Acetals, relative stability

Fig. 4 Acetals relative stability depends on at least four dipoles.

Enamines derived from 1-azabicycloalkanes, readily accessible by mercuric acetate oxidation of saturated bases (112), have been extensively studied recently (113-115). Since an immonium salt is formed during dehydrogenation, the composition of the liberated enamine mixture shows the relative stability of the various possible isomers. The study of infrared and NMR spectra has shown that the position of the enamine double bond is determined by factors similar to those determining the relative stability of simple olefins. 

Both thermodynamic and kinetic factors need to be considered. Take, for instance, acetic acid. The liquid contains mostly dimer but the crystal contains the catemer and no (polymorphic) dimer crystal has ever been obtained. Various computations (R. S. Payne, R. J. Roberts, R. C. Rowe, R. Docherty, Generation of crystal structures of acetic acid and its halogenated analogs , J. Comput. Chem, 1998, 19,1-20 W. T. M. Mooij, B. P. van Eijck, S. L. Price, P. Verwer, J. Kroon, Crystal structure predictions for acetic acid , J. Comput. Chem., 1998, 19, 459-474) show the relative stability of the dimer. Perhaps the dimer is not formed in the crystal because it is 0-dimensional and as such, not able to propagate so easily to the bulk crystal as say, the 1-dimensional catemer. 

As conformational anomeric effects represent only a fraction (—1 to —3 kcal/mol) of the global enthalpic anomeric effect or gem-dioxy stabilizing effect (—6 to —17 kcal/mol), additional factors have to be taken into account. Depending on substitution, steric factors can affect the relative stability of acetal conformers. Dubois et al.2S have demonstrated that in furanose pyranose derivatives a bulky substituent at the furanose... 

With due consideration of the explanations just presented for the observed, relative stabilities of cyclic acetals derived from polyols, in terms of their constitution and conformation, nearly all of the following observations on the selective hydrolysis of cyclic acetals of alditols and dialkyl dithioacetals may be readily understood. 

Hydrolytic studies on 1,2.4,5-di-O-isopropylidene-D-mannitol showed56 that the two acetal rings are cleaved at similar rates, suggesting for the relative stabilities of the two rings, a = a-erythro. 

The major mechanistic and structural aspect of the acetalation process is its orientation toward derivatives obtained either under thermodynamically controlled conditions or under kinetically controlled conditions. We will not discuss here all structural factors concerning the relative stabilities of acyclic and cyclic acetals of polyols and monosaccharides, because such a discussion has been extensively reviewed and adequately commented on [8,10,12 -14]. However, it is important to focus here on the main consequences of these relative stabilities in relation to the various experimental conditions to orientate the choice of specific conditions, particularly for the most important monosaccharides (D-glucose, D-mannose, and D-galactose). 

Acetals and Related Functions 5 B, and respectively. The remaining conformers , , and F possess a plane of symmetry. Consequently, the acetal function can in principle exist in the six different conformations A-F. Experimental evidence will be described which shows that the relative stabilities of these various conformers depend on stereoelectronic effects and the standard steric interactions. 

Fig. 10) has been carried out (52) and the result was compared with those of the monomethyl acetals 50 and 1, and the gem-dimethyl acetals 52 and 53. Each pair of isomers gave essentially an identical result, i,e. 33% of the equatorial and 77% of the axial isomer ( 2%) after equilibration in benzene at 70°C. This corresponds to a aG of =0.8 kcal/mol, a value close to that predicted for the relative stability of rotamer A over E (0.6 kcal/mol). 

In an ab initio study of the substituent effects on the relative stability of the E and Z conformers of 4-X-substituted phenyl acetates (16 R = Me) and trifluoroacetates (16 R = CF3) (Scheme 6), it was shown that increasing electron-withdrawing ability... 

Concerning the relative stabilities of the two forms, we have already seen (Section IV) that the two tautomers are probably of comparable intrinsic stability. Without forgetting the difficulties of drawing conclusions about relative stabilities from studies of chemical reactivity, it may nevertheless be added that this situation is to some extent corroborated by chemical evidence indicating, for example, the formation in almost equal amounts of 7- and 9-acetylpurine on treatment of the base under appropriate conditions with acetic anhydride, the 7 position being slightly favored.165 The explanation... 

The Paterno-Buchi photocycloaddition of silyl 0,X-ketene acetals (with X—O, S, Se) and aromatic aldehydes was intensively investigated by Abe and coworkers in the last decade [62]. The regioselectivity of the reaction (71 vs. 72) is highly affected by the heteroatom (Sch. 19) [63,64]. The regioselectivity is rationalized by (a) the relative stability of the 1,4-biradicals and (b) the relative nucleophilicity of sp2-carbons in the respective 0,X-ketene acetal. 

Experiments on the bromination of equilibrated ketone-acetal systems in methanol were also recently performed for substituted acetophenones (El-Alaoui, 1979 Toullec and El-Alaoui, 1979). Lyonium catalytic constants fit (57), but for most of the substituents the (fcA)m term is negligible and cannot be obtained with accuracy. However, the relative partial rates for the bromination of equilibrated ketone-acetal systems can be estimated. For a given water concentration, it was observed that the enol path is more important for 3-nitroacetophenone than for 4-methoxyacetophenone. In fact, the smaller the proportion of free ketone at equilibrium, the more the enol path is followed. From these results, it can be seen that the enol-ether path is predominant even if the acetal form is of minor importance. The proportions of the two competing routes must only depend on (i) the relative stabilities of the hydroxy-and alkyoxycarbenium ions, (ii) the relative reactivities of these two ions yielding enol and enol ether, respectively, and (iii) the ratio of alcohol and water concentrations which determines the relative concentrations of the ions at equilibrium. Since acetal formation is a dead-end in the mechanism, the amount of acetal has no bearing on the relative rates. Bromination, isotope exchange or another reaction can occur via the enol ether even in secondary and tertiary alcohols, i.e. when the acetal is not stable at all because of steric hindrance. 

Monomers that yield radicals in which the unpaired electron isextensively delocalized have ground state structures that are themselves resonance stabilized. The important factor is the relative stability of the product radical, however, because a single electron is more easily delocalized than one in a C=C double bond. Thus resonance stabilization causes as increase in monomer reactivity and a decrease in reactivity of the resulting polymer radical. Styrene is more reactive toward polymerization than vinyl acetate, for example, and the propagation rate in the former polymerization is much slower than in the radical synthesis of polyfvinyl acetate). 

The conformational flexibility postulated for the 2,4 3,5-diacetals of mannitol would disappear in acetals prepared from higher aldehydes and therefore possessing substituents at the acetal carbon atoms, but the status of such acetals is at present obscure., Two isomers of l,6-dichloro-l,6-dideoxy-2,3,4,5-di-O-ethylidene-D-mannitol are known, but the structures have not been determined. Two stereoisomers with the 2,4 3,5-structure and related to the H-inside and 0-inside conformations LVI and LVII, respectively, are clearly possible, but the structures 2,3 4,5 and 2,5 3,4 must also be considered. There is considerable scope for more work on 2,3,4,5-diacetals of mannitol, and confirmation of the structure XLVII advanced by Ness, Hann and Hudson for their mono-O-benzylidene-mono-O-methylene derivative is a necessary step in the assessment of the relative stability of the different structural types. 

One of the most fascinating areas of metal-cyclopentadienyl chemistry in recent years concerns the formation and relative stabilities of metal-locenyl-carbonium ions. The ability of iron to stabilize cationic centers in certain ferrocene compounds was noted by Weliky and Gould 243) in 1957. In 1959 Richards and Hill 218y 214, ) reported the results of some kinetic studies relating to the relative rates of solvolysis of metal-locenylmethylcarbinyl acetates. They determined that these solvolyses proceeded via a carbonium ion mechanism and found that the ferrocenyl acetate, for example, solvolyzed nearly seven times faster than did tri-phenylmethyl acetate under the same conditions. 

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