Isomers, different reaction

Many aryhydrazones provide two or more isomers when subjected to the conditions of the Fischer indole cyclization. The product ratio and the direction of indolization can also be affected by different reaction conditions (i.e. catalysts and solvents), which is attributed, at least in part, to the relative stabilities of the two possible tautomeric ene-hydrazine intermediates. Generally, strongly acidic conditions favor formation of the least substituted ene-hydrazine, while cyclization carried out in weak acids favors the most substituted ene-hydrazine. Eaton s acid (10% P2O5 in MeSOsH) has been demonstrated to be an effective catalyst for the preparation of 3-unsubstituted indoles from methyl ketones under strongly acidic conditions. Many comprehensive reviews on this topic have appeared. ... 

Brown has proposed that the importance of meta substitution in the alkylation is related to the activity of the attacking species, and he utilized the relative reactivity of toluene and benzene in the reaction under consideration as a measure of this activity.It might be possible that the amounts of 3-isomer formed may similarly be related to the relative reactivity of thiophene and benzene in different reactions. 

Examples from preparative chemistry indicate the possible occurrence of special alpha-e ects in several other cases. The different reaction of 2- and 4-ethoxy quinolines with a thiol yielding carbostyril and a 4-arylthio derivative, respectively, may indicate the intervention of structures such as 17 in the transition state of the 2-isomer. 

By studying the NMR spectra of the products, Jensen and co-workers were able to establish that the alkylation of (the presumed) [Co (DMG)2py] in methanol by cyclohexene oxide and by various substituted cyclohexyl bromides and tosylates occurred primarily with inversion of configuration at carbon i.e., by an 8 2 mechanism. A small amount of a second isomer, which must have been formed by another minor pathway, was observed in one case (95). Both the alkylation of [Co (DMG)2py] by asymmetric epoxides 129, 142) and the reduction of epoxides to alcohols by cobalt cyanide complexes 105, 103) show preferential formation of one isomer. In addition, the ratio of ketone to alcohol obtained in the reaction of epoxides with [Co(CN)5H] increases with pH and this has been ascribed to differing reactions with the hydride (reduction to alcohol) and Co(I) (isomerization to ketone) 103) (see also Section VII,C). 

In this context, 14 different reaction conditions were investigated in 14 hours. By software-supported process optimization (factorial design), the initial yield of 49% could be improved to 78% with a simultaneous increase in keto/enol isomer ratio (A B) of65 35 to95 5. 

The study of the transformation of 5-alkoxyalkyl-5-alkyl-l,3-dioxanes provided the first experimental evidence that the conformation of the reactant molecule plays a determining role regarding the direction of the catalytic reaction. The reason for the differing reaction directions clearly indicates that the conformers adsorb in different ways.32 The 5-alkoxyethyl isomers can exist in their chair conformations (1 and 2 in Scheme 4.12). The main reaction of the adsorbed surface species is the formation of an ester (3) by the rupture of the C-O bond in the ring. In one of the two isomers (1) the R2-0 group can also be adsorbed and this adsorption leads to a smaller ester molecule (4). 

In the case of 1,2,4-trithiolanes, both els and trans isomers are obtained with a different reaction mechanism involving a molecule of thioketone and a molecule of thiosulfine.50... 

A complicating factor associated with experimental application of the Skell Hypothesis is that triplet carbenes abstract hydrogen atoms from many olefins more rapidly than they add to them. Also, in general, the two cyclopropanes that can be formed are diastereomers, and thus there is no reason to expect that they will be formed from an intermediate with equal efficiency. To allay these problems, stereospecifically deuteriated a-methyl-styrene has been employed as a probe for the multiplicity of the reacting carbene. In this case, one bond formation from the triplet carbene is expected to be rapid since it generates a particularly well-stabilized 1,3-biradical. Also, the two cyclopropane isomers differ only in isotopic substitution and this is anticipated to have only a small effect on the efficiencies of their formation. The expected non-stereospecific reaction of the triplet carbene is shown in (15) and its stereospecific counterpart in (16). 

Zirconocene-catalyzed kinetic resolution of dihydrofurans is also possible, as illustrated in Scheme 6.8 [18]. Unlike their six-membered ring counterparts, both of the heterocycle enantiomers react readily, albeit through distinctly different reaction pathways, to afford — with high diastereomeric and enantiomeric purities — constitutional isomers that are readily separable (the first example of parallel kinetic resolution involving an organome-tallic agent). A plausible reason for the difference in the reactivity pattern of pyrans and furans is that, in the latter class of compounds, both olefmic carbons are adjacent to a C—O bond C—Zr bond formation can take place at either end of the C—C 7T-system. The furan substrate and the (ebthi)Zr-alkene complex (R)-3 interact such that unfavorable... 

This stereospecific oxidation does not occur for all dioximes, probably due to isomerisation of the dioxime during the reaction or to different reaction mechanisms involved in the use of different oxidants. When the lipophilic-hydrophilic balance of the two furoxan isomers is appropriate, they are easily separated by chromatography or fractional crystallisation. For example, the synthesis of 4-hydroxymethyl-3-furoxancarboxamide (CAS 1609), one of the most promising furoxancarboxamide vasodilators (see later), passes through the intermediate formation of a mixture of the two isomeric methyl hydroxymethylfuroxancarboxylic esters, which can easily be separated by recrystallisation from isopropyl acetate [18]. 

Ortho-Effect. The ortho-effect is one of the most widely known structural phenomena in organic chemistry. It is widely used in organic chemistry for synthetic purposes. The mass spectra of the majority of ort/jo-substituted aromatic compounds possess significant differences in comparison with the spectra of their meta- and para-isomers. A classic example of the ortho-effect in mass spectrometry involves fragmentation of alkylsalicylates. The intense peaks of [M - ROH]+ ions dominate in the El spectra of these compounds. These peaks are absent in the spectra of their meta- and para-isomers. The reaction leading to these ions may be represented by Scheme 5.12. 

Since our first model system is achiral, we did not need to consider different diastereomeric manifolds. However, we did need to follow four different reaction pathways corresponding to the four possible cis -dihydride isomers (Figure 5). Intermediates with traits phosphorus orientations were not considered because the catalysts of interest have chelating diphosphine ligands. 

The concept of "selectivity" must be clearly distinguished from the term "specificity" [4][5], Specific, applied to a reaction, means that two (or more) isomeric starting materials give -under the same reaction conditions- different reaction products which are also isomers. Depending upon the isomers we may be considering, we may refer to "regiospecificity" (structural isomers) or to "stereospecificity" (either diastereospecificity or enantiospecificity). For instance, the formation of we5o-2,3-dibromobutane by addition of bromine to ( )-2-butene, in contrast with the formation of the d,l - 2,3-dibromobutene from the (Z)-2-butene, is a case of diastereospecificity. 

The successful mechanism for a reaction is a theory that correlates the many facts which have been discovered and is fruitful for the prediction of new experiments (1). One approach to mechanism is the study of stereochemistry which seeks information concerning the geometrical relationships between the reactants at the critical stages in the reaction. Information is gleaned from the examination of the products, if several isomers differing only in configuration may be formed, or from a study of the reactivity of closely related substances whose molecular shapes are varied in a specific manner. Occasionally a stereochemical fact places a considerable restraint upon the allowable mechanistic postulates, but the most effective employment of stereochemistry generally depends upon its detailed correlation with other experimental methods. 

The catalysts exhibiting the highest activity are the high-surface sodium on activated-alumina catalysts. They were used by O Grady et al. (12), to obtain equilibrium distributions of olefin isomers in short contact times and at relatively low temperatures, as shown in Table I. Since the same composition was reached with different starting materials and different reaction times, equilibrium distributions of products are easily obtained. The preparation of these high-surface sodium catalysts has been described (15). 

Although the catalyst results in the rapid formation of an equilibrium isomer distribution, Haag and Pines (11), as well as O Grady et al. (12), have found that the rate of formation of isomers differs considerably. Eunetic studies of the reaction of 1-butene over sodium on alumina show that in short contact times 1-butene is predominantly converted into cfs-2-butene rather than into the thermodynamically favored trans-2-butene. On longer contact with the catalyst, cfs-2-butene is converted to trans-2-butene in the amount expected from the equilibrium distribution. By assuming that the isomerization reaction is first order in each isomer, a... 

Desimoni and co-workers were able to produce either enantiomer (2R or 25) of the Diels-Alder cycloadduct using the same isomer of phe-box ligand ent-6 under different reaction conditions (Fig. 9.25, Table 9.11)." " They found that when using magnesium(II) perchlorate as the metal source, the reaction produced cycloadduct in >98% yield with an endo/exo ratio of 93 7 and an endo ee of 70% for the (25) isomer. In contrast, when magnesium(II) perchlorate was used in the presence of 2 equiv of water, the reaction afforded the cycloadduct again in >98% yield with an endo/exo ratio of 93 7, but in this instance, the endo ee was 65% for the (2R) isomer. This selectivity difference was explained by a change in... 

The ketene valence isomers of mesoionic pyrimidinylium olates have been shown to undergo three different reaction pathways (Scheme 1) either retro-ene-type fragmentation to give C3O2 and 2-aminopyridine (path A), electrocycli-zation to a naphthyridine (path B), or cycloreversion to 2-pyridyl isocyanate and a ketene (path C) <2000J(P2)1841>. The exact nature of the product varied with the substituents present. 

When the same substituents are at each end of the double or triple bond, it is called symmetrical. Unsymmetrical means different substituents are at each end of the double or triple bond. Electrophilic addition of unsymmetrical reagents to unsymmetrical double or triple bonds follows Markovnikov s rule. According to Markovnikov s rule, addition of unsymmetrical reagents, e.g. HX, H2O or ROH, to an unsymmetrical alkene proceeds in a way that the hydrogen atom adds to the carbon that already has the most hydrogen atoms. The reaction is not stereoselective since it proceeds via a planar carbocation intermediate. However, when reaction proceeds via a cyclic carbocation intermediate, it produces regiospecific and stereospecific product (see below). A regioselective reaction is a reaction that can potentially yield two or more constitutional isomers, but actually produces only one isomer. A reaction in which one stereoisomer is formed predominantly is called a stereoselective reaction. 

The two isomers CH3O and CH2OH have quite different reaction characteristics. The hydroxymethyl radical mainly reacts with oxygen to form the second desired end product, formaldehyde ... 

The tris chelates are enantiomeric and the racemic mixture can be conveniently resolved into the optical isomers by reaction with the optically active tartrate by virtue of the different solubility properties of the two enantiomers.840 The enantiomers of [Ni(phen)3]2+ are more... 

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