Formation of isomerization

Industrially, vitamin is prepared from the chromic acid oxidation of 2-methylnaphthalene (56). Although the yields are low, the process is economical owing to the low cost and availabiUty of the starting material and the oxidizing agent. However, the process is compHcated by the formation of isomeric 6-meth5l-l,4-naphthoquinone. As a result, efforts have been directed to develop process technology to faciUtate the separation of the isomeric naphthoquinone and to improve selectivity of the oxidation. 

A-norsteroids (59) of the cholestane, androstane, pregnane and hydrocortisone series have been prepared from the diols (61)." However, preparation of these 1,2-diols is complicated by concomitant formation of isomeric 4,5-diols, which are usually difficult to separate. The sequence (58) -> (59) appears to be the most practical route to A-norsteroids (59), provided that diosphenol (58) is readily available. 

Tnfluoroacetic anhydnde in a mixture with sulfuric acid is an efficient reagent for the sulfonylation of aromatic compounds [44] The reaction of benzene with this system in nitromethane at room temperature gives diphenyl sulfone in 61% yield Alkyl and alkoxy benzenes under similar conditions form the corresponding diaryl sulfones in almost quantitative yield, whereas yields of sulfones from deactivated arenes such as chlorobenzene are substantially lower [44] The same reagent (tnfluoroacetic anhydride-sulfunc acid) reacts with adamantane and its derivatives with formation of isomeric adamantanols, adamantanones, and cyclic sultones [45]... 

In the case of the direct irradiation, the singlet excited state is populated, hence the formation of the Dewar furan is energetically possible (Fig. 1). This result is in agreement both with the evidence for the formation of the Dewar furan in the direct irradiation and with the formation of isomeric furans. 

Although the Paterno-Buchi reaction is of high synthetic potential, its use in organic synthesis is still not far developed. In recent years some promising applications in the synthesis of natural products have been reported. The scarce application in synthesis may be due to the non-selective formation of isomeric products that can be difficult to separate—e.g. 6 and 7—as well as to the formation of products by competitive side-reactions such as Norrish type-I- and type-II fragmentations. 

The formation of isomeric aldehydes is caused by cobalt organic intermediates, which are formed by the reaction of the olefin with the cobalt carbonyl catalyst. These cobalt organic compounds isomerize rapidly into a mixture of isomer position cobalt organic compounds. The primary cobalt organic compound, carrying a terminal fixed metal atom, is thermodynamically more stable than the isomeric internal secondary cobalt organic compounds. Due to the less steric hindrance of the terminal isomers their further reaction in the catalytic cycle is favored. Therefore in the hydroformylation of an olefin the unbranched aldehyde is the main reaction product, independent of the position of the double bond in the olefinic educt ( contrathermodynamic olefin isomerization) [49]. 

As shown in Figure 9.31, butane is formed electrocatalytically (Ab t < 1) since no gaseous H2 is supplied, thus Abutis restricted to its electrocatalysis limits (tA negative potential region of electrocatalysis, electrochemically promoted formation of isomerization products continues with large A and p values (Fig. 9.31). 

Bis and tris(pyrazolyl)hydroborato ligands are generally prepared as the potassium derivatives by the direct reaction of KBH4 with the appropriate pyrazole (RR pzH), as illustrated in Scheme 1 for the parent system (13). The reaction is extremely general and has been applied to a number of different derivatives, which have included the incorporation of chiral (14, 15) and tethered (16) substituents. However, if the alkyl substituents of the pyrazole are inequivalent (i.e., R + R ), a potential problem may be encountered with the formation of isomeric products, in which either of the R and R substituents may occupy the 3-position of the poly(pyrazolyl)borato ligand. Nevertheless, if the difference in steric demands of R and R is large, there is a strong... 

It has been shown e that two mechanisms, elimination-addition (benzyne) and SN2 displacement, are operative in the liquid-phase hydrolysis of halogenatcd aromatic compounds. The formation of isomeric phenols as a result of the availability of the benzyne route makes the reaction of limited synthetic value. The incorporation of the copper-cuprous oxide system suppresses reaction via the benzyne route, so that the present method has general utility for the preparation of isomer-free phenols. For example, >-cresol is the only cresol formed from -bromotoluene under the conditions of this preparation. 

Table 54. Experimental heats of formation of isomeric compounds...

Fig. 29 Formation of isomeric decalins [71 ]—[73] by cyclization of a terpenoid alcohol catalysed by antibody HA5-19A4 raised to hapten [82]. The transition state [83a] has the leaving group in the equatorial position, as favoured by the Stork-Eschenmoser...

ESR spectra for, 22 294, 301 as high-energy fuels, 18 2-4 hydrogenation course of, 18 6-8 equilibria, 18 7, 8 kinetic processes, 18 6, 7 experimental procedures, 18 19, 20 apparatus and methods, 18 20 materials, 18 20 mechanism of, 18 21-45 formation of isomeric decahydro-naphthalenes, 18 23-30 deuterogena-tion of - -octalin, 18 29 routes to trans isomers, 18 26-30 selectivity to trons-decalin, 18 24, 25 olefin intermediates, 18 30-45 dihydro-and hexahydronaphthalenes, 18 32, 33 analysis of products, 18 33 oc-tahydronaphthalenes, 18 34-45 analysis of products, 18 34 deu-... 

Much is known about the lifetimes of carbocation intermediates of solvolysis, and these data have proven critical in the design of experiments to estimate absolute rate constants for reorganization of ion pairs. Consider reorganization of an ion-pair reaction intermediate that exchanges the positions of the nucleophilic atoms of the leaving group (, Scheme 9) and that occurs in competition with diffusional separation to free ions (k-d) which is much faster than addition of solvent to the ion pair. Ion-pair separation is irreversible and will result in formation of solvolysis reaction products s ). Reorganization of the ion pair will result in formation of isomerization reaction product and the yield of this reaction product will provide a measure of the relative rate constant... 

The formation of isomeric products can be reconciled by consideration of the mechanistic pathway. Deprotonation of the more acidic methyl group leads to formation of iminium salt 88 that can then form two ylides, 89 and 90, furnishing a mixture of products after subsequent cycloaddition (Scheme 3.24). 

Although limited by formation of isomeric mixtures, this method represents the only method for the preparation of simple 2- and 3-silyl- and gcrmyl-substituted cyclobutanones. The [2 + 2] cycloadditions of silyl- and germylketenes with alkenes do not take place readily. 

The cycloaddition was accomplished by heating 100 in HMPTA21,23,24 or in silicon oil,146 or by treating with potassium fe/Y-butoxide in DMF147 as well as in triethylamine and/or pyridine.23,148 The cyclization 100 -> 101 was found occasionally accompanied by the formation of isomeric thiophenes.21... 

Studies have been devoted to further elucidate the effects of structural changes of zirconia on activity and selectivity in isosynthesis and to acquire information on the mechanism of the formation of isomeric hydrocarbons. 

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