Enthalpic anomeric effect

The gem-dioxy substitution stabilizing effect is called the enthalpic anomeric effect .5 If one takes alkanes as reference compounds, one finds... 

Table 1 Standard heats of formation [AH°f (gas) in kcal/mol] of selected compounds. Evidence of the stabilizing g-em-dioxy-substituent effects (= enthalpic anomeric effect)"... 

Taken from NIST Chemistry Webbook. b Estimated from CH2(CN)2 adding -7 kcal/mol (exchange of a CH2 group by a CHMe group). c Enthalpy difference indicates a destabilization effect for gem-disubstitution with respect to alkanes. d Enthalpy difference shows a stabilization effect for gem-dialkoxy-disubstitution with respect to alkanes. c Enthalpy difference shows increases of enthalpic anomeric effect for tertiary acetals compared to secondary acetals. 

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... 

Recently, Juaristi and Cuevas (55) presented an application of Franck s methodology based on temperature-independent enthalpy AH° values (see Section Il.F., Eqs. [17] and [18]) to estimate the enthalpic contribution to the C—S—C—P(0) anomeric effect as A// g= 14kJ/mol [llkJ/mol in terms (56), but see also Section V.A]. 

The approach originated by Booth is becoming increasingly accepted (12,18,113). Booth et al. have recently presented an estimation of the enthalpic anomeric effect in 2-methoxytetrahydropyran (39) (p. 186), 2-(2, 2, 2 -trifluoro-ethoxy)tetrahydropyran (24) (112) (Table 3), and carbomethoxy (COOMe) and cyano (CN) derivatives of tetrahydropyran (30 and 33, respectively. Table 4) and piperidine (31 and 34, respectively) (42). Unfortunately Franck s suggestion (Section II.C) has been overlooked in this endeavor, and AH values have been evaluated (42, 50,71, 112) according to Eq. [17]. 

Very recently Juaristi and Cuevas (55) estimated the enthalpic contribution AW g to the S—C—P(0) anomeric effect following Franck s method (according to Eq. [17], given by Eq. [18] see also Section II.C) ... 

Roversi E, Scopelliti R, Solari E, Estoppey R, Vogel P, Brafia P, Menendes B, Sordo JA. The hetero-Diels-Alder addition of sulfur dioxide to 1-fluorohuta-l,3-dienes the sofa conformations preferred by 6-fluorosultines (6-fluoro-3,6-dihydro-l,2-oxathiin-2-oxides) enjoy enthalpic and conformational anomeric effects. Chem. Eur. J. 2002 8... 

Free radical chemistry continues to be of importance in the field and a survey has been published on polar and enthalpic effects in free radical reactions. A further short review dealt with free radicals, carbenes and nitrenes at the anomeric centre of carbohydrates. A useful survey has appeared of the application of carbohydrate based chiral auxilliaries in stereoselective syntheses which covers a range of reactions, for example cycloaddition processes, reductions and Strecker reactions. ... 

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