Antiperiplanar lone pair hypothesis theory

The antiperiplanar, lone-pair hypothesis is based on the presumption that the lifetime of a tetrahedral intermediate in hydrolysis is shorter than the average time of rotation about the C-O bond that is, the barrier of conformational transition exceeds the reaction barrier. However, a simple intermediate (hydrogen orthoester) has been detected, and on this basis, the antiperiplanar, lone-pair theory was questioned.Similarly, studies on the hydrolysis of saccharide derivatives 4os indicated that the departure of... 

Figure 3.10 Ideas behind the theory of stereoelectronic control or antiperiplanar lone pair hypothesis (ALPH) Antiperiplanar (sp ) lone pairs are shown shaded.

If it is assumed that effects on the ground state are amplified in the transition state, this purely empirical thermodynamic anomeric effect is transformed into a predictive kinetic anomeric effect (Kirby, 1983). In the transition state for breakdown of a tetrahedral species containing the X—C—X—C fragment, breakdown of the isomer with the scissile bond antiperiplanar to a heteroatom lone pair will proceed more rapidly, under stereoelectronic acceleration (Scheme 17). The theory has obvious utility in predicting product distribution when a single species may break down by a number of possible pathways (Deslongchamps, 1983). However, the hypothesis has not been without its detractors. Some of the experimental work used... 

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