Isomerization product distributions

Observation (iii) above, taken in the context of the triad annihilation in Scheme 12, indicates that the more or less statistical o/p pattern is diagnostic of the homolytic pathway (66) since it will clearly dominate the competition for TOL+- at the high concentrations of added N02 (Scheme 16). Indeed this conclusion is supported by observation (i), in which essentially the same isomeric product distribution (i.e. ortho meta para 70 2 28%) is achieved when the pyridine competition is thwarted for the sterically hindered 2,6-lutidine, an ineffective nucleophile (Schlesener et al., 1984). According to the formulation in Scheme 16, the isomeric product distribution is established from the sterically hindered Me2PyNOj during the homolytic annihilation of TOL+- by N02, most favourably at the ortho and para... 

Isomeric product distributions. Isomeric product distributions obtained from toluene and anisole have been the subject of considerable mechanistic discussion in electrophilic aromatic nitration (Schofield, 1980 Olah et al., 1989). As applied to nitrations with iV-nitropyridinium ion, the yellow colour of the EDA complex immediately attendant upon the mixing of toluene and PyN02 in acetonitrile persists for about a day (in the dark), whereas the charge-transfer colour of toluene and Me2PyNOj is discharged within 10 min at 25°C. Both bleached solutions afford an identical product mixture (81), consisting of o- (62%), m- (4%) and p-nitrotoluenes (34%)... 

In view of the striking similarities that are consistently delineated in all three aspects dealing with (a) the isomeric product distributions, (b) nuclear versus side-chain nitration and (c) ipso adducts, the most direct formulation of electrophilic nitration invokes the production of the same intermediates (as those in the reactive triad in Scheme 10) via a purely thermal process (Scheme 19). 

The use of iodoacetic acid as an aryl radical trapping agent has confirmed the intermediacy of aryl radicals in some hydrodediazoniation reactions, whether these are initiated or not.4 Spontaneous hydrodediazoniation of aryldiazonium fluoroborates occurs in warm dimethylformamide (DMF). Detailed study5 of the conversion of the 4-nitro derivative into nitrobenzene indicates a homolytic mechanism in which H-atom abstraction occurs from both sites in DMF with a formyl methyl preference of 3.5 1.0. High yields of mixed perfluorinated biaryls may be obtained by the catalytic dediazoniation of pentafluorobenzenediazonium ions in acetonitrile containing aromatic substrates and small amounts of iodide salts. The catalytic role of iodide and the isomeric product distributions indicate that arylation proceeds through the pentafluorophenyl radical in an efficient homolytic chain process.6... 

However, the isomeric product distributions obtained upon photolysis of phenyl-methyl and various arylthiophenes did not conform to this simple mechanism. These results together with deuterium and C-labelling experiments led to the conclusion that the sequential order of the ring carbon atoms has changed during rearrangement. An attractive mechanism was proposed in which a valene thiophene is first formed, followed by collapse to a symmetrical Ladenburg structure, viz. 

It should be noted that the isomeric product distribution of the hydroxycyclohexadienyl radicals is not necessarily identical for the two isoelectronic reaction pathways. The product pattern seems to be governed by the charge distribution on the radical cation and the electron density distribution on the corresponding substituted benzene. [5-7]... 

Scheme 1. Isomeric product distribution resulting from [3 + 2] cycloaddition of diazomethane or a substituted azide to C70, and extrusion-rearrangement sequence to the respective 6-6 closed or 6-5 open structures...

The potential of microreaction processes to influence isomeric product distributions was demonstrated by Loebbecke et cd. [20], They reported on the nitration of naphthalene at moderate temperatures in different types of microreactors to ensure isothermal processing and precise control of residence time (Scheme 4.8). A deliberate synthesis of either mono- or dinitro-substituted naphthalene with high selectivities was achieved, in contrast to a broad product spectrum obtained rmder macroscopic batch conditions. In particular, isomeric ratios of 1- to 2-mononitro-naphthalene and also 1,5- to 1,8-dinitronaphthalene could be significantly increased compared with batch processes. 

Radical decomposition is one of the most important types of reactions. In this case, a larger radical decomposes to an olefin and a smaller radical. Radicals usually decompose at the beta position of the radical center where the C—C bond is the weakest. In the case of naphthenes and aromatics this may not be the case, and C—H bond may be the weakest. Radical isomerization frequently occurs for large radicals, and explains to a large extent the observed product distribution. 

Cesium fluonde in dimethylformamide catalyzes the isomerization offluori-nated cyclobutenes, perfluorobipyntmdines, and their oligomers to products with expanded rings [30, 31, 32] The product distribution in cobalt tnfluonde fluorina-tion depends strongly on the temperature of the reaction [33] Fluorinated 1-dimethylamino-5,6,7,8-tetrafluoro-l,4-dihydro-l,4-ethenonaphthalene rearranges in protic media to a biphenyl derivative [34] (equation 8)... 

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. 

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... 

The activity of CoSx-MoSx/NaY (2. IMo/SC) is shown in Fig.5 for the HYD of butadiene as a function of the Co/Mo atomic ratio. The HYD activity decreased slightly on the addition of Co up to Co/Mo = ca. 1, followed by a steep decrease at a further incorporation of Co. The HYD/HDS activity ratio decreased with increasing Co content and reached the ratio for CoSx/NaY at the Co/Mo atomic ratio of the maximum HDS activity (Fig.3). The product selectivity in the HYD of butadiene shifted from t-2-butene rich distribution to 1-butene rich one on the addition of Co, as presented in Fig.6. It is worthy of noting that at the Co/Mo ratio of the maximum HDS activity, the butene distribution is close to that for CoSx/NaY. It should be noted, however, that these product distributions are not the initial distributions of the HYD over the catalyst but the distributions modified by successive isomerization reactions. It was found that MoSx/NaY showed high isomerization activities of butenes even in the... 

Reactions over chromium oxide catalysts are often carried out without the addition of hydrogen to the reaction mixture, since this addition tends to reduce the catalytic activity. Thus, since chromium oxide is highly active for dehydrogenation, under the usual reaction conditions (temperature >500°C) extensive olefin formation occurs. In the following discussion we shall, in the main, be concerned only with skeletally distinguished products. Information about reaction pathways has been obtained by a study of the reaction product distribution from unlabeled (e.g. 89, 3, 118, 184-186, 38, 187) as well as from 14C-labeled reactants (89, 87, 88, 91-95, 98, 188, 189). The main mechanistic conclusions may be summarized. Although some skeletal isomerization occurs, chromium oxide catalysts are, on the whole, less efficient for skeletal isomerization than are platinum catalysts. Cyclic C5 products are of never more than very minor impor-... 

In considering ways to rationalize the differences in product distribution in Table IV, it is speculated that the ability of the surface atoms of certain metals to exhibit different valence states may be important (23). It has previously been suggested by Boudart and Ptak (27) that the ability of platinum and iridium to catalyze the isomerization of neopentane could be a consequence of the variable surface valency of these metals proposed by... 

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