From isomerization

Conjugation as well as geometric and positional isomerization occur when an alkadienoic acid such as linoleic acid is treated with a strong base at an elevated temperature. CycHc fatty acids result from isomerization of linolenic acid ia strong base at about 250°C (58). Conjugated fatty acids undergo the Diels-Alder reaction with many dienophiles including ethylene, propylene, acryUc acid, and maleic anhydride. 

The C4H7+ cation shown as the first entry in Scheme 5.5 is a particularly interesting example. It is a bridged ion which can be reached from isomeric cyclopropylmethyl and cyclobutyl irms. 

There are conflicting reports regarding the direction of elimination from isomeric 12-methyl-12-hydroxy compounds. Levine found little dependence on hydroxyl orientation, whereas Coxon found the tra -diaxial requirement overwhelming. 

Azabicyclo[4.1.0]hept-3-enes, e. g. 13, are available from 1,4-dihydrobenzenes 12 by the route indicated, and are useful precursors for the synthesis of specifically substituted 1//-azepines free from isomeric contamination.61 For example, low temperature addition of bromine to bicycle 13 yields the dibromo derivative 14 which, with powdered sodium methoxide in tet-rahydrofuran, undergoes a double dehydrobromination followed by electrocyclic ring opening of the resulting 7-azabicyclo[4.1.0]hepta-2,4-diene 15 to give methyl l//-azepine-l-carboxylate (16). 

Characteristic fragment ions Aliphatic ketones also give abundant McLafferty rearrangement ions at m/z 58, 72, 86, and so forth. Methyl ketones produce an abundant ion at m/z 43. Low-intensity ions at m/z 31, 45, 59, 73, and so on reveal oxygen in the unknown ketone and are especially useful in distinguishing ketone spectra from isomeric paraffin spectra. Subtract 43 from the mass of the rearrangement ion to determine R. 

For the reason given above and for other reasons, it is unlikely that the encounter complex is a n complex, but just what kind of attraction exists between Y+ and ArH is not known, other than the presumption that they are together within a solvent cage (see also p. 694). There is evidence (from isomerizations occurring in the alkyl group, as well as other observations) that n complexes are present on the pathway from substrate to arenium ion in the gas-phase protonation of alkylbenzenes. ... 

Aside from isomerization, transformation of the 5,6-epoxy to the 5,8-furanoid group is a common alteration during heating treatments of carotenoids. Violaxanthin was found to be the major carotenoid in mangoes however, in commercially processed mango juice, violaxanthin was not detected while auroxanthin, not present in the... 

Coupled intermolecular hydroaminations and intramolecular cycUzations give pyrroles resulting from isomerization of the initially formed exomethylene dihydropyrrole derivatives (Eq. 4.85) [137]. 

Structural rearrangements are not encountered with saturated Grignard reagents, but allylic and homoallylic systems can give products resulting from isomerization. NMR studies indicate that allylmagnesium bromide exists as a CT-bonded structure in which there is rapid equilibration of the two terminal carbons.101 Similarly,... 

Isomer Distributions in Initial Products from Hydrogenolysis of Methylcyclopentane and from Isomerization of Hexanes over Platinum Catalysts ... 

Proportions of Isotopically Labeled Products from Isomerization of Hexanes over 0.2% Platinum /Alumina Catalyst at 273°C ... 

In the Pd-catalyzed cross-coupling reactions of acylzirconocene chlorides with allylic halides and/or acetates (Section 5.4.4.4), the isolation of the expected p,y-unsaturated ketone is hampered by the formation of the a, P-un saturated ketone, which arises from isomerization of the p,y-double bond. This undesirable formation of the unsaturated ketone can be avoided by the use of a Cu(I) catalyst instead of a Pd catalyst [35], Most Cu(I) salts, with the exception of CuBr - SMe2, can be used as efficient catalysts Thus the reactions of acylzirconocene chlorides with allyl compounds (Table 5 8 and Scheme 5 30) or propargyl halides (Table 5.9) in the presence of a catalytic amount (10 mol%) of Cu(I) in DMF or THF are completed within 1 h at 0°C to give ffie acyl--allyl or acyl-allenyl coupled products, respectively, in good yields. ill... 

Several Sonogashira adducts of heteroaromatics including some pyridines (see Section 4.3) and pyrimidines underwent an unexpected isomerization [69]. This observed isomerization appeared to be idiosyncratic, and substrate-dependent. The normal Sonogashira adduct 100 was obtained when 2-methylthio-5-iodo-6-methylpyrimidine (99) was reacted with but-3-yn-ol, whereas chalcone 101, derived from isomerization of the normal Sonogashira adduct, was the major product when the reaction was carried out with l-phenylprop-2-yn-l-ol. 

From the FIA—MS overview spectrum, speculation that there can be more than just one structurally defined molecule type behind an observable signal i.e. the presence of isobaric compounds, cannot be excluded whenever one signal defined by the m/z-ratio is examined in FIA-MS spectra. Consequently, the information obtained by FIA-MS is quite limited whenever we deal with complex mixtures of environmental pollutants rather than the analysis of pure products or formulations with a known range of ingredients. LC separation is inevitable when mixtures of isomeric compounds should be identified with MS-MS. Therefore, in FIA-MS-MS special attention has to be paid to avoid the generation of mixed product ion spectra from isomeric parent compounds. This would block identification by library search and may lead to misinterpretations of product ion spectra because of the fragmentation behaviour observed. 

Ketones and aldehydes possess the highest a-acidity, many examples being known. In fact, it is usually very difficult to keep a terminal propargyl ketone from isomerizing to the allenyl ketone. Thus the oxidation of a homopropargylic alcohol 54, a typical precursor, in most of these cases directly delivers the allenyl ketones 56 rather than the propargyl ketones 55 in high yields [92-109] (Scheme 1.23). 

Except for the addition products to 1,3-butadiene, the initial products isomerize slowly to the 1,4-adducts13. Although small amounts of these compounds have been found among the addition of 4-chlorobenzenesulfenyl chloride to methyl substituted 1,3-butadienes (equation 95), it was not possible to establish whether they were formed under kinetic control or resulted from isomerization of the initially formed adducts. Therefore it is generally reported that arenesulfenyl chlorides react with dienes to give exclusively 1,2-adducts. 

The MM method proved to be remarkably useful for elucidating the complex mechanisms of the famous syntheses of adamantane and diamantane as well as other diamondoid molecules from isomeric polycyclic hydrocarbons under Lewis acid catalysis in Schleyer s laboratory (128,258,264-266),... 

There are many examples of reactions of diynes with metal species that give mono or binuclear products containing ligands other than simple this section and there are obvious connections with that presented earlier as many of the products are derived from isomerization, rearrangement, and bond-forming reactions of initially formed diyne complexes. 

Hydrogenolysis of butane was performed using Pt and PtRh nanowires and particles/FSM-16 [23, 32] Table 15.6 summarizes the results. In this reaction, methane, ethane and propane were formed from hydrogenolysis of butane, and isobutane from isomerization (15.1) ... 

The cleanest product composition may be effected by decomposition of the pure hydroperoxide or solutions in the injection block of the gas chromatograph. In carbon tetrachloride solution only methyl vinyl ketone and methyl vinyl carbinol were produced, the ratio of ketone to alcohol being 2.9. No definite traces of products from isomerized hydroperoxide were observed. 

Allylic halides, propargylic halides and benzylic halides complicated mixtures of products resulting from isomerization of the initial coupling products in the case of ArCH Br formation of ArCHjC Ar is also possible [58,59]. 

Complexes were also considered for other carbenes and even suggested for reactions in alkane solution( ) A counterproposal, backed by the inability of theory to find support for such stable complexes, held that the second source was not the carbene-alkene complex but instead was the diazo compound, formed from isomerization of excited diazirine. " Other LFP studies reinforced the need for two intermediates, but could not finally resolve the question of carbene complexes ver-sus diazo compound. However, the question is now settled in this case,... 

---