Isomeric inversion

Steric acceleration of the isomerization can apparently occur it is almost certainly Cla of 65 which is removed electrolytically, yet the major electrolysis product is 66(66 67 = 3.2 1) 27>, probably because steric compression between Clb and the nearby aromatic ring in 65 can be relieved through isomerization (inversion). 

Racemization and isomerization (inversion at the sulfmyl sulfur) between 16 and 17 has been reported as indicated in Scheme 3 <1997T12203>. A retardation of the isomerization between 16 and 17 has been observed by adding a small amount of the radical scavenger, l,l-diphenyl-2-picrylhydrazyl (DPPFl), and the authors suggest that the isomerization is caused by a radical contaminant. 

The second case, called Isomeric inversion , is illustrated by Fig. 1.21. Under classical conditions Pi alone is obtained whereas rapid heating or microwave heating leads to P2. This situation is more interesting than the first because it forecasts reactivity changes induced by heating rate despite the same chemical operating conditions (reagent concentrations). 

Isomerization + Inversion Isomerization + Inversion Some isomerization + Some inversion... 

As is inversely proportional to solvent viscosity, in sufficiently viscous solvents the rate constant k becomes equal to k y. This concerns, for example, reactions such as isomerizations involving significant rotation around single or double bonds, or dissociations requiring separation of fragments, altiiough it may be difficult to experimentally distinguish between effects due to local solvent structure and solvent friction. 

Figure B2.4.2. Eyring plot of log(rate/7) versus (1/7), where Jis absolute temperature, for the cis-trans isomerism of the aldehyde group in fiirfiiral. Rates were obtained from tln-ee different experiments measurements (squares), bandshapes (triangles) and selective inversions (circles). The line is a linear regression to the data. The slope of the line is A H IR, and the intercept at 1/J = 0 is A S IR, where R is the gas constant. A and A are the enthalpy and entropy of activation, according to equation (B2.4.1)...

Note 2. Propargyl bromide itself also reacts with ammonia and therefore the interval between the addition of this compound and its conversion with the enethiolate should be kept as short as possible. Inverse addition was applied if propargyl bromide is added to the enthiolate solution, the primary product partly isomerizes into H2C=C(5C2Hs)SCH=C=CH2 under the catalytic influence of the enethiolate. 

Furthermore, the catalytic allylation of malonate with optically active (S)-( )-3-acetoxy-l-phenyl-1-butene (4) yields the (S)-( )-malonates 7 and 8 in a ratio of 92 8. Thus overall retention is observed in the catalytic reaction[23]. The intermediate complex 6 is formed by inversion. Then in the catalytic reaction of (5 )-(Z)-3-acetoxy-l-phenyl-l-butene (9) with malonate, the oxidative addition generates the complex 10, which has the sterically disfavored anti form. Then the n-a ir rearrangement (rotation) of the complex 10 moves the Pd from front to the rear side to give the favored syn complex 6, which has the same configuration as that from the (5 )-( )-acetate 4. Finally the (S)-( )-mal-onates 7 and 8 are obtained in a ratio of 90 10. Thus the reaction of (Z)-acetate 9 proceeds by inversion, n-a-ir rearrangement and inversion of configuration accompanied by Z to isomerization[24]. 

Nitrile ylides derived from the photolysis of 1-azirines have also been found to undergo a novel intramolecular 1,1-cycloaddition reaction (75JA3862). Irradiation of (65) gave a 1 1 mixture of azabicyclohexenes (67) and (68). On further irradiation (67) was quantitatively isomerized to (68). Photolysis of (65) in the presence of excess dimethyl acetylenedicar-boxylate resulted in the 1,3-dipolar trapping of the normal nitrile ylide. Under these conditions, the formation of azabicyclohexenes (67) and (68) was entirely suppressed. The photoreaction of the closely related methyl-substituted azirine (65b) gave azabicyclohexene (68b) as the primary photoproduct. The formation of the thermodynamically less favored endo isomer, i.e. (68b), corresponds to a complete inversion of stereochemistry about the TT-system in the cycloaddition process. 

H-Azepine, 2-(o-hydroxyphenyl)-synthesis, 7, 538 3 H-Azepine, methyl- H NMR, 7, 495 3H-Azepine, 3-methylring inversion barrier, 7, 14 3 H-Azepine, 2-methylene-isomerization, 7, 505 3H-Azepine, 7-(N-phthalimido) synthesis, 7, 538 4H-Azepine, 4,5-dihydro-cyclization, 7, 524... 

Azetidine, 7V-bromo-, 7, 240 Azetidine, AT-r-butyl- N NMR, 7, 11 Azetidine, AT-t-butyl-3-chloro-transannular nucleophilic attack, 7, 25 Azetidine, 3-chloro-isomerization, 7, 42 Azetidine, AT-chloro-, 7, 240 dehydrohalogenation, 7, 275 Azetidine, 7V-chloro-2-methyl-inversion, 7, 7 Azetidine, 3-halo-synthesis, 7, 246 Azetidine, AT-halo-synthesis, 7, 246 Azetidine, AT-hydroxy-synthesis, 7, 271 Azetidine, 2-imino-stability, 7, 256 Azetidine, 2-methoxy-synthesis, 7, 246 Azetidine, 2-methyl-circular dichroism, 7, 239 optical rotatory dispersion, 7, 239 Azetidine, AT-nitroso-deoxygenation, 7, 241 oxidation, 7, 240 synthesis, 7, 246 Azetidine, thioacyl-ring expansion, 7, 241 Azetidine-4-carboxylic acid, 2-oxo-oxidative decarboxylation, 7, 251 Azetidine-2-carboxylic acids absolute configuration, 7, 239 azetidin-2-ones from, 7, 263 synthesis, 7, 246... 

Aziridine, 2,3-diphenyl-l-(2,4,6-trinitrophenyl)-irradiation, 7, 61 Aziridine, 1,2-divinyl-rearrangement, 7, 539 Aziridine, 2,3-divinyl-rearrangement, 7, 42, 65, 539 Aziridine, N-ethyl-inversion, 7, 6 Aziridine, 2-halo-reactions, 7, 74 Aziridine, A/-halo-invertomers, 7, 6 Aziridine, 2-methyl- N NMR, 7, 11 Aziridine, methylene-ring-ring valence isomerizations, 7, 22 synthesis, 7, 92 Aziridine, iV-nitroso-reactions, 7, 74 Aziridine, iV-phosphino-inversion, 7, 7 Aziridine, 1-phthalimido-UV irradiation, 7, 62-63 Aziridine, l-(3-thienyl)-2-vinyl-rearrangement, 4, 746 Aziridine, 7V-trimethylsilyl-inversion, 7, 7 Aziridine, 1,2,3-triphenyl-irradiation, 7, 61 Aziridine, vinyl-isomerization, S, 287 Aziridinecarboxylic acid ring expansion, 7, 262 Aziridine-2,2-dicarboxylic acid, 1-methoxy-diethyl ester... 

Fig. 1. Examples of temperature dependence of the rate constant for the reactions in which the low-temperature rate-constant limit has been observed 1. hydrogen transfer in the excited singlet state of the molecule represented by (6.16) 2. molecular reorientation in methane crystal 3. internal rotation of CHj group in radical (6.25) 4. inversion of radical (6.40) 5. hydrogen transfer in halved molecule (6.16) 6. isomerization of molecule (6.17) in excited triplet state 7. tautomerization in the ground state of 7-azoindole dimer (6.1) 8. polymerization of formaldehyde in reaction (6.44) 9. limiting stage (6.45) of (a) chain hydrobromination, (b) chlorination and (c) bromination of ethylene 10. isomerization of radical (6.18) 11. abstraction of H atom by methyl radical from methanol matrix [reaction (6.19)] 12. radical pair isomerization in dimethylglyoxime crystals [Toriyama et al. 1977].

This reaction converts ribulose-5-P to another ketose, namely, xylulose-5-P. This reaction also proceeds by an enediol intermediate, but involves an inversion at C-3 (Figure 23.31). In the reaction, an acidic proton located a- to a carbonyl carbon is removed to generate the enediolate, but the proton is added back to the same carbon from the opposite side. Note the distinction in nomenclature here. Interchange of groups on a single carbon is an epimerization, and interchange of groups between carbons is referred to as an isomerization. 

Asano and co-workers have reported die kinetic effects of pressure, solvent, and substituent on geometric isomerization about die carbon-nitrogen double bond for pyrazol-3-one azomethines 406 (R = H), 406 (R = NO2) and 407, (Scheme 93). The results demonstrate the versatility of die inversion mechanism. The rotation mechanism has been invalidated. First-wder rate constants and activating volumes for diermal E-Z isomerization for 406 (R = H) and 406 (R = NO2) are given at 25°C in benzene and methanol (89JOC379). 

Hydrogenation of epoxides lends itself well to both synthetic applications and mechanistic studies. The reaction is complex, for either carbon-oxygen bond may break with or without inversion of configuration, and the product may contain deoxygenated products (92,93) as well as ketones derived by isomerization (26). The reaction is especially sensitive to both catalyst and environment (74). 

Variable-temperature HNMR spectroscopic studies on l-phenyl-3/7-naphtho[l,2-c]azepine and the isomeric l-phenyl-37/-naphtho[2,l-c]azepine indicate that in the [l,2-c]-isomer severe steric interactions between the phenyl group and the adjacent naphthalene ring prevent ring inversion at room temperature.39... 

The Diels-Alder reaction of cyclopropenes with 1,2,4,5-tetrazines (see Vol.E9c, p 904), a reaction with inverse electron demand, gives isolable 3,4-diazanorcaradienes 1, which are converted into 4H-1,2-diazepines 2 on heating. The transformation involves a symmetry allowed [1,5] sigmatropic shift of one of the bonds of the three-membered ring, a so-called walk rearrangement , followed by valence isomerization.106,107... 

A comparison of the configuration of the substrates and reaction products shows that the oxiranyl anions arc configurationally stable under the reaction conditions. Only one example is known in which isomerization was observed. When the ci.v-tm-butyl-substituted epoxysilane27 was metalated and quenched with 2-cyclohexenone, addition product 27 was obtained under inversion of the anionic center. Presumably the strain created in forcing the ter/-butyl and the trimethylsilyl group cis on the oxirane ring facilitates the isomerization process13. 

When Wiberg and Pracht (1972b) synthesized 3,3-di-(trimethylsilyl)-l-phenyltri-azene by reacting benzenediazonium chloride with sodium di-(trimethylsilyl)amide they found a faintly yellow compound if the reaction was carried out at -78 °C and an orange form at — 20 °C. NMR spectra were consistent with (Z)/( )-stereoiso-merism. Measurement of the isomerization rates at various temperatures in ether and in pentane indicates that the mechanism involves an inversion transition state (13.5) and not a rotation, because the free reaction enthalpies are independent of the polarity of the solvent. 

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