Stability of intermediates

While the thermodynamic stability may be deduced from experimental determinations of the concentration of intermediates, these concentrations are hard to determine in situ. In the numerical models the thermodynamic stability of the intermediates may easily be calculated. Actually, accurate calculations of the thermodynamic stability of the intermediates is a necessity for approaches starting at the description of the intermediates at the atomic level. 

The calculated enthalpy and Gibbs free energy for the intermediates [396] are illustrated in Fig. 2.4 and 2.5. One arrives at somewhat different conclusions on the relative stability of the intermediates depending whether-one bases the conclusions on the enthalpy or on the Gibbs free energy. As a consequence there is no simple relation between the heat of formation and the equilibrium concentration. 

As the model by Stoltze and Norskov is based on a description of the spectroscopic properties of the intermediates, it is straightforward to use data 

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Cobalt trifluoride fluorination corresponds to the electron-transfer mechanism via a radical cation. RF groups attached to the ring enhance the stability of intermediate dienes and monoenes. Perfluoroalkyl pyridines, pyrazines, and pyrimidines were successfully fluorinated but pyridazines eliminated nitrogen. The lack of certain dienes was attributed to the difference in stability of FC=C and RFC=C and steric effects [81JCS(P1)2059]. 

The temperature regimes for the stability of intermediates is different for various transition metals. For example on Fe(lll) the adsorbed ethylene decomposes partially at 200 K, while the conversion to surface carbon is complete at 370 K. Similarly, on nickel faces molecular chemisorption of ethylene is restricted to temperatures below ambient. At temperatures between approximately 290 K and 450 K ethylene chemisorption on nickel... 

Stabilization of intermediates by strong adsorption will frequently be a necessary precondition for synthesis. Thus, in the case of the Kolbe reaction, further oxidation of the radicals is prevented the formation of metal-carbon bonds in the reduction of alkyl halides (Fleischmann et al., 1971a Galli and Olivani, 1970) or oxidation of Grignard reagents (Fleischmann et al., 1972c) is shown by the isolation of organometallic... 

This formation of smaller polyhedra when larger substituents are present is not surprising when the stability of intermediate silanols formed in these reactions toward polycondensation is taken into account. The methylsilanols condense... 

This reaction type has been intensely studied °. The application of highly polar solvents, catalysis with tertiary amines" or with acids mesomeric stabilization of intermediate carbenium ions " (allylic and benzylic systems propargylic systems" ) as well as derivatives of sulfinic acids with increasing acidity - usually indicate an ionic pathway (intra- and/or inter-molecular) ... 

That resonance stabilization of intermediate biradicals is important in determining the efficiency of decarbonylation follows from the following examples yielding benzyl radicals upon loss of carbon monoxide(57) ... 

As a result, diastereo- and enantiomerically pure functionalized nitro derivative (205) was synthesized in satisfactory yield. This compound was used in the ISOC procedure, which gave diastereo- and enantiomerically pure isoxazolidine (206) in good yield. The latter compound can be considered as a possible reagent for asymmetric synthesis. It should be noted that silylation in this procedure, like that described above (Scheme 3.145), was performed with the use of BSA as the silylating agent in the presence of a small amount of Hunig s base, the latter being evidently added for acceleration of silylation and stabilization of intermediate silyl nitronate. 

Retro-[4 +2]-cycloaddition of dihydrooxazines of type C was considered in Section 3.5.1.3. An analogous process was described for N-alkyl- (490) or N-trimethylsily 1-27/ - dihydrooxazines (491). However, 2H- dihydrooxazines dealt with in studies (490, 491) could be isolated or, at least, characterized. At the same time, accumulation of heteroatoms bound to each other dramatically decreases the stability of intermediates C. 

Hg(OAc)2 easily reacts with allene to yield methoxymercuration products, i.e. vinylmercury compound 41, with the trans isomer being the major product. The chirality of allene was transferred into the final products, indicating the intermediacy of a cr-bridged mercurinium ion [23-26], The stereoselectivity of this reaction was determined by the relative stability of intermediates 40 and 42 and steric hindrance for the incoming methoxy group. 

Further work (10) with acid effects in the radiolysis of binary mixtures such as benzene-methanol and pyridine-methanol indicates that the acid phenomenon is more complicated than the simple H atom model originally developed ( ). These more recent experiments (10) show that whilst increased hydrogen atom yields in the presence of acid enhance the overall grafting yield, other mechanisms also contribute to this acid effect. Thus the acid stability of intermediate radicals (I-III) and also analogous species involving the trunk polymer are important. With radicals (I-III), at low styrene concentrations in methanol, these intermediates (MR-) will predominantly react with other available... 

Hammerum, S. Formation and Stabilization of Intermediate Ion-Neutral Complexes in Radical Cation Dissociation Reactions. J. Chem. Soc., Chem. Commun. 1988, 858-859. 

Numerous suggestions have been made that enzymes might owe part of their catalytic efficiency to the opportunity they afford for stabilization of intermediates or transition states by hydrogen bonding to functional groups near the active site. For example, in the case of (x-chymotrypsin this might be represented as in [43] where... 

This may be rationalized by considering the stability of intermediate addition cations. When the electrophile attacks at C-5 or C-8, the intermediate cation is stabilized by resonance, each having two favourable forms that do not perturb the aromaticity of the pyridinium system. In contrast, for attack at C-6 or C-7 there is only one such resonance form. We used similar reasoning to explain why naphthalene... 

Because many of them are nearly inert, ionic liquids have been used to stabilize highly polar or ionic transition states. Ionic liquids provide favorable media for the formation and stabilization of intermediates in reactions that proceed through charged intermediates. An example is the Baylis-Hillman reaction catalyzed by 1,4-diazabicyclo (222). octane (DABCO) (Scheme 8) (162). 

In a molecule, fluorine atoms influence bond energies, electronic distribution, acidity, hydrogen bonds, steric interactions, and the stability of intermediate entities in a transformation. These factors, which have great influence on chemical reactivity, are examined. 

Solvolyses go by an S l mechanism. Relative rates of different reactants in S l reactions depend on the stabilities of intermediate carbocations. H2C=CHCH2C1 is more reactive because... 

Decreasing resonance stabilization of intermediates by altering the position of amine or replacing electron-conducting intercyclic linkages... 

Proof of concept for this route was established by activating acid 9 with N,N -carbonyldi-imidazole (CDl) and treating it with Meldrum s acid to afford adduct 24 (Scheme 5.14). Despite its relatively high instability, adduct 24 was isolated by crystallization after aqueous work-up. Isolated 24 was easily converted to 25 in moderate yield by treatment with triazole salt 3 and Hunig s base. Because of the limited stability of intermediate 24 and in order to maximize the efficiency of the... 

The performance of a number of single oxides of transition metals was studied by Skorbilina et al. [295] using a differential reactor. As usual, o-tolualdehyde, phthalic anhydride and carbon oxides are the main reaction products. The initial selectivity with respect to partial oxidation products decreases in the order Co > Ti > V > Mo > Ni > Mn > Fe > Cu from 71% to 33%. The relatively high initial selectivities demonstrated by the deep oxidation catalysts (e.g. Co, Ni, Mn) indicates that the primary activation is probably the same for all these catalysts, while the differences that actually determine the character of the catalyst are connected with the stability of intermediates and products. 

The relatively easy decarboxylation of a- (682) and y-carboxylic acids is a result of inductive stabilization of intermediate ylides of type (683) (cf. Section 3.2.1.8.2). By carrying out the decarboxylation in the presence of aldehydes or ketones, products of type (684) are formed (Hammick Reaction). 

Stabilization of Intermediate Nitrenes Nitrenes Reaction products... 

Styrene, unlike propylene, produces an alkyl intermediate which is stable enough to be readily detectable, and the branched nitrile product 20 is strongly favored over the linear one. This unusual behavior can be attributed to stabilization of intermediate 21 through donation of ring electrons to the... 

Display the electrostatic potential map for 2-methyl anisole. For which ring site, para to methyl or para to methoxy, is the electrostatic potential more negative Where do you expect electrophilic attack to occur Is your result consistent with the relative stabilities of intermediate benzenium ions formed upon addition of the electrophile Compare energies for 3-methyl-4-methoxybenzenium ion and 4-methyl-3-methoxybenzenium ion. 

Tokuyama Soda commercialized a catalytic process for propylene hydration catalyzed by aqueous solutions of heteropolyacids such as H3PW12O40 (165). In Table XV, the activities of various acids are compared at a constant proton concentration. H3PW12O40 is two or three times more active than H2S04 or H3PO4. The reason for the high activity is assumed to be the stabilization of intermediate propyl cations by coordination. 

In the first step the free hydroxy group is protected as a ten-butyl dimethyl si lyl ether Then the lactone is reduced to a lactol, which is the cyclic herniacetal of an aldehyde. Overreduction to an alcohol can be prevented by stoichiometric addition of the reducing reagent at low temperature. Selectivity here depends on the relative stability of intermediate 19, which decomposes only in the course of workups (see Chapter 3). 

Among metal salts other than MHaln, metal carboxylates (and especially acetates) are sometimes used in the synthesis of metal alkoxides. Their advantage in reactions with M OR lies in considerably lower stability of intermediates — metal alkoxoacetates in comparison with metal alkoxohalogenides. 

The potential surface for die gradient path addition of ethylene to silene and the possible existence and stability of intermediates in the thermal decomposition reaction of silacyclobutane has been explored.38 The energy maximum of die multi-step process corresponds to a cyclic transition state leading on one side to a planar silacyclobutane transition state which falls to ground-state puckered silacyclobutane and on the other side to a trans diradical which fragments to ethylene and silene. 

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