Steric and stereoelectronic effects in saturated heterocycles I. Small molecular fragment constituents. Theory vs. experiment

Stereoelectronic Effects in Negatively and Positively (Protonated) Charged Species. Ab Initio Studies of the Anomeric Effect in 1,3-Dioxa Systems

“…The relative stabilities of the protonation products, i.e., the conformers of methoxymethanol show that the gg form is the most stable one, followed by g - g + , ag + , and (high up) aa. This is in accord with earlier findings 2j, except that no minimum was observed for the g - a form, which rather converged to the g - g + form. In methoxymethoxide, the gauche form is considerably favored over the anti form (5.8 kcal/mol), due both to appreciable OC−H---O - hydrogen bonding (the H---O distance is 2.46 Å) and to the double anomeric effect: strong O - n−σ* C-O interactions in both conformers but an On π −σ* C-O - interaction only in the gauche form.…”

“…The stereoelectronic behavior of C−X−C−Y−C-containing molecular systems, known as the anomeric effect , has been extensively studied and documented, − including by ourselves, and is fairly well understood. It is now well accepted 1 that the anomeric effect in a R−X−C−Y−R‘ system consists of a difference of properties and behavior between its anti ( a ) and gauche ( g ) forms. This is due to an Xn π −σ* C-Y two-electron−two-orbital interaction 3 (or a negative hyperconjugation 4 in valence bond terms) and is manifest in (1) structural parameters, e.g., shorter anomeric bonds at the donor O−C and longer ones at the acceptor C−O and wider anomeric bond angles; (2) relative energy, i.e., greater stability of gauche (axial) forms over anti (equatorial) ones; and (3) stereoselective reactivity, i.e., variation of rates of attack at or around the anomeric center, all those as a function of torsion angles in R−X−C−Y−R‘.…”

Mechanism of Formation and Stabilities of the New Dioxadiazadecalin Systems. Ring−Chain Tautomerism1

“…It can be easily interpreted as a consequence of the above ring−chain tautomerism, especially in the second stage of DODAD closure. One should, however, remember that we deal here with systems bearing, R−N−C−O−R‘ units, which are prone to the known anomeric effect . , In fact, two unequal anomeric effects may operate in the appropriate conformation: a weak lp O −σ* C - N one, accompanying the strong, synergized lp N −σ* C - O anomeric interaction, due to the good lp donor (N) anti positioned to an excellent acceptor (O) (see below). Notably, the valence bond representation of this molecular orbital mixing, is a double bond−no bond resonance, the thermodynamic counterpart of which is the above-discussed ring−chain tautomerism.…”

Conformational analysis of trans‐2‐halocyclohexanols and their methyl ethers: a 1H NMR, theoretical and solvation approach

“…The energy difference between the conformations should not be explained only in terms of dipole-dipole and steric effects, but also as being due to an attractive interaction between the two heteroatoms, known as the 'gauche effect.' 1,11,[22][23][24][25][26][27][28] For the halohydrins, the main factor governing the conformational equilibrium is intramolecular hydrogen bonding, mainly when the compounds are diluted in non-polar solvents, such as in CCl 4 solution.…”