Gauche-oxygen effects

The gauche-oxygen effect in 1,2-DME can be described approximately by the one- and two-center terms for the OCH2—CH2O fragment. Although the electrostatic interaction, mainly of the 0---0 pair, disfavors the G-conformation. 

It is quite clear from the data that there is substantial "excess attractive interaction in compound 4 i.e. the slight experimental preference for the axial conformation contrasts with a substantial calculated preference for the equatorial one the difference has been ascribed to the "gauche attractive effect" between oxygens (14,17). In contrast, for compounds 1 and 2 there is a substantial "excess" repulsion, i.e. the equatorial conformation is preferred more than calculated. The effect, which has been called "gauche repulsive effect" (14) is especially marked for gauche interactions between sulfur atoms, as in 2. Only in the case of compound 3 is the difference between calculated and observed preference for the equatorial position sufficiently small to make any conclusion uncertain, especially in view of the fact that, in the absence of energy minimization, the calculated energy difference (1.50 kcal/mol) may be somewhat overestimated. 

The preference for the gauche arrangement is an example of the anomeric effect. An oxygen lone pair is anti to fluorine in the stable conformation but not in the unstable conformation. 

Stereoelectronic effects and nonbonded interactions are non-cooperative in the reactions of (E)-allylboronates and x-heteroatom-substituted aldehydes. Thus, while transition state 8 experiences the fewest nonbonded interactions (gauche pentane type, to the extent that X has a lower steric requirement than R3), transition state 9 is expected to benefit from favorable stereoelectronic activation (Felkin-type)58f. This perhaps explains why the reaction of 2,3-[iso-propylidenebis(oxy)]propanal and ( >2-butenylboronate proceeds with a modest preference (55%) by way ol transition state 9. This result is probably a special case, how ever, since C-3 of 2.3-[isopropylidenebis(oxy)]propanal is not very stcrically demanding in 9 owing to the acetonide unit that ties back the oxygen substituent, thereby minimizing interactions with the... 

Stereoelectronic factors are also important in the conformational dynamics of acyclic acetals [6] (Cosse-Barbi and Dubois, 1986). Here the usual preference for staggered conformations is supplemented by the anomeric effect (Kirby, 1983), which favours the gauche stereochemistry ( = 60°) about both central C-O bonds, mainly because this allows optimal n-rr overlap between an oxygen lone pair and the antibonding (cr ) orbital of the C-0 bond. Thus the pathway for conformational isomerization suggested by... 

The picture is less clear when free electron pairs on other heteroatoms (O, S, etc.) are involved. There is only one clear-cut case providing evidence concerning endocyclic sulfoxides The unusually large shieldings of carbon atoms in gauche position relative to an axial oxygen atom in thiane oxides 79, 80, 82, and 83 (p. 252) (200,201) have been partially ascribed to an upfield-shifting effect of the equatorial antiperiplanar lone pair (cf. Section III-C). 

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