Equilibria configurational

Compounds in which conformational, rather than configurational, equilibria are influenced by the anomeric effect are depicted in entries 4—6. Single-crystal X-ray dilfiaction studies have unambiguously established that all the chlorine atoms of trans, cis, ira j-2,3,5,6-tetrachloro-l,4-dioxane occupy axial sites in the crystal. Each chlorine in die molecule is bonded to an anomeric carbon and is subject to the anomeric effect. Equally striking is the observation that all the substituents of the tri-0-acetyl-/ -D-xylopyranosyl chloride shown in entry 5 are in the axial orientation in solution. Here, no special crystal packing forces can be invoked to rationalize the preferred conformation. The anomeric effect of a single chlorine is sufficient to drive the equilibrium in favor of the conformation that puts the three acetoxy groups in axial positions. 

Because the three-electron-bonded radicals are formed at the cost of the removal of the nitrogen p-electron, such cation-radicals should be considered as p-acids. Of course, the formation and behavior of these p-acids have to be dependent on steric factors. Works by Tomilin et al. (1996, 2000), Bietti et al. (1998), Dombrowski et al. (2005), and Yu et al. (2007) should be mentioned as describing stereoelectronic requirements to formations and configurational equilibria of A-alkyl-substituted cation-radicals. 

Position of Configurational Equilibria of 2-iPr-5-R-SuBSTiTUTED 1, 3-Dioxanes in Cyclohexane (77JOC1533)... 

Other workers have studied conformational and configurational equilibria by nuclear magnetic resonance spectroscopy. Horton and coworkers295ribopyranose tetraacetate exists in acetone-d6 at room temperature in the CE and CA conformations, in rapid equilibrium, in the ratio of 9 11. At —84°, the ratio of the CE and CA conformers was 2 1 at —60°, the rate of interconversion of the two conformers was found to be 130 times per second. The equilibrium... 

Thus, the most stable configuration of N-sulfinyl compounds is apparently (Z)-3 and for sulfur diimides, ( ,Z)-7 or -8. Interactions within a specific molecule can alter the relative stability of the isomeric forms so that in solution configurational equilibria may exist. 

Nickel(n) forms a large number of complexes encompassing coordination numbers 4, 5 and 6, and all the main structural types, viz., octahedral, trigonal bipyramidal, square-pyramidal, tetrahedral, and square. Moreover, it is characteristic of Ni" complexes that complicated equilibria, which are generally temperature-dependent and sometimes concentration-dependent, often exist between these structural types. In this Section we shall describe the characteristics of the individual structural types separately, using as examples mainly those compounds that exist completely or almost completely in one form or another. In the next Section we shall discuss the configurational equilibria. 

Compounds in which conformational, rather than configurational, equilibria are influenced by the anomeric effect are depicted in Entries 4 to 6. X-ray diffraction studies have unambiguously established that all the chlorine atoms of trans, cis, trans-2,3,5,6-tetrachloro-l,4-dioxane occupy axial sites in the crystal (Entry 4). Each chlorine... 

Consideration of the anomeric equilibria of various acetylated, 2,3-un saturated, pyranoid sugars led to the conclusion that polar, allylic substituents favor the quasi-axial orientation (0.8 kcal.mole for the acetoxyl group). Such an allylic effect was found to exert significant control over the conformational tendency and configurational equilibria of unsaturated, pyranoid sugar derivatives. Thus, tri-O-acetyl-2-hydroxy-D-xylal (41) adopts the H (d) conformation. In... 

A Study on the consequences of substitution in the aglycon on the configurational equilibria of neutral or protonated iV-aryl 5-thioglucopyranosylamines 35 concluded that there is no evidence in support of a generalized reverse anomeric effect. 

The question of the existence of the generalized RAE was addressed by systematic examination of substituent and solvent effects on the configurational equilibria of Ai-aryl-5-thioglucopyranosylamines 88-95 and Ai-arylglucopyranosylamines 96-98 (Fig. 2.32) and the corresponding protonated species [85]. 

Randell KD, Johnston BD, Green DF, Pinto BM (2000) Is there a generalized reverse anomeric effect Substituent and solvent effects on the configurational equilibria of neutral and protonated A-arylglucopyranosyl amines and W-aryl-5-thioglucopyranosylamines. J Org Chem 65 220-226... 

CONFIGURATIONAL EQUILIBRIA OF SOME TRANSITION METAL COMPLEXES IN SOLUTION. 

C3 configurational EQUILIBRIA OF SOME TRANSITION METAL COMPLEXES IN SOLUTION, H.C.A. King, E. K6r6s,... 

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