nitrogen undergoes the initial condensation step. When R 7 H, the nucleophilic addition step determines the structure of the product and, in fact, isomeric mixtures of quinoxaline-1,4-dioxides are often observed. One report suggests that N-3 of the more stable tautomer is the site of nucleophilic attack in accord with observed reaction products. However, a later study concludes that the product distribution can be best rationalized by invoking the ortho-dinitrosobenzene form 10 as the reactive intermediate. 

In 1971, a short communication was published [54] by Kumada and co-workers reporting the formation of di- and polysilanes from dihydrosilanes by the action of a platinum complex. Also the Wilkinson catalyst (Ph3P)3RhCl promotes hydrosilation. If no alkenes are present, formation of chain silanes occurs. A thorough analysis of the product distribution shows a high preference for polymers (without a catalyst, disproportionation reactions of the silanes prevail). Cross experiments indicate the formation of a silylene complex as intermediate and in solution, free silylenes could also be trapped by Et3SiH [55, 56],... 

It is clear from the results that there is no kinetic isotope effect when deuterium is substituted for hydrogen in various positions in hydrazobenzene and 1,1 -hydrazonaphthalene. This means that the final removal of hydrogen ions from the aromatic rings (which is assisted either by the solvent or anionic base) in a positively charged intermediate or in a concerted process, is not rate-determining (cf. most electrophilic aromatic substitution reactions47). The product distribution... 

Sect. 2.1.1) and [3C+2S] cyclopentene derivatives. The product distribution can be controlled by choosing the appropriate reaction conditions [72]. Moreover, the cyclopentene derivatives are the exclusive products from the coupling of fi-pyrrolyl-substituted carbene complexes [72b,c] (Scheme 25). The crucial intermediate chromacyclobutane is formed in an initial step by a [2+2] cycloaddition. This chromacyclobutane rearranges to give the rf-complex when non-coordinating solvents are used. Finally, a reductive elimination leads to the formal [3C+2S] cyclopentene derivatives. 

The positive charge should reside on a complex entity, and there is no ready means for assessing the products of the neutralization process. Although we know that neutralization must yield 3.8 intermediates/100 e.v., there is no chemical evidence for their contribution to the product distribution. This cannot be interpreted by neutralization yielding predominantly hydrogen atoms, ethyl radicals, or methyl radicals. One can quantitatively account for these intermediates on the basis of the distribution of primary species and second- and third-order ion-molecule reactions (36). 

There is no structure for an elongated core intermediate between that shown in Fig. 11, with three inner-core spher-ons, and that shown in Fig. 12, with four. The transition between these structures is calculated by use of Eq. 1, with n, = 22, to occur at nt = 69, that is, at N = 138. It is accordingly an expectation from the close-packed-spheron theory that, as observed, 90Ac13a2- 7 (formed by bombardment of Re- 20 with 11-Mev protons) gives a three-humped fission product distribution curve (23), which has been interpreted (24) as showing that both symmetric fission and asymmetric fission occur. 

The current-potential relationship indicates that the rate determining step for the Kolbe reaction in aqueous solution is most probably an irreversible 1 e-transfer to the carboxylate with simultaneous bond breaking leading to the alkyl radical and carbon dioxide [8]. However, also other rate determining steps have been proposed [10]. When the acyloxy radical is assumed as intermediate it would be very shortlived and decompose with a half life of t 10" to carbon dioxide and an alkyl radical [89]. From the thermochemical data it has been concluded that the rate of carbon dioxide elimination effects the product distribution. Olefin formation is assumed to be due to reaction of the carboxylate radical with the alkyl radical and the higher olefin ratio for propionate and butyrate is argued to be the result of the slower decarboxylation of these carboxylates [90]. 

Reversible formation of ionic intermediates in halogenated solvents has been suggested to be due to the weakly nucleophilic character of the counteranion, the tribromide ion, which should dissociate into nucleophilic bromide and free bromine before reacting with the bromonium ion (refs. 11,25,26). In order to check this hypothesis the product distribution of the c/s-stilbene bromination in chloroform was investigated (ref. 27). In the latter solvent the formation constant of Br3 is considerably lower than in DCE, Kf = 2.77 (0.13) x 10 against > 2 x 107 M 1. (ref. 28). As a consequence, at 10 3 M [Br2] relevant amounts of bromide ions are present as counteranion of the bromonium intermediate. Nevertheless, the same trend for the isomerization of cis- to rran -stilbene, as well as an increase of... 

It should be pointed out however that there is no proof for the intermediate formation of free radicals and other mechanisms may explain the product distribution equally well. Such an alternative may be an ionic mechanism, possibly catalyzed by impurities or the walls of the container. In the above experiments Pyrex tubes were used [64]. 

Katsumura, Kitaura and their coworkers [74] found and discussed the high reactivity of vinylic vs allylic hydrogen in the photosensitized reactions of twisted 1,3-dienes in terms of the interaction in the perepoxide structure. Yoshioka and coworkers [75] investigated the effects of solvent polarity on the product distribution in the reaction of singlet oxygen with enolic tautomers of 1,3-diketones and discussed the role of the perepoxide intermediate or the perepoxide-Uke transition state to explain their results. A recent review of the ene reactions of was based on the significant intervention of the perepoxide structure [76], which can be taken as a quasi-intermediate. 

The catalytic pyrolysis of R22 over metal fluoride catalysts was studied at 923K. The catalytic activities over the prepared catalysts were compared with those of a non-catalytic reaction and the changes of product distribution with time-on-stream (TOS) were investigated. The physical mixture catalysts showed the highest selectivity and yield for TFE. It was found that the specific patterns of selectivity with TOS are probably due to the modification of catalyst surface. Product profiles suggest that the secondary reaction of intermediate CF2 with HF leads to the formation of R23. 

To conclusively disprove the involvement of the chromanol methide radical, the reaction of a-tocopherol with dibenzoyl peroxide was conducted in the presence of a large excess of ethyl vinyl ether used as a solvent component. If 5a-a-tocopheryl benzoate (11) was formed homolytically according to Fig. 6.6, the presence of ethyl vinyl ether should have no large influence on the product distribution. However, if (11) was formed heterolytically according to Fig. 6.9, the intermediate o-QM 3 would be readily trapped by ethyl vinyl ether in a hetero-Diels-Alder process with inverse electron demand,27 thus drastically reducing the amount of 11 formed. Exactly the latter outcome was observed experimentally. In fact, using a 10-fold excess of ethyl vinyl ether relative to a-tocopherol and azobis(isobutyronitrile) (AIBN) as radical... 

In the synthesis of the tetracyclic intermediates for the synthesis of isoarborinol and its CDE-antipode femenol, the stereochemistry of the Diels-Alder reaction can be varied using various Lewis-acid catalysts in aqueous media (Eq. 12.36).97 Their results show that the hydrophobic effects play an important role in enhancing reaction rates and can control product distribution. Novel 2,4-dialkyl-1-alkylideneamino-3-(methoxycarbonylmethyl)azetidines were obtained from aldazines and... 

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